MMP-7 cleaves the NR1 NMDA receptor subunit and modifies NMDA receptor function

Extracellular Metalloproteinases in the Plasticity of Excitatory and Inhibitory Synapses

Long-term synaptic plasticity is shaped by the controlled reorganization of the synaptic proteome. A key component of this process is local proteolysis performed by the family of extracellular matrix metalloproteinases (MMPs). In recent years, considerable progress was achieved in identifying extracellular proteases involved in neuroplasticity phenomena and their protein substrates. Perisynaptic metalloproteinases regulate plastic changes at synapses through the processing of extracellular and membrane proteins. MMP9 was found to play a crucial role in excitatory synapses by controlling the NMDA-dependent LTP component. In addition, MMP3 regulates the L-type calcium channel-dependent form of LTP as well as the plasticity of neuronal excitability. Both MMP9 and MMP3 were implicated in memory and learning. Moreover, altered expression or mutations of different MMPs are associated with learning deficits and psychiatric disorders, including schizophrenia, addiction, or stress response. Contrary to excitatory drive, the investigation into the role of extracellular proteolysis in inhibitory synapses is only just beginning. Herein, we review the principal mechanisms of MMP involvement in the plasticity of excitatory transmission and the recently discovered role of proteolysis in inhibitory synapses. We discuss how different matrix metalloproteinases shape dynamics and turnover of synaptic adhesome and signal transduction pathways in neurons. Finally, we discuss future challenges in exploring synapse- and plasticity-specific functions of different metalloproteinases.

Matrix metalloproteinases shape the oligodendrocyte (niche) during development and upon demyelination

The oligodendrocyte lineage cell is crucial to proper brain function. During central nervous system development, oligodendrocyte progenitor cells (OPCs) migrate and proliferate to populate the entire brain and spinal cord, and subsequently differentiate into mature oligodendrocytes that wrap neuronal axons in an insulating myelin layer. When damage occurs to the myelin sheath, OPCs are activated and recruited to the demyelinated site, where they differentiate into oligodendrocytes that remyelinate the denuded axons. The process of OPC attraction and differentiation is influenced by a multitude of factors from the cell's niche. Matrix metalloproteinases (MMPs) are powerful and versatile enzymes that do not only degrade extracellular matrix proteins, but also cleave cell surface receptors, growth factors, signaling molecules, proteases and other precursor proteins, leading to their activation or degradation. MMPs are markedly upregulated during brain development and upon demyelinating injury, where their broad functions influence the behavior of neural progenitor cells (NPCs), OPCs and oligodendrocytes. In this review, we focus on the role of MMPs in (re)myelination. We will start out in the developing brain with describing the effects of MMPs on NPCs, OPCs and eventually oligodendrocytes. Then, we will outline their functions in oligodendrocyte process extension and developmental myelination. Finally, we will review their potential role in demyelination, describe their significance in remyelination and discuss the evidence for a role of MMPs in remyelination failure, focusing on multiple sclerosis. In conclusion, MMPs shape the oligodendrocyte (niche) both during development and upon demyelination, and thus are important players in directing the fate and behavior of oligodendrocyte lineage cells throughout their life cycle.

Effects of matrix metalloproteinase inhibitors on N-methyl-D-aspartate receptor and contribute to long-term potentiation in the anterior cingulate cortex of adult mice

Matrix metalloproteinases (MMPs) have been suggested to contribute to long-term potentiation, behavioral learning, and memory. In the dorsal horn of spinal cord, MMPs were reported to contribute to injury-related changes, and inhibitors of MMPs have been proposed as potential analgesics. However, it is unclear whether MMP inhibitors produce these effects by inhibiting the function of N-methyl-D-aspartate receptor (NMDAR), a key receptor for the induction of long-term potentiation. In this study, we wanted to examine if MMP inhibitors affect NMDAR-mediated excitatory postsynaptic currents in the anterior cingulate cortex of adult mice. Among different subtype inhibitors we used, we found that MMP-9 and MMP-2/9 inhibitors did not change NMDAR-mediated excitatory postsynaptic currents. However, MMP-3 and broad-spectrum MMP inhibitors reduced the NMDAR-mediated excitatory postsynaptic currents. Consistently, MMP-9 and MMP-2/9 inhibitors had no effect on NMDAR-dependent long-term potentiation, but MMP-3 and broad-spectrum MMP inhibitors inhibited the induction of long-term potentiation. Our results suggest that MMP inhibitors may produce their effects by inhibiting NMDAR functions in central synapses.

Matrix Metalloproteases as Influencers of the Cells' Social Media

Matrix metalloproteinases (MMPs) have been studied in the context of cancer due to their ability to increase cell invasion, and were initially thought to facilitate metastasis solely through the degradation of the extracellular matrix (ECM). MMPs have also been investigated in the context of their ECM remodeling activity in several acute and chronic inflammatory diseases. However, after several MMP inhibitors failed in phase III clinical trials, a global reassessment of their biological functions was undertaken, which has revealed multiple unanticipated functions including the processing of chemokines, cytokines, and cell surface receptors. Despite what their name suggests, the matrix aspect of MMPs could contribute to a lesser part of their physiological functions in inflammatory diseases, as originally anticipated. Here, we present examples of MMP substrates implicated in cell signaling, independent of their ECM functions, and discuss the impact for the use of MMP inhibitors.

Matrix metalloproteinases in the CNS: interferons get nervous

Matrix metalloproteinases (MMPs) have been investigated in context of chronic inflammatory diseases and demonstrated to degrade multiple components of the extracellular matrix (ECM). However, following several disappointing MMP clinical trials, recent studies have demonstrated unexpected novel functions of MMPs in viral infections and autoimmune inflammatory diseases in unanticipated locations. Thus, MMPs play additional functions in inflammation than just ECM degradation. They can regulate the activity of chemokines and cytokines of the immune response by precise proteolytic processing resulting in activation or inactivation of signaling pathways. MMPs have been demonstrated to cleave multiple substrates of the central nervous systems (CNS) and contribute to promoting and dampening diseases of the CNS. Initially, believed to be solely promoting pathologies, more than 10 MMPs to date have been shown to have protective functions. Here, we present some of the beneficial and destructive roles of MMPs in CNS pathologies and discuss strategies for the use of MMP inhibitors.

