Skip to content

Mitosomes of Entamoeba histolytica are abundant mitochondrion-related remnant organelles that lack a detectable organellar genome (Leon-Avila and Tovar, 2004)

This article from Tovar’s group shows that mitosomes contain no DNA using the mitosome marker Cpn60 and a DNA marker. Seen from the perspective that mitochondria were derived from the eukaryotic genome (nucleus), it can be argued that mitochondrial-like organelles without DNA, but with protein targeting and biochemical function, are possible. The step to incorporation of some DNA seems small. Of course, Tovar sees it from the assumption that it is an indisputable fact that mitochondria were once an endosymbiont. The conclusion that EhKOs and mitosomes are distinct and unrelated cellular structures seems unjustified, it is merely conjecture and does not ‘demonstrate unequivocally’. This seems to be the strategy for the Martin/Tovar groups: just shout that it is the absolute truth.

Abstract: The existence of mitochondrion-related relict organelles (mitosomes) in the amitochondrial human pathogen Entamoeba histolytica and the detection of extranuclear DNA-containing cytoplasmic structures (EhKOs) has led to the suggestion that a remnant genome from the original mitochondrial endosymbiont might have been retained in this organism. This study reports on the mutually exclusive distribution of Cpn60 and extranuclear DNA in E. histolytica and on the distribution of Cpn60-containing mitosomes in this parasite. In situ nick-translation coupled to immunofluorescence microscopy failed to detect the presence of DNA in mitosomes, either in fixed parasite trophozoites or in partially purified organellar fractions. These results indicate that a remnant organellar genome has not been retained in E. histolytica mitosomes and demonstrate unequivocally that EhKOs and mitosomes are distinct and unrelated cellular structures.

The first paragraph of the introduction makes some claims that are mainly wishful thinking, and are basically speculation upon speculation. This makes it difficult to tell fact from fiction in their articles. 

Over the past few years mitochondrial remnant organelles (mitosomes) have been identified in a number of amitochondrial parasitic protozoa (e.g. Giardia intestinalis, Entamoeba histolytica, Trachipleistophora hominis and Cryptosporidium parvum) (Mai et al., 1999; Tovar et al., 1999, 2003; Williams et al., 2002; Riordan et al., 2003), disproving the Archezoa hypothesis, which postulated that amitochondrial eukaryotic organisms were the direct descendants of the nucleated cell that hosted the original mitochondrial endosymbiont (Cavalier-Smith, 1983, 1998). It is now apparent that eukaryotic microbes that lack typical mitochondria are not primitively amitochondrial but are highly derived descendants of mitochondrioncontaining ancestors whose capacity for aerobic respiration has been gradually lost during the course of evolution.

Mitosomes have a function in biosynthesis of Fe-S clusters and the proteins involved have been identified in other amitochondriate organism, indicating that they were always nuclear-encoded. They seem to favor a story in which all these genes were transferred to the nucleus from the original endosymbiont, and that somehow the mitochindria were rduced to a mitosome.

Perhaps the most significant finding in relation to the biology of mitosomes is the direct demonstration that Giardia mitosomes function in the biosynthesis of molecular iron–sulphur (Fe–S) clusters and in their subsequent incorporation into functional Fe–S proteins (Tovar et al., 2003). Genes encoding several proteins involved in Fe–S cluster metabolism have also been identified in the genomes of several other amitochondrial organisms, including Encephalitozoon cuniculi, Entamoeba histolytica, C. parvum and Trichomonas vaginalis (Katinka et al., 2001; Tachezy et al., 2001; LaGier et al., 2003; Bankier et al., 2003; and, leading to the suggestion that the requirement for Fe–S proteins probably represents the selective pressure driving the retention of the original mitochondrial endosymbiont in all eukaryotic lineages (Tovar et al., 2003; van der Giezen & Tovar, 2004; Embley et al., 2003b).

But in the next paragraph the evidence upon which all their data is based is rather thin (prokaryotic phylogenetic affinity?)

The presence of a remnant organellar genome with prokaryotic phylogenetic affinity is perhaps the strongest evidence for the bacterial ancestry of mitochondria.

The EhKOs are very interesting and I have to read more about them. Apparently, it is clear that these structure emanate from the nucleus whereas mitosomes are derived from aputative endosymbiont. Beats me how they can conclude the latter, also given the fact (see Intro) that mitosomes have been just discovered. EhKO’s are in line with hypthesis that organelles are derived from nucleus and acquired relative independence.

All known endosymbiosis-derived organelles, i.e. mitochondria, chloroplasts, mitosomes and hydrogenosomes, are self-replicating entities that segregate faithfully into each daughter cell following asexual cell division. In contrast, the proportion of the parasite population harbouring EhKOs appears to be cell-cycle-dependent, with as few as 10 and 20 % of the population displaying these structures at G2 and S phases, respectively (Rodríguez et al., 1998). In experiments with unsynchronized parasite trophozoites the percentage of the cell population harbouring extranuclear DNA does not exceed 50 % (Orozco et al., 1997; Rodríguez et al., 1998; Luna-Arias et al., 1999), a distribution not consistent with that of an essential cellular organelle carrying out vital cellular functions. EhKOs have been shown to emanate from the cell nucleus, to be cell-cycle-regulated and to contain eukaryotic-type rRNA, a pyruvate : ferredoxin oxidoreductase-like protein and a number of eukaryotic-type transcriptional regulators, including a TATA box-binding protein, an enhancer-binding protein and a tumour-suppressor protein (Orozco et al., 1997; Rodríguez et al., 1998; Luna-Arias et al., 1999; Solis et al., 2002; Marchat et al., 2003; Mendoza et al., 2003).

The presence of eukaryotic-type nuclear components and the recent demonstration that EhKOs emanate from the cell nucleus confirm the autogenous nuclear origin of EhKOs and invalidate the suggestion that these cellular structures might be self-replicating organelles of endosymbiotic origin. The apparent biogenesis of minute vesicles from the cell nucleus in the diplomonad G. intestinalis (Benchimol, 2002) – akin to the process observed in E. histolytica (Solis et al., 2002) – betrays the existence of an enigmatic biological phenomenon which may be unique to these parasites and which deserves further investigation. Given the different origins, cellular distribution and molecular composition of mitosomes and EhKOs, we conclude that mitosomes and EhKOs of E. histolytica are distinct and unrelated subcellular structures.

Post a Comment

You must be logged in to post a comment.