Long-term functional and morphological assessment of a standardized rat sciatic nerve crush injury with a non-serrated clamp

We have recently described the sequence of functional and morphologic changes occurring after a standardized sciatic nerve crush injury. An 8-week post-injury time was used because this end point is the far most used. Unexpectedly, both functional and morphological data revealed that animals had still not recovered to normal pre-injury levels. Therefore, the present study was designed in order to prolong the observation up to 12 weeks. Functional recovery was evaluated using sciatic functional index (SFI), static sciatic index (SSI), extensor postural thrust (EPT), withdrawal reflex latency (WRL) and ankle kinematics. In addition, quantitative morphology was carried out on regenerated nerve fibers. A full functional recovery was predicted by SFI/SSI, EPT and WRL but not all ankle kinematics parameters. Moreover, only two morphological parameters (myelin thickness/axon diameter ratio and fiber/axon diameter ratio) returned to normal values. Data presented in this paper provide a baseline for selecting the adequate end-point and methods of recovery assessment for a rat sciatic nerve crush study and suggest that the combined use of functional and morphological analysis should be recommended in this experimental model.

Activation of MAPK ERK in peripheral nerve after injury

Background

Activation of extracellular signal-regulated protein kinase (ERK), a member of mitogen-activated protein kinase (MAPK) family, has been proposed to mediate neurite outgrowth-promoting effects of several neurotrophic factors in vitro. However, the precise activity of ERK during axonal regeneration in vivo remains unclear. Peripheral axotomy has been shown to activate ERK in the cell bodies of primary afferent neurons and associated satellite cells. Nevertheless, whether ERK is also activated in the axons and surrounded Schwann cells which also play a key role in the regeneration process has not been clarified.

Results

Phosphorylation of ERK in the sciatic nerve in several time-points after crush injury has been examined. Higher phosphorylation of ERK was observed in the proximal and distal nerve stumps compared to the contralateral intact nerve from one day to one month after crush. The activation of ERK was mainly localized in the axons of the proximal segments. In the distal segments, however, active ERK was predominantly found in Schwann cells forming Bungner's bands.

Conclusion

The findings indicate that ERK is activated in both the proximal and distal nerve stumps following nerve injury. The role of activated ERK in Wallerian degeneration and subsequent regeneration in vivo remains to be elucidated.

An identified central pattern-generating neuron co-ordinates sensory-motor components of respiratory behavior in Lymnaea

Defining the attributes of individual central pattern-generating (CPG) neurons underlying various rhythmic behaviors are fundamental to our understanding of how the brain controls motor programs, such as respiration and locomotion. To this end, we have explored a simple invertebrate preparation in which the neuronal basis of respiratory rhythmogenesis can be investigated from the whole animal to a single cell level. An identified dopaminergic neuron, termed right pedal dorsal 1 (RPeD1), is a component of the CPG network which controls hypoxia-driven, aerial respiration in the fresh water snail Lymnaea stagnalis. Using intact, semi-intact and isolated brain preparations, we have discovered that in addition to its role as a respiratory CPG neuron, RPeD1 co-ordinates sensory-motor input from the pneumostome (the respiratory orifice) at the water/air interface to initiate respiratory rhythm generation. An additional, novel role of RPeD1 was also found. Specifically, direct intracellular stimulation of RPeD1 induced pneumostome openings, in the absence of motor neuronal activity. To determine further the role of RPeD1 in the respiratory behavior of intact animals, either its axon was severed or the soma selectively killed. Many components of the respiratory behavior in the intact animals were found to be perturbed following RPeD1 axotomy or 'somatomy' (soma removed). Taken together, the data presented provide a direct demonstration that RPeD1 is a multifunctional CPG neuron, which also serves many additional roles in the control of breathing behavior, ranging from co-ordination of mechanosensory input to the motor control of the respiratory orifice.

