Semaphorin III can repulse and inhibit adult sensory afferents in vivo

Cell adhesion molecule L1 modulates nerve-growth-factor-induced CGRP-IR fiber sprouting

Overexpression of nerve growth factor (NGF) using adenoviruses (Adts) after spinal cord injury induces extensive regeneration and sprouting of calcitonin-gene-related peptide immunoreactive (CGRP-IR) fibers, whereas overexpression of cell adhesion molecules (CAMs) has no effect on the normal distribution of these fibers. Interestingly, co-expression of cell adhesion molecule L1 and NGF significantly decreases (p<0.0001) CGRP-IR fiber sprouting within the spinal cord, when compared to NGF alone. Co-expression of cell adhesion molecules NCAM or N-cadherin had no effect on NGF-induced CGRP-IR fiber sprouting. These data demonstrate that reduced sprouting is specific to L1 co-expression and not other cell adhesion molecules. In vitro studies carried out to address potential mechanisms show that neurite outgrowth over astrocytes overexpressing L1 in the presence of NGF is comparable to controls, indicating that other factors present in vivo might be involved in the L1-mediated reduction in sprouting. One potential factor is semaphorin 3A (sema3A), which mediates growth cone collapse of CGRP-positive axons. Recent studies have shown that L1 is important in sema3A receptor signaling for cortical neurons. In our study, co-expression of sema3A indeed reduces neurite outgrowth from DRG neurons by about 40% on L1-expressing astrocytes. Based on these results, we hypothesize that overexpression of L1 potentiates sema3A signaling resulting in reduced sprouting.

Chemorepellent axon guidance molecules in spinal cord injury

Regenerating axons stop growing when they reach the border of the glial-fibrotic scar, presumably because they encounter a potent molecular barrier inhibiting growth cone advance. Chemorepulsive axon guidance molecules provide a non-permissive environment restricting and channeling axon growth in the developing nervous system. These molecules could also act as growth-inhibitory molecules in the regenerating nervous system. The receptors for repulsive guidance cues are expressed in the mature nervous system, suggesting that adult neurons are sensitive to the activity of developmentally active repulsive proteins. In this review, we summarize recent observations on semaphorins, ephrins, and slits in the injured brain and spinal cord, providing evidence that these proteins are major players in inhibiting axonal regeneration and establishing the glial-fibrotic scar.

MICAL flavoprotein monooxygenases: expression during neural development and following spinal cord injuries in the rat

MICALs comprise of a family of phylogenetically conserved, multidomain cytosolic flavoprotein monooxygenases. Drosophila (D-)MICAL binds the neuronal Sema1a receptor PlexA, and D-MICAL-PlexA interactions are required in vivo for Sema1a-induced axon repulsion. The biological functions of vertebrate MICAL proteins, however, remain unknown. Here, we describe three rodent MICAL genes and analyze their expression in the intact rat nervous system and in two models of spinal cord injury. MICAL-1, -2, and -3 expression patterns in the embryonic, postnatal, and adult nervous system support the idea that MICALs play roles in neural development and plasticity. In addition, MICAL expression is elevated in oligodendrocytes and in meningeal fibroblasts at sites of spinal cord injury but is unchanged in lesioned corticospinal tract neurons. Furthermore, we find that the selective monooxygenase inhibitor EGCG attenuates the repulsive effects of Sema3A and Sema3F in vitro, but not those of several other repulsive cues and substrates. These results implicate MICALs in neuronal regeneration and support the possibility of employing EGCG to attenuate Sema3-mediated axon repulsion in the injured spinal cord.

Particle-mediated gene transfer of opioid growth factor receptor cDNA regulates cell proliferation of the corneal epithelium

PURPOSE

This study was designed to determine at the molecular level whether interactions between the opioid growth factor (OGF) and OGF receptor (OGFr) play a role in regulating DNA synthesis in the homeostasis of the corneal epithelium.

METHODS

The plasmid pcDNA3.1+OGFr-HA, carrying the rat OGFr cDNA epitope-tagged with a C-terminal hemagglutinin (HA), or the empty-vector (pcDNA3.1+), was delivered twice by the Helios Gene Gun System at 300 psi to the cornea of anesthetized rats. The contralateral (untreated) cornea served as the naive specimen. BrdU was used to determine whether the recombinant OGFr was effective in regulating DNA synthesis in the rat peripheral corneal epithelium.

RESULTS

Within 18 hours of transfection, positive HA staining was apparent in both the basal and suprabasal layers (efficiency > 90% of the cells) throughout the central and peripheral cornea. Quantitative immunohistochemistry with rhodamine-conjugated anti-OGFr antibodies revealed twofold more OGFr expression in the central and peripheral epithelium of transfected corneas relative to naive corneas. The number of BrdU-positive basal cells in the peripheral epithelium of the transfected cornea was one-third of that in the naive cornea.

CONCLUSIONS

These data demonstrate the direct role of the OGF-OGFr system in determining cellular renewal in the mammalian corneal epithelium. Moreover, the successful establishment of a novel delivery system of cDNAs to the ocular surface suggests a therapeutic role for gene therapy in the eye.

Immunomodulation by topical particle-mediated administration of cytokine plasmid DNA suppresses herpetic stromal keratitis without impairment of antiviral defense

BACKGROUND

We investigated whether the course of herpetic stromal keratitis (HSK) in BALB/c mice could be altered by topical gene-gun-mediated administration of interleukin (IL)-4 or IL-10 plasmid DNA.

METHODS

Corneas of BALB/c mice were transfected with plasmids expressing beta-galactosidase (beta-gal), IL-4, IL-10, granulocyte-macrophage colony-stimulating factor (GM-CSF), and pCR3.1 (control) 2 days before Herpes simplex virus-1 (HSV-1; KOS) infection. Development of keratitis and cell infiltration were studied. HSV-1 replication was monitored by plaque assay. Expression of cytokines was detected by enzyme-linked immunosorbent assay. HSV-specific proliferation in the regional lymph nodes and spleens was measured. HSV-1 neutralizing antibody titers and IgG2A/IgG1 ratios were determined.

