CNS integrins switch growth factor signalling to promote target-dependent survival

Cell reprogramming for oligodendrocytes: A review of protocols and their applications to disease modeling and cell-based remyelination therapies

Oligodendrocytes are a type of glial cells that produce a lipid-rich membrane called myelin. Myelin assembles into a sheath and lines neuronal axons in the brain and spinal cord to insulate them. This not only increases the speed and efficiency of nerve signal transduction but also protects the axons from damage and degradation, which could trigger neuronal cell death. Demyelination, which is caused by a loss of myelin and oligodendrocytes, is a prominent feature of many neurological conditions, including Multiple sclerosis (MS), spinal cord injuries (SCI), and leukodystrophies. Demyelination is followed by a time of remyelination mediated by the recruitment of endogenous oligodendrocyte precursor cells, their migration to the injury site, and differentiation into myelin-producing oligodendrocytes. Unfortunately, endogenous remyelination is not sufficient to overcome demyelination, which explains why there are to date no regenerative-based treatments for MS, SCI, or leukodystrophies. To better understand the role of oligodendrocytes and develop cell-based remyelination therapies, human oligodendrocytes have been derived from somatic cells using cell reprogramming. This review will detail the different cell reprogramming methods that have been developed to generate human oligodendrocytes and their applications to disease modeling and cell-based remyelination therapies. Recent developments in the field have seen the derivation of brain organoids from pluripotent stem cells, and protocols have been devised to incorporate oligodendrocytes within the organoids, which will also be reviewed.

Development of myelinating glia: An overview

Myelin is essential to nervous system function, playing roles in saltatory conduction and trophic support. Oligodendrocytes (OLs) and Schwann cells (SCs) form myelin in the central and peripheral nervous systems respectively and follow different developmental paths. OLs are neural stem-cell derived and follow an intrinsic developmental program resulting in a largely irreversible differentiation state. During embryonic development, OL precursor cells (OPCs) are produced in distinct waves originating from different locations in the central nervous system, with a subset developing into myelinating OLs. OPCs remain evenly distributed throughout life, providing a population of responsive, multifunctional cells with the capacity to remyelinate after injury. SCs derive from the neural crest, are highly dependent on extrinsic signals, and have plastic differentiation states. SC precursors (SCPs) are produced in early embryonic nerve structures and differentiate into multipotent immature SCs (iSCs), which initiate radial sorting and differentiate into myelinating and non-myelinating SCs. Differentiated SCs retain the capacity to radically change phenotypes in response to external signals, including becoming repair SCs, which drive peripheral regeneration. While several transcription factors and myelin components are common between OLs and SCs, their differentiation mechanisms are highly distinct, owing to their unique lineages and their respective environments. In addition, both OLs and SCs respond to neuronal activity and regulate nervous system output in reciprocal manners, possibly through different pathways. Here, we outline their basic developmental programs, mechanisms regulating their differentiation, and recent advances in the field.

Chronic oligodendrocyte injury in central nervous system pathologies

Myelin, the membrane surrounding neuronal axons, is critical for central nervous system (CNS) function. Injury to myelin-forming oligodendrocytes (OL) in chronic neurological diseases (e.g. multiple sclerosis) ranges from sublethal to lethal, leading to OL dysfunction and myelin pathology, and consequent deleterious impacts on axonal health that drive clinical impairments. This is regulated by intrinsic factors such as heterogeneity and age, and extrinsic cellular and molecular interactions. Here, we discuss the responses of OLs to injury, and perspectives for therapeutic targeting. We put forward that targeting mature OL health in neurological disease is a promising therapeutic strategy to support CNS function.

The molecular regulation of oligodendrocyte development and CNS myelination by ECM proteins

Oligodendrocytes are myelin-forming cells in the central nervous system (CNS). The development of oligodendrocytes is regulated by a large number of molecules, including extracellular matrix (ECM) proteins that are relatively less characterized. Here, we review the molecular functions of the major ECM proteins in oligodendrocyte development and pathology. Among the ECM proteins, laminins are positive regulators in oligodendrocyte survival, differentiation, and/or myelination in the CNS. Conversely, fibronectin, tenascin-C, hyaluronan, and chondroitin sulfate proteoglycans suppress the differentiation and myelination. Tenascin-R shows either positive or negative functions in these activities. In addition, the extracellular domain of the transmembrane protein teneurin-4, which possesses the sequence homology with tenascins, promotes the differentiation of oligodendrocytes. The activities of these ECM proteins are exerted through binding to the cellular receptors and co-receptors, such as integrins and growth factor receptors, which induces the signaling to form the elaborated and functional structure of myelin. Further, the ECM proteins dynamically change their structures and functions at the pathological conditions as multiple sclerosis. The ECM proteins are a critical player to serve as a component of the microenvironment for oligodendrocytes in their development and pathology.

Thyroid hormone-dependent oligodendroglial cell lineage genomic and non-genomic signaling through integrin receptors

Multiple sclerosis (MS) is a heterogeneous autoimmune disease whereby the pathological sequelae evolve from oligodendrocytes (OLs) within the central nervous system and are targeted by the immune system, which causes widespread white matter pathology and results in neuronal dysfunction and neurological impairment. The progression of this disease is facilitated by a failure in remyelination following chronic demyelination. One mediator of remyelination is thyroid hormone (TH), whose reliance on monocarboxylate transporter 8 (MCT8) was recently defined. MCT8 facilitates the entry of THs into oligodendrocyte progenitor cell (OPC) and pre-myelinating oligodendrocytes (pre-OLs). Patients with MS may exhibit downregulated MCT8 near inflammatory lesions, which emphasizes an inhibition of TH signaling and subsequent downstream targeted pathways such as phosphoinositide 3-kinase (PI3K)-Akt. However, the role of the closely related mammalian target of rapamycin (mTOR) in pre-OLs during neuroinflammation may also be central to the remyelination process and is governed by various growth promoting signals. Recent research indicates that this may be reliant on TH-dependent signaling through β1-integrins. This review identifies genomic and non-genomic signaling that is regulated through mTOR in TH-responsive pre-OLs and mature OLs in mouse models of MS. This review critiques data that implicates non-genomic Akt and mTOR signaling in response to TH-dependent integrin receptor activation in pre-OLs. We have also examined whether this can drive remyelination in the context of neuroinflammation and associated sequelae. Importantly, we outline how novel therapeutic small molecules are being designed to target integrin receptors on oligodendroglial lineage cells and whether these are viable therapeutic options for future use in clinical trials for MS.

Pinch2 regulates myelination in the mouse central nervous system

The extensive morphological changes of oligodendrocytes during axon ensheathment and myelination involve assembly of the Ilk-Parvin-Pinch (IPP) heterotrimeric complex of proteins to relay essential mechanical and biochemical signals between integrins and the actin cytoskeleton. Binding of Pinch1 and Pinch2 isoforms to Ilk is mutually exclusive and allows the formation of distinct IPP complexes with specific signaling properties. Using tissue-specific conditional gene ablation in mice, we reveal an essential role for Pinch2 during central nervous system myelination. Unlike Pinch1 gene ablation, loss of Pinch2 in oligodendrocytes results in hypermyelination and in the formation of pathological myelin outfoldings in white matter regions. These structural changes concur with inhibition of Rho GTPase RhoA and Cdc42 activities and phenocopy aspects of myelin pathology observed in corresponding mouse mutants. We propose a dual role for Pinch2 in preventing an excess of myelin wraps through RhoA-dependent control of membrane growth and in fostering myelin stability via Cdc42-dependent organization of cytoskeletal septins. Together, these findings indicate that IPP complexes containing Pinch2 act as a crucial cell-autonomous molecular hub ensuring synchronous control of key signaling networks during developmental myelination.

