Abstract:
Lens fibre cells communicate with each other through an intercellular network of gap
junction channels. A reduction of the intracellular pH normally closes gap junction
channels. Yet, the channels remain open in the lens core region despite its acidic pH
which is caused by the accumulation of lactate as a product of glycolysis. The solution to
this enigma lies in the post-translational processing of the gap junction proteins
connexin46 (Cx46) and connexin50 (Cx50). Cx46 and Cx50 both have their carboxyl
peptide portion cleaved upon fibre cell maturation. The calcium activated neutral protease
calpain was identified as the enzyme which accomplishes this cleavage. The amino -
terminal 38 kD portion of both connexins remained embedded in the membrane as
channels which were fully permeable to tracer dyes. The cleaved carboxyl tail of Cx50
was recovered, and using protein sequencing, the cleavage site was identified in the Cx50
molecule.
Full length and truncated bovine Cx50 were cloned and expressed in Xenopus oocytes.
The truncation mutant produced channels with voltage gating properties similar to full
length Cx50. However, when the intracellular pH was lowered by exposing the oocytes
to CO2, channels composed of truncated Cx50 did not close whereas normal Cx50
channels did. My results suggest that the carboxyl tail of Cx50 contributes to pH gating,
and provide for the first time, an explanation for the ability of the lens to maintain
intercellular communication in all regions of this avascular tissue. This ability is of critical
importance for the maintenance of lens homeostasis and transparency. A new model of
diabetic cataractogenesis is proposed based on these results.