Repulsive factors and axon regeneration in the CNS

Neural tissue engineering: strategies for repair and regeneration

Nerve regeneration is a complex biological phenomenon. In the peripheral nervous system, nerves can regenerate on their own if injuries are small. Larger injuries must be surgically treated, typically with nerve grafts harvested from elsewhere in the body. Spinal cord injury is more complicated, as there are factors in the body that inhibit repair. Unfortunately, a solution to completely repair spinal cord injury has not been found. Thus, bioengineering strategies for the peripheral nervous system are focused on alternatives to the nerve graft, whereas efforts for spinal cord injury are focused on creating a permissive environment for regeneration. Fortunately, recent advances in neuroscience, cell culture, genetic techniques, and biomaterials provide optimism for new treatments for nerve injuries. This article reviews the nervous system physiology, the factors that are critical for nerve repair, and the current approaches that are being explored to aid peripheral nerve regeneration and spinal cord repair.

The effects of treatment with antibodies to transforming growth factor β1 and β2 following spinal cord damage in the adult rat

We recently showed axonal ingrowth into fibronectin (FN) mats implanted into the spinal cord. However, little axonal growth was found from FN mats into intact spinal cord. Previous research has shown that this is due in part to astrocytosis around an area of CNS damage. Antibodies to transforming growth factor beta (TGFbeta) can diminish this astrocytosis. TGFbeta also has effects on macrophages and Schwann cells, both of which infiltrate the spinal cord following damage. We examined the axonal, Schwann cell, and macrophage infiltration into FN mats as well as the level of astrocytosis and chondroitin sulfate proteoglycan NG2 around FN implants incubated in TGFbeta antibodies and implanted into a lesion cavity in the spinal cord. We also examined the effects of applying TGFbeta antibodies to a spinal cord hemisection site. Anti-TGFbeta1 within FN mats resulted in extensive cavitation, with the area of damage being larger than the original lesion. Cavitation was also seen following application of anti-TGFbeta1 to a spinal cord hemisection site. No cavitation was seen following saline, non-immune IgG or anti-TGFbeta2 treatment. However, anti-TGFbeta2 treatment did result in diminished axonal growth and Schwann cell and macrophage infiltration. Around the implant site, anti-TGFbeta2 treatment resulted in a reduction in the level of astrocytosis but had not effect on levels of NG2. Similar effects were seen following anti-TGFbeta2 application to spinal cord hemisection sites. The results suggest that anti-TGFbeta1 exacerbates secondary damage by preventing the anti-inflammatory effect of endogenous TGFbeta1. Anti-TGFbeta2 did not enhance axonal regeneration in this model but did slightly reduce astrocytosis.

Basic advances and new avenues in therapy of spinal cord injury

The prospects for successful clinical trials of neuroprotective and neurorestorative interventions for patients with acute and chronic myelopathies depend on preclinical animal models of injury and repair that reflect the human condition. Remarkable progress continues in the attempt to promote connections between the brain and the sensory and motor neurons below a spinal cord lesion. Recent experiments demonstrate the potential for biological therapies to regenerate or remyelinate axons and to incorporate new neural cells into the milieu of a traumatic spinal cord injury. The computational flexibility and plasticity of the sensorimotor systems of the brain, spinal cord, and motor unit make functional use of new circuitry feasible in patients. To incorporate residual and new pathways, neural repair strategies must be coupled to rehabilitation therapies that drive activity-dependent plasticity for walking, for reaching and grasping, and for bowel and bladder control. Prevention of pain and dysautonomia are also clinical targets. Research aims to define the temporal windows of opportunity for interventions, test the safety and efficacy of delivery systems of agents and cells, and provide a better understanding of the cascades of gene expression and cell interactions both acutely and chronically after injury. These bench-to-bedside studies are defining the neurobiology of spinal cord injury rehabilitation.

Neurotrophic effects on dorsal root regeneration into the spinal cord

Dorsal root ganglion neurons exhibit a robust and generally successful regenerative response following injury of their peripheral processes. Regeneration fails, however, after section of their central processes in the dorsal roots or dorsal columns. Experiments characterizing the attenuated response of these neurons to injury, and the inhibition of regeneration exerted by astrocytes and oligodendrocytes within the dorsal root entry zone and spinal cord, have contributed important insights into the failure of regeneration after injury to the central nervous system (CNS). Interventions that have enhanced the metabolic response of injured dorsal root ganglion neurons, and altered the inhospitable environment, have increased sensory afferent regeneration and recovery. There is reason to expect that these strategies will help to develop clinically applicable treatments of CNS injuries.

