The role of repulsive guidance molecules in the embryonic and adult vertebrate central nervous system

Development and validation of an ultra-performance liquid chromatography with tandem mass spectrometry method for determination of soluble repulsive guidance molecule A in human serum and cerebrospinal fluid

Aim: Repulsive guidance molecule A (RGMa) is upregulated in neurodegenerative diseases. To assess RGMa levels in human serum and cerebrospinal fluid (CSF), a quantification method was developed and validated according to ICH M10 guideline.Methods & results: Sample preparation consisted of immunoprecipitation (IP, only for serum), digestion and purification followed by MS.Conclusion: An UPLC-MS/MS method was established and used to assess normal range of soluble RGMa levels in serum and CSF of healthy controls, and patients with mild cognitive impairment or Alzheimer's disease. The normal range was between 13.0-44.8 ng/ml (CSF) and 9.9-20.9 ng/ml (serum) in healthy controls. In the CSF of patients with mild cognitive impairment and Alzheimer's disease, total soluble RGMa was twofold lower while unchanged in serum.

Repulsive guidance molecules b (RGMb): molecular mechanism, function and role in diseases

Repulsive guidance molecule b (RGMb), a glycosylphosphatidylinositol-anchored member of the RGM family, is initially identified as a co-receptor of bone morphogenetic protein (BMP) in the nervous system. The expression of RGMb is transcriptionally regulated by dorsal root ganglion 11 (DRG11), which is a transcription factor expressed in embryonic DRG and dorsal horn neurons and plays an important role in the development of sensory circuits. RGMb is involved in important physiological processes such as embryonic development, immune response, intercellular adhesion and tumorigenesis. Furthermore, RGMb is mainly involved in the regulation of RGMb-neogenin-Rho and BMP signalling pathways. The recent discovery of programmed death-ligand 2 (PD-L2)-RGMb binding reveals that the cell signalling network and functional regulation centred on RGMb are extremely complex. The latest report suggests that down-regulation of the PD-L2-RGMb pathway in the gut microbiota promotes an anti-tumour immune response, which defines a potentially effective immune strategy. However, the biological function of RGMb in a variety of human diseases has not been fully determined, and will remain an active research field. This article reviews the properties and functions of RGMb, focusing on its role under various physiological and pathological conditions.

Neutralizing RGMa with Elezanumab Promotes Cerebroprotection and Recovery in Rabbit Middle Cerebral Artery Occlusion

Repulsive guidance molecule A (RGMa) is an inhibitor of neuronal growth and survival which is upregulated in the damaged central nervous system following acute spinal cord injury (SCI), traumatic brain injury, acute ischemic stroke (AIS), and other neuropathological conditions. Neutralization of RGMa is neuroprotective and promotes neuroplasticity in several preclinical models of neurodegeneration and injury including multiple sclerosis, AIS, and SCI. Given the limitations of current treatments for AIS due to narrow time windows to intervention (TTI), and restrictive patient selection criteria, there is significant unmet need for therapeutic agents that enable tissue survival and repair following acute ischemic damage for a broader population of stroke patients. In this preclinical study, we evaluated whether elezanumab, a human anti-RGMa monoclonal antibody, could improve neuromotor function and modulate neuroinflammatory cell activation following AIS with delayed intervention times up to 24 h using a rabbit embolic permanent middle cerebral artery occlusion model (pMCAO). In two replicate 28-day pMCAO studies, weekly intravenous infusions of elezanumab, over a range of doses and TTIs of 6 and 24 h after stroke, significantly improved neuromotor function in both pMCAO studies when first administered 6 h after stroke. All elezanumab treatment groups, including the 24 h TTI group, had significantly less neuroinflammation as assessed by microglial and astrocyte activation. The novel mechanism of action and potential for expanding TTI in human AIS make elezanumab distinct from current acute reperfusion therapies, and support evaluation in clinical trials of acute CNS damage to determine optimal dose and TTI in humans.

Genetic basis of sow hyperprolificacy and litter size optimization based on a genome-wide association study

Over the last few decades, there has been a constant increase in sow litter size, the consequences of which include parturition duration extension, an increase in the percentage of stillborn and hypoxic piglets, and increased variation in piglet birth weight, which reduces their vitality. As such, it seems clear that further increasing sow fertility will generate difficulties and costs in rearing numerous litters with low birth weights. Therefore, the current study aimed to analyze the genetic background of sow hyperprolifcacy using a genome-wide association study (GWAS). The research included 144 sows in the maternal component, divided into two equal groups. The first group (control) consisted of females giving birth to the optimal number of piglets in their third and fourth litters (14-16), while the second group (cases) included those with excessive litter size (>16). The analyzed sows were genotyped using Illumina's PorcineSNP60v2 BeadChip microarray, comprising 64,232 single nucleotide polymorphisms (SNPs). Statistical analysis using R included quality control of genotyping data and GWAS analysis based on five logistic regression models (dominant, codominant, overdominant, recessive, and log-additive) with a single SNP marker as the explanatory variable. On this basis, one SNP (SIRI0000069) was identified on chromosome seven within the EFCAB11 (EF-hand calcium binding domain 11) gene that had a statistically significant effect on sow hyperprolificacy. Additionally, ten SNPs (INRA0007631, ALGA0011600, ALGA0043433, ALGA0043428, M1GA0010535 ALGA00443338, ALGA0087116, MARC0056787, ALGA0112928, and ALGA0089047) had a relationship with the analyzed feature at a level close to significance, set at 1-5. These SNPs appear important since they are located on chromosomes on which a large number of quantitative trait loci (QTLs) and SNPs associated with reproductive characteristics, including litter size, have been identified.

