Central nervous system remyelination in culture--a tool for multiple sclerosis research

Aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders: progress of experimental models based on disease pathogenesis

Neuromyelitis optica spectrum disorders are neuroinflammatory demyelinating disorders that lead to permanent visual loss and motor dysfunction. To date, no effective treatment exists as the exact causative mechanism remains unknown. Therefore, experimental models of neuromyelitis optica spectrum disorders are essential for exploring its pathogenesis and in screening for therapeutic targets. Since most patients with neuromyelitis optica spectrum disorders are seropositive for IgG autoantibodies against aquaporin-4, which is highly expressed on the membrane of astrocyte endfeet, most current experimental models are based on aquaporin-4-IgG that initially targets astrocytes. These experimental models have successfully simulated many pathological features of neuromyelitis optica spectrum disorders, such as aquaporin-4 loss, astrocytopathy, granulocyte and macrophage infiltration, complement activation, demyelination, and neuronal loss; however, they do not fully capture the pathological process of human neuromyelitis optica spectrum disorders. In this review, we summarize the currently known pathogenic mechanisms and the development of associated experimental models in vitro, ex vivo, and in vivo for neuromyelitis optica spectrum disorders, suggest potential pathogenic mechanisms for further investigation, and provide guidance on experimental model choices. In addition, this review summarizes the latest information on pathologies and therapies for neuromyelitis optica spectrum disorders based on experimental models of aquaporin-4-IgG-seropositive neuromyelitis optica spectrum disorders, offering further therapeutic targets and a theoretical basis for clinical trials.

Complementary strategies to be used in conjunction with animal models for multiple sclerosis drug discovery: adapting preclinical validation of drug candidates to the need of remyelinating strategies

INTRODUCTION

The quest for novel MS therapies focuses on promoting remyelination and neuroprotection, necessitating innovative drug design paradigms and robust preclinical validation methods to ensure efficient clinical translation. The complexity of new drugs action mechanisms is strengthening the need for solid biological validation attempting to address all possible pitfalls and biases precluding access to efficient and safe drugs.

AREAS COVERED

In this review, the authors describe the different in vitro and in vivo models that should be used to create an integrated approach for preclinical validation of novel drugs, including the evaluation of the action mechanism. This encompasses 2D, 3D in vitro models and animal models presented in such a way to define the appropriate use in a global process of drug screening and hit validation.

EXPERT OPINION

None of the current available tests allow the concomitant evaluation of anti-inflammatory, immune regulators or remyelinating agents with sufficient reliability. Consequently, the collaborative efforts of academia, industry, and regulatory agencies are essential for establishing standardized protocols, validating novel methodologies, and translating preclinical findings into clinically meaningful outcomes.

RXR nuclear receptor signaling modulates lipid metabolism and triggers lysosomal clearance of alpha-synuclein in neuronal models of synucleinopathy

Disease-modifying strategies for Parkinson disease (PD), the most common synucleinopathy, represent a critical unmet medical need. Accumulation of the neuronal protein alpha-synuclein (αS) and abnormal lipid metabolism have each been implicated in PD pathogenesis. Here, we elucidate how retinoid-X-receptor (RXR) nuclear receptor signaling impacts these two aspects of PD pathogenesis. We find that activated RXR differentially regulates fatty acid desaturases, significantly reducing the transcript levels of the largely brain-specific desaturase SCD5 in human cultured neural cells and PD patient-derived neurons. This was associated with reduced perilipin-2 protein levels in patient neurons, reversal of αS-induced increases in lipid droplet (LD) size, and a reduction of triglyceride levels in human cultured cells. With regard to αS proteostasis, our study reveals that RXR agonism stimulates lysosomal clearance of αS. Our data support the involvement of Polo-like kinase 2 activity and αS S129 phosphorylation in mediating this benefit. The lowering of cellular αS levels was associated with reduced cytotoxicity. Compared to RXR activation, the RXR antagonist HX531 had the opposite effects on LD size, SCD, αS turnover, and cytotoxicity, all supporting pathway specificity. Together, our findings show that RXR-activating ligands can modulate fatty acid metabolism and αS turnover to confer benefit in cellular models of PD, including patient neurons. We offer a new paradigm to investigate nuclear receptor ligands as a promising strategy for PD and related synucleinopathies.

Human post-mortem organotypic brain slice cultures: a tool to study pathomechanisms and test therapies

Human brain experimental models recapitulating age- and disease-related characteristics are lacking. There is urgent need for human-specific tools that model the complex molecular and cellular interplay between different cell types to assess underlying disease mechanisms and test therapies. Here we present an adapted ex vivo organotypic slice culture method using human post-mortem brain tissue cultured at an air-liquid interface to also study brain white matter. We assessed whether these human post-mortem brain slices recapitulate the in vivo neuropathology and if they are suitable for pathophysiological, experimental and pre-clinical treatment development purposes, specifically regarding leukodystrophies. Human post-mortem brain tissue and cerebrospinal fluid were obtained from control, psychiatric and leukodystrophy donors. Slices were cultured up to six weeks, in culture medium with or without human cerebrospinal fluid. Human post-mortem organotypic brain slice cultures remained viable for at least six weeks ex vivo and maintained tissue structure and diversity of (neural) cell types. Supplementation with cerebrospinal fluid could improve slice recovery. Patient-derived organotypic slice cultures recapitulated and maintained known in vivo neuropathology. The cultures also showed physiologic multicellular responses to lysolecithin-induced demyelination ex vivo, indicating their suitability to study intrinsic repair mechanisms upon injury. The slice cultures were applicable for various experimental studies, as multi-electrode neuronal recordings. Finally, the cultures showed successful cell-type dependent transduction with gene therapy vectors. These human post-mortem organotypic brain slice cultures represent an adapted ex vivo model suitable for multifaceted studies of brain disease mechanisms, boosting translation from human ex vivo to in vivo. This model also allows for assessing potential treatment options, including gene therapy applications. Human post-mortem brain slice cultures are thus a valuable tool in preclinical research to study the pathomechanisms of a wide variety of brain diseases in living human tissue.

Ageing impairs the regenerative capacity of regulatory T cells in mouse central nervous system remyelination

Myelin regeneration (remyelination) is essential to prevent neurodegeneration in demyelinating diseases such as Multiple Sclerosis, however, its efficiency declines with age. Regulatory T cells (Treg) recently emerged as critical players in tissue regeneration, including remyelination. However, the effect of ageing on Treg-mediated regenerative processes is poorly understood. Here, we show that expansion of aged Treg does not rescue age-associated remyelination impairment due to an intrinsically diminished capacity of aged Treg to promote oligodendrocyte differentiation and myelination in male and female mice. This decline in regenerative Treg functions can be rescued by a young environment. We identified Melanoma Cell Adhesion Molecule 1 (MCAM1) and Integrin alpha 2 (ITGA2) as candidates of Treg-mediated oligodendrocyte differentiation that decrease with age. Our findings demonstrate that ageing limits the neuroregenerative capacity of Treg, likely limiting their remyelinating therapeutic potential in aged patients, and describe two mechanisms implicated in Treg-driven remyelination that may be targetable to overcome this limitation.

