Function of Sphingoid Bases in Brain Health and Disease

Project Summary Sphingolipid metabolism is fundamental for the function of the nervous system. Mutations in the enzymes of sphingolipid metabolism lead to severe neurological disorders such as sphingolipidoses and hereditary neuropathy, underscoring the essential roles of sphingolipid metabolism in the brain. Ceramide synthase 1 (CerS1) is the main neuronal isoform of the enzyme that synthesizes the common precursor for all sphingolipids, i.e., ceramide. Catalytically inactive CerS1 mutations in human and mice lead to serious neurological disorders, myoclonic epilepsy with mental retardation and cerebellar neurodegeneration with ataxia. Our previous work has shown that neurotoxicity in CerS1 deficiency is due to the increased levels of sphingolipid bases, dihydrosphingosine and sphingosine and their phosphates, dihydrosphingosine-1-phosphate and sphingosine- 1-phosphate. Importantly, increased levels of the sphingolipid bases were shown in complex neurological disorders such as Alzheimer’s disease. A critical knowledge gap and barrier to progress is the lack of a thorough understanding of the mechanism underlying toxicity of those sphingolipid metabolites. To address this shortcoming, we developed an innovative transgenic mouse model to investigate the mechanism underlying sphingoid base-induced neurotoxicity in neurodegenerative diseases. In this transgenic model, a different ceramide synthase isoform, CerS2, is ectopically expressed in the neurons of the CerS1-deficient mice. The CerS2 transgene rescued the neurodegenerative phenotype and restored the physiological levels of the sphingoid bases and phosphates. The levels of the individual ceramide species in the CerS2 transgenic, on the other hand, were the same as in the CerS1 mutant. Using this mouse model, we will pursue complementary in vivo and in vitro approaches to dissect autonomous neuronal effects of sphingoid bases elevation on gene expression and sphingolipid metabolism from trans-effects on glia, both of which can contribute to neurodegeneration. In this proposal we will address the hypothesis that neuronal homeostasis of sphingoid bases and their phosphate derivatives is essential for neuronal health and, if dysregulated, induces neuronal dysfunction and activation of glia. We will: a) test that upregulation of the brain levels of sphingoid bases and phosphates in CerS1-deficient mutant mice alters the transcriptomes of neurons and glia over time by affecting neuronal maturation leading to defective neuronal differentiation and the activation of glia (Aim 1); b) test how CerS1 deficiency and sphingolipid base upregulation in neurons affect their intracellular membrane and lipid trafficking (Aim 2); and c) test that neuronal-induced sphingoid base phosphate signaling leads to the activation of glia (Aim 3).