Regulation of NLGN3 and the Synaptic Rho-GEF Signaling Pathway by CDK5.

Neuroligins (NLGNs) are postsynaptic cell adhesion molecules that are involved in synapse assembly and function. The NLGN gene family consists of 5 genes (NLGN1-3, 4X, and 4Y). NLGN3 forms heterodimers with other NLGNs and is expressed at both excitatory and inhibitory synapses, although the distinct role at different synapses is not fully understood. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that targets various neuronal substrates to impact neuronal migration, neurite outgrowth, synaptic transmission, and plasticity. Both NLGNs and their presynaptic binding partners neurexins are highly associated with neurodevelopmental disorders. The NLGN3 gene is on the X chromosome and variants in NLGN3 have been linked to the pathophysiology in neurodevelopmental disorders. To better understand the endogenous modulation of NLGN3, we generated an HA-tagged knock-in mouse. We found that Cdk5 associates with NLGN3 in vivo and phosphorylates NLGN3 on serine 725 (S725) in the knock-in mouse of either sex. The phosphorylation affects the NLGN3 association with Kalirin-7, a postsynaptic guanine nucleotide exchange factors for Rho GTPase family proteins. We further observed that the phosphorylation modulates NLGN3 surface expression and NLGN3-mediated synaptic currents in cultured rat neurons. Thus, we characterized NLGN3 as a novel Cdk5 substrate and revealed the functional consequences of NLGN3 S725 phosphorylation in neurons. Our study provides a novel molecular mechanism underlying Cdk5-mediated regulation of postsynaptic cell adhesion molecules.SIGNIFICANCE STATEMENT NLGN3 is involved in synapse assembly and function at both excitatory and inhibitory synapses and has been associated with the pathophysiology of neurodevelopmental disorders. Cdk5 has brain-specific activity and is involved in neuronal transmission, synapse function, and plasticity. Here, we characterize NLGN3 as a Cdk5 substrate for the first time and show that Cdk5-mediated phosphorylation regulates NLGN3 function. We demonstrate that NLGN3 S725 is a Cdk5 phosphorylation site, and reveal that the site is important for NLGN3 association with Kalirin-7, NLGN3 surface expression, and NLGN3-mediated synaptic transmission.

Autism Spectrum Disorder/Intellectual Disability-Associated Mutations in Trio Disrupt Neuroligin 1-Mediated Synaptogenesis.

We recently identified an autism spectrum disorder/intellectual disability (ASD/ID)-related de novo mutation hotspot in the Rac1-activating GEF1 domain of the protein Trio. Trio is a Rho guanine nucleotide exchange factor (RhoGEF) that is essential for glutamatergic synapse function. An ASD/ID-related mutation identified in Trio's GEF1 domain, Trio D1368V, produces a pathologic increase in glutamatergic synaptogenesis, suggesting that Trio is coupled to synaptic regulatory mechanisms that govern glutamatergic synapse formation. However, the molecular mechanisms by which Trio regulates glutamatergic synapses are largely unexplored. Here, using biochemical methods, we identify an interaction between Trio and the synaptogenic protein Neuroligin 1 (NLGN1) in the brain. Molecular biological approaches were then combined with super-resolution dendritic spine imaging and whole-cell voltage-clamp electrophysiology in hippocampal slices from male and female rats to examine the impact ASD/ID-related Trio mutations have on NLGN1-mediated synaptogenesis. We find that an ASD/ID-related mutation in Trio's eighth spectrin repeat region, Trio N1080I, inhibits Trio's interaction with NLGN1 and prevents Trio D1368V-mediated synaptogenesis. Inhibiting Trio's interaction with NLGN1 via Trio N1080I blocked NLGN1-mediated synaptogenesis and increases in synaptic NMDA Receptor function but not NLGN1-mediated increases in synaptic AMPA Receptor function. Finally, we show that the aberrant synaptogenesis produced by Trio D1368V is dependent on NLGN signaling. Our findings demonstrate that ASD/ID-related mutations in Trio are able to pathologically increase as well as decrease NLGN-mediated effects on glutamatergic neurotransmission, and point to an NLGN1-Trio interaction as part of a key pathway involved in ASD/ID etiology.SIGNIFICANCE STATEMENT A number of genes have been implicated in the development of autism spectrum disorder/intellectual disability (ASD/ID) in humans. It is now important to identify relationships between these genes to uncover specific cellular regulatory pathways that contribute to these disorders. In this study, we discover that two glutamatergic synapse regulatory proteins implicated in ASD/ID, Trio and Neuroligin 1, interact with one another to promote glutamatergic synaptogenesis. We also identify ASD/ID-related mutations in Trio that either inhibit or augment Neuroligin 1-mediated glutamatergic synapse formation. Together, our results identify a synaptic regulatory pathway that, when disrupted, likely contributes to the development of ASD/ID. Going forward, it will be important to determine whether this pathway represents a point of convergence of other proteins implicated in ASD/ID.

