Plasma lipid abnormalities in the abyssinian cat with a hereditary rod-cone degeneration

Alternative splicing in CEP290 mutant cats results in a milder phenotype than LCACEP290 patients

PURPOSE

The rdAc cat has an intronic mutation in the centrosomal 290 kDa (CEP290) gene resulting in a frameshift and a premature stop codon (c.6960 + 9 T > G, p.Ile2321AlafsTer3) predicted to truncate the protein by 157 amino acids. CEP290 mutations in human patients cause a range or phenotypes including syndromic conditions and severe childhood loss of vision while the rdAc cat has a milder phenotype. We sought to further characterize the effect of rdAc mutation on CEP290 expression.

METHODS

TaqMan quantitative real-time polymerase chain reaction assays were used to compare wildtype and truncated transcript levels. Relative protein abundance was analyzed by Western blot. Immunohistochemistry (IHC) was performed to detect CEP290 protein.

RESULTS

CEP290 mutant cats show low-level (17.4% of wildtype cats) use of the wildtype splice site and usage of the mutant splice site. Western analysis shows retina from cats homozygous for the mutation has CEP290 protein that likely comprises a combination of both wildtype and truncated protein. IHC detects CEP290 in affected and control retina labeling the region of the interconnecting cilium.

CONCLUSIONS

The comparably milder phenotype of CEP290 mutant cats is likely due to the retained production of some full-length CEP290 protein with possible functional contributions from presence of truncated protein.

The use of canine models of inherited retinal degeneration to test novel therapeutic approaches

Inherited retinal degenerations (RDs) are a common cause of blindness in dogs and in humans. Over the past two decades numerous genes causally associated with these diseases have been identified and several canine models have been used to improve our understanding of the molecular mechanisms of RDs, as well as to test the proof of principle and safety of novel therapies. This review briefly summarizes the drug delivery approaches and therapeutic strategies that have been and are currently tested in dogs, with a particular emphasis on corrective gene therapy, and retinal neuroprotection.

Haploinsufficiency of RanBP2 is neuroprotective against light-elicited and age-dependent degeneration of photoreceptor neurons

Prolonged light exposure is a determinant factor in inducing neurodegeneration of photoreceptors by apoptosis. Yet, the molecular bases of the pathways and components triggering this cell death event are elusive. Here, we reveal a prominent age-dependent increase in the susceptibility of photoreceptor neurons to undergo apoptosis under light in a mouse model. This is accompanied by light-induced subcellular changes of photoreceptors, such as dilation of the disks at the tip of the outer segments, prominent vesiculation of nascent disks, and autophagy of mitochondria into large multilamellar bodies. Notably, haploinsufficiency of Ran-binding protein-2 (RanBP2) suppresses apoptosis and most facets of membrane dysgenesis observed with age upon light-elicited stress. RanBP2 haploinsufficiency promotes decreased levels of free fatty acids in the retina independent of light exposure and turns the mice refractory to weight gain on a high-fat diet, whereas light promotes an increase in hydrogen peroxide regardless of the genotype. These studies demonstrate the presence of age-dependent and RanBP2-mediated pathways modulating membrane biogenesis of the outer segments and light-elicited neurodegeneration of photoreceptors. Furthermore, the findings support a mechanism whereby the RanBP2-dependent production of free fatty acids, metabolites thereof or the modulation of a cofactor dependent on any of these, promote apoptosis of photoreceptors in concert with the light-stimulated production of reactive oxygen species.

Alpha-tocopherol protects against oxidative damage to lipids of the rod outer segments of the equine retina

Oxidative stress is a possible risk factor for eye diseases. Lipid peroxidation is one of the major events induced by oxidative stress and is particularly active in polyunsaturated fatty acid (PUFA)-rich biomembranes. This work evaluated endogenous lipid antioxidants, in vitro non-enzymatic lipid peroxidation of rod outer segment membranes (ROS), the fatty acid composition during oxidative damage of total lipids from equine retina and ROS, and the protective action of alpha-tocopherol (alpha-Toc). The major lipid soluble antioxidant was alpha-Toc followed by retinoids and carotenoids. The retina contained a high percentage of PUFAs, mainly docosahexaenoic acid (22:6n-3) and arachidonic acid (20:4n-6). Lipid peroxidation of the equine ROS, induced by Fe(2+)-ascorbate, was monitored using chemiluminescence (CL) with or without pre-treatment with alpha-Toc. With alpha-Toc pre-treatment, CL values were significantly decreased. The most abundant fatty acid was 22:6n-3. After 3h incubation, 95% of total PUFAs were destroyed by peroxidation, whereas in alpha-Toc pre-treated ROS the percentage was significantly decreased. The results show that the retina has an endogenous lipid soluble antioxidant system. ROS were highly sensitive to oxidative damage, since their fatty acid composition was markedly modified during the lipid peroxidation process. The protective role of alpha-Toc as an antioxidant was evident and it could be used in the treatment of equine ocular diseases in which free radicals are involved.

