Subcellular Localization of Connexin 26 in Cardiomyocytes and in Cardiomyocyte-Derived Extracellular Vesicles.

Connexins (Cxs) are a family of membrane-spanning proteins, expressed in vertebrates and named according to their molecular weight. They are involved in tissue homeostasis, and they function by acting at several communication levels. Cardiac Cxs are responsible for regular heart function and, among them, Cx26 and Cx43 are widely expressed throughout the heart. Cx26 is present in vessels, as well as in cardiomyocytes, and its localization is scattered all over the cell aside from at the intercalated discs as is the case for the other cardiac Cxs. However, having been found in cardiomyocytes only recently, both its subcellular localization and its functional characterization in cardiomyocytes remain poorly understood. Therefore, in this study we aimed to obtain further data on the localization of Cx26 at the subcellular level. Our TEM immunogold analyses were performed on rat heart ventricles and differentiated H9c2 cardiac cell sections as well as on differentiated H9c2 derived extracellular vesicles. The results confirmed the absence of Cx26 at intercalated discs and showed the presence of Cx26 at the level of different subcellular compartments. The peculiar localization at the level of extracellular vesicles suggested a specific role for cardiac Cx26 in inter-cellular communication in an independent gap junction manner.

Expression pattern of Connexin 26 and Connexin 30 in mature cochlea of the monkey.

Connexin26 (Cx26) and Cx30 are the predominant connexin subtypes found in the cochlea. They play an essential role in the cochlear functions. However, most studies use mice and the data on the cochlear expression profiles of the two Cxs in higher animals (e.g., humans) are scarce. Studies using the cochleae from non-human primate other than mice may provide information needed to narrow this gap. Here we studied cellular distributions of Cx26 and Cx30 in the adult monkey and guinea pig cochleae by immunofluorescent labeling and confocal microscopy observations. We detected Cx26 and Cx30 expressions in the type I, II& V fibrocytes in the spiral ligament, fibrocytes of the spiral limbus, in the supporting cells of organ of Corti, inner and outer sulcus cells, and in the basal cells of the stria vascularis. Both Cx26 and Cx30 were not detected in hair cells, in mesenchymal cells under the basilar membrane and cells lining the scala vestibule. Cells of the Reissner's membrane and spiral ganglion neurons are also negative. These findings demonstrate that cochlear expressions of Cx26 and Cx30 in the adult mouse, guinea pig and non-human primate have a common cellular pattern.

Transforming Growth Factor Beta 1 Drives a Switch in Connexin Mediated Cell-to-Cell Communication in Tubular Cells of the Diabetic Kidney.

BACKGROUND/AIMS: Changes in cell-to-cell communication have been linked to several secondary complications of diabetes, but the mechanism by which connexins affect disease progression in the kidney is poorly understood. This study examines a role for glucose-evoked changes in the beta1 isoform of transforming growth factor (TGFß1), on connexin expression, gap-junction mediated intercellular communication (GJIC) and hemi-channel ATP release from tubular epithelial cells of the proximal renal nephron. METHODS: Biopsy material from patients with and without diabetic nephropathy was stained for connexin-26 (CX26) and connexin-43 (CX43). Changes in expression were corroborated by immunoblot analysis in human primary proximal tubule epithelial cells (hPTECs) and model epithelial cells from human renal proximal tubules (HK2) cultured in either low glucose (5mmol/L) ± TGFß1 (2-10ng/ml) or high glucose (25mmol/L) for 48h or 7days. Secretion of the cytokine was determined by ELISA. Paired whole cell patch clamp recordings were used to measure junctional conductance in control versus TGFß1 treated (10ng/ml) HK2 cells, with carboxyfluorescein uptake and ATP-biosensing assessing hemi-channel function. A downstream role for ATP in mediating the effects of TGF-ß1 on connexin mediated cell communication was assessed by incubating cells with ATPγS (1-100µM) or TGF-ß1 +/- apyrase (5 Units/ml). Implications of ATP release were measured through immunoblot analysis of interleukin 6 (IL-6) and fibronectin expression. RESULTS: Biopsy material from patients with diabetic nephropathy exhibited increased tubular expression of CX26 and CX43 (P<0.01, n=10), data corroborated in HK2 and hPTEC cells cultured in TGFß1 (10ng/ml) for 7days (P<0.001, n=3). High glucose significantly increased TGFß1 secretion from tubular epithelial cells (P<0.001, n=3). The cytokine (10ng/ml) reduced junctional conductance between HK2 cells from 4.5±1.3nS in control to 1.15±0.9nS following 48h TGFß1 and to 0.42±0.2nS after 7days TGFß1 incubation (P<0.05, n=5). Acute (48h) and chronic (7day) challenge with TGFß1 produced a carbenoxolone (200µM)-sensitive increase in carboxyfluorescein loading, matched by an increase in ATP release from 0.29±0.06µM in control to 1.99±0.47µM after 48hr incubation with TGFß1 (10ng/ml; P<0.05, n=3). TGF-ß1 (2-10ng/ml) and ATPγs (1-100µM) increased expression of IL-6 (P<0.001 n=3) and fibronectin (P<0.01 n=3). The effect of TGF-ß1 on IL-6 and fibronectin expression was partially blunted when preincubated with apyrase (n=3). CONCLUSION: These data suggest that chronic exposure to glucose-evoked TGFß1 induce an increase in CX26 and CX43 expression, consistent with changes observed in tubular epithelia from patients with diabetic nephropathy. Despite increased connexin expression, direct GJIC communication decreases, whilst hemichannel expression/function and paracrine release of ATP increases, changes that trigger increased levels of expression of interleukin 6 and fibronectin. Linked to inflammation and fibrosis, local increases in purinergic signals may exacerbate disease progression and highlight connexin mediated cell communication as a future therapeutic target for diabetic nephropathy.