Newly Identified Deficiencies in the Multiple Sclerosis Central Nervous System and Their Impact on the Remyelination Failure

The pathogenesis of multiple sclerosis (MS) remains enigmatic and controversial. Myelin sheaths in the central nervous system (CNS) insulate axons and allow saltatory nerve conduction. MS brings about the destruction of myelin sheaths and the myelin-producing oligodendrocytes (ODCs). The conundrum of remyelination failure is, therefore, crucial in MS. In this review, the roles of epidermal growth factor (EGF), normal prions, and cobalamin in CNS myelinogenesis are briefly summarized. Thereafter, some findings of other authors and ourselves on MS and MS-like models are recapitulated, because they have shown that: (a) EGF is significantly decreased in the CNS of living or deceased MS patients; (b) its repeated administration to mice in various MS-models prevents demyelination and inflammatory reaction; (c) as was the case for EGF, normal prion levels are decreased in the MS CNS, with a strong correspondence between liquid and tissue levels; and (d) MS cobalamin levels are increased in the cerebrospinal fluid, but decreased in the spinal cord. In fact, no remyelination can occur in MS if these molecules (essential for any form of CNS myelination) are lacking. Lastly, other non-immunological MS abnormalities are reviewed. Together, these results have led to a critical reassessment of MS pathogenesis, partly because EGF has little or no role in immunology.

Laminin regulates oligodendrocyte development and myelination

Oligodendrocytes are the cells that myelinate axons and provide trophic support to neurons in the CNS. Their dysfunction has been associated with a group of disorders known as demyelinating diseases, such as multiple sclerosis. Oligodendrocytes are derived from oligodendrocyte precursor cells, which differentiate into premyelinating oligodendrocytes and eventually mature oligodendrocytes. The development and function of oligodendrocytes are tightly regulated by a variety of molecules, including laminin, a major protein of the extracellular matrix. Accumulating evidence suggests that laminin actively regulates every aspect of oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination. How can laminin exert such diverse functions in oligodendrocytes? It is speculated that the distinct laminin isoforms, laminin receptors, and/or key signaling molecules expressed in oligodendrocytes at different developmental stages are the reasons. Understanding molecular targets and signaling pathways unique to each aspect of oligodendrocyte biology will enable more accurate manipulation of oligodendrocyte development and function, which may have implications in the therapies of demyelinating diseases. Here in this review, we first introduce oligodendrocyte biology, followed by the expression of laminin and laminin receptors in oligodendrocytes and other CNS cells. Next, the functions of laminin in oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination, are discussed in detail. Last, key questions and challenges in the field are discussed. By providing a comprehensive review on laminin's roles in OL lineage cells, we hope to stimulate novel hypotheses and encourage new research in the field.

The extracellular matrix as modifier of neuroinflammation and remyelination in multiple sclerosis

Remyelination failure contributes to axonal loss and progression of disability in multiple sclerosis. The failed repair process could be due to ongoing toxic neuroinflammation and to an inhibitory lesion microenvironment that prevents recruitment and/or differentiation of oligodendrocyte progenitor cells into myelin-forming oligodendrocytes. The extracellular matrix molecules deposited into lesions provide both an altered microenvironment that inhibits oligodendrocyte progenitor cells, and a fuel that exacerbates inflammatory responses within lesions. In this review, we discuss the extracellular matrix and where its molecules are normally distributed in an uninjured adult brain, specifically at the basement membranes of cerebral vessels, in perineuronal nets that surround the soma of certain populations of neurons, and in interstitial matrix between neural cells. We then highlight the deposition of different extracellular matrix members in multiple sclerosis lesions, including chondroitin sulphate proteoglycans, collagens, laminins, fibronectin, fibrinogen, thrombospondin and others. We consider reasons behind changes in extracellular matrix components in multiple sclerosis lesions, mainly due to deposition by cells such as reactive astrocytes and microglia/macrophages. We next discuss the consequences of an altered extracellular matrix in multiple sclerosis lesions. Besides impairing oligodendrocyte recruitment, many of the extracellular matrix components elevated in multiple sclerosis lesions are pro-inflammatory and they enhance inflammatory processes through several mechanisms. However, molecules such as thrombospondin-1 may counter inflammatory processes, and laminins appear to favour repair. Overall, we emphasize the crosstalk between the extracellular matrix, immune responses and remyelination in modulating lesions for recovery or worsening. Finally, we review potential therapeutic approaches to target extracellular matrix components to reduce detrimental neuroinflammation and to promote recruitment and maturation of oligodendrocyte lineage cells to enhance remyelination.

The Current Challenges for Drug Discovery in CNS Remyelination

The myelin sheath wraps around axons, allowing saltatory currents to be transmitted along neurons. Several genetic, viral, or environmental factors can damage the central nervous system (CNS) myelin sheath during life. Unless the myelin sheath is repaired, these insults will lead to neurodegeneration. Remyelination occurs spontaneously upon myelin injury in healthy individuals but can fail in several demyelination pathologies or as a consequence of aging. Thus, pharmacological intervention that promotes CNS remyelination could have a major impact on patient’s lives by delaying or even preventing neurodegeneration. Drugs promoting CNS remyelination in animal models have been identified recently, mostly as a result of repurposing phenotypical screening campaigns that used novel oligodendrocyte cellular models. Although none of these have as yet arrived in the clinic, promising candidates are on the way. Many questions remain. Among the most relevant is the question if there is a time window when remyelination drugs should be administrated and why adult remyelination fails in many neurodegenerative pathologies. Moreover, a significant challenge in the field is how to reconstitute the oligodendrocyte/axon interaction environment representative of healthy as well as disease microenvironments in drug screening campaigns, so that drugs can be screened in the most appropriate disease-relevant conditions. Here we will provide an overview of how the field of in vitro models developed over recent years and recent biological findings about how oligodendrocytes mature after reactivation of their staminal niche. These data have posed novel questions and opened new views about how the adult brain is repaired after myelin injury and we will discuss how these new findings might change future drug screening campaigns for CNS regenerative drugs.

Effects of the extracellular matrix on myelin development and regeneration in the central nervous system

Extracellular matrix (ECM) is a collection of extracellular molecules secreted by cells, providing structural and biochemical support for surrounding tissues. The ECM exerts biological effects by interacting with growth factors, signal receptors or adhesion molecules. In the case of myelin formation and regeneration, the combination of ECM and its receptors (for example, integrins) modulates signaling pathways such as PI3K, MAPK, etc., which in turn induces complex biological effects throughout various stages of myelination and regeneration. Studies have also found that myelin injury would cause changes in ECM composition and thus affecting the myelin regeneration process. Research on the ECM will provide a better understanding of how myelin is formed and regenerated, which will help to develop new therapies for demyelinating diseases. Future progress in this field will provide important information on how to modify the ECM to promote proliferation and differentiation of oligodendrocyte precursor cells (OPC), thereby stimulating myelin formation and regeneration and restoring normal neural function.

