Schwann cell p75NTR prevents spontaneous sensory reinnervation of the adult spinal cord

Effects of Sphingosine-1-Phosphate on the Facilitation of Peripheral Nerve Regeneration

This study aims to explore the role of sphingosine-1-phosphate (S1P) in peripheral nerve regeneration after injury. S1P is a crucial metabolite involved in cell migration, inflammation, and nerve regeneration. In this research, six-week-old male Sprague-Dawley rats (total n=18) underwent transection of the inferior alveolar nerve (IAN) and were divided into three groups: S1PR agonist (FTY720) (n=6), saline control (n=6), and S1P1R antagonist (n=6). Regeneration was assessed using immunostaining and retrograde tracing. Results showed that the S1PR agonist group had superior axonal and Schwann cell regeneration compared to controls. Additionally, the combination with S1P1R antagonists inhibited the effects of the agonists, further confirming the potential role of S1P1R in nerve repair. Our results suggest that mediating S1P1R signaling could facilitate the regeneration of peripheral nerves after injury.

The phenotypic changes of Schwann cells promote the functional repair of nerve injury

Functional recovery after nerve injury is a significant challenge due to the complex nature of nerve injury repair and the non-regeneration of neurons. Schwann cells (SCs), play a crucial role in the nerve injury repair process because of their high plasticity, secretion, and migration abilities. Upon nerve injury, SCs undergo a phenotypic change and redifferentiate into a repair phenotype, which helps in healing by recruiting phagocytes, removing myelin fragments, promoting axon regeneration, and facilitating myelin formation. However, the repair phenotype can be unstable, limiting the effectiveness of the repair. Recent research has found that transplantation of SCs can be an effective treatment option, therefore, it is essential to comprehend the phenotypic changes of SCs and clarify the related mechanisms to develop the transplantation therapy further.

Strain-induced bands of Büngner formation promotes axon growth in 3D tissue-engineered constructs

Treatment of peripheral nerve lesions remains a major challenge due to poor functional recovery; hence, ongoing research efforts strive to enhance peripheral nerve repair. In this study, we aimed to establish three-dimensional tissue-engineered bands of Büngner constructs by subjecting Schwann cells (SCs) embedded in fibrin hydrogels to mechanical stimulation. We show for the first time that the application of strain induces (i) longitudinal alignment of SCs resembling bands of Büngner, and (ii) the expression of a pronounced repair SC phenotype as evidenced by upregulation of BDNF, NGF, and p75NTR. Furthermore, we show that mechanically aligned SCs provide physical guidance for migrating axons over several millimeters in vitro in a co-culture model with rat dorsal root ganglion explants. Consequently, these constructs hold great therapeutic potential for transplantation into patients and might also provide a physiologically relevant in vitro peripheral nerve model for drug screening or investigation of pathologic or regenerative processes.

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.

Dexmedetomidine Alleviates Lipopolysaccharide-Induced Acute Kidney Injury by Inhibiting p75NTR-Mediated Oxidative Stress and Apoptosis

Oxidative stress and apoptosis play a key role in the pathogenesis of sepsis-associated acute kidney injury (AKI). Dexmedetomidine (DEX) may present renal protective effects in sepsis. Therefore, we studied antioxidant effects and the mechanism of DEX in an inflammatory proximal tubular epithelial cell model and lipopolysaccharide- (LPS-) induced AKI in mice. Methods. We assessed renal function (creatinine, urea nitrogen), histopathology, oxidative stress (malondialdehyde (MDA) and superoxide dismutase (SOD)), and apoptosis (TUNEL staining and Cleaved caspase-3) in mice. In vitro experiments including Cleaved caspase-3 and p75NTR/p38MAPK/JNK signaling pathways were evaluated using western blot. Reactive oxidative species (ROS) production and apoptosis were determined using flow cytometry. Results. DEX significantly improved renal function and kidney injury and also revert the substantially increased level of MDA concentrations as well as the reduction of the SOD enzyme activity found in LPS-induced AKI mice. In parallel, DEX treatment also reduced the apoptosis and Cleaved caspase-3 expression evoked by LPS. The expression of p75NTR was increased in kidney tissues of mice with AKI but decreased after treatment with DEX. In cultured human renal tubular epithelial cell line (HK-2 cells), DEX inhibited LPS-induced apoptosis and generation of ROS, but this was reversed by overexpression of p75NTR. Furthermore, pretreatment with DEX significantly downregulated phosphorylation of JNK and p38MAPK in LPS-stimulated HK-2 cells, and this effect was abolished by overexpression of p75NTR. Conclusion. DEX ameliorated AKI in mice with sepsis by partially reducing oxidative stress and apoptosis through regulation of p75NTR/p38MAPK/JNK signaling pathways.

