In vivo neuronal function of the fragile X mental retardation protein is regulated by phosphorylation

Drosophila melanogaster as a Model to Study Fragile X-Associated Disorders

Fragile X syndrome (FXS) is a global neurodevelopmental disorder caused by the expansion of CGG trinucleotide repeats (≥200) in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene. FXS is the hallmark of Fragile X-associated disorders (FXD) and the most common monogenic cause of inherited intellectual disability and autism spectrum disorder. There are several animal models used to study FXS. In the FXS model of Drosophila, the only ortholog of FMR1, dfmr1, is mutated so that its protein is missing. This model has several relevant phenotypes, including defects in the circadian output pathway, sleep problems, memory deficits in the conditioned courtship and olfactory conditioning paradigms, deficits in social interaction, and deficits in neuronal development. In addition to FXS, a model of another FXD, Fragile X-associated tremor/ataxia syndrome (FXTAS), has also been established in Drosophila. This review summarizes many years of research on FXD in Drosophila models.

The Drosophila fragile X mental retardation protein modulates the neuronal cytoskeleton to limit dendritic arborization

Dendritic arbor development is a complex, highly regulated process. Post-transcriptional regulation mediated by RNA-binding proteins plays an important role in neuronal dendrite morphogenesis by delivering on-site, on-demand protein synthesis. Here, we show how the Drosophila fragile X mental retardation protein (FMRP), a conserved RNA-binding protein, limits dendrite branching to ensure proper neuronal function during larval sensory neuron development. FMRP knockdown causes increased dendritic terminal branch growth and a resulting overelaboration defect due, in part, to altered microtubule stability and dynamics. FMRP also controls dendrite outgrowth by regulating the Drosophila profilin homolog chickadee (chic). FMRP colocalizes with chic mRNA in dendritic granules and regulates its dendritic localization and protein expression. Whereas RNA-binding domains KH1 and KH2 are both crucial for FMRP-mediated dendritic regulation, KH2 specifically is required for FMRP granule formation and chic mRNA association, suggesting a link between dendritic FMRP granules and FMRP function in dendrite elaboration. Our studies implicate FMRP-mediated modulation of both the neuronal microtubule and actin cytoskeletons in multidendritic neuronal architecture, and provide molecular insight into FMRP granule formation and its relevance to FMRP function in dendritic patterning.

Intercepting IRE1 kinase-FMRP signaling prevents atherosclerosis progression

Fragile X Mental Retardation protein (FMRP), widely known for its role in hereditary intellectual disability, is an RNA‐binding protein (RBP) that controls translation of select mRNAs. We discovered that endoplasmic reticulum (ER) stress induces phosphorylation of FMRP on a site that is known to enhance translation inhibition of FMRP‐bound mRNAs. We show ER stress‐induced activation of Inositol requiring enzyme‐1 (IRE1), an ER‐resident stress‐sensing kinase/endoribonuclease, leads to FMRP phosphorylation and to suppression of macrophage cholesterol efflux and apoptotic cell clearance (efferocytosis). Conversely, FMRP deficiency and pharmacological inhibition of IRE1 kinase activity enhances cholesterol efflux and efferocytosis, reducing atherosclerosis in mice. Our results provide mechanistic insights into how ER stress‐induced IRE1 kinase activity contributes to macrophage cholesterol homeostasis and suggests IRE1 inhibition as a promising new way to counteract atherosclerosis.

Regulation of AMPAR expression by microRNAs

AMPA receptors (AMPARs) are the major excitatory neurotransmitter receptor in the brain, and their expression at synapses is a critical determinant of synaptic transmission and therefore brain function. Synaptic plasticity involves increases or decreases in synaptic strength, caused by changes in the number or subunit-specific subtype of AMPARs expressed at synapses, and resulting in modifications of functional connectivity of neuronal circuits, a process which is thought to underpin learning and the formation or loss of memories. Furthermore, numerous neurological disorders involve dysregulation of excitatory synaptic transmission or aberrant recruitment of plasticity processes. MicroRNAs (miRNAs) repress the translation of target genes by partial complementary base pairing with mRNAs, and are the core component of a mechanism widely used in a range of cell processes for regulating protein translation. MiRNA-dependent translational repression can occur locally in neuronal dendrites, close to synapses, and can also result in relatively rapid changes in protein expression. MiRNAs are therefore well-placed to regulate synaptic plasticity via the local control of AMPAR subunit synthesis, and can also result in synaptic dysfunction in the event of dysregulation in disease. Here, I will review the miRNAs that have been identified as playing a role in physiological or pathological changes in AMPAR subunit expression at synapses, focussing on miRNAs that target mRNAs encoding AMPAR subunits, and on miRNAs that target AMPAR accessory proteins involved in AMPAR trafficking and hence the regulation of AMPAR synaptic localisation.

