Oligodendrocytes and the control of myelination in vivo: new insights from the rat anterior medullary velum

Anat-SFSeg: Anatomically-guided superficial fiber segmentation with point-cloud deep learning

Diffusion magnetic resonance imaging (dMRI) tractography is a critical technique to map the brain's structural connectivity. Accurate segmentation of white matter, particularly the superficial white matter (SWM), is essential for neuroscience and clinical research. However, it is challenging to segment SWM due to the short adjacent gyri connection in a U-shaped pattern. In this work, we propose an Anatomically-guided Superficial Fiber Segmentation (Anat-SFSeg) framework to improve the performance on SWM segmentation. The framework consists of a unique fiber anatomical descriptor (named FiberAnatMap) and a deep learning network based on point-cloud data. The spatial coordinates of fibers represented as point clouds, as well as the anatomical features at both the individual and group levels, are fed into a neural network. The network is trained on Human Connectome Project (HCP) datasets and tested on the subjects with a range of cognitive impairment levels. One new metric named fiber anatomical region proportion (FARP), quantifies the ratio of fibers in the defined brain regions and enables the comparison with other methods. Another metric named anatomical region fiber count (ARFC), represents the average fiber number in each cluster for the assessment of inter-subject differences. The experimental results demonstrate that Anat-SFSeg achieves the highest accuracy on HCP datasets and exhibits great generalization on clinical datasets. Diffusion tensor metrics and ARFC show disorder severity associated alterations in patients with Alzheimer's disease (AD) and mild cognitive impairments (MCI). Correlations with cognitive grades show that these metrics are potential neuroimaging biomarkers for AD. Furthermore, Anat-SFSeg could be utilized to explore other neurodegenerative, neurodevelopmental or psychiatric disorders.

U-fiber diffusion kurtosis and susceptibility characteristics in relapsing-remitting multiple sclerosis may be related to cognitive deficits and neurodegeneration

OBJECTIVES

To evaluate the diffusion kurtosis and susceptibility change in the U-fiber region of patients with relapsing-remitting multiple sclerosis (pwRRMS) and their correlations with cognitive status and degeneration.

MATERIALS AND METHODS

Mean kurtosis (MK), axial kurtosis (AK), radial kurtosis (RK), kurtosis fractional anisotropy (KFA), and the mean relative quantitative susceptibility mapping (mrQSM) values in the U-fiber region were compared between 49 pwRRMS and 48 healthy controls (HCs). The U-fiber were divided into upper and deeper groups based on the location. The whole brain volume, gray and white matter volume, and cortical thickness were obtained. The correlations between the mrQSM values, DKI-derived metrics in the U-fiber region and clinical scale scores, brain morphologic parameters were further investigated.

RESULTS

The decreased MK, AK, RK, KFA, and increased mrQSM values in U-fiber lesions (p < 0.001, FDR corrected), decreased RK, KFA, and increased mrQSM values in U-fiber non-lesions (p = 0.034, p < 0.001, p < 0.001, FDR corrected) were found in pwRRMS. There were differences in DKI-derived metrics and susceptibility values between the upper U-fiber region and the deeper one for U-fiber non-lesion areas of pwRRMS and HCs (p < 0.05), but not for U-fiber lesions in DKI-derived metrics. The DKI-derived metrics and susceptibility values were widely related with cognitive tests and brain atrophy.

CONCLUSION

RRMS patients show abnormal diffusion kurtosis and susceptibility characteristics in the U-fiber region, and these underlying tissue abnormalities are correlated with cognitive deficits and degeneration.

CLINICAL RELEVANCE STATEMENT

The macroscopic and microscopic tissue damages of U-fiber help to identify cognitive impairment and brain atrophy in multiple sclerosis and provide underlying pathophysiological mechanism.

KEY POINTS

• Diffusion kurtosis and susceptibility changes are present in the U-fiber region of multiple sclerosis. • There are gradients in diffusion kurtosis and susceptibility characteristics in the U-fiber region. • Tissue damages in the U-fiber region are correlated with cognitive impairment and brain atrophy.

Superficial white matter integrity in neuromyelitis optica spectrum disorder and multiple sclerosis

Background

Superficial white matter (SWM) is a particularly vulnerable area of white matter adjacent to cerebral cortex that was shown to be a sensitive marker of disease severity in several neurological and psychiatric disorders, including multiple sclerosis (MS), but has not been studied in neuromyelitis optica spectrum disorder (NMOSD).

Objective

To compare the integrity of SWM between MS patients, NMOSD patients and healthy controls, and explore the correlation of SWM integrity with cognitive performance and overall disability.

Methods

Forty NMOSD patients, 48 MS patients and 52 healthy controls were included in the study. Mean diffusivity (MD) values obtained by diffusion tensor imaging were used as a measure of SWM integrity. Cognitive performance and overall disability were assessed with standardized tests.

Results

Superficial white matter MD was increased in MS patients compared to healthy controls. Higher MD was associated with poorer spatial memory (most prominently in right temporal and right limbic lobe) and poorer information processing speed in MS patients. After adjusting for age, no significant differences of SWM MD were observed between NMOSD patients and healthy controls.

Conclusion

Integrity of SWM is compromised in MS, but not in NMOSD, and can serve as a sensitive marker of disease severity.

Aberrant frontal lobe "U"-shaped association fibers in first-episode schizophrenia: A 7-Tesla Diffusion Imaging Study

Schizophrenia is believed to be a developmental disorder with one hypothesis suggesting that symptoms arise due to abnormal interactions (or disconnectivity) between different brain regions. While some major deep white matter pathways have been extensively studied (e.g. arcuate fasciculus), studies of short-ranged, "U"-shaped tracts have been limited in patients with schizophrenia, in part due to the sheer abundance of tracts present and due to the spatial variations across individuals that defy probabilistic characterization in the absence of reliable templates. In this study, we use diffusion magnetic resonance imaging (dMRI) to investigate frontal lobe superficial white matter that are present in the majority of study participants, comparing healthy controls and minimally treated patients with first-episode schizophrenia (<3 median days of lifetime treatment). Through group comparisons, 3 out of 63 frontal lobe "U"-shaped tracts were found to demonstrate localized aberrations affecting the microstructural tissue properties (via diffusion tensor metrics) in this early stage of disease. No associations were found in patients between aberrant segments of affected tracts and clinical or cognitive variables. Aberrations in the frontal lobe "U"-shaped tracts in early untreated stages of psychosis occur irrespective of symptom burden, and are distributed across critical functional networks associated with executive function and salience processing. While we limited the investigation to the frontal lobe, a framework has been developed to study such connections in other brain regions, enabling further extensive investigations jointly with the major deep white matter pathways.

Depth-wise profiles of iron and myelin in the cortex and white matter using χ-separation: A preliminary study

The in-vivo profiling of iron and myelin across cortical depths and underlying white matter has important implications for advancing knowledge about their roles in brain development and degeneration. Here, we utilize χ-separation, a recently-proposed advanced susceptibility mapping that creates positive (χ_pos) and negative (χ_neg) susceptibility maps, to generate the depth-wise profiles of χ_pos and χ_neg as surrogate biomarkers for iron and myelin, respectively. Two regional sulcal fundi of precentral and middle frontal areas are profiled and compared with findings from previous studies. The results show that the χ_pos profiles peak at superificial white matter (SWM), which is an area beneath cortical gray matter known to have the highest accumulation of iron within the cortex and white matter. On the other hand, the χ_neg profiles increase in SWM toward deeper white matter. These characteristics in the two profiles are in agreement with histological findings of iron and myelin. Furthermore, the χ_neg profiles report regional differences that agree with well-known distributions of myelin concentration. When the two profiles are compared with those of QSM and R₂*, different shapes and peak locations are observed. This preliminary study offers an insight into one of the possible applications of χ-separation for exploring microstructural information of the human brain, as well as clinical applications in monitoring changes of iron and myelin in related diseases.

