Lithium ameliorates altered glycogen synthase kinase-3 and behavior in a mouse model of fragile X syndrome

Adaptive group behavior of Fragile X mice in unfamiliar environments

Fragile X Syndrome (FXS) stands out as a prominent cause of inherited intellectual disability and a prevalent disorder closely linked to autism. FXS is characterized by substantial alterations in social behavior, encompassing social withdrawal, avoidance of eye contact, heightened social anxiety, increased arousal levels, language deficits, and challenges in regulating emotions. Conventional behavioral assessments primarily focus on short-term interactions within controlled settings. In this study, we conducted a comprehensive examination of the adaptive group behavior of Fmr1 KO male mice over a three-day period, without introducing experimental interventions or task-based evaluations. The data unveiled intricate behavioral anomalies, with the most significant changes manifesting during the initial adaptation to unfamiliar environments. Notably, certain behaviors exhibited a gradual return to typical patterns over time. This dynamic Fmr1 KO phenotype exhibited heightened activity, featuring increased exploration, amplified social interest, and an unconventional approach to social interactions characterized by a higher frequency of shorter engagements. These findings contribute to the growing understanding of social behavior in individuals with FXS and underscore the significance of comprehending their adaptive responses in various environmental contexts.

MARK1 regulates dendritic spine morphogenesis and cognitive functions in vivo

Dendritic spines play a pivotal role in synaptic communication and are crucial for learning and memory processes. Abnormalities in spine morphology and plasticity are observed in neurodevelopmental and neuropsychiatric disorders, yet the underlying signaling mechanisms remain poorly understood. The microtubule affinity regulating kinase 1 (MARK1) has been implicated in neurodevelopmental disorders, and the MARK1 gene shows accelerated evolution in the human lineage suggesting a role in cognition. However, the in vivo role of MARK1 in synaptogenesis and cognitive functions remains unknown. Here we show that forebrain-specific conditional knockout (cKO) of Mark1 in mice causes defects in dendritic spine morphogenesis in hippocampal CA1 pyramidal neurons with a significant reduction in spine density. In addition, we found loss of MARK1 causes synaptic accumulation of GKAP and GluA2. Furthermore, we found that MARK1 cKO mice show defects in spatial learning in the Morris water maze and reduced anxiety-like behaviors in the elevated plus maze. Taken together, our data show a novel role for MARK1 in regulating dendritic spine morphogenesis and cognitive functions in vivo.

Ketogenic Diet Affects Sleep Architecture in C57BL/6J Wild Type and Fragile X Mice

Nearly half of children with fragile X syndrome experience sleep problems including trouble falling asleep and frequent nighttime awakenings. The goals here were to assess sleep-wake cycles in mice in response to Fmr1 genotype and a dietary intervention that reduces hyperactivity. Electroencephalography (EEG) results were compared with published rest-activity patterns to determine if actigraphy is a viable surrogate for sleep EEG. Specifically, sleep-wake patterns in adult wild type and Fmr1KO littermate mice were recorded after EEG electrode implantation and the recordings manually scored for vigilance states. The data indicated that Fmr1KO mice exhibited sleep-wake patterns similar to wild type littermates when maintained on a control purified ingredient diet. Treatment with a high-fat, low-carbohydrate ketogenic diet increased the percentage of non-rapid eye movement (NREM) sleep in both wild type and Fmr1KO mice during the dark cycle, which corresponded to decreased activity levels. Treatment with a ketogenic diet flattened diurnal sleep periodicity in both wild type and Fmr1KO mice. Differences in several sleep microstructure outcomes (number and length of sleep and wake bouts) supported the altered sleep states in response to a ketogenic diet and were correlated with altered rest-activity cycles. While actigraphy may be a less expensive, reduced labor surrogate for sleep EEG during the dark cycle, daytime resting in mice did not correlate with EEG sleep states.

Progranulin is an FMRP target that influences macroorchidism but not behaviour in a mouse model of Fragile X Syndrome

A growing body of evidence has implicated progranulin in neurodevelopment and indicated that aberrant progranulin expression may be involved in neurodevelopmental disease. Specifically, increased progranulin expression in the prefrontal cortex has been suggested to be pathologically relevant in male Fmr1 knockout (Fmr1 KO) mice, a mouse model of Fragile X Syndrome (FXS). Further investigation into the role of progranulin in FXS is warranted to determine if therapies that reduce progranulin expression represent a viable strategy for treating patients with FXS. Several key knowledge gaps remain. The mechanism of increased progranulin expression in Fmr1 KO mice is poorly understood and the extent of progranulin's involvement in FXS-like phenotypes in Fmr1 KO mice has been incompletely explored. To this end, we have performed a thorough characterization of progranulin expression in Fmr1 KO mice. We find that the phenomenon of increased progranulin expression is post-translational and tissue-specific. We also demonstrate for the first time an association between progranulin mRNA and FMRP, suggesting that progranulin mRNA is an FMRP target. Subsequently, we show that progranulin over-expression in Fmr1 wild-type mice causes reduced repetitive behaviour engagement in females and mild hyperactivity in males but is largely insufficient to recapitulate FXS-associated behavioural, morphological, and electrophysiological abnormalities. Lastly, we determine that genetic reduction of progranulin expression on an Fmr1 KO background reduces macroorchidism but does not alter other FXS-associated behaviours or biochemical phenotypes.

The autism-associated loss of δ-catenin functions disrupts social behavior

δ-catenin is expressed in excitatory synapses and functions as an anchor for the glutamatergic AMPA receptor (AMPAR) GluA2 subunit in the postsynaptic density. The glycine 34 to serine (G34S) mutation in the δ-catenin gene has been found in autism spectrum disorder (ASD) patients and results in loss of δ-catenin functions at excitatory synapses, which is presumed to underlie ASD pathogenesis in humans. However, how the G34S mutation causes loss of δ-catenin functions to induce ASD remains unclear. Here, using neuroblastoma cells, we identify that the G34S mutation increases glycogen synthase kinase 3β (GSK3β)-dependent δ-catenin degradation to reduce δ-catenin levels, which likely contributes to the loss of δ-catenin functions. Synaptic δ-catenin and GluA2 levels in the cortex are significantly decreased in mice harboring the δ-catenin G34S mutation. The G34S mutation increases glutamatergic activity in cortical excitatory neurons while it is decreased in inhibitory interneurons, indicating changes in cellular excitation and inhibition. δ-catenin G34S mutant mice also exhibit social dysfunction, a common feature of ASD. Most importantly, pharmacological inhibition of GSK3β activity reverses the G34S-induced loss of δ-catenin function effects in cells and mice. Finally, using δ-catenin knockout mice, we confirm that δ-catenin is required for GSK3β inhibition-induced restoration of normal social behavior in δ-catenin G34S mutant animals. Taken together, we reveal that the loss of δ-catenin functions arising from the ASD-associated G34S mutation induces social dysfunction via alterations in glutamatergic activity and that GSK3β inhibition can reverse δ-catenin G34S-induced synaptic and behavioral deficits.