RESULTS

Expression of beta-gal was found in the treated corneas, but not in other tissues. IL-4 or IL-10 plasmid administration induced cytokine production in the corneas. After treatment with 300 psi, the severity of HSK was attenuated (each P<0.05), and the numbers of infiltrating inflammatory cells were lower than in the pCR3.1-treated controls (P<0.001). IL-6, but not IL-1alpha, expression in the cornea was reduced after treatment with IL-4 or IL-10 plasmid DNA. The HSV-1-specific DTH response, corneal Th1 cytokine profile, IgG/IgG2a/IgG1 ratio, neutralizing antibody titers, and virus clearance did not differ between the groups.

CONCLUSIONS

Thus, topically administered IL-4 and IL-10 plasmid DNA can lead to a milder course of HSK without impeding viral clearance. The gene gun technique for corneal delivery of plasmid cytokine DNA may be useful for modulating local immune responses without affecting antiviral defense.

Role of class 3 semaphorins in the development and maturation of the septohippocampal pathway

In examining the role of Class 3 secreted semaphorins in the prenatal and postnatal development of the septohippocampal pathway, we found that embryonic (E14-E16) septal axons were repelled by the cingulate cortex and the striatum. We also found that the hippocampus exerts chemorepulsion on dorsolateral septal fibers, but not on fibers arising in the medial septum/diagonal band complex, which is the source of septohippocampal axons. These data indicate that endogenous chemorepellents prevent the growth of septal axons in nonappropriate brain areas and direct septohippocampal fibers to the target hippocampus. The embryonic septum expressed np-1 and np-2 mRNAs, and the striatum and cerebral cortex expressed sema 3A and sema 3F. Experiments with recombinant semaphorins showed that Sema 3A and 3F, but not Sema 3C or 3E, induce chemorepulsion of septal axons. Sema 3A and 3F also induce growth cone collapse of septal axons. This indicates that these factors are endogenous cues for the early guidance of septohippocampal fibers, including cholinergic and gamma-aminobutyric acid (GABA)ergic axons, during the embryonic stages. During postnatal stages, when target cell selection and synaptogenesis take place, np-1 and np-2 were expressed by septohippocampal neurons at all ages tested. In the target hippocampus, pyramidal and granule cells expressed sema 3E and sema 3A, whereas most interneurons expressed sema 3C, but few expressed sema 3E or 3A. Combined tracing and expression studies showed that GABAergic septohippocampal fibers terminated preferentially onto sema 3C-positive interneurons. In contrast, cholinergic septohippocampal fibers terminated onto sema 3E and sema 3A-expressing pyramidal and granule cells. The data suggest that Class 3 secreted semaphorins are involved in postnatal development. Moreover, because GABAergic and cholinergic axons terminate onto neurons expressing distinct, but overlapping, patterns of semaphorin expression, semaphorin functions may be regulated by different signaling mechanisms at postnatal stages.

Genetic background regulates semaphorin gene expression and epileptogenesis in mouse brain after kainic acid status epilepticus

The host response to neural injury, which can include axonal sprouting and synaptic reorganization is likely to be under tight genetic regulatory control at the level of the genome and may be implicated in epileptogenesis. Despite its importance, however, the molecular basis of synaptic reorganization is unclear. We have studied the development of synaptic reorganization, semaphorin gene expression, and epileptogenesis in hippocampus of epileptogenic sensitive (FVB/NJ) and epileptogenic resistant (C57BL/6J) mice (i.e. distinct genetic backgrounds) after kainic acid-induced status epilepticus. Our results support the hypothesis that disruption of transcriptional regulation of axon guidance genes leads to a differential loss of tonic neuropilin-2 dependent activation of semaphorin 3F receptors on hippocampal neurons on distinct genetic backgrounds. This results in rearranged synaptic circuitry and thus promotes epileptogenesis. These findings may define biologic principles underlying the role of semaphorin signaling which may broadly apply to other systems undergoing neural regeneration.

Functional repair after dorsal root rhizotomy using nerve conduits and neurotrophic molecules

Functional recovery after large excision of dorsal roots is absent because of both the limited regeneration capacity of the transected root, and the inability of regenerating sensory fibers to traverse the dorsal root entry zone. In this study, bioresorbable guidance conduits were used to repair 6-mm dorsal root lesion gaps in rats, while neurotrophin-encoding adenoviruses were used to elicit regeneration into the spinal cord. Polyester conduits with or without microfilament bundles were implanted between the transected ends of lumbar dorsal roots. Four weeks later, adenoviruses encoding NGF or GFP were injected into the spinal cord along the entry zone of the damaged dorsal roots. Eight weeks after injury, nerve regeneration was observed through both types of implants, but those containing microfilaments supported more robust regeneration of calcitonin gene-related peptide (CGRP)-positive nociceptive axons. NGF overexpression induced extensive regeneration of CGRP(+) fibers into the spinal cord from implants showing nerve repair. Animals that received conduits containing microfilaments combined with spinal NGF virus injections showed the greatest recovery in nociceptive function, approaching a normal level by 7-8 weeks. This recovery was reversed by recutting the dorsal root through the centre of the conduit, demonstrating that regeneration through the implant, and not sprouting of intact spinal fibers, restored sensory function. This study demonstrates that a combination of PNS guidance conduits and CNS neurotrophin therapy can promote regeneration and restoration of sensory function after severe dorsal root injury.

Semaphorin and neuropilin expression in motoneurons after intraspinal motoneuron axotomy

We have examined mRNA and protein distribution for the axon guidance molecules semaphorin3A, 3F, 4F and semaphorin receptors neuropilin-1 and 2, 1-21 days after intramedullary axotomy of rat lumbar spinal cord motoneurons. We show that semaphorin3A mRNA and protein are up-regulated in the scar and in motoneurons from 3 days and upto 3 weeks after injury. Neuropilin-1 mRNA showed no changed expression in axotomized motoneurons. Semaphorin3F mRNA expression was found in ventral roots after ventral funiculus lesion (VFL) and neuropilin-2 mRNA was found in affected motoneurons from 1 day after injury throughout the examined period. Semaphorin4F mRNA was first found in motoneurons 3 weeks after lesion. These results suggest semaphorin/neuropilin involvement in the injury response of intramedullary axotomized motoneurons.

