Very long chain polyunsaturated fatty acids and rod cell structure and function

Retina and RPE lipid profile changes linked with ABCA4 associated Stargardt's maculopathy

Stargardt maculopathy, caused predominantly by mutations in the ABCA4 gene, is characterized by an accumulation of non-degradable visual pigment derivative, lipofuscin, in the retinal pigment epithelium (RPE) - resulting in RPE atrophy. RPE is a monolayer tissue located adjacent to retinal photoreceptors and regulates their health and functioning; RPE atrophy triggers photoreceptor cell death and vision loss in Stargardt patients. Previously, ABCA4 mutations in photoreceptors were thought to be the major contributor to lipid homeostasis defects in the eye. Recently, we demonstrated that ABCA4 loss of function in the RPE leads to cell-autonomous lipid homeostasis defects. Our work underscores that an incomplete understanding of lipid metabolism and lipid-mediated signaling in the retina and RPE are potential causes for lacking treatments for this disease. Here we report altered lipidomic in mouse and human Stargardt models. This work provides the basis for therapeutics that aim to restore lipid homeostasis in the retina and the RPE.

Influence of very-long-chain polyunsaturated fatty acids on membrane structure and dynamics

The unique attributes of very-long-chain polyunsaturated fatty acids (VLC-PUFAs), their long carbon chains (n > 24) and high degree of unsaturation, impart unique chemical and physical properties to this class of fatty acids. The changes imparted by VLC-PUFA 32:6 n-3 on lipid packing and the compression moduli of model membranes were evaluated from π-A isotherms of VLC-PUFA in 1,2-distearoyl-sn-3-glycero-phosphocholine (DSPC) lipid monolayers. To compare the attractive or repulsive forces between VLC-PUFA and DSPC lipid monolayers, the measured mean molecular areas (MMAs) were compared with the calculated MMAs of an ideal mixture of VLC-PUFA and DSPC. The presence of 0.1, 1, and 10 mol % VLC-PUFA shifted the π-A isotherm to higher MMAs of the lipids comprising the membrane and the observed positive deviations from ideal behavior of the mixed VLC-PUFA:DSPC monolayers correspond to repulsive forces between VLC-PUFAs and DSPC. The MMA of the VLC-PUFA component was estimated using the measured MMAs of DSPC of 47.1 ± 0.7 Å²/molecule, to be 15,000, 1100, and 91 Å²/molecule at 0.1, 1, and 10 mol % VLC-PUFA:DSPC mixtures, respectively. The large MMAs of VLC-PUFA suggest that the docosahexaenoic acid tail reinserts into the membrane and adopts a nonlinear structure in the membrane, which is most pronounced at 0.1 mol % VLC-PUFA. The presence of 0.1 mol % VLC-PUFA:DSPC also significantly increased the compression modulus of the membrane by 28 mN/m compared with a pure DSPC membrane. The influence of VLC-PUFA on lipid "flip-flop" was investigated by sum-frequency vibrational spectroscopy. The incorporation of 0.1 mol % VLC-PUFA increased the DSPC flip-flop rate fourfold. The fact that VLC-PUFA promotes lipid translocation is noteworthy as retinal membranes require a high influx of retinoids which may be facilitated by lipid flip-flop.

The Elovl4 Spinocerebellar Ataxia-34 Mutation 736T>G (p.W246G) Impairs Retinal Function in the Absence of Photoreceptor Degeneration

