Chondroitin sulfate proteoglycans in demyelinated lesions impair remyelination

Oligodendrocyte Progenitors in Glial Scar: A Bet on Remyelination

Oligodendrocyte progenitor cells (OPCs) represent a subtype of glia, giving rise to oligodendrocytes, the myelin-forming cells in the central nervous system (CNS). While OPCs are highly proliferative during development, they become relatively quiescent during adulthood, when their fate is strictly influenced by the extracellular context. In traumatic injuries and chronic neurodegenerative conditions, including those of autoimmune origin, oligodendrocytes undergo apoptosis, and demyelination starts. Adult OPCs become immediately activated; they migrate at the lesion site and proliferate to replenish the damaged area, but their efficiency is hampered by the presence of a glial scar-a barrier mainly formed by reactive astrocytes, microglia and the deposition of inhibitory extracellular matrix components. If, on the one hand, a glial scar limits the lesion spreading, it also blocks tissue regeneration. Therapeutic strategies aimed at reducing astrocyte or microglia activation and shifting them toward a neuroprotective phenotype have been proposed, whereas the role of OPCs has been largely overlooked. In this review, we have considered the glial scar from the perspective of OPCs, analysing their behaviour when lesions originate and exploring the potential therapies aimed at sustaining OPCs to efficiently differentiate and promote remyelination.

Astrocytes: Lessons Learned from the Cuprizone Model

A diverse array of neurological and psychiatric disorders, including multiple sclerosis, Alzheimer's disease, and schizophrenia, exhibit distinct myelin abnormalities at both the molecular and histological levels. These aberrations are closely linked to dysfunction of oligodendrocytes and alterations in myelin structure, which may be pivotal factors contributing to the disconnection of brain regions and the resulting characteristic clinical impairments observed in these conditions. Astrocytes, which significantly outnumber neurons in the central nervous system by a five-to-one ratio, play indispensable roles in the development, maintenance, and overall well-being of neurons and oligodendrocytes. Consequently, they emerge as potential key players in the onset and progression of a myriad of neurological and psychiatric disorders. Furthermore, targeting astrocytes represents a promising avenue for therapeutic intervention in such disorders. To gain deeper insights into the functions of astrocytes in the context of myelin-related disorders, it is imperative to employ appropriate in vivo models that faithfully recapitulate specific aspects of complex human diseases in a reliable and reproducible manner. One such model is the cuprizone model, wherein metabolic dysfunction in oligodendrocytes initiates an early response involving microglia and astrocyte activation, culminating in multifocal demyelination. Remarkably, following the cessation of cuprizone intoxication, a spontaneous process of endogenous remyelination occurs. In this review article, we provide a historical overview of studies investigating the responses and putative functions of astrocytes in the cuprizone model. Following that, we list previously published works that illuminate various aspects of the biology and function of astrocytes in this multiple sclerosis model. Some of the studies are discussed in more detail in the context of astrocyte biology and pathology. Our objective is twofold: to provide an invaluable overview of this burgeoning field, and, more importantly, to inspire fellow researchers to embark on experimental investigations to elucidate the multifaceted functions of this pivotal glial cell subpopulation.

Prominent elevation of extracellular matrix molecules in intracerebral hemorrhage

Background

Intracerebral hemorrhage (ICH) is the predominant type of hemorrhagic stroke with high mortality and disability. In other neurological conditions, the deposition of extracellular matrix (ECM) molecules is a prominent obstacle for regenerative processes and an enhancer of neuroinflammation. Whether ECM molecules alter in composition after ICH, and which ECM members may inhibit repair, remain largely unknown in hemorrhagic stroke.

Methods

The collagenase-induced ICH mouse model and an autopsied human ICH specimen were investigated for expression of ECM members by immunofluorescence microscopy. Confocal image z-stacks were analyzed with Imaris 3D to assess the association of immune cells and ECM molecules. Sections from a mouse model of multiple sclerosis were used as disease and staining controls. Tissue culture was employed to examine the roles of ECM members on oligodendrocyte precursor cells (OPCs).

Results

Among the lectican chondroitin sulfate proteoglycan (CSPG) members, neurocan but not aggrecan, versican-V1 and versican-V2 was prominently expressed in perihematomal tissue and lesion core compared to the contralateral area in murine ICH. Fibrinogen, fibronectin and heparan sulfate proteoglycan (HSPG) were also elevated after murine ICH while thrombospondin and tenascin-C was not. Confocal microscopy with Imaris 3D rendering co-localized neurocan, fibrinogen, fibronectin and HSPG molecules to Iba1+ microglia/macrophages or GFAP+ astrocytes. Marked differentiation from the multiple sclerosis model was observed, the latter with high versican-V1 and negligible neurocan. In culture, purified neurocan inhibited adhesion and process outgrowth of OPCs, which are early steps in myelination in vivo. The prominent expression of neurocan in murine ICH was corroborated in human ICH sections.

Conclusion

ICH caused distinct alterations in ECM molecules. Among CSPG members, neurocan was selectively upregulated in both murine and human ICH. In tissue culture, neurocan impeded the properties of oligodendrocyte lineage cells. Alterations to the ECM in ICH may adversely affect reparative outcomes after stroke.

The extracellular matrix as modifier of neuroinflammation and recovery in ischemic stroke and intracerebral hemorrhage

Stroke is the second leading cause of death worldwide and has two major subtypes: ischemic stroke and hemorrhagic stroke. Neuroinflammation is a pathological hallmark of ischemic stroke and intracerebral hemorrhage (ICH), contributing to the extent of brain injury but also in its repair. Neuroinflammation is intricately linked to the extracellular matrix (ECM), which is profoundly altered after brain injury and in aging. In the early stages after ischemic stroke and ICH, immune cells are involved in the deposition and remodeling of the ECM thereby affecting processes such as blood-brain barrier and cellular integrity. ECM components regulate leukocyte infiltration into the central nervous system, activate a variety of immune cells, and induce the elevation of matrix metalloproteinases (MMPs) after stroke. In turn, excessive MMPs may degrade ECM into components that are pro-inflammatory and injurious. Conversely, in the later stages after stroke, several ECM molecules may contribute to tissue recovery. For example, thrombospondin-1 and biglycan may promote activity of regulatory T cells, inhibit the synthesis of proinflammatory cytokines, and aid regenerative processes. We highlight these roles of the ECM in ischemic stroke and ICH and discuss their potential cellular and molecular mechanisms. Finally, we discuss therapeutics that could be considered to normalize the ECM in stroke. Our goal is to spur research on the ECM in order to improve the prognosis of ischemic stroke and ICH.

Recombinant Erythropoietin Induces Oligodendrocyte Progenitor Cell Proliferation After Traumatic Brain Injury and Delayed Hypoxemia

Traumatic brain injury (TBI) can result in axonal loss and demyelination, leading to persistent damage in the white matter. Demyelinated axons are vulnerable to pathologies related to an abnormal myelin structure that expose neurons to further damage. Oligodendrocyte progenitor cells (OPCs) mediate remyelination after recruitment to the injury site. Often this process is inefficient due to inadequate OPC proliferation. To date, no effective treatments are currently available to stimulate OPC proliferation in TBI. Recombinant human erythropoietin (rhEPO) is a pleiotropic neuroprotective cytokine, and its receptor is present in all stages of oligodendroglial lineage cell differentiation. Therefore, we hypothesized that rhEPO administration would enhance remyelination after TBI through the modulation of OPC response. Utilizing a murine model of controlled cortical impact and a primary OPC culture in vitro model, we characterized the impact of rhEPO on remyelination and proliferation of oligodendrocyte lineage cells. Myelin black gold II staining of the peri-contusional corpus callosum revealed an increase in myelinated area in association with an increase in BrdU-positive oligodendrocytes in injured mice treated with rhEPO. Furthermore, morphological analysis of OPCs showed a decrease in process length in rhEPO-treated animals. RhEPO treatment increased OPC proliferation after in vitro CSPG exposure. Erythropoietin receptor (EPOr) gene knockdown using siRNA prevented rhEPO-induced OPC proliferation, demonstrating that the rhEPO effect on OPC response is EPOr activation dependent. Together, our findings demonstrate that rhEPO administration may promote myelination by increasing oligodendrocyte lineage cell proliferation after TBI.

