Exploring oligodendrocyte guidance: 'to boldly go where no cell has gone before'

The Emerging Role of Extracellular Vesicle Derived From Neurons/Neurogliocytes in Central Nervous System Diseases: Novel Insights Into Ischemic Stroke

Neurons and neurogliocytes (oligodendrocytes, astrocytes, and microglia) are essential for maintaining homeostasis of the microenvironment in the central nervous system (CNS). These cells have been shown to support cell-cell communication via multiple mechanisms, most recently by the release of extracellular vesicles (EVs). Since EVs carry a variety of cargoes of nucleic acids, lipids, and proteins and mediate intercellular communication, they have been the hotspot of diagnosis and treatment. The mechanisms underlying CNS disorders include angiogenesis, autophagy, apoptosis, cell death, and inflammation, and cell-EVs have been revealed to be involved in these pathological processes. Ischemic stroke is one of the most common causes of death and disability worldwide. It results in serious neurological and physical dysfunction and even leads to heavy economic and social burdens. Although a large number of researchers have reported that EVs derived from these cells play a vital role in regulating multiple pathological mechanisms in ischemic stroke, the specific interactional relationships and mechanisms between specific cell-EVs and stroke treatment have not been clearly described. This review aims to summarize the therapeutic effects and mechanisms of action of specific cell-EVs on ischemia. Additionally, this study emphasizes that these EVs are involved in stroke treatment by inhibiting and activating various signaling pathways such as ncRNAs, TGF-β1, and NF-κB.

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.

Disruption of Sema3A/Plexin-A1 inhibitory signalling in oligodendrocytes as a therapeutic strategy to promote remyelination

Current treatments in multiple sclerosis (MS) are modulating the inflammatory component of the disease, but no drugs are currently available to repair lesions. Our study identifies in MS patients the overexpression of Plexin‐A1, the signalling receptor of the oligodendrocyte inhibitor Semaphorin 3A. Using a novel type of peptidic antagonist, we showed the possibility to counteract the Sema3A inhibitory effect on oligodendrocyte migration and differentiation in vitro when antagonizing Plexin‐A1. The use of this compound in vivo demonstrated a myelin protective effect as shown with DTI‐MRI and confirmed at the histological level in the mouse cuprizone model of induced demyelination/remyelination. This effect correlated with locomotor performances fully preserved in chronically treated animals. The administration of the peptide also showed protective effects, leading to a reduced severity of demyelination in the context of experimental autoimmune encephalitis (EAE). Hence, the disruption of the inhibitory microenvironmental molecular barriers allows normal myelinating cells to exert their spontaneous remyelinating capacity. This opens unprecedented therapeutic opportunity for patients suffering a disease for which no curative options are yet available.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human 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.

Regulation of oligodendrocyte precursor migration during development, in adulthood and in pathology

Oligodendrocytes are the myelin-forming cells in the central nervous system (CNS). These cells originate from oligodendrocyte precursor cells (OPCs) during development, and they migrate extensively from oligodendrogliogenic niches along the neural tube to colonise the entire CNS. Like many other such events, this migratory process is precisely regulated by a battery of positional and signalling cues that act via their corresponding receptors and that are expressed dynamically by OPCs. Here, we will review the cellular and molecular basis of this important event during embryonic and postnatal development, and we will discuss the relevance of the substantial number of OPCs existing in the adult CNS. Similarly, we will consider the behaviour of OPCs in normal and pathological conditions, especially in animal models of demyelination and of the demyelinating disease, multiple sclerosis. The spontaneous remyelination observed after damage in demyelinating pathologies has a limited effect. Understanding the cellular and molecular mechanisms underlying the biology of OPCs, particularly adult OPCs, should help in the design of neuroregenerative strategies to combat multiple sclerosis and other demyelinating diseases.

Expression of Semaphorin 4F in neurons and brain oligodendrocytes and the regulation of oligodendrocyte precursor migration in the optic nerve

Semaphorins are secreted or membrane-anchored proteins that play critical roles in neural development and adult brain plasticity. Sema4F is a transmembrane semaphorin found on glutamatergic synapses, in which it is attached to the PSD-95-scaffolding protein. Here we further examined the expression of Sema4F by raising specific antibodies. We show that Sema4F protein is widely expressed by neurons during neural development and in the adult brain. We also demonstrate a preferential localization of this protein in postsynaptic dendrites. Moreover, Sema4F is expressed not only by neurons but also by oligodendrocyte precursors in the optic nerve and along the migratory pathways of oligodendroglial cells, and also by subsets of postnatal oligodendroglial cells in the brain. Finally, in vitro experiments demonstrate that endogenous Sema4F expressed by brain cells of oligodendroglial lineage regulates the outgrowth migration of oligodendrocyte precursors and promotes their differentiation. The present data extend our knowledge about the expression of Sema4F and uncover a novel function in the control of oligodendrocyte precursor migration in the developing brain.

