**1. Introduction**

The cell death mechanisms used by the parasite *Tritrichomonas foetus* is a matter of an ongoing debate. Many mechanisms have been studied in different treatments, but much remains to be elucidated with respect to the protein machinery developed by these organisms with regard to death pathways. This review summarizes the current knowledge about cell death of *T. foetus* by showing all models. The aim is to show, on the one hand, that there is too much data requiring one or more explanatory model (s), but, the authors proposed specific models, on the other hand that the present data is not sufficient to definitely proof programmed cell death for this organism. Furthermore, we would like to point out perspectives on the proteomic of programmed cell death in this protist.

Besides the significance of the parasite as an etiologic agent, *T. foetus* has been used as a model for the study of drug carriers such as graphene oxide and carbon nanotube oxide (GCN-O) composite. It is still considered fascinating to study the mode of cell death since they do not have mitochondria but possess an unusual anaerobic membrane-bound organelle named the hydrogenosome [1–3].

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 [4]. 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 *T. foetus*, do not have a known usual programmed death machinery [5].

Despite apoptosis has been shown to be the major mechanisms of death observed in *T. foetus* [6], different studies suggest the existence of more than one mechanism of cell death and, it's the type of stimuli and/or the degree of stimuli that determines the mechanism of death chosen by this cell.

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 [7]. It has been assumed that the machinery of PCD is absent in amitochondrial organism, like trichomonads, however, recent studies show that *T. foetus* have the capacity of choose which way will take among many forms of cell death caspases dependent or independent, depending on the stimuli of this parasite, once individuals in the same culture can take different pathways, either inside of the parasite can occur more than one mechanism of cellular death [4]. It has become increasingly apparent that the mechanisms of cell death

#### **Figure 1.**

*Proposed model for possible executioners pathways during* T. foetus *cell death, which includes the presence of possible apoptosis, autophagy, paraptosis and necrosis. Abbreviations: 1 = apoptosis, 2 = autophagy, 3 = necrosis, 4 = Paraptosis, DF = death receptor, PS \* = phosphatidylserine exposure, CAD = caspase-activated DNase, ICAD = inhibitor of CAD.*

*Cell Death after Photodynamic Therapy Treatment in Unicellular Protozoan Parasite… DOI: http://dx.doi.org/10.5772/intechopen.94140*

show a large diversity of phenotypes and cellular mechanisms, and, apparently, a modulation mechanism of cell death may lead to another [8]. Besides, the conservation of the molecular mechanism is relevant to the functional role of PCD process in the biology of protozoa since studies confirm the existence of this process in unicellular eukaryotes of different phylogenetic origins [9]. Hypotheses of alternative pathways of trichomonads cell death are suggested in **Figure 1**.

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) [11], *Entamoeba histolytica*, *Giardia lamblia*, *Trichomonas vaginalis*, *Tritrichomonas foetus* and rumen ciliates, they have hydrogenosomes instead of mitochondria. Hydrogenosomes of trichomonads are involved by two membrane layers, as mitochondria, and are organelles related to oxidation of pyruvate and the synthesis of ATP, as well as the storage of Ca2+ [12, 13]. As shown previously [14], Succinyl-coenzyme A synthetase (SCS) catalyzes the formation of ATP via substrate-level phosphorylation in hydrogenosome as it does, in mitochondria. Both organelles, mitochondria and hydrogenosome, use pyruvate as a major substrate and form acetyl-CoA (**Figure 2a, b**). Hydrogenosomes convert pyruvate quantitatively to acetate, malate, CO2, and H2, with acetate as the major product. The electrons from pyruvate:ferredoxin oxidoreductase (PFO) pass through ferredoxin and are transferred to NAD or NADP by ferredoxin:NAD(P) oxidoreductase. This process is accompanied by the phosphorylation of ADP to ATP in presence of

#### **Figure 2.**

*Comparison of oxidation metabolism of pyruvate between hydrogenosome and mitochondria. (a) In* T. foetus *and probably in all other trichomonad flagellates acetate is formed by the sucessive action of ASCoA (reaction acetyl-CoA+succinate = acetate + succinyl-CoA) and ST (reaction succinyl-CoA+ADP+Pi = succinate + ATP). ASCoA = acetate:succinate CoA transferase; PFO = pyruvate:ferredoxin oxidoreductase; ST = succinate thiokinase. (b) Piruvate oxidation in mitochondria precedes the Krebs cycle. PDC = pyruvate dehydrogenase. SCS = succinyl CoA sintetase.*

succinate and acetate:succinate CoA-tranferase. The production of H2 is catalyzed by a hydrogenase which transfers electrons to protons H+ , a process not available to organisms without hidrogenosomo (**Figure 2a**) [11].

