Activity-dependent endocytic sorting of kainate receptors to recycling or degradation pathways

GluK2 Q/R editing regulates kainate receptor signaling and long-term potentiation of AMPA receptors

Q/R editing of the kainate receptor (KAR) subunit GluK2 radically alters recombinant KAR properties, but the effects on endogenous KARs in vivo remain largely unexplored. Here, we compared GluK2 editing-deficient mice that express ∼95% unedited GluK2(Q) to wild-type counterparts that express ∼85% edited GluK2(R). At mossy fiber-CA3 (MF-CA3) synapses GluK2(Q) mice displayed increased postsynaptic KAR function and KAR-mediated presynaptic facilitation, demonstrating enhanced ionotropic function. Conversely, GluK2(Q) mice exhibited reduced metabotropic KAR function, assessed by KAR-mediated inhibition of slow after-hyperpolarization currents (ISAHP). GluK2(Q) mice also had fewer GluA1-and GluA3-containing AMPA receptors (AMPARs) and reduced postsynaptic AMPAR currents at both MF-CA3 and CA1-Schaffer collateral synapses. Moreover, long-term potentiation of AMPAR-mediated transmission at CA1-Schaffer collateral synapses was reduced in GluK2(Q) mice. These findings suggest that GluK2 Q/R editing influences ionotropic/metabotropic balance of KAR signaling to regulate synaptic expression of AMPARs and plasticity.

The TMEM106B T186S coding variant increases neurite arborization and synaptic density in primary hippocampal neurons

The lysosomal protein TMEM106B was identified as a risk modifier of multiple dementias including frontotemporal dementia and Alzheimer's disease. The gene comes in two major haplotypes, one associated with disease risk, and by comparison, the other with resilience. Only one coding polymorphism distinguishes the two alleles, a threonine-to-serine substitution at residue 185 (186 in mouse), that is inherited in disequilibrium with multiple non-coding variants. Transcriptional studies suggest synaptic, neuronal, and cognitive preservation in human subjects with the protective haplotype, while murine in vitro studies reveal dramatic effects of TMEM106B deletion on neuronal development. Despite this foundation, the field has not yet resolved whether coding variant is biologically meaningful, and if so, whether it has any specific effect on neuronal phenotypes. Here we studied how loss of TMEM106B or expression of the lone coding variant in isolation affected transcriptional signatures in the mature brain and neuronal structure during development in primary neurons. Homozygous expression of the TMEM106B T186S variant in knock-in mice increased cortical expression of genes associated with excitatory synaptic function and axon outgrowth, and promoted neurite branching, dendritic spine density, and synaptic density in primary hippocampal neurons. In contrast, constitutive TMEM106B deletion affected transcriptional signatures of myelination without altering neuronal development in vitro. Our findings show that the T186S variant is functionally relevant and may contribute to disease resilience during neurodevelopment.

Aggregation-prone Tau impairs mitochondrial import, which affects organelle morphology and neuronal complexity

Mitochondrial protein import is essential for organellar biogenesis, and thereby for the sufficient supply of cytosolic ATP - which is particularly important for cells with high energy demands like neurons. This study explores the prospect of import machinery perturbation as a cause of neurodegeneration instigated by the accumulation of aggregating proteins linked to disease. We found that the aggregation-prone Tau variant (TauP301L) reduces the levels of components of the import machinery of the outer (TOM20, encoded by TOMM20) and inner membrane (TIM23, encoded by TIMM23) while associating with TOM40 (TOMM40). Intriguingly, this interaction affects mitochondrial morphology, but not protein import or respiratory function; raising the prospect of an intrinsic rescue mechanism. Indeed, TauP301L induced the formation of tunnelling nanotubes (TNTs), potentially for the recruitment of healthy mitochondria from neighbouring cells and/or the disposal of mitochondria incapacitated by aggregated Tau. Consistent with this, inhibition of TNT formation (and rescue) reveals Tau-induced import impairment. In primary neuronal cultures, TauP301L induced morphological changes characteristic of neurodegeneration. Interestingly, these effects were mirrored in cells where the import sites were blocked artificially. Our results reveal a link between aggregation-prone Tau and defective mitochondrial import relevant to disease.

