Cyclin-dependent Kinases and Cardiac Growth

Schneider, Michael

Abstract

This is a competing renewal of the Applicant's investigation of cyclin-dependent protein kinases (Cdks) and their functional role in cardiac growth. Hypertrophic growth is a risk factor for mortality in heart diseases. Information is still lacking to explain this global increase in RNA and protein per cell. Hypertrophic signals cause phosphorylation of the RNA polymerase II (RNAPII) carboxyl-terminal domain (CTD), which is required in other biological settings for transcript elongation. CTD kinases include components of two basal transcription factors, TFIIH (cyclin HCdk7) and positive transcription elongation factor b (P-TEFb, cyclin T-Cdk9). During the past period of support, our interest in cardiac Cdks led us to discover the activation of Cdk7 and Cdk9 in hypertrophy triggered by the signaling proteins Gaq and calcineurin or chronic mechanical stress. Only Cdk9 was activated by acute load or, in culture, by the hypertrophic agonist endothelin (ET-1). A preferential role for Cdk9 was shown in CTD phosphorylation and growth induced by ET-1, using pharmacological inhibition and dominant-negative proteins. ET-1 did not increase expression or assembly of cyclin T-Cdk9 but, rather, caused dissociation of 7SK snRNA, an endogenous Cdk9 inhibitor. Activity of Cdk9 was proven to be limiting for cardiac growth, by suppressing its inhibitor (7SK) in cultured myocytes and preventing downregulation of its activator (cyclin T1) in mouse myocardium. Based on these findings and additional interim work, we propose to study the function and fundamental mechanisms of the cyclin T-Cdk9-RNAPII cascade in cardiac muscle: (1) To study gain-of-function mutations for cyclin T1 and Cdk9 in transgenic mice, singly and in combination. (2) To define the essential functions of Cdk9, using conditional dominant-negative and loss-of-function mutations. (3) To test the prediction that hypertrophic signals, through cyclin T-Cdk9, induce promoter escape by RNAPII.