Growth and guidance cues for regenerating axons: where have they gone?

Both attractive and repellent cues are required to guide developing axons to their targets in the central nervous system. Critical guidance molecules in the developing brain include the semaphorins, netrins, slits, and ephrins. Current research indicates that many of these molecules and their receptors are expressed in the adult central nervous system (CNS), and that injury can alter the levels of these ligands/receptors. Recent studies have begun the process of elucidating the functions of these receptors in adult mammals, and the effects that they have on the regeneration of adult neurons. This review addresses our current knowledge with respect to the response of adult CNS neurons to axonal injury, interventions for enhancing the survival and regeneration of injured neurons, and the expression of developmental axon guidance cues in the injured mature CNS, with specific focus on the retino-tectal projection.

Semaphorin3A inhibits nerve growth factor-induced sprouting of nociceptive afferents in adult rat spinal cord

Increased expression of NGF after spinal cord injury induces sprouting of primary nociceptive axons. Exogenous application of NGF also results in extensive sprouting of these axons and causes chronic pain in uninjured animals. During development, semaphorin3A is thought to act as a repulsive guidance cue for NGF-responsive nociceptive afferents, restricting their projections to the superficial dorsal horn. We investigated the ability of semaphorin3A to selectively reduce NGF-induced sprouting and neuropathic pain in adult rats. The chemorepulsive effect of virus-mediated semaphorin3A expression was shown to counteract the sprouting induced by NGF in a dose-dependent manner, both in vitro and in adult rat spinal cords. Coexpression of semaphorin3A and NGF at moderate to low concentrations within the adult spinal cord reduced sprouting of calcitonin gene-related peptide and substance P-containing axons compared with GFP and NGF coexpression controls. At high expression levels of NGF, there was no difference in sprouting between the semaphorin3A-treated and control groups. The distribution of endogenous primary nociceptive afferents in the spinal cord appeared to be unaffected by semaphorin3A treatment in these experiments. Behavioral assessment shows that semaphorin3A coexpression with NGF led to decreased mechanical allodynia but no significant reductions in thermal hyperalgesia. These findings demonstrate directly that mature sensory afferents maintain their responsiveness to semaphorin3A, suggesting that this molecule might be used therapeutically to control aberrant sensory sprouting involved in pain or autonomic dysfunction.

Meningeal cell-derived semaphorin 3A inhibits neurite outgrowth

The neural scar that forms after injury to the mammalian central nervous system is a barrier to sprouting and regenerating axons. In addition to reactive astrocytes that are present throughout the lesion site, leptomeningeal fibroblasts invade the lesion core. When isolated in vitro, these cells form a very poor substrate for growing neurites, even more so than reactive astrocytes. Nevertheless the molecular mechanisms involved in this growth inhibition are not well understood. Semaphorins have been reported to be upregulated in meningeal cells (MCs) on mechanical injury to the brain and spinal cord. In the present study, we show that Sema3A mRNA and active protein are produced by cultured meningeal cells. A protein extract from these cells induces the collapse of embryonic dorsal root ganglion (DRG) growth cones. This collapsing activity is partially blocked by neuropilin-1 antibodies and is absent in meningeal cells derived from Sema3A-knockout mice. In addition to growth cone collapse, recombinant Sema3A but not Sema3C inhibits neurite outgrowth of embryonic DRGs. Consistent with this result we find that the inhibitory effect of meningeal cells on neurite outgrowth is partially overcome on Sema3A-deficient MCs. Furthermore we show that the inhibitory effect of MC-derived Sema3A on neurite outgrowth is modulated by nerve growth factor. Our results show that Sema3A, a chemorepellent during nervous system development, is a major neurite growth-inhibitory molecule in meningeal fibroblasts and is therefore likely to contribute to the inhibitory properties of the neural scar.

Sema3A regulates the timing of target contact by cranial sensory axons

The trigeminal ganglion provides the somatosensory innervation for the anterior rat tongue. At early embryonic stages (embryonic day [E] 12-13) pre-tongue explants repel trigeminal axon outgrowth, and this is mediated by Sema3A (Rochlin and Farbman [1998] J. Neurosci. 18:6840-6852; Rochlin et al. [2000] J. Comp. Neurol. 422:579-593). Despite a decrease in repulsion by E14 and older tongue explants, Sema3A mRNA persists throughout the dorsal epithelium through E18, after axons have begun to penetrate papilla epithelium. We investigated the hypothesis that Sema3A continues to act as a repellent and that subpopulations of trigeminal axons that penetrate the epithelium become unresponsive to Sema3A. Sema3A repelled trigeminal axons in vitro regardless of the neurotrophic factor used to stimulate axon outgrowth, but the minimum level of Sema3A required to repel depended on the neurotrophic factor. Thus, in vitro, trigeminal axons are repelled by Sema3A when they would be penetrating the Sema3A-mRNA rich epithelium in vivo. Whereas dorsal epithelium on tongue explants dissected at stages preceding target contact (E15) repelled trigeminal axons in vitro, explants dissected at later stages (E18), after axons would have penetrated the epithelium in vivo, were not repellent. To determine whether Sema3A prevents premature target penetration in vivo, we assessed the timing of target contact by sensory axons in Sema3A-/minus; and +/+ mice. Contact of the epithelium occurs prematurely in Sema3A-/minus; mice, but not penetration. Taken together, our data imply that Sema3A acts as a short-range repellent that regulates the timing of target contact by trigeminal axons.

