Distinct effects of p75 in mediating actions of neurotrophins on basal forebrain oligodendrocytes

CXCL1-CXCR2 axis mediates inflammatory response after sciatic nerve injury by regulating macrophage infiltration

Macrophages play a crucial role in the inflammatory response following sciatic nerve injury. Studies have demonstrated that C-X-C motif chemokine (CXCL) 1 recruit macrophages by binding to C-X-C chemokine receptor (CXCR) 2 and participates in the inflammatory response of various diseases. Based on these findings, we aimed to explore the role of the CXCL1-CXCR2 axis in the repair process after peripheral nerve injury. Initially, we simulated sciatic nerve injury and observed an increased expression of CXCL1 and CXCR2 in the nerves of the injury group. Both in vivo and in vitro experiments confirmed that the heightened CXCL1 expression occurs in Schwann cells and is secreted, while the elevated CXCR2 is expressed by recruited macrophages. In addition, in vitro experiments demonstrated that the binding of CXCL1 to CXCR2 can activate the NLRP3 inflammasome and promote the production of interleukin-1 beta (IL-1β) in macrophages. However, after mice were subjected to sciatic nerve injury, the number of macrophages and the expression of inflammatory factors in the sciatic nerve were reduced following treatment with the CXCR2 inhibitor SB225002. Simultaneously, we evaluated the sciatic nerve function index, the expression of p75 neurotrophic factor receptor (p75NTR), and myelin proteins, and all of these results were improved with the use of SB225002. Thus, our results suggest that after sciatic nerve injury, the CXCL1-CXCR2 axis mediates the inflammatory response by promoting the recruitment and activation of macrophages, which is detrimental to the repair of the injured nerves. In contrast, treatment with SB225002 promotes the repair of injured sciatic nerves.

Dynamics of myelin deficits in the 5xFAD mouse model for Alzheimer's disease and the protective role of BDNF

Recent findings highlight myelin breakdown as a decisive early event in Alzheimer's Disease (AD) acting as aggravating factor of its progression. However, it is still unclear whether myelin loss is attributed to increased oligodendrocyte vulnerability, reduced repairing capacity or toxic stimuli. In the present study, we sought to clarify the starting point of myelin disruption accompanied with Oligodendrocyte Progenitor Cell (OPC) elimination in the brain of the 5xFAD mouse model of AD at 6 months of age in Dentate Gyrus of the hippocampus in relation to neurotrophin system. Prominent inflammation presence was detected since the age of 6 months playing a key role in myelin disturbance and AD progression. Expression analysis of neurotrophin receptors in OPCs was performed to identify new targets that could increase myelination in health and disease. OPCs in both control and 5xFAD mice express TrkB, TrkC and p75 receptors but not TrkA. Brain-derived neurotrophic factor (BDNF) that binds to TrkB receptor is well-known about its pro-myelination effect, promoting oligodendrocytes proliferation and differentiation, so we focused our investigation on its effects in OPCs under neurodegenerative conditions. Our in vitro results showed that BDNF rescues OPCs from death and promotes their proliferation and differentiation in presence of the toxic Amyloid-β 1-42. Collectively, our results indicate that BDNF possess an additional neuroprotective role through its actions on oligodendrocytic component and its use could be proposed as a drug-based myelin-enhancing strategy, complementary to amyloid and tau centered therapies in AD.

A Brief Overview on BDNF-Trk Pathway in the Nervous System: A Potential Biomarker or Possible Target in Treatment of Multiple Sclerosis?

The growing incidence of neurodegenerative disorders in our populations is leading the research to identify potential biomarkers and targets for facilitating their early management and treatments. Biomarkers represent the crucial indicators of both physiological and pathological processes. Specific changes in molecular and cellular mechanisms of physiological processes result in biochemical alterations at systemic level, which can give us comprehensive information regarding the nature of any disease. In addition, any disease biomarker should be specific and reliable, able to consent of distinguishing the physiological condition of a tissue, organ, or system from disease, and be diverse among the various diseases, or subgroups or phenotypes of them. Accordingly, biomarkers can predict chances for diseases, facilitate their early diagnosis, and set guidelines for the development of new therapies for treating diseases and disease-making process. Here, we focus our attention on brain neurotrophic factor (BDNF)–tropomyosin receptor kinase (Trk) pathway, describing its multiple roles in the maintenance of central nervous system (CNS) health, as well as its implication in the pathogenesis of multiple sclerosis (MS). In addition, we also evidence the features of such pathway, which make of it a potential MS biomarker and therapeutic target.

Brain-derived neurotrophic factor increases cell number of neural progenitor cells derived from human induced pluripotent stem cells

Background

Several pieces of evidence from in vitro studies showed that brain-derived neurotrophic factor (BDNF) promotes proliferation and differentiation of neural stem/progenitor cells (NSCs) into neurons. Moreover, the JAK2 pathway was proposed to be associated with mouse NSC proliferation. BDNF could activate the STAT-3 pathway and induce proliferation in mouse NSCs. However, its effects on proliferation are not fully understood and JAK/STAT pathway was proposed to play a role in this activity.

Methods

In the present study, the effects of BDNF on cell proliferation and neurite outgrowth of Alzheimer’s disease (AD) induced pluripotent stem cells (iPSCs)-derived human neural progenitor cells (hNPCs) were examined. Moreover, a specific signal transduction pathway important in cell proliferation was investigated using a JAK2 inhibitor (AG490) to clarify the role of that pathway.

Results

The proliferative effect of BDNF was remarkably observed as an increase in Ki-67 positive cells. The cell number of hNPCs was significantly increased after BDNF treatment represented by cellular metabolic activity of the cells measured by MTT assay. This noticeable effect was statistically shown at 20 ng/ml of BDNF treatment. BDNF, however, did not promote neurite outgrowth but increased neuronal cell number. It was found that AG490 suppressed hNPCs proliferation. However, this inhibitor partially decreased BDNF-induced hNPCs proliferation. These results demonstrated the potential role of BDNF for the amelioration of AD through the increase of AD-derived hNPCs number.

Brain-Derived Neurotrophic Factor in Central Nervous System Myelination: A New Mechanism to Promote Myelin Plasticity and Repair

Brain-derived neurotrophic factor (BDNF) plays vitally important roles in neural development and plasticity in both health and disease. Recent studies using mutant mice to selectively manipulate BDNF signalling in desired cell types, in combination with animal models of demyelinating disease, have demonstrated that BDNF not only potentiates normal central nervous system myelination in development but enhances recovery after myelin injury. However, the precise mechanisms by which BDNF enhances myelination in development and repair are unclear. Here, we review some of the recent progress made in understanding the influence BDNF exerts upon the myelinating process during development and after injury, and discuss the cellular and molecular mechanisms underlying its effects. In doing so, we raise new questions for future research.

