New roles for old proteins in adult CNS axonal regeneration

Enhancing structural plasticity of PC12 neurons during differentiation and neurite regeneration with a catalytically inactive mutant version of the zRICH protein

BACKGROUND

Studies of the molecular mechanisms of nerve regeneration have led to the discovery of several proteins that are induced during successful nerve regeneration. RICH proteins were identified as proteins induced during the regeneration of the optic nerve of teleost fish. These proteins are 2',3'-cyclic nucleotide, 3'-phosphodiesterases that can bind to cellular membranes through a carboxy-terminal membrane localization domain. They interact with the tubulin cytoskeleton and are able to enhance neuronal structural plasticity by promoting the formation of neurite branches.

RESULTS

PC12 stable transfectant cells expressing a fusion protein combining a red fluorescent protein with a catalytically inactive mutant version of zebrafish RICH protein were generated. These cells were used as a model to analyze effects of the protein on neuritogenesis. Differentiation experiments showed a 2.9 fold increase in formation of secondary neurites and a 2.4 fold increase in branching points. A 2.2 fold increase in formation of secondary neurites was observed in neurite regeneration assays.

CONCLUSIONS

The use of a fluorescent fusion protein facilitated detection of expression levels. Two computer-assisted morphometric analysis methods indicated that the catalytically inactive RICH protein induced the formation of branching points and secondary neurites both during differentiation and neurite regeneration. A procedure based on analysis of random field images provided comparable results to classic neurite tracing methods.

Inhibition of Sema-3A Promotes Cell Migration, Axonal Growth, and Retinal Ganglion Cell Survival

Purpose

Semaphorin 3A (Sema-3A) is a secreted protein that deflects axons from inappropriate regions and induces neuronal cell death. Intravitreal application of polyclonal antibodies against Sema-3A prevents loss of retinal ganglion cells ensuing from axotomy of optic nerves. This suggested a therapeutic approach for neuroprotection via inhibition of the Sema-3A pathway.

Methods

To develop potent and specific Sema-3A antagonists, we isolated monoclonal anti-Sema-3A antibodies from a human antibody phage display library and optimized low-molecular weight Sema-3A signaling inhibitors. The best inhibitors were identified using in vitro scratch assays and semiquantitative repulsion assays.

Results

A therapeutic approach for neuroprotection must have a long duration of action. Therefore, antibodies and low-molecular weight inhibitors were formulated in extruded implants to allow controlled and prolonged release. Following release from the implants, Sema-3A inhibitors antagonized Sema-3A effects in scratch and repulsion assays and protected retinal ganglion cells in animal models of optic nerve injury, retinal ischemia, and glaucoma.

Conclusions and Translational Relevance

Collectively, our findings indicate that the identified Sema-3A inhibitors should be further evaluated as therapeutic candidates for the treatment of Sema-3A-driven central nervous system degenerative processes.

The Mechanisms of Peripheral Nerve Preconditioning Injury on Promoting Axonal Regeneration

Two major factors contribute to the failure of axonal regrowth in the central nervous system (CNS), namely, the neuronal intrinsic regenerative capacity and the extrinsic local inhibitory microenvironments. However, a preconditioning peripheral nerve lesion could substantially enhance the regeneration of central axons following a subsequent spinal cord injury. In the present review, we summarize the molecular mechanisms of the preconditioning injury effect on promoting axonal regeneration. The injury signal transduction resulting from preconditioning peripheral nerve injury regulates the RAG expression to enhance axonal regeneration. Importantly, preconditioning peripheral nerve injury triggers interactions between neurons and nonneuronal cells to amplify and maintain their effects. Additionally, the preconditioning injury impacts mitochondria, protein, and lipid synthesis. All these coordinated changes endow axonal regeneration.

Schwann Cell Transplantation Improves Reticulospinal Axon Growth and Forelimb Strength after Severe Cervical Spinal Cord Contusion

Schwann cell (SC) implantation alone has been shown to promote the growth of propriospinal and sensory axons, but not long-tract descending axons, after thoracic spinal cord injury (SCI). In the current study, we examined if an axotomy close to the cell body of origin (so as to enhance the intrinsic growth response) could permit supraspinal axons to grow onto SC grafts. Adult female Fischer rats received a severe (C5) cervical contusion (1.1 mm displacement, 3 KDyn). At 1 week postinjury, 2 million SCs ex vivo transduced with lentiviral vector encoding enhanced green fluorescent protein (EGFP) were implanted within media into the injury epicenter; injury-only animals served as controls. Animals were tested weekly using the BBB score for 7 weeks postimplantation and received at end point tests for upper body strength: self-supported forelimb hanging, forearm grip force, and the incline plane. Following behavioral assessment, animals were anterogradely traced bilaterally from the reticular formation using BDA-Texas Red. Stereological quantification revealed a twofold increase in the numbers of preserved NeuN+ neurons rostral and caudal to the injury/graft site in SC implanted animals, corroborating previous reports of their neuroprotective efficacy. Examination of labeled reticulospinal axon growth revealed that while rarely an axon was present within the lesion site of injury-only controls, numerous reticulospinal axons had penetrated the SC implant/lesion milieu. This has not been observed following implantation of SCs alone into the injured thoracic spinal cord. Significant behavioral improvements over injury-only controls in upper limb strength, including an enhanced grip strength (a 296% increase) and an increased self-supported forelimb hanging, accompanied SC-mediated neuroprotection and reticulospinal axon growth. The current study further supports the neuroprotective efficacy of SC implants after SCI and demonstrates that SCs alone are capable of supporting modest supraspinal axon growth when the site of axon injury is closer to the cell body of the axotomized neuron.

Knockdown of Nogo gene by short hairpin RNA interference promotes functional recovery of spinal cord injury in a rat model

The specific myelin component Nogo protein is one of the major inhibitory molecules of spinal cord axonal outgrowth following spinal cord injury. The present study aimed to investigate the effects of silencing Nogo protein with shRNA interference on the promotion of functional recovery in a rat model with spinal cord hemisection. Nogo-A short hairpin RNAs (Nogo shRNAs) were constructed and transfected into rats with spinal cord hemisection by adenovirus-mediated transfection. Reverse transcription‑polymerase chain reaction and western blotting were performed to analyze the expression of Nogo-A and Growth Associated Protein 43 (GAP-43). In addition, Basso Beattie Bresnahan (BBB) scores were used to assess the functional recovery of rats following spinal cord injury. The results demonstrated that expression of the Nogo‑A gene was observed to be downregulated following transfection and GAP‑43 expression was observed to increase. The BBB scores were increased following treatment with Nogo shRNAs, indicating functional recovery of the injured nerves. Thus, Nogo-A shRNA interference can knockdown Nogo gene expression and upregulate GAP-43 to promote the functional recovery of spinal cord injury in rats. This finding may advance progress toward assisting the regeneration of injured neurons through the use of Nogo-A shRNA.

