Mice lacking basic fibroblast growth factor showed faster sensory recovery

Neurotrophic factors have been shown to stimulate and support peripheral nerve repair. One of these factors is basic fibroblast growth factor (FGF-2), which is up-regulated after peripheral nerve injury and influences early sciatic nerve regeneration by regulating Schwann cell proliferation. Our previous study on FGF-2 deficient mice indicated that FGF-2 is important for axonal maturation and remyelination one week after sciatic nerve crush (Jungnickel, J., Claus, P., Gransalke, K., Timmer, M. and Grothe, C., 2004. Targeted disruption of the FGF-2 gene affects the response to peripheral nerve injury. Mol. Cell. Neurosci. 25, 444-452). However, the functional impact of these effects on sensory and motor fibers was not clear. After performing pinch test, walking track analysis and rotarod, we found faster recovery of mechanosensory but not of motor function in mutant mice. To elucidate the role of FGF-2 on structural recovery, we analyzed FGF-2 deficient mice and wild-type littermates 2 and 4 weeks after sciatic nerve crush. Two weeks after peripheral nerve injury, regenerating fibers of mutant mice showed both significantly increased axon and myelin size, but no difference in the number of myelinated and unmyelinated fibers. Molecular analysis indicated that the expression level of myelin protein zero was significantly enhanced in lesioned nerves in the absence of FGF-2. These results suggest that loss of FGF-2 could positively influence restoration of mechanosensory function by accelerating structural recovery transiently.     

Physiological function and putative therapeutic impact of the FGF-2 system in peripheral nerve regeneration--lessons from in vivo studies in mice and rats

Diffusible and substratum-bound molecules regulate development and regeneration of the peripheral nervous system. The understanding of physiological function of these factors could have an impact on the development of new therapeutic strategies to stimulate nerve regeneration across long gaps. Within the group of trophic factors, basic fibroblast growth factor (FGF-2) and its high-affinity receptors are expressed in the intact peripheral nervous system and regulated following nerve injury. After exogenous application, FGF-2 promotes neuronal survival and neurite outgrowth in vitro and in vivo. In this review, animal studies on the physiological role of the endogenous FGF-2 system and the regenerative capacity after exogenous FGF-2 administration are summarized. The concept of FGF-2 function is discussed in context with other growth factors that are also physiologically relevant in the peripheral nervous system. Studies of sciatic nerve axotomy in FGF-2- and FGF receptor (R) 3-deleted mice, respectively, strongly suggested that FGF-2 binding to FGFR3 is involved in injury-induced neuronal apoptosis. At the lesion site, inhibition of myelination and stimulation of Schwann cell proliferation by FGF-2 via FGFR1/2 is suggested from rat and mouse studies, whereas neurite formation is very likely enhanced via FGFR3 activation. Additionally to these demonstrated physiological functions of endogenous FGF-2, administration of FGF-2 isoforms in the rat model of nerve regeneration across long gaps revealed a role of the high molecular weight isoforms of FGF-2 on sensory recovery. Within the group of physiologically relevant trophic factors, the FGF-2 system seems to be crucially involved in the scenario of peripheral nerve development and regeneration.     

Myelinated fiber regeneration after sciatic nerve crush: morphometric observations in young adult and aging mice and the effects of macrophage suppression and conditioning lesions.

To study myelinated nerve fiber regeneration during aging, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two, 4, and 8 weeks later, both groups of mice were perfused. The sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. Two weeks after axotomy, fascicles of aging mice contained significantly fewer regenerated myelinated fibers than those of young adults. After 4 weeks, the difference in the number of myelinated fibers was less. However, measurements of myelinated fibers in fascicles of aging mice showed that areas of Schwann cell cytoplasm and myelin were significantly reduced at all intervals. In contrast, although axon diameters in aging mice were somewhat less 2 weeks after crushing, the difference decreased with time, suggesting that in nerves of aging mice, regenerative responses of Schwann cells were more affected than those of axons. Other experiments in young mice showed that myelinated fiber regeneration could be retarded by suppressing macrophage responses and was not significantly changed by conditioning lesions before crush injury.     