The autism-associated loss of δ-catenin functions disrupts social behaviors

δ-catenin is expressed in excitatory synapses and functions as an anchor for the glutamatergic AMPA receptor (AMPAR) GluA2 subunit in the postsynaptic density. The glycine 34 to serine (G34S) mutation in the δ-catenin gene is found in autism spectrum disorder (ASD) patients and induces loss of δ-catenin functions at excitatory synapses, which is presumed to underlie ASD pathogenesis in humans. However, how the G34S mutation causes loss of δ-catenin functions to induce ASD remains unclear. Here, using neuroblastoma cells, we discover that the G34S mutation generates an additional phosphorylation site for glycogen synthase kinase 3β (GSK3β). This promotes δ-catenin degradation and causes the reduction of δ-catenin levels, which likely contributes to the loss of δ-catenin functions. Synaptic δ-catenin and GluA2 levels in the cortex are significantly decreased in mice harboring the δ-catenin G34S mutation. The G34S mutation increases glutamatergic activity in cortical excitatory neurons while it is decreased in inhibitory interneurons, indicating changes in cellular excitation and inhibition. δ-catenin G34S mutant mice also exhibit social dysfunction, a common feature of ASD. Most importantly, inhibition of GSK3β activity reverses the G34S-induced loss of δ-catenin function effects in cells and mice. Finally, using δ-catenin knockout mice, we confirm that δ-catenin is required for GSK3β inhibition-induced restoration of normal social behaviors in δ-catenin G34S mutant animals. Taken together, we reveal that the loss of δ-catenin functions arising from the ASD-associated G34S mutation induces social dysfunction via alterations in glutamatergic activity and that GSK3β inhibition can reverse δ-catenin G34S-induced synaptic and behavioral deficits.

Significance Statement

δ-catenin is important for the localization and function of glutamatergic AMPA receptors at synapses in many brain regions. The glycine 34 to serine (G34S) mutation in the δ-catenin gene is found in autism patients and results in the loss of δ-catenin functions. δ-catenin expression is also closely linked to other autism-risk genes involved in synaptic structure and function, further implying that it is important for the autism pathophysiology. Importantly, social dysfunction is a key characteristic of autism. Nonetheless, the links between δ-catenin functions and social behaviors are largely unknown. The significance of the current research is thus predicated on filling this gap by discovering the molecular, cellular, and synaptic underpinnings of the role of δ-catenin in social behaviors.

Targeted Treatments for Fragile X Syndrome

The histories of targeted treatment trials in fragile X syndrome (FXS) are reviewed in animal studies and human trials. Advances in understanding the neurobiology of FXS have identified a number of pathways that are dysregulated in the absence of FMRP and are therefore pathways that can be targeted with new medication. The utilization of quantitative outcome measures to assess efficacy in multiple studies has improved the quality of more recent trials. Current treatment trials including the use of cannabidiol (CBD) topically and metformin orally have positive preliminary data, and both of these medications are available clinically. The use of the phosphodiesterase inhibitor (PDE4D), BPN1440, which raised the level of cAMP that is low in FXS has very promising results for improving cognition in adult males who underwent a controlled trial. There are many more targeted treatments that will undergo trials in FXS, so the future looks bright for new treatments.

Inhibition of glycogen synthase kinase 3 by lithium, a mechanism in search of specificity

Inhibition of Glycogen synthase kinase 3 (GSK3) is a popular explanation for the effects of lithium ions on mood regulation in bipolar disorder and other mental illnesses, including major depression, cyclothymia, and schizophrenia. Contribution of GSK3 is supported by evidence obtained from animal and patient derived model systems. However, the two GSK3 enzymes, GSK3α and GSK3β, have more than 100 validated substrates. They are thus central hubs for major biological functions, such as dopamine-glutamate neurotransmission, synaptic plasticity (Hebbian and homeostatic), inflammation, circadian regulation, protein synthesis, metabolism, inflammation, and mitochondrial functions. The intricate contributions of GSK3 to several biological processes make it difficult to identify specific mechanisms of mood stabilization for therapeutic development. Identification of GSK3 substrates involved in lithium therapeutic action is thus critical. We provide an overview of GSK3 biological functions and substrates for which there is evidence for a contribution to lithium effects. A particular focus is given to four of these: the transcription factor cAMP response element-binding protein (CREB), the RNA-binding protein FXR1, kinesin subunits, and the cytoskeletal regulator CRMP2. An overview of how co-regulation of these substrates may result in shared outcomes is also presented. Better understanding of how inhibition of GSK3 contributes to the therapeutic effects of lithium should allow for identification of more specific targets for future drug development. It may also provide a framework for the understanding of how lithium effects overlap with those of other drugs such as ketamine and antipsychotics, which also inhibit brain GSK3.

Towards Kinase Inhibitor Therapies for Fragile X Syndrome: Tweaking Twists in the Autism Spectrum Kinase Signaling Network

Absence of the Fragile X Mental Retardation Protein (FMRP) causes autism spectrum disorders and intellectual disability, commonly referred to as the Fragile X syndrome. FMRP is a negative regulator of protein translation and is essential for neuronal development and synapse formation. FMRP is a target for several post-translational modifications (PTMs) such as phosphorylation and methylation, which tightly regulate its cellular functions. Studies have indicated the involvement of FMRP in a multitude of cellular pathways, and an absence of FMRP was shown to affect several neurotransmitter receptors, for example, the GABA receptor and intracellular signaling molecules such as Akt, ERK, mTOR, and GSK3. Interestingly, many of these molecules function as protein kinases or phosphatases and thus are potentially amendable by pharmacological treatment. Several treatments acting on these kinase-phosphatase systems have been shown to be successful in preclinical models; however, they have failed to convincingly show any improvements in clinical trials. In this review, we highlight the different protein kinase and phosphatase studies that have been performed in the Fragile X syndrome. In our opinion, some of the paradoxical study conclusions are potentially due to the lack of insight into integrative kinase signaling networks in the disease. Quantitative proteome analyses have been performed in several models for the FXS to determine global molecular processes in FXS. However, only one phosphoproteomics study has been carried out in Fmr1 knock-out mouse embryonic fibroblasts, and it showed dysfunctional protein kinase and phosphatase signaling hubs in the brain. This suggests that the further use of phosphoproteomics approaches in Fragile X syndrome holds promise for identifying novel targets for kinase inhibitor therapies.

The GSK-3 Inhibitor CT99021 Enhances the Acquisition of Spatial Learning and the Accuracy of Spatial Memory

Glycogen synthase kinase 3 (GSK-3) is a Ser/Thr protein kinase that regulates many cellular processes, including synaptic plasticity. Previously, we reported that inhibition of GSK-3 prevents the induction of one of the major forms of synaptic plasticity, N-methyl-D-aspartate receptor (NMDAR)-dependent long-term depression (LTD), in hippocampal slices. In the present study, we have investigated the effects of inhibiting GSK-3 on learning and memory in healthy naïve animals. Systemic administration of a highly selective GSK-3 inhibitor, CT99021, reversibly blocked NMDAR-dependent LTD in the CA1 region of the hippocampus in anesthetized adult mice. In behavioral tasks, CT99021 had no effect on locomotor activity, anxiety, hippocampus-dependent contextual fear memory, and hippocampus-dependent reversal learning. However, CT99021 facilitated the rate of learning in the Morris water maze (MWM) and T-maze and enhanced the accuracy of long-term spatial memory in the MWM. These findings suggest that GSK-3 regulates the accuracy of spatial memory acquisition and recall.

EEG as a translational biomarker and outcome measure in fragile X syndrome

Targeted treatments for fragile X syndrome (FXS) have frequently failed to show efficacy in clinical testing, despite success at the preclinical stages. This has highlighted the need for more effective translational outcome measures. EEG differences observed in FXS, including exaggerated N1 ERP amplitudes, increased resting gamma power and reduced gamma phase-locking in the sensory cortices, have been suggested as potential biomarkers of the syndrome. These abnormalities are thought to reflect cortical hyper excitability resulting from an excitatory (glutamate) and inhibitory (GABAergic) imbalance in FXS, which has been the target of several pharmaceutical remediation studies. EEG differences observed in humans also show similarities to those seen in laboratory models of FXS, which may allow for greater translational equivalence and better predict clinical success of putative therapeutics. There is some evidence from clinical trials showing that treatment related changes in EEG may be associated with clinical improvements, but these require replication and extension to other medications. Although the use of EEG characteristics as biomarkers is still in the early phases, and further research is needed to establish its utility in clinical trials, the current research is promising and signals the emergence of an effective translational biomarker.

