Inhibition of sensory neuron apoptosis and prevention of loss by NT-3 administration following axotomy

Electrospun fiber-mediated delivery of neurotrophin-3 mRNA for neural tissue engineering applications

Aligned electrospun fibers provide topographical cues and local therapeutic delivery to facilitate robust peripheral nerve regeneration. mRNA delivery enables transient expression of desired proteins that promote axonal regeneration. However, no prior work delivers mRNA from electrospun fibers for peripheral nerve regeneration applications. Here, we developed the first aligned electrospun fibers to deliver pseudouridine-modified (Ψ) neurotrophin-3 (NT-3) mRNA (ΨNT-3mRNA) to primary Schwann cells and assessed NT-3 secretion and bioactivity. We first electrospun aligned poly(L-lactic acid) (PLLA) fibers and coated them with the anionic substrates dextran sulfate sodium salt (DSS) or poly(3,4-dihydroxy-L-phenylalanine) (pDOPA). Cationic lipoplexes containing ΨNT-3mRNA complexed to JetMESSENGER® were then immobilized to the fibers, resulting in detectable ΨNT-3mRNA release for 28 days from all fiber groups investigated (PLLA+mRNA, 0.5DSS4h+mRNA, and 2pDOPA4h+mRNA). The 2pDOPA4h+mRNA group significantly increased Schwann cell secretion of NT-3 for 21 days compared to control PLLA fibers (p < 0.001-0.05) and, on average, increased Schwann cell secretion of NT-3 by ≥ 2-fold compared to bolus mRNA delivery from the 1µgBolus+mRNA and 3µgBolus+mRNA groups. The 2pDOPA4h+mRNA fibers supported Schwann cell secretion of NT-3 at levels that significantly increased dorsal root ganglia (DRG) neurite extension by 44% (p < 0.0001) and neurite area by 64% (p < 0.001) compared to control PLLA fibers. The data show that the 2pDOPA4h+mRNA fibers enhance the ability of Schwann cells to promote neurite growth from DRG, demonstrating this platform's potential capability to improve peripheral nerve regeneration. STATEMENT OF SIGNIFICANCE: Aligned electrospun fibers enhance axonal regeneration by providing structural support and guidance cues, but further therapeutic stimulation is necessary to improve functional outcomes. mRNA delivery enables the transient expression of therapeutic proteins, yet achieving local, sustained delivery remains challenging. Previous work shows that genetic material delivery from electrospun fibers improves regeneration; however, mRNA delivery has not been explored. Here, we examine mRNA delivery from aligned electrospun fibers to enhance neurite outgrowth. We show that immobilization of NT-3mRNA/JetMESSENGER® lipoplexes to aligned electrospun fibers functionalized with pDOPA enables local, sustained NT-3mRNA delivery to Schwann cells, increasing Schwann cell secretion of NT-3 and enhancing DRG neurite outgrowth. This study displays the potential benefits of electrospun fiber-mediated mRNA delivery platforms for neural tissue engineering.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Governor Vessel Electro-Acupuncture Promotes the Intrinsic Growth Ability of Spinal Neurons through Activating Calcitonin Gene-Related Peptide/α-Calcium/Calmodulin-Dependent Protein Kinase/Neurotrophin-3 Pathway after Spinal Cord Injury

Spinal cord injury (SCI) invariably results in neuronal death and failure of axonal regeneration. This is attributed mainly to the hostile microenvironment and the poor intrinsic regrowth capacity of the injured spinal neurons. We have reported previously that electro-acupuncture on Governor Vessel acupoints (GV-EA) can promote neuronal survival and axonal regeneration of injured spinal cord. However, the underlying mechanism for this has remained uncertain. The present study aimed to explore the neural afferent pathway of GV-EA stimulation and the possible mechanism by which GV-EA can activate the intrinsic growth ability of injured spinal neurons. By cholera toxin B (CTB) retrograde labeling, immunostaining, and enzyme-linked immunosorbent assay (ELISA), we showed here that GV-EA could stimulate the spinal nerve branches of the dorsal root ganglion cells. This would then increase the release of calcitonin gene-related peptide (CGRP) from the afferent terminals in the spinal cord. It is of note that the effect was abrogated after dorsal rhizotomy. Additionally, both in vivo and in vitro results showed that CGRP would act on the post-synaptic spinal cord neurons and triggered the synthesis and secretion of neurotrophin-3 (NT-3) by activating the calcitonin gene-related peptide (CGRP)/ receptor activity-modifying protein (RAMP)1/calcium/calmodulin-dependent protein kinase (αCaMKII) pathway. Remarkably, the observed effect was prevented by the dorsal rhizotomy and the blockers of the CGRP/RAMP1/αCaMKII pathway. More importantly, increase in NT-3 promoted the survival, axonal regrowth, and synaptic maintenance of spinal cord neurons in the injured spinal cord. Therefore, it is concluded that increase in NT-3 production is one of the mechanisms by which GV-EA can activate the intrinsic growth ability of spinal neurons after SCI. The experimental results have reinforced the theoretical basis of GV-EA for its clinical efficacy in patients with SCI.