Myelinated fiber regeneration after crush injury is retarded in sciatic nerves of aging mice

To compare nerve regeneration in young adult and aging mice, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two weeks later, both groups of mice were perfused with an aldehyde solution, and, after additional fixation, the sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. The same level was sectioned in unoperated contralateral nerves; these nerves served as controls. Electron micrographs and the Bioquant Image Analysis System IV were used to measure areas of posterior tibial fascicles and count the number of myelinated axons, the number of unmyelinated axons, and their frequency in Schwann cell units. In aging mice, the total number of regenerating myelinated axons was significantly reduced, but totals of regenerating unmyelinated axons in aging and young adults did not differ significantly. In aging mice, the frequency of Schwann cells that contained a single unmyelinated axon was greater, suggesting that before myelination began, Schwann cell ensheathment of axons also was slowed. After axotomy by a crush injury, the area of the posterior tibial fascicle was less than that in young adults and the distal disintegration of myelin sheath remnants also appeared to be retarded. The results indicate that responses of neurons, axons, and Schwann cells could be important in slowing the regeneration of myelinated fibers found in sciatic nerves from aging mice.     

Fibroblast growth factor receptor 3 signaling regulates injury-related effects in the peripheral nervous system

Fibroblast growth factor receptor (FGFR) signaling is crucial for neural development and regeneration. Here we investigated the L5 spinal ganglion and the sciatic nerve of intact Fgfr3-deficient mice after nerve injury. Quantification of sensory neurons in the L5 spinal ganglion revealed no significant differences between wild-type and Fgfr3-deficient mice. Seven days after nerve lesion, the normally occurring neuron loss in wild-type mice was not found in Fgfr3-deficient animals, suggesting that FGFR3 signaling is involved in the cell death process. Morphometric analysis of the sciatic nerve showed similar numbers of myelinated axons, but the axonal and myelin diameter was significantly smaller in Fgfr3-deficient mice compared to the wild types. Evaluation of regenerating myelinated axons of the sciatic nerve revealed no differences between both mouse strains 7 days after crush injury. Our results suggest that FGFR3 signaling seems to be involved in processes of damage-induced neuron death and axonal development.     

Biodegradable magnesium wire promotes regeneration of compressed sciatic nerves

Magnesium(Mg) wire has been shown to be biodegradable and have anti-inflammatory properties. It can induce Schwann cells to secrete nerve growth factor and promote the regeneration of nerve axons after central nervous system injury. We hypothesized that biodegradable Mg wire may enhance compressed peripheral nerve regeneration. A rat acute sciatic nerve compression model was made, and AZ31 Mg wire(3 mm diameter; 8 mm length) bridged at both ends of the nerve. Our results demonstrate that sciatic functional index, nerve growth factor, p75 neurotrophin receptor, and tyrosine receptor kinase A m RNA expression are increased by Mg wire in Mg model. The numbers of cross section nerve fibers and regenerating axons were also increased. Sciatic nerve function was improved and the myelinated axon number was increased in injured sciatic nerve following Mg treatment. Immunofluorescence histopathology showed that there were increased vigorous axonal regeneration and myelin sheath coverage in injured sciatic nerve after Mg treatment. Our findings confirm that biodegradable Mg wire can promote the regeneration of acute compressed sciatic nerves.     

Hereditary hypertrophic neuropathy in the Trembler mouse: Part 2. Histopathological studies: Electron microscopy

The Trembler mouse suffers from a dominantly inherited hypertrophic neuropathy. Electron microscopy, including a quantitative analysis of myelination was performed on the nerves of Trembler mice from birth to senility and compared with the findings in control mice. Axons in adult Trembler nerves were thinly myelinated and were surrounded by very few myelin lamellae which in turn were often uncompact circumferentially and longitudinally. Schwann cell cytoplasm was copious and had a normal content of organelles. Well-developed “onion-bulb” formations which consisted of thinly myelinated axons surrounded by empty membrane configurations were frequently seen.

The initiation of myelination was studied. The diameter distribution of promyelin fibres of control and Trembler sciatic nerve at ages day 2, 4, and 7 was calculated Myelination in Trembler nerves commenced on axons of larger diameters than controls.

The effectiveness of myelination was studied by relating the number of turns of myelin to the axon area of control and Trembler sciatic nerves from age 2 days to adult mice. At all ages Trembler axons were less well myelinated than controls and the difference was more pronounced with age.

Schwann cell activity was examined by relating the area of the Schwann cell cytoplasm to the area of the axon it invests. The relative amount of Schwann cell cytoplasm decreased progressively in control axons with age and as the axon became better myelinated. By contrast, that of Tremblers did not undergo a similar reduction as the animal matured and the relative amount of Schwann cell cytoplasm was markedly increased in adult Tremblers when compared with controls.

