Expression of nerve growth factor receptor mRNA is developmentally regulated and increased after axotomy in rat spinal cord motoneurons

Biofluid Biomarkers in the Prognosis of Amyotrophic Lateral Sclerosis: Recent Developments and Therapeutic Applications

Amyotrophic lateral sclerosis is a neurodegenerative disease characterized by the degeneration of motor neurons for which effective therapies are lacking. One of the most explored areas of research in ALS is the discovery and validation of biomarkers that can be applied to clinical practice and incorporated into the development of innovative therapies. The study of biomarkers requires an adequate theoretical and operational framework, highlighting the "fit-for-purpose" concept and distinguishing different types of biomarkers based on common terminology. In this review, we aim to discuss the current status of fluid-based prognostic and predictive biomarkers in ALS, with particular emphasis on those that are the most promising ones for clinical trial design and routine clinical practice. Neurofilaments in cerebrospinal fluid and blood are the main prognostic and pharmacodynamic biomarkers. Furthermore, several candidates exist covering various pathological aspects of the disease, such as immune, metabolic and muscle damage markers. Urine has been studied less often and should be explored for its possible advantages. New advances in the knowledge of cryptic exons introduce the possibility of discovering new biomarkers. Collaborative efforts, prospective studies and standardized procedures are needed to validate candidate biomarkers. A combined biomarkers panel can provide a more detailed disease status.

P75 neurotrophin receptor as a therapeutic target for drug development to treat neurological diseases

The interaction of neurotrophins with their receptors is involved in the pathogenesis and progression of various neurological diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury and acute and chronic cerebral damage. The p75 neurotrophin receptor (p75NTR) plays a pivotal role in the development of neurological dysfunctions as a result of its high expression, abnormal processing and signalling. Therefore, p75NTR represents as a vital therapeutic target for the treatment of neurodegeneration, neuropsychiatric disorders and cerebrovascular insufficiency. This review summarizes the current research progress on the p75NTR signalling in neurological deficits. We also summarize the present therapeutic approaches by genetically and pharmacologically targeting p75NTR for the attenuation of pathological changes. Based on the evolving knowledge, the role of p75NTR in the regulation of tau hyperphosphorylation, Aβ metabolism, the degeneration of motor neurons and dopaminergic neurons has been discussed. Its position as a biomarker to evaluate the severity of diseases and as a druggable target for drug development has also been elucidated. Several prototype small molecule compounds were introduced to be crucial in neuronal survival and functional recovery via targeting p75NTR. These small molecule compounds represent desirable agents in attenuating neurodegeneration and cell death as they abolish activation-induced neurotoxicity of neurotrophins via modulating p75NTR signalling. More comprehensive and in-depth investigations on p75NTR-based drug development are required to shed light on effective treatment of numerous neurological disorders.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Engineered extracellular vesicles derived from primary M2 macrophages with anti-inflammatory and neuroprotective properties for the treatment of spinal cord injury

Background

Uncontrollable inflammation and nerve cell apoptosis are the most destructive pathological response after spinal cord injury (SCI). So, inflammation suppression combined with neuroprotection is one of the most promising strategies to treat SCI. Engineered extracellular vesicles with anti-inflammatory and neuroprotective properties are promising candidates for implementing these strategies for the treatment of SCI.

Results

By combining nerve growth factor (NGF) and curcumin (Cur), we prepared stable engineered extracellular vesicles of approximately 120 nm from primary M2 macrophages with anti-inflammatory and neuroprotective properties (Cur@EVs^−cl−NGF). Notably, NGF was coupled with EVs by matrix metalloproteinase 9 (MMP9)-a cleavable linker to release at the injured site accurately. Through targeted experiments, we found that these extracellular vesicles could actively and effectively accumulate at the injured site of SCI mice, which greatly improved the bioavailability of the drugs. Subsequently, Cur@EVs^−cl−NGF reached the injured site and could effectively inhibit the uncontrollable inflammatory response to protect the spinal cord from secondary damage; in addition, Cur@EVs^−cl−NGF could release NGF into the microenvironment in time to exert a neuroprotective effect against nerve cell damage.

Conclusions

A series of in vivo and in vitro experiments showed that the engineered extracellular vesicles significantly improved the microenvironment after injury and promoted the recovery of motor function after SCI. We provide a new method for inflammation suppression combined with neuroprotective strategies to treat SCI.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01123-9.

Structural basis of NF-κB signaling by the p75 neurotrophin receptor interaction with adaptor protein TRADD through their respective death domains

The p75 neurotrophin receptor (p75NTR) is a critical mediator of neuronal death and tissue remodeling and has been implicated in various neurodegenerative diseases and cancers. The death domain (DD) of p75NTR is an intracellular signaling hub and has been shown to interact with diverse adaptor proteins. In breast cancer cells, binding of the adaptor protein TRADD to p75NTR depends on nerve growth factor and promotes cell survival. However, the structural mechanism and functional significance of TRADD recruitment in neuronal p75NTR signaling remain poorly understood. Here we report an NMR structure of the p75NTR-DD and TRADD-DD complex and reveal the mechanism of specific recognition of the TRADD-DD by the p75NTR-DD mainly through electrostatic interactions. Furthermore, we identified spatiotemporal overlap of p75NTR and TRADD expression in developing cerebellar granule neurons (CGNs) at early postnatal stages and discover the physiological relevance of the interaction between TRADD and p75NTR in the regulation of canonical NF-κB signaling and cell survival in CGNs. Our results provide a new structural framework for understanding how the recruitment of TRADD to p75NTR through DD interactions creates a membrane-proximal platform, which can be efficiently regulated by various neurotrophic factors through extracellular domains of p75NTR, to propagate downstream signaling in developing neurons.

Neuroprotective Effect of Vascular Endothelial Growth Factor on Motoneurons of the Oculomotor System

Vascular endothelial growth factor (VEGF) was initially characterized as a potent angiogenic factor based on its activity on the vascular system. However, it is now well established that VEGF also plays a crucial role as a neuroprotective factor in the nervous system. A deficit of VEGF has been related to motoneuronal degeneration, such as that occurring in amyotrophic lateral sclerosis (ALS). Strikingly, motoneurons of the oculomotor system show lesser vulnerability to neurodegeneration in ALS compared to other motoneurons. These motoneurons presented higher amounts of VEGF and its receptor Flk-1 than other brainstem pools. That higher VEGF level could be due to an enhanced retrograde input from their target muscles, but it can also be produced by the motoneurons themselves and act in an autocrine way. By contrast, VEGF’s paracrine supply from the vicinity cells, such as glial cells, seems to represent a minor source of VEGF for brainstem motoneurons. In addition, ocular motoneurons experiment an increase in VEGF and Flk-1 level in response to axotomy, not observed in facial or hypoglossal motoneurons. Therefore, in this review, we summarize the differences in VEGF availability that could contribute to the higher resistance of extraocular motoneurons to injury and neurodegenerative diseases.