Inflammation, degeneration and regeneration in the injured spinal cord: insights from DNA microarrays

GeneChip microarrays have recently been introduced to the field of neurobiology to identify and monitor the expression levels of thousands of genes simultaneously. This powerful technique is now used for studying the pathophysiology of CNS injuries including spinal cord lesions. Early stages after injury are characterized by the strong upregulation of genes involved in transcription and inflammation and a general downregulation of structural proteins and proteins involved in neurotransmission. Later, an increase in the expression of growth factors, axonal guidance factors, extracellular matrix molecules and angiogenic factors reflects the attempts for repair, while upregulation of stress genes and proteases and downregulation of cytoskeletal and synaptic mRNA reflect the struggle of the tissue to survive. DNA microarrays have the potential to aid discovery of new targets for neuroprotective or restorative therapeutic approaches

Steroid Effects on Glial Cells

Repair of damage and recovery of function are fundamental endeavors for recuperation of patients and experimental animals with spinal cord injury. Steroid hormones, such as progesterone (PROG), show regenerative and myelinating properties following injury of the peripheral and central nervous system. In this work, we studied PROG effects on glial cells of the normal and transected (TRX) spinal cord, to complement previous studies in motoneurons. Both neurons and glial cells expressed the classical PROG receptor (PR), suggesting that genomic mechanisms participated in PROG action. In TRX rats, PROG treatment stimulated the number of NADPH-diaphorase (nitric oxide synthase) active astrocytes, whereas the number of astrocytes expressing the glial fibrillary acidic protein (GFAP) was stimulated in control but not in TRX rats. PROG also stimulated the immunocytochemical staining for myelin-basic protein (MBP) and the number of oligodendrocyte precursor cells expressing the chondroitin sulfate proteoglycan NG2 in TRX rats. In terms of beneficial or detrimental consequences, these PROG effects may be supportive of neuronal recuperation, as shown for several neuronal functional parameters that were normalized by PROG treatment of spinal cord injured animals. Thus, PROG effects on glial cells go in parallel with morphological and biochemical evidence of survival of damaged motoneurons.

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.

Fibroblast growth factor-inducible-14 is induced in axotomized neurons and promotes neurite outgrowth

For successful nerve regeneration, a coordinated shift in gene expression pattern must occur in axotomized neurons. To identify genes participating in axonal regeneration, we characterized mRNA expression profiles in dorsal root ganglia (DRG) before and after sciatic nerve transection. Dozens of genes are differentially expressed after sciatic nerve injury by microarray analysis. Induction of SOX11, FLRT3, myosin-X, and fibroblast growth factor-inducible-14 (Fn14) mRNA in axotomized DRG neurons was verified by Northern analysis and in situ hybridization. The Fn14 gene encodes a tumor necrosis-like weak inducer of apoptosis (TWEAK) receptor and is dramatically induced in DRG neurons after nerve damage, despite low expression in developing DRG neurons. Fn14 expression in PC12 cells is also upregulated by nerve growth factor treatment. Overexpression of Fn14 promotes growth cone lamelipodial formation and increases neurite outgrowth in PC12 cells. These Fn14 effects are independent of the ligand, TWEAK. Fn14 colocalizes with the Rho family GTPases, Cdc42 and Rac1. Furthermore, Fn14 physically associates with Rac1 GTPase in immunoprecipitation studies. The neurite outgrowth-promoting effect of Fn14 is enhanced by Rac1 activation and suppressed by Rac1 inactivation. These findings suggest that Fn14 contributes to nerve regeneration via a Rac1 GTPase-dependent mechanism.

ARNO and ARF6 regulate axonal elongation and branching through downstream activation of phosphatidylinositol 4-phosphate 5-kinase alpha

In the developing nervous system, controlled neurite extension and branching are critical for the establishment of connections between neurons and their targets. Although much is known about the regulation of axonal development, many of the molecular events that regulate axonal extension remain unknown. ADP-ribosylation factor nucleotide-binding site opener (ARNO) and ADP-ribosylation factor (ARF)6 have important roles in the regulation of the cytoskeleton as well as membrane trafficking. To investigate the role of these molecules in axonogenesis, we expressed ARNO and ARF6 in cultured rat hippocampal neurons. Expression of catalytically inactive ARNO or dominant negative ARF6 resulted in enhanced axonal extension and branching and this effect was abrogated by coexpression of constitutively active ARF6. We sought to identify the downstream effectors of ARF6 during neurite extension by coexpressing phosphatidyl-inositol-4-phosphate 5-Kinase alpha [PI(4)P 5-Kinase alpha] with catalytically inactive ARNO and dominant negative ARF6. We found that PI(4)P 5-Kinase alpha plays a role in neurite extension and branching downstream of ARF6. Also, expression of inactive ARNO/ARF6 depleted the actin binding protein mammalian ena (Mena) from the growth cone leading edge, indicating that these effects on axonogenesis may be mediated by changes in cytoskeletal dynamics. These results suggest that ARNO and ARF6, through PI(4)P 5-Kinase alpha, regulate axonal elongation and branching during neuronal development.

Ex vivo adenoviral vector-mediated neurotrophin gene transfer to olfactory ensheathing glia: effects on rubrospinal tract regeneration, lesion size, and functional recovery after implantation in the injured rat spinal cord

The present study uniquely combines olfactory ensheathing glia (OEG) implantation with ex vivo adenoviral (AdV) vector-based neurotrophin gene therapy in an attempt to enhance regeneration after cervical spinal cord injury. Primary OEG were transduced with AdV vectors encoding rat brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or bacterial marker protein beta-galactosidase (LacZ) and subsequently implanted into adult Fischer rats directly after unilateral transection of the dorsolateral funiculus. Implanted animals received a total of 2 x 105 OEG that were subjected to transduction with neurotrophin-encoding AdV vector, AdV-LacZ, or no vector, respectively. At 4 months after injury, lesion volumes were smaller in all OEG implanted rats and significantly reduced in size after implantation of neurotrophin-encoding AdV vector-transduced OEG. All OEG grafts were filled with neurofilament-positive axons, and AdV vector-mediated expression of BDNF by implanted cells significantly enhanced regenerative sprouting of the rubrospinal tract. Behavioral analysis revealed that OEG-implanted rats displayed better locomotion during horizontal rope walking than unimplanted lesioned controls. Recovery of hind limb function was also improved after implantation of OEG that were transduced with a BDNF- or NT-3-encoding AdV vector. Hind limb performance during horizontal rope locomotion did directly correlate with lesion size, suggesting that neuroprotective effects of OEG implants contributed to the level of functional recovery. Thus, our results demonstrate that genetic engineering of OEG not only resulted in a cell that was more effective in promoting axonal outgrowth but could also lead to enhanced recovery after injury, possibly by sparing of spinal tissue.