Canadian Association of Neuroscience Review: Axonal Regeneration in the Peripheral and Central Nervous Systems – Current Issues and Advances

Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS.

Cortical PKC inhibition promotes axonal regeneration of the corticospinal tract and forelimb functional recovery after cervical dorsal spinal hemisection in adult rats

Our previous study shows that conventional protein kinases C (cPKCs) are key signaling mediators that are activated by extracellular inhibitory molecules. Inhibition of cPKC by intrathecal infusion of a cPKC inhibitor, GÖ6976, into the site of dorsal hemisection (DH) induces regeneration of lesioned dorsal column sensory, but not corticospinal tract (CST), axons. Here, we investigated whether a direct cortical delivery of GÖ6976 into the soma of corticospinal neurons promotes regeneration of CST and the recovery of forelimb function in rats with cervical spinal cord injuries. We report that cortical delivery of GÖ6976 reduced injury-induced activation of conventional PKCα and PKCβ1 in CST neurons, promoted regeneration of CST axons through and beyond a cervical DH at C4, formed new synapses on target neurons caudal to the injury, and enhanced forelimb functional recovery in adult rats. When combined with lenti-Chondroitinase ABC treatment, cortical administration of GÖ6976 promoted even greater CST axonal regeneration and recovery of forelimb function. Thus, this study has demonstrated a novel strategy that can promote anatomical regeneration of damaged CST axons and partial recovery of forelimb function. Importantly, such an effect is critically dependent on the efficient blockage of injury-induced PKC activation in the soma of layer V CST neurons.

zRICH, a protein induced during optic nerve regeneration in zebrafish, promotes neuritogenesis and interacts with tubulin

Mammals do not regenerate axons in their central nervous system (CNS) spontaneously. This phenomenon is the cause of numerous medical conditions after damage to nerve fibers in the CNS of humans. The study of the mechanisms of nerve regeneration in other vertebrate animals able to spontaneously regenerate axons in their CNS is essential for understanding nerve regeneration from a scientific point of view, and for developing therapeutic approaches to enhance nerve regeneration in the CNS of humans. RICH proteins are a novel group of proteins implicated in nerve regeneration in the CNS of teleost fish, yet their mechanisms of action are not well understood. A number of mutant versions of the zebrafish RICH (zRICH) protein were generated and characterized at biochemical and cellular levels in our laboratory. With the aim of understanding the effects of RICH proteins in neuronal axon outgrowth, stable transfectants derived from the neuronal model PC12 cell line expressing zRICH Wild-Type or mutant versions of zRICH were studied. Results from differentiation experiments suggest that RICH proteins enhance neuronal plasticity by facilitating neurite branching. Biochemical co-purification results have demonstrated that zRICH binds to the cytoskeletal protein tubulin. The central domain of the protein is sufficient for tubulin binding, but a mutant version of the protein lacking the terminal domains, which cannot bind to the plasma membrane, was not able to enhance neurite branching. RICH proteins may facilitate axon regeneration by regulating the axonal cytoskeleton and facilitating the formation of new neurite branches.

In vivo evidence that TRAF4 is required for central nervous system myelin homeostasis

Tumor Necrosis Factor Receptor-Associated Factors (TRAFs) are major signal transducers for the TNF and interleukin-1/Toll-like receptor superfamilies. However, TRAF4 does not fit the paradigm of TRAF function in immune and inflammatory responses. Its physiological and molecular functions remain poorly understood. Behavorial analyses show that TRAF4-deficient mice (TRAF4-KO) exhibit altered locomotion coordination typical of ataxia. TRAF4-KO central nervous system (CNS) ultrastructure shows strong myelin perturbation including disorganized layers and disturbances in paranode organization. TRAF4 was previously reported to be expressed by CNS neurons. Using primary cell culture, we now show that TRAF4 is also expressed by oligodendrocytes, at all stages of their differentiation. Moreover, histology and electron microscopy show degeneration of a high number of Purkinje cells in TRAF4-KO mice, that was confirmed by increased expression of the Bax pro-apoptotic marker (immunofluorescence), TUNEL analysis, and caspase-3 activation and PARP1 cleavage (western blotting). Consistent with this phenotype, MAG and NogoA, two myelin-induced neurite outgrowth inhibitors, and their neuron partners, NgR and p75NTR were overexpressed (Q-RT-PCR and western blotting). The strong increased phosphorylation of Rock2, a RhoA downstream target, indicated that the NgR/p75NTR/RhoA signaling pathway, known to induce actin cytoskeleton rearrangement that favors axon regeneration inhibition and neuron apoptosis, is activated in the absence of TRAF4 (western blotting). Altogether, these results provide conclusive evidence for the pivotal contribution of TRAF4 to myelination and to cerebellar homeostasis, and link the loss of TRAF4 function to demyelinating or neurodegenerative diseases.

Inducing alignment in astrocyte tissue constructs by surface ligands patterned on biomaterials

Planar substrates with patterned ligands were used to induce astrocyte alignment whereas substrates with uniform fields of ligand were used to produce random cell orientation. DRG neurons plated on top of oriented astrocyte monolayers exhibited directional outgrowth along aligned astrocytes, demonstrating that purely biological cues provided by the oriented astrocytes were sufficient to provide guidance cues. Antibody blocking studies demonstrated that astrocyte associated FN played a major mechanistic role in directing engineered neurite extension. Our results show that nanometer level surface cues are sufficient to direct nerve outgrowth through an intervening organized astrocyte cell layer. In other studies, we showed that patterned ligands were able to transmit organization cues through multiple cell layers to control the overall alignment of an astrocyte tissue construct, demonstrating how natural scar tissue may develop in situ into potent barriers. In such constructs the spatial organization of astrocyte derived FN maintained its organizational anisotropy throughout the thickness of multilayered astrocyte constructs. These in vitro studies suggest possible roles for such constructs as bridging substrates for neuroregenerative applications.

