Synaptic targeting of PSD-Zip45 (Homer 1c) and its involvement in the synaptic accumulation of F-actin

Super-Resolution Three-Dimensional Imaging of Actin Filaments in Cultured Cells and the Brain via Expansion Microscopy

Actin is an essential protein in almost all life forms. It mediates diverse biological functions, ranging from controlling the shape of cells and cell movements to cargo transport and the formation of synaptic connections. Multiple diseases are closely related to the dysfunction of actin or actin-related proteins. Despite the biological importance of actin, super-resolution imaging of it in tissue is still challenging, as it forms very dense networks in almost all cells inside the tissue. In this work, we demonstrate multiplexed super-resolution volumetric imaging of actin in both cultured cells and mouse brain slices via expansion microscopy (ExM). By introducing a simple labeling process, which enables the anchoring of an actin probe, phalloidin, to a swellable hydrogel, the multiplexed ExM imaging of actin filaments was achieved. We first showed that this technique could visualize the nanoscale details of actin filament organizations in cultured cells. Then, we applied this technique to mouse brain slices and visualized diverse actin organizations, such as the parallel actin filaments along the long axis of dendrites and dense actin structures in postsynaptic spines. We examined the postsynaptic spines in the mouse brain and showed that the organizations of actin filaments are highly diverse. This technique, which enables the high-throughput 60 nm resolution imaging of actin filaments and other proteins in cultured cells and thick tissue slices, would be a useful tool to study the organization of actin filaments in diverse biological circumstances and how they change under pathological conditions.

Whole-Neuron Synaptic Mapping Reveals Spatially Precise Excitatory/Inhibitory Balance Limiting Dendritic and Somatic Spiking

The balance between excitatory and inhibitory (E and I) synapses is thought to be critical for information processing in neural circuits. However, little is known about the spatial principles of E and I synaptic organization across the entire dendritic tree of mammalian neurons. We developed a new open-source reconstruction platform for mapping the size and spatial distribution of E and I synapses received by individual genetically-labeled layer 2/3 (L2/3) cortical pyramidal neurons (PNs) in vivo. We mapped over 90,000 E and I synapses across twelve L2/3 PNs and uncovered structured organization of E and I synapses across dendritic domains as well as within individual dendritic segments. Despite significant domain-specific variation in the absolute density of E and I synapses, their ratio is strikingly balanced locally across dendritic segments. Computational modeling indicates that this spatially precise E/I balance dampens dendritic voltage fluctuations and strongly impacts neuronal firing output.

PSD-Zip70 Deficiency Causes Prefrontal Hypofunction Associated with Glutamatergic Synapse Maturation Defects by Dysregulation of Rap2 Activity

Dysregulation of synapse formation and plasticity is closely related to the pathophysiology of psychiatric and neurodevelopmental disorders. The prefrontal cortex (PFC) is particularly important for executive functions such as working memory, cognition, and emotional control, which are impaired in the disorders. PSD-Zip70 (Lzts1/FEZ1) is a postsynaptic density (PSD) protein predominantly expressed in the frontal cortex, olfactory bulb, striatum, and hippocampus. Here we found that PSD-Zip70 knock-out (PSD-Zip70KO) mice exhibit working memory and cognitive defects, and enhanced anxiety-like behaviors. These abnormal behaviors are caused by impaired glutamatergic synapse transmission accompanied by tiny-headed immature dendritic spines in the PFC, due to aberrant Rap2 activation, which has roles in synapse formation and plasticity. PSD-Zip70 modulates the Rap2 activity by interacting with SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) in the postsynapse. Furthermore, suppression of the aberrant Rap2 activation in the PFC rescued the behavioral defects in PSD-Zip70KO mice. Our data demonstrate a critical role for PSD-Zip70 in Rap2-dependent spine synapse development in the PFC and underscore the importance of this regulation in PFC-dependent behaviors.

SIGNIFICANCE STATEMENT

PSD-Zip70 deficiency causes behavioral defects in working memory and cognition, and enhanced anxiety due to prefrontal hypofunction. This study revealed that PSD-Zip70 plays essential roles in glutamatergic synapse maturation via modulation of the Rap2 activity in the PFC. PSD-Zip70 interacts with both SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) and modulates the Rap2 activity in postsynaptic sites. Our results provide a novel Rap2-specific regulatory mechanism in synaptic maturation involving PSD-Zip70.

High affinity receptor labeling based on basic leucine zipper domain peptides conjugated with pH-sensitive fluorescent dye: Visualization of AMPA-type glutamate receptor endocytosis in living neurons

