Focally administered nerve growth factor suppresses molecular regenerative responses of axotomized peripheral afferents in rats

New Frontiers in Peripheral Nerve Regeneration: Concerns and Remedies

Topical advances in studying molecular and cellular mechanisms responsible for regeneration in the peripheral nervous system have highlighted the ability of the nervous system to repair itself. Still, serious injuries represent a challenge for the morphological and functional regeneration of peripheral nerves, calling for new treatment strategies that maximize nerve regeneration and recovery. This review presents the canonical view of the basic mechanisms of nerve regeneration and novel data on the role of exosomes and their transferred microRNAs in intracellular communication, regulation of axonal growth, Schwann cell migration and proliferation, and stromal cell functioning. An integrated comprehensive understanding of the current mechanistic underpinnings will open the venue for developing new clinical strategies to ensure full regeneration in the peripheral nervous system.

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.

Putative roles of soluble trophic factors in facial nerve regeneration, target reinnervation, and recovery of vibrissal whisking

It is well-known that, after nerve transection and surgical repair, misdirected regrowth of regenerating motor axons may occur in three ways. The first way is that the axons enter into endoneurial tubes that they did not previously occupy, regenerate through incorrect fascicles and reinnervate muscles that they did not formerly supply. Consequently the activation of these muscles results in inappropriate movements. The second way is that, in contrast with the precise target-directed pathfinding by elongating motor nerves during embryonic development, several axons rather than a single axon grow out from each transected nerve fiber. The third way of misdirection occurs by the intramuscular terminal branching (sprouting) of each regenerating axon to culminate in some polyinnervation of neuromuscular junctions, i.e. reinnervation of junctions by more than a single axon. Presently, "fascicular" or "topographic specificity" cannot be achieved and hence target-directed nerve regeneration is, as yet, unattainable. Nonetheless, motor and sensory reinnervation of appropriate endoneurial tubes does occur and can be promoted by brief nerve electrical stimulation. This review considers the expression of neurotrophic factors in the neuromuscular system and how this expression can promote functional recovery, with emphasis on the whisking of vibrissae on the rat face in relationship to the expression of the factors. Evidence is reviewed for a role of neurotrophic factors as short-range diffusible sprouting stimuli in promoting complete functional recovery of vibrissal whisking in blind Sprague Dawley (SD)/RCS rats but not in SD rats with normal vision, after facial nerve transection and surgical repair. Briefly, a complicated time course of growth factor expression in the nerves and denervated muscles include (1) an early increase in FGF2 and IGF2, (2) reduced NGF between 2 and 14days after nerve transection and surgical repair, (3) a late rise in BDNF and (4) reduced IGF1 protein in the denervated muscles at 28days. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of nerve injury-associated neurotrophic factors and cytokines at the neuromuscular junctions of denervated muscles. In particular, the increase of FGF2 and concomittant decrease of NGF during the first week after facial nerve-nerve anastomosis in SD/RCS blind rats may prevent intramuscular axon sprouting and, in turn, reduce poly-innervation of the neuromuscular junction.

Nerve growth factor and basic fibroblast growth factor promote reinnervation by nerve-muscle-endplate grafting

INTRODUCTION

This study was designed to test whether exogenous application of nerve growth factor (NGF) and basic fibroblast growth factor (FGF-2) to muscles reinnervated with nerve-muscle-endplate band grafting (NMEG) could promote specific outcomes.

METHODS

The right sternomastoid muscle in adult rats was experimentally denervated and immediately reinnervated by implanting an NMEG pedicle from the ipsilateral sternohyoid muscle. A fibrin sealant containing NGF and FGF-2 was focally applied to the implantation site. Maximal tetanic force, muscle weight, regenerated axons, and motor endplates were analyzed 3 months after treatment.

RESULTS

Mean tetanic force, wet muscle weight, and number of regenerated axons in the treated muscles were 91%, 92%, and 84% of the contralateral controls, respectively. The majority of endplates (86%) in the treated muscles were reinnervated by regenerated axons.

