Regulation of dendritic spine morphology and synaptic function by Shank and Homer

Local calcium signaling in neurons

Transient rises in the cytoplasmic concentration of calcium ions serve as second messenger signals that control many neuronal functions. Selective triggering of these functions is achieved through spatial localization of calcium signals. Several qualitatively different forms of local calcium signaling can be distinguished by the location of open calcium channels as well as by the distance between these channels and the calcium binding proteins that serve as the molecular targets of calcium action. Local calcium signaling is especially prominent at presynaptic active zones and postsynaptic densities, structures that are distinguished by highly organized macromolecular arrays that yield precise spatial arrangements of calcium signaling proteins. Similar forms of local calcium signaling may be employed throughout the nervous system, though much remains to be learned about the molecular underpinnings of these events.

Differential distribution of Rac1 and Rac3 GTPases in the developing mouse brain: implications for a role of Rac3 in Purkinje cell differentiation

Rac3 is one of the three known Rac GTPases in vertebrates. Rac3 shows high sequence homology to Rac1, and its transcript is specifically expressed in the developing nervous system, where its localization and function are unknown. By using Rac3-specific antibodies, we show that the endogenous Rac3 protein is differentially expressed during mouse brain development, with a peak of expression at times of neuronal maturation and synaptogenesis. Comparison with Rac1 shows clear-cut differences in the overall distribution of the two GTPases in the developing brain, and in their subcellular distribution in regions of the brain where both proteins are expressed. At P7, Rac3 staining is particularly marked in the deep cerebellar nuclei and in the pons, where it shows a discontinuous distribution around the neuronal cell bodies, in contrast with the diffuse staining of Rac1. Rac3 does not evidently co-localize with pre- and post-synaptic markers, nor with GFAP-positive astrocytes, but it clearly co-localizes with actin filaments, and with the terminal portions of calbindin-positive Purkinje cell axons in the deep cerebellar nuclei. Our data implicate Rac3 in neuronal differentiation, and support a specific role of this GTPase in actin-mediated remodelling of Purkinje cell neuritic terminals at time of synaptogenesis.

Regulation of spine morphology and synaptic function by LIMK and the actin cytoskeleton

Filamentous actin (F-actin) is highly enriched in the dendritic spine, a specialized postsynaptic structure on which the great majority of the excitatory synapses are formed in the mammalian central nervous system (CNS). The protein kinases of the Lim-kinase (LIMK) family are potent regulators of actin dynamics in many cell types and they are abundantly expressed in the CNS, including the hippocampus. Using a combination of genetic manipulations and electrophysiological recordings in mice, we have demonstrated that LIMK-1 signaling is important in vivo in the regulation of the actin cytoskeleton, spine morphology, and synaptic function, including hippocampal long-term potentiation (LTP), a prominent form of long lasting synaptic plasticity thought to be critical to memory formation. Our results provide strong genetic evidence that LIMK and its substrate ADF/cofilin are involved in spine morphology and synaptic properties and are consistent with the notion that the Rho family small GTPases and the actin cytoskeleton are critical to spine structure and synaptic regulation.

PI3 kinase enhancer–Homer complex couples mGluRI to PI3 kinase, preventing neuronal apoptosis

Phosphoinositide 3 kinase enhancer (PIKE) is a recently identified nuclear GTPase that activates nuclear phosphoinositide 3-kinase (PI3 kinase). We have identified, cloned and characterized a new form of PIKE, designated PIKE-L, which, unlike the nuclear PIKE-S, localizes to both the cytoplasm and the nucleus. We demonstrate physiologic binding of PIKE-L to Homer, an adaptor protein known to link metabotropic glutamate receptors to multiple intracellular targets including the inositol 1,4,5-trisphosphate receptor (IP3R). We show that activation of group I metabotropic glutamate receptors (mGluRIs) enhances formation of an mGluRI-Homer-PIKE-L complex, leading to activation of PI3 kinase activity and prevention of neuronal apoptosis. Our findings indicate that this complex mediates the well-known ability of agonists of mGluRI to prevent neuronal apoptosis.

Drebrin-dependent actin clustering in dendritic filopodia governs synaptic targeting of postsynaptic density-95 and dendritic spine morphogenesis

Dendritic spines have two major structural elements: postsynaptic densities (PSDs) and actin cytoskeletons. PSD proteins are proposed to regulate spine morphogenesis. However, other molecular mechanisms should govern spine morphogenesis, because the initiation of spine morphogenesis precedes the synaptic clustering of these proteins. Here, we show that synaptic clustering of drebrin, an actin-binding protein highly enriched in dendritic spines, governs spine morphogenesis. We immunocytochemically analyzed developing hippocampal neurons of low-density cultures. Filopodia-like dendritic protrusions were classified into two types: diffuse-type filopodia, which have diffuse distribution of drebrin, and cluster-type filopodia, which have drebrin clusters with filamentous actin (F-actin). Most cluster-type filopodia were synaptic filopodia. Postsynaptic drebrin clusters were found in both most synaptic filopodia and spines. Postsynaptic PSD-95 clusters, however, were found in only one-half of synaptic filopodia but in most spines. These data indicate that cluster-type filopodia are not mature spines but their precursors. Suppression of the upregulation of drebrin adult isoform (drebrin A) by antisense oligonucleotides against it attenuated synaptic clustering of PSD-95, as well as clustering of drebrin and F-actin. Furthermore, the restoration of drebrin A expression by injection of the expression vectors of drebrin A tagged with green fluorescent protein into the neurons treated with the antisense oligonucleotides induced synaptic reclustering of PSD-95 on clusters of the labeled drebrin A. These data indicated that the synaptic clustering of drebrin is necessary for that of PSD-95 in developing neurons. Together, these data suggest that synaptic clustering of drebrin is an essential step for spine morphogenesis.

