Fungal accessory enzymes for the cleavage of hemicellulose-derived residues.
\r\n\tThe sense of proprioception includes various aspects or submodalities such as position sense, motion sense (kinaesthesia; including the duration, direction, amplitude, speed, acceleration and timing of movements), force tension sense, and change in velocity sense.
\r\n\r\n\tProprioception is mediated by proprioceptors, a specialized subset of about 10-15% of mechanosensory neurons localized in dorsal root ganglia that convey information about the stretch and tension of muscles, tendons, joints and perhaps the skin. So, the neurological basis of proprioception originates from proprioceptors with contact specialized sensory organs in muscles (muscle spindles), tendons (Golgi tendon organs), joints (different morphotypes of sensory corpuscles including Ruffini’s corpuscles and Pacinian corpuscles) and the skin (cutaneous mechanoreceptors). Thereafter, the information originated in the proprioceptors forming complex nerve pathways reach the central nervous system at the level of the spinal cord, the cerebellum and the cerebral cortex for processing. Hence, proprioception can be regarded as a continuous loop of feedforward and feedback inputs between sensory receptors throughout the body and the nervous system.
\r\n\r\n\tIn limb and axial muscles, the proprioception originates in the muscle spindles. Nevertheless, the cephalic muscles, with the exception of the extraocular muscles and those innervated by the mandibular branch of the trigeminal nerve, lack muscle spindles. But the facial or pharyngeal proprioception plays key roles in the regulation and coordination of facial musculature and diverse reflexes. At the basis of these functional characteristics are the multiple communications between cranial nerves. Substituting muscle spindles by other kinds of proprioceptors might be at the basis.
\r\n\tOn the other hand, since the stimuli for proprioceptors are mechanical (stretch, tension, and so) proprioception can be regarded as a modality of mechanosensitivity. During the last decade progress has been made to understanding the molecular basis of mechanosensitivity. However, identity of mechanotransducers is poorly know. The mechanogated ion channels acid-sensing ion channel 2 (ASIC2), transient receptor potential vanilloid 4 (TRPV4) and PIEZO2 have been related to mechanotransduction and have been detected in proprioceptors innervating muscle spindles and Golgi tendon organs in mice. Also, mice lacking Piezo2 showed severely uncoordinated body movements and abnormal limb positions.
\r\n\tFinally, the lesion of the proprioception receptors, proprioceptors or the nerve center and pathways related to proprioception result in poor proprioception. Importantly, age-related changes also affect proprioception due to a combination of natural age-related changes to the central nervous system, nerves, joints, and muscles. Acute and long-term impairment can be related to toxicological, medical or injury conditions, but also with neuromuscular and central nervous system diseases.
\r\n\tBased on the above comments this book intends to provide a comprehensive update an overview of the anatomical, structural and molecular basis of proprioception as well as of the main causes of proprioception impairment and possible treatments.
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He completed his postdoctoral training at the universities of Brno and Prague (Czech Republic), 'La Sapienza' and 'Tor Vergata' in Rome specializing in peripheral nervous system and growth factors of the neurotrophin family. Currently he is a Professor of Anatomy and Human Embryology of the Department of Morphology and Cell Biology at the University of Oviedo, and Head of the research group SINPOS (Sensory Organs and Peripheral Nervous System) at the University of Oviedo. He has taught as a contracted professor, at the Universities of Messina, 'Federico II' of Naples, Rome 'La Sapienza' and Rome 'Tor Vergata', Sassari, Barí, and CEU-San Pablo at Madrid. 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In this context, biofuels are emerging as a new source of energy derived from biomass. The production of biofuels could decrease effectively the impact of pollutants in the atmosphere, in addition to assisting in the management for tons of biomass waste. Biomass (plant matter) can be referred to "traditional biomass", which is used in inefficient ways such as the highly pollutant primitive cooking stoves (wood), and "modern biomass" that refers to biomass produced in a sustainable way and used for electricity generation, heat production and transportation of liquid fuels [1]. In addition to these definitions, The International Energy Agency (IEA) defines biomass as any plant matter that could be used directly as fuel or converted into fuels, electricity or heat. Therefore, in order to provide useful management of biomass, it is clear that one needs to learn how to extract energy from plants.
Plant cells are mainly composed by lignocellulosic material, which includes cellulose, hemicellulose and lignin (lignocellulosic complex). The hydrolysis of lignocellulose to glucose is a major bottleneck in cellulosic biofuel production processes [2]. In nature, microorganisms, especially fungi, are able to degrade the plant cell wall through a set of acting synergistically enzymes. This phenomenon leads to glucose being released in a free form, which can enter the metabolism of the microorganism, providing its energy. A great challenge is to modify the architecture of the plant cell walls and/or the ability of the microorganisms to degrade it, by modifying their genomes. For instance, researchers can generate genetically engineered microorganisms able to degrade efficiently the polymers in the plant cells, producing sugars monomers that can be fermented directly by yeasts, generating ethanol. This chapter will describe the composition of plant cell walls and how microorganisms cope with the lignocellulosic material. The main focus will be on fungi cell wall degrading enzymes (CWDEs) and their genetic regulation. The aim of this chapter is to guide scientists in order to genetically improve microorganisms that can be able to efficiently degrade the plant biomass. Also, new strains could be great producers of CWDEs, providing enzymatic cocktails that can be introduced commercially. Finally, the future perspectives will demonstrate how far we are from cellulosic ethanol and other biomass-derived chemical compounds, regarding to research of microorganisms.
Plant cell wall [6] polysaccharides are the most abundant organic compounds found in nature. These compounds consist mainly of polysaccharides such as cellulose, hemicelluloses and pectin, as well as the phenolic polymer lignin. Together, the polysaccharides and lignin provide high complexity and rigidity to the plant cell wall. Cellulose, the major constituent of plant cell wall consists of ß-1,4 linked D-glucose units that form linear polymeric chains of about 8000-12 000 glucose units. In its crystalline form, cellulose consists of chains that are packed together by hydrogen bonds to form highly insoluble structures, called microfibrils. In addition to the crystalline structure, cellulose contains amorphous regions within the microfibrils (noncrystalline structure) [3].
Hemicelluloses are heterogeneous polysaccharides consisted by different units of sugars, being the second most abundant polysaccharides in plant cell wall. Hemicelluloses are usually classified according to the main residues of sugars present in the backbone of the structural polymer. Xylan, the most abundant hemicellulose polymer in cereals and hardwood, is composed by ß-1,4-linked D-xylose units in the main backbone, and can be substituted by different side groups such as D-galactose, L-arabinose, glucuronic acid, acetyl, feruloyl, and
Pectins are another family of plant cell wall heteropolysaccharides, containing a backbone of α-1,4-linked D-galacturonic acid. The polymers usually contain two different types of regions. The so-called "smooth" region of pectins contains residues of D-galacturonic acids that can be methylated or acetylated. In the other region, referred as "hairy" region, the backbone of D-galacturonic acids residues is interrupted by α-1,2-linked L-rhamnose residues. In the hairy region, long side chains of L-arabinose and D-galactose residues can be attached to the rhamnose residues [5]. Figure 2 depicts a schematic representation of the hairy region of pectins.
Lignin is a phenolic polymer that confers strength to plant cell wall. Lignin is a highly insoluble complex branched polymer of substituted phenylpropane units, which are joined together by ether and carbon-carbon linkages, forming an extensive cross-linked network within the cell wall. The cross-linking between the different polymers described above confers the complexity and rigidity of the plant cell wall, which is responsible for the protection of plant cell as a whole. In addition to offer protection against mechanical stress and osmotic lysis, the plant cell wall is an effective barrier against pathogens, including many microorganisms. However, during the course of evolution some microorganisms, in order to survive, developed efficient strategies to degrade plant cell wall components, mainly the polysaccharides [6].