Running and swimming prevent the deregulation of the BDNF/TrkB neurotrophic signalling at the neuromuscular junction in mice with amyotrophic lateral sclerosis

Nerve-induced muscle contraction regulates the BDNF/TrkB neurotrophic signalling to retrogradely modulate neurotransmission and protect the neuromuscular junctions and motoneurons. In muscles with amyotrophic lateral sclerosis, this pathway is strongly misbalanced and neuromuscular junctions are destabilized, which may directly cause the motoneuron degeneration and muscular atrophy observed in this disease. Here, we sought to demonstrate (1) that physical exercise, whose recommendation has been controversial in amyotrophic lateral sclerosis, would be a good option for its therapy, because it normalizes and improves the altered neurotrophin pathway and (2) a plausible molecular mechanism underlying its positive effect. SOD1-G93A mice were trained following either running or swimming-based protocols since the beginning of the symptomatic phase (day 70 of age) until day 115. Next, the full BDNF pathway, including receptors, downstream kinases and proteins related with neurotransmission, was characterized and motoneuron survival was analysed. The results establish that amyotrophic lateral sclerosis-induced damaging molecular changes in the BDNF/TrkB pathway are reduced, prevented or even overcompensated by precisely defined exercise protocols that modulate TrkB isoforms and neurotransmission regulatory proteins and reduce motoneuron death. Altogether, the maintenance of the BDNF/TrkB signalling and the downstream pathway, particularly after the swimming protocol, adds new molecular evidence of the benefits of physical exercise to reduce the impact of amyotrophic lateral sclerosis. These results are encouraging since they reveal an improvement even starting the therapy after the onset of the disease.

NT3P75-2 gene-modified bone mesenchymal stem cells improve neurological function recovery in mouse TBI model

BACKGROUND

The attainment of extensive neurological function recovery remains the key challenge for the treatment of traumatic brain injury (TBI). Transplantation of bone marrow-derived mesenchymal stem cells (BMSCs) has been shown to improve neurological function recovery after TBI. However, the survival of BMSCs after transplantation in early-stage TBI is limited, and much is unknown about the mechanisms mediating this neurological function recovery. Secretion of neurotrophic factors, including neurotrophin 3 (NT3), is one of the critical factors mediating BMSC neurological function recovery. Gene mutation of NT3 (NT3^P75-2) has been shown to enhance the biological function of NT3 via the reduction of the activation of the P75 signal pathway. Thus, we investigated whether NT3^P75-2 gene-modified BMSCs could enhance the survival of BMSCs and further improve neurological function recovery after TBI.

METHODS

The ability of NT3^P75-2 induction to improve cell growth rate of NSC-34 and PC12 cells in vitro was first determined. BMSCs were then infected with three different lentiviruses (green fluorescent protein (GFP), GFP-NT3, or GFP-NT3^P75-2), which stably express GFP, GFP-NT3, or GFP-NT3^P75-2. At 24 h post-TBI induction in mice, GFP-labeled BMSCs were locally transplanted into the lesion site. Immunofluorescence and histopathology were performed at 1, 3, and/or 7 days after transplantation to evaluate the survival of BMSCs as well as the lesion volume. A modified neurological severity scoring system and the rotarod test were chosen to evaluate the functional recovery of the mice. Cell growth rate, glial activation, and signaling pathway analyses were performed to determine the potential mechanisms of NT3^P75-2 in functional recovery after TBI.

