Apoptosis in Trypanosomatids: Evolutionary and phylogenetic considerations

MINI-REVIEW

Apoptosis in Trypanosomatids: Evolutionary and phylogenetic considerations

Marcello A. Barcinski

Departamento de Parasitologia, ICB/USP, Av. Prof. Lineu Prestes, 1374, 05508-900 São Paulo, SP, Brasil. Tel.: [phone]. Fax: [phone]. E.mail: [e-mail].

ABSTRACT

Programmed cell death (PCD) or apoptosis, an active process of cell death, plays a central role in normal tissue development and organogenesis, as well as in the pathogenesis of different diseases. Although it occurs in diverse cells and tissues under the influence of a remarkable variety of inducing agents, the resultant ultrastructural and biochemical changes are extremely monotonous, indicating the existence of a common biological mechanism underlying its occurrence. It is generally accepted that a developmental program leading to cell death cannot be advantageous to unicellular organisms and that PCD appeared in evolution to fulfill the organizational needs of multicellular life. However, the recent description of apoptotic death occurring in three different species of pathogenic kinetoplastids suggests that the evolutionary origin of PCD precedes the appearence of multicellular organisms. The present study proposes that a population of pathogenic Trypanosomatids is socially organized and that PCD is a prerequisite for this organization and for the fulfillment of the demands of a heteroxenic lifestyle. This proposal includes possible roles for PCD in the development of the parasite in the insect vector and/or in its mammalian host and suggests experimental strategies to localize the evolutionary origin of PCD within the kinetoplastids.

INTRODUCTION

Programmed cell death (PCD) or apoptosis is a process of genetically controlled cell death. It has been clearly shown that active cell death under the control of PCD plays a pivotal role in normal development, maturation and organogenesis in a wide range of animal species. PCD is responsible for the physiological elimination of unnecessary, superfluous or potentially harmful cells during embryogenesis and in processes of continuous cell renewal systems in adult organisms (White, 1996). Aberrant, misplaced or extemporaneous induction or inhibition of PCD can lead to different diseases. The possibility of manipulating, under controlled conditions, the occurrence of PCD has enormous therapeutic potentialities (Thompson, 1995). All these reasons have raised great excitement about PCD among the biomedical community. The selection process for life or death pathways as well as signalization, control and execution steps involved in PCD requires the operation of different elements of an intercellular communication network. This fact added to the biased a priori belief that unicellular organisms cannot benefit from an autoinflicting death program has led some authors to consider PCD as a prerogative of multicellular organisms and thus with an evolutionary origin necessarily downstream of the onset of multicellularity (Vaux et al., 1994; Evan, 1994).