Diffusion tensor imaging of fetal brain: principles, potential and limitations of promising technique

Human brain development is a complex process that begins in the third week of gestation. During early development, the fetal brain undergoes dynamic morphological changes. These changes result from events such as neurogenesis, neuronal migration, synapse formation, axonal growth and myelination. Disruption of any of these processes is thought to be responsible for a wide array of different pathologies. Recent advances in magnetic resonance imaging, especially diffusion-weighted imaging and diffusion tensor imaging (DTI), have enabled characterization and evaluation of brain development in utero. In this review, aimed at practitioners involved in fetal medicine and high-risk pregnancies, we provide a comprehensive overview of fetal DTI studies focusing on characterization of early normal brain development as well as evaluation of brain pathology in utero. We also discuss the reliability and limitations of fetal brain DTI. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.

Older age, male sex, and cerebral microbleeds predict white matter loss after traumatic brain injury

Little is known on how mild traumatic brain injury affects white matter based on age at injury, sex, cerebral microbleeds, and time since injury. Here, we study the fractional anisotropy of white matter to study these effects in 109 participants aged 18-77 (46 females, age μ ± σ = 40 ± 17 years) imaged within [Formula: see text] 1 week and [Formula: see text] 6 months post-injury. Age is found to be linearly associated with white matter degradation, likely due not only to injury but also to cumulative effects of other pathologies and to their interactions with injury. Age is associated with mean anisotropy decreases in the corpus callosum, middle longitudinal fasciculi, inferior longitudinal and occipitofrontal fasciculi, and superficial frontal and temporal fasciculi. Over [Formula: see text] 6 months, the mean anisotropies of the corpus callosum, left superficial frontal fasciculi, and left corticospinal tract decrease significantly. Independently of other predictors, age and cerebral microbleeds contribute to anisotropy decrease in the callosal genu. Chronically, the white matter of commissural tracts, left superficial frontal fasciculi, and left corticospinal tract degrade appreciably, independently of other predictors. Our findings suggest that large commissural and intra-hemispheric structures are at high risk for post-traumatic degradation. This study identifies detailed neuroanatomic substrates consistent with brain injury patients' age-dependent deficits in information processing speed, interhemispheric communication, motor coordination, visual acuity, sensory integration, reading speed/comprehension, executive function, personality, and memory. We also identify neuroanatomic features underlying white matter degradation whose severity is associated with the male sex. Future studies should compare our findings to functional measures and other neurodegenerative processes.

Evidence That ITPR2-Mediated Intracellular Calcium Release in Oligodendrocytes Regulates the Development of Carbonic Anhydrase II + Type I/II Oligodendrocytes and the Sizes of Myelin Fibers

Myelination of neuronal axons in the central nervous system (CNS) by oligodendrocytes (OLs) enables rapid saltatory conductance and axonal integrity, which are crucial for normal brain functioning. Previous studies suggested that different subtypes of oligodendrocytes in the CNS form different types of myelin determined by the diameter of axons in the unit. However, the molecular mechanisms underlying the developmental association of different types of oligodendrocytes with different fiber sizes remain elusive. In the present study, we present the evidence that the intracellular Ca²⁺ release channel associated receptor (Itpr2) contributes to this developmental process. During early development, Itpr2 is selectively up-regulated in oligodendrocytes coinciding with the initiation of myelination. Functional analyses in both conventional and conditional Itpr2 mutant mice revealed that Itpr2 deficiency causes a developmental delay of OL differentiation, resulting in an increased percentage of CAII⁺ type I/II OLs which prefer to myelinate small-diameter axons in the CNS. The increased percentage of small caliber myelinated axons leads to an abnormal compound action potentials (CAP) in the optic nerves. Together, these findings revealed a previously unrecognized role for Itpr2-mediated calcium signaling in regulating the development of different types of oligodendrocytes.

Chronic Exposure to Hypoxia Inhibits Myelinogenesis and Causes Motor Coordination Deficits in Adult Mice

Exposure to chronic hypoxia is considered to be a risk factor for deficits in brain function in adults, but the underlying mechanisms remain largely unknown. Since active myelinogenesis persists in the adult central nervous system, here we aimed to investigate the impact of chronic hypoxia on myelination and the related functional consequences in adult mice. Using a transgenic approach to label newly-generated myelin sheaths (NG2-CreERTM; Tau-mGFP), we found that myelinogenesis was highly active in most brain regions, such as the motor cortex and corpus callosum. After exposure to hypoxia (10% oxygen) 12 h per day for 4 weeks, myelinogenesis was largely inhibited in the 4-month old brain and the mice displayed motor coordination deficits revealed by the beam-walking test. To determine the relationship between the inhibited myelination and functional impairment, we induced oligodendroglia-specific deletion of the transcription factor Olig2 by tamoxifen (NG2-CreERTM; Tau-mGFP; Olig2 fl/fl) in adult mice to mimic the decreased myelinogenesis caused by hypoxia. The deletion of Olig2 inhibited myelinogenesis and consequently impaired motor coordination, suggesting that myelinogenesis is required for motor function in adult mice. To understand whether enhancing myelination could protect brain functions against hypoxia, we treated hypoxic mice with the myelination-enhancing drug-clemastine, which resulted in enhanced myelogenesis and improved motor coordination. Taken together, our data indicate that chronic hypoxia inhibits myelinogenesis and causes functional deficits in the brain and that enhancing myelinogenesis protects brain functions against hypoxia-related deficits.

Development of type I/II oligodendrocytes regulated by teneurin-4 in the murine spinal cord

In the spinal cord, the axonal tracts with various caliber sizes are myelinated by oligodendrocytes and function as high-velocity ways for motor and sensory nerve signals. In some neurological disorders, such as multiple sclerosis, demyelination of small caliber axons is observed in the spinal cord. While type I/II oligodendrocytes among the four types are known to myelinate small diameter axons, their characteristics including identification of regulating molecules have not been understood yet. Here, we first found that in the wild-type mouse spinal cord, type I/II oligodendrocytes, positive for carbonic anhydrase II (CAII), were located in the corticospinal tract, fasciculus gracilis, and the inside part of ventral funiculus, in which small diameter axons existed. The type I/II oligodendrocytes started to appear between postnatal day (P) 7 and 11. We further analyzed the type I/II oligodendrocytes in the mutant mice, whose small diameter axons were hypomyelinated due to the deficiency of teneurin-4. In the teneurin-4 deficient mice, type I/II oligodendrocytes were significantly reduced, and the onset of the defect was at P11. Our results suggest that CAII-positive type I/II oligodendrocytes myelinate small caliber axons in the spinal cord and teneurin-4 is the responsible molecule for the generation of type I/II oligodendrocytes.

Engineering biomaterial microenvironments to promote myelination in the central nervous system

Promoting remyelination and/or minimizing demyelination are key therapeutic strategies under investigation for diseases and injuries like multiple sclerosis (MS), spinal cord injury, stroke, and virus-induced encephalopathy. Myelination is essential for efficacious neuronal signaling. This myelination process is originated by oligodendrocyte progenitor cells (OPCs) in the central nervous system (CNS). Resident OPCs are capable of both proliferation and differentiation, and also migration to demyelinated injury sites. OPCs can then engage with these unmyelinated or demyelinated axons and differentiate into myelin-forming oligodendrocytes (OLs). However this process is frequently incomplete and often does not occur at all. Biomaterial strategies can now be used to guide OPC and OL development with the goal of regenerating healthy myelin sheaths in formerly damaged CNS tissue. Growth and neurotrophic factors delivered from such materials can promote proliferation of OPCs or differentiation into OLs. While cell transplantation techniques have been used to replace damaged cells in wound sites, they have also resulted in poor transplant cell viability, uncontrollable differentiation, and poor integration into the host. Biomaterial scaffolds made from extracellular matrix (ECM) mimics that are naturally or synthetically derived can improve transplanted cell survival, support both transplanted and endogenous cell populations, and direct their fate. In particular, stiffness and degradability of these scaffolds are two parameters that can influence the fate of OPCs and OLs. The future outlook for biomaterials research includes 3D in vitro models of myelination / remyelination / demyelination to better mimic and study these processes. These models should provide simple relationships of myelination to microenvironmental biophysical and biochemical properties to inform improved therapeutic approaches.