Metabolic implications of axonal demyelination and its consequences for synchronized network activity: An in silico and in vitro study

Myelination enhances the conduction velocity of action potentials (AP) and increases energy efficiency. Thick myelin sheaths are typically found on large-distance axonal connections or in fast-spiking interneurons, which are critical for synchronizing neuronal networks during gamma-band oscillations. Loss of myelin sheath is associated with multiple alterations in axonal architecture leading to impaired AP propagation. While numerous studies are devoted to the effects of demyelination on conduction velocity, the metabolic effects and the consequences for network synchronization have not been investigated. Here we present a unifying computational model for electrophysiology and metabolism of the myelinated axon. The computational model suggested that demyelination not only decreases the AP speed but AP propagation in demyelinated axons requires compensatory processes like mitochondrial mass increase and a switch from saltatory to continuous propagation to rescue axon functionality at the cost of reduced AP propagation speed and increased energy expenditure. Indeed, these predictions were proven to be true in a culture model of demyelination where the pharmacologically-induced loss of myelin was associated with increased oxygen consumption rates, and a significant broadening of bandwidth as well as a decrease in the power of gamma oscillations.

Validation of Recombinant Heparan Sulphate Reagents for CNS Repair

Therapies that target the multicellular pathology of central nervous system (CNS) disease/injury are urgently required. Modified non-anticoagulant heparins mimic the heparan sulphate (HS) glycan family and have been proposed as therapeutics for CNS repair since they are effective regulators of numerous cellular processes. Our in vitro studies have demonstrated that low-sulphated modified heparan sulphate mimetics (LS-mHeps) drive CNS repair. However, LS-mHeps are derived from pharmaceutical heparin purified from pig intestines, in a supply chain at risk of shortages and contamination. Alternatively, cellular synthesis of heparin and HS can be achieved using mammalian cell multiplex genome engineering, providing an alternative source of recombinant HS mimetics (rHS). TEGA Therapeutics (San Diego) have manufactured rHS reagents with varying degrees of sulphation and we have validated their ability to promote repair in vitro using models that mimic CNS injury, making comparisons to LS-mHep7, a previous lead compound. We have shown that like LS-mHep7, low-sulphated rHS compounds promote remyelination and reduce features of astrocytosis, and in contrast, highly sulphated rHS drive neurite outgrowth. Cellular production of heparin mimetics may, therefore, offer potential clinical benefits for CNS repair.

ANGPTL2 binds MAG to efficiently enhance oligodendrocyte differentiation

BACKGROUND

Oligodendrocytes have robust regenerative ability and are key players in remyelination during physiological and pathophysiological states. However, the mechanisms of brain microenvironmental cue in regulation of the differentiation of oligodendrocytes still needs to be further investigated.

RESULTS

We demonstrated that myelin-associated glycoprotein (MAG) was a novel receptor for angiopoietin-like protein 2 (ANGPTL2). The binding of ANGPTL2 to MAG efficiently promoted the differentiation of oligodendrocytes in vitro, as evaluated in an HCN cell line. Angptl2-null mice had a markedly impaired myelination capacity in the early stage of oligodendrocyte development. These mice had notably decreased remyelination capacities and enhanced motor disability in a cuprizone-induced demyelinating mouse model, which was similar to the Mag-null mice. The loss of remyelination ability in Angptl2-null/Mag-null mice was similar to the Angptl2-WT/Mag-null mice, which indicated that the ANGPTL2-mediated oligodendrocyte differentiation effect depended on the MAG receptor. ANGPTL2 bound MAG to enhance its phosphorylation level and recruit Fyn kinase, which increased Fyn phosphorylation levels, followed by the transactivation of myelin regulatory factor (MYRF).

CONCLUSION

Our study demonstrated an unexpected cross-talk between the environmental protein (ANGPTL2) and its surface receptor (MAG) in the regulation of oligodendrocyte differentiation, which may benefit the treatment of many demyelination disorders, including multiple sclerosis.

Cholinergic control of Th17 cell pathogenicity in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a mouse model of multiple sclerosis (MS) in which Th17 cells have a crucial but unclear function. Here we show that choline acetyltransferase (ChAT), which synthesizes acetylcholine (ACh), is a critical driver of pathogenicity in EAE. Mice with ChAT-deficient Th17 cells resist disease progression and show reduced brain-infiltrating immune cells. ChAT expression in Th17 cells is linked to strong TCR signaling, expression of the transcription factor Bhlhe40, and increased Il2, Il17, Il22, and Il23r mRNA levels. ChAT expression in Th17 cells is independent of IL21r signaling but dampened by TGFβ, implicating ChAT in controlling the dichotomous nature of Th17 cells. Our study establishes a cholinergic program in which ACh signaling primes chronic activation of Th17 cells, and thereby constitutes a pathogenic determinant of EAE. Our work may point to novel targets for therapeutic immunomodulation in MS.

Cell-based experimental strategies for myelin repair in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune demyelinating disorder of the central nervous system (CNS), diagnosed at a mean age of 32 years. CNS glia are crucial players in the onset of MS, primarily involving astrocytes and microglia that can cause/allow massive oligodendroglial cells death, without immune cell infiltration. Current therapeutic approaches are aimed at modulating inflammatory reactions during relapsing episodes, but lack the ability to induce very significant repair mechanisms. In this review article, different experimental approaches based mainly on the application of different cell types as therapeutic strategies applied for the induction of myelin repair and/or the amelioration of the disease are discussed. Regarding this issue, different cell sources were applied in various experimental models of MS, with different results, both in significant improvements in remyelination and the reduction of neuroinflammation and glial activation, or in neuroprotection. All cell types tested have advantages and disadvantages, which makes it difficult to choose a better option for therapeutic application in MS. New strategies combining cell-based treatment with other applications would result in further improvements and would be good candidates for MS cell therapy and myelin repair.

Significant Association of rs77493513 Polymorphism in 3'-UTR of the NRG1 Gene with the Risk of Multiple Sclerosis Disease

Multiple sclerosis (MS) is a chronic inflammatory and autoimmune disease characterized by demyelination of the central nervous system (CNS). Neuregulin 1 (NRG1) is a signaling protein that plays an important role in a variety of biological processes, including potentiate oligodendrocyte differentiation and myelination in the CNS, immune response regulation, and inflammation. Single nucleotide polymorphism (SNP) rs77493513 is located in the untranslated region of the 3' mRNA (3'-UTR) of the NRG1 gene, which is predicted to be the binding site of several microRNAs and may play an important role in post-transcriptional regulation. Study aimed to investigate the association of SNP rs77493513 in the NRG1 gene with the risk of MS disease. In this study, genomic DNA was extracted from whole blood samples of 182 patients with relapsing-remitting multiple sclerosis (RRMS) and 198 controls. Different genotypes of rs77493513 polymorphism were determined using RFLP-PCR technique. Statistical analysis was performed using SPSS 21.0 software and by t, χ2 and logistic regression tests. Our data showed that genotypes AC (OR=3.63, CI= 1.93-6.81, p<0.001) and CC (OR=7.90, CI= 4.13-15.11, p<0.001) significantly increased the risk of MS disease and C allele is risk allele. Also, AC (OR=0.16, CI= 0.04-0.63, p= 0.009) and CC (OR=0.14, CI= 0.03-0.53, p=0.04) genotypes significantly decrease the age of onset of the disease. The results show that allele C of rs77493513 polymorphism in the NRG1 gene can be a risk factor for MS.