The STEP61 interactome reveals subunit-specific AMPA receptor binding and synaptic regulation.

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific protein phosphatase that regulates a variety of synaptic proteins, including NMDA receptors (NAMDRs). To better understand STEP's effect on other receptors, we used mass spectrometry to identify the STEP61 interactome. We identified a number of known interactors, but also ones including the GluA2 subunit of AMPA receptors (AMPARs). We show that STEP61 binds to the C termini of GluA2 and GluA3 as well as endogenous AMPARs in hippocampus. The synaptic expression of GluA2 and GluA3 is increased in STEP-KO mouse brain, and STEP knockdown in hippocampal slices increases AMPAR-mediated synaptic currents. Interestingly, STEP61 overexpression reduces the synaptic expression and synaptic currents of both AMPARs and NMDARs. Furthermore, STEP61 regulation of synaptic AMPARs is mediated by lysosomal degradation. Thus, we report a comprehensive list of STEP61 binding partners, including AMPARs, and reveal a central role for STEP61 in differentially organizing synaptic AMPARs and NMDARs.

PSD-95 binding dynamically regulates NLGN1 trafficking and function.

PSD-95 is a scaffolding protein that regulates the synaptic localization of many receptors, channels, and signaling proteins. The NLGN gene family encodes single-pass transmembrane postsynaptic cell adhesion molecules that are important for synapse assembly and function. At excitatory synapses, NLGN1 mediates transsynaptic binding with neurexin, a presynaptic cell adhesion molecule, and also binds to PSD-95, although the relevance of the PSD-95 interaction is not clear. We now show that disruption of the NLGN1 and PSD-95 interaction decreases surface expression of NLGN1 in cultured neurons. Furthermore, PKA phosphorylates NLGN1 on S839, near the PDZ ligand, and dynamically regulates PSD-95 binding. A phosphomimetic mutation of NLGN1 S839 significantly reduced PSD-95 binding. Impaired NLGN1/PSD-95 binding diminished synaptic NLGN1 expression and NLGN1-mediated synaptic enhancement. Our results establish a phosphorylation-dependent molecular mechanism that regulates NLGN1 and PSD-95 binding and provides insights into excitatory synaptic development and function.

Regulation of glutamate receptors by striatal-enriched tyrosine phosphatase 61 (STEP61 ).

