Zinc metalloproteinase-mediated cleavage of the human Nogo-66 receptor

MT3-MMP Promotes Excitatory Synapse Formation by Promoting Nogo-66 Receptor Ectodomain Shedding

Cell-surface molecules are dynamically regulated at the synapse to assemble and disassemble adhesive contacts that are important for synaptogenesis and for tuning synaptic transmission. Metalloproteinases dynamically regulate cellular behaviors through the processing of cell surface molecules. In the present study, we evaluated the role of membrane-type metalloproteinases (MT-MMPs) in excitatory synaptogenesis. We find that MT3-MMP and MT5-MMP are broadly expressed in the mouse cerebral cortex and that MT3-MMP loss-of-function interferes with excitatory synapse development in dissociated cortical neurons and in vivo We identify Nogo-66 receptor (NgR1) as an MT3-MMP substrate that is required for MT3-MMP-dependent synapse formation. Introduction of the shed ectodomain of NgR1 is sufficient to accelerate excitatory synapse formation in dissociated cortical neurons and in vivo Together, our findings support a role for MT3-MMP-dependent shedding of NgR1 in regulating excitatory synapse development.SIGNIFICANCE STATEMENT In this study, we identify MT3-MMP, a membrane-bound zinc protease, to be necessary for the development of excitatory synapses in cortical neurons. We identify Nogo-66 receptors (NgR1) as a downstream target of MT3-MMP proteolytic activity. Furthermore, processing of surface NgR1 by MT3-MMP generates a soluble ectodomain fragment that accelerates the formation of excitatory synapses. We propose that MT3-MMP activity and NgR1 shedding could stimulate circuitry remodeling in the adult brain and enhance functional connectivity after brain injury.

Cerebral ischemia/repefusion injury: From bench space to bedside

While stroke research represents the primary interface between circulation and brain research, the hemostasis system also carries a pivotal role in the mechanism of vascular brain injury. The complex interrelated events triggered by the energy crisis have a specific spatial and temporal pattern arching from the initial damage to the final events of brain repair. The complexity of the pathophysiology make it difficult to model this disease, therefore it is challenging to find appropriate therapeutic targets. The ever-persistent antagonism between the positive results of drug candidates in the experimental stroke models and the failures of the clinical trials prompts changes in the research strategy, especially in the field of potential neuroprotective therapies. System biology approach could initiate new directions in the future for both preclinical and clinical research. Incentive methods aimed at anti-apoptosis mechanisms and the augmentation of post-ischemic brain repair could benefit the facts, that these processes can be targeted much longer following the cell-necrosis in the hyper-acute phase. Sequential monitoring of candidate genes and proteins responsible for stroke progression and post-stroke repair seems to be useful both in therapeutic target-identification, and in clinical testing. Understanding the mechanism behind the effect of selegiline and other drugs capable of activating the anti-apoptotic gene expression could help to find new approaches to enhance the regenerative potential in the remodeling of neuronal and microvascular networks.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Delayed Gelatinase Inhibition Induces Reticulon 4 Receptor Expression in the Peri-Infarct Cortex

Matrix metalloproteinase (MMP) inhibition can potentially prevent hemorrhagic transformation following cerebral infarction; however, delayed-phase MMP activity is also necessary for functional recovery after experimental stroke. We sought to identify potential mechanisms responsible for the impaired recovery associated with subacute MMP inhibition in a transient middle cerebral artery occlusion model of focal ischemia in CD rats. Gelatinase inhibition was achieved by intracerebral injection of the Fn-439 MMP inhibitor 7 days after stroke. Treatment efficacy was determined on day 9 by in situ gelatin zymography. The peri-infarct cortex was identified by triphenyl tetrazolium chloride staining, and tissue samples were dissected for TaqMan array gene-expression study. Of 84 genes known to influence poststroke regeneration, we found upregulation of mRNA for the reticulon 4 receptor (Rtn4r), a major inhibitor of regenerative nerve growth in the adult CNS, and borderline expression changes for 3 additional genes (DCC, Jun, and Ngfr). Western blot confirmed increased Rtn4r protein in the peri-infarct cortex of treated animals, and double immunolabeling showed colocalization primarily with the S100 astrocyte marker. These data suggest that increased Rtn4 receptor expression in the perilesional cortex may contribute to the impaired regeneration associated with MMP inhibition in the subacute phase of cerebral infarction.