Glycogen Synthase Kinase-3 Inhibitors: Preclinical and Clinical Focus on CNS-A Decade Onward

The protein kinase, GSK-3, participates in diverse biological processes and is now recognized a promising drug discovery target in treating multiple pathological conditions. Over the last decade, a range of newly developed GSK-3 inhibitors of diverse chemotypes and inhibition modes has been developed. Even more conspicuous is the dramatic increase in the indications that were tested from mood and behavior disorders, autism and cognitive disabilities, to neurodegeneration, brain injury and pain. Indeed, clinical and pre-clinical studies were largely expanded uncovering new mechanisms and novel insights into the contribution of GSK-3 to neurodegeneration and central nerve system (CNS)-related disorders. In this review we summarize new developments in the field and describe the use of GSK-3 inhibitors in the variety of CNS disorders. This remarkable volume of information being generated undoubtedly reflects the great interest, as well as the intense hope, in developing potent and safe GSK-3 inhibitors in clinical practice.

Converging purinergic and immune signaling pathways drive IL-6 secretion by Fragile X cortical astrocytes via STAT3

The symptoms of Fragile X syndrome (FXS) are driven in part by abnormal glial-mediated function. FXS astrocytes release elevated levels of immune-related factors interleukin-6 (IL-6) and tenascin C (TNC), and also demonstrate increased purinergic signaling, a pathway linked to signaling factor release. Here, in cortical astrocytes from the Fmr1 knockout (KO) FXS mouse model, purinergic agonism enhanced TNC secretion and STAT3 phosphorylation, two processes linked to elevated IL-6 secretion in FXS, while STAT3 knockdown and TLR4 antagonism normalized Fmr1 KO IL-6 release. We therefore suggest that purinergic signaling and immune regulatory pathways converge to drive FXS cortical pro-inflammatory responses.

Testing Fmr1 KO Phenotypes in Response to GSK3 Inhibitors: SB216763 versus AFC03127

Glycogen synthase kinase 3 (GSK3) is a proline-directed serine-threonine kinase that is associated with several neurological disorders, including Alzheimer’s disease and fragile X syndrome (FXS). We tested the efficacy of a novel GSK3 inhibitor AFC03127, which was developed by Angelini Pharma, in comparison to the metabotropic glutamate receptor 5 inhibitor 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and the GSK3 inhibitor SB216763 in in vivo and in vitro assays in Fmr1^KO mice, a mouse model useful for the study of FXS. The in vivo assay tested susceptibility to audiogenic-induced seizures (AGS) whereas the in vitro assays assessed biomarker expression and dendritic spine length and density in cultured primary neurons as a function of drug dose. MPEP and SB216763 attenuated AGS in Fmr1^KO mice, whereas AFC03127 did not. MPEP and AFC03127 significantly reduced dendritic expression of amyloid-beta protein precursor (APP). All drugs rescued spine length and the ratio of mature dendritic spines. Spine density was not statistically different between vehicle and GSK3 inhibitor-treated cells. The drugs were tested over a wide concentration range in the in vitro assays to determine dose responses. A bell-shaped dose response decrease in APP expression was observed in response to AFC03127, which was more effective than SB216763. These findings confirm previous studies demonstrating differential effects of various GSK3 inhibitors on AGS propensity in Fmr1^KO mice and confirm APP as a downstream biomarker that is responsive to GSK3 activity.

The novel potent GSK3 inhibitor AF3581 reverts fragile X syndrome phenotype

Glycogen synthase kinase 3 (GSK3) is a kinase mediating phosphorylation on serine and threonine amino acid residues of several target molecules. The enzyme is involved in the regulation of many cellular processes and aberrant activity of GSK3 has been linked to several disease conditions such as Fragile X Syndrome (FXS). Recent evidences demonstrating an increased activity of GSK3 in murine models of FXS, suggest that dysregulation/hyperactivation of the GSK3 path should contribute to FXS development. A likely possibility could be that in FXS there is a functional impairment of the upstream inhibitory input over GSK3 thus making overactive the kinase. Since GSK3 signaling is a central regulatory node for critical neurodevelopmental pathways, understanding the contribution of GSK3 dysregulation to FXS, may provide novel targets for therapeutic interventions for this disease. In this study we used AF3581, a potent GSK3 inhibitor that we recently discovered, in an in vivo FXS mouse model to elucidate the crucial role of GSK3 in specific behavioral patterns (locomotor activity, sensorimotor gating and social behavior) associated with this disease. All the behavioral alterations manifested by Fmr1 knockout mice were reverted after a chronic treatment with our GSK3 inhibitor, confirming the importance of this pathway as a therapeutic target.

Posttranslational modifications & lithium's therapeutic effect-Potential biomarkers for clinical responses in psychiatric & neurodegenerative disorders

Several neurodegenerative diseases and neuropsychiatric disorders display aberrant posttranslational modifications (PTMs) of one, or many, proteins. Lithium treatment has been used for mood stabilization for many decades, and is highly effective for large subsets of patients with diverse neurological conditions. However, the differential effectiveness and mode of action are not fully understood. In recent years, studies have shown that lithium alters several protein PTMs, altering their function, and consequently neuronal physiology. The impetus for this review is to outline the links between lithium's therapeutic mode of action and PTM homeostasis. We first provide an overview of the principal PTMs affected by lithium. We then describe several neuropsychiatric disorders in which PTMs have been implicated as pathogenic. For each of these conditions, we discuss lithium's clinical use and explore the putative mechanism of how it restores PTM homeostasis, and thereby cellular physiology. Evidence suggests that determining specific PTM patterns could be a promising strategy to develop biomarkers for disease and lithium responsiveness.

Deciphering the roles of glycogen synthase kinase 3 (GSK3) in the treatment of autism spectrum disorder and related syndromes

Autism spectrum disorder (ASD) is a complex and multifactorial neurodevelopmental disorder characterized by the presence of restricted interests and repetitive behaviors besides deficits in social communication. Syndromic ASD is a subset of ASD caused by underlying genetic disorders, most commonly Fragile X Syndrome (FXS) and Rett Syndrome (RTT). Various mutations and consequent malfunctions in core signaling pathways have been identified in ASD, including glycogen synthase kinase 3 (GSK3). A growing body of evidence suggests a key role of GSK3 dysregulation in the pathogenesis of ASD and its related disorders. Here, we provide a synopsis of the implication of GSK3 in ASD, FXS, and RTT as a promising therapeutic target for the treatment of ASD.

Heterogeneity in Fragile X Syndrome Highlights the Need for Precision Medicine-Based Treatments

Fragile X syndrome (FXS) is the most frequent monogenic cause of autism or intellectual disability, and research on its pathogenetic mechanisms has provided important insights on this neurodevelopmental condition. Nevertheless, after 30 years of intense research, efforts to develop treatments have been mostly unsuccessful. The aim of this review is to compile evidence from existing research pointing to clinical, genetic, and therapeutic response heterogeneity in FXS and highlight the need of implementing precision medicine-based treatments. We comment on the high genetic and phenotypic heterogeneity present in FXS, as a contributing factor to the difficulties found during drug development. Given that several clinical trials have showed a non-negligeable fraction of positive responders to drugs targeting core FXS symptoms, we propose that success of clinical trials can be achieved by tackling the underlying heterogeneity in FXS by accurately stratifying patients into drug-responder subpopulations. These precision medicine-based approaches, which can be first applied to well-defined monogenic diseases such as FXS, can also serve to define drug responder profiles based on specific biomarkers or phenotypic features that can associate patients with different genetic backgrounds to a same candidate drug, thus repositioning a same drug for a larger number of patients with NDDs.

