Nerve growth factor and neurotrophin-3 enhance neurite outgrowth and up-regulate the levels of messenger RNA for growth-associated protein GAP-43 and T alpha 1 alpha-tubulin in cultured adult rat sensory neurones

Porous Silicon Nanoparticles Embedded in Poly(lactic-co-glycolic acid) Nanofiber Scaffolds Deliver Neurotrophic Payloads to Enhance Neuronal Growth

Scaffolds made from biocompatible polymers provide physical cues to direct the extension of neurites and to encourage repair of damaged nerves. The inclusion of neurotrophic payloads in these scaffolds can substantially enhance regrowth and repair processes. However, many promising neurotrophic candidates are excluded from this approach due to incompatibilities with the polymer or with the polymer processing conditions. This work provides one solution to this problem by incorporating porous silicon nanoparticles (pSiNPs) that are pre-loaded with the therapeutic into a polymer scaffold during fabrication. The nanoparticle-drug-polymer hybrids are prepared in the form of oriented poly(lactic-co-glycolic acid) nanofiber scaffolds. We test three different therapeutic payloads: bpV(HOpic), a small molecule inhibitor of phosphatase and tensin homolog (PTEN); an RNA aptamer specific to tropomyosin-related kinase receptor type B (TrkB); and the protein nerve growth factor (NGF). Each therapeutic is loaded using a loading chemistry that is optimized to slow the rate of release of these water-soluble payloads. The drug-loaded pSiNP-nanofiber hybrids release approximately half of their TrkB aptamer, bpV(HOpic), or NGF payload in 2, 10, and >40 days, respectively. The nanofiber hybrids increase neurite extension relative to drug-free control nanofibers in a dorsal root ganglion explant assay.

Emerging intersections between neuroscience and glioma biology

The establishment of neuronal and glial networks in the brain depends on the activities of neural progenitors, which are influenced by cell-intrinsic mechanisms, interactions with the local microenvironment and long-range signaling. Progress in neuroscience has helped identify key factors in CNS development. In parallel, studies in recent years have increased our understanding of molecular and cellular factors in the development and growth of primary brain tumors. To thrive, glioma cells exploit pathways that are active in normal CNS progenitor cells, as well as in normal neurotransmitter signaling. Furthermore, tumor cells of incurable gliomas integrate into communicating multicellular networks, where they are interconnected through neurite-like cellular protrusions. In this Review, we discuss evidence that CNS development, organization and function share a number of common features with glioma progression and malignancy. These include mechanisms used by cells to proliferate and migrate, interact with their microenvironment and integrate into multicellular networks. The emerging intersections between the fields of neuroscience and neuro-oncology considered in this review point to new research directions and novel therapeutic opportunities.

Short-term plasticity after partial deafferentation in the oculomotor system

Medial rectus motoneurons are innervated by two main pontine inputs. The specific function of each of these two inputs remains to be fully understood. Indeed, selective partial deafferentation of medial rectus motoneurons, performed by the lesion of either the vestibular or the abducens input, initially induces similar changes in motoneuronal discharge. However, at longer time periods, the responses to both lesions are dissimilar. Alterations on eye movements and motoneuronal discharge induced by vestibular input transection recover completely 2 months post-lesion, whereas changes induced by abducens internuclear lesion are more drastic and permanent. Functional recovery could be due to some kind of plastic process, such as reactive synaptogenesis, developed by the remaining intact input, which would occupy the vacant synaptic spaces left after lesion. Herein, by means of confocal microscopy, immunocytochemistry and retrograde labeling, we attempt to elucidate the possible plastic processes that take place after partial deafferentation of medial rectus motoneuron. 48 h post-injury, both vestibular and abducens internuclear lesions produced a reduced synaptic coverage on these motoneurons. However, 96 h after vestibular lesion, there was a partial recovery in the number of synaptic contacts. This suggests that there was reactive synaptogenesis. This recovery was preceded by an increase in somatic neurotrophin content, suggesting a role of these molecules in presynaptic axonal sprouting. The rise in synaptic coverage might be due to terminal sprouting performed by the remaining main input, i.e., abducens internuclear neurons. The present results may improve the understanding of this apparently redundant input system.

Effects of Selective Deafferentation on the Discharge Characteristics of Medial Rectus Motoneurons

Medial rectus motoneurons receive two main pontine inputs: abducens internuclear neurons, whose axons course through the medial longitudinal fasciculus (MLF), and neurons in the lateral vestibular nucleus, whose axons project through the ascending tract of Deiters (ATD). Abducens internuclear neurons are responsible for conjugate gaze in the horizontal plane, whereas ATD neurons provide medial rectus motoneurons with a vestibular input comprising mainly head velocity. To reveal the relative contribution of each input to the oculomotor physiology, single-unit recordings from medial rectus motoneurons were obtained in the control situation and after selective deafferentation from cats with unilateral transection of either the MLF or the ATD. Both MLF and ATD transection produced similar short-term alterations in medial rectus motoneuron firing pattern, which were more drastic in MLF of animals. However, long-term recordings revealed important differences between the two types of lesion. Thus, while the effects of the MLF section were permanent, 2 months after ATD lesioning all motoneuronal firing parameters were similar to the control. These findings indicated a more relevant role of the MLF pathway in driving motoneuronal firing and evidenced compensatory mechanisms following the ATD lesion. Confocal immunocytochemistry revealed that MLF transection produced also a higher loss of synaptic boutons, mainly at the dendritic level. Moreover, 2 months after ATD transection, we observed an increase in synaptic coverage around motoneuron cell bodies compared with short-term data, which is indicative of a synaptogenic compensatory mechanism of the abducens internuclear pathway that could lead to the observed firing and morphological recovery.SIGNIFICANCE STATEMENT Eye movements rely on multiple neuronal circuits for appropriate performance. The abducens internuclear pathway through the medial longitudinal fascicle (MLF) and the vestibular neurons through the ascending tract of Deiters (ATD) are a dual system that supports the firing of medial rectus motoneurons. We report the effect of sectioning the MLF or the ATD pathway on the firing of medial rectus motoneurons, as well as the plastic mechanisms by which one input compensates for the lack of the other. This work shows that while the effects of MLF transection are permanent, the ATD section produces transitory effects. A mechanism based on axonal sprouting and occupancy of the vacant synaptic space due to deafferentation is the base for the mechanism of compensation on the medial rectus motoneuron.

