ProSAP/Shank postsynaptic density proteins interact with insulin receptor tyrosine kinase substrate IRSp53

Oligonol enhances brain cognitive function in high-fat diet-fed mice

Oligonol, a low-molecular-weight polyphenol derived from lychee fruit, is well recognized for its antioxidant properties, blood glucose regulation, and fat mass reduction capability. However, its effect on the central nervous system remains unclear. Here, we investigated the effects of oligonol on brain in a high-fat diet (HFD) fed mouse model, and SH-SY5Y neuronal cells and primary cultured cortical neuron under insulin resistance conditions. HFD mice were orally administered oligonol (20 mg/kg) daily, and SH-SY5Y cells and primary cortical neurons were pretreated with 500 ng/mL oligonol under in vitro insulin resistance conditions. Our findings revealed that oligonol administration reduced blood glucose levels and improved spatial memory function in HFD mice. In vitro data demonstrated that oligonol protected neuronal cells and enhanced neural structure against insulin resistance. We confirmed RNA sequencing in the oligonol-pretreated insulin-resistant SH-SY5Y neuronal cells. Our RNA-sequencing data indicated that oligonol contributes to metabolic signaling and neurite outgrowth. In conclusion, our study provides insights into therapeutic potential of oligonol with respect to preventing neuronal cell damage and improving neural structure and cognitive function in HFD mice.

Structural deficits in key domains of Shank2 lead to alterations in postsynaptic nanoclusters and to a neurodevelopmental disorder in humans

Postsynaptic scaffold proteins such as Shank, PSD-95, Homer and SAPAP/GKAP family members establish the postsynaptic density of glutamatergic synapses through a dense network of molecular interactions. Mutations in SHANK genes are associated with neurodevelopmental disorders including autism and intellectual disability. However, no SHANK missense mutations have been described which interfere with the key functions of Shank proteins believed to be central for synapse formation, such as GKAP binding via the PDZ domain, or Zn2+-dependent multimerization of the SAM domain. We identify two individuals with a neurodevelopmental disorder carrying de novo missense mutations in SHANK2. The p.G643R variant distorts the binding pocket for GKAP in the Shank2 PDZ domain and prevents interaction with Thr(-2) in the canonical PDZ ligand motif of GKAP. The p.L1800W variant severely delays the kinetics of Zn2+-dependent polymerization of the Shank2-SAM domain. Structural analysis shows that Trp1800 dislodges one histidine crucial for Zn2+ binding. The resulting conformational changes block the stacking of helical polymers of SAM domains into sheets through side-by-side contacts, which is a hallmark of Shank proteins, thereby disrupting the highly cooperative assembly process induced by Zn2+. Both variants reduce the postsynaptic targeting of Shank2 in primary cultured neurons and alter glutamatergic synaptic transmission. Super-resolution microscopy shows that both mutants interfere with the formation of postsynaptic nanoclusters. Our data indicate that both the PDZ- and the SAM-mediated interactions of Shank2 contribute to the compaction of postsynaptic protein complexes into nanoclusters, and that deficiencies in this process interfere with normal brain development in humans.

Potential molecular mechanism of exercise reversing insulin resistance and improving neurodegenerative diseases

Neurodegenerative diseases are debilitating nervous system disorders attributed to various conditions such as body aging, gene mutations, genetic factors, and immune system disorders. Prominent neurodegenerative diseases include Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Insulin resistance refers to the inability of the peripheral and central tissues of the body to respond to insulin and effectively regulate blood sugar levels. Insulin resistance has been observed in various neurodegenerative diseases and has been suggested to induce the occurrence, development, and exacerbation of neurodegenerative diseases. Furthermore, an increasing number of studies have suggested that reversing insulin resistance may be a critical intervention for the treatment of neurodegenerative diseases. Among the numerous measures available to improve insulin sensitivity, exercise is a widely accepted strategy due to its convenience, affordability, and significant impact on increasing insulin sensitivity. This review examines the association between neurodegenerative diseases and insulin resistance and highlights the molecular mechanisms by which exercise can reverse insulin resistance under these conditions. The focus was on regulating insulin resistance through exercise and providing practical ideas and suggestions for future research focused on exercise-induced insulin sensitivity in the context of neurodegenerative diseases.

The Shank/ProSAP N-Terminal (SPN) Domain of Shank3 Regulates Targeting to Postsynaptic Sites and Postsynaptic Signaling

Members of the Shank family of postsynaptic scaffold proteins (Shank1-3) link neurotransmitter receptors to the actin cytoskeleton in dendritic spines through establishing numerous interactions within the postsynaptic density (PSD) of excitatory synapses. Large Shank isoforms carry at their N-termini a highly conserved domain termed the Shank/ProSAP N-terminal (SPN) domain, followed by a set of Ankyrin repeats. Both domains are involved in an intramolecular interaction which is believed to regulate accessibility for additional interaction partners, such as Ras family G-proteins, αCaMKII, and cytoskeletal proteins. Here, we analyze the functional relevance of the SPN-Ank module; we show that binding of active Ras or Rap1a to the SPN domain can differentially regulate the localization of Shank3 in dendrites. In Shank1 and Shank3, the linker between the SPN and Ank domains binds to inactive αCaMKII. Due to this interaction, both Shank1 and Shank3 exert a negative effect on αCaMKII activity at postsynaptic sites in mice in vivo. The relevance of the SPN-Ank intramolecular interaction was further analyzed in primary cultured neurons; here, we observed that in the context of full-length Shank3, a closed conformation of the SPN-Ank tandem is necessary for proper clustering of Shank3 on the head of dendritic spines. Shank3 variants carrying Ank repeats which are not associated with the SPN domain lead to the atypical formation of postsynaptic clusters on dendritic shafts, at the expense of clusters in spine-like protrusions. Our data show that the SPN-Ank tandem motif contributes to the regulation of postsynaptic signaling and is also necessary for proper targeting of Shank3 to postsynaptic sites. Our data also suggest how missense variants found in autistic patients which alter SPN and Ank domains affect the synaptic function of Shank3.

A lissencephaly-associated BAIAP2 variant causes defects in neuronal migration during brain development

Lissencephaly is a neurodevelopmental disorder characterized by a loss of brain surface convolutions caused by genetic variants that disrupt neuronal migration. However, the genetic origins of the disorder remain unidentified in nearly one-fifth of people with lissencephaly. Using whole-exome sequencing, we identified a de novo BAIAP2 variant, p.Arg29Trp, in an individual with lissencephaly with a posterior more severe than anterior (P>A) gradient, implicating BAIAP2 as a potential lissencephaly gene. Spatial transcriptome analysis in the developing mouse cortex revealed that Baiap2 is expressed in the cortical plate and intermediate zone in an anterior low to posterior high gradient. We next used in utero electroporation to explore the effects of the Baiap2 variant in the developing mouse cortex. We found that Baiap2 knockdown caused abnormalities in neuronal migration, morphogenesis and differentiation. Expression of the p.Arg29Trp variant failed to rescue the migration defect, suggesting a loss-of-function effect. Mechanistically, the variant interfered with the ability of BAIAP2 to localize to the cell membrane. These results suggest that the functions of BAIAP2 in the cytoskeleton, cell morphogenesis and migration are important for cortical development and for the pathogenesis of lissencephaly in humans.