Could the inhibitor of DNA binding 2 and 4 play a role in white matter injury?

White matter injury (WMI) prevents the normal development of myelination, leading to central nervous system myelination disorders and the production of chronic sequelae associated with WMI, such as chronic dyskinesia, cognitive impairment and cerebral palsy. This results in a large emotional and socioeconomic burden. Decreased myelination in preterm infant WMI is associated with the delayed development or destruction of oligodendrocyte (OL) lineage cells, particularly oligodendrocyte precursor cells (OPCs). The development of cells from the OL lineage involves the migration, proliferation and different stages of OL differentiation, finally leading to myelination. A series of complex intrinsic, extrinsic and epigenetic factors regulate the OPC cell cycle withdrawal, OL lineage progression and myelination. We focus on the inhibitor of DNA binding 2 (ID2), because it is widely involved in the different stages of OL differentiation and genesis. ID2 is a key transcription factor for the normal development of OL lineage cells, and the pathogenesis of WMI is closely linked with OL developmental disorders. ID4, another family member of the IDs protein, also plays a similar role in OL differentiation and genesis. ID2 and ID4 belong to the helix-loop-helix family; they lack the DNA-binding sequences and inhibit oligodendrogenesis and OPC differentiation. In this review, we mainly discuss the roles of ID2 in OL development, especially during OPC differentiation, and summarize the ID2-mediated intracellular and extracellular signaling pathways that regulate these processes. We also discuss ID4 in relation to bone morphogenetic protein signaling and oligodendrogenesis. It is likely that these developmental mechanisms are also involved in the myelin repair or remyelination in human neurological diseases.

Superficial white matter damage in anti-NMDA receptor encephalitis

BACKGROUND

Clinical brain MRI is normal in the majority of patients with anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis. However, extensive deep white matter damage wasrecently identifiedin these patients using diffusion weighted imaging. Here, our aim was to study a particularly vulnerable brain compartment, the late myelinating superficial white matter.

METHODS

Forty-six patients with anti-NMDAR encephalitis were included. Ten out of these were considered neurologically recovered (modified Rankin scale of zero), while 36 patients were non-recovered. In addition, 30 healthy controls were studied. MRI data were collected from all subjects and superficial white matter mean diffusivity derived from diffusion tensor imaging was compared between groups in whole brain, lobar and vertex-based analyses. Patients underwent comprehensive cognitive testing, and correlation analyses were performed between cognitive performance and superficial white matter integrity.

RESULTS

Non-recovered patients showed widespread superficial white matter damage in comparison to recovered patients and healthy controls. Vertex-based analyses revealed that damage predominated in frontal and temporal lobes. In contrast, the superficial white matter was intact in recovered patients. Importantly, persistent cognitive impairments in working memory, verbal memory, visuospatial memory and attention significantly correlated with damage of the superficial white matter in patients.

CONCLUSIONS

Anti-NMDAR encephalitis is associated with extensive superficial white matter damage in patients with incomplete recovery. The strong association with impairment in several cognitive domains highlights the clinical relevance of white matter damage in this disorder and warrants investigations of the underlying pathophysiological mechanisms.

Plasticity in deep and superficial white matter: a DTI study in world class gymnasts

Brain white matter (WM) could be generally categorized into two types, deep and superficial WM. Studies combining these two types WM are important for a better understanding of brain plasticity induced by motor training. In this study, we applied both univariate and multivariate approaches to study gymnastic training-induced plasticity in brain WM. Specifically, we acquired diffusion tensor imaging data from 13 world class gymnasts and 14 non-athlete normal controls, reconstructed brain deep and superficial WM tracts, estimated and compared their fractional anisotropy (FA) difference between the two groups. Taking FA values as the features, we applied logistic regression and support vector machine to distinguish the gymnasts from the controls. Compared to the controls, the gymnasts showed lower FA in four regional deep WM tracts, including the occipital lobe portion of left inferior fronto-occipital fasciculus (IFOF.L), occipital and temporal lobe portion of right inferior longitudinal fasciculus (ILF.R), insular cortex portion of right uncinate fasciculus (UF.R), and parietal lobe portion of right arcuate fasciculus (AF.R). Meanwhile, we found lower FA in the superficial U-shaped tracts within the frontal lobe in the gymnasts compared to the controls. In addition, we detected that mean FA in either the AF.R or the U-shaped tracts connecting the left pars triangularis and superior frontal gyrus was negatively correlated with years of training in the gymnasts. Classification analyses indicated FA in deep WM hold higher potential to distinguish the gymnasts from the controls. Overall, our findings provide a more complete picture of training-induced plasticity in brain WM.

A mutation in the Tubb4a gene leads to microtubule accumulation with hypomyelination and demyelination

OBJECTIVE

Our goal was to define the genetic cause of the profound hypomyelination in the taiep rat model and determine its relevance to human white matter disease.

METHODS

Based on previous localization of the taiep mutation to rat chromosome 9, we tested whether the mutation resided within the Tubb4a (β-tubulin 4A) gene, because mutations in the TUBB4A gene have been described in patients with central nervous system hypomyelination. To determine whether accumulation of microtubules led to progressive demyelination, we analyzed the spinal cord and optic nerves of 2-year-old rats by light and electron microscopy. Cerebral white matter from a patient with TUBB4A Asn414Lys mutation and magnetic resonance imaging evidence of severe hypomyelination were studied similarly.

RESULTS

As the taiep rat ages, there is progressive loss of myelin in the brain and dorsal column of the spinal cord associated with increased oligodendrocyte numbers with accumulation of microtubules. This accumulation involved the entire cell body and distal processes of oligodendrocytes, but there was no accumulation of microtubules in axons. A single point mutation in Tubb4a (p.Ala302Thr) was found in homozygous taiep samples. A similar hypomyelination associated with increased oligodendrocyte numbers and arrays of microtubules in oligodendrocytes was demonstrated in the human patient sample.

INTERPRETATION

The taiep rat is the first animal model of TUBB4 mutations in humans and a novel system in which to test the mechanism of microtubule accumulation. The finding of microtubule accumulation in a patient with a TUBB4A mutation and leukodystrophy confirms the usefulness of taiep as a model of the human disease. Ann Neurol 2017;81:690-702.

Major Superficial White Matter Abnormalities in Huntington's Disease

BACKGROUND

The late myelinating superficial white matter at the juncture of the cortical gray and white matter comprising the intracortical myelin and short-range association fibers has not received attention in Huntington's disease. It is an area of the brain that is late myelinating and is sensitive to both normal aging and neurodegenerative disease effects. Therefore, it may be sensitive to Huntington's disease processes.

METHODS

Structural MRI data from 25 Pre-symptomatic subjects, 24 Huntington's disease patients and 49 healthy controls was run through a cortical pattern-matching program. The surface corresponding to the white matter directly below the cortical gray matter was then extracted. Individual subject's Diffusion Tensor Imaging (DTI) data was aligned to their structural MRI data. Diffusivity values along the white matter surface were then sampled at each vertex point. DTI measures with high spatial resolution across the superficial white matter surface were then analyzed with the General Linear Model to test for the effects of disease.