The periodicity of control and Trembler compact myelin was compared. The myelin period of Trembler mouse was significantly greater than that of controls. The defect in Trembler peripheral nerves was considered to be that of dysmyelinogenesis. The Schwann cell was active but ineffective in the synthesis, compaction and maintenance of myelin.     

Endogenous BDNF is required for myelination and regeneration of injured sciatic nerve in rodents

Following a peripheral nerve injury, brain-derived neurotrophic factor (BDNF) and the p75 neurotrophin receptor are upregulated in Schwann cells of the Wallerian degenerating nerves. However, it is not known whether the endogenous BDNF is critical for the functions of Schwann cells and regeneration of injured nerve. Treatment with BDNF antibody was shown to retard the length of the regenerated nerve from injury site by 24%. Histological and ultrastructural examination showed that the number and density of myelinated axons in the distal side of the lesion in the antibody-treated mice was reduced by 83%. In the BDNF antibody-treated animals, there were only distorted and disorganized myelinated fibres in the injured nerve where abnormal Schwann cells and phagocytes were present. As a result of nerve degeneration in BDNF antibody-treated animals, subcellular organelles, such as mitochondria, disappeared or were disorganized and the laminal layers of the myelin sheath were loosened, separated or collapsed. Our in situ hybridization revealed that BDNF mRNA was expressed in Schwann cells in the distal segment of lesioned nerve and in the denervated muscle fibres. These results indicate that Schwann cells and muscle fibres may contribute to the sources of BDNF during regeneration and that the deprivation of endogenous BDNF results in an impairment in regeneration and myelination of regenerating axons. It is concluded that endogenous BDNF is required for peripheral nerve regeneration and remyelination after injury.     

Numbers of regenerating axons in parent and tributary peripheral nerves in the rat

This study is concerned with numerical parameters of axonal regeneration in peripheral nerves. Our first finding is that the number of axons that regenerate into the distal stump of a somatic nerve at a particular time after transection is partially dependent on the type of lesion used to interrupt the axons. The second question concerns the proportion of axons that regenerate into the distal stump of a parent nerve compared to the proportions that regenerate into tributary nerves that arise from the parent. The proportions of regenerated myelinated axons in the nerve to the medial gastrocnemius muscle and myelinated and unmyelinated axons in the sural nerve are the same as the proportions of myelinated and unmyelinated axons that regenerate into the distal stump of the sciatic nerve for the crush, 0 and 4 mm gap transections. Proportionally fewer axons regenerate into the tributary nerves following the 8 mm gap transection, however. This implies that the length of the gap has an influence on whether or not axons in tributary nerves regenerate in concert with axons in the distal stump of the parent nerve. The unmyelinated fibers in the nerve to the medial gastrocnemius muscle are different because they do not regenerate in proportion to those in the distal stump of the sciatic nerve. We also provide evidence to indicate that myelinated axons branch whereas unmyelinated fibers end blindly when they enter the distal stump after crossing a sciatic nerve transection. Finally the normal arrangement of perineurial cells seems to be disrupted after the sciatic nerve regenerates across a gap.     

Degeneration of non-myelinated axons in the rat sciatic nerve following lysolecithin injection

Summary Lysolecthin has been used in many studies to induce demyelination in peripheral nerves. In the present investigation lysolecithin (lysophosphatidyl choline) was injected into rat sciatic nerves at a dose of 2–3 m of a 10 mg/ml solution in order to study the effects of this lipid on cellular elements other than myelin within the nerve. Twenty-four hours after injection, there was splitting of myelin, lysis of Schwann cells, and complete loss of non-myelinated axons and their Schwann cells at the site of injection. Numerous swollen non-myelinated axons containing accumulated organelles were seen just proximal to the site of injection at 48 h. Loss of non-myelinated axons from the distal part of the nerve was also noted at 3 days after injection but by 7 days regenerating non-myelinated axons had re-appeared in the distal part of the nerve. Although demyelination, followed by remyelination was a prominent feature in the injected segment of the nerve, no damage to myelinated axons was detected. These results suggest that the presence of the myelin sheath protects the large myelinated axons against the action of lysolecithin, but with lysis of Schwann cells, the non-myelinated axons are exposed to the action of lysolecithin. Apart from selective damage to non-myelinated fibres with subsequent degeneration, it is also possible that lysolecithin interferes with axoplasmic flow in non-myelinated axons.     

