Severe Intellectual Disability and Enhanced Gamma-Aminobutyric Acidergic Synaptogenesis in a Novel Model of Rare RASopathies

Understanding copy number variations through their genes: a molecular view on 16p11.2 deletion and duplication syndromes

Neurodevelopmental disorders (NDDs) include a broad spectrum of pathological conditions that affect >4% of children worldwide, share common features and present a variegated genetic origin. They include clinically defined diseases, such as autism spectrum disorders (ASD), attention-deficit/hyperactivity disorder (ADHD), motor disorders such as Tics and Tourette's syndromes, but also much more heterogeneous conditions like intellectual disability (ID) and epilepsy. Schizophrenia (SCZ) has also recently been proposed to belong to NDDs. Relatively common causes of NDDs are copy number variations (CNVs), characterised by the gain or the loss of a portion of a chromosome. In this review, we focus on deletions and duplications at the 16p11.2 chromosomal region, associated with NDDs, ID, ASD but also epilepsy and SCZ. Some of the core phenotypes presented by human carriers could be recapitulated in animal and cellular models, which also highlighted prominent neurophysiological and signalling alterations underpinning 16p11.2 CNVs-associated phenotypes. In this review, we also provide an overview of the genes within the 16p11.2 locus, including those with partially known or unknown function as well as non-coding RNAs. A particularly interesting interplay was observed between MVP and MAPK3 in modulating some of the pathological phenotypes associated with the 16p11.2 deletion. Elucidating their role in intracellular signalling and their functional links will be a key step to devise novel therapeutic strategies for 16p11.2 CNVs-related syndromes.

Developmental effect of RASopathy mutations on neuronal network activity on a chip

RASopathies are a group of genetic disorders caused by mutations in genes encoding components and regulators of the RAS/MAPK signaling pathway, resulting in overactivation of signaling. RASopathy patients exhibit distinctive facial features, cardiopathies, growth and skeletal abnormalities, and varying degrees of neurocognitive impairments including neurodevelopmental delay, intellectual disabilities, or attention deficits. At present, it is unclear how RASopathy mutations cause neurocognitive impairment and what their neuron-specific cellular and network phenotypes are. Here, we investigated the effect of RASopathy mutations on the establishment and functional maturation of neuronal networks. We isolated cortical neurons from RASopathy mouse models, cultured them on multielectrode arrays and performed longitudinal recordings of spontaneous activity in developing networks as well as recordings of evoked responses in mature neurons. To facilitate the analysis of large and complex data sets resulting from long-term multielectrode recordings, we developed MATLAB-based tools for data processing, analysis, and statistical evaluation. Longitudinal analysis of spontaneous network activity revealed a convergent developmental phenotype in neurons carrying the gain-of-function Noonan syndrome-related mutations Ptpn11 D61Y and Kras V14l. The phenotype was more pronounced at the earlier time points and faded out over time, suggesting the emergence of compensatory mechanisms during network maturation. Nevertheless, persistent differences in excitatory/inhibitory balance and network excitability were observed in mature networks. This study improves the understanding of the complex relationship between genetic mutations and clinical manifestations in RASopathies by adding insights into functional network processes as an additional piece of the puzzle.

Structural interhemispheric connectivity defects in mouse models of BBSOAS: Insights from high spatial resolution 3D white matter tractography

White matter (WM) tract formation and axonal pathfinding are major processes in brain development allowing to establish precise connections between targeted structures. Disruptions in axon pathfinding and connectivity impairments will lead to neural circuitry abnormalities, often associated with various neurodevelopmental disorders (NDDs). Among several neuroimaging methodologies, Diffusion Tensor Imaging (DTI) is a magnetic resonance imaging (MRI) technique that has the advantage of visualizing in 3D the WM tractography of the whole brain non-invasively. DTI is particularly valuable in unpinning structural tract connectivity defects of neural networks in NDDs. In this study, we used 3D DTI to unveil brain-specific tract defects in two mouse models lacking the Nr2f1 gene, which mutations in patients have been proven to cause an emerging NDD, called Bosch-Boonstra-Schaaf Optic Atrophy (BBSOAS). We aimed to investigate the impact of the lack of cortical Nr2f1 function on WM morphometry and tract microstructure quantifications. We found in both mutant mice partial loss of fibers and severe misrouting of the two major cortical commissural tracts, the corpus callosum, and the anterior commissure, as well as the two major hippocampal efferent tracts, the post-commissural fornix, and the ventral hippocampal commissure. DTI tract malformations were supported by 2D histology, 3D fluorescent imaging, and behavioral analyses. We propose that these interhemispheric connectivity impairments are consistent in explaining some cognitive defects described in BBSOAS patients, particularly altered information processing between the two brain hemispheres. Finally, our results highlight 3DDTI as a relevant neuroimaging modality that can provide appropriate morphometric biomarkers for further diagnosis of BBSOAS patients.