Post-translational modifications of the Fragile X Mental Retardation Protein in neuronal function and dysfunction

The Fragile X Mental Retardation Protein (FMRP) is an RNA-binding protein essential to the regulation of local translation at synapses. In the mammalian brain, synapses are constantly formed and eliminated throughout development to achieve functional neuronal networks. At the molecular level, thousands of proteins cooperate to accomplish efficient neuronal communication. Therefore, synaptic protein levels and their functional interactions need to be tightly regulated. FMRP generally acts as a translational repressor of its mRNA targets. FMRP is the target of several post-translational modifications (PTMs) that dynamically regulate its function. Here we provide an overview of the PTMs controlling the FMRP function and discuss how their spatiotemporal interplay contributes to the physiological regulation of FMRP. Importantly, FMRP loss-of-function leads to Fragile X syndrome (FXS), a rare genetic developmental condition causing a range of neurological alterations including intellectual disability (ID), learning and memory impairments, autistic-like features and seizures. Here, we also explore the possibility that recently reported missense mutations in the FMR1 gene disrupt the PTM homoeostasis of FMRP, thus participating in the aetiology of FXS. This suggests that the pharmacological targeting of PTMs may be a promising strategy to develop innovative therapies for patients carrying such missense mutations.

Modelling Learning and Memory in Drosophila to Understand Intellectual Disabilities

Neurodevelopmental disorders (NDDs) include a large number of conditions such as Fragile  X  syndrome, autism spectrum disorders and Down syndrome, among others. They are characterized by limitations in adaptive and social behaviors, as well as intellectual disability (ID). Whole-exome and whole-genome sequencing studies have highlighted a large number of NDD/ID risk genes. To dissect the genetic causes and underlying biological pathways, in vivo experimental validation of the effects of these mutations is needed. The fruit fly, Drosophila melanogaster, is an ideal model to study NDDs, with highly tractable genetics, combined with simple behavioral and circuit assays, permitting rapid medium-throughput screening of NDD/ID risk genes. Here, we review studies where the use of well-established assays to study mechanisms of learning and memory in Drosophila has permitted insights into molecular mechanisms underlying IDs. We discuss how technologies in the fly model, combined with a high degree of molecular and physiological conservation between flies and mammals, highlight the Drosophila system as an ideal model to study neurodevelopmental disorders, from genetics to behavior.

Structural and Functional Abnormalities in the Olfactory System of Fragile X Syndrome Models

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability. It is produced by mutation of the Fmr1 gene that encodes for the Fragile Mental Retardation Protein (FMRP), an important RNA-binding protein that regulates the expression of multiple proteins located in neuronal synapses. Individuals with FXS exhibit abnormal sensory information processing frequently leading to hypersensitivity across sensory modalities and consequently a wide array of behavioral symptoms. Insects and mammals engage primarily their sense of smell to create proper representations of the external world and guide adequate decision-making processes. This feature in combination with the exquisitely organized neuronal circuits found throughout the olfactory system (OS) and the wide expression of FMRP in brain regions that process olfactory information makes it an ideal model to study sensory alterations in FXS models. In the last decade several groups have taken advantage of these features and have used the OS of fruit fly and rodents to understand neuronal alteration giving rise to sensory perception issues. In this review article, we will discuss molecular, morphological and physiological aspects of the olfactory information processing in FXS models. We will highlight the decreased inhibitory/excitatory synaptic balance and the diminished synaptic plasticity found in this system resulting in behavioral alteration of individuals in the presence of odorant stimuli.

Fragile X Mental Retardation Protein positively regulates PKA anchor Rugose and PKA activity to control actin assembly in learning/memory circuitry

Recent work shows Fragile X Mental Retardation Protein (FMRP) drives the translation of very large proteins (>2000 aa) mediating neurodevelopment. Loss of function results in Fragile X syndrome (FXS), the leading heritable cause of intellectual disability (ID) and autism spectrum disorder (ASD). Using the Drosophila FXS disease model, we discover FMRP positively regulates the translation of the very large A-Kinase Anchor Protein (AKAP) Rugose (>3000 aa), homolog of ASD-associated human Neurobeachin (NBEA). In the central brain Mushroom Body (MB) circuit, where Protein Kinase A (PKA) signaling is necessary for learning/memory, FMRP loss reduces Rugose levels and targeted FMRP overexpression elevates Rugose levels. Using a new in vivo transgenic PKA activity reporter (PKA-SPARK), we find FMRP loss reduces PKA activity in MB Kenyon cells whereas FMRP overexpression elevates PKA activity. Consistently, loss of Rugose reduces PKA activity, but Rugose overexpression has no independent effect. A well-established PKA output is regulation of F-actin cytoskeleton dynamics. In the FXS disease model, F-actin is aberrantly accumulated in MB lobes and single MB Kenyon cells. Consistently, Rugose loss results in similar F-actin accumulation. Moreover, targeted FMRP, Rugose and PKA overexpression all result in increased F-actin accumulation in the MB circuit. These findings uncover a FMRP-Rugose-PKA mechanism regulating actin cytoskeleton. This study reveals a novel FMRP mechanism controlling neuronal PKA activity, and demonstrates a shared mechanistic connection between FXS and NBEA associated ASD disease states, with a common link to PKA and F-actin misregulation in brain neural circuits. SIGNIFICANCE STATEMENT: Autism spectrum disorder (ASD) arises from a wide array of genetic lesions, and it is therefore critical to identify common underlying molecular mechanisms. Here, we link two ASD states; Neurobeachin (NBEA) associated ASD and Fragile X syndrome (FXS), the most common inherited ASD. Using established Drosophila disease models, we find Fragile X Mental Retardation Protein (FMRP) positively regulates translation of NBEA homolog Rugose, consistent with a recent advance showing FMRP promotes translation of very large proteins associated with ASD. FXS exhibits reduced cAMP induction, a potent activator of PKA, and Rugose/NBEA is a PKA anchor. Consistently, we find brain PKA activity strikingly reduced in both ASD models. We discover this pathway regulation controls actin cytoskeleton dynamics in brain neural circuits.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