Rescue and regeneration of injured peripheral nerve axons by intrathecal insulin

Insulin peptide, acting through tyrosine kinase receptor pathways, contributes to nerve development or repair. In this work, we examined the direction, impact and repertoire of insulin signaling in vivo during peripheral nerve regeneration in rats. First, we demonstrated that insulin receptor is expressed on lumbar dorsal root ganglia neuronal perikarya using immunohistochemistry. Immunoblots and polymerase chain reactions confirmed the presence of both alpha and beta insulin receptor subunits in dorsal root ganglia. In vivo and in vitro assessment of dorsal root ganglion neurons showed preferential localization of insulin receptor to perikaryal sites. In vivo, intrathecal delivery of fluorescein isothiocyanate-labeled insulin identified localization around dorsal root ganglia neurons. The direction and impact of potential insulin signaling was evaluated by concurrently delivering insulin or carrier over a 2 week period using mini-osmotic pumps, either intrathecally, near nerve, or with both deliveries, following a selective sural nerve crush injury. Only intrathecal insulin increased the number and maturity of regenerating sensory sural nerve axons distal to the crush site. As well, only intrathecal insulin rescued retrograde loss of sural axons after crush. In a separate experiment, insulin also rescued retrograde loss and atrophy of deep peroneal, largely motor, axons post-injury. Intrathecal insulin increased the expression of calcitonin-gene-related peptide in regenerating sprouts, increased the number of visualized regenerating fiber clusters, and reduced downregulation of calcitonin-gene-related peptide in dorsal root ganglia neurons. Insulin delivered intrathecally does not appear to influence expression of insulin-like growth factor-1 at dorsal root ganglion neurons or near peripheral nerve injury, but was associated with upregulation of insulin receptor alpha subunit in dorsal root ganglia. Intrathecal insulin delivery was associated with greater recovery of thermal sensation and longer distances to stimulus response with the pinch test following sural nerve crush. Insulin signaling at neuron perikarya can drive distal sensory axon regrowth, rescue retrograde alterations of axons and alter axon peptide expression. Moreover, such actions are associated with upregulation of its own receptor.

Heat shock protein 27 rescues motor neurons following nerve injury and preserves muscle function

Heat shock proteins (HSPs) are a family of ubiquitously expressed proteins that are up-regulated in response to a range of stresses and play an important role in cellular defence mechanisms. In previous studies, we demonstrated that overexpression of heat shock protein 27 (HSP27) in transgenic mice has significant cytoprotective properties in vivo, reducing caspase-3-mediated cell death in the hippocampus associated with limbic seizures and reducing infarct size in cardiac ischaemia. In motor neurons, HSP27 is also implicated as a survival promoting factor; however, it remains to be established whether HSP27 is able to exert long-term neuroprotective effects following neonatal nerve injury. We now show that, following neonatal nerve crush, HSP27 overexpression in vivo provides a substantial rescue of motor neurons 5-6 months following nerve injury. Furthermore, in vivo isometric tension recordings demonstrate that surviving motor neurons were able to regenerate, resulting in a 90% improvement (P < 0.0005) in motor unit number in HSP27 mice. Moreover, this increase in motor unit number was associated with improved muscle weight, muscle force, contractile speeds, and histochemical markers of muscle activity. These properties of HSP27 therefore have considerable potential for improving long-term muscle function in motor neuron disorders.

Axotomy of sympathetic neurons activates the metalloproteinase-2 enzymatic pathway

We have previously shown that intraganglionic synapse disassembly consequent on superior cervical ganglion (SCG) neuron axotomy was preceded by the loss of the dystroglycan beta subunit (beta-DG) localized at the postsynaptic specializations. Because DG, a transmembrane molecular complex bridging the extracellular matrix to the cortical cytoskeleton, could be a physiological target of metalloproteinases (MMPs) 2 and 9, we investigated their possible involvement in the injury-induced intraganglionic synapse disassembly. In rat SCG, only MMP-2 was present and localized in both neurons and nonneuronal cells. After ganglion neuron axotomy, both MMP-2 activity and protein level increased, whereas the level of its mRNA was unchanged, suggesting prominent MMP-2 posttranslational regulation. mRNA and protein levels of the enzymes involved in the MMP-2 activation pathway, the membrane-type 1-MMP (MT1-MMP), and the tissue inhibitor of metalloproteinase-2 (TIMP-2) also increased after injury with a time course that correlated with that of MMP-2 activation. In addition, postganglionic nerve crush induced an increase in the beta-DG 30-kDa fragment produced by the MMP-dependent degradation of DG. These data suggest that MMP-2 activated during SCG neuron reaction to axotomy may degrade postsynaptic DG, contributing to the disruption of the molecular bridge between pre- and postsynaptic elements and disassembly of the intraganglionic synapses.

