WAVE1 is required for oligodendrocyte morphogenesis and normal CNS myelination

Molecular Motors in Myelination and Their Misregulation in Disease

Molecular motors are cellular components involved in the intracellular transport of organelles and materials to ensure cell homeostasis. This is particularly relevant in neurons, where the synaptic components synthesized in the soma need to travel over long distances to their destination. They can walk on microtubules (kinesins and dyneins) or actin filaments (myosins), the major components of cell cytoskeleton. While kinesins mostly perform the anterograde transport of intracellular components toward the plus ends of microtubules located distally in cell processes, cytoplasmic dyneins allow the retrograde flux of intracellular cargo toward the minus ends of microtubules located at the cell soma. Axon myelination represents a major aspect of neuronal maturation and is essential for neuronal function, as it speeds up the transmission of electrical signals. Increasing evidence supports a role for molecular motors in the homeostatic control of myelination. This role includes the trafficking of myelin components along the processes of myelinating cells and local regulation of pathways that ensure axon wrapping. Dysfunctional control of the intracellular transport machinery has therefore been linked to several brain pathologies, including demyelinating diseases. These disorders include a broad spectrum of conditions characterized by pathological demyelination of axons within the nervous system, ultimately leading to axonal degeneration and neuronal death, with multiple sclerosis representing the most prevalent and studied condition. This review highlights the involvement of molecular motors in the homeostatic control of myelination. It also discusses studies that have yielded insights into the dysfunctional activity of molecular motors in the pathophysiology of multiple sclerosis.

RhoA regulates oligodendrocyte differentiation and myelination by orchestrating cortical and membrane tension

Timely differentiation and myelin formation by oligodendrocytes are essential for the physiological functioning of the central nervous system (CNS). While the Rho GTPase RhoA has been hinted as a negative regulator of myelin sheath formation, the precise in vivo mechanisms have remained elusive. Here we show that RhoA controls the timing and progression of myelination by oligodendrocytes through a fine-tuned balance between cortical tension, membrane tension and cell shape. Using a conditional mouse model, we observe that Rhoa ablation results in the acceleration of myelination driven by hastened differentiation and facilitated through membrane expansion induced by changes in MLCII activity and in F-actin redistribution and turnover within the cell. These findings reveal RhoA as a central molecular integrator of alterations in actin cytoskeleton, actomyosin contractility and membrane tension underlying precise morphogenesis of oligodendrocytes and normal myelination of the CNS.

The WAVE complex in developmental and adulthood brain disorders

Actin polymerization and depolymerization are fundamental cellular processes required not only for the embryonic and postnatal development of the brain but also for the maintenance of neuronal plasticity and survival in the adult and aging brain. The orchestrated organization of actin filaments is controlled by various actin regulatory proteins. Wiskott‒Aldrich syndrome protein-family verprolin-homologous protein (WAVE) members are key activators of ARP2/3 complex-mediated actin polymerization. WAVE proteins exist as heteropentameric complexes together with regulatory proteins, including CYFIP, NCKAP, ABI and BRK1. The activity of the WAVE complex is tightly regulated by extracellular cues and intracellular signaling to execute its roles in specific intracellular events in brain cells. Notably, dysregulation of the WAVE complex and WAVE complex-mediated cellular processes confers vulnerability to a variety of brain disorders. De novo mutations in WAVE genes and other components of the WAVE complex have been identified in patients with developmental disorders such as intellectual disability, epileptic seizures, schizophrenia, and/or autism spectrum disorder. In addition, alterations in the WAVE complex are implicated in the pathophysiology of Alzheimer's disease and Parkinson's disease, as well as in behavioral adaptations to psychostimulants or maladaptive feeding.

Myosin superfamily members during myelin formation and regeneration

Myelin is an insulator that forms around axons that enhance the conduction velocity of nerve fibers. Oligodendrocytes dramatically change cell morphology to produce myelin throughout the central nervous system (CNS). Cytoskeletal alterations are critical for the morphogenesis of oligodendrocytes, and actin is involved in cell differentiation and myelin wrapping via polymerization and depolymerization, respectively. Various protein members of the myosin superfamily are known to be major binding partners of actin filaments and have been intensively researched because of their involvement in various cellular functions, including differentiation, cell movement, membrane trafficking, organelle transport, signal transduction, and morphogenesis. Some members of the myosin superfamily have been found to play important roles in the differentiation of oligodendrocytes and in CNS myelination. Interestingly, each member of the myosin superfamily expressed in oligodendrocyte lineage cells also shows specific spatial and temporal expression patterns and different distributions. In this review, we summarize previous findings related to the myosin superfamily and discuss how these molecules contribute to myelin formation and regeneration by oligodendrocytes.

Antagonistic actions of PAK1 and NF2/Merlin drive myelin membrane expansion in oligodendrocytes

In the central nervous system, the formation of myelin by oligodendrocytes (OLs) relies on the switch from the polymerization of the actin cytoskeleton to its depolymerization. The molecular mechanisms that trigger this switch have yet to be elucidated. Here, we identified P21-activated kinase 1 (PAK1) as a major regulator of actin depolymerization in OLs. Our results demonstrate that PAK1 accumulates in OLs in a kinase-inhibited form, triggering actin disassembly and, consequently, myelin membrane expansion. Remarkably, proteomic analysis of PAK1 binding partners enabled the identification of NF2/Merlin as its endogenous inhibitor. Our findings indicate that Nf2 knockdown in OLs results in PAK1 activation, actin polymerization, and a reduction in OL myelin membrane expansion. This effect is rescued by treatment with a PAK1 inhibitor. We also provide evidence that the specific Pak1 loss-of-function in oligodendroglia stimulates the thickening of myelin sheaths in vivo. Overall, our data indicate that the antagonistic actions of PAK1 and NF2/Merlin on the actin cytoskeleton of the OLs are critical for proper myelin formation. These findings have broad mechanistic and therapeutic implications in demyelinating diseases and neurodevelopmental disorders.

Novel insight into the role of A-kinase anchoring proteins (AKAPs) in ischemic stroke and therapeutic potentials

Ischemic stroke, a devastating disease associated with high mortality and disability worldwide, has emerged as an urgent public health issue. A-kinase anchoring proteins (AKAPs) are a group of signal-organizing molecules that compartmentalize and anchor a wide range of receptors and effector proteins and have a major role in stabilizing mitochondrial function and promoting neurodevelopmental development in the central nervous system (CNS). Growing evidence suggests that dysregulation of AKAPs expression and activity is closely associated with oxidative stress, ion disorder, mitochondrial dysfunction, and blood-brain barrier (BBB) impairment in ischemic stroke. However, the underlying mechanisms remain inadequately understood. This review provides a comprehensive overview of the composition and structure of A-kinase anchoring protein (AKAP) family members, emphasizing their physiological functions in the CNS. We explored in depth the molecular and cellular mechanisms of AKAP complexes in the pathological progression and risk factors of ischemic stroke, including hypertension, hyperglycemia, lipid metabolism disorders, and atrial fibrillation. Herein, we highlight the potential of AKAP complexes as a pharmacological target against ischemic stroke in the hope of inspiring translational research and innovative clinical approaches.