The present study proposes that multicellularity is indeed a prerequisite for the operation of PCD but not necessarily organized as a single multicellular organism. According to this proposal, PCD can occur in any situation where living cells display features of a structured organization whose members have developed patterns of relationships and division of labor through interactions among themselves and/or with elements of their environment. This definition includes protozoan parasites growing and differentiating in culture, their insect vectors, or their mammalian host. Such organization can generate a new level of individuality, equally amenable as any multicellular organism to natural selection. In a population of multicellular organisms, the action sites of natural selection are individuals displaying a certain degree of variability within themselves. Evolution proceeds by positively selecting individuals to whom a specific phenotypic trait confers an adaptive advantage. The mutated phenotype is usually expressed in a specific tissue or organ of a multicellular organism, but can only be selected if it interferes, within a specific environment, with the fitness of the individual bearing the mutation. In a population of unicellular organisms, a mutated phenotype, expressed in part of the population, can be selected if, as in the aforementioned situation, it increases the fitness of the entire population. In the case of infective unicellular protozoa, a developmental feature displayed by some of its members (e.g. apoptosis) may eventually enable an entire population to survive and multiply in different hosts and be transmitted by different vectors. Evolution of a selected population can occur independent of the mechanism by which their genes are transmitted. These ideas emerged from the recent description of PCD in Dictyostelium discoideum (Cornillon et al., 1994), an ameboid fungus whose cell cycle includes multiplication as a unicellular protist and development of a multicellular structure under special conditions, and especially in the bona-fide unicellular organisms Trypanosoma cruzi (Ameisen et al., 1995), Trypanosoma brucei (Welburn et al., 1996), and Leishmania (L) amazonensis (Moreira et al., 1996), three examples of vector-transmitted heteroxenic pathogenic kinetoplastid parasites. That unicellular organisms can live communally and organize themselves as multicellular structures for performing specific tasks is not a new theory. Several examples, in different bacteria, have been described since the beginning of this century (Shapiro, 1988), and recently reviewed (Shapiro, 1995). According to this author, in spite of descriptions of multicellular organizations in bacterial populations, the perception of bacteria as functional unicellular organisms persists as a heritage of the medical origin of microbiology. The demonstration that pure cultures derived from single pathogenic bacteria can, upon inoculation, reproduce the disease has strongly imprinted this idea. This notion permeates through parasitology, a science also derived from medical practice. Pathogenic protozoan parasites are generally viewed as non-gregarious cells able to autonomously fulfill all the requirements of a heteroxenic lifecycle and pathogenicity to their hosts. Several examples, however, strongly suggest that this is not the case, and that intercellular signalization among themselves and with cells and molecules from their hosts and vectors plays a role in their differentiation and virulence (Barcinski and Costa Moreira, 1994; Welburn et al., 1997). The three examples of heteroxenic pathogenic protozoa display apoptotic features much closely resembling cells of higher eukaryotes than the ones displayed by D. discoideum, a free living slime mold.

Independent of which alternative explanation one accepts for the origin of parasitism among the kinetoplastids, the appearance of heteroxenic forms which alternate between vertebrate and invertebrate hosts has coevolved with the acquisition of hematophagic competence by some invertebrate species (Simpson and Maslov, 1994).

Evolutionary trees based on different molecular markers have shown that Leishmania spp. and Endotrypanum belong to a cluster that branched much later than T. brucei and T. cruzi. Also, Phytomonas spp. - heteroxenic parasite of plants-belongs to a different evolutionary cluster than the other four species (Fernandes et al., 1993). This fact implies that a heteroxenic life cycle originated more than once in independent evolutionary events (Fernandes et al., 1993). Within this scenario, where apoptosis has already been described in three different species of heteroxenic mammalian pathogenic parasites, several questions can be asked: is apoptosis displayed by free-living kinetoplastids or is it a specific feature of a parasitic lifestyle? Has apoptosis coevolved with a heteroxenic lifecycle? If so, is this phenotype an adaptation for parasite multiplication in the insect vector, for infectivity in the definitive host, or both? Is apoptosis a prerequisite for infectivity in a mammalian host?

These questions can be more or less completely answered by looking for the occurrence of apoptosis in free-living kinetoplastids as well as monoxenic and plant Trypanosomatids. Whether apoptosis in these unicellular organisms is regulated by similar molecular mechanisms as those used from nematodes to man (Yuan, 1996) is not a less fascinating question, equally amenable to experimental approaches.