Peripheral nerve injury induced changes in the spinal cord and strategies to counteract/enhance the changes to promote nerve regeneration

Peripheral nerve injury leads to morphological, molecular and gene expression changes in the spinal cord and dorsal root ganglia, some of which have positive impact on the survival of neurons and nerve regeneration, while the effect of others is the opposite. It is crucial to take prompt measures to capitalize on the positive effects of these reactions and counteract the negative impact after peripheral nerve injury at the level of spinal cord, especially for peripheral nerve injuries that are severe, located close to the cell body, involve long distance for axons to regrow and happen in immature individuals. Early nerve repair, exogenous supply of neurotrophic factors and Schwann cells can sustain the regeneration inductive environment and enhance the positive changes in neurons. Administration of neurotrophic factors, acetyl-L-carnitine, N-acetyl-cysteine, and N-methyl-D-aspartate receptor antagonist MK-801 can help counteract axotomy-induced neuronal loss and promote regeneration, which are all time-dependent. Sustaining and reactivation of Schwann cells after denervation provides another effective strategy. FK506 can be used to accelerate axonal regeneration of neurons, especially after chronic axotomy. Exploring the axotomy-induced changes after peripheral nerve injury and applying protective and promotional measures in the spinal cord which help to retain a positive functional status for neuron cell bodies will inevitably benefit regeneration of the peripheral nerve and improve functional outcomes.

The regulatory roles of non-coding RNAs in nerve injury and regeneration

Non-coding RNAs (ncRNAs), especially microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), have attracted much attention since their regulatory roles in diverse cell processes were recognized. Emerging studies demonstrate that many ncRNAs are differentially expressed after injury to the nervous system, significantly affecting nerve regeneration. In this review, we compile the miRNAs and lncRNAs that have been reported to be dysregulated following a variety of central and peripheral nerve injuries, including acquired brain injury, spinal cord injury, and peripheral nerve injury. We also list investigations on how these miRNAs and lncRNAs exert the regulatory actions in neurodegenerative and neuroregenerative processes through different mechanisms involving their interaction with target coding genes. We believe that comprehension of the expression profiles and the possible functions of ncRNAs during the processes of nerve injury and regeneration will help understand the molecular mechanisms responsible for post-nerve-injury changes, and may contribute to the potential use of ncRNAs as a diagnostic marker and therapeutic target for nerve injury.

Withdrawal and restoration of central vagal afferents within the dorsal vagal complex following subdiaphragmatic vagotomy

Vagotomy, a severing of the peripheral axons of the vagus nerve, has been extensively utilized to determine the role of vagal afferents in viscerosensory signaling. Vagotomy is also an unavoidable component of some bariatric surgeries. Although it is known that peripheral axons of the vagus nerve degenerate and then regenerate to a limited extent following vagotomy, very little is known about the response of central vagal afferents in the dorsal vagal complex to this type of damage. We tested the hypothesis that vagotomy results in the transient withdrawal of central vagal afferent terminals from their primary central target, the nucleus of the solitary tract (NTS). Sprague-Dawley rats underwent bilateral subdiaphragmatic vagotomy and were sacrificed 10, 30, or 60 days later. Plastic changes in vagal afferent fibers and synapses were investigated at the morphological and functional levels by using a combination of an anterograde tracer, synapse-specific markers, and patch-clamp electrophysiology in horizontal brain sections. Morphological data revealed that numbers of vagal afferent fibers and synapses in the NTS were significantly reduced 10 days following vagotomy and were restored to control levels by 30 days and 60 days, respectively. Electrophysiology revealed transient decreases in spontaneous glutamate release, glutamate release probability, and the number of primary afferent inputs. Our results demonstrate that subdiaphragmatic vagotomy triggers transient withdrawal and remodeling of central vagal afferent terminals in the NTS. The observed vagotomy-induced plasticity within this key feeding center of the brain may be partially responsible for the response of bariatric patients following gastric bypass surgery.

Double gene therapy with granulocyte colony-stimulating factor and vascular endothelial growth factor acts synergistically to improve nerve regeneration and functional outcome after sciatic nerve injury in mice

Peripheral-nerve injuries are a common clinical problem and often result in long-term functional deficits. Reconstruction of peripheral-nerve defects is currently undertaken with nerve autografts. However, there is a limited availability of nerves that can be sacrificed and the functional recovery is never 100% satisfactory. We have previously shown that gene therapy with vascular endothelial growth factor (VEGF) significantly improved nerve regeneration, neuronal survival, and muscle activity. Our hypothesis is that granulocyte colony-stimulating factor (G-CSF) synergizes with VEGF to improve the functional outcome after sciatic nerve transection. The left sciatic nerves and the adjacent muscle groups of adult mice were exposed, and 50 or 100 μg (in 50 μl PBS) of VEGF and/or G-CSF genes was injected locally, just below the sciatic nerve, and transferred by electroporation. The sciatic nerves were transected and placed in an empty polycaprolactone (PCL) nerve guide, leaving a 3-mm gap to challenge nerve regeneration. After 6 weeks, the mice were perfused and the sciatic nerve, the dorsal root ganglion (DRG), the spinal cord and the gastrocnemius muscle were processed for light and transmission electron microscopy. Treated animals showed significant improvement in functional and histological analyses compared with the control group. However, the best results were obtained with the G-CSF+VEGF-treated animals: quantitative analysis of regenerated nerves showed a significant increase in the number of myelinated fibers and blood vessels, and the number of neurons in the DRG and motoneurons in the spinal cord was significantly higher. Motor function also showed that functional recovery occurred earlier in animals receiving G-CSF+VEGF-treatment. The gastrocnemius muscle showed an increase in weight and in the levels of creatine phosphokinase, suggesting an improvement of reinnervation and muscle activity. These results suggest that these two factors acted synergistically and optimized the nerve repair potential, improving regeneration after a transection lesion.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

