Generation of Cx57-Cre mice and exclusive Cre expression in horizontal cells.

<p><b><i>A</i></b>, Targeting scheme. The coding region of the wild-type <i>Cx57</i> gene is located in exon 2 and 3. After electroporating the targeting vector (pKW-DTR-fret-Cre), a large part of exon 2 was replaced by a DTR-eGFP construct, flanked by <i>frt</i> sites, and the coding sequence for the NLS-Cre recombinase followed by a neomycin resistance cassette (<i>neoR</i>). After Flp- mediated recombination, the coding sequence for the NLS-Cre recombinase gets under the control of the Cx57 promoter. DTR-eGFP: fusion protein of the diphtheria toxin receptor (DTR) and eGFP; NLS: nuclear localization signal. <b><i>B</i></b>, Southern blot analysis of ScaI-digested genomic DNA to test for correct recombination. A signal of 7.2 kb confirms the correct targeting of the Cx57-DTRfrtCre construct to the <i>Cx57</i> locus. The functioning of the <i>frt</i> sites and successful Flp-mediated excision of DTR were verified by a signal of 8.7 kb. <b><i>C-E</i></b>, Projections of confocal stacks of vertical retina sections from Cx57Cre/Cre mice labeled with antibodies against calbindin (magenta; C, E), a marker for mouse horizontal cells, and Cre recombinase (green; D, E). Only calbindin-positive horizontal cells show Cre recombinase immunoreactivity in their nuclei (arrows) in the distal INL; no other retinal cell type is Cre-positive. <b><i>F</i></b>, The Nomarski image shows the normal retinal layering. ONL/INL, outer/inner nuclear layer; OPL/IPL, outer/inner plexiform layer; GCL, ganglion cell layer. Scale bar: C-F, 20 µm.