Proteomics, ultrastructure, and physiology of hippocampal synapses in a fragile X syndrome mouse model reveal presynaptic phenotype

Delayed recruitment of activity-dependent bulk endocytosis in Fmr1 knockout neurons

The presynapse performs an essential role in brain communication via the activity-dependent release of neurotransmitters. However, the sequence of events through which a presynapse acquires functionality is relatively poorly understood, which is surprising, since mutations in genes essential for its operation are heavily implicated in neurodevelopmental disorders. We addressed this gap in knowledge by determining the developmental trajectory of synaptic vesicle (SV) recycling pathways in primary cultures of rat hippocampal neurons. Exploiting a series of optical and morphological assays, we revealed that the majority of nerve terminals displayed activity-dependent calcium influx from 3 days in vitro (DIV), immediately followed by functional evoked exocytosis and endocytosis, although the number of responsive nerve terminals continued to increase until the second week in vitro. However, the most intriguing discovery was that activity-dependent bulk endocytosis (ADBE) was only observed from DIV 14 onwards. Importantly, optimal ADBE recruitment was delayed until DIV 21 in Fmr1 knockout neurons, which model Fragile X Syndrome (FXS). This implicates the delayed recruitment of ADBE as a potential contributing factor in the development of circuit dysfunction in FXS, and potentially other neurodevelopmental disorders.

Region-Related Differences in Short-Term Synaptic Plasticity and Synaptotagmin-7 in the Male and Female Hippocampus of a Rat Model of Fragile X Syndrome

Fragile X syndrome (FXS) is an intellectual developmental disorder characterized, inter alia, by deficits in the short-term processing of neural information, such as sensory processing and working memory. The primary cause of FXS is the loss of fragile X messenger ribonucleoprotein (FMRP), which is profoundly involved in synaptic function and plasticity. Short-term synaptic plasticity (STSP) may play important roles in functions that are affected by FXS. Recent evidence points to the crucial involvement of the presynaptic calcium sensor synaptotagmin-7 (Syt-7) in STSP. However, how the loss of FMRP affects STSP and Syt-7 have been insufficiently studied. Furthermore, males and females are affected differently by FXS, but the underlying mechanisms remain elusive. The aim of the present study was to investigate possible changes in STSP and the expression of Syt-7 in the dorsal (DH) and ventral (VH) hippocampus of adult males and females in a Fmr1-knockout (KO) rat model of FXS. We found that the paired-pulse ratio (PPR) and frequency facilitation/depression (FF/D), two forms of STSP, as well as the expression of Syt-7, are normal in adult KO males, but the PPR is increased in the ventral hippocampus of KO females (6.4 ± 3.7 vs. 18.3 ± 4.2 at 25 ms in wild type (WT) and KO, respectively). Furthermore, we found no gender-related differences, but did find robust region-dependent difference in the STSP (e.g., the PPR at 50 ms: 50.0 ± 5.5 vs. 17.6 ± 2.9 in DH and VH of WT male rats; 53.1 ± 3.6 vs. 19.3 ± 4.6 in DH and VH of WT female rats; 48.1 ± 2.3 vs. 19.1 ± 3.3 in DH and VH of KO male rats; and 51.2 ± 3.3 vs. 24.7 ± 4.3 in DH and VH of KO female rats). AMPA receptors are similarly expressed in the two hippocampal segments of the two genotypes and in both genders. Also, basal excitatory synaptic transmission is higher in males compared to females. Interestingly, we found more than a twofold higher level of Syt-7, not synaptotagmin-1, in the dorsal compared to the ventral hippocampus in the males of both genotypes (0.43 ± 0.1 vs. 0.16 ± 0.02 in DH and VH of WT male rats, and 0.6 ± 0.13 vs. 0.23 ± 0.04 in DH and VH of KO male rats) and in the WT females (0.97 ± 0.23 vs. 0.31 ± 0.09 in DH and VH). These results point to the susceptibility of the female ventral hippocampus to FMRP loss. Importantly, the different levels of Syt-7, which parallel the higher score of the dorsal vs. ventral hippocampus on synaptic facilitation, suggest that Syt-7 may play a pivotal role in defining the striking differences in STSP along the long axis of the hippocampus.

Proteomics insights into fragile X syndrome: Unraveling molecular mechanisms and therapeutic avenues

Fragile X Syndrome (FXS) is a neurodevelopment disorder characterized by cognitive impairment, behavioral challenges, and synaptic abnormalities, with a genetic basis linked to a mutation in the FMR1 (Fragile X Messenger Ribonucleoprotein 1) gene that results in a deficiency or absence of its protein product, Fragile X Messenger Ribonucleoprotein (FMRP). In recent years, mass spectrometry (MS) - based proteomics has emerged as a powerful tool to uncover the complex molecular landscape underlying FXS. This review provides a comprehensive overview of the proteomics studies focused on FXS, summarizing key findings with an emphasis on dysregulated proteins associated with FXS. These proteins span a wide range of cellular functions including, but not limited to, synaptic plasticity, RNA translation, and mitochondrial function. The work conducted in these proteomic studies provides a more holistic understanding to the molecular pathways involved in FXS and considerably enhances our knowledge into the synaptic dysfunction seen in FXS.

Septotemporal Variation of Information Processing in the Hippocampus of Fmr1 KO Rat

INTRODUCTION

Fragile X messenger ribonucleoprotein (FMRP) is a protein involved in many neuronal processes in the nervous system including the modulation of synaptic transmission. The loss of FMRP produces the fragile X syndrome (FXS), a neurodevelopmental disorder affecting synaptic and neuronal function and producing cognitive impairments. However, the effects of FXS on short-term processing of synaptic inputs and neuronal outputs in the hippocampus have not yet been sufficiently clarified. Furthermore, it is not known whether dorsal and ventral hippocampi are affected similarly or not in FXS.

METHOD

We used an Fmr1 knockout (KO) rat model of FXS and recordings of evoked field potentials from the CA1 field of transverse slices from both the dorsal and the ventral hippocampi of adult rats.

RESULTS

Following application of a frequency stimulation protocol consisting of a ten-pulse train and recordings of fEPSP, we found that the dorsal but not ventral KO hippocampus shows altered short-term synaptic plasticity. Furthermore, applying the frequency stimulation protocol and recordings of population spikes, both segments of the KO hippocampus display altered short-term neuronal dynamics.

CONCLUSIONS

These data suggest that short-term processing of synaptic inputs is affected in the dorsal, not ventral, FXS hippocampus, while short-term processing of neuronal output is affected in both segments of the FXS hippocampus in a similar way. These FXS-associated changes may have significant impact on the functions of the dorsal and ventral hippocampi in individuals with FXS.

On-Tissue Spatial Proteomics Integrating MALDI-MS Imaging with Shotgun Proteomics Reveals Soy Consumption-Induced Protein Changes in a Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS), the leading cause of inherited intellectual disability and autism, is caused by the transcriptional silencing of the FMR1 gene, which encodes the fragile X messenger ribonucleoprotein (FMRP). FMRP interacts with numerous brain mRNAs that are involved in synaptic plasticity and implicated in autism spectrum disorders. Our published studies indicate that single-source, soy-based diets are associated with increased seizures and autism. Thus, there is an acute need for an unbiased protein marker identification in FXS in response to soy consumption. Herein, we present a spatial proteomics approach integrating mass spectrometry imaging with label-free proteomics in the FXS mouse model to map the spatial distribution and quantify levels of proteins in the hippocampus and hypothalamus brain regions. In total, 1250 unique peptides were spatially resolved, demonstrating the diverse array of peptidomes present in the tissue slices and the broad coverage of the strategy. A group of proteins that are known to be involved in glycolysis, synaptic transmission, and coexpression network analysis suggest a significant association between soy proteins and metabolic and synaptic processes in the Fmr1KO brain. Ultimately, this spatial proteomics work represents a crucial step toward identifying potential candidate protein markers and novel therapeutic targets for FXS.

