Entamoeba histolytica: A Mystery in Evolutionary History

By CS

Figure 1: Life cycle of Entamoeba histolytica 

The enteric protozoan, Entamoeba histolytica, is a major amoebic pathogen of humans, infecting approximately 50 million people and causing about 100,000 deaths every year ^[1,2].  The parasite is endemic in developing countries and is usually contracted from fecal-contaminated water or food ^[3].  Infection begins with the ingestion of a dormant cyst, which can release the motile trophozoite form in the intestine of the host (Fig.1) ^[1].   While these trophozoites are often asymptomatic, in 10% of infected individuals these forms are invasive, causing colitis by breaching the colon or inducing abscesses in the liver ^[1]. Not only has research regarding its pathology been beneficial, but the absence of typical eukaryotic cell construction in this microbe has also contributed to interesting insights on protozoan evolution and cell biology.

Introductory biology classes tend to harp on the differences between prokaryotic and eukaryotic organisms and focus on the increased complexity in cellular components of the latter.  The development of these cellular discrepancies is then placed on a timeline to infer an evolution towards increasingly intricate organisms. This tendency gives the false notion that evolution only works one way, towards a more complex organism.  It is not surprising, then, that seemingly simple eukaryotes are initially thought to have diverged early from the first nucleus-containing cell.  E. histolytica fell victim to such misplacement.  Cellular structures in the protozoan are largely absent; in the past these parasites have been described as “bags of cytoplasm with a nucleus” ^[4].  E. histolytica, unlike other protozoa, lack typical mitochondria, the Golgi apparatus, rough endoplasmic reticulum (ER), and peroxisomes ^[4,5].  These observations led to the conclusion that E. histolytica had diverged before the development of these organelles ^[5].  However, recent research has introduced the possibility that the parasite lost these structures, suggesting a later divergence along the timeline (Fig.2).

Figure 2: Evolutionary timeline. E. histolytica was previously
believed to have diverged soon after the first nucleated cell.
Upon re-examination, divergence likely occurred more recently.

Prior to 1997, most research on E. histolytica yielded no evidence of an endoplasmic reticulum or Golgi complex ^[6]. Instead, what researchers did see were large vacuoles within the protozoan ^[6]. To identify these compartments, one research group probed the vacuoles utilizing dyes and markers specific for the detection of ER and Golgi components ^[6]. This research found that some of the vesicles were reacting positively to the markers, suggesting that the vacuoles corresponded to remnants of the ER and Golgi ^[6]. More recently, it has been proposed that E. histolytica contains a continuous ER as opposed to separate ER-like vesicles. Making use of a green fluorescent protein (GFP) recombinant protein that could be retained within a typical ER, researchers in Vermont attempted to visualize ER-like components in the protozoan. Their results illustrated that the protein fluoresced in a reticular pattern, was excluded from cytoplasmic regions of the protozoan, and co-localized with a known ER chaperone protein. The study concluded that a continuous ER-like compartment housed the GFP fusion protein ^[5]. This discovery shows the similarity between E. histolytica and the text-book eukaryotic cell, which also contains a continuous ER organelle. Perhaps this similarity suggests a more recent degeneration of the ER compared to other lost organelles.

Because it lacks obvious mitochondria, E. histolytica were initially believed to have diverged before the origin of this structure ^[1]. However, in 1999, small organelles called mitosomes were discovered in the protozoan ^[1]. These structures have also been found in Giardia intestinalis, another intestine-infecting protozoan ^[7]. Like the hydrogenosomes of the protozoan Trichomonas vaginalis, mitoses are hypothesized to be a degenerated form of mitochondria ^[1]. Unlike classical mitochondria, these structures lack any traces of DNA and play no apparent role in energy production ^[7,8]. Pyruvate oxidation via the Krebs cycle and electron transport proteins, both of which are central characteristics of text-book mitochondria, are also absent within the mitosome ^[8]. Because it is ‘power-house’ deficient, the glycolytic pathway of E. histolytica produces a very limited amount of ATP, forcing the parasite to scrounge for less convenient forms of energy ^[8].