Plant cell wall degradation mechanisms are pivotal for the lifestyle of many microorganisms, once they should be able to degrade the plant polymers to acquire nutrients from plants. For instance, saprophytic fungi inhabit dead organic materials like decaying wood and leaves. In order to take energy from these materials, these fungi need to produce enzymes capable of degrading the majority of plant cell wall polysaccharides present in the biomass. The main mechanism through which fungi and other microorganisms degrade plant biomass consists of production and secretion of enzymes acting synergistically in the plant cell wall, releasing monomers that can be used by the microorganism as chemical energy. The next section will discuss mechanisms of cell wall degrading enzymes (CWDE) production by fungi, the most important producers of carbohydrate-active enzymes.
Schematic representation of the three major hemicellulose structures. A. Xylan. B. Xyloglucan. C. Galactomannan (upper left) and Galactoglucomannan (lower right).
Schematic view of the hairy region of pectin.
In order to survive, microorganisms developed, during the course of evolution, physiological mechanisms to cope with a variety of environmental factors. The acquirement of nutrients represents a challenge for all living organisms, especially for microorganisms. Saprophytism, one of the most common lifestyle of microorganisms, involves living in dead or decaying organic matter, mainly composed by plant biomass. In this context, microorganisms developed cellular mechanisms in order to take energy from plant biomass, and one of this mechanisms involves the production and secretion of carbohydrate-active enzymes. These enzymes degrade the plant cell wall, releasing sugars monomers that can be used as substrates for the metabolism of the microorganism. The microbial use of plant biomass is pivotal for life on Earth, because it is responsible for large portions of carbon flux in the biosphere. In addition, plant cell wall-degrading enzymes (CWDEs) have a broad range of industrial applications, such as within the food and feed industry and for sustainable production of many chemicals and fuels.
The capacity to degrade lignocellulose is mainly distributed among fungi and bacteria. Cellulolytic bacteria can be found in different genus such as
Concerning to lignin degradation, many white-rot basidiomycetes and some actinomycetes are able to produce lignin-degrading enzymes, especially peroxidases. For instance,
The fungi
The plant cell wall-degrading machinery of aerobic and anaerobic microorganisms differs significantly, regarding to its macromolecular organization. The cellulase/hemicellulase apparatus of anaerobic bacteria is frequently assembled into a large multienzyme complex, named cellulosomes [14, 15]. This complex contains enzymes with a variety of activities such as polysaccharide lyases, carbohydrate esterases and glycoside hydrolases [16-18]. Basically, the catalytic components of the cellulosomes include a structure named dockerins, which are noncatalytic modules that bind to cohesin modules, located in a large noncatalytic protein acting as scaffold [15]. The protein-protein interaction between dockerins and cohesins allows the integration of the hydrolytic enzymes into the complex [19, 20]. It has been demostrated that scaffoldins are also responsible for the anchoring of the whole complex onto crystalline cellulose, through a noncatalytic carbohydrate-binding module (CBM) [21]. The main studies concerning cellulosomes are being focused on anaerobic bacteria, especially from
Fermentative production of ethanol is largely performed nowadays through the use of starch or sucrose provided by agricultural crops such as wheat, corn or sugarcane. In Brazil, for instance, the ethanol production through yeast fermentation of substrates from sugarcane is a well-known and consolidated process. However, the improvement of fermentative processes towards utilization of lower-value substrates such as lignocellulosic residues is emerging as a valuable approach for reducing the production cost and consequently increasing the use of ethanol as biofuel. In sugarcane mills, for instance, a large quantity of sugarcane bagasse, which is a great source of lignocellulosic residue, is produced as a by-product of the industrial process. The sugarcane bagasse can be used as a lower-value substrate to produce the so-called second generation ethanol, in other words the ethanol generated from lignocellulosic material. The conversion of lignocellulose to ethanol requires challenging biological processes that includes: (i) delignification in order to release free cellulose and hemicellulose from the lignocellulosic material; (ii) depolymerization of the carbohydrates polymers from the cellulose and hemicellulose to generate free sugars; and (iii) fermentation of mixed hexose and pentose sugars to finally produce ethanol [25]. Glucose presents approximately 60% of the total sugars available in cellulosic biomass. The yeast
In summary, many microorganisms are able to produce and secrete hemicellulolytic enzymes, but fungi are pointed as the most important microorganisms concerning the biomass degradation. The significance of secreted enzymes in the life of these organisms and the biotechnological importance of filamentous fungi and their enzymes prompted an interest towards understanding the mechanisms of expression and regulation of the extracellular enzymes, as well as the characterization of the transcription factors involved. The next sections of this chapter will discuss the fungal enzyme sets for lignocellulosic degradation and the gene expression regulation of these enzymes.
Fungi play a central role in the degradation of plant biomass, producing an extensive array of carbohydrate-active enzymes responsible for polysaccharide degradation. The enzyme sets for plant cell wall degradation differ between many fungal species, and our understanding about fungal diversity with respect to degradation of plant matter is essential for the improvement of new strains and the development of enzymatic cocktails for industrial applications.
Carbohydrate-active enzymes are usually classified in different families, based on amino acid sequence of the related catalytic module. An extensive and detailed database presenting these hydrolytic enzymes can be found at www.cazy.org (CAZymes,
Cellulose, a polysaccharide consisted of linear β-1,4-linked D-glucopyranose chains, requires three classes of enzymes for its degradation: β-1,4-endoglucanases (EGL), exoglucanases/cellobiohydrolases (CBH), and β-glucosidase (BGL). The endoglucanases cleave cellulose chains internally mainly from the amorphous region, releasing units to be degraded by CBHs and/or BGLs. The cellobiohydrolases cleave celobiose units (the cellulose-derived disaccahride) from the end of the polysaccharide chains [6]. Finally, β-glucosidases hydrolise cellobiose to glucose, the monomeric readily metabolisable carbon source for fungi [35]. These three classes of enzymes need to act synergistically and sequentially in order to degrade completely the cellulose matrix. After endo- and exo-cleaving (performed by EGLs and CBHs, respectively), the BGLs degrade the remaining oligosaccharides to glucose. A schematic view of cellulose degradation is depicted in the Figure 3.
The most efficient cellulose-degrading fungi is
Schematic view of cellulose degradation. Endoglucanases hydrolise cellulose bonds internally, while cellobiohydrolases cleave celobiose units from the ends of the polysaccharide chains. The released cellobiose units (disaccharide) are finally hydrolyzed by β-glucosidases, releasing glucose, the main carbon source readily metabolisable by fungi.
Hemicellulose is a complex polysaccharide matrix composed of different residues branched in three kinds of backbones, named xylan, xyloglucan and mannan. The complexity of hemicellulose requires a concerted action of endo-enzymes cleaving internally the main chain, exo-enzymes releasing monomeric sugars, and accessory enzymes cleaving the side chains of the polymers or associated oligosaccharides, leading to the release of various mono- and disaccharides depending on hemicellulose type.
Xylan, a polymer composed by ß-1,4-linked D-xylose units, is degraded through the action of ß-1,4-endoxylanase, which cleaves the xylan backbone into smaller oligosaccharides, and ß-1,4-xylosidase, which cleaves the oligosaccharides into xylose. Fungal ß-1,4-endoxylanase are classified as GH10 or GH11 [40], differing from each other in substrate specificty [41]. Endoxylanases belonging to family GH10 usually have broader substrate specificity than endoxylanases from family GH11 [33]. GH10 endoxylanases are known to degrade xylan backbones with a high degree of substitutions and smaller xylo-oligosaccharides in addition to degrade linear chains of 1,4-linked D-xylose residues. Thus, GH10 endoxylanases are necessary to degrade completely substituted xylans [42]. ß-Xylosidases are highly specific for small unsubstituted xylose olygosaccharides and they are important for the complete degradation of xylan. Some ß-xylosidases have been shown transxylosylation activity, suggesting a role for these enzymes in the synthesis of specific oligosaccharides [43, 44].
