Purpose: Frizzled3 (Fzd3), a member of the core planar cell polarity (PCP) family in mammals, contributes to visual development by guiding axonal projections of some retinal ganglion cells. However, its other functions in the maturation of the visual system, especially the retina, remain elusive. The present study explores the role of Fzd3 in retinal development by focusing on rod bipolar cells (RBCs).
Methods: Frizzled3 was conditionally removed from the retina of Isl1-Cre;Fzd3f/- mice using the Cre-loxP system. Electroretinograms (ERGs) were performed to measure the light response of retinas. Frizzled3 expression was monitored by β-galactosidase (β-gal) staining and anti-β-gal immunostaining. Immunofluorescence was used to examine cellular distribution and morphology during development, and electron microscopy was applied to visualize the dendritic invaginations of RBCs.
Results: Electroretinograms showed decreased b-wave amplitudes, and lower b- to a-wave ratios in Isl1-Cre;Fzd3f/- than in control (Isl1-Cre;Fzd3f/+) mice. In RBCs, where Fzd3 was expressed and inactivated, the planar organization, shape, and orientation of somas were disrupted. From P10, dendrites of these RBCs displayed reduced arborization with mistargeting. Furthermore, their dendritic invaginations into rod terminals were suppressed, and the density of rod ribbons in the OPL was reduced.
Conclusions: Frizzled3 is required to shape the pattern of RBC somas and dendrites, and the structural and functional connectivity between rods and RBCs. Our results highlight novel functions for Fzd3 in regulating retinal development.