Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intracellular pool of AMPA receptors in rat brain. By velocity gradient centrifugation of microsomal light membranes from rat brain, we identified a membrane fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA receptors. This membrane compartment sedimented more slowly than synaptosomes but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 and syntaxin-13. Morphological examination of this fraction revealed round and tubular vesicles ranging from approximately 50 to 300 nm in diameter. Immunocytochemistry of cultured hippocampal neurons showed that a significant portion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associated with tubulovesicular recycling endosomes) and with transferrin receptors. Taken together, these results suggest that a pool of intracellular GluR2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA receptors.

[Effect of radix angelicae sinensis decoction for supplementing blood on inhibiting the increase of endothelial cell monolayer permeability induced by hypoosmotic solution]

OBJECTIVE

To study effect of radix Angelicae sinensis decoction for supplementing blood (RASDSB) on inhibiting the increase of endothelial cell monolayer permeability induced by hypoosmotic solution.

METHODS

The endothelial cells isolated from newborn bovine aorta were cultured on polycarbonate microporous filter membrane to develop compact endothelial monolayer. Fluid filtration coefficient (Kf), filtration volume (Jv) and osmotic reflective coefficient (sigma) to protein of the endothelial monolayer were measured treated by hypoosmotic solution (changing concentration of serum in M199 solution from 20% to 2%) for 120 min or by hypoosmotic solution containing 10(-4) g x ml(-1) RASDSB for 120 min after perfused Hanks balanced salt solution containing 5 g x L(-1) albumin.

RESULTS

Kf and Jv of the endothelial monolayer treated by hypoosmotic solution increased and sigma of that decreased. RASDSB could inhibit above-mentioned change. Morphological analysis demonstrated that RASDSB could inhibit widening of intercellular distance and enlargement of cellular area in the endothelial monolayer induced by hypoosmolality.

CONCLUSION

Hypoosmotic solution could increase endothelial cell monolayer permeability and RASDSB could inhibit the increase.

PDZ domains and the organization of supramolecular complexes

PDZ domains are modular protein interaction domains that bind in a sequence-specific fashion to short C-terminal peptides or internal peptides that fold in a beta-finger. The diversity of PDZ binding specificities can be explained by variable amino acids lining the peptide-binding groove of the PDZ domain. Abundantly represented in Caenorhabditis elegans, Drosophila melanogaster, and mammalian genomes, PDZ domains are frequently found in multiple copies or are associated with other protein-binding motifs in multidomain scaffold proteins. PDZ-containing proteins are typically involved in the assembly of supramolecular complexes that perform localized signaling functions at particular subcellular locations. Organization around a PDZ-based scaffold allows the stable localization of interacting proteins and enhances the rate and fidelity of signal transduction within the complex. Some PDZ-containing proteins are more dynamically regulated in distribution and may also be involved in the trafficking of interacting proteins within the cell.

Subunit-specific temporal and spatial patterns of AMPA receptor exocytosis in hippocampal neurons

Using a thrombin cleavage assay in cultured hippocampal neurons, we studied the kinetics, regulation and site of AMPA receptor surface delivery. Surface insertion of the GluR1 subunit occurs slowly in basal conditions and is stimulated by NMDA receptor activation and insulin, whereas GluR2 exocytosis is constitutively rapid. Although both subunits ultimately concentrate in synapses, GluR1 and GluR2 show different spatial patterns of surface accumulation, consistent with GluR1 being inserted initially at extrasynaptic sites and GluR2 being inserted more directly at synapses. The spatiotemporal pattern of surface accumulation is determined by the cytoplasmic tails of GluR subunits, and in heteromeric receptors, GluR1 acts dominantly over GluR2. We propose that GluR1 controls the exocytosis and GluR2/3, the recycling and endocytosis of AMPA receptors.

Regulation of dendritic spine morphology by SPAR, a PSD-95-associated RapGAP

The PSD-95/SAP90 family of scaffold proteins organizes the postsynaptic density (PSD) and regulates NMDA receptor signaling at excitatory synapses. We report that SPAR, a Rap-specific GTPase-activating protein (RapGAP), interacts with the guanylate kinase-like domain of PSD-95 and forms a complex with PSD-95 and NMDA receptors in brain. In heterologous cells, SPAR reorganizes the actin cytoskeleton and recruits PSD-95 to F-actin. In hippocampal neurons, SPAR localizes to dendritic spines and causes enlargement of spine heads, many of which adopt an irregular appearance with putative multiple synapses. Dominant negative SPAR constructs cause narrowing and elongation of spines. The effects of SPAR on spine morphology depend on the RapGAP and actin-interacting domains, implicating Rap signaling in the regulation of postsynaptic structure.