Gene expression profile in rat dorsal root ganglion following sciatic nerve injury and systemic neurotrophin-3 administration

Following sciatic nerve transection in adult rats, a proportion of injured dorsal root ganglion (DRG) neurons die, through apoptosis, over the following 6 months. Previous studies showed that axotomy and neurotrophin-3 administration may have effects on expression of neurotrophins and their receptors in DRG. In the current study, the fourth and fifth lumbar DRGs of rats were examined 2 weeks after right sciatic nerve transection and ligation. The effects of axotomy and systemic NT-3 treatment on neuronal genes were investigated by microarray. The results demonstrated that bone morphogenetic protein (BMP) and Janus protein tyrosine kinase signaling pathways are induced in axotomized DRG, and PI-3 kinase and BMP pathways and genes controlling various cellular functions were induced after axotomy and NT-3 administration.

Inhibition of MAPK ERK impairs axonal regeneration without an effect on neuronal loss after nerve injury

Activation of extracellular signal-regulated protein kinase (ERK), a member of the mitogen-activated protein kinase family, has been shown to mediate neurite outgrowth-promoting effects of various neurotrophic factors in vitro. Moreover, in vivo, ERK is activated in the primary sensory neurons and associated glial cells after nerve injury. However, the precise role of ERK in nerve regeneration remains unclear.

OBJECTIVE

This work was aimed to investigate the effects of ERK inhibition on axonal regeneration and neuronal loss after axotomy.

METHODS

Unilateral sciatic nerve crush was performed, and inhibition of ERK was achieved by intraperitoneal injection of 300 microg kg(-1) day(-1) of u0126 for 2 weeks in the inhibitor group. For the control group, only the vehicle was given with the same schedule.

RESULTS

ERK was activated in the crushed sciatic nerve, and this was significantly reduced by the inhibitor. In contrast, there was no activation of ERK in the L4/L5 spinal ganglia. Morphological analysis revealed the similar extent of neuronal loss in the two groups. In addition, the mean regeneration distance in the inhibitor group was lower than that of the control group.

CONCLUSION

These results suggest the crucial role of ERK in nerve regeneration but not sensory neuronal loss after trauma.

Cigarette smoking decreases neurotrophin-3 expression in rat hippocampus after transient forebrain ischemia

Stroke is a common cause of death and severe disability among adults in developed countries. Cigarette smoking adversely affects human health in many ways and is considered to be a risk factor for a stroke. However, the mechanism that determines the relative importance of neurotrophins in this process remains unclear. To study the effect of chronic cigarette smoking on ischemic stroke, in situ hybridization and immunohistochemistry were employed to detect the mRNA and protein expression of neurotrophin-3 (NT-3), respectively, which is thought to play a critical role in protection against neuronal death in brain ischemia. Rats, with or without chronic cigarette smoking, were subjected to 20 min of transient forebrain ischemia. Distribution and quantification of mRNA and protein of NT-3 in the whole hippocampus and the cell death in the hippocampal CA1-CA3 regions were determined in these rats. Experimental results show that chronic cigarette smoking produces a significantly delay and persistent down-regulation of ischemia-induced NT-3 mRNA and protein changes at 6-24h post-ischemia, and seemingly increases neuron death 7 days after reperfusion. These experimental results indicate that by influencing NT-3 expression, directly or indirectly, chronic cigarette smoking has a potentially harmful effect when acute brain ischemia attacks.

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.

The reduction of proprioceptors in the mesencephalic trigeminal tract nucleus after neonatal masseteric nerve transection; effect of brain-derived neurotrophic factor

The effect of neonatal masseteric nerve transection on primary proprioceptors was examined in the mesencephalic trigeminal tract nucleus (Mes5) of the rat. At 72 h to 21 days after the injury, the number of Mes5 neurons decreased on the side ipsilateral to the transection. The means+/-SD of percentage proportion of ipsilateral/contralateral neurons at 72 h and 21 days were 69.9+/-7.5% and 58.2+/-14.6%, respectively. The application of brain-derived neurotrophic factor to the proximal stump of the masseteric nerve delayed the loss of Mes5 neurons at 72 h after the injury; the mean numbers+/-SD of ipsilateral and contralateral Mes5 neurons in injured animals with BDNF application was 553.6+/-61.9 and 558.4+/-55.3, respectively. Saline application had no effect on the injury-induced loss of Mes5 neurons; i.e., the mean numbers+/-SD of ipsilateral and contralateral Mes5 neurons were 367.3+/-72.5 and 543+/-33.5, respectively. These findings indicate that trigeminal primary proprioceptors are sensitive to the neonatal injury. The survival of proprioceptors during early postnatal period is probably dependent upon brain-derived neurotrophic factor in the trigeminal nervous system.