Techniques to visualize receptor trafficking in living neurons are important, but currently available methods are limited in their labeling efficiency, specificity and reliability. Here we report a method for receptor labeling with a basic leucine zipper domain peptide (ZIP) and a binding cassette specific to ZIP. Receptors are tagged with a ZIP-binding cassette at their extracellular domain. Tagged receptors expressed in cultured cells were labeled with exogenously applied fluorescently labeled ZIP with low background and high affinity. To test if ZIP labeling is useful in monitoring endocytosis and intracellular trafficking, we next conjugated ZIP with a pH-sensitive dye RhP-M (ZIP-RhP-M). ZIP binding to its binding cassette was pH-resistant and RhP-M fluorescence dramatically increased in acidic environment. Thus AMPA-type glutamate receptors (AMPARs) labeled by ZIP-RhP-M can report receptor endocytosis and subsequent intracellular trafficking. Application of ZIP-RhP-M to cultured hippocampal neurons expressing AMPARs tagged with a ZIP-binding cassette resulted in appearance of fluorescent puncta in PSD-95-positive large spines, suggesting local endocytosis and acidification of AMPARs in individual mature spines. This spine pool of AMPARs in acidic environment was distinct from the early endosomes labeled by transferrin uptake. These results suggest that receptor labeling by ZIP-RhP-M is a useful technique for monitoring endocytosis and intracellular trafficking. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Stress-induced sensitization to cocaine: actin cytoskeleton remodeling within mesocorticolimbic nuclei

This study investigated the consequence of repeated stress on actin cytoskeleton remodeling in the nucleus accumbens (NAc) and prefrontal cortex (Pfc), and the involvement of this remodeling in the expression of stress-induced motor cross-sensitization with cocaine. Wistar rats were restrained daily (2 h) for 7 days and, 3 weeks later, their NAc and Pfc were dissected 45 min after acute saline or cocaine (30 mg/kg i.p.). F-actin, actin-binding proteins (ABP) and GluR1 were quantified by Western blotting, and dendritic spines and postsynaptic density (PSD) size measured by electron microscopy. In the NAc from the stress plus cocaine group we observed a decrease in the phosphorylation of two ABPs, cofilin and cortactin, and an increase in the PSD size and the surface expression of GluR1, consistent with a more highly branched actin cytoskeleton. The Pfc also showed evidence of increased actin polymerization after stress as an increase was observed in Arp2, and in the number of spines. Inhibiting actin cycling and polymerization with latrunculin A into the NAc, but not the Pfc, inhibited the expression of cross-sensitization to cocaine (15 mg/kg i.p.) and restored the expression of GluR1 to control levels. This study shows that a history of repeated stress alters the ability of a subsequent cocaine injection to modulate dendritic spine morphology, actin dynamics and GluR1 expression in the NAc. Furthermore, by regulating GluR1 expression in the NAc, elevated actin cycling contributes to the expression of cross-sensitization between stress and cocaine, while stress-induced changes in the Pfc were not associated with cross-sensitization.

Deconstructing signal transduction pathways that regulate the actin cytoskeleton in dendritic spines

Dendritic spines are the sites of most excitatory synapses in the central nervous system. Recent studies have shown that spines function independently of each other, and they are currently the smallest known processing units in the brain. Spines exist in an array of morphologies, and spine structure helps dictate synaptic function. Dendritic spines are rich in actin, and actin rearrangements are critical regulators of spine morphology and density. In this review, we discuss the importance of actin in regulating dendritic spine morphogenesis, and discuss the upstream signal transduction pathways that either foster or inhibit actin polymerization. The understanding of actin regulatory pathways is best conceptualized as a hierarchical network in which molecules function in discrete levels defined by their molecular distance to actin. To this end, we focus on several classes of molecules, including guanine nucleotide exchange factors, small GTPases, small GTPase effectors, and actin binding proteins. We discuss how individual proteins in these molecular classes impact spine morphogenesis, and reveal the biochemical interactions in these networks that are responsible for shaping actin polymerization. Finally, we discuss the importance of these actin regulatory pathways in neuropsychiatric disorders.

Role of syntaxin 4 in activity-dependent exocytosis and synaptic plasticity in hippocampal neurons

Activity-dependent exocytosis of recycling endosomes that contain AMPA receptors in postsynaptic regions of hippocampal neurons occurs at microdomains enriched in the target SNARE [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor] syntaxin 4 (Stx4). These Stx4-enriched domains are located near the postsynaptic density, and disrupting SNARE interactions involving Stx4 prevents the fusion of recycling endosomes that contain AMPA receptors in dendritic spines. AMPA receptor trafficking is important for long-term potentiation; thus, Stx4 is an essential postsynaptic component for synaptic plasticity in hippocampal neurons.

Cutting edge: association with I kappa B kinase beta regulates the subcellular localization of Homer3

The signaling and adaptor protein Homer3 plays a role in controlling immune homeostasis and self-reactivity. Homer3 is recruited to the immune synapse (IS) following TCR ligation, although the mechanisms regulating this subcellular localization are unknown. We show that Homer3 specifically associates with a novel ubiquitin-like domain in the IkappaB kinase (IKK) beta subunit of the IKK complex. Homer3 associates with IKKbeta in T cells and colocalizes with the IKK complex at the IS. However, Homer3 is not required for IKK activation, as NF-kappaB signaling is intact in Homer3-deficient T cells. Instead, the IKK complex recruits Homer3 to the IS following TCR engagement, and we present evidence that this association regulates actin dynamics in T cells. These findings identify a novel interaction between two major signaling proteins and reveal an unexpected NF-kappaB-independent function for the IKK complex in regulating the subcellular localization of Homer3.