Matrix Metalloproteinases During Axonal Regeneration, a Multifactorial Role from Start to Finish

By proteolytic cleavage, matrix metalloproteinases (MMPs) not only remodel the extracellular matrix (ECM) but they also modify the structure and activity of other proteinases, growth factors, signaling molecules, cell surface receptors, etc. Their vast substrate repertoire adds a complex extra dimension of biological control and turns MMPs into important regulatory nodes in the protease web. In the central nervous system (CNS), the detrimental impact of elevated MMP activities has been well-described for traumatic injuries and many neurodegenerative diseases. Nonetheless, there is ample proof corroborating MMPs as fine regulators of CNS physiology, and well-balanced MMP activity is instrumental to development, plasticity, and repair. In this manuscript, we review the emerging evidence for MMPs as beneficial modulators of axonal regeneration in the mammalian CNS. By exploring the multifactorial causes underlying the inability of mature axons to regenerate, and describing how MMPs can help to overcome these hurdles, we emphasize the benign actions of these Janus-faced proteases.

New Insights into the Roles of Nogo-A in CNS Biology and Diseases

Nogos have become a hot topic for its well-known number Nogo-A's big role in clinical matters. It has been recognized that the expression of Nogo-A and the receptor NgR1 inhibit the neuron's growth after CNS injuries or the onset of the MS. The piling evidence supports the notion that the Nogo-A is also involved in the synaptic plasticity, which was shown to negatively regulate the strength of synaptic transmission. The occurrence of significant schizophrenia-like behavioral phenotypes in Nogo-A KO rats also added strong proof to this conclusion. This review mainly focuses on the structure of Nogo-A and its corresponding receptor-NgR1, its intra- and extra-cellular signaling, together with its major physiological functions such as regulation of migration and distribution and its related diseases like stroke, AD, ALS and so on.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Regulation of ECM degradation and axon guidance by growth cone invadosomes

Invadopodia and podosomes, collectively referred to as invadosomes, are F-actin-rich basal protrusions of cells that provide sites of attachment to and degradation of the extracellular matrix. Invadosomes promote the invasion of cells, ranging from metastatic cancer cells to immune cells, into tissue. Here, we show that neuronal growth cones form protrusions that share molecular, structural and functional characteristics of invadosomes. Growth cones from all neuron types and species examined, including a variety of human neurons, form invadosomes both in vitro and in vivo. Growth cone invadosomes contain dynamic F-actin and several actin regulatory proteins, as well as Tks5 and matrix metalloproteinases, which locally degrade the matrix. When viewed using three-dimensional super-resolution microscopy, F-actin foci often extended together with microtubules within orthogonal protrusions emanating from the growth cone central domain. Finally, inhibiting the function of Tks5 both reduced matrix degradation in vitro and disrupted motoneuron axons from exiting the spinal cord and extending into the periphery. Taken together, our results suggest that growth cones use invadosomes to target protease activity during axon guidance through tissues.

Where no synapses go: gatekeepers of circuit remodeling and synaptic strength

Growth inhibitory molecules in the adult mammalian central nervous system (CNS) have been implicated in the blocking of axonal sprouting and regeneration following injury. Prominent CNS regeneration inhibitors include Nogo-A, oligodendrocyte myelin glycoprotein (OMgp), and chondroitin sulfate proteoglycans (CSPGs), and a key question concerns their physiological role in the naïve CNS. Emerging evidence suggests novel functions in dendrites and at synapses of glutamatergic neurons. CNS regeneration inhibitors target the neuronal actin cytoskeleton to regulate dendritic spine maturation, long-term synapse stability, and Hebbian forms of synaptic plasticity. This is accomplished in part by antagonizing plasticity-promoting signaling pathways activated by neurotrophic factors. Altered function of CNS regeneration inhibitors is associated with mental illness and loss of long-lasting memory, suggesting unexpected and novel physiological roles for these molecules in brain health.

