Optimal Lens Epithelial Cell Proliferation Is Dependent on the Connexin Isoform Providing Gap Junctional Coupling

Abstract

Gap junctions between epithelial cells are essential for normal lens growth. In mice, knockout of Cx50 or targeted replacement of Cx50 with Cx46 (knockin) caused smaller lenses because of decreased epithelial cell proliferation. However, it remains unclear whether Cx50 functionally contributes to lens epithelial coupling during maximal proliferation on postnatal day 2 (P2) and P3. To determine which connexins functionally contribute to epithelial cell coupling and proliferation, junctional coupling from epithelial cells of wild-type and knockin mice was examined. Epithelial cells were isolated from wild-type or knockin mice at different developmental ages. Junctional currents were measured by dual whole cell voltage clamp. Cell proliferation was assayed by BrdU incorporation. Connexins were immunolocalized using specific antibodies. Junctional currents between lens epithelial cells exhibited a developmentally regulated sensitivity to quinine, a drug that blocks Cx50 gap junctions, but not Cx43 or Cx46. Single-channel currents had a unitary conductance of 210 pS, typical of Cx50. Immunocytochemical staining showed Cx43 and Cx50 were abundantly expressed in wild-type cells, and Cx46 replaced Cx50 in knockin cells. A correlation between functional activity of Cx50 and maximal proliferation was also found. In epithelial cells from P3 wild-type mice, there was a high density of BrdU-labeled nuclei in both the central epithelium and the equatorial epithelium, and 60% or more of total coupling was provided by Cx50. In older cells, proliferation was greatly reduced, and the contribution of Cx50 to total coupling was progressively reduced (45% or less on P12; 25% or less on P28). Coupling between epithelial cells of Cx46 knockin mice was similar in magnitude to that of wild-type mice but had pharmacologic and biophysical characteristics of Cx46. This functional replacement of Cx50 with Cx46 was correlated with 71% and 13% reductions in BrdU-labeled cells in the P3 central epithelium and equatorial epithelium, respectively. These results reconcile previous genetic studies showing that Cx50 influences epithelial cell proliferation, with numerous studies suggesting that Cx43 was the principal epithelial cell connexin. They further show that the contribution of Cx50 is highest during peak postnatal proliferation but progressively declines with age thereafter.