ORMDL3-mediated SPT regulation Coordinates Myelin Sphingolipid and Protein Synthesis in Oligodendrocytes.

Myelin is an essential and highly specialized membrane in the central and peripheral nervous systems that enwraps axons to accelerate electrical transmission and support neuronal health. The generation of this multilamellar structure requires a tightly coordinated synthesis of specific proteins and lipids. Among these are sphingolipids (SLs), which are major components of myelin. SL production is initiated by the serine palmitoyltransferase (SPT) enzyme complex, the rate-limiting enzyme in the de novo SL biosynthesis pathway. ORMDL proteins (ORMDL1-3) are the regulatory subunits of SPT, which, by sensing ceramide levels, tune SL flux. Although ORMDL3 has been linked to asthma and peripheral myelination, its role in CNS myelination and the CNS myelin-making oligodendrocytes (OLGs) remains unclear. We therefore assessed the function of ORMDL3 in OLs by generating a Cnp-Cre -driven, oligodendrocyte-specific Ormdl3 conditional knockout (cKO) mouse model. Loss of Ormdl3 selectively increased myelin SLs, particularly long-chain sulfatides, without altering ceramide or galactosylceramide abundance. These changes are most prominent around postnatal day 35, a developmental period of active myelin turnover. Ultrastructural analysis of optic nerves shows a thicker myelin sheath and increased axon caliber in cKO mice. Unexpectedly, deletion of Ormdl3 also increases levels of major myelin proteins (MBP, MOG, and PLP) and is accompanied by dynamic, region- and sex-dependent regulation of enzymes involved in sulfatide biosynthesis, without altering OLGs number or maturation. Together, for the first time, these findings identify Ormdl3 as a key regulator of SL homeostasis during developmental myelination and suggest that it helps synchronize lipid synthesis with myelin protein expression.