Ultrasound imaging of the rabbit peroneal nerve

Ultrasound imaging of peripheral nerves is increasingly used in the clinic for a wide range of applications. Although yet unapplied for experimental neuroscience, it also has potential value in this research area. This study explores the feasibility, possibilities and limitations of this technique in rabbits, with special focus on peripheral nerve regeneration after trauma. The peroneal nerve of 25 New Zealand White rabbits was imaged at varying time intervals after a crush lesion. The ultrasonic appearance of the nerve was determined, and recordings were validated with in vivo anatomy. Nerve swelling at the lesion site was estimated from ultrasound images and compared with anatomical parameters. The peroneal nerve could reliably be identified in all animals, and its course and anatomical variations agreed perfectly with anatomy. Nerve diameters from ultrasound were related to in vivo diameters (p < 0.001, R(2) = 77%), although the prediction interval was rather wide. Nerve thickenings could be visualized and preliminary results indicate that ultrasound can differentiate between neuroma formation and external nerve thickening. The value of the technique for experimental neuroscience is discussed. We conclude that ultrasound imaging of the rabbit peroneal nerve is feasible and that it is a promising tool for different research areas within the field of experimental neuroscience.

Increased growth factor expression and cell proliferation after contusive spinal cord injury

The damage caused by traumatic central nervous system (CNS) injury can be divided into two phases: primary and secondary. The initial injury destroys many of the local neurons and glia and triggers secondary mechanisms that result in further cell loss. Approximately 50% of the astrocytes and oligodendrocytes in the spared white matter of the epicenter die by 24 h after spinal cord injury (SCI), but their densities return to normal levels by 6 weeks. This repopulation is largely due to the proliferation of local progenitors that divide in response of CNS injury. Previous studies indicate that the secondary events that cause cell death after SCI also increase the local levels of several growth factors that stimulate the proliferation of these endogenous progenitors. We compared the spatial pattern of the post-injury up-regulation of the pro-mitotic growth factors with that of 5-bromodeoxyuridine (BrdU) incorporation to determine if each could play a role in proliferation. Three days after a standard contusive SCI or laminectomy, animals received intraperitoneal BrdU injections to label dividing cells and were perfused 2 h after the last injection. Immunohistochemistry for BrdU and basic fibroblast growth factor (FGF2) and in situ hybridization for ciliary neurotrophic factor (CNTF) and glial growth factor (GGF2) mRNA were used to compare the number of dividing cells with growth factor levels in sections 2 and 4 mm from the epicenter. All three growth factors are significantly up-regulated 3 days after SCI, when cell proliferation is maximal. The increase in GGF2 and FGF2 levels is highest in sections 2 mm rostral to the epicenter, mimicking BrdU incorporation. Addition of rhGGF2 to cultured cells isolated from the spinal cord 3 days after SCI increased the number of NG2+ glial progenitors. These data suggest that FGF2 and GGF2 may contribute to the spontaneous recovery observed after SCI by stimulating the proliferation of local progenitors that help repopulate the injured cord.