Postnatal neuronal Bace1 deletion impairs neuroblast and oligodendrocyte maturation

Beta amyloid cleaving enzyme 1 (BACE1) is largely expressed by neurons and is the sole β-secretase for initiating the production of neuronal β-amyloid peptides (Aβ). To fully understand the physiological functions of neuronal BACE1, we used mouse genetic approach coupled with unbiased single nucleus RNA sequencing (snRNAseq) to investigate how targeted deletion of Bace1 in neurons, driven by Thy-1-Cre recombinase, would affect functions in the nervous system. Our transcriptome results revealed that BACE1 is essential for maturation of neural precursor cells and oligodendrocytes in mice. RNA velocity analysis confirmed deficit in the trajectory of neuroblasts in reaching the immature granule neuron state in young Bace1fl/fl; Thy1-cre mice. Further analysis of differential gene expression indicated changes in genes important for SNARE signaling, tight junction signaling, synaptogenesis and insulin secretion pathways. Morphological studies revealed a hypomyelination in Bace1fl/fl;Thy1-cre sciatic nerves, but no detectable myelination changes in the corpus callosum, despite clear reduction in myelination proteins in the brain. Functional studies showed reduction in long-term potential, defects in synaptogenesis and learning behavioral. Altogether, our results show that neuronal BACE1 is critical for optimal development of central and peripheral nervous system, and inhibition of neuronal BACE1 will result in deficits in synaptic functions and cognitive behaviors.

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.

Expression of the human herpesvirus 6A latency-associated transcript U94A impairs cytoskeletal functions in human neural cells

Many neurodegenerative diseases have a multifactorial etiology and variable course of progression that cannot be explained by current models. Neurotropic viruses have long been suggested to play a role in these diseases, although their exact contributions remain unclear. Human herpesvirus 6A (HHV-6A) is one of the most common viruses detected in the adult brain, and has been clinically associated with multiple sclerosis (MS), and, more recently, Alzheimer's disease (AD). HHV-6A is a ubiquitous viral pathogen capable of infecting glia and neurons. Primary infection in childhood is followed by the induction of latency, characterized by expression of the U94A viral transcript in the absence of viral replication. Here we examine the effects of U94A on cells of the central nervous system. We found that U94A expression inhibits the migration and impairs cytoplasmic maturation of human oligodendrocyte precursor cells (OPCs) without affecting their viability, a phenotype that may contribute to the failure of remyelination seen in many patients with MS. A subsequent proteomics analysis of U94A expression OPCs revealed altered expression of genes involved in tubulin associated cytoskeletal regulation. As HHV-6A seems to significantly be associated with early AD pathology, we extended our initially analysis of the impact of U94A on human derived neurons. We found that U94A expression inhibits neurite outgrowth of primary human cortical neurons and impairs synapse maturation. Based on these data we suggest that U94A expression by latent HHV-6A in glial cells and neurons renders them susceptible to dysfunction and degeneration. Therefore, latent viral infections of the brain represent a unique pathological risk factor that may contribute to disease processes.

Ablation of arginyl-tRNA-protein transferase in oligodendrocytes impairs central nervous system myelination

Addition of arginine (Arg) from tRNA can cause major alterations of structure and function of protein substrates. This post-translational modification, termed protein arginylation, is mediated by the enzyme arginyl-tRNA-protein transferase 1 (Ate1). Arginylation plays essential roles in a variety of cellular processes, including cell migration, apoptosis, and cytoskeletal organization. Ate1 is associated with neuronal functions such as neurogenesis and neurite growth. However, the role of Ate1 in glial development, including oligodendrocyte (OL) differentiation and myelination processes in the central nervous system, is poorly understood. The present study revealed a peak in Ate1 protein expression during myelination process in primary cultured OLs. Post-transcriptional downregulation of Ate1 reduced the number of OL processes, and branching complexity, in vitro. We conditionally ablated Ate1 from OLs in mice using 2',3'-cyclic nucleotide 3'-phosphodiesterase-Cre promoter ("Ate1-KO" mice), to assess the role of Ate1 in OL function and axonal myelination in vivo. Immunostaining for OL differentiation markers revealed a notable reduction of mature OLs in corpus callosum of 14-day-old Ate1-KO, but no changes in spinal cord, in comparison with wild-type controls. Local proliferation of OL precursor cells was elevated in corpus callosum of 21-day-old Ate1-KO, but was unchanged in spinal cord. Five-month-old Ate1-KO displayed reductions of mature OL number and myelin thickness, with alterations of motor behaviors. Our findings, taken together, demonstrate that Ate1 helps maintain proper OL differentiation and myelination in corpus callosum in vivo, and that protein arginylation plays an essential role in developmental myelination.

Manipulating oligodendrocyte intrinsic regeneration mechanism to promote remyelination

In demyelinated lesions, astrocytes, activated microglia and infiltrating macrophages secrete several factors regulating oligodendrocyte precursor cells' behaviour. What appears to be the initiation of an intrinsic mechanism of myelin repair is only leading to partial recovery and inefficient remyelination, a process worsening over the course of the disease. This failure is largely due to the concomitant accumulation of inhibitory cues in and around the lesion sites opposing to growth promoting factors. Here starts a complex game of interactions between the signalling pathways controlling oligodendrocytes migration or differentiation. Receptors of positive or negative cues are modulating Ras, PI3K or RhoGTPases pathways acting on oligodendrocyte cytoskeleton remodelling. From the description of this intricate signalling network, this review addresses the extent to which the modulation of the global response to inhibitory cues may pave the route towards novel therapeutic approaches for myelin repair.

Human Glial Progenitor Cells Effectively Remyelinate the Demyelinated Adult Brain

Neonatally transplanted human glial progenitor cells (hGPCs) can myelinate the brains of myelin-deficient shiverer mice, rescuing their phenotype and survival. Yet, it has been unclear whether implanted hGPCs are similarly able to remyelinate the diffusely demyelinated adult CNS. We, therefore, ask if hGPCs could remyelinate both congenitally hypomyelinated adult shiverers and normal adult mice after cuprizone demyelination. In adult shiverers, hGPCs broadly disperse and differentiate as myelinating oligodendrocytes after subcortical injection, improving both host callosal conduction and ambulation. Implanted hGPCs similarly remyelinate denuded axons after cuprizone demyelination, whether delivered before or after demyelination. RNA sequencing (RNA-seq) of hGPCs back from cuprizone-demyelinated brains reveals their transcriptional activation of oligodendrocyte differentiation programs, while distinguishing them from hGPCs not previously exposed to demyelination. These data indicate the ability of transplanted hGPCs to disperse throughout the adult CNS, to broadly myelinate regions of dysmyelination, and also to be recruited as myelinogenic oligodendrocytes later in life, upon demyelination-associated demand.

