A critical role for the cholesterol-associated proteolipids PLP and M6B in myelination of the central nervous system

CRISPR/Cas9-mediated deletion of a GA-repeat in human GPM6B leads to disruption of neural cell differentiation from NT2 cells

The human neuron-specific gene, GPM6B (Glycoprotein membrane 6B), is considered a key gene in neural cell functionality. This gene contains an exceptionally long and strictly monomorphic short tandem repeat (STR) of 9-repeats, (GA)9. STRs in regulatory regions, may impact on the expression of nearby genes. We used CRISPR-based tool to delete this GA-repeat in NT2 cells, and analyzed the consequence of this deletion on GPM6B expression. Subsequently, the edited cells were induced to differentiate into neural cells, using retinoic acid (RA) treatment. Deletion of the GA-repeat significantly decreased the expression of GPM6B at the RNA (p < 0.05) and protein (40%) levels. Compared to the control cells, the edited cells showed dramatic decrease of the astrocyte and neural cell markers, including GFAP (0.77-fold), TUBB3 (0.57-fold), and MAP2 (0.2-fold). Subsequent sorting of the edited cells showed an increased number of NES (p < 0.01), but a decreased number of GFAP (p < 0.001), TUBB3 (p < 0.05), and MAP2 (p < 0.01), compared to the control cells. In conclusion, CRISPR/Cas9-mediated deletion of a GA-repeat in human GPM6B, led to decreased expression of this gene, which in turn, disrupted differentiation of NT2 cells into neural cells.

Glycoprotein M6A upregulation detected by transcriptome analysis controls the proliferation of keloidal fibroblasts

Hypertrophic scars and keloids are fibroproliferative disorders caused by abnormal wound healing. Their exact cause has not been found, but abnormalities during the wound healing process including inflammatory, immune, genetic, and other factors are thought to predispose an individual to excessive scarring. In the present study, we performed transcriptome analysis of established keloid cell lines (KEL FIB), focusing on gene expression analysis and fusion gene detection for the first time. For gene expression analysis, fragments per kilobase per million map read values were calculated, which were validated by real-time PCR and immunohistochemistry. Fusion genes were predicted by transcriptome sequence, and validated by Sanger sequence and G-banding. As a result, GPM6A was shown in the expression analysis to be upregulated in KEL FIB compared with normal fibroblasts. The GPM6A upregulation in KEL FIB was confirmed by real-time PCR, and GPM6A messenger ribonucleic acid expression was consistently significantly elevated in the tissues of hypertrophic scar and keloid compared to normal skin. Immunohistochemistry also revealed that the number of fibroblast-like spindle-shaped cells positive for GPM6A was significantly increased in keloidal tissues. GPM6A inhibition by small interfering ribonucleic acid significantly reduced the number of KEL FIB. On the other hand, although we hypothesized that fusion genes are involved in the pathogenesis of keloids, the transcriptome analysis could not prove the presence of fusion genes in KEL FIB. Taken together, GPM6A upregulation may have an inducible effect on cell proliferation in keloidal fibroblasts. GPM6A can be a novel therapeutic target in hypertrophic scars and keloids. The inflammatory nature may be more prominent in the pathogenesis of keloids, rather than being skin tumors, as proposed by Ogawa et al. Future studies using several cell lines will be required.

The Bidirectional Interplay of α-Synuclein with Lipids in the Central Nervous System and Its Implications for the Pathogenesis of Parkinson's Disease

The alteration and aggregation of alpha-synuclein (α-syn) play a crucial role in neurodegenerative diseases collectively termed as synucleinopathies, including Parkinson's disease (PD). The bidirectional interaction of α-syn with lipids and biomembranes impacts not only α-syn aggregation but also lipid homeostasis. Indeed, lipid composition and metabolism are severely perturbed in PD. One explanation for lipid-associated alterations may involve structural changes in α-syn, caused, for example, by missense mutations in the lipid-binding region of α-syn as well as post-translational modifications such as phosphorylation, acetylation, nitration, ubiquitination, truncation, glycosylation, and glycation. Notably, different strategies targeting the α-syn-lipid interaction have been identified and are able to reduce α-syn pathology. These approaches include the modulation of post-translational modifications aiming to reduce the aggregation of α-syn and modify its binding properties to lipid membranes. Furthermore, targeting enzymes involved in various steps of lipid metabolism and exploring the neuroprotective potential of lipids themselves have emerged as novel therapeutic approaches. Taken together, this review focuses on the bidirectional crosstalk of α-syn and lipids and how alterations of this interaction affect PD and thereby open a window for therapeutic interventions.

Targeted inactivation of the Septin2 and Septin9 genes in myelinating Schwann cells of mice

The formation of axon-enwrapping myelin sheaths by oligodendrocytes in the central nervous system involves the assembly of a scaffolding septin filament comprised of the subunits SEPTIN2, SEPTIN4, SEPTIN7 and SEPTIN8. Conversely, in the peripheral nervous system (PNS), myelin is synthesized by a different cell type termed Schwann cells, and it remained unknown if septins also assemble as a multimer in PNS myelin. According to prior proteome analysis, PNS myelin comprises the subunits SEPTIN2, SEPTIN7, SEPTIN8, SEPTIN9, and SEPTIN11, which localize to the paranodal and abaxonal myelin subcompartments. Here, we use the Cre/loxP-system to delete the Septin9-gene specifically in Schwann cells, causing a markedly reduced abundance of SEPTIN9 in sciatic nerves, implying that Schwann cells are the main cell type expressing SEPTIN9 in the nerve. However, Septin9-deficiency in Schwann cells did not affect the abundance or localization of other septin subunits. In contrast, when deleting the Septin2-gene in Schwann cells the abundance of all relevant septin subunits was markedly reduced, including SEPTIN9. Notably, we did not find evidence that deleting Septin2 or Septin9 in Schwann cells impairs myelin biogenesis, nerve conduction velocity or motor/sensory capabilities, at least at the assessed timepoints. Our data thus show that SEPTIN2 but not SEPTIN9 is required for the formation or stabilization of a septin multimer in PNS myelin in vivo; however, its functional relevance remains to be established.

Mechanisms of axonal support by oligodendrocyte-derived extracellular vesicles

Extracellular vesicles (EVs) have recently emerged as versatile elements of cell communication in the nervous system, mediating tissue homeostasis. EVs influence the physiology of their target cells via horizontal transfer of molecular cargo between cells and by triggering signalling pathways. In this Review, we discuss recent work revealing that EVs mediate interactions between oligodendrocytes and neurons, which are relevant for maintaining the structural integrity of axons. In response to neuronal activity, myelinating oligodendrocytes release EVs, which are internalized by neurons and provide axons with key factors that improve axonal transport, stress resistance and energy homeostasis. Glia-to-neuron transfer of EVs is thus a crucial facet of axonal preservation. When glial support is impaired, axonal integrity is progressively lost, as observed in myelin-related disorders, other neurodegenerative diseases and with normal ageing. We highlight the mechanisms that oligodendroglial EVs use to sustain axonal integrity and function.

