Intercalated Disc-Associated Protein mXina Influences Surface Expresession of Ito and _K1 Currents in Ventricular Myocytes

Mouse Xina (mXina) encodes a Xin repeat-containing, actin-binding protein localized to the intercalated disc (ICD). Ablation of mXinaprogressively leads to disrupted ICD structure, cardiac hypertrophy and cardiomyopathy with conduction defects during adulthood. Such conduction defects could be due to ICD structural defects and/or cell electrophysiological property changes. Here, we showed that despite the normal ICD structure, juvenile mXin α-null cardiomyocytes (from 3~4-week-old mice) exhibited a significant reduction in the Ito, similar to adult mutant cells. Juvenile but not adult mutant cardiomyocytes also had a significant reduction in the I_K. In contrast, the mutant adult ventricular myocytes had a significant reduction in the I_K1 on hyperpolarization. These together could account for the prolongation of APD and developing EAD observed in juvenile mXina-null cells. Interestingly, juvenile mXina-null cardiomyocytes had a notable decrease in the amplitude of intracellular Ca²⁺ transient and no change in the h.L, suggesting that the prolonged APD did not promote an increase in [Ca²⁺]_I for cardiac hypertrophy. Juvenile mXina-null ventricles had reduced levels of membrane-associated KChIP2, an auxiliary subunit of I_to, and filamin, an actin cross-linking protein. We further showed that mXin α interacted with both proteins, providing a novel mechanism for I_to surface expression.