Cellular signaling impacts upon GABAergic cortical interneuron development

The development and maturation of cortical GABAergic interneurons has been extensively studied, with much focus on nuclear regulation via transcription factors. While these seminal events are critical for the establishment of interneuron developmental milestones, recent studies on cellular signaling cascades have begun to elucidate some potential contributions of cell signaling during development. Here, we review studies underlying three broad signaling families, mTOR, MAPK, and Wnt/beta-catenin in cortical interneuron development. Notably, each pathway harbors signaling factors that regulate a breadth of interneuron developmental milestones and properties. Together, these events may work in conjunction with transcriptional mechanisms and other events to direct the complex diversity that emerges during cortical interneuron development and maturation.

Safrole oxide induced 5-HT neuron-like cell differentiation of bone marrow mesenchymal stem cells by elevating G9a

In our previous study, we found that safrole oxide (SFO) could induce bone marrow mesenchymal stem cell differentiation into neuron-like cells. However, which kind of neuron cells was induced by SFO was unknown. Here, we found that SFO could induce BMSC differentiation into 5-hydroxytryptamine (5-HT) neuron-like cells. Microarray analysis of BMSCs treated with SFO for 6 h revealed a total of 35 genes changed more than twice. We selected G9a, a histone methyltransferase for further study. The upregulation of G9a was confirmed by RT-PCR and Western blot analysis. Small interfering RNA knockdown of G9a blocked SFO-induced BMSC differentiation. These results demonstrated that G9a was the pivotal factor in SFO-medicated 5-HT neuronal differentiation of BMSCs. Our findings provide a new clue for further investigating the gene control of BMSC differentiation into 5-HT neuron-like cells and provide a putative strategy for depression diseases therapies.

Açai Berry Mitigates Parkinson's Disease Progression Showing Dopaminergic Neuroprotection via Nrf2-HO1 Pathways

The current pharmacological treatment for Parkinson's disease (PD) is focused on symptom alleviation rather than disease prevention. In this study, we look at a new strategy to neuroprotection that focuses on nutrition, by a supplementation with Açai berry in an experimental models of PD. Daily orally supplementation with Açai berry dissolved in saline at the dose of 500 mg/kg considerably reduced motor and non-motor symptom and neuronal cell death of the dopaminergic tract induced by 4 injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Furthermore, Açai berry administration reduced α-synuclein aggregation in neurons, enhanced tyrosine hydroxylase and dopamine transporter activities, and avoided dopamine depletion. Moreover, Açai berry administration was able to reduce astrogliosis and microgliosis as well as neuronal death. Its beneficial effects could be due to its bioactive phytochemical components that are able to stimulate nuclear factor erythroid 2-related factor 2 (Nrf2) by counteracting the oxidative stress and neuroinflammation that are the basis of this neurodegenerative disease.

