Toll signal transduction pathway in bivalves: Complete cds of intermediate elements and related gene transcription levels in hemocytes of immune stimulated Mytilus galloprovincialis

Abstract

a b s t r a c tBased on protein domain structure and organization deduced from mRNA contigs, 15 transcripts of the Toll signaling pathway have been identified in the bivalve, Mytilus galloprovincialis. Identical searches performed on publicly available Mytilus edulis ESTs revealed 11 transcripts, whereas searches performed in genomic and new transcriptome sequences of the Pacific oyster, Crassostrea gigas, identified 21 Toll-related transcripts. The remarkable molecular diversity of TRAF and IKK coding sequences of C. gigas, suggests that the sequence data inferred from Mytilus cDNAs may not be exhaustive. Most of the Toll pathway genes were constitutively and ubiquitously expressed in M. galloprovincialis, although at different levels, and clearly induced after in vivo injection with bacteria. Such over-transcription was more rapid and intense with Gram-negative than with Gram-positive bacteria. Injection of a fungus modulated the transcription of few Toll pathway genes, with the induction levels of TLR/MyD88 complex being always less intense. Purified LPS and b-glucans had marginal effect whereas peptidoglycans were ineffective. At the moment, we found no evidence of an IMD transcript in bivalves. In conclusion, mussels possess a complete Toll pathway which can be triggered either by Gram-positive or Gram-negative bacteria.

Characterization of the <i>TRAF3IP1</i> gene in Yesso scallop (<i>Patinopecten yessoensis</i>) and its expression in response to bacterial challenge

“…Unlike the upregulation of PyTRAF3IP1 in response to bacterial challenge, the expression of P. yessoensis TRAF3 (PyTRAF3) was not significantly up-or even down-regulated during 0-24 h of bacterial infection . A similar expression pattern of TRAF3 under bacterial invasion was also reported in Mediterranean mussel (M. galloprovincialis) (Toubiana et al, 2014) and pearl oyster (P. fucata) (Huang et al, 2012). Interestingly, there are two TRAF3 isoforms in C. gigas, which display contrary expression responses to Gram-negative bacterial challenge: CgTRAF3-L is significantly up-regulated, whereas CgTRAF3-S is significantly down-regulated (Huang et al, 2014).…”

“…With innate immunity, they have evolved elaborate mechanisms to protect themselves against pathogenic invasion. As an important gene that functions in innate immunity, TRAF3 has been identified and characterized in several marine bivalves, such as Pinctada fucata (Huang et al, 2012), Crassostrea gigas (Huang et al, 2014), Patinopecten yessoensis and Mytilus galloprovincialis (Toubiana et al, 2014). The regulated expression of bivalve TRAF3s in response to bacterial challenge that has been revealed in these studies implies the involvement of TRAF3 and its related molecules in innate immunity of shellfish.…”

“…A similar pattern has also been reported in several bivalve species. For instance, PyRel and PyTollip in P. yessoensis Zhang et al, 2015), CgTLRs and CgMyD88 in C. gigas and MgIKK-2 in M. galloprovincialis (Toubiana et al, 2014) all displayed higher expression levels after infection by Gram-negative bacteria than by Gram-positive bacteria. Interestingly, the second strong response of PyTRAF3IP1 at 24 h post V. anguillarum challenge has also been reported for CfRel (Zhou et al, 2013) and PyTollip .…”

An updated molecular basis for mussel immunity

“…The subsequent identification of 15 downstream elements provided further evidence about the existence of a complete signaling pathway similar to the Drosophila Toll and to the mammalian TLR pathways [99]. Besides MyD88, one or more transcript variants denoting TOLLIP, IRAKs, TRAFs, TAK1, IKK, IKKg/ NEMO, IkB, Relish/p65 and NF-kB were comparatively identified in M. galloprovincialis, M. edulis and C. gigas (see Fig.…”

“…Nevertheless, no plausible evidence could be found for the first key adaptor protein downstream to PGRP-LC, the IMD/RIP protein. Given that the Toll pathway was observed to be responsive to both Gramþ and Gram-bacteria, the existence of an IMD pathway was reported as uncertain in mussel [99].…”