Identification of the C. sordellii lethal toxin receptor elucidates principles of receptor specificity in clostridial toxins

Abstract

Clostridium sordellii lethal toxin (TcsL) is responsible for an almost invariably lethal toxic shock syndrome associated with gynecological C. sordellii infections. Here, using CRISPR/Cas9 screening, we identify semaphorins SEMA6A and SEMA6B as the cellular receptors for TcsL and demonstrate that soluble extracellular SEMA6A can protect mice from TcsL-induced edema. A 3.3 Å cryo-EM structure shows that TcsL binds SEMA6A with the same region that the highly related C. difficile TcdB toxin uses to bind structurally unrelated Frizzled receptors. Remarkably, reciprocal mutations in this evolutionarily divergent surface are sufficient to switch receptor specificity between the toxins. Our findings establish semaphorins as physiologically relevant receptors for TcsL, and reveal the molecular basis for the difference in tissue targeting and disease pathogenesis between highly related toxins.Clostridium sordellii is an anaerobic gram-positive bacterium found in soil and in the gastrointestinal and vaginal tracts of animals and humans. C. sordellii is present in the rectal or vaginal tract of 3-4% of women, but vaginal colonization rate after childbirth or abortion is as high as 29% (1, 2). While the majority of carriers are asymptomatic, pathogenic C. sordellii infections arise rapidly and are highly lethal. Most C. sordellii infections occur in women following childbirth, medically induced abortion, or miscarriage, leading to a toxic shock syndrome with almost 100% mortality within days (1-4). The primary cause of the high mortality associated with C. sordellii infections is the lethal toxin TcsL (5), which belongs to the large clostridial toxin (LCT) family together with toxins from related species such as Clostridium difficile (6). LCTs enter the host cell by receptor-mediated endocytosis followed by translocation into the cytoplasm, where they potently modulate host cell function by glucosylating small Ras family GTPases (6).Although all LCTs are highly similar at the sequence level, they differ in their tissue specificity and in their effects on cell morphology, physiology, and viability. TcsL is most closely related to the C. difficile cytotoxin TcdB, sharing almost 90% sequence similarity. TcdB binds Frizzled family receptors FZD1/FZD2/FZD7 expressed in the colonic epithelium (7,8), the primary site of C. difficile infection. C. sordellii, however, does not infect or damage colonic epithelium, suggesting that TcsL binds a different cell-surface receptor.

Populational genomic insights of Paraclostridium bifermentans as an emerging human pathogen

“…As a member of clostridial cluster XI that is phylogenomically close to P. bifermentans, P. sordellii is commonly found in the rectal or vaginal tract of 3-4% of women, with the majority of carriers remaining asymptomatic (Aldape et al, 2016;Chong et al, 2016). Nevertheless, when pathogenic P. sordellii infections occur, they can rapidly progress and are associated with high mortality rates (~70%) due to the production of the lethal toxin protein TcsL, a member of the large clostridial toxin (LCT) family (Lee et al, 2020). It is worth noting that C. difficile also produces a toxin of the LCT family, namely TcdB, sharing 90% sequence identity with TcsL, which a major virulence factor responsible for C. difficile infections (Chen et al, 2018).…”