A Phase 2 Study of AMO-02 (Tideglusib) in Congenital and Childhood-Onset Myotonic Dystrophy Type 1 (DM1)

BACKGROUND

GSK3β is an intracellular regulatory kinase that is dysregulated in multiple tissues in type 1 myotonic dystrophy, a rare neuromuscular disorder that manifests at any age. AMO-02 (tideglusib) inhibits GSK3β activity in preclinical models of type 1 myotonic dystrophy and promotes cellular maturation as well as normalizes aberrant molecular and behavioral phenotypes. This phase 2 study assessed the pharmacokinetics, safety and tolerability, and preliminary efficacy of AMO-02 in adolescents and adults with congenital and childhood-onset type 1 myotonic dystrophy.

METHODS

Sixteen subjects (aged 13 to 34 years) with congenital and childhood-onset type 1 myotonic dystrophy received 12 weeks of single-blind fixed-dose oral treatment with either 400 mg (n = 8) or 1000 mg (n = 8) AMO-02 (NCT02858908). Blood samples were obtained for pharmacokinetic assessment. Safety assessments, such as laboratory tests and electrocardiograms, as well as efficacy assessments of syndromal, cognitive, and muscular functioning, were obtained.

RESULTS

AMO-02 plasma concentrations conformed to a two-compartment model with first-order absorption and elimination, and dose-dependent increases in exposure (area under the curve) were observed. AMO-02 was generally safe and well-tolerated. No early discontinuations due to adverse events or dose adjustments of AMO-02 occurred. The majority of subjects manifested clinical improvement in their central nervous system and neuromuscular symptoms after 12 weeks of treatment compared with the placebo baseline, with a larger response noted at the 1000 mg/day dose level. AMO-02 exposure (cumulative area under the curve) was significantly correlated (P < 0.01) with change from baseline on several key efficacy assessments.

CONCLUSION

AMO-02 has favorable pharmacokinetic and clinical risk/benefit profiles meriting further study as a potential treatment for congenital and childhood-onset type 1 myotonic dystrophy.

Haploinsufficiency of Cyfip2 Causes Lithium-Responsive Prefrontal Dysfunction

OBJECTIVE

Genetic variants of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) encoding an actin-regulatory protein are associated with brain disorders, including intellectual disability and epilepsy. However, specific in vivo neuronal defects and potential treatments for CYFIP2-associated brain disorders remain largely unknown. Here, we characterized Cyfip2 heterozygous (Cyfip2^+/- ) mice to understand their neurobehavioral phenotypes and the underlying pathological mechanisms. Furthermore, we examined a potential treatment for such phenotypes of the Cyfip2^+/- mice and specified a neuronal function mediating its efficacy.

METHODS

We performed behavioral analyses of Cyfip2^+/- mice. We combined molecular, ultrastructural, and in vitro and in vivo electrophysiological analyses of Cyfip2^+/- prefrontal neurons. We also selectively reduced CYFIP2 in the prefrontal cortex (PFC) of mice with virus injections.

RESULTS

Adult Cyfip2^+/- mice exhibited lithium-responsive abnormal behaviors. We found increased filamentous actin, enlarged dendritic spines, and enhanced excitatory synaptic transmission and excitability in the adult Cyfip2^+/- PFC that was restricted to layer 5 (L5) neurons. Consistently, adult Cyfip2^+/- mice showed increased seizure susceptibility and auditory steady-state responses from the cortical electroencephalographic recordings. Among the identified prefrontal defects, lithium selectively normalized the hyperexcitability of Cyfip2^+/- L5 neurons. RNA sequencing revealed reduced expression of potassium channel genes in the adult Cyfip2^+/- PFC. Virus-mediated reduction of CYFIP2 in the PFC was sufficient to induce L5 hyperexcitability and lithium-responsive abnormal behavior.

INTERPRETATION

These results suggest that L5-specific prefrontal dysfunction, especially hyperexcitability, underlies both the pathophysiology and the lithium-mediated amelioration of neurobehavioral phenotypes in adult Cyfip2^+/- mice, which can be implicated in CYFIP2-associated brain disorders. ANN NEUROL 2020;88:526-543.

Selective inhibition of glycogen synthase kinase 3α corrects pathophysiology in a mouse model of fragile X syndrome

Fragile X syndrome is caused by FMR1 gene silencing and loss of the encoded fragile X mental retardation protein (FMRP), which binds to mRNA and regulates translation. Studies in the Fmr1-/y mouse model of fragile X syndrome indicate that aberrant cerebral protein synthesis downstream of metabotropic glutamate receptor 5 (mGluR5) signaling contributes to disease pathogenesis, but clinical trials using mGluR5 inhibitors were not successful. Animal studies suggested that treatment with lithium might be an alternative approach. Targets of lithium include paralogs of glycogen synthase kinase 3 (GSK3), and nonselective small-molecule inhibitors of these enzymes improved disease phenotypes in a fragile X syndrome mouse model. However, the potential therapeutic use of GSK3 inhibitors has been hampered by toxicity arising from inhibition of both α and β paralogs. Recently, we developed GSK3 inhibitors with sufficient paralog selectivity to avoid a known toxic consequence of dual inhibition, that is, increased β-catenin stabilization. We show here that inhibition of GSK3α, but not GSK3β, corrected aberrant protein synthesis, audiogenic seizures, and sensory cortex hyperexcitability in Fmr1-/y mice. Although inhibiting either paralog prevented induction of NMDA receptor-dependent long-term depression (LTD) in the hippocampus, only inhibition of GSK3α impaired mGluR5-dependent and protein synthesis-dependent LTD. Inhibition of GSK3α additionally corrected deficits in learning and memory in Fmr1-/y mice; unlike mGluR5 inhibitors, there was no evidence of tachyphylaxis or enhanced psychotomimetic-induced hyperlocomotion. GSK3α selective inhibitors may have potential as a therapeutic approach for treating fragile X syndrome.

β-amyloid redirects norepinephrine signaling to activate the pathogenic GSK3β/tau cascade

The brain noradrenergic system is critical for normal cognition and is affected at early stages in Alzheimer's disease (AD). Here, we reveal a previously unappreciated direct role of norepinephrine signaling in connecting β-amyloid (Aβ) and tau, two key pathological components of AD pathogenesis. Our results show that Aβ oligomers bind to an allosteric site on α2A adrenergic receptor (α2AAR) to redirect norepinephrine-elicited signaling to glycogen synthase kinase 3β (GSK3β) activation and tau hyperphosphorylation. This norepinephrine-dependent mechanism sensitizes pathological GSK3β/tau activation in response to nanomolar accumulations of extracellular Aβ, which is 50- to 100-fold lower than the amount required to activate GSK3β by Aβ alone. The significance of our findings is supported by in vivo evidence in two mouse models, human tissue sample analysis, and longitudinal clinical data. Our study provides translational insights into mechanisms underlying Aβ proteotoxicity, which might have strong implications for the interpretation of Aβ clearance trial results and future drug design and for understanding the selective vulnerability of noradrenergic neurons in AD.

A DNAzyme based knockdown model for Fragile-X syndrome in zebrafish reveals a critical window for therapeutic intervention

INTRODUCTION

FXS is the leading cause of intellectual disabilities in males and a major monogenic cause of ASD (Autism spectrum disorders). It occurs due to the loss of FMRP, whose role in early development is not well understood. In this study, we have used a novel DNAzyme based approach to create a larval model of FXS in zebrafish with specific focus on the early developmental window.

METHODS

Fmr1specific DNAzymes were electroporated into embryos to create the knockdown. Changes in RNA and protein levels of FMRP and relevant biomarkers were measured in the 0-7dpf window. Behavioral tests to measure anxiety, cognitive impairments and irritability in the larvae were conducted at the 7dpf stage. Drug treatment was carried out at various time points in the 0-7dpf window to identify the critical window for pharmacological intervention.