Comparability of Elezanumab Safety, Tolerability, and Pharmacokinetics in Healthy Japanese, Chinese, and White Participants

Elezanumab is a fully human monoclonal antibody, which is directed against repulsive guidance molecule A. The safety, tolerability, pharmacokinetics (PK), and immunogenicity of elezanumab were assessed in 2 Phase 1 clinical studies. The objective of this study was to assess the PK, safety, tolerability, and immunogenicity following intravenous infusion of elezanumab in healthy adult Japanese, Han Chinese, and Caucasian participants as well as Western participants from the single-ascending-dose study. Elezanumab exposures were approximately 20% higher in Japanese and Han Chinese participants compared to White participants without controlling for body weight. After statistically controlling for body weight by including it as a covariate, the PK of elezanumab in White participants were comparable to those in Japanese and Han Chinese participants. The clinical implications of these exposure differences are yet to be determined. All adverse events were assessed by the investigator as having no reasonable possibility of being related to the study drugs and were mild in severity. No positive immunogenicity effect was observed that impacted elezanumab exposure or safety.

Phase 1 Evaluation of Elezanumab (Anti-Repulsive Guidance Molecule A Monoclonal Antibody) in Healthy and Multiple Sclerosis Participants

OBJECTIVE

This study was undertaken to describe the safety, tolerability, pharmacokinetics, and immunogenicity of elezanumab (ABT-555), a fully human monoclonal antibody (mAb) directed against repulsive guidance molecule A (RGMa), in healthy and multiple sclerosis (MS) study participants.

METHODS

The single-center, first-in-human, single ascending dose (SAD) study evaluated elezanumab (50-1,600mg intravenous [IV] and 150mg subcutaneous) in 47 healthy men and women. The multicenter multiple ascending dose (MAD; NCT02601885) study evaluated elezanumab (150mg, 600mg, and 1,800mg) in 20 adult men and women with MS, receiving either maintenance or no immunomodulatory treatment.

RESULTS

No pattern of study drug-related adverse events was identified for either the SAD or MAD elezanumab regimens. Across both studies, the T_max occurred within 4 hours of elezanumab IV infusion, and the harmonic mean of t_1/2 ranged between 18.6 and 67.7 days. Following multiple dosing, elezanumab C_max , area under the curve, and C_trough increased dose-proportionally and resulted in dose-dependent increases in elezanumab cerebrospinal fluid (CSF) concentrations. Elezanumab CSF penetration was 0.1% to 0.4% across both studies, with CSF levels of free RGMa decreased by >40%. Changes in CSF interleukin-10 (IL-10) and free RGMa demonstrated dose/exposure-dependence.

INTERPRETATION

The elezanumab pharmacokinetic profile supports monthly, or bimonthly, administration of up to 1,800mg with the option of a loading dose of 3,600mg. Elezanumab partitioning into CSF is within the range expected for mAbs. Reduced CSF levels of free RGMa demonstrate central nervous system target binding of elezanumab with an apparent maximal effect at 1,800mg IV. Exposure-associated increases in CSF IL-10, an anti-inflammatory cytokine with neuroprotective/neurorestorative properties, support potential pathway modulation in MS participants. ANN NEUROL 2023;93:285-296.

Elezanumab, a clinical stage human monoclonal antibody that selectively targets repulsive guidance molecule A to promote neuroregeneration and neuroprotection in neuronal injury and demyelination models

Repulsive guidance molecule A (RGMa) is a potent inhibitor of axonal growth and a regulator of neuronal cell death. It is up-regulated following neuronal injury and accumulates in chronic neurodegenerative diseases. Neutralizing RGMa has the potential to promote neuroregeneration and neuroprotection. Previously we reported that a rat anti-N terminal RGMa (N-RGMa) antibody r5F9 and its humanized version h5F9 (ABT-207) promote neuroprotection and neuroregeneration in preclinical neurodegenerative disease models. However, due to its cross-reactivity to RGMc/hemojuvelin, ABT-207 causes iron accumulation in vivo, which could present a safety liability. Here we report the generation and characterization of a novel RGMa-selective anti-N-RGMa antibody elezanumab, which is currently under Phase 2 clinical evaluation in multiple disease indications. Elezanumab, a human monoclonal antibody generated by in vitro PROfusion mRNA display technology, competes with ABT-207 in binding to N-RGMa but lacks RGMc cross-reactivity with no impact on iron metabolism. It neutralizes repulsive activity of soluble RGMa in vitro and blocks membrane RGMa mediated BMP signaling. In the optic nerve crush and optic neuritis models, elezanumab promotes axonal regeneration and prevents retinal nerve fiber layer degeneration. In the spinal targeted experimental autoimmune encephalomyelitis (EAE) model, elezanumab promotes axonal regeneration and remyelination, decreases inflammatory lesion area and improves functional recovery. Finally, in the mouse cuprizone model, elezanumab reduces demyelination, which is consistent with its inhibitory effect on BMP signaling. Taken together, these preclinical data demonstrate that elezanumab has neuroregenerative and neuroprotective activities without impact on iron metabolism, thus providing a compelling rationale for its clinical development in neurodegenerative diseases.

