Regulation of peripheral nerve regeneration by insulin-like growth factors

Sustained IGF-1 delivery ameliorates effects of chronic denervation and improves functional recovery after peripheral nerve injury and repair

Functional recovery following peripheral nerve injury is limited by progressive atrophy of denervated muscle and Schwann cells (SCs) that occurs during the long regenerative period prior to end-organ reinnervation. Insulin-like growth factor 1 (IGF-1) is a potent mitogen with well-described trophic and anti-apoptotic effects on neurons, myocytes, and SCs. Achieving sustained, targeted delivery of small protein therapeutics remains a challenge. We hypothesized that a novel nanoparticle (NP) delivery system can provide controlled release of bioactive IGF-1 targeted to denervated muscle and nerve tissue to achieve improved motor recovery through amelioration of denervation-induced muscle atrophy and SC senescence and enhanced axonal regeneration. Biodegradable NPs with encapsulated IGF-1/dextran sulfate polyelectrolyte complexes were formulated using a flash nanoprecipitation method to preserve IGF-1 bioactivity and maximize encapsulation efficiencies. Under optimized conditions, uniform PEG-b-PCL NPs were generated with an encapsulation efficiency of 88.4%, loading level of 14.2%, and a near-zero-order release of bioactive IGF-1 for more than 20 days in vitro. The effects of locally delivered IGF-1 NPs on denervated muscle and SCs were assessed in a rat median nerve transection-without- repair model. The effects of IGF-1 NPs on axonal regeneration, muscle atrophy, reinnervation, and recovery of motor function were assessed in a model in which chronic denervation is induced prior to nerve repair. IGF-1 NP treatment resulted in significantly greater recovery of forepaw grip strength, decreased denervation-induced muscle atrophy, decreased SC senescence, and improved neuromuscular reinnervation.

Insulin-Like Growth Factor-1: A Promising Therapeutic Target for Peripheral Nerve Injury

Patients who sustain peripheral nerve injuries (PNIs) are often left with debilitating sensory and motor loss. Presently, there is a lack of clinically available therapeutics that can be given as an adjunct to surgical repair to enhance the regenerative process. Insulin-like growth factor-1 (IGF-1) represents a promising therapeutic target to meet this need, given its well-described trophic and anti-apoptotic effects on neurons, Schwann cells (SCs), and myocytes. Here, we review the literature regarding the therapeutic potential of IGF-1 in PNI. We appraised the literature for the various approaches of IGF-1 administration with the aim of identifying which are the most promising in offering a pathway toward clinical application. We also sought to determine the optimal reported dosage ranges for the various delivery approaches that have been investigated.

Improvement of Eye Alignment in Adult Strabismic Monkeys by Sustained IGF-1 Treatment

Purpose

The goal of this study was to determine if continuous application of insulin-like growth factor-1 (IGF-1) could improve eye alignment of adult strabismic nonhuman primates and to assess possible mechanisms of effect.

Methods

A continuous release pellet of IGF-1 was placed on one medial rectus muscle in two adult nonhuman primates (M1, M2) rendered exotropic by the alternating monocular occlusion method during the first months of life. Eye alignment and eye movements were recorded for 3 months, after which M1 was euthanized, and the lateral and medial rectus muscles were removed for morphometric analysis of fiber size, nerve, and neuromuscular density.

Results

Monkey 1 showed a 40% reduction in strabismus angle, a reduction of exotropia of approximately 11° to 14° after 3 months. Monkey 2 showed a 15% improvement, with a reduction of its exotropia by approximately 3°. The treated medial rectus muscle of M1 showed increased mean myofiber cross-sectional areas. Increases in myofiber size also were seen in the contralateral medial rectus and lateral rectus muscles. Similarly, nerve density increased in the contralateral medial rectus and yoked lateral rectus.

Conclusions

This study demonstrates that in adult nonhuman primates with a sensory-induced exotropia in infancy, continuous IGF-1 treatment improves eye alignment, resulting in muscle fiber enlargement and altered innervational density that includes the untreated muscles. This supports the view that there is sufficient plasticity in the adult ocular motor system to allow continuous IGF-1 treatment over months to produce improvement in eye alignment in early-onset strabismus.

