Activity-dependent proteolytic cleavage of cell adhesion molecules regulates excitatory synaptic development and function

Multiomics profiling of human plasma and cerebrospinal fluid reveals ATN-derived networks and highlights causal links in Alzheimer's disease

INTRODUCTION

This study employed an integrative system and causal inference approach to explore molecular signatures in blood and CSF, the amyloid/tau/neurodegeneration [AT(N)] framework, mild cognitive impairment (MCI) conversion to Alzheimer's disease (AD), and genetic risk for AD.

METHODS

Using the European Medical Information Framework (EMIF)-AD cohort, we measured 696 proteins in cerebrospinal fluid (n = 371), 4001 proteins in plasma (n = 972), 611 metabolites in plasma (n = 696), and genotyped whole-blood (7,778,465 autosomal single nucleotide epolymorphisms, n = 936). We investigated associations: molecular modules to AT(N), module hubs with AD Polygenic Risk scores and APOE4 genotypes, molecular hubs to MCI conversion and probed for causality with AD using Mendelian randomization (MR).

RESULTS

AT(N) framework associated with protein and lipid hubs. In plasma, Proprotein Convertase Subtilisin/Kexin Type 7 showed evidence for causal associations with AD. AD was causally associated with Reticulocalbin 2 and sphingomyelins, an association driven by the APOE isoform.

DISCUSSION

This study reveals multi-omics networks associated with AT(N) and causal AD molecular candidates.

Activity-Dependent Synapse Refinement: From Mechanisms to Molecules

The refinement of immature neuronal networks into efficient mature ones is critical to nervous system development and function. This process of synapse refinement is driven by the neuronal activity-dependent competition of converging synaptic inputs, resulting in the elimination of weak inputs and the stabilization of strong ones. Neuronal activity, whether in the form of spontaneous activity or experience-evoked activity, is known to drive synapse refinement in numerous brain regions. More recent studies are now revealing the manner and mechanisms by which neuronal activity is detected and converted into molecular signals that appropriately regulate the elimination of weaker synapses and stabilization of stronger ones. Here, we highlight how spontaneous activity and evoked activity instruct neuronal activity-dependent competition during synapse refinement. We then focus on how neuronal activity is transformed into the molecular cues that determine and execute synapse refinement. A comprehensive understanding of the mechanisms underlying synapse refinement can lead to novel therapeutic strategies in neuropsychiatric diseases characterized by aberrant synaptic function.

γ-secretase promotes Drosophila postsynaptic development through the cleavage of a Wnt receptor

Developing synapses mature through the recruitment of specific proteins that stabilize presynaptic and postsynaptic structure and function. Wnt ligands signaling via Frizzled (Fz) receptors play many crucial roles in neuronal and synaptic development, but whether and how Wnt and Fz influence synaptic maturation is incompletely understood. Here, we show that Fz2 receptor cleavage via the γ-secretase complex is required for postsynaptic development and maturation. In the absence of γ-secretase, Drosophila neuromuscular synapses fail to recruit postsynaptic scaffolding and cytoskeletal proteins, leading to behavioral deficits. Introducing presenilin mutations linked to familial early-onset Alzheimer's disease into flies leads to synaptic maturation phenotypes that are identical to those seen in null alleles. This conserved role for γ-secretase in synaptic maturation and postsynaptic development highlights the importance of Fz2 cleavage and suggests that receptor processing by proteins linked to neurodegeneration may be a shared mechanism with aspects of synaptic development.

The epilepsy-associated protein PCDH19 undergoes NMDA receptor-dependent proteolytic cleavage and regulates the expression of immediate-early genes

Protocadherin-19 (PCDH19) is a synaptic cell-adhesion molecule encoded by X-linked PCDH19, a gene linked with epilepsy. Here, we report a synapse-to-nucleus signaling pathway through which PCDH19 bridges neuronal activity with gene expression. In particular, we describe the NMDA receptor (NMDAR)-dependent proteolytic cleavage of PCDH19, which leads to the generation of a PCDH19 C-terminal fragment (CTF) able to enter the nucleus. We demonstrate that PCDH19 CTF associates with chromatin and with the chromatin remodeler lysine-specific demethylase 1 (LSD1) and regulates expression of immediate-early genes (IEGs). Our results are consistent with a model whereby PCDH19 favors maintenance of neuronal homeostasis via negative feedback regulation of IEG expression and provide a key to interpreting PCDH19-related hyperexcitability.