RESULTS

The DNAzyme based knockdown approach led to a significant knockdown of FMRP in the zebrafish embryos, accompanied by increased anxiety, irritability and cognitive impairments at 7dpf, thus creating a robust larval model of FXS. Treatment with the Mavoglurant was able to rescue the behavioral phenotypes in the FXS larvae, and found to be more efficacious in the 0-3dpf window.

DISCUSSION

The results from this study have revealed that a) a DNAzyme based knockdown approach can be used to create robust larval zebrafish model of disease, in a high-throughput manner and b) optimal window for therapeutic intervention for FXS as well as other pediatric diseases with a monogenic cause can be identified using such a model.

Molecular Biomarkers in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability (ID) and a known monogenic cause of autism spectrum disorder (ASD). It is a trinucleotide repeat disorder, in which more than 200 CGG repeats in the 5' untranslated region (UTR) of the fragile X mental retardation 1 (FMR1) gene causes methylation of the promoter with consequent silencing of the gene, ultimately leading to the loss of the encoded fragile X mental retardation 1 protein, FMRP. FMRP is an RNA binding protein that plays a primary role as a repressor of translation of various mRNAs, many of which are involved in the maintenance and development of neuronal synaptic function and plasticity. In addition to intellectual disability, patients with FXS face several behavioral challenges, including anxiety, hyperactivity, seizures, repetitive behavior, and problems with executive and language performance. Currently, there is no cure or approved medication for the treatment of the underlying causes of FXS, but in the past few years, our knowledge about the proteins and pathways that are dysregulated by the loss of FMRP has increased, leading to clinical trials and to the path of developing molecular biomarkers for identifying potential targets for therapies. In this paper, we review candidate molecular biomarkers that have been identified in preclinical studies in the FXS mouse animal model and are now under validation for human applications or have already made their way to clinical trials.

Brain Penetrable Histone Deacetylase 6 Inhibitor SW-100 Ameliorates Memory and Learning Impairments in a Mouse Model of Fragile X Syndrome

Disease-modifying therapies are needed for Fragile X Syndrome (FXS), as at present there are no effective treatments or cures. Herein, we report on a tetrahydroquinoline-based selective histone deacetylase 6 (HDAC6) inhibitor SW-100, its pharmacological and ADMET properties, and its ability to improve upon memory performance in a mouse model of FXS, Fmr1^-/- mice. This small molecule demonstrates good brain penetrance, low-nanomolar potency for the inhibition of HDAC6 (IC₅₀ = 2.3 nM), with at least a thousand-fold selectivity over all other class I, II, and IV HDAC isoforms. Moreover, through its inhibition of the α-tubulin deacetylase domain of HDAC6 (CD2), in cells SW-100 upregulates α-tubulin acetylation with no effect on histone acetylation and selectively restores the impaired acetylated α-tubulin levels in the hippocampus of Fmr1^-/- mice. Lastly, SW-100 ameliorates several memory and learning impairments in Fmr1^-/- mice, thus modeling the intellectual deficiencies associated with FXS, and hence providing a strong rationale for pursuing HDAC6-based therapies for the treatment of this rare disease.

Molecular Mechanisms of Synaptic Dysregulation in Fragile X Syndrome and Autism Spectrum Disorders

Fragile X syndrome (FXS) is the most common form of monogenic hereditary cognitive impairment. FXS patient exhibit a high comorbidity rate with autism spectrum disorders (ASDs). This makes FXS a model disease for understanding how synaptic dysregulation alters neuronal excitability, learning and memory, social behavior, and more. Since 1991, with the discovery of fragile X mental retardation 1 (FMR1) as the sole gene that is mutated in FXS, thousands of studies into the function of the gene and its encoded protein FMR1 protein (FMRP), have been conducted, yielding important information regarding the pathophysiology of the disease, as well as insight into basic synaptic mechanisms that control neuronal networking and circuitry. Among the most important, are molecular mechanisms directly involved in plasticity, including glutamate and γ-aminobutyric acid (GABA) receptors, which can control synaptic transmission and signal transduction, including short- and long-term plasticity. More recently, several novel mechanisms involving growth factors, enzymatic cascades and transcription factors (TFs), have been proposed to have the potential of explaining some of the synaptic dysregulation in FXS. In this review article, I summarize the main mechanisms proposed to underlie synaptic disruption in FXS and ASDs. I focus on studies conducted on the Fmr1 knock-out (KO) mouse model and on FXS-human pluripotent stem cells (hPSCs), emphasizing the differences and even contradictions between mouse and human, whenever possible. As FXS and ASDs are both neurodevelopmental disorders that follow a specific time-course of disease progression, I highlight those studies focusing on the differential developmental regulation of synaptic abnormalities in these diseases.

Home-cage hypoactivity in mouse genetic models of autism spectrum disorder

Genome-wide association and whole exome sequencing studies from Autism Spectrum Disorder (ASD) patient populations have implicated numerous risk factor genes whose mutation or deletion results in significantly increased incidence of ASD. Behavioral studies of monogenic mutant mouse models of ASD-associated genes have been useful for identifying aberrant neural circuitry. However, behavioral results often differ from lab to lab, and studies incorporating both males and females are often not performed despite the significant sex-bias of ASD. In this study, we sought to investigate the simple, passive behavior of home-cage activity monitoring across multiple 24-h days in four different monogenic mouse models of ASD: Shank3b^-/-, Cntnap2^-/-, Pcdh10^+/-, and Fmr1 knockout mice. Relative to sex-matched wildtype (WT) littermates, we discovered significant home-cage hypoactivity, particularly in the dark (active) phase of the light/dark cycle, in male mice of all four ASD-associated transgenic models. For Cntnap2^-/- and Pcdh10^+/- mice, these activity alterations were sex-specific, as female mice did not exhibit home-cage activity differences relative to sex-matched WT controls. These home-cage hypoactivity alterations differ from activity findings previously reported using short-term activity measurements in a novel open field. Despite circadian problems reported in human ASD patients, none of the mouse models studied had alterations in free-running circadian period. Together, these findings highlight a shared phenotype across several monogenic mouse models of ASD, outline the importance of methodology on behavioral interpretation, and in some genetic lines parallel the male-enhanced phenotypic presentation observed in human ASDs.

Loss of Lgl1 Disrupts the Radial Glial Fiber-guided Cortical Neuronal Migration and Causes Subcortical Band Heterotopia in Mice

Radial glial cells (RGCs) are neuronal progenitors and function as scaffolds for neuronal radial migration in the developing cerebral cortex. These functions depend on a polarized radial glial scaffold, which is of fundamental importance for brain development. Lethal giant larvae 1 (Lgl1), a key regulator for cell polarity from Drosophila to mammals, plays a key role in tumorigenesis and brain development. To overcome neonatal lethality in Lgl1-null mice and clarify the role of Lgl1 in mouse cerebral cortex development and function, we created Lgl1 dorsal telencephalon-specific knockout mice mediated by Emx1-Cre. Lgl1^Emx1 conditional knockout (CKO) mice had normal life spans and could be used for function research. Histology results revealed that the mutant mice displayed an ectopic cortical mass in the dorsolateral hemispheric region between the normotopic cortex and the subcortical white matter, resembling human subcortical band heterotopia (SBH). The Lgl1^Emx1 CKO cortex showed disrupted adherens junctions (AJs), which were accompanied by ectopic RGCs and intermediate progenitors, and disorganization of the radial glial fiber system. The early- and late-born neurons failed to reach the destined position along the disrupted radial glial fiber scaffold and instead accumulated in ectopic positions and formed SBH. Additionally, the absence of Lgl1 led to severe abnormalities in RGCs, including hyperproliferation, impaired differentiation, and increased apoptosis. Lgl1^Emx1 CKO mice also displayed deficiencies in anxiety-related behaviors. We concluded that Lgl1 is essential for RGC development and neural migration during cerebral cortex development.