Characterization of canine dental pulp cells and their neuroregenerative potential

Dental pulp cells (DPCs) of various species have been studied for their potentials of differentiation into functional neurons and secretion of neurotrophic factors. In canine, DPCs have only been studied for cell surface markers and differentiation, but there is little direct evidence for therapeutic potentials for neurological disorders. The present study aimed to further characterize canine DPCs (cDPCs), particularly focusing on their neuroregenerative potentials. It was also reported that superparamagnetic iron oxide (SPIO) particles were useful for labeling of MSCs and tracking with magnetic resonance imaging (MRI). Our data suggested that cDPCs hold higher proliferation capacity than bone marrow stromal cells, the other type of mesenchymal stem cells which have been the target of intensive research. Canine DPCs constitutively expressed neural markers, suggesting a close relationship to the nervous system in their developmental origin. Canine DPCs promoted neuritogenesis of PC12 cells, most likely through secretion of neurotrophic factors. Furthermore, SPIO nanoparticles could be effectively transported to cDPCs without significant cytotoxicity and unfavorable effects on neuritogenesis. SPIO-labeled cDPCs embedded in agarose spinal cord phantoms were successfully visualized with a magnetic resonance imaging arousing a hope for noninvasive cell tracking in transplantation studies.

Impaired axonal regeneration by isolectin B4-binding dorsal root ganglion neurons in vitro

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, alpha7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding alpha7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both alpha7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

Colostrinin-Driven Neurite Outgrowth Requires p53 Activation in PC12 Cells

1. Colostrinin (CLN) induces maturation and differentiation of murine thymocytes, promotes proliferation of peripheral blood leukocytes, induces immunomodulator cytokines, and ameliorates oxidative stress-mediated activation of c-Jun NH2-terminal kinases. 2. Here we report that upon treatment with CLN, medullary pheochromocytoma (PC12) cells ceased to proliferate and extend neurites. 3. The arrest of CLN-treated PC12 cells in the G1 phase of the cell cycle was due to an increase in the phosphorylation of p53 at serine(15) (p53ser15) and expression of p21WAF1. PC12 cells treated with inhibitory oligonucleotides to p53 lacked p53ser15 and p21WAF1 expression, and did not show morphological changes after CLN exposure. Transfection with inhibitory oligonucleotides to p21WAF1 had no effect on p53 activation; however, cells failed to arrest or extend neurites. An oligonucleotide inhibiting luciferase expression had no effect on CLN-mediated p53 activation, p21WAF1 expression, growth arrest, or neurite outgrowth. 4. We conclude that CLN induces delicate cassettes of signaling pathways common to cell proliferation and differentiation, and mediates activities that are similar to those of hormones and neurotrophins, leading to neurite outgrowth.

Conditioning injury-induced spinal axon regeneration fails in interleukin-6 knock-out mice

Regeneration of injured adult sensory neurons within the CNS is essentially abortive, attributable in part to lesion-induced or revealed inhibitors such as the chondroitin sulfate proteoglycans and the myelin inhibitors (Nogo-A, MAG, and OMgp). Much of this inhibition may be overcome by boosting the growth status of sensory neurons by delivering a conditioning lesion to their peripheral branches. Here, we identify a key role for the lesion-induced cytokine interleukin-6 (IL-6) in mediating conditioning lesion-induced enhanced regeneration of injured dorsal column afferents. In adult mice, conditioning injury to the sciatic nerve 1 week before bilateral dorsal column crush resulted in regeneration of dorsal column axons up to and beyond the injury site into host CNS tissue. This enhanced growth state was accompanied by an increase in the expression of the growth-associated protein GAP43 in preinjured but not intact dorsal root ganglia (DRGs). Preconditioning injury of the sciatic nerve in IL-6 -/- mice resulted in the total failure in regeneration of dorsal column axons consequent on the lack of GAP43 upregulation after a preconditioning injury. DRGs cell counts and cholera toxin beta subunit labeling revealed that impaired regeneration in knock-out mice was unrelated to cell loss or a deficit in tracer transport. In vitro, exogenous IL-6 boosted sensory neuron growth status as evidenced by enhanced neurite extension. This effect required NGF or NT-3 but not soluble IL-6 receptor as cofactors. Evidence of conditioning lesion-enhanced growth status of DRGs cells can also be observed in vitro as an earlier and enhanced rate of neurite extension; this phenomenon fails in IL-6 -/- mice preinjured 7 d in vivo. We suggest that injury-induced IL-6 upregulation is required to promote regeneration within the CNS. Our results indicate that this is achieved through a boosted growth state of dorsal column projecting sensory neurons.

Regulation of axonal growth and guidance by the neurotrophin family of neurotrophic factors

1. The neurotrophins play an important role during development to stimulate and guide axonal growth for the establishment of a correctly wired and functional neural system. Neurotrophins can also regulate adult nervous system plasticity by promoting neuronal survival and stimulating nerve regrowth following injury. 2. Therefore, the potential exists for these neurotrophic factors to be used as therapeutic agents for the treatment of neurodegenerative disorders. However, in order to realize the full capacity of neurotrophic factors as therapeutic agents, it is important to understand the mechanisms by which they elicit their survival and regenerative effects. 3. The present paper reviews some of the ways in which neurotrophins regulate axonal growth and guidance.

Oleic acid induces GAP-43 expression through a protein kinase C-mediated mechanism that is independent of NGF but synergistic with NT-3 and NT-4/5

We have recently shown that the presence of albumin in astrocytes triggers the synthesis and release of oleic acid, which behaves as a neurotrophic factor for neurons. Thus, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth-associated protein, GAP-43. In this work we show that oleic acid upregulates GAP-43 expression by a protein kinase C (PKC)-dependent mechanism. Since GAP-43 expression has been shown to be upregulated by several neurotrophins, we investigated the relationship between the effect of oleic acid and that of NGF, neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5) on GAP-43 expression. Our results indicate that NGF is not involved in the neurotrophic effect of oleic acid because the addition of NGF did not modify the effect of oleic acid on GAP-43 expression. Neither NT-3 nor NT-4/5 alone modified GAP-43 expression. However, NT-3 and NT-4/5 acted synergistically with oleic acid to increase GAP-43 expression. The lack of effect of NGF as compared to other neurotrophins is not unexpected since we have not found TrkA expression under our experimental conditions. The effect of oleic acid on GAP-43 expression must be independent of autocrine factors synthesized by neurons because this effect was also observed at low cellular densities. In conclusion, our results indicate that oleic acid behaves as a neurotrophic factor, inducing GAP-43 expression through a PKC-mediated mechanism that is not mediated by other neurotrophic factors but that is strongly synergized by NT-3 and NT-4/5.

