Murine spinal cord explants: A model for evaluating axonal growth and myelination in vitro

Impact of microchannel width on axons for brain-on-chip applications

Technologies for axon guidance for in vitro disease models and bottom up investigations are increasingly being used in neuroscience research. One of the most prevalent patterning methods is using polydimethylsiloxane (PDMS) microstructures due to compatibility with microscopy and electrophysiology which enables systematic tracking of axon development with precision and efficiency. Previous investigations of these guidance platforms have noted axons tend to follow edges and avoid sharp turns; however, the specific impact of spatial constraints remains only partially explored. We investigated the influence of microchannel width beyond a constriction point, as well as the number of available microchannels, on axon growth dynamics. Further, by manipulating the size of micron/submicron-sized PDMS tunnels we investigated the space restriction that prevents growth cone penetration showing that restrictions smaller than 350 nm were sufficient to exclude axons. This research offers insights into the interplay of spatial constraints, axon development, and neural behavior. The findings are important for designing in vitro platforms and in vivo neural interfaces for both fundamental neuroscience and translational applications in rapidly evolving neural implant technologies.

SARM1 Depletion Slows Axon Degeneration in a CNS Model of Neurotropic Viral Infection

Zika virus (ZIKV) is a neurotropic flavivirus recently linked to congenital ZIKV syndrome in children and encephalitis and Guillain-Barré syndrome in adults. Neurotropic viruses often use axons to traffic to neuronal or glial cell somas where they either remain latent or replicate and proceed to infect new cells. Consequently, it has been suggested that axon degeneration could represent an evolutionarily conserved mechanism to limit viral spread. Whilst it is not known if ZIKV transits in axons, we previously reported that ZIKV infection of glial cells in a murine spinal cord-derived cell culture model of the CNS is associated with a profound loss of neuronal cell processes. This, despite that postmitotic neurons are relatively refractory to infection and death. Here, we tested the hypothesis that ZIKV-associated degeneration of neuronal processes is dependent on activation of Sterile alpha and armadillo motif-containing protein 1 (SARM1), an NADase that acts as a central executioner in a conserved axon degeneration pathway. To test this, we infected wild type and Sarm1 homozygous or heterozygous null cell cultures with ZIKV and examined NAD⁺ levels as well as the survival of neurons and their processes. Unexpectedly, ZIKV infection led to a rapid SARM1-independent reduction in NAD⁺. Nonetheless, the subsequent profound loss of neuronal cell processes was SARM1-dependent and was preceded by early changes in the appearance of β-tubulin III staining. Together, these data identify a role for SARM1 in the pathogenesis of ZIKV infection, which may reflect SARM1's conserved prodegenerative function, independent of its NADase activity.

Río-Hortega's drawings revisited with fluorescent protein defines a cytoplasm-filled channel system of CNS myelin

A century ago this year, Pío del Río-Hortega (1921) coined the term 'oligodendroglia' for the 'interfascicular glia' with very few processes, launching an extensive discovery effort on his new cell type. One hundred years later, we review his original contributions to our understanding of the system of cytoplasmic channels within myelin in the context of what we observe today using light and electron microscopy of genetically encoded fluorescent reporters and immunostaining. We use the term myelinic channel system to describe the cytoplasm-delimited spaces associated with myelin; being the paranodal loops, inner and outer tongues, cytoplasm-filled spaces through compact myelin and further complex motifs associated to the sheath. Using a central nervous system myelinating cell culture model that contains all major neural cell types and produces compact myelin, we find that td-tomato fluorescent protein delineates the myelinic channel system in a manner reminiscent of the drawings of adult white matter by Río-Hortega, despite that he questioned whether some cytoplasmic figures he observed represented artefact. Together, these data lead us to propose a slightly revised model of the 'unrolled' sheath. Further, we show that the myelinic channel system, while relatively stable, can undergo subtle dynamic shape changes over days. Importantly, we capture an under-appreciated complexity of the myelinic channel system in mature myelin sheaths.

Novel in vitro Experimental Approaches to Study Myelination and Remyelination in the Central Nervous System

Myelin is the lipidic insulating structure enwrapping axons and allowing fast saltatory nerve conduction. In the central nervous system, myelin sheath is the result of the complex packaging of multilamellar extensions of oligodendrocyte (OL) membranes. Before reaching myelinating capabilities, OLs undergo a very precise program of differentiation and maturation that starts from OL precursor cells (OPCs). In the last 20 years, the biology of OPCs and their behavior under pathological conditions have been studied through several experimental models. When co-cultured with neurons, OPCs undergo terminal maturation and produce myelin tracts around axons, allowing to investigate myelination in response to exogenous stimuli in a very simple in vitro system. On the other hand, in vivo models more closely reproducing some of the features of human pathophysiology enabled to assess the consequences of demyelination and the molecular mechanisms of remyelination, and they are often used to validate the effect of pharmacological agents. However, they are very complex, and not suitable for large scale drug discovery screening. Recent advances in cell reprogramming, biophysics and bioengineering have allowed impressive improvements in the methodological approaches to study brain physiology and myelination. Rat and mouse OPCs can be replaced by human OPCs obtained by induced pluripotent stem cells (iPSCs) derived from healthy or diseased individuals, thus offering unprecedented possibilities for personalized disease modeling and treatment. OPCs and neural cells can be also artificially assembled, using 3D-printed culture chambers and biomaterial scaffolds, which allow modeling cell-to-cell interactions in a highly controlled manner. Interestingly, scaffold stiffness can be adopted to reproduce the mechanosensory properties assumed by tissues in physiological or pathological conditions. Moreover, the recent development of iPSC-derived 3D brain cultures, called organoids, has made it possible to study key aspects of embryonic brain development, such as neuronal differentiation, maturation and network formation in temporal dynamics that are inaccessible to traditional in vitro cultures. Despite the huge potential of organoids, their application to myelination studies is still in its infancy. In this review, we shall summarize the novel most relevant experimental approaches and their implications for the identification of remyelinating agents for human diseases such as multiple sclerosis.

