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

总结 蛋白质唾液酸化是由唾液酸转移酶家族的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糖蛋白的调节 在多种病原体靶向的内在稳态机制中的丰度和功能。 环境因素在人类炎症性肠病（IBD）的起源中占主导地位， 季节性细菌感染也有牵连。因此，我们开发了一种小鼠模型， 人类食物中毒，包括革兰氏阴性杆菌的复发性低滴度非致死性胃感染 细菌病原体沙门氏菌肠伤寒（ST），人类食源性疾病的主要原因 国际吧在这种独特的模型中，ST病原体被宿主迅速清除，然而， 严重和持续性结肠炎的发展类似于溃疡性结肠炎（UC）。我们证明了发病机制 与宿主体内涉及抗炎肠道的保护机制的失效有关。 肠上皮细胞产生的碱性磷酸酶（IAP）糖蛋白酶。ST段感染复发导致 Toll样受体-4（Tlr 4）诱导神经氨酸酶（Neu）活性和宿主Neu 3丰度与新生IAP 去唾液酸化和内吞降解，从而降低IAP半衰期、丰度和功能。IAP 在缺乏ST 3Gal 6唾液酸转移酶的小鼠中类似地获得缺乏，导致自发性结肠炎。 这两种情况下的疾病机制都是Tlr 4依赖性的，与去磷酸化减少有关， 对结肠细菌产生的脂多糖-磷酸进行解毒。在人类中， 在结肠炎中已经报道了IAP和Neu 3的类似调节，IAP的遗传缺陷引起结肠炎， 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，在这方面， Neu 3的关键参与仍有待确定。本文提出的研究将直接测试 在Neu 3缺失小鼠中，宿主Neu 3在结肠炎的发作和进展中的作用。此外，其他可能性 肠道革兰氏阴性病原体同样引起IAP缺乏症，将随着 使用相关革兰氏阴性肠道病原体的其他复发性非致死性胃感染模型，包括 高毒力猪霍乱沙门氏菌我们最近测量到Neu活性升高， 复发性ST感染期间结肠中Neu 3蛋白丰度与粘蛋白去唾液酸化相关- 2（Muc 2），保护性粘蛋白屏障的主要糖蛋白组分。侵蚀机理的研究 结肠炎中的粘蛋白屏障仍然未知，但在发病机制中起重要作用。我们最近联系了 粘蛋白屏障的侵蚀导致Muc 2蛋白唾液酸化的减少，可能是通过ST 3Gal 6。值得注意的是，Neu 处理增加了Muc 2蛋白水解，降低了Muc 2丰度。本文提出的研究将 确定Muc 2被ST 3Gal 6唾液酸化在产生和维持保护性粘蛋白屏障中的作用。 这些研究将为结肠炎的预防和治疗提供重要进展。

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.