Role of group I metabotropic glutamate receptors and NMDA receptors in homocysteine-evoked acute neurodegeneration of cultured cerebellar granule neurones

Hyperhomocysteinemia is a risk factor in neurodegeneration. It has been suggested that apart from disturbances in methylation processes, the mechanisms of this effect may include excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptors. In this study we demonstrate that apart from NMDA receptors, also group I metabotropic glutamate receptors participate in acute homocysteine (Hcy)-induced neurotoxicity in cultured rat cerebellar granule neurones. Primary neuronal cultures were incubated for 30 min in the Mg(2+)-free ionic medium containing homocysteine and other ligands, and neurodegenerative changes were assessed 24h later using propidium iodide staining. D,L-Homocysteine given alone appeared to be a weak neurotoxin, with EC(50) of 17.4mM, whereas EC(50) for L-glutamate was 0.17 mM. Addition of 50 microM glycine enhanced homocysteine neurotoxicity, and only that portion of neurotoxicity was abolished by 0.5 microM MK-801, an uncompetitive NMDA receptor antagonist. The net stimulation of 45Ca uptake by granule cells incubated in the presence of 25 mM D,L-homocysteine with 50 microM glycine was only 3% of the net uptake evoked by 1mM glutamate. Application of an antagonist of group I metabotropic glutamate receptors (mGluRs) LY367385 at 25 and 250 microM concentrations, induced a dose-dependent partial neuroprotection, whereas given together with MK-801 completely prevented neurotoxicity. In the absence of glycine, LY367385 and MK-801 given alone failed to induce neuroprotection, while applied together completely prevented homocysteine neurotoxicity. Agonist of group I mGluRs, 10 trans-azetidine-2,3-dicarboxylic acid (t-ADA) induced significant neurotoxicity. This study shows for the first time that acute homocysteine-induced neurotoxicity is mediated both by group I mGluRs and NMDA receptors, and is not accompanied by massive influx of extracellular Ca(2+) to neurones.

Inhibition of dendritic spine morphogenesis and synaptic transmission by activity-inducible protein Homer1a

The postsynaptic density (PSD) proteins Shank and Homer cooperate to induce the maturation and enlargement of dendritic spines (Sala et al., 2001). Homer1a is an activity-inducible short-splice variant of Homer that lacks dimerization capacity. Here, we show that Homer1a reduces the density and size of dendritic spines in cultured hippocampal neurons in correlation with an inhibition of Shank targeting to synapses. Expression of Homer1a also decreases the size of PSD-95 clusters, the number of NMDA receptor clusters, and the level of surface AMPA receptors, implying a negative effect on the growth of synapses. In parallel with the morphological effects on synapses, Homer1a-expressing neurons show diminished AMPA and NMDA receptor postsynaptic currents. All of these outcomes required the integrity of the Ena/VASP Homology 1 domain of Homer1a that mediates binding to the PPXXF motif in Shank and other binding partners. Overexpression of the C-terminal region of Shank containing the Homer binding site causes effects similar to those of Homer1a. These data indicate that an association between Shank and the constitutively expressed long-splice variants of Homer (e.g., Homer1b/c) is important for maintaining dendritic-spine structure and synaptic function. Because Homer1a expression is induced by synaptic activity, our results suggest that this splice variant of Homer operates in a negative feedback loop to regulate the structure and function of synapses in an activity-dependent manner.

Induction of dendritic spines by an extracellular domain of AMPA receptor subunit GluR2

Synaptic transmission from excitatory nerve cells in the mammalian brain is largely mediated by AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors located at the surface of dendritic spines. The abundance of postsynaptic AMPA receptors correlates with the size of the synapse and the dimensions of the dendritic spine head. Moreover, long-term potentiation is associated with the formation of dendritic spines as well as synaptic delivery of AMPA receptors. The molecular mechanisms that coordinate AMPA receptor delivery and spine morphogenesis are unknown. Here we show that overexpression of the glutamate receptor 2 (GluR2) subunit of AMPA receptors increases spine size and density in hippocampal neurons, and more remarkably, induces spine formation in GABA-releasing interneurons that normally lack spines. The extracellular N-terminal domain (NTD) of GluR2 is responsible for this effect, and heterologous fusion proteins of the NTD of GluR2 inhibit spine morphogenesis. We propose that the NTD of GluR2 functions at the cell surface as part of a receptor-ligand interaction that is important for spine growth and/or stability.

Lipid- and protein-mediated multimerization of PSD-95: implications for receptor clustering and assembly of synaptic protein networks

Postsynaptic density protein 95 (PSD-95/SAP-90) is a palmitoylated membrane-associated guanylate kinase that oligomerizes and clusters ion channels and associated signaling machinery at excitatory synapses in brain. However, the mechanism for PSD-95 oligomerization and its relationship to ion channel clustering remain uncertain. Here, we find that multimerization of PSD-95 is determined by only its first 13 amino acids, which also have a remarkable capacity to oligomerize heterologous proteins. Multimerization does not involve a covalent linkage but rather palmitoylation of two cysteine residues in the 13 amino acid motif. This lipid-mediated oligomerization is a specific property of the PSD-95 motif, because it is not observed with other palmitoylated domains. Clustering K+ channel Kv1.4 requires interaction of palmitoylated PSD-95 with tetrameric K+ channel subunits but, surprisingly, does not require multimerization of PSD-95. Finally, disrupting palmitoylation with 2-bromopalmitate disperses PSD-95/K+-channel clusters. These data suggest new models for K+ channel clustering by PSD-95 - a reversible process regulated by protein palmitoylation.