Evaluation of the long-term regenerative potential in an experimental nerve amputee model

In this study, we evaluated the long-term maintenance of regenerated axons in an experimental nerve amputee model. The sciatic nerve of adult rats was transected and repaired with a silicone tube leaving a short gap; the distal nerve segment was again transected 10 mm distally and the distal stump either introduced in a capped silicone chamber (amputee group) or connected to denervated targets (tibial branch into the gastrocnemius muscle and peroneal nerve apposed to skin) (reinnervation group). Morphological studies were performed at 2.5, 6, and 9 months after surgery. In all cases, axons regenerated across the silicone tube and grew in the distal nerve segment. In the amputee group, the morphological results show the expected features of a neuroma that is formed when regenerating axons are prevented from reaching the end organs, with a large number of axonal profiles indicative of regenerative sprouting. The number of myelinated axons counted at the distal nerve was sustained over 9 months follow-up, indicating that regenerated axons are maintained chronically. Immunohistochemical labeling showed maintained expression of choline acetyltransferase, calcitonin gene-related peptide, and growth-related peptides 43 in the distal neuroma at 6 and 9 months. Reconnection of the distal nerve to foreign targets mildly improved the pattern of nerve regeneration, decreasing the number of excessive sprouts. These results indicate that axons regenerated may be eventually interfaced with external input-output systems over long time, even if ending in the absence of distal targets as will occur in amputee limbs.

Neurotrophin-3 administration alters neurotrophin, neurotrophin receptor and nestin mRNA expression in rat dorsal root ganglia following axotomy

In the months following transection of adult rat peripheral nerve some sensory neurons undergo apoptosis. Two weeks after sciatic nerve transection some neurons in the L4 and L5 dorsal root ganglia begin to show immunoreactivity for nestin, a filament protein expressed by neuronal precursors and immature neurons, which is stimulated by neurotrophin-3 (NT-3) administration. The aim of this study was to examine whether NT-3 administration could be compensating for decreased production of neurotrophins or their receptors after axotomy, and to determine the effect on nestin synthesis. The levels of mRNA in the ipsilateral and contralateral L4 and L5 dorsal root ganglia were analyzed using real-time polymerase chain reaction, 1 day, 1, 2 and 4 weeks after unilateral sciatic nerve transection and NT-3 or vehicle administration via s.c. micro-osmotic pumps. In situ hybridization was used to identify which cells and neurons expressed mRNAs of interest, and the expression of full-length trkC and p75NTR protein was investigated using immunohistochemistry. Systemic NT-3 treatment increased the expression of brain-derived neurotrophic factor, nestin, trkA, trkB and trkC mRNA in ipsilateral ganglia compared with vehicle-treated animals. Some satellite cells surrounding neurons expressed trkA and trkC mRNA and trkC immunoreactivity. NT-3 administration did not affect neurotrophin mRNA levels in the contralateral ganglia, but decreased the expression of trkA mRNA and increased the expression of trkB mRNA and p75NTR mRNA and protein. These data suggest that systemically administered NT-3 may counteract the decrease, or even increase, neurotrophin responsiveness in both ipsi- and contralateral ganglia after nerve injury.

Isolation and Characterization of Neural Crest Progenitors from Adult Dorsal Root Ganglia

After peripheral nerve injury, the number of sensory neurons in the adult dorsal root ganglia (DRG) is initially reduced but recovers to a normal level several months later. The mechanisms underlying the neuronal recovery after injury are not clear. Here, we showed that in the DRG explant culture, a subpopulation of cells that emigrated out from adult rat DRG expressed nestin and p75 neurotrophin receptor and formed clusters and spheres. They differentiated into neurons, glia, and smooth muscle cells in the presence or absence of serum and formed secondary and tertiary neurospheres in cloning assays. Molecular expression analysis demonstrated the characteristics of neural crest progenitors and their potential for neuronal differentiation by expressing a set of well-defined genes related to adult stem cells niches and neuronal fate decision. Under the influence of neurotrophic factors, some of these progenitors gave rise to neuropeptide-expressing cells and protein zero-expressing Schwann cells. In a 5-bromo-2'-deoxyuridine chasing study, we showed that these progenitors likely originate from satellite glial cells. Our study suggests that a subpopulation of glia in adult DRG is likely to be progenitors for neurons and glia and may play a role in neurogenesis after nerve injury. Disclosure of potential conflicts of interest is found at the end of this article.

X-Linked inhibitor of apoptosis protein gene-based neuroprotection for the peripheral nervous system

OBJECTIVE

The recently discovered X-linked inhibitor of apoptosis protein (XIAP) is among the most potent inhibitors of programmed cell death. In the current experiment, we examine the potential of adenoviral XIAP gene delivery to protect neurons of the peripheral nervous system using in vitro models of amyotrophic lateral sclerosis (ALS) and diabetic neuropathy.

METHODS

XIAP complementary deoxyribonucleic acid was fused in frame with the green fluorescent protein sequence and cloned into a first generation adenoviral vector. The impact of XIAP gene expression on glutamate-induced apoptosis was measured in the neuronal SH-SY5Y cell line with immunohistochemistry for active caspase-3 and with cell density assays. Next, the effect of XIAP expressing neurons on the survival of uninfected neighboring neurons was measured. Finally, the impact of XIAP gene expression on glutamate-induced apoptosis was assessed in embryonic motor neuron and dorsal root ganglion cultures.