Modified recombinant human IgG1-Fc is superior to natural intravenous immunoglobulin at inhibiting immune-mediated demyelination

Intravenous immunoglobulin (IVIG) is an established treatment for numerous autoimmune conditions. Although Fc fragments derived from IVIG have shown efficacy in controlling immune thrombocytopenia in children, the mechanisms of action are unclear and controversial. The aim of this study was to dissect IVIG effector mechanisms using further adapted Fc fragments on demyelination in an ex vivo model of the central nervous system-immune interface. Using organotypic cerebellar slice cultures (OSCs) from transgenic mice, we induced extensive immune-mediated demyelination and oligodendrocyte loss with an antibody specific for myelin oligodendrocyte glycoprotein (MOG) and complement. Protective effects of adapted Fc fragments were assessed by live imaging of green fluorescent protein expression, immunohistochemistry and confocal microscopy. Cysteine- and glycan-adapted Fc fragments protected OSC from demyelination in a dose-dependent manner where equimolar concentrations of either IVIG or control Fc were ineffective. The protective effects of the adapted Fc fragments are partly attributed to interference with complement-mediated oligodendroglia damage. Transcriptome analysis ruled out signatures associated with inflammatory or innate immune responses. Taken together, our findings show that recombinant biomimetics can be made that are at least two hundred-fold more effective than IVIG in controlling demyelination by anti-MOG antibodies.

Zika Virus Infection Leads to Demyelination and Axonal Injury in Mature CNS Cultures

Understanding how Zika virus (Flaviviridae; ZIKV) affects neural cells is paramount in comprehending pathologies associated with infection. Whilst the effects of ZIKV in neural development are well documented, impact on the adult nervous system remains obscure. Here, we investigated the effects of ZIKV infection in established mature myelinated central nervous system (CNS) cultures. Infection incurred damage to myelinated fibers, with ZIKV-positive cells appearing when myelin damage was first detected as well as axonal pathology, suggesting the latter was a consequence of oligodendroglia infection. Transcriptome analysis revealed host factors that were upregulated during ZIKV infection. One such factor, CCL5, was validated in vitro as inhibiting myelination. Transferred UV-inactivated media from infected cultures did not damage myelin and axons, suggesting that viral replication is necessary to induce the observed effects. These data show that ZIKV infection affects CNS cells even after myelination-which is critical for saltatory conduction and neuronal function-has taken place. Understanding the targets of this virus across developmental stages including the mature CNS, and the subsequent effects of infection of cell types, is necessary to understand effective time frames for therapeutic intervention.

Cerebrospinal fluid replacement solutions promote neuroglia migratory behaviors and spinal explant outgrowth in microfluidic culture

Disorders of the nervous system (NS) impact millions of adults, worldwide, as a consequence of traumatic injury, genetic illness, or chronic health conditions. Contemporary studies have begun to incorporate neuroglia into emerging NS therapies to harness the regenerative potential of glial-mediated synapses in the brain and spinal cord. However, the role of cerebrospinal fluid (CSF) that surrounds neuroglia and interfaces with their associated synapses remains only partially explored. The flow of CSF within subarachnoid spaces (SAS) circulates essential polypeptides, metabolites, and growth factors that directly impact neural response and recovery via signaling with healthy glia. Despite the availability of artificial CSF solutions used in neurosurgery and NS treatments, tissue engineering projects continue to use cell culture media, such as Neurobasal (NB) and Dulbecco's Modified Eagle Medium (DMEM), for development and characterization of many transplantable cells, matrixes, and integrated cellular systems. The current study examined in vitro behaviors of glial Schwann cells (ShC) and spinal cord explants (SCE) within a CSF replacement solution, Elliott's B Solution (EBS), used widely in the treatment of NS disorders. Our tests used EBS to create defined chemical microenvironments of extracellular factors within a glial line (gLL) microfluidic device, previously described by our group. The gLL is comparable in scale to the in vivo SAS that envelopes endogenous CSF and enables molecular transport via mechanisms of convective diffusion. Our results illustrate that EBS solutions facilitate ShC survival, morphology, and proliferation similar to those measured in traditional DMEM, and additionally support glial chemotactic behaviors in response to brain-derived growth factor (BDNF). Our data indicates that ShC undergo significant chemotaxis toward high and low concentration gradients of BDNF with statistical differences between gradients formed within diluents of EBS and DMEM solutions. Moreover, SCE cultured with EBS solutions facilitated measurement of neurite explant extension commensurate with reported in vivo measurements. This data highlights the translational significance and advantages of incorporating CSF replacement fluids to interrogate cellular behaviors and advance regenerative NS therapies.