RESULTS

Overall, NT3^P75-2 improved cell growth rate of NSC-34 and PC12 cells in vitro. In addition, NT3^P75-2 significantly improved the survival of transplanted BMSCs and neurological function recovery after TBI. Overexpression of NT3^P75-2 led to a significant reduction in the activation of glial cells, brain water content, and brain lesion volume after TBI. This was associated with a reduced activation of the p75 neurotrophin receptor (P75NTR) and the c-Jun N-terminal kinase (JNK) signal pathway due to the low affinity of NT3^P75-2 for the receptor.

CONCLUSIONS

Taken together, our results demonstrate that administration of NT3^P75-2 gene-modified BMSCs dramatically improves neurological function recovery after TBI by increasing the survival of BMSCs and ameliorating the inflammatory environment, providing a new promising treatment strategy for TBI.

Peripheral Nerve Regeneration Is Independent From Schwann Cell p75NTR Expression

Schwann cell reprogramming and differentiation are crucial prerequisites for neuronal regeneration and re-myelination to occur following injury to peripheral nerves. The neurotrophin receptor p75NTR has been identified as a positive modulator for Schwann cell myelination during development and implicated in promoting nerve regeneration after injury. However, most studies base this conclusion on results obtained from complete p75NTR knockout mouse models and cannot dissect the specific role of p75NTR expressed by Schwann cells. In this present study, a conditional knockout model selectively deleting p75NTR expression in Schwann cells was generated, where p75NTR expression is replaced with that of an mCherry reporter. Silencing of Schwann cell p75NTR expression was confirmed in the sciatic nerve in vivo and in vitro, without altering axonal expression of p75NTR. No difference in sciatic nerve myelination during development or following sciatic nerve crush injury was observed, as determined by quantification of both myelinated and unmyelinated nerve fiber densities, myelinated axonal diameter and myelin thickness. However, the absence of Schwann cell p75NTR reduced motor nerve conduction velocity after crush injury. Our data indicate that the absence of Schwann cell p75NTR expression in vivo is not critical for axonal regrowth or remyelination following sciatic nerve crush injury, but does play a key role in functional recovery. Overall, this represents the first step in redefining the role of p75NTR in the peripheral nervous system, suggesting that the Schwann cell-axon unit functions as a syncytium, with the previous published involvement of p75NTR in remyelination most likely depending on axonal/neuronal p75NTR and/or mutual glial-axonal interactions.

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.

Fabrication, characterization and biocompatibility of collagen/oxidized regenerated cellulose-Ca composite scaffold for carrying Schwann cells

Schwann cell (SC) is the primary structural and functional part of the peripheral nervous system, and it plays a key role in the repair and regeneration of peripheral nerve. In order to develop a suitable scaffold for SC nerve tissue engineering, three kinds of scaffolds, including pristine collagen, pure oxidized regenerated cellulose-Ca (ORCCa) and collagen/ORC-Ca composite scaffolds, have been fabricated for carrying SC in this study. SC is then seeded on the scaffolds to form SC-scaffold nerve tissue engineering composites and evaluate their biocompatibility. The chemical and physical structure of the scaffolds are investigated by FTIR, NMR and SEM. The wettability of the collagen/ORC-Ca composite scaffold is close to that of pristine collagen, and the tensile strength of the composite scaffold (0.58 MPa) is better than that of pristine collagen (0.36 MPa). Cytotoxicity, cell proliferation, cell adhesion and western blotting assays are conducted to evaluate the biocompatibility and properties of different scaffolds. The results show that the three scaffolds exhibit no toxicity, and the proliferation rate of SC on the collagen/ORC-Ca composite scaffold is significantly higher than that of the other scaffolds (p < 0.05). The number of the adhesion cells on the composite scaffold (244.67 ± 13.02) is much more than that in the pure ORC-Ca group (p < 0.01). Furthermore, the expression of N-Cadheri and PMP22 proteins in the collagen/ORC-Ca composite scaffold is significantly superior to the other two scaffolds (both p < 0.01). Therefore, it could be concluded that the collagen/ORC-Ca composite is a promising candidate as a scaffold for carrying SC to form nerve tissue engineering composites in order to assist the peripheral nervous in the repair and regeneration.

