Phosphoregulation of the titin-cap protein telethonin in cardiac myocytes.

Telethonin (also known as titin-cap or t-cap) is a muscle-specific protein whose mutation is associated with cardiac and skeletal myopathies through unknown mechanisms. Our previous work identified cardiac telethonin as an interaction partner for the protein kinase D catalytic domain. In this study, kinase assays used in conjunction with MS and site-directed mutagenesis confirmed telethonin as a substrate for protein kinase D and Ca(2+)/calmodulin-dependent kinase II in vitro and identified Ser-157 and Ser-161 as the phosphorylation sites. Phosphate affinity electrophoresis and MS revealed endogenous telethonin to exist in a constitutively bis-phosphorylated form in isolated adult rat ventricular myocytes and in mouse and rat ventricular myocardium. Following heterologous expression in myocytes by adenoviral gene transfer, wild-type telethonin became bis-phosphorylated, whereas S157A/S161A telethonin remained non-phosphorylated. Nevertheless, both proteins localized predominantly to the sarcomeric Z-disc, where they partially replaced endogenous telethonin. Such partial replacement with S157A/S161A telethonin disrupted transverse tubule organization and prolonged the time to peak of the intracellular Ca(2+) transient and increased its variance. These data reveal, for the first time, that cardiac telethonin is constitutively bis-phosphorylated and suggest that such phosphorylation is critical for normal telethonin function, which may include maintenance of transverse tubule organization and intracellular Ca(2+) transients.

Four-and-a-half LIM domains proteins are novel regulators of the protein kinase D pathway in cardiac myocytes.

PKD (protein kinase D) is a serine/threonine kinase implicated in multiple cardiac roles, including the phosphorylation of the class II HDAC5 (histone deacetylase isoform 5) and thereby de-repression of MEF2 (myocyte enhancer factor 2) transcription factor activity. In the present study we identify FHL1 (four-and-a-half LIM domains protein 1) and FHL2 as novel binding partners for PKD in cardiac myocytes. This was confirmed by pull-down assays using recombinant GST-fused proteins and heterologously or endogenously expressed PKD in adult rat ventricular myocytes or NRVMs (neonatal rat ventricular myocytes) respectively, and by co-immunoprecipitation of FHL1 and FHL2 with GFP-PKD1 fusion protein expressed in NRVMs. In vitro kinase assays showed that neither FHL1 nor FHL2 is a PKD1 substrate. Selective knockdown of FHL1 expression in NRVMs significantly inhibited PKD activation and HDAC5 phosphorylation in response to endothelin 1, but not to the α1-adrenoceptor agonist phenylephrine. In contrast, selective knockdown of FHL2 expression caused a significant reduction in PKD activation and HDAC5 phosphorylation in response to both stimuli. Interestingly, neither intervention affected MEF2 activation by endothelin 1 or phenylephrine. We conclude that FHL1 and FHL2 are novel cardiac PKD partners, which differentially facilitate PKD activation and HDAC5 phosphorylation by distinct neurohormonal stimuli, but are unlikely to regulate MEF2-driven transcriptional reprogramming.

Neurohormonal regulation of cardiac histone deacetylase 5 nuclear localization by phosphorylation-dependent and phosphorylation-independent mechanisms.

RATIONALE: Myocyte enhancer factor 2 (MEF2) transcription factors drive the genetic reprogramming that precipitates pathological cardiac hypertrophy and remodeling. Class II histone deacetylase (HDAC) isoforms, such as HDAC5, act as signal-responsive repressors of MEF2 activity in cardiac myocytes and their nuclear export provides a key mechanism for the neurohormonal induction of such activity. OBJECTIVE: To delineate the mechanism(s) through which 2 clinically relevant neurohormonal stimuli, endothelin-1 (ET1) and the ß-adrenergic receptor (ß-AR) agonist isoproterenol (ISO), may regulate HDAC5 nuclear localization in adult cardiac myocytes. METHODS AND RESULTS: ET1 induced HDAC5 phosphorylation and nuclear export in ventricular myocytes from the adult rat heart. Use of a novel, highly selective protein kinase D (PKD) inhibitor and a nonphosphorylatable HDAC5 mutant revealed that PKD-mediated phosphorylation was necessary for ET1-induced HDAC5 nuclear export. In contrast, ISO reduced HDAC5 phosphorylation in the presence or absence of ET1 but still induced HDAC5 nuclear export. ISO-induced HDAC5 nuclear export occurred through a ß(1)-AR-mediated oxidative process that was independent of PKD, protein kinase A, and Ca(2+)/calmodulin-dependent kinase II activities. Although ET1 and ISO shared a similar ability to induce HDAC5 nuclear export, albeit through distinct phosphorylation-dependent versus phosphorylation-independent mechanisms, ISO induced a significantly greater increase in MEF2 activity. CONCLUSIONS: PKD-mediated HDAC5 phosphorylation and nuclear export are unlikely to be of major importance in regulating MEF2-driven cardiac remodeling in the presence of sympathetic activity with intact ß(1)-AR signaling, which would not only counteract HDAC5 phosphorylation but also induce HDAC5 nuclear export through a novel phosphorylation-independent, oxidation-mediated mechanism. Inhibition of this mechanism may contribute to the therapeutic efficacy of ß(1)-AR antagonists in heart failure.