A systematic review of Drosophila short-term-memory genetics: Meta-analysis reveals robust reproducibility

Geneticists use olfactory conditioning in Drosophila to identify learning genes; however, little is known about how these genes are integrated into short-term memory (STM) pathways. Here, we investigated the hypothesis that the STM evidence base is weak. We performed systematic review and meta-analysis of the field. Using metrics to quantify variation between discovery articles and follow-up studies, we found that seven genes were both highly replicated, and highly reproducible. However, ∼80% of STM genes have never been replicated. While only a few studies investigated interactions, the reviewed genes could account for >1000% memory. This large summed effect size could indicate irreproducibility, many shared pathways, or that current assay protocols lack the specificity needed to identify core plasticity genes. Mechanistic theories of memory will require the convergence of evidence from system, circuit, cellular, molecular, and genetic experiments; systematic data synthesis is an essential tool for integrated neuroscience.

Joining the dots - protein-RNA interactions mediating local mRNA translation in neurons

Establishing and maintaining the complex network of connections required for neuronal communication requires the transport and in situ translation of large groups of mRNAs to create local proteomes. In this Review, we discuss the regulation of local mRNA translation in neurons and the RNA-binding proteins that recognise RNA zipcode elements and connect the mRNAs to the cellular transport networks, as well as regulate their translation control. However, mRNA recognition by the regulatory proteins is mediated by the combinatorial action of multiple RNA-binding domains. This increases the specificity and affinity of the interaction, while allowing the protein to recognise a diverse set of targets and mediate a range of mechanisms for translational regulation. The structural and molecular understanding of the interactions can be used together with novel microscopy and transcriptome-wide data to build a mechanistic framework for the regulation of local mRNA translation.

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.

ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

The leading cause of heritable intellectual disability (ID) and autism spectrum disorders (ASD), Fragile X syndrome (FXS), is caused by loss of the mRNA-binding translational suppressor Fragile X Mental Retardation Protein (FMRP). In the Drosophila FXS disease model, we found FMRP binds shrub mRNA (human Chmp4) to repress Shrub expression, causing overexpression during the disease state early-use critical period. The FXS hallmark is synaptic overelaboration causing circuit hyperconnectivity. Testing innervation of a central brain learning/memory center, we found FMRP loss and Shrub overexpression similarly increase connectivity. The ESCRT-III core protein Shrub has a central role in endosome-to-multivesicular body membrane trafficking, with synaptic requirements resembling FMRP. Consistently, we found FMRP loss and Shrub overexpression similarly elevate endosomes and result in the arrested accumulation of enlarged intraluminal vesicles within synaptic boutons. Importantly, genetic correction of Shrub levels in the FXS model prevents synaptic membrane trafficking defects and strongly restores innervation. These results reveal a new molecular mechanism underpinning the FXS disease state.

Mammalian FMRP S499 Is Phosphorylated by CK2 and Promotes Secondary Phosphorylation of FMRP

The fragile X mental retardation protein (FMRP) is an mRNA-binding regulator of protein translation that associates with 4-6% of brain transcripts and is central to neurodevelopment. Autism risk genes’ transcripts are overrepresented among FMRP-binding mRNAs, and FMRP loss-of-function mutations are responsible for fragile X syndrome, the most common cause of monogenetic autism. It is thought that FMRP-dependent translational repression is governed by the phosphorylation of serine residue 499 (S499). However, recent evidence suggests that S499 phosphorylation is not modulated by metabotropic glutamate receptor class I (mGluR-I) or protein phosphatase 2A (PP2A), two molecules shown to regulate FMRP translational repression. Moreover, the mammalian FMRP S499 kinase remains unknown. We found that casein kinase II (CK2) phosphorylates murine FMRP S499. Further, we show that phosphorylation of FMRP S499 permits phosphorylation of additional, nearby residues. Evidence suggests that these nearby residues are modulated by mGluR-I and PP2A pathways. These data support an alternative phosphodynamic model of FMRP that is harmonious with prior studies and serves as a framework for further investigation.

Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.