A detection method for neuronal death indicates abnormalities in intracellular membranous components in neuronal cells that underwent delayed death

Acute neuronal degeneration is always preceded under the light and electron microscopes by a stage called microvacuolation, which is characterized by a finely vacuolar alteration in the cytoplasm of the neurons destined to death. In this study, we reported a method for detecting neuronal death using two membrane-bound dyes, rhodamine R6 and DiOC₆(3), which may be associated with the so-called microvacuolation. This new method produced a spatiotemporally similar staining pattern to Fluoro-Jade B in kainic acid-damaged brains in mice. Further experiments showed that increased staining of rhodamine R6 and DiOC₆(3) was observed only in degenerated neurons, but not in glia, erythrocytes, or meninges. Different from Fluoro-Jade-related dyes, rhodamine R6 and DiOC₆(3) staining is highly sensitive to solvent extraction and detergent exposure. Staining with Nile red for phospholipids and filipin III for non-esterified cholesterol supports that the increased staining of rhodamine R6 and DiOC₆(3) might be associated with increased levels of phospholipids and free cholesterol in the perinuclear cytoplasm of damaged neurons. In addition to kainic acid-injected neuronal death, rhodamine R6 and DiOC₆(3) were similarly useful for detecting neuronal death in ischemic models either in vivo or in vitro. As far as we know, the staining with rhodamine R6 or DiOC₆(3) is one of a few histochemical methods for detecting neuronal death whose target molecules have been well defined and therefore may be useful for explaining experimental results as well as exploring the mechanisms of neuronal death. (250 words).

Bidirectional regulation of synaptic SUMOylation by Group 1 metabotropic glutamate receptors

SUMOylation is a post-translational modification essential to cell homeostasis. A tightly controlled equilibrium between SUMOylation and deSUMOylation processes is also critical to the neuronal function including neurotransmitter release and synaptic transmission and plasticity. Disruption of the SUMOylation homeostasis in neurons is associated with several neurological disorders. The balance between the SUMOylation and deSUMOylation of substrate proteins is maintained by a group of deSUMOylation enzymes called SENPs. We previously showed that the activation of type 5 metabotropic glutamate receptors (mGlu5R) first triggers a rapid increase in synaptic SUMOylation and then upon the sustained activation of these receptors, the deSUMOylase activity of SENP1 allows the increased synaptic SUMOylation to get back to basal levels. Here, we combined the use of pharmacological tools with subcellular fractionation and live-cell imaging of individual hippocampal dendritic spines to demonstrate that the synaptic accumulation of the deSUMOylation enzyme SENP1 is bidirectionally controlled by the activation of type 1 mGlu1 and mGlu5 receptors. Indeed, the pharmacological blockade of mGlu1R activation during type 1 mGluR stimulation leads to a faster and greater accumulation of SENP1 at synapses indicating that mGlu1R acts as a brake to the mGlu5R-dependent deSUMOylation process at the post-synapse. Altogether, our findings reveal that type 1 mGluRs work in opposition to dynamically tune the homeostasis of SUMOylation at the mammalian synapse.

Sustained postsynaptic kainate receptor activation downregulates AMPA receptor surface expression and induces hippocampal LTD

It is well established that long-term depression (LTD) can be initiated by either NMDA or mGluR activation. Here we report that sustained activation of GluK2 subunit-containing kainate receptors (KARs) leads to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) endocytosis and induces LTD of AMPARs (KAR-LTD_AMPAR) in hippocampal neurons. The KAR-evoked loss of surface AMPARs is blocked by the ionotropic KAR inhibitor UBP 310 indicating that KAR-LTD_AMPAR requires KAR channel activity. Interestingly, however, blockade of PKC or PKA also reduces GluA2 surface expression and occludes the effect of KAR activation. In acute hippocampal slices, kainate application caused a significant loss of GluA2-containing AMPARs from synapses and long-lasting depression of AMPAR excitatory postsynaptic currents in CA1. These data, together with our previously reported KAR-LTP_AMPAR, demonstrate that KARs can bidirectionally regulate synaptic AMPARs and synaptic plasticity via different signaling pathways.

