Regulation of dendritic spine morphogenesis by insulin receptor substrate 53, a downstream effector of Rac1 and Cdc42 small GTPases

Restoring social deficits in IRSp53-deleted mice: chemogenetic inhibition of ventral dentate gyrus Emx1-expressing cells

IRSp53 is a synaptic scaffold protein reported to be involved in schizophrenia, autism spectrum disorders, and social deficits in knockout mice. Identifying critical brain regions and cells related to IRSp53 deletion is expected to be of great help in the treatment of psychiatric problems. In this study, we performed chemogenetic inhibition within the ventral dentate gyrus (vDG) of mice with IRSp53 deletion in Emx1-expressing cells (Emx1-Cre;IRSp53 flox/flox). We observed the recovery of social deficits after chemogenetic inhibition within vDG of Emx1-Cre;IRSp53 flox/flox mice. Additionally, chemogenetic activation induced social deficits in Emx1-Cre mice. CRHR1 expression increased in the hippocampus of Emx1-Cre;IRSp53 flox/flox mice, and CRHR1 was reduced by chemogenetic inhibition. Htd2, Ccn1, and Atp61l were decreased in bulk RNA sequencing, and Eya1 and Ecrg4 were decreased in single-cell RNA sequencing of the hippocampus in Emx1-Cre;IRSp53 flox/flox mice compared to control mice. This study determined that the vDG is a critical brain region for social deficits caused by IRSp53 deletion. Social deficits in Emx1-Cre;IRSp53 flox/flox mice were recovered through chemogenetic inhibition, providing clues for new treatment methods for psychiatric disorders accompanied by social deficits.

Preliminary Study on Clinical Characteristics and Pathogenesis of IQSEC2 Mutations Patients

Background

The IQ motif and Sec7 domain ArfGEF 2 (IQSEC2), an X-linked gene that encodes the BRAG1 protein, is a guanine nucleotide exchange factor for the ADP ribosylation factor (ARF) protein family in the small guanosine triphosphate (GTP) binding protein. Mutations in this gene result in disorders such as intellectual disability (ID) and epilepsy. In this study, we analyze the clinical features of two patients with IQSEC2-mutation-related disease and discuss their possible pathogenesis.

Methods

The two patients were diagnosed with ID and epilepsy. Genetic testing was performed using whole-exome sequencing, and the three-dimensional protein structure was analyzed. UCSC Genome Browser was used to analyze the conservation of IQSEC2 in different species. We compared IQSEC2 expression in the proband families with that in a control group, as well as the expression of the postsynaptic identity protein 95 (PSD-95), synapse-associated protein 97 (SAP97), ADP ribosylation factor 6 (ARF-6), and insulin receptor substrate 53kDa (IRSP53) genes interacting with IQSEC2.

Results

We identified two semi-zygote mutations located in conserved positions in different species: an unreported de novo mutation, C.3576C>A (p. Tyr1192*), and a known mutation, c.2983C>T (p. Arg995Trp). IQSEC2 mutations resulted in significant changes in the predicted three-dimensional protein structure, while its expression in the two probands was significantly lower than that in the age-matched control group, and IQSEC2 expression in proband 1 was lower than that in his family members. The expression levels of PSD-95, ARF-6, and SAP97, IRSP 53, which interact with IQSEC2, were also significantly different from those in the family members and age-matched healthy children.

Conclusion

The clinical phenotype resulting from IQSEC2 mutations can be explained by the significant decrease in its expression, loss of function of the mutant protein, and change in the expression of related genes. Our results provide novel insights into the molecular phenotype conferred by the IQSEC2 variants.

C-terminal truncations in IQSEC2: implications for synaptic localization, guanine nucleotide exchange factor activity, and neurological manifestations

IQSEC2 gene on chromosome Xq11.22 encodes a member of guanine nucleotide exchange factor (GEF) protein that is implicated in the activation of ADP-ribosylation factors (Arfs) at the postsynaptic density (PSD), and plays a crucial role in synaptic transmission and dendritic spine formation. Alterations in IQSEC2 have been linked to X-linked intellectual developmental disorders including epilepsy and behavioral abnormalities. Of interest, truncating variants at the C-terminus of IQSEC2 can cause severe phenotypes, akin to truncating variants located in other regions. Here, we present a 5-year-old boy with severe intellectual disability and progressive epilepsy. The individual carried a nonsense variant p.Q1227* in the last exon of the IQSEC2 gene that was supposed to escape nonsense-mediated mRNA decay, thereby leading to a translation of C-terminus truncated IQSEC2 protein with residual activity. The functional analyses showed that the GEF activity of IQSEC2 Q1227* was compromised, and that the IQSEC2 Q1227* lacked preferential synaptic localization due to the absence of functional domains for binding to scaffolding proteins in the PSD. The impaired GEF activity and disrupted synaptic localization of the mutant IQSEC2 protein could impact dendritic and spine development in neurons, potentially explaining the patient's severe neurological manifestations. Our findings indicate that C-terminal truncations in IQSEC2, previously not well-characterized, may have significant pathogenic implications.

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.

Tunnelling nanotube formation is driven by Eps8/IRSp53-dependent linear actin polymerization

Tunnelling nanotubes (TNTs) connect distant cells and mediate cargo transfer for intercellular communication in physiological and pathological contexts. How cells generate these actin-mediated protrusions to span lengths beyond those attainable by canonical filopodia remains unknown. Through a combination of micropatterning, microscopy, and optical tweezer-based approaches, we demonstrate that TNTs formed through the outward extension of actin achieve distances greater than the mean length of filopodia and that branched Arp2/3-dependent pathways attenuate the extent to which actin polymerizes in nanotubes, thus limiting their occurrence. Proteomic analysis using epidermal growth factor receptor kinase substrate 8 (Eps8) as a positive effector of TNTs showed that, upon Arp2/3 inhibition, proteins enhancing filament turnover and depolymerization were reduced and Eps8 instead exhibited heightened interactions with the inverted Bin/Amphiphysin/Rvs (I-BAR) domain protein IRSp53 that provides a direct connection with linear actin polymerases. Our data reveals how common protrusion players (Eps8 and IRSp53) form tunnelling nanotubes, and that when competing pathways overutilizing such proteins and monomeric actin in Arp2/3 networks are inhibited, processes promoting linear actin growth dominate to favour tunnelling nanotube formation.