PAK1 Positively Regulates Oligodendrocyte Morphology and Myelination

The actin cytoskeleton is crucial for oligodendrocyte differentiation and myelination. Here we show that p21-activated kinase 1 (PAK1), a well-known actin regulator, promotes oligodendrocyte morphologic change and myelin production in the CNS. A combination of in vitro and in vivo models demonstrated that PAK1 is expressed throughout the oligodendrocyte lineage with highest expression in differentiated oligodendrocytes. Inhibiting PAK1 early in oligodendrocyte development decreased oligodendrocyte morphologic complexity and altered F-actin spreading at the tips of oligodendrocyte progenitor cell processes. Constitutively activating AKT in oligodendrocytes in male and female mice, which leads to excessive myelin wrapping, increased PAK1 expression, suggesting an impact of PAK1 during active myelin wrapping. Furthermore, constitutively activating PAK1 in oligodendrocytes in zebrafish led to an increase in myelin internode length while inhibiting PAK1 during active myelination decreased internode length. As myelin parameters influence conduction velocity, these data suggest that PAK1 may influence communication within the CNS. These data support a model in which PAK1 is a positive regulator of CNS myelination.SIGNIFICANCE STATEMENT Myelin is a critical component of the CNS that provides metabolic support to neurons and also facilitates communication between cells in the CNS. Recent data demonstrate that actin dynamics drives myelin wrapping, but how actin is regulated during myelin wrapping is unknown. The authors investigate the role of the cytoskeletal modulator PAK1 during differentiation and myelination by oligodendrocytes, the myelinating cells of the CNS. They demonstrate that PAK1 promotes oligodendrocyte differentiation and myelination by modulating the cytoskeleton and thereby internode length, thus playing a critical role in the function of the CNS.

WAVE1 and WAVE2 have distinct and overlapping roles in controlling actin assembly at the leading edge

SCAR/WAVE proteins and Arp2/3 complex assemble branched actin networks at the leading edge. Two isoforms of SCAR/WAVE, WAVE1 and WAVE2, reside at the leading edge, yet it has remained unclear whether they perform similar or distinct roles. Further, there have been conflicting reports about the Arp2/3-independent biochemical activities of WAVE1 on actin filament elongation. To investigate this in vivo, we knocked out WAVE1 and WAVE2 genes, individually and together, in B16-F1 melanoma cells. We demonstrate that WAVE1 and WAVE2 are redundant for lamellipodia formation and motility. However, there is a significant decrease in the rate of leading edge actin extension in WAVE2 KO cells, and an increase in WAVE1 KO cells. The faster rates of actin extension in WAVE1 KO cells are offset by faster retrograde flow, and therefore do not translate into faster lamellipodium protrusion. Thus, WAVE1 restricts the rate of actin extension at the leading edge, and appears to couple actin networks to the membrane to drive protrusion. Overall, these results suggest that WAVE1 and WAVE2 have redundant roles in promoting Arp2/3-dependent actin nucleation and lamellipodia formation, but distinct roles in controlling actin network extension and harnessing network growth to cell protrusion.

N-Wasp Regulates Oligodendrocyte Myelination

Oligodendrocyte myelination depends on actin cytoskeleton rearrangement. Neural Wiskott-Aldrich syndrome protein(N-Wasp) is an actin nucleation factor that promotes polymerization of branched actin filaments. N-Wasp activity is essential for myelin membrane wrapping by Schwann cells, but its role in oligodendrocytes and CNS myelination remains unknown. Here we report that oligodendrocytes-specific deletion of N-Wasp in mice of both sexes resulted in hypomyelination (i.e., reduced number of myelinated axons and thinner myelin profiles), as well as substantial focal hypermyelination reflected by the formation of remarkably long myelin outfolds. These myelin outfolds surrounded unmyelinated axons, neuronal cell bodies, and other myelin profiles. The latter configuration resulted in pseudo-multimyelin profiles that were often associated with axonal detachment and degeneration throughout the CNS, including in the optic nerve, corpus callosum, and the spinal cord. Furthermore, developmental analysis revealed that myelin abnormalities were already observed during the onset of myelination, suggesting that they are formed by aberrant and misguided elongation of the oligodendrocyte inner lip membrane. Our results demonstrate that N-Wasp is required for the formation of normal myelin in the CNS. They also reveal that N-Wasp plays a distinct role in oligodendrocytes compared with Schwann cells, highlighting a difference in the regulation of actin dynamics during CNS and PNS myelination.SIGNIFICANCE STATEMENT Myelin is critical for the normal function of the nervous system by facilitating fast conduction of action potentials. During the process of myelination in the CNS, oligodendrocytes undergo extensive morphological changes that involve cellular process extension and retraction, axonal ensheathment, and myelin membrane wrapping. Here we present evidence that N-Wasp, a protein regulating actin filament assembly through Arp2/3 complex-dependent actin nucleation, plays a critical role in CNS myelination, and its absence leads to several myelin abnormalities. Our data provide an important step into the understanding of the molecular mechanisms underlying CNS myelination.

Neuron-oligodendroglia interactions: Activity-dependent regulation of cellular signaling

Oligodendrocyte lineage cells (oligodendroglia) and neurons engage in bidirectional communication throughout life to support healthy brain function. Recent work shows that changes in neuronal activity can modulate proliferation, differentiation, and myelination to support the formation and function of neural circuits. While oligodendroglia express a diverse collection of receptors for growth factors, signaling molecules, neurotransmitters and neuromodulators, our knowledge of the intracellular signaling pathways that are regulated by neuronal activity remains largely incomplete. Many of the pathways that modulate oligodendroglia behavior are driven by changes in intracellular calcium signaling, which may differentially affect cytoskeletal dynamics, gene expression, maturation, integration, and axonal support. Additionally, activity-dependent neuron-oligodendroglia communication plays an integral role in the recovery from demyelinating injuries. In this review, we summarize the modalities of communication between neurons and oligodendroglia and explore possible roles of activity-dependent calcium signaling in mediating cellular behavior and myelination.