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

The Shank family of proteins interacts with NMDA receptor and metabotropic glutamate receptor complexes in the postsynaptic density (PSD). Targeted to the PSD by a PDZ-dependent mechanism, Shank promotes the maturation of dendritic spines and the enlargement of spine heads via its ability to recruit Homer to postsynaptic sites. Shank and Homer cooperate to induce accumulation of IP3 receptors in dendritic spines and formation of putative multisynapse spines. In addition, postsynaptic expression of Shank enhances presynaptic function, as measured by increased minifrequency and FM4-64 uptake. These data suggest a central role for the Shank scaffold in the structural and functional organization of the dendritic spine and synaptic junction.

Antibodies in haystacks: how selection strategy influences the outcome of selection from molecular diversity libraries

Antibodies against most antigens can be isolated from high quality phage antibody libraries. However, not all antibodies binding a particular antigen are necessarily found when standard selections are performed. Here we investigate the effect of two different selection strategies on the isolation of antibodies against a number of different antigens, and find that these different strategies tend to select different antibodies, with little overlap between them. This indicates that the full diversity of these libraries is not tapped by a single selection strategy and that each selection strategy imposes different selective criteria in addition to that of antigen binding. To fully exploit such libraries, therefore, many different selection strategies should probably be employed for each antigen. The use of alternative strategies should be considered when selection apparently fails, or when the number of different antibodies recognizing an antigen needs to be maximised. Furthermore, the microtitre selection strategy developed is likely to prove useful in the application of phage antibody libraries to the human genome project, allowing the high throughput selection of antibodies against multiple antigens simultaneously.

Molecular organization of the postsynaptic specialization

A specific set of molecules including glutamate receptors is targeted to the postsynaptic specialization of excitatory synapses in the brain, gathering in a structure known as the postsynaptic density (PSD). Synaptic targeting of glutamate receptors depends on interactions between the C-terminal tails of receptor subunits and specific PDZ domain-containing scaffold proteins in the PSD. These scaffold proteins assemble a specialized protein complex around each class of glutamate receptor that functions in signal transduction, cytoskeletal anchoring, and trafficking of the receptors. Among the glutamate receptor subtypes, the N-methyl-d-aspartate receptor is relatively stably integrated in the PSD, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor moves in and out of the postsynaptic membrane in highly dynamic fashion. The distinctive cell biological behaviors of N-methyl-d-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors can be explained by their differential interactions with cytoplasmic proteins.

Bipartite interaction between neurofibromatosis type I protein (neurofibromin) and syndecan transmembrane heparan sulfate proteoglycans

The neurofibromatosis type 1 (NF1) gene encodes a large tumor suppressor protein (neurofibromin). Although it is known to possess Ras GTPase-activating protein (GAP) activity, the cellular role of neurofibromin remains unclear. Here we used yeast two-hybrid screening to identify neurofibromin-interacting proteins. Syndecan-2, a transmembrane heparan sulfate proteoglycan (HSPG), was isolated as a binding partner for two distinct regions of the neurofibromin protein. We subsequently found that neurofibromin can bind all four mammalian syndecans. NF1 interaction requires the transmembrane domain and a membrane-proximal region of the cytoplasmic tail of syndecan, but not the C terminus of syndecan known to bind to CASK, a membrane-associated guanylate kinase (MAGUK). Neurofibromin, syndecans, and CASK have overlapping subcellular distributions in axons and synapses of neurons, as shown by biochemical fractionation and immunostaining. Moreover, neurofibromin exists in a complex with syndecan and CASK in vivo, as evidenced by their coimmunoprecipitation from rat brain. Our findings suggest that interaction with different members of the syndecan family may be a mechanism for localizing neurofibromin to specialized domains of the plasma membrane.