Phosphorylation regulates glutamate receptor trafficking. The cytosolic C-terminal domains of both NMDA receptors (NMDARs) and AMPA receptors (AMPARs) have distinct motifs, which are substrates for serine/threonine and tyrosine phosphorylation. Decades of research have shown how phosphorylation of glutamate receptors mediates protein binding and receptor trafficking, ultimately controlling synaptic transmission and plasticity. STEP is a protein tyrosine phosphatase (also known as PTPN5), with several isoforms resulting from alternative splicing. Targets of STEP include a variety of important synaptic substrates, among which are the tyrosine kinase Fyn and glutamate receptors. In particular, STEP61 , the longest isoform, dephosphorylates the NMDAR subunit GluN2B and strongly regulates the expression of NMDARs at synapses. This interplay between STEP, Fyn and GluN2B-containing NMDARs has been characterized by multiple groups. More recently, STEP61 was shown to bind to AMPARs in a subunit-specific manner and differentially regulate synaptic NMDARs and AMPARs. Because of its many effects on synaptic proteins, STEP has been implicated in regulating excitatory synapses during plasticity and playing a role in synaptic dysfunction in a variety of neurological disorders. In this review, we will highlight the ways in which STEP61 differentially regulates NMDARs and AMPARs, as well as its role in plasticity and disease.

Truncating variants in the SHANK1 gene are associated with a spectrum of neurodevelopmental disorders.

PURPOSE: In this study, we aimed to characterize the clinical phenotype of a SHANK1-related disorder and define the functional consequences of SHANK1 truncating variants. METHODS: Exome sequencing (ES) was performed for six individuals who presented with neurodevelopmental disorders. Individuals were ascertained with the use of GeneMatcher and Database of Chromosomal Imbalance and Phenotype in Humans Using Ensembl Resources (DECIPHER). We evaluated potential nonsense-mediated decay (NMD) of two variants by making knock-in cell lines of endogenous truncated SHANK1, and expressed the truncated SHANK1 complementary DNA (cDNA) in HEK293 cells and cultured hippocampal neurons to examine the proteins. RESULTS: ES detected de novo truncating variants in SHANK1 in six individuals. Evaluation of NMD resulted in stable transcripts, and the truncated SHANK1 completely lost binding with Homer1, a linker protein that binds to the C-terminus of SHANK1. These variants may disrupt protein-protein networks in dendritic spines. Dispersed localization of the truncated SHANK1 variants within the spine and dendritic shaft was also observed when expressed in neurons, indicating impaired synaptic localization of truncated SHANK1. CONCLUSION: This report expands the clinical spectrum of individuals with truncating SHANK1 variants and describes the impact these variants may have on the pathophysiology of neurodevelopmental disorders.

Neurolastin, a dynamin family GTPase, translocates to mitochondria upon neuronal stress and alters mitochondrial morphology in vivo.

Neurolastin is a dynamin family GTPase that also contains a RING domain and exhibits both GTPase and E3 ligase activities. It is specifically expressed in the brain and is important for synaptic transmission, as neurolastin knockout animals have fewer dendritic spines and exhibit a reduction in functional synapses. Our initial study of neurolastin revealed that it is membrane-associated and partially co-localizes with endosomes. Using various biochemical and cell-culture approaches, we now show that neurolastin also localizes to mitochondria in HeLa cells, cultured neurons, and brain tissue. We found that the mitochondrial localization of neurolastin depends upon an N-terminal mitochondrial targeting sequence and that neurolastin is imported into the mitochondrial intermembrane space. Although neurolastin was only partially mitochondrially localized at steady state, it displayed increased translocation to mitochondria in response to neuronal stress and mitochondrial fragmentation. Interestingly, inactivation or deletion of neurolastin's RING domain also increased its mitochondrial localization. Using EM, we observed that neurolastin knockout animals have smaller but more numerous mitochondria in cerebellar Purkinje neurons, indicating that neurolastin regulates mitochondrial morphology. We conclude that the brain-specific dynamin GTPase neurolastin exhibits stress-responsive localization to mitochondria and is required for proper mitochondrial morphology.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits.

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca2+ ) into the post-synaptic compartment. Ca2+ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

DLG2 variants in patients with pubertal disorders.