Microbiome-derived carnitine mimics as previously unknown mediators of gut-brain axis communication

Alterations to the gut microbiome are associated with various neurological diseases, yet evidence of causality and identity of microbiome-derived compounds that mediate gut-brain axis interaction remain elusive. Here, we identify two previously unknown bacterial metabolites 3-methyl-4-(trimethylammonio)butanoate and 4-(trimethylammonio)pentanoate, structural analogs of carnitine that are present in both gut and brain of specific pathogen-free mice but absent in germ-free mice. We demonstrate that these compounds are produced by anaerobic commensal bacteria from the family Lachnospiraceae (Clostridiales) family, colocalize with carnitine in brain white matter, and inhibit carnitine-mediated fatty acid oxidation in a murine cell culture model of central nervous system white matter. This is the first description of direct molecular inter-kingdom exchange between gut prokaryotes and mammalian brain cells, leading to inhibition of brain cell function.

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.

Fluorescent Reporter Mice for Nerve Guidance Conduit Assessment: A High-Throughput in vivo Model

OBJECTIVE

Use of cell culture and conventional in vivo mammalian models to assess nerve regeneration across guidance conduits is resource-intensive. Herein we describe a high-throughput platform utilizing transgenic mice for stain-free axon visualization paired with rapid cryosection techniques for low-cost screening of novel bioengineered nerve guidance conduit performance.

METHODS

Interposition repair of sciatic nerve transection in mice expressing yellow fluorescent protein in peripheral neurons (Thy1.2 YFP-16) was performed with various bioengineered neural conduit compositions using a rapid sutureless entubulation technique under isoflurane anesthesia. Axonal ingrowth was assessed at 3 and 6 weeks using epifluorescent microscopy following cryosectioning.

RESULTS

Mean procedure time (incision-to-closure) was less than 2½ minutes. Direct operational costs of a 3-week experiment was calculated at $21.47 per animal. Tissue processing steps were minimized to aldehyde fixation, cryoprotection and sectioning, and rapid fluorescent dye staining for conduit visualization. Fluorescent microscopy readily resolved robust axonal sprouting at 3 weeks, with clear elucidation of ingrowth-permissive, semipermissive, or restrictive nerve guidance conduit environments.

CONCLUSION

A rapid and cost-efficient in vivo platform for screening of nerve guidance conduit performance has been described.

LEVEL OF EVIDENCE

NA. Laryngoscope, E392-E392, 2018.

Axonal abnormalities in vanishing white matter

Objective

We aimed to study the occurrence and development of axonal pathology and the influence of astrocytes in vanishing white matter.

Methods

Axons and myelin were analyzed using electron microscopy and immunohistochemistry on Eif2b4 and Eif2b5 single‐ and double‐mutant mice and patient brain tissue. In addition, astrocyte‐forebrain co‐culture studies were performed.

Results

In the corpus callosum of Eif2b5‐mutant mice, myelin sheath thickness, axonal diameter, and G‐ratio developed normally up to 4 months. At 7 months, however, axons had become thinner, while in control mice axonal diameters had increased further. Myelin sheath thickness remained close to normal, resulting in an abnormally low G‐ratio in Eif2b5‐mutant mice. In more severely affected Eif2b4‐Eif2b5 double‐mutants, similar abnormalities were already present at 4 months, while in milder affected Eif2b4 mutants, few abnormalities were observed at 7 months. Additionally, from 2 months onward an increased percentage of thin, unmyelinated axons and increased axonal density were present in Eif2b5‐mutant mice. Co‐cultures showed that Eif2b5 mutant astrocytes induced increased axonal density, also in control forebrain tissue, and that control astrocytes induced normal axonal density, also in mutant forebrain tissue. In vanishing white matter patient brains, axons and myelin sheaths were thinner than normal in moderately and severely affected white matter. In mutant mice and patients, signs of axonal transport defects and cytoskeletal abnormalities were minimal.

Interpretation

In vanishing white matter, axons are initially normal and atrophy later. Astrocytes are central in this process. If therapy becomes available, axonal pathology may be prevented with early intervention.

BrainPhys® increases neurofilament levels in CNS cultures, and facilitates investigation of axonal damage after a mechanical stretch-injury in vitro

Neurobasal®/B27 is a gold standard culture media used to study primary neurons in vitro. An alternative media (BrainPhys®/SM1) was recently developed which robustly enhances neuronal activity vs. Neurobasal® or DMEM. To the best of our knowledge BrainPhys® has not been explored in the setting of neuronal injury. Here we characterized the utility of BrainPhys® in a model of in vitro mechanical-stretch injury.

METHODS/RESULTS

Primary rat cortical neurons were maintained in classic Neurobasal®, or sequentially maintained in Neurocult® followed by BrainPhys® (hereafter simply referred to as "BrainPhys® maintained neurons"). The levels of axonal markers and proteins involved in neurotransmission were compared on day in vitro 10 (DIV10). BrainPhys® maintained neurons had higher levels of GluN2B, GluR1, Neurofilament light/heavy chain (NF-L & NF-H), and protein phosphatase 2 A (PP2A) vs. neurons in Neurobasal®. Mechanical stretch-injury (50ms/54% biaxial stretch) to BrainPhys® maintained neurons modestly (albeit significantly) increased 24h lactate dehydrogenase (LDH) levels but markedly decreased axonal NF-L levels post-injury vs. uninjured controls or neurons given a milder 38% stretch-injury. Furthermore, two 54% stretch-injuries (in tandem) exacerbated 24h LDH release, increased α-spectrin breakdown products (SBDPs), and decreased Tau levels. Also, BrainPhys® maintained cultures had decreased markers of cell damage 24h after a single 54% stretch-injury vs. neurons in Neurobasal®. Finally, we tested the hypothesis that lentivirus mediated overexpression of the pro-death protein RBM5 exacerbates neuronal and/or axonal injury in primary CNS cultures. RBM5 overexpression vs. empty-vector controls increased 24h LDH release, and SBDP levels, after a single 54% stretch-injury but did not affect NF-L levels or Tau.