Early regeneration of axons following peripheral nerve injury is enhanced if p75NTR is eliminated from the surrounding pathway

The common neurotrophin receptor, p75^NTR , has been proposed to be an inhibitor of axon regeneration after peripheral nerve injury, but whether this effect is on the regenerating axons, immune cells migrating into the injury site, or cells in the pathway surrounding the axons is not clear. Cut nerves in mice expressing fluorescent proteins in axons were repaired with grafts from non-fluorescent hosts to study axon elongation when p75^NTR was eliminated separately from axons and immune cells in the proximal stump of cut nerves, from cells in the regeneration pathway, or both. Two weeks later, axons from wild type mice regenerating into grafts devoid of p75^NTR had elongated more than twice as far as axons in grafts from wild type mice. No enhancement of regeneration of axons in p75^NTR knockout mice was observed, whether nerves were repaired with grafts from wild type mice or from p75^NTR knockout mice. To evaluate whether inhibition of p75^NTR could be used to improve regeneration, nerves in wild type mice repaired without grafts were exposed to a specific inhibitor of the p75^NTR receptor, LM11A-31, at the time of nerve repair. This local blockade of p75^NTR resulted in successful regeneration of axons of nearly three times as many motoneurons and reinnervation of twice as many muscle fibers by regenerating motor axons as untreated controls. Expression of p75^NTR surrounding regenerating axons contributes to poor regeneration during the first 2 weeks after peripheral nerve injury. Inhibition of p75^NTR might be a therapeutic target for treatments of peripheral nerve injuries.

The p75NTR neurotrophin receptor is required to organize the mature neuromuscular synapse by regulating synaptic vesicle availability

The coordinated movement of organisms relies on efficient nerve-muscle communication at the neuromuscular junction. After peripheral nerve injury or neurodegeneration, motor neurons and Schwann cells increase the expression of the p75NTR pan-neurotrophin receptor. Even though p75NTR targeting has emerged as a promising therapeutic strategy to delay peripheral neuronal damage progression, the effects of long-term p75NTR inhibition at the mature neuromuscular junction have not been elucidated. We performed quantitative neuroanathomical analyses of the neuromuscular junction in p75NTR null mice by laser confocal and electron microscopy, which were complemented with electromyography, locomotor tests, and pharmacological intervention studies. Mature neuromuscular synapses of p75NTR null mice show impaired postsynaptic organization and ultrastructural complexity, which correlate with altered synaptic function at the levels of nerve activity-induced muscle responses, muscle fiber structure, force production, and locomotor performance. Our results on primary myotubes and denervated muscles indicate that muscle-derived p75NTR does not play a major role on postsynaptic organization. In turn, motor axon terminals of p75NTR null mice display a strong reduction in the number of synaptic vesicles and active zones. According to the observed pre and postsynaptic defects, pharmacological acetylcholinesterase inhibition rescued nerve-dependent muscle response and force production in p75NTR null mice. Our findings revealing that p75NTR is required to organize mature neuromuscular junctions contribute to a comprehensive view of the possible effects caused by therapeutic attempts to target p75NTR.

Regulation of BACE1 expression after injury is linked to the p75 neurotrophin receptor

BACE1 is a transmembrane aspartic protease that cleaves various substrates and it is required for normal brain function. BACE1 expression is high during early development, but it is reduced in adulthood. Under conditions of stress and injury, BACE1 levels are increased; however, the underlying mechanisms that drive BACE1 elevation are not well understood. One mechanism associated with brain injury is the activation of injurious p75 neurotrophin receptor (p75), which can trigger pathological signals. Here we report that within 72 h after controlled cortical impact (CCI) or laser injury, BACE1 and p75 are increased and tightly co-expressed in cortical neurons of mouse brain. Additionally, BACE1 is not up-regulated in p75 null mice in response to focal cortical injury, while p75 over-expression results in BACE1 augmentation in HEK-293 and SY5Y cell lines. A luciferase assay conducted in SY5Y cell line revealed that BACE1 expression is regulated at the transcriptional level in response to p75 transfection. Interestingly, this effect does not appear to be dependent upon p75 ligands including mature and pro-neurotrophins. In addition, BACE1 activity on amyloid precursor protein (APP) is enhanced in SY5Y-APP cells transfected with a p75 construct. Lastly, we found that the activation of c-jun n-terminal kinase (JNK) by p75 contributes to BACE1 up-regulation. This study explores how two injury-induced molecules are intimately connected and suggests a potential link between p75 signaling and the expression of BACE1 after brain injury.

Spatiotemporal Differences in Gene Expression Between Motor and Sensory Autografts and Their Effect on Femoral Nerve Regeneration in the Rat

To improve the outcome after autologous nerve grafting in the clinic, it is important to understand the limiting variables such as distinct phenotypes of motor and sensory Schwann cells. This study investigated the properties of phenotypically different autografts in a 6 mm femoral nerve defect model in the rat, where the respective femoral branches distally of the inguinal bifurcation served as homotopic, or heterotopic autografts. Axonal regeneration and target reinnervation was analyzed by gait analysis, electrophysiology, and wet muscle mass analysis. We evaluated regeneration-associated gene expression between 5 days and 10 weeks after repair, in the autografts as well as the proximal, and distal segments of the femoral nerve using qRT-PCR. Furthermore we investigated expression patterns of phenotypically pure ventral and dorsal roots. We identified highly significant differences in gene expression of a variety of regeneration-associated genes along the central – peripheral axis in healthy femoral nerves. Phenotypically mismatched grafting resulted in altered spatiotemporal expression of neurotrophic factor BDNF, GDNF receptor GFRα1, cell adhesion molecules Cadm3, Cadm4, L1CAM, and proliferation associated Ki67. Although significantly higher quadriceps muscle mass following homotopic nerve grafting was measured, we did not observe differences in gait analysis, and electrophysiological parameters between treatment paradigms. Our study provides evidence for phenotypic commitment of autologous nerve grafts after injury and gives a conclusive overview of temporal expression of several important regeneration-associated genes after repair with sensory or motor graft.

