Enhanced Prefrontal Neuronal Activity and Social Dominance Behavior in Postnatal Forebrain Excitatory Neuron-Specific Cyfip2 Knock-Out Mice

Recent advances in CYFIP2-associated neurodevelopmental disorders: From human genetics to molecular mechanisms and mouse models

Cytoplasmic FMR1-interacting protein 2 (CYFIP2) is an evolutionarily conserved protein with a critical role in brain development and function. As a key component of the WAVE regulatory complex, CYFIP2 regulates actin cytoskeleton dynamics, essential for maintaining proper neuronal morphology and circuit formation. Recent studies have also shown that CYFIP2 interacts with various RNA-binding proteins, suggesting its involvement in mRNA processing and translation in neurons. Since 2018, de novo CYFIP2 variants have been identified in patients with neurodevelopmental disorders, particularly developmental and epileptic encephalopathy and West syndrome, characterized by early-onset intractable seizures, intellectual disability, microcephaly, and developmental delay. This review summarizes these CYFIP2 variants and examines their potential impact on the molecular functions of CYFIP2, focusing on its roles in regulating actin dynamics and mRNA processing/translation. Additionally, we review various Cyfip2 mouse models, highlighting the insights they offer into CYFIP2 function, dysfunction, and clinical relevance. Finally, we discuss future research directions aimed at better understanding CYFIP2-associated neurodevelopmental disorders and potential therapeutic strategies.

Neurodevelopmental disorder-associated CYFIP2 regulates membraneless organelles and eIF2α phosphorylation via protein interactors and actin cytoskeleton

De novo variants in the Cytoplasmic FMR1-interacting protein 2 (CYFIP2) have been repeatedly associated with neurodevelopmental disorders and epilepsy, underscoring its critical role in brain development and function. While CYFIP2's role in regulating actin polymerization as part of the WAVE regulatory complex (WRC) is well-established, its additional molecular functions remain relatively unexplored. In this study, we performed unbiased quantitative proteomic analysis, revealing 278 differentially expressed proteins (DEPs) in the forebrain of Cyfip2 knock-out embryonic mice compared to wild-type mice. Unexpectedly, these DEPs, in conjunction with previously identified CYFIP2 brain interactors, included not only other WRC components but also numerous proteins associated with membraneless organelles (MLOs) involved in mRNA processing and translation within cells, including the nucleolus, stress granules, and processing bodies. Additionally, single-cell transcriptomic analysis of the Cyfip2 knock-out forebrain revealed gene expression changes linked to cellular stress responses and MLOs. We also observed morphological changes in MLOs in Cyfip2 knock-out brains and CYFIP2 knock-down cells under basal and stress conditions. Lastly, we demonstrated that CYFIP2 knock-down in cells, potentially through WRC-dependent actin regulation, suppressed the phosphorylation levels of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α), thereby enhancing protein synthesis. These results suggest a physical and functional connection between CYFIP2 and various MLO proteins and also extend CYFIP2's role within the WRC from actin regulation to influencing eIF2α phosphorylation and protein synthesis. With these dual functions, CYFIP2 may fine-tune the balance between MLO formation/dynamics and protein synthesis, a crucial aspect of proper mRNA processing and translation.

The mutated cytoplasmic fragile X messenger ribonucleoprotein 1 (FMR1)-interacting protein 2 (CYFIP2 S968F) regulates cocaine-induced reward behaviour and plasticity in the nucleus accumbens

BACKGROUND AND PURPOSE

Cytoplasmic fragile X messenger ribonucleoprotein 1 (FMR1)-interacting protein 2 (CYFIP2), as a component of the Wiskott-Aldrich syndrome protein family verprolin-homologous protein (WAVE) regulatory complex, is involved in actin polymerization, contributing to neuronal development and structural plasticity. Mutating serine-968 to phenylalanine (S968F) in CYFIP2 causes an altered cocaine response in mice. The neuronal mechanisms underlying this response remain unknown.

