Nogo-A interacts with the Nogo-66 receptor through multiple sites to create an isoform-selective subnanomolar agonist

Development of neural repair therapy for chronic spinal cord trauma: soluble Nogo receptor decoy from discovery to clinical trial

PURPOSE OF REVIEW

After traumatic spinal cord injury (SCI), neurological deficits persist due to the disconnection of surviving neurons. While repair of connectivity may restore function, no medical therapy exists today.This review traces the development of the neural repair-based therapeutic AXER-204 from animal studies to the recent clinical trial for chronic cervical SCI.

RECENT FINDINGS

Molecular studies reveal a Nogo-66 Receptor 1 (NgR1, RTN4R) pathway inhibiting axon regeneration, sprouting, and plasticity in the adult mammalian central nervous system (CNS). Rodent and nonhuman primate studies demonstrate that the soluble receptor decoy NgR(310)ecto-Fc or AXER-204 promotes neural repair and functional recovery in transection and contusion SCI. Recently, this biological agent completed a first-in-human and randomized clinical trial for chronic cervical SCI. The intervention was safe and well tolerated. Across all participants, upper extremity strength did not improve with treatment. However, posthoc and biomarker analyses suggest that AXER-204 may benefit treatment-naïve patients with incomplete SCI in the chronic stage.

SUMMARY

NgR1 signaling restricts neurological recovery in animal studies of CNS injury. The recent clinical trial of AXER-204 provides encouraging signals supporting future focused trials of this neural repair therapeutic. Further, AXER-204 studies provide a roadmap for the development of additional and synergistic therapies for chronic SCI.

Strategies to neutralize RhoA/ROCK pathway after spinal cord injury

Regeneration is bungled following CNS injuries, including spinal cord injury (SCI). Inherent decay of permissive conditions restricts the regrowth of the mature CNS after an injury. Hypertrophic scarring, insignificant intrinsic axon-growth activity, and axon-growth inhibitory molecules such as myelin inhibitors and scar inhibitors constitute a significant hindrance to spinal cord repair. Besides these molecules, a combined absence of various mechanisms responsible for axonal regeneration is the main reason behind the dereliction of the adult CNS to regenerate. The neutralization of specific inhibitors/proteins by stymieing antibodies or encouraging enzymatic degradation results in improved axon regeneration. Previous efforts to induce regeneration after SCI have stimulated axonal development in or near lesion sites, but not beyond them. Several pathways are responsible for the axonal growth obstruction after a CNS injury, including SCI. Herein, we summarize the axonal, glial, and intrinsic factor which impedes the regeneration. We have also discussed the methods to stabilize microtubules and through this to maintain the proper cytoskeletal dynamics of growth cone as disorganized microtubules lead to the failure of axonal regeneration. Moreover, we primarily focus on diverse inhibitors of axonal growth and molecular approaches to counteract them and their downstream intracellular signaling through the RhoA/ROCK pathway.

Identification of a new functional domain of Nogo-A that promotes inflammatory pain and inhibits neurite growth through binding to NgR1

Nogo-A is a key inhibitory molecule to axon regeneration, and plays diverse roles in other pathological conditions, such as stroke, schizophrenia, and neurodegenerative diseases. Nogo-66 and Nogo-Δ20 fragments are two known functional domains of Nogo-A, which act through the Nogo-66 receptor (NgR1) and sphingosine-1-phosphate receptor 2 (S1PR2), respectively. Here, we reported a new functional domain of Nogo-A, Nogo-A aa 846-861, was identified in the Nogo-A-specific segment that promotes complete Freund's adjuvant (CFA)-induced inflammatory pain. Intrathecal injection of its antagonist peptide 846-861PE or the specific antibody attenuated the CFA-induced inflammatory heat hyperalgesia. The 846-861 PE reduced the content of transient receptor potential vanilloid subfamily member 1 (TRPV1) in dorsal root ganglia (DRG) and decreased the response of DRG neurons to capsaicin. These effects were accompanied by a reduction in LIMK/cofilin phosphorylation and actin polymerization. GST pull-down and fluorescence resonance energy transfer (FRET) assays both showed that Nogo-A aa 846-861 bound to NgR1. Moreover, we demonstrated that Nogo-A aa 846-861 inhibited neurite outgrowth from cortical neurons and DRG explants. We concluded that Nogo-A aa 846-861 is a novel ligand of NgR1, which activates the downstream signaling pathways that inhibit axon growth and promote inflammatory pain.

Can We Design a Nogo Receptor-Dependent Cellular Therapy to Target MS?

The current landscape of therapeutics designed to treat multiple sclerosis (MS) and its pathological sequelae is saturated with drugs that modify disease course and limit relapse rates. While these small molecules and biologicals are producing profound benefits to patients with reductions in annualized relapse rates, the repair or reversal of demyelinated lesions with or without axonal damage, remains the principle unmet need for progressive forms of the disease. Targeting the extracellular pathological milieu and the signaling mechanisms that drive neurodegeneration are potential means to achieve neuroprotection and/or repair in the central nervous system of progressive MS patients. The Nogo-A receptor-dependent signaling mechanism has raised considerable interest in neurological disease paradigms since it can promulgate axonal transport deficits, further demyelination, and extant axonal dystrophy, thereby limiting remyelination. If specific therapeutic regimes could be devised to directly clear the Nogo-A-enriched myelin debris in an expedited manner, it may provide the necessary CNS environment for neurorepair to become a clinical reality. The current review outlines novel means to achieve neurorepair with biologicals that may be directed to sites of active demyelination.