The Complex Work of Proteases and Secretases in Wallerian Degeneration: Beyond Neuregulin-1

After damage, axons in the peripheral nervous system (PNS) regenerate and regrow following a process termed Wallerian degeneration, but the regenerative process is often incomplete and usually the system does not reach full recovery. Key steps to the creation of a permissive environment for axonal regrowth are the trans-differentiation of Schwann cells and the remodeling of the extracellular matrix (ECM). In this review article, we will discuss how proteases and secretases promote effective regeneration and remyelination. We will detail how they control neuregulin-1 (NRG-1) activity at the post-translational level, as the concerted action of alpha, beta and gamma secretases cooperates to balance activating and inhibitory signals necessary for physiological myelination and remyelination. In addition, we will discuss the role of other proteases in nerve repair, among which A Disintegrin And Metalloproteinases (ADAMs) and gamma-secretases substrates. Moreover, we will present how matrix metalloproteinases (MMPs) and proteases of the blood coagulation cascade participate in forming newly synthetized myelin and in regulating axonal regeneration. Overall, we will highlight how a deeper comprehension of secretases and proteases mechanism of action in Wallerian degeneration might be useful to develop new therapies with the potential of readily and efficiently improve the regenerative process.

Death domain of p75 neurotrophin receptor: a structural perspective on an intracellular signalling hub

The death domain (DD) is a globular protein motif with a signature feature of an all-helical Greek-key motif. It is a primary mediator of a variety of biological activities, including apoptosis, cell survival and cytoskeletal changes, which are related to many neurodegenerative diseases, neurotrauma, and cancers. DDs exist in a wide range of signalling proteins including p75 neurotrophin receptor (p75^NTR ), a member of the tumour necrosis factor receptor superfamily. The specific signalling mediated by p75^NTR in a given cell depends on the type of ligand engaging the extracellular domain and the recruitment of cytosolic interactors to the intracellular domain, especially the DD, of the receptor. In solution, the p75^NTR -DDs mainly form a symmetric non-covalent homodimer. In response to extracellular signals, conformational changes in the p75^NTR extracellular domain (ECD) propagate to the p75^NTR -DD through the disulfide-bonded transmembrane domain (TMD) and destabilize the p75^NTR -DD homodimer, leading to protomer separation and exposure of binding sites on the DD surface. In this review, we focus on recent advances in the study of the structural mechanism of p75^NTR -DD signalling through recruitment of diverse intracellular interactors for the regulation and control of diverse functional outputs.

The Role of BDNF in Peripheral Nerve Regeneration: Activity-Dependent Treatments and Val66Met

Despite the ability of peripheral nerves to spontaneously regenerate after injury, recovery is generally very poor. The neurotrophins have emerged as an important modulator of axon regeneration, particularly brain derived neurotrophic factor (BDNF). BDNF regulation and signaling, as well as its role in activity-dependent treatments including electrical stimulation, exercise, and optogenetic stimulation are discussed here. The importance of a single nucleotide polymorphism in the BDNF gene, Val66Met, which is present in 30% of the human population and may hinder the efficacy of these treatments in enhancing regeneration after injury is considered. Preliminary data are presented on the effectiveness of one such activity-dependent treatment, electrical stimulation, in enhancing axon regeneration in mice expressing the met allele of the Val66Met polymorphism.

Comparison of two different media for maturation rate of neural progenitor cells to neuronal and glial cells emphasizing on expression of neurotrophins and their respective receptors

Neural cells derived from embryonic stem cells (ESCs) have potential usefulness for the treatment of neurodegenerative disorders. Modulation of intrinsic growth factors expression such as neurotrophins and their respective receptors by these cells is necessary to obtain functional neural cells for transplantation. In present study, we compared neural differentiation potential of two different media, NB + 5%ES-FBS + N2B27 and Ko-DMEM + 5%ES-FBS for conversion of mESC derived neural progenitors (NPs) into mature neural cells with emphasis on effect of the these two media on neurotrophins and their respective receptors expression. Immunofluorescence staining, RT-qPCR and western blot analysis showed that the expression of neuronal specific markers, MAP2 and Tuj-1, in NB + 5%ES-FBS + N2B27 medium was significantly higher than the other medium. Western blot assay revealed that the expression of BDNF and NGF increased significantly in mature neural cells obtained from NB + 5%ES-FBS + N2B27 medium but decreased in neural cells from Ko-DMEM + 5%ES-FBS medium compared to mESCs. TrkB protein was not detectable in mESCs but its expression increased in neural cells obtained from both media although its expression in NB + 5%ES-FBS + N2B27 medium was significantly higher than the other medium. In contrast to TrkB, p75NTR protein was detectable in mESCs and is remained constant in neural cells cultured in NB + 5%ES-FBS + N2B27 medium but decreased significantly in the other medium. In conclusion, our results indicated that NB + 5%ES-FBS + N2B27 medium promoted neural differentiation process of mESCs and caused enhancement of neurotrophins protein expression in addition to their cognate receptors.

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.

Cellular Trafficking of Amyloid Precursor Protein in Amyloidogenesis Physiological and Pathological Significance

The accumulation of excess intracellular or extracellular amyloid beta (Aβ) is one of the key pathological events in Alzheimer's disease (AD). Aβ is generated from the cleavage of amyloid precursor protein (APP) by beta secretase-1 (BACE1) and gamma secretase (γ-secretase) within the cells. The endocytic trafficking of APP facilitates amyloidogenesis while at the cell surface, APP is predominantly processed in a non-amyloidogenic manner. Several adaptor proteins bind to both APP and BACE1, regulating their trafficking and recycling along the secretory and endocytic pathways. The phosphorylation of APP at Thr668 and BACE1 at Ser498, also influence their trafficking. Neurotrophins and proneurotrophins also influence APP trafficking through their receptors. In this review, we describe the molecular trafficking pathways of APP and BACE1 that lead to Aβ generation, the involvement of different signaling molecules or adaptor proteins regulating APP and BACE1 subcellular localization. We have also discussed how neurotrophins could modulate amyloidogenesis through their receptors.

