Repulsive factors and axon regeneration in the CNS

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.

Emerging cues mediating astroglia lineage restriction of progenitor cells in the injured/diseased adult CNS

Other than specific neurogenic regions, the adult central nervous system (CNS) is not conducive for neuronal regeneration and neurogenesis, particularly at sites of injury or neurodegeneration. Engraftment of neural stem/progenitor cells into non-neurogenic regions or sites of injury/disease invariably results mainly in astroglia differentiation. The reasons for such a lineage restriction have not been well defined. Recent findings have brought to light some underlying novel mechanistic basis for this preferential differentiation into astroglia. The more oxidized state of pathological brain tissue leads to upregulation of the protein deacetylase sirtuin 1 (Sirt1). Sirt1 appears to stabilize a co-repressor complex of Hairy/enhancer of split (Hes)1, thereby suppressing expression of the proneuronal transcription factor Mash1, and directs progenitor cell differentiation towards the glia lineage. Sirt1 upregulated by CNS inflammation may also inhibit neuronal differentiation. Myelin-associated inhibitors such as Nogo, acting through the Nogo-66 receptor (NgR), also appear to promote neural stem/progenitor cell differentiation into astrocytes. Understanding the molecular basis of glia lineage restriction of neural progenitors in the injured or diseased CNS would provide handles to improving the success of stem cell-based transplantation therapy.

Real-time multi-site multi-parametric monitoring of rat brain subjected to traumatic brain injury

INTRODUCTION

Traumatic brain injury (TBI) is one of the major causes of death in the world, with at least ten million serious traumatic brain injuries occurring annually; nevertheless, the pathophysiologic events taking place immediately after the injury are not yet fully known.

OBJECTIVE

To study the effects of TBI on brain hemodynamic, metabolic and ionic homeostasis using the multi-parametric monitoring system. This system enables real-time monitoring of cerebral blood flow (CBF), mitochondrial NADH redox state, extracellular levels of K+, H+, DC potential, ECoG and ICP.

METHODS

In order to find the best brain location for the monitoring device in relation to the fluid percussion injury site, we used the multi-site multi-parametric monitoring system. Two groups of rats were connected to four monitoring probes at four different locations near the injury site, two in each hemisphere. We monitored CBF, NADH redox state, tissue reflectance and DC steady potential in each of the four sites.

RESULTS

Under anoxia, the initial CBF decrease was followed by an increase, NADH level increased, the reflectance decreased and dc potential showed a biphasic response, in all 4 locations. However, following fluid percussion injury, there was a significant variability in the responses in each of the 4 monitored locations.

CONCLUSION

The advantage of the multi-parametric-monitoring approach for enhanced understanding of the injured brain was indicated. Moreover, we showed that contralateral monitoring of the injured brain gives good indication for the events taking place following fluid percussion brain injury.

NMR structure and dynamics of human ephrin-B2 ectodomain: the functionally critical C-D and G-H loops are highly dynamic in solution

Eph receptors and ephrins constitute the largest family of receptor tyrosine kinases with 15 individual receptors and nine ligands. Its ectodomains represent attractive targets not only for understanding fundamental mechanisms underlying axon guidance, cell migration, segmentation, tumorigenesis, and bone remodeling, but also for drug screening/design to treat cancers, bone diseases and viral infection. So far no NMR study on the ephrin ectodomains is available and as such their properties in solution still remain unknown. In this study, we presented the first NMR structure and dynamics of the human ephrin-B2 ectodomain as well as its interaction with the receptor EphB2. Strikingly, the NMR study reveals a picture different from those previously obtained by X-ray crystallography. Although in solution it still adopts the same Greek key fold, with the central beta-barrel ( approximately 30% of the molecule) highly similar to that in crystal structures, the other regions are highly dynamic and accessible to the bulk solvent. In particular, the functionally critical C-D and G-H loops of the ephrin-B2 ectodomain are highly flexible as reflected by several NMR probes including hydrogen exchange and (15)N backbone relaxation data. Nevertheless, as revealed by ITC and NMR, the ephrin-B2 ectodomain binds to EphB2 with a K(d) of 22.3 nM to form a tight complex in which the tip of the C-D loop and the C-terminus still remain largely flexible. The present results may bear critical implications in understanding the molecular details as well as designing antagonists of therapeutic interest for Eph-ephrin interactions.

Two faces of chondroitin sulfate proteoglycan in spinal cord repair: a role in microglia/macrophage activation

BACKGROUND

Chondroitin sulfate proteoglycan (CSPG) is a major component of the glial scar. It is considered to be a major obstacle for central nervous system (CNS) recovery after injury, especially in light of its well-known activity in limiting axonal growth. Therefore, its degradation has become a key therapeutic goal in the field of CNS regeneration. Yet, the abundant de novo synthesis of CSPG in response to CNS injury is puzzling. This apparent dichotomy led us to hypothesize that CSPG plays a beneficial role in the repair process, which might have been previously overlooked because of nonoptimal regulation of its levels. This hypothesis is tested in the present study.

METHODS AND FINDINGS

We inflicted spinal cord injury in adult mice and examined the effects of CSPG on the recovery process. We used xyloside to inhibit CSPG formation at different time points after the injury and analyzed the phenotype acquired by the microglia/macrophages in the lesion site. To distinguish between the resident microglia and infiltrating monocytes, we used chimeric mice whose bone marrow-derived myeloid cells expressed GFP. We found that CSPG plays a key role during the acute recovery stage after spinal cord injury in mice. Inhibition of CSPG synthesis immediately after injury impaired functional motor recovery and increased tissue loss. Using the chimeric mice we found that the immediate inhibition of CSPG production caused a dramatic effect on the spatial organization of the infiltrating myeloid cells around the lesion site, decreased insulin-like growth factor 1 (IGF-1) production by microglia/macrophages, and increased tumor necrosis factor alpha (TNF-alpha) levels. In contrast, delayed inhibition, allowing CSPG synthesis during the first 2 d following injury, with subsequent inhibition, improved recovery. Using in vitro studies, we showed that CSPG directly activated microglia/macrophages via the CD44 receptor and modulated neurotrophic factor secretion by these cells.

CONCLUSIONS

Our results show that CSPG plays a pivotal role in the repair of injured spinal cord and in the recovery of motor function during the acute phase after the injury; CSPG spatially and temporally controls activity of infiltrating blood-borne monocytes and resident microglia. The distinction made in this study between the beneficial role of CSPG during the acute stage and its deleterious effect at later stages emphasizes the need to retain the endogenous potential of this molecule in repair by controlling its levels at different stages of post-injury repair.