Brain Dysfunction in LAMA2-Related Congenital Muscular Dystrophy: Lessons From Human Case Reports and Mouse Models

Laminin α2 gene (LAMA2)-related Congenital Muscular Dystrophy (CMD) was distinguished by a defining central nervous system (CNS) abnormality—aberrant white matter signals by MRI—when first described in the 1990s. In the past 25 years, researchers and clinicians have expanded our knowledge of brain involvement in LAMA2-related CMD, also known as Congenital Muscular Dystrophy Type 1A (MDC1A). Neurological changes in MDC1A can be structural, including lissencephaly and agyria, as well as functional, including epilepsy and intellectual disability. Mouse models of MDC1A include both spontaneous and targeted LAMA2 mutations and range from a partial loss of LAMA2 function (e.g., dy^2J/dy^2J), to a complete loss of LAMA2 expression (dy^3K/dy^3K). Diverse cellular and molecular changes have been reported in the brains of MDC1A mouse models, including blood-brain barrier dysfunction, altered neuro- and gliogenesis, changes in synaptic plasticity, and decreased myelination, providing mechanistic insight into potential neurological dysfunction in MDC1A. In this review article, we discuss selected studies that illustrate the potential scope and complexity of disturbances in brain development in MDC1A, and as well as highlight mechanistic insights that are emerging from mouse models.

Crosstalk between Epidermal Growth Factor Receptors (EGFR) and integrins in resistance to EGFR tyrosine kinase inhibitors (TKIs) in solid tumors

Cell adhesion to the extracellular matrix (ECM) is important in a variety of physiological and pathologic processes, including development, tumor invasion, and metastasis. Integrin-mediated attachment to ECM proteins has emerged to cue events primitively important for the transformed phenotype of human cancer cells. Cross-talk between integrins and growth factor receptors takes an increasingly prominent role in defining adhesion, motility, and cell growth. This functional interaction has expanded beyond to link integrins with resistance to Tyrosine kinase inhibitors (TKIs) of Epidermal Growth Factor Receptors (EGFRs). In this regard, integrin-mediated adhesion has two separate functions one as a clear collaborator with growth factor receptor signaling and the second as a basic mechanism contributing in Epithelial to Mesenchymal Transition (EMT) which affects response to chemotherapy. This review provides an overview of these mechanisms and describes treatment options for selectively targeting and disrupting integrin interaction to EGFR for cancer therapy.

Matrix metalloproteinases shape the oligodendrocyte (niche) during development and upon demyelination

The oligodendrocyte lineage cell is crucial to proper brain function. During central nervous system development, oligodendrocyte progenitor cells (OPCs) migrate and proliferate to populate the entire brain and spinal cord, and subsequently differentiate into mature oligodendrocytes that wrap neuronal axons in an insulating myelin layer. When damage occurs to the myelin sheath, OPCs are activated and recruited to the demyelinated site, where they differentiate into oligodendrocytes that remyelinate the denuded axons. The process of OPC attraction and differentiation is influenced by a multitude of factors from the cell's niche. Matrix metalloproteinases (MMPs) are powerful and versatile enzymes that do not only degrade extracellular matrix proteins, but also cleave cell surface receptors, growth factors, signaling molecules, proteases and other precursor proteins, leading to their activation or degradation. MMPs are markedly upregulated during brain development and upon demyelinating injury, where their broad functions influence the behavior of neural progenitor cells (NPCs), OPCs and oligodendrocytes. In this review, we focus on the role of MMPs in (re)myelination. We will start out in the developing brain with describing the effects of MMPs on NPCs, OPCs and eventually oligodendrocytes. Then, we will outline their functions in oligodendrocyte process extension and developmental myelination. Finally, we will review their potential role in demyelination, describe their significance in remyelination and discuss the evidence for a role of MMPs in remyelination failure, focusing on multiple sclerosis. In conclusion, MMPs shape the oligodendrocyte (niche) both during development and upon demyelination, and thus are important players in directing the fate and behavior of oligodendrocyte lineage cells throughout their life cycle.

A Fyn biosensor reveals pulsatile, spatially localized kinase activity and signaling crosstalk in live mammalian cells

Cell behavior is controlled through spatio-temporally localized protein activity. Despite unique and often contradictory roles played by Src-family-kinases (SFKs) in regulating cell physiology, activity patterns of individual SFKs have remained elusive. Here, we report a biosensor for specifically visualizing active conformation of SFK-Fyn in live cells. We deployed combinatorial library screening to isolate a binding-protein (F29) targeting activated Fyn. Nuclear-magnetic-resonance (NMR) analysis provides the structural basis of F29 specificity for Fyn over homologous SFKs. Using F29, we engineered a sensitive, minimally-perturbing fluorescence-resonance-energy-transfer (FRET) biosensor (FynSensor) that reveals cellular Fyn activity to be spatially localized, pulsatile and sensitive to adhesion/integrin signaling. Strikingly, growth factor stimulation further enhanced Fyn activity in pre-activated intracellular zones. However, inhibition of focal-adhesion-kinase activity not only attenuates Fyn activity, but abolishes growth-factor modulation. FynSensor imaging uncovers spatially organized, sensitized signaling clusters, direct crosstalk between integrin and growth-factor-signaling, and clarifies how compartmentalized Src-kinase activity may drive cell fate.

Remodeling of the interstitial extracellular matrix in white matter multiple sclerosis lesions: Implications for remyelination (failure)

The extracellular matrix (ECM) provides protection, rigidity, and structure toward cells. It consists, among others, of a wide variety of glycoproteins and proteoglycans, which act together to produce a complex and dynamic environment, most relevant in transmembrane events. In the brain, the ECM occupies a notable proportion of its volume and maintains the homeostasis of central nervous system (CNS). In addition, remodeling of the ECM, that is transient changes in ECM proteins regulated by matrix metalloproteinases (MMPs), is an important process that modulates cell behavior upon injury, thereby facilitating recovery. Failure of ECM remodeling plays an important role in the pathogenesis of multiple sclerosis (MS), a neurodegenerative demyelinating disease of the CNS with an inflammatory response against protective myelin sheaths that surround axons. Remyelination of denuded axons improves the neuropathological conditions of MS, but this regeneration process fails over time, leading to chronic disease progression. In this review, we uncover abnormal ECM remodeling in MS lesions by discussing ECM remodeling in experimental demyelination models, that is when remyelination is successful, and compare alterations in ECM components to the ECM composition and MMP expression in the parenchyma of demyelinated MS lesions, that is when remyelination fails. Inter- and intralesional differences in ECM remodeling in the distinct white matter MS lesions are discussed in terms of consequences for oligodendrocyte behavior and remyelination (failure). Hence, the review will aid to understand how abnormal ECM remodeling contributes to remyelination failure in MS lesions and assists in developing therapeutic strategies to promote remyelination.

Laminin α2, α4, and α5 Chains Positively Regulate Migration and Survival of Oligodendrocyte Precursor Cells

In the developing central nervous system (CNS), oligodendrocyte precursor cells (OPCs) migrate along blood vessels and are widely distributed in the CNS. Meanwhile, OPCs require survival factors from the extracellular microenvironment. In other tissues, laminins, heterotrimetric (αβγ) extracellular matrix proteins, promote cell migration and survival. However, the expression pattern and functions of laminins in OPC development remain poorly understood. In the present study, we first investigated the expression of laminin α chains, which bind to cell surface receptors such as integrins, in the postnatal murine brain. We found that laminin α1, α2, α4, and α5 chains were expressed around blood vessels and OPCs attached the laminin α chain-positive vessels. We then evaluated the effect of these laminins on OPCs activity using recombinant laminin E8s (LME8s) that are minimally active fragments of the laminin isoforms. OPCs attached on LM211E8, LM411E8, and LM511E8, containing laminin α2, α4, and α5 chains, respectively, through integrin β1. Further, these three LME8s promoted migration of OPCs, and OPC survival was prolonged on either LM411E8 or LM511E8 via the activation of focal adhesion kinase. Together, our findings suggest that laminins expressed surrounding blood vessels positively regulate migration and survival of OPCs through the integrin β1-FAK pathway.