In vitro and in vivo characterization of a novel semaphorin 3A inhibitor, SM-216289 or xanthofulvin

SM-216289 (xanthofulvin) isolated from the fermentation broth of a fungal strain, Penicillium sp. SPF-3059, was identified as a strong semaphorin 3A (Sema3A) inhibitor. Sema3A-induced growth cone collapse of dorsal root ganglion neurons in vitro was completely abolished in the presence of SM-216289 at levels less than 2 mum (IC50 = 0.16 mum). When dorsal root ganglion explants were co-cultured with Sema3A-producing COS7 cells in a collagen gel matrix, SM-216289 enabled neurites to grow toward the COS7 cells. SM-216289 diminished the binding of Sema3A to its receptor neuropilin-1 in vitro, suggesting a direct interference of receptor-ligand association. Moreover, our data suggest that SM-216289 interacted with Sema3A directly and blocked the binding of Sema3A to its receptor. We examined the efficacy of SM-216289 in vivo using a rat olfactory nerve axotomy model, in which strong Sema3A induction has been reported around regenerating axons. The regeneration of olfactory nerves was significantly accelerated by a local administration of SM-216289 in the lesion site, suggesting the involvement of Sema3A in neural regeneration as an inhibitory factor. SM-216289 is an excellent molecular probe to investigate the function of Sema3A, in vitro and in vivo, and may be useful for the treatment of traumatic neural injuries.

Temporal specific patterns of semaphorin gene expression in rat brain after kainic acid-induced status epilepticus

Mossy fiber sprouting and other forms of synaptic reorganization may form the basis for a recurrent excitatory network in epileptic foci. Four major classes of axon guidance molecules--the ephrins, netrins, slits, and semaphorins--provide targeting information to outgrowing axons along predetermined pathways during development. These molecules may also play a role in synaptic reorganization in the adult brain and thereby promote epileptogenesis. We studied semaphorin gene expression, as assessed by in situ hybridization, using riboprobes generated from rat cDNA in an adult model of synaptic reorganization, kainic acid (KA)-induced status epilepticus (SE). Within the first week after KA-induced SE, semaphorin 3C, a class III semaphorin, mRNA content is decreased in the CA1 area of the hippocampus and is increased in the upper layers of cerebral cortex. Another class III semaphorin, semaphorin 3F, is also decreased in CA1 and CA3 of hippocampus within the first week after KA-SE. These changes in gene expression are principally confined to neurons. By contrast, there was little change in the semaphorin 4C mRNA content of CA1 neurons at this time. No changes in expression of semaphorin 3A and 4C genes were detected 28 days after KA-induced SE. Regulation of semaphorin gene expression after KA-induced SE suggests that neurons may regulate the expression of axonal guidance molecules and thereby contribute to synaptic reorganization after injury of the mature brain. The anatomic locale of the altered semaphorin gene expression may serve as a marker for specific networks undergoing synaptic reorganization in the epileptic brain.

Neuropilin and class 3 semaphorins in nervous system regeneration

Injury to the mature mammalian central nervous system (CNS) is often accompanied by permanent loss of function of the damaged neural circuits. The failure of injured CNS axons to regenerate is thought to be caused, in part, by neurite outgrowth inhibitory factors expressed in and around the lesion. These include several myelin associated inhibitors, proteoglycans, and tenascin-R. Recent studies have documented the presence of class 3 semaphorins in fibroblast-like meningeal cells present in the core of the neural scar formed following CNS injury. Class 3 semaphorins display neurite growth-inhibitory effects on growing axons during embryonic development. The induction of the expression of class 3 semaphorins in the neural scar and the persistent expression of their receptors, the neuropilins and plexins, by injured CNS neurons suggest that they contribute to the regenerative failure of CNS neurons. Neuropilins are also expressed in the neural scar in a subpopulation of meningeal fibroblast and in neurons in the vicinity of the scar. Semaphorin/neuropilin signaling might therefore also be important for cell migration, angiogenis and neuronal cell death in or around neural scars. In contrast to neurons in the CNS, neuropilin/plexin positive neurons in the PNS do display long distance regeneration following injury. Injured PNS neurons do not encounter a semaphorin positive neural scar. Furthermore, Semaphorin 3A is downregulated in the regenerating spinal motor neurons themselves. This was accompanied by a transient upregulation of Semaphorin 3A in the target muscle. These observations suggest that the injury induced regulation of Semaphorin 3A in the PNS contributes to successful regeneration and target reinnervation. Future studies in genetically modified mice should provide more insight into the mechanisms by which neuropilins and semaphorins influence nervous system regeneration and degeneration.

Gradients and growth cone guidance of grasshopper neurons

The generation of a functional nervous system is dependent on precise pathfinding of axons during development. This pathfinding is directed by the distribution of local and long-range guidance cues, the latter of which are believed to be distributed in gradients. Gradients of guidance cues have been associated with growth cone function for over a hundred years. However, little is known about the mechanisms used by growth cones to respond to these gradients, in part owing to the lack of identifiable gradients in vivo. In the developing grasshopper limb, two gradients of the semaphorin Sema-2a are necessary for correct neuronal pathfinding in vivo. The gradients are found in regions where growth cones make critical steering decisions. Observations of different growth cone behaviors associated with these gradients have provided some insights into how growth cones respond to them. Growth cones appear to respond more faithfully to changes in concentration, rather than absolute levels, of Sema-2a expression, whereas the absolute levels may regulate growth cone size.

Anti-semaphorin 3A antibodies rescue retinal ganglion cells from cell death following optic nerve axotomy

Damage to the optic nerve in mammals induces retrograde degeneration and apoptosis of the retinal ganglion cell (RGC) bodies. The mechanisms that mediate the response of the neuronal cells to the axonal injury are still unknown. We have previously shown that semaphorins, axon guidance molecules with repulsive cues, are capable of mediating apoptosis in cultured neuronal cells (Shirvan, A., Ziv, I., Fleminger, G., Shina, R., He, Z., Brudo, I., Melamed, E., and Brazilai, A. (1999) J. Neurochem. 73, 961-971). In this study, we examined the involvement of semaphorins in an in vivo experimental animal model of complete axotomy of the rat optic nerve. We demonstrate that a marked induction of type III semaphorin proteins takes place in ipsilateral retinas at early stages following axotomy, well before any morphological signs of RGC apoptosis can be detected. Time course analysis revealed that a peak of expression occurred after 2-3 days and then declined. A small conserved peptide derived from semaphorin 3A that was previously shown to induce neuronal death in culture was capable of inducing RGC loss upon its intravitreous injection into the rat eye. Moreover, we demonstrate a marked inhibition of RGC loss when axotomized eyes were co-treated by intravitreous injection of function-blocking antibodies against the semaphorin 3A-derived peptide. Marked neuronal protection from degeneration was also observed when the antibodies were applied 24 h post-injury. We therefore suggest that semaphorins are key proteins that modulate the cell fate of axotomized RGC. Neutralization of the semaphorin repulsive function may serve as a promising new approach for treatment of traumatic injury in the adult mammalian central nervous system or of ophthalmologic diseases such as glaucoma and ischemic optic neuropathy that induce apoptotic RGC death.