CONCLUSION

BrainPhys® is a promising new reagent which facilities the investigation of molecular targets involved in axonal and/or neuronal injury in vitro.

Neurofilament light as an immune target for pathogenic antibodies

Antibodies to neuronal antigens are associated with many neurological diseases including paraneoplastic neurological disorders, epilepsy, amyotrophic lateral sclerosis and multiple sclerosis. Immunization with neuronal antigens such as neurofilament light (NF-L), a neuronal intermediate filament in axons, has been shown to induce neurological disease and spasticity in mice. Also, although antibodies to NF-L are widely used as surrogate biomarkers of axonal injury in amyotrophic lateral sclerosis and multiple sclerosis, it remains to be elucidated if antibodies to NF-L contribute to neurodegeneration and neurological disease. To address this, we examined the pathogenic role of antibodies directed to NF-L in vitro using spinal cord co-cultures and in vivo in experimental autoimmune encephalomyelitis (EAE) and optic neuritis animal models of multiple sclerosis. Here we show that peripheral injections of antibodies to NF-L augmented clinical signs of neurological disease in acute EAE, increased retinal ganglion cell loss in experimental optic neuritis and induced neurological signs following intracerebral injection into control mice. The pathogenicity of antibodies to NF-L was also observed in spinal cord co-cultures where axonal loss was induced. Taken together, our results reveal that as well as acting as reliable biomarkers of neuronal damage, antibodies to NF-L exacerbate neurological disease, suggesting that antibodies to NF-L generated during disease may also be pathogenic and play a role in the progression of neurodegeneration.

Zika virus tropism and interactions in myelinating neural cell cultures: CNS cells and myelin are preferentially affected

The recent global outbreak of Zika virus (ZIKV) infection has been linked to severe neurological disorders affecting the peripheral and central nervous systems (PNS and CNS, respectively). The pathobiology underlying these diverse clinical phenotypes are the subject of intense research; however, even the principal neural cell types vulnerable to productive Zika infection remain poorly characterised. Here we used CNS and PNS myelinating cultures from wild type and Ifnar1 knockout mice to examine neuronal and glial tropism and short-term consequences of direct infection with a Brazilian variant of ZIKV. Cell cultures were infected pre- or post-myelination for various intervals, then stained with cell-type and ZIKV-specific antibodies. In bypassing systemic immunity using ex vivo culture, and the type I interferon response in Ifnar1 deficient cells, we were able to evaluate the intrinsic infectivity of neural cells. Through systematic quantification of ZIKV infected cells in myelinating cultures, we found that ZIKV infection is enhanced in the absence of the type I interferon responses and that CNS cells are considerably more susceptible to infection than PNS cells. In particular, we demonstrate that CNS axons and myelinating oligodendrocytes are especially vulnerable to injury. These results have implications for understanding the pathobiology of neurological symptoms associated with ZIKV infection. Furthermore, we provide a quantifiable ex vivo infection model that can be used for fundamental and therapeutic studies on viral neuroinvasion and its consequences.

Transplanted miR-219-overexpressing oligodendrocyte precursor cells promoted remyelination and improved functional recovery in a chronic demyelinated model

Oligodendrocyte precursor cells (OPCs) have the ability to repair demyelinated lesions by maturing into myelin-producing oligodendrocytes. Recent evidence suggests that miR-219 helps regulate the differentiation of OPCs into oligodendrocytes. We performed oligodendrocyte differentiation studies using miR-219-overexpressing mouse embryonic stem cells (miR219-mESCs). The self-renewal and multiple differentiation properties of miR219-mESCs were analyzed by the expression of the stage-specific cell markers Nanog, Oct4, nestin, musashi1, GFAP, Tuj1 and O4. MiR-219 accelerated the differentiation of mESC-derived neural precursor cells (NPCs) into OPCs. We further transplanted OPCs derived from miR219-mESCs (miR219-OPCs) into cuprizone-induced chronically demyelinated mice to observe remyelination, which resulted in well-contained oligodendrocyte grafts that migrated along the corpus callosum and matured to express myelin basic protein (MBP). Ultrastructural studies further confirmed the presence of new myelin sheaths. Improved cognitive function in these mice was confirmed by behavioral tests. Importantly, the transplanted miR219-OPCs induced the proliferation of endogenous NPCs. In conclusion, these data demonstrate that miR-219 rapidly transforms mESCs into oligodendrocyte lineage cells and that the transplantation of miR219-OPCs not only promotes remyelination and improves cognitive function but also enhances the proliferation of host endogenous NPCs following chronic demyelination. These results support the potential of a therapeutic role for miR-219 in demyelinating diseases.

In vitro models of axon regeneration

A variety of in vitro models have been developed to understand the mechanisms underlying the regenerative failure of central nervous system (CNS) axons, and to guide pre-clinical development of regeneration-promoting therapeutics. These range from single-cell based assays that typically focus on molecular mechanisms to organotypic assays that aim to recapitulate in vivo behavior. By utilizing a combination of models, researchers can balance the speed, convenience, and mechanistic resolution of simpler models with the biological relevance of more complex models. This review will discuss a number of models that have been used to build our understanding of the molecular mechanisms of CNS axon regeneration.