Cellular transport and membrane dynamics of the glycine receptor

Regulation of synaptic transmission is essential to tune individual-to-network neuronal activity. One way to modulate synaptic strength is to regulate neurotransmitter receptor numbers at postsynaptic sites. This can be achieved either through plasma membrane insertion of receptors derived from intracellular vesicle pools, a process depending on active cytoskeleton transport, or through surface membrane removal via endocytosis. In parallel, lateral diffusion events along the plasma membrane allow the exchange of receptor molecules between synaptic and extrasynaptic compartments, contributing to synaptic strength regulation. In recent years, results obtained from several groups studying glycine receptor (GlyR) trafficking and dynamics shed light on the regulation of synaptic GlyR density. Here, we review (i) proteins and mechanisms involved in GlyR cytoskeletal transport, (ii) the diffusion dynamics of GlyR and of its scaffolding protein gephyrin that control receptor numbers, and its relationship with synaptic plasticity, and (iii) adaptative changes in GlyR diffusion in response to global activity modifications, as a homeostatic mechanism.

Activity-driven mobilization of post-synaptic proteins

Synapses established during central nervous system development can be modified through synapse elimination and formation. These processes are, in part, activity dependent and require regulated trafficking of post-synaptic components. Here, we investigate the activity-driven remodeling of cultured rat hippocampal neurons at 14 days in vitro, focusing on the post-synaptic proteins PSD-95, Shank, neuroligin (NL)1 and actin. Using live imaging and photoconductive stimulation, we found that high-frequency activity altered the trajectory, but not velocity, of PSD-95-GFP and Shank-YFP clusters, whereas it reduced the speed and increased the number of NL1 clusters. Actin-CFP reorganized into puncta following activity and approximately 50% of new puncta colocalized with NL1 clusters. Actin reorganization was enhanced by the overexpression of NL1 and decreased by the expression of an NL1 mutant, NL1-R473C. These results demonstrate activity-dependent changes that may result in the formation of new post-synaptic sites and suggest that NL1 modulates actin reorganization. The results also suggest that a common mechanism underlies both the developmental and activity-dependent remodeling of excitatory synapses.

TRPC1 channels are critical for hypertrophic signaling in the heart

RATIONALE

Cardiac muscle adapts to increase workload by altering cardiomyocyte size and function resulting in cardiac hypertrophy. G protein-coupled receptor signaling is known to govern the hypertrophic response through the regulation of ion channel activity and downstream signaling in failing cardiomyocytes.

OBJECTIVE

Transient receptor potential canonical (TRPC) channels are G protein-coupled receptor operated channels previously implicated in cardiac hypertrophy. Our objective of this study is to better understand how TRPC channels influence cardiomyocyte calcium signaling.

METHODS AND RESULTS

Here, we used whole cell patch clamp of adult cardiomyocytes to show upregulation of a nonselective cation current reminiscent of TRPC channels subjected to pressure overload. This TRPC current corresponds to the increased TRPC channel expression noted in hearts of mice subjected to pressure overload. Importantly, we show that mice lacking TRPC1 channels are missing this putative TRPC current. Moreover, Trpc1(-)(/)(-) mice fail to manifest evidence of maladaptive cardiac hypertrophy and maintain preserved cardiac function when subjected to hemodynamic stress and neurohormonal excess. In addition, we provide a mechanistic basis for the protection conferred to Trpc1(-)(/)(-) mice as mechanosensitive signaling through calcineurin/NFAT, mTOR and Akt is altered in Trpc1(-)(/)(-) mice.

CONCLUSIONS

From these studies, we suggest that TRPC1 channels are critical for the adaptation to biomechanical stress and TRPC dysregulation leads to maladaptive cardiac hypertrophy and failure.

Control of the postsynaptic membrane viscosity

The physical properties of the postsynaptic membrane (PSM), including its viscosity, determine its capacity to regulate the net flux of synaptic membrane proteins such as neurotransmitter receptors. To address these properties, we studied the lateral diffusion of glycophosphatidylinositol-anchored green fluorescent protein and cholera toxin bound to the external leaflet of the plasma membrane. Relative to extrasynaptic regions, their mobility was reduced at synapses and even more at inhibitory than at excitatory ones. This indicates a higher density of obstacles and/or higher membrane viscosity at inhibitory contacts. Actin depolymerization reduced the confinement and accelerated a population of fast, mobile molecules. The compaction of obstacles thus depends on actin cytoskeleton integrity. Cholesterol depletion increased the mobility of the slow diffusing molecules, allowing them to diffuse more rapidly through the crowded PSM. Thus, the PSM has lipid-raft properties, and the density of obstacles to diffusion depends on filamentous actin. Therefore, lipid composition and actin-dependent protein compaction regulate viscosity of the PSM and, consequently, the molecular flow in and out of synapses.