EXPERIMENTAL APPROACH

We performed cocaine reward-related behavioural tests and examined changes in synaptic protein phenotypes and neuronal morphology in the nucleus accumbens (NAc), using CYFIP2 S968F knock-in mice to investigate the role of CYFIP2 in regulating cocaine reward.

KEY RESULTS

CYFIP2 S968F mutation attenuated cocaine-induced behavioural sensitization and conditioned place preference. Cocaine-induced c-Fos was not observed in the NAc of CYFIP2 S968F knock-in mice. However, c-Fos induction was still evident in the medial prefrontal cortex (mPFC). CYFIP2 S968F mutation altered cocaine-associated CYFIP2 signalling, glutamatergic protein expression and synaptic density in the NAc following cocaine exposure. To further determine the role of CYFIP2 in NAc neuronal activity and the mPFC projecting to the NAc activity-mediating reward response, we used optogenetic tools to stimulate the NAc or mPFC-NAc pathway and observed that optogenetic activation of the NAc or mPFC-NAc pathway induced reward-related behaviours. This effect was not observed in the S968F mutation in CYFIP2.

CONCLUSION AND IMPLICATIONS

These results suggest that CYFIP2 plays a role in controlling cocaine-mediated neuronal function and structural plasticity in the NAc, and that CYFIP2 could serve as a target for regulating cocaine reward.

Mesocorticolimbic circuit mechanisms of social dominance behavior

Social animals, including rodents, primates, and humans, partake in competition for finite resources, thereby establishing social hierarchies wherein an individual's social standing influences diverse behaviors. Understanding the neurobiological underpinnings of social dominance is imperative, given its ramifications for health, survival, and reproduction. Social dominance behavior comprises several facets, including social recognition, social decision-making, and actions, indicating the concerted involvement of multiple brain regions in orchestrating this behavior. While extensive research has been dedicated to elucidating the neurobiology of social interaction, recent studies have increasingly delved into adverse social behaviors such as social competition and hierarchy. This review focuses on the latest advancements in comprehending the mechanisms of the mesocorticolimbic circuit governing social dominance, with a specific focus on rodent studies, elucidating the intricate dynamics of social hierarchies and their implications for individual well-being and adaptation.

Cell-autonomous reduction of CYFIP2 changes dendrite length, dendritic protrusion morphology, and inhibitory synapse density in the hippocampal CA1 pyramidal neurons of 17-month-old mice

The cytoplasmic FMR1-interacting protein 2 (CYFIP2) have diverse molecular functions in neurons, including the regulation of actin polymerization, mRNA translation, and mitochondrial morphology and function. Mutations in the CYFIP2 gene are associated with early-onset epilepsy and neurodevelopmental disorders, while decreases in its protein levels are linked to Alzheimer's disease (AD). Notably, previous research has revealed AD-like phenotypes, such as dendritic spine loss, in the hippocampal CA1 pyramidal neurons of 12-month-old Cyfip2 heterozygous mice but not of age-matched CA1 pyramidal neuron-specific Cyfip2 conditional knock-out (cKO) mice. This study aims to investigate whether dendritic spine loss in Cyfip2 cKO mice is merely delayed compared to Cyfip2 heterozygous mice, and to explore further neuronal phenotypes regulated by CYFIP2 in aged mice. We characterized dendrite and dendritic protrusion morphologies, along with excitatory/inhibitory synapse densities in CA1 pyramidal neurons of 17-month-old Cyfip2 cKO mice. Overall dendritic branching was normal, with a reduction in the length of basal, not apical, dendrites in CA1 pyramidal neurons of Cyfip2 cKO mice. Furthermore, while dendritic protrusion density remained normal, alterations were observed in the length of mushroom spines and the head volume of stubby spines in basal, not apical, dendrites of Cyfip2 cKO mice. Although excitatory synapse density remained unchanged, inhibitory synapse density increased in apical, not basal, dendrites of Cyfip2 cKO mice. Consequently, a cell-autonomous reduction of CYFIP2 appears insufficient to induce dendritic spine loss in CA1 pyramidal neurons of aged mice. However, CYFIP2 is required to maintain normal dendritic length, dendritic protrusion morphology, and inhibitory synapse density.

