discs large 1


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Miscellaneous PDZ domain interactions and other PDZ domain proteins

In C. elegans, genes involved in targeting LET-23 RTK to the basolateral membrane domain can be identified in genetic screens for vulvaless mutants. Vulval signaling requires basolateral expression of LET-23 RTK, since cells that lack LET-23 RTK in the basolateral membrane domain presumably cannot respond to LIN-3 EGF in the basal extracellular space. Mutations in lin-2, lin-7, and lin-10 were initially isolated because they decrease vulval signaling and cause a vulvaless phenotype. Subsequently, these mutations were shown to result in apical mislocalization of LET-23 RTK. LIN-7, LIN-2, and LIN-10 each contain protein domains that may mediate interactions with other proteins. LIN-7 contains a single PDZ domain, and PDZ domains are known to mediate protein-protein interactions with C-terminal tails of transmembrane proteins as well as with other PDZ domains. LIN-2 contains a CaM kinase domain, a calmodulin-binding domain, a PDZ domain, an SH3 domain, and a guanylate kinase domain. LIN-2 is highly similar to mammalian Lin2/CASK (52% identical overall). Lin2/CASK is expressed in epithelia and neurons and has been shown to bind to neurexin, syndecan, and protein 4.1. LIN-2 is related to a family of proteins called membrane-associated guanylate kinases (MAGUKs) that includes discs-large (DlgA) and PSD-95. In mammals, DlgA binds the cytoplasmic tail of the Shaker-type K+ channel; in Drosophila, dlg mutations prevent synaptic localization of the Shaker channel. In C. elegans, the LET-23 receptor tyrosine kinase is localized to the basolateral membranes of polarized vulval epithelial cells. lin-2, lin-7, and lin-10 are required for basolateral localization of LET-23, since LET-23 is mislocalized to the apical membrane in lin-2, lin-7, and lin-10 mutants. Yeast two-hybrid, in vitro binding, and in vivo coimmunoprecipitation experiments show that LIN-2, LIN-7, and LIN-10 form a protein complex. Furthermore, compensatory mutations in lin-7 and let-23 exhibit allele-specific suppression of apical mislocalization and signaling-defective phenotypes. These results present a mechanism for basolateral localization of LET-23 receptor tyrosine kinase by direct binding to the LIN-2/LIN-7/LIN-10 complex. Each of the binding interactions within this complex is conserved, suggesting that this complex may also mediate basolateral localization in mammals (Kaech, 1998).

Since the C. elegans PDZ proteins LIN-2, LIN-7, and LIN-10 define a basolateral localization pathway for epithelial proteins, the hypothesis that this same pathway is utilized in neurons to localize postsynaptic proteins was tested. The pathways for localizing basolateral and postsynaptic proteins in C. elegans were examined. GLR-1 glutamate receptors are localized to postsynaptic elements of central synapses and, when ectopically expressed, to basolateral membranes of epithelial cells. Proper localization of GLR-1 in both neurons and epithelia requires the PDZ protein LIN-10, defining LIN-10 as a shared component of the basolateral and postsynaptic localization pathways. Changing the GLR-1 carboxy-terminal sequence from a group I PDZ-binding consensus (-TAV) to a group II consensus (-FYV) restores GLR-1 synaptic localization in lin-10 mutants. Thus, these interneurons utilize at least two separate postsynaptic localization pathways. lin-10 encodes an ortholog of the mammalian PDZ protein X11/Mint. X11/Mint is abundantly expressed in the brain, but its physiological function is not known (Rongo, 1998).

The PDZ target motifs located in the C-terminal end of many receptors and ion channels mediate protein-protein interactions by binding to specific PDZ-containing proteins. These interactions are involved in the localization of surface proteins on specialized membrane domains of neuronal and epithelial cells. However, the molecular mechanism responsible for this PDZ protein-dependent polarized localization is still unclear. The epithelial gamma-aminobutyric acid (GABA) transporter (BGT-1) contains a PDZ target motif that mediates the interaction with the PDZ protein LIN-7 in Madin-Darby canine kidney (MDCK) cells. The role of this interaction in the basolateral localization of the transporter has been investigated. Although the transporters from which the PDZ target motif is deleted are still targeted to the basolateral surface, they are not retained but internalized in an endosomal recycling compartment. Furthermore, an interfering BGT peptide determines the intracellular relocation of the native transporter. These data indicate that interactions with PDZ proteins determine the polarized surface localization of target proteins by means of retention and not targeting mechanisms. PDZ proteins may, therefore, act as a sort of membrane protein sorting machinery which, by recognizing retention signals (the PDZ target sequences), prevents protein internalization (Perego, 1999).