Single, high-dose intraspinal injection of chondroitinase reduces glycosaminoglycans in injured spinal cord and promotes corticospinal axonal regrowth after hemisection but not contusion

Chondroitin sulfate proteoglycans (CSPGs) inhibit axonal growth, and treatment with chondroitinase ABC promotes axonal regeneration in some models of central nervous system (CNS) injury. The aims of this study were (1) to compare the spatiotemporal appearance of CSPG expression between spinal cord contusion and hemisection models, and (2) to evaluate chondroitinase treatment effects on axonal regrowth in the two injury models. After hemisection, CSPG-immunoreactivity (IR) in the injury site rose to peak levels at 18 days but then decreased dramatically by 49 days; in contrast, CSPG-IR remained high for at least 49 days after contusion. After hemisection, many anterogradely labeled corticospinal tract (CST) axons remained close to CSPG-rich lesion sites, but after contusion, most CST axons retracted by approximately 1 mm rostral from the rostral-most CSPG-rich cyst. Intraspinal injection of chondroitinase at 0, 1, 2, and 4 weeks following injury dramatically reduced CSPG-IR in both injury models within 4 days, and CSPG-IR remained low for at least 3 weeks. After the chondroitinase treatment, many axons grew around the lesion site in hemisected spinal cords but not in contused spinal cords. We propose that improved axonal growth in hemisected spinal cords is due to decreased inhibition resulting from degradation of CSPGs located adjacent to severed CST axons. However, in spinal cord contusions, retracted CST axons fail to grow across gliotic regions that surround CSPG-rich injury sites despite efficient degradation with chondroitinase, suggesting that other inhibitors of axonal growth persist in the gliotic regions.

Cell autonomous function of Nogo and reticulons: The emerging story at the endoplasmic reticulum

The myelin-associated membrane protein reticulon-4 (RTN4)/Nogo has been extensively studied with regards to its neurite outgrowth inhibitory function, both in limiting plasticity in the healthy adult brain and regeneration during central nervous system injury. These activities are presumably associated with Nogo splice isoforms expressed on the cell surface and function largely in trans, exerting an influence as an intercellular membrane-bound ligand. Nogo, and other reticulon paralogues and orthologues, are however mainly localized to the endoplasmic reticulum (ER), and are likely to have cell autonomous functions that are not yet clear. Emerging evidence suggests that Nogo may have a role in modulating the morphology and functions of the ER. This role is apparently not essential for cell viability under normal growth conditions, but may be manifested under certain stress conditions.

The myelin-associated glycoprotein inhibitor BENZ induces outgrowth and survival of rat dorsal root ganglion cell cultures

The novel myelin-associated glycoprotein (MAG) inhibitor BENZ binds to the N-acetylneuraminic acid (Neu5Ac) portion of the N-terminal Ig-like domain of MAG. Treatment of rat dorsal root ganglion (DRG) cell cultures with BENZ-induced outgrowth of neurofilament 200-positive neurites improved survival of neurons and increased the number of GFAP-positive cells, as determined by fluorescence and confocal laser microscopy and by Western immunoblotting. Furthermore, treatment of DRG cell cultures with BENZ repressed gene and protein expression of the small GTPase RhoA but induced expression of Rho GTP-activating proteins 5 and 24, likely to counteract protein kinase A activity. Specifically, expression of inhibitors of neurite outgrowth, for example, Rock2 and PAK4, was repressed, but cofilin 1, a promoter of axonal growth, was induced. We propose that the MAG inhibitor BENZ abrogates the RhoA-ROCK-cofilin pathway to promote neurite outgrowth. Our findings require confirmation by in vivo animal studies.

Harvested human neurons engineered as live nervous tissue constructs: implications for transplantation. Laboratory investigation

OBJECT

Although neuron transplantation to repair the nervous system has shown promise in animal models, there are few practical sources of viable neurons for clinical application and insufficient approaches to bridge extensive nerve damage in patients. Therefore, the authors sought a clinically relevant source of neurons that could be engineered into transplantable nervous tissue constructs. The authors chose to evaluate human dorsal root ganglion (DRG) neurons due to their robustness in culture.

METHODS

Cervical DRGs were harvested from 16 live patients following elective ganglionectomies, and thoracic DRGs were harvested from 4 organ donor patients. Following harvest, the DRGs were digested in a dispase-collagenase treatment to dissociate neurons for culture. In addition, dissociated human DRG neurons were placed in a specially designed axon expansion chamber that induces continuous mechanical tension on axon fascicles spanning 2 populations of neurons originally plated approximately 100 microm apart.

RESULTS

The adult human DRG neurons, positively identified by neuronal markers, survived at least 3 months in culture while maintaining the ability to generate action potentials. Stretch-growth of axon fascicles in the expansion chamber occurred at the rate of 1 mm/day to a length of 1 cm, creating the first engineered living human nervous tissue constructs.

CONCLUSIONS

These data demonstrate the promise of adult human DRG neurons as an alternative transplant material due to their availability, viability, and capacity to be engineered. Also, these data show the feasibility of harvesting DRGs from living patients as a source of neurons for autologous transplant as well as from organ donors to serve as an allograft source of neurons.

The changes in neurotrophic properties of the peripheral nerves extracts following blocking of BDNF activity

OBJECTIVES

Retinal ganglion cells (RGCs) of adult rats are unable to regenerate their axons after optic nerve injury and soon after they enter the pathway of apoptosis. They may, however, survive and regenerate new axons in response to application of specific peripheral nerve extracts that presumably contain a range of neurotrophic substances. One of the recognized substances of proven neurotrophic activity is brain-derived neurotrophic factor (BDNF). We have investigated whether blocking the BDNF activity in post-microsomal fractions obtained from 7 day pre-degenerated peripheral nerves would affect its neurotrophic properties towards RGCs after optic nerve transection in adult rats.

METHODS

Autologous connective tissue chambers sutured to the distal end of transected optic nerve served as active substances containers. Surviving RGCs were visualized using Dil. The number of myelinated outgrowing fibers within the chambers was evaluated in histologic sections.

RESULTS

BDNF and 7 day pre-degenerated nerve extracts, and also extracts with blocked BDNF activity, enhanced RGC fibers outgrowth. The regeneration was significantly weaker in the control group. Blocking the BDNF activity in the 7 day pre-degenerated peripheral nerve extract reduced its neurotrophic effects but the differences were insignificant in comparison with non-blocked extracts.

DISCUSSION

The regeneration intensities in groups receiving 7 day pre-degenerated peripheral nerve extracts (PD7) and BDNF were comparable. The number of surviving cells was higher in the PD7 group and there were more regenerating fibers in the BDNF group, which may be explained by the strong BDNF effect on axonal collateralization and sprouting.

Transplanted neural progenitor cells survive and differentiate but achieve limited functional recovery in the lesioned adult rat spinal cord

Endogenous repair after injury in the adult CNS is limited by a number of factors including cellular loss, inflammation, cavitation and glial scarring. Spinal cord neural progenitor cells (SCNPCs) may provide a valuable cellular source for promoting repair following spinal cord injury. SCNPCs are multipotent, can be expanded in vitro, have the capacity to differentiate into CNS cell lineages and are capable of long-term survival following transplantation.

AIMS & METHOD

To determine the extent to which SCNPCs may contribute to spinal cord repair SCNPCs isolated from rat fetal spinal cord were expanded ex vivo and transplanted into the adult rat spinal cord after a dorsal column crush lesion.

RESULTS

The survival and distribution of transplanted cells were examined at 24 h, 1, 2 and 6 weeks after injury. Transplanted cells were identified at all time points, located mainly at the lesion perimeter, indicating good post-transplant cell survival. Furthermore, SCNPCs maintained their ability to differentiate in vivo, with approximately 40% differentiating into cells with a glial morphology, whilst 8% displayed a neural morphology. Transplanted animals were also assessed on a number of behavioral tasks measuring sensorimotor and proprioceptive function to determine the extent to which SCNPC transplants might attenuate lesion-induced functional deficits. SCNPCs failed to promote significant functional recovery, with a small improvement observed in only one of the four tasks employed, primarily related to improvements in sensory function. Tracing of the corticospinal tract and ascending dorsal column pathway revealed no regeneration of the axons beyond the lesion site.