Presynaptic Protein Synthesis in Brain Function and Disease

Local protein synthesis in mature brain axons regulates the structure and function of presynaptic boutons by adjusting the presynaptic proteome to local demands. This crucial mechanism underlies experience-dependent modifications of brain circuits, and its dysregulation may contribute to brain disorders, such as autism and intellectual disability. Here, we discuss recent advancements in the axonal transcriptome, axonal RNA localization and translation, and the role of presynaptic local translation in synaptic plasticity and memory.

The potential role of ribonucleic acid methylation in the pathological mechanisms of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited cause of intellectual disabilities and single-gene cause of autism spectrum disorder (ASD), resulting from the loss of functional fragile X messenger ribonucleoprotein (FMRP), an RNA-binding protein (RBP) encoded by the fragile X messenger ribonucleoprotein 1 (FMR1) gene. Ribonucleic acid (RNA) methylation can lead to developmental diseases, including FXS, through various mechanisms mediated by 5-hydroxymethylcytosine, 5-methylcytosine, N6-methyladenosine, etc. Emerging evidence suggests that modifications of some RNA species have been linked to FXS. However, the underlying pathological mechanism has yet to be elucidated. In this review, we reviewed the implication of RNA modification in FXS and summarized its specific characteristics for facilitating the identification of new therapeutic targets.

Novel presynaptic assay system revealed that metformin ameliorates exaggerated synaptic release and Munc18-1 accumulation in presynapses of neurons from Fragile X syndrome mouse model

Fragile X syndrome (FXS) is a developmental disorder characterized by intellectual disability and autistic-like behaviors. These symptoms are supposed to result from dysregulated translation in pre- and postsynapses, resulting in aberrant synaptic plasticity. Although most drug development research on FXS has focused on aberrant postsynaptic functions by excess translation in postsynapses, the effect of drug candidates on FXS in presynaptic release is largely unclear. In this report, we developed a novel assay system using neuron ball culture with beads to induce presynapse formation, allowing for the analysis of presynaptic phenotypes, including presynaptic release. Metformin, which is shown to rescue core phenotypes in FXS mouse model by normalizing dysregulated translation, ameliorated the exaggerated presynaptic release of neurons of FXS model mouse using this assay system. Furthermore, metformin suppressed the excess accumulation of the active zone protein Munc18-1, which is supposed to be locally translated in presynapses. These results suggest that metformin rescues both postsynaptic and presynaptic phenotypes by inhibiting excess translation in FXS neurons.

The activation of mGluR4 rescues parallel fiber synaptic transmission and LTP, motor learning and social behavior in a mouse model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS), the most common inherited intellectual disability, is caused by the loss of expression of the Fragile X Messenger Ribonucleoprotein (FMRP). FMRP is an RNA-binding protein that negatively regulates the expression of many postsynaptic as well as presynaptic proteins involved in action potential properties, calcium homeostasis and neurotransmitter release. FXS patients and mice lacking FMRP suffer from multiple behavioral alterations, including deficits in motor learning for which there is currently no specific treatment.

METHODS

We performed electron microscopy, whole-cell patch-clamp electrophysiology and behavioral experiments to characterise the synaptic mechanisms underlying the motor learning deficits observed in Fmr1KO mice and the therapeutic potential of positive allosteric modulator of mGluR4.

RESULTS

We found that enhanced synaptic vesicle docking of cerebellar parallel fiber to Purkinje cell Fmr1KO synapses was associated with enhanced asynchronous release, which not only prevents further potentiation, but it also compromises presynaptic parallel fiber long-term potentiation (PF-LTP) mediated by β adrenergic receptors. A reduction in extracellular Ca2+ concentration restored the readily releasable pool (RRP) size, basal synaptic transmission, β adrenergic receptor-mediated potentiation, and PF-LTP. Interestingly, VU 0155041, a selective positive allosteric modulator of mGluR4, also restored both the RRP size and PF-LTP in mice of either sex. Moreover, when injected into Fmr1KO male mice, VU 0155041 improved motor learning in skilled reaching, classical eyeblink conditioning and vestibuloocular reflex (VOR) tests, as well as the social behavior alterations of these mice.

LIMITATIONS

We cannot rule out that the activation of mGluR4s via systemic administration of VU0155041 can also affect other brain regions. Further studies are needed to stablish the effect of a specific activation of mGluR4 in cerebellar granule cells.

CONCLUSIONS

Our study shows that an increase in synaptic vesicles, SV, docking may cause the loss of PF-LTP and motor learning and social deficits of Fmr1KO mice and that the reversal of these changes by pharmacological activation of mGluR4 may offer therapeutic relief for motor learning and social deficits in FXS.

Axonal and presynaptic FMRP: Localization, signal, and functional implications

Fragile X mental retardation protein (FMRP) binds a selected set of mRNAs and proteins to guide neural circuit assembly and regulate synaptic plasticity. Loss of FMRP is responsible for Fragile X syndrome, a neuropsychiatric disorder characterized with auditory processing problems and social difficulty. FMRP actions in synaptic formation, maturation, and plasticity are site-specific among the four compartments of a synapse: presynaptic and postsynaptic neurons, astrocytes, and extracellular matrix. This review summarizes advancements in understanding FMRP localization, signals, and functional roles in axons and presynaptic terminals.

Excessive proteostasis contributes to pathology in fragile X syndrome

In fragile X syndrome (FX), the leading monogenic cause of autism, excessive neuronal protein synthesis is a core pathophysiology; however, an overall increase in protein expression is not observed. Here, we tested whether excessive protein synthesis drives a compensatory rise in protein degradation that is protective for FX mouse model (Fmr1^-/y) neurons. Surprisingly, although we find a significant increase in protein degradation through ubiquitin proteasome system (UPS), this contributes to pathological changes. Normalizing proteasome activity with bortezomib corrects excessive hippocampal protein synthesis and hyperactivation of neurons in the inferior colliculus (IC) in response to auditory stimulation. Moreover, systemic administration of bortezomib significantly reduces the incidence and severity of audiogenic seizures (AGS) in the Fmr1^-/y mouse, as does genetic reduction of proteasome, specifically in the IC. Together, these results identify excessive activation of the UPS pathway in Fmr1^-/y neurons as a contributor to multiple phenotypes that can be targeted for therapeutic intervention.

Local mRNA translation and cytoskeletal reorganization: Mechanisms that tune neuronal responses

Neurons are highly polarized cells with significantly long axonal and dendritic extensions that can reach distances up to hundreds of centimeters away from the cell bodies in higher vertebrates. Their successful formation, maintenance, and proper function highly depend on the coordination of intricate molecular networks that allow axons and dendrites to quickly process information, and respond to a continuous and diverse cascade of environmental stimuli, often without enough time for communication with the soma. Two seemingly unrelated processes, essential for these rapid responses, and thus neuronal homeostasis and plasticity, are local mRNA translation and cytoskeletal reorganization. The axonal cytoskeleton is characterized by high stability and great plasticity; two contradictory attributes that emerge from the powerful cytoskeletal rearrangement dynamics. Cytoskeletal reorganization is crucial during nervous system development and in adulthood, ensuring the establishment of proper neuronal shape and polarity, as well as regulating intracellular transport and synaptic functions. Local mRNA translation is another mechanism with a well-established role in the developing and adult nervous system. It is pivotal for axonal guidance and arborization, synaptic formation, and function and seems to be a key player in processes activated after neuronal damage. Perturbations in the regulatory pathways of local translation and cytoskeletal reorganization contribute to various pathologies with diverse clinical manifestations, ranging from intellectual disabilities (ID) to autism spectrum disorders (ASD) and schizophrenia (SCZ). Despite the fact that both processes are essential for the orchestration of pathways critical for proper axonal and dendritic function, the interplay between them remains elusive. Here we review our current knowledge on the molecular mechanisms and specific interaction networks that regulate and potentially coordinate these interconnected processes.

