New Insights into the Roles of Xin Repeat-Containing Proteins in Cardiac Development, Function, and Disease

Since the discovery of Xin repeat-containing proteins in 1996, the importance of Xin proteins in muscle development, function, regeneration, and disease has been continuously implicated. Most Xin proteins are localized to myotendinous junctions of the skeletal muscle and also to intercalated discs (ICDs) of the heart. The Xin gene is only found in vertebrates, which are characterized by a true chambered heart. This suggests that the evolutionary origin of the Xin gene may have played a key role in vertebrate origins. Diverse vertebrates including mammals possess two paralogous genes, Xin alpha (or Xirp1) and Xin beta (or Xirp2), and this review focuses on the role of their encoded proteins in cardiac muscles. Complete loss of mouse Xin beta (mXin beta) results in the failure of forming ICD, severe growth retardation, and early postnatal lethality. Deletion of mouse Xina (mXin alpha) leads to late-onset cardiomyopathy with conduction defects. Molecular studies have identified three classes of mXin alpha-interacting proteins: catenins, actin regulators/modulators, and ion-channel subunits. Thus, mXin alpha acts as a scaffolding protein modulating the N-cadherin-mediated adhesion and ion-channel surface expression. Xin expression is significantly upregulated in early stages of stressed hearts, whereas Xin expression is downregulated in failing hearts from various human cardiomyopathies. Thus, mutations in these Xin loci may lead to diverse cardiomyopathies and heart failure.