Homer-1a/Vesl-1S enhances hippocampal synaptic transmission

Homer/Vesl proteins are involved in regulating metabotropic glutamate receptors, synaptogenesis, dendritic spine development and axonal pathfinding. We investigated the potential modulation of glutamatergic synaptic transmission by the immediate early gene product Homer-1a/Vesl-1S and by the constitutively expressed long-form Homer-1c/Vesl-1L in CA1 pyramidal cells from cultured rat hippocampal slices. Semliki Forest virus vector-mediated overexpression of Homer-1a enhanced alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function, but did not detectably affect N-methyl-d-aspartate (NMDA) receptor function and presynaptic glutamate release. Overexpression of Homer-1c, by contrast, did not alter synaptic transmission. To corroborate our electrophysiological results obtained in slice cultures, we performed quantitative immunocytochemistry in cultures of dissociated hippocampal neurons. Homer-1a also increased synaptic clustering of AMPA but not NMDA receptors, whereas Homer-1c had no detectable effect. Our results show that Homer-1a potentiates synaptic AMPA receptor function, supporting a critical role for Homer-1a in hippocampal synaptic plasticity.

Molecular characterisation of the 22q13 deletion syndrome supports the role of haploinsufficiency of SHANK3/PROSAP2 in the major neurological symptoms

METHODS

The 22q13 deletion syndrome (MIM 606232) is characterised by moderate to profound mental retardation, delay/absence of expressive speech, hypotonia, normal to accelerated growth, and mild dysmorphic features. We have determined the deletion size and parent of origin in 56 patients with this syndrome.

RESULTS

Similar to other terminal deletion syndromes, there was an overabundance of paternal deletions. The deletions vary widely in size, from 130 kb to over 9 Mb; however all 45 cases that could be specifically tested for the terminal region at the site of SHANK3 were deleted for this gene. The molecular structure of SHANK3 was further characterised. Comparison of clinical features to deletion size showed few correlations. Some measures of developmental assessment did correlate to deletion size; however, all patients showed some degree of mental retardation and severe delay or absence of expressive speech, regardless of deletion size.

CONCLUSION

Our analysis therefore supports haploinsufficiency of the gene SHANK3, which codes for a structural protein of the postsynaptic density, as a major causative factor in the neurological symptoms of 22q13 deletion syndrome.

The 'magic tail' of G protein-coupled receptors: an anchorage for functional protein networks

All cell types express a great variety of G protein-coupled receptors (GPCRs) that are coupled to only a limited set of G proteins. This disposition favors cross-talk between transduction pathways. However, GPCRs are organized into functional units. They promote specificity and thus avoid unsuitable cross-talk. New methodologies (mostly yeast two-hybrid screens and proteomics) have been used to discover more than 50 GPCR-associated proteins that are involved in building these units. In addition, these protein networks participate in the trafficking, targeting, signaling, fine-tuning and allosteric regulation of GPCRs. To date, proteins that interact with the GPCR C-terminus are the most abundant and are the focus of this review.

Organization of signaling complexes by PDZ-domain scaffold proteins

Transduction of biological signals from receptors at the plasma membrane to their targets in cytoplasm and nucleus relies on specific protein-protein interactions. A common strategy used by cells is to organize proteins in the same signaling cascade into large molecular weight, multiprotein complexes. PDZ domain proteins have been shown to play important roles in assembling various signaling complexes. Here, we first present biophysical basis of the advantages of organizing proteins in a signaling cascade into a clustered multiprotein complex. We then discuss the structure, ligand binding, and function of PDZ domains in organizing synaptic signaling complexes.

Synapse-associated protein-97 isoform-specific regulation of surface AMPA receptors and synaptic function in cultured neurons

Members of the synapse-associated protein-97 (SAP97) family of scaffold proteins have been implicated as central organizers of synaptic junctions to build macromolecular signaling complexes around specific postsynaptic neurotransmitter receptors. In this regard, SAP97 has been suggested to regulate the synaptic localization of glutamate receptor type 1 subunits of the AMPA-type glutamate receptors. To test this hypothesis directly, we assessed the effects of SAP97 overexpression on surface expression of synaptic AMPA receptors. We find that recombinant SAP97 not only becomes concentrated at synaptic junctions but also leads to an increase in synaptic AMPA receptors, spine enlargement, and an increase in miniature EPSC (mEPSC) frequency, indicating that SAP97 has both postsynaptic and presynaptic effects on synaptic transmission. Synaptic targeting of SAP97, increased surface AMPA receptors, and increased mEPSC frequency are dependent on the presence of specific alternatively spliced sequences in SAP97 that encode a protein 4.1 binding site. These results suggest that SAP97 can affect the synaptic recruitment of AMPA receptors and spine morphology and that these effects may be regulated by alternative splicing.

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.

Lipid rafts in the maintenance of synapses, dendritic spines, and surface AMPA receptor stability

Cholesterol/sphingolipid microdomains (lipid rafts) in the membrane are involved in protein trafficking, formation of signaling complexes, and regulation of actin cytoskeleton. Here, we show that lipid rafts exist abundantly in dendrites of cultured hippocampal neurons, in which they are associated with several postsynaptic proteins including surface AMPA receptors. Depletion of cholesterol/sphingolipid leads to instability of surface AMPA receptors and gradual loss of synapses (both inhibitory and excitatory) and dendritic spines. The remaining synapses and spines in raft-depleted neurons become greatly enlarged. The importance of lipid rafts for normal synapse density and morphology could explain why cholesterol promotes synapse maturation in retinal ganglion cells (Mauch et al., 2001) and offers a potential link between disordered cholesterol metabolism and the synapse loss seen in neurodegenerative disease.

The Shank family of postsynaptic density proteins interacts with and promotes synaptic accumulation of the beta PIX guanine nucleotide exchange factor for Rac1 and Cdc42

The Shank/ProSAP family of multidomain proteins is known to play an important role in organizing synaptic multiprotein complexes. Here we report a novel interaction between Shank and beta PIX, a guanine nucleotide exchange factor for the Rac1 and Cdc42 small GTPases. This interaction is mediated by the PDZ domain of Shank and the C-terminal leucine zipper domain and the PDZ domain-binding motif at the extreme C terminus of beta PIX. Shank colocalizes with beta PIX at excitatory synaptic sites in cultured neurons. In brain, Shank forms a complex with beta PIX and beta PIX-associated signaling molecules including p21-associated kinase (PAK), an effector kinase of Rac1/Cdc42. Importantly, overexpression of Shank in cultured neurons promotes synaptic accumulation of beta PIX and PAK. Considering the involvement of Rac1 and PAK in spine dynamics, these results suggest that Shank recruits beta PIX and PAK to spines for the regulation of postsynaptic structure.

