Impaired synaptic incorporation of AMPA receptors in a mouse model of fragile X syndrome.

Fragile X syndrome (FXS) is the most common monogenetic cause of inherited intellectual disability and autism in humans. One of the well-characterized molecular phenotypes of Fmr1 KO mice, a model of FXS, is increased translation of synaptic proteins. Although this upregulation stabilizes in adulthood, abnormalities during the critical period of plasticity have long-term effects on circuit formation and synaptic properties. Using high-resolution quantitative proteomics of synaptoneurosomes isolated from the adult, developed brains of Fmr1 KO mice, we show a differential abundance of proteins regulating the postsynaptic receptor activity of glutamatergic synapses. We investigated the AMPA receptor composition and shuttling in adult Fmr1 KO and WT mice using a variety of complementary experimental strategies such as surface protein crosslinking, immunostaining of surface receptors, and electrophysiology. We discovered that the activity-dependent synaptic delivery of AMPARs is impaired in adult Fmr1 KO mice. Furthermore, we show that Fmr1 KO synaptic AMPARs contain more GluA2 subunits that can be interpreted as a switch in the synaptic AMPAR subtype toward an increased number of Ca2+-impermeable receptors in adult Fmr1 KO synapses.

How dendritic spines shape is determined by MMP-9 activity in FXS.

Matrix metalloproteinase-9 (MMP-9) belongs to the family of endopeptidases expressed in neurons and secreted at the synapse in response to neuronal activity. It regulates the pericellular environment by cleaving its protein components. MMP9 is involved in activity-dependent reorganization of spine architecture. In the mouse model of fragile X syndrome (FXS), the most common inherited intellectual disability and the most common single-gene cause of autism, increased synaptic expression of MMP-9 is responsible for the observed dendritic spine abnormalities. In this chapter, I summarize the current data on the molecular regulatory pathways responsible for synaptic MMP-9 expression and discuss the fact that MMP-9 is extracellularly localized, making it a particularly attractive potential target for therapeutic pharmacological intervention in FXS.

TIAR and FMRP shape pro-survival nascent proteome of leukemia cells in the bone marrow microenvironment.

Chronic myeloid leukemia (CML) cells circulate between blood and bone marrow niche, representing different microenvironments. We studied the role of the two RNA-binding proteins, T-cell-restricted intracellular antigen (TIAR), and the fragile X mental retardation protein (FMRP) in the regulation of protein translation in CML cells residing in settings mimicking peripheral blood microenvironment (PBM) and bone marrow microenvironment (BMM). The outcomes showed how conditions shaped the translation process through TIAR and FMRP activity, considering its relevance in therapy resistance. The QuaNCAT mass-spectrometric approach revealed that TIAR and FMRP have a discrete modulatory effect on protein synthesis and thus affect distinct aspects of leukemic cells functioning in the hypoxic niche. In the BMM setup, FMRP impacted metabolic adaptation of cells and TIAR substantially supported the resistance of CML cells to translation inhibition by homoharringtonine. Overall, our results demonstrated that targeting post-transcriptional control should be considered when designing anti-leukemia therapeutic solutions.

Destabilization of mutated human PUS3 protein causes intellectual disability.

Pseudouridine (Ψ) is an RNA base modification ubiquitously found in many types of RNAs. In humans, the isomerization of uridine is catalyzed by different stand-alone pseudouridine synthases (PUS). Genomic mutations in the human pseudouridine synthase 3 gene (PUS3) have been identified in patients with neurodevelopmental disorders. However, the underlying molecular mechanisms that cause the disease phenotypes remain elusive. Here, we utilize exome sequencing to identify genomic variants that lead to a homozygous amino acid substitution (p.[(Tyr71Cys)];[(Tyr71Cys)]) in human PUS3 of two affected individuals and a compound heterozygous substitution (p.[(Tyr71Cys)];[(Ile299Thr)]) in a third patient. We obtain wild-type and mutated full-length human recombinant PUS3 proteins and characterize the enzymatic activity in vitro. Unexpectedly, we find that the p.Tyr71Cys substitution neither affect tRNA binding nor pseudouridylation activity in vitro, but strongly impair the thermostability profile of PUS3, while the p.Ile299Thr mutation causes protein aggregation. Concomitantly, we observe that the PUS3 protein levels as well as the level of PUS3-dependent Ψ levels are strongly reduced in fibroblasts derived from all three patients. In summary, our results directly illustrate the link between the identified PUS3 variants and reduced Ψ levels in the patient cells, providing a molecular explanation for the observed clinical phenotypes.

