Peter J. Mohler

We report identification of an ankyrin-B-based macromolecular complex of Na/K ATPase (alpha 1 and alpha 2 isoforms), Na/Ca exchanger 1, and InsP₃ receptor that is localized in cardiomyocyte T-tubules in discrete microdomains distinct from classic dihydropyridine receptor/ryanodine receptor “dyads.” E1425G mutation of ankyrin-B, which causes human cardiac arrhythmia, also blocks binding of ankyrin-B to all three components of the complex. The ankyrin-B complex is markedly reduced in adult ankyrin-B^+/− cardiomyocytes, which may explain elevated [Ca²⁺]_i transients in these cells. Thus, loss of the ankyrin-B complex provides a molecular basis for cardiac arrhythmia in humans and mice. T-tubule-associated ankyrin-B, Na/Ca exchanger, and Na/K ATPase are not present in skeletal muscle, where ankyrin-B is expressed at 10-fold lower levels than in heart. Ankyrin-B also is not abundantly expressed in smooth muscle. We propose that the ankyrin-B-based complex is a specialized adaptation of cardiomyocytes with a role for cytosolic Ca²⁺ modulation.

This review addresses a new mechanism for arrhythmia due to abnormal cellular localization of membrane ion channels and transporters. The focus is on ankyrins, a family of proteins that localize diverse membrane ion channels and transporters, and recent evidence that mutations affecting functions of ankyrins result in cardiac arrhythmia. A loss-of-function mutation of ankyrin-B in humans and a null mutation in mice result in a dominantly-inherited fatal cardiac arrhythmia initially classified as type 4 long QT syndrome. Characterization of additional probands suggests ankyrin-B mutations cause a new cardiac arrhythmia syndrome associated with sinus node dysfunction that is distinct from long QT syndrome. Ankyrin-B mutation results in elevated calcium transients in cardiomyocytes accompanied by loss of cellular targeting of Na/K ATPase, Na/Ca exchanger, and InsP3 receptor (all ankyrin-binding proteins) to cardiomyocyte membrane domains. The principal voltage-gated Na channel in heart, Nav1.5, is directly associated with ankyrin-G, which is encoded by a distinct gene from ankyrin-B. Mutation of Nav1.5 causing loss of binding to ankyrin-G results in Brugada syndrome and loss of targeting of Nav1.5 to the cell surface of cardiomyocytes. Ankyrin-B and ankyrin-G are recently recognized constituents of the heart that target diverse ion channels/pumps/transporters to physiologic sites of action in cardiomyocytes. Mutations of ankyrin-B cause a newly defined cardiac arrhythmia syndrome associated with abnormal calcium homeostasis in a mouse model. Ankyrin-G associates with the principal voltage-gated Na channel in the heart, and loss of this interaction due to mutation of Nav1.5 results in Brugada syndrome.

Ankyrins are a family of multivalent membrane-adaptor proteins first identified in the erythrocyte over 25 years ago as a link between the anion exchanger and the spectrin-based cytoskeleton. Since their initial discovery, ankyrin function has been linked to protein targeting and membrane domain organization in a variety of cell types including erythrocytes, neurons, epithelial cells, and cardiomyocytes. Recent findings demonstrate that dysfunction in ankyrin-based cellular pathways in the heart leads to human ventricular arrhythmia and sudden cardiac death. This review will present an overview of the ankyrin family with special emphasis on the recently identified roles of ankyrin polypeptides in ion channel and transporter targeting in the heart.

Ankyrins are a ubiquitously expressed family of membrane-adaptor proteins found in most vertebrate tissues. Since the first ankyrin polypeptide was identified over 25 years ago, studies in humans, mice, and lower organisms have implicated critical roles for ankyrins in normal metazoan physiology. This review will provide an overview of the ankyrin family and highlight seminal findings in the field which have linked dysfunction in ankyrin-based pathways with defects in metazoan physiology and human disease.

We identify a human mutation (E1053K) in the ankyrin-binding motif of Na_v1.5 that is associated with Brugada syndrome, a fatal cardiac arrhythmia caused by altered function of Na_v1.5. The E1053K mutation abolishes binding of Na_v1.5 to ankyrin-G, and also prevents accumulation of Na_v1.5 at cell surface sites in ventricular cardiomyocytes. Ankyrin-G and Na_v1.5 are both localized at intercalated disc and T-tubule membranes in cardiomyocytes, and Na_v1.5 coimmunoprecipitates with 190-kDa ankyrin-G from detergent-soluble lysates from rat heart. These data suggest that Na_v1.5 associates with ankyrin-G and that ankyrin-G is required for Na_v1.5 localization at excitable membranes in cardiomyocytes. Together with previous work in neurons, these results in cardiomyocytes suggest that ankyrin-G participates in a common pathway for localization of voltage-gated Na_vchannels at sites of function in multiple excitable cell types.