Key Words: plakophilin-2 Ⅲ intercalated disc Ⅲ arrhythmogenic right ventricular cardiomyopathy Ⅲ cardiac desmosomes A high-resolution image of the site of end-end contact between cardiomyocytes reveals an electron-dense organization called "the intercalated disc." Its classic definition involves 3 structures: desmosomes and adherens junctions, providing mechanical coupling; and gap junctions, allowing electric/metabolic synchronization between cells. Recent studies show that other molecules, not directly involved in intercellular coupling, also reside preferentially at the intercalated disc. Among them is Na V 1.5, the major ␣ subunit of the cardiac sodium channel. 1 Here, we ask whether Na v 1.5 and the desmosomal protein plakophilin-2 (PKP2) coexist in the same molecular complex and whether loss of PKP2 expression affects (1) the amplitude and kinetics of the sodium current and (2) action potential propagation in a monolayer of cardiomyocytes. Our data demonstrate a functional crosstalk between a protein defined in the context of intercellular junctions (PKP2) and another protein that is fundamental to the electrical behavior of the single myocyte.

Regulation of cell-cell communication by the gap junction protein connexin43 can be modulated by a variety of connexin-associating proteins. In particular, c-Src can disrupt the connexin43 (Cx43)-zonula occludens-1 (ZO-1) interaction, leading to down-regulation of gap junction intercellular communication. The binding sites for ZO-1 and c-Src correspond to widely separated Cx43 domains (ϳ100 residues apart); however, little is known about the structural modifications that may allow information to be transferred over this distance. Here, we have characterized the structure of the connexin43 carboxyl-terminal domain (Cx43CT) to assess its ability to interact with domains from ZO-1 and c-Src. NMR data indicate that the Cx43CT exists primarily as an elongated random coil, with two regions of ␣-helical structure. NMR titration experiments determined that the ZO-1 PDZ-2 domain affected the last 19 Cx43CT residues, a region larger than that reported to be required for Cx43CT-ZO-1 binding. The c-Src SH3 domain affected Cx43CT residues Lys-264 -Lys-287, Ser-306 -Glu-316, His-331-Phe-337, Leu-356 -Val-359, and Ala-367-Ser-372. Only region Lys-264 -Lys-287 contains the residues previously reported to act as an SH3 binding domain. The specificity of these interactions was verified by peptide competition experiments. Finally, we demonstrated that the SH3 domain could partially displace the Cx43CT-PDZ-2 complex. These studies represent the first structural characterization of a connexin domain when integrated in a multimolecular complex. Furthermore, we demonstrate that the structural characteristics of a disordered Cx43CT are advantageous for signaling between different binding partners that may be important in describing the mechanism of channel closure or internalization in response to pathophysiological stimuli.Gap junction channels serve to directly interconnect the cytoplasm of neighboring cells, allowing the passage of moderately small ions, metabolites, and signaling molecules. Mammalian gap junction channels are formed by as many as 21 different connexin proteins (1). Of these, connexin43 (Cx43) 1 is the most completely characterized in terms of channel gating properties (2-4), phosphorylation sites (5-7), mechanisms of pH sensitivity (8 -11), and overall molecular structure (12). Cx43 is the most abundant gap junction protein in various tissues, including heart and brain. Cx43 null mice have been extensively investigated, with important differences being found as compared with wild types with regard to numerous processes, including cardiac developmental abnormalities, electrical synchrony in the heart, spreading depression in brain, as well as global gene expression changes in heart and astrocytes (13)(14)(15)(16)(17)(18)(19)(20).Connexin molecules are tetraspan membrane proteins, with both amino and carboxyl termini within the cytoplasm. Although the structure of the membrane-spanning portions of Cx43 has been solved to a resolution of about 7.5 Å (in the membrane plane) using electron crystallography (12), a constr...

Abstract-Desmosomes and gap junctions are distinct structural components of the cardiac intercalated disc. Here, we asked whether the presence of plakophilin (PKP)2, a component of the desmosome, is essential for the proper function and distribution of the gap junction protein connexin (Cx)43. We used RNA silencing technology to decrease the expression of PKP2 in cardiac cells (ventricular myocytes, as well as epicardium-derived cells) obtained from neonatal rat hearts.We evaluated the content, distribution, and function of Cx43 gap junctions. Our results show that loss of PKP2 expression led to a decrease in total Cx43 content, a significant redistribution of Cx43 to the intracellular space, and a decrease in dye coupling between cells. Separate experiments showed that Cx43 and PKP2 can coexist in the same macromolecular complex. Our results support the notion of a molecular crosstalk between desmosomal and gap junction proteins is an inherited disease that presents with sustained monomorphic ventricular tachycardia and sudden cardiac death. The disease is characterized by progressive fibrofatty infiltration of the myocardium, most prominent in the free wall of the right ventricle. 1 Recent studies have linked ARVC with mutations in proteins of the cardiac desmosome, 2 a component of the intercalated disc essential for mechanical coupling between cardiac cells. 3 It is estimated that as many as 70% of the mutations linked to familial ARVC are in the gene coding for plakophilin (PKP)2, 4 a 98-kDa desmosomal protein. PKP2 interacts with plakoglobin, desmoplakin, and the desmosomal cadherins via its amino terminal ("head") domain. [5][6] Loss of PKP2 destabilizes the desmosome, 7 and its genetic deletion in mice leads to rupture of the myocardial wall during the embryonic stage. 7 Loss of desmosomal integrity could lead to disruption of mechanical function in hearts afflicted with ARVC; yet, the latter does not directly explain the highly arrhythmogenic nature of the disease, particularly in cases in which lifethreatening arrhythmias occur in the absence of severe displacement of myocardium with fatty or fibrous tissue. 8 Recently, Saffitz and colleagues proposed that disruption of mechanical coupling may lead to loss of gap junctionmediated electrical communication between cells. 8 -10 This hypothesis awaits confirmation in a cellular model in which protein expression can be manipulated and intercellular communication can be assessed directly.Here, we used small interfering (si)RNA technology to silence PKP2 expression in neonatal cardiac cells, and we explored the effect of loss of PKP2 expression on the distribution and function of gap junctions. Our studies focused primarily on 2 cell populations: cardiac myocytes and epicardium-derived cells (EPDCs). Although the importance of cardiac myocytes in the context of ARVC and arrhythmias seems self-evident, a possible role for EPDCs in ARVC has not been described. Yet, as progenitors of the cardiac fibroblast cell lineage, the function of EPDCs deserves atte...

pH-induced closure of connexin43 (Cx43) channels involves interaction of the Cx43 carboxyl-terminal (Cx43CT) with a separate "receptor" domain. The receptor location and structure and whether the interaction is directly intramolecular are unknown. Here we show resonant mirror technology, enzyme-linked sorbent assays, and nuclear magnetic resonance (NMR) experiments demonstrating pH-dependent binding of Cx43CT to region 119 -144 of Cx43 (Cx43L2), which we propose is the receptor. NMR showed that acidification induced ␣-helical order in Cx43L2, whereas only a minor modification in Cx43CT structure was detected. These data provide the first demonstration of chemically induced structural order and binding between cytoplasmic connexin domains.