MMPs in learning and memory and neuropsychiatric disorders

Matrix metalloproteinases (MMPs) are a group of over twenty proteases, operating chiefly extracellularly to cleave components of the extracellular matrix, cell adhesion molecules as well as cytokines and growth factors. By virtue of their expression and activity patterns in animal models and clinical investigations, as well as functional studies with gene knockouts and enzyme inhibitors, MMPs have been demonstrated to play a paramount role in many physiological and pathological processes in the brain. In particular, they have been shown to influence learning and memory processes, as well as major neuropsychiatric disorders such as schizophrenia, various kinds of addiction, epilepsy, fragile X syndrome, and depression. A possible link connecting all those conditions is either physiological or aberrant synaptic plasticity where some MMPs, e.g., MMP-9, have been demonstrated to contribute to the structural and functional reorganization of excitatory synapses that are located on dendritic spines. Another common theme linking the aforementioned pathological conditions is neuroinflammation and MMPs have also been shown to be important mediators of immune responses.

Multiplex Matrix Metalloproteinases Analysis in the Cerebrospinal Fluid Reveals Potential Specific Patterns in Multiple Sclerosis Patients

Background: Matrix metalloproteinases (MMPs) are pleiotropic enzymes involved in extracellular protein degradation and turnover. MMPs are implicated in the pathogenesis of many neurological diseases, including multiple sclerosis (MS).

Objective: To search the level of MMPs in the cerebrospinal fluid (CSF) of MS patients and detect possible disease-specific patterns.

Methods: CSF samples from 32 MS patients and, from 15 control subjects with other inflammatory neurological diseases (OIND) were analyzed. The Bio-Plex Pro Human MMP 9-Plex Panel (Bio-Rad) was used for the quantification of MMP-1, MMP-2, MMP-3, MMP-7, MMP-8, MMP-9, MMP-10, MMP-12, and MMP-13.

Results: CSF MMP-1 and MMP-12 levels were significantly reduced in MS as compared with OIND. In MS patients' CSF: (i) MMP-1 levels were significantly higher in women vs. men; (ii) MMP-10 concentrations were higher in patients with CSF-restricted IgG oligoclonal bands, and (iii) MMP-7 levels were increased in patients with longer disease duration. In the OIND group MMP-7 and MMP-12 levels significantly and directly correlated with age.

Conclusions: Our study contributes to investigating the role of MMPs in MS, with regard to CSF immunological features and disease duration. Sex-specific differences were also detected in MMPs CSF levels.

New thrombolytic strategy providing neuroprotection in experimental ischemic stroke: MMP10 alone or in combination with tissue-type plasminogen activator

Aims

Early reperfusion with tissue-type plasminogen activator (tPA) is an effective therapeutic strategy to treat acute ischemic stroke, but only 1/3 of tPA-treated patients recover and are free from disability. tPA has also shown neurotoxicity in experimental models of cerebral ischemia. Considering that MMP-10 improves stroke injury, we have examined the therapeutic and protective effect of MMP10 and tPA/MMP10 as clot-dissolving and neuroprotective agent in an experimental model of ischemic stroke and studied in vitro the molecular pathways involved in MMP10-mediated effects.

Methods and results

Cerebral ischemia was induced by the local injection of thrombin into the middle cerebral artery followed by reperfusion with MMP10 (6.5 µg/kg) and tPA (10 mg/kg) alone or in combination with MMP10. Cell cultures were also performed to determine the effect of MMP10 and tPA/MMP10 on brain endothelial cells and neurons. tPA/MMP10 significantly reduced the infarct size in the ischemic stroke model compared with tPA alone (P < 0.05). In vitro, MMP10 reduced the tPA-promoted endothelial ionic permeability, preserved the expression of claudin-5 and decreased ERK1/2 activation. Moreover, combination of tPA/MMP10 prevented tPA-mediated neuronal excitotoxicity and calcium influx. These effects were reversed by blocking MMP10 activity with a monoclonal antibody.

Conclusion

These results show that MMP10, either alone or in combination with tPA, might represent a new strategy for thrombolysis in ischemic stroke, providing higher protection against cerebrovascular damage.

Multifaceted Roles of Metzincins in CNS Physiology and Pathology: From Synaptic Plasticity and Cognition to Neurodegenerative Disorders

The extracellular matrix (ECM) and membrane proteolysis play a key role in structural and functional synaptic plasticity associated with development and learning. A growing body of evidence underscores the multifaceted role of members of the metzincin superfamily, including metalloproteinases (MMPs), A Disintegrin and Metalloproteinases (ADAMs), A Disintegrin and Metalloproteinase with Thrombospondin Motifs (ADAMTSs) and astacins in physiological and pathological processes in the central nervous system (CNS). The expression and activity of metzincins are strictly controlled at different levels (e.g., through the regulation of translation, limited activation in the extracellular space, the binding of endogenous inhibitors and interactions with other proteins). Thus, unsurprising is that the dysregulation of proteolytic activity, especially the greater expression and activation of metzincins, is associated with neurodegenerative disorders that are considered synaptopathies, especially Alzheimer's disease (AD). We review current knowledge of the functions of metzincins in the development of AD, mainly the proteolytic processing of amyloid precursor protein, the degradation of amyloid β (Aβ) peptide and several pathways for Aβ clearance across brain barriers (i.e., blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB)) that contain specific receptors that mediate the uptake of Aβ peptide. Controlling the proteolytic activity of metzincins in Aβ-induced pathological changes in AD patients' brains may be a promising therapeutic strategy.