Translating the Role of mTOR- and RAS-Associated Signalopathies in Autism Spectrum Disorder: Models, Mechanisms and Treatment

Mutations affecting mTOR or RAS signaling underlie defined syndromes (the so-called mTORopathies and RASopathies) with high risk for Autism Spectrum Disorder (ASD). These syndromes show a broad variety of somatic phenotypes including cancers, skin abnormalities, heart disease and facial dysmorphisms. Less well studied are the neuropsychiatric symptoms such as ASD. Here, we assess the relevance of these signalopathies in ASD reviewing genetic, human cell model, rodent studies and clinical trials. We conclude that signalopathies have an increased liability for ASD and that, in particular, ASD individuals with dysmorphic features and intellectual disability (ID) have a higher chance for disruptive mutations in RAS- and mTOR-related genes. Studies on rodent and human cell models confirm aberrant neuronal development as the underlying pathology. Human studies further suggest that multiple hits are necessary to induce the respective phenotypes. Recent clinical trials do only report improvements for comorbid conditions such as epilepsy or cancer but not for behavioral aspects. Animal models show that treatment during early development can rescue behavioral phenotypes. Taken together, we suggest investigating the differential roles of mTOR and RAS signaling in both human and rodent models, and to test drug treatment both during and after neuronal development in the available model systems.

A Rational Design of α-Helix-Shaped Peptides Employing the Hydrogen-Bond Surrogate Approach: A Modulation Strategy for Ras-RasGRF1 Interaction in Neuropsychiatric Disorders

In the last two decades, abnormal Ras (rat sarcoma protein)-ERK (extracellular signal-regulated kinase) signalling in the brain has been involved in a variety of neuropsychiatric disorders, including drug addiction, certain forms of intellectual disability, and autism spectrum disorder. Modulation of membrane-receptor-mediated Ras activation has been proposed as a potential target mechanism to attenuate ERK signalling in the brain. Previously, we showed that a cell penetrating peptide, RB3, was able to inhibit downstream signalling by preventing RasGRF1 (Ras guanine nucleotide-releasing factor 1), a neuronal specific GDP/GTP exchange factor, to bind Ras proteins, both in brain slices and in vivo, with an IC₅₀ value in the micromolar range. The aim of this work was to mutate and improve this peptide through computer-aided techniques to increase its inhibitory activity against RasGRF1. The designed peptides were built based on the RB3 peptide structure corresponding to the α-helix of RasGRF1 responsible for Ras binding. For this purpose, the hydrogen-bond surrogate (HBS) approach was exploited to maintain the helical conformation of the designed peptides. Finally, residue scanning, MD simulations, and MM-GBSA calculations were used to identify 18 most promising α-helix-shaped peptides that will be assayed to check their potential activity against Ras-RasGRF1 and prevent downstream molecular events implicated in brain disorders.

The 4-Hole-Board Test for Assessment of Long-Term Spatial Memory in Mice

The hole-board test has been used in rodents since the early 60s to measure exploratory behavior, locomotor activity and cognitive function. The test is based on rodents' natural curiosity and attraction for novelty. Basically, the hole-board consists of a small square arena with an extractable platform as floor, which has a set of equally spaced circular holes on its surface. In this article, we describe the protocol of a 4-hole-board test allowing the assessment of long-term spatial memory in mice without the employment of water or food restriction, painful stimuli (as electrical shocks) or any aversive condition (as forced swimming or exposure to intense light). Four holes are present on the floor of the square arena (one for each of its four quadrants). Mice released in the arena spontaneously approach the holes and explore them by briefly inserting the snout inside, a behavior defined as nose-poking (or head-dipping). If, after 24 hr, rodents are re-exposed to the hole-board, the novelty of the holes decreases. Animals with an intact long-term memory will show a reduction of the frequency of nose-poking into the holes. The total number of nose-pokes on day 1 is an index of exploration, while the percentage of decrease in nose-poking on day 2 represents an index of long-term spatial memory. Number of quadrant crossings is scored as a control measure for locomotor activity, which with the present protocol should remain stable across the days of testing. Indeed, the 4-hole-board test represents a stress-free and animal-friendly option to evaluate long-term spatial memory. In the present paper, we provide detailed description of the hole-board apparatus and step-by-step protocol for assessment of spatial memory in mice. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Validation of the 4-hole-board Basic Protocol 2: Evaluation of long-term spatial memory through the 4-hole-board test.

