Activity-induced synaptic capture and exocytosis of the neuronal serine protease neurotrypsin

The role of agrin in synaptic development, plasticity and signaling in the central nervous system

Development of the neuromuscular junction (NMJ) requires secretion of specific isoforms of the proteoglycan agrin by motor neurons. Secreted agrin is widely expressed in the basal lamina of various tissues, whereas a transmembrane form is highly expressed in the brain. Expression in the brain is greatest during the period of synaptogenesis, but remains high in regions of the adult brain that show extensive synaptic plasticity. The well-established role of agrin in NMJ development and its presence in the brain elicited investigations of its possible role in synaptogenesis in the brain. Initial studies on the embryonic brain and neuronal cultures of agrin-null mice did not reveal any defects in synaptogenesis. However, subsequent studies in culture demonstrated inhibition of synaptogenesis by agrin antisense oligonucleotides or agrin siRNA. More recently, a substantial loss of excitatory synapses was found in the brains of transgenic adult mice that lacked agrin expression everywhere but in motor neurons. The mechanisms by which agrin influences synapse formation, maintenance and plasticity may include enhancement of excitatory synaptic signaling, activation of the "muscle-specific" receptor tyrosine kinase (MuSK) and positive regulation of dendritic filopodia. In this article I will review the evidence that agrin regulates synapse development, plasticity and signaling in the brain and discuss the evidence for the proposed mechanisms.

The serine protease subtilisin suppresses epileptiform activity in rat hippocampal slices and neocortex in vivo

Serine proteases of the S8A family and those belonging to the subtilase group generate a long-lasting inhibition of hippocampal evoked potentials, which shows little recovery and resembles long-term depression. The present work investigates the effects of subtilisin A on epileptiform activity induced in hippocampal slices. Interictal bursts were generated by perfusion with 4-aminopyridine in magnesium-free medium, whereas ictal bursts were produced by the addition of baclofen. Subtilisin A superfused for 10 min at concentrations of 50 nM and above reduced the duration of ictal bursts, whereas higher concentrations reduced the frequency of interictal activity with little or no recovery, indicating similarity with the long-term depression reported previously. The anti-epileptiform activity was not prevented by inhibitors of phosphatases or several kinases, but the inhibition of ictal activity was selectively reduced by the tyrosine kinase inhibitor genistein. The rho-activated coiled-coil kinase (ROCK) inhibitor Y-27632 had no effect on the suppression of ictal or interictal bursts. Subtilisin applied at nanomolar concentrations to the surface of the cerebral cortex in vivo also suppressed epileptiform spikes induced by bicuculline. It is concluded that serine proteases of the subtilase group are highly potent inhibitors of epileptiform activity, especially ictal bursts, and that tyrosine kinases may be involved in that inhibition. The mechanism of inhibition is different from the long-lasting depression of evoked potentials, which is partly mediated via ROCK.

Activity-dependent plasticity and gene expression modifications in the adult CNS

Information processing, memory formation, or functional recovery after nervous system damage depend on the ability of neurons to modify their functional properties or their connections. At the cellular/molecular level, structural modifications of neural circuits are finely regulated by intrinsic neuronal properties and growth-regulatory cues in the extracellular milieu. Recently, it has become clear that stimuli coming from the external world, which comprise sensory inflow, motor activity, cognitive elaboration, or social interaction, not only provide the involved neurons with instructive information needed to shape connection patterns to sustain adaptive function, but also exert a powerful influence on intrinsic and extrinsic growth-related mechanisms, so to create permissive conditions for neuritic remodeling. Here, we present an overview of recent findings concerning the effects of experience on molecular mechanisms underlying CNS structural plasticity, both in physiological conditions and after damage, with particular focus on activity-dependent modulation of growth-regulatory genes and epigenetic modifications.