The multifaceted role of metalloproteinases in physiological and pathological conditions in embryonic and adult brains

Matrix metalloproteinases (MMPs) are a large family of ubiquitous extracellular endopeptidases, which play important roles in a variety of physiological and pathological conditions, from the embryonic stages throughout adult life. Their extraordinary physiological "success" is due to concomitant broad substrate specificities and strict regulation of their expression, activation and inhibition levels. In recent years, MMPs have gained increasing attention as significant effectors in various aspects of central nervous system (CNS) physiology. Most importantly, they have been recognized as main players in a variety of brain disorders having different etiologies and evolution. A common aspect of these pathologies is the development of acute or chronic neuroinflammation. MMPs play an integral part in determining the result of neuroinflammation, in some cases turning its beneficial outcome into a harmful one. This review summarizes the most relevant studies concerning the physiology of MMPs, highlighting their involvement in both the developing and mature CNS, in long-lasting and acute brain diseases and, finally, in nervous system repair. Recently, a concerted effort has been made in identifying therapeutic strategies for major brain diseases by targeting MMP activities. However, from this revision of the literature appears clear that MMPs have multifaceted functional characteristics, which modulate physiological processes in multiple ways and with multiple consequences. Therefore, when choosing MMPs as possible targets, great care must be taken to evaluate the delicate balance between their activation and inhibition and to determine at which stage of the disease and at what level they become active in order maximize chances of success.

The Human Ether-a-go-go-related Gene (hERG) Potassium Channel Represents an Unusual Target for Protease-mediated Damage

The human ether-a-go-go-related gene (hERG) encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium channel (IKr), which is important for cardiac repolarization. Dysfunction of hERG causes long QT syndrome and sudden death, which occur in patients with cardiac ischemia. Cardiac ischemia is also associated with activation, up-regulation, and secretion of various proteolytic enzymes. Here, using whole-cell patch clamp and Western blotting analysis, we demonstrate that the hERG/IKr channel was selectively cleaved by the serine protease, proteinase K (PK). Using molecular biology techniques including making a chimeric channel between protease-sensitive hERG and insensitive human ether-a-go-go (hEAG), as well as application of the scorpion toxin BeKm-1, we identified that the S5-pore linker of hERG is the target domain for proteinase K cleavage. To investigate the physiological relevance of the unique susceptibility of hERG to proteases, we show that cardiac ischemia in a rabbit model was associated with a reduction in mature ERG expression and an increase in the expression of several proteases, including calpain. Using cell biology approaches, we found that calpain-1 was actively released into the extracellular milieu and cleaved hERG at the S5-pore linker. Using protease cleavage-predicting software and site-directed mutagenesis, we identified that calpain-1 cleaves hERG at position Gly-603 in the S5-pore linker of hERG. Clarification of protease-mediated damage of hERG extends our understanding of hERG regulation. Damage of hERG mediated by proteases such as calpain may contribute to ischemia-associated QT prolongation and sudden cardiac death.

Matrix Metalloprotease 3 Activity Supports Hippocampal EPSP-to-Spike Plasticity Following Patterned Neuronal Activity via the Regulation of NMDAR Function and Calcium Flux

Matrix metalloproteases (MMPs) comprise a family of endopeptidases that are involved in remodeling the extracellular matrix and play a critical role in learning and memory. At least 24 different MMP subtypes have been identified in the human brain, but less is known about the subtype-specific actions of MMP on neuronal plasticity. The long-term potentiation (LTP) of excitatory synaptic transmission and scaling of dendritic and somatic neuronal excitability are considered substrates of memory storage. We previously found that MMP-3 and MMP-2/9 may be differentially involved in shaping the induction and expression of excitatory postsynaptic potential (EPSP)-to-spike (E-S) potentiation in hippocampal brain slices. MMP-3 and MMP-2/9 proteolysis was previously shown to affect the integrity or mobility of synaptic N-methyl-d-aspartate receptors (NMDARs) in vitro. However, the functional outcome of such MMP-NMDAR interactions remains largely unknown. The present study investigated the role of these MMP subtypes in E-S plasticity and NMDAR function in mouse hippocampal acute brain slices. The temporal requirement for MMP-3/NMDAR activity in E-S potentiation within the CA1 field largely overlapped, and MMP-3 but not MMP-2/9 activity was crucial for the gain-of-function of NMDARs following LTP induction. Functional changes in E-S plasticity following MMP-3 inhibition largely correlated with the expression of cFos protein, a marker of activity-related gene transcription. Recombinant MMP-3 promoted a gain in NMDAR-mediated field potentials and somatodendritic Ca²⁺ waves. These results suggest that long-term hippocampal E-S potentiation requires transient MMP-3 activity that promotes NMDAR-mediated postsynaptic Ca²⁺ entry that is vital for the activation of downstream signaling cascades and gene transcription.

Two classes of matrix metalloproteinases reciprocally regulate synaptogenesis

Synaptogenesis requires orchestrated intercellular communication between synaptic partners, with trans-synaptic signals necessarily traversing the extracellular synaptomatrix separating presynaptic and postsynaptic cells. Extracellular matrix metalloproteinases (Mmps) regulated by secreted tissue inhibitors of metalloproteinases (Timps), cleave secreted and membrane-associated targets to sculpt the extracellular environment and modulate intercellular signaling. Here, we test the roles of Mmp at the neuromuscular junction (NMJ) model synapse in the reductionist Drosophila system, which contains just two Mmps (secreted Mmp1 and GPI-anchored Mmp2) and one secreted Timp. We found that all three matrix metalloproteome components co-dependently localize in the synaptomatrix and show that both Mmp1 and Mmp2 independently restrict synapse morphogenesis and functional differentiation. Surprisingly, either dual knockdown or simultaneous inhibition of the two Mmp classes together restores normal synapse development, identifying a reciprocal suppression mechanism. The two Mmp classes co-regulate a Wnt trans-synaptic signaling pathway modulating structural and functional synaptogenesis, including the GPI-anchored heparan sulfate proteoglycan (HSPG) Wnt co-receptor Dally-like protein (Dlp), cognate receptor Frizzled-2 (Frz2) and Wingless (Wg) ligand. Loss of either Mmp1 or Mmp2 reciprocally misregulates Dlp at the synapse, with normal signaling restored by co-removal of both Mmp classes. Correcting Wnt co-receptor Dlp levels in both Mmp mutants prevents structural and functional synaptogenic defects. Taken together, these results identify an Mmp mechanism that fine-tunes HSPG co-receptor function to modulate Wnt signaling to coordinate synapse structural and functional development.