The Nogo-66 receptor family in the intact and diseased CNS

The Nogo-66 receptor family (NgR) consists in three glycophosphatidylinositol (GPI)-anchored receptors (NgR1, NgR2 and NgR3), which are primarily expressed by neurons in the central and peripheral mammalian nervous system. NgR1 was identified as serving as a high affinity binding protein for the three classical myelin-associated inhibitors (MAIs) Nogo-A, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMgp), which limit axon regeneration and sprouting in the injured brain. Recent studies suggest that NgR signaling may also play an essential role in the intact adult CNS in restricting axonal and synaptic plasticity and are involved in neurodegenerative diseases, particularly in Alzheimer's disease pathology through modulation of β-secretase cleavage. Here, we outline the biochemical properties of NgRs and their functional roles in the intact and diseased CNS.

Membrane-type matrix metalloproteinase-3 regulates neuronal responsiveness to myelin through Nogo-66 receptor 1 cleavage

Nogo-66 receptor 1 (NgR1) is a glycosylphosphatidylinositol-anchored receptor for myelin-associated inhibitors that restricts plasticity and axonal regrowth in the CNS. NgR1 is cleaved from the cell surface of SH-SY5Y neuroblastoma cells in a metalloproteinase-dependent manner; however, the mechanism and physiological consequence of NgR1 shedding have not been explored. We now demonstrate that NgR1 is shed from multiple populations of primary neurons. Through a loss-of-function approach, we found that membrane-type matrix metalloproteinase-3 (MT3-MMP) regulates endogenous NgR1 shedding in primary neurons. Neuronal knockdown of MT3-MMP resulted in the accumulation of NgR1 at the cell surface and reduced the accumulation of the NgR1 cleavage fragment in medium conditioned by cortical neurons. Recombinant MT1-, MT2-, MT3-, and MT5-MMPs promoted NgR1 shedding from the surface of primary neurons, and this treatment rendered neurons resistant to myelin-associated inhibitors. Introduction of a cleavage-resistant form of NgR1 reconstitutes the neuronal response to these inhibitors, demonstrating that specific metalloproteinases attenuate neuronal responses to myelin in an NgR1-dependent manner.

Matrix metalloproteinases and neurotrauma: evolving roles in injury and reparative processes

Matrix metalloproteinases (MMPs) are involved in a wide range of proteolytic events in fetal development and normal tissue remodeling as well as wound healing and inflammation. In the CNS, they have been implicated in a variety of neurodegenerative diseases ranging from multiple sclerosis to Alzheimer disease and are integral to stroke-related cell damage. Although studies implicate increased activity of MMPs in pathogenesis in the CNS, there is also a growing literature to support their participation in events that support recovery processes. Here the authors provide a brief overview of MMPs and their regulation, address their complex roles following traumatic injuries to the adult and developing CNS, and consider their time- and context-dependent signatures that influence both injury and reparative processes.

Molecular basis of the interactions of the Nogo-66 receptor and its homolog NgR2 with myelin-associated glycoprotein: development of NgROMNI-Fc, a novel antagonist of CNS myelin inhibition

