Functional expression of connexin57 in horizontal cells of the mouse retina

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.