Mass spectrometry methods for analysis of extracellular matrix components in neurological diseases

The brain extracellular matrix (ECM) is a highly glycosylated environment and plays important roles in many processes including cell communication, growth factor binding, and scaffolding. The formation of structures such as perineuronal nets (PNNs) is critical in neuroprotection and neural plasticity, and the formation of molecular networks is dependent in part on glycans. The ECM is also implicated in the neuropathophysiology of disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and Schizophrenia (SZ). As such, it is of interest to understand both the proteomic and glycomic makeup of healthy and diseased brain ECM. Further, there is a growing need for site-specific glycoproteomic information. Over the past decade, sample preparation, mass spectrometry, and bioinformatic methods have been developed and refined to provide comprehensive information about the glycoproteome. Core ECM molecules including versican, hyaluronan and proteoglycan link proteins, and tenascin are dysregulated in AD, PD, and SZ. Glycomic changes such as differential sialylation, sulfation, and branching are also associated with neurodegeneration. A more thorough understanding of the ECM and its proteomic, glycomic, and glycoproteomic changes in brain diseases may provide pathways to new therapeutic options.

The Heterogeneous Multiple Sclerosis Lesion: How Can We Assess and Modify a Degenerating Lesion?

Multiple sclerosis (MS) is a heterogeneous disease of the central nervous system that is governed by neural tissue loss and dystrophy during its progressive phase, with complex reactive pathological cellular changes. The immune-mediated mechanisms that promulgate the demyelinating lesions during relapses of acute episodes are not characteristic of chronic lesions during progressive MS. This has limited our capacity to target the disease effectively as it evolves within the central nervous system white and gray matter, thereby leaving neurologists without effective options to manage individuals as they transition to a secondary progressive phase. The current review highlights the molecular and cellular sequelae that have been identified as cooperating with and/or contributing to neurodegeneration that characterizes individuals with progressive forms of MS. We emphasize the need for appropriate monitoring via known and novel molecular and imaging biomarkers that can accurately detect and predict progression for the purposes of newly designed clinical trials that can demonstrate the efficacy of neuroprotection and potentially neurorepair. To achieve neurorepair, we focus on the modifications required in the reactive cellular and extracellular milieu in order to enable endogenous cell growth as well as transplanted cells that can integrate and/or renew the degenerative MS plaque.

Remyelination in animal models of multiple sclerosis: finding the elusive grail of regeneration

Remyelination biology and the therapeutic potential of restoring myelin sheaths to prevent neurodegeneration and disability in multiple sclerosis (MS) has made considerable gains over the past decade with many regeneration strategies undergoing tested in MS clinical trials. Animal models used to investigate oligodendroglial responses and regeneration of myelin vary considerably in the mechanism of demyelination, involvement of inflammatory cells, neurodegeneration and capacity for remyelination. The investigation of remyelination in the context of aging and an inflammatory environment are of considerable interest for the potential translation to progressive multiple sclerosis. Here we review how remyelination is assessed in mouse models of demyelination, differences and advantages of these models, therapeutic strategies that have emerged and current pro-remyelination clinical trials.

Roles and regulation of microglia activity in multiple sclerosis: insights from animal models

As resident macrophages of the CNS, microglia are critical immune effectors of inflammatory lesions and associated neural dysfunctions. In multiple sclerosis (MS) and its animal models, chronic microglial inflammatory activity damages myelin and disrupts axonal and synaptic activity. In contrast to these detrimental effects, the potent phagocytic and tissue-remodelling capabilities of microglia support critical endogenous repair mechanisms. Although these opposing capabilities have long been appreciated, a precise understanding of their underlying molecular effectors is only beginning to emerge. Here, we review recent advances in our understanding of the roles of microglia in animal models of MS and demyelinating lesions and the mechanisms that underlie their damaging and repairing activities. We also discuss how the structured organization and regulation of the genome enables complex transcriptional heterogeneity within the microglial cell population at demyelinating lesions.

It takes two to remyelinate: A bioengineered platform to study astrocyte-oligodendrocyte crosstalk and potential therapeutic targets in remyelination

The loss of the myelin sheath insulating axons is the hallmark of demyelinating diseases. These pathologies often lead to irreversible neurological impairment and patient disability. No effective therapies are currently available to promote remyelination. Several elements contribute to the inadequacy of remyelination, thus understanding the intricacies of the cellular and signaling microenvironment of the remyelination niche might help us to devise better strategies to enhance remyelination. Here, using a new in vitro rapid myelinating artificial axon system based on engineered microfibres, we investigated how reactive astrocytes influence oligodendrocyte (OL) differentiation and myelination ability. This artificial axon culture system enables the effective uncoupling of molecular cues from the biophysical properties of the axons, allowing the detailed study of the astrocyte-OL crosstalk. Oligodendrocyte precursor cells (OPCs) were cultured on poly(trimethylene carbonate-co-ε-caprolactone) copolymer electrospun microfibres that served as surrogate axons. This platform was then combined with a previously established tissue engineered glial scar model of astrocytes embedded in 1 % (w/v) alginate matrices, in which astrocyte reactive phenotype was acquired using meningeal fibroblast conditioned medium. OPCs were shown to adhere to uncoated engineered microfibres and differentiate into myelinating OL. Reactive astrocytes were found to significantly impair OL differentiation ability, after six and eight days in a co-culture system. Differentiation impairment was seen to be correlated with astrocytic miRNA release through exosomes. We found significantly reduction on the expression of pro-myelinating miRNAs (miR-219 and miR-338) and an increase in anti-myelinating miRNA (miR-125a-3p) content between reactive and quiescent astrocytes. Additionally, we show that OPC differentiation inhibition could be reverted by rescuing the activated astrocytic phenotype with ibuprofen, a chemical inhibitor of the small rhoGTPase RhoA. Overall, these findings show that modulating astrocytic function might be an interesting therapeutic avenue for demyelinating diseases. The use of these engineered microfibres as an artificial axon culture system will enable the screening for potential therapeutic agents that promote OL differentiation and myelination while providing valuable insight on the myelination/remyelination processes.

Aging microglia

Microglia are the tissue-resident macrophage population of the brain, specialized in supporting the CNS environment and protecting it from endogenous and exogenous insults. Nonetheless, their function declines with age, in ways that remain to be fully elucidated. Given the critical role played by microglia in neurodegenerative diseases, a better understanding of the aging microglia phenotype is an essential prerequisite in designing better preventive and therapeutic strategies. In this review, we discuss the most recent literature on microglia in aging, comparing findings in rodent models and human subjects.

