Nogo signaling and non-physical injury-induced nervous system pathology

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.

Assessment of Nogo-66 receptor 1 function in vivo after spinal cord injury

BACKGROUND

Neuronal Nogo-66 receptor 1 (NgR1) has attracted attention as a converging point for mediating the effects of myelin-associate inhibitory ligands in the central nervous system, establishing the growth-restrictive environment, and limiting axon regeneration after traumatic injury.

OBJECTIVE

To investigate the factors that may be contributing to the discrepancy in the importance of NgR1, which has been undermined by several studies that have shown the lack of substantial axon regeneration after spinal cord injury (SCI) in NgR1-knockout or -knockdown animal models.

METHODS

We used mice carrying either a homozygous or heterozygous null mutation in the NgR1 gene and subjected them to either a moderate or severe SCI.

RESULTS

Locomotor function assessments revealed that the level of functional recovery is affected by the degree of injury suffered. NgR1 ablation enhanced local collateral sprouting in the mutant mice. Reactive astrocytes and chondroitin sulfate proteoglycans (CSPGs) are upregulated surrounding the injury site. Matrix metalloproteinase-9, which has been shown to degrade CSPGs, was significantly upregulated in the homozygous mutant mice compared with the heterozygous or wild-type mice. However, CSPG levels remained higher in the homozygous compared with the heterozygous mice, suggesting that CSPG-degrading activity of matrix metalloproteinase-9 may require the presence of NgR1.

CONCLUSION

Genetic ablation of NgR1 may lead to significant recovery in locomotor function after SCI. The difference in locomotor recovery we observed between the groups that suffered various degrees of injury suggests that injury severity may be a confounding factor in functional recovery after SCI.

Impact of RTN3 deficiency on expression of BACE1 and amyloid deposition

Reticulon 3 (RTN3) has previously been shown to interact with BACE1 and negatively regulate BACE1 activity. To what extent RTN3 deficiency affects BACE1 activity is an intriguing question. In this study, we aimed to address this by generating RTN3-null mice. Mice with complete deficiency of RTN3 grow normally and have no obviously discernible phenotypes. Morphological analyses of RTN3-null mice showed no significant alterations in cellular structure, although RTN3 is recognized as a protein contributing to the shaping of tubular endoplasmic reticulum. Biochemical analysis revealed that RTN3 deficiency increased protein levels of BACE1. This elevation of BACE1 levels correlated with enhanced processing of amyloid precursor protein at the β-secretase site. We also demonstrated that RTN3 deficiency in Alzheimer's mouse models facilitates amyloid deposition, further supporting an in vivo role of RTN3 in the regulation of BACE1 activity. Since it has been shown that RTN3 monomer is reduced in brains of Alzheimer's patients, our results suggest that long-lasting reduction of RTN3 levels has adverse effects on BACE1 activity and may contribute to Alzheimer's pathogenesis.

Zuo-Gui and You-Gui pills, two traditional Chinese herbal formulas, downregulated the expression of NogoA, NgR, and RhoA in rats with experimental autoimmune encephalomyelitis

ETHNOPHARMACOLOGICAL RELEVANCE

Zuo-Gui pills (ZGPs) and You-Gui pills (YGPs) are 2 traditional Chinese herbal formulas used for treating multiple sclerosis (MS) in the clinical setting and have been shown to have neuroprotective effects in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. The aim of this study was to explore the mechanisms underlying the neuroprotective functions of ZGPs and YGPs.

MATERIALS AND METHODS

Female Lewis rats were randomly divided into normal control, EAE model, 2g/kg ZGP-treated EAE, 3g/kg YGP-treated EAE, and prednisone acetate-treated groups. EAE model was induced by subcutaneous injection of MBP68-86 antigen. The neurological function scores were estimated. Histological structures of the brains and spinal cords were observed, and myelinated and axons imaged. NogoA, Nogo receptor (NgR), and RhoA transcript and protein levels were measured by real-time quantitative RT-PCR and western blotting on postimmunization (PI) days 14 (acute stage) and 28 (remission stage).

