Dominant Cataracts Result from Inappropriate Mixing of Wildtype Lens Connexins

Abstract

Abstract: : Purpose: Connexins mediate ionic and biochemical coupling between cells by assembling into gap junctions. Our previous studies have demonstrated that the lens requires two connexins for proper clarity and growth. Knockout of Cx46 produced severe cataracts, while knockout of Cx50 resulted in a growth defect and mild cataracts (Cell 91:833, J. Cell Biol 143:815, Development 129:167). Complete genetic replacement of Cx50 with Cx46 by knock-in rescued the cataract defect but not growth (Science 295:319). To further evaluate the role of connexins in normal lens growth and homeostasis, we generated heterozygous knock-in and knock-over animals. Methods: Knock-in mice were mated with wildtype and knockout mice to generate heterozygous knock-in and knock-over animals. Lens growth and clarity were analyzed anatomically, microscopically and biochemically. Lens communication was quantified by impedance techniques. Results: Heterozygous replacement of Cx50 with Cx46 [Cx46 (+/+) Cx50 (+/46)] rescued normal lens growth, but resulted in "dominant-like" cataracts despite producing robust levels of ionic gap junctional coupling. This cataract phenotype was independent of the presence of the endogenous Cx46 gene, Cx46 (+/-) Cx50 (+/46) or Cx46 (-/-) Cx50 (+/46) lenses developed similar cataracts. Knock-over lenses, where native Cx46 was deleted and completely knocked into the Cx50 locus [Cx46 (-/-) Cx50 (46/46)], displayed deficient growth but normal clarity. Conclusions: Forced mixing of Cx46 and Cx50 on the Cx50 gene did not reduce the magnitude of ionic coupling between lens cells, or perturb normal growth, but instead produced a dominant cataract phenotype. Cx46 alone on the Cx50 locus was able to restore lens clarity, but not growth. Thus, lens growth control and clarity are modulated by distinct modes of biochemical coupling, and inappropriate mixing of connexins perturbs lens clarity, but not growth.  

Keywords: animal model • cataract • gap junctions/coupling