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.

FMRP Sustains Presynaptic Function via Control of Activity-Dependent Bulk Endocytosis

Synaptic vesicle (SV) recycling is essential for the maintenance of neurotransmission, with a number of neurodevelopmental disorders linked to defects in this process. Fragile X syndrome (FXS) results from a loss of fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. Hyperexcitability of neuronal circuits is a key feature of FXS, therefore we investigated whether SV recycling was affected by the absence of FMRP during increased neuronal activity. We revealed that primary neuronal cultures from male Fmr1 knock-out (KO) rats display a specific defect in activity-dependent bulk endocytosis (ADBE). ADBE is dominant during intense neuronal activity, and this defect resulted in an inability of Fmr1 KO neurons to sustain SV recycling during trains of high-frequency stimulation. Using a molecular replacement strategy, we also revealed that a human FMRP mutant that cannot bind BK channels failed to correct ADBE dysfunction in KO neurons, however this dysfunction was corrected by BK channel agonists. Therefore, FMRP performs a key role in sustaining neurotransmitter release via selective control of ADBE, suggesting intervention via this endocytosis mode may correct the hyperexcitability observed in FXS.SIGNIFICANCE STATEMENT Loss of fragile X mental retardation protein (FMRP) results in fragile X syndrome (FXS), however whether its loss has a direct role in neurotransmitter release remains a matter of debate. We demonstrate that neurons lacking FMRP display a specific defect in a mechanism that sustains neurotransmitter release during intense neuronal firing, called activity-dependent bulk endocytosis (ADBE). This discovery provides key insights into mechanisms of brain communication that occur because of loss of FMRP function. Importantly it also reveals ADBE as a potential therapeutic target to correct the circuit hyperexcitability observed in FXS.

Correction of amygdalar dysfunction in a rat model of fragile X syndrome

Fragile X syndrome (FXS), a commonly inherited form of autism and intellectual disability, is associated with emotional symptoms that implicate dysfunction of the amygdala. However, current understanding of the pathogenesis of the disease is based primarily on studies in the hippocampus and neocortex, where FXS defects have been corrected by inhibiting group I metabotropic glutamate receptors (mGluRs). Here, we observe that activation, rather than inhibition, of mGluRs in the basolateral amygdala reverses impairments in a rat model of FXS. FXS rats exhibit deficient recall of auditory conditioned fear, which is accompanied by a range of in vitro and in vivo deficits in synaptic transmission and plasticity. We find presynaptic mGluR5 in the amygdala, activation of which reverses deficient synaptic transmission and plasticity, thereby restoring normal fear learning in FXS rats. This highlights the importance of modifying the prevailing mGluR-based framework for therapeutic strategies to include circuit-specific differences in FXS pathophysiology.

Reciprocal regulation of spontaneous synaptic vesicle fusion by Fragile X mental retardation protein and group I metabotropic glutamate receptors

Fragile X mental retardation protein (FMRP) is a neuronal protein mediating multiple functions, with its absence resulting in one of the most common monogenic causes of autism, Fragile X syndrome (FXS). Analyses of FXS pathophysiology have identified a range of aberrations in synaptic signaling pathways and plasticity associated with group I metabotropic glutamate (mGlu) receptors. These studies, however, have mostly focused on the post-synaptic functions of FMRP and mGlu receptor activation, and relatively little is known about their presynaptic effects. Neurotransmitter release is mediated via multiple forms of synaptic vesicle (SV) fusion, each of which contributes to specific neuronal functions. The impacts of mGlu receptor activation and loss of FMRP on these SV fusion events remain unexplored. Here we combined electrophysiological and fluorescence imaging analyses on primary hippocampal cultures prepared from an Fmr1 knockout (KO) rat model. Compared to wild-type (WT) hippocampal neurons, KO neurons displayed an increase in the frequency of spontaneous excitatory post-synaptic currents (sEPSCs), as well as spontaneous SV fusion events. Pharmacological activation of mGlu receptors in WT neurons caused a similar increase in spontaneous SV fusion and sEPSC frequency. Notably, this increase in SV fusion was not observed when spontaneous activity was blocked using the sodium channel antagonist tetrodotoxin. Importantly, the effect of mGlu receptor activation on spontaneous SV fusion was occluded in Fmr1 KO neurons. Together, our results reveal that FMRP represses spontaneous presynaptic SV fusion, whereas mGlu receptor activation increases this event. This reciprocal control appears to be mediated via their regulation of intrinsic neuronal excitability.

Reduced mGluR5 Activity Modulates Mitochondrial Function

The metabotropic glutamate receptor 5 (mGluR5) is an essential modulator of synaptic plasticity, learning and memory; whereas in pathological conditions, it is an acknowledged therapeutic target that has been implicated in multiple brain disorders. Despite robust pre-clinical data, mGluR5 antagonists failed in several clinical trials, highlighting the need for a better understanding of the mechanisms underlying mGluR5 function. In this study, we dissected the molecular synaptic modulation mediated by mGluR5 using genetic and pharmacological mouse models to chronically and acutely reduce mGluR5 activity. We found that next to dysregulation of synaptic proteins, the major regulation in protein expression in both models concerned specific processes in mitochondria, such as oxidative phosphorylation. Second, we observed morphological alterations in shape and area of specifically postsynaptic mitochondria in mGluR5 KO synapses using electron microscopy. Third, computational and biochemical assays suggested an increase of mitochondrial function in neurons, with increased level of NADP/H and oxidative damage in mGluR5 KO. Altogether, our observations provide diverse lines of evidence of the modulation of synaptic mitochondrial function by mGluR5. This connection suggests a role for mGluR5 as a mediator between synaptic activity and mitochondrial function, a finding which might be relevant for the improvement of the clinical potential of mGluR5.

Neuronal BIN1 Regulates Presynaptic Neurotransmitter Release and Memory Consolidation

BIN1, a member of the BAR adaptor protein family, is a significant late-onset Alzheimer disease risk factor. Here, we investigate BIN1 function in the brain using conditional knockout (cKO) models. Loss of neuronal Bin1 expression results in the select impairment of spatial learning and memory. Examination of hippocampal CA1 excitatory synapses reveals a deficit in presynaptic release probability and slower depletion of neurotransmitters during repetitive stimulation, suggesting altered vesicle dynamics in Bin1 cKO mice. Super-resolution and immunoelectron microscopy localizes BIN1 to presynaptic sites in excitatory synapses. Bin1 cKO significantly reduces synapse density and alters presynaptic active zone protein cluster formation. Finally, 3D electron microscopy reconstruction analysis uncovers a significant increase in docked and reserve pools of synaptic vesicles at hippocampal synapses in Bin1 cKO mice. Our results demonstrate a non-redundant role for BIN1 in presynaptic regulation, thus providing significant insights into the fundamental function of BIN1 in synaptic physiology relevant to Alzheimer disease.