Adrenergic regulation of bone metabolism: possible involvement of sympathetic innervation of osteoblastic and osteoclastic cells

It has been demonstrated that human osteoblastic as well as osteoclastic cells are equipped with adrenergic receptors and neuropeptide receptors and that they constitutively express diffusible axon guidance molecules that are known to function as a chemoattractant and/or chemorepellent for growing nerve fibers. These findings suggest that the extension of axons of sympathetic and peripheral sensory neurons to osteoblastic and osteoclastic cells is required for the dynamic neural regulation of local bone metabolism. Recently, bone resorption modulated by sympathetic stimulation was demonstrated to be associated with ODF (osteoclast differentiation factor) and OCIF (osteoclastogenesis inhibitory factor) produced by osteoblasts/stromal cells. This review summarizes the evidence implicating sympathetic neuron action in bone metabolism. The possible function of osteoclastogenesis, which could result in the initiation of sympathomimetic bone resorption, is also discussed.

Cell type-specific expression of neuropilins in an MCA-occlusion model in mice suggests a potential role in post-ischemic brain remodeling

Neuropilin-1 and -2 (NP-1/NP-2) are transmembrane receptors that play a role in axonal guidance by binding of class III semaphorins, and in angiogenesis by binding of the vascular endothelial growth factor isoform VEGF165 and placenta growth factor (PLGF). We investigated the expression pattern of NP-1/NP-2, their co-receptors, vascular endothelial growth factor receptor-1 and -2 (VEGFR-1, VEGFR-2), and their ligands, class III semaphorins, VEGF and PLGF, following experimental cerebral ischemia in mice. By means of in situ hybridization and immunohistochemistry we observed loss of expression of class III semaphorins in neurons in the infarct/peri-infarct area. In contrast, we observed high expression of NP-1 in vessels, neurons, and astrocytes surrounding the infarct. VEGF and PLGF were upregulated in different cell types following stroke. Our results suggest a shift in the balance between semaphorins and VEGF/PLGF, which compete for NP-binding. Possibly, the loss of semaphorins facilitates binding of the competing ligands (VEGF/PLGF), thus inducing angiogenesis. In addition, the observed expression patterns further suggest a neurotrophic/neuroprotective role of VEGF/PLGF.

Temporal regulation of neuropilin-1 expression and sensitivity to semaphorin 3A in NGF- and NT3-responsive chick sensory neurons

The extracellular molecule semaphorin 3A (Sema3A) is proposed to be a negative guidance cue that participates in patterning DRG sensory axons in the developing chick spinal cord. During development Sema3A is first expressed throughout the spinal cord gray matter, but Sema3A expression later disappears from the dorsal horn, where small-caliber cutaneous afferents terminate. Sema3A expression remains in the ventral horn, where large-muscle proprioceptive afferents terminate. It has been proposed that temporal changes in the sensitivity of different classes of sensory afferents to Sema3A contribute to the different pathfinding of these sensory afferents. This study compared the expression of the semaphorin 3A receptor subunit, neuropilin-1, and the collapse response of growth cones to semaphorin 3A for NGF (cutaneous)- and NT3 (proprioceptive)-dependent sensory axons extended from E6-E10 chick embryos. Growth cones extended from E6 DRGs in NT3-containing medium expressed neuropilin-1 and collapsed in response to Sema3A. From E7 until E10 NT3-responsive growth cones expressed progressively lower levels of neuropilin-1, and were less sensitive to Sema3A. On the other hand, growth cones extended from DRGs in NGF-containing medium expressed progressively higher levels of neuropilin-1 and higher levels of collapse response to Sema3A over the period from E6-E10. Thus, developmental patterning of sensory terminals in the chick spinal cord may arise from changes in both Sema3A expression in the developing spinal cord and accompanying changes in neuronal expression of the Sema3A receptor subunit, neuropilin-1.

Knowing how to navigate: mechanisms of semaphorin signaling in the nervous system

Neuronal connections are made during embryonic development with astonishing precision to ultimately form the physical basis for the central nervous system's main capacity: information processing. Over the past few decades, much has been learned about the general principles of axon guidance. A key finding to emerge is that extracellular cues play decisive roles in establishing the connections. One family of such cues, the semaphorin proteins, was first identified as repellents for navigating axons during brain wiring. Recent studies have implicated these molecules in many other processes of neuronal development, including axonal fasciculation, target selection, neuronal migration, and dendritic guidance, as well as in the remodeling and repair of the adult nervous system. It appears that responding neuronal processes sense these semaphorin signals by a family of transmembrane molecules, namely the plexins, even though neuropilins were also found to be required for mediating the interaction between plexins and class 3 semaphorins. Our understanding of the intracellular signaling machinery linking the receptors to the cytoskeleton machinery is still incomplete, but several molecules have been implicated in mediating or modulating semaphorin-induced responses. Adding to the complexity of semaphorin biology, new findings implicate semaphorins in functioning not only as signaling ligands, but also as signal-transducing receptors. Thus, semaphorins may serve as important probes for exploring the mechanisms of intercellular communication during the development and function of the nervous system.