Metabotropic Glutamate Receptor and Fragile X Signaling in a Female Model of Escalated Aggression

BACKGROUND

Escalated aggression is a behavioral sign of numerous psychiatric disorders characterized by a loss of control. The neurobiology underlying escalated aggression is unknown and is particularly understudied in females. Research in our laboratory demonstrated that repeated aggressive experience in female hamsters resulted in an escalated response to future aggressive encounters and an increase in dendritic spine density on nucleus accumbens (NAc) neurons. We hypothesized that the activation of group I metabotropic glutamate receptor signaling though the fragile X mental retardation protein (FMRP) pathway may underlie synaptic plasticity associated with aggression escalation.

METHODS

Female hamsters were given five daily aggression tests with or without prior treatment with the metabotropic glutamate receptor 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)-pyridine. Following aggression testing, messenger RNA expression and protein levels were measured in the nucleus accumbens for postsynaptic density protein 95 (PSD-95) and SAP90/PSD-95-associated protein 3, as well as the levels of phosphorylated FMRP.

RESULTS

Experience-dependent escalation of aggression in female hamsters depends on activation of mGluR5 receptors. Furthermore, aggressive experience decreases phosphorylation of FMRP in the NAc, which is coupled to a long-term increase in the expression of the synaptic scaffolding proteins PSD-95 and SAP90/PSD-95-associated protein 3. Finally, the experience-dependent increase in PSD-95 is prevented by antagonism of the mGluR5 receptor.

CONCLUSIONS

Activation of the FMRP pathway by group I metabotropic glutamate receptors is involved in regulating synaptic plasticity following aggressive experience. The NAc is a novel target for preclinical studies of the treatment of escalated aggression, with the added benefit that emerging therapeutic approaches are likely to be effective in treating pathologic aggression in both female and male subjects.

A fully automated Drosophila olfactory classical conditioning and testing system for behavioral learning and memory assessment

BACKGROUND

Aversive olfactory classical conditioning has been the standard method to assess Drosophila learning and memory behavior for decades, yet training and testing are conducted manually under exceedingly labor-intensive conditions. To overcome this severe limitation, a fully automated, inexpensive system has been developed, which allows accurate and efficient Pavlovian associative learning/memory analyses for high-throughput pharmacological and genetic studies.

NEW METHOD

The automated system employs a linear actuator coupled to an odorant T-maze with airflow-mediated transfer of animals between training and testing stages. Odorant, airflow and electrical shock delivery are automatically administered and monitored during training trials. Control software allows operator-input variables to define parameters of Drosophila learning, short-term memory and long-term memory assays.

RESULTS

The approach allows accurate learning/memory determinations with operational fail-safes. Automated learning indices (immediately post-training) and memory indices (after 24h) are comparable to traditional manual experiments, while minimizing experimenter involvement.

COMPARISON WITH EXISTING METHODS

The automated system provides vast improvements over labor-intensive manual approaches with no experimenter involvement required during either training or testing phases. It provides quality control tracking of airflow rates, odorant delivery and electrical shock treatments, and an expanded platform for high-throughput studies of combinational drug tests and genetic screens. The design uses inexpensive hardware and software for a total cost of ∼$500US, making it affordable to a wide range of investigators.

CONCLUSIONS

This study demonstrates the design, construction and testing of a fully automated Drosophila olfactory classical association apparatus to provide low-labor, high-fidelity, quality-monitored, high-throughput and inexpensive learning and memory behavioral assays.

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

Therapeutic Strategies in Fragile X Syndrome: From Bench to Bedside and Back

Fragile X syndrome (FXS), an inherited intellectual disability often associated with autism, is caused by the loss of expression of the fragile X mental retardation protein. Tremendous progress in basic, preclinical, and translational clinical research has elucidated a variety of molecular-, cellular-, and system-level defects in FXS. This has led to the development of several promising therapeutic strategies, some of which have been tested in larger-scale controlled clinical trials. Here, we will summarize recent advances in understanding molecular functions of fragile X mental retardation protein beyond the well-known role as an mRNA-binding protein, and will describe current developments and emerging limitations in the use of the FXS mouse model as a preclinical tool to identify therapeutic targets. We will review the results of recent clinical trials conducted in FXS that were based on some of the preclinical findings, and discuss how the observed outcomes and obstacles will inform future therapy development in FXS and other autism spectrum disorders.

Learning and behavioral deficits associated with the absence of the fragile X mental retardation protein: what a fly and mouse model can teach us

The Fragile X syndrome (FXS) is the most frequent form of inherited mental disability and is considered a monogenic cause of autism spectrum disorder. FXS is caused by a triplet expansion that inhibits the expression of the FMR1 gene. The gene product, the Fragile X Mental Retardation Protein (FMRP), regulates mRNA metabolism in brain and nonneuronal cells. During brain development, FMRP controls the expression of key molecules involved in receptor signaling, cytoskeleton remodeling, protein synthesis and, ultimately, spine morphology. Symptoms associated with FXS include neurodevelopmental delay, cognitive impairment, anxiety, hyperactivity, and autistic-like behavior. Twenty years ago the first Fmr1 KO mouse to study FXS was generated, and several years later other key models including the mutant Drosophila melanogaster, dFmr1, have further helped the understanding of the cellular and molecular causes behind this complex syndrome. Here, we review to which extent these biological models are affected by the absence of FMRP, pointing out the similarities with the observed human dysfunction. Additionally, we discuss several potential treatments under study in animal models that are able to partially revert some of the FXS abnormalities.