The Actin Cytoskeleton in Myelinating Cells

Myelinating cells of both the peripheral and central nervous systems (CNSs) undergo dramatic cytoskeletal reorganization in order to differentiate and produce myelin. Myelinating oligodendrocytes in the CNS show a periodic actin pattern, demonstrating tight regulation of actin. Furthermore, recent data demonstrate that actin polymerization drives early cell differentiation and that actin depolymerization drives myelin wrapping. Dysregulation of the actin cytoskeleton in myelinating cells is seen in some disease states. This review highlights the cytoskeletal molecules that regulate differentiation of and myelination by cells of the PNS and CNS, informing our understanding of neural development, in particular myelination.

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.

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

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

Myosin XVI Regulates Actin Cytoskeleton Dynamics in Dendritic Spines of Purkinje Cells and Affects Presynaptic Organization

The actin cytoskeleton is crucial for function and morphology of neuronal synapses. Moreover, altered regulation of the neuronal actin cytoskeleton has been implicated in neuropsychiatric diseases such as autism spectrum disorder (ASD). Myosin XVI is a neuronally expressed unconventional myosin known to bind the WAVE regulatory complex (WRC), a regulator of filamentous actin (F-actin) polymerization. Notably, the gene encoding the myosin’s heavy chain (MYO16) shows genetic association with neuropsychiatric disorders including ASD. Here, we investigated whether myosin XVI plays a role for actin cytoskeleton regulation in the dendritic spines of cerebellar Purkinje cells (PCs), a neuronal cell type crucial for motor learning, social cognition and vocalization. We provide evidence that both myosin XVI and the WRC component WAVE1 localize to PC spines. Fluorescence recovery after photobleaching (FRAP) analysis of GFP-actin in cultured PCs shows that Myo16 knockout as well as PC-specific Myo16 knockdown, lead to faster F-actin turnover in the dendritic spines of PCs. We also detect accelerated F-actin turnover upon interference with the WRC, and upon inhibition of Arp2/3 that drives formation of branched F-actin downstream of the WRC. In contrast, inhibition of formins that are responsible for polymerization of linear actin filaments does not cause faster F-actin turnover. Together, our data establish myosin XVI as a regulator of the postsynaptic actin cytoskeleton and suggest that it is an upstream activator of the WRC-Arp2/3 pathway in PC spines. Furthermore, ultra-structural and electrophysiological analyses of Myo16 knockout cerebellum reveals the presence of reduced numbers of synaptic vesicles at presynaptic terminals in the absence of the myosin. Therefore, we here define myosin XVI as an F-actin regulator important for presynaptic organization in the cerebellum.

De novo hotspot variants in CYFIP2 cause early-onset epileptic encephalopathy

OBJECTIVE

The cytoplasmic fragile X mental retardation 1 interacting proteins 2 (CYFIP2) is a component of the WASP-family verprolin-homologous protein (WAVE) regulatory complex, which is involved in actin dynamics. An obvious association of CYFIP2 variants with human neurological disorders has never been reported. Here, we identified de novo hotspot CYFIP2 variants in neurodevelopmental disorders and explore the possible involvement of the CYFIP2 mutants in the WAVE signaling pathway.

METHODS

We performed trio-based whole-exome sequencing (WES) in 210 families and case-only WES in 489 individuals with epileptic encephalopathies. The functional effect of CYFIP2 variants on WAVE signaling was evaluated by computational structural analysis and in vitro transfection experiments.

RESULTS

We identified three de novo CYFIP2 variants at the Arg87 residue in 4 unrelated individuals with early-onset epileptic encephalopathy. Structural analysis indicated that the Arg87 residue is buried at an interface between CYFIP2 and WAVE1, and the Arg87 variant may disrupt hydrogen bonding, leading to structural instability and aberrant activation of the WAVE regulatory complex. All mutant CYFIP2 showed comparatively weaker interactions to the VCA domain than wild-type CYFIP2. Immunofluorescence revealed that ectopic speckled accumulation of actin and CYFIP2 was significantly increased in cells transfected with mutant CYFIP2.

INTERPRETATION

Our findings suggest that de novo Arg87 variants in CYFIP2 have gain-of-function effects on the WAVE signaling pathway and are associated with severe neurological disorders. Ann Neurol 2018;83:794-806.

Cyfip1 haploinsufficient rats show white matter changes, myelin thinning, abnormal oligodendrocytes and behavioural inflexibility

The biological basis of the increased risk for psychiatric disorders seen in 15q11.2 copy number deletion is unknown. Previous work has shown disturbances in white matter tracts in human carriers of the deletion. Here, in a novel rat model, we recapitulated low dosage of the candidate risk gene CYFIP1 present within the 15q11.2 interval. Using diffusion tensor imaging, we first showed extensive white matter changes in Cyfip1 mutant rats, which were most pronounced in the corpus callosum and external capsule. Transmission electron microscopy showed that these changes were associated with thinning of the myelin sheath in the corpus callosum. Myelin thinning was independent of changes in axon number or diameter but was associated with effects on mature oligodendrocytes, including aberrant intracellular distribution of myelin basic protein. Finally, we demonstrated effects on cognitive phenotypes sensitive to both disruptions in myelin and callosal circuitry.

People with a genetic deletion of the 15q11.2 locus are at increased risk for psychiatric disorders and white matter disturbances, but the gene(s) responsible are unclear. Here, the authors show that low dosage of CYFIP1, present in the human 15q11.2 region, alters white matter structure and cognition in rats.

Characterization of a long noncoding RNA Pcdh17it as a novel marker for immature premyelinating oligodendrocytes

Oligodendrocyte precursors (OPs) proliferate and differentiate into oligodendrocytes (OLs) during postnatal development and into adulthood in the central nervous system (CNS). Following the initiation of differentiation, OPs give rise to immature, premyelinating OLs, which undergo further differentiation and mature into myelin-forming OLs. We identified an immature OL-specific long noncoding RNA, named Pcdh17it. Through co-localization analysis and morphological characterization of OLs, we found that Pcdh17it is a specific marker for newly born immature OLs in the developing and adult forebrain of mice, and we used this new marker to analyze OL generation over the lifespan of mice. Pcdh17it is an effective tool for monitoring newly born OLs in adult brain, allowing detailed study of the dynamics of OP differentiation into OLs in the normal and pathological CNS.

CYFIP1 overexpression increases fear response in mice but does not affect social or repetitive behavioral phenotypes

Background

CYFIP1, a protein that interacts with FMRP and regulates protein synthesis and actin dynamics, is overexpressed in Dup15q syndrome as well as autism spectrum disorder (ASD). While CYFIP1 heterozygosity has been rigorously studied due to its loss in 15q11.2 deletion, Prader-Willi and Angelman syndrome, the effects of CYFIP1 overexpression, as is observed in patients with CYFIP1 duplication, are less well understood.