Effect of diet on the fatty acid and molecular species composition of dog retina phospholipids

Dogs were born to mothers fed commercial diets low or enriched in n-3 fatty acids and raised on those diets until they were about 50 d old. Retinas were removed, lipids were extracted, and total phospholipids were analyzed for fatty acid and molecular species composition. Animals from the low n-3 group had significantly lower retinal levels of 22:6n-3 and higher levels of n-6 fatty acids, especially 20:4n-6 and 22:5n-6. There was no difference in the retinal levels of 18:2n-6, and only small differences were found in saturated and monounsaturated fatty acids. The most dramatic differences in molecular species occurred in 22:6n-3-22:6n-3 (4.7 vs. 0.8%) and 18:0-22:6n-3 (27.6 vs. 14.4%); total molecular species containing 22:6n-3 were significantly lower in the low n-3 group (45.5 vs. 24.0%). Molecular species containing 20:4n-6 and 22:5n-6 were greater in the low n-3 animals (13.0 vs. 25.7%), as were molecular species containing only saturated and monounsaturated fatty acids (40.8 vs. 35.4%). These results show that modest differences in the amount of n-3 fatty acids in the diets of dogs can have profound effects on the fatty acid and molecular species composition of their retinas.

Ultrastructural changes of the retina and the retinal pigment epithelium in Briard dogs with hereditary congenital night blindness and partial day blindness

The offspring of two Briard dogs (brother and sister) with congenital, clinically stationary night blindness showed an aggravation of the disease with severe impairment of day vision in addition to night blindness. This ultrastructural study was performed on four such second generation puppies at the age of 4 months. The neuroretina and retinal pigment epithelium (RPE) from four locations were studied: the central area (immediately temporal to the optic disc); the centre of the tapetal area; the upper periphery (border of tapetal area); and the lower periphery (non-tapetal area). The RPE showed large inclusions, seemingly lipid in nature, mainly in the central and tapetal areas of the retina. Small, membrane bound, electron-dense inclusions were scattered in the RPE cytoplasm in all areas examined. The small inclusions were found to be less numerous in normal than in affected dogs and may be lysosomal in nature. Forty to fifty percent of the rod outer segments in the tapetal area showed disorientation of the disc membranes, whereas the corresponding figures were 20-40% in the central and lower peripheral areas and 6-25% in the upper peripheral area. No structural abnormalities were found in the rod inner segments or synaptic bodies. The cones were better preserved. The inner retina appeared normal. These electron microscopic findings seem to correspond to a previously published electrophysiologic evaluation, indicating a defective and delayed rod function (virtually no scotopic a- and b-waves), a better preserved cone function (photopic flicker responses present, although reduced) and impaired RPE activity (a prominent, slow negative potential of long latency at the site of the c-wave). It appears that these Briard dogs, showing structural changes of the rod outer segments in addition to pigment epithelial inclusions, mainly located in the posterior pole, comprise a pigment epitheliopathy and retinopathy morphologically different from other hereditary canine retinopathies that have been described earlier in the literature and different from animal models of congenital night blindness.

Conservation of docosahexaenoic acid in the retina

Over the last several years, evidence has accumulated that n-3 fatty acids, particularly 22:6n-3, are essential for the development of the structure and function of the visual system. The importance of 22:6n-3 is reflected in the tenacious manner in which the retina conserves this fatty acid during n-3 deficiency. We have shown that conservation is achieved by recycling 22:6n-3 within the retina or between the retina and the pigment epithelium. Within the retina, recycling could be accomplished by deacylation-reacylation reactions (Louie et al., 1991; Zimmerman and Keys, 1988). Recycling between the retina and the RPE may be achieved through specific transport proteins, possibly interphotoreceptor retinoid-binding protein (Bazan et al., 1985) and/or apolipoprotein E (Bazan et al., 1991).