Candida albicans: Pathogenesis and Secretory Pathways

Abstract

Candida albicans is a member of the human host’s microbiome composition; therefore, it is recognized as a portion of the human host body’s normal flora in a homeostasis condition. However, when the host develops an abnormal condition, e.g., immune deficiency, C. albicans acts as an opportunistic pathogen. C. albicans has an effective arsenal of a wide range of virulence factors. Due to this knowledge, the enzymes construct a significant portion of substantial fungal virulence factors, which are made of proteins and play an essential role in fungal invasion, fungal-hyphal growth, and biofilm formation. An active secreted protein should be processed via the fungal secretion system, such as the endoplasmic reticulum (ER) and/or Golgi apparatus (GA). In other words, an active protein that acts as a fungal virulence factor should undergo several vital and pivotal maturation processes, including glycosylation and folding. In this chapter, we have a rigorous look at these processes, which directly determine the pathogenesis of C. albicans.

1. Introduction

Fungi, including Candida albicans (C. albicans), as significant members of eukaryotes, can adapt to different environmental factors. Their flexible ability to adapt to other conditions is associated with vast metabolic pathways, morphogenetic characteristics, and life cycle diversities detected in fungal populations, e.g., C. albicans. Based on the reported results from previous investigations, the eukaryotic kingdom of fungi is estimated to possess 1.5 to 5 million species. The genome size of fungal species amazingly differs between 2 and 180 million nucleotides, and the proteome size of fungal species seems to range from 2 to 35 thousand [1, 2]. Although there is a wide range of fungal species, only a few fungi are known as primary or opportunistic pathogens. Among these pathogenic fungi, Candida spp., e.g., C. albicans, C. auris, C. glabrata, and C. tropicalis are identified as important fungal etiologic agents of human infectious diseases such as urogenital infections (vulvovaginal candidiasis, candidal balanitis), Candida onychomycosis, oropharyngeal candidiasis, skin candidiasis, and systemic candidiasis [2, 3, 4, 5, 6, 7, 8]. We know the human host is armed with many antifungal immune responses and signaling pathways; however, C. albicans also recruits its virulence factors to effectively oppose the human host defense system. C. albicans, as a dimorphic fungus, can adapt to the human host temperature, so it grows in its adhesive and invasive filamentous form. The filamentous form of C. albicans is the fungal invasive form that makes it capable of taking up nutrients from the human host body, charging its effective growth and progression, and escaping the human host immune system. Furthermore, several antifungal drug-resistant strains can easily resist antifungal therapeutics [2, 8]. Due to this knowledge, we will closely examine C. albicans’ pathogenesis and secretory pathways.

2. Pan-genome of Candida albicans

In comparative genomic analyses, both eukaryotes and prokaryotes possess genomic pools composed of core- and accessory genomes. Usually, the core genome comprises those genes (housekeeping genes) that contribute to essential life activities, including reproduction, metabolism, cell division, virulence, and pathogenesis [9, 10]. On the other hand, the accessory (flexible, dispensable, and adaptive) genome contains genes that participate in specific functions, e.g., antibiotic resistance, specific metabolic pathways, and particular virulence, for the cell adaptation to its environment [9, 10]. The accessory genes are not present in all genomes but are usually detectable in ≥2 genomes. Hence, the genomic plasticity of the organisms is directly associated with mobile genetic elements [9, 10, 11, 12, 13]. Evolutionary biology studies depict two groups of genes, including analogous and homologous genes. The comparable genes appear through an independent converged evolutionary process, while the homologous genes appear through an effective evolutionary process with identical origination from a similar ancestor. Furthermore, homologous genes are classified into two groups: paralogous genes, the outcome of the mutation feature, and orthologous genes – the outcome of evolutionary speciation [9, 10]. The results show that C. albicans is usually a diploid-genome yeast with 6189 genes in its pan-genomic pool. Six thousand sixty-nine genes constitute the core genomic pool. The 120 genes left form the flexible genome. The genomic investigations depict a close similarity (>80%) in nucleotide level among 5363 orthologous genes, which are shared between C. albicans and C. dubliniensis (a non-Candida albicans Candida (NCAC)) [14]. In this regard, it is estimated that >200 genes contribute to the trafficking of proteins within a limited secretory pathway of C. albicans (between different interior sections of the cell and the exterior cell surface). It does not include the proteins involved in post-translational modifications, protein folding, quality control, and those situated in the endoplasmic reticulum (ER) and Golgi apparatus (GA) (Figure 1). According to comparative genomic investigations, the secretory machinery constitution is conserved in eukaryotic cells, e.g., C. albicans. Moreover, the introns are detectable in only 8% of the genes in the secretory system [15, 16, 17, 18, 19, 20, 21].