Extracellular proteolysis in structural and functional plasticity of mossy fiber synapses in hippocampus

Brain is continuously altered in response to experience and environmental changes. One of the underlying mechanisms is synaptic plasticity, which is manifested by modification of synapse structure and function. It is becoming clear that regulated extracellular proteolysis plays a pivotal role in the structural and functional remodeling of synapses during brain development, learning and memory formation. Clearly, plasticity mechanisms may substantially differ between projections. Mossy fiber synapses onto CA3 pyramidal cells display several unique functional features, including pronounced short-term facilitation, a presynaptically expressed long-term potentiation (LTP) that is independent of NMDAR activation, and NMDA-dependent metaplasticity. Moreover, structural plasticity at mossy fiber synapses ranges from the reorganization of projection topology after hippocampus-dependent learning, through intrinsically different dynamic properties of synaptic boutons to pre- and postsynaptic structural changes accompanying LTP induction. Although concomitant functional and structural plasticity in this pathway strongly suggests a role of extracellular proteolysis, its impact only starts to be investigated in this projection. In the present report, we review the role of extracellular proteolysis in various aspects of synaptic plasticity in hippocampal mossy fiber synapses. A growing body of evidence demonstrates that among perisynaptic proteases, tissue plasminogen activator (tPA)/plasmin system, β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) and metalloproteinases play a crucial role in shaping plastic changes in this projection. We discuss recent advances and emerging hypotheses on the roles of proteases in mechanisms underlying mossy fiber target specific synaptic plasticity and memory formation.

Diverse impact of acute and long-term extracellular proteolytic activity on plasticity of neuronal excitability

Learning and memory require alteration in number and strength of existing synaptic connections. Extracellular proteolysis within the synapses has been shown to play a pivotal role in synaptic plasticity by determining synapse structure, function, and number. Although synaptic plasticity of excitatory synapses is generally acknowledged to play a crucial role in formation of memory traces, some components of neural plasticity are reflected by nonsynaptic changes. Since information in neural networks is ultimately conveyed with action potentials, scaling of neuronal excitability could significantly enhance or dampen the outcome of dendritic integration, boost neuronal information storage capacity and ultimately learning. However, the underlying mechanism is poorly understood. With this regard, several lines of evidence and our most recent study support a view that activity of extracellular proteases might affect information processing in neuronal networks by affecting targets beyond synapses. Here, we review the most recent studies addressing the impact of extracellular proteolysis on plasticity of neuronal excitability and discuss how enzymatic activity may alter input-output/transfer function of neurons, supporting cognitive processes. Interestingly, extracellular proteolysis may alter intrinsic neuronal excitability and excitation/inhibition balance both rapidly (time of minutes to hours) and in long-term window. Moreover, it appears that by cleavage of extracellular matrix (ECM) constituents, proteases may modulate function of ion channels or alter inhibitory drive and hence facilitate active participation of dendrites and axon initial segments (AISs) in adjusting neuronal input/output function. Altogether, a picture emerges whereby both rapid and long-term extracellular proteolysis may influence some aspects of information processing in neurons, such as initiation of action potential, spike frequency adaptation, properties of action potential and dendritic backpropagation.

A Metabotropic-Like Flux-Independent NMDA Receptor Regulates Ca2+ Exit from Endoplasmic Reticulum and Mitochondrial Membrane Potential in Cultured Astrocytes

Astrocytes were long thought to be only structural cells in the CNS; however, their functional properties support their role in information processing and cognition. The ionotropic glutamate N-methyl D-aspartate (NMDA) receptor (NMDAR) is critical for CNS functions, but its expression and function in astrocytes is still a matter of research and debate. Here, we report immunofluorescence (IF) labeling in rat cultured cortical astrocytes (rCCA) of all NMDAR subunits, with phenotypes suggesting their intracellular transport, and their mRNA were detected by qRT-PCR. IF and Western Blot revealed GluN1 full-length synthesis, subunit critical for NMDAR assembly and transport, and its plasma membrane localization. Functionally, we found an iCa2+ rise after NMDA treatment in Fluo-4-AM labeled rCCA, an effect blocked by the NMDAR competitive inhibitors D(-)-2-amino-5-phosphonopentanoic acid (APV) and Kynurenic acid (KYNA) and dependent upon GluN1 expression as evidenced by siRNA knock down. Surprisingly, the iCa2+ rise was not blocked by MK-801, an NMDAR channel blocker, or by extracellular Ca2+ depletion, indicating flux-independent NMDAR function. In contrast, the IP3 receptor (IP3R) inhibitor XestosponginC did block this response, whereas a Ryanodine Receptor inhibitor did so only partially. Furthermore, tyrosine kinase inhibition with genistein enhanced the NMDA elicited iCa2+ rise to levels comparable to those reached by the gliotransmitter ATP, but with different population dynamics. Finally, NMDA depleted the rCCA mitochondrial membrane potential (mΔψ) measured with JC-1. Our results demonstrate that rCCA express NMDAR subunits which assemble into functional receptors that mediate a metabotropic-like, non-canonical, flux-independent iCa2+ increase.