BIN1 is a significant risk factor for late-onset Alzheimer disease. BIN1 has a general role in endocytosis and membrane dynamics in non-neuronal cells. De Rossi et al. show that BIN1 localizes to presynaptic terminals and plays an indispensable role in excitatory synaptic transmission by regulating neurotransmitter vesicle dynamics.

Ythdf is a N6-methyladenosine reader that modulates Fmr1 target mRNA selection and restricts axonal growth in Drosophila

N6‐methyladenosine (m⁶A) regulates a variety of physiological processes through modulation of RNA metabolism. This modification is particularly enriched in the nervous system of several species, and its dysregulation has been associated with neurodevelopmental defects and neural dysfunctions. In Drosophila, loss of m⁶A alters fly behavior, albeit the underlying molecular mechanism and the role of m⁶A during nervous system development have remained elusive. Here we find that impairment of the m⁶A pathway leads to axonal overgrowth and misguidance at larval neuromuscular junctions as well as in the adult mushroom bodies. We identify Ythdf as the main m⁶A reader in the nervous system, being required to limit axonal growth. Mechanistically, we show that the m⁶A reader Ythdf directly interacts with Fmr1, the fly homolog of Fragile X mental retardation RNA binding protein (FMRP), to inhibit the translation of key transcripts involved in axonal growth regulation. Altogether, this study demonstrates that the m⁶A pathway controls development of the nervous system and modulates Fmr1 target transcript selection.

ATP Synthase c-Subunit Leak Causes Aberrant Cellular Metabolism in Fragile X Syndrome

Loss of the gene (Fmr1) encoding Fragile X mental retardation protein (FMRP) causes increased mRNA translation and aberrant synaptic development. We find neurons of the Fmr1^-/y mouse have a mitochondrial inner membrane leak contributing to a "leak metabolism." In human Fragile X syndrome (FXS) fibroblasts and in Fmr1^-/y mouse neurons, closure of the ATP synthase leak channel by mild depletion of its c-subunit or pharmacological inhibition normalizes stimulus-induced and constitutive mRNA translation rate, decreases lactate and key glycolytic and tricarboxylic acid (TCA) cycle enzyme levels, and triggers synapse maturation. FMRP regulates leak closure in wild-type (WT), but not FX synapses, by stimulus-dependent ATP synthase β subunit translation; this increases the ratio of ATP synthase enzyme to its c-subunit, enhancing ATP production efficiency and synaptic growth. In contrast, in FXS, inability to close developmental c-subunit leak prevents stimulus-dependent synaptic maturation. Therefore, ATP synthase c-subunit leak closure encourages development and attenuates autistic behaviors.

Input-Output Relationship of CA1 Pyramidal Neurons Reveals Intact Homeostatic Mechanisms in a Mouse Model of Fragile X Syndrome

Cellular hyperexcitability is a salient feature of fragile X syndrome animal models. The cellular basis of hyperexcitability and how it responds to changing activity states is not fully understood. Here, we show increased axon initial segment length in CA1 of the Fmr1^-/y mouse hippocampus, with increased cellular excitability. This change in length does not result from reduced AIS plasticity, as prolonged depolarization induces changes in AIS length independent of genotype. However, depolarization does reduce cellular excitability, the magnitude of which is greater in Fmr1^-/y neurons. Finally, we observe reduced functional inputs from the entorhinal cortex, with no genotypic difference in the firing rates of CA1 pyramidal neurons. This suggests that AIS-dependent hyperexcitability in Fmr1^-/y mice may result from adaptive or homeostatic regulation induced by reduced functional synaptic connectivity. Thus, while AIS length and intrinsic excitability contribute to cellular hyperexcitability, they may reflect a homeostatic mechanism for reduced synaptic input onto CA1 neurons.

Presynaptic dysfunction in neurodevelopmental disorders: Insights from the synaptic vesicle life cycle

The activity-dependent fusion, retrieval and recycling of synaptic vesicles is essential for the maintenance of neurotransmission. Until relatively recently it was believed that most mutations in genes that were essential for this process would be incompatible with life, because of this fundamental role. However, an ever-expanding number of mutations in this very cohort of genes are being identified in individuals with neurodevelopmental disorders, including autism, intellectual disability and epilepsy. This article will summarize the current state of knowledge linking mutations in presynaptic genes to neurodevelopmental disorders by sequentially covering the various stages of the synaptic vesicle life cycle. It will also discuss how perturbations of specific stages within this recycling process could translate into human disease. Finally, it will also provide perspectives on the potential for future therapy that are targeted to presynaptic function.

Cellular localization of the FMRP in rat retina

The fragile X mental retardation protein (FMRP) is a regulator of local translation through its mRNA targets in the neurons. Previous studies have demonstrated that FMRP may function in distinct ways during the development of different visual subcircuits. However, the localization of the FMRP in different types of retinal cells is unclear. In this work, the FMRP expression in rat retina was detected by Western blot and immunofluorescence double labeling. Results showed that the FMRP expression could be detected in rat retina and that the FMRP had a strong immunoreaction (IR) in the ganglion cell (GC) layer, inner nucleus layer (INL), and outer plexiform layer (OPL) of rat retina. In the outer retina, the bipolar cells (BCs) labeled by homeobox protein ChX10 (ChX10) and the horizontal cells (HCs) labeled by calbindin (CB) were FMRP-positive. In the inner retina, GABAergic amacrine cells (ACs) labeled by glutamate decarbonylase colocalized with the FMRP. The dopaminergic ACs (tyrosine hydroxylase marker) and cholinergic ACs (choline acetyltransferase (ChAT) marker) were co-labeled with the FMRP. In most GCs (labeled by Brn3a) and melanopsin-positive intrinsically photosensitive retinal GCs (ipRGCs) were also FMRP-positive. The FMRP expression was observed in the cellular retinal binding protein-positive Müller cells. These results suggest that the FMRP could be involved in the visual pathway transmission.

FMRP ribonucleoprotein complexes and RNA homeostasis

The Fragile Mental Retardation 1 gene (FMR1), at Xq27.3, encodes the fragile mental retardation protein (FMRP), and displays in its 5'-untranslated region a series of polymorphic CGG triplet repeats that may undergo dynamic mutation. Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability among men, and is most frequently due to FMR1 full mutation and consequent transcription repression. FMR1 premutations may associate with at least two other clinical conditions, named fragile X-associated primary ovarian insufficiency (FXPOI) and tremor and ataxia syndrome (FXTAS). While FXPOI and FXTAS appear to be mediated by FMR1 mRNA accumulation, relative reduction of FMRP, and triplet repeat translation, FXS is due to the lack of the RNA-binding protein FMRP. Besides its function as mRNA translation repressor in neuronal and stem/progenitor cells, RNA editing roles have been assigned to FMRP. In this review, we provide a brief description of FMR1 transcribed microsatellite and associated clinical disorders, and discuss FMRP molecular roles in ribonucleoprotein complex assembly and trafficking, as well as aspects of RNA homeostasis affected in FXS cells.