Xyloglucan consists of ß-1,4-linked D-glucose backbone substituted mainly by D-xylose and therefore requires endoglucanases (xyloglucanases) and ß-glucosidases action in order to be degraded. Some endoglucanases are specific for the substituted xyloglucan backbone, and they are not able to hydrolise cellulose [45]. Xyloglucan-active endoglucanases have specific modes of action. For instance, a xyloglucanase from
Mannans, also referred to galacto(gluco)mannans, consist of a backbone of ß-1,4-linked D-mannose (mannans) and D-glucose (glucomannans) residues with D-galactose side chains. The degradation of this type of hemicellulose is performed by the action of ß-endomannanases (ß-mannanases) and ß-mannosidases, commonly expressed by aspergilli [48]. The ß-mannanases cleave the backbone of galacto(gluco)mannans, releasing mannooligosaccharides. Several structural features in the polymer determine the ability of ß-mannanases to hydrolise the mannan backbone, such as the ratio of glucose to mannose and the number and distribution of substituents on the backbone [49]. It has been shown that ß-mannanase is most active on galactomannans with a low substitution of the backbone [50], and the presence of galactose residues on the mannan backbone significantly prevents ß-mannanase activity [51]. The main products of ß-mannanase activity on mannan are mannobiose and mannotriose. ß-Mannosidases act on the nonreducing ends of mannooligosaccharides, releasing mannose. As shown by substrate specificity studies, ß-Mannosidase is able to completely release terminal mannose residues when one or more adjacent unsubstituted mannose residues are present [52].
The complete degradation of hemicellulose is only achieved after release of all substitutions present on the main backbone. The high degree of substitution in the hemicellulose polymers requires the action of various accessory enzymes able to release all these substitutions from the polysaccharide. At least nine different enzyme activities distributed along 12 GH and 4 CE families are required to completely degrade the hemicellulose substituents [33].
Arabinose is one of the most common sugar residues in hemicellulose and is present in arabinose-substituted xyloglucan and (arabino-)xylan. The release of arabinose from the polymer is performed by α-arabinofuranosidases and arabinoxylan arabinofuranohydrolases. α-Arabinofuranosidases are mainly found in GH 51 and 54 families, and the differences in the substrate specificity between these enzymes could be exemplified by two arabinofuranosidases of
Another type of substituent present in hemicellulose is D-xylose. Hydrolases responsible for the release of D-xylose residues from the xyloglucan backbone are referred to α-xylosidases. These enzymes can differ with respect to the type of glycoside they can hydrolize. For instance, α-xylosidase II (AxhII) from
There are many other possible substituents in hemicellulose, such as L-fucose, α-linked D-galactose, D-glucuronic acid, acetyl group and
\n\t\t\t\t | \n\t\t\t\n\t\t\t\t | \n\t\t\t\n\t\t\t\t | \n\t\t
Xyloglucan/xylan | \n\t\t\tL-arabinose | \n\t\t\tα-arabinofuranosidases | \n\t\t
\n\t\t\t | \n\t\t\t | arabinoxylan arabinofuranohydrolases | \n\t\t
Xyloglucan | \n\t\t\tD-xylose | \n\t\t\tα-xylosidases | \n\t\t
Xyloglucan | \n\t\t\tL-fucose | \n\t\t\tα-fucosidases | \n\t\t
Xylan/galactomannans | \n\t\t\tD-galactose | \n\t\t\tα-galactosidases | \n\t\t
Xylan | \n\t\t\tD-glucuronic acid | \n\t\t\tα-glucuronidases | \n\t\t
Xylan | \n\t\t\tacetyl group | \n\t\t\tacetyl xylan esterases | \n\t\t
Xylan | \n\t\t\t\n\t\t\t\t | \n\t\t\t\n\t\t\t\t | \n\t\t
Xylan | \n\t\t\tferulic acid | \n\t\t\tferuloyl esterases | \n\t\t
Fungal accessory enzymes for the cleavage of hemicellulose-derived residues.
Pectins are composed of a main backbone of α-1,4-linked D-galacturonic acid, and consist of two regions: the "smooth" region and the "hairy" region. The "smooth” region contains residues of D-galacturonic acids that can be methylated or acetylated, while in the "hairy" region, the backbone of D-galacturonic acids residues is interrupted by α-1,2-linked L-rhamnose residues. Moreover, in the hairy region, long side chains of L-arabinose and D-galactose residues can be attached to the rhamnose residues (Figure 2). As observed for cellulose and hemicellulose, degradation of pectins also requires a set of hydrolytic enzymes to degrade completely the polymer. Glycoside hydrolases (GHs) and polysaccharide lyases (PLs) are the two classes of hydrolytic enzymes required for pectin backbone degradation.
The GHs involved in pectin backbone degradation include endo- and exo-polygalacturonases, which cleave the backbone of smooth regions, while the intricate hairy regions are further degraded by endo- and exo-rhamnogalacturonases, xylogalacturonases, α-rhamnosidases, unsaturated glucuronyl hydrolases, and unsaturated rhamnogalacturonan hydrolases [33]. Endo- and exo-polygalacturonases are able to cleave α-1,4-glycosidic bonds of α-galacturonic acids. Rhamnogalacturonases cleave α-1,2-glycosidic bonds between D-galacturonic acid and L-rhamnose residues in the hairy region of the pectin backbone [57]. An endo-xylogalacturonase from
Schematic view on a hemicellulolytic system, degradation of arabinoxylan is depicted. The arrows represent each enzyme active for a determined substrate.
The fungal PLs pectin and pectate lyases hydrolyze α-1,4-linked D-galacturonic acid residues in the smooth regions of pectin backbone [59]. Pectin lyases have preference for substrates with a high degree of methylesterification, whereas pectate lyases prefer substrates with a low degree of esterification. Moreover, pectate lyases require Ca2+ ions for catalysis while pectin lyases lack such ion requirement to catalysis [60]. The PL rhamnogalacturonan lyase cleaves within the hairy region of pectin and appears to be structurally different from pectin and pectate lyases. As presented by nailing reviews [33, 48], the pectin structures xylogalacturonan and rhamnogalacturonan require a repertoire of accessory enzymes to remove the side chains, providing access for the main-chain pectinolytic enzymes. The accessory enzymes endoarabinanases, exoarabinanases, β-endogalactanases, and several esterases are specific for pectin degradation, while α-arabinofuranosidases, β-galactosidases, and β-xylosidases are also required for hemicellulose degradation.
Lignin, a highly insoluble complex branched polymer of substituted phenylpropane units joined by carbon-carbon and ether linkages, provides an extensive cross-linked network within the cell wall, and it is known to increase the strength and recalcitrance of the plant cell wall. Microbial lignin degradation is often complicated, once the microbe needs to cope with three major challenges related to lignin structure: (i) enzymatic system to degrade the lignin polymer needs to be essentially extracellular, because lignin is a large polymer, (ii) the mechanism of enzymatic degradation should be oxidative and not hydrolytic, since the lignin structure comprises carbon-carbon and ether bonds, and (iii) lignin stereochemistry is irregular, requiring enzymes with less specificity than hydrolytic enzymes required for cellulose/hemicellulose degradation [61]. The most well characterized enzymes able to degrade the lignin polymer are lignin peroxidase (LiP), laccase (Lac), manganese peroxidase (MnP), versatile peroxidase, and H2O2-generating enzymes such as glyoxal oxidase (GLOX) and aryl alcohol oxidase (AAO).
Lignin and manganese peroxidases (LiP and MnP, respectively) catalyse a variety of oxidative reactions dependent on H2O2. LiP oxidizes non-phenolic units of lignin (mainly Cα-Cβ bonds) by removing one electron and creating cation radicals that decompose chemically [62]. MnPs differ significantly from LiPs, once they cannot oxidize directly non-phenolic lignin-related structures [63]. In order to oxidize non-phenolic lignin-related components, the oxidizing power of MnPs is transferred to Mn3+, a product of the MnP reaction: 2 Mn(II) + 2H+ + H2O2 → 2 Mn(III) + 2H2O [64]. In this way, Mn3+ diffuses into the lignified cell wall, attacking it from the inside [63].