Disrupting interaction between miR-132 and Mmp9 3'UTR improves synaptic plasticity and memory in mice.

As microRNAs have emerged to be important regulators of molecular events occurring at the synapses, the new questions about their regulatory effect on the behavior have araised. In the present study, we show for the first time that the dysregulated specific targeting of miR132 to Mmp9 mRNA in the mouse brain results in the increased level of Mmp9 protein, which affects synaptic plasticity and has an effect on memory formation. Our data points at the importance of complex and precise regulation of the Mmp9 level by miR132 in the brain.

Long-term mitochondrial stress induces early steps of Tau aggregation by increasing reactive oxygen species levels and affecting cellular proteostasis.

Accumulating evidence indicates that mitochondrial dysfunction is involved in the pathogenesis of neurodegenerative diseases. Both of these conditions are often associated with an increase in protein aggregation. However, still unknown are the specific defects of mitochondrial biology that play a critical role in the development of Alzheimer's disease, in which Tau protein aggregates are observed in the brains of some patients. Here, we report that long-term mitochondrial stress triggered Tau dimerization, which is the first step of protein aggregation. Mitochondrial dysfunction was induced in HEK293T cells that received prolonged treatment with rotenone and in HEK293T cells with the knockout of NDUFA11 protein. To monitor changes in Tau protein aggregation, we took advantage of the bimolecular fluorescence complementation assay using HEK293T cells that were transfected with plasmids that encoded Tau. Inhibition of the ISR with ISRIB induced Tau dimerization, whereas ISR activation with salubrinal, guanabenz, and sephin1 partially reversed this process. Cells that were treated with ROS scavengers, N-acetyl-l-cysteine or MitoQ, significantly reduced the amount of ROS and Tau dimerization, indicating the involvement of oxidative stress in Tau aggregation. Our results indicate that long-term mitochondrial stress may induce early steps of Tau protein aggregation by affecting oxidative balance and cellular proteostasis.

PTPN4 germline variants result in aberrant neurodevelopment and growth.

Protein-tyrosine phosphatases (PTPs) are pleomorphic regulators of eukaryotic cellular responses to extracellular signals that function by modulating the phosphotyrosine of specific proteins. A handful of PTPs have been implicated in germline and somatic human disease. Using exome sequencing, we identified missense and truncating variants in PTPN4 in six unrelated individuals with varying degrees of intellectual disability or developmental delay. The variants occurred de novo in all five subjects in whom segregation analysis was possible. Recurring features include postnatal growth deficiency or excess, seizures, and, less commonly, structural CNS, heart, or skeletal anomalies. PTPN4 is a widely expressed protein tyrosine phosphatase that regulates neuronal cell homeostasis by protecting neurons against apoptosis. We suggest that pathogenic variants in PTPN4 confer risk for growth and cognitive abnormalities in humans.

Chronic fluoxetine treatment impairs motivation and reward learning by affecting neuronal plasticity in the central amygdala.