Kainate receptors and synaptic plasticity

Synaptic plasticity has classically been characterized to involve the NMDA and AMPA subtypes of glutamate receptors, with NMDA receptors providing the key trigger for the induction of long-term plasticity leading to changes in AMPA receptor expression. Here we review the more subtle roles played by kainate receptors, which contribute critical postsynaptic signalling as well as playing major presynaptic auto-receptor roles. We focus on two research areas: plasticity of kainate receptors themselves and the contribution they make to the plasticity of synaptic transmission. This article is part of the special issue on Glutamate Receptors - Kainate receptors.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

Fragment-linking peptide design yields a high-affinity ligand for microtubule-based transport

Synthetic peptides are attractive candidates to manipulate protein-protein interactions inside the cell as they mimic natural interactions to compete for binding. However, protein-peptide interactions are often dynamic and weak. A challenge is to design peptides that make improved interactions with the target. Here, we devise a fragment-linking strategy-"mash-up" design-to deliver a high-affinity ligand, KinTag, for the kinesin-1 motor. Using structural insights from natural micromolar-affinity cargo-adaptor ligands, we have identified and combined key binding features in a single, high-affinity ligand. An X-ray crystal structure demonstrates interactions as designed and reveals only a modest increase in interface area. Moreover, when genetically encoded, KinTag promotes transport of lysosomes with higher efficiency than natural sequences, revealing a direct link between motor-adaptor binding affinity and organelle transport. Together, these data demonstrate a fragment-linking strategy for peptide design and its application in a synthetic motor ligand to direct cellular cargo transport.

Missense mutation of Fmr1 results in impaired AMPAR-mediated plasticity and socio-cognitive deficits in mice

Fragile X syndrome (FXS) is the most frequent form of inherited intellectual disability and the best-described monogenic cause of autism. CGG-repeat expansion in the FMR1 gene leads to FMR1 silencing, loss-of-expression of the Fragile X Mental Retardation Protein (FMRP), and is a common cause of FXS. Missense mutations in the FMR1 gene were also identified in FXS patients, including the recurrent FMRP-R138Q mutation. To investigate the mechanisms underlying FXS caused by this mutation, we generated a knock-in mouse model (Fmr1R138Q) expressing the FMRP-R138Q protein. We demonstrate that, in the hippocampus of the Fmr1R138Q mice, neurons show an increased spine density associated with synaptic ultrastructural defects and increased AMPA receptor-surface expression. Combining biochemical assays, high-resolution imaging, electrophysiological recordings, and behavioural testing, we also show that the R138Q mutation results in impaired hippocampal long-term potentiation and socio-cognitive deficits in mice. These findings reveal the functional impact of the FMRP-R138Q mutation in a mouse model of FXS.

Sorting nexin-27 regulates AMPA receptor trafficking through the synaptic adhesion protein LRFN2

The endosome-associated cargo adaptor sorting nexin-27 (SNX27) is linked to various neuropathologies through sorting of integral proteins to the synaptic surface, most notably AMPA receptors. To provide a broader view of SNX27-associated pathologies, we performed proteomics in rat primary neurons to identify SNX27-dependent cargoes, and identified proteins linked to excitotoxicity, epilepsy, intellectual disabilities, and working memory deficits. Focusing on the synaptic adhesion molecule LRFN2, we established that SNX27 binds to LRFN2 and regulates its endosomal sorting. Furthermore, LRFN2 associates with AMPA receptors and knockdown of LRFN2 results in decreased surface AMPA receptor expression, reduced synaptic activity, and attenuated hippocampal long-term potentiation. Overall, our study provides an additional mechanism by which SNX27 can control AMPA receptor-mediated synaptic transmission and plasticity indirectly through the sorting of LRFN2 and offers molecular insight into the perturbed function of SNX27 and LRFN2 in a range of neurological conditions.

Phosphorylation of Syntaxin-1a by casein kinase 2α regulates pre-synaptic vesicle exocytosis from the reserve pool

The t-soluble NSF-attachment protein receptor protein Syntaxin-1a (Stx-1a) is abundantly expressed at pre-synaptic terminals where it plays a critical role in the exocytosis of neurotransmitter-containing synaptic vesicles. Stx-1a is phosphorylated by Casein kinase 2α (CK2α) at Ser14, which has been proposed to regulate the interaction of Stx-1a and Munc-18 to control of synaptic vesicle priming. However, the role of CK2α in synaptic vesicle dynamics remains unclear. Here, we show that CK2α over-expression reduces evoked synaptic vesicle release. Furthermore, shRNA-mediated knockdown of CK2α in primary hippocampal neurons strongly enhanced vesicle exocytosis from the reserve pool, with no effect on the readily releasable pool of primed vesicles. In neurons in which endogenous Stx-1a was knocked down and replaced with a CK2α phosphorylation-deficient mutant, Stx-1a(D17A), vesicle exocytosis was also increased. These results reveal a previously unsuspected role of CK2α phosphorylation in specifically regulating the reserve synaptic vesicle pool, without changing the kinetics of release from the readily releasable pool.

