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

Overexpression of Motopsin, an Extracellular Serine Protease Related to Intellectual Disability, Promotes Adult Neurogenesis and Neuronal Responsiveness in the Dentate Gyrus

Motopsin, a serine protease encoded by PRSS12, is secreted by neuronal cells into the synaptic clefts in an activity-dependent manner, where it induces synaptogenesis by modulating Na+/K+-ATPase activity. In humans, motopsin deficiency leads to severe intellectual disability and, in mice, it disturbs spatial memory and social behavior. In this study, we investigated mice that overexpressed motopsin in the forebrain using the Tet-Off system (DTG-OE mice). The elevated agrin cleavage or the reduced Na+/K+-ATPase activity was not detected. However, motopsin overexpression led to a reduction in spine density in hippocampal CA1 basal dendrites. While motopsin overexpression decreased the ratio of mature mushroom spines in the DG, it increased the ratio of immature thin spines in CA1 apical dendrites. Female DTG-OE mice showed elevated locomotor activity in their home cages. DTG-OE mice showed aberrant behaviors, such as delayed latency to the target hole in the Barnes maze test and prolonged duration of sniffing objects in the novel object recognition test (NOR), although they retained memory comparable to that of TRE-motopsin littermates, which normally express motopsin. After NOR, c-Fos-positive cells increased in the dentate gyrus (DG) of DTG-OE mice compared with that of DTG-SO littermates, in which motopsin overexpression was suppressed by the administration of doxycycline, and TRE-motopsin littermates. Notably, the numbers of doublecortin- and 5-bromo-2'-deoxyuridine-labeled cells significantly increased in the DG of DTG-OE mice, suggesting increased adult neurogenesis. Importantly, our results revealed a new function in addition to modulating neuronal responsiveness and spine morphology in the DG: the regulation of neurogenesis.

Modeling Methamphetamine Use Disorder and Relapse in Animals: Short- and Long-term Epigenetic, Transcriptional., and Biochemical Consequences in the Rat Brain

Methamphetamine use disorder (MUD) is a neuropsychiatric disorder characterized by binge drug taking episodes, intervals of abstinence, and relapses to drug use even during treatment. MUD has been modeled in rodents and investigators are attempting to identify its molecular bases. Preclinical experiments have shown that different schedules of methamphetamine self-administration can cause diverse transcriptional changes in the dorsal striatum of Sprague-Dawley rats. In the present review, we present data on differentially expressed genes (DEGs) identified in the rat striatum following methamphetamine intake. These include genes involved in transcription regulation, potassium channel function, and neuroinflammation. We then use the striatal data to discuss the potential significance of the molecular changes induced by methamphetamine by reviewing concordant or discordant data from the literature. This review identified potential molecular targets for pharmacological interventions. Nevertheless, there is a need for more research on methamphetamine-induced transcriptional consequences in various brain regions. These data should provide a more detailed neuroanatomical map of methamphetamine-induced changes and should better inform therapeutic interventions against MUD.

Mice deficient in synaptic protease neurotrypsin show impaired spaced long-term potentiation and blunted learning-induced modulation of dendritic spines

Neurotrypsin (NT) is a neuronal trypsin-like serine protease whose mutations cause severe mental retardation in humans. NT is activated in vitro by Hebbian-like conjunction of pre- and postsynaptic activities, which promotes the formation of dendritic filopodia via proteolytic cleavage of the proteoglycan agrin. Here, we investigated the functional importance of this mechanism for synaptic plasticity, learning, and extinction of memory. We report that juvenile neurotrypsin-deficient (NT^-/-) mice exhibit impaired long-term potentiation induced by a spaced stimulation protocol designed to probe the generation of new filopodia and their conversion into functional synapses. Behaviorally, juvenile NT^-/- mice show impaired contextual fear memory and have a sociability deficit. The latter persists in aged NT^-/- mice, which, unlike juvenile mice, show normal recall but impaired extinction of contextual fear memories. Structurally, juvenile mutants exhibit reduced spine density in the CA1 region, fewer thin spines, and no modulation in the density of dendritic spines following fear conditioning and extinction in contrast to wild-type littermates. The head width of thin spines is reduced in both juvenile and aged NT^-/- mice. In vivo delivery of adeno-associated virus expressing an NT-generated fragment of agrin, agrin-22, but not a shorter agrin-15, elevates the spine density in NT^-/- mice. Moreover, agrin-22 co-aggregates with pre- and postsynaptic markers and increases the density and size of presynaptic boutons and presynaptic puncta, corroborating the view that agrin-22 supports the synaptic growth.

Deconstruction of Neurotrypsin Reveals a Multi-factorially Regulated Activity Affecting Myotube Formation and Neuronal Excitability

Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.

Motopsin deficiency imparts partial insensitivity to doxorubicin-induced hippocampal impairments in adult mice

Motopsin is a serine protease that plays a crucial role in synaptic functions. Loss of motopsin function causes severe intellectual disability in humans. In this study, we evaluated the role of motopsin in the neuropathological development of cognitive impairments following chemotherapy, also known as chemobrain. Motopsin knockout (KO) and wild-type (WT) mice were intravenously injected with doxorubicin (Dox) or saline four times every 8 days and were evaluated for open field, novel object recognition, and passive avoidance tests. Parvalbumin-positive neurons in the hippocampus were immunohistochemically analyzed. Dox administration significantly decreased the total distance in the open field test in both WT and motopsin KO mice without affecting the duration spent in the center square. A significant interaction between the genotype and drug treatment was detected in the recognition index (the rate to investigate a novel object) in the novel object recognition test, although Dox treatment did not affect the total investigation time. Additionally, Dox treatment significantly decreased the recognition index in WT mice, whereas it tended to increase the recognition index in motopsin KO mice. Dox treatment did not affect the latency to enter a dark compartment in either WT or motopsin KO mice in the passive avoidance test. Interestingly, Dox treatment increased the parvalbumin-positive neurons in the stratum oriens of the hippocampus CA1 region of only WT mice, not motopsin KO mice. Our data suggest that motopsin deficiency imparted partial insensitivity to Dox-induced hippocampal impairments. Alternatively, motopsin may contribute to the neuropathology of chemobrain.

