Modifying lipid rafts promotes regeneration and functional recovery

The liver and muscle secreted HFE2-protein maintains central nervous system blood vessel integrity

Liver failure causes breakdown of the Blood CNS Barrier (BCB) leading to damages of the Central-Nervous-System (CNS), however the mechanisms whereby the liver influences BCB-integrity remain elusive. One possibility is that the liver secretes an as-yet to be identified molecule(s) that circulate in the serum to directly promote BCB-integrity. To study BCB-integrity, we developed light-sheet imaging for three-dimensional analysis. We show that liver- or muscle-specific knockout of Hfe2/Rgmc induces BCB-breakdown, leading to accumulation of toxic-blood-derived fibrinogen in the brain, lower cortical neuron numbers, and behavioral deficits in mice. Soluble HFE2 competes with its homologue RGMa for binding to Neogenin, thereby blocking RGMa-induced downregulation of PDGF-B and Claudin-5 in endothelial cells, triggering BCB-disruption. HFE2 administration in female mice with experimental autoimmune encephalomyelitis, a model for multiple sclerosis, prevented paralysis and immune cell infiltration by inhibiting RGMa-mediated BCB alteration. This study has implications for the pathogenesis and potential treatment of diseases associated with BCB-dysfunction.

Decoding the molecular crosstalk between grafted stem cells and the stroke-injured brain

Stem cell therapy shows promise for multiple disorders; however, the molecular crosstalk between grafted cells and host tissue is largely unknown. Here, we take a step toward addressing this question. Using translating ribosome affinity purification (TRAP) with sequencing tools, we simultaneously decode the transcriptomes of graft and host for human neural stem cells (hNSCs) transplanted into the stroke-injured rat brain. Employing pathway analysis tools, we investigate the interactions between the two transcriptomes to predict molecular pathways linking host and graft genes; as proof of concept, we predict host-secreted factors that signal to the graft and the downstream molecular cascades they trigger in the graft. We identify a potential host-graft crosstalk pathway where BMP6 from the stroke-injured brain induces graft secretion of noggin, a known brain repair factor. Decoding the molecular interplay between graft and host is a critical step toward deciphering the molecular mechanisms of stem cell action.

The Plasma Membrane Polarity Is Higher in the Neuronal Growth Cone than in the Cell Body of Hippocampal and Cerebellar Granule Neurons

The polarity of the biological membrane, or lipid order, regulates many cellular events. It is generally believed that the plasma membrane polarity is regulated according to cell type and function, sometimes even within a cell. Neurons have a variety of functionally specialized subregions, each of which bears distinct proteins and lipids, and the membrane polarity of the subregions may differ accordingly. However, no direct experimental evidence of it has been presented to date. In the present study, we used a cell-impermeable solvatochromic membrane probe NR12A to investigate the local polarity of the plasma membrane of neurons. Both in hippocampal and cerebellar granule neurons, growth cones have higher membrane polarity than the cell body. In addition, the overall variation in the polarity value of each pixel was greater in the growth cone than in cell bodies, suggesting that the lateral diffusion and/or dynamics of the growth cone membrane are greater than other parts of the neuron. These tendencies were much less notably observed in the lamellipodia of a non-neuronal cell. Our results suggest that the membrane polarity of neuronal growth cones is unique and this characteristic may be important for its structure and function.

Increase in repulsive guidance molecule-a (RGMa) in lacunar and cortical stroke patients is related to the severity of the insult

Repulsive guidance molecule-a (RGMa) inhibits angiogenesis and increases inflammation. Animal models of cerebral ischemia have shown that an increased expression of RGMa leads to larger infarction and its inhibition attenuates effects of ischemia. We report on the relationship of RGMa to stroke types and severity. This is a prospective study in patients admitted to the stroke service in Qatar. We collected the clinical determinants, including NIHSS at admission, imaging and outcome at discharge and 90-days. RGMa levels were determined by measuring mRNA levels extracted from peripheral blood mononuclear cells (PBMCs) within 24 h of onset and at 5 days. There were 90 patients (lacunar: 64, cortical: 26) and 35 age-matched controls. RGMa mRNA levels were significantly higher in the stroke patients: day 1: 1.007 ± 0.13 versus 2.152 ± 0.19 [p < 0.001] and day-5: 3.939 ± 0.36 [p < 0.0001]) and significantly higher in patients with severe stroke (NIHSS ≥ 8) compared to milder symptoms (NIHSS < 8) at day 1 (NIHSS ≥ 8: 2.563 ± 0.36; NIHSS < 8: 1.947 ± 0.2) and day 5 (NIHSS ≥ 8: 5.25 ± 0.62; NIHSS < 8: 3.259 ± 0.419). Cortical stroke patients had marginally higher RGMa mRNA levels compared to lacunar stroke at day 1 (cortical stroke: 2.621 ± 0.46 vs lacunar stroke: 1.961 ± 0.19) and day 5 (cortical stroke: 4.295 ± 0.76 vs lacunar stroke: 3.774 ± 0.39). In conclusion, there is an increase in the level of RGMa mRNA in patients with acute stroke and seen in patients with lacunar and cortical stroke. The increase in RGMa mRNA levels is related to the severity of the stroke and increases over the initial 5 days. Further studies are required to determine the effects of the increase in RGMa on stroke recovery.

Delayed administration of elezanumab, a human anti-RGMa neutralizing monoclonal antibody, promotes recovery following cervical spinal cord injury

Spinal cord injury (SCI) elicits a cascade of degenerative events including cell death, axonal degeneration, and the upregulation of inhibitory molecules which limit repair. Repulsive guidance molecule A (RGMa) is an axon growth inhibitor which is also involved in neuronal cell death and differentiation. SCI causes upregulation of RGMa in the injured rodent, non-human primate, and human spinal cord. Recently, we showed that delayed administration of elezanumab, a high affinity human RGMa-specific monoclonal antibody, promoted neuroprotective and regenerative effects following thoracic SCI. Since most human traumatic SCI is at the cervical level, and level-dependent anatomical and molecular differences may influence pathophysiological responses to injury and treatment, we examined the efficacy of elezanumab and its therapeutic time window of administration in a clinically relevant rat model of cervical impact-compression SCI. Pharmacokinetic analysis of plasma and spinal cord tissue lysate showed comparable levels of RGMa antibodies with delayed administration following cervical SCI. At 12w after SCI, elezanumab promoted long term benefits including perilesional sparing of motoneurons and increased neuroplasticity of key descending pathways involved in locomotion and fine motor function. Elezanumab also promoted growth of corticospinal axons into spinal cord gray matter and enhanced serotonergic innervation of the ventral horn to form synaptic connections caudal to the cervical lesion. Significant recovery in grip and trunk/core strength, locomotion and gait, and spontaneous voiding ability was found in rats treated with elezanumab either immediately post-injury or at 3 h post-SCI, and improvements in specific gait parameters were found when elezanumab was delayed to 24 h post-injury. We also developed a new locomotor score, the Cervical Locomotor Score, a simple and sensitive measure of trunk/core and limb strength and stability during dynamic locomotion.