Nogo-A promotes inflammatory heat hyperalgesia by maintaining TRPV-1 function in the rat dorsal root ganglion neuron

Nogo-A is a key inhibitory molecule of axon regeneration in oligodendrocytes. However, little is known about its role in adult neurons. In this study, we showed an important function of Nogo-A on regulation of inflammatory pain in dorsal root ganglion (DRG) neurons. In adult rats with complete Freund's adjuvant (CFA) hind paw inflammation, DRG neurons showed a significant increase in Nogo-A expression. Disruption of Nogo-A signaling with Nogo-66 receptor antagonist peptide, Nogo-A blocking antibody, Nogo-A short hairpin RNA, or Nogo-A gene knockout attenuated CFA-induced inflammatory heat hyperalgesia. Moreover, disruption of Nogo-A signaling suppressed the function and expression in DRG neurons of the transient receptor potential vanilloid subfamily member (TRPV)-1 channel, which is known to be the endogenous transducer of noxious heat during inflammation. These effects were accompanied with a reduction in LIM domain kinase (LIMK)/cofilin phosphorylation and actin polymerization. Similar disruption of actin filament architecture by direct action of Latrunculin A reduced the TRPV-1 activity and up-regulation of TRPV-1 protein caused by CFA. We conclude that Nogo-A plays an essential role in the development of inflammatory heat hyperalgesia, partly through maintaining TRPV-1 function via activation of the LIMK/cofilin pathway, which regulates actin filament dynamics. These findings support a therapeutic potential of modulating Nogo-A signaling in pain management.-Hu, F., Liu, H.-C., Su, D.-Q., Chen, H.-J., Chan, S.-O., Wang, Y., Wang, J. Nogo-A promotes inflammatory heat hyperalgesia by maintaining TRPV-1 function in the rat dorsal root ganglion neuron.

The Involvement of the Myelin-Associated Inhibitors and Their Receptors in CNS Plasticity and Injury

The limited capacity for the central nervous system (CNS) to repair itself was first described over 100 years ago by Spanish neuroscientist Ramon Y. Cajal. However, the exact mechanisms underlying this failure in neuronal regeneration remain unclear and, as such, no effective therapeutics yet exist. Numerous studies have attempted to elucidate the biochemical and molecular mechanisms that inhibit neuronal repair with increasing evidence suggesting that several inhibitory factors and repulsive guidance cues active during development actually persist into adulthood and may be contributing to the inhibition of repair. For example, in the injured adult CNS, there are various inhibitory factors that impede the outgrowth of neurites from damaged neurons. One of the most potent of these neurite outgrowth inhibitors is the group of proteins known as the myelin-associated inhibitors (MAIs), present mainly on the membranes of oligodendroglia. Several studies have shown that interfering with these proteins can have positive outcomes in CNS injury models by promoting neurite outgrowth and improving functional recovery. As such, the MAIs, their receptors, and downstream effectors are valid drug targets for the treatment of CNS injury. This review will discuss the current literature on MAIs in the context of CNS development, plasticity, and injury. Molecules that interfere with the MAIs and their receptors as potential candidates for the treatment of CNS injury will additionally be introduced in the context of preclinical and clinical trials.

Multiple Roles for Nogo Receptor 1 in Visual System Plasticity

During the developmental critical period for visual plasticity, discordant vision alters the responsiveness of neurons in visual cortex. The subsequent closure of the critical period not only consolidates neural function but also limits recovery of acuity from preceding abnormal visual experience. Despite species-specific differences in circuitry of the visual system, these characteristics are conserved. The nogo-66 receptor 1 (ngr1) is one of only a small number of genes identified thus far that is essential to closing the critical period. Mice lacking a functional ngr1 gene retain developmental visual plasticity as adults and their visual acuity spontaneously improves after prolonged visual deprivation. Experiments employing conditional mouse genetics have revealed that ngr1 restricts plasticity within distinct circuits for ocular dominance and visual acuity. However, the mechanisms by which NgR1 limits plasticity have not been elucidated, in part because the subcellular localization and signal transduction of the protein are only partially understood. Here we explore potential mechanisms for NgR1 function in relation to manipulations that reactivate visual plasticity in adults and propose lines of investigation to address relevant gaps in knowledge.

Intravitreal delivery of human NgR-Fc decoy protein regenerates axons after optic nerve crush and protects ganglion cells in glaucoma models

PURPOSE

Glaucoma is a major cause of vision loss due to retinal ganglion cell (RGC) degeneration. Therapeutic intervention controls increased IOP, but neuroprotection is unavailable. NogoReceptor1 (NgR1) limits adult central nervous system (CNS) axonal sprouting and regeneration. We examined NgR1 blocking decoy as a potential therapy by defining the pharmacokinetics of intravitreal NgR(310)-Fc, its promotion of RGC axonal regeneration following nerve crush, and its neuroprotective effect in a microbead glaucoma model.

METHODS

Human NgR1(310)-Fc was administered intravitreally, and levels were monitored in rat vitreal humor and retina. Axonal regeneration after optic nerve crush was assessed by cholera toxin β anterograde labeling. In a microbead model of glaucoma with increased IOP, the number of surviving and actively transporting RGCs was determined after 4 weeks by retrograde tracing with Fluro-Gold (FG) from the superior colliculus.

RESULTS

After intravitreal bolus administration, the terminal half-life of NgR1(310)-Fc between 1 and 7 days was approximately 24 hours. Injection of 5 μg protein once per week after optic nerve crush injury significantly increased RGCs with regenerating axons. Microbeads delivered to the anterior chamber increased pressure, and caused 15% reduction in FG-labeled RGCs of control rats, with a 40% reduction in large diameter RGCs. Intravitreal treatment with NgR1(310)-Fc did not reduce IOP, but maintained large diameter RGC density at control levels.

CONCLUSIONS

Human NgR1(310)-Fc has favorable pharmacokinetics in the vitreal space and rescues large diameter RGC counts from increased IOP. Thus, the NgR1 blocking decoy protein may have efficacy as a disease-modifying therapy for glaucoma.

