Elevated insulin-like growth factor (IGF) gene expression in sciatic nerves during IGF-supported nerve regeneration

Motor Neuron Diseases and Neuroprotective Peptides: A Closer Look to Neurons

Motor neurons (MNs) are specialized neurons responsible for muscle contraction that specifically degenerate in motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS), spinal and bulbar muscular atrophy (SBMA), and spinal muscular atrophy (SMA). Distinct classes of MNs degenerate at different rates in disease, with a particular class named fast-fatigable MNs (FF-MNs) degenerating first. The etiology behind the selective vulnerability of FF-MNs is still largely under investigation. Among the different strategies to target MNs, the administration of protective neuropeptides is one of the potential therapeutic interventions. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with beneficial effects in many neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and more recently SBMA. Another neuropeptide that has a neurotrophic effect on MNs is insulin-like growth factor 1 (IGF-1), also known as somatomedin C. These two peptides are implicated in the activation of neuroprotective pathways exploitable in the amelioration of pathological outcomes related to MNDs.

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.

Evidence for a Role of Nerve Injury in Painful Intervertebral Disc Degeneration: A Cross-Sectional Proteomic Analysis of Human Cerebrospinal Fluid

Intervertebral disc degeneration (DD) is a cause of low back pain (LBP) in some individuals. However, although >30% of adults have DD, LBP only develops in a subset of individuals. To gain insight into the mechanisms underlying nonpainful versus painful DD, human cerebrospinal fluid (CSF) was examined using differential expression shotgun proteomic techniques comparing healthy control participants, subjects with nonpainful DD, and patients with painful DD scheduled for spinal fusion surgery. Eighty-eight proteins were detected, 27 of which were differentially expressed. Proteins associated with DD tended to be related to inflammation (eg, cystatin C) regardless of pain status. In contrast, most differentially expressed proteins in DD-associated chronic LBP patients were linked to nerve injury (eg, hemopexin). Cystatin C and hemopexin were selected for further examination using enzyme-linked immunosorbent assay in a larger cohort. While cystatin C correlated with DD severity but not pain or disability, hemopexin correlated with pain intensity, physical disability, and DD severity. This study shows that CSF can be used to study mechanisms underlying painful DD in humans, and suggests that while painful DD is associated with nerve injury, inflammation itself is not sufficient to develop LBP.

PERSPECTIVE

CSF was examined for differential protein expression in healthy control participants, pain-free adults with asymptomatic intervertebral DD, and LBP patients with painful intervertebral DD. While DD was related to inflammation regardless of pain status, painful degeneration was associated with markers linked to nerve injury.

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.

CRMP1 Interacted with Spy1 During the Collapse of Growth Cones Induced by Sema3A and Acted on Regeneration After Sciatic Nerve Crush

CRMP1, a member of the collapsin response mediator protein family (CRMPs), was reported to regulate axon outgrowth in Sema3A signaling pathways via interactions with its co-receptor protein neuropilin-1 and plexin-As through the Fyn-cyclin-dependent kinase 5 (CDK5) cascade and the sequential phosphorylation of CRMP1 by lycogen synthase kinase-3β (GSK-3β). Using yeast two-hybrid, we identified a new molecule, Speedy A1 (Spy1), a member of the Speedy/RINGO family, with an interaction with CRMP1. Besides, for the first time, we observed the association of CRMP1 with actin. Based on this, we wondered the association of them and their function in Sema3A-induced growth cones collapse and regeneration process after SNC. During our study, we constructed overexpression plasmid and short hairpin RNA (shRNA) to question the relationship of CRMP1/Spy1 and CRMP1/actin. We observed the interactions of CRMP1/Spy1 and CRMP1/actin. Besides, we found that Spy1 could affect CRMP1 phosphorylation actived by CDK5 and that enhanced CRMP1 phosphorylation might disturb the combination of CRMP1 and actin, which would contribute to abnormal of Sema3A-induced growth cones collapse and finally lead to influent regeneration process after rat sciatic nerve crush. Through rat walk footprint test, we also observed the variance during regeneration progress, respectively. We speculated that CRMP1 interacted with Spy1 which would disturb the association of CRMP1 with actin and was involved in the collapse of growth cones induced by Sema3A and regeneration after sciatic nerve crush.

Adaptive and maladaptive motor axonal sprouting in aging and motoneuron disease

Motor unit (MU) enlargement by sprouting is an important compensatory mechanism for loss of functional MUs during normal aging and neuromuscular disease. Perisynaptic Schwann cells at neuromuscular junctions extend processes that bridge between denervated and reinnervated endplates, and guide axonal sprouts to reinnervate the denervated endplates. In a rat model of partial denervation, high levels of daily neuromuscular activity have been shown to inhibit the outgrowth of sprouts by preventing Schwann cell bridging. In this review, we consider (1) the relative roles of increasing levels of oxidative stress and neuromuscular activity to the destabilization of neuromuscular junctions with age and disease, and (2) how a progressive increase in the neuromuscular activity of declining numbers of functional MUs contributes to the progressive failure of adaptive sprouting and, in turn, to the progressive muscle weakness in the motoneuron diseases of post-polio syndrome and amyotrophic lateral sclerosis. We conclude that there is a time-related progression of MU loss, adaptive sprouting followed by maladaptive sprouting, and continuing recession of terminals during normal aging. The progression is accelerated in motoneuron disease, progressing more rapidly in the post-polio syndrome after prolonged denervation and extremely rapidly in ALS.

The pros and cons of growth factors and cytokines in peripheral axon regeneration

Injury to a peripheral nerve induces a complex cellular and molecular response required for successful axon regeneration. Proliferating Schwann cells organize into chains of cells bridging the lesion site, which is invaded by macrophages. Approximately half of the injured neuron population sends out axons that enter the glial guidance channels in response to secreted neurotrophic factors and neuropoietic cytokines. These lesion-associated polypeptides create an environment that is highly supportive for axon regrowth, particularly after acute injury, and ensure that the vast majority of regenerating axons are directed toward the distal nerve stump. Unfortunately, most neurotrophic factors and neuropoietic cytokines are also strong stimulators of axonal sprouting. Although some of the axonal branches will withdraw at later stages, the sprouting effect contributes to the misdirection of reinnervation that results in the lack of functional recovery observed in many patients with peripheral nerve injuries. Here, we critically review the role of neuronal growth factors and cytokines during axon regeneration in the peripheral nervous system. Their differential effects on axon elongation and sprouting were elucidated in various studies on intraneuronal signaling mechanisms following nerve lesion. The present data define a goal for future therapeutic strategies, namely, to selectively stimulate a Ras/Raf/ERK-mediated axon elongation program over an intrinsic PI3K-dependent axonal sprouting program in lesioned motor and sensory neurons. Instead of modulating growth factor or cytokine levels at the lesion site, targeting specific intraneuronal molecules, such as the negative feedback inhibitors of ERK signaling, has been shown to promote long-distance regeneration while avoiding sprouting of regenerating axons until they have reached their target areas.