Alpinia oxyphylla Miq. fruit extract activates IGFR-PI3K/Akt signaling to induce Schwann cell proliferation and sciatic nerve regeneration

BACKGROUND

It is known that the medicinal herb Alpinia oxyphylla Miq. is widely used as a remedy for diarrhea as well as the symptoms accompanying hypertension and cerebrovascular disorders. Moreover, it has also been reported that Alpinia oxyphylla Miq. has beneficial effects on anti-senescence and neuro-protection. This study focuses on the molecular mechanisms by which the Alpinia oxyphylla Miq. fruits promote neuron regeneration.

METHODS

A piece of silicone rubber was guided across a 15 mm gap in the sciatic nerve of a rat. This nerve gap was then filled with various doses of Alpinia oxyphylla Miq. fruits to assess their regenerative effect on damaged nerves. Further, we investigated the role of Alpinia oxyphylla Miq. fruits in RSC96 Schwann cell proliferation.

RESULTS

Our current results showed that treatment with the extract of Alpinia oxyphylla Miq. fruits triggers the phosphorylated insulin-like growth factor-1 receptor- phosphatidylinositol 3-kinase/serine-threonine kinase pathway, and up-regulated the proliferating cell nuclear antigen in a dose-dependent manner. Cell cycle analysis on RSC96 Schwann cells showed that, after exposure to Alpinia oxyphylla Miq. fruit extract, the transition from the first gap phase to the synthesis phase occurs in 12-18 h. The expression of the cell cycle regulatory proteins cyclin D1, cyclin E and cyclin A increased in a dose-dependent manner. Transfection with a small interfering RNA blocked the expression of phosphatidylinositol 3-kinase and induced down-regulation both on the mRNA and protein levels, which resulted in a reduction of the expression of the survival factor B-cell lymphoma 2.

CONCLUSION

We provide positive results that demonstrate that Alpinia oxyphylla Miq. fruits facilitate the survival and proliferation of RSC96 cells via insulin-like growth factor-1 signaling.

IGF-1 and Chondroitinase ABC Augment Nerve Regeneration after Vascularized Composite Limb Allotransplantation

Impaired nerve regeneration and inadequate recovery of motor and sensory function following peripheral nerve repair remain the most significant hurdles to optimal functional and quality of life outcomes in vascularized tissue allotransplantation (VCA). Neurotherapeutics such as Insulin-like Growth Factor-1 (IGF-1) and chondroitinase ABC (CH) have shown promise in augmenting or accelerating nerve regeneration in experimental models and may have potential in VCA. The aim of this study was to evaluate the efficacy of low dose IGF-1, CH or their combination (IGF-1+CH) on nerve regeneration following VCA. We used an allogeneic rat hind limb VCA model maintained on low-dose FK506 (tacrolimus) therapy to prevent rejection. Experimental animals received neurotherapeutics administered intra-operatively as multiple intraneural injections. The IGF-1 and IGF-1+CH groups received daily IGF-1 (intramuscular and intraneural injections). Histomorphometry and immunohistochemistry were used to evaluate outcomes at five weeks. Overall, compared to controls, all experimental groups showed improvements in nerve and muscle (gastrocnemius) histomorphometry. The IGF-1 group demonstrated superior distal regeneration as confirmed by Schwann cell (SC) immunohistochemistry as well as some degree of extrafascicular regeneration. IGF-1 and CH effectively promote nerve regeneration after VCA as confirmed by histomorphometric and immunohistochemical outcomes.

Therapeutic augmentation of the growth hormone axis to improve outcomes following peripheral nerve injury

INTRODUCTION

Peripheral nerve injuries often result in debilitating motor and sensory deficits. There are currently no therapeutic agents that are clinically available to enhance the regenerative process. Following surgical repair, axons often must regenerate long distances to reach and reinnervate distal targets. Progressive atrophy of denervated muscle and Schwann cells (SCs) prior to reinnervation contributes to poor outcomes. Growth hormone (GH)-based therapies have the potential to accelerate axonal regeneration while at the same time limiting atrophy of muscle and the distal regenerative pathway prior to reinnervation.