FMRP S499 is phosphorylated independent of mTORC1-S6K1 activity

Hyperactive mammalian target of rapamycin (mTOR) is associated with cognitive deficits in several neurological disorders including tuberous sclerosis complex (TSC). The phosphorylation of the mRNA-binding protein FMRP reportedly depends on mTOR complex 1 (mTORC1) activity via p70 S6 kinase 1 (S6K1). Because this phosphorylation is thought to regulate the translation of messages important for synaptic plasticity, we explored whether FMRP phosphorylation of the S6K1-dependent residue (S499) is altered in TSC and states of dysregulated TSC-mTORC1 signaling. Surprisingly, we found that FMRP S499 phosphorylation was unchanged in heterozygous and conditional Tsc1 knockout mice despite significantly elevated mTORC1-S6K1 activity. Neither up- nor down-regulation of the mTORC1-S6K1 axis in vivo or in vitro had any effect on phospho-FMRP S499 levels. In addition, FMRP S499 phosphorylation was unaltered in S6K1-knockout mice. Collectively, these data strongly suggest that FMRP S499 phosphorylation is independent of mTORC1-S6K1 activity and is not altered in TSC.

Fragile X mental retardation protein regulates translation by binding directly to the ribosome

Fragile X syndrome (FXS) is the most common form of inherited mental retardation, and it is caused by loss of function of the fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that is involved in the translational regulation of several neuronal mRNAs. However, the precise mechanism of translational inhibition by FMRP is unknown. Here, we show that FMRP inhibits translation by binding directly to the L5 protein on the 80S ribosome. Furthermore, cryoelectron microscopic reconstruction of the 80S ribosome⋅FMRP complex shows that FMRP binds within the intersubunit space of the ribosome such that it would preclude the binding of tRNA and translation elongation factors on the ribosome. These findings suggest that FMRP inhibits translation by blocking the essential components of the translational machinery from binding to the ribosome.

Drosophila models of early onset cognitive disorders and their clinical applications

The number of genes known to cause human monogenic diseases is increasing rapidly. For the extremely large, genetically and phenotypically heterogeneous group of intellectual disability (ID) disorders, more than 600 causative genes have been identified to date. However, knowledge about the molecular mechanisms and networks disrupted by these genetic aberrations is lagging behind. The fruit fly Drosophila has emerged as a powerful model organism to close this knowledge gap. This review summarizes recent achievements that have been made in this model and envisions its future contribution to our understanding of ID genetics and neuropathology. The available resources and efficiency of Drosophila place it in a position to tackle the main challenges in the field: mapping functional modules of ID genes to provide conceptually novel insights into the genetic control of cognition, tailored functional studies to improve 'next-generation' diagnostics, and identification of reversible ID phenotypes and medication. Drosophila's behavioral repertoire and powerful genetics also open up perspectives for modeling genetically complex forms of ID and neuropsychiatric disorders, which overlap in their genetic etiologies. In conclusion, Drosophila provides many opportunities to advance future medical genomics of early onset cognitive disorders.

Impaired activity-dependent neural circuit assembly and refinement in autism spectrum disorder genetic models

Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics, transgenic calcium reporters, highly-targeted transgenic drivers for individually-identified neurons, and a vastly improved connectome of the brain are now being combined to provide unparalleled opportunities to both manipulate and monitor A-D processes during critical period brain development in defined neural circuits. The field is now poised to exploit this new Drosophila transgenic toolbox for the systematic dissection of A-D mechanisms in normal versus ASD brain development, particularly utilizing the well-established Drosophila FXS disease model.

