Choline-dependent metabolism in PNS myelination

ABSTRACT Cells have a limited capacity to synthesize choline, thus cells depend on protein transporters to import choline. Choline is used to synthesize phosphatidylcholine, from which structural lipid components of myelin are synthesized. Phosphatidylcholine is also metabolized to generate phosphotidylinositols, whose phosphorylated derivatives are important signaling lipids that regulate myelination. Choline is involved in synthesis of the universal methyl donor, S-adenosylmethionine (SAM) for histone and DNA methylation, thus regulating gene expression. Considering the position of choline at the crossroad for the biosynthesis of phospholipids and epigenetic regulation, we have very little to no understanding of the regulation of choline import and choline-dependent metabolism in myelinating glial cells. Choline transporter for Schwann cells has not been identified. We have identified choline-like-transporter 1 (CTL1) as an important regulator of Schwann cell myelination. CTL1 deletion in Schwann cells (CTL1sc-KO) results in early onset of focal hyper-myelination in the PNS. Biochemical analysis revealed an overall decrease in choline-derived phospholipids in the myelin. Furthermore, CTL1 loss impaired myelin gene expression and exhibited altered DNA modifications in Schwann cells. From these observations, we hypothesize that CTL1 is a Schwann cell choline transporter. We also hypothesize that choline-dependent metabolism feeds into the phospholipid signaling and epigenetic modifications that are important for myelination. To this end, we will investigate three aspects of choline metabolism in Schwann cell myelination. Aim 1 will test the hypothesis that CTL1 is a Schwann cell choline transporter. MALDI-TOF and tandem mass spectrometry will be performed to directly measure choline import into CTL1sc-KO Schwann cells. Impact of CTL1 loss on phosphatidylcholine synthesis will also be analyzed. In Aim 2, we will test the hypothesis that myelin defects in CTL1sc-KO mice results from imbalance in PI(3,5)P2 and PI(3,4,5)P3 synthesis. This is based on the observation that phosphatidylinositol contents are altered in CTL1sc-KO nerve and the myelination defects resemble those seen in mice with dysregulated PI(3,5)P2 and PI(3,4,5)P3 synthesis. Aim 3 will test the hypothesis that CTL1 loss alters gene expression in Schwann cells by modulating histone and DNA methylation. Perturbed lipid metabolism, including choline, is an underlying mechanism in many hereditary diseases associated with PNS myelination defects. Furthermore, dietary supplement of phospholipids has been considered as a potential therapeutic option for treating PNS neuropathies. Therefore, results from this study will provide important insights into understanding the implication of choline metabolism in developing therapeutic strategies to treat PNS neuropathies.