PURPOSE: Impaired function of gonadotropin-releasing hormone (GnRH) neurons can cause a phenotypic spectrum ranging from delayed puberty to isolated hypogonadotropic hypogonadism (IHH). We sought to identify a new genetic etiology for these conditions. METHODS: Exome sequencing was performed in an extended family with autosomal dominant, markedly delayed puberty. The effects of the variant were studied in a GnRH neuronal cell line. Variants in the same gene were sought in a large cohort of individuals with IHH. RESULTS: We identified a rare missense variant (F900V) in DLG2 (which encodes PSD-93) that cosegregated with the delayed puberty. The variant decreased GnRH expression in vitro. PSD-93 is an anchoring protein of NMDA receptors, a type of glutamate receptor that has been implicated in the control of puberty in laboratory animals. The F900V variant impaired the interaction between PSD-93 and a known binding partner, Fyn, which phosphorylates NMDA receptors. Variants in DLG2 that also decreased GnRH expression were identified in three unrelated families with IHH. CONCLUSION: The findings indicate that variants in DLG2/PSD-93 cause autosomal dominant delayed puberty and may also contribute to IHH. The findings also suggest that the pathogenesis involves impaired NMDA receptor signaling and consequently decreased GnRH secretion.

Isoform-specific cleavage of neuroligin-3 reduces synapse strength.

The assembly and maintenance of synapses are dynamic processes that require bidirectional contacts between the pre- and postsynaptic structures. A network of adhesion molecules mediate this physical interaction between neurons. How synapses are disassembled and if there are distinct mechanisms that govern the removal of specific adhesion molecules remain unclear. Here, we report isoform-specific proteolytic cleavage of neuroligin-3 in response to synaptic activity and protein kinase C signaling resulting in reduced synapse strength. Although neuroligin-1 and neuroligin-2 are not directly cleaved by this pathway, when heterodimerized with neuroligin-3, they too undergo proteolytic cleavage. Thus protein kinase C-dependent cleavage is mediated through neuroligin-3. Recent studies on glioma implicate the neuroligin-3 ectodomain as a mitogen. Here we demonstrate: (1) there are mechanisms governing specific adhesion molecule remodeling; (2) neuroligin-3 is a key regulator of neuroligin cleavage events; and (3) there are two cleavage pathways; basal and activity-dependent that produce the mitogenic form of neuroligin-3.

Metabotropic glutamate receptor trafficking.

The metabotropic glutamate receptors (mGlu receptors) are G protein-coupled receptors that bind to the excitatory neurotransmitter glutamate and are important in the modulation of neuronal excitability, synaptic transmission, and plasticity in the central nervous system. Trafficking of mGlu receptors in and out of the synaptic plasma membrane is a fundamental mechanism modulating excitatory synaptic function through regulation of receptor abundance, desensitization, and signaling profiles. In this review, we cover the regulatory mechanisms determining surface expression and endocytosis of mGlu receptors, with particular focus on post-translational modifications and receptor-protein interactions. The literature we review broadens our insight into the precise events defining the expression of functional mGlu receptors at synapses, and will likely contribute to the successful development of novel therapeutic targets for a variety of developmental, neurological, and psychiatric disorders.

PSD-95 stabilizes NMDA receptors by inducing the degradation of STEP61.

Phosphorylation regulates surface and synaptic expression of NMDA receptors (NMDARs). Both the tyrosine kinase Fyn and the tyrosine phosphatase striatal-enriched protein tyrosine phosphatase (STEP) are known to target the NMDA receptor subunit GluN2B on tyrosine 1472, which is a critical residue that mediates NMDAR endocytosis. STEP reduces the surface expression of NMDARs by promoting dephosphorylation of GluN2B Y1472, whereas the synaptic scaffolding protein postsynaptic density protein 95 (PSD-95) stabilizes the surface expression of NMDARs. However, nothing is known about a potential functional interaction between STEP and PSD-95. We now report that STEP61 binds to PSD-95 but not to other PSD-95 family members. We find that PSD-95 expression destabilizes STEP61 via ubiquitination and degradation by the proteasome. Using subcellular fractionation, we detect low amounts of STEP61 in the PSD fraction. However, STEP61 expression in the PSD is increased upon knockdown of PSD-95 or in vivo as detected in PSD-95-KO mice, demonstrating that PSD-95 excludes STEP61 from the PSD. Importantly, only extrasynaptic NMDAR expression and currents were increased upon STEP knockdown, as is consistent with low STEP61 localization in the PSD. Our findings support a dual role for PSD-95 in stabilizing synaptic NMDARs by binding directly to GluN2B but also by promoting synaptic exclusion and degradation of the negative regulator STEP61.

