Steven M. Taffet

Intramolecular interactions mediate pH regulation of connexin43 channels.

We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins.

Connexin43 Remodeling Caused by Inhibition of Plakophilin-2 Expression in Cardiac Cells

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. The results are discussed in the context of arrhythmogenic right ventricular cardiomyopathy, an inherited disease involving mutations in desmosomal proteins, including PKP2.

A structural basis for the unequal sensitivity of the major cardiac and liver gap junctions to intracellular acidification: the carboxyl tail length

The regulation of junctional conductance (Gi) of the major cardiac (connexin43; Cx43) and liver (connexin32; Cx32) gap junction proteins by intracellular hydrogen ion concentration (pH; pHi), as well as well as that of a truncation mutant of Cx43 (M257) with 125 amino acids deleted from the COOH terminus, was characterized in pairs of Xenopus laevis oocytes expressing homologous channels. Oocytes were injected with 40 nl mRNAs (2 micrograms/microliters) encoding the respective proteins; subsequently, cells were stripped, paired, and incubated for 20-24 h. Gj was measured in oocyte pairs using the dual electrode voltage-clamp technique, while pHi was recorded simultaneously in the unstimulated cell by means of a proton-selective microelectrode. Because initial experiments showed that the pH-sensitive microelectrode responded more appropriately to acetate than to CO2 acidification, oocytes expressing Cx32 and wild type and mutant Cx43 were exposed to a sodium acetate saline, which was balanced to various levels of pH using NaOH and HCl. pH was changed in a stepwise manner, and quasi-steady-state Gj -pHi relationships were constructed from data collected at each step after both Gj and pHi had reached their respective asymptotic values. A moderate but significant increase of Gj was observed in Cx43 pairs as pHi decreased from 7.2 to 6.8. In both Cx32 and M257 pairs, Gj increased significantly over a wider pH range (i.e., between 7.2 and 6.3). Further acidification reversibly reduced Gj to zero in all oocyte pairs. Pooled data for the individual connexins obtained during uncoupling were fitted by the Hill equation; apparent 50%-maximum (pK;pKa) values were 6.6 and 6.1 for Cx43 and Cx32, respectively, and Hill coefficients were 4.2 for Cx43 and 6.2 for Cx32. Like Cx32, M257 had a more acidic pKa (6.1) and steeper Hill coefficient (6.0) than wild type Cx43. The pKa and Hill coefficient of M257 were very similar to those of Cx32. These experiments provide the first direct comparison of the effects of acidification on Gj in oocyte pairs expressing Cx43 or Cx32. The results indicate that structural differences in the connexins are the basis for their unequal sensitivity to intracellular acidification in vivo. The data further suggest that a common pH gating mechanism may exist between amino acid residues 1 and 256 in both Cx32 and Cx43. However, the longer carboxyl tail of Cx43 relative to Cx32 or M257 provides additional means to facilitate acidification-induced gating; its presence shifts the pKa from 6.1 (Cx32 and M257) to 6.6 (Cx43) in the conductance of these channels.

Interactions Between Ankyrin-G, Plakophilin-2, and Connexin43 at the Cardiac Intercalated Disc

Rationale: The early description of the intercalated disc defined 3 structures, all of them involved in cell-cell communication: desmosomes, gap junctions, and adherens junctions. Current evidence demonstrates that molecules not involved in providing a physical continuum between cells also populate the intercalated disc. Key among them is the voltage-gated sodium channel complex. An important component of this complex is the cytoskeletal adaptor protein Ankyrin-G (AnkG). Objective: To test the hypothesis that AnkG partners with desmosome and gap junction molecules and exerts a functional effect on intercellular communication in the heart. Methods and Results: We used a combination of microscopy, immunochemistry, patch-clamp, and optical mapping to assess the interactions between AnkG, Plakophilin-2, and Connexin43. Coimmunoprecipitation studies from rat heart lysate demonstrated associations between the 3 molecules. With the use of siRNA technology, we demonstrated that loss of AnkG expression caused significant changes in subcellular distribution and/or abundance of PKP2 and Connexin43 as well as a decrease in intercellular adhesion strength and electric coupling. Regulation of AnkG and of Nav1.5 by Plakophilin-2 was also demonstrated. Finally, optical mapping experiments in AnkG-silenced cells demonstrated a shift in the minimal frequency at which rate-dependence activation block was observed. Conclusions: These experiments support the hypothesis that AnkG is a key functional component of the intercalated disc at the intersection of 3 complexes often considered independent: the voltage-gated sodium channel, gap junctions, and the cardiac desmosome. Possible implications to the pathophysiology of inherited arrhythmias (such as arrhythmogenic right ventricular cardiomyopathy) are discussed.