RESULTS

XIAP gene expression reduced the percentage of active caspase-3 positive SH-SY5Y neurons and preserved cell density after glutamate exposure. In heterogeneously infected cultures, cells infected with XIAP were protected, but uninfected neighboring cells were not. In primary E15 models, inhibition of proapoptotic effects was demonstrated after glutamate insult in motor neurons and glucose insult in dorsal root ganglion cells.

CONCLUSION

XIAP gene delivery through the neurosurgical delivery of viral vectors may provide a means for neuroprotection in ALS and diabetic neuropathy.

Tyrosine kinase receptor immunoreactivity in trigeminal mesencephalic and motor neurons following transection of masseteric nerve of the rat

Neurotrophins are known to promote survival after neural injury. To determine the relative importance of tyrosine kinase receptors on the survival of axotomized trigeminal nuclear neurons, we examined the temporal expression profile of tyrosine kinase A, tyrosine kinase B and tyrosine kinase C receptors in the mesencephalic trigeminal nucleus and the motor trigeminal nucleus following transection of the masseteric nerve in rats. Axotomized neurons in these nuclei were retrogradely identified with FluoroGold. We found increase in tyrosine kinase A-immunoreactive mesencephalic trigeminal nucleus neurons in the second week after axotomy but no change in the number of tyrosine kinase A-immunoreactive motor trigeminal nucleus neurons. There was no change in the number of tyrosine kinase B-immunoreactive mesencephalic trigeminal nucleus neurons but the significant increase of tyrosine kinase B-immunoreactive motor trigeminal nucleus neurons throughout the period of observation (3 weeks) peaked at approximately 1 week after axotomy. There was no alteration in the number of tyrosine kinase C-immunoreactive mesencephalic trigeminal nucleus neurons but significant increase in tyrosine kinase C-immunoreactive motor trigeminal nucleus neurons observable by 4 days post-axotomy was followed by decline to levels lower than the control in 2 weeks. Temporal changes in the expression of individual tyrosine kinase receptors in mesencephalic trigeminal nucleus and motor trigeminal nucleus neurons following transection of the masseteric nerve suggest differential contribution of tyrosine kinase-specific neurotrophins to the survival of these neurons after axotomy.

Oxaliplatin causes selective atrophy of a subpopulation of dorsal root ganglion neurons without inducing cell loss

Peripheral neuropathy is induced by multiple doses of oxaliplatin and interferes with the clinical utility of the drug in patients with colorectal cancer. In this study, we sought to determine whether cell loss or selective neuronal damage was the basis for the peripheral neuropathy caused by oxaliplatin. Adult female rats were given 1.85 mg/kg oxaliplatin twice per week for 8 weeks. Nerve conduction and L5 dorsal root ganglia (DRG) were studied 1 week after the completion of all treatment. No mortality occurred during oxaliplatin treatment, but the rate of body weight gain was reduced compared to age-matched vehicle-treated controls. Oxaliplatin slowed conduction velocity and delayed conduction times in peripheral sensory nerves, without affecting central or motor nerve conduction. In L5 DRG, total numbers of neurons were unchanged by oxaliplatin, but there were significant reductions in neuronal size distribution, ganglion volume, average cell size and the relative frequency of large cells. In addition, the relative frequency of small DRG cells was increased by oxaliplatin. Oxaliplatin significantly altered the size distribution and average cell body area of the predominantly large parvalbumin-immunoreactive DRG neurons without affecting the frequency of parvalbumin staining. On the contrary, neither the staining frequency nor the size distribution of the predominantly small substance P-immunoreactive DRG neurons was changed by oxaliplatin. In conclusion, oxaliplatin causes selective atrophy of a subpopulation of DRG neurons with predominantly large parvalbumin-expressing cells without inducing neuronal loss. Because DRG cell body size and axonal conduction velocity are positively correlated, neuronal atrophy may be the morphological basis for the development of decreased sensory nerve conduction velocity that characterizes oxaliplatin-induced peripheral neuropathy.

Effects of systemically administered NT-3 on sensory neuron loss and nestin expression following axotomy

Previous work has shown that administration of the neurotrophin NT-3 intrathecally or to the proximal stump can prevent axotomy-induced sensory neuron loss and that NT-3 can stimulate sensory neuron differentiation in vitro. We have examined the effect of axotomy and systemic NT-3 administration on neuronal loss, apoptosis (defined by morphology and activated caspase-3 immunoreactivity), and nestin expression (a protein expressed by neuronal precursor cells) in dorsal root ganglia (DRG) following axotomy of the adult rat sciatic nerve. Systemic administration of 1.25 or 5 mg of NT-3 over 1 month had no effect on the incidence of apoptotic neurons but prevented the overall loss of neurons seen at 4 weeks in vehicle-treated animals. Nestin-immunoreactive neurons began to appear 2 weeks after sciatic transection in untreated animals and steadily increased in incidence over the next 6 weeks. NT-3 administration increased the number of nestin-immunoreactive neurons at 1 month by two- to threefold. Nestin-IR neurons had a mean diameter of 20.78 +/- 2.5 microm and expressed the neuronal markers neurofilament 200, betaIII-tubulin, protein gene product 9.5, growth associated protein 43, trkA, and calcitonin gene-related peptide. Our results suggest that the presence of nestin in DRG neurons after nerve injury is due to recent differentiation and that exogenous NT-3 may prevent neuron loss by stimulating this process, rather than preventing neuron death.

