Deciphering the Role of Long-chain Acyl-CoA Synthetase 6 in Brain Lipid Metabolism and Neuroprotection

The omega-3 fatty acid, docosahexaenoic acid (DHA), is highly enriched in the central nervous system (CNS) and thought to protect against neurological dysfunction. Due to its neuroprotective nature, DHA is widely considered a medicinal preventative strategy and treatment. However, while dietary DHA supplementation increases DHA levels in peripheral tissues, it often fails to increase brain DHA levels. The indirect relationship between dietary-DHA and brain-DHA enrichment highlights the existence of unique mechanisms that allow DHA to be metabolized in the brain. The mechanisms regulating DHA enrichment in the brain remain unclear. The purpose of this dissertation was to answer fundamental questions about the mechanisms that regulate brain DHA metabolism by testing the hypothesis that a specific fatty acid metabolizing enzyme, long-chain acyl-CoA synthetase isoform 6 (ACSL6), is required for enrichment of DHA into the brain. As reviewed in my first-author literature review published in PLEFA in September 2020, ACSL6 is one of a large family of 26 acyl-CoA synthetase (ACS) enzymes that catalyze the initial step in cellular fatty acid metabolism and serve as potential master regulators of brain phospholipid acyl-chain diversity. Of all the ACSs, ACSL6 is almost exclusively expressed in the CNS and has been shown to have substrate preference for DHA. To test our hypothesis, we genetically targeted ACSL6 to create a novel conditional ACSL6-deficient mouse (Acsl6-/-). We discovered that whole-body Acsl6 deletion results in a significant 35-72% reduction in DHA-containing phospholipids in the CNS. Our initial findings for this animal model were published as my first first-author research article in PNAS in October of 2018, and our findings were of such great interest that the article was mentioned on the cover of the issue and was accompanied by a complementary commentary highlighting the importance of our discovery. We then went on to combine in situ fluorescence hybridization, several genetically targeted mouse models, lipidomics, and MALDI lipid imaging to demonstrated that ACSL6 in neurons regulates the enrichment of DHA in the brain. We also have shown that ACSL6 expressed in astrocytes does alter membrane acyl-chain content, but does not impact DHA, which we mechanistically explain is due to cell-type specific expression of ACSL6 variants that contain altered fatty acid binding domains. After many behavioral assessments, we found that ACSL6 knockout mice were hyporesponsive to auditory and foot-shock sensory stimulation, exhibited hyperlocomotion, and showed altered short-term working spatial memory. Acsl6-/- mice displayed exacerbated astrogliosis in an age-dependent manner and accompanied with increased inflammation and gliosis. This neuroinflammation occurred independent of changes in pro- and anti-inflammatory lipid mediators suggesting there is an alternative mechanism triggering the neuroinflammation. RNA-seq data revealed downregulation of synaptic proteins' gene expression in Acsl6-/- mice as early as 2 months of age. Thus, we hypothesized that ACSL6-mediated DHA deficiency results in gradual synaptic function deregulation that contributes to increased neuroinflammation during aging. These age-dependent findings are currently under revisions for publication. Together, these data identified ACSL6 in neurons as an important regulator of DHA metabolism that plays a neuroprotective role during aging and on memory and motor function. This dissertation work has led to the discovery of an important and fundamental mechanism by which the parenchyma of the central nervous system controls fatty acid metabolism, particularly for the neuroprotective omega-3 fatty acid DHA