Insulin and insulin-like growth factor prevent brain atrophy and cognitive impairment in diabetic rats

There are an estimated 36 million dementia patients worldwide. The anticipated tripling of this number by year 2050 will negatively impact the capacity to deliver quality health care. The epidemic in diabetes is particularly troubling, because diabetes is a substantial risk factor for dementia independently of cerebrovascular disease. There is an urgent need to elucidate the pathogenesis of progressive brain atrophy, the cause of dementia, to allow rational design of new therapeutic interventions. This review summarizes recent tests of the hypothesis that the concomitant loss of insulin and insulin-like growth factors (IGFs) is the dominant cause for age-dependent, progressive brain atrophy with degeneration and cognitive decline. These tests are the first to show that insulin and IGFs regulate adult brain mass by maintaining brain protein content. Insulin and IGF levels are reduced in diabetes, and replacement of both ligands can prevent loss of total brain protein, widespread cell degeneration, and demyelination. IGF alone prevents retinal degeneration in diabetic rats. It supports synapses and is required for learning and memory. Replacement doses in diabetic rats can cross the blood-brain barrier to prevent hippocampus-dependent memory impairment. Insulin and IGFs are protective despite unabated hyperglycemia in diabetic rats, severely restricting hyperglycemia and its consequences as dominant pathogenic causes of brain atrophy and impaired cognition. These findings have important implications for late-onset alzheimer's disease (LOAD) where diabetes is a major risk factor, and concomitant decline in insulin and IGF activity suggest a similar pathogenesis for brain atrophy and dementia.

Insulin-like growth factors in the peripheral nervous system

Insulin-like growth factors (IGFs) play an integral role in development, growth, and survival. This article details the current understanding of the effects of IGFs in the peripheral nervous system (PNS) during health and disease, and introduces how the IGF system regulates PNS development and impacts growth and survival of PNS cells. Also discussed are implications of IGF signaling in neurodegeneration and the status and prospects of IGF therapies for PNS conditions. There is substantial support for the application of IGF therapies in the treatment of PNS injury and disease.

IRS2 signalling is required for the development of a subset of sensory spinal neurons

Insulin and insulin-like growth factor-I play important roles in the development and maintenance of neurons and glial cells of the nervous system. Both factors activate tyrosine kinase receptors, which signal through adapter proteins of the insulin receptor substrate (IRS) family. Although insulin and insulin-like growth factor-I receptors are expressed in dorsal root ganglia (DRG), the function of IRS-mediated signalling in these structures has not been studied. Here we address the role of IRS2-mediated signalling in murine DRG. Studies in cultured DRG neurons from different embryonic stages indicated that a subset of nerve growth factor-responsive neurons is also dependent on insulin for survival at very early time points. Consistent with this, increased apoptosis during gangliogenesis resulted in a partial loss of trkA-positive neurons in DRG of Irs2 mutant embryos. Analyses in adult Irs2(-/-) mice revealed that unmyelinated fibre afferents, which express calcitonin gene-related peptide/substance P and isolectin B4, as well as some myelinated afferents to the skin were affected by the mutation. The diminished innervation of glabrous skin in adult Irs2(-/-) mice correlated with longer paw withdrawal latencies in the hot-plate assay. Collectively, these findings indicate that IRS2 signalling is required for the proper development of spinal sensory neurons involved in the perception of pain.

Neurotrophins: potential therapeutic tools for the treatment of spinal cord injury

Spinal cord injury permanently disrupts neuroanatomical circuitry and can result in severe functional deficits. These functional deficits, however, are not immutable and spontaneous recovery occurs in some patients. It is highly likely that this recovery is dependent upon spared tissue and the endogenous plasticity of the central nervous system. Neurotrophic factors are mediators of neuronal plasticity throughout development and into adulthood, affecting proliferation of neuronal precursors, neuronal survival, axonal growth, dendritic arborization and synapse formation. Neurotrophic factors are therefore excellent candidates for enhancing axonal plasticity and regeneration after spinal cord injury. Understanding growth factor effects on axonal growth and utilizing them to alter the intrinsic limitations on regenerative growth will provide potent tools for the development of translational therapeutic interventions for spinal cord injury.

Harvey Cushing's innovative attempt at xenotransplanting a rabbit spinal cord in a patient after resection of a peripheral nerve tumor in 1902

OBJECTIVE

A review of Harvey Cushing's surgical cases at Johns Hopkins Hospital revealed new information about his early work using nerve xenografts to repair peripheral nerve injuries.

METHODS

The Johns Hopkins Hospital surgical records from 1896 to 1912 were reviewed. A single case in which Cushing used a xenograft to repair a peripheral nerve defect was selected for further study.

RESULTS

In August 1902, a 23-year-old woman presented with tingling and numbness in her left foot and focal tenderness in the popliteal region. Cushing performed an exploratory operation, revealing an encapsulated tumor originating from the internal popliteal nerve. After resecting the segment of involved nerve, Cushing harvested the spinal cord from a rabbit and used it to span the 18-cm defect. At a 5-month postoperative follow-up, according to Cushing's clinical notes, the patient had partially regained some sensation in her leg and foot. Seven months later, the patient presented with a local tumor recurrence, and her leg was amputated. An examination of the amputated specimen demonstrated that the xenograft was still intact without gross evidence of xenograft rejection.

CONCLUSION

Despite its questionable functional success, we report a previously unpublished operative case by Harvey Cushing in which a rabbit spinal cord was transplanted into a human to bridge a sciatic nerve gap. This sheds light on the potential for animal tissue as a source for the treatment of neurological disease in humans.

