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

Characterization of zebrafish PSD-95 gene family members

The PSD-95 family of membrane- associated guanylate kinases (MAGUKs) are thought to act as molecular scaffolds that regulate the assembly and function of the multiprotein signaling complex found at the postsynaptic density of excitatory synapses. Genetic analysis of PSD-95 family members in the mammalian nervous system has so far been difficult, but the zebrafish is emerging as an ideal vertebrate system for studying the role of particular genes in the developing and mature nervous system. Here we describe the cloning of the zebrafish orthologs of PSD-95, PSD-93, and two isoforms of SAP-97. Using in situ hybridization analysis we show that these zebrafish MAGUKs have overlapping but distinct patterns of expression in the developing nervous system and craniofacial skeleton. Using a pan-MAGUK antibody we show that MAGUK proteins localize to neurons within the developing hindbrain, cerebellum, visual and olfactory systems, and to skin epithelial cells. In the olfactory and visual systems MAGUK proteins are expressed strongly in synaptic regions, and the onset of expression in these areas coincides with periods of synapse formation. These data are consistent with the idea that PSD-95 family members are involved in synapse assembly and function, and provide a platform for future functional studies in vivo in a highly tractable model organism.

Association of potassium channel Kv3.4 subunits with pre- and post-synaptic structures in brainstem and spinal cord

Voltage-gated K+ channels (Kv) are divided into eight subfamilies (Kv1-8) and play a major role in determining the excitability of neurones. Members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique pattern of expression, although single neurones can express more than one subtype. Of the Kv3 subunits relatively little is known of the Kv3.4 subunit distribution in the nervous system, particularly in the brainstem and spinal cord of the rat. We performed immunohistochemistry to determine both the cellular and sub-cellular distribution of the Kv3.4 subunit in these areas. Kv3.4 subunit immunoreactivity (Kv3.4-IR) was widespread, with dense, punctate staining in many regions including the intermediolateral cell column (IML) and the dorsal vagal nucleus (DVN), nucleus ambiguus (NA) and nucleus tractus solitarius (NTS). In the ventral horn a presynaptic location was confirmed by co-localization of Kv3.4-IR with the synaptic vesicle protein, SV2 and also with the glutamate vesicle markers vesicular glutamate transporter (VGluT) 1, VGluT2 or the glycine transporter GlyT2, suggesting a role for the channel in both excitatory and inhibitory neurotransmission. Electron microscopy confirmed a presynaptic terminal location of Kv3.4-IR in the VH, IML, DVN, NA and NTS. Interestingly however, patches of Kv3.4-IR were also revealed postsynaptically in dendritic and somatic structures throughout these areas. This staining was striking due to its localization at synaptic junctions at terminals with morphological features consistent with excitatory functions, suggesting an association with the postsynaptic density. Therefore the pre and postsynaptic localization of Kv3.4-IR suggests a role both in the control of transmitter release and in regulating neuronal excitability.

PDZ domain proteins of synapses

PDZ domains are protein-interaction domains that are often found in multi-domain scaffolding proteins. PDZ-containing scaffolds assemble specific proteins into large molecular complexes at defined locations in the cell. In the postsynaptic density of neuronal excitatory synapses, PDZ proteins such as PSD-95 organize glutamate receptors and their associated signalling proteins and determine the size and strength of synapses. PDZ scaffolds also function in the dynamic trafficking of synaptic proteins by assembling cargo complexes for transport by molecular motors. As key organizers that control synaptic protein composition and structure, PDZ scaffolds are themselves highly regulated by synthesis and degradation, subcellular distribution and post-translational modification.

Presynaptic trafficking of synaptotagmin I is regulated by protein palmitoylation

Protein palmitoylation plays a critical role in sorting and targeting of several proteins to pre- and postsynaptic sites. In this study, we have analyzed the role of palmitoylation in trafficking of synaptotagmin I and its modulation by synaptic activity. We found that palmitoylation of N-terminal cysteines contributed to sorting of synaptotagmin I to an intracellular vesicular compartment at the presynaptic terminal. Presynaptic targeting is a unique feature of N-terminal sequences of synaptotagmin I because the palmitoylated N terminus of synaptotagmin VII failed to localize to presynaptic sites. We also found that palmitate was stably associated with both synaptotagmin I and SNAP-25 and that rapid neuronal depolarization did not affect palmitate turnover on these proteins. However, long-term treatment with drugs that either block synaptic activity or disrupt SNARE complex assembly modulated palmitoylation and accumulation of synaptotagmin I at presynaptic sites. We conclude that palmitoylation is involved in trafficking of specific elements involved in transmitter release and that distinct mechanisms regulate addition and removal of palmitate on select neuronal proteins.

Interaction of PSD-95 with potassium channels visualized by fluorescence lifetime-based resonance energy transfer imaging

Resonance energy transfer (RET) has been extensively used to estimate the distance between two different fluorophores. This study demonstrates how protein-protein interactions can be visualized and quantified in living cells by time-correlated single-photon counting (TCSPC) imaging techniques that exploit the RET between appropriate fluorescent labels. We used this method to investigate the association of the potassium inward rectifier channel Kir2.1 and the neuronal PDZ protein PSD-95, which has been implicated in subcellular targeting and clustering of ion channels. Our data show that the two proteins not only colocalize within clusters but also interact with each other. Moreover, the data allow a spatially resolved quantification of this protein-protein interaction with respect to the relative number and the proximity between interacting molecules. Depending on the subcellular localization, a fraction of 20 to 60% of PSD-95 molecules interacted with Kir2.1 channels, approximating their fluorescent labels by less than 5 nm.