The oligodendrocyte growth cone and its actin cytoskeleton: A fundamental element for progenitor cell migration and CNS myelination

Cells of the oligodendrocyte (OLG) lineage engage in highly motile behaviors that are crucial for effective central nervous system (CNS) myelination. These behaviors include the guided migration of OLG progenitor cells (OPCs), the surveying of local environments by cellular processes extending from differentiating and pre-myelinating OLGs, and during the process of active myelin wrapping, the forward movement of the leading edge of the myelin sheath's inner tongue along the axon. Almost all of these motile behaviors are driven by actin cytoskeletal dynamics initiated within a lamellipodial structure that is located at the tip of cellular OLG/OPC processes and is structurally as well as functionally similar to the neuronal growth cone. Accordingly, coordinated stoichiometries of actin filament (F-actin) assembly and disassembly at these OLG/OPC growth cones have been implicated in directing process outgrowth and guidance, and the initiation of myelination. Nonetheless, the functional importance of the OLG/OPC growth cone still remains to be fully understood, and, as a unique aspect of actin cytoskeletal dynamics, F-actin depolymerization and disassembly start to predominate at the transition from myelination initiation to myelin wrapping. This review provides an overview of the current knowledge about OLG/OPC growth cones, and it proposes a model in which actin cytoskeletal dynamics in OLG/OPC growth cones are a main driver for morphological transformations and motile behaviors. Remarkably, these activities, at least at the later stages of OLG maturation, may be regulated independently from the transcriptional gene expression changes typically associated with CNS myelination.

Galectin-4, a Negative Regulator of Oligodendrocyte Differentiation, Is Persistently Present in Axons and Microglia/Macrophages in Multiple Sclerosis Lesions

Neuron-derived molecules are potent regulators of oligodendrocyte differentiation and myelination during brain development and upon demyelination. Their analysis will thus contribute to understanding remyelination failure in demyelinating diseases, such as multiple sclerosis (MS). Previously, we have identified neuronal galectin-4 as a novel negative soluble regulator in the timing of developmental myelination. Here, we investigated whether galectin-4 is re-expressed in axons upon demyelination to regulate the timing of remyelination. Our findings revealed that galectin-4 is transiently localized to axons in demyelinated areas upon cuprizone-induced demyelination. In contrast, in chronic demyelinated MS lesions, where remyelination fails, galectin-4 is permanently present on axons. Remarkably, microglia/macrophages in cuprizone-demyelinated areas also harbor galectin-4, as also observed in activated microglia/macrophages that are present in active MS lesions and in inflammatory infiltrates in chronic-relapsing experimental autoimmune encephalomyelitis. In vitro analysis showed that galectin-4 is effectively endocytosed by macrophages, and may scavenge galectin-4 from oligodendrocytes, and that endogenous galectin-4 levels are increased in alternatively interleukin-4-activated macrophages and microglia. Hence, similar to developmental myelination, the (re)expressed galectin-4 upon demyelination may act as factor in the timing of oligodendrocyte differentiation, while the persistent presence of galectin-4 on demyelinated axons may disrupt this fine-tuning of remyelination.

IgM Natural Autoantibodies in Physiology and the Treatment of Disease

Antibodies are vital components of the adaptive immune system for the recognition and response to foreign antigens. However, some antibodies recognize self-antigens in healthy individuals. These autoreactive antibodies may modulate innate immune functions. IgM natural autoantibodies (IgM-NAAs) are a class of primarily polyreactive immunoglobulins encoded by germline V-gene segments which exhibit low affinity but broad specificity to both foreign and self-antigens. Historically, these autoantibodies were closely associated with autoimmune disease. Nevertheless, not all human autoantibodies are pathogenic and compelling evidence indicates that IgM-NAAs may exert a spectrum of effects from injurious to protective depending upon cellular and molecular context. In this chapter, we review the current state of knowledge regarding the potential physiological and therapeutic roles of IgM-NAAs in different disease conditions such as atherosclerosis, cancer, and autoimmune disease. We also describe the discovery of two reparative IgM-NAAs by our laboratory and delineate their proposed mechanisms of action in central nervous system (CNS) disease.

Brain-Derived Neurotrophic Factor in Central Nervous System Myelination: A New Mechanism to Promote Myelin Plasticity and Repair

Brain-derived neurotrophic factor (BDNF) plays vitally important roles in neural development and plasticity in both health and disease. Recent studies using mutant mice to selectively manipulate BDNF signalling in desired cell types, in combination with animal models of demyelinating disease, have demonstrated that BDNF not only potentiates normal central nervous system myelination in development but enhances recovery after myelin injury. However, the precise mechanisms by which BDNF enhances myelination in development and repair are unclear. Here, we review some of the recent progress made in understanding the influence BDNF exerts upon the myelinating process during development and after injury, and discuss the cellular and molecular mechanisms underlying its effects. In doing so, we raise new questions for future research.

Injury-induced perivascular niche supports alternative differentiation of adult rodent CNS progenitor cells

Following CNS demyelination, oligodendrocyte progenitor cells (OPCs) are able to differentiate into either remyelinating oligodendrocytes (OLs) or remyelinating Schwann cells (SCs). However, the signals that determine which type of remyelinating cell is generated and the underlying mechanisms involved have not been identified. Here, we show that distinctive microenvironments created in discrete niches within demyelinated white matter determine fate decisions of adult OPCs. By comparative transcriptome profiling we demonstrate that an ectopic, injury-induced perivascular niche is enriched with secreted ligands of the BMP and Wnt signalling pathways, produced by activated OPCs and endothelium, whereas reactive astrocyte within non-vascular area express the dual BMP/Wnt antagonist Sostdc1. The balance of BMP/Wnt signalling network is instructive for OPCs to undertake fate decision shortly after their activation: disruption of the OPCs homeostasis during demyelination results in BMP4 upregulation, which, in the absence of Socstdc1, favours SCs differentiation.

To Be or Not to Be: Environmental Factors that Drive Myelin Formation during Development and after CNS Trauma

Oligodendrocytes are specialized glial cells that myelinate central nervous system (CNS) axons. Historically, it was believed that the primary role of myelin was to compactly ensheath axons, providing the insulation necessary for rapid signal conduction. However, mounting evidence demonstrates the dynamic importance of myelin and oligodendrocytes, including providing metabolic support to neurons and regulating axon protein distribution. As such, the development and maintenance of oligodendrocytes and myelin are integral to preserving CNS homeostasis and supporting proper functioning of widespread neural networks. Environmental signals are critical for proper oligodendrocyte lineage cell progression and their capacity to form functional compact myelin; these signals are markedly disturbed by injury to the CNS, which may compromise endogenous myelin repair capabilities. This review outlines some key environmental factors that drive myelin formation during development and compares that to the primary factors that define a CNS injury milieu. We aim to identify developmental factors disrupted after CNS trauma as well as pathogenic factors that negatively impact oligodendrocyte lineage cells, as these are potential therapeutic targets to promote myelin repair after injury or disease.