Time dependent integration of matrix metalloproteinases and their targeted substrates directs axonal sprouting and synaptogenesis following central nervous system injury

Over the past two decades, many investigators have reported how extracellular matrix molecules act to regulate neuroplasticity. The majority of these studies involve proteins which are targets of matrix metalloproteinases. Importantly, these enzyme/substrate interactions can regulate degenerative and regenerative phases of synaptic plasticity, directing axonal and dendritic reorganization after brain insult. The present review first summarizes literature support for the prominent role of matrix metalloproteinases during neuroregeneration, followed by a discussion of data contrasting adaptive and maladaptive neuroplasticity that reveals time-dependent metalloproteinase/substrate regulation of postinjury synaptic recovery. The potential for these enzymes to serve as therapeutic targets for enhanced neuroplasticity after brain injury is illustrated with experiments demonstrating that metalloproteinase inhibitors can alter adaptive and maladaptive outcome. Finally, the complexity of metalloproteinase role in reactive synaptogenesis is revealed in new studies showing how these enzymes interact with immune molecules to mediate cellular response in the local regenerative environment, and are regulated by novel binding partners in the brain extracellular matrix. Together, these different examples show the complexity with which metalloproteinases are integrated into the process of neuroregeneration, and point to a promising new angle for future studies exploring how to facilitate brain plasticity.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

Long-term depression-inducing stimuli promote cleavage of the synaptic adhesion molecule NGL-3 through NMDA receptors, matrix metalloproteinases and presenilin/γ-secretase

Long-term depression (LTD) reduces the functional strength of excitatory synapses through mechanisms that include the removal of AMPA glutamate receptors from the postsynaptic membrane. LTD induction is also known to result in structural changes at excitatory synapses, including the shrinkage of dendritic spines. Synaptic adhesion molecules are thought to contribute to the development, function and plasticity of neuronal synapses largely through their trans-synaptic adhesions. However, little is known about how synaptic adhesion molecules are altered during LTD. We report here that NGL-3 (netrin-G ligand-3), a postsynaptic adhesion molecule that trans-synaptically interacts with the LAR family of receptor tyrosine phosphatases and intracellularly with the postsynaptic scaffolding protein PSD-95, undergoes a proteolytic cleavage process. NGL-3 cleavage is induced by NMDA treatment in cultured neurons and low-frequency stimulation in brain slices and requires the activities of NMDA glutamate receptors, matrix metalloproteinases (MMPs) and presenilin/γ-secretase. These results suggest that NGL-3 is a novel substrate of MMPs and γ-secretase and that NGL-3 cleavage may regulate synaptic adhesion during LTD.

A prospective evaluation of the temporal matrix metalloproteinase response after severe traumatic brain injury in humans

Abstract Accumulating pre-clinical data suggests that matrix metalloproteinase (MMP) expression plays a critical role in the pathophysiology of secondary brain injury. We conducted a prospective multimodal monitoring study in order to characterize the temporal MMP response after severe traumatic brain injury (TBI) in eight critically ill humans and its relationship with outcomes. High-cutoff, cerebral microdialysis (n=8); external ventricular drainage (n=3); and arterial and jugular venous bulb catheters were used to collect microdialysate, cerebrospinal fluid, and arterial and jugular bulb blood over 6 days. Levels of MMP-8 and -9 were initially high in microdialysate and then gradually declined over time. After these MMPs decreased, a spike in the microdialysate levels of MMP-2 and -3 occurred, followed by a gradual rise in the microdialysate concentration of MMP-7. Use of generalized estimating equations suggested that MMP-8 concentration in microdialysate was associated with mortality (p=0.019) and neurological outcome at hospital discharge (p=0.013). Moreover, the mean microdialysate concentration of MMP-8 was 2.4-fold higher among those who died after severe TBI than in those who survived. Mean microdialysate levels of MMP-8 also rose with increasing intracranial pressure (ICP), whereas those of MMP-7 decreased with increasing cerebral perfusion pressure (CPP). Significant changes in the mean microdialysate concentrations of MMP-1, -2, -3, and -9 and MMP-1, -2, -3, -7, and -9 also occurred with increases in microdialysate glucose and the lactate/pyruvate ratio, respectively. These results imply that monitoring of MMPs following severe TBI in humans is feasible, and that their expression may be associated with clinical outcomes, ICP, CPP, and cerebral metabolism.

Matrix metalloproteinases regulate the formation of dendritic spine head protrusions during chemically induced long-term potentiation

Dendritic spines are are small membranous protrusions that extend from neuronal dendrites and harbor the majority of excitatory synapses. Increasing evidence has shown that matrix metalloproteinases (MMPs), a family of extracellularly acting and Zn²⁺-dependent endopeptidases, are able to rapidly modulate dendritic spine morphology. Spine head protrusions (SHPs) are filopodia-like processes that extend from the dendritic spine head, representing a form of postsynaptic structural remodeling in response to altered neuronal activity. Herein, we show that chemically induced long-term potentiation (cLTP) in dissociated hippocampal cultures upregulates MMP-9 activity that controls the formation of SHPs. Blocking of MMPs activity or microtubule dynamics abolishes the emergence of SHPs. In addition, autoactive recombinant MMP-9, promotes the formation of SHPs in organotypic hippocampal slices. Furthermore, spines with SHPs gained postsynaptic α-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) receptors upon cLTP and the synaptic delivery of AMPA receptors was controlled by MMPs. The present results strongly imply that MMP-9 is functionally involved in the formation of SHPs and the control of postsynaptic receptor distribution upon cLTP.

