Cos2/Kif7 and Osm-3/Kif17 regulate onset of outer segment development in zebrafish photoreceptors through distinct mechanisms

Zebrafish morphants of osm-3/kif17, a kinesin-2 family member and intraflagellar transport motor, have photoreceptor outer segments that are dramatically reduced in number and size. However, two genetic mutant lines, osm-3/kif17(sa0119) and osm-3/kif17(sa18340), reportedly lack any observable morphological outer segment defects. In this work, we use TALENs to generate an independent allele, osm-3/kif17(mw405), and show that both osm-3/kif17(sa0119) and osm-3/kif17(mw405) have an outer segment developmental delay in both size and density that is fully recovered by 6 days post-fertilization. Additionally, we use CRISPRs to generate cos2/kif7(mw406), a mutation in the kinesin-4 family member cos2/kif7 that has been implicated in controlling ciliary architecture and Hedgehog signaling to test whether it may be functioning redundantly with osm-3/kif17. We show that cos2/kif7(mw406) has an outer segment developmental delay similar to the osm-3/kif17 mutants. Using a three-dimensional mathematical model of outer segments, we show that while cos2/kif7(mw406) and osm-3/kif17(mw405) outer segments are smaller throughout the first 6 days of development, the volumetric rates of outer segment morphogenesis are not different among wild-type, cos2/kif7(mw406), and osm-3/kif17(mw405) after 60hpf. Instead, our model suggests that cos2/kif7(mw406) and osm-3/kif17(mw405) impact outer segment morphogenesis through upstream events that that are different for each motor. In the case of cos2/kif7(mw406) mutants, we show that early defects in Hedgehog signaling lead to a general, non-photoreceptor-specific delay of retinal neurogenesis, which in turn causes the secondary phenotype of delayed outer segment morphogenesis. In contrast, the osm-3/kif17(mw405) outer segment morphogenesis delays are linked specifically to initial disc morphogenesis of photoreceptors rather than an upstream event. Further, we show that osm-3/kif17 mutant mice also exhibit a similarly delayed outer segment development, suggesting a role for osm-3/kif17 in early outer segment development that is conserved across species. In conclusion, we show that both osm-3/kif17 and cos2/kif7 have comparable outer segment developmental delays, although through independent mechanisms.