Myelin-associated glycoprotein (MAG) is a sialic acid-binding Ig-family lectin that functions in neuronal growth inhibition and stabilization of axon-glia interactions. The ectodomain of MAG is comprised of five Ig-like domains and uses neuronal cell-type-specific mechanisms to signal growth inhibition. We show that the first three Ig-like domains of MAG bind with high affinity and in a sialic acid-dependent manner to the Nogo-66 receptor-1 (NgR1) and its homolog NgR2. Domains Ig3-Ig5 of MAG are sufficient to inhibit neurite outgrowth but fail to associate with NgR1 or NgR2. Nogo receptors are sialoglycoproteins comprised of 8.5 canonical leucine-rich repeats (LRR) flanked by LRR N-terminal (NT) and C-terminal (CT)-cap domains. The LRR cluster is connected through a stalk region to a membrane lipid anchor. The CT-cap domain and stalk region of NgR2, but not NgR1, are sufficient for MAG binding, and when expressed in neurons, exhibit constitutive growth inhibitory activity. The LRR cluster of NgR1 supports binding of Nogo-66, OMgp, and MAG. Deletion of disulfide loop Cys(309)-Cys(336) of NgR1 selectively increases its affinity for Nogo-66 and OMgp. A chimeric Nogo receptor variant (NgR(OMNI)) in which Cys(309)-Cys(336) is deleted and followed by a 13 aa MAG-binding motif of the NgR2 stalk, shows superior binding of OMgp, Nogo-66, and MAG compared with wild-type NgR1 or NgR2. Soluble NgR(OMNI) (NgR(OMNI)-Fc) binds strongly to membrane-bound inhibitors and promotes neurite outgrowth on both MAG and CNS myelin substrates. Thus, NgR(OMNI)-Fc may offer therapeutic opportunities following nervous system injury or disease where myelin inhibits neuronal regeneration.

Phosphorylation of Nogo receptors suppresses Nogo signaling, allowing neurite regeneration

The myelin-associated proteins Nogo-A, MAG, and OMgp transmit signals from oligodendrocytes into neurons through binding to Nogo receptors. Nogo signaling has critical roles in development and maintenance of the central nervous system (CNS). It can inhibit differentiation, migration, and neurite outgrowth of neurons, causing poor recovery of the adult CNS from damage. Here, I show that phosphorylation of Nogo receptors by casein kinase II (CK2) inhibits binding of the myelin-associated proteins. Brain-derived neurotrophic factor stimulates the phosphorylation, suppressing Nogo-dependent inhibition of neurite outgrowth from neuroblastoma-derived neural cells. Similarly, in rat adult neurons, extracellular CK2 treatment overcomes inhibition of neurite outgrowth by the myelin-associated proteins. These findings provide new strategies to control Nogo signaling and hence neuronal regeneration.

Gamma-secretase represents a therapeutic target for the treatment of invasive glioma mediated by the p75 neurotrophin receptor

The multifunctional signaling protein p75 neurotrophin receptor (p75^NTR) is a central regulator and major contributor to the highly invasive nature of malignant gliomas. Here, we show that neurotrophin-dependent regulated intramembrane proteolysis (RIP) of p75^NTR is required for p75^NTR-mediated glioma invasion, and identify a previously unnamed process for targeted glioma therapy. Expression of cleavage-resistant chimeras of p75^NTR or treatment of animals bearing p75^NTR-positive intracranial tumors with clinically applicable γ-secretase inhibitors resulted in dramatically decreased glioma invasion and prolonged survival. Importantly, proteolytic processing of p75^NTR was observed in p75^NTR-positive patient tumor specimens and brain tumor initiating cells. This work highlights the importance of p75^NTR as a therapeutic target, suggesting that γ-secretase inhibitors may have direct clinical application for the treatment of malignant glioma.

Despite technical advances, clinical prognosis of patients with malignant glioma, with an average survival of less than one year, has not changed. The highly invasive nature of these tumors, together with the recently identified brain tumor-initiating cells, provide disease reservoirs that render these tumors incurable by conventional therapies. Here, we present the first evidence to our knowledge that regulated intramembrane proteolysis of the neurotrophin receptor p75^NTR is a critical regulator of glioma invasion. Inhibition of this process by clinically relevant γ-secretase inhibitors dramatically impairs the highly invasive nature of genetically distinct glioblastomas and brain tumor-initiating cells and prolongs survival. These data highlight regulated intramembrane proteolysis as a therapeutic target of malignant glioma and implicate the application of γ-secretase inhibitors in the treatment of these devastating tumors.

