Abstract-Specific mutations in GJA1, the gene encoding the gap junction protein connexin43 (Cx43), cause an autosomal dominant disorder called oculodentodigital dysplasia (ODDD). Here, we characterize the effects of 8 of these mutations on Cx43 function. Immunochemical studies have shown that most of the mutant proteins formed gap junction plaques at the sites of cell-cell apposition. However, 2 of the mutations (a codon duplication in the first extracellular loop, F52dup, and a missense mutation in the second extracellular loop, R202H, produced full-length connexins that failed to properly form gap junction plaques. Cx43 proteins containing ODDD mutations found in the N-terminus (Y17S), first transmembrane domain (G21R, A40V), second transmembrane domain (L90V), and cytoplasmic loop (I130T, K134E) do form gap junction plaques but show compromised channel function. L90V, I130T, and K134E demonstrated a significant decrease in junctional conductance relative to Cx43WT. Mutations Y17S, G21R, and A40V demonstrated a complete lack of functional electrical coupling even in the presence of significant plaque formation between paired cells. Heterologous channels formed by coexpression of Cx43WT and mutation R202H resulted in electrically functional gap junctions that were not permeable to Lucifer yellow. Therefore, the mutations found in ODDD not only cause phenotypic variability, but also result in various functional consequences. Overall, our data show an extensive range of molecular phenotypes, consistent with the pleiotropic nature of the clinical syndrome as a whole. (Circ Res. 2005;96:e83-e91.)

Abstract-Haplodeficient mice expressing carboxyl-terminally truncated Cx43 (K258stop/KO), instead of the wild-type Cx43 isoform, reach adulthood and reveal no abnormalities in heart morphology. Here, we have analyzed the expression of K258stop protein and the morphology of gap junctions in adult hearts of these mice. Coimmunofluorescence analysis revealed reduced juxtaposition of K258stop with other junctional proteins at the intercalated disc. Immunoprecipitation studies documented changes in the interaction with previously described Cx43 binding proteins. Quantitative transmission electron and confocal microscopy confirmed the localization of K258stop gap junctions to the periphery of the intercalated disc and further revealed an increase in the size of K258stop gap junction plaques and a reduction in their number. Dual whole cell patch clamp analysis confirmed that K258stop gap junctions were functional, with single channel properties similar to those described in exogenous systems. We conclude that the carboxyl-terminal domain of Cx43 (Cx43CT)

Abstract-The carboxyl-terminal domain of connexin43 (Cx43CT) is involved in various intra-and intermolecular interactions that regulate gap junctions. Here, we used phage display to identify novel peptidic sequences that bind Cx43CT and modify Cx43 regulation. We found that Cx43CT binds preferentially to peptides containing a sequence RXP, where X represents any amino acid and R and P correspond to the amino acids arginine and proline, respectively.A biased "RXP library" led to the identification of a peptide (dubbed "RXP-E") that bound Cx43CT with high affinity. Nuclear magnetic resonance data showed RXP-E-induced shifts in the resonance peaks of residues 343 to 346 and 376 to 379 of Cx43CT. Patch-clamp studies revealed that RXP-E partially prevented octanol-induced and acidificationinduced uncoupling in Cx43-expressing cells. Moreover, RXP-E increased mean open time of Cx43 channels. The full effect of RXP-E was dependent on the integrity of the CT domain. These data suggest that RXP-based peptides could serve as tools to help determine the role of Cx43 as a regulator of function in conditions such as ischemia-induced arrhythmias. (Circ Res. 2006;98:1365-1372.)Key Words: Cx43CT Ⅲ connexin Ⅲ particle-receptor interaction Ⅲ gap junctions C onnexins are integral membrane proteins that oligomerize to form intercellular channels called gap junctions. The most abundant gap junction protein in a number of mammalian systems is connexin43 (Cx43). Our previous work has suggested that regulation of Cx43 channels results from the association of the carboxyl-terminal (CT) domain, acting as a gating particle, and a separate region of the connexin molecule, acting as a receptor for the gating particle. 1,2 Additional studies have shown that this intramolecular interaction can be modulated by other intermolecular interactions in the microenvironment of the gap junction plaque. 3 Thus, the emerging picture of a gap junction plaque is that of a macromolecular complex in which proteins act in concert to modulate intercellular communication. At the center of these interactions is the CT domain, which acts as a substrate for a number of kinases, 4 a ligand for noncatalytic proteins, and a gating particle to modify coupling between cells. 5 As a key player in the regulation of gap junctions, CT presents itself as a target of chemical 6,7 or genetic manipulation intended to modify function. 8 Here, we sought to disrupt the regulation of Cx43 by chemical means. Our rationale was based on the knowledge that Cx43CT is capable of interacting with other proteins. We reasoned that this "stickiness" of Cx43CT can be used to "adhere" peptidic sequences to it. We further speculated that the interaction of Cx43CT with small peptides can modify the behavior of the gap junction channel. This rationale was supported by previous work showing that peptides can modify both the chemical and voltage-gating behavior of Cx43. 6,7 In the present study, we used a highthroughput phage display screening to find peptidic sequences that bind Cx43CT. Furth...