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PCDH19 undergoes NMDAR-dependent cleavage by ADAM10 and possibly gamma secretase

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In the nucleus, PCDH19 C-terminal fragment (CTF) associates with the chromatin remodeler LSD1

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PCDH19 CTF favors immediate-early gene (IEG) repression

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PCDH19 downregulation affects LSD1 splicing by NOVA1 and increases IEG expression

Shed CNTNAP2 ectodomain is detectable in CSF and regulates Ca2+ homeostasis and network synchrony via PMCA2/ATP2B2

Although many neuronal membrane proteins undergo proteolytic cleavage, little is known about the biological significance of neuronal ectodomain shedding (ES). Here, we show that the neuronal sheddome is detectable in human cerebrospinal fluid (hCSF) and is enriched in neurodevelopmental disorder (NDD) risk factors. Among shed synaptic proteins is the ectodomain of CNTNAP2 (CNTNAP2-ecto), a prominent NDD risk factor. CNTNAP2 undergoes activity-dependent ES via MMP9 (matrix metalloprotease 9), and CNTNAP2-ecto levels are reduced in the hCSF of individuals with autism spectrum disorder. Using mass spectrometry, we identified the plasma membrane Ca2+ ATPase (PMCA) extrusion pumps as novel CNTNAP2-ecto binding partners. CNTNAP2-ecto enhances the activity of PMCA2 and regulates neuronal network dynamics in a PMCA2-dependent manner. Our data underscore the promise of sheddome analysis in discovering neurobiological mechanisms, provide insight into the biology of ES and its relationship with the CSF, and reveal a mechanism of regulation of Ca2+ homeostasis and neuronal network synchrony by a shed ectodomain.

The nucleolar δ isoform of adapter protein SH2B1 enhances morphological complexity and function of cultured neurons

The adapter protein SH2B1 is recruited to neurotrophin receptors, including TrkB (also known as NTRK2), the receptor for brain-derived neurotrophic factor (BDNF). Herein, we demonstrate that the four alternatively spliced isoforms of SH2B1 (SH2B1α-SH2B1δ) are important determinants of neuronal architecture and neurotrophin-induced gene expression. Primary hippocampal neurons from Sh2b1-/- [knockout (KO)] mice exhibit decreased neurite complexity and length, and BDNF-induced expression of the synapse-related immediate early genes Egr1 and Arc. Reintroduction of each SH2B1 isoform into KO neurons increases neurite complexity; the brain-specific δ isoform also increases total neurite length. Human obesity-associated variants, when expressed in SH2B1δ, alter neurite complexity, suggesting that a decrease or increase in neurite branching may have deleterious effects that contribute to the severe childhood obesity and neurobehavioral abnormalities associated with these variants. Surprisingly, in contrast to SH2B1α, SH2B1β and SH2B1γ, which localize primarily in the cytoplasm and plasma membrane, SH2B1δ resides primarily in nucleoli. Some SH2B1δ is also present in the plasma membrane and nucleus. Nucleolar localization, driven by two highly basic regions unique to SH2B1δ, is required for SH2B1δ to maximally increase neurite complexity and BDNF-induced expression of Egr1, Arc and FosL1.