Thyroxin Protects White Matter from Hypoxic-Ischemic Insult in the Immature Sprague⁻Dawley Rat Brain by Regulating Periventricular White Matter and Cortex BDNF and CREB Pathways

Background: Periventricular white-matter (WM) injury is a prominent feature of brain injury in preterm infants. Thyroxin (T4) treatment reduces the severity of hypoxic-ischemic (HI)-mediated WM injury in the immature brain. This study aimed to delineate molecular events underlying T4 protection following periventricular WM injury in HI rats. Methods: Right common-carotid-artery ligation, followed by hypoxia, was performed on seven-day-old rat pups. The HI pups were injected with saline, or 0.2 or 1 mg/kg of T4 at 48–96 h postoperatively. Cortex and periventricular WM were dissected for real-time (RT)-quantitative polymerase chain reactions (PCRs), immunoblotting, and for immunofluorescence analysis of neurotrophins, myelin, oligodendrocyte precursors, and neointimal. Results: T4 significantly mitigated hypomyelination and oligodendrocyte death in HI pups, whereas angiogenesis of periventricular WM, observed using antiendothelium cell antibody (RECA-1) immunofluorescence and vascular endothelium growth factor (VEGF) immunoblotting, was not affected. T4 also increased the brain-derived neurotrophic factors (BDNFs), but not the nerve growth factor (NGF) expression of injured periventricular WM. However, phosphorylated extracellular signal regulated kinase (p-ERK) and phosphorylated cyclic adenosine monophosphate response element-binding protein (p-CREB) concentrations, but not the BDNF downstream pathway kinases, p38, c-Jun amino-terminal kinase (c-JNK), or Akt, were reduced in periventricular WM with T4 treatment. Notably, T4 administration significantly increased BDNF and phosphorylated CREB in the overlying cortex of the HI-induced injured cortex. Conclusion: Our findings reveal that T4 reversed BNDF signaling to attenuate HI-induced WM injury by activating ERK and CREB pathways in the cortex, but not directly in periventricular WM. This study offers molecular insight into the neuroprotective actions of T4 in HI-mediated WM injury in the immature brain.

p75 Neurotrophin Receptor: A Double-Edged Sword in Pathology and Regeneration of the Central Nervous System

The low-affinity nerve growth factor receptor p75^NTR is a major neurotrophin receptor involved in manifold and pleiotropic functions in the developing and adult central nervous system (CNS). Although known for decades, its entire functions are far from being fully elucidated. Depending on the complex interactions with other receptors and on the cellular context, p75^NTR is capable of performing contradictory tasks such as mediating cell death as well as cell survival. In parallel, as a prototype marker for certain differentiation stages of Schwann cells and related CNS aldynoglial cells, p75^NTR has recently gained increasing notice as a marker for cells with proposed regenerative potential in CNS diseases, such as demyelinating disease and traumatic CNS injury. Besides its pivotal role as a marker for transplantation candidate cells, recent studies in canine neuroinflammatory CNS conditions also highlight a spontaneous endogenous occurrence of p75^NTR-positive glia, which potentially play a role in Schwann cell-mediated CNS remyelination. The aim of the present communication is to review the pleiotropic functions of p75^NTR in the CNS with a special emphasis on its role as an immunohistochemical marker in neuropathology. Following a brief illustration of the expression of p75^NTR in neurogenesis and in developed neuronal populations, the implications of p75^NTR expression in astrocytes, oligodendrocytes, and microglia are addressed. A special focus is put on the role of p75^NTR as a cell marker for specific differentiation stages of Schwann cells and a regeneration-promoting CNS population, collectively referred to as aldynoglia.

To Be or Not to Be: Environmental Factors that Drive Myelin Formation during Development and after CNS Trauma

Oligodendrocytes are specialized glial cells that myelinate central nervous system (CNS) axons. Historically, it was believed that the primary role of myelin was to compactly ensheath axons, providing the insulation necessary for rapid signal conduction. However, mounting evidence demonstrates the dynamic importance of myelin and oligodendrocytes, including providing metabolic support to neurons and regulating axon protein distribution. As such, the development and maintenance of oligodendrocytes and myelin are integral to preserving CNS homeostasis and supporting proper functioning of widespread neural networks. Environmental signals are critical for proper oligodendrocyte lineage cell progression and their capacity to form functional compact myelin; these signals are markedly disturbed by injury to the CNS, which may compromise endogenous myelin repair capabilities. This review outlines some key environmental factors that drive myelin formation during development and compares that to the primary factors that define a CNS injury milieu. We aim to identify developmental factors disrupted after CNS trauma as well as pathogenic factors that negatively impact oligodendrocyte lineage cells, as these are potential therapeutic targets to promote myelin repair after injury or disease.

Candidate molecular pathways of white matter vulnerability in the brain of normal aging rhesus monkeys

Mammalian aging is associated with decline in cognitive functions. Studies searching for a cause of cognitive aging initially focused on neuronal loss but quantitative investigations of rat, monkey, and human brain using stereology demonstrated that in normal aging, unlike in neurodegenerative disease, neurons are not lost. Instead, electron microscopic and MRI studies in normal aging monkeys revealed age-related damage to myelin sheaths, loss of axons, and reduction in white matter volume which correlates with cognitive impairments. However, little is known about the cause of myelin defects or associated axon loss. The present study investigates the effect of age on signaling pathways between oligodendroglia and neurons using a custom PCR array to assess the expression of 87 genes of interest in cortical gray matter and white matter from the inferior parietal lobe (IPL) of normal rhesus monkeys ranging in age from 4.2 to 30.4 years old. From this array data, five target genes of interest were selected for further analysis to confirm gene expression and measure protein expression. The most interesting target gene identified is brain-derived neurotrophic factor (BDNF), which was the only gene that was altered at both mRNA and protein levels. In gray matter, BDNF mRNA was decreased. While the level of the mature form of the protein was unchanged, there was a specific decrease in the precursor form of BDNF. These alterations in the BDNF in gray matter could contribute to the vulnerability and loss of the axons with age.

Bad wrap: Myelin and myelin plasticity in health and disease

Human central nervous system myelin development extends well into the fourth decade of life, and this protracted period underscores the potential for experience to modulate myelination. The concept of myelin plasticity implies adaptability in myelin structure and function in response to experiences during development and beyond. Mounting evidence supports this concept of neuronal activity-regulated changes in myelin-forming cells, including oligodendrocyte precursor cell proliferation, oligodendrogenesis and modulation of myelin microstructure. In healthy individuals, myelin plasticity in associative white matter structures of the brain is implicated in learning and motor function in both rodents and humans. Activity-dependent changes in myelin-forming cells may influence the function of neural networks that depend on the convergence of numerous neural signals on both a temporal and spatial scale. However, dysregulation of myelin plasticity can disadvantageously alter myelin microstructure and result in aberrant circuit function or contribute to pathological cell proliferation. Emerging roles for myelin plasticity in normal neurological function and in disease are discussed. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 123-135, 2018.

