An aminophenothiazine inhibitor of the NCS-1/Ric8a complex regulates synaptic function in fragile X syndrome

The protein complex formed by the Ca2+ sensor neuronal calcium sensor 1 (NCS-1) and the guanine exchange factor protein Ric8a coregulates synapse number and probability of neurotransmitter release, emerging as a potential therapeutic target for diseases affecting synapses, such as fragile X syndrome (FXS), the most common heritable autism disorder. Using crystallographic data and the virtual screening of a chemical library, we identified a set of heterocyclic small molecules as potential inhibitors of the NCS-1/Ric8a interaction. The aminophenothiazine FD44 interferes with NCS-1/Ric8a binding, and it restores normal synapse number and associative learning in a Drosophila FXS model. The synaptic effects elicited by FD44 feeding are consistent with the genetic manipulation of NCS-1. The crystal structure of NCS-1 bound to FD44 and the structure–function studies performed with structurally close analogs explain the FD44 specificity and the mechanism of inhibition, in which the small molecule stabilizes a mobile C-terminal helix inside a hydrophobic crevice of NCS-1 to impede Ric8a interaction. Our study shows the drugability of the NCS-1/Ric8a interface and uncovers a suitable region in NCS-1 for development of additional drugs of potential use on FXS and related synaptic disorders.

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Effects of the sigma-1 receptor agonist blarcamesine in a murine model of fragile X syndrome: neurobehavioral phenotypes and receptor occupancy

Scientific Reports ◽

10.1038/s41598-021-94079-7 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Samantha T. Reyes ◽

Robert M. J. Deacon ◽

Scarlett G. Guo ◽

Francisco J. Altimiras ◽

Jessa B. Castillo ◽

...

Keyword(s):

Fragile X Syndrome ◽

Neurodevelopmental Disorders ◽

Fragile X ◽

Regional Distribution ◽

Receptor Occupancy ◽

Autism Spectrum ◽

Contextual Fear Conditioning ◽

Synaptic Function ◽

Sigma 1 Receptor ◽

Sigma 1

AbstractFragile X syndrome (FXS), a disorder of synaptic development and function, is the most prevalent genetic form of intellectual disability and autism spectrum disorder. FXS mouse models display clinically-relevant phenotypes, such as increased anxiety and hyperactivity. Despite their availability, so far advances in drug development have not yielded new treatments. Therefore, testing novel drugs that can ameliorate FXS’ cognitive and behavioral impairments is imperative. ANAVEX2-73 (blarcamesine) is a sigma-1 receptor (S1R) agonist with a strong safety record and preliminary efficacy evidence in patients with Alzheimer’s disease and Rett syndrome, other synaptic neurodegenerative and neurodevelopmental disorders. S1R’s role in calcium homeostasis and mitochondrial function, cellular functions related to synaptic function, makes blarcamesine a potential drug candidate for FXS. Administration of blarcamesine in 2-month-old FXS and wild type mice for 2 weeks led to normalization in two key neurobehavioral phenotypes: open field test (hyperactivity) and contextual fear conditioning (associative learning). Furthermore, there was improvement in marble-burying (anxiety, perseverative behavior). It also restored levels of BDNF, a converging point of many synaptic regulators, in the hippocampus. Positron emission tomography (PET) and ex vivo autoradiographic studies, using the highly selective S1R PET ligand [18F]FTC-146, demonstrated the drug’s dose-dependent receptor occupancy. Subsequent analyses also showed a wide but variable brain regional distribution of S1Rs, which was preserved in FXS mice. Altogether, these neurobehavioral, biochemical, and imaging data demonstrates doses that yield measurable receptor occupancy are effective for improving the synaptic and behavioral phenotype in FXS mice. The present findings support the viability of S1R as a therapeutic target in FXS, and the clinical potential of blarcamesine in FXS and other neurodevelopmental disorders.

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Hyperexcitability and Homeostasis in Fragile X Syndrome

Frontiers in Molecular Neuroscience ◽

10.3389/fnmol.2021.805929 ◽

2022 ◽

Vol 14 ◽

Author(s):

Xiaopeng Liu ◽

Vipendra Kumar ◽

Nien-Pei Tsai ◽

Benjamin D. Auerbach

Keyword(s):

Fragile X Syndrome ◽

Fragile X ◽

Inhibitory Neuron ◽

Cell Types ◽

Developmental Time ◽

Brain Regions ◽

Synaptic Function ◽

Homeostatic Plasticity ◽

Neuron Activity ◽

The Impact

Fragile X Syndrome (FXS) is a leading inherited cause of autism and intellectual disability, resulting from a mutation in the FMR1 gene and subsequent loss of its protein product FMRP. Despite this simple genetic origin, FXS is a phenotypically complex disorder with a range of physical and neurocognitive disruptions. While numerous molecular and cellular pathways are affected by FMRP loss, there is growing evidence that circuit hyperexcitability may be a common convergence point that can account for many of the wide-ranging phenotypes seen in FXS. The mechanisms for hyperexcitability in FXS include alterations to excitatory synaptic function and connectivity, reduced inhibitory neuron activity, as well as changes to ion channel expression and conductance. However, understanding the impact of FMR1 mutation on circuit function is complicated by the inherent plasticity in neural circuits, which display an array of homeostatic mechanisms to maintain activity near set levels. FMRP is also an important regulator of activity-dependent plasticity in the brain, meaning that dysregulated plasticity can be both a cause and consequence of hyperexcitable networks in FXS. This makes it difficult to separate the direct effects of FMR1 mutation from the myriad and pleiotropic compensatory changes associated with it, both of which are likely to contribute to FXS pathophysiology. Here we will: (1) review evidence for hyperexcitability and homeostatic plasticity phenotypes in FXS models, focusing on similarities/differences across brain regions, cell-types, and developmental time points; (2) examine how excitability and plasticity disruptions interact with each other to ultimately contribute to circuit dysfunction in FXS; and (3) discuss how these synaptic and circuit deficits contribute to disease-relevant behavioral phenotypes like epilepsy and sensory hypersensitivity. Through this discussion of where the current field stands, we aim to introduce perspectives moving forward in FXS research.