Endogenous Glycoprotein GPM6a Is Involved in Neurite Outgrowth in Rat Dorsal Root Ganglion Neurons

The peripheral nervous system (PNS) has a unique ability for self-repair. Dorsal root ganglion (DRG) neurons regulate the expression of different molecules, such as neurotrophins and their receptors, to promote axon regeneration after injury. However, the molecular players driving axonal regrowth need to be better defined. The membrane glycoprotein GPM6a has been described to contribute to neuronal development and structural plasticity in central-nervous-system neurons. Recent evidence indicates that GPM6a interacts with molecules from the PNS, although its role in DRG neurons remains unknown. Here, we characterized the expression of GPM6a in embryonic and adult DRGs by combining analysis of public RNA-seq datasets with immunochemical approaches utilizing cultures of rat DRG explants and dissociated neuronal cells. M6a was detected on the cell surfaces of DRG neurons throughout development. Moreover, GPM6a was required for DRG neurite elongation in vitro. In summary, we provide evidence on GPM6a being present in DRG neurons for the first time. Data from our functional experiments support the idea that GPM6a could contribute to axon regeneration in the PNS.

Myelin lipid metabolism and its role in myelination and myelin maintenance

Myelin is a specialized cell membrane indispensable for rapid nerve conduction. The high abundance of membrane lipids is one of myelin's salient features that contribute to its unique role as an insulator that electrically isolates nerve fibers across their myelinated surface. The most abundant lipids in myelin include cholesterol, glycosphingolipids, and plasmalogens, each playing critical roles in myelin development as well as function. This review serves to summarize the role of lipid metabolism in myelination and myelin maintenance, as well as the molecular determinants of myelin lipid homeostasis, with an emphasis on findings from genetic models. In addition, the implications of myelin lipid dysmetabolism in human diseases are highlighted in the context of hereditary leukodystrophies and neuropathies as well as acquired disorders such as Alzheimer's disease.

1DE-MS Profiling for Proteoform-Correlated Proteomic Analysis, by Combining SDS-PAGE, Whole-Gel Slicing, Quantitative LC-MS/MS, and Reconstruction of Gel Distributions of Several Thousands of Proteins

SDS-PAGE has often been used in proteomic analysis, but generally for sample prefractionation although the technique separates proteins by molecular masses (M_ws) and the information would contribute to proteoform-level analysis. Here, we report a method that combines SDS-PAGE, whole-gel slicing, and quantitative LC-MS/MS for establishing gel distributions of several thousand proteins in a proteome. A previously obtained data set on rat cerebral cortex with cerebral ischemia-reperfusion injury¹ was analyzed, and the gel distributions of 5906 proteins were reconstructed. These distributions, referred to as 1DE-MS profiles, revealed that about 30% of the proteins had more than one proteoform detected in the gels. The profiles were categorized into six types by distribution (narrow, dispersed, or broad) and relative deviations between the abundance-peak apparent M_ws and calculated M_ws. Only 56% of the proteins showed narrow distributions and matched M_ws, while the others had rather complex profiles. Bioinformatic analysis on example profiles showed the resolved proteoforms involved alternative splicing, proteolytic processing, glycosylation and ubiquitination, fragmentation, and probably transmembrane structures. Profile-based differential analysis revealed that many of the disease-caused changes were proteoform dependent. This work provided a proteome-scale view of protein distributions in SDS-PAGE gels, and the method would be useful to obtain proteoform-correlated information for in-depth proteomics.

Human myelin proteolipid protein structure and lipid bilayer stacking

The myelin sheath is an essential, multilayered membrane structure that insulates axons, enabling the rapid transmission of nerve impulses. The tetraspan myelin proteolipid protein (PLP) is the most abundant protein of compact myelin in the central nervous system (CNS). The integral membrane protein PLP adheres myelin membranes together and enhances the compaction of myelin, having a fundamental role in myelin stability and axonal support. PLP is linked to severe CNS neuropathies, including inherited Pelizaeus-Merzbacher disease and spastic paraplegia type 2, as well as multiple sclerosis. Nevertheless, the structure, lipid interaction properties, and membrane organization mechanisms of PLP have remained unidentified. We expressed, purified, and structurally characterized human PLP and its shorter isoform DM20. Synchrotron radiation circular dichroism spectroscopy and small-angle X-ray and neutron scattering revealed a dimeric, α-helical conformation for both PLP and DM20 in detergent complexes, and pinpoint structural variations between the isoforms and their influence on protein function. In phosphatidylcholine membranes, reconstituted PLP and DM20 spontaneously induced formation of multilamellar myelin-like membrane assemblies. Cholesterol and sphingomyelin enhanced the membrane organization but were not crucial for membrane stacking. Electron cryomicroscopy, atomic force microscopy, and X-ray diffraction experiments for membrane-embedded PLP/DM20 illustrated effective membrane stacking and ordered organization of membrane assemblies with a repeat distance in line with CNS myelin. Our results shed light on the 3D structure of myelin PLP and DM20, their structure-function differences, as well as fundamental protein-lipid interplay in CNS compact myelin.

Conservation and divergence of myelin proteome and oligodendrocyte transcriptome profiles between humans and mice

Human myelin disorders are commonly studied in mouse models. Since both clades evolutionarily diverged approximately 85 million years ago, it is critical to know to what extent the myelin protein composition has remained similar. Here, we use quantitative proteomics to analyze myelin purified from human white matter and find that the relative abundance of the structural myelin proteins PLP, MBP, CNP, and SEPTIN8 correlates well with that in C57Bl/6N mice. Conversely, multiple other proteins were identified exclusively or predominantly in human or mouse myelin. This is exemplified by peripheral myelin protein 2 (PMP2), which was specific to human central nervous system myelin, while tetraspanin-2 (TSPAN2) and connexin-29 (CX29/GJC3) were confined to mouse myelin. Assessing published scRNA-seq-datasets, human and mouse oligodendrocytes display well-correlating transcriptome profiles but divergent expression of distinct genes, including Pmp2, Tspan2, and Gjc3. A searchable web interface is accessible via www.mpinat.mpg.de/myelin. Species-dependent diversity of oligodendroglial mRNA expression and myelin protein composition can be informative when translating from mouse models to humans.

The Human Myelin Proteome and Sub-Metalloproteome Interaction Map: Relevance to Myelin-Related Neurological Diseases

Myelin in humans is composed of about 80% lipids and 20% protein. Initially, myelin protein composition was considered low, but various recent proteome analyses have identified additional myelin proteins. Although, the myelin proteome is qualitatively and quantitatively identified through complementary proteomic approaches, the corresponding Protein–Protein Interaction (PPI) network of myelin is not yet available. In the present work, the PPI network was constructed based on available experimentally supported protein interactions of myelin in PPI databases. The network comprised 2017 PPIs between 567 myelin proteins. Interestingly, structure-based in silico analysis revealed that 20% of the myelin proteins that are interconnected in the proposed PPI network are metal-binding proteins/enzymes that construct the main sub-PPI network of myelin proteome. Finally, the PPI networks of the myelin proteome and sub-metalloproteome were analyzed ontologically to identify the biochemical processes of the myelin proteins and the interconnectivity of myelin-associated diseases in the interactomes. The presented PPI dataset could provide a useful resource to the scientific community to further our understanding of human myelin biology and serve as a basis for future studies of myelin-related neurological diseases and particular autoimmune diseases such as multiple sclerosis where myelin epitopes are implicated.

