The cardiac response to chronic stress involves the activation of a myocyte signal transduction network that in disease promotes pathological cardiac remodeling and heart failure. Underlying these cellular and pathophysiological changes is the altered transcription of genes that determine cardiac phenotype. Class IIa histone deacetylases are transcriptional repressors whose nuclear export is associated with the induction of pathological remodeling. These HDACs are regulated by multiple, functionally opposing post-translational modifications, including phosphorylation by PKD and PKA that promote nuclear export and import, respectively. The scaffold protein mAKAPβ orchestrates signaling in the cardiac myocyte required for pathological cardiac remodeling. Whereas mAKAPβ-bound PKD was required for HDAC5 nuclear export in response to α-adrenergic receptor stimulation of cultured neonatal rat ventricular myocytes, mAKAPβ-bound PKA conferred β-adrenergic (βAR) inhibition of that process. βAR signaling promoting class IIa HDAC retention in the nucleus is, however, only upon acute βAR stimulation. Chronic βAR stimulation as present in disease resulted in HDAC5 nuclear export, also by a mAKAPβ-dependent mechanism. In this project, we will investigate the bidirectional control of class IIa HDAC phosphorylation and nuclear export by mAKAPβ signalosomes. We propose that this switch in signaling is due in part to the presence of PKA-inducible protein phosphatase 2A (PP2A) and salt-inducible kinase I (SIK1) activity in mAKAPβ signalosomes. Specific Aim 1: Requirement for mAKAPβ - class IIa HDAC complexes in pathological remodeling. In this Aim we will characterize the structure and function of mAKAPβ- complexes containing HDAC5 and the HDAC target MEF2D, as well as their importance for catecholamine- induced hypertrophy of adult myocytes in vitro. Using adeno-associated virus to deliver the disruptor peptide to the cardiac myocyte in vivo, we will test whether inhibited complex formation will prevent the pathological remodeling and heart failure induced by pressure overload. Specific Aim 2: Elucidation of the mechanism for mAKAPβ-dependent β-adrenergic inhibition of HDAC5 nuclear export. Using specific mAKAP mutant proteins and anchoring disruptor proteins, we will dissect the relative roles of PKA-dependent HDAC5 phosphorylation and PP2A activation at mAKAPβ in inhibiting GqPCR-induced HDAC5 nuclear export. Specific Aim 3: Requirement for SIK1 in HDAC nuclear export and pathological remodeling. We now reveal that the HDAC kinase SIK1 binds mAKAPβ and that mAKAPβ-bound PKA is required for SIK1 induction in myocytes. We will study whether SIK1 and its phosphorylation by mAKAPβ-bound PKA is required for HDAC5 nuclear export in vitro and test the relevance of SIK1 to the adult cardiac myocyte in vivo using a conditional knock-out mouse model. These Aims will elucidate how mAKAPβ signalosomes bidirectionally coordinate type IIa HDAC function in myocytes. In additio...