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.

Dynamic induction of the myelin-associated growth inhibitor Nogo-A in perilesional plasticity regions after human spinal cord injury

The myelin-associated inhibitor Nogo-A (Reticulon 4, RTN4) restricts axonal outgrowth, plasticity, and neural circuitry formation in experimental models of spinal cord injury (SCI) and is targeted in clinical interventions starting treatment within 4 weeks post-SCI. Specifically, Nogo-A expressed by oligodendroglia restricts compensatory neurite sprouting. To interrogate the hypothesis of an inducible, lesion reactive Nogo-A expression over time, we analyzed the spatiotemporal Nogo-A expression at the spinal lesion core (region of tissue necrosis and axonal damage/pruning) and perilesional rim (region of plasticity formation). Spinal cord specimens of SCI subjects (n = 22) were compared to neuropathologically unaltered controls (n = 9). Nogo-A expression was investigated ranging from acute (0-3 days), early subacute (4-21 days), late subacute (22-90 days) to early chronic-chronic (91 days to 1.5 years after SCI) stages after SCI. Nogo-A expression in controls is confined to motoneurons in the anterior horn and to oligodendrocytes in gray and white matter. After SCI, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes (i) inclined at the organizing perilesional rim specifically, (ii) increased further over time, and (iii) peaked at chronic stages after SCI. By contrast, at the lesion core, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes did not increase. Increasing numbers of Nogo-A+ oligodendrocytes coincided with oligodendrogenesis corroborated by Nogo-A coexpression of Ki67+ , TPPP/p25+ proliferating oligodendrocytes. Nogo-A oligodendrocyte expression emerges at perilesional (plasticity) regions over time and suggests an extended therapeutical window for anti-Nogo-A pathway targeting interventions beyond 4 weeks in patients after SCI.

Sustained dual drug delivery of anti-inhibitory molecules for treatment of spinal cord injury

Myelin-associated inhibitors (MAIs) and chondroitin sulfate proteoglycans (CSPGs) are major contributors to axon growth inhibition following spinal cord injury and limit functional recovery. The NEP1-40 peptide competitively binds the Nogo receptor and partially blocks inhibition from MAIs, while chondroitinase ABC (ChABC) enzymatically digests CSPGs, which are upregulated at the site of injury. In vitro studies showed that the combination of ChABC and NEP1-40 increased neurite extension compared to either treatment alone when dissociated embryonic dorsal root ganglia were seeded onto inhibitory substrates containing both MAIs and CSPGs. Furthermore, the ability to provide sustained delivery of biologically active ChABC and NEP1-40 from biomaterial scaffolds was achieved by loading ChABC into lipid microtubes and NEP1-40 into poly (lactic-co-glycolic acid) (PLGA) microspheres, obviating the need for invasive intrathecal pumps or catheters. Fibrin scaffolds embedded with the drug delivery systems (PLGA microspheres and lipid microtubes) were capable of releasing active ChABC for up to one week and active NEP1-40 for over two weeks in vitro. In addition, the loaded drug delivery systems in fibrin scaffolds decreased CSPG deposition and development of a glial scar, while also increasing axon growth after spinal cord injury in vivo. Therefore, the sustained, local delivery of ChABC and NEP1-40 within the injured spinal cord may block both myelin and CSPG-associated inhibition and allow for improved axon growth.

Nogo-A is involved in secondary axonal degeneration of thalamus in hypertensive rats with focal cortical infarction

We investigate whether Nogo-A is involved in the secondary axonal degeneration in the thalamus after distal middle cerebral artery occlusion (MCAO) in stroke-prone renovascular hypertensive rats (RHRSP). The expression of Nogo-A in ipsilateral ventroposterior nucleus (VPN) of the thalamus in RHRSP was observed at 1, 2 and 4 weeks after distal MCAO. In addition, intracerebroventricular infusion of NEP1-40, a Nogo-66 receptor (NgR) antagonist peptide, was administered starting 24 h after MCAO and continued for 1, 2 and 4 weeks, respectively. Axonal damage and regeneration were evaluated by analysis of the immunoreactivity (IR) of amyloid betaA4 precursor protein (APP), growth associated protein 43 (GAP-43) and microtubule associated protein 2 (MAP-2) in ipsilateral VPN of the thalamus at 1, 2 and 4 weeks after distal MCAO. Following ischemia, the expression of Nogo-A in oligodendrocytes increased persistently and its localization became redistributed around damaged axons and dendrites. Administration of NEP1-40 downregulated the expression of Nogo-A, reduced axonal injury and enhanced axonal regeneration. Our data suggest that Nogo-A is involved in secondary axonal degeneration and that inhibition of Nogo-A can reduce neuronal damage in the thalamus after distal MCAO.

