Functional expression of connexin57 in horizontal cells of the mouse retina

Abstract

Horizontal cells are interneurons of the vertebrate retina that exhibit strong electrical and tracer coupling but the identity of the channel-forming connexins has remained elusive. Here we show that horizontal cells of the mouse retina express connexin57 (Cx57). We have generated Cx57-deficient mice by replacing the Cx57 coding region with a lacZ reporter gene, expressed under control of the endogenous Cx57 promoter. These mice were fertile and showed no obvious anatomical or behavioural abnormalities. Cx57 mRNA was expressed in the retina of wild-type littermates but was absent from the retina of Cx57-deficient mice. Previously reported results that the Cx57 gene was very weakly expressed in several other mouse tissues turned out to be unspecific. Cx57 mRNA is abundantly expressed in the retina and weakly in the thymus of adult mice but absent in all other adult tissues tested, including brain. Furthermore, Cx57 is expressed in embryonic kidney at E16.5 to E18.5 days post-conception, as indicated by the pattern of lacZ expression. Within the retina, lacZ signals were assigned exclusively to horizontal cells based on co-localization with cell-type-specific marker proteins. Microinjection of Neurobiotin into horizontal cells of isolated retinae revealed less than 1% of tracer coupling in Cx57-deficient retinae compared with wild-type controls. Cx57 is the first connexin identified in mammalian horizontal cells and the first connexin whose expression is apparently restricted to only one type of neuron.

Screening of gap junction antagonists on dye coupling in the rabbit retina

“…AII amacrine cells express Cx36 (Feigenspan et al, 2001;Mills et al, 2001), Atype HCs express Cx50 (O'Brien et al, 2006) and the mouse homolog of the rabbit B-type HC may use Cx57 (Hombach et al, 2004). MFA, 18-β-GA, mefloquine and 2-APB blocked coupling in all these networks, and so we conclude they have little selectivity between different connexins, at least within the group of neuronal connexins such as Cx36, Cx50 and Cx57 in the retina.…”

“…For example, A-type and B-type horizontal cells have distinct coupling properties, probably because they express different connexins. Atype HCs are coupled via Cx50 gap junctions while B-type HCs may be coupled by Cx57 (Hombach et al, 2004). In the inner retina, AII amacrine cells are coupled by Cx36 gap junctions (Feigenspan et al, 2001;Mills et al, 2001).…”

“…In mammals, four neuronal connexins have been identified from this gene family. Cx36 appears to be the dominant neuronal connexin, but horizontal cells (HCs) express Cx50 or Cx57 and other inner retinal neurons express Cx45 (Massey et al, 2003;Hombach et al, 2004;Sohl et al, 2005;O'Brien et al, 2006). Gap junctions are abundant throughout the retina.…”

“…A-type HCs are the best coupled, approximately 20× better than B-type HCs (Mills & Massey, 1998) and express Cx50 gap junctions, including giant plaques between major dendrites (O'Brien et al, 2006). B-type HCs in the rabbit retina are axon bearing (Dacheux & Raviola, 1982) and the homologous cell type in the mouse retina expresses Cx57 (Hombach et al, 2004). Furthermore in the Cx57 knockout mouse, there is no HC coupling and the size of the receptive field is reduced (Hombach et al, 2004).…”

“…B-type HCs in the rabbit retina are axon bearing (Dacheux & Raviola, 1982) and the homologous cell type in the mouse retina expresses Cx57 (Hombach et al, 2004). Furthermore in the Cx57 knockout mouse, there is no HC coupling and the size of the receptive field is reduced (Hombach et al, 2004). The size of horizontal cells, their extensive coupling and the ease with which HCs can be dye injected makes them suitable to test gap junction antagonists.…”