Transcript profiling of different types of multiple sclerosis lesions yields FGF1 as a promoter of remyelination

Chronic demyelination is a pathological hallmark of multiple sclerosis (MS). Only a minority of MS lesions remyelinates completely. Enhancing remyelination is, therefore, a major aim of future MS therapies. Here we took a novel approach to identify factors that may inhibit or support endogenous remyelination in MS. We dissected remyelinated, demyelinated active, and demyelinated inactive white matter MS lesions, and compared transcript levels of myelination and inflammation-related genes using quantitative PCR on customized TaqMan Low Density Arrays. In remyelinated lesions, fibroblast growth factor (FGF) 1 was the most abundant of all analyzed myelination-regulating factors, showed a trend towards higher expression as compared to demyelinated lesions and was significantly higher than in control white matter. Two MS tissue blocks comprised lesions with adjacent de- and remyelinated areas and FGF1 expression was higher in the remyelinated rim compared to the demyelinated lesion core. In functional experiments, FGF1 accelerated developmental myelination in dissociated mixed cultures and promoted remyelination in slice cultures, whereas it decelerated differentiation of purified primary oligodendrocytes, suggesting that promotion of remyelination by FGF1 is based on an indirect mechanism. The analysis of human astrocyte responses to FGF1 by genome wide expression profiling showed that FGF1 induced the expression of the chemokine CXCL8 and leukemia inhibitory factor, two factors implicated in recruitment of oligodendrocytes and promotion of remyelination. Together, this study presents a transcript profiling of remyelinated MS lesions and identified FGF1 as a promoter of remyelination. Modulation of FGF family members might improve myelin repair in MS.

Myelinating cocultures of purified oligodendrocyte lineage cells and retinal ganglion cells

In this article, we introduce methods for generating rapidly myelinating cocultures with reaggregates of purified retinal ganglion cells and optic nerve oligodendrocyte precursor cells. This coculture system facilitates the study of complex central nervous system neuronal-glial interactions and myelination. It enables control of the extracellular environment and allows the use of transfected, virally infected, mutant, or knockout neurons and/or glial cell types. It is therefore possible to assess the role of various signaling pathways and genes in myelination and node of Ranvier formation.

A phenotypic culture system for the molecular analysis of CNS myelination in the spinal cord

Studies of central nervous system myelination lack defined in vitro models which would effectively dissect molecular mechanisms of myelination that contain cells of the correct phenotype. Here we describe a co-culture of purified motoneurons and oligodendrocyte progenitor cells, isolated from rat embryonic spinal cord using a combination of immunopanning techniques. This model illustrates differentiation of oligodendrocyte progenitors into fully functional mature oligodendrocytes that myelinate axons. It also illustrates a contribution of axons to the rate of oligodendrocyte maturation and myelin gene expression. The defined conditions used allow molecular analysis of distinct stages of myelination and precise manipulation of inductive cues affecting axonal-oligodendrocyte interactions. This phenotypic in vitro myelination model can provide valuable insight into our understanding of demyelinating disorders, such as multiple sclerosis and traumatic diseases such as spinal cord injury where demyelination represents a contributing factor to the pathology of the disorder.

Astroglial-axonal interactions during early stages of myelination in mixed cultures using in vitro and ex vivo imaging techniques

BACKGROUND

[corrected] Myelination is a very complex process that requires the cross talk between various neural cell types. Previously, using cytosolic or membrane associated GFP tagged neurospheres, we followed the interaction of oligodendrocytes with axons using time-lapse imaging in vitro and ex vivo and demonstrated dynamic changes in cell morphology. In this study we focus on GFP tagged astrocytes differentiated from neurospheres and their interactions with axons.

RESULTS

We show the close interaction of astrocyte processes with axons and with oligodendrocytes in mixed mouse spinal cord cultures with formation of membrane blebs as previously seen for oligodendrocytes in the same cultures. When GFP-tagged neurospheres were transplanted into the spinal cord of the dysmyelinated shiverer mouse, confirmation of dynamic changes in cell morphology was provided and a prevalence for astrocyte differentiation compared with oligodendroglial differentiation around the injection site. Furthermore, we were able to image GFP tagged neural cells in vivo after transplantation and the cells exhibited similar membrane changes as cells visualised in vitro and ex vivo.

CONCLUSION

These data show that astrocytes exhibit dynamic cell process movement and changes in their membrane topography as they interact with axons and oligodendrocytes during the process of myelination, with the first demonstration of bleb formation in astrocytes.

Temporal oligodendrocyte lineage progression: in vitro models of proliferation, differentiation and myelination

Oligodendrocytes are neuroglial cells responsible, within the central nervous system, for myelin sheath formation that provides an electric insulation of axons and accelerate the transmission of electrical signals. In order to be able to produce myelin, oligodendrocytes progress through a series of differentiation steps from oligodendrocyte precursor cells to mature oligodendrocytes (migration, increase in morphologic complexity and expression pattern of specific markers), which are modulated by cross talk with other nerve cells. If during the developmental stage any of these mechanisms is affected by toxic or external stimuli it may result into impaired myelination leading to neurological deficits. Such being the case, several approaches have been developed to evaluate how oligodendrocyte development and myelination may be impaired. The present review aims to summarize changes that oligodendrocytes suffer from precursor cells to mature ones, and to describe and discuss the different in vitro models used to evaluate not only oligodendrocyte development (proliferation, migration, differentiation and ability to myelinate), but also their interaction with neurons and other glial cells. First we discuss the temporal oligodendrocyte lineage progression, highlighting the differences between human and rodent, usually used as tissue supply for in vitro cultures. Second we describe how to perform and characterize the different in vitro cultures, as well as the methodologies to evaluate oligodendrocyte functionality in each culture system, discussing their advantages and disadvantages. Finally, we briefly discuss the current status of in vivo models for oligodendrocyte development and myelination.

