Ventral root avulsion leads to downregulation of GluR2 subunit in spinal motoneurons in adult rats

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Up-regulation of heat shock protein 27 inhibits apoptosis in lumbosacral nerve root avulsion-induced neurons

Lumbosacral nerve root avulsion leads to widespread death of neurons in the anterior horn area of the injured spinal cord, which results in dysfunction in the lower extremities. Heat shock protein 27 (Hsp27) has been found to play cytoprotective roles under adverse conditions. However, the role of Hsp27 in neurons after lumbosacral nerve root avulsion is unknown. The aim of the present study was to investigate the effects and mechanism of action of Hsp27 on neurons after lumbosacral nerve root avulsion. It was found that Hsp27 expression was elevated in the anterior horn area of the injured spinal cord and the up-regulation of Hsp27 protected neurons against apoptosis after lumbosacral nerve root avulsion. In addition, Hsp27 plays an anti-apoptotic role by suppressing oxidative stress reactions. These findings indicated that Hsp27 may play a key role in resistance to lumbosacral nerve root avulsion-induced neuron apoptosis and may prove to be a potential strategy for improving prognosis after lumbosacral nerve root avulsion.

Clinical and neurobiological advances in promoting regeneration of the ventral root avulsion lesion

Root avulsions due to traction to the brachial plexus causes complete and permanent loss of function. Until fairly recent, such lesions were considered impossible to repair. Here we review clinical repair strategies and current progress in experimental ventral root avulsion lesions. The current gold standard in patients with a root avulsion is nerve transfer, whereas reimplantation of the avulsed root into the spinal cord has been performed in a limited number of cases. These neurosurgical repair strategies have significant benefit for the patient but functional recovery remains incomplete. Developing new ways to improve the functional outcome of neurosurgical repair is therefore essential. In the laboratory, the molecular and cellular changes following ventral root avulsion and the efficacy of intervention strategies have been studied at the level of spinal motoneurons, the ventral spinal root and peripheral nerve, and the skeletal muscle. We present an overview of cell-based pharmacological and neurotrophic factor treatment approaches that have been applied in combination with surgical reimplantation. These interventions all demonstrate neuroprotective effects on avulsed motoneurons, often accompanied with various degrees of axonal regeneration. However, effects on survival are usually transient and robust axon regeneration over long distances has as yet not been achieved. Key future areas of research include finding ways to further extend the post-lesion survival period of motoneurons, the identification of neuron-intrinsic factors which can promote persistent and long-distance axon regeneration, and finally prolonging the pro-regenerative state of Schwann cells in the distal nerve.

A new cervical nerve root avulsion model using a posterior extra-vertebral approach in rats

BACKGROUND

The nerve root avulsion injury causes decrease of motor neurons in the spinal ventral horn. To investigate the motoneuron death after avulsion injury in rats, the intradural root avulsion procedure is usually used, although it is technically demanding and associated with a risk of unexpected spinal cord damage. We have developed a new cervical nerve root avulsion procedure in rats and investigated the validity of our procedure.

METHODS

Our procedure is using a posterior approach and pulling the C6 nerve root outside the vertebral foramen without intradural procedures. The lateral third of the lateral mass is needed to be resected before pulling the nerve root. The accomplishment of our procedure is judged by confirmation of the bifurcated stump of the avulsed nerve root and the leakage of the spinal fluid from vertebral foramen. At first, four Sprague-Dawley (SD) rats were used for the examination of C6 motor neuron distribution in the normal spinal cord. Then, 40 SD rats were divided into following four groups and the survival rate of motor neuron was examined. (A) an intradural avulsion group, (B) an intradural rhizotomy group, (C) our extravertebral avulsion group, and (D) an extravertebral rupture group. Another 26 SD rats were used for the examination of histomorphorogic changes in the spinal cord after our extra-vertebral avulsion procedure.

RESULTS

At 28 days after injury, the percentage of surviving motor neurons in groups A (39.0 ± 2.1%) and C (47.5 ± 7.1%) were significantly lower than those in groups B (77.1 ± 12.3%) and D (98.9 ± 9.9%). Compared with other groups, our procedure was easier and associated with less unexpected spinal cord damage. Although the length of the distal stump of the extravertebrally avulsed ventral rootlets was varied between 1.5 and 3.2 mm, this difference did not affect motoneuron death. The extravertebral avulsion injury showed intraspinal bleeding along the motoneuron axons, glial reaction and macrophage infiltration in the lesioned side of the ventral horn.

CONCLUSIONS

Our extravertebral avulsion procedure is simple and reproducible. It would become a useful tool for the study of cervical nerve root avulsion injury.

