The Genetic Control of Stoichiometry Underlying Autism

Opto-CLIP reveals dynamic FMRP regulation of mRNAs upon CA1 neuronal activation

Neuronal diversity and function are intricately linked to the dynamic regulation of RNA metabolism, including splicing, localization, and translation. Electrophysiologic studies of synaptic plasticity, models for learning and memory, are disrupted in Fragile X Syndrome (FXS). FXS is characterized by the loss of FMRP, an RNA-binding protein (RBP) known to bind and suppress translation of specific neuronal RNAs. Since molecular studies have demonstrated that synaptic plasticity in CA1 excitatory hippocampal neurons is protein-synthesis dependent, together these observations have suggested a potential role for FMRP in synaptic plasticity in FXS. To explore this model, we developed a new experimental platform, Opto-CLIP, to integrate optogenetics with cell-type specific FMRP CLIP and RiboTag in CA1 hippocampal neurons, allowing investigation of FMRP-regulated dynamics after neuronal activation. We tracked changes in FMRP binding and ribosome-associated RNA profiles 30 minutes after neuronal activation. Our findings reveal a significant reduction in FMRP-RNA binding to transcripts encoding nuclear proteins, suggesting FMRP translational inhibition may be de-repressed to allow rapid translational responses required for neuronal homeostasis. In contrast, FMRP binding to transcripts encoding synaptic targets were generally stable after activation, but all categories of targets demonstrated variability in FMRP translational control. Opto-CLIP revealed differential regulation of subsets of transcripts within CA1 neurons rapidly after depolarization, and offers promise as a generally useful platform to uncover mechanisms of RBP-mediated RNA regulation in the context of synaptic plasticity.

A general principle governing neuronal evolution reveals a human-accelerated neuron type potentially underlying the high prevalence of autism in humans

The remarkable ability of a single genome sequence to encode a diverse collection of distinct cell types, including the thousands of cell types found in the mammalian brain, is a key characteristic of multicellular life. While it has been observed that some cell types are far more evolutionarily conserved than others, the factors driving these differences in evolutionary rate remain unknown. Here, we hypothesized that highly abundant neuronal cell types may be under greater selective constraint than rarer neuronal types, leading to variation in their rates of evolution. To test this, we leveraged recently published cross-species single-nucleus RNA-sequencing datasets from three distinct regions of the mammalian neocortex. We found a strikingly consistent relationship where more abundant neuronal subtypes show greater gene expression conservation between species, which replicated across three independent datasets covering >106 neurons from six species. Based on this principle, we discovered that the most abundant type of neocortical neurons-layer 2/3 intratelencephalic excitatory neurons-has evolved exceptionally quickly in the human lineage compared to other apes. Surprisingly, this accelerated evolution was accompanied by the dramatic down-regulation of autism-associated genes, which was likely driven by polygenic positive selection specific to the human lineage. In sum, we introduce a general principle governing neuronal evolution and suggest that the exceptionally high prevalence of autism in humans may be a direct result of natural selection for lower expression of a suite of genes that conferred a fitness benefit to our ancestors while also rendering an abundant class of neurons more sensitive to perturbation.

Early postnatal development of the MDGA2+/- mouse model of synaptic dysfunction

Synaptic dysfunction underlies many neurodevelopmental disorders (NDDs). The membrane-associated mucin domain-containing glycosylphosphatidylinositol anchor proteins (MDGAs) regulate synaptic development by modulating neurexin-neuroligin complex formation. Since understanding the neurodevelopmental profile and the sex-based differences in the manifestation of the symptoms of NDDs is important for their early diagnosis, we tested a mouse model haploinsufficient for MDGA2 (MDGA2+/-) on a neurodevelopmental test battery, containing sensory, motor, and cognitive measures, as well as ultrasonic vocalizations. When male and female MDGA2+/- and wildtype (WT) C57BL/6 J mice were examined from 2 to 23 days of age using this test battery, genotype and sex differences in body weight, sensory-motor processes, and ultrasonic vocalizations were observed. The auditory startle reflex appeared earlier in the MDGA2+/- than in WT mice and the MDGA2+/- mice produced fewer ultrasonic vocalizations. The MDGA2+/- mice showed reduced locomotion and rearing than WT mice in the open field after 17 days of age and spent less time investigating a novel object than WT mice at 21 days of age. Female MDGA2+/- mice weighed less than WT females and showed lower grip strength, indicating a delay in sensory-motor development in MDGA2+/- mice, which appears to be more pronounced in females than males. The behavioural phenotypes resulting from MDGA2 haploinsufficiency suggests that it shows delayed development of motor behaviour, grip strength and exploratory behaviour, non-social phenotypes of NDDs.