Laccases oxidize phenolic compounds and reduce molecular oxygen to water. Lac catalyses the formation of phenoxyl radicals and their unspecific reactions, leading finally to Cα-hydroxyl oxidation to ketone, alkyl-aryl cleavage, demethoxylation and Cα-Cβ cleavage in phenolic substructures [61]. Versatile peroxidases (VPs) are able to oxidize phenolic and non-phenolic aromatic compounds, as well as Mn2+ [64].
In order to degrade lignin, microbes require sources of extracellular H2O2, to support the oxidative turnover of LiPs and MnPs responsible for ligninolysis. The hydrogen peroxide is provided by extracellular oxidases that reduce molecular oxygen to H2O2, with the synergistic oxidation of a cosubstrate. The most well characterized extracellular H2O2-generating enzymes are glyoxal oxidase (GLOX) and aryl alcohol oxidase (AAO).
Most studies on enzymatic lignin degradation rely on white-rot fungi, which can mineralise lignin to CO2 and H2O in pure cultures [65, 66]. Among these fungi are
In summary, microbial degradation of lignocellulosic material requires a concerted action of a variety of enzymes arranged in an enzymatic complex, depending on the biomass to be degraded. The gene expression, production and secretion of plant cell wall-degrading enzymes demand energy from the microbial cells and therefore the overall process is highly regulated. There is an intense cross-talk in induction of expression of the genes encoding different classes of enzymes. The control of the regulation of CWDEs production could be the key for the development of new microbial strains that efficiently produce and secrete CWDEs. The regulation of genes encoding polysaccharide-degrading enzymes will be the subject of the next section of this chapter.
The production of CWDEs by fungi is an energy-consuming process. The fine-tuned regulation of genes encoding CWDEs ensures that these enzymes will be produced only under conditions in which the fungus requires plant polymers as carbon source. Readily metabolizable carbohydrates repress the synthesis of enzymes related to catabolism of alternative carbon sources such as plant cell wall polysaccharides. In this way, preferential utilization of the most favored carbon source prevails, and one of the regulatory mechanisms involved in this adaptation is carbon catabolite repression (CCR). The CCR is activated by many carbon sources, depending on the lifestyle of the microorganism, but usually glucose is the most repressive molecule [75]. Nowadays, the search for microorganisms able to efficiently degrade lignocellulosic biomass is pivotal for the establishment of sustainable production of biomass-derived ethanol and other biocompounds. In this context, CCR appears as a major challenge to overcome, once this mechanism is responsible for enzymatic exclusion of less preferred carbon source such as lignocellulose-derived sugars. Hence, the comprehension of molecular mechanisms behind CCR, as well as the transcriptional control of cell wall derived enzymes are prerequisite in order to develop new microbial strains for lignocellulose degradation. In this section, the induction of expression of cellulases and hemicellulases, the transcriptional control of genes encoding CWDEs and the overall mechanism behind CCR will be discussed.
Although the biochemistry of the process behind lignocellulosic degradation has been studied in detail, the mechanism by which filamentous fungi sense the substrate and initiate the overall process of hydrolases production is still unsolved. Some researchers have been proposed that a low constitutive level of cellulase expression is responsible for the formation of an inducer from cellulose, amplifying the signal. Another group of scientists suggest that the fungus initiates a starvation process, which could in turn activates cellulase/hemicellulase expression. Also, it is possible that an inducing sugar derived from carbohydrates released somehow from the fungal cell wall could be the derepressing molecules for hydrolase induction. Despite of the fact that the true mechanism behind natural cellulase/hemicellulase induction is still lacking, some individual molecules are known to induce these hydrolases.
The fungus
Cellobiose (two β-1,4-linked glucose units) appears to induce cellulase expression in many species of fungi. Cellobiose is formed as the end product of cellobiohydrolases activity, and it has been show to induce cellulase expression in
Lactose (1,4-
Moreover, induction of cellulase genes could be achieved in
Hemicellulase expression has been studied mostly in Aspergilli and
The monosaccharide D-xylose is a well-known inducer of xylanolytic enzymes in
The genes encoding enzymes responsible for the degradation of arabinoxylan in
Regarding to pectinolytic enzymes, D-galacturonic acid, polygalacturonate and sugar beet pectin have been shown to induce virtually all the genes encoding for pectin degradation enzymes in
In
Complex mixtures of polysaccharides have been shown to induce a wide range of cellulases/hemicellulases genes in
The ligninolytic system of many fungi appears to be induced under nutrient deprivation, mainly nitrogen, carbon and sulphur. Therefore, the expression of most of the ligninolytic genes is regarded as a stress response to nutrient deprivation. Also, the presence of Mn(II) is required for induction of manganese peroxidase (MnPs) genes in the white-rot fungi
Laccases are multicopper oxidase proteins, and therefore can be induced by copper, although other metals can induce the expression of laccase genes as well, such as manganese and cadmium [105, 106]. Many natural and xenobiotic aromatic compounds, which are often related to lignin or humic substances, were shown to induce genes related to laccases [107]. In general, it has been postulated that laccases are the first enzymes degrading lignin, and possible further degradation products released from the polymer could act as inducers to amplify laccase expression, and subsequently induce other ligninolytic genes [105].
A number of genes encoding plant cell wall degrading enzymes appears to present in their promoter regions regulatory elements for binding of transcriptional activators. The filamentous fungi
Complementation by transformation of an
The XlnR transcriptional activator belongs to the class of zinc binuclear cluster domain proteins (PF00172) [113]. The DNA-binding domain is found in the XlnR at the N-terminal of the protein and, in addition to this domain, a fungal specific transcription factor domain is also present (PF04082) [110]. Functional studies have been demonstrated that a putative coiled-coil domain is important for the XlnR function, as the disruption of the α-helix structure (Leu650Pro mutation) lead to cytoplasmic localization and loss of function of XlnR, due to a loss of transcription of the structural genes of the regulon [114]. As demonstrated by the same study, a C-terminal portion of XlnR appeared to be involved in transcriptional regulation, as a deletion of some amino acids of the C-terminus increased the expression of XlnR target genes, even under D-glucose repression conditions [114]. Efforts have been done in order to evaluate the behavior of XlnR regulon to optimize the expression of target genes. For instance, a modeling study for the observation of XlnR regulon dynamics under D-xylose induction was performed. In this study, it was demonstrated that regulation of the
In
Ace2 belongs to a zinc-binuclear cluster DNA-binding protein and appears to be an activator of cellulase and hemicellulase genes in cellulose-induced cultures of
The enzymatic system responsible for plant polysaccharide degradation is induced and commonly amplified after releasing of the plant cell wall polymers components. Two important components of the plant cell wall are D-xylose and L-arabinose, present in the polymers arabinan, arabinogalactan, xyloglucan and xylan. In Aspergilli, D-xylose and L-arabinose are catabolized through the pentose catabolic pathway, PCP [120], consisting of a series of reversible reductase/dehydrogenase steps culminating with the formation of D-xylulose-5-phosphate, which enters the pentose phosphate pathway (PPP). In
Furthermore, it was found that in
AmyR is another transcriptional activator found in Aspergilli. AmyR was first described as a transcriptional regulator of genes encoding enzymes involved in starch and maltose hydrolysis [129]. Nowadays, studies have been demonstrated a broader role for AmyR, which appears to regulate another gene expression systems. High levels of both α- and β-glucosidase as well as α- and β-galactosidase in the
The filamentous ascomycete fungus
In fact, studies assessing a near-full genome deletion strain set in
As briefly described above, microorganisms are known to adjust their carbon metabolism in order to minimize energy demands. One of these regulatory mechanisms is the carbon catabolite repression (CCR). Readily metabolizable carbon sources, such as glucose, are preferably catabolized and, in general, suppress the utilization of alternative carbon sources, repressing mainly the enzymatic system required for the catabolism of less favorable carbohydrates. For general carbon catabolite repression in some Aspergilli species, the DNA-binding Cys2His2 zinc-finger repressor CreA is absolutely necessary [75]. In general, the negative effect of this regulatory system depends on the concentration of the preferable carbon source (elicitor). For instance, higher concentrations of the elicitor usually induce stronger transcriptional repression [137]. The presence of the repressing elicitors initiates signal transduction pathways to result in transcriptional repression of the catabolism of poor carbon sources. In this context, the molecular mechanisms leading to CCR is well known for the ethanol utilization in