Synergistic effects of brain-derived neurotrophic factor and chondroitinase ABC on retinal fiber sprouting after denervation of the superior colliculus in adult rats

Damage to the adult CNS often causes devastating and permanent deficits because of the limited capacity of the brain for anatomical reorganization. The finding that collateral sprouting of uninjured fiber tracts mediates recovery of function prompts the search for experimental strategies that stimulate axonal plasticity after CNS trauma. Here we characterize treatments that promote the sprouting of undamaged retinal afferents into the denervated superior colliculus (SC) after a partial retinal lesion in the adult rat. Delivery of brain-derived neurotrophic factor (BDNF) was performed to enhance the intrinsic potential of retinal ganglion cells to reelongate their axons. Reduction of the neurite growth-inhibitory properties of the adult SC was accomplished via treatment with chondroitinase ABC (C-ABC), which degrades chondroitin sulfate proteoglycans. Retinal axons were labeled via intraocular injections of fluorescently tagged cholera toxin B subunit, and fiber sprouting within the denervated SC was measured by quantitative laser-scanning confocal microscopy 1 week after the retinal lesion. We found that both the administration of BDNF and the injection of C-ABC induce significant sprouting of retinal afferents into the collicular scotoma. Remarkably, the combined treatment with BDNF and C-ABC showed synergistic effects on axon growth. Colocalization analysis with anti-synapsin antibodies demonstrated synapse formation by the sprouting axons. These results suggest that the combined treatment with BDNF and C-ABC can be relevant in therapies for the repair of the damaged adult CNS.

Osmotic swelling induces p75 neurotrophin receptor (p75NTR) expression via nitric oxide

Brain injuries by physical trauma, epileptic seizures, or microbial infection upset the osmotic homeostasis resulting in cell swelling (cerebral edema), inflammation, and apoptosis. Expression of the neurotrophin receptor p75NTR is increased in the injured tissue and axon regeneration is repressed by the Nogo receptor using p75NTR as the signal transducer. Hence, p75NTR seems central to the injury response and we wished to determine the signals that regulate its expression. Here, we demonstrate that tonicity mediated cell swelling rapidly activates transcription of the endogenous p75NTR gene and of a p75NTR promoter-reporter gene in various cell types. Transcription activation is independent of de novo protein synthesis and requires the activities of phospholipase C, protein kinase C, and nitric-oxide synthase. Hence, p75NTR is a nitric oxide effector gene regulated by osmotic swelling, thereby providing a strategy for therapeutic intervention to modulate p75NTR functions following injury.

Robust neural integration from retinal transplants in mice deficient in GFAP and vimentin

With recent progress in neuroscience and stem-cell research, neural transplantation has emerged as a promising therapy for treating CNS diseases. The success of transplantation has been limited, however, by the restricted ability of neural implants to survive and establish neuronal connections with the host. Little is known about the mechanisms responsible for this failure. Neural implantation triggers reactive gliosis, a process accompanied by upregulation of intermediate filaments in astrocytes and formation of astroglial scar tissue. Here we show that the retinas of adult mice deficient in glial fibrillary acidic protein and vimentin, and consequently lacking intermediate filaments in reactive astrocytes and Müller cells, provide a permissive environment for grafted neurons to migrate and extend neurites. The transplanted cells integrated robustly into the host retina with distinct neuronal identity and appropriate neuronal projections. Our results indicate an essential role for reactive astroglial cells in preventing neural graft integration after transplantation.

Nogo-C is sufficient to delay nerve regeneration

Axonal regeneration succeeds in the peripheral but not central nervous system of adult mammals. Peripheral clearance of myelin coupled with selective CNS expression of axon growth inhibitors, such as Nogo, may account for this reparative disparity. To assess the sufficiency of Nogo for limiting axonal regeneration, we generated transgenic mice expressing Nogo-C in peripheral Schwann cells. Nogo-C includes the panisoform inhibitory Nogo-66 domain, but not a second Nogo-A-specific inhibitory domain, allowing a selective consideration of the Nogo-66 region. The oct-6::nogo-c transgenic mice regenerate axons less rapidly than do wild-type mice after mid-thigh sciatic nerve crush. The delayed axonal regeneration is associated with a decreased recovery rate for motor function after sciatic nerve injury. Thus, expression of the Nogo-66 domain by otherwise permissive myelinating cells is sufficient to hinder axonal reextension after trauma.

