Secreted Proteins in Microsporidian Parasites: A Functional and Evolutionary Perspective on Host-parasite Interactions.

Abstract

The Microsporidia form a phylum of obligate intracellular parasites known to cause disease in humans and a diverse range of economically important animal species. Once classified as ‘primitive’ eukaryotes, it is now recognised that the peculiarities of microsporidian genomics and cell biology are, in fact, the consequence of extreme reduction allowed by an intimate relationship with the host cell. Excluding survival as an extracellular spore, microsporidia are in direct contact with the host throughout their developmental lifecycle, from entry to egress. Host cell manipulations have been described in morphological terms, but despite this, characterisation of such processes at the molecular level remains challenging. The logistics of the microsporidian lifecycle suggest secreted proteins and membrane proteins with extracellular domains may be involved in virulence and implicated in host cell manipulation.

This study employs bioinformatic tools to predict secreted proteins in diverse microsporidia and comparative genomics to identify conserved proteins which may be required for host cell manipulation, pathogenicity and lifecycle progression. The protein complement secreted into the extracellular environment during microsporidian spore germination, a lifecycle stage required for host cell invasion, is identified experimentally. This analysis suggests that novel microsporidian specific hypothetical proteins, that is, proteins with no functional annotation or domain, play a significant role during parasite invasion of the host and provides the first identification of potential microsporidian effector proteins. Aiming to address microsporidian pathogenicity during intracellular stages, candidate virulence factor proteins, namely a hemolysin and a protein tyrosine phosphatase are also characterised and localised in situ.

Lastly, an animal-derived horizontal gene transfer event is used in conjunction with both the fossil record and molecular dating approaches to add timescale to the microsporidian diversification. This work suggests that microsporidia radiated recently, achieving extreme cellular diversity, acquiring a novel infection mechanism and undergoing vast speciation in a short evolutionary timescale, likely within the last 200 million years.