The roles of microglia and astrocytes in myelin phagocytosis in the central nervous system

Myelination is an important process in the central nervous system (CNS). Oligodendrocytes (OLs) extend multiple layers to densely sheath on axons, composing the myelin to achieve efficient electrical signal conduction. The myelination during developmental stage maintains a balanced state. However, numerous CNS diseases including neurodegenerative and cerebrovascular diseases cause demyelination and disrupt the homeostasis, resulting in inflammation and white matter deficits. Effective clearance of myelin debris is needed in the region of demyelination, which is a key step for remyelination and tissue regeneration. Microglia and astrocytes are the major resident phagocytic cells in the brain, which may play different or collaborative roles in myelination. Microglia and astrocytes participate in developmental myelination through engulfing excessive unneeded myelin. They are also involved in the clearance of degenerated myelin debris for accelerating remyelination, or engulfing healthy myelin sheath for inhibiting remyelination. This review focuses on the roles of microglia and astrocytes in phagocytosing myelin in the developmental brain and diseased brain. In addition, the interaction between microglia and astrocytes to mediate myelin engulfment is also summarized.

TREM2 mediates physical exercise-promoted neural functional recovery in rats with ischemic stroke via microglia-promoted white matter repair

BACKGROUND

The repair of white matter injury is of significant importance for functional recovery after ischemic stroke, and the up-regulation of triggering receptors expressed on myeloid cells 2 (TREM2) after ischemic stroke is neuroprotective and implicated in remyelination. However, the lack of effective therapies calls for the need to investigate the regenerative process of remyelination and the role of rehabilitation therapy. This study sought to investigate whether and how moderate physical exercise (PE) promotes oligodendrogenesis and remyelination in rats with transient middle cerebral artery occlusion (tMCAO).

METHODS

Male Sprague-Dawley rats (weighing 250-280 g) were subjected to tMCAO. AAV-shRNA was injected into the lateral ventricle to silence the Trem2 gene before the operation. The rats in the physical exercise group started electric running cage training at 48 h after the operation. The Morris water maze and novel object recognition test were used to evaluate cognitive function. Luxol fast blue staining, diffusion tensor imaging, and electron microscopy were used to observe myelin injury and repair. Immunofluorescence staining was applied to observe the proliferation and differentiation of oligodendrocyte precursor cells (OPCs). Expression of key molecules were detected using immunofluorescence staining, quantitative real-time polymerase chain reaction, Western blotting, and Enzyme-linked immunosorbent assay, respectively.

RESULTS

PE exerted neuroprotective efects by modulating microglial state, promoting remyelination and recovery of neurological function of rats over 35 d after stroke, while silencing Trem2 expression in rats suppressed the aforementioned effects promoted by PE. In addition, by leveraging the activin-A neutralizing antibody, we found a direct beneficial effect of PE on microglia-derived activin-A and its subsequent role on oligodendrocyte differentiation and remyelination mediated by the activin-A/Acvr axis.

CONCLUSIONS

The present study reveals a novel regenerative role of PE in white matter injury after stroke, which is mediated by upregulation of TREM2 and microglia-derived factor for oligodendrocytes regeneration. PE is an effective therapeutic approach for improving white matter integrity and alleviating neurological function deficits after ischemic stroke.

The role of immune cells and associated immunological factors in the immune response to spinal cord injury

Spinal cord injury (SCI) is a devastating neurological condition prevalent worldwide. Where the pathological mechanisms underlying SCI are concerned, we can distinguish between primary injury caused by initial mechanical damage and secondary injury characterized by a series of biological responses, such as vascular dysfunction, oxidative stress, neurotransmitter toxicity, lipid peroxidation, and immune-inflammatory response. Secondary injury causes further tissue loss and dysfunction, and the immune response appears to be the key molecular mechanism affecting injured tissue regeneration and functional recovery from SCI. Immune response after SCI involves the activation of different immune cells and the production of immunity-associated chemicals. With the development of new biological technologies, such as transcriptomics, the heterogeneity of immune cells and chemicals can be classified with greater precision. In this review, we focus on the current understanding of the heterogeneity of these immune components and the roles they play in SCI, including reactive astrogliosis and glial scar formation, neutrophil migration, macrophage transformation, resident microglia activation and proliferation, and the humoral immunity mediated by T and B cells. We also summarize findings from clinical trials of immunomodulatory therapies for SCI and briefly review promising therapeutic drugs currently being researched.

Modified black phosphorus quantum dots promotes spinal cord injury repair by targeting the AKT signaling pathway

Spinal cord injury (SCI) is a serious refractory disease of the central nervous system (CNS), which mostly caused by high-energy trauma. Existing interventions such as hormone shock and surgery are insufficient options, which relate to the secondary inflammation and neuronal dysfunction. Hydrogel with neuron-protective behaviors attracts tremendous attention, and black phosphorus quantum dots (BPQDs) encapsulating with Epigallocatechin-3-gallate (EGCG) hydrogels (E@BP) is designed for inflammatory modulation and SCI treatment in this study. E@BP displays good stability, biocompatibility and safety profiles. E@BP incubation alleviates lipopolysaccharide (LPS)-induced inflammation of primary neurons and enhances neuronal regeneration in vitro. Furthermore, E@BP reconstructs structural versus functional integrity of spinal cord tracts, which promotes recovery of motor neuron function in SCI rats after transplantation. Importantly, E@BP restarts the cell cycle and induces nerve regeneration. Moreover, E@BP diminishes local inflammation of SCI tissues, characterized by reducing accumulation of astrocyte, microglia, macrophages, and oligodendrocytes. Indeed, a common underlying mechanism of E@BP regulating neural regenerative and inflammatory responses is to promote the phosphorylation of key proteins related to AKT signaling pathway. Together, E@BP probably repairs SCI by reducing inflammation and promoting neuronal regeneration via the AKT signaling pathway.

Combination Therapy of Mesenchymal Stem Cell Transplantation and Astrocyte Ablation Improve Remyelination in a Cuprizone-Induced Demyelination Mouse Model

Astrocytes display an active, dual, and controversial role in multiple sclerosis (MS), a chronic inflammatory demyelination disorder. However, mesenchymal stem cells (MSCs) can affect myelination in demyelinating disorders. This study aimed to investigate the effect of single and combination therapies of astrocyte ablation and MSC transplantation on remyelination in the cuprizone (CPZ) model of MS. C57BL/6 mice were fed 0.2% CPZ diet for 12 weeks. Astrocytes were ablated twice by L-a-aminoadipate (L-AAA) at the beginning of weeks 13 and 14 whereas MSCs were injected in the corpus callosum at the beginning of week 13. Motor coordination and balance were assessed through rotarod test whereas myelin content was evaluated by Luxol-fast blue (LFB) staining and transmission electron microscopy (TEM). Glial cells were assessed by immunofluorescence staining while mRNA expression was evaluated by quantitative real-time PCR. Combination treatment of ablation of astrocytes and MSC transplantation (CPZ + MSC + L-AAA) significantly decreased motor coordination deficits better than single treatments (CPZ + MSCs or CPZ + L-AAA), in comparison to CPZ mice. In addition, L-AAA and MSCs treatment significantly enhanced remyelination compared to CPZ group. Moreover, combination therapy caused a significant decrease in the number of GFAP⁺ and Iba-1⁺ cells, whereas oligodendrocytes were significantly increased in comparison to CPZ mice. Finally, MSC administration resulted in a significant upregulation of BDNF and NGF mRNA expression levels. Our data indicate that transient ablation of astrocytes along with MSCs treatment improve remyelination through enhancing oligodendrocytes and attenuating gliosis in a chronic demyelinating mouse model of MS.