RESULTS

There was an overall increase in the axial and radial diffusivity across much of the superficial white matter (p < 0.001) in Pre-symptomatic subjects compared to controls. In Huntington's disease patients increased diffusivity covered essentially the whole brain (p < 0.001). Changes are correlated with genotype (CAG repeat number) and disease burden (p < 0.001).

CONCLUSIONS

This study showed broad abnormalities in superficial white matter even before symptoms are present in Huntington's disease. Since, the superficial white matter has a unique microstructure and function these abnormalities suggest it plays an important role in the disease.

The superficial white matter in Alzheimer's disease

White matter abnormalities have been shown in the large deep fibers of Alzheimer's disease patients. However, the late myelinating superficial white matter comprised of intracortical myelin and short-range association fibers has not received much attention. To investigate this area, we extracted a surface corresponding to the superficial white matter beneath the cortex and then applied a cortical pattern-matching approach which allowed us to register and subsequently sample diffusivity along thousands of points at the interface between the gray matter and white matter in 44 patients with Alzheimer's disease (Age: 71.02 ± 5.84, 16M/28F) and 47 healthy controls (Age 69.23 ± 4.45, 19M/28F). In patients we found an overall increase in the axial and radial diffusivity across most of the superficial white matter (P < 0.001) with increases in diffusivity of more than 20% in the bilateral parahippocampal regions and the temporal and frontal lobes. Furthermore, diffusivity correlated with the cognitive deficits measured by the Mini-Mental State Examination scores (P < 0.001). The superficial white matter has a unique microstructure and is critical for the integration of multimodal information during brain maturation and aging. Here we show that there are major abnormalities in patients and the deterioration of these fibers relates to clinical symptoms in Alzheimer's disease.

Neuregulin-1 controls an endogenous repair mechanism after spinal cord injury

Spontaneous remyelination after spinal cord injury is mediated largely by Schwann cells of unknown origin. Bartus et al. show that neuregulin-1 promotes differentiation of spinal cord-resident precursor cells into PNS-like Schwann cells, which remyelinate central axons and promote functional recovery. Targeting the neuregulin-1 system could enhance endogenous regenerative processes.

Following traumatic spinal cord injury, acute demyelination of spinal axons is followed by a period of spontaneous remyelination. However, this endogenous repair response is suboptimal and may account for the persistently compromised function of surviving axons. Spontaneous remyelination is largely mediated by Schwann cells, where demyelinated central axons, particularly in the dorsal columns, become associated with peripheral myelin. The molecular control, functional role and origin of these central remyelinating Schwann cells is currently unknown. The growth factor neuregulin-1 (Nrg1, encoded by NRG1) is a key signalling factor controlling myelination in the peripheral nervous system, via signalling through ErbB tyrosine kinase receptors. Here we examined whether Nrg1 is required for Schwann cell-mediated remyelination of central dorsal column axons and whether Nrg1 ablation influences the degree of spontaneous remyelination and functional recovery following spinal cord injury. In contused adult mice with conditional ablation of Nrg1, we found an absence of Schwann cells within the spinal cord and profound demyelination of dorsal column axons. There was no compensatory increase in oligodendrocyte remyelination. Removal of peripheral input to the spinal cord and proliferation studies demonstrated that the majority of remyelinating Schwann cells originated within the injured spinal cord. We also examined the role of specific Nrg1 isoforms, using mutant mice in which only the immunoglobulin-containing isoforms of Nrg1 (types I and II) were conditionally ablated, leaving the type III Nrg1 intact. We found that the immunoglobulin Nrg1 isoforms were dispensable for Schwann cell-mediated remyelination of central axons after spinal cord injury. When functional effects were examined, both global Nrg1 and immunoglobulin-specific Nrg1 mutants demonstrated reduced spontaneous locomotor recovery compared to injured controls, although global Nrg1 mutants were more impaired in tests requiring co-ordination, balance and proprioception. Furthermore, electrophysiological assessments revealed severely impaired axonal conduction in the dorsal columns of global Nrg1 mutants (where Schwann cell-mediated remyelination is prevented), but not immunoglobulin-specific mutants (where Schwann cell-mediated remyelination remains intact), providing robust evidence that the profound demyelinating phenotype observed in the dorsal columns of Nrg1 mutant mice is related to conduction failure. Our data provide novel mechanistic insight into endogenous regenerative processes after spinal cord injury, demonstrating that Nrg1 signalling regulates central axon remyelination and functional repair and drives the trans-differentiation of central precursor cells into peripheral nervous system-like Schwann cells that remyelinate spinal axons after injury. Manipulation of the Nrg1 system could therefore be exploited to enhance spontaneous repair after spinal cord injury and other central nervous system disorders with a demyelinating pathology.

Oligodendrocytes: Myelination and Axonal Support

Myelinated nerve fibers have evolved to enable fast and efficient transduction of electrical signals in the nervous system. To act as an electric insulator, the myelin sheath is formed as a multilamellar membrane structure by the spiral wrapping and subsequent compaction of the oligodendroglial plasma membrane around central nervous system (CNS) axons. Current evidence indicates that the myelin sheath is more than an inert insulating membrane structure. Oligodendrocytes are metabolically active and functionally connected to the subjacent axon via cytoplasmic-rich myelinic channels for movement of macromolecules to and from the internodal periaxonal space under the myelin sheath. This review summarizes our current understanding of how myelin is generated and also the role of oligodendrocytes in supporting the long-term integrity of myelinated axons.

Function of microglia and macrophages in secondary damage after spinal cord injury

Spinal cord injury (SCI) is a devastating type of neurological trauma with limited therapeutic opportunities. The pathophysiology of SCI involves primary and secondary mechanisms of injury. Among all the secondary injury mechanisms, the inflammatory response is the major contributor and results in expansion of the lesion and further loss of neurologic function. Meanwhile, the inflammation directly and indirectly dominates the outcomes of SCI, including not only pain and motor dysfunction, but also preventingneuronal regeneration. Microglia and macrophages play very important roles in secondary injury. Microglia reside in spinal parenchyma and survey the microenvironment through the signals of injury or infection. Macrophages are derived from monocytes recruited to injured sites from the peripheral circulation. Activated resident microglia and monocyte-derived macrophages induce and magnify immune and inflammatory responses not only by means of their secretory moleculesand phagocytosis, but also through their influence on astrocytes, oligodendrocytes and demyelination. In this review, we focus on the roles of microglia and macrophages in secondary injury and how they contribute to the sequelae of SCI.

Glial cell development and function in zebrafish

The zebrafish is a premier vertebrate model system that offers many experimental advantages for in vivo imaging and genetic studies. This review provides an overview of glial cell types in the central and peripheral nervous system of zebrafish. We highlight some recent work that exploited the strengths of the zebrafish system to increase the understanding of the role of Gpr126 in Schwann cell myelination and illuminate the mechanisms controlling oligodendrocyte development and myelination. We also summarize similarities and differences between zebrafish radial glia and mammalian astrocytes and consider the possibility that their distinct characteristics may represent extremes in a continuum of cell identity. Finally, we focus on the emergence of zebrafish as a model for elucidating the development and function of microglia. These recent studies have highlighted the power of the zebrafish system for analyzing important aspects of glial development and function.