Amyloid precursor protein and amyloid precursor-like protein 2 have distinct roles in modulating myelination,demyelination, and remyelination of axons

The identification of factors that regulate myelination provides important insight into the molecular mechanisms that coordinate nervous system development and myelin regeneration after injury. In this study, we investigated the role of amyloid precursor protein (APP) and its paralogue amyloid precursor-like protein 2 (APLP2) in myelination using APP and APLP2 knockout (KO) mice. Given that BACE1 regulates myelination and myelin sheath thickness in both the peripheral and central nervous systems, we sought to determine if APP and APLP2, as alternate BACE1 substrates, also modulate myelination, and therefore provide a better understanding of the events regulating axonal myelination. In the peripheral nervous system, we identified that adult, but not juvenile KO mice, have lower densities of myelinated axons in their sciatic nerves while in the central nervous system, axons within both the optic nerves and corpus callosum of both KO mice were significantly hypomyelinated compared to wild-type (WT) controls. Biochemical analysis demonstrated significant increases in BACE1 and myelin oligodendrocyte glycoprotein and decreased NRG1 and proteolipid protein levels in both KO brain tissue. The acute cuprizone model of demyelination/remyelination revealed that whereas axons in the corpus callosum of WT and APLP2-KO mice underwent similar degrees of demyelination and subsequent remyelination, the myelinated callosal axons in APP-KO mice were less susceptible to cuprizone-induced demyelination and showed a failure in remyelination after cuprizone withdrawal. These data identified APP and APLP2 as modulators of normal myelination and demyelination/remyelination conditions. Deletion of APP and APLP2 identifies novel interplays between the BACE1 substrates in the regulation of myelination.     

Skeletal muscle-derived cells repair peripheral nerve defects in mice

Skeletal muscle-derived cells have strong secretory function,while skeletal muscle-derived stem cells,which are included in muscle-derived cells,can differentiate into Schwann cell-like cells and other cell types.However,the effect of muscle-derived cells on peripheral nerve defects has not been reported.In this study,5-mm-long nerve defects were created in the right sciatic nerves of mice to construct a peripheral nerve defect model.Adult female C57BL/6 mice were randomly divided into four groups.For the muscle-derived cell group,muscle-derived cells were injected into the catheter after the cut nerve ends were bridged with a polyurethane catheter.For external oblique muscle-fabricated nerve conduit and polyurethane groups,an external oblique muscle-fabricated nerve conduit or polyurethane catheter was used to bridge the cut nerve ends,respectively.For the sham group,the sciatic nerves on the right side were separated but not excised.At 8 and 12 weeks post-surgery,distributions of axons and myelin sheaths were observed,and the nerve diameter was calculated using immunofluorescence staining.The number,diameter,and thickness of myelinated nerve fibers were detected by toluidine blue staining and transmission electron microscopy.Muscle fiber area ratios were calculated by Masson’s trichrome staining of gastrocnemius muscle sections.Sciatic functional index was recorded using walking footprint analysis at 4,8,and 12 weeks after operation.The results showed that,at 8 and 12 weeks after surgery,myelin sheaths and axons of regenerating nerves were evenly distributed in the muscle-derived cell group.The number,diameter,and myelin sheath thickness of myelinated nerve fibers,as well as gastrocnemius muscle wet weight and muscle area ratio,were significantly higher in the muscle-derived cell group compared with the polyurethane group.At 4,8,and 12 weeks post-surgery,sciatic functional index was notably increased in the muscle-derived cell group compared with the polyurethane group.These criteria of the muscle-derived cell group were not significantly different from the external oblique muscle-fabricated nerve conduit group.Collectively,these data suggest that muscle-derived cells effectively accelerated peripheral nerve regeneration.This study was approved by the Animal Ethics Committee of Plastic Surgery Hospital,Chinese Academy of Medical Sciences(approval No.040)on September 28,2016.     