NGF Attenuates High Glucose-Induced ER Stress, Preventing Schwann Cell Apoptosis by Activating the PI3K/Akt/GSK3β and ERK1/2 Pathways

Diabetic peripheral neuropathy (DPN) is one of the most common and troublesome complications of diabetes mellitus. It has been demonstrated that nerve growth factor (NGF) exerts a pivotal role in the regulation of neuronal growth and the promotion of DPN recovery. However, the exact molecular mechanisms are not well understood. Recent studies have indicated that as a novel therapeutic target, endoplasmic reticulum (ER) stress participates in the onset and progression of DPN. In the present study, it has been demonstrated that NGF prevents the sciatic nerve from degeneration and demyelination in DPN rats. Thus, RSC 96 cells, which retain the characteristic features of Schwann cells (SCs), were cultured in medium containing 30 mM glucose (high glucose, HG) to mimic SCs in DPN mice. The 50-ng/ml dose of NGF was identified to be the optimal concentration for treating an excessive ER stress level under HG conditions for 24 h. We found that NGF treatment significantly inhibits HG-induced ER stress and subsequently suppresses ER-related apoptosis. Further, NGF administration also activates the upstream signaling pathway of ER stress, PI3K/Akt/GSK3β signaling and ERK1/2 signaling. Co-treatment with the PI3K inhibitor LY294002 or ERK1/2 inhibitor U0126 significantly reverses the protective role of NGF on HG-induced excessive ER stress and subsequent apoptosis. These observations suggest that the neuroprotective role of NGF in DPN is mediated by the inhibition of excessive ER stress via the activation of the PI3K/Akt/GSK3β and ERK1/2 signaling pathways.

Multiple uses of fibrin sealant for nervous system treatment following injury and disease

Lesions to the nervous system often produce hemorrhage and tissue loss that are difficult, if not impossible, to repair. Therefore, scar formation, inflammation and cavitation take place, expanding the lesion epicenter. This significantly worsens the patient conditions and impairment, increasing neuronal loss and glial reaction, which in turn further decreases the chances of a positive outcome. The possibility of using hemostatic substances that also function as a scaffold, such as the fibrin sealant, reduces surgical time and improve postoperative recovery. To date, several studies have demonstrated that human blood derived fibrin sealant produces positive effects in different interventions, becoming an efficient alternative to suturing. To provide an alternative to homologous fibrin sealants, the Center for the Study of Venoms and Venomous Animals (CEVAP, Brazil) has proposed a new bioproduct composed of certified animal components, including a thrombin-like enzyme obtained from snake venom and bubaline fibrinogen. Thus, the present review brings up to date literature assessment on the use of fibrin sealant for nervous system repair and positions the new heterologous bioproduct from CEVAP as an alternative to the commercial counterparts. In this way, clinical and pre-clinical data are discussed in different topics, ranging from central nervous system to peripheral nervous system applications, specifying positive results as well as future enhancements that are necessary for improving the use of fibrin sealant therapy.

