Impairing committed cholesterol biosynthesis in white matter astrocytes, but not grey matter astrocytes, enhances in vitro myelination

Abstract

Remyelination is a regenerative process that is essential to recover saltatory conduction and to prevent neurodegeneration upon demyelination. The formation of new myelin involves the differentiation of oligodendrocyte progenitor cells (OPCs) toward oligodendrocytes and requires high amounts of cholesterol. Astrocytes (ASTRs) modulate remyelination by supplying lipids to oligodendrocytes. Remarkably, remyelination is more efficient in grey matter (GM) than in white matter (WM), which may relate to regional differences in ASTR subtype. Here, we show that a feeding layer of gmASTRs was more supportive to in vitro myelination than a feeding layer of wmASTRs. While conditioned medium from both gmASTRs and wmASTRs accelerated gmOPC differentiation, wmOPC differentiation is enhanced by secreted factors from gmASTRs, but not wmASTRs. In vitro analyses revealed that gmASTRs secreted more cholesterol than wmASTRs. Cholesterol efflux from both ASTR types was reduced upon exposure to pro‐inflammatory cytokines, which was mediated via cholesterol transporter ABCA1, but not ABCG1, and correlated with a minor reduction of myelin membrane formation by oligodendrocytes. Surprisingly, a wmASTR knockdown of Fdft1 encoding for squalene synthase (SQS), an enzyme essential for the first committed step in cholesterol biosynthesis, enhanced in vitro myelination. Reduced secretion of interleukin‐1β likely by enhanced isoprenylation, and increased unsaturated fatty acid synthesis, both pathways upstream of SQS, likely masked the effect of reduced levels of ASTR‐derived cholesterol. Hence, our findings indicate that gmASTRs export more cholesterol and are more supportive to myelination than wmASTRs, but specific inhibition of cholesterol biosynthesis in ASTRs is beneficial for wmASTR‐mediated modulation of myelination.

Macroglial diversity: white and grey areas and relevance to remyelination

“…Hence, while myelination in WM relies more on lipids supplied by ASTRs, gmASTRs actually appear better equipped for the supply of cholesterol. Surprisingly, inhibition of committed cholesterol biosynthesis in wmAS-TRs but not gmASTRs, increases in vitro myelination [257]. As cholesterol biosynthesis is intertwined with unsaturated fatty acid and non-sterol isoprenoid biosynthesis [258][259][260], their upregulated synthesis upon blocking committed cholesterol synthesis may have obscured the effect of decreased cholesterol levels.…”

“…Therefore, when studying the effect of potential remyelination promoting therapeutics, research should take into account regionspecific outcomes. For example, modulating committed cholesterol synthesis is supportive for in vitro myelination when blocked in wmASTRs while having no effect when blocked in gmASTRs [257]. Although not studied yet, this may imply that enhancing committed cholesterol synthesis in ASTRs may actually be detrimental for remyelination in WM but not in GM.…”

“…Strikingly, when blocking lipid biosynthesis in ASTRs during development, hypomyelination is more evident in WM than in GM [61], indicating that developmental myelination in WM depends more on ASTR-derived lipids. In addition, primary gmASTRs export more cholesterol and are more supportive for in vitro myelination than wmASTRs [257]. Hence, while myelination in WM relies more on lipids supplied by ASTRs, gmASTRs actually appear better equipped for the supply of cholesterol.…”

“…As cholesterol biosynthesis is intertwined with unsaturated fatty acid and non-sterol isoprenoid biosynthesis [258][259][260], their upregulated synthesis upon blocking committed cholesterol synthesis may have obscured the effect of decreased cholesterol levels. In fact, an increase in non-sterol isoprenoid synthesis increases isoprenylation, which reduces the release of pro-inflammatory cytokine IL1β from cells, including ASTRs [257,261] and likely also the release of other cytokines [262]. Therefore, modulating lipid biosynthesis in wmASTRs but not gmASTRs, alters the inflammatory microenvironment in WM, which affects wmOPC differentiation.…”

Glial Cells Promote Myelin Formation and Elimination

“…Intriguingly, lipid availability may vary with regional astrocyte heterogeneity. Gray matter astrocytes may secrete more cholesterol than white matter astrocytes, and inhibiting cholesterol synthesis from white matter-derived astrocytes improved in vitro myelination ( Werkman et al, 2020 ), raising the possibility that different populations of astrocytes provide different support for nearby oligodendrocytes. Together, these data extend the known role of astrocytes in providing cholesterol to neurons to promote synapse formation ( Mauch et al, 2001 ) to include oligodendrocytes to enable myelin formation.…”