Novel Peptide NT/K-CRS Derived from Kringle Structure of Neurotrypsin Inhibits Neovascularization In Vitro and In Vivo

Purpose: To assess the anti-neovascularization effect of a novel peptide NT/K-CRS derived from the kringle domain of neurotrypsin in vitro and in vivo. Methods: Primary human umbilical vein endothelial cells (HUVECs) were treated with vascular endothelial growth factor (VEGF) in advance. Cell migration, lumen formation, and cell proliferation assays were performed to determine the anti-neovascularization effect of NT/K-CRS in HUVECs. TUNEL and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium tests were conducted to evaluate cell viability. Chick chorioallantoic membrane and oxygen-induced retinopathy model were established to assess the anti-angiogenic role of NT/K-CRS in vivo. Results: The in vitro results showed that NT/K-CRS effectively decreased VEGF-induced cell migration and endothelial tube formation, with no significant effect on cell proliferation and cell viability. In addition, NT/K-CRS showed great efficacy in angiogenesis inhibition in chicken embryos. The cytokine release syndrome (CRS) peptide also inhibited retinal neovascularization and improved retinal blood perfusion in oxygen-treated mouse pups through intravitreal injection. Conclusions: NT/K-CRS peptide derived from the kringle domain of neurotrypsin can strongly inhibit neovascularization in vitro and vivo. This novel peptide may become a promising therapeutic agent for neovascular-related ocular diseases.

Novel Peptide NT/K-CFY Derived from Kringle Structure of Neurotrypsin Inhibits Neovascularization

Purpose: To assess the anti-neovascularization effect of a novel peptide NT/K-CFY derived from the kringle domain of neurotrypsin.Materials and Methods: Cell migration, lumen formation and cell proliferation assays were performed to determine the anti-neovascularization effect of NT/K-CFY in primary human umbilical vein endothelial cells (HUVECs). Chick chorioallantoic membrane (CAM) and oxygen-induced retinopathy (OIR) models were established to assess the anti-angiogenic role of NT/K-CFY in vivo. The retinal expression of vascular endothelial growth factor (VEGF) and pigment epithelium-derived factor (PEDF) was examined by western blot and real-time PCR in OIR model.Results: The in vitro results showed that NT/K-CFY effectively and safely decreased VEGF-induced cell migration, cell proliferation and tube formation in HUVECs. In addition, NT/K-CFY showed certain efficacy in angiogenesis inhibition in chicken embryos and oxygen-treated mouse pups. Moreover, the CFY peptide also improved retinal blood perfusion and reversed the abnormal expression of VEGF and PEDF in OIR mouse model.Conclusion: NT/K-CFY peptide strongly inhibits neovascularization in vitro and vivo. This novel peptide may become a promising therapeutic agent for ocular angiogenesis-related diseases.

Microglia regulate synaptic development and plasticity

Synapses are fundamental structures of neural circuits that transmit information between neurons. Thus, the process of neural circuit formation via proper synaptic connections shapes the basis of brain functions and animal behavior. Synapses continuously undergo repeated formation and elimination throughout the lifetime of an organism, reflecting the dynamics of neural circuit function. The structural transformation of synapses has been described mainly in relation to neural activity-dependent strengthening and weakening of synaptic functions, that is, functional plasticity of synapses. An increasing number of studies have unveiled the roles of microglia, brain-resident immune cells that survey the brain parenchyma with highly motile processes, in synapse formation and elimination as well as in regulating synaptic function. Over the past 15 years, the molecular mechanisms underlying microglia-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of microglia-dependent regulation causes synaptic dysfunction that leads to brain diseases. In this review, we will broadly introduce studies that report the roles of microglia in synaptic plasticity and the possible underlying molecular mechanisms.

Fast Regulation of GABAAR Diffusion Dynamics by Nogo-A Signaling

Precisely controlling the excitatory and inhibitory balance is crucial for the stability and information-processing ability of neuronal networks. However, the molecular mechanisms maintaining this balance during ongoing sensory experiences are largely unclear. We show that Nogo-A signaling reciprocally regulates excitatory and inhibitory transmission. Loss of function for Nogo-A signaling through S1PR2 rapidly increases GABA_AR diffusion, thereby decreasing their number at synaptic sites and the amplitude of GABAergic mIPSCs at CA3 hippocampal neurons. This increase in GABA_AR diffusion rate is correlated with an increase in Ca²⁺ influx and requires the calcineurin-mediated dephosphorylation of the γ2 subunit at serine 327. These results suggest that Nogo-A signaling rapidly strengthens inhibitory GABAergic transmission by restricting the diffusion dynamics of GABA_ARs. Together with the observation that Nogo-A signaling regulates excitatory transmission in an opposite manner, these results suggest a crucial role for Nogo-A signaling in modulating the excitation and inhibition balance to restrict synaptic plasticity.

SNARE-Mediated Exocytosis in Neuronal Development

The formation of the nervous system involves establishing complex networks of synaptic connections between proper partners. This developmental undertaking requires the rapid expansion of the plasma membrane surface area as neurons grow and polarize, extending axons through the extracellular environment. Critical to the expansion of the plasma membrane and addition of plasma membrane material is exocytic vesicle fusion, a regulated mechanism driven by soluble N-ethylmaleimide-sensitive factor attachment proteins receptors (SNAREs). Since their discovery, SNAREs have been implicated in several critical neuronal functions involving exocytic fusion in addition to synaptic transmission, including neurite initiation and outgrowth, axon specification, axon extension, and synaptogenesis. Decades of research have uncovered a rich variety of SNARE expression and function. The basis of SNARE-mediated fusion, the opening of a fusion pore, remains an enigmatic event, despite an incredible amount of research, as fusion is not only heterogeneous but also spatially small and temporally fast. Multiple modes of exocytosis have been proposed, with full-vesicle fusion (FFV) and kiss-and-run (KNR) being the best described. Whereas most in vitro work has reconstituted fusion using VAMP-2, SNAP-25, and syntaxin-1; there is much to learn regarding the behaviors of distinct SNARE complexes. In the past few years, robust heterogeneity in the kinetics and fate of the fusion pore that varies by cell type have been uncovered, suggesting a paradigm shift in how the modes of exocytosis are viewed is warranted. Here, we explore both classic and recent work uncovering the variety of SNAREs and their importance in the development of neurons, as well as historical and newly proposed modes of exocytosis, their regulation, and proteins involved in the regulation of fusion kinetics.

Listeria monocytogenes exploits host exocytosis to promote cell-to-cell spread

The facultative intracellular pathogen Listeria monocytogenes uses an actin-based motility process to spread within human tissues. Filamentous actin from the human cell forms a tail behind bacteria, propelling microbes through the cytoplasm. Motile bacteria remodel the host plasma membrane into protrusions that are internalized by neighboring cells. A critical unresolved question is whether generation of protrusions by Listeria involves stimulation of host processes apart from actin polymerization. Here we demonstrate that efficient protrusion formation in polarized epithelial cells involves bacterial subversion of host exocytosis. Confocal microscopy imaging indicated that exocytosis is up-regulated in protrusions of Listeria in a manner that depends on the host exocyst complex. Depletion of components of the exocyst complex by RNA interference inhibited the formation of Listeria protrusions and subsequent cell-to-cell spread of bacteria. Additional genetic studies indicated important roles for the exocyst regulators Rab8 and Rab11 in bacterial protrusion formation and spread. The secreted Listeria virulence factor InlC associated with the exocyst component Exo70 and mediated the recruitment of Exo70 to bacterial protrusions. Depletion of exocyst proteins reduced the length of Listeria protrusions, suggesting that the exocyst complex promotes protrusion elongation. Collectively, these results demonstrate that Listeria exploits host exocytosis to stimulate intercellular spread of bacteria.