The role of reticulons in neurodegenerative diseases

Reticulons (RTNs) are a group of membrane-associated proteins mainly responsible for shaping the tubular endoplasmic reticulum network, membrane trafficking, inhibition of axonal growth, and apoptosis. These proteins share a common sequence feature, the reticulon homology domain, which consists of paired hydrophobic stretches that are believed to induce membrane curvature by acting as a wedge in bilayer membranes. RTNs are ubiquitously expressed in all tissues, but each RTN member exhibits a unique expression pattern that prefers certain tissues or even cell types. Recently, accumulated evidence has suggested additional and unexpected roles for RTNs, including those on DNA binding, autophagy, and several inflammatory-related functions. These manifold actions of RTNs account for their ever-growing recognition of their involvement in neurodegenerative diseases like Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as hereditary spastic paraplegia. This review summarizes the latest discoveries on RTNs in human pathophysiology, and the engagement of these in neurodegeneration, along with the implications of these findings for a better understanding of the molecular events triggered by RTNs and their potential exploitation as next-generation therapeutics.

Experience-dependent expression of Nogo-A and Nogo receptor in the developing rat visual cortex

Nogo-A and Nogo receptor (NgR) expression in the visual cortex following a critical developmental period (postnatal days 20-60) has been previously shown. However, little is known regarding Nogo-A and NgR expression between postnatal day 0 and initiation of the critical period. The present study analyzed Nogo-A and NgR expression at four different time points: postnatal day 0 (P0), before critical period (P14), during critical period (P28), and after critical period (P60). Results showed significantly increased Nogo-A mRNA and protein expression levels in the visual cortex following birth, and expression levels remained steady between P28 and P60. NgR mRNA or protein expression was dramatically upregulated with age and peaked at P14 or P28, respectively, and maintained high expression to P60. In addition, Nogo-A and NgR expression was analyzed in each visual cortex layer in normal developing rats and rats with monocular deprivation. Monocular deprivation decreased Nogo-A and NgR mRNA and protein expression in the rat visual cortex, in particular in layers II-III and IV in the visual cortex contralateral to the deprived eye. These findings suggested that Nogo-A and NgR regulated termination of the critical period in experience- dependent visual cortical plasticity.

Arabidopsis RTNLB1 and RTNLB2 Reticulon-like proteins regulate intracellular trafficking and activity of the FLS2 immune receptor

Receptors localized at the plasma membrane are critical for the recognition of pathogens. The molecular determinants that regulate receptor transport to the plasma membrane are poorly understood. In a screen for proteins that interact with the FLAGELIN-SENSITIVE2 (FLS2) receptor using Arabidopsis thaliana protein microarrays, we identified the reticulon-like protein RTNLB1. We showed that FLS2 interacts in vivo with both RTNLB1 and its homolog RTNLB2 and that a Ser-rich region in the N-terminal tail of RTNLB1 is critical for the interaction with FLS2. Transgenic plants that lack RTNLB1 and RTNLB2 (rtnlb1 rtnlb2) or overexpress RTNLB1 (RTNLB1ox) exhibit reduced activation of FLS2-dependent signaling and increased susceptibility to pathogens. In both rtnlb1 rtnlb2 and RTNLB1ox, FLS2 accumulation at the plasma membrane was significantly affected compared with the wild type. Transient overexpression of RTNLB1 led to FLS2 retention in the endoplasmic reticulum (ER) and affected FLS2 glycosylation but not FLS2 stability. Removal of the critical N-terminal Ser-rich region or either of the two Tyr-dependent sorting motifs from RTNLB1 causes partial reversion of the negative effects of excess RTNLB1 on FLS2 transport out of the ER and accumulation at the membrane. The results are consistent with a model whereby RTNLB1 and RTNLB2 regulate the transport of newly synthesized FLS2 to the plasma membrane.

Cell-specific aptamers as emerging therapeutics

Aptamers are short nucleic acids that bind to defined targets with high affinity and specificity. The first aptamers have been selected about two decades ago by an in vitro process named SELEX (systematic evolution of ligands by exponential enrichment). Since then, numerous aptamers with specificities for a variety of targets from small molecules to proteins or even whole cells have been selected. Their applications range from biosensing and diagnostics to therapy and target-oriented drug delivery. More recently, selections using complex targets such as live cells have become feasible. This paper summarizes progress in cell-SELEX techniques and highlights recent developments, particularly in the field of medically relevant aptamers with a focus on therapeutic and drug-delivery applications.

A multi-domain fragment of Nogo-A protein is a potent inhibitor of cortical axon regeneration via Nogo receptor 1

Nogo-A limits axon regeneration and functional recovery after central nervous system injury in adult mammals. Three regions of Nogo-A (Nogo-A-24, Nogo-66, and Nogo-C39) interact with the neuronal Nogo-66 receptor 1 (NgR1). Nogo-66 also interacts with a structurally unrelated cell surface receptor, paired immunoglobulin-like receptor (PirB). We show here that the other two NgR1-interacting domains, Nogo-A-24 and Nogo-C39, also bind to PirB with high affinity. A purified 22-kDa protein containing all three NgR1- and PirB-interacting domains (Nogo-22) is a substantially more potent growth cone-collapsing molecule than Nogo-66 for chick dorsal root ganglion neurons and mature cortical neurons. Moreover, Nogo-22 inhibits axon regeneration of mature cortical neurons in vitro more potently than does Nogo-66. Although all three NgR1-interacting domains of Nogo-A also interact with PirB, expression of PirB in mature cortical cultures is nearly undetectable. Consistent with a relatively minor role for PirB in mature cortical neurons, Nogo-22 inhibition of axon regeneration is abolished by genetic deletion of NgR1. Thus, NgR1 is the predominant receptor for Nogo-22 in regenerating cortical neurons.