Gamma-secretase inhibitors in clinical trials for patients with Alzheimer disease can be used to block the highly invasive behavior of malignant glioma and prolong survival.

Role of extracellular factors in axon regeneration in the CNS: implications for therapy

The glial scar that forms after an injury to the CNS contains molecules that are inhibitory to axon growth. Understanding of the mechanisms of inhibition has allowed the development of therapeutic strategies aimed at promoting axon regeneration. Promising results have been obtained in animal models, and some therapies are undergoing clinical trials. This offers great hope for achievement of functional recovery after CNS injury.

Metalloproteases and gamma-secretase: new membrane partners regulating p75 neurotrophin receptor signaling?

Signaling by the p75 neurotrophin receptor (p75) has been implicated in diverse neuronal responses, including the control of neuronal survival versus death and axonal regeneration and growth cone collapse, involving p75 in different neuropathological conditions. There are different levels of complexity regulating p75-mediated signaling. First, p75 can interact with different ligands and co-receptors in the plasma membrane, forming tripartite complexes, whose activation result in different cellular outcomes. Moreover, it was recently described that trafficking capacities of p75 in neurons are regulating, in addition to p75 downstream interactions, also the sequential cleavage of p75. The proteolytical processing of p75 involves, first, a shedding event that releases a membrane-bound carboxiterminal fragment (p75-CTF), followed by a gamma-secretase mediated cleavage, generating a soluble intracellular domain (p75-ICD) with signaling capabilities. The first shedding event, generating a p75-CTF, is the key step to regulating the production of p75-ICD, and although the generation of p75-ICD is important for both p75-mediated control of neuronal survival and the control of neurite outgrowth, little is known how both cleavage events are regulated. In this review, we argue that both sheddases and gamma-secretase are key membrane components regulating p75-mediated signaling transduction; therefore, further attention should be paid to their roles as p75 signaling regulators.

Targeting MMPs in acute and chronic neurological conditions

The matrix metalloproteinases (MMPs) are important enzymes that regulate developmental processes, maintain normal physiology in adulthood and have reparative roles at specific stages after an insult to the nervous system. Conversely, the concordant presence and significant upregulation of several MMP members in virtually all neurological conditions result in pathology. Thus, the MMPs have diverse functions, capable of mediating repair and recovery on the one hand and being involved in producing injury on the other. Therefore, targeting MMPs in neurological conditions has become a complicated challenge. This article highlights the beneficial roles of MMPs in normal and reparative processes within the nervous system and discusses the detriments of MMPs encountered in pathology. We review the availability of MMP inhibitors for clinical use and propose that an important consideration for these inhibitors is timing and duration of their use. With acute injuries where a massive upregulation of several MMPs are observed in the early periods after the insult, early and short-term use of broad spectrum MMP inhibitors would seem logical. In chronic conditions where recurrent insults to the CNS are accompanied by prolonged upregulation of MMPs, thereby necessitating the chronic use of medications, the beneficial effects of MMPs in repair may be compromised by the long-term application of MMP inhibitors. In this review we have used spinal cord injury and multiple sclerosis as examples of acute and chronic neurological conditions, respectively, and we consider the use of MMP inhibitors in these states.

Metalloproteinases: mediators of pathology and regeneration in the CNS

The matrix metalloproteinases and related A disintegrin and metalloproteinase enzymes are implicated in various diseases of the nervous system. However, metalloproteinases are increasingly being recognized as having beneficial roles during nervous system development and following injury. This review discusses general principles that govern the expression of metalloproteinases in the nervous system and their detrimental outcomes. It then focuses on the roles of metalloproteinases and their mechanisms in regulating neurogenesis, myelin formation and axonal growth. It is clear that metalloproteinases are important determinants in enabling recovery from injury to the nervous system.

