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LINGO-1 is a functional member of the Nogo66 receptor (NgR1)/p75 and NgR1/TROY signaling complexes that prevent axonal regeneration through RhoA in the central nervous system. LINGO-1 also promotes cell death after neuronal injury and spinal cord injury. The authors sought to examine whether blocking LINGO-1 function with LINGO-1 antagonists promotes retinal ganglion cell (RGC) survival after ocular hypertension and optic nerve transection. An experimental ocular hypertension model was induced in adult rats using an argon laser to photocoagulate the episcleral and limbal veins. LINGO-1 expression in the retinas was investigated using immunohistochemistry and Western blotting. Soluble LINGO-1 protein (LINGO-1-Fc) and anti-LINGO-1 mAb 1A7 were injected into the vitreous body to examine their effects on RGC survival after ocular hypertension and optic nerve transection. Signal transduction pathways mediating neuroprotective LINGO-1-Fc effects were characterized using Western blotting and specific kinase inhibitors. LINGO-1 was expressed in RGCs and up-regulated after intraocular pressure elevation. Blocking LINGO-1 function with LINGO-1 antagonists, LINGO-1-Fc and 1A7 significantly reduced RGC loss 2 and 4 weeks after ocular hypertension and also promoted RGC survival after optic nerve transection. LINGO-1-Fc treatment blocked the RhoA, JNK pathway and promoted Akt activation. LINGO-1-Fc induced Akt phosphorylation, and the survival effect of LINGO-1 antagonists was abolished by Akt phosphorylation inhibitor. The authors demonstrated that blocking LINGO-1 function with LINGO-1 antagonists rescues RGCs from cell death after ocular hypertension and optic nerve transection. They also delineated the RhoA and PI-3K/Akt pathways as the predominant mediator of LINGO-1-Fc neuroprotection in this paradigm of RGC death. Show less

Lingo-1 (also known as Lern1) is a component of the Nogo receptor complex that mediates intracellular signaling in response to myelin associated inhibitors (MAIs): NogoA, MAG, and Omgp. Signaling through Nogo receptor extends to more than its well known role in preventing axon regeneration after lesion in the CNS, being implicated in neuronal functional maturation. Using Lingo-1-deficient mice, it has been demonstrated that Lingo-1 plays relevant roles in oligodendrocyte differentiation during brain development, and that treatment with Lingo-1 antagonists can improve axon regeneration after lesion in adult mice by decreasing MAI mediated signaling. However, a detailed description of the pattern of expression of Lingo-1 protein in correlation with the other partners of Nogo receptor is missing. Here, we show that components of the Nogo receptor complex, Lingo-1, NgR1, p75, and TROY coexist in mouse brain in a defined time window only at later postnatal stages. We have also determined the Lingo-1 distribution showing expression in particular subsets of neurons, but not in myelinating mature oligodendrocytes. Surprisingly, Lingo-1 is expressed at early developmental stages without NgR1, which supports the notion that Lingo-1 may participate in other activities in developing neurons different from oligodendrocyte maturation or axon extension inhibition in the adult. Finally, we propose that the intracellular domain of Lingo-1 contributes to signaling and show that it interacts with the postmitotic neuronal specific zinc finger protein Myt1l, suggesting that Lingo-1 may regulate Myt1l transcription factor activity by affecting its subcellular localization. Show less

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

Despite intense research efforts, the physiological function and molecular environment of the amyloid precursor protein has remained enigmatic. Here we describe the application of time-controlled transcardiac perfusion cross-linking, a method for the in vivo mapping of protein interactions in intact tissue, to study the interactome of the amyloid precursor protein (APP). To gain insights into the specificity of reported protein interactions the study was extended to the mammalian amyloid precursor-like proteins (APLP1 and APLP2). To rule out sampling bias as an explanation for differences in the individual datasets, a small scale quantitative iTRAQ (isobaric tags for relative and absolute quantitation)-based comparison of APP, APLP1, and APLP2 interactomes was carried out. An interactome map was derived that confirmed eight previously reported interactions of APP and revealed the identity of more than 30 additional proteins that reside in spatial proximity to APP in the brain. Subsequent validation studies confirmed a physiological interaction between APP and leucine-rich repeat and Ig domain-containing protein 1, demonstrated a strong influence of Ig domain-containing protein 1 on the proteolytic processing of APP, and consolidated similarities in the biology of APP and p75. Show less