AREAS COVERED

In this review, we discuss the potential mechanisms by which GH-based therapies act on the multiple tissue types involved in peripheral nerve regeneration to ultimately enhance outcomes, and review the pertinent mechanistic and translational studies that have been performed. We also address potential secondary benefits of GH-based therapies pertaining to improved bone, tendon and wound healing in the setting of peripheral nerve injury.

EXPERT OPINION

GH-based therapies carry great promise for the treatment of peripheral nerve injuries, given the multi-modal mechanism of action not seen with other experimental therapies. A number of FDA-approved drugs that augment the GH axis are currently available, which may facilitate clinical translation.

RSC96 Schwann Cell Proliferation and Survival Induced by Dilong through PI3K/Akt Signaling Mediated by IGF-I

Schwann cell proliferation is critical for the regeneration of injured nerves. Dilongs are widely used in Chinese herbal medicine to remove stasis and stimulate wound-healing functions. Exactly how this Chinese herbal medicine promotes tissue survival remains unclear. The aim of the present study was to investigate the molecular mechanisms by which Dilong promote neuron regeneration. Our results show that treatment with extract of Dilong induces the phosphorylation of the insulin-like growth factor-I (IGF-I)-mediated phosphatidylinositol 3-kinase/serine-threonine kinase (PI3K/Akt) pathway, and activates protein expression of cell nuclear antigen (PCNA) in a time-dependent manner. Cell cycle analysis showed that G₁ transits into the S phase in 12–16 h, and S transits into the G₂ phase 20 h after exposure to earthworm extract. Strong expression of cyclin D1, cyclin E and cyclin A occurs in a time-dependent manner. Small interfering RNA (siRNA)-mediated knockdown of PI3K significantly reduced PI3K protein expression levels, resulting in Bcl₂ survival factor reduction and a marked blockage of G₁ to S transition in proliferating cells. These results demonstrate that Dilong promotes the proliferation and survival of RSC96 cells via IGF-I signaling. The mechanism is mainly dependent on the PI3K protein.

IGF-1 stimulates de novo fatty acid biosynthesis by Schwann cells during myelination

Schwann cell (SC) differentiation to the myelinating phenotype is characterized by the elaboration of a lipid-rich membrane and the expression of myelin-specific proteins. Insulin-like growth factor-1 (IGF-1) has been identified as a growth factor that stimulates the early events of myelination in SCs that signals via the PI3K/Akt pathway. Given the role of IGF-1 in promoting myelination, we performed studies to determine if the fatty acid biosynthetic pathway was a target of IGF-1 signaling in the formation of myelin membrane in dorsal root ganglion neuron/Schwann cell (DRG/SC) cocultures. We report that the fatty acid profile of lipid extracts of cocultures treated with IGF-1 match that reported for native myelin membrane by electrospray mass spectroscopy analysis. We also demonstrate de novo fatty acid biosynthesis in response to IGF-1 treatment in DRG/SC cocultures metabolically labeled with (13)C-acetate as a carbon source for fatty acid synthesis. Consistent with this finding, Western blot analysis of lysates from both cocultures and purified SCs reveal that IGF-1 stimulates two key fatty acid synthesizing enzymes. Additionally, we show that stimulation of fatty acid synthesizing enzymes is mediated by the PI3K/Akt signaling pathway. We also show that the fatty acid synthesizing enzymes and associated signaling pathways are elevated during the period of myelin membrane formation in sciatic nerve. Collectively, these findings demonstrate that IGF-1 plays an important regulatory function during myelin membrane formation.

Neural regeneration protein is a novel chemoattractive and neuronal survival-promoting factor

Neurogenesis and neuronal migration are the prerequisites for the development of the central nervous system. We have identified a novel rodent gene encoding for a neural regeneration protein (NRP) with an activity spectrum similar to the chemokine stromal-derived factor (SDF)-1, but with much greater potency. The Nrp gene is encoded as a forward frameshift to the hypothetical alkylated DNA repair protein AlkB. The predicted protein sequence of NRP contains domains with homology to survival-promoting peptide (SPP) and the trefoil protein TFF-1. The Nrp gene is first expressed in neural stem cells and expression continues in glial lineages. Recombinant NRP and NRP-derived peptides possess biological activities including induction of neural migration and proliferation, promotion of neuronal survival, enhancement of neurite outgrowth and promotion of neuronal differentiation from neural stem cells. NRP exerts its effect on neuronal survival by phosphorylation of the ERK1/2 and Akt kinases, whereas NRP stimulation of neural migration depends solely on p44/42 MAP kinase activity. Taken together, the expression profile of Nrp, the existence in its predicted protein structure of domains with similarities to known neuroprotective and migration-inducing factors and the high potency of NRP-derived synthetic peptides acting in femtomolar concentrations suggest it to be a novel gene of relevance in cellular and developmental neurobiology.