Elezanumab, a human anti-RGMa monoclonal antibody, promotes neuroprotection, neuroplasticity, and neurorecovery following a thoracic hemicompression spinal cord injury in non-human primates

Spinal cord injury (SCI) is a devastating condition characterized by loss of function, secondary to damaged spinal neurons, disrupted axonal connections, and myelin loss. Spontaneous recovery is limited, and there are no approved pharmaceutical treatments to reduce ongoing damage or promote repair. Repulsive guidance molecule A (RGMa) is upregulated following injury to the central nervous system (CNS), where it is believed to induce neuronal apoptosis and inhibit axonal growth and remyelination. We evaluated elezanumab, a human anti-RGMa monoclonal antibody, in a novel, newly characterized non-human primate (NHP) hemicompression model of thoracic SCI. Systemic intravenous (IV) administration of elezanumab over 6 months was well tolerated and associated with significant improvements in locomotor function. Treatment of animals for 16 weeks with a continuous intrathecal infusion of elezanumab below the lesion was not efficacious. IV elezanumab improved microstructural integrity of extralesional tissue as reflected by higher fractional anisotropy and magnetization transfer ratios in treated vs. untreated animals. IV elezanumab also reduced SCI-induced increases in soluble RGMa in cerebrospinal fluid, and membrane bound RGMa rostral and caudal to the lesion. Anterograde tracing of the corticospinal tract (CST) from the contralesional motor cortex following 20 weeks of IV elezanumab revealed a significant increase in the density of CST fibers emerging from the ipsilesional CST into the medial/ventral gray matter. There was a significant sprouting of serotonergic (5-HT) fibers rostral to the injury and in the ventral horn of lower thoracic regions. These data demonstrate that 6 months of intermittent IV administration of elezanumab, beginning within 24 h after a thoracic SCI, promotes neuroprotection and neuroplasticity of key descending pathways involved in locomotion. These findings emphasize the mechanisms leading to improved recovery of neuromotor functions with elezanumab in acute SCI in NHPs.

Inhibition of repulsive guidance molecule-a protects dopaminergic neurons in a mouse model of Parkinson's disease

Repulsive guidance molecule-a (RGMa), a glycosylphosphatidylinositol-anchored membrane protein, has diverse functions in axon guidance, cell patterning, and cell survival. Inhibition of RGMa attenuates pathological dysfunction in animal models of central nervous system (CNS) diseases including spinal cord injury, multiple sclerosis, and neuromyelitis optica. Here, we examined whether antibody-based inhibition of RGMa had therapeutic effects in a mouse model of Parkinson's disease (PD). We treated mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and found increased RGMa expression in the substantia nigra (SN). Intraventricular, as well as intravenous, administration of anti-RGMa antibodies reduced the loss of tyrosine hydroxylase (TH)-positive neurons and accumulation of Iba1-positive microglia/macrophages in the SN of MPTP-treated mice. Selective expression of RGMa in TH-positive neurons in the SN-induced neuronal loss/degeneration and inflammation, resulting in a progressive movement disorder. The pathogenic effects of RGMa overexpression were attenuated by treatment with minocycline, which inhibits microglia and macrophage activation. Increased RGMa expression upregulated pro-inflammatory cytokine expression in microglia. Our observations suggest that the upregulation of RGMa is associated with the PD pathology; furthermore, inhibitory RGMa antibodies are a potential therapeutic option.

The Cord Blood Insulin and Mitochondrial DNA Content Related Methylome

Mitochondrial dysfunction seems to play a key role in the etiology of insulin resistance. At birth, a link has already been established between mitochondrial DNA (mtDNA) content and insulin levels in cord blood. In this study, we explore shared epigenetic mechanisms of the association between mtDNA content and insulin levels, supporting the developmental origins of this link. First, the association between cord blood insulin and mtDNA content in 882 newborns of the ENVIRONAGE birth cohort was assessed. Cord blood mtDNA content was established via qPCR, while cord blood levels of insulin were determined using electrochemiluminescence immunoassays. Then the cord blood DNA methylome and transcriptome were determined in 179 newborns, using the human 450K methylation Illumina and Agilent Whole Human Genome 8 × 60 K microarrays, respectively. Subsequently, we performed an epigenome-wide association study (EWAS) adjusted for different maternal and neonatal variables. Afterward, we focused on the 20 strongest associations based on p-values to assign transcriptomic correlates and allocate corresponding pathways employing the R packages ReactomePA and RDAVIDWebService. On the regional level, we examined differential methylation using the DMRcate and Bumphunter packages in R. Cord blood mtDNA content and insulin were significantly correlated (r = 0.074, p = 0.028), still showing a trend after additional adjustment for maternal and neonatal variables (p = 0.062). We found an overlap of 33 pathways which were in common between the association with cord blood mtDNA content and insulin levels, including pathways of neurodevelopment, histone modification, cytochromes P450 (CYP)-metabolism, and biological aging. We further identified a DMR annotated to Repulsive Guidance Molecule BMP Co-Receptor A (RGMA) linked to cord blood insulin as well as mtDNA content. Metabolic variation in early life represented by neonatal insulin levels and mtDNA content might reflect or accommodate alterations in neurodevelopment, histone modification, CYP-metabolism, and aging, indicating etiological origins in epigenetic programming. Variation in metabolic hormones at birth, reflected by molecular changes, might via these alterations predispose children to metabolic diseases later in life. The results of this study may provide important markers for following targeted studies.

Inhibition of neogenin promotes neuronal survival and improved behavior recovery after spinal cord injury

Following spinal cord trauma, axonal regeneration in the mammalian spinal cord does not occur and functional recovery may be further impeded by retrograde neuronal death. By contrast, lampreys recover after spinal cord injury (SCI) and axons re-connected to their targets in spinal cord. However, the identified reticulospinal (RS) neurons located in the lamprey brain differ in their regenerative capacities - some are good regenerators, and others are bad regenerators - despite the fact that they have analogous projection pathways. Previously, we reported that axonal guidance receptor Neogenin involved in regulation of axonal regeneration after SCI and downregulation of Neogenin synthesis by morpholino oligonucleotides (MO) enhanced the regeneration of RS neurons. Incidentally, the bad regenerating RS neurons often undergo a late retrograde apoptosis after SCI. Here we report that, after SCI, expression of RGMa mRNA was upregulated around the transection site, while its receptor Neogenin continued to be synthesized almost inclusively in the "bad-regenerating" RS neurons. Inhibition of Neogenin by MO prohibited activation of caspases and improved the survival of RS neurons at 10 weeks after SCI. These data provide new evidence in vivo that Neogenin is involved in retrograde neuronal death and failure of axonal regeneration after SCI.