Multiple sclerosis (MS) is an inflammatory disease of the CNS that causes progressive neurological disability in most patients. Certain alleles of immunity-associated genes increase risk of MS, confirming a role for autoimmune mechanisms in pathogenesis. Activated mononuclear cells infiltrate the CNS and trigger an inflammatory cascade, resulting in demyelination and axonal injury. Non-inflammatory mechanisms also appear to be involved in axonal degeneration but are not fully elucidated. Current therapies are anti-inflammatory, and no available therapy is known to promote myelin repair or maintenance. Leucine-rich repeats and Ig domain-containing, neurite outgrowth inhibitor (Nogo) receptor-interacting protein-1 (LINGO-1) is a potent negative regulator of axonal myelination. This article provides an overview of the available data on the effects of LINGO-1 antagonists on oligodendrocyte differentiation and remyelination. LINGO-1 is a potential target for neuroprotective therapy in that antagonists may promote remyelination in diseases such as MS. Show less

Nogo-A is one of the most potent oligodendrocyte-derived inhibitors for axonal regrowth in the injured adult CNS. However, the physiological function of Nogo-A in development and in healthy oligodendrocytes is still unknown. In the present study, we investigated the role of Nogo-A for myelin formation in the developing optic nerve. By quantitative real-time PCR, we found that the expression of Nogo-A increased faster in differentiating oligodendrocytes than that of the major myelin proteins MBP (myelin basic protein), PLP (proteolipid protein)/DM20, and CNP (2',3'-cyclic nucleotide 3'-phosphodiesterase). The analysis of optic nerves and cerebella of mice deficient for Nogo-A (Nogo-A(-/-)) revealed a marked delay of oligodendrocyte differentiation, myelin sheath formation, and axonal caliber growth within the first postnatal month. The combined deletion of Nogo-A and MAG caused a more severe transient hypomyelination. In contrast to MAG(-/-) mice, Nogo-A(-/-) mutants did not present abnormalities in the structure of myelin sheaths and Ranvier nodes. The common binding protein for Nogo-A and MAG, NgR1, was exclusively upregulated in MAG(-/-) animals, whereas the level of Lingo-1, a coreceptor, remained unchanged. Together, our results demonstrate that Nogo-A and MAG are differently involved in oligodendrocyte maturation in vivo, and suggest that Nogo-A may influence also remyelination in pathological conditions such as multiple sclerosis. Show less

LINGO-1 has been critically implicated in the central regulation of CNS axon regeneration and oligodendrocyte maturation. We have recently demonstrated that pretreatment with LINGO-1 antagonist (LINGO-1-Fc) inhibited low potassium-induced cerebellar granular neurons (CGNs) apoptosis. In the present study, we examined the neuroprotective mechanism of LINGO-1-Fc by Western blot and in situ GST pull-down assay. CGN cultures were preincubated in medium with LINGO-1-Fc or control protein at the concentration of 10 mug/ml for 2 h and then switched to low potassium medium in the presence of corresponding proteins. Cultures were harvested at indicated time intervals for successive analysis. Several apoptosis-associated signaling factors, GSK-3beta, ERK1/2, and Rho GTPases, were observed to be activated in response to potassium deprivation and the activation/dephosphorylation of GSK-3beta was suppressed by LINGO-1-Fc pretreatment compared with control group. Besides, the endogenous LINGO-1 expression level of CGN cultures was augmented by low potassium stimuli and restrained by LINGO-1 antagonist treatment. Although the protein level of p75(NTR) and Nogo-A were down-regulated in different patterns during apoptosis, neither of them was affected by LINGO-1-Fc application. Taken together, these results suggest a new mechanism of LINGO-1 antagonist regulated neuronal survival involving protein synthesis of LINGO-1 and inactivation of GSK-3 pathway. Show less

The regenerative capacity of the adult central nervous system (CNS) is severely limited and although partial regeneration can be observed in the CNS of multiple sclerosis (MS) patients, these attempts at repair have been universally unsuccessful in preventing the accumulation of irreversible neurological deficits. Novel therapies to treat MS must therefore take into account the need for both immunomodulation and neuroprotection and, as such, multifaceted treatment strategies are required. Two complimentary approaches that aim to regenerate an incapacitated CNS have recently emerged. Firstly, targeting degraded myelin growth inhibitory molecules released as a consequence of the inflammatory process provides a unique opportunity to manipulate the microenvironment of the degenerating CNS. Proof of concept studies have established that this therapeutic approach has tremendous potential in regenerating damaged axons as demonstrated in models of spinal cord injury (SCI) and experimental autoimmune encephalomyelitis (EAE), an animal model for MS. In addition, stem cell based therapies offer a means of modulating inflammatory immune cells and promoting tissue repair as shown in a number of allogeneic transplant and autoimmune settings. This review attempts to summarise some of these approaches. Show less

