Age-associated decline in regeneration capacity limits the restoration of nervous system functionality after injury. In a model for demyelination, we found that old mice fail to resolve the inflammatory response initiated after myelin damage. Aged phagocytes accumulated excessive amounts of myelin debris, which triggered cholesterol crystal formation and phagolysosomal membrane rupture and stimulated inflammasomes. Myelin debris clearance required cholesterol transporters, including apolipoprotein E. Stimulation of reverse cholesterol transport was sufficient to restore the capacity of old mice to remyelinate lesioned tissue. Thus, cholesterol-rich myelin debris can overwhelm the efflux capacity of phagocytes, resulting in a phase transition of cholesterol into crystals and thereby inducing a maladaptive immune response that impedes tissue regeneration.

The repair of inflamed, demyelinated lesions as in multiple sclerosis necessitates the clearance of cholesterol-rich myelin debris by microglia/macrophages and the switch from a pro-inflammatory to an anti-inflammatory lesion environment. Subsequently, oligodendrocytes increase cholesterol levels as a prerequisite for synthesizing new myelin membranes. We hypothesized that lesion resolution is regulated by the fate of cholesterol from damaged myelin combined with oligodendroglial sterol synthesis. By integrating expression profiling, genetics, and comprehensive phenotyping, we found that paradoxically sterol synthesis in myelin-phagocytosing microglia/macrophages determines repair of acutely demyelinated lesions. Rather than producing cholesterol, microglia/macrophages synthesized desmosterol, the immediate cholesterol precursor. Desmosterol activated LXR-signaling to resolve inflammation, creating a permissive environment for oligodendrocyte differentiation. Moreover, LXR-target gene products facilitated the efflux of lipid/cholesterol from lipid-laden microglia/macrophages to support remyelination by oligodendrocytes. Consequently, pharmacological stimulation of sterol synthesis boosted repair of demyelinated lesions, suggesting novel therapeutic strategies for myelin repair in multiple sclerosis.

Multiple Sclerosis (MS) is an inflammatory demyelinating disorder in which remyelination failure contributes to persistent disability. Cholesterol is rate-limiting for myelin biogenesis in the developing CNS; however, whether cholesterol insufficiency contributes to remyelination failure in MS, is unclear. Here, we show the relationship between cholesterol, myelination and neurological parameters in mouse models of demyelination and remyelination. In the cuprizone model, acute disease reduces serum cholesterol levels that can be restored by dietary cholesterol. Concomitant with blood-brain barrier impairment, supplemented cholesterol directly supports oligodendrocyte precursor proliferation and differentiation, and restores the balance of growth factors, creating a permissive environment for repair. This leads to attenuated axon damage, enhanced remyelination and improved motor learning. Remarkably, in experimental autoimmune encephalomyelitis, cholesterol supplementation does not exacerbate disease expression. These findings emphasize the safety of dietary cholesterol in inflammatory diseases and point to a previously unrecognized role of cholesterol in promoting repair after demyelinating episodes.

‘Tomacula’ and myelin outfoldings are striking neuropathological features of a diverse group of inherited demyelinating neuropathies. Whereas the underlying genetic defects are well known, the molecular mechanisms of tomacula formation have remained obscure. We hypothesized that they are caused by uncontrolled, excessive myelin membrane growth, a process, which is regulated in normal development by neuregulin-1/ErbB2, PI3 Kinase signalling and ERK/MAPK signalling. Here, we demonstrate by targeted disruption of Pten in Schwann cells that hyperactivation of the endogenous PI3 Kinase pathway causes focal hypermyelination, myelin outfoldings and tomacula, even when induced in adult animals by tamoxifen, and is associated with progressive peripheral neuropathy. Activated AKT kinase is associated with PtdIns(3,4,5)P3 at paranodal loops and Schmidt–Lanterman incisures. This striking myelin pathology, with features of human CMT type 4B1 and HNPP, is dependent on AKT/mTOR signalling, as evidenced by a significant amelioration of the pathology in mice treated with rapamycin. We suggest that regions of non-compact myelin are under lifelong protection by PTEN against abnormal membrane outgrowth, and that dysregulated phosphoinositide levels play a critical role in the pathology of tomaculous neuropathies.

Volume microscopy at high resolution is increasingly required to better understand cellular functions in the context of three-dimensional assemblies. Focused ion beam (FIB) milling for serial block face imaging in the scanning electron microscope (SEM)is an efficient and fast method to generate such volume data for 3D analysis. Here, we apply this technique at cryo-conditions to image fully hydrated frozen specimen of mouse optic nerves and Bacillus subtilis spores obtained by high-pressure freezing (HPF). We established imaging conditions to directly visualize the ultrastructure in the block face at −150°C by using an in-lens secondary electron (SE) detector. By serial sectioning with a focused ion beam and block face imaging of the optic nerve we obtained a volume as large as X=7.72 µm, Y=5.79 µm and Z=3.81 µm with a lateral pixel size of 7.5 nm and a slice thickness of 30 nm in Z. The intrinsic contrast of membranes was sufficient to distinguish structures like Golgi cisternae, vesicles, endoplasmic reticulum and cristae within mitochondria and allowed for a threedimensional reconstruction of different types of mitochondria within an oligodendrocyte and an astrocytic process. Applying this technique to dormant Bacillus subtilis spores we obtained volumes containing numerous spores and discovered a bright signal in the core, which can not be related to any known structure so far. In summary, we describe the use of cryo FIB-SEM as a tool for direct and fast 3D cryo-imaging of large native frozen samples including tissues.

Duplication of PLP1 (proteolipid protein gene 1) and the subsequent overexpression of the myelin protein PLP (also known as DM20) in oligodendrocytes is the most frequent cause of Pelizaeus-Merzbacher disease (PMD), a fatal leukodystrophy without therapeutic options. PLP binds cholesterol and is contained within membrane lipid raft microdomains. Cholesterol availability is the rate-limiting factor of central nervous system myelin synthesis. Transgenic mice with extra copies of the Plp1 gene are accurate models of PMD. Dysmyelination followed by demyelination, secondary inflammation and axon damage contribute to the severe motor impairment in these mice. The finding that in Plp1-transgenic oligodendrocytes, PLP and cholesterol accumulate in late endosomes and lysosomes (endo/lysosomes), prompted us to further investigate the role of cholesterol in PMD. Here we show that cholesterol itself promotes normal PLP trafficking and that dietary cholesterol influences PMD pathology. In a preclinical trial, PMD mice were fed a cholesterol-enriched diet. This restored oligodendrocyte numbers and ameliorated intracellular PLP accumulation. Moreover, myelin content increased, inflammation and gliosis were reduced and motor defects improved. Even after onset of clinical symptoms, cholesterol treatment prevented disease progression. Dietary cholesterol did not reduce Plp1 overexpression but facilitated incorporation of PLP into myelin membranes. These findings may have implications for therapeutic interventions in patients with PMD.

SummaryBreakdown of myelin sheaths is a pathological hallmark of several autoimmune diseases of the nervous system. We employed autoantibody-mediated animal models of demyelinating diseases, including a rat model of neuromyelitis optica (NMO), to target myelin and found that myelin lamellae are broken down into vesicular structures at the innermost region of the myelin sheath. We demonstrated that myelin basic proteins (MBP), which form a polymer in between the myelin membrane layers, are targeted in these models. Elevation of intracellular Ca2+ levels resulted in MBP network disassembly and myelin vesiculation. We propose that the aberrant phase transition of MBP molecules from their cohesive to soluble and non-adhesive state is a mechanism triggering myelin breakdown in NMO and possibly in other demyelinating diseases.

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