BACKGROUND AND PURPOSE: The therapeutic effects of fluoxetine are believed to be due to increasing neuronal plasticity and reversing some learning deficits. Nevertheless, a growing amount of evidence shows adverse effects of this drug on cognition and some forms of neuronal plasticity. EXPERIMENTAL APPROACH: To study the effects of chronic fluoxetine treatment, we combine an automated assessment of motivation and learning in mice with an investigation of neuronal plasticity in the central amygdala and basolateral amygdala. We use immunohistochemistry to visualize neuronal types and perineuronal nets, along with DI staining to assess dendritic spine morphology. Gel zymography is used to test fluoxetine's impact on matrix metalloproteinase-9, an enzyme involved in synaptic plasticity. KEY RESULTS: We show that chronic fluoxetine treatment in non-stressed mice increases perineuronal nets-dependent plasticity in the basolateral amygdala, while impairing MMP-9-dependent plasticity in the central amygdala. Further, we illustrate how the latter contributes to anhedonia and deficits of reward learning. Behavioural impairments are accompanied by alterations in morphology of dendritic spines in the central amygdala towards an immature state, most likely reflecting animals' inability to adapt. We strengthen the link between the adverse effects of fluoxetine and its influence on MMP-9 by showing that behaviour of MMP-9 knockout animals remains unaffected by the drug. CONCLUSION AND IMPLICATIONS: Chronic fluoxetine treatment differentially affects various forms of neuronal plasticity, possibly explaining its opposing effects on brain and behaviour. These findings are of immediate clinical relevance since reported side effects of fluoxetine pose a potential threat to patients.

Mitochondrial protein biogenesis in the synapse is supported by local translation.

Synapses are the regions of the neuron that enable the transmission and propagation of action potentials on the cost of high energy consumption and elevated demand for mitochondrial ATP production. The rapid changes in local energetic requirements at dendritic spines imply the role of mitochondria in the maintenance of their homeostasis. Using global proteomic analysis supported with complementary experimental approaches, we show that an essential pool of mitochondrial proteins is locally produced at the synapse indicating that mitochondrial protein biogenesis takes place locally to maintain functional mitochondria in axons and dendrites. Furthermore, we show that stimulation of synaptoneurosomes induces the local synthesis of mitochondrial proteins that are transported to the mitochondria and incorporated into the protein supercomplexes of the respiratory chain. Importantly, in a mouse model of fragile X syndrome, Fmr1 KO mice, a common disease associated with dysregulation of synaptic protein synthesis, we observed altered morphology and respiration rates of synaptic mitochondria. That indicates that the local production of mitochondrial proteins plays an essential role in synaptic functions.

Neuroligin 1, 2, and 3 Regulation at the Synapse: FMRP-Dependent Translation and Activity-Induced Proteolytic Cleavage.

Neuroligins (NLGNs) are cell adhesion molecules located on the postsynaptic side of the synapse that interact with their presynaptic partners neurexins to maintain trans-synaptic connection. Fragile X syndrome (FXS) is a common neurodevelopmental disease that often co-occurs with autism and is caused by the lack of fragile X mental retardation protein (FMRP) expression. To gain an insight into the molecular interactions between the autism-related genes, we sought to determine whether FMRP controls the synaptic levels of NLGNs. We show evidences that FMRP associates with Nlgn1, Nlgn2, and Nlgn3 mRNAs in vitro in both synaptoneurosomes and neuronal cultures. Next, we confirm local translation of Nlgn1, Nlgn2, and Nlgn3 mRNAs to be synaptically regulated by FMRP. As a consequence of elevated Nlgns mRNA translation Fmr1 KO mice exhibit increased incorporation of NLGN1 and NLGN3 into the postsynaptic membrane. Finally, we show that neuroligins synaptic level is precisely and dynamically regulated by their rapid proteolytic cleavage upon NMDA receptor stimulation in both wild type and Fmr1 KO mice. In aggregate, our study provides a novel approach to understand the molecular basis of FXS by linking the dysregulated synaptic expression of NLGNs with FMRP.

Novel calcineurin A (PPP3CA) variant associated with epilepsy, constitutive enzyme activation and downregulation of protein expression.