Extrasynaptic vesicle recycling in mature hippocampal neurons

Fast neuronal signalling relies on highly regulated vesicle fusion and recycling at specialized presynaptic terminals. Recently, examples of non-classical neurotransmission have also been reported, where fusion of vesicles can occur at sites remote from conventional synapses. This has potentially broad biological implications, but the underlying mechanisms are not well established. Here we show that a complete vesicle recycling pathway can occur at discrete axonal sites in mature hippocampal neurons and that extrasynaptic fusion is a robust feature of native tissue. We demonstrate that laterally mobile vesicle clusters trafficking between synaptic terminals become transiently stabilized by evoked action potentials and undergo complete but delayed Ca(2+)-dependent fusion along axons. This fusion is associated with dynamic actin accumulation and, subsequently, vesicles can be locally recycled, re-acidified and re-used. Immunofluorescence and ultrastructural work demonstrates that extrasynaptic fusion sites can have apposed postsynaptic specializations, suggesting that mobile vesicle recycling may underlie highly dynamic neuron-neuron communication.

Casting a net on dendritic spines: the extracellular matrix and its receptors

Dendritic spines are dynamic structures that accommodate the majority of excitatory synapses in the brain and are influenced by extracellular signals from presynaptic neurons, glial cells, and the extracellular matrix (ECM). The ECM surrounds dendritic spines and extends into the synaptic cleft, maintaining synapse integrity as well as mediating trans-synaptic communications between neurons. Several scaffolding proteins and glycans that compose the ECM form a lattice-like network, which serves as an attractive ground for various secreted glycoproteins, lectins, growth factors, and enzymes. ECM components can control dendritic spines through the interactions with their specific receptors or by influencing the functions of other synaptic proteins. In this review, we focus on ECM components and their receptors that regulate dendritic spine development and plasticity in the normal and diseased brain.

Anatomical localization of protease-activated receptor-1 and protease-mediated neuroglial crosstalk on peri-synaptic astrocytic endfeet

We studied the localization, activation and function of protease-activated receptor 1 (PAR-1) at the CNS synapse utilizing rat brain synaptosomes and slices. Confocal immunofluoresence and transmission electron microscopy in brain slices with pre-embedding diaminobenzidine (DAB) immunostaining found PAR-1 predominantly localized to the peri-synaptic astrocytic endfeet. Structural confocal immunofluorescence microscopy studies of isolated synaptosomes revealed spherical structures stained with anti-PAR-1 antibody which co-stained mainly for glial-filament acidic protein compared with the neuronal markers synaptophysin and PSD-95. Immunoblot studies of synaptosomes demonstrated an appropriate major band corresponding to PAR-1 and activation of the receptor by a specific agonist peptide (SFLLRN) significantly modulated phosphorylated extracellular signal-regulated kinase. A significant membrane potential depolarization was produced by thrombin (1 U/mL) and the PAR-1 agonist (100 μM) and depolarization by high K(+) elevated extracellular thrombin-like activity in the synaptosomes preparation. The results indicate PAR-1 localized to the peri-synaptic astrocytic endfeet is most likely activated by synaptic proteases and induces cellular signaling and modulation of synaptic electrophysiology. A protease mediated neuron-glia pathway may be important in both physiological and pathological regulation of the synapse.

Serine proteases, serine protease inhibitors, and protease-activated receptors: roles in synaptic function and behavior

Serine proteases, serine protease inhibitors, and protease-activated receptors have been intensively investigated in the periphery and their roles in a wide range of processes-coagulation, inflammation, and digestion, for example-have been well characterized (see Coughlin, 2000; Macfarlane et al., 2001; Molinari et al., 2003; Wang et al., 2008; Di Cera, 2009 for reviews). A growing number of studies demonstrate that these protein systems are widely expressed in many cell types and regions in mammalian brains. Accumulating lines of evidence suggest that the brain has co-opted the activities of these interesting proteins to regulate various processes underlying synaptic activity and behavior. In this review, we discuss emerging roles for serine proteases in the regulation of mechanisms underlying synaptic plasticity and memory formation.