CONCLUSIONS

These data indicate that, although transplanted SCNPCs show good survival in the spinal cord injury environment, combination with other treatment strategies is likely to be required for these cells to fully exert their therapeutic potential.

Regulation and guidance of cell behavior for tissue regeneration via the siRNA mechanism

RNA intereference and short-interfering RNA (siRNA) have been proven to be effective at decreasing the expression of target genes and provide a valuable tool for promoting and directing the growth of functional tissues for repair and reconstructive tissue engineering applications. siRNA is a gene-silencing mechanism that involves double-stranded RNA-mediated sequence-specific mRNA degradation and is a powerful mechanism for controlling cell behavior. The use of siRNA to reduce the expression of a target gene can induce the expression of one or more tissue-inductive factors, direct the differentiation of stem or progenitor cells, or remove a factor that inhibits regeneration, which can be useful in fundamental studies of tissue formation or in applications to promote in vivo regeneration. The potential of siRNA is illustrated through specific examples within the fields of angiogenesis, bone and nerve regeneration, and wound healing. In addition, challenges to deliver siRNA effectively for tissue engineering applications are addressed. siRNA represents a powerful tool to investigate and/or promote tissue formation, and numerous opportunities exist for identifying targets that promote regeneration of tissue and developing effective delivery systems.

The pivotal role of RhoA GTPase in the molecular signaling of axon growth inhibition after CNS injury and targeted therapeutic strategies

The dogma that the adult central nervous system (CNS) is nonpermissive to axonal regeneration is beginning to fall in the face of increased understanding of the molecular and cellular biology of axon outgrowth. It is now appreciated that axon growth is regulated by a combination of extracellular factors related to the milieu of the developing or adult CNS and the presence of injury, and intracellular factors related to the "growth state" of the developing or regenerating neuron. Several critical points of convergence within the developing or regenerating neuron for mediating intracellular cell signaling effects on the growth cone cytoskeleton have been identified, and their modulation has produced marked increases in axon outgrowth within the "nonpermissive" milieu of the adult injured CNS. One such critical convergence point is the small GTPase RhoA, which integrates signaling events produced by both myelin-associated inhibitors (e.g., NogoA) and astroglial-derived inhibitors (chondroitin sulfate proteoglycans) and regulates the activity of downstream effectors that modulate cytoskeletal dynamics within the growth cone mediating axon outgrowth or retraction. Inhibition of RhoA has been associated with increased outgrowth on nonpermissive substrates in vitro and increased axon regeneration in vivo. We are developing lentiviral vectors that modulate RhoA activity, allowing more long-term expression than is possible with current approaches. These vectors may be useful in regenerative strategies for spinal cord injury, brain injury, and neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and Huntington's disease.

One-year follow-up after bone marrow stromal cell treatment in middle-aged female rats with stroke

BACKGROUND AND PURPOSE

We sought to evaluate the long-term effects of bone marrow stromal cell (BMSC) treatment on retired breeder rats with stroke.

METHODS

Female retired breeder rats were subjected to 2-hour middle cerebral artery occlusion (MCAO) followed by an injection of 2 x 10(6) male BMSCs (n=8) or phosphate-buffered saline (n=11) into the ipsilateral internal carotid artery at 1 day after stroke. The rats were humanely killed 1 year later. Functional tests, in situ hybridization, and histochemical and immunohistochemical staining were performed.

RESULTS

Significant recovery of neurological deficits was found in BMSC-treated rats beginning 2 weeks after cell injection compared with control animals. The beneficial effects of cell transplantation persisted for at least 1 year (P<0.01). In situ hybridization for the Y chromosome showed that donor cells survived in the brains of recipient rats, among which 22.3+/-1.95% of cells expressed the astrocyte marker glial fibrillary acidic protein, 16.8+/-2.13% expressed the neuronal marker microtubule-associated protein 2, and 5.5+/-0.42% and <1% of cells colocalized with the microglial marker IB4 and the endothelial cell marker von Willebrand factor, respectively. Only very few BMSCs, however, were found in peripheral organs such as the heart, lung, liver, spleen, and kidney in recipient rats. BMSCs significantly reduced axonal loss (P<0.01), the thickness of the lesion scar wall (P<0.01), and the number of Nogo-A-positive cells (P<0.05) along the scar border; meanwhile, synaptophysin expression (P<0.05) was significantly increased in BMSC-treated ischemic brains compared with control untreated brains.

CONCLUSIONS

The beneficial effects of BMSCs on ischemic brain tissue persisted for at least 1 year. Most surviving BMSCs were present in the ischemic brain, but very few were found in other organs. The long-term improvement in functional outcome may be related to the structural and molecular changes induced by BMSCs.

A role for Nogo receptor in macrophage clearance from injured peripheral nerve

We report a role for Nogo receptors (NgRs) in macrophage efflux from sites of inflammation in peripheral nerve. Increasing numbers of macrophages in crushed rat sciatic nerves express NgR1 and NgR2 on the cell surface in the first week after injury. These macrophages show reduced binding to myelin and MAG in vitro, which is reversed by NgR siRNA knockdown and by inhibiting Rho-associated kinase. Fourteen days after sciatic nerve crush, regenerating nerves with newly synthesized myelin have fewer macrophages than cut/ligated nerves that lack axons and myelin. Almost all macrophages in the cut/ligated nerves lie within the Schwann cell basal lamina, while in the crushed regenerating nerves the majority migrate out. Furthermore, crush-injured nerves of NgR1- and MAG-deficient mice and Y-27632-treated rats show impaired macrophage efflux from Schwann cell basal lamina containing myelinated axons. These data have implications for the resolution of inflammation in peripheral nerve and CNS pathologies.

DSD-1-Proteoglycan/Phosphacan and Receptor Protein Tyrosine Phosphatase-Beta Isoforms during Development and Regeneration of Neural Tissues

Interactions between neurons and glial cells play important roles in regulating key events of development and regeneration of the CNS. Thus, migrating neurons are partly guided by radial glia to their target, and glial scaffolds direct the growth and directional choice of advancing axons, e.g., at the midline. In the adult, reactive astrocytes and myelin components play a pivotal role in the inhibition of regeneration. The past years have shown that astrocytic functions are mediated on the molecular level by extracellular matrix components, which include various glycoproteins and proteoglycans. One important, developmentally regulated chondroitin sulfate proteoglycan is DSD-1-PG/phosphacan, a glial derived proteoglycan which represents a splice variant of the receptor protein tyrosine phosphatase (RPTP)-beta (also known as PTP-zeta). Current evidence suggests that this proteoglycan influences axon growth in development and regeneration, displaying inhibitory or stimulatory effects dependent on the mode of presentation, and the neuronal lineage. These effects seem to be mediated by neuronal receptors of the Ig-CAM superfamily.