PPP3CA encodes calmodulin-binding catalytic subunit of calcineurin, a ubiquitously expressed calcium/calmodulin-regulated protein phosphatase. Recently de novo PPP3CA variants were reported as a cause of disease in 12 subjects presenting with epileptic encephalopathy and dysmorphic features. We describe a boy with similar phenotype and severe early onset epileptic encephalopathy in whom a novel de novo c.1324C>T (p.(Gln442Ter)) PPP3CA variant was found by whole exome sequencing. Western blot experiments in patient's cells (EBV transformed lymphocytes and neuronal cells derived through reprogramming) indicate that despite normal mRNA abundance the protein expression level is strongly reduced both for the mutated and wild-type protein. By in vitro studies with recombinant protein expressed in E. coli we show that c.1324C>T (p.(Gln442Ter)) results in constitutive activation of the enzyme. Our results confirm the role of PPP3CA defects in pathogenesis of a distinct neurodevelopmental disorder including severe epilepsy and dysmorphism and provide further functional clues regarding the pathogenic mechanism.

The level of microRNA 21 is upregulated by rapamycin in serum of tuberous sclerosis complex patients and subependymal giant cell astrocytoma (SEGA)-derived cell cultures.

Tuberous sclerosis complex (TSC) represents a genetic condition, in which the clinical manifestations are caused by the disinhibition of the mammalian target of rapamycin (mTOR) pathway due to mutations in the TSC1 (hamartin) or TSC2 (tuberin) genes. The deregulated mTOR activity leads to multi-site tumors, including subependymal giant cell astrocytoma (SEGA). SEGA is a brain tumor that affects around 15% of TSC patients. The aim of the study was to evaluate miR-21 expression in the serum of two groups of TSC patients: with or without SEGA tumors. We found no differences in the level of miR-21 depending on the presence of SEGA. Next, we studied the influence of prolonged rapamycin administration on miR-21 level in the blood serum of TSC patients (6-12 months of rapamycin) and in primary cultures of SEGA-derived cells treated with rapamycin in vitro. Here we show that rapamycin treatment leads to the upregulation of miR-21 in both patients' serum and in primary SEGA tumor cells in the culture indicating the regulatory relationship between rapamycin treatment and miR-21 expression.

Neurodevelopmental phenotype caused by a de novo PTPN4 single nucleotide variant disrupting protein localization in neuronal dendritic spines.

Protein tyrosine phosphatase non-receptor type 4 (PTPN4) encodes non-receptor protein tyrosine phosphatase implicated in synaptic plasticity and innate immune response. The only report of PTPN4-associated disease described a neurodevelopmental disorder associated with a whole gene deletion. We describe a child with developmental delay, autistic features, hypotonia, increased immunoglobulin E and dental problems with a novel mosaic de novo variant in PTPN4 (hg19 chr2:g.120620188 T > C, NM_002830.3:p.[Leu72Ser]/c.215T>C) located in domain that controls protein subcellular distribution. Studies in mouse hippocampal neurons transfected with non-mutated or mutated human PTPN4 showed that despite their similar expression in neurons the mutated protein was absent from dendritic spines. Next, we studied patient's primary blood mononuclear cells' response to lipopolysaccharide stimulation and found no difference from control in phosphorylation of TBK1 and IRF3 (involved in Toll-like receptor 4 signaling) and induction of cytokines' messenger RNA. We conclude that the PTPN4 p.(Leu72Ser) variant is a likely cause of neurodevelopmental symptoms of our proband whereas its role in immune dysfunction requires further studies.

Rapid, experience-dependent translation of neurogranin enables memory encoding.

Experience induces de novo protein synthesis in the brain and protein synthesis is required for long-term memory. It is important to define the critical temporal window of protein synthesis and identify newly synthesized proteins required for memory formation. Using a behavioral paradigm that temporally separates the contextual exposure from the association with fear, we found that protein synthesis during the transient window of context exposure is required for contextual memory formation. Among an array of putative activity-dependent translational neuronal targets tested, we identified one candidate, a schizophrenia-associated candidate mRNA, neurogranin (Ng, encoded by the Nrgn gene) responding to novel-context exposure. The Ng mRNA was recruited to the actively translating mRNA pool upon novel-context exposure, and its protein levels were rapidly increased in the hippocampus. By specifically blocking activity-dependent translation of Ng using virus-mediated molecular perturbation, we show that experience-dependent translation of Ng in the hippocampus is required for contextual memory formation. We further interrogated the molecular mechanism underlying the experience-dependent translation of Ng, and found that fragile-X mental retardation protein (FMRP) interacts with the 3'UTR of the Nrgn mRNA and is required for activity-dependent translation of Ng in the synaptic compartment and contextual memory formation. Our results reveal that FMRP-mediated, experience-dependent, rapid enhancement of Ng translation in the hippocampus during the memory acquisition enables durable context memory encoding.