Evidences indicate that hydrogenosomes are anaerobic forms of mitochondria [15] or a specialized form of mitochondria useful in lower O2 environments [16]. According to Martin (2005) [16] hydrogenosomes and mitochondria are, respectively, anaerobic and aerobic manifestations of the same organelle. Although, unlike mitochondria, the hydrogenosomes lack the DNA [15].

Trichomonad hydrogenosomes possess many proteins in common with mitochondria [15]. Translocation studies using hydrogenosomal ADP/ATP carrier of *T. vaginalis* revealed compatibility in membrane protein import between mitochondria and hydrogenosomes. These hydrogenosomal ADP/ATP carriers utilize the same translocation pathway for translocation into mitochondrial inner membrane [17]. Hydrogenosomes also contain heat-shock proteins which are known to participate in protein translocation and folding in mitochondria [17].

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 *T. vaginalis* genome DNA that in other eukaryotes codes for a protein located in mitochondria. This suggests that trichomonads have had mitochondria in their early history, and this nuclear sequence could be the result of an ancient gene transfer from mitochondria to nucleus [18]. Many components of classical mitochondria are absent in hydrogenosomes, and they generate molecular hydrogen instead of consume oxygen. It is interesting to study whether hydrogenosomes are involved in the cell death at all [1].

During hydrogenosome formation, they have different forms, and then acquire a spherical structure, which can be changed in stress conditions [19]. Studies proposed that *T. foetus* under treatment with hydroxyurea, zinc or under serum deprivation present endoplasmic reticulum cisternae surrounded by abnormal hydrogenosomes, which have bigger size enlarged peripheral vesicles, and sometimes presenting a degraded aspect [20, 21].

Hydrogenosomes also exhibited altered size and shape and they were randomly distributed within parasites cells after lycorine treatment [2]. 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 [20]. *T. foetus* treated with taxol, nocodazole or colchicine showed modifications in size, shape and distribution of the hydrogenosomes [22]. The presence of hydrogenosomes with altered morphologies was also observed in parasites incubated with different concentrations of thiabendazole [23]. Recent studies showed that alterations in lysosomes and hydrogenosomes were also observed in *T. foetus* after proteossome inhibitors treatment [24]. Other studies suggest that tetracycline disrupts hydrogenosomal function since it reduced the hydrogenosomal energy metabolism efficiency in *T. vaginalis* [25].

#### **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 [7]. PCD It now generally refers to any cell death that is mediated by the intracellular death program, no matter what triggers it

*Cell Death after Photodynamic Therapy Treatment in Unicellular Protozoan Parasite… DOI: http://dx.doi.org/10.5772/intechopen.94140*

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 nonapoptotic 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 [28].

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.

### **3.1 Apoptosis**

*APOPTOSIS* is characterized by specific morphological and biochemical changes of dying cells, including cell shrinkage, nuclear condensation and fragmentation, dynamic membrane blebbing and loss of adhesion to neighbors or to extracellular matrix. The biochemical changes include chromosomal DNA cleavage into internucleosomal fragments, phosphatidylserine externalization and a number of intracellular substrate cleavages by specific proteolysis [33].

#### **3.2 Autophagic**

*AUTOPHAGIC* is characterized by the sequestration of cytoplasm and organelles in double or multimembrane vesicles and delivery to the cells own lysosomes for subsequent degradation, exhibits extensive degradation of Golgi apparatus, polyribosomes and endoplasmatic reticulum, which precedes nuclear destruction [27, 34]. Later, swelling of cavities was observed and the dying cells were ultimately fragmented and were phagocytosed by the neighboring cells. The process of autophagy depends on both continuous protein synthesis and the continuous presence of ATP. The molecular mechanisms have been extensively studied in yeast and mammalian orthologues continue to be elucidated [35, 36]. In PDT treatments, a process employing UV-light and photosensitizes to kill cancer cells, as well as accumulation of lysosomotropic agents within the organelle, a process also triggers the lysosomal pathway of cell death [37, 38].

#### **3.3 Citoplasmatic death or programmed necrosis**

*CITOPLASMATIC DEATH or PROGRAMMED NECROSIS* is characterized by the swelling of cavities with a membrane border, such as mitochondria, followed by extensive fragmentation of the cells into fragments so small that cell debris can no longer be observed. This type of cell death occurred without the lysosomal system taking part and with recognizable reaction of the neighboring cells and was observed in regions of vacuolated cartilage during mineralization [27].

There is evidence that modulation of one form of cell death may lead to another [7]. Under some circumstances, apoptosis and autophagy can exert synergetic effects, whereas in other situations autophagy can be triggered only when apoptosis is suppressed [33].
