Selective expression ofLDLR andVLDLR in myelinating oligodendrocytes

Mediator MED23 controls oligodendrogenesis and myelination by modulating Sp1/P300-directed gene programs

Gaining the molecular understanding for myelination development and regeneration has been a long-standing goal in neurological research. Mutations in the transcription cofactor Mediator Med23 subunit are often associated with intellectual disability and white matter defects, although the precise functions and mechanisms of Mediator in myelination remain unclear. In this study, we generated a mouse model carrying an Med23Q649R mutation that has been identified in a patient with hypomyelination features. The MED23Q649R mouse model develops white matter thinning and cognitive decline, mimicking common clinical phenotypes. Further, oligodendrocyte-lineage specific Med23 knockout mice verified the important function of MED23 in regulating central nervous system myelination and postinjury remyelination. Utilizing the in vitro cellular differentiation assay, we found that the oligodendrocyte progenitor cells, either carrying the Q649R mutation or lacking Med23, exhibit significant deficits in their capacity to differentiate into mature oligodendrocytes. Gene profiling combined with reporter assays demonstrated that Mediator Med23 controls Sp1-directed gene programs related to oligodendrocyte differentiation and cholesterol metabolism. Integrative analysis demonstrated that Med23 modulates the P300 binding to Sp1-targeted genes, thus orchestrating the H3K27 acetylation and enhancer activation for the oligodendrocyte lineage progression. Collectively, our findings identified the critical role for the Mediator Med23 in oligodendrocyte fate determination and provide mechanistic insights into the myelination pathogenesis associated with MED23 mutations.

Possible roles of deep cortical neurons and oligodendrocytes in the neural basis of human sociality

Sociality is an instinctive property of organisms that live in relation to others and is a complex characteristic of higher order brain functions. However, the evolution of the human brain to acquire higher order brain functions, such as sociality, and the neural basis for executing these functions and their control mechanisms are largely unknown. Several studies have attempted to evaluate how human sociality was acquired during the course of evolution and the mechanisms controlling sociality from a neurodevelopment viewpoint. This review discusses these findings in the context of human brain evolution and the pathophysiology of autism spectrum disorder (ASD). Comparative genomic studies of postmortem primate brains have demonstrated human-specific regulatory mechanisms underlying higher order brain functions, providing evidence for the contribution of oligodendrocytes to human brain function. Functional analyses of the causative genes of ASD in animal models have demonstrated that the neural basis of social behavior is associated with layer 6 (L6) of the neocortex and oligodendrocytes. These findings demonstrate that both neurons and oligodendrocytes contribute to the neural basis and molecular mechanisms underlying human brain evolution and social functioning. This review provides novel insights into sociability and the corresponding neural bases of brain disorders and evolution.

Identification of Key Genes and Regulatory Pathways in Multiple Sclerosis Brain Samples: A Meta-Analysis of Micro-Array Datasets

Multiple sclerosis (MS) is an autoimmune disorder of the central nervous system (CNS) whose aetiology is only partly understood. Investigating the intricate transcriptional changes occurring in MS brains is critical to unravel novel pathogenic mechanisms and therapeutic targets. Unfortunately, this process is often hindered by the difficulty in retrieving an adequate number of samples. However, by merging data from publicly available datasets, it is possible to identify alterations in gene expression profiles and regulatory pathways that were previously overlooked. Here, we merged microarray gene expression profiles obtained from CNS white matter samples taken from MS donors to identify novel differentially expressed genes (DEGs) linked with MS. Data from three independent datasets (GSE38010, GSE32915, and GSE108000) were combined and used to detect novel DEGs using the Stouffer's Z-score method. Corresponding regulatory pathways were analysed using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway databases. Finally, top up- and down-regulated transcripts were validated by real-time quantitative PCR (qPCR) using an independent set of white matter tissue samples obtained from MS donors with different disease subtypes. There were a total of 1446 DEGs, of which 742 were up-regulated and 704 genes were down-regulated. DEGs were associated with several myelin-related pathways and protein metabolism pathways. Validation studies of selected top up- or down-regulated genes highlighted MS subtype-specific differences in the expression of some of the identified genes, underlining a more complex scenario of white matter pathology amongst people afflicted by this devastating disease.

