Cataracts are caused by alterations of a critical N-terminal positive charge in Connexin50

¹ Dept. of Pediatrics, University of Chicago, Chicago, Illinois, United States
² Dept. of Physiology and Biophysics, State University of New York, Stony Brook, New York, United States
³ Dept. of Pediatrics, University of Chicago, 5841 S. Maryland Ave, Chicago, Illinois, 60637-1470, United States

^* To whom correspondence should be addressed. E-mail: ebeyer{at}peds.bsd.uchicago.edu.

	Abstract

Purpose: To elucidate the basis of the autosomal dominant congenital nuclear cataracts caused by the connexin50 mutant, CX50R23T, by determining its cellular distribution and functional behavior and the consequences of substituting other amino acids for Arginine-23. Methods: Connexin50 (CX50) mutants were generated by PCR and transfected into HeLa or N2a cells. Expressed CX50 protein was detected by immunoblotting and localized by immunofluorescence. Intercellular communication was assessed by microinjection of neurobiotin or by double whole cell patch clamp recording. Results: HeLa cells stably transfected with CX50R23T or wild type CX50 produced immunoreactive CX50 bands of identical electrophoretic mobility. While HeLa cells stably expressing CX50 contained abundant gap junction plaques, CX50R23T localized predominantly in the cytoplasm. HeLa cells expressing wild type CX50 showed large gap junctional conductances and extensive transfer of neurobiotin, but those expressing CX50R23T did not show significant intercellular communication by either assay. Moreover, CX50R23T inhibited the function of co-expressed wild type CX50. Three CX50R23 substitution mutants (CX50R23K, CX50R23L, and CX50R23W) formed gap junction plaques, while two mutants with substitutions to negatively charged residues (CX50R23D, CX50R23E) did not form detectable plaques. Only the mutant with a positive charge substitution (CX50R23K) allowed neurobiotin transfer at levels similar to wild type CX50; none of the other mutants induced transfer. Conclusions: These results suggest that substitution of amino acid 23 in CX50 by any residue that is not positively charged would lead to cataract formation.

Key Words: connexins, gap junctions, cataractogenesis, membrane channels, Intercellular communication

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