The Role of Cdc42 in the Insulin and Leptin Pathways Contributing to the Development of Age-Related Obesity

Age-related obesity significantly increases the risk of chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, and certain cancers. The insulin-leptin axis is crucial in understanding metabolic disturbances associated with age-related obesity. Rho GTPase Cdc42 is a member of the Rho family of GTPases that participates in many cellular processes including, but not limited to, regulation of actin cytoskeleton, vesicle trafficking, cell polarity, morphology, proliferation, motility, and migration. Cdc42 functions as an integral part of regulating insulin secretion and aging. Some novel roles for Cdc42 have also been recently identified in maintaining glucose metabolism, where Cdc42 is involved in controlling blood glucose levels in metabolically active tissues, including skeletal muscle, adipose tissue, pancreas, etc., which puts this protein in line with other critical regulators of glucose metabolism. Importantly, Cdc42 plays a vital role in cellular processes associated with the insulin and leptin signaling pathways, which are integral elements involved in obesity development if misregulated. Additionally, a change in Cdc42 activity may affect senescence, thus contributing to disorders associated with aging. This review explores the complex relationships among age-associated obesity, the insulin-leptin axis, and the Cdc42 signaling pathway. This article sheds light on the vast molecular web that supports metabolic dysregulation in aging people. In addition, it also discusses the potential therapeutic implications of the Cdc42 pathway to mitigate obesity since some new data suggest that inhibition of Cdc42 using antidiabetic drugs or antioxidants may promote weight loss in overweight or obese patients.

A mechanosensing mechanism controls plasma membrane shape homeostasis at the nanoscale

As cells migrate and experience forces from their surroundings, they constantly undergo mechanical deformations which reshape their plasma membrane (PM). To maintain homeostasis, cells need to detect and restore such changes, not only in terms of overall PM area and tension as previously described, but also in terms of local, nanoscale topography. Here, we describe a novel phenomenon, by which cells sense and restore mechanically induced PM nanoscale deformations. We show that cell stretch and subsequent compression reshape the PM in a way that generates local membrane evaginations in the 100 nm scale. These evaginations are recognized by I-BAR proteins, which triggers a burst of actin polymerization mediated by Rac1 and Arp2/3. The actin polymerization burst subsequently re-flattens the evagination, completing the mechanochemical feedback loop. Our results demonstrate a new mechanosensing mechanism for PM shape homeostasis, with potential applicability in different physiological scenarios.

Orchestration of synaptic functions by WAVE regulatory complex-mediated actin reorganization

The WAVE regulatory complex (WRC), composed of five components-Cyfip1/Sra1, WAVE/Scar, Abi, Nap1/Nckap1, and Brk1/HSPC300-is essential for proper actin cytoskeletal dynamics and remodeling in eukaryotic cells, likely by matching various patterned signals to Arp2/3-mediated actin nucleation. Accumulating evidence from recent studies has revealed diverse functions of the WRC in neurons, demonstrating its crucial role in dictating the assembly of molecular complexes for the patterning of various trans-synaptic signals. In this review, we discuss recent exciting findings on the physiological role of the WRC in regulating synaptic properties and highlight the involvement of WRC dysfunction in various brain disorders.

Mechanical regulation of synapse formation and plasticity

Dendritic spines are small protrusions arising from dendrites and constitute the major compartment of excitatory post-synapses. They change in number, shape, and size throughout life; these changes are thought to be associated with formation and reorganization of neuronal networks underlying learning and memory. As spines in the brain are surrounded by the microenvironment including neighboring cells and the extracellular matrix, their protrusion requires generation of force to push against these structures. In turn, neighboring cells receive force from protruding spines. Recent studies have identified BAR-domain proteins as being involved in membrane deformation to initiate spine formation. In addition, forces for dendritic filopodium extension and activity-induced spine expansion are generated through cooperation between actin polymerization and clutch coupling. On the other hand, force from expanding spines affects neurotransmitter release from presynaptic terminals. Here, we review recent advances in our understanding of the physical aspects of synapse formation and plasticity, mainly focusing on spine dynamics.

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.

Metabolic Syndrome as a Risk Factor for Alzheimer's Disease: A Focus on Insulin Resistance

Alzheimer's disease (AD) is the main type of dementia and is a disease with a profound socioeconomic burden due to the lack of effective treatment. In addition to genetics and environmental factors, AD is highly associated with metabolic syndrome, defined as the combination of hypertension, hyperlipidemia, obesity and type 2 diabetes mellitus (T2DM). Among these risk factors, the connection between AD and T2DM has been deeply studied. It has been suggested that the mechanism linking both conditions is insulin resistance. Insulin is an important hormone that regulates not only peripheral energy homeostasis but also brain functions, such as cognition. Insulin desensitization, therefore, could impact normal brain function increasing the risk of developing neurodegenerative disorders in later life. Paradoxically, it has been demonstrated that decreased neuronal insulin signalling can also have a protective role in aging and protein-aggregation-associated diseases, as is the case in AD. This controversy is fed by studies focused on neuronal insulin signalling. However, the role of insulin action on other brain cell types, such as astrocytes, is still unexplored. Therefore, it is worthwhile exploring the involvement of the astrocytic insulin receptor in cognition, as well as in the onset and/or development of AD.

Expression of actin- and oxidative phosphorylation-related transcripts across the cortical visuospatial working memory network in unaffected comparison and schizophrenia subjects

Visuospatial working memory (vsWM), which is impaired in schizophrenia (SZ), is mediated by a distributed cortical network. In one node of this network, the dorsolateral prefrontal cortex (DLPFC), altered expression of transcripts for actin assembly and mitochondrial oxidative phosphorylation (OXPHOS) have been reported in SZ. To understand the relationship between these processes, and the extent to which similar alterations are present in other regions of vsWM network in SZ, a subset of actin- (CDC42, BAIAP2, ARPC3, and ARPC4) and OXPHOS-related (ATP5H, COX4I1, COX7B, and NDUFB3) transcripts were quantified in DLPFC by RNA sequencing in 139 SZ and unaffected comparison (UC) subjects, and in DLPFC and three other regions of the cortical vsWM network by qPCR in 20 pairs of SZ and UC subjects. By RNA sequencing, levels of actin- and OXPHOS-related transcripts were significantly altered in SZ, and robustly correlated in both UC and SZ subject groups. By qPCR, cross-regional expression patterns of these transcripts in UC subjects were consistent with greater actin assembly in DLPFC and higher OXPHOS activity in primary visual cortex (V1). In SZ, CDC42 and ARPC4 levels were lower in all regions, BAIAP2 levels higher only in V1, and ARPC3 levels unaltered across regions. All OXPHOS-related transcript levels were lower in SZ, with the disease effect decreasing from posterior to anterior regions. The differential alterations in markers of actin assembly and energy production across regions of the cortical vsWM network in SZ suggest that each region may make specific contributions to vsWM impairments in the illness.

