SAPAP Scaffold Proteins: From Synaptic Function to Neuropsychiatric Disorders

Allosteric modulation of Grb2 drives ligand-dependent signal responses

Adaptor proteins play a crucial role in signal transduction by facilitating the assembly of protein complexes at specific subcellular domains. These multifunctional molecules contain multiple binding modules that enhance the efficiency and flexibility of cellular signaling pathways, thereby orchestrating complex responses. Among these proteins, Grb2 (growth factor receptor-bound protein 2) emerges as a key regulator owing to its unique "sandwich" structure. Despite lacking intrinsic enzymatic activity, recent investigations have revealed that Grb2 acts not merely as a passive bridge but also utilizes intramolecular allosteric communication to modulate binding specificity. In this study, we compared the kinetic binding properties of SH2-SH3 belonging to Grb2 with Gab2 and the same experiment with bound states of the SH2 domain using two different peptides that mimics the physiological ligands of SH2. Our results demonstrate that the SH2 domain plays a critical regulatory role, exhibiting remarkably distinct behaviors in free and bound states, and depending on the ligand it binds to. This suggests how selectivity can be modulated by intradomain allostery. In vitro functional assays measuring the activation levels of the target protein further supported our hypothesis.

Premorbid characteristics of the SAPAP3 mouse model of obsessive-compulsive disorder: behavior, neuroplasticity, and psilocybin treatment

BACKGROUND

SAPAP3-knockout (SAPAP3-KO) mice develop excessive self-grooming behavior at 4-6 months of age, serving as a model for obsessive-compulsive disorder (OCD). Given that anxiety often precedes OCD diagnosis in humans, this study investigated whether juvenile SAPAP3-KO mice exhibit anxiety-like behaviors before developing the self-grooming phenotype, and whether such behaviors respond to psilocybin (PSIL) treatment. The study also examined 4 key neuroplasticity-related synaptic proteins-GAP43, PSD95, synaptophysin, and SV2A-as SAPAP3 is a postsynaptic scaffold protein that interacts with PSD95 and may affect synaptic function.

METHODS

Two studies were conducted using male and female juvenile (10-13 weeks) SAPAP3-KO mice. Study 1 compared behavioral phenotypes between homozygous (HOM), heterozygous, and wild-type (WT) mice. Study 2 evaluated a different sample of HOM and WT mice and assessed the effect of PSIL (4.4 mg/kg) on identified behavioral differences. Both studies included comprehensive behavioral testing focused on anxiety-like behavior, social interaction, and cognitive function. Additionally, levels of 4 synaptic proteins were measured by western blots in the frontal cortex, hippocampus, amygdala, and striatum of juvenile and adult SAPAP3-KO mice.

RESULTS

In both studies, juvenile HOM SAPAP3-KO mice showed significant anxiety-like behaviors compared to WT mice, spending less time in open field center, and elevated plus maze open arms. They also buried fewer marbles and found fewer buried Oreos than WT mice. Psilocybin treatment did not improve these behavioral manifestations. Analysis of synaptic proteins revealed significant increases in GAP43, synaptophysin, and SV2A across multiple brain regions in adult male HOM mice and of SV2A in the frontal cortex of HOM females compared to WT, but not in juvenile mice of either sex.

CONCLUSIONS

Juvenile SAPAP3-KO mice exhibit anxiety-like behaviors before developing the characteristic excessive self-grooming phenotype, paralleling the prodromal anxiety often seen in human OCD. Unlike in adult SAPAP3-KO mice, these manifestations were not responsive to PSIL treatment. The age-dependent increases in synaptic proteins observed in adult (but not juvenile) male SAPAP3-KO mice HOM for the deletion and to a lesser extent in female homozygotes, may represent compensatory plasticity changes in response to the phenotype. These results provide insights into the developmental trajectory of OCD-like behaviors and associated neuroplastic adaptations.

Synaptic editing of frontostriatal circuitry prevents excessive grooming in SAPAP3-deficient mice

Synaptic dysfunction has been implicated as a key mechanism underlying the pathophysiology of psychiatric disorders. Most pharmacological therapeutics for schizophrenia, autism spectrum disorder, obsessive-compulsive disorder, and major depressive disorder temporarily augment chemical synapse function. Nevertheless, medication non-compliance is a major clinical challenge, and behavioral dysfunction often returns following pharmacotherapeutic discontinuation. Here, we deployed a designer electrical synapse to edit a single class of chemical synapses in a genetic mouse model of obsessive-compulsive disorder (OCD). Editing these synapses in juvenile mice normalized circuit function and prevented the emergence of pathological repetitive behavior in adulthood. Thus, we establish precision circuit editing as a putative strategy for preventative psychotherapeutics.

