The pseudoprotease iRhom1 controls ectodomain shedding of membrane proteins in the nervous system

Proteolytic ectodomain shedding of membrane proteins is a fundamental mechanism to control the communication between cells and their environment. A key protease for membrane protein shedding is ADAM17, which requires a non-proteolytic subunit, either inactive Rhomboid 1 (iRhom1) or iRhom2 for its activity. While iRhom1 and iRhom2 are coexpressed in most tissues and appear to have largely redundant functions, the brain is an organ with predominant expression of iRhom1. Yet, little is known about the spatio-temporal expression of iRhom1 in mammalian brain and about its function in controlling membrane protein shedding in the nervous system. Here, we demonstrate that iRhom1 is expressed in mouse brain from the prenatal stage to adulthood with a peak in early postnatal development. In the adult mouse brain iRhom1 was widely expressed, including in cortex, hippocampus, olfactory bulb and cerebellum. Proteomic analysis of the secretome of primary neurons and of cerebrospinal fluid (CSF), obtained from iRhom1-deficient and control mice, identified several membrane proteins that require iRhom1 for their shedding in vitro or in vivo. One of these proteins was the ‘multiple-EGF-like-domains protein 10’ (MEGF10), a phagocytic receptor in the brain that is linked to the removal of amyloid  and apoptotic neurons. MEGF10 was further validated as an ADAM17 substrate using ADAM17-deficient mouse embryonic fibroblasts. Taken together, this study discovers a role for iRhom1 in controlling membrane protein shedding in the mouse brain, establishes MEGF10 as an iRhom1-dependent ADAM17 substrate and demonstrates that iRhom1 is widely expressed in murine brain.

膜蛋白的蛋白酶解外域脱落是调控细胞与其环境之间沟通的一种基本机制。ADAM17作为一种关键的膜蛋白脱落蛋白酶，其活性依赖于非蛋白酶解亚基，即非活性罗盘状蛋白1（iRhom1）或iRhom2。尽管iRhom1和iRhom2在大多数组织中共同表达，且功能上存在大量冗余，但大脑作为一种主要表达iRhom1的器官，关于iRhom1在大脑中的时空表达及其在神经系统中调控膜蛋白脱落的功能，了解甚少。本研究揭示了iRhom1在从胚胎到成年的小鼠大脑中均有表达，并在出生后早期达到峰值。在成年小鼠大脑中，iRhom1的表达广泛，包括皮层、海马体、嗅球和小脑。通过对iRhom1缺陷型和对照组小鼠的原代神经元分泌物和脑脊液（CSF）进行蛋白质组学分析，鉴定出数种需要iRhom1进行脱落作用的膜蛋白。其中之一是“多EGF样结构域蛋白10”（MEGF10），这是一种与清除淀粉样β和凋亡神经元相关的脑内吞噬受体。通过使用ADAM17缺陷型小鼠胚胎成纤维细胞进一步验证了MEGF10作为ADAM17底物的地位。综上所述，本研究揭示了iRhom1在控制小鼠大脑膜蛋白脱落中的作用，确立了MEGF10作为iRhom1依赖的ADAM17底物的地位，并证明了iRhom1在小鼠大脑中的广泛表达。