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.

Assessing the Potential of Molecular Imaging for Myelin Quantification in Organotypic Cultures

Ex vivo models for the noninvasive study of myelin-related diseases represent an essential tool to understand the mechanisms of diseases and develop therapies against them. Herein, we assessed the potential of multimodal imaging traceable myelin-targeting liposomes to quantify myelin in organotypic cultures. Methods: MRI testing was used to image mouse cerebellar tissue sections and organotypic cultures. Demyelination was induced by lysolecithin treatment. Myelin-targeting liposomes were synthetized and characterized, and their capacity to quantify myelin was tested by fluorescence imaging. Results: Imaging of freshly excised tissue sections ranging from 300 µm to 1 mm in thickness was achieved with good contrast between white (WM) and gray matter (GM) using T2w MRI. The typical loss of stiffness, WM structures, and thickness of organotypic cultures required the use of diffusion-weighted methods. Designed myelin-targeting liposomes allowed for semiquantitative detection by fluorescence, but the specificity for myelin was not consistent between assays due to the unspecific binding of liposomes. Conclusions: With respect to the sensitivity, imaging of brain tissue sections and organotypic cultures by MRI is feasible, and myelin-targeting nanosystems are a promising solution to quantify myelin ex vivo. With respect to specificity, fine tuning of the probe is required. Lipid-based systems may not be suitable for this goal, due to unspecific binding to tissues.

A 3D disease and regeneration model of peripheral nervous system-on-a-chip

Demyelinating diseases involve loss of myelin sheaths and eventually lead to neurological problems. Unfortunately, the precise mechanisms remain unknown, and there are no effective therapies. To overcome these limitations, a reliable and physiologically relevant in vitro model is required. Here, we present a three-dimensional peripheral nervous system (PNS) microfluidic platform that recapitulates the full spectrum of myelination, demyelination, and remyelination using primary Schwann cells (SCs) and motor neurons (MNs). The platform enables reproducible hydrogel patterning and long-term stable coculture of MNs and SCs over 40 days in vitro based on three distinct design factors. Furthermore, the on-demand detachable substrate allows in-depth biological analysis. We demonstrated the possibility of mimicking segmental demyelination by lysophosphatidylcholine, and recovery of myelin structure by application of two drugs: benzatropine or methylcobalamin. This 3D PNS disease-on-a-chip may serve as a potential platform for understanding the pathophysiology of demyelination and screening drugs for remyelination.

Pro-inflammatory activation following demyelination is required for myelin clearance and oligodendrogenesis

Remyelination requires innate immune system function, but how exactly microglia and macrophages clear myelin debris after injury and tailor a specific regenerative response is unclear. Here, we asked whether pro-inflammatory microglial/macrophage activation is required for this process. We established a novel toxin-based spinal cord model of de- and remyelination in zebrafish and showed that pro-inflammatory NF-κB–dependent activation in phagocytes occurs rapidly after myelin injury. We found that the pro-inflammatory response depends on myeloid differentiation primary response 88 (MyD88). MyD88-deficient mice and zebrafish were not only impaired in the degradation of myelin debris, but also in initiating the generation of new oligodendrocytes for myelin repair. We identified reduced generation of TNF-α in lesions of MyD88-deficient animals, a pro-inflammatory molecule that was able to induce the generation of new premyelinating oligodendrocytes. Our study shows that pro-inflammatory phagocytic signaling is required for myelin debris degradation, for inflammation resolution, and for initiating the generation of new oligodendrocytes.

Myelin detection in fluorescence microscopy images using machine learning

BACKGROUND

The myelin sheath produced by glial cells insulates the axons, and supports the function of the nervous system. Myelin sheath degeneration causes neurodegenerative disorders, such as multiple sclerosis (MS). There are no therapies for MS that promote remyelination. Drug discovery frequently involves screening thousands of compounds. However, this is not feasible for remyelination drugs, since myelin quantification is a manual labor-intensive endeavor. Therefore, the development of assistive software for expedited myelin detection is instrumental for MS drug discovery by enabling high-content image-based drug screens.

NEW METHOD

In this study, we developed a machine learning based expedited myelin detection approach in fluorescence microscopy images. Multi-channel three-dimensional microscopy images of a mouse stem cell-based myelination assay were labeled by experts. A spectro-spatial feature extraction method was introduced to represent local dependencies of voxels both in spatial and spectral domains. Feature extraction yielded two data set of over forty-seven thousand annotated images in total.

RESULTS

Myelin detection performances of 23 different supervised machine learning techniques including a customized-convolutional neural network (CNN), were assessed using various train/test split ratios of the data sets. The highest accuracy values of 98.84±0.09% and 98.46±0.11% were achieved by Boosted Trees and customized-CNN, respectively.

COMPARISON WITH EXISTING METHODS

Our approach can detect myelin in a common experimental setup. Myelin extending in any orientation in 3 dimensions is segmented from 3 channel z-stack fluorescence images.

CONCLUSIONS

Our results suggest that the proposed expedited myelin detection approach is a feasible and robust method for remyelination drug screening.

Bruton's Tyrosine Kinase Inhibition Promotes Myelin Repair

Background:

Microglia are the resident macrophages of the central nervous system (CNS). In multiple sclerosis (MS) and related experimental models, microglia have either a pro-inflammatory or a pro-regenerative/pro-remyelinating function. Inhibition of Bruton’s tyrosine kinase (BTK), a member of the Tec family of kinases, has been shown to block differentiation of pro-inflammatory macrophages in response to granulocyte–macrophage colony-stimulating factor in vitro. However, the role of BTK in the CNS is unknown.

Methods:

Our aim was to investigate the effect of BTK inhibition on myelin repair in ex vivo and in vivo experimental models of demyelination and remyelination. The remyelination effect of a BTK inhibitor (BTKi; BTKi-1) was then investigated in LPC-induced demyelinated cerebellar organotypic slice cultures and metronidazole-induced demyelinated Xenopus MBP-GFP-NTR transgenic tadpoles.

Results:

Cellular detection of BTK and its activated form BTK-phospho-Y223 (p-BTK) was determined by immunohistochemistry in organotypic cerebellar slice cultures, before and after lysophosphatidylcholine (LPC)-induced demyelination. A low BTK signal detected by immunolabeling under normal conditions in cerebellar slices was in sharp contrast to an 8.5-fold increase in the number of BTK-positive cells observed in LPC-demyelinated slice cultures. Under both conditions, approximately 75% of cells expressing BTK and p-BTK were microglia and 25% were astrocytes. Compared with spontaneous recovery, treatment of demyelinated slice cultures and MTZ-demyelinated transgenic tadpoles with BTKi resulted in at least a 1.7-fold improvement of remyelination.

Conclusion:

Our data demonstrate that BTK inhibition is a promising therapeutic strategy for myelin repair.