Distribution and development of peripheral glial cells in the human fetal cochlea

The adult human cochlea contains various types of peripheral glial cells that envelop or myelinate the three different domains of the spiral ganglion neurons: the central processes in the cochlear nerve, the cell bodies in the spiral ganglia, and the peripheral processes in the osseous spiral lamina. Little is known about the distribution, lineage separation and maturation of these peripheral glial cells in the human fetal cochlea. In the current study, we observed peripheral glial cells expressing SOX10, SOX9 and S100B as early as 9 weeks of gestation (W9) in all three neuronal domains. We propose that these cells are the common precursor to both mature Schwann cells and satellite glial cells. Additionally, the peripheral glial cells located along the peripheral processes expressed NGFR, indicating a phenotype distinct from the peripheral glial cells located along the central processes. From W12, the spiral ganglion was gradually populated by satellite glial cells in a spatiotemporal gradient. In the cochlear nerve, radial sorting was accomplished by W22 and myelination started prior to myelination of the peripheral processes. The developmental dynamics of the peripheral glial cells in the human fetal cochlea is in support of a neural crest origin. Our study provides the first overview of the distribution and maturation of peripheral glial cells in the human fetal cochlea from W9 to W22.

Stem cell salvage of injured peripheral nerve

We previously developed a collagen tube filled with autologous skin-derived stem cells (SDSCs) for bridging long rat sciatic nerve gaps. Here we present a case report describing a compassionate use of this graft for repairing the polyinjured motor and sensory nerves of the upper arms of a patient. Preclinical assessment was performed with collagen/SDSC implantation in rats after sectioning the sciatic nerve. For the patient, during the 3-year follow-up period, functional recovery of injured median and ulnar nerves was assessed by pinch gauge test and static two-point discrimination and touch test with monofilaments, along with electrophysiological and MRI examinations. Preclinical experiments in rats revealed rescue of sciatic nerve and no side effects of patient-derived SDSC transplantation (30 and 180 days of treatment). In the patient treatment, motor and sensory functions of the median nerve demonstrated ongoing recovery postimplantation during the follow-up period. The results indicate that the collagen/SDSC artificial nerve graft could be used for surgical repair of larger defects in major lesions of peripheral nerves, increasing patient quality of life by saving the upper arms from amputation.

Distinct changes in synaptic protein composition at neuromuscular junctions of extraocular muscles versus limb muscles of ALS donors

The pathophysiology of amyotrophic lateral sclerosis (ALS) is very complex and still rather elusive but in recent years evidence of early involvement of the neuromuscular junctions (NMJs) has accumulated. We have recently reported that the human extraocular muscles (EOMs) are far less affected than limb muscles at the end-stage of ALS from the same donor. The present study aimed to compare the differences in synaptic protein composition at NMJ and in nerve fibers between EOM and limb muscles from ALS donors and controls. Neurofilament light subunit and synaptophysin decreased significantly at NMJs and in nerve fibers in limb muscles with ALS whereas they were maintained in ALS EOMs. S100B was significantly decreased at NMJs and in nerve fibers in both EOMs and limb muscles of ALS donors, but other markers confirmed the presence of terminal Schwann cells in these NMJs. p75 neurotrophin receptor was present in nerve fibers but absent at NMJs in ALS limb muscles. The EOMs were able to maintain the integrity of their NMJs to a very large extent until the end-stage of ALS, in contrast to the limb muscles. Changes in Ca²⁺ homeostasis, reflected by altered S100B distribution, might be involved in the breakdown of nerve-muscle contact at NMJs in ALS.

Extrinsic cellular and molecular mediators of peripheral axonal regeneration

The ability of injured peripheral nerves to regenerate and reinnervate their original targets is a characteristic feature of the peripheral nervous system (PNS). On the other hand, neurons of the central nervous system (CNS), including retinal ganglion cell (RGC) axons, are incapable of spontaneous regeneration. In the adult PNS, axonal regeneration after injury depends on well-orchestrated cellular and molecular processes that comprise a highly reproducible series of degenerative reactions distal to the site of injury. During this fine-tuned process, named Wallerian degeneration, a remodeling of the distal nerve fragment prepares a permissive microenvironment that permits successful axonal regrowth originating from the proximal nerve fragment. Therefore, a multitude of adjusted intrinsic and extrinsic factors are important for surviving neurons, Schwann cells, macrophages and fibroblasts as well as endothelial cells in order to achieve successful regeneration. The aim of this review is to summarize relevant extrinsic cellular and molecular determinants of successful axonal regeneration in rodents that contribute to the regenerative microenvironment of the PNS.