Cyfip2 controls the acoustic startle threshold through FMRP, actin polymerization, and GABAB receptor function

Animals process a constant stream of sensory input, and to survive they must detect and respond to dangerous stimuli while ignoring innocuous or irrelevant ones. Behavioral responses are elicited when certain properties of a stimulus such as its intensity or size reach a critical value, and such behavioral thresholds can be a simple and effective mechanism to filter sensory information. For example, the acoustic startle response is a conserved and stereotyped defensive behavior induced by sudden loud sounds, but dysregulation of the threshold to initiate this behavior can result in startle hypersensitivity that is associated with sensory processing disorders including schizophrenia and autism. Through a previous forward genetic screen for regulators of the startle threshold a nonsense mutation in Cytoplasmic Fragile X Messenger Ribonucleoprotein (FMRP)-interacting protein 2 (cyfip2) was found that causes startle hypersensitivity in zebrafish larvae, but the molecular mechanisms by which Cyfip2 establishes the acoustic startle threshold are unknown. Here we used conditional transgenic rescue and CRISPR/Cas9 to determine that Cyfip2 acts though both Rac1 and FMRP pathways, but not the closely related FXR1 or FXR2, to establish the acoustic startle threshold during early neurodevelopment. To identify proteins and pathways that may be downstream effectors of Rac1 and FMRP, we performed a candidate-based drug screen that indicated that Cyfip2 can also act acutely to maintain the startle threshold branched actin polymerization and N-methyl D-aspartate receptors (NMDARs). To complement this approach, we used unbiased discovery proteomics to determine that loss of Cyfip2 alters cytoskeletal and extracellular matrix components while also disrupting oxidative phosphorylation and GABA receptor signaling. Finally, we functionally validated our proteomics findings by showing that activating GABAB receptors, which like NMDARs are also FMRP targets, restores normal startle sensitivity in cyfip2 mutants. Together, these data reveal multiple mechanisms by which Cyfip2 regulates excitatory/inhibitory balance in the startle circuit to control the processing of acoustic information.

Metabolome subtyping reveals multi-omics characteristics and biological heterogeneity in major psychiatric disorders

Growing evidence suggests that major psychiatric disorders (MPDs) share common etiologies and pathological processes. However, the diagnosis is currently based on descriptive symptoms, which ignores the underlying pathogenesis and hinders the development of clinical treatments. This highlights the urgency of characterizing molecular biomarkers and establishing objective diagnoses of MPDs. Here, we collected untargeted metabolomics, proteomics and DNA methylation data of 327 patients with MPDs, 131 individuals with genetic high risk and 146 healthy controls to explore the multi-omics characteristics of MPDs. First, differential metabolites (DMs) were identified and we classified MPD patients into 3 subtypes based on DMs. The subtypes showed distinct metabolomics, proteomics and DNA methylation signatures. Specifically, one subtype showed dysregulation of complement and coagulation proteins, while the DNA methylation showed abnormalities in chemical synapses and autophagy. Integrative analysis in metabolic pathways identified the important roles of the citrate cycle, sphingolipid metabolism and amino acid metabolism. Finally, we constructed prediction models based on the metabolites and proteomics that successfully captured the risks of MPD patients. Our study established molecular subtypes of MPDs and elucidated their biological heterogeneity through a multi-omics investigation. These results facilitate the understanding of pathological mechanisms and promote the diagnosis and prevention of MPDs.

Analysis of networks in the dorsolateral prefrontal cortex in chronic schizophrenia: Relevance of altered immune response