Resolution of disulfide heterogeneity in Nogo receptor I fusion proteins by molecular engineering

NgRI (Nogo-66 receptor) is part of a signalling complex that inhibits axon regeneration in the central nervous system. Truncated soluble versions of NgRI have been used successfully to promote axon regeneration in animal models of spinal-cord injury, raising interest in this protein as a potential therapeutic target. The LRR (leucine-rich repeat) regions in NgRI are flanked by N- and C-terminal disulfide-containing 'cap' domains (LRRNT and LRRCT respectively). In the present work we show that, although functionally active, the NgRI(310)-Fc fusion protein contains mislinked and heterogeneous disulfide patterns in the LRRCT domain, and we report the generation of a series of variant molecules specifically designed to prevent this heterogeneity. Using these variants we explored the effects of modifying the NgRI truncation site or the spacing between the NgRI and Fc domains, or replacing cysteines within the NgRI or IgG hinge regions. One variant, which incorporates replacements of Cys²⁶⁶ and Cys³⁰⁹ with alanine residues, completely eliminated disulfide scrambling while maintaining functional in vitro and in vivo efficacy. This modified NgRI-Fc molecule represents a significantly improved candidate for further pharmaceutical development, and may serve as a useful model for the optimization of other IgG fusion proteins made from LRR proteins.

Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin

VIDEO ABSTRACT

The most common inherited form of Frontotemporal Lobar Degeneration (FTLD) known stems from Progranulin (GRN) mutation and exhibits TDP-43 plus ubiquitin aggregates. Despite the causative role of GRN haploinsufficiency in FTLD-TDP, the neurobiology of this secreted glycoprotein is unclear. Here, we examined PGRN binding to the cell surface. PGRN binds to cortical neurons via its C terminus, and unbiased expression cloning identifies Sortilin (Sort1) as a binding site. Sort1⁻/⁻ neurons exhibit reduced PGRN binding. In the CNS, Sortilin is expressed by neurons and PGRN is most strongly expressed by activated microglial cells after injury. Sortilin rapidly endocytoses and delivers PGRN to lysosomes. Mice lacking Sortilin have elevations in brain and serum PGRN levels of 2.5- to 5-fold. The 50% PGRN decrease causative in FTLD-TDP cases is mimicked in GRN+/⁻ mice, and is fully normalized by Sort1 ablation. Sortilin-mediated PGRN endocytosis is likely to play a central role in FTLD-TDP pathophysiology.

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.

MAG and OMgp synergize with Nogo-A to restrict axonal growth and neurological recovery after spinal cord trauma

Functional recovery after adult CNS damage is limited in part by myelin inhibitors of axonal regrowth. Three molecules, Nogo-A, MAG, and OMgp, are produced by oligodendrocytes and share neuronal receptor mechanisms through NgR1 and PirB. While each has an axon-inhibitory role in vitro, their in vivo interactions and relative potencies have not been defined. Here, we compared mice singly, doubly, or triply mutant for these three myelin inhibitor proteins. The myelin extracted from Nogo-A mutant mice is less inhibitory for axons than is that from wild-type mice, but myelin lacking MAG and OMgp is indistinguishable from control. However, myelin lacking all three inhibitors is less inhibitory than Nogo-A-deficient myelin, uncovering a redundant and synergistic role for all three proteins in axonal growth inhibition. Spinal cord injury studies revealed an identical in vivo hierarchy of these three myelin proteins. Loss of Nogo-A allows corticospinal and raphespinal axon growth above and below the injury, as well as greater behavioral recovery than in wild-type or heterozygous mutant mice. In contrast, deletion of MAG and OMgp stimulates neither axonal growth nor enhanced locomotion. The triple-mutant mice exhibit greater axonal growth and improved locomotion, consistent with a principal role for Nogo-A and synergistic actions for MAG and OMgp, presumably through shared receptors. These data support the hypothesis that targeting all three myelin ligands, as with NgR1 decoy receptor, provides the optimal chance for overcoming myelin inhibition and improving neurological function.

Identification and characterization of alternative promoters of zebrafish Rtn-4/Nogo genes in cultured cells and zebrafish embryos

In mammals, the Nogo family consists of Nogo-A, Nogo-B and Nogo-C. However, there are three Rtn-4/Nogo-related transcripts were identified in zebrafish. In addition to the common C-terminal region, the N-terminal regions of Rtn4-n/Nogo-C1, Rtn4-m/Nogo-C2 and Rtn4-l/Nogo-B, respectively, contain 9, 25 and 132 amino acid residues. In this study, we isolated the 5'-upstream region of each gene from a BAC clone and demonstrated that the putative promoter regions, P1-P3, are functional in cultured cells and zebrafish embryos. A transgenic zebrafish Tg(Nogo-B:GFP) line was generated using P1 promoter region to drive green fluorescent protein (GFP) expression through Tol2-mediated transgenesis. This line recapitulates the endogenous expression pattern of Rtn4-l/Nogo-B mRNA in the brain, brachial arches, eyes, muscle, liver and intestines. In contrast, GFP expressions by P2 and P3 promoters were localized to skeletal muscles of zebrafish embryos. Several GATA and E-box motifs are found in these promoter regions. Using morpholino knockdown experiments, GATA4 and GATA6 were involved in the control of P1 promoter activity in the liver and intestine, while Myf5 and MyoD for the control of P1 and P3 promoter activities in muscles. These data demonstrate that zebrafish Rtn4/Nogo transcripts might be generated by coupling mechanisms of alternative first exons and alternative promoter usage.

Reticulon 3 is an interacting partner of the SALM family of adhesion molecules

Synaptic adhesion-like molecules (SALMs) are a recently discovered family of adhesion molecules that is widely distributed in the central nervous system and has been implicated in neurite outgrowth and synapse formation. To identify proteins that interact with extracellular domains of SALMs, we carried out yeast two-hybrid screening using the extracellular domain of SALM1 as bait. A clone encoding full-length reticulon 3A1 was isolated. This interaction was shown to occur through the LRR domain, which is found on all SALMs. To determine whether this relationship also occurs in brain, we performed immunoprecipitation using antibodies to SALMs 1-4. A 19-kDa band, identified as reticulon 3C, bound to all four SALMs, whereas a 90-kDa band, which did not comigrate with any known reticulon 3 variant, bound to SALMs 2 and 3. These results show that reticulon 3 may play a role in the trafficking of the SALM family of adhesion molecules.

