克罗恩病（CD）是一种临床常见的肠道慢性炎症反应性疾病，肠稳态失衡是导致其发病的关键，目前缺少有效治疗方法。我们前期发现一种新的鼠李糖乳杆菌LGG效应蛋白HM0539能有效保护小鼠CD，但具体机制不明。最近，我们发现HM0539可水解肽聚糖，并通过其水解产物保护CD。随后分析表明HM0539水解肽聚糖的主要产物是MDP（一种NOD2配体）。鉴于MDP-NOD2通路调节肠稳态的重要性及该通路受损在CD发病中的关键性，我们提出假说：HM0539通过水解肠道中的肽聚糖产生MDP，激活NOD2维持肠稳态以保护CD。本研究拟先构建和利用肽聚糖水解酶活性突变的HM0539，明确水解肽聚糖产生MDP是介导HM0539保护CD的关键；然后利用识别MDP缺陷的NOD2移码突变小鼠，阐明HM0539通过MDP-NOD2通路调节肠稳态保护CD的机制。本研究可为开发CD有效防治方法提供新思路，并有望实现临床转化。

Crohn's disease (CD) is a chronic inflammatory disorder of the gastrointestinal tract with no effective therapy, which is believed to result from the disturbance of intestinal homeostasis. Previously, we found a novel secreted protein from LGG, named HM0539, with a significant protective effect against TNBS-induced colitis in mice, but the precise mechanism is unclear. Further study suggested that the product of HM0539-hydrolysed peptidoglycan was sufficient for protection. ANTS analysis showed that the major fragment of HM0539-hydrolysed peptidoglycan is MDP, a defined ligand of NOD2. Given that MDP-NOD2 signaling plays a critical role in modulating gut homeostasis and its disturbance is involved in the pathogenesis of CD, we speculated that the protective mechanism of HM0539 against CD may be driven by initiation of MDP-NOD2-mediated gut homeostasis. The aim of this proposal is to verify this hypothesis. We will first create an enzymatic mutant of HM0539, to determine whether MDP is the major fragment of HM0539-hydrolysed peptidoglycan, and whether MDP is required for HM0539-mediated protective effect. Then, we will generate a NOD2 frameshift mutant mice with MDP-sensing defect, to investigate whether MDP-NOD2-mediated gut homeostasis is involved in HM0539-mediated protective role. The completion of this proposal will provide novel directions for clinical treatment of CD.