An unbiased cDNA library prepared from isolated Aplysia sensory neuron processes is enriched for cytoskeletal and translational mRNAs

Local protein synthesis is required for long-lasting synapse-specific plasticity in cultured Aplysia sensorimotor synapses. To identify synaptically localized mRNAs, we prepared a cDNA library from isolated sensory neurites. By sequence analysis, we estimate that the library contains 263 distinct mRNAs, with 98 of these mRNAs constituting 70% of all clones. The localized transcripts are enriched for mRNAs encoding cytoskeletal elements and components of the translational machinery. In situ hybridization confirms that the mRNAs for at least eight of these transcripts are present in distal neurites. Immunocytochemistry reveals that serotonin regulates the translation of one of the localized mRNAs, that encoding alpha1-tubulin. Our identification of mRNAs encoding cytoskeletal elements suggests that local protein synthesis is required for the growth of new synaptic connections associated with persistent synaptic strengthening. Our finding of mRNAs encoding components of the translational machinery suggests that local protein synthesis serves to increase the translational capacity of synapses.

Persistent TrkA activity is necessary to maintain transcription in neuronally differentiated PC12 cells

Neurotrophins are required for the differentiation and survival of several different neuronal subpopulations in the developing nervous system. The PC12 cell line responds to nerve growth factor (NGF) by withdrawing from the cell cycle and acquiring a sympathetic neuron-like phenotype. Previous studies have shown that the activation kinetics of the NGF receptor, TrkA, and downstream protein kinases appear rapid and seemingly transient after NGF treatment of naive PC12 cells. However, maintenance of the neuronal phenotype and survival of differentiated PC12 cells under serum-free conditions require constant NGF exposure. In this study we have addressed the mechanisms that NGF uses to maintain neuronal PC12 cells. We show that TrkA remains phosphorylated at a basal level throughout differentiation of the PC12 cells. The phospho-TrkA levels in the differentiated PC12 cells were diminished by both complete NGF withdrawal and pharmacological inhibition of Trk kinase activity. Intracellular sequestration of the majority of TrkA molecules (both phosphorylated and non-phosphorylated TrkA) and persistent dephosphorylation of the small pool of cell surface TrkA renders the persistent phospho-TrkA signal in the differentiated PC12 cells resistant to partial NGF withdrawal as well as exposure to additional NGF. NGF regulated both extracellular-regulated kinases 1/2 and Akt activity in the differentiated PC12 cells via sustained TrkA activity. Moreover, analysis of transcription using activating protein 1-, serum response element-, and cyclic AMP response element-Luc reporter constructs showed that NGF regulated these promoters through TrkA activity in differentiated PC12 cells. Interestingly, the initial response of the cyclic AMP response element promoter to NGF was delayed, becoming Trk-dependent well beyond the peaks in TrkA and downstream protein kinase signal transduction.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

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.

Response of GAP-43 and p75 in human neuromas over time after traumatic injury

OBJECTIVE

GAP-43 and p75 are proteins that promote growth cone and neurite formation, elongation, and arborization in regenerating nerve axons. The objectives of this study were to determine whether GAP-43 and the low-affinity nerve growth factor receptor p75 are elevated in traumatic neuromas and whether there is a correlation between the relative amount of GAP-43 or p75 and demographic characteristics such as time elapsed between injury and repair.

METHODS

Traumatic neuromas from 21 randomly selected patients were studied, and the charts were reviewed. Specimens were collected at the time of nerve resection and grafting. Immunohistochemical analysis was performed on each sample and normal human nerve with antibodies to GAP-43 and p75. Western blot and computerized gel analyses were performed.

RESULTS

All neuroma specimens harvested within 13 months of injury exhibited markedly elevated GAP-43 levels compared with normal nerve. Specimens harvested at 14 months or more after injury showed precipitously lower GAP-43 levels, similar to or less than those of normal nerve. The correlation between the amount of intra-axonal GAP-43 and postinjury time interval was statistically significant, P = 0.0038. High GAP-43 levels were also correlated with transection injury, high postoperative sensory grade, and pain. p75 levels were elevated, without consistent variation in our population.

CONCLUSION

These preliminary data suggest that the expression of intra-axonal GAP-43 may vary over time after injury, remaining elevated for approximately the first year, then decreasing abruptly to normal or subnormal levels. These results correlate with clinical experience, indicating that peripheral nerves should be repaired relatively early if repair is indicated.

Leukemia inhibitory factor determines the growth status of injured adult sensory neurons

Conditioning injury to adult mammalian sensory neurons enhances their regeneration potential. Here we show that leukemia inhibitory factor (LIF) is a fundamental component of the conditioning response. Conditioning injury in vivo significantly increases the intrinsic growth capacity of sensory neurons in vitro from LIF+/+ mice. This conditioning effect is significantly blunted in sensory neurons from LIF-/- mice. Enhanced growth is rescued in vitro in LIF-/- mice by the addition of exogenous LIF, and the effect blocked by human LIF-05, an LIF receptor antagonist. Furthermore, we demonstrate that LIF promotes elongating but not arborizing neurite outgrowth in vitro and is required for normal regeneration of injured adult sensory neurons in vivo. LIF is also functionally protective to peptidergic sensory neurons after nerve damage in vivo. Our results indicate that the alteration in intrinsic growth status of injured sensory neurons depends, at least in part, on LIF.

Estrogen effects on neurite outgrowth and cytoskeletal gene expression in ERalpha-transfected PC12 cell lines

The potential of gonadal steroids like estrogen (E) to promote neurite sprouting is of interest in development and aging, as well as after neural trauma. The specific roles of the two main estrogen receptors, ERalpha and ERbeta, in neuronal sprouting are not yet well understood. We examined the hypothesis that E can enhance nerve growth factor (NGF)-stimulated neurite sprouting in an ERalpha-dependent manner. PC12 cells that were stably transfected with the full-length rat ERalpha gene (PCER) and a control line of cells transfected with vector DNA alone (PCCON) were compared. Both cell lines vigorously differentiate neurites when treated with NGF. We determined that both lines show basal expression of ERbeta mRNA, but only the PCER cells express ERalpha mRNA. Estrogen treatment markedly enhanced NGF-stimulated neurite outgrowth from PCER but not from PCCON cells. Significantly larger proportions of PCER cells (34 and 53% at 24 and 48 h, respectively) had neurites than did the PCCON cells (17 and 26% at 24 and 48 h) after E plus NGF treatment. We also examined the effects of E and NGF treatment of PCER and PCCON cells on peripherin, alpha-tubulin, and tau mRNA expression. In undifferentiated PCER cells, E treatment increased peripherin, reduced alpha-tubulin, and did not alter tau mRNA levels. No changes in these mRNAs were observed in the controls (undifferentiated PCCON cells) after E treatment. NGF treatment markedly stimulated expression of peripherin, alpha-tubulin, and tau mRNAs in both PCER and PCCON cells. From these observations we conclude that E synergizes with NGF and stimulates neurite sprouting and also modulates expression of several cytoskeletal mRNAs through ERalpha.