MEK inhibition ameliorates social behavior phenotypes in a Spred1 knockout mouse model for RASopathy disorders

BACKGROUND

RASopathies are a group of disorders that result from mutations in genes coding for proteins involved in regulating the Ras-MAPK signaling pathway, and have an increased incidence of autism spectrum disorder (ASD). Legius syndrome is a rare RASopathy caused by loss-of-function mutations in the SPRED1 gene. The patient phenotype is similar to, but milder than, Neurofibromatosis type 1-another RASopathy caused by loss-of-function mutations in the NF1 gene. RASopathies exhibit increased activation of Ras-MAPK signaling and commonly manifest with cognitive impairments and ASD. Here, we investigated if a Spred1-/- mouse model for Legius syndrome recapitulates ASD-like symptoms, and whether targeting the Ras-MAPK pathway has therapeutic potential in this RASopathy mouse model.

METHODS

We investigated social and communicative behaviors in Spred1-/- mice and probed therapeutic mechanisms underlying the observed behavioral phenotypes by pharmacological targeting of the Ras-MAPK pathway with the MEK inhibitor PD325901.

RESULTS

Spred1-/- mice have robust increases in social dominance in the automated tube test and reduced adult ultrasonic vocalizations during social communication. Neonatal ultrasonic vocalization was also altered, with significant differences in spectral properties. Spred1-/- mice also exhibit impaired nesting behavior. Acute MEK inhibitor treatment in adulthood with PD325901 reversed the enhanced social dominance in Spred1-/- mice to normal levels, and improved nesting behavior in adult Spred1-/- mice.

LIMITATIONS

This study used an acute treatment protocol to administer the drug. It is not known what the effects of longer-term treatment would be on behavior. Further studies titrating the lowest dose of this drug that is required to alter Spred1-/- social behavior are still required. Finally, our findings are in a homozygous mouse model, whereas patients carry heterozygous mutations. These factors should be considered before any translational conclusions are drawn.

CONCLUSIONS

These results demonstrate for the first time that social behavior phenotypes in a mouse model for RASopathies (Spred1-/-) can be acutely reversed. This highlights a key role for Ras-MAPK dysregulation in mediating social behavior phenotypes in mouse models for ASD, suggesting that proper regulation of Ras-MAPK signaling is important for social behavior.

Spatiotemporal expression of Rap1 and Ras mediates the acquisition and reinstatement of methamphetamine-induced conditioned place preference in mice via extracellular signal-regulated kinase activation

Drug addiction is a chronic recurrent brain disease characterized by compulsive drug use and a high tendency to relapse. We previously reported that the Ras-extracellular signal-regulated kinase (ERK)-ΔFosB pathway in the caudate putamen (CPu) was involved in methamphetamine-induced behavioral sensitization. Rap1, as an antagonist of Ras originally, was found to participate in neuronal synaptic plasticity recently, but the role of Rap1 in methamphetamine addiction is unclear. First, in this study, we constructed the acquisition, extinction and reinstatement of methamphetamine-induced conditioned place preference (CPP) in mice, respectively. Then, protein levels of Rap1, Ras and pERK/ERK in the prefrontal cortex (PFc), CPu and hippocampus of CPP mice on three phases were detected. We found that protein levels of Rap1, Ras and pERK/ERK in the CPu were significantly increased after repeated methamphetamine administration, as well as Rap1 and pERK/ERK in the hippocampus. However, protein levels of Rap1 and pERK/ERK in the CPu were decreased on the reinstatement of CPP mice. Therefore, Rap1 and Ras in the CPu and Rap1 in the hippocampus may participate in the regulation of the acquisition of methamphetamine-induced CPP in mice by activating ERK. Moreover, Rap1-ERK cascade in the CPu contributes to both the acquisition and reinstatement of methamphetamine-induced CPP in mice.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Impaired instrumental learning in Spred1-/- mice, a model for a rare RASopathy