Mitochondrial impairment induced by 3-nitropropionic acid is enhanced by endogenous metalloprotease activity inhibition in cultured rat striatal neurons

Metalloproteases from the metzincin family mediate molecule processing at the cell membrane termed ectodomain shedding (ES). This mechanism enables the generation of intracellular and extracellular fragments from cell membrane molecules that exert additional functions involved in cell processes including cell death, beyond those of full length molecules. Micotoxin 3-nitropropionic acid (3-NP) induces striatal neuronal degeneration in vivo and in vitro through mitochondrial complex II inhibition. In this study, we hypothesized that metalloproteases regulate mitochondrial activity in cultured rat striatal neurons undergoing degeneration. To test this idea, striatal neuronal cultures characterized by NeuN and GAD-67 expression were treated with 3-NP together with the metalloprotease inhibitor GM6001 and their mitochondrial activity was evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Our results showed that metalloprotease inhibition potentiated mitochondrial activity impairment induced by 3-NP whereas the inhibitor alone had no effect. These results indicate that metalloproteases regulate and promote mitochondrial functionality in striatal neurons undergoing degeneration induced by 3-NP. Since NMDA receptor is involved in the excitotoxic neuronal death triggered by 3-NP and is known to undergo ES, we analyzed NMDAR subunit NR1 phenotypic distribution by immunofluorescence. 3-NP and GM6001 induced abnormal perinuclear NR1 accumulation that was not observed with 3-NP or GM6001 alone. This observation suggests that metalloproteases are involved in NR1 cellular reorganization induced by 3-NP, and that their inhibition results in abnormal NR1 distribution. Together results indicate that endogenous metalloproteases are activated during striatal neurodegeneration induced by 3-NP eliciting an adaptative or compensatory response that protects mitochondrial functionality.

Synaptic circuit remodelling by matrix metalloproteinases in health and disease

Matrix metalloproteinases (MMPs) are extracellularly acting enzymes that have long been known to have deleterious roles in brain injury and disease. In particular, widespread and protracted MMP activity can contribute to neuronal loss and synaptic dysfunction. However, recent studies show that rapid and focal MMP-mediated proteolysis proactively drives synaptic structural and functional remodelling that is crucial for ongoing cognitive processes. Deficits in synaptic remodelling are associated with psychiatric and neurological disorders, and aberrant MMP expression or function may contribute to the molecular mechanisms underlying these deficits. This Review explores the paradigm shift in our understanding of the contribution of MMPs to normal and abnormal synaptic plasticity and function.

Selected statins produce rapid spinal motor neuron loss in vitro

BACKGROUND

Hmg-CoA reductase inhibitors (statins) are widely used to prevent disease associated with vascular disease and hyperlipidemia. Although side effects are uncommon, clinical observations suggest statin exposure may exacerbate neuromuscular diseases, including peripheral neuropathy and amyotrophic lateral sclerosis. Although some have postulated class-effects, prior studies of hepatocytes and myocytes indicate that the statins may exhibit differential effects. Studies of neuronal cells have been limited.

METHODS

We examined the effects of statins on cultured neurons and Schwann cells. Cultured spinal motor neurons were grown on transwell inserts and assessed for viability using immunochemical staining for SMI-32. Cultured cortical neurons and Schwann cells were assessed using dynamic viability markers.

RESULTS

7 days of exposure to fluvastatin depleted spinal motor neurons in a dose-dependent manner with a KD of < 2 μM. Profound neurite loss was observed after 4 days exposure in culture. Other statins were found to produce toxic effects at much higher concentrations. In contrast, no such toxicity was observed for cultured Schwann cells or cortical neurons.

CONCLUSIONS

It is known from pharmacokinetic studies that daily treatment of young adults with fluvastatin can produce serum levels in the single micromolar range. We conclude that specific mechanisms may explain neuromuscular disease worsening with statins and further study is needed.

Cleavage of the NR2B subunit amino terminus of N-methyl-D-aspartate (NMDA) receptor by tissue plasminogen activator: identification of the cleavage site and characterization of ifenprodil and glycine affinities on truncated NMDA receptor

Thrombolysis using tissue plasminogen activator (tPA) has been the key treatment for patients with acute ischemic stroke for the past decade. Recent studies, however, suggest that this clot-busting protease also plays various roles in brain physiological and pathophysiological glutamatergic-dependent processes, such as synaptic plasticity and neurodegeneration. In addition, increasing evidence implicates tPA as an important neuromodulator of the N-methyl-d-aspartate (NMDA) receptors. Here, we demonstrate that recombinant human tPA cleaves the NR2B subunit of NMDA receptor. Analysis of NR2B in rat brain lysates and cortical neurons treated with tPA revealed concentration- and time-dependent degradation of NR2B proteins. Peptide sequencing studies performed on the cleaved-off products obtained from the tPA treatment on a recombinant fusion protein of the amino-terminal domain of NR2B revealed that tPA-mediated cleavage occurred at arginine 67 (Arg(67)). This cleavage is tPA-specific, plasmin-independent, and removes a predicted ~4-kDa fragment (Arg(27)-Arg(67)) from the amino-terminal domain of the NR2B protein. Site-directed mutagenesis of putative cleavage site Arg(67) to Ala(67) impeded tPA-mediated degradation of recombinant protein. This analysis revealed that NR2B is a novel substrate of tPA and suggested that an Arg(27)-Arg(67)-truncated NR2B-containing NMDA receptor could be formed. Heterologous expression of NR2B with Gln(29)-Arg(67) deleted is functional but exhibits reduced ifenprodil inhibition and increased glycine EC(50) with no change in glutamate EC(50). Our results confirmed NR2B as a novel proteolytic substrate of tPA, where tPA may directly interact with NR2B subunits leading to a change in pharmacological properties of NR2B-containing NMDA receptors.