The role of insulin/IGF1 signalling in neurodevelopmental and neuropsychiatric disorders - Evidence from human neuronal cell models

Insulin and insulin-like growth factor 1 (IGF1) signalling play a central role in the development and maintenance of neurons in the brain, and human neurodevelopmental as well as neuropsychiatric disorders have been linked to impaired insulin and IGF1 signalling. This review focuses on the impairments of the insulin and IGF1 signalling cascade in the context of neurodevelopmental and neuropsychiatric disorders, based on evidence from human neuronal cell models. Clear evidence was obtained for impaired insulin and IGF1 receptor downstream signalling in neurodevelopmental disorders, while the evidence for its role in neuropsychiatric disorders was less substantial. Human neuronal model systems can greatly add to our knowledge about insulin/IGF1 signalling in the brain, its role in restoring dendritic maturity, and complement results from clinical studies and animal models. Moreover, they represent a useful model for the development of new therapeutic strategies. Further research is needed to systematically investigate the exact role of the insulin/IGF1 signalling cascades in neurodevelopmental and neuropsychiatric disorders, and to elucidate the respective therapeutic implications.

Hippocampal BAIAP2 prevents chronic mild stress-induced depression-like behaviors in mice

Background

The pathogenesis of depression is closely related to changes in hippocampal synaptic plasticity; however, the underlying mechanism is still unclear. Brain-specific angiogenesis inhibitor 1-associated protein 2 (BAIAP2), a postsynaptic scaffold protein in excitatory synapses important for synaptic plasticity, is highly expressed in the hippocampus and has been implicated in several psychiatric disorders. However, the role of BAIAP2 in depression remains poorly understood.

Methods

In the present study, a mouse model of depression was established via exposure to chronic mild stress (CMS). An adeno-associated virus (AAV) vector expressing BAIAP2 was injected into the hippocampal brain region of mice and a BAIAP2 overexpression plasmid was transfected into HT22 cells to upregulate BAIAP2 expression. Depression- and anxiety-like behaviors and dendritic spine density were examined in mice using behavioral tests and Golgi staining, respectively. In vitro, hippocampal HT22 cells were treated with corticosterone (CORT) to simulate the stress state, and the effect of BAIAP2 on CORT-induced cell injury was explored. Reverse transcription-quantitative PCR and western blotting were employed to determine the expression levels of BAIAP2 and those of the synaptic plasticity-related proteins glutamate receptor ionotropic, AMPA 1 (GluA1), and synapsin 1 (SYN1).

Results

Mice exposed to CMS exhibited depression- and anxiety-like behaviors accompanied by decreased levels of BAIAP2 in the hippocampus. In vitro, the overexpression of BAIAP2 increased the survival rate of CORT-treated HT22 cells and upregulated the expression of GluA1 and SYN1. Consistent with the in vitro data, the AAV-mediated overexpression of BAIAP2 in the hippocampus of mice significantly inhibited CMS-induced depression-like behavior, concomitant with increases in dendritic spine density and the expression of GluA1 and SYN1 in hippocampal regions.

Conclusion

Our findings indicate that hippocampal BAIAP2 can prevent stress-induced depression-like behavior and may be a promising target for the treatment of depression or other stress-related diseases.

Oxidative stress: The nexus of obesity and cognitive dysfunction in diabetes

Obesity has been associated with oxidative stress. Obese patients are at increased risk for diabetic cognitive dysfunction, indicating a pathological link between obesity, oxidative stress, and diabetic cognitive dysfunction. Obesity can induce the biological process of oxidative stress by disrupting the adipose microenvironment (adipocytes, macrophages), mediating low-grade chronic inflammation, and mitochondrial dysfunction (mitochondrial division, fusion). Furthermore, oxidative stress can be implicated in insulin resistance, inflammation in neural tissues, and lipid metabolism disorders, affecting cognitive dysfunction in diabetics.

Type 2 Diabetes and Parkinson's Disease: A Focused Review of Current Concepts

Highly reproducible epidemiological evidence shows that type 2 diabetes (T2D) increases the risk and rate of progression of Parkinson's disease (PD), and crucially, the repurposing of certain antidiabetic medications for the treatment of PD has shown early promise in clinical trials, suggesting that the effects of T2D on PD pathogenesis may be modifiable. The high prevalence of T2D means that a significant proportion of patients with PD may benefit from personalized antidiabetic treatment approaches that also confer neuroprotective benefits. Therefore, there is an immediate need to better understand the mechanistic relation between these conditions and the specific molecular pathways affected by T2D in the brain. Although there is considerable evidence that processes such as insulin signaling, mitochondrial function, autophagy, and inflammation are involved in the pathogenesis of both PD and T2D, the primary aim of this review is to highlight the evidence showing that T2D-associated dysregulation of these pathways occurs not only in the periphery but also in the brain and how this may facilitate neurodegeneration in PD. We also discuss the challenges involved in disentangling the complex relationship between T2D, insulin resistance, and PD, as well as important questions for further research. © 2022 International Parkinson and Movement Disorder Society.

Diabetes: Risk factor and translational therapeutic implications for Alzheimer's disease

Type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) commonly co-occur. T2DM increases the risk for AD by approximately twofold. Animal models provide one means of interrogating the relationship of T2DM to AD and investigating brain insulin resistance in the pathophysiology of AD. Animal models show that persistent hyperglycaemia results in chronic low-grade inflammation that may contribute to the development of neuroinflammation and accelerate the pathobiology of AD. Epidemiological studies suggest that patients with T2DM who received treatment with specific anti-diabetic agents have a decreased risk for the occurrence of AD and all-cause dementia. Agents such as metformin ameliorate T2DM and may have other important systemic effects that lower the risk of AD. Glucagon-like peptide 1 (GLP-1) agonists have been associated with a decreased risk for AD in patients with T2DM. Both insulin and non-insulin anti-diabetic treatments have been evaluated for the treatment of AD in clinical trials. In most cases, patients included in the trials have clinical features of AD but do not have T2DM. Many of the trials were conducted prior to the use of diagnostic biomarkers for AD. Trials have had a wide range of durations and population sizes. Many of the agents used to treat T2DM do not cross the blood brain barrier, and the effects are posited to occur via lowering of peripheral hyperglycaemia and reduction of peripheral and central inflammation. Clinical trials of anti-diabetic agents to treat AD are ongoing and will provide insight into the therapeutic utility of these agents.

Adult re-expression of IRSp53 rescues NMDA receptor function and social behavior in IRSp53-mutant mice

IRSp53 (or BAIAP2) is an abundant excitatory postsynaptic scaffolding/adaptor protein that is involved in actin regulation and has been implicated in autism spectrum disorders, schizophrenia, and attention-deficit/hyperactivity disorder. IRSp53 deletion in mice leads to enhanced NMDA receptor (NMDAR) function and social deficits that are responsive to NMDAR inhibition. However, it remains unclear whether IRSp53 re-expression in the adult IRSp53-mutant mouse brain after the completion of brain development could reverse these synaptic and behavioral dysfunctions. Here we employed a brain-blood barrier (BBB)-penetrant adeno-associated virus (AAV) known as PHP.eB to drive adult IRSp53 re-expression in IRSp53-mutant mice. The adult IRSp53 re-expression normalized social deficits without affecting hyperactivity or anxiety-like behavior. In addition, adult IRSp53 re-expression normalized NMDAR-mediated excitatory synaptic transmission in the medial prefrontal cortex. Our results suggest that adult IRSp53 re-expression can normalize synaptic and behavioral deficits in IRSp53-mutant mice and that BBB-penetrant adult gene re-expression has therapeutic potential.

