Gap Junctional Coupling in Lenses from α8 Connexin Knockout Mice

Abstract

Lens fiber cell gap junctions contain α₃ (Cx46) and α₈ (Cx50) connexins. To examine the roles of the two different connexins in lens physiology, we have genetically engineered mice lacking either α₃ or α₈ connexin. Intracellular impedance studies of these lenses were used to measure junctional conductance and its sensitivity to intracellular pH. In Gong et al. 1998, we described results from α₃ connexin knockout lenses. Here, we present original data from α₈ connexin knockout lenses and a comparison with the previous results. The lens has two functionally distinct domains of fiber cell coupling. In wild-type mouse lenses, the outer shell of differentiating fibers (see 1, DF) has an average coupling conductance per area of cell–cell contact of ∼1 S/cm², which falls to near zero when the cytoplasm is acidified. In the inner core of mature fibers (see 1, MF), the average coupling conductance is ∼0.4 S/cm², and is insensitive to acidification of the cytoplasm. Both connexin isoforms appear to contribute about equally in the DF since the coupling conductance for either heterozygous knockout (+/−) was ∼70% of normal and 30–40% of the normal for both −/− lenses. However, their contribution to the MF was different. About 50% of the normal coupling conductance was found in the MF of α₃ +/− lenses. In contrast, the coupling of MF in the α₈ +/− lenses was the same as normal. Moreover, no coupling was detected in the MF of α₃ −/− lenses. Together, these results suggest that α₃ connexin alone is responsible for coupling MF. The pH- sensitive gating of DF junctions was about the same in wild-type and α₃ connexin −/− lenses. However, in α₈ −/− lenses, the pure α₃ connexin junctions did not gate closed in the response to acidification. Since α₃ connexin contributes about half the coupling conductance in DF of wild-type lenses, and that conductance goes to zero when the cytoplasmic pH drops, it appears α₈ connexin regulates the gating of α₃ connexin. Both connexins are clearly important to lens physiology as lenses null for either connexin lose transparency. Gap junctions in the MF survive for the lifetime of the organism without protein turnover. It appears that α₃ connexin provides the long-term communication in MF. Gap junctions in DF may be physiologically regulated since they are capable of gating when the cytoplasm is acidified. It appears α₈ connexin is required for gating in DF.