FKBP51 is Involved in Epileptic Seizure by Regulating PSD95 in a PTZ-Induced Epileptic Mouse Model

BACKGROUND

Epilepsy, the world's third most prevalent chronic brain disorder, significantly affects patients' quality of life and increases the economic burden on families and society. Previous studies have demonstrated that FK506-binding protein 51 (FKBP51) plays a crucial role in synaptic plasticity. However, FKBP51 exhibits different functions under various physiological and pathological conditions. Our study explored the relationship between FKBP51 and epilepsy and its possible mechanism of action. We also analyzed the expression levels of postsynaptic density-95 (PSD95) and synaptophysin (SYP) in the hippocampus to examine the pathophysiology of epilepsy.

METHODS

A chronic epileptic kindling model was established by injecting pentylenetetrazole (PTZ) intraperitoneally, and a spontaneous seizure model was created by injecting kainic acid (KA) into the dentate gyrus using a stereotaxic apparatus. Endogenous FKBP51 expression was inhibited using adeno-associated virus (AAV)-FKBP51-Small hairpin RNAs (shRNA). The expression of FKBP51, PSD95, and SYP in the hippocampus and synaptosomes was measured through western blotting. Golgi staining and electron microscopy were used to examine spines and synaptic structures.

RESULTS

The results showed a significant increase in FKBP51 expression in the hippocampal tissue of the PTZ- and KA-induced epilepsy model groups. Inhibition of FKBP51 expression through AAV-FKBP51-shRNA resulted in a shorter latency and an elevated seizure grade score in mice. Moreover, the suppression of FKBP51 expression enhanced the expression of synaptic plasticity-related proteins, increased the density of dendritic spines, and elevated the quantity of spherical synaptic vesicles in the presynaptic membrane in the hippocampus.

CONCLUSIONS

FKBP51 may serve as an endogenous protective factor in epilepsy by regulating the expression of the synaptic plasticity-related protein PSD95, the density of dendritic spines, and the number of synaptic vesicles in the hippocampal CA1.

Altered odor perception in Dlgap2 mutant mice, a mouse model of autism spectrum disorder

Olfactory dysfunction has been observed in patients with Autism Spectrum Disorder (ASD). A microdeletion at the 8p23 terminal regions of chromosome 8p23 was identified in a Taiwanese patient with ASD, suggesting a potential association with mutations in the DLGAP2 gene. DLGAP2 is expressed in the olfactory bulb in rodents. The current study investigated olfactory phenotypes of Dlgap2 mutant mice. The results indicated that odor detection capabilities were comparable between wild-type (WT) and Dlgap2 mutant mice. However, homozygous mutant (Homo) mice showed less interest in sniffing odors of banana and almond but greater sniffing activity in response to bedding from unfamiliar cages. Notably, exposure to banana odor elicited significant c-fos expression in most olfaction-related brain regions of WT mice, while Homo mice did not show much increase in c-fos levels in major olfactory areas, which may correlate with their diminished sniffing behavior. Bedding stimuli induced pronounced c-fos expression in WT brains and some olfaction-related regions, including the olfactory bulb, amygdala, hypothalamus, and medial prefrontal cortex, in Homo mice. These mutants may still process olfactory signals from the bedding through a relatively narrow channel, which might elicit their interest, leading to increased sniffing behaviors that may compensate for their olfactory deficits. The DLGAP2 protein was absent in the olfactory bulb of Homo mice, and the levels of PSD95 and CaMKIIβ were also affected, indicating alterations in synaptic transmission and signaling within the olfactory system. This study evaluated olfactory perception in a mouse model of ASD, which may advance diagnostic and therapeutic strategies.

Resilience mechanisms underlying Alzheimer's disease

Alzheimer's disease (AD) consists of two main pathologies, which are the deposition of amyloid plaque as well as tau protein aggregation. Evidence suggests that not everyone who carries the AD-causing genes displays AD-related symptoms; they might never acquire AD as well. These individuals are referred to as non-demented individuals with AD neuropathology (NDAN). Despite the presence of extensive AD pathology in their brain, it was found that NDAN had better cognitive function than was expected, suggesting that they were more resilient (better at coping) to AD due to differences in their brains compared to other demented or cognitively impaired patients. Thus, identification of the mechanisms underlying resilience is crucial since it represents a promising therapeutic strategy for AD. In this review, we will explore the molecular mechanisms underpinning the role of genetic and molecular resilience factors in improving resilience to AD. These include protective genes and proteins such as APOE2, BDNF, RAB10, actin network proteins, scaffolding proteins, and the basal forebrain cholinergic system. A thorough understanding of these resilience mechanisms is crucial for not just comprehending the development of AD but may also open new treatment possibilities for AD by enhancing the neuroprotective pathway and targeting the pathogenic process.

