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Biomedical subjects

Morgan Sheng

Publications and source records attributed to Morgan Sheng.

At least 19 recordsLinked to original sources
PubMed2003

Targeted protein degradation and synapse remodeling by an inducible protein kinase.

Synaptic plasticity involves the reorganization of synapses at the protein and the morphological levels. Here, we report activity-dependent remodeling of synapses by serum-inducible kinase (SNK). SNK was induced in hippocampal neurons by synaptic activity and was targeted to dendritic spines. SNK bound to and phosphorylated spine-associated Rap guanosine triphosphatase activating protein (SPAR), a postsynaptic actin regulatory protein, leading to degradation of SPAR. Induction of SNK in hippocampal neurons eliminated SPAR protein, depleted postsynaptic density-95 and Bassoon clusters, and caused loss of mature dendritic spines. These results implicate SNK as a mediator of activity-dependent change in the molecular composition and morphology of synapses.

Actins↗
PubMed2003

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.

Animals↗
PubMed2003

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.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Interaction of the deafness-dystonia protein DDP/TIMM8a with the signal transduction adaptor molecule STAM1.

The Mohr-Tranebjaerg-Jensen deafness-dystonia-optic atrophy protein DDP/TIMM8a is translated on cytoplasmic ribosomes but targeted ultimately to the mitochondrial intermembrane space, where it is involved in mitochondrial protein import. STAM1 is a cytoplasmic signal-transducing adaptor molecule implicated in cytokine signaling. We report here a direct interaction between DDP and STAM1, identified by yeast two-hybrid screening and confirmed by co-immunoprecipitation, fusion protein "pull downs," and nuclear redistribution assays. DDP coordinates Zn(2+), and Zn(2+) was found to stimulate the DDP-STAM1 interaction in vitro. Endogenous STAM1 localizes predominantly to early endosomes, and we found no evidence that STAM1 is imported into mitochondria in vitro. Thus, the DDP-STAM1 interaction likely occurs in the cytoplasm or at the mitochondrial outer membrane. The DDP-STAM1 interaction requires a coiled-coil region in STAM1 that overlaps with the immunoreceptor tyrosine-based activation motif (ITAM), a region previously shown to be important for interaction with Jak2/3 and hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs). Thus, DDP binding may alter the interactions of STAM1 with several cytoplasmic proteins involved in cell signaling and endosomal trafficking.

Amino Acid Sequence↗
PubMed2003

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.

Animals↗
PubMed2003

Supramodular structure and synergistic target binding of the N-terminal tandem PDZ domains of PSD-95.

PDZ domain proteins play critical roles in binding, clustering and subcellular targeting of membrane receptors and ion channels. PDZ domains in multi-PDZ proteins often are arranged in groups with highly conserved spacing and intervening sequences; however, the functional significance of such tandem arrangements of PDZs is unclear. We have solved the three-dimensional structure of the first two PDZ domains of postsynaptic density protein-95 (PSD-95 PDZ1 and PDZ2), which are closely linked to each other in the PSD-95 family of scaffold proteins. The two PDZs have limited freedom of rotation and their C-terminal peptide-binding grooves are aligned with each other with an orientation preference for binding to pairs of C termini extending in the same direction. Increasing the spacing between PDZ1 and PDZ2 resulted in decreased binding between PDZ12 and its dimeric targets. The same mutation impaired the functional ability of PSD-95 to cluster Kv1.4 potassium channels in heterologous cells. The data presented provide a molecular basis for preferential binding of PSD-95 to multimeric membrane proteins with appropriate C-terminal sequences.

Amino Acid Sequence↗
PubMed2003

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.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Interaction between liprin-alpha and GIT1 is required for AMPA receptor targeting.

Liprin-alpha is a multidomain protein that interacts with the LAR family of receptor protein tyrosine phosphatases and the GRIP/ABP family of AMPA receptor-interacting proteins. Previous studies have indicated that liprin-alpha regulates the development of presynaptic active zones and that the association of liprin-alpha with GRIP is required for postsynaptic targeting of AMPA receptors. However, the underlying molecular mechanisms are not well understood. Here we report that liprin-alpha directly interacts with GIT1, a multidomain protein with GTPase-activating protein activity for the ADP-ribosylation factor family of small GTPases known to regulate protein trafficking and the actin cytoskeleton. Electron microscopic analysis indicates that GIT1 distributes to the region of postsynaptic density (PSD) as well as presynaptic active zones. GIT1 is enriched in PSD fractions and forms a complex with liprin-alpha, GRIP, and AMPA receptors in brain. Expression of dominant-negative constructs interfering with the GIT1-liprin-alpha interaction leads to a selective and marked reduction in the dendritic and surface clustering of AMPA receptors in cultured neurons. These results suggest that the GIT1-liprin-alpha interaction is required for AMPA receptor targeting and that GIT1 may play an important role in the organization of presynaptic and postsynaptic multiprotein complexes.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Association of the kinesin motor KIF1A with the multimodular protein liprin-alpha.

