Protein variation in blood-dwelling schistosome worms generated by differential splicing of micro-exon gene transcripts

Abstract

Schistosoma mansoni is a well-adapted blood-dwelling parasitic helminth, persisting for decades in its human host despite being continually exposed to potential immune attack. Here, we describe in detail micro-exon genes (MEG) in S. mansoni, some present in multiple copies, which represent a novel molecular system for creating protein variation through the alternate splicing of short (#36 bp) symmetric exons organized in tandem. Analysis of three closely related copies of one MEG family allowed us to trace several evolutionary events and propose a mechanism for micro-exon generation and diversification. Microarray experiments show that the majority of MEGs are up-regulated in life cycle stages associated with establishment in the mammalian host after skin penetration. Sequencing of RT-PCR products allowed the description of several alternate splice forms of micro-exon genes, highlighting the potential use of these transcripts to generate a complex pool of protein variants. We obtained direct evidence for the existence of such pools by proteomic analysis of secretions from migrating schistosomula and mature eggs. Whole-mount in situ hybridization and immunolocalization showed that MEG transcripts and proteins were restricted to glands or epithelia exposed to the external environment. The ability of schistosomes to produce a complex pool of variant proteins aligns them with the other major groups of blood parasites, but using a completely different mechanism. We believe that our data open a new chapter in the study of immune evasion by schistosomes, and their ability to generate variant proteins could represent a significant obstacle to vaccine development.

Towards an understanding of the epigenetics of schistosomes: a comparative epigenomic study

“…It is expressed in eggs and miracidia and has been shown to be glycosylated and secreted in eggs. Its potential role in the context of immune evasion strategy has been previously discussed (DeMarco et al 2010). It is of particular interest that MEG-2 shares many common features with the SmPoMuc genes, previously described as major molecular determinants for the compatibility polymorphism (Roger et al 2008b).…”

“…These MEG-type genes have only been described in S. mansoni (DeMarco et al 2010). They are composed of 18 different family members and are a molecular system for creating protein variation through alternate splicing of short exons.…”

The cell biology of schistosomes: a window on the evolution of the early metazoa

“…Ingested dextran was endocytosed at the luminal surface of the gut epithelium, being concentrated into micron sized vesicles by 24 h after feeding. The uncoating process is intriguing because it takes place at the exact point in the oesophagus where the only authenticated product of the oesophageal gland, Ag10.3/MEG-4.1, is released into the lumen (DeMarco et al 2010). It is tempting to conclude that this protein plays a key role as the uncoating agent.…”

“…It is tempting to conclude that this protein plays a key role as the uncoating agent. The gene is encoded by microexons and has a repeating modular structure, the number units expressed being controlled by alternative splicing (DeMarco et al 2010). The molecular weight on 2D gels is much higher than predicted from its amino acid sequence, suggesting that it may be heavily glycosylated.…”

Non-coding RNAs in schistosomes: an unexplored world

“…It is also known that the parasite has a sophisticated alternative splicing mechanism for genes encoding secreted proteins such as micro-exon genes (MEGs) (Demarco et al 2010), polymorphic mucins genes (SmPoMucs) (Roger et al 2008) and venom allergen-like (SmVALs) genes (Chalmers et al 2008). These mechanisms are supposed to increase the repertoire of parasite proteins (Verjovski-Almeida and Demarco 2011), possibly helping to evade the immune response.…”