Microglial transglutaminase-2 drives myelination and myelin repair via GPR56/ADGRG1 in oligodendrocyte precursor cells

In the central nervous system (CNS), myelin formation and repair are regulated by oligodendrocyte (OL) lineage cells, which sense and integrate signals from their environment, including from other glial cells and the extracellular matrix (ECM). The signaling pathways that coordinate this complex communication, however, remain poorly understood. The adhesion G protein-coupled receptor ADGRG1 (also known as GPR56) is an evolutionarily conserved regulator of OL development in humans, mice, and zebrafish, although its activating ligand for OL lineage cells is unknown. Here, we report that microglia-derived transglutaminase-2 (TG2) signals to ADGRG1 on OL precursor cells (OPCs) in the presence of the ECM protein laminin and that TG2/laminin-dependent activation of ADGRG1 promotes OPC proliferation. Signaling by TG2/laminin to ADGRG1 on OPCs additionally improves remyelination in two murine models of demyelination. These findings identify a novel glia-to-glia signaling pathway that promotes myelin formation and repair, and suggest new strategies to enhance remyelination.

BDNF haploinsufficiency exerts a transient and regionally different influence upon oligodendroglial lineage cells during postnatal development

Brain-Derived Neurotrophic Factor (BDNF) plays important roles in promoting myelination in the developing central nervous system (CNS), however the influence it exerts on oligodendrocyte development in vivo remains unclear. As BDNF knockout mice die in the perinatal period, we undertook a systematic developmental analysis of oligodendroglial lineage cells within multiple CNS regions of BDNF heterozygous (HET) mice. Our data identify that BDNF heterozygosity results in transient reductions in oligodendroglial lineage cell density and progression that are largely restricted to the optic nerve, whereas the corpus callosum, cerebral cortex, basal forebrain and spinal cord white matter tracts are unaffected. In the first two postnatal weeks, BDNF HET mice exhibit reductions in the density of oligodendroglial lineage cells, oligodendrocyte precursor cells (OPCs) and postmitotic oligodendrocytes selectively in the optic nerve, but not in the brain or spinal cord white matter tracts. However, this normalizes later in development. The overall proportion of OPCs and mature oligodendrocytes remains unchanged from P9 to P30 in all CNS regions. This study identifies that BDNF exerts transient effects on oligodendroglial lineage cells selectively in the optic nerve during postnatal development. Taken together, this provides compelling evidence that BDNF haploinsufficiency exerts modest effects upon oligodendroglial cell density and lineage progression in vivo, suggesting its major role is restricted to promoting oligodendrocyte myelination.

Engineered 3D-printed artificial axons

Myelination is critical for transduction of neuronal signals, neuron survival and normal function of the nervous system. Myelin disorders account for many debilitating neurological diseases such as multiple sclerosis and leukodystrophies. The lack of experimental models and tools to observe and manipulate this process in vitro has constrained progress in understanding and promoting myelination, and ultimately developing effective remyelination therapies. To address this problem, we developed synthetic mimics of neuronal axons, representing key geometric, mechanical, and surface chemistry components of biological axons. These artificial axons exhibit low mechanical stiffness approaching that of a human axon, over unsupported spans that facilitate engagement and wrapping by glial cells, to enable study of myelination in environments reflecting mechanical cues that neurons present in vivo. Our 3D printing approach provides the capacity to vary independently the complex features of the artificial axons that can reflect specific states of development, disease, or injury. Here, we demonstrate that oligodendrocytes' production and wrapping of myelin depend on artificial axon stiffness, diameter, and ligand coating. This biofidelic platform provides direct visualization and quantification of myelin formation and myelinating cells' response to both physical cues and pharmacological agents.

Myelination is delayed during postnatal brain development in the mdx mouse model of Duchenne muscular dystrophy

BACKGROUND

In Duchenne muscular dystrophy (DMD), the loss of the dystrophin component of the dystrophin-glycoprotein complex (DGC) compromises plasma membrane integrity in skeletal muscle, resulting in extensive muscle degeneration. In addition, many DMD patients exhibit brain deficits in which the cellular etiology remains poorly understood. We recently found that dystroglycan, a receptor component of the DGC that binds intracellularly to dystrophin, regulates the development of oligodendrocytes, the myelinating glial cells of the brain.

RESULTS

We investigated whether dystrophin contributes to oligodendroglial function and brain myelination. We found that oligodendrocytes express up to three dystrophin isoforms, in conjunction with classic DGC components, which are developmentally regulated during differentiation and in response to extracellular matrix engagement. We found that mdx mice, a model of DMD lacking expression of the largest dystrophin isoform, have delayed myelination and inappropriate oligodendrocyte progenitor proliferation in the cerebral cortex. When we prevented the expression of all oligodendroglial dystrophin isoforms in cultured oligodendrocytes using RNA interference, we found that later stages of oligodendrocyte maturation were significantly delayed, similar to mdx phenotypes in the developing brain.

CONCLUSIONS

We find that dystrophin is expressed in oligodendrocytes and influences developmental myelination, which provides new insight into potential cellular contributors to brain dysfunction associated with DMD.

Differential Regulation of MeCP2 Phosphorylation by Laminin in Oligodendrocytes

Oligodendrocytes (OLGs) are the myelinating cells of the central nervous system (CNS), and its proper differentiation is crucial for normal functioning of neurons. Methyl-CpG-binding protein 2 (MeCP2) is a multifunctional methylated DNA binding protein; mutation of which causes Rett syndrome, a severe neurodevelopmental disorder. Previously, we reported that MeCP2 is expressed in all the stages of oligodendrocyte development, and also shown the role of MeCP2 as a transcription regulator of myelin genes in OLGs. The expression and function of MeCP2 phosphorylation at S80 (pS80MeCP2) has been well studied in neurons and astrocytes; however, there is no data so far available in OLGs regarding pS80MeCP2. Certain developmental stimuli such as growth factors and extracellular matrix (ECM) protein play important role in OLG development. In the present study, we have examined the effects of external stimuli (growth factors (GFs) and extracellular matrix (ECMs)) on S80 phosphorylation of MeCP2 in N19 oligodendroglial cells (N19 OLGs). This study provides the first evidence that laminin (LN) differentially regulates the expression of pS80MeCP2 in immature and mature N19 OLGs. Thus, MeCP2 is phosphorylated in a stimulus-dependent manner during oligodendrocyte development, and thereby, it may regulate the oligodendrocyte behavior.

Fibronectin on extracellular vesicles from microvascular endothelial cells is involved in the vesicle uptake into oligodendrocyte precursor cells

We previously reported transplantation of brain microvascular endothelial cells (MVECs) into cerebral white matter infarction model improved the animal's behavioral outcome by increasing the number of oligodendrocyte precursor cells (OPCs). We also revealed extracellular vesicles (EVs) derived from MVECs promoted survival and proliferation of OPCs in vitro. In this study, we investigated the mechanism how EVs derived from MVECs contribute to OPC survival and proliferation. Protein mass spectrometry and enzyme-linked immunosorbent assay revealed fibronectin was abundant on the surface of EVs from MVECs. As fibronectin has been reported to promote OPC survival and proliferation via integrin signaling pathway, we blocked the binding between fibronectin and integrins using RGD sequence mimics. Blocking the binding, however, did not attenuate the survival and proliferation promoting effect of EVs on OPCs. Flow cytometric and imaging analyses revealed fibronectin on EVs mediates their internalization into OPCs by its binding to heparan sulfate proteoglycan on OPCs. OPC survival and proliferation promoted by EVs were attenuated by blocking the internalization of EVs into OPCs. These lines of evidence suggest that fibronectin on EVs mediates their internalization into OPCs, and the cargo of EVs promotes survival and proliferation of OPCs, independent of integrin signaling pathway.