DISCUSSION

Focal administration of NGF and FGF-2 promotes efficacy of the NMEG technique. Muscle Nerve 57: 449-459, 2018.

Preferential Enhancement of Sensory and Motor Axon Regeneration by Combining Extracellular Matrix Components with Neurotrophic Factors

After peripheral nerve injury, motor and sensory axons are able to regenerate but inaccuracy of target reinnervation leads to poor functional recovery. Extracellular matrix (ECM) components and neurotrophic factors (NTFs) exert their effect on different neuronal populations creating a suitable environment to promote axonal growth. Here, we assessed in vitro and in vivo the selective effects of combining different ECM components with NTFs on motor and sensory axons regeneration and target reinnervation. Organotypic cultures with collagen, laminin and nerve growth factor (NGF)/neurotrophin-3 (NT3) or collagen, fibronectin and brain-derived neurotrophic factor (BDNF) selectively enhanced sensory neurite outgrowth of DRG neurons and motor neurite outgrowth from spinal cord slices respectively. For in vivo studies, the rat sciatic nerve was transected and repaired with a silicone tube filled with a collagen and laminin matrix with NGF/NT3 encapsulated in poly(lactic-co-glycolic acid) (PLGA) microspheres (MP) (LM + MP.NGF/NT3), or a collagen and fibronectin matrix with BDNF in PLGA MPs (FN + MP.BDNF). Retrograde labeling and functional tests showed that LM + MP.NGF/NT3 increased the number of regenerated sensory neurons and improved sensory functional recovery, whereas FN + MP.BDNF preferentially increased regenerated motoneurons and enhanced motor functional recovery. Therefore, combination of ECM molecules with NTFs may be a good approach to selectively enhance motor and sensory axons regeneration and promote appropriate target reinnervation.

Comparison of trophic factors' expression between paralyzed and recovering muscles after facial nerve injury. A quantitative analysis in time course

After peripheral nerve injury, recovery of motor performance negatively correlates with the poly-innervation of neuromuscular junctions (NMJ) due to excessive sprouting of the terminal Schwann cells. Denervated muscles produce short-range diffusible sprouting stimuli, of which some are neurotrophic factors. Based on recent data that vibrissal whisking is restored perfectly during facial nerve regeneration in blind rats from the Sprague Dawley (SD)/RCS strain, we compared the expression of brain derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), insulin growth factors 1 and 2 (IGF1, IGF2) and nerve growth factor (NGF) between SD/RCS and SD-rats with normal vision but poor recovery of whisking function after facial nerve injury. To establish which trophic factors might be responsible for proper NMJ-reinnervation, the transected facial nerve was surgically repaired (facial-facial anastomosis, FFA) for subsequent analysis of mRNA and proteins expressed in the levator labii superioris muscle. A complicated time course of expression included (1) a late rise in BDNF protein that followed earlier elevated gene expression, (2) an early increase in FGF2 and IGF2 protein after 2 days with sustained gene expression, (3) reduced IGF1 protein at 28 days coincident with decline of raised mRNA levels to baseline, and (4) reduced NGF protein between 2 and 14 days with maintained gene expression found in blind rats but not the rats with normal vision. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of lesion-associated neurotrophic factors and cytokines in denervated muscles. The increase of FGF-2 protein and concomittant decrease of NGF (with no significant changes in BDNF or IGF levels) during the first week following FFA in SD/RCS blind rats possibly prevents the distal branching of regenerating axons resulting in reduced poly-innervation of motor endplates.

Improved Peripheral Nerve Regeneration in Streptozotocin-Induced Diabetic Rats by Oral Lumbrokinase

We assessed the therapeutic effects of lumbrokinase, a group of enzymes extracted from the earthworm, on peripheral-nerve regeneration using well-defined sciatic nerve lesion paradigms in diabetic rats induced by the injection of streptozotocin (STZ). We found that lumbrokinase therapy could improve the rats' circulatory blood flow and promote the regeneration of axons in a silicone rubber conduit after nerve transection. Lumbrokinase treatment could also improve the neuromuscular functions with better nerve conductive performances. Immunohistochemical staining showed that lumbrokinase could dramatically promote calcitonin gene-related peptide (CGRP) expression in the lamina I–II regions in the dorsal horn ipsilateral to the injury and cause a marked increase in the number of macrophages recruited within the distal nerve stumps. In addition, the lumbrokinase could stimulate the secretion of interleukin-1 (IL-1), nerve growth factor (NGF), platelet-derived growth factor (PDGF), and transforming growth factor-β (TGF-β) in dissected diabetic sciatic nerve segments. In conclusion, the administration of lumbrokinase after nerve repair surgery in diabetic rats was found to have remarkable effects on promoting peripheral nerve regeneration and functional recovery.