A Rare Variant Identified Within the GluN2B C-Terminus in a Patient with Autism Affects NMDA Receptor Surface Expression and Spine Density.

NMDA receptors (NMDARs) are ionotropic glutamate receptors that are crucial for neuronal development and higher cognitive processes. NMDAR dysfunction is involved in a variety of neurological and psychiatric diseases; however, the mechanistic link between the human pathology and NMDAR dysfunction is poorly understood. Rare missense variants within NMDAR subunits have been identified in numerous patients with mental or neurological disorders. We specifically focused on the GluN2B NMDAR subunit, which is highly expressed in the hippocampus and cortex throughout development. We analyzed several variants located in the GluN2B C terminus and found that three variants in patients with autism (S1415L) or schizophrenia (L1424F and S1452F) (S1413L, L1422F, and S1450F in rodents, respectively) displayed impaired binding to membrane-associated guanylate kinase (MAGUK) proteins. In addition, we observed a deficit in surface expression for GluN2B S1413L. Furthermore, there were fewer dendritic spines in GluN2B S1413L-expressing neurons. Importantly, synaptic NMDAR currents in neurons transfected with GluN2B S1413L in GluN2A/B-deficient mouse brain slices revealed only partial rescue of synaptic current amplitude. Functional properties of GluN2B S1413L in recombinant systems revealed no change in receptor properties, consistent with synaptic defects being the result of reduced trafficking and targeting of GluN2B S1413L to the synapse. Therefore, we find that GluN2B S1413L displays deficits in NMDAR trafficking, synaptic currents, and spine density, raising the possibility that this mutation may contribute to the phenotype in this autism patient. More broadly, our research demonstrates that the targeted study of certain residues in NMDARs based on rare variants identified in patients is a powerful approach to studying receptor function.SIGNIFICANCE STATEMENT We have used a "bedside-to-bench" approach to investigate the functional regulation of NMDA receptors (NMDARs). Using information from deep sequencing of patients with neurological or psychiatric disorders, we investigated missense variants identified in the intracellular C-terminal domain of the GluN2B NMDAR subunit. We found several variants that displayed altered properties. In particular, one variant identified in a patient with autism, human GluN2B S1415L, displayed reduced surface expression and binding to PSD-95. Furthermore expression of GluN2B S1415L (S1413L in mouse) showed a deficit in rescue of synaptic NMDAR currents and fewer dendritic spines, consistent with other reports of spine abnormalities being associated with autism. More broadly, we demonstrate that using patient data is an effective approach to probing the structure/function relationship of NMDARs.

Phosphorylation of the kainate receptor (KAR) auxiliary subunit Neto2 at serine 409 regulates synaptic targeting of the KAR subunit GluK1.