Platelet-Activating Factor Deteriorates Lysophosphatidylcholine-Induced Demyelination Via Its Receptor-Dependent and -Independent Effects

Accumulating evidence suggests that platelet-activating factor (PAF) increases the inflammatory response in demyelinating diseases such as multiple sclerosis. However, PAF receptor (PAFR) antagonists do not show therapeutic efficacy for MS, and its underlying mechanisms remain poorly understood. In the present study, we investigated the effects of PAF on an ex vivo demyelination cerebellar model following lysophosphatidylcholine (LPC, 0.5 mg/mL) application using wild-type and PAFR conventional knockout (PAFR-KO) mice. Demyelination was induced in cerebellar slices that were cultured with LPC for 18 h. Exogenous PAF (1 μM) acting on cerebellar slices alone did not cause demyelination but increased the severity of LPC-induced demyelination in both wild-type and PAFR-KO mice. LPC inhibited the expression of PAF-AH, MBP, TNF-α, and TGF-β1 but facilitated the expression of IL-1β and IL-6 in wild-type preparations. Of note, exogenous PAF stimulated microglial activation in both wild-type and PAFR-KO mice. The subsequent inflammatory cytokines TNFα, IL-1β, and IL-6 as well as the anti-inflammatory cytokine TGF-β1 demonstrated a diverse transcriptional profile with or without LPC treatment. PAF promoted TNF-α expression and suppressed TGF-β1 expression indiscriminately in wild-type and knockout slices; however, transcription of IL-1β and IL-6 was not significantly affected in both slices. The syntheses of IL-1β and IL-6 were significantly increased in LPC-induced demyelination preparations without PAF but showed a redundancy in PAF-treated wild-type and knockout slices. These data suggest that PAF can play a detrimental role in LPC-induced demyelination probably due to a redundant response of PAFR-dependent and PAFR-independent effects on inflammatory cytokines.

Dynamic CCN3 expression in the murine CNS does not confer essential roles in myelination or remyelination

Remyelination is a natural regenerative process driven by oligodendrocytes that occurs following myelin damage. Understanding this process holds therapeutic value for demyelinating diseases such as multiple sclerosis, in which remyelination can fail. CCN3 is a matricellular protein previously reported to enhance oligodendrocyte progenitor differentiation and myelination in vitro and ex vivo. Here, we show that despite extensive and dynamic expression in the murine CNS in homeostasis and following toxin-induced myelin damage, CCN3 is not required for myelination or remyelination in vivo. Yet, the anatomically distinct expression pattern suggests unidentified roles of CCN3 in a range of neurological processes. This investigation provides a framework for future investigations of the expression and role of CCN proteins in the CNS.

CCN3 is a matricellular protein that promotes oligodendrocyte progenitor cell differentiation and myelination in vitro and ex vivo. CCN3 is therefore a candidate of interest in central nervous system (CNS) myelination and remyelination, and we sought to investigate the expression and role of CCN3 during these processes. We found CCN3 to be expressed predominantly by neurons in distinct areas of the CNS, primarily the cerebral cortex, hippocampus, amygdala, suprachiasmatic nuclei, anterior olfactory nuclei, and spinal cord gray matter. CCN3 was transiently up-regulated following demyelination in the brain of cuprizone-fed mice and spinal cord lesions of mice injected with lysolecithin. However, CCN3^−/− mice did not exhibit significantly different numbers of oligodendroglia or differentiated oligodendrocytes in the healthy or remyelinating CNS, compared to WT controls. These results suggest that despite robust and dynamic expression in the CNS, CCN3 is not required for efficient myelination or remyelination in the murine CNS in vivo.

In vitro Models of Neurodegenerative Diseases

Neurodegenerative diseases are progressive degenerative conditions characterized by the functional deterioration and ultimate loss of neurons. These incurable and debilitating diseases affect millions of people worldwide, and therefore represent a major global health challenge with severe implications for individuals and society. Recently, several neuroprotective drugs have failed in human clinical trials despite promising pre-clinical data, suggesting that conventional cell cultures and animal models cannot precisely replicate human pathophysiology. To bridge the gap between animal and human studies, three-dimensional cell culture models have been developed from human or animal cells, allowing the effects of new therapies to be predicted more accurately by closely replicating some aspects of the brain environment, mimicking neuronal and glial cell interactions, and incorporating the effects of blood flow. In this review, we discuss the relative merits of different cerebral models, from traditional cell cultures to the latest high-throughput three-dimensional systems. We discuss their advantages and disadvantages as well as their potential to investigate the complex mechanisms of human neurodegenerative diseases. We focus on in vitro models of the most frequent age-related neurodegenerative disorders, such as Parkinson’s disease, Alzheimer’s disease and prion disease, and on multiple sclerosis, a chronic inflammatory neurodegenerative disease affecting young adults.

Development of the first in vivo GPR17 ligand through an iterative drug discovery pipeline: A novel disease-modifying strategy for multiple sclerosis

The GPR17 receptor, expressed on oligodendroglial precursors (OPCs, the myelin producing cells), has emerged as an attractive target for a pro-myelinating strategy in multiple sclerosis (MS). However, the proof-of-concept that selective GPR17 ligands actually exert protective activity in vivo is still missing. Here, we exploited an iterative drug discovery pipeline to prioritize novel and selective GPR17 pro-myelinating agents out of more than 1,000,000 compounds. We first performed an in silico high-throughput screening on GPR17 structural model to identify three chemically-diverse ligand families that were then combinatorially exploded and refined. Top-scoring compounds were sequentially tested on reference pharmacological in vitro assays with increasing complexity, ending with myelinating OPC-neuron co-cultures. Successful ligands were filtered through in silico simulations of metabolism and pharmacokinetics, to select the most promising hits, whose dose and ability to target the central nervous system were then determined in vivo. Finally, we show that, when administered according to a preventive protocol, one of them (named by us as galinex) is able to significantly delay the onset of experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. This outcome validates the predictivity of our pipeline to identify novel MS-modifying agents.

TLR3 agonists induce fibronectin aggregation by activated astrocytes: a role of pro-inflammatory cytokines and fibronectin splice variants

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system which eventually results in axonal loss mainly due to failure of remyelination. Previously we have shown that the persistent presence of stable astrocyte-derived fibronectin aggregates in MS lesions impairs OPC differentiation, and thereby remyelination. Here we set out to discern whether and, if so, how inflammatory mediators as present in MS lesions trigger astrocytes to form fibronectin aggregates. Our findings revealed that in slice cultures only upon demyelination, the TLR3 agonist Poly(I:C) evoked astrocytes to form fibronectin aggregates. Consistently, pro-inflammatory cytokine-pretreated astrocytes were more susceptible to Poly(I:C)-induced fibronectin aggregation, indicating that astrocytes form fibronectin aggregates upon a double hit by inflammatory mediators. The underlying mechanism involves disrupted fibronectin fibrillogenesis at the cell surface as a result of a cytokine-induced increase in relative mRNA levels of EIIIA^pos-Fn over EIIIB^pos-Fn and a Poly(I:C)-mediated decrease in integrin affinity. Remarkably, fibronectin aggregation is exacerbated by white matter astrocytes compared to grey matter astrocytes, which may be a reflection of higher expression levels of EIIIA^pos-fibronectin in white matter astrocytes. Hence, interfering with alternative fibronectin splicing and/or TLR3-mediated signaling may prevent fibronectin aggregation and overcome remyelination failure in MS lesions.