Characterization of signal transduction pathway in neurotropic action of angiotensin II in brain neurons

Interaction of angiotensin II with the neuronal angiotensin type 1 receptor stimulates the PI3K signaling pathway. Our objective in this study was to investigate the hypothesis that the PI3K cascade regulates the neurotropic actions of angiotensin II in rat brain neurons. We followed growth associated protein-43 expression and neurite extension as markers of neurotropic activity. Angiotensin II, through its interaction with the angiotensin type 1 receptor, increased growth associated protein-43 expression and neurite extension. These effects were abolished by pretreatment of neurons with wortmannin and rapamycin, but not by PD 98059. Antisense oligonucleotides specific for p70(S6) kinase also inhibited angiotensin II-stimulated neurotropic activity. These data confirm the involvement of PI3K and p70(S6) kinase in angiotensin II-mediated neurotropic action. Further support for this was provided by the observation that angiotensin II caused a time-dependent stimulation of p70(S6) kinase by an angiotensin type 1 receptor-mediated process. We also found that the neurotropic actions of angiotensin II are mediated by plasminogen activator inhibitor-1. Evidence for this includes 1) angiotensin II-stimulated neuronal plasminogen activator inhibitor-1 gene expression, 2) potent neurotropic action of exogenous plasminogen activator inhibitor-1, and 3) inhibitory neurotropic effect of angiotensin II by antisense oligonucleotide-mediated depletion of plasminogen activator inhibitor-1. Finally, we found that the neurotropic action of plasminogen activator inhibitor-1 is not blocked by either angiotensin type 1 receptor antagonist or inhibitors of PI3K or p70(S6) kinase, indicating that the plasminogen activator inhibitor-1 step is downstream from the p70(S6) kinase. These observations demonstrate that angiotensin II is a neurotropic hormone that engages a distinct PI3K-p70(S6) kinase-plasminogen activator inhibitor-1 signaling pathway for this action.

Adult neuronal regeneration induced by transgenic integrin expression

In a variety of adult CNS injury models, embryonic neurons exhibit superior regenerative performance when compared with adult neurons. It is unknown how young neurons extend axons in the injured adult brain, in which adult neurons fail to regenerate. This study shows that cultured adult neurons do not adapt to conditions that are characteristic of the injured adult CNS: low levels of growth-promoting molecules and the presence of inhibitory proteoglycans. In contrast, young neurons readily adapt to these same conditions, and adaptation is accompanied by an increase in the expression of receptors for growth-promoting molecules (receptors of the integrin family). Surprisingly, the regenerative performance of adult neurons can be restored to that of young neurons by gene transfer-mediated expression of a single alpha-integrin.

Neurofilament cytoskeleton disruption does not modify accumulation of trophic factor mRNA

Previously we described a transgenic mouse model in which neurofilaments are sequestered in neuronal cell bodies and withheld from the axonal compartment. This model and other transgenic models with disrupted neurofilaments are used widely to investigate the role of the neurofilament cytoskeleton in normal neurons and in inherited or acquired diseases. To interpret such studies, it is important to establish whether the maldistribution of neurofilaments has major secondary consequences on the cell biology of the affected neurons. Notably, multiple perturbations of the nervous system simultaneously affect both the neuronal cytoskeleton and neurotrophin expression. To determine whether the expression of neurotrophic factors or their receptors is perturbed by a primary disruption in neurofilaments, we compared the accumulated mRNA levels for ciliary neuroptrophic factor (CNTF), nerve growth factor, neurotrophin 3, and the alpha CNTF receptor in mature transgenic mice and their littermate controls. Consistently with the prolonged survival of neurons expressing atypical or abnormally distributed neurofilaments, no obvious changes were observed for any of the mRNA species examined.

Effects of red nucleus ablation and exogenous neurotrophin-3 on corticospinal axon terminal distribution in the adult rat

Collateral sprouting of undamaged descending axons is one potential mechanism for recovery of function after incomplete spinal cord injury. In this study, we have investigated whether terminals of the intact corticospinal tract in the rat would sprout following ablation of a parallel descending pathway, the rubrospinal tract. No sprouting was detected after this injury alone. However, the combination of rubrospinal tract ablation with administration of 100ng neurotrophin-3 to neurons of the corticospinal tract resulted in marked increased density of corticospinal innervation in the superficial dorsal horn. There was no effect of administration of neurotrophin-3 alone and increase in axon density was not detected in the deep dorsal horn. These results imply that spontaneous sprouting of undamaged corticospinal axons does not occur following ablation of a parallel tract system, although collateral sprouting can be induced through a combination of the lesion plus exogenous growth factor. Induced change in corticospinal terminal density is detected in the superficial dorsal horn only, supporting the hypothesis that this is an area particularly supportive of circuit reorganisation.

Neurite growth promotion by nerve growth factor and insulin-like growth factor-1 in cultured adult sensory neurons: role of phosphoinositide 3-kinase and mitogen activated protein kinase

Although neurons of the PNS no longer require neurotrophins such as Nerve Growth Factor (NGF) for their survival, such factors are involved in regulating axonal sprouting and regeneration after injury. In addition to the neurotrophin receptors, sensory neurons are reported to express IGF-1, EGF and FGF receptors. To investigate the influence of growth factors in addition to NGF, we examined the effects of IGF-1 EGF and FGF on neurite growth from adult rat dorsal root ganglion sensory neurons in both dissociated cultures and in compartmented cultures. As expected, NGF elicited robust neuritic growth in both the dissociated and compartmented cultures. The growth response to IGF-1 was similar, although there was minimal neurite growth in response to EGF or FGF. In addition, IGF-1 (but neither FGF nor EGF), when applied to cell bodies in compartmented cultures, potentiated the distal neurite growth into NGF-containing side compartments. This potentiation was not seen when these factors were provided along with NGF in the side compartments of compartmented cultures, or in the dissociated cultures. To determine the contribution of signaling intermediates downstream of receptor activation, we used inhibitors of the potential effectors and Western blotting. The PI 3-kinase inhibitor, LY294002 attenuated neurite growth evoked by NGF, IGF and EGF in dissociated cultures, although the MAP kinase kinase (MEK) inhibitor PD098059 diminished the growth in only IGF. Immunoprecipitation and Western blotting results demonstrated differential activation of MAPK, PI 3-kinase, PLCgamma1 and SNT by the different factors. Activation of PI 3-kinase and SNT by both NGF and IGF-1 correlated with their effects on neurite growth. These results support the hypothesis that the PI 3-kinase pathway plays an important role in neuritogenesis.