p75 neurotrophin receptor interacts with and promotes BACE1 localization in endosomes aggravating amyloidogenesis

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deposition of amyloid beta (Aβ) and dysregulation of neurotrophic signaling, causing synaptic dysfunction, loss of memory, and cell death. The expression of p75 neurotrophin receptor is elevated in the brain of AD patients, suggesting its involvement in this disease. However, the exact mechanism of its action is not yet clear. Here, we show that p75 interacts with beta-site amyloid precursor protein cleaving enzyme-1 (BACE1), and this interaction is enhanced in the presence of Aβ. Our results suggest that the colocalization of BACE1 and amyloid precursor protein (APP) is increased in the presence of both Aβ and p75 in cortical neurons. In addition, the localization of APP and BACE1 in early endosomes is increased in the presence of Aβ and p75. An increased phosphorylation of APP-Thr668 and BACE1-Ser498 by c-Jun N-terminal kinase (JNK) in the presence of Aβ and p75 could be responsible for this localization. In conclusion, our study proposes a potential involvement in amyloidogenesis for p75, which may represent a future therapeutic target for AD. Cover Image for this Issue: doi. 10.1111/jnc.14163.

Functional recovery from sciatic nerve crush injury is delayed because of increased distal atrophy in mice lacking the p75 receptor

Peripheral nerve injuries are becoming more common, but without effective treatment, the outcome is often very poor. Recent research shows that p75 plays an important role in nerve regeneration, but its mechanisms of action during behavioral recovery and axon regrowth remain unclear. To investigate these mechanisms, we examined recovery from sciatic nerve crush injury in wild-type and p75 knockout mice. We found that sciatic nerve crush injury upregulates mRNA and protein expressions of p75 and p75 deficiency alters gene and protein expression of molecules associated with distal portion atrophy. However, p75 deletion did not alter gene and protein expression in the spinal cord of molecules related to neuronal intrinsic growth capacity. Behavioral testing showed that functional recovery was delayed in mice lacking p75. These results suggest that p75 regulates gene and protein expression that limits the distal atrophy after sciatic nerve injury, thereby regulating axonal growth and functional recovery.

Microtubule stabilization promoted axonal regeneration and functional recovery after spinal root avulsion

A spinal root avulsion injury disconnects spinal roots with the spinal cord. The rampant motoneuron death, inhibitory CNS/PNS transitional zone (TZ) for axonal regrowth and limited regeneration speed together lead to motor dysfunction. Microtubules rearrange to assemble a new growth cone and disorganized microtubules underline regeneration failure. It has been shown that microtubule-stabilizing drug, Epothilone B, enhanced axonal regeneration and attenuated fibrotic scaring after spinal cord injury. Here, we are reporting that after spinal root avulsion+ re-implantation in adult rats, EpoB treatment improved motor functional recovery and potentiated electrical responses of motor units. It facilitated axons to cross the TZ and promoted more and bigger axons in the peripheral nerve. Neuromuscular junctions were reformed with better preserved postsynaptic structure, and muscle atrophy was prevented by EpoB administration. Our study showed that EpoB was a promising therapy for promoting axonal regeneration after peripheral nerve injury.

Urinary p75ECD: A prognostic, disease progression, and pharmacodynamic biomarker in ALS

Objective:

To evaluate urinary neurotrophin receptor p75 extracellular domain (p75^ECD) levels as disease progression and prognostic biomarkers in amyotrophic lateral sclerosis (ALS).

Methods:

The population in this study comprised 45 healthy controls and 54 people with ALS, 31 of whom were sampled longitudinally. Urinary p75^ECD was measured using an enzyme-linked immunoassay and validation included intra-assay and inter-assay coefficients of variation, effect of circadian rhythm, and stability over time at room temperature, 4°C, and repeated freeze-thaw cycles. Longitudinal changes in urinary p75^ECD were examined by mixed model analysis, and the prognostic value of baseline p75^ECD was explored by survival analysis.

Results:

Confirming our previous findings, p75^ECD was higher in patients with ALS (5.6 ± 2.2 ng/mg creatinine) compared to controls (3.6 ± 1.4 ng/mg creatinine, p < 0.0001). Assay reproducibility was high, with p75^ECD showing stability across repeated freeze-thaw cycles, at room temperature and 4°C for 2 days, and no diurnal variation. Urinary p75^ECD correlated with the revised ALS Functional Rating Scale at first evaluation (r = −0.44, p = 0.008) and across all study visits (r = −0.36, p < 0.0001). p75^ECD also increased as disease progressed at an average rate of 0.19 ng/mg creatinine per month (p < 0.0001). In multivariate prognostic analysis, bulbar onset (hazard ratio [HR] 3.0, p = 0.0035), rate of disease progression from onset to baseline (HR 4.4, p < 0.0001), and baseline p75^ECD (HR 1.3, p = 0.0004) were predictors of survival.

Conclusions:

The assay for urinary p75^ECD is analytically robust and shows promise as an ALS biomarker with prognostic, disease progression, and potential pharmacodynamic application. Baseline urinary p75^ECD provides prognostic information and is currently the only biological fluid–based biomarker of disease progression.

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Proneurotrophin-3 promotes cell cycle withdrawal of developing cerebellar granule cell progenitors via the p75 neurotrophin receptor

Cerebellar granule cell progenitors (GCP) proliferate extensively in the external granule layer (EGL) of the developing cerebellum prior to differentiating and migrating. Mechanisms that regulate the appropriate timing of cell cycle withdrawal of these neuronal progenitors during brain development are not well defined. The p75 neurotrophin receptor (p75(NTR)) is highly expressed in the proliferating GCPs, but is downregulated once the cells leave the cell cycle. This receptor has primarily been characterized as a death receptor for its ability to induce neuronal apoptosis following injury. Here we demonstrate a novel function for p75(NTR) in regulating proper cell cycle exit of neuronal progenitors in the developing rat and mouse EGL, which is stimulated by proNT3. In the absence of p75(NTR), GCPs continue to proliferate beyond their normal period, resulting in a larger cerebellum that persists into adulthood, with consequent motor deficits.