Axonal degeneration in an in vitro model of ischemic white matter injury

Ischemic white matter injuries underlie cognitive decline in the elderly and vascular dementia. Ischemia in the subcortical white matter is caused by chronic reduction of blood flow due to narrowing of small arterioles. However, it remains unclear how chronic ischemia leads to white matter pathology. We aimed to develop an in vitro model of ischemic white matter injury using organotypic slice cultures. Cultured cerebellar slices preserved fully myelinated white matter tracts that were amenable to chronic hypoxic insult. Prolonged hypoxia caused progressive morphological evidence of axonal degeneration with focal constrictions and swellings. In contrast, myelin sheaths and oligodendrocytes exhibited remarkable resilience to hypoxia. The cytoskeletal degradation of axons was accompanied by mitochondrial shortening and lysosomal activation. Multiple pharmacological manipulations revealed that the AMPA glutamate receptor, calpain proteolysis, and lysosomal proteases were independently implicated in hypoxia-induced axonal degeneration in our model. Thus, our in vitro model would be a novel experimental system to explore molecular mechanisms of ischemic white matter injury. Furthermore, we verified that the in vitro assay could be successfully utilized to screen for molecules that can ameliorate hypoxia/ischemia-induced axonal degeneration.

Alkyne-Tagged PLGA Allows Direct Visualization of Nanoparticles In Vitro and Ex Vivo by Stimulated Raman Scattering Microscopy

Polymeric nanoparticles (NPs) are attractive candidates for the controlled and targeted delivery of therapeutics in vitro and in vivo. However, detailed understanding of the uptake, location, and ultimate cellular fate of the NPs is necessary to satisfy safety concerns, which is difficult because of the nanoscale size of these carriers. In this work, we show how small chemical labels can be appended to poly(lactic acid-co-glycolic acid) (PLGA) to synthesize NPs that can then be imaged by stimulated Raman scattering microscopy, a vibrational imaging technique that can elucidate bond-specific information in biological environments, such as the identification of alkyne signatures in modified PLGA terpolymers. We show that both deuterium and alkyne labeled NPs can be imaged within primary rat microglia, and the alkyne NPs can also be imaged in ex vivo cortical mouse brain tissue. Immunohistochemical analysis confirms that the NPs localize in microglia in the mouse brain tissue, demonstrating that these NPs have the potential to deliver therapeutics selectively to microglia.

Cryogel scaffolds for regionally constrained delivery of lysophosphatidylcholine to central nervous system slice cultures: A model of focal demyelination for multiple sclerosis research

The pathology of multiple sclerosis (MS) is typified by focal demyelinated areas of the brain and spinal cord, which results in axonal degeneration and atrophy. Although the field has made much progress in developing immunomodulatory therapies to reduce the occurrence of these focal lesions, there is a conspicuous lack of licensed effective therapies to reduce axonal degeneration or promote repair. Remyelination, carried out by oligodendrocytes, does occur in MS, and is protective against axonal degeneration. Unfortunately, remyelination is not very efficient, and ultimately fails and so there is a research focus to generate new therapeutics to enhance remyelination leading to neuroprotection. To develop these therapies, we need preclinical models that well reflect remyelination in MS. We have previously characterized an ex vivo model that uses lysophosphatidylcholine (LPC) to cause acute and global demyelination of tissue slices, followed by spontaneous remyelination, which has been widely used as a surrogate for in vivo rodent models of demyelination. However, this ex vivo model lacks the focal demyelinated lesions seen in MS, surrounded by normal tissue from which the repairing oligodendrocytes are derived. Therefore, to improve the model, we have developed and characterized small macroporous cryogel scaffolds for controlled/regional delivery of LPC with diameters of either 0.5, 1 or 2 mm. Placement of LPC loaded scaffolds adjacent to ex vivo cultured mouse brain and spinal cord slices induced focal areas of demyelination in proximity to the scaffold. To the best of our knowledge, this is the first such report of spatial mimicry of the in vivo condition in ex vivo tissue culture. This will allow not only the investigation into focal lesions, but also provides a better platform technology with which to test remyelination-promoting therapeutics. STATEMENT OF SIGNIFICANCE: This manuscript is the first report of using macroporous hydrogels (cryogels) as a research tool for lysophosphatidylcholine (LPC) delivery, in order to create an ex vivo model of focal demyelination in the brain and spinal cord, which is of great relevance to multiple sclerosis research. Here, we transform an existing ex vivo model of demyelination by delivering LPC to focal regions of brain and spinal cord slice cultures. We have developed an easy-to-handle cylindrical and macroporous PEG-based sponge-like scaffold material (cryogel) that can deliver LPC only to a small area of the slice. Such cryogels are ideal as a delivery system in this culture model as they exhibit a soft but robust nature, with high mechanical deformability in their dry and swollen state, with no need to stay permanently hydrated. In addition, the synthesis of these cryogels is simple and easy to reproduce via photochemical cryopolymerisation using a PEG-diacrylate monomer and a photoinitiator, which are both commercially available. This more accurate model of demyelination will not only allow researchers to gain a better understanding of the CNS remyelination process in diseases such as MS, but also provides a platform technology, which could be utilized to screen and test pro-remyelination compounds which may help to find new therapeutics for progressive MS.

Inhibitory milieu at the multiple sclerosis lesion site and the challenges for remyelination

Regeneration of myelin, following injury, can occur within the central nervous system to reinstate proper axonal conductance and provide trophic support. Failure to do so renders the axons vulnerable, leading to eventual degeneration, and neuronal loss. Thus, it is essential to understand the mechanisms by which remyelination or failure to remyelinate occur, particularly in the context of demyelinating and neurodegenerative disorders. In multiple sclerosis, oligodendrocyte progenitor cells (OPCs) migrate to lesion sites to repair myelin. However, during disease progression, the ability of OPCs to participate in remyelination diminishes coincident with worsening of the symptoms. Remyelination is affected by a broad range of cues from intrinsic programming of OPCs and extrinsic local factors to the immune system and other systemic elements including diet and exercise. Here we review the literature on these diverse inhibitory factors and the challenges they pose to remyelination. Results spanning several disciplines from fundamental preclinical studies to knowledge gained in the clinic will be discussed.

D-Aspartate treatment attenuates myelin damage and stimulates myelin repair

Glutamate signaling may orchestrate oligodendrocyte precursor cell (OPC) development and myelin regeneration through the activation of glutamate receptors at OPC‐neuron synapses. D‐Aspartate is a D‐amino acid exerting modulatory actions at glutamatergic synapses. Chronic administration of D‐Aspartate has been proposed as therapeutic treatment in diseases related to myelin dysfunction and NMDA receptors hypofunction, including schizophrenia and cognitive deficits. Here, we show, by using an in vivo remyelination model, that administration of D‐Aspartate during remyelination improved motor coordination, accelerated myelin recovery, and significantly increased the number of small‐diameter myelinated axons. Chronically administered during demyelination, D‐Aspartate also attenuated myelin loss and inflammation. Interestingly, D‐Aspartate exposure stimulated OPC maturation and accelerated developmental myelination in organotypic cerebellar slices. D‐Aspartate promoting effects on OPC maturation involved the activation of glutamate transporters, AMPA and NMDA receptors, and the Na⁺/Ca²⁺ exchanger NCX3. While blocking NMDA or NCX3 significantly prevented D‐Aspartate‐induced [Ca²⁺]_i oscillations, blocking AMPA and glutamate transporters prevented both the initial and oscillatory [Ca²⁺]_i response as well as D‐Aspartate‐induced inward currents in OPC. Our findings reveal that D‐Aspartate treatment may represent a novel strategy for promoting myelin recovery.