Polymer and nano-technology applications for repair and reconstruction of the central nervous system

The hydrophilic polymer PEG and its related derivatives, have served as therapeutic agents to reconstruct the phospholipid bilayers of damaged cell membranes by erasing defects in the plasmalemma. The special attributes of hydrophilic polymers when in contact with cell membranes have been used for several decades since these well-known properties have been exploited in the manufacture of monoclonal antibodies. However, while traditional therapeutic efforts to combat traumatic injuries of the central nervous system (CNS) have not been successful, nanotechnology-based drug delivery has become a new emerging strategy with the additional promise of targeted membrane repair. As such, this potential use of nanotechnology provides new avenues for nanomedicine that uses nanoparticles themselves as the therapeutic agent in addition to their other functionalities. Here we will specifically address new advances in experimental treatment of Spinal Cord and Traumatic Brain injury (SCI and TBI respectively). We focus on the concept of repair of the neurolemma and axolemma in the acute stage of injury, with less emphasis on the worthwhile, and voluminous, issues concerning regenerative medicine/nanomedicine. It is not that the two are mutually exclusive - they are not. However, the survival of the neuron and the tissues of white matter are critical to any further success in what will likely be a multi-component therapy for TBI and SCI. This review includes a brief explanation of the characteristics of traumatic spinal cord injury SCI, the biological basis of the injuries, and the treatment opportunities of current polymer-based therapies. In particular, we update our own progress in such applications for CNS injuries with various suggestions and discussion, primarily nanocarrier-based drug delivery systems. The application of nanoparticles as drug-delivery vehicles to the CNS may likely be advantageous over existing molecular-based therapies. As a "proof-of-concept", we will discuss the recent investigations that have preferentially facilitated repair and functional recovery from breaches in neural membranes via rapid sealing and reassembly of the compromised site with silica or chitosan nanoparticles.

Development of an in vitro model to evaluate the regenerative capacity of adult brain-derived tyrosine hydroxylase-expressing dopaminergic neurons

The loss of nigral dopaminergic (DA) neurons is the disease-defining pathological change responsible for progressive motor dysfunction in Parkinson's disease. In this study, we sought to establish a culture method for adult rat tyrosine hydroxylase (TH)-immunoreactive DA neurons. In this context, we investigated the role of fibroblast growth factor 2 (FGF2), brain-derived neurotrophic factor (BDNF), transforming growth factor-β3 (TGF-β3), glial-derived neurotrophic factor (GDNF) and dibutyryl-cyclic AMP (dbcAMP) in these cultures. Culturing in the presence of FGF2, BDNF and GDNF enhanced the survival of DA neurons by 15-fold and promoted neurite growth. In contrast, dbcAMP promoted neurite growth in all neurons but did not enhance DA cell survival. This study demonstrates that long-term cultures of DA neurons can be established from the mature rat brain and that survival and regeneration of DA neurons can be manipulated by epigenetic factors such as growth factors and intracellular cAMP pathways.

Functions of Nogo proteins and their receptors in the nervous system

The membrane protein Nogo-A was initially characterized as a CNS-specific inhibitor of axonal regeneration. Recent studies have uncovered regulatory roles of Nogo proteins and their receptors--in precursor migration, neurite growth and branching in the developing nervous system--as well as a growth-restricting function during CNS maturation. The function of Nogo in the adult CNS is now understood to be that of a negative regulator of neuronal growth, leading to stabilization of the CNS wiring at the expense of extensive plastic rearrangements and regeneration after injury. In addition, Nogo proteins interact with various intracellular components and may have roles in the regulation of endoplasmic reticulum (ER) structure, processing of amyloid precursor protein and cell survival.

Oligodendrocyte precursor cells differentially expressing Nogo-A but not MAG are more permissive to neurite outgrowth than mature oligodendrocytes

Grafting oligodendrocyte precursor cells (OPCs) has been used as a strategy to repair demyelination of the central nervous system (CNS). Whether OPCs can promote CNS axonal regeneration remains to be tested. If so, they should be permissive to axonal growth and may express less inhibitory molecules on their surface. Here we examined the expression of two oligodendrocyte-associated myelin inhibitors Nogo-A and myelin-associated glycoprotein (MAG) during oligodendrogliogenesis and tested their abilities to promote neurite outgrowth in vitro. Whereas the intracellular domain of Nogo-A was consistently expressed throughout oligodendrocyte differentiation, MAG was expressed only at later stages. Furthermore, the membrane-associated extracellular domain of Nogo-A was not expressed in OPCs but expressed in mature oligodendrocytes. In a dorsal root ganglion (DRG) and OPC/oligodendrocyte co-culture model, significantly greater DRG neurite outgrowth onto OPC monolayer than mature oligodendrocyte was found (1042+/-123 vs. 717+/-342 micrometer; p=0.011). Moreover, DRG neurites elongated as fasciculated fiber tracts and contacted directly on OPCs (133+/-37 cells/fascicle). In contrast, few, if any, direct contacts were found between DRG neurites and mature oligodendrocytes (5+/-3 cells/fascicle, p<0.001). In fact, acellular spaces were found between neurites and surrounding mature oligodendrocytes in contrast to the lack of such spaces in OPC/DRG coculture (51.1+/-16.5 vs. 2.4+/-3.9 micrometer; p<0.001). Thus, OPCs expressing neither extracellular domain of Nogo-A nor MAG are significantly more permissive than mature oligodendrocytes expressing both. Grafting OPCs may thus represent a feasible strategy to foster CNS axonal regeneration.

Effect of different regions of Nogo-A on the differentiation of neural progenitors

Nogo-A is an inhibitor of neurite outgrowth and axonal regeneration after CNS injury. Several functional regions including Nogo-66 were identified to mediate the inhibitory effect of Nogo-A. We have reported that Nogo-66 could promote neural progenitors to differentiate into glial cells. Here we exam three other regions of Nogo-A and show two of them also mediate the differentiation of neural progenitors. A 172-residues N-terminal region and a 37-residues C-terminal region of Nogo-A both could inhibit neuronal differentiation and promoted glial cell formation. This study illustrated that Nogo-A had multiple functional domains on the behavior of neuronal cells. The inhibitory effect of neural differentiation of Nogo-A may also contribute to its restraint of CNS repair.

Neurotrophic activity of extracts from distal stumps of pre-degenerated peripheral rat nerves varies according to molecular mass spectrum

OBJECTIVE

We investigated neurotrophic activity of extracts from pre-degenerated and non-pre-degenerated peripheral nerves (complete extracts and extracts with fractions of narrower range of molecular weight) on the injured hippocampus.

