Blood-brain barrier permeability in astrocyte-free regions of the central nervous system remyelinated by Schwann cells

ErbB receptor signaling directly controls oligodendrocyte progenitor cell transformation and spontaneous remyelination after spinal cord injury

We recently discovered a novel role for neuregulin‐1 (Nrg1) signaling in mediating spontaneous regenerative processes and functional repair after spinal cord injury (SCI). We revealed that Nrg1 is the molecular signal responsible for spontaneous functional remyelination of dorsal column axons by peripheral nervous system (PNS)‐like Schwann cells after SCI. Here, we investigate whether Nrg1/ErbB signaling controls the unusual transformation of centrally derived progenitor cells into these functional myelinating Schwann cells after SCI using a fate‐mapping/lineage tracing approach. Specific ablation of Nrg1‐ErbB receptors in central platelet‐derived growth factor receptor alpha (PDGFRα)‐derived lineage cells (using PDGFRαCreERT2/Tomato‐red reporter mice crossed with ErbB3fl/fl/ErbB4fl/fl mice) led to a dramatic reduction in P0‐positive remyelination in the dorsal columns following spinal contusion injury. Central myelination, assessed by Olig2 and proteolipid protein expression, was unchanged. Loss of ErbB signaling in PDGFRα lineage cells also significantly impacted the degree of spontaneous locomotor recovery after SCI, particularly in tests dependent on proprioception. These data have important implications, namely (a) cells from the PDGFRα‐expressing progenitor lineage (which are presumably oligodendrocyte progenitor cells, OPCs) can differentiate into remyelinating PNS‐like Schwann cells after traumatic SCI, (b) this process is controlled by ErbB tyrosine kinase signaling, and (c) this endogenous repair mechanism has significant consequences for functional recovery after SCI. Thus, ErbB tyrosine kinase receptor signaling directly controls the transformation of OPCs from the PDGFRα‐expressing lineage into PNS‐like functional remyelinating Schwann cells after SCI.

Neuregulin-1 promotes remyelination and fosters a pro-regenerative inflammatory response in focal demyelinating lesions of the spinal cord

Oligodendroglial cell death and demyelination are hallmarks of neurotrauma and multiple sclerosis that cause axonal damage and functional impairments. Remyelination remains a challenge as the ability of endogenous precursor cells for oligodendrocyte replacement is hindered in the unfavorable milieu of demyelinating conditions. Here, in a rat model of lysolecithin lysophosphatidyl-choline (LPC)-induced focal demyelination, we report that Neuregulin-1 (Nrg-1), an important factor for oligodendrocytes and myelination, is dysregulated in demyelinating lesions and its bio-availability can promote oligodendrogenesis and remyelination. We delivered recombinant human Nrg-1β1 (rhNrg-1β1) intraspinally in the vicinity of LPC demyelinating lesion in a sustained manner using poly lactic-co-glycolic acid microcarriers. Availability of Nrg-1 promoted generation and maturation of new oligodendrocytes, and accelerated endogenous remyelination by both oligodendrocyte and Schwann cell populations in demyelinating foci. Importantly, Nrg-1 enhanced myelin thickness in newly remyelinated spinal cord axons. Our complementary in vitro studies also provided direct evidence that Nrg-1 significantly promotes maturation of new oligodendrocytes and facilitates their transition to a myelinating phenotype. Nrg-1 therapy remarkably attenuated the upregulated expression chondroitin sulfate proteoglycans (CSPGs) specific glycosaminoglycans in the extracellular matrix of demyelinating foci and promoted interleukin-10 (IL-10) production by immune cells. CSPGs and IL-10 are known to negatively and positively regulate remyelination, respectively. We found that Nrg-1 effects are mediated through ErbB2 and ErbB4 receptor activation. Our work provides novel evidence that dysregulated levels of Nrg-1 in demyelinating lesions of the spinal cord pose a challenge to endogenous remyelination, and appear to be an underlying cause of myelin thinning in newly remyelinated axons.

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.