Oligodendrocyte Lineage Cells in Chronic Demyelination of Multiple Sclerosis Optic Nerve

Reports that chronically demyelinated multiple sclerosis brain and spinal cord lesions contained immature oligodendrocyte lineage cells have generated major interest aimed at the potential for promotion of endogenous repair. Despite the prominence of the optic nerve as a lesion site and its importance in clinical disease assessment, no detailed studies of multiple sclerosis‐affected optic nerve exist. This study aims to provide insight into the cellular pathology of chronic demyelination in multiple sclerosis through direct morphological and immunohistochemical analysis of optic nerve in conjunction with observations from an experimental cat optic nerve model of successful remyelination. Myelin staining was followed by immunohistochemistry to differentially label neuroglia. Digitally immortalized sections were then analyzed to generate quantification data and antigenic phenotypes including maturational stages within the oligodendrocyte lineage. It was found that some chronically demyelinated multiple sclerosis optic nerve lesions contained oligodendroglial cells and that heterogeneity existed in the presence of myelin sheaths, oligodendrocyte maturational stages and extent of axonal investment. The findings advance our understanding of oligodendrocyte activity in chronically demyelinated human optic nerve and may have implications for studies aimed at enhancement of endogenous repair in multiple sclerosis.

Human intraretinal myelination: axon diameters and axon/myelin thickness ratios

Purpose:

Human intraretinal myelination of ganglion cell axons occurs in about 1% of the population. We examined myelin thickness and axon diameter in human retinal specimens containing myelinated retinal ganglion cell axons.

Materials and Methods:

Two eyes containing myelinated patches were prepared for electron microscopy. Two areas were examined in one retina and five in the second retina. Measurements were compared to normal retinal and optic nerve samples and the rabbit retina, which normally contains myelinated axons. Measurements were made using a graphics tablet.

Results:

Mean axon diameter of myelinated axons at all locations were significantly larger than unmyelinated axons (P ≤ 0.01). Myelinated axons within the patches were significantly larger than axons within the optic nerve (P < 0.01). The relationship between axon diameter/fiber diameter (the G-ratio) seen in the retinal sites differed from that in the nerve. G-ratios were higher and myelin thickness was positively correlated to axon diameter (P < 0.01) in the retina but negatively correlated to axon diameter in the nerve (P < 0.001).

Conclusion:

Intraretinally myelinated axons are larger than non-myelinated axons from the same population and suggests that glial cells can induce diameter changes in retinal axons that are not normally myelinated. This effect is more dramatic on intraretinal axons compared with the normal transition zone as axons enter the optic nerve and these changes are abnormal. Whether intraretinal myelin alters axonal conduction velocity or blocks axonal conduction remains to be clarified and these issues may have different clinical outcomes.

Immunological demyelination triggers macrophage/microglial cells activation without inducing astrogliosis

The glial scar formed by reactive astrocytes and axon growth inhibitors associated with myelin play important roles in the failure of axonal regeneration following central nervous system (CNS) injury. Our laboratory has previously demonstrated that immunological demyelination of the CNS facilitates regeneration of severed axons following spinal cord injury. In the present study, we evaluate whether immunological demyelination is accompanied with astrogliosis. We compared the astrogliosis and macrophage/microglial cell responses 7 days after either immunological demyelination or a stab injury to the dorsal funiculus. Both lesions induced a strong activated macrophage/microglial cells response which was significantly higher within regions of immunological demyelination. However, immunological demyelination regions were not accompanied by astrogliosis compared to stab injury that induced astrogliosis which extended several millimeters above and below the lesions, evidenced by astroglial hypertrophy, formation of a glial scar, and upregulation of intermediate filaments glial fibrillary acidic protein (GFAP). Moreover, a stab or a hemisection lesion directly within immunological demyelination regions did not induced astrogliosis within the immunological demyelination region. These results suggest that immunological demyelination creates a unique environment in which astrocytes do not form a glial scar and provides a unique model to understand the putative interaction between astrocytes and activated macrophage/microglial cells.

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

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

Propofol-induced apoptosis of neurones and oligodendrocytes in fetal and neonatal rhesus macaque brain

BACKGROUND

Exposure of the fetal or neonatal non-human primate (NHP) brain to isoflurane or ketamine for 5 h causes widespread apoptotic degeneration of neurones, and exposure to isoflurane also causes apoptotic degeneration of oligodendrocytes (OLs). The present study explored the apoptogenic potential of propofol in the fetal and neonatal NHP brain.

METHOD

Fetal rhesus macaques at gestational age 120 days were exposed in utero, or postnatal day 6 rhesus neonates were exposed directly for 5 h to propofol anaesthesia (n=4 fetuses; and n=4 neonates) or to no anaesthesia (n=4 fetuses; n=5 neonates), and the brains were systematically evaluated 3 h later for evidence of apoptotic degeneration of neurones or glia.

RESULTS

Exposure of fetal or neonatal NHP brain to propofol caused a significant increase in apoptosis of neurones, and of OLs at a stage when OLs were just beginning to myelinate axons. Apoptotic degeneration affected similar brain regions but to a lesser extent than we previously described after isoflurane. The number of OLs affected by propofol was approximately equal to the number of neurones affected at both developmental ages. In the fetus, neuroapoptosis affected particularly subcortical and caudal regions, while in the neonate injury involved neocortical regions in a distinct laminar pattern and caudal brain regions were less affected.

CONCLUSIONS

Propofol anaesthesia for 5 h caused death of neurones and OLs in both the fetal and neonatal NHP brain. OLs become vulnerable to the apoptogenic action of propofol when they are beginning to achieve myelination competence.

Alcohol-induced apoptosis of oligodendrocytes in the fetal macaque brain

Background

In utero exposure of the fetal non-human primate (NHP) brain to alcohol on a single occasion during early or late third-trimester gestation triggers widespread acute apoptotic death of cells in both gray and white matter (WM) regions of the fetal brain. In a prior publication, we documented that the dying gray matter cells are neurons, and described the regional distribution and magnitude of this cell death response. Here, we present new findings regarding the magnitude, identity and maturational status of the dying WM cells in these alcohol-exposed fetal NHP brains.

Results

Our findings document that the dying WM cells belong to the oligodendrocyte (OL) lineage. OLs become vulnerable when they are just beginning to generate myelin basic protein in preparation for myelinating axons, and they remain vulnerable throughout later stages of myelination. We found no evidence linking astrocytes, microglia or OL progenitors to this WM cell death response. The mean density (profiles per mm³) of dying WM cells in alcohol-exposed brains was 12.7 times higher than the mean density of WM cells dying by natural apoptosis in drug-naive control brains.

Conclusions

In utero exposure of the fetal NHP brain to alcohol on a single occasion triggers widespread acute apoptotic death of neurons (previous study) and of OLs (present study) throughout WM regions of the developing brain. The rate of OL apoptosis in alcohol-exposed brains was 12.7 times higher than the natural OL apoptosis rate. OLs become sensitive to the apoptogenic action of alcohol when they are just beginning to generate constituents of myelin in their cytoplasm, and they remain vulnerable throughout later stages of myelination. There is growing evidence for a similar apoptotic response of both neurons and OLs following exposure of the developing brain to anesthetic and anticonvulsant drugs. Collectively, this body of evidence raises important questions regarding the role that neuro and oligo apoptosis may play in the human condition known as fetal alcohol spectrum disorder (FASD), and also poses a question whether other apoptogenic drugs, although long considered safe for pediatric/obstetric use, may have the potential to cause iatrogenic FASD-like developmental disability syndromes.

Teneurin-4 is a novel regulator of oligodendrocyte differentiation and myelination of small-diameter axons in the CNS

Myelination is essential for proper functioning of the CNS. In this study, we have identified a mouse mutation, designated furue, which causes tremors and hypomyelination in the CNS, particularly in the spinal cord, but not in the sciatic nerve of the PNS. In the spinal cord of the furue mice, myelination of small-diameter axons was dramatically reduced, and differentiation of oligodendrocytes, the myelin-forming cells in the CNS, was inhibited. We subsequently found that the furue mutation was associated with a transgene insertion into the teneurin-4 (Ten-4, Ten-m4/Odz4) gene, encoding a transmembrane protein of unknown function. Ten-4 was strongly expressed in the spinal cord of wild-type mice and was induced during normal oligodendrocyte differentiation. In contrast, in the furue mice, the expression of Ten-4 was absent. Differentiation and cellular process formation of oligodendrocytes were inhibited in primary cell culture from the furue mice. Cell differentiation and process formation were also inhibited in the oligodendrocyte progenitor cell line CG-4 after suppression of Ten-4 expression by shRNA. Furthermore, Ten-4 positively regulated focal adhesion kinase, an essential signaling molecule for oligodendrocyte process formation and myelination of small-diameter axons. These findings suggest that Ten-4 is a novel regulator of oligodendrocyte differentiation and that it plays a critical role in the myelination of small-diameter axons in the CNS.