Figure 1.

The secretory pathway and the glycosylation processing including ER and GA (biorender.com).

3. Translocation

The secretory pathway fully mediates the virulence and the pathogenesis of pathogenic fungi like C. albicans. The protein secretion system participates in superficial adhesin secretion, immune system interacting proteins production, hydrolytic enzymes expression, phenotypic switching proteins secretion, and yeast-hypha trans-figurative protein expression determines the capability, functionality, and dynamics of a cell [8, 15, 21]. The expressed proteins are initially inactive and should be activated. Hence, the biosynthesized ribosomal proteins should get the related modifications (such as glycosylation) and folding within the ER and then get packaged in the GA (Figure 1) [21].

4. Glycosylation

The glycosylation process begins with the quality control of a protein within the ER, which is the first section of the secretory pathway to check and mature the related proteins that are supposed to be glycosylated. On the other hand, the GA has been recognized as the leading center for glycosylation [22]. Indeed, the GA, recognized as the central hub of the cellular secretory machinery system, is made of different cisternae comprising the cis-Golgi network, cis, medial, trans, and trans-Golgi network with their topological characteristics and structures (Figure 1). Protein molecules get post-translational modifications within the GA cisternae and then depart to their final destination. The GA trafficking and homeostasis are directly associated with the vesicular trafficking machinery system (VTMS). The VTMS comprises tethers, small GTPase enzymes, soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs), and vesicular coats [23, 24]. The secreted and cell-surface proteins contribute to continuous and coordinated signaling between the cells within a multicellular organism. These proteins undergo post-translational modifications or glycosylation for the most to bear branched sugar polymers of the glycans with covalent bonds. The reported records show that glycome is involved in various biological activities, e.g., adhesion, protein folding, migration, cell stability, intracellular communication, and host-pathogen interactions. Depending on glycans’ attachment to amino acids’ hydroxyl group or amide group, two main types of glycoproteins including O-linked (attached glycans to threonines, serines, or hydroxylysines subsets)- and N-linked (attached glycans to asparagine residues; e.g., Asn-X(X is any amino acid excluding proline)-Ser/Thr) glycoproteins are recognized [25, 26, 27, 28]. In addition, the attachment of glycosylphosphatidylinositol (GPI) to the cell wall and cell membrane proteins is known as post-translational modification, too. In other words, the GPI molecules anchor the cell wall and cell membrane proteins through covalent bonds. The biosynthetic pathway of GPI is a conserved metabolic pathway that occurs through several phases [28]. The GPI-anchored proteins support eukaryotes’ growth and viability and are simultaneously critical factors of pathogenicity and virulence in eukaryotic pathogens like C. albicans. The central portion of the plasma membrane and dynamic structure of the cell wall in fungi such as C. albicans is formed by glycoproteins. Hence, the permeability, flexibility, plasticity, and suitable molecular scaffold in strong and impenetrable fungal cell walls are directly associated with glycans in the form of glycoproteins. According to previous studies, we now know that glycoproteins are pivotal biomolecules that contribute to different biological activities and functionalities. This feature results in effective morphogenesis, viability, and cell stability [27, 28, 29, 30]. The structure and composition of the dynamic organelle of the fungal cell wall continuously interact and interchange with environmental factors and stresses [30]. That is why the fungal cell wall in pathogenic fungi can trigger the human host immune system and make the fungal cell capable of escaping from both innate and adaptive immune responses [31]. As previous investigations show, 20% (1200/6000 genes) of the genomic pool in Saccharomyces cerevisiae participates in the cell wall biosynthesis pathway [32]. In toto, the cell wall in fungi has a similar structure and composition because of its role and function. As mentioned earlier, the cell wall interacts with various peripheral factors, such as immune biomolecules in the human host. This layered organelle is supported by a conserved skeletal structure composed of a core section formed by glucan (branched ß-(1,3) with covalent linkages), interchain (3–4%), and chitin. Moreover, interchain hydrogen bonds between chitin and glucan result in microfilaments that cover the cell [30]. The branched glucan is linked to different biomolecules of proteins and or polysaccharides. The composition of proteins and polysaccharides may differ from one species to another among fungal populations [30]. ß-(1,3) and ß-(1,6) glycosidic bonds are identifiable within the linear molecules of D-glucose. The ß-(1,3) glucan contributes to the helical backbone, and the ß-(1,6) glucan is involved in branching structures through cross-linking with molecules of mannoproteins and chitins. This feature is supported by the GPI anchor attached to ß-(1,3) glucan [33].

