Regulation of Cx43 gap junctions: the gatekeeper and the password

Molecular dynamics and in vitro analysis of Connexin43: A new 14-3-3 mode-1 interacting protein

The interaction of cellular proteins with the gap junction protein Connexin43 (Cx43) is thought to form a dynamic scaffolding complex that functions as a platform for the assembly of signaling, structural, and cytoskeletal proteins. A high stringency Scansite search of rat Cx43 identified the motif containing Ser373 (S373) as a 14-3-3 binding site. The S373 motif and the second best mode-1 motif, containing Ser244 (S244), are conserved in rat, mouse, human, chicken, and bovine, but not in Xenopus or zebrafish Cx43. Docking studies of a mouse/rat 14-3-3 homology model with the modeled phosphorylated S373 or S244 peptide ligands or their serine-to-alanine mutants, S373A or S244A, revealed that the pS373 motif facilitated a greater number of intermolecular contacts than the pS244 motif, thus supporting a stronger 14-3-3 binding interaction with the pS373 motif. The alanine substitution also reduced more than half the number of intermolecular contacts between 14-3-3 and the S373 motif, emphasizing the phosphorylation dependence of this interaction. Furthermore, the ability of the wild-type or the S244A GST-Cx43 C-terminal fusion protein, but not the S373A fusion protein, to interact with either 14-3-3 or 14-3-3zeta in GST pull-down experiments clearly demonstrated that the S373 motif mediates the direct interaction between Cx43 and 14-3-3 proteins. Blocking growth factor-induced Akt activation and presumably any Akt-mediated phosphorylation of the S373 motif in ROSE 199 cells did not prevent the down-regulation of Cx43-mediated cell-cell communication, suggesting that an Akt-mediated interaction with 14-3-3 was not involved in the disruption of Cx43 function.

Histone deacetylase inhibitor 4-phenylbutyrate modulates glial fibrillary acidic protein and connexin 43 expression, and enhances gap-junction communication, in human glioblastoma cells

Human glioblastoma cell cultures were established and the expression of glial fibrillary acidic protein (GFAP) and the gap-junction protein connexin 43 (Cx43) was confirmed by Western blot. Following treatment with 4-phenylbutyrate (4-PB), increased concentrations of non-phosphorylated GFAP were seen, while phosphorylated isoforms remained intact. Immunocytochemical staining of glioblastoma cells revealed an intracellular redistribution of GFAP. In addition to cytoplasmic immunostaining, GFAP immunoreactivity was also associated with the nucleus and/or the nuclear membrane. Phosphorylated and non-phosphorylated Cx43 proteins were increased 2- to 5-fold following 4-PB treatment, and were redistributed to areas of the cell surface, participating in cell-to-cell contacts. In addition, functional gap-junction coupling was amplified, as indicated by increased fluorescent dye transfer, and elevated levels of Cx43 protein were detected in parallel with enhanced gap-junction communication. Induced cell differentiation, with improved functional coupling of tumour cells, may be of importance for therapeutic strategies involving intercellular transport of low molecular-weight compounds.

Besides affecting intracellular calcium signaling, 2-APB reversibly blocks gap junctional coupling in confluent monolayers, thereby allowing measurement of single-cell membrane currents in undissociated cells

2-aminoethoxydiphenyl borate (2-APB) has been widely used as a blocker of the IP3 receptor and TRP channels, including store-operated calcium channels. We now show in monolayers of normal rat kidney cells (NRK/49F) that 2-APB completely and reversibly blocks gap junctional intercellular communication at concentrations similar to that required for inhibition of PGF2alpha-induced increases in intracellular calcium. Gap junctional conductances between NRK cells were estimated with single-electrode patch-clamp measurements and were fully blocked by 2-APB (50 microM), when applied extracellularly but not via the patch pipette. Half maximal inhibition (IC50) of electrical coupling in NRK cells was achieved at 5.7 microM. Similar results were obtained for human embryonic kidney epithelial cells (HEK293/tsA201) with an IC50 of 10.3 microM. Using 2-APB as an electrical uncoupler of monolayer cells, we could thus measure inward rectifier potassium, L-type calcium, and calcium-dependent chloride membrane currents in confluent NRK monolayers, with properties similar to those in dissociated NRK cells in the absence of 2-APB. The electrical uncoupling action described here is a new 2-APB property that promises to provide a powerful pharmacological tool to study single-cell properties in cultured confluent monolayers and intact tissues by electrical and chemical uncoupling of the cells without the need of prior dissociation.