Transcriptome Profiling of the Hippocampal Seizure Network Implicates a Role for Wnt Signaling during Epileptogenesis in a Mouse Model of Temporal Lobe Epilepsy

Mesial temporal lobe epilepsy (mTLE) is a life-threatening condition characterized by recurrent hippocampal seizures. mTLE can develop after exposure to risk factors such as febrile seizure, trauma, and infection. Within the latent period between exposure and onset of epilepsy, pathological remodeling events occur that contribute to epileptogenesis. The molecular mechanisms responsible are currently unclear. We used the mouse intrahippocampal kainite model of mTLE to investigate transcriptional dysregulation in the ipsilateral and contralateral dentate gyrus (DG), representing the epileptogenic zone (EZ) and peri-ictal zone (PIZ). DG were analyzed after 3, 7, and 14 days by RNA sequencing. In both the EZ and PIZ, transcriptional dysregulation was dynamic over the epileptogenic period with early expression of genes representing cell signaling, migration, and proliferation. Canonical Wnt signaling was upregulated in the EZ and PIZ at 3 days. Expression of inflammatory genes differed between the EZ and PIZ, with early expression after 3 days in the PIZ and delayed expression after 7-14 days in the EZ. This suggests that critical gene changes occur early in the hippocampal seizure network and that Wnt signaling may play a role within the latent epileptogenic period. These findings may help to identify novel therapeutic targets that could prevent epileptogenesis.

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.

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.

Dysmetabolism and Neurodegeneration: Trick or Treat?

Accumulating evidence suggests the existence of a strong link between metabolic syndrome and neurodegeneration. Indeed, epidemiologic studies have described solid associations between metabolic syndrome and neurodegeneration, whereas animal models contributed for the clarification of the mechanistic underlying the complex relationships between these conditions, having the development of an insulin resistance state a pivotal role in this relationship. Herein, we review in a concise manner the association between metabolic syndrome and neurodegeneration. We start by providing concepts regarding the role of insulin and insulin signaling pathways as well as the pathophysiological mechanisms that are in the genesis of metabolic diseases. Then, we focus on the role of insulin in the brain, with special attention to its function in the regulation of brain glucose metabolism, feeding, and cognition. Moreover, we extensively report on the association between neurodegeneration and metabolic diseases, with a particular emphasis on the evidence observed in animal models of dysmetabolism induced by hypercaloric diets. We also debate on strategies to prevent and/or delay neurodegeneration through the normalization of whole-body glucose homeostasis, particularly via the modulation of the carotid bodies, organs known to be key in connecting the periphery with the brain.

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.

Early adversity and insulin: neuroendocrine programming beyond glucocorticoids

Exposure to direct or contextual adversities during early life programs the functioning of the brain and other biological systems, contributing to the development of physical as well as mental health issues in the long term. While the role of glucocorticoids in mediating the outcomes of early adversity has been explored for many years, less attention has been given to insulin. Beyond its metabolic effects in the periphery, central insulin action affects synaptic plasticity, brain neurotransmission, and executive functions. Knowledge about the interactions between the peripheral metabolism and brain function from a developmental perspective can contribute to prevention and diagnosis programs, as well as early interventions for vulnerable populations.

Dendrite tapering actuates a self-organizing signaling circuit for stochastic filopodia initiation in neurons

How signaling units spontaneously arise from a noisy cellular background is not well understood. Here, we show that stochastic membrane deformations can nucleate exploratory dendritic filopodia, dynamic actin-rich structures used by neurons to sample its surroundings for compatible transcellular contacts. A theoretical analysis demonstrates that corecruitment of positive and negative curvature-sensitive proteins to deformed membranes minimizes the free energy of the system, allowing the formation of long-lived curved membrane sections from stochastic membrane fluctuations. Quantitative experiments show that once recruited, curvature-sensitive proteins form a signaling circuit composed of interlinked positive and negative actin-regulatory feedback loops. As the positive but not the negative feedback loop can sense the dendrite diameter, this self-organizing circuit determines filopodia initiation frequency along tapering dendrites. Together, our findings identify a receptor-independent signaling circuit that employs random membrane deformations to simultaneously elicit and limit formation of exploratory filopodia to distal dendritic sites of developing neurons.

The PPARg System in Major Depression: Pathophysiologic and Therapeutic Implications

To an exceptional degree, and through multiple mechanisms, the PPARg system rapidly senses cellular stress, and functions in the CNS in glial cells, neurons, and cerebrovascular endothelial cell in multiple anti-inflammatory and neuroprotective ways. We now know that depression is associated with neurodegeneration in the subgenual prefrontal cortex and hippocampus, decreased neuroplasticity, and defective neurogenesis. Brain-derived neurotrophic factor (BDNF) is markedly depleted in these areas, and is thought to contribute to the neurodegeneration of the subgenual prefrontal cortex and the hippocampus. The PPARg system strongly increases BDNF levels and activity in these brain areas. The PPARg system promotes both neuroplasticity and neurogenesis, both via effects on BDNF, and through other mechanisms. Ample evidence exists that these brain areas transduce many of the cardinal features of depression, directly or through their projections to sites such as the amygdala and nucleus accumbens. Behaviorally, these include feelings of worthlessness, anxiety, dread of the future, and significant reductions in the capacity to anticipate and experience pleasure. Physiologically, these include activation of the CRH and noradrenergic system in brain and the sympathetic nervous system and hypothalamic-pituitary-adrenal axis in the periphery. Patients with depression are also insulin-resistant. The PPARg system influences each of these behavioral and physiological in ways that would ameliorate the manifestations of depressive illness. In addition to the cognitive and behavioral manifestations of depression, depressive illness is associated with the premature onsets of coronary artery disease, stroke, diabetes, and osteoporosis. As a consequence, patients with depressive illness lose approximately seven years of life. Inflammation and insulin resistance are two of the predominant processes that set into motion these somatic manifestations. PPARg agonists significantly ameliorate both pathological processes. In summary, PPARg augmentation can impact positively on multiple significant pathological processes in depression. These include loss of brain tissue, defective neuroplasticity and neurogenesis, widespread inflammation in the central nervous system and periphery, and insulin resistance. Thus, PPARg agonists could potentially have significant antidepressant effects.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