Synchronized formation and remodeling of postsynaptic densities: long-term visualization of hippocampal neurons expressing postsynaptic density proteins tagged with green fluorescent protein

To explore mechanisms governing the formation and remodeling of postsynaptic density (PSD), we used dissociated cultures of hippocampal neurons isolated from transgenic embryos expressing green fluorescent protein (GFP)-tagged PSD proteins PSD-Zip45 (Homer 1c) and PSD-95. Expression of GFP-tagged PSD molecules was stable, and the remodeling process of PSDs could be followed for >1 week. A higher expression level of GFP-PSD-Zip45 enabled us to quantitatively analyze the amount of PSD-Zip45 clusters during development. Repetitive imaging of the same cell populations between 11 and 17 d in culture revealed an increase of the average PSD-Zip45 cluster density from 0.32 to 0.73/microm. Newly generated dendrites rapidly acquired GFP-PSD-Zip45 clusters, and their density reached the level of parental dendrites within a few days. Temporal profiles of GFP-PSD-Zip45 cluster density showed a variety of patterns. Some dendrites showed a monotonous increase of clusters, whereas others showed complex patterns, including short decremental stages. Analysis of long-term remodeling of PSD-95-GFP clusters confirmed that the decremental stages were not specific to the PSD-Zip45 clusters. Comparison of the temporal profiles of the cluster density among neurons indicated synchronization of both GFP-PSD-Zip45 and PSD-95 clustering within individual cells. Furthermore, activation of cAMP-dependent protein kinase suppressed the decremental stages of cluster remodeling. These observations suggest the presence of signaling mechanisms that can induce synchronized addition or elimination of PSD proteins throughout dendritic arborization of a single neuron.

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

PSD-Zip45/Homer1c, which contains an enabled/VASP homology 1 (EVH1) domain and leucine zipper motifs, is a postsynaptic density (PSD) scaffold protein that interacts with metabotropic glutamate receptors and the shank family. We studied the molecular mechanism underlying the synaptic targeting of PSD-Zip45 in cultured hippocampal neurons. The EVH1 domain and the extreme C-terminal leucine zipper motif were molecular determinants for its synaptic targeting. The overexpression of the mutant of the EVH1 domain or deletion of the extreme C-terminal leucine zipper motif markedly suppressed the synaptic localization of endogenous shank but not PSD-95 or GKAP. In contrast, an overexpressed GKAP mutant lacking shank binding activity had no effect on the synaptic localization of shank. Actin depolymerization by latrunculin A reduced the synaptic localization of PSD-Zip45, shank, and F-actin but not of PSD-95 or GKAP. Overexpression of PSD-Zip45 enhanced the accumulation of synaptic F-actin. Additionally, overexpression of PSD-Zip45 and an isoform of shank induced synaptic enlargement in association with the further accumulation of synaptic F-actin. The EVH1 domain and extreme C-terminal leucine zipper motif of PSD-Zip45 were also critical for these events. Thus, these data suggest that the PSD-Zip45-shank and PSD-95-GKAP complexes form different synaptic compartments, and PSD-Zip45 alone or PSD-Zip45-shank is involved in the synaptic accumulation of F-actin.

Coincidence in dendritic clustering and synaptic targeting of homer proteins and NMDA receptor complex proteins NR2B and PSD95 during development of cultured hippocampal neurons

Homer is a scaffold protein that binds glutamate receptor complexes and actin cytoskeleton in postsynapses. The present study analyzed developmental changes in subcellular localization of Homer proteins in cultured hippocampal neurons. All three Homer family proteins, Homer 1b/c, Cupidin/Homer 2, and Homer 3, not only form heteromeric coclusters, but also localize close to the NMDA receptor complex including the NR2B subunit and PSD95 throughout dendritic and synaptic differentiation. Synaptic clustering of Homer proteins is enhanced by simultaneous blockade of NMDA receptor and cAMP phosphodiesterase activities, as is clustering of NMDA receptors. Homer proteins colocalize with actin-cytoskeletal proteins F-actin and Drebrin partially during the middle stage and to a greater extent in the late stage, and with the GluR1 subunit of AMPA receptors only in the late stage. Clustering sites of Homer are not synaptic in early-middle stages, but become synaptic in the late stage, as deduced from synaptic targeting of Bassoon, Synaptophysin, and N-cadherin. Our results indicate a coincidence in dendritic clustering in addition to developmental and activity-regulated synaptic targeting between Homer and the NMDA receptor complex.

Association of the type 1 inositol (1,4,5)-trisphosphate receptor with 4.1N protein in neurons

The type 1 inositol (1,4,5)-trisphosphate receptor (InsP(3)R1) is an intracellular calcium (Ca(2+)) release channel that plays an important role in neuronal function. In yeast two-hybrid screen of rat brain cDNA library with the InsP(3)R1 carboxy-terminal bait we isolated multiple clones of neuronal cytoskeletal protein 4.1N. We mapped the 4.1N-interaction site to a short fragment (50 amino acids) within the carboxy-terminal tail of the InsP(3)R1 and the InsP(3)R1-interaction site to the carboxy-terminal domain (CTD) of 4.1N. We established that InsP(3)R1 carboxy-terminal binds selectively to the CTDDelta alternatively spliced form of the 4.1N protein. In biochemical experiments we demonstrated that 4.1N and InsP(3)R1 specifically associate in vitro. We showed that both 4.1N and InsP(3)R1 were enriched in synaptic locations and immunoprecipitated the 4.1N-InsP(3)R1 complex from rat brain synaptosomes. In biochemical experiments we demonstrated a possibility of InsP(3)R1-4.1N-CASK-syndecan-2 quaternary complex formation. From our findings we hypothesize that InsP(3)R1-4.1N association may play a role in InsP(3)R1 localization or Ca(2+) signaling in neurons.

