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Christin Hanke-Gogokhia, Zhijian Wu, Jeanne M Frederick, Houbin Zhang, Wolfgang Baehr; The function of ARL3 in mouse photoreceptors. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© 2017 Association for Research in Vision and Ophthalmology.
Arf-like protein 3 (ARL3) is a ubiquitous protein expressed in ciliated cells, e.g. photoreceptors. Germline deletion of Arl3 in mouse caused syndromic ciliopathy reminiscent of Bardet-Biedl or Joubert syndromes. Mouse photoreceptors were used as a primary cilium model system to explore the function of ARL3 by generating rod photoreceptor (prefix rod) and retina-specific (prefix ret) Arl3 deletions.
EUCOMM gene trap was inserted in intron 1 of the mouse Arl3 gene. Rod- and retina-specific Arl3 conditional knockout mice were obtained by crossing with Flp-mice followed by mating with iCre75+ or Six3Cre+ transgenic mice, respectively. ERG and OptoMotry were used to test photoreceptor function, whereas progress of retinal degeneration was evaluated by optical coherence tomography, confocal immunohistochemistry and histology. AAV particles were injected in the subretinal space to rescue Arl3-/- photoreceptors.
In predegenerate rodArl3-/- mice, lipidated phototransduction proteins (PDE6, GRK1 and transducin) show deficient trafficking to the outer segments (OS), consistent with ARL3’s role as a cargo release factor of lipid binding proteins (PDE6D and UNC119). Transmembrane proteins (rhodopsin and GC1) traffic normally. Its recently discovered GEF, ARL13b, appears unaffected and localizes to ROS of both wild-type and mutant mice. By contrast, retArl3-/- rod photoreceptors expressing Cre during embryonic development fail to form connecting cilia and outer segments, and degenerate rapidly as shown by ERG, OCT and histology. Farnesylated INPP5E, reportedly in primary cilia and associated with Joubert Syndrome, is present exclusively in Golgi membranes of wildtype photoreceptors and significantly reduced in retArl3-/- as revealed by immunohistochemistry and neonatal electroporation. Absence of connecting cilia in retina-specific knockout photoreceptors implicates ARL3 in intraflagellar transport (IFT) and axoneme formation in the central retina. Photoreceptor degeneration can be rescued in part by subretinal injection of AAV particles expressing ARL3-EGFP before eye opening; viral expression of ARL3-EGFP preserved ONL thickness and ROS-like structure at age 2 months.
Our results identified two ARL3 functions: one as a regulator of photoreceptor ciliogenesis and IFT, and another as a cargo dislocation factor in trafficking of lipidated protein to the rod outer segment.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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