In vivo control of NMDA receptor transcript level in motoneurons by viral transduction of a short antisense gene

Glutamate receptors play critical roles in normal and pathological processes. We developed an antisense gene delivery strategy to modulate the NMDA type of glutamate receptor. Using transient transfection in vitro and viral mediated gene transfer in vitro and in vivo, the effect of expression of an antisense gene fragment (60 bp) of the NR1 subunit was tested. Immunoblot analysis showed an antisense-concentration-dependent reduction in the NR1 subunit upon transient co-transfection of a plasmid expressing a sense NR1 gene and a plasmid expressing the antisense fragment into COS-7 cells. After recombination into an adenoviral vector, this antisense fragment reduced the amount of endogenous NR1 protein in PC12 cells. Finally, direct intraparenchymal injection of the viral vector into rat spinal cord resulted in diminished NR1 in motor neurons. Our results demonstrate the efficacy of this approach, which combines antisense with viral gene delivery to control the expression of specific genes in vivo. This approach may also be useful in reducing excitatory neurotransmission in vivo, with implications for the treatment of spinal disorders such as amyotrophic lateral sclerosis or chronic pain.

Age-dependent effects of secreted Semaphorins 3A, 3F, and 3E on developing hippocampal axons: in vitro effects and phenotype of Semaphorin 3A (-/-) mice

We studied the role of Semaphorins in the formation of hippocampal connections at embryonic and early postnatal stages. We show that the embryonic entorhinal cortex has a repulsive effect on embryonic hippocampal axons that disappears gradually at postnatal stages. Such chemorepulsion is blocked by Neuropilin-1 and -2 blocking antibodies. However, at perinatal stages, the inner layers of the entorhinal cortex attract CA1 axons. At these stages, Sema3A and Sema3F bind commissural and entorhinal axons. Sema3A and Sema3F repel hippocampal axons at E14-P2, but not at E13. A similar spatiotemporal pattern of chemorepulsion is observed for Sema3A on entorhinal axons, in contrast to Sema3F, which repels these axons only at postnatal ages. Sema3E also repels hippocampal axons but exclusively at E14. We show that Sema3A and Sema3F can induce the collapse of hippocampal growth cones and that membrane-bound Sema3A and Sema3F can guide hippocampal axons in the stripe assay. In sema3A (-/-) mice, the entorhinohippocampal projection is largely normal although single axons innervate aberrantly the stratum radiatum and the hilus. Thus, the chemorepulsion evoked by Sema3A, Sema3E, and Sema3F is dynamically regulated in the developing hippocampal formation.

Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.

Comparison and evaluation of gene therapy and epigenetic approaches for wound healing

During the past decade considerable evidence has mounted concerning the importance of growth factors in the wound healing process both for cell replication and for stimulating reparative cells to synthesize and secrete extracellular matrix components. During normal wound healing the growth factor concentration has to be maintained at a certain level. If the growth factor concentration is too low, normal healing fails to occur. Whereas if the growth factor concentration is too high due to either over-expression of the growth factor or too much growth factor being applied to the wound, aberrant wound healing will occur. One approach for controlling the amount of growth factor at the wound site during normal healing is through gene therapy and the titration of gene dosage. However if a narrow window exists between the beneficial therapeutic effect and toxic effects with increasing gene dosage, an agent may be necessary to give in combination with gene therapy to regulate the over-expression of growth factor. In addition to genetic approaches to regulate wound healing, epigenetic approaches also exist. Antisense oligodeoxynucleotides have been shown to regulate wound repair in certain model systems and to determine the protein(s) necessary for normal wound healing. A novel approach to regulate the activity of collagen genes, thereby affecting fibrosis, is to use a sense oligodeoxynucleotide having the same sequence of the cis element which regulates the promoter activity of a particular collagen gene. This exogenous oligodeoxynucleotide will compete with the cis element in the collagen gene for the trans-acting factor which regulates promoter activity. These epigenetic approaches afford the opportunity to regulate over-expression of growth factor and therefore preclude the potential toxic effects of gene therapy. Both genetic and epigenetic approaches for regulating the wound healing process, either normal or aberrant wound healing, have certain advantages and disadvantages which are discussed in the present article.

Sensitivity of NGF-responsive dorsal root ganglion neurons to semaphorin D is maintained in both neonatal and adult mice

Using a coculture assay of DRG neurons and aggregates of cells transfected with individual semaphorins, we have investigated the ability of semaphorins A, D, and E to inhibit axonal growth from DRG neurons. We show that axons of these neurons that grow in response to NGF remain responsive to semaphorin D in neonatal and in adult mice, although sensitivity may decline in the latter. Consistent with these findings, expression of the semaphorin receptor, neuropilin-1, is maintained in the DRGs of adult mice.

The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages

Our objective was to investigate sympathetic and sensory nerve fibers in synovial tissue in rheumatoid arthritis (RA) and osteoarthritis (OA) in relation to histological inflammation and synovial cytokine and norepinephrine (NE) secretion. Immunohistochemistry was used to detect nerve fibers and inflammatory parameters. A superfusion technique of synovial tissue pieces was used to investigate cytokine and NE secretion. In RA, we detected 0.2 +/- 0.04 tyrosine hydroxylase-positive (TH-positive=sympathetic) nerve fibers/mm2 as compared to 4.4 +/- 0. 8 nerve fibers/mm2 in OA (P<0.001). In RA, there was a negative correlation between the number of TH-positive nerve fibers and inflammation index (RRank=-0.705, P=0.002) and synovial IL-6 secretion (RRank=-0.630, P=0.009), which was not found in OA. Substance P-positive (=sensory) nerve fibers were increased in RA as compared to OA (3.5+/-0.2 vs. 2.3+/-0.3/mm2, P=0.009). Despite lower numbers of sympathetic nerve fibers in RA than in OA, NE release was similar at baseline (RA vs. OA: 152+/-36 vs. 106+/-21 pg/ml, n.s.). Basal synovial NE secretions correlate with the number of TH-positive CD 163+ synovial macrophages (RA: RRank=0.622, P=0.031; OA: RRank=0.299, n.s.), and synovial macrophages have been shown to produce NE in vitro. Whereas sympathetic innervation is reduced, sensory innervation is increased in the synovium from patients with longstanding RA when compared to the synovium from OA patients. The differential patterns of innervation are dependent on the severity of the inflammation. However, NE secretion from the synovial tissue is maintained by synovial macrophages. This demonstrates a loss of the influence of the sympathetic nervous system on the inflammation, accompanied by an up-regulation of the sensory inputs into the joint, which may contribute to the maintenance of the disease.