Recognition molecules and neural repair

Neural recognition molecules were discovered and characterized initially for their functional roles in cell adhesion as regulators of affinity between cells and the extracellular matrix in vitro. They were then recognized as mediators or co-receptors which trigger signal transduction mechanisms affecting cell adhesion and de-adhesion. Their involvement in contact attraction and repulsion relies on cell-intrinsic properties that are modulated by the spatial contexts of their expression at particular stages of ontogenetic development, in synaptic plasticity and during regeneration after injury. The functional roles of recognition molecules in cell proliferation and migration, determination of developmental fate, growth cone guidance, and synapse formation, stabilization and modulation have been well documented not only by in vitro, but also by in vivo studies that have been greatly aided by generation of genetically altered mice. More recently, the functions of recognition molecules have been investigated under conditions of neural repair and manipulated using a broad range of genetic and pharmacological approaches to achieve a beneficial outcome. The principal aim of most therapeutically oriented approaches has been to neutralize inhibitory factors. However, less attention has been paid to enhancing repair by stimulating the stimulatory factors. When considering potential therapeutic strategies, it is worth considering that a single recognition molecule can possess domains that are conducive or repellent and that the spatial distribution of recognition molecules can determine the overall function: Recognition molecules may be repellent for neurite outgrowth when presented as barriers or steep-concentration gradients and conducive when presented as uniform substrates. The focus of this review will be on the more recent attempts to study the conducive mechanisms with the expectation that they may be able to tip the balance from a regeneration inhospitable to a hospitable environment. It is likely that a combination of the two principles, as multifactorial as each principle may be in itself, will be of therapeutic value in humans.

Delayed Nogo receptor therapy improves recovery from spinal cord contusion

OBJECTIVE

Myelin-associated inhibitors play a role in limiting axonal growth in the adult central nervous system. Blocking these inhibitors may promote neurological recovery from spinal cord contusion.

METHODS

The soluble Nogo-66 receptor (NgR(310)ecto-Fc) protein, which can neutralize three myelin inhibitors, was infused into rats after spinal cord contusion for 28 days. Treatment was initiated intrathecally at the time of injury or 3 days after injury by the intracerebroventricular route at a dose of 0.29 mg/kg/day. Recovery of locomotion and of axonal growth was assessed. Some animals received combination therapy with NgR(310)ecto-Fc plus rolipram, a cyclic adenosine monophosphate phosphodiesterase inhibitor.

RESULTS

Seven weeks after spinal injury, the Basso-Beattie Bresnahan locomotor scores were significantly improved in the 3-day delayed NgR(310)ecto-Fc treatment group (9.5 +/- 0.7; n = 16) versus the vehicle-treated group, (6.75 +/- 0.7; n = 15) (p < or = 0.01, analysis of variance). The percentage of NgR(310)ecto-Fc-treated animals able to support their weight was twice that in the control group. Delayed therapy was as efficacious as acute therapy. Addition of rolipram did not alter recovery. The beneficial behavioral effects of NgR(310)ecto-Fc correlated with sprouting of raphespinal axons in the caudal spinal cord and of corticospinal axons in the rostral spinal cord.

INTERPRETATION

NgR(310)ecto-Fc treatment improves outcome in a rodent model that closely mimicked human spinal cord injury.

Sprouting, regeneration and circuit formation in the injured spinal cord: factors and activity

Central nervous system (CNS) injuries are particularly traumatic, owing to the limited capabilities of the mammalian CNS for repair. Nevertheless, functional recovery is observed in patients and experimental animals, but the degree of recovery is variable. We review the crucial characteristics of mammalian spinal cord function, tract development, injury and the current experimental therapeutic approaches for repair. Regenerative or compensatory growth of neurites and the formation of new, functional circuits require spontaneous and experimental reactivation of developmental mechanisms, suppression of the growth-inhibitory properties of the adult CNS tissue and specific targeted activation of new connections by rehabilitative training.

Structural insight into the binding diversity between the human Nck2 SH3 domains and proline-rich proteins

Human Nck2 (hNck2) is a 380-residue adapter protein consisting of three SH3 domains and one SH2 domain. Nck2 plays a pivotal role in connecting and integrating signaling networks constituted by transmembrane receptors such as ephrinB and effectors critical for cytoskeletonal dynamics and remodeling. In this study, we aimed to determine the NMR structures and dynamic properties of the hNck2 SH3 domains and to define their ligand binding preferences with nine proline-rich peptides derived from Wire, CAP-1, CAP-2, Prk, Wrch1, Wrch2, and Nogo. The results indicate (1) the first hNck2 SH3 domain is totally insoluble. On the other hand, although the second and third hNck2 SH3 domains adopt a conserved SH3 fold, they exhibit distinctive dynamic properties. Interestingly, the third SH3 domain has a far-UV CD spectrum typical of a largely unstructured protein but exhibits {1H}-15N steady-state NOE values larger than 0.7 for most residues. (2) The HSQC titrations revealed that the two SH3 domains have differential ligand preferences. The second SH3 domain seems to prefer a consensus sequence of APx#PxR, while the third SH3 domain prefers PxAPxR. (3) Several high-affinity bindings were identified for hNck2 SH3 domains by isothermal titration calorimetry. In particular, the binding of SH3-3 with the Nogo-A peptide was discovered and shown to exhibit a Kd of 5.7 microM. Interestingly, of the three SH3-binding motifs carried by Wrch1, only the middle one was capable of binding SH3-2. Our results provide valuable clues for further functional investigations into the Nck2-mediated signaling networks.

Progesterone increases the expression of myelin basic protein and the number of cells showing NG2 immunostaining in the lesioned spinal cord

It is now widely accepted that progesterone (PROG) brings neuroprotection in lesions of the peripheral and central nervous system. Spinal cord trauma leads to neuronal degeneration, astrogliosis, demyelination, and proliferation of oligodendrocyte-precursor cells (OPCs). In this work, we studied the effects of PROG on myelin-related parameters in rats with complete spinal cord transection (TRX). To this end, sham-operated controls and rats with TRX at thoracic level T10 received vehicle or PROG (4 mg/kg/day) during 3 days. Three variables were measured in the lumbar L4 region below the lesion: (1) expression of myelin basic protein (MBP) at the protein and mRNA levels; (2) density of NG2-immunopositive cells as markers for OPCs; and (3) number of cells immunopositive for RIP, an antibody staining mature oligodendrocytes. TRX decreased MBP immunostaining in the corticospinal tract (CST) and dorsal ascending tract (DAT) but not the ventral funiculus (VF). NG2+ cells, which were detected in low number in controls, increased after TRX in the gray and white matter. RIP-positive cell number, however, remained unchanged. PROG treatment of rats with TRX enhanced the expression of MBP protein and mRNA in CST and DAT, but not VF and highly stimulated the number of cells showing NG2 immunostaining over untreated lesioned rats. Instead, density of RIP positive cells was similar in the PROG-treated and untreated lesioned groups. We propose that PROG effects on expression of MBP and the number of NG2 immunopositive cells may contribute to neuroprotection, as they go in parallel with previous results showing enhanced biochemical and morphological parameters of motoneurons of animals with TRX receiving PROG treatment.

Strategies of medical intervention in the management of acute spinal cord injury

STUDY DESIGN

: Literature review.

OBJECTIVE

: The purpose of this paper is to review clinical treatment strategies and future developments in the treatment of acute spinal cord injury.