Neurotrophin-3 antisense oligonucleotide attenuates nerve injury-induced Abeta-fibre sprouting

It is proposed that following peripheral nerve injury abnormal sprouting of Abeta-fibre primary afferent neurons in the spinal cord contributes to the allodynia that often occurs with such injury. Allodynia is characterized as pain due to a stimulus which is normally non-noxious. Our recent in vivo experiments show that intrathecal administration of neurotrophin-3 (NT-3), in normal animals, induces allodynia and sprouting of Abeta-fibres. In this study, we examine whether intrathecal administration of NT-3 antisense oligonucleotides (50 microM), via an osmotic pump for 14 days, attenuates nerve injury-induced sprouting and allodynia. The oligonucleotides used in this study were phosphorothioate modified and control experiments, using an ELISA, confirm that intrathecal administration of the antisense induces a significant decrease in NT-3 levels in the spinal cord. All surgery was conducted on anaesthetized Wistar rats (sodium pentobarbitone, i.p. 50 mg/kg). Consistent with previous studies, transganglionic labelling of Abeta-fibres with choleragenoid-horseradish peroxidase (C-HRP) shows that complete transection of the sciatic nerve induces an expansion of C-HRP label into lamina II of the spinal dorsal horn. Using image analysis, we find that intrathecal administration of NT-3 antisense attenuates the density of C-HRP labelling in lamina II in nerve injured animals. A NT-3 sense oligonucleotide (50 microM) has no effect. To test the effect of NT-3 antisense on allodynia, the nociceptive flexion reflex is examined, using an Ugo Basile Analgesymeter, in animals with partial sciatic nerve ligation. Intrathecal administration of 50 microM NT-3 antisense significantly attenuates nerve injury-induced allodynia, whereas the sense oligonucleotide has no effect. These results provide further evidence that endogenous NT-3 contributes to both nerve injury-induced Abeta-fibre sprouting and allodynia and demonstrates the potential of neurotrophin-3 antisense oligonucleotides as therapeutic agents for neuropathic pain.

Cyclic AMP prevents an increase in GAP-43 but promotes neurite growth in cultured adult rat dorsal root ganglion neurons

High expression of the growth-associated protein GAP-43 in neurons is correlated with developmental and regenerative axon growth. It has been postulated that during development and after injury, GAP-43 expression is elevated due to the unavailability of a target-derived repressive signal, but that GAP-43 expression then declines upon target contact. Here we examine the cyclic AMP second messenger signaling pathway to determine if it might mediate retrograde transmission of a signal which represses GAP-43 expression and inhibits growth. Cultures of adult rat dorsal root ganglia were chronically exposed to membrane-permeable analogs of cyclic AMP and activators of adenyl cyclase. These treatments caused GAP-43 protein levels to decrease in a dose-dependent manner, although neuronal survival was not affected. GAP-43 mRNA was also decreases by cyclic AMP. GAP-43 protein levels were not repressed by neurotrophins, cytokines, or other agents. Surprisingly, cyclic AMP caused an increase in the rate of neurite outgrowth, even though the neurons were partially depleted of GAP-43. Growth stimulation was quickly inducible and reversible, could occur in the presence of transcription inhibitors, and did not entail alterations in branching pattern. These findings suggest that axon growth involving high levels of GAP-43 is distinct from the growth stimulation which is rapidly induced by cyclic AMP.

Prosaposin-derived peptides enhanced sprouting of sensory neurons in vitro and induced sprouting at motor endplates in vivo

Prosaposin exhibits neurotrophic factor properties that are localized to a 12-amino acid sequence located in the amino terminal portion of the saposin C domain. Prosaptides are peptides derived from the neurotrophic portion of prosaposin; these have been previously reported to be bioactive in neuroblastoma cell lines in vitro. We report that prosaptides were also bioactive in explants of adult primary sensory neurons by dose-dependently increasing both the number (3- to 4-fold) and elongation of these neurites by 50%. Local injection of prosaptides into the gluteus muscle of adult mice also induced sprouting at the motor endplate. Our results indicate that prosaptides are potent neuritogenic factors for both sensory and motor neurons of adult peripheral nerve.

Growth responses of different subpopulations of adult sensory neurons to neurotrophic factors in vitro

Different subpopulations of adult primary sensory neurons in the dorsal root ganglia express receptors for different trophic factors, and are therefore potentially responsive to distinct trophic signals. We have compared the effect of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and NT-3, and of glial cell line-derived neurotrophic factor (GDNF) on neurite outgrowth in dissociated cultures of sensory neurons from the lumbar ganglia of young adult rats, and attempted to establish subset-specific effects of these trophic factors. We analysed three parameters of neurite growth (percentage of process-bearing neurons, length of longest neurite and total neurite length), which may correlate with particular types of axon growth in vivo, and may therefore respond differently to trophic factor presence. Our results showed that percentage of process-bearing neurons and total neurite length were influenced by trophic factors, whilst the length of the longest neurite was trophic factor independent. Only NGF and GDNF were found to enhance significantly the proportion of process-bearing neurons in vitro. GDNF was more effective than NGF on small, IB4- neurons, which are known to develop GDNF responsiveness early in postnatal development. NGF, and to a much lesser extent GDNF, enhanced the total length of the neurites produced by neurons in culture. BDNF exerted an inhibitory effect on growth, and both BDNF and NT-3 could partially block some of the growth-promoting effects of NGF on specific neuronal subpopulations.

Increased expression of GAP-43 in small sensory neurons after stimulation by NGF indicative of neuroregeneration in capsaicin-treated rats

Intraplantar injections of human recombinant nerve growth factor (rhNGF-beta) into the hind paw of capsaicin-treated adult rats are known to lead to a recovery of depleted peptide transmitter substances, to the immunohistochemical reappearance of peptidergic innervation in the skin and in the dorsal horn of the spinal cord, as well as to a recovery of the function of capsaicin-lesioned neurons. In the present study a marker peptide for neuronal regeneration and outgrowth, growth associated protein 43 (GAP-43), was investigated in lumbar dorsal root ganglia (DRGs) and in the hindpaw skin, in order to differentiate which population of the sensory neurons responds with a neuroregenerative behaviour. In situ hybridization histochemistry (ISH) revealed that at day 8 after the capsaicin treatment GAP-43 expression was significantly increased in small DRG cells as compared to control animals, and treatment with NGF in capsaicinized rats lead to an even more pronounced increase of GAP-43 expression in the small-sized cell population. Intraepidermal labelling of GAP-43 peptide was observed in the skin of control animals, but was markedly reduced in the animals that were treated with capsaicin alone. However, intraepidermal GAP-43 immunoreactive (GAP-43-IR) fibres nearly fully recovered in the capsaicin + NGF-treated group. These results indicate that the population of small DRG cells shows spontaneous regenerative activity after a capsaicin lesion which does not lead to a successful recovery of nerve terminals in the skin. Only after an additional NGF treatment small DRG cells show an even stronger regenerative response which now also involves structural reorganization of neuron membranes and axogenesis in the periphery.