Involvement of the guanine nucleotide exchange factor Vav3 in central nervous system development and plasticity

Small GTP-hydrolyzing enzymes (GTPases) of the RhoA family play manifold roles in cell biology and are regulated by upstream guanine nucleotide exchange factors (GEFs). Herein, we focus on the GEFs of the Vav subfamily. Vav1 was originally described as a proto-oncogene of the hematopoietic lineage. The GEFs Vav2 and Vav3 are more broadly expressed in various tissues. In particular, the GEF Vav3 may play important roles in the developing nervous system during the differentiation of neural stem cells into the major lineages, namely neurons, oligodendrocytes and astrocytes. We discuss its putative regulatory roles for progenitor differentiation in the developing retina, polarization of neurons and formation of synapses, migration of oligodendrocyte progenitors and establishment of myelin sheaths. We propose that Vav3 mediates the response of various neural cell types to environmental cues.

Adhesion G-protein coupled receptors and extracellular matrix proteins: Roles in myelination and glial cell development

Adhesion G protein-coupled receptors (aGPCRs) are a large family of transmembrane proteins that play important roles in many processes during development, primarily through cell-cell and cell-extracellular matrix (ECM) interactions. In the nervous system, they have been linked to the complex process of myelination, both in the central and peripheral nervous system. GPR126 is essential in Schwann cell-mediated myelination in the peripheral nervous system (PNS), while GPR56 is involved in oligodendrocyte development central nervous system (CNS) myelination. VLGR1 is another aGPCR that is associated with the expression of myelin-associated glycoprotein (MAG) which has inhibitory effects on the process of nerve repair. The ECM is composed of a vast array of structural proteins, three of which interact specifically with aGPCRs: collagen III/GPR56, collagen IV/GPR126, and laminin-211/GPR126. As druggable targets, aGPCRs are valuable in their ability to unlock treatment for a wide variety of currently debilitating myelin disorders. Developmental Dynamics 246:275-284, 2017. © 2016 Wiley Periodicals, Inc.

Extracellular cues influencing oligodendrocyte differentiation and (re)myelination

There is an increasing number of neurologic disorders found to be associated with loss and/or dysfunction of the CNS myelin sheath, ranging from the classic demyelinating disease, multiple sclerosis, through CNS injury, to neuropsychiatric diseases. The disabling burden of these diseases has sparked a growing interest in gaining a better understanding of the molecular mechanisms regulating the differentiation of the myelinating cells of the CNS, oligodendrocytes (OLGs), and the process of (re)myelination. In this context, the importance of the extracellular milieu is becoming increasingly recognized. Under pathological conditions, changes in inhibitory as well as permissive/promotional cues are thought to lead to an overall extracellular environment that is obstructive for the regeneration of the myelin sheath. Given the general view that remyelination is, even though limited in human, a natural response to demyelination, targeting pathologically 'dysregulated' extracellular cues and their downstream pathways is regarded as a promising approach toward the enhancement of remyelination by endogenous (or if necessary transplanted) OLG progenitor cells. In this review, we will introduce the extracellular cues that have been implicated in the modulation of (re)myelination. These cues can be soluble, part of the extracellular matrix (ECM) or mediators of cell-cell interactions. Their inhibitory and permissive/promotional roles with regard to remyelination as well as their potential for therapeutic intervention will be discussed.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

G protein-coupled receptor 37 is a negative regulator of oligodendrocyte differentiation and myelination

While the formation of myelin by oligodendrocytes is critical for the function of the central nervous system, the molecular mechanism controlling oligodendrocyte differentiation remains largely unknown. Here we identify G protein-coupled receptor 37 (GPR37) as an inhibitor of late-stage oligodendrocyte differentiation and myelination. GPR37 is enriched in oligodendrocytes and its expression increases during their differentiation into myelin forming cells. Genetic deletion of Gpr37 does not affect the number of oligodendrocyte precursor cells, but results in precocious oligodendrocyte differentiation and hypermyelination. The inhibition of oligodendrocyte differentiation by GPR37 is mediated by suppression of an exchange protein activated by cAMP (EPAC)-dependent activation of Raf-MAPK-ERK1/2 module and nuclear translocation of ERK1/2. Our data suggest that GPR37 regulates central nervous system myelination by controlling the transition from early-differentiated to mature oligodendrocytes.

The molecular mechanism controlling oligodendrocyte differentiation is not fully understood. Here the authors show that G protein coupled receptor 37 acts as a negative regulator of CNS myelination, and this effect is mediated by suppression of ERK signalling.

A new in vitro mouse oligodendrocyte precursor cell migration assay reveals a role for integrin-linked kinase in cell motility

Background

The decline of remyelination in chronic multiple sclerosis (MS) is in part attributed to inadequate oligodendrocyte precursor cell (OPC) migration, a process governed by the extracellular matrix (ECM). Elucidating the mechanisms underlying OPC migration is therefore an important step towards developing new therapeutic strategies to promote myelin repair. Many seminal OPC culture methods were established using rat-sourced cells, and these often need modification for use with mouse OPCs due to their sensitive nature. It is of interest to develop mouse OPC assays to leverage the abundant transgenic lines. To this end, we developed a new OPC migration method specifically suited for use with mouse-derived cells.

Results

To validate its utility, we combined the new OPC migration assay with a conditional knockout approach to investigate the role of integrin-linked kinase (ILK) in OPC migration. ILK is a focal adhesion protein that stabilizes cellular adhesions to the extracellular matrix (ECM) by mediating a linkage between matrix-bound integrin receptors and the cytoskeleton. We identified ILK as a regulator of OPC migration on three permissive substrates. ILK loss produced an early, albeit transient, deficit in OPC migration on laminin matrix, while migration on fibronectin and polylysine was heavily reliant on ILK expression.

Conclusions

Inclusively, our work provides a new tool for studying mouse OPC migration and highlights the role of ILK in its regulation on ECM proteins relevant to MS.

The Orphan G Protein-coupled Receptor GPR17 Negatively Regulates Oligodendrocyte Differentiation via Gαi/o and Its Downstream Effector Molecules

Recent studies have recognized G protein-coupled receptors as important regulators of oligodendrocyte development. GPR17, in particular, is an orphan G protein-coupled receptor that has been identified as oligodendroglial maturation inhibitor because its stimulation arrests primary mouse oligodendrocytes at a less differentiated stage. However, the intracellular signaling effectors transducing its activation remain poorly understood. Here, we use Oli-neu cells, an immortalized cell line derived from primary murine oligodendrocytes, and primary rat oligodendrocyte cultures as model systems to identify molecular targets that link cell surface GPR17 to oligodendrocyte maturation blockade. We demonstrate that stimulation of GPR17 by the small molecule agonist MDL29,951 (2-carboxy-4,6-dichloro-1H-indole-3-propionic acid) decreases myelin basic protein expression levels mainly by triggering the Gαi/o signaling pathway, which in turn leads to reduced activity of the downstream cascade adenylyl cyclase-cAMP-PKA-cAMP response element-binding protein (CREB). In addition, we show that GPR17 activation also diminishes myelin basic protein abundance by lessening stimulation of the exchange protein directly activated by cAMP (EPAC), thus uncovering a previously unrecognized role for EPAC to regulate oligodendrocyte differentiation. Together, our data establish PKA and EPAC as key downstream effectors of GPR17 that inhibit oligodendrocyte maturation. We envisage that treatments augmenting PKA and/or EPAC activity represent a beneficial approach for therapeutic enhancement of remyelination in those demyelinating diseases where GPR17 is highly expressed, such as multiple sclerosis.