Assembly and plasticity of the glutamatergic postsynaptic specialization

Glutamate mediates most excitatory synaptic transmission in the brain. Synaptic strength at glutamatergic synapses shows a remarkable degree of use-dependent plasticity and such modifications may represent a physiological correlate to learning and memory. Glutamate receptors and downstream enzymes are organized at synapses by cytoskeletal proteins containing multiple protein-interacting domains. Recent studies demonstrate that these 'scaffolding' proteins within the postsynaptic specialization have the capacity to promote synaptic maturation, influence synapse size, and modulate glutamate receptor function.

Rapid induction of dendritic spine morphogenesis by trans-synaptic ephrinB-EphB receptor activation of the Rho-GEF kalirin

The morphogenesis of dendritic spines, the major sites of excitatory synaptic transmission in the brain, is important in synaptic development and plasticity. We have identified an ephrinB-EphB receptor trans-synaptic signaling pathway which regulates the morphogenesis and maturation of dendritic spines in hippocampal neurons. Activation of the EphB receptor induces translocation of the Rho-GEF kalirin to synapses and activation of Rac1 and its effector PAK. Overexpression of dominant-negative EphB receptor, catalytically inactive kalirin, or dominant-negative Rac1, or inhibition of PAK eliminates ephrin-induced spine development. This novel signal transduction pathway may be critical for the regulation of the actin cytoskeleton controlling spine morphogenesis during development and plasticity.

The postsynaptic density and dendritic raft localization of PSD-Zip70, which contains an N-myristoylation sequence and leucine-zipper motifs

The postsynaptic site of the excitatory synapse, which is composed of the postsynaptic density (PSD) attached to the postsynaptic membrane, is a center for synaptic plasticity. To reveal the molecular organization and functional regulation of the postsynaptic site, we cloned a 70 kDa protein that is concentrated in PSDs using a monoclonal antibody against the PSD. This protein, named PSD-Zip70, is highly homologous to the human FEZ1/LZTS1 gene product. PSD-Zip70 contains an N-myristoylation consensus sequence, a polybasic cluster in the N-terminal region and four leucine-zipper motifs in the C-terminal region. Light and electron microscopy showed that this protein was localized to the dendritic spines, especially in the PSD and the postsynaptic membrane. Fractionation of the synaptic plasma membrane demonstrated that PSD-Zip70 was localized to the PSD and the dendritic raft. In Madin-Darby canine kidney (MDCK) cells, exogenous PSD-Zip70 was targeted to the apical plasma membrane of microvilli, and its N-myristoylation was necessary for this targeting. In hippocampal neurons, N-myristoylation was also required for the membrane localization and the C-terminal region was critically involved in the synaptic targeting. These results suggest that PSD-Zip70 may be involved in the dynamic properties of the structure and function of the postsynaptic site.

Birth, growth and elimination of a single synapse

Synapses are functional units regulating information flows in the neuronal circuits. How synaptic junctions are formed and remodelled is a fundamental question in developmental neurobiology. In recent years, it has become possible to visualize the formation, maintenance and remodelling of a single synapse by using new imaging methods. These studies, identifying synaptic structures by lipophilic dye markers and genetically modified synaptic molecules with fluorescent proteins, provided new insights into synapse development and maturation. Experimental evidence indicates very rapid assembly of both presynaptic and postsynaptic marker proteins at newly formed synaptic junctions. Morphological expansion of the synaptic junctional membrane is tightly coupled to both efficacy of the presynaptic neurotransmitter release and postsynaptic receptor distribution. The elimination process of pre-existing synapses has also been reported, and evidence for persistent remodelling of synaptic junctions has been provided. Information regarding birth, maturation and elimination of a single synapse is accumulating and will influence our concepts about how neuronal circuits are organized and maintained.

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

The ProSAP/Shank family of multidomain proteins of the postsynaptic density (PSD) can either directly or indirectly interact with NMDA-type and metabotropic glutamate receptors and the actin-based cytoskeleton. In a yeast two hybrid screen utilizing a proline-rich domain that is highly conserved among the ProSAP/Shank family members, we isolated several cDNA clones coding for the insulin receptor substrate IRSp53. The specificity of this interaction was confirmed in transfected COS cells. Co-immunoprecipitation of IRSp53 and ProSAP2 solubilized from rat brain membranes indicates that the interaction occurs in vivo. The C-terminal SH3 domain of IRSp53 is responsible for the interaction with a novel proline-rich consensus sequence of ProSAP/Shank that was characterized by mutational analysis. IRSp53 is a substrate for the insulin receptor in the brain and acts downstream of small GTPases of the Rho family. Binding of Cdc42Hs to IRSp53 induces actin filament assembly, reorganization and filopodia outgrowth in neuronal cell lines. Our data suggest that IRSp53 can be recruited to the PSD via its ProSAP/Shank interaction and may contribute to the morphological reorganization of spines and synapses after insulin receptor and/or Cdc42Hs activation.