Apolipoprotein E ε4 disrupts oligodendrocyte differentiation by interfering with astrocyte-derived lipid transport

Carriers of the APOE4 (apolipoprotein E ε4) variant of the APOE gene are subject to several age-related health risks, including Alzheimer's disease (AD). The deficient lipid and cholesterol transport capabilities of the APOE4 protein are one reason for the altered risk profile. In particular, APOE4 carriers are at elevated risk for sporadic AD. While deposits o misfolded proteins are present in the AD brain, white matter (WM) myelin is also disturbed. As myelin is a lipid- and cholesterol-rich structure, the connection to APOE makes considerable biological sense. To explore the APOE-WM connection, we have analyzed the impact of human APOE4 on oligodendrocytes (OLs) of the mouse both in vivo and in vitro. We find that APOE proteins is enriched in astrocytes but sparse in OL. In human APOE4 (hAPOE4) knock-in mice, myelin lipid content is increased but the density of major myelin proteins (MBP, MAG, and PLP) is largely unchanged. We also find an unexpected but significant reduction of cell density of the OL lineage (Olig2+ ) and an abnormal accumulation of OL precursors (Nkx 2.2+ ), suggesting a disruption of OL differentiation. Gene ontology analysis of an existing RNA-seq dataset confirms a robust transcriptional response to the altered chemistry of the hAPOE4 mouse brain. In culture, the uptake of astrocyte-derived APOE during Lovastatin-mediated depletion of cholesterol synthesis is sufficient to sustain OL differentiation. While endogenous hAPOE protein isoforms have no effects on OL development, exogenous hAPOE4 abolishes the ability of very low-density lipoprotein to restore myelination in Apoe-deficient, cholesterol-depleted OL. Our data suggest that APOE4 impairs myelination in the aging brain by interrupting the delivery of astrocyte-derived lipids to the oligodendrocytes. We propose that high myelin turnover and OL exhaustion found in APOE4 carriers is a likely explanation for the APOE-dependent myelin phenotypes of the AD brain.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer's disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell-cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Nimodipine Exerts Beneficial Effects on the Rat Oligodendrocyte Cell Line OLN-93

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS). Therapy is currently limited to drugs that interfere with the immune system; treatment options that primarily mediate neuroprotection and prevent neurodegeneration are not available. Here, we studied the effects of nimodipine on the rat cell line OLN-93, which resembles young mature oligodendrocytes. Nimodipine is a dihydropyridine that blocks the voltage-gated L-type calcium channel family members Cav1.2 and Cav1.3. Our data show that the treatment of OLN-93 cells with nimodipine induced the upregulation of myelin genes, in particular of proteolipid protein 1 (Plp1), which was confirmed by a significantly greater expression of PLP1 in immunofluorescence analysis and the presence of myelin structures in the cytoplasm at the ultrastructural level. Whole-genome RNA sequencing additionally revealed the upregulation of genes that are involved in neuroprotection, remyelination, and antioxidation pathways. Interestingly, the observed effects were independent of Cav1.2 and Cav1.3 because OLN-93 cells do not express these channels, and there was no measurable response pattern in patch-clamp analysis. Taking into consideration previous studies that demonstrated a beneficial effect of nimodipine on microglia, our data support the notion that nimodipine is an interesting drug candidate for the treatment of MS and other demyelinating diseases.

Circulating factors that influence the central nervous system remyelination

Injury in the central nervous system leads to neurological deficits, depending on the disruption of neural networks. Remyelination, which occurs partially and spontaneously, is a critical process in the regeneration of neural networks to recover from neurological deficits. Remyelination depends on the development of oligodendrocytes, including the proliferation of oligodendrocyte precursor cells (OPCs) and the differentiation of OPCs into mature oligodendrocytes to form myelin. OPC proliferation and differentiation are regulated by intracellular and extracellular mechanisms, and recent studies have demonstrated that circulating factors secreted from peripheral organs or infiltrated immune cells play a key role in controlling oligodendrocyte development following remyelination in adult mammals. In this review, we describe the beneficial and detrimental effects of systemic environments, such as circulating factors derived from peripheral organs and immune cells, on CNS remyelination.