PRP-1 Protective Effect against Central and Peripheral Neurodegeneration following n. ischiadicus Transection

We investigated the action of the new hypothalamic proline-rich peptide (PRP-1), normally produced by neurosecretory cells of hypothalamic nuclei (NPV and NSO), 3 and 4 weeks following rat sciatic nerve transection. The impulse activity flow of interneurons (IN) and motoneurons (MN) on stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius--G) and extensor (n. peroneus communis--P) nerves of both injured and symmetric intact sides of spinal cord (SC) was recorded in rats with daily administration of PRP-1 (for a period of 3 weeks) and without it (control). On the injured side of SC in control, there were no responses of IN and MN on ipsilateral G and P stimulation, while responses were elicited on contralateral nerve stimulation. The neuron responses on the intact side of SC were revealed in a reverse ratio. Thus, there were no effects upon stimulation of the injured nerve distal stump in the control because of the absence of fusion between transected nerve stumps. This was also testified by the atrophy of the distal stump and the absence of motor activity of the affected limb. In PRP-1-treated animals, the responses of SC IN and MN in postaxotomy 3 weeks on the injured side of SC at ipsilateral nerve stimulation and on the intact side at contralateral nerve stimulation were recorded because of the obvious fusion of the severed nerve stumps. The histochemical data confirmed the electrophysiological findings. Complete coalescence of transected fibers together with restoration of the motor activity of the affected limb provided evidence for reinnervation on the injured side. Thus, it may be concluded that PRP-1 promotes nerve regeneration and may be used clinically to improve the outcome of peripheral nerve primary repair.

Frozen-section fluorescence microscopy and stereology in the quantification of neuronal death within dorsal root ganglia

Histochemical and morphological research increasingly relies upon quantification of complex biological systems. For such investigations to be meaningful, quantification techniques must meet the seemingly conflicting requirements of being theoretically robust, yet sufficiently practical to facilitate widespread applicability. Validity ought to be enhanced by theoretical simplicity, use of measured rather than assumed variables, and minimising observer interpretation. Practicality is facilitated by simplifying and reducing measurements, broadening applicability, and reducing costs and analysis time. As a result, quantification systems that rely upon sampling and estimation have been favoured over serial reconstruction techniques. To provide reliable estimates, sampling must be valid at all levels from tissue harvest, to the selection of microscope fields in which quantification is performed by techniques that account for the anisotropic distribution, and variable size of many elements in biological systems. These principles are embodied in the development of a stereological approach to the quantification of neuronal death within dorsal root ganglia after peripheral nerve injury. This frozen section technique is efficient and flexible, since it permits simultaneous morphological examination, TUNEL, or standard fluorescence immunohistochemistry, broadening its applicability. Section shrinkage is minimal, and counting by optical disection has proved to be time-efficient and sufficiently reproducible to reliably detect losses in the order of 5, with minimal inter-observer variation. As is discussed, stereology has not yet met with universal acceptance, but by balancing theoretical validity with practical applicability, it has proved an excellent approach to the investigation of neuronal death within dorsal root ganglia. Frozen-section fluorescence microscopy and stereology in the quantification of neuronal death within dorsal root ganglia.

Selective reactions of cutaneous and muscle afferent neurons to peripheral nerve transection in rats

To determine whether peripheral nerve injury has similar effects on all functional types of afferent neuron, we retrogradely labeled populations of neurons projecting to skin and to muscle with FluoroGold and lesioned various peripheral nerves in the rat. Labeled neurons were counted after different periods and related to immunohistochemically identified ectopic terminals and satellite cells in lumbar dorsal root ganglia. After 10 weeks, 30% of cutaneous afferent somata labeled from transected sural nerves had disappeared but, if all other branches of the sciatic nerve had also been cut, 60% of cutaneous neurons were lost. Small-diameter sural neurons preferentially disappeared. In contrast, the number of muscle afferent somata was not affected by transection of various nerves. p75 was downregulated in axotomized cutaneous neurons but in not axotomized muscle neurons. Conversely, p75 was upregulated in satellite cells around cutaneous but not muscle neurons. Consistent with this, perineuronal rings containing tyrosine hydroxylase, calcitonin gene-related peptide, galanin, or synaptophysin were formed preferentially around cutaneous neurons. Selective lesions of predominantly cutaneous nerves triggered the formation of rings, but none were detected after selective lesions of muscle nerves. We conclude that cutaneous neurons are both more vulnerable and more associated with ectopic nerve terminals than muscle neurons in dorsal root ganglia after transection and ligation of peripheral nerves.