Altered mitochondrial function in cells carrying a premutation or unmethylated full mutation of the FMR1 gene

Fragile X-related disorders are due to a dynamic mutation of the CGG repeat at the 5' UTR of the FMR1 gene, coding for the RNA-binding protein FMRP. As the CGG sequence expands from premutation (PM, 56-200 CGGs) to full mutation (> 200 CGGs), FMRP synthesis decreases until it is practically abolished in fragile X syndrome (FXS) patients, mainly due to FMR1 methylation. Cells from rare individuals with no intellectual disability and carriers of an unmethylated full mutation (UFM) produce slightly elevated levels of FMR1-mRNA and relatively low levels of FMRP, like in PM carriers. With the aim of clarifying how UFM cells differ from CTRL and FXS cells, a comparative proteomic approach was undertaken, from which emerged an overexpression of SOD2 in UFM cells, also confirmed in PM but not in FXS. The SOD2-mRNA bound to FMRP in UFM more than in the other cell types. The high SOD2 levels in UFM and PM cells correlated with lower levels of superoxide and reactive oxygen species (ROS), and with morphological anomalies and depolarization of the mitochondrial membrane detected through confocal microscopy. The same effect was observed in CTRL and FXS after treatment with MC2791, causing SOD2 overexpression. These mitochondrial phenotypes reverted after knock-down with siRNA against SOD2-mRNA and FMR1-mRNA in UFM and PM. Overall, these data suggest that in PM and UFM carriers, which have high levels of FMR1 transcription and may develop FXTAS, SOD2 overexpression helps to maintain low levels of both superoxide and ROS with signs of mitochondrial degradation.

iPSC modeling of rare pediatric disorders

Discerning the underlying pathological mechanisms and the identification of therapeutic strategies to treat individuals affected with rare neurological diseases has proven challenging due to a host of factors. For instance, rare diseases affecting the nervous system are inherently lacking in appropriate patient sample availability compared to more common diseases, while animal models often do not accurately recapitulate specific disease phenotypes. These challenges impede research that may otherwise illuminate aspects of disease initiation and progression, leading to the ultimate identification of potential therapeutics. The establishment of induced pluripotent stem cells (iPSCs) as a human cellular model with defined genetics has provided the unique opportunity to study rare diseases within a controlled environment. iPSC models enable researchers to define mutational effects on specific cell types and signaling pathways within increasingly complex systems. Among rare diseases, pediatric diseases affecting neurodevelopment and neurological function highlight the critical need for iPSC-based disease modeling due to the inherent difficulty associated with collecting human neural tissue and the complexity of the mammalian nervous system. Rare neurodevelopmental disorders are therefore ideal candidates for utilization of iPSC-based in vitro studies. In this review, we address both the state of the iPSC field in the context of their utility and limitations for neurodevelopmental studies, as well as speculating about the future applications and unmet uses for iPSCs in rare diseases.

Visual Behavior Impairments as an Aberrant Sensory Processing in the Mouse Model of Fragile X Syndrome

Fragile X Syndrome (FXS), the most common inherited form of human intellectual disability (ID) associated with autistic-like behaviors, is characterized by dys-sensitivity to sensory stimuli, especially vision. In the absence of Fragile Mental Retardation Protein (FMRP), both retinal and cerebral structures of the visual pathway are impaired, suggesting that perception and integration of visual stimuli are altered. However, behavioral consequences of these defects remain unknown. In this study, we used male Fmr1^−/y mice to further define visual disturbances from a behavioral perspective by focusing on three traits characterizing visual modality: perception of depth, contrasts and movements. We performed specific tests (Optomotor Drum, Visual Cliff) to evaluate these visual modalities, their evolution from youth to adulthood, and to assess their involvement in a cognitive task. We show that Fmr1^−/y mice exhibit alteration in their visual skills, displaying impaired perspective perception, a drop in their ability to understand a moving contrasted pattern, and a defect in contrasts discrimination. Interestingly, Fmr1^−/y phenotypes remain stable over time from adolescence to late adulthood. Besides, we report that color and shape are meaningful for the achievement of a cognitive test involving object recognition. Altogether, these results underline the significance of visual behavior alterations in FXS conditions and relevance of assessing visual skills in neuropsychiatric models before performing behavioral tasks, such as cognitive assessments, that involve visual discrimination.

Mass Spectrometry for the Study of Autism and Neurodevelopmental Disorders

Mass spectrometry (MS) has been increasingly used to study central nervous system (CNS) disorders, including autism spectrum disorders (ASDs). The first studies of ASD using MS focused on the identification of external toxins, but current research is more directed at understanding endogenous protein changes that occur in ASD (ASD proteomics). This chapter focuses on how MS has been used to study ASDs, with particular focus on proteomic analysis. Other neurodevelopmental disorders have been investigated using this technique, including genetic syndromes associated with autism such as fragile X syndrome (FXS) and Smith-Lemli-Opitz Syndrome (SLOS).

Altered steady state and activity-dependent de novo protein expression in fragile X syndrome

Whether fragile X mental retardation protein (FMRP) target mRNAs and neuronal activity contributing to elevated basal neuronal protein synthesis in fragile X syndrome (FXS) is unclear. Our proteomic experiments reveal that the de novo translational profile in FXS model mice is altered at steady state and in response to metabotropic glutamate receptor (mGluR) stimulation, but the proteins expressed differ under these conditions. Several altered proteins, including Hexokinase 1 and Ras, also are expressed in the blood of FXS model mice and pharmacological treatments previously reported to ameliorate phenotypes modify their abundance in blood. In addition, plasma levels of Hexokinase 1 and Ras differ between FXS patients and healthy volunteers. Our data suggest that brain-based de novo proteomics in FXS model mice can be used to find altered expression of proteins in blood that could serve as disease-state biomarkers in individuals with FXS.

Elevated protein synthesis, and dysregulated mGluR signalling, are documented in fragile X syndrome (FXS) Here the authors use proteomic analysis in a mouse model of FXS, and following mGluR5 stimulation, to identify potential biomarkers for the 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.

Role of Microtubule-Associated Protein in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social interaction and communication, along with repetitive and restrictive patterns of behaviors or interests. Normal brain development is crucial to behavior and cognition in adulthood. Abnormal brain development, such as synaptic and myelin dysfunction, is involved in the pathogenesis of ASD. Microtubules and microtubule-associated proteins (MAPs) are important in regulating the processes of brain development, including neuron production and synaptic formation, as well as myelination. Increasing evidence suggests that the level of MAPs are changed in autistic patients and mouse models of ASD. Here, we discuss the roles of MAPs.

Human iPS Cell-Derived Patient Tissues and 3D Cell Culture Part 1: Target Identification and Lead Optimization

Human-induced pluripotent stem cells (HiPSCs), and new technologies to culture them into functional cell types and tissues, are now aiding drug discovery. Patient-derived HiPSCs can provide disease models that are more clinically relevant and so more predictive than the currently available animal-derived or tumor cell-derived cells. These cells, consequently, exhibit disease phenotypes close to the human pathology, particularly when cultured under conditions that allow them to recapitulate the tissue architecture in three-dimensional (3D) systems. A key feature of HiPSCs is that they can be cultured under conditions that favor formation of multicellular spheroids or organoids. By culturing and differentiating in systems mimicking the human tissue in vivo, the HiPSC microenvironment further reflects patient in vivo physiology, pathophysiology, and ultimately pharmacological responsiveness. We assess the rationale for using HiPSCs in several phases of preclinical drug discovery, specifically in disease modeling, target identification, and lead optimization. We also discuss the growing use of HiPSCs in compound lead optimization, particularly in profiling compounds for their potential metabolic liability and off-target toxicities. Collectively, we contend that both approaches, HiPSCs and 3D cell culture, when used in concert, have exciting potential for the development of novel medicines.

