TLR processing at low pH

Endosome maturation (acidification) is required for both the activation of TLR9 and TLR7 through proteolytic cleavage and the disassembly of pathogens, thereby releasing the TLR ligands within them. TLR7 and TLR9 are cleaved within their ectodomains by pH-sensitive cysteine endopeptidases. Cathepsins (CTS) B, K, L, and S, and asparagine endopeptidase (AEP, also known as legumain) have been implicated in endolysosomal TLR processing, however, several groups have reported somewhat controversial results on the role of specific proteases (Matsumoto F et al 2008, Park B et al 2008, Ewald SE et al 2008, Ewald SE et al 2011, Sepulveda FE et al 2009).<p>One study showed that TLR9 proteolysis is a multistep process with the initial cleavage that can be mediated by AEP or multiple members of the cathepsin family. The second event is mediated exclusively by cathepsins. TLR7 and TLR3 were reported to be cleaved in a similar manner (Ewald SE et al 2011). Cleavage of TLR3 is not shown in this reaction, since other studies demonstrated that the N-terminal region of TLR3 ectodomain was implicated in ligand binding, suggesting that TLR3 may function as a full-length receptor (Liu L et al 2008, Tokisue T et al 2008).</p> <p>Both full-length receptor and cleaved fragment corresponding to the C-terminal part of TLR9 were capable to bind ligand, however only the processed form (TLR9 C-ter, aa 471-1032) was shown to bind MyD88 and induce signaling in different mouse cells (Ewald SE et al 2008).

内体成熟（酸化）对于通过蛋白水解裂解激活TLR9和TLR7以及解聚病原体，从而释放其内的TLR配体至关重要。TLR7和TLR9在其外域内被pH敏感的半胱氨酸内肽酶裂解。半胱氨酸内肽酶B、K、L和S，以及天冬酰胺内肽酶（AEP，亦称豆蔻酶）已被归咎于内溶酶体TLR处理，然而，多个研究小组报告了关于特定蛋白酶作用的有争议的结果（Matsumoto F等，2008年，Park B等，2008年，Ewald SE等，2008年，Ewald SE等，2011年，Sepulveda FE等，2009年）。<p>一项研究表明，TLR9的蛋白水解是一个多步骤的过程，最初的裂解可能由AEP或半胱氨酸内肽酶家族的多个成员介导。第二个事件则完全由半胱氨酸内肽酶介导。有报告指出，TLR7和TLR3以类似的方式进行裂解（Ewald SE等，2011年）。TLR3的裂解在此反应中未显示，因为其他研究证明TLR3外域的N端区域与配体结合有关，这表明TLR3可能作为全长受体发挥作用（Liu L等，2008年，Tokisue T等，2008年）。<p>全长受体及其对应TLR9C端的部分裂解片段均能结合配体，然而，只有经过处理的TLR9C端（氨基酸471-1032）被证明能与MyD88结合并在不同的小鼠细胞中诱导信号传导（Ewald SE等，2008年）。