A human B-lymphocyte 100-kDa protein shares 60% amino acid identity with Drosophila Discs large. This human homolog known as hDlg contains a C-terminal domain homologous to the known guanylate kinases, a Src homology 3 region motif, and three repeats known as the Dlg repeat after Drosophila Discs large. Two nonhomologous domains that can contain in-frame insertions result in at least four alternatively spliced isoforms of hDlg. Several hDlg RNA transcripts are widely distributed in human and murine tissues, and the protein is localized to regions of cell-cell contact. Protein 4.1 (Drosophila homolog: Coracle), the defining member of a family that includes talin and merlin/schwannomin, has the same cellular localization as hDlg, and two sites within hDlg associate in vitro with the 30-kDa N-terminal domain of protein 4.1 (Lue, 1994).

hDlg, a human homolog of the Drosophila Dlg tumor suppressor, contains two binding sites for protein 4.1, one within a domain containing three PSD-95/Dlg/ZO-1 (PDZ) repeats and another within the alternatively spliced I3 domain. The PDZ-protein 4.1 interaction has been further defined in vitro and the functional role of both 4.1 binding sites in situ is shown. A single protease-resistant structure formed by the entirety of both PDZ repeats 1 and 2 (PDZ1-2) contains the protein 4.1-binding site. Both this PDZ1-2 site and the I3 domain associate with a 30-kD NH2-terminal domain of protein 4.1 that is conserved in ezrin/radixin/moesin (ERM) proteins. Both protein 4.1 and the ezrin ERM protein interact with the murine form of hDlg in a coprecipitating immune complex. In permeabilized cells and tissues, either the PDZ1-2 domain or the I3 domain alone are sufficient for proper subcellular targeting of exogenous hDlg. In situ, PDZ1-2-mediated targeting involves interactions with both 4.1/ERM proteins and proteins containing the COOH-terminal T/SXV motif. I3-mediated targeting depends exclusively on interactions with 4.1/ERM proteins. These data elucidate the multivalent nature of membrane-associated guanylate kinase homolog (MAGUK) targeting, thus beginning to define those protein interactions that are critical in MAGUK function (Lue, 1996).

The human homolog (hDlg) of the Drosophila Discs large tumor suppressor (Dlg) is a multidomain protein consisting of a carboxyl-terminal guanylate kinase-like domain, an SH3 domain, and three slightly divergent copies of the PDZ (DHR/GLGF) domain. The structural organization of the three PDZ domains of hDlg were examined using a combination of protease digestion and in vitro binding measurements. The PDZ domains are organized into two conformationally stable modules: one (PDZ1+2), consists of PDZ domains 1 and 2, and the other (PDZ3) corresponds to the third PDZ domain. Using amino acid sequencing and mass spectrometry, the boundaries of the PDZ domains after digestion with endoproteinase Asp-N, trypsin, and alpha-chymotrypsin were determined. The purified PDZ1+2, but not the PDZ3 domain, contains a high affinity binding site for the cytoplasmic domain of Shaker-type K+ channels. Similarly, the PDZ1+2 domain can also specifically bind to ATP. The binding of ATP to the PDZ domains may have several important functional implications:

The results suggest a mechanism by which PDZ domain-binding proteins may be coupled to ATP and the membrane cytoskeleton via hDlg (Marfatia, 1996).