RASopathies are neuro-cardio-facio-cutaneous disorders stemming from mutations in genes regulating the RAS-MAPK pathway. Legius syndrome is a rare RASopathy disorder caused by mutations in the SPRED1 gene. SPRED1 protein negatively regulates activation of Ras by inhibiting RAS/RAF and by its interaction with neurofibromin, a Ras GTPase-activating protein (RAS-GAP). Cognitive impairments have been reported in Legius syndrome as well as in other RASopathy disorders. Modelling these cognitive deficits in a Spred1 mouse model for Legius syndrome has demonstrated spatial learning and memory deficits, but other cognitive domains remained unexplored. Here, we attempted to utilize a cognitive touchscreen battery to investigate if Spred1^-/- mice exhibit deficits in other cognitive domains. We show that Spred1^-/- mice had heterogeneous performance in instrumental operant learning, with a large subgroup (n = 9/20) failing to reach the standard criterion on touchscreen operant pretraining, precluding further cognitive testing. To examine whether targeting the RAS-MAPK signalling pathway could rescue these cognitive impairments, Spred1^-/- mice were acutely treated with the clinically relevant mitogen-activated protein kinase (MEK) inhibitor PD325901. However, MEK inhibition did not improve their instrumental learning. We conclude that Spred1^-/- mice can model severe cognitive impairments that cannot be reversed in adulthood.

TBL1XR1 Ensures Balanced Neural Development Through NCOR Complex-Mediated Regulation of the MAPK Pathway

TBL1XR1 gene is associated with multiple developmental disorders presenting several neurological aspects. The relative protein is involved in the modulation of important cellular pathways and master regulators of transcriptional output, including nuclear receptor repressors, Wnt signaling, and MECP2 protein. However, TBL1XR1 mutations (including complete loss of its functions) have not been experimentally studied in a neurological context, leaving a knowledge gap in the mechanisms at the basis of the diseases. Here, we show that Tbl1xr1 knock-out mice exhibit behavioral and neuronal abnormalities. Either the absence of TBL1XR1 or its point mutations interfering with stability/regulation of NCOR complex induced decreased proliferation and increased differentiation in neural progenitors. We suggest that this developmental unbalance is due to a failure in the regulation of the MAPK cascade. Taken together, our results broaden the molecular and functional aftermath of TBL1XR1 deficiency associated with human disorders.

Neuron type-specific expression of a mutant KRAS impairs hippocampal-dependent learning and memory

KRAS mutations are associated with rare cases of neurodevelopmental disorders that can cause intellectual disabilities. Previous studies showed that mice expressing a mutant KRAS have impaired the development and function of GABAergic inhibitory neurons, which may contribute to behavioural deficits in the mutant mice. However, the underlying cellular mechanisms and the role of excitatory neurons in these behavioural deficits in adults are not fully understood. Herein, we report that neuron type-specific expression of a constitutively active mutant KRAS^G12V in either excitatory or inhibitory neurons resulted in spatial memory deficits in adult mice. In inhibitory neurons, KRAS^G12V induced ERK activation and enhanced GABAergic synaptic transmission. Expressing KRAS^G12V in inhibitory neurons also impaired long-term potentiation in the hippocampal Shaffer-collateral pathway, which could be rescued by picrotoxin treatment. In contrast, KRAS^G12V induced ERK activation and neuronal cell death in excitatory neurons, which might have contributed to the severe behavioural deficits. Our results showed that both excitatory and inhibitory neurons are involved in mutant KRAS-associated learning deficits in adults via distinct cellular mechanisms.

p140Cap Regulates GABAergic Synaptogenesis and Development of Hippocampal Inhibitory Circuits

The neuronal scaffold protein p140Cap was investigated during hippocampal network formation. p140Cap is present in presynaptic GABAergic terminals and its genetic depletion results in a marked alteration of inhibitory synaptic activity. p140Cap-/- cultured neurons display higher frequency of miniature inhibitory postsynaptic currents (mIPSCs) with no changes of their mean amplitude. Consistent with a potential presynaptic alteration of basal GABA release, p140Cap-/- neurons exhibit a larger synaptic vesicle readily releasable pool, without any variation of single GABAA receptor unitary currents and number of postsynaptic channels. Furthermore, p140Cap-/- neurons show a premature and enhanced network synchronization and appear more susceptible to 4-aminopyridine-induced seizures in vitro and to kainate-induced seizures in vivo. The hippocampus of p140Cap-/- mice showed a significant increase in the number of both inhibitory synapses and of parvalbumin- and somatostatin-expressing interneurons. Specific deletion of p140Cap in forebrain interneurons resulted in increased susceptibility to in vitro epileptic events and increased inhibitory synaptogenesis, comparable to those observed in p140Cap-/- mice. Altogether, our data demonstrate that p140Cap finely tunes inhibitory synaptogenesis and GABAergic neurotransmission, thus regulating the establishment and maintenance of the proper hippocampal excitatory/inhibitory balance.

