Cardiac Myocyte Response to Mechanical or Chemical Unloading

This thesis tests the overall hypothesis that external and internal generation of tension have different outcomes on cardiac myocyte remodeling. Physiological remodeling occurs in response to external changes in tension and load sensed through mechanosignaling pathways. Direct, internal tension generation in the sarcomere can also cause remodeling, perhaps while bypassing some of the signal pathways. Though hypertrophy of the loaded heart has been studied extensively, less is understood about the mechanisms for sarcomere disassembly when workload is reduced. A clearer picture of the disassembly process may inform the future pursuit of therapeutics for heart failure, particularly those with desired outcomes of unloading and reverse remodeling. Histone deacetylase 3 (HDAC3) has been shown to bind to the Z-disk of sarcomeres; this localization is reduced by chemical hypertrophic signals such as phenylephrine, suggesting a potential role for HDAC3 in non-hypertrophic conditions. -actinin, a key component of the Z-disk, contains lysine residues available for HDAC activity. The present study tests the specific hypothesis that in external unloading conditions, the disassembly of sarcomeres is facilitated by deacetylase activity of HDAC3 at the Z-disk; and furthermore, that direct, internal unloading of the actomyosin complex does not involve HDAC3 in disassembly. External unloading was tested in cultured neonatal rat cardiomyocytes by subjecting them to cyclic strain followed by a 6-hour rest period; for internal unloading, cells were treated for 2 or 6 hours with a myosin inhibitor. Unloaded cells were also treated with general and specific HDAC inhibitors. Results were assessed with live-cell recordings, immunofluorescence, and protein quantification to analyze contractile function, sarcomere content and HDAC3 distribution. Inhibition of HDAC3 caused an attenuation of the normal response to external loading and unloading. Internal unloading by pharmacologic attenuation of actomyosin tension showed no changes in HDAC3 distribution, nor effects of HDAC3 inhibition, despite evidence of comparable sarcomere disassembly. Though further research is needed, together these results support a potential role for cardiac HDAC3 in the endogenous reverse-remodeling pathway, while also highlighting important differences between two methods of artificially unloading the heart.