LINGO-1 is selectively expressed in the CNS on both oligodendrocyte precursor cells (OPCs) and neurons. Its expression is developmentally regulated in the normal CNS, as well as up-regulated in human or rat models of neuropathologies. LINGO-1 functions as a negative regulator of oligodendrocyte differentiation and myelination, neuronal survival and axonal regeneration. Across diverse animal CNS disease models, targeted LINGO-1 inhibition was found to promote neuron and oligodendrocyte survival, axon regeneration, oligodendrocyte differentiation, remyelination and improved functional recovery. The targeted inhibition of LINGO-1 therefore presents a novel therapeutic approach for the treatment of neurological diseases. Show less

We have analysed the expression during mouse development of the four member Lingo/LERN gene family which encodes type 1 transmembrane proteins containing 12 extracellular leucine rich repeats, an immunoglobulin C2 domain and a short intracellular tail. Each family member has a distinct pattern of expression in the mouse embryo as is the case for the related NLRR, FLRT and LRRTM gene families. Lingo1/LERN1 is expressed in the developing trigeminal, facio-acoustic and dorsal root ganglia. An interesting expression pattern is also observed in the somites with expression localising to the inner surface of the dermomyotome in the ventro-caudal lip. Further expression is seen in lateral cells of the hindbrain and midbrain, lateral cells in the motor horn of the neural tube, the otic vesicle epithelium and epithelium associated with the developing gut. Lingo3/LERN2 is expressed in a broad but specific pattern in many tissues across the embryo. Lingo2/LERN3 is seen in a population of cells lying adjacent to the epithelial lining of the olfactory pit while Lingo4/LERN4 is expressed in the neural tube in a subset of progenitors adjacent to the motor neurons. Expression of all Lingo/LERN genes increases as the embryo develops but is low in the adult with only Lingo1/LERN1 and Lingo2/LERN3 being detectable in adult brain. Show less

Functional recovery following acute CNS injury in humans, such as spinal cord injury and stroke, is exceptionally limited, leaving the affected individual with life-long neurological deficits such as loss of limb movement and sensation leading to a compromised quality of life. As yet, there is no effective treatment on the market for such injuries. This lack of functional recovery can at least in part be attributed to the restriction of axonal regeneration and neuroplasticity by several CNS myelin proteins that have been shown to be potent inhibitors of neurite outgrowth in vitro, namely myelin-associated glycoprotein (MAG), Nogo-A and oligodendrocyte myelin glycoprotein (OMgp). Nogo-A contains multiple neurite outgrowth inhibitory domains exposed on the surface of myelinating oligodendrocytes located within its amino-terminal region (amino-Nogo-A) and C-terminal region (Nogo-66). Although structurally dissimilar; Nogo-66, MAG and OMgp exert their inhibitory effects by binding the GPI-linked neuronal Nogo-66 receptor (NgR) that transduces the inhibitory signal to the cell interior via transmembrane co-receptors LINGO-1 and p75(NTR)or TROY. Although the receptor(s) for amino-Nogo-A are unknown, amino-Nogo-A and NgR ligands mutually activate the small GTPase RhoA. Consistent with their neurite outgrowth inhibitory function, approaches counter-acting Nogo-A using function-blocking antibodies, NgR using peptide antagonists and receptor bodies or RhoA using deactivating enzymes have been shown to significantly enhance axonal regeneration and neuroplasticity leading to improved functional recovery in animal models of acute CNS injury. These in vivo findings thus provide a sound basis for the development of an effective treatment for acute CNS injuries in humans. Show less

Myelination is necessary both for rapid salutatory conduction and the long-term survival of the axon. In the CNS the myelin sheath is formed by the oligodendrocytes. Each oligodendrocyte myelinates several axons and, as the number of wraps around each axon is determined precisely by the axon diameter, this requires a close, highly regulated interaction between the axons and each of the oligodendrocyte processes. Adhesion molecules are likely to play an important role in the bi-directional signalling between axon and oligodendrocyte that underlies this interaction. Here we review the current knowledge of the function of adhesion molecules in the different phases of oligodendrocyte differentiation and myelination, and discuss how the properties of these proteins defined by other cell biological systems indicates potential roles in oligodendrocytes. We show how the function of a number of different adhesion and cell-cell interaction molecules such as polysialic acid neural cell adhesion molecule, Lingo-1, Notch, neuregulin, integrins and extracellullar matrix proteins provide negative and positive signals that coordinate the formation of the myelin membrane. Compiling this information from a number of different cell biological and genetic experiments helps us to understand the pathology of multiple sclerosis and direct new areas of research that might eventually lead to potential drug targets to increase remyelination. Show less