Heart failure (HF) is accompanied by complex alterations in myocardial energy metabolism. Up to 40% of HF patients have dyssynchronous ventricular contraction, which is an independent indicator of mortality. We hypothesized that electromechanical dyssynchrony significantly affects metabolic remodeling in the course of HF. We used a canine model of tachypacing-induced HF. Animals were paced at 200 bpm for 6 weeks either in the right atrium (synchronous HF, SHF) or in the right ventricle (dyssynchronous HF, DHF). We collected biopsies from left ventricular apex and performed comprehensive metabolic pathway analysis using multi-platform metabolomics (GC/MS; MS/MS; HPLC) and LC-MS/MS label-free proteomics. We found important differences in metabolic remodeling between SHF and DHF. As compared to Control, ATP, phosphocreatine (PCr), creatine, and PCr/ATP (prognostic indicator of mortality in HF patients) were all significantly reduced in DHF, but not SHF. In addition, the myocardial levels of carnitine (mitochondrial fatty acid carrier) and fatty acids (12:0, 14:0) were significantly reduced in DHF, but not SHF. Carnitine parmitoyltransferase I, a key regulatory enzyme of fatty acid ß-oxidation, was significantly upregulated in SHF but was not different in DHF, as compared to Control. Both SHF and DHF exhibited a reduction, but to a different degree, in creatine and the intermediates of glycolysis and the TCA cycle. In contrast to this, the enzymes of creatine kinase shuttle were upregulated, and the enzymes of glycolysis and the TCA cycle were predominantly upregulated or unchanged in both SHF and DHF. These data suggest a systemic mismatch between substrate supply and demand in pacing-induced HF. The energy deficit observed in DHF, but not in SHF, may be associated with a critical decrease in fatty acid delivery to the ß-oxidation pipeline, primarily due to a reduction in myocardial carnitine content.

The use of voltage-sensitive fluorescent dyes (VSD) for noninvasive measurement of the action potential (AP) in isolated cells has been hindered by low-photon yield of the preparation, dye toxicity, and photodynamic damage. Here we used a new red-shifted VSD, di-4-ANBDQBS, and a fast electron-multiplied charge-coupled device camera for optical AP (OAP) recording in guinea pig cardiac myocytes. Loading di-4-ANBDQBS did not alter APs recorded with micropipette. With short laser exposures (just enough to record one OAP every 1-5 min), di-4-ANBDQBS yielded fluorescent signals with very high signal-to-background ratios (change in fluorescence on depolarization/fluorescence at resting potential: 19.2 ± 4.1%) and signal-to-noise ratios (40 ± 13.2). Quantum chemical calculations comparing the ANBDQ chromophore to the conventional ANEP chromophore showed that the higher wavelength and the greater voltage sensitivity of the former have the same electro-optical origin: a longer path for electron redistribution in the excited state. OAP closely tracked simultaneously recorded electrical APs, permitting measurement of AP duration within 1% error. Prolonged laser exposure caused progressive AP duration prolongation and instability. However, these effects were alleviated or abolished by reducing the dye concentration and by perfusion with antioxidants. Thus the presented technique provides a unique opportunity for noninvasive AP recording in single cardiomyocytes.