Endocytosis, trafficking and exocytosis of intact full-length botulinum neurotoxin type a in cultured rat neurons

Botulinum toxin A (BoNT/A) is a potent neurotoxin that acts primarily by silencing synaptic transmission by blocking neurotransmitter release. BoNT/A comprises a light chain (LC/A) intracellular protease and a heavy chain (HC/A) composed of a receptor binding domain (HCC/A) and a translocation domain (HCN/A) that mediates cell entry. Following entry into the neuron, the disulphide bond linking the two peptide chains is reduced to release the LC/A. To gain better insight into the trafficking and fate of BoNT/A before dissociation we have used a catalytically inactive, non-toxic full-length BoNT/A(0) mutant. Our data confirm that BoNT/A(0) enters cortical neurons both in an activity-dependent manner and via a pathway dependent on fibroblast growth factor receptor 3 (Fgfr3) signalling. We demonstrate that both dynamin-dependent endocytosis and lipid rafts are involved in BoNT/A internalisation and that full-length BoNT/A(0) traffics to early endosomes. Furthermore, while a proportion of BoNT/A remains stable in neurons for 3 days, BoNT/A degradation is primarily mediated by the proteasome. Finally, we demonstrate that a fraction of the endocytosed full-length BoNT/A(0) is capable of exiting the cell to intoxicate other neurons. Together, our data shed new light on the entry routes, trafficking and degradation of BoNT/A, and confirm that trafficking properties previously described for the isolated HCC/A receptor binding domain of are also applicable to the intact, full-length toxin.

The C-terminal helix 9 motif in rat cannabinoid receptor type 1 regulates axonal trafficking and surface expression

Cannabinoid type one receptor (CB1R) is only stably surface expressed in axons, where it downregulates neurotransmitter release. How this tightly regulated axonal surface polarity is established and maintained is unclear. To address this question, we used time-resolved imaging to determine the trafficking of CB1R from biosynthesis to mature polarised localisation in cultured rat hippocampal neurons. We show that the secretory pathway delivery of CB1R is axonally biased and that surface expressed CB1R is more stable in axons than in dendrites. This dual mechanism is mediated by the CB1R C-terminus and involves the Helix 9 (H9) domain. Removal of the H9 domain increases secretory pathway delivery to dendrites and decreases surface stability. Furthermore, CB1R^ΔH9 is more sensitive to agonist-induced internalisation and less efficient at downstream signalling than CB1R^WT. Together, these results shed new light on how polarity of CB1R is mediated and indicate that the C-terminal H9 domain plays key roles in this process.

The brain contains around 100 billion neurons that are in constant communication with one another. Each consists of a cell body, plus two components specialized for exchanging information. These are the axon, which delivers information, and the dendrites, which receive it. This exchange takes place at contact points between neurons called synapses. To send a message, a neuron releases chemicals called neurotransmitters from its axon terminals into the synapse. The neurotransmitters cross the synapse and bind to receptor proteins on the dendrites of another neuron. In doing so, they pass on the message.

Cannabinoid type 1 receptors (CB1Rs) help control the flow of information at synapses. They do this by binding neurotransmitters called endocannabinoids, which are unusual among neurotransmitters. Rather than sending messages from axons to dendrites, endocannabinoids send them in the opposite direction. Thus, it is dendrites that release endocannabinoids, which then bind to CB1Rs in axon terminals. This backwards, or 'retrograde', signalling dampens the release of other neurotransmitters. This slows down brain activity, and gives rise to the 'mellow' sensation that recreational cannabis users often describe.

