The identification of nearly a dozen ion channel genes involved in the genesis of human atrial and ventricular arrhythmias has been critical for the diagnosis and treatment of fatal cardiovascular diseases. In contrast, very little is known about the genetic and molecular mechanisms underlying human sinus node dysfunction (SND). Here, we report a genetic and molecular mechanism for human SND. We mapped two families with highly penetrant and severe SND to the human ANK2 (ankyrin-B/AnkB) locus. Mice heterozygous for AnkB phenocopy human SND displayed severe bradycardia and rate variability. AnkB is essential for normal membrane organization of sinoatrial node cell channels and transporters, and AnkB is required for physiological cardiac pacing. Finally, dysfunction in AnkB-based trafficking pathways causes abnormal sinoatrial node (SAN) electrical activity and SND. Together, our findings associate abnormal channel targeting with human SND and highlight the critical role of local membrane organization for sinoatrial node excitability.calcium ͉ trafficking ͉ arrhythmia ͉ cytoskeleton

In eukaryotic cells, ankyrins serve as adaptor proteins that link membrane proteins to the underlying cytoskeleton. These adaptor proteins form protein complexes consisting of integral membrane proteins, signalling molecules and cytoskeletal components. With their modular architecture and ability to interact with many proteins, ankyrins organize and stabilize these protein networks, thereby establishing the infrastructure of membrane domains with specialized functions. To this end, ankyrin collaborates with a number of proteins including cytoskeletal proteins, cell adhesion molecules and large structural proteins. This review addresses the targeting and stabilization of protein networks related to ankyrin interactions with the cytoskeletal protein β-spectrin, L1-cell adhesion molecules and the large myofibrillar protein obscurin. The significance of these interactions for differential targeting of cardiac proteins and neuronal membrane formation is also presented. Finally, this review concludes with a discussion about ankyrin dysfunction in human diseases such as haemolytic anaemia, cardiac arrhythmia and neurological disorders.

Bhasin N, Cunha SR, Mudannayake M, Gigena MS, Rogers TB, Mohler PJ. Molecular basis for PP2A regulatory subunit B56␣ targeting in cardiomyocytes. Am J Physiol Heart Circ Physiol 293: H109-H119, 2007. First published April 6, 2007; doi:10.1152/ajpheart.00059.2007.-Protein phosphatase 2A (PP2A) is a multifunctional protein phosphatase with critical roles in excitable cell signaling. In the heart, PP2A function is linked with modulation of ␤-adrenergic signaling and has been suggested to regulate key ion channels and transporters including Na/Ca exchanger, ryanodine receptor, inositol 1,4,5-trisphosphate receptor, and Na/K ATPase. Although many of the functional roles and molecular targets for PP2A in heart are known, little is established regarding the cellular pathways that localize specific PP2A isoform activities to subcellular sites. We report that the PP2A regulatory subunit B56␣ is an in vivo binding partner for ankyrin-B, an adapter protein required for normal subcellular localization of the Na/Ca exchanger, Na/K ATPase, and inositol 1,4,5-trisphosphate receptor. Ankyrin-B and B56␣ are colocalized and coimmunoprecipitate in primary cardiomyocytes. Using multiple strategies, we identified the structural requirements on B56␣ for ankyrin-B association as a 13 residue motif in the B56␣ COOH terminus not present in other B56 family polypeptides. Finally, we report that reduced ankyrin-B expression in primary ankyrin-B ϩ/Ϫ cardiomyocytes results in disorganized distribution of B56␣ that can be rescued by exogenous expression of ankyrin-B. These new data implicate ankyrin-B as a critical targeting component for PP2A in heart and identify a new class of signaling proteins targeted by ankyrin polypeptides. cytoskeleton; trafficking; phosphatase EFFICIENT EXCITABLE CELL FUNCTION requires precisely orchestrated signaling pathways to modulate the function of diverse membrane, cytosolic, and nuclear proteins. Specifically, the collaboration between finely tuned protein phosphorylation/ dephosphorylation pathways is critical for neuronal communication and cardiac excitation-contraction coupling. Findings over the past decade clearly demonstrate that a local organization of kinase and phosphatase proteins with specific effector protein pathways facilitates efficient signaling. This local organization may occur by direct interaction of kinases and phosphatases with anchoring or scaffolding proteins, cytoskeleton, or even by direct interaction with target ion channels, transporters, or receptors (21,36,42,44,67). However, the cellular pathways underlying the biogenesis/targeting of kinases or phosphatases to specialized membrane protein complexes are not clearly resolved.Protein phosphatase 2A (PP2A) is a multifunctional serine/ threonine phosphatase with critical roles in ion channel/transporter regulation (3, 34, 41, 88), Wnt signaling (39, 63, 72, 84), transformation (2, 6, 27, 75), cell polarization (23), circadian rhythm (5, 68, 85), and cell survival and apoptosis (37,40,64,73,76,81). In heart, PP2A function is neces...

Background Atrial fibrillation (AF) is the most common cardiac arrhythmia, affecting over two million patients in the US alone. Despite decades of research, surprisingly little is known regarding the molecular pathways underlying the pathogenesis of AF. ANK2 encodes ankyrin-B, a multifunctional adapter molecule implicated in membrane targeting of ion channels, transporters, and signaling molecules in excitable cells. Methods and Results Here, we report early-onset AF in patients harboring loss-of-function mutations in ANK2. In mice, we show that ankyrin-B-deficiency results in atrial electrophysiological dysfunction and increased susceptibility to AF. Moreover, ankyrin-B+/− atrial myocytes display shortened action potentials, consistent with human AF. Ankyrin-B is expressed in atrial myocytes, and we demonstrate its requirement for the membrane targeting and function of a subgroup of voltage-gated Ca2+ channels (Cav1.3) responsible for low-voltage activated L-type Ca2+current. Ankyrin-B directly associates with Cav1.3, and this interaction is regulated by a short, highly-conserved motif specific to Cav1.3. Moreover, loss of ankyrin-B in atrial myocytes results in decreased Cav1.3 expression, membrane localization, and function sufficient to produce shortened atrial action potentials and arrhythmias. Finally, we demonstrate reduced ankyrin-B expression in atrial samples of patients with documented AF, further supporting an association between ankyrin-B and AF. Conclusions These findings support that reduced ankyrin-B expression or mutations in ANK2 are associated with atrial fibrillation. Additionally, our data demonstrate a novel pathway for ankyrin-B-dependent regulation of Cav1.3 channel membrane targeting and regulation in atrial myocytes.