The Serotonin 5-HT2A and 5-HT2C Receptors Interact with Specific Sets of PDZ Proteins

The 5-hydroxytryptamine type 2A (5-HT(2A)) receptor and the 5-HT(2C) receptor are closely related members of the G-protein-coupled receptors activated by serotonin that share very similar pharmacological profiles and cellular signaling pathways. These receptors express a canonical class I PDZ ligand (SXV) at their C-terminal extremity. Here, we have identified proteins that interact with the PDZ ligand of the 5-HT(2A) and 5-HT(2C) receptors by a proteomic approach associating affinity chromatography using immobilized synthetic peptides encompassing the PDZ ligand and mass spectrometry. We report that both receptor C termini interact with specific sets of PDZ proteins in vitro. The 5-HT(2C) receptor but not the 5-HT(2A) receptor binds to the Veli-3.CASK.Mint1 ternary complex and to SAP102. In addition, the 5-HT(2C) receptor binds more strongly to PSD-95 and MPP-3 than the 5-HT(2A) receptor. In contrast, a robust interaction between the 5-HT(2A) receptor and the channel-interacting PDZ protein CIPP was found, whereas CIPP did not significantly associate with the 5-HT(2C) receptor. We also show that residues located at the -1 position and upstream the PDZ ligand in the C terminus of the 5-HT(2A) and 5-HT(2C) receptors are major determinants in their interaction with specific PDZ proteins. Immunofluorescence and electron microscopy studies strongly suggested that these specific interactions also take place in living cells and that the 5-HT(2) receptor-PDZ protein complexes occur in intracellular compartments. The interaction of the 5-HT(2A) and the 5-HT(2C) receptor with specific sets of PDZ proteins may contribute to their different signal transduction properties.

Intracellular association of glycine receptor with gephyrin increases its plasma membrane accumulation rate

Gephyrin, a tubulin-binding protein, is the core of inhibitory postsynaptic scaffolds stabilizing glycine receptors (GlyRs) and/or GABA(A) receptors. Previous ultrastructural studies in vivo and in vitro have reported a localization of gephyrin to intracellular cisternas during development or after glycinergic denervation (Seitanidou et al., 1992; Colin et al., 1996, 1998). These data were compatible with a traffic of this cytoplasmic, but membrane-associated, protein together with membrane proteins such as GlyR after exocytosis and/or endocytosis pathways. We have now investigated the consequences of a GlyR-gephyrin interaction on the localization and the dynamics of these two molecules in African green monkey kidney cells (COS-7) cells and in neurons transfected with green fluorescent protein-tagged-gephyrin and myc-tagged GlyR alpha1 subunits. In these experiments, myc-tagged GlyR alpha1 contained, or did not contain, the gephyrin-binding sequence (betagb) of the GlyR beta subunit. We report here that GlyR-gephyrin interaction localizes gephyrin to GlyR-containing organelles. Videomicroscopy and nocodazole treatment indicate that the movements of these vesicles are microtubule dependent. Expressing GlyR alpha1 with a thrombin cleavage site between the myc-tag and the N terminal of the GlyR alpha1 subunit (Rosenberg et al., 2001) allowed monitoring of newly inserted receptors in the cell surface. Using temperature changes to block GlyR in, and then release it from, the trans-Golgi network, we show that gephyrin accelerates the accumulation of GlyR at the cell surface. Therefore, our data strongly suggest that some GlyR clusters are associated with gephyrin on their way to the cell surface and that this association increases the accumulation of GlyR at the plasma membrane.

Differential binding of the AP-2 adaptor complex and PSD-95 to the C-terminus of the NMDA receptor subunit NR2B regulates surface expression

NMDA receptor expression on the plasma membrane and at synaptic sites is tightly regulated. We have recently shown that the NMDA receptor subunit NR2B has an endocytic motif contained within its C-terminus. We now identify this motif as a consensus tyrosine-based motif (YEKL) and demonstrate that this sequence binds directly to the medium chain of the AP-2 adaptor, a protein complex that links internalized proteins to clathrin. Although the AP-2 binding site on NR2B is adjacent to the PSD-95 binding site, it is distinct, as mutation of tyrosine 1472 of the endocytic motif disrupts AP-2 binding but not binding to PSD-95. Internalization assays reveal that like PSD-95, both SAP97 and PSD-93 inhibit NR2B-mediated endocytosis. Furthermore, we find that co-expression of a PSD-95 mutant that is unable to cluster NMDA receptors also inhibits NR2B-mediated endocytosis. Together, these data demonstrate that AP-2 and PSD-95 bind to unique sites on the C-terminus of NR2B and have antagonistic functional consequences that are independent of the ability of the PSD-95 to cluster receptors on the plasma membrane.