Electrotonic myofibroblast-to-myocyte coupling increases propensity to reentrant arrhythmias in two-dimensional cardiac monolayers.

In pathological conditions such as ischemic cardiomyopathy and heart failure, differentiation of fibroblasts into myofibroblasts may result in myocyte-fibroblast electrical coupling via gap junctions. We hypothesized that myofibroblast proliferation and increased heterocellular coupling significantly alter two-dimensional cardiac wave propagation and reentry dynamics. Co-cultures of myocytes and myofibroblasts from neonatal rat ventricles were optically mapped using a voltage-sensitive dye during pacing and sustained reentry. The myofibroblast/myocyte ratio was changed systematically, and junctional coupling of the myofibroblasts was reduced or increased using silencing RNAi or adenoviral overexpression of Cx43, respectively. Numerical simulations in two-dimensional models were used to quantify the effects of heterocellular coupling on conduction velocity (CV) and reentry dynamics. In both simulations and experiments, reentry frequency and CV diminished with larger myofibroblast/myocyte area ratios; complexity of propagation increased, resulting in wave fractionation and reentry multiplication. The relationship between CV and coupling was biphasic: an initial decrease in CV was followed by an increase as heterocellular coupling increased. Low heterocellular coupling resulted in fragmented and wavy wavefronts; at high coupling wavefronts became smoother. Heterocellular coupling alters conduction velocity, reentry stability, and complexity of wave propagation. The results provide novel insight into the mechanisms whereby electrical myocyte-myofibroblast interactions modify wave propagation and the propensity to reentrant arrhythmias.

Cholinergic atrial fibrillation: IK,ACh gradients determine unequal left/right atrial frequencies and rotor dynamics

OBJECTIVE We tested the hypothesis that left atrial (LA) myocytes are more sensitive to acetylcholine (ACh) than right atrial (RA) myocytes, which results in a greater dose-dependent increase in LA than RA rotor frequency, increased LA-to-RA frequency gradient and increased incidence of wavelet formation during atrial fibrillation (AF). METHODS AND RESULTS AF was induced in seven Langendorff-perfused sheep hearts in the presence of ACh (0.1-4.0 microM) and studied using optical mapping and bipolar recordings. Dominant frequencies (DFs) were determined in optical and electrical signals and phase movies were used to identify rotors and quantify their dynamics. DFs in both atria increased monotonically with ACh concentration until saturation, but the LA frequency predominated at all concentrations. Rotors were also seen more often in the LA, and although their life span decreased, their frequency and number of rotations increased. Patch-clamp studies demonstrated that ACh-activated potassium current (I(K,ACh)) density was greater in LA than RA sheep myocytes. Additionally, ribonuclease protection assay demonstrated that Kir3.4 and Kir3.1 mRNAs were more abundant in LA than in RA. CONCLUSIONS A greater abundance of Kir3.x channels and higher I(K,ACh) density in LA than RA myocytes result in greater ACh-induced speeding-up of rotors in the LA than in the RA, which explains the ACh dose-dependent changes in overall AF frequency and wavelet formation.

Unique Kir2.x Properties Determine Regional and Species Differences in the Cardiac Inward Rectifier K+ Current

The inwardly rectifying potassium (Kir) 2.x channels mediate the cardiac inward rectifier potassium current (IK1). In addition to differences in current density, atrial and ventricular IK1 have differences in outward current profiles and in extracellular potassium ([K+]o) dependence. The whole-cell patch-clamp technique was used to study these properties in heterologously expressed Kir2.x channels and atrial and ventricular IK1 in guinea pig and sheep hearts. Kir2.x channels showed distinct rectification profiles: Kir2.1 and Kir2.2 rectified completely at potentials more depolarized than −30 mV (I≈0 pA). In contrast, rectification was incomplete for Kir2.3 channels. In guinea pig atria, which expressed mainly Kir2.1, IK1 rectified completely. In sheep atria, which predominantly expressed Kir2.3 channels, IK1 did not rectify completely. Single-channel analysis of sheep Kir2.3 channels showed a mean unitary conductance of 13.1±0.1 pS in 15 cells, which corresponded with IK1 in sheep atria (9.9±0.1 pS in 32 cells). Outward Kir2.1 currents were increased in 10 mmol/L [K+]o, whereas Kir2.3 currents did not increase. Correspondingly, guinea pig (but not sheep) atrial IK1 showed an increase in outward currents in 10 mmol/L [K+]o. Although the ventricles of both species expressed Kir2.1 and Kir2.3, outward IK1 currents rectified completely and increased in high [K+]o-displaying Kir2.1-like properties. Likewise, outward current properties of heterologously expressed Kir2.1-Kir2.3 complexes in normal and 10 mmol/L [K+]o were similar to Kir2.1 but not Kir2.3. Thus, unique properties of individual Kir2.x isoforms, as well as heteromeric Kir2.x complexes, determine regional and species differences of IK1 in the heart.