Learning Induces Transient Upregulation of Brevican in the Auditory Cortex during Consolidation of Long-Term Memories

It is a daily challenge for our brains to establish new memories via learning while providing stable storage of remote memories. In the adult vertebrate brain, bimodal regulation of the extracellular matrix (ECM) may regulate the delicate balance of learning-dependent plasticity and stable memory formation. Here, we trained adult male mice in a cortex-dependent auditory discrimination task and measured the abundance of ECM proteins brevican (BCN) and tenascin-R over the course of acquisition learning, consolidation, and long-term recall in two learning-relevant brain regions; the auditory cortex and hippocampus. Although early training led to a general downregulation of total ECM proteins, successful retrieval correlated with a region-specific and transient upregulation of BCN levels in the auditory cortex. No other parameter such as arousal or stress could account for the transient and region-specific BCN upregulation. This performance-dependent biphasic regulation of the ECM may assist transient plasticity to facilitate initial learning and subsequently promote the long-term consolidation of memory.SIGNIFICANCE STATEMENT The capacity to learn throughout life and at the same time guarantee lifelong storage and remote recall of established memories is a daily challenge. Emerging evidence suggests an important function of the extracellular matrix (ECM), a conglomerate of secreted proteins and polysaccharides in the adult vertebrate brain. We trained mice in an auditory long-term memory task and measured learning-related dynamic changes of the ECM protein brevican. Specifically, in the auditory cortex brevican is downregulated during initial learning and subsequently upregulated in exclusively those animals that have learned the task, suggesting a performance-dependent regulation in the service of memory consolidation and storage. Our data may provide novel therapeutic implications for several neuropsychiatric diseases involving dysregulation of the ECM.

Transient Confinement of CaV2.1 Ca2+-Channel Splice Variants Shapes Synaptic Short-Term Plasticity

The precision and reliability of synaptic information transfer depend on the molecular organization of voltage-gated calcium channels (VGCCs) within the presynaptic membrane. Alternative splicing of exon 47 affects the C-terminal structure of VGCCs and their affinity to intracellular partners and synaptic vesicles (SVs). We show that hippocampal synapses expressing VGCCs either with exon 47 (Ca_V2.1₊₄₇) or without (Ca_V2.1_Δ47) differ in release probability and short-term plasticity. Tracking single channels revealed transient visits (∼100 ms) of presynaptic VGCCs in nanodomains (∼80 nm) that were controlled by neuronal network activity. Surprisingly, despite harboring prominent binding sites to scaffold proteins, Ca_V2.1₊₄₇ persistently displayed higher mobility within nanodomains. Synaptic accumulation of Ca_V2.1 was accomplished by optogenetic clustering, but only Ca_V2.1₊₄₇ increased transmitter release and enhanced synaptic short-term depression. We propose that exon 47-related alternative splicing of Ca_V2.1 channels controls synapse-specific release properties at the level of channel mobility-dependent coupling between VGCCs and SVs.

Pool size estimations for dense-core vesicles in mammalian CNS neurons

Neuropeptides are essential signaling molecules transported and secreted by dense-core vesicles (DCVs), but the number of DCVs available for secretion, their subcellular distribution, and release probability are unknown. Here, we quantified DCV pool sizes in three types of mammalian CNS neurons in vitro and in vivo Super-resolution and electron microscopy reveal a total pool of 1,400-18,000 DCVs, correlating with neurite length. Excitatory hippocampal and inhibitory striatal neurons in vitro have a similar DCV density, and thalamo-cortical axons in vivo have a slightly higher density. Synapses contain on average two to three DCVs, at the periphery of synaptic vesicle clusters. DCVs distribute equally in axons and dendrites, but the vast majority (80%) of DCV fusion events occur at axons. The release probability of DCVs is 1-6%, depending on the stimulation. Thus, mammalian CNS neurons contain a large pool of DCVs of which only a small fraction can fuse, preferentially at axons.

Shaping Synapses by the Neural Extracellular Matrix

Accumulating data support the importance of interactions between pre- and postsynaptic neuronal elements with astroglial processes and extracellular matrix (ECM) for formation and plasticity of chemical synapses, and thus validate the concept of a tetrapartite synapse. Here we outline the major mechanisms driving: (i) synaptogenesis by secreted extracellular scaffolding molecules, like thrombospondins (TSPs), neuronal pentraxins (NPs) and cerebellins, which respectively promote presynaptic, postsynaptic differentiation or both; (ii) maturation of synapses via reelin and integrin ligands-mediated signaling; and (iii) regulation of synaptic plasticity by ECM-dependent control of induction and consolidation of new synaptic configurations. Particularly, we focused on potential importance of activity-dependent concerted activation of multiple extracellular proteases, such as ADAMTS4/5/15, MMP9 and neurotrypsin, for permissive and instructive events in synaptic remodeling through localized degradation of perisynaptic ECM and generation of proteolytic fragments as inducers of synaptic plasticity.

Vulnerability or resilience of motopsin knockout mice to maternal separation stress depending on adulthood behaviors

BACKGROUND

Both environmental and genetic conditions contribute to the robust development of neuronal circuits and adulthood behaviors. Loss of motopsin gene function causes severe intellectual disability in humans and enhanced social behavior in mice. Furthermore, childhood maltreatment is a risk factor for some psychiatric disorders, and children with disabilities have a higher risk of abuse than healthy children.

MATERIALS AND METHODS

In this study, we investigated the effects of maternal separation (MS) on adulthood behaviors of motopsin knockout (KO) and wild-type (WT) mice.

RESULTS

The MS paradigm decreased the duration that WT mice stayed in the center area of an open field, but not for motopsin KO mice; however, it decreased the novel object recognition index in both genotypes. In the marble burying test, motopsin KO mice buried fewer marbles than WT mice, regardless of the rearing conditions. The MS paradigm slightly increased and reduced open arm entry in the elevated plus maze by WT and motopsin KO mice, respectively. In the three-chamber test, the rate of sniffing the animal cage was increased by the MS paradigm only for motopsin KO mice. After the three-chamber test, motopsin KO mice had fewer cFos-positive cells in the prelimbic cortex, which is involved in emotional response, than WT mice. In the infralimbic cortex, the MS paradigm decreased the number of cFos-positive cells in motopsin KO mice.

CONCLUSION

Our results suggest that motopsin deficiency and childhood adversity independently affect some behaviors, but they may interfere with each other for other behaviors. Defective neuronal circuits in the prefrontal cortex may add to this complexity.