Brain uptake of multivalent and multi-specific DVD-Ig proteins after systemic administration

Therapeutic monoclonal antibodies and endogenous IgG antibodies show limited uptake into the central nervous system (CNS) due to the blood-brain barrier (BBB), which regulates and controls the selective and specific transport of both exogenous and endogenous materials to the brain. The use of natural transport mechanisms, such as receptor-mediated transcytosis (RMT), to deliver antibody therapeutics into the brain have been studied in rodents and monkeys. Recent successful examples include monovalent bispecific antibodies and mono- or bivalent fusion proteins; however, these formats do not have the capability to bind to both the CNS target and the BBB transport receptor in a bivalent fashion as a canonical antibody would. Dual-variable-domain immunoglobulin (DVD-Ig) proteins offer a bispecific format where monoclonal antibody-like bivalency to both the BBB receptor and the therapeutic target is preserved, enabling independent engineering of binding affinity, potency, valency, epitope and conformation, essential for successful generation of clinical candidates for CNS applications with desired drug-like properties. Each of these parameters can affect the binding and transcytosis ability mediated by different receptors on the brain endothelium differentially, allowing exploration of diverse properties. Here, we describe generation and characterization of several different DVD-Ig proteins, specific for four different CNS targets, capable of crossing the BBB through transcytosis mediated by the transferrin receptor 1 (TfR1). After systemic administration of each DVD-Ig, we used two independent methods in parallel to observe specific uptake into the brain. An electrochemiluminescent-based sensitive quantitative assay and a semi-quantitative immunohistochemistry technique were used for brain concentration determination and biodistribution/localization in brain, respectively. Significantly enhanced brain uptake and retention was observed for all TfR1 DVD-Ig proteins regardless of the CNS target or the systemic administration route selected.

Repulsive Guidance Molecule a (RGMa) Induces Neuropathological and Behavioral Changes That Closely Resemble Parkinson's Disease

Repulsive guidance molecule member a (RGMa) is a membrane-associated or released guidance molecule that is involved in axon guidance, cell patterning, and cell survival. In our previous work, we showed that RGMa is significantly upregulated in the substantia nigra of patients with Parkinson's disease. Here we demonstrate the expression of RGMa in midbrain human dopaminergic (DA) neurons. To investigate whether RGMa might model aspects of the neuropathology of Parkinson's disease in mouse, we targeted RGMa to adult midbrain dopaminergic neurons using adeno-associated viral vectors. Overexpression of RGMa resulted in a progressive movement disorder, including motor coordination and imbalance, which is typical for a loss of DA release in the striatum. In line with this, RGMa induced selective degeneration of dopaminergic neurons in the substantia nigra (SN) and affected the integrity of the nigrostriatal system. The degeneration of dopaminergic neurons was accompanied by a strong microglia and astrocyte activation. The behavioral, molecular, and anatomical changes induced by RGMa in mice are remarkably similar to the clinical and neuropathological hallmarks of Parkinson's disease. Our data indicate that dysregulation of RGMa plays an important role in the pathology of Parkinson's disease, and antibody-mediated functional interference with RGMa may be a disease modifying treatment option.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is a neurodegenerative disease characterized by severe motor dysfunction due to progressive degeneration of mesencephalic dopaminergic (DA) neurons in the substantia nigra. To date, there is no regenerative treatment available. We previously showed that repulsive guidance molecule member a (RGMa) is upregulated in the substantia nigra of PD patients. Adeno-associated virus-mediated targeting of RGMa to mouse DA neurons showed that overexpression of this repulsive axon guidance and cell patterning cue models the behavioral and neuropathological characteristics of PD in a remarkable way. These findings have implications for therapy development as interfering with the function of this specific axon guidance cue may be beneficial to the survival of DA neurons.

The roles of RGMa-neogenin signaling in inflammation and angiogenesis

Repulsive guidance molecule (RGM) is a glycosylphosphatidylinositol (GPI)-anchored glycoprotein that has diverse functions in the developing and pathological central nervous system (CNS). The binding of RGM to its receptor neogenin regulates axon guidance, neuronal differentiation, and survival during the development of the CNS. In the pathological state, RGM expression is induced after spinal cord injury, and the inhibition of RGM promotes axon growth and functional recovery. Furthermore, RGM expression is also observed in immune cells, and RGM regulates inflammation and neurodegeneration in autoimmune encephalomyelitis. RGMa induces T cell activation in experimental autoimmune encephalomyelitis (EAE), which is the animal model of multiple sclerosis (MS). RGM is expressed in pathogenic Th17 cells and induces neurodegeneration by binding to neogenin. Angiogenesis is an additional key factor involved in the pathophysiology of EAE. Via neogenin, treatment with RGMa can suppress endothelial tube formation; this finding indicates that RGMa inhibits neovascularization. These observations suggest the feasibility of utilizing the RGMa-neogenin signaling pathway as a therapeutic target to overcome inflammation and neurodegeneration. This review focuses on the molecular mechanisms of inflammation and angiogenesis via RGM-neogenin signaling.