Fyn is an intermediate kinase that BDNF utilizes to promote oligodendrocyte myelination

Fyn, a member of the Src family of nonreceptor tyrosine kinases, promotes central nervous system myelination during development; however the mechanisms mediating this effect remain unknown. Here we show that Fyn phosphorylation is modulated by BDNF in vivo. Concordant with this, we find that BDNF stimulates Fyn phosphorylation in myelinating cocultures, an effect dependent on oligodendroglial expression of TrkB. Importantly, PP2, a pharmacological inhibitor of Src family kinases, not only abrogated the promyelinating influence of BDNF in vitro, but also attenuated BDNF-induced phosphorylation of Erk1/2 in oligodendrocytes. Over-expression of Fyn in oligodendrocytes significantly promotes phosphorylation of Erk1/2, and promotes myelination to the extent that exogenous BDNF exerts no additive effect in vitro. In contrast, expression of a kinase-dead mutant of Fyn in oligodendrocytes significantly inhibited BDNF-induced activation of Erk1/2 and abrogated the promyelinating effect of BDNF. Analysis of white matter tracts in vivo revealed that phosphorylated Fyn primarily colocalized with mature oligodendrocytes, and was rarely observed in oligodendrocyte progenitor cells, a profile that closely parallels the detection of phosphorylated Erk1/2 in the developing central nervous system. Taken together, these data identify that Fyn kinase exerts a key role in mediating the promyelinating influence of BDNF. Here we identify a pathway in which BDNF activation of oligodendroglial TrkB receptors stimulates the phosphorylation of Fyn, a necessary step required to potentiate the phosphorylation of Erk1/2, which in turn regulates oligodendrocyte myelination.

Disturbed macro-connectivity in schizophrenia linked to oligodendrocyte dysfunction: from structural findings to molecules

Schizophrenia is a severe psychiatric disorder with multi-factorial characteristics. A number of findings have shown disrupted synaptic connectivity in schizophrenia patients and emerging evidence suggests that this results from dysfunctional oligodendrocytes, the cells responsible for myelinating axons in white matter to promote neuronal conduction. The exact cause of this is not known, although recent imaging and molecular profiling studies of schizophrenia patients have identified changes in white matter tracts connecting multiple brain regions with effects on protein signaling networks involved in the myelination process. Further understanding of oligodendrocyte dysfunction in schizophrenia could lead to identification of novel drug targets for this devastating disease.

Axo-Glia Interaction Preceding CNS Myelination Is Regulated by Bidirectional Eph-Ephrin Signaling

In the central nervous system, myelination of axons is required to ensure fast saltatory conduction and for survival of neurons. However, not all axons are myelinated, and the molecular mechanisms involved in guiding the oligodendrocyte processes toward the axons to be myelinated are not well understood. Only a few negative or positive guidance clues that are involved in regulating axo-glia interaction prior to myelination have been identified. One example is laminin, known to be required for early axo-glia interaction, which functions through α6β1 integrin. Here, we identify the Eph-ephrin family of guidance receptors as novel regulators of the initial axo-glia interaction, preceding myelination. We demonstrate that so-called forward and reverse signaling, mediated by members of both Eph and ephrin subfamilies, has distinct and opposing effects on processes extension and myelin sheet formation. EphA forward signaling inhibits oligodendrocyte process extension and myelin sheet formation, and blocking of bidirectional signaling through this receptor enhances myelination. Similarly, EphB forward signaling also reduces myelin membrane formation, but in contrast to EphA forward signaling, this occurs in an integrin-dependent manner, which can be reversed by overexpression of a constitutive active β1-integrin. Furthermore, ephrin-B reverse signaling induced by EphA4 or EphB1 enhances myelin sheet formation. Combined, this suggests that the Eph-ephrin receptors are important mediators of bidirectional signaling between axons and oligodendrocytes. It further implies that balancing Eph-ephrin forward and reverse signaling is important in the selection process of axons to be myelinated.

Laminin promotes metalloproteinase-mediated dystroglycan processing to regulate oligodendrocyte progenitor cell proliferation

The cell surface receptor dystroglycan mediates interactions between oligodendroglia and laminin-211, an extracellular matrix protein that regulates timely oligodendroglial development. However, dystroglycan's precise role in oligodendroglial development and the potential mechanisms to regulate laminin-dystroglycan interactions remain unknown. Here we report that oligodendroglial dystroglycan is cleaved by metalloproteinases, thereby uncoupling oligodendroglia from laminin binding. Dystroglycan cleavage is selectively stimulated by oligodendrocyte progenitor cell attachment to laminin-211, but not laminin-111 or poly-D-lysine. In addition, dystroglycan cleavage occurs most prominently in oligodendrocyte progenitor cells, with limited dystroglycan cleavage observed in differentiating oligodendrocytes. When dystroglycan cleavage is blocked by metalloproteinase inhibitors, oligodendrocyte progenitor cell proliferation is substantially decreased. Conversely, expression of the intracellular portion of cleaved dystroglycan results in increased oligodendrocyte progenitor cell proliferation, suggesting that endogenous dystroglycan cleavage may promote oligodendrocyte progenitor cell cycle progression. Intriguingly, while matrix metalloproteinase-2 and/or -9 have been reported to be responsible for dystroglycan cleavage, we find that these two metalloproteinases are neither necessary nor sufficient for cleavage of oligodendroglial dystroglycan. In summary, laminin-211 stimulates metalloproteinase-mediated dystroglycan cleavage in oligodendrocyte progenitor cells (but not in differentiated oligodendrocytes), which in turn promotes oligodendrocyte progenitor cell proliferation. This novel regulation of oligodendroglial laminin-dystroglycan interactions may have important consequences for oligodendroglial differentiation, both during development and during disease when metalloproteinase levels become elevated.

The roles of extracellular related-kinases 1 and 2 signaling in CNS myelination

Substantial progress has been made in identifying the intracellular signaling pathways that regulate central nervous system myelination. Recently, the mitogen activated protein kinase pathway, in particular the extracellular signal-related kinase 1 (Erk1) and Erk2, have been identified as critically important in mediating the effects of several growth factors that regulate oligodendroglial development and myelination. Here we will review the recent studies that identify the key role that Erk1/2 signaling plays in regulating oligodendroglial development, myelination and remyelination, discuss the potential mechanisms that Erk1/2 may utilize to influence myelination, and highlight some questions for further research. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Drosophila PS2 and PS3 integrins play distinct roles in retinal photoreceptors-glia interactions

Cellular migration and differentiation are important developmental processes that require dynamic cellular adhesion. Integrins are heterodimeric transmembrane receptors that play key roles in adhesion plasticity. Here, we explore the developing visual system of Drosophila to study the roles of integrin heterodimers in glia development. Our data show that αPS2 is essential for retinal glia migration from the brain into the eye disc and that glial cells have a role in the maintenance of the fenestrated membrane (Laminin-rich ECM layer) in the disc. Interestingly, the absence of glial cells in the eye disc did not affect the targeting of retinal axons to the optic stalk. In contrast, αPS3 is not required for retinal glia migration, but together with Talin, it functions in glial cells to allow photoreceptor axons to target the optic stalk. Thus, we present evidence that αPS2 and αPS3 integrin have different and specific functions in the development of retinal glia.