Although glucan (α-(1,3)) is a practical construction in fungal cell wall structure and is associated with human fungal pathogens, it is absent in the cell wall structure of C. albicans and S. cerevisiae yeasts [30]. Because of fewer cell wall layers in the bud scar position, the yeast cell wall organelle encompasses a thinner diameter from the outside. Therefore, the inner side of the cell wall has been constructed by glucan (ß-(1,3)) and chitin composition [34, 35]. The fungal outer skeletal layers are very varied compared with the cell wall’s inner layers. In both C. albicans and S. cerevisiae, the inner cell wall is entirely covered by a solid outer cell wall. A significant amount of mannosylated glycoproteins enriches the outer cell wall. The mannosylation process (recruitment of guanosine diphosphate (GDP)-mannose to create α- and ß-linked oligo mannosyl) is achieved via mannosyltransferase enzymes in C. albicans and S. cerevisiae [30, 36].

By recorded reports, a portion of the cell wall dry weight in C. albicans belongs to chitin molecules (up to ~2%), ß-(1,6) glucan (up to ~20%), and ß-(1,3) glucan [37, 38].

5. Protein glycosylation pathways

As aforementioned, the cell wall biosynthetic pathways in C. albicans are effective points of view in human-pathogen interactions because the glycosylated proteins and other fungal cell wall compositions (Figure 2), including structural polysaccharides such as chitin, chitosan, glucans, and O-, N-, and GPI-linked glycoproteins interact directly or indirectly with human host immune system. These fungal molecules are significant targets in producing new antifungal agents and immunotherapy methodologies [28, 39, 40, 41, 42].

Figure 2.

The cell wall composition, structure, and the virulence factors associated with the pathogenesis in C. albicans.

The biosynthetic pathway of O-linked glycosylation may produce and assemble linear oligosaccharides made of up to seven α-1,2-linked mannose residues. Although the feature of O-glycosylation is achieved within the GA [43], the ER lumen is where the attachment of α-linked mannose residues to threonine/serine residues takes place [44]. This process begins within the ER through the contribution of Dolichol (Dol)-P-Man, which acts as a sugar resource (Figure 3). The presence of protein mannosyltransferases achieves the sugar donation by Dol-P-Man. Protein mannosyltransferase enzymes are encoded by the gene family of PMT1, PMT3, PMT4, PMT5, and PMT6 [45, 46, 47].

Figure 3.

Protein glycosylation in the endoplasmic reticulum (biorender.com).

