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JC Besharse, SA Baker, JA Deane, DG Cole, BL Dickert, JL Rosenbaum, GB Witman, GJ Pazour; IFT88, an Intraflagellar Transport Protein, is Required for Photoreceptor Outer Segment Assembly in the Mouse . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2713.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Intraflagellar transport (IFT) in which a large protein complex is moved along ciliary microtubules by the kinesin II and cytoplasmic dynein (1b/2) motors, is required for flagellar and sensory cilium formation in Chlamydomonas and C. elegans. IFT has been proposed as a general mechanism for delivery to ciliary structures, including the sensory cilium/outer segment of photoreceptors. As a test of the hypothesis that a component of IFT is required for outer segment assembly, we have studied photoreceptor development in a mouse line (orpk/Tg737) carrying an insertional mutation in the IFT88 subunit of the IFT protein complex. Methods: The Tg737 mouse line was homozygous for the recessive Pdebrd1 retinal degeneration defect, precluding analysis of the effect of the Tg737 mutation. Therefore, heterozygous Tg737 mutant mice on the FVB/N (+/TgN737Rpw, Pdebrd1/Pdebrd1) background were bred to wild-type C57B6/J (+/+, +/+) animals and the offspring were sibling mated to produce F2 animals, or backcrossed to the heterozygous Tg737 FVB/N parent. The Tg737 and Pdebrd1 were genotyped by PCR. Results: Mice homozygous for the IFT88 mutation (IFT88-/-) exhibited aberrant outer segment development detected at post-natal day 10 (p10). Discs of both rods and cones were poorly aligned, often larger in diameter than those of littermate controls, or completely vesiculated. Although the outer nuclear layer attained normal thickness, outer segments never achieved adult dimensions. At p21 and later, extracellular vesicles accumulated in the space around photoreceptors. Immunogold localization revealed that opsin was transported into aberrant outer segments, but also accumulated in the inner segment plasma membrane and in the extracellular vesicles. Progressive loss of photoreceptors occurred after p21; by p45 =50% of the ONL was lost and only a few rows of nuclei remained at p77-84. Retinal anomalies outside of the photoreceptor layer were not seen . The IFT88-/- phenotype was present in mice that were both heterozygous and homozygous wildtype at the Pdebrd1 locus. Conclusion: The degeneration in Tg737 mice suggests a failure of normal outer segment formation. By analogy to its role in cilia and flagella, the mutation in IFT88 may disrupt transport of essential components into the cilium/outer segment.
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