RGMa Participates in the Blood-Brain Barrier Dysfunction Through BMP/BMPR/YAP Signaling in Multiple Sclerosis

The infiltration of inflammatory cells into the central nervous system (CNS) through the dysfunctional blood–brain barrier (BBB) was critical in the early stages of MS. However, the mechanisms underlying BBB dysfunction remain unknown. Repulsive guidance molecule-a (RGMa) is involved in the pathogenesis of multiple sclerosis (MS), but its role needs to be further explored. This study aimed to evaluate whether RMGa regulates BBB permeability in endothelial cells and MS, and if so, what mechanism may be involved. We created an experimental autoimmune encephalomyelitis (EAE) model in C57BL/6 mice and a human brain microvascular endothelial cell (HBMEC) culture. The permeability of the BBB is measured in response to various interventions. Our results showed that RGMa is expressed in the endothelial cells in HBMECs and EAE mice. RGMa and its signaling counterpart, bone morphogenetic protein 2 (BMP2)/bone morphogenetic protein receptor type II (BMPRII), were gradually increased as the disease progressed. Moreover, as EAE progressed and the BBB was disrupted, the downstream effector, yes-associated protein (YAP), as well as the tight junctional proteins zonula occludens 1 (ZO-1) and claudin-5, decreased significantly. The permeability assay revealed that lentivirus-induced RGMa overexpression in HBMECs caused a significant breakdown of the BBB, whereas RGMa knockdown significantly strengthens the integrity of the BBB. Furthermore, specifically activating BMPR II or inhibiting YAP based on RGMa knockdown results in a significant decrease of ZO-1 and claudin-5 in vitro. On the contrary, inhibition of BMPR II or activation of YAP after upregulating RGMa prevents the downregulation of ZO-1 and claudin-5 in HBMECs. In addition, serum-soluble RGMa (sRGMa) levels were significantly higher in MS patients, particularly in MS patients with Gd⁺ lesions, indicating that the BBB has been disrupted. In conclusion, this study shows that RGMa causes BBB dysfunction in endothelial cells via BMP2/BMPR II/YAP, resulting in BBB integrity disruption in MS and that it could be a novel therapeutic target for BBB permeability in MS.

Cholesterol synthesis inhibition or depletion in axon regeneration

Cholesterol is biosynthesized by all animal cells. Beyond its metabolic role in steroidogenesis, it is enriched in the plasma membrane where it has key structural and regulatory functions. Cholesterol is thus presumably important for post-injury axon regrowth, and this notion is supported by studies showing that impairment of local cholesterol reutilization impeded regeneration. However, several studies have also shown that statins, inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, are enhancers of axon regeneration, presumably acting through an attenuation of the mevalonate isoprenoid pathway and consequent reduction in protein prenylation. Several recent reports have now shown that cholesterol depletion, as well as inhibition of cholesterol synthesis per se, enhances axon regeneration. Here, I discussed these findings and propose some possible underlying mechanisms. The latter would include possible disruptions to axon growth inhibitor signaling by lipid raft-localized receptors, as well as other yet unclear neuronal survival signaling process enhanced by cholesterol lowering or depletion.

Repulsive guidance molecule acts in axon branching in Caenorhabditis elegans

Repulsive guidance molecules (RGMs) are evolutionarily conserved proteins implicated in repulsive axon guidance. Here we report the function of the Caenorhabditis elegans ortholog DRAG-1 in axon branching. The axons of hermaphrodite-specific neurons (HSNs) extend dorsal branches at the region abutting the vulval muscles. The drag-1 mutants exhibited defects in HSN axon branching in addition to a small body size phenotype. DRAG-1 expression in the hypodermal cells was required for the branching of the axons. Although DRAG-1 is normally expressed in the ventral hypodermis excepting the vulval region, its ectopic expression in vulval precursor cells was sufficient to induce the branching. The C-terminal glycosylphosphatidylinositol anchor of DRAG-1 was important for its function, suggesting that DRAG-1 should be anchored to the cell surface. Genetic analyses suggested that the membrane receptor UNC-40 acts in the same pathway with DRAG-1 in HSN branching. We propose that DRAG-1 expressed in the ventral hypodermis signals via the UNC-40 receptor expressed in HSNs to elicit branching activity of HSN axons.

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.

Transcriptome profiling implicated in beneficiary actions of kimchi extracts against Helicobacter pylori infection

Dietary intervention to prevent Helicobacter pylori (H. pylori)-gastric cancer might be ideal because of no risk of bacterial resistance, safety, and rejuvenating action of atrophic gastritis. We have published data about the potential of fermented kimchi as nutritional approach for H. pylori. Hence recent advances in RNAseq analysis lead us to investigate the transcriptome analysis to explain these beneficiary actions of kimchi. gastric cells were infected with either H. pylori or H. pylori plus kimchi. 943 genes were identified as significantly increased or decreased genes according to H. pylori infection and 68 genes as significantly changed between H. pylori infection and H. pylori plus kimchi (p<0.05). Gene classification and Medline database showed DLL4, FGF18, PTPRN, SLC7A11, CHAC1, FGF21, ASAN, CTH, and CREBRF were identified as significantly increased after H. pylori, but significantly decreased with kimchi and NEO1, CLDN8, KLRG1, and IGFBP1 were identified as significantly decreased after H. pylori, but increased with kimchi. After KEGG and STRING-GO analysis, oxidative stress, ER stress, cell adhesion, and apoptosis genes were up-regulated with H. pylori infection but down-regulated with kimchi, whereas tissue regeneration, cellular anti-oxidative response, and anti-inflammation genes were reversely regulated with kimchi (p<0.01). Conclusively, transcriptomes of H. pylori plus kimchi showed significant biological actions.

Liquid-liquid phase separation as a common organizing principle of intracellular space and biomembranes providing dynamic adaptive responses

This work is devoted to the phenomenon of liquid-liquid phase separation (LLPS), which has come to be recognized as fundamental organizing principle of living cells. We distinguish separation processes with different dimensions. Well-known 3D-condensation occurs in aqueous solution and leads to membraneless organelle (MLOs) formation. 2D-films may be formed near membrane surfaces and lateral phase separation (membrane rafts) occurs within the membranes themselves. LLPS may also occur on 1D structures like DNA and the cyto- and nucleoskeleton. Phase separation provides efficient transport and sorting of proteins and metabolites, accelerates the assembly of metabolic and signaling complexes, and mediates stress responses. In this work, we propose a model in which the processes of polymerization (1D structures), phase separation in membranes (2D structures), and LLPS in the volume (3D structures) influence each other. Disordered proteins and whole condensates may provide membrane raft separation or polymerization of specific proteins. On the other hand, 1D and 2D structures with special composition or embedded IDRs can nucleate condensates. We hypothesized that environmental change may trigger a LLPS which can propagate within the cell interior moving along the cytoskeleton or as an autowave. New phase propagation quickly and using a low amount of energy adjusts cell signaling and metabolic systems to new demands. Cumulatively, the interconnected phase separation phenomena in different dimensions represent a previously unexplored system of intracellular communication and regulation which cannot be ignored when considering both physiological and pathological cell processes.