RESULTS

ZGPs and YGPs significantly reduced neurological functions scores and abrogated inflammatory infiltrates, demyelination, and axonal damage. Furthermore, treatment with ZGPs and YGPs inhibited NogoA, NgR, and RhoA mRNA and protein expression in rats at both the acute and remission stages. ZGPs exhibited stronger effects on NogoA and RhoA expressions, as well as neurological function, during the acute stage of EAE, while YGPs caused greater reductions in NogoA expression during the remission stage.

CONCLUSIONS

Our findings suggested that ZGPs and YGPs exerted neuroprotective effects by downregulation of NogoA, NgR, and RhoA pathways, with differences in response times and targets observed between ZGPs and YGPs.

Nogo-receptor 1 deficiency has no influence on immune cell repertoire or function during experimental autoimmune encephalomyelitis

The potential role of Nogo-66 Receptor 1 (NgR1) on immune cell phenotypes and their activation during neuroinflammatory diseases such as multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), is unclear. To further understand the function of this receptor on haematopoietically-derived cells, phenotypic and functional analyses were performed using NgR1-deficient (ngr1-/-) animals. Flow cytometry-based phenotypic analyses performed on blood, spleen, thymus, lymph nodes, bone marrow and central nervous-system (CNS)-infiltrating blood cells revealed no immunological defects in naïve ngr1-/- animals versus wild-type littermate (WTLM) controls. EAE was induced by either recombinant myelin oligodendrocyte glycoprotein (rMOG), a model in which B cells are considered to contribute pathogenically, or by MOG_35–55 peptide, a B cell-independent model. We have demonstrated that in ngr1-/- mice injected with MOG_35–55, a significant reduction in the severity of EAE correlated with reduced axonal damage present in the spinal cord when compared to their WTLM controls. However, despite a reduction in axonal damage observed in the CNS of ngr1-/- mice at the chronic stage of disease, no clinical differences could be attributed to a specific genotype when rMOG was used as the encephalitogen. Following MOG_35–55-induction of EAE, we could not derive any major changes to the immune cell populations analyzed between ngr1-/- and WTLM mice. Collectively, these data demonstrate that NgR1 has little if any effects on the repertoire of immune cells, their activation and trafficking to the CNS.

Increased expression of reticulon 3 in neurons leads to reduced axonal transport of β site amyloid precursor protein-cleaving enzyme 1

BACE1 is the sole enzyme responsible for cleaving amyloid precursor protein at the β-secretase site, and this cleavage initiates the generation of β-amyloid peptide (Aβ). Because amyloid precursor protein is predominantly expressed by neurons and deposition of Aβ aggregates in the human brain is highly correlated with the Aβ released at axonal terminals, we focused our investigation of BACE1 localization on the neuritic region. We show that BACE1 was not only enriched in the late Golgi, trans-Golgi network, and early endosomes but also in both axons and dendrites. BACE1 was colocalized with the presynaptic vesicle marker synaptophysin, indicating the presence of BACE1 in synapses. Because the excessive release of Aβ from synapses is attributable to an increase in amyloid deposition, we further explored whether the presence of BACE1 in synapses was regulated by reticulon 3 (RTN3), a protein identified previously as a negative regulator of BACE1. We found that RTN3 is not only localized in the endoplasmic reticulum but also in neuritic regions where no endoplasmic reticulum-shaping proteins are detected, implicating additional functions of RTN3 in neurons. Coexpression of RTN3 with BACE1 in cultured neurons was sufficient to reduce colocalization of BACE1 with synaptophysin. This reduction correlated with decreased anterograde transport of BACE1 in axons in response to overexpressed RTN3. Our results in this study suggest that altered RTN3 levels can impact the axonal transport of BACE1 and demonstrate that reducing axonal transport of BACE1 in axons is a viable strategy for decreasing BACE1 in axonal terminals and, perhaps, reducing amyloid deposition.