Thyroid hormone reduces the loss of axotomized sensory neurons in dorsal root ganglia after sciatic nerve transection in adult rat

We have shown that a local administration of thyroid hormones (T3) at the level of transected rat sciatic nerve induced a significant increase in the number of regenerated axons. To address the question of whether local administration of T3 rescues the axotomized sensory neurons from death, in the present study we estimated the total number of surviving neurons per dorsal root ganglion (DRG) in three experimental group animals. Forty-five days following rat sciatic nerve transection, the lumbar (L4 and L5) DRG were removed from PBS-control, T3-treated as well as from unoperated rats, and serial sections (1 microm) were cut. The physical dissector method was used to estimate the total number of sensory neurons in the DRGs. Our results revealed that in PBS-control rats transection of sciatic nerve leads to a significant (P < 0.001) decrease in the mean number of sensory neurons (8743.8 +/- 748.6) compared with the number of neurons in nontransected ganglion (mean 13,293.7 +/- 1368.4). However, administration of T3 immediately after sciatic nerve transection rescues a great number of axotomized neurons so that their mean neuron number (12,045.8 +/- 929.8) is not significantly different from the mean number of neurons in the nontransected ganglion. In addition, the volume of ganglia showed a similar tendency. These results suggest that T3 rescues a high number of axotomized sensory neurons from death and allows these cells to grow new axons. We believe that the relative preservation of neurons is important in considering future therapeutic approaches of human peripheral nerve lesion and sensory neuropathy.

Neurotrophin-3 prevents the proximal accumulation of neurofilament proteins in sensory neurons of streptozocin-induced diabetic rats

The relation between neurofilament expression and/or phosphorylation in the proximal versus distal components of the sensory peripheral neuraxis was studied and related to disorders in structure and function of the distal axon of streptozocin (STZ)-induced diabetic rats studied for 14 weeks. The ability of neurotrophin-3 (NT-3) to prevent abnormalities in neurofilament biology was also investigated. Compared with age-matched controls, neurofilament heavy (NF-H) (3.3-fold) and neurofilament medium (NF-M) (2.5-fold), but not neurofilament light (NF-L), subunits accumulated in the proximal axon of sensory neurons of the lumbar dorsal root ganglia (DRG) in untreated diabetic rats. Neurofilament accumulation was prevented by NT-3. Small- and large-diameter sensory neurons exhibited elevated levels of NF-H protein accumulation and phosphorylation in the DRG of untreated diabetic rats, levels that were ameliorated by NT-3. The sural nerve of untreated diabetic rats showed a 50% decrease in the levels of NF-H and NF-M, but not NF-L, subunits; NT-3 only partially normalized the defect in NF-M expression. These observations were associated with significant lowering of motor and sensory nerve conduction velocity but no alteration in the mean axonal diameter of myelinated axons in the sural nerve in untreated diabetic rats. It is proposed that the accumulation of NF-H and NF-M subunits in the proximal axon is an etiologic factor in the distal axon degeneration observed in diabetes.

Damage-induced neuronal endopeptidase (DINE/ECEL) expression is regulated by leukemia inhibitory factor and deprivation of nerve growth factor in rat sensory ganglia after nerve injury

Damage-induced neuronal endopeptidase (DINE) is a novel metallopeptidase and is expressed in response to various neuronal injuries. The expression regulation of DINE mRNA in the dorsal root ganglia (DRGs) after sciatic nerve injury is examined. A substantial increase of DINE mRNA expression was observed in relatively small-sized DRG neurons after nerve injury. The expression was observed in isolectin B4-negative and partly TrkA-positive neurons, and the expression profile was fairly similar to that of the neuropeptide galanin. More than 80% of DINE mRNA-positive neurons simultaneously demonstrated galanin immunoreactivity after nerve injury. In cultured DRG, DINE mRNA expression was enhanced by leukemia inhibitory factor (LIF) but not by other growth factors and cytokines. LIF treatment to rat sciatic nerve induced DINE mRNA expression in DRG without nerve injury, and, conversely, the intranerve injection of anti-gp130 antibody after sciatic nerve injury significantly inhibited the upregulation of DINE mRNA in DRG. Furthermore, nerve growth factor (NGF) deprivation, which can induce galanin expression, also enhanced DINE mRNA expression in vitro and in vivo. Both LIF application and NGF deprivation additively enhanced DINE expression in vitro. These results suggest that DINE gene expression is regulated separately by both LIF and NGF deprivation, and this regulation pattern is similar to that of galanin gene expression. Because both DINE and galanin have a neuroprotective function, their simultaneous induction may provide more successful protection for injured sensory neurons.

On the use of fast blue, fluoro-gold and diamidino yellow for retrograde tracing after peripheral nerve injury: uptake, fading, dye interactions, and toxicity

The usefulness of three retrograde fluorescent dyes for tracing injured peripheral axons was investigated. The rat sciatic was transected bilaterally and the proximal end briefly exposed to either Fast Blue (FB), Fluoro-Gold (FG) or to Diamidino Yellow (DY) on the right side, and to saline on the left side, respectively. The nerves were then resutured and allowed to regenerate. Electrophysiological tests 3 months later showed similar latencies and amplitudes of evoked muscle and nerve action potentials between tracer groups. The nerves were then cut distal to the original injury and exposed to a second (different) dye. Five days later, retrogradely labelled neurones were counted in the dorsal root ganglia (DRGs) and spinal cord ventral horn. The number of neurones labelled by the first tracer was similar for all three dyes in the DRG and ventral horn except for FG, which labelled fewer motoneurones. When used as second tracer, DY labelled fewer neurones than FG and FB in some experimental situations. The total number of neurones labelled by the first and/or second tracer was reduced by about 30% compared with controls. The contributions of cell death as well as different optional tracer combinations for studies of nerve regeneration are discussed.

