ALTERATION IN CONNEXIN-43 GAP JUNCTION GENE DOSAGE IMPAIRS CONOTRUNCAL HEART DEVELOPMENT

Connexin 43 (Cx43) knockout mice and transgenic mice (CMV43) overexpre ssing the Cx43 gap junction gene exhibit heart defects involving the c onotruncus and right ventricle. Based on the heart phenotype and Cx43 gene and transgene expression pattern, we previously proposed that the heart defects may reflect a role for gap junctions in the modulation of cardiac neural crest development. To further elucidate the mechanis m by which these heart defects may arise, fetal heart structure and fu nction in these transgenic and knockout mice were examined by magnetic resonance microscopy and Doppler echocardiography. Magnetic resonance microscopy of E14.5 fetuses revealed an enlargement of the right vent ricular chamber in the heterozygous Cx43 knockout and CMV43 transgenic mice. This was accompanied by thinning of the chamber wall. In the ho mozygous Cx43 knockout mouse, heart malformation was also restricted t o the right ventricle. This was generally characterized by two pouches at the base of the pulmonary outflow tract, but occasionally hearts w ith a single pouch were found. Magnetic resonance microscopy showed in some of the CMV43 and Cx43 knockout mice an attenuation of the ductus arteriosus, a phenotype which may be indicative of outflow tract obst ruction. This was confirmed by the in utero Doppler echocardiography, which showed increased outflow velocity in E12.5 to 14.5 CMV43 and Cx4 3 knockout fetuses. In some of these fetuses, Doppler analysis also re vealed arrhythmia and absence of isovolemic contraction time. Further examination of these hearts by histology and immunohistochemistry show ed abnormal myocardial development in the conotruncus. Particularly in teresting was the presence of abundant subendocardial fibrous tissue e xpressing smooth muscle actin. In the developing heart, such mesenchym e in the outflow tract is usually considered neural crest-derived tiss ue. Together, these results confirm the importance of Cx43 gene dosage in conotruncal heart development and suggest that this likely involve s a role for Cx43 gap junctions in cardiac crest development. In futur e studies, these transgenic mice may serve as valuable animal models f or further studying the role of gap junctions and cardiac crest cells in conotruncal heart development. (C) 1998 Academic Press.