Gap junctional coupling in lenses from alpha(8) connexin knockout mice

Lens fiber cell gap junctions contain as (Cx46) and alpha (8) (Cx50) connex ins. To examine the roles of the two different connexins in lens physiology , we have genetically engineered mice lacking either alpha (3) or alpha (8) connexin. Intracellular impedance studies of these lenses were used to mea sure junctional conductance and its sensitivity to intracellular pH. In Gon g et al. (1998), we described results from a3 connexin knockout lenses. Her e, we present original data from a3 connexin knockout lenses and a comparis on with the previous results. The lens has two functionally distinct domain s of fiber cell coupling. In wild-type mouse lenses, the outer shell of dif ferentiating fibers (see Fig. 1, DF) has a average coupling conductance per area of cell-cell contact of similar to1 S/cm(2), which falls to near zero when the cytoplasm is acidified. In the inner core of mature fibers (see F ig. 1, MIT), the average coupling conductance is similar to0.4 S/cm(2), and is insensitive to acidification of the cytoplasm. Both connexin isoforms a ppear to contribute about equally in the DF since the coupling conductance for either heterozygous knockout (+/-) was similar to 70% of normal and 30- 40% of the normal for both -/- lenses. However, their contribution to the M F was different. About 50% of the normal coupling conductance was found in the MF of alpha (3) +/- lenses. In contrast, the coupling of MF in the as /- lenses was the same as normal. Moreover, no coupling was detected in the MF of alpha (3) -/- lenses. Together, these results suggest that alpha (3) connexin alone is responsible for coupling MF. The pH-sensitive gating of DF junctions was about the same in wild-type and alpha (3)-connexin -/- len ses. However, in as -/- lenses, the pure alpha (3) connexin junctions did n ot gate closed in the response to acidification. Since alpha (3) connexin c ontributes about half the coupling conductance in DF of wild-type lenses, a nd that conductance goes to zero when the cytoplasmic pH drops, it appears alpha (8) connexin regulates the gating of alpha (3) connexin. Both connexi ns are clearly important to lens physiology as lenses null for either conne xin lose transparency: Gap junctions in the MF survive for the lifetime of the organism without protein turnover. It appears that alpha (3) connexin p rovides the long-terra communication in MF Gap junctions in DF may be physi ologically regulated since they are capable of gating when the cytoplasm is acidified. It appears alpha (8) connexin is required for gating in DF.