Ultrastructural changes leading to cell death pathways on
Programmed cell death in T. foetus does not seem to make sense at first sight; however, different mechanisms of cellular death in this unicellular organism have been observed. This review summarizes the available data related to programmed cell death already published for the cattle parasite T. foetus and attempts to clarify some crucial points to understand this mechanism found in non-mitochondriates parasites, as well as assist in future research. Important results with different treatments showed that the T. foetus can choose among different pathways how to initiate cell death. Thus, a major challenge for cellular death research remains the identification of the molecular cell death machinery of this protist, such as caspases pathway, nuclear abnormalities, morphology cell changes, cellular death in this parasite and the prospects in the future research. Although, the possibility of the existence of different pathways to cell death in trichomonads is discussed and a model for possible executioners pathways during T. foetus cell death is proposed.
- cell death mechanism
- amitochondrial protozoan
- photodynamic therapy
The cell death mechanisms used by the parasite
Besides the significance of the parasite as an etiologic agent,
Cell death has been studied in many organisms: in mitochondriate organisms there are multiple forms of cell death, including the “programmed cellular death” (PCD) types, that will be described below, and depends or not on the presence of family of proteins, which control the mitochondrial membrane permeabilization and the release of some mitochondrial proteins to cytosol, like observed mainly in apoptosis . Besides, other types of programmed death accompanied by changes in morphological and biochemical features like autophagic cell death, for exemple, have been studied. The amitochondriate organisms, like
Despite apoptosis has been shown to be the major mechanisms of death observed in
Photodynamic Therapy (PDT) and drugs administration used for cancer chemotherapy results in DNA damage in some cells. A variety of injurious stimuli such as heat, radiation, hypoxia and cytotoxic anticancer drugs can induce apoptosis in low doses or result in necrosis at higher doses . It has been assumed that the machinery of PCD is absent in amitochondrial organism, like trichomonads, however, recent studies show that
Other types of cell death may also be considered to be forms of PCD, because they require gene activation and function in an energy dependent manner. PCD is a genetically regulated physiological process, fundamental for multicellular organism development and homeostasis. Studies show that depending on the damage infringed, the cells seem to “choose” how to die [8, 10].
2. Trichomonad hydrogenosomes
According to Müller (1988) ,
Evidences indicate that hydrogenosomes are anaerobic forms of mitochondria  or a specialized form of mitochondria useful in lower O2 environments . According to Martin (2005)  hydrogenosomes and mitochondria are, respectively, anaerobic and aerobic manifestations of the same organelle. Although, unlike mitochondria, the hydrogenosomes lack the DNA .
Trichomonad hydrogenosomes possess many proteins in common with mitochondria . Translocation studies using hydrogenosomal ADP/ATP carrier of
The most accept hypothesis for the origin of hydrogenosomes and mitochondria is that both organelles share a common ancestral. Phylogenetic studies demonstrated the existence of a typical Hsp 70 gene in the
During hydrogenosome formation, they have different forms, and then acquire a spherical structure, which can be changed in stress conditions . Studies proposed that
Hydrogenosomes also exhibited altered size and shape and they were randomly distributed within parasites cells after lycorine treatment . The sequence of alterations during the degradation of hydrogenosomes after treatment with lycorine included: matrix swelling, rupture of outer membrane, appearance of flocculent densities, and fragmentation of all membranous structures except the peripheral vesicle .
3. Morphological features to define programmed cellular death
PCD is not confined to apoptosis but that cells use different pathways for active self-destruction: condensation prominent or apoptosis; autophagy prominent, etc. [26, 27]. Although, there is some resistance to the exclusive use of the term PCD to specifically describe apoptosis . PCD It now generally refers to any cell death that is mediated by the intracellular death program, no matter what triggers it and whether or not it displays all of the characteristic features of apoptosis It has become increasingly apparent that cell death mechanisms include a highly diverse array of phenotypes and molecular mechanisms. Because other types of cell death may require gene activation and function in an energy dependent manner, they are also considered to be forms of PCD. There is evidence of other forms of non-apoptotic programmed cell death that should also be considered since they may lead to new insights into cell death programs and reveal their potentially unique roles in development, homeostasis, neoplasia and degeneration. It is probable that all normal cell deaths, as well as many pathological cell deaths, utilize this evolutionarily conserved death program .
Apoptosis, autophagy and necrosis was previously named as ‘type I, II and III cell death’, respectively [29, 30]. Although, several critiques are related to this clear-cut distinction [31, 32]. According to morphological criteria, the cell death modalities during tissue development and homeostasis can be defined with three distinct routes of cellular catabolism.