SUMMARY OF BACKGROUND DATA

: The treatment of acute spinal cord injury continues to be supportive. The search for specialized pharmacologic agents to prevent secondary injury and promote repair or regeneration remains heated.

METHODS

: Medline search from 1996 to present limited to clinical research and basic science review articles in the English Language.

RESULTS

: Steroids continue to be administered in the clinical setting of acute spinal cord injury primarily out of peer pressure and fear of litigation. Basic science experiments suggest that modulation of post-traumatic inflammation may provide the best opportunity to arrest the secondary injury cascade. Protein kinase and metalloproteinase inhibition are promising treatment strategies. Regeneration techniques are concentrating on cell transplantation and manipulating glial receptors and protein production. Clinical investigations are limited to Phase III trials on a very select few of these drugs.

CONCLUSIONS

: While many advances in the basic science of spinal cord injury provide optimism for future treatments, clinical science lags. At present, there are no pharmacologic strategies of proven benefit. Although steroids continue to be given to patients with spinal cord injury in many institutions, evidence of deleterious effects continues to accumulate. Current standard of care management includes support of arterial oxygenation and spinal cord perfusion pressure.

Mice lacking L1 cell adhesion molecule have deficits in locomotion and exhibit enhanced corticospinal tract sprouting following mild contusion injury to the spinal cord

L1 is a member of the immunoglobulin superfamily of cell adhesion molecules that is associated with axonal growth, including formation of the corticospinal tract (CST). The present study describes the effects of L1 deletion on hindlimb function in locomotion, and examines the role of L1 in recovery and remodeling after contusive spinal cord injury (SCI) in mice. Uninjured adult L1 knockout (Y/-) mice had impaired performance on locomotor tests compared with their wild-type littermates (Y/+). Anterograde tracing demonstrated that CST axons project to thoracic, but not lumbar, levels of the spinal cord of Y/- mice, and revealed a diversion of these fibers from their position in the base of the dorsal columns. Retrograde tracing also revealed reduced numbers of descending projections from paraventricular hypothalamus and red nuclei to the lumbar spinal cord in Y/- mice. SCI at the mid-thoracic level produced a lesion encompassing the center of the spinal cord, including the site of the dorsal CST and surrounding gray matter (GM). The injury caused lasting deficits in fine aspects of locomotion. There was no effect of genotype on final lesion size or the growth of axons into the lesion area. However, injured Y/- mice demonstrated a robust expansion of CST projections throughout the GM of the cervical and thoracic spinal cord rostral to the lesion compared with Y/+ littermates. Thus, L1 is important for the development of multiple spinal projections and also contributes to the restriction of CST sprouting rostral to the site of a SCI in adults.

Long-Term Care of Paraplegic Laboratory Mammals

Repair of spinal cord injuries (SCIs) is still a major clinical challenge. Several attempts have been made to find a cure for this condition in experimental animals that could be extrapolated to humans. A key for success seems the availability of optimum animal models for testing different therapies. Complete spinal cord lesion in mammals is considered the most accurate injury model. In addition, long-term survival of animals seems more appropriate, as this increases the efficacy of the repair strategies. However, paraplegic animals require special care and treatment for proper longterm maintenance, and to date, there are no published protocols. This lack of available information has discouraged scientists from working with this injury model. Over the past 7 years, we have tested the repair efficacy of olfactory ensheathing glia in paraplegic rats for survival periods of more than 8 months. To keep these animals healthy for this long time, we adapted and administered treatments used in people with paraplegia. These same protocols (developed for rodents in our group) are being applied to paraplegic monkeys. In this review, we provide an overview of the proper handling and care of paraplegic adult laboratory mammals for long periods. This information might help other groups to optimize the outcome obtained and to better evaluate the prospect of a given experimental repair strategy. In addition, the use of human treatments in paraplegic animals provides a more realistic model for a later transfer to the clinical arena.

Nogo-B is a new physiological substrate for MAPKAP-K2

The neurite outgrowth inhibitor protein Nogo is one of 300 proteins that contain a reticulon homology domain, which is responsible for their association with the endoplasmic reticulum. Here we have found that the Nogo-B spliceform becomes phosphorylated at Ser107 in response to lipopolysaccharide in RAW264 macrophages or anisomycin in HeLa cells. The phosphorylation is prevented by SB 203580, an inhibitor of SAPK2a (stress-activated protein kinase 2a)/p38a and SAPK2b/p38b, and does not occur in embryonic fibroblasts generated from SAPK2a/p38a-deficient mice. Nogo-B is phosphorylated at Ser107 in vitro by MAPKAP-K2 [MAPK (mitogen-activated protein kinase)-activated protein kinase-2] or MAPKAP-K3, but not by other protein kinases that are known to be activated by SAPK2a/p38a. The anisomycin-induced phosphorylation of Ser107 in HeLa cells can be prevented by 'knockdown' of MAPKAP-K2 using siRNA (small interfering RNA). Taken together, our results identify Nogo-B as a new physiological substrate of MAPKAP-K2.

Retinoic acid receptor β2 promotes functional regeneration of sensory axons in the spinal cord

The embryonic CNS readily undergoes regeneration, unlike the adult CNS, which has limited axonal repair after injury. Here we tested the hypothesis that retinoic acid receptor beta2 (RARbeta2), critical in development for neuronal growth, may enable adult neurons to grow in an inhibitory environment. Overexpression of RARbeta2 in adult rat dorsal root ganglion cultures increased intracellular levels of cyclic AMP and stimulated neurite outgrowth. Stable RARbeta2 expression in DRG neurons in vitro and in vivo enabled their axons to regenerate across the inhibitory dorsal root entry zone and project into the gray matter of the spinal cord. The regenerated neurons enhanced second-order neuronal activity in the spinal cord, and RARbeta2-treated rats showed highly significant improvement in sensorimotor tasks. These findings show that RARbeta2 induces axonal regeneration programs within injured neurons and may thus offer new therapeutic opportunities for CNS regeneration.

Expression analysis of neuroleukin, calmodulin, cortactin, and Rho7/Rnd2 in the intact and injured mouse brain

Subtracted cDNA libraries from the mouse developing inferior colliculus were previously constructed between postnatal day (P) 6 and 10. In the P10-P6 subtracted library, neuroleukin, calmodulin I, cortactin, and Rho7 were identified. The goal of the present study was to analyze their distribution, at the mRNA and protein levels, in both the adult and the developing mouse brain. The four molecules showed a wide expression throughout the brain, with a neuronal-enriched localization in structures such as the cortex, the hippocampus, the cerebellum, and the inferior colliculus. The level of expression of their corresponding mRNAs increased during brain postnatal development. The expression of these molecules was also investigated 2 weeks after a mechanical lesion in the adult cerebral cortex. Neuroleukin and cortactin were found to be expressed by reactive astrocytes, while there were no changes in the expression of calmodulin and Rho7. The expression of neuroleukin, calmodulin, cortactin, and Rho7 is discussed in the context of their putative role in the maturation of the brain and in the axonal regeneration process.

