Abstract 8: GTPase-Activating Protein for Rho Associated with FAK (GRAF) Regulates Cardiomyocyte Membrane Integrity
Cardiac myocytes are unique in their requirement to sustain continuous repetitive contraction in the setting of intense mechanical stress while simultaneously maintaining high membrane integrity for an appropriate electrical gradient. The consequence of failure of the membrane repair response has been highlighted in recent reports linking cardiomyocyte membrane fragility with cardiac degeneration in patients as well as in their analogous mouse models. Herein, we describe a novel role for GTPase activator for Rho associated with Focal Adhesion Kinase (GRAF) in regulating cardiomyocyte membrane integrity. We previously published that disruption of GRAF in Xenopus laevis resulted in progressive skeletal muscle degeneration. We now show that GRAF-depleted tadpoles exhibit defective cardiac formation and function. Interestingly, damage of muscle cells in vivo and in vitro led to a translocation of GRAF to the sarcolemma, suggesting that GRAF may be an important component of the cardiac membrane repair machinery. To further explore this possibility, we generated GRAF hypomorphic mice that exhibit greater than 99% reduction of endogenous GRAF expression. While GRAF deficient mice show normal Mendelian birth distribution and are viable, they exhibit a modest skeletal muscle pathology. Although baseline cardiac integrity was not compromised in GRAF deficient mice, treatment either with cardiotoxin or intraperitoneal injection of isoproterenol led to elevated cardiomyocyte membrane damage (assessed by Evan’s blue dye uptake) in GRAF deficient compared to control mice (19% vs 2% of myocytes within afflicted ventricular area for cardiotoxin, 18% vs 8% for isoproterenol respectively). Moreover, cultured GRAF null myocytes exhibited a significantly attenuated membrane resealing response following laser-mediated disruption compared to GRAF-containing control cells as assessed by accumulation of the membrane impermeable dye, FM-143. As well, the survival rate after injury of GRAF-deficient cells was markedly attenuated (20% vs 85% in control cells). While cardiac cell membrane damage is likely a frequent and important event, the repair process is currently understudied, and this is the first report to implicate a Rho regulator in this response.
- © 2012 by American Heart Association, Inc.