pH gating of lens fibre connexins

Abstract

Chemical gating of gap junction channels by intracellular pH may be an important mechanism for the physiological regulation of cell-cell coupling. In the ocular lens, pH gating of gap junction channels has been implicated as a possible cause of cataract in diabetics. To address this question further, I determined the pH dependence of the rat connexin (Cx)-46 and ovine Cx49 in transfected HeLa cells using the pH-clamp technique during dual whole-cell recording. pH gating for both connexins was fast and reversible. The apparent p K(a) (p K(a,app)) was 6.66 +/- 0.01 and the Hill coefficient ( n) 6.8 +/- 1.8 for Cx49, and for Cx46 6.8 +/- 0.01 and 2.2 +/- 0.15, respectively. C-terminal truncation of Cx46 by 163 aa did not abolish the pH sensitivity but shifted the p K(a,app) to 6.6 +/- 0.01. This finding is inconsistent with the ball-and-chain model proposed for Cx43. Voltage gating of Cx46 channels was also not altered by truncation or acidic pH, indicating that the two gating mechanisms are functionally and possibly structurally separate. The data also imply a significant role of pH gating for lens pathophysiology. For the normal pH range in the lens cortex (pH 6.8-7.2) most gap junction channels will be open. However, mild acidification will reduce gap junctional coupling significantly, especially for Cx50 channels. Localized closure of gap junction channels will disrupt lens transport and thus may contribute to the tissue damage observed in diabetic lenses.

Comparative Analysis of Human γD-Crystallin Aggregation under Physiological and Low pH Conditions

“…This may not come as a surprise when one considers that the lowest pH of 6.5 detected in the nucleus [20], [72] is not too different from the neutral pH of 7.0 explored by our study, which also induces amorphous aggregation in HγD-crys. While our experimental condition is an over simplified model of what’s going on in the lens, it is undeniable that the inherent aggregation property of HγD-crys under neutral pH is of the amorphous kind.…”

“…One involves the possibility of partially degraded HγD-crys forming fibrillar aggregates in the low pH environment of lysozomal compartment during lens fiber cell differentiation, which may be involved in the early stages of cataract formation [18]. Another hypothesis has to do with the decreased pH in the lens nucleus overtime leading to loss of α-crystallin chaperone capability to protect HγD-crys from aggregation, thus resulting in senile nuclear cataract formation [20], [21]. Regardless of the mechanisms of cataract formation, the only accepted form of treatment currently available is the surgical removal of the opaque lens and replacement with an artificial lens.…”

Lens ion homeostasis relies on the assembly and/or stability of large connexin 46 gap junction plaques on the broad sides of differentiating fiber cells

“…Gap junctions composed of Cx46 and/or Cx50 contribute to intercellular coupling in differentiating lens fibers, while only Cx46 channels are functional in mature fibers (2,19,21,37,65). In DKO lenses, we observed a ϳ50% decrease in conductance in DKO lens fibers compared with the control.…”

Lens Gap Junctions in Growth, Differentiation, and Homeostasis

“…DeRosa et al (47) showed COOH termini cleaved Cx50 channels, when exogenously expressed in oocytes, went to the plasma membrane and formed plaques, but the constituent channels were rarely open. Exogenous expression of COOH termini cleaved Cx46 channels showed no differences in coupling from expression of intact protein (60). Putting all of these results together suggests the following hypothesis: Cx46 and Cx50 form mostly homomeric channels in the plaques coupling the DF, where each contributes about equally to coupling conductance.…”