METHODS

The experiment was carried out on male Wistar C rats. The complete extracts or fractions with different ranges of molecular weight were introduced to the site of injury with the autologous connective tissue chambers. We examined DiI-labeled hippocampal cell and AChE-positive nerve endings to assess the regeneration intensity.

RESULTS

The highest number of labeled hippocampal cells was observed in the group treated with fraction of molecular weight 10-100 kDa (72.5 +/- 13.7) obtained from pre-degenerated nerves. We observed the presence of AChE-positive fibers inside all examined chambers.

DISCUSSION

These results demonstrate that suitable modification of CNS environments by introducing the protein fractions obtained from peripheral nerves can initiate the regeneration of the damaged hippocampal structure in adult rats. Moreover, it is possible to intensify their neurotrophic effect by former pre-degeneration of peripheral nerves and extraction from the entire extract proteins of molecular weight of 10-100 kDa.

Benefit of chondroitinase ABC on sensory axon regeneration in a laceration model of spinal cord injury in the rat

BACKGROUND

Chondroitin sulfate proteoglycans are up-regulated in the spinal cord after SCI, creating a molecular barrier inhibitory to axon growth. Chondroitinase ABC degrades CSPGs in vitro and in vivo.

METHODS

We studied whether IT ChABC promotes axonal regeneration in a laceration model of SCI. Three groups of Sprague-Dawley rats were used: control and rats treated with low-dose and high-dose IT ChABC. Chondroitin sulfate proteoglycan breakdown products were measured by 2-B-6 expression, and intact CSPGs by CS-56 expression. Sensory axonal regeneration was traced after CTB injection into the median, ulnar, and sciatic nerves.

RESULTS

CS-56 expression was down-regulated and 2-B-6 expression was increased in the groups treated with IT ChABC but not in the control. Laminin and GFAP immunoreactivity was unaltered in the ChABC groups. The number of axons growing into the scar was 3.1 times greater (P < .01) in the high-dose ChABC group and 2.1 times greater (P < .01) in the low-dose group compared with the controls. The length of axonal growth after high- and low-dose ChABC was 9.9 (P < .01) and 8.3 (P < .01) times greater, respectively, than in the control group. Axons extended across the lesion gap and into the distal spinal cord stump in 2 of 8 (low dose) and in 3 of 9 (high dose) rats compared with none in the control group.

CONCLUSIONS

Intrathecal ChABC administration caused a slight decrease in CSPGs in the scar after a laceration SCI with a minimal increase in sensory axonal regeneration into and across the laceration gap.

Nogo-66 promotes the differentiation of neural progenitors into astroglial lineage cells through mTOR-STAT3 pathway

BACKGROUND

Neural stem/progenitor cells (NPCs) can differentiate into neurons, astrocytes and oligodendrocytes. NPCs are considered valuable for the cell therapy of injuries in the central nervous system (CNS). However, when NPCs are transplanted into the adult mammalian spinal cord, they mostly differentiate into glial lineage. The same results have been observed for endogenous NPCs during spinal cord injury. However, little is known about the mechanism of such fate decision of NPCs.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we have found that myelin protein and Nogo-66 promoted the differentiation of NPCs into glial lineage. NgR and mTOR-Stat3 pathway were involved in this process. Releasing NgR from cell membranes or blocking mTOR-STAT3 could rescue the enhanced glial differentiation by Nogo-66.

CONCLUSIONS/SIGNIFICANCE

These results revealed a novel function of Nogo-66 in the fate decision of NPCs. This discovery could have profound impact on the understanding of CNS development and could improve the therapy of CNS injuries.

Peripherally-derived BDNF promotes regeneration of ascending sensory neurons after spinal cord injury

Background

The blood brain barrier (BBB) and truncated trkB receptor on astrocytes prevent the penetration of brain derived neurotrophic factor (BDNF) applied into the peripheral (PNS) and central nervous system (CNS) thus restrict its application in the treatment of nervous diseases. As BDNF is anterogradely transported by axons, we propose that peripherally derived and/or applied BDNF may act on the regeneration of central axons of ascending sensory neurons.

Methodology/Principal Findings

The present study aimed to test the hypothesis by using conditioning lesion of the sciatic nerve as a model to increase the expression of endogenous BDNF in sensory neurons and by injecting exogenous BDNF into the peripheral nerve or tissues. Here we showed that most of regenerating sensory neurons expressed BDNF and p-CREB but not p75NTR. Conditioning-lesion induced regeneration of ascending sensory neuron and the increase in the number of p-Erk positive and GAP-43 positive neurons was blocked by the injection of the BDNF antiserum in the periphery. Enhanced neurite outgrowth of dorsal root ganglia (DRG) neurons in vitro by conditioning lesion was also inhibited by the neutralization with the BDNF antiserum. The delivery of exogenous BDNF into the sciatic nerve or the footpad significantly increased the number of regenerating DRG neurons and regenerating sensory axons in the injured spinal cord. In a contusion injury model, an injection of BDNF into the footpad promoted recovery of motor functions.

Conclusions/Significance

Our data suggest that endogenous BDNF in DRG and spinal cord is required for the enhanced regeneration of ascending sensory neurons after conditioning lesion of sciatic nerve and peripherally applied BDNF may have therapeutic effects on the spinal cord injury.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Gangliosides and Nogo receptors independently mediate myelin-associated glycoprotein inhibition of neurite outgrowth in different nerve cells

In the injured nervous system, myelin-associated glycoprotein (MAG) on residual myelin binds to receptors on axons, inhibits axon outgrowth, and limits functional recovery. Conflicting reports identify gangliosides (GD1a and GT1b) and glycosylphosphatidylinositol-anchored Nogo receptors (NgRs) as exclusive axonal receptors for MAG. We used enzymes and pharmacological agents to distinguish the relative roles of gangliosides and NgRs in MAG-mediated inhibition of neurite outgrowth from three nerve cell types, dorsal root ganglion neurons (DRGNs), cerebellar granule neurons (CGNs), and hippocampal neurons. Primary rat neurons were cultured on control substrata and substrata adsorbed with full-length native MAG extracted from purified myelin. The receptors responsible for MAG inhibition of neurite outgrowth varied with nerve cell type. In DRGNs, most of the MAG inhibition was via NgRs, evidenced by reversal of inhibition by phosphatidylinositol-specific phospholipase C (PI-PLC), which cleaves glycosylphosphatidylinositol anchors, or by NEP1-40, a peptide inhibitor of NgR. A smaller percentage of MAG inhibition of DRGN outgrowth was via gangliosides, evidenced by partial reversal by addition of sialidase to cleave GD1a and GT1b or by P4, an inhibitor of ganglioside biosynthesis. Combining either PI-PLC and sialidase or NEP1-40 and P4 was additive. In contrast to DRGNs, in CGNs MAG inhibition was exclusively via gangliosides, whereas inhibition of hippocampal neuron outgrowth was mostly reversed by sialidase or P4 and only modestly reversed by PI-PLC or NEP1-40 in a non-additive fashion. A soluble proteolytic fragment of native MAG, dMAG, also inhibited neurite outgrowth. In DRGNs, dMAG inhibition was exclusively NgR-dependent, whereas in CGNs it was exclusively ganglioside-dependent. An inhibitor of Rho kinase reversed MAG-mediated inhibition in all nerve cells, whereas a peptide inhibitor of the transducer p75(NTR) had cell-specific effects quantitatively similar to NgR blockers. Our data indicate that MAG inhibits axon outgrowth via two independent receptors, gangliosides and NgRs.