Methods

We developed and validated a mouse model of human CYFIP1 overexpression (CYFIP1 OE) using qPCR and western blot analysis. We performed a large battery of behavior testing on these mice, including ultrasonic vocalizations, three-chamber social assay, home-cage behavior, Y-maze, elevated plus maze, open field test, Morris water maze, fear conditioning, prepulse inhibition, and the hot plate assay. We also performed RNA sequencing and analysis on the basolateral amygdala.

Results

Extensive behavioral testing in CYFIP1 OE mice reveals no changes in the core behaviors related to ASD: social interactions and repetitive behaviors. However, we did observe mild learning deficits and an exaggerated fear response. Using RNA sequencing of the basolateral amygdala, a region associated with fear response, we observed changes in pathways related to cytoskeletal regulation, oligodendrocytes, and myelination. We also identified GABA-A subunit composition changes in basolateral amygdala neurons, which are essential components of the neural fear conditioning circuit.

Conclusion

Overall, this research identifies the behavioral and molecular consequences of CYFIP1 overexpression and how they contribute to the variable phenotype seen in Dup15q syndrome and in ASD patients with excess CYFIP1.

Independent and cooperative roles of the Mek/ERK1/2-MAPK and PI3K/Akt/mTOR pathways during developmental myelination and in adulthood

Multiple extracellular and intracellular signals regulate the functions of oligodendrocytes as they progress through the complex process of developmental myelination and then maintain a functionally intact myelin sheath throughout adult life, preserving the integrity of the axons. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination. However, it remains poorly understood whether they function independently, sequentially, or converge using a common mechanism to facilitate oligodendrocyte differentiation, myelin growth, and maintenance. To address these questions, we analyzed multiple genetically modified mice and asked whether the deficits due to the conditional loss-of-function of ERK1/2 or mTOR could be abrogated by simultaneous constitutive activation of PI3K/Akt or Mek, respectively. From these studies, we concluded that while PI3K/Akt, not Mek/ERK1/2, plays a key role in promoting oligodendrocyte differentiation and timely initiation of myelination through mTORC1 signaling, Mek/ERK1/2-MAPK functions largely independently of mTORC1 to preserve the integrity of the myelinated axons during adulthood. However, to promote the efficient growth of the myelin sheath, these two pathways cooperate with each other converging at the level of mTORC1, both in the context of normal developmental myelination or following forced reactivation of the myelination program during adulthood. Thus, Mek/ERK1/2-MAPK and the PI3K/Akt/mTOR signaling pathways work both independently and cooperatively to maintain a finely tuned, temporally regulated balance as oligodendrocytes progress through different phases of developmental myelination into adulthood. Therapeutic strategies aimed at targeting remyelination in demyelinating diseases are expected to benefit from these findings.

Heterogeneity of Oligodendrocytes and Their Precursor Cells

While oligodendrocytes have been thought to be homogenous, a number of reports have indicated evidences of the heterogeneity of oligodendrocytes and their precursor cells, OPCs. Almost a century ago, Del Río Hortega found three and four types of oligodendrocytes with regions where they exist and their morphologies, respectively. Interfascicular oligodendrocytes are one of the three regional dependent types and are the most typical oligodendendroglial cells that myelinate axonal fibers in the white matter tracts. In the other two, perineuronal oligodendrocyes function as reserve cells for remyelination and regulate neuronal excitability, whereas perivascular oligodendrocytes may play a role in metabolic support of axons. Among the four morphological categories, type I and II oligodendrocytes form many myelin sheaths on small-diameter axons and specific signal is required for the myelination of small-diameter axons. Type III and IV oligodendrocytes myelinate a few number of axons/or one axon, whose diameters are large. A recent comprehensive gene expression analysis with single-cell RNA sequencing identifies six different populations in mature oligodendrocytes and only one population in OPCs. However, OPCs are not uniformed developmentally and regionally. Further, the capacity of OPC differentiation depends on the environments and conditions of the tissues. Taken together, oligodendrocytes and OPCs are diverse as the other cell types in the CNS. The orchestration of these cells with their specialized functions is critical for proper functioning of the CNS.

Familial t(1;11) translocation is associated with disruption of white matter structural integrity and oligodendrocyte-myelin dysfunction

Although the underlying neurobiology of major mental illness (MMI) remains unknown, emerging evidence implicates a role for oligodendrocyte-myelin abnormalities. Here, we took advantage of a large family carrying a balanced t(1;11) translocation, which substantially increases risk of MMI, to undertake both diffusion tensor imaging and cellular studies to evaluate the consequences of the t(1;11) translocation on white matter structural integrity and oligodendrocyte-myelin biology. This translocation disrupts among others the DISC1 gene which plays a crucial role in brain development. We show that translocation-carrying patients display significant disruption of  white matter integrity compared with familial controls. At a cellular level, we observe dysregulation of key pathways controlling oligodendrocyte development and morphogenesis in induced pluripotent stem cell (iPSC) derived case oligodendrocytes. This is associated with reduced proliferation and a stunted morphology in vitro. Further, myelin internodes in a humanized mouse model that recapitulates the human translocation as well as after transplantation of t(1;11) oligodendrocyte progenitors were significantly reduced when  compared with controls. Thus we provide evidence that the t(1;11) translocation has biological effects at both the systems and cellular level that together suggest oligodendrocyte-myelin dysfunction.

Evolvability of the actin cytoskeleton in oligodendrocytes during central nervous system development and aging

The organization of actin filaments into a wide range of subcellular structures is a defining feature of cell shape and dynamics, important for tissue development and homeostasis. Nervous system function requires morphological and functional plasticity of neurons and glial cells, which is largely determined by the dynamic reorganization of the actin cytoskeleton in response to intrinsic and extracellular signals. Oligodendrocytes are specialized glia that extend multiple actin-based protrusions to form the multilayered myelin membrane that spirally wraps around axons, increasing conduction speed and promoting long-term axonal integrity. Myelination is a remarkable biological paradigm in development, and maintenance of myelin is essential for a healthy adult nervous system. In this review, we discuss how structure and dynamics of the actin cytoskeleton is a defining feature of myelinating oligodendrocytes' biology and function. We also review "old and new" concepts to reflect on the potential role of the cytoskeleton in balancing life and death of myelin membranes and oligodendrocytes in the aging central nervous system.

Advances in myelinating glial cell development

In the vertebrate nervous system, the fast conduction of action potentials is potentiated by the myelin sheath, a multi-lamellar, lipid-rich structure that also provides vital trophic and metabolic support to axons. Myelin is elaborated by the plasma membrane of specialized glial cells, oligodendrocytes in the central nervous system (CNS) and Schwann cells (SCs) in the peripheral nervous system (PNS). The diseases that result from damage to myelin or glia, including multiple sclerosis and Charcot-Marie-Tooth disease, underscore the importance of these cells for human health. Therefore, an understanding of glial development and myelination is crucial in addressing the etiology of demyelinating diseases and developing patient therapies. In this review, we discuss new insights into the roles of mechanotransduction and cytoskeletal rearrangements as well as activity dependent myelination and axonal maintenance by glia. Together, these discoveries advance our knowledge of myelin and glia in nervous system health and plasticity throughout life.