Intravenously Infused F3.Olig2 Improves Memory Deficits via Restoring Myelination in the Aged Hippocampus following Experimental Ischemic Stroke

Oligodendrocytes play a crucial role in creating the myelin sheath that is an important component in neural transmission. In an animal model of transient cerebral ischemia, application of oligodendrocyte progenitor cells (OPCs) has not yet been reported. In this study, the effects of F3.Olig2 transplantation on memory and cognitive dysfunction were investigated in the aged gerbil in which ischemic stroke was induced. To investigate the possible mechanisms underlying repair, changes in the expression of myelin basic protein (MBP), oligodendrocyte-specific protein (OSP), and brain-derived neurotrophic factor (BDNF) were examined. Experimental ischemic stroke was induced by occlusion of bilateral common carotid arteries in aged gerbils. Gerbils ( n = 31 per group) were randomly divided into three groups: (1) vehicle sham group, (2) vehicle ischemia group, and (3) F3.Olig2 ischemia group. After 1, 3, and 7 days of ischemia–reperfusion (I-R), saline or F3.Olig2 cells (1 × 10 6 cells in 100 μl) were injected into the gerbils intravenously. The gerbils were sacrificed 10 days after I-R for identification of grafted F3.Olig2 cells, and 15 and 30 days after I-R for tissue analysis after conducting passive avoidance and novel object recognition test. Injected F3.Olig2 cells and MBP, OSP, and BDNF were detected by specific antibodies using immunohistochemistry and/or Western blots. Memory and cognition were significantly increased in the F3.Olig2 ischemia group compared with the vehicle ischemia group. In the F3.Olig2 ischemia group, the neurons were not protected from ischemic damage; however, MBP, OSP, and BDNF expressions were significantly increased. Our results show that injection of F3.Olig2 cells significantly improved impaired memory and cognition, which might be related to increased MBP expression via increasing OSP and BDNF expression in the aged gerbil hippocampus following transient cerebral ischemia.

Intracerebroventricular administration of α-ketoisocaproic acid decreases brain-derived neurotrophic factor and nerve growth factor levels in brain of young rats

Maple syrup urine disease (MSUD) is an inherited aminoacidopathy resulting from dysfunction of the branched-chain keto acid dehydrogenase complex, leading to accumulation of the branched-chain amino acids (BCAA) leucine, isoleucine and valine as well as their corresponding transaminated branched-chain α-ketoacids. This disorder is clinically characterized by ketoacidosis, seizures, coma, psychomotor delay and mental retardation whose pathophysiology is not completely understood. Recent studies have shown that oxidative stress may be involved in neuropathology of MSUD. However, the effect of accumulating α-ketoacids in MSUD on neurotrophic factors has not been investigated. Thus, the objective of the present study was to evaluate the effects of acute intracerebroventricular administration of α-ketoisocaproic acid (KIC) on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) levels in the brains of young male rats. Ours results showed that intracerebroventricular administration of KIC decreased BDNF levels in hippocampus, striatum and cerebral cortex, without induce a detectable change in pro-BDNF levels. Moreover, NGF levels in the hippocampus were reduced after intracerebroventricular administration of KIC. In conclusion, these data suggest that the effects of KIC on demyelination and memory processes may be mediated by reduced trophic support of BDNF and NGF. Moreover, lower levels of BDNF and NGF are consistent with the hypothesis that a deficit in this neurotrophic factor may contribute to the structural and functional alterations of brain underlying the psychopathology of MSUD, supporting the hypothesis of a neurodegenerative process in MSUD.

Patterns of anterior versus posterior white matter fractional anistotropy concordance in adult nonhuman primates: Effects of early life stress

INTRODUCTION

Functional neuroimaging studies report global prefrontal dysconnectivity in mood disorders, supporting the notion of widespread disruptions in brain networks. Microscopic alterations in white matter (WM) tracts - which possess neuroplastic properties and play a central role in brain connectivity - are interrogated herein in the context of brain dysconnectivity. Early life stress (ELS), an antecedent to human mood disorders, induces WM alterations in volumetrics and integrity. We hypothesized that nonhuman primate infants exposed to ELS would exhibit persistent impairments in both frontal and posterior concordance of WM integrity, therefore contributing to global brain dysconnectivity.

METHODS

Using a 3T MRI, diffusion tensor imaging (DTI) was performed on 21 adult male Bonnet macaques, 12 of whom had been raised under variable foraging demand (VFD) conditions and nine of whom had been raised under normative conditions (Non-VFD). As representative of anterior regions, fractional anisotropy (FA) concordance between anterior corpus callosum (ACorpusC) and anterior limb of the internal capsule (ALIC) was examined. For posterior regions, FA concordance between posterior corpus callosum (PCorpusC) and posterior limb of the internal capsule (PLICA) and between PCorpusC and occipital WM was examined. Examination of posterior FA was explored in the context of frontal markers of neuroplasticity.

RESULTS

A concordant relationship for FA between left ALIC and ACorpusC was evident in Non-VFD-reared subjects, but significantly absent in VFD-reared subjects. For left posterior regions, FA concordance between PLICA and PCorpusC and occipital WM and PCorpusC was evident in VFD-reared and not Non-VFD-reared subjects. The posterior concordance in VFD was significantly distinguishable from the deficit in anterior concordance FA in VFD.

CONCLUSIONS

The findings support the view that disrupted emotional integrity of the maternal-infant attachment process affects normative synchronous development of frontal white matter tracts but creates errant posterior concordance and also disrupts an inverse relationship between posterior white matter tracts and markers of neuroplasticity. We provide preliminary evidence that a concordant relationship between capsular-callosal FA may become discordant, providing a putative mechanism for prefrontal functional brain dysconnectivity.

PSA-NCAM positive neural progenitors stably expressing BDNF promote functional recovery in a mouse model of spinal cord injury

Background

Neural stem cells for the treatment of spinal cord injury (SCI) are of particular interest for future therapeutic use. However, until now, stem cell therapies are often limited due to the inhibitory environment following the injury. Therefore, in this study, we aimed at testing a combinatorial approach with BDNF (brain-derived neurotrophic factor) overexpressing early neural progenitors derived from mouse embryonic stem cells. BDNF is a neurotrophin, which both facilitates neural differentiation of stem cells and favors regeneration of damaged axons.