A Perspective on the Potential of Opsins as an Integral Mechanism of Photobiomodulation: It's Not Just the Eyes

Objective: To investigate the potential relationship between opsins and photobiomodulation. Background: Opsins and other photoreceptors occur in all phyla and are important in light-activated signaling and organism homeostasis. In addition to the visual opsin systems of the retina (OPN1 and OPN2), there are several non-visual opsins found throughout the body tissues, including encephalopsin/panopsin (OPN3), melanopsin (OPN4), and neuropsin (OPN5), as well as other structures that have light-sensitive properties, such as enzymes, ion channels, particularly those located in cell membranes, lysosomes, and neuronal structures such as the nodes of Ranvier. The influence of these structures on exposure to light, including self-generated light within the body (autofluorescence), on circadian oscillators, and circadian and ultradian rhythms have become increasingly reported. The visual and non-visual phototransduction cascade originating from opsins and other structures has potential significant mechanistic effects on tissues and health. Methods: A PubMed and Google Scholar search was made using the search terms "photobiomodulation", "light", "neuron", "opsins", "neuropsin", "melanopsin", "encephalopsin", "rhodopsin", and "chromophore". Results: This review was examined the influence of neuropsin (also known as kallikrein 8), encephalopsin, and melanopsin specifically on ion channel function, and more broadly on the central and peripheral nervous systems. The relationship between opsins 3, 4, and 5 and photobiomodulation mechanisms was evaluated, along with a proposed role of photobiomodulation through opsins and light-sensitive organelles as potential alleviators of symptoms and accelerators of beneficial regenerative processes. The potential clinical implications of this in musculoskeletal conditions, wounds, and in the symptomatic management of neurodegenerative disease was also examined. Conclusions: Systematic research into the pleotropic therapeutic role of photobiomodulation, mediated through its action on opsins and other light-sensitive organelles may assist in the future execution of safe, low-risk precision medicine for a variety of chronic and complex disease conditions, and for health maintenance in aging.

Identification of plasma proteins relating to brain neurodegeneration and vascular pathology in cognitively normal individuals

INTRODUCTION

This study aims to first discover plasma proteomic biomarkers relating to neurodegeneration (N) and vascular (V) damage in cognitively normal individuals and second to discover proteins mediating sex-related difference in N and V pathology.

METHODS

Five thousand and thirty-two plasma proteins were measured in 1061 cognitively normal individuals (628 females and 433 males), nearly 90% of whom had magnetic resonance imaging measures of hippocampal volume (as N) and white matter hyperintensities (as V).

RESULTS

Differential protein expression analysis and co-expression network analysis revealed different proteins and modules associated with N and V, respectively. Furthermore, causal mediation analysis revealed four proteins mediated sex-related difference in N and one protein mediated such difference in V damage.

DISCUSSION

Once validated, the identified proteins could help to select cognitively normal individuals with N and V pathology for Alzheimer's disease clinical trials and provide targets for further mechanistic studies on brain sex differences, leading to sex-specific therapeutic strategies.

Heparan Sulfated Glypican-4 Is Released from Astrocytes by Proteolytic Shedding and GPI-Anchor Cleavage Mechanisms

Astrocytes provide neurons with diffusible factors that promote synapse formation and maturation. In particular, glypican-4/GPC4 released from astrocytes promotes the maturation of excitatory synapses. Unlike other secreted factors, GPC4 contains the C-terminal GPI-anchorage signal. However, the mechanism by which membrane-tethered GPC4 is released from astrocytes is unknown. Using mouse primary astrocyte cultures and a quantitative luciferase-based release assay, we show that GPC4 is expressed on the astrocyte surface via a GPI-anchorage. Soluble GPC4 is robustly released from the astrocytes largely by proteolytic shedding and, to a lesser extent, by GPI-anchor cleavage, but not by vesicular release. Pharmacological, overexpression, and loss of function screens showed that ADAM9 in part mediates the release of GPC4 from astrocytes. The released GPC4 contains the heparan sulfate side chain, suggesting that these release mechanisms provide the active form that promotes synapse maturation and function. Overall, our studies identified the release mechanisms and the major releasing enzyme of GPC4 in astrocytes and will provide insights into understanding how astrocytes regulate synapse formation and maturation.