In this context, CreA appears as a sole transcriptional repressor of the system, exerting its function in the presence of a co-repressor [139, 142 - 143]. It is well known that in the
A variety of studies have been demonstrated the mechanisms through which CreA represses some polysaccharide-degrading enzymatic systems in fungi. It was shown that CreA appears to repress
F-box proteins are proteins containing at least one F-box domain in their structures. The F-box domain is a protein structural motif of about 50 amino acids that mediates protein-protein interactions [147]. Usually, F-box proteins mediate ubiquitination of proteins targeted for degradation by the proteasome, but these proteins have also been associated with cellular functions such as signal transduction and regulation of cell-cycle [148]. A study that performed a screening of 42
In summary, an intricate and fine-tuned regulation network exists in order to control the expression of plant cell-wall degradation genes in fungi. A variety of transcriptional regulators are able to respond to different nutritional requirements of the fungus, depending on its lifestyle. In general, readily metabolizable carbon sources such as glucose represses the transcription of genes responsible for the poor carbon source catabolism, via different mechanisms. The carbon catabolite repression in fungi is a common mechanism of regulation through which the organism adapts to nutritional availability in their environment. For instance, in
In order to degrade efficiently plant biomass, a microorganism should possess characteristics that make the process economically viable. For cellulosic ethanol production, for instance, an efficient microorganism should produce high yields of the desired product, must have a broad substrate range and high ethanol tolerance and it has to be tolerant to the inhibitors present in lignocellulosic hydrolysates. Therefore, engineering microbial strains for improvement of effectiveness in industrial applications is not a simple task. Concerning to bioethanol production, the most promising organism for genetic bioengineering is the yeast
However, the metabolism of
While most biological routes being studied for the processing of lignocellulosic biomass focused on the separate production of hydrolytic enzymes, in a process that usually comprises several steps, another approach is suggested to achieve this goal. This approach, termed consolidated bioprocessing (CBP) involves the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars in a single stage via microorganisms or a consortium [157]. CBP appears to offer very large costs reduction if microorganisms can be developed that possess the required combination of substrate utilization and product formation properties [158]. In a 2006 report in biomass conversion to biofuels, the U.S. Department of Energy endorsed the view that CBP technology is "the ultimate low-cost configuration for cellulose hydrolysis and fermentation" (DOE Joint Task Force, 2006; energy.gov). Currently, CBP technology is developing fast, especially due to partnerships with venture capital investors and researchers. The main challenge of CBP is to generate engineered microorganisms able to produce the saccharolytic enzymes and converting the sugars released by those enzymes into the desired end-products. In addition, CBP microorganisms need to be able to perform these tasks rapidly and efficiently under challenging, industrial processes. A successful microbial platform for production of bioethanol from microalgae is currently available, and demonstrates an application of the CBP [159]. A DNA fragment encoding enzymes for alginate transport and metabolism from
The approach required for generation of CBP microorganisms involves the knowledge of many topics discussed in this chapter, concerning to fundamental principles of microbial cellulose utilization and its regulation. Moreover, the principles of synthetic bioengineering discussed above can be applied to the development of new strains for CBP technology, and therefore the generation of new microbial platforms able to uptake and metabolize completely the lignocellulosic biomass.
A large quantity of lignocellulosic residues is accumulating over the world, mainly due to the expansion of industrial processes, but other sources such as wood, grass, agricultural, forestry and urban solid wastes contribute to accumulation of lignocellulosic material. These residues constitute a renewable resource from which many useful biological and chemical products can be derived. The natural ability of fungi and other microorganisms to degrade lignocellulosic biomass, due to highly efficient enzymatic systems, is very attractive for the development of new strategies concerning industrial processes. Paper manufacture, composting, human and animal feeding, economically important chemical compounds and biomass fuel production are among some industrial applications derived from microbial lignocellulosic degradation.
Global climate change and future energy demands initiate a race in order to achieve sustainable fuels derived from biomass residues. Conversion of sugars to ethanol is already currently done at very low cost from sugarcane in Brazil, and from corn, in United States. However, the challenge is how to obtain the biofuel from the wastes generated from the mills producing ethanol. Residues such as sugarcane bagasse and corncobs contain large amounts of lignocellulosic material and therefore can be transformed into biofuels. A major advantage of using residues to produce biofuels is to reduce the competition between fuels and food. In this context, hydrolytic enzymes such as cellulases contribute for the large cost of cellulosic ethanol nowadays. The great bottleneck to achieve cellulosic biofuels is the plant biomass recalcitrance, and overcome such barrier is the key for the development of feasible industrial processes for biofuels production. For instance, it was recently demonstrated that
The comprehension of the machinery behind the enzymatic systems of fungi able to degrade plant cell wall polysaccharides favors the use of the microorganisms in industrial applications. Currently, through advanced molecular techniques, it is possible to engineer new microbial strains by insertion or deletion of genes involved in important metabolic pathways responsible for biomass degradation. The useful host cells to develop the synthetic bioengineering should have versatile genetic tools, resources and suitability for bio-refinery processes, such as stress tolerance. Therefore, a strain development in future requires insertion, deletion and expression controls of multiple genes and it is a difficult task to achieve. However, integrated advanced techniques could be able to overcome these challenges, including computational simulation of metabolic pathways, genome synthesis, directed evolution and minimum genome factory. The synthesis of the whole genome has already been done [160, 162] and, as discussed in reference [153], in a near future the synthesis of very large fragments of DNA will make it possible to design a whole yeast artificial chromosome (YAC) encoding a number of genes. According to these authors, the
As said by Lee Lynd, a pioneering researcher in the field of biomass: "the first step toward realizing currently improbable futures is to show that they are possible". These technologies described above are currently available for scientific community and, along with advances in industrial processes, endorse the possibility to take energy from plant biomass using microorganisms. Thus, the Humanity has never been so close to use new and sustainable ways of energy.
The concept of frailty is frequently mentioned in studies related to the elderly population—health status, self-care dependence, healthcare resources or even the configuration of the wards where care is provided. Looking at the scientific knowledge and clinical practice, frailty in the elderly is considered a relevant dimension of quality of life. Moreover, there is a tendency to accept that individuals with severe frailty have to be considered vulnerable and should be protected.
Frailty has been viewed as a cornerstone of geriatric medicine and a platform of biological vulnerability to a host of other geriatric syndromes and adverse health outcomes [1], such as long-term nursing home stay, injurious falls and death, in community-dwelling older adults independent of medical comorbidities and age. The expression “frailty elderly” was used for the first time in 1970, by researchers from the Federal Council on Aging (FCA) of the United States, with the purpose of describing elderly people who lived in unfavourable socioeconomic conditions and presented physical weakness and cognitive deficit that, with advancing age, began to demand more care; in the 1980s, frailty in the elderly people was understood mainly as synonymous of disability or the presence of a disease, chronic or extreme condition linked with ageing [2]. In 1990, the expression “frailty elderly” was referred for the first time on the
The term “frailty” started to be used frequently in terms of diagnosis, clinical decisions and provision of care. Frailty and cognitive and functional decline are relatively common in older dependent people with health problems. One of the challenges for researchers today has been to study the physical characteristics and psychological symptoms of frailty and to relate them to adverse health outcomes. In this chapter, we intend to analyse the matters that have most attracted the attention of researchers and health professionals who deal with people in situations of frailty.