Molecular signals of plasticity at the tetrapartite synapse

The emergence of astroglia as an important participant of the synaptic machinery has led to the 'tripartite synapse' hypothesis. Recent findings suggest that synaptic signaling also involves the surrounding extracellular matrix (ECM). The ECM can incorporate and store molecular traces of both neuronal and glial activities. It can also modulate function of local receptors or ion channels and send diffuse molecular signals using products of its use-dependent proteolytic cleavage. Recent experimental findings implicate the ECM in mechanisms of synaptic plasticity and glial remodeling, thus lending support to the 'tetrapartite synapse' concept. This inclusive view might help to understand better the mechanisms underlying signal integration and novel forms of long-term homeostatic regulation in the brain.

The role of myosin V in exocytosis and synaptic plasticity

In neuroscience, myosin V motor proteins have attracted attention since they are highly expressed in brain, and absence of myosin Va in man leads to a severe neurological disease called Griscelli syndrome. While in some cells myosin V is described to act as a vesicle transport motor, an additional role in exocytosis has emerged recently. In neurons, myosin V has been linked to exocytosis of secretory vesicles and recycling endosomes. Through these functions, it is implied in regulating important brain functions including the release of neuropeptides by exocytosis of large dense-core vesicles and the insertion of neurotransmitter receptors into post-synaptic membranes. This review focuses on the role of myosin V in (i) axonal transport and stimulated exocytosis of large dense-core vesicles to regulate the secretion of neuroactive substances, (ii) tethering of the endoplasmic reticulum at cerebellar synapses to permit long-term depression, (iii) recycling of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors at hippocampal synapses during long-term potentiation, and (iv) recycling of nicotinic acetylcholine receptors at the neuromuscular junction. Myosin V is thus discussed as an important modulator of synaptic plasticity.

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.

Memory impairment in rats by hippocampal administration of the serine protease subtilisin

Since the serine protease subtilisin has been reported to generate a novel form of long-term depression (LTD) in rat hippocampal slices, the present work was designed to determine whether it has any effect on learning and memory processes. Rats were used to examine the effects of subtilisin, injected directly into the dorsal hippocampus, on task performance in a step-through inhibitory avoidance of a mild footshock. The administration of 100 ng of subtilisin into each hippocampus, immediately after training, was sufficient to induce a detectable learning deficit with a footshock stimulus of 0.5 mA. Higher doses produced dose-related impairments in memory consolidation. These effects were not the result of irreversible toxicity, since rats trained with a higher amplitude footshock (0.75 mA) were able to perform as control animals; therefore, the amnesic effect was not further evident. Furthermore, the administration of subtilisin before avoidance training did not produce any detectable effect on performance during the training or test sessions, indicating that neither acquisition nor consolidation was affected. It is concluded that the post-training administration of a serine protease inhibitor is able to produce robust deficits of memory consolidation consistent with its ability to generate LTD, raising the possibility that related molecules could play physiological or pathological roles in the modulation of learning and memory.

Building and remodeling synapses

Synaptic junctions are generated by adhesion proteins that bridge the synaptic cleft to firmly anchor pre- and postsynaptic membranes. Several cell adhesion molecule (CAM) families localize to synapses, but it is not yet completely understood how each synaptic CAM family contributes to synapse formation and/or structure, and whether or how smaller groups of CAMs serve as minimal, functionally cooperative adhesive units upon which structure is based. Synapse structure and function evolve over the course of development, and in mature animals, synapses are composed of a greater number of proteins, surrounded by a stabilizing extracellular matrix, and often contacted by astrocytic processes. Thus, in mature networks undergoing plasticity, persistent changes in synapse strength, morphology, or number must be accompanied by selective and regulated remodeling of the neuropil. Recent work indicates that regulated, extracellular proteolysis may be essential for this, and rather than simply acting permissively to enable synapse plasticity, is more likely playing a proactive role in driving coordinated synaptic structural and functional modifications that underlie persistent changes in network activity.

A vesicle superpool spans multiple presynaptic terminals in hippocampal neurons

Synapse-specific vesicle pools have been widely characterized at central terminals. Here, we demonstrate a vesicle pool that is not confined to a synapse but spans multiple terminals. Using fluorescence imaging, correlative electron microscopy, and modeling of vesicle dynamics, we show that some recycling pool vesicles at synapses form part of a larger vesicle "superpool." The vesicles within this superpool are highly mobile and are rapidly exchanged between terminals (turnover: approximately 4% of total pool/min), significantly changing vesicular composition at synapses over time. In acute hippocampal slices we show that the mobile vesicle pool is also a feature of native brain tissue. We also demonstrate that superpool vesicles are available to synapses during stimulation, providing an extension of the classical recycling pool. Experiments using focal BDNF application suggest the involvement of a local TrkB-receptor-dependent mechanism for synapse-specific regulation of presynaptic vesicle pools through control of vesicle release and capture to or from the extrasynaptic pool.