Myelin-associated glycoprotein modulates apoptosis of motoneurons during early postnatal development via NgR/p75(NTR) receptor-mediated activation of RhoA signaling pathways

Myelin-associated glycoprotein (MAG) is a minor constituent of nervous system myelin, selectively expressed on the periaxonal myelin wrap. By engaging multiple axonal receptors, including Nogo-receptors (NgRs), MAG exerts a nurturing and protective effect the axons it ensheaths. Pharmacological activation of NgRs has a modulatory role on p75^NTR-dependent postnatal apoptosis of motoneurons (MNs). However, it is not clear whether this reflects a physiological role of NgRs in MN development. NgRs are part of a multimeric receptor complex, which includes p75^NTR, Lingo-1 and gangliosides. Upon ligand binding, this multimeric complex activates RhoA/ROCK signaling in a p75^NTR-dependent manner. The aim of this study was to analyze a possible modulatory role of MAG on MN apoptosis during postnatal development. A time course study showed that Mag-null mice suffer a loss of MNs during the first postnatal week. Also, these mice exhibited increased susceptibility in an animal model of p75^NTR-dependent MN apoptosis induced by nerve-crush injury, which was prevented by treatment with a soluble form of MAG (MAG-Fc). The protective role of MAG was confirmed in in vitro models of p75^NTR-dependent MN apoptosis using the MN1 cell line and primary cultures. Lentiviral expression of shRNA sequences targeting NgRs on these cells abolished protection by MAG-Fc. Analysis of RhoA activity using a FRET-based RhoA biosensor showed that MAG-Fc activates RhoA. Pharmacological inhibition of p75^NTR/RhoA/ROCK pathway, or overexpression of a p75^NTR mutant unable to activate RhoA, completely blocked MAG-Fc protection against apoptosis. The role of RhoA/ROCK signaling was further confirmed in the nerve-crush model, where pretreatment with ROCK inhibitor Y-27632 blocked the pro-survival effect of MAG-Fc. These findings identify a new protective role of MAG as a modulator of apoptosis of MNs during postnatal development by a mechanism involving the p75^NTR/RhoA/ROCK signaling pathway. Also, our results highlight the relevance of the nurture/protective effects of myelin on neurons.

Dynamic nature of the p75 neurotrophin receptor in response to injury and disease

Neurotrophins and their respective tropomyosin related kinase (Trk) receptors (TrkA, TrkB, and TrkC) and the p75 neurotrophin receptor (p75(NTR)) play a fundamental role in the development and maintenance of the nervous system making them important targets for treatment of neurodegenerative diseases. Whereas Trk receptors are directly activated by specific neurotrophins, the p75(NTR) is a multifunctional receptor that exerts its effects via heterodimeric interactions with TrkA, TrkB, TrkC, sortilin or the Nogo receptor to regulate a wide array of cellular functions. By partnering with different receptors the p75(NTR) regulates binding of mature versus pro-neurotrophins and activation of different signaling pathways with outcomes ranging from growth and survival to cell death. While the developmental downregulation of the p75(NTR) has raised questions regarding its role in the mature nervous system, recent data have revealed widespread expression of low levels, a role in synaptic plasticity and adult neurogenesis and upregulation in response to injury or disease. Studies are needed to better understand these processes, particularly in the damaged nervous system, but will be complicated by expression of p75(NTR) on immune cells including macrophages and microglia that are intimately involved in disease and repair processes. Recent approaches that regulate p75(NTR) function with small non-peptide ligands have demonstrated potent neuroprotection in models of injury and neurodegenerative diseases that highlight the importance of the p75(NTR) as a therapeutic target. Future studies hold the promise of revealing a wealth of information on the multifaceted actions of the p75(NTR) that will inform the design of new neurotrophin-based therapies.

Incorporation of chitosan microspheres into collagen-chitosan scaffolds for the controlled release of nerve growth factor

Background

Artifical nerve scaffold can be used as a promising alternative to autologous nerve grafts to enhance the repair of peripheral nerve defects. However, current nerve scaffolds lack efficient microstructure and neurotrophic support.

Methods

Microsphere–Scaffold composite was developed by incorporating chitosan microspheres loaded with nerve growth factor (NGF–CMSs) into collagen-chitosan scaffolds (CCH) with longitudinally oriented microchannels (NGF–CMSs/CCH). The morphological characterizations, in vitro release kinetics study, neurite outgrowth assay, and bioactivity assay were evaluated. After that, a 15-mm-long sciatic nerve gap in rats was bridged by the NGF–CMSs/CCH, CCH physically absorbed NGF (NGF/CCH), CCH or nerve autograft. 16 weeks after implantation, electrophysiology, fluoro-gold retrograde tracing, and nerve morphometry were performed.

Results

The NGF–CMSs were evenly distributed throughout the longitudinally oriented microchannels of the scaffold. The NGF–CMSs/CCH was capable of sustained release of bioactive NGF within 28 days as compared with others in vitro. In vivo animal study demonstrated that the outcomes of NGF–CMSs/CCH were better than those of NGF/CCH or CCH.

Conclusion

Our findings suggest that incorporation of NGF–CMSs into the CCH may be a promising tool in the repair of peripheral nerve defects.

Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75(NTR) and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.

Cell biology in neuroscience: Death of developing neurons: new insights and implications for connectivity

The concept that target tissues determine the survival of neurons has inspired much of the thinking on neuronal development in vertebrates, not least because it is supported by decades of research on nerve growth factor (NGF) in the peripheral nervous system (PNS). Recent discoveries now help to understand why only some developing neurons selectively depend on NGF. They also indicate that the survival of most neurons in the central nervous system (CNS) is not simply regulated by single growth factors like in the PNS. Additionally, components of the cell death machinery have begun to be recognized as regulators of selective axonal degeneration and synaptic function, thus playing a critical role in wiring up the nervous system.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Alteration in BDNF and its receptors, full-length and truncated TrkB and p75(NTR) following penetrating traumatic brain injury