A systems level approach to temporal expression dynamics in Drosophila reveals clusters of long term memory genes

The ability to integrate experiential information and recall it in the form of memory is observed in a wide range of taxa, and is a hallmark of highly derived nervous systems. Storage of past experiences is critical for adaptive behaviors that anticipate both adverse and positive environmental factors. The process of memory formation and consolidation involve many synchronized biological events including gene transcription, protein modification, and intracellular trafficking: However, many of these molecular mechanisms remain illusive. With Drosophila as a model system we use a nonassociative memory paradigm and a systems level approach to uncover novel transcriptional patterns. RNA sequencing of Drosophila heads during and after memory formation identified a number of novel memory genes. Tracking the dynamic expression of these genes over time revealed complex gene networks involved in long term memory. In particular, this study focuses on two functional gene clusters of signal peptides and proteases. Bioinformatics network analysis and prediction in combination with high-throughput RNA sequencing identified previously unknown memory genes, which when genetically knocked down resulted in behaviorally validated memory defects.

A Novel Egr-1-Agrin Pathway and Potential Implications for Regulation of Synaptic Physiology and Homeostasis at the Neuromuscular Junction

Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the AGRN gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1−/−) display a marked increase in AGRN mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1−/− mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1−/− vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1−/− mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.

Physiological Concentrations of Amyloid Beta Regulate Recycling of Synaptic Vesicles via Alpha7 Acetylcholine Receptor and CDK5/Calcineurin Signaling

Despite the central role of amyloid β (Aβ) peptide in the etiopathogenesis of Alzheimer’s disease (AD), its physiological function in healthy brain is still debated. It is well established that elevated levels of Aβ induce synaptic depression and dismantling, connected with neurotoxicity and neuronal loss. Growing evidence suggests a positive regulatory effect of Aβ on synaptic function and cognition; however the exact cellular and molecular correlates are still unclear. In this work, we tested the effect of physiological concentrations of Aβ species of endogenous origin on neurotransmitter release in rat cortical and hippocampal neurons grown in dissociated cultures. Modulation of production and degradation of the endogenous Aβ species as well as applications of the synthetic rodent Aβ₄₀ and Aβ₄₂ affected efficacy of neurotransmitter release from individual presynapses. Low picomolar Aβ₄₀ and Aβ₄₂ increased, while Aβ depletion or application of low micromolar concentration decreased synaptic vesicle recycling, showing a hormetic effect of Aβ on neurotransmitter release. These Aβ-mediated modulations required functional alpha7 acetylcholine receptors as well as extracellular and intracellular calcium, involved regulation of CDK5 and calcineurin signaling and increased recycling of synaptic vesicles. These data indicate that Aβ regulates neurotransmitter release from presynapse and suggest that failure of the normal physiological function of Aβ in the fine-tuning of SV cycling could disrupt synaptic function and homeostasis, which would, eventually, lead to cognitive decline and neurodegeneration.

Activity Induces Fmr1-Sensitive Synaptic Capture of Anterograde Circulating Neuropeptide Vesicles

Synaptic neuropeptide and neurotrophin stores are maintained by constitutive bidirectional capture of dense-core vesicles (DCVs) as they circulate in and out of the nerve terminal. Activity increases DCV capture to rapidly replenish synaptic neuropeptide stores following release. However, it is not known whether this is due to enhanced bidirectional capture. Here experiments at the Drosophila neuromuscular junction, where DCVs contain neuropeptides and a bone morphogenic protein, show that activity-dependent replenishment of synaptic neuropeptides following release is evident after inhibiting the retrograde transport with the dynactin disruptor mycalolide B or photobleaching DCVs entering a synaptic bouton by retrograde transport. In contrast, photobleaching anterograde transport vesicles entering a bouton inhibits neuropeptide replenishment after activity. Furthermore, tracking of individual DCVs moving through boutons shows that activity selectively increases capture of DCVs undergoing anterograde transport. Finally, upregulating fragile X mental retardation 1 protein (Fmr1, also called FMRP) acts independently of futsch/MAP-1B to abolish activity-dependent, but not constitutive, capture. Fmr1 also reduces presynaptic neuropeptide stores without affecting activity-independent delivery and evoked release. Therefore, presynaptic motoneuron neuropeptide storage is increased by a vesicle capture mechanism that is distinguished from constitutive bidirectional capture by activity dependence, anterograde selectivity, and Fmr1 sensitivity. These results show that activity recruits a separate mechanism than used at rest to stimulate additional synaptic capture of DCVs for future release of neuropeptides and neurotrophins.

SIGNIFICANCE STATEMENT

Synaptic release of neuropeptides and neurotrophins depends on presynaptic accumulation of dense-core vesicles (DCVs). At rest, DCVs are captured bidirectionally as they circulate through Drosophila motoneuron terminals by anterograde and retrograde transport. Here we show that activity stimulates further synaptic capture that is distinguished from basal capture by its selectivity for anterograde DCVs and its inhibition by overexpression of the fragile X retardation protein Fmr1. Fmr1 dramatically lowers DCV numbers in synaptic boutons. Therefore, activity-dependent anterograde capture is a major determinant of presynaptic peptide stores.

Dependence receptor involvement in subtilisin-induced long-term depression and in long-term potentiation

The serine protease subtilisin induces a form of long-term depression (LTD) which is accompanied by a reduced expression of the axo-dendritic guidance molecule Unco-ordinated-5C (Unc-5C). One objective of the present work was to determine whether a loss of Unc-5C function contributed to subtilisin-induced LTD by using Unc-5C antibodies in combination with the pore-forming agents Triton X-100 (0.005%) or streptolysin O in rat hippocampal slices. In addition we have assessed the effect of subtilisin on the related dependence receptor Deleted in Colorectal Cancer (DCC) and used antibodies to this protein for functional studies. Field excitatory postsynaptic potentials (fEPSPs) were analyzed in rat hippocampal slices and protein extracts were used for Western blotting. Subtilisin produced a greater loss of DCC than of Unc-5C, but the antibodies had no effect on resting excitability or fEPSPs and did not modify subtilisin-induced LTD. However, antibodies to DCC but not Unc-5C did reduce the amplitude of theta-burst long-term potentiation (LTP). In addition, two inhibitors of endocytosis - dynasore and tat-gluR2(3Y) - were tested and, although the former compound had no effect on neurophysiological responses, tat-gluR2(3Y) did reduce the amplitude of subtilisin-induced LTD without affecting the expression of DCC or Unc-5C but with some loss of PostSynaptic Density Protein-95. The results support the view that the dependence receptor DCC may be involved in LTP and suggest that the endocytotic removal of a membrane protein or proteins may contribute to subtilisin-induced LTD, although it appears that neither Unc-5C nor DCC are involved in this process.