BDNF abolishes the survival effect of NT-3 in axotomized Clarke neurons of adult rats

Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) have previously been shown to support survival and axonal regeneration in various types of neurons. Also, synergistic neuroprotective effects of these neurotrophins have been reported in descending rubrospinal neurons after cervical spinal cord injury (Novikova et al., [2000] Eur. J. Neurosci. 12:776-780). The present study investigates the effects of intrathecally delivered NT-3 and BDNF on the survival and atrophy of ascending spinocerebellar neurons of Clarke nucleus (CN) after cervical spinal cord injury in adult rats. At 8 weeks after cervical spinal cord hemisection, 40% of the axotomized CN neurons had been lost, and the remaining cells exhibited marked atrophy. Microglial activity was significantly increased in CN of the operated side. Intrathecal infusion of NT-3 for 8 weeks postoperatively resulted in 91% cell survival and a reduction in cell atrophy, but did not reduce microglial activity. In spite of the fact that the CN neurons expressed both TrkC and TrkB receptors, only NT-3 had a neuroprotective effect, whereas BDNF was ineffective. Furthermore, when a combination of BDNF and NT-3 was administered, the neuroprotective effect of NT-3 was lost. The present results indicate a therapeutic potential for NT-3 in the treatment of spinal cord injury, but also demonstrate that in certain neuronal populations the neuroprotection obtained by a combination of neurotrophic factors may be less than that of a single neurotrophin.

Neurotrophin-3 antisense oligonucleotide attenuates nerve injury-induced Abeta-fibre sprouting

It is proposed that following peripheral nerve injury abnormal sprouting of Abeta-fibre primary afferent neurons in the spinal cord contributes to the allodynia that often occurs with such injury. Allodynia is characterized as pain due to a stimulus which is normally non-noxious. Our recent in vivo experiments show that intrathecal administration of neurotrophin-3 (NT-3), in normal animals, induces allodynia and sprouting of Abeta-fibres. In this study, we examine whether intrathecal administration of NT-3 antisense oligonucleotides (50 microM), via an osmotic pump for 14 days, attenuates nerve injury-induced sprouting and allodynia. The oligonucleotides used in this study were phosphorothioate modified and control experiments, using an ELISA, confirm that intrathecal administration of the antisense induces a significant decrease in NT-3 levels in the spinal cord. All surgery was conducted on anaesthetized Wistar rats (sodium pentobarbitone, i.p. 50 mg/kg). Consistent with previous studies, transganglionic labelling of Abeta-fibres with choleragenoid-horseradish peroxidase (C-HRP) shows that complete transection of the sciatic nerve induces an expansion of C-HRP label into lamina II of the spinal dorsal horn. Using image analysis, we find that intrathecal administration of NT-3 antisense attenuates the density of C-HRP labelling in lamina II in nerve injured animals. A NT-3 sense oligonucleotide (50 microM) has no effect. To test the effect of NT-3 antisense on allodynia, the nociceptive flexion reflex is examined, using an Ugo Basile Analgesymeter, in animals with partial sciatic nerve ligation. Intrathecal administration of 50 microM NT-3 antisense significantly attenuates nerve injury-induced allodynia, whereas the sense oligonucleotide has no effect. These results provide further evidence that endogenous NT-3 contributes to both nerve injury-induced Abeta-fibre sprouting and allodynia and demonstrates the potential of neurotrophin-3 antisense oligonucleotides as therapeutic agents for neuropathic pain.

Downregulation of TrkA expression in primary sensory neurons after unilateral lumbar spinal nerve transection and some rescuing effects of nerve growth factor infusion

Peripheral nerve injury results in sprouting of sympathetic and sensory nerve terminals around large diameter neurons in the dorsal root ganglia (DRG), but the underlying mechanism is not clear. Current study sought to examine changes of the nerve growth factor (NGF) receptor TrkA in DRG and spinal cord after a spinal nerve transection by an immunohistochemical technique and to investigate effects of NGF on the expression of TrkA protein in the same animal model. In the control rat, TrkA immunoreactivity was localized to about 55 +/ -1% of total neurons in DRG and to laminae I and II of the spinal cord. The percentage of TrkA immunoreactive neurons in DRG and TrkA staining intensity of spinal cord were reduced 1 week after the nerve lesion. The changes became maximal 2 weeks, but recovered partially 4 weeks after the lesion. The size of TrkA immunoreactive neurons dramatically shifted to smaller sizes, becoming more remarkable 4 weeks after the lesion. In the contralateral DRG, the percentage of TrkA immunoreactive neurons also decreased significantly. Exogenous NGF delivered to DRG for 2 weeks partially reversed the reduction of TrkA expression as well as atrophy of TrkA immunoreactive neurons. No TrkA immunoreactive basket was found around neuronal somata. Our data show that unilateral peripheral nerve injury results in dynamic downregulation of TrkA in sensory neurons in bilateral DRG and spinal cord, and that TrkA expression in sensory neurons is partially regulated by target-derived NGF.