Amino-Nogo-A antagonizes reactive oxygen species generation and protects immature primary cortical neurons from oxidative toxicity

Nogo-A is originally identified as an inhibitor of axon regeneration from the CNS myelin. Nogo-A is mainly expressed by oligodendrocytes, and also by some neuronal subpopulations, particularly in the developing nervous system. Although extensive studies have uncovered regulatory roles of Nogo-A in neurite outgrowth inhibition, precursor migration, neuronal homeostasis, plasticity and neurodegeneration, its cell-autonomous functions in neurons are largely uncharacterized. Here, we show that HIV-1 trans-activating-mediated amino-Nogo-A protein transduction into cultured primary cortical neurons achieves an almost complete neuroprotection against oxidative stress induced by exogenous hydrogen peroxide (H(2)O(2)). Endogenously expressed neuronal Nogo-A is significantly downregulated upon H(2)O(2) treatment. Furthermore, knockdown of Nogo-A results in more susceptibility to acute oxidative insults and markedly increases neuronal death. Interacting with peroxiredoxin 2 (Prdx2), amino-Nogo-A reduces reactive oxygen species (ROS) generation and extracellular signal-regulated kinase phosphorylation to exert neuroprotective effects. Structure-function mapping experiments reveal that, out of NiG-Δ20, a novel region comprising residues 290-562 of amino-Nogo-A is indispensable for preventing oxidative neuronal death. Moreover, mutagenesis analysis confirms that cysteine residues 424, 464 and 559 are involved in the inhibition of ROS generation and neuroprotective role of amino-Nogo-A. Our data suggest that neuronal Nogo-A might play a cell-autonomous role in improving neuronal survival against oxidative insult through interacting with Prdx2 and scavenging of ROS.

ALS genetic modifiers that increase survival of SOD1 mice and are suitable for therapeutic development

Amyotrophic lateral sclerosis (ALS) is a frequently fatal motor neuron disease without any cure. To find molecular therapeutic targets, several studies crossed transgenic ALS murine models with animals transgenic for some ALS target genes. We aimed to revise the new discoveries and new works in this field. We selected the 10 most promising genes, according to their capability when down-regulated or up-regulated in ALS animal models, for increasing life span and mitigating disease progression: XBP-1, NogoA and NogoB, dynein, heavy and medium neurofilament, NOX1 and NOX2, MLC-mIGF-1, NSE-VEGF, and MMP-9. Interestingly, some crucial modifier genes have been described as being involved in common pathways, the most significant of which are inflammation and cytoskeletal activities. The endoplasmic reticulum also seems to play an important role in ALS pathogenesis, as it is involved in different selected gene pathways. In addition, these genes have evident links to each other, introducing the hypothesis of a single unknown, common pathway involving all of these identified genes and others to be discovered.

Functional outcome is impaired following traumatic brain injury in aging Nogo-A/B-deficient mice