CaMKK-CaMK1a, a new post-traumatic signalling pathway induced in mouse somatosensory neurons

Neurons innervating peripheral tissues display complex responses to peripheral nerve injury. These include the activation and suppression of a variety of signalling pathways that together influence regenerative growth and result in more or less successful functional recovery. However, these responses can be offset by pathological consequences including neuropathic pain. Calcium signalling plays a major role in the different steps occurring after nerve damage. As part of our studies to unravel the roles of injury-induced molecular changes in dorsal root ganglia (DRG) neurons during their regeneration, we show that the calcium calmodulin kinase CaMK1a is markedly induced in mouse DRG neurons in several models of mechanical peripheral nerve injury, but not by inflammation. Intrathecal injection of NRTN or GDNF significantly prevents the post-traumatic induction of CaMK1a suggesting that interruption of target derived factors might be a starter signal in this de novo induction. Inhibition of CaMK signalling in injured DRG neurons by pharmacological means or treatment with CaMK1a siRNA resulted in decreased velocity of neurite growth in vitro. Altogether, the results suggest that CaMK1a induction is part of the intrinsic regenerative response of DRG neurons to peripheral nerve injury, and is thus a potential target for therapeutic intervention to improve peripheral nerve regeneration.

Effect of Arecoline on Regeneration of Injured Peripheral Nerves

The present study provides in vitro and in vivo evaluation of arecoline on peripheral nerve regeneration. In the in vitro study, we found that arecoline at 50 μg/ml could significantly promote the survival and outgrowth of cultured Schwann cells as compared to the controls treated with culture medium only. In the in vivo study, we evaluated peripheral nerve regeneration across a 10-mm gap in the sciatic nerve of the rat, using a silicone rubber nerve chamber filled with the arecoline solution. In the control group, the chambers were filled with normal saline only. At the end of the fourth week, morphometric data revealed that the arecoline-treated group at 5 μg/ml significantly increased the number and the density of myelinated axons as compared to the controls. Immunohistochemical staining in the arecoline-treated animals at 5 μg/ml also showed their neural cells in the L4 and L5 dorsal root ganglia ipsilateral to the injury were strongly retrograde-labeled with fluorogold and lamina I–II regions in the dorsal horn ipsilateral to the injury were significantly calcitonin gene-related peptide-immunolabeled compared with the controls. In addition, we found that the number of macrophages recruited in the distal sciatic nerve was increased as the concentration of arecoline was increased. Electrophysiological measurements showed the arecoline-treated groups at 5 and 50 μg/ml had a relatively larger nerve conductive velocity of the evoked muscle action potentials compared to the controls. These results indicate that arecoline could stimulate local inflammatory conditions, improving the recovery of a severe peripheral nerve injury.

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.

Puerarin Accelerates Peripheral Nerve Regeneration

This study investigates the effect of puerarin (PR) on peripheral nerve regeneration in vitro and in vivo. PR at concentrations of 1, 10, and 100 μM significantly promoted survival and outgrowth of cultured Schwann cells, as compared to the controls treated with culture medium only. in vivo study, peripheral nerve regeneration was evaluated across a 15-mm gap in the sciatic nerve of rats using a silicone rubber nerve chamber filled with PR solution. The control group chambers were filled with normal saline only. At the end of eight weeks, animals in the PR groups, especially at a concentration of 1 μM, had a significantly higher density of myelinated axons, greater evoked action potential area, and a larger nerve conductive velocity, as compared to the controls. All experimental results indicate that PR treatment promotes nerve growth and is a promising herbal medicine for recovery of regenerating peripheral nerves.