The intellectual disability protein PAK3 regulates oligodendrocyte precursor cell differentiation

Oligodendrocyte and myelin deficits have been reported in mental/psychiatric diseases. The p21-activated kinase 3 (PAK3), a serine/threonine kinase, whose activity is stimulated by the binding of active Rac and Cdc42 GTPases is affected in these pathologies. Indeed, many mutations of Pak3 gene have been described in non-syndromic intellectual disability diseases. Pak3 is expressed mainly in the brain where its role has been investigated in neurons but not in glial cells. Here, we showed that PAK3 is highly expressed in oligodendrocyte precursors (OPCs) and its expression decreases in mature oligodendrocytes. In the developing white matter of the Pak3 knockout mice, we found defects of oligodendrocyte differentiation in the corpus callosum and to a lesser extent in the anterior commissure, which were compensated at the adult stage. In vitro experiments in OPC cultures, derived from Pak3 knockout and wild type brains, support a developmental and cell-autonomous role for PAK3 in regulating OPC differentiation into mature oligodendrocytes. Moreover, we did not detect any obvious alterations of the proliferation or migration of Pak3 null OPCs compared to wild type. Overall, our data highlight PAK3 as a new regulator of OPC differentiation.

Knockdown of Unconventional Myosin ID Expression Induced Morphological Change in Oligodendrocytes

Myelin is a special multilamellar structure involved in various functions in the nervous system. In the central nervous system, the oligodendrocyte (OL) produces myelin and has a unique morphology. OLs have a dynamic membrane sorting system associated with cytoskeletal organization, which aids in the production of myelin. Recently, it was reported that the assembly and disassembly of actin filaments is crucial for myelination. However, the partner myosin molecule which associates with actin filaments during the myelination process has not yet been identified. One candidate myosin is unconventional myosin ID (Myo1d) which is distributed throughout central nervous system myelin; however, its function is still unclear. We report here that Myo1d is expressed during later stages of OL differentiation, together with myelin proteolipid protein (PLP). In addition, Myo1d is distributed at the leading edge of the myelin-like membrane in cultured OL, colocalizing mainly with actin filaments, 2',3'-cyclic nucleotide phosphodiesterase and partially with PLP. Myo1d-knockdown with specific siRNA induces significant morphological changes such as the retraction of processes and degeneration of myelin-like membrane, and finally apoptosis. Furthermore, loss of Myo1d by siRNA results in the impairment of intracellular PLP transport. Together, these results suggest that Myo1d may contribute to membrane dynamics either in wrapping or transporting of myelin membrane proteins during formation and maintenance of myelin.

The Activators of Cyclin-Dependent Kinase 5 p35 and p39 Are Essential for Oligodendrocyte Maturation, Process Formation, and Myelination

The regulation of oligodendrocyte development and myelin formation in the CNS is poorly defined. Multiple signals influence the rate and extent of CNS myelination, including the noncanonical cyclin-dependent kinase 5 (Cdk5) whose functions are regulated by its activators p35 and p39. Here we show that selective loss of either p35 or p39 perturbed specific aspects of oligodendrocyte development, whereas loss of both p35 and p39 completely inhibited the development of mature oligodendrocytes and myelination. In the absence of p35, oligodendrocyte differentiation was delayed, process outgrowth was truncated in vitro, and the patterning and extent of myelination were perturbed in the CNS of p35(-/-) mice. In the absence of p39, oligodendrocyte maturation was transiently affected both in vitro and in vivo. However, loss of both p35 and p39 in oligodendrocyte lineage cells completely inhibited oligodendrocyte progenitor cell differentiation and myelination both in vitro and after transplantation into shiverer slice cultures. Loss of p35 and p39 had a more profound effect on oligodendrocyte development than simply the loss of Cdk5 and could not be rescued by Cdk5 overexpression. These data suggest p35 and p39 have specific and overlapping roles in oligodendrocyte development, some of which may be independent of Cdk5 activation.

WAVE1 and Regulation of Actin Nucleation in Myelination

The myelin sheath can be compared to the neuronal growth cone in that the unfurled sheath looks like a giant lamellum. The authors recently tested this hypothesis by examining the importance of WAVE1, a regulator of lamellipodia formation in neurons and other cells, in myelinogenesis. They found that WAVE1 is critical for formation of oligodendrocyte lamellae and myelin sheaths. They review the regulation of WAVE1 and how WAVE1 is transported and localized to lamellipodia. Because they found that some but not all myelination was impaired by knockout of WAVE1 function, they hypothesize that other regulators of actin nucleation help oligodendrocytes produce myelin in parallel with WAVE1 function. Interestingly, they found that oligodendrocyte maturation also is disturbed with WAVE1 knockout and propose that proper localization and transport of signaling molecules relevant to the integrin signaling cascade are disrupted by loss of WAVE1 function. NEUROSCIENTIST 13(5):486—491, 2007. DOI: 10.1177/1073858407299423

Cytoskeletal Linker Protein Dystonin Is Not Critical to Terminal Oligodendrocyte Differentiation or CNS Myelination

Oligodendrocyte differentiation and central nervous system myelination require massive reorganization of the oligodendrocyte cytoskeleton. Loss of specific actin- and tubulin-organizing factors can lead to impaired morphological and/or molecular differentiation of oligodendrocytes, resulting in a subsequent loss of myelination. Dystonin is a cytoskeletal linker protein with both actin- and tubulin-binding domains. Loss of function of this protein results in a sensory neuropathy called Hereditary Sensory Autonomic Neuropathy VI in humans and dystonia musculorum in mice. This disease presents with severe ataxia, dystonic muscle and is ultimately fatal early in life. While loss of the neuronal isoforms of dystonin primarily leads to sensory neuron degeneration, it has also been shown that peripheral myelination is compromised due to intrinsic Schwann cell differentiation abnormalities. The role of this cytoskeletal linker in oligodendrocytes, however, remains unclear. We sought to determine the effects of the loss of neuronal dystonin on oligodendrocyte differentiation and central myelination. To address this, primary oligodendrocytes were isolated from a severe model of dystonia musculorum, Dstdt-27J, and assessed for morphological and molecular differentiation capacity. No defects could be discerned in the differentiation of Dstdt-27J oligodendrocytes relative to oligodendrocytes from wild-type littermates. Survival was also compared between Dstdt-27J and wild-type oligodendrocytes, revealing no significant difference. Using a recently developed migration assay, we further analysed the ability of primary oligodendrocyte progenitor cell motility, and found that Dstdt-27J oligodendrocyte progenitor cells were able to migrate normally. Finally, in vivo analysis of oligodendrocyte myelination was done in phenotype-stage optic nerve, cerebral cortex and spinal cord. The density of myelinated axons and g-ratios of Dstdt-27J optic nerves was normal, as was myelin basic protein expression in both cerebral cortex and spinal cord. Together these data suggest that, unlike Schwann cells, oligodendrocytes do not have an intrinsic requirement for neuronal dystonin for differentiation and myelination.

