TMX2 is a key regulator of cellular redox state and its dysfunction causes severe brain developmental abnormalities.

The redox state of the neural progenitors regulates physiological processes such as neuronal differentiation, dendritic and axonal growth. The relevance of ER-associated oxidoreductases in these processes is largely unexplored. We describe a severe neurological disorder caused by biallelic loss of function variants in the Thioredoxin (TRX)-Related Transmembrane-2 (TMX2) gene, detected by exome sequencing in ten affected individuals from seven unrelated families presenting with congenital microcephaly, cortical polymicrogyria and other migration disorders. TMX2 encodes one of the five TMX proteins of the Protein Disulfide Isomerase family and is the first to be linked to human brain disease. Our mechanistic studies on protein function show that TMX2 localizes to the ER Mitochondria-Associated-Membranes (MAMs), is involved in posttranslational modification and protein folding and undergoes physical interaction with the MAM associated and ER folding chaperone calnexin and ER calcium pump SERCA2. These interactions are functionally relevant because TMX2-deficient fibroblasts show decreased mitochondrial respiratory reserve capacity and compensatory increased basal glycolytic activity. Intriguingly, under basal conditions TMX2 occurs in both reduced and oxidized monomeric form, while it forms a stable dimer under treatment with hydrogen peroxide, recently recognized as signaling molecule in neural morphogenesis and axonal pathfinding. Exogenous expression of the pathogenic TMX2 variants or of variants with in vitro mutagenized TRX domain induces a constitutive TMX2 polymerization, mimicking increased oxidative state. Altogether these data uncover TMX2 as a sensor in the MAM-regulated redox state and identify it as a key adaptive regulator of neuronal proliferation, migration and organization in the developing brain.

神经祖细胞的氧化还原状态调控神经元分化、树突与轴突生长等生理过程。目前学界对内质网相关氧化还原酶在上述过程中的作用尚鲜有系统探索。我们报道了一例由硫氧还蛋白（Thioredoxin, TRX）相关跨膜蛋白2（Thioredoxin-Related Transmembrane-2, TMX2）基因双等位基因功能丧失变异导致的重症神经系统疾病：通过外显子测序，在7个无关家系的10名受累患者中检出该变异，这些患者均表现为先天性小头畸形、脑皮质多小脑回及其他神经元迁移障碍。TMX2属于蛋白质二硫键异构酶家族的5种TMX蛋白之一，是首个与人类脑部疾病相关的该家族成员。我们针对其蛋白功能的机制研究显示，TMX2定位于内质网-线粒体相关膜（Endoplasmic Reticulum Mitochondria-Associated Membranes, MAMs），参与翻译后修饰与蛋白质折叠，并可与定位于MAMs的内质网折叠伴侣钙连蛋白（calnexin）以及内质网钙泵SERCA2发生物理相互作用。上述相互作用具有明确的功能相关性：TMX2缺陷的成纤维细胞表现出线粒体呼吸储备能力下降，以及代偿性的基础糖酵解活性升高。值得注意的是，在基础生理状态下，TMX2同时以还原型和氧化型单体形式存在；而在过氧化氢（一种近年被证实参与神经形态发生与轴突导向的信号分子）处理下，TMX2会形成稳定的二聚体。体外诱变TRX结构域的变异或致病性TMX2变异体的外源表达，均可诱导组成型TMX2聚合，模拟氧化状态升高的细胞表型。综上，本研究数据证实TMX2是MAMs调控的氧化还原状态的感受器，并确定其为发育大脑中神经元增殖、迁移与组织构建的关键适应性调控因子。