Novel mechanism underlying fibrinogen biogenesis in the endoplasmic reticulum

许多疾病现在被认为是“构象疾病”，是由蛋白质错误折叠和随后的聚集引起的。一个例子是肝纤维蛋白原储存病（HFSD），其潜在原因是内质网（ER）保留和纤维蛋白原突变体聚集，导致肝损伤、低纤维蛋白原血症和大出血。然而，野生型和突变型纤维蛋白原在内质网中的生物发生和质量控制背后的分子事件仍然未知。在本应用程序的初步数据中，我们偶然发现纤维蛋白原的生物发生是由主要内质网质量控制机制，内质网相关降解（ERAD）的Sel1L-Hrd1蛋白复合物调节的。Sel1L-Hrd1 ERAD是ERAD进化上最保守的分支，针对错误折叠的内质网蛋白进行细胞质蛋白酶体降解。肝细胞特异性sell缺陷小鼠在肝细胞内质网中表现出含纤维蛋白原的内含物，肝损伤和低纤维蛋白原血症，与人类HFSD患者相似。事实上，无论是野生型还是致病突变型的纤维蛋白原都可以被Sel1L-Hrd1 ERAD降解。这些数据表明肝细胞Sel1L-Hrd1 ERAD在纤维蛋白原生物生成和活体内平衡中起着关键作用。因此，本应用的总体假设是，Sel1L-Hrd1 ERAD是纤维蛋白原生物生成、凝血和肝脏稳态的关键调控机制，通过靶向错误折叠（无论是野生型还是突变型）的纤维蛋白原蛋白进行蛋白酶体降解。ERAD对纤维蛋白原的作用阻止了肝脏中有毒纤维蛋白原包涵体和聚集体的形成。我们将实现两个目标：(1)揭示肝脏Sel1L-Hrd1 ERAD在凝血和纤维蛋白原生物生成中的意义和分子机制；(2)阐明Sel1L-Hrd1 ERAD在HFSD中的病理意义。这些研究的完成不仅将揭示ERAD功能在纤维蛋白原早期生物发生中的意义和分子机制，还将对其他与蛋白质错误折叠相关的构象疾病的机制和治疗策略产生更广泛的影响。

A large number of diseases are now recognized as ‘conformational diseases’, caused by protein misfolding and subsequent aggregation. One example is hepatic fibrinogen storage disease (HFSD), where the underlying cause is the endoplasmic reticulum (ER) retention and aggregation of fibrinogen mutants, leading to liver damage, hypofibrinogenemia, and excessive bleeding. However, the molecular events underlying the biogenesis and quality control of, both wildtype and mutant fibrinogen, in the ER remain unknown. In the preliminary data of this application, we serendipitously found that the biogenesis of fibrinogen is regulated by the Sel1L-Hrd1 protein complex of the principal ER quality-control machinery, ER-associated degradation (ERAD). Sel1L-Hrd1 ERAD represents the most evolutionarily conserved branch of ERAD and targets misfolded ER proteins for cytosolic proteasomal degradation. Hepatocyte-specific Sel1L-deficient mice exhibit fibrinogen-containing inclusions in the ER of hepatocytes, hepatic damage and hypofibrinogenemia, resembling human patients with HFSD. Indeed, both wildtype and disease-causing mutants of fibrinogens are degraded by Sel1L-Hrd1 ERAD. These data point to a critical role of hepatocyte Sel1L-Hrd1 ERAD in fibrinogen biogenesis and live homeostasis. Hence, the overarching hypothesis of this application is that Sel1L-Hrd1 ERAD is a critical regulatory mechanism for fibrinogen biogenesis, coagulation and liver homeostasis by targeting misfolded, either wildtype or mutant, fibrinogen proteins for proteasomal degradation. This action of ERAD towards fibrinogen prevents the formation of toxic fibrinogen inclusions and aggregates in the liver. We will accomplish two Aims: (1) Demonstrate the significance and molecular mechanism of hepatic Sel1L-Hrd1 ERAD in coagulation and fibrinogen biogenesis; and (2) Delineate the pathological importance of Sel1L-Hrd1 ERAD in HFSD. Completion of these studies will not only delineate the significance and molecular mechanism underlying the ERAD function in early biogenesis of fibrinogen, but also have broader impact on the mechanism and therapeutical strategy for other conformational diseases associated with protein misfolding.