Hyaluronan-based extracellular matrix under conditions of homeostatic plasticity

Neuronal networks are balanced by mechanisms of homeostatic plasticity, which adjusts synaptic strength via molecular and morphological changes in the pre- and post-synapse. Here, we wondered whether the hyaluronic acid-based extracellular matrix (ECM) of the brain is involved in mechanisms of homeostatic plasticity. We hypothesized that the ECM, being rich in chondroitin sulfate proteoglycans such as brevican, which are suggested to stabilize synapses by their inhibitory effect on structural plasticity, must be remodelled to allow for structural and molecular changes during conditions of homeostatic plasticity. We found a high abundance of cleaved brevican fragments throughout the hippocampus and cortex and in neuronal cultures, with the strongest labelling in perineuronal nets on parvalbumin-positive interneurons. Using an antibody specific for a brevican fragment cleaved by the matrix metalloprotease ADAMTS4, we identified the enzyme as the main brevican-processing protease. Interestingly, we found ADAMTS4 largely associated with synapses. After inducing homeostatic plasticity in neuronal cell cultures by prolonged network inactivation, we found increased brevican processing at inhibitory as well as excitatory synapses, which is in line with the ADAMTS4 subcellular localization. Thus, the ECM is remodelled in conditions of homeostatic plasticity, which may liberate synapses to allow for a higher degree of structural plasticity.

Detection of muscle wasting in patients with chronic heart failure using C-terminal agrin fragment: results from the Studies Investigating Co-morbidities Aggravating Heart Failure (SICA-HF)

AIMS

Skeletal muscle wasting affects 20% of patients with chronic heart failure and has serious implications for their activities of daily living. Assessment of muscle wasting is technically challenging. C-terminal agrin-fragment (CAF), a breakdown product of the synaptically located protein agrin, has shown early promise as biomarker of muscle wasting. We sought to investigate the diagnostic properties of CAF in muscle wasting among patients with heart failure.

METHODS AND RESULTS

We assessed serum CAF levels in 196 patients who participated in the Studies Investigating Co-morbidities Aggravating Heart Failure (SICA-HF). Muscle wasting was identified using dual-energy X-ray absorptiometry (DEXA) in 38 patients (19.4%). Patients with muscle wasting demonstrated higher CAF values than those without (125.1 ± 59.5 pmol/L vs. 103.8 ± 42.9 pmol/L, P = 0.01). Using receiver operating characteristics (ROC), we calculated the optimal CAF value to identify patients with muscle wasting as >87.5 pmol/L, which had a sensitivity of 78.9% and a specificity of 43.7%. The area under the ROC curve was 0.63 (95% confidence interval 0.56-0.70). Using simple regression, we found that serum CAF was associated with handgrip (R = - 0.17, P = 0.03) and quadriceps strength (R = - 0.31, P < 0.0001), peak oxygen consumption (R = - 0.5, P < 0.0001), 6-min walk distance (R = - 0.32, P < 0.0001), and gait speed (R = - 0.2, P = 0.001), as well as with parameters of kidney and liver function, iron metabolism and storage.

CONCLUSION

CAF shows good sensitivity for the detection of skeletal muscle wasting in patients with heart failure. Its assessment may be useful to identify patients who should undergo additional testing, such as detailed body composition analysis. As no other biomarker is currently available, further investigation is warranted.

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.

Synaptic neuropeptide release by dynamin-dependent partial release from circulating vesicles

Neurons release neuropeptides, enzymes, and neurotrophins by exocytosis of dense-core vesicles (DCVs). Peptide release from individual DCVs has been imaged in vitro with endocrine cells and at the neuron soma, growth cones, neurites, axons, and dendrites but not at nerve terminals, where peptidergic neurotransmission occurs. Single presynaptic DCVs have, however, been tracked in native terminals with simultaneous photobleaching and imaging (SPAIM) to show that DCVs undergo anterograde and retrograde capture as they circulate through en passant boutons. Here dynamin (encoded by the shibire gene) is shown to enhance activity-evoked peptide release at the Drosophila neuromuscular junction. SPAIM demonstrates that activity depletes only a portion of a single presynaptic DCV's content. Activity initiates exocytosis within seconds, but subsequent release occurs slowly. Synaptic neuropeptide release is further sustained by DCVs undergoing multiple rounds of exocytosis. Synaptic neuropeptide release is surprisingly similar regardless of anterograde or retrograde DCV transport into boutons, bouton location, and time of arrival in the terminal. Thus vesicle circulation and bidirectional capture supply synapses with functionally competent DCVs. These results show that activity-evoked synaptic neuropeptide release is independent of a DCV's past traffic and occurs by slow, dynamin-dependent partial emptying of DCVs, suggestive of kiss-and-run exocytosis.

Identification of biological markers for better characterization of older subjects with physical frailty and sarcopenia

Population aging is rapidly accelerating worldwide; however, longer life expectancy is not the only public health goal. Indeed, extended lifetime involves maintaining function and the capacity of living independently. Sarcopenia and physical frailty are both highly relevant entities with regards to functionality and autonomy of older adults. The concepts and definitions of frailty and sarcopenia have largely been revised over the years. Sarcopenia is an age-related progressive and generalized loss of skeletal muscle mass and strength. On the other hand, frailty is a state of increased vulnerability to stressors, responsible for exposing the older person to enhanced risk of adverse outcomes. Physical frailty and sarcopenia substantially overlap and several adverse outcomes of frailty are likely mediated by sarcopenia. Indeed, the concepts of sarcopenia and physical frailty can be perceived as related to the same target organ (i.e., skeletal muscle) and it may be possible to combine them into a unique definition. The biological background of such a close relationship needs to be explored and clarified as it can potentially provide novel and pivotal insights for the assessment and treatment of these conditions in old age. The aim of this paper is to indicate and discuss possible biological markers to be considered in the framing of physical frailty and sarcopenia.

Synaptic activity controls localization and function of CtBP1 via binding to Bassoon and Piccolo

Persistent experience-driven adaptation of brain function is associated with alterations in gene expression patterns, resulting in structural and functional neuronal remodeling. How synaptic activity-in particular presynaptic performance-is coupled to gene expression in nucleus remains incompletely understood. Here, we report on a role of CtBP1, a transcriptional co-repressor enriched in presynapses and nuclei, in the activity-driven reconfiguration of gene expression in neurons. We demonstrate that presynaptic and nuclear pools of CtBP1 are interconnected and that both synaptic retention and shuttling of CtBP1 between cytoplasm and nucleus are co-regulated by neuronal activity. Finally, we show that CtBP1 is targeted and/or anchored to presynapses by direct interaction with the active zone scaffolding proteins Bassoon and Piccolo. This association is regulated by neuronal activity via modulation of cellular NAD/NADH levels and restrains the size of the CtBP1 pool available for nuclear import, thus contributing to the control of activity-dependent gene expression. Our combined results reveal a mechanism for coupling activity-induced molecular rearrangements in the presynapse with reconfiguration of neuronal gene expression.

