Kif17 phosphorylation regulates photoreceptor outer segment turnover

Joseph C. Besharse,Sean R. Kundinger,Tylor R. Lewis,Christine Insinna,Brian A. Link

doi:10.1186/s12860-018-0177-9

Abstract

BackgroundKIF17, a kinesin-2 motor that functions in intraflagellar transport, can regulate the onset of photoreceptor outer segment development. However, the function of KIF17 in a mature photoreceptor remains unclear. Additionally, the ciliary localization of KIF17 is regulated by a C-terminal consensus sequence (KRKK) that is immediately adjacent to a conserved residue (mouse S1029/zebrafish S815) previously shown to be phosphorylated by CaMKII. Yet, whether this phosphorylation can regulate the localization, and thus function, of KIF17 in ciliary photoreceptors remains unknown.ResultsUsing transgenic expression in zebrafish photoreceptors, we show that phospho-mimetic KIF17 has enhanced localization along the cone outer segment. Importantly, expression of phospho-mimetic KIF17 is associated with greatly enhanced turnover of the photoreceptor outer segment through disc shedding in a cell-autonomous manner, while genetic mutants of kif17 in zebrafish and mice have diminished disc shedding. Lastly, cone expression of constitutively active tCaMKII leads to a kif17-dependent increase in disc shedding.ConclusionsTaken together, our data support a model in which phosphorylation of KIF17 promotes its photoreceptor outer segment localization and disc shedding, a process essential for photoreceptor maintenance and homeostasis. While disc shedding has been predominantly studied in the context of the mechanisms underlying phagocytosis of outer segments by the retinal pigment epithelium, this work implicates photoreceptor-derived signaling in the underlying mechanisms of disc shedding.

Highlights

KIF17, a kinesin-2 motor that functions in intraflagellar transport, can regulate the onset of photoreceptor outer segment development
Phospho-mimetic mouse KIF17 has enhanced ciliary localization Published data suggest that the conserved nuclear localization signal (NLS) in the C-terminus of KIF17 can regulate the ciliary localization of KIF17 through a classical nuclear import mechanism [8]
The frequency of ciliary localization of the phospho-mimetic KIF17(S1029D)-mCherry was significantly increased between two- to four-fold over the phospho-deficient KIF17(S1029A)-mCherry in all cell lines studied (Additional file 1: Figure S1B)

Summary

Introduction

KIF17, a kinesin-2 motor that functions in intraflagellar transport, can regulate the onset of photoreceptor outer segment development. The ciliary localization of KIF17 is regulated by a C-terminal consensus sequence (KRKK) that is immediately adjacent to a conserved residue (mouse S1029/zebrafish S815) previously shown to be phosphorylated by CaMKII. Whether this phosphorylation can regulate the localization, and function, of KIF17 in ciliary photoreceptors remains unknown. Calcium/calmodulin-dependent protein kinase II, or CaMKII, has been shown to phosphorylate the C-terminus of KIF17 to regulate binding and release of Mint, a scaffolding protein for motor cargo, in neurons [16] This conserved phosphorylation site lies immediately adjacent to the NLS in KIF17 (Fig. 1a). Given the accumulation of Kif at the ciliary tip of photoreceptors [7], we hypothesized that Kif phosphorylation could regulate a possible role of Kif in disc shedding

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