Progressive axonopathy when oligodendrocytes lack the myelin protein CMTM5

Oligodendrocytes facilitate rapid impulse propagation along the axons they myelinate and support their long-term integrity. However, the functional relevance of many myelin proteins has remained unknown. Here, we find that expression of the tetraspan-transmembrane protein CMTM5 (chemokine-like factor-like MARVEL-transmembrane domain containing protein 5) is highly enriched in oligodendrocytes and central nervous system (CNS) myelin. Genetic disruption of the Cmtm5 gene in oligodendrocytes of mice does not impair the development or ultrastructure of CNS myelin. However, oligodendroglial Cmtm5 deficiency causes an early-onset progressive axonopathy, which we also observe in global and tamoxifen-induced oligodendroglial Cmtm5 mutants. Presence of the Wld^S mutation ameliorates the axonopathy, implying a Wallerian degeneration-like pathomechanism. These results indicate that CMTM5 is involved in the function of oligodendrocytes to maintain axonal integrity rather than myelin biogenesis.

A new perspective on cholesterol in pediatric health: association of vitamin D metabolism, respiratory diseases, and mental health problems

Cholesterol, the main structural molecule of cell membranes, is involved in essential functions of the human body. Dyslipidemia is an established risk factor for cardiovascular diseases (CVDs) that is observed even in childhood. To reduce the risk of CVDs in children, several clinical guidelines have been published for the management of pediatric dyslipidemia. However, pediatric dyslipidemia is also associated with several health problems other than CVDs. This article reviews the current data on dyslipidemia-related pediatric health issues. There is strong evidence that low serum vitamin D levels, asthma, and mental health problems may be associated with dyslipidemia in the pediatric population regardless of body mass index. This review also highlights the need for further large-scale population-based studies in the Korean pediatric population to establish effective strategies for promoting children's health.

Predominant monomorphism of the RIT2 and GPM6B exceptionally long GA blocks in human and enriched divergent alleles in the disease compartment

Across human protein-coding genes, the human neuron-specific genes, RIT2 and GPM6B, contain the two longest GA short tandem repeats (STRs) of 11 and 9-repeats, respectively, the length ranges of which are functional, and result in gene expression alteration. Here we sequenced the RIT2 and GPM6B STRs in 600 human subjects, consisting of late-onset neurocognitive disorder (n = 200), multiple sclerosis (n = 200), and controls (n = 200). Furthermore, we selected two large human databases, including the general-population-based gnomAD ( https://gnomad.broadinstitute.org ) and a mainly disease-phenotype-archiving database, TOPMed ( https://www.nhlbiwgs.org ), to compare allele frequencies in the general populations vs. the disease compartment. The RIT2 and GPM6B GA-repeats were monomorphic in the human subjects studied, at lengths of 11 and 9-repeats, respectively, and were predominantly human-specific in formula. Exception included a 9/11 genotype of the RIT2 GA-STR in an isolate case of female multiple sclerosis. Exceedingly rare alleles of the two GA repeats were significantly enriched in TOPMed vs. the gnomAD. We report prime instances of predominant monomorphism for specific lengths of STRs in human, and possible enrichment of rare divergent alleles in the disease phenotype compartment. While STRs are most attended because of their high polymorphic nature, STR monomorphism is an underappreciated feature, which may have a link with natural selection and disease.

Consumption of Breast Milk Is Associated with Decreased Prevalence of Autism in Fragile X Syndrome

Breastfeeding is associated with numerous health benefits, but early life nutrition has not been specifically studied in the neurodevelopmental disorder fragile X syndrome (FXS). Herein, I evaluate associations between the consumption of breast milk during infancy and the prevalence of autism, allergies, diabetes, gastrointestinal (GI) problems and seizures in FXS. The study design was a retrospective survey of families enrolled in the Fragile X Online Registry and Accessible Research Database (FORWARD). There was a 1.7-fold reduction in the prevalence of autism in FXS participants who were fed breast milk for 12 months or longer. There were strong negative correlations between increased time the infant was fed breast milk and the prevalence of autism and seizures and moderate negative correlations with the prevalence of GI problems and allergies. However, participants reporting GI problems or allergies commenced these comorbidities significantly earlier than those not fed breast milk. Parsing the data by sex indicated that males exclusively fed breast milk exhibited decreased prevalence of GI problems and allergies. These data suggest that long-term or exclusive use of breast milk is associated with reduced prevalence of key comorbidities in FXS, although breast milk is associated with the earlier development of GI problems and allergies.

Neuronal Glycoprotein M6a: An Emerging Molecule in Chemical Synapse Formation and Dysfunction

The cellular and molecular mechanisms underlying neuropsychiatric and neurodevelopmental disorders show that most of them can be categorized as synaptopathies-or damage of synaptic function and plasticity. Synaptic formation and maintenance are orchestrated by protein complexes that are in turn regulated in space and time during neuronal development allowing synaptic plasticity. However, the exact mechanisms by which these processes are managed remain unknown. Large-scale genomic and proteomic projects led to the discovery of new molecules and their associated variants as disease risk factors. Neuronal glycoprotein M6a, encoded by the GPM6A gene is emerging as one of these molecules. M6a has been involved in neuron development and synapse formation and plasticity, and was also recently proposed as a gene-target in various neuropsychiatric disorders where it could also be used as a biomarker. In this review, we provide an overview of the structure and molecular mechanisms by which glycoprotein M6a participates in synapse formation and maintenance. We also review evidence collected from patients carrying mutations in the GPM6A gene; animal models, and in vitro studies that together emphasize the relevance of M6a, particularly in synapses and in neurological conditions.

Ensheathment and Myelination of Axons: Evolution of Glial Functions

Myelination of axons provides the structural basis for rapid saltatory impulse propagation along vertebrate fiber tracts, a well-established neurophysiological concept. However, myelinating oligodendrocytes and Schwann cells serve additional functions in neuronal energy metabolism that are remarkably similar to those of axon-ensheathing glial cells in unmyelinated invertebrates. Here we discuss myelin evolution and physiological glial functions, beginning with the role of ensheathing glia in preventing ephaptic coupling, axoglial metabolic support, and eliminating oxidative radicals. In both vertebrates and invertebrates, axoglial interactions are bidirectional, serving to regulate cell fate, nerve conduction, and behavioral performance. One key step in the evolution of compact myelin in the vertebrate lineage was the emergence of the open reading frame for myelin basic protein within another gene. Several other proteins were neofunctionalized as myelin constituents and help maintain a healthy nervous system. Myelination in vertebrates became a major prerequisite of inhabiting new ecological niches.