Utilizing molecular details of the pain system to illustrate biochemical principles

To capture student interest and show clinical relevance, molecular details from the pain system can be used as supplemental examples to basic biochemistry lectures. Lecture topics include glutamate, substance P, calmodulin-dependent protein kinase II, synaptic proteases, calcitonin gene-related peptide, and neuronal protein synthesis. These topics are utilized to illustrate basic biochemical issues and are linked to pain-related topics such as pain transmission, synaptic plasticity, long-term potentiation, and central sensitization. For analysis, a brief survey was administered to evaluate student attitudes toward a representative lecture segment. Survey results support the premise that utilizing the pain system is an effective tool to engage chiropractic students during basic biochemistry lectures.

A mental retardation gene, motopsin/neurotrypsin/prss12, modulates hippocampal function and social interaction

Motopsin is a mosaic serine protease secreted from neuronal cells in various brain regions, including the hippocampus. The loss of motopsin function causes nonsyndromic mental retardation in humans and impairs long-term memory formation in Drosophila. To understand motopsin's function in the mammalian brain, motopsin knockout (KO) mice were generated. Motopsin KO mice did not have significant deficits in memory formation, as tested using the Morris water maze, passive avoidance and Y-maze tests. A social recognition test showed that the motopsin KO mice had the ability to recognize two stimulator mice, suggesting normal social memory. In a social novelty test, motopsin KO mice spent a longer time investigating a familiar mouse than wild-type (WT) mice did. In a resident-intruder test, motopsin KO mice showed prolonged social interaction as compared with WT mice. Consistent with the behavioral deficit, spine density was significantly decreased on apical dendrites, but not on basal dendrites, of hippocampal pyramidal neurons of motopsin KO mice. In contrast, pyramidal neurons at the cingulate cortex showed normal spine density. Spatial learning and social interaction induced the phosphorylation of cAMP-responsive element-binding protein (CREB) in hippocampal neurons of WT mice, whereas the phosphorylation of CREB was markedly decreased in mutant mouse brains. Our results indicate that an extracellular protease, motopsin, preferentially affects social behaviors, and modulates the functions of hippocampal neurons.

The process-inducing activity of transmembrane agrin requires follistatin-like domains

Clustering or overexpression of the transmembrane form of the extracellular matrix proteoglycan agrin in neurons results in the formation of numerous highly motile filopodia-like processes extending from axons and dendrites. Here we show that similar processes can be induced by overexpression of transmembrane-agrin in several non-neuronal cell lines. Mapping of the process-inducing activity in neurons and non-neuronal cells demonstrates that the cytoplasmic part of transmembrane agrin is dispensable and that the extracellular region is necessary for process formation. Site-directed mutagenesis reveals an essential role for the loop between beta-sheets 3 and 4 within the Kazal subdomain of the seventh follistatin-like domain of TM-agrin. An aspartic acid residue within this loop is critical for process formation. The seventh follistatin-like domain could be functionally replaced by the first and sixth but not by the eighth follistatin-like domain, demonstrating a functional redundancy among some follistatin-like domains of agrin. Moreover, a critical distance of the seventh follistatin-like domain to the plasma membrane appears to be required for process formation. These results demonstrate that different regions within the agrin protein are responsible for synapse formation at the neuromuscular junction and for process formation in central nervous system neurons and suggest a role for agrin's follistatin-like domains in the developing central nervous system.