FMRP recruitment of β-catenin to the translation pre-initiation complex represses translation

Canonical Wnt/β-catenin signaling is an essential regulator of various cellular functions throughout development and adulthood. Aberrant Wnt/β-catenin signaling also contributes to various pathologies including cancer, necessitating an understanding of cell context-dependent mechanisms regulating this pathway. Since protein-protein interactions underpin β-catenin function and localization, we sought to identify novel β-catenin interacting partners by affinity purification coupled with tandem mass spectrometry in vascular smooth muscle cells (VSMCs), where β-catenin is involved in both physiological and pathological control of cell proliferation. Here, we report novel components of the VSMC β-catenin interactome. Bioinformatic analysis of the protein networks implies potentially novel functions for β-catenin, particularly in mRNA translation, and we confirm a direct interaction between β-catenin and the fragile X mental retardation protein (FMRP). Biochemical studies reveal a basal recruitment of β-catenin to the messenger ribonucleoprotein and translational pre-initiation complex, fulfilling a translational repressor function. Wnt stimulation antagonizes this function, in part, by sequestering β-catenin away from the pre-initiation complex. In conclusion, we present evidence that β-catenin fulfills a previously unrecognized function in translational repression.

Rescue of Fmr1KO phenotypes with mGluR5 inhibitors: MRZ-8456 versus AFQ-056

Metabotropic glutamate receptor 5 (mGluR₅) is a drug target for central nervous system disorders such as fragile X syndrome that involve excessive glutamate-induced excitation. We tested the efficacy of a novel negative allosteric modulator of mGluR₅ developed by Merz Pharmaceuticals, MRZ-8456, in comparison to MPEP and AFQ-056 (Novartis, a.k.a. mavoglurant) in both in vivo and in vitro assays in a mouse model of fragile X syndrome, Fmr1^KO mice. The in vivo assays included susceptibility to audiogenic-induced seizures and pharmacokinetic measurements of drug availability. The in vitro assays included dose response assessments of biomarker expression and dendritic spine length and density in cultured primary neurons. Both MRZ-8456 and AFQ-056 attenuated wild running and audiogenic-induced seizures in Fmr1^KO mice with similar pharmacokinetic profiles. Both drugs significantly reduced dendritic expression of amyloid-beta protein precursor (APP) and rescued the ratio of mature to immature dendritic spines. These findings demonstrate that MRZ-8456, a drug being developed for the treatment of motor complications of L-DOPA in Parkinson's disease and which completed a phase I clinical trial, is effective in attenuating both well-established (seizures and dendritic spine maturity) and exploratory biomarker (APP expression) phenotypes in a mouse model of fragile X syndrome.

Fragile X syndrome and fragile X-associated tremor ataxia syndrome

Fragile X-associated disorders encompass several conditions, which are caused by expansion mutations in the fragile X mental retardation 1 (FMR1) gene. Fragile X syndrome is the most common inherited etiology of intellectual disability and results from a full mutation or >200 CGG repeats in FMR1. It is associated with developmental delay, autism spectrum disorder, and seizures. Fragile X-associated tremor/ataxia syndrome is a progressive neurodegenerative disease that occurs in premutation carriers of 55-200 CGG repeats in FMR1 and is characterized by kinetic tremor, gait ataxia, parkinsonism, executive dysfunction, and neuropathy. Fragile X-associated primary ovarian insufficiency also occurs in premutation carrier women and manifests with infertility and early menopause. The diseases constituting fragile X-associated disorders differ mechanistically, due to the distinct molecular properties of premutation versus full mutations. Fragile X syndrome occurs when there is a lack of fragile X mental retardation protein (FMRP) due to FMR1 methylation and silencing. In fragile X-associated tremor ataxia syndrome, a toxic gain of function is postulated with the production of excess CGG repeat-containing FMR1 mRNA, abnormal translation of the repeat sequence leading to production of polyglycine, polyalanine, and other polypeptides and to outright deficits in translation leading to reduced FMRP at larger premutation sizes. The changes in underlying brain chemistry due to FMR1 mutations have led to therapeutic studies in these disorders, with some progress being made in fragile X syndrome. This paper also summarizes indications for testing, genetic counseling issues, and what the future holds for these disorders.

Are FXR Family Proteins Integrators of Dopamine Signaling and Glutamatergic Neurotransmission in Mental Illnesses?

Dopamine receptors and related signaling pathways have long been implicated in pathophysiology and treatment of mental illnesses, including schizophrenia and bipolar disorder. Dopamine signaling may impact neuronal activity by modulation of glutamate neurotransmission. Recent evidence indicates a direct and/or indirect involvement of fragile X-related family proteins (FXR) in the regulation and mediation of dopamine receptor functions. FXRs consists of fragile X mental retardation protein 1 (Fmr1/FMRP) and its autosomal homologs Fxr1 and Fxr2. These RNA-binding proteins are enriched in the brain. Loss of function mutation in human FMR1 is the major genetic contributor to Fragile X mental retardation syndrome. Therefore, the role of FXR proteins has mostly been studied in the context of autism spectrum disorders. However, recent genome-wide association studies have linked this family to schizophrenia, bipolar disorders, and mood regulation pointing toward a broader involvement in mental illnesses. FXR family proteins play an important role in the regulation of glutamate-mediated neuronal activity and plasticity. Here, we discuss the brain-specific functions of FXR family proteins by focusing on the regulation of dopamine receptor functions, ionotropic glutamate receptors-mediated synaptic plasticity and contribution to mental illnesses. Based on recent evidence, we propose that FXR proteins are potential integrators of dopamine signaling and ionotropic glutamate transmission.

Paradoxical effect of baclofen on social behavior in the fragile X syndrome mouse model

INTRODUCTION

Fragile X syndrome (FXS) is a common monogenetic cause of intellectual disability, autism spectrum features, and a broad range of other psychiatric and medical problems. FXS is caused by the lack of the fragile X mental retardation protein (FMRP), a translational regulator of specific mRNAs at the postsynaptic compartment. The absence of FMRP leads to aberrant synaptic plasticity, which is believed to be caused by an imbalance in excitatory and inhibitory network functioning of the synapse. Evidence from studies in mice demonstrates that GABA, the major inhibitory neurotransmitter in the brain, and its receptors, is involved in the pathogenesis of FXS. Moreover, several FXS phenotypes, including social behavior deficits, could be corrected in Fmr1 KO mice after acute treatment with GABAB agonists.

METHODS

As FXS would probably require a lifelong treatment, we investigated the effect of chronic treatment with the GABAB agonist baclofen on social behavior in Fmr1 KO mice on two behavioral paradigms for social behavior: the automated tube test and the three-chamber sociability test.

RESULTS

Unexpectedly, chronic baclofen treatment resulted in worsening of the FXS phenotypes in these behavior tests. Strikingly, baclofen treatment also affected wild-type animals in both behavioral tests, inducing a phenotype similar to that of untreated Fmr1 KO mice.

CONCLUSION

Altogether, the disappointing results of recent clinical trials with the R-baclofen enantiomer arbaclofen and our current results indicate that baclofen should be reconsidered and further evaluated before its application in targeted treatment for FXS.