Synaptic strength at excitatory synapses is determined by the presence of glutamate receptors (i.e. AMPA, NMDA, and kainate receptors) at the synapse. Synaptic strength is modulated by multiple factors including assembly of different receptor subunits, interaction with auxiliary subunits, and post-translational modifications of either the receptors or their auxiliary subunits. Using mass spectrometry, we found that the intracellular region of neuropilin and tolloid-like proteins (Neto) 1 and Neto2, the auxiliary subunits of kainate receptor (KARs), are phosphorylated by multiple kinases in vitro Specifically, Neto2 was phosphorylated at serine 409 (Ser-409) by Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein kinase A (PKA) both in vitro and in heterologous cells. Interestingly, we observed a substantial increase in Neto2 Ser-409 phosphorylation in the presence of CaMKII, and this phosphorylation was reduced in the presence of the KAR subunit GluK1 or GluK2. We also found endogenous phosphorylation of Neto2 at Ser-409 in the brain. Moreover, Neto2 Ser-409 phosphorylation inhibited synaptic targeting of GluK1 because, unlike WT Neto2 and the phosphodeficient mutant Neto2 S409A, the Neto2 S409D phosphomimetic mutant impeded GluK1 trafficking to synapses. These results support a molecular mechanism by which Neto2 phosphorylation at Ser-409 helps restrict GluK1 targeting to the synapse.

Autism-associated mutation inhibits protein kinase C-mediated neuroligin-4X enhancement of excitatory synapses.

Autism spectrum disorders (ASDs) comprise a highly heritable, multifarious group of neurodevelopmental disorders, which are characterized by repetitive behaviors and impairments in social interactions. Point mutations have been identified in X-linked Neuroligin (NLGN) 3 and 4X genes in patients with ASDs and all of these reside in their extracellular domains except for a single point mutation in the cytoplasmic domain of NLGN4X in which an arginine is mutated to a cysteine (R704C). Here we show that endogenous NLGN4X is robustly phosphorylated by protein kinase C (PKC) at T707, and R704C completely eliminates T707 phosphorylation. Endogenous NLGN4X is intensely phosphorylated on T707 upon PKC stimulation in human neurons. Furthermore, a phospho-mimetic mutation at T707 has a profound effect on NLGN4X-mediated excitatory potentiation. Our results now establish an important interplay between a genetic mutation, a key posttranslational modification, and robust synaptic changes, which can provide insights into the synaptic dysfunction of ASDs.

Dynamic Regulation of N-Methyl-d-aspartate (NMDA) and α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors by Posttranslational Modifications.

Many molecular mechanisms underlie the changes in synaptic glutamate receptor content that are required by neuronal networks to generate cellular correlates of learning and memory. During the last decade, posttranslational modifications have emerged as critical regulators of synaptic transmission and plasticity. Notably, phosphorylation, ubiquitination, and palmitoylation control the stability, trafficking, and synaptic expression of glutamate receptors in the central nervous system. In the current review, we will summarize some of the progress made by the neuroscience community regarding our understanding of phosphorylation, ubiquitination, and palmitoylation of the NMDA and AMPA subtypes of glutamate receptors.

TARP γ-8 glycosylation regulates the surface expression of AMPA receptors.

TARP [transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory protein] γ-8 is an auxiliary subunit of AMPA receptors that is widely distributed in the hippocampus. It has been shown that TARP γ-8 promotes surface expression of AMPA receptors; however, how TARP γ-8 regulates the expression of AMPA receptors remains unclear. In the present study, we examined the effect of TARP glycosylation on AMPA receptor trafficking. We first showed that TARP γ-8 is an N-glycosylated protein, which contains two glycosylation sites, Asn53 and Asn56, and compared this with the glycosylation of TARP γ-2 and the AMPA receptor auxiliary protein CNIH-2 (cornichon homologue 2). We next examine the effect of TARP glycosylation on TARP trafficking and also on AMPA receptor surface expression. We find that TARP γ-8 glycosylation is critical for surface expression of both TARP γ-8 and GluA1 in heterologous cells and neurons. Specifically, knockdown of TARP γ-8 causes a decrease in both total and surface AMPA receptors. We find that the expression of unglycosylated TARP γ-8 in cultured neurons is unable to restore GluA1 expression fully. Furthermore, when the maturation of TARP γ-8 is impaired, a large pool of immature GluA1 is retained intracellularly. Taken together, our data reveal an important role for the maturation of TARP γ-8 in the trafficking and function of the AMPA receptor complex.