Like most other proteins, CB1Rs are built inside the cell body. So, how do these receptors end up in the axon terminals where they are needed? Are they initially sent to both axons and dendrites, with the CB1Rs that travel to dendrites being rerouted back to axons? Or do the receptors travel directly to the axon itself? Fletcher-Jones et al. tracked newly made CB1Rs in rat neurons growing in a dish. The results revealed that the receptors go directly to the axon, before moving on to the axon terminals. A specific region of the CB1R protein is crucial for sending the receptors to the axon, and for ensuring that they do not get diverted to the dendrite surface. This region stabilizes CB1Rs at the axon surface, and helps to make the receptors available to bind endocannabinoids.

CB1Rs also respond to medical marijuana, a topic that continues to generate interest as well as controversy. Activating CB1Rs could help treat a wide range of diseases, such as chronic pain, epilepsy and multiple sclerosis. Future studies should build on our understanding of CB1Rs to explore and optimize new therapeutic approaches.

ADAR2-mediated Q/R editing of GluK2 regulates kainate receptor upscaling in response to suppression of synaptic activity

Kainate receptors (KARs) regulate neuronal excitability and network function. Most KARs contain the subunit GluK2 (also known as GRIK2), and the properties of these receptors are determined in part by ADAR2 (also known as ADARB1)-mediated mRNA editing of GluK2, which changes a genomically encoded glutamine residue into an arginine residue (Q/R editing). Suppression of synaptic activity reduces ADAR2-dependent Q/R editing of GluK2 with a consequential increase in GluK2-containing KAR surface expression. However, the mechanism underlying this reduction in GluK2 editing has not been addressed. Here, we show that induction of KAR upscaling, a phenomenon in which surface expression of receptors is increased in response to a chronic decrease in synaptic activity, results in proteasomal degradation of ADAR2, which reduces GluK2 Q/R editing. Because KARs incorporating unedited GluK2(Q) assemble and exit the ER more efficiently, this leads to an upscaling of KAR surface expression. Consistent with this, we demonstrate that partial ADAR2 knockdown phenocopies and occludes KAR upscaling. Moreover, we show that although the AMPA receptor (AMPAR) subunit GluA2 (also known as GRIA2) also undergoes ADAR2-dependent Q/R editing, this process does not mediate AMPAR upscaling. These data demonstrate that activity-dependent regulation of ADAR2 proteostasis and GluK2 Q/R editing are key determinants of KAR, but not AMPAR, trafficking and upscaling.This article has an associated First Person interview with the first author of the paper.

Erythropoietin Induces Homeostatic Plasticity at Hippocampal Synapses

The cytokine erythropoietin (EPO) is the master regulator of erythropoiesis. Intriguingly, many studies have shown that the cognitive performance of patients receiving EPO for its hematopoietic effects is enhanced, which prompted the growing interest in the use of EPO-based strategies to treat neuropsychiatric disorders. EPO plays key roles in brain development and maturation, but also modulates synaptic transmission. However, the mechanisms underlying the latter have remained elusive. Here, we show that acute (40-60 min) exposure to EPO presynaptically downregulates spontaneous and afferent-evoked excitatory transmission, without affecting basal firing of action potentials. Conversely, prolonged (3 h) exposure to EPO, if followed by a recovery period (1 h), is able to elicit a homeostatic increase in excitatory spontaneous, but not in evoked, synaptic transmission. These data lend support to the emerging view that segregated pathways underlie spontaneous and evoked neurotransmitter release. Furthermore, we show that prolonged exposure to EPO facilitates a form of hippocampal long-term potentiation that requires noncanonical recruitment of calcium-permeable AMPA receptors for its maintenance. These findings provide important new insight into the mechanisms by which EPO enhances neuronal function, learning, and memory.

An Integrative Neuroscience Framework for the Treatment of Chronic Pain: From Cellular Alterations to Behavior