The role of hair follicle nestin-expressing stem cells during whisker sensory-nerve growth in long-term 3D culture

We have previously reported that nestin-expressing hair follicle stem cells can differentiate into neurons, Schwann cells, and other cell types. In the present study, vibrissa hair follicles, including their sensory nerve stump, were excised from transgenic mice in which the nestin promoter drives green fluorescent protein (ND-GFP mice), and were placed in 3D histoculture supported by Gelfoam®. β-III tubulin-positive fibers, consisting of ND-GFP-expressing cells, extended up to 500 µm from the whisker nerve stump in histoculture. The growing fibers had growth cones on their tips expressing F-actin. These findings indicate that β-III tubulin-positive fibers elongating from the whisker follicle sensory nerve stump were growing axons. The growing whisker sensory nerve was highly enriched in ND-GFP cells which appeared to play a major role in its elongation and interaction with other nerves in 3D culture, including the sciatic nerve, the trigeminal nerve, and the trigeminal nerve ganglion. The results of the present report suggest a major function of the nestin-expressing stem cells in the hair follicle is for growth of the follicle sensory nerve.

Nestin-Expressing Stem Cells Promote Nerve Growth in Long-Term 3-Dimensional Gelfoam®-Supported Histoculture

We have previously reported that hair follicles contain multipotent stem cells which express nestin. The nestin-expressing cells form the hair follicle sensory nerve. In vitro, the nestin-expressing hair follicle cells can differentiate into neurons, Schwann cells, and other cell types. In the present study, the sciatic nerve was excised from transgenic mice in which the nestin promoter drives green fluorescent protein (ND-GFP mice). The ND-GFP cells of the sciatic nerve were also found to be multipotent as the ND-GFP cells in the hair follicle. When the ND-GFP cells in the mouse sciatic nerve cultured on Gelfoam® and were imaged by confocal microscopy, they were observed forming fibers extending the nerve. The fibers consisted of ND-GFP-expressing spindle cells, which co-expressed the neuron marker β-III tubulin, the immature Schwann-cell marker p75^NTR and TrkB which is associated with neurons. The fibers also contain nestin-negative spherical cells expressing GFAP, a Schwann-cell marker. The β-III tubulin-positive fibers had growth cones on their tips expressing F-actin, indicating they are growing axons. When the sciatic nerve from mice ubiquitously expressing red fluorescent protein (RFP) was co-cultured on Gelfoam® with the sciatic nerve from ND-GFP transgenic mice, the interaction of nerves was observed. Proliferating nestin-expressing cells in the injured sciatic nerve were also observed in vivo. Nestin-expressing cells were also observed in posterior nerves but not in the spinal cord itself, when placed in 3-D Gelfoam® culture. The results of the present report suggest a critical function of nestin-expressing cells in peripheral nerve growth and regeneration.

Time-lapse imaging of the dynamics of CNS glial-axonal interactions in vitro and ex vivo

Background

Myelination is an exquisite and dynamic example of heterologous cell-cell interaction, which consists of the concentric wrapping of multiple layers of oligodendrocyte membrane around neuronal axons. Understanding the mechanism by which oligodendrocytes ensheath axons may bring us closer to designing strategies to promote remyelination in demyelinating diseases. The main aim of this study was to follow glial-axonal interactions over time both in vitro and ex vivo to visualize the various stages of myelination.

Methodology/Principal Findings

We took two approaches to follow myelination over time: i) time-lapse imaging of mixed CNS myelinating cultures generated from mouse spinal cord to which exogenous GFP-labelled murine cells were added, and ii) ex vivo imaging of the spinal cord of shiverer (Mbp mutant) mice, transplanted with GFP-labelled murine neurospheres. We demonstrate that oligodendrocyte-axonal interactions are dynamic events with continuous retraction and extension of oligodendroglial processes. Using cytoplasmic and membrane-GFP labelled cells to examine different components of the myelin-like sheath, we provide evidence from time-lapse fluorescence microscopy and confocal microscopy that the oligodendrocytes' cytoplasm-filled processes initially spiral around the axon in a corkscrew-like manner. This is followed subsequently by focal expansion of the corkscrew process to form short cuffs, which then extend longitudinally along the axons. We predict from this model that these spiral cuffs must extend over each other first before extending to form internodes of myelin.

Conclusion

These experiments show the feasibility of visualizing the dynamics of glial-axonal interaction during myelination over time. Moreover, these approaches complement each other with the in vitro approach allowing visualization of an entire internodal length of myelin and the ex vivo approach validating the in vitro data.

