No abstract

Cadherins function in the adhesion of presynaptic and postsynaptic membranes at excitatory synapses. Here we show that the cadherinassociated protein neural plakophilin-related arm protein (NPRAP; also called ␦-catenin) binds via a postsynaptic density-95 (PSD-95)/ discs large/zona occludens-1 (PDZ) interaction to AMPA receptor (AMPAR)-binding protein (ABP) and the related glutamate receptor (GluR)-interacting protein (GRIP), two multi-PDZ proteins that bind the GluR2 and GluR3 AMPAR subunits. The resulting cadherin-NPRAP-ABP/GRIP complexes serve as anchorages for AMPARs. Exogenous NPRAP that was bound to cadherins at adherens junctions of Madin-Darby canine kidney cells recruited ABP from the cytosol to form cadherin-NPRAP-ABP complexes, dependent on NPRAP interaction with the ABP PDZ domain 2. The cadherin-NPRAP-ABP complexes also bound GluR2. In cultured hippocampal neurons, dominant-negative mutants of NPRAP designed to disrupt tethering of ABP to NPRAP-cadherin complexes reduced surface levels of endogenous GluR2, indicating that interaction with cadherin-NPRAP-ABP complexes stabilized GluR2 at the neuronal plasma membrane. Cadherins, NPRAP, GRIP, and GluR2 copurified in the fractionation of synaptosomes and the postsynaptic density, two fractions enriched in synaptic proteins. Furthermore, synaptosomes contain NPRAP-GRIP complexes, and NPRAP localizes with the postsynaptic marker PSD-95 and with AMPARs and GRIP at spines of hippocampal neurons. Thus, tethering is likely to take place at synaptic or perisynaptic sites. NPRAP also binds PSD-95, which is a scaffold for NMDA receptors, for AMPARs in complexes with auxiliary subunits, the TARPs (transmembrane AMPA receptor regulator proteins), and for adhesion molecules. Thus, the interaction of scaffolding proteins with cadherin-NPRAP complexes may anchor diverse signaling and adhesion molecules at cadherins.

Ca2ϩ channel inactivation is a key element in controlling the level of Ca 2ϩ entry through voltage-gated Ca 2ϩ channels. Interaction between the pore-forming ␣ 1 subunit and the auxiliary ␤ subunit is known to be a strong modulator of voltage-dependent inactivation. Here, we demonstrate that an N-terminal membrane anchoring site (MAS) of the ␤ 2a subunit strongly reduces ␣ 1A (Ca V 2.1) Ca 2ϩ channel inactivation. This effect can be mimicked by the addition of a transmembrane segment to the N terminus of the ␤ 2a subunit. Inhibition of inactivation by ␤ 2a also requires a link between MAS and another important molecular determinant, the ␤ interaction domain (BID). Our data suggest that mobility of the Ca 2ϩ channel I-II loop is necessary for channel inactivation. Interaction of this loop with other identified intracellular channel domains may constitute the basis of voltage-dependent inactivation. We thus propose a conceptually novel mechanism for slowing of inactivation by the ␤ 2a subunit, in which the immobilization of the channel inactivation gate occurs by means of MAS and BID.

No abstract

GABA B receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. Whereas heterodimerization between GABA B receptor GABA B R1 and GABA B R2 subunits is essential for functional expression, how neurons coordinate the assembly of these critical receptors remains to be established. Here we have identified Marlin-1, a novel GABA B receptor-binding protein that associates specifically with the GABA B R1 subunit in yeast, tissue culture cells, and neurons. Marlin-1 is expressed in the brain and exhibits a granular distribution in cultured hippocampal neurons. Marlin-1 binds different RNA species including the 3-untranslated regions of both the GABA B R1 and GABA B R2 mRNAs in vitro and also associates with RNA in cultured neurons. Inhibition of Marlin-1 expression via small RNA interference technology results in enhanced intracellular levels of the GABA B R2 receptor subunit without affecting the level of GABA B R1. Together our results suggest that Marlin-1 functions to regulate the cellular levels of GABA B R2 subunits, which may have significant effects on the production of functional GABA B receptor heterodimers. Therefore, our observations provide an added level of regulation for the control of GABA B receptor expression and for the efficacy of inhibitory synaptic transmission.It has become widely accepted that protein-protein interactions are responsible for the formation and maintenance of the signaling platforms that organize local transduction units (1). Protein associations have also been shown to modulate the synthesis, targeting, and stabilization of membrane receptors (2). The study of the spatial and temporal compartmentalization of G protein-coupled receptors (GPCRs) 1 is essential to understand the mechanisms for neuronal integration of multiple stimuli. The identification of signaling partners has been well documented for ion channels (3-5), but less is known about the proteins that assist GPCRs (6, 7). GABA B receptors mediate the slow and prolonged phase of synaptic inhibition (8) and, unlike other GPCRs, they require the formation of a heterodimer between GABA B R1 and GAB-A B R2 (9). These two subunits display high homology to metabotropic glutamate, Ca 2ϩ -sensing, vomeronasal, and putative pheromone receptors, and recombinant GABA B R1/GABA B R2 receptors mimic the effector-coupling and pharmacological properties of native receptors (10). Clinically, GABA B receptors have been implicated in depression, neuroprotection, and cognition, and the production of GABA B R1 knock-out mice has confirmed their role in epilepsy and pain (11,12). Furthermore, recent studies indicate that their activation may be beneficial in the treatment of withdrawal symptoms from addictive drugs (13). Given their critical role in synaptic transmission and their potential therapeutic implications, it is fundamental to understand all aspects of GABA B receptor signaling.The modulation of neuronal GABA B receptors is likely to occur at multiple levels rangi...