The evidence that BDNF is involved in neuroprotection, neuronal repair and recovery after traumatic brain injury (TBI) is substantial. We have previously shown that the polymorphism of the human BDNF gene predicts cognitive recovery and outcome following penetrating TBI. The distribution of expression of BDNF and its receptors after penetrating TBI has not been investigated. In this study we examined the expression of these genes in a rat model of penetrating TBI. The injury is produced by a controlled penetration of a 2mm thick needle-shaped object, which is accelerated with a pellet from an air gun. We used in situ hybridization and investigated the mRNA expression of BDNF and its receptors: the full-length and the truncated TrkB and p75(NTR), from 1 day to 8 weeks following penetrating TBI. In addition, the protein level of BDNF in frontal cortex and hippocampus was measured by reverse phase protein microarray (RPPM). The mRNA expression of BDNF and its receptors decreased in the hippocampus in the border zone ipsilateral to the injury while there was an increase in mRNA expression at the contralateral side. The increase in BDNF mRNA expression in the hippocampus was sustained for 2 weeks following injury, with the highest expression noted in the CA3 cell layer. Furthermore, the protein analysis by RPPM showed increased levels of BDNF in the frontal cortex and the hippocampus up to 2 weeks after TBI. At 8 weeks following injury there was an intense labeling of the truncated TrkB receptor and the p75(NTR) in the area surrounding the cavity. Our study is the first report on the expression of BDNF and its receptors following penetrating TBI and suggests that their expression is altered long after the acute phase of injury. Further studies are needed to investigate if the late expressions of these receptors are beneficial or deleterious. In either case it indicates the possibility to influence the recovery after brain injury during the chronic phase and the development of treatments that may improve the outcome of TBI patients.

Death Receptors in the Selective Degeneration of Motoneurons in Amyotrophic Lateral Sclerosis

While studies on death receptors have long been restricted to immune cells, the last decade has provided a strong body of evidence for their implication in neuronal death and hence neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS). ALS is a fatal paralytic disorder that primarily affects motoneurons in the brain and spinal cord. A neuroinflammatory process, associated with astrocyte and microglial activation as well as infiltration of immune cells, accompanies motoneuron degeneration and supports the contribution of non-cell-autonomous mechanisms in the disease. Hallmarks of Fas, TNFR, LT-βR, and p75^NTR signaling have been observed in both animal models and ALS patients. This review summarizes to date knowledge of the role of death receptors in ALS and the link existing between the selective loss of motoneurons and neuroinflammation. It further suggests how this recent evidence could be included in an ultimate multiapproach to treat patients.

Neuroprotective effects of NGF, BDNF, NT-3 and GDNF on axotomized extraocular motoneurons in neonatal rats

Neurotrophic factors delivered from target muscles are essential for motoneuronal survival, mainly during development and early postnatal maturation. It has been shown that the disconnection between motoneurons and their innervated muscle by means of axotomy produces a vast neuronal death in neonatal animals. In the present work, we have evaluated the effects of different neurotrophic factors on motoneuronal survival after neonatal axotomy, using as a model the motoneurons innervating the extraocular eye muscles. With this purpose, neonatal rats were monocularly enucleated at the day of birth (postnatal day 0) and different neurotrophic treatments (NGF, BDNF, NT-3, GDNF and the mixture of BDNF+GDNF) were applied intraorbitally by means of a Gelfoam implant (a single dose of 5 μg of each factor). We first demonstrated that extraocular eye muscles of neonatal rats expressed these neurotrophic factors and therefore constituted a natural source of retrograde delivery for their innervating motoneurons. By histological and immunocytochemical methods we determined that all treatments significantly rescued extraocular motoneurons from axotomy-induced cell death. For the dose used, NGF and GDNF were the most potent survival factors for these motoneurons, followed by BDNF and lastly by NT-3. The simultaneous administration of BDNF and GDNF did not increase the survival-promoting effects above those obtained by GDNF alone. Interestingly, the rescue effects of all neurotrophic treatments persisted even 30 days after lesion. The administration of these neurotrophic factors, with the exception of NT-3, also prevented the loss of the cholinergic phenotype observed by 10 days after axotomy. At the dosage applied, NGF and GDNF were revealed again as the most effective neuroprotective agents against the axotomy-induced decrease in ChAT. Two remarkable findings highlighted in the present work that contrasted with other motoneuronal types after neonatal axotomy: first, the extremely high efficacy of NGF as a neuroprotective agent and, second, the long-lasting effects of neurotrophic administration on cell survival and ChAT expression in extraocular motoneurons.

A novel in vivo method for isolating antibodies from a phage display library by neuronal retrograde transport selectively yields antibodies against p75(NTR.)

The neurotrophin receptor p75(NTR) is utilized by a variety of pathogens to gain entry into the central nervous system (CNS). We tested if this entry portal might be exploited using a phage display library to isolate internalizing antibodies that target the CNS in vivo. By applying a phage library that expressed human single chain variable fragment (scFv) antibodies on their surface to a transected sciatic nerve, we showed that (1) phage conjugated to anti-p75(NTR) antibody or phage scFv library pre-panned against p75(NTR) are internalized by neurons expressing p75(NTR); (2) subsequent retrograde axonal transport separates internalized phage from the applied phage; and, (3) internalized phage can be recovered from a proximal ligature made on a nerve. This approach resulted in 13-fold increase in the number of phage isolated from the injured nerve compared with the starting population, and isolation of 18 unique internalizing p75(NTR) antibodies that were transported from the peripheral nerve into the spinal cord, through the blood-brain barrier. In addition, antibodies recognizing other potentially internalized antigens were identified through in vivo selection using a fully diverse library. Because p75(NTR) expression is upregulated in motor neurons in response to injury and in disease, the p75(NTR) antibodies may have substantial potential for cell-targeted drug/gene delivery. In addition, this novel selection method provides the potential to generate panels of antibodies that could be used to identify further internalization targets, which could aid drug delivery across the blood-brain barrier.

The pros and cons of growth factors and cytokines in peripheral axon regeneration

Injury to a peripheral nerve induces a complex cellular and molecular response required for successful axon regeneration. Proliferating Schwann cells organize into chains of cells bridging the lesion site, which is invaded by macrophages. Approximately half of the injured neuron population sends out axons that enter the glial guidance channels in response to secreted neurotrophic factors and neuropoietic cytokines. These lesion-associated polypeptides create an environment that is highly supportive for axon regrowth, particularly after acute injury, and ensure that the vast majority of regenerating axons are directed toward the distal nerve stump. Unfortunately, most neurotrophic factors and neuropoietic cytokines are also strong stimulators of axonal sprouting. Although some of the axonal branches will withdraw at later stages, the sprouting effect contributes to the misdirection of reinnervation that results in the lack of functional recovery observed in many patients with peripheral nerve injuries. Here, we critically review the role of neuronal growth factors and cytokines during axon regeneration in the peripheral nervous system. Their differential effects on axon elongation and sprouting were elucidated in various studies on intraneuronal signaling mechanisms following nerve lesion. The present data define a goal for future therapeutic strategies, namely, to selectively stimulate a Ras/Raf/ERK-mediated axon elongation program over an intrinsic PI3K-dependent axonal sprouting program in lesioned motor and sensory neurons. Instead of modulating growth factor or cytokine levels at the lesion site, targeting specific intraneuronal molecules, such as the negative feedback inhibitors of ERK signaling, has been shown to promote long-distance regeneration while avoiding sprouting of regenerating axons until they have reached their target areas.