Proteolysis of glutamate receptor-interacting protein by calpain in rat brain: implications for synaptic plasticity

Activation of the calcium-dependent protease calpain has been proposed to be a key step in synaptic plasticity in the hippocampus. However, the exact pathway through which calpain mediates or modulates changes in synaptic function remains to be clarified. Here we report that glutamate receptor-interacting protein (GRIP) is a substrate of calpain, as calpain-mediated GRIP degradation was demonstrated using three different approaches: (i) purified calpain I digestion of synaptic membranes, (ii) calcium treatment of frozen-thawed brain sections, and (iii) NMDA-stimulated organotypic hippocampal slice cultures. More importantly, calpain activation resulted in the disruption of GRIP binding to the GluR2 subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Because GRIP has been proposed to function as an AMPA receptor-targeting and synaptic-stabilizing protein, as well as a synaptic-organizing molecule, calpain-mediated degradation of GRIP and disruption of AMPA receptor anchoring are likely to play important roles in the structural and functional reorganization accompanying synaptic modifications in long-term potentiation and long-term depression.

The 8-kDa dynein light chain binds to its targets via a conserved (K/R)XTQT motif

Cytoplasmic dynein is a large, multisubunit molecular motor that translocates cargoes toward the minus ends of microtubules. Proper functioning of the dynein motor requires precise assembly of its various subunits. Using purified recombinant proteins, we show that the highly conserved 8-kDa light chain (DLC8) binds to the intermediate chain of the dynein complex. The DLC8-binding region was mapped to a highly conserved 10-residue fragment (amino acid sequence SYSKETQTPL) C-terminal to the second alternative splicing site of dynein intermediate chain. Yeast two-hybrid screening using DLC8 as bait identified numerous additional DLC8-binding proteins. Biochemical and mutational analysis of selected DLC8-binding proteins revealed that DLC8 binds to a consensus sequence containing a (K/R)XTQT motif. The (K/R)XTQT motif interacts with the common target-accepting grooves of DLC8 dimer. The role of each conserved amino acid residue in this pentapeptide motif in supporting complex formation with DLC8 was systematically studied using site-directed mutagenesis.

Tbr1 regulates differentiation of the preplate and layer 6

During corticogenesis, early-born neurons of the preplate and layer 6 are important for guiding subsequent neuronal migrations and axonal projections. Tbr1 is a putative transcription factor that is highly expressed in glutamatergic early-born cortical neurons. In Tbr1-deficient mice, these early-born neurons had molecular and functional defects. Cajal-Retzius cells expressed decreased levels of Reelin, resulting in a reeler-like cortical migration disorder. Impaired subplate differentiation was associated with ectopic projection of thalamocortical fibers into the basal telencephalon. Layer 6 defects contributed to errors in the thalamocortical, corticothalamic, and callosal projections. These results show that Tbr1 is a common genetic determinant for the differentiation of early-born glutamatergic neocortical neurons and provide insights into the functions of these neurons as regulators of cortical development.

Sharpin, a novel postsynaptic density protein that directly interacts with the shank family of proteins

The Shank family of proteins (also termed CortBP, ProSAP, or Synamon) is highly enriched in the postsynaptic density (PSD) of excitatory synapses in brain. Shank contains multiple domains for protein-protein interactions, including ankyrin repeats, SH3 domain, PDZ domain, SAM domain, and an extensive proline-rich region. We have identified a novel protein, termed Sharpin, that directly interacts with the ankyrin repeats of Shank. Sharpin is enriched in the PSD and forms a complex with Shank in heterologous cells and brain. Immunostaining reveals the presence of Sharpin at excitatory synapses and its colocalization with Shank. While the C-terminal half of Sharpin interacts with Shank, the N-terminal half of Sharpin mediates homomultimerization. Considering the fact that the ankyrin repeats and the SH3 domain of Shank can be truncated by alternative splicing, these results define Sharpin as a novel PSD protein that may regulate the complexity of the Shank-based protein network in an alternative splicing-dependent manner.

Differential cellular and subcellular localization of ampa receptor-binding protein and glutamate receptor-interacting protein

Excitatory synaptic currents in the mammalian brain are typically mediated by the neurotransmitter glutamate, acting at AMPA receptors. We used immunocytochemistry to investigate the distribution of AMPA receptor-binding protein (ABP) in the cerebral neocortex. ABP was most prominent in pyramidal neurons, although it was also present (at lower levels) in interneurons. ABP and its putative binding partners, the GluR2/3 subunits of the AMPA receptor, exhibited prominent cellular colocalization. Under appropriate processing conditions, colocalization could also be documented in puncta, many of which could be recognized as dendritic spines. However, a sizable minority of GluR2/3-positive puncta were immunonegative for ABP. Because glutamate receptor-interacting protein (GRIP) may also anchor GluR2, we studied the relative distribution of ABP and GRIP. There was extensive colocalization of these two antigens at the cellular level, although GRIP, unlike ABP, was strongest in nonpyramidal neurons. Different parts of a single dendrite could stain selectively for ABP or GRIP. To further characterize this heterogeneity, we investigated punctate staining of neuropil using synaptophysin and the membrane tracer DiA to identify probable synapses. Some puncta were comparably positive for both ABP and GRIP, but the majority were strongly positive for one antigen and only weakly positive or immunonegative for the other. This heterogeneity could be seen even within adjacent spines of a single dendrite. These data suggest that ABP may act as a scaffold for AMPA receptors either in concert with or independently from GRIP.