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.

One Raft to Guide Them All, and in Axon Regeneration Inhibit Them

Central nervous system damage caused by traumatic injuries, iatrogenicity due to surgical interventions, stroke and neurodegenerative diseases is one of the most prevalent reasons for physical disability worldwide. During development, axons must elongate from the neuronal cell body to contact their precise target cell and establish functional connections. However, the capacity of the adult nervous system to restore its functionality after injury is limited. Given the inefficacy of the nervous system to heal and regenerate after damage, new therapies are under investigation to enhance axonal regeneration. Axon guidance cues and receptors, as well as the molecular machinery activated after nervous system damage, are organized into lipid raft microdomains, a term typically used to describe nanoscale membrane domains enriched in cholesterol and glycosphingolipids that act as signaling platforms for certain transmembrane proteins. Here, we systematically review the most recent findings that link the stability of lipid rafts and their composition with the capacity of axons to regenerate and rebuild functional neural circuits after damage.

Pharmacological management of secondary spinal cord injury

Introduction: Secondary spinal cord injury (SCI) sets on immediately after trauma and, despite prompt treatment, may become chronic. SCI is a complex condition and presents numerous challenges to patients and physicians alike, also considering the lack of an approved pharmacological therapy.Areas covered: This review describes the pathophysiological mechanisms leading to secondary SCI to highlight possible targets for pharmacological therapy. Furthermore, an extensive search of the literature on different databases (PubMed, Google scholar, Embase, and Scopus) and of the current clinical trials (clinicaltrials.gov) was performed to investigate the current outlook for the pharmacological management of SCI. Only drugs with performed or ongoing clinical trials were considered.Expert opinion: Pharmacological therapy aims to improve motor and sensory function in patients. Overall, drugs are divided into neuroprotective compounds, which aim to limit the damage induced by the pro-inflammatory and pro-apoptotic milieu of SCI, and neuroregenerative drugs, which induce neuronal and axonal regrowth. While many compounds have been trialed with promising results, none has yet completed a stage III trial and has been approved for the pharmacological management of SCI.

Simultaneous binding of Guidance Cues NET1 and RGM blocks extracellular NEO1 signaling

During cell migration or differentiation, cell surface receptors are simultaneously exposed to different ligands. However, it is often unclear how these extracellular signals are integrated. Neogenin (NEO1) acts as an attractive guidance receptor when the Netrin-1 (NET1) ligand binds, but it mediates repulsion via repulsive guidance molecule (RGM) ligands. Here, we show that signal integration occurs through the formation of a ternary NEO1-NET1-RGM complex, which triggers reciprocal silencing of downstream signaling. Our NEO1-NET1-RGM structures reveal a "trimer-of-trimers" super-assembly, which exists in the cell membrane. Super-assembly formation results in inhibition of RGMA-NEO1-mediated growth cone collapse and RGMA- or NET1-NEO1-mediated neuron migration, by preventing formation of signaling-compatible RGM-NEO1 complexes and NET1-induced NEO1 ectodomain clustering. These results illustrate how simultaneous binding of ligands with opposing functions, to a single receptor, does not lead to competition for binding, but to formation of a super-complex that diminishes their functional outputs.

Strategies for Oligodendrocyte and Myelin Repair in Traumatic CNS Injury

A major consequence of traumatic brain and spinal cord injury is the loss of the myelin sheath, a cholesterol-rich layer of insulation that wraps around axons of the nervous system. In the central nervous system (CNS), myelin is produced and maintained by oligodendrocytes. Damage to the CNS may result in oligodendrocyte cell death and subsequent loss of myelin, which can have serious consequences for functional recovery. Demyelination impairs neuronal function by decelerating signal transmission along the axon and has been implicated in many neurodegenerative diseases. After a traumatic injury, mechanisms of endogenous remyelination in the CNS are limited and often fail, for reasons that remain poorly understood. One area of research focuses on enhancing this endogenous response. Existing techniques include the use of small molecules, RNA interference (RNAi), and monoclonal antibodies that target specific signaling components of myelination for recovery. Cell-based replacement strategies geared towards replenishing oligodendrocytes and their progenitors have been utilized by several groups in the last decade as well. In this review article, we discuss the effects of traumatic injury on oligodendrocytes in the CNS, the lack of endogenous remyelination, translational studies in rodent models promoting remyelination, and finally human clinical studies on remyelination in the CNS after injury.

Cholesterol synthesis inhibition promotes axonal regeneration in the injured central nervous system

Neuronal regeneration in the injured central nervous system is hampered by multiple extracellular proteins. These proteins exert their inhibitory action through interactions with receptors that are located in cholesterol rich compartments of the membrane termed lipid rafts. Here we show that cholesterol-synthesis inhibition prevents the association of the Neogenin receptor with lipid rafts. Furthermore, we show that cholesterol-synthesis inhibition enhances axonal growth both on inhibitory -myelin and -RGMa substrates. Following optic nerve injury, lowering cholesterol synthesis with both drugs and siRNA-strategies allows for robust axonal regeneration and promotes neuronal survival. Cholesterol inhibition also enhanced photoreceptor survival in a model of Retinitis Pigmentosa. Our data reveal that Lovastatin leads to several opposing effects on regenerating axons: cholesterol synthesis inhibition promotes regeneration whereas altered prenylation impairs regeneration. We also show that the lactone prodrug form of lovastatin has differing effects on regeneration when compared to the ring-open hydroxy-acid form. Thus the association of cell surface receptors with lipid rafts contributes to axonal regeneration inhibition, and blocking cholesterol synthesis provides a potential therapeutic approach to promote neuronal regeneration and survival in the diseased Central Nervous System. SIGNIFICANCE STATEMENT: Statins have been intensively used to treat high levels of cholesterol in humans. However, the effect of cholesterol inhibition in both the healthy and the diseased brain remains controversial. In particular, it is unclear whether cholesterol inhibition with statins can promote regeneration and survival following injuries. Here we show that late stage cholesterol inhibition promotes robust axonal regeneration following optic nerve injury. We identified distinct mechanisms of action for activated vs non-activated Lovastatin that may account for discrepancies found in the literature. We show that late stage cholesterol synthesis inhibition alters Neogenin association with lipid rafts, thereby i) neutralizing the inhibitory function of its ligand and ii) offering a novel opportunity to promote CNS regeneration and survival following injuries.