Preparation of polysomal fractions from mouse brain synaptoneurosomes and analysis of polysomal-bound mRNAs.

BACKGROUND: Here we describe a detailed, reliable protocol for isolation of polysomal fractions from mouse brain synaptoneurosomes. This method is an important tool to study local protein synthesis in neurons. NEW METHOD: We combined rapid preparation of synaptoneurosomes by filtration with polysome profiling. We provide a detailed protocol highlighting difficulties and critical steps of: i) preparation of synaptoneurosomes; ii) polyribosome fractionation from synaptoneurosomes; iii) extraction of proteins and RNA from sucrose gradient fractions. RESULTS: and Comparison with Existing Methods We fractionated polyribosomes from synaptoneurosomes and detected the association of Mmp9, Camk2a and Stx1B mRNA with polysomes in the unstimulated conditions. Synaptic stimulation led to increased levels of Mmp9 and Camk2a mRNA in the heavy polysomal fractions. We compared our protocol with existing methods CONCLUSIONS: We have developed a reliable, effective method to prepare polyribosomal fractions from synaptoneurosomes to study polyribosomal binding of mRNAs as an aspect of synaptic translation in vitro.

A normal genetic variation modulates synaptic MMP-9 protein levels and the severity of schizophrenia symptoms.

Matrix metalloproteinase 9 (MMP-9) has recently emerged as a molecule that contributes to pathological synaptic plasticity in schizophrenia, but explanation of the underlying mechanisms has been missing. In the present study, we performed a phenotype-based genetic association study (PGAS) in > 1,000 schizophrenia patients from the Göttingen Research Association for Schizophrenia (GRAS) data collection and found an association between the MMP-9 rs20544 C/T single-nucleotide polymorphism (SNP) located in the 3'untranslated region (UTR) and the severity of a chronic delusional syndrome. In cultured neurons, the rs20544 SNP influenced synaptic MMP-9 activity and the morphology of dendritic spines. We demonstrated that Fragile X mental retardation protein (FMRP) bound the MMP-9 3'UTR We also found dramatic changes in RNA structure folding and alterations in the affinity of FMRP for MMP-9 RNA, depending on the SNP variant. Finally, we observed greater sensitivity to psychosis-related locomotor hyperactivity in Mmp-9 heterozygous mice. We propose a novel mechanism that involves MMP-9-dependent changes in dendritic spine morphology and the pathophysiology of schizophrenia, providing the first mechanistic insights into the way in which the single base change in the MMP-9 gene (rs20544) influences gene function and results in phenotypic changes observed in schizophrenia patients.

miR-132 Regulates Dendritic Spine Structure by Direct Targeting of Matrix Metalloproteinase 9 mRNA.

Mir-132 is a neuronal activity-regulated microRNA that controls the morphology of dendritic spines and neuronal transmission. Similar activities have recently been attributed to matrix metalloproteinase-9 (MMP-9), an extrasynaptic protease. In the present study, we provide evidence that miR-132 directly regulates MMP-9 mRNA in neurons to modulate synaptic plasticity. With the use of luciferase reporter system, we show that miR-132 binds to the 3'UTR of MMP-9 mRNA to regulate its expression in neurons. The overexpression of miR-132 in neurons reduces the level of endogenous MMP-9 protein secretion. In synaptoneurosomes, metabotropic glutamate receptor (mGluR)-induced signaling stimulates the dissociation of miR-132 from polyribosomal fractions and shifts it towards the messenger ribonucleoprotein (mRNP)-containing fraction. Furthermore, we demonstrate that the overexpression of miR-132 in the cultured hippocampal neurons from Fmr1 KO mice that have increased synaptic MMP-9 level provokes enlargement of the dendritic spine heads, a process previously implicated in enhanced synaptic plasticity. We propose that activity-dependent miR-132 regulates structural plasticity of dendritic spines through matrix metalloproteinase 9.