Characterizing Semaphorin-Mediated Effects on Sensory and Motor Axon Pathfinding and Connectivity During Embryonic Development

How are precise connectivity to peripheral targets and corresponding sensory-motor networks established during developmental innervation of the vertebrate extremities? The formation of functional sensory-motor circuits requires highly appropriate temporal and spatial regulation of axon growth which is achieved through the combination of different molecular mechanisms such as communication between heterotypic fiber systems, axon-environment, or axon-glia interactions that ensure proper fasciculation and accurate pathfinding to distal targets. Family members of the class 3 semaphorins and their cognate receptors, the neuropilins, were shown to govern various events during wiring of central and peripheral circuits, with mice lacking Sema3-Npn signaling showing deficits in timing of growth, selective fasciculation, guidance fidelity, and coupling of sensory axon growth to motor axons at developmental time points. Given the accuracy with which these processes have to interact in a stepwise manner, deficiency of the smallest cog in the wheel may impact severely on the faithful establishment and functionality of peripheral circuitries, ultimately leading to behavioral impairments or even cause the death of the animal. Reliable quantitative analyses of sensory-motor fasciculation, extension, and guidance of axons to their cognate target muscles and the skin during development, but also assessment of physiological and behavioral consequences at adult age, are therefore a necessity to extend our understanding of the molecular mechanisms of peripheral circuit formation. In this chapter we provide a detailed methodology to characterize class 3 semaphorin-mediated effects on peripheral sensory and motor axon pathfinding and connectivity during embryonic development.

RhoA/Rho kinase in spinal cord injury

A spinal cord injury refers to an injury to the spinal cord that is caused by a trauma instead of diseases. Spinal cord injury includes a primary mechanical injury and a much more complex secondary injury process involving inflammation, oxidation, excitotoxicity, and cell death. During the secondary injury, many signal pathways are activated and play important roles in mediating the pathogenesis of spinal cord injury. Among them, the RhoA/Rho kinase pathway plays a particular role in mediating spinal degeneration and regeneration. In this review, we will discuss the role and mechanism of RhoA/Rho kinase-mediated spinal cord pathogenesis, as well as the potential of targeting RhoA/Rho kinase as a strategy for promoting both neuroprotection and axonal regeneration.

Targeting repulsive guidance molecule A to promote regeneration and neuroprotection in multiple sclerosis

Repulsive guidance molecule A (RGMa) is a potent inhibitor of neuronal regeneration and a regulator of cell death, and it plays a role in multiple sclerosis (MS). In autopsy material from progressive MS patients, RGMa was found in active and chronic lesions, as well as in normal-appearing gray and white matter, and was expressed by cellular meningeal infiltrates. Levels of soluble RGMa in the cerebrospinal fluid were decreased in progressive MS patients successfully treated with intrathecal corticosteroid triamcinolone acetonide (TCA), showing functional improvements. In vitro, RGMa monoclonal antibodies (mAbs) reversed RGMa-mediated neurite outgrowth inhibition and chemorepulsion. In animal models of CNS damage and MS, RGMa antibody stimulated regeneration and remyelination of damaged nerve fibers, accelerated functional recovery, and protected the retinal nerve fiber layer as measured by clinically relevant optic coherence tomography. These data suggest that targeting RGMa is a promising strategy to improve functional recovery in MS patients.

An Automated Strategy for Unbiased Morphometric Analyses and Classifications of Growth Cones In Vitro

During neural circuit development, attractive or repulsive guidance cue molecules direct growth cones (GCs) to their targets by eliciting cytoskeletal remodeling, which is reflected in their morphology. The experimental power of in vitro neuronal cultures to assay this process and its molecular mechanisms is well established, however, a method to rapidly find and quantify multiple morphological aspects of GCs is lacking. To this end, we have developed a free, easy to use, and fully automated Fiji macro, Conographer, which accurately identifies and measures many morphological parameters of GCs in 2D explant culture images. These measurements are then subjected to principle component analysis and k-means clustering to mathematically classify the GCs as “collapsed” or “extended”. The morphological parameters measured for each GC are found to be significantly different between collapsed and extended GCs, and are sufficient to classify GCs as such with the same level of accuracy as human observers. Application of a known collapse-inducing ligand results in significant changes in all parameters, resulting in an increase in ‘collapsed’ GCs determined by k-means clustering, as expected. Our strategy provides a powerful tool for exploring the relationship between GC morphology and guidance cue signaling, which in particular will greatly facilitate high-throughput studies of the effects of drugs, gene silencing or overexpression, or any other experimental manipulation in the context of an in vitro axon guidance assay.

Repulsive Guidance Molecules a, b and c Are Skeletal Muscle Proteins, and Repulsive Guidance Molecule a Promotes Cellular Hypertrophy and Is Necessary for Myotube Fusion

Repulsive guidance molecules (RGMs) compose a family of glycosylphosphatidylinositol (GPI)-anchored axon guidance molecules and perform several functions during neural development. New evidence has suggested possible new roles for these axon guidance molecules during skeletal muscle development, which has not been investigated thus far. In the present study, we show that RGMa, RGMb and RGMc are all induced during skeletal muscle differentiation in vitro. Immunolocalization performed on adult skeletal muscle cells revealed that RGMa, RGMb and RGMc are sarcolemmal proteins. Additionally, RGMa was found to be a sarcoplasmic protein with a surprisingly striated pattern. RGMa colocalization with known sarcoplasmic proteins suggested that this axon guidance molecule is a skeletal muscle sarcoplasmic protein. Western blot analysis revealed two RGMa fragments of 60 and 33 kDa, respectively, in adult skeletal muscle samples. RGMa phenotypes in skeletal muscle cells (C2C12 and primary myoblasts) were also investigated. RGMa overexpression produced hypertrophic cells, whereas RGMa knockdown resulted in the opposite phenotype. RGMa knockdown also blocked myotube formation in both skeletal muscle cell types. Our results are the first to show an axon guidance molecule as a skeletal muscle sarcoplasmic protein and to include RGMa in a system that regulates skeletal muscle cell size and differentiation.

