Role of fatty acid metabolism in optic nerve hypoplasia

ABSTRACT Optic nerve hypoplasia (ONH) is a very common congenital optic nerve (ON) disorder and is the leading cause of childhood blindness in developed nations. ONH incidence has increased ~8-fold over the last two decades. ONH is characterized by a thin, underdeveloped ON that often results from secondary loss of retinal ganglion cells (RGCs). The most common prenatal determinants of ONH are a young primiparous mother, an unhealthy maternal lifestyle including alcohol abuse, and nutritional deprivation. We have developed and published murine models of ONH by manipulating the X-linked gene CASK, since CASK mutations in humans are associated with ONH. The ONH pathology of CASK mutant mice recapitulates human ONH, including the timing of pathology onset (after RGC development; i.e., secondary loss) and the non-progressive nature of the pathology. Biochemical experiments show that CASK interacts with metabolic proteins and modulates mitochondrial function. CASK deficiency leads to increased fatty acid oxidation and a deficit of the ω-6 fatty acid arachidonic acid (ARA) in the central nervous system (CNS). ARA deficiency is also observed in other conditions associated with ONH. We hypothesize that ONH results from an early ARA deficit, thus ONH can be exacerbated by perturbing brain ARA metabolism (via astrocyte dysfunction) and ameliorated by dietary ARA supplementation. During the third trimester, ARA is exclusively obtained from the mother; in neonates, brain ARA is also obtained from the diet until adequate enzymatic activity (conversion of the essential fatty acid linoleic acid into ARA) is reached. This post-neonatal shift in ARA acquisition from diet to synthesis may contribute to ONH’s non-progressive nature. In the CNS, fatty acid metabolism (including ARA uptake and production) occurs predominantly in astrocytes. In this proposal we plan to test our hypothesis in two independent ONH mouse models: 1) CASK(+/-) heterozygous knockout mice, and 2) a previously published fetal alcohol syndrome (FAS) mouse model. With these models, we will examine mitochondrial metabolism, oxidative damage and fatty acid metabolic defects in the retina, ON and brain. We will also quantify levels of two ω-fatty acids (docosahexaenoic acid and ARA), as well as phospholipids in the ON of both types of ONH mice. Next, we will genetically disrupt the function of astrocytes (crucial for brain ARA metabolism) in a CASK hypomorph ONH model by complete deletion of CASK in astrocytes. We will investigate if this manipulation exacerbates the metabolic defect and ONH as assessed both morphologically and functionally, using a visual behavioral assay and an innovative electrophysiological tool called Network Response to Visual Excitation (NeRVE). Finally, we will test if ARA supplementation ameliorates ONH in the two models described above. Our study is likely to identify ARA deficiency as the final common pathway that explains ONH’s association with nutritional deprivation, maternal diabetes, infantile cholestasis and FAS. Positive results from ARA supplementation will be readily translatable.