Hepatocyte growth factor incorporated into herpes simplex virus vector accelerates facial nerve regeneration after crush injury

Hepatocyte growth factor (HGF) promotes regeneration of the central nervous system, but its effects on the peripheral nervous system remain unclear. This study was conducted to elucidate the effect of HGF on regeneration of the murine facial nerve after crush injury. To do so, a replication-defective herpes simplex virus vector that incorporated HGF was prepared (HSV-HGF). The main trunk of the facial nerve was compressed by mosquito hemostats, and HSV-HGF, control vector or medium was then applied to the compressed nerve. We found that mice in the HGF group required significantly fewer days for complete recovery from nerve compression. Furthermore, the amplitude of the evoked buccinator muscle compound action potential increased following HSV-HGF application. HGF expression in and around the compressed nerve was demonstrated by enzyme-linked immunoassay and immunohistochemistry. In addition, HSV-HGF introduction around the damaged nerve significantly accelerated recovery of function of the facial nerve. These data suggest a possible role of HGF in promoting facial nerve regeneration after nerve damage. Furthermore, this viral delivery method may be applied clinically for many types of severe facial palsy during facial nerve decompression surgery.

Conditioning lesions enhance growth state only in sensory neurons lacking calcitonin gene-related peptide and isolectin B4-binding

A conditioning lesion improves regeneration of central and peripheral axons of dorsal root ganglion (DRG) neurons after a subsequent injury by enhancing intrinsic growth capacity. This enhanced growth state is also observed in cultured DRG neurons, which support a more sparsely and rapidly elongating mode of growth after a prior conditioning lesion in vivo. Here we examined differences in the capacity or requirements of specific types of sensory neurons for regenerative growth, which has important consequences for development of strategies to improve recovery after injury. We showed that after partial or complete injury of the sciatic nerve in mice, an elongating mode of growth in vitro was activated only in DRG neurons that did not express calcitonin gene-related peptide (CGRP) or bind Bandeiraea simplicifolia I-isolectin B4 (IB4). We also directly examined the response of conditioned sensory neurons to nerve growth factor (NGF), which does not enhance growth in injured peripheral nerves in vivo. We showed that after partial injury, NGF stimulated a highly branched and linearly restricted rather than elongating mode of growth. After complete injury, the function of NGF was impaired, which immunohistochemical studies of DRG indicated was at least partly due to downregulation of the NGF receptor, tropomyosin-related kinase A (TrkA). These results suggest that, regardless of the type of conditioning lesion, each type of DRG neuron has a distinct intrinsic capacity or requirement for the activation of rapidly elongating growth, which does not appear to be influenced by NGF.

Expression of TrkA receptor for neurotrophins in trigeminal neurons innervating the rat gingivomucosal tissue

The purpose of this study was to characterize and evaluate the expression of TrkA receptor in trigeminal ganglion (TG) neurons that innervate the rat gingivomucosal tissue. A retrograde nerve tracer Fluorogold (FG) was injected into the gingiva (group 1) or applied into the gingival sulcus (group 2) of the first right maxillary molar to identify the neurons in TG that innervate the gingivomucosa. After 10 days TG were dissected and FG fluorescence in neurons was observed under UV light microscope. To draw a comparison, approximately 1000 neurons per ganglion from the entire TG (group 3) and approximately 350 neurons per ganglion from the maxillary region in TG (group 4), were analyzed. Expression of TrkA receptor in TG neurons was investigated by immunohistochemistry. About 70% of neurons in groups 1 and 2 contained TrkA receptor, which was statistically significantly more than in groups 3 (41%) and 4 (38%). FG-labeled TrkA-immunopositive neurons were predominantly small or medium-sized (less than 1200microm(2)). However, the neurons innervating the rat gingivomucosa were on average larger than the neurons in the entire TG or in the maxillary region. In conclusion, the majority of neurons in TG that innervate the rat gingivomucosa are small or medium-sized, contain TrkA receptor and are most probably nociceptive.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Neurotrophins and peripheral neuropathies