Directly Reprogrammed Neurons as a Tool to Assess Neurotoxicity of the Contaminant 4-Hydroxy-2',3,5,5'-tetrachlorobiphenyl (4'OH-CB72) in Melon-Headed Whales

Whales accumulate high levels of environmental pollutants. Exposure to polychlorinated biphenyls (PCBs) and their metabolites (OH-PCBs) could be linked to abnormal behavior, which may lead to mass stranding of marine mammals. Whales may thus suffer from adverse effects such as neuronal dysfunction, yet testing the neurotoxicity of these compounds has never been feasible for these species. This study established neurons chemically reprogrammed from fibroblasts of mass stranded melon-headed whales (Peponocephala electra) and used them for in vitro neurotoxicity assays. Exposure to 4-hydroxy-2',3,5,5'-tetrachlorobiphenyl (4'OH-CB72), a metabolite of PCBs, caused apoptosis in the reprogrammed neurons. Transcriptome analysis of 4'OH-CB72-treated whale neurons showed altered expressions of genes associated with oxidative phosphorylation, chromatin degradation, axonal transport, and neurodegenerative diseases. These results suggest that 4'OH-CB72 exposure may induce neurodegeneration through disrupted apoptotic processes. A comparison of the results with human reprogrammed neurons revealed the specific effects on the whale neurons. Our noninvasive approach using fibroblast-derived neurons is useful for hazard and risk assessments of neurotoxicity in whales.

Destroy the old to build the new: Activity-dependent lysosomal exocytosis in neurons

Lysosomes are organelles that support diverse cellular functions such as terminal degradation of macromolecules and nutrient recycling. Additionally, lysosomes can fuse with the plasma membrane, a phenomenon referred to as lysosomal exocytosis, to release their contents, including hydrolytic enzymes and cargo proteins. Recently, neuronal activity has been shown to induce lysosomal exocytosis in dendrites and axons. Secreted lysosomal enzyme cathepsin B induces and stabilizes synaptic structural changes by degrading the local extracellular matrix. Extracellular matrix reorganization could also enhance the lateral diffusion of the co-released synaptic organizer Cbln1 along the surface of axons to facilitate new synapse formation. Similarly, lateral diffusion of dendritic AMPA-type glutamate receptors could be facilitated to enhance functional synaptic plasticity. Therefore, lysosomal exocytosis is a powerful way of building new cellular structures through the coordinated destruction of the old environment. Understanding the mechanisms by which lysosomal exocytosis is regulated in neurons is expected to lead to the development of new therapeutics for neuronal plasticity following spinal cord injury or neurodegenerative disease.

Long-term plasticity of inhibitory synapses in the hippocampus and spatial learning depends on matrix metalloproteinase 3

Learning and memory are known to depend on synaptic plasticity. Whereas the involvement of plastic changes at excitatory synapses is well established, plasticity mechanisms at inhibitory synapses only start to be discovered. Extracellular proteolysis is known to be a key factor in glutamatergic plasticity but nothing is known about its role at GABAergic synapses. We reveal that pharmacological inhibition of MMP3 activity or genetic knockout of the Mmp3 gene abolishes induction of postsynaptic iLTP. Moreover, the application of exogenous active MMP3 mimics major iLTP manifestations: increased mIPSCs amplitude, enlargement of synaptic gephyrin clusters, and a decrease in the diffusion coefficient of synaptic GABAA receptors that favors their entrapment within the synapse. Finally, we found that MMP3 deficient mice show faster spatial learning in Morris water maze and enhanced contextual fear conditioning. We conclude that MMP3 plays a key role in iLTP mechanisms and in the behaviors that presumably in part depend on GABAergic plasticity.

Cyclin Y, a novel actin-binding protein, regulates spine plasticity through the cofilin-actin pathway

While positive regulators of hippocampal long-term potentiation (LTP) have extensively been investigated, relatively little is known about the inhibitory regulators of LTP. We previously reported that Cyclin Y (CCNY), a member of cyclin family generally known to function in proliferating cells, is a novel postsynaptic protein that serves as a negative regulator of functional LTP. However, whether CCNY plays a role in structural LTP, which is mechanistically linked to functional LTP, and which mechanisms are involved in the CCNY-mediated suppression of LTP at the molecular level remain elusive. Here, we report that CCNY negatively regulates the plasticity-induced changes in spine morphology through the control of actin dynamics. We observed that CCNY directly binds to filamentous actin and interferes with LTP-induced actin polymerization as well as depolymerization by blocking the activation of cofilin, an actin-depolymerizing factor, thus resulting in less plastic spines and the impairment of structural LTP. These data suggest that CCNY acts as an inhibitory regulator for both structural and functional LTP by modulating actin dynamics through the cofilin-actin pathway. Collectively, our findings provide a mechanistic insight into the inhibitory modulation of hippocampal LTP by CCNY, highlighting a novel function of a cyclin family protein in non-proliferating neuronal cells.