The cellular prion protein in multiple sclerosis: A potential target for neurotherapeutics?

Multiple sclerosis (MS) is a debilitating disease that affects millions. There is no known cure for the disease and neither is the cause of the disease known. Recent studies have indicated that it is a multi-factorial disease with several genes involved. Importantly, sunlight and vitamin D have been implicated in the progression of the disease. The pathogenesis of MS chiefly involves loss of oligodendrocytes, which in addition to being killed by inflammatory mediators in the CNS, also succumbs to loss of trophic support from astrocytes. Neurotrophins play an important role in myelination and the cellular prion protein (PrPC) is a key player in this process. Although the physiological roles of PrPC remain to be fully understood, increasing evidence suggests multiple roles for PrPC in regulation of cellular immunity and for its interaction with several neurotrophins that are necessary for homeostasis of the nervous system. This mini-review focuses on the findings establishing a crucial role for PrPC in the neuropathogenesis of MS, emphasizing its neuroprotective role. Since MS is a multi-factorial disease with unknown etiology and no cure, this review aims to highlight endogenous repair mechanisms mediated by PrPC that might contribute to functional recovery in MS patients.

Cells of the oligodendroglial lineage, myelination, and remyelination

Myelin is critical in maintaining electrical impulse conduction in the central nervous system. The oligodendrocyte is the cell type responsible for myelin production within this compartment. The mutual supply of trophic support between oligodendrocytes and the underlying axons may indicate why demyelinated axons undergo degeneration more readily; the latter contributes to the neural decline in multiple sclerosis (MS). Myelin repair, termed remyelination, occurs in acute inflammatory lesions in MS and is associated with functional recovery and clinical remittances. Animal models have demonstrated that remyelination is mediated by oligodendrocyte progenitor cells (OPCs) which have responded to chemotactic cues, migrated into the lesion, proliferated, differentiated into mature oligodendrocytes, and ensheathed demyelinated axons. The limited remyelination observed in more chronic MS lesions may reflect intrinsic properties of neural cells or extrinsic deterrents. Therapeutic strategies currently under development include transplantation of exogenous OPCs and promotion of remyelination by endogenous OPCs. All currently approved MS therapies are aimed at dampening the immune response and are not directly targeting neural processes.

Modes and regulation of glial migration in vertebrates and invertebrates

Neurons and glial cells show mutual interdependence in many developmental and functional aspects of their biology. To establish their intricate relationships with neurons, glial cells must migrate over what are often long distances. In the CNS glial cells generally migrate as single cells, whereas PNS glial cells tend to migrate as cohorts of cells. How are their journeys initiated and directed, and what stops the migratory phase once glial cells are aligned with their neuronal counterparts? A deeper understanding of glial migration and the underlying neuron-glia interactions may contribute to the development of therapeutics for demyelinating diseases or glial tumours.

Phosphodiesterase-Ialpha/autotaxin's MORFO domain regulates oligodendroglial process network formation and focal adhesion organization

Development of a complex process network by maturing oligodendrocytes is a critical but currently poorly characterized step toward myelination. Here, we demonstrate that the matricellular oligodendrocyte-derived protein phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX) and especially its MORFO domain are able to promote this developmental step. In particular, the single EF hand-like motif located within PD-Ialpha/ATX's MORFO domain was found to stimulate the outgrowth of higher order branches but not process elongation. This motif was also observed to be critical for the stimulatory effect of PD-Ialpha/ATX's MORFO domain on the reorganization of focal adhesions located at the leading edge of oligodendroglial protrusions. Collectively, our data suggest that PD-Ialpha/ATX promotes oligodendroglial process network formation and expansion via the cooperative action of multiple functional sites located within the MORFO domain and more specifically, a novel signaling pathway mediated by the single EF hand-like motif and regulating the correlated events of process outgrowth and focal adhesion organization.

