Regulation of gap junction intercellular Communication by pH in MC3T3-E1 osteoblastic cells

Abstract

Gap junction intercellular communication (GJIC) may be related to coordinating the function of osteoblasts during bone mineralization. Since an alkaline pH supports mineral deposition while an acidic pH promotes mineral dissolution, it was investigated whether GJIC is altered by changes in extracellular pH (pHo) Functional GJIC was assessed by fluorescent dye transfer after microinjection, and connexin protein abundance was examined by immunoprecipitation and immunoblotting in MC3T3-E1 cells, a model of osteoblast-like cells. The percent of cells coupled by GJIC was found to be 40.7% (24 of 59 injected cells) at pH 6.9, 72.2% (26 of 36) at pH 7.2, and 92.8% (26 of 28) at pH 7.6. A decrease in GJIC was detectable by 30-60 minutes of exposure to a pHo of 6.9. Decreased gap junction communication was also found in cells after 3, 8, and 24 h of incubation in a bicarbonate-CO2 system at an ambient pH of 6.9. Connexin protein abundance experiments showed that at after exposure to a pH of 6.9 for 2.75 h, the specific band(s) at 41-43 kD were fainter compared with these same band(s) at pH 7.2 and 7.6. There was no significant difference in band densities at pH 7.2 and 7.6. Determination of intracellular pH (pHi) showed that it was similar to pHo after 2.75 h of incubation at each ambient pH. When pHi was clamped at 6.9 or 7.2, there was a time-dependent decrease in the gap junction coupling frequency at a pHi of 6.9 when pHo was 7.2. Steady-state mRNA levels were decreased at pHo 6.9 but were unchanged at either pHo 7.2 or 7.6. Our conclusions are that (1) longer incubations ( > or = 2.75 h) at low pHo decrease GJIC which in part may be due to a decrease in connexin protein abundance perhaps as a result of a decrease in connexin steady-state mRNA expression; (2) GJIC inhibition or augmentation found at low and high pHo, respectively, suggests that gating of the GJ channel by pH may also occur; (3) pho-induced alterations in GJIC in the MC3T3-E1 osteoblastic model are related to concomitant changes in pHi.