Understanding frailty has become crucial for caring for the elderly. In older people with dementia, the assessment of frailty is more important than determining the degree of dementia, since it is crucial to develop appropriate care people need; there are old people with moderate dementia but with a severe level of frailty.
In this chapter, we intend to review the concepts of frailty, operationalization strategies and assessment tools and clarify some ideas from the debate on what frailty is.
The concept of frailty has grown in importance because of a need to evaluate the health status of older persons and a need to prevent or at least delay the onset of late-life disability and its adverse consequences [3]. There is to date no clear consensus regarding the definition of frailty; some definitions have been proposed, each with their own strengths and weaknesses [3].
Frailty is a multidimensional concept and can be defined as a dynamic state that affects an individual with declines in one or more domains, such as physical, cognitive, social, attention or senses [4]. There is usually a dependence on self-care and need of support from others. Elderly does not mean frailty, but the ageing process led to frailty, which means that there are changes that reflect ageing-related alterations and involve intrinsic and extrinsic factors which are typical of ageing.
The occurrence of frailty is mainly a state of vulnerability resulting from comorbidities and the overall decline in organ functions. The progression to later stages of dementia often signals a loss of autonomy, dependence and reduction in physical and cognitive function. Frailty of people is positively related with their caregiver burden and associated with higher levels of depression on the caregiver. A lack of understanding about frailty has been identified as a barrier to providing optimal care to elderly people, for example, people with advanced dementia [4].
Frailty is an emerging concept used in the field of geriatrics and gerontology, to make reference to the clinical condition of the elderly. There is a deficit of information regarding the incidence and prevalence of frailty in the elderly, mainly due to the lack of consensus definition that can be used as reference in different populations. There is usually a “clinical sense” about what is frailty and what a frail elderly person is, but there is no agreement, a standard definition regarding this concept, that can assist in the diagnosis of frailty condition. As mentioned above, frailty is often considered an inherent condition of ageing, an attitude that can cause late interventions with minimal potential for prevention or reversing the consequences and adverse effects from the problem.
The concept of frailty, widely used in the recent years, focuses primarily on the physical dimensions. That is why it is understood that the criteria for assessing presence/absence are the physical signs and symptoms, sedentary behaviour, weight loss, exhaustion, slowed gait, decreased muscle strength, with three or more of these five criteria we are facing physical frailty and the presence of one or two criteria indicates pre-physical frailty [5].
The diagnosis of frailty relies currently on the assessment of a small subset of easily measurable clinical markers. Just as conceptual disagreements arise about what frailty means, there are also disagreements about how to evaluate it. While recognizing the multifactorial nature of frailty, it is important to develop an “operational definition” of frailty that is simple enough to be used clinically and to guide prevention and care [3].
Frailty among older persons appears in the investigation as a dynamic process, characterized by frequent changes over time. The evolution of frailty incorporates quantitative and qualitative data, which motivated researchers to invest in modelling. Recent studies have highlighted age, medical factors and higher socioeconomic status to be protective [6]. In the study carried out by the
Andrade et al. [2] state that currently, two research groups have distinguished in the pursuit of consensus on the definition of frailty in the elderly: one of them in the United States, at the Johns Hopkins University, and the other one in Canada, the Canadian Initiative on Frailty and Aging (CIF-A). The group of researchers from the Johns Hopkins University produced an operational definition of frailty in the elderly and proposed measurable and objective criteria to the phenomenon. This operational definition starts from the hypothesis that the term is a geriatric syndrome and it can be identified by means of a phenotype that includes five measurable components: (a) unintentional weight loss, greater than 4.5 kg or more than 5% of body weight in the last year; (b) signs of fatigue; (c) reduction of handgrip strength, assessed with a specific instrument and adjusted to the person’s sex and body mass; (d) little physical activity assessed by calorie consumption (measured in kcal), adjusted by sex; and (e) reduction of march activity in seconds, distance of 4.5 m adjusted by gender and height [2].
A second definition was formulated by researchers from the CIF-A, indicated above. This is based on a multidimensional construct—frailty was defined using a more holistic approach, which emphasizes the complex aetiology of the phenomenon, understood as a not optimal condition in elderly, multifactorial and dynamic in nature, relating it to its history or trajectory of life [2]. The indicated trajectory can be shaped by biological, psychological and social, whose interactions result in resources and/or individual deficits in a given context. A tool was developed to measure frailty in the elderly—the Edmonton Frail Scale (EFS)—contemplating nine domains: (I) cognition, (II) general state of (III) functional independence, (IV) support, (V) medication use, (VI) nutrition, (VII) humour, (VIII) continence and (IX) functional performance. These authors consider this scale more comprehensive, especially considering aspects of cognition, humour and social support [2].
Some definitions of frailty promote a multidimensional approach based on an evaluation according to “frailty indexes”, which are calculated considering the accumulation of possible deficits, such as the presence of diseases, abnormal laboratory values, signs and symptoms or disabilities [7, 8].
It is difficult to establish a typology of frailty, given its multidimensional nature. On the one hand, frailty results from an articulation of factors of a physical and psychological nature. On the other hand, it is possible to assess frailty to highlight one or another aspect. Also, the investigation indicates that emotional management strategies can interfere with the signs and symptoms of frailty and with the ability to adjust to different disabilities.
Given the definitive trends in frailty, and although the creation of a typology is sometimes an academic task, we will try to describe four types of frailty in the elderly, on the assumption that they intersect and present common dimensions: physical, cognitive, social and emotional.
Frailty is a clinical situation known for the great vulnerability of the person in terms of the different physiological systems. In addition to the physical dimension, frailty is characterized by problems at the social, emotional and cognitive levels, despite the possibility of delaying its evolution in early stages [3, 9]. Fried et al. [10] proposed a clinical phenotype of frailty, defining it as a situation of increased vulnerability in the person for homeostatic resolution after pronounced distress. This growing vulnerability increases the risk of adverse outcomes, such as falls, fractures, hospitalization and ultimately mortality in elderly people living in organizations in the community or in their own homes.
Four main mechanisms can be identified in the progression of frailty: atherosclerosis, sarcopenia, cognitive deterioration and malnutrition [11]. It has been proven that malnutrition can be the cause of cognitive and functional decline and that the lack of some nutrients can cause cognitive frailty and vascular dementia [11].
There is an evident relationship between functionality and cognition, as evidenced by research evidence and some assessment tools (e.g., Clinical Dementia Rating). Many cross-sectional studies demonstrated the relationship between general cognitive function, emotions and physical frailty [12]. However, it is important to keep in mind that the decline in cognition and capacity of emotional management, given its functions and nature, evokes so many limitations to functionality that it becomes relevant to consider a cognitive frailty as a specific type.
Many studies have focused on the proposed entity of “cognitive frailty” to describe a clinical condition that is characterized by simultaneous occurrence of physical frailty and cognitive impairment in the absence of overt dementia [13]. Alzheimer’s disease is characterized by an association between physical and cognitive decline, but in the opposite direction, people with physical limitations are more predisposed to suffer emotional and cognitive problems. However, it should be noted that in recent years studies are more focused on physical frailty, with a relative paucity of data available for concomitant transitions in cognitive status [6].
An International Consensus Group studied the “cognitive frailty” condition. “Cognitive frailty”, although so defined, implies the presence of physical and cognitive decline. The key symptoms to characterize cognitive frailty are as follows: (1) presence of physical frailty and cognitive impairment and (2) exclusion from the concomitant presence of any type of dementia [14]. At the same time, the group indicated that “cognitive frailty” implies a rigorous diagnosis in terms of memory performance but also of other cognitive functions [14].
