Alteration in Connexin 43 Gap Junction Gene Dosage Impairs Conotruncal Heart Development

Abstract

Connexin 43 (Cx43) knockout mice and transgenic mice (CMV43) overexpressing the Cx43 gap junction gene exhibit heart defects involving the conotruncus 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 mechanism by which these heart defects may arise, fetal heart structure and function 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 ventricular chamber in the heterozygous Cx43 knockout and CMV43 transgenic mice. This was accompanied by thinning of the chamber wall. In the homozygous Cx43 knockout mouse, heart malformation was also restricted to the right ventricle. This was generally characterized by two pouches at the base of the pulmonary outflow tract, but occasionally hearts with 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 obstruction. This was confirmed by the in utero Doppler echocardiography, which showed increased outflow velocity in E12.5 to 14.5 CMV43 and Cx43 knockout fetuses. In some of these fetuses, Doppler analysis also revealed arrhythmia and absence of isovolemic contraction time. Further examination of these hearts by histology and immunohistochemistry showed abnormal myocardial development in the conotruncus. Particularly interesting was the presence of abundant subendocardial fibrous tissue expressing smooth muscle actin. In the developing heart, such mesenchyme in the outflow tract is usually considered neural crest-derived tissue. Together, these results confirm the importance of Cx43 gene dosage in conotruncal heart development and suggest that this likely involves a role for Cx43 gap junctions in cardiac crest development. In future studies, these transgenic mice may serve as valuable animal models for further studying the role of gap junctions and cardiac crest cells in conotruncal heart development.

Connexins, cell motility, and the cytoskeleton

“…Supporting in vivo data showed more CNCs in the right outflow tract of the heart after Cx43 overexpression and less CNCs for the Cx43 KO mouse embryos [Huang et al, 1998b]. The authors propose a model where GJs may help coordinating CNC deployment to the heart by allowing intercellular transfer of second messengers and signaling molecules that regulate cell motility.…”

Controlled levels of canonical Wnt signaling are required for neural crest migration

“…As a support for this observation an excess of the gap junction protein connexin-43α1 (Cx43α1) has been shown to significantly increase the migration rate of mouse cardiac NC cells. On the other hand, downregulation of Cx43α1 impairs migration of human glioblastoma cells, which share similarities with migrating NC cells (Huang et al, 1998). The cadherindependent adhesion, a major player of cell interactions is also subjected to modifications.…”

Homocysteine enhances cardiac neural crest cell attachment in vitro by increasing intracellular calcium levels

“…In cultured mouse cardiac NC cells, elevated Hcys increases connexin-43 expression and increases connexin-43 localization with NC cell processes (Boot et al, 2006). Because perturbing connexin-43 expression genetically in mouse NC cells results in neurocristopathies (Huang et al, 1998;, it is possible the effects of Hcys on NC morphogenesis may involve changes in gap junctional protein expression and localization. However, the precise mechanisms by which Hcys perturbs NC cell formation and migration are unknown.…”