Experimental alcohol-related peripheral neuropathy: role of insulin/IGF resistance

The mechanisms of alcohol-related peripheral neuropathy (ALPN) are poorly understood. We hypothesize that, like alcohol-related liver and brain degeneration, ALPN may be mediated by combined effects of insulin/IGF resistance and oxidative stress. Adult male Long Evans rats were chronically pair-fed with diets containing 0% or 37% ethanol (caloric), and subjected to nerve conduction studies. Chronic ethanol feeding slowed nerve conduction in the tibial (p = 0.0021) motor nerve, and not plantar sensory nerve, but it did not affect amplitude. Histological studies of the sciatic nerve revealed reduced nerve fiber diameters with increased regenerative sprouts, and denervation myopathy in ethanol-fed rats. qRT-PCR analysis demonstrated reduced mRNA levels of insulin, IGF-1, and IGF-2 polypeptides, IGF-1 receptor, and IRS2, and ELISAs revealed reduced immunoreactivity for insulin and IGF-1 receptors, IRS-1, IRS-4, myelin-associated glycoprotein, and tau in sciatic nerves of ethanol-fed rats (all p < 0.05 or better). The findings suggest that ALPN is characterized by (1) slowed conduction velocity with demyelination, and a small component of axonal degeneration; (2) impaired trophic factor signaling due to insulin and IGF resistance; and (3) degeneration of myelin and axonal cytoskeletal proteins. Therefore, ALPN is likely mediated by molecular and signal transduction abnormalities similar to those identified in alcoholic liver and brain degeneration.

Neuroglialpharmacology: myelination as a shared mechanism of action of psychotropic treatments

Current psychiatric diagnostic schema segregate symptom clusters into discrete entities, however, large proportions of patients suffer from comorbid conditions that fit neither diagnostic nor therapeutic schema. Similarly, psychotropic treatments ranging from lithium and antipsychotics to serotonin reuptake inhibitors (SSRIs) and acetylcholinesterase inhibitors have been shown to be efficacious in a wide spectrum of psychiatric disorders ranging from autism, schizophrenia (SZ), depression, and bipolar disorder (BD) to Alzheimer's disease (AD). This apparent lack of specificity suggests that psychiatric symptoms as well as treatments may share aspects of pathophysiology and mechanisms of action that defy current symptom-based diagnostic and neuron-based therapeutic schema. A myelin-centered model of human brain function can help integrate these incongruities and provide novel insights into disease etiologies and treatment mechanisms. Available data are integrated herein to suggest that widely used psychotropic treatments ranging from antipsychotics and antidepressants to lithium and electroconvulsive therapy share complex signaling pathways such as Akt and glycogen synthase kinase-3 (GSK3) that affect myelination, its plasticity, and repair. These signaling pathways respond to neurotransmitters, neurotrophins, hormones, and nutrition, underlie intricate neuroglial communications, and may substantially contribute to the mechanisms of action and wide spectra of efficacy of current therapeutics by promoting myelination. Imaging and genetic technologies make it possible to safely and non-invasively test these hypotheses directly in humans and can help guide clinical trial efforts designed to correct myelination abnormalities. Such efforts may provide insights into novel avenues for treatment and prevention of some of the most prevalent and devastating human diseases.

Individual axons regulate the myelinating potential of single oligodendrocytes in vivo

The majority of axons in the central nervous system (CNS) are eventually myelinated by oligodendrocytes, but whether the timing and extent of myelination in vivo reflect intrinsic properties of oligodendrocytes, or are regulated by axons, remains undetermined. Here, we use zebrafish to study CNS myelination at single-cell resolution in vivo. We show that the large caliber Mauthner axon is the first to be myelinated (shortly before axons of smaller caliber) and that the presence of supernumerary large caliber Mauthner axons can profoundly affect myelination by single oligodendrocytes. Oligodendrocytes that typically myelinate just one Mauthner axon in wild type can myelinate multiple supernumerary Mauthner axons. Furthermore, oligodendrocytes that exclusively myelinate numerous smaller caliber axons in wild type can readily myelinate small caliber axons in addition to the much larger caliber supernumerary Mauthner axons. These data indicate that single oligodendrocytes can myelinate diverse axons and that their myelinating potential is actively regulated by individual axons.

Spatial and temporal profiles of growth factor expression during CNS demyelination reveal the dynamics of repair priming

Demyelination is the cause of disability in various neurological disorders. It is therefore crucial to understand the molecular regulation of oligodendrocytes, the myelin forming cells in the CNS. Growth factors are known to be essential for the development and maintenance of oligodendrocytes and are involved in the regulation of glial responses in various pathological conditions. We employed the well established murine cuprizone model of toxic demyelination to analyze the expression of 13 growth factors in the CNS during de- and remyelination. The temporal mRNA expression profile during demyelination and the subsequent remyelination were analyzed separately in the corpus callosum and cerebral cortex using laser microdissection and real-time PCR techniques. During demyelination a similar pattern of growth factor mRNA expression was observed in both areas with a strong up-regulation of NRG1 and GDNF and a slight increase of CNTF in the first week of cuprizone treatment. HGF, FGF-2, LIF, IGF-I, and TGF-ß1 were up-regulated mainly during peak demyelination. In contrast, during remyelination there were regional differences in growth factor mRNA expression levels. GDNF, CNTF, HGF, FGF-2, and BDNF were elevated in the corpus callosum but not in the cortex, suggesting tissue differences in the molecular regulation of remyelination in the white and grey matter. To clarify the cellular source we isolated microglia from the cuprizone lesions. GDNF, IGF-1, and FGF mRNA were detected in the microglial fraction with a temporal pattern corresponding to that from whole tissue PCR. In addition, immunohistochemical analysis revealed IGF-1 protein expression also in the reactive astrocytes. CNTF was located in astrocytes. This study identified seven different temporal expression patterns for growth factors in white and grey matter and demonstrated the importance of early tissue priming and exact orchestration of different steps during callosal and cortical de- and remyelination.

Oligodendrocyte progenitor cells proliferate and survive in an immature state following treatment with an axolemma-enriched fraction

The ability of an AEF (axolemma-enriched fraction) to influence the proliferation, survival and differentiation of OPC (oligodendrocyte progenitor cells) was evaluated. Following addition of AEF to cultured OPC, the AEF associated with the outer surface of OPC so that subsequent metabolic events were likely mediated by direct AEF-OPC contact. Addition of AEF to the cultured OPC resulted in a dose- and time-dependent increase in proliferation that was partially dependent on Akt (protein kinase B) and MAPK (mitogen-activated protein kinase) activation. The major mitogen in an AEF-SE (soluble 2.0 M NaCl extract of the AEF) was identified as aFGF (acidic fibroblast growth factor) and accounted for 50% of the mitogenicity. The remaining 50% of the mitogenicity had properties consistent with bFGF (basic fibroblast growth factor) but was not unequivocally identified. Under conditions that limit the survival of OPC in culture, AEF treatment prolonged the survival of the OPC. Antigenic and morphological examination of the AEF-treated OPC indicated that the AEF treatment helped the OPC survive in a more immature state. The potential downstream metabolic pathways potentially activated in OPC by AEF and the consequences of these activated pathways are discussed. The results of these studies are consistent with the view that direct contact of axons with OPC stimulates their proliferation and survival while preventing their differentiation.