Chronic pain can result from many pain syndromes including complex regional pain syndrome (CRPS), phantom limb pain and chronic low back pain, among others. On a molecular level, chronic pain syndromes arise from hypersensitization within the dorsal horn of the spinal cord, a process known as central sensitization. Central sensitization involves an upregulation of ionotropic and metabotropic glutamate receptors (mGluRs) similar to that of long-term potentiation (LTP). Regions of the brain in which LTP occurs, such as the amygdala and hippocampus, are implicated in fear- and memory-related brain circuity. Chronic pain dramatically influences patient quality of life. Individuals with chronic pain may develop pain-related anxiety and pain-related fear. The syndrome also alters functional connectivity in the default-mode network (DMN) and salience network. On a cellular/molecular level, central sensitization may be reversed through degradative glutamate receptor pathways. This, however, rarely happens. Instead, cortical brain regions may serve in a top-down regulatory capacity for the maintenance or alleviation of pain. Specifically, the medial prefrontal cortex (mPFC), which plays a critical role in fear-related brain circuits, the DMN, and salience network may be the driving forces in this process. On a cellular level, the mPFC may form new neural circuits through LTP that may cause extinction of pre-existing pain pathways found within fear-related brain circuits, the DMN, and salience network. In order to promote new LTP connections between the mPFC and other key brain structures, such as the amygdala and insula, we propose a holistic rehabilitation program including cognitive behavioral therapy (CBT) and revolving around: (1) cognitive reappraisals; (2) mindfulness meditation; and (3) functional rehabilitation. Unlike current medical interventions focusing upon pain-relieving medications, we do not believe that chronic pain treatment should focus on reversing the effects of central sensitization. Instead, we propose here that it is critical to focus on non-invasive efforts to promote new neural circuits originating from the mPFC.

The where, what, and when of membrane protein degradation in neurons

Membrane protein turnover and degradation are required for the function and health of all cells. Neurons may live for the entire lifetime of an organism and are highly polarized cells with spatially segregated axonal and dendritic compartments. Both longevity and morphological complexity represent challenges for regulated membrane protein degradation. To investigate how neurons cope with these challenges, an increasing number of recent studies investigated local, cargo-specific protein sorting, and degradation at axon terminals and in dendritic processes. In this review, we explore the current answers to the ensuing questions of where, what, and when membrane proteins are degraded in neurons. © 2017 The Authors Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 283-297, 2018.

Assembly, Secretory Pathway Trafficking, and Surface Delivery of Kainate Receptors Is Regulated by Neuronal Activity

Ionotropic glutamate receptor (iGluR) trafficking and function underpin excitatory synaptic transmission and plasticity and shape neuronal networks. It is well established that the transcription, translation, and endocytosis/recycling of iGluRs are all regulated by neuronal activity, but much less is known about the activity dependence of iGluR transport through the secretory pathway. Here, we use the kainate receptor subunit GluK2 as a model iGluR cargo to show that the assembly, early secretory pathway trafficking, and surface delivery of iGluRs are all controlled by neuronal activity. We show that the delivery of de novo kainate receptors is differentially regulated by modulation of GluK2 Q/R editing, PKC phosphorylation, and PDZ ligand interactions. These findings reveal that, in addition to short-term regulation of iGluRs by recycling/endocytosis and long-term modulation by altered transcription/translation, the trafficking of iGluRs through the secretory pathway is under tight activity-dependent control to determine the numbers and properties of surface-expressed iGluRs.

Sumo Modification of Ion Channels

Recently, a role for SUMO modification outside of the nucleus has emerged. Although the number of extranuclear proteins known to be sumoylated is comparatively small, ion channels represent one important new class of these proteins. Ion channels are responsible for the control of membrane excitability and therefore are critical for fundamental physiological processes such as muscle contraction, neuronal firing, and cellular homeostasis. As such, these ion-conducting proteins are subject to precise regulation. Recently, several studies have identified sumoylation as a novel mechanism of modulating ion channel function. These studies expand the list of known functions of sumoylation and reveal that, in addition to its more established role in the regulation of nuclear proteins, this modification plays important roles at the cytoplasmic face of membranes.

Proteolysis, synaptic plasticity and memory

Protein degradation has many critical functions in the nervous system such as refinement of synaptic connections during development and synaptic plasticity and memory in the adult organisms. A major cellular machinery of proteolysis is the ubiquitin-proteasome pathway (UPP). The UPP precisely regulates proteolysis by covalently attaching ubiquitin, a small protein, to substrates through sequential enzymatic reactions and the proteins marked with the ubiquitin tag are degraded by complex containing many subunits called the proteasome. Research over the years has shown a role for the UPP in regulating presynaptic and postsynaptic proteins critical for neurotransmission and synaptic plasticity. Studies have also revealed a role for the UPP in various forms of memory. Mechanistic investigations suggest that the function of the UPP in neurons is not homogenous and is subject to local regulation in different neuronal sub-compartments. In both invertebrate and vertebrate model systems, local roles have been found for enzymes that attach ubiquitin to substrate proteins as well as for enzymes that remove ubiquitin from substrates. The proteasome also has disparate functions in different parts of the neuron. In addition to the UPP, proteolysis by the lysosome and autophagy play a role in synaptic plasticity and memory. This review details the functions of proteolysis in synaptic plasticity and summarizes the findings on the connection between proteolysis and memory mainly focusing on the UPP including its local roles.