Impacts of lesion severity and tyrosine kinase receptor B deficiency on functional outcome of femoral nerve injury assessed by a novel single-frame motion analysis in mice

Functional recovery after peripheral nerve injury is often poor. Comprehension of cellular and molecular mechanisms limiting or promoting restoration of function and design of efficient therapeutic approaches remain serious challenges for neuroscience and medicine. Progress has been restricted by the lack of reliable methods for evaluation of motor functions in laboratory animals. We describe a novel approach for assessment of muscle function in mice after femoral nerve damage, an injury causing impairment of knee extension. The functional deficit can be precisely estimated by angle and distance measurements on single video frames recorded during movements of the animals with or without body weight support. Using this method we describe here the precise time-course and degree of functional recovery after femoral nerve crush and transection. In addition, we show that restoration of function is considerably impaired in mice with a reduced expression level of the tyrosine kinase receptor B, a cognate receptor for the neurotrophin brain-derived neurotrophic factor. This finding is consistent with known functions of brain-derived neurotrophic factor and tyrosine kinase receptor B and demonstrates the potential of the method. The principles of the approach are highly relevant for the development of novel functional assays in other peripheral and, in particular, central nervous system injury paradigms.

Nonviral HVJ (hemagglutinating virus of Japan) liposome-mediated retrograde gene transfer of human hepatocyte growth factor into rat nervous system promotes functional and histological recovery of the crushed nerve

Hepatocyte growth factor (HGF) is well known to be involved in many biological functions, such as organ regeneration and angiogenesis, and to exert neurotrophic effects on motor, sensory, and parasympathetic neurons. In this study, we gave repeated intramuscular injections of the human HGF gene, using nonviral HVJ (hemagglutinating virus of Japan) liposome method, to examine whether transfection of the rat nervous system with this gene is able to exert neurotrophic effects facilitating recovery of a crushed nerve. The expression of HGF protein and HGF mRNA indicated that gene transfer into the nervous system did occur via retrograde axonal transport. At 4 weeks after crush, electrophysiological examination of the crushed nerve showed a significantly shorter mean latency and a significantly greater mean maximum M-wave amplitude with repeated injections of HGF gene. Furthermore, histological findings showed that the mean diameter of the axons, the axon number and the axon population were significantly larger in the group with repeated injections of HGF gene. The above results show that repeated human HGF gene transfer into the rat nervous system is able to promote crushed-nerve recovery, both electrophysiologically and histologically, and suggest that HGF gene transfer has potential for the treatment of crushed nerve.

Schwann cell engraftment into injured peripheral nerve prevents changes in action potential properties

Peripheral nerve injury results in changes in action potential waveform, ion channel organization, and firing properties of primary afferent neurons. It has been suggested that these changes are the result of reduction in basal trophic support from skin targets. Subcutaneous injections of Fluro-Gold (FG) in the hind limb of the rat were used to identify cutaneous primary afferent neurons. Five days after FG injection, sciatic nerves were ligated and encapsulated in a silicon tube allowing neuroma formation. Green fluorescent protein (GFP)-expressing Schwann cells (SCs) were injected proximal to the cut end of the nerve. Thirteen to 22 days after injury and SC injection, the L4 and L5 dorsal root ganglia (DRG) were prepared for acute culture. Whole cell patch-clamp recordings in current clamp mode were obtained and action potential properties of medium-sized (34-45 microm) FG+ DRG neurons were characterized. In the neuroma group without cell transplantation, action potential duration and spike inflections were reduced as were the amplitude and duration of spike afterhyperpolarizations. These changes were not observed after transection by nerve crush where axons were allowed to regenerate to distal peripheral targets. In the transplantation group, GFP(+)-SCs were extensively distributed throughout the neuroma, and oriented longitudinally along axons proximal to the neuroma. Changes in action potential properties were attenuated in the GFP(+)-SC group. Thus the engrafted SC procedure ameliorated the changes in action potential waveform of cutaneous primary afferents associated with target disconnection and neuroma formation.

Nerve injury affects the capillary supply in rat slow and fast muscles differently