Aging restricts the ability of mesenchymal stem cells to promote the generation of oligodendrocytes during remyelination

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS) that leads to severe neurological deficits. Due to their immunomodulatory and neuroprotective activities and their ability to promote the generation of oligodendrocytes, mesenchymal stem cells (MSCs) are currently being developed for autologous cell therapy in MS. As aging reduces the regenerative capacity of all tissues, it is of relevance to investigate whether MSCs retain their pro-oligodendrogenic activity with increasing age. We demonstrate that MSCs derived from aged rats have a reduced capacity to induce oligodendrocyte differentiation of adult CNS stem/progenitor cells. Aging also abolished the ability of MSCs to enhance the generation of myelin-like sheaths in demyelinated cerebellar slice cultures. Finally, in a rat model for CNS demyelination, aging suppressed the capability of systemically transplanted MSCs to boost oligodendrocyte progenitor cell (OPC) differentiation during remyelination. Thus, aging restricts the ability of MSCs to support the generation of oligodendrocytes and consequently inhibits their capacity to enhance the generation of myelin-like sheaths. These findings may impact on the design of therapies using autologous MSCs in older MS patients.

The Anti-Inflammatory Effect of Sulforaphane in Mice with Experimental Autoimmune Encephalomyelitis

BACKGROUND

Multiple sclerosis (MS) is an immune-associated inflammatory disorder of the central nervous system and results in serious disability. Although many disease-modifying therapy drugs have been developed, these drugs have shown limited clinical efficacy and some adverse effects in previous studies, therefore, there has been reasonable need for less harmful and cost-effective therapeutics. Herein, we tested the anti-inflammatory effect of sulforaphane (SFN) in a mouse model of experimental autoimmune encephalomyelitis (EAE).

METHODS

The EAE mice were randomly assigned into two experimental groups: the phosphate-buffered saline (PBS)-treated EAE group and SFN-treated EAE group. After EAE mice induction by auto-immunization against the myelin oligodendrocyte glycoprotein peptide, we evaluated EAE symptom scores and biochemical analyses such as infiltration of inflammatory cells and demyelination of the spinal cord. Furthermore, western blotting was performed using the spinal cords of EAE mice.

RESULTS

In the behavioral study, the SFN-treated EAE mice showed favorable clinical scores compared with PBS-treated EAE mice at the 13th day (1.30 ± 0.15 vs. 1.90 ± 0.18; P = 0.043) and 14th day (1.80 ± 0.13 vs. 2.75 ± 0.17; P = 0.003). Additionally, the biochemical studies revealed that SFN treatment inhibited the inflammatory infiltration, demyelinating injury of the spinal cords, and the up-regulation of inducible nitric oxide synthase in the EAE mice.

CONCLUSION

The SFN treatment showed anti-inflammatory and anti-oxidative effects in the EAE mice. Conclusively, this study suggests that SFN has neuroprotective effects via anti-inflammatory processing, so it could be a new therapeutic or nutritional supplement for MS.

Sex Differences in the Effect of Alcohol Drinking on Myelinated Axons in the Anterior Cingulate Cortex of Adolescent Rats

Cognitive deficits associated with teenage drinking may be due to disrupted myelination of prefrontal circuits. To better understand how alcohol affects myelination, male and female Wistar rats (n = 7-9/sex/treatment) underwent two weeks of intermittent operant self-administration of sweetened alcohol or sweetened water early in adolescence (postnatal days 28-42) and we tested for macro- and microstructural changes to myelin. We previously reported data from the males of this study showing that alcohol drinking reduced myelinated fiber density in layers II-V of the anterior cingulate division of the medial prefrontal cortex (Cg1); herein, we show that myelinated fiber density was not significantly altered by alcohol in females. Alcohol drinking patterns were similar in both sexes, but males were in a pre-pubertal state for a larger proportion of the alcohol exposure period, which may have contributed to the differential effects on myelinated fiber density. To gain more insight into how alcohol impacts myelinated axons, brain sections from a subset of these animals (n = 6/sex/treatment) were used for microstructural analyses of the nodes of Ranvier. Confocal analysis of nodal domains, flanked by immunofluorescent-labeled contactin-associated protein (Caspr) clusters, indicated that alcohol drinking reduced nodal length-to-width ratios in layers II/III of the Cg1 in both sexes. Despite sex differences in the underlying cause (larger diameter axons after alcohol in males vs. shorter nodal lengths after alcohol in females), reduced nodal ratios could have important implications for the speed and integrity of neural transmission along these axons in both males and females. Alcohol-induced changes to myelinated axonal populations in the Cg1 may contribute to long-lasting changes in prefrontal function associated with early onset drinking.

Enhanced remyelination during late pregnancy: involvement of the GABAergic system

Pregnant women with MS experience fewer relapses, especially during the third trimester. In this study, we explore the cellular and molecular events that bring about the protective effect of late pregnancy on the course of de/remyelination in rats. Using cellular, molecular, and ultrastructural methods, we explored remyelination in response to a focal demyelination in the corpus callosum of late pregnant, virgin, and postpartum rats. We further explored the role of GABAA receptor (GABAAR) in the promyelinating effect observed during late pregnancy. Remyelination in response to a gliotoxin-induced demyelination in the corpus callosum was enhanced in late pregnant rats when compared to that seen in virgin and postpartum rats. This pregnancy-associated promyelinating effect was lost when either the GABAAR was blocked or when 5α-reductase, the rate limiting enzyme for the endogenous GABAAR activator allopregnanolone, was inhibited. Taken together, these data suggest that the pregnancy-associated pro-myelination operates, at least in part, through a GABAergic activated system.

Uncovering the biology of myelin with optical imaging of the live brain

Myelin has traditionally been considered a static structure that is produced and assembled during early developmental stages. While this characterization is accurate in some contexts, recent studies have revealed that oligodendrocyte generation and patterns of myelination are dynamic and potentially modifiable throughout life. Unique structural and biochemical properties of the myelin sheath provide opportunities for the development and implementation of multimodal label-free and fluorescence optical imaging approaches. When combined with genetically encoded fluorescent tags targeted to distinct cells and subcellular structures, these techniques offer a powerful methodological toolbox for uncovering mechanisms of myelin generation and plasticity in the live brain. Here, we discuss recent advances in these approaches that have allowed the discovery of several forms of myelin plasticity in developing and adult nervous systems. Using these techniques, long-standing questions related to myelin generation, remodeling, and degeneration can now be addressed.