Fibroblast growth factors in schizophrenia

A large association study by O'Donovan et al recently suggested that genetic variation in fibroblast growth factor receptor (FGFR) 2 increases the risk for developing schizophrenia. Fibroblast growth factors (FGFs) are part of the family of glial growth factors; they control the growth and patterning of specific brain structures and regulate the maintenance and repair of neuronal tissues. In addition, a direct interaction was recently found between FGFRs and adenosine A(2A) receptors, leading to corticostriatal plasticity and antagonizing the signaling pathway of dopamine D(2) receptors. These findings make FGFs plausible candidate genes for schizophrenia. Here, we review the role of FGFs in schizophrenia and combine evidence from studies on variations in FGF genes, RNA expression, protein levels, and FGF administration, as well as the effects of medication and environmental risk factors for schizophrenia. These data suggest that changes in the FGF system contribute to schizophrenia and possibly to a wider range of psychiatric disorders. The role of FGFs in schizophrenia and related disorders needs to be studied in more detail.

Corpus callosum and inferior forebrain white matter microstructure are related to functional outcome from raised intracranial pressure in child traumatic brain injury

In severe paediatric traumatic brain injury (TBI), a common focus of treatment is raised intracranial pressure (ICP). We have previously reported frontal cerebral vulnerability with executive deficits from raised ICP in paediatric TBI. Now, using diffusion tensor imaging (DTI) in a different population, we have examined fractional anisotropy (FA), and mean, axial and radial diffusivity (MD, AD, RD) in 4 regions of the corpus callosum (CC) and in both inferior frontal regions. Our aim was to examine during the chronic phase of TBI whether the CC cross-sectional area correlated with regional DTI metrics of white matter microstructure, with global outcome ratings of function (Functional Independence Measure and Multiattribute Health Status Classification) and with performance in the Rey-Osterrieth Complex Figure (ROCF) test. We examined 33 paediatric TBI cases who were followed, on average, 4.9 years after severe injury. All cases had received mechanical ventilation during their acute treatment and, a priori, they were assigned to a non-ICP or a raised ICP group. Twenty-two participants had mainly right-sided injury at the time of acute ictus. The findings confirm that severe TBI in childhood, complicated by intracranial hypertension, results in CC vulnerability. In the chronic phase of recovery, it is reduced in the cross-sectional area, it is more compact and thinned, and the anterior region is disproportionately small. Late after raised ICP, we have also found that individuals exhibit regional microstructural abnormality with combined reduced FA and increased MD, AD and RD. Smaller size and such microstructural changes in the anterior CC were associated with similar right-sided (rather than left-sided) frontal microstructural changes in the ICP group. Taken together, this evidence points to an interaction between raised ICP-related brain tissue perturbation and focal frontal extracallosal injury, leading to anterior CC regional vulnerability, most likely wallerian degeneration. At long-term follow-up, this lack of white matter integrity in the anterior CC is correlated with functional outcome, particularly in aspects of social interaction and the copy component of the ROCF test, which suggests that the CC-to-forebrain function warrants further study in chronic TBI.

Axons and astrocytes release ATP and glutamate to evoke calcium signals in NG2-glia

NG2-glia are an abundant population of cells in the adult CNS that make up a novel glial cell type. Here, we have examined calcium signals in NG2-glia identified by expression of the fluorescent protein DsRed under the control of the NG2 promoter in the white matter of the mouse optic nerve. We focused on mice aged postnatal day (P)12-16, after the main period of oligodendrocyte generation. Using fluo-4 and fura-2 calcium imaging in isolated intact nerves, we show that glutamate and ATP evoke Ca(2+) signals in NG2-glia in situ, acting on AMPA-type glutamate receptors and P2Y(1) and P2X(7) purine receptors; NMDA evoked a weak Ca(2+) signal in a small proportion of NG2-glia. We show that axonal action potentials and mechanical stimulation of astrocytes effect the release of glutamate and ATP to act on NG2-glia; ATP alone evokes robust Ca(2+) signals, whereas glutamate did not unless AMPA receptor desensitization was blocked with cyclothiazide. We identify the precise contacts that NG2-glia form with axons at nodes of Ranvier, and the intricate bipartite sheaths formed between the processes of NG2-glia and astrocytes. In addition, we provide evidence that NG2-glia express synaptophysin, indicating they have mechanisms for transmitting as well as receiving signals. This study places NG2-glia within a neuron-glial network, and identifies roles for glutamate and ATP in communication with astrocytes as well as axons.

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.

Respiratory rhythm of brainstem-spinal cord preparations: Effects of maturation, age, mass and oxygenation

We examined the effect of age, mass and the presence of the pons on the longevity (length of time spontaneous respiratory-related activity is produced) of brainstem-spinal cord preparations of neonatal rodents (rats and hamsters) and the level of oxygenation in the medulla respiratory network in these preparations. We found the longevity of the preparations from both species decreased with increasing postnatal age. Physical removal of the pons increased respiratory frequency and the longevity of the preparation. However, tissue oxygenation at the level of the medullary respiratory network was not affected by removal of the pons or increasing postnatal age (up to postnatal day 4). Taken together, these data suggest that the effect of removing the pons on respiratory frequency and the longevity of brainstem-spinal cord preparations with increasing postnatal age are primarily due to postnatal development and appear to be unrelated to mass or changes in levels of tissue oxygenation.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

Changes in PDGF expression in spared dorsal root ganglia and associated spinal dorsal horns in cats subjected to partial dorsal root ganglionectomy

Changes in the platelet derived growth factor (PDGF) in the spared dorsal root ganglia (DRG) and associated spinal dorsal horns were evaluated in cats subjected to unilateral removal of L1-L5 and L7-S2 DRG, sparing the L6 DRG. The number of PDGF immunopositive neurons and protein expression decreased significantly in the spared DRG and associated dorsal horns of the L3 and L6 cord segments at 3 days post-operation (dpo). It bottomed to the lowest level at 7 dpo in the DRG, then returned to the control level at 14 dpo; while in the L6 dorsal horn, it rapidly increased at 7 dpo and exceeded the control level at 14 dpo. This showed a significant upregulation in the spared DRG and associated spinal dorsal horns, especially in the L6 cord segment following a transient decrease. Meanwhile, a significant upregulation of PDGF mRNA was also seen in L6 DRG and L3 and L6 dorsal horns at 3 dpo. The upregulation of the endogenous PDGF in the said structures indicated a potential role of this factor in spinal cord plasticity after partial dorsal root ganglia removal in cats.

Effects of Electro-acupuncture on PDGF Expression in Spared Dorsal Root Ganglion and Associated Dorsal Horn Subjected to Partial Dorsal Root Ganglionectomy in Cats

The effects of electro-acupuncture (EA) on the expression of platelet derived growth factor (PDGF) in spared dorsal root ganglion (DRG) and associated dorsal horns were evaluated in cats subjected to bilateral removal of L1-L5 and L7-S2 DRG, while sparing L6 DRG and were demonstrated using Immunohistochemistry, Western blot and RT-PCR techniques. On the acupunctured side, there was a significant increase in the total number of PDGF positive neurons. Large neurons of the L6 DRG at 7 days post operation (dpo), and small to medium-sized neurons at 14 dpo, as well as in the lamina II of the L6 spinal cord at 14 dpo was observed. The expression of PDGF protein increased significantly in the L6 DRG at 7 and 14 dpo and in the dorsal horn of the L6 spinal cord at 14 dpo while the upregulation of PDGF mRNA was seen at 3 dpo in the L6 DRG and the dorsal horn of the L3 and L6 spinal cord. These findings demonstrate that intrinsic PDGF has been upregulated in cats subjected to partial dorsal root ganglionectomy following EA, indicating endogenous PDGF is involved in promoting spinal plasticity following EA.