Animal Models of Central Nervous System Immune-Mediated Diseases: Therapeutic Interventions with Bioactive Peptides and Mimetics

Experimental allergic encephalomyelitis (EAE) is a T helper 1 (Th1) mediated autoimmune disease and the principal animal model for multiple sclerosis (MS). Like MS, EAE is characterized by a coordinated inflammatory attack on the myelin sheath in the central nervous system (CNS), with damage to axons. No matter whether the ideal animal model is not yet available, much knowledge concerning the pathogenesis of MS has been achieved through studies on EAE. Dissecting the underlying immune mechanisms provided recognition of several myelin antigens that are vulnerable in autoimmune attack. The beneficial effect and the mechanism of action of a number of the currently used immunomodulating agents in MS therapy were first indicated in EAE. Altered peptide ligands (APL) can modulate T-cell responses to native peptide antigens implicated in the pathogenesis of autoimmune diseases such as MS and EAE. However, peptide therapy is hindered due to the sensitivity of peptides to proteolytic enzymes as well as due to some immune-mediated side effects. A number of cyclic myelin peptide analogs seem to be potential candidates in maintaining the biological function of the original peptide and effective in controlling inflammation in EAE. Additional data regarding the immunomodulating and neuroprotective effect of these much promising agents is required. Based on the data from studies on EAE models, clinical trials should also be designed in order to elucidate the impact of such APL-induced immune responses in MS disease activity. These clinical trials should carefully incorporate monitoring of both clinical, neuroimaging and immunological parameters.

Immunization with myelin or recombinant Nogo-66/MAG in alum promotes axon regeneration and sprouting after corticospinal tract lesions in the spinal cord

We have shown previously that immunization with myelin in incomplete Freund's adjuvant (IFA) is able to promote robust regeneration of corticospinal tract fibers in adult mice. In the present study the effectiveness of such immunization with myelin was compared to that of a combination of two axon growth inhibitors in myelin, Nogo-66 (the 66-amino-acid inhibitory region of Nogo-A) and myelin-associated glycoprotein (MAG). The effectiveness of two adjuvants, IFA and aluminum hydroxide (Alum), was also compared, the latter being one that can be used in humans. In addition, larger dorsal overhemisections were made at the lower thoracic level, which resulted in a larger scar. These studies were carried out in SJL/J mice, a mouse strain that is susceptible to autoimmune experimental allergic encephalomyelitis (EAE). None of the immunized mice developed EAE. Long-distance axon regeneration and sprouting of the corticospinal tract was seen in myelin and Nogo-66/MAG immunized mice. Alum was as effective or better than IFA as the adjuvant. Overall, the robustness of axon growth and sprouting was greater in mice immunized with myelin. The abundance of this growth was less than in our earlier work in which smaller lesions were made, pointing to the possible influence of inhibitors in the scar. This work shows, however, that axon growth inhibitors in myelin can be selectively blocked using this immunization approach to promote long-distance axon regeneration in the spinal cord.

Delayed systemic Nogo-66 receptor antagonist promotes recovery from spinal cord injury

Traumatized axons possess an extremely limited ability to regenerate within the adult mammalian CNS. The myelin-derived axon outgrowth inhibitors Nogo, oligodendrocyte-myelin glycoprotein, and myelin-associated glycoprotein, all bind to an axonal Nogo-66 receptor (NgR) and at least partially account for this lack of CNS repair. Although the intrathecal application of an NgR competitive antagonist at the time of spinal cord hemisection induces significant regeneration of corticospinal axons, such immediate local therapy may not be as clinically feasible for cases of spinal cord injury. Here, we consider whether this approach can be adapted to systemic therapy in a postinjury therapeutic time window. Subcutaneous treatment with the NgR antagonist peptide NEP1-40 (Nogo extracellular peptide, residues 1-40) results in extensive growth of corticospinal axons, sprouting of serotonergic fibers, upregulation of axonal growth protein SPRR1A (small proline-rich repeat protein 1A), and synapse re-formation. Locomotor recovery after thoracic spinal cord injury is enhanced. Furthermore, delaying the initiation of systemic NEP1-40 administration for up to 1 week after cord lesions does not limit the degree of axon sprouting and functional recovery. This indicates that the regenerative capacity of transected corticospinal tract axons persists for weeks after injury. Systemic Nogo-66 receptor antagonists have therapeutic potential for subacute CNS axonal injuries such as spinal cord trauma.