Methods

Mouse embryonic stem cells, modified to stably express BDNF-GFP, were differentiated into PSA-NCAM positive progenitors, which were enriched, and SSEA1 depleted by a sequential procedure of magnetic-activated and fluorescence-activated cell sorting. Purified cells were injected into the lesion core seven days after contusion injury of the spinal cord in mice, and the Basso mouse scale (BMS) test to evaluate motor function was performed for 5 weeks after transplantation. To analyze axonal regeneration the anterograde tracer biotinylated dextran amine was injected into the sensorimotor cortex two weeks prior to tissue analysis. Cellular differentiation was analyzed by immunohistochemistry of spinal cord sections.

Results

Motor function was significantly improved in animals obtaining transplanted BDNF-GFP-overexpressing cells as compared to GFP-expressing cells and vehicle controls. Stem cell differentiation in vivo revealed an increase of neuronal and oligodendrocytic lineage differentiation by BDNF as evaluated by immunohistochemistry of the neuronal marker MAP2 (microtubule associated protein 2) and the oligodendrocytic markers ASPA (aspartoacylase) and Olig2 (oligodendrocyte transcription factor 2). Furthermore, axonal tracing showed a significant increase of biotin dextran amine positive corticospinal tract fibers in BDNF-GFP-cell transplanted animals caudally to the lesion site.

Conclusions

The combinatorial therapy approach by transplanting BDNF-overexpressing neural progenitors improved motor function in a mouse contusion model of SCI. Histologically, we observed enhanced neuronal and oligodendrocytic differentiation of progenitors as well as enhanced axonal regeneration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0268-x) contains supplementary material, which is available to authorized users.

Selective reduction of striatal mature BDNF without induction of proBDNF in the zQ175 mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder characterized by massive loss of medium spiny neurons in the striatum. However, the mechanisms by which mutant huntingtin leads to this selective neuronal death remain incompletely understood. Brain-derived neurotrophic factor (BDNF) has been shown to be neuroprotective on HD striatal neurons both in vitro and in vivo. ProBDNF, the precursor of mature BDNF (mBDNF), also can be secreted but promotes apoptosis of neurons expressing p75(NTR) and sortilin receptors. Although a reduction of total striatal BDNF protein has been reported in HD patients and mouse models, it remains unclear whether conversion of proBDNF to mBDNF is altered in HD, and whether the proBDNF receptors, p75(NTR) and sortilin are dysregulated, leading to impaired striatal neuron survival. To test these hypotheses, we generated bdnf-HA knock-in (KI) mice on the zQ175 HD background to accurately quantitate the levels of both proBDNF and mBDNF in the HD striatum. In aged zQ175 HD mice, we observed a significant loss of mBDNF and decreased TrkB activation, but no increase of proBDNF or p75(NTR) levels either in the sensorimotor cortex or the striatum. However, immunoreactivities of p75(NTR) and sortilin receptor are both increased in immature striatal oligodendrocytes, which associate with significant myelin defects in the HD striatum. Taken together, the present study indicates that diminished mature BDNF trophic signaling through the TrkB receptor, rather than an induction in proBDNF, is a main contributing factor to the vulnerability of striatal neurons in the zQ175 HD mouse model.

18β-glycyrrhetinic acid suppresses experimental autoimmune encephalomyelitis through inhibition of microglia activation and promotion of remyelination

Microglia are intrinsic immune cells in the central nervous system (CNS). The under controlled microglia activation plays important roles in inflammatory demyelination diseases, such as multiple sclerosis (MS). However, the means to modulate microglia activation as a therapeutic modality and the underlying mechanisms remain elusive. Here we show that administration of 18β-glycyrrhetinic acid (GRA), by using both preventive and therapeutic treatment protocols, significantly suppresses disease severity of experimental autoimmune encephalomyelitis (EAE) in C57BL/6 mice. The treatment effect of GRA on EAE is attributed to its regulatory effect on microglia. GRA-modulated microglia significantly decreased pro-inflammatory profile in the CNS through suppression of MAPK signal pathway. The ameliorated CNS pro-inflammatory profile prevented the recruitment of encephalitogenic T cells into the CNS, which alleviated inflammation-induced demyelination. In addition, GRA treatment promoted remyelination in the CNS of EAE mice. The induced remyelination can be mediated by the overcome of inflammation-induced blockade of brain-derived neurotrophic factor expression in microglia, as well as enhancing oligodendrocyte precursor cell proliferation. Collectively, our results demonstrate that GRA-modulated microglia suppresses EAE through inhibiting microglia activation-mediated CNS inflammation, and promoting neuroprotective effect of microglia, which represents a potential therapeutic strategy for MS and maybe other neuroinflammatory diseases associated with microglia activation.

The thrombin receptor is a critical extracellular switch controlling myelination

Hemorrhagic white matter injuries in the perinatal period are a growing cause of cerebral palsy yet no neuroprotective strategies exist to prevent the devastating motor and cognitive deficits that ensue. We demonstrate that the thrombin receptor (protease-activated receptor 1, PAR1) exhibits peak expression levels in the spinal cord at term and is a critical regulator of the myelination continuum from initiation to the final levels achieved. Specifically, PAR1 gene deletion resulted in earlier onset of spinal cord myelination, including substantially more Olig2-positive oligodendrocytes, more myelinated axons, and higher proteolipid protein (PLP) levels at birth. In vitro, the highest levels of PAR1 were observed in oligodendrocyte progenitor cells (OPCs), being reduced with differentiation. In parallel, the expression of PLP and myelin basic protein (MBP), in addition to Olig2, were all significantly higher in cultures of PAR1-/- oligodendroglia. Moreover, application of a small molecule inhibitor of PAR1 (SCH79797) to OPCs in vitro increased PLP and MBP expression. Enhancements in myelination associated with PAR1 genetic deletion were also observed in adulthood as evidenced by higher amounts of MBP and thickened myelin sheaths across large, medium, and small diameter axons. Enriched spinal cord myelination in PAR1-/- mice was coupled to increases in extracellular-signal-regulated kinase 1/2 and AKT signaling developmentally. Nocturnal ambulation and rearing activity were also elevated in PAR1-/- mice. These studies identify the thrombin receptor as a powerful extracellular regulatory switch that could be readily targeted to improve myelin production in the face of white matter injury and disease.

