Molecular cloning of three zebrafish lin7 genes and their expression patterns in the retina.

The vertebrate retina develops from an undifferentiated sheet of neuroepithelial cells, whose differentiation requires the generation and maintenance of the correct cellular polarity. To examine the role of cell polarity in retinal development, we cloned three zebrafish lin7 genes (lin7a, lin7b, and lin7c), which each encodes a protein candidate that is required for generation/maintenance of neuroepithelial cell junctions. These three zebrafish Lin7 proteins share over 78% amino acid identity and contain both L27 and PDZ domains that are present in all Lin7 homologs. Immunoblots revealed that the Lin7b and Lin7c proteins were first expressed in the developing eye by 24hr postfertilization (hpf), while Lin7a was not detected in the eye until 72 hpf. At 33 hpf, the Lin7 proteins localized at, or slightly apical of, the actin-associated adherens junctions in the retinal neuroepithelium. This subcellular distribution required the expression of the Nok protein. In the absence of Nok, the Lin7 proteins failed to localize to either the ectopic adherens junctions or the cell membrane. At 4 days postfertilization, in situ hybridisation revealed that all three lin7 genes were expressed in both the ganglion cell layer and the bipolar cell region of the inner nuclear layer. The lin7a gene was also expressed in the amacrine and horizontal cell regions of the inner nuclear layer, while lin7c was also expressed in the outer nuclear layer. In the adult retina, where Lin7a is the predominant form expressed, the Lin7 proteins were localized to the outer and inner plexiform layers, the bipolar and horizontal cells of the inner nuclear layer, and the ganglion cells. These results suggest that the three zebrafish Lin7 proteins possess partially redundant, yet essential, roles in retinal development.