Homer proteins and InsP(3) receptors co-localise in the longitudinal sarcoplasmic reticulum of skeletal muscle fibres

Striated muscle represents one of the best models for studies on Ca(2+) signalling. However, although much is known on the localisation and molecular interactions of the ryanodine receptors (RyRs), far less is known on the localisation and on the molecular interactions of the inositol trisphosphate receptors (InsP(3)Rs) in striated muscle cells. Recently, members of the Homer protein family have been shown to cluster type 1 metabotropic glutamate receptors (mGluR1) in the plasma membrane and to interact with InsP(3)R in the endoplasmic reticulum of neurons. Thus, these scaffolding proteins are good candidates for organising plasma membrane receptors and intracellular effector proteins in signalosomes involved in intracellular Ca(2+) signalling. Homer proteins are also expressed in skeletal muscle, and the type 1 ryanodine receptor (RyR1) contains a specific Homer-binding motif. We report here on the relative sub-cellular localisation of InsP(3)Rs and Homer proteins in skeletal muscle cells with respect to the localisation of RyRs. Immunofluorescence analysis showed that both Homer and InsP(3)R proteins present a staining pattern indicative of a localisation at the Z-line, clearly distinct from that of RyR1. Consistent herewith, in sub-cellular fractionation experiments, Homer proteins and InsP(3)R were both found in the fractions enriched in longitudinal sarcoplasmic reticulum (LSR) but not in fractions of terminal cisternae that are enriched in RyRs. Thus, in skeletal muscle, Homer proteins may play a role in the organisation of a second Ca(2+) signalling compartment containing the InsP(3)R, but are apparently not involved in the organisation of RyRs at triads.

Changing views of Cajal's neuron: the case of the dendritic spine

Ever since dendritic spines were first described in detail by Santiago Ramón y Cajal, they were assumed to underlie the physical substrate of long term memory in the brain. Recent time-lapse imaging of dendritic spines in live tissue, using confocal microscopy, have revealed an amazingly plastic structure, which undergoes continuous changes in shape and size, not intuitively related to its assumed role in long term memory. Functionally, the spine is shown to be an independent cellular compartment, able to regulate calcium concentration independently of its parent dendrite. The shape of the spine is instrumental in regulating the link between the synapse and the parent dendrite such that longer spines have less impact on the dendrite than shorter ones. The spine can be formed, change its shape and disappear in response to afferent stimulation, in a dynamic fashion, indicating that spine morphology is an important vehicle for structuring synaptic interactions. While this role is crucial in the developing nervous system, large variations in spine densities in the adult brain indicate that tuning of synaptic impact may be a role of spines throughout the life of a neuron.

Postsynaptic targeting of alternative postsynaptic density-95 isoforms by distinct mechanisms

Members of the postsynaptic density-95 (PSD95)/synapse-associated protein-90 (SAP90) family of scaffolding proteins contain a common set of modular protein interaction motifs including PDZ (postsynaptic density-95/Discs large/zona occludens-1), Src homology 3, and guanylate kinase domains, which regulate signaling and plasticity at excitatory synapses. We report that N-terminal alternative splicing of PSD95 generates an isoform, PSD95beta that contains an additional "L27" motif, which is also present in SAP97. Using yeast two hybrid and coimmunoprecipitation assays, we demonstrate that this N-terminal L27 domain of PSD-95beta, binds to an L27 domain in the membrane-associated guanylate kinase calcium/calmodulin-dependent serine kinase, and to Hrs, an endosomal ATPase that regulates vesicular trafficking. By transfecting heterologous cells and hippocampal neurons, we find that interactions with the L27 domain regulate synaptic clustering of PSD95beta. Disrupting Hrs-regulated early endosomal sorting in hippocampal neurons selectively blocks synaptic clustering of PSD95beta but does not interfere with trafficking of the palmitoylated isoform, PSD95alpha. These studies identify molecular and functional heterogeneity in synaptic PSD95 complexes and reveal critical roles for L27 domain interactions and Hrs regulated vesicular trafficking in postsynaptic protein clustering.

Metabotropic glutamate receptor 1-induced upregulation of NMDA receptor current: mediation through the Pyk2/Src-family kinase pathway in cortical neurons

The mechanism underlying the upregulation of NMDA receptor function by group I metabotropic glutamate receptors (mGluRs), including mGluR1 and 5, is not known. Here we show that in cortical neurons, brief selective activation of group I mGluRs with (S)-3,5-dihydroxy-phenylglycine (DHPG) induced a Ca(2+)-calmodulin-dependent activation of Pyk2/CAKbeta and the Src-family kinases Src and Fyn that was independent of protein kinase C (PKC). Activation of Pyk2 and Src/Fyn kinases led to increased tyrosine phosphorylation of NMDA receptor subunits 2A and B (NR2A/B) and was blocked by a selective mGluR1 antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, but not an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine. Functional linkage between mGluR1 activation and NR2A tyrosine phosphorylation through Pyk2 and Src was also demonstrated after expression of these elements in human embryonic kidney 293 cells. Supporting functional consequences, selective activation of mGluR1 by DHPG induced a potentiation of NMDA receptor-mediated currents that was blocked by inhibiting mGluR1 or Src-family kinases. Furthermore, antagonizing calmodulin or mGluR1, but not PKC, reduced the basal tyrosine phosphorylation levels of Pyk2 and Src, suggesting that mGluR1 may control the basal activity of these kinases and thus the tyrosine phosphorylation levels of NMDA receptors.

A new technique to co-localise membrane proteins with Homer/vesl

The minimal requirements were defined as necessary for cluster formation of the group 1 metabotropic glutamate receptor (mGluR), which is regulated by the Homer/vesl family of scaffolding proteins [Curr. Opin. Neurobiol. 10 (2000) 370]. Cluster formation of G-protein-coupled receptors (GPCRs) plays a fundamental role in signal transduction, particularly at the neuronal synapse. To understand the interaction of mGluR with PSD-Zip45, a Homer/vesl family member, we designed a series of chimeric receptor proteins, consisting of C-terminal mGluR1alpha sequences that were fused to endothelin B receptors (ET(B)Rs). In vitro and in vivo studies revealed that an extended 20 amino acid long C-terminal mGluR1alpha peptide, including the proline-rich core motif PPXXF, is sufficient to induce clustering of chimeric ET(B)R/mGluR1alpha receptors by PSD-Zip45. This result is especially important because it constitutes the basis for a new approach to form two-dimensional crystals of membrane proteins in situ, which may render unstable membrane proteins amenable to electron crystallographic structure determination.

Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice

In vitro studies indicate a role for the LIM kinase family in the regulation of cofilin phosphorylation and actin dynamics. In addition, abnormal expression of LIMK-1 is associated with Williams syndrome, a mental disorder with profound deficits in visuospatial cognition. However, the in vivo function of this family of kinases remains elusive. Using LIMK-1 knockout mice, we demonstrate a significant role for LIMK-1 in vivo in regulating cofilin and the actin cytoskeleton. Furthermore, we show that the knockout mice exhibited significant abnormalities in spine morphology and in synaptic function, including enhanced hippocampal long-term potentiation. The knockout mice also showed altered fear responses and spatial learning. These results indicate that LIMK-1 plays a critical role in dendritic spine morphogenesis and brain function.

Cadherin regulates dendritic spine morphogenesis

Synaptic remodeling has been postulated as a mechanism underlying synaptic plasticity, and cadherin adhesion molecules are thought to be a regulator of such a process. We examined the effects of cadherin blockage on synaptogenesis in cultured hippocampal neurons. This blockade resulted in alterations of dendritic spine morphology, such as filopodia-like elongation of the spine and bifurcation of its head structure, along with concomitant disruption of the distribution of postsynaptic proteins. The accumulation of synapsin at presynaptic sites and synaptic vesicle recycling were also perturbed, although these synaptic responses to the cadherin blockade became less evident upon the maturation of the synapses. These findings suggest that cadherin regulates dendritic spine morphogenesis and related synaptic functions, presumably cooperating with cadherin-independent adhesive mechanisms to maintain spine-axon contacts.

Head, neck, and spines: a role for LIMK-1 in the hippocampus

LIM kinase 1 regulates actin filament dynamics through inhibition of ADF/cofilins. Surprisingly, nervous system development in LIM kinase 1 knockout mice is grossly normal, but the animals have deficits in spatial learning, alterations in LTP, and abnormalities in hippocampal dendritic spine structure. The findings are consistent with a role for LIMK-1 in synapse formation and function.

Homer as both a scaffold and transduction molecule

Increasing evidence shows that scaffold proteins not only control membrane assembly of receptors and channels, but also modulate intracellular signaling by assembled receptors. The Homer family of proteins act as scaffolds to bind clusters of proteins and glutamate receptors at postsynaptic sites. We review results of cloning and gene expression of this protein family, and summarize roles in glutamate receptor function and intracellular signaling in neurons. Homer proteins trigger the localization of metabotropic glutamate receptor subtype 5 (mGlu5 receptor) to the postsynaptic plasma membrane. They can also alter the kinetics and peak amplitude of the intracellular Ca2+ responses of mGlu1 and mGlu5 receptors. Homer proteins can either prevent or promote spontaneous activation of these receptors, depending on the type of Homer protein isoform expressed.

ProSAP/Shank proteins - a family of higher order organizing molecules of the postsynaptic density with an emerging role in human neurological disease

The postsynaptic density (PSD) is a specialized electron-dense structure underneath the postsynaptic plasmamembrane of excitatory synapses. It is thought to anchor and cluster glutamate receptors exactly opposite to the presynaptic neurotransmitter release site. Various efforts to study the molecular structure of the PSD identified several new proteins including membrane receptors, cell adhesion molecules, components of signalling cascades, cytoskeletal elements and adaptor proteins with scaffolding functions to interconnect these PSD components. The characterization of a novel adaptor protein family, the ProSAPs or Shanks, sheds new light on the basic structural organization of the PSD. ProSAPs/Shanks are multidomain proteins that interact directly or indirectly with receptors of the postsynaptic membrane including NMDA-type and metabotropic glutamate receptors, and the actin-based cytoskeleton. These interactions suggest that ProSAP/Shanks may be important scaffolding molecules of the PSD with a crucial role in the assembly of the PSD during synaptogenesis, in synaptic plasticity and in the regulation of dendritic spine morphology. Moreover the analysis of a patient with 22q13.3 distal deletion syndrome revealed a balanced translocation with a breakpoint in the human ProSAP2/Shank3 gene. This ProSAP2/Shank3 haploinsufficiency may cause a syndrome that is characterized by severe expressive language delay, mild mental retardation and minor facial dysmorphisms.

Nuclear calcium signaling evoked by cholinergic stimulation in hippocampal CA1 pyramidal neurons

The cholinergic system is thought to play an important role in hippocampal-dependent learning and memory. However, the mechanism of action of the cholinergic system in these actions in not well understood. Here we examined the effect of muscarinic receptor stimulation in hippocampal CA1 pyramidal neurons using whole-cell recordings in acute brain slices coupled with high-speed imaging of intracellular calcium. Activation of muscarinic acetylcholine receptors by synaptic stimulation of cholinergic afferents or application of muscarinic agonist in CA1 pyramidal neurons evoked a focal rise in free calcium in the apical dendrite that propagated as a wave into the soma and invaded the nucleus. The calcium rise to a single action potential was reduced during muscarinic stimulation. Conversely, the calcium rise during trains of action potentials was enhanced during muscarinic stimulation. The enhancement of free intracellular calcium was most pronounced in the soma and nuclear regions. In many cases, the calcium rise was distinguished by a clear inflection in the rising phase of the calcium transient, indicative of a regenerative response. Both calcium waves and the amplification of action potential-induced calcium transients were blocked the emptying of intracellular calcium stores or by antagonism of inositol 1,4,5-trisphosphate receptors with heparin or caffeine. Ryanodine receptors were not essential for the calcium waves or enhancement of calcium responses. Because rises in nuclear calcium are known to initiate the transcription of novel genes, we suggest that these actions of cholinergic stimulation may underlie its effects on learning and memory.