Aberrant oligodendroglial LDL receptor orchestrates demyelination in chronic cerebral ischemia

Oligodendrocytes express low-density lipoprotein receptor (LDLR) to endocytose cholesterol for the maintenance of adulthood myelination. However, the potential role of LDLR in chronic cerebral ischemia-related demyelination remains unclear. We used bilateral carotid artery stenosis (BCAS) to induce sustained cerebral ischemia in mice. This hypoxic-ischemic injury caused a remarkable decrease in oligodendroglial LDLR, with impaired oligodendroglial differentiation and survival. Oligodendroglial cholesterol levels, however, remained unchanged. Mouse miR-344e-3p and the human homolog miR-410-3p, 2 miRNAs directly targeting Ldlr, were identified in experimental and clinical leukoaraiosis and were thus implicated in the LDLR reduction. Lentiviral delivery of LDLR ameliorated demyelination following chronic cerebral ischemia. By contrast, Ldlr-/- mice displayed inadequate myelination in the corpus callosum. Ldlr-/- oligodendrocyte progenitor cells (OPCs) exhibited reduced ability to differentiate and myelinate axons in vitro. Transplantation with Ldlr-/- OPCs could not rescue the BCAS-induced demyelination. Such LDLR-dependent myelin restoration might involve a physical interaction of the Asn-Pro-Val-Tyr (NPVY) motif with the phosphotyrosine binding domain of Shc, which subsequently activated the MEK/ERK pathway. Together, our findings demonstrate that the aberrant oligodendroglial LDLR in chronic cerebral ischemia impairs myelination through intracellular signal transduction. Preservation of oligodendroglial LDLR may provide a promising approach to treat ischemic demyelination.

TDP-43 mediates SREBF2-regulated gene expression required for oligodendrocyte myelination

Cholesterol metabolism operates autonomously within the central nervous system (CNS), where the majority of cholesterol resides in myelin. We demonstrate that TDP-43, the pathological signature protein for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), influences cholesterol metabolism in oligodendrocytes. TDP-43 binds directly to mRNA of SREBF2, the master transcription regulator for cholesterol metabolism, and multiple mRNAs encoding proteins responsible for cholesterol biosynthesis and uptake, including HMGCR, HMGCS1, and LDLR. TDP-43 depletion leads to reduced SREBF2 and LDLR expression, and cholesterol levels in vitro and in vivo. TDP-43-mediated changes in cholesterol levels can be restored by reintroducing SREBF2 or LDLR. Additionally, cholesterol supplementation rescues demyelination caused by TDP-43 deletion. Furthermore, oligodendrocytes harboring TDP-43 pathology from FTD patients show reduced HMGCR and HMGCS1, and coaggregation of LDLR and TDP-43. Collectively, our results indicate that TDP-43 plays a role in cholesterol homeostasis in oligodendrocytes, and cholesterol dysmetabolism may be implicated in TDP-43 proteinopathies-related diseases.

Brain cell type-specific endocytosis of arylsulfatase A identifies limitations of enzyme-based therapies for metachromatic leukodystrophy

Enzyme replacement therapies, allogeneic bone marrow transplantation and gene therapies are treatment options for lysosomal storage diseases caused by inherited deficiencies of soluble lysosomal enzymes. Independent from the approach, the enzyme must be delivered to lysosomes of deficient patient cells. Little is known about the dissemination of enzyme within a tissue where cells compete for uptake via different receptor systems, binding affinities and endocytic rates. To evaluate dissemination and lysosomal targeting of a lysosomal enzyme in the CNS, we analysed receptor-mediated endocytosis of arylsulfatase A (ASA) by different types of brain-derived cell lines and primary murine brain cells. For ASA expressed by chinese hamster ovary cells for enzyme replacement therapy of metachromatic leukodystrophy, endocytic rates decline from microglia to neurons and astrocytes and to oligodendrocytes. Only immature oligodendrocytes endocytose significant amounts of enzyme. Uptake by non-microglial cells is due to mannose 6-phosphate receptors, whereas several receptor systems participate in endocytosis by microglial cells. Interestingly, ASA expressed by microglial cells cannot be taken up in a mannose 6-phosphate dependent manner. The resulting failure to correct non-microglial cells corroborates in vivo data and indicates that therapeutic effects of allogeneic bone marrow transplantation and hematopoietic stem cell gene therapy on metachromatic leukodystrophy are independent of metabolic cross-correction of neurons, astrocytes and oligodendrocytes by receptor-mediated endocytosis.