Liprin-alpha/SYD-2 is a multimodular scaffolding protein important for presynaptic differentiation and postsynaptic targeting of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid glutamate receptors. However, the molecular mechanisms underlying these functions remain largely unknown. Here we report that liprin-alpha interacts with the neuron-specific kinesin motor KIF1A. KIF1A colocalizes with liprin-alpha in various subcellular regions of neurons. KIF1A coaccumulates with liprin-alpha in ligated sciatic nerves. KIF1A cofractionates and coimmunopreciptates with liprin-alpha and various liprin-alpha-associated membrane, signaling, and scaffolding proteins including alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors, GRIP/ABP, RIM, GIT1, and beta PIX. These results suggest that liprin-alpha functions as a KIF1A receptor, linking KIF1A to various liprin-alpha-associated proteins for their transport in neurons.

Adaptor Proteins, Signal Transducing↗
PubMed2003

AMPA receptor trafficking and synaptic plasticity: major unanswered questions.

Dynamic movements of AMPA receptors in and out of the postsynaptic membrane account for, at least in part, the expression of NMDA receptor-dependent changes in synaptic efficacy such as long-term potentiation and long-term depression. Recently some of key molecules and subunit rules involved in AMPA receptor trafficking have been identified. In this update article, we try to highlight what we believe to be the major conceptual problems and unanswered questions in this rapidly moving field of neuroscience.

Animals↗
PubMed2002

Crystal structure of GRIP1 PDZ6-peptide complex reveals the structural basis for class II PDZ target recognition and PDZ domain-mediated multimerization.

PDZ domains bind to short segments within target proteins in a sequence-specific fashion. Glutamate receptor-interacting protein (GRIP)/ABP family proteins contain six to seven PDZ domains and interact via the sixth PDZ domain (class II) with the C termini of various proteins including liprin-alpha. In addition the PDZ456 domain mediates the formation of homo- and heteromultimers of GRIP proteins. To better understand the structural basis of peptide recognition by a class II PDZ domain and PDZ-mediated multimerization, we determined the crystal structures of the GRIP1 PDZ6 domain alone and in complex with a synthetic C-terminal octapeptide of human liprin-alpha at resolutions of 1.5 and 1.8 A, respectively. Remarkably, unlike other class II PDZ domains, Ile-736 at alphaB5 rather than conserved Leu-732 at alphaB1 makes a direct hydrophobic contact with the side chain of the Tyr at the -2 position of the ligand. Moreover, the peptide-bound structure of PDZ6 shows a slight reorientation of helix alphaB, indicating that the second hydrophobic pocket undergoes a conformational adaptation to accommodate the bulkiness of the Tyr side chain, and forms an antiparallel dimer through an interface located at a site distal to the peptide-binding groove. This configuration may enable formation of GRIP multimers and efficient clustering of GRIP-binding proteins.

Adaptor Proteins, Signal Transducing↗
PubMed2002

Clathrin adaptor AP2 and NSF interact with overlapping sites of GluR2 and play distinct roles in AMPA receptor trafficking and hippocampal LTD.

Proteins that bind to the cytoplasmic tails of AMPA receptors control receptor trafficking and thus the strength of postsynaptic responses. Here we show that AP2, a clathrin adaptor complex important for endocytosis, associates with a region of GluR2 that overlaps the NSF binding site. Peptides used previously to interfere with NSF binding also antagonize GluR2-AP2 interaction. Using GluR2 mutants and peptide variants that dissociate NSF and AP2 interaction, we find that AP2 is involved specifically in NMDA receptor-induced (but not ligand-dependent) internalization of AMPA receptors, and is essential for hippocampal long-term depression (LTD). NSF function, on the other hand, is needed to maintain synaptic AMPA receptor responses, but is not directly required for NMDA receptor-mediated internalization and LTD.

Adaptor Protein Complex 2↗
PubMed2002

Postsynaptic signaling and plasticity mechanisms.

In excitatory synapses of the brain, specific receptors in the postsynaptic membrane lie ready to respond to the release of the neurotransmitter glutamate from the presynaptic terminal. Upon stimulation, these glutamate receptors activate multiple biochemical pathways that transduce signals into the postsynaptic neuron. Different kinds of synaptic activity elicit different patterns of postsynaptic signals that lead to short- or long-lasting strengthening or weakening of synaptic transmission. The complex molecular mechanisms that underlie postsynaptic signaling and plasticity are beginning to emerge.

Animals↗
PubMed2002

Gephyrin interacts with Dynein light chains 1 and 2, components of motor protein complexes.

The clustering of glycine receptors and major subtypes of GABA(A) receptors at inhibitory synapses is mediated by the tubulin-binding protein gephyrin. In an attempt to identify additional components of inhibitory postsynaptic specializations, we performed a yeast two-hybrid screen using gephyrin as bait. Multiple positive clones encoded either the dynein light chain-1 (Dlc-1), also known as dynein LC8 and protein inhibitor of neuronal nitric oxide synthase, or its homolog Dlc-2. Dlc-1 protein bound efficiently to gephyrin in in vitro binding assays and colocalized with gephyrin during coexpression in HEK293 cells. The binding site for Dlc was mapped to a fragment of 63 amino acids within the central linker domain of gephyrin. In hippocampal neurons, endogenous Dlc protein was enriched at synaptic sites identified by synaptophysin and gephyrin immunostaining. Immunoelectron microscopy in spinal cord sections revealed Dlc immunoreactivity at the edges of postsynaptic differentiations, in close contact with cytoskeletal structures and at the periphery of the Golgi apparatus. Because Dlc-1 and Dlc-2 have been described as stoichiometric components of cytoplasmic dynein and myosin-Va complexes, our results suggest that motor proteins are involved in the subcellular localization of gephyrin.

Amino Acid Sequence↗