Deep learning for high-throughput quantification of oligodendrocyte ensheathment at single-cell resolution

High-throughput quantification of oligodendrocyte myelination is a challenge that, if addressed, would facilitate the development of therapeutics to promote myelin protection and repair. Here, we established a high-throughput method to assess oligodendrocyte ensheathment in-vitro, combining nanofiber culture devices and automated imaging with a heuristic approach that informed the development of a deep learning analytic algorithm. The heuristic approach was developed by modeling general characteristics of oligodendrocyte ensheathments, while the deep learning neural network employed a UNet architecture and a single-cell training method to associate ensheathed segments with individual oligodendrocytes. Reliable extraction of multiple morphological parameters from individual cells, without heuristic approximations, allowed the UNet to match the accuracy of expert-human measurements. The capacity of this technology to perform multi-parametric analyses at the level of individual cells, while reducing manual labor and eliminating human variability, permits the detection of nuanced cellular differences to accelerate the discovery of new insights into oligodendrocyte physiology.

Combination Therapy With Fingolimod and Neural Stem Cells Promotes Functional Myelination in vivo Through a Non-immunomodulatory Mechanism

Myelination, which occurs predominantly postnatally and continues throughout life, is important for proper neurologic function of the mammalian central nervous system (CNS). We have previously demonstrated that the combination therapy of fingolimod (FTY720) and transplanted neural stem cells (NSCs) had a significantly enhanced therapeutic effect on the chronic stage of experimental autoimmune encephalomyelitis, an animal model of CNS autoimmunity, compared to using either one of them alone. However, reduced disease severity may be secondary to the immunomodulatory effects of FTY720 and NSCs, while whether this therapy directly affects myelinogenesis remains unknown. To investigate this important question, we used three myelination models under minimal or non-inflammatory microenvironments. Our results showed that FTY720 drives NSCs to differentiate into oligodendrocytes and promotes myelination in an ex vivo brain slice culture model, and in the developing CNS of healthy postnatal mice in vivo. Elevated levels of neurotrophic factors, e.g., brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor, were observed in the CNS of the treated infant mice. Further, FTY720 and NSCs efficiently prolonged the survival and improved sensorimotor function of shiverer mice. Together, these data demonstrate a direct effect of FTY720, beyond its known immunomodulatory capacity, in NSC differentiation and myelin development as a novel mechanism underlying its therapeutic effect in demyelinating diseases.

An in vitro model for studying CNS white matter: functional properties and experimental approaches

The normal development and maintenance of CNS white matter, and its responses to disease and injury, are defined by synergies between axons, oligodendrocytes, astrocytes and microglia, and further influenced by peripheral components such as the gut microbiome and the endocrine and immune systems. Consequently, mechanistic insights, therapeutic approaches and safety tests rely ultimately on in vivo models and clinical trials. However, in vitro models that replicate the cellular complexity of the CNS can inform these approaches, reducing costs and minimising the use of human material or experimental animals; in line with the principles of the 3Rs. Using electrophysiology, pharmacology, time-lapse imaging, and immunological assays, we demonstrate that murine spinal cord-derived myelinating cell cultures recapitulate spinal-like electrical activity and innate CNS immune functions, including responses to disease-relevant myelin debris and pathogen associated molecular patterns (PAMPs).  Further, we show they are (i) amenable to siRNA making them suitable for testing gene-silencing strategies; (ii) can be established on microelectrode arrays (MEAs) for electrophysiological studies; and (iii) are compatible with multi-well microplate formats for semi-high throughput screens, maximising information output whilst further reducing animal use. We provide protocols for each of these. Together, these advances increase the utility of this in vitro tool for studying normal and pathological development and function of white matter, and for screening therapeutic molecules or gene targets for diseases such as multiple sclerosis, motor neuron disease or spinal cord injury, whilst avoiding in vivo approaches on experimental animals.

Toxin-Based Models to Investigate Demyelination and Remyelination

Clinical myelin diseases, and our best experimental approximations, are complex entities in which demyelination and remyelination proceed unpredictably and concurrently. These features can make it difficult to identify mechanistic details. Toxin-based models offer lesions with predictable spatiotemporal patterns and relatively discrete phases of damage and repair: a simpler system to study the relevant biology and how this can be manipulated. Here, we discuss the most widely used toxin-based models, with a focus on lysolecithin, ethidium bromide, and cuprizone. This includes an overview of their respective mechanisms, strengths, and limitations and step-by-step protocols for their use.

Ex Vivo Slice Cultures to Study Myelination, Demyelination, and Remyelination in Mouse Brain and Spinal Cord

In vitro culture systems have been invaluable in understanding the cell biology of oligodendrocytes; the monoculture of primary oligodendroglia has helped characterize different stages of oligodendrocyte maturation in the absence of neurons. However, oligodendrocyte monocultures do not model the interaction of oligodendrocytes with neurons where they form myelin wraps. To circumvent this problem, coculture systems were developed; oligodendrocytes and neurons are cultured together, facilitating the study of myelin wraps and the interaction between the two cell types. However, this coculture system also has limitations, as other cells are not present and it does not represent the three-dimensional multicellular structure seen in vivo. Some of these limitations are resolved by using ex vivo slice cultures to serve as a three-dimensional culture system that is more similar to in vivo and can be used to study myelination, demyelination, and remyelination, over extended periods of time. Slice cultures are economical compared to in vivo studies and live imaging using them is less challenging. The focus of this chapter is to describe how to culture brain and spinal cord slices of mice and use them to study myelination, demyelination, and remyelination.

Modulation of proteoglycan receptor PTPσ enhances MMP-2 activity to promote recovery from multiple sclerosis

Multiple Sclerosis (MS) is characterized by focal CNS inflammation leading to the death of oligodendrocytes (OLs) with subsequent demyelination, neuronal degeneration, and severe functional deficits. Inhibitory chondroitin sulfate proteoglycans (CSPGs) are increased in the extracellular matrix in the vicinity of MS lesions and are thought to play a critical role in myelin regeneration failure. We here show that CSPGs curtail remyelination through binding with their cognate receptor, protein tyrosine phosphatase σ (PTPσ) on oligodendrocyte progenitor cells (OPCs). We report that inhibition of CSPG/PTPσ signaling by systemically deliverable Intracellular Sigma Peptide (ISP), promotes OPC migration, maturation, remyelination, and functional recovery in animal models of MS. Furthermore, we report a downstream molecular target of PTPσ modulation in OPCs involving upregulation of the protease MMP-2 that allows OPCs to enzymatically digest their way through CSPGs. In total, we demonstrate a critical role of PTPσ/CSPG interactions in OPC remyelination in MS.

Demyelination failure in multiple sclerosis (MS) may contribute to the disease progression. This study shows that chondroitin sulfate proteoglycans (CSPGs) can inhibit remyelination in an animal model of MS via CSPG binding with the receptor PTPσ on oligodendrocyte progenitor cells, and disruption of this interaction can promote recovery in the animal models of MS.