Semaphorin function in the developing invertebrate peripheral nervous system

Different members of the semaphorin family of secreted and transmembrane guidance molecules play important and diverse roles during neuronal development. Within the developing grasshopper limb bud, two semaphorins are expressed in relatively non-overlapping and distinct expression patterns. The establishment of the tibial sensory projection within the limb bud relies on the combinatorial action of both semaphorins. In this review, we describe the function of the two semaphorins in axonal guidance and propose that a hierarchy of cues guide sensory neurons in the developing peripheral nervous system.Key words: semaphorin, axon guidance, grasshopper, peripheral nervous system, review.

Keeping in touch: sensory neurone regeneration in the CNS

Adult neurones fail to regenerate when injured in the CNS, which leads to severe and irreversible functional deficits. Several important advances in understanding the reasons for this failure have been gained from the use of primary sensory neurones as a model system. The peripherally and centrally projecting branches of sensory neurones are differentially capable of regeneration, which is why these cells are ideally situated to elucidate the mechanisms that underlie regeneration failure. Such mechanisms include both a hostile environment within the spinal cord and a poor growth response following injury. For successful functional regeneration to occur, it is likely that both of these barriers will have to be surmounted, along with the challenge of guiding regrowing axons to appropriate postsynaptic targets. The contribution that the study of primary sensory neurones has made to the attainment of this goal will be reviewed.

Expression of mRNA for axon guidance molecules, such as semaphorin-III, netrins and neurotrophins, in human osteoblasts and osteoclasts

In the present study, we demonstrated the constitutive expression of diffusible axon guidance molecules such as neurotrophins, semaphorin-III, netrin-1, and netrin-2-like protein, which are known to function as a chemoattractant and/or chemorepellent for growing nerve fibers, in human osteoblastic and osteoclastic cells. The findings, obtained by RT-PCR, ELISA, and Western blot analysis suggest the extension of axons of peripheral sensory and sympathetic neurons to osteoblastic and osteoclastic cells and the possible neural regulation of bone metabolism in these osteogenic cells.

Pathology of nerve terminal degeneration in the skin

To characterize the pathology of epidermal nerve degeneration and regeneration, we investigated temporal and spatial changes in skin innervation of the mouse footpad. Within 24 hours after sciatic nerve axotomy, terminals of epidermal nerves appeared swollen and there was a mild reduction in epidermal nerve density (5.7 +/- 2.8 vs 12.7 +/- 2.2 fibers/mm, p < 0.04). Epidermal nerves completely disappeared by 48 hours (0.2 +/- 0.2 vs 14.2 +/- 0.9 fibers/mm, p < 0.001). Concomitant with the disappearance of epidermal nerves, the immunocytochemical pattern of the subepidermal nerve plexus became fragmented. At the electron microscopic level, the axoplasm of degenerating dermal nerves was distended with organelles and later became amorphous. Beginning from day 28 after axotomy, collateral sprouts from the adjacent saphenous nerve territory extended into the denervated area with a beaded appearance. They never penetrated the epidermal-dermal junction to innervate the epidermis. In contrast, 3 months after nerve crushing, the epidermis on the surgery side resumed a normal innervation pattern as the epidermis on the control side (10.3 +/- 3.9 vs 10.6 +/- 1.5 fibers/mm, p = 0.1). This study demonstrates the characteristics of degenerating and regenerating nerves, and suggests that successful reinnervation mainly originates from regenerating nerves of the original nerve trunks. All these findings provide qualitative and quantitative information for interpreting the pathology of cutaneous nerves.

Axon regeneration: Vaccinating against spinal cord injury

Myelin is a potent inhibitor of axon regeneration, but has been viewed as just one of many factors that prevent regeneration after injury. So it comes as a surprise that immunization against myelin has been found to allow extensive axon regeneration after injury, without apparent autoimmune-induced demyelination.

Semaphorins and their receptors in vertebrates and invertebrates

The semaphorins are a family of intercellular signaling proteins that has grown to include 19 identified members in higher vertebrates. Several of its members act as axonal guidance molecules. One participates in signaling in the immune system. The majority, however, do not yet have known biological functions. Recent studies have shown that neuropilins and plexins act as receptors for semaphorins. The most important challenge for the future is to define the biological roles of semaphorins in vivo.

Neuropilin-1 is expressed on adult mammalian dorsal root ganglion neurons and mediates semaphorin3a/collapsin-1-induced growth cone collapse by small diameter sensory afferents

Neuropilin-1 on the growth cones of NGF-dependent embryonic dorsal root ganglion (DRG) neurons mediates the repulsive effects of secreted semaphorin3a, but its role in adult neurons is unknown. Here we show that most adult rat DRG neurons, regardless of cell diameter/afferent phenotype, express neuropilin-1 protein in vitro. However, the response of growth cones belonging to these neurons (induced by recombinant collapsin-1/semaphorin3a and blocked by the anti-neuropilin-1 antibody) was restricted to those of small cell body diameter (<30 microm), corresponding primarily to nociceptive sensory afferents. Neurotrophic factors had a differential effect on neuropilin-1 expression in vitro, with DRG neurons cultured in either NGF or GDNF expressing the highest levels on their neurites. These findings suggest that neuropilin-1-mediated repellent effects of semaphorins may regulate the behavior of nociceptive sensory axons in the adult as well as the embryonic peripheral nervous system.

Semaphorins as mediators of neuronal apoptosis

Shrinkage and collapse of the neuritic network are often observed during the process of neuronal apoptosis. However, the molecular and biochemical basis for the axonal damage associated with neuronal cell death is still unclear. We present evidence for the involvement of axon guidance molecules with repulsive cues in neuronal cell death. Using the differential display approach, an up-regulation of collapsin response mediator protein was detected in sympathetic neurons undergoing dopamine-induced apoptosis. A synchronized induction of mRNA of the secreted collapsin-1 and the intracellular collapsin response mediator protein that preceded commitment of neurons to apoptosis was detected. Antibodies directed against a conserved collapsin-derived peptide provided marked and prolonged protection of several neuronal cell types from dopamine-induced apoptosis. Moreover, neuronal apoptosis was inhibited by antibodies against neuropilin-1, a putative component of the semaphorin III/collapsin-1 receptor. Induction of neuronal apoptosis was also caused by exposure of neurons to semaphorin III-alkaline phosphatase secreted from 293EBNA cells. Anti-collapsin-1 antibodies were effective in blocking the semaphorin III-induced death process. We therefore suggest that, before their death, apoptosis-destined neurons may produce and secrete destructive axon guidance molecules that can affect their neighboring cells and thus transfer a "death signal" across specific and susceptible neuronal populations.