The Secreted Glycoprotein Reelin Suppresses the Proliferation and Regulates the Distribution of Oligodendrocyte Progenitor Cells in the Embryonic Neocortex

Oligodendrocyte (OL) progenitor cells (OPCs) are generated, proliferate, migrate, and differentiate in the developing brain. Although the development of OPCs is prerequisite for normal brain function, the molecular mechanisms regulating their development in the neocortex are not fully understood. Several molecules regulate the tangential distribution of OPCs in the developing neocortex, but the cue molecule(s) that regulate their radial distribution remains unknown. Here, we demonstrate that the secreted glycoprotein Reelin suppresses the proliferation of OPCs and acts as a repellent for their migration in vitro These functions rely on the binding of Reelin to its receptors and on the signal transduction involving the intracellular protein Dab1. In the late embryonic neocortex of mice with attenuated Reelin signaling [i.e., Reelin heterozygote-deficient, Dab1 heterozygote-deficient mutant, or very low-density lipoprotein receptor (VLDLR)-deficient mice], the number of OPCs increased and their distribution shifted toward the superficial layers. In contrast, the number of OPCs decreased and they tended to distribute in the deep layers in the neocortex of mice with abrogated inactivation of Reelin by proteolytic cleavage, namely a disintegrin and metalloproteinase with thrombospondin type 1 motifs 3 (ADAMTS-3)-deficient mice and cleavage-resistant Reelin knock-in mice. Both male and female animals were used. These data indicate that Reelin-Dab1 signaling regulates the proliferation and radial distribution of OPCs in the late embryonic neocortex and that the regulation of Reelin function by its specific proteolysis is required for the normal development of OPCs.SIGNIFICANCE STATEMENT Here, we report that Reelin-Dab1 signaling regulates the proliferation and radial distribution of OPCs in the late embryonic mouse neocortex. Oligodendrocyte (OL) progenitor cells (OPCs) express Reelin signaling molecules and respond to Reelin stimulation. Reelin-Dab1 signaling suppresses the proliferation of OPCs both in vitro and in vivo Reelin repels OPCs in vitro, and the radial distribution of OPCs is altered in mice with either attenuated or augmented Reelin-Dab1 signaling. This is the first report identifying the secreted molecule that plays a role in the radial distribution of OPCs in the late embryonic neocortex. Our results also show that the regulation of Reelin function by its specific proteolysis is important for the normal development of OPCs.

Erythropoietin Increases Myelination in Oligodendrocytes: Gene Expression Profiling Reveals Early Induction of Genes Involved in Lipid Transport and Metabolism

Several studies have shown that erythropoietin (EPO) has neuroprotective or neuroreparative actions on diseases of the nervous system and that improves oligodendrocyte (OL) differentiation and myelination in vivo and in vitro. This study aims at investigating the early molecular mechanisms for the pro-myelinating action of EPO at the gene expression level. For this purpose, we used a differentiating OL precursor cell line, rat central glia-4 cells. Cells were differentiated or not, and then treated with EPO for 1 or 20 h. RNA was extracted and changes in the gene expression profile were assessed using microarray analysis. Experiments were performed in biological replicates of n = 4. Differentiation alone changed the expression of 11% of transcripts (2,663 out of 24,272), representing 2,436 genes, half of which were upregulated and half downregulated. At 20 h of treatment, EPO significantly affected the expression of 99 genes that were already regulated by differentiation and of 150 genes that were not influenced by differentiation alone. Analysis of the transcripts most upregulated by EPO identified several genes involved in lipid transport (e.g., Cd36) and lipid metabolism (Ppargc1a/Pgc1alpha, Lpin1, Pnlip, Lpin2, Ppard, Plin2) along with Igf1 and Igf2, growth factors known for their pro-myelinating action. All these genes were only induced by EPO and not by differentiation alone, except for Pnlip which was highly induced by differentiation and augmented by EPO. Results were validated by quantitative PCR. These findings suggest that EPO might increase remyelination by inducing insulin-like growth factors and increasing lipid metabolism.