Synergistic effect of neural stem cells and olfactory ensheathing cells on repair of adult rat spinal cord injury

Spinal cord injury (SCI) is a common clinical disease that places a heavy burden on families and society. Cellular therapy provides a method of giving a supplement of cells lost in the injury and promoting functional recovery after SCI. Neural stem cells (NSCs) and olfactory ensheathing cells (OECs) are two most promising cell types. NSCs have the potential of differentiating into neurons and glial cells, and OECs could help the axons of neurons pass through the glial scar to promote functional recovery. NSCs were isolated from the cortices of fetal rats on days 12-14 of embryonic development and OECs were isolated from the olfactory bulbs of adult rats. In vitro coculture studies demonstrated OECs could promote NSCs to differentiate into neurons. Four groups of rats that had been 3/4 spinal cord transectioned at T9 were injected with DMEM/F12 solution, NSCs, OECs, and NSCs + OECs, respectively, 7 days post-SCI. Twelve weeks postoperation, the hindlimb locomotor function of rats in the cotransplantation group was significantly improved compared with that in the other three groups. Histological observation and immunohistochemical staining of NF-200 both showed new nerve fibers across the injured region. Cotransplantation of NSCs and OECs might have a synergistic effect on promoting neural regeneration and improving the recovery of locomotion function. Cotransplantation of NSCs and OECs was better than a single graft of either NSCs or OECs. These findings have provided a new way of thinking in the treatment of SCI.

Rho-associated kinase II (ROCKII) limits axonal growth after trauma within the adult mouse spinal cord

Rho GTPases are thought to mediate the action of several axonal growth inhibitors in the adult brain and spinal cord. RhoA has been targeted pharmacologically in both humans and animals to promote neurite outgrowth and functional recovery following CNS trauma. However, rat spinal cord injury studies suggest a complicated and partial benefit of inhibiting Rho or its downstream effector, Rho-associated kinase (ROCKII). This limited benefit may reflect inhibition of other kinases, poor access, or a minimal role of ROCKII in vivo. Therefore, we studied ROCKII mutant mice to probe this pathway genetically. ROCKII(-/-) dorsal root ganglion neurons are less sensitive to inhibition by Nogo protein or by chondroitin sulfate proteoglycan in vitro. We examined adult ROCKII(-/-) mice in two injury paradigms, cervical multilevel dorsal rhizotomy and midthoracic dorsal spinal cord hemisection. After dorsal root crush injury, the ROCKII(-/-) mice recovered use of the affected forepaw more quickly than did controls. Moreover, multiple classes of sensory axons regenerated across the dorsal root entry zone into the spinal cord of mice lacking ROCKII. After the spinal cord injury, ROCKII(-/-) mice showed enhanced local growth of raphespinal axons in the caudal spinal cord and corticospinal axons into the lesion site. Improved functional recovery was not observed by Basso Mouse Scale score following dorsal hemisection, likely due to developmental defects in the nervous system. Together, these findings demonstrate that the ROCKII gene product limits axonal growth after CNS trauma.

The growth-inhibitory protein Nogo is involved in midline routing of axons in the mouse optic chiasm

We have investigated the role of Nogo, a protein that inhibits regenerating axons in the adult central nervous system, on axon guidance in the developing optic chiasm of mouse embryos. Nogo protein is expressed by radial glia in the midline within the optic chiasm where uncrossed axons turn, and the Nogo receptor (NgR) is expressed on retinal neurites and growth cones. In vitro neurite outgrowth from both dorsonasal and ventrotemporal retina was inhibited by Nogo protein, and this inhibition was abolished by blocking NgR activity. In slice cultures of the optic pathway, blocking NgR with a peptide antagonist produced significant reduction in the uncrossed projection but had no effect on the crossing axons. This result was confirmed by treating cultures with an anti-Nogo functional blocking antibody. In vitro coculture assays of retina and optic chiasm showed that NgR was selectively reduced on neurites and growth cones from dorsonasal retina when they contacted chiasm cells, but not on those from ventrotemporal retina. These findings provide evidence that Nogo signaling is involved in directing the growth of axons in the mouse optic chiasm and that this process relies on a differential regulation of NgR on axons from the dorsonasal and ventrotemporal retina.

Axon regeneration in the peripheral and central nervous systems

Axon regeneration in the mature mammalian central nervous system (CNS) is extremely limited after injury. Consequently, functional deficits persist after spinal cord injury (SCI), traumatic brain injury, stroke, and related conditions that involve axonal disconnection. This situation differs from that in the mammalian peripheral nervous system (PNS), where long-distance axon regeneration and substantial functional recovery can occur in the adult. Both extracellular molecules and the intrinsic growth capacity of the neuron influence regenerative success. This chapter discusses determinants of axon regeneration in the PNS and CNS.

Nogo-A and Nogo-66 receptor in amyotrophic lateral sclerosis

Nogo/reticulon (RTN)-4 has been strongly implicated as a disease marker for the motor neuron disease amyotrophic lateral sclerosis (ALS). Nogo isoforms, including Nogo-A, are ectopically expressed in the skeletal muscle of ALS mouse models and patients and their levels correlate with the disease severity. The notion of a direct involvement of Nogo-A in ALS aetiology is supported by the findings that Nogo-A deletion in mice reduces muscle denervation and prolongs survival, whereas overexpression of Nogo-A destabilizes motor nerve terminals and promotes denervation. Another intriguing, and somewhat paradoxical, recent finding revealed that binding of the Nogo-66 receptor (NgR) by either agonistic or antagonistic Nogo-66-derived peptides protects against p75 neurotrophin receptor (p75(NTR))-dependent motor neuron death. Ligand binding by NgR could result in subsequent engagement of p75(NTR), and this association could preclude pro-apoptotic signalling by the latter. Understanding the intricate interplay among Nogo isoforms, NgR and p75(NTR) in ALS disease progression may provide important, therapeutically exploitable information.

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.