The role of neurotrophic factors in nerve regeneration

This review considers the 2 sources of neurotrophic factors in the peripheral nervous system (PNS), the neurons and the nonneuronal cells in the denervated distal nerve stumps, and their role in axon regeneration. Morphological assessment of regenerative success in response to administration of exogenous growth factors after nerve injury and repair has indicated a role of the endogenous neurotrophic factors from Schwann cells in the distal nerve stump. However, the increased number of axons may reflect more neurons regenerating their axons and/or increased numbers of axon sprouts from the same number of neurons. Using fluorescent dyes to count neurons that regenerated their axons across a suture site and into distal nerve stumps, brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) were found not to increase the number of neurons that regenerated their axons after immediate nerve repair. Nevertheless, the factors did reverse the deleterious effect of delayed nerve repair, indicating that the axons that regenerate into the distal nerve stump normally have access to sufficient levels of endogenous neurotrophic factors to sustain their regeneration, while neurons that do not have access to these factors require exogenous factors to sustain axon regeneration. Neurons upregulate neurotrophic factors after axotomy. The upregulation is normally slow, beginning after 7 days and occurring in association with a protracted period of axonal regeneration in which axons grow out from the proximal nerve stump across a suture site over a period of 1 month in rodents. This staggered axon regeneration across the suture site is accelerated by a 1-hour period of low-frequency electrical stimulation that simultaneously accelerates the expression of BDNF and its trkB receptor in the neurons. Elevation of the level of BDNF after 2 days to > 3 times that found in unstimulated neurons was accompanied by elevation of the level of cAMP and followed by accelerated upregulation of growth-associated genes, tubulin, actin, and GAP-43 and downregulation of neurofilament protein. Elevation of cAMP levels via rolipram inhibition of phosphodiesterase 4 mimicked the effect of the low-frequency electrical stimulation. In conclusion, the enhanced upregulation of neurotrophic factors in the electrically stimulated axotomized neurons accelerates axon outgrowth into the distal nerve stumps where endogenous sources of growth factors in the Schwann cells support the regeneration of the axons toward the denervated targets. The findings provide strong support for endogenous neurotrophic factors of axotomized neurons and of denervated Schwann cells playing a critical role in supporting axon regeneration in the PNS.

Effects of insulin-like growth factor-I and platelet-rich plasma on sciatic nerve crush injury in a rat model

OBJECT

Local administration of insulin-like growth factor-I (IGF-I) has been shown to increase the rate of axon regeneration in crush-injured and freeze-injured rat sciatic nerves. Local administration of platelet-rich plasma (PRP) has been also shown to have a measurable effect on facial nerve regeneration after transection in a rat model. The objective of the study was to compare the effects of locally administered IGF-I and PRP on the parameters of the Sciatic Function Index (SFI), sensory function (SF), axon count, and myelin thickness/axon diameter ratio (G-ratio) in a rat model of crush-injured sciatic nerves.

METHODS

The right sciatic nerve of Wistar albino rats (24 animals) was crushed using a Yasargil-Phynox aneurysm clip for 45 minutes. All animals were randomly divided into 3 groups: Group 1 (control group) was treated with saline, Group 2 was treated with IGF-I, and Group 3 was treated with PRP. Injections were performed using the tissue expander's injection port with a connecting tube directed at the crush-injured site. Functional recovery was assessed with improvement in the SFI. Recovery of sensory function was using the pinch test. Histopathological examination was performed 3 months after the injury.

RESULTS

The SFI showed an improved functional recovery in the IGF-I-treated animals (Group 2) compared with the saline-treated animals (Group 1) 30 days after the injury. In IGF-I-treated rats, sensory function returned to the baseline level significantly faster than in saline-treated and PRP-treated rats as shown in values between SF-2 and SF-7. The G-ratios were found to be significantly higher in both experimental groups than in the control group.

CONCLUSIONS

This study suggests that the application of IGF-I to the crush-injured site may expedite the functional recovery of paralyzed muscle by increasing the rate of axon regeneration.

Guidance molecules in axon regeneration

The regenerative capacity of injured adult mammalian central nervous system (CNS) tissue is very limited. Disease or injury that causes destruction or damage to neuronal networks typically results in permanent neurological deficits. Injury to the spinal cord, for example, interrupts vital ascending and descending fiber tracts of spinally projecting neurons. Because neuronal structures located proximal or distal to the injury site remain largely intact, a major goal of spinal cord injury research is to develop strategies to reestablish innervation lost as a consequence of injury. The growth inhibitory nature of injured adult CNS tissue is a major barrier to regenerative axonal growth and sprouting. An increasing complexity of molecular players is being recognized. CNS inhibitors fall into three general classes: members of canonical axon guidance molecules (e.g., semaphorins, ephrins, netrins), prototypic myelin inhibitors (Nogo, MAG, and OMgp) and chondroitin sulfate proteoglycans (lecticans, NG2). On the other end of the spectrum are molecules that promote neuronal growth and sprouting. These include growth promoting extracellular matrix molecules, cell adhesion molecules, and neurotrophic factors. In addition to environmental (extrinsic) growth regulatory cues, cell intrinsic regulatory mechanisms exist that greatly influence injury-induced neuronal growth. Various degrees of growth and sprouting of injured CNS neurons have been achieved by lowering extrinsic inhibitory cues, increasing extrinsic growth promoting cues, or by activation of cell intrinsic growth programs. More recently, combination therapies that activate growth promoting programs and at the same time attenuate growth inhibitory pathways have met with some success. In experimental animal models of spinal cord injury (SCI), mono and combination therapies have been shown to promote neuronal growth and sprouting. Anatomical growth often correlates with improved behavioral outcomes. Challenges ahead include testing whether some of the most promising treatment strategies in animal models are also beneficial for human patients suffering from SCI.

Fibroblast growth factor and insulin-like growth factor rescue growth cones of sensory neurites from collapse after tetracaine-induced injury

BACKGROUND

Basic fibroblast growth factor (bFGF) and insulin-like growth factor (IGF)-1 have multiple effects on cells, including proliferation, differentiation, and survival. In this study, we investigated the effects of different concentrations of IGF and bFGF on the morphology of growth cones of the developing sensory neurons after tetracaine-induced injury in vitro.

METHODS

Dorsal root ganglia were isolated from chick embryos on embryonic day 7 or 8 and cultured for 24 hours. Tissues were then exposed to 100 mumol/L tetracaine for 60 minutes. The media were replaced by tetracaine-free media containing different concentrations of IGF, bFGF, or combination of IGF 50 ng/mL and bFGF 5 ng/mL and incubated for a further 24 hours. Growth cone collapse assays were then performed to assess regeneration of neurons.

RESULTS

Exposure of dorsal root ganglia explants to tetracaine 100 mumol/L for 1 hour caused significant growth cone collapse 24 hours after washing out tetracaine (P < 0.01). It was found that adding bFGF (5, 10, 20, and 50 ng/mL) or IGF (50 and 100 ng/mL) to the replacement media significantly decreased growth cone collapse percentage at 24 hours after washout (P < 0.01); however, the low concentrations of bFGF (2 ng/mL) or IGF (25 ng/mL) did not cause significant change. Growth cone collapse after simultaneous addition of 5 ng/mL bFGF and 50 ng/mL IGF was statistically lower than the values after adding 5 ng/mL bFGF (P < 0.01), and it was marginally lower than 50 ng/mL IGF.

