In vivo effect of mutant ELOVL4 on the expression and function of wild-type ELOVL4

PURPOSE

Mutations in the elongation of very long chain fatty acids 4 (ELOVL4) gene cause human Stargardt's macular dystrophy 3 (STGD3), a juvenile onset dominant form of macular degeneration. To understand the role of the ELOVL4 protein in retinal function, several mouse models have been developed by using transgenic (TG), knock-in (Elovl4(+/mut)), and knockout (Elovl4(+/-)) approaches. Here we analyzed quantitatively the ELOVL4 protein and its enzymatic products (very long chain saturated fatty acid [VLC-FA] and VLC-polyunsaturated fatty acid [VLC-PUFA]) in the retinas of 8 to 10-week-old TG1(+), TG2(+), and Elovl4(+/mut) mice that harbor the mutant ELOVL4 and compared them to their wild-type littermates and Elovl4(+/-) that do not express the mutant protein. We also analyzed skin from these mice to gain insight into the pathogenesis resulting from the ELOVL4 mutation.

METHODS

ELOVL4 protein localization in the retina was determined by immunohistochemistry. Levels of wild-type ELOVL4 protein in skin and retinas were determined by Western blotting. Total lipids from skin and retinas were measured by gas chromatography-mass spectrometry (GC-MS). Retinal glycerophosphatidylcholines (PC) were analyzed by tandem mass spectrometry.

RESULTS

Immunohistochemical and Western analysis indicated that wild-type ELOVL4 protein was reduced in heterozygous Elovl4(+/mut) and Elovl4(+/-) retinas, but not in TG2(+) retinas. We found that VLC-FA was reduced by 50% in the skin of Elovl4(+/-) and by 60% to 65% in Elovl4(+/mut). We found VLC-PUFA levels at ∼ 50% in both the retinas, and wild-type levels of VLC-PUFA in TG2(+) retinas.

CONCLUSIONS

We conclude that the presence of the mutant ELOVL4 does not affect the function of wild-type ELOVL4 in the fully developed 8- to 10-week-old retinas.

Inhibition of de novo ceramide biosynthesis by FTY720 protects rat retina from light-induced degeneration

Light-induced retinal degeneration (LIRD) in albino rats causes apoptotic photoreceptor cell death. Ceramide is a second messenger for apoptosis. We tested whether increases in ceramide mediate photoreceptor apoptosis in LIRD and if inhibition of ceramide synthesis protects the retina. Sprague-Dawley rats were exposed to 2,700 lux white light for 6 h, and the retinal levels of ceramide and its intermediary metabolites were measured by GC-MS or electrospray ionization tandem mass spectrometry. Enzymes of the de novo biosynthetic and sphingomyelinase pathways of ceramide generation were assayed, and gene expression was measured. The dosage and temporal effect of the ceramide synthase inhibitor FTY720 on the LIRD retina were measured by histological and functional analyses. Retinal ceramide levels increased coincident with the increase of dihydroceramide at various time points after light stress. Light stress in retina induces ceramide generation predominantly through the de novo pathway, which was prevented by systemic administration of FTY720 (10 mg/kg) leading to the protection of retinal structure and function. The neuroprotection of FTY720 was independent of its immunosuppressive action. We conclude that ceramide increase by de novo biosynthesis mediates photoreceptor apoptosis in the LIRD model and that inhibition of ceramide production protects the retina against light stress.

Expression and localization of CERKL in the mammalian retina, its response to light-stress, and relationship with NeuroD1 gene

Mutations in the Ceramide kinase like (CERKL) gene are associated with retinitis pigmentosa (RP26) and cone-rod dystrophy. CERKL is homologous to Ceramide kinase (CERK), and its function is still unknown. The purpose of this study was to test the expression and distribution of this gene and its protein in rat and in mouse tissues, in light-stressed rat retinas and in the retinas of NeuroD1 knock-out mice to understand the role of CERKL in the retina. Expression of Cerkl and Cerk mRNA was determined by quantitative RT-PCR. Expression of the protein was determined by Western blotting with anti-CERKL antibody. Localization of the protein was determined by using immunofluorescence microscopy. With qRT-PCR, we revealed that the relative mRNA expression of Cerkl was the highest in the retina among all the rat tissue tested; it was >10-fold higher than in the brain. On the other hand, Cerk has ubiquitous expression and its relative abundance is >2 fold of Cerkl in the retina. Cerkl was expressed minimally in the developing mouse eyes and reached a peak at retinal maturity at 2 months. Western blots of retinal tissues revealed two major CERKL protein bands: 59 kDa (C1) and 37 kDa (C2). However, only C2 CERKL was found in the rat retinal rod outer segment (ROS) at level of that was not changed in light vs. dark adaptation. In the light-stressed retina, expression of Cerkl mRNA increased significantly, which was reflected in only on C2 CERKL protein. The CERKL protein localized prominently to the ganglion cells, inner nuclear layers (INL), retinal pigment epithelial (RPE) cells, and photoreceptor inner segments in the retinal sections. Nuclear localization of CERKL was not affected in RPE, INL and the ganglion cell layers in the light-stressed retina; however, the perinuclear and outer segment locations appear to be altered. In the NeuroD1 knock-out mouse retina, the expression of Cerkl mRNA and protein decreased and that decrease also pertains to C2 CERKL. In conclusion, the retina had the highest level of Cerkl mRNA and protein expression, which reached its maximum in the adult retina; CERKL localized to ROS and RPE cells and the light-adaptation did not change the level of CERKL in ROS; light-stress induced Cerkl expression in the retina; and its expression decreased in NeuroD1 knock-out retina. Thus, CERKL may be important for the stress responses and protection of photoreceptor cells.

