Developmental regulation and extracellular release of a VSG expression-site-associated gene product from Trypanosoma brucei bloodstream forms

Abstract

SummaryTrypanosomes evade host immunity by exchanging variant surface glycoprotein (VSG) coats. VSG genes are transcribed from telomeric expression sites, which contain a diverse family of expression-site-associated genes (ESAGs). We have discovered that the mRNAs for one ESAG family, ESAG9, are strongly developmentally regulated, being enriched in stumpy forms, a life-cycle stage in the mammalian bloodstream that is important for the maintenance of chronic parasite infections and for tsetse transmission. ESAG9 gene sequences are highly diverse in the genome and encode proteins with weak similarity to the massively diverse MASP proteins in Trypanosoma cruzi. We demonstrate that ESAG9 proteins are modified by N-glycosylation and can be shed to the external milieu, this being dependent upon coexpression with at least one other family member. The expression profile and extracellular release of ESAG9 proteins represents a novel and unexpected aspect of the transmission biology of trypanosomes in their mammalian host. We suggest that these molecules might interact with the external environment, with possible implications for infection chronicity or parasite transmission.

A short bifunctional element operates to positively or negatively regulateESAG9expression in different developmental forms ofTrypanosoma brucei

“…and 84% amino acid similarity to ESAG9u and ESAG9c (Florent et al, 1991), respectively], and also highly related to an expression site linked copy in T. brucei s427 identified by TAR cloning (Young et al, 2008) (88% similarity). This gene was also confirmed as being stumpy-enriched in different strains of T. brucei (Barnwell et al, 2010), with Digital SAGE analysis indicating fivefold higher mRNA expression in stumpy forms than in slender forms (data not shown). Initially, a CAT reporter gene assay approach was used to assess the contributions of the ESAG9 39UTR sequence to the control of gene expression (Fig.…”

“…These genes are highly diverse and rarely found as components of conventional vsg expression sites, most copies being located within polycistronic transcription units positioned at chromosomal internal and sub-telomeric regions (Barnwell et al, 2010;Hertz-Fowler et al, 2008). The protein products of ESAG9 genes are of unknown function, although they are secreted by bloodstream form parasites, potentially to assist infection chronicity or transmission (Barnwell et al, 2010).…”

“…These genes are highly diverse and rarely found as components of conventional vsg expression sites, most copies being located within polycistronic transcription units positioned at chromosomal internal and sub-telomeric regions (Barnwell et al, 2010;Hertz-Fowler et al, 2008). The protein products of ESAG9 genes are of unknown function, although they are secreted by bloodstream form parasites, potentially to assist infection chronicity or transmission (Barnwell et al, 2010). Most interestingly, however, ESAG9 genes show developmental expression in the mammalian bloodstream, being expressed only upon development of the bloodstream parasites toward stumpy forms (Barnwell et al, 2010;Jensen et al, 2009), transmission stages that accumulate at the peak of each wave of parasitaemia .…”

“…The protein products of ESAG9 genes are of unknown function, although they are secreted by bloodstream form parasites, potentially to assist infection chronicity or transmission (Barnwell et al, 2010). Most interestingly, however, ESAG9 genes show developmental expression in the mammalian bloodstream, being expressed only upon development of the bloodstream parasites toward stumpy forms (Barnwell et al, 2010;Jensen et al, 2009), transmission stages that accumulate at the peak of each wave of parasitaemia . This differentiation event occurs in response to a density sensing mechanism that the parasites use to monitor the parasitaemia and thereby ensure the production of transmission stages Vassella et al, 1997).…”

“…In vivo, the upregulation of ESAG9 transcripts is one of the earliest events during stumpy formation, with these genes being among the most highly expressed mRNAs during the transition from intermediate to stumpy cells. This involves the elevated expression of many different ESAG9 family members and so is not restricted to expression associated copies of the gene (Barnwell et al, 2010). Hence, the ESAG9 genes provide an example of a stringently and coordinately regulated gene family that is developmentally expressed in response to the parasites' quorum-sensing signal.…”