GPM6B Inhibit PCa Proliferation by Blocking Prostate Cancer Cell Serotonin Absorptive Capacity

Prostate cancer is currently one of the most common fatal tumor types in men. Although multiple treatments can alleviate some cases, advanced prostate cancer, especially CRPC, still has a very poor prognosis. Therefore, early detection and diagnosis of prostate cancer have a very important role in the prognosis of patients. Glycoprotein M6B (GPM6B) is a transmembrane protein that belongs to the proteolipid protein family. GPM6B has been proved and can be used as a biomarker for gynecological malignancies and breast carcinoma. However, there are no studies that explored the functions of GPM6B in PCa. We explored differentially expressed genes in prostate cancer by analyzing TCGA data and found GPM6B downregulated in PCa tissues compared to that in normal prostate tissues. The GPM6B expression in PCa patient's tumor tissues was significantly related to clinical stage, T classification, lymph node metastasis, and distant metastasis, but not significantly related to age and Gleason score. Also, patients with highGPM6B expression had a better prognosis. The overexpression of GPM6B in prostate cancer cells could inhibit cell proliferation. Serotonin treatment could enhance the proliferation of PCa cell lines; moreover, fluoxetine could reverse this result. In conclusion, we identified GPM6B as a tumor suppressor in PCa. In mechanism, it can regulate the uptaking of serotonin and inhibit the growth of prostate cancer. These results suggested the potential function of GPM6B as a diagnostic marker of PCa and provided clues for the development of new treatment targets for PCa.

Structural myelin defects are associated with low axonal ATP levels but rapid recovery from energy deprivation in a mouse model of spastic paraplegia

In several neurodegenerative disorders, axonal pathology may originate from impaired oligodendrocyte-to-axon support of energy substrates. We previously established transgenic mice that allow measuring axonal ATP levels in electrically active optic nerves. Here, we utilize this technique to explore axonal ATP dynamics in the Plp^null/y mouse model of spastic paraplegia. Optic nerves from Plp^null/y mice exhibited lower and more variable basal axonal ATP levels and reduced compound action potential (CAP) amplitudes, providing a missing link between axonal pathology and a role of oligodendrocytes in brain energy metabolism. Surprisingly, when Plp^null/y optic nerves are challenged with transient glucose deprivation, both ATP levels and CAP decline slower, but recover faster upon reperfusion of glucose. Structurally, myelin sheaths display an increased frequency of cytosolic channels comprising glucose and monocarboxylate transporters, possibly facilitating accessibility of energy substrates to the axon. These data imply that complex metabolic alterations of the axon–myelin unit contribute to the phenotype of Plp^null/y mice.

Imaging of ATP dynamics in the optic nerve axons of mice lacking the major myelin protein PLP (a model of spastic paraplegia) reveals complex alterations in the metabolic interaction between oligodendrocytes and axons, associated with structural deficits of myelin.

The Dynamic Proteome of Oligodendrocyte Lineage Differentiation Features Planar Cell Polarity and Macroautophagy Pathways

Background

Generation of oligodendrocytes is a sophisticated multistep process, the mechanistic underpinnings of which are not fully understood and demand further investigation. To systematically profile proteome dynamics during human embryonic stem cell differentiation into oligodendrocytes, we applied in-depth quantitative proteomics at different developmental stages and monitored changes in protein abundance using a multiplexed tandem mass tag-based proteomics approach.

Findings

Our proteome data provided a comprehensive protein expression profile that highlighted specific expression clusters based on the protein abundances over the course of human oligodendrocyte lineage differentiation. We identified the eminence of the planar cell polarity signalling and autophagy (particularly macroautophagy) in the progression of oligodendrocyte lineage differentiation—the cooperation of which is assisted by 106 and 77 proteins, respectively, that showed significant expression changes in this differentiation process. Furthermore, differentially expressed protein analysis of the proteome profile of oligodendrocyte lineage cells revealed 378 proteins that were specifically upregulated only in 1 differentiation stage. In addition, comparative pairwise analysis of differentiation stages demonstrated that abundances of 352 proteins differentially changed between consecutive differentiation time points.

Conclusions

Our study provides a comprehensive systematic proteomics profile of oligodendrocyte lineage cells that can serve as a resource for identifying novel biomarkers from these cells and for indicating numerous proteins that may contribute to regulating the development of myelinating oligodendrocytes and other cells of oligodendrocyte lineage. We showed the importance of planar cell polarity signalling in oligodendrocyte lineage differentiation and revealed the autophagy-related proteins that participate in oligodendrocyte lineage differentiation.

A role of oligodendrocytes in information processing

Myelinating oligodendrocytes enable fast propagation of action potentials along the ensheathed axons. In addition, oligodendrocytes play diverse non-canonical roles including axonal metabolic support and activity-dependent myelination. An open question remains whether myelination also contributes to information processing in addition to speeding up conduction velocity. Here, we analyze the role of myelin in auditory information processing using paradigms that are also good predictors of speech understanding in humans. We compare mice with different degrees of dysmyelination using acute multiunit recordings in the auditory cortex, in combination with behavioral readouts. We find complex alterations of neuronal responses that reflect fatigue and temporal acuity deficits. We observe partially discriminable but similar deficits in well myelinated mice in which glial cells cannot fully support axons metabolically. We suggest a model in which myelination contributes to sustained stimulus perception in temporally complex paradigms, with a role of metabolically active oligodendrocytes in cortical information processing.

Lipids in regulating oligodendrocyte structure and function

Oligodendrocytes enwrap central nervous system axons with myelin, a lipid enriched highly organized multi-layer membrane structure that allows for fast long-distance saltatory conduction of neuronal impulses. Myelin has an extremely high lipid content (∼80 % of its dry weight) and a peculiar lipid composition, with a 2:2:1 cholesterol:phospholipid:glycolipid ratio. Inherited neurodegenerative diseases of the lipids (caused by mutations in lipogenic enzymes) often present oligodendrocyte and/or myelin defects which contribute to the overall disease pathophysiology. These phenomena triggered an increasing number of studies over the functions lipid exert to shape and maintain myelin, and brought to the finding that lipids are more than only structural building blocks. They act as signaling molecules to drive proliferation and differentiation of oligodendrocyte progenitor cells, as well as proliferation of premyelinating oligodendrocytes, and their maturation into myelinating ones. Here, we summarize key findings in these areas, while presenting the main related human diseases. Despite many advances in the field, various questions remain open which we briefly discuss. This article is part of a special issue entitled "Role of Lipids in CNS Cell Physiology and Pathology".

The CNS Myelin Proteome: Deep Profile and Persistence After Post-mortem Delay

Myelin membranes are dominated by lipids while the complexity of their protein composition has long been considered to be low. However, numerous additional myelin proteins have been identified since. Here we revisit the proteome of myelin biochemically purified from the brains of healthy c56Bl/6N-mice utilizing complementary proteomic approaches for deep qualitative and quantitative coverage. By gel-free, label-free mass spectrometry, the most abundant myelin proteins PLP, MBP, CNP, and MOG constitute 38, 30, 5, and 1% of the total myelin protein, respectively. The relative abundance of myelin proteins displays a dynamic range of over four orders of magnitude, implying that PLP and MBP have overshadowed less abundant myelin constituents in initial gel-based approaches. By comparisons with published datasets we evaluate to which degree the CNS myelin proteome correlates with the mRNA and protein abundance profiles of myelin and oligodendrocytes. Notably, the myelin proteome displays only minor changes if assessed after a post-mortem delay of 6 h. These data provide the most comprehensive proteome resource of CNS myelin so far and a basis for addressing proteomic heterogeneity of myelin in mouse models and human patients with white matter disorders.