The Mechanics and Thermodynamics of Tubule Formation in Biological Membranes

Membrane tubulation is a ubiquitous process that occurs both at the plasma membrane and on the membranes of intracellular organelles. These tubulation events are known to be mediated by forces applied on the membrane either due to motor proteins, by polymerization of the cytoskeleton, or due to the interactions between membrane proteins binding onto the membrane. The numerous experimental observations of tube formation have been amply supported by mathematical modeling of the associated membrane mechanics and have provided insights into the force-displacement relationships of membrane tubes. Recent advances in quantitative biophysical measurements of membrane-protein interactions and tubule formation have necessitated the need for advances in modeling that will account for the interplay of multiple aspects of physics that occur simultaneously. Here, we present a comprehensive review of experimental observations of tubule formation and provide context from the framework of continuum modeling. Finally, we explore the scope for future research in this area with an emphasis on iterative modeling and experimental measurements that will enable us to expand our mechanistic understanding of tubulation processes in cells.

Neural differentiation medium for human pluripotent stem cells to model physiological glucose levels in human brain

Cortical neurospheres (NSPs) derived from human pluripotent stem cells (hPSC), have proven to be a successful platform to investigate human brain development and neuro-related diseases. Currently, many of the standard hPSC neural differentiation media, use concentrations of glucose (approximately 17.5-25 mM) and insulin (approximately 3.2 μM) that are much greater than the physiological concentrations found in the human brain. These culture conditions make it difficult to analyse perturbations of glucose or insulin on neuronal development and differentiation. We established a new hPSC neural differentiation medium that incorporated physiological brain concentrations of glucose (2.5 mM) and significantly reduced insulin levels (0.86 μM). This medium supported hPSC neural induction and formation of cortical NSPs. The revised hPSC neural differentiation medium, may provide an improved platform to model brain development and to investigate neural differentiation signalling pathways impacted by abnormal glucose and insulin levels.

Insulin-Dependent Maturation of Newly Generated Olfactory Sensory Neurons after Injury

Loss of olfactory sensory neurons (OSNs) after injury to the olfactory epithelium (OE) triggers the generation of OSNs that are incorporated into olfactory circuits to restore olfactory sensory perception. This study addresses how insulin receptor-mediated signaling affects the functional recovery of OSNs after OE injury. Insulin levels were reduced in mice by ablating the pancreatic β cells via streptozotocin (STZ) injections. These STZ-induced diabetic and control mice were then intraperitoneally injected with the olfactotoxic drug methimazole to selectively ablate OSNs. The OE of diabetic and control mice regenerated similarly until day 14 after injury. Thereafter, the OE of diabetic mice contained fewer mature and more apoptotic OSNs than control mice. Functionally, diabetic mice showed reduced electro-olfactogram (EOG) responses and their olfactory bulbs (OBs) had fewer c-Fos-active cells following odor stimulation, as well as performed worse in an odor-guided task compared with control mice. Insulin administered intranasally during days 8-13 after injury was sufficient to rescue recovery of OSNs in diabetic mice compared with control levels, while insulin administration between days 1 and 6 did not. During this critical time window on days 8-13 after injury, insulin receptors are highly expressed and intranasal application of an insulin receptor antagonist inhibits regeneration. Furthermore, an insulin-enriched environment could facilitate regeneration even in non-diabetic mice. These results indicate that insulin facilitates the regeneration of OSNs after injury and suggest a critical stage during recovery (8-13 d after injury) during which the maturation of newly generated OSNs is highly dependent on and promoted by insulin.

Role of Lipids in Morphogenesis of T-Cell Microvilli

T cells communicate with the environment via surface receptors. Cooperation of surface receptors regulates T-cell responses to diverse stimuli. Recently, finger-like membrane protrusions, microvilli, have been demonstrated to play a role in the organization of receptors and, hence, T-cell activation. However, little is known about the morphogenesis of dynamic microvilli, especially in the cells of immune system. In this review, I focus on the potential role of lipids and lipid domains in morphogenesis of microvilli. Discussed is the option that clustering of sphingolipids with phosphoinositides at the plasma membrane results in dimpling (curved) domains. Such domains can attract phosphoinositide-binding proteins and stimulate actin cytoskeleton reorganization. This process triggers cortical actin opening and bundling of actin fibres to support the growing of microvilli. Critical regulators of microvilli morphogenesis in T cells are unknown. At the end, I suggest several candidates with a potential to organize proteins and lipids in these structures.

Identification of secreted factors in dental pulp cell-conditioned medium optimized for neuronal growth

With their potent regenerative and protective capacities, stem cell-derived conditioned media emerged as an effective alternative to cell therapy, and have a prospect to be manufactured as pharmaceutical products for tissue regeneration applications. Our study investigates the neuroregenerative potential of human dental pulp cells (DPCs) conditioned medium (CM) and defines an optimization strategy of DPC-CM for enhanced neuronal outgrowth. Primary sensory neurons from mouse dorsal root ganglia were cultured with or without DPC-CM, and the lengths of βIII-tubulin positive neurites were measured. The impacts of several manufacturing features as the duration of cell conditioning, CM storage, and preconditioning of DPCs with some factors on CM functional activity were assessed on neurite length. We observed that DPC-CM significantly enhanced neurites outgrowth of sensory neurons in a concentration-dependent manner. The frozen storage of DPC-CM had no impact on experimental outcomes and 48 h of DPC conditioning is optimal for an effective activity of CM. To further understand the regenerative feature of DPC-CM, we studied DPC secretome by human growth factor antibody array analysis and revealed the presence of several factors involved in either neurogenesis, neuroprotection, angiogenesis, and osteogenesis. The conditioning of DPCs with the B-27 supplement enhanced significantly the neuroregenerative effect of their secretome by changing its composition in growth factors. Here, we show that DPC-CM significantly stimulate neurite outgrowth in primary sensory neurons. Moreover, we identified secreted protein candidates that can potentially promote this promising regenerative feature of DPC-CM.