Multiple sclerosis as a by-product of the failure to sustain protective autoimmunity: a paradigm shift

Autoimmune diseases are traditionally viewed as an outcome of a chaotic situation in which an individual's immune system reacts against the body's own proteins. In multiple sclerosis, a disease of the white matter of the central nervous system (CNS), the immune attack is directed against myelin proteins. In this article, the authors propose a paradigm shift in the perception of autoimmune disease. They suggest that an autoimmune disease may be viewed as a by-product of the malfunctioning of a physiological autoimmune response whose purpose is protective. The proposed view is based on observations by their group suggesting that an autoimmune response is the body's own mechanism for coping with CNS damage. According to this view, all individuals are endowed with the potential ability to evoke an autoimmune response to CNS injuries. However, the inherent ability to control this response so that its beneficial effect will be expressed is limited and is correlated with the individual's inherent ability to resist autoimmune disease induction. The same autoimmune T cells are responsible for neuroprotection and for disease development. In patients with CNS trauma or neurodegenerative disorders, it might be possible to gain maximal autoimmune protection and avoid autoimmune disease induction by boosting the immune response, using myelin-associated peptides that are nonpathogenic or antigens that simulate the activities of such peptides. In patients with multiple sclerosis and other neurodegenerative diseases, where the aim is to block the autoimmune disorder while deriving the potential benefit of the autoimmune response, the effect of treatment should be immunomodulatory rather than immunosuppressive. In this article, the authors present a novel concept of protective autoimmunity and propose that autoimmune disease is a by-product of failure to sustain it. They summarize the basic findings that led them to formulate the new concept and offer an explanation for the commonly observed presence of cells and antibodies directed against self-components in healthy individuals. The therapeutic implications of the new concept and their experimental findings are discussed.

Nogo-66 receptor antagonist peptide promotes axonal regeneration

Myelin-derived axon outgrowth inhibitors, such as Nogo, may account for the lack of axonal regeneration in the central nervous system (CNS) after trauma in adult mammals. A 66-residue domain of Nogo (Nogo-66) is expressed on the surface of oligodendrocytes and can inhibit axonal outgrowth through an axonal Nogo-66 receptor (NgR). The IN-1 monoclonal antibody recognizes Nogo-A and promotes corticospinal tract regeneration and locomotor recovery; however, the undefined nature of the IN-1 epitope in Nogo, the limited specificity of IN-1 for Nogo, and nonspecific anti-myelin effects have prevented a firm conclusion about the role of Nogo-66 or NgR. Here, we identify competitive antagonists of NgR derived from amino-terminal peptide fragments of Nogo-66. The Nogo-66(1 40) antagonist peptide (NEP1 40) blocks Nogo-66 or CNS myelin inhibition of axonal outgrowth in vitro, demonstrating that NgR mediates a significant portion of axonal outgrowth inhibition by myelin. Intrathecal administration of NEP1 40 to rats with mid-thoracic spinal cord hemisection results in significant axon growth of the corticospinal tract, and improves functional recovery. Thus, Nogo-66 and NgR have central roles in limiting axonal regeneration after CNS injury, and NEP1-40 provides a potential therapeutic agent.

Antigen-specific tolerance induction and the immunotherapy of experimental autoimmune disease

Antigen-specific tolerance induction is the ultimate goal for specific immunotherapy of autoimmune diseases. Here we will discuss recent experiments designed to induce tolerance following mucosal administration of antigens in a mouse model of experimental autoimmune encephalomyelitis (EAE). We were unable to induce oral tolerance either with whole myelin, myelin basic protein (MBP) or the immunodominant peptide antigen. Oral tolerance was possible, however, with an analogue of the immunodominant peptide modified to increase its affinity for the restricting major histocompatibility complex (MHC) antigen. By contrast, intranasal deposition of peptide antigen proved highly effective for both prevention and treatment of EAE. Prevention of disease was directly related to the antigenic property of the peptide which, in itself, was related to affinity for MHC. Notably, administration of a single peptide was shown to inhibit disease involving multiple epitopes. We investigated the resulting bystander regulation by studying the cellular basis of peripheral tolerance in a transgenic model. These studies indicate that bystander regulation may be the consequence of selective cytokine secretion.

