Downregulation of connexin 45 gene products during mouse heart development

The electrical activity in heart is generated in the sinoatrial node and th en propagates to the atrial and ventricular tissues. The gap junction chann els that couple the myocytes are responsible for this propagation process. The gap junction channels are dodecamers of transmembrane proteins of the c onnexin (Cx) family. Three members of this family have been demonstrated to be synthesized in the cardiomyocytes: Cx40, Cx43, and Cx45. In addition, e ach of them has been shown to form channels with unique and specific electr ophysiological properties. Understanding the conduction phenomenon requires detailed knowledge of the spatiotemporal expression pattern of these Cxs i n heart. The expression patterns of Cx40 and Cx43 have been previously desc ribed in the adult heart and during its development. Here we report the exp ression of Cx45 gene products in mouse heart from the stage of the first co ntractions (8.5 days postcoitum [dpc]) to the adult stage. The Cx45 gene tr anscript was demonstrated by reverse transcriptase-polymerase chain reactio n experiments to be present in heart at all stages investigated. Between 8. 5 and 10.5 dpc it was shown by in situ hybridization to be expressed in low amounts in all cardiac compartments (including the inflow and outflow trac ts and the atrioventricular canal) and then to be downregulated from 11 to 12 dpc onward. At subsequent fetal stages, the transcript was weakly detect ed in the ventricles, with the most distinct expression in the outflow trac t. Cx45 protein was demonstrated by immunofluorescence microscopy to be exp ressed in the myocytes of young embryonic hearts (8.5 to 9.5 dpc). However, beyond 10.5 dpc the protein was no longer detected with this technique in the embryonic, fetal, or neonatal working myocardium, although it could be shown by immunoblotting that the protein was still synthesized in neonatal heart. In the major part of adult heart, Cx45 was undetectable. It was, how ever, clearly seen in the anterior regions of the interventricular septum a nd in trace amounts in some small foci dispersed in the ventricular free wa lls. Cx45 gene is the first Cx gene so far demonstrated to be activated in heart at the stage of the first contractions. The coordination of myocytes during the slow peristaltic contractions that occur at this stage would thu s appear to be controlled by the Cx45 channels.