Peripheral nerve regeneration is delayed in neuropilin 2-deficient mice

Peripheral nerve transection or crush induces expression of class 3 semaphorins by epineurial and perineurial cells at the injury site and of the neuropilins neuropilin-1 and neuropilin-2 by Schwann and perineurial cells in the nerve segment distal to the injury. Neuropilin-dependent class 3 semaphorin signaling guides axons during neural development, but the significance of this signaling system for regeneration of adult peripheral nerves is not known. To test the hypothesis that neuropilin-2 facilitates peripheral-nerve axonal regeneration, we crushed sciatic nerves of adult neuropilin-2-deficient and littermate control mice. Axonal regeneration through the crush site and into the distal nerve segment, repression by the regenerating axons of Schwann cell p75 neurotrophin receptor expression, remyelination of the regenerating axons, and recovery of normal gait were all significantly slower in the neuropilin-2-deficient mice than in the control mice. Thus, neuropilin-2 facilitates peripheral-nerve axonal regeneration.     

Cholesterol reutilization during myelination of regenerating PNS axons is impaired in Niemann-Pick disease type C mice

Niemann-Pick C (NPC) disease is an autosomal recessive lipidosis characterized by lysosomal accumulations of unesterified cholesterol. Cultured NPC cells exhibit a defect in the intracellular trafficking of LDL-derived cholesterol that leads to lysosomal accumulations of unesterified cholesterol. We found in a preliminary study that the myelination of regenerating axons was retarded in the NPC mouse following sciatic nerve crush. Because lipoprotein-mediated cholesterol transport is involved in myelination during nerve regeneration, we investigated whether this cholesterol reutilization pathway was perturbed in the NPC mouse. Mice received intraneural injections of [³H]acetate to label myelin cholesterol, and 2 weeks later the injected nerves were crushed above the injection site. Four weeks after crush, the nerves were examined by electron microscopic autoradiography. In normal mice, regeneration was well advanced, with thick myelin sheaths surrounding the regenerated axons and very little myelin debris remaining. The new myelin sheaths were well labeled, indicative of efficient cholesterol reutilization. In NPC mice, the new myelin sheaths were thinner and contained little label, indicative of retarded regeneration and little or no cholesterol reutilization. These data suggest the possibility of a causal link between compromised cholesterol reutilization and delayed or slowed regeneration of myelin sheaths. J. Neurosci. Res. 49:389–392, 1997. © 1997 Wiley-Liss, Inc.     

Bone marrow stromal cells and resorbable collagen guidance tubes enhance sciatic nerve regeneration in mice

We evaluated peripheral nerve regeneration using a tubular nerve guide of resorbable collagen filled with either bone marrow-derived cells (BMDCs) in Dulbecco's cell culture medium (DMEM) or with DMEM alone (control). The control group received just the culture medium (vehicle). The left sciatic nerves of ten isogenic mice were transected and the tubular nerve guides were sutured to the end of the proximal and distal nerve stumps. Motor function was tested at 2, 4 and 6 weeks after surgery using the walking track test. The pawprints were analyzed and the print lengths (PL) were measured to evaluate functional recovery. After 6 weeks, mice were anesthetized, perfused transcardially with fixative containing aldehydes, and the sciatic nerves and tubes were dissected and processed for scanning and transmission electron microscopy. Scanning electron microscopy of the collagen tube revealed that the tube wall became progressively thinner after surgery, proving that the tube can be resorbed in vivo. Quantitative analysis of the regenerating nerves showed that the number of myelinated fibers and the myelin area were significantly increased in the experimental group. Also, motor function recovery was faster in animals that received the cell grafts. These results indicate that the collagen tube filled with BMDCs provided an adequate and favorable environment for the growth and myelination of regenerating axons compared to the collagen tube alone.     

Peripheral nerve abnormalities of mutant (PMA) mouse--myelinated fiber counts of sciatic, peroneal, sural and tibial nerves

A mutant mouse characterized by peroneal muscular atrophy and congenital absence of the peroneal nerve has been described by Esaki et al. and investigated as a possible animal model of Charcot-Marie-Tooth disease or spinal muscular atrophy. However, the nature of the peripheral nerve abnormality of this mutant mouse has not been precisely defined yet. In this study, in addition to the qualitative evaluation of teased fiber and Epon-embedded preparations, the total transverse fascicular area and the total numbers of myelinated fibers per nerve in sciatic, peroneal (proximal and distal), sural (proximal and distal) and tibial (proximal and distal) nerves on both right and left sides were compared between six peroneal muscular atrophy (pma) mice with autosomal recessive gene manifesting mainly the peroneal muscular atrophy and their six control mice to understand and define the peripheral nerve abnormalities. The pma mice showed pes equinovarus bilaterally and their peroneal nerves were absent. No myelinated fibers showing axonal degeneration or segmental demyelination were found on teased fiber preparation. Onion bulb, demyelinated or remyelinated axons and myelin ovoids were not observed in Epon-embedded sections. The mean total transverse fascicular area and mean total number of myelinated fibers per nerve in the sciatic nerve in pma mice was significantly less (p less than 0.02) than that in control mice. On the other hand they were significantly greater (p less than 0.0001) in the sural nerve in pma mice than in control mice.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differences in peripheral nerve degeneration/regeneration between wild-type and neuronal nitric oxide synthase knockout mice

