Dysfunction of the ß2-spectrin-based pathway in human heart failure.

ß2-Spectrin is critical for integrating membrane and cytoskeletal domains in excitable and nonexcitable cells. The role of ß2-spectrin for vertebrate function is illustrated by dysfunction of ß2-spectrin-based pathways in disease. Recently, defects in ß2-spectrin association with protein partner ankyrin-B were identified in congenital forms of human arrhythmia. However, the role of ß2-spectrin in common forms of acquired heart failure and arrhythmia is unknown. We report that ß2-spectrin protein levels are significantly altered in human cardiovascular disease as well as in large and small animal cardiovascular disease models. Specifically, ß2-spectrin levels were decreased in atrial samples of patients with atrial fibrillation compared with tissue from patients in sinus rhythm. Furthermore, compared with left ventricular samples from nonfailing hearts, ß2-spectrin levels were significantly decreased in left ventricle of ischemic- and nonischemic heart failure patients. Left ventricle samples of canine and murine heart failure models confirm reduced ß2-spectrin protein levels. Mechanistically, we identify that ß2-spectrin levels are tightly regulated by posttranslational mechanisms, namely Ca(2+)- and calpain-dependent proteases. Furthermore, consistent with this data, we observed Ca(2+)- and calpain-dependent loss of ß2-spectrin downstream effector proteins, including ankyrin-B in heart. In summary, our findings illustrate that ß2-spectrin and downstream molecules are regulated in multiple forms of cardiovascular disease via Ca(2+)- and calpain-dependent proteolysis.

Dysfunction in the ßII spectrin-dependent cytoskeleton underlies human arrhythmia.

BACKGROUND: The cardiac cytoskeleton plays key roles in maintaining myocyte structural integrity in health and disease. In fact, human mutations in cardiac cytoskeletal elements are tightly linked to cardiac pathologies, including myopathies, aortopathies, and dystrophies. Conversely, the link between cytoskeletal protein dysfunction and cardiac electric activity is not well understood and often overlooked in the cardiac arrhythmia field. METHODS AND RESULTS: Here, we uncover a new mechanism for the regulation of cardiac membrane excitability. We report that ßII spectrin, an actin-associated molecule, is essential for the posttranslational targeting and localization of critical membrane proteins in heart. ßII spectrin recruits ankyrin-B to the cardiac dyad, and a novel human mutation in the ankyrin-B gene disrupts the ankyrin-B/ßII spectrin interaction, leading to severe human arrhythmia phenotypes. Mice lacking cardiac ßII spectrin display lethal arrhythmias, aberrant electric and calcium handling phenotypes, and abnormal expression/localization of cardiac membrane proteins. Mechanistically, ßII spectrin regulates the localization of cytoskeletal and plasma membrane/sarcoplasmic reticulum protein complexes, including the Na/Ca exchanger, ryanodine receptor 2, ankyrin-B, actin, and αII spectrin. Finally, we observe accelerated heart failure phenotypes in ßII spectrin-deficient mice. CONCLUSIONS: Our findings identify ßII spectrin as critical for normal myocyte electric activity, link this molecule to human disease, and provide new insight into the mechanisms underlying cardiac myocyte biology.