Fragile X mental retardation protein regulates accumulation of the active zone protein Munc18-1 in presynapses via local translation in axons during synaptogenesis

Fragile X mental retardation protein (FMRP), a causative gene (FMR1) product of Fragile X syndrome (FXS), is an RNA-binding protein to regulate local protein synthesis in dendrites for postsynaptic functions. However, involvement of FMRP in local protein synthesis in axons for presynaptic functions remains unclear. Here we investigated role of FMRP in local translation of the active zone protein Munc18-1 during presynapse formation. We found that leucine-rich repeat transmembrane neuronal 2 (LRRTM2)-conjugated beads, which promotes synchronized presynapse formation, induced simultaneous accumulation of FMRP and Munc18-1 in presynapses of axons of mouse cortical neurons in neuronal cell aggregate culture. The LRRTM2-induced accumulation of Munc18-1 in presynapses was observed in axons protein-synthesis-dependently, even physically separated from cell bodies. The accumulation of Munc18-1 was enhanced in Fmr1-knockout (KO) axons as compared to wild type (WT), suggesting FMRP-regulated suppression for local translation of Munc18-1 in axons during presynapse formation. Using naturally formed synapses of dissociated culture, structured illumination microscope revealed that accumulation of Munc18-1 puncta in Fmr1-KO neurons increased significantly at 19 days in vitro, as compared to WT. Our findings lead the possibility that excessive accumulation of Munc18-1 in presynapses at early stage of synaptic development in Fmr1-KO neurons may have a critical role in impaired presynaptic functions in FXS.

Quantitative proteomics of forebrain subcellular fractions in fragile X mental retardation 1 knockout mice following acute treatment with 2-Methyl-6-(phenylethynyl)pyridine: Relevance to developmental study of schizophrenia

The fragile X mental retardation 1 knockout (Fmr1 KO) mouse replicates behavioral deficits associated with autism, fragile X syndrome, and schizophrenia. Less is known whether protein expression changes are consistent with findings in subjects with schizophrenia. In the current study, we used liquid chromatography tandem mass spectrometry (LC-MS/MS) proteomics to determine the protein expression of four subcellular fractions in the forebrains of Fmr1 KO mice vs. C57BL/6 J mice and the effect of a negative allosteric modulator of mGluR5-2-Methyl-6-(phenylethynyl)pyridine (MPEP)-on protein expression. Strain- and treatment-specific differential expression of proteins was observed, many of which have previously been observed in the brains of subjects with schizophrenia. Western blotting verified the direction and magnitude of change for several proteins in different subcellular fractions as follows: neurofilament light protein (NEFL) and 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) in the total homogenate; heterogeneous nuclear ribonucleoproteins C1/C2 (HNRNPC) and heterogeneous nuclear ribonucleoprotein D0 (HNRNPD) in the nuclear fraction; excitatory amino acid transporter 2 (EAAT2) and ras-related protein rab 3a (RAB3A) in the synaptic fraction; and ras-related protein rab 35 (RAB35) and neuromodulin (GAP43) in the rough endoplasmic reticulum fraction. Individuals with FXS do not display symptoms of schizophrenia. However, the biomarkers that have been identified suggest that the Fmr1 KO model could potentially be useful in the study of schizophrenia.

Synaptic proteomics as a means to identify the molecular basis of mental illness: Are we getting there?

Synapses are centrally involved in many brain disorders, particularly in psychiatric and neurodevelopmental ones. However, our current understanding of the proteomic alterations affecting synaptic performance in the majority of mental illnesses is limited. As a result, novel pharmacotherapies with improved neurological efficacy have been scarce over the past decades. The main goal of synaptic proteomics in the context of mental illnesses is to identify dysregulated molecular mechanisms underlying these conditions. Here we reviewed and performed a meta-analysis of previous neuroproteomic research to identify proteins that may be consistently dysregulated in one or several mental disorders. Notably, we found very few proteins reproducibly altered among independent experiments for any given condition or between conditions, indicating that we are still far from identifying key pathophysiological mechanisms of mental illness. We suggest that future research in the field will require higher levels of standardization and larger-scale experiments to address the challenge posed by biological and methodological variability. We strongly believe that more resources should be placed in this field as the need to identify the molecular roots of mental illnesses is highly pressing.

Alterations in CA1 hippocampal synapses in a mouse model of fragile X syndrome

Fragile X Syndrome (FXS) is the major cause of inherited mental retardation and the leading genetic cause of Autism spectrum disorders. FXS is caused by mutations in the Fragile X Mental Retardation 1 (Fmr1) gene, which results in transcriptional silencing of Fragile X Mental Retardation Protein (FMRP). To elucidate cellular mechanisms involved in the pathogenesis of FXS, we compared dendritic spines in the hippocampal CA1 region of adult wild-type (WT) and Fmr1 knockout (Fmr1-KO) mice. Using diolistic labeling, confocal microscopy, and three-dimensional electron microscopy, we show a significant increase in the diameter of secondary dendrites, an increase in dendritic spine density, and a decrease in mature dendritic spines in adult Fmr1-KO mice. While WT and Fmr1-KO mice had the same mean density of spines, the variance in spine density was three times greater in Fmr1-KO mice. Reduced astrocyte participation in the tripartite synapse and less mature post-synaptic densities were also found in Fmr1-KO mice. We investigated whether the increase in synaptic spine density was associated with altered synaptic pruning during development. Our data are consistent with reduced microglia-mediated synaptic pruning in the CA1 region of Fmr1-KO hippocampi when compared with WT littermates at postnatal day 21, which is the peak period of synaptic pruning in the mouse hippocampus. Collectively, these results support abnormal synaptogenesis and synaptic remodeling in mice deficient in FMRP. Deficits in the maturation and distribution of synaptic spines on dendrites of CA1 hippocampal neurons may play a role in the intellectual disabilities associated with FXS.

Proteomic Profiling of Brain and Testis Reveals the Diverse Changes in Ribosomal Proteins in fmr1 Knockout Mice

Fragile X syndrome (FXS), the leading cause of inherited forms of mental retardation and autism, is caused by the transcriptional silencing of fmr1 encoding the fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that is a widely expressed, but primarily in the brain and testis, and associated approximately 4% of transcripts. Macro-orchidism is a common symptom associated with FXS both in humans and mice. Thus, we analyze the pooled samples of cerebral cortex, hippocampus and testis from both the fmr1-KO and wild-type mice by a LC-MS/MS proteomic study. Among the identified proteins, most of those showing significant changes in expression were up- or downregulated in the absence of FMRP. Proteins (FMRP, RPS8, RPL23a and ATPIF1, RPL6, GAP43, MTCH2 and MPZ in brain, and FMRP, CAH3, AKR1B7 and C9 in testis) identified by MS/MS were also verified by Western blotting. The Gene Ontology and WikiPathways analysis revealed that the differentially expressed proteins were clustered in the polyribosome and RNA-binding protein categories in both cerebral cortex and hippocampus, but not in testis. Although this study was limited by the little number of samples, our results provide detailed insights into the ribosomal protein profiles of cerebral cortex, hippocampus and testis in the absence of FMRP. Our studies also provide a better understanding of protein profile changes and the underlying dysregulated pathways arising from fmr1 silencing in FXS.

