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D. L. Stenkamp, T. Sherpa, S. S. Hunter; Optimizing Retinal Regeneration: A Role for Sonic Hedgehog. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4309.
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Teleost fish can regenerate their retinas after damage. We demonstrated recovery of visual function in zebrafish subjected to widespread retinal damage inflicted by intraocular injection of ouabain (Sherpa et al., 2008; Dev Neurobiol 68:166-81). Functional recovery required 98 days, matching the time of optic nerve head (ONH) restoration. Later recovery times were characterized by overproduction of cells in the retinal ganglion cell (RGC) layer and ONH hypertrophy. Here we determine whether a selective lesion to inner retinal layers will result in faster visual recovery and fewer histological errors, and identify the molecular basis of any differences in regeneration following broad vs. selective retinal damage.
Zebrafish were injected intraocularly with high or low doses of ouabain, creating widespread retinal damage, or damage limited to the inner retinal layers, respectively (Fimbel et al., 2007; J Neurosci 27:1712-24). Visual function was monitored by a place-preference assay. Radial sections of the retina were used to measure the ONH, or were stained with anti-HuC/D antibodies to facilitate counting of cells in the RGC layer and assessment of laminar pattern. QRT-PCR was used to measure the expression of selected genes in both lesion models as a function of recovery time.
Following selective retinal damage, recovery of visual function was observed at 58 days post-lesion, matching the time of ONH restoration (60 days; Fimbel et al., 2007). Eyes evaluated at later recovery times did not show ONH hypertrophy, did not contain supernumerary cells in the GCL, and the lamination errors seen at early recovery times became virtually nonexistent. In contrast, visual recovery following broad retinal damage was slower (Sherpa et al., 2008), and was accompanied by increasing retinal disorganization as a function of recovery time. In both damage models, several genes related to RGC development (Brn3b, ath5, fgf8) were significantly downregulated immediately following the lesion, then expression returned to normal, adult levels. However, although the sonic hedgehog (shh) gene was downregulated in both models, expression returned to normal, adult levels only in the selective lesion model, remaining at reduced levels in the broad lesion model.
Regeneration of RGCs in a manner that restores vision is optimized by the presence of other, undamaged retinal cells. Delays in visual recovery, overproduction of RGCs, retinal disorganization, and overgrowth of the ONH are correlated with errors in shh expression during retinal regeneration. An experiment to assess retinal regeneration in conditions of reduced shh signaling is underway.
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