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Ping Song; arl13a functions on the growth of the retina photoreceptors and amacrine cells by regulating cell division. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1490.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in ARL13B lead the ciliopathy Joubert Syndrome and cause photoreceptor degeneration in humans. Loss of zebrafish arl13b cause a slow, progressive retinal degeneration that may reflect compensation by arl13a, a zebrafish paralog of arl13b. The purpose of this study is to determine if arl13a partially compensates for arl13b during early photoreceptor outer segment development.
Antisense morphoinos were used to knockdown arl13a in zebrafish. Immunohistochemistry was performed to examine the photoreceptors and amacrine cells. The whole mount in situ was taken to analyze early gastrulation movements during embryonic development.
Strong expression of the arl13a was observed at 50% epiboly, with higher expression in head region, especially at optic vesicle after 8 somite stage. At 25 hpf stage, the expression is mostly restricted in eyes and midbrain, and then shifted to heart and hatching gland region at 2 dpf stage. In 3-5 dpf larva, expression was only detectable at the lip (4 dpf) and jaw ( 5 dpf). Morpholino knockdown of zebrafish arl13a resulted in microphthalmia, shortened jaw, and a curved body axis. Immunohistochemistry showed that amacrine cells were reduced at 3 dpf, whereas rods and cones were missing at 3 hpf and significantly reduced in arl13a morphants by 4 dpf. TUNEL staining revealed significant apoptosis in morphant retinas. Staining for PCNA, a marker of proliferating cells, labeled the ciliary marginal zone of wild-type retinas between 50-96 hpf, but labeled cells throughout the inner and outer nuclear layers of morphant retinas. Finally, arl13a morphants exhibited developmental delays as revealed by defects in early gastrulation movements.
<br /> Whereas mutation of arl13b results in widespread defects in ciliogenesis, morpholino knockdown of arl13a blocked differentiation of amacrine cells and photoreceptors. Retinal progenitors remained PNCA positive but cell death was also observed throughout the inner and outer retina, suggesting a failure to properly differentiate. The high expression of arl13a in retinal progenitors, but not the differentiated retina, did not mimic that of arl13b and suggests a role distinct from arl13b. Taken together, the results indicate that the arl13a does not compensate for arl13b in zebrafish photoreceptors and likely does not play a role in ciliogenesis.
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