Contact of myeloma cells induces a characteristic transcriptome signature in skeletal precursor cells -Implications for myeloma bone disease

Physical interaction of skeletal precursors with multiple myeloma cells has been shown to suppress their osteogenic potential while favoring their tumor-promoting features. Although several transcriptome analyses of myeloma patient-derived mesenchymal stem cells have displayed differences compared to their healthy counterparts, these analyses insufficiently reflect the signatures mediated by tumor cell contact, vary due to different methodologies, and lack results in lineage-committed precursors. To determine tumor cell contact-mediated changes on skeletal precursors, we performed transcriptome analyses of mesenchymal stem cells and osteogenic precursor cells cultured in contact with the myeloma cell line INA-6. Comparative analyses confirmed dysregulation of genes which code for known disease-relevant factors and additionally revealed upregulation of genes that are associated with plasma cell homing, adhesion, osteoclastogenesis, and angiogenesis. Osteoclast-derived coupling factors, a dysregulated adipogenic potential, and an imbalance in favor of anti-anabolic factors may play a role in the hampered osteoblast differentiation potential of mesenchymal stem cells. Angiopoietin-Like 4 (ANGPTL4) was selected from a list of differentially expressed genes as a myeloma cell contact-dependent target in skeletal precursor cells which warranted further functional analyses. Adhesion assays with full-length ANGPTL4-coated plates revealed a potential role of this protein in INA-6 cell attachment. This study expands knowledge of the myeloma cell contact-induced signature in the stromal compartment of myelomatous bones and thus offers potential targets that may allow detection and treatment of myeloma bone disease at an early stage.

Developmental delay in motor skill acquisition in Niemann-Pick C1 mice reveals abnormal cerebellar morphogenesis

Niemann-Pick type C1 (NPC1) disease is a lysosomal storage disorder caused by defective intracellular trafficking of exogenous cholesterol. Purkinje cell (PC) degeneration is the main sign of cerebellar dysfunction in both NPC1 patients and animal models. It has been recently shown that a significant decrease in Sonic hedgehog (Shh) expression reduces the proliferative potential of granule neuron precursors in the developing cerebellum of Npc1^−/− mice. Pursuing the hypothesis that this developmental defect translates into functional impairments, we have assayed Npc1-deficient pups belonging to the milder mutant mouse strain Npc1^nmf164 for sensorimotor development from postnatal day (PN) 3 to PN21. Npc1^nmf164/ Npc1^nmf164 pups displayed a 2.5-day delay in the acquisition of complex motor abilities compared to wild-type (wt) littermates, in agreement with the significant disorganization of cerebellar cortex cytoarchitecture observed between PN11 and PN15. Compared to wt, Npc1^nmf164 homozygous mice exhibited a poorer morphological differentiation of Bergmann glia (BG), as indicated by thicker radial shafts and less elaborate reticular pattern of lateral processes. Also BG functional development was defective, as indicated by the significant reduction in GLAST and Glutamine synthetase expression. A reduced VGluT2 and GAD65 expression also indicated an overall derangement of the glutamatergic/GABAergic stimulation that PCs receive by climbing/parallel fibers and basket/stellate cells, respectively. Lastly, Npc1-deficiency also affected oligodendrocyte differentiation as indicated by the strong reduction of myelin basic protein. Two sequential 2-hydroxypropyl-β-cyclodextrin administrations at PN4 and PN7 counteract these defects, partially preventing functional impairment of BG and fully restoring the normal patterns of glutamatergic/GABAergic stimulation to PCs.

These findings indicate that in Npc1^nmf164 homozygous mice the derangement of synaptic connectivity and dysmyelination during cerebellar morphogenesis largely anticipate motor deficits that are typically observed during adulthood.

Central Nervous System Demyelination and Remyelination is Independent from Systemic Cholesterol Level in Theiler's Murine Encephalomyelitis

High dietary fat and/or cholesterol intake is a risk factor for multiple diseases and has been debated for multiple sclerosis. However, cholesterol biosynthesis is a key pathway during myelination and disturbances are described in demyelinating diseases. To address the possible interaction of dyslipidemia and demyelination, cholesterol biosynthesis gene expression, composition of the body's major lipid repositories and Paigen diet-induced, systemic hypercholesterolemia were examined in Theiler's murine encephalomyelitis (TME) using histology, immunohistochemistry, serum clinical chemistry, microarrays and high-performance thin layer chromatography. TME-virus (TMEV)-infected mice showed progressive loss of motor performance and demyelinating leukomyelitis. Gene expression associated with cholesterol biosynthesis was overall down-regulated in the spinal cord of TMEV-infected animals. Spinal cord levels of galactocerebroside and sphingomyelin were reduced on day 196 post TMEV infection. Paigen diet induced serum hypercholesterolemia and hepatic lipidosis. However, high dietary fat and cholesterol intake led to no significant differences in clinical course, inflammatory response, astrocytosis, and the amount of demyelination and remyelination in the spinal cord of TMEV-infected animals. The results suggest that down-regulation of cholesterol biosynthesis is a transcriptional marker for demyelination, quantitative loss of myelin-specific lipids, but not cholesterol occurs late in chronic demyelination, and serum hypercholesterolemia exhibited no significant effect on TMEV infection.