Enhancing cognition through pharmacological and environmental interventions: Examples from preclinical models of neurodevelopmental disorders

In this review we discuss the role of environmental and pharmacological treatments to enhance cognition with special regards to neurodevelopmental related disorders and aging. How the environment influences brain structure and function, and the interactions between rearing conditions and gene expression, are fundamental questions that are still poorly understood. We propose a model that can explain some of the discrepancies in findings for effects of environmental enrichment on outcome measures. Evidence of a direct causal correlation of nootropics and treatments that enhanced cognition also will be presented, and possible molecular mechanisms that include neurotrophin signaling and downstream pathways underlying these processes are discussed. Finally we review recent findings achieved with a wide set of behavioral and cognitive tasks that have translational validity to humans, and should be useful for future work on devising appropriate therapies. As will be discussed, the collective findings suggest that a combinational therapeutic approach of environmental enrichment and nootropics could be particularly successful for improving learning and memory in both developmental disorders and normal aging.

ROCK/PKA Inhibition Rescues Hippocampal Hyperexcitability and GABAergic Neuron Alterations in a Oligophrenin-1 Knock-Out Mouse Model of X-Linked Intellectual Disability

Oligophrenin-1 (Ophn1) encodes a Rho GTPase activating protein whose mutations cause X-linked intellectual disability (XLID) in humans. Loss of function of Ophn1 leads to impairments in the maturation and function of excitatory and inhibitory synapses, causing deficits in synaptic structure, function and plasticity. Epilepsy is a frequent comorbidity in patients with Ophn1-dependent XLID, but the cellular bases of hyperexcitability are poorly understood. Here we report that male mice knock-out (KO) for Ophn1 display hippocampal epileptiform alterations, which are associated with changes in parvalbumin-, somatostatin- and neuropeptide Y-positive interneurons. Because loss of function of Ophn1 is related to enhanced activity of Rho-associated protein kinase (ROCK) and protein kinase A (PKA), we attempted to rescue Ophn1-dependent pathological phenotypes by treatment with the ROCK/PKA inhibitor fasudil. While acute administration of fasudil had no impact on seizure activity, seven weeks of treatment in adulthood were able to correct electrographic, neuroanatomical and synaptic alterations of Ophn1 deficient mice. These data demonstrate that hyperexcitability and the associated changes in GABAergic markers can be rescued at the adult stage in Ophn1-dependent XLID through ROCK/PKA inhibition.SIGNIFICANCE STATEMENT In this study we demonstrate enhanced seizure propensity and impairments in hippocampal GABAergic circuitry in Ophn1 mouse model of X-linked intellectual disability (XLID). Importantly, the enhanced susceptibility to seizures, accompanied by an alteration of GABAergic markers were rescued by Rho-associated protein kinase (ROCK)/protein kinase A (PKA) inhibitor fasudil, a drug already tested on humans. Because seizures can significantly impact the quality of life of XLID patients, the present data suggest a potential therapeutic pathway to correct alterations in GABAergic networks and dampen pathological hyperexcitability in adults with XLID.

Differential contribution of pyramidal cells and interneurons to activity-dependent gene transcription changes

The type of neuronal activity determines the outcome of gene expression. Hence, the characterization of underlying mechanisms in transcriptome alterations may serve as a biomarker and provide new intervention methods for the treatment of pathologic conditions. Parrish et al. (Parrish RR, Codadu NK, Racca C, Trevelyan AJ. J Neurophysiol 120: 2358-2367, 2018) show that the changes in interneuronal gene transcription are correlated with the type of the activated neuronal population and that the initiation route of Ras/ERK MAPK pathway determines the polarity of the gene expression.