Elongation of very long chain fatty acids-4 (ELOVL4) is essential for synthesis of very long chain polyunsaturated and saturated fatty acids (VLC-PUFA and VLC-SFA, respectively) of chain length greater than 26 carbons. Mutations in the ELOVL4 gene cause several distinct neurodegenerative diseases including Stargardt-like macular dystrophy (STGD3), spinocerebellar ataxia 34 (SCA34), and a neuro-ichthyotic syndrome with severe seizures and spasticity, as well as erythrokeratitis variabilis (EKV), a skin disorder. However, the relationship between ELOVL4 mutations, its VLC-PUFA and VLC-SFA products, and specific neurological symptoms remains unclear. We generated a knock-in rat line (SCA34-KI) that expresses the 736T>G (p.W246G) form of ELOVL4 that causes human SCA34. Lipids were analyzed by gas chromatography and mass spectrometry. Retinal function was assessed using electroretinography. Retinal integrity was assessed by histology, optical coherence tomography, and immunolabeling. Analysis of retina and skin lipids showed that the W246G mutation selectively impaired synthesis of VLC-SFA, but not VLC-PUFA. Homozygous SCA34-KI rats showed reduced ERG a- and b-wave amplitudes by 90 days of age, particularly for scotopic responses. Anatomical analyses revealed no indication of neurodegeneration in heterozygote or homozygote SCA34-KI rats out to 6-7 months of age. These studies reveal a previously unrecognized role for VLC-SFA in regulating retinal function, particularly transmission from photoreceptors to the inner retina, in the absence of neurodegeneration. Furthermore, these findings suggest that the tissue specificity and symptoms associated with disease-causing ELOVL4 mutations likely arise from selective differences in the ability of the mutant ELOVL4 enzymes to support synthesis of VLC-PUFA and/or VLC-SFA.

Long-term follow-up of autosomal dominant Stargardt macular dystrophy (STGD3) subjects enrolled in a fish oil supplement interventional trial

BACKGROUND

Earlier studies have raised the notion that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) supplementation could be a useful intervention in autosomal dominant Stargardt macular dystrophy (STGD3). We sought to assess whether fish oil supplementation has a beneficial effect on the clinical course of STGD3 secondary to a mutation in the ELOVL4 gene.

MATERIALS AND METHODS

Eleven patients with STGD3 were enrolled in an 8-year open-label, clinical interventional study of over-the-counter fish oil supplements at a recommended daily dose of 650 mg EPA and 350 mg DHA (NCT00420602). Subjects had annual eye examinations with complete imaging, visual function testing, and blood lipid analyses. Compliance with therapy was measured by periodic patient self-report and with serum and red blood cell biomarkers of lipid consumption. Paired sample t-tests were used to measure differences in mean values of visual acuity, lipid biomarkers, and contrast sensitivity obtained at baseline and the last follow-up.

RESULTS

All subjects showed progression of their maculopathy, and we could not discern a beneficial effect of the intervention. Compliance with the recommended fish oil supplement intervention was poor as assessed by patient self-report and biomarkers of lipid consumption.

CONCLUSIONS

Our inability to detect a benefit of fish oil could be the result of small subject numbers, poor compliance, or intervention too late in the course of the disease. We still advise STGD3 patients to consume fish or fish oil regularly, and we recommend that pre-symptomatic children with ELOVL4 mutations should be especially targeted for these interventions.

Elovl4 5-bp deletion does not accelerate cone photoreceptor degeneration in an all-cone mouse

Mutations in the elongation of very long chain fatty acid 4 (ELOVL4) gene cause Stargardt macular dystrophy 3 (STGD3), a rare, juvenile-onset, autosomal dominant form of macular degeneration. Although several mouse models have already been generated to investigate the link between the three identified disease-causing mutations in the ELOVL4 gene, none of these models recapitulates the early-onset cone photoreceptor cell death observed in the macula of STGD3 patients. To address this specifically, we investigated the effect of mutant ELOVL4 in a mouse model with an all-cone retina. Hence, we bred mice carrying the heterozygously mutated Elovl4 gene on the R91W;Nrl^-/- all-cone background and analyzed the retinal lipid composition, morphology, and function over the course of 1 year. We observed a reduction of total phosphatidylcholine-containing very long chain-polyunsaturated fatty acids (PC-VLC-PUFAs) by 39% in the R91W;Nrl^-/-;Elovl4 mice already at 6 weeks of age with a pronounced decline of the longest forms of PC-VLC-PUFAs. Total levels of shorter-chain fatty acids (< C26) remained unaffected. However, this reduction in PC-VLC-PUFA content in the all-cone retina had no impact on morphology or function and did not accelerate retinal degeneration in the R91W;Nrl^-/-;Elovl4 mice. Taken together, mutations in the ELOVL4 gene lead to cone degeneration in humans, whereas mouse models expressing the mutant Elovl4 show predominant rod degeneration. The lack of a phenotype in the all-cone retina expressing the mutant form of the protein supports the view that aberrant function of ELOVL4 is especially detrimental for rods in mice and suggests a more subtle role of VLC-PUFAs for cone maintenance and survival.