Neurotrophic factors in peripheral neuropathies: therapeutic implications

Neurotrophic factors are proteins which promote the survival of specific neuronal populations. Many have other physiological effects on neurons such as inducing morphological differentiation, enhancing nerve regeneration, stimulating neurotransmitter expression, and otherwise altering the physiological characteristics of neurons. These properties suggest that neurotrophic factors are highly promising as potential therapeutic agents for neurological disease. Neurotrophic factors will most likely be applied to the peripheral nervous system initially, since there are fewer problems for large proteins to gain access to peripheral neurons. Many of the most intensively studied factors are active in the peripheral nervous system. These include the neurotrophins (nerve growth factor, brain derived neurotrophic factor, neurotrophin-3, neurotrophin-4/5), the insulin like growth factors, ciliary neurotrophic factor, and glial cell derived neurotrophic factor and its related proteins. The biology of these factors and their receptors in the peripheral nervous system is reviewed here. We also review data suggesting that abnormal availability of some factors may contribute towards the pathogenesis of certain types of peripheral neuropathy. Finally, the pre-clinical data suggesting that individual factors might be effective in treating neuropathy is reviewed, along with data relating to possible side effects of neurotrophic factor therapy. Several factors have already entered clinical trials with variable success. The data from these trials is reviewed as well.

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.

Regulation of membrane expansion at the nerve growth cone

Exocytotic incorporation of plasmalemmal precursor vesicles (PPVs) into the cell surface is necessary for neurite extension and is known to occur mainly at the growth cone. This report examines whether this is a regulated event controlled by growth factors. The Golgi complex and nascent PPVs of hippocampal neurons in culture were pulse-labeled with fluorescent ceramide. We studied the dynamics of labeled PPVs upon arrival at the axonal growth cone. In controls and cultures stimulated with brain-derived neurotrophic factor (BDNF), PPV clusters persisted in growth cones with a half-life (t(1/2)) of >14 minutes. Upon challenge with IGF-1, however, fluorescent elements cleared from the growth cones with a t(1/2) of only 6 minutes. Plasmalemmal expansion was measured directly as externalization of membrane glycoconjugates in resealed growth cone particles (GCPs) isolated from fetal forebrain. These assays demonstrated that membrane expansion could be stimulated by IGF-1 in a dose-dependent manner but not by BDNF, even though intact, functional BDNF receptor was present on GCPs. Because both BDNF and IGF-1 are known to enhance neurite growth, but BDNF did not stimulate membrane expansion at the growth cone, we studied the effect of BDNF on the IGF-1 receptor. BDNF was found to cause the translocation of the growth-cone-specific IGF-1 receptor subunit beta(gc) to the distal axon, in a KIF2-dependent manner. We conclude that IGF-1 stimulates axonal assembly at the growth cone, and that this occurs via regulated exocytosis of PPVs. This mechanism is affected by BDNF only indirectly, by regulation of the beta(gc) level at the growth cone.