The dorsolateral prefrontal cortex (DLPFC) has a crucial role in cognitive functioning and negative symptoms in schizophrenia. However, limited information of altered protein networks is available in this region in schizophrenia. We performed a proteomic analysis using single-shot liquid chromatography-tandem mass spectrometry of grey matter of postmortem DLPFC in chronic schizophrenia subjects (n = 20) and unaffected subjects (n = 20) followed by bioinformatic analysis to identify altered protein networks in schizophrenia (PXD024939 identifier in ProteomeXchange repository). Our results displayed a proteome profile in the DLPFC of 1989 proteins. 43 proteins were found significantly altered in schizophrenia. Analysis of this panel showed an enrichment of biological processes implicated in vesicle-mediated transport, processing and antigen presentation via MHC class II, intracellular transport and selenium metabolism. The enriched identified pathways were MHC class II antigen presentation, vesicle-mediated transport, Golgi ER retrograde transport, Nef mediated CD8 downregulation and the immune system. All these enriched categories were found to be downregulated. Furthermore, our network analyses showed crosstalk between proteins involved in MHC class II antigen presentation, membrane trafficking, Golgi-to-ER retrograde transport, Nef-mediated CD8 downregulation and the immune system with only one module built by 13 proteins. RAB7A showed eight interactions with proteins of all these pathways. Our results provide an altered molecular network involved in immune response in the DLPFC in schizophrenia with a central role of RAB7A. These results suggest that RAB7A or other proteins of this network could be potential targets for novel pharmacological strategies in schizophrenia for improving cognitive and negative symptoms.

CYFIP2 p.Arg87Cys Causes Neurological Defects and Degradation of CYFIP2

Here, we report the generation and comprehensive characterization of a knockin mouse model for the hotspot p.Arg87Cys variant of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) gene, which was recently identified in individuals diagnosed with West syndrome, a developmental and epileptic encephalopathy. The Cyfip2^+/R87C mice recapitulated many neurological and neurobehavioral phenotypes of the patients, including spasmlike movements, microcephaly, and impaired social communication. Age-progressive cytoarchitectural disorganization and gliosis were also identified in the hippocampus of Cyfip2^+/R87C mice. Beyond identifying a decrease in CYFIP2 protein levels in the Cyfip2^+/R87C brains, we demonstrated that the p.Arg87Cys variant enhances ubiquitination and proteasomal degradation of CYFIP2. ANN NEUROL 2023;93:155-163.

Cell-autonomous reduction of CYFIP2 is insufficient to induce Alzheimer's disease-like pathologies in the hippocampal CA1 pyramidal neurons of aged mice

Cytoplasmic FMR1-interacting protein 2 (CYFIP2) is an evolutionarily conserved multifunctional protein that regulates the neuronal actin cytoskeleton, mRNA translation and transport, and mitochondrial morphology and function. Supporting its critical roles in proper neuronal development and function, human genetic studies have repeatedly identified variants of the CYFIP2 gene in individuals diagnosed with neurodevelopmental disorders. Notably, a few recent studies have also suggested a mechanistic link between reduced CYFIP2 level and Alzheimer's disease (AD). Specifically, in the hippocampus of 12-month-old Cyfip2 heterozygous mice, several AD-like pathologies were identified, including increased levels of Tau phosphorylation and gliosis, and loss of dendritic spines in CA1 pyramidal neurons. However, detailed pathogenic mechanisms, such as cell types and their circuits where the pathologies originate, remain unknown for AD-like pathologies caused by CYFIP2 reduction. In this study, we aimed to address this issue by examining whether the cell-autonomous reduction of CYFIP2 in CA1 excitatory pyramidal neurons is sufficient to induce AD-like phenotypes in the hippocampus. We performed immunohistochemical, morphological, and biochemical analyses in 12-month-old Cyfip2 conditional knock-out mice, which have postnatally reduced CYFIP2 expression level in CA1, but not in CA3, excitatory pyramidal neurons of the hippocampus. Unexpectedly, we could not find any significant AD-like phenotype, suggesting that the CA1 excitatory neuron-specific reduction of CYFIP2 level is insufficient to lead to AD-like pathologies in the hippocampus. Therefore, we propose that CYFIP2 reduction in other neurons and/or their synaptic connections with CA1 pyramidal neurons may be critically involved in the hippocampal AD-like phenotypes of Cyfip2 heterozygous mice.