Troy/Taj and its role in CNS axon regeneration

Binding of myelin inhibitors to the NgR1/p75/LINGO-1 signaling complex activates RhoA to mediate the inhibition of axonal outgrowth. The nerve growth factor receptor p75, a TNF family receptor, is absent or poorly expressed in certain types of neurons that respond to myelin inhibitors, thereby prompting speculation that other TNF family receptors are involved in the NgR1 complex. Troy/Taj is an orphan TNF family receptor that is broadly expressed in postnatal and adult neurons. Troy binds to NgR1 and can functionally replace p75 in the p75/NgR1/LINGO-1 complex to activate RhoA and block neurite outgrowth in the presence of myelin inhibitors. Neurons from Troy-deficient mice are more resistant to the suppressive action of the myelin inhibitors. The discovery of TROY function in axon growth is an important step for understanding the complex regulation of axonal regeneration by diverse members of the TNF receptor family.

The reticulons: a family of proteins with diverse functions

The reticulon family is a diverse group of proteins that mostly localize to the endoplasmic reticulum and may be important in neurodegenerative diseases.

The reticulon family is a large and diverse group of membrane-associated proteins found throughout the eukaryotic kingdom. All of its members contain a carboxy-terminal reticulon homology domain that consists of two hydrophobic regions flanking a hydrophilic loop of 60-70 amino acids, but reticulon amino-terminal domains display little or no similarity to each other. Reticulons principally localize to the endoplasmic reticulum, and there is evidence that they influence endoplasmic reticulum-Golgi trafficking, vesicle formation and membrane morphogenesis. However, mammalian reticulons have also been found on the cell surface and mammalian reticulon 4 expressed on the surface of oligodendrocytes is an inhibitor of axon growth both in culture and in vivo. There is also growing evidence that reticulons may be important in neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. The diversity of structure, topology, localization and expression patterns of reticulons is reflected in their multiple, diverse functions in the cell.

Developmental neurotoxicity testing in vitro: models for assessing chemical effects on neurite outgrowth

In vitro models may be useful for the rapid toxicological screening of large numbers of chemicals for their potential to produce toxicity. Such screening could facilitate prioritization of resources needed for in vivo toxicity testing towards those chemicals most likely to result in adverse health effects. Cell cultures derived from nervous system tissue have proven to be powerful tools for elucidating cellular and molecular mechanisms of nervous system development and function, and have been used to understand the mechanism of action of neurotoxic chemicals. Recently, it has been suggested that in vitro models could be used to screen for chemical effects on critical cellular events of neurodevelopment, including differentiation and neurite growth. This review examines the use of neuronal cell cultures as an in vitro model of neurite outgrowth. Examples of the cell culture systems that are commonly used to examine the effects of chemicals on neurite outgrowth are provided, along with a description of the methods used to quantify this neurodevelopmental process in vitro. Issues relating to the relevance of the methods and models currently used to assess neurite outgrowth are discussed in the context of hazard identification and chemical screening. To demonstrate the utility of in vitro models of neurite outgrowth for the evaluation of large numbers of chemicals, efforts should be made to: (1) develop a set of reference chemicals that can be used as positive and negative controls for comparing neurite outgrowth between model systems, (2) focus on cell cultures of human origin, with emphasis on the emerging area of neural progenitor cells, and (3) use high-throughput methods to quantify endpoints of neurite outgrowth.

The N-terminal domain of Nogo-A inhibits cell adhesion and axonal outgrowth by an integrin-specific mechanism

Myelin-derived Nogo-A protein limits axonal growth after CNS injury. One domain binds to the Nogo-66 receptor to inhibit axonal outgrowth, whereas a second domain, Amino-Nogo, inhibits axonal outgrowth and cell adhesion through unknown mechanisms. Here, we show that Amino-Nogo inhibition depends strictly on the composition of the extracellular matrix, suggesting that Amino-Nogo inhibits the function of certain integrins. Amino-Nogo inhibition can be partially overcome by antibodies that activate integrin beta1 or by the addition of Mn2+, an integrin activator. Furthermore, Amino-Nogo reduces focal adhesion kinase activation by fibronectin. Analysis of various cell lines reveals that alpha(v)beta3, alpha5, and alpha4 integrins are sensitive to Amino-Nogo, but alpha6 integrin is not. Both alpha(v) and alpha5 integrins have widespread expression in adult brain and are found in axonal growth cones. Thus, inhibition of integrin signaling by Amino-Nogo contributes to the failure of CNS axon regeneration.

Mechanisms of CNS myelin inhibition: evidence for distinct and neuronal cell type specific receptor systems

Following injury to the adult mammalian central nervous system, regenerative growth of severed axons is very limited. The lack of neuronal repair is often associated with significant functional deficits, and depending on the severity of injury, may result in permanent paralysis distal to the site of injury. A detailed understanding of the molecular mechanisms that limit neuronal growth in the injured spinal cord is an important step toward the development of specific strategies aimed at restoring functional connectivity lost as a consequence of injury. While rapid progress is being made in defining the molecular identity of CNS growth inhibitory constituents, comparatively little is known about their receptors and downstream signaling mechanisms. Emerging new evidence suggests that the mechanisms for myelin inhibition are likely to be complex, involving multiple and distinct receptor systems that may operate in a redundant manner. Furthermore, the relative contribution of a specific ligand-receptor system to bring about growth inhibition may greatly vary among different neuronal cell types. Myelin-associated glycoprotein (MAG), for example, employs different mechanisms to inhibit neurite outgrowth of cerebellar, sensory, and retinal ganglion neurons in vitro. Nogo-A harbors distinct growth inhibitory regions, which employ different signaling mechanisms. The Nogo-66 receptor 1 (NgR1), a shared ligand binding component in a receptor complex for Nogo-66, MAG, and OMgp, participates in neuronal growth cone collapse to acutely presented myelin inhibitors, but is dispensable for longitudinal neurite outgrowth inhibition on substrate-bound Nogo-66, MAG, OMgp, or crude CNS myelin in vitro. Consistent with the idea of cell-type specific mechanisms for myelin inhibition, different types of CNS neurons possess very different regenerative capacities and respond differently to experimental treatment strategies in vivo. We speculate that differences in regenerative axonal growth among different fiber systems are a reflection of their intrinsic ability to elongate axons and their distinct cell surface receptor profiles to respond to the growth inhibitory extracellular milieu. The existence of cell type specific mechanisms to impair regenerative axonal growth in the CNS may have important implications for the development of treatment strategies. Depending on the fiber tract injured, different ligand-receptor systems may need to be targeted in order to elicit robust and long-distance regenerative axonal growth.