Neogenin neutralization prevents photoreceptor loss in inherited retinal degeneration

Inherited retinal degenerations (IRDs) are characterized by the progressive loss of photoreceptors and represent one of the most prevalent causes of blindness among working-age populations. Cyclic nucleotide dysregulation is a common pathological feature linked to numerous forms of IRD, yet the precise mechanisms through which this contributes to photoreceptor death remain elusive. Here we demonstrate that cAMP induced upregulation of the dependence receptor neogenin in the retina. Neogenin levels were also elevated in both human and murine degenerating photoreceptors. We found that overexpressing neogenin in mouse photoreceptors was sufficient to induce cell death, whereas silencing neogenin in degenerating murine photoreceptors promoted survival, thus identifying a pro-death signal in IRDs. A possible treatment strategy is modeled whereby peptide neutralization of neogenin in Rd1, Rd10, and Rho P23H-knockin mice promotes rod and cone survival and rescues visual function as measured by light-evoked retinal ganglion cell recordings, scotopic/photopic electroretinogram recordings, and visual acuity tests. These results expose neogenin as a critical link between cAMP and photoreceptor death, and identify a druggable target for the treatment of retinal degeneration.

The effect of periaqueductal gray's metabotropic glutamate receptor subtype 8 activation on locomotor function following spinal cord injury

Background and aims The pathophysiology of spinal cord injury is very complex. One of the debilitating aspects of spinal cord injury in addition to pain is a defect in motor function below the lesion surface. In this study, we tried to assess the modulatory effect of (S)-3,4-Dicarboxyphenylglycine (DCPG), a metabotropic glutamate receptor subtype 8 (mGluR8) agonist, on animal's locomotor functions in a model of compression spinal cord injury. Methods We used a contusion method (T6-T8) for induction of spinal cord injury. Male Wistar rats were randomly assigned to five equal groups (n = 10 per group). Clips compression injury model was used to induce spinal cord injury. Three weeks post injury DCPG, siRNA (small interfering Ribonucleic Acid) and normal saline (vehicle) were administered intra-ventrolaterally to the periaqueductal gray (PAG) region. Motor function, were assessed through BBB (Basso, Beattie, and Bresnahan Locomotor Rating Scale) and ladder walking test. In addition, the effects of DCPG on axonal regeneration in corticospinal tract were evaluated. Results We found that DCPG could improve motor function and axonal regeneration in corticospinal tract when compared to siRNA group. Conclusions The results revealed that activation of mGluR8 in PAG is capable to improve motor function and of axonal regeneration due to the inhibitory effect on glutamate transmission on the spinal cord surface and also the elimination of the deleterious effect of glutamate on the regeneration of the injured area as an excitatory neurotransmitter. Implications Our findings in this study showed that, more attention should be paid to glutamate and its receptors in spinal cord injury studies, whether at the spinal or cerebral level, especially in the field of motor function after spinal cord injury.

Growth cone repulsion to Netrin-1 depends on lipid raft microdomains enriched in UNC5 receptors

During brain development, Uncoordinated locomotion 5 (UNC5) receptors control axonal extension through their sensing of the guidance molecule Netrin-1. The correct positioning of receptors into cholesterol-enriched membrane raft microdomains is crucial for the efficient transduction of the recognized signals. However, whether such microdomains are required for the appropriate axonal guidance mediated by UNC5 receptors remains unknown. Here, we combine the use of confocal microscopy, live-cell FRAP analysis and single-particle tracking PALM to characterize the distribution of UNC5 receptors into raft microdomains, revealing differences in their membrane mobility properties. Using pharmacological and genetic approaches in primary neuronal cultures and brain cerebellar explants we further demonstrate that disrupting raft microdomains inhibits the chemorepulsive response of growth cones and axons against Netrin-1. Together, our findings indicate that the distribution of all UNC5 receptors into cholesterol-enriched raft microdomains is heterogeneous and that the specific localization has functional consequences for the axonal chemorepulsion against Netrin-1.

Repulsive guidance molecules lock growth differentiation factor 5 in an inhibitory complex

Repulsive guidance molecules (RGMs) are cell surface proteins that regulate the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. They control fundamental biological processes, such as migration and differentiation by direct interaction with the Neogenin (NEO1) receptor and function as coreceptors for the bone morphogenetic protein (BMP)/growth differentiation factor (GDF) family. We determined crystal structures of all three human RGM family members in complex with GDF5, as well as the ternary NEO1-RGMB-GDF5 assembly. Surprisingly, we show that all three RGMs inhibit GDF5 signaling, which is in stark contrast to RGM-mediated enhancement of signaling observed for other BMPs, like BMP2. Despite their opposite effect on GDF5 signaling, RGMs occupy the BMP type 1 receptor binding site similar to the observed interactions in RGM-BMP2 complexes. In the NEO1-RGMB-GDF5 complex, RGMB physically bridges NEO1 and GDF5, suggesting cross-talk between the GDF5 and NEO1 signaling pathways. Our crystal structures, combined with structure-guided mutagenesis of RGMs and BMP ligands, binding studies, and cellular assays suggest that RGMs inhibit GDF5 signaling by competing with GDF5 type 1 receptors. While our crystal structure analysis and in vitro binding data initially pointed towards a simple competition mechanism between RGMs and type 1 receptors as a possible basis for RGM-mediated GDF5 inhibition, further experiments utilizing BMP2-mimicking GDF5 variants clearly indicate a more complex mechanism that explains how RGMs can act as a functionality-changing switch for two structurally and biochemically similar signaling molecules.

Axon growth and synaptic function: A balancing act for axonal regeneration and neuronal circuit formation in CNS trauma and disease

Axons in the adult mammalian central nervous system (CNS) fail to regenerate inside out due to intrinsic and extrinsic neuronal determinants. During CNS development, axon growth, synapse formation, and function are tightly regulated processes allowing immature neurons to effectively grow an axon, navigate toward target areas, form synaptic contacts and become part of information processing networks that control behavior in adulthood. Not only immature neurons are able to precisely control the expression of a plethora of genes necessary for axon extension and pathfinding, synapse formation and function, but also non-neuronal cells such as astrocytes and microglia actively participate in sculpting the nervous system through refinement, consolidation, and elimination of synaptic contacts. Recent evidence indicates that a balancing act between axon regeneration and synaptic function may be crucial for rebuilding functional neuronal circuits after CNS trauma and disease in adulthood. Here, we review the role of classical and new intrinsic and extrinsic neuronal determinants in the context of CNS development, injury, and disease. Moreover, we discuss strategies targeting neuronal and non-neuronal cell behaviors, either alone or in combination, to promote axon regeneration and neuronal circuit formation in adulthood.