Structural, ultrastructural, microstructural, metabolic, molecular, and clinical functional data indicated that prolonged continuous-flow LVAD unloading does not induce hypertrophy regression to the point of atrophy and degeneration. These findings may be useful in designing future investigations that combine LVAD unloading and pharmaceutical therapies as a bridge to recovery of the failing heart.

Previous studies indicate that the carboxyl terminal of connexin43 (Cx43CT) is involved in fast transjunctional voltage gating. Separate studies support the notion of an intramolecular association between Cx43CT and a region of the cytoplasmic loop (amino acids 119-144; referred to as "L2"). Structural analysis of L2 shows two alpha-helical domains, each with a histidine residue in its sequence (H126 and H142). Here, we determined the effect of H142 replacement by lysine, alanine, and glutamate on the voltage gating of Cx43 channels. Mutation H142E led to a significant reduction in the frequency of occurrence of the residual state and a prolongation of dwell open time. Macroscopically, there was a large reduction in the fast component of voltage gating. These results resembled those observed for a mutant lacking the carboxyl terminal (CT) domain. NMR experiments showed that mutation H142E significantly decreased the Cx43CT-L2 interaction and disrupted the secondary structure of L2. Overall, our data support the hypothesis that fast voltage gating involves an intramolecular particle-receptor interaction between CT and L2. Some of the structural constrains of fast voltage gating may be shared with those involved in the chemical gating of Cx43.

Calcium/calmodulin-dependent protein kinase II (CaMKII) regulates the principle ion channels mediating cardiac excitability and conduction, but how this regulation translates to the normal and ischemic heart remains unknown. Diverging results on CaMKII regulation of Na channels further prevent predicting how CaMKII activity regulates excitability and conduction in the intact heart. To address this deficiency, we tested the effects of the CaMKII blocker KN93 (1 and 2.75 μM) and its inactive analog KN92 (2.75 μM) on conduction and excitability in the left (LV) and right (RV) ventricles of rabbit hearts during normal perfusion and global ischemia. We used optical mapping to determine local conduction delays and the optical action potential (OAP) upstroke velocity (d/d). At baseline, local conduction delays were similar between RV and LV, whereas the OAP d/d was lower in RV than in LV. At 2.75 μM, KN93 heterogeneously slowed conduction and reduced d/d, with the largest effect in the RV outflow tract (RVOT). This effect was further exacerbated by ischemia, leading to recurrent conduction block in the RVOT and early ventricular fibrillation (at 6.7 ± 0.9 vs. 18.2 ± 0.8 min of ischemia in control, < 0.0001). Neither KN92 nor 1 μM KN93 depressed OAP d/d or conduction. Rabbit cardiomyocytes isolated from RVOT exhibited a significantly lower d/d than those isolated from the LV. KN93 (2.75 μM) significantly reduced d/d in cells from both locations. This led to frequency-dependent intermittent activation failure occurring predominantly in RVOT cells. Thus CaMKII blockade exacerbates intrinsically lower excitability in the RVOT, which is proarrhythmic during ischemia. We show that calcium/calmodulin-dependent protein kinase II (CaMKII) blockade exacerbates intrinsically lower excitability in the right ventricular outflow tract, which causes highly nonuniform chamber-specific slowing of conduction and facilitates ventricular fibrillation during ischemia. Constitutive CaMKII activity is necessary for uniform and safe ventricular conduction, and CaMKII block is potentially proarrhythmic.