Radixin deficiency causes conjugated hyperbilirubinemia with loss of Mrp2 from bile canalicular membranes

ERM家族由三个密切相关的蛋白组成，ezrin， radixin和moesins，它们是肌动蛋白丝和整体膜蛋白之间的一般交联剂。在这里，我们产生了缺乏放射素（基因符号RDX）的小鼠，放射素是野生型肝脏中主要的ERM蛋白，集中在胆管膜（bcm）上。Rdx^<-1->小鼠出生时正常，但4周龄左右，血清结合胆红素水平开始逐渐升高，未见组织学异常；8周龄后观察到轻度肝损伤。这种表型类似于杜宾-约翰逊综合征的人偶联高胆红素血症，杜宾-约翰逊综合征是由多药耐药蛋白2 （Mrp2，基因符号ABCC2）突变引起的，尽管这种综合征与明显的肝损伤无关。在野生型小鼠中，Mrp2集中于bcm，将结合的胆红素分泌到胆汁中。从Rdx^<-1->小鼠出生开始，与其他BCM蛋白如二肽基肽酶IV（CD26）和p -糖蛋白相比，Mrp2在BCM中似乎有选择性地降低（降至约20%）。此外，几条线索的证据表明，放射素与Mrp2的c端细胞质结构域直接相关。这些发现表明，通过支持Mrp2在bcm上的定位，放射素是结合胆红素分泌所必需的。本研究为胆汁分泌和结合性高胆红素血症的分子机制，以及ERM蛋白在决定整体膜蛋白定位中的作用提供了新的见解。

The ERM family consists of three closely-related proteins, ezrin, radixin, and moesins, which function as general cross-linkers between actin filaments and integral membrane proteins. Here, we generated mice lacking radixin (gene symbol RDX), which is the dominant ERM protein in the wildtype liver, and is concentrated at bile canalicular membranes (BCMs). Rdx^<-1-> mice were normal at birth, but around 4 weeks of age, the serum levels of conjugated bilirubin began to increase gradually without showing any histological abnormality; after 8 weeks of age, mild liver injury was observed. This phenotype is similar to human conjugated hyperbilirubinemia in Dubin-Johnson syndrome, which is caused by mutations in the multi-drug resistance protein 2 (Mrp2, gene symbol ABCC2), although this syndrome is not associated with overt liver injury. In wildtype mice, Mrp2 is concentrated at BCMs to secrete conjugated bilirubin into bile. From birth in Rdx^<-1-> mice, Mrp2 appeared to be selectively decreased (down to 〜20%) in BCMs compared to other BCM proteins such as dipeptidyl peptidase IV(CD26) and P-glycoproteins (down to 〜70%). Furthermore, several lines of evidence have shown a direct association of radixin with the C-terminal cytoplasmic domain of Mrp2. These findings indicate that radixin is required for conjugated bilirubin secretion through its support of Mrp2 localization at BCMs. This study provides new insights into the molecular mechanisms of bile secretion and conjugated hyperbilirubinemia, and the involvement of ERM proteins in determining the localization of integral membrane proteins.