Peripheral neuropathy is a common disorder seen in general neurology and neuromuscular specialty clinics. Treatment options directed at the underlying cause can only be offered in a handful of conditions, such as those with possible autoimmune etiology. The remainder fall into the idiopathic or genetic category with no known treatment. This review surveys the evidence supporting the rationale for the therapeutic use of neurotrophins and other neurotrophic factors in these disorders in relationship to the underlying pathobiological process. Previous clinical trials are assessed, and increasingly better understood and appreciated therapeutic potential of neurotrophins is emphasized.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Time course of substance P expression in dorsal root ganglia following complete spinal nerve transection

Recent evidence suggests that substance P (SP) is up-regulated in primary sensory neurons following axotomy and that this change occurs in larger neurons that do not usually produce SP. If this is so, then the up-regulation may allow normally neighboring, uninjured, and nonnociceptive dorsal root ganglion (DRG) neurons to become effective in activating pain pathways. By using immunohistochemistry, we performed a unilateral L5 spinal nerve transection on male Wistar rats and measured SP expression in ipsilateral L4 and L5 DRGs and contralateral L5 DRGs at 1-14 days postoperatively (dpo) and in control and sham-operated rats. In normal and sham-operated DRGs, SP was detectable almost exclusively in small neurons (< or =800 microm2). After surgery, the mean size of SP-positive neurons from the axotomized L5 ganglia was greater at 2, 4, 7, and 14 dpo. Among large neurons (>800 microm2) from the axotomized L5, the percentage of SP-positive neurons increased at 2, 4, 7, and 14 dpo. Among small neurons from the axotomized L5, the percentage of SP-positive neurons was increased at 1 and 3 dpo but was decreased at 7 and 14 dpo. Thus, SP expression is affected by axonal damage, and the time course of the expression is different between large and small DRG neurons. These data support a role for SP-producing, large DRG neurons in persistent sensory changes resulting from nerve injury.

The role of astragaloside in regeneration of the peripheral nerve system

The aim of this study was to evaluate peripheral nerve regeneration across a 15-mm gap in the sciatic nerve of the rat, using a silicone rubber nerve guide filled with different concentrations of astragaloside (0, 50, 100, and 200 microM). Collagen was also filled in the chambers to prevent the astragaloside from leakage. At the end of 8 weeks, animals from the group treated with astragaloside, especially at the concentration of 50 microM, had a higher rate of successful regeneration across the wide gap, a significantly larger number of myelinated axons, and a greater evoked action potential than the control group. However, the high-dose astragaloside (200 microM) completely reversed this positive effect of growth-promoting capability and inhibited nerve regeneration. Thus, astragaloside plays a dual role in anastomosis, being salutary in aiding the growth of axons in peripheral nerve but also detrimental, terminating the nerve regenerative processes if improperly applied.

Both positive and negative factors regulate gene expression following chronic facial nerve resection

Previously, we reported that following a chronic nerve resection, removal of the neuroma reversed the atrophy, increased the number of countable motoneurons and resulted in the re-expression of GAP-43 and alpha tubulin mRNA. In the present study, we questioned whether this response was due to the removal of the neuroma, or a result of factors such as neurotrophins, produced at the injury site. To test this hypothesis, 10 weeks after axotomy, the axonal transport blocker colchicine or, glial derived neurotrophic factor (GDNF) was injected proximal to the neuroma. The injection of GDNF or colchicine elicited an increase in motoneuron size and in GAP-43, but not alpha tubulin, mRNA. These data suggest that in addition to factors produced at the injury site, the neuroma acts as a source of target-like repressive signals that when removed results in an increase in gene expression and motoneuron size. To analyze the regenerative potential of chronically resected motoneurons, mice without a previous nerve injury and mice with a chronic resection received a pre-degenerated segment of sciatic nerve attached to the proximal facial nerve stump. Axons from both the chronic and acute groups grew into the grafts, however, significantly more retrogradely labeled motoneurons were counted in the acute group compared to the chronic resection group. No difference in motoneuron cell size was observed between the two groups of regenerated neurons. Therefore, despite severe atrophy, many of the surviving mouse facial motoneurons retain the propensity to extend their axons when provided with the appropriate environment.