Inside the diabetic brain: Insulin resistance and molecular mechanism associated with cognitive impairment and its possible therapeutic strategies

Type 2 diabetes mellitus (T2DM) the most prevalent metabolic disease that has evolved into a major public health issue. Concerning about its secondary complications, a growing body of evidence links T2DM to cognitive impairment and neurodegenerative disorders. The underlying pathology behind this secondary complication disease is yet to be fully known. Nonetheless, they are likely to be associated with poor insulin signaling as a result of insulin resistance. We have combed through a rising body of literature on insulin signaling in the normal and diabetic brains along with various factors like insulin resistance, hyperglycemia, obesity, oxidative stress, neuroinflammation and Aβ plaques which can act independently or synergistically to link T2DM with cognitive impairments. Finally, we explored several pharmacological and non-pharmacological methods in the hopes of accelerating the rational development of medications for cognitive impairment in T2DM by better understanding these shared pathways.

IRSp53 promotes postsynaptic density formation and actin filament bundling

IRSp53 (aka BAIAP2) is a scaffold protein that couples membranes with the cytoskeleton in actin-filled protrusions such as filopodia and lamellipodia. The protein is abundantly expressed in excitatory synapses and is essential for synapse development and synaptic plasticity, although with poorly understood mechanisms. Here we show that specific multivalent interactions between IRSp53 and its binding partners PSD-95 or Shank3 drive phase separation of the complexes in solution. IRSp53 can be enriched to the reconstituted excitatory PSD (ePSD) condensates via bridging to the core and deeper layers of ePSD. Overexpression of a mutant defective in the IRSp53/PSD-95 interaction perturbs synaptic enrichment of IRSp53 in mouse cortical neurons. The reconstituted PSD condensates promote bundled actin filament formation both in solution and on membranes, via IRSp53-mediated actin binding and bundling. Overexpression of mutants that perturb IRSp53-actin interaction leads to defects in synaptic maturation of cortical neurons. Together, our studies provide potential mechanistic insights into the physiological roles of IRSp53 in synapse formation and function.

Activity dependent dissociation of the Homer1 interactome

Neurons encode information by rapidly modifying synaptic protein complexes, which changes the strength of specific synaptic connections. Homer1 is abundantly expressed at glutamatergic synapses, and is known to alter its binding to metabotropic glutamate receptor 5 (mGlu5) in response to synaptic activity. However, Homer participates in many additional known interactions whose activity-dependence is unclear. Here, we used co-immunoprecipitation and label-free quantitative mass spectrometry to characterize activity-dependent interactions in the cerebral cortex of wildtype and Homer1 knockout mice. We identified a small, high-confidence protein network consisting of mGlu5, Shank2 and 3, and Homer1–3, of which only mGlu5 and Shank3 were significantly reduced following neuronal depolarization. We identified several other proteins that reduced their co-association in an activity-dependent manner, likely mediated by Shank proteins. We conclude that Homer1 dissociates from mGlu5 and Shank3 following depolarization, but our data suggest that direct Homer1 interactions in the cortex may be more limited than expected.

Revisiting I-BAR Proteins at Central Synapses

Dendritic spines, the distinctive postsynaptic feature of central nervous system (CNS) excitatory synapses, have been studied extensively as electrical and chemical compartments, as well as scaffolds for receptor cycling and positioning of signaling molecules. The dynamics of the shape, number, and molecular composition of spines, and how they are regulated by neural activity, are critically important in synaptic efficacy, synaptic plasticity, and ultimately learning and memory. Dendritic spines originate as outward protrusions of the cell membrane, but this aspect of spine formation and stabilization has not been a major focus of investigation compared to studies of membrane protrusions in non-neuronal cells. We review here one family of proteins involved in membrane curvature at synapses, the BAR (Bin-Amphiphysin-Rvs) domain proteins. The subfamily of inverse BAR (I-BAR) proteins sense and introduce outward membrane curvature, and serve as bridges between the cell membrane and the cytoskeleton. We focus on three I-BAR domain proteins that are expressed in the central nervous system: Mtss2, MIM, and IRSp53 that promote negative, concave curvature based on their ability to self-associate. Recent studies suggest that each has distinct functions in synapse formation and synaptic plasticity. The action of I-BARs is also shaped by crosstalk with other signaling components, forming signaling platforms that can function in a circuit-dependent manner. We discuss another potentially important feature-the ability of some BAR domain proteins to impact the function of other family members by heterooligomerization. Understanding the spatiotemporal resolution of synaptic I-BAR protein expression and their interactions should provide insights into the interplay between activity-dependent neural plasticity and network rewiring in the CNS.

SHANK3 conformation regulates direct actin binding and crosstalk with Rap1 signaling

Actin-rich cellular protrusions direct versatile biological processes from cancer cell invasion to dendritic spine development. The stability, morphology, and specific biological functions of these protrusions are regulated by crosstalk between three main signaling axes: integrins, actin regulators, and small guanosine triphosphatases (GTPases). SHANK3 is a multifunctional scaffold protein, interacting with several actin-binding proteins and a well-established autism risk gene. Recently, SHANK3 was demonstrated to sequester integrin-activating small GTPases Rap1 and R-Ras to inhibit integrin activity via its Shank/ProSAP N-terminal (SPN) domain. Here, we demonstrate that, in addition to scaffolding actin regulators and actin-binding proteins, SHANK3 interacts directly with actin through its SPN domain. Molecular simulations and targeted mutagenesis of the SPN-ankyrin repeat region (ARR) interface reveal that actin binding is inhibited by an intramolecular closed conformation of SHANK3, where the adjacent ARR domain covers the actin-binding interface of the SPN domain. Actin and Rap1 compete with each other for binding to SHANK3, and mutation of SHANK3, resulting in reduced actin binding, augments inhibition of Rap1-mediated integrin activity. This dynamic crosstalk has functional implications for cell morphology and integrin activity in cancer cells. In addition, SHANK3-actin interaction regulates dendritic spine morphology in neurons and autism-linked phenotypes in vivo.

SHANK2 mutations impair apoptosis, proliferation and neurite outgrowth during early neuronal differentiation in SH-SY5Y cells

SHANK2 mutations have been identified in individuals with neurodevelopmental disorders, including intellectual disability and autism spectrum disorders (ASD). Using CRISPR/Cas9 genome editing, we obtained SH-SY5Y cell lines with frameshift mutations on one or both SHANK2 alleles. We investigated the effects of the different SHANK2 mutations on cell morphology, cell proliferation and differentiation potential during early neuronal differentiation. All mutant cell lines showed impaired neuronal differentiation marker expression. Cells with bi-allelic SHANK2 mutations revealed diminished apoptosis and increased proliferation, as well as decreased neurite outgrowth during early neuronal differentiation. Bi-allelic SHANK2 mutations resulted in an increase in p-AKT levels, suggesting that SHANK2 mutations impair downstream signaling of tyrosine kinase receptors. Additionally, cells with bi-allelic SHANK2 mutations had lower amyloid precursor protein (APP) expression compared to controls, suggesting a molecular link between SHANK2 and APP. Together, we can show that frameshift mutations on one or both SHANK2 alleles lead to an alteration of neuronal differentiation in SH-SY5Y cells, characterized by changes in cell growth and pre- and postsynaptic protein expression. We also provide first evidence that downstream signaling of tyrosine kinase receptors and amyloid precursor protein expression are affected.

