Functional characteristics of skate connexin35, a member of the γ subfamily of connexins expressed in the vertebrate retina

Retinal neurons are coupled by electrical synapses that have been studied extensively in situ and in isolated cell pairs. Although many unique gating properties have been identified, the connexin composition of retinal gap junctions is not well defined. We have functionally characterized connexin35 (Cx35), a recently cloned connexin belonging to the gamma subgroup expressed in the skate retina, and compared its biophysical properties with those obtained from electrically coupled retinal cells. Injection of Cx35 RNA into pairs of Xenopus oocytes induced intercellular conductances that were voltage-gated at transjunctional potentials greater than or equal to 60 mV, and that were also closed by intracellular acidification. In contrast, Cx35 was unable to functionally interact with rodent connexins from the alpha or beta subfamilies. Voltage-activated hemichannel currents were also observed in single oocytes expressing Cx35, and superfusing these oocytes with medium containing 100 mu M quinine resulted in a 1.8-fold increase in the magnitude of the outward currents, but did not change the threshold of voltage activation (membrane potential = +20 mV). Cx35 intercellular channels between paired oocytes were insensitive to quinine treatment. Both hemichannel activity and its modulation by quinine were seen previously in recordings from isolated skate horizontal cells. Voltage-activated currents of Cx46 hemichannels were also enhanced 1.6-fold following quinine treatment, whereas Cx43-injected oocytes showed no hemichannel activity in the presence, or absence, of quinine. Although the cellular localization of Cx35 is unknown, the functional characteristics of Cx35 in Xenopus oocytes are consistent with the hemichannel and intercellular channel properties of skate horizontal cells.