Recovery of Calcium Activity and Contraction in Models of Dilated Cardiomyopathy

Abstract

Dilated Cardiomyopathy (DCM) is the most common form of systolic heart failure, causing progressive ventricular dilation and loss of systolic function. A transgenic mouse model of DCM, with a missense mutation in alpha-tropomyosin (D230N-Tm), is characteristic of the defects in contraction and calcium handling. Calcium transients and contraction are also depressed in cardiomyocytes from infarcted hearts. We have previously shown that dATP improves contraction of heart muscle from dogs with DCM. We have also shown that small increases in intracellular dATP, achieved by over-expressing ribonucleotide reductase (RNR) via an adeno-associated virus type 6 (AAV6-RNR), significantly improves cardiomyocyte contraction and left ventricular function of infarcted rodent and pig hearts. This study evaluated whether RNR over-expression can improve contraction and calcium handling in the D230N-Tm and infarct models of DCM. Four-month-old D230N-Tm mice were systemically injected with AAV6-RNR. Four months post-injection, cardiomyocytes were isolated to measure contraction, calcium handling, and dATP content using LC-MS. Western blots confirmed over-expression of RNR. We found a significant increase in sarcomere fractional shortening of cells isolated from injected mice compared to their transgenic, un-injected littermates (8.62±0.4 vs. 6.99±0.4 %, P<0.05). Calcium transient amplitude was decreased in DCM compared to WT mice (1.60±0.03 vs. 1.74±0.02, P<0.05), and was restored post-injection (1.69±0.02 vs. 1.60±0.03, P<0.05). In separate studies, four weeks following myocardial infarction we isolated cardiomyocytes from rat hearts and transduced them with AV-RNR vectors. Similar to the transgenic model, infarction reduced fractional shortening and calcium transients. AV-RNR rescued calcium transients to non-infarcted levels and fractional shortening was increased beyond that of non-infarcted levels. Our data demonstrates that over-expression of RNR recovers both contraction and calcium handling in these DCM disease models and suggests that, in addition to myosin, a second target of dATP may be SERCA2a.