CONCLUSION

bFGF and bIGF decreased growth cone collapse after tetracaine-induced injury in vitro.

Decreased motoneuron survival in Igf2 null mice after sciatic nerve transection

The search for therapeutic targets to prevent neurons from dying is ongoing and involves the exploration of a long list of neurotrophic factors. Insulin-like growth factor 2 (IGF2) is a member of the insulin family with known neurotrophic properties. In this study, we used Igf2 knockout (Igf2) neonate mice to determine whether Igf2 deficiency is detrimental to motor neuron survival after axonal injury. Results show that Igf2 neonatal mice are more susceptible to motor neuron damage than Igf2 mice, as they have a significantly lower percentage of motor neuron survival after a sciatic nerve transection. Neuronal survival was significantly improved in Igf2 mice when IGF2 was administered. These results support the role of IGF2 in neonatal motor neuron survival.

Recovery of whisking function promoted by manual stimulation of the vibrissal muscles after facial nerve injury requires insulin-like growth factor 1 (IGF-1)

Recently, we showed that manual stimulation (MS) of denervated vibrissal muscles enhanced functional recovery following facial nerve cut and suture (FFA) by reducing poly-innervation at the neuro-muscular junctions (NMJ). Although the cellular correlates of poly-innervation are established, with terminal Schwann cells (TSC) processes attracting axon sprouts to "bridge" adjacent NMJ, molecular correlates are poorly understood. Since quantitative RT-PCR revealed a rapid increase of IGF-1 mRNA in denervated muscles, we examined the effect of daily MS for 2 months after FFA in IGF-1(+/-) heterozygous mice; controls were wild-type (WT) littermates including intact animals. We quantified vibrissal motor performance and the percentage of NMJ bridged by S100-positive TSC. There were no differences between intact WT and IGF-1(+/-) mice for vibrissal whisking amplitude (48 degrees and 49 degrees ) or the percentage of bridged NMJ (0%). After FFA and handling alone (i.e. no MS) in WT animals, vibrissal whisking amplitude was reduced (60% lower than intact) and the percentage of bridged NMJ increased (42% more than intact). MS improved both the amplitude of vibrissal whisking (not significantly different from intact) and the percentage of bridged NMJ (12% more than intact). After FFA and handling in IGF-1(+/-) mice, the pattern was similar (whisking amplitude 57% lower than intact; proportion of bridged NMJ 42% more than intact). However, MS did not improve outcome (whisking amplitude 47% lower than intact; proportion of bridged NMJ 40% more than intact). We conclude that IGF-I is required to mediate the effects of MS on target muscle reinnervation and recovery of whisking function.

Insulin-like growth factors in the peripheral nervous system

IGF-I and -II are potent neuronal mitogens and survival factors. The actions of IGF-I and -II are mediated via the type I IGF receptor (IGF-IR) and IGF binding proteins regulate the bioavailability of the IGFs. Cell viability correlates with IGF-IR expression and intact IGF-I/IGF-IR signaling pathways, including activation of MAPK/phosphatidylinositol-3 kinase. The expression of IGF-I and -II, IGF-IR, and IGF binding proteins are developmentally regulated in the central and peripheral nervous system. IGF-I therapy demonstrates mixed therapeutic results in the treatment of peripheral nerve injury, neuropathy, and motor neuron diseases such as amyotrophic lateral sclerosis. In this review we discuss the role of IGFs during peripheral nervous system development and the IGF signaling system as the potential therapeutic target for the treatment of nerve injury and motor neuron diseases.

Potential utility of rhIGF-1 in neuromuscular and/or degenerative disease

Neuromuscular/neurodegenerative disorders, such as the death of spinal cord motor neurons in amyotrophic lateral sclerosis (ALS) or the degeneration of spinal cord motor neuron axons in certain peripheral neuropathies, present a unique opportunity for therapeutic intervention with neurotrophic proteins. We have found that in mixed rat embryonic spinal cord cultures or in purified motor neuron preparations, recombinant human insulin-like growth factor 1 (rhIGF-1) enhances the survival of motor neurons at EC50 concentrations of 2 nM, consistent with an interaction at the tyrosine kinase-coupled rhIGF-1 receptor. In a model of programmed cell death in ovo, administration of rhIGF-1 produces a marked survival of motor neurons. In a variety of models of predominantly motor neuron or nerve injury in rodents, administration of rhIGF-1 prevents the death of motor neurons in neonatal facial nerve lesions, attenuates the loss of cholinergic phenotype in adult hypoglossal nerve axotomy and hastens recovery from sciatic nerve crush in mice. In a genetic model of motor neuron compromise, the wobbler mouse, rhIGF-1 (1 mg/kg s.c. daily) delayed the deterioration of grip strength and provided for a more normal distribution of fibre types. In addition, rhIGF-1 (0.3-1.0 mg/kg s.c. daily) prevents the motor and/or sensory neuropathy in rodents caused by vincristine, cisplatinum or Taxol. These combined data indicate that rhIGF-1 has marked effects on the survival of compromised motor neurons and the maintenance of their axons and functional connections. They also suggest the potential utility of rhIGF-1 for the treatment of diseases such as ALS and certain neuropathies.

Ganglioside 9-O-acetyl GD3 expression is upregulated in the regenerating peripheral nerve

Evidence accumulates suggesting that 9-O-acetylated gangliosides, recognized by a specific monoclonal antibody (Jones monoclonal antibody), are involved in neuronal migration and axonal growth. These molecules are expressed in rodent embryos during the period of axon extension of peripheral nerves and are absent in adulthood. We therefore aimed at verifying if these molecules are re-expressed in adult rats during peripheral nerve regeneration. In this work we studied the time course of ganglioside 9-O-acetyl GD3 expression during regeneration of the crushed sciatic nerve and correlated this expression with the time course of axonal regeneration as visualized by immunohistochemistry for neurofilament 200 in the nerve. We have found that the ganglioside 9-O-acetyl GD3 is re-expressed during the period of regeneration and this expression correlates spatio-temporally with the arrival of axons to the lesion site. Confocal analysis of double and triple labeling experiments allowed the localization of this ganglioside to Schwann cells encircling growing axons in the sciatic nerve. Explant cultures of peripheral nerves also revealed ganglioside expressing reactive Schwann cells migrating from the normal and previously crushed nerve. Ganglioside 9-O-acetyl GD3 is also upregulated in DRG neurons and motoneurons of the ventral horn of spinal cord showing that the reexpression of this molecule is not restricted to Schwann cells. These results suggest that ganglioside 9-O-acetyl GD3 may be involved in the regrowth of sciatic nerve axons after crush being upregulated in both neurons and glia.