Neurobiology. A stargazer foretells the way to the synapse

How do AMPA receptors that are made in the cytoplasm of excitatory neurons travel to and become localized in the distant postsynaptic membranes of dendrites? Nakagawa and Sheng, in a Perspective, suggest that the answer may lie in the stargazin protein that has now been found to interact with AMPA receptors, guiding them to the postsynaptic membrane.

Distinct molecular mechanisms and divergent endocytotic pathways of AMPA receptor internalization

Internalization of postsynaptic AMPA receptors depresses excitatory transmission, but the underlying dynamics and mechanisms of this process are unclear. Using immunofluorescence and surface biotinylation, we characterized and quantified basal and regulated AMPA receptor endocytosis in cultured hippocampal neurons, in response to synaptic activity, AMPA and insulin. AMPA-induced AMPA receptor internalization is mediated in part by secondary activation of voltage-dependent calcium channels, and in part by ligand binding independent of receptor activation. Although both require dynamin, insulin- and AMPA-induced AMPA receptor internalization are differentially dependent on protein phosphatases and sequence determinants within the cytoplasmic tails of GluR1 and GluR2 subunits. AMPA receptors internalized in response to AMPA stimulation enter a recycling endosome system, whereas those internalized in response to insulin diverge into a distinct compartment. Thus, the molecular mechanisms and intracellular sorting of AMPA receptors are diverse, and depend on the internalizing stimulus.

Association of dystrophin-related protein 2 (DRP2) with postsynaptic densities in rat brain

The fundamental function of the membrane-associated cytoskeletal proteins dystrophin and utrophin remains unclear. To gain further insights into the dystrophin family of proteins, we have studied dystrophin-related protein 2 (DRP2), whose expression is largely confined to the vertebrate central nervous system. Both human and rat DRP2 are expressed from two alternative but neighboring transcriptional start sites and have simple transcript structures. Antibodies raised against DRP2 detect a characteristic quartet of bands ( approximately 100-120 kDa) in Western blots of rat brain. The DRP2 protein is associated with brain membrane fractions and highly enriched in the postsynaptic density. Immunohistochemistry shows DRP2 to be widely distributed in a punctate pattern on neuronal dendrites and in neuropil, with particular concentration in regions of the brain involved in cholinergic synaptic transmission. Given the presence of utrophin in the cholinergic neuromuscular junction, and perturbations of cholinergic transmission in dystrophin-deficient nematodes, our findings may suggest a role for DRP2 in the organization of central cholinergic synapses.

Ligand-gated ion channel interactions with cytoskeletal and signaling proteins

In recent years, it has become apparent that ligand-gated ion channels (ionotropic receptors) in the neuronal plasma membrane interact via their cytoplasmic domains with a multitude of intracellular proteins. Different classes of ligand-gated channels associate with distinct sets of intracellular proteins, often through specialized scaffold proteins containing PDZ domains. These specific interactions link the receptor channel to the cortical cytoskeleton and to appropriate signal transduction pathways in the cell. Thus ionotropic receptors are components of extensive protein complexes that are likely involved in the subcellular targeting, cytoskeletal anchoring, and localized clustering of the receptors at specific sites on the neuronal surface. In addition to structural functions, receptor-associated proteins can play important roles as activity modulators or downstream effectors of ligand-gated channels.

[Cataract extraction with rotated-chipping phacoemulsification in the capsule]

OBJECTIVE

To observe the clinical effects of rotated-chipping phacoemulsification in the capsule.

METHODS

The operation of continuous circular capsulectomy, nuclear hydrodissection, cataract extraction with rotated-chipping phacoemulsification in capsule and intraocular lens implantation was performed on 215 eyes.

RESULTS

Rotated-chipping technique was successfully completed in 215 eyes. In the operation 2 to 4 circles were rotated with the ultrasonic energy 19.3% for 12 seconds on average. The rate of corneal endothelium loss was 7.5%.