3.3 Citoplasmatic death or programmed necrosis
There is evidence that modulation of one form of cell death may lead to another . Under some circumstances, apoptosis and autophagy can exert synergetic effects, whereas in other situations autophagy can be triggered only when apoptosis is suppressed .
4. Biochimical and morphological features to define cell death in parasite trichomonas
4.1 Caspases pathway
Caspases are essential proteins involved in cellular death that exist in cytosol of most cells in its inactive form as a polypeptide. They are, activated by cleavage, and apoptosis is considered a consequence of their activation cascade .
In some cases caspases can induce cellular death and in others they seem to be irrelevant in decision between death and life. In both situations, caspases participate in morphology of apoptosis . According to Mariante et al. , caspases dependent cell death in trichomonads can occur through different known mechanisms, either by death receptors pathway, or through unknown signaling pathways, like the release of hydrogenosomes molecules, which may have analogous functions of mitochondrial proteins.
Although it is stablished that the apoptosis regulated by caspases are an important form of PCD, in many instances, PCD is caspase independent and non-apoptotic. Necrosis-like might or might not require caspases to activate cell death, while paraptosis and autophagic/vacuolar cell death traditionally do not call for the participation of caspases [10, 40]. Despite, some studies in mammalian cells indicate that caspases can regulate both apoptotic and nonapoptotic cell death, as autophagy .
In eukaryotic organisms, it is known that the caspases possess a fundamental role during the process of cellular death, especially on apoptosis, being the primary site of interaction with Bcl-2 proteins family. However, despite of the lack of mitochondria in trichomonads, Mariante et al.  confirm the participation of proteases of the Caspase-3 family in
4.2 Nuclear abnormalities
Trichomonads treated with different concentrations of H2O2 showed severe nuclear changes like unusual DNA condensation. Peripheral heterochromatin masses and nuclear DNA fragmentation can be observed in the nucleus of some cells, probably due to the activation of different endonucleases. One the other hand, in mammals the same treatment lead a different nuclear organization . Nuclear changes was observed in
4.3 Morphology cell changes
5. Possible cell death modalities in trichomonas
The biochemical basis for alternative forms of cell death morphologically distinct from PCD continues unknown. Understanding the mechanisms for these forms has implications for the understanding of evolutionary aspects of cell death programs, developmental cell death, neurodegeneration, and cancer therapeutics and for the design of novel therapeutic agents for diseases featuring these alternative forms of cell death .
The absence of apoptotic bodies and the characterization of a non-apoptotic-like cell death which fails to fulfill the criteria for apoptosis suggest paraptosis mechanism, once the cell death features shown by lycorine treatment in
(lack of chromatin condensation)
|PS Externalization||YES||NO||Licorine (||NO|||
Mepoxomicin and Bertezomid
|Cellular shapped changes||YES||H2O2|
Mepoxomicin and Bertezomid
|Lysossomal enzime release||YES||YES||PDT||YES|||
Taxol, Nacodazole and Colchicine
|Plasmatic Membrane Projections (Blebs)||YES||NO||NO||H2O2|
Taxol, Nacodazole and Colchicine
|Hydrogenosomes shaped changes||*||*||*||H2O2|
Taxol, Nacodazole and Colchicine
|Citoplasmatic components degradetion||YES|
(Fragmention of citoplasm)
Mepoxomicin and Bertezomid
Although, recent studies showed the “ladder pattern” compatible internucleosomic genomic DNA fragmentation characteristic of apoptosis, in
The different treatments and the different results obtained with
6. Implications and future directions
The evidence of that alternative, non-apoptotic, PCD in unicellular organisms has important implications for understanding cell dynamics. The environmental stimuli can produce different types of cell death depending on the intensity of stimulus, and that classic apoptosis and necrosis may represent only two extremes of a continuum intermediate form of cell death, applicable to also unicellular organisms . Comparatives analyses of proteome maps from parasites exhibiting such pathogenic characteristics may provide valuable data to understand the pathogenic mechanisms involved in urogenital trichomoniasis. Besides, the metabolic pathways that are different from those of their mammal hosts, given that Trichomonads possesses a hydrogenosomal/cytosolic compartmentalization of metabolism and metabolic pathway, the identification of proteins involved in such metabolic pathways could reveal good targets for drugs development [1, 48].
Several laboratories have contributed to understand the protein expression of Trichomonads, but despite numerous research and efforts to unravel the mechanisms of cell death, detailed description of the molecular mechanisms is still unknown. Identification of proteins related to the machinery of death of these cells should be the main focus of studies in the coming years. Studies related to molecular biology and biochemistry are still needed because little is known about the overall proteomic expression profiling of this parasite.
We gratefully acknowledge the Brazilian agency FAPESP (São Paulo Research Foundation) (2013/20054-8) for financial support.
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