Zebrafish semaphorin Z1b inhibits growing motor axons in vivo

Zebrafish semaphorin 1b (sema Z1b) is a new member of the semaphorin family, related to mammalian sema D/III. It is expressed in rhombomeres three and five, and in the posterior half of newly formed somites which is avoided by ventrally extending motor axons. Embryos injected at the 1-2 cell stage with synthetic sema Z1b mRNA developed normally but many (63%) showed missing or severely stunted ventral motor nerves. Other axons, somites, and hindbrain rhombomeres were not affected. No abnormalities were seen in control embryos injected with lacZ mRNA. Sema Z1b might normally influence the midsegmental pathway choice of the ventrally extending motor axons by contributing to a repulsive domain in the posterior somite.

Growth cone guidance: first steps towards a deeper understanding

Growth cones, the hand-like structures at the tip of growing neurites, possess remarkable abilities to detect directional cues. On their way to their targets they traverse a dense jungle of many different cells, expressing a variety of different molecular guidance cues. Proper reading and integration of these cues is essential for precise wiring of different parts of the peripheral and central nervous systems. Guidance cues have been classified according to the response they elicit as either attractive or repulsive. Recent work, however, suggests that this might not represent an absolute distinction and that the internal state of the growth cone can dictate whether it detects a cue as repulsive or attractive. This article reviews some new experimental approaches to understanding growth cone signal transduction mechanisms induced by extracellular guidance cues.

Distribution of semaphorin IV in adult human brain

The semaphorins comprise a family of secreted and membrane bound proteins that influence development of the nervous system as well as non-neural organs. H.SemaIV was originally isolated from a homozygously deleted region involving a subset of small cell lung cancers, a neuroendocrine derived neoplasm. To investigate H.SemaIV expression, specific polyclonal antibodies directed against a unique polypeptide (amino acids 758-773) were developed and their specificity confirmed. In cell lines, H.SemaIV staining was observed in cytoplasmic granules. In the normal adult human brain, we noted three general characteristics of H.SemaIV expression. H.SemaIV was strongly present in specific nuclei or in neuronal regions arranged in defined subnuclear structures. It was also present in neurons but not glial cells or ependymocytes. Lastly, H.SemaIV was not present in cell bodies, but rather in fibers and nerve terminals. Interestingly, an altered pattern of staining was detected in brains of three patients with Alzheimer's disease.

Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissue formed following injuries of adult but not neonatal CNS

This study evaluates the expression of the chemorepellent semaphorin III (D)/collapsin-1 (sema III) following lesions to the rat CNS. Scar tissue, formed after penetrating injuries to the lateral olfactory tract (LOT), cortex, perforant pathway, and spinal cord, contained numerous spindle-shaped cells expressing high levels of sema III mRNA. The properties of these cells were investigated in detail in the lesioned LOT. Most sema III mRNA-positive cells were located in the core of the scar and expressed proteins characteristic for fibroblast-like cells. Neuropilin-1, a sema III receptor, was expressed in injured neurons with projections to the lesion site, in a subpopulation of scar-associated cells and in blood vessels around the scar. In contrast to lesions made in the mature CNS, LOT transection in neonates did not induce sema III mRNA expression within cells in the lesion and was followed by vigorous axonal regeneration. The concomitant expression of sema III and its receptor neuropilin-1 in the scar suggests that sema III/neuropilin-1-mediated mechanisms are involved in CNS scar formation. The expression of the secreted chemorepellent sema III following CNS injury provides the first evidence that chemorepulsive semaphorins may contribute to the inhibitory effects exerted by scars on the outgrowth of injured CNS neurites. The vigorous regrowth of injured axons in the absence of sema III following early neonatal lesions is consistent with this notion. The inactivation of sema III in scar tissue by either antibody perturbation or by genetic or pharmacological intervention could be a powerful means to promote long-distance regeneration in the adult CNS.

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.

Regulation of semaphorin III/collapsin-1 gene expression during peripheral nerve regeneration

The competence of neurons to regenerate depends on their ability to initiate a program of gene expression supporting growth and on the growth-permissive properties of glial cells in the distal stump of the injured nerve. Most studies on intrinsic molecular mechanisms governing peripheral nerve regeneration have focussed on the lesion-induced expression of proteins promoting growth cone motility, neurite extension, and adhesion. However, little is known about the expression of intrinsic chemorepulsive proteins and their receptors, after peripheral nerve injury and during nerve regeneration. Here we report the effect of peripheral nerve injury on the expression of the genes encoding sema III/coll-1 and its receptor neuropilin-1, which are known to be expressed in adult sensory and/or motor neurons. We have shown that peripheral nerve crush or transection results in a decline in sema III/coll-1 mRNA expression in injured spinal and facial motor neurons. This decline was paralleled by an induction in the expression of the growth-associated protein B-50/GAP-43. As sema III/coll-1 returned to normal levels following nerve crush, B-50/GAP-43 returned to precrush levels. Thus, the decline in sema III/coll-1 mRNA coincided with sensory and motor neuron regeneration. A sustained decline in sema III/coll-1 mRNA expression was found when regeneration was blocked by nerve transection and ligation. No changes were observed in neuropilin-1 mRNA levels after injury to sensory and motor neurons, suggesting that regenerating peripheral neurons continue to be sensitive to sema III/coll-1. Therefore we propose that a decreased expression of sema III/coll-1, one of the major ligands for neuropilin-1, during peripheral nerve regeneration is an important molecular event that is part of the adaptive response related to the success of regenerative neurite outgrowth occurring following peripheral nerve injury.