Up-regulation of connexin43 in glomerular podocytes in response to injury

Podocyte injury or podocyte loss in the renal glomerulus has been proposed as the crucial mechanism in the development of focal segmental glomerulosclerosis. However, it is poorly understood how podocytes respond to injury. In this study, glomerular expression of connexin43 (Cx43) gap junction protein was examined at both protein and transcript levels in an experimental model of podocyte injury, puromycin aminonucleoside (PAN) nephrosis. A striking increase in the number of immunoreactive dots with anti-Cx43 antibody was demonstrated along the glomerular capillary wall in the early to nephrotic stage of PAN nephrosis. The conspicuous change was not detected in the other areas including the mesangium and Bowman's capsule. Immunoelectron microscopy showed that the immunogold particles for Cx43 along the capillary wall were localized predominantly at the cell-cell contact sites of podocytes. Consistently, Western blotting and ribonuclease protection assay revealed a distinct increase of Cx43 protein, phosphorylation, and transcript in glomeruli during PAN nephrosis. The changes were detected by 6 hours after PAN injection. These findings indicate that the increase of Cx43 expression is one of the earliest responses that have ever been reported in podocyte injury. To show the presence of functional gap junctional intercellular communication (GJIC) in podocytes, GJIC was assessed in podocytes in the primary culture by transfer of fluorescent dye, Lucifer yellow, after a single-cell microinjection. Diffusion of the dye into adjacent cells was observed frequently in the cultured podocytes, but scarcely in cultured parietal epithelial cells of Bowman's capsule, which was compatible with their Cx43 staining. Thus, it is concluded that Cx43-mediated GJIC is present between podocytes, suggesting that podocytes may respond to injury as an integrated epithelium on a glomerulus rather than individually as a separate cell.