The Joubert Syndrome protein Ahi1 is required for photoreceptor outer segment formation in zebrafish

摘要

Purpose : Joubert syndrome (JBTS) is an autosomal recessive ciliopathy characterized by retinal degeneration. The Abelson helper integration site 1 (AHI1) gene has been implicated in JBTS. The purpose of this study was to establish mutant alleles of ahi1 in zebrafish using transcription activator-like effector nucleases (TALENs) and to investigate the resulting phenotypes. Methods : Zebrafish ahi1 mutants were generated by TALEN-mediated gene editing. Mutant alleles were identified by high-resolution melt analysis and confirmed by direct sequencing. Visual function was assessed by the optokinetic response (OKR) at 5 days post fertilization (dpf). Photoreceptor morphology and degeneration was investigated using light microscopy and immunohistochemistry. Kidney cilia were evaluated by whole-mount antibody staining and fluorescence microscopy on 36 hours post fertilization (hpf) zebrafish embryos. Results : Three ahi1 mutant alleles were generated using TALENs. The ahi1LRI46 mutation has a 7 base-pair (bp) deletion and the ahi1LRI53 mutation has a 4bp deletion, both generate a stop codon at amino acid (aa) 240 and 241 respectively. The ahi1LRI47 mutation has a complex mutation consisting of a 6bp deletion and a 15bp insertion, which results in 4 new amino acids and a termination codon at aa 241. The ahi1LRI46 mutant was used for subsequent analysis. At 4 dpf, ahi1LRI46 mutants have curvy tails, cardiac edema and small eyes. Kidney cysts were observed in ~15% of mutants. At 5 dpf homozygous mutants had normal OKR function, but smaller eyes with preserved retinal lamination. The outer nuclear layer of ahi1 mutants lacked the ordered, columnar organization observed in wild-type siblings. PNA staining revealed that mutants had shorter cones outer segments. Rhodopsin staining in rod photoreceptors showed no differences between samples. Whole mount immunolabeling on 36 hpf embryos showed lower pronephric cilia density, although cilia lengths were conserved. Conclusions : The ahi1LRI46 zebrafish mutant results in early lethality and does not affect visual behavior at 5 dpf. Labeling of cones showed shortening of outer segments. No defects in rhodopsin trafficking were observed. Pronephric cilia density is diminished but cilia length is unaffected. Taken together, these results suggest that loss of ahi1 results in slow cone degeneration in zebrafish maybe due to a perturbation of the polarized vesicle trafficking. This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.