Dominant-negative myosin Va impairs retrograde but not anterograde axonal transport of large dense core vesicles

Axonal transport of peptide and hormone-containing large dense core vesicles (LDCVs) is known to be a microtubule-dependent process. Here, we suggest a role for the actin-based motor protein myosin Va specifically in retrograde axonal transport of LDCVs. Using live-cell imaging of transfected hippocampal neurons grown in culture, we measured the speed, transport direction, and the number of LDCVs that were labeled with ectopically expressed neuropeptide Y fused to EGFP. Upon expression of a dominant-negative tail construct of myosin Va, a general reduction of movement in both dendrites and axons was observed. In axons, it was particularly interesting that the retrograde speed of LDCVs was significantly impaired, although anterograde transport remained unchanged. Moreover, particles labeled with the dominant-negative construct often moved in the retrograde direction but rarely in the anterograde direction. We suggest a model where myosin Va acts as an actin-dependent vesicle motor that facilitates retrograde axonal transport.

Brain extracellular matrix affects AMPA receptor lateral mobility and short-term synaptic plasticity

Many synapses in the mature CNS are wrapped by a dense extracellular matrix (ECM). Using single-particle tracking and fluorescence recovery after photobleaching, we found that this net-like ECM formed surface compartments on rat primary neurons that acted as lateral diffusion barriers for AMPA-type glutamate receptors. Enzymatic removal of the ECM increased extrasynaptic receptor diffusion and the exchange of synaptic AMPA receptors. Whole-cell patch-clamp recording revealed an increased paired-pulse ratio as a functional consequence of ECM removal. These results suggest that the surface compartments formed by the ECM hinder lateral diffusion of AMPA receptors and may therefore modulate short-term synaptic plasticity.

Dynein light chain regulates axonal trafficking and synaptic levels of Bassoon

Bassoon and the related protein Piccolo are core components of the presynaptic cytomatrix at the active zone of neurotransmitter release. They are transported on Golgi-derived membranous organelles, called Piccolo-Bassoon transport vesicles (PTVs), from the neuronal soma to distal axonal locations, where they participate in assembling new synapses. Despite their net anterograde transport, PTVs move in both directions within the axon. How PTVs are linked to retrograde motors and the functional significance of their bidirectional transport are unclear. In this study, we report the direct interaction of Bassoon with dynein light chains (DLCs) DLC1 and DLC2, which potentially link PTVs to dynein and myosin V motor complexes. We demonstrate that Bassoon functions as a cargo adapter for retrograde transport and that disruption of the Bassoon-DLC interactions leads to impaired trafficking of Bassoon in neurons and affects the distribution of Bassoon and Piccolo among synapses. These findings reveal a novel function for Bassoon in trafficking and synaptic delivery of active zone material.

Coincident pre- and postsynaptic activation induces dendritic filopodia via neurotrypsin-dependent agrin cleavage

The synaptic serine protease neurotrypsin is essential for cognitive function, as its deficiency in humans results in severe mental retardation. Recently, we demonstrated the activity-dependent release of neurotrypsin from presynaptic terminals and proteolytical cleavage of agrin at the synapse. Here we show that the activity-dependent formation of dendritic filopodia is abolished in hippocampal neurons from neurotrypsin-deficient mice. Administration of the neurotrypsin-dependent 22 kDa fragment of agrin rescues the filopodial response. Detailed analyses indicated that presynaptic action potential firing is necessary for the release of neurotrypsin, whereas postsynaptic NMDA receptor activation is necessary for the neurotrypsin-dependent cleavage of agrin. This contingency characterizes the neurotrypsin-agrin system as a coincidence detector of pre- and postsynaptic activation. As the resulting dendritic filopodia are thought to represent precursors of synapses, the neurotrypsin-dependent cleavage of agrin at the synapse may be instrumental for a Hebbian organization and remodeling of synaptic circuits in the CNS.

NMR solution structure of the neurotrypsin Kringle domain

Neurotrypsin is a multidomain protein that serves as a brain-specific serine protease. Here we report the NMR structure of its kringle domain, NT/K. The data analysis was performed with the BACUS (Bayesian analysis of coupled unassigned spins) algorithm. This study presents the first application of BACUS to the structure determination of a 13C unenriched protein for which no prior experimental 3D structure was available. NT/K adopts the kringle fold, consisting of an antiparallel beta-sheet bridged by an overlapping pair of disulfides. The structure reveals the presence of a surface-exposed left-handed polyproline II helix that is closely packed to the core beta-structure. This feature distinguishes NT/K from other members of the kringle fold and points toward a novel functional role for a kringle domain. Functional divergence among kringle domains is discussed on the basis of their surface and electrostatic characteristics.