Subcutaneous Nogo receptor removes brain amyloid-beta and improves spatial memory in Alzheimer's transgenic mice

The production and aggregation of cerebral amyloid-beta (Abeta) peptide are thought to play a causal role in Alzheimer's disease (AD). Previously, we found that the Nogo-66 receptor (NgR) interacts physically with both Abeta and the amyloid precursor protein (APP). The inverse correlation of Abeta levels with NgR levels within the brain may reflect regulation of Abeta production and/or Abeta clearance. Here, we assess the potential therapeutic benefit of peripheral NgR-mediated Abeta clearance in APPswe/PSEN-1deltaE9 transgenic mice. Through site-directed mutagenesis, we demonstrate that the central 15-28 aa of Abeta associate with specific surface-accessible patches on the leucine-rich repeat concave side of the solenoid structure of NgR. In transgenic mice, subcutaneous NgR(310)ecto-Fc treatment reduces brain Abeta plaque load while increasing the relative levels of serum Abeta. These changes in Abeta are correlated with improved spatial memory in the radial arm water maze. The benefits of peripheral NgR administration are evident when therapy is initiated after disease onset. Thus, the peripheral association of NgR(310)ecto-Fc with central Abeta residues provides an effective therapeutic approach for AD.

Epitope mapping of the neuronal growth inhibitor Nogo-A for the Nogo receptor and the cognate monoclonal antibody IN-1 by means of the SPOT technique

Nogo-A is a potent inhibitor of axonal outgrowth in the central nervous system of adult mammals, where it is expressed as a membrane protein on oligodendrocytes and in myelin. Here we describe an attempt to identify linear peptide epitopes in its sequence that are responsible for the interaction either with the Nogo receptor (NgR) or with the neutralizing monoclonal antibody IN-1. Analysis of an array of immobilized overlapping 15 mer peptides covering the entire amino acid sequence of human Nogo-A (1192 residues) revealed a single epitope with prominent binding activity both towards the recombinant NgR and the IN-1 F(ab) fragment. Further truncation and substitution analysis yielded the minimal epitope sequence 'IKxLRRL' (x not equal to P), which occurs within the so-called Nogo66 region (residues 1054-1120) of Nogo-A. The bacterially produced Nogo66 fragment exhibited binding activity both for the recombinant NgR and for the IN-1 F(ab) fragment on the Western blot as well as in ELISA. Unexpectedly, the synthetic epitope peptide and the recombinant Nogo66 showed cross-reactivity with the 8-18C5 F(ab) fragment, which is directed against myelin oligodendrocyte glycoprotein (MOG) as a structurally unrelated target. On the other hand, the recombinant N-terminal domain of Nogo-A (residues 334-966) was shown to specifically interact on the Western blot and in an ELISA with the IN-1 F(ab) fragment but not with the recombinant NgR, which is in agreement with previous results. Hence, our data suggest that there is a distinct binding site for the Nogo receptor in the Nogo66 region of Nogo-A, whereas its interaction with NgR is less specific than anticipated before. Although there probably exists a non-linear epitope for the neutralizing antibody IN-1 in the N-terminal region of Nogo-A, which is likely to be accessible from outside the cell, a previously postulated second binding site for NgR in this region (called Nogo-A-24) remains elusive.

Characterization of myelin ligand complexes with neuronal Nogo-66 receptor family members

Nogo, MAG, and OMgp are myelin-associated proteins that bind to a neuronal Nogo-66 receptor (NgR/NgR1) to limit axonal regeneration after central nervous system injury. Within Nogo-A, two separate domains are known interact with NgR1. NgR1 is the founding member of the three-member NgR family, whereas Nogo-A (RTN4A) belongs to a four-member reticulon family. Here, we systematically mapped the interactions between these superfamilies, demonstrating novel nanomolar interactions of RTN2 and RTN3 with NgR1. Because RTN3 is expressed in spinal cord white matter, it may have a role in myelin inhibition of axonal growth. Further analysis of the Nogo-A and NgR1 interactions revealed a novel third interaction site between the proteins, suggesting a trivalent Nogo-A interaction with NgR1. We also confirmed here that MAG binds to NgR2, but not to NgR3. Unexpectedly, we found that OMgp interacts with MAG with a higher affinity compared with NgR1. To better define how these multiple structurally distinct ligands bind to NgR1, we examined a series of Ala-substituted NgR1 mutants for ligand binding activity. We found that the core of the binding domain is centered in the middle of the concave surface of the NgR1 leucine-rich repeat domain and surrounded by differentially utilized residues. This detailed knowledge of the molecular interactions between NgR1 and its ligands is imperative when assessing options for development of NgR1-based therapeutics for central nervous system injuries.

The Nogo-Nogo receptor pathway limits a spectrum of adult CNS axonal growth

The hypothesis that Nogo-A (Reticulon 4A) and Nogo-66 receptor (NgR1) limit adult CNS axonal growth after injury is supported by both in vitro experiments and in vivo pharmacological studies. However, genetic assessment of the role of Nogo-A in corticospinal tract (CST) axons after spinal cord dorsal hemisection has yielded conflicting results. CST regeneration is detected in homozygous nogo-ab(trap/trap) mice, but not in nogo-ab(atg/atg) mice. CST regeneration is also present after pharmacological NgR blockade, but not in ngr1(-/-) mice. To assess the nogo-ab(atg) and ngr1-null alleles for other axon growth phenotypes, we created unilateral pyramidotomies and monitored the uninjured CST. There is robust pyramidotomy-induced growth of nogo-ab(atg/atg) and ngr1(-/-) CST axons into denervated cervical gray matter. This fiber growth correlates with recovery of fine motor skill in the affected forelimb. Thus nogo-ab and ngr1 play a modulated role in limiting CNS axonal growth across a spectrum of different tracts in various lesion models.