Cholesterol in myelin biogenesis and hypomyelinating disorders

The largest pool of free cholesterol in mammals resides in myelin membranes. Myelin facilitates rapid saltatory impulse propagation by electrical insulation of axons. This function is achieved by ensheathing axons with a tightly compacted stack of membranes. Cholesterol influences myelination at many steps, from the differentiation of myelinating glial cells, over the process of myelin membrane biogenesis, to the functionality of mature myelin. Cholesterol emerged as the only integral myelin component that is essential and rate-limiting for the development of myelin in the central and peripheral nervous system. Moreover, disorders that interfere with sterol synthesis or intracellular trafficking of cholesterol and other lipids cause hypomyelination and neurodegeneration. This review summarizes recent results on the roles of cholesterol in CNS myelin biogenesis in normal development and under different pathological conditions. This article is part of a Special Issue entitled Brain Lipids.

Localization of reelin signaling pathway components in murine midbrain and striatum

We investigated the distribution patterns of the extracellular matrix protein Reelin and of crucial Reelin signaling components in murine midbrain and striatum. The cellular distribution of the Reelin receptors VLDLr and ApoER2, the intracellular downstream mediator Dab1, and the alternative Reelin receptor APP were analyzed at embryonic day 16, at postnatal stage 15 (P15), and in 3-month-old mice. Reelin was expressed intracellularly and extracellularly in midbrain mesencephalic dopaminergic (mDA) neurons of newborns. In the striatum, Calbindin D-28k⁺ neurons exhibited Reelin intracellularly at E16 and extracellularly at P15 and 3 months. ApoER2 and VLDLr were expressed in mDA neurons at E16 and P15 and in oligodendrocytes at 3 months, whereas Dab1 and APP immunoreactivity was observed in mDA at all stages analyzed. In the striatum, Calbindin D-28k⁺/GAD67⁺ inhibitory neurons expressed VLDLr, ApoER2, and Dab1 at P15, but only Dab1 at E16 and 3 months. APP was always expressed in mouse striatum in which it colocalized with Calbindin D-28k. Our data underline the importance of Reelin signalling during embryonic development and early postnatal maturation of the mesostriatal and mesocorticolimbic system, and suggest that the striatum and not the midbrain is the primary source of Reelin for midbrain neurons. The loss of ApoER2 and VLDLr expression in the mature midbrain and striatum implies that Reelin functions are restricted to migratory events and early postnatal maturation and are dispensable for the maintenance of dopaminergic neurons.

Control of astrocyte progenitor specification, migration and maturation by Nkx6.1 homeodomain transcription factor

Although astrocytes are the most abundant cell type in the central nervous system (CNS), little is known about their molecular specification and differentiation. It has previously been reported that transcription factor Nkx6.1 is expressed in neuroepithelial cells that give rise to astrocyte precursors in the ventral spinal cord. In the present study, we systematically investigated the function of Nkx6.1 in astrocyte development using both conventional and conditional Nkx6.1 mutant mice. At early postnatal stages, Nkx6.1 was expressed in a subpopulation of astrocytes in the ventral spinal cord. In the conventional Nkx6.1KO spinal cord, the initial specification of astrocyte progenitors was affected by the mutation, and subsequent migration and differentiation were disrupted in newborn mice. In addition, the development of VA2 subtype astrocytes was also inhibited in the white matter. Further studies with Nkx6.1 conditional mutants revealed significantly delayed differentiation and disorganized arrangement of fibrous astrocytes in the ventral white matter. Together, our studies indicate that Nkx6.1 plays a vital role in astrocyte specification and differentiation in the ventral spinal cord.

mTORC1 is essential for early steps during Schwann cell differentiation of amniotic fluid stem cells and regulates lipogenic gene expression