In the adult mammalian central nervous system (CNS), it is well known that injured axons exhibit very limited regeneration ability. Due to this lack of appropriate axonal regeneration, a traumatic damage to the adult brain and spinal cord frequently causes permanent neuronal deficits such as paralysis. Several axon growth inhibitors, including myelin-associated glycoprotein, Nogo, and oligodensrocyte myelin glycoprotein, in the CNS have been identified in the myelin. Receptor complex comprising of the Nogo receptor, the p75 receptor, and LINGO-1 transduces the signals from all of these inhibitors in vitro. Downstream of these inhibitors, activation of small GTPase RhoA and its effector Rho-kinase has been shown to be a key element for neurite growth inhibition and growth cone collapse elicited by these inhibitors. Consistent with these findings in vitro, inhibition of RhoA or Rho-kinase in vivo promotes axon growth and functional recovery after spinal cord injury. Recently, several developmental guidance proteins, including repulsive guidance molecules, semaphorin, and ephrin are suggested to be involved in axon growth inhibition after injury to the CNS. Thus, multiple axon growth inhibitors seem to contribute to inability of the injured axons to regenerate, and therapeutic strategy to block the multiple axon growth inhibitors may provide efficient tools that produce functional regeneration following injuries to the CNS. In addition, it is noted that synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibers are important components of the spontaneous recovery process. The molecular mechanism of this phenomenon is poorly understood, and elucidation of this will contribute to enhancement of functional recovery after incomplete injury to the CNS. I will summarize recent findings regarding these issues. Show less

Cortical sensory maps can reorganize in the adult brain in an experience-dependent manner. We monitored somatosensory cortical reorganization after sensory deafferentation using functional magnetic resonance imaging (fMRI) in rats subjected to complete transection of the mid-thoracic spinal cord. Cortical representation in response to spared forelimb stimulation was observed to enlarge and invade adjacent sensory-deprived hind limb territory in the primary somatosensory cortex as early as 3 days after injury. Functional MRI also demonstrated long-term cortical plasticity accompanied by increased thalamic activation. To support the notion that alterations of cortical neuronal circuitry after spinal cord injury may underlie the fMRI changes, we quantified transcriptional activities of several genes related to cortical plasticity including the Nogo receptor (NgR), its co-receptor LINGO-1 and brain derived neurotrophic factor (BDNF), using in situ hybridization. We demonstrate that NgR and LINGO-1 are down-regulated specifically in cortical areas deprived of sensory input and in adjacent cortex from 1 day after injury, while BDNF is up-regulated. Our results demonstrate that cortical neurons react to sensory deprivation by decreasing transcriptional activities of genes encoding the Nogo receptor components in the sensory deprived and the anatomically adjacent non-deprived area. Combined with the BDNF up-regulation, these changes presumably allow structural changes in the neuropil. Our observations therefore suggest an involvement of Nogo signalling in cortical activity-dependent plasticity in the somatosensory system. In spinal cord injury, cortical reorganization as shown here can become a disadvantage, much like the situation in amblyopia or phantom sensation. Successful strategies to repair sensory pathways at the spinal cord level may not lead to proper reestablishment of cortical connections, once deprived hind limb cortical areas have been reallocated to forelimb use. In such situations, methods to control cortical plasticity, possibly by targeting Nogo signalling, may become helpful. Show less

Demyelinating diseases, such as multiple sclerosis, are characterized by the loss of the myelin sheath around neurons, owing to inflammation and gliosis in the central nervous system (CNS). Current treatments therefore target anti-inflammatory mechanisms to impede or slow disease progression. The identification of a means to enhance axon myelination would present new therapeutic approaches to inhibit and possibly reverse disease progression. Previously, LRR and Ig domain-containing, Nogo receptor-interacting protein (LINGO-1) has been identified as an in vitro and in vivo negative regulator of oligodendrocyte differentiation and myelination. Here we show that loss of LINGO-1 function by Lingo1 gene knockout or by treatment with an antibody antagonist of LINGO-1 function leads to functional recovery from experimental autoimmune encephalomyelitis. This is reflected biologically by improved axonal integrity, as confirmed by magnetic resonance diffusion tensor imaging, and by newly formed myelin sheaths, as determined by electron microscopy. Antagonism of LINGO-1 or its pathway is therefore a promising approach for the treatment of demyelinating diseases of the CNS. Show less