GABAergic circuit dysfunction in the Drosophila Fragile X syndrome model

Fragile X syndrome (FXS), caused by loss of FMR1 gene function, is the most common heritable cause of intellectual disability and autism spectrum disorders. The FMR1 protein (FMRP) translational regulator mediates activity-dependent control of synapses. In addition to the metabotropic glutamate receptor (mGluR) hyperexcitation FXS theory, the GABA theory postulates that hypoinhibition is causative for disease state symptoms. Here, we use the Drosophila FXS model to assay central brain GABAergic circuitry, especially within the Mushroom Body (MB) learning center. All 3 GABAA receptor (GABAAR) subunits are reportedly downregulated in dfmr1 null brains. We demonstrate parallel downregulation of glutamic acid decarboxylase (GAD), the rate-limiting GABA synthesis enzyme, although GABAergic cell numbers appear unaffected. Mosaic analysis with a repressible cell marker (MARCM) single-cell clonal studies show that dfmr1 null GABAergic neurons innervating the MB calyx display altered architectural development, with early underdevelopment followed by later overelaboration. In addition, a new class of extra-calyx terminating GABAergic neurons is shown to include MB intrinsic α/β Kenyon Cells (KCs), revealing a novel level of MB inhibitory regulation. Functionally, dfmr1 null GABAergic neurons exhibit elevated calcium signaling and altered kinetics in response to acute depolarization. To test the role of these GABAergic changes, we attempted to pharmacologically restore GABAergic signaling and assay effects on the compromised MB-dependent olfactory learning in dfmr1 mutants, but found no improvement. Our results show that GABAergic circuit structure and function are impaired in the FXS disease state, but that correction of hypoinhibition alone is not sufficient to rescue a behavioral learning impairment.

Biophysical characterization of G-quadruplex forming FMR1 mRNA and of its interactions with different fragile X mental retardation protein isoforms

Fragile X syndrome, the most common form of inherited mental impairment in humans, is caused by the absence of the fragile X mental retardation protein (FMRP) due to a CGG trinucleotide repeat expansion in the 5'-untranslated region (UTR) and subsequent translational silencing of the fragile x mental retardation-1 (FMR1) gene. FMRP, which is proposed to be involved in the translational regulation of specific neuronal messenger RNA (mRNA) targets, contains an arginine-glycine-glycine (RGG) box RNA binding domain that has been shown to bind with high affinity to G-quadruplex forming mRNA structures. FMRP undergoes alternative splicing, and the binding of FMRP to a proposed G-quadruplex structure in the coding region of its mRNA (named FBS) has been proposed to affect the mRNA splicing events at exon 15. In this study, we used biophysical methods to directly demonstrate the folding of FMR1 FBS into a secondary structure that contains two specific G-quadruplexes and analyze its interactions with several FMRP isoforms. Our results show that minor splice isoforms, ISO2 and ISO3, created by the usage of the second and third acceptor sites at exon 15, bind with higher affinity to FBS than FMRP ISO1, which is created by the usage of the first acceptor site. FMRP ISO2 and ISO3 cannot undergo phosphorylation, an FMRP post-translational modification shown to modulate the protein translation regulation. Thus, their expression has to be tightly regulated, and this might be accomplished by a feedback mechanism involving the FMRP interactions with the G-quadruplex structures formed within FMR1 mRNA.

Deletion of PTEN produces deficits in conditioned fear and increases fragile X mental retardation protein

The phosphatase and tensin homolog detected on chromosome 10 (PTEN) gene product modulates activation of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway. The PI3K pathway has been found to be involved in the regulation of the fragile X mental retardation protein, which is important for long-term depression and in the formation of new memories. We used delayed fear conditioning and trace fear conditioning to determine learning and memory deficits in neuron subset-specific Pten (NS-Pten) conditional knockout (KO) mice. We found that NS-Pten KO mice had deficits in contextual learning and trace conditioning, but did not have deficits in the ability to learn a conditioned stimulus. Furthermore, we found increased levels in the total and phosphorylated forms of the fragile X mental retardation protein (FMRP) in the hippocampus of NS-Pten KO mice.

Fruitful research: drug target discovery for neurodegenerative diseases in Drosophila

INTRODUCTION

Although vertebrate model systems have obvious advantages in the study of human disease, invertebrate organisms have contributed enormously to this field as well. The conservation of genome structure and physiology among organisms poses unexpected peculiarities, and the redundancy in certain gene families or the presence of polymorphisms that can slightly alter gene expression can, in certain instances, bring invertebrate systems, such as Drosophila, closer to humans than mice and vice versa. This necessitates the analysis of disease pathways in multiple model organisms.

AREAS COVERED

The author highlights findings from Drosophila models of neurodegenerative diseases that have occurred in the past few years. She also highlights and discusses various molecular, genetic and genomic tools used in flies, as well as methods for generating disease models. Finally, the author describes Drosophila models of Alzheimer's, Parkinson's tri-nucleotide repeat diseases, and Fragile X syndrome and summarizes insights in disease mechanisms that have been discovered directly in fly models.

EXPERT OPINION

Full genome genetic screens in Drosophila can lead to the rapid identification of drug target candidates that can be subsequently validated in a vertebrate system. In addition, the Drosophila models of neurodegeneration may often show disease phenotypes that are absent in equivalent mouse models. The author believes that the extensive contribution of Drosophila to both new disease drug target discovery, in addition to target validation, makes them indispensible to drug discovery and development.

RNA-binding proteins and translational regulation in axons and growth cones

RNA localization and regulation play an important role in the developing and adult nervous system. In navigating axons, extrinsic cues can elicit rapid local protein synthesis that mediates directional or morphological responses. The mRNA repertoire in axons is large and dynamically changing, yet studies suggest that only a subset of these mRNAs are translated after cue stimulation, suggesting the need for a high level of translational regulation. Here, we review the role of RNA-binding proteins (RBPs) as local regulators of translation in developing axons. We focus on their role in growth, guidance, and synapse formation, and discuss the mechanisms by which they regulate translation in axons.

