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C. Insinna, J.C. Besharse; Fibrocystin, the ARPKD Protein, Is Expressed in the Photoreceptor Connecting Cilium . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5529.
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Recently, several proteins initially associated with polycystic kidney disease (PKD) including IFT88/polaris and polycystins I and II have been found to be widely expressed in multiple types of cilia in different tissues. Many of these ciliary defects causing cystic kidney disease, like Senior–Loken syndrome, are associated with retinal degeneration. Fibrocystin, a protein encoded by PKHDl, responsible for the recessive form of PKD, is expressed in kidney and liver primary cilia and basal bodies. Fibrocystin is a 450 kDa glycoprotein of unknown function. We have tested the hypothesis that fibrocystin is also localized in the basal body and cilium of photoreceptors.
We used a combination of RT–PCR from whole retina RNA, immunoprecitation followed by Western blotting from retinal extracts, and Western blotting and immunofluorescence on isolated photoreceptor axonemes to test the hypothesis that fibrocystin is associated with photoreceptor cilia or basal bodies .
Fibrocystin expression was confirmed in retina by both RT–PCR and by Western blotting. It was also found in association with ciliary axonemes isolated from crude photoreceptor outer segments. Double label immunofluorescence microscopy of whole–mounted axonemes with a rabbit antibody to fibrocystin and the K26 Mab, a marker for the connecting cilium, revealed that fibrocystin was localized primarily at the level basal body adjacent to the connecting cilium.
These findings indicate that fibrocystin is expressed in the photoreceptor connecting cilium basal body and are consistent with the idea of a common molecular composition and transport pathways within multiple distinct ciliary types. Investigation of the function of fibrocystin in the photoreceptor connecting cilium under normal and pathologic conditions will likely offer new insights on ARPKD pathogenesis and new approaches to rational therapeutics.
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