Late development of OCD-like phenotypes in Dlgap1 knockout mice

RATIONALE

Despite variants in the Dlgap1 gene having the two lowest p-value in a genome-wide association study of obsessive compulsive disorder (OCD), previous studies reported the absence of OCD-like phenotypes in Dlgap1 knockout (KO) mice. Since these studies observed behavioral phenotypes only for a short period, development of OCD-like phenotypes in these mice at older ages was still plausible.

OBJECTIVE

To examine the presence or absence of development of OCD-like phenotypes in Dlgap1 KO mice and their responsiveness to fluvoxamine.

METHODS AND RESULTS

Newly produced Dlgap1 KO mice were observed for a year. Modified SHIRPA primary screen in 2-month-old homozygous mutant mice showed only weak signs of anxiety, stress conditions and aggression. At older ages, however, these mutant mice exhibited excessive self-grooming characterized by increased scratching which led to skin lesions. A significant sex difference was observed in this scratching behavior. The penetrance of skin lesions reached 50% at 6-7 months of age and 90% at 12 months of age. In the open-field test performed just after the appearance of these lesions, homozygous mutant mice spent significantly less time in the center, an anxiety-like behavior, than did their wild-type and heterozygous littermates, none and less than 10% of which showed skin lesions at 1 year, respectively. The skin lesions and excessive self-grooming were significantly alleviated by two-week treatment with fluvoxamine.

CONCLUSION

Usefulness of Dlgap1 KO mice as a tool for investigating the pathogenesis of OCD-like phenotypes and its translational relevance was suggested.

Molecular Modeling and In Vitro Functional Analysis of the RGS12 PDZ Domain Variant Associated with High-Penetrance Familial Bipolar Disorder

Bipolar disorder's etiology involves genetics, environmental factors, and gene-environment interactions, underlying its heterogeneous nature and treatment complexity. In 2020, Forstner and colleagues catalogued 378 sequence variants co-segregating with familial bipolar disorder. A notable candidate was an R59Q missense mutation in the PDZ (PSD-95/Dlg1/ZO-1) domain of RGS12. We previously demonstrated that RGS12 loss removes negative regulation on the kappa opioid receptor, disrupting basal ganglia dopamine homeostasis and dampening responses to dopamine-eliciting psychostimulants. Here, we investigated the R59Q variation in the context of potential PDZ domain functional alterations. We first validated a new target for the wildtype RGS12 PDZ domain-the SAPAP3 C-terminus-by molecular docking, surface plasmon resonance (SPR), and co-immunoprecipitation. While initial molecular dynamics (MD) studies predicted negligible effects of the R59Q variation on ligand binding, SPR showed a significant reduction in binding affinity for the three peptide targets tested. AlphaFold2-generated models predicted a modest reduction in protein-peptide interactions, which is consistent with the reduced binding affinity observed by SPR, suggesting that the substituted glutamine side chain may weaken the affinity of RGS12 for its in vivo binding targets, likely through allosteric changes. This difference may adversely affect the CNS signaling related to dynorphin and dopamine in individuals with this R59Q variation, potentially impacting bipolar disorder pathophysiology.

Experience-dependent grooming microstructure alterations and gastrointestinal dysfunction in the SAPAP3 knockout mouse model of compulsive behaviour

BACKGROUND

Compulsive- and anxiety-like behaviour can be efficiently modelled in SAPAP3 knockout (KO) mice, a preclinical model of relevance to obsessive-compulsive disorder (OCD). Although there is emerging evidence in the clinical literature of gastrointestinal dysfunction in OCD, no previous studies have investigated gut function in preclinical models of relevance to OCD. Similarly, the effects of voluntary exercise (EX) or environmental enrichment (EE) have not yet been explored in this context.

METHOD

We comprehensively phenotyped the SAPAP3 KO mouse model, including the assessment of grooming microstructure, anxiety- and depressive-like behaviour, and gastrointestinal function. Mice were exposed to either standard housing (SH), exercise (EX, provided by giving mice access to running wheels), or environmental enrichment (EE) for 4 weeks to investigate the effects of enriched housing conditions in this animal model relevant to OCD.