Neurotrophic factor changes in the rat thick skin following chronic constriction injury of the sciatic nerve

BACKGROUND

Cutaneous peripheral neuropathies have been associated with changes of the sensory fiber innervation in the dermis and epidermis. These changes are mediated in part by the increase in local expression of trophic factors. Increase in target tissue nerve growth factor has been implicated in the promotion of peptidergic afferent and sympathetic efferent sprouting following nerve injury. The primary source of nerve growth factor is cells found in the target tissue, namely the skin. Recent evidence regarding the release and extracellular maturation of nerve growth factor indicate that it is produced in its precursor form and matured in the extracellular space. It is our hypothesis that the precursor form of nerve growth factor should be detectable in those cell types producing it. To date, limitations in available immunohistochemical tools have restricted efforts in obtaining an accurate distribution of nerve growth factor in the skin of naïve animals and those with neuropathic pain lesions. It is the objective of this study to delineate the distribution of the precursor form of nerve growth factor to those cell types expressing it, as well as to describe its distribution with respect to those nerve fibers responsive to it.

RESULTS

We observed a decrease in peptidergic fiber innervation at 1 week after the application of a chronic constriction injury (CCI) to the sciatic nerve, followed by a recovery, correlating with TrkA protein levels. ProNGF expression in CCI animals was significantly higher than in sham-operated controls from 1-4 weeks post-CCI. ProNGF immunoreactivity was increased in mast cells at 1 week post-CCI and, at later time points, in keratinocytes. P75 expression within the dermis and epidermis was significantly higher in CCI-operated animals than in controls and these changes were localized to neuronal and non-neuronal cell populations using specific markers for each.

CONCLUSIONS

We describe proNGF expression by non-neuronal cells over time after nerve injury as well as the association of NGF-responsive fibers to proNGF-expressing target tissues. ProNGF expression increases following nerve injury in those cell types previously suggested to express it.

Wallerian degeneration: gaining perspective on inflammatory events after peripheral nerve injury

In this review, we first provide a brief historical perspective, discussing how peripheral nerve injury (PNI) may have caused World War I. We then consider the initiation, progression, and resolution of the cellular inflammatory response after PNI, before comparing the PNI inflammatory response with that induced by spinal cord injury (SCI).In contrast with central nervous system (CNS) axons, those in the periphery have the remarkable ability to regenerate after injury. Nevertheless, peripheral nervous system (PNS) axon regrowth is hampered by nerve gaps created by injury. In addition, the growth-supportive milieu of PNS axons is not sustained over time, precluding long-distance regeneration. Therefore, studying PNI could be instructive for both improving PNS regeneration and recovery after CNS injury. In addition to requiring a robust regenerative response from the injured neuron itself, successful axon regeneration is dependent on the coordinated efforts of non-neuronal cells which release extracellular matrix molecules, cytokines, and growth factors that support axon regrowth. The inflammatory response is initiated by axonal disintegration in the distal nerve stump: this causes blood-nerve barrier permeabilization and activates nearby Schwann cells and resident macrophages via receptors sensitive to tissue damage. Denervated Schwann cells respond to injury by shedding myelin, proliferating, phagocytosing debris, and releasing cytokines that recruit blood-borne monocytes/macrophages. Macrophages take over the bulk of phagocytosis within days of PNI, before exiting the nerve by the circulation once remyelination has occurred. The efficacy of the PNS inflammatory response (although transient) stands in stark contrast with that of the CNS, where the response of nearby cells is associated with inhibitory scar formation, quiescence, and degeneration/apoptosis. Rather than efficiently removing debris before resolving the inflammatory response as in other tissues, macrophages infiltrating the CNS exacerbate cell death and damage by releasing toxic pro-inflammatory mediators over an extended period of time. Future research will help determine how to manipulate PNS and CNS inflammatory responses in order to improve tissue repair and functional recovery.