Regulated transport and local translation of mRNA in neurons are critical for modulating synaptic strength, maintaining proper neural circuitry, and establishing long term memory. Neuronal RNA granules are ribonucleoprotein particles that serve to transport mRNA along microtubules and control local protein synthesis in response to synaptic activity. Studies suggest that neuronal RNA granules share similar structures and functions with somatic P-bodies. We recently reported that the Huntington disease protein huntingtin (Htt) associates with Argonaute (Ago) and localizes to cytoplasmic P-bodies, which serve as sites of mRNA storage, degradation, and small RNAmediated gene silencing. Here we report that wild-type Htt associates with Ago2 and components of neuronal granules and cotraffics with mRNA in dendrites. Htt was found to co-localize with RNA containing the 3-untranslated region sequence of known dendritically targeted mRNAs. Knockdown of Htt in neurons caused altered localization of mRNA. When tethered to a reporter construct, Htt down-regulated reporter gene expression in a manner dependent on Ago2, suggesting that Htt may function to repress translation of mRNAs during transport in neuronal granules.

Novel mRNA isoforms for two members of the group III metabotropic glutamate receptors (mGluRs), called mGluR7b and mGluR8b, were identified from rat brain cerebral cortex and hippocampus. In both cases, the alternative splicing is generated by a similar out-of-frame insertion in the carboxyl-terminus that results in the replacement of the last 16 amino acids of mGluR7 and mGluR8 by 23 and 16 different amino acids, respectively. Distribution analysis for mGluR7 and mGluR8 isoforms revealed that the two splice variants are generally coexpressed in the same brain areas. The few exceptions were the olfactory bulb, in which only the mGluR7a form could be detected by reverse transcription-polymerase chain reaction, and the lateral reticular and ambiguous nuclei, which showed only mGluR8a labelling. Despite expression in the same regions, different mRNA abundance for the two variants of each receptor were observed. When transiently coexpressed in HEK 293 cells with the phospholipase C-activating chimeric G alpha qi9-G-protein, the a and b forms for both receptor subtypes showed a similar pharmacological profile. The rank order of potencies for both was: DL-amino-4-phosphonobutyrate > L-serine-O-phosphate > glutamate. However, the agonist potencies were significantly higher for mGluR8a, b compared with mGluR7a,b. In Xenopus oocytes, glutamate evoked currents only with mGluR8 when coexpressed with Kir 3.1 and 3.4. Glutamate-induced currents were antagonized by the group II/III antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. In conclusion, the two isoforms of each receptor have identical pharmacological profiles when expressed in heterologous systems, despite structural differences in the carboxyl-terminal domains.

The proteoloytic machinery comprising metalloproteases and γ-secretase, an intramembrane aspartyl protease involved in Alzheimer’s disease, cleaves several substrates besides the extensively studied amyloid precursor protein (APP). Some of these substrates, such as N-cadherin, are synaptic proteins involved in synapse remodeling and maintenance. Here we show, in rat and mice that metalloproteases and γ-secretase are physiologic regulators of synapses. Both proteases are synaptic, with γ-secretase tethered at the synapse by δ-catenin, a synaptic scafolding protein which also binds to N-cadherin and, through scaffolds, to α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR) and a metalloprotease. Activity-dependent proteolysis by metalloproteases and γ-secretase takes place at both sides of the synapse, with the metalloprotease cleavage being N-methyl-D-aspartic acid receptor (NMDAR)-dependent. This proteolysis decreases levels of synaptic proteins and diminishes synaptic transmission. Our results suggest that activity-dependent substrate cleavage by synaptic metalloproteases and γ-secretase modifies synaptic transmission, providing a novel form of synaptic autoregulation.