Neurogenesis, exercise, and cognitive late effects of pediatric radiotherapy

Brain cancer is a common type of childhood malignancy, and radiotherapy (RT) is a mainstay of treatment. RT is effective for tumor eradication, and survival rates are high. However, RT damages the brain and disrupts ongoing developmental processes, resulting in debilitating cognitive “late” effects that may take years to fully manifest. These late effects likely derive from a long-term decrement in cell proliferation, combined with a neural environment that is hostile to plasticity, both of which are induced by RT. Long-term suppression of cell proliferation deprives the brain of the raw materials needed for optimum cognitive performance (such as new neurons in the hippocampus and new glia in frontal cortex), while chronic inflammation and dearth of trophic substances (such as growth hormone) limit neuroplastic potential in existing circuitry. Potential treatments for cognitive late effects should address both of these conditions. Exercise represents one such potential treatment, since it has the capacity to enhance cell proliferation, as well as to promote a neural milieu permissive for plasticity. Here, we review the evidence that cognitive late effects can be traced to RT-induced suppression of cell proliferation and hostile environmental conditions, as well as emerging evidence that exercise may be effective as an independent or adjuvant therapy.

Identification of endothelin 2 as an inflammatory factor that promotes central nervous system remyelination

The development of new regenerative therapies for multiple sclerosis is hindered by the lack of potential targets for enhancing remyelination. The study of naturally regenerative processes such as the innate immune response represents a powerful approach for target discovery to solve this problem. By 'mining' these processes using transcriptional profiling we can identify candidate factors that can then be tested individually in clinically-relevant models of demyelination and remyelination. Here, therefore, we have examined a previously described in vivo model of the innate immune response in which zymosan-induced macrophage activation in the retina promotes myelin sheath formation by oligodendrocytes generated from transplanted precursor cells. While this model is not itself clinically relevant, it does provide a logical starting point for this study as factors that promote myelination must be present. Microarray analysis of zymosan-treated retinae identified several cytokines (CXCL13, endothelin 2, CCL20 and CXCL2) to be significantly upregulated. When tested in a cerebellar slice culture model, CXCL13 and endothelin 2 promoted myelination and endothelin 2 also promoted remyelination. In studies to identify the receptor responsible for this regenerative effect of endothelin 2, analysis of both remyelination following experimental demyelination and of different stages of multiple sclerosis lesions in human post-mortem tissue revealed high levels of endothelin receptor type B in oligodendrocyte lineage cells. Confirming a role for this receptor in remyelination, small molecule agonists and antagonists of endothelin receptor type B administered in slice cultures promoted and inhibited remyelination, respectively. Antagonists of endothelin receptor type B also inhibited remyelination of experimentally-generated demyelination in vivo. Our work therefore identifies endothelin 2 and the endothelin receptor type B as a regenerative pathway and suggests that endothelin receptor type B agonists represent a promising therapeutic approach to promote myelin regeneration.

CNS pericytes: concepts, misconceptions, and a way out

Rouget, in 1873, was the first to describe a population of cells surrounding capillaries, which he regarded as contractile elements. Fifty years later, Zimmermann termed these cells "pericytes" and distinguished three subtypes along the vascular tree. Since then, the discussion concerning the contractile ability of pericytes has never ceased. Current concepts of pericyte biology rather suggest critical roles in the maintenance of homeostasis, blood-brain barrier (BBB) integrity, angiogenesis, and neovascularization. In addition, data from models of brain pathology suggest that novel pericytes are recruited from the bone marrow, but their respective precursor remains enigmatic. Recent data also suggest an important role in the regulation of cerebral blood flow, thus confirming Rouget's original idea. However, comparison of data from different studies is often constrained by the fact that pericytes were questionably identified. Although a clear-cut definition exists, defining pericytes as part of the vascular wall being enclosed in its basement membrane, pericytes are often mixed up with adjacent cell types of the vascular wall, the perivascular space, and the juxtavascular parenchyma. In fact, their identification is difficult-if not impossible-in standard histological sections. An unambiguous distinction, however, is possible at the ultrastructural level and in semi-thin sections, where their location within the vascular basement membrane can be displayed. Using these techniques in combination with immunological staining methods allows demarking their unique morphology and location. Here, we review original papers describing pericytes, briefly outline their topography within the vascular compartments, describe methods for their identification, and summarize current concepts of their function.