Regulatory expression of Neurensin-1 in the spinal motor neurons after mouse sciatic nerve injury

Axonal regeneration after crush injury of the sciatic nerve has been intensely studied for the elucidation of molecular and cellular mechanisms. Neurite extension factor1 (Nrsn1) is a unique membranous protein that has a microtubule-binding domain and is specifically expressed in neurons. Our studies have shown that Nrsn1 is localized particularly in actively extending neurites, thus playing a role in membrane transport to the growing distal ends of extending neurites. To elucidate the possible role of Nrsn1 during peripheral axonal regeneration, we examined the expression of Nrsn1 mRNA by in situ hybridization and Nrsn1 localization by immunocytochemistry, using a mouse model. The results revealed that during the early phase of axonal regeneration of motor nerves, Nrsn1 mRNA is upregulated in the injured motor neuron. Nrsn1 is localized in the cell bodies of motor neurons and at the growing distal ends of regenerating axons. These results indicate that Nrsn1 plays an active role in axonal regeneration as well as in embryonic development.

Insulin-like growth factor I plasmid therapy promotes in vivo angiogenesis

Angiogenesis, the formation of neovessels from the endothelium of preexisting vessels, is stimulated by soluble angiogenic factors. Insulin-like growth factor I (IGF-I) stimulates myogenesis and induces nerve regeneration after injury, and it has been shown to stimulate angiogenesis. However, the in vivo angiogenic effects of IGF-I in regenerating and diabetic muscle have yet to be described. Therefore, we studied the effects of human IGF-I (hIGF-I) delivered by a plasmid-mediated therapy on angiogenesis in mouse models of these two conditions. Plasmid hIGF-I was delivered to the injured tibialis muscle by direct intramuscular injection followed by electroporation. Initial experiments compared two muscle-specific hIGF-I-expressing constructs containing either a skeletal actin 3'UTR (pAV2001) or a human growth hormone (GH) 3'UTR (pAV2002). Skeletal actin 3'UTR mediates sequestration of hIGF-I in the muscle and was more active, while the GH 3'UTR mediated release of IGF-I into the circulation. Treatment of regenerating muscle with pAV2001 and sequestration of IGF-I in muscle led to increased expression of vascular endothelial growth factor (VEGF) and VEGF receptors fetal liver kinase-1 and FmS-like tyrosine kinase receptor-1, as well as platelet endothelial cell adhesion molecule-1, on endothelial cells. These results indicate that IGF-I can amplify angiogenic responses in regenerating muscle. In a mouse diabetic model, plasmid-mediated IGF-I therapy reversed diabetic microangiopathy, as shown by increased angiogenesis and arterial flow as analyzed by Doppler imaging. These studies show that plasmid IGF-I delivery and sequestration in muscle can augment angiogenesis in regenerating muscle and increase blood flow and angiogenesis in the diabetic limb.

Oxidized galectin-1 stimulates macrophages to promote axonal regeneration in peripheral nerves after axotomy

Various neurotrophic factors that promote axonal regeneration have been investigated in vivo, but the signals that prompt neurons to send out processes in peripheral nerves after axotomy are not well understood. Previously, we have shown oxidized galectin-1 (GAL-1/Ox) promotes initial axonal growth after axotomy in peripheral nerves. However, the mechanism by which GAL-1/Ox promotes axonal regeneration remains unclear and is the subject of the present study. To identify possible target cells of GAL-1/Ox, a fluorescently labeled recombinant human GAL-1/Ox (rhGAL-1/Ox) was incubated with DRG neurons, Schwann cells, and intraperitoneal macrophages from adult rats. Only the cell surfaces of intraperitoneal macrophages bound the rhGAL-1/Ox, suggesting that these cells possess a receptor for GAL-1/Ox. Experiments examining tyrosine phosphorylation revealed that rhGAL-1/Ox stimulated changes in signal transduction pathways in these macrophages. These changes caused macrophages to secrete an axonal growth-promoting factor. This was demonstrated when conditioned media of macrophages stimulated with rhGAL-1/Ox in 48 hr culture strongly enhanced axonal regeneration from transected-nerve sites of DRG explants. Furthermore, activated macrophage-conditioned media also improved Schwann cell migration from the transected-nerve sites. From these results, we propose that axonal regeneration occurs in axotomized peripheral nerves as a result of cytosolic reduced galectin-1 being released from Schwann cells and injured axons, which then becomes oxidized in the extracellular space. Oxidized galectin-1 then stimulates macrophages to secrete a factor that promotes axonal growth and Schwann cell migration, thus enhancing peripheral nerve regeneration.

Systemic insulin-like growth factor-I administration prevents cognitive impairment in diabetic rats, and brain IGF regulates learning/memory in normal adult rats

Diabetic patients have impaired learning/memory, brain atrophy, and two-fold increased risk of dementia. The cause of cognitive disturbances that progress to dementia is unknown. Because neurotrophic insulin-like growth factor (IGF) levels are reduced in diabetic patients and rodents, and IGF can cross the blood-central nervous system barrier (B-CNS-B), the hypothesis was tested that IGF administered systemically can prevent cognitive disturbances, independently of hyperglycemia and a generalized catabolic state. Latency to escape to a hidden platform in the Morris Water Maze is used widely to test spatial memory, a hippocampus-dependent task. Adult rats were rendered diabetic with streptozotocin and implanted 4 weeks later with subcutaneous pumps that released either vehicle (D + Veh) or 20 microg/day IGF-I (D + IGF). Latency to escape to the hidden platform was prolonged in (D + Veh) versus non-diabetic rats (P < 0.003) 10.5 weeks after the onset of diabetes. Such prolongation was prevented in (D + IGF) versus (D + Veh) rats (P < 0.03). The data show that IGF-I can act across the B-CNS-B to prevent loss of cognition-related performance in the water maze independently of ongoing hyperglycemia and reduction in brain (P < 0.001) and whole body weight (P < 0.001) in diabetic rats. The hypothesis that brain IGF contributes to learning/memory was tested. An anti-IGF antibody, or preimmune serum, was infused into the lateral ventricles in non-diabetic rats. Learning in a passive avoidance task was impaired significantly in the IGF antibody versus preimmune serum-treated groups on test Days 1, 2, and 3 (P = 0.04, 0.02 and 0.004, respectively). The data together are consistent with a model in which brain IGF is essential for learning/memory, and a loss of IGF activity due to diabetes may contribute to cognitive disturbances.