The RNA-binding protein, ZC3H14, is required for proper poly(A) tail length control, expression of synaptic proteins, and brain function in mice

A number of mutations in genes that encode ubiquitously expressed RNA-binding proteins cause tissue specific disease. Many of these diseases are neurological in nature revealing critical roles for this class of proteins in the brain. We recently identified mutations in a gene that encodes a ubiquitously expressed polyadenosine RNA-binding protein, ZC3H14 (Zinc finger CysCysCysHis domain-containing protein 14), that cause a nonsyndromic, autosomal recessive form of intellectual disability. This finding reveals the molecular basis for disease and provides evidence that ZC3H14 is essential for proper brain function. To investigate the role of ZC3H14 in the mammalian brain, we generated a mouse in which the first common exon of the ZC3H14 gene, exon 13 is removed (Zc3h14Δex13/Δex13) leading to a truncated ZC3H14 protein. We report here that, as in the patients, Zc3h14 is not essential in mice. Utilizing these Zc3h14Δex13/Δex13mice, we provide the first in vivo functional characterization of ZC3H14 as a regulator of RNA poly(A) tail length. The Zc3h14Δex13/Δex13 mice show enlarged lateral ventricles in the brain as well as impaired working memory. Proteomic analysis comparing the hippocampi of Zc3h14+/+ and Zc3h14Δex13/Δex13 mice reveals dysregulation of several pathways that are important for proper brain function and thus sheds light onto which pathways are most affected by the loss of ZC3H14. Among the proteins increased in the hippocampi of Zc3h14Δex13/Δex13 mice compared to control are key synaptic proteins including CaMK2a. This newly generated mouse serves as a tool to study the function of ZC3H14 in vivo.

Autism spectrum disorder: neuropathology and animal models

Autism spectrum disorder (ASD) has a major impact on the development and social integration of affected individuals and is the most heritable of psychiatric disorders. An increase in the incidence of ASD cases has prompted a surge in research efforts on the underlying neuropathologic processes. We present an overview of current findings in neuropathology studies of ASD using two investigational approaches, postmortem human brains and ASD animal models, and discuss the overlap, limitations, and significance of each. Postmortem examination of ASD brains has revealed global changes including disorganized gray and white matter, increased number of neurons, decreased volume of neuronal soma, and increased neuropil, the last reflecting changes in densities of dendritic spines, cerebral vasculature and glia. Both cortical and non-cortical areas show region-specific abnormalities in neuronal morphology and cytoarchitectural organization, with consistent findings reported from the prefrontal cortex, fusiform gyrus, frontoinsular cortex, cingulate cortex, hippocampus, amygdala, cerebellum and brainstem. The paucity of postmortem human studies linking neuropathology to the underlying etiology has been partly addressed using animal models to explore the impact of genetic and non-genetic factors clinically relevant for the ASD phenotype. Genetically modified models include those based on well-studied monogenic ASD genes (NLGN3, NLGN4, NRXN1, CNTNAP2, SHANK3, MECP2, FMR1, TSC1/2), emerging risk genes (CHD8, SCN2A, SYNGAP1, ARID1B, GRIN2B, DSCAM, TBR1), and copy number variants (15q11-q13 deletion, 15q13.3 microdeletion, 15q11-13 duplication, 16p11.2 deletion and duplication, 22q11.2 deletion). Models of idiopathic ASD include inbred rodent strains that mimic ASD behaviors as well as models developed by environmental interventions such as prenatal exposure to sodium valproate, maternal autoantibodies, and maternal immune activation. In addition to replicating some of the neuropathologic features seen in postmortem studies, a common finding in several animal models of ASD is altered density of dendritic spines, with the direction of the change depending on the specific genetic modification, age and brain region. Overall, postmortem neuropathologic studies with larger sample sizes representative of the various ASD risk genes and diverse clinical phenotypes are warranted to clarify putative etiopathogenic pathways further and to promote the emergence of clinically relevant diagnostic and therapeutic tools. In addition, as genetic alterations may render certain individuals more vulnerable to developing the pathological changes at the synapse underlying the behavioral manifestations of ASD, neuropathologic investigation using genetically modified animal models will help to improve our understanding of the disease mechanisms and enhance the development of targeted treatments.

Cell-Type-Specific Translation Profiling Reveals a Novel Strategy for Treating Fragile X Syndrome

Excessive mRNA translation downstream of group I metabotropic glutamate receptors (mGlu_1/5) is a core pathophysiology of fragile X syndrome (FX); however, the differentially translating mRNAs that contribute to altered neural function are not known. We used translating ribosome affinity purification (TRAP) and RNA-seq to identify mistranslating mRNAs in CA1 pyramidal neurons of the FX mouse model (Fmr1^−/y) hippocampus, which exhibit exaggerated mGlu_1/5-induced long-term synaptic depression (LTD). In these neurons, we find that the Chrm4 transcript encoding muscarinic acetylcholine receptor 4 (M₄) is excessively translated, and synthesis of M₄ downstream of mGlu₅ activation is mimicked and occluded. Surprisingly, enhancement rather than inhibition of M₄ activity normalizes core phenotypes in the Fmr1^−/y, including excessive protein synthesis, exaggerated mGluR-LTD, and audiogenic seizures. These results suggest that not all excessively translated mRNAs in the Fmr1^−/y brain are detrimental, and some may be candidates for enhancement to correct pathological changes in the FX brain.

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TRAP-seq reveals altered translation of >120 mRNAs in Fmr1^−/y CA1 pyramidal neurons

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Muscarinic receptor M₄ is excessively translated in Fmr1^−/y hippocampus

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Enhancement, not inhibition, of M₄ corrects core phenotypes in the Fmr1^−/y mouse

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Not all excessively translating mRNAs are detrimental to Fmr1^−/y brain function

Fragile X Mental Retardation Protein Requirements in Activity-Dependent Critical Period Neural Circuit Refinement

Activity-dependent synaptic remodeling occurs during early-use critical periods, when naive juveniles experience sensory input. Fragile X mental retardation protein (FMRP) sculpts synaptic refinement in an activity sensor mechanism based on sensory cues, with FMRP loss causing the most common heritable autism spectrum disorder (ASD), fragile X syndrome (FXS). In the well-mapped Drosophila olfactory circuitry, projection neurons (PNs) relay peripheral sensory information to the central brain mushroom body (MB) learning/memory center. FMRP-null PNs reduce synaptic branching and enlarge boutons, with ultrastructural and synaptic reconstitution MB connectivity defects. Critical period activity modulation via odorant stimuli, optogenetics, and transgenic tetanus toxin neurotransmission block show that elevated PN activity phenocopies FMRP-null defects, whereas PN silencing causes opposing changes. FMRP-null PNs lose activity-dependent synaptic modulation, with impairments restricted to the critical period. We conclude that FMRP is absolutely required for experience-dependent changes in synaptic connectivity during the developmental critical period of neural circuit optimization for sensory input.

The implication of AMPA receptor in synaptic plasticity impairment and intellectual disability in fragile X syndrome

Fragile X syndrome (FXS) is the most frequently inherited form of intellectual disability and prevalent single-gene cause of autism. A priority of FXS research is to determine the molecular mechanisms underlying the cognitive and social functioning impairments in humans and the FXS mouse model. Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate a majority of fast excitatory neurotransmission in the central nervous system and are critically important for nearly all aspects of brain function, including neuronal development, synaptic plasticity, and learning and memory. Both preclinical and clinical studies have indicated that expression, trafficking, and functions of AMPARs are altered and result in altered synapse development and plasticity, cognitive impairment, and poor mental health in FXS. In this review, we discuss the contribution of AMPARs to disorders of FXS by highlighting recent research advances with a specific focus on change in AMPARs expression, trafficking, and dependent synaptic plasticity. Since changes in synaptic strength underlie the basis of learning, development, and disease, we suggest that the current knowledge base of AMPARs has reached a unique point to permit a comprehensive re-evaluation of their roles in FXS.