Remyelination of dorsal column axons by endogenous Schwann cells restores the normal pattern of Nav1.6 and Kv1.2 at nodes of Ranvier

Demyelination of CNS axons occurs in a number of pathological conditions, including multiple sclerosis and contusion-type spinal cord injury. The demyelination can be repaired by remyelination in both humans and rodents, and even within the CNS remyelination can be achieved by endogenous and/or exogenous Schwann cells, the myelinating cells of the PNS. Remyelinated axons can often conduct impulses securely, but the organization of ion channels at long-term remyelinated nodes is not known. In the present study, the expression of voltage-gated sodium (Na(v)) and potassium (K(v)) channels along central axons remyelinated by endogenous Schwann cells has been studied in lesions induced more than 1 year previously by the intraspinal injection of ethidium bromide (EB). The expression of the channels at long-term nodes formed by Schwann cell remyelination has been compared with that present in nascent nodes formed in the adult at 18 and 23 days post-EB injection. Immunohistochemical studies revealed that long-term nodes formed by Schwann cell remyelination exhibit a clustering of Na(v)1.6 sodium channels within the nodal membrane, with the Shaker-type potassium channel K(v)1.2 segregated within the juxtaparanodal region, similar to the arrangement at normal mature CNS nodes. Na(v)1.2 was not detected at nodes formed by Schwann cells at any stage of their development. Moreover, Na(v)1.6, but not Na(v)1.2, was clustered at nascent nodes formed by remyelinating Schwann cells 18 and 23 days following EB injection. These observations show that endogenous Schwann cells can establish and maintain nodes of Ranvier on central axons for over one year, and that the nodes exhibit an apparently normal distribution of sodium and potassium channels, with Na(v)1.6 the predominant subtype of sodium channel present at such nodes at all stages of their development.

Inflammation and primary demyelination induced by the intraspinal injection of lipopolysaccharide

Inflammation is a prominent feature of several disorders characterized by primary demyelination, but it is not clear whether a relationship exists between inflammation and myelin damage. We have found that substantial demyelination results from the focal inflammatory lesion caused by the injection of lipopolysaccharide (LPS; 200 ng) directly into the rat dorsal funiculus. Within 24 h, such injections caused a focal inflammatory response consisting of a substantial number of polymorphonuclear cells and ED1-positive and inducible nitric oxide synthase (iNOS)-positive macrophages/microglia. The number of inflammatory cells was substantially reduced by day 7. OX-52-positive T-cells were less frequently observed but were present in the meninges at 8 h, reached a maximum in the dorsal funiculus at 7 days, and were rare at 14 days. The inflammation was followed by the appearance of a large lesion of primary demyelination that encompassed up to ∼75% of the cross-sectional area of the dorsal funiculus. Treatment with dexamethasone significantly reduced the number of cells expressing iNOS, but did not prevent the demyelination. By 28 days the lesions were largely remyelinated, usually by Schwann cells. These changes were not observed in control, saline-injected animals. We conclude that the intraspinal injection of LPS results in inflammation and subsequently in prominent demyelination. The mechanisms underlying the demyelination are not clear, but it is notable that it typically begins with disruption of the adaxonal myelin. Indeed, there is an early loss of myelin-associated glycoprotein within the lesion, despite the persistence of proteolipid protein. This combination is a feature of the pattern III lesion recently described in multiple sclerosis (Lucchinetti et al., 2000), and we therefore suggest that LPS-induced demyelination may serve as the first experimental model available for the study of this type of multiple sclerosis lesion.