Matrix-dependent local retention of secretory vesicle cargo in cortical neurons

Neurons secrete many diffusible signals from synaptic and other secretory vesicles. We characterized secretion of guidance cues, neuropeptides, neurotrophins, and proteases from single secretory vesicles using pHluorin-tagged cargo in cortical neurons. Stimulation triggered transient and persistent fusion events. Transient events represented full release followed by cargo diffusion or incomplete release followed by vesicle retrieval, as previously observed in neuroendocrine cells. Unexpectedly, we also observed that certain cargo, such as Semaphorin 3A (Sema3A), was delivered at the cell surface as stable deposits. Stable deposits and transient events were observed for single cargo and both were SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and calcium dependent. The ratio between stable and transient events did not depend on cargo size, subcellular localization (synaptic vs extrasynaptic secretion), or the presence of the extracellular matrix. Instead, the ratio is cargo specific and depends on an interaction with the vesicle matrix through a basic domain in the cargo protein. Inhibition of this interaction through deletion of the basic domain in Sema3A abolished stable deposits and rendered all events transient. Strikingly, cargo favoring transient release was stably deposited after corelease with cargo favoring stable deposit. These data argue against cargo diffusion after exocytosis as a general principle. Instead, the vesicle matrix retains secreted signals, probably for focal signaling at the cell surface.

Agrin in the nervous system: synaptogenesis and beyond

The heparan sulfate proteoglycan agrin is best known for its essential role during formation, maintenance and regeneration of the neuromuscular junction. Mutations in agrin-interacting proteins are the genetic basis for a number of neuromuscular disorders. However, agrin is widely expressed in many tissues including neurons and glial cells of the brain, where its precise function is much less understood. Fewer synapses develop in brains that lack agrin, consistent with a function of agrin during CNS synaptogenesis. Recently, a specific transmembrane form of agrin (TM-agrin) was identified that is concentrated at that interneuronal synapses in the brain. Clustering or overexpression of TM-agrin leads to the formation of filopodia-like processes, which might be precursors for CNS synapses. Agrin is subject to defined and activity-dependent proteolytic cleavage by neurotrypsin at synapses and dysregulation of agrin processing might contribute to the development of mental retardation. This review summarizes what is known about the role of agrin during synapse formation at the neuromuscular junction and in the developing CNS and will discuss additional functions of agrin in the adult CNS, in particular during BBB formation, during recovery after traumatic brain injury and in the etiology of diseases, including Alzheimer’s disease and mental retardation.

Presynaptic ryanodine receptor-CamKII signaling is required for activity-dependent capture of transiting vesicles

Activity elicits capture of dense-core vesicles (DCVs) that transit through resting Drosophila synaptic boutons to produce a rebound in presynaptic neuropeptide content following release. The onset of capture overlaps with an increase in the mobility of DCVs already present in synaptic boutons. Vesicle mobilization requires Ca(2+)-induced Ca2+ release by presynaptic endoplasmic reticulum (ER) ryanodine receptors (RyRs) that in turn stimulates Ca2+/calmodulin-dependent kinase II (CamKII). Here we show that the same signaling is required for activity-dependent capture of transiting DCVs. Specifically, the CamKII inhibitor KN-93, but not its inactive analog KN-92, eliminated the rebound replacement of neuropeptidergic DCVs in synaptic boutons. Furthermore, pharmacologically or genetically inhibiting neuronal sarco-endoplasmic reticulum calcium ATPase to deplete presynaptic ER Ca2+ stores or directly inhibiting RyRs prevented the capture response. These results show that the presynaptic RyR-CamKII pathway, which triggers mobilization of resident synaptic DCVs to facilitate exocytosis, also mediates activity-dependent capture of transiting DCVs to replenish neuropeptide stores.