The nervous system-specific leucine-rich repeat Ig-containing protein LINGO-1 is associated with the Nogo-66 receptor complex and is endowed with a canonical EGF receptor (EGFR)-like tyrosine phosphorylation site. Our studies indicate that LINGO-1 expression is elevated in the substantia nigra of Parkinson's disease (PD) patients compared with age-matched controls and in animal models of PD after neurotoxic lesions. LINGO-1 expression is present in midbrain dopaminergic (DA) neurons in the human and rodent brain. Therefore, the role of LINGO-1 in cell damage responses of DA neurons was examined in vitro and in experimental models of PD induced by either oxidative (6-hydroxydopamine) or mitochondrial (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) toxicity. In LINGO-1 knockout mice, DA neuron survival was increased and behavioral abnormalities were reduced compared with WT. This neuroprotection was accompanied by increased Akt phosphorylation (p-Akt). Similar neuroprotective in vivo effects on midbrain DA neurons were obtained in WT mice by blocking LINGO-1 activity using LINGO-1-Fc protein. Neuroprotection and enhanced neurite growth were also demonstrated for midbrain DA neurons in vitro. LINGO-1 antagonists (LINGO-1-Fc, dominant negative LINGO-1, and anti-LINGO-1 antibody) improved DA neuron survival in response to MPP+ in part by mechanisms that involve activation of the EGFR/Akt signaling pathway through a direct inhibition of LINGO-1's binding to EGFR. These results show that inhibitory agents of LINGO-1 activity can protect DA neurons against degeneration and indicate a role for the leucine-rich repeat protein LINGO-1 and related classes of proteins in the pathophysiological responses of midbrain DA neurons in PD. Show less

Nogo-A has been considered as one of the most important myelin-associated axonal regeneration inhibitors in the central nervous system. Recent studies have demonstrated various additional physiological roles of Nogo family members. To understand the possible effect of Nogo-A on the differentiation of oligodendrocytes, antibodies against distinct extracellular domains of Nogo-A were applied in cell cultures. Oligodendrocyte precursor cells from P2 rat cortex were grown in the presence of monoclonal antibody against the N-terminal inhibitory domain of Nogo-A or the C-terminal 66 amino acid loop of Nogo-A for 3 days, and the antibody treatment resulted in stunted process extension and inhibited differentiation of oligodendrocytes. Concomitant with morphology changes, Rho GTPases activity was greatly increased upon the antibody treatment and the expression level of LINGO-1, which was recently shown to be a negative regulator for the oligodendrocyte maturation, was upregulated in the process of antibody treatment. These results indicate that endogenous Nogo-A expressed in oligodendrocyte may act though Rho GTPase and LINGO-1 to influence the morphological differentiation of oligodendrocytes and will help us to understand the physiology role of Nogo-A in oligodendrocyte biology. Show less

Neuronal Nogo66 receptor-1 (NgR1) binds the myelin inhibitors NogoA, OMgp, and myelin-associated glycoprotein (MAG) and has been proposed to function as the ligand-binding component of a receptor complex that also includes Lingo-1, p75(NTR), or TROY. In this study, we use Vibrio cholerae neuraminidase (VCN) and mouse genetics to probe the molecular composition of the MAG receptor complex in postnatal retinal ganglion cells (RGCs). We find that VCN treatment is not sufficient to release MAG inhibition of RGCs; however, it does attenuate MAG inhibition of cerebellar granule neurons. Furthermore, the loss of p75(NTR) is not sufficient to release MAG inhibition of RGCs, but p75(NTR-/-) dorsal root ganglion neurons show enhanced growth on MAG compared to wild-type controls. Interestingly, TROY is not a functional substitute for p75(NTR) in RGCs. Finally, NgR1(-/-) RGCs are strongly inhibited by MAG. In the presence of VCN, however, NgR1(-/-) RGCs exhibit enhanced neurite growth. Collectively, our experiments reveal distinct and cell type-specific mechanisms for MAG-elicited growth inhibition. Show less

Although CNS neurons have the potential to regenerate their axons after injury, myelin debris carrying axon growth inhibitors rapidly induce growth cone collapse. Receptors (NgR1, NgR2) and coreceptors (LINGO-1, p75(NTR), TROY) for these inhibitors have been characterized and transduction pathways partially identified. However, little is known about the expression of these receptors in intact and lesioned supraspinal projection neurons. Using in situ hybridization, immunohistochemistry and neuronal tract-tracing, we found that NgR1, NgR2 and LINGO-1 are strongly expressed in several neuronal populations of the adult mouse brain projecting to the spinal cord, including neurons projecting through the corticospinal, rubrospinal, caerulospinal, reticulospinal, raphespinal and vestibulospinal tracts. As expected, p75(NTR) expression was restricted to neuronal descending pathways from the brainstem. TROY was absent from most brain regions and from all neuronal projection systems, suggesting that additional signal-transducing coreceptors exist. Qualitative and quantitative analyses revealed that brain expression for these receptors was not affected by a severe T10 spinal cord contusion. Show less