Cellular and molecular characterization of multipolar Map5-expressing cells: a subset of newly generated, stage-specific parenchymal cells in the mammalian central nervous system

Although extremely interesting in adult neuro-glio-genesis and promising as an endogenous source for repair, parenchymal progenitors remain largely obscure in their identity and physiology, due to a scarce availability of stage-specific markers. What appears difficult is the distinction between real cell populations and various differentiation stages of the same population. Here we focused on a subset of multipolar, polydendrocyte-like cells (mMap5 cells) expressing the microtubule associated protein 5 (Map5), which is known to be present in most neurons. We characterized the morphology, phenotype, regional distribution, proliferative dynamics, and stage-specific marker expression of these cells in the rabbit and mouse CNS, also assessing their existence in other mammalian species. mMap5 cells were never found to co-express the Ng2 antigen. They appear to be a population of glial cells sharing features but also differences with Ng2+progenitor cells. We show that mMap5 cells are newly generated, postmitotic parenchymal elements of the oligodendroglial lineage, thus being a stage-specific population of polydendrocytes. Finally, we report that the number of mMap5 cells, although reduced within the brain of adult/old animals, can increase in neurodegenerative and traumatic conditions.

Fragile X mental retardation protein and synaptic plasticity

Loss of the translational repressor FMRP causes Fragile X syndrome. In healthy neurons, FMRP modulates the local translation of numerous synaptic proteins. Synthesis of these proteins is required for the maintenance and regulation of long-lasting changes in synaptic strength. In this role as a translational inhibitor, FMRP exerts profound effects on synaptic plasticity.

Characterization of fragile X mental retardation protein recruitment and dynamics in Drosophila stress granules

The RNA-binding protein Fragile X Mental Retardation (FMRP) is an evolutionarily conserved protein that is particularly abundant in the brain due to its high expression in neurons. FMRP deficiency causes fragile X mental retardation syndrome. In neurons, FMRP controls the translation of target mRNAs in part by promoting dynamic transport in and out neuronal RNA granules. We and others have previously shown that upon stress, mammalian FMRP dissociates from translating polysomes to localize into neuronal-like granules termed stress granules (SG). This localization of FMRP in SG is conserved in Drosophila. Whether FMRP plays a key role in SG formation, how FMRP is recruited into SG, and whether its association with SG is dynamic are currently unknown. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMR1 (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple model, we assessed the role of dFMRP in SG formation and defined the determinants required for its recruitment in SG as well as its dynamics in SG. We show that dFMRP is dispensable for SG formation in vitro and ex vivo. FRAP experiments showed that dFMRP shuttles in and out SG. The shuttling activity of dFMRP is mediated by a protein-protein interaction domain located at the N-terminus of the protein. This domain is, however, dispensable for the localization of dFMRP in SG. This localization of dFMRP in SG requires the KH and RGG motifs which are known to mediate RNA binding, as well as the C-terminal glutamine/asparagine rich domain. Our studies thus suggest that the mechanisms controlling the recruitment of FMRP into SG and those that promote its shuttling between granules and the cytosol are uncoupled. To our knowledge, this is the first demonstration of the regulated shuttling activity of a SG component between RNA granules and the cytosol.

RNA-binding proteins involved in RNA localization and their implications in neuronal diseases

Very often, developmental abnormalities or subtle disturbances of neuronal function may yield brain diseases even if they become obvious only late in life. It is therefore our intention to highlight fundamental mechanisms of neuronal cell biology with a special emphasis on dendritic mRNA localization including local protein synthesis at the activated synapse. Furthermore, we would like to point out possible links to neuronal or synaptic dysfunction. In particular, we will focus on a series of well-known RNA-binding proteins that are involved in these processes and outline how their dysfunction might yield neurodevelopmental, neurodegenerative or neuropsychiatric disorders. We are convinced that increasing our understanding of RNA biology in general and the mechanisms underlying mRNA transport and subsequent translation at the synapse will ultimately generate important novel RNA-based tools in the near future that will allow us to hopefully treat some of these devastating diseases.

Characterization of Fragile X Mental Retardation Protein granules formation and dynamics in Drosophila

FMRP is an evolutionarily conserved protein that is highly expressed in neurons and its deficiency causes fragile X mental retardation syndrome. FMRP controls the translation of target mRNAs in part by promoting their dynamic transport in neuronal RNA granules. We have previously shown that high expression of mammalian FMRP induces formation of granules termed FMRP granules. These RNA granules are reminiscent of neuronal granules, of stress granules, as well as of the recently described in vitro-assembled granules. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMRP (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple organism, we investigated formation and dynamics of FMRP granules. We found that increased expression of dFMRP in Drosophila cells induces the formation of dynamic dFMRP RNA granules. Mutagenesis studies identified the N-terminal protein–protein domain of dFMRP as a key determinant for FMRP granules formation. The RGG RNA binding motif of dFMRP is dispensable for dFMRP granules formation since its deletion does not prevent formation of those granules. Deletion of the RGG motif reduced, however, dFMRP trafficking between FMRP granules and the cytosol. Similarly, deletion of a large part of the KH RNA binding motif of dFMRP had no effect on formation of dFMRP-granules, but diminished the shuttling activity of dFMRP. Our results thus suggest that the mechanisms controlling formation of RNA granules and those promoting their dynamics are uncoupled. This study opens new avenues to further elucidate the molecular mechanisms controlling FMRP trafficking with its associated mRNAs in and out of RNA granules.