Targeting Homer genes using adeno-associated viral vector: lessons learned from behavioural and neurochemical studies

Over a decade of in-vitro data support a critical role for members of the Homer family of postsynaptic scaffolding proteins in regulating the functional architecture of glutamate synapses. Earlier studies of Homer knockout mice indicated a necessary role for Homer gene products in normal mesocorticolimbic glutamate transmission and behaviours associated therewith. The advent of adeno-associated viral vectors carrying cDNA for, or short hairpin RNA against, specific Homer isoforms enabled the site-directed targeting of Homers to neurons in the brain. This approach has allowed our groups to address developmental issues associated with conventional knockout mice, to confirm active roles for distinct Homer isoforms in regulating glutamate transmission in vivo, as well as in mediating a variety of behavioural processes. This review summarizes the existing data derived from our studies using adeno-associated viral vector-mediated neuronal targeting of Homer in rodents, implicating this family of proteins in drug and alcohol addiction, learning/memory and emotional processing.

Activity-independent and subunit-specific recruitment of functional AMPA receptors at neurexin/neuroligin contacts

A combination of cell culture and animal studies has recently shown that adhesion between neurexins and neuroligins played important roles in synapse initiation, maturation, and function. Binding of neurexin-1beta to neuroligin-1 triggers the postsynaptic clustering of the scaffold postsynaptic density protein 95, but the composition and timing of accumulation of glutamate receptors at those nascent contacts remain unclear. Using glutamate iontophoresis and patch-clamp recordings, we identified functional AMPA receptors (AMPARs) and NMDA receptors at postsynaptic density protein 95 clusters induced by neurexin-1beta coated microspheres on primary hippocampal neurons. The recruitment of AMPARs occurred as early as 2 h after initial contact, and was not blocked by TTX/2-amino-5-phosphovaleric acid (APV) treatment. The differential recruitment of recombinant subunits GluR1 and GluR2, as well as the absence of rectification in voltage/current curves, further indicate that neurexin/neuroligin contacts primarily recruit GluR2-containing AMPARs. Finally, by using glutamate un-caging and calcium imaging, we show that AMPARs participate in calcium entry at neurexin-1beta induced post-synapses, most likely through the activation of voltage-gated calcium channels. Such rapid and activity-independent accumulation of functional AMPARs at neurexin-1beta-induced postsynapses points to a new role of AMPARs in synaptogenesis.

Developmental roles for Homer: more than just a pretty scaffold

Homer proteins are best known as scaffold proteins at the post-synaptic density where they facilitate synaptic signalling and are thought to be required for learning and memory. Evidence implicating Homer proteins in the development of the nervous system is also steadily accumulating. Homer is highly conserved and is expressed at key developmental time points in the nervous system of several species. Homer regulates intracellular calcium homeostasis, clustering and trafficking of receptors and proteins at the cytosolic surface of the plasma membrane, transcription and translation, and cytoskeletal organization. Each of these functions has obvious potential to regulate neuronal development, and indeed Homer is implicated in several pathologies associated with the developing nervous system. Current data justify more critical experimental approaches to the role of Homer in the developing nervous system and related neurological disorders.

Mice lacking Homer 1 exhibit a skeletal myopathy characterized by abnormal transient receptor potential channel activity

Transient receptor potential (TRP) channels are nonselective cation channels, several of which are expressed in striated muscle. Because the scaffolding protein Homer 1 has been implicated in TRP channel regulation, we hypothesized that Homer proteins play a significant role in skeletal muscle function. Mice lacking Homer 1 exhibited a myopathy characterized by decreased muscle fiber cross-sectional area and decreased skeletal muscle force generation. Homer 1 knockout myotubes displayed increased basal current density and spontaneous cation influx. This spontaneous cation influx in Homer 1 knockout myotubes was blocked by reexpression of Homer 1b, but not Homer 1a, and by gene silencing of TRPC1. Moreover, diminished Homer 1 expression in mouse models of Duchenne's muscular dystrophy suggests that loss of Homer 1 scaffolding of TRP channels may contribute to the increased stretch-activated channel activity observed in mdx myofibers. These findings provide direct evidence that Homer 1 functions as an important scaffold for TRP channels and regulates mechanotransduction in skeletal muscle.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

More than just synaptic building blocks: scaffolding proteins of the post-synaptic density regulate dendritic patterning

The dendritic arbor is responsible for receiving and consolidating neuronal input. Outgrowth and morphogenesis of the arbor are complex stages of development that are poorly understood. However, recent findings have identified synaptic scaffolding proteins as novel regulators of these important events. Scaffolding proteins are enriched in the post-synaptic density where they bind and bring into close proximity neurotransmitter receptors, signaling molecules, and regulators of the actin cytoskeleton. This property is important for dendritic spine morphogenesis and maintenance in the mature neuron. Scaffolding proteins are now being described as key regulators of neurite outgrowth, dendritic development, and pattern formation in immature neurons. These proteins, which include post-synaptic-95, Shank and Densin-180, as well as many of their interacting partners, appear to regulate both the microtubule and actin cytoskeleton to influence dendrite morphology. Through a large array of protein-protein interaction domains, scaffolding proteins are able to form large macromolecular complexes that include cytoskeletal motor proteins as well as microtubule and actin regulatory molecules. Together, the new findings form a persuasive argument that scaffolding proteins deliver critical regulatory elements to sites of dendritic outgrowth and branching to modulate the formation and maintenance of the dendritic arbor.

