Cholesterol Biosynthesis Supports Myelin Gene Expression and Axon Ensheathment through Modulation of P13K/Akt/mTor Signaling

Myelin, which ensheaths and insulates axons, is a specialized membrane highly enriched with cholesterol. During myelin formation, cholesterol influences membrane fluidity, associates with myelin proteins such as myelin proteolipid protein, and assembles lipid-rich microdomains within membranes. Surprisingly, cholesterol also is required by oligodendrocytes, glial cells that make myelin, to express myelin genes and wrap axons. How cholesterol mediates these distinct features of oligodendrocyte development is not known. One possibility is that cholesterol promotes myelination by facilitating signal transduction within the cell, because lipid-rich microdomains function as assembly points for signaling molecules. Signaling cascades that localize to cholesterol-rich regions of the plasma membrane include the PI3K/Akt pathway, which acts upstream of mechanistic target of rapamycin (mTOR), a major driver of myelination. Through manipulation of cholesterol levels and PI3K/Akt/mTOR signaling in zebrafish, we discovered that mTOR kinase activity in oligodendrocytes requires cholesterol. Drawing on a combination of pharmacological and rescue experiments, we provide evidence that mTOR kinase activity is required for cholesterol-mediated myelin gene expression. On the other hand, cholesterol-dependent axon ensheathment is mediated by Akt signaling, independent of mTOR kinase activity. Our data reveal that cholesterol-dependent myelin gene expression and axon ensheathment are facilitated by distinct signaling cascades downstream of Akt. Because mTOR promotes cholesterol synthesis, our data raise the possibility that cholesterol synthesis and mTOR signaling engage in positive feedback to promote the formation of myelin membrane.