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

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

Mice lacking tissue plasminogen activator and urokinase plasminogen activator genes show attenuated matrix metalloproteases activity after sciatic nerve crush

Plasminogen activators (PAs), tissue PA (tPA) and urokinase PA (uPA), have been shown to be induced in sensory neurons after sciatic nerve crush. These findings suggested that PAs facilitate peripheral nerve regeneration by digesting adhesive cell contacts and by activation of other proteases, thereby initiating a proteolytic cascade. Both tPA and uPA activate some matrix metalloproteases (MMPs), indirectly via plasminogen activation or directly, such as the uPA activation of MMP-2. In this study, we demonstrated, by using tPA and uPA knockout mice, that a lack of a plasminogen activator affected MMP-9 and MMP-2 activity after crushing of the sciatic nerve. These findings show that the PAs are important for MMP-9 and MMP-2 activity at the crush site.

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.

Muscle reinnervation and IGF-I synthesis are affected by exposure to heparin: an effect partially antagonized by anti-growth hormone-releasing hormone

Sciatic nerve crush was performed in 2-day-old rats, then reinnervation of the extensor digitorum longus muscle, motor neuron survival, and muscle IGF-I production were monitored. In saline-treated rats, the extent of reinnervation was around 50% and the number of EDL reinnervating motor neurons was significantly reduced. In heparin-treated rats the extent of muscle reinnervation, the recovery of nerve-evoked muscle twitch tension, and the number of motor neurons reinnervating the extensor digitorum longus muscle were greatly enhanced compared to saline-treated rats. In addition, treatment with heparin increased markedly insulin-like growth factor-I levels in denervated muscles. The concomitant exposure to anti-growth hormone releasing hormone partially abolished the stimulatory action of heparin on muscle reinnervation and prevented the increase of insulin-like growth factor-I muscle levels.

Influence of insulin-like growth factor-I (IGF-I) on nerve autografts and tissue-engineered nerve grafts

To overcome the problems of limited donor nerves for nerve reconstruction, we established nerve grafts made from cultured Schwann cells and basal lamina from acellular muscle and used them to bridge a 2-cm defect of the rat sciatic nerve. Due to their basal lamina and to viable Schwann cells, these grafts allow regeneration that is comparable to autologous nerve grafts. In order to enhance regeneration, insulin-like growth factor (IGF-I) was locally applied via osmotic pumps. Autologous nerve grafts with and without IGF-I served as controls. Muscle weight ratio was significantly increased in the autograft group treated with IGF-I compared to the group with no treatment; no effect was evident in the tissue-engineered grafts. Autografts with IGF-I application revealed a significantly increased axon count and an improved g-ratio as indicator for "maturity" of axons compared to autografts without IGF-I. IGF-I application to the engineered grafts resulted in a decreased axon count compared to grafts without IGF-I. The g-ratio, however, revealed no significant difference between the groups. Local administration of IGF-I improves axonal regeneration in regular nerve grafts, but not in tissue-engineered grafts. Seemingly, in these grafts the interactive feedback mechanisms of neuron, glial cell, and extracellular matrix are not established, and IGF-I cannot exert its action as a pleiotrophic signal.

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.

Glycosaminoglycan-promoted muscle reinnervation and insulin-like growth factor-I levels are affected by anti-growth hormone-releasing hormone exposure

The present study shows that exposure to antibodies to growth hormone-releasing hormone (GHRH) partially counteracted the promoting effects of treatment with glycosaminoglycans (GAGs) on muscle reinnervation. Sciatic nerve crush was performed in 2-day-old rats, and reinnervation of the extensor digitorum longus muscle was monitored. The extent of reinnervation was rather poor in saline-treated rats, whereas in GAG-treated rats the extent of muscle reinnervation, the recovery of nerve-evoked muscle twitch tension, and the number of motor neurons reinnervating the extensor digitorum longus muscle were greatly enhanced. In addition, treatment with glycosaminoglycans increased markedly insulin-like growth factor-I (IGF-I) levels in denervated muscles. Both types of stimulatory action exerted by GAGs were affected by concomitant exposure to anti-GHRH, with abolition of IGF-I muscle increase and a smaller enhancement in muscle reinnervation.

Induction of the plasminogen activator system accompanies peripheral nerve regeneration after sciatic nerve crush

Peripheral nerve regeneration is dependent on the ability of regenerating neurites to migrate through cellular debris and altered extracellular matrix at the injury site, grow along the residual distal nerve sheath conduit, and reinnervate synaptic targets. In cell culture, growth cones of regenerating axons secrete proteases, specifically plasminogen activators (PAs), which are believed to facilitate growth cone movement by digesting extracellular matrices and cell adhesions. In this study, the PA system was shown to be specifically activated in sensory neurons after sciatic nerve crush in adult mice. The number of sensory neurons expressing urokinase PA receptor (uPAR) mRNA levels increased above sham levels by 8 hr after crush, whereas the number of sensory neurons expressing uPA and tissue PA (tPA) mRNAs was significantly increased by 3 d after crush. PA mRNA levels were also increased at the crush site, with uPA mRNA elevated by 8 hr after crush and tPA and uPAR mRNA levels markedly increased by 7 d. PA-dependent enzymatic activity was significantly increased from 1 to 7 d after crush in nerves that had been crushed compared with uncrushed nerves. Immunohistochemistry showed that tPA was localized within regenerating axons of the sciatic nerve. There were no significant changes in plasminogen activator inhibitor 1 activity between crush and sham after the injury. These results clearly demonstrated that after injury the PA system was rapidly induced in sensory neurons, where it may play an important role in nerve regeneration in vivo.