Depolarization-dependent Induction of Site-specific Changes in Sialylation on N-linked Glycoproteins in Rat Nerve Terminals

Synaptic transmission leading to release of neurotransmitters in the nervous system is a fast and highly dynamic process. Previously, protein interaction and phosphorylation have been thought to be the main regulators of synaptic transmission. Here we show that sialylation of N-linked glycosylation is a novel potential modulator of neurotransmitter release mechanisms by investigating depolarization-dependent changes of formerly sialylated N-linked glycopeptides. We suggest that negatively charged sialic acids can be modulated, similarly to phosphorylation, by the action of sialyltransferases and sialidases thereby changing local structure and function of membrane glycoproteins. We characterized site-specific alteration in sialylation on N-linked glycoproteins in isolated rat nerve terminals after brief depolarization using quantitative sialiomics. We identified 1965 formerly sialylated N-linked glycosites in synaptic proteins and found that the abundances of 430 glycosites changed after 5 s depolarization. We observed changes on essential synaptic proteins such as synaptic vesicle proteins, ion channels and transporters, neurotransmitter receptors and cell adhesion molecules. This study is to our knowledge the first to describe ultra-fast site-specific modulation of the sialiome after brief stimulation of a biological system.

Neuropsin in mental health

Neuropsin is a brain-expressed extracellular matrix serine protease that governs synaptic plasticity through activity-induced proteolytic cleavage of synaptic proteins. Its substrates comprise several molecules central to structural synaptic plasticity, and studies in rodents have documented its role in cognition and the behavioral and neurobiological response to stress. Intriguingly, differential usage of KLK8 (neuropsin gene) splice forms in the fetal and adult brain has only been reported in humans, suggesting that neuropsin may serve a specialized role in human neurodevelopment. Through systematic interrogation of large-scale genetic data, we review KLK8 regulation in the context of mental health and provide a summary of clinical and preclinical evidence supporting a role for neuropsin in the pathogenesis of mental illness.

Neuron-oligodendroglia interactions: Activity-dependent regulation of cellular signaling

Oligodendrocyte lineage cells (oligodendroglia) and neurons engage in bidirectional communication throughout life to support healthy brain function. Recent work shows that changes in neuronal activity can modulate proliferation, differentiation, and myelination to support the formation and function of neural circuits. While oligodendroglia express a diverse collection of receptors for growth factors, signaling molecules, neurotransmitters and neuromodulators, our knowledge of the intracellular signaling pathways that are regulated by neuronal activity remains largely incomplete. Many of the pathways that modulate oligodendroglia behavior are driven by changes in intracellular calcium signaling, which may differentially affect cytoskeletal dynamics, gene expression, maturation, integration, and axonal support. Additionally, activity-dependent neuron-oligodendroglia communication plays an integral role in the recovery from demyelinating injuries. In this review, we summarize the modalities of communication between neurons and oligodendroglia and explore possible roles of activity-dependent calcium signaling in mediating cellular behavior and myelination.

Pathophysiology of Trans-Synaptic Adhesion Molecules: Implications for Epilepsy

Chemical synapses are specialized interfaces between neurons in the brain that transmit and modulate information, thereby integrating cells into multiplicity of interacting neural circuits. Cell adhesion molecules (CAMs) might form trans-synaptic complexes that are crucial for the appropriate identification of synaptic partners and further for the establishment, properties, and dynamics of synapses. When affected, trans-synaptic adhesion mechanisms play a role in synaptopathies in a variety of neuropsychiatric disorders including epilepsy. This review recapitulates current understanding of trans-synaptic interactions in pathophysiology of interneuronal connections. In particular, we discuss here the possible implications of trans-synaptic adhesion dysfunction for epilepsy.