Solution structure of the catalytic domain of RICH protein from goldfish

Regeneration-induced CNPase homolog (RICH) is an axonal growth-associated protein, which is induced in teleost fish upon optical nerve injury. RICH consists of a highly acidic N-terminal domain, a catalytic domain with 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) activity and a C-terminal isoprenylation site. In vitro RICH and mammalian brain CNPase specifically catalyze the hydrolysis of 2',3'-cyclic nucleotides to produce 2'-nucleotides, but the physiologically relevant in vivo substrate remains unknown. Here, we report the NMR structure of the catalytic domain of goldfish RICH and describe its binding to CNPase inhibitors. The structure consists of a twisted nine-stranded antiparallel beta-sheet surrounded by alpha-helices on both sides. Despite significant local differences mostly arising from a seven-residue insert in the RICH sequence, the active site region is highly similar to that of human CNPase. Likewise, refinement of the catalytic domain of rat CNPase using residual dipolar couplings gave improved agreement with the published crystal structure. NMR titrations of RICH with inhibitors point to a similar catalytic mechanism for RICH and CNPase. The results suggest a functional importance for the evolutionarily conserved phosphodiesterase activity and hint of a link with pre-tRNA splicing.

Recovery from paralysis in adult rats using embryonic stem cells

OBJECTIVE

We explored the potential of embryonic stem cell-derived motor neurons to functionally replace those cells destroyed in paralyzed adult rats.

METHODS

We administered a phosphodiesterase type 4 inhibitor and dibutyryl cyclic adenosine monophosphate to overcome myelin-mediated repulsion and provided glial cell-derived neurotrophic factor within the sciatic nerve to attract transplanted embryonic stem cell-derived axons toward skeletal muscle targets.

RESULTS

We found that these strategies significantly increased the success of transplanted axons extending out of the spinal cord into ventral roots. Furthermore, transplant-derived axons reached muscle, formed neuromuscular junctions, were physiologically active, and mediated partial recovery from paralysis.

INTERPRETATION

We conclude that restoration of functional motor units by embryonic stem cells is possible and represents a potential therapeutic strategy for patients with paralysis. To our knowledge, this is the first report of the anatomical and functional replacement of a motor neuron circuit within the adult mammalian host.

Differential effects of Th1, monocyte/macrophage and Th2 cytokine mixtures on early gene expression for immune-related molecules by central nervous system mixed glial cell cultures

Cytokines secreted within the central nervous system (CNS) are important in the development of multiple sclerosis (MS) lesions. The balance between Th1, monocyte/macrophage (M/M) and Th2 cytokines in the CNS may be pivotal in determining the outcome of lesion development. We examined the effects of mixtures of cytokines on gene expression by CNS glial cells, as mixtures of cytokines are present in MS lesions, which in turn contain mixtures of glial cells. In this initial analysis by gene array, we examined changes at 6 hours to identify early changes in gene expression that represent primary responses to the cytokines. Rat glial cells were incubated with mixtures of Th1, M/M and Th2 cytokines for 6 hours and examined for changes in early gene expression employing microarray gene chip technology. A minimum of 814 genes were differentially regulated by one or more of the cytokine mixtures in comparison to controls, including changes in expression in a large number of genes for immune system-related proteins. Expression of the proteins for these genes likely influences development and inhibition of MS lesions as well as protective and regenerative processes. Analysing gene expression for the effects of various combinations of exogenous cytokines on glial cells in the absence of the confounding effects of inflammatory cells themselves should increase our understanding of cytokine-induced pathways in the CNS.

Characterization of the domains of zRICH, a protein induced during optic nerve regeneration in zebrafish

Teleost fish show a remarkable capability of nerve regeneration in their CNS, while injuries to axon fibers in the CNS of mammals result in degeneration and loss of function. Understanding this difference has biomedical consequences to humans. Both extrinsic factors from the neuronal environment and intrinsic neuronal factors seem to play a role in successful nerve regeneration. Among the intrinsic factors, a number of proteins termed axonal growth associated proteins (GAPs) are strongly induced during axon regeneration. RICH proteins are axonal GAPs that show homology to mammalian myelin marker proteins termed CNPases. Sequence analysis distinguishes three domains in these proteins. In this report, mutant versions of zebrafish RICH proteins were generated to study the roles of the domains of the protein at biochemical and cellular levels. The central CNPase homology domain was sufficient for catalytic activity. The amino terminal acidic domain causes the anomalous electrophoretic migration observed for RICH proteins. The small C-terminal domain bears an isoprenylation motif and is necessary for the interaction of zRICH with cellular membranes. At the cellular level, expression of wild-type zRICH protein in PC12 cells did not induce neurite generation. Additionally, neither the expression of wild-type zRICH nor the expression of mutant versions of the protein interfered with the levels of differentiation of PC12 cells induced by nerve growth factor, suggesting that, at least in this model of neuronal differentiation, zRICH proteins do not participate in the process of generation of neurites.

Polyamine and aminoguanidine treatments to promote structural and functional recovery in the adult mammalian brain after injury: a brief literature review and preliminary data about their combined administration

The regeneration potential of the adult mammalian central nervous system (CNS) is very modest, due to, among other factors, the presence of either a glial scar, or myelin-associated regeneration inhibitors such as Nogo-A, MAG and OMgp, which all interact with the same receptor (NgR). After a brief review of the key proteins (Rho and PKC) implicated in NgR-mediated signalling cascades, we will tackle the implications of cAMP and Arginase I in overcoming myelin growth-inhibitory influence, and then will focus on the effects of polyamines and aminoguanidine to propose (and to briefly support this proposal by our own preliminary data) that their association might be a potent way to enable functionally-relevant regeneration in the adult mammalian CNS.