Delayed administration of the human anti-RGMa monoclonal antibody elezanumab promotes functional recovery including spontaneous voiding after spinal cord injury in rats

Spinal cord injury (SCI) often results in permanent functional loss due to a series of degenerative events including cell death, axonal damage, and the upregulation of inhibitory proteins that impede regeneration. Repulsive Guidance Molecule A (RGMa) is a potent inhibitor of axonal growth that is rapidly upregulated following injury in both the rodent and human central nervous system (CNS). Previously, we showed that monoclonal antibodies that specifically block inhibitory RGMa signaling promote neuroprotective and regenerative effects when administered acutely in a clinically relevant rat model of thoracic SCI. However, it is unknown whether systemic administration of RGMa blocking antibodies are effective for SCI after delayed administration. Here, we administered elezanumab, a human monoclonal antibody targeting RGMa, intravenously either acutely or at 3 h or 24 h following thoracic clip impact-compression SCI. Rats treated with elezanumab acutely and at 3 h post-injury showed improvements in overground locomotion and fine motor function and gait. Rats treated 24 h post-SCI trended towards better recovery demonstrating significantly greater stride length and swing speed. Treated rats also showed greater tissue preservation with reduced lesion areas. As seen with acute treatment, delayed administration of elezanumab at 3 h post-SCI also increased perilesional neuronal sparing and serotonergic and corticospinal axonal plasticity. In addition, all elezanumab treated rats showed earlier spontaneous voiding ability and less post-trauma bladder wall hypertrophy. Together, our data demonstrate the therapeutic efficacy of delayed systemic administration of elezanumab in a rat model of SCI, and uncovers a new role for RGMa inhibition in bladder recovery following SCI.

The role of hepatocyte growth factor in mesenchymal stem cell-induced recovery in spinal cord injured rats

Background

Mesenchymal stem cells (MSCs) have become a promising treatment for spinal cord injury (SCI) due to the fact that they provide a favorable environment. Treatment using MSCs results in a better neurological functional improvement through the promotion of nerve cell regeneration and the modulation of inflammation. Many studies have highlighted that the beneficial effects of MSCs are more likely associated with their secreted factors. However, the identity of the factor that plays a key role in the MSC-induced neurological functional recovery following SCI as well as its molecular mechanism still remains unclear.

Methods

A conditioned medium (collected from the MSCs) and hepatocyte growth factor (HGF) were used to test the effects on the differentiation of neural stem cells (NSCS) in the presence of BMP4 with or without a c-Met antibody. In SCI rats, Western blot, ELISA, immunohistochemistry, and hematoxylin-eosin staining were used to investigate the biological effects of MSC-conditioned medium and HGF on nerve cell regeneration and inflammation with or without the pre-treatment using a c-Met antibody. In addition, the possible molecular mechanism (cross-talk between HGF/c-Met and the BMP/Smad 1/5/8 signaling pathway) was also detected by Western blot both in vivo and in vitro.

Results

The conditioned medium from bone marrow-derived MSCs (BMSCs) was able to promote the NSC differentiation into neurons in vitro and the neurite outgrowth in the scar boundary of SCI rats by inhibiting the BMP/Smad signaling pathway as well as reduces the secondary damage through the modulation of the inflammatory process. The supplementation of HGF showed similar biological effects to those of BMSC-CM, whereas a functional blocking of the c-Met antibody or HGF knockdown in BMSCs significantly reversed the functional improvement mediated by the BMSC-CM.

Conclusions

The MSC-associated biological effects on the recovery of SCI rats mainly depend on the secretion of HGF.

Nystatin Regulates Axonal Extension and Regeneration by Modifying the Levels of Nitric Oxide

Nystatin is a pharmacological agent commonly used for the treatment of oral, mucosal and cutaneous fungal infections. Nystatin has also been extensively applied to study the cellular function of cholesterol-enriched structures because of its ability to bind and extract cholesterol from mammalian membranes. In neurons, cholesterol level is tightly regulated, being essential for synapse and dendrite formation, and axonal guidance. However, the action of Nystatin on axon regeneration has been poorly evaluated. Here, we examine the effect of Nystatin on primary cultures of hippocampal neurons, showing how acute dose (minutes) of Nystatin increases the area of growth cones, and chronic treatment (days) enhances axon length, axon branching, and axon regeneration post-axotomy. We describe two alternative signaling pathways responsible for the observed effects and activated at different concentrations of Nystatin. At elevated concentrations, Nystatin promotes growth cone expansion through phosphorylation of Akt; whereas, at low concentrations, Nystatin enhances axon length and regrowth by increasing nitric oxide levels. Together, our findings indicate new signaling pathways of Nystatin and propose this compound as a novel regulator of axon regeneration.

Multi-Omic Analyses of Growth Cones at Different Developmental Stages Provides Insight into Pathways in Adult Neuroregeneration

Growth cones (GCs) are structures associated with growing neurons. GC membrane expansion, which necessitates protein-lipid interactions, is critical to axonal elongation in development and in adult neuritogenesis. We present a multi-omic analysis that integrates proteomics and lipidomics data for the identification of GC pathways, cell phenotypes, and lipid-protein interactions, with an analytic platform to facilitate the visualization of these data. We combine lipidomic data from GC and adult axonal regeneration following optic nerve crush. Our results reveal significant molecular variability in GCs across developmental ages that aligns with the upregulation and downregulation of lipid metabolic processes and correlates with distinct changes in the lipid composition of GC plasmalemma. We find that these processes also define the transition into a growth-permissive state in the adult central nervous system. The insight derived from these analyses will aid in promoting adult regeneration and functional innervation in devastating neurodegenerative diseases.

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Simultaneous proteomics and lipidomics analyses of developmental growth cones

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Combined multi-omics analyses of regenerating optic nerves and growth cones

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Integrating protein-protein with protein-lipid interactions in growth cones

Dynamin and reverse-mode sodium calcium exchanger blockade confers neuroprotection from diffuse axonal injury

Mild traumatic brain injury (mTBI) is a frequently overlooked public health concern that is difficult to diagnose and treat. Diffuse axonal injury (DAI) is a common mTBI neuropathology in which axonal shearing and stretching induces breakdown of the cytoskeleton, impaired axonal trafficking, axonal degeneration, and cognitive dysfunction. DAI is becoming recognized as a principal neuropathology of mTBI with supporting evidence from animal model, human pathology, and neuroimaging studies. As mitochondrial dysfunction and calcium overload are critical steps in secondary brain and axonal injury, we investigated changes in protein expression of potential targets following mTBI using an in vivo controlled cortical impact model. We show upregulated expression of sodium calcium exchanger1 (NCX1) in the hippocampus and cortex at distinct time points post-mTBI. Expression of dynamin-related protein1 (Drp1), a GTPase responsible for regulation of mitochondrial fission, also changes differently post-injury in the hippocampus and cortex. Using an in vitro model of DAI previously reported by our group, we tested whether pharmacological inhibition of NCX1 by SN-6 and of dynamin1, dynamin2, and Drp1 by dynasore mitigates secondary damage. Dynasore and SN-6 attenuate stretch injury-induced swelling of axonal varicosities and mitochondrial fragmentation. In addition, we show that dynasore, but not SN-6, protects against H2O2-induced damage in an organotypic oxidative stress model. As there is currently no standard treatment to mitigate cell damage induced by mTBI and DAI, this work highlights two potential therapeutic targets for treatment of DAI in multiple models of mTBI and DAI.