Evidence for a systemic regulation of neurotrophin synthesis in response to peripheral nerve injury

Up-regulation of neurotrophin synthesis is an important mechanism of peripheral nerve regeneration after injury. Neurotrophin expression is regulated by a complex series of events including cell interactions and multiple molecular stimuli. We have studied neurotrophin synthesis at 2 weeks time-point in a transvertebral model of unilateral or bilateral transection of sciatic nerve in rats. We have found that unilateral sciatic nerve transection results in the elevation of nerve growth factor (NGF) and NT-3, but not glial cell-line derived neurotrophic factor or brain-derived neural factor, in the uninjured nerve on the contralateral side, commonly considered as a control. Bilateral transection further increased NGF but not other neurotrophins in the nerve segment distal to the transection site, as compared to the unilateral injury. To further investigate the distinct role of NGF in regeneration and its potential for peripheral nerve repair, we transduced isogeneic Schwann cells with NGF-encoding lentivirus and transplanted the over-expressing cells into the distal segment of a transected nerve. Axonal regeneration was studied at 2 weeks time-point using pan-neuronal marker NF-200 and found to directly correlate with NGF levels in the regenerating nerve.

Motoneuron programmed cell death in response to proBDNF

Motoneurons (MN) as well as most neuronal populations undergo a temporally and spatially specific period of programmed cell death (PCD). Several factors have been considered to regulate the survival of MNs during this period, including availability of muscle-derived trophic support and activity. The possibility that target-derived factors may also negatively regulate MN survival has been considered, but not pursued. Neurotrophin precursors, through their interaction with p75(NTR) and sortilin receptors have been shown to induce cell death during development and following injury in the CNS. In this study, we find that muscle cells produce and secrete proBDNF. ProBDNF through its interaction with p75(NTR) and sortilin, promotes a caspase-dependent death of MNs in culture. We also provide data to suggest that proBDNF regulates MN PCD during development in vivo.

Early regenerative effects of NGF-transduced Schwann cells in peripheral nerve repair

Peripheral nerve injury leads to a rapid and robust increase in the synthesis of neurotrophins which guide and support regenerating axons. To further optimize neurotrophin supply at the earliest stages of regeneration, we over-expressed NGF in Schwann cells (SCs) by transducing these cells with a lentiviral vector encoding NGF (NGF-SCs). Transplantation of NGF-SCs in a rat sciatic nerve transection/repair model led to significant increase of NGF levels 2weeks after injury and correspondingly to substantial improvement in axonal regeneration. Numbers of NF200, ChAT and CGRP-positive axon profiles, as well as the gastrocnemius muscle weights, were significantly higher in the NGF-Schwann cell group compared to the animals that received control SCs transduced with a lentiviral vector encoding GFP (GFP-SCs). Comparison with other models of NGF application signifies the important role of this neurotrophin during the early stages of regeneration, and supports the importance of developing combined gene and cell therapy for peripheral nerve repair.

Necdin protects embryonic motoneurons from programmed cell death

NECDIN belongs to the type II Melanoma Associated Antigen Gene Expression gene family and is located in the Prader-Willi Syndrome (PWS) critical region. Necdin-deficient mice develop symptoms of PWS, including a sensory and motor deficit. However, the mechanisms underlying the motor deficit remain elusive. Here, we show that the genetic ablation of Necdin, whose expression is restricted to post-mitotic neurons in the spinal cord during development, leads to a loss of 31% of specified motoneurons. The increased neuronal loss occurs during the period of naturally-occurring cell death and is not confined to specific pools of motoneurons. To better understand the role of Necdin during the period of programmed cell death of motoneurons we used embryonic spinal cord explants and primary motoneuron cultures from Necdin-deficient mice. Interestingly, while Necdin-deficient motoneurons present the same survival response to neurotrophic factors, we demonstrate that deletion of Necdin leads to an increased susceptibility of motoneurons to neurotrophic factor deprivation. We show that by neutralizing TNFα this increased susceptibility of Necdin-deficient motoneurons to trophic factor deprivation can be reduced to the normal level. We propose that Necdin is implicated through the TNF-receptor 1 pathway in the developmental death of motoneurons.

Neurotrophin receptors TrkA and TrkC cause neuronal death whereas TrkB does not

Neurons of the peripheral nervous system have long been known to require survival factors to prevent their death during development. But why they selectively become dependent on secretory molecules has remained a mystery, as is the observation that in the central nervous system, most neurons do not show this dependency. Using engineered embryonic stem cells, we show here that the neurotrophin receptors TrkA and TrkC (tropomyosin receptor kinase A and C, also known as Ntrk1 and Ntrk3, respectively) instruct developing neurons to die, both in vitro and in vivo. By contrast, TrkB (also known as Ntrk2), a closely related receptor primarily expressed in the central nervous system, does not. These results indicate that TrkA and TrkC behave as dependence receptors, explaining why developing sympathetic and sensory neurons become trophic-factor-dependent for survival. We suggest that the expansion of the Trk gene family that accompanied the segregation of the peripheral from the central nervous system generated a novel mechanism of cell number control.

