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J S. Zigler, Jr., C Zhang, R Grebe, S Hose, D Sinha; Synergistic Effect of Two Independent Spontaneous Mutations on the Optic Nerve of the Rat. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2671.
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To investigate the effects of the Nuc1 and frogleg mutations on the optic nerve and retina of the Sprague-Dawley rat.
Mutant rats in a mapping population of Sprague-Dawley/Lewis hybrids, were identified initially by their phenotypic characteristics; genotypes were confirmed by PCR analysis using markers flanking the genetic locus of each mutation. Animals were perfused with 4% paraformaldehyde and post-fixed in 3% glutaraldehyde prior to electron microscopic analysis of the optic nerve. Whole eyes were sectioned and stained with Hematoxylin and eosin. Eyes fixed in 2% paraformaldehyde were used for immunohistochemistry with specific markers for astrocytes (GFAP), oligodendrocytes (APC and olig-2), and axons (SMI 312).
The Nuc1 mutation in the βA3/A1-crystallin gene causes retinal developmental abnormalities arising, at least in part, from expression of the mutant protein in astrocytes. The frogleg mutation arose spontaneously in the Nuc1 breeding colony, and is characterized by impairment of hindlimb function; the phenotype appears to be neurological in origin. We have recently found that rats homozygous for both mutations may have severe pathology of the optic nerves, whereas animals homozygous for either mutation alone, have only minor effects on optic nerve structure. Transmission electron microscopy of optic nerves from double mutants revealed obvious structural abnormalities in both glial cell populations, astrocytes and oligodendrocytes. In addition, there was marked decrease in, and severe disorganization of, myelin sheaths in these optic nerves. These findings were confirmed by immunohistochemical analysis with specific markers for astrocytes, oligodendrocytes, axons, and myelin (MBP). Staining for MBP and for oligodendrocytes was similar in the wildtype and Nuc1 optic nerves, but was modestly decreased in frogleg. In the double mutants, the decrease in these parameters could be as great as 90%. In the eyes with severely affected optic nerves, the retina is thinner than normal with marked degeneration of the inner retina. Persistence of the fetal vasculature, a characteristic of Nuc1, may also be extremely severe in the double mutants.
The data suggest that the frogleg mutation, for which the gene remains to be identified, affects the oligodendrocytes of the optic nerve. This effect is magnified in animals also homozygous for the Nuc1 mutation, which is known to affect astrocyte structure and function. The basis for the synergistic effect in the double mutants where both glial cell populations are affected remains to be determined.
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