Neurons and glia share a mutual dependence in establishing a functional relationship, and none is more evident than the process by which axons control myelination. Here, we identify LRR and Ig domain-containing, Nogo receptor-interacting protein (LINGO-1) as a potent axonal inhibitor of oligodendrocyte differentiation and myelination that is regulated by nerve growth factor and its cognate receptor TrkA in a dose-dependent manner. Whereas LINGO-1 expressed by oligodendrocyte progenitor cells was previously identified as an inhibitor of differentiation, we demonstrate that axonal expression of LINGO-1 inhibits differentiation with equal potency. Disruption of LINGO-1 on either cell type is sufficient to overcome the inhibitory action and promote differentiation and myelination, independent of axon diameter. Furthermore, these results were recapitulated in transgenic mice overexpressing the full length LINGO-1 under the neuronal promoter synapsin. Myelination was greatly inhibited in the presence of enforced axonal LINGO-1. The implications of these results relate specifically to the development of potential therapeutics targeting extrinsic growth factors that may regulate the axonal expression of modulators of oligodendrocyte development. Show less

Nogo constitutes a family of neurite outgrowth inhibitors contributing to a failure of axonal regeneration in the adult central nervous system (CNS). Nogo-A is expressed exclusively on oligodendrocytes where Nogo-66 segment binds to Nogo receptor (NgR) expressed on neuronal axons. NgR signalling requires a coreceptor p75(NTR) or TROY in combination with an adaptor LINGO-1. To characterize the cell types expressing the NgR complex in the human CNS, we studied demyelinating lesions of multiple sclerosis (MS) brains by immunohistochemistry. TROY and LINGO-1 were identified in subpopulations of reactive astrocytes, macrophages/microglia and neurones but not in oligodendrocytes. TROY was up-regulated, whereas LINGO-1 was reduced in MS brains by Western blot. These results suggest that the ternary complex of NgR/TROY/LINGO-1 expressed on astrocytes, macrophages/microglia and neurones, by interacting with Nogo-A on oligodendrocytes, might modulate glial-neuronal interactions in demyelinating lesions of MS. Show less

After binding, central nervous system (CNS) myelin-derived axon growth inhibitory ligands, the Nogo-66 receptor (NgR), complexes with LINGO-1 and either the low-affinity neurotrophin receptor (p75(NTR)) or TROY to initiate growth cone collapse via a Rho-A inhibitory signaling pathway and/or Ca(2+)-dependent activation of epidermal growth factor receptor (EGFR) through an unknown signaling pathway. We have shown that axon growth through CNS myelin is disinhibited after neurotrophic factor administration by 1) initiating intramembranous proteolysis (RIP) of p75(NTR), leading to cleavage of the extracellular (p75(ECD)) and intracellular domains (p75(ICD)) by alpha- and gamma-secretase, respectively, thereby paralyzing inhibitory signaling; 2) shedding of soluble NgR(ECD), which acts as a competitive antagonist to NgR for binding of inhibitory ligands; and 3) antagonizing NgR/p75(NTR) clustering by competitive p75(ECD)/NgR interaction. Here, we report that TNF-alpha converting enzyme (TACE) (a disintegrin and metalloproteinase 17, ADAM17) induces disinhibition of FGF2-stimulated neurite outgrowth of dorsal root ganglion neurons (DRGN) cultured in the presence of a predetermined concentration of inhibitory CNS myelin-derived ligands. After addition of TACE (which has alpha-secretase activity) to mitotically arrested adult rat mixed DRG cultures, we demonstrate 1) NgR(ECD) shedding; 2) release of p75(ECD) and p75(ICD) by RIP of p75(NTR); 3) blockade of Rho-A activation; 4) reduced EGFR phosphorylation; and 5) increased FGF2-stimulated DRGN neurite outgrowth and branching in the presence of CNS myelin-derived inhibitory ligands. Thus, TACE-induced cleavage of NgR and RIP of p75(NTR) abrogates axon growth inhibitory signaling, thereby disinhibiting CNS axon/neurite growth. Show less