A CRHR1 antagonist prevents synaptic loss and memory deficits in a trauma-induced delirium-like syndrome

Older patients with severe physical trauma are at high risk of developing neuropsychiatric syndromes with global impairment of cognition, attention, and consciousness. We employed a thoracic trauma (TxT) mouse model and thoroughly analyzed age-dependent spatial and temporal posttraumatic alterations in the central nervous system. Up to 5 days after trauma, we observed a transient 50% decrease in the number of excitatory synapses specifically in hippocampal pyramidal neurons accompanied by alterations in attention and motor activity and disruption of contextual memory consolidation. In parallel, hippocampal corticotropin-releasing hormone (CRH) expression was highly upregulated, and brain-derived neurotrophic factor (BDNF) levels were significantly reduced. In vitro experiments revealed that CRH application induced neuronal autophagy with rapid lysosomal degradation of BDNF via the NF-κB pathway. The subsequent synaptic loss was rescued by BDNF as well as by specific NF-κB and CRH receptor 1 (CRHR1) antagonists. In vivo, the chronic application of a CRHR1 antagonist after TxT resulted in reversal of the observed histological, molecular, and behavioral alterations. The data suggest that neuropsychiatric syndromes (i.e., delirium) after peripheral trauma might be at least in part due to the activation of the hippocampal CRH/NF-κB/BDNF pathway, which results in a dramatic loss of synaptic contacts. The successful rescue by stress hormone receptor antagonists should encourage clinical trials focusing on trauma-induced delirium and/or other posttraumatic syndromes.

An autism-linked missense mutation in SHANK3 reveals the modularity of Shank3 function

Genome sequencing has revealed an increasing number of genetic variations that are associated with neuropsychiatric disorders. Frequently, studies limit their focus to likely gene-disrupting mutations because they are relatively easy to interpret. Missense variants, instead, have often been undervalued. However, some missense variants can be informative for developing a more profound understanding of disease pathogenesis and ultimately targeted therapies. Here we present an example of this by studying a missense variant in a well-known autism spectrum disorder (ASD) causing gene SHANK3. We analyzed Shank3's in vivo phosphorylation profile and identified S685 as one phosphorylation site where one ASD-linked variant has been reported. Detailed analysis of this variant revealed a novel function of Shank3 in recruiting Abelson interactor 1 (ABI1) and the WAVE complex to the post-synaptic density (PSD), which is critical for synapse and dendritic spine development. This function was found to be independent of Shank3's other functions such as binding to GKAP and Homer. Introduction of this human ASD mutation into mice resulted in a small subset of phenotypes seen previously in constitutive Shank3 knockout mice, including increased allogrooming, increased social dominance, and reduced pup USV. Together, these findings demonstrate the modularity of Shank3 function in vivo. This modularity further indicates that there is more than one independent pathogenic pathway downstream of Shank3 and correcting a single downstream pathway is unlikely to be sufficient for clear clinical improvement. In addition, this study illustrates the value of deep biological analysis of select missense mutations in elucidating the pathogenesis of neuropsychiatric phenotypes.

IRSp53 is a novel interactor of SHIP2: A role of the actin binding protein Mena in their cellular localization in breast cancer cells

A tight control of the machineries regulating membrane bending and actin dynamics is very important for the generation of membrane protrusions, which are crucial for cell migration and invasion. Protein/protein and protein/phosphoinositides complexes assemble and disassemble to coordinate these mechanisms, the scaffold properties of the involved proteins playing a prominent role in this organization. The PI 5-phosphatase SHIP2 is a critical enzyme modulating PI(3,4,5)P₃, PI(4,5)P₂ and PI(3,4)P₂ content in the cell. The scaffold properties of SHIP2 contribute to the specific targeting or retention of the protein in particular subcellular domains. Here, we identified IRSp53 as a new binding interactor of SHIP2 proline-rich domain. Both proteins are costained in HEK293T cells protrusions, upon transfection. We showed that the SH3-binding polyproline motif recognized by IRSp53 in SHIP2 is different from the regions targeted by other PRR binding partners i.e., CIN85, ITSN or even Mena a common interactor of both SHIP2 and IRSp53. We presented evidence that IRSp53 phosphorylation on S366 did not influence its interaction with SHIP2 and that Mena is not necessary for the association of SHIP2 with IRSp53 in MDA-MB-231 cells. The absence of Mena in MDA-MB-231 cells decreased the intracellular content in F-actin and modified the subcellular localization of SHIP2 and IRSp53 by increasing their relative content at the plasma membrane. Together our data suggest that SHIP2, through interaction with the cell protrusion regulators IRSp53 and Mena, participate to the formation of multi-protein complexes. This ensures the appropriate modulations of PIs which is important for regulation of membrane dynamics.

Truncating mutations in SHANK3 associated with global developmental delay interfere with nuclear β-catenin signaling

Mutations in SHANK3, coding for a large scaffold protein of excitatory synapses in the CNS, are associated with neurodevelopmental disorders including autism spectrum disorders and intellectual disability (ID). Several cases have been identified in which the mutation leads to truncation of the protein, eliminating C-terminal sequences required for post-synaptic targeting of the protein. We identify here a patient with a truncating mutation in SHANK3, affected by severe global developmental delay and intellectual disability. By analyzing the subcellular distribution of this truncated form of Shank3, we identified a nuclear localization signal (NLS) in the N-terminal part of the protein which is responsible for targeting Shank3 fragments to the nucleus. To determine the relevance of Shank3 for nuclear signaling, we analyze how it affects signaling by β-catenin, a component of the Wnt pathway. We show that full length as well as truncated variants of Shank3 interact with β-catenin via the PDZ domain of Shank3, and the armadillo repeats of β-catenin. As a result of this interaction, truncated forms of Shank3 and β-catenin strictly co-localize in small intra-nuclear bodies both in 293T cells and in rat hippocampal neurons. On a functional level, the sequestration of both proteins in these nuclear bodies is associated with a strongly repressed transcriptional activation by β-catenin owing to interaction with the truncated Shank3 fragment found in patients. Our data suggest that truncating mutations in SHANK3 may not only lead to a reduction in Shank3 protein available at postsynaptic sites but also negatively affect the Wnt signaling pathway.

Metabolic Syndrome, Brain Insulin Resistance, and Alzheimer's Disease: Thioredoxin Interacting Protein (TXNIP) and Inflammasome as Core Amplifiers

Empirical evidence indicates a strong association between insulin resistance and pathological alterations related to Alzheimer's disease (AD) in different cerebral regions. While cerebral insulin resistance is not essentially parallel with systemic metabolic derangements, type 2 diabetes mellitus (T2DM) has been established as a risk factor for AD. The circulating "toxic metabolites" emerging in metabolic syndrome may engage several biochemical pathways to promote oxidative stress and neuroinflammation leading to impair insulin function in the brain or "type 3 diabetes". Thioredoxin-interacting protein (TXNIP) as an intracellular amplifier of oxidative stress and inflammasome activation may presumably mediate central insulin resistance. Emerging data including those from our recent studies has demonstrated a sharp TXNIP upregulation in stroke, aging and AD and well underlining the significance of this hypothesis. With the main interest to illustrate TXNIP place in type 3 diabetes, the present review primarily briefs the potential mechanisms contributing to cerebral insulin resistance in a metabolically deranged environment. Then with a particular focus on plausible TXNIP functions to drive and associate with AD pathology, we present the most recent evidence supporting TXNIP as a promising therapeutic target in AD as an age-associated dementia.