Activity-controlled proteolytic cleavage at the synapse

Activity-controlled enzymatic cleavage of proteins on the surface of synaptic membranes or in the synaptic or perisynaptic interstitial compartment represents a direct way to regulate synaptic structure, function, and number. Extracellular proteolysis at synapses was initially understood to be plasticity enabling by freeing synapses from the constraints provided by the extracellular matrix. However, recent observations indicate that at least part of the extracellular protein cleavage results in activation of previously cryptic functions that regulate adaptive changes of synapses and neuronal circuits. Here, we focus on peptidases with distinct localization and function at synapses combined with regulation by neuronal and synaptic activity, and evaluate their function in the context of developmental and/or adult synaptic plasticity.

Biomarkers of muscle quality: N-terminal propeptide of type III procollagen and C-terminal agrin fragment responses to resistance exercise training in older adults

BACKGROUND

N-terminal peptide of procollagen type III (P3NP) and C-terminal agrin fragment (CAF) are circulating biomarkers that are related to lean body mass in older adults. P3NP is a circulating marker reflective of muscular structural remodeling while CAF is a circulating marker of neuromuscular remodeling. As resistance exercise is an established intervention that can effectively improve muscle quality, we sought to evaluate circulating biomarker changes corresponding to a resistance exercise intervention in older adults.

METHODS

Twenty-three older adults (aged 61 to 85 years) were randomized into an intervention (6-week resistance training) or control group. Resting circulating P3NP, CAF, lean body mass (LBM), muscle cross-sectional area (CSA), muscle strength, and muscle quality were determined at baseline and after the intervention or control period by enzyme-linked immunosorbent assay, dual-energy X-ray absorptiometry, ultrasound, leg extension, and relative strength, respectively. Changes in circulating biomarkers and measures of muscle mass and quality were evaluated with repeated-measures analysis of variance; clinical interpretations were made with magnitude-based inferences, and relationships between variables were evaluated with bivariate correlations.

RESULTS

The short-term resistance exercise intervention was effective at improving muscle quality by 28 % (p < 0.001) despite no significant changes in lean body mass. Baseline circulating P3NP was somewhat lower in older women (4.15 ± 1.9 ng/mL) compared with older men (4.81 ± 2.1 ng/mL). The exercise intervention tended to increase circulating P3NP (baseline = 4.53 ± 1.80 to post = 4.88 ± 1.86) and was significantly correlated with changes in LBM (r = 0.422, p = 0.045). At baseline, women (3.91 ± 1.12 pg/mL) had somewhat higher circulating CAF than men (3.47 ± 1.37 pg/mL). Circulating CAF increased by 10.4 % (3.59 to 4.00 pg/ml) in older adults following 6 weeks of resistance exercise training. Changes in circulating CAF were significantly related to changes in CSA of the vastus lateralis (r = 0.542, p = 0.008).

CONCLUSIONS

Assessment of P3NP and CAF from blood samples may provide minimally invasive and clinically informative measures of skeletal muscle status in older adults. Circulating CAF appears to increase in response to short-term resistance exercise training in older adults to a clinically meaningful magnitude. Changes in circulating P3NP in response to the intervention were less clear but appear to reflect muscle hypertrophy. Further research is needed to elucidate whether P3NP, CAF, or other biomarkers can reflect muscle qualitative adaptations with larger and longer studies.

AAV vector-mediated secretion of chondroitinase provides a sensitive tracer for axonal arborisations

As part of a project to express chondroitinase ABC (ChABC) in neurons of the central nervous system, we have inserted a modified ChABC gene into an adeno-associated viral (AAV) vector and injected it into the vibrissal motor cortex in adult rats to determine the extent and distribution of expression of the enzyme. A similar vector for expression of green fluorescent protein (GFP) was injected into the same location. For each vector, two versions with minor differences were used, giving similar results. After 4 weeks, the brains were stained to show GFP and products of chondroitinase digestion. Chondroitinase was widely expressed, and the AAV-ChABC and AAV-GFP vectors gave similar expression patterns in many respects, consistent with the known projections from the directly transduced neurons in vibrissal motor cortex and adjacent cingulate cortex. In addition, diffusion of vector to deeper neuronal populations led to labelling of remote projection fields which was much more extensive with AAV-ChABC than with AAV-GFP. The most notable of these populations are inferred to be neurons of cortical layer 6, projecting widely in the thalamus, and neurons of the anterior pole of the hippocampus, projecting through most of the hippocampus. We conclude that, whereas GFP does not label the thinnest axonal branches of some neuronal types, chondroitinase is efficiently secreted from these arborisations and enables their extent to be sensitively visualised. After 12 weeks, chondroitinase expression was undiminished.

Enhanced cognitive flexibility in reversal learning induced by removal of the extracellular matrix in auditory cortex

During brain maturation, the occurrence of the extracellular matrix (ECM) terminates juvenile plasticity by mediating structural stability. Interestingly, enzymatic removal of the ECM restores juvenile forms of plasticity, as for instance demonstrated by topographical reconnectivity in sensory pathways. However, to which degree the mature ECM is a compromise between stability and flexibility in the adult brain impacting synaptic plasticity as a fundamental basis for learning, lifelong memory formation, and higher cognitive functions is largely unknown. In this study, we removed the ECM in the auditory cortex of adult Mongolian gerbils during specific phases of cortex-dependent auditory relearning, which was induced by the contingency reversal of a frequency-modulated tone discrimination, a task requiring high behavioral flexibility. We found that ECM removal promoted a significant increase in relearning performance, without erasing already established-that is, learned-capacities when continuing discrimination training. The cognitive flexibility required for reversal learning of previously acquired behavioral habits, commonly understood to mainly rely on frontostriatal circuits, was enhanced by promoting synaptic plasticity via ECM removal within the sensory cortex. Our findings further suggest experimental modulation of the cortical ECM as a tool to open short-term windows of enhanced activity-dependent reorganization allowing for guided neuroplasticity.

ECM receptors in neuronal structure, synaptic plasticity, and behavior

During central nervous system development, extracellular matrix (ECM) receptors and their ligands play key roles as guidance molecules, informing neurons where and when to send axonal and dendritic projections, establish connections, and form synapses between pre- and postsynaptic cells. Once stable synapses are formed, many ECM receptors transition in function to control the maintenance of stable connections between neurons and regulate synaptic plasticity. These receptors bind to and are activated by ECM ligands. In turn, ECM receptor activation modulates downstream signaling cascades that control cytoskeletal dynamics and synaptic activity to regulate neuronal structure and function and thereby impact animal behavior. The activities of cell adhesion receptors that mediate interactions between pre- and postsynaptic partners are also strongly influenced by ECM composition. This chapter highlights a number of ECM receptors, their roles in the control of synapse structure and function, and the impact of these receptors on synaptic plasticity and animal behavior.