“Cognitive frailty” could represent a cognitive entity with specific neuropsychological patterns (executive and selective attention) [14]. The mechanisms in action and how deterioration occurs are not yet fully understood.
The loss of emotional management capacities and of establishing social interactions generates potential situations of frailty. It is also evident that any types of frailty (physical or psychological) also interfere with the emotional and social spheres. Usually, people with frailty (with cognitive impairment) experienced high levels of emotional discomfort and behavioural changes. Even without significant cognitive changes, symptoms usually emerge that emphasize the importance of emotions and social interactions: sadness, loneliness, nervousness, concern for oneself, self-concept, self-care and sense of hope.
The relationship between emotions, behaviour and frailty emerges in studies that explore this association. Emotion, which can be considered positive or negative, interferes with the perception of self-efficacy and the subjective sense of well-being. Furthermore, studies conducted in older adults found that positive emotions were associated with lower disability in the execution of daily living activities, higher levels of mobility, less physical dependence and major likelihood of survival, as well as higher level of adjustment to chronic health problems; on the other hand, negative emotions are correlated with stress sensations and poor coping abilities [15].
Clark and Watson [16] emphasize the relationship between emotions and functionality, which is understood by the well-known association between emotions and behaviour. They concluded, in a study carried out with older adults, that positive emotions may be associated with lower disability in the execution of daily living activities, better mobility, good functional status and major likelihood of survival; on the contrary, negative emotions can be correlated with distress and poor coping abilities. Mulasso et al. [15] provide empirical evidence to the multidimensional theorization and definition of frailty, hypothesizing that a reduced level of positive emotions and high level of negative emotions may contribute to increases in the severity of frailty condition; on the other hand, they highlighted the role of emotion experience in interventions for the prevention of frailty, such as interventions of physical exercise or cognitive training associated with frequent experience of positive emotions.
Simultaneously, studies emphasize also the need to identify risks for frailty [4, 6, 9]. All dimensions that constitute limitations on functionality, carrying out activities of daily living, cognitive impairment and social isolation can and should be considered risks for frailty [4]. There are currently models, mathematical equations and Bayesian networks that allow identifying these risks and even predicting them, conjugating certain variables. Usually, these models take into account demographic, social and clinical variables. These models can have good performance, isolated or conjugated with other evaluation tools. Moreover, they can predict frailty evolution and enable dependent persons to be identified for further specific assessment or interventions.
There are many studies that explore frailty, types of frailty and predictors of frailty every year. The relationship between frailty and functionality and the psychological sphere and relationship between the frailty of the recipient of care and burden on the caregiver are increasingly studied.
Armstrong et al. [17] used of a large database (n = 23,952) with comprehensive health information on home care clients (aged 65+) of eight Community Care Access Centres (CCACs) in Ontario, Canada. In this large cohort of older home care clients, they found that greater evidence of frailty as defined by each of the three measures was associated with greater risk of adverse outcomes. This result additionally confirmed the potential utility of a frailty concept for identifying vulnerable individuals within the home healthcare sector. They concluded that mathematical models can utilize data collected during clinical assessments to provide a quantitative indicator of a client’s level of frailty.
Dudzińska-Griszek, Szuster and Szewieczek [18] developed a study whose aim was to assess conditions that influence grip strength in geriatric inpatients. A comprehensive geriatric assessment was complemented with assessment for the frailty phenotype. Functional assessment included Barthel Index of Activities of Daily Living (Barthel Index), Instrumental Activities of Daily Living Scale and Mini-Mental State Examination. The conclusion was that cognitive function, somatic comorbidity and medical treatment affect grip strength as a measure of physical frailty in geriatric inpatients.
A retrospective cohort study on 18,341 Medicare Advantage enrollees aged 65+ was conducted by Anzaldi et al. [19] in Massachusetts. When analysing the clinical information systems, they identified the presence of 10 syndromes commonly found in the elderly (falls, malnutrition, dementia, severe urinary incontinence, absence of faecal control, visual impairment, walking impairment, pressure ulcers, lack of social support and weight loss), as well as references to the presence of frailty identified in the natural language processing (NLP) algorithm. The main conclusion was that patients identified as “frail” by providers in clinical notes have higher rates of healthcare utilization and more geriatric syndromes than other patients. Certain geriatric syndromes were more highly correlated with descriptions of frailty than others.
Shimada et al. [20] studied the cognitive frailty in 4570 older adults. The aim of the study was to analyse the extent to which a new perspective of cognitive frailty could be considered as a predictor of dementia. There are 2326 women and the average age was 71.9 ± 5.5 years. Physical frailty was defined as the presence of more than one of these symptoms: slow walking speed and muscle weakness. Cognitive frailty was defined as comorbid physical frailty and cognitive impairment. They concluded that cognitive impairment and cognitive frailty could be considered risk factors for dementia. Findings showed clearly that individuals with comorbid physical frailty and cognitive impairment could have a higher risk of dementia than healthy older adults or older adults with either physical frailty or cognitive impairment alone.
The estimation of the prevalence of frailty in patients admitted to intensive care unit (ICU) and its impact on intra-ICU mortality, at 1 month and at 6 months, was developed by Cuenca et al. [21]. A prospective cohort study was conducted. Frailty was present in 35% of patients admitted to the ICU, associated with higher rates of mortality.
Ma et al. [22] carried out a study to determine social frailty status via developing a simple self-reported screening tool, termed the HALFT scale, and to examine the association between social frailty and physical functioning, cognition, depression and mortality among community-dwelling older adults. They state that social frailty is related to adverse health-related outcomes. Moreover, they added that research into the relationship between social frailty and physical functioning remains limited. A prospective cohort study was carried out, with 1697 community-dwelling adults aged ≥60 years from Beijing. The scale developed was based on five items: unhelpful to others, limited social participation, loneliness, financial difficulty and not having anyone to talk to.
The prevalence of social frailty in the participants was 7.7%. Social frailty was positively associated with physical frailty, low levels of physical activity and poor physical functioning. Researchers also found that social frailty was associated with dementia, memory decline, depression and cognitive impairment. Having experienced a negative or traumatic event was also associated with social frailty. Additionally, social frailty was associated with physical functioning, cognition and depression and predicts mortality; they emphasize that interventions aimed at preventing or delaying social frailty are warranted.
In a cross-sectional study carried out by Mulasso et al. [15] the association between frailty and emotional experience was studied in a sample of Italian community-dwelling older adults. Participants consisted of 104 older adults (age 76 ± 8 years; 59.6% women) living in Italy. Frailty and emotion perception were measured with appropriate and valid tools. The Mini-Mental State Examination was used as a screening tool for cognitive functions (people with a score ≤ 20 points were excluded). The researchers stated that frailty increases individual vulnerability to external stressors and involves high risk for adverse geriatric outcomes [15]; findings demonstrate that emotion perception may influence frailty, which is really relevant for the evaluation and prevention of frailty in older adults.
A theoretical study based on research studies that equate the role of nutrition and nutrients in cognitive and functional decline was developed by Gomez-Gomez and Sapico [23]. They state that one of the most important factors to consider in the development of cognitive deterioration is oxidative stress. Consequently, they added that increasing antioxidants in the diet may be one of the therapeutic strategies in the management of these patients.
Some studies were analysed, mainly those that showed the effectiveness of antioxidants in the adjustment of oxidative stress, given their function as free radical scavengers, or factors that potentiate the antioxidant effect. Anyway, the studies emphasized that the inappropriate use of antioxidants could have side effects and become toxic at high doses. Given the multiplicity and some divergence in the results, additional studies are required as well as clinical trials to increase the clinical effectiveness [23].
Several studies were analysed, namely, those that have shown the effectiveness of antioxidants in the adjustment of oxidative stress, either by their function as free radical scavengers or potentiating the antioxidant effect. Studies showed that the inappropriate use of antioxidants could have side effects and toxicity at high doses. However, it was indicated that additional studies are required as well as clinical trials to increase the clinical effectiveness [23].