The Homer family proteins

The Homer family of adaptor proteins consists of three members in mammals, and homologs are also known in other animals but not elsewhere. They are predominantly localized at the postsynaptic density in mammalian neurons and act as adaptor proteins for many postsynaptic density proteins. As a result of alternative splicing each member has several variants, which are classified primarily into the long and short forms. The long Homer forms are constitutively expressed and consist of two major domains: the amino-terminal target-binding domain, which includes an Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) homology 1 (EVH1) domain, and the carboxy-terminal self-assembly domain containing a coiled-coil structure and leucine zipper motif. Multimers of long Homer proteins, coupled through their carboxy-terminal domains, are thought to form protein clusters with other postsynaptic density proteins, which are bound through the amino-terminal domains. Such Homer-mediated clustering probably regulates or facilitates signal transduction or cross-talk between target proteins. The short Homer forms lack the carboxy-terminal domain; they are expressed in an activity-dependent manner as immediate-early gene products, possibly disrupting Homer clusters by competitive binding to target proteins. Homer proteins are also involved in diverse non-neural physiological functions.

Transmitting on actin: synaptic control of dendritic architecture

Excitatory synaptic transmission in the central nervous system mainly takes place at dendritic spines, highly motile protrusions on the dendritic surface. Depending on the stimuli received, dendritic spines undergo rapid actin-based changes in their morphology. This plasticity appears to involve signaling through numerous proteins that control the organization of the actin cytoskeleton (actin regulators). At least in part, recruitment and activation of these depends on neurotransmitter receptors at the post-synapse, which directly link neurotransmission to changes in dendritic spine architecture. However, other, non-neurotransmitter-receptors present at dendritic spines also participate. It is likely that several receptor types can control the activity of a single actin-regulatory pathway and it is the complex integration of numerous signals that determines the overall architecture of a dendritic spine.

Differential control of postsynaptic density scaffolds via actin-dependent and -independent mechanisms

Organization and dynamic remodeling of postsynaptic density (PSD) are thought to be critical in postsynaptic signal transduction, but the underlying molecular mechanisms are not well understood. We show here that four major scaffolding molecules, PSD-95, GKAP, Shank, and PSD-Zip45, show distinct instability in total molecular content per synapse. Fluorescence recovery after photobleaching also confirmed their distinct turnover rates. Among the PSD molecules examined, PSD-95 was most stable, but its elimination did not influence the dynamics of its direct binding partner GKAP. Multiple interactions of scaffolding molecules with the actin cytoskeleton have suggested their importance in both maintenance and remodeling of the PSD. Indeed, acute pharmacological disruption of F-actin rapidly eliminated the dynamic fraction of GKAP, Shank, and PSD-Zip45, without changing synaptic localization of PSD-95. GKAP content in synapses increased after pharmacological enhancement of neuronal activity, whereas Shank and PSD-Zip45 content showed reduction. Inhibition of F-actin dynamics prevented activity-dependent redistribution of all three scaffolds. We also assessed involvement of glutamate receptors in the regulation of PSD dynamics. Genetic manipulations eliminating either NMDA receptors or metabotropic glutamate receptors did not primarily influence mobility of their binding scaffolds. These results collectively indicate a critical role of filamentous actin in determining the extent of dynamic reorganization in PSD molecular composition.

NMDA receptor-dependent synaptic translocation of insulin receptor substrate p53 via protein kinase C signaling

The activity-dependent remodeling of postsynaptic structure is a fundamental process underlying learning and memory. Insulin receptor substrate p53 (IRSp53), a key player in cytoskeletal dynamics, is enriched in the postsynaptic density (PSD) fraction, but its significance in synaptic functions remains unclear. We report here that IRSp53 is accumulated rapidly at the postsynaptic sites of cultured hippocampal neurons after glutamate or NMDA stimulation in an actin cytoskeleton-dependent manner. Pharmacological profiles showed that a PKC inhibitor, but not other kinase inhibitors, specifically suppressed the synaptic translocation of IRSp53 in response to NMDA, and the selective activation of PKC with phorbol ester markedly induced the synaptic translocation. Reverse transcriptase-PCR and Western blotting showed that IRSp53-S is the major isoform expressed in cultured hippocampal neurons. The synaptic targeting of IRSp53-S was found to be mediated through N-terminal coiled-coil domain and the PDZ (PSD-95/Discs large/zona occludens-1)-binding sequence at its C-terminal end and regulated by the PKC phosphorylation of its N terminus. In electrophysiological experiments, overexpression of IRSp53-S wild type and IRSp53-S mutant that is spontaneously accumulated at the postsynaptic sites enhanced the postsynaptic function as detected by an increased miniature EPSC amplitude. These data suggest that IRSp53 is involved in NMDA receptor-linked synaptic plasticity via PKC signaling.