FMRP regulates mRNAs encoding distinct functions in the cell body and dendrites of CA1 pyramidal neurons

Neurons rely on translation of synaptic mRNAs in order to generate activity-dependent changes in plasticity. Here, we develop a strategy combining compartment-specific crosslinking immunoprecipitation (CLIP) and translating ribosome affinity purification (TRAP) in conditionally tagged mice to precisely define the ribosome-bound dendritic transcriptome of CA1 pyramidal neurons. We identify CA1 dendritic transcripts with differentially localized mRNA isoforms generated by alternative polyadenylation and alternative splicing, including many that have altered protein-coding capacity. Among dendritic mRNAs, FMRP targets were found to be overrepresented. Cell-type-specific FMRP-CLIP and TRAP in microdissected CA1 neuropil revealed 383 dendritic FMRP targets and suggests that FMRP differentially regulates functionally distinct modules in CA1 dendrites and cell bodies. FMRP regulates ~15-20% of mRNAs encoding synaptic functions and 10% of chromatin modulators, in the dendrite and cell body, respectively. In the absence of FMRP, dendritic FMRP targets had increased ribosome association, consistent with a function for FMRP in synaptic translational repression. Conversely, downregulation of FMRP targets involved in chromatin regulation in cell bodies suggests a role for FMRP in stabilizing mRNAs containing stalled ribosomes in this compartment. Together, the data support a model in which FMRP regulates the translation and expression of synaptic and nuclear proteins within different compartments of a single neuronal cell type.

An mtDNA mutant mouse demonstrates that mitochondrial deficiency can result in autism endophenotypes

Autism spectrum disorders (ASDs) have increasingly been associated with mitochondrial dysfunction, corroborated by mitochondrial DNA (mtDNA) germline and somatic variants being found in ASD patients. If mitochondrial defects can generate ASD, then specific mtDNA mutations should induce ASD endophenotypes in mice. We tested this prediction by introduction of an mtDNA ND6 gene missense mutation (ND6^P25L) into the mouse germline and found ASD endophenotypes. The ND6^P25L mice exhibit impaired social interaction, compulsive behavior, and increased anxiety. They have reduced electroencephalographic delta and theta wave power, increased predilection to seizures, but without diminution of hippocampal interneurons. These endophenotypes correlate with impaired cortical and hippocampal mitochondrial respiration and increased reactive oxygen species production. Thus, mitochondrial defects can be sufficient to produce ASD phenotypes.

Autism spectrum disorders (ASDs) are characterized by a deficit in social communication, pathologic repetitive behaviors, restricted interests, and electroencephalogram (EEG) aberrations. While exhaustive analysis of nuclear DNA (nDNA) variation has revealed hundreds of copy number variants (CNVs) and loss-of-function (LOF) mutations, no unifying hypothesis as to the pathophysiology of ASD has yet emerged. Based on biochemical and physiological analyses, it has been hypothesized that ASD may be the result of a systemic mitochondrial deficiency with brain-specific manifestations. This proposal has been supported by recent mitochondrial DNA (mtDNA) analyses identifying both germline and somatic mtDNA variants in ASD. If mitochondrial defects do predispose to ASD, then mice with certain mtDNA mutations should present with autism endophenotypes. To test this prediction, we examined a mouse strain harboring an mtDNA ND6 gene missense mutation (P25L). This mouse manifests impaired social interactions, increased repetitive behaviors and anxiety, EEG alterations, and a decreased seizure threshold, in the absence of reduced hippocampal interneuron numbers. EEG aberrations were most pronounced in the cortex followed by the hippocampus. Aberrations in mitochondrial respiratory function and reactive oxygen species (ROS) levels were also most pronounced in the cortex followed by the hippocampus, but absent in the olfactory bulb. These data demonstrate that mild systemic mitochondrial defects can result in ASD without apparent neuroanatomical defects and that systemic mitochondrial mutations can cause tissue-specific brain defects accompanied by regional neurophysiological alterations.