Endogenous neurotrophic factors enhance neurite growth by bag cell neurons of Aplysia

Mechanisms that regulate neurite outgrowth are phylogenetically conserved, including the signaling molecules involved. Here, we describe neurotrophic effects on isolated bag cell neurons (BCNs) of substrate-bound growth factors endogenous to the sea slug Aplysia californica. Sheath cells dissociated from the pleural-visceral connectives of the Aplysia CNS and arterial cells dissociated from the anterior aorta enhance neurite outgrowth when compared to controls, i.e., BCNs grown in defined medium alone. In addition, the substrate remaining after sheath cells or arterial cells are killed significantly enhances growth, relative to all other conditions tested. For instance, primary neurites are more numerous and greater in length for BCNs cultured on substrate produced by arterial cells. These results suggest that sheath and arterial cells produce growth-promoting factors, some of which are found in the substrates produced by these cell types. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), we found that Aplysia collagen-like peptides are produced by dissociated arterial cells, and therefore likely contribute to the observed growth effects. Collagen-like peptides and other factors produced by sheath and arterial cells likely influence neurite growth in the Aplysia CNS during development, learning and memory, and regeneration after injury.

A role for semaphorins and neuropilins in oligodendrocyte guidance

Oligodendrocytes develop in defined CNS regions as progenitor cells, which migrate to their final destinations, encountering soluble and membrane-bound signals that influence their differentiation and potential to myelinate axonal projections. To identify the regulatory genes that may be involved in this process, microarray analysis of developing oligodendroglia was performed. Several neural guidance genes, including members of the neuropilin (NP) and semaphorin families were detected. These findings were verified and expanded upon using RT-PCR with RNA from fluorescent activated cell sorted A2B5+ oligodendrocyte progenitors and O4+ pro-oligodendrocytes isolated from in vitro and in vivo sources. RT-PCR, western and immunocytochemical analyses revealed that oligodendrocytes expressed NP1, several alternatively spliced isoforms of NP2, and a broad spectrum of both soluble (Class 3), membrane-spanning (Class 4-6), and membrane-tethered (Class 7) semaphorin ligands. Class 3 semaphorins, in a modified stripe assay, caused the collapse of oligodendrocyte progenitor growth cones, redirection of processes, and altered progenitor migration. Our data support a role for neuropilins and semaphorins in orchestrating the migration patterns of developing oligodendrocytes in the CNS.

Control of growth cone motility and morphology by LIM kinase and Slingshot via phosphorylation and dephosphorylation of cofilin

Growth cone motility and morphology are based on actin-filament dynamics. Cofilin plays an essential role for the rapid turnover of actin filaments by severing and depolymerizing them. The activity of cofilin is repressed by phosphorylation at Ser3 by LIM kinase (LIMK, in which LIM is an acronym of the three gene products Lin-11, Isl-1, and Mec-3) and is reactivated by dephosphorylation by phosphatases, termed Slingshot (SSH). We investigated the roles of cofilin, LIMK, and SSH in the growth cone motility and morphology and neurite extension by expressing fluorescence protein-labeled cofilin, LIMK1, SSH1, or their mutants in chick dorsal root ganglion (DRG) neurons and then monitoring live images of growth cones by time-lapse video fluorescence microscopy. The expression of LIMK1 remarkably repressed growth cone motility and neurite extension, whereas the expression of SSH1 or a nonphosphorylatable S3A mutant of cofilin enhanced these events. The fan-like shape of growth cones was disorganized by the expression of any of these proteins. The repressive effects on growth cone behavior by LIMK1 expression were significantly rescued by the coexpression of S3A-cofilin or SSH1. These findings suggest that LIMK1 and SSH1 play critical roles in controlling growth cone motility and morphology and neurite extension by regulating the activity of cofilin and may be involved in signaling pathways that regulate stimulus-induced growth cone guidance. Using various mutants of cofilin, we also obtained evidence that the actin-filament-severing activity of cofilin is critical for growth cone motility and neurite extension.

Axon regeneration in young adult mice lacking Nogo-A/B

After injury, axons of the adult mammalian brain and spinal cord exhibit little regeneration. It has been suggested that axon growth inhibitors, such as myelin-derived Nogo, prevent CNS axon repair. To investigate this hypothesis, we analyzed mice with a nogo mutation that eliminates Nogo-A/B expression. These mice are viable and exhibit normal locomotion. Corticospinal tract tracing reveals no abnormality in uninjured nogo-A/B(-/-) mice. After spinal cord injury, corticospinal axons of young adult nogo-A/B(-/-) mice sprout extensively rostral to a transection. Numerous fibers regenerate into distal cord segments of nogo-A/B(-/-) mice. Recovery of locomotor function is improved in these mice. Thus, Nogo-A plays a role in restricting axonal sprouting in the young adult CNS after injury.

Nogo (Reticulon 4) expression in innervated and denervated mouse skeletal muscle

The nogo gene encodes at least three different proteins, which share a high C-terminal homology with other members of the Reticulon family. Nogo (Reticulon 4) expression has been studied in innervated and denervated mouse hind-limb and hemidiaphragm muscles. A common Nogo A, B, and C probe hybridized to three transcripts, in accordance with human and rat data. Denervation caused decreased Nogo C and increased Nogo A mRNA expression, while Nogo B was not substantially altered. Western blots and immunohistochemistry confirmed the presence of Nogo A-like and Nogo B-like immunoreactivity in muscle. Nogo A-like immunoreactivity increased after denervation and was also present in intramuscular nerves in both innervated and denervated muscle. Nogo B-like immunoreactivity was observed in connective tissue surrounding muscle fibres and nerves. The different Nogo transcripts are produced by both alternative splicing (A and B) and alternative promoter usage (C); both mechanisms seem to be under neural control in skeletal muscle.