Consistent with the multi-domain organization of MAGUKs, hDlg consists of three copies of the PDZ (PSD-95/Discs large/ZO-1) domain, an SH3 motif, and a guanylate kinase-like domain. In addition, the hDlg contains an amino-terminal proline-rich domain that is absent in other MAGUKs. To explore the role of hDlg in cell signaling pathways, human T lymphocytes were used as a model system to investigate interaction of hDlg with known tyrosine kinases. In human T lymphocyte cell lines, binding properties of hDlg were studied by immunoprecipitation, immunoblotting, and immune complex kinase assays. Protein tyrosine kinase activity is associated with the immunoprecipitates of hDlg. Immunoblotting experiments reveals that the immunoprecipitates of hDlg contain p56(lck), a member of the Src family of tyrosine kinases. The specificity of the interaction is demonstrated by the lack of p59(fyn) tyrosine kinase and phosphotidylinositol 3-kinase in the hDlg immunoprecipitates. Direct interaction between hDlg and p56(lck) is demonstrated using glutathione S-transferase fusion proteins of hDlg and recombinant p56(lck) expressed in the baculovirus-infected Sf9 cells. The p56(lck) binding site was localized within the amino-terminal segment of the hDlg-containing proline-rich domain. In addition, hDlg associates in vitro with the shaker type Kv1.3 channel, which was expressed in T lymphocytes as an epitope-tagged protein using a vaccinia virus expression system. Taken together, these results provide the first evidence of a direct interaction between hDlg and p56(lck) tyrosine kinase and suggest a novel function of hDlg in coupling tyrosine kinase and voltage-gated potassium channel in T lymphocytes (Hanada, 1997).

During synaptic development, proteins aggregate at specialized pre- and post-synaptic structures. Mechanisms that mediate protein clustering at these sites remain unknown. To investigate this process, synaptic targeting of a postsynaptic density protein, PSD-95, was analyzed by expressing green fluorescent protein- (GFP-) tagged PSD-95 in cultured hippocampal neurons. Postsynaptic clustering relies on three elements of PSD-95: N-terminal palmitoylation, the first two PDZ domains, and a C-terminal targeting motif. In contrast, disruptions of PDZ3, SH3, or guanylate kinase (GK) domains do not affect synaptic targeting. Palmitoylation is sufficient to target the diffusely expressed SAP-97 to synapses, and palmitoylation cannot be replaced with alternative membrane association motifs, suggesting that a specialized synaptic lipid environment mediates postsynaptic clustering. The requirements for PDZ domains and a C-terminal domain of PSD-95 indicate that protein-protein interactions cooperate with lipid interactions in synaptic targeting (Craven, 1999).

The par genes are required to establish polarity in the Caenorhabditis elegans embryo. Three of the PAR proteins themselves exhibit asymmetric distributions. PAR-1, a putative serine/threonine kinase, and PAR-2, a protein containing a putative ATP-binding site and a zinc-binding motif of the RING finger class, are localized to the posterior periphery, whereas PAR-3, a novel protein containing three PDZ domains, is localized to the anterior periphery of a 1-cell embryo. Mutations in two par genes, par-3 and par-6, exhibit similar phenotypes. A third gene, pkc-3, gives a similar phenotype when the protein is depleted by RNA interference. PAR-3 and PKC-3 protein are colocalized to the anterior periphery of asymmetrically dividing cells of the germline lineage. The peripheral localizations of both proteins depends upon the activity of par-6. The molecular cloning of par-6 and the immunolocalization of PAR-6 protein are reported. par-6 encodes a PDZ-domain-containing protein and has homologs in mammals and flies. The PDZ domain of PAR-6 shares most similarity to the PDZ domain of Tax clone 40, a human protein that interacts with the Tax protein of human T-cell leukemia virus (TLV). The alignment of PAR-6 PDZ with PSD-95 PDZ3 shows that amino acids forming the beta-sheets and alpha-helix structures in PSD-95 PDZ3 are well conserved in PAR-6 PDZ. The conservation is greatest among these homologs over a 115 amino acid region containing the PDZ domain; worm PAR-6 is 88% and 80% identical to the fly (Fly EST #LD08317) and mouse proteins (#440139), respectively. PAR-6 colocalizes with PAR-3; par-3 and pkc-3 activity are both required for the peripheral localization of PAR-6. The localization of both PAR-3 and PAR-6 proteins is affected identically by mutations in the par-2, par-4 and par-5 genes. The co-dependence of PAR-3, PAR-6 and PKC-3 for peripheral localization and the overlap in their distributions lead to a proposal that they act in a protein complex (Hung, 1999).