Modulation of the Microglial Nogo-A/NgR Signaling Pathway as a Therapeutic Target for Multiple Sclerosis

Current therapeutics targeting chronic phases of multiple sclerosis (MS) are considerably limited in reversing the neural damage resulting from repeated inflammation and demyelination insults in the multi-focal lesions. This inflammation is propagated by the activation of microglia, the endogenous immune cell aiding in the central nervous system homeostasis. Activated microglia may transition into polarized phenotypes; namely, the classically activated proinflammatory phenotype (previously categorized as M1) and the alternatively activated anti-inflammatory phenotype (previously, M2). These transitional microglial phenotypes are dynamic states, existing as a continuum. Shifting microglial polarization to an anti-inflammatory status may be a potential therapeutic strategy that can be harnessed to limit neuroinflammation and further neurodegeneration in MS. Our research has observed that the obstruction of signaling by inhibitory myelin proteins such as myelin-associated inhibitory factor, Nogo-A, with its receptor (NgR), can regulate microglial cell function and activity in pre-clinical animal studies. Our review explores the microglial role and polarization in MS pathology. Additionally, the potential therapeutics of targeting Nogo-A/NgR cellular mechanisms on microglia migration, polarization and phagocytosis for neurorepair in MS and other demyelination diseases will be discussed.

CNS remyelination and inflammation: From basic mechanisms to therapeutic opportunities

Remyelination, the myelin regenerative response that follows demyelination, restores saltatory conduction and function and sustains axon health. Its declining efficiency with disease progression in the chronic autoimmune disease multiple sclerosis (MS) contributes to the currently untreatable progressive phase of the disease. Although some of the bona fide myelin regenerative medicine clinical trials have succeeded in demonstrating proof-of-principle, none of these compounds have yet proceeded toward approval. There therefore remains a need to increase our understanding of the fundamental biology of remyelination so that existing targets can be refined and new ones discovered. Here, we review the role of inflammation, in particular innate immunity, in remyelination, describing its many and complex facets and discussing how our evolving understanding can be harnessed to translational goals.

The role of extracellular matrix alterations in mediating astrocyte damage and pericyte dysfunction in Alzheimer's disease: A comprehensive review

The brain is a highly vascularized tissue protected by the blood-brain barrier (BBB), a complex structure allowing only necessary substances to pass through into the brain while limiting the entrance of harmful toxins. The BBB comprises several components, and the most prominent features are tight junctions between endothelial cells (ECs), which are further wrapped in a layer of pericytes. Pericytes are multitasked cells embedded in a thick basement membrane (BM) that consists of a fibrous extracellular matrix (ECM) and are surrounded by astrocytic endfeet. The primary function of astrocytes and pericytes is to provide essential blood supply and vital nutrients to the brain. In Alzheimer's disease (AD), long-term neuroinflammatory cascades associated with infiltration of harmful neurotoxic proteins may lead to BBB dysfunction and altered ECM components resulting in brain homeostatic imbalance, synaptic damage, and declined cognitive functions. Moreover, BBB structure and functional integrity may be lost due to induced ECM alterations, astrocyte damage, and pericytes dysfunction, leading to amyloid-beta (Aβ) hallmarks deposition in different brain regions. Herein, we highlight how BBB, ECM, astrocytes, and pericytes dysfunction can play a leading role in AD's pathogenesis and discuss their impact on brain functions.

Tissue-Engineered Models of the Human Brain: State-of-the-Art Analysis and Challenges

Neurological disorders affect billions of people across the world, making the discovery of effective treatments an important challenge. The evaluation of drug efficacy is further complicated because of the lack of in vitro models able to reproduce the complexity of the human brain structure and functions. Some limitations of 2D preclinical models of the human brain have been overcome by the use of 3D cultures such as cell spheroids, organoids and organs-on-chip. However, one of the most promising approaches for mimicking not only cell structure, but also brain architecture, is currently represented by tissue-engineered brain models. Both conventional (particularly electrospinning and salt leaching) and unconventional (particularly bioprinting) techniques have been exploited, making use of natural polymers or combinations between natural and synthetic polymers. Moreover, the use of induced pluripotent stem cells (iPSCs) has allowed the co-culture of different human brain cells (neurons, astrocytes, oligodendrocytes, microglia), helping towards approaching the central nervous system complexity. In this review article, we explain the importance of in vitro brain modeling, and present the main in vitro brain models developed to date, with a special focus on the most recent advancements in tissue-engineered brain models making use of iPSCs. Finally, we critically discuss achievements, main challenges and future perspectives.

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.

CAQK, a peptide associating with extracellular matrix components targets sites of demyelinating injuries

The destruction of the myelin sheath that encircles axons leads to impairments of nerve conduction and neuronal dysfunctions. A major demyelinating disorder is multiple sclerosis (MS), a progressively disabling disease in which immune cells attack the myelin. To date, there are no therapies to target selectively myelin lesions, repair the myelin or stop MS progression. Small peptides recognizing epitopes selectively exposed at sites of injury show promise for targeting therapeutics in various pathologies. Here we show the selective homing of the four amino acid peptide, cysteine-alanine-lysine glutamine (CAQK), to sites of demyelinating injuries in three different mouse models. Homing was assessed by administering fluorescein amine (FAM)-labeled peptides into the bloodstream of mice and analyzing sites of demyelination in comparison with healthy brain or spinal cord tissue. FAM-CAQK selectively targeted demyelinating areas in all three models and was absent from healthy tissue. At lesion sites, the peptide was primarily associated with the fibrous extracellular matrix (ECM) deposited in interstitial spaces proximal to reactive astrocytes. Association of FAM-CAQK was detected with tenascin-C although tenascin depositions made up only a minor portion of the examined lesion sites. In mice on a 6-week cuprizone diet, FAM-CAQK peptide crossed the nearly intact blood-brain barrier and homed to demyelinating fiber tracts. These results demonstrate the selective targeting of CAQK to demyelinating injuries under multiple conditions and confirm the previously reported association with the ECM. This work sets the stage for further developing CAQK peptide targeting for diagnostic and therapeutic applications aimed at localized myelin repair.

Targeting Fibronectin to Overcome Remyelination Failure in Multiple Sclerosis: The Need for Brain- and Lesion-Targeted Drug Delivery

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease with unknown etiology that can be characterized by the presence of demyelinated lesions. Prevailing treatment protocols in MS rely on the modulation of the inflammatory process but do not impact disease progression. Remyelination is an essential factor for both axonal survival and functional neurological recovery but is often insufficient. The extracellular matrix protein fibronectin contributes to the inhibitory environment created in MS lesions and likely plays a causative role in remyelination failure. The presence of the blood–brain barrier (BBB) hinders the delivery of remyelination therapeutics to lesions. Therefore, therapeutic interventions to normalize the pathogenic MS lesion environment need to be able to cross the BBB. In this review, we outline the multifaceted roles of fibronectin in MS pathogenesis and discuss promising therapeutic targets and agents to overcome fibronectin-mediated inhibition of remyelination. In addition, to pave the way for clinical use, we reflect on opportunities to deliver MS therapeutics to lesions through the utilization of nanomedicine and discuss strategies to deliver fibronectin-directed therapeutics across the BBB. The use of well-designed nanocarriers with appropriate surface functionalization to cross the BBB and target the lesion sites is recommended.

