Functional properties, developmental regulation, and chromosomal localization of murine connexin36, a gap-junctional protein expressed preferentiallyin retina and brain

Retinal neurons of virtually every type are coupled by gap-junctional chann els whose pharmacological and gating properties have been studied extensive ly, We have begun to identify the molecular composition and functional prop erties of the connexins that form these 'electrical synapses,' and have clo ned several that constitute a new subclass (gamma) of the connexin family e xpressed predominantly in retina and brain. in this paper, we present a ser ies of experiments characterizing connexin36 (Cx36), a member of the gamma subclass that was cloned from a mouse retinal cDNA library. Cx36 has been l ocalized to mouse chromosome 2, in a region syntenic to human chromosome 5, and immunocytochemistry showed strong labeling in the ganglion cell and in ner nuclear layers of the mouse retina. Comparison of the developmental tim e course of Cx36 expression in mouse retina with the genesis of the various classes of retinal cells suggests that the expression of Cx36 occurs prima rily after cellular differentiation is complete. Because photic stimulation can affect the gap-junctional coupling between retinal neurons, we determi ned whether lighting conditions might influence the steady state levels of Cx36 transcript in the mouse retina. Steady-state levels of Cx36 transcript were significantly higher in animals reared under typical cyclic-light con ditions; exposure either to constant darkness or to continuous illumination reduced the steady-state level of mRNA approximately 40%. Injection of Cx3 6 cRNA into pairs of Xenopus oocytes induced intercellular conductances tha t were relatively insensitive to transjunctional voltage, a property shaved with other members of the gamma subclass of connexins. Like skate Cx35, mo use Cx36 was unable to form heterotypic gap-junctional channels when paired with two other rodent connexins. In addition, mouse Cx36 failed to form vo ltage-activated hemichannels, whereas both skate and perch Cx35 displayed q uinine-sensitive hemichannel activity. The conservation of intercellular ch annel gating contrasts with the failure of Cx36 to make hemichannels, sugge sting that the voltage-gating mechanisms of hemichannels may be distinct fr om those of intact intercellular channels. J. Neurosci. Res. 59: 813-826, 2 000. (C) 2000 Wiley-Liss, Inc.