Anti-repulsive Guidance Molecule C (RGMc) Antibodies Increases Serum Iron in Rats and Cynomolgus Monkeys by Hepcidin Downregulation

High levels of hepcidin, the main regulator of systemic iron metabolism, lead to various diseases. Targeting hepcidin and lowering its concentration is a possible form of intervention in order to treat these diseases. High turnover rate of hepcidin is a major drawback of therapies directly targeting this peptide. We developed two monoclonal antibodies ABT-207 and h5F9-AM8 which inhibit hemojuvelin/repulsive guidance molecule C (RGMc) and downregulate hepcidin. We conducted single-application and dose response studies to understand the antibodies' mechanism and subchronic toxicology studies to exclude safety-related concerns. Investigation was carried out at different biological levels through qPCR, Affymetrix, liquid chromatography coupled with mass spectrometry (LC-MS/MS), histopathology, serum iron, unsaturated iron binding capacity (UIBC), and drug concentration measurements. After a single application of these antibodies, hepcidin expression in liver and its serum protein levels were reduced. Serum iron increased for several weeks. The RGMc antibodies show a pronounced dose response relationship in rats with h5F9-AM8 having an IC50 (UIBC) of approximately 80-fold higher than ABT-207. When hepcidin levels were downregulated, iron deposition in the liver was visible histologically 1 week post application. These antibody-mediated iron depositions were not associated with any adverse toxicologically relevant effect at the doses and time points evaluated. Iron depositions seen after 14 weekly treatments with ABT-207 were reversible in rats and in cynomolgus monkeys. Due to their long-lasting effects and excellent safety profile, both RGMc-blocking antibodies ABT-207 and h5F9-AM8 are favorable clinical candidates for diseases characterized by high serum hepcidin levels like anemia of chronic disease.

Axon growth inhibition by RhoA/ROCK in the central nervous system

Rho kinase (ROCK) is a serine/threonine kinase and a downstream target of the small GTPase Rho. The RhoA/ROCK pathway is associated with various neuronal functions such as migration, dendrite development, and axonal extension. Evidence from animal studies reveals that RhoA/ROCK signaling is involved in various central nervous system (CNS) diseases, including optic nerve and spinal cord injuries, stroke, and neurodegenerative diseases. Given that RhoA/ROCK plays a critical role in the pathophysiology of CNS diseases, the development of therapeutic agents targeting this pathway is expected to contribute to the treatment of CNS diseases. The RhoA/ROCK pathway mediates the effects of myelin-associated axon growth inhibitors—Nogo, myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgp), and repulsive guidance molecule (RGM). Blocking RhoA/ROCK signaling can reverse the inhibitory effects of these molecules on axon outgrowth, and promotes axonal sprouting and functional recovery in animal models of CNS injury. To date, several RhoA/ROCK inhibitors have been under development or in clinical trials as therapeutic agents for neurological disorders. In this review, we focus on the RhoA/ROCK signaling pathway in neurological disorders. We also discuss the potential therapeutic approaches of RhoA/ROCK inhibitors for various neurological disorders.

Role of DAPK in neuronal cell death

Neuronal cell death happens as a result of the normal physiological process that occurs during development, or as part of the pathological process that occurs during disease. Death-associated protein kinase (DAPK) is an intracellular protein that mediates cell death by its serine/threonine kinase activity, and transmits apoptotic cell death signals in various cells, including neurons. DAPK is elevated in injured neurons in acute models of injury such as ischemia and seizure. The absence of DAPK has been shown to protect neurons from a wide variety of acute toxic insults. Moreover, DAPK also regulates neuronal cell death during central nervous system development. Neurons are initially overproduced in the developing nervous system, following which approximately one-half of the original cell population dies. This "naturally-occurring" or "programmed" cell death is essential for the construction of the developing nervous system. In this review, we focus on the role of DAPK in neuronal cell death after neuronal injury. The participation of DAPK in developmental neuronal death is also explained.

Decreased levels of repulsive guidance molecule A in association with beneficial effects of repeated intrathecal triamcinolone acetonide application in progressive multiple sclerosis patients

Repeated intrathecal application of the sustained release steroid triamcinolone acetonide is beneficial in progressive multiple sclerosis patients. Its putative regenerative effect may involve regulation of the repulsive guidance molecule A synthesis. This protein inhibits axonal regeneration and functional recovery. Objectives were to demonstrate the efficacy of four triamcinolone applications every other day in association with repulsive guidance molecule A levels in cerebrospinal fluid. Clinical evaluation was performed at baseline and on each day after a triamcinolone administration in 25 progressive multiple sclerosis patients. Repulsive guidance molecule A concentrations were determined before each triamcinolone application by western blot analysis with quantification. Clinical scores for multiple sclerosis improved, and the maximum walking distance and speed ameliorated in 17 patients. Repulsive guidance molecule A levels declined in these responders. The remaining patients showed no prompt clinical benefit and no decrease of repulsive guidance molecule A concentrations. Decline of repulsive guidance molecule A may reflect regeneration and functional recovery by triamcinolone in progressive multiple sclerosis patients.

RGMa and RGMb expression pattern during chicken development suggest unexpected roles for these repulsive guidance molecules in notochord formation, somitogenesis, and myogenesis

BACKGROUND

Repulsive guidance molecules (RGM) are high-affinity ligands for the Netrin receptor Neogenin, and they are crucial for nervous system development including neural tube closure; neuronal and neural crest cell differentiation and axon guidance. Recent studies implicated RGM molecules in bone morphogenetic protein signaling, which regulates a variety of developmental processes. Moreover, a role for RGMc in iron metabolism has been established. This suggests that RGM molecules may play important roles in non-neural tissues.