Transcriptional Regulation of Brain-Derived Neurotrophic Factor (BDNF) by Methyl CpG Binding Protein 2 (MeCP2): a Novel Mechanism for Re-Myelination and/or Myelin Repair Involved in the Treatment of Multiple Sclerosis (MS)

Multiple sclerosis (MS) is a chronic progressive, neurological disease characterized by the targeted immune system-mediated destruction of central nervous system (CNS) myelin. Autoreactive CD4+ T helper cells have a key role in orchestrating MS-induced myelin damage. Once activated, circulating Th1-cells secrete a variety of inflammatory cytokines that foster the breakdown of blood-brain barrier (BBB) eventually infiltrating into the CNS. Inside the CNS, they become reactivated upon exposure to the myelin structural proteins and continue to produce inflammatory cytokines such as tumor necrosis factor α (TNFα) that leads to direct activation of antibodies and macrophages that are involved in the phagocytosis of myelin. Proliferating oligodendrocyte precursors (OPs) migrating to the lesion sites are capable of acute remyelination but unable to completely repair or restore the immune system-mediated myelin damage. This results in various permanent clinical neurological disabilities such as cognitive dysfunction, fatigue, bowel/bladder abnormalities, and neuropathic pain. At present, there is no cure for MS. Recent remyelination and/or myelin repair strategies have focused on the role of the neurotrophin brain-derived neurotrophic factor (BDNF) and its upstream transcriptional repressor methyl CpG binding protein (MeCP2). Research in the field of epigenetic therapeutics involving histone deacetylase (HDAC) inhibitors and lysine acetyl transferase (KAT) inhibitors is being explored to repress the detrimental effects of MeCP2. This review will address the role of MeCP2 and BDNF in remyelination and/or myelin repair and the potential of HDAC and KAT inhibitors as novel therapeutic interventions for MS.

Role of p75 neurotrophin receptor in stem cell biology: more than just a marker

p75(NTR), the common receptor for both neurotrophins and proneurotrophins, has been widely studied because of its role in many tissues, including the nervous system. More recently, a close relationship between p75(NTR) expression and pluripotency has been described. p75(NTR) was shown to be expressed in various types of stem cells and has been used to prospectively isolate stem cells with different degrees of potency. Here, we give an overview of the current knowledge on p75(NTR) in stem cells, ranging from embryonic to adult stem cells, and cancer stem cells. In an attempt to address its potential role in the control of stem cell biology, the molecular mechanisms underlying p75(NTR) signaling in different models are also highlighted. p75(NTR)-mediated functions include survival, apoptosis, migration, and differentiation, and depend on cell type, (pro)neurotrophin binding, interacting transmembrane co-receptors expression, intracellular adaptor molecule availability, and post-translational modifications, such as regulated proteolytic processing. It is therefore conceivable that p75(NTR) can modulate cell-fate decisions through its highly ramified signaling pathways. Thus, elucidating the potential implications of p75(NTR) activity as well as the underlying molecular mechanisms of p75(NTR) will shed new light on the biology of both normal and cancer stem cells.

α-Synuclein impairs oligodendrocyte progenitor maturation in multiple system atrophy

Multiple system atrophy (MSA), an atypical parkinsonian disorder, is characterized by α-synuclein (α-syn(+)) cytoplasmatic inclusions in mature oligodendrocytes. Oligodendrocyte progenitor cells (OPCs) represent a distinct cell population with the potential to replace dysfunctional oligodendrocytes. However, the role of OPCs in MSA and their potential to replace mature oligodendrocytes is still unclear. A postmortem analysis in MSA patients revealed α-syn within OPCs and an increased number of striatal OPCs. In an MSA mouse model, an age-dependent increase of dividing OPCs within the striatum and the cortex was detected. Despite of myelin loss, there was no reduction of mature oligodendrocytes in the corpus callosum or the striatum. Dissecting the underlying molecular mechanisms an oligodendroglial cell line expressing human α-syn revealed that α-syn delays OPC maturation by severely downregulating myelin-gene regulatory factor and myelin basic protein. Brain-derived neurotrophic factor was reduced in MSA models and its in vitro supplementation partially restored the phenotype. Taken together, efficacious induction of OPC maturation may open the window to restore glial and neuronal function in MSA.

TAp73 knockout mice show morphological and functional nervous system defects associated with loss of p75 neurotrophin receptor

Total and N-terminal isoform selective p73 knockout mice show a variety of central nervous system defects. Here we show that TAp73 is a transcriptional activator of p75 neurotrophin receptor (p75(NTR)) and that p75(NTR) mRNA and protein levels are strongly reduced in the central and peripheral nervous systems of p73 knockout mice. In parallel, primary cortical neurons from p73 knockout mice showed a reduction in neurite outgrowth and in nerve growth factor-mediated neuronal differentiation, together with reduced miniature excitatory postsynaptic current frequencies and behavioral defects. p73 null mice also have impairments in the peripheral nervous system with reduced thermal sensitivity, axon number, and myelin thickness. At least some of these morphological and functional impairments in p73 null cells can be rescued by p75(NTR) re-expression. Together, these data demonstrate that loss of p75(NTR) contributes to the neurological phenotype of p73 knockout mice.

proNGF inhibits proliferation and oligodendrogenesis of postnatal hippocampal neural stem/progenitor cells through p75NTR in vitro

Neural stem/progenitor cells (NSCs) proliferate and differentiate under tight regulation by various factors in the stem cell niche. Recent studies have shown that the precursor of nerve growth factor (NGF), proNGF, abounds in the central nervous system (CNS) and that its expression level in the brain is substantially elevated with aging as well as in several types of CNS disorders. In this study, we found for the first time that proNGF inhibited the proliferation of NSCs isolated from postnatal mouse hippocampus and caused cell cycle arrest in the G0/G1 phase without affecting apoptosis. In addition, proNGF reduced the differentiation of NSCs to oligodendrocytes. The effects of proNGF were blocked by the fusion protein of p75 neurotrophin receptor extracellular domain and human IgG Fc fragment (p75NTR/Fc), and by p75NTR knockout, suggesting that proNGF/p75NTR interaction was involved in the effects of proNGF on NSC proliferation and differentiation. proNGF decreased the phosphorylation level of extracellular signal responsive kinase 1/2 (ERK 1/2) in a p75NTR-dependent manner under both self-renewal and differentiation conditions. The inhibition of ERK 1/2 phosphorylation by U0126 significantly reduced the proliferation and oligodendrogenesis of NSCs, indicating that ERK 1/2 inhibition by proNGF partially explains its effects on NSC proliferation and oligodendrogenesis. These results suggest that the proNGF/p75NTR signal plays a key role in the regulation of NSCs' behavior.