Nitric oxide (NO), a unique biological messenger molecule, is synthesized by three isoforms of the enzyme NO synthase (NOS) and diffuses from the site of production across cellular membranes. A postulated role for NO in degeneration and regeneration of peripheral nerves has been explored in a sciatic nerve model comparing wild-type mice and mice lacking neuronal NOS after transection and microsurgical repair. In NOS knockout mice, regenerative delay was observed, preceded by a decelerated Wallerian degeneration (WD). In the regenerated nerve, pruning of uncontrolled sprouts was disturbed, leading to an enhanced number of axons, whereas remyelination seemed to be less affected. Delayed regeneration was associated with a delayed recovery of sensor and motor function. In such a context, possible NO targets are neurofilaments and myelin sheaths of the interrupted axon, filopodia of the growth cone, newly formed neuromuscular endplates, and Schwann cells in the distal nerve stump. The results presented suggest that 1) local release of NO following peripheral nerve injury is a crucial factor in degeneration/regeneration, 2) success of fiber regeneration in the peripheral nervous system depends on a regular WD, and 3) manipulation of NO supply may offer interesting therapeutic options for treatment of peripheral nerve lesions.     

Single vs multiple somatic nerve crushes in the rat

Nerve lesions modify regenerative responses to subsequent lesions. Some of the modifications might be useful. To increase our understanding of these modifications, the present study determines myelinated and unmyelinated axon numbers in the distal part of rat sciatic nerve and in 2 smaller branches, the nerve to the medial gastrocnemius muscle and the sural nerve, 8 weeks and 9 months following either single or the last of 3 crushes to the rat sciatic nerve. For myelinated axons, there is a significant and proportional increase distal to the crush in the sciatic nerve and in its smaller tributaries following both single and triple crushes. These increased axons persist. We interpret these data to indicate that some of the regenerating myelinated axons branch at the site of lesion, pass without branching into the tributary nerves, and then presumably find attachments at the periphery. If true, single or multiple crushes might be useful in conditions where it would be desirable to increase numbers of processes from surviving neurons. The major differences between single and triple crushes are that myelinated axons are increased more after triple crush and increase significantly between 8 weeks and 9 months after triple crush but not after single crush. Thus not only myelinated axon numbers, but the timing of the myelination process seems to change if regeneration following single crush is compared to similar regeneration following multiple crushes. Unmyelinated axons do not regenerate in the same way as the myelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin as an in vivo growth factor

Insulin peptide has been identified to promote regeneration of axons in culture and in some in vivo model systems. Such actions have been linked to direct actions of insulin, or to cross occupation of closely linked IGF-1 receptors. In this work, we examined insulin support of peripheral nerve regenerative events in mice. Systemic insulin administration accelerated the reinnervation of foot interosseous endplates by motor axons after sciatic nerve transection and enhanced recovery of functional mouse hindpaw function. Similarly, insulin accelerated the regeneration-related maturation of myelinated fibers regrowing beyond a sciatic nerve crush injury. That such benefits might occur through direct signaling on axons was supported by immunohistochemical studies of expression with an antibody directed to the beta insulin receptor (IR) subunit. The proportion of sensory neurons expressing IRbeta increased ipsilateral to a similar sciatic crush injury in the L4 and L5 dorsal root ganglia. Insulin receptors, although widely expressed in axons, were also preferentially and intensely expressed on axons regrowing just beyond a peripheral nerve crush injury zone. The findings indicate that insulin imparts a substantial impact on regenerating peripheral nerve axons through upregulation of its expression following injury. Although the findings do not exclude insulin coactivating IGF-1 receptors during regeneration, its own receptors are present and available for action on injured nerves.