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FJ Martinez-Wittinghan, TW White, GJ Baldo, C Sellitto, V Valiunas, PR Brink, RT Mathias; Gap Junctional Coupling and pH Gating of Lenses from "Cx46 for Cx50" Knock-In Mice . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1919.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To study gap junctional coupling conductance of fiber cells in lenses from wild type (WT two Cx50 and two Cx46 allels) and Knock-in (KI) mice where Cx46 replaces Cx50 (heterozygous KI: three Cx46 alleles and one Cx50 allele; KI(50/46) or homozygous KI: four Cx46 alleles; KI(46/46)). Methods: In the KI mice, one or two alleles of Cx50 have been deleted and replaced with those of Cx46, maintaining the promoter for Cx50 (See White et al, ARVO abstracts 2002). Gap junctional coupling of fiber cells in intact lenses was determined with impedance studies; pH gating was evaluated by bubbling the bath solution with 100% carbon dioxide. Results: Previous studies have suggested gap junctional coupling is quantitatively and functionally different in the outer shell of differentiating fibers (DF) vs. the inner core of mature fibers (MF). The coupling conductance per unit area of cell contact (S/sq cm) in DF was: WT = 0.95, KI(50/46) =0.72, KI(46/46) =0.55. The opposite trend was seen in MF: WT =0.44, KI(50/46) =0.49, KI(46/46) =1.33. In addition, acidification of fiber cell cytoplasm in KI(46/46) lenses causes uncoupling of DF similar to that seen in WT lenses and in contrast to the previously observed lack of pH gating in Cx50 knockout (KO) mice. Conclusions: KI lenses provide information about gap junctional coupling of lens fibers when there is no diversity of connexins but the amount remains nearly unchanged. Our previous studies of Cx46 or Cx50 KO lenses suggested Cx50 may not contribute to MF coupling conductance, probably because of cleavage of Cx50 in the DF to MF transition. The results on MF coupling conductance presented here are consistent with this hypothesis, since it increases as Cx46 replaces Cx50. The decrease in DF coupling conductance when Cx46 replaces Cx50 may be due to the lower single channel conductance reported for Cx46. Surprisingly, in KI(46/46) mouse lenses, the MF conductance appeared to be higher than that of DF. This does not seem to be due to changes in open probability or unitary conductance due to cleavage of Cx46, since single hemichannel studies showed no difference in these parameters in Cx46 vs. its C-terminus cleaved form. Lastly, Cx46 in vivo is capable of gating in absence of Cx50 suggesting that connexin amount may influence pH gating.
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