Abstract 3638: AKAP5 is Essential for the Response of Cardiomyocytes to Beta-Adrenergic Stimulation
Sympathetic stimulation of the heart through β-adrenergic receptors increases the force of contraction and accelerates the rate of relaxation by modulating the release and reuptake of calcium from the sarcoplasmic reticulum. In response to elevated cAMP, PKA phosphorylates key proteins involved in calcium-induced calcium release including the L-type Ca2+channel (Cav1.2), ryanodine receptors, and phospholamban. Multiple AKAPs (A Kinase Anchoring Proteins) are expressed in heart but the overall physiological significance of each in calcium-induced calcium release is not well understood. Adult cardiomyocytes from AKAP5 KO mice fail to respond to β-adrenergic stimulation as measured by calcium imaging but their response to exogenous cAMP analogs remains intact. We show that AKAP5 (also known as AKAP150) is required for isoproterenol stimulation of the calcium transient amplitude as well as the phosphorylation of phospholamban and ryanodine receptors by PKA. AKAP5 participates in a protein complex that includes adenylyl cyclase 5/6, PKA, calcineurin, Cav1.2 channels, β-adrenergic receptors, and caveolin-3 (CAV3). We can functionally identify two populations of Cav1.2 channels, those that are associated with the CAV3-containing complexes and are phosphorylated in response to isoproterenol stimulation and those channels that are not associated with CAV3 and are not phosphorylated by isoproterenol in wild type cells. In the AKAP5 KO, the phosphorylation of Cav1.2 associated with CAV3 is lost and this correlates with the delocalization of adenylyl cyclase 5/6 away from the T-tubules. Our results demonstrate a key role for AKAP5 in cardiac responses to sympathetic stimulation and suggest that the localization of adenylyl cyclase 5/6 to the CAV3 complex by AKAP5 is responsible for creating a highly-localized microdomain of cAMP that mediates functional changes in nearby calcium channels as well as ryanodine receptors and phospholamban.