Extracellular phosphorylation drives the formation of neuronal circuitry

Until recently, the existence of extracellular kinase activity was questioned. Many proteins of the central nervous system are targeted, but it remains unknown whether, or how, extracellular phosphorylation influences brain development. Here we show that the tyrosine kinase vertebrate lonesome kinase (VLK), which is secreted by projecting retinal ganglion cells, phosphorylates the extracellular protein repulsive guidance molecule b (RGMb) in a dorsal-ventral descending gradient. Silencing of VLK or RGMb causes aberrant axonal branching and severe axon misguidance in the chick optic tectum. Mice harboring RGMb with a point mutation in the phosphorylation site also display aberrant axonal pathfinding. Mechanistic analyses show that VLK-mediated RGMb phosphorylation modulates Wnt3a activity by regulating LRP5 protein gradients. Thus, the secretion of VLK by projecting neurons provides crucial signals for the accurate formation of nervous system circuitry. The dramatic effect of VLK on RGMb and Wnt3a signaling implies that extracellular phosphorylation likely has broad and profound effects on brain development, function and disease.

Rgma-Induced Neo1 Proteolysis Promotes Neural Tube Morphogenesis

Neuroepithelial cell (NEC) elongation is one of several key cell behaviors that mediate the tissue-level morphogenetic movements that shape the neural tube (NT), the precursor of the brain and spinal cord. However, the upstream signals that promote NEC elongation have been difficult to tease apart from those regulating apico-basal polarity and hingepoint formation, due to their confounding interdependence. The Repulsive Guidance Molecule a (Rgma)/Neogenin 1 (Neo1) signaling pathway plays a conserved role in NT formation (neurulation) and is reported to regulate both NEC elongation and apico-basal polarity, through signal transduction events that have not been identified. We examine here the role of Rgma/Neo1 signaling in zebrafish (sex unknown), an organism that does not use hingepoints to shape its hindbrain, thereby enabling a direct assessment of the role of this pathway in NEC elongation. We confirm that Rgma/Neo1 signaling is required for microtubule-mediated NEC elongation, and demonstrate via cell transplantation that Neo1 functions cell autonomously to promote elongation. However, in contrast to previous findings, our data do not support a role for this pathway in establishing apical junctional complexes. Last, we provide evidence that Rgma promotes Neo1 glycosylation and intramembrane proteolysis, resulting in the production of a transient, nuclear intracellular fragment (NeoICD). Partial rescue of Neo1a and Rgma knockdown embryos by overexpressing neoICD suggests that this proteolytic cleavage is essential for neurulation. Based on these observations, we propose that RGMA-induced NEO1 proteolysis orchestrates NT morphogenesis by promoting NEC elongation independently of the establishment of apical junctional complexes.SIGNIFICANCE STATEMENT The neural tube, the CNS precursor, is shaped during neurulation. Neural tube defects occur frequently, yet underlying genetic risk factors are poorly understood. Neuroepithelial cell (NEC) elongation is essential for proper completion of neurulation. Thus, connecting NEC elongation with the molecular pathways that control this process is expected to reveal novel neural tube defect risk factors and increase our understanding of NT development. Effectors of cell elongation include microtubules and microtubule-associated proteins; however, upstream regulators remain controversial due to the confounding interdependence of cell elongation and establishment of apico-basal polarity. Here, we reveal that Rgma-Neo1 signaling controls NEC elongation independently of the establishment of apical junctional complexes and identify Rgma-induced Neo1 proteolytic cleavage as a key upstream signaling event.

Regulation of Brain Cholesterol: What Role Do Liver X Receptors Play in Neurodegenerative Diseases?

Liver X Receptors (LXR) alpha and beta are two members of nuclear receptor superfamily documented as endogenous cholesterol sensors. Following conversion of cholesterol in oxysterol, both LXR isoforms detect intracellular concentrations and act as transcription factors to promote expression of target genes. Among their numerous physiological roles, they act as central cholesterol-lowering factors. In the central nervous system (CNS), cholesterol has been shown to be an essential determinant of brain function, particularly as a major constituent of myelin and membranes. In the brain, LXRs act as cholesterol central regulators, and, beyond this metabolic function, LXRs have additional roles such as providing neuroprotective effects and lowering neuroinflammation. In many neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and multiple sclerosis (MS), dysregulations of cholesterol and oxysterol have been reported. In this paper, we propose to focus on recent advances in the knowledge of the LXRs roles on brain cholesterol and oxysterol homeostasis, neuroinflammation, neuroprotection, and their putative involvement in neurodegenerative disorders. We will discuss their potential use as candidates for both molecular diagnosis and as promising pharmacological targets in the treatment of ALS, AD, or MS patients.

Cholesterol Depletion Regulates Axonal Growth and Enhances Central and Peripheral Nerve Regeneration

Axonal growth during normal development and axonal regeneration rely on the action of many receptor signaling systems and complexes, most of them located in specialized raft membrane microdomains with a precise lipid composition. Cholesterol is a component of membrane rafts and the integrity of these structures depends on the concentrations present of this compound. Here we explored the effect of cholesterol depletion in both developing neurons and regenerating axons. First, we show that cholesterol depletion in vitro in developing neurons from the central and peripheral nervous systems increases the size of growth cones, the density of filopodium-like structures and the number of neurite branching points. Next, we demonstrate that cholesterol depletion enhances axonal regeneration after axotomy in vitro both in a microfluidic system using dissociated hippocampal neurons and in a slice-coculture organotypic model of axotomy and regeneration. Finally, using axotomy experiments in the sciatic nerve, we also show that cholesterol depletion favors axonal regeneration in vivo. Importantly, the enhanced regeneration observed in peripheral axons also correlated with earlier electrophysiological responses, thereby indicating functional recovery following the regeneration. Taken together, our results suggest that cholesterol depletion per se is able to promote axonal growth in developing axons and to increase axonal regeneration in vitro and in vivo both in the central and peripheral nervous systems.

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

Exosomes Mediate Mobilization of Autocrine Wnt10b to Promote Axonal Regeneration in the Injured CNS

Developing strategies that promote axonal regeneration within the injured CNS is a major therapeutic challenge, as axonal outgrowth is potently inhibited by myelin and the glial scar. Although regeneration can be achieved using the genetic deletion of PTEN, a negative regulator of the mTOR pathway, this requires inactivation prior to nerve injury, thus precluding therapeutic application. Here, we show that, remarkably, fibroblast-derived exosomes (FD exosomes) enable neurite growth on CNS inhibitory proteins. Moreover, we demonstrate that, upon treatment with FD exosomes, Wnt10b is recruited toward lipid rafts and activates mTOR via GSK3β and TSC2. Application of FD exosomes shortly after optic nerve injury promoted robust axonal regeneration, which was strongly reduced in Wnt10b-deleted animals. This work uncovers an intercellular signaling pathway whereby FD exosomes mobilize an autocrine Wnt10b-mTOR pathway, thereby awakening the intrinsic capacity of neurons for regeneration, an important step toward healing the injured CNS.