Connexin 43 Interacts with Zona Occludens-1 and -2 Proteins in a Cell Cycle Stage-specific Manner

Gap junction channels play an important role in cell growth control, secretion and embryonic development. Gap junctional communication and channel assembly can be regulated by protein-protein interaction with kinases and phosphatases. We have utilized tandem mass spectrometry (MS/MS) sequence analysis as a screen to identify proteins from cell lysates that interact with the C-terminal cytoplasmic region of connexin 43 (Cx43). MS/MS analysis of tryptic fragments yielded several proteins including zona occludens-1 (ZO-1), a structural protein previously identified to interact with Cx43, and ZO-2, a potential novel interacting partner. We confirmed the interaction of ZO-2 with Cx43 by using a combination of fusion protein “pull down,” co-immunoprecipitation, and co-localization experiments. We show that the C-terminal region of Cx43 is necessary for interaction with the PDZ2 domain of ZO-2. Far Western analysis revealed that ZO-2 can directly bind to Cx43 independent of other interacting partners. Immunofluorescence studies indicate that both ZO-1 and ZO-2 can co-localize with Cx43 within the plasma membrane at apparent gap junctional structures. We examined Cx43 interaction with ZO-1 and ZO-2 at different stages of the cell cycle and found that Cx43 had a strong preference for interaction with ZO-1 during G0, whereas ZO-2 interaction occurred approximately equally during G0 and S phases. Since essentially all of the Cx43 in G0 cells is assembled into Triton X-100-resistant junctions, Cx43-ZO-1 interaction may contribute to their stability.

Regulation of tumor necrosis factor expression in a macrophage-like cell line by lipopolysaccharide and cyclic AMP☆

Bacterial lipopolysaccharide (LPS, 1 microgram/ml) induced the rapid production of tumor necrosis factor (TNF-alpha) mRNA in the RAW264 macrophage-like cell line. TNF-alpha mRNA peaked within 45 min of LPS treatment and remained high for greater than 3 hr. Transcription of TNF-alpha mRNA was increased within 15 min of LPS treatment. The quantity of TNF-alpha mRNA in LPS-stimulated cells was reduced to basal levels by treatment with cAMP, cAMP analogs, or agents which raise intracellular cAMP. This was not a general effect on all mRNA levels as the expression of a second gene, ornithine decarboxylase, was enhanced by cAMP treatment. cAMP did not have an effect on the stability of TNF-alpha mRNA. This is in contrast to the protein synthesis inhibitor, cycloheximide, which leads to a stabilization of TNF-alpha mRNA. Our results suggest that the primary regulation of tumor necrosis factor by cAMP and LPS occurs at the transcriptional level.

High Incidence of Cardiac Malformations in Connexin40-Deficient Mice

Abstract— Gap junctions are intercellular channels formed by oligomerization of a protein called connexin (Cx). The heart expresses at least three connexin isotypes: Cx40, Cx43, and Cx45. A possible role for Cx40 in cardiac morphogenesis remains to be determined. We have characterized the anatomy and histology of fetal and newborn hearts obtained from crossing Cx40-deficient mice of mixed genetic background (C57BL/6×129Sv). Hearts were serial-sectioned (5 &mgr;m) along the coronal plane, stained with hematoxylin-eosin, and visualized by conventional light microscopy. Cardiac malformations in mice lacking Cx40 in one allele (Cx40+/−) included bifid atrial appendage, ventricular septal defect, tetralogy of Fallot (TOF), and an aortic arch abnormality. In Cx40−/− mice resulting from crossing of Cx40+/− mice, the most common cardiac malformations were double-outlet right ventricle (DORV), TOF, and endocardial cushion defects. Overall incidence of cardiac malformations was 6/33 (18%) in Cx40+/− mice and 4/12 (33%) in Cx40−/− mice. No cardiac malformations were observed in 15 wild-type mice studied. In addition, we examined 39 hearts from offspring of Cx40−/− matings. Frequency of cardiac malformations was even higher in this group (44%). Over one third of the hearts (14 of 39) showed conotruncal malformations corresponding to either DORV or TOF. Endocardial cushion defects were found in 3 out of 39 hearts. Our results suggest that Cx40 participates in cardiac morphogenesis, likely in association with other (unknown) products whose expression may vary with the genetic background of the mice.