Shank3 Mice Carrying the Human Q321R Mutation Display Enhanced Self-Grooming, Abnormal Electroencephalogram Patterns, and Suppressed Neuronal Excitability and Seizure Susceptibility

Shank3, a postsynaptic scaffolding protein involved in regulating excitatory synapse assembly and function, has been implicated in several brain disorders, including autism spectrum disorders (ASD), Phelan-McDermid syndrome, schizophrenia, intellectual disability, and mania. Here we generated and characterized a Shank3 knock-in mouse line carrying the Q321R mutation (Shank3 ^Q321R mice) identified in a human individual with ASD that affects the ankyrin repeat region (ARR) domain of the Shank3 protein. Homozygous Shank3 ^Q321R/Q321R mice show a selective decrease in the level of Shank3a, an ARR-containing protein variant, but not other variants. CA1 pyramidal neurons in the Shank3 ^Q321R/Q321R hippocampus show decreased neuronal excitability but normal excitatory and inhibitory synaptic transmission. Behaviorally, Shank3 ^Q321R/Q321R mice show moderately enhanced self-grooming and anxiolytic-like behavior, but normal locomotion, social interaction, and object recognition and contextual fear memory. In addition, these mice show abnormal electroencephalogram (EEG) patterns and decreased susceptibility to induced seizures. These results indicate that the Q321R mutation alters Shank3 protein stability, neuronal excitability, repetitive and anxiety-like behavior, EEG patterns, and seizure susceptibility in mice.

IRSp53 coordinates AMPK and 14-3-3 signaling to regulate filopodia dynamics and directed cell migration

Filopodia are actin-filled membrane protrusions that play essential roles in cell motility and cell–cell communication and act as precursors of dendritic spines. IRSp53 is an essential regulator of filopodia formation, which couples Rho-GTPase signaling to actin cytoskeleton and membrane remodeling. IRSp53 has three major domains: an N-terminal inverse-BAR (I-BAR) domain, a Cdc42- and SH3-binding CRIB-PR domain, and an SH3 domain that binds downstream cytoskeletal effectors. Phosphorylation sites in the region between the CRIB-PR and SH3 domains mediate the binding of 14-3-3. Yet the mechanism by which 14-­3-3 regulates filopodia formation and dynamics and its role in cell migration are poorly understood. Here, we show that phosphorylation-dependent inhibition of IRSp53 by 14-3-3 counters activation by Cdc42 and cytoskeletal effectors, resulting in down-regulation of filopodia dynamics and cancer cell migration. In serum-starved cells, increased IRSp53 phosphorylation triggers 14-3-3 binding, which inhibits filopodia formation and dynamics, irrespective of whether IRSp53 is activated by Cdc42 or downstream effectors (Eps8, Ena/VASP). Pharmacological activation or inhibition of AMPK, respectively, increases or decreases the phosphorylation of two of three sites in IRSp53 implicated in 14-3-3 binding. Mutating these phosphorylation sites reverses 14-3-3-dependent inhibition of filopodia dynamics and cancer cell chemotaxis.

Mechanism of IRSp53 inhibition by 14-3-3

Filopodia are precursors of dendritic spines and polarized cell migration. The I-BAR-domain protein IRSp53 is a key regulator of filopodia dynamics that couples Rho-GTPase signaling to cytoskeleton and membrane remodeling, playing essential roles in neuronal development and cell motility. Here, we describe the structural-functional basis for 14-3-3-dependent inhibition of IRSp53. Phosphoproteomics, quantitative binding and crystallographic studies demonstrate that 14-3-3 binds to two pairs of phosphorylation sites in IRSp53. Using bicistronic expression, we obtain an IRSp53 heterodimer in which only one subunit is phosphorylated, and show that each subunit of IRSp53 independently binds one 14-3-3 dimer. A FRET-sensor assay using natively phosphorylated IRSp53 reveals opposite conformational changes upon binding of activatory (Cdc42, Eps8) or inhibitory (14-3-3) inputs. Finally, we show that 14-3-3 inhibits IRSp53 binding to membranes. Collectively, our findings support a mechanism whereby phosphorylation-dependent inhibition of IRSp53 by 14-3-3 counters membrane binding and interactions with Cdc42 and downstream cytoskeletal effectors.

IRSp53 is a key regulator of filopodia formation and cell migration. Here, the authors elucidate a mechanism of phosphorylation-dependent inhibition of IRSp53 by 14-3-3, which impedes the interactions of IRSp53 with membranes and downstream cytoskeletal effectors.

Dendritic spine actin cytoskeleton in autism spectrum disorder

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the shape and size of dendritic spines correlate with the functional changes in excitatory synapses and are heavily dependent on the remodeling of the underlying actin cytoskeleton. Recent evidence implicates synapses at dendritic spines as important substrates of pathogenesis in neuropsychiatric disorders, including autism spectrum disorder (ASD). Although synaptic perturbations are not the only alterations relevant for these diseases, understanding the molecular underpinnings of the spine and synapse pathology may provide insight into their etiologies and could reveal new drug targets. In this review, we will discuss recent findings of defective actin regulation in dendritic spines associated with ASD.

Cell-Type-Specific Shank2 Deletion in Mice Leads to Differential Synaptic and Behavioral Phenotypes

Shank2 is an excitatory postsynaptic scaffolding protein implicated in synaptic regulation and psychiatric disorders including autism spectrum disorders. Conventional Shank2-mutant (Shank2^-/-) mice display several autistic-like behaviors, including social deficits, repetitive behaviors, hyperactivity, and anxiety-like behaviors. However, cell-type-specific contributions to these behaviors have remained largely unclear. Here, we deleted Shank2 in specific cell types and found that male mice lacking Shank2 in excitatory neurons (CaMKII-Cre;Shank2^fl/fl) show social interaction deficits and mild social communication deficits, hyperactivity, and anxiety-like behaviors. In particular, male mice lacking Shank2 in GABAergic inhibitory neurons (Viaat-Cre;Shank2^fl/fl) display social communication deficits, repetitive self-grooming, and mild hyperactivity. These behavioral changes were associated with distinct changes in hippocampal and striatal synaptic transmission in the two mouse lines. These results indicate that cell-type-specific deletions of Shank2 in mice lead to differential synaptic and behavioral abnormalities.SIGNIFICANCE STATEMENT Shank2 is an abundant excitatory postsynaptic scaffolding protein implicated in the regulation of excitatory synapses and diverse psychiatric disorders including autism spectrum disorders. Previous studies have reported in vivo functions of Shank2 mainly using global Shank2-null mice, but it remains largely unclear how individual cell types contribute to Shank2-dependent regulation of neuronal synapses and behaviors. Here, we have characterized conditional Shank2-mutant mice carrying the Shank2 deletion in excitatory and inhibitory neurons. These mouse lines display distinct alterations of synaptic transmission in the hippocampus and striatum that are associated with differential behavioral abnormalities in social, repetitive, locomotor, and anxiety-like domains.