Functional evidence for epitope spreading in the relapsing pathology of experimental autoimmune encephalomyelitis

The role of epitope spreading in the pathology of relapsing-remitting experimental autoimmune encephalomyelitis (R-EAE) was examined. Using peripherally induced immunologic tolerance as a probe to analyze the neuropathologic T cell repertoire, we show that the majority of the immunopathologic reactivity during the acute phase of R-EAE in SJL/J mice induced by active immunization with the intact proteolipid (PLP) molecule is directed at the PLP139-151 epitope and that responses to secondary encephalitogenic PLP epitopes may contribute to the later relapsing phases of disease. Intermolecular epitope spreading was demonstrated by showing the development of T cell responses to PLP139-151 after acute disease in mice in which R-EAE was initiated by the transfer of T cells specific for the non-cross-reactive MBP84-104 determinant. Intramolecular epitope spreading was demonstrated by showing that endogenous host T cells specific for a secondary encephalitogenic PLP epitope (PLP178-191) are demonstrable by both splenic T cell proliferative and in vivo delayed-type hypersensitivity responses in mice in which acute central nervous system damage was initiated by T cells reactive with the immunodominant, non-cross-reactive PLP139-151 sequence. The PLP178-191-specific responses are activated as a result of and correlate with the degree of acute tissue damage, since they do not develop in mice tolerized to the initiating epitope before expression of acute disease. Most importantly, we show that the PLP178-191-specific responses are capable of mediating R-EAE upon adoptive secondary transfer to naive recipient mice. Furthermore, induction of tolerance to intact PLP (which inhibits responses to both the initiating PLP139-151 epitope and to the PLP178-191 epitope) after the acute disease episode is sufficient to prevent relapsing disease. These results strongly support a contributory role of T cell responses to epitopes released as a result of acute tissue damage to the immunopathogenesis of relapsing clinical episodes and have important implications for the design of antigen-specific immunotherapies for the treatment of chronic autoimmune disorders in humans.

Suppression of experimental autoimmune encephalomyelitis by oral administration of myelin antigens: IV. Suppression of chronic relapsing disease in the Lewis rat and strain 13 guinea pig

Oral administration of proteins is a long-recognized method of inducing antigen-specific peripheral immune tolerance. We previously showed that oral administration of myelin basic protein suppresses monophasic experimental autoimmune encephalomyelitis in the Lewis rat when it is given in association with immunization and prior to disease onset. As a potential therapy for human autoimmune disease, it is crucial to determine whether oral tolerance can ameliorate an ongoing immune response. We therefore asked whether oral administration of myelin antigens, after sensitization and disease expression has occurred, could affect immunological, clinical, or pathological features of experimental autoimmune encephalomyelitis. Chronic relapsing experimental autoimmune encephalomyelitis was induced in the Lewis rat and strain 13 guinea pig by immunization with whole guinea pig cord homogenate, complete Freund's adjuvant, and Mycobacterium tuberculosis. Following recovery from the first attack, animals were orally given bovine myelin, guinea pig myelin, or guinea pig myelin basic protein three times per week for up to 3 months. Animals receiving myelin products orally had decreased severity and frequency of clinical relapses, decreased delayed-type hypersensitivity responses to myelin antigens, diminished inflammation in the central nervous system (CNS), and decreased areas of CNS demyelination. In the rat, guinea pig myelin basic protein was as effective as guinea pig myelin in ameliorating the disease and also resulted in decreased serum anti-myelin basic protein antibody levels. No exacerbation of disease or worsening of pathological findings occurred in the animals given myelin products. These results demonstrate that oral administration of myelin antigens can suppress chronic relapsing experimental autoimmune encephalomyelitis and have direct relevance to therapy of human demyelinating disorders such as multiple sclerosis.

Theiler's virus encephalomyelitis is unaffected by treatment with myelin components

Treatment of SJL/J mice with myelin components prior to infection with Theiler's picornavirus did not effect the development of inflammatory demyelinating lesions characteristic of Theiler's mouse encephalomyelitis. These results suggest that the pathogenesis of this disease differs from experimental autoimmune encephalomyelitis, which can be suppressed by such a treatment.

Antigen-induced suppression of experimental allergic neuritis in the guinea pig

Treatment of guinea pigs suffering from a fatal form of experimental allergic neuritis (EAN) using P2 basic protein of PNS myelin reduced the clinical severity, the overall mortality and the incidence of respiratory problems. The large basic protein (MBP) common to the CNS and PNS had no suppressive effect on EAN. Cyanogen bromide cleavage of P2 destroyed its suppressive activity. P0 protein of PNS myelin had marginal suppressive activity but Cop 1, a synthetic copolymer effective in suppressing EAE, was without significant effect on EAN. The suppression with P2 protein of a milder form of EAN (induced with a reduced amount of PNS myelin) was less marked. Increasing the treatment period from 10 to 15 days did not increase the suppressive effect of P2 in the milder form of EAN.