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

Effects of delivery of nerve growth factor, from a catheterized osmotic mini-pump to the proximal stump of a transected sciatic nerve, were compared with the effects of normal saline. A pilot measured retrograde axonal transport of nerve growth factor to determine a pump concentration which raised axonal transport ipsilaterally, but not contralaterally. The effects of this delivery over 12 days were then determined on expression of growth-associated protein-43, trkA, p75NTR and preprotachykinin A ipsilateral and contralateral to the pump in dorsal root ganglia at L4 and L5 (pooled). Ganglionic expression was measured both as messenger RNA and protein. Axotomy (saline pumps) increased growth-associated protein-43 messenger RNA (318 +/- 14%: all changes are percent of contralateral, non-axotomized ganglia with saline pumps) and immunoreactivity (431 +/- 43%). The increase was significantly less (P < 0.001) ipsilateral to nerve growth factor pumps (191 +/- 45%). Axotomy reduced expression of p75NTR (messenger RNA: 52 +/- 17%, P < 0.01; immunoreactivity: 74 +/- 3%, P < 0.05). These decreases were converted to increases by nerve growth factor delivery (respectively 143 +/- 40% and 281 +/- 67%; both P < 0.01). With trkA, axotomy decreased the expression of the messenger RNA (68 +/- 40%, P < 0.01) and of the primary translation product--110,000 mol. wt protein (55 +/- 12%, P < 0.01)--but not the fully glycosylated trkA protein (mol. wt 145,000). Nerve growth factor delivery did not affect trkA expression. Axotomy reduced messenger RNA for the substance P precursor, preprotachykinin A, to 42 +/- 17% (P < 0.01) and this reduction was prevented by nerve growth factor treatment. We suggest that the primary effect of nerve growth factor on axotomized C-fibres is not to promote regeneration, although that may be its secondary effect via an action on Schwann cells. It is possible that reduced neuronal sensitivity to nerve growth factor during regeneration is advantageous in suppressing nociception.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Effect of sciatic nerve crush on local and target tissue production of neurotrophin-3 transcripts in rats

The effect of sciatic nerve crush in adult rats on neurotrophin-3 (NT-3) mRNA expression at the site of crush and in ipsilateral foot skin was studied using competitive reverse transcription-polymerase chain reaction (cRT-PCR). Mid-sciatic nerve crush resulted in a significant reduction in the expression of NT-3 mRNA in nerve segments distal to the injury site at 3 and 7 days (approximate 60% decrease; P < 0.01). The reduced NT-3 rnRNA expression started to increase at days 14 post-crush and returned towards control levels at 21 days following the crush. The nerve segment proximal to the crush site showed a similar changed pattern of NT-3 mRNA expression. The effects of denervation on NT-3 mRNA expression in foot skin were also studied. Reduced expression of NT-3 was observed by 7 days post-crush, with a 25% decrease observed by 14 days (P < 0.002). Levels of NT-3 mRNA had returned to normal by 21 days post-crush. The results show that changes in axon-Schwann cell contact do not account for the nerve crush induced loss, or subsequent recovery, of NT-3 mRNA expression in nerve.

BDNF and NT-4/5 prevent atrophy of rat rubrospinal neurons after cervical axotomy, stimulate GAP-43 and Talpha1-tubulin mRNA expression, and promote axonal regeneration

Rubrospinal neurons (RSNs) undergo a marked atrophy in the second week after cervical axotomy. This delayed atrophy is accompanied by a decline in the expression of regeneration-associated genes such as GAP-43 and Talpha1-tubulin, which are initially elevated after injury. These responses may reflect a deficiency in the trophic support of axotomized RSNs. To test this hypothesis, we first analyzed the expression of mRNAs encoding the trk family of neurotrophin receptors. In situ hybridization revealed expression of full-length trkB receptors in virtually all RSNs, which declined 7 d after axotomy. Full-length trkC mRNA was expressed at low levels. Using RT-PCR, we found that mRNAs encoding trkC isoforms with kinase domain inserts were present at levels comparable to that for the unmodified receptor. TrkA mRNA expression was not detected in RSNs, and the expression of p75 was restricted to a small subpopulation of axotomized cells. In agreement with the pattern of trk receptor expression, infusion of recombinant human BDNF or NT-4/5 into the vicinity of the axotomized RSNs, between days 7 and 14 after axotomy, fully prevented their atrophy. This effect was still evident 2 weeks after the termination of BDNF treatment. Moreover, BDNF or NT-4/5 treatment stimulated the expression of GAP-43 and Talpha1-tubulin mRNA and maintained the level of trkB expression. Vehicle, NGF, or NT-3 treatment had no significant effect on cell size or GAP-43 and Talpha1-tubulin expression. In a separate experiment, infusion of BDNF also was found to increase the number of axotomized RSNs that regenerated into a peripheral nerve graft. Thus, in BDNF-treated animals, the prevention of neuronal atrophy and the stimulation GAP-43 and Talpha1-tubulin expression is correlated with an increased regenerative capacity of axotomized RSNs.

Effects of the neurotrophins nerve growth factor, neurotrophin-3, and brain-derived neurotrophic factor (BDNF) on neurite growth from adult sensory neurons in compartmented cultures

We used compartmented cultures to study the regulation of adult sensory neurite growth by neurotrophins. We examined the effects of the neurotrophins nerve growth factor (NGF), neurotrophin-3 (NT3), and BDNF on distal neurite elongation from adult rat dorsal root ganglion (DRG) neurons. Neurons were plated in the center compartments of three-chambered dishes in the absence of neurotrophin, and neurite extension into the distal (side) compartments containing NGF, BDNF, or NT3 was quantitated. Initial proximal neurite growth did not require any of the neurotrophins, while subsequent elongation into distal compartments required NGF. After neurites had extended into NGF-containing distal compartments, removal of NGF by treatment with anti-NGF resulted in the cessation of growth with minimal neurite retraction. In contrast to the effects of NGF, no distal neurite elongation was observed into compartments with BDNF or NT3. To examine possible additive influences, neurite extension into compartments containing BDNF plus NGF or NT3 plus NGF was quantitated. There was no increased neurite extension into NGF plus NT3 compartments, while the combination of BDNF plus NGF resulted in an inhibition of neurite extension compared with NGF alone. We then investigated whether the regrowth of neurites that had originally grown into NGF subsequent to in vitro axotomy still required NGF. The results demonstrated that unlike adult sensory nerve regeneration in vivo, the in vitro regrowth did require NGF, and neither BDNF nor NT3 was able to substitute for NGF. Since the initial growth from neurons after dissociation (which is also a regenerative response) did not require NGF, it would appear that neuritic growth and regrowth of adult DRG neurons in vitro includes both NGF-independent and NGF-dependent components. The compartmented culture system provides a unique model to further study aspects of this differential regulation of neurite growth.