Structural Plasticity of Dendritic Spines Requires GSK3α and GSK3ß.

Although memories appear to be elusive phenomena, they are stored in the network of physical connections between neurons. Dendritic spines, which are actin-rich dendritic protrusions, serve as the contact points between networked neurons. The spines' shape contributes to the strength of signal transmission. To acquire and store information, dendritic spines must remain plastic, i.e., able to respond to signals, by changing their shape. We asked whether glycogen synthase kinase (GSK) 3α and GSK3ß, which are implicated in diseases with neuropsychiatric symptoms, such as Alzheimer's disease, bipolar disease and schizophrenia, play a role in a spine structural plasticity. We used Latrunculin B, an actin polymerization inhibitor, and chemically induced Long-Term Depression to trigger fast spine shape remodeling in cultured hippocampal neurons. Spine shrinkage induced by either stimulus required GSK3α activity. GSK3ß activity was only important for spine structural changes after treatment with Latrunculin B. Our results indicate that GSK3α is an essential component for short-term spine structural plasticity. This specific function should be considered in future studies of neurodegenerative diseases and neuropsychiatric conditions that originate from suboptimal levels of GSK3α/ß activity.

HAX-1: a novel p-body protein.

HAX-1, a multifunctional protein involved in the regulation of apoptosis, cell migration, and calcium homeostasis, binds the 3' untranslated region motifs of specific transcripts. This suggests that HAX-1 plays a role in post-transcriptional regulation, at the level of mRNA stability/transport or translation. In this study, we analyze in detail HAX-1 colocalization with processing bodies (P-bodies) and its dependence on mRNA availability. Endogenous P-body markers DCP1 and Rck/p54 were shown to colocalize with endogenous HAX-1, but in case of the overexpressed proteins, only DCP1 displayed unperturbed colocalization with HAX-1. HAX-1 colocalization with DCP1 was observed in most of the cell lines studied, but its presence was not required for P-body formation, and its silencing caused an increase in P-body number. Preliminary mapping suggested that HAX-1 has more than one short P-body-targeting sequence. The pools of P-body-localized HAX-1 and cytosolic HAX-1 were demonstrated to dynamically exchange, suggesting steady flow of the protein. Active transcription was shown to be a factor in the localization of HAX-1 to P-bodies. Also, it was observed that HAX-1 localizes to some unidentified foci, which do not contain DCP1. In addition, it was demonstrated that HAX-1 status influences vimentin expression levels. Overall, HAX-1 was shown to colocalize with P-body markers and influence P-body number per cell in a manner dependent on mRNA availability. Presented data support the hypothesis that HAX-1 is involved in mRNA processing as an element of P-body interaction network.

The Fragile X mental retardation protein regulates matrix metalloproteinase 9 mRNA at synapses.

Activity-dependent protein synthesis at synapses is dysregulated in the Fragile X syndrome (FXS). This process contributes to dendritic spine dysmorphogenesis and synaptic dysfunction in FXS. Matrix Metalloproteinase 9 (MMP-9) is an enzyme involved in activity-dependent reorganization of dendritic spine architecture and was shown to regulate spine morphology in a mouse model of FXS, the Fmr1 knock-out mice. Here we show that MMP-9 mRNA is part of the FMRP complex and colocalizes in dendrites. In the absence of FMRP MMP-9 mRNA translation is increased at synapses, suggesting that this mechanism contributes to the increased metalloproteinase level at synapses of Fmr1 knock-out mice. We propose that such a local effect can contribute to the aberrant dendritic spine morphology observed in the Fmr1 knock-out mice and in patients with FXS.