Update on the treatment of spinal cord injury

Acute spinal cord injury (SCI) is a devastating neurological disorder that can affect any individual at a given instance. Current treatment options for SCI include the use of high dose methylprednisolone sodium succinate, a corticosteroid, surgical interventions to stabilize and decompress the spinal cord, intensive multisystem medical management, and rehabilitative care. While utility of these therapeutic options provides modest benefits, there is a critical need to identify novel approaches to treat or repair the injured spinal cord in hope to, at the very least, improve upon the patient's quality of life. Thankfully, several discoveries at the preclinical level are now transitioning into the clinical arena. These include the Surgical Treatment for Acute Spinal Cord Injury Study (STASCIS) Trial to evaluate the role and timing of surgical decompression for acute SCI, neuroprotection with the semisynthetic second generation tetracycline derivative, minocycline; aiding axonal conduction with the potassium channel blockers, neuroregenerative/neuroprotective approaches with the Rho antagonist, Cethrin; the use of anti-NOGO monoclonal antibodies to augment plasticity and regeneration; as well as cell-mediated repair with stem cells, bone marrow stromal cells, and olfactory ensheathing cells. This review overviews the pathobiology of SCI and current treatment choices before focusing the rest of the discussion on the variety of promising neuroprotective and cell-based approaches that have recently moved, or are very close, to clinical testing.

Matrix metalloproteinases and proteoglycans in axonal regeneration

After an injury to the adult mammalian central nervous system (CNS), a variety of growth-inhibitory molecules are upregulated. A glial scar forms at the site of injury and is composed of numerous molecular substances, including chondroitin sulfate proteoglycans (CSPGs). These proteoglycans inhibit axonal growth in vitro and in vivo. Matrix metalloproteinases (MMPs) can degrade the core protein of some CSPGs as well as other growth-inhibitory molecules such as Nogo and tenascin-C. MMPs have been shown to facilitate axonal regeneration in the adult mammalian peripheral nervous system (PNS). This review will focus on the various roles of proteoglycans and MMPs within the injured nervous system. First, we will present a general background on the injured central nervous system and explore the roles that proteoglycans play in the injured PNS and CNS. Second, we will discuss the various functions of MMPs within the injured PNS and CNS. Special attention will be paid to the possibility of how MMPs might modify the growth-inhibitory extracellular environment of the injured adult mammalian spinal cord and facilitate axonal regeneration in the CNS.

Nogo-66 receptor at the gap junctions between pituicytes of the rat

Nogo-66 receptor was first identified in neurons. Recently, it was demonstrated in glial cells as well. Our previous study on the expression of Nogo-66 receptor in the cerebellum of the rat surprisingly found its location at the glial gap junctions. Here, we present our study on Nogo-66 receptor in the rat posterior pituitary, which is densely packed with pituicytes, a special type of astrocyte, and is known to be rich in gap junctions. We were able to demonstrate with immunohistochemistry and immuno-electron microscopy abundant Nogo-66 receptor immunoreactive gap junctions between pituicytes. This study, together with our prior one, strongly suggests that the Nogo-66 receptor may play a role in regulating the function of the gap junctions.

TACE‐induced cleavage of NgR and p75 NTR in dorsal root ganglion cultures disinhibits outgrowth and promotes branching of neurites in the presence of inhibitory CNS myelin

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.

