Highlights
- •Neurons increase cholesterol synthesis in chronic myelin disease and multiple sclerosis
- •Remyelination is facilitated by neuronal and oligodendroglial cholesterol synthesis
- •Neuronal cholesterol augments proliferation of oligodendrocyte precursor cells
Summary
Astrocyte-derived cholesterol supports brain cells under physiological conditions. However, in demyelinating lesions, astrocytes downregulate cholesterol synthesis, and the cholesterol that is essential for remyelination has to originate from other cellular sources. Here, we show that repair following acute versus chronic demyelination involves distinct processes. In particular, in chronic myelin disease, when recycling of lipids is often defective, de novo neuronal cholesterol synthesis is critical for regeneration. By gene expression profiling, genetic loss-of-function experiments, and comprehensive phenotyping, we provide evidence that neurons increase cholesterol synthesis in chronic myelin disease models and in patients with multiple sclerosis (MS). In mouse models, neuronal cholesterol facilitates remyelination specifically by triggering oligodendrocyte precursor cell proliferation. Our data contribute to the understanding of disease progression and have implications for therapeutic strategies in patients with MS.
Introduction
During normal brain development, cholesterol is produced locally by de novo synthesis involving all CNS cells (Berghoff et al., 2021; Camargo et al., 2012; Fünfschilling et al., 2012). Neuronal cholesterol is essential during neurogenesis (Fünfschilling et al., 2012), but the highest rates of cholesterol synthesis in the brain are achieved by oligodendrocytes during post-natal myelination (Dietschy, 2009). The resulting cholesterol-rich myelin enwraps, shields, and insulates axons to enable rapid conduction of neuronal impulses. In the adult brain, cholesterol synthesis is attenuated to low steady-state levels (Dietschy and Turley, 2004).Destruction of lipid-rich myelin in demyelinating diseases such as multiple sclerosis (MS) likely impairs neuronal function by disrupting the axon-myelin unit (Stassart et al., 2018). Remyelination is considered crucial for limiting axon damage and slowing progressive clinical disability. Statin-mediated inhibition of the cholesterol synthesis pathway impairs remyelination (Miron et al., 2009). Previously, we showed that following an acute demyelinating episode, oligodendrocytes import cholesterol for new myelin membranes from damaged myelin that has been recycled by phagocytic microglia (Berghoff et al., 2021). In contrast, oligodendroglial cholesterol synthesis contributes to remyelination following chronic demyelination (Berghoff et al., 2021; Voskuhl et al., 2019). Notably, astrocytes reduce expression of cholesterol synthesis genes following demyelination (Berghoff et al., 2021; Itoh et al., 2018). As in the healthy brain, astrocytes support neurons by providing cholesterol in apolipoprotein E (ApoE)-containing lipoproteins (Dietschy, 2009), and the lack of this support in the diseased CNS contributes to the disruption of CNS cholesterol homeostasis. However, neuronal responses to myelin degeneration with regard to cholesterol metabolism, as well as the role of neuronal cholesterol in remyelination, remain unknown.Here, we show that during chronic myelin disease, neurons increase cholesterol synthesis. Similarly, neurons in MS brain upregulate a gene profile related to cholesterol synthesis and metabolism in non-lesion areas. Our data support the essential role of neuronal cholesterol for remyelination, a role that is likely relevant for MS disease progression.
Results
Loss of Fdft1 in neurons alters white matter cholesterol metabolism
In the adult brain, neuronal synthesis and horizontal transfer from glial cells meet neuronal cholesterol demands. To evaluate neuronal versus glial cholesterol metabolism, we acutely isolated neurons, astrocytes, and oligodendrocytes from cortex or subcortical white matter of adult mice (Figure 1A). The abundance of neuronal mRNA transcripts related to cholesterol metabolism was compared with oligodendrocyte and astrocyte profiles obtained previously (Berghoff et al., 2021). Compared with oligodendrocytes and astrocytes, neurons showed low steady-state expression levels of cholesterol synthesis genes (Figure 1B; Table S1). In contrast, several gene transcripts related to cholesterol import (Apobr, Scarb1, Lrp1), storage (Soat1), and brain export (Cyp46a1) were higher in relative abundance. To assess the relevance of cell-type-specific cholesterol synthesis, we genetically inactivated squalene synthase (SQS; Fdft1 gene), an essential enzyme of the sterol biosynthesis pathway, in adult oligodendrocytes (OL, OLcKO), oligodendrocyte precursor cells (OPC, OPCcKO), astrocytes (AcKO), or neurons (NcKO) (Figures 1C, S1A, and S1B) (Berghoff et al., 2021; Fünfschilling et al., 2012; Saher et al., 2005). Comparable with oligodendroglial and astrocyte mutants (Berghoff et al., 2021), loss of cholesterol synthesis in neurons did not affect peripheral serum cholesterol level or body weight (Figure S1C). In an open-field test, only OLcKO animals showed signs of anxiety (Figures 1D, S1D, and S1E), which were enhanced in OPC/oligodendrocyte double mutants (Figure S1F). Notably, these behavioral changes occurred in the absence of overt myelin/oligodendrocyte deficits (Figure S1G).