Genetic approaches in Drosophila for the study neurodevelopmental disorders

The fruit fly Drosophila melanogaster is one of the premier genetic model organisms used in biomedical research today owing to the extraordinary power of its genetic tool-kit. Made famous by numerous seminal discoveries of basic developmental mechanisms and behavioral genetics, the power of fruit fly genetics is becoming increasingly applied to questions directly relevant to human health. In this review we discuss how Drosophila research is applied to address major questions in neurodevelopmental disorders. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Evidence for a fragile X mental retardation protein-mediated translational switch in metabotropic glutamate receptor-triggered Arc translation and long-term depression

Group 1 metabotropic glutamate receptor (mGluR)-stimulated protein synthesis and long-term synaptic depression (mGluR-LTD) are altered in the mouse model of fragile X syndrome, Fmr1 knock-out (KO) mice. Fmr1 encodes fragile X mental retardation protein (FMRP), a dendritic RNA binding protein that functions, in part, as a translational suppressor. It is unknown whether and how FMRP acutely regulates LTD and/or the rapid synthesis of new proteins required for LTD, such as the activity-regulated cytoskeletal-associated protein (Arc). The protein phosphatase PP2A dephosphorylates FMRP, which contributes to translational activation of some target mRNAs. Here, we report that PP2A and dephosphorylation of FMRP at S500 are required for an mGluR-induced, rapid (5 min) increase in dendritic Arc protein and LTD in rat and mouse hippocampal neurons. In Fmr1 KO neurons, basal, dendritic Arc protein levels and mGluR-LTD are enhanced, but mGluR-triggered Arc synthesis is absent. Lentiviral-mediated expression of wild-type FMRP in Fmr1 KO neurons suppresses basal dendritic Arc levels and mGluR-LTD, and restores rapid mGluR-triggered Arc synthesis. A phosphomimic of FMRP (S500D) suppresses steady-state dendritic Arc levels but does not rescue mGluR-induced Arc synthesis. A dephosphomimic of FMRP (S500A) neither suppresses dendritic Arc nor supports mGluR-induced Arc synthesis. Accordingly, S500D-FMRP expression in Fmr1 KO neurons suppresses mGluR-LTD, whereas S500A-FMRP has no effect. These data support a model in which phosphorylated FMRP functions to suppress steady-state translation of Arc and LTD. Upon mGluR activation of PP2A, FMRP is rapidly dephosphorylated, which contributes to rapid new synthesis of Arc and mGluR-LTD.

Matrix metalloproteinases and minocycline: therapeutic avenues for fragile X syndrome

Fragile X syndrome (FXS) is the most common known genetic form of intellectual disability and autism spectrum disorders. FXS patients suffer a broad range of other neurological symptoms, including hyperactivity, disrupted circadian activity cycles, obsessive-compulsive behavior, and childhood seizures. The high incidence and devastating effects of this disease state make finding effective pharmacological treatments imperative. Recently, reports in both mouse and Drosophila FXS disease models have indicated that the tetracycline derivative minocycline may hold great therapeutic promise for FXS patients. Both models strongly suggest that minocycline acts on the FXS disease state via inhibition of matrix metalloproteinases (MMPs), a class of zinc-dependent extracellular proteases important in tissue remodeling and cell-cell signaling. Recent FXS clinical trials indicate that minocycline may be effective in treating human patients. In this paper, we summarize the recent studies in Drosophila and mouse FXS disease models and human FXS patients, which indicate that minocycline may be an effective FXS therapeutic treatment, and discuss the data forming the basis for the proposed minocycline mechanism of action as an MMP inhibitor.

The pathophysiology of fragile X (and what it teaches us about synapses)

Fragile X is the most common known inherited cause of intellectual disability and autism, and it typically results from transcriptional silencing of FMR1 and loss of the encoded protein, FMRP (fragile X mental retardation protein). FMRP is an mRNA-binding protein that functions at many synapses to inhibit local translation stimulated by metabotropic glutamate receptors (mGluRs) 1 and 5. Recent studies on the biology of FMRP and the signaling pathways downstream of mGluR1/5 have yielded deeper insight into how synaptic protein synthesis and plasticity are regulated by experience. This new knowledge has also suggested ways that altered signaling and synaptic function can be corrected in fragile X, and human clinical trials based on this information are under way.