Nutritional and developmental programming effects of insulin

The discovery of insulin in 1921 was a major breakthrough in medicine and for therapy in patients with diabetes. The dramatic rise in the prevalence of overweight and obesity has been tightly linked to an increased prevalence of gestational diabetes mellitus (GDM), which poses major health concerns. Babies born to GDM mothers are more likely to develop obesity, type 2 diabetes and cardiovascular disease later in life. Evidence accumulated during the past two decades has revealed that high levels insulin, such as those observed during GDM, can have a widespread effect on the development and function of a variety of organs. This review summarises our current knowledge on the role of insulin in the placenta, cardiovascular system and brain during critical periods of development, as well as how it can contribute to lifelong metabolic regulation. We also discuss possible intervention strategies to ameliorate and hopefully reverse the developmental defects associated with obesity and GDM.

Insulin Receptor Substrate p53 Ameliorates High-Glucose-Induced Activation of NF-κB and Impaired Mobility of HUVECs

Diabetes-related macrovascular and microvascular complications lead to poor prognosis. Insulin receptor substrate p53 (IRSp53) is known to act as a substrate for the insulin receptor tyrosine kinase, but its role in endothelial dysfunction remains unclear. Human umbilical vein endothelial cells (HUVECs) treated with D-glucose at different concentrations and a streptozocin-induced rat diabetes mellitus (DM) model were used to investigate the effects of hyperglycemia on the expression levels of IRSp53 and galectin-3 (gal-3) and the inflammatory state and mobility of HUVECs. Thereafter, IRSp53-overexpressing HUVECs and IRSp53-knockdown HUVECs were established using IRSp53-overexpressing lentivirus or IRSp53-siRNA to explore the role of IRSp53 in the HUVEC inflammatory state and HUVEC mobility. D-glucose at high concentration (HG) and hyperglycemia were found to induce downregulation of IRSp53 and upregulation of gal-3 in vitro and in vivo. Treatment with HG resulted in activation of NF-κB in HUVECs and impaired HUVEC mobility. Insulin restored HG-induced changes in the expression levels of IRSp53 and gal-3 in HUVECs and protected the cells from NF-κB activation and impaired mobility. Overexpression of IRSp53 inhibited the activation of NF-κB in HUVECs and strengthened HUVEC migration. Knockdown of IRSp53 facilitated the activation of NF-κB in HUVECs and decreased HUVEC migration. However, neither overexpression nor knockdown of IRSp53 altered the effects of insulin on HG-induced detrimental changes in HUVECs. HG and hyperglycemia resulted in downregulation of IRSp53 in vitro and in vivo. IRSp53 is concluded to inhibit the activation of NF-κB in HUVECs and to strengthen HUVEC migration.

Hippocampal insulin resistance and the Sirtuin 1 signaling pathway in diabetes-induced cognitive dysfunction

In the peripheral nervous system, the activation of Sirtuin 1 can improve insulin resistance; however, the role played by Sirtuin 1 in the central nervous system remains unknown. In this study, rat models of diabetes mellitus were generated by a single injection of streptozotocin. At 8 weeks after streptozotocin injection, the Morris water maze test and western blot assays confirmed that the diabetic model rats had learning and memory deficits, insulin resistance, and Sirtuin 1 expression could be detected in the hippocampus. Insulin and the insulin receptor inhibitor S961 were intranasally administered to investigate the regulatory effects of insulin signaling on Sirtuin 1. The results showed that insulin administration improved the impaired cognitive function of diabetic model rats and increased the expression levels of phosphorylated insulin receptor, phosphorylated insulin receptor substrate 1, and Sirtuin 1 in the hippocampus. Conversely, S961 administration resulted in more severe cognitive dysfunction and reduced the expression levels of phosphorylated insulin receptor, phosphorylated insulin receptor substrate 1, and Sirtuin 1. The Sirtuin 1 activator SRT2104 and the inhibitor Sirtinol were injected into the lateral ventricle, which revealed that the activation of Sirtuin 1 increased the expression levels of target of rapamycin complex 1, phosphorylated cAMP-response element-binding protein, and brain-derived neurotrophic factor. Hippocampal dendritic length and spine density also increased in response to Sirtuin 1 activation. In contrast, Sirtinol decreased the expression levels of target of rapamycin complex 1, phosphorylated cAMP-response element-binding protein, and brain-derived neurotrophic factor and damaged the dendritic structure. These findings suggest that the Sirtuin 1 signaling pathway plays an important role in the development of insulin resistance-related cognitive deficits in diabetic rats. This study was approved by the Animal Ethics Welfare Committee of the First Affiliated Hospital of Hunan University of Chinese Medicine (approval No. ZYFY201811207) in November 2018.

The Ways of Actin: Why Tunneling Nanotubes Are Unique Cell Protrusions

Actin remodeling is at the heart of the response of cells to external or internal stimuli, allowing a variety of membrane protrusions to form. Fifteen years ago, tunneling nanotubes (TNTs) were identified, bringing a novel addition to the family of actin-supported cellular protrusions. Their unique property as conduits for cargo transfer between distant cells emphasizes the unique nature of TNTs among other protrusions. While TNTs in different pathological and physiological scenarios have been described, the molecular basis of how TNTs form is not well understood. In this review, we discuss the role of several actin regulators in the formation of TNTs and suggest potential players based on their comparison with other actin-based protrusions. New perspectives for discovering a distinct TNT formation pathway would enable us to target them in treating the increasing number of TNT-involved pathologies.