Myelin-Associated MAL and PLP Are Unusual among Multipass Transmembrane Proteins in Preferring Ordered Membrane Domains

Eukaryotic membranes can be partitioned into lipid-driven membrane microdomains called lipid rafts, which function to sort lipids and proteins in the plane of the membrane. As protein selectivity underlies all functions of lipid rafts, there has been significant interest in understanding the structural and molecular determinants of raft affinity. Such determinants have been described for lipids and single-spanning transmembrane proteins; however, how multipass transmembrane proteins (TMPs) partition between ordered and disordered phases has not been widely explored. Here we used cell-derived giant plasma membrane vesicles (GPMVs) to systematically measure multipass TMP partitioning to ordered membrane domains. Across a set of 24 structurally and functionally diverse multipass TMPs, the large majority (92%) had minimal raft affinity. The only exceptions were two myelin-associated four-pass TMPs, myelin and lymphocyte protein (MAL), and proteo lipid protein (PLP). We characterized the potential mechanisms for their exceptional raft affinity and observed that PLP requires cholesterol and sphingolipids for optimal association with ordered membrane domains and that PLP and MAL appear to compete for cholesterol-mediated raft affinity. These observations suggest broad conclusions about the composition of ordered membrane domains in cells and point to previously unrecognized drivers of raft affinity for multipass transmembrane proteins.

Subcellular diversion of cholesterol by gain- and loss-of-function mutations in PMP22

Abnormalities of the peripheral myelin protein 22 (PMP22) gene, including duplication, deletion and point mutations are a major culprit in Type 1 Charcot-Marie-Tooth (CMT) diseases. The complete absence of PMP22 alters cholesterol metabolism in Schwann cells, which likely contributes to myelination deficits. Here, we examined the subcellular trafficking of cholesterol in distinct models of PMP22-linked neuropathies. In Schwann cells from homozygous Trembler J (TrJ) mice carrying a Leu16Pro mutation, cholesterol was retained with TrJ-PMP22 in the Golgi, alongside a corresponding reduction in its plasma membrane level. PMP22 overexpression, which models CMT1A caused by gene duplication, triggered cholesterol sequestration to lysosomes, and reduced ATP-binding cassette transporter-dependent cholesterol efflux. Conversely, lysosomal targeting of cholesterol by U18666A treatment increased wild type (WT)-PMP22 levels in lysosomes. Mutagenesis of a cholesterol recognition motif, or CRAC domain, in human PMP22 lead to increased levels of PMP22 in the ER and Golgi compartments, along with higher cytosolic, and lower membrane-associated cholesterol. Importantly, cholesterol trafficking defects observed in PMP22-deficient Schwann cells were rescued by WT but not CRAC-mutant-PMP22. We also observed that myelination deficits in dorsal root ganglia explants from heterozygous PMP22-deficient mice were improved by cholesterol supplementation. Collectively, these findings indicate that PMP22 is critical in cholesterol metabolism, and this mechanism is likely a contributing factor in PMP22-linked hereditary neuropathies. Our results provide a basis for understanding how altered expression of PMP22 impacts cholesterol metabolism.

Multiple Sclerosis: Lipids, Lymphocytes, and Vitamin D

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. We review the two core MS features, myelin instability, fragmentation, and remyelination failure, and dominance of pathogenic CD4+ Th17 cells over protective CD4+ Treg cells. To better understand myelin pathology, we describe myelin biosynthesis, structure, and function, then highlight stearoyl-CoA desaturase (SCD) in nervonic acid biosynthesis and nervonic acid's contribution to myelin stability. Noting that vitamin D deficiency decreases SCD in the periphery, we propose it also decreases SCD in oligodendrocytes, disrupting the nervonic acid supply and causing myelin instability and fragmentation. To better understand the distorted Th17/Treg cell balance, we summarize Th17 cell contributions to MS pathogenesis, then highlight how 1,25-dihydroxyvitamin D3 signaling from microglia to CD4+ T cells restores Treg cell dominance. This signaling rapidly increases flux through the methionine cycle, removing homocysteine, replenishing S-adenosyl-methionine, and improving epigenetic marking. Noting that DNA hypomethylation and inappropriate DRB1*1501 expression were observed in MS patient CD4+ T cells, we propose that vitamin D deficiency thwarts epigenetic downregulation of DRB1*1501 and Th17 cell signature genes, and upregulation of Treg cell signature genes, causing dysregulation within the CD4+ T cell compartment. We explain how obesity reduces vitamin D status, and how estrogen and vitamin D collaborate to promote Treg cell dominance in females. Finally, we discuss the implications of this new knowledge concerning myelin and the Th17/Treg cell balance, and advocate for efforts to address the global epidemics of obesity and vitamin D deficiency in the expectation of reducing the impact of MS.

Pathology of myelinated axons in the PLP-deficient mouse model of spastic paraplegia type 2 revealed by volume imaging using focused ion beam-scanning electron microscopy

Advances in electron microscopy including improved imaging techniques and state-of-the-art detectors facilitate imaging of larger tissue volumes with electron microscopic resolution. In combination with genetic tools for the generation of mouse mutants this allows assessing the three-dimensional (3D) characteristics of pathological features in disease models. Here we revisited the axonal pathology in the central nervous system of a mouse model of spastic paraplegia type 2, the Plp-/Y mouse. Although PLP is a bona fide myelin protein, the major hallmark of the disease in both SPG2 patients and mouse models are axonal swellings comprising accumulations of numerous organelles including mitochondria, gradually leading to irreversible axonal loss. To assess the number and morphology of axonal mitochondria and the overall myelin preservation we evaluated two sample preparation techniques, chemical fixation or high-pressure freezing and freeze substitution, with respect to the objective of 3D visualization. Both methods allowed visualizing distribution and morphological details of axonal mitochondria. In Plp-/Y mice the number of mitochondria is 2-fold increased along the entire axonal length. Mitochondria are also found in the excessive organelle accumulations within axonal swellings. In addition, organelle accumulations were detected within the myelin sheath and the inner tongue. We find that 3D electron microscopy is required for a comprehensive understanding of the size, content and frequency of axonal swellings, the hallmarks of axonal pathology.

Cholesterol and triglyceride levels in first-episode patients with major depressive disorder: A meta-analysis of case-control studies

BACKGROUND

Lipid profile disturbances are frequently observed in major depressive disorder (MDD) and constitute to high mortality rates. However, less is known about whether this risk is present in patients with first-episode MDD. Therefore, this meta-analysis was conducted to examine if lipid parameters differed between healthy controls and first-episode MDD patients.

METHODS

Cochrane Library, PubMed, PsycINFO, EMBASE, Chinese Journal Net, and WanFang databases were searched from inception to October 23, 2018. The primary outcomes were triglycerides, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and total cholesterol levels.