Diversity Matters: A Revised Guide to Myelination

The evolutionary success of the vertebrate nervous system is largely due to a unique structural feature--the myelin sheath, a fatty envelope that surrounds the axons of neurons. By increasing the speed by which electrical signals travel along axons, myelin facilitates neuronal communication between distant regions of the nervous system. We review the cellular and molecular mechanisms that regulate the development of myelin as well as its homeostasis in adulthood. We discuss how finely tuned neuron-oligodendrocyte interactions are central to myelin formation during development and in the adult, and how these interactions can have profound implications for the plasticity of the adult brain. We also speculate how the functional diversity of both neurons and oligodendrocytes may impact on the myelination process in both health and disease.

Myelination: actin disassembly leads the way

The mechanisms that drive the spiral wrapping of the myelin sheath around axons are poorly understood. Two papers in this issue of Developmental Cell demonstrate that actin disassembly, rather than actin assembly, predominates during oligodendrocyte maturation and is critical for the genesis of the central myelin sheath.

Oligodendrocytes in a Nutshell

Oligodendrocytes are the myelinating cells of the central nervous system (CNS). While the phrase is oft repeated and holds true, the last few years have borne witness to radical change in our understanding of this unique cell type. Once considered static glue, oligodendrocytes are now seen as plastic and adaptive, capable of reacting to a changing CNS. This review is intended as a primer and guide, exploring how the past 5 years have fundamentally altered our appreciation of oligodendrocyte development and CNS myelination.

Actin filament turnover drives leading edge growth during myelin sheath formation in the central nervous system

During CNS development, oligodendrocytes wrap their plasma membrane around axons to generate multilamellar myelin sheaths. To drive growth at the leading edge of myelin at the interface with the axon, mechanical forces are necessary, but the underlying mechanisms are not known. Using an interdisciplinary approach that combines morphological, genetic, and biophysical analyses, we identified a key role for actin filament network turnover in myelin growth. At the onset of myelin biogenesis, F-actin is redistributed to the leading edge, where its polymerization-based forces push out non-adhesive and motile protrusions. F-actin disassembly converts protrusions into sheets by reducing surface tension and in turn inducing membrane spreading and adhesion. We identified the actin depolymerizing factor ADF/cofilin1, which mediates high F-actin turnover rates, as an essential factor in this process. We propose that F-actin turnover is the driving force in myelin wrapping by regulating repetitive cycles of leading edge protrusion and spreading.

CNS myelin wrapping is driven by actin disassembly

Myelin is essential in vertebrates for the rapid propagation of action potentials, but the molecular mechanisms driving its formation remain largely unknown. Here we show that the initial stage of process extension and axon ensheathment by oligodendrocytes requires dynamic actin filament assembly by the Arp2/3 complex. Unexpectedly, subsequent myelin wrapping coincides with the upregulation of actin disassembly proteins and rapid disassembly of the oligodendrocyte actin cytoskeleton and does not require Arp2/3. Inducing loss of actin filaments drives oligodendrocyte membrane spreading and myelin wrapping in vivo, and the actin disassembly factor gelsolin is required for normal wrapping. We show that myelin basic protein, a protein essential for CNS myelin wrapping whose role has been unclear, is required for actin disassembly, and its loss phenocopies loss of actin disassembly proteins. Together, these findings provide insight into the molecular mechanism of myelin wrapping and identify it as an actin-independent form of mammalian cell motility.

Myelination of the nervous system: mechanisms and functions

Myelination of axons in the nervous system of vertebrates enables fast, saltatory impulse propagation, one of the best-understood concepts in neurophysiology. However, it took a long while to recognize the mechanistic complexity both of myelination by oligodendrocytes and Schwann cells and of their cellular interactions. In this review, we highlight recent advances in our understanding of myelin biogenesis, its lifelong plasticity, and the reciprocal interactions of myelinating glia with the axons they ensheath. In the central nervous system, myelination is also stimulated by axonal activity and astrocytes, whereas myelin clearance involves microglia/macrophages. Once myelinated, the long-term integrity of axons depends on glial supply of metabolites and neurotrophic factors. The relevance of this axoglial symbiosis is illustrated in normal brain aging and human myelin diseases, which can be studied in corresponding mouse models. Thus, myelinating cells serve a key role in preserving the connectivity and functions of a healthy nervous system.

Daam2-PIP5K is a regulatory pathway for Wnt signaling and therapeutic target for remyelination in the CNS

Wnt signaling plays an essential role in developmental and regenerative myelination of the CNS; however, contributions of proximal regulators of the Wnt receptor complex to these processes remain undefined. To identify components of the Wnt pathway that regulate these processes, we applied a multifaceted discovery platform and found that Daam2-PIP5K comprise a novel pathway regulating Wnt signaling and myelination. Using dorsal patterning of the chick spinal cord we found that Daam2 promotes Wnt signaling and receptor complex formation through PIP5K-PIP2. Analysis of Daam2 function in oligodendrocytes (OLs) revealed that it suppresses OL differentiation during development, after white matter injury (WMI), and is expressed in human white matter lesions. These findings suggest a pharmacological strategy to inhibit Daam2-PIP5K function, application of which stimulates remyelination after WMI. Put together, our studies integrate information from multiple systems to identify a novel regulatory pathway for Wnt signaling and potential therapeutic target for WMI.

