Morphological characterization of the retinal degeneration in three strains of mice carrying the rd-3 mutation

Abstract

Retinal development in 3 strains of rd-3/rd-3 mutant mice, previously shown to have different rates of degeneration, was studied using light, electron, and immunofluorescence microscopy. The time course and phenotype of the degeneration as well as details on the mechanism of massive photoreceptor cell loss are compared with other known retinal degenerations in mice. Up until postnatal day (P) 10, the retinas of all three strains (RBF, 4Bnr, In-30) develop similarly to those of pigmented and nonpigmented controls. TUNEL-positive cells appear in the outer nuclear layer (ONL) by P14, and reach a maximum in all three mutant strains around P21. Scattered rods and cones form a loose, monolayered ONL by 8 weeks in the albino RBF strain, by 10 weeks in the albino 4Bnr strain, and by 16 weeks in the pigmented In-30 strain. Though the initial degeneration begins in the central retina, there is no preferred gradient of cell death between central and peripheral photoreceptors. Rods and cones are present at all ages examined. During development, stacks of outer segments (OS) form in all three strains though they never achieve full adult lengths, and often have disorganized, atypical OS. Rod opsin is expressed in the developing OS but is redistributed into plasma membrane as OS degeneration proceeds. Retinal pigment epithelial (RPE) cells of all mutant strains contain packets of phagocytosed OS, and their apical processes associate with the distal ends of the OS. At their synaptic sites, photoreceptor terminals contain ribbons apposed to apparently normal postsynaptic triads. As photoreceptors are lost, Müller cells fill in space in the ONL but they do not appear to undergo significant hypertrophy or migration, though during the degeneration, glial fibrillary acidic protein (GFAP) expression is gradually upregulated. Macrophage-like cells are found frequently in the subretinal space after the onset of photoreceptor apoptosis. As OS disappear, the RPE apical processes revert to simple microvilli. Late in the degeneration, some RPE cells die and neighboring cells appear to flatten as if to maintain confluence. In regions of RPE cell loss that happen to lie above retina where the ONL is gone, cells of the inner nuclear layer (INL), wrapped by Müller cell processes, may front directly on Bruch's membrane.

Union Makes Strength: A Worldwide Collaborative Genetic and Clinical Study to Provide a Comprehensive Survey of RD3 Mutations and Delineate the Associated Phenotype

“…Consistently, the retina of the animal develops normally but both rod and cone photoreceptors start degenerating by postnatal day 14 - i.e. when eyes open – and the degeneration is almost complete by the age of 2–4 months [21], [38], [44]. Similarly, the rd3 frameshift mutation (p.Pro139Alafs*) of the rcd2 collie [40] is likely to produce an unstable, non-functional rd3 protein leading to the loss of GC expression and trafficking.…”

Characterization of a Canine Model of Autosomal Recessive Retinitis Pigmentosa due to aPDE6AMutation

“…Activation of Müller glial cells, reflected by increased GFAP immunoreactivity, is an early feature of the degeneration. This is a common occurrence in degenerating retinas 23, 42–44 and is likely to be secondary to photoreceptor loss; a recent study indicated that damaged photoreceptors release endothelin 2, which in turn stimulates the reactivity of Müller glia. 45 …”

Insights into the role of RD3 in guanylate cyclase trafficking, photoreceptor degeneration, and Leber congenital amaurosis

“…The rate of degeneration varies with the background strain (Linberg et al, 2005; Friedman et al, 2006; Danciger et al, 2008; Molday et al, 2013). The albino RBF/DnJ strain shows the fastest rate of degeneration with only a monolayer of photoreceptor nuclei present 8 weeks after birth.…”