Bi-directional regulation of postsynaptic cortactin distribution by BDNF and NMDA receptor activity

Abstract Cortactin is an F-actin-associated protein which interacts with the postsynaptic scaffolding protein Shank at the SH3 domain and is localized within the dendritic spine in the mouse neuron. Green fluorescent protein (GFP)-based time-lapse imaging revealed cortactin redistribution from dendritic cytoplasm to postsynaptic sites by application of brain-derived neurotrophic factor (BDNF). This response was mediated by mitogen-activated protein (MAP) kinase activation and was dependent on the C-terminal SH3 domain. In contrast, activation of N-methyl-D-aspartate (NMDA) receptors induced loss of cortactin from postsynaptic sites. This NMDA-dependent redistribution was blocked by an Src family kinase inhibitor. Conversely, increasing Src family kinase activity induced cortactin phosphorylation and loss of cortactin from the postsynaptic sites. Finally, blocking of endogenous BDNF reduced the amount of cortactin at the postsynaptic sites and an NMDA receptor antagonist prevented this reduction. These results indicate the importance of counterbalance between BDNF and NMDA receptor-mediated signalling in the reorganization of the postsynaptic actin cytoskeleton during neuronal development.

Behavioral and neurochemical phenotyping of Homer1 mutant mice: possible relevance to schizophrenia

Homer proteins are involved in the functional assembly of postsynaptic density proteins at glutamatergic synapses and are implicated in learning, memory and drug addiction. Here, we report that Homer1-knockout (Homer1-KO) mice exhibit behavioral and neurochemical abnormalities that are consistent with the animal models of schizophrenia. Relative to wild-type mice, Homer1-KO mice exhibited deficits in radial arm maze performance, impaired prepulse inhibition, enhanced 'behavioral despair', increased anxiety in a novel objects test, enhanced reactivity to novel environments, decreased instrumental responding for sucrose and enhanced MK-801- and methamphetamine-stimulated motor behavior. No-net-flux in vivo microdialysis revealed a decrease in extracellular glutamate content in the nucleus accumbens and an increase in the prefrontal cortex. Moreover, in Homer1-KO mice, cocaine did not stimulate a rise in frontal cortex extracellular glutamate levels, suggesting hypofrontality. These behavioral and neurochemical data derived from Homer1 mutant mice are consistent with the recent association of schizophrenia with a single-nucleotide polymorphism in the Homer1 gene and suggest that the regulation of extracellular levels of glutamate within limbo-corticostriatal structures by Homer1 gene products may be involved in the pathogenesis of this neuropsychiatric disorder.

Determination of absolute protein numbers in single synapses by a GFP-based calibration technique

To build a quantitative model of molecular organization of neurons, it is essential to have information about the number of protein molecules at individual synapses. Here we developed a method to estimate absolute numbers of individual proteins at actual excitatory synapses by calibrating the fluorescence intensity of microspheres with single EGFP molecules. In cultured hippocampal neurons, we observed a monotonous increase of postsynaptic protein numbers per single synapse during neuronal differentiation and subsequent stabilization. At maturity we calculated that a single excitatory postsynaptic site contains 100-450 of individual postsynaptic proteins, such as PSD-95, GKAP, Shank and Homer. This narrow range of postsynaptic protein content suggests relatively simple stoichiometry of postsynaptic molecular organization. The EGFP-based calibration technique provides an unprecedented general method for estimating the amounts of proteins in macromolecular complexes.

Homer 1b/c expression correlates with zebrafish olfactory system development

The zebrafish, (Danio rerio) is an important model organism for the analysis of molecular mechanisms that govern neuronal circuit development. The neuronal circuitry that mediates olfaction is crucial for the development and survival of all teleost fishes. In concert with other sensory systems, olfaction is functional at early stages in zebrafish development and mediates important behavioral and survival strategies in the developing larva. Odorant cues are transduced by an array of signaling molecules from receptors in olfactory sensory neurons. The scaffolding protein family known as Homer is well placed to orchestrate this signaling cascade by interacting with and coupling membrane bound receptors to cytosolic signaling partners. To date, Homer has not been demonstrated in the zebrafish. Here we report that the Homer 1b/c isoform was prominent in the olfactory system from the earliest stages of differentiation. We describe the spatial and temporal distribution of Homer in the zebrafish olfactory system. At 24 hours post fertilization (hpf), Homer expression delineated the boundary of the presumptive olfactory placode. Subsequent expression steadily increased throughout the developing olfactory placode, with a prominent localization to the dendritic knobs of the olfactory sensory neurons. Homer expression in the developing olfactory bulb was punctate and prominent in the glomeruli, displaying an apparent synaptic localization. This work supports the hypothesis that Homer is an important molecule in neuronal circuit development, necessary for crucial behaviors required for development and survival.

Collaboration of PSD-Zip70 with its binding partner, SPAR, in dendritic spine maturity

Recent studies have reported on the molecular mechanisms underlying dendritic spine (spine) dynamics. Because most of these studies investigated spine dynamics by overexpressing constitutively active or dominant-negative PSD (postsynaptic density) proteins in cultured mature neurons, the results represent the enlargement of mature spines or their return to an immature state. Here, we developed the technique of in utero electroporation to investigate spine dynamics. Using this technique, we demonstrated the suppression of spine maturation by the C-terminal variants of PSD-Zip70 in vitro and in vivo. Transient overexpression of the C terminus of PSD-Zip70 and knock-down of PSD-Zip70 also displayed the destabilization of mature spines. We further found the PSD-Zip70 and SPAR (spine-associated RapGAP) interaction via the short C-terminal region of PSD-Zip70 and the GK-binding domain of SPAR. In association with immature spines induced by overexpression of the PSD-Zip70 C terminus or knock-down of PSD-Zip70, SPAR lost its spine localization. Overexpression of the GK-binding domain of SPAR also induced to form immature spines without affecting the localization of PSD-Zip70 in the small heads of filopodial spines. Our results suggest that PSD-Zip70 in collaboration with SPAR is critically involved in spine maturity, especially in the mature spine formation and the maintenance of spine maturity.