Age-Dependent Protein Aggregation Initiates Amyloid-β Aggregation

Aging is the most important risk factor for neurodegenerative diseases associated with pathological protein aggregation such as Alzheimer's disease. Although aging is an important player, it remains unknown which molecular changes are relevant for disease initiation. Recently, it has become apparent that widespread protein aggregation is a common feature of aging. Indeed, several studies demonstrate that 100s of proteins become highly insoluble with age, in the absence of obvious disease processes. Yet it remains unclear how these misfolded proteins aggregating with age affect neurodegenerative diseases. Importantly, several of these aggregation-prone proteins are found as minor components in disease-associated hallmark aggregates such as amyloid-β plaques or neurofibrillary tangles. This co-localization raises the possibility that age-dependent protein aggregation directly contributes to pathological aggregation. Here, we show for the first time that highly insoluble proteins from aged Caenorhabditis elegans or aged mouse brains, but not from young individuals, can initiate amyloid-β aggregation in vitro. We tested the seeding potential at four different ages across the adult lifespan of C. elegans. Significantly, protein aggregates formed during the early stages of aging did not act as seeds for amyloid-β aggregation. Instead, we found that changes in protein aggregation occurring during middle-age initiated amyloid-β aggregation. Mass spectrometry analysis revealed several late-aggregating proteins that were previously identified as minor components of amyloid-β plaques and neurofibrillary tangles such as 14-3-3, Ubiquitin-like modifier-activating enzyme 1 and Lamin A/C, highlighting these as strong candidates for cross-seeding. Overall, we demonstrate that widespread protein misfolding and aggregation with age could be critical for the initiation of pathogenesis, and thus should be targeted by therapeutic strategies to alleviate neurodegenerative diseases.

Towards the Molecular Foundations of Glutamatergic-targeted Antidepressants

BACKGROUND

Depression affects over 120 million individuals of all ages and is the leading cause of disability worldwide. The lack of objective diagnostic criteria, together with the heterogeneity of the depressive disorder itself, makes it challenging to develop effective therapies. The accumulation of preclinical data over the past 20 years derived from a multitude of models using many divergent approaches, has fueled the resurgence of interest in targeting glutamatergic neurotransmission for the treatment of major depression.

OBJECTIVE

The emergence of mechanistic studies are advancing our understanding of the molecular underpinnings of depression. While clearly far from complete and conclusive, they offer the potential to lead to the rational design of more specific therapeutic strategies and the development of safer and more effective rapid acting, long lasting antidepressants.

METHODS

The development of comprehensive omics-based approaches to the dysregulation of synaptic transmission and plasticity that underlies the core pathophysiology of MDD are reviewed to illustrate the fundamental elements.

RESULTS

This review frames the rationale for the conceptualization of depression as a "pathway disease". As such, it culminates in the call for the development of novel state-of-the-art "-omics approaches" and neurosystems biological techniques necessary to advance our understanding of spatiotemporal interactions associated with targeting glutamatergic-triggered signaling in the CNS.

CONCLUSION

These technologies will enable the development of novel psychiatric medications specifically targeted to impact specific, critical intracellular networks in a more focused manner and have the potential to offer new dimensions in the area of translational neuropsychiatry.

Delayed bradykinin postconditioning modulates intrinsic neuroprotective enzyme expression in the rat CA1 region after cerebral ischemia: a proteomic study

Pyramidal cells in the CA1 brain region exhibit an ischemic tolerance after delayed postconditioning; therefore, this approach seems to be a promising neuroprotective procedure in cerebral postischemic injury improvement. However, little is known about the effect of postconditioning on protein expression patterns in the brain, especially in the affected hippocampal neurons after global cerebral ischemia. This study is focused on the examination of the ischemia-vulnerable CA1 neuronal layer and on the acquisition of protection from delayed neuronal death after ischemia. Ischemic-reperfusion injury was induced in Wistar rats and bradykinin was applied 2 days after the ischemic insult in an attempt to overcome delayed cell death. Analysis of complex peptide CA1 samples was performed by automated two dimensional liquid chromatography (2D-LC) fractionation coupled to tandem matrix assisted laser desorption/ionization time-of-flight (MALDI TOF/TOF) mass spectrometry instrumentation. We devoted our attention to differences in protein expression mapping in ischemic injured CA1 neurons in comparison with equally affected neurons, but with bradykinin application. Proteomic analysis identified several proteins occurring only after postconditioning and control, which could have a potentially neuroprotective influence on ischemic injured neurons. Among them, the prominent position occupies a regulator of glutamate level aspartate transaminase AATC, a scavenger of glutamate in brain neuroprotection after ischemia-reperfusion. We identified this enzyme in controls and after postconditioning, but AATC presence was not detected in the ischemic injured CA1 region. This finding was confirmed by two-dimensional differential electrophoresis followed by MALDI-TOF/TOF MS identification. Results suggest that bradykinin as delayed postconditioning may be associated with modulation of protein expression after ischemic injury and thus this procedure can be involved in neuroprotective metabolic pathways.

Dynamics of the mouse brain cortical synaptic proteome during postnatal brain development

Development of the brain involves the formation and maturation of numerous synapses. This process requires prominent changes of the synaptic proteome and potentially involves thousands of different proteins at every synapse. To date the proteome analysis of synapse development has been studied sparsely. Here, we analyzed the cortical synaptic membrane proteome of juvenile postnatal days 9 (P9), P15, P21, P27, adolescent (P35) and different adult ages P70, P140 and P280 of C57Bl6/J mice. Using a quantitative proteomics workflow we quantified 1560 proteins of which 696 showed statistically significant differences over time. Synaptic proteins generally showed increased levels during maturation, whereas proteins involved in protein synthesis generally decreased in abundance. In several cases, proteins from a single functional molecular entity, e.g., subunits of the NMDA receptor, showed differences in their temporal regulation, which may reflect specific synaptic development features of connectivity, strength and plasticity. SNARE proteins, Snap 29/47 and Stx 7/8/12, showed higher expression in immature animals. Finally, we evaluated the function of Cxadr that showed high expression levels at P9 and a fast decline in expression during neuronal development. Knock down of the expression of Cxadr in cultured primary mouse neurons revealed a significant decrease in synapse density.

Widespread alterations in the synaptic proteome of the adolescent cerebral cortex following prenatal immune activation in rats

An increasing number of studies have revealed associations between pre- and perinatal immune activation and the development of schizophrenia and autism spectrum disorders (ASDs). Accordingly, neuroimmune crosstalk has a considerably large impact on brain development during early ontogenesis. While a plethora of heterogeneous abnormalities have already been described in established maternal immune activation (MIA) rodent and primate animal models, which highly correlate to those found in human diseases, the underlying molecular background remains obscure. In the current study, we describe the long-term effects of MIA on the neocortical pre- and postsynaptic proteome of adolescent rat offspring in detail. Molecular differences were revealed in sub-synaptic fractions, which were first thoroughly characterized using independent methods. The widespread proteomic examination of cortical samples from offspring exposed to maternal lipopolysaccharide administration at embryonic day 13.5 was conducted via combinations of different gel-based proteomic techniques and tandem mass spectrometry. Our experimentally validated proteomic data revealed more pre- than postsynaptic protein level changes in the offspring. The results propose the relevance of altered synaptic vesicle recycling, cytoskeletal structure and energy metabolism in the presynaptic region in addition to alterations in vesicle trafficking, the cytoskeleton and signal transduction in the postsynaptic compartment in MIA offspring. Differing levels of the prominent signaling regulator molecule calcium/calmodulin-dependent protein kinase II in the postsynapse was validated and identified specifically in the prefrontal cortex. Finally, several potential common molecular regulators of these altered proteins, which are already known to be implicated in schizophrenia and ASD, were identified and assessed. In summary, unexpectedly widespread changes in the synaptic molecular machinery in MIA rats were demonstrated which might underlie the pathological cortical functions that are characteristic of schizophrenia and ASD.