Degradation of Id2 by the anaphase-promoting complex couples cell cycle exit and axonal growth

In the developing nervous system, Id2 (inhibitor of DNA binding 2, also known as inhibitor of differentiation 2) enhances cell proliferation, promotes tumour progression and inhibits the activity of neurogenic basic helix-loop-helix (bHLH) transcription factors. The anaphase promoting complex/cyclosome and its activator Cdh1 (APC/C(Cdh1)) restrains axonal growth but the targets of APC/C(Cdh1) in neurons are unknown. Id2 and other members of the Id family are very unstable proteins that are eliminated as cells enter the quiescent state, but how they are targeted for degradation has remained elusive. Here we show that Id2 interacts with the core subunits of APC/C and Cdh1 in primary neurons. APC/C(Cdh1) targets Id2 for degradation through a destruction box motif (D box) that is conserved in Id1 and Id4. Depletion of Cdh1 stabilizes Id proteins in neurons, whereas Id2 D-box mutants are impaired for Cdh1 binding and remain stable in cells that exit from the cell cycle and contain active APC/C(Cdh1). Mutants of the Id2 D box enhance axonal growth in cerebellar granule neurons in vitro and in the context of the cerebellar cortex, and overcome the myelin inhibitory signals for growth. Conversely, activation of bHLH transcription factors induces a cluster of genes with potent axonal inhibitory functions including the gene coding for the Nogo receptor, a key transducer of myelin inhibition. Degradation of Id2 in neurons permits the accumulation of the Nogo receptor, thereby linking APC/C(Cdh1) activity with bHLH target genes for the inhibition of axonal growth. These findings indicate that deregulated Id activity might be useful to reprogramme quiescent neurons into the axonal growth mode.

Sensory neurite outgrowth on white matter astrocytes is influenced by intracellular and extracellular S100A4 protein

The central nervous system (CNS) is considered a nonpermissive environment for axonal regeneration because of the presence of myelin and associated repulsive molecules. However, neural cells transplanted to the CNS preferably migrate and extend their fibers in white matter areas. We previously showed that white matter astrocytes in vivo express the calcium-binding protein S100A4, which is strongly up-regulated in areas of white matter degeneration. To investigate the role of white matter astrocytes and their specific protein S100A4 in axonal regeneration, we developed white matter astrocyte cultures with strong S100A4 expression and grew dissociated adult dorsal root ganglion (DRG) cells on top of astrocytes for 24 hr. By using small interfering S100A4 RNA, we were able to eliminate S100A4 expression and compare growth of DRG cell neurites on S100A4-silenced and S100A4-expressing astrocytes. In addition, we studied whether extracellular S100A4 has an effect on neurite growth from adult DRG cells cultured on S100A4-expressing white matter astrocytes. Our data show that white matter astrocytes are permissive for neurite growth, although high levels of S100A4 in white matter astrocytes have a negative effect on this growth. Extracellular application of S100A4 induced extensive growth of DRG cell neurites on white matter astrocytes. These findings suggest that white matter astrocytes are able to support axonal regeneration and, furthermore, that administration of extracellular S100A4 provides strong additional support for axonal regeneration.

Some principles of regeneration in mammalian systems

This article presents some general principles underlying regenerative phenomena in vertebrates, starting with the epimorphic regeneration of the amphibian limb and continuing with tissue and organ regeneration in mammals. Epimorphic regeneration following limb amputation involves wound healing, followed shortly by a phase of dedifferentiation that leads to the formation of a regeneration blastema. Up to the point of blastema formation, dedifferentiation is guided by unique regenerative pathways, but the overall developmental controls underlying limb formation from the blastema generally recapitulate those of embryonic limb development. Damaged mammalian tissues do not form a blastema. At the cellular level, differentiation follows a pattern close to that seen in the embryo, but at the level of the tissue and organ, regeneration is strongly influenced by conditions inherent in the local environment. In some mammalian systems, such as the liver, parenchymal cells contribute progeny to the regenerate. In others, e.g., skeletal muscle and bone, tissue-specific progenitor cells constitute the main source of regenerating cells. The substrate on which regeneration occurs plays a very important role in determining the course of regeneration. Epimorphic regeneration usually produces an exact replica of the structure that was lost, but in mammalian tissue regeneration the form of the regenerate is largely determined by the mechanical environment acting on the regenerating tissue, and it is normally an imperfect replica of the original. In organ hypertophy, such as that occurring after hepatic resection, the remaining liver mass enlarges, but there is no attempt to restore the original form.

Upregulation of EphA3 Receptor after Spinal Cord Injury

Spinal cord injury (SCI) releases a cascade of events that leads to the onset of an inhibitory milieu for axonal regeneration. Some of these changes result from the presence of repulsive factors that may restrict axonal outgrowth after trauma. The Eph receptor tyrosine kinase (RTK) family has emerged as a key repellent cue known to be involved in neurite outgrowth, synapse formation, and axonal pathfinding during development. Given the nonpermissive environment for axonal regeneration after SCI, we questioned whether re-expression of one of these molecules occurs during regenerative failure. We examined the expression profile of EphA3 at the mRNA and protein levels after SCI, using the NYU contusion model. There is a differential distribution of this molecule in the adult spinal cord and EphA3 showed an increase in expression after several injury models like optic nerve and brain injury. Standardized semi-quantitative RT-PCR analysis demonstrated a time-dependent change in EphA3 mRNA levels, without alterations in beta-actin levels. The basal level of EphA3 mRNA in the adult spinal cord is low and its expression was induced 2 days after trauma (the earliest time point analyzed) and this upregulation persisted for 28 days post-injury (the latest time point examined). These results were corroborated at the protein level by immunohistochemical analysis and the cell phenotype identified by double labeling studies. In control animals, EphA3 immunoreactivity was observed in motor neurons of the ventral horn but not in lesioned animals. In addition, GFAP-positive cells were visualized in the ventral region of injured white matter. These results suggest that upregulation of EphA3 in reactive astrocytes may contribute to the repulsive environment for neurite outgrowth and may be involved in the pathophysiology generated after SCI.