(Make) stick and cut loose--disintegrin metalloproteases in development and disease

"A disintegrin and metalloprotease" (ADAM) proteases form a still growing family of about 40 type 1 transmembrane proteins. They are defined by a common modular ectodomain architecture that combines cell deadhesion/adhesion and fusion motifs (disintegrin and cysteine-rich domains), with a Zn-protease domain capped by a large prodomain. Their ectodomain thus strikingly resembles snake venom disintegrin proteases, which by combined integrin blocking and extracellular proteolysis, can cause extensive tissue damage after snake bites. A surprisingly large proportion (13 ADAMs) is exclusively expressed in the male gonads, and only a minority can be found throughout all tissues. As predicted by their amino acid sequence, a major proportion of this family has not maintained a functional protease domain, most probably rendering them into pure adhesion and/or fusion proteins. For most ADAMs, the respective key function has remained elusive. Despite their overall conserved ectodomain structure, ADAMs appear to be subdivided into those with a predominant role in direct adhesion (e.g., ADAMs 1, 2, and 3) and those mainly acting as proteases (e.g., ADAMs 10 and 17). Only for a few of them are functions of more than one domain documented (e.g., ADAM9 in cell fusion and proteolysis). Several ADAMs exist in both membrane-resident and secreted isoforms; the functional significance of this dichotomy is in most cases still unclear. Knockout phenotypes have been informative only in a few cases (ADAMs 1, 2, 10, 12, 15, 17, and 19) and are mainly related to their protease function. A common denominator of ADAM-mediated proteolysis is the ectodomain shedding of a broad spectrum of substrates, including paracrine growth factors like epidermal growth factor receptor (EGFR) ligands, cell adhesion molecules like CD44 or cadherins, and the initiation of regulated intramembrane proteolysis (RIP), whereby the transmembrane fragment of the respective substrate is further cleaved by an intramembrane cleaving protease to release an intracellular domain acting as a nuclear transcription regulator. Most ADAMs feature a significant overlap of substrate specificities, explaining why an inactivation of individual ADAMs only rarely causes major phenotypes.

Neurotrophic factor synergy is required for neuronal survival and disinhibited axon regeneration after CNS injury

The therapeutic effects of individual neurotrophic factors (NTF) have proved disappointing in clinical trials for neuronal repair and axon regeneration. Here, we demonstrate NTF synergistic neuronal responses after a combination of basic fibroblast growth factor, neurotrophin-3 and brain derived growth factor delivered to the somata of retinal ganglion cells promoted greater survival and axon growth than did the sum of the effects of each NTF alone. Triple and not single NTF treatments potentiated regulated intramembraneous proteolysis of p75(NTR), and ectodomain shedding of Nogo receptor, correlated with a 30% decrease in activation of Rho-A, a key signalling molecule in the axon growth inhibitory cascade. Thus, combinatorial NTF administration synergistically enhanced neuronal survival, disinhibited axon growth and promoted axon regeneration through the hostile CNS environment without the intervention of scar tissue at the lesion site.

Why do Nogo/Nogo-66 receptor gene knockouts result in inferior regeneration compared to treatment with neutralizing agents?

IN-1, the monoclonal antibody against the exon 3-encoded N-terminal domain of Nogo-A, and the Nogo-66 receptor (NgR) antagonist NEP1-40 have both shown efficacy in promoting regeneration in animal spinal cord injury models, the latter even when administered subcutaneously 1 week after injury. These results are supportive of the hypothesis that the Nogo-NgR axis is a major path for inhibition of spinal cord axonal regeneration and uphold the promises of these neutralizing agents in clinical applications. However, mice with targeted disruption of Nogo and NgR have, surprisingly, only modest regenerative capacity (if any) compared with treatment with IN-1 or NEP1-40. Disruption of the Nogo gene by various groups yielded results ranging from significant regenerative improvement in young mice to no improvement. Likewise, knockout of NgR produced some improvement in raphespinal and rubrospinal axonal regeneration, but not that of corticospinal neurons. Other than invoking possible differences in genetic background, we suggest here some possible and testable explanations for the above phenomena. These possibilities include effects of IN-1 and NEP1-40 on the CNS beyond neutralization of Nogo and NgR functions, and the latter's possible role in the CNS beyond that of neuronal growth inhibition.