Effects of glial cell line-derived neurotrophic factor on axonal growth and apoptosis in adult mammalian sensory neurons in vitro

The effects of glial cell line-derived neurotrophic factor on axonal outgrowth and apoptosis were studied in vitro using explanted dorsal root ganglia-peripheral nerve preparations of adult mice. In gels of matrigel or collagen type 1, glial cell line-derived neurotrophic factor increased both the numbers and lengths of axons growing out of explanted preparations, although less effectively than nerve growth factor. Stimulation of axonal outgrowth by glial cell line-derived neurotrophic factor was unaffected by K252a, a protein kinase inhibitor which blocks the effects of nerve growth factor and other neurotrophins acting through trk receptors. To determine the phenotype of the axons responding to glial cell line-derived neurotrophic factor, preparations were stained using antibodies to trkA, calcitonin gene-related peptide, 200,000 mol. wt phosphorylated neurofilaments (monoclonal antibody RT97) and the lectin Bandeiraea simplicifolia 1B4. RT97 recognizes large diameter neurons whilst 1B4 labels small diameter neurons which broadly do not express neurotrophin receptors. In preparations cultured with glial cell line-derived neurotrophic factor, significant increases in the numbers of outgrowing axons labelled with RT97 and 1B4 were observed but the numbers of calcitonin gene-related peptide-positive axons were not significantly increased and their staining intensity was generally faint. In separate preparations it was found that in the presence of glial cell line-derived neurotrophic factor, the majority of the 1B4 labelled axons were trkA negative, indicating that this factor can stimulate axonal growth in this population of neurons which do not respond to the neurotrophins. Spontaneous apoptosis in neurons and satellite cells occurs in explanted preparations of the type used in the present investigations, but in cryostat sections of preparations cultured in the presence of glial cell line-derived neurotrophic factor, the incidence of apoptosis was lower than in control preparations which had been cultured in the absence of this factor. This suggests that glial cell line-derived neurotrophic factor may promote survival of some adult sensory neurons in vitro.

Effects of neuronal proteoglycans on activity-dependent growth responses of fetal hippocampal neurons

Excitatory amino-acid (EAA) neurotransmitters act as molecular signals influencing the structure of neurons during development. However, the signal transduction and effector mechanisms responsible for these effects have yet to be fully elucidated. We have previously provided evidence that EAA agonists induce the synthesis and release of proteoglycans (PGs) with neurite-promoting activity from fetal hippocampal neurons. In the present studies exposure of fetal hippocampal neurons to glutamate (100 microM) for 5 min resulted in increases in the neuron-specific growth-associated genes T alpha 1 alpha-tubulin (T alpha 1), microtubule-associated protein-2 (MAP-2) and growth-associated protein-43 (GAP-43). mRNA levels peaked at between 8 and 12 h following exposure as determined by competitive reverse transcription polymerase chain reaction (RT-PCR). Increases in neurite growth as measured by axonal length, the total length of dendrites, the number of branches per axon, the total length of branches per axon and the total neurite length were also observed 48 h after glutamate exposure. The increase in T alpha 1, MAP-2 and GAP-43 mRNA levels following glutamate exposure was mediated via both N-methyl-D-aspartate and metabotropic receptor activation. Heparin, which inhibits the neurite growth-promoting effects of PGs in vitro, and heparitinase, which catalyzes the cleavage of heparan sulphate, also inhibited the glutamate-dependent induction of T alpha 1, MAP-2 and GAP-43 mRNA expression and neurite growth when added to culture medium following glutamate exposure. Chondroitin sulphate and chondroitinase AC had no effects on the mRNA levels tested or on neurite growth. Therefore, these studies suggest that neuronal PGs regulated by activation of EAA receptors mediate neuronal growth responses.

The role of axonal cytoskeleton in diabetic neuropathy

The neuropathy associated with diabetes includes well documented impairment of axonal transport, a reduction in axon calibre and a reduced capacity for nerve regeneration. All of those aspects of nerve function rely on the integrity of the axonal cytoskeleton. Alterations in the axonal cytoskeleton in experimental diabetes include an insulin-dependent non-enzymatic glycation of actin that is reflected in increased glycation of platelet actin in the clinical situation. There is a reduced synthesis of mRNA for the isoforms of tubulin that are associated with nerve growth and regeneration and an elevated non-enzymatic glycation of peripheral nerve tubulin in both diabetic patients and diabetic animals. mRNAs for neurofilament proteins are selectively reduced in the diabetic rat and post-translational modification of at least one of the neurofilament proteins is altered. There is some evidence that altered expression of isoforms of protein kinases may contribute to these changes.

Quantitative analysis of GAP-43 expression by neurons in microcultures using cell-ELISA

A cell-ELISA technique is described which allows the quantification of GAP-43 protein in a large number of microcultures of adult dorsal root ganglion neurons. GAP-43 is measured in the 1-10 ng range, corresponding to the amount of GAP-43 present in fewer than 500 DRG neurons. Specificity of the assay is confirmed using Western blotting and immunocytochemistry. The GAP-43 content of adult DRG microcultures rises during 2 weeks in culture, although the number of surviving neurons decreases. The GAP-43 content of cultured adult DRG neurons is not increased by chronic exposure to added nerve growth factor after 7 days in vitro. However, GAP-43 is increased in DRG taken from animals with prior peripheral nerve injury, and is decreased by chronic exposure to dibutyryl cyclic AMP after 7 days in vitro. The method affords the sensitivity and statistical power to document modest changes in GAP-43 protein abundance in complex cultures.

Regulation of rat testis gonocyte proliferation by platelet-derived growth factor and estradiol: identification of signaling mechanisms involved

To determine what factors regulate gonocyte proliferation in newborn rats, we first examined the expression of several signal transduction molecules by immunocytochemistry in 3-day-old rat testis sections. We found that gonocytes specifically expressed the iota and zeta isoforms of protein kinase (PK) C (PKC) and the phosphatidylinositol 3-kinase (PI3-K). Because both the zeta PKC and PI 3-K have been shown to play a role in platelet-derived growth factor (PDGF)-induced cell proliferation, we examined the effects of PDGF on gonocytes. For this, we developed a method to obtain highly purified and viable gonocytes in culture. After enzymatic digestion, differential adhesion, and two successive gradient fractionations, the gonocyte suspension obtained was over 90% pure, as assessed by light microscopy. The viability of cultured gonocytes exceeded 90% after 48 h in the presence of 2.5% FBS used as a survival factor. Immunodetection studies showed that isolated gonocytes expressed zeta PKC, PI 3-K, and the PDGF receptor. Treatment with 10 ng/ml PDGF induced a 4-fold increase of bromodeoxyuridine incorporation into gonocytes (from 5% proliferative gonocytes under basal conditions to 20% in the presence of PDGF). Because neonatal Sertoli cells secrete high levels of the growth promoting steroid, 17 beta-estradiol, we also tested its effect and found that it induced gonocyte proliferation at a level comparable with that of PDGF and that this effect was blocked by the estrogen receptor antagonist, ICI 164384. The combination of PDGF and estradiol, however, was not additive, suggesting that their effects were mediated by common molecular target(s). These results demonstrate that PDGF and estradiol activate gonocyte proliferation in vitro, suggesting that they may act as the physiological regulators of gonocyte development in vivo.