Astrocyte-associated fibronectin is critical for axonal regeneration in adult white matter

Although it has been suggested that astroglia guide pioneering axons during development, the cellular and molecular substrates that direct axon regeneration in adult white matter have not been elucidated. We show that although adult cortical neurons were only able to elaborate very short, highly branched, dendritic-like processes when seeded onto organotypic slice cultures of postnatal day 35 (P35) rat brain containing the corpus callosum, adult dorsal root ganglion (DRG) neurons were able to regenerate lengthy axons within the reactive glial environment of this degenerating white matter tract. The callosum in both P35 slices and adult rat brain was rich in fibronectin, but not laminin. Furthermore, the fibronectin was intimately associated with the intratract astrocytes. Blockade of fibronectin function in situ with an anti-fibronectin antibody dramatically decreased outgrowth of DRG neurites, suggesting that fibronectin plays an important role in axon regeneration in mature white matter. The critical interaction between regrowing axons and astroglial-associated fibronectin in white matter may be an additional factor to consider when trying to understand regeneration failure and devising strategies to promote regeneration.

Integration of retrograde axonal and nuclear transport mechanisms in neurons: implications for therapeutics

The elongated morphology of neuronal processes imposes a significant challenge for effective intracellular communication between the neurites and the cell body. This problem is especially acute upon injury, when the cell body must receive accurate and timely information on the site and extent of axonal damage to mount an appropriate response. Recent work has demonstrated that nuclear import factors from the importin (karyopherin) alpha and beta families provide a mechanism for retrograde injury signaling. Importins are found throughout axons and dendrites at significant distances from the cell body, and importin beta protein is increased after nerve lesion by local translation of axonal mRNA. This leads to formation of a high-affinity nuclear localization signal (NLS) binding complex that traffics retrogradely due to an interaction of importin alpha with the motor protein dynein. Disruption of the complex with excess NLS peptides delays regeneration of injured sensory neurons. The dual role of importins in retrograde transport in axons and nuclear import in cell bodies suggests new avenues for manipulating intrinsic regeneration mechanisms in the nervous system and may provide a novel route for drug delivery to the CNS.

Modulation of Rho GTPase activity alleviates chondroitin sulfate proteoglycan-dependent inhibition of neurite extension

The central nervous system (CNS) fails to regenerate after injury. A glial scar forms at the injury site, contributing to regenerative failure partly resulting from the chondroitin sulfate proteoglycans (CSPGs) in the glial scar. The family of Rho GTPases, which includes Cdc42, Rac1, and RhoA, is involved in growth cone dynamics. Although the response of neural cells to the inactivation of Rho when contacting myelin-related substrates, or CSPG, has been investigated, Rac1's and Cdc42's abilities to modulate CSPG-dependent inhibition have yet to be explored. In this study, a stripe assay was utilized to examine the effects of modulating all three Rho GTPases on neurite extension across inhibitory CSPG lanes. Alternating laminin (LN) and CSPG lanes were created and NG108-15 cells and E9 chick dorsal root ganglia (DRGs) were cultured on the lanes. By using the protein delivery agent Chariot, the neuronal response to exposure of constitutively active (CA) and dominant negative (DN) mutants of the Rho GTPases, along with the bacterial toxin C3, was determined by quantifying the percentage ratio of neurites crossing the CSPG lanes. CA-Cdc42, CA-Rac1, and C3 transferase significantly increased the number of neurites crossing into the CSPG lanes compared with the negative controls for both the NG108-15 cells and the E9 chick DRGs. We also show that these mutant proteins require the delivery vehicle, Chariot, to enter the neurons and affect neurite extension. Therefore, activation of Cdc42 and Rac, as well as inhibition of Rho, helps overcome the CSPG-dependent inhibition of neurite extension.

Negative regulation of T cell receptor signaling by Siglec-7 (p70/AIRM) and Siglec-9

Siglec-7 (p70/AIRM) and Siglec-9 are "CD33"-related siglecs expressed on natural killer (NK) cells and subsets of peripheral T cells. Like other inhibitory NK cell receptors, they contain immunoglobulin receptor family tyrosine-based inhibitory motifs in their cytoplasmic domains, and Siglec-7 has been demonstrated to negatively regulate NK cell activation. Based on reports of the presence of these siglecs on T cells, we sought to determine if they are capable of modulating T cell receptor (TCR) signaling using Jurkat T cells stably and transiently transfected with Siglec-7 or Siglec-9. Following either pervanadate stimulation or TCR engagement, both Siglecs exhibited increased tyrosine phosphorylation and recruitment of SHP-1. Effects of Siglec-7 and -9 were also evident in downstream events in the signaling pathway. Both siglecs reduced phosphorylation of Tyr319 on ZAP-70, known to play a pivotal role in up-regulation of gene transcription following TCR stimulation. There was also a corresponding decreased transcriptional activity of nuclear factor of activated T cells (NFAT) as determined using a luciferase reporter gene. Like all siglecs, Siglec-7 and -9 recognize sialic acid-containing glycans of glycoproteins and glycolipids as ligands. Mutation of the conserved Arg in the ligand binding site of Siglec-7 (Arg124) or Siglec-9 (Arg120) resulted in reduced inhibitory function in the NFAT/luciferase transcription assay, suggesting that ligand binding is required for optimal inhibition of TCR signaling. The combined results demonstrate that both Siglec-7 and Siglec-9 are capable of negative regulation of TCR signaling and that ligand binding is required for optimal activity.

Stem cell therapy for neurodegenerative diseases: the issue of transdifferentiation

In the past few years research on stem cells has exploded as a tool to develop potential therapies to treat incurable neurodegenerative diseases. Stem cell transplantation has been effective in several animal models, but the underlying restorative mechanisms are still unknown. Several events such as cell fusion, neurotrophic factor release, endogenous stem cell proliferation, and transdifferentiation (adult cell acquisition of new unexpected identities) may explain therapeutic success, in addition to replacement of lost cells. This issue needs to be clarified further to maximize the potential for effective therapies. Preliminary stem transplantation trials have already been performed for some neurodegenerative diseases. There is no effective pharmacological treatment for amyotrophic lateral sclerosis, but recent preliminary data both in experimental and clinical settings have targeted it as an ideal candidate disease for the development of stem cell therapy in humans. This review summarizes recent advances gained in stem cell research applied to neurodegenerative diseases with a special emphasis to the criticisms put forward.