The goal of this study was to determine the acute effects of permanent denervation on the length density of the capillary network in rat slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles and the effect of short-lasting reinnervation in slow muscle only. Denervation was performed by cutting the sciatic nerve. Both muscles were excised 2 weeks later. Reinnervation was studied 4 weeks after nerve crush in SOL muscle only. Capillaries and muscle fibres were visualised by triple immunofluorescent staining with antibodies against CD31 and laminin and with fluorescein-labelled Griffonia (Bandeira) simplicifolia lectin. A recently developed stereological approach allowing the estimation of the length of capillaries adjacent to each individual fibre (Lcap/Lfib) was employed. Three-dimensional virtual test grids were applied to stacks of optical images captured with a confocal microscope and their intersections with capillaries and muscle fibres were counted. Interrelationships among capillaries and muscle fibres were demonstrated with maximum intensity projection of the acquired stacks of optical images. The course of capillaries in EDL seemed to be parallel to the fibre axes, whereas in SOL, their preferential direction deviated from the fibre axes and formed more cross-connections among neighbouring capillaries. Lcap/Lfib was clearly reduced in denervated SOL but remained unchanged in EDL, although the muscle fibres significantly atrophied in both muscle types. When soleus muscle was reinnervated, capillary length per unit fibre length was completely restored. The physiological background for the different responses of the capillary network in slow and fast muscle is discussed.

Spatiotemporal quantification of recruit and resident macrophages after crush nerve injury utilizing immunohistochemistry

The purpose of this study was to investigate quantitatively the temporal and spatial regulation and the morphological changes of the recruit and resident macrophages in the sciatic nerve during Wallerian degeneration and the following regeneration using immunohistochemistry. Sciatic nerves in Sprague-Dawley (SD) rats were examined after nerve crush. The rats were anesthetized with 100 mg of ketamine and 20 mg of xylazine in a dose of 1 ml/kg by intraperitoneal injection. Anti-ED-1 antibody was used to detect phagocytic macrophage and anti-OX-6 antibody was used to detect MHC class II cells. Few ED-1-immunopositive cells were seen within the normal sciatic nerve. After crush injury the number and the size of ED-1-immunopositive cells started to increase in all the segments distal to the crush site 3 days after injury and the number and size reached its peak on day 14 when the population of macrophage was 150 times higher in all the segments compared to controls. However, the number of ED-1-immunopositive cells and the size of the cells remains significantly high even after day 56 when functional recovery and axonal regeneration were complete. OX-6-immunopositive cells were observed within the control sciatic nerves. The number decreases significantly 3 days after injury in all the segments distal to the crush site but showed no significant difference thereafter. There were also no significant differences in the cell areas. ED-1-immunopositive phagocytic macrophages show significant differences temporally in both the cell number and the size even after axonal regeneration.

Contralateral cytokine gene induction after peripheral nerve lesions: dependence on the mode of injury and NMDA receptor signaling

There is increasing evidence that unilateral nerve injury evokes contralateral responses, but the underlying mechanisms are largely unknown. In the present investigation, we analyzed cytokine and chemokine gene induction in contralateral, non-lesioned nerves after sciatic nerve crush and chronic constriction injury (CCI) by quantitative reverse transcriptase polymerase chain reaction in mice. After sciatic nerve crush, contralateral changes in cytokine gene expression were restricted to interleukin (IL)-1beta, which showed a monophasic peak at the first postoperative day. Following CCI, contralateral transcripts for IL-1beta, IL-10 and monocyte chemoattractant protein-1 (MCP-1) were significantly increased already at day 1 and upregulation persisted over the next 4 weeks. In contrast, tumor necrosis factor alpha (TNF-alpha) levels remained unchanged. Contralateral gene induction was restricted to the homonymous opposite sciatic nerve, but spared the femoral nerve. NMDA receptor blockade completely abolished contralateral cytokine expression after CCI on the mRNA level. In contralateral dorsal root ganglia, only IL-10 mRNA levels were modified after nerve injury. Sham operation significantly increased the cytokine and chemokine gene expression at the ipsilateral side, but could not mediate contralateral effects. Our study confirms that nerve injury evokes contralateral responses and identifies NMDA-mediated signaling as one underlying mechanism.