The Nogo-66 receptor homolog NgR2 is a sialic acid-dependent receptor selective for myelin-associated glycoprotein

The Nogo-66 receptor (NgR1) is a promiscuous receptor for the myelin inhibitory proteins Nogo/Nogo-66, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp). NgR1, an axonal glycoprotein, is the founding member of a protein family composed of the structurally related molecules NgR1, NgR2, and NgR3. Here we show that NgR2 is a novel receptor for MAG and acts selectively to mediate MAG inhibitory responses. MAG binds NgR2 directly and with greater affinity than NgR1. In neurons NgR1 and NgR2 support MAG binding in a sialic acid-dependent Vibrio cholerae neuraminidase-sensitive manner. Forced expression of NgR2 is sufficient to impart MAG inhibition to neonatal sensory neurons. Soluble NgR2 has MAG antagonistic capacity and promotes neuronal growth on MAG and CNS myelin substrate in vitro. Structural studies have revealed that the NgR2 leucine-rich repeat cluster and the NgR2 "unique" domain are necessary for high-affinity MAG binding. Consistent with its role as a neuronal MAG receptor, NgR2 is an axonassociated glycoprotein. In postnatal brain NgR1 and NgR2 are strongly enriched in Triton X-100-insoluble lipid rafts. Neural expression studies of NgR1 and NgR2 have revealed broad and overlapping, yet distinct, distribution in the mature CNS. Taken together, our studies identify NgRs as a family of receptors (or components of receptors) for myelin inhibitors and provide insights into how interactions between MAG and members of the Nogo receptor family function to coordinate myelin inhibitory responses.

Nogo-A and nogo receptor expression in demyelinating lesions of multiple sclerosis

A myelin-associated neurite outgrowth inhibitor, Nogo-A, plays a key role in inhibition of axonal regeneration following injury and ischemia in the central nervous system (CNS). Because axonal injury is a pathologic hallmark of multiple sclerosis (MS), we have investigated the expression of Nogo-A and its receptor NgR in four MS and 12 non-MS control brains by immunohistochemistry. Nogo-A expression was markedly upregulated in surviving oligodendrocytes at the edge of chronic active demyelinating lesions of MS and ischemic lesions of acute and old cerebral infarction, whereas NgR expression was greatly enhanced in reactive astrocytes and microglia/macrophages in these lesions when compared with their expression in the brains of neurologically normal controls. Nogo-A and NgR were also identified in a subpopulation of neurons. In contrast, Nogo-A was undetectable in reactive astrocytes and microglia/macrophages and NgR was not expressed on oligodendrocytes in any cases examined. Western blot analysis and double labeling immunocytochemistry identified the constitutive expression of NgR in cultured human astrocytes. These results suggest that Nogo-A expressed on oligodendrocytes might interact with NgR presented by reactive astrocytes and microglia/macrophages in active demyelinating lesions of MS, although biologic effects caused by Nogo-A/NgR interaction among glial cells remain unknown.

Ectodomain shedding of human Nogo-66 receptor homologue-1 by zinc metalloproteinases

The Nogo-66 receptor (NgR) plays a pivotal role in the inhibition of neuroregeneration as the receptor for multiple neurite outgrowth inhibitors such as Nogo-A. We have previously shown that NgR undergoes zinc metalloproteinase-mediated ectodomain shedding in neuroblastoma cells. Here, we demonstrate that the NgR-related protein NgR homologue-1 is released from neuroblastoma cells as a full-length ectodomain (NgRH1-ecto) and an N-terminal fragment (NTF-NgRH1) containing the leucine-rich repeat region of the protein. Inhibitors of the major protease classes failed to block the release of NgRH1-ecto, suggesting that this occurs via a protease-independent mechanism, presumably by a phospholipase-like enzyme. The release of NTF-NgRH1 was blocked by a hydroxamate-based zinc metalloproteinase inhibitor and tissue inhibitor of metalloproteinases-2 and -3, but not -1, implicating the involvement of membrane-type matrix metalloproteinases in this process. Our findings thus highlight the parallels between the ectodomain shedding of NgRH1 and that previously described for NgR.