RESULTS

To explore which tissues and processed may be regulated by RGM molecules, we systematically investigated the expression of RGMa and RGMb, the only RGM molecules currently known for avians, in the chicken embryo.

CONCLUSIONS

Our study suggests so far unknown roles of RGM molecules in notochord, somite and skeletal muscle development.

RNA interference against repulsive guidance molecule A improves axon sprout and neural function recovery of rats after MCAO/reperfusion

Repulsive guidance molecule a (RGMa) is a neurite growth inhibitor that is of great interest in the study of CNS neuronal regeneration. We adopted RNA interference (RNAi) as a means of suppressing the expression of RGMa and observed the improvement in axonal regeneration and neurological function of rats after cerebral ischemic injury. Recombinant adenovirus rAd5-shRNA-RGMa was constructed and prepared for animal experimentation. RGMa and neurofilament protein 200 (NF200) in the ischemic cortex and ipsilateral hippocampus were detected by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. The ischemic regions were examined by triphenyltetrazolium chloride (TTC) staining and the newborn neurite branches by Biotinylated Dextran Amine (BDA) neuronal tracing. Behavior tests were adopted to evaluate neurologic function recovery. Results showed RGMa was down-regulated and axonal growth was improved in the RNAi treated group (P<0.01). The number of axonal sprouts of corticospinal tract from the uninjured side to the ischemic side in the RNAi treated group was increased (P<0.01). Behavior test scores in the RNAi treated group were significantly better than other groups after 6 weeks (P<0.01). RGMa in rat brains after middle cerebral artery occlusion (MCAO) can be down-regulated by RNAi successfully, which may lead to improved axonal growth and neural anatomy plasticity, as well as neuron functional recovery.

Identification of the neogenin-binding site on the repulsive guidance molecule A

Repulsive guidance molecule (RGM) is a membrane-bound protein that was originally identified as an axon guidance molecule in the chick retinotectal system. RGMa, one of the 3 isoforms found in mammals, is involved in laminar patterning, cephalic neural tube closure, axon guidance, and inhibition of axonal regeneration. In addition to its roles in the nervous system, RGMa plays a role in enhancing helper T-cell activation. Binding of RGM to its receptor, neogenin, is considered necessary to transduce these signals; however, information on the binding of RGM to neogenin is limited. Using co-immunoprecipitation studies, we have identified that the RGMa region required for binding to neogenin contains amino acids (aa) 259–295. Synthesized peptide consisting of aa 284–293 directly binds to the extracellular domain (ECD) of recombinant neogenin, and addition of this peptide inhibits RGMa-induced growth cone collapse in mouse cortical neurons. Thus, we propose that this peptide is a promising lead in finding reagents capable of inhibiting RGMa signaling.

Repulsive guidance molecule-A (RGM-A) inhibits leukocyte migration and mitigates inflammation

Directed cell migration is a prerequisite not only for the development of the central nervous system, but also for topically restricted, appropriate immune responses. This is crucial for host defense and immune surveillance. Attracting environmental cues guiding leukocyte cell traffic are likely to be complemented by repulsive cues, which actively abolish cell migration. One such a paradigm exists in the developing nervous system, where neuronal migration and axonal path finding is balanced by chemoattractive and chemorepulsive cues, such as the neuronal repulsive guidance molecule-A (RGM-A). As expressed at the inflammatory site, the role of RGM-A within the immune response remains unclear. Here we report that RGM-A (i) is expressed by epithelium and leukocytes (granulocytes, monocytes, and T/B lymphocytes); (ii) inhibits leukocyte migration by contact repulsion and chemorepulsion, depending on dosage, through its receptor neogenin; and (iii) suppresses the inflammatory response in a model of zymosan-A-induced peritonitis. Systemic application of RGM-A attenuates the humoral proinflammatory response (TNF-α, IL-6, and macrophage inflammatory protein 1α), infiltration of inflammatory cell traffic, and edema formation. In contrast, the demonstrated anti-inflammatory effect of RGM-A is absent in mice homozygous for a gene trap mutation in the neo1 locus (encoding neogenin). Thus, our results suggest that RGM-A is a unique endogenous inhibitor of leukocyte chemotaxis that limits inflammatory leukocyte traffic and creates opportunities to better understand and treat pathologies caused by exacerbated or misdirected inflammatory responses.

Effects of age and retinal degeneration on the expression of proprotein convertases in the visual cortex

Proprotein convertases (PCs) comprise a large family of subtilisin-like, eukaryotic, serine endoproteases that process substrates important in the development, homeostasis, and pathology of the nervous system. Despite important interactions with these substrates, including neurotrophins, PC expression throughout normal postnatal development and disease progression in the brain remains unknown. The primary objective of this study was to determine whether the expression profiles of widely expressed and tissue-specific PCs varied during normal brain development or neurological disorders. We examined the expression of mRNAs for seven PCs in the visual cortex of normal and visually impaired mice at 10 postnatal developmental time points between Week 1 and Week 35. Widely expressed PCs (furin, PACE4, PC5, and PC7) all exhibited a similar expression profile. High mRNA levels were seen at Week 1 with levels generally lower over the next 5-6 weeks. In visually impaired mice, widely expressed PCs again all exhibited a similar expression profile, but it was dramatically different than observed in normal mice. The temporal expression of tissue-specific PCs varied in wild-type mice. Interestingly, this variability was sharply reduced in visually impaired mice. Overall, these data suggest a timetable of altered PC expression that corresponds closely with the formation of functional visual maps in the visual cortex. The implications of these findings are discussed in the context of neurotrophin processing and synaptogenesis in the developing visual cortex.