Brain insulin resistance in type 2 diabetes and Alzheimer disease: concepts and conundrums

Considerable overlap has been identified in the risk factors, comorbidities and putative pathophysiological mechanisms of Alzheimer disease and related dementias (ADRDs) and type 2 diabetes mellitus (T2DM), two of the most pressing epidemics of our time. Much is known about the biology of each condition, but whether T2DM and ADRDs are parallel phenomena arising from coincidental roots in ageing or synergistic diseases linked by vicious pathophysiological cycles remains unclear. Insulin resistance is a core feature of T2DM and is emerging as a potentially important feature of ADRDs. Here, we review key observations and experimental data on insulin signalling in the brain, highlighting its actions in neurons and glia. In addition, we define the concept of 'brain insulin resistance' and review the growing, although still inconsistent, literature concerning cognitive impairment and neuropathological abnormalities in T2DM, obesity and insulin resistance. Lastly, we review evidence of intrinsic brain insulin resistance in ADRDs. By expanding our understanding of the overlapping mechanisms of these conditions, we hope to accelerate the rational development of preventive, disease-modifying and symptomatic treatments for cognitive dysfunction in T2DM and ADRDs alike.

Synaptic Actin Dysregulation, a Convergent Mechanism of Mental Disorders?

Actin polymerization governs activity-dependent modulation of excitatory synapses, including their morphology and functionality. It is clear from human genetics that neuropsychiatric and neurodevelopmental disturbances are multigenetic in nature, highlighting the need to better understand the critical neural pathways associated with these disorders and how they are altered by genetic risk alleles. One such signaling pathway that is heavily implicated by candidate genes for psychiatric and neurodevelopmental disorders are regulators of signaling to the actin cytoskeleton, suggesting that its disruption and the ensuring abnormalities of spine structures and postsynaptic complexes is a commonly affected pathway in brain disorders. This review will discuss recent experimental findings that strongly support genetic evidence linking the synaptic cytoskeleton to mental disorders, such as schizophrenia and autism spectrum disorders.

Homer1 (VesL-1) in the rat esophagus: focus on myenteric plexus and neuromuscular junction

Homer1, a scaffolding protein of the postsynaptic density (PSD), enriched at excitatory synapses is known to anchor and modulate group I metabotropic glutamate receptors (mGluRs) and different channel- and receptor-proteins. Homer proteins are expressed in neurons of different brain regions, but also in non-neuronal tissues like skeletal muscle. Occurrence and location of Homer1 and mGluR5 in myenteric plexus and neuromuscular junctions (NMJ) of rat esophagus have yet not been characterized. We located Homer1 and mGluR5 immunoreactivity (-iry) in rat esophagus and focused on myenteric neurons, intraganglionic laminar endings (IGLEs) and NMJs, using double- and triple-label immunohistochemistry and confocal laser scanning microscopy. Homer1-iry was found in a subpopulation of vesicular glutamate transporter 2 (VGLUT2) positive IGLEs and cholinergic varicosities within myenteric ganglia, but neither in nitrergic nor cholinergic myenteric neuronal cell bodies. Homer1-iry was detected in 63% of esophageal and, for comparison, in 35% of sternomastoid NMJs. Besides the location in the PSD, Homer1-iry colocalized with cholinergic markers, indicating a presynaptic location in coarse VAChT/CGRP/NF200- immunoreactive (-ir) terminals of nucleus ambiguus neurons supplying striated esophageal muscle. mGluR5-iry was found in subpopulations of myenteric neuronal cell bodies, VGLUT2-ir IGLEs and cholinergic varicosities within the myenteric neuropil and NMJs of esophagus and sternomastoid muscles. Thus, Homer1 may anchor mGluR5 at presynaptic sites of cholinergic boutons at esophageal motor endplates, in a small subpopulation of VGLUT2-ir IGLEs and cholinergic varicosities within myenteric ganglia possibly modulating Ca²⁺-currents and neurotransmitter release.

Actin-Dependent Alterations of Dendritic Spine Morphology in Shankopathies

Shank proteins (Shank1, Shank2, and Shank3) act as scaffolding molecules in the postsynaptic density of many excitatory neurons. Mutations in SHANK genes, in particular SHANK2 and SHANK3, lead to autism spectrum disorders (ASD) in both human and mouse models. Shank3 proteins are made of several domains-the Shank/ProSAP N-terminal (SPN) domain, ankyrin repeats, SH3 domain, PDZ domain, a proline-rich region, and the sterile alpha motif (SAM) domain. Via various binding partners of these domains, Shank3 is able to bind and interact with a wide range of proteins including modulators of small GTPases such as RICH2, a RhoGAP protein, and βPIX, a RhoGEF protein for Rac1 and Cdc42, actin binding proteins and actin modulators. Dysregulation of all isoforms of Shank proteins, but especially Shank3, leads to alterations in spine morphogenesis, shape, and activity of the synapse via altering actin dynamics. Therefore, here, we highlight the role of Shank proteins as modulators of small GTPases and, ultimately, actin dynamics, as found in multiple in vitro and in vivo models. The failure to mediate this regulatory role might present a shared mechanism in the pathophysiology of autism-associated mutations, which leads to dysregulation of spine morphogenesis and synaptic signaling.

Novel Therapeutic Approach for Autism Spectrum Disorder: Focus on SHANK3

SHANK3 is a synaptic scaffolding protein and plays an important role in neuronal development. SHANK3 interacts with various synaptic molecules, including post-synaptic density-95 (PSD-95), homer and GluR1 AMPA receptor. SHANK3 gene is a causable gene of the Phelan- McDermid syndrome (also known as the 22q13.3 deletion syndrome), whose manifestation is global developmental delay and autistic behavior, especially shows severe speech and language deficit. Additionally since cumulative gene analysis in autistic subjects identified several mutations in SHANK3 gene, including deletion and duplication in a particular region, abnormality of SHANK3 gene is thought the be related with the neuropathology of autism spectrum disorder (ASD). We here review the recent findings in regard to the roles of SHANK3 in higher brain functions, molecular-biologic studies of the complex expression of Shank3 transcripts and production of SHANK3 isoforms, and behavioral studies of Shank3-mutant mice, including our recent findings, and discuss a novel therapeutic approach for ASD.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

Selective Localization of Shanks to VGLUT1-Positive Excitatory Synapses in the Mouse Hippocampus

Members of the Shank family of multidomain proteins (Shank1, Shank2, and Shank3) are core components of the postsynaptic density (PSD) of excitatory synapses. At synaptic sites Shanks serve as scaffolding molecules that cluster neurotransmitter receptors as well as cell adhesion molecules attaching them to the actin cytoskeleton. In this study we investigated the synapse specific localization of Shank1-3 and focused on well-defined synaptic contacts within the hippocampal formation. We found that all three family members are present only at VGLUT1-positive synapses, which is particularly visible at mossy fiber contacts. No costaining was found at VGLUT2-positive contacts indicating that the molecular organization of VGLUT2-associated PSDs diverges from classical VGLUT1-positive excitatory contacts in the hippocampus. In light of SHANK mutations in neuropsychiatric disorders, this study indicates which glutamatergic networks within the hippocampus will be primarily affected by shankopathies.