Estrogen receptor β ligand therapy activates PI3K/Akt/mTOR signaling in oligodendrocytes and promotes remyelination in a mouse model of multiple sclerosis

The identification of a drug that stimulates endogenous myelination and spares axon degeneration during multiple sclerosis (MS) could potentially reduce the rate of disease progression. Using experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, we have previously shown that prophylactic administration of the estrogen receptor (ER) β ligand 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN) decreases clinical disease, is neuroprotective, stimulates endogenous myelination, and improves axon conduction without altering peripheral cytokine production or reducing central nervous system (CNS) inflammation. Here, we assessed the effects of therapeutic DPN treatment during peak EAE disease, which represents a more clinically relevant treatment paradigm. In addition, we investigated the mechanism of action of DPN treatment-induced recovery during EAE. Given that prophylactic and therapeutic treatments with DPN during EAE improved remyelination-induced axon conduction, and that ER (α and β) and membrane (m)ERs are present on oligodendrocyte lineage cells, a direct effect of treatment on oligodendrocytes is likely. DPN treatment of EAE animals resulted in phosphorylated ERβ and activated the phosphatidylinositol 3-kinase (PI3K)/serine-threonine-specific protein kinase (Akt)/mammalian target of rapamycin (mTOR) signaling pathway, a pathway required for oligodendrocyte survival and axon myelination. These results, along with our previous studies of prophylactic DPN treatment, make DPN and similar ERβ ligands immediate and favorable therapeutic candidates for demyelinating disease.

LINGO-1, a transmembrane signaling protein, inhibits oligodendrocyte differentiation and myelination through intercellular self-interactions

Overcoming remyelination failure is a major goal of new therapies for demyelinating diseases like multiple sclerosis. LINGO-1, a key negative regulator of myelination, is a transmembrane signaling protein expressed in both neurons and oligodendrocytes. In neurons, LINGO-1 is an integral component of the Nogo receptor complex, which inhibits axonal growth via RhoA. Because the only ligand-binding subunit of this complex, the Nogo receptor, is absent in oligodendrocytes, the extracellular signals that inhibit myelination through a LINGO-1-mediated mechanism are unknown. Here we show that LINGO-1 inhibits oligodendrocyte terminal differentiation through intercellular interactions and is capable of a self-association in trans. Consistent with previous reports, overexpression of full-length LINGO-1 inhibited differentiation of oligodendrocyte precursor cells (OPCs). Unexpectedly, treatment with a soluble recombinant LINGO-1 ectodomain also had an inhibitory effect on OPCs and decreased myelinated axonal segments in cocultures with neurons from dorsal root ganglia. We demonstrated LINGO-1-mediated inhibition of OPCs through intercellular signaling by using a surface-bound LINGO-1 construct expressed ectopically in astrocytes. Further investigation showed that the soluble LINGO-1 ectodomain can interact with itself in trans by binding to CHO cells expressing full-length LINGO-1. Finally, we observed that soluble LINGO-1 could activate RhoA in OPCs. We propose that LINGO-1 acts as both a ligand and a receptor and that the mechanism by which it negatively regulates OPC differentiation and myelination is mediated by a homophilic intercellular interaction. Disruption of this protein-protein interaction could lead to a decrease of LINGO-1 inhibition and an increase in myelination.

Levels of BDNF impact oligodendrocyte lineage cells following a cuprizone lesion

Previous work in culture has shown that basal forebrain (BF) oligodendrocyte (OLG) lineage cells respond to BDNF by increasing DNA synthesis and differentiation. Further, in the BF in vivo, reduced levels of BDNF as seen in BDNF(+/-) mice result in reduced numbers of NG2+ cells and deficits in myelin proteins throughout development and in the adult, suggesting that BDNF impacts the proliferating population of OLGs as well as differentiation in vivo. In this study, to investigate the roles BDNF may play in the repair of a demyelinating lesion, the cuprizone model was used and the corpus callosum was examined. BDNF protein levels were reduced after cuprizone treatment, suggesting that the demyelinating lesion itself elicits a decrease in BDNF. To analyze the effects of a further reduction of BDNF on OLG lineage cells following cuprizone, BDNF(+/-) mice were evaluated. These mice exhibited a blunted increase in the NG2 response at 4 and 5 weeks of cuprizone treatment. In addition, BDNF(+/-) mice exhibited decreased levels of myelin proteins during the demyelination and remyelination processes with no change in the total number of OLGs. These effects appear to be relatively specific to OLG lineage cells as comparable changes in CD11b+ microglia, GFAP+ astrocytes, and SMI32+ injured axons were not observed. These data indicate that BDNF may play a role following a demyelinating lesion by regulating the numbers of progenitors and the abilities of demyelinating and differentiating cells to express myelin proteins.

Brain-derived neurotrophic factor promotes central nervous system myelination via a direct effect upon oligodendrocytes

The extracellular factors that are responsible for inducing myelination in the central nervous system (CNS) remain elusive. We investigated whether brain-derived neurotrophic factor (BDNF) is implicated, by first confirming that BDNF heterozygous mice exhibit delayed CNS myelination during early postnatal development. We next established that the influence of BDNF upon myelination was direct, by acting on oligodendrocytes, using co-cultures of dorsal root ganglia neurons and oligodendrocyte precursor cells. Importantly, we found that BDNF retains its capacity to enhance myelination of neurons or by oligodendrocytes derived from p75NTR knockout mice, indicating the expression of p75NTR is not necessary for BDNF-induced myelination. Conversely, we observed that phosphorylation of TrkB correlated with myelination, and that inhibiting TrkB signalling also inhibited the promyelinating effect of BDNF, suggesting that BDNF enhances CNS myelination via activating oligodendroglial TrkB-FL receptors. Together, our data reveal a previously unknown role for BDNF in potentiating the normal development of CNS myelination, via signalling within oligodendrocytes.

Direct and indirect effects of neuropeptide Y and neurotrophin 3 on myelination in the neonatal brains

Neuropeptide Y (NPY) is expressed in the developing central nervous system, however, its role in the brain development remains unclear. In this study, C57/B6 mice were intraperitoneally administered 1 nmol/capita/day of NPY, 10 nmol/capita/day of an NPY-receptor 1-specific antagonist (Y1R-A), or NPY and Y1R-A simultaneously (NPY+Y1R-A) from postnatal day (P) 7 to P14. Recombinant NPY reached the P14 cerebrum in 1 hour. These treatments didn't significantly affect body weight gain or P14 brain weight. The ratio of myelinated axons to total axons in the parietal cerebrum was significantly higher in the NPY group than in the control group. The expression of myelin basic protein (MBP)-mRNA in the cerebrum was significantly higher in the NPY group than in the control group and was significantly lower in the NPY+Y1R-A group than in the NPY group, while it was significantly higher in the NPY+Y1R-A group than in the control group. In cultured oligodendroglioma-derived B12 cells, NPY didn't influence the MBP-mRNA expression, while neurotrophin 3 (NT3) increased MBP mRNA via receptor-type tyrosine kinase type C (Trk C). NPY administration significantly increased NT3-mRNA expression in the P14 cerebrum as deduced by quantitative real-time PCR. The change in phosphorylated Trk C (P-Trk C) was proportional to that of the NT3-mRNA expression, and the proportion of P-Trk C was higher in the NPY group than in the control group. These results suggest that NPY, partially via Y1R, induces NT3 which, via Trk C phosphorylation, accelerates myelination by oligodendrocytes in the mouse brain during the neonatal period.