Promotion of axonal regeneration in the injured CNS

Molecules that are found in the extracellular environment at a CNS lesion site, or that are associated with myelin, inhibit axon growth. In addition, neuronal changes--such as an age-dependent reduction in concentrations of cyclic AMP--render the neuron less able to respond to axotomy by a rapid, forward, actin-dependent movement. An alternative mechanism, based on the protrusive forces generated by microtubule elongation or the anterograde transport of cytoskeletal elements, may underlie a slower form of axon elongation that happens during regeneration in the mature CNS. Therapeutic approaches that restore the extracellular CNS environment or the neuron's characteristics back to a more embryonic state increase axon regeneration and improve functional recovery after injury. These advances in the understanding of regeneration in the CNS have major implications for neurorehabilitation and for the use of axonal regeneration as a therapeutic approach to disorders of the CNS such as spinal-cord injury.

The Rho/ROCK pathway mediates neurite growth-inhibitory activity associated with the chondroitin sulfate proteoglycans of the CNS glial scar

Axons fail to regenerate in the central nervous system after injury. Chondroitin sulfate proteoglycans (CSPG) expressed in the scar significantly contribute to the nonpermissive properties of the central nervous system environment. To examine the inhibitory activity of a CSPG mixture on retina ganglion cell (RGC) axon growth, we employed both a stripe assay and a nerve fiber outgrowth assay. We show that the inhibition exerted by CSPGs in vitro can be blocked by application of either C3 transferase, a specific inhibitor of the Rho GTPase, or Y27632, a specific inhibitor of the Rho kinase. These results demonstrate that CSPG-associated inhibition of neurite outgrowth is mediated by the Rho/ROCK signaling pathway. Consistent with these results, we found that retina ganglion cell axon growth on glial scar tissue was enhanced in the presence of C3 transferase and Y27632, respectively. In addition, we show that the recently identified inhibitory CSPG Te38 is upregulated in the lesioned spinal cord.

Suppression of Rho-kinase activity promotes axonal growth on inhibitory CNS substrates

Several molecules inhibit axonal growth cones and may account for the failure of central nervous system regeneration, including myelin proteins and various chondroitan sulfate proteoglycans expressed at the site of injury. Axonal growth inhibition by myelin and chondroitan sulfate proteoglycans may in part be controlled by Rho-GTPase, which mediates growth cone collapse. Here, we tested in vitro whether pharmacological inhibition of a major downstream effector of Rho, Rho-kinase, promotes axonal outgrowth from dorsal root ganglia grown on aggrecan. Aggrecan substrates stimulated Rho activity and were inhibitory to axonal growth. Y-27632 treatment promoted the growth of axons by 5- to 10-fold and induced "steamlined" growth cones with longer filopodia and smaller lamellipodia. Interestingly, more actin bundles reminiscent of stress fibers in the central domain of the growth cone were observed when grown on aggrecan compared to laminin. In addition, Y-27632 significantly promoted axonal growth on both myelin and adult rat spinal cord cryosections. Our data suggest that suppression of Rho-kinase activity may enhance axonal regeneration in the central nervous system.

Olfactory ensheathing glia transplantation: a therapy to promote repair in the mammalian central nervous system

A therapy to treat injuries to the central nervous system (CNS) is, to date, a major clinical challenge. The devastating functional consequences they cause in human patients have encouraged many scientists to search, in animal models, for a repair strategy that could, in the future, be applied to humans. However, although several experimental approaches have obtained some degree of success, very few have been translated into clinical trials. Traumatic and demyelinating lesions of the spinal cord have attracted several groups with the same aim: to find a way to promote axonal regeneration, remyelination, and functional recovery, by using a simple, safe, effective, and viable procedure. During the past decade, olfactory ensheathing glia (OEG) transplantation has emerged as a very promising experimental therapy to promote repair of spinal cords, after different types of injuries. Transplants of these cells promoted axonal regeneration and functional recovery after partial and complete spinal cord lesions. Moreover, olfactory ensheathing glia were able to form myelin sheaths around demyelinated axons. In this article, we review these recent advances and discuss to what extent olfactory ensheathing glia transplantation might have a future as a therapy for different spinal cord affections in humans.

The axonal chemorepellant semaphorin 3A is increased in the cerebellum in schizophrenia and may contribute to its synaptic pathology

The neuropathological features of schizophrenia are suggestive of a developmentally induced impairment of synaptic connectivity. Semaphorin 3A (sema3A) might contribute to this process because it is a secreted chemorepellant which regulates axonal guidance. We have investigated sema3A in the cerebellum (an area in which expression persists in adulthood), and measured its abundance in 16 patients with schizophrenia and 16 controls. In adults, sema3A was predominantly localized to the inner part of the molecular layer neuropil, whereas infants and rats showed greater labelling of Purkinje cell bodies. Sema3A was increased in schizophrenia, as shown by enzyme-linked immunosorbent assay (+28%; P<0.05) and immunohistochemistry (+45%; P<0.01). We also measured reelin mRNA, since reelin is involved in related developmental processes and is decreased in other brain regions in schizophrenia. Reelin mRNA showed a trend reduction in the subjects with schizophrenia (-26%; P=0.07) and, notably, was negatively correlated with sema3A. Sema3A also correlated negatively with synaptophysin and complexin II mRNAs. The results show that sema3A is elevated in schizophrenia, and is associated with downregulation of genes involved in synaptic formation and maintenance. In this respect, sema3A appears to contribute to the synaptic pathology of schizophrenia, perhaps via ongoing effects of persistent sema3A elevation on synaptic plasticity. The findings are consistent with an early neurodevelopmental origin for the disorder, and the reciprocal changes in sema3A and reelin may be indicative of a pathogenic mechanism that affects the balance between trophic and inhibitory factors regulating synaptogenesis.