Shank3 is localized in axons and presynaptic specializations of developing hippocampal neurons and involved in the modulation of NMDA receptor levels at axon terminals

Autism-related Shank1, Shank2, and Shank3 are major postsynaptic scaffold proteins of excitatory glutamatergic synapses. A few studies, however, have already indicated that within a neuron, the presence of Shank family members is not limited to the postsynaptic density. By separating axons from dendrites of developing hippocampal neurons in microfluidic chambers, we show that RNA of all three Shank family members is present within axons. Immunostaining confirms these findings as all three Shanks are indeed found within separated axons and further co-localize with well-known proteins of the presynaptic specialization in axon terminals. Therefore, Shank proteins might not only serve as postsynaptic scaffold proteins, but also play a crucial role during axonal outgrowth and presynaptic development and function. This is supported by our findings that shRNA-mediated knockdown of Shank3 results in up-regulation of the NMDA receptor subunit GluN1 in axon terminals. Taken together, our findings will have major implications for the future analysis of neuronal Shank biology in both health and disease. Shank1, Shank2, and Shank3 are major postsynaptic scaffold proteins of excitatory glutamatergic synapses strongly related to several neuropsychiatric disorders. However, a few studies have already implicated a functional role of the Shanks beyond the postsynaptic density (PSD). We here show that all three Shanks are localized in both axons and pre-synaptic specializiations of developing hippocampal neurons in culture. We further provide evidence that Shank3 is involved in the modulation of NMDA receptor levels at axon terminals. Taken together, our study will open up novel avenues for the future analysis of neuronal Shank biology in both health and disease.

Emerging Roles of BAI Adhesion-GPCRs in Synapse Development and Plasticity

Synapses mediate communication between neurons and enable the brain to change in response to experience, which is essential for learning and memory. The sites of most excitatory synapses in the brain, dendritic spines, undergo rapid remodeling that is important for neural circuit formation and synaptic plasticity. Abnormalities in synapse and spine formation and plasticity are associated with a broad range of brain disorders, including intellectual disabilities, autism spectrum disorders (ASD), and schizophrenia. Thus, elucidating the mechanisms that regulate these neuronal processes is critical for understanding brain function and disease. The brain-specific angiogenesis inhibitor (BAI) subfamily of adhesion G-protein-coupled receptors (adhesion-GPCRs) has recently emerged as central regulators of synapse development and plasticity. In this review, we will summarize the current knowledge regarding the roles of BAIs at synapses, highlighting their regulation, downstream signaling, and physiological functions, while noting the roles of other adhesion-GPCRs at synapses. We will also discuss the relevance of BAIs in various neurological and psychiatric disorders and consider their potential importance as pharmacological targets in the treatment of these diseases.

Severe learning deficits of IRSp53 mutant mice are caused by altered NMDA receptor-dependent signal transduction

Learning and memory is dependent on postsynaptic architecture and signaling processes in forebrain regions. The insulin receptor substrate protein of 53 kDa (IRSp53, also known as Baiap2) is a signaling and adapter protein in forebrain excitatory synapses. Mice deficient in IRSp53 display enhanced levels of postsynaptic N-methyl-D-aspartate receptors (NMDARs) and long-term potentiation (LTP) associated with severe learning deficits. In humans, reduced IRSp53/Baiap2 expression is associated with a variety of neurological disorders including autism, schizophrenia, and Alzheimer's disease. Here, we analyzed mice lacking one copy of the gene coding for IRSp53 using behavioral tests including contextual fear conditioning and the puzzle box. We show that a 50% reduction in IRSp53 levels strongly affects the performance in fear-evoking learning paradigms. This correlates with increased targeting of NMDARs to the postsynaptic density (PSD) in hippocampi of both heterozygous and knock out (ko) mice at the expense of extrasynaptic NMDARs. As hippocampal NMDAR-dependent LTP is enhanced in IRSp53-deficient mice, we investigated signaling cascades important for the formation of fear-evoked memories. Here, we observed a dramatic increase in cAMP response element-binding protein-dependent signaling in heterozygous and IRSp53-deficient mice, necessary for the transcriptional dependent phase of LTP. In contrast, activation of the MAPK and Akt kinase pathways required for translation-dependent phase of LTP are reduced. Our data suggest that loss or even the reduction in IRSp53 increases NMDAR-dependent cAMP responsive element-binding protein activation in the hippocampus, and interferes with the ability of mice to learn upon anxiety-related stimuli. We show here that a moderate reduction in the postsynaptic protein IRSp53 in mice leads to an increase in postsynaptic NMDA receptors. Both in heterozygous and IRSp53 deficient mice, this is associated with altered postsynaptic signal transduction, and poor performance of mice in fear-associated learning paradigms, indicating that precise control of postsynaptic NMDA receptor density is essential for memory formation.

IRSp53/BAIAP2 in dendritic spine development, NMDA receptor regulation, and psychiatric disorders

IRSp53 (also known as BAIAP2) is a multi-domain scaffolding and adaptor protein that has been implicated in the regulation of membrane and actin dynamics at subcellular structures, including filopodia and lamellipodia. Accumulating evidence indicates that IRSp53 is an abundant component of the postsynaptic density at excitatory synapses and an important regulator of actin-rich dendritic spines. In addition, IRSp53 has been implicated in diverse psychiatric disorders, including autism spectrum disorders, schizophrenia, and attention deficit/hyperactivity disorder. Mice lacking IRSp53 display enhanced NMDA (N-methyl-d-aspartate) receptor function accompanied by social and cognitive deficits, which are reversed by pharmacological suppression of NMDA receptor function. These results suggest the hypothesis that defective actin/membrane modulation in IRSp53-deficient dendritic spines may lead to social and cognitive deficits through NMDA receptor dysfunction. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Proteogenomic Analysis Identifies a Novel Human SHANK3 Isoform

Mutations of the SHANK3 gene have been associated with autism spectrum disorder. Individuals harboring different SHANK3 mutations display considerable heterogeneity in their cognitive impairment, likely due to the high SHANK3 transcriptional diversity. In this study, we report a novel interaction between the Mutated in colorectal cancer (MCC) protein and a newly identified SHANK3 protein isoform in human colon cancer cells and mouse brain tissue. Hence, our proteogenomic analysis identifies a new human long isoform of the key synaptic protein SHANK3 that was not predicted by the human reference genome. Taken together, our findings describe a potential new role for MCC in neurons, a new human SHANK3 long isoform and, importantly, highlight the use of proteomic data towards the re-annotation of GC-rich genomic regions.

Postsynaptic distribution of IRSp53 in spiny excitatory and inhibitory neurons

The 53 kDa insulin receptor substrate protein (IRSp53) is highly enriched in the brain. Despite evidence that links mutations of IRSp53 with autism and other neuropsychiatric problems, the functional significance of this protein remains unclear. We used light and electron microscopic immunohistochemistry to demonstrate that IRSp53 is expressed throughout the adult rat brain. Labeling concentrated selectively in dendritic spines, where it was associated with the postsynaptic density (PSD). Surprisingly, its organization within the PSD of spiny excitatory neurons of neocortex and hippocampus differed from that within spiny inhibitory neurons of neostriatum and cerebellar cortex. The present data support previous suggestions that IRSp53 is involved in postsynaptic signaling, while hinting that its signaling role may differ in different types of neurons.