Nogo A, B and C expression in schizophrenia, depression and bipolar frontal cortex, and correlation of Nogo expression with CAA/TATC polymorphism in 3′-UTR

Schizophrenia may result from altered gene expression leading to abnormal neurodevelopment. In a search for genes with altered expression in schizophrenia, our previous work on human frontal cerebral cortex found the mRNA of Nogo, a myelin-associated protein which inhibits the outgrowth of neurites and nerve terminals, to be overexpressed in schizophrenia. Because those earlier results did not examine tissues for the separate Nogo A, B and C isoforms from age- and sex-matched individuals, we repeated the study for all three isoforms, using a new set of tissues from matched individuals, and using the more accurate method of quantitative real-time PCR (polymerase chain reaction). We found Nogo C to be overexpressed by 26% in the schizophrenia tissues, which is in accordance with our earlier results. The expression of Nogo B was statistically significantly reduced by 17% in the frontal cortices from individuals who had been diagnosed as having had severe depression. Furthermore, we show that there is a direct correlation between the expression of Nogo A and C and the presence of alleles with a CAA insert, irrespective of disease status. While upregulation of Nogo C expression may play a role in schizophrenia, altered Nogo B may contribute to the clinical condition of depression. Nogo A showed a statistically non-significant increase in expression in schizophrenia.

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.

Extracellular regulators of axonal growth in the adult central nervous system

Robust axonal growth is required during development to establish neuronal connectivity. However, stable fibre patterns are necessary to maintain adult mammalian central nervous system (CNS) function. After adult CNS injury, factors that maintain axonal stability limit the recovery of function. Extracellular molecules play an important role in preserving the stability of the adult CNS axons and in restricting recovery from pathological damage. Adult axonal growth inhibitors include a group of proteins on the oligodendrocyte, Nogo-A, myelin-associated glycoprotein, oligodendrocyte-myelin glycoprotein and ephrin-B3, which interact with axonal receptors, such as NgR1 and EphA4. Extracellular proteoglycans containing chondroitin sulphates also inhibit axonal sprouting in the adult CNS, particularly at the sites of astroglial scar formation. Therapeutic perturbations of these extracellular axonal growth inhibitors and their receptors or signalling mechanisms provide a degree of axonal sprouting and regeneration in the adult CNS. After CNS injury, such interventions support a partial return of neurological function.

Expression of Nogo-A and NgR in the developing rat brain after hypoxia-ischemia

Nogo-A and its receptor, NgR, have been shown to inhibit neurite growth in the adult rat. Therefore, we hypothesized that Nogo-A and NgR will be upregulated and thus play a similar role in the damage in developing rat brain following hypoxia-ischemia (HI). To test this hypothesis, we subjected postnatal day 7 (P7) rats to HI by permanently ligating the right common carotid artery, followed by exposure to 8%O2/92% N2 for 3 h. Rat brains at 0 h, 6 h, 12 h, 24 h and 72 h after HI, as well as from sham controls, were collected to determine histopathological damage and expression levels of Nogo-A and NgR using hematoxylin and eosin (H&E) staining, immunohistochemistry, fluorescence immunolabeling, Western blot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR). We found neuronal degeneration and edema in the ischemic cortex, becoming most prominent at 24 h following HI in this model. Accordingly, the expression of Nogo-A and NgR protein was significantly upregulated at 24 h compared with the sham controls (p<0.01). The upregulated Nogo-A and NgR immunoreactive cells were mainly located in the core of the ischemic cortex and colocalized to neurons. Meanwhile, we found the expression of both Nogo-A and NgR mRNA was increased at 6 h and peaked at 12 h in the ischemic cortex after HI, compared with sham controls. Our findings of upregulation of neurite growth inhibitor Nogo-A and its receptor NgR in ischemic cortex suggest that Nogo-A and NgR may participate in the pathology seen after HI in neonatal rats.

The Nogo receptor complex: confining molecules to molecular mechanisms

Myelin inhibitory ligands of the Nogo-66 receptor (NgR1) limit axon regeneration in the adult CNS. Recent findings have identified additional co-receptors (functional homologues) of the trimeric NgR1 complex, post-translational modifications of the co-receptors within the cell membrane and novel Ca(2+)-dependent cytoplasmic-protein phosphorylation mechanisms. Such unique signalling pathways provide the potential to transduce myelin-derived growth inhibitory signals to the axonal cytoskeleton, and have been areas of intense investigation in recent years. Here, we summarize current understanding of the molecular basis of myelin-derived axon-growth inhibition in the CNS.

Experimental strategies to promote spinal cord regeneration--an integrative perspective

Detailed pathophysiological findings of secondary damage phenomena after spinal cord injury (SCI) as well as the identification of inhibitory and neurotrophic proteins have yielded a plethora of experimental therapeutic approaches. Main targets are (i) to minimize secondary damage progression (neuroprotection), (ii) to foster axon conduction (neurorestoration) and (iii) to supply a permissive environment to promote axonal sprouting (neuroregenerative therapies). Pre-clinical studies have raised hope in functional recovery through the antagonism of growth inhibitors, application of growth factors, cell transplantation, and vaccination strategies. To date, even though based on successful pre-clinical animal studies, results of clinical trials are characterized by dampened effects attributable to difficulties in the study design (patient heterogeneity) and species differences. A combination of complementary therapeutic strategies might be considered pre-requisite for future synergistic approaches. Here, we line out pre-clinical interventions resulting in improved functional neurological outcome after spinal cord injury and track them on their intended way to bedside.

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.