Mice lacking tPA, uPA, or plasminogen genes showed delayed functional recovery after sciatic nerve crush

Axonal outgrowth during peripheral nerve regeneration relies on the ability of growth cones to traverse through an environment that has been altered structurally and along a basal lamina sheath to reinnervate synaptic targets. To promote migration, growth cones secrete proteases that are thought to dissolve cell-cell and cell-matrix adhesions. These proteases include the plasminogen activators (PAs), tissue PA (tPA) and urokinase PA (uPA), and their substrate, plasminogen. PA expression and secretion are upregulated in regenerating mammalian sensory neurons in culture. After sciatic nerve crush in mice, there was an induction of PA mRNAs in the sensory neurons contributing to the crushed nerve and an upregulation of PA-dependent activity in crushed nerve compared with sham counterparts during nerve regeneration. To further assess the role of the PA system during peripheral nerve regeneration, PA-dependent activity as well as recovery of sensory and motor function in the injured hindlimb were assessed in wild-type, tPA, uPA, and plasminogen knock-out mice. Protease activity visualized by gel zymography showed that after nerve crush, the upregulation of PA activity in the tPA and uPA knock-out mice was delayed compared with wild-type mice. Recovery of sensory function was assessed by toe pinch, footpad prick, and the toe-spreading reflex. All knock-out mice demonstrated a significant delay in hindlimb response to these sensory stimuli compared with wild-type mice. For each modality tested, the uPA knock-out mice were the most dramatically affected, showing the longest delay to initiate a response. These studies clearly showed that PAs were necessary for timely functional recovery by regenerating peripheral nerves.

Identification of oxidized galectin-1 as an initial repair regulatory factor after axotomy in peripheral nerves

Various neurotrophic factors that promote axonal regeneration have been investigated in vivo, but the signals that prompt the axons to send out processes in peripheral nerves after axotomy are not well understood. We have shown using two specific strategies that galectin-1 can play an important role in this initial stage. One used an in vitro nerve regeneration model that allowed us to monitor the initial axon and support cell outgrowth from the proximal nerve stump comparable to the initial stages of nerve repair. The other strategy was to clarify the axonal regeneration-promoting factor from kidney-derived cells. Using these strategies, we discovered that oxidized galectin-1 from the cell (COS1 cell) conditioned media acts as an axonal regeneration-promoting factor without the lectin activity. Oxidized recombinant human galectin-1 (rhGAL-1/Ox) showed the same activity at low concentrations (pg/ml range). A similarly low concentration also effectively promoted axonal regeneration in both transection and crush experiments in vivo. Moreover, the application of functional anti-galectin-1 antibody strongly inhibited the regeneration in vivo. Since galectin-1was shown to be secreted and localized in the regenerating sciatic nerve, this suggests that secreted galectin-1 may be oxidized and change its molecular structure to regulate initial repair after axotomy as a kind of cytokine.

IGF-I expression is decreased in LIF-deficient mice after peripheral nerve injury

We investigated the regulation of insulin-like growth factor 1 (IGF-1) expression after sciatic nerve crush using leukemia inhibitory factor (LIF)-deficient mice. One day post-crush, IGF-1 mRNA levels were lower in the LIF-deficient mouse nerve than in the wild type nerve. IGF-1 protein, analyzed by immunohistochemistry, was also decreased 1 day post-crush in LIF-deficient nerves relative to wild type nerves. By 3 days post-crush, IGF-1 immunoreactivity was induced in Schwann cells to equivalent levels in both types of nerve. After crush, IGF-1 expression was also found in mast cells, and these were initially decreased in the LIF-deficient mice. Thus, LIF appears to regulate IGF-1 expression in the peripheral nerve basally and early in the regeneration response in vivo.

Uptake of circulating insulin-like growth factor-I into the cerebrospinal fluid of normal and diabetic rats and normalization of IGF-II mRNA content in diabetic rat brain

Brain injury has been prevented recently by systemic administration of human insulin-like growth factor-I (hIGF-I). It is widely believed that protein neurotrophic factors do not enter the brain from blood, and the mechanism by which circulating hIGF-I may be neuroprotective is uncertain. This investigation tested the hypothesis that hIGF-I is taken up into cerebrospinal fluid (CSF) from the circulation. (125)I-hIGF-I was injected subcutaneously into rats. The (125)I-IGF-I recovered from CSF and plasma were indistinguishable in size from authentic (125)I-hIGF-I on SDS-PAGE. An ELISA was used that detected immunoreactive hIGF-I, but not rat IGF-I, rat IGF-II, human IGF-II, or insulin. Osmotic minipumps were implanted for constant subcutaneous infusion of various hIGF-I doses. Uptake into CSF reached a plateau at plasma concentrations above approximately 150 ng/ml hIGF-I; the plateau was consistent with carrier-mediated uptake. The plasma, but not CSF, hIGF-I level was significantly reduced in streptozotocin diabetic vs. nondiabetic rats, and uptake of hIGF-I into CSF was nonlinear with respect to plasma hIGF-I concentrations. Nonlinear uptake excluded leakage or transmembrane diffusion of IGF-I from blood into CSF as a dominant route for entry, but the site and mechanism of uptake remain to be established. The IGF-II mRNA content per milligram brain (P < 0.02) as well as per poly(A)(+) RNA (P < 0.05) was significantly increased towards normal in diabetic rats treated by subcutaneous administration of hIGF-I vs. vehicle. This effect of circulating hIGF-I may have been due to regulation of IGF-II gene expression in the choroid plexus and leptomeninges, structures at least in part outside of the blood-central nervous system barrier. These data support the hypothesis that circulating IGF-I supports the brain indirectly through regulation of IGF-II gene expression as well as by uptake into the CSF.

Galectin-1 regulates initial axonal growth in peripheral nerves after axotomy

The signals that prompt the axons to send out processes in peripheral nerves after axotomy are not well understood. Here, we report that galectin-1 can play an important role in this initial stage. We developed an in vitro nerve regeneration model that allows us to monitor the initial axon and support cell outgrowth from the proximal nerve stump, which is comparable to the initial stages of nerve repair. We isolated a factor secreted from COS1 cells that enhanced axonal regeneration, and we identified the factor as galectin-1. Recombinant human galectin-1 (rhGAL-1) showed the same activity at low concentrations (50 pg/ml) that are two orders of magnitude lower than those of lectin activity. A similarly low concentration was also effective in in vivo experiments of axonal regeneration with migrating reactive Schwann cells to a grafted silicone tube after transection of adult rat peripheral nerve. Moreover, the application of functional anti-rhGAL-1 antibody strongly inhibited the regeneration in vivo as well as in vitro. The same effect of rhGAL-1 was confirmed in crush/freeze experiments of the adult mouse sciatic nerve. Because galectin-1 is expressed in the regenerating sciatic nerves as well as in both sensory neurons and motor neurons, we suggest that galectin-1 may regulate initial repair after axotomy. This high activity of the factor applied under nonreducing conditions suggests that galectin-1 may work as a cytokine, not as a lectin.