Distinct patterns of glia repair and remyelination in antibody-mediated demyelination models of multiple sclerosis and neuromyelitis optica

Multiple sclerosis (MS) and neuromyelitis optica (NMO) are inflammatory demyelinating disorders of the central nervous system with evidence of antibody-mediated pathology. Using ex vivo organotypic mouse cerebellar slice cultures, we have demonstrated that recombinant antibodies (rAbs) cloned from cerebrospinal fluid plasmablasts of MS and NMO patients target myelin- and astrocyte-specific antigens to induce disease-specific oligodendrocyte loss and myelin degradation. In this study, we examined glial cell responses and myelin integrity during recovery from disease-specific antibody-mediated injury. Following exposure to MS rAb and human complement (HC) in cerebellar explants, myelinating oligodendrocytes repopulated the demyelinated tissue and formed new myelin sheaths along axons. Remyelination was accompanied by pronounced microglial activation. In contrast, following treatment with NMO rAb and HC, there was rapid regeneration of astrocytes and pre-myelinating oligodendrocytes but little formation of myelin sheaths on preserved axons. Deficient remyelination was associated with progressive axonal loss and the return of microglia to a resting state. Our results indicate that antibody-mediated demyelination in MS and NMO show distinct capacities for recovery associated with differential injury to adjacent axons and variable activation of microglia. Remyelination was rapid in MS rAb plus HC-induced demyelination. By contrast, oligodendrocyte maturation and remyelination failed following NMO rAb-mediated injury despite the rapid restoration of astrocytes and preservation of axons in early lesions.

Activin receptors regulate the oligodendrocyte lineage in health and disease

The most prevalent neurological disorders of myelin include perinatal brain injury leading to cerebral palsy in infants and multiple sclerosis in adults. Although these disorders have distinct etiologies, they share a common neuropathological feature of failed progenitor differentiation into myelin-producing oligodendrocytes and lack of myelin, for which there is an unmet clinical need. Here, we reveal that a molecular pathology common to both disorders is dysregulation of activin receptors and that activin receptor signaling is required for the majority of myelin generation in development and following injury. Using a constitutive conditional knockout of all activin receptor signaling in oligodendrocyte lineage cells, we discovered this signaling to be required for myelination via regulation of oligodendrocyte differentiation and myelin compaction. These processes were found to be dependent on the activin receptor subtype Acvr2a, which is expressed during oligodendrocyte differentiation and axonal ensheathment in development and following myelin injury. During efficient myelin regeneration, Acvr2a upregulation was seen to coincide with downregulation of Acvr2b, a receptor subtype with relatively higher ligand affinity; Acvr2b was shown to be dispensable for activin receptor-driven oligodendrocyte differentiation and its overexpression was sufficient to impair the abovementioned ligand-driven responses. In actively myelinating or remyelinating areas of human perinatal brain injury and multiple sclerosis tissue, respectively, oligodendrocyte lineage cells expressing Acvr2a outnumbered those expressing Acvr2b, whereas in non-repairing lesions Acvr2b+ cells were increased. Thus, we propose that following human white matter injury, this increase in Acvr2b expression would sequester ligand and consequently impair Acvr2a-driven oligodendrocyte differentiation and myelin formation. Our results demonstrate dysregulated activin receptor signaling in common myelin disorders and reveal Acvr2a as a novel therapeutic target for myelin generation following injury across the lifespan.

Pericytes Stimulate Oligodendrocyte Progenitor Cell Differentiation during CNS Remyelination

The role of the neurovascular niche in CNS myelin regeneration is incompletely understood. Here, we show that, upon demyelination, CNS-resident pericytes (PCs) proliferate, and parenchymal non-vessel-associated PC-like cells (PLCs) rapidly develop. During remyelination, mature oligodendrocytes were found in close proximity to PCs. In Pdgfb^ret/ret mice, which have reduced PC numbers, oligodendrocyte progenitor cell (OPC) differentiation was delayed, although remyelination proceeded to completion. PC-conditioned medium accelerated and enhanced OPC differentiation in vitro and increased the rate of remyelination in an ex vivo cerebellar slice model of demyelination. We identified Lama2 as a PC-derived factor that promotes OPC differentiation. Thus, the functional role of PCs is not restricted to vascular homeostasis but includes the modulation of adult CNS progenitor cells involved in regeneration.

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CNS-resident PCs react to demyelination and are found close to differentiating OPCs

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PC-deficient mice show delayed OPC differentiation during CNS remyelination

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PC-conditioned medium accelerates OPC differentiation and enhances remyelination

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PC-derived LAMA2 induces OPC differentiation

Characterization of a murine mixed neuron-glia model and cellular responses to regulatory T cell-derived factors

One of the unmet clinical needs in demyelinating diseases such as Multiple Sclerosis (MS) is to provide therapies that actively enhance the process of myelin regeneration (remyelination) in the central nervous system (CNS). Oligodendrocytes, the myelinating cells of the CNS, play a central role in remyelination and originate from oligodendrocyte progenitor cells (OPCs). We recently showed that depletion of regulatory T cells (Treg) impairs remyelination in vivo, and that Treg-secreted factors directly enhance oligodendrocyte differentiation. Here we aim to further characterize the dynamics of Treg-enhanced oligodendrocyte differentiation as well as elucidate the cellular components of a murine mixed neuron-glia model.

Murine mixed neuron-glia cultures were generated from P2–7 C57BL/6 mice and characterized for percentage of neuronal and glial cell populations prior to treatment at 7 days in vitro (div) as well as after treatment with Treg-conditioned media at multiple timepoints up to 12 div.

Mixed neuron-glia cultures consisted of approximately 30% oligodendroglial lineage cells, 20% neurons and 10% microglia. Furthermore, a full layer of astrocytes, that could not be quantified, was present. Treatment with Treg-conditioned media enhanced the proportion of MBP⁺ oligodendrocytes and decreased the proportion of PDGFRα⁺ OPCs, but did not affect OPC proliferation or survival. Treg-enhanced oligodendrocyte differentiation was not caused by Treg polarizing factors, was dependent on the number of activation cycles Treg underwent and was robustly achieved by using 5% conditioned media.

These studies provide in-depth characterization of a murine mixed neuron-glia model as well as further insights into the dynamics of Treg-enhanced oligodendrocyte differentiation.

Correlated Heterospectral Lipidomics for Biomolecular Profiling of Remyelination in Multiple Sclerosis

Analyzing lipid composition and distribution within the brain is important to study white matter pathologies that present focal demyelination lesions, such as multiple sclerosis. Some lesions can endogenously re-form myelin sheaths. Therapies aim to enhance this repair process in order to reduce neurodegeneration and disability progression in patients. In this context, a lipidomic analysis providing both precise molecular classification and well-defined localization is crucial to detect changes in myelin lipid content. Here we develop a correlated heterospectral lipidomic (HSL) approach based on coregistered Raman spectroscopy, desorption electrospray ionization mass spectrometry (DESI-MS), and immunofluorescence imaging. We employ HSL to study the structural and compositional lipid profile of demyelination and remyelination in an induced focal demyelination mouse model and in multiple sclerosis lesions from patients ex vivo. Pixelwise coregistration of Raman spectroscopy and DESI-MS imaging generated a heterospectral map used to interrelate biomolecular structure and composition of myelin. Multivariate regression analysis enabled Raman-based assessment of highly specific lipid subtypes in complex tissue for the first time. This method revealed the temporal dynamics of remyelination and provided the first indication that newly formed myelin has a different lipid composition compared to normal myelin. HSL enables detailed molecular myelin characterization that can substantially improve upon the current understanding of remyelination in multiple sclerosis and provides a strategy to assess remyelination treatments in animal models.