Effects of gabapentin on spontaneous discharges and subthreshold membrane potential oscillation of type A neurons in injured DRG

Ectopic spontaneous discharges play a critical role for both initiation and maintenance of the neuropathic pain state. Gabapentin (GBP) has been shown to be effective in animal models of neuropathic pain as well as in chronic pain patients. To investigate the peripheral mechanisms of GBP, the effects of GBP on spontaneous discharges and subthreshold membrane potential oscillation (SMPO) of chronically compressed dorsal root ganglion (DRG) were examined electrophysiolocally in vitro. The rate of spontaneous discharges was transitorily enhanced when GBP was applied to the DRG. When the concentration was under 5microM, only enhanced effect was observed, while spontaneous discharges were completely suppressed when the concentration of GBP was beyond 5microM. The similar doses of GBP blocking the spontaneous discharges failed to block the propagation of impulses by electrical nerve stimulation. Furthermore, we found that the SMPO of injured DRG cells can be selectively abolished by GBP without interrupting spike propagation. The results suggest that the inhibitory effect of GBP on SMPO might be one of the membrane mechanisms of action of GBP. This may partially explain the antinociceptive action of GBP by directly suppression nociceptive afferent input to the spinal cord.

Unilateral optic nerve crush induces bilateral retinal glial cell proliferation

The post-injury responses of retinal ganglion cells elicit a number of glial reactions which have not been completely understood. The bilateral pattern of non-neuronal retinal cell proliferation was examined in association with the differential fates of unilaterally injured adult retinal ganglion cells by means of bromodeoxyuridine (BrdU) immunocytochemistry. Lateralization of the glioproliferative events was studied by analysing both the experimental and the uninjured contralateral as well as matched retinas of sham-operated animals. Control adult rat retina included very few BrdU-positive cells within the nerve fibre and ganglion cell layers; however, experimental retinas of degenerating groups exhibited statistically significantly higher densities of newborn cells in most layers. Clusters of labelled cells were found in the inner plexiform layer related to OX-42 staining, indicating their microglial nature. Indeed, double-labelling experiments, after short-term unilateral optic nerve crushing, identified proliferating retinal glial cells in vivo. Both types of glia, astroglial and microglial cells, exhibited BrdU-positive labelling in injured as well as uninjured experimental rat retinas. Moreover, microglial proliferating cells were also identified in explanted retinal pieces after 2 days in culture. Affected and contralateral retinas responded similarly to the unilateral experimental manipulations applied with respect to BrdU labelling. The acute glial responses observed suggest that bilateral glial proliferation might represent a common response related to degeneration events in both retinas, i.e. ipsi- and contralateral to the experimental injury.

Use of muscle fibrillation for tracking nerve regeneration

Individual muscle fibers in denervated muscle demonstrate repetitive, spontaneous contraction. Such fibrillation activity disappears in denervated muscle if reinnervation occurs, but this relationship has not been formally studied. To test whether the disappearance of fibrillation can be used to track nerve regeneration, we quantified the presence and subsequent disappearance of electromyographic (EMG) fibrillation potentials and fibrillation-related movement in the rat tongue after unilateral hypoglossal nerve crush at two locations. In mice, fibrillation movement of vibrissae were monitored after unilateral facial nerve crush and compared with the return of symmetrical vibrissae sweeping movements. In both of these rodent cranial motor systems, there was a conspicuous loss of fibrillation at a time when reinnervation is known to take place, suggesting that the visual appearance of fibrillation-related movement can be used as a simple, noninvasive means of tracking nerve regeneration in these popular experimental motor systems.

Assessment of nerve degeneration by gadofluorine M-enhanced magnetic resonance imaging

Nerve injury represents a major cause of disability. In the peripheral nervous system, nerves have the capacity to regrow but within weeks after injury, it is impossible to clarify whether proper regeneration is under way or is failing. In this experimental study, we report on a novel tool to assess nerve outgrowth in vivo. After systemic application, the novel gadolinium-based magnetic resonance (MR) contrast agent Gadofluorine M (Gf) selectively accumulated and persisted in nerve fibers undergoing Wallerian degeneration causing bright contrast on T1-weighted MR images. Gf enhancement on MR imaging was present already at 48 hours within the entire nerve segments undergoing Wallerian degeneration, and subsequently disappeared from proximal to distal parts in parallel to regrowth of nerve fibers. Most importantly, Gf enhancement persisted in nonregenerating, permanently transected nerves. Our novel Gf-based MR imaging methodology holds promise for clinical use to bridge the diagnostic gap between nerve injury and completed nerve regeneration, and to determine the necessity for neurolysis and engraftment if spontaneous regeneration is not successful.