Developing Schwann cells acquire the ability to survive without axons by establishing an autocrine circuit involving insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB

Although Schwann cell precursors from early embryonic nerves die in the absence of axonal signals, Schwann cells in older nerves can survive in the absence of axons in the distal stump of transected nerves. This is crucially important, because successful axonal regrowth in a damaged nerve depends on interactions with living Schwann cells in the denervated distal stump. Here we show that Schwann cells acquire the ability to survive without axons by establishing an autocrine survival loop. This mechanism is absent in precursors. We show that insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB are important components of this autocrine survival signal. The secretion of these factors by Schwann cells has significant implications for cellular communication in developing nerves, in view of their known ability to regulate survival and differentiation of other cells including neurons.

A role for insulin-like growth factor-I in the regulation of Schwann cell survival

During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one.

Abnormalities of protein kinases in neurodegenerative diseases

In neurodegenerative diseases such as ALS and AD there is evidence for abnormal regulation of protein kinases. In these diseases, altered activities and protein levels of several specific kinases suggest that abnormal phosphorylation is present and this aberrant phosphorylation may be involved in the pathogenesis of these diseases. The observation that regulation of the NMDA receptor ion channel is altered in tissue from ALS patients may arise from the abnormal phosphorylation state of the protein kinase regulating NMDA receptor function. Whether the abnormalities of these protein kinases is a primary event leading to altered receptor regulation or vice versa is still poorly understood. The seemingly multiple pathogenic mechanisms of ALS and AD create complexity in assessing a primary cause that may lead to cell death. The mechanisms causing cell death (apoptosis or necrosis) may be overlapping with integrated events among the components interacting and contributing to a final pathway for neuron death. Thus, evidence of impairment in protein kinase signalling in these diseases may be a primary cause, a secondary event, or a compensatory mechanism. To further study this issue, different model systems could be beneficial to obtain a better understanding of these diseases.

Time-dependent alteration of insulin-like growth factor gene expression during nerve regeneration in regions of muscle enriched with neuromuscular junctions

Insulin-like growth factors (IGFs) increase the rate of motor axon elongation, prevent motoneuron death, and may support the reestablishment of synapses following nerve injury. In situ hybridization was used in the present study to examine the temporal and spatial distribution of IGF gene expression in soleus muscle following sciatic nerve crush in rats. In intact muscle, IGF-II gene expression was generally low, and localized to interstitial cells, possibly fibroblast and Schwann cells. These cells were found in the middle of muscle which is enriched in neuromuscular junctions. IGF-II gene expression, 4-6 days postcrush, was increased in interstitial cells. Thereafter, IGF-II gene expression was also increased in muscle cells or cells closely associated with muscle fibers, such as satellite cells. IGF-II gene expression was increased to a much greater extent in the midregion of muscle enriched in end-plates than in the two ends of muscle, but returned towards normal following the reestablishment of functional synapses. On the other hand, IGF-I gene expression was only slightly increased following nerve crush, and this increase was associated with interstitial, but not muscle cells. These results show that the IGF-I and IGF-II genes are regulated by independent signals and may play separate roles during nerve regeneration. For example, a regional increase in IGF-II gene expression may support preferential nerve terminal sprouting in the middle of muscle enriched in neuromuscular junctions, thereby increasing the probability for the reestablishment of synapses.

Acute treatment with corticosteroids decreases IGF-1 and IGF-2 expression in the rat diaphragm and gastrocnemius

Massive doses of methylprednisolone (M) or triamcinolone (T) induced diaphragmatic type IIx/b atrophy, resulting in a leftward shift of the force-frequency curve in rats (). To examine the role of insulin-like growth factors (IGFs) in these changes, IGF mRNA content was measured in costal diaphragm, gastrocnemius, and liver removed from 32 rats treated daily during 5 d either with saline (control, C and pair-fed, PF), M, or T (80 mg/kg). Blood samples were taken to measure IGF-1 serum levels. RNA levels were measured by Northern and dot-blots after hybridization with rat IGF-1 or IGF-2 cDNA probes labeled with alpha-32P. Compared with C (845 +/- 128 ng/ml), IGF-1 serum levels were significantly decreased in M (699 +/- 90 ng/ml, p < 0.001 versus C) and PF animals (505 +/- 33 ng/ml, p < 0.001 versus others) and even more so, in T-treated animals (273 +/- 134 ng/ml, p < 0.001 versus others). Along the same lines, IGF-1 expression in the liver was depressed after corticosteroid treatment and in PF, whereas IGF-2 mRNA content remained unchanged. Compared with C, the relative expression of IGF-1 mRNA in the diaphragm was depressed by 44% and 69% in the M and T groups, respectively (p < 0.0001 versus C), while it was unchanged in PF animals. In the gastrocnemius, IGF-1 expression was reduced after M and T (-51% and -59%, respectively, p < 0.0001 versus C) as well as in PF animals (-40%, p < 0.001 versus C). For IGF-2, a similar pattern of expression was found in the diaphragm and the gastrocnemius. Indeed, IGF-2 mRNA tended to decrease in corticosteroid-treated rats (NS) whereas it was unchanged in PF rats. We conclude that decreased IGF expression after corticosteroid treatment was similar in diaphragm and gastrocnemius and may be responsible for the diaphragmatic changes observed after steroid treatment.

Insulin-like growth factor-II increases and IGF is required for postnatal rat spinal motoneuron survival following sciatic nerve axotomy

The prolonged disconnection of nerve from muscle results in the death of motoneurons and permanent paralysis. Because clinical nerve injuries generally involve postbirth motoneurons, there is interest in uncovering factors that may support their survival. A rich history of research dating back to the time of Santiago Ramon y Cajal and Viktor Hamburger supports the inference that there are soluble neurotrophic factors associated with nerve and muscle. However, the endogenous factors normally required for motoneuron survival following nerve injury have eluded identification. Two interrelated hypotheses were tested: (1) administration of insulin-like growth factor-II (IGF-II) can support the survival of postbirth motoneurons, and (2) endogenous IGFs are essential for motoneuron survival following nerve injury. We report that IGF-II locally administered close to the proximal nerve stump prevented the death of motoneurons (estimated by relative numbers of neuronal profiles) which ordinarily follows sciatic nerve transection in neonatal rats. By contrast, anti-IGF antiserum, as well as IGF binding proteins-4 and -6, significantly increased (P < 0.01) motoneuron death. This report shows that IGF-II can support survival, and contains the novel observation that endogenous IGF activity in or near nerves is required for motoneuron survival. Other studies have determined that IGF gene and protein expression are increased in nerve and muscle following sciatic nerve crush, and that IGFs are required for nerve regeneration. Taken together, these data show that IGFs are nerve- and muscle-derived soluble factors that support motoneuron survival as well as nerve regeneration.