Inhibitors of neuronal regeneration: mediators and signaling mechanisms

Neuritogenesis and its inhibition are opposite and balancing processes during development as well as pathological states of adult neuron. In particular, the inability of adult central nervous system (CNS) neurons to regenerate upon injury has been attributed to both a lack of neuritogenic ability and the presence of neuronal growth inhibitors in the CNS environment. I review here recent progress in our understanding of neuritogenic inhibitors, with particular emphasis on those with a role in the inhibition of neuronal regeneration in the CNS, their signaling cascades and signal mediators. Neurotrophines acting through the tropomyosin-related kinase (Trk) family and p75 receptors promote neuritogenesis, which appears to require sustained activation of the mitogen activated protein (MAP) kinase pathway, and/or the activation of phosphotidylinositol 3-kinase (PI3 kinase). During development, a plethora of guidance factors and their receptors navigate the growing axon. However, much remained to be learned about the signaling receptors and pathways that mediate the activity of inhibitors of CNS regeneration. There is growing evidence that neuronal guidance molecules, particularly semaphorins, may also have a role as inhibitors of CNS regeneration. Although direct links have not yet been established in many cases, signals from these agents may ultimately converge upon the modulators and effectors of the Rho-family GTPases. Rho-family GTPases and their effectors modulate the activities of actin modifying molecules such as cofilin and profilin, resulting in cytoskeletal changes associated with growth cone extension or retraction.

A p75(NTR) and Nogo receptor complex mediates repulsive signaling by myelin-associated glycoprotein

Myelin-associated glycoprotein (MAG), an inhibitor of axon regeneration, binds with high affinity to the Nogo-66 receptor (NgR). Here we report that the p75 neurotrophin receptor (p75(NTR)) is a co-receptor of NgR for MAG signaling. In cultured human embryonic kidney (HEK) cells expressing NgR, p75(NTR) was required for MAG-induced intracellular Ca2+ elevation. Co-immunoprecipitation showed an association of NgR with p75(NTR) that can be disrupted by an antibody against p75(NTR) (NGFR5), and extensive coexpression was observed in the developing rat nervous system. Furthermore, NGFR5 abolished MAG-induced repulsive turning of Xenopus axonal growth cones and Ca2+ elevation, both in neurons and in NgR/p75(NTR)-expressing HEK cells. Thus we conclude that p75(NTR) is a co-receptor of NgR for MAG signaling and a potential therapeutic target for promoting nerve regeneration.

Myelin-associated glycoprotein as a functional ligand for the Nogo-66 receptor

Axonal regeneration in the adult central nervous system (CNS) is limited by two proteins in myelin, Nogo and myelin-associated glycoprotein (MAG). The receptor for Nogo (NgR) has been identified as an axonal glycosyl-phosphatidyl-inositol (GPI)-anchored protein, whereas the MAG receptor has remained elusive. Here, we show that MAG binds directly, with high affinity, to NgR. Cleavage of GPI-linked proteins from axons protects growth cones from MAG-induced collapse, and dominant-negative NgR eliminates MAG inhibition of neurite outgrowth. MAG-resistant embryonic neurons are rendered MAG-sensitive by expression of NgR. MAG and Nogo-66 activate NgR independently and serve as redundant NgR ligands that may limit axonal regeneration after CNS injury.

Expression of Nogo protein by growing axons in the developing nervous system

We produced monoclonal antibody NG1 that strongly binds growing axons in the developing nervous system of mice. This antibody intensely labeled the growth cone of cultured neurons. Although these immunostaining patterns suggested the association of growing axons with the antigen recognized by this antibody, the antigen was identified as Nogo protein, an axonal repulsive factor isolated from the myelin. On the basis of this unexpected finding, we discuss the possible functions of Nogo in the developing nervous system.

Localization of Nogo-A and Nogo-66 receptor proteins at sites of axon-myelin and synaptic contact

Axon regeneration in the adult CNS is limited by the presence of inhibitory proteins. An interaction of Nogo on the oligodendrocyte surface with Nogo-66 Receptor (NgR) on axons has been suggested to play an important role in limiting axonal growth. Here, we compare the localization of these two proteins immunohistochemically as a test of this hypothesis. Throughout much of the adult CNS, Nogo-A is detected on oligodendrocyte processes surrounding myelinated axons, including areas of axon-oligodendrocyte contact. The NgR protein is detected selectively in neurons and is present throughout axons, indicating that Nogo-A and its receptor are juxtaposed along the course of myelinated fibers. NgR protein expression is restricted to postnatal neurons and their axons. In contrast, Nogo-A is observed in myelinating oligodendrocytes, embryonic muscle, and neurons, suggesting that Nogo-A has additional physiologic roles unrelated to NgR binding. After spinal cord injury, Nogo-A is upregulated to a moderate degree, whereas NgR levels are maintained at constant levels. Taken together, these data confirm the apposition of Nogo ligand and NgR receptor in situations of limited axonal regeneration and support the hypothesis that this system regulates CNS axonal plasticity and recovery from injury.