The intracellular protein-tyrosine phosphatase PTPL1 has five PDZ domains; one of them, PDZ 2, has previously been shown to interact with the C-terminal tail of Fas, a member of the tumor necrosis factor receptor family. Not only PDZ 2 but also PDZ 4 of PTPL1 interacts with high affinity with peptides derived from the C terminus of Fas. The five most C-terminal amino acid residues of Fas influence the affinity of the interaction. Whereas the glutamine and isoleucine residues in the 4th and 5th positions from the C terminus affect the interaction in a negative and positive manner, respectively, the three C-terminal amino acid residues (SLV) are necessary and sufficient for a high affinity interaction to occur. Both the carboxyl group and side chain of the valine residue at the C terminus of Fas are essential, and the leucine and serine residues in the 2nd and 3rd positions, respectively, from the C terminus are important for the interactions with PDZ 2 and PDZ 4 of PTPL1 (Saras, 1997).

Fyn, a member of the Src-family protein-tyrosine kinase (PTK), is implicated in learning and memory that involves N-methyl-D-aspartate (NMDA) receptor function. Analysis of the physical and functional interaction between Fyn and NMDA receptors was carried out to see how Fyn participates in synaptic plasticity. Tyrosine phosphorylation of NR2A, one of the NMDA receptor subunits, is reduced in fyn-mutant mice. NR2A is tyrosine-phosphorylated in 293T cells when coexpressed with Fyn. Therefore, NR2A would be a substrate for Fyn in vivo. Results also show that PSD-95, which directly binds to and coclusters with NMDA receptors, promotes Fyn-mediated tyrosine phosphorylation of NR2A. Different regions of PSD-95 associate with NR2A and Fyn, respectively; therefore, PSD-95 could mediate complex formation of Fyn with NR2A. PSD-95 also associates with other Src-family PTKs: Src, Yes, and Lyn. These results suggest that PSD-95 is critical for regulation of NMDA receptor activity by Fyn and other Src-family PTKs, serving as a molecular scaffold for anchoring these PTKs to NR2A (Tezuka, 1999).

NE-dlg/SAP102, a neuronal and endocrine tissue-specific membrane-associated guanylate kinase family protein, is known to bind to C-terminal ends of N-methyl-D-aspartate receptor 2B (NR2B) through its PDZ (PSD-95/Dlg/ZO-1) domains. NE-dlg/SAP102 and NR2B colocalize at synaptic sites in cultured rat hippocampal neurons, and their expressions increase in parallel with the onset of synaptogenesis. NE-dlg/SAP102 interacts with calmodulin in a Ca2+-dependent manner. The binding site for calmodulin has been determined to lie at the putative basic alpha-helix region located around the src homology 3 (SH3) domain of NE-dlg/SAP102. Using a surface plasmon resonance measurement system, specific binding of recombinant NE-dlg/SAP102 to the immobilized calmodulin, with a Kd value of 44 nM, was detected. However, the binding of Ca2+/calmodulin to NE-dlg/SAP102 does not modulate the interaction between PDZ domains of NE-dlg/SAP102 and the C-terminal end of rat NR2B. The region near the calmodulin binding site of NE-dlg/SAP102 interacts with the GUK-like domain of PSD-95/SAP90 by two-hybrid screening. A pull down assay revealed that NE-dlg/SAP102 can interact with PSD-95/SAP90 in the presence of both Ca2+ and calmodulin. These findings suggest that the Ca2+/calmodulin modulates interaction of neuronal membrane-associated guanylate kinase proteins and regulates clustering of neurotransmitter receptors at central synapses (Masuko, 1999).

