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

Abstract

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/kif17sa0119 and osm-3/kif17sa18340, reportedly lack any observable morphological outer segment defects. In this work, we use TALENs to generate an independent allele, osm-3/kif17mw405, and show that both osm-3/kif17sa0119 and osm-3/kif17mw405 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/kif7mw406, 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/kif7mw406 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/kif7mw406 and osm-3/kif17mw405 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/kif7mw406, and osm-3/kif17mw405 after 60hpf. Instead, our model suggests that cos2/kif7mw406 and osm-3/kif17mw405 impact outer segment morphogenesis through upstream events that that are different for each motor. In the case of cos2/kif7mw406 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/kif17mw405 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.

Acute inhibition of heterotrimeric kinesin-2 function reveals mechanisms of intraflagellar transport in mammalian cilia

“…Expression of neither the heterotrimeric motor KIF3A/KIF3C/KAP nor the homodimeric motor KIF17 was able to rescue ciliogenesis in the absence of heterotrimeric KIF3A/KIF3B/KAP function. This finding is consistent with the fact that loss of KIF17 or KIF3C expression has minimal phenotypes in mouse models [23, 24, 71, 72]. Recent work in zebrafish suggested that the heterotrimeric KIF3A/KIF3C/KAP motor is sufficient to drive IFT in a subset of ciliated cells [40].…”

“…7 ). This model would explain why Kif17 mutant zebrafish display only subtle defects in olfactory cilium structure and a delay in photoreceptor outer segment development [72, 78].…”

Kif17 phosphorylation regulates its ciliary localization and photoreceptor outer segment turnover

“…While we have previously shown a developmental role of KIF17 in regulating the onset of OS morphogenesis [14], there is no evidence of a role of KIF17 in mature photoreceptors [15]. We hypothesized that investigating the temporal expression patterns of Kif17 in mature photoreceptors throughout the diurnal light:dark cycle could shed light on a potential function of KIF17.…”

“…KIF17 is expressed in photoreceptors [12], which have a modified primary cilium called the outer segment (OS) that contains the components required for phototransduction [13]. Although KIF17 may regulate the onset of OS morphogenesis [14], there has been no evidence of a role for KIF17 in a mature photoreceptor [15]. As KIF17 can accumulate at the ciliary tip of photoreceptors [16], like mammalian cell lines [6, 7, 15, 17], it is possible that KIF17 may have some role in distal tip maintenance.…”

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

“…For example, in C. elegans amphid-channel sensory cilia heterotrimeric and homodimeric kinesin-2 motors cooperate in assembling the proximal axoneme, whereas homodimeric kinesin-2 (OSM-3) alone mediates assembly of distal axonemal microtubule singlets [79,81,120,121]. In zebrafish and mouse, homodimeric kinesin-2 (KIF17) seems dispensable for axoneme assembly in most tissues [122][123][124][125][126], even though kif17 mutant animals displayed subtle structural defects in olfactory cilia [125] and a slight delay in photoreceptor outer segment development [126]. In contrast, genetic inactivation of heterotrimeric kinesin-2 motor subunits in the same organisms usually leads to a dramatic loss of cilia in most tissues [123,[127][128][129], but some genetic redundancy may exist between different motor subunits, e.g.…”