Nerve growth factor regulates the firing patterns and synaptic composition of motoneurons

Target-derived neurotrophins exert powerful synaptotrophic actions in the adult brain and are involved in the regulation of different forms of synaptic plasticity. Target disconnection produces a profound synaptic stripping due to the lack of trophic support. Consequently, target reinnervation leads to synaptic remodeling and restoration of cellular functions. Extraocular motoneurons are unique in that they normally express the TrkA neurotrophin receptor in the adult, a feature not seen in other cranial or spinal motoneurons, except after lesions such as axotomy or in neurodegenerative diseases like amyotrophic lateral sclerosis. We investigated the effects of nerve growth factor (NGF) by retrogradely delivering this neurotrophin to abducens motoneurons of adult cats. Axotomy reduced the density of somatic boutons and the overall tonic and phasic firing modulation. Treatment with NGF restored synaptic inputs and firing modulation in axotomized motoneurons. When K252a, a selective inhibitor of tyrosine kinase activity, was applied to specifically test TrkA effects, the NGF-mediated restoration of synapses and firing-related parameters was abolished. Discharge variability and recruitment threshold were, however, increased by NGF compared with control or axotomized motoneurons. Interestingly, these parameters returned to normal following application of REX, an antibody raised against neurotrophin receptor p75 (p75(NTR)). In conclusion, NGF, acting retrogradely through TrkA receptors, supports afferent boutons and regulates the burst and tonic signals correlated with eye movements. On the other hand, p75(NTR) activation regulates recruitment threshold, which impacts on firing regularity. To our knowledge, this is the first report showing powerful synaptotrophic effects of NGF on motoneurons in vivo.

Proneurotrophins, seizures, and neuronal apoptosis

Neurons respond to numerous factors in their environment that influence their survival and function during development and in the mature brain. Among these factors, the neurotrophins have been shown to support neuronal survival and function, acting primarily through the Trk family of receptor tyrosine kinases. However, recent studies have established that the uncleaved neurotrophin precursors, the proneurotrophins, can be secreted and induce apoptosis via the p75 neurotrophin receptor, suggesting that the balance of secreted mature and proneurotrophins has a critical impact on neuronal survival or death. Epileptic seizures elicit increases in both proneurotrophin secretion and p75(NTR) expression, shifting the balance of these factors toward signaling cell death. This review will discuss the evidence that this ligand-receptor system plays an important role in neuronal loss following seizures.

NRAGE, a p75NTR adaptor protein, is required for developmental apoptosis in vivo

NRAGE (also known as Maged1, Dlxin) is a member of the MAGE gene family that may play a role in the neuronal apoptosis that is regulated by the p75 neurotrophin receptor (p75NTR). To test this hypothesis in vivo, we generated NRAGE knockout mice and found that NRAGE deletion caused a defect in developmental apoptosis of sympathetic neurons of the superior cervical ganglia, similar to that observed in p75NTR knockout mice. Primary sympathetic neurons derived from NRAGE knockout mice were resistant to apoptosis induced by brain-derived neurotrophic factor (BDNF), a pro-apoptotic p75NTR ligand, and NRAGE-deficient sympathetic neurons show attenuated BDNF-dependent JNK activation. Hair follicle catagen is an apoptosis-like process that is dependent on p75NTR signaling; we show that NRAGE and p75NTR show regulated co-expression in the hair follicle and that identical defects in hair follicle catagen are present in NRAGE and p75NTR knockout mice. Interestingly, NRAGE knockout mice have severe defects in motoneuron apoptosis that are not observed in p75NTR knockout animals, raising the possibility that NRAGE may facilitate apoptosis induced by receptors other than p75NTR. Together, these studies demonstrate that NRAGE plays an important role in apoptotic-signaling in vivo.

Nerve repair: experimental and clinical evaluation of neurotrophic factors in peripheral nerve regeneration

Neurotrophic factors are a family of polypeptides required for survival of discrete neuronal populations. In the normal state such factors are mostly synthesised by target tissues and are used for the viability of the nerve-cell bodies. After nerve injury, neurotrophic factors (NFs) are synthesised by non-neuronal (Schwann cells and fibroblasts) in the nerve trunk, and act to support the outgrowth of axons. NFs can be classified into three major groups: (1) neurotrophins; (2) neurokines; and (3) the transforming growth factor beta (TGF)-beta superfamily.

P75 nerve growth factor receptor is expressed in regenerating human nerve grafts

BACKGROUND

The purpose of this study was to elucidate the expression of p75 nerve growth factor receptor (p75NGFR) in human cross-facial nerve grafts and to compare the immunohistological findings with patient data and the functional outcome in facial reanimation.

MATERIALS AND METHODS

The study comprised 37 sural nerve graft specimens. All of the patients had long-lasting complete facial paralysis and were operated on by the standard two-stage procedure involving cross-facial nerve grafts and microneurovascular muscle transfer. Nerve biopsies were taken 4 to 20 months (mean, 8 months) after the cross-facial nerve grafting. Immunohistochemistry for p75NGFR as well as for Schwann cells (S-100; Dako, Glostrup, Denmark) and for Neurofilament-200 (NF-200; Boehringer, Mannheim, Germany) was performed.

RESULTS

In graft biopsies, the mean number of NF-200-positive axons amounted to 38% (range, 6-81%) of that in control samples. Further, regenerated axons were thinner than in control samples. Morphologically, the grafted nerves were characterized by fibrosis and invasion of inflammatory cells. A longer time between cross-facial nerve grafting and biopsy sampling correlated with a higher number of viable axons (NF-200) (P = 0.002). In all cases, expression of p75NGF receptor was clearly higher at the distal end of the grafted nerve. Expression of p75NGFR was lower in older than in younger patients (P = 0.003). A high expression of p75NGFR was often seen with better function of the transplanted muscle.

CONCLUSION

Increased expression of p75NGFR in human nerve grafts was noted, especially in younger patients. We suggest that p75NGFR expression might be a contributing factor in a successful axonal regeneration and eventual recovery of muscle function.

Neurotrophins and their receptors: roles in plasticity, neurodegeneration and neuroprotection

It is beyond doubt that the neurotrophin family of proteins plays key roles in determining the fate of the neuron, not only during embryonic development, but also in the adult brain. Neurotrophins such as NGF (nerve growth factor) and BDNF (brain-derived neurotrophic factor) can play dual roles: first, in neuronal survival and death, and, secondly, in activity-dependent plasticity. The neurotrophins manifest their effects by binding to two discrete receptor subtypes: the Trk (tropomyosin receptor kinase) family of RTKs (receptor tyrosine kinases) and the p75NTR (p75 neurotrophin receptor). The differential activation of these receptors by the mature neurotrophins and their precursors, the proneurotrophins, renders analysis of the biological functions of these receptors in the adult brain highly complex. Here, we briefly give a broad review of current knowledge of the roles of neurotrophins in the adult brain, including expression of hippocampal plasticity, neurodegeneration and exercise-induced neuroprotection.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.