Repurposing available drugs for neurodevelopmental disorders: The fragile X experience

Many available drugs have been repurposed as treatments for neurodevelopmental disorders. In the specific case of fragile X syndrome, many clinical trials of available drugs have been conducted with the goal of disease modification. In some cases, detailed understanding of basic disease mechanisms has guided the choice of drugs for clinical trials, and several notable successes in fragile X clinical trials have led to common use of drugs such as minocycline in routine medical practice. Newer technologies like Disease-Gene Expression Matching (DGEM) may allow for more rapid identification of promising repurposing candidates. A DGEM study predicted that sulindac could be therapeutic for fragile X, and subsequent preclinical validation studies have shown promising results. The use of combinations of available drugs and nutraceuticals has the potential to greatly expand the options for repurposing, and may even be a viable business strategy. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Of Men and Mice: Modeling the Fragile X Syndrome

The Fragile X Syndrome (FXS) is one of the most common forms of inherited intellectual disability in all human societies. Caused by the transcriptional silencing of a single gene, the fragile x mental retardation gene FMR1, FXS is characterized by a variety of symptoms, which range from mental disabilities to autism and epilepsy. More than 20 years ago, a first animal model was described, the Fmr1 knock-out mouse. Several other models have been developed since then, including conditional knock-out mice, knock-out rats, a zebrafish and a drosophila model. Using these model systems, various targets for potential pharmaceutical treatments have been identified and many treatments have been shown to be efficient in preclinical studies. However, all attempts to turn these findings into a therapy for patients have failed thus far. In this review, I will discuss underlying difficulties and address potential alternatives for our future research.

Disparate Effects of Lithium and a GSK-3 Inhibitor on Neuronal Oscillatory Activity in Prefrontal Cortex and Hippocampus

Glycogen synthase kinase-3 (GSK-3) plays a critical role in cognitive dysfunction associated with Alzheimer’s disease (AD), yet the mechanism by which GSK-3 alters cognitive processes in other disorders, such as schizophrenia, remains unknown. In the present study, we demonstrated a role for GSK-3 in the direct regulation of neuronal oscillations in hippocampus (HIP) and prelimbic cortex (PL). A comparison of the GSK-3 inhibitors SB 216763 and lithium demonstrated disparate effects of the drugs on spatial memory and neural oscillatory activity in HIP and PL. SB 216763 administration improved spatial memory whereas lithium treatment had no effect. Analysis of neuronal local field potentials in anesthetized animals revealed that whereas both repeated SB 216763 (2.5 mg/kg) and lithium (100 mg/kg) induced a theta frequency spike in HIP at approximately 10 Hz, only SB 216763 treatment induced an overall increase in theta power (4–12 Hz) compared to vehicle. Acute administration of either drug suppressed slow (32–59 Hz) and fast (61–100 Hz) gamma power. In PL, both drugs induced an increase in theta power. Repeated SB 216763 increased HIP–PL coherence across all frequencies except delta, whereas lithium selectively suppressed delta coherence. These findings demonstrate that GSK-3 plays a direct role in the regulation of theta oscillations in regions critically involved in cognition, and highlight a potential mechanism by which GSK-3 may contribute to cognitive decline in disorders of cognitive dysfunction.

Drug development for neurodevelopmental disorders: lessons learned from fragile X syndrome

Neurodevelopmental disorders such as fragile X syndrome (FXS) result in lifelong cognitive and behavioural deficits and represent a major public health burden. FXS is the most frequent monogenic form of intellectual disability and autism, and the underlying pathophysiology linked to its causal gene, FMR1, has been the focus of intense research. Key alterations in synaptic function thought to underlie this neurodevelopmental disorder have been characterized and rescued in animal models of FXS using genetic and pharmacological approaches. These robust preclinical findings have led to the implementation of the most comprehensive drug development programme undertaken thus far for a genetically defined neurodevelopmental disorder, including phase IIb trials of metabotropic glutamate receptor 5 (mGluR5) antagonists and a phase III trial of a GABA_B receptor agonist. However, none of the trials has been able to unambiguously demonstrate efficacy, and they have also highlighted the extent of the knowledge gaps in drug development for FXS and other neurodevelopmental disorders. In this Review, we examine potential issues in the previous studies and future directions for preclinical and clinical trials. FXS is at the forefront of efforts to develop drugs for neurodevelopmental disorders, and lessons learned in the process will also be important for such disorders.

MDM2 inhibition rescues neurogenic and cognitive deficits in a mouse model of fragile X syndrome

Fragile X syndrome, the most common form of inherited intellectual disability, is caused by loss of the fragile X mental retardation protein (FMRP). However, the mechanism remains unclear, and effective treatment is lacking. We show that loss of FMRP leads to activation of adult mouse neural stem cells (NSCs) and a subsequent reduction in the production of neurons. We identified the ubiquitin ligase mouse double minute 2 homolog (MDM2) as a target of FMRP. FMRP regulates Mdm2 mRNA stability, and loss of FMRP resulted in elevated MDM2 mRNA and protein. Further, we found that increased MDM2 expression led to reduced P53 expression in adult mouse NSCs, leading to alterations in NSC proliferation and differentiation. Treatment with Nutlin-3, a small molecule undergoing clinical trials for treating cancer, specifically inhibited the interaction of MDM2 with P53, and rescued neurogenic and cognitive deficits in FMRP-deficient mice. Our data reveal a potential regulatory role for FMRP in the balance between adult NSC activation and quiescence, and identify a potential new treatment for fragile X syndrome.

Metabotropic glutamate receptors as emerging research targets in bipolar disorder

Glutamatergic dysregulation is implicated in the neuropathology of bipolar disorder (BD). There is increasing interest in investigating the role of metabotropic glutamate receptors (mGluRs) in BD and as a target for treatment intervention. Bipolar mGluR studies (published January 1992-April 2016) were identified via PubMed, Embase, Web of Science, and Scopus. Full-text screening, data extraction, and quality appraisal were conducted in duplicate, with strict inclusion and exclusion criteria. The initial literature search for mGluRs in BD, including non-bipolar mood disorders and primary psychotic disorders, identified 1544 articles. 61 abstracts were selected for relevance, 16 articles met full inclusion criteria, and three additional articles were found via citations. Despite limited literature, studies demonstrated: single nucleotide polymorphisms (SNPs) associated with BD, including a GRM3 SNP associated with greater likelihood of psychosis (rs6465084), mRNA binding protein Fragile X Mental Retardation Protein associated with altered mGluR1/5 activity in BD populations, and lithium decreasing mGluR5 expression and mGluR-mediated intracellular calcium signaling. Limited research has been performed on the role of mGluRs in BD, but results highlight the importance of ongoing study. Future directions for research of mGluRs in BD include GRM polymorphisms, epigenetic regulation, intracellular proteins, and pharmacologic interactions.

Targets for Drug Therapy for Autism Spectrum Disorder: Challenges and Future Directions

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by persistent deficits in social communication and interaction and restricted, repetitive patterns of behavior, interests, and activities. Various factors are involved in the etiopathogenesis of ASD, including genetic factors, environmental toxins and stressors, impaired immune responses, mitochondrial dysfunction, and neuroinflammation. The heterogeneity in the phenotype among ASD patients and the complex etiology of the condition have long impeded the advancement of the development of pharmacological therapies. In the recent years, the integration of findings from mouse models to human genetics resulted in considerable progress toward the understanding of ASD pathophysiology. Currently, strategies to treat core symptoms of ASD are directed to correct synaptic dysfunctions, abnormalities in central oxytocin, vasopressin, and serotonin neurotransmission, and neuroinflammation. Here, we present a survey of the studies that have suggested molecular targets for drug development for ASD and the state-of-the-art of medicinal chemistry efforts in related areas.