Clinical Evidence of Antidepressant Effects of Insulin and Anti-Hyperglycemic Agents and Implications for the Pathophysiology of Depression-A Literature Review

Close connections between depression and type 2 diabetes (T2DM) have been suggested by many epidemiological and experimental studies. Disturbances in insulin sensitivity due to the disruption of various molecular pathways cause insulin resistance, which underpins many metabolic disorders, including diabetes, as well as depression. Several anti-hyperglycemic agents have demonstrated antidepressant properties in clinical trials, probably due to their action on brain targets based on the shared pathophysiology of depression and T2DM. In this article, we review reports of clinical trials examining the antidepressant effect of these medications, including insulin, metformin, glucagon like peptide-1 receptor agonists (GLP-1RA), and peroxisome proliferator-activated receptor (PPAR)-γ agonists, and briefly consider possible molecular mechanisms underlying the associations between amelioration of insulin resistance and improvement of depressive symptoms. In doing so, we intend to suggest an integrative perspective for understanding the pathophysiology of depression.

Molecular architecture of postsynaptic Interactomes

The postsynaptic density (PSD) plays an essential role in the organization of the synaptic signaling machinery. It contains a set of core scaffolding proteins that provide the backbone to PSD protein-protein interaction networks (PINs). These core scaffolding proteins can be seen as three principal layers classified by protein family, with DLG proteins being at the top, SHANKs along the bottom, and DLGAPs connecting the two layers. Early studies utilizing yeast two hybrid enabled the identification of direct protein-protein interactions (PPIs) within the multiple layers of scaffolding proteins. More recently, mass-spectrometry has allowed the characterization of whole interactomes within the PSD. This expansion of knowledge has further solidified the centrality of core scaffolding family members within synaptic PINs and provided context for their role in neuronal development and synaptic function. Here, we discuss the scaffolding machinery of the PSD, their essential functions in the organization of synaptic PINs, along with their relationship to neuronal processes found to be impaired in complex brain disorders.

Angiotensin-Converting Enzyme Inhibitor Rapidly Ameliorates Depressive-Type Behaviors via Bradykinin-Dependent Activation of Mammalian Target of Rapamycin Complex 1

BACKGROUND

Angiotensin-converting enzyme inhibitors (ACEIs) are widely prescribed antihypertensive agents. Intriguingly, case reports and clinical trials have indicated that ACEIs, including captopril and lisinopril, may have a rapid mood-elevating effect in certain patients, but few experimental studies have investigated their value as fast-onset antidepressants.

METHODS

The present study consisted of a series of experiments using biochemical assays, immunohistochemistry, and behavioral techniques to examine the effect and mechanism of captopril on depressive-like behavior in 2 animal models, the chronic unpredictable stress model and the chronic social defeat stress model.

RESULTS

Captopril (19.5 or 39 mg/kg, intraperitoneal injection) exerted rapid antidepressant activity in mice treated under the chronic unpredictable stress model and mice treated under the chronic social defeat stress model. Pharmacokinetic analysis revealed that captopril crossed the blood-brain barrier and that lisinopril, another ACEI with better blood-brain barrier permeability, exerted a faster and longer-lasting effect at a same molar equivalent dose. This antidepressant effect seemed to be independent of the renin-angiotensin system, but dependent on the bradykinin (BK) system, since the decreased BK detected in the stressed mice could be reversed by captopril. The hypofunction of the downstream effector of BK, Cdc42 (cell division control protein 42) homolog, contributed to the stress-induced loss of dendritic spines, which was rapidly reversed by captopril via activating the mTORC1 (mammalian target of rapamycin complex 1) pathway.

CONCLUSIONS

Our findings indicate that the BK-dependent activation of mTORC1 may represent a promising mechanism underlying antidepressant pharmacology. Considering their affordability and availability, ACEIs may emerge as a novel fast-onset antidepressant, especially for patients with comorbid depression and hypertension.

Hyperactive ACC-MDT Pathway Suppresses Prepulse Inhibition in Mice

Altered prepulse inhibition (PPI) is an endophenotype associated with multiple brain disorders, including schizophrenia. Circuit mechanisms that regulate PPI have been suggested, but none has been demonstrated through direct manipulations. IRSp53 is an abundant excitatory postsynaptic scaffold implicated in schizophrenia, autism spectrum disorders, and attention-deficit/hyperactivity disorder. We found that mice lacking IRSp53 in cortical excitatory neurons display decreased PPI. IRSp53-mutant layer 6 cortical neurons in the anterior cingulate cortex (ACC) displayed decreased excitatory synaptic input but markedly increased neuronal excitability, which was associated with excessive excitatory synaptic input in downstream mediodorsal thalamic (MDT) neurons. Importantly, chemogenetic inhibition of mutant neurons projecting to MDT normalized the decreased PPI and increased excitatory synaptic input onto MDT neurons. In addition, chemogenetic activation of MDT-projecting layer 6 neurons in the ACC decreased PPI in wild-type mice. These results suggest that the hyperactive ACC-MDT pathway suppresses PPI in wild-type and IRSp53-mutant mice.

Brain insulin resistance impairs hippocampal plasticity

Nutrient-related signals have been demonstrated to influence brain development and cognitive functions. In particular, insulin signaling has been shown to impact on molecular cascades underlying hippocampal plasticity, learning and memory. Alteration of brain insulin signaling interferes with the maintenance of neural stem cell niche and neuronal activity in the hippocampus. Brain insulin resistance is also emerging as key factor causing the cognitive impairment observed in metabolic and neurodegenerative diseases. Here, we review the molecular mechanisms involved in the insulin modulation of both adult neurogenesis and synaptic activity in the hippocampus. We also summarize the effects of altered insulin sensitivity on hippocampal plasticity. Finally, we reassume the experimental and epidemiological evidence highlighting the critical role of brain insulin resistance at the crossroad between type 2 diabetes and Alzheimer's disease.

Palmitoylated Proteins in Dendritic Spine Remodeling

Activity-responsive changes in the actin cytoskeleton are required for the biogenesis, motility, and remodeling of dendritic spines. These changes are governed by proteins that regulate the polymerization, depolymerization, bundling, and branching of actin filaments. Thus, processes that have been extensively characterized in the context of non-neuronal cell shape change and migration are also critical for learning and memory. In this review article, we highlight actin regulatory proteins that associate, at least transiently, with the dendritic plasma membrane. All of these proteins have been shown, either in directed studies or in high-throughput screens, to undergo palmitoylation, a potentially reversible, and stimulus-dependent cysteine modification. Palmitoylation increases the affinity of peripheral proteins for the membrane bilayer and contributes to their subcellular localization and recruitment to cholesterol-rich membrane microdomains.