Dose and duration of nerve growth factor (NGF) administration determine the extent of behavioral recovery following peripheral nerve injury in the rat

Nerve growth factor (NGF) has been previously shown to support neuron survival and direct neurite outgrowth in vitro, and to enhance axonal regeneration in vivo. However, a systematic analysis of NGF dose and dose duration on behavioral recovery following peripheral nerve injury in rodents has not been previously investigated. Here, we show that NGF promotes a bell shaped dose-response, with an optimal threshold effect occurring at 800 pg/μl. High dose NGF inhibited regeneration. However, this effect could be reversed through functional blockade of p75 receptors, thus implicating these receptors as mediators of the inhibitory response. Longer term evaluation showed that animals administered NGF at 80 ng/day for 3 weeks had greater sensorimotor recovery compared to all other treatment groups. These animals made significantly fewer errors during skilled locomotion, and displayed both increased vertical and fore-aft ground reaction forces during flat surface locomotion. Furthermore, terminal electrophysiological and myological assessments (EMG, wet gastrocnemius muscle weights) corroborated the behavioral data. Overall, these data support the hypothesis that both appropriate dose and duration of NGF are important determinants of behavioral recovery following nerve injury in the rat.

Neuronal plasticity and seasonal reproduction in sheep

Seasonal reproduction represents a naturally occurring example of functional plasticity in the adult brain as it reflects changes in neuroendocrine pathways controlling gonadotropin-releasing hormone (GnRH) secretion and, in particular, the responsiveness of GnRH neurons to estradiol negative feedback. Structural plasticity within this neural circuitry may, in part, be responsible for seasonal switches in the negative feedback control of GnRH secretion that underlie annual reproductive transitions. We review evidence for structural changes in the circuitry responsible for seasonal inhibition of GnRH secretion in sheep. These include changes in synaptic inputs onto GnRH neurons, as well as onto dopamine neurons in the A15 cell group, a nucleus that plays a key role in estradiol negative feedback. We also present preliminary data suggesting a role for neurotrophins and neurotrophin receptors as an early mechanistic step in the plasticity that accompanies seasonal reproductive transitions in sheep. Finally, we review recent evidence suggesting that kisspeptin cells of the arcuate nucleus constitute a critical intermediary in the control of seasonal reproduction. Although a majority of the data for a role of neuronal plasticity in seasonal reproduction has come from the sheep model, the players and principles are likely to have relevance for reproduction in a wide variety of vertebrates, including humans, and in both health and disease.

Transplantation of Olfactory Ensheathing Cells as Adjunct Cell Therapy for Peripheral Nerve Injury

Traumatic events, such as work place trauma or motor vehicle accident violence, result in a significant number of severe peripheral nerve lesions, including nerve crush and nerve disruption defects. Transplantation of myelin-forming cells, such as Schwann cells (SCs) or olfactory ensheathing cells (OECs), may be beneficial to the regenerative process because the applied cells could mediate neurotrophic and neuroprotective effects by secretion of chemokines. Moreover, myelin-forming cells are capable of bridging the repair site by establishing an environment permissive to axonal regeneration. The cell types that are subject to intense investigation include SCs and OECs either derived from the olfactory bulb or the olfactory mucosa, stromal cells from bone marrow (mesenchymal stem cells, MSCs), and adipose tissue-derived cells. OECs reside in the peripheral and central nervous system and have been suggested to display unique regenerative properties. However, so far OECs were mainly used in experimental studies to foster central regeneration and it was not until recently that their regeneration-promoting activity for the peripheral nervous system was recognized. In the present review, we summarize recent experimental evidence regarding the regenerative effects of OECs applied to the peripheral nervous system that may be relevant to design novel autologous cell transplantation therapies.