Considering the already described cases - T. brucei, T. cruzi and Leishmania mexicana amazonensis - PCD can be envisioned as one of the requisites for a parasite population to infect, survive, multiply and differentiate inside an insect vector and/or a mam- malian host. In the insect, PCD can aid in keeping the number of parasites constant, as described in the infected tsetse fly (Maudlin and Welburn, 1994). In this situation, PCD can act by minimizing the harmful effects that parasite overloading can have on vector fitness, increasing the transmission rate. As already stated, PCD can only be a selective advantage if its occurrence is limited to part of the infecting population. This limitation precludes clearance of the parasites and characterizes a clear-cut division of tasks and altruistic behavior of some of its members. This has been postulated to occur in the development process of Leishmania spp. in suprapapylaria vectors (Vickerman, 1994; Schlein, 1993). Some parasites, maybe from a specific sub-population, produce chitinase enabling the population to evade the peritrophic membrane and destroy the estomodal valve (Schlein, 1993). Evasion of the peritrophic membrane allows promastigote migration to the vector midgut and develop further into the infective stage after differentially glycosylating their LPG (Sacks et al., 1994). Destruction of the estomodal valves is considered a prerequisite for the vector to inoculate infective parasites in the mammalian host (Schlein, 1993). As a hypothesis one can speculate that activation of Leishmania at different stages of development by vector-derived ligands can differentially induce promastigote proliferation or its death by apoptosis. Promastigote’s positive and negative selection in the insect vector is compatible with the demonstration of a differential sensitivity of different morphotypes to vector saliva (Charlab et al., 1995). If, within the kinetoplastids, apoptosis turns out to be an exclusive trait of heteroxenic species, it is reasonable to postulate that it had coevolved with insect hematophagic habits as exemplified by sensitivity of promastigotes to hemoglobin (Schlein, 1993) and hemin (Charlab et al., 1995), T. cruzi epimastigotes to a globin fragment (Flawia et al., 1997), and the procyclic forms of T. brucei to low-density lipoprotein (LDL) (Bastin et al., 1996). This last example is extremely informative since a receptor for LDL has been found throughout the kinetoplastid order. Leishmania donovani, the monoxenic parasite Crithydia luciliae, and the plant parasite Phytomonas characias accumulate significantly less LDL than bloodstream or culture forms of T. brucei. Also, a panel of monoclonal antibodies raised against the T. brucei receptor displayed different affinities when tested against different species. These results may indicate that the homologues of T. brucei LDL receptor display minor structural and functional differences in other species of kinetoplastids. Apoptosis gene(s) expression, developmentally regulated by factor(s) of vector origin, could explain the tightly regulated specificity of vector competence. This hypothesis demands that the heteroxenic pathogenic Trypanosomatids be monoclonal in nature as shown to be the case (Tibayrenc et al., 1990). In the mammalian host, apoptosis could be useful for the parasite population in situations where an inflammatory reaction induced by dying parasites is to be avoided. Also, as an absolute speculation, antigens from an infective parasite dying from apoptosis could induce a host response preferentially of the CD4⁺ Th2 type. Both of these situations would facilitate progression of the infection induced by the nondying forms, characterizing a form of cooperativity within the infective population.

ACKNOWLEDGMENTS

Research supported by CNPq/PRONEX and FAPESP. The author wishes to thank Drs. Bernardo Beiguelman, Erney Camargo, Henrique Krieger, and Jeffrey Shaw for useful discussions. Publication supported by FAPESP.

RESUMO

A morte celular programada (PCD) ou apoptose, um processo ativo de morte celular, desempenha um papel fundamental no desenvolvimento tecidual normal e na organogênese, assim como na patogênese de diferentes doenças. Embora este processo ocorra em uma gama variada de diferentes células e tecidos, sob a influência dos mais diversos agentes indutores, a resultante morfológica e bioquímica do processo é extremamente monótona, sugerindo que um mecanismo único opere em todas as situações. Era consensualmente aceito que um programa de morte programada não poderia ser vantajoso para organismos unicelulares e que a PCD teria surgido na evolução para servir às necessidades organizacionais dos seres multicelulares. No entanto, a descrição de PCD em três diferentes espécies de tripanosomatídeos patogênicos indica que a origem da PCD é anterior à da multicelularidade. A hipótese colocada neste trabalho é a de que uma população de tripanosomatídeos tem uma organização social e que esta é necessária ao cumprimento das exigências biológicas de um parasita heterôxênico. A proposta inclui possíveis papéis biológicos para a PCD no desenvolvimento do parasita no inseto vetor e/ou no hospedeiro mamífero e sugere abordagens experimentais para se localizar a origem evolutiva da PCD entre os cinetoplastídeos.

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(Received October 15, 1997)

Publication Dates

• Publication in this collection
  06 Jan 1999
• Date of issue
  Mar 1998

History

• Received
  15 Oct 1997