Inhibition of CSPG receptor PTPσ promotes migration of newly born neuroblasts, axonal sprouting, and recovery from stroke

In addition to neuroprotective strategies, neuroregenerative processes could provide targets for stroke recovery. However, the upregulation of inhibitory chondroitin sulfate proteoglycans (CSPGs) impedes innate regenerative efforts. Here, we examine the regulatory role of PTPσ (a major proteoglycan receptor) in dampening post-stroke recovery. Use of a receptor modulatory peptide (ISP) or Ptprs gene deletion leads to increased neurite outgrowth and enhanced NSCs migration upon inhibitory CSPG substrates. Post-stroke ISP treatment results in increased axonal sprouting as well as neuroblast migration deeply into the lesion scar with a transcriptional signature reflective of repair. Lastly, peptide treatment post-stroke (initiated acutely or more chronically at 7 days) results in improved behavioral recovery in both motor and cognitive functions. Therefore, we propose that CSPGs induced by stroke play a predominant role in the regulation of neural repair and that blocking CSPG signaling pathways will lead to enhanced neurorepair and functional recovery in stroke.

Chondroitin Sulphate Proteoglycan Axonal Coats in the Human Mediodorsal Thalamic Nucleus

Mounting evidence supports a key involvement of the chondroitin sulfate proteoglycans (CSPGs) NG2 and brevican (BCAN) in the regulation of axonal functions, including axon guidance, fasciculation, conductance, and myelination. Prior work suggested the possibility that these functions may, at least in part, be carried out by specialized CSPG structures surrounding axons, termed axonal coats. However, their existence remains controversial. We tested the hypothesis that NG2 and BCAN, known to be associated with oligodendrocyte precursor cells, form axonal coats enveloping myelinated axons in the human brain. In tissue blocks containing the mediodorsal thalamic nucleus (MD) from healthy donors (n = 5), we used dual immunofluorescence, confocal microscopy, and unbiased stereology to characterize BCAN and NG2 immunoreactive (IR) axonal coats and measure the percentage of myelinated axons associated with them. In a subset of donors (n = 3), we used electron microscopy to analyze the spatial relationship between axons and NG2- and BCAN-IR axonal coats within the human MD. Our results show that a substantial percentage (∼64%) of large and medium myelinated axons in the human MD are surrounded by NG2- and BCAN-IR axonal coats. Electron microscopy studies show NG2- and BCAN-IR axonal coats are interleaved with myelin sheets, with larger axons displaying greater association with axonal coats. These findings represent the first characterization of NG2 and BCAN axonal coats in the human brain. The large percentage of axons surrounded by CSPG coats, and the role of CSPGs in axonal guidance, fasciculation, conductance, and myelination suggest that these structures may contribute to several key axonal properties.

Emerging therapies to target CNS pathophysiology in multiple sclerosis

The rapidly evolving therapeutic landscape of multiple sclerosis (MS) has contributed to paradigm shifts in our understanding of the biological mechanisms that contribute to CNS injury and in treatment philosophies. Opportunities remain to further improve treatment of relapsing-remitting MS, but two major therapeutic gaps are the limiting of progressive disease mechanisms and the repair of CNS injury. In this Review, we provide an overview of selected emerging therapies that predominantly target processes within the CNS that are thought to be involved in limiting non-relapsing, progressive disease injury or promoting tissue repair. Among these, we consider agents that modulate adaptive and innate CNS-compartmentalized inflammation, which can be mediated by infiltrating immune cells and/or resident CNS cells, including microglia and astrocytes. We also discuss agents that target degenerative disease mechanisms, agents that might confer neuroprotection, and agents that create a more favourable environment for or actively contribute to oligodendrocyte precursor cell differentiation, remyelination and axonal regeneration. We focus on agents that are novel for MS, that are known to or are presumed to penetrate the CNS, and that have already entered early stages of development in MS clinical trials.

The Extracellular Matrix Proteins Tenascin-C and Tenascin-R Retard Oligodendrocyte Precursor Maturation and Myelin Regeneration in a Cuprizone-Induced Long-Term Demyelination Animal Model

Oligodendrocytes are the myelinating cells of the central nervous system. The physiological importance of oligodendrocytes is highlighted by diseases such as multiple sclerosis, in which the myelin sheaths are degraded and the axonal signal transmission is compromised. In a healthy brain, spontaneous remyelination is rare, and newly formed myelin sheaths are thinner and shorter than the former ones. The myelination process requires the migration, proliferation, and differentiation of oligodendrocyte precursor cells (OPCs) and is influenced by proteins of the extracellular matrix (ECM), which consists of a network of glycoproteins and proteoglycans. In particular, the glycoprotein tenascin-C (Tnc) has an inhibitory effect on the differentiation of OPCs and the remyelination efficiency of oligodendrocytes. The structurally similar tenascin-R (Tnr) exerts an inhibitory influence on the formation of myelin membranes in vitro. When Tnc knockout oligodendrocytes were applied to an in vitro myelination assay using artificial fibers, a higher number of sheaths per single cell were obtained compared to the wild-type control. This effect was enhanced by adding brain-derived neurotrophic factor (BDNF) to the culture system. Tnr^−/− oligodendrocytes behaved differently in that the number of formed sheaths per single cell was decreased, indicating that Tnr supports the differentiation of OPCs. In order to study the functions of tenascin proteins in vivo Tnc^−/− and Tnr^−/− mice were exposed to Cuprizone-induced demyelination for a period of 10 weeks. Both Tnc^−/− and Tnr^−/− mouse knockout lines displayed a significant increase in the regenerating myelin sheath thickness after Cuprizone treatment. Furthermore, in the absence of either tenascin, the number of OPCs was increased. These results suggest that the fine-tuning of myelin regeneration is regulated by the major tenascin proteins of the CNS.

Comparative Profiling of TG2 and Its Effectors in Human Relapsing Remitting and Progressive Multiple Sclerosis

Multiple Sclerosis (MS) is a chronic CNS autoimmune disease characterized by immune-mediated demyelination, axon loss, and disability. Dysregulation of transglutaminase-2 (TG2) has been implicated in disease initiation and progression. Herein, TG2 expression in post-mortem human brain tissue from Relapsing Remitting MS (RRMS) or Progressive MS (PMS) individuals were examined and correlated with the presence of TG2 binding partners and effectors implicated in the processes of inflammation, scar formation, and the antagonism of repair. Tissues from Relapsing-Remitting Multiple Sclerosis (RRMS; n = 6), Progressive Multiple Sclerosis (PMS; n = 5), and non-MS control (n = 6) patients underwent immunohistochemistry for TG2, PLA2, COX-2, FN, CSPG, and HSPG. TG2 was strongly upregulated in active RRMS and PMS lesions, within blood vessels and the perivascular tissue of sclerotic plaques. TG2 colocalization was observed with GFAP+ astrocytes and ECM, including FN, HSPG, and CSPG, which also increased in either RRMS or PMS lesions. Although TG2 was not colocalized with inflammatory mediators COX-2 and PLA2, or the macrophage-microglia marker Iba1, its increased expression correlated with their elevation in active RRMS and PMS lesions. In summary, the correlation of strong TG2 induction in either RRMS or PMS with some of its binding partners but not others implicates potentially different roles for TG2 in disparate MS forms that may warrant further investigation.