Neural ECM proteases in learning and synaptic plasticity

Recent studies implicate extracellular proteases in synaptic plasticity, learning, and memory. The data are especially strong for such serine proteases as thrombin, tissue plasminogen activator, neurotrypsin, and neuropsin as well as matrix metalloproteinases, MMP-9 in particular. The role of those enzymes in the aforementioned phenomena is supported by the experimental results on the expression patterns (at the gene expression and protein and enzymatic activity levels) and functional studies, including knockout mice, specific inhibitors, etc. Counterintuitively, the studies have shown that the extracellular proteolysis is not responsible mainly for an overall degradation of the extracellular matrix (ECM) and loosening perisynaptic structures, but rather allows for releasing signaling molecules from the ECM, transsynaptic proteins, and latent form of growth factors. Notably, there are also indications implying those enzymes in the major neuropsychiatric disorders, probably by contributing to synaptic aberrations underlying such diseases as schizophrenia, bipolar, autism spectrum disorders, and drug addiction.

Activity-dependent adaptations in inhibitory axons

Synaptic connections in our brains change continuously and throughout our lifetime. Despite ongoing synaptic changes, a healthy balance between excitation and inhibition is maintained by various forms of homeostatic and activity-dependent adaptations, ensuring stable functioning of neuronal networks. In this review we summarize experimental evidence for activity-dependent changes occurring in inhibitory axons, in cultures as well as in vivo. Axons form many presynaptic terminals, which are dynamic structures sharing presynaptic material along the axonal shaft. We discuss how internal (e.g., vesicle sharing) and external factors (e.g., binding of cell adhesion molecules or secreted factors) may affect the formation and plasticity of inhibitory synapses.

Mental retardation-related protease, motopsin (prss12), binds to the BRICHOS domain of the integral membrane protein 2a

Motopsin (prss12), a mosaic serine protease secreted by neuronal cells, is believed to be important for cognitive function, as the loss of its function causes severe nonsyndromic mental retardation. To understand the molecular role of motopsin, we identified the integral membrane protein 2a (Itm2a) as a motopsin-interacting protein using a yeast two-hybrid system. A pull-down assay showed that the BRICHOS domain of Itm2a was essential for this interaction. Motopsin and Itm2a co-localized in COS cells and in cultured neurons when transiently expressed in these cells. Both proteins were co-immunoprecipitated from lysates of these transfected COS cells. Itm2a was strongly detected in a brain lysate prepared between postnatal day 0 and 10, during which period motopsin protein was also enriched in the brain. Immunohistochemistry detected Itm2a as patchy spots along endothelial cells of brain capillaries (which also expressed myosin II regulatory light chain [RLC]), and on glial fibrillary acidic protein (GFAP)-positive processes in the developing cerebral cortex. The data raise the possibility that secreted motopsin interacts with endothelial cells in the developing brain.

Zymogen activation of neurotrypsin and neurotrypsin-dependent agrin cleavage on the cell surface are enhanced by glycosaminoglycans

The serine peptidase neurotrypsin is stored in presynaptic nerve endings and secreted in an inactive zymogenic form by synaptic activity. After activation, which requires activity of postsynaptic NMDA (N-methyl-D-aspartate) receptors, neurotrypsin cleaves the heparan sulfate proteoglycan agrin at active synapses. The resulting C-terminal 22-kDa fragment of agrin induces dendritic filopodia, which are considered to be precursors of new synapses. In the present study, we investigated the role of GAGs (glycosaminoglycans) in the activation of neurotrypsin and neurotrypsin-dependent agrin cleavage. We found binding of neurotrypsin to the GAG side chains of agrin, which in turn enhanced the activation of neurotrypsin by proprotein convertases and resulted in enhanced agrin cleavage. A similar enhancement of neurotrypsin binding to agrin, neurotrypsin activation and agrin cleavage was induced by the four-amino-acid insert at the y splice site of agrin, which is crucial for the formation of a heparin-binding site. Non-agrin GAGs also contributed to binding and activation of neurotrypsin and, thereby, to agrin cleavage, albeit to a lesser extent. Binding of neurotrypsin to cell-surface glycans locally restricts its conversion from zymogen into active peptidase. This provides the molecular foundation for the local action of neurotrypsin at or in the vicinity of its site of synaptic secretion. By its local action at synapses with correlated pre- and post-synaptic activity, the neurotrypsin-agrin system fulfils the requirements for a mechanism serving experience-dependent modification of activated synapses, which is essential for adaptive structural reorganizations of neuronal circuits in the developing and/or adult brain.

Regulation of presynaptic anchoring of the scaffold protein Bassoon by phosphorylation-dependent interaction with 14-3-3 adaptor proteins

The proper organization of the presynaptic cytomatrix at the active zone is essential for reliable neurotransmitter release from neurons. Despite of the virtual stability of this tightly interconnected proteinaceous network it becomes increasingly clear that regulated dynamic changes of its composition play an important role in the processes of synaptic plasticity. Bassoon, a core component of the presynaptic cytomatrix, is a key player in structural organization and functional regulation of presynaptic release sites. It is one of the most highly phosphorylated synaptic proteins. Nevertheless, to date our knowledge about functions mediated by any one of the identified phosphorylation sites of Bassoon is sparse. In this study, we have identified an interaction of Bassoon with the small adaptor protein 14-3-3, which depends on phosphorylation of the 14-3-3 binding motif of Bassoon. In vitro phosphorylation assays indicate that phosphorylation of the critical Ser-2845 residue of Bassoon can be mediated by a member of the 90-kDa ribosomal S6 protein kinase family. Elimination of Ser-2845 from the 14-3-3 binding motif results in a significant decrease of Bassoon's molecular exchange rates at synapses of living rat neurons. We propose that the phosphorylation-induced 14-3-3 binding to Bassoon modulates its anchoring to the presynaptic cytomatrix. This regulation mechanism might participate in molecular and structural presynaptic remodeling during synaptic plasticity.

Extracellular matrix molecules, their receptors, and secreted proteases in synaptic plasticity

Neural cells secrete diverse molecules, which accumulate in the extracellular space and form the extracellular matrix (ECM). Interactions between cells and the ECM are well recognized to play the crucial role in cell migration and guidance of growing axons, whereas formation of mature neural ECM in the form of perineuronal nets is believed to restrict certain forms of developmental plasticity. On the other hand, major components of perineuronal nets and other ECM molecules support induction of functional plasticity, the most studied form of which is long-term potentiation. Here, we review the underlying mechanisms by which ECM molecules, their receptors and remodeling proteases regulate the induction and maintenance of synaptic modifications. In particular, we highlight that activity-dependent secretion and activation of proteases leads to a local cleavage of the ECM and release of signaling proteolytic fragments. These molecules regulate transmitter receptor trafficking, actin cytoskeleton, growth of dendritic spines, and formation of dendritic filopodia.

