Regulation of nerve growth factor in the heart: the role of the calcineurin-NFAT pathway.

A heightened sympathetic tone accelerates the development of lethal arrhythmias after myocardial infarction (MI) and the progression of heart failure (HF). Cardiomyocytes control their local neural milieu by expression of nerve growth factor (NGF), which triggers sympathetic neural growth (sympathetic nerve sprouting: SNS). The molecular mechanisms that regulate NGF expression are largely unknown. During HF or MI the myocytes are exposed to increased mechanical load and adrenergic stimulation. Both stimuli induce myocyte hypertrophy. The angiotensin-II-calcineurin-NFAT (nuclear factor of activated t-cells) pathway is a well characterized signaling cascade in the pathogenesis of myocyte hypertrophy. The present study aims to investigate the molecular mechanisms by which mechanical stretch and/or alpha-1-adrenergic stimulation affect NGF expression in neonatal rat ventricular myocytes. Both stimuli resulted in a down-regulation of NGF gene and protein expression. Angiotensin-II type 1 receptor blockade with losartan blunted the stretch-induced NGF down-regulation. Specific calcineurin inhibition with cyclosporine A and FK506 or NFAT inhibition with 11R-VIVIT reversed the stretch or alpha-1-adrenergic induced decrease of NGF. Calcineurin over-expression increased NFAT-DNA binding activity and decreased NGF expression. The magnitude of NGF decrease was sufficient to reduce neurite outgrowth of cultured sympathetic neurons. In conclusion, mechanical stretch and alpha-1-adrenergic stimulation contribute to a decrease of cardiomyocyte NGF expression via the calcineurin-NFAT pathway. To evaluate if the calcineurin-NFAT is critically involved in the pathogenesis of SNS further in-vivo studies in models of HF and MI are required. Nevertheless, the calcineurin-NFAT pathway may provide promising starting points for new pharmacological strategies to prevent SNS in the heart.