Versican promotes T helper 17 cytotoxic inflammation and impedes oligodendrocyte precursor cell remyelination

Remyelination failure in multiple sclerosis (MS) contributes to progression of disability. The deficient repair results from neuroinflammation and deposition of inhibitors including chondroitin sulfate proteoglycans (CSPGs). Which CSPG member is repair-inhibitory or alters local inflammation to exacerbate injury is unknown. Here, we correlate high versican-V1 expression in MS lesions with deficient premyelinating oligodendrocytes, and highlight its selective upregulation amongst CSPG members in experimental autoimmune encephalomyelitis (EAE) lesions modeling MS. In culture, purified versican-V1 inhibits oligodendrocyte precursor cells (OPCs) and promotes T helper 17 (Th17) polarization. Versican-V1-exposed Th17 cells are particularly toxic to OPCs. In NG2^CreER:MAPT^mGFP mice illuminating newly formed GFP⁺ oligodendrocytes/myelin, difluorosamine (peracetylated,4,4-difluoro-N-acetylglucosamine) treatment from peak EAE reduces lesional versican-V1 and Th17 frequency, while enhancing GFP⁺ profiles. We suggest that lesion-elevated versican-V1 directly impedes OPCs while it indirectly inhibits remyelination through elevating local Th17 cytotoxic neuroinflammation. We propose CSPG-lowering drugs as potential dual pronged repair and immunomodulatory therapeutics for MS.

Ghorbani and colleagues describe versican-V1 as an inhibitor of remyelination using transgenic mice that illuminate new GFP ⁺ oligodendrocytes. Mechanisms of versican-V1 include the direct inhibition of oligodendrocytes, and elevating Th17 cells.

Emerging therapeutic role of chondroitinase (ChABC) in neurological disorders and cancer

Abstract:

Proteoglycans are essential biomacromolecules that participate in matrix structure and organization, cell proliferation and migration, and cell surface signal transduction. However, their roles in physiology, particularly in CNS remain incompletely deciphered. Numerous studies highlight the elevated levels of chondroitin sulphate proteoglycans (CSPGs) in various diseases like cancers and neurological disorders like spinal cord injury (SCI), traumatic brain damage, neurodegenerative diseases, and are mainly implicated to hinder tissue repair. In such a context, chondroitinase ABC (ChABC), a therapeutic enzyme has shown immense hope to treat these diseases in several preclinical studies, primarily attributed to the digestion of the side chains of the proteoglycan chondroitin sulphate (CS) molecule. Despite extensive research, the progress in evolving the concept of therapeutic targeting of proteoglycans is still in its infancy. This review thus provides fresh insights into the emerging therapeutic applications of ChABC in various diseases apart from SCI and the underlying mechanisms.

Tenascins Interfere With Remyelination in an Ex Vivo Cerebellar Explant Model of Demyelination

Oligodendrocytes form myelin membranes and thereby secure the insulation of axons and the rapid conduction of action potentials. Diseases such as multiple sclerosis highlight the importance of this glial cell population for brain function. In the adult brain, efficient remyelination following the damage to oligodendrocytes is compromised. Myelination is characterized by proliferation, migration, and proper integration of oligodendrocyte precursor cells (OPCs). These processes are among others controlled by proteins of the extracellular matrix (ECM). As a prominent representative ECM molecule, tenascin-C (Tnc) exerts an inhibitory effect on the migration and differentiation of OPCs. The structurally similar paralogue tenascin-R (Tnr) is known to promote the differentiation of oligodendrocytes. The model of lysolecithin-induced demyelination of cerebellar slice cultures represents an important tool for the analysis of the remyelination process. Ex vivo cerebellar explant cultures of Tnc^−/− and Tnr^−/− mouse lines displayed enhanced remyelination by forming thicker myelin membranes upon exposure to lysolecithin. The inhibitory effect of tenascins on remyelination could be confirmed when demyelinated wildtype control cultures were exposed to purified Tnc or Tnr protein. In that approach, the remyelination efficiency decreased in a dose-dependent manner with increasing concentrations of ECM molecules added. In order to examine potential roles in a complex in vivo environment, we successfully established cuprizone-based acute demyelination to analyze the remyelination behavior after cuprizone withdrawal in SV129, Tnc^−/−, and Tnr^−/− mice. In addition, we documented by immunohistochemistry in the cuprizone model the expression of chondroitin sulfate proteoglycans that are inhibitory for the differentiation of OPCs. In conclusion, inhibitory properties of Tnc and Tnr for myelin membrane formation could be demonstrated by using an ex vivo approach.

Oligodendrocyte and Extracellular Matrix Contributions to Central Nervous System Motor Function: Implications for Dystonia

The quest to elucidate nervous system function and dysfunction in disease has focused largely on neurons and neural circuits. However, fundamental aspects of nervous system development, function, and plasticity are regulated by nonneuronal elements, including glial cells and the extracellular matrix (ECM). The rapid rise of genomics and neuroimaging techniques in recent decades has highlighted neuronal-glial interactions and ECM as a key component of nervous system development, plasticity, and function. Abnormalities of neuronal-glial interactions have been understudied but are increasingly recognized to play a key role in many neurodevelopmental disorders. In this report, we consider the role of myelination and the ECM in the development and function of central nervous system motor circuits and the neurodevelopmental disease dystonia. © 2022 International Parkinson and Movement Disorder Society.

Microglia as therapeutic targets for central nervous system remyelination

Failed remyelination underpins neurodegeneration and central nervous system (CNS) dysfunction with aging and progression of neurological diseases, such as multiple sclerosis and Alzheimer's disease. Existing therapies have shown limited efficacy in halting disease progression in humans, highlighting the need to identify pro-remyelination treatments. Microglia are CNS-resident macrophages with critical roles in the regulation of remyelination, representing a promising therapeutic target. However, there are currently no therapeutics which specifically target microglia. Recent studies have revealed that microglia are a heterogenous population with distinct transcriptional states in health and disease conditions, including during remyelination, suggesting functional differences between states. Here, we discuss the potential contributions of different microglia states to degenerative and regenerative processes, examine the potential to target microglia in a state-specific manner to promote remyelination and consider the key issues to be addressed before such therapies can be clinically applied.

Chondroitin 6-sulfate-binding peptides improve recovery in spinal cord-injured mice

The role of glycosaminoglycan sulfation patterns, particularly in regard to scar formation and inhibition of neuritogenesis, has been mainly studied in cell culture with a focus on chondroitin 4-sulfate. In this study, we investigated chondroitin 6-sulfate (C6S) and found that it also inhibits neurite outgrowth of mouse cerebellar granule neurons in vitro. To examine whether the inhibitory activity of C6S could be neutralized, seven previously characterized high-affinity C6S-binding peptides were tested, among which three peptides neutralized the inhibitory functions of C6S. We further investigated these peptides in a mouse model of spinal cord injury, since upregulation of C6S expression in the glial scar following injury has been associated with reduced axonal regrowth and functional recovery. We here subjected mice to severe compression injury at thoracic levels T7-T9, immediately followed by inserting gelfoam patches soaked in C6S-binding peptides or in a control peptide. Application of C6S-binding peptides led to functional recovery after injury and prevented fibrotic glial scar formation, as seen by decreased activation of astrocytes and microglia/macrophages. Decreased expression of several lecticans and deposition of fibronectin at the site of injury were also observed. Application of C6S-binding peptides led to axonal regrowth and inhibited the C6S-mediated activation of RhoA/ROCK and decrease of PI3K-Akt-mTOR signaling pathways. Taken together, these results indicate that treatment with C6S-binding peptides improves functional recovery in a mouse model of spinal cord injury.

