This revised application seeks support to investigate the basic mechanisms of aging in the heart. We previously observed that in response to stress, the compensatory hypertrophy, protein synthesis, and UNA synthesis are reduced in old compared to young adult hearts. This decreased myocardial reserve in old hearts is associated with an altered program of gene expression. We observed that the basal binding activity to the c-fos serum response element (SRE) in the heart was increased with age, and that serum response factor (SRF), the major SRE binding protein, was also slightly increased in old compared with young adult hearts. Inasmuch as SRF is a key transcription factor in many muscle genes, it is plausible that SRF contributes to the altered expression of these genes. In a genetically modified mouse that was generated with cardiac-specific, moderate overexpression of the wild type SRF gene, there was significant cardiomyocyte hypertrophy, cardiomyopathy and early mortality. The SRF null is embryonically lethal. A dominant negative, mutant SRF transgenic mouse was generated in which the mutations are in the DNA binding domain, thereby severely compromising the mutant SRF's ability to bind to the c-fos SRE; these newborn dominant negative transgenic mice had dilated hearts with abnormally small cardiomyocytes, poor postnatal growth and development, and all died within 12 days after birth. These findings suggest that SRF is essential for cardiac development, growth and maintenance, but that a moderate increase or decrease in cardiac SRF activity can have potentially disastrous consequences in the animal. We sought to determine the effect of mild changes in SRF activity on the heart, and found that in a transgenic mouse with mildly increased SRF activity, there were mild cardiac changes in young adulthood which appeared to be similar to those usually observed in old age. These findings are compatible with the intriguing notion that a transcription factor such as SRF may be involved in mediating, in part, the process of myocardial aging. However, a link between the activity of any gene and aging in the heart has not been previously established. We therefore propose to conduct a detailed investigation of this novel concept, which holds significant potential for the future development of new targets of therapy. The central theme of our proposal is that alterations in a single transcription factor such as SRF may underlie, in part, the aging changes that are commonly observed in the heart. We propose to test this hypothesis and to elucidate the mechanism(s) underlying the aging process of the heart.
Our specific aims are: l) To test the hypothesis that the process of cardiac aging might be partly attributable to altered transcription regulation of genes such as the serum response factor (SRF); 2) To test the hypothesis that a mildly reduced level of SRF in the old heart would be associated with delayed cardiac aging; 3) To test the hypothesis that SRF might mediate, in part, aspects of altered calcium handling that has been observed during normal adult cardiac aging.