RESULTS

A total of 11 case-control studies compared 690 subjects with first-episode MDD and 614 healthy controls were included and analyzed. Compared to healthy controls, patients with first-episode MDD were significantly associated with higher triglyceride (SMD = 0.29, 95% CI: 0.09, 0.48, P = 0.004) and lower HDL cholesterol levels (SMD = -0.54, 95% CI: -0.86, -0.22, P = 0.001). Subgroup analyses revealed that first-episode MDD patients with higher triglyceride and lower HDL levels were found only in Chinese and plasma group when compared to healthy controls (P < 0.05). Meta-regression analysis showed that the significant heterogeneity for triglyceride and HDL cholesterol was partly explained by the quality of study. No significant difference was found in LDL cholesterol and total cholesterol levels between the two groups.

LIMITATIONS

Heterogeneity was relatively high among the included studies.

CONCLUSIONS

Elevated triglyceride and decreased HDL cholesterol levels may be associated with first-episode MDD. Findings support early lipid monitoring and interventions targeting healthy lifestyle.

Cellular Signal-Regulated Schwann Cell Myelination and Remyelination

Increasing studies have demonstrated multiple signaling molecules responsible for oligodendrocytes and Schwann cells development such as migration, differentiation, myelination, and axo-glial interaction. However, complicated roles in these events are still poorly understood. This chapter focuses on well established intracellular signaling transduction and recent topics that control myelination and are elucidated from accumulating evidences. The underlying molecular mechanisms, which involved in membrane trafficking through small GTPase Arf6 and its activator cytohesins, demonstrate the crosstalk between well established intracellular signaling transduction and a new finding signaling pathway in glial cells links to physiological phenotype and essential role in peripheral nerve system (PNS). Since Arf family proteins affect the expression levels of myelin protein zero (MPZ) and Krox20, which is a transcription factor regulatory factor in early developmental stages of Schwann cells, Arf proteins likely to be key regulator for Schwann cells development. Herein, we discuss how intracellular signaling transductions in Schwann cells associate with myelination in CNS and PNS.

Gallyas Silver Impregnation of Myelinated Nerve Fibers

In the nervous system of vertebrates, nerve impulse propagation is accelerated by the ensheathment of neuronal axons with myelin. Myelin sheaths are molecularly specialized, lipid-rich plasma membrane extensions of Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system (CNS). To visualize myelinated nerve fibers and to allow for the morphological analyses of myelin in the brain and the spinal cord, an efficient method for silver impregnation of myelin has originally been developed by Ferenc Gallyas in 1979, referred to as Gallyas silver impregnation. Gallyas' method is based on the agyrophilic characteristic of myelin to form and bind silver particles, while this process is suppressed in tissues other than myelin. The silver particles are finally enhanced in a developing step ("physical developer"). The main advantage of this method is that it efficiently visualizes both large myelinated fiber tracts and individual myelinated axons. Here we provide our laboratory protocol that is suitable for paraffin embedded sections and the use of light microscopy based on Gallyas' original protocol and subsequent modifications by Pistorio and colleagues.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATP-binding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1.SIGNIFICANCE STATEMENT Understanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ∼80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.

Anillin facilitates septin assembly to prevent pathological outfoldings of central nervous system myelin

Myelin serves as an axonal insulator that facilitates rapid nerve conduction along axons. By transmission electron microscopy, a healthy myelin sheath comprises compacted membrane layers spiraling around the cross-sectioned axon. Previously we identified the assembly of septin filaments in the innermost non-compacted myelin layer as one of the latest steps of myelin maturation in the central nervous system (CNS) (Patzig et al., 2016). Here we show that loss of the cytoskeletal adaptor protein anillin (ANLN) from oligodendrocytes disrupts myelin septin assembly, thereby causing the emergence of pathological myelin outfoldings. Since myelin outfoldings are a poorly understood hallmark of myelin disease and brain aging we assessed axon/myelin-units in Anln-mutant mice by focused ion beam-scanning electron microscopy (FIB-SEM); myelin outfoldings were three-dimensionally reconstructed as large sheets of multiple compact membrane layers. We suggest that anillin-dependent assembly of septin filaments scaffolds mature myelin sheaths, facilitating rapid nerve conduction in the healthy CNS.

Maintenance of high proteolipid protein level in adult central nervous system myelin is required to preserve the integrity of myelin and axons

Proteolipid protein (PLP) is the most abundant integral membrane protein in central nervous system (CNS) myelin. Expression of the Plp-gene in oligodendrocytes is not essential for the biosynthesis of myelin membranes but required to prevent axonal pathology. This raises the question whether the exceptionally high level of PLP in myelin is required later in life, or whether high-level PLP expression becomes dispensable once myelin has been assembled. Both models require a better understanding of the turnover of PLP in myelin in vivo. Thus, we generated and characterized a novel line of tamoxifen-inducible Plp-mutant mice that allowed us to determine the rate of PLP turnover after developmental myelination has been completed, and to assess the possible impact of gradually decreasing amounts of PLP for myelin and axonal integrity. We found that 6 months after targeting the Plp-gene the abundance of PLP in CNS myelin was about halved, probably reflecting that myelin is slowly turned over in the adult brain. Importantly, this reduction by 50% was sufficient to cause the entire spectrum of neuropathological changes previously associated with the developmental lack of PLP, including myelin outfoldings, lamellae splittings, and axonal spheroids. In comparison to axonopathy and gliosis, the infiltration of cytotoxic T-cells was temporally delayed, suggesting a corresponding chronology also in the genetic disorders of PLP-deficiency. High-level abundance of PLP in myelin throughout adult life emerges as a requirement for the preservation of white matter integrity.

Myelin: Methods for Purification and Proteome Analysis

Molecular characterization of myelin is a prerequisite for understanding the normal structure of the axon/myelin-unit in the healthy nervous system and abnormalities in myelin-related disorders. However, reliable molecular profiles necessitate very pure myelin membranes, in particular when considering the power of highly sensitive "omics"-data acquisition methods. Here, we recapitulate the history and recent applications of myelin purification. We then provide our laboratory protocols for the biochemical isolation of a highly pure myelin-enriched fraction from mouse brains and for its proteomic analysis. We also supply methodological modifications when investigating posttranslational modifications, RNA, or myelin from peripheral nerves. Notably, technical advancements in solubilizing myelin are beneficial for gel-based and gel-free myelin proteome analyses. We conclude this article by exemplifying the exceptional power of label-free proteomics in the mass-spectrometric quantification of myelin proteins.

Glycoprotein M6B Interacts with TβRI to Activate TGF-β-Smad2/3 Signaling and Promote Smooth Muscle Cell Differentiation

Smooth muscle cells (SMCs), which form the walls of blood vessels, play an important role in vascular development and the pathogenic process of vascular remodeling. However, the molecular mechanisms governing SMC differentiation remain poorly understood. Glycoprotein M6B (GPM6B) is a four-transmembrane protein that belongs to the proteolipid protein family and is widely expressed in neurons, oligodendrocytes, and astrocytes. Previous studies have revealed that GPM6B plays a role in neuronal differentiation, myelination, and osteoblast differentiation. In the present study, we found that the GPM6B gene and protein expression levels were significantly upregulated during transforming growth factor-β1 (TGF-β1)-induced SMC differentiation. The knockdown of GPM6B resulted in the downregulation of SMC-specific marker expression and repressed the activation of Smad2/3 signaling. Moreover, GPM6B regulates SMC Differentiation by Controlling TGF-β-Smad2/3 Signaling. Furthermore, we demonstrated that similar to p-Smad2/3, GPM6B was profoundly expressed and coexpressed with SMC differentiation markers in embryonic SMCs. Moreover, GPM6B can regulate the tightness between TβRI, TβRII, or Smad2/3 by directly binding to TβRI to activate Smad2/3 signaling during SMC differentiation, and activation of TGF-β-Smad2/3 signaling also facilitate the expression of GPM6B. Taken together, these findings demonstrate that GPM6B plays a crucial role in SMC differentiation and regulates SMC differentiation through the activation of TGF-β-Smad2/3 signaling via direct interactions with TβRI. This finding indicates that GPM6B is a potential target for deriving SMCs from stem cells in cardiovascular regenerative medicine. Stem Cells 2018 Stem Cells 2019;37:190-201.