Nanoscale segregation of actin nucleation and elongation factors determines dendritic spine protrusion

Actin dynamics drive morphological remodeling of neuronal dendritic spines and changes in synaptic transmission. Yet, the spatiotemporal coordination of actin regulators in spines is unknown. Using single protein tracking and super-resolution imaging, we revealed the nanoscale organization and dynamics of branched F-actin regulators in spines. Branched F-actin nucleation occurs at the PSD vicinity, while elongation occurs at the tip of finger-like protrusions. This spatial segregation differs from lamellipodia where both branched F-actin nucleation and elongation occur at protrusion tips. The PSD is a persistent confinement zone for IRSp53 and the WAVE complex, an activator of the Arp2/3 complex. In contrast, filament elongators like VASP and formin-like protein-2 move outwards from the PSD with protrusion tips. Accordingly, Arp2/3 complexes associated with F-actin are immobile and surround the PSD. Arp2/3 and Rac1 GTPase converge to the PSD, respectively, by cytosolic and free-diffusion on the membrane. Enhanced Rac1 activation and Shank3 over-expression, both associated with spine enlargement, induce delocalization of the WAVE complex from the PSD. Thus, the specific localization of branched F-actin regulators in spines might be reorganized during spine morphological remodeling often associated with synaptic plasticity.

NMDA-induced accumulation of Shank at the postsynaptic density is mediated by CaMKII

Shank is a specialized scaffold protein present in high abundance at the postsynaptic density (PSD). Using pre-embedding immunogold electron microscopy on cultured hippocampal neurons, we had previously demonstrated further accumulation of Shank at the PSD under excitatory conditions. Here, using the same experimental protocol, we demonstrate that a cell permeable CaMKII inhibitor, tatCN21, blocks NMDA-induced accumulation of Shank at the PSD. Furthermore we show that NMDA application changes the distribution pattern of Shank at the PSD, promoting a 7-10 nm shift in the median distance of Shank labels away from the postsynaptic membrane. Inhibition of CaMKII with tatCN21 also blocks this shift in the distribution of Shank. Altogether these results imply that upon activation of NMDA receptors, CaMKII mediates accumulation of Shank, preferentially at the distal regions of the PSD complex extending toward the cytoplasm.

The Potential Role of Insulin on the Shank-Postsynaptic Platform in Neurodegenerative Diseases Involving Cognition

Loss of synaptic function is critical in the pathogenesis of Alzheimer's disease (AD) and other central nervous system (CNS) degenerations. A promising candidate in the regulation of synaptic function is Shank, a protein that serves as a scaffold for excitatory synaptic receptors and proteins. Loss of Shank alters structure and function of the postsynaptic density (PSD). Shank proteins are associated with N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor loss at the PSD in AD; mutations in Shank also lead to autism spectrum disorders (ASDs) and schizophrenia, both of which affect cognition, suggesting that Shank may play a common pathologic role in AD, ASD, and schizophrenia. Shank protein directly associates with insulin receptor substrate protein p53 in PSD. Insulin and insulin sensitizers have been used in clinical trials for these diseases; this suggests that insulin signals may alter protein homeostasis at the shank-postsynaptic platform in PSDs; insulin could improve the function of synapses in these diseases.

High abundance of BDNF within glutamatergic presynapses of cultured hippocampal neurons

In the mammalian brain, the neurotrophin brain-derived neurotrophic factor (BDNF) has emerged as a key factor for synaptic refinement, plasticity and learning. Although BDNF-induced signaling cascades are well known, the spatial aspects of the synaptic BDNF localization remained unclear. Recent data provide strong evidence for an exclusive presynaptic location and anterograde secretion of endogenous BDNF at synapses of the hippocampal circuit. In contrast, various studies using BDNF overexpression in cultured hippocampal neurons support the idea that postsynaptic elements and other dendritic structures are the preferential sites of BDNF localization and release. In this study we used rigorously tested anti-BDNF antibodies and achieved a dense labeling of endogenous BDNF close to synapses. Confocal microscopy showed natural BDNF close to many, but not all glutamatergic synapses, while neither GABAergic synapses nor postsynaptic structures carried a typical synaptic BDNF label. To visualize the BDNF distribution within the fine structure of synapses, we implemented super resolution fluorescence imaging by direct stochastic optical reconstruction microscopy (dSTORM). Two-color dSTORM images of neurites were acquired with a spatial resolution of ~20 nm. At this resolution, the synaptic scaffold proteins Bassoon and Homer exhibit hallmarks of mature synapses and form juxtaposed bars, separated by a synaptic cleft. BDNF imaging signals form granule-like clusters with a mean size of ~60 nm and are preferentially found within the fine structure of the glutamatergic presynapse. Individual glutamatergic presynapses carried up to 90% of the synaptic BDNF immunoreactivity, and only a minor fraction of BDNF molecules was found close to the postsynaptic bars. Our data proof that hippocampal neurons are able to enrich and store high amounts of BDNF in small granules within the mature glutamatergic presynapse, at a principle site of synaptic plasticity.

High fat diet produces brain insulin resistance, synaptodendritic abnormalities and altered behavior in mice

Insulin resistance and other features of the metabolic syndrome are increasingly recognized for their effects on cognitive health. To ascertain mechanisms by which this occurs, we fed mice a very high fat diet (60% kcal by fat) for 17days or a moderate high fat diet (HFD, 45% kcal by fat) for 8weeks and examined changes in brain insulin signaling responses, hippocampal synaptodendritic protein expression, and spatial working memory. Compared to normal control diet mice, cerebral cortex tissues of HFD mice were insulin-resistant as evidenced by failed activation of Akt, S6 and GSK3β with ex-vivo insulin stimulation. Importantly, we found that expression of brain IPMK, which is necessary for mTOR/Akt signaling, remained decreased in HFD mice upon activation of AMPK. HFD mouse hippocampus exhibited increased expression of serine-phosphorylated insulin receptor substrate 1 (IRS1-pS(616)), a marker of insulin resistance, as well as decreased expression of PSD-95, a scaffolding protein enriched in post-synaptic densities, and synaptopodin, an actin-associated protein enriched in spine apparatuses. Spatial working memory was impaired as assessed by decreased spontaneous alternation in a T-maze. These findings indicate that HFD is associated with telencephalic insulin resistance and deleterious effects on synaptic integrity and cognitive behaviors.

The BAI subfamily of adhesion GPCRs: synaptic regulation and beyond

The brain-specific angiogenesis inhibitors 1-3 (BAI1-3) comprise a subfamily of adhesion G-protein-coupled receptors (GPCRs). These receptors are highly expressed in the brain and were first studied for their ability to inhibit angiogenesis and tumor formation. Subsequently, BAI1 was found to play roles in apoptotic cell phagocytosis and myoblast fusion. Until recently, however, little was known about the physiological importance of the BAI subfamily in the context of normal brain function. Recent work has provided evidence for key roles of BAI1-3 in the regulation of synaptogenesis and dendritic spine formation. In this review, we summarize the current understanding of the BAI subfamily with regard to downstream signaling pathways, physiological actions, and potential importance as novel drug targets in the treatment of psychiatric and neurological diseases.