The phosphodiesterase inhibitor rolipram delivered after a spinal cord lesion promotes axonal regeneration and functional recovery

Although there is no spontaneous regeneration of mammalian spinal axons after injury, they can be enticed to grow if cAMP is elevated in the neuronal cell bodies before the spinal axons are cut. Prophylactic injection of cAMP, however, is useless as therapy for spinal injuries. We now show that the phosphodiesterase 4 (PDE4) inhibitor rolipram (which readily crosses the blood-brain barrier) overcomes inhibitors of regeneration in myelin in culture and promotes regeneration in vivo. Two weeks after a hemisection lesion at C3/4, with embryonic spinal tissue implanted immediately at the lesion site, a 10-day delivery of rolipram results in considerable axon regrowth into the transplant and a significant improvement in motor function. Surprisingly, in rolipram-treated animals, there was also an attenuation of reactive gliosis. Hence, because rolipram promotes axon regeneration, attenuates the formation of the glial scar, and significantly enhances functional recovery, and because it is effective when delivered s.c., as well as post-injury, it is a strong candidate as a useful therapy subsequent to spinal cord injury.

Axonal growth of embryonic stem cell-derived motoneurons in vitro and in motoneuron-injured adult rats

We generated spinal motoneurons from embryonic stem (ES) cells to determine the developmental potential of these cells in vitro and their capacity to replace motoneurons in the adult mammalian spinal cord. ES cell-derived motoneurons extended long axons, formed neuromuscular junctions, and induced muscle contraction when cocultured with myoblasts. We transplanted motoneuron-committed ES cells into the spinal cords of adult rats with motoneuron injury and found that approximately 3,000 ES cell-derived motoneurons (25% of input) survived for >1 month in the spinal cord of each animal. ES cell-derived axonal growth was inhibited by myelin, and this inhibition was overcome by administration of dibutyryl cAMP (dbcAMP) or a Rho kinase inhibitor in vitro and in vivo. In transplanted rats infused with dbcAMP, approximately 80 ES cell-derived motor axons were observed within the ventral roots of each animal, whereas none were observed in transplanted rats not treated with dbcAMP. Because these cells replicate many of the developmental and mature features of true motoneurons, they are an important biological tool to understand formation of motor units in vitro and a potential therapeutic tool to reconstitute neural circuits in vivo.

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.

Suppression of p75NTR does not promote regeneration of injured spinal cord in mice

The neurotrophin receptor p75NTR is the coreceptor for Nogo receptor, mediating growth cone collapse in vitro by MAG, myelin oligodendrocyte glycoprotein (Omgp), and Nogo. Whether p75NTR plays any role in the failure of nerve regeneration in vivo is not known. Immunohistochemical data showed that p75NTR was expressed in only a very small subset of ascending sensory axons but not in any corticospinal axons in the dorsal column of either normal or injured spinal cord. Using p75NTR-deficient mice, we showed that the depletion of the functional p75NTR did not promote the regeneration of the descending corticospinal tract and ascending sensory neurons in the spinal cord 2 weeks after spinal cord injury. Local administration of p75NTR-Fc fusion molecule, the dominant-negative receptor to block the function of neurite outgrowth inhibitors, did not improve regeneration of ascending sensory neurons in the injured spinal cord. Our results suggest that p75NTR may not be a critical molecule mediating the function of myelin-associated inhibitory factors in vivo.

PKC mediates inhibitory effects of myelin and chondroitin sulfate proteoglycans on axonal regeneration

Successful axon regeneration in the mammalian central nervous system (CNS) is at least partially compromised due to the inhibitors associated with myelin and glial scar. However, the intracellular signaling mechanisms underlying these inhibitory activities are largely unknown. Here we provide biochemical and functional evidence that conventional isoforms of protein kinase C (PKC) are key components in the signaling pathways that mediate the inhibitory activities of myelin components and chondroitin sulfate proteoglycans (CSPGs), the major class of inhibitors in the glial scar. Both the myelin inhibitors and CSPGs induce PKC activation. Blocking PKC activity pharmacologically and genetically attenuates the ability of CNS myelin and CSPGs to activate Rho and inhibit neurite outgrowth. Intrathecal infusion of a PKC inhibitor, Gö6976, into the site of dorsal hemisection promotes regeneration of dorsal column axons across and beyond the lesion site in adult rats. Thus, perturbing PKC activity could represent a therapeutic approach to stimulating axon regeneration after brain and spinal cord injuries.

Myelin associated glycoprotein cross-linking triggers its partitioning into lipid rafts, specific signaling events and cytoskeletal rearrangements in oligodendrocytes

Myelin-associated glycoprotein (MAG) has been implicated in inhibition of nerve regeneration in the CNS. This results from interactions between MAG and the Nogo receptor and gangliosides on the apposing axon, which generates intracellular inhibitory signals in the neuron. However, because myelin-axon signaling is bidirectional, we undertook an analysis of potential MAG-activated signaling in oligodendrocytes (OLs). In this study, we show that antibody cross-linking of MAG on the surface of OLs (to mimic axonal binding) leads to the redistribution of MAG into detergent (TX-100)-insoluble complexes, hyperphosphorylation of Fyn, dephosphorylation of serine and threonine residues in specific proteins, including lactate dehydrogenase and the beta subunit of the trimeric G-protein-complex, and cleavage of alpha-fodrin followed by a transient depolymerization of actin. We propose that these changes are part of a signaling cascade in OLs associated with MAG function as a mediator of axon-glial communication which might have implications for the mutual regulation of the formation and stability of axons and myelin.

Axonal Regeneration of Fish Optic Nerve after Injury

Since Sperry's work in the 1950s, it has been known that the central nervous system (CNS) neurons of lower vertebrates such as fish and amphibians can regenerate after axotomy, whereas the CNS neurons of mammals become apoptotic after axotomy. The goldfish optic nerve (ON) is one of the most studied animal models of CNS regeneration. Morphological changes in the goldfish retina and tectum after ON transection were first researched in the 1970s-1980s. Many biochemical studies of neurite outgrowth-promoting substances were then carried out in the 1980s-1990s. Many factors have been reported to be active substances that show increased levels during fish ON regeneration, as shown by using various protein chemistry techniques. However, there are very few molecular cloning techniques for studying ON regeneration after injury. In this review article, we summarize the neurite outgrowth-promoting factors reported by other researchers and describe our strategies for searching for ON regenerating molecules using a differential hybridization technique in the goldfish visual system. The process of goldfish ON regeneration after injury is very long. It takes about half a year from the start of axonal regrowth to complete restoration of vision. The process has been classified into three stages: early, middle and late. We screened for genes with increased expression during regeneration using axotomized goldfish retinal and tectal cDNA libraries and obtained stage-specific cDNA clones that were upregulated in the retina and tectum. We further discuss functional roles of these molecules in the regeneration processes of goldfish ON.