Expression of axotomy-inducible and apoptosis-related genes in sensory nerves of rats with experimental diabetes

In diabetes, peripheral nerves suffer deficient neurotrophic support-a situation which resembles axotomy. This raises the question: does inappropriate establishment of an axotomised neuronal phenotype contribute to diabetic neuropathy, and in extremis, does this provoke apoptosis? We hybridized reverse-transcribed RNA, from the dorsal root ganglia (DRG) of 8-week streptozotocin (STZ)-induced diabetic rats, to Affymetrix Rat Genome U34A chips and scanned the array for expression of (a) genes that are upregulated by axotomy, (b) proapoptotic and (c) anti-apoptotic genes. Expression of the axotomy-responsive genes coding for growth-associated protein 43 (GAP-43), galanin, neuropeptide Y (NPY), pre-pro-vasoactive intestinal polypeptide (pre-pro-VIP), neuronal nitric oxide synthase (nNOS), protease nexin 1, heat-shock protein 27 (HSP 27) and myosin light chain kinase II (MLCK II) was unaffected in ganglia from diabetic rats compared to controls; thus, no axotomised phenotype was established. The expression of the majority of proapoptotic genes in the DRG was also unaltered (bax, bad, bid, bok, c-Jun, p38, TNFR1, caspase 3 and NOS2). Similarly there was no change in expression of the majority of antiapoptotic genes (bcl2, bcl-xL, bcl-w, NfkappaB). These alterations in gene expression make it clear that neither axotomy nor apoptotic phenotypes are established in neurones in this model of diabetes.

The facial nerve axotomy model

Experimental models such as the facial nerve axotomy paradigm in rodents allow the systematic and detailed study of the response of neurones and their microenvironment to various types of challenges. Well-studied experimental examples include peripheral nerve trauma, the retrograde axonal transport of neurotoxins and locally enhanced inflammation following the induction of experimental autoimmune encephalomyelitis in combination with axotomy. These studies have led to novel insights into the regeneration programme of the motoneurone, the role of microglia and astrocytes in synaptic plasticity and the biology of glial cells. Importantly, many of the findings obtained have proven to be valid in other functional systems and even across species barriers. In particular, microglial expression of major histocompatibility complex molecules has been found to occur in response to various types of neuronal damage and is now regarded as a characteristic component of "glial inflammation". It is found in the context of numerous neurodegenerative disorders including Parkinson's and Alzheimer's disease. The detachment of afferent axonal endings from the surface membrane of regenerating motoneurones and their subsequent displacement by microglia ("synaptic stripping") and long-lasting insulation by astrocytes have also been confirmed in humans. The medical implications of these findings are significant. Also, the facial nerve system of rats and mice has become the best studied and most widely used test system for the evaluation of neurotrophic factors.

Locally administered nerve growth factor suppresses ginsenoside Rb1-enhanced peripheral nerve regeneration

A high-dose of nerve growth factor (NGF) mixed with ginsenoside Rb1 (GRb1) was encapsulated by collagen and placed in silicone rubber chambers, which were used to repair dissected Sprague-Dawley rat sciatic nerves with 15 mm gaps. Six weeks after surgery, no axons or Schwann cells were seen in these chambers. By comparison, nerves treated with collagen-GRb1 alone had regenerated axons and Schwann cells in their endoneurial areas. We suggest that excessive NGF may not promote but, rather, suppress developing nerves.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Effect of bilobalide on peripheral nerve regeneration

The present study provides in vivo trials of silicone rubber chambers filled with different concentrations of bilobalide (0, 50, 100, 200, 400 microM) to bridge a 15 mm sciatic nerve defect in rats. Collagen was also filled in the chambers to prevent the bilobalide from leakage. Histological and electrophysiological techniques were used to evaluate the functional recovery of the nerve. At the conclusion of 8 weeks, animals from the group treated with the bilobalide, especially at the concentration of 200 microM, had a higher rate (40%) of successful regeneration across the wide gap and a significantly larger number of myelinated axons (4094 +/- 1555), compared to only 10% and 2485 in the control group. However, the high dose bilobalide (400 microM) completely reversed this positive effect of growth-promoting capability and inhibited nerve regeneration. Only 10% of the animals treated with the high dose bilobalide had regenerated cables within the silicone rubber chambers. These results indicated that bilobalide could be involved in both positive and negative effects on regenerating nerves. Therefore, whether a proper dosage of bilobalide is used or not plays a critical factor in deciding if it can sustain nerve regeneration over long gaps.