X-box binding protein 1 is essential for the anti-oxidant defense and cell survival in the retinal pigment epithelium

Damage to the retinal pigment epithelium (RPE) is an early event in the pathogenesis of age-related macular degeneration (AMD). X-box binding protein 1 (XBP1) is a key transcription factor that regulates endoplasmic reticulum (ER) homeostasis and cell survival. This study aimed to delineate the role of endogenous XBP1 in the RPE. Our results show that in a rat model of light-induced retinal degeneration, XBP1 activation was suppressed in the RPE/choroid complex, accompanied by decreased anti-oxidant genes and increased oxidative stress. Knockdown of XBP1 by siRNA resulted in reduced expression of SOD1, SOD2, catalase, and glutathione synthase and sensitized RPE cells to oxidative damage. Using Cre/LoxP system, we generated a mouse line that lacks XBP1 only in RPE cells. Compared to wildtype littermates, RPE-XBP1 KO mice expressed less SOD1, SOD2, and catalase in the RPE, and had increased oxidative stress. At age 3 months and older, these mice exhibited apoptosis of RPE cells, decreased number of cone photoreceptors, shortened photoreceptor outer segment, reduced ONL thickness, and deficit in retinal function. Electron microscopy showed abnormal ultrastructure, Bruch's membrane thickening, and disrupted basal membrane infolding in XBP1-deficient RPE. These results indicate that XBP1 is an important gene involved in regulation of the anti-oxidant defense in the RPE, and that impaired activation of XBP1 may contribute to RPE dysfunction and cell death during retinal degeneration and AMD.

Retinal sphingolipids and their very-long-chain fatty acid-containing species

PURPOSE

Recent evidence suggests that ceramide metabolism plays an important role in retinal photoreceptor cell survival and apoptosis. The purpose of this study was to characterize sphingolipids in the retina with special emphasis on the very-long-chain-containing saturated (VLC-FA) and polyunsaturated (VLC-PUFA) fatty acid-containing species. The VLC-FAs and VLC-PUFAs are synthesized by the ELOVL4 protein, which is involved in human Stargardt's macular dystrophy type 3 (STGD3).

METHODS

Total lipids were extracted from retina and other tissues, and different sphingolipid classes were isolated and purified using various combinations of liquid- and solid-phase separation. Purified sphingolipids were analyzed by high-performance thin layer chromatography (HPTLC), gas chromatography (GC), and GC-MS (GC-mass spectrometry).

RESULTS

Nonsialylated sphingolipids (NSLs) comprised approximately 3.5% of total retinal lipids of which 70% was sphingomyelin. Ceramide and glycosylceramides (GCs) constituted<or=1% of total retinal lipids. Gangliosides (GGs), on the other hand, comprised approximately 3.0% of total retinal lipids. Fatty acid analysis of retinal NSLs indicated an abundance of saturated fatty acids, with the presence of VLC-FAs but not of VLC-PUFAs beyond 24 carbons. However, GG had significant levels of unsaturated, polyunsaturated, and VLC-PUFAs. Retinal rod outer segments (ROS) contained approximately 1% each of NSL and GG, and their fatty acid profile was not very different from whole retinal NSL and GG, respectively.

CONCLUSIONS

Retina has a total of 6% to 7% fatty acids that are N-linked to a sphingosine, which would be 11 to 13 mole % in comparison to phospholipids. The presence of VLC-FAs and VLC-PUFAs in retinal sphingolipids indicates that they may play role in ELOVL4-mediated Stargardt 3.

Curcumin protects retinal cells from light-and oxidant stress-induced cell death

Age-related macular degeneration (AMD) is a complex disease that has potential involvement of inflammatory and oxidative stress-related pathways in its pathogenesis. In search of effective therapeutic agents, we tested curcumin, a naturally occurring compound with known anti-inflammatory and antioxidative properties, in a rat model of light-induced retinal degeneration (LIRD) and in retina-derived cell lines. We hypothesized that any compound effective against LIRD, which involves significant oxidative stress and inflammation, would be a candidate for further characterization for its potential application in AMD. We observed significant retinal neuroprotection in rats fed diets supplemented with curcumin (0.2% in diet) for 2 weeks. The mechanism of retinal protection from LIRD by curcumin involves inhibition of NF-kappaB activation and down-regulation of cellular inflammatory genes. When tested on retina-derived cell lines (661W and ARPE-19), pretreatment of curcumin protected these cells from H(2)O(2)-induced cell death by up-regulating cellular protective enzymes, such as HO-1, thioredoxin. Since, curcumin with its pleiotropic activities can modulate the expression and activation of many cellular regulatory proteins such as NF-kappaB, AKT, NRF2, and growth factors, which in turn inhibit cellular inflammatory responses and protect cells; we speculate that curcumin would be an effective nutraceutical compound for preventive and augmentative therapy of AMD.