Indeed, Dolichol, composed of isoprene units, contributes to protein glycosylation modifications as a lipidic portion of the intermediates. The oligosaccharide of Dol-PP-GlcNAc₂Man₅ is produced by a combination of the sugar of mannose (Man) and N-acetyl-D-glucosamine (GlcNAc). In this biosynthetic pathway, GDP-Man and UDP-GlcNAc act as donor substrates or sugar donors [48, 49, 50].

The glycosylated proteins within the ER lumen move to GA to complete the glycosylation process. In this regard, the newly arrived glycoproteins into the Golgi complex undergo a new glycosylation process in which further mannose residues are linked. To catalyze this process, the Golgi-related α-1,2-mannosyltransferase enzymes -known as GDP-mannose-dependent mannosyltransferases – contribute to this catalytic reaction. The genes of MNT1 and MNT2 encode the Golgi-located α-1,2-mannosyltransferase enzymes. The product of the MNT1 gene (Mnt1) is involved in adding the second mannose residue, while the product of the MNT2 gene (Mnt2) participates in the attachment of the third mannose residue to the O-linked glycan molecules. Both Mnt1 and Mnt2 are recognized as fungal virulence elements. The phosphomannosylation of O-linked glycans may be occurred by the Mnt3 and Mnt5 enzymes [46, 51, 52].

The fungal N-linked glycosylation pathway in C. albicans is known as an effective process because the initial phases of N-linked protein glycosylation are recognized as conserved eukaryotic processes that are observed in different eukaryotic cells from fungi to plants and from mammals to protozoa. The biosynthesis of N-linked glycans contains a collection of enzymatic reactions and modifications that should occur in ER and GA. In this regard, the essential enzymes of glycosyltransferases and glycosidases have pivotal keys in this biosynthetic pathway. In the first step, the ER (rough ER (RER))-located glycosyltransferase enzymes contribute to an assemblage of an oligosaccharide molecule upon a targeted isoprenoid lipid. In the second step, the ER-located glycosidase enzymes and Golgi-located glycosyltransferase enzymes catalyze additional modifications relating to N-linked glycan molecules [28, 53]. Glycosidase enzymes are glycoside hydrolyses that contribute to glycosidic bond hydrolysis in glycosides. These glycoside molecules are detectable in various substances, such as glycoproteins, fungi, and other organisms. The Carbohydrate-Active EnZyme database http://www.cazy.org/) (December 4, 2023) present 187 families of glycoside hydrolase enzymes (http://www.cazy.org/Glycoside-Hydrolases.html), 117 families relating to glycosyltransferases (http://www.cazy.org/GlycosylTransferases.html), 43 families in association with polysaccharide lyase enzymes (http://www.cazy.org/Polysaccharide-Lyases.html), 20 families regarding carbohydrate esterases (http://www.cazy.org/Carbohydrate-Esterases.html), and 17 families associated with auxiliary activities (http://www.cazy.org/Auxiliary-Activities.html) [54, 55].

6. Conclusion

The secretory system of C. albicans involves many fungal cell structures and functions. In this regard, many structural compositions and activities are identified as a double-edged sword. However, the cell wall compositions are recognized as a pathogenic arsenal and virulence factor in medical mycology. For instance, glucans participate in C. albicans biofilm formation, and the production of these cell wall materials may be promoted during this feature. Formation of biofilm may increase the feature of antifungal drug resistance, and this unfavored occurrence is known as a big challenge in the public healthcare system and medicine.

Acknowledgments

The authors have special thanks to IntechOpen publisher for the kind invitation. We also appreciate Antonija Grgec, the Publishing Process Manager of the present book project, for her brilliant support. Figures 1 and 3 are directly taken from biorender.com. Hence, the authors are obliged to biorender.com, too.

Conflict of interest

The authors declare no conflicts of interest.

Author contribution

The authors have contributed to the present chapter. All the authors have read and approved the latest version of the chapter.