The Xin repeat-containing protein, mXinβ, initiates the maturation of the intercalated discs during postnatal heart development

The intercalated disc (ICD) is a unique structure to the heart and plays vital roles in communication and signaling among cardiomyocytes. ICDs are formed and matured during postnatal development through a profound redistribution of the intercellular junctions, as well as recruitment and assembly of more than 200 proteins at the termini of cardiomyocytes. The molecular mechanism underlying this process is not completely understood. The mouse orthologs (mXina and mXin beta) of human cardiomyopathy-associated (CMYA)/Xin actin-binding repeat-containing protein (XIRP) genes (CMYA1/XIRP1 and CMYA3/XIRP2, respectively) encode proteins localized to ICDs. Ablation of mXina results in adult late-onset cardiomyopathy with conduction defects and up-regulation of mXin beta. ICD structural defects are found in adult but not juvenile mXin beta-null hearts. On the other hand, loss of mXin)(3 leads to ICD defects at postnatal day 16.5, a developmental stage when the heart is forming ICDs, suggesting mXin beta is required for ICD formation. Using quantitative Western blot, we showed in this study that mXin beta but not mXin alpha, was uniquely up-regulated during the redistribution of intercellular junction from the lateral membrane of cardiomyocytes to their termini. In the absence of mXin beta, the intercellular junctions failed to be restricted to the termini of the cells, and the onset of such defect correlated with the peak expression of mXin beta. Immunofluorescence staining and subcellular fractionation showed that mXin beta preferentially associated with the forming ICDs, further suggesting that mXin beta functioned locally to promote ICD maturation. In contrast, the spatiotemporal expression profile of mXin alpha and the lack of more severe ICD defects in mXin beta-/-;mXin beta-/- double knockout hearts than in mXin beta-/-hearts suggested that mXin alpha, was not essential for the postnatal formation of ICDs. A two-step model for the development of ICD is proposed where mXin beta is essential for the redistribution of intercellular junction components from the lateral puncta to the cell termini. (C) 2012 Elsevier Inc. All rights reserved.