Activation of sodium-dependent glutamate transporters regulates the morphological aspects of oligodendrocyte maturation via signaling through calcium/calmodulin-dependent kinase IIβ's actin-binding/-stabilizing domain

Signaling via the major excitatory amino acid glutamate has been implicated in the regulation of various aspects of the biology of oligodendrocytes, the myelinating cells of the central nervous system (CNS). In this respect, cells of the oligodendrocyte lineage have been described to express a variety of glutamate-responsive transmembrane proteins including sodium-dependent glutamate transporters. The latter have been well characterized to mediate glutamate clearance from the extracellular space. However, there is increasing evidence that they also mediate glutamate-induced intracellular signaling events. Our data presented here show that the activation of oligodendrocyte expressed sodium-dependent glutamate transporters, in particular GLT-1 and GLAST, promotes the morphological aspects of oligodendrocyte maturation. This effect was found to be associated with a transient increase in intracellular calcium levels and a transient phosphorylation event at the serine (S)(371) site of the calcium sensor calcium/calmodulin-dependent kinase type IIβ (CaMKIIβ). The potential regulatory S(371) site is located within CaMKIIβ's previously defined actin-binding/-stabilizing domain, and phosphorylation events within this domain were identified in our studies as a requirement for sodium-dependent glutamate transporter-mediated promotion of oligodendrocyte maturation. Furthermore, our data provide good evidence for a role of these phosphorylation events in mediating detachment of CaMKIIβ from filamentous (F)-actin, and hence allowing a remodeling of the oligodendrocyte's actin cytoskeleton. Taken together with our recent findings, which demonstrated a crucial role of CaMKIIβ in regulating CNS myelination in vivo, our data strongly suggest that a sodium-dependent glutamate transporter-CaMKIIβ-actin cytoskeleton axis plays an important role in the regulation of oligodendrocyte maturation and CNS myelination.

Accelerated repair of demyelinated CNS lesions in the absence of non-muscle myosin IIB

The oligodendrocyte (OL), the myelinating cell of the central nervous system, undergoes dramatic changes in the organization of its cytoskeleton as it differentiates from a precursor (oligodendrocyte precursor cells) to a myelin-forming cell. These changes include an increase in its branching cell processes, a phenomenon necessary for OL to myelinate multiple axon segments. We have previously shown that levels and activity of non-muscle myosin II (NMII), a regulator of cytoskeletal contractility, decrease as a function of differentiation and that inhibition of NMII increases branching and myelination of OL in coculture with neurons. We have also found that mixed glial cell cultures derived from NMIIB knockout mice display an increase in mature myelin basic protein-expressing OL compared with wild-type cultures. We have now extended our studies to investigate the role of NMIIB ablation on myelin repair following focal demyelination by lysolecithin. To this end, we generated an oligodendrocyte-specific inducible knockout model using a Plp-driven promoter in combination with a temporally activated CRE-ER fusion protein. Our data indicate that conditional ablation of NMII in adult mouse brain, expedites lesion resolution and remyelination by Plp+ oligodendrocyte-lineage cells when compared with that observed in control brains. Taken together, these data validate the function of NMII as that of a negative regulator of OL myelination in vivo and provide a novel target for promoting myelin repair in conditions such as multiple sclerosis.

The antagonistic modulation of Arp2/3 activity by N-WASP, WAVE2 and PICK1 defines dynamic changes in astrocyte morphology

Astrocytes exhibit a complex, branched morphology, allowing them to functionally interact with numerous blood vessels, neighboring glial processes and neuronal elements, including synapses. They also respond to central nervous system (CNS) injury by a process known as astrogliosis, which involves morphological changes, including cell body hypertrophy and thickening of major processes. Following severe injury, astrocytes exhibit drastically reduced morphological complexity and collectively form a glial scar. The mechanistic details behind these morphological changes are unknown. Here, we investigate the regulation of the actin-nucleating Arp2/3 complex in controlling dynamic changes in astrocyte morphology. In contrast to other cell types, Arp2/3 inhibition drives the rapid expansion of astrocyte cell bodies and major processes. This intervention results in a reduced morphological complexity of astrocytes in both dissociated culture and in brain slices. We show that this expansion requires functional myosin II downstream of ROCK and RhoA. Knockdown of the Arp2/3 subunit Arp3 or the Arp2/3 activator N-WASP by siRNA also results in cell body expansion and reduced morphological complexity, whereas depleting WAVE2 specifically reduces the branching complexity of astrocyte processes. By contrast, knockdown of the Arp2/3 inhibitor PICK1 increases astrocyte branching complexity. Furthermore, astrocyte expansion induced by ischemic conditions is delayed by PICK1 knockdown or N-WASP overexpression. Our findings identify a new morphological outcome for Arp2/3 activation in restricting rather than promoting outwards movement of the plasma membrane in astrocytes. The Arp2/3 regulators PICK1, and N-WASP and WAVE2 function antagonistically to control the complexity of astrocyte branched morphology, and this mechanism underlies the morphological changes seen in astrocytes during their response to pathological insult.

CaMKIIβ regulates oligodendrocyte maturation and CNS myelination

CNS myelination and the maturation of the myelinating cells of the CNS, namely oligodendrocytes, are thought to be regulated by molecular mechanisms controlling the actin cytoskeleton. However, the exact nature of these mechanisms is currently only poorly understood. Here we assessed the role of calcium/calmodulin-dependent kinase type II (CaMKII), in particular CaMKIIβ, in oligodendrocyte maturation and CNS myelination. Using in vitro culture studies, our data demonstrate that CaMKIIβ is critical for the proper morphological maturation of differentiating oligodendrocytes, an aspect of oligodendrocyte maturation that is mediated to a large extent by changes in the cellular cytoskeleton. Furthermore, our data provide evidence for an actin-cytoskeleton-stabilizing role of CaMKIIβ in differentiating oligodendrocytes. Using Camk2b knock-out and Camk2b(A303R) mutant mice, our data revealed an in vivo functional role of CaMKIIβ in regulating myelin thickness that may be mediated by a non-kinase-catalytic activity. Our data point toward a critical role of CaMKIIβ in regulating oligodendrocyte maturation and CNS myelination via an actin-cytoskeleton-regulatory mechanism.

Integrin-linked kinase regulates process extension in oligodendrocytes via control of actin cytoskeletal dynamics

Integrin-linked kinase (ILK) is a major structural adaptor protein governing signaling complex formation and cytoskeletal dynamics. Here, through the use of conditional knock-out mice, we demonstrate a requirement for ILK in oligodendrocyte differentiation and axonal myelination in vivo. In conjunction, ILK-deficient primary oligodendrocytes are defined by a failure in process extension and an inability to form myelin membrane upon axonal contact. Surprisingly, phosphorylation of the canonical downstream targets Akt and GSK3β is unaffected following ILK loss. Rather, the defects are due in part to actin cytoskeleton dysregulation with a correspondent increase in active RhoA levels. Morphological rescue is possible following Rho kinase inhibition in an oligodendrocyte subset. Furthermore, phenotypic severity correlates with environmental complexity; oligodendrocytes are severely malformed in vitro (a relatively simple environment), but undergo phenotypic recovery in the context of the whole animal. Together, our work demonstrates ILK as necessary for normal oligodendrocyte development, reinforces its role as a bridge between the actin cytoskeleton and cell membrane, and highlights the overarching compensatory capacity of oligodendrocytes in response to cellular milieu.