GSK-3β deletion in dentate gyrus excitatory neuron impairs synaptic plasticity and memory

Increasing evidence suggests that glycogen synthase kinase-3β (GSK-3β) plays a crucial role in neurodegenerative/psychiatric disorders, while pan-neural knockout of GSK-3β also shows detrimental effects. Currently, the function of GSK-3β in specific type of neurons is elusive. Here, we infused AAV-CaMKII-Cre-2A-eGFP into GSK-3β^lox/lox mice to selectively delete the kinase in excitatory neurons of hippocampal dentate gyrus (DG), and studied the effects on cognitive/psychiatric behaviors and the molecular mechanisms. We found that mice with GSK-3β deletion in DG excitatory neurons displayed spatial and fear memory defects with an anti-anxiety behavior. Further studies demonstrated that GSK-3β deletion in DG subset inhibited hippocampal synaptic transmission and reduced levels of GluN1, GluN2A and GluN2B (NMDAR subunits), GluA1 (AMPAR subunit), PSD93 and drebrin (postsynaptic structural proteins), and synaptophysin (presynaptic protein). GSK-3β deletion also suppressed the activity-dependent neural activation and calcium/calmodulin-dependent protein kinase II (CaMKII)/CaMKIV-cAMP response element binding protein (CREB) signaling. Our data suggest that GSK-3β in hippocampal DG excitatory neurons is essential for maintaining synaptic plasticity and memory.

Fragile X targeted pharmacotherapy: lessons learned and future directions

Our understanding of fragile X syndrome (FXS) pathophysiology continues to improve and numerous potential drug targets have been identified. Yet, current prescribing practices are only symptom-based in order to manage difficult behaviors, as no drug to date is approved for the treatment of FXS. Drugs impacting a diversity of targets in the brain have been studied in recent FXS-specific clinical trials. While many drugs have focused on regulation of enhanced glutamatergic or deficient GABAergic neurotransmission, compounds studied have not been limited to these mechanisms. As a single-gene disorder, it was thought that FXS would have consistent drug targets that could be modulated with pharmacotherapy and lead to significant improvement. Unfortunately, despite promising results in FXS animal models, translational drug treatment development in FXS has largely failed. Future success in this field will depend on learning from past challenges to improve clinical trial design, choose appropriate outcome measures and age range choices, and find readily modulated drug targets. Even with many negative placebo-controlled study results, the field continues to move forward exploring both the new mechanistic drug approaches combined with ways to improve trial execution. This review summarizes the known phenotype and pathophysiology of FXS and past clinical trial rationale and results, and discusses current challenges facing the field and lessons from which to learn for future treatment development efforts.

Excitation/Inhibition Imbalance in Animal Models of Autism Spectrum Disorders

Imbalances between excitation and inhibition in synaptic transmission and neural circuits have been implicated in autism spectrum disorders. Excitation and inhibition imbalances are frequently observed in animal models of autism spectrum disorders, and their correction normalizes key autistic-like phenotypes in these animals. These results suggest that excitation and inhibition imbalances may contribute to the development and maintenance of autism spectrum disorders and represent an important therapeutic target.

Hyperactive locomotion in a Drosophila model is a functional readout for the synaptic abnormalities underlying fragile X syndrome

Fragile X syndrome (FXS) is the most common cause of heritable intellectual disability and autism and affects ~1 in 4000 males and 1 in 8000 females. The discovery of effective treatments for FXS has been hampered by the lack of effective animal models and phenotypic readouts for drug screening. FXS ensues from the epigenetic silencing or loss-of-function mutation of the fragile X mental retardation 1 (FMR1) gene, which encodes an RNA binding protein that associates with and represses the translation of target mRNAs. We previously found that the activation of LIM kinase 1 (LIMK1) downstream of augmented synthesis of bone morphogenetic protein (BMP) type 2 receptor (BMPR2) promotes aberrant synaptic development in mouse and Drosophila models of FXS and that these molecular and cellular markers were correlated in patients with FXS. We report that larval locomotion is augmented in a Drosophila FXS model. Genetic or pharmacological intervention on the BMPR2-LIMK pathway ameliorated the synaptic abnormality and locomotion phenotypes of FXS larvae, as well as hyperactivity in an FXS mouse model. Our study demonstrates that (i) the BMPR2-LIMK pathway is a promising therapeutic target for FXS and (ii) the locomotion phenotype of FXS larvae is a quantitative functional readout for the neuromorphological phenotype associated with FXS and is amenable to the screening novel FXS therapeutics.

Intranasal siRNA administration reveals IGF2 deficiency contributes to impaired cognition in Fragile X syndrome mice

Molecular mechanisms underlying learning and memory remain imprecisely understood, and restorative interventions are lacking. We report that intranasal administration of siRNAs can be used to identify targets important in cognitive processes and to improve genetically impaired learning and memory. In mice modeling the intellectual deficiency of Fragile X syndrome, intranasally administered siRNA targeting glycogen synthase kinase-3β (GSK3β), histone deacetylase-1 (HDAC1), HDAC2, or HDAC3 diminished cognitive impairments. In WT mice, intranasally administered brain-derived neurotrophic factor (BDNF) siRNA or HDAC4 siRNA impaired learning and memory, which was partially due to reduced insulin-like growth factor-2 (IGF2) levels because the BDNF siRNA- or HDAC4 siRNA-induced cognitive impairments were ameliorated by intranasal IGF2 administration. In Fmr1^-/- mice, hippocampal IGF2 was deficient, and learning and memory impairments were ameliorated by IGF2 intranasal administration. Therefore intranasal siRNA administration is an effective means to identify mechanisms regulating cognition and to modulate therapeutic targets.

Pharmacotherapy for Fragile X Syndrome: Progress to Date

To date, no drug is approved for the treatment of Fragile X Syndrome (FXS) although many drugs are used to manage challenging behaviors from a symptomatic perspective in this population. While our understanding of FXS pathophysiology is expanding, efforts to devise targeted FXS-specific treatments have had limited success in placebo-controlled trials. Compounds aimed at rectifying excessive glutamate and deficient gamma-aminobutyric acid (GABA) neurotransmission, as well as other signaling pathways known to be affected by Fragile X Mental Retardation Protein (FMRP) are under various phases of development in FXS. With the failure of several metabotropic glutamate receptor subtype 5 (mGlur5) selective antagonists under clinical investigation, no clear single treatment appears to be greatly effective. These recent challenges call into question various aspects of clinical study design in FXS. More objective outcome measures are under development and validation. Future trials will likely be aimed at correcting multiple pathways known to be disrupted by the loss of FMRP. This review offers a brief summary of the prevalence, phenotypic characteristics, genetic causes and molecular functions of FMRP in the brain (as these have been extensively reviewed elsewhere), discusses the most recent finding in FXS drug development, and summarizes FXS trials utilizing symptomatic treatment.

Wnt signaling networks in autism spectrum disorder and intellectual disability

Background

Genetic factors play a major role in the risk for neurodevelopmental disorders such as autism spectrum disorders (ASDs) and intellectual disability (ID). The underlying genetic factors have become better understood in recent years due to advancements in next generation sequencing. These studies have uncovered a vast number of genes that are impacted by different types of mutations (e.g., de novo, missense, truncation, copy number variations).

Abstract

Given the large volume of genetic data, analyzing each gene on its own is not a feasible approach and will take years to complete, let alone attempt to use the information to develop novel therapeutics. To make sense of independent genomic data, one approach is to determine whether multiple risk genes function in common signaling pathways that identify signaling “hubs” where risk genes converge. This approach has led to multiple pathways being implicated, such as synaptic signaling, chromatin remodeling, alternative splicing, and protein translation, among many others. In this review, we analyze recent and historical evidence indicating that multiple risk genes, including genes denoted as high-confidence and likely causal, are part of the Wingless (Wnt signaling) pathway. In the brain, Wnt signaling is an evolutionarily conserved pathway that plays an instrumental role in developing neural circuits and adult brain function.

Conclusions

We will also review evidence that pharmacological therapies and genetic mouse models further identify abnormal Wnt signaling, particularly at the synapse, as being disrupted in ASDs and contributing to disease pathology.