Schwann cell development is hallmarked by the induction of a lipogenic profile. Here we used amniotic fluid stem (AFS) cells and focused on the mechanisms occurring during early steps of differentiation along the Schwann cell lineage. Therefore, we initiated Schwann cell differentiation in AFS cells and monitored as well as modulated the activity of the mechanistic target of rapamycin (mTOR) pathway, the major regulator of anabolic processes. Our results show that mTOR complex 1 (mTORC1) activity is essential for glial marker expression and expression of Sterol Regulatory Element-Binding Protein (SREBP) target genes. Moreover, SREBP target gene activation by statin treatment promoted lipogenic gene expression, induced mTORC1 activation and stimulated Schwann cell differentiation. To investigate mTORC1 downstream signaling we expressed a mutant S6K1, which subsequently induced the expression of the Schwann cell marker S100b, but did not affect lipogenic gene expression. This suggests that S6K1 dependent and independent pathways downstream of mTORC1 drive AFS cells to early Schwann cell differentiation and lipogenic gene expression. In conclusion our results propose that future strategies for peripheral nervous system regeneration will depend on ways to efficiently induce the mTORC1 pathway.

Dynamic expression of secreted Frizzled-related protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia

Wnt proteins have been implicated in regulating a variety of developmental processes in the CNS. Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of the spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression ofsFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.

NDRG1 functions in LDL receptor trafficking by regulating endosomal recycling and degradation

N-myc downstream-regulated gene 1 (NDRG1) mutations cause Charcot-Marie-Tooth disease type 4D (CMT4D). However, the cellular function of NDRG1 and how it causes CMT4D are poorly understood. We report that NDRG1 silencing in epithelial cells results in decreased uptake of low-density lipoprotein (LDL) due to reduced LDL receptor (LDLR) abundance at the plasma membrane. This is accompanied by the accumulation of LDLR in enlarged EEA1-positive endosomes that contain numerous intraluminal vesicles and sequester ceramide. Concomitantly, LDLR ubiquitylation is increased but its degradation is reduced and ESCRT (endosomal sorting complex required for transport) proteins are downregulated. Co-depletion of IDOL (inducible degrader of the LDLR), which ubiquitylates the LDLR and promotes its degradation, rescues plasma membrane LDLR levels and LDL uptake. In murine oligodendrocytes, Ndrg1 silencing not only results in reduced LDL uptake but also in downregulation of the oligodendrocyte differentiation factor Olig2. Both phenotypes are rescued by co-silencing of Idol, suggesting that ligand uptake through LDLR family members controls oligodendrocyte differentiation. These findings identify NDRG1 as a novel regulator of multivesicular body formation and endosomal LDLR trafficking. The deficiency of functional NDRG1 in CMT4D might impair lipid processing and differentiation of myelinating cells.

Apolipoprotein E Mimetic Promotes Functional and Histological Recovery in Lysolecithin-Induced Spinal Cord Demyelination in Mice

Objective

Considering demyelination is the pathological hallmark of multiple sclerosis (MS), reducing demyelination and/or promoting remyelination is a practical therapeutic strategy to improve functional recovery for MS. An apolipoprotein E (apoE)-mimetic peptide COG112 has previously demonstrated therapeutic efficacy on functional and histological recovery in a mouse experimental autoimmune encephalomyelitis (EAE) model of human MS. In the current study, we further investigated whether COG112 promotes remyelination and improves functional recovery in lysolecithin induced focal demyelination in the white matter of spinal cord in mice.

Methods

A focal demyelination model was created by stereotaxically injecting lysolecithin into the bilateral ventrolateral funiculus (VLF) of T8 and T9 mouse spinal cords. Immediately after lysolecithin injection mice were treated with COG112, prefix peptide control or vehicle control for 21 days. The locomotor function of the mice was measured by the beam walking test and Basso Mouse Scale (BMS) assessment. The nerve transmission of the VLF of mice was assessed in vivo by transcranial magnetic motor evoked potentials (tcMMEPs). The histological changes were also examined by by eriochrome cyanine staining, immunohistochemistry staining and electron microscopy (EM) method.

Results

The area of demyelination in the spinal cord was significantly reduced in the COG112 group. EM examination showed that treatment with COG112 increased the thickness of myelin sheaths and the numbers of surviving axons in the lesion epicenter. Locomotor function was improved in COG112 treated animals when measured by the beam walking test and BMS assessment compared to controls. TcMMEPs also demonstrated the COG112-mediated enhancement of amplitude of evoked responses.