Static stretch affects neural stem cell differentiation in an extracellular matrix-dependent manner

Neural stem and progenitor cell (NSPC) fate is strongly influenced by mechanotransduction as modulation of substrate stiffness affects lineage choice. Other types of mechanical stimuli, such as stretch (tensile strain), occur during CNS development and trauma, but their consequences for NSPC differentiation have not been reported. We delivered a 10% static equibiaxial stretch to NSPCs and examined effects on differentiation. We found static stretch specifically impacts NSPC differentiation into oligodendrocytes, but not neurons or astrocytes, and this effect is dependent on particular extracellular matrix (ECM)-integrin linkages. Generation of oligodendrocytes from NSPCs was reduced on laminin, an outcome likely mediated by the α6 laminin-binding integrin, whereas similar effects were not observed for NSPCs on fibronectin. Our data demonstrate a direct role for tensile strain in dictating the lineage choice of NSPCs and indicate the dependence of this phenomenon on specific substrate materials, which should be taken into account for the design of biomaterials for NSPC transplantation.

Lipid membrane domains in the brain

The brain is characterized by the presence of cell types with very different functional specialization, but with the common trait of a very high complexity of structures originated by their plasma membranes. Brain cells bear evident membrane polarization with the creation of different morphological and functional subcompartments, whose formation, stabilization and function require a very high level of lateral order within the membrane. In other words, the membrane specialization of brain cells implies the presence of distinct membrane domains. The brain is the organ with the highest enrichment in lipids like cholesterol, glycosphingolipids, and the most recently discovered brain membrane lipid, phosphatidylglucoside, whose collective behavior strongly favors segregation within the membrane leading to the formation of lipid-driven membrane domains. Lipid-driven membrane domains function as dynamic platforms for signal transduction, protein processing, and membrane turnover. Essential events involved in the development and in the maintenance of the functional integrity of the brain depend on the organization of lipid-driven membrane domains, and alterations in lipid homeostasis, leading to deranged lipid-driven membrane organization, are common in several major brain diseases. In this review, we summarize the forces behind the formation of lipid membrane domains and their biological roles in different brain cells. This article is part of a Special Issue entitled Brain Lipids.

The major myelin-resident protein PLP is transported to myelin membranes via a transcytotic mechanism: involvement of sulfatide

Myelin membranes are sheet-like extensions of oligodendrocytes that can be considered membrane domains distinct from the cell's plasma membrane. Consistent with the polarized nature of oligodendrocytes, we demonstrate that transcytotic transport of the major myelin-resident protein proteolipid protein (PLP) is a key element in the mechanism of myelin assembly. Upon biosynthesis, PLP traffics to myelin membranes via syntaxin 3-mediated docking at the apical-surface-like cell body plasma membrane, which is followed by subsequent internalization and transport to the basolateral-surface-like myelin sheet. Pulse-chase experiments, in conjunction with surface biotinylation and organelle fractionation, reveal that following biosynthesis, PLP is transported to the cell body surface in Triton X-100 (TX-100)-resistant microdomains. At the plasma membrane, PLP transiently resides within these microdomains and its lateral dissipation is followed by segregation into 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS)-resistant domains, internalization, and subsequent transport toward the myelin membrane. Sulfatide triggers PLP's reallocation from TX-100- into CHAPS-resistant membrane domains, while inhibition of sulfatide biosynthesis inhibits transcytotic PLP transport. Taking these findings together, we propose a model in which PLP transport to the myelin membrane proceeds via a transcytotic mechanism mediated by sulfatide and characterized by a conformational alteration and dynamic, i.e., transient, partitioning of PLP into distinct membrane microdomains involved in biosynthetic and transcytotic transport.

The EIIIA domain from astrocyte-derived fibronectin mediates proliferation of oligodendrocyte progenitor cells following CNS demyelination

Central nervous system remyelination by oligodendrocyte progenitor cells (OPCs) ultimately fails in the majority of multiple sclerosis (MS) lesions. Remyelination benefits from transient expression of factors that promote migration and proliferation of OPCs, which may include fibronectin (Fn). Fn is present in demyelinated lesions in two major forms; plasma Fn (pFn), deposited following blood‐brain barrier disruption, and cellular Fn, synthesized by resident glial cells and containing alternatively spliced domains EIIIA and EIIIB. Here, we investigated the distinctive roles that astrocyte‐derived Fn (aFn) and pFn play in remyelination. We used an inducible Cre‐lox recombination strategy to selectively remove pFn, aFn or both from mice, and examined the impact on remyelination of toxin‐induced demyelinated lesions of spinal cord white matter. This approach revealed that astrocytes are a major source of Fn in demyelinated lesions. Furthermore, following aFn conditional knockout, the number of OPCs recruited to the demyelinated lesion decreased significantly, whereas OPC numbers were unaltered following pFn conditional knockout. However, remyelination completed normally following conditional knockout of aFn and pFn. Both the EIIIA and EIIIB domains of aFn were expressed following demyelination, and in vitro assays demonstrated that the EIIIA domain of aFn mediates proliferation of OPCs, but not migration. Therefore, although the EIIIA domain from aFn mediates OPC proliferation, aFn is not essential for successful remyelination. Since previous findings indicated that astrocyte‐derived Fn aggregates in chronic MS lesions inhibit remyelination, aFn removal may benefit therapeutic strategies to promote remyelination in MS. GLIA 2015;63:242–256

We investigated the roles of cellular and plasma fibronectin in remyelination, and found that astrocyte‐derived fibronectin mediates oligodendrocyte progenitor cell proliferation in a toxin‐induced demyelination model, likely via the EIIIA domain.

The lateral membrane organization and dynamics of myelin proteins PLP and MBP are dictated by distinct galactolipids and the extracellular matrix

In the central nervous system, lipid-protein interactions are pivotal for myelin maintenance, as these interactions regulate protein transport to the myelin membrane as well as the molecular organization within the sheath. To improve our understanding of the fundamental properties of myelin, we focused here on the lateral membrane organization and dynamics of peripheral membrane protein 18.5-kDa myelin basic protein (MBP) and transmembrane protein proteolipid protein (PLP) as a function of the typical myelin lipids galactosylceramide (GalC), and sulfatide, and exogenous factors such as the extracellular matrix proteins laminin-2 and fibronectin, employing an oligodendrocyte cell line, selectively expressing the desired galactolipids. The dynamics of MBP were monitored by z-scan point fluorescence correlation spectroscopy (FCS) and raster image correlation spectroscopy (RICS), while PLP dynamics in living cells were investigated by circular scanning FCS. The data revealed that on an inert substrate the diffusion rate of 18.5-kDa MBP increased in GalC-expressing cells, while the diffusion coefficient of PLP was decreased in sulfatide-containing cells. Similarly, when cells were grown on myelination-promoting laminin-2, the lateral diffusion coefficient of PLP was decreased in sulfatide-containing cells. In contrast, PLP's diffusion rate increased substantially when these cells were grown on myelination-inhibiting fibronectin. Additional biochemical analyses revealed that the observed differences in lateral diffusion coefficients of both proteins can be explained by differences in their biophysical, i.e., galactolipid environment, specifically with regard to their association with lipid rafts. Given the persistence of pathological fibronectin aggregates in multiple sclerosis lesions, this fundamental insight into the nature and dynamics of lipid-protein interactions will be instrumental in developing myelin regenerative strategies.