Matrix metalloproteinase-9 and -2 enhance the ligand sensitivity of photoreceptor cyclic nucleotide-gated channels

Photoreceptor cyclic nucleotide-gated (CNG) channels are the principal ion channels responsible for transduction of the light-induced change in cGMP concentration into an electrical signal. The ligand sensitivity of photoreceptor CNG channels is subject to regulation by intracellular signaling effectors, including calcium-calmodulin, tyrosine kinases and phosphoinositides. Little is known, however, about regulation of channel activity by modification to extracellular regions of CNG channel subunits. Extracellular proteases MMP9 and -2 are present in the interphotoreceptor matrix adjacent to photoreceptor outer segments. Given that MMPs have been implicated in retinal dysfunction and degeneration, we hypothesized that MMP activity may alter the functional properties of photoreceptor CNG channels. For heterologously expressed rod and cone CNG channels, extracellular exposure to MMPs dramatically increased the apparent affinity for cGMP and the efficacy of cAMP. These changes to ligand sensitivity were not prevented by destabilization of the actin cytoskeleton or by disruption of integrin mediated cell adhesion, but could be attenuated by inhibition of MMP catalytic activity. MMP-mediated gating changes exhibited saturable kinetic properties consistent with enzymatic processing of the CNG channels. In addition, exposure to MMPs decreased the abundance of full-length expressed CNGA3 subunits, with a concomitant increase in putative degradation products. Similar gating effects and apparent proteolysis were observed also for native rod photoreceptor CNG channels. Furthermore, constitutive apparent proteolysis of retinal CNGA1 and retinal MMP9 levels were both elevated in aged mice compared with young mice. Together, these results provide evidence that MMP-mediated proteolysis can regulate the ligand sensitivity of CNG channels.

Matrix metalloproteinase-9 controls proliferation of NG2+ progenitor cells immediately after spinal cord injury

We have demonstrated that overcoming matrix metalloproteinase (MMP)-mediated suppression of glial proliferation stimulates axonal regeneration in the peripheral nervous system. The regenerative capacity of the adult CNS in response to injury and demyelination depends on the ability of multipotent glial NG2+ progenitor cells to proliferate and mature, mainly into oligodendrocytes. Herein, we have established the important role of MMPs, specifically MMP-9, in regulation of NG2+ cell proliferation in injured spinal cord. Targeting transiently induced MMP-9 using acute MMP-9/2 inhibitor (SB-3CT) therapy for two days after T9-10 spinal cord dorsal hemisection produced a significant increase in mitosis (assessed by bromodeoxyuridine incorporation) of NG2+ cells but not GFAP+astrocytes and Iba-1+ microglia and/or macrophages. Acute MMP-9/2 blockade reduced the shedding of the NG2 proteoglycan and of the NR1 subunit of the N-methyl D-aspartate (NMDA) receptor, whose decline is believed to accompany NG2+ cell maturation into OLs. Increase in post-mitotic oligodendrocytes during remyelination and improved myelin neuropathology in the hemisected spinal cord were accompanied by locomotion and somatosensory recovery after acute MMP-9/2 inhibition. Collectively, these data establish a novel role for MMPs in regulation of NG2+ cell proliferation in the damaged CNS, and a long-term benefit of acute MMP-9 block after SCI.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Matrix metalloproteinase-dependent shedding of intercellular adhesion molecule-5 occurs with long-term potentiation

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that can be released or activated in a neuronal activity dependent manner. Although pathologically elevated levels of MMPs may be synaptotoxic, physiologically appropriate levels of MMPs may instead enhance synaptic transmission. MMP inhibitors can block long term potentiation (LTP), and at least one family member can affect an increase in the volume of dendritic spines. While the mechanism by which MMPs affect these changes is not completely understood, one possibility is that the cleavage of specific synaptic cell adhesion molecules plays a role. In the present study, we have examined the ability of neuronal activity to stimulate rapid MMP dependent shedding of the intercellular adhesion molecule-5 (ICAM-5), a synaptic adhesion molecule that is thought to inhibit the maturation and enlargement of dendritic spines. Since such cleavage would likely occur within minutes if it were relevant to a process such as LTP, we focused on post stimulus time points of 30 min or less. We show that NMDA can stimulate rapid shedding of ICAM-5 from cortical neurons in dissociated cell cultures and that such shedding is diminished by pretreatment of cultures with inhibitors that target MMP-3 and -9, proteases thought to influence synaptic plasticity. Additional studies suggest that MMP mediated cleavage of ICAM-5 occurs at amino acid 780, so that the major portion of the ectodomain is released. Since reductions in ICAM-5 have been linked to changes in dendritic spine morphology that are associated with LTP, we also examined the possibility that MMP dependent ICAM-5 shedding occurs following high frequency tetanic stimulation of murine hippocampal slices. Results show that the shedding of ICAM-5 occurs in association with LTP, and that both LTP and the associated ICAM-5 shedding are reduced when slices are pretreated with an MMP inhibitor. Together, these findings suggest that neuronal activity is linked to the shedding of a molecule that may inhibit dendritic spine enlargement and that MMPs can affect this change. While further studies will be necessary to determine the extent to which cleavage of ICAM-5 in particular contributes to MMP dependent LTP, our data support an emerging body of literature suggesting that MMPs are critical mediators of synaptic plasticity.

Matrix metalloproteinases, synaptic injury, and multiple sclerosis

Multiple sclerosis (MS) is a disease of the central nervous system in which immune mediated damage to myelin is characteristic. For an overview of this condition and its pathophysiology, please refer to one of many excellent published reviews (Sorensen and Ransohoff, 1998; Weiner, 2009). To follow, is a discussion focused on the possibility that synaptic injury occurs in at least a subset of patients, and that matrix metalloproteinases (MMPs) play a role in such.

Activation of microglia by neuronal activity: results from a new in vitro paradigm based on neuronal-silicon interfacing technology

Cognition and behavior primarily arise from the communication that occurs between brain cells. By using photoconductive stimulation to trigger localized regions of neuronal action potentials and astrocyte Ca(2+) waves in dissociated rat hippocampal cultures, we can directly study microglia behavior in response to physiological and pathological levels of activity. Connections between neurons can be modified by microglia, which regulate gap junctions and synapses through secretion of proteins such as cytokines, proteases and neurotrophic factors. Activated microglia participate in bidirectional communication with the excitable tissues that they support. Through feedback from the many ion channels and surface receptors they express, microglia are informed of neuronal and astrocytic activity that may indicate disruption in the homeostasis of the CNS. Such disturbances alert microglia to locations of such activity and promote their transformation into a reactive state, in which they perform adaptive functions that can be either neuroprotective, neurotoxic, or neuromodulatory. Under physiological conditions, normal brain activity has the effect of suppressing microglia inflammatory responses. This report summarizes available data about the interaction of microglia and brain activity and presents a new in vitro paradigm to study the mechanisms involved. We propose that photoconductive stimulation is a powerful tool for studying the cellular and molecular mechanisms underlying the dynamic interactions between neurons, astrocytes and microglia.