Semaphorins in development and adult brain: Implication for neurological diseases

As a group, Semaphorins are expressed in most tissues and this distribution varies considerably with age. Semaphorins are dynamically expressed during embryonic development and their expression is often associated with growing axons. This expression decreases with maturity and several observations support the idea that in adult brain the expression of secreted Semaphorins is sensitive to electrical activity and experience. The functional role of Semaphorins in guiding axonal projections is well established and more recent evidence points to additional roles in the development, function and reorganization of synaptic complexes. Semaphorins exert the majority of their effects by binding to cognate receptor proteins through their extracellular domains. A common theme is that Semaphorin-triggered signalling induces the rearrangement of the actin and microtubule cytoskeleton. Mutations in Semaphorin genes are linked to several human diseases associated with neurological changes, but their actual influence in the pathogenesis of these diseases remains to be demonstrated. In addition, Semaphorins and their receptors are likely to mediate cross-talk between neurons and other cell types, including in pathological situations where their influence can be damaging or favourable depending on the context. We discuss how the manipulation of Semaphorin function might be crucial for future clinical studies.

Activation of sphingosine-1-phosphate receptor S1P5 inhibits oligodendrocyte progenitor migration

Sphingosine-1-phosphate (S1P) acts as an extracellular ligand for a family of G-protein coupled receptors that are crucial in cell migration. S1P5 is exclusively expressed in oligodendrocytes and oligodendrocyte precursor cells (OPCs), which migrate considerable distances during brain development. The current studies suggest a physiological role for S1P and S1P5 in regulation of OPC migration. mRNA expression levels of S1P2 and S1P5 are comparable in OPCs, but S1P binding specifically to the S1P5 receptor blocked OPC migration (IC50=29 nM). Thus, knocking down S1P5 using siRNA prevented the S1P-induced decrease in OPC migration, whereas knocking down S1P2 did not have any effect. S1P-induced modulation of OPC migration was insensitive to pertussis toxin, suggesting that S1P5-initiated signaling is not mediated by the G alpha(i)-protein coupled pathway. Furthermore, S1P5 appears to engage the G alpha(12/13) protein coupled Rho/ROCK signaling pathway to impede OPC migration. To modulate OPC motility, extracellular S1P could be derived from the export of intracellular S1P generated in response to glutamate treatment of OPCs. These studies suggest that S1P could be a part of the neuron-oligodendroglial communication network regulating OPC migration and may provide directional guidance cues for migrating OPCs in the developing brain.

Plexin-A4 is expressed in oligodendrocyte precursor cells and acts as a mediator of semaphorin signals

Class 3 semaphorin acts as a guidance clue for both cell migration and nerve fiber projection. The signal of class 3 semaphorin travels via a receptor complex consisting of neuropilins and Plexin-A subfamily. Although it has been reported that class 3 semaphorin acts as a repellent for oligodendrocyte precursor cells (OPCs), which migrate actively during brain development, the expression of Plexin-A subfamily has not been reported in OPCs yet. Therefore, it is currently unclear how semaphorin signals can travel in OPCs. In the present study, the expression of Plexin-A4 (PlexA4) was first demonstrated in a newly established OPC line and OPCs in developing brain. In the OPC line, repulsion for process extension was caused by both Sema3A and Sema6A, and the effect of the semaphorins was diminished in cells expressing PlexA4 lacking the cytoplasmic domain. These results strongly suggest that PlexA4 expressed in OPCs acts as a mediator of semaphorin signals.

Growth conelike sensorimotor structures are characteristic features of postmigratory, premyelinating oligodendrocytes

During development, postmigratory, premyelinating oligodendrocytes extend processes that navigate through the central nervous system (CNS) environment, where they recognize a number of extracellular cues, including axonal segments to be myelinated. Ultimately this recognition event leads to the formation of the CNS myelin sheath. However, the morphological structures and molecular mechanisms that control such oligodendroglial pathfinding are poorly understood. Here we show that postmigratory, premyelinating oligodendrocyte processes possess at their distal tips expansions that ultrastructurally resemble growth cones of postmigratory neurons and that we will refer to as OLG-growth cones. OLG-growth cones are highly motile and capable of mediating process outgrowth, retraction, and branching. In addition, they express regulators of cytoskeletal organization, GAP43 and cofilin, that are known to mediate neuronal growth cone navigation. In a choice situation, processes of postmigratory, premyelinating oligodendrocytes and their OLG-growth cones have the ability to selectively avoid a nonpermissive substrate, that is, collagen IV. Thus, our findings provide, for the first time, a detailed characterization of sensorimotor structures present at the tips of postmigratory, premyelinating oligodendrocyte processes. Furthermore, the data presented here suggest that, although the cellular mechanisms involved in growth cone steering may be similar for postmigratory neuronal and oligodendroglial cells, extracellular cues may be interpreted in a cell-type-specific fashion.