Signature of Glycylglutamic Acid Structure

BACKGROUND

Glutamate (Glu) is of great interest in biomedical research. It is considered a biomarker in diabetes, which may potentially contribute to the development of autism in genetically vulnerable human populations, and it is found in relation to advanced glycation end products (AGEs) [1]. Additionally, Glu plays an active role in the function of ligand-gated ion channel glutamate receptors, chloride channels capable of filtering glutamate, as well as Potassium (K+)-channel [2]. Glu attains α [3] and β [4] crystal forms and Cβ-CH2 show asymmetric 1H signal pattern in NMR spectra.

OBJECTIVES

The current study was undertaken to understand the signal patterns of Cβ-CH2 in Glu of the smallest dipeptide, Glycylglutamic Acid (GlyGlu), as well as the order, and planarity of the amide bond in the molecule.

MATERIALS AND METHODS

NMR spectra of GlyGlu were measured in D2O to deduce 1H and 13C chemical shifts and coupling constants. GlyGlu was crystallized from MeOH and the structure was determined by single crystal X-ray diffraction techniques.

RESULTS

The sidechain of Glu in the dipeptide dissimilates the β form. The amino group of Gly (Glycine) is protonated and exhibits hydrogen bonding with the main chain carboxylate group of a symmetry-related Glu that is deprotonated in the crystal packing of GlyGlu. The deprotonated main chain carboxylate of Glu is also in hydrogen-bonding distance from the side chain carboxylic acid group that is in the protonated form of a symmetry-related Glu of the dipeptide. The Cβ-CH2 geminal protons on the side chain of Glu have different chemical shifts and splitting pattern in 1H NMR reflecting their dissymmetric environment.

CONCLUSION

The results reported will be useful for monitoring changes that Glu and/or molecules in connection to Glu may undergo in in vivo, in situ, and in vitro conditions. This provides a valuable metric which will enable the examination of the metabolites relevant to the detection and diagnosis of disease or developmental conditions, as well as scrutinizing the effectiveness of treatment options.

Cerebral Expression of Metabotropic Glutamate Receptor Subtype 5 in Idiopathic Autism Spectrum Disorder and Fragile X Syndrome: A Pilot Study

Multiple lines of evidence suggest that dysfunction of the metabotropic glutamate receptor subtype 5 (mGluR5) plays a role in the pathogenesis of autism spectrum disorder (ASD). Yet animal and human investigations of mGluR5 expression provide conflicting findings about the nature of dysregulation of cerebral mGluR5 pathways in subtypes of ASD. The demonstration of reduced mGluR5 expression throughout the living brains of men with fragile X syndrome (FXS), the most common known single-gene cause of ASD, provides a clue to examine mGluR5 expression in ASD. We aimed to (A) compare and contrast mGluR5 expression in idiopathic autism spectrum disorder (IASD), FXS, and typical development (TD) and (B) show the value of positron emission tomography (PET) for the application of precision medicine for the diagnosis and treatment of individuals with IASD, FXS, and related conditions. Two teams of investigators independently administered 3-[18F]fluoro-5-(2-pyridinylethynyl)benzonitrile ([18F]FPEB), a novel, specific mGluR5 PET ligand to quantitatively measure the density and the distribution of mGluR5s in the brain regions, to participants of both sexes with IASD and TD and men with FXS. In contrast to participants with TD, mGluR5 expression was significantly increased in the cortical regions of participants with IASD and significantly reduced in all regions of men with FXS. These results suggest the feasibility of this protocol as a valuable tool to measure mGluR5 expression in clinical trials of individuals with IASD and FXS and related conditions.

Reduced Expression of Cerebral Metabotropic Glutamate Receptor Subtype 5 in Men with Fragile X Syndrome

Glutamatergic receptor expression is mostly unknown in adults with fragile X syndrome (FXS). Favorable behavioral effects of negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 5 (mGluR5) in fmr1 knockout (KO) mouse models have not been confirmed in humans with FXS. Measurement of cerebral mGluR5 expression in humans with FXS exposed to NAMs might help in that effort. We used positron emission tomography (PET) to measure the mGluR5 density as a proxy of mGluR5 expression in cortical and subcortical brain regions to confirm target engagement of NAMs for mGluR5s. The density and the distribution of mGluR5 were measured in two independent samples of men with FXS (N = 9) and typical development (TD) (N = 8). We showed the feasibility of this complex study including MRI and PET, meaning that this challenging protocol can be accomplished in men with FXS with an adequate preparation. Analysis of variance of estimated mGluR5 expression showed that mGluR5 expression was significantly reduced in cortical and subcortical regions of men with FXS in contrast to age-matched men with TD.