Neuroprotective action of hypothalamic peptide PRP-1 at various time survivals following spinal cord hemisection

The purpose of the present study was to evaluate the neuroprotective action of proline-rich peptide-1 (PRP-1) produced by hypothalamic nuclei cells (nuclei paraventricularis and supraopticus) following lateral hemisection of spinal cord (SC). The dynamics of rehabilitative shifts were investigated at various periods of postoperative survival (1-2, 3, and 4 weeks), both with administration of PRP-1 and without it (control). We registered evoked spike flow activity in both interneurons and motoneurons of the same segment of transected and symmetric intact sides of SC and below it on the stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius) and extensor (n. peroneus communis) nerves. In the control group (administration of 0.9% saline as placebo), no significant decrease of post-stimulus activity of neurons was observed on the transected side by the 2nd week. This activity strongly decreased by week 3 postaxotomy, with some increase on the intact side, possibly of compensatory origin. No shifts occurred by the 4th week. Regardless of the period of administration, PRP-1 increased neuronal activity on the transected side, with the same activation levels on both SC sides. These data were confirmed by histochemical investigation. PRP-1 administration, both daily and every other day, for a period of 2-3 weeks led to prevention of scar formation and promotion of the re-growth of white matter nerve fibers in the damaged area. It also resulted in prevention of neuroglial elements degeneration and reduction in gliosis expression in the lesion supporting neuronal survival. Thus, PRP-1 achieved protection against "tissue stress", which was also confirmed by the registration of activity on the level of transection and restoration of the motor activity on the injured side. The obtained data propose the possibility of PRP-1 application in clinical practice for prevention of neurodegeneration of traumatic origin.

Osteonectin is a Schwann cell-secreted factor that promotes retinal ganglion cell survival and process outgrowth

We have investigated the factors made by Schwann cells (SCs) that stimulate survival and neurite outgrowth from postnatal rat retinal ganglion cells (RGCs). These effects are preserved under K252a blockade of the Trk family of neurotrophin receptors and are not fully mimicked by the action of a number of known trophic factors. To identify novel factors responsible for this regenerative activity, we have used a radiolabelling assay. Proteins made by SCs were labelled radioactively and then fed to purified RGCs. The proteins taken up by the RGCs were then isolated and further characterized. Using this assay we have identified a major 40 kDa factor taken up by RGCs, which was microsequenced and shown to be the matricellular protein osteonectin (ON). Using an in vitro assay of purified RGCs we show that ON promotes both survival and neurite outgrowth. We conclude that ON has a potential new role in promoting CNS repair.

Prolonged lesional expression of RhoA and RhoB following spinal cord injury

Inhibition of the small GTPase ras homology protein (Rho) or its downstream target, the Rho-associated kinase (ROCK), has been shown to promote axon regeneration and to improve functional recovery following spinal cord injury (SCI) in the adult rat. Here, we have analyzed the expression of RhoA and RhoB following spinal cord injury in order to assess whether Rho is a possible target for late pharmacological intervention. In control spinal cords, RhoA(+) cells were almost absent, whereas RhoB was localized to some ependymal cells, a few microglia, and some dissociated neurons. In injured spinal cords, RhoA(+) and RhoB(+)cells accumulated at perilesional areas and in the developing necrotic core early after injury at day 1. After reaching their maximum levels (RhoA at day 3; RhoB at day 1), RhoA(+) and RhoB(+) cell numbers remained significantly elevated until day 28. In areas remote from the lesion (> or =0.75 mm), a more discrete accumulation of RhoA(+) and RhoB(+) cells was observed, primarily in areas of ongoing Wallerian degeneration. RhoA and RhoB were predominantly expressed by polymorphonuclear granulocytes, ED1(+) microglia/macrophages, oligodendrocytes, some neurons, and swollen axons/neurites. Furthermore, expression was located to lesional, reactive astrocytes and fibroblastoid cells confined to areas of scar formation. Our experiments have determined that most RhoA(+) and RhoB(+) cells (>70%) are of mononuclear origin. The persistent presence of lesional RhoA(+) and RhoB(+) axon/neurite fibers over a period of 4 weeks after injury suggests that Rho inhibition is a putative therapeutic concept also for delayed intervention after SCI.

Re-establishing the regenerative potential of central nervous system axons in postnatal mice

At a certain point in development, axons in the mammalian central nervous system lose their ability to regenerate after injury. Using the optic nerve model, we show that this growth failure coincides with two developmental events: the loss of Bcl-2 expression by neurons and the maturation of astrocytes. Before postnatal day 4, when astrocytes are immature, overexpression of Bcl-2 alone supported robust and rapid optic nerve regeneration over long distances, leading to innervation of brain targets by day 4 in mice. As astrocytes matured after postnatal day 4, axonal regeneration was inhibited in mice overexpressing Bcl-2. Concurrent induction of Bcl-2 and attenuation of reactive gliosis reversed the failure of CNS axonal re-elongation in postnatal mice and led to rapid axonal regeneration over long distances and reinnervation of the brain targets by a majority of severed optic nerve fibers up to 2 weeks of age. These results suggest that an early postnatal downregulation of Bcl-2 and post-traumatic reactive gliosis are two important elements of axon regenerative failure in the CNS.

The role of complement in immunological demyelination of the mammalian spinal cord

STUDY DESIGN

Specificity of serum complement component to elicit immunological demyelination.

OBJECTIVES

To assess the role of complement components and pathways in experimental immunological demyelination of the adult rat spinal cord.

SETTING

ICORD, University of British Columbia, Vancouver, Canada.

SUBJECTS

We used 32 adult male Sprague-Dawley rats, of approximately 220 g weight.

METHODS

Rats received intraspinal infusions of demyelinating reagents, delivered by osmotic minipump, for a 7-day infusion at 0.5 microl/h. Reagents consisted of a polyclonal antibody to galactocerebroside and human serum complement. Complement sera deficient for a single component were used to assess the role of the alternative pathway, the classical pathway, and the membrane attack complex. Demyelination was assessed, at 7 days, ultrastructurally.

RESULTS

Removal of C3 protein, common to classical and alternative complement pathways, or C4 protein, a classical pathway protein, resulted in no demyelination. However, complement deficient in Factor B, an alternative pathway protein, produced effective demyelination. Upon removal of C5 or C6, membrane attack complex proteins, demyelination was also observed.

CONCLUSION

This suggests that the classical pathway is sufficient for the protocol to demyelinate the adult rat spinal cord, and that the membrane attack complex is also not required.

XTRPC1-dependent chemotropic guidance of neuronal growth cones

Calcium arising through release from intracellular stores and from influx across the plasma membrane is essential for signalling by specific guidance cues and by factors that inhibit axon regeneration. The mediators of calcium influx in these cases are largely unknown. Transient receptor potential channels (TRPCs) belong to a superfamily of Ca2+-permeable, receptor-operated channels that have important roles in sensing and responding to changes in the local environment. Here we report that XTRPC1, a Xenopus homolog of mammalian TRPC1, is required for proper growth cone turning responses of Xenopus spinal neurons to microscopic gradients of netrin-1, brain-derived neurotrophic factor and myelin-associated glycoprotein, but not to semaphorin 3A. Furthermore, XTRPC1 is required for midline guidance of axons of commissural interneurons in the developing Xenopus spinal cord. Thus, members of the TRPC family may serve as a key mediator for the Ca2+ influx that regulates axon guidance during development and inhibits axon regeneration in adulthood.