FINDINGS

Our study is the first to assess grooming microstructure, perseverative locomotor activity, and gastrointestinal function in SAPAP3 KO mice. We are also the first to report a sexually dimorphic effect of grooming in young-adult SAPAP3 KO mice; along with changes to grooming patterning and indicators of gut dysfunction, which occurred in the absence of gut dysbiosis in this model. Overall, we found no beneficial effects of voluntary exercise or environmental enrichment interventions in this mouse model; and unexpectedly, we revealed a deleterious effect of wheel-running exercise on grooming behaviour. We suspect that the detrimental effects of experimental housing in our study may be indicative of off-target effects of stress-a conclusion that warrants further investigation into the effects of chronic stress in this preclinical model of compulsive behaviour.

Key Synaptic Pathology in Autism Spectrum Disorder: Genetic Mechanisms and Recent Advances

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interactions and verbal communication, accompanied by symptoms of restricted and repetitive patterns of behavior or interest. Over the past 30 years, the morbidity of ASD has increased in most areas of the world. Although the pathogenesis of ASD is not fully understood, it has been associated with over 1000 genes or genomic loci, indicating the importance and complexity of the genetic mechanisms involved. This review focuses on the synaptic pathology of ASD and particularly on genetic variants involved in synaptic structure and functions. These include SHANK, NLGN, NRXN, FMR1, and MECP2 as well as other potentially novel genes such as CHD8, CHD2, and SYNGAP1 that could be core elements in ASD pathogenesis. Here, we summarize several pathological pathways supporting the hypothesis that synaptic pathology caused by genetic mutations may be the pathogenic basis for ASD.

The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents

BORLAND, J.M., The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents, NEUROSCI BIOBEH REV 21(1) XXX-XXX, 2024.-Sociality shapes an organisms' life. The nucleus accumbens is a critical brain region for mental health. In the following review, the effects of different types of social interactions on the physiology of neurons in the nucleus accumbens is synthesized. More specifically, the effects of sex behavior, aggression, social defeat, pair-bonding, play behavior, affiliative interactions, parental behaviors, the isolation from social interactions and maternal separation on measures of excitatory synaptic transmission, intracellular signaling and factors of transcription and translation in neurons in the nucleus accumbens in rodent models are reviewed. Similarities and differences in effects depending on the type of social interaction is then discussed. This review improves the understanding of the molecular and synaptic mechanisms of sociality.

Mapping proteomic composition of excitatory postsynaptic sites in the cerebellar cortex

Functions of the cerebellar cortex, from motor learning to emotion and cognition, depend on the appropriate molecular composition at diverse synapse types. Glutamate receptor distributions have been partially mapped using immunogold electron microscopy. However, information is lacking on the distribution of many other components, such as Shank2, a postsynaptic scaffolding protein whose cerebellar dysfunction is associated with autism spectrum disorders. Here, we used an adapted Magnified Analysis of the Proteome, an expansion microscopy approach, to map multiple glutamate receptors, scaffolding and signaling proteins at single synapse resolution in the cerebellar cortex. Multiple distinct synapse-selective distribution patterns were observed. For example, AMPA receptors were most concentrated at synapses on molecular layer interneurons and at climbing fiber synapses, Shank1 was most concentrated at parallel fiber synapses on Purkinje cells, and Shank2 at both climbing fiber and parallel fiber synapses on Purkinje cells but little on molecular layer interneurons. Our results are consistent with gene expression data but also reveal input-selective targeting within Purkinje cells. In specialized glomerular structures of the granule cell layer, AMPA receptors as well as most other synaptic components preferentially targeted to synapses. However, NMDA receptors and the synaptic GTPase activating protein SynGAP preferentially targeted to extrasynaptic sites. Thus, glomeruli may be considered integrative signaling units through which mossy fibers differentially activate synaptic AMPA and extrasynaptic NMDA receptor complexes. Furthermore, we observed NMDA receptors and SynGAP at adherens junctions, suggesting a role in structural plasticity of glomeruli. Altogether, these data contribute to mapping the cerebellar 'synaptome'.

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

S-SCAM is essential for synapse formation

Synapse formation is critical for the wiring of neural circuits in the developing brain. The synaptic scaffolding protein S-SCAM/MAGI-2 has important roles in the assembly of signaling complexes at post-synaptic densities. However, the role of S-SCAM in establishing the entire synapse is not known. Here, we report significant effects of RNAi-induced S-SCAM knockdown on the number of synapses in early stages of network development in vitro. In vivo knockdown during the first three postnatal weeks reduced the number of dendritic spines in the rat brain neocortex. Knockdown of S-SCAM in cultured hippocampal neurons severely reduced the clustering of both pre- and post-synaptic components. This included synaptic vesicle proteins, pre- and post-synaptic scaffolding proteins, and cell adhesion molecules, suggesting that entire synapses fail to form. Correspondingly, functional and morphological characteristics of developing neurons were affected by reducing S-SCAM protein levels; neurons displayed severely impaired synaptic transmission and reduced dendritic arborization. A next-generation sequencing approach showed normal expression of housekeeping genes but changes in expression levels in 39 synaptic signaling molecules in cultured neurons. These results indicate that S-SCAM mediates the recruitment of all key classes of synaptic molecules during synapse assembly and is critical for the development of neural circuits in the developing brain.