RGMa mediates reactive astrogliosis and glial scar formation through TGFβ1/Smad2/3 signaling after stroke

In response to stroke, astrocytes become reactive astrogliosis and are a major component of a glial scar. This results in the formation of both a physical and chemical (production of chondroitin sulfate proteoglycans) barrier, which prevent neurite regeneration that, in turn, interferes with functional recovery. However, the mechanisms of reactive astrogliosis and glial scar formation are poorly understood. In this work, we hypothesized that repulsive guidance molecule a (RGMa) regulate reactive astrogliosis and glial scar formation. We first found that RGMa was strongly expressed by reactive astrocytes in the glial scar in a rat model of middle cerebral artery occlusion/reperfusion. Genetic or pharmacologic inhibition of RGMa in vivo resulted in a strong reduction of reactive astrogliosis and glial scarring as well as in a pronounced improvement in functional recovery. Furthermore, we showed that transforming growth factor β1 (TGFβ1) stimulated RGMa expression through TGFβ1 receptor activin-like kinase 5 (ALK5) in primary cultured astrocytes. Knockdown of RGMa abrogated key steps of reactive astrogliosis and glial scar formation induced by TGFβ1, including cellular hypertrophy, glial fibrillary acidic protein upregulation, cell migration, and CSPGs secretion. Finally, we demonstrated that RGMa co-immunoprecipitated with ALK5 and Smad2/3. TGFβ1-induced ALK5-Smad2/3 interaction and subsequent phosphorylation of Smad2/3 were impaired by RGMa knockdown. Taken together, we identified that after stroke, RGMa promotes reactive astrogliosis and glial scar formation by forming a complex with ALK5 and Smad2/3 to promote ALK5-Smad2/3 interaction to facilitate TGFβ1/Smad2/3 signaling, thereby inhibiting neurological functional recovery. RGMa may be a new therapeutic target for stroke.

Retrograde Activation of the Extrinsic Apoptotic Pathway in Spinal-Projecting Neurons after a Complete Spinal Cord Injury in Lampreys

Spinal cord injury (SCI) is a devastating condition that leads to permanent disability because injured axons do not regenerate across the trauma zone to reconnect to their targets. A prerequisite for axonal regeneration will be the prevention of retrograde degeneration that could lead to neuronal death. However, the specific molecular mechanisms of axotomy-induced degeneration of spinal-projecting neurons have not been elucidated yet. In lampreys, SCI induces the apoptotic death of identifiable descending neurons that are “bad regenerators/poor survivors” after SCI. Here, we investigated the apoptotic process activated in identifiable descending neurons of lampreys after SCI. For this, we studied caspase activation by using fluorochrome-labeled inhibitors of caspases, the degeneration of spinal-projecting neurons using Fluro-Jade C staining, and the involvement of the intrinsic apoptotic pathway by means of cytochrome c and Vα double immunofluorescence. Our results provide evidence that, after SCI, bad-regenerating spinal cord-projecting neurons slowly degenerate and that the extrinsic pathway of apoptosis is involved in this process. Experiments using the microtubule stabilizer Taxol showed that caspase-8 signaling is retrogradely transported by microtubules from the site of axotomy to the neuronal soma. Preventing the activation of this process could be an important therapeutic approach after SCI in mammals.

RGMa inhibition with human monoclonal antibodies promotes regeneration, plasticity and repair, and attenuates neuropathic pain after spinal cord injury

Traumatic spinal cord injury (SCI) causes a cascade of degenerative events including cell death, axonal damage, and the upregulation of inhibitory molecules which prevent regeneration and limit recovery. Repulsive guidance molecule A (RGMa) is a potent neurite growth inhibitor in the central nervous system, exerting its repulsive activity by binding the Neogenin receptor. Here, we show for the first time that inhibitory RGMa is markedly upregulated in multiple cell types after clinically relevant impact-compression SCI in rats, and importantly, also in the injured human spinal cord. To neutralize inhibitory RGMa, clinically relevant human monoclonal antibodies were systemically administered after acute SCI, and were detected in serum, cerebrospinal fluid, and in the injured tissue. Rats treated with RGMa blocking antibodies showed significantly improved recovery of motor function and gait. Furthermore, RGMa blocking antibodies promoted neuronal survival, and enhanced the plasticity of descending serotonergic pathways and corticospinal tract axonal regeneration. RGMa antibody also attenuated neuropathic pain responses, which was associated with fewer activated microglia and reduced CGRP expression in the dorsal horn caudal to the lesion. These results show the therapeutic potential of the first human RGMa antibody for SCI and uncovers a new role for the RGMa/Neogenin pathway on neuropathic pain.

RGMs: Structural Insights, Molecular Regulation, and Downstream Signaling

Although originally discovered as neuronal growth cone-collapsing factors, repulsive guidance molecules (RGMs) are now known as key players in many fundamental processes, such as cell migration, differentiation, iron homeostasis, and apoptosis, during the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. Furthermore, three RGMs (RGMa, RGMb/DRAGON, and RGMc/hemojuvelin) have been linked to the pathogenesis of various disorders ranging from multiple sclerosis (MS) to cancer and juvenile hemochromatosis (JHH). While the molecular details of these (patho)biological effects and signaling modes have long remained unknown, recent studies unveil several exciting and novel aspects of RGM processing, ligand-receptor interactions, and downstream signaling. In this review, we highlight recent advances in the mechanisms-of-action and function of RGM proteins.

Lipid-Free Apolipoprotein A-I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

BACKGROUND

Atherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high-density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high-density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low-dose lipid-free apolipoprotein A-I (apoA-I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid-free apoA-I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high-density lipoprotein cholesterol concentrations.

METHODS AND RESULTS

Ldlr^-/- and Ldlr^-/- apoA-I^-/- mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid-free human apoA-I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid-free apoA-I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin-treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA-I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane.

CONCLUSIONS

ApoA-I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid-free apoA-I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.

A plasma membrane microdomain compartmentalizes ephrin-generated cAMP signals to prune developing retinal axon arbors

The development of neuronal circuits is controlled by guidance molecules that are hypothesized to interact with the cholesterol-enriched domains of the plasma membrane termed lipid rafts. Whether such domains enable local intracellular signalling at the submicrometre scale in developing neurons and are required for shaping the nervous system connectivity in vivo remains controversial. Here, we report a role for lipid rafts in generating domains of local cAMP signalling in axonal growth cones downstream of ephrin-A repulsive guidance cues. Ephrin-A-dependent retraction of retinal ganglion cell axons involves cAMP signalling restricted to the vicinity of lipid rafts and is independent of cAMP modulation outside of this microdomain. cAMP modulation near lipid rafts controls the pruning of ectopic axonal branches of retinal ganglion cells in vivo, a process requiring intact ephrin-A signalling. Together, our findings indicate that lipid rafts structure the subcellular organization of intracellular cAMP signalling shaping axonal arbors during the nervous system development.