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Modulation of dendritic spines and synaptic function by Rac1: A possible link to Fragile X syndrome pathology

Brain Research ◽

10.1016/j.brainres.2011.05.020 ◽

2011 ◽

Vol 1399 ◽

pp. 79-95 ◽

Cited By ~ 53

Author(s):

Odelia Y.N. Bongmba ◽

Luis A. Martinez ◽

Mary E. Elhardt ◽

Karlis Butler ◽

Maria V. Tejada-Simon

Keyword(s):

Fragile X Syndrome ◽

Dendritic Spines ◽

Fragile X ◽

Synaptic Function

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Homeostatic Inhibitory Control of Cortical Hyperexcitability in Fragile X Syndrome

10.1101/459511 ◽

2018 ◽

Author(s):

C.A. Cea-Del Rio ◽

A. Nunez-Parra ◽

S. Freedman ◽

D. Restrepo ◽

M.M. Huntsman

Keyword(s):

Synaptic Plasticity ◽

Mouse Model ◽

Somatosensory Cortex ◽

Fragile X Syndrome ◽

Fragile X ◽

Synaptic Function ◽

Network Activity ◽

Inhibitory Input ◽

Long Term Depression

AbstractIn mouse models of Fragile X Syndrome (FXS), cellular and circuit hyperexcitability are a consequence of altered brain development [reviewed in (Contractor et al., 2015)]. Mechanisms that favor or hinder plasticity of synapses could affect neuronal excitability. This includes inhibitory long term depression (I-LTD) – a heterosynaptic form of plasticity that requires the activation of metabotropic glutamate receptors (mGluRs). Differential circuit maturation leads to shifted time points for critical periods of synaptic plasticity across multiple brain regions (Harlow et al., 2010; He et al., 2014), and disruptions of the development of excitatory and inhibitory synaptic function are also observed both during development and into adulthood (Vislay et al., 2013). However, little is known about how this hyperexcitable environment affects inhibitory synaptic plasticity. Our results demonstrate that the somatosensory cortex of the Fmr1 KO mouse model of FXS exhibits increased GABAergic spontaneous activity, a faulty mGluR-mediated inhibitory input and impaired plasticity processes. We find the overall diminished mGluR activation in the Fmr1 KO mice leads to both a decreased spontaneous inhibitory postsynaptic input to principal cells and also to a disrupted form of inhibitory long term depression (I-LTD). In cortical synapses, this I-LTD is dependent on mGluR activation and the mobilization endocannabinoids (eCBs). Notably, these data suggest enhanced hyperexcitable phenotypes in FXS may be homeostatically counterbalanced by the inhibitory drive of the network and its altered response to mGluR modulation.Significance StatementFragile X Syndrome is a pervasive neurodevelopmental disorder characterized by intellectual disability, autism, epilepsy, anxiety and altered sensory sensitivity. In both in vitro and in vivo recordings in the somatosensory cortex of the Fmr1 knockout mouse model of Fragile X Syndrome we show that hyperexcitable network activity contributes to ineffective synaptic plasticity at inhibitory synapses. This increased excitability prevents cortical circuits from adapting to sensory information via ineffective plasticity mechanisms.

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Normal CA1 place fields but discoordinated network discharge in a Fmr1-null mouse model of fragile X syndrome

10.1101/152462 ◽

2017 ◽

Cited By ~ 2

Author(s):

Fraser Todd Sparks ◽

Zoe Nicole Talbot ◽

Dino Dvorak ◽

Bridget Mary Curran ◽

Juan Marcos Alarcon ◽

...

Keyword(s):

Single Cell ◽

Fragile X Syndrome ◽

Neural Coding ◽

Fragile X ◽

Field Potential ◽

Synaptic Function ◽

Null Mouse ◽

Wild Type ◽

Place Fields ◽

Cognitive Inflexibility

SummarySilence of FMR1 causes loss of fragile X mental retardation protein (FMRP) and dysregulated translation at synapses, resulting in the intellectual disability and autistic symptoms of Fragile X Syndrome (FXS). Synaptic dysfunction hypotheses for how intellectual disabilities like cognitive inflexibility arise in FXS, predict impaired neural coding in the absence of FMRP. We tested the prediction by comparing hippocampus place cells in wild-type and FXS-model mice. Experience-driven CA1 synaptic function and synaptic plasticity changes are excessive in Fmr1-null mice, but CA1 place fields are normal. However, Fmr1-null discharge relationships to local field potential oscillations are abnormally weak, stereotyped, and homogeneous; also discharge coordination within Fmr1-null place cell networks is weaker and less reliable than wild-type. Rather than disruption of single-cell neural codes, these findings point to invariant tuning of single-cell responses and inadequate discharge coordination within neural ensembles as a pathophysiological basis of cognitive inflexibility in FXS.

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