FIGURE 8.

Hypothetical model of ift122 function in photoreceptor cell. A, upper panel, at 4 dpf elongation of photoreceptor OS begins in the wild-type retina. Opsin produced in photoreceptor cell bodies is transported to the OS (red triangles) and accumulated in the OS. Middle panel, in ift122 mutant photoreceptors, smaller OS and shorter cilia were observed at 4 dpf. A certain amount of opsin is transported to the OS; however, probably because of insufficient transport capacity of IFT, opsins are accumulated in the photoreceptor cell bodies. The moderate ectopic opsin accumulation causes the late onset photoreceptor cell death around 7–10 dpf. Lower panel, in the IFT complex B mutant retina (e.g. elipsa) photoreceptors, cilia, and OS are not formed, because the anterograde IFT system is completely disrupted. Opsin accumulates in the photoreceptor cell bodies in the IFT-B mutant retina. The ectopic opsin accumulation causes early photoreceptor cell death around 5 dpf. B, left panel, in wild-type photoreceptor cells, the IFT complex B mediates efficient cargo transport from inner segments to outer segments along ciliary axonemes and are recycled by retrograde transport machinery. Right panel, in ift122 mutant photoreceptors, the IFT complex B is accumulated in the tip of axonemes of connecting cilia due to the loss of IFT recycling. Even under such conditions, certain amount of cargos are transported to outer segments and small outer segments are formed in ift122 mutant photoreceptors.