Leucine-rich repeats (LRR) are protein-protein interaction domains (20-29 amino acid residues in length) found in proteins with diverse structure and functions. We note here an emerging group of central nervous system-enriched, type I surface proteins with an ectodomain containing LRR repeats and motifs found in cell adhesion molecules. Members of this group include the Amphoterin-induced gene and ORF-1 (AMIGO-1), LRR and Ig domain containing Nogo Receptor interacting protein I (LINGO-1) and the netrin-G1 ligand NGL-1. The above proteins carry, in addition to the LRR repeats, an immunoglobin (Ig)-like segment in their ectodomain. Two other related families of molecules, the NLRRs and the FLRTs, have in addition, a fibronectin type III repeat. The LRR domain distinguishes these molecules from the more extensively studied Ig-like family of cell adhesion molecules, and the transmembrane domain differentiate them from the family of secreted extracellular proteoglycans with LRRs. Functionally, many members of this group of proteins could modulate neurite outgrowth of neurons, at least in vitro. LINGO-1, initially discovered as a component of the Nogo-66 receptor complex which inhibits neurite growth, also regulates oligodendrocyte differentiation and myelination. Current knowledge and recent findings pertaining to the functions of this interesting group of proteins in the nervous system are discussed. Show less

LINGO-1 is a CNS-specific protein and a functional component of the NgR1/p75/LINGO-1 and NgR1/TAJ(TROY)/LINGO-1 signaling complexes that mediate inhibition of axonal outgrowth. These receptor complexes mediate the axonal growth inhibitory effects of Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (OMgp) via RhoA activation. Soluble LINGO-1 (LINGO-1-Fc), which acts as an antagonist of these pathways by blocking LINGO-1 binding to NgR1, was administered to rats after dorsal or lateral hemisection of the spinal cord. LINGO-1-Fc treatment significantly improved functional recovery, promoted axonal sprouting and decreased RhoA activation and increased oligodendrocyte and neuronal survival after either rubrospinal or corticospinal tract transection. These experiments demonstrate an important role for LINGO-1 in modulating axonal outgrowth in vivo and that treatment with LINGO-1-Fc can significantly enhance recovery after spinal cord injury. Show less

Current concepts of Attention-Deficit/Hyperactivity Disorder (ADHD) emphasize the role of higher-order cognitive functions and reinforcement processes attributed to structural and biochemical anomalies in cortical and limbic neural networks innervated by the monoamines, dopamine, noradrenaline and serotonin. However, these explanations do not account for the ubiquitous findings in ADHD of intra-individual performance variability, particularly on tasks that require continual responses to rapid, externally-paced stimuli. Nor do they consider attention as a temporal process dependent upon a continuous energy supply for efficient and consistent function. A consideration of this feature of intra-individual response variability, which is not unique to ADHD but is also found in other disorders, leads to a new perspective on the causes and potential remedies of specific aspects of ADHD. We propose that in ADHD, astrocyte function is insufficient, particularly in terms of its formation and supply of lactate. This insufficiency has implications both for performance and development: H1) In rapidly firing neurons there is deficient ATP production, slow restoration of ionic gradients across neuronal membranes and delayed neuronal firing; H2) In oligodendrocytes insufficient lactate supply impairs fatty acid synthesis and myelination of axons during development. These effects occur over vastly different time scales: those due to deficient ATP (H1) occur over milliseconds, whereas those due to deficient myelination (H2) occur over months and years. Collectively the neural outcomes of impaired astrocytic release of lactate manifest behaviourally as inefficient and inconsistent performance (variable response times across the lifespan, especially during activities that require sustained speeded responses and complex information processing). Multi-level and multi-method approaches are required. These include: 1) Use of dynamic strategies to evaluate cognitive performance under conditions that vary in duration, complexity, speed, and reinforcement; 2) Use of sensitive neuroimaging techniques such as diffusion tensor imaging, magnetic resonance spectroscopy, electroencephalography or magnetoencephalopathy to quantify developmental changes in myelination in ADHD as a potential basis for the delayed maturation of brain function and coordination, and 3) Investigation of the prevalence of genetic markers for factors that regulate energy metabolism (lactate, glutamate, glucose transporters, glycogen synthase, glycogen phosphorylase, glycolytic enzymes), release of glutamate from synaptic terminals and glutamate-stimulated lactate production (SNAP25, glutamate receptors, adenosine receptors, neurexins, intracellular Ca2+), as well as astrocyte function (alpha1, alpha2 and beta-adrenoceptors, dopamine D1 receptors) and myelin synthesis (lactate transporter, Lingo-1, Quaking homolog, leukemia inhibitory factor, and Transferrin). The hypothesis extends existing theories of ADHD by proposing a physiological basis for specific aspects of the ADHD phenotype - namely frequent, transient and impairing fluctuations in functioning, particularly during performance of speeded, effortful tasks. The immediate effects of deficient ATP production and slow restoration of ionic gradients across membranes of rapidly firing neurons have implications for daily functioning: For individuals with ADHD, performance efficacy would be enhanced if repetitive and lengthy effortful tasks were segmented to reduce concurrent demands for speed and accuracy of response (introduction of breaks into lengthy/effortful activities such as examinations, motorway driving, assembly-line production). Also, variations in task or modality and the use of self- rather than system-paced schedules would be helpful. This would enable energetic demands to be distributed to alternate neural resources, and energy reserves to be re-established. Longer-term effects may manifest as reduction in regional brain volumes since brain areas with the highest energy demand will be most affected by a restricted energy supply and may be reduced in size. Novel forms of therapeutic agent and delivery system could be based on factors that regulate energy production and myelin synthesis. Since the phenomena and our proposed basis for it are not unique to ADHD but also manifests in other disorders, the implications of our hypotheses may be relevant to understanding and remediating these other conditions as well. Show less

