HEART-SPECIFIC TARGETING OF FIREFLY LUCIFERASE BY THE MYOSIN LIGHT CHAIN-2 PROMOTER AND DEVELOPMENTAL REGULATION IN TRANSGENIC MICE

Based on hybridization studies indicating constitutive expression leve ls of the endogenous myosin light chain-2 (MLC-2) gene in embryonic, f etal, and adult myocardium, a model system for selective targeting of genes to the heart of transgenic mice has been developed. A 2.1-kb DNA fragment of the 5' flanking region of the rat cardiac MLC-2 gene was fused to the firefly luciferase reporter gene and introduced into fert ilized mouse oocytes. In four independent transgenic mouse lines, the expression of the MLC-2-luciferase fusion gene was found exclusively i n heart muscle. In contrast to the endogenous MLC-2 gene, no luciferas e activity was detectable in slow-twitch skeletal muscle or any other tissue of transgenic mice. This result suggests that the 2.1-kb DNA fr agment of the 5' flanking region of the cardiac MLC-2 gene contains th e regulatory elements required for selective gene expression in cardia c myocytes in vivo. In contrast to the endogenous steady-state MLC-2 e xpression during development, transgenic luciferase activity was 10-fo ld higher during embryogenesis, when formation of the ventricular loop and septum takes place. The enhanced luciferase activity in early hea rt development may suggest a growth-dependent control mechanism, invol ving either transcriptional or posttranscriptional regulation. In conc lusion, this model system with the 2.1-kb ventricle-specific MLC-2 pro moter sequence should facilitate the overexpression of gene products i n the developing and mature heart muscle and further elucidate molecul ar mechanisms of myocardial diseases such as cardiomyopathies.