The wmN1 enhancer region in intron 1 is required for expression of human PLP1

The myelin proteolipid protein gene (PLP1) encodes the most abundant protein present in myelin from the central nervous system (CNS). Its expression must be tightly controlled as evidenced by mutations that alter PLP1 dosage; both overexpression (elevated PLP1 copy number) and lack thereof (PLP1 deletion) result in X-linked genetic disorders in man. However, not much is known about the mechanisms that govern expression of the human gene. To address this, transgenic mice were generated which utilize human PLP1 (hPLP1) sequences (proximal 6.2 kb of 5'-flanking DNA to the first 38 bp of exon 2) to drive expression of a lacZ reporter cassette. LoxP sites were incorporated around a 1.5-kb section of hPLP1 intron 1 since it contains sequence orthologous to the wmN1 region from mouse which, previously, was shown to augment expression of a minimally-promoted transgene coincident with the active myelination period of CNS development. Eight transgenic lines were generated with the parental, 6.2hPLP(+)Z/FL, transgene. All lines expressed the transgene appropriately in brain as evidenced by staining with X-gal in white matter regions and olfactory bulb. Removal of the "wmN1" region from 6.2hPLP(+)Z/FL with a ubiquitously expressed Cre-driver caused a dramatic reduction in transgene activity. These results demonstrate for the first time that the wmN1 enhancer region: (1) is functional in hPLP1; (2) works in collaboration with its native promoter-not just a basal heterologous promoter; (3) is required for high levels of hPLP1 gene activity; (4) has a broader effect, both spatially and temporally, than originally projected with mPlp1.

Glial M6B stabilizes the axonal membrane at peripheral nodes of Ranvier

Glycoprotein M6B and the closely related proteolipid protein regulate oligodendrocyte myelination in the central nervous system, but their role in the peripheral nervous system is less clear. Here we report that M6B is located at nodes of Ranvier in peripheral nerves where it stabilizes the nodal axolemma. We show that M6B is co-localized and associates with gliomedin at Schwann cell microvilli that are attached to the nodes. Developmental analysis of sciatic nerves, as well as of myelinating Schwann cells/dorsal root ganglion neurons cultures, revealed that M6B is already present at heminodes, which are considered the precursors of mature nodes of Ranvier. However, in contrast to gliomedin, which accumulates at heminodes with or prior to Na+ channels, we often detected Na+ channel clusters at heminodes without any associated M6B, indicating that it is not required for initial channel clustering. Consistently, nodal cell adhesion molecules (NF186, NrCAM), ion channels (Nav1.2 and Kv7.2), cytoskeletal proteins (AnkG and βIV spectrin), and microvilli components (pERM, syndecan3, gliomedin), are all present at both heminodes and mature nodes of Ranvier in Gpm6b null mice. Using transmission electron microscopy, we show that the absence of M6B results in progressive appearance of nodal protrusions of the nodal axolemma, that are often accompanied by the presence of enlarged mitochondria. Our results reveal that M6B is a Schwann cell microvilli component that preserves the structural integrity of peripheral nodes of Ranvier.

Single-cell transcriptomics of the developing lateral geniculate nucleus reveals insights into circuit assembly and refinement

Neurons and nonneuronal cells in the developing brain dynamically regulate gene expression as neural connectivity is established. However, the specific gene programs activated in distinct cell populations during the assembly and refinement of many intact neuronal circuits have not been thoroughly characterized. In this study, we take advantage of recent advances in transcriptomic profiling techniques to characterize gene expression in the postnatal developing lateral geniculate nucleus (LGN) at single-cell resolution. Our data reveal that genes involved in brain development are dynamically regulated in all major cell types of the LGN, suggesting that the establishment of neural connectivity depends upon functional collaboration between multiple neuronal and nonneuronal cell types in this brain region.

Coordinated changes in gene expression underlie the early patterning and cell-type specification of the central nervous system. However, much less is known about how such changes contribute to later stages of circuit assembly and refinement. In this study, we employ single-cell RNA sequencing to develop a detailed, whole-transcriptome resource of gene expression across four time points in the developing dorsal lateral geniculate nucleus (LGN), a visual structure in the brain that undergoes a well-characterized program of postnatal circuit development. This approach identifies markers defining the major LGN cell types, including excitatory relay neurons, oligodendrocytes, astrocytes, microglia, and endothelial cells. Most cell types exhibit significant transcriptional changes across development, dynamically expressing genes involved in distinct processes including retinotopic mapping, synaptogenesis, myelination, and synaptic refinement. Our data suggest that genes associated with synapse and circuit development are expressed in a larger proportion of nonneuronal cell types than previously appreciated. Furthermore, we used this single-cell expression atlas to identify the Prkcd-Cre mouse line as a tool for selective manipulation of relay neurons during a late stage of sensory-driven synaptic refinement. This transcriptomic resource provides a cellular map of gene expression across several cell types of the LGN, and offers insight into the molecular mechanisms of circuit development in the postnatal brain.

Subchronic olanzapine exposure leads to increased expression of myelination-related genes in rat fronto-medial cortex

Schizophrenia is a psychotic disorder with severe and disabling symptoms, such as hallucinations, delusions, blunted affect and social withdrawal. The neuropathology remains elusive, but disturbances in immunity-related processes, neuronal connectivity and myelination have consistently been linked to schizophrenia. Antipsychotic drugs can be efficient in reducing symptoms, acting primarily on the dopamine system, but additional biological targets are likely to exist. Here we have screened for novel mechanisms of action in an animal model, using adult rats exposed to long-acting olanzapine, achieving stable and clinically relevant antipsychotic drug concentrations. By microarray-based examination of global gene expression in the fronto-medial cortex, at the single gene- and gene-set level, we observed downregulation of two neuropeptide-encoding genes, Vgf and Cort (fold change -1,25 and -1,48, respectively) in response to olanzapine exposure. Furthermore, we demonstrated significant upregulation of five out of ~2000 GO predefined gene sets after olanzapine exposure. Strikingly, all were linked to myelination and oligodendrocyte development; "Ensheathment of neurons", "Axon ensheathment", "Myelination", "Myelin sheath" and "Oligodendrocyte development" (FDR-values < 25). Sixteen of the leading edge genes in these gene sets were analysed independently by qPCR, of which 11 genes displayed significant upregulation, including Plp1, Mal, Mag and Cnp (fold change: 1,30, 1,50, 1,30 and 1,15, respectively). Several of the upregulated genes (e.g. MAG, MAL and CNP) have previously been reported as downregulated in post-mortem brain samples from schizophrenia patients. Although caution needs to be taken when extrapolating results from animal studies to humans, the data suggest a role for olanzapine in alleviating myelination-related dysfunction in schizophrenia.