Homer expression in the Xenopus tadpole nervous system

Homer proteins are integral components of the postsynaptic density and are thought to function in synaptogenesis and plasticity. In addition, overexpression of Homer in the developing Xenopus retinotectal system results in axonal pathfinding errors. Here we report that Xenopus contains the homer1 gene, expressed as the isoform, xhomer1b, which is highly homologous to the mammalian homer1b. The mammalian homer1 gene is expressed as three isoforms, the truncated or short form homer1a and the long forms homer1b and -1c. For Xenopus, we cloned three very similar variants of homer1b, identified as Xenopus xhomer1b.1, xhomer1b.2, and xhomer1b.3, which display up to 98% homology with each other and 90% similarity to mammalian homer1b. Furthermore, we demonstrate that Xenopus also contains a truncated form of the Homer1 protein, which could be induced by kainic acid injection and is likely homologous to the mammalian Homer1a. xHomer1b expression was unaffected by neuronal activity levels but was developmentally regulated. Within the brain, the spatial and temporal distributions of both Homer isoforms were similar in the neuropil and cell body regions. Homer1 was detected in motor axons. Differential distribution of the two isoforms was apparent: Homer1b immunoreactivity was prominent at junctions between soma and the ventricular surface; in the retina, the Mueller radial glia were immunoreactive for Homer1, but not Homer1b, suggesting the retinal glia contain only the Homer1a isoform. Homer1b expression in muscle was prominent throughout development and was aligned with the actin striations in skeletal muscle. The high level of conservation of the xhomer1 gene and the protein expression in the developing nervous system suggest that Homer1 expression may be important for normal neuronal circuit development.

A functional role of postsynaptic density-95-guanylate kinase-associated protein complex in regulating Shank assembly and stability to synapses

Postsynaptic density (PSD) proteins include scaffold, cytoskeletal, and signaling proteins that structurally and functionally interact with glutamate receptors and other postsynaptic membrane proteins. The molecular mechanisms regulating the assembly of PSD proteins and their associations with synapses are still widely unknown. We investigated the molecular mechanisms of Shank1 targeting and synapse assembly by looking at the function of guanylate kinase-associated protein (GKAP) and PSD-95 interactions. Shank1 when it is not associated to GKAP, which binds to the Shank PSD-95-Discs Large-zona occludens-1 domain, forms filamentous and fusiform structures in which the Src homology 3 domain specifically interacts with the ankyrin repeat domain, thus allowing its multimerization via a novel form of intermolecular interaction. Surprisingly, in both COS-7 cells and hippocampal neurons, GKAP forms insoluble aggregates with Shank that colocalize with heat shock protein 70 and neurofilaments, two markers of the aggresomes in which misfolded proteins accumulate. However, the two proteins are organized in clusters in COS cells and synaptic clusters in neurons when both are overexpressed and associated with wild-type PSD-95, but not with palmitoylation-deficient PSD-95. Synaptic activity in neurons induces the formation of Shank and GKAP intracellular aggregation and degradation. Similarly, the overexpression of a GKAP mutant that is incapable of binding PSD-95 induces Shank aggregation and degradation in neurons. Our data suggest a possible functional and structural role of the PSD-95-GKAP complex in Shank and PSD protein assembly and stability to synapses.

Homer proteins regulate sensitivity to cocaine

Drug addiction involves complex interactions between pharmacology and learning in genetically susceptible individuals. Members of the Homer gene family are regulated by acute and chronic cocaine administration. Here, we report that deletion of Homer1 or Homer2 in mice caused the same increase in sensitivity to cocaine-induced locomotion, conditioned reward, and augmented extracellular glutamate in nucleus accumbens as that elicited by withdrawal from repeated cocaine administration. Moreover, adeno-associated virus-mediated restoration of Homer2 in the accumbens of Homer2 KO mice reversed the cocaine-sensitized phenotype. Further analysis of Homer2 KO mice revealed extensive additional behavioral and neurochemical similarities to cocaine-sensitized animals, including accelerated acquisition of cocaine self-administration and altered regulation of glutamate by metabotropic glutamate receptors and cystine/glutamate exchange. These data show that Homer deletion mimics the behavioral and neurochemical phenotype produced by repeated cocaine administration and implicate Homer in regulating addiction to cocaine.