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.

Structural insight into the binding diversity between the Tyr-phosphorylated human ephrinBs and Nck2 SH2 domain

The binding interaction between the Nck2 SH2 domain and the phosphorylated ephrinB initiates a critical pathway for the reverse signaling network mediated by Eph receptor-ephrinB. Previously, the NMR structure and Tyr phosphorylations of the human ephrinB cytoplasmic domain have been studied. To obtain a complete story, it would be of significant interest to determine the structure of the Nck2 SH2 domain that shows a low sequence identity to other SH2 domains with known structures. Here, we report the determination of the solution structure of the human Nck2 SH2 domain and investigate its interactions with three phosphorylated ephrinB fragments by NMR spectroscopy. The results indicate that: 1) although the human Nck2 SH2 domain adopts a core tertiary fold common to all SH2 domains, it owns some unique properties such as a shorter C-terminal helix and unusual electrostatic potential surface. However, the most striking finding is that the C-terminal tail of the human Nck2 SH2 domain adopts a short antiparallel beta-sheet that, to the best of our knowledge, has never been identified in other SH2 domains. The truncation study suggests that one function of the C-terminal tail is to control the folding/solubility of the SH2 domain. 2) In addition to [Tyr(P)304]ephrinB2(301-322) and [Tyr(P)316]ephrinB2(301-322), here we identified [Tyr(P)330]ephrinB2(324-333) also capable of binding to the SH2 domain. The detailed NMR study indicated that the binding mechanisms for the three ephrinB fragments might be different. The binding with [Tyr(P)304]-ephrinB2(301-322) and [Tyr(P)316]ephrinB2(301-322) might be mostly involved in the residues over the N-half of the SH2 domain and provoked a significant increase in the backbone and side chain dynamics of the SH2 domain on the microsecond-millisecond time scale. In contrast, binding with [Tyr(P)330]ephrinB2(324-333) might have most residues over both halves engaged but induced less profound conformational dynamics on the mus-ms time scale.

Potent glycan inhibitors of myelin-associated glycoprotein enhance axon outgrowth in vitro

Myelin-associated glycoprotein (MAG, Siglec-4) is one of several endogenous axon regeneration inhibitors that limit recovery from central nervous system injury and disease. Molecules that block such inhibitors may enhance axon regeneration and functional recovery. MAG, a member of the Siglec family of sialic acid-binding lectins, binds to sialoglycoconjugates on axons and particularly to gangliosides GD1a and GT1b, which may mediate some of the inhibitory effects of MAG. In a prior study, we identified potent monovalent sialoside inhibitors of MAG using a novel screening platform. In the current study, the most potent of these were tested for their ability to reverse MAG-mediated inhibition of axon outgrowth from rat cerebellar granule neurons in vitro. Monovalent sialoglycans enhanced axon regeneration in proportion to their MAG binding affinities. The most potent glycoside was disialyl T antigen (NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc-R), followed by 3-sialyl T antigen (NeuAcalpha2-3Galbeta1-3GalNAc-R), structures expressed on O-linked glycoproteins as well as on gangliosides. Prior studies indicated that blocking gangliosides reversed MAG inhibition. In the current study, blocking O-linked glycoprotein sialylation with benzyl-alpha-GalNAc had no effect. The ability to reverse MAG inhibition with monovalent glycosides encourages further exploration of glycans and glycan mimetics as blockers of MAG-mediated axon outgrowth inhibition.

Inhibition of Nogo: a key strategy to increase regeneration, plasticity and functional recovery of the lesioned central nervous system

In the adult central nervous system (CNS) myelin and oligodendrocytes, Nogo-A exerts a growth inhibitory function leading to restricted axonal regeneration. After development of different anti-Nogo-A antibodies and other Nogo-A blocking reagents their application has recently been studied in various in vivo animal models of spinal cord injury and stroke. These studies show that intracerebral application of Nogo-A-inactivating reagents leads to enhanced regeneration and compensatory sprouting, structural reorganization or plasticity, and functional recovery as seen in different behavioural analyses.

Structural characterization of the human Nogo-A functional domains. Solution structure of Nogo-40, a Nogo-66 receptor antagonist enhancing injured spinal cord regeneration

The recent discovery of the Nogo family of myelin inhibitors and the Nogo-66 receptor opens up a very promising avenue for the development of therapeutic agents for treating spinal cord injury. Nogo-A, the largest member of the Nogo family, is a multidomain protein containing at least two regions responsible for inhibiting central nervous system (CNS) regeneration. So far, no structural information is available for Nogo-A or any of its structural domains. We have subcloned and expressed two Nogo-A fragments, namely the 182 residue Nogo-A(567-748) and the 66 residue Nogo-66 in Escherichia coli. CD and NMR characterization indicated that Nogo-A(567-748) was only partially structured while Nogo-66 was highly insoluble. Nogo-40, a truncated form of Nogo-66, has been previously shown to be a Nogo-66 receptor antagonist that is able to enhance CNS neuronal regeneration. Detailed NMR examinations revealed that a Nogo-40 peptide had intrinsic helix-forming propensity, even in an aqueous environment. The NMR structure of Nogo-40 was therefore determined in the presence of the helix-stabilizing solvent trifluoroethanol. The solution structure of Nogo-40 revealed two well-defined helices linked by an unstructured loop, representing the first structure of Nogo-66 receptor binding ligands. Our results provide the first structural insights into Nogo-A functional domains and may have implications in further designs of peptide mimetics that would enhance CNS neuronal regeneration.

Segregation of Nogo66 receptors into lipid rafts in rat brain and inhibition of Nogo66 signaling by cholesterol depletion

NogoA, a myelin-associated component, inhibits neurite outgrowth. Nogo66, a portion of NogoA, binds to Nogo66 receptor (NgR) and induces the inhibitory signaling. LINGO-1 and p75 neurotrophin receptor (p75), the low-affinity nerve growth factor receptor, are also required for NogoA signaling. However, signaling mechanisms downstream to Nogo receptor remain poorly understood. Here, we observed that NgR and p75 were colocalized in low-density membrane raft fractions derived from forebrains and cerebella as well as from cerebellar granule cells. NgR interacted with p75 in lipid rafts. In addition, disruption of lipid rafts by beta-methylcyclodextrin, a cholesterol-binding reagent, reduced the Nogo66 signaling. Our results suggest an important role of lipid rafts in facilitating the interaction between NgRs and provide insight into mechanisms underlying the inhibition of neurite outgrowth by NogoA.

Adult neural stem cells and repair of the adult central nervous system

Neural stem cells are present not only in the developing nervous systems, but also in the adult central nervous system of mammals, including humans. The mature central nervous system has been traditionally regarded as an unfavorable environment for the regeneration of damaged axons of mature neurons and the generation of new neurons. In the adult central nervous system, however, newly generated neurons from adult neural stem cells in specific regions exhibit a striking ability to migrate, send out long axonal and dendritic projections, integrate into pre-existing neuronal circuits, and contribute to normal brain functions. Adult stem cells with potential neural capacity recently have been isolated from various neural and nonneural sources. Rapid advances in the stem cell biology have raised exciting possibilities of replacing damaged or lost neurons by activation of endogenous neural stem cells and/or transplantation of in vitro-expanded stem cells and/or their neuronal progeny. Before the full potential of adult stem cells can be realized for regenerative medicine, we need to identify the sources of stem cells, to understand mechanisms regulating their proliferation, fate specification, and, most importantly in the case of neuronal lineages, to characterize their functional properties. Equally important, we need to understand the neural development processes in the normal and diseased adult central nervous system environment, which is quite different from the embryonic central nervous system, where neural development has been traditionally investigated. Here we will review some recent progress of adult neural stem cell research that is applicable to developmental neurobiology and also has potential implications in clinical neuroscience.