Plasticity of Spine Structure: Local Signaling, Translation and Cytoskeletal Reorganization

Dendritic spines are small protrusive structures on dendritic surfaces, and function as postsynaptic compartments for excitatory synapses. Plasticity of spine structure is associated with many forms of long-term neuronal plasticity, learning and memory. Inside these small dendritic compartments, biochemical states and protein-protein interactions are dynamically modulated by synaptic activity, leading to the regulation of protein synthesis and reorganization of cytoskeletal architecture. This in turn causes plasticity of structure and function of the spine. Technical advances in monitoring molecular behaviors in single dendritic spines have revealed that each signaling pathway is differently regulated across multiple spatiotemporal domains. The spatial pattern of signaling activity expands from a single spine to the nearby dendritic area, dendritic branch and the nucleus, regulating different cellular events at each spatial scale. Temporally, biochemical events are typically triggered by short Ca²⁺ pulses (~10–100 ms). However, these signals can then trigger activation of downstream protein cascades that can last from milliseconds to hours. Recent imaging studies provide many insights into the biochemical processes governing signaling events of molecular assemblies at different spatial localizations. Here, we highlight recent findings of signaling dynamics during synaptic plasticity and discuss their roles in long-term structural plasticity of dendritic spines.

Tyrosine phosphorylation of the transmembrane protein SIRPα: Sensing synaptic activity and regulating ectodomain cleavage for synapse maturation

Synapse maturation is a neural activity-dependent process during brain development, in which active synapses preferentially undergo maturation to establish efficient neural circuits in the brain. Defects in this process are implicated in various neuropsychiatric disorders. We have previously reported that a postsynaptic transmembrane protein, signal regulatory protein-α (SIRPα), plays an important role in activity-dependently directing synapse maturation. In the presence of synaptic activity, the ectodomain of SIRPα is cleaved and released and then acts as a retrograde signal to induce presynaptic maturation. However, how SIRPα detects synaptic activity to promote its ectodomain cleavage and synapse maturation is unknown. Here, we show that activity-dependent tyrosine phosphorylation of SIRPα is critical for SIRPα cleavage and synapse maturation. We found that during synapse maturation and in response to neural activity, SIRPα is highly phosphorylated on its tyrosine residues in the hippocampus, a structure critical for learning and memory. Tyrosine phosphorylation of SIRPα was necessary for SIRPα cleavage and presynaptic maturation, as indicated by the fact that a phosphorylation-deficient SIRPα variant underwent much less cleavage and could not drive presynaptic maturation. However, SIRPα phosphorylation did not affect its synaptic localization. Finally, we show that inhibitors of the Src and JAK kinase family suppress neural activity-dependent SIRPα phosphorylation and cleavage. Together, our results indicate that SIRPα phosphorylation serves as a mechanism for detecting synaptic activity and linking it to the ectodomain cleavage of SIRPα, which in turn drives synapse maturation in an activity-dependent manner.

Fragile X Mental Retardation Protein Regulates Activity-Dependent Membrane Trafficking and Trans-Synaptic Signaling Mediating Synaptic Remodeling