CONCLUSION

The technique of rotated-chipping phacoemulsification in capsule is safe and rapid which can be used for cataracts with nuclei of various degrees in sclerosis.

alpha-actinin-2 in rat striatum: localization and interaction with NMDA glutamate receptor subunits

Alpha-actinin (alpha-actinin-2) is a protein which links the NR1 and NR2B subunits of N-methyl-D-aspartate (NMDA) glutamate receptors to the actin cytoskeleton. Because of the importance of NMDA receptors in modulating the function of the striatum, we have examined the localization of alpha-actinin-2 protein and mRNA in striatal neurons, and its biochemical interaction with NMDA receptor subunits present in the rat striatum. Using an alpha-actinin-2-specific antibody, we found intense immunoreactivity in the striatal neuropil and within striatal neurons that also expressed parvalbumin, calretinin and calbindin. Conversely, alpha-actinin-2 immunoreactivity was not detected in neurons expressing choline acetyltransferase and neuronal nitric oxide synthase. Dual-label in situ hybridization revealed that the highest expression of alpha-actinin-2 mRNA is in substance P-containing striatal projection neurons. The alpha-actinin-2 mRNA is also present in enkephalinergic projection neurons and interneurons expressing parvalbumin, choline acetyl transferase and the 67-kDa isoform of glutamic acid decarboxylase, but was not detected in somatostatin-expressing interneurons. Immunoprecipitation of membrane protein extracts showed that alpha-actinin-2 is present in heteromeric complexes of NMDA subunits, but is not associated with AMPA receptors in the striatum. A subunit-specific anti-NR1 antibody co-precipitated major fractions of NR2A and NR2B subunits, but only a minor fraction of striatal alpha-actinin-2. Conversely, alpha-actinin-2 antibody immunoprecipitated only modest fractions of striatal NR1, NR2A and NR2B subunits. These data demonstrate that alpha-actinin-2 is a very abundant striatal protein, but exhibits cellular specificity in its expression, with very high levels in substance-P-containing projection neurons, and very low levels in somatostatin and neuronal nitric oxide synthase interneurons. Despite the high expression of this protein in the striatum, only a minority of NMDA receptors are linked to alpha-actinin-2. This interaction may identify a subset of receptors with distinct anatomical and functional properties.

Interaction of the postsynaptic density-95/guanylate kinase domain-associated protein complex with a light chain of myosin-V and dynein

NMDA receptors interact directly with postsynaptic density-95 (PSD-95), a scaffold protein that organizes a cytoskeletal- signaling complex at the postsynaptic membrane. The molecular mechanism by which the PSD-95-based protein complex is trafficked to the postsynaptic site is unknown but presumably involves specific motor proteins. Here we demonstrate a direct interaction between the PSD-95-associated protein guanylate kinase domain-associated protein (GKAP) and dynein light chain (DLC), a light chain subunit shared by myosin-V (an actin-based motor) and cytoplasmic dynein (a microtubule-based motor). A yeast two-hybrid screen with GKAP isolated DLC2, a novel protein 93% identical to the previously cloned 8 kDa dynein light chain (DLC1). A complex containing PSD-95, GKAP, DLC, and myosin-V can be immunoprecipitated from rat brain extracts. DLC colocalizes with PSD-95 and F-actin in dendritic spines of cultured neurons and is enriched in biochemical purifications of PSD. Immunogold electron microscopy reveals a concentration of DLC in the postsynaptic compartment of asymmetric synapses of brain in which it is associated with the PSD and the spine apparatus. We discuss the possibility that the GKAP/DLC interaction may be involved in trafficking of the PSD-95 complex by motor proteins.

The Shank family of scaffold proteins

Shank proteins make up a new family of scaffold proteins recently identified through their interaction with a variety of membrane and cytoplasmic proteins. Shank polypeptides contain multiple sites for protein-protein interaction, including ankyrin repeats, an SH3 domain, a PDZ domain, a long proline-rich region, and a SAM domain. Binding partners for most of these domains have been identified: for instance, the PDZ domain of Shank proteins interacts with GKAP (a postsynaptic-density protein) as well as several G-protein-coupled receptors. The specific localization of Shank proteins at postsynaptic sites of brain excitatory synapses suggests a role for this family of proteins in the organization of cytoskeletal/ signaling complexes at specialized cell junctions.