The impact of RASopathy-associated mutations on CNS development in mice and humans

The RAS signaling pathway is involved in the regulation of developmental processes, including cell growth, proliferation, and differentiation, in the central nervous system (CNS). Germline mutations in the RAS signaling pathway genes are associated with a group of neurodevelopmental disorders, collectively called RASopathy, which includes neurofibromatosis type 1, Noonan syndrome, cardio-facio-cutaneous syndrome, and Costello syndrome. Most mutations associated with RASopathies increase the activity of the RAS-ERK signaling pathway, and therefore, most individuals with RASopathies share common phenotypes, such as a short stature, heart defects, facial abnormalities, and cognitive impairments, which are often accompanied by abnormal CNS development. Recent studies using mouse models of RASopathies demonstrated that particular mutations associated with each disorder disrupt CNS development in a mutation-specific manner. Here, we reviewed the recent literatures that investigated the developmental role of RASopathy-associated mutations using mutant mice, which provided insights into the specific contribution of RAS-ERK signaling molecules to CNS development and the subsequent impact on cognitive function in adult mice.

Brain mitochondrial proteome alteration driven by creatine deficiency suggests novel therapeutic venues for creatine deficiency syndromes

Creatine (Cr) is a small metabolite with a central role in energy metabolism and mitochondrial function. Creatine deficiency syndromes are inborn errors of Cr metabolism causing Cr depletion in all body tissues and particularly in the nervous system. Patient symptoms involve intellectual disability, language and behavioral disturbances, seizures and movement disorders suggesting that brain cells are particularly sensitive to Cr depletion. Cr deficiency was found to affect metabolic activity and structural abnormalities of mitochondrial organelles; however a detailed analysis of molecular mechanisms linking Cr deficit, energy metabolism alterations and brain dysfunction is still missing. Using a proteomic approach we evaluated the proteome changes of the brain mitochondrial fraction induced by the deletion of the Cr transporter (CrT) in developing mutant mice. We found a marked alteration of the mitochondrial proteomic landscape in the brain of CrT deficient mice, with the overexpression of many proteins involved in energy metabolism and response to oxidative stress. Moreover, our data suggest possible abnormalities of dendritic spines, synaptic function and plasticity, network excitability and neuroinflammatory response. Intriguingly, the alterations occurred in coincidence with the developmental onset of neurological symptoms. Thus, cerebral mitochondrial alterations could represent an early response to Cr deficiency that could be targeted for therapeutic intervention.

Excitatory neuron-specific SHP2-ERK signaling network regulates synaptic plasticity and memory

Mutations in RAS signaling pathway components cause diverse neurodevelopmental disorders, collectively called RASopathies. Previous studies have suggested that dysregulation in RAS-extracellular signal-regulated kinase (ERK) activation is restricted to distinct cell types in different RASopathies. Some cases of Noonan syndrome (NS) are associated with gain-of-function mutations in the phosphatase SHP2 (encoded by PTPN11); however, SHP2 is abundant in multiple cell types, so it is unclear which cell type(s) contribute to NS phenotypes. Here, we found that expressing the NS-associated mutant SHP2^D61G in excitatory, but not inhibitory, hippocampal neurons increased ERK signaling and impaired both long-term potentiation (LTP) and spatial memory in mice, although endogenous SHP2 was expressed in both neuronal types. Transcriptomic analyses revealed that the genes encoding SHP2-interacting proteins that are critical for ERK activation, such as GAB1 and GRB2, were enriched in excitatory neurons. Accordingly, expressing a dominant-negative mutant of GAB1, which reduced its interaction with SHP2^D61G, selectively in excitatory neurons, reversed SHP2^D61G-mediated deficits. Moreover, ectopic expression of GAB1 and GRB2 together with SHP2^D61G in inhibitory neurons resulted in ERK activation. These results demonstrate that RAS-ERK signaling networks are notably different between excitatory and inhibitory neurons, accounting for the cell type-specific pathophysiology of NS and perhaps other RASopathies.