BDNF+/- mice exhibit deficits in oligodendrocyte lineage cells of the basal forebrain

Previous work indicated that brain-derived neurotrophic factor (BDNF), through the trkB receptor, increases DNA synthesis in oligodendrocyte (OLG) progenitor cells (OPCs) and differentiation of postmitotic OLGs of the basal forebrain (BF). In the present studies, BDNF knockout animals were used to investigate BDNF's effects on OLG lineage cells (OLCs) in vivo. OLCs of the BF were found to express the trkB receptor, suggesting they are responsive to BDNF. Immunohistochemistry using NG2 and CC1 antibodies was utilized to examine the numbers of NG2+ OPCs and CC1+ postmitotic BF OLGs. At embryonic day 17 (E17), BDNF-/- animals display reduced NG2+ cells. This reduction was also observed in BDNF+/- mice at E17 and at postnatal day 1 (P1), P14, and adult stage, suggesting that BDNF plays a role in OPC development. BDNF+/- mice do not exhibit deficits in numbers of CC1+ OLGs. However, myelin basic protein, myelin associated glycoprotein, and proteolipid protein are reduced in BDNF+/- mice, suggesting that BDNF plays a role in differentiation. These data indicate that progenitor cells and myelin proteins may be affected in vivo by a decrease in BDNF.

Intravitreal NGF administration counteracts retina degeneration after permanent carotid artery occlusion in rat

Background

The neurotrophin nerve growth factor (NGF) is produced by different cell types in the anterior and posterior eye, exerting a neuroprotective role in the adult life. The visual system is highly sensitive to NGF and the retina and optic nerve provides suitable subjects for the study of central nervous system degeneration. The model of bilateral carotid occlusion (two-vessel occlusion, 2VO) is a well-established model for chronic brain hypoperfusion leading to brain capillary pathology, to retina and optic nerve degeneration. In order to study if a single intravitreal injection of NGF protects the retina and the optic nerve from degeneration during systemic circulatory diseases, we investigated morphological and molecular changes occurring in the retina and optic nerve of adult rats at different time-points (8, 30 and 75 days) after bilateral carotid occlusion.

Results

We demonstrated that a single intravitreal injection of NGF (5 μg/3 μl performed 24 hours after 2VO ligation) has a long-lasting protective effect on retina and optic nerve degeneration. NGF counteracts retinal ganglion cells degeneration by early affecting Bax/Bcl-2 balance- and c-jun- expression (at 8 days after 2VO). A single intravitreal NGF injection regulates the demyelination/remyelination balance after ischemic injury in the optic nerve toward remyelination (at 75 days after 2VO), as indicated by the MBP expression regulation, thus preventing optic nerve atrophy and ganglion cells degeneration. At 8 days, NGF does not modify 2VO-induced alteration in VEFG and related receptors mRNA expression.

Conclusion

The protective effect of exogenous NGF during this systemic circulatory disease seems to occur also by strengthening the effect of endogenous NGF, the synthesis of which is increased by vascular defect and also by the mechanical lesion associated with NGF or even vehicle intraocular delivery.

Brain-derived neurotrophic factor effects on oligodendrocyte progenitors of the basal forebrain are mediated through trkB and the MAP kinase pathway

Previous work has indicated that BDNF increases the differentiation of basal forebrain (BF) oligodendrocytes (OLGs) in culture through the mediation of trkB and the MAPK pathway (Du et al. [ 2006a, b] Mol. Cell. Neurosci. 31:366-375; J. Neurosci. Res. 84:1692-1702). In the present work, effects of BDNF on BF OLG progenitor cells (OPCs) were examined. BDNF increased DNA synthesis of OPCs, as assessed by thymidine and bromodeoxyuridine incorporation. Effects of BDNF on DNA synthesis were mediated through the trkB receptor and not the p75 receptor, as shown by inhibitors that block neurotrophin binding to the receptors and by the phosphorylation of trkB. TrkB can activate the mitogen- activated protein kinase (MAPK), phosphatidylinositol-3 kinase (PI3-K), and phospholipase C-gamma (PLC-gamma) pathways. BDNF elicited the phosphorylation of MAPK and Akt, a kinase downstream of PI3K, but not PLC-gamma in OPCs. Through the use of specific inhibitors to the MAPK and PI3-K pathways, it was found that the MAPK pathway was responsible for the effect of BDNF on DNA synthesis. These data indicate that BDNF affects OPC proliferation and development through the mediation of trkB and the MAPK pathway.

Role of p75NTR in female rat urinary bladder with cyclophosphamide-induced cystitis

Previous studies demonstrated changes in urinary bladder neurotrophin content and upregulation of neurotrophin receptors, TrkA and the p75 neurotrophin receptor (p75(NTR)), in micturition reflex pathways after cyclophosphamide (CYP)-induced cystitis. p75(NTR) can bind nerve growth factor (NGF) and modulate NGF-TrkA binding and signaling. We examined p75(NTR) expression and the role of p75(NTR) in the micturition reflex in control and CYP-treated rats. p75(NTR) Immunoreactivity was present throughout the urinary bladder. CYP-induced cystitis (4 h, 48 h, chronic) increased (P < or = 0.05) p75(NTR) expression in whole urinary bladder as shown by Western blotting. The role of p75(NTR) in bladder function in control and CYP-treated rats was determined using conscious cystometry and immunoneutralization or PD90780, a compound known to specifically block NGF binding to p75(NTR). An anti-p75(NTR) monoclonal antibody or PD90780 was infused intravesically and cystometric parameters were evaluated. Both methods of p75(NTR) blockade significantly (P < or = 0.05) decreased the intercontraction interval and void volume in control and CYP-treated rats. Intravesical infusion of PD90780 also significantly (P < or = 0.001) increased intravesical pressure and increased the number of nonvoiding contractions during the filling phase. Control intravesical infusions of isotype-matched IgG and vehicle were without effect. Intravesical instillation of PD90780 significantly (P < or = 0.01) reduced the volume threshold to elicit a micturition contraction in control rats (no inflammation) and CYP-treated in a closed urinary bladder system. These studies demonstrate 1) ubiquitous p75(NTR) expression in urinary bladder and increased expression with CYP-induced cystitis and 2) p75(NTR) blockade at the level of the urinary bladder produces bladder hyperreflexia in control and CYP-treated rats. The overall activity of the urinary bladder reflects the balance of NGF-p75(NTR) and NGF-TrkA signaling.