Myelin development, plasticity, and pathology in the auditory system

Myelin allows for the rapid and precise timing of action potential propagation along neuronal circuits and is essential for healthy auditory system function. In this article, we discuss what is currently known about myelin in the auditory system with a focus on the timing of myelination during auditory system development, the role of myelin in supporting peripheral and central auditory circuit function, and how various myelin pathologies compromise auditory information processing. Additionally, in keeping with the increasing recognition that myelin is dynamic and is influenced by experience throughout life, we review the growing evidence that auditory sensory deprivation alters myelin along specific segments of the brain's auditory circuit. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 80-92, 2018.

Does Sirt2 Regulate Cholesterol Biosynthesis During Oligodendroglial Differentiation In Vitro and In Vivo?

Sirtuin2 (SIRT2) is a deacetylase enzyme predominantly expressed in myelinating glia of the central nervous system (CNS). We have previously demonstrated that Sirt2 expression enhances oligodendrocyte (OL) differentiation and arborization in vitro, but the molecular targets of SIRT2 in OLs remain speculative. SIRT2 has been implicated in cholesterol biosynthesis by promoting the nuclear translocation of sterol regulatory element binding protein (SREBP)-2. We investigated this further in CNS myelination by examining the role of Sirt2 in cholesterol biosynthesis in vivo and in vitro employing Sirt2 ^-/- mice, primary OL cells and CG4-OL cells. Our results demonstrate that expression of cholesterol biosynthetic genes in the CNS white matter or cholesterol content in purified myelin fractions did not differ between Sirt2 ^-/- and age-matched wild-type mice. Cholesterol biosynthetic gene expression profiles and total cholesterol content were not altered in primary OLs from Sirt2 ^-/- mice and in CG4-OLs when Sirt2 was either down-regulated with RNAi or overexpressed. In addition, Sirt2 knockdown or overexpression in CG4-OLs had no effect on SREBP-2 nuclear translocation. Our results indicate that Sirt2 does not impact the expression of genes encoding enzymes involved in cholesterol biosynthesis, total cholesterol content, or nuclear translocation of SREBP-2 during OL differentiation and myelination.

Partial Immunoblotting of 2D-Gels: A Novel Method to Identify Post-Translationally Modified Proteins Exemplified for the Myelin Acetylome

Post-translational modifications (PTMs) play a key role in regulating protein function, yet their identification is technically demanding. Here, we present a straightforward workflow to systematically identify post-translationally modified proteins based on two-dimensional gel electrophoresis. Upon colloidal Coomassie staining the proteins are partially transferred, and the investigated PTMs are immunodetected. This strategy allows tracking back the immunopositive antigens to the corresponding spots on the original gel, from which they are excised and mass spectrometrically identified. Candidate proteins are validated on the same membrane by immunodetection using a second fluorescence channel. We exemplify the power of partial immunoblotting with the identification of lysine-acetylated proteins in myelin, the oligodendroglial membrane that insulates neuronal axons. The excellent consistency of the detected fluorescence signals at all levels allows the differential comparison of PTMs across multiple conditions. Beyond PTM screening, our multi-level workflow can be readily adapted to clinical applications such as identifying auto-immune antigens or host-pathogen interactions.

Oligodendroglial membrane dynamics in relation to myelin biogenesis

In the central nervous system, oligodendrocytes synthesize a specialized membrane, the myelin membrane, which enwraps the axons in a multilamellar fashion to provide fast action potential conduction and to ensure axonal integrity. When compared to other membranes, the composition of myelin membranes is unique with its relatively high lipid to protein ratio. Their biogenesis is quite complex and requires a tight regulation of sequential events, which are deregulated in demyelinating diseases such as multiple sclerosis. To devise strategies for remedying such defects, it is crucial to understand molecular mechanisms that underlie myelin assembly and dynamics, including the ability of specific lipids to organize proteins and/or mediate protein-protein interactions in healthy versus diseased myelin membranes. The tight regulation of myelin membrane formation has been widely investigated with classical biochemical and cell biological techniques, both in vitro and in vivo. However, our knowledge about myelin membrane dynamics, such as membrane fluidity in conjunction with the movement/diffusion of proteins and lipids in the membrane and the specificity and role of distinct lipid-protein and protein-protein interactions, is limited. Here, we provide an overview of recent findings about the myelin structure in terms of myelin lipids, proteins and membrane microdomains. To give insight into myelin membrane dynamics, we will particularly highlight the application of model membranes and advanced biophysical techniques, i.e., approaches which clearly provide an added value to insight obtained by classical biochemical techniques.

Septin/anillin filaments scaffold central nervous system myelin to accelerate nerve conduction

Myelination of axons facilitates rapid impulse propagation in the nervous system. The axon/myelin-unit becomes impaired in myelin-related disorders and upon normal aging. However, the molecular cause of many pathological features, including the frequently observed myelin outfoldings, remained unknown. Using label-free quantitative proteomics, we find that the presence of myelin outfoldings correlates with a loss of cytoskeletal septins in myelin. Regulated by phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P₂)-levels, myelin septins (SEPT2/SEPT4/SEPT7/SEPT8) and the PI(4,5)P₂-adaptor anillin form previously unrecognized filaments that extend longitudinally along myelinated axons. By confocal microscopy and immunogold-electron microscopy, these filaments are localized to the non-compacted adaxonal myelin compartment. Genetic disruption of these filaments in Sept8-mutant mice causes myelin outfoldings as a very specific neuropathology. Septin filaments thus serve an important function in scaffolding the axon/myelin-unit, evidently a late stage of myelin maturation. We propose that pathological or aging-associated diminishment of the septin/anillin-scaffold causes myelin outfoldings that impair the normal nerve conduction velocity.

DOI:http://dx.doi.org/10.7554/eLife.17119.001

Normal communication within the brain or between the brain and other parts of the body requires information to flow quickly around the nervous system. This information travels along nerve cells in the form of electrical signals. To speed up the signals, a part of the nerve cell called the axon is frequently wrapped in an electrically insulating sheath made up of a membrane structure called myelin.

The myelin sheath becomes impaired as a result of disease or ageing. In order to understand what might produce these changes, Patzig et al. have used biochemical and microscopy techniques to study mice that had similar defects in their myelin sheaths.

The study reveals that forming a normal myelin sheath around an axon requires a newly identified ‘scaffold’ made of a group of proteins called the septins. Combining with another protein called anillin, septins assemble into filaments in the myelin sheath. These filaments then knit together into a scaffold that grows lengthways along the myelin-wrapped axon. Without this scaffold, the myelin sheath grew defects known as outfoldings. Axons transmitted electrical signals much more slowly than normal when the septin scaffold was missing from the myelin sheath.

Future studies are needed to understand the factors that control how the septin scaffold forms. This could help to reveal ways of reversing the changes that alter the myelin sheath during ageing and disease.

DOI:http://dx.doi.org/10.7554/eLife.17119.002