The second extracellular domain of connexin 50 is important for in cell adhesion, lens differentiation, and adhesion molecule expression.
Connexin (Cx)-forming channels play essential roles in maintaining lens homeostasis and transparency. We showed here channel-independent roles of Cx50 in cell-cell adhesion and confirmed the second extracellular (E2) domain as a critical domain for cell adhesion function. We found that cell adhesion decreased in cells expressing chimeric Cx50 in which the E2 domain was swapped with the E2 domain of either Cx43 or Cx46. In contrast, adhesion increased in cells expressing chimeric Cx43 and Cx46 with the Cx50 (E2) domain. This function is Cx channel-independent and Cx50 E2 domain-dependent cell adhesion acting in both homotypic and heterotypic manners. In addition, we generated eight site mutations of unique residues between Cx50 and the other two lens Cxs and found that mutation of any one of the residues abolished the adhesive function. Moreover, expression of adhesive-impaired mutants decreased adhesion-related proteins, N-cadherin and β-catenin. Expression of the adhesion-impaired Cx50W188P mutant in embryonic chick lens caused enlarged extracellular spaces, distorted fiber organization, delayed nuclear condensation, and cortical cataracts. In summary, the results from both in vitro and in vivo studies demonstrate the importance of the adhesive function of Cx50 in the lens.