Persistence of PAD and presynaptic inhibition of muscle spindle afferents after peripheral nerve crush

Two to twelve weeks after crushing a muscle nerve, still before the damaged afferents reinnervate the muscle receptors, conditioning stimulation of group I fibers from flexor muscles depolarizes the damaged afferents [M. Enriquez, I. Jimenez, P. Rudomin, Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush. Exp. Brain Res., 107 (1996), 405-420]. It is not known, however, if this primary afferent depolarization (PAD) is indeed related to presynaptic inhibition. We now show in the cat that 2-12 weeks after crushing the medial gastrocnemius nerve (MG), conditioning stimulation of group I fibers from flexors increases the excitability of the intraspinal terminals of both the intact lateral gastrocnemius plus soleus (LGS) and of the previously damaged MG fibers ending in the motor pool, because of PAD. The PAD is associated with the depression of the pre- and postsynaptic components of the extracellular field potentials (EFPs) evoked in the motor pool by stimulation of either the intact LGS or of the previously damaged MG nerves. These observations indicate, in contrast to what has been reported for crushed cutaneous afferents [K.W. Horch, J.W. Lisney, Changes in primary afferent depolarization of sensory neurones during peripheral nerve regeneration in the cat, J. Physiol., 313 (1981), 287-299], that shortly after damaging their peripheral axons, the synaptic efficacy of group I spindle afferents remains under central control. Presynaptic inhibitory mechanisms could be utilized to adjust the central actions of muscle afferents not fully recovered from peripheral lesions.

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (WldS) nerves

BACKGROUND

The progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (WldS) mutant mice.

RESULTS

In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37-42 h after transection and 40-44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all WldS axons were partially fragmented 15-25 days after nerve lesion, WldS axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks.

CONCLUSIONS

We conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that WldS axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate.

Axonal transport of the cellular prion protein is increased during axon regeneration

The cellular prion protein, PrPc, is a glycosylphosphatidylinositol-anchored cell surface glycoprotein and a protease-resistant conformer of the protein may be the infectious agent in transmissible spongiform encephalopathies. PrPc is localized on growing axons in vitro and along fibre bundles that contain elongating axons in developing and adult brain. To determine whether the growth state of axons influenced the expression and axonal transport of PrPc, we examined changes in the protein following post-traumatic regeneration in the hamster sciatic nerve. Our results show (1) that PrPc in nerve is significantly increased during nerve regeneration; (2) that this increase involves an increase in axonally transported PrPc; and (3) that the PrPc preferentially targeted for the newly formed portions of the regenerating axons consists of higher molecular weight glycoforms. These results raise the possibility that PrPc may play a role in the growth of axons in vivo, perhaps as an adhesion molecule interacting with the extracellular environment through specialized glycosylation.

mRNAs encoding theAplysia homologues of fasciclin-I and β-thymosin are expressed only in the second phase of nerve injury and are differentially segregated in axons regenerating in vitro and in vivo

Studies using Aplysia californica have demonstrated that transcription after nerve injury occurs during a rapid, transient first phase and a delayed, prolonged second phase. Although the second phase is especially important for regeneration, the mRNAs produced during this phase have not been identified. We characterized two such mRNAs following axotomy. One encodes a novel fasciclin-I homologue, Aplysia fasciclin-like protein (apFasP), and the other encodes Aplysia beta-thymosin (apbetaT). In addition to mRNA synthesis, proteins required for regeneration must be available at the site of growth, and the transport and local translation of certain extrasomatic mRNAs aids in this process. We found apbetaT and apFasP proteins and mRNA at growth cones in vitro. However, only the mRNA for apbetaT was present in regenerating axons in vivo. This implies that the membrane protein apFasP is supplied by rapid transport from the soma, whereas the soluble apbetaT is synthesized locally.