Neuropathology and pathogenesis of diabetic autonomic neuropathy

Autonomic neuropathy is a significant complication of diabetes resulting in increased patient morbidity and mortality. A number of studies, which have shown correspondence between neuropathologic findings in experimental animals and human subjects, have demonstrated that axonal and dendritic pathology in sympathetic ganglia in the absence of significant neuron loss represents a neuropathologic hallmark of diabetic autonomic neuropathy. A recurring theme in sympathetic ganglia, as well as in the pot-ganglionic autonomic innervation of various end organs, is the involvement of distal portions of axons and nerve terminals by degenerative or dystrophic changes. In both animals and humans, there is a surprising selectivity of the diabetic process for subpopulations of autonomic ganglia, nerve terminals within sympathetic ganglia and end organs, from end organ to end organ, and between vascular and other targets within individual end organs. Although the involvement or autonomic axons in somatic nerves may reflect an ischemic pathogenesis, the selectivity of the diabetic process confounds simple global explanations of diabetic autonomic neuropathy as the result of diminished blood flow with resultant tissue hypoxia. A single unifying pathogenetic hypothesis has not yet emerged from clinical and experimental animal studies, and it is likely that diabetic autonomic neuropathy will be shown to have multiple causative mechanisms, which will interact to result in the variety of presentations of autonomic injury in diabetes. Some of these mechanisms will be shared with aging changes in the autonomic nervous system. The role of various neurotrophic substances and the polyol pathway in the pathogenesis and treatment of diabetic neuropathy likely represents a two-edged sword with both salutary and exacerbating effects. The basic neurobiologic process underlying the diabetes-induced development of neuroaxonal dystrophy, synaptic dysplasia, defective axonal regeneration, and alterations in neurotrophic substance may be mechanistically related.

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

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

Effects of Systemically Applied IGF-1 on Motor Nerve Recovery After Peripheral Nerve Transection and Repair in the Rat - A Functional Study

The trophic effects of systemically applied Rh insulin-like growth factor-1 (rhIGF-1) on peripheral motor nerve regeneration following transection and epineural repair in rats median nerve have been examined. RhIGF-1 (0.5 mg/kg/rat) was administered subcutaneously to the neck region of the repaired side for 14 days post-operation. Motor recovery was tested with the grasping test that is an objective quantitative behavioural assessment of regeneration of the rats median nerve. Muscle twitch tension and muscle weight were measured in the flexor digitorum sublimus muscle. No significant differences between experimental and control animals regarding onset of muscle function, recovery of muscle power, and muscle weight were found. These results demonstrate that subcutaneously applied rhIGF-1 cannot improve functional motor recovery after nerve transection and repair in the rat as has been demonstrated after nerve crushing injury. This is regarded as a consequence of specificity failure during reinnervation, which occurs after nerve transection and repair, whereas after crushing injury specific reinnervation is a common feature.

Insulin-like growth factor I reverses experimental diabetic autonomic neuropathy

Recent studies have suggested a role for neurotrophic substances in the pathogenesis and treatment of diabetic neuropathy. In this study, the effect of insulin-like growth factor I (IGF-I) on diabetic sympathetic autonomic neuropathy was examined in an experimental streptozotocin-induced diabetic rat model. Two months of IGF-I treatment of chronically diabetic rats with established neuroaxonal dystrophy (the neuropathological hallmark of the disease) involving the superior mesenteric ganglion and ileal mesenteric nerves resulted in nearly complete normalization of the frequency of neuroaxonal dystrophy in both sites without altering the severity of diabetes. Treatment with low-dose insulin (to control for the transient glucose-lowering effects of IGF-I) failed to affect the frequency of ganglionic or mesenteric nerve neuroaxonal dystrophy or the severity of diabetes. The striking improvement in the severity of diabetic autonomic neuropathy shown with IGF-I treatment in these studies and the fidelity of the rat model to findings in diabetic human sympathetic ganglia provide promise for the development of new clinical therapeutic strategies.

Neurotrophic factors in the therapy of diabetic neuropathy

A large number of neurotrophic factors that exert effects on specific neuronal populations in the peripheral nervous system have been discovered. Some of these factors may prove useful for the treatment of diabetic peripheral neuropathy. Among the most promising are members of the neurotrophin gene family (nerve growth factor [NGF], brain-derived neurotrophic factor, neurotrophin [NT]-3, and NT-4/5), insulin-like growth factor (IGF)-I and IGF-II, and glial cell-derived neurotrophic factor. Of these, NGF and the IGFs have been tested most extensively in animal models of diabetic neuropathy, with encouraging results. Recombinant human nerve growth factor (rhNGF) has been tested in phase II clinical trials for treatment of patients with diabetes, and the results have been encouraging. Phase III trials of rhNGF have been completed, and clinical trials of other neurotrophic factors are likely to be conducted in the next few years.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

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.