Densin-180 (Drosophila homolog: Scribbled), a brain-specific protein highly concentrated at the postsynaptic density (PSD), belongs to the LAP [leucine-rich repeats and PSD-95/Dlg-A/ZO-1 (PDZ) domains] family of proteins, some of which play fundamental roles in the establishment of cell polarity. To identify new Densin-180-interacting proteins, a yeast two-hybrid library was screened using the COOH-terminal fragment of Densin-180 containing the PDZ domain as bait, and MAGUIN-1 was isolated as a Densin-180-binding protein. MAGUIN-1, a mammalian homologue of Drosophila connector enhancer of KSR (CNK), is known to interact with PSD-95 and has a short isoform, MAGUIN-2. The Densin-180 PDZ domain binds to the COOH-terminal PDZ domain-binding motif of MAGUIN-1. Densin-180 co-immunoprecipitates with MAGUIN-1 as well as with PSD-95 from the rat brain. In dissociated hippocampal neurones Densin-180 co-localizes with MAGUINs and PSD-95, mainly at neuritic spines. In transfected cells, Densin-180 forms a ternary complex with MAGUIN-1 and PSD-95, whereas no association was detected between Densin-180 and PSD-95 in the absence of MAGUIN-1. MAGUIN-1 forms a dimer or multimer via the COOH-terminal leucine-rich region which is present in MAGUIN-1 but not in MAGUIN-2. Among the PDZ domains of PSD-95, the first is sufficient for interaction with MAGUIN-1. These results suggest that the potential to dimerize or multimerize allows MAGUIN-1 to bind simultaneously to both Densin-180 and PSD-95, leading to the ternary complex assembly of these proteins at the postsynaptic membrane (Ohtakara, 2002).

PSD-95 and synaptic function

To identify the molecular mechanisms underlying psychostimulant-elicited plasticity in the brain reward system, a phenotype-driven approach was undertaken using genome-wide microarray profiling of striatal transcripts from three genetic and one pharmacological mouse models of psychostimulant or dopamine supersensitivity. A small set of co-affected genes was identified. One of these genes, encoding the synaptic scaffolding protein PSD-95, is downregulated in the striatum of all three mutants and in chronically, but not acutely, cocaine-treated mice. At the synaptic level, enhanced long-term potentiation (LTP) of the frontocortico-accumbal glutamatergic synapses correlates with PSD-95 reduction in every case. Finally, targeted deletion of PSD-95 in an independent line of mice enhances LTP, augments the acute locomotor-stimulating effects of cocaine, but leads to no further behavioral plasticity in response to chronic cocaine. These findings uncover a previously unappreciated role of PSD-95 in psychostimulant action and identify a molecular and cellular mechanism shared between drug-related plasticity and learning (Yao, 2004).

Single-particle electron microscopy (EM) combined with biochemical measurements revealed the molecular shape of SAP97 (also known as hDlg) and a monomer-dimer transition that depends on the N-terminal L27 domain. Overexpression of SAP97 drives GluR1 to synapses, potentiates AMPA receptor (AMPAR) excitatory postsynaptic currents (EPSCs), and occludes LTP. Synaptic potentiation and GluR1 delivery are dissociable by L27 domain mutants that inhibit multimerization of SAP97. Loss of potentiation is correlated with faster turnover of monomeric SAP97 mutants in dendritic spines. It is proposed that L27-mediated interactions of SAP97 with itself or other proteins regulate the synaptic delivery of AMPARs. RNAi knockdown of endogenous PSD-95 depletes surface GluR1 and impaires AMPA EPSCs. In contrast, RNAi knockdown of endogenous SAP97 reduces surface expression of both GluR1 and GluR2 and inhibits both AMPA and NMDA EPSCs. Thus SAP97 has a broader role than its close relative, PSD-95, in the maintenance of synaptic function (Nakagawa, 2004).