Regulation of Nogo and Nogo receptor during the development of the entorhino-hippocampal pathway and after adult hippocampal lesions

Axonal regeneration in the adult CNS is limited by the presence of several inhibitory proteins associated with myelin. Nogo-A, a myelin-associated inhibitor, is responsible for axonal outgrowth inhibition in vivo and in vitro. Here we study the onset and maturation of Nogo-A and Nogo receptor in the entorhino-hippocampal formation of developing and adult mice. We also provide evidence that Nogo-A does not inhibit embryonic hippocampal neurons, in contrast to other cell types such as cerebellar granule cells. Our results also show that Nogo and Nogo receptor mRNA are expressed in the adult by both principal and local-circuit hippocampal neurons, and that after lesion, Nogo-A is also transiently expressed by a subset of reactive astrocytes. Furthermore, we analyzed their regulation after kainic acid (KA) treatment and in response to the transection of the entorhino-hippocampal connection. We found that Nogo-A and Nogo receptor are differentially regulated after kainic acid or perforant pathway lesions. Lastly, we show that the regenerative potential of lesioned entorhino-hippocampal organotypic slice co-cultures is increased after blockage of Nogo-A with two IN-1 blocking antibodies. In conclusion, our results show that Nogo and its receptor might play key roles during development of hippocampal connections and that they are implicated in neuronal plasticity in the adult.

Neuron-binding human monoclonal antibodies support central nervous system neurite extension

Two human IgMs (sHIgM12 and sHIgM42) were identified that supported in vitro central nervous system (CNS) neurite extension equal to the potent neurite stimulatory molecule laminin. Both IgMs bound to multiple cell types in unfixed CNS tissue and to the surface of neurons in culture. Both monoclonal antibodies (mAbs) overrode the inhibitory effect of CNS mouse myelin on granule cell neurite extension. Neither mAb bound to the surface of mature oligodendrocytes or strictly colocalized with myelin proteins. Sialidase treatment eliminated the neuronal surface binding of both mAbs, whereas blocking sphingolipid synthesis with Fumonisin B1 or removing GPI-linked proteins with PIPLC did not. When used as substrates for mixed neuron/glia aggregates, sHIgM12 and sHIgM42 supported robust neurite extension while astrocytes remained in the aggregates. In contrast, laminin supported astrocyte migration and spreading. Human mAbs that support neurite extension are novel factors that may be of use in encouraging axon repair following injury while minimizing glial cell infiltration. Both human mAbs were isolated from individuals with monoclonal gammopathy. Each individual has carried high mAb titers in circulation for years without detriment. sHIgM12 and sHIgM42 are therefore unlikely to be systemically pathogenic.

Lesional Expression of RhoA and RhoB following Traumatic Brain Injury in Humans

Inhibition of the small GTPase Rho or of its downstream target Rho-associated kinase (ROCK) has been shown to promote axon regeneration and to improve functional recovery following traumatic CNS lesions in the adult rat. In order to determine the expression pattern of RhoA and RhoB following human traumatic brain injury (TBI) and to assess whether Rho is a possible target for pharmacological intervention in humans, we investigated expression patterns of RhoA and RhoB in brain specimens from 25 patients who died after closed TBI in comparison to brain tissue derived from four neuropathologically unaffected control patients by immunohistochemistry. A highly significant lesional upregulation of both RhoA and RhoB was observed beginning several hours after the traumatic event and continuing for months after TBI. The cellular sources of both molecules included polymorphonuclear granulocytes, monocytes/macrophages, and reactive astrocytes. Additionally, expression of RhoA was also detected in neuronal cells in some of the cases. From our data, we conclude that inhibition of Rho is a promising mechanism for the development of new pharmacological interventions in human TBI. As the observed upregulation of RhoA and RhoB was still detectable months after TBI, we speculate that even delayed treatment with Rho inhibitors might be a therapeutic option.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

Star-cross'd neurons: astroglial effects on neural repair in the adult mammalian CNS

Astroglia have long been thought to play merely a supporting role in the life of the neuron. However, these star-shaped cells have recently been the focus of intense study that has begun to emphasize remarkable and novel roles for these amazing cells. While astroglia play positive roles in the life of the neuron, they can simultaneously exert negative influences. Kinouchi et al. convincingly demonstrate and characterize an inhibitory role played by astroglia after neuronal transplantation. These findings remind us that astroglia exert positive and negative influences on neuronal survival, migration, neurite outgrowth and functional integration. Here, we review the complementary and often contradictory roles of astroglia during neuronal integration.

Oligodendrocyte myelin glycoprotein (OMgp): evolution, structure and function

The oligodendrocyte myelin glycoprotein (OMgp) is a glycosylphosphatidylinositol-anchored protein expressed by neurons and oligodendrocytes in the central nervous system (CNS). Although the precise function of OMgp is yet to be determined in vivo, recent in vitro studies suggested roles for this protein in both the developing and adult central nervous system. In vitro experiments demonstrated the participation of OMgp in growth cone collapse and inhibition of neurite outgrowth through its interaction with NgR, the receptor for Nogo. This function requires its leucine-rich repeat domain, a highly conserved region in OMgp during mammal evolution. OMgp leucine-rich repeat domain is also implicated in the inhibition of cell proliferation. Based on its developmental expression, localization and structure, OMgp may also be involved in the formation and maintenance of myelin sheaths. Cell proliferation, neuronal sprouting and myelination are crucial processes involved in brain development and regeneration after injury. Here, we review the information available on the structure and evolution of OMgp, summarize its tissue expression and discuss its putative role(s) during the development and in adult CNS.

Transection of the adult rat spinal cord upregulates EphB3 receptor and ligand expression

Eph receptors and ligands represent two families of proteins that control axonal guidance during development. Recent work has shown that several Eph receptors are expressed postnatally. Because the Eph molecules represent a class of axon guidance molecules that are mainly inhibitory to axonal growth, we investigated whether EphB3 expression was upregulated in both spinal cord and four supraspinal nuclei (locus coeruleus, vestibular, raphe pallidus, and red) 1 week after a complete spinal cord thoracic transection. Injured rats had a significant increase in EphB3 mRNA and protein expression in the spinal cord. The increased EphB3 expression was colocalized with GFAP staining and indicated that astrocytes play a role in EphB3 expression after spinal cord injury. No change in EphB3 expression was seen in supraspinal brain nuclei, which further demonstrated that changes in expression were due to changes in the local microenvironment at the injury site. The expression of EphB3 was colocalized to regions of the CNS that had a high level of EphB3 binding ligands. These data indicate upregulation of EphB3 expression after injury may also contribute to an environment in the spinal cord that is inhibitory to axonal regeneration.