Neuropeptide Y2 receptor protein is present in peptidergic and nonpeptidergic primary sensory neurons of the mouse

The localization of the neuropeptide tyrosine (NPY) Y2 receptor (Y2R) protein was studied in mouse dorsal root ganglia (DRGs) and spinal cord, by using a recently developed rabbit anti-Y2R antibody and a sensitive immunohistochemical method. Y2R-like immunoreactivity (-LI) was observed in about 10% of the small/medium-sized lumbar DRG neurons. Among these, about 44% were calcitonin gene-related peptide-immunoreactive, and about 38% bound isolectin B4. In the dorsal horn of the spinal cord, an intense Y2R-LI was seen in the most superficial layers, mostly restricted to laminae I-II. This immunoreactivity was completely abolished by dorsal rhizotomy. Y2R-L1 was also detected on the skin, more abundantly in hairy than glabrous skin. Specificity experiments showed complete disappearance of the Y2R-LI described above after incubation with antibody preadsorbed with the immunogenic peptide. Furthermore, Y2R-LI was also absent in a Y2R knockout mouse. These results demonstrate that the NPY Y2R is associated mainly with both peptidergic and nonpeptidergic small, presumably nociceptive, neurons projecting to the superficial layers of the dorsal horn. The results also support a role for this receptor and NPY in pain mechanisms.

Modulation of muscle spindle innervation by neurotrophin-3 following nerve injury

Muscle spindles monitor changes in muscle length and are innervated by groups Ia and II sensory axons as well as gamma motor axons. Ia sensory axons respond to neurotrophin-3 (NT-3), which plays an important role in sculpting proprioceptive development. Previously, transgenic mice were generated that overexpress NT-3 in muscle (mlc/NT-3 mice). These mice have alterations in proprioceptive elements due to the developmental actions of NT-3 and neuroprotective effects on Ia axons following nerve injury (Taylor, M.D., Vancura, R., Williams, J.M., Riekhof, J.T., Taylor, B.K., Wright, D.E., 2001. Overexpression of neurotrophin-3 in skeletal muscle alters normal and injury-induced limb control. Somatosens. Motor Res. 18 (4), 286-294.) Here, we investigated the actions of NT-3 on each class of injured axons innervating spindles and explored the mechanisms by which NT-3 acts. Immunohistochemical assessment of muscle spindle innervation following crush revealed that the degeneration of Ia axons innervating spindles in mlc/NT-3 mice was substantially reduced, and overall spindle innervation by group II and gamma fibers was greatly improved at later stages. Mlc/NT-3 mice also displayed a significant reduction in the expression of the injury-induced transcription factor ATF3 by retrogradely labeled muscle afferent neurons. The effects of transgenic NT-3 overexpression on spindle innervation could be mimicked if wild-type mice were treated intramuscularly with recombinant NT-3 prior to but not following injury, suggesting that NT-3's actions were due to preexposure to NT-3. This view was supported by in vitro experiments in which large DRG neurons from mlc/NT-3 mice grew significantly longer neurites than wild-type neurons. The results reveal that improved Ia-spindle interactions after injury may enhance spindle innervation by group II and gamma fibers. Finally, exposure of muscle afferent fibers to NT-3 prior to injury alters axonal responses both in vitro and in vivo.

Galectin-1 in regenerating motoneurons

The exogenous application of recombinant galectin-1 has recently been shown to promote the rate of peripheral nerve regeneration. Endogenous neuronal galectin-1 expression has recently been demonstrated to increase after axotomy. Here we demonstrate a significant increase in the endogenous neuronal expression of galectin-1 mRNA in facial motoneurons after either a nerve resection or crush injury in mice. This increase in galectin-1 expression was due in part to the loss of target-derived factor(s) as indicated by both the return of galectin-1 expression to control levels following target re-innervation and the increase in galectin-1 expression after blockade of axonal transport by an interneuronal colchicine injection. Furthermore, interneuronal injections of glial-derived neurotrophic factor into the uninjured nerve also increased galectin-1 mRNA expression within facial motoneurons suggesting that positive signals may also be involved in the regulation of galectin-1 expression. Galectin-1 null mutant mice showed an attenuated rate of functional recovery of whisking movement after a facial nerve crush.