The Neurobiological Underpinnings of Obsessive-Compulsive Symptoms in Psychosis, Translational Issues for Treatment-Resistant Schizophrenia

Almost 25% of schizophrenia patients suffer from obsessive-compulsive symptoms (OCS) considered a transdiagnostic clinical continuum. The presence of symptoms pertaining to both schizophrenia and obsessive-compulsive disorder (OCD) may complicate pharmacological treatment and could contribute to lack or poor response to the therapy. Despite the clinical relevance, no reviews have been recently published on the possible neurobiological underpinnings of this comorbidity, which is still unclear. An integrative view exploring this topic should take into account the following aspects: (i) the implication for glutamate, dopamine, and serotonin neurotransmission as demonstrated by genetic findings; (ii) the growing neuroimaging evidence of the common brain regions and dysfunctional circuits involved in both diseases; (iii) the pharmacological modulation of dopaminergic, serotoninergic, and glutamatergic systems as current therapeutic strategies in schizophrenia OCS; (iv) the recent discovery of midbrain dopamine neurons and dopamine D1- and D2-like receptors as orchestrating hubs in repetitive and psychotic behaviors; (v) the contribution of N-methyl-D-aspartate receptor subunits to both psychosis and OCD neurobiology. Finally, we discuss the potential role of the postsynaptic density as a structural and functional hub for multiple molecular signaling both in schizophrenia and OCD pathophysiology.

Dysregulated Signaling at Postsynaptic Density: A Systematic Review and Translational Appraisal for the Pathophysiology, Clinics, and Antipsychotics' Treatment of Schizophrenia

Emerging evidence from genomics, post-mortem, and preclinical studies point to a potential dysregulation of molecular signaling at postsynaptic density (PSD) in schizophrenia pathophysiology. The PSD that identifies the archetypal asymmetric synapse is a structure of approximately 300 nm in diameter, localized behind the neuronal membrane in the glutamatergic synapse, and constituted by more than 1000 proteins, including receptors, adaptors, kinases, and scaffold proteins. Furthermore, using FASS (fluorescence-activated synaptosome sorting) techniques, glutamatergic synaptosomes were isolated at around 70 nm, where the receptors anchored to the PSD proteins can diffuse laterally along the PSD and were stabilized by scaffold proteins in nanodomains of 50-80 nm at a distance of 20-40 nm creating "nanocolumns" within the synaptic button. In this context, PSD was envisioned as a multimodal hub integrating multiple signaling-related intracellular functions. Dysfunctions of glutamate signaling have been postulated in schizophrenia, starting from the glutamate receptor's interaction with scaffolding proteins involved in the N-methyl-D-aspartate receptor (NMDAR). Despite the emerging role of PSD proteins in behavioral disorders, there is currently no systematic review that integrates preclinical and clinical findings addressing dysregulated PSD signaling and translational implications for antipsychotic treatment in the aberrant postsynaptic function context. Here we reviewed a critical appraisal of the role of dysregulated PSD proteins signaling in the pathophysiology of schizophrenia, discussing how antipsychotics may affect PSD structures and synaptic plasticity in brain regions relevant to psychosis.

Synapse Dysfunctions in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory disease of the central nervous system (CNS) affecting nearly three million humans worldwide. In MS, cells of an auto-reactive immune system invade the brain and cause neuroinflammation. Neuroinflammation triggers a complex, multi-faceted harmful process not only in the white matter but also in the grey matter of the brain. In the grey matter, neuroinflammation causes synapse dysfunctions. Synapse dysfunctions in MS occur early and independent from white matter demyelination and are likely correlates of cognitive and mental symptoms in MS. Disturbed synapse/glia interactions and elevated neuroinflammatory signals play a central role. Glutamatergic excitotoxic synapse damage emerges as a major mechanism. We review synapse/glia communication under normal conditions and summarize how this communication becomes malfunctional during neuroinflammation in MS. We discuss mechanisms of how disturbed glia/synapse communication can lead to synapse dysfunctions, signaling dysbalance, and neurodegeneration in MS.