Accumulation of AMPA receptors at synapses is a fundamental feature of glutamatergic synaptic transmission. Stargazin, a member of the TARP family, is an AMPAR auxiliary subunit allowing interaction of the receptor with scaffold proteins of the postsynaptic density, such as PSD-95. How PSD-95 and Stargazin regulate AMPAR number in synaptic membranes remains elusive. Using single quantum dot and FRAP imaging in live hippocampal neurons, it has been shown that exchange of AMPAR by lateral diffusion between extrasynaptic and synaptic sites mostly depends on the interaction of Stargazin with PSD-95 and not upon the GluR2 AMPAR subunit C terminus. Disruption of interactions between Stargazin and PSD-95 strongly increases AMPAR surface diffusion, preventing AMPAR accumulation at postsynaptic sites. Furthermore, AMPARs and Stargazin diffuse as complexes in and out synapses. These results propose a model in which the Stargazin-PSD-95 interaction plays a key role to trap and transiently stabilize diffusing AMPARs in the postsynaptic density (Bats, 2007).

The NMDA receptor, brain-derived neurotrophic factor (BDNF), postsynaptic density protein 95 (PSD-95) and phosphatidylinositol 3-kinase (PI3K) have all been implicated in long-term potentiation. This study shows that these molecules are involved in a single pathway for synaptic potentiation. In visual cortical neurons in young rodents, the neurotrophin receptor TrkB is associated with PSD-95. When BDNF is applied to cultured visual cortical neurons, PSD-95-labeled synaptic puncta enlarge, and fluorescent recovery after photobleaching (FRAP) reveals increased delivery of green fluorescent protein-tagged PSD-95 to the dendrites. The recovery of fluorescence requires TrkB, signaling through PI3K and the serine-threonine kinase Akt, and an intact Golgi apparatus. Stimulation of NMDARs mimics the PSD-95 trafficking that is induced by BDNF but requires active BDNF and PI3K. Furthermore, local dendritic contact with a BDNF-coated microsphere induces PSD-95 FRAP throughout the dendrites of the stimulated neuron, suggesting that this mechanism induces rapid neuron-wide synaptic increases in PSD-95 and refinement whenever a few robust inputs activate the NMDAR-BDNF-PI3K pathway (Yoshii, 2007).

The scaffold protein PSD-95 promotes the maturation and strengthening of excitatory synapses, functions that require proper localization of PSD-95 in the postsynaptic density (PSD). Phosphorylation of ser-295 enhances the synaptic accumulation of PSD-95 and the ability of PSD-95 to recruit surface AMPA receptors and potentiate excitatory postsynaptic currents. Evidence is presented that a Rac1-JNK1 signaling pathway mediates ser-295 phosphorylation and regulates synaptic content of PSD-95. Ser-295 phosphorylation is suppressed by chronic elevation, and increased by chronic silencing, of synaptic activity. Rapid dephosphorylation of ser-295 occurs in response to NMDA treatment that causes chemical long-term depression (LTD). Overexpression of a phosphomimicking mutant (S295D) of PSD-95 inhibits NMDA-induced AMPA receptor internalization and blocks the induction of LTD. The data suggest that synaptic strength can be regulated by phosphorylation-dephosphorylation of ser-295 of PSD-95 and that synaptic depression requires the dephosphorylation of ser-295 (Kim, 2007).

The activity-dependent regulation of AMPA-type glutamate receptors and the stabilization of synapses are critical to synaptic development and plasticity. One candidate molecule implicated in maturation, synaptic strengthening, and plasticity is PSD-95. This study found that acute knockdown of PSD-95 in brain slice cultures by RNAi arrests the normal development of synaptic structure and function that is driven by spontaneous activity. Surprisingly, PSD-95 is not necessary for the induction and early expression of long-term potentiation (LTP). However, knockdown of PSD-95 leads to smaller increases in spine size after chemically induced LTP. Furthermore, although at this age spine turnover is normally low and LTP produces a transient increase, in cells with reduced PSD-95 spine turnover is high and remains increased after LTP. Taken together, these data support a model in which appropriate levels of PSD-95 are required for activity-dependent synapse stabilization after initial phases of synaptic potentiation (Ehrlich, 2007).