Physiological Tau Interactome in Brain and Its Link to Tauopathies

Alzheimer's disease (AD) and most of the other tauopathies are incurable neurodegenerative diseases with unpleasant symptoms and consequences. The common hallmark of all of these diseases is tau pathology, but its connection with disease progress has not been completely understood so far. Therefore, uncovering novel tau-interacting partners and pathology affected molecular pathways can reveal the causes of diseases as well as potential targets for the development of AD treatment. Despite the large number of known tau-interacting partners, a limited number of studies focused on in vivo tau interactions in disease or healthy conditions are available. Here, we applied an in vivo cross-linking approach, capable of capturing weak and transient protein-protein interactions, to a unique transgenic rat model of progressive tau pathology similar to human AD. We have identified 175 potential novel and known tau-interacting proteins by MALDI-TOF mass spectrometry. Several of the most promising candidates for possible drug development were selected for validation by coimmunoprecipitation and colocalization experiments in animal and cellular models. Three proteins, Baiap2, Gpr37l1, and Nptx1, were confirmed as novel tau-interacting partners, and on the basis of their known functions and implications in neurodegenerative or psychiatric disorders, we proposed their potential role in tau pathology.

Antagonistic regulation by insulin-like peptide and activin ensures the elaboration of appropriate dendritic field sizes of amacrine neurons

Establishing appropriate sizes and shapes of dendritic arbors is critical for proper wiring of the central nervous system. Here we report that Insulin-like Peptide 2 (DILP2) locally activates transiently expressed insulin receptors in the central dendrites of Drosophila Dm8 amacrine neurons to positively regulate dendritic field elaboration. We found DILP2 was expressed in L5 lamina neurons, which have axonal terminals abutting Dm8 dendrites. Proper Dm8 dendrite morphogenesis and synapse formation required insulin signaling through TOR (target of rapamycin) and SREBP (sterol regulatory element-binding protein), acting in parallel with previously identified negative regulation by Activin signaling to provide robust control of Dm8 dendrite elaboration. A simulation of dendritic growth revealed trade-offs between dendritic field size and robustness when branching and terminating kinetic parameters were constant, but dynamic modulation of the parameters could mitigate these trade-offs. We suggest that antagonistic DILP2 and Activin signals from different afferents appropriately size Dm8 dendritic fields.

Neurodegeneration in Alzheimer's disease and glaucoma: overlaps and missing links

The eye is said to be the window into the brain. Alzheimer’s disease (AD) and glaucoma both being diseases of the elderly, have several epidemiological and histological overlaps in pathogenesis. Both these diseases are neurodegenerative conditions. Over the years, a consensus has developed that both may be two ends of a singular spectrum of diseases. Epidemiological studies have shown that more Alzheimer’s patients may be suffering from glaucoma than general healthy population. Retinal ganglion cell damage is a characteristic of both diseases, along with discovery of amyloid-β and tau protein deposition in the retina and aqueous humor of eye. The latter two proteins are known to be pathognomonic of AD. Other pathways such as the insulin receptor pathway also seem to be affected in both diseases similarly. In spite of these overlaps, there are few missing links which still need more evidence, namely, intraocular pressure mechanisms, cerebrospinal fluid pressure and trans-lamina cribrosa pressure gradients, vascular autoregulation factors, etc. Several factors point towards a common pathogenesis at some level for both diseases and prospective studies are necessary to study the natural course of both diseases.

BRAG1/IQSEC2 as a regulator of small GTPase-dependent trafficking

Precise trafficking events, such as those that underlie synaptic transmission and plasticity, require complex regulation. G-protein signaling plays an essential role in the regulation of membrane and protein trafficking. However, it is not well understood how small GTPases and their regulatory proteins coordinate such specific events. Our recent publication focused on a highly abundant synaptic GEF, BRAG1, whose physiologic relevance was unknown. We find that BRAG1s GEF activity is required for activity-dependent trafficking of AMPARs. Moreover, BRAG1 bidirectionally regulates synaptic transmission in a manner independent of this activity. In addition to the GEF domain, BRAG1 contains several functional domains whose roles are not yet understood but may mediate protein-protein interactions and regulatory effects necessary for its role in regulation of AMPAR trafficking. In this commentary, we explore the potential for BRAG1 to provide specificity of small GTPase signaling, coordinating activity-dependent activation of small GTPase activity with signaling and scaffolding molecules involved in trafficking through its GEF activity and other functional domains.

Quantitative investigation of negative membrane curvature sensing and generation by I-BARs in filopodia of living cells

Membrane curvature has recently been recognized as an active regulator of cellular function, with several protein families identified as sensors and generators of membrane curvature. Amongst them, the inverse Bin/Amphiphysin/Rvs (I-BAR) domain family has been implicated in the sensing and generation of membrane structures with negative membrane curvature e.g. filopodia or dendritic spines. However, to date, quantitative biophysical investigations of I-BAR domains have mostly taken place in reconstitution. Here, we use fluorescence microscopy to quantitatively investigate membrane curvature sensing and generation by I-BARs in filopodia of living cells. As a model system, we selected two prototypic members of the I-BAR family, the insulin receptor substrate p53 and missing-in-metastasis. Our data demonstrated how I-BARs sense negative membrane curvature in the complex environment of live cells by revealing a dependence on membrane curvature for both their binding affinity to membranes and their saturation density. The non-monotonic dependence of protein sorting with negative membrane curvature allowed us to apply previously developed thermodynamic models to provide estimates of the effective intrinsic curvature and bending rigidity of the two I-BARs bound at the plasma membrane. Our results agree with studies performed on the insulin receptor substrate p53 in reconstitution. To quantitate membrane curvature generation by I-BARs we measured how their overexpression reduces the peak and the width of the size distribution of filopodia, resulting in filopodia populations with smaller and more uniform diameters. Our findings provide a quantitative biophysical insight in the ability of I-BARs to sense and generate negative membrane curvature in the crowded environment of living cells.

