Residual Cx45 and its relationship to Cx43 in murine ventricular myocardium.

Gap junction channels in ventricular myocardium are required for electrical and metabolic coupling between cardiac myocytes and for normal cardiac pump function. Although much is known about expression patterns and remodeling of cardiac connexin(Cx)43, little is known about the less abundant Cx45, which is required for embryonic development and viability, is downregulated in adult hearts, and is pathophysiologically upregulated in human end-stage heart failure. We applied quantitative immunoblotting and immunoprecipitation to native myocardial extracts, immunogold electron microscopy to cardiac tissue and membrane sections, electrophysiological recordings to whole hearts, and high-resolution tandem mass spectrometry to Cx45 fusion protein, and developed two new tools, anti-Cx45 antisera and Cre(+);Cx45 floxed mice, to facilitate characterization of Cx45 in adult mammalian hearts.

Identification of CaMKII phosphorylation sites in Connexin43 by high-resolution mass spectrometry.

Connexin43 (Cx43) is a major cardiac gap junction channel protein required for normal electrical and contractile activity. Gap junction channel assembly, function, and turnover are regulated by phosphorylation under both normal and disease conditions. The carboxyl terminus (CT) of Cx43 contains numerous amino acid residues that are phosphorylated by protein kinases.

Connexin43 expression levels influence intercellular coupling and cell proliferation of native murine cardiac fibroblasts.

Little is known about connexin expression and function in murine cardiac fibroblasts. The authors isolated native ventricular fibroblasts from adult mice and determined that although they expressed both connexin43 (Cx43) and connexin45 (Cx45), the relative abundance of Cx45 was greater than that of Cx43 in fibroblasts compared to myocytes, and the electrophoretic mobility of both Cx43 and Cx45 differed in fibroblasts and in myocytes. Increasing Cx43 expression by adenoviral infection increased intercellular coupling, whereas decreasing Cx43 expression by genetic ablation decreased coupling.

Diminished zonula occludens-1 expression in the failing human heart.

Background: Reduced expression of the major gap junction protein connexin 43 (Cx43) in the failing human heart may lead to arrhythmias and sudden cardiac death. Cx43 interacts with the actin binding protein, zonula occludens-1 (ZO-1), and it has recently been demonstrated that ZO-1 regulates the formation and function of Cx43 gap junctions. We hypothesize that normal expression of ZO-1 and its interaction with Cx43 are required for appropriate assembly and function of Cx43 gap junctions in the heart.

ZO-1 alters the plasma membrane localization and function of Cx43 in osteoblastic cells.

ZO-1 is the major connexin-interacting protein in ROS 17/2.8 (ROS) osteoblastic cells. We examined the role of ZO-1 in Cx43-mediated gap junction formation and function in ROS cells that expressed the connexin-interacting fragment of ZO-1 (ROS/ZO-1dn) cells.

Pathways for degradation of connexins and gap junctions.

Gap junctional proteins, connexins, and gap junctional plaques are short-lived. Three pathways for their degradation have been proposed: (1) misfolded/abnormally oligomerized connexins are retrogradely translocated and degraded by the proteasome through endoplasmic reticulum-associated degradation; (2) connexins (as monomers or oligomers) may traffic directly from an early secretory compartment to the lysosome for degradation without reaching the plasma membrane; (3) connexins within gap junction plaques are degraded by the lysosome after endocytotic internalization. Degradation of gap junction plaques is proteasome-dependent in some cell types.

Redistribution of connexin45 in gap junctions of connexin43-deficient hearts.

Objective: Adult ventricular myocytes express two gap junction channel proteins: connexin43 (Cx43) and connexin45 (Cx45). Cx43-deficient mice exhibit slow ventricular epicardial conduction, suggesting that Cx43 plays an important role in intercellular coupling in the ventricle. Cx45 is much less abundant than Cx43 in working ventricular myocytes.