Intercalated Disc-Associated Protein mXin-alpha influences surface expression of ITO currents in ventricular myocytes

Abstract

Mouse Xin-alpha (mXin-alpha) encodes a Xin repeat-containing, actin-binding protein localized to the intercalated disc (ICD). Ablation of mXin-alpha progressively 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 mXina-null cardiomyocytes (from 3~4-week-old mice) exhibited a significant reduction in the transient outward K+ current (ITO), similar to adult mutant cells. Juvenile but not adult mutant cardiomyocytes also had a significant reduction in the delayed rectifier K+ current. In contrast, the mutant adult ventricular myocytes had a significant reduction in the inward rectifier K+ current (IK1) on hyperpolarization. These together could account for the prolongation of action potential duration (APD) and the ease of developing early afterdepolarization observed in juvenile mXinalpha-null cells. Interestingly, juvenile mXin-alpha-null cardiomyocytes had a notable decrease in the amplitude of intracellular Ca2+ transient and no change in the L-type Ca2+ current, suggesting that the prolonged APD did not promote an increase in intracellular Ca2+ for cardiac hypertrophy. Juvenile mXin-alpha-null ventricles had reduced levels of membrane-associated Kv channel interacting protein 2, an auxiliary subunit of ITO, and filamin, an actin cross-linking protein. We further showed that mXin-alpha interacted with both proteins, providing a novel mechanism for ITO surface expression Keywords Developmental Changes; Electrophysiology; KChIP2; transient outward K + current; L-type Ca 2+ current Send correspondence to: Cheng-I Lin, Department of Physiology & Biophysics, National Defense Medical Center, No 161, Minchuan East Rd, Sec. 6, Neihu 114, Taipei, Taiwan, Tel: 886-2-87924854, Fax: 886-2-87924860, bme03@ndmctsgh.edu.tw. NIH Public Access Author ManuscriptFront Biosci (Elite Ed). Author manuscript; available in PMC 2012 February 14. Published in final edited form as:Front Biosci (Elite Ed). ; 3: 1425-1442. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript INTRODUCTIONStriated muscle-specific Xin repeat-containing proteins are downstream targets of transcription factors Nkx2.5 and Mef2, localize to the intercalated disc (ICD) of the heart, and are important in cardiac morphogenesis and function (1-7). In mammals, two families of Xin repeat-containing proteins, Xinα and Xinβ, exist (8). Through alternative splicing of messages, each family of Xin gene encodes at least 2 Xin repeat-containing protein variants (major, mXinα or mXinβ, as well as minor, mXinα-a or mXinβ-a in the mouse) (5,7,9). In addition, a third variant (called Xin C) from Xinα gene has been detected in mouse and human heart, however, this variant, if it exists in vivo, did not contain Xin repeats (9). Loss of both mXinα and mXinα-a in the mouse (referred to mXinα-null mouse) heart result...