Homer–PIKE complex: a novel link between mGluRI and PI 3-kinase

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, are thought to signal through the phospholipase and inositol (1,4,5)-trisphosphate receptor system, or through the adenylyl cyclase and protein kinase C system. Rong et al. have recently identified a new phosphoinositide (PI) 3-kinase enhancer (PIKE-L) that links group I mGlu receptors (mGluRI) to PI 3-kinase through Homer proteins, adaptors that bind mGluRI. mGluRI agonists enhanced mGluRI-Homer-PIKE-L complex formation, leading to activation of PI 3-kinase and inhibition of staurosporine-induced neuronal apoptosis. These results reveal a novel anti-apoptotic signaling mechanism that involves formation of an mGluRI signaling complex.

Characterization of an ankyrin repeat-containing Shank2 isoform (Shank2E) in liver epithelial cells

Shank proteins are a family of multidomain scaffolding proteins best known for their role in organizing the postsynaptic density region in neurons. Unlike Shank1 and Shank3, Shank2 [also known as Pro-SAP1 (proline-rich synapse-associated protein 1), CortBP1 (cortactin binding protein 1) or Spank-3] has been described as a truncated family member without an N-terminal ankyrin repeat domain. The present study utilized bioinformatics to demonstrate the presence of exons encoding ankyrin repeats in the region preceding the previously described Shank2 gene. cDNA sequencing of mRNA from epithelial cells revealed a novel spliceoform of Shank2, termed Shank2E, that encodes a predicted 200 kDa protein with six N-terminal ankyrin repeats. Shank2 mRNA from epithelial tissues was larger than transcripts in brain. Likewise, the apparent mass of Shank2 protein was larger in epithelial tissues (230 kDa) when compared with brain (165/180 kDa). Immunofluorescence and membrane fractionation found Shank2E concentrated at the apical membrane of liver epithelial cells. In cultured cholangiocytes, co-immunoprecipitation and detergent solubility studies revealed Shank2E complexed with actin and co-distributed with actin in detergent-insoluble lipid rafts. These findings indicate epithelial cells express an ankyrin repeat-containing Shank2 isoform, termed Shank2E, that is poised to co-ordinate actin-dependent events at the apical membrane.

Homer/Vesl proteins and their roles in CNS neurons

Since their initial discovery in 1997, Homer/Vesl proteins have become increasingly investigated as putative regulators of receptor and ion-channel function in the central nervous system. Within a relatively brief period, numerous research reports have described manifold effects of Homer proteins, including the modulation of the trafficking of type I metabotropic glutamate receptors (mGluRs), axonal pathfinding, mGluR coupling to calcium and potassium channels, agonist-independent mGluR activity, ryanodine receptor regulation, locomotor activity, and behavioral plasticity. This review summarizes our current knowledge on the induction, expression, and structure of the various forms of Homer proteins, as well as their roles in neuronal function. In addition, we provide an outlook on novel developments with regard to the involvement of Homer-1a in hippocampal synaptic function.

Locus ceruleus control of state-dependent gene expression

Wakefulness and sleep are accompanied by changes in behavior and neural activity, as well as by the upregulation of different functional categories of genes. However, the mechanisms responsible for such state-dependent changes in gene expression are unknown. Here we investigate to what extent state-dependent changes in gene expression depend on the central noradrenergic (NA) system, which is active in wakefulness and reduces its firing during sleep. We measured the levels of approximately 5000 transcripts expressed in the cerebral cortex of control rats and in rats pretreated with DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine], a neurotoxin that removes the noradrenergic innervation of the cortex. We found that NA depletion reduces the expression of approximately 20% of known wakefulness-related transcripts. Most of these transcripts are involved in synaptic plasticity and in the cellular response to stress. In contrast, NA depletion increased the expression of the sleep-related gene encoding the translation elongation factor 2. These results indicate that the activity of the central NA system during wakefulness modulates neuronal transcription to favor synaptic potentiation and counteract cellular stress, whereas its inactivity during sleep may play a permissive role to enhance brain protein synthesis.

Activity-inducible protein Homer1a/Vesl-1S promotes redistribution of postsynaptic protein Homer1c/Vesl-1L in cultured rat hippocampal neurons

In cultured rat hippocampal neurons, overexpression of Homer1a/Vesl-1S, an inducible protein upregulated by seizure or long-term potentiation, caused a reduction of punctate distribution of a postsynaptic protein Homer1c/Vesl-1L, without significant decrease in its total amount. Clusters of F-actin were also decreased. Treatments of cells with BDNF or a proteasome inhibitor, which cause increase in the expression level of endogenous Homer1a, also resulted in the reduction of Homer1c puncta. These results indicate that the accumulation of Homer1a, either exogenously expressed or endogenously induced, caused redistribution and dispersion of postsynaptic clusters of Homer1c and F-actin, suggesting an important role of Homer1a in synaptic remodeling.

The dynamic organization of postsynaptic proteins: translocating molecules regulate synaptic function

Physiological roles for postsynaptic molecules in synaptogenesis and plasticity are under intense investigation. Recent imaging experiments, including GFP-based and single-particle tracking strategies, reveal rapid movement of synaptic components to and from the postsynaptic sites. Furthermore, specific patterns of neuronal activity and/or activation of specific transmitter receptors trigger selective translocation of postsynaptic components. These emerging dynamic properties of synaptic specializations add another layer of complexity to the signaling mechanisms of CNS synapses.