Pharmacological Inhibition of ERK Signaling Rescues Pathophysiology and Behavioral Phenotype Associated with 16p11.2 Chromosomal Deletion in Mice

The human 16p11.2 microdeletion is one of the most common gene copy number variations linked to autism, but the pathophysiology associated with this chromosomal abnormality is largely unknown. The 593 kb deletion contains the ERK1 gene and other genes that converge onto the ERK/MAP kinase pathway. Perturbations in ERK signaling are linked to a group of related neurodevelopmental disorders hallmarked by intellectual disability, including autism. We report that mice harboring the 16p11.2 deletion exhibit a paradoxical elevation of ERK activity, cortical cytoarchitecture abnormalities and behavioral deficits. Importantly, we show that treatment with a novel ERK pathway inhibitor during a critical period of brain development rescues the molecular, anatomical and behavioral deficits in the 16p11.2 deletion mice. The ERK inhibitor treatment administered to adult mice ameliorates a subset of these behavioral deficits. Our findings provide evidence for potential targeted therapeutic intervention in 16p11.2 deletion carriers.SIGNIFICANCE STATEMENT The ERK/MAPK pathway is genetically linked to autism spectrum disorders and other syndromes typified by intellectual disability. We provide direct evidence connecting the ERK/MAP kinases to the developmental abnormalities in neurogenesis and cortical cytoarchitecture associated with the 16p11.2 chromosomal deletion. Most importantly, we demonstrate that treatment with a novel ERK-specific inhibitor during development rescues aberrant cortical cytoarchitecture and restores normal levels of cell-cycle regulators during cortical neurogenesis. These treatments partially reverse the behavioral deficits observed in the 16p11.2del mouse model, including hyperactivity, memory as well as olfaction, and maternal behavior. We also report a rescue of a subset of these deficits upon treatment of adult 16p11.2del mice. These data provide a strong rationale for therapeutic approaches to this disorder.

Mechanisms underlying cognitive deficits in a mouse model for Costello Syndrome are distinct from other RASopathy mouse models

RASopathies, characterized by germline mutations in genes encoding proteins of the RAS-ERK signaling pathway, show overlapping phenotypes, which manifest themselves with a varying severity of intellectual disability. However, it is unclear to what extent they share the same downstream pathophysiology that underlies the cognitive deficits. Costello syndrome (CS) is a rare RASopathy caused by activating mutations in the HRAS gene. Here we investigated the mechanisms underlying the cognitive deficits of HRas G12V/G12V mice. HRas G12V/G12V mice showed robust upregulation of ERK signaling, neuronal hypertrophy, increased brain volume, spatial learning deficits, and impaired mGluR-dependent long-term depression (LTD). In contrast, long-term potentiation (LTP), which is affected in other RASopathy mouse models was unaffected. Treatment with lovastatin, a HMG-CoA-Reductase inhibitor which has been shown to rescue the behavioral phenotypes of mouse models of NF1 and Noonan syndrome, was unable to restore ERK signaling and the cognitive deficits of HRas G12V/G12V mice. Administration of a potent mitogen-activated protein kinase (MEK) inhibitor rescued the ERK upregulation and the mGluR-LTD deficit of HRas G12V/G12V mice, but failed to rescue the cognitive deficits. Taken together, this study indicates that the fundamental molecular and cellular mechanisms underlying the cognitive aspects of different RASopathies are remarkably distinct, and may require disease specific treatments.

Impairment of cocaine-mediated behaviours in mice by clinically relevant Ras-ERK inhibitors

Ras-ERK signalling in the brain plays a central role in drug addiction. However, to date, no clinically relevant inhibitor of this cascade has been tested in experimental models of addiction, a necessary step toward clinical trials. We designed two new cell-penetrating peptides - RB1 and RB3 - that penetrate the brain and, in the micromolar range, inhibit phosphorylation of ERK, histone H3 and S6 ribosomal protein in striatal slices. Furthermore, a screening of small therapeutics currently in clinical trials for cancer therapy revealed PD325901 as a brain-penetrating drug that blocks ERK signalling in the nanomolar range. All three compounds have an inhibitory effect on cocaine-induced ERK activation and reward in mice. In particular, PD325901 persistently blocks cocaine-induced place preference and accelerates extinction following cocaine self-administration. Thus, clinically relevant, systemically administered drugs that attenuate Ras-ERK signalling in the brain may be valuable tools for the treatment of cocaine addiction.

DOI:http://dx.doi.org/10.7554/eLife.17111.001