EXPRESSION PATTERN OF CONNEXIN GENE-PRODUCTS AT THE EARLY DEVELOPMENTAL STAGES OF THE MOUSE CARDIOVASCULAR-SYSTEM

The synchronized contraction of myocytes in cardiac muscle requires th e structural and functional integrity of the gap junctions present bet ween these cells. Gap junctions are clusters of intercellular channels formed by transmembrane proteins of the connexin (Cx) family. Product s of several Cx genes have been identified in the mammalian heart (eg, Cx45, Cx43, Cx40, and Cx37), and their expression was shown to be reg ulated during the development of the myocardium. Cx43, Cx40, and Cx45 are components of myocyte gap junctions, and it has also been demonstr ated that Cx40 was expressed in the endothelial cells of the blood ves sels. The aim of the present work was to investigate the expression an d regulation of Cx40, Cx43, and Cx37 during the early stages of mouse heart maturation, between 8.5 days post coitum (dpc), when the first r hythmic contractions appear, and 14.5 dpc, when the four-chambered hea rt is almost completed. At 8.5 dpc, only the reverse-transcriptase pol ymerase chain reaction technique has allowed identification of Cx43, C x40, and Cx37 gene transcripts in mouse heart, suggesting a very low a ctivity level of these genes. From 9.5 dpc, all three transcripts beca me detectable in whole-mount in situ-hybridized embryos, and the most obvious result was the labeling of the vascular system with Cx40 and C x37 anti-sense riboprobes. Cx40 and Cx37 gene products (transcript and /or protein) were demonstrated to be expressed in the vascular endothe lial cells at all stages examined. By contrast, only Cx37 gene product s were found in the endothelial cells of the endocardium. In heart, Cx 37 was expressed exclusively in these cells, which rules out any direc t involvement of this Cx in the propagation of electrical activity bet ween myocytes and the synchronization of contractions. Between 9.5 and 11.5 dpc, Ck40 gene activation in myocytes was demonstrated to procee d according to a caudorostral gradient involving first the primitive a trium and the common ventricular chamber (9.5 dpc) and then the right ventricle (11.5 dpc). During this period of heart morphogenesis, there is clearly a temporary and asymmetrical regionalization of the Cx40 g ene expression that is superimposed on the functional regionalization. In addition, comparison of Cx40 and Cx43 distribution at the above de velopmental stages has shown that these Cxs have overlapping (left ven tricle) or complementary (atrial tissue and right ventricle) expressio n patterns.