Calsyntenin-1 shelters APP from proteolytic processing during anterograde axonal transport

Endocytosis of amyloid-β precursor protein (APP) is thought to represent the major source of substrate for the production of the amyloidogenic Aβ peptide by the β-secretase BACE1. The irreversible nature of proteolytic cleavage implies the existence of an efficient replenishment route for APP from its sites of synthesis to the cell surface. We recently found that APP exits the trans-Golgi network in intimate association with calsyntenin-1, a transmembrane cargo-docking protein for Kinesin-1-mediated vesicular transport. Here we characterized the function of calsyntenin-1 in neuronal APP transport using selective immunoisolation of intracellular trafficking organelles, immunocytochemistry, live-imaging, and RNAi. We found that APP is co-transported with calsyntenin-1 along axons to early endosomes in the central region of growth cones in carriers that exclude the α-secretase ADAM10. Intriguingly, calsyntenin-1/APP organelles contained BACE1, suggesting premature cleavage of APP along its anterograde path. However, we found that APP contained in calsyntenin-1/APP organelles was stable. We further analyzed vesicular trafficking of APP in cultured hippocampal neurons, in which calsyntenin-1 was reduced by RNAi. We found a markedly increased co-localization of APP and ADAM10 in axons and growth cones, along with increased proteolytic processing of APP and Aβ secretion in these neurons. This suggested that the reduced capacity for calsyntenin-1-dependent APP transport resulted in mis-sorting of APP into additional axonal carriers and, therefore, the premature encounter of unprotected APP with its ectodomain proteases. In combination, our results characterize calsyntenin-1/APP organelles as carriers for sheltered anterograde axonal transport of APP.

The brain's extracellular matrix and its role in synaptic plasticity

The extracellular matrix (ECM) of the brain has important roles in regulating synaptic function and plasticity. A juvenile ECM supports the wiring of neuronal networks, synaptogenesis, and synaptic maturation. The closure of critical periods for experience-dependent shaping of neuronal circuits coincides with the implementation of a mature form of ECM that is characterized by highly elaborate hyaluronan-based structures, the perineuronal nets (PNN), and PNN-like perisynaptic ECM specializations. In this chapter, we will focus on some recently reported aspects of ECM functions in brain plasticity. These include (a) the discovery that the ECM can act as a passive diffusion barrier for cell surface molecules including neurotransmitter receptors and in this way compartmentalize cell surfaces, (b) the specific functions of ECM components in actively regulating synaptic plasticity and homeostasis, and (c) the shaping processes of the ECM by extracellular proteases and in turn the activation particular signaling pathways.

Peripheral nervous system genes expressed in central neurons induce growth on inhibitory substrates

Trauma to the spinal cord and brain can result in irreparable loss of function. This failure of recovery is in part due to inhibition of axon regeneration by myelin and chondroitin sulfate proteoglycans (CSPGs). Peripheral nervous system (PNS) neurons exhibit increased regenerative ability compared to central nervous system neurons, even in the presence of inhibitory environments. Previously, we identified over a thousand genes differentially expressed in PNS neurons relative to CNS neurons. These genes represent intrinsic differences that may account for the PNS's enhanced regenerative ability. Cerebellar neurons were transfected with cDNAs for each of these PNS genes to assess their ability to enhance neurite growth on inhibitory (CSPG) or permissive (laminin) substrates. Using high content analysis, we evaluated the phenotypic profile of each neuron to extract meaningful data for over 1100 genes. Several known growth associated proteins potentiated neurite growth on laminin. Most interestingly, novel genes were identified that promoted neurite growth on CSPGs (GPX3, EIF2B5, RBMX). Bioinformatic approaches also uncovered a number of novel gene families that altered neurite growth of CNS neurons.

Genetic suppression of agrin reduces mania-like behavior in Na+ , K+ -ATPase α3 mutant mice

Myshkin mice heterozygous for an inactivating mutation in the neuron-specific Na(+) ,K(+) -ATPase α3 isoform show behavior analogous to mania, including an abnormal endogenous circadian period. Agrin is a proteoglycan implicated as a regulator of synapses that has been proposed to inhibit activity of Na(+) ,K(+) -ATPase α3. We examined whether the mania-related behavior of Myshkin mice could be rescued by a reduction in the expression of agrin through genetic knockout. The suppression of agrin reduced hyperambulation and holeboard exploration, restored anxiety-like behavior (or reduced risk-taking behavior), improved prepulse inhibition and shortened the circadian period. Hence, agrin is important for regulating mania-like behavior and circadian rhythms. In Myshkin mice, the suppression of agrin increased brain Na(+) ,K(+) -ATPase activity by 11 ± 4%, whereas no effect on Na(+) ,K(+) -ATPase activity was detected when agrin was suppressed in mice without the Myshkin mutation. These results introduce agrin as a potential therapeutic target for the treatment of mania and other neurological disorders associated with reduced Na(+) ,K(+) -ATPase activity and neuronal hyperexcitability.

Molecular changes associated with hippocampal long-lasting depression induced by the serine protease subtilisin-A

The serine protease subtilisin-A (SubA) induces a form of long-term depression (LTD) of synaptic transmission in the rat hippocampus, and molecular changes associated with SubA-induced LTD (SubA-LTD) were explored by using recordings of evoked postsynaptic potentials and immunoblotting. SubA-LTD was prevented by a selective inhibitor of SubA proteolysis, but the same inhibitor did not affect LTD induced by electrical stimulation or activation of metabotropic glutamate receptors. SubA-LTD was reduced by the protein kinase inhibitors genistein and lavendustin A, although not by inhibitors of p38 mitogen-activated protein kinase, glycogen synthase kinase-3, or protein phosphatases. It was also reduced by (RS)-α-methyl-4-carboxyphenylglycine, a broad-spectrum antagonist at metabotropic glutamate receptors. Inhibition of the Rho kinase enzyme Rho-associated coiled-coil kinase reduced SubA-LTD, although inhibitors of the RhoGTPase-activating enzymes farnesyl transferase and geranylgeranyl transferase did not. In addition, a late phase of SubA-LTD was dependent on new protein synthesis. There was a small, non-significant difference in SubA-LTD between wild-type and RhoB(-/-) mice. Marked decreases were seen in the levels of Unc-5H3, a protein that is intimately involved in the development and plasticity of glutamatergic synapses. Smaller changes were noted, at higher concentrations of SubA, in Unc-5H1, vesicle-associated membrane protein-1 (synaptobrevin), and actin, with no changes in the levels of synaptophysin, synaptotagmin, RhoA, or RhoB. None of these changes was associated with LTD induced electrically or by the metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine. These results indicate that SubA induces molecular changes that overlap with other forms of LTD, but that the overall molecular profile of SubA-LTD is quite different.