TNF-α triggers rapid membrane insertion of Ca(2+) permeable AMPA receptors into adult motor neurons and enhances their susceptibility to slow excitotoxic injury

Excitotoxicity (caused by over-activation of glutamate receptors) and inflammation both contribute to motor neuron (MN) damage in amyotrophic lateral sclerosis (ALS) and other diseases of the spinal cord. Microglial and astrocytic activation in these conditions results in release of inflammatory mediators, including the cytokine, tumor necrosis factor-alpha (TNF-α). TNF-α has complex effects on neurons, one of which is to trigger rapid membrane insertion of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors, and in some cases, specific insertion of GluA2 lacking, Ca(2+) permeable AMPA receptors (Ca-perm AMPAr). In the present study, we use a histochemical stain based upon kainate stimulated uptake of cobalt ions ("Co(2+) labeling") to provide the first direct demonstration of the presence of substantial numbers of Ca-perm AMPAr in ventral horn MNs of adult rats under basal conditions. We further find that TNF-α exposure causes a rapid increase in the numbers of these receptors, via a phosphatidylinositol 3 kinase (PI3K) and protein kinase A (PKA) dependent mechanism. Finally, to assess the relevance of TNF-α to slow excitotoxic MN injury, we made use of organotypic spinal cord slice cultures. Co(2+) labeling revealed that MNs in these cultures possess Ca-perm AMPAr. Addition of either a low level of TNF-α, or of the glutamate uptake blocker, trans-pyrrolidine-2,4-dicarboxylic acid (PDC) to the cultures for 48 h resulted in little MN injury. However, when combined, TNF-α+PDC caused considerable MN degeneration, which was blocked by the AMPA/kainate receptor blocker, 2,3-Dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline (NBQX), or the Ca-perm AMPAr selective blocker, 1-naphthyl acetylspermine (NASPM). Thus, these data support the idea that prolonged TNF-α elevation, as may be induced by glial activation, acts in part by increasing the numbers of Ca-perm AMPAr on MNs to enhance injurious excitotoxic effects of deficient astrocytic glutamate transport.

Involvement of brain-derived neurotrophic factor and sonic hedgehog in the spinal cord plasticity after neurotoxic partial removal of lumbar motoneurons

Adult mammals could spontaneously achieve a partial sensory-motor recovery after spinal cord injury, by mechanisms including synaptic plasticity. We previously showed that this recovery is associated to the expression of synapsin-I, and that sonic hedgehog and Notch-1 could be also involved in plasticity. The role of brain-derived neurotrophic factor and glutamate receptors in regulating synaptic efficacy has been explored in the last decade but, although these mechanisms are now well-defined in the brain, the molecular mechanisms underlying the so called "spinal learning" are still less clear. Here, we measured the expression levels of choline acetyltransferase, synapsin-I, sonic hedgehog, Notch-1, glutamate receptor subunits (GluR1, GluR2, GluR4, NMDAR1) and brain-derived neurotrophic factor, in a motoneuron-depleted mouse spinal lesion model obtained by intramuscular injection of cholera toxin-B saporin. The lesion caused the down-regulation of the majority of analysed proteins. Moreover, we found that in lesioned but not in control spinal tissue, synapsin-I expression is associated to that of both brain-derived neurotrophic factor and sonic hedgehog, whereas GluR2 expression is linked to that of Shh. These results suggest that brain-derived neurotrophic factor and sonic hedgehog could collaborate in modulating synaptic plasticity after the removal of motoneurons, by a mechanism involving both pre- and post-synaptic processes. Interestingly, the involvement of sonic hedgehog showed here is novel, and offers new routes to address spinal cord plasticity and repair.

Ca permeable AMPA channels in diseases of the nervous system

Since the discovery and molecular characterization of Ca(2+)-permeable AMPA channels just over two decades ago, a large body of evidence has accumulated implicating contributions of these unusual glutamate activated channels to selective neurodegeneration in certain conditions, including ischemia and amyotrophic lateral sclerosis. Factors likely involved in their contributions to disease include their distinct patterns of expression in certain neuronal populations, their upregulation via various mechanisms in response to disease associated stresses, and their high permeability to Zn(2+) as well as to Ca(2+). However, full characterization of their contributions to certain diseases as well as development of therapeutics has been limited by the lack of selective and bioavailable blockers of these channels that can be employed in animals or humans. This review summarizes some of the clues that have emerged over recent years to the contributions of these channels in disease.