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

Microglia shape the synaptic environment in health and disease, but synapses do not exist in a vacuum. Instead, pre- and postsynaptic terminals are surrounded by extracellular matrix (ECM), which together with glia comprise the four elements of the contemporary tetrapartite synapse model. While research in this area is still just beginning, accumulating evidence points toward a novel role for microglia in regulating the ECM during normal brain homeostasis, and such processes may, in turn, become dysfunctional in disease. As it relates to synapses, microglia are reported to modify the perisynaptic matrix, which is the diffuse matrix that surrounds dendritic and axonal terminals, as well as perineuronal nets (PNNs), specialized reticular formations of compact ECM that enwrap neuronal subsets and stabilize proximal synapses. The interconnected relationship between synapses and the ECM in which they are embedded suggests that alterations in one structure necessarily affect the dynamics of the other, and microglia may need to sculpt the matrix to modify the synapses within. Here, we provide an overview of the microglial regulation of synapses, perisynaptic matrix, and PNNs, propose candidate mechanisms by which these structures may be modified, and present the implications of such modifications in normal brain homeostasis and in disease.

Inhibition of Autophagy Flux Promotes Secretion of Chondroitin Sulfate Proteoglycans in Primary Rat Astrocytes

Following spinal cord injury (SCI), reactive astrocytes in the glial scar produce high levels of chondroitin sulfate proteoglycans (CSPGs), which are known to inhibit axonal regeneration. Transforming growth factor beta (TGFβ) is a well-known factor that induces the production of CSPGs, and in this study, we report a novel mechanism underlying TGFβ's effects on CSPG secretion in primary rat astrocytes. We observed increased TGFβ-induced secretion of the CSPGs neurocan and brevican, and this occurred simultaneously with inhibition of autophagy flux. In addition, we show that neurocan and brevican levels are further increased when TGFβ is administered in the presence of an autophagy inhibitor, Bafilomycin-A1, while they are reduced when cells are treated with a concentration of rapamycin that is not sufficient to induce autophagy. These findings suggest that TGFβ mediates its effects on CSPG secretion through autophagy pathways. They also represent a potential new approach to reduce CSPG secretion in vivo by targeting autophagy pathways, which could improve axonal regeneration after SCI.

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.

THAP1 modulates oligodendrocyte maturation by regulating ECM degradation in lysosomes

Mechanisms controlling myelination during central nervous system (CNS) maturation play a pivotal role in the development and refinement of CNS circuits. The transcription factor THAP1 is essential for timing the inception of myelination during CNS maturation through a cell-autonomous role in the oligodendrocyte lineage. Here, we demonstrate that THAP1 modulates the extracellular matrix (ECM) composition by regulating glycosaminoglycan (GAG) catabolism within oligodendrocyte progenitor cells (OPCs). Thap1 ^-/- OPCs accumulate and secrete excess GAGs, inhibiting their maturation through an autoinhibitory mechanism. THAP1 controls GAG metabolism by binding to and regulating the GusB gene encoding β-glucuronidase, a GAG-catabolic lysosomal enzyme. Applying GAG-degrading enzymes or overexpressing β-glucuronidase rescues Thap1 ^-/- OL maturation deficits in vitro and in vivo. Our studies establish lysosomal GAG catabolism within OPCs as a critical mechanism regulating oligodendrocyte development.

In vivo imaging reveals mature Oligodendrocyte division in adult Zebrafish

Whether mature oligodendrocytes (mOLs) participate in remyelination has been disputed for several decades. Recently, some studies have shown that mOLs participate in remyelination by producing new sheaths. However, whether mOLs can produce new oligodendrocytes by asymmetric division has not been proven. Zebrafish is a perfect model to research remyelination compared to other species. In this study, optic nerve crushing did not induce local mOLs death. After optic nerve transplantation from olig2:eGFP fish to AB/WT fish, olig2⁺ cells from the donor settled and rewrapped axons in the recipient. After identifying these rewrapping olig2⁺ cells as mOLs at 3 months posttransplantation, in vivo imaging showed that olig2⁺ cells proliferated. Additionally, in vivo imaging of new olig2⁺ cell division from mOLs was also captured within the retina. Finally, fine visual function was renewed after the remyelination program was completed. In conclusion, our in vivo imaging results showed that new olig2⁺ cells were born from mOLs by asymmetric division in adult zebrafish, which highlights the role of mOLs in the progression of remyelination in the mammalian CNS.

Molecular signature of extracellular matrix pathology in schizophrenia

Growing evidence points to a critical involvement of the extracellular matrix (ECM) in the pathophysiology of schizophrenia (SZ). Decreases of perineuronal nets (PNNs) and altered expression of chondroitin sulphate proteoglycans (CSPGs) in glial cells have been identified in several brain regions. GWAS data have identified several SZ vulnerability variants of genes encoding for ECM molecules. Given the potential relevance of ECM functions to the pathophysiology of this disorder, it is necessary to understand the extent of ECM changes across brain regions, their region- and sex-specificity and which ECM components contribute to these changes. We tested the hypothesis that the expression of genes encoding for ECM molecules may be broadly disrupted in SZ across several cortical and subcortical brain regions and include key ECM components as well as factors such as ECM posttranslational modifications and regulator factors. Gene expression profiling of 14 neocortical brain regions, caudate, putamen and hippocampus from control subjects (n = 14/region) and subjects with SZ (n = 16/region) was conducted using Affymetrix microarray analysis. Analysis across brain regions revealed widespread dysregulation of ECM gene expression in cortical and subcortical brain regions in SZ, impacting several ECM functional key components. SRGN, CD44, ADAMTS1, ADAM10, BCAN, NCAN and SEMA4G showed some of the most robust changes. Region-, sex- and age-specific gene expression patterns and correlation with cognitive scores were also detected. Taken together, these findings contribute to emerging evidence for large-scale ECM dysregulation in SZ and point to molecular pathways involved in PNN decreases, glial cell dysfunction and cognitive impairment in SZ.

Progress in mimicking brain microenvironments to understand and treat neurological disorders

Neurological disorders including traumatic brain injury, stroke, primary and metastatic brain tumors, and neurodegenerative diseases affect millions of people worldwide. Disease progression is accompanied by changes in the brain microenvironment, but how these shifts in biochemical, biophysical, and cellular properties contribute to repair outcomes or continued degeneration is largely unknown. Tissue engineering approaches can be used to develop in vitro models to understand how the brain microenvironment contributes to pathophysiological processes linked to neurological disorders and may also offer constructs that promote healing and regeneration in vivo. In this Perspective, we summarize features of the brain microenvironment in normal and pathophysiological states and highlight strategies to mimic this environment to model disease, investigate neural stem cell biology, and promote regenerative healing. We discuss current limitations and resulting opportunities to develop tissue engineering tools that more faithfully recapitulate the aspects of the brain microenvironment for both in vitro and in vivo applications.

Astrocytes in Multiple Sclerosis-Essential Constituents with Diverse Multifaceted Functions

In multiple sclerosis (MS), astrocytes respond to the inflammatory stimulation with an early robust process of morphological, transcriptional, biochemical, and functional remodeling. Recent studies utilizing novel technologies in samples from MS patients, and in an animal model of MS, experimental autoimmune encephalomyelitis (EAE), exposed the detrimental and the beneficial, in part contradictory, functions of this heterogeneous cell population. In this review, we summarize the various roles of astrocytes in recruiting immune cells to lesion sites, engendering the inflammatory loop, and inflicting tissue damage. The roles of astrocytes in suppressing excessive inflammation and promoting neuroprotection and repair processes is also discussed. The pivotal roles played by astrocytes make them an attractive therapeutic target. Improved understanding of astrocyte function and diversity, and the mechanisms by which they are regulated may lead to the development of novel approaches to selectively block astrocytic detrimental responses and/or enhance their protective properties.

