Transceptors, solute transporters that facilitate intracellular entry of molecules and also initiate intracellular signaling events, have been primarily studied in lower-order species. Ammonia, a cytotoxic endogenous metabolite, is converted to urea in hepatocytes for urinary excretion in mammals. During hyperammonemia, when hepatic metabolism is impaired, nonureagenic ammonia disposal occurs primarily in skeletal muscle. Increased ammonia uptake in skeletal muscle is mediated by a membrane-bound, 12 transmembrane domain solute transporter, Rhesus blood group-associated B glycoprotein (RhBG). We show that in addition to its transport function, RhBG interacts with myeloid differentiation primary response-88 (MyD88) to initiate an intracellular signaling cascade that culminates in activation of NFκB. We also show that ammonia-induced MyD88 signaling is independent of the canonical toll-like receptor-initiated mechanism of MyD88-dependent NFκB activation. In silico, in vitro, and in situ experiments show that the conserved cytosolic J-domain of the RhBG protein interacts with the Toll-interleukin-1 receptor (TIR) domain of MyD88. In skeletal muscle from human patients, human-induced pluripotent stem cell-derived myotubes, and myobundles show an interaction of RhBG–MyD88 during hyperammonemia. Using complementary experimental and multiomics analyses in murine myotubes and mice with muscle-specific RhBG or MyD88 deletion, we show that the RhBG–MyD88 interaction is essential for the activation of NFkB but not ammonia transport. Our studies show a paradigm of substrate-dependent regulation of transceptor function with the potential for modulation of cellular responses in mammalian systems by decoupling transport and signaling functions of transceptors.

中文翻译：

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氨转运蛋白 RhBG 通过激活 NFκB 启动下游信号传导和功能反应


受体，即促进分子进入细胞内并启动细胞内信号传导事件的溶质转运蛋白，主要在低等物种中进行研究。氨是一种细胞毒性内源性代谢物，在哺乳动物的肝细胞中转化为尿素，通过尿液排泄。在高氨血症期间，当肝脏代谢受损时，非尿素氨处理主要发生在骨骼肌中。骨骼肌中氨吸收的增加是由膜结合的 12 次跨膜结构域溶质转运蛋白、恒河猴血型相关 B 糖蛋白 (RhBG) 介导的。我们发现，除了转运功能外，RhBG 还与骨髓分化初级反应 88 (MyD88) 相互作用，启动细胞内信号级联反应，最终激活 NFκB。我们还表明，氨诱导的 MyD88 信号传导独立于典型的 Toll 样受体启动的 MyD88 依赖性 NFκB 激活机制。计算机模拟、体外和原位实验表明，RhBG 蛋白的保守胞质 J 结构域与 MyD88 的 Toll-interleukin-1 受体 (TIR) 结构域相互作用。在人类患者的骨骼肌中，人类诱导的多能干细胞衍生的肌管和肌束在高氨血症期间显示出 RhBG-MyD88 的相互作用。通过对小鼠肌管和肌肉特异性 RhBG 或 MyD88 缺失的小鼠进行补充实验和多组学分析，我们发现 RhBG-MyD88 相互作用对于激活 NFkB 但不是氨转运至关重要。我们的研究展示了底物依赖性转运蛋白功能调节的范例，具有通过解耦转运蛋白和信号转导功能来调节哺乳动物系统中细胞反应的潜力。