Slitrk2 controls excitatory synapse development via PDZ-mediated protein interactions

Members of the Slitrk (Slit- and Trk-like protein) family of synaptic cell-adhesion molecules control excitatory and inhibitory synapse development through isoform-dependent extracellular interactions with leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs). However, how Slitrks participate in activation of intracellular signaling pathways in postsynaptic neurons remains largely unknown. Here we report that, among the six members of the Slitrk family, only Slitrk2 directly interacts with the PDZ domain-containing excitatory scaffolds, PSD-95 and Shank3. The interaction of Slitrk2 with PDZ proteins is mediated by the cytoplasmic COOH-terminal PDZ domain-binding motif (Ile-Ser-Glu-Leu), which is not found in other Slitrks. Mapping analyses further revealed that a single PDZ domain of Shank3 is responsible for binding to Slitrk2. Slitrk2 forms in vivo complexes with membrane-associated guanylate kinase (MAGUK) family proteins in addition to PSD-95 and Shank3. Intriguingly, in addition to its role in synaptic targeting in cultured hippocampal neurons, the PDZ domain-binding motif of Slitrk2 is required for Slitrk2 promotion of excitatory synapse formation, transmission, and spine development in the CA1 hippocampal region. Collectively, our data suggest a new molecular mechanism for conferring isoform-specific regulatory actions of the Slitrk family in orchestrating intracellular signal transduction pathways in postsynaptic neurons.

Alternative 3' UTRs direct localization of functionally diverse protein isoforms in neuronal compartments

The proper subcellular localization of RNAs and local translational regulation is crucial in highly compartmentalized cells, such as neurons. RNA localization is mediated by specific cis-regulatory elements usually found in mRNA 3′UTRs. Therefore, processes that generate alternative 3′UTRs—alternative splicing and polyadenylation—have the potential to diversify mRNA localization patterns in neurons. Here, we performed mapping of alternative 3′UTRs in neurites and soma isolated from mESC-derived neurons. Our analysis identified 593 genes with differentially localized 3′UTR isoforms. In particular, we have shown that two isoforms of Cdc42 gene with distinct functions in neuronal polarity are differentially localized between neurites and soma of mESC-derived and mouse primary cortical neurons, at both mRNA and protein level. Using reporter assays and 3′UTR swapping experiments, we have identified the role of alternative 3′UTRs and mRNA transport in differential localization of alternative CDC42 protein isoforms. Moreover, we used SILAC to identify isoform-specific Cdc42 3′UTR-bound proteome with potential role in Cdc42 localization and translation. Our analysis points to usage of alternative 3′UTR isoforms as a novel mechanism to provide for differential localization of functionally diverse alternative protein isoforms.

Proteomics Analysis Identifies IRSp53 and Fascin as Critical for PRV Egress and Direct Cell-Cell Transmission

Pseudorabies virus (PRV) has been widely used as a live trans-synaptic tracer for mapping neuronal circuits. Systematically identifying mature PRV virion proteomes and defining co-purified host proteins are necessary to fully understand the detailed mechanism underlying PRV transmission processes. Here, a PRV virion purification strategy based on sorting with flow cytometry is developed and the mature extracellular and intracellular PRV virion proteomes using LC coupled with MS/MS are characterized. In addition to viral proteins, a large number of host proteins are also identified, including proteins related to actin cytoskeletal dynamics and membrane protrusion. How many of these host proteins are true virion components are unknown and the majority of these may not be. Through functional analysis, it is found that IRSp53 and fascin are critical for the egress process and play a role in direct cell-cell transmission. Moreover, it is shown that CDC42 and Rac1 are also involved in the production of mature extracellular virions. The results suggest that the formation of the filopodia-like cytoskeleton and the rearrangement of the membrane, which are both associated with IRSp53 and fascin, may be important for the transmission of viruses used in neuronal tracing.

Brain Insulin Resistance and Hippocampal Plasticity: Mechanisms and Biomarkers of Cognitive Decline

In the last decade, much attention has been devoted to the effects of nutrient-related signals on brain development and cognitive functions. A turning point was the discovery that brain areas other than the hypothalamus expressed receptors for hormones related to metabolism. In particular, insulin signaling has been demonstrated to impact on molecular cascades underlying hippocampal plasticity, learning and memory. Here, we summarize the molecular evidence linking alteration of hippocampal insulin sensitivity with changes of both adult neurogenesis and synaptic plasticity. We also review the epidemiological studies and experimental models emphasizing the critical role of brain insulin resistance at the crossroad between metabolic and neurodegenerative disease. Finally, we brief novel findings suggesting how biomarkers of brain insulin resistance, involving the study of brain-derived extracellular vesicles and brain glucose metabolism, may predict the onset and/or the progression of cognitive decline.

Shank3 regulates striatal synaptic abundance of Cyld, a deubiquitinase specific for Lys63-linked polyubiquitin chains

The SH3 and multiple ankyrin repeat domains 3 (Shank3) proteins are core organizers of the postsynaptic density in neuronal excitatory synapses, and their defects cause various neurodevelopmental and neuropsychiatric disorders. Mechanistically, Shank3 directly and indirectly interacts with hundreds of synaptic proteins with diverse functions and potentially exerts its regulatory roles in synaptic development and function via these interactors. However, Shank3-dependent regulation of synaptic abundance has been validated in vivo for only a few Shank3 interactors. Here, using a quantitative proteomic analysis, we identified 136 proteins with altered synaptic abundance in the striatum of Shank3-overexpressing transgenic (TG) mice. By comparing these proteins with those found in a previous analysis of the postsynaptic density of Shank3 knock-out (KO) striatum, we identified and confirmed that cylindromatosis-associated deubiquitinase (Cyld), a deubiquitinase specific for Lys63-linked polyubiquitin chains, was up- and down-regulated in Shank3 TG and KO striatal synapses, respectively. Consistently, we found that the synaptic levels of Lys63-linked polyubiquitin chains were down- and up-regulated in the Shank3 TG and KO striata, respectively. Furthermore, by isolating and analyzing the synaptic Cyld complex, we generated a Cyld interactome consisting of 103 proteins, which may include Cyld substrates. Bioinformatic analyses suggested associations of the Cyld interactome with a few brain disorders and synaptic functions. Taken together, these results suggest that Shank3 regulates the synaptic abundance of Cyld in the mouse striatum and, thereby, potentially modulates the Lys63-linked polyubiquitination of striatal synaptic proteins.