Increasing age is associated with a poor prognosis following traumatic brain injury (TBI). CNS axons may recover poorly following TBI due to expression of myelin-derived inhibitors to axonal outgrowth such as Nogo-A. To study the role of Nogo-A/B in the pathophysiological response of the elderly to TBI, 1-year-old mice deficient in Nogo-A/B (Nogo-A/B homozygous(-/-) mice), Nogo-A/B heterozygous(-/+) mice, and age-matched wild-type (WT) littermate controls were subjected to a controlled cortical impact (CCI) TBI. Sham-injured WT mice (7 months old) and 12 month old naïve Nogo-A/B(-/-) and Nogo-A/B(-/+) served as controls. Neurological motor function was evaluated up to 3 weeks, and cognitive function, hemispheric tissue loss, myelin staining and hippocampal beta-amyloid (A beta) immunohistochemistry were evaluated at 4 weeks post-injury. In WT littermates, TBI significantly impaired learning ability at 4 weeks and neurological motor function up to 2 weeks post-injury and caused a significant loss of hemispheric tissue. Following TBI, Nogo-A/B(-/-) mice showed significantly less recovery from neurological motor and cognitive deficits compared to brain-injured WT mice. Naïve Nogo-A/B(-/-) and Nogo-A/B(-/+) mice quickly learned the MWM task in contrast to brain-injured Nogo-A/B(-/-) mice who failed to learn the MWM task at 4 weeks post-injury. Hemispheric tissue loss and cortical lesion volume were similar among the brain-injured genotypes. Neither TBI nor the absence of NogoA/B caused an increased A beta expression. Myelin staining showed a reduced area and density in the corpus callosum in brain-injured Nogo-A/B(-/-) animals compared to their littermate controls. These novel and unexpected behavioral results demonstrate that the absence of Nogo-A/B may negatively influence outcome, possibly related to hypomyelination, following TBI in mice and suggest a complex role for this myelin-associated axonal growth inhibitor following TBI.

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.

Novel modulators of amyloid-beta precursor protein processing

Proteolytic processing of the amyloid precursor protein (APP) is modulated by the action of enzymes alpha-, beta- and gamma-secretases, with the latter two mediating the amyloidogenic production of amyloid-beta (Abeta). Cellular modulators of APP processing are well known from studies of genetic mutations (such as those found in APP and presenilins) or polymorphisms (such as the apolipoprotein E4 epsilon-allele) that predisposes an individual to early or late-onset Alzheimer's disease. In recent years, several classes of molecule with modulating functions in APP processing and Abeta secretion have emerged. These include the neuronal Munc-18 interacting proteins (Mints)/X11s, members of the reticulon family (RTN-3 and RTN-4/Nogo-B), the Nogo-66 receptor (NgR), the peptidyl-prolyl isomerase Pin1 and the Rho family GTPases and their effectors. Mints and NgR bind to APP directly, while RTN3 and Nogo-B interact with the beta-secretase BACE1. Phosphorylated APP is a Pin1 substrate, which binds to its phosphor-Thr668-Pro motif. These interactions by and large resulted in a reduction of Abeta generation both in vitro and in vivo. Inhibition of Rho and Rho-kinase (ROCK) activity may underlie the ability of non-steroidal anti-inflammatory drugs and statins to reduce Abeta production, a feat which could also be achieved by Rac1 inhibition. Detailed understanding of the underlying mechanisms of action of these novel modulators of APP processing, as well as insights into the molecular neurological basis of how Abeta impairs leaning and memory, will open up multiple avenues for the therapeutic intervention of Alzheimer's disease.

Nogo in multiple sclerosis: Growing roles of a growth inhibitor

In recent years, knowledge about the physiological functions of the Nogo-A protein has grown considerably, and this molecule has evolved from being one of the most important axonal regrowth inhibitors present in central nervous system (CNS) myelin, to several other potentially important roles in different areas such as nervous system development, epilepsy, vascular physiology, muscle pathology and CNS tumors. Therapeutically, targeting the Nogo-A protein by means of the immune response has been tried in an attempt to block neurite growth inhibition and promote regeneration in spinal cord injury models; the immune response to Nogo-A, however, has not been extensively studied. We propose to review recent evidence that Nogo-A may also play an important role in autoimmune demyelinating diseases such as experimental autoimmune encephalomyelitis and multiple sclerosis, including that Nogo-66 derived epitopes are encephalitogenic antigens in susceptible mouse strains, and that the immune response to Nogo-66 antigens includes both strong T cell and B cell activation, with epitope spreading of the antibody response to other myelin molecules. In CNS immunotherapy, careful targeting of neural self-antigens is a prerequisite in order to avoid unexpected deleterious effects, and increasing knowledge about the immune response to Nogo-A may provide a safe basis for the development of relevant therapeutic alternatives for several neurological conditions.