Long-term potentiation is accompanied by dendritic spine growth and changes in the composition of the postsynaptic density (PSD). Activity-dependent growth of apical spines of CA1 pyramidal neurons is accompanied by destabilization of the PSD that results in transient loss and rapid replacement of PSD-95 and SHANK2. Signaling through PSD-95 is required for activity-dependent spine growth and trafficking of SHANK2. N-terminal PDZ and C-terminal guanylate kinase domains of PSD-95 are required for both processes, indicating that PSD-95 coordinates multiple signals to regulate morphological plasticity. Activity-dependent trafficking of PSD-95 is triggered by phosphorylation at serine 73, a conserved calcium/calmodulin-dependent protein kinase II (CaMKII) consensus phosphorylation site, which negatively regulates spine growth and potentiation of synaptic currents. It is proposed that PSD-95 and CaMKII act at multiple steps during plasticity induction to initially trigger and later terminate spine growth by trafficking growth-promoting PSD proteins out of the active spine (Steiner, 2008).

Diacylglycerol (DAG) is an important lipid signalling molecule that exerts an effect on various effector proteins including protein kinase C. A main mechanism for DAG removal is to convert it to phosphatidic acid (PA) by DAG kinases (DGKs). However, it is not well understood how DGKs are targeted to specific subcellular sites and tightly regulates DAG levels. The neuronal synapse is a prominent site of DAG production. This study shows that DGKzeta is targeted to excitatory synapses through its direct interaction with the postsynaptic PDZ scaffold PSD-95. Overexpression of DGKzeta in cultured neurons increases the number of dendritic spines, which receive the majority of excitatory synaptic inputs, in a manner requiring its catalytic activity and PSD-95 binding. Conversely, DGKzeta knockdown reduces spine density. Mice deficient in DGKzeta expression show reduced spine density and excitatory synaptic transmission. Time-lapse imaging indicates that DGKzeta is required for spine maintenance but not formation. It is proposed that PSD-95 targets DGKzeta to synaptic DAG-producing receptors to tightly couple synaptic DAG production to its conversion to PA for the maintenance of spine density (Kim, 2009).

Phase Transition in Postsynaptic Densities Underlies Formation of Synaptic Complexes and Synaptic Plasticity

Postsynaptic densities (PSDs) are membrane semi-enclosed, submicron protein-enriched cellular compartments beneath postsynaptic membranes, which constantly exchange their components with bulk aqueous cytoplasm in synaptic spines. Formation and activity-dependent modulation of PSDs is considered as one of the most basic molecular events governing synaptic plasticity in the nervous system. This study discovered that SynGAP, one of the most abundant PSD proteins and a Ras/Rap GTPase activator, forms a homo-trimer and binds to multiple copies of PSD-95 (see Drosophila Dlg1). Binding of SynGAP to PSD-95 induces phase separation of the complex, forming highly concentrated liquid-like droplets reminiscent of the PSD. The multivalent nature of the SynGAP/PSD-95 complex is critical for the phase separation to occur and for proper activity-dependent SynGAP dispersions from the PSD. In addition to revealing a dynamic anchoring mechanism of SynGAP at the PSD, these results also suggest a model for phase-transition-mediated formation of PSD (Zeng, 2016).

Discs large regulates the generation of memory T cells

Mammalian ortholog of Drosophila cell polarity protein, Dlg1, plays a critical role in neural synapse formation, epithelial cell homeostasis, and urogenital development. More recently, it has been proposed that Dlg1 may also be involved in the regulation of T-cell proliferation, migration, and Ag-receptor signaling. However, a requirement for Dlg1 in development and function of T lineage cells remains to be established. This study investigated a role for Dlg1 during T-cell development and function using a combination of conditional Dlg1 KO and two different Cre expression systems where Dlg1 deficiency is restricted to the T-cell lineage only, or all hematopoietic cells. Using three different TCR models, it was shown that Dlg1 is not required during development and selection of thymocytes bearing functionally rearranged TCR transgenes. Moreover, Dlg1 is dispensable in the activation and proliferative expansion of Ag-specific TCR-transgenic CD4(+) and CD8(+) T cells in vitro and in vivo. Surprisingly, however, it was showm that Dlg1 is required for normal generation of memory T cells during endogenous response to cognate Ag. Thus, Dlg1 is not required for the thymocyte selection or the activation of primary T cells, however it is involved in the generation of memory T cells (Gmyrek, 2013).

Table of contents

discs large 1: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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