Endogenous ciliary neurotrophic factor is a lesion factor for axotomized motoneurons in adult mice

Ciliary neurotrophic factor (CNTF) is an abundant cytosolic molecule in myelinating Schwann cells of adult rodents. In newborn animals in which CNTF is not yet expressed, exogenous CNTF that is locally administered very effectively protects motoneurons from degeneration by axotomy. To evaluate whether endogenous CNTF, released after nerve injury from the cytosol of Schwann cells, supports motoneuron survival, we transected the facial nerve in 4-week-old pmn mice. In this mouse mutant a rapidly progressing degenerative disease of motoneurons starts by the third postnatal week at the hindlimbs and progresses to the anterior parts of the body, leading to death by the seventh to eighth week. Apoptotic death of motoneurons can be observed during this period, as revealed by TUNEL staining. In 6-week-old unlesioned pmn mice approximately 40% of facial motoneurons have degenerated. Facial nerve lesion dramatically increased the number of surviving motoneurons in pmn mice. This protective effect was absent in pmn mice lacking endogenous CNTF. Quantitative analysis of leukemia inhibitory factor (LIF) mRNA expression revealed that the dramatic upregulation seen in wild-type mice after peripheral nerve lesion did not occur in pmn mice. Therefore, endogenous LIF cannot compensate for the lack of CNTF in pmn crossbred with CNTF knock-out mice. Thus, endogenous CNTF released from lesioned Schwann cells supports the survival of axotomized motoneurons under conditions in which motoneurons are in the process of rapid degeneration.

Suppression of KIF2 in PC12 cells alters the distribution of a growth cone nonsynaptic membrane receptor and inhibits neurite extension

In the present study, we present evidence about the cellular functions of KIF2, a kinesin-like superfamily member having a unique structure in that its motor domain is localized at the center of the molecule (Noda Y., Y. Sato-Yoshitake, S. Kondo, M. Nangaku, and N. Hirokawa. 1995. J. Cell Biol. 129:157-167.). Using subcellular fractionation techniques, isopicnic sucrose density centrifugation of microsomal fractions from developing rat cerebral cortex, and immunoisolation with KIF2 antibodies, we have now identified a type of nonsynaptic vesicle that associates with KIF2. This type of organelle lacks synaptic vesicle markers (synapsin, synaptophysin), amyloid precursor protein, GAP-43, or N-cadherin. On the other hand, it contains betagc, which is a novel variant of the beta subunit of the IGF-1 receptor, which is highly enriched in growth cone membranes. Both betagc and KIF2 are upregulated by NGF in PC12 cells and highly concentrated in growth cones of developing neurons. We have also analyzed the consequences of KIF2 suppression by antisense oligonucleotide treatment on nerve cell morphogenesis and the distribution of synaptic and nonsynaptic vesicle markers. KIF2 suppression results in a dramatic accumulation of betagc within the cell body and in its complete disappearance from growth cones; no alterations in the distribution of synapsin, synaptophysin, GAP-43, or amyloid percursor protein are detected in KIF2-suppressed neurons. Instead, all of them remained highly enriched at nerve terminals. KIF2 suppression also produces a dramatic inhibition of neurite outgrowth; this phenomenon occurs after betagc has disappeared from growth cones. Taken collectively, our results suggest an important role for KIF2 in neurite extension, a phenomenon that may be related with the anterograde transport of a type of nonsynaptic vesicle that contains as one of its components a growth cone membrane receptor for IGF-1, a growth factor implicated in nerve cell development.

Gel matrix vehicles for growth factor application in nerve gap injuries repaired with tubes: a comparison of biomatrix, collagen, and methylcellulose

The repair of nerve gap injuries with tubular nerve guides has been used extensively as an in vivo test model in identifying substances which may enhance nerve regeneration. The model has also been used clinical nerve repair. The objective of this study was to compare three different gel matrix-forming materials as potential vehicles for growth factors in this system. The vehicles included a laminin containing extracellular matrix preparation (Biomatrix), collagen, and a 2% methylcellulose gel. The growth factor test substance consisted of a combination of platelet-derived growth factor BB (PDGF-BB) and insulin-like growth factor I (IGF-I). An 8-mm gap in rat sciatic nerve was repaired with a silicone tube containing each of the vehicles alone or with a combination of each vehicle plus PDGF-BB and IGF-I. At 4 weeks after injury, the application of the growth factor combination significantly stimulated axonal regeneration when applied in methylcellulose or collagen, but not in Biomatrix. A similar trend was present between the vehicle control groups. By 8 weeks after injury, nerves repaired with methylcellulose as a vehicle had significantly greater conduction velocity than either collagen or Biomatrix. It was concluded that a 2% methylcellulose gel was the best of the three matrices tested, both in its effects on nerve regeneration and flexibility of formulation.

