Although ultraviolet (UV) sensitivity is widespread among animals it is considered rare in mammals, being restricted to the few species that have a visual pigment maximally sensitive (λmax) below 400 nm. However, even animals without such a pigment will be UV-sensitive if they have ocular media that transmit these wavelengths, as all visual pigments absorb significant amounts of UV if the energy level is sufficient. Although it is known that lenses of diurnal sciurid rodents, tree shrews and primates prevent UV from reaching the retina, the degree of UV transmission by ocular media of most other mammals without a visual pigment with λmax in the UV is unknown. We examined lenses of 38 mammalian species from 25 families in nine orders and observed large diversity in the degree of short-wavelength transmission. All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity. Other mammals, however, such as hedgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315–400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.
Asymmetric segregation of cell-fate determinants during cytokinesis plays an important part in controlling cell-fate choice in invertebrates. During Drosophila neurogenesis, for example, asymmetric segregation of the Numb protein, which inhibits Notch signaling, is necessary for the two daughter cells of a division to have different fates. In vertebrates, the role of asymmetric segregation of cell-fate determinants is uncertain, and the way the process might be regulated is unknown. We have studied the orientation of cell divisions and the distribution of Numb in the developing rat retina. We show that, whereas most retinal neuroepithelial cells divide with their mitotic spindles oriented parallel to the plane of the neuroepithelium, a substantial minority divides with their spindles oriented perpendicularly. The proportion of these vertically dividing cells changes during development, peaking around the day of birth. Numb appears to be inherited only by the apical daughter cell when a neuroepithelial cell divides vertically. Similarly, in dissociated cell cultures, some retinal neuroepithelial cells divide asymmetrically and distribute Numb to only one of the two daughter cells, suggesting that the dissociated cells can retain their polarity in vitro. Using retinal explant cultures, we find that the retinal pigment epithelium apparently promotes vertical divisions in the neural retina. To our knowledge, this is the first evidence that asymmetric segregation of cell-fate determinants may contribute to cell diversification in the mammalian retina and that an epithelium controls this process by influencing the plane of division in the adjacent neural retina.
Inflammation is an umbrella feature of ageing. It is present in the aged retina and many retinal diseases including age-related macular degeneration (AMD). In ageing and in AMD mitochondrial function declines. In normal ageing this can be manipulated by brief exposure to 670 nm light on the retina, which increases mitochondrial membrane potential and reduces inflammation. Here we ask if 670 nm exposure has the same ability in an aged mouse model of AMD, the complement factor H knockout (CFH−/−) where inflammation is a key feature. Further, we ask whether this occurs when 670 nm is delivered briefly in environmental lighting rather than directly focussed on the retina. Mice were exposed to 670 nm for 6 minutes twice a day for 14 days in the form of supplemented environmental light. Exposed animals had significant increase in cytochrome c oxidase (COX), which is a mitochondrial enzyme regulating oxidative phosphorylation.There was a significant reduction in complement component C3, an inflammatory marker in the outer retina. Vimetin and glial fibrillary acidic protein (GFAP) expression, which reflect retinal stress in Muller glia, were also significantly down regulated. There were also significant changes in outer retinal macrophage morphology. However, amyloid beta (Aβ) load, which also increases with age in the outer retina and is pro-inflammatory, did not change. Hence, 670 nm is effective in reducing inflammation probably via COX activation in mice with a genotype similar to that in 50% of AMD patients even when brief exposures are delivered via environmental lighting. Further, inflammation can be reduced independent of Aβ. The efficacy revealed here supports current early stage clinical trials of 670 nm in AMD patients.
"Cone dystrophy with supernormal rod electroretinogram (ERG)" is an autosomal recessive disorder that causes lifelong visual loss combined with a supernormal ERG response to a bright flash of light. We have linked the disorder to a 0.98-cM (1.5-Mb) region on chromosome 9p24, flanked by rs1112534 and rs1074449, using homozygosity mapping in one large consanguineous pedigree. Analysis of one gene within this region, KCNV2, showed a homozygous nonsense mutation. Mutations were also found in 17 alleles of 10 other unrelated families with the same disorder. In situ hybridization demonstrated KCNV2 expression in human rod and cone photoreceptors. The precise function of KCNV2 in human photoreceptors remains to be determined, although this work suggests that mutations might perturb or abrogate I(KX), the potassium current within vertebrate photoreceptor inner segments, which has been shown to set their resting potential and voltage response.
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