Abreu et al. [4] examined the healthcare needs of community-dwelling older people, trying to understand the relationship between frailty, functional dependence and healthcare needs among community-dwelling people with moderate to severe dementia. A sample of 83 participants was recruited. The Edmonton Frail Scale was used to evaluate frailty, in addition to tools that were chosen to collect data on other variables. A set of 26 healthcare needs was defined to support the assessment. There was a significant association between “severe frailty” and “severe dementia” and “fully dependent” and “severely or fully dependent in the activities of daily living”. The most prevalent healthcare needs in the sample were food preparation, medication/taking pills, looking after their home, toilet use, sensory problems, communication/interaction, bladder, bowels, eating and drinking, memory, sleeping and fall prevention. In particular, the study shows a set of needs that are present simultaneously in both frailty and dementia stages, according to their severity. They found in the study that 16.7% of people with moderate dementia were also diagnosed with severe frailty. Concerning the needs assessment, the authors state that the concept of “severe dementia” is clearly a limiter in the matter of frailty. As an alternative, they suggest the expression of “advanced dementia”, encompassing people with severe dementia and people with moderate dementia but who also have severe frailty.
Usually, scales assess some domains of frailty in old people (cognition, general health status, functional independence, social support, medication usage, nutrition, mood, continence and functional performance). These tools are important on clinical point of view, for research and decision-making. Several tools that evaluate functionality and cognition also evaluate several dimensions that we are traditionally including in frailty.
Armstrong et al. [17] indicate, in the scope of their study, three conceptually different approaches to the measurement of frailty: (1) Changes in Health, End-Stage Disease and Signs and Symptoms (CHESS) scale, (2) Edmonton Frail Scale (EFS), (3) the frailty index (FI) and the Tilburg Frailty Indicator (TFI).
The CHESS scale is a tool that uses information from the person’s clinical assessment, which is used to calculate the person’s level of decline. The tool was developed using statistical methods, based on the items available in the inter-RAI instruments. It is not a tool for objectively assessing frailty, but it allows assessing the “instability” of health status, which is also a predictor of mortality [17]. The scores ranging from 0 (meaning no instability) to 5 (for the highest level of instability) have been demonstrated to be a strong predictor of mortality (P < 0.0001) in continuing care patients [24].
The EFS is a brief multidimensional clinical measure, widely used and designed to use in both inpatient and outpatient settings [25]. The scale assesses nine domains of frailty in old people (cognition, general health status, functional independence, social support, medication usage, nutrition, mood, continence and functional performance) [25]. Total score can vary from 0 to 17. The participants were classified into categories, and a higher score represents a higher degree of frailty. Severe frail and non-frail participants were defined according of the EFS score from not frail (0–5), vulnerable (6–7), mild frailty (8–9), moderate frailty (10–11) and severe frailty (12–17). The EFS is a measure of frailty compared to the clinical impression of specialists after their more comprehensive assessment. A larger part of the assessment tools is focused primarily on determining the person’s level of functioning in terms of managing activities of daily living and instrumental activities of daily living. In post-operative older adults, high scores on the EFS have been shown to be associated with increased complications and a lower chance of being discharged home after surgery [17].
The FI was developed by Rockwood and Mitnitski based on an idea of “accumulation of deficits” [17]. The FI is based on the view that frailty is a non-specific multifactorial state, best characterized by the quantity, rather than the quality, of the health deficits that the person accumulates during the course of life [26]. The FI is thus calculated as the proportion of potential deficits present in the person and can be calculated from the information present in most previous systems of clinical data (databases) [17].
The TFI is a tool widely used to assess 3 frailty domains and their 15 components. It is a user-friendly questionnaire and has good psychometric properties assessed in the initial validation process, constituting a good strategy for multidimensional assessment of frailty in community settings [27]. The instrument consists of two parts. Part A includes life-course determinants of frailty (sex, age and marital status), and part B assesses 15 components of frailty. The score on total frailty has a range of 0–15; people with a score ≥ 5 are considered frail; for physical, psychological and social frailty, the score ranges are 0–8, 0–4 and 0–3, respectively [28].
Studies carried out in different countries have demonstrated that these tools have in general good psychometric properties and are reliable and valid instruments for assessing frailty in community-dwelling older people [4, 17, 24, 25, 26, 27, 29, 30].
Frailty’s assessment is inseparable from an objective and competent evaluation of healthcare needs. Frailty is a multidimensional concept and can be defined as a dynamic state that affects an individual with declines in one or more domains, such as physical, cognitive, social, attention or senses. The assessment of frailty is of limited interest if healthcare professionals do not invest in assessing the needs of frailty people in healthcare. This assessment must be multidimensional, multifactorial, longitudinal and comprehensive, covering all activities of life.
There are many debates on what are health needs assessment and problem identification. What is important to note is that care needs assessment is a systematic and sequential process, conducted by a care professional, which begins with the assessment of dependency focus, accounts for the presence and efficacy of current help, recognizes perceived need and finally determines the type of intervention needed to meet those needs [31].
It has been recognized that needs in the elderly should be patient-centred; holistic; analysed on by dependent people, caregivers and professionals; communicated to other professionals; and met in order to achieve better coordination between leading disciplines; needs assessment enhances the patient and carers experience and leads to more accurate information, but the level of reassessment by other professionals and the incidence of service duplication should also be reduced [31].
Care needs assessment has to promote an objective, competent evaluation of the self-care deficits. A self-care deficit is an inability to perform certain daily activities dependent on health and well-being. Common activities of daily living are the following: eating, bathing, getting dressed, toileting, transferring and continence. Self-care deficits can arise from physical or mental impairments. In elderly people, some of these problems accumulate and comorbidities appear. Health professionals play an important role when it comes to addressing self-care deficits through assessment and intervention. For assessment, evaluation of needs and identification of focuses of attention are necessary. Intervention can include, but is not limited to, helping patients to manage signs and symptoms, adhere to the therapeutic regime, adjust to deficits and strive to preserve, as far as possible, their self-care capacity.
With the ageing of the population and increased longevity, the need to provide palliative care is emphasized. However, this increased need is not usually accompanied by the availability of beds, which requires the use of indicators to manage the availability of palliative care provision. When to begin palliative care is a troublesome question for patients, families and healthcare providers [32]. Severe frailty is a relevant marker, along with functional dependence, cognitive impairment, symptom distress and family support for beginning palliative care. Frailty, independent of specific diseases, can be associated with a limited life expectancy and therefore is an important indication for palliative care [32]. Frailty is an essential model for palliative care in older adults as optimal medical treatment for the frail patient typically includes preventive, life-prolonging, rehabilitative and palliative measures in varying proportion and intensity based on the individual patient’s needs and preferences [33].
Frailty elderly usually have dependence on self-care and need of support from others. Elderly does not mean frailty, but the ageing process led to frailty, which means that there are changes that reflect ageing-related alterations and involve intrinsic and extrinsic factors which are typical of ageing [4]. Usually, scales assess some domains of frailty in old people (cognition, general health status, functional independence, social support, medication usage, nutrition, mood, continence and functional performance). The occurrence of frailty is mainly a state of vulnerability resulting from comorbidities and the overall decline in organ functions. The progression to later stages of frailty often signals a loss of autonomy, dependence and reduction in physical and cognitive function.
Frailty is commonly positively related with caregiver burden and associated with higher levels of depression on the caregiver. A lack of understanding about frailty has been identified as a barrier to providing optimal care to elderly people. Self-care deficit theories suggest people are better able to recover when they maintain some independence over their own self-care. The evaluation of frailty is closely linked to the identification of dependencies in self-care. The use of frailty and self-care dependence assessment helps to determine the focus of attention, to respect vulnerability, to limit dependence as much as possible and to provide quality, safety and competent care.
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