Expression and distribution of IGF-1 receptors containing a beta-subunit variant (betagc) in developing neurons

Betagc is a beta-subunit variant of the insulin-like growth factor-1 (IGF-1) receptor highly enriched in growth cone membranes prepared by subcellular fractionation of fetal rat brain (). The present study is focused on the expression and on the cellular and subcellular distribution of betagc in developing neurons and differentiating PC12 cells. In the developing cerebral cortex and, at least at early stages, in cultured primary neurons, betagc expression was found to be correlated with neurite outgrowth. In PC12 cells betagc expression was nerve growth factor (NGF)-dependent and also paralleled neurite outgrowth. In contrast, beta-subunits of the insulin receptor and/or of other IGF-1 receptors ("betaP5"; detected with antibody AbP5) were downregulated as betagc expression increased. Immunofluorescence studies confirmed the enrichment of betagc at growth cones and demonstrated morphologically its spatial separation from betaP5, which is confined to the perikaryon. At the growth cone, betagc colocalizes and associates in a proximal region with microtubules, but it seems independent of the more peripheral microfilaments. Some betagc immunoreactivity is detected in the perinuclear region of PC12 cells, most likely the Golgi complex and its vicinity. betagc seems to emerge from the periphery of this structure in an apparently vesicular compartment distinct from that carrying synaptophysin to the growth cones. The facts that (1) betagc expression is correlated closely with neurite outgrowth, that (2) it is regulated in PC12 cells by a neurotrophin, NGF, and that (3) betagc is concentrated in the proximal growth cone region raise new questions regarding a possible role of IGF-1 receptors containing betagc in the regulation of neurite growth.

IGF-I: a mitogen also involved in differentiation processes in mammalian cells

The main source of insulin-like growth factor I (IGF-I) postnatally is the liver, under growth hormone stimulation, although IGF-I is already present in embryonic tissues and in fetal serum, when its expression is independent of growth hormone. The extracellular alpha-subunit of the IGF-I receptor (IGF-IR) contains an IGF-I binding domain, and the beta-subunit possesses tyrosine kinase activity, which is greatly enhanced when IGF-I binds to the alpha-subunit and leads to its autophosphorylation. Insulin receptor substrate 1 (IRS-1) is the most well characterized cellular substrate for IGF-I, containing at least 20 potential tyrosine phosphorylation sites. The tyrosine phosphorylated form of IRS-1 acts as a docking protein by associating SH2-containing proteins including the p85 regulatory subunit of phosphatidylinositol-3-kinase (P13-kinase), the protein tyrosine phosphatase SH-PTP2, the SH2- and SH3-containing adaptor protein Nck and the growth factor receptor-bound protein-2 (Grb2/Sem5) protein. Grb2 is found associated with mSOS, a GTP/GDP exchange factor involved in converting the inactive Ras-GDP to the active Ras-GTP. The p85 regulatory subunit of PI3-kinase can be also a direct in vitro substrate of the IGF-IR. Although IRS-1 is the major substrate of the IGF-IR, there is another early phosphotyrosine substrate termed SHC, which also activates Ras via Grb2-mSos complex. Activation of p21-Ras induces a serine/threonine kinase cascade leading to the activation of MAP-kinases. The importance of IGF-I as a mitogen throughout development has been clearly demonstrated in IGF-I and IGF-IR knockout mouse studies and also in transgenic mice over-expressing IGF-I. IGF-I is a mitogen in many cell types in culture such as T lymphocytes, chondrocytes or osteoblasts and it is considered to be a progression factor in mouse fibroblasts. IGF-I is also involved in muscle, neurons and adipogenic differentiation of mesenchymal cells. However, IGF-I induces proliferation and differentiation in fetal brown adipocytes, suggesting that both cellular processes are not necessarily mutually exclusive in fetal cells.