Peripheral axon crush elevates transport of p75NTR in the central projection of sensory neurones of rats

The effect of peripheral axon crush on the axonal transport of the neurotrophin receptors, p75(NTR) and trkA, was studied in dorsal roots of adult rats. Lumbar dorsal roots were crushed for 3-6 h to cause accumulation of p75(NTR) and trkA. Immunohistochemistry showed the presence of the NGF receptors in axons, indicating retrograde and anterograde axonal transport in the dorsal root. Western blots confirmed that the time course of accumulation of p75(NTR) was consistent with fast axonal transport. However, trkA accumulation was too low to indicate significant levels of axonal transport. Sciatic nerve crush induced a 2-fold increase (P<0.05) in the bidirectional axonal transport of p75(NTR) in the dorsal root while trkA transport remained below detectable levels.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

FK506 enhances reinnervation by regeneration and by collateral sprouting of peripheral nerve fibers

We examined the effects of FK506 administration on the degree of target reinnervation by regenerating axons (following sciatic nerve crush) and by collateral sprouts of the intact saphenous nerve (after sciatic nerve resection) in the mouse. FK506-treated animals received either 0.2 or 5 mg/kg/day, dosages previously found to maximally increase the rate of axonal regeneration in the mouse. Functional reinnervation of motor, sensory, and sweating activities was assessed by noninvasive methods in the hind paw over a 1-month period following lesion. Morphometric analysis of the regenerated nerves and immunohistochemical labeling of the paw pads were performed at the end of follow-up. In the sciatic nerve crush model, FK506 administration shortened the time until target reinnervation and increased the degree of functional and morphological reinnervation achieved. The recovery achieved by regeneration was greater overall with the 5 mg/kg dose than with the dose of 0.2 mg/kg of FK506. In the collateral sprouting model, reinnervation by nociceptive and sudomotor axons was enhanced by FK506. Here, the field expansion followed a faster course between 4 and 14 days in FK506-treated animals. In regard to dose, while collateral sprouting of nociceptive axons was similarly increased at both dosages (0.2 and 5 mg/kg), sprouting of sympathetic axons was more extensive at the high dose. This suggests that the efficacy of FK506 varies between subtypes of neurons. Taken together, our findings indicate that, in addition to an effect on rate of axonal elongation, FK506 improves functional recovery of denervated targets by increasing both regenerative and collateral reinnervation.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

An injectable nerve regeneration chamber for studies of unstable soluble growth factors

Modern surgical techniques cannot guarantee functional recovery following peripheral nerve injuries. Research into factors that may influence nerve regeneration has therefore assumed a prominent potential therapeutic role. We report here on the development of an approach to allow for direct manipulation of the microenvironment of regenerating peripheral nerve axons. We show that solutions can be delivered directly to this local milieu in vivo and that such a delivery can be performed multiple times over an extended period, potentially facilitating studies of multiple molecular players that act locally. We also demonstrate that the bundle of regenerated axons are amenable to morphological analysis by 21 days and that the injection system remains patent for at least 21 days.

