Wnt/β-catenin signaling plays a major role in the development of the nervous system and contributes to neuronal plasticity. However, its role in myelination remains unclear. Here, we identify the Wnt/β-catenin pathway as an essential driver of myelin gene expression. The selective inhibition of Wnt components by small interfering RNA or dominant-negative forms blocks the expression of myelin protein zero (MPZ) and peripheral myelin protein 22 (PMP22) in mouse Schwann cells and proteolipid protein in mouse oligodendrocytes. Moreover, the activation of Wnt signaling by recombinant Wnt1 ligand increases by threefold the transcription of myelin genes and enhances the binding of β-catenin to T-cell factor/lymphoid-enhancer factor transcription factors present in the vicinity of the MPZ and PMP22 promoters. Most important, loss-of-function analyses in zebrafish embryos show,in vivo, a key role for Wnt/β-catenin signaling in the expression of myelin genes and in myelin sheath compaction, both in the peripheral and central nervous systems. Inhibition of Wnt/β-catenin signaling resulted in hypomyelination, without affecting Schwann cell and oligodendrocyte generation or axonal integrity. The present findings attribute to Wnt/β-catenin pathway components an essential role in myelin gene expression and myelinogenesis.

Significance: Liver X receptors (LXRs) α and β are the two major receptors of oxysterols, oxygenated derivatives of cholesterol. They control the homeostasis of cholesterol, an important lipid constituent of myelin sheaths. In the central nervous system, these insulating structures are generated by oligodendrocytes and are stabilized by myelin proteins. Here, we provide evidence of a new role of LXRs in the myelin physiology of the cerebellum. Mice invalidated for both LXRs exhibit alteration in motor coordination and spatial learning linked with myelination deficits. We demonstrated that LXRs intervene both in oligodendroglial cell maturation and in the transcriptional control of myelin gene expression during (re)myelination processes.

Glycogen synthase kinase 3β (GSK3β) inhibitors, especially the mood stabilizer lithium chloride, are also used as neuroprotective or anti-inflammatory agents. We studied the influence of LiCl on the remyelination of peripheral nerves. We showed that the treatment of adult mice with LiCl after facial nerve crush injury stimulated the expression of myelin genes, restored the myelin structure, and accelerated the recovery of whisker movements. LiCl treatment also promoted remyelination of the sciatic nerve after crush. We also demonstrated that peripheral myelin gene MPZ and PMP22 promoter activities, transcripts, and protein levels are stimulated by GSK3β inhibitors (LiCl and SB216763) in Schwann cells as well as in sciatic and facial nerves. LiCl exerts its action in Schwann cells by increasing the amount of β-catenin and provoking its nuclear localization. We showed by ChIP experiments that LiCl treatment drives β-catenin to bind to T-cell factor/lymphoid-enhancer factor response elements identified in myelin genes. Taken together, our findings open perspectives in the treatment of nerve demyelination by administering GSK3β inhibitors such as lithium.

Oxysterols are reactive molecules generated from the oxidation of cholesterol. Their implication in cholesterol homeostasis and in the progression of neurodegenerative disorders is well known, but few data are available for their functions in the peripheral nervous system. Our aim was to study the influence of oxysterols on myelin gene expression and myelin sheath formation in peripheral nerves. We show by gas chromatography/mass spectrometry that Schwann cells and sciatic nerves contain 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol and that they express their biosynthetic enzymes and receptors (liver X receptors LXRα and LXRβ). We demonstrate that oxysterols inhibit peripheral myelin gene expression [myelin protein zero (MPZ) and peripheral myelin protein-22 (PMP22)] in a Schwann cell line. This downregulation is mediated by either LXRα or LXRβ, depending on the promoter context, as suggested by siRNA strategy and chromatin immunoprecipitation assays in Schwann cells and in the sciatic nerve of LXR knock-out mice. Importantly, the knock-out of LXR in mice results in thinner myelin sheaths surrounding the axons. Oxysterols repress myelin genes via two mechanisms: by binding of LXRs to myelin gene promoters and by inhibiting the Wnt/β-catenin pathway that is crucial for the expression of myelin genes. The Wnt signaling components (Disheveled, TCF/LEF, β-catenin) are strongly repressed by oxysterols. Furthermore, the recruitment of β-catenin at the levels of the MPZ and PMP22 promoters is decreased. Our data reveal new endogenous mechanisms for the negative regulation of myelin gene expression, highlight the importance of oxysterols and LXR in peripheral nerve myelination, and open new perspectives of treating demyelinating diseases with LXR agonists.

In several neurodegenerative diseases of the CNS, oligodendrocytes are implicated in an inflammatory process associated with altered levels of oxysterols and inflammatory enzymes such as secreted phospholipase A2 (sPLA2). In view of the scarce literature related to this topic, we investigated oxysterol effects on these myelinating glial cells. Natural oxysterol 25‐hydroxycholesterol (25‐OH; 1 and 10 μM) altered oligodendrocyte cell line (158N) morphology and triggered apoptosis (75% of apoptosis after 72 h). These effects were mimicked by 22(S)‐OH (1 and 10 μM) which does not activate liver X receptor (LXR) but not by a synthetic LXR ligand (T0901317). Therefore, oxysterol‐induced apoptosis appears to be independent of LXR. Interestingly, sPLA2 type IIA (sPLA2‐IIA) over‐expression partially rescued 158N cells from oxysterol‐induced apoptosis. In fact, 25‐OH, 24(S)‐OH, and T0901317 stimulated sPLA2‐IIA promoter and sPLA2 activity in oligodendrocyte cell line. Accordingly, administration of T0901317 to mice enhanced sPLA2 activity in brain extracts by twofold. Short interfering RNA strategy allowed to establish that stimulation of sPLA2‐IIA is mediated by pregnane X receptor (PXR) at high oxysterol concentration (10 μM) and by LXR β at basal oxysterol concentration. Finally, GC coupled to mass spectrometry established that oligodendrocytes contain oxysterols and express their biosynthetic enzymes, suggesting that they may act through autocrine/paracrine mechanism. Our results show the diversity of oxysterol signalling in the CNS and highlight the positive effects of the LXR/PXR pathway which may open new perspectives in the treatment of demyelinating and neurodegenerative diseases.