Distribution of ELOVL4 in the Developing and Adult Mouse Brain

ELOngation of Very Long chain fatty acids (ELOVL)-4 is essential for the synthesis of very long chain-fatty acids (fatty acids with chain lengths ≥ 28 carbons). The functions of ELOVL4 and its very long-chain fatty acid products are poorly understood at present. However, mutations in ELOVL4 cause neurodevelopmental or neurodegenerative diseases that vary according to the mutation and inheritance pattern. Heterozygous inheritance of different ELOVL4 mutations causes Stargardt-like Macular Dystrophy or Spinocerebellar Ataxia type 34. Homozygous inheritance of ELOVL4 mutations causes more severe disease characterized by seizures, intellectual disability, ichthyosis, and premature death. To better understand ELOVL4 and very long chain fatty acid function in the brain, we examined ELOVL4 expression in the mouse brain between embryonic day 18 and postnatal day 60 by immunolabeling using ELOVL4 and other marker antibodies. ELOVL4 was widely expressed in a region- and cell type-specific manner, and was restricted to cell bodies, consistent with its known localization to endoplasmic reticulum. ELOVL4 labeling was most prominent in gray matter, although labeling also was present in some cells located in white matter. ELOVL4 was widely expressed in the developing brain by embryonic day 18 and was especially pronounced in regions underlying the lateral ventricles and other neurogenic regions. The basal ganglia in particular showed intense ELOVL4 labeling at this stage. In the postnatal brain, cerebral cortex, hippocampus, cerebellum, thalamus, hypothalamus, midbrain, pons, and medulla all showed prominent ELOVL4 labeling, although ELOVL4 distribution was not uniform across all cells or subnuclei within these regions. In contrast, the basal ganglia showed little ELOVL4 labeling in the postnatal brain. Double labeling studies showed that ELOVL4 was primarily expressed by neurons, although presumptive oligodendrocytes located in white matter tracts also showed labeling. Little or no ELOVL4 labeling was present in astrocytes or radial glial cells. These findings suggest that ELOVL4 and its very long chain fatty acid products are important in many parts of the brain and that they are particularly associated with neuronal function. Specific roles for ELOVL4 and its products in oligodendrocytes and myelin and in cellular proliferation, especially during development, are possible.

Mouse Models of Stargardt 3 Dominant Macular Degeneration

Stargardt type 3 macular degeneration is dependent on a dominant defect in a single gene, ELOVL4 (elongase of very long chain fatty acids 4). The encoded enzyme, ELOVL4, is required for the synthesis of very long chain polyunsaturated fatty acids (VLC-PUFAs), a rare class of > C24 lipids. In vitro expression studies suggest that mutated ELOVL4(STGD3) proteins fold improperly, resulting in ER stress and formation of cytosolic aggresomes of wild type and mutant ELOVL4. Although a number of mouse models have been developed to determine whether photoreceptor cell loss in STGD3 results from depletion of VLC-PUFAs, aggresome-dependent cell stress or a combination of these two factors, none of these models adequately recapitulates the disease phenotype in humans. Thus, the precise molecular mechanism by which ELOVL4 mutation causes photoreceptor degeneration in mice and in human patients remains to be characterized. This mini review compares and evaluates current STGD3 mouse models and determines what conclusions can be drawn from past work.