Ubiquitin ligase RNF167 regulates AMPA receptor-mediated synaptic transmission.

AMPA receptors (AMPARs) mediate the majority of fast excitatory neurotransmission, and their density at postsynaptic sites determines synaptic strength. Ubiquitination is a posttranslational modification that dynamically regulates the synaptic expression of many proteins. However, very few of the ubiquitinating enzymes implicated in the process have been identified. In a screen to identify transmembrane RING domain-containing E3 ubiquitin ligases that regulate surface expression of AMPARs, we identified RNF167. Predominantly lysosomal, a subpopulation of RNF167 is located on the surface of cultured neurons. Using a RING mutant RNF167 or a specific shRNA to eliminate endogenous RNF167, we demonstrate that AMPAR surface expression increases in hippocampal neurons with disrupted RNF167 activity and that RNF167 is involved in activity-dependent ubiquitination of AMPARs. In addition, RNF167 regulates synaptic AMPAR currents, whereas synaptic NMDAR currents are unaffected. Therefore, our study identifies RNF167 as a selective regulator of AMPAR-mediated neurotransmission and expands our understanding of how ubiquitination dynamically regulates excitatory synapses.

Distance-dependent scaling of AMPARs is cell-autonomous and GluA2 dependent.

The extensive dendritic arbor of a pyramidal cell introduces considerable complexity to the integration of synaptic potentials. Propagation of dendritic potentials is largely passive, in contrast to regenerative axonal potentials that are maintained by voltage-gated sodium channels, leading to a declination in amplitude as dendritic potentials travel toward the soma in a manner that disproportionally affects distal synaptic inputs. To counteract this amplitude filtering, Schaffer collateral synapses onto CA1 pyramidal cells contain a varying number of AMPA receptors (AMPARs) per synapse that increases with distance from the soma, a phenomenon known as distance-dependent scaling. Here, we undertake an investigation into the molecular mechanisms of distance-dependent scaling. Using dendritic recordings from rat pyramidal neurons, we confirm the basic scaling phenomenon and find that it is expressed and can be manipulated cell autonomously. Finally, we show that it depends on the presence of both a reserve pool of AMPARs and the AMPAR subunit GluA2.

Regulation of metabotropic glutamate receptor 7 (mGluR7) internalization and surface expression by Ser/Thr protein phosphatase 1.

The metabotropic glutamate receptor type 7 (mGluR7) is the predominant group III mGluR in the presynaptic active zone, where it serves as an autoreceptor to inhibit neurotransmitter release. Our previous studies show that PKC phosphorylation of mGluR7 on Ser-862 is a key mechanism controlling constitutive and activity-dependent surface expression of mGluR7 by regulating a competitive interaction of calmodulin and protein interacting with C kinase (PICK1). As receptor phosphorylation and dephosphorylation are tightly coordinated through the precise action of protein kinases and phosphatases, dephosphorylation by phosphatases is likely to play an active role in governing the activity-dependent or agonist-induced changes in mGluR7 receptor surface expression. In the present study, we find that the serine/threonine protein phosphatase 1 (PP1) has a crucial role in the constitutive and agonist-induced dephosphorylation of Ser-862 on mGluR7. Treatment of neurons with PP1 inhibitors leads to a robust increase in Ser-862 phosphorylation and increased surface expression of mGluR7. In addition, Ser-862 phosphorylation of both mGluR7a and mGluR7b is a target of PP1. Interestingly, agonist-induced dephosphorylation of mGluR7 is regulated by PP1, whereas NMDA-mediated activity-induced dephosphorylation is not, illustrating there are multiple signaling pathways that affect receptor phosphorylation and trafficking. Importantly, PP1γ1 regulates agonist-dependent Ser-862 dephosphorylation and surface expression of mGluR7.