Direct live monitoring of heterotypic axon-axon interactions in vitro

This protocol describes an optimized method for direct in vitro monitoring of homo- and heterotypic axon-axon interactions involved in the developmental assembly of neural circuits. The assay exploits a classical example of heterotypic axonal interactions by modeling the sequential extension of spinal motor and somatosensory neuron axons, but the procedure should be readily adaptable to other neuron types. The protocol is based on the rapid isolation and primary culture of genetically identified motor neurons combined with straightforward vital dye labeling and culture of dorsal root ganglion sensory neurons. Subsequently, axonal interactions are directly monitored via live fluorescence microscopy, whereas axon type identities can be unambiguously delineated throughout the experiments. Through chemical compound application or by using neurons derived from genetically engineered mice, the protocol facilitates the dissection of molecular pathways driving the axonal interactions that are crucial for neural pathway and circuit assembly. The whole procedure can be completed in 3 d.

Functional duality of astrocytes in myelination

Astrocytes undergo major phenotypic changes in response to injury and disease that directly influence repair in the CNS, but the mechanisms involved are poorly understood. Previously, we have shown that neurosphere-derived rat astrocytes plated on poly-L-lysine (PLL-astrocytes) support myelination in dissociated rat spinal cord cultures (myelinating cultures). It is hypothesized that astrocyte reactivity can affect myelination, so we have exploited this culture system to ascertain how two distinct astrocyte phenotypes influence myelination. Astrocytes plated on tenascin C (TnC-astrocytes), a method to induce quiescence, resulted in less myelinated fibers in the myelinating cultures when compared with PLL-astrocytes. In contrast, treatment of myelinating cultures plated on PLL-astrocytes with ciliary neurotrophic factor (CNTF), a cytokine known to induce an activated astrocyte phenotype, promoted myelination. CNTF could also reverse the effect of quiescent astrocytes on myelination. A combination of microarray gene expression analysis and quantitative real-time PCR identified CXCL10 as a potential candidate for the reduction in myelination in cultures on TnC-astrocytes. The effect of TnC-astrocytes on myelination was eliminated by neutralizing CXCL10 antibodies. Conversely, CXCL10 protein inhibited myelination on PLL-astrocytes. Furthermore, CXCL10 treatment of purified oligodendrocyte precursor cells did not affect proliferation, differentiation, or process extension compared with untreated controls, suggesting a role in glial/axonal ensheathment. These data demonstrate a direct correlation of astrocyte phenotypes with their ability to support myelination. This observation has important implications with respect to the development of therapeutic strategies to promote CNS remyelination in demyelinating diseases.

In vitro modeling of central nervous system myelination and remyelination

This review aims to summarize the current techniques to study myelination and remyelination in culture systems. We attempt to put these into historical context, and to identify the strengths and weaknesses of each approach, which vary depending on the experimental question to be tested. We discuss the difficulty and importance of quantification of myelination and in particular remyelination. Finally, we provide our predictions of how these techniques will and should develop in the future.

Restoring the balance between disease and repair in multiple sclerosis: insights from mouse models

Multiple sclerosis (MS) is considered an autoimmune-mediated demyelinating disease that targets the central nervous system (CNS). Despite considerable research efforts over multiple decades, our understanding of the basic biological processes that are targeted in the disease and the mechanisms of pathogenesis are poorly understood. Consequently, current therapies directed at controlling the progression of the disease are limited in their effectiveness. Historically, the primary focus of MS research has been to define the cellular and molecular basis of the immunological pathogenic mechanisms. Recently, however, it has become clear that long-term functional recovery in MS will require the development of strategies that facilitate myelin repair in lesion areas. The emerging evidence that the adult vertebrate CNS retains the capacity to regenerate neural cells that have been lost to disease or damage has provoked intensive research focused on defining the mechanisms of myelin repair. Unfortunately, the existing animal models of MS are poorly equipped to assess myelin repair, and new validated strategies to identify therapeutics targeted at promoting myelin repair are badly needed. This Commentary will review established murine models of MS, and discuss emerging technologies that promise to provide insights into the mechanisms of myelin repair.

Oligodendrocyte-spinal cord explant co-culture: an in vitro model for the study of myelination

The in vitro models developed to investigate the growth and myelination of axons, such as dorsal root ganglion (DRG)-Schwann cell co-culture, DRG-oligodendrocyte co-culture and central nervous system (CNS) neuron-oligodendrocyte co-culture, have provided an effective way to reveal the mechanisms that underlie the interaction between neurons and myelin-forming cells. In order to better understand the complex process of myelination during CNS development and spinal cord repair, we established a rat spinal cord neuron-oligodendrocyte co-culture model. In this co-culture system, the spinal cord explants were used as the source of neurons, and the oligodendrocytes were induced from GFP-oligodendrocyte precursor cells (GFP-OPCs). The results showed that the GFP-oligodendrocytes that differentiated from GFP-OPCs in co-culture attached to the neurites growing out from the spinal cord explants and formed myelin structures. As the oligodendrocytes expressed GFP, and the neuron somas remained in the explants, the interaction between oligodendrocytes and neurites in co-culture were observed clearly and dynamically without immunostaining.