Sigma receptor agonist 2-(4-morpholinethyl)1 phenylcyclohexanecarboxylate (Pre084) increases GDNF and BiP expression and promotes neuroprotection after root avulsion injury

Spinal root avulsion leads to a progressive loss of axotomized motoneurons (MNs). Nowadays, there is no effective treatment to prolong MN survival that could permit recovery as a result of delayed surgical repair. Administration of Sigma-1 receptor (Sig-1R) ligands has been reported to promote beneficial effects after several types of neural injury. In order to shed light of whether Sig-1R ligands could promote MN survival after root avulsion, L4-L5 spinal roots were unilaterally avulsed in adult rats and the Sig-1R agonist Pre084 was administered at different doses. The ventral spinal cords of the animals were studied from 3 to 21 days post-operation (DPO) by using histological, immunohistochemical, and Western blot techniques. Daily treatment with 0.25 mg/kg Pre084 significantly promoted MN survival (68% vs 43% in untreated rats) at 21 DPO, an effect that was antagonized by coadministration of BD1063, an antagonist of Sig-1R. There was a reduction in astroglial- associated immunoreactivity in rats treated with Pre084. Moreover, Pre084 produced an increase in the Sig-1R co-chaperone BiP within MNs, and an increase of GDNF expression by astrocytes in the ventral horn early after injury. Although the mechanisms promoting MN survival by Pre084 remain unclear, we hypothesize that it is mediated at least in part through the increase in these cytoprotective factors. Therefore, early application of Sig-1R agonist appears to be a promising therapy to improve MN survival after root avulsion.

The AMPA receptor as a therapeutic target: current perspectives and emerging possibilities

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a subtype of the ionotropic glutamate receptors that plays a prominent role in neurotransmission and is widespread throughout the CNS. Because of this, its malfunction can result in a multitude of nervous system diseases. This review looks at compounds that are able to modulate AMPAR function by binding to one of several sites on the receptor that either downregulate its function (competitive, noncompetitive and uncompetitive antagonists) or upregulate its function (positive modulators). It will also give an account of the various diseases that have implicated AMPAR dysfunction and how specific types of AMPAR modulator may be beneficial in their treatment. The AMPAR remains an unexploited but important therapeutic target.

Neuroregenerative effects of lentiviral vector-mediated GDNF expression in reimplanted ventral roots

Traumatic avulsion of spinal nerve roots causes complete paralysis of the affected limb. Reimplantation of avulsed roots results in only limited functional recovery in humans, specifically of distal targets. Therefore, root avulsion causes serious and permanent disability. Here, we show in a rat model that lentiviral vector-mediated overexpression of glial cell line-derived neurotrophic factor (GDNF) in reimplanted nerve roots completely prevents motoneuron atrophy after ventral root avulsion and stimulates regeneration of axons into reimplanted roots. However, over the course of 16 weeks neuroma-like structures are formed in the reimplanted roots, and regenerating axons are trapped at sites with high levels of GDNF expression. A high local concentration of GDNF therefore impairs long distance regeneration. These observations show the feasibility of combining neurosurgical repair of avulsed roots with gene-therapeutic approaches. Our data also point to the importance of developing viral vectors that allow regulated expression of neurotrophic factors.

Calcium-permeable AMPA channels in neurodegenerative disease and ischemia

Compelling evidence supports contributions of glutamate receptor overactivation ('excitotoxicity') to neurodegeneration in both acute conditions, such as stroke, and chronic neurodegenerative conditions, such as amyotrophic lateral sclerosis. However, anti-excitotoxic therapeutic trials, which have generally targeted highly Ca2+ permeable NMDA-type glutamate channels, have to date failed to demonstrate impressive efficacy. Whereas most AMPA type glutamate channels are Ca2+ impermeable, an evolving body of evidence supports the contention that relatively unusual Ca2+ permeable AMPA channels might be crucial contributors to injury in these conditions. These channels are preferentially expressed in discrete neuronal subpopulations, and their numbers appear to be upregulated in amyotrophic lateral sclerosis and stroke. In addition, unlike NMDA channels, Ca2+ permeable AMPA channels are not blocked by Mg2+, but are highly permeable to another potentially harmful endogenous cation, Zn2+. The targeting of these channels might provide efficacious new avenues in the therapy of certain neurological diseases.

Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats

Section of motor nerve fibers (axotomy) elicits a variety of morphofunctional responses in the motoneurons in the motor nuclei. Later than the fifth post-operational day after section of the facial nerve, synapse elimination occurs in the facial motoneuron pool, leading to gradual abolishment of synaptic input-driven activities of the axotomized motoneurons. However, it remains unknown how the amount of synaptic input changes during this period between the axotomy and the synaptic elimination. Here we examined a hypothesis that axotomy of the motoneurons itself modifies the synaptic inputs to the motoneurons. One day after axotomy, the postsynaptic currents, mostly mediated by non-N-methyl-D-aspartic acid (non-NMDA) receptors, recorded from the axotomized facial motoneurons in the acute slice preparations of the rats were of higher frequency and larger amplitude than those in the intact motoneurons. This difference was not observed after the third post-operational day and appeared earlier than the changes in the electrophysiological properties and increase in the number of dead neurons in the axotomized motor nucleus. The larger postsynaptic current frequency of the axotomized motoneurons was observed both in the absence and in the presence of tetrodotoxin citrate, suggesting that increased excitability and facilitated release underlie the postsynaptic current frequency increase. These results suggest that synaptic re-organization occurs in the synapses of motoneurons at an early stage following axotomy.