The RGM/DRAGON family of BMP co-receptors

The BMP signaling pathway controls a number of cell processes during development and in adult tissues. At the cellular level, ligands of the BMP family act by binding a hetero-tetrameric signaling complex, composed of two type I and two type II receptors. BMP ligands make use of a limited number of receptors, which in turn activate a common signal transduction cascade at the intracellular level. A complex regulatory network is required in order to activate the signaling cascade at proper times and locations, and to generate specific downstream effects in the appropriate cellular context. One such regulatory mechanism is the repulsive guidance molecule (RGM) family of BMP co-receptors. This article reviews the current knowledge regarding the structure, regulation, and function of RGMs, focusing on known and potential roles of RGMs in physiology and pathophysiology.

Stimulating neuroregeneration as a therapeutic drug approach for traumatic brain injury

Traumatic brain injury, a silent epidemic of modern societies, is a largely neglected area in drug development and no drug is currently available for the treatment of patients suffering from brain trauma. Despite this grim situation, much progress has been made over the last two decades in closely related medical indications, such as spinal cord injury, giving rise to a more optimistic approach to drug development in brain trauma. Fundamental insights have been gained with animal models of central nervous system (CNS) trauma and spinal cord injury. Neuroregenerative drug candidates have been identified and two of these have progressed to clinical development for spinal cord injury patients. If successful, these drug candidates may be used to treat brain trauma patients. Significant progress has also been made in understanding the fundamental molecular mechanism underlying irreversible axonal growth arrest in the injured CNS of higher mammals. From these studies, we have learned that the axonal retraction bulb, previously regarded as a marker for failure of regenerative growth, is not static but dynamic and, therefore, amenable to pharmacotherapeutic approaches. With the development of modified magnetic resonance imaging methods, fibre tracts can be visualised in the living human brain and such imaging methods will soon be used to evaluate the neuroregenerative potential of drug candidates. These significant advances are expected to fundamentally change the often hopeless situation of brain trauma patients and will be the first step towards overcoming the silent epidemic of brain injury.

LIM-only protein 4 interacts directly with the repulsive guidance molecule A receptor Neogenin

Repulsive guidance molecule A (RGM A) was recently described as a potent inhibitor of neuroregeneration in a rat spinal cord injury model. The receptor mediating RGM A's repulsive activity was shown to be Neogenin, a member of the Deleted in Colorectal Cancer (DCC) family of netrin receptors. Binding of RGM A to Neogenin induces activation of the small GTPase RhoA and of its effector Rho-kinase by an unknown mechanism. Here we show, that the cytoplasmic tail of Neogenin interacts directly with the transcriptional coactivator LIM domain only 4 (LMO4) in human SH-SY5Y cells, human Ntera neurons, and in embryonic rat cortical neurons. RGM A binding to Neogenin but not binding of Netrin-1, induces release of LMO4 from Neogenin. Down-regulation of LMO4 neutralizes the repulsive activity of RGM A in neuronal cell lines and embryonic rat cortical neurons and prevents RhoA activation. These results show for the first time that an interaction of Neogenin with LMO4 is involved in the RGM A - Neogenin signal transduction pathway for RhoA activation.

A developmental classification of malformations of the brainstem

OBJECTIVE

With advances in imaging and genetics, malformations of the brainstem are being more commonly identified. We describe and classify brainstem anomalies in 138 patients ascertained over a period of 10 years

METHODS

Magnetic resonance imaging studies and, where available, clinical records of the patients were retrospectively reviewed. Malformations were segregated according to magnetic resonance findings and classified when possible by embryological mechanisms

RESULTS

The most common location for anomalies was the pons, which was involved in 114 patients. The midbrain was involved in 45 patients, whereas the medulla was involved in 14. In 53 patients, more than 1 region was affected (all 3 regions in 6 patients, midbrain and pons in 39, and medulla and pons in 8). The malformations were divided into four groups: (1) malformations with abnormal brainstem segmentation, (2) malformations with segmental hypoplasia, (3) postsegmentation malformations, and (4) malformations associated with abnormal cortical organization

INTERPRETATION

The malformations of the brainstem identified in this study were diverse and complex. This proposed classification organizes them into groupings based on known genetics and embryological events. Use of this system will help clinicians and scientists to better understand these disorders and, ultimately, to better counsel families of affected patients.

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

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

The detection and quantification of growth cone collapsing activities

Growth cone guidance during development, as well as axonal extension in neural repair and plasticity, is strongly regulated by both attractive (growth-promoting) and repulsive (growth-inhibiting) guidance molecules. The growth cone collapse assay has been widely and successfully used for the identification and purification of molecules that are repulsive to growth cones or inhibit axonal outgrowth. Here we provide a detailed description of the assay, which uses the morphology of the growth cone after exposure to a test protein as the readout. With the modifications detailed in this protocol, this assay can be used for the biochemical enrichment of proteins with a collapsing activity and for the identification of a collapsing activity of a known protein or gene. This assay does not require very specialized equipment and can be established by every lab with experience in neuronal cell culture. It can be completed in 3 d.

Neogenin and repulsive guidance molecule signaling in the central nervous system

The repulsive guidance molecule (RGM) is a membrane-bound protein that was originally identified as an axon guidance molecule in the visual system. Functional studies have revealed that it has roles in axon guidance and laminar patterning in Xenopus and chick embryos, and in controlling cephalic neural tube closure in mouse embryos. The recent identification of neogenin as a receptor for RGM has provided evidence of the diverse functions of this ligand-receptor pair. Re-expression of RGM is observed after injury in the adult human and rat central nervous systems. Inhibition of RGM enhances growth of injured axons and promotes functional recovery after spinal cord injury in rats. Thus, re-expression of embryonic repulsive cues in adult tissues contributes to failure of axon regeneration in the central nervous system.