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Ping Song, Lynn Dudinsky, Brian D Perkins, ; Rapid Photoreceptor Degeneration Occurs in Zebrafish arl13b Mutants Following Suppression of PCP Signaling. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4380.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in ARL13b are linked to Joubert Syndrome, a ciliopathy characterized by a distinctive midbrain-hindbrain malformation, kidney cysts, and varying degrees of retinal degeneration. Arl13b localizes to cilia but its function(s) are poorly understood. Mice and zebrafish with Arl13b mutations exhibit cilia defects but retinal phenotypes have not been explored. As genetic interactions between mutant alleles of cilia loci can affect pathological expression, ARL13b may interact with other loci to promote retinal degeneration. Morpholino suppression of arl13b leads to phenotypes in zebrafish consistent with defects in planar cell polarity (PCP). We hypothesize that Arl13b and PCP components function together to enhance the arl13b phenotypic spectrum within photoreceptors.
Photoreceptor and cilia structure was examined by light and electron microscopy. Immunohistochemistry was performed to examine localization of rhodopsin and ciliary markers. Genetic interactions were tested by injecting morpholinos against vangl2, bbs8 or fuzzy into arl13b mutants or morphants and assessed by in situ hybridization. Genetic mosaic animals were generated by blastula transplantation using arl13b:Tg(TaC:GFP) transgenic as donors.
Zebrafish arl13b mutant photoreceptors did not exhibit overt signs of retinal degeneration by electron microscopy or immunohistochemistry at 5 days post fertilization (dpf). Starting at 14 dpf and continuing through 30 dpf, cells lacking Arl13b died following transplantation into wild-type host animals, whereas control transplanted cells survived. Full morpholino knockdown of arl13b phenocopied arl13b mutants. Co-injection of sub-phenotypic doses of morpholinos against arl13b and PCP genes, such as vangl2, fuzzy, or bbs8 resulted in much stronger PCP phenotypes in early embryos and shorter photoreceptor outer segments and cilia in older larvae.
Arl13b is required for normal ciliary function and is believed to regulate protein trafficking. Loss of arl13b leads to slow photoreceptor degeneration following cell transplantation. Furthermore, arl13b genetically interacts with bbs8, a ciliopathy gene associated with Bardet-Biedl Syndrome. Genetic interactions between arl13b and bbs8 or components of the PCP pathway, suggests that modifying alleles of PCP genes may enhance photoreceptor degeneration in ciliopathies.
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