Interference of commissural connections through the genu of the corpus callosum specifically impairs sensorimotor gating

White matter abnormalities in schizophrenic patients are characterized as regional tract-specific. Myelin loss at the genu of the corpus callosum (GCC) is one of the most consistent findings in schizophrenic patients across the different populations. We characterized the axons that pass through the GCC by stereotactically injecting an anterograde axonal tracing viral vector into the forceps minor of the corpus callosum in one hemisphere, and identified the homotopic brain structures that have commissural connections in the two hemispheres of the prefrontal cortex, including the anterior cingulate area, the prelimbic area, the secondary motor area, and the dorsal part of the agranular insular area, along with commissural connections with the primary motor area, caudoputamen, and claustrum. To investigate whether dysmyelination in these commissural connections is critical for the development of schizophrenia symptoms, we generated a mouse model with focal demyelination at the GCC by stereotactically injecting demyelinating agent lysolecithin into this site, and tested these mice in a battery of behavioral tasks that are used to model the schizophrenia-like symptom domains. We found that demyelination at the GCC influenced neither the social interest or mood state, nor the locomotive activity or motor coordination. Nevertheless, it specifically reduced the prepulse inhibition of acoustic startle that is a well-known measure of sensorimotor gating. This study advances our understanding of the pathophysiological contributions of the GCC-specific white matter lesion to the related disease, and demonstrates an indispensable role of interhemispheric communication between the frontal cortices for the top-down regulation of the sensorimotor gating.

A Novel Lysolecithin Model for Visualizing Damage in vivo in the Larval Zebrafish Spinal Cord

Background: Lysolecithin is commonly used to induce demyelinating lesions in the spinal cord and corpus callosum of mammalian models. Although these models and clinical patient samples are used to study neurodegenerative diseases, such as multiple sclerosis (MS), they do not allow for direct visualization of disease-related damage in vivo. To overcome this limitation, we created and characterized a focal lysolecithin injection model in zebrafish that allows us to investigate the temporal dynamics underlying lysolecithin-induced damage in vivo. Results: We injected lysolecithin into 4-6 days post-fertilization (dpf) zebrafish larval spinal cords and, coupled with in vivo, time-lapse imaging, observed hallmarks consistent with mammalian models of lysolecithin-induced demyelination, including myelinating glial cell loss, myelin perturbations, axonal sparing, and debris clearance. Conclusion: We have developed and characterized a lysolecithin injection model in zebrafish that allows us to investigate myelin damage in a living, vertebrate organism. This model may be a useful pre-clinical screening tool for investigating the safety and efficacy of novel therapeutic compounds that reduce damage and/or promote repair in neurodegenerative disorders, such as MS.

In vivo tensor-valued diffusion MRI of focal demyelination in white and deep grey matter of rodents

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We performed in-vivo tensor-valued diffusion MRI in demyelinating rodents.

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Lysolecithin was injected in white and deep grey matter to cause focal demyelination.

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Focal demyelination reduced microscopic fractional anisotropy (µFA).

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Isotropic kurtosis may be particularly sensitive to deep grey matter lesions.

Background

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease leading to damage of white matter (WM) and grey matter (GM). Magnetic resonance imaging (MRI) is the modality of choice to assess brain damage in MS, but there is an unmet need in MRI for achieving higher sensitivity and specificity to MS-related microstructural alterations in WM and GM.

Objective

To explore whether tensor-valued diffusion MRI (dMRI) can yield sensitive microstructural read-outs for focal demyelination in cerebral WM and deep GM (DGM).

Methods

Eight rats underwent L-α-Lysophosphatidylcholine (LPC) injections in the WM and striatum to introduce focal demyelination. Multimodal MRI was performed at 7 Tesla after 7 days. Tensor-valued dMRI was complemented by diffusion tensor imaging, quantitative MRI and proton magnetic resonance spectroscopy (MRS).

Results

Quantitative MRI and MRS confirmed that LPC injections caused inflammatory demyelinating lesions in WM and DGM. Tensor-valued dMRI illustrated a significant decline of microscopic fractional anisotropy (µFA) in both LPC-treated WM and DGM (P < 0.005) along with a marked increase of isotropic kurtosis (MK_I) in DGM (P < 0.0001).

Conclusion

Tensor-valued dMRI bears considerable potential for microstructural imaging in MS, suggesting a regional µFA decrease may be a sensitive indicator of MS lesions, while a regional MK_I increase may be particularly sensitive in detecting DGM lesions of MS.

Myelin Repair: From Animal Models to Humans

It is widely thought that brain repair does not occur, but myelin regeneration provides clear evidence to the contrary. Spontaneous remyelination may occur after injury or in multiple sclerosis (MS). However, the efficiency of remyelination varies considerably between MS patients and between the lesions of each patient. Myelin repair is essential for optimal functional recovery, so a profound understanding of the cells and mechanisms involved in this process is required for the development of new therapeutic strategies. In this review, we describe how animal models and modern cell tracing and imaging methods have helped to identify the cell types involved in myelin regeneration. In addition to the oligodendrocyte progenitor cells identified in the 1990s as the principal source of remyelinating cells in the central nervous system (CNS), other cell populations, including subventricular zone-derived neural progenitors, Schwann cells, and even spared mature oligodendrocytes, have more recently emerged as potential contributors to CNS remyelination. We will also highlight the conditions known to limit endogenous repair, such as aging, chronic inflammation, and the production of extracellular matrix proteins, and the role of astrocytes and microglia in these processes. Finally, we will present the discrepancies between observations in humans and in rodents, discussing the relationship of findings in experimental models to myelin repair in humans. These considerations are particularly important from a therapeutic standpoint.

Harnessing the Benefits of Neuroinflammation: Generation of Macrophages/Microglia with Prominent Remyelinating Properties

Excessive inflammation within the CNS is injurious, but an immune response is also required for regeneration. Macrophages and microglia adopt different properties depending on their microenvironment, and exposure to IL4 and IL13 has been used to elicit repair. Unexpectedly, while LPS-exposed macrophages and microglia killed neural cells in culture, the addition of LPS to IL4/IL13-treated macrophages and microglia profoundly elevated IL10, repair metabolites, heparin binding epidermal growth factor trophic factor, antioxidants, and matrix-remodeling proteases. In C57BL/6 female mice, the generation of M(LPS/IL4/IL13) macrophages required TLR4 and MyD88 signaling, downstream activation of phosphatidylinositol-3 kinase/mTOR and MAP kinases, and convergence on phospho-CREB, STAT6, and NFE2. Following mouse spinal cord demyelination, local LPS/IL4/IL13 deposition markedly increased lesional phagocytic macrophages/microglia, lactate and heparin binding epidermal growth factor, matrix remodeling, oligodendrogenesis, and remyelination. Our data show that a prominent reparative state of macrophages/microglia is generated by the unexpected integration of pro- and anti-inflammatory activation cues. The results have translational potential, as the LPS/IL4/IL13 mixture could be locally applied to a focal CNS injury to enhance neural regeneration and recovery.SIGNIFICANCE STATEMENT The combination of LPS and regulatory IL4 and IL13 signaling in macrophages and microglia produces a previously unknown and particularly reparative phenotype devoid of pro-inflammatory neurotoxic features. The local administration of LPS/IL4/IL13 into spinal cord lesion elicits profound oligodendrogenesis and remyelination. The careful use of LPS and IL4/IL13 mixture could harness the known benefits of neuroinflammation to enable repair in neurologic insults.