Adiponectin Receptor-1 C-Terminal Fragment (CTF) in Plasma: Putative Biomarker for Diabetes

Introduction

Polypeptide fragments from cell surface receptors when found in plasma may be indicators of receptor regulation in disease conditions. It is known that subjects with diabetes have significantly lower plasma concentrations of adiponectin, a hormone released by adipose tissue, compared with nondiabetic controls. This hormone interacts with cell surface receptors in muscle (AdipoR1) and liver (AdipoR2).

Methods

We analyzed the relative distribution of specific fragments of AdipoR1 in healthy and diabetic individuals using an immunoaffinity mass spectrometry approach. We used antibodies raised against AdipoR1 immobilized on pre-activated protein chip surfaces to determine the molecular weights of bound polypeptide fragments using immunomass spectrometry (immuno-MS).

Results

Initially, immuno-MS analyses using a polyclonal antibody revealed two peaks (m/z 3,902 and 7,812) in plasma from normal, healthy individuals (n = 5) that were not present in the plasma of diabetics (n = 5). To confirm the detection of these fragments, a monoclonal antibody was developed against the last 25 amino acids of the AdipoR1 C-terminal fragment (CTF). Using the immuno-MS method, the monoclonal antibody detected the AdipoR1 CTF (m/z 3475) in all healthy controls (n = 10), but did not detect these fragments in the diabetic patients (n = 10).

Discussion

These preliminary observations suggest that the plasma levels of this receptor fragment may serve as an indicator of diabetic condition.

An endoplasmic reticulum retention signal located in the extracellular amino-terminal domain of the NR2A subunit of N-Methyl-D-aspartate receptors

N-Methyl-d-aspartate (NMDA) receptors play critical roles in complex brain functions as well as pathogenesis of neurodegenerative diseases. There are many NMDA isoforms and subunit types that, together with subtype-specific assembly, give rise to significant functional heterogeneity of NMDA receptors. Conventional NMDA receptors are obligatory heterotetramers composed of two glycine-binding NR1 subunits and two glutamate-binding NR2 subunits. When individually expressed in heterogeneous cells, most of the NR1 splice variants and the NR2 subunits remain in the endoplasmic reticulum (ER) and do not form homomeric channels. The mechanisms underlying NMDA receptor trafficking and functional expression remain uncertain. Using truncated and chimeric NMDA receptor subunits expressed in heterogeneous cells and hippocampal neurons, together with immunostaining, biochemical, and functional analyses, we found that the NR2A amino-terminal domain (ATD) contains an ER retention signal, which can be specifically masked by the NR1a ATD. Interestingly, no such signal was found in the ATD of the NR2B subunit. We further identified the A2 segment of the NR2A ATD to be the primary determinant of ER retention. These findings indicate that NR2A-containing NMDA receptors may undergo a different ER quality control process from NR2B-containing NMDA receptors.

Matrix metalloproteinase-9 controls NMDA receptor surface diffusion through integrin beta1 signaling

Matrix metalloproteinase-9 (MMP-9) has emerged as a physiological regulator of NMDA receptor (NMDAR)-dependent synaptic plasticity and memory. The pathways by which MMP-9 affects NMDAR signaling remain, however, elusive. Using single quantum dot tracking, we demonstrate that MMP-9 enzymatic activity increases NR1-NMDAR surface trafficking but has no influence on AMPA receptor mobility. The mechanism of MMP-9 action on NMDAR is not mediated by change in overall extracellular matrix structure nor by direct cleavage of NMDAR subunits, but rather through an integrin beta1-dependent pathway. These findings describe a new target pathway for MMP-9 action in key physiological and pathological brain processes.

Inhibition of matrix metalloproteinase activity disrupts reconsolidation but not consolidation of a fear memory

The reconsolidation hypothesis posits that memories that have been reactivated can be either enhanced or disrupted by pharmacological manipulation. Synaptic plasticity is presumed to underlie the reconsolidation process. Matrix metalloproteinases are proteins that regulate the extracellular matrix involved in plasticity events, and these proteins have recently been shown to influence learning and memory. However, all studies on the role of matrix metalloproteinases in learning and memory have employed tasks that rely on contextual cues. The goal of this study was to determine the extent to which FN-439 would disrupt the consolidation and/or reconsolidation of a fear memory associated with a conditioned stimulus that signaled tone-shock pairings and that was independent of contextual cues. Male Sprague-Dawley rats were given infusions of FN-439 (35 microg intracerebroventricular) 30 min prior to conditioning (tone-shock paired association) or 30 min prior to a single reactivation session given 24h after conditioning. Administration of FN-439 did not disrupt consolidation of the freezing response when the tone (conditioned stimulus) was presented. In contrast, FN-439 infusion disrupted reconsolidation of the fear memory in a reactivation-dependent manner. The reduced freezing behavior was not due to a decrease in general anxiety levels, since FN-439 had no effect on the percent of open-arm time or open-arm entries in an elevated-plus maze task. Thus, we demonstrated for the first time that matrix metalloproteinase inhibition in the brain is capable of disrupting the reconsolidation of a tone-shock association memory that does not depend on contextual cues. The finding that a fear response to a previously paired conditioned stimulus can be disrupted by treatment with an MMP inhibitor during a single reactivation session suggests that this class of compounds may have therapeutic potential for posttraumatic stress disorder and/or simple phobias.