Protein Sialylation and De-Sialyation in Cell Surface Glycoprotein Homeostasis and Disease

摘要 蛋白质唾液酸化是唾液酸转移酶家族成员产生的翻译后修饰。 我们最近发现，由此产生的唾液酸键参与了糖蛋白的调节。 在多种病原体靶向的内在动态平衡机制中的丰度和功能。 环境因素是人类炎症性肠病(IBD)和 季节性细菌感染也有牵连。因此，我们开发了一种重复的小鼠模型 人类食物中毒由反复低滴度非致死性胃革兰氏阴性菌感染组成 鼠伤寒沙门氏菌(ST)，人类食源性疾病的主要原因 全世界。在这个独特的模型中，ST病原体被宿主迅速清除，但逐渐 严重和持续性结肠炎的发展类似于溃疡性结肠炎(UC)。我们证明了这种致病机制 与宿主体内涉及抗炎肠道的保护机制的失效有关 肠细胞产生的碱性磷酸酶(IAP)糖蛋白酶。反复的ST感染导致 Toll样受体-4(TLR4)诱导新生IAP的神经氨酸酶(Neu)活性和宿主Neu3丰度 去唾液酸化和内吞降解，从而减少IAP的半衰期、丰度和功能。IAP 缺乏ST3Gal6唾液酸转移酶导致自发性结肠炎的小鼠也同样获得了缺乏ST3Gal6唾液酸转移酶的能力。 这两种情况下的疾病机制都是TLR4依赖的，并与去磷酸化和 结肠共生菌产生的脂多糖-磷酸的解毒作用。在人类身上， IAP和Neu3在结肠炎中的调节相似，IAP的遗传缺陷会导致结肠炎， 支持正在进行的IAP增强和神经氨酸酶抑制临床试验的基本原理。然而， Neu3的关键参与仍有待确定。这里提出的研究将直接测试 宿主Neu3在Neu3基因缺失小鼠结肠炎的发生和发展中的作用此外，其他人有可能 同样引起IAP缺乏症的肠道革兰氏阴性病原菌将随着 其他使用相关革兰氏阴性肠道病原体的复发性非致死性胃感染模型，包括 超强毒力肠炎沙门氏菌血清型。我们最近测量了神经细胞活动的升高和 复发ST感染期间结肠中Neu3蛋白的含量与粘蛋白的去唾液酸化有关 2(Muc2)，保护粘蛋白屏障的主要糖蛋白成分。泥沙侵蚀的机理 粘蛋白屏障在结肠炎中的作用仍不清楚，但在发病机制中起着重要作用。我们最近联系到了 粘蛋白屏障的侵蚀，可能是ST3Gal6减少了Muc2蛋白的唾液酸化。值得注意的是，Neu 处理增加了Muc2的蛋白分解，降低了Muc2的丰度。本文提出的研究将 确定ST3Gal6对Muc2唾液酸化在产生和维持保护性粘蛋白屏障中的作用。 总之，这些研究将为结肠炎的预防和治疗提供重要的进展。

SUMMARY Protein sialylation is a post-translational modification produced by members of a family of sialyltransferases. We recently discovered that the resulting sialic acid linkages are involved in the regulation of glycoprotein abundance and function in an intrinsic homeostatic mechanism that is targeted by multiple pathogens. Environmental factors are dominant in the origins of the human Inflammatory Bowel Diseases (IBDs) and seasonal bacterial infections have been implicated. We therefore developed a mouse model of repeated human food poisoning comprised of recurrent low-titer non-lethal gastric infections of the Gram-negative bacterial pathogen Salmonella enterica Typhimurium (ST), a leading cause of human foodborne illness worldwide. In this unique model, the ST pathogen was rapidly cleared by the host, however a progressively severe and persistent colitis developed similar to Ulcerative Colitis (UC). We demonstrated that pathogenesis was linked to the disabling of a protective mechanism in the host involving the anti-inflammatory Intestinal Alkaline Phosphatase (IAP) glycoprotein enzyme produced by enterocytes. Recurrent ST infections resulted in Toll-like receptor-4 (Tlr4) induction of neuraminidase (Neu) activity and host Neu3 abundance with nascent IAP de-sialylation and endocytic degradation, thereby reducing IAP half-life, abundance, and function. IAP deficiency was similarly acquired in mice lacking the ST3Gal6 sialyltransferase resulting in spontaneous colitis. The disease mechanism in both cases was Tlr4-dependent and linked to reduced dephosphorylation and detoxification of the lipopolysaccharide-phosphate produced by commensal bacteria of the colon. In humans, similar modulation of IAP and Neu3 have been reported in colitis, and genetic deficiency of IAP causes colitis, supporting the rationale for ongoing clinical trials of IAP augmentation and neuraminidase inhibition. However, the key involvement of Neu3 remains to be established. Research proposed herein will directly test the role of host Neu3 in the onset and progression of colitis among Neu3-null mice. In addition, the possibility that other enteric Gram-negative pathogens similarly provoke IAP deficiency will be investigated with the development of other recurrent non-lethal gastric infection models using related Gram-negative enteric pathogens including the hypervirulent Salmonella enterica Choleraesuis serovar. We have recently measured elevated Neu activity and Neu3 protein abundance in the colon during recurrent ST infection associated with the de-sialylation of Mucin– 2 (Muc2), the major glycoprotein component of the protective mucin barrier. The mechanism of erosion of the mucin barrier in colitis remains unknown but plays a large role in pathogenesis. We have recently linked erosion of the mucin barrier to reductions of Muc2 protein sialylation, likely by ST3Gal6. Remarkably, Neu treatment increases Muc2 proteolysis with reduced Muc2 abundance. Research proposed herein will determine the role of Muc2 sialylation by ST3Gal6 in generating and maintaining the protective mucin barrier. Together these studies will provide important advances pertaining to the prevention and treatment of colitis.