Suppression of experimental allergic neuritis with P2 protein of PNS myelin

Experimental allergic neuritis was induced in the guinea pig with bovine PNS myelin. Treatment with P2 protein at the onset of the disease markedly reduced the severity of the clinical signs compared with untreated controls and animals treated with CNS myelin basic protein. The mortality was reduced from 94% in the combined control group to 50% in the treated group. Saline was a better vehicle for administration of the suppressive inoculation than incomplete Freund's adjuvant.

Chronic relapsing experimental allergic encephalomyelitis: CNS plaque development in unsuppressed and suppressed animals

Central nervous system (CNS) lesion morphology has been studied in inbred Strain 13 guinea pigs sensitized for chronic relapsing EAE in which the disease was either left to develop (unsuppressed) or was suppressed with injections containing myelin basic protein (MBP). Pathologic changes correlated well with clinical activity. In unsuppressed chronic EAE animals, active clinical disease was invariably matched by acute inflammation in the CNS. In more chronic states, the CNS displayed fibrosis and remyelination while relapses showed the CNS to contain recent changes superimposed upon old lesions. In animals in which the disease was suppressed by injections of MBP, clinical signs did not develop. However, some early subclinical changes were seen morphologically. These lesions were able to remyelinate early on and there was no progression in lesion formation. Apparently, therefore, MBP had a beneficial effect upon the course of the disease and had promoted structural repair. It thus appears that MBP therapy might be one effective approach for the prevention of chronic relapsing EAE. The findings should prove relevant to future MBP trials in multiple sclerosis.

[Therapeutic effect of myelin basic protein and synthetic encephalitogenic peptide in guinea pigs with experimental allergic encephalomyelitis]

Effect of antigenic factors of experimental allergic encephalomyelitis (alkaline protein of myelin and synthetic encephalitogenic peptide) was studied in guinea pigs with distinct manifestations of the experimental allergic encephalomyelitis. The animals recovered after prolonged administration of alkaline protein of myelin within 14 days, if they were sensibilized either by bovine alkaline protein of myelin or by synthetic encephalitogenic peptide. Synthetic encephalitogenic peptide was only effective in treatment of the disease caused by the peptide.

Suppression of acute and chronic experimental allergic encephalomyelitis in Strain 13 guinea pigs. A clinical and pathological study

Adult inbred Strain 13 guinea pigs develop an acute, fatal form of experimental allergic encephalomyelitis (EAE) about 2 weeks after a single injection of isologous spinal cord in complete Freund's adjuvant (CFA), but similarly injected juveniles develop a delayed, rarely fatal chronic form. Thirty-seven sensitised adult Strain 13 animals were separated into 2 groups. One group was permitted to develop acute EAE. The other group was injected intramuscularly with 1 mg of guinea pig or bovine myelin basic protein (MBP) in incomplete Freund's adjuvant (IFA) on day 2, 7 or 10 post-inoculation (PI) followed by 0.2 mg in IFA every third day for a total of 10 doses. Animals in the unsuppressed group succumbed to acute EAE 13-16 days post-sensitisation. No animal in the suppressed group died during this period. Animals treated with MBP beginning 2 days PI showed no clinical signs, but mild clinical manifestations occurred in animals suppressed from days 7 and 10 PI. These signs remitted by 21 days post-sensitisation. One suppressed animal (out of 21) died during the fourth week postsensitisation. The other 20 suppressed animals appeared clinically normal towards the end of the course of MBP injections and remained so for the 6 months of study. Morphological examination revealed that CNS lesions occurred in all animals. In animals suppressed with MBP beginning on day 2 PI, lesions consisted only of a few meningeal inflammatory cells. Animals given MBP beginning on day 7 or 10 PI and sampled 1-2 weeks later, had lesions which could not be distinguished from those occurring in the non-suppressed acute EAE group. In time, the suppressed animals developed lesions which were typical of chronic EAE with remyelination as a predominant feature. Preliminary experiments on the suppression of chronic EAE in 5 juvenile Strain 13 guinea pigs have revealed that 3 MBP-injected animals failed to develop clinical disease over a 28-week period of study although lesions typical of chronic EAE were present. Simultaneously, 2 non-suppressed juvenile animals developed clinical signs by 12 weeks. These were associated with both acute inflammation and demyelination superimposed upon regions of chronic demyelinative activity.