Involvement of Cajal-Retzius cells in robust and layer-specific regeneration of the entorhino-hippocampal pathways

Severed adult CNS axons can extend over long distances when a permissive 'milieu', such as grafted Schwann cells or ensheathing cells, is provided. Moreover, functional blocking of endogenous inhibitory factors, such as Nogo-A or proteoglycans, enhances the regeneration of axotomized neurons. Here we examine whether guidance cues available during the development of axonal pathways could also potentiate the regeneration of lesioned adult circuits. The Cajal-Retzius cells in the hippocampus are transient pioneer neurons that guide entorhino-hippocampal afferents to their target layers. By using an in vitro model of axotomy of the entorhino-hippocampal pathway we show that Cajal-Retzius cells triggered the regeneration of the axotomized entorhino-hippocampal pathway. Furthermore, the regrowth induced by Cajal-Retzius cells was robust and its pattern was indistinguishable from that of the unlesioned entorhino-hippocampal pathway. Thus, regenerating axons regrew in a layer-specific fashion towards the appropriate target layers, making synaptic contacts with target pyramidal neurons. Interestingly, the ability of lesioned entorhinal axons to regrow was maintained for at least 9 days after axotomy. These results show that the growth-promoting cells controlling the development of neural circuits will be a relevant approach to promoting the regeneration of lesioned adult CNS pathways.

Inhibition of axon growth by oligodendrocyte precursor cells

The glial scar that forms at the site of injury is thought to be a biochemical and physical barrier to successful regeneration, although the molecules responsible for this barrier function are not well understood. Glia scars contain large numbers of oligodendrocyte precursor cells (OPCs) and these cells can produce several different growth-inhibitory chondroitin sulfate proteoglycans (CSPGs), including NG2, neurocan, and phosphacan. Here, we used membrane-based assays to show that the surface of OPCs is both nonpermissive and inhibitory for neurite outgrowth. Inhibition of growth by OPC is reversed by treatment with antibodies against the NG2 CSPG and the expression of NG2 is sufficient to change a growth-permissive cell surface to a nonpermissive surface. These result suggest that the OPCs that accumulate rapidly at sites of CNS injury can contribute to the creation of an environment that inhibits nerve regeneration and that NG2 is a necessary feature of that environment.

Cytokines and neurotrophic factors fail to affect Nogo-A mRNA expression in differentiated human neurones: implications for inflammation-related axonal regeneration in the central nervous system

Nogo is a novel myelin-associated inhibitor of neurite outgrowth which regulates stable neuronal connections during axonal regeneration following injury in the adult mammalian central nervous system (CNS). Because cytokines and neurotrophic factors play a key role in inflammation-related axonal regeneration, we investigated: (i) the constitutive expression of Nogo and the Nogo receptor (NgR) mRNA in human neural cell lines; (ii) Nogo and NgR mRNA levels in the NTera2 human teratocarcinoma cell line during retinoic acid (RA)-induced neuronal differentiation; and (iii) their regulation in NTera2-derived differentiated neurones (NTera2-N) after exposure to a battery of cytokines and growth factors potentially produced by activated glial cells at post-traumatic inflammatory lesions in the CNS. By reverse transcriptase-polymerase chain reaction analysis, the constitutive expression of Nogo-A, the longest isoform of three distinct Nogo transcripts and NgR mRNA was identified in a wide variety of human neural and non-neural cell lines. By Northern blot analysis, the levels of Nogo-A mRNA were elevated markedly in NTera2 cells following RA-induced neuronal differentiation, accompanied by an increased expression of the neurite growth-associated protein GAP-43 mRNA. In contrast, Nogo-A, Nogo-B, NgR and GAP-43 mRNA levels were unaltered in NTera2-N cells by exposure to basic fibroblast growth factor, brain-derived neurotrophic factor, glia-derived neurotrophic factor, tumour necrosis factor-alpha, interleukin-1beta, dibutyryl cyclic AMP or phorbol 12-myristate 13-acetate. These results indicate that both Nogo-A and NgR mRNA are coexpressed in various human cell types, including differentiated neurones, where their expression is unaffected by exposure to a panel of cytokines and neurotrophic factors which might be involved in inflammation-related axonal regeneration in the CNS.

Nogos and the Nogo-66 receptor: factors inhibiting CNS neuron regeneration

The recently cloned gene Nogo, whose alternative splice products correspond to the antigenic target of the central nervous system (CNS) regeneration enhancing monoclonal antibody IN-1, codes for membrane proteins enriched in brain, particularly in oligodendrocytes. The 66-amino acid extracellular domain of Nogo (Nogo-66) interacts with a high-affinity receptor (NgR), a glycosylphosphatidylinositol (GPI)-linked protein with multiple leucine-rich repeats. The amino terminal cytoplasmic domain of Nogo appears to have a general cellular growth inhibitory effect. Nogo-66, on the other hand, specifically retards neurite outgrowth and induces growth cone collapse, possibly through its interaction with NgR and as yet unidentified transmembrane coreceptors. Recent results also suggest that Nogo expression may induce apoptosis in tumor cells. Together, these proteins provide new molecular handles for the design of therapeutic interventions for CNS injuries and neurodegenerative diseases, as well as possible leads to anticancer strategies.