Molecular mechanisms of CNS synaptogenesis

Synapses of the mammalian CNS are asymmetric sites of cell-cell adhesion between nerve cells. They are designed to mediate the rapid and efficient transmission of signals from the presynaptic bouton of one neuron to the postsynaptic plasma membrane of a second neuron. Significant progress has been made in the characterization of the structural, functional and developmental assembly of CNS synapses. Recent progress has been made in understanding the molecular and cellular mechanisms that underlie synaptogenesis, in particular that of glutamatergic synapses of the CNS.

Modulation of synaptic signalling complexes by Homer proteins

The number of neurotransmitter receptors in the postsynaptic membrane and their functional coupling to intracellular signalling cascades are important determinants of synaptic strength--and hence potential targets for plasticity related modulation. In this context, Homer/Vesl proteins have gained particular interest for three main reasons: (i) they constitute part of the molecular scaffold at postsynaptic densities of excitatory synapses in the mammalian brain; (ii) they physically link type-I metabotropic glutamate receptors to the postsynaptic density and to inositol 1,4,5-triphosphate receptors in the subsynaptic endoplasmic reticulum; and (iii) Homer-1a, which has been categorized as an immediate early gene isoform, exerts dominant-negative activity, suggesting that it is involved in activity dependent rearrangements at synaptic junctions. Although these fundamental aspects have been reviewed previously by Xiao et al., this review will address primarily more recent studies on the regulation of Homer 1a expression and on the role of Homer/Vesl proteins in spine morphogenesis and receptor targeting and signalling.

Rapid turnover of actin in dendritic spines and its regulation by activity

Dendritic spines are motile structures that contain high concentrations of filamentous actin. Using hippocampal neurons expressing fluorescent actin and the method of fluorescence recovery after photobleaching, we found that 85 +/- 2% of actin in the spine was dynamic, with a turnover time of 44.2 +/- 4.0 s. The rapid turnover is not compatible with current models invoking a large population of stable filaments and static coupling of filaments to postsynaptic components. Low-frequency stimulation known to induce long-term depression in these neurons stabilized nearly half the dynamic actin in the spine. This effect depended on the activation of N-methyl-D-aspartate (NMDA) receptors and the influx of calcium. In neurons from mice lacking gelsolin, a calcium-dependent actin-binding protein, activity-dependent stabilization of actin was impaired. Our studies provide new information on the kinetics of actin turnover in spines, its regulation by neural activity and the mechanisms involved in this regulation.

Synaptic activity-induced conversion of intronic to exonic sequence in Homer 1 immediate early gene expression

Three Homer genes regulate the activity of metabotropic glutamate receptors mGluR1a and mGluR5 and their coupling to releasable intracellular Ca2+ pools and ion channels. Only the Homer 1 gene evolved bimodal expression of constitutive (Homer 1b and c) and immediate early gene (IEG) products (Homer 1a and Ania 3). The IEG forms compete functionally with the constitutive Homer proteins. The complex expression of the Homer 1 gene, unique for IEGs, focused our attention on the gene organization. In contrast to most IEGs, which have genes that are <5 kb, the Homer 1 gene was found to span approximately 100 kb. The constitutive Homer 1b/c forms are encoded by exons 1-10, whereas the IEG forms are encoded by exons 1-5 and parts of intron 5. RNase protection demonstrated a >10-fold activity-dependent increase in mRNA levels exclusively for the IEG forms. Moreover, fluorescent in situ hybridization documented that new primary Homer 1 transcripts are induced in neuronal nuclei within a few minutes after seizure, typical of IEGs, and that Homer 1b-specific exons are excluded from the activity-induced transcripts. Thus, at the resting state of the neurons, the entire gene is constitutively transcribed at low levels to yield Homer 1b/c transcripts. Neuronal activity sharply increases the rate of transcription initiation, with most transcripts now ending within the central intron. These coordinate transcriptional events rapidly convert a constitutive gene to an IEG and regulate the expression of functionally different Homer 1 proteins.

Doing (F/L)PPPPs: EVH1 domains and their proline-rich partners in cell polarity and migration

Actin filament assembly is a tightly regulated process that functions in many aspects of cell physiology. Members of the Ena/VASP (Drosophila Enabled/vasodilator-stimulated phosphoprotein) family are key players in regulating actin filament assembly, in many cases through their association with binding partners that display a particular proline-rich motif, FPPPP. Ena/VASP proteins interact with these partners via the highly conserved Ena/VASP homology 1 (EVH1) domain. The diverse array of binding partners for EVH1 domains, including cytoskeletal proteins such as zyxin, transmembrane guidance receptors such as Roundabout, and the T-cell signaling protein Fyb/SLAP, shows that these interactions are likely to be important in a number of cellular processes that require regulated actin filament assembly.

Mutation of Drosophila homer disrupts control of locomotor activity and behavioral plasticity

Homer proteins have been proposed to play a role in synaptogenesis, synapse function, receptor trafficking, and axon pathfinding. Here we report the isolation and characterization of the Drosophila gene homer, the single Homer-related gene in fly. Using anti-Homer antibody we show that Homer is expressed in a broad range of tissues but is highly enriched in the CNS. Similarly to its mammalian counterpart, the Drosophila Homer localizes to the dendrites and the endoplasmic reticulum (ER). This subcellular distribution is dependent on an intact Enabled/Vasp homology 1 domain, suggesting that Homer must bind to one or more of its partners for proper localization. We have created a mutation of homer and show that flies homozygous for this mutation are viable and show coordinated locomotion, suggesting that Homer is not essential for basic neurotransmission. However, we found that homer mutants display defects in behavioral plasticity and the control of locomotor activity. Our results argue that in the CNS, Homer-related proteins operate in the ER and in dendrites to regulate the development and function of neural networks underlying locomotor control and behavioral plasticity.

New building blocks for the dendritic spine

Two recent studies by Sheng and associates (Pak et al., 2001; Sala et al., 2001) provide an elegant molecular analysis of the role of a spine-specific protein, SPAR, and the synaptic proteins Shank and Homer, in regulating dendritic spine morphology, and the possible functional consequences of this regulation.