REGIONALIZED TRANSCRIPTIONAL DOMAINS OF MYOSIN LIGHT-CHAIN 3F TRANSGENES IN THE EMBRYONIC MOUSE HEART - MORPHOGENETIC IMPLICATIONS

Within the embryonic heart, five segments can be distinguished: two fa st-conducting atrial and ventricular compartments flanked by slow-cond ucting segments, the inflow tract, the atrioventricular canal, and the outflow tract. These compartments assume morphological identity as a result of looping of the linear heart tube. Subsequently, the formatio n of interatrial, interventricular, and outflow tract septa generates a four-chambered heart. The lack of markers that distinguish right and left compartments within the heart has prevented a precise understand ing of these processes. Transgenic mice carrying an nlacZ reporter gen e under transcriptional control of regulatory sequences from the MLC1F /3F gene provide specific markers to investigate such regionalization. Our results show that transgene expression is restricted to distinct regions of the myocardium: beta-galactosidase activity in 3F-nlacZ-2E mice is confined predominantly to the embryonic right atrium, atrioven tricular canal, and left ventricle, whereas, in 3E-nlacZ-9 mice, the t ransgene is expressed in both atrial and ventricular segments (right/l eft) and in the atrioventricular canal, but not in the inflow and outf low tracts. These lines of mice illustrate that distinct embryonic car diac regions have different transcriptional specificities and provide early markers of myocardial subdivisions. Regional differences in tran sgene expression are not detected in the linear heart tube but become apparent as the heart begins to loop. Subsequent regionalization of tr ansgene expression provides new insights into later morphogenetic even ts, including the development of the atrioventricular canal and the fa te of the outflow tract. (C) 1997 Academic Press.