Distinct stages of myelination regulated by gamma-secretase and astrocytes in a rapidly myelinating CNS coculture system

Mechanistic studies of CNS myelination have been hindered by the lack of a rapidly myelinating culture system. Here, we describe a versatile CNS coculture method that allows time-lapse microscopy and molecular analysis of distinct stages of myelination. Employing a culture architecture of reaggregated neurons fosters extension of dense beds of axons from purified retinal ganglion cells. Seeding of oligodendrocyte precursor cells on these axons results in differentiation and ensheathment in as few as 3 days, with generation of compact myelin within 6 days. This technique enabled (1) the demonstration that oligodendrocytes initiate new myelin segments only during a brief window early in their differentiation, (2) identification of a contribution of astrocytes to the rate of myelin wrapping, and (3) molecular dissection of the role of oligodendrocyte gamma-secretase activity in controlling the ensheathment of axons. These insights illustrate the value of this defined system for investigating multiple aspects of CNS myelination.

Identification of Tmem10/Opalin as an oligodendrocyte enriched gene using expression profiling combined with genetic cell ablation

Oligodendrocytes form an insulating multilamellar structure of compact myelin around axons, which allows efficient and rapid propagation of action potentials. However, little is known about the molecular mechanisms operating at the onset of myelination and during maintenance of the myelin sheath in the adult. Here we use a genetic cell ablation approach combined with Affymetrix GeneChip microarrays to identify a number of oligodendrocyte-enriched genes that may play a key role in myelination. One of the "oligogenes" we cloned using this approach is Tmem10/Opalin, which encodes for a novel transmembrane glycoprotein. In situ hybridization and RT-PCR analysis revealed that Tmem10 is selectively expressed by oligodendrocytes and that its expression is induced during their differentiation. Developmental immunofluorescence analysis demonstrated that Tmem10 starts to be expressed in the white matter tracks of the cerebellum and the corpus callosum at the onset of myelination after the appearance of other myelin genes such as MBP. In contrast to the spinal cord and brain, Tmem10 was not detected in myelinating Schwann cells, indicating that it is a CNS-specific myelin protein. In mature oligodendrocytes, Tmem10 was present at the cell soma and processes, as well as along myelinated internodes, where it was occasionally concentrated at the paranodes. In myelinating spinal cord cultures, Tmem10 was detected in MBP-positive cellular processes that were aligned with underlying axons before myelination commenced. These results suggest a possible role of Tmem10 in oligodendrocyte differentiation and CNS myelination.

Distribution of mitochondria along small-diameter myelinated central nervous system axons

Small-diameter myelinated CNS axons are preferentially affected in multiple sclerosis (MS) and in the hereditary spastic paraplegias (HSP), in which the distal axon degenerates. Mitochondrial dysfunction has been implicated in the pathogenesis of these and other disorders involving axonal degeneration. The aim of this study was to determine whether the frequency of axonal mitochondria changes along the length of small-diameter fibers and whether there is a preferential localization to the region of the node of Ranvier. We find that mitochondrial numbers do not change along the length of a myelinated small-diameter fiber, and, in contrast to the peripheral nervous system, there is no tendency for mitochondrial numbers to increase at the node.

Myelinated, synapsing cultures of murine spinal cord--validation as an in vitro model of the central nervous system

Research in central nervous system (CNS) biology and pathology requires in vitro models, which, to recapitulate the CNS in vivo, must have extensive myelin and synapse formation under serum-free (defined) conditions. However, finding such a model has proven difficult. The technique described here produces dense cultures of myelinated axons, with abundant synapses and nodes of Ranvier, that are suitable for both morphological and biochemical analysis. Cellular and molecular events were easily visualised using conventional microscopy. Ultrastructurally, myelin sheaths were of the appropriate thickness relative to axonal diameter (G-ratio). Production of myelinated axons in these cultures was consistent and repeatable, as shown by statistical analysis of multiple experimental repeats. Myelinated axons were so abundant that from one litter of embryonic mice, myelin was produced in amounts sufficient for bulk biochemical analysis. This culture method was assessed for its ability to generate an in vitro model of the CNS that could be used for both neurobiological and neuropathological research. Myelin protein kinetics were investigated using a myelin fraction isolated from the cultures. This fraction was found to be superior, quantitatively and qualitatively, to the fraction recovered from standard cultures of dissociated oligodendrocytes, or from brain slices. The model was also used to investigate the roles of specific molecules in the pathogenesis of inflammatory CNS diseases. Using the defined conditions offered by this culture system, dose-specific, inhibitory effects of inflammatory cytokines on myelin formation were demonstrated, unequivocally. The method is technically quick, easy and reliable, and should have wide application to CNS research.

Long-term neurite orientation on astrocyte monolayers aligned by microtopography

After spinal cord injury neuronal connections are not easily re-established. Success has been hampered by the lack of orientation of neurites inside scar tissue and a lack of neurites crossing out of the site of injury. Oriented scaffolds in biodegradable polymers could be an excellent way to support both the orientation of neurites within the injury site as well as aiding their crossing out of the lesion. To establish the validity of using grooved micro-topography in polycaprolactone in combination with glia we have studied the long-term (3 weeks) orientation of neuronal cells on monolayers of astrocytes on the top of grooved topographies of various dimensions. We find that neurites are significantly aligned by groove/ridge type topographies which are "buried" under a monolayer of astrocytes for up to 3 weeks. This alignment is significantly lower than that of neurites growing directly on the topography, but these neurons do not survive on the poly-l-lysine coated polymer for more than a week. The alignment of neurites on the astrocyte layer to the underlying topography decreases over time, and with groove width. Topographies with 12.5 or 25 microm lateral dimension appear optimal for the long-term alignment and can support myelination. We have shown for the first time that micro-topography can act through an overlaid astrocyte layer and results in aligned neurites in long-term culture and that these can be myelinated by endogenous oligodendrocytes.