Fragile X syndrome (FXS) is the leading monogenic cause of autism and intellectual disability. The disease arises through loss of fragile X mental retardation protein (FMRP), which normally exhibits peak expression levels in early-use critical periods, and is required for activity-dependent synaptic remodeling during this transient developmental window. FMRP canonically binds mRNA to repress protein translation, with targets that regulate cytoskeleton dynamics, membrane trafficking, and trans-synaptic signaling. We focus here on recent advances emerging in these three areas from the Drosophila disease model. In the well-characterized central brain mushroom body (MB) olfactory learning/memory circuit, FMRP is required for activity-dependent synaptic remodeling of projection neurons innervating the MB calyx, with function tightly restricted to an early-use critical period. FMRP loss is phenocopied by conditional removal of FMRP only during this critical period, and rescued by FMRP conditional expression only during this critical period. Consistent with FXS hyperexcitation, FMRP loss defects are phenocopied by heightened sensory experience and targeted optogenetic hyperexcitation during this critical period. FMRP binds mRNA encoding Drosophila ESCRTIII core component Shrub (human CHMP4 homolog) to restrict Shrub translation in an activity-dependent mechanism only during this same critical period. Shrub mediates endosomal membrane trafficking, and perturbing Shrub expression is known to interfere with neuronal process pruning. Consistently, FMRP loss and Shrub overexpression targeted to projection neurons similarly causes endosomal membrane trafficking defects within synaptic boutons, and genetic reduction of Shrub strikingly rescues Drosophila FXS model defects. In parallel work on the well-characterized giant fiber (GF) circuit, FMRP limits iontophoretic dye loading into central interneurons, demonstrating an FMRP role controlling core neuronal properties through the activity-dependent repression of translation. In the well-characterized Drosophila neuromuscular junction (NMJ) model, developmental synaptogenesis and activity-dependent synaptic remodeling both require extracellular matrix metalloproteinase (MMP) enzymes interacting with the heparan sulfate proteoglycan (HSPG) glypican dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling, with FXS synaptogenic defects alleviated by both MMP and HSPG reduction. This new mechanistic axis spanning from activity to FMRP to HSPG-dependent MMP regulation modulates activity-dependent synaptogenesis. We discuss future directions for these mechanisms, and intersecting research priorities for FMRP in glial and signaling interactions.

Neuronal activity drives FMRP- and HSPG-dependent matrix metalloproteinase function required for rapid synaptogenesis

Matrix metalloproteinase (MMP) functions modulate synapse formation and activity-dependent plasticity. Aberrant MMP activity is implicated in fragile X syndrome (FXS), a disease caused by the loss of the RNA-binding protein FMRP and characterized by neurological dysfunction and intellectual disability. Gene expression studies in Drosophila suggest that Mmps cooperate with the heparan sulfate proteoglycan (HSPG) glypican co-receptor Dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling and that synaptogenic defects in the fly model of FXS are alleviated by either inhibition of Mmp or genetic reduction of Dlp. We used the Drosophila neuromuscular junction (NMJ) glutamatergic synapse to test activity-dependent Dlp and Mmp intersections in the context of FXS. We found that rapid, activity-dependent synaptic bouton formation depended on secreted Mmp1. Acute neuronal stimulation reduced the abundance of Mmp2 but increased that of both Mmp1 and Dlp, as well as enhanced the colocalization of Dlp and Mmp1 at the synapse. Dlp function promoted Mmp1 abundance, localization, and proteolytic activity around synapses. Dlp glycosaminoglycan (GAG) chains mediated this functional interaction with Mmp1. In the FXS fly model, activity-dependent increases in Mmp1 abundance and activity were lost but were restored by reducing the amount of synaptic Dlp. The data suggest that neuronal activity-induced, HSPG-dependent Mmp regulation drives activity-dependent synaptogenesis and that this is impaired in FXS. Thus, exploring this mechanism further may reveal therapeutic targets that have the potential to restore synaptogenesis in FXS patients.

Effect of Clozapine on DNA Methylation in Peripheral Leukocytes from Patients with Treatment-Resistant Schizophrenia

Clozapine is an atypical antipsychotic, that is established as the treatment of choice for treatment-resistant schizophrenia (SCZ). To date, no study investigating comprehensive DNA methylation changes in SCZ patients treated with chronic clozapine has been reported. The purpose of the present study is to reveal the effects of clozapine on DNA methylation in treatment-resistant SCZ. We conducted a genome-wide DNA methylation profiling in peripheral leukocytes (485,764 CpG dinucleotides) from treatment-resistant SCZ patients treated with clozapine (n = 21) in a longitudinal study. Significant changes in DNA methylation were observed at 29,134 sites after one year of treatment with clozapine, and these genes were enriched for “cell substrate adhesion” and “cell matrix adhesion” gene ontology (GO) terms. Furthermore, DNA methylation changes in the CREBBP (CREB binding protein) gene were significantly correlated with the clinical improvements. Our findings provide insights into the action of clozapine in treatment-resistant SCZ.