Protein interactome and cell-type expression analyses reveal that cytoplasmic FMR1-interacting protein 1 (CYFIP1), but not CYFIP2, associates with astrocytic focal adhesion

The two members of the cytoplasmic FMR1-interacting protein family, CYFIP1 and CYFIP2, are evolutionarily conserved multifunctional proteins whose defects are associated with distinct types of brain disorders. Even with high sequence homology between CYFIP1 and CYFIP2, several lines of evidence indicate their different functions in the brain; however, the underlying mechanisms remain largely unknown. Here, we performed reciprocal immunoprecipitation experiments using CYFIP1-2×Myc and CYFIP2-3×Flag knock-in mice and found that CYFIP1 and CYFIP2 are not significantly co-immunoprecipitated with each other in the knock-in brains compared to negative control wild-type brains. Moreover, CYFIP1 and CYFIP2 showed different size distributions by size-exclusion chromatography of wild-type mouse brains. Specifically, mass spectrometry-based analysis of CYFIP1-2×Myc knock-in brains identified 131 proteins in the CYFIP1 interactome. Comparison of the CYFIP1 interactome with the previously identified brain region- and age-matched CYFIP2 interactome, consisting of 140 proteins, revealed only eight common proteins. Investigations using single-cell RNA-sequencing databases suggested non-neuronal cell- and neuron-enriched expression of Cyfip1 and Cyfip2, respectively. At the protein level, CYFIP1 was detected in both neurons and astrocytes, while CYFIP2 was detected only in neurons, suggesting the predominant expression of CYFIP1 in astrocytes. Bioinformatic characterization of the CYFIP1 interactome, and co-expression analysis of Cyfip1 with astrocytic genes, commonly linked CYFIP1 with focal adhesion proteins. Immunocytochemical analysis and proximity ligation assay suggested partial co-localization of CYFIP1 and focal adhesion proteins in cultured astrocytes. Together, these results suggest a CYFIP1-specific association with astrocytic focal adhesion, which may contribute to the different brain functions and dysfunctions of CYFIP1 and CYFIP2.

Gut microbiota shapes social dominance through modulating HDAC2 in the medial prefrontal cortex

Social dominance is a ubiquitous phenomenon among social animals, including humans. To date, individual attributes leading to dominance (after a contest) remain largely elusive. Here, we report that socially dominant rats can be distinguished from subordinates based on their intestinal microbiota. When dysbiosis is induced, rats are predisposed to a subordinate state, while dysbiotic rats reclaim social dominance following microbiota transplantation. Winning hosts are characterized by core microbes, a majority of which are associated with butyrate production, and the sole colonization of Clostridium butyricum is sufficient to restore dominance. Regarding molecular aspects, a histone deacetylase, HDAC2, is responsive to microbial status and mediates competition outcome; however, this occurs only in a restricted population of cells in the medial prefrontal cortex (mPFC). Furthermore, HDAC2 acts by modulating synaptic activity in mPFC. Together, these findings uncover a link between commensals and host dominance, providing insight into the gut-brain mechanisms underlying dominance determination.

Cytoplasmic FMR1 interacting protein (CYFIP) family members and their function in neural development and disorders

In humans, the cytoplasmic FMR1 interacting protein (CYFIP) family is composed of CYFIP1 and CYFIP2. Despite their high similarity and shared interaction with many partners, CYFIP1 and CYFIP2 act at different points in cellular processes. CYFIP1 and CYFIP2 have different expression levels in human tissues, and knockout animals die at different time points of development. CYFIP1, similar to CYFIP2, acts in the WAVE regulatory complex (WRC) and plays a role in actin dynamics through the activation of the Arp2/3 complex and in a posttranscriptional regulatory complex with the fragile X mental retardation protein (FMRP). Previous reports have shown that CYFIP1 and CYFIP2 may play roles in posttranscriptional regulation in different ways. While CYFIP1 is involved in translation initiation via the 5'UTR, CYFIP2 may regulate mRNA expression via the 3'UTR. In addition, this CYFIP protein family is involved in neural development and maturation as well as in different neural disorders, such as intellectual disabilities, autistic spectrum disorders, and Alzheimer's disease. In this review, we map diverse studies regarding the functions, regulation, and implications of CYFIP proteins in a series of molecular pathways. We also highlight mutations and their structural effects both in functional studies and in neural diseases.