Central and peripheral nerve regeneration by transplantation of Schwann cells and transdifferentiated bone marrow stromal cells

In contrast to the peripheral nervous system (PNS), little structural and functional regeneration of the central nervous system (CNS) occurs spontaneously following injury in adult mammals. The inability of the CNS to regenerate is mainly attributed to its own inhibitorial environment such as glial scar formation and the myelin sheath of oligodendrocytes. Therefore, one of the strategies to promote axonal regeneration of the CNS is to experimentally modify the environment to be similar to that of the PNS. Schwann cells are the myelinating glial cells in the PNS, and are known to play a key role in Wallerian degeneration and subsequent regeneration. Central nervous system regeneration can be elicited by Schwann cell transplantation, which provides a suitable environment for regeneration. The underlying cellular mechanism of regeneration is based upon the cooperative interactions between axons and Schwann cells involving the production of neurotrophic factors and other related molecules. Furthermore, tight and gap junctional contact between the axon and Schwann cell also mediates the molecular interaction and linking. In this review, the role of the Schwann cell during the regeneration of the sciatic (representing the PNS) and optic (representing the CNS) nerves is explained. In addition, the possibility of optic nerve reconstruction by an artificial graft of Schwann cells is also described. Finally, the application of cells not of neuronal lineage, such as bone marrow stromal cells (MSCs), in nerve regeneration is proposed. Marrow stromal cells are known as multipotential stem cells that, under specific conditions, differentiate into several kinds of cells. The strategy to transdifferentiate MSCs into the cells with a Schwann cell phenotype and the induction of sciatic and optic nerve regeneration are described.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Novel peptide-based biomaterial scaffolds for tissue engineering

Biomaterial scaffolds are components of cell-laden artificial tissues and transplantable biosensors. Some of the most promising new synthetic biomaterial scaffolds are composed of self-assembling peptides that can be modified to contain biologically active motifs. Peptide-based biomaterials can be fabricated to form two- and three-dimensional structures. Recent studies show that biomaterial promotion of multi-dimensional cell-cell interactions and cell density are crucial for proper cellular differentiation and for subsequent tissue formation. Other refinements in tissue engineering include the use of stem cells, cell pre-selection and growth factor pre-treatment of cells that are used for seeding scaffolds. These cell-culture technologies, combined with improved processes for defining the dimensions of peptide-based scaffolds, might lead to further improvements in tissue engineering. Novel peptide-based biomaterial scaffolds seeded with cells show promise for tissue repair and for other medical applications.

Vaccination with a Nogo-A-derived peptide after incomplete spinal-cord injury promotes recovery via a T-cell-mediated neuroprotective response: comparison with other myelin antigens

The myelin-associated protein Nogo-A has received more research attention than any other inhibitor of axonal regeneration in the injured central nervous system (CNS). Circumvention of its inhibitory effect, by using antibodies specific to Nogo-A, has been shown to promote axonal regrowth. Studies in our laboratory have demonstrated that active or passive immunization of CNS-injured rats or mice with myelin-associated peptides induces a T-cell-mediated protective autoimmune response, which promotes recovery by reducing posttraumatic degeneration. Here, we show that neuronal degeneration after incomplete spinal-cord contusion in rats was substantially reduced, and hence recovery was significantly promoted, by posttraumatic immunization with p472, a peptide derived from Nogo-A. The observed effect seemed to be mediated by T cells and could be reproduced by passive transfer of a T cell line directed against the Nogo-A peptide. Thus, it seems that after incomplete spinal-cord injury, immunization with a variety of myelin-associated peptides, including those derived from Nogo-A, can be used to evoke a T cell-mediated response that promotes recovery. The choice of peptide(s) for clinical treatment of spinal-cord injuries should be based on safety considerations; in particular, the likelihood that the chosen peptide will not cause an autoimmune disease or interfere with essential functions of this peptide or other proteins. From a therapeutic point of view, the fact that the active cellular agents are T cells rather than antibodies is an advantage, as T cell production commences within the time window required for a protective effect after spinal-cord injury, whereas antibody production takes longer.

Analysis of gene expression following sciatic nerve crush and spinal cord hemisection in the mouse by microarray expression profiling

1. The responses of periphery (PNS) and central nervous systems (CNS) towards nerve injury are different: while injured mammalian periphery nerons can successfully undergo regeneration, axons in the central nervous system are usually not able to regenerate. 2. In the present study, the genes which were differentially expressed in the PNS and CNS following nerve injury were identified and compared by microarray profiling techniques. 3. Sciatic nerve crush and hemisection of the spinal cord of adult mice were used as the models for nerve injury in PNS and CNS respectivey. 4. It was found that of all the genes examined, 14% (80/588) showed changes in expression following either PNS or CNS injury, and only 3% (18/588) showed changes in both types of injuries. 5. Among all the differentially expressed genes, only 8% (6/80) exhibited similar changes in gene expression (either up- or down-regulation) following injury in both PNS and CNS nerve injuries. 6. Our results indicated that microarray expression profiling is an efficient and useful method to identify genes that are involved in the regeneration process following nerve injuries, and several genes which are differentially expressed in the PNS and/or CNS following nerve injuries were identified in the present study.