Liver X receptors: from cholesterol regulation to neuroprotection-a new barrier against neurodegeneration in amyotrophic lateral sclerosis?

Cholesterol plays a central role in numerous nervous system functions. Cholesterol is the major constituent of myelin sheaths, is essential for synapse and dendrite formation, axon guidance as well as neurotransmission. Among regulators of cholesterol homeostasis, liver X receptors (LXRs), two members of the nuclear receptor superfamily, play a determinant role. LXRs act as cholesterol sensors and respond to high intracellular cholesterol concentration by decreasing plasmatic and intracellular cholesterol content. Beyond their cholesterol-lowering role, LXRs have been proposed as regulators of immunity and anti-inflammatory factors. Dysregulation of cholesterol metabolism combined to neuroinflammatory context have been described in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). ALS is characterized by the progressive loss of motoneurons in the brain and spinal cord, leading to severe paralytic condition and death of patients in a median time of 3 years. Motoneuron degeneration is accompanied by chronic neuroinflammatory response, involving microglial and astrocytic activation, infiltration of blood-derived immune cells and release of pro-inflammatory factors. We propose to discuss here the role of LXRs as a molecular link between the central nervous system cholesterol metabolism, neuroinflammation, motoneuron survival and their potential as promising therapeutic candidates for ALS therapy.

Lrig2 Negatively Regulates Ectodomain Shedding of Axon Guidance Receptors by ADAM Proteases

Many guidance receptors are proteolytically cleaved by membrane-associated metalloproteases of the ADAM family, leading to the shedding of their ectodomains. Ectodomain shedding is crucial for receptor signaling and function, but how this process is controlled in neurons remains poorly understood. Here, we show that the transmembrane protein Lrig2 negatively regulates ADAM-mediated guidance receptor proteolysis in neurons. Lrig2 binds Neogenin, a receptor for repulsive guidance molecules (RGMs), and prevents premature Neogenin shedding by ADAM17 (TACE). RGMa reduces Lrig2-Neogenin interactions, providing ADAM17 access to Neogenin and allowing this protease to induce ectodomain shedding. Regulation of ADAM17-mediated Neogenin cleavage by Lrig2 is required for neurite growth inhibition by RGMa in vitro and for cortical neuron migration in vivo. Furthermore, knockdown of Lrig2 significantly improves CNS axon regeneration. Together, our data identify a unique ligand-gated mechanism to control receptor shedding by ADAMs and reveal functions for Lrigs in neuron migration and regenerative failure.

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.

ϒ-secretase and LARG mediate distinct RGMa activities to control appropriate layer targeting within the optic tectum

While a great deal of progress has been made in understanding the molecular mechanisms that regulate retino-tectal mapping, the determinants that target retinal projections to specific layers of the optic tectum remain elusive. Here we show that two independent RGMa-peptides, C- and N-RGMa, activate two distinct intracellular pathways to regulate axonal growth. C-RGMa utilizes a Leukemia-associated RhoGEF (LARG)/Rho/Rock pathway to inhibit axonal growth. N-RGMa on the other hand relies on ϒ-secretase cleavage of the intracellular portion of Neogenin to generate an intracellular domain (NeICD) that uses LIM-only protein 4 (LMO4) to block growth. In the developing tectum (E18), overexpression of C-RGMa and dominant-negative LARG (LARG-PDZ) induced overshoots in the superficial tectal layer but not in deeper tectal layers. In younger embryos (E12), C-RGMa and LARG-PDZ prevented ectopic projections toward deeper tectal layers, indicating that C-RGMa may act as a barrier to descending axons. In contrast both N-RGMa and NeICD overexpression resulted in aberrant axonal-paths, all of which suggests that it is a repulsive guidance molecule. Thus, two RGMa fragments activate distinct pathways resulting in different axonal responses. These data reveal how retinal projections are targeted to the appropriate layer in their target tissue.

Emerging physiological and pathological implications of tunneling nanotubes formation between cells

Cell-to-cell communication is a critical requirement to coordinate behaviors of the cells in a community and thereby achieve tissue homeostasis and conservation of the multicellular organisms. Tunneling nanotubes (TNTs), as a cell-to-cell communication over long distance, allow for bi- or uni-directional transfer of cellular components between cells. Identification of inducing agents and the cell and molecular mechanism underling the formation of TNTs and their structural and functional features may lead to finding new important roles for these intercellular bridges in vivo and in vitro. During the last decade, research has shown TNTs have different structural and functional properties, varying between and within cell systems. In this review, we will focus on TNTs and their cell and molecular mechanism of formation. Moreover, the latest findings into their functional roles in physiological and pathological processes, such as signal transduction, micro and nano-particles delivery, immune responses, embryogenesis, cellular reprogramming, apoptosis, cancer, and neurodegenerative diseases initiation and progression and pathogens transfer, will be discussed.

Repulsive guidance molecule is a structural bridge between neogenin and bone morphogenetic protein

Repulsive guidance molecules (RGMs) control crucial processes including cell motility, adhesion, immune-cell regulation and systemic iron metabolism. RGMs signal via the neogenin (NEO1) and the bone morphogenetic protein (BMP) pathways. Here, we report crystal structures of the N-terminal domains of all human RGM family members in complex with the BMP ligand BMP2, revealing a new protein fold and a conserved BMP-binding mode. Our structural and functional data suggest a pH-linked mechanism for RGM-activated BMP signaling and offer a rationale for RGM mutations causing juvenile hemochromatosis. We also determined the crystal structure of the ternary BMP2-RGM-NEO1 complex, which, along with solution scattering and live-cell super-resolution fluorescence microscopy, indicates BMP-induced clustering of the RGM-NEO1 complex. Our results show how RGM acts as the central hub that links BMP and NEO1 and physically connects these fundamental signaling pathways.

Uncoupling Neogenin association with lipid rafts promotes neuronal survival and functional recovery after stroke

The dependence receptor Neogenin and its ligand, the repulsive guidance molecule a (RGMa), regulate apoptosis and axonal growth in the developing and the adult central nervous system (CNS). Here, we show that this pathway has also a critical role in neuronal death following stroke, and that providing RGMa to neurons blocks Neogenin-induced death. Interestingly, the Neogenin pro-death function following ischemic insult depends on Neogenin association with lipid rafts. Thus, a peptide that prevents Neogenin association with lipid rafts increased neuronal survival in several in vitro stroke models. In rats, a pro-survival effect was also observed in a model of ocular ischemia, as well as after middle cerebral artery occlusion (MCAO). Treatments that prevented Neogenin association with lipid rafts improved neuronal survival and the complexity of the neuronal network following occlusion of the middle artery. Toward the development of a treatment for stroke, we developed a human anti-RGMa antibody that also prevents Neogenin association with lipid rafts. We show that this antibody also protected CNS tissue from ischemic damage and that its application resulted in a significant functional improvement even when administrated 6 h after artery occlusion. Thus, our results draw attention to the role of Neogenin and lipid rafts as potential targets following stroke.