Apoptosis of cardiac myocytes in Gsalpha transgenic mice.
-The stimulatory GTP-binding protein Gsalpha transmits signals from catecholamine receptors to activate adenylyl cyclase and thereby initiate a cascade leading to cardiac chronotropy and inotropy. Transgenic mice overexpressing the Gs alpha subunit (Gsalpha) selectively in their hearts exhibit increased cardiac contractility in response to beta-adrenergic receptor stimulation. However, with aging, these mice develop a cardiomyopathy. This study sought morphological and biochemical evidence that overexpression of Gsalpha is associated with increased myocyte apoptosis in the older animals and to determine whether such overexpression can promote apoptosis of isolated neonatal cardiac myocytes exposed to beta-adrenergic receptor agonists. In the hearts of 15- to 18-month-old Gsalpha transgenic mice, histochemistry and electron microscopy illustrated the existence of numerous myocytes with abnormal nuclei embedded in collagen-rich connective tissue. Terminal deoxyribonucleotide transferase-mediated dUTP nick-end labeling (TUNEL, for in situ labeling of DNA breaks) demonstrated that approximately 0.6% of myocyte nuclei contained fragmented DNA. Agarose gel electrophoresis provided further biochemical evidence of apoptosis by showing internucleosomal DNA fragmentation. Cultured cardiac myocytes from newborn Gsalpha transgenic mice showed increased TUNEL staining and internucleosomal DNA fragmentation compared with wild-type controls when treated with the beta-agonist isoproterenol. Thus, enhanced activation of beta-adrenergic signaling by overexpression of Gsalpha in the hearts of transgenic mice induces apoptosis of cardiac myocytes. This represents a potential mechanism that may contribute to the development of cardiomyopathy in this model.