Filamentous actin is concentrated in specific subpopulations of neuronal and glial structures in rat central nervous system

This paper is the second in a series of studies on the light and electron microscopic distribution of filamentous actin (F-actin) in the rat central nervous system (CNS) using phalloidin tagged with the fluorophore eosin followed by fluorescence photooxidation. A previous report described the selective localization of high concentrations of F-actin in subpopulations of dendritic spines in hippocampus, cerebellum and neostriatum. Dendritic spines were the most intensely stained structures in the CNS, but several other structures were notable for their consistent staining for F-actin. Although the majority of cell bodies, axons and large dendrites were unlabeled, mossy fibers and Schaffer collaterals in the hippocampal formation, basket cell axons in the cerebellar pinceau, and granule cell dendrites in the glomeruli of the cerebellar cortex routinely showed strong F-actin labeling. Staining was observed in all three glial cell types. Labeling was consistently observed in the astrocytic processes surrounding the Purkinje cell soma and primary dendrite. Intense but sporadic staining was observed in the perinodal glia of the Node of Ranvier. A few examples of labeled oligodendrocyte processes were also seen in the neostriatum. Labeling was observed in microglia in every brain region examined, although the labeling was present in the lumen of the endoplasmic reticulum and the nuclear membrane, leading to questions about its specificity. Perycites apposed to the blood vessels also showed very consistent labeling. Our results suggest that selected structures in the adult CNS in addition to dendritic spines are enriched in F-actin.

Effects of 4-aminopyridine on demyelinated axons, synapses and muscle tension

Several clinical trials have demonstrated that 4-amino-pyridine (4-AP), a potassium channel-blocking agent, improves symptoms in some patients with multiple sclerosis. The beneficial effects have typically been attributed to the restoration of conduction to demyelinated axons, since this effect was previously demonstrated experimentally. However, the clinical dose is approximately 250-1000 times lower than that used experimentally, potentially making extrapolation of the experimental findings unreliable. To examine the action(s) of 4-AP in demyelinating disorders, the drug was administered at clinical doses, both in vivo and in vitro, to rat dorsal column axons which had been experimentally demyelinated by the intraspinal injection of ethidium bromide. 4-AP had no consistent effect in restoring conduction to demyelinated axons, even to axons which were held just on the verge of conducting by adjusting the lesion temperature. However, 4-AP had prominent effects that did not involve demyelinated axons, including the potentiation of synaptic transmission and an increase in skeletal muscle twitch tension. We propose that these latter effects may be largely responsible for the beneficial action of 4-AP in multiple sclerosis patients. If so, the dominant effects of 4-AP in multiple sclerosis patients are independent of demyelination, and it follows that 4-AP may be beneficial in other neurological disorders in which function is diminished.

Role of the CNS microvascular pericyte in the blood-brain barrier

Pericytes are a very important cellular constituent of the blood-brain barrier. They play a regulatory role in brain angiogenesis, endothelial cell tight junction formation, blood-brain barrier differentiation, as well as contribute to the microvascular vasodynamic capacity and structural stability. Central nervous system pericytes express macrophage functions and are actively involved in the neuroimmune network operating at the blood-brain barrier. They exhibit unique functional characteristics critical for the pathogenesis of a number of cerebrovascular, neurodegenerative, and neuroimmune diseases.

Conduction in segmentally demyelinated mammalian central axons

The prominent symptoms associated with central demyelinating diseases such as multiple sclerosis (MS) are primarily caused by conduction deficits in affected axons. The symptoms may go into remission, but the mechanisms underlying remissions are uncertain. One factor that could be important is the restoration of conduction to affected axons, but it is not known whether demyelinated central axons resemble their peripheral counterparts in being able to conduct in the absence of repair by remyelination. In the present study we have made intra-axonal recordings from central axons affected by a demyelinating lesion, and then the axons have been labeled ionophoretically to permit their subsequent identification. Ultrastructural examination of 23 labeled preparations has established that some segmentally demyelinated central axons can conduct, and that they can do so over continuous lengths of demyelination exceeding several internodes (2500 micron). Such segmentally demyelinated central axons were found to conduct with the anticipated reduction in velocity and a refractory period of transmission (RPT) as much as 34 times the value obtained from the nondemyelinated portion of the same axon; the RPT was typically prolonged to 2-5 times the normal value. We conclude that some segmentally demyelinated central axons can conduct, and we propose that the restoration of conduction to such axons is likely to contribute to the remissions commonly observed in diseases such as MS.