The function of p75NTR in glia

The p75 neurotrophin receptor (p75(NTR)) is expressed on many cell types and can influence a variety of cellular functions. This receptor can mediate cell survival or cell death, can promote or inhibit axonal growth and can facilitate or attenuate proliferation, depending on the cell context. The emerging picture regarding p75(NTR) indicates that it can partner with different coreceptors to dictate specific responses. It then signals by recruiting intracellular binding proteins to activate different signaling pathways. The function of p75(NTR) has mainly been studied in neurons; however, it is also expressed in a variety of glial populations, especially during development and after injury, where its roles have been poorly defined. In this review, we will examine the potential roles for p75(NTR) in glial function.

The role of neurotrophins during early development

The effects of neurotrophins during the middle and late stages of development are well known. It was previously thought that neurotrophins had no role during early development, but this is not the case and is the subject of this review article. The earliest neurotrophin receptor expressed is that for neurotrophin-3 (NT-3). TrkC is detected in the neural plate and is present in the neural tube. Initially, the distribution of TrkC is homogenous, but it becomes localized to specific regions of the neural tube as the neural tube differentiates. The receptor for brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5), TrkB, is detected somewhat later than TrkC in the neural tube where it is also differentially localized. In contrast, the NGF receptor, TrkA, was not detected during early development. Both NT-3 and BDNF have been shown to have effects in vitro during early development. NT-3 caused an increase in neurite outgrowth and apoptosis in neural plate explants, and promoted differentiation of progenitors into motoneurons. BDNF increased the number of motoneurons in neural tube explants. These data suggest that NT-3 and BDNF may play a role during early development in vivo.

Growth factor regulation of remyelination: behind the growing interest in endogenous cell repair of the CNS

Remyelination facilitates recovery of saltatory conduction along demyelinated axons and may help prevent axon damage in patients with demyelinating diseases, such as multiple sclerosis. The extent of remyelination in multiple sclerosis lesions varies dramatically, indicating a capacity for repair that is not fulfilled in lesions with poor remyelination. In experimental models of demyelinating disease, remyelination is limited by chronic disease that depletes the oligodendrocyte progenitor (OP) population, inhibits OP differentiation into remyelinating oligodendrocytes and/or perturbs cell survival in the lesion environment. Manipulating the activity of growth factor signaling pathways significantly improves the ability of endogenous OP cells to accomplish extensive remyelination. Specifically, growth factors have been identified that can regulate OP proliferation, differentiation and survival in demyelinated lesions. Therefore, growth factors may be key signals for strategies to improve conditions with poor remyelination.

Effects of acute and repeated exposure to lipopolysaccharide on cytokine and corticosterone production during remyelination

Chronic exposure to the copper-chelating agent, cuprizone (CPZ), is an increasingly popular model for producing demyelination. More importantly, cessation of cuprizone exposure allows for full remyelination, which represents a window of opportunity for determining the influence of environmental factors on regenerative processes. In the present study, CPZ-treated animals were assessed for functional status of systemic and central cytokine responsiveness to LPS, as well as assessment for signs of body weight changes. Exposure of male C57BL/6J mice to 5 weeks of 0.2% CPZ in the diet was optimal in producing demyelination and microglial activation, as measured by myelin basic protein, CD11b, and CD45 immunohistochemistry. Acute challenge with LPS at the end of 5 weeks CPZ treatment did not alter IL-1beta, IL-6, nor TNFalpha responses in the spleen and corpus callosum. Similarly, repeated exposure to LPS during the remyelination phase (CPZ removal) did not influence these measures to LPS. Plasma corticosterone was unaffected following acute challenge of CPZ-pretreated animals, but after repeated LPS treatment, there was a significant augmentation of the corticosterone response in CPZ-pretreated mice. Interestingly, the basal concentration of IL-1beta in the corpus callosum of CPZ treated animals was significantly increased, which was in keeping with the increase in activated microglial cells. In conclusion, the cuprizone model of demyelination and remyelination does not appear to influence the systemic nor central IL-1, IL-6, and TNF responses to acute nor repeated LPS. This opens up the possibility for studying the contribution of systemic inflammatory processes on remyelination after cessation of CPZ treatment.

Mitogen-activated protein kinase pathway mediates effects of brain-derived neurotrophic factor on differentiation of basal forebrain oligodendrocytes

Previous studies indicate that brain-derived neurotrophic factor (BDNF), through the mediation of the trkB receptor, modulates the expression of differentiated traits in basal forebrain (BF) oligodendrocytes (OLGs). Specifically, BDNF up-regulates the expression of myelin basic protein (MBP), proteolipid protein (PLP), and myelin associated glycoprotein (MAG; Du et al. [2006] Mol. Cell. Neurosci. 31:366-375). However, the signaling cascades mediating the effects of BDNF have not been defined. The current study employs biochemical and molecular biological approaches to examine the involvements of the mitogen-activated protein kinase (MAPK) pathway, the phosphatidylinositol-3 kinase (PI3K) pathway, and the phospholipase C-gamma (PLC-gamma) pathway. Our results indicate that, in BF OLGs, BDNF activates the MAPK pathway and the PLC-gamma pathway but not the PI3K-Akt signaling cascade. By using specific inhibitors and mutated dominant negative or constitutively active forms of MAPK kinase, we demonstrate that the MAPK pathway is mediating the effects of BDNF on expression of differentiated traits in BF OLGs.

The quest for remyelination: a new role for neurotrophins and their receptors

The formation of myelin is dependent on a reciprocal and intimate relationship between neurons and the myelin-forming glia. Recently, the neurotrophin family of growth factors has been shown to regulate the complex cell-cell interactions that control myelination. Neurotrophins and their receptors influence myelin formation via two distinct mechanisms, either by acting on the neurons, changing the axonal signals that control myelination, or by acting directly on the myelin-forming glia. In this review, we will discuss research highlighting the ability of neurotrophins to both promote and inhibit the myelination process. As reflected in the work presented here, these effects are dependent on a delicate balance of which neurotrophins are expressed, and what receptors are activated. Additionally, we examine an emerging model in which the growth factors that promote the early survival and differentiation of particular sets of neurons later play important roles as key regulators in glial development. Characterizing the temporal expression and the cellular targets of neurotrophins, both during development and after injury, represents a pivotal step in developing a greater understanding of the myelination process, contributing to the development of effective treatments for demyelinating conditions. We conclude this review by discussing the potential for neurotrophins as therapeutics in the quest for remyelination.