Complete recovery by nerve growth factor of neuropeptide content and function in capsaicin-impaired sensory neurons

In the present study the ability of nerve growth factor (NGF) to facilitate the recovery of peptidergic primary sensory C-fibers after an acute capsaicin treatment (50 mg/kg s.c.) was investigated in adult rats. NGF (4 micrograms 1/day for 3 days) was injected into the plantar of one hind paw starting 24 h after the capsaicin treatment. Without NGF, there was a significant reduction of calcitonin gene-related peptide (CGRP) and substance P content of the paw skin and the sciatic nerve. CGRP and substance P levels were completely replenished in the NGF-treated paw skin and in the innervating sciatic nerve they even increased over control levels as determined 40 h after the last injection of NGF. CGRP levels also recovered in the contralateral paw and sciatic nerve, but no recovery was observed in other tissues such as the front paw, the auricle, or the urinary bladder. Mustard oil-induced neurogenic plasma extravasation, taken as a functional parameter for peptidergic primary sensory C-fibers, was significantly decreased after the capsaicin treatment and showed a complete recovery by NGF in the injected paw as well as in the contralateral paw skin. These results show that NGF not only was able to reverse the decrease of transmitter content caused by capsaicin but also restored the peripheral function of primary afferent neurons.

Effects of intraventricular transplantation of NGF-secreting cells on cholinergic basal forebrain neurons after partial immunolesion

The aim of the present study was to examine the effects of nerve growth factor on brain cholinergic function after a partial immunolesion to the rat cholinergic basal forebrain neurons (CBFNs) by 192 IgG-saporin. Two weeks after intraventricular injections of 1.3 micrograms of 192 IgG-saporin, about 50% of CBFNs were lost which was associated with 40-60% reductions of choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU) activities throughout the basal forebrain cholinergic system. Two groups of lesioned animals received intraventricular transplantations of mouse 3T3 fibroblasts retrovirally transfected with either the rat NGF gene (3T3NGF+) or the retrovirus alone (3T3NGF-) and were sacrificed eight weeks later. In vivo production of NGF by 3T3NGF+ cells was confirmed by NGF immunohistochemistry on the grafts and NGF immunoassay on cerebrospinal fluid (CSF) samples. Both ChAT and HACU activities returned to normal control levels in the basal forebrain and cortex after 3T3NGF+ transplants, whereas no recovery was observed in 3T3NGF- transplanted animals. There was a 25% increase in the size of remaining CBFNs and an increased staining intensity for NGF immunoreactivity in these cells after NGF treatments. Acetylcholinesterase (AChE) histochemistry revealed that the optical density of AChE-positive fibers in the cerebral cortex and hippocampus were reduced by about 60% in immunolesioned rats which were completely restored by 3T3NGF+ grafts. In addition, decreases in growth-associated protein (GAP)-43 immunoreactivity after immunolesion and increases in synaptophysin immunoreactivity after 3T3NGF+ grafts were observed in the hippocampus. Our results further confirm the notion that transfected NGF-secreting cells are useful in long-term in vivo NGF treatment and NGF can upregulate CBFN function. They also highly suggest that NGF induces terminal sprouting from remaining CBFNs.

Estrogen increases GAP-43 (neuromodulin) mRNA in the preoptic area of aged rats

Estrogen has been shown to affect the growth, differentiation, and survival of brain neurons and to modulate processes involved in synapse formation and connectivity. These trophic effects are diminished with aging as secretion of estrogen declines. The growth associated protein GAP-43 is found concentrated in axonal growth cones and is implicated in neuronal growth and regeneration. Previous studies have established that expression of GAP-43 can be modulated by estrogen in the preoptic area of developing and adult rat brain. This study was undertaken to determine whether this estrogenic regulation of GAP-43 mRNA is retained in aged rat brain. Young (3 months) and aged (24 months) rats were ovariectomized to remove endogenous estrogen and GAP-43 mRNA in the preoptic area was evaluated using in situ hybridization to compare estrogen and vehicle treatments between age groups. The results demonstrate an age-related decline in GAP-43 mRNA hybridization signal that can be restored to levels comparable to that seen in young animals with estrogen treatment.

Delayed kindling development after rapidly recurring seizures: relation to mossy fiber sprouting and neurotrophin, GAP-43 and dynorphin gene expression

Development of kindling and mossy fiber sprouting, and changes of gene expression were studied after 40 seizures produced during about 3 h by electrical stimulation every 5 min in the ventral hippocampus. As assessed by 5 test stimulations, enhanced responsiveness was present already after 6-24 h but from 1 week post-seizure increased gradually up to 4 weeks without additional stimuli. Sprouting of mossy fibers in the dentate gyrus was demonstrated only at 4 weeks with Timm's staining. In situ hybridization showed a transient increase (maximum at 2 h) of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), TrkB and TrkC mRNA levels and reduction (maximum at 12-24 h) of neurotrophin-3 (NT-3) mRNA expression in dentate granule cells after the seizures. In addition, BDNF mRNA levels were elevated in CA1 and CA3 regions, amygdala and piriform cortex. Marked increases of mRNA for growth-associated protein (GAP-43), with maximum expression at 12-24 h, were observed in dentate granule cells and in amygdala-piriform cortex. Dynorphin mRNA levels showed biphasic changes in dentate granule cells with an increase at 2 h followed by a decrease at 24 h. No long-term alterations of gene expression were observed. These findings indicate that increased responsiveness develops rapidly after recurring seizures but that the kindled state is reached gradually in about 4 weeks. Mossy fiber sprouting occurs in parallel to epileptogenesis and may play a causative role. Short-term changes of neurotrophin and Trk, GAP-43 and dynorphin mRNA levels and the assumed alterations of the corresponding proteins could trigger structural rearrangements underlying kindling but might also contribute to the initial increase of seizure susceptibility.

Developmental regulation of alpha-tubulin mRNAs during the differentiation of cultured cerebellar granule cells

T alpha 1 and T26 alpha-tubulin mRNA expression was examined during the differentiation of rat cerebellar granule cells in vitro and in situ. High levels of T alpha 1 transcript correlated with neurons extending processes and hence may implicate T alpha 1 with neuritogenesis. In comparison, T26 labeling was much less prominent, appeared more constitutive and was possibly associated with cell proliferation. Such profiles indicate that the different isotypes play different roles in the assembly and function of microtubules during neuronal differentiation.