Petersen Lara, Stroh Svenja, Schöttelndreier Dennis, Grassl Guntram A, Rottner Klemens, Brakebusch Cord, Fahrer Jörg, Genth Harald
Institute for Toxicology, Hannover Medical School, Hannover, Germany.
Department of Toxicology, University Medical Center Mainz, Mainz, Germany.
Front Microbiol. 2022 Mar 7;13:846215. doi: 10.3389/fmicb.2022.846215. eCollection 2022.
infection (CDI) in humans causes pseudomembranous colitis (PMC), which is a severe pathology characterized by a loss of epithelial barrier function and massive colonic inflammation. PMC has been attributed to the action of two large protein toxins, Toxin A (TcdA) and Toxin B (TcdB). TcdA and TcdB mono-O-glucosylate and thereby inactivate a broad spectrum of Rho GTPases and (in the case of TcdA) also some Ras GTPases. Rho/Ras GTPases promote G1-S transition through the activation of components of the ERK, AKT, and WNT signaling pathways. With regard to CDI pathology, TcdB is regarded of being capable of inhibiting colonic stem cell proliferation and colonic regeneration, which is likely causative for PMC. In particular, it is still unclear, the glucosylation of which substrate Rho-GTPase is critical for TcdB-induced arrest of G1-S transition. Exploiting SV40-immortalized mouse embryonic fibroblasts (MEFs) with deleted Rho subtype GTPases, evidence is provided that Rac1 (not Cdc42) positively regulates Cyclin D1, an essential factor of G1-S transition. TcdB-catalyzed Rac1 glucosylation results in Cyclin D1 suppression and arrested G1-S transition in MEFs and in human colonic epithelial cells (HCEC), Remarkably, Rac1 MEFs are insensitive to TcdB-induced arrest of G1-S transition, suggesting that TcdB arrests G1-S transition in a Rac1 glucosylation-dependent manner. Human intestinal organoids (HIOs) specifically expressed Cyclin D1 (neither Cyclin D2 nor Cyclin D3), which expression was suppressed upon TcdB treatment. In sum, Cyclin D1 expression in colonic cells seems to be regulated by Rho GTPases (most likely Rac1) and in turn seems to be susceptible to TcdB-induced suppression. With regard to PMC, toxin-catalyzed Rac1 glucosylation and subsequent G1-S arrest of colonic stem cells seems to be causative for decreased repair capacity of the colonic epithelium and delayed epithelial renewal.
人类艰难梭菌感染（CDI）会引发伪膜性结肠炎（PMC），这是一种严重的病理状况，其特征为上皮屏障功能丧失和结肠大量炎症。PMC被认为是由两种大型蛋白质毒素，即毒素A（TcdA）和毒素B（TcdB）的作用所致。TcdA和TcdB对多种Rho GTP酶进行单-O-糖基化修饰，从而使其失活，并且（就TcdA而言）还会使一些Ras GTP酶失活。Rho/Ras GTP酶通过激活ERK、AKT和WNT信号通路的组分来促进G1-S期转换。关于CDI病理，TcdB被认为能够抑制结肠干细胞增殖和结肠再生，这可能是PMC的病因。特别是，目前仍不清楚TcdB诱导G1-S期转换停滞时，哪种底物Rho-GTP酶的糖基化是关键的。利用缺失Rho亚型GTP酶的SV40永生化小鼠胚胎成纤维细胞（MEF），有证据表明Rac1（而非Cdc42）正向调节细胞周期蛋白D1，这是G1-S期转换的一个关键因子。TcdB催化的Rac1糖基化导致MEF和人结肠上皮细胞（HCEC）中细胞周期蛋白D1受到抑制以及G1-S期转换停滞。值得注意的是，Rac1基因敲除的MEF对TcdB诱导的G1-S期转换停滞不敏感，这表明TcdB以Rac1糖基化依赖的方式使G1-S期转换停滞。人肠道类器官（HIO）特异性表达细胞周期蛋白D1（而非细胞周期蛋白D2或细胞周期蛋白D3），TcdB处理后其表达受到抑制。总之，结肠细胞中细胞周期蛋白D1的表达似乎受Rho GTP酶（很可能是Rac1）调节，进而似乎易受TcdB诱导的抑制作用影响。关于PMC，毒素催化的Rac1糖基化以及随后结肠干细胞的G1-S期停滞似乎是结肠上皮修复能力下降和上皮更新延迟的原因。
