The Mitochondrion-Related Organelles of Blastocystis

Blastocystis is an anaerobic microbial eukaryote that inhabits the gut of various animals. While it was considered to be a parasite, its pathogenicity is controversial, since recent estimations suggest that Blastocystis could be present in more than one billion humans. Blastocystis belongs to the stramenopiles (heterokonts), a diverse group of eukaryotes that also include unicellular diatoms and giant multicellular kelps. The Blastocystis cell lacks typical features of other stramenopiles, its genome has laterally acquired many genes from other prokaryotes and eukaryotes, resulting in having exceptional biochemistry and unique adaptations of its mitochondria. For example, Blastocystis mitochondrion-related organelles (MROs) have characteristics of typical mitochondria, including Complex I and II from electron transport chain, mitochondrial DNA, Fe-S cluster assembly and amino acid metabolism, but also proteins that are typically present in obligate anaerobes including FeFe-Hydrogenase, pyruvate metabolism and alternative oxidase. Some of the pathways have been localized and biochemically characterized, providing a better understanding on the functions of these organelles. In this chapter, we will present and examine the status quo regarding the biology and morphology of Blastocystis MROs and we will discuss future avenues on exploring even further adaptations of these organelles

The Mitochondrion-Related Organelles of Cryptosporidium Species

Amongst all apicomplexans, the mitochondrion-related organelle of Cryptosporidium species is the least studied. To date, most of our knowledge on this tiny organelle stems from observations on the remnant mitochondrion, mitosome, of Cryptosporidium parvum. In C. parvum the mitosome is structurally distinguished from the hydrogenosomes and mitosomes of other anaerobic protists by its (1) close association with the crystalloid body, an organelle unique to this apicomplexan and the function of which is currently unknown; (2) close association with the outer nuclear membrane, and possibly nuclear pores; (3) envelopment by rough endoplasmic reticulum, and in some cases an apparent direct tethering to ribosomes; and (4) atypical internal membranous compartments that lack well-defined crista junctions with the mitochondrial inner membrane, a characteristic that defines most aerobic mitochondria. Like most hydrogenosome- and other mitosome-bearing anaerobic protists, however, C. parvum lacks a mitochondrial genome, i.e. proteins are encoded by the nucleus and targeted back to the mitosome. As a consequence of this reductive evolution, there are no genes for electron transport or oxidative phosphorylation, and the only function so far ascribed to this tiny organelle is one common to all eukaryotic mitochondria, the assembly and maturation of iron-sulphur clusters. The ultrastructure and tomography of the C. parvum mitosome and crystalloid body, as well as the probable functions of these organelles, are the primary topics herein. An overview of iron-sulphur cluster biosynthesis, likely mechanisms for import into and export from the mitosome, as well as core carbohydrate and energy metabolism are also discussed. Similarities and differences in the structure and function of both organelles in the genus Cryptosporidium, with anaerobic protists in general, and with other apicomplexans specifically, are described