The Nogo system has recently been implicated not only in regeneration but also in modulating plasticity. One reason for declining memory functions in aging may be altered plasticity in the aged hippocampus and cortex cerebri. Therefore, we have examined the levels of mRNA encoding Nogo, OMgp and MAG, as well as the receptor components NgR, Lingo-1 and Troy in cortex and hippocampus of young (4 months), middle aged (16 months) and old (24 months) Fisher 344 rats. No significant changes of receptor components or the ligands OMgp or MAG were observed. Nogo mRNA, however, was significantly decreased in hippocampal subregions of aged animals. The specific decrease of Nogo mRNA levels in hippocampus and possibly cortex cerebri may relate to age-dependent decline of brain plasticity. Show less

Nogo receptor (NgR)-mediated control of axon growth relies on the central nervous system-specific type I transmembrane protein Lingo-1. Interactions between Lingo-1 and NgR, along with a complementary co-receptor, result in neurite and axonal collapse. In addition, the inhibitory role of Lingo-1 is particularly important in regulation of oligodendrocyte differentiation and myelination, suggesting that pharmacological modulation of Lingo-1 function could be a novel approach for nerve repair and remyelination therapies. Here we report on the crystal structure of the ligand-binding ectodomain of human Lingo-1 and show it has a bimodular, kinked structure composed of leucine-rich repeat (LRR) and immunoglobulin (Ig)-like modules. The structure, together with biophysical analysis of its solution properties, reveals that in the crystals and in solution Lingo-1 persistently associates with itself to form a stable tetramer and that it is its LRR-Ig-composite fold that drives such assembly. Specifically, in the crystal structure protomers of Lingo-1 associate in a ring-shaped tetramer, with each LRR domain filling an open cleft in an adjacent protomer. The tetramer buries a large surface area (9,200 A2) and may serve as an efficient scaffold to simultaneously bind and assemble the NgR complex components during activation on a membrane. Potential functional binding sites that can be identified on the ectodomain surface, including the site of self-recognition, suggest a model for protein assembly on the membrane. Show less

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

A major obstacle for successful axon regeneration in the adult central nervous system (CNS) arises from inhibitory molecules in CNS myelin, which signal through a common receptor complex on neurons consisting of the ligand-binding Nogo-66 receptor (NgR) and two transmembrane coreceptors, p75 and LINGO-1. However, p75 expression is only detectable in subpopulations of mature neurons, raising the question of how these inhibitory signals are transduced in neurons lacking p75. In this study, we demonstrate that TROY (also known as TAJ), a TNF receptor family member selectively expressed in the adult nervous system, can form a functional receptor complex with NgR and LINGO-1 to mediate cellular responses to myelin inhibitors. Also, both overexpressing a dominant-negative TROY or presence of a soluble TROY protein can efficiently block neuronal response to myelin inhibitors. Our results implicate TROY in mediating myelin inhibition, offering new insights into the molecular mechanisms of regeneration failure in the adult nervous system. Show less

Myelin-associated inhibitory factors (MAIFs) are inhibitors of CNS axonal regeneration following injury. The Nogo receptor complex, composed of the Nogo-66 receptor 1 (NgR1), neurotrophin p75 receptor (p75), and LINGO-1, represses axon regeneration upon binding to these myelin components. The limited expression of p75 to certain types of neurons and its temporal expression during development prompted speculation that other receptors are involved in the NgR1 complex. Here, we show that an orphan receptor in the TNF family called TAJ, broadly expressed in postnatal and adult neurons, binds to NgR1 and can replace p75 in the p75/NgR1/LINGO-1 complex to activate RhoA in the presence of myelin inhibitors. In vitro exogenously added TAJ reversed neurite outgrowth caused by MAIFs. Neurons from Taj-deficient mice were more resistant to the suppressive action of the myelin inhibitors. Given the limited expression of p75, the discovery of TAJ function is an important step for understanding the regulation of axonal regeneration. Show less