Glial cell line-derived neurotrophic factor released by synthetic guidance channels promotes facial nerve regeneration in the rat

Regeneration of the human facial nerve after lesion is often limited, leading to severe functional impairments, in particular when repair is delayed for several months, when cross-facial nerve grafts have to be performed, or in elderly patients. To improve the outcome, the potential accelerating and maturating effects of the neurotrophic factors glial cell line-derived neurotrophic factor (GDNF) and neurotrophin-3 (NT-3) on nerve regeneration were assessed using an axotomy model of the rat facial nerve. One-centimeter-long synthetic guidance channels releasing the neurotrophic factors over several weeks were used to bridge an 8 mm nerve gap, a distance that does not allow regeneration in the absence of growth factors. Nerve cables regenerated in the presence of GDNF showed a large number of myelinated axons 6 weeks after grafting (871 +/- 373, n = 5), whereas only 106 +/- 86 (n = 5) myelinated axons were counted in the presence of NT-3. Retrograde labeling with fluorogold revealed 981 +/- 450 (n = 5) and 53 +/- 38 (n = 5) retrogradely labeled motoneurons in the facial nucleus in the presence of GDNF and NT-3, respectively. No regenerated axons or retrogradely labeled cells were observed in the absence of growth factors (n = 6). These results demonstrate that GDNF, as previously described for the sciatic nerve, a mixed sensory and motor nerve, is also very efficient in promoting regeneration of the facial nerve, an essentially pure motor nerve. GDNF may therefore be useful in improving facial nerve regeneration in the clinic.

Intrathecal administration of nerve growth factor delays GAP 43 expression and early phase regeneration of adult rat peripheral nerve

Whether nerve growth factor (NGF) promotes peripheral nerve regeneration in vivo, in particular in adults, is controversial. We therefore examined the effect of exogenous NGF on nerve regeneration and the expression of GAP 43 (growth-associated protein 43) in adult rats. NGF was infused intrathecally via an osmotic mini-pump, while control rats received artificial cerebrospinal fluid. Two days after the infusion was initiated, the right sciatic nerves were transected or crushed, and the animals allowed to survive for 3 to 11 days. The right DRG, the right proximal stump of the transected sciatic nerve, and the posterior horn of the spinal cord were examined by Western blotting, immunohistochemistry, and electron microscopy. GAP 43 immunoreactivity in the NGF-treated animals was significantly lower than in the aCSF-treated controls. Electron microscopy showed that the number of myelinated and unmyelinated axons decreased significantly in the NGF-treated rats as compared with the controls. These findings are indicative that exogenous NGF delayed GAP 43 induction and the early phase of peripheral nerve regeneration and supports the hypothesis that the loss of NGF supply from peripheral targets via retrograde transport caused by axotomy serves as a signal for DRG neurons to invoke regenerative responses. NGF administered intrathecally may delay the neurons' perception of the reduction of the endogenous NGF, causing a delay in conversion of DRG neurons from the normal physiological condition to regrowth state.

Focal application of neutralizing antibodies to soluble neurotrophic factors reduces collateral axonal branching after peripheral nerve lesion

A major reason for the insufficient recovery of function after motor nerve injury are the numerous axonal branches which often re-innervate muscles with completely different functions. We hypothesized that a neutralization of diffusable neurotrophic factors at the lesion site in rats could reduce the branching of transected axons. Following analysis of local protein expression by immunocytochemistry and by in situ hybridization, we transected the facial nerve trunk of adult rats and inserted both ends into a silicon tube containing (i) collagen gel with neutralizing concentrations of antibodies to NGF, BDNF, bFGF, IGF-I, CNTF and GDNF; (ii) five-fold higher concentrations of the antibodies and (iii) combination of antibodies. Two months later, retrograde labelling was used to estimate the portion of motoneurons the axons of which had branched and projected into three major branches of the facial trunk. After control entubulation in collagen gel containing non-immune mouse IgG 85% of all motoneurons projecting along the zygomatic branch sprouted and sent at least one twin axon to the buccal and/or marginal-mandibular branches of the facial nerve. Neutralizing concentrations of anti-NGF, anti-BDNF and anti-IGF-I significantly reduced sprouting. The most pronounced effect was achieved after application of anti-BDNF, which reduced the portion of branched neurons to 18%. All effects after a single application of antibodies were concentration-dependent and superior to those observed after combined treatment. This first report on improved quality of reinnervation by antibody-therapy implies that, in rats, the post-transectional collateral axonal branching can be reduced without obvious harmful effects on neuronal survival and axonal elongation.