Although it lacks many characteristics of the classical mitochondrion, certain properties of the mitosome provide evidence for its mitochondrial origin. The presence of a double membrane, several mitochondrial-like proteins, and functional protein import pathways indicate that these structures could be derived from mitochondria (Fig.3) ^[7,8].  This finding further suggests that E. histolytica underwent secondary loss of typical organelles and is not as primitive as initially believed. The development of the mitosome may be attributable to the fact that this parasite thrives in the anaerobic conditions within a host’s digestive tract upon infection ^[7].  In such an environment, oxidative phosphorylation, a process requiring oxygen and occurring in the mitochondria, would not be able to run. Thus, if this mitochondrial mechanism does not work where E. histolytica likes to play, the parasite can afford to dump the program.  Although degeneration might not be this simple, these environmental circumstances may partly explain the loss of key mitochondrial components.

Figure 3. Reduced complexity of mitochondrial import proteins
in the E. histolytica mitosome membranes compared to those
found in standard mitochondria. 

Some researchers have also observed the presence of small DNA containing vesicles in E. histolytica. Two types of these vesicles have been described in literature: cryptons and EhkOs ^[9]. The function and origin of these structures is still largely a mystery as researchers have found opposing data regarding their properties. It is also important to note that these compartments as well as the mitosome were all discovered and named by different research groups. Thus, there is some confusion as to whether some compartments are identical. For example, some researchers believe that cryptons and EhkOs constitute the same vesicle ^[9]. Others, skeptical over whether mitosomes actually lack DNA, believe that EhkOs are the same as mitoses. Debates also question whether each structure is derived from the nucleus or a product of a degenerate mitochondrion. Obtaining concurring data on the origins as well as the function of these compartments would help in understanding the developments within E. histolytica throughout its evolutionary history. 

Secondary loss of organelles has also been found in other human parasites such as the anaerobic protozoans G. intestinalis, Blastocystis hominis and T. vaginalis as well as the intracellular microsporidians Antonospora locustae and Trachipleistophora hominis ^[1,7]. Perhaps organelle degeneration reflects a transitioning process by which microorganisms become increasingly dependent on another organism to survive. Thousands of years from now, will a modern day microbe slowly diverge in a similar fashion, evolving into a future human parasite? In the case of Entamoeba histolytica, the rejection of eukaryotic cellular complexity to develop a simpler internal structure seems to be of benefit, even if it risks being dubbed “primitive” ^[10].

References:

1. Morf, Laura, and Upinder Singh. 2012. Entamoeba histolytica: A Snapshot of Current Research and Methods for Genetic Analysis. Current Opinion in Microbiology 15 (4) (8): 469-75.

2. Ali, I. K., Haque, R, Siddique, A., Kabir, M., Serman, N.E., Gray, S. A., Cangelosi, G. A., and Petri, W. A., Jr. 2012. Proteomic Analysis of the Cyst Stage of Entamoeba histolytica. PLoS Neglected Trop. Dis. 6, e1643.

3. Sehgal D. Bhattacharya A. Bhattacharya S. 1996. Pathogenesis of Infection by Entamoeba histolytic. Journal of Biosciences 21 423–432.

4. Clark, C. G. 2000. The Evolution of Entamoeba, a Cautionary Tale. Res. Microbiol. 151:599–603.

5. Teixeira, J. E., and C. D. Huston. 2008. Evidence of a Continuous Endoplasmic Reticulum in the Protozoan Parasite Entamoeba histolytica. Eukaryot. Cell 7:1222-1226.

6. Mazzuco, A., M. Benchimol, and W. De Souza. 1997. Endoplasmic Reticulum and Golgi-like Elements in Entamoeba. Micron 28 (3) (6): 241-7.

7. Dolezal P, et al. 2010. The Essentials of Protein Import in the Degenerate Mitochondrion of Entamoeba histolytica. PloS Pathog. 6:e1000812.

8. Aguilera, Penelope, Tara Barry, and Jorge Tovar. 2008. Entamoeba histolytica Mitosomes: Organelles in Search of a Function. Experimental Parasitology 118 (1) (1): 10-6.

9. Herrera-Aguirre ME, Luna-Arias JP, Labra-Barrios ML, Orozco E. 2010. Identification of four Entamoeba histolytic Organellar DNA polymerases of the family B and Cellular Localization of the Ehodp1 Gene and EhODP1 Protein. J Biomed Biotechnol, 2010: 734898.

10. Bakker-Grunwald, T., and C. Wöstmann. 1993. Entamoeba histolytica as a Model for the Primitive Eukaryotic Cell. Parasitology Today 9 (1) (1): 27-31.