The proneural gene Mash1 specifies an early population of telencephalic oligodendrocytes

The bHLH (basic helix-loop-helix) transcription factor Mash1 is best known for its role in the regulation of neurogenesis. However, Mash1 is also expressed in oligodendrocyte precursors and has recently been shown to promote the generation of oligodendrocytes in cell culture, suggesting that it may regulate oligodendrogenesis as well. Here, we show that in the developing ventral forebrain, Mash1 is expressed by a subset of oligodendrocyte precursors (OPCs) as soon as they are generated in the ventricular zone. Using reporter mice, we demonstrate that a subset of OPCs in both the embryonic and postnatal forebrain originate from Mash1-positive progenitors, including a large fraction of adult NG2-positive OPCs. Using Mash1 null mutant mice, we show that Mash1 is required for the generation of an early population of OPCs in the ventral forebrain between embryonic day 11.5 (E11.5) and E13.5, whereas OPCs generated later in embryonic development are not affected. Overexpression of Mash1 in the dorsal telencephalon induces expression of PDGFRalpha (platelet-derived growth factor receptor alpha) but not other OPC markers, suggesting that Mash1 specifies oligodendrogenesis in cooperation with other factors. Analysis of double-mutant mice suggests that Olig2 is one of the factors that cooperate with Mash1 for generation of OPCs. Together, our results show for the first time that Mash1 cooperates in vivo with Olig2 in oligodendrocyte specification, demonstrating an essential role for Mash1 in the generation of a subset of oligodendrocytes and revealing a genetic heterogeneity of oligodendrocyte lineages in the mouse forebrain.

Contrasting effects of mitogenic growth factors on myelination in neuron-oligodendrocyte co-cultures

Mitogenic growth factors play an important role in the initial stages of oligodendrocyte development, but their roles in the process of myelination itself remain less well defined. In order to study directly the effects of different growth factors on myelination, we used a purified in vitro co-culture system with dorsal root ganglion neurons and oligodendrocytes. Extensive myelination had occurred in these cultures 14 days after oligodendrocyte precursors (OPCs) were added, with the relationship between neurite density and the percentage of oligodendrocytes forming myelin sheaths providing a robust and straightforward means of quantifying myelination. Addition of soluble neuregulin (Nrg1), a mitogen for oligodendroglial cells that also provides an axonal signal implicated in oligodendrocyte survival, increased myelination. Conversely, the OPC mitogens FGF-2 and PDGF inhibited myelination. The inhibitory effect of these mitogens was reversible, as inhibition of PDGF allowed myelination to proceed. Taken together, these data indicate that different mitogenic growth factors can regulate myelination by oligodendrocytes in addition to their well-described effects on earlier stages of oligodendroglial development. Moreover, the results highlight important differences between the growth factors.

Erythropoietin-mediated preservation of the white matter in rat spinal cord injury

We investigated the effect of a single administration of recombinant human erythropoietin (rhEPO) on the preservation of the ventral white matter of rats at 4 weeks after contusive spinal cord injury (SCI), a time at which functional recovery is significantly improved in comparison to the controls [Gorio A, Necati Gokmen N, Erbayraktar S, Yilmaz O, Madaschi L, Cichetti C, Di Giulio AM, Enver Vardar E, Cerami A, Brines M (2002) Recombinant human erythropoietin counteracts secondary injury and markedly enhances neurological recovery from experimental spinal cord trauma. Proc Natl Acad Sci U S A 99:9450-9455; Gorio A, Madaschi L, Di Stefano B, Carelli S, Di Giulio AM, De Biasi S, Coleman T, Cerami A, Brines M (2005) Methylprednisolone neutralizes the beneficial effects of erythropoietin in experimental spinal cord injury. Proc Natl Acad Sci U S A 102:16379-16384]. Specifically, we examined, by morphological and cytochemical methods combined with light, confocal and electron microscopy, i) myelin preservation, ii) activation of adult oligodendrocyte progenitors (OPCs) identified for the expression of NG2 transmembrane proteoglycan, iii) changes in the amount of the chondroitin sulfate proteoglycans neurocan, versican and phosphacan and of their glycosaminoglycan component labeled with Wisteria floribunda lectin, and iv) ventral horn density of the serotonergic plexus as a marker of descending motor control axons. Injured rats received either saline or a single dose of rhEPO within 30 min after SCI. The results showed that the significant improvement of functional outcome observed in rhEPO-treated rats was associated with a better preservation of myelin in the ventral white matter. Moreover, the significant increase of both the number of NG2-positive OPCs and the labeling for Nogo-A, a marker of differentiated oligodendrocytes, suggested that rhEPO treatment could result in the generation of new myelinating oligodendrocytes. Sparing of fiber tracts in the ventral white matter was confirmed by the increased density of the serotonergic plexus around motor neurons. As for chondroitin sulfate proteoglycans, only phosphacan, increased in saline-treated rats, returned to normal levels in rhEPO group, probably reflecting a better maintenance of glial-axolemmal relationships along nerve fibers. In conclusion, this investigation expands previous studies supporting the pleiotropic neuroprotective effect of rhEPO on secondary degenerative response and its therapeutic potential for the treatment of SCI and confirms that the preservation of the ventral white matter, which contains descending motor pathways, may be critical for limiting functional deficit.

"Skittish" Abca2 knockout mice display tremor, hyperactivity, and abnormal myelin ultrastructure in the central nervous system

The ATP-binding cassette transporter 2 (ABCA2) is an endolysosomal protein most highly expressed in the central and peripheral nervous system tissues and macrophages. Previous studies indicated its role in cholesterol/steroid (estramustine, estradiol, and progesterone) trafficking/sequestration, oxidative stress response, and Alzheimer's disease. Developmental studies have shown its expression during macrophage and oligodendrocyte differentiation, processes requiring membrane growth. To determine the in vivo function(s) of this transporter, we generated a knockout mouse from a gene-targeted disruption of the murine ABCA2 gene. Knockout males and females are viable and fertile. However, a non-Mendelian inheritance pattern was shown among male progeny of heterozygous crosses. Compared to wild-type and heterozygous littermates, knockout mice displayed a tremor without ataxia, hyperactivity, and reduced body weight; the latter two phenotypes were more marked in females than in males. This sexual disparity suggests a role for ABCA2 in hormone-dependent neurological and/or developmental pathways. Myelin sheath thickness in the spinal cords of knockout mice was greatly increased compared to that in wild-type mice, while a significant reduction in myelin membrane periodicity (compaction) was observed in both spinal cords and cerebra of knockout mice. Loss of ABCA2 function in vivo resulted in abnormal myelin compaction in spinal cord and cerebrum, an ultrastructural defect that we propose to be the cause of the phenotypic tremor.

Human embryonic stem cell-derived oligodendrocyte progenitor cell transplants remyelinate and restore locomotion after spinal cord injury

Demyelination contributes to loss of function after spinal cord injury, and thus a potential therapeutic strategy involves replacing myelin-forming cells. Here, we show that transplantation of human embryonic stem cell (hESC)-derived oligodendrocyte progenitor cells (OPCs) into adult rat spinal cord injuries enhances remyelination and promotes improvement of motor function. OPCs were injected 7 d or 10 months after injury. In both cases, transplanted cells survived, redistributed over short distances, and differentiated into oligodendrocytes. Animals that received OPCs 7 d after injury exhibited enhanced remyelination and substantially improved locomotor ability. In contrast, when OPCs were transplanted 10 months after injury, there was no enhanced remyelination or locomotor recovery. These studies document the feasibility of predifferentiating hESCs into functional OPCs and demonstrate their therapeutic potential at early time points after spinal cord injury.