Abstract
Purpose: :
To characterize the cellular and functional properties of two Cx50 mutations, Cx50G46V (G46V) and Cx50W45S (W45S), located at adjacent positions at the interface between the first transmembrane domain and the first extracellular loop, identified in people with congenital cataracts.
Methods: :
GFP-tagged and untagged versions of the connexins were generated by PCR. Connexin protein distribution was studied by immunofluorescence in transfected HeLa cells. Formation of functional channels and hemichannels was assessed by two-microelectrode voltage-clamp in cRNA-injected Xenopus oocytes.
Results: :
Wild type (wt) Cx50, W45S, and G46V showed similar localization to gap junction plaques in transfected HeLa cells. Oocyte pairs injected with wt Cx50 or G46V were well coupled, but oocyte pairs injected with W45S cRNA were not coupled. Co-expression of W45S decreased the junctional conductance induced by wt Cx50 or wt Cx46 by 61% and 66%, respectively suggesting that it was a dominant-negative inhibitor of both connexins. Both G46V and wt Cx50 cRNA-injected oocytes developed hemichannel currents (in KCl solution containing zero added calcium, pH 8.0); in contrast, oocytes injected with W45S cRNA did not develop detectable hemichannel currents. The hemichannel currents in G46V cRNA-injected oocytes were much larger than those in wt Cx50 cRNA-injected oocytes. The increase in hemichannel activity of G46V did not appear to be the result of altered sensitivity to external calcium, monovalent cations or [pH]o. This increase in hemichannel activity persisted when G46V was co-expressed with wild-type lens connexins. Oocyte viability experiments showed that oocytes expressing G46V alone or in combination with wild-type lens connexins exhibited increased cell death.
Conclusions: :
Mutations at adjacent residues at the TM1/E1 boundary in Cx50 have different effects on hemichannel and gap junction channel function when expressed alone or together with the wild type connexins. W45S causes dominant loss of function, and G46V causes dominant gain of hemichannel function. These results suggest that these two mutants lead to cataracts through different mechanisms
Keywords: cataract • gap junctions/coupling • mutations