Conclusion

The apoE-mimetic COG112 demonstrates a favorable combination of activities in suppressing inflammatory response, mitigating demyelination and in promoting remyelination and associated functional recovery in animal model of CNS demyelination. These data support that apoE-mimetic strategy may represent a promising therapy for MS and other demyelination disorders.

Liver X receptors regulate cholesterol homeostasis in oligodendrocytes

Cholesterol synthesis and transport in oligodendrocytes are essential for optimal myelination and remyelination in pathological conditions such as multiple sclerosis. However, little is known about cholesterol homeostasis in the myelin-forming oligodendrocytes. Liver X receptors (LXRs) are nuclear oxysterol receptors that regulate genes involved in cholesterol homeostasis and may therefore play an important role in de- and remyelination. We investigated whether LXRs regulate cholesterol homeostasis in oligodendrocytes. mRNA expression of genes encoding LXR-α and LXR-β and their target genes (ABCA1, ABCG1, ABCG4, apoE, and LDLR) was detected in oligodendrocytes derived from both neonatal and adult rats using quantitative real-time PCR. The expression of LXR-β and several target genes was increased during oligodendrocyte differentiation. We further demonstrated that treatment of primary neonatal rat oligodendrocytes with the synthetic LXR agonist T0901317 induced the expression of several established LXR target genes, including ABCA1, ABCG1, apoE, and LDLR. Treatment of oligodendrocytes with T0901317 resulted in an enhanced cholesterol efflux in the presence of apolipoprotein A-I or high-density lipoprotein particles. These data show that LXRs are involved in regulating cholesterol homeostasis in oligodendrocytes.

Cholesterol: a novel regulatory role in myelin formation

Myelin consists of tightly compacted membranes that form an insulating sheath around axons. The function of myelin for rapid saltatory nerve conduction is dependent on its unique composition, highly enriched in glycosphingolipids and cholesterol. Cholesterol emerged as the only integral myelin component that is essential and rate limiting for the development of CNS and PNS myelin. Experiments with conditional mouse mutants that lack cholesterol biosynthesis in oligodendrocytes revealed that only minimal changes of the CNS myelin lipid composition are tolerated. In Schwann cells of the PNS, protein trafficking and myelin compaction depend on cholesterol. In this review, the authors summarize the role of cholesterol in myelin biogenesis and myelin disease.

Lipid metabolism in myelinating glial cells: lessons from human inherited disorders and mouse models

The integrity of central and peripheral nervous system myelin is affected in numerous lipid metabolism disorders. This vulnerability was so far mostly attributed to the extraordinarily high level of lipid synthesis that is required for the formation of myelin, and to the relative autonomy in lipid synthesis of myelinating glial cells because of blood barriers shielding the nervous system from circulating lipids. Recent insights from analysis of inherited lipid disorders, especially those with prevailing lipid depletion and from mouse models with glia-specific disruption of lipid metabolism, shed new light on this issue. The particular lipid composition of myelin, the transport of lipid-associated myelin proteins, and the necessity for timely assembly of the myelin sheath all contribute to the observed vulnerability of myelin to perturbed lipid metabolism. Furthermore, the uptake of external lipids may also play a role in the formation of myelin membranes. In addition to an improved understanding of basic myelin biology, these data provide a foundation for future therapeutic interventions aiming at preserving glial cell integrity in metabolic disorders.

Cholesterol regulates the endoplasmic reticulum exit of the major membrane protein P0 required for peripheral myelin compaction

Rapid impulse conduction requires electrical insulation of axons by myelin, a cholesterol-rich extension of the glial cell membrane with a characteristic composition of proteins and lipids. Mutations in several myelin protein genes cause endoplasmic reticulum (ER) retention and disease, presumably attributable to failure of misfolded proteins to pass the ER quality control. Because many myelin proteins partition into cholesterol-rich membrane rafts, their interaction with cholesterol could potentially be part of the ER quality control system. Here, we provide in vitro and in vivo evidence that the major peripheral myelin protein P0 requires cholesterol for exiting the ER and reaching the myelin compartment. Cholesterol dependency of P0 trafficking in heterologous cells is mediated by a cholesterol recognition/interaction amino acid consensus (CRAC) motif. Mutant mice lacking cholesterol biosynthesis in Schwann cells suffer from severe hypomyelination with numerous uncompacted myelin stretches. This demonstrates that high-level cholesterol coordinates P0 export with myelin membrane synthesis, which is required for the correct stoichiometry of myelin components and for myelin compaction.