Sumoylation in Synaptic Function and Dysfunction

Sumoylation has recently emerged as a key post-translational modification involved in many, if not all, biological processes. Small Ubiquitin-like Modifier (SUMO) polypeptides are covalently attached to specific lysine residues of target proteins through a dedicated enzymatic pathway. Disruption of the SUMO enzymatic pathway in the developing brain leads to lethality indicating that this process exerts a central role during embryonic and post-natal development. However, little is still known regarding how this highly dynamic protein modification is regulated in the mammalian brain despite an increasing number of data implicating sumoylated substrates in synapse formation, synaptic communication and plasticity. The aim of this review is therefore to briefly describe the enzymatic SUMO pathway and to give an overview of our current knowledge on the function and dysfunction of protein sumoylation at the mammalian synapse.

VPS26A-SNX27 Interaction-Dependent mGluR5 Recycling in Dorsal Horn Neurons Mediates Neuropathic Pain in Rats

Retromer, which crucially contributes to endosomal sorting machinery through the retrieval and recycling of signaling receptors away from degradation, has been identified as a critical element for glutamatergic-receptor-dependent neural plasticity at excitatory synapses. We observed it accompanied by behavioral allodynia; neuropathic injury time-dependently enhanced VPS26A and SNX27 expression; VPS26A-SNX27 coprecipitation; and VPS26A-positive, SNX27-positive, and VPS26A-SNX27 double-labeled immunoreactivity in the dorsal horn of Sprague Dawley rats that were all sufficiently ameliorated through the focal knock-down of spinal VPS26A expression. Although the knock-down of spinal SNX27 expression exhibited similar effects, spinal nerve ligation (SNL)-enhanced VPS26A expression remained unaffected. Moreover, SNL also increased membrane-bound and total mGluR5 abundance, VPS26A-bound SNX27 and mGluR5 and mGluR5-bound VPS26A and SNX27 coprecipitation, and mGluR5-positive and VPS26A/SNX27/mGluR5 triple-labeled immunoreactivity in the dorsal horn, and these effects were all attenuated through the focal knock-down of spinal VPS26A and SNX27 expression. Although administration with MPEP adequately ameliorated SNL-associated allodynia, mGluR5 expression, and membrane insertion, SNL-enhanced VPS26A and SNX27 expression were unaffected. Together, these results suggested a role of spinal VPS26A-SNX27-dependent mGluR5 recycling in the development of neuropathic pain. This is the first study that links retromer-associated sorting machinery with the spinal plasticity underlying pain hypersensitivity and proposes the possible pathophysiological relevance of endocytic recycling in pain pathophysiology through the modification of glutamatergic mGluR5 recycling.

SIGNIFICANCE STATEMENT

VPS26A-SNX27-dependent mGluR5 recycling plays a role in the development of neuropathic pain. The regulation of the VPS26A-SNX27 interaction that modifies mGluR5 trafficking and expression in the dorsal horn provides a novel therapeutic strategy for pain relief.

SUMOylation of Syntaxin1A regulates presynaptic endocytosis

Neurotransmitter release from the presynaptic terminal is under very precise spatial and temporal control. Following neurotransmitter release, synaptic vesicles are recycled by endocytosis and refilled with neurotransmitter. During the exocytosis event leading to release, SNARE proteins provide most of the mechanical force for membrane fusion. Here, we show one of these proteins, Syntaxin1A, is SUMOylated near its C-terminal transmembrane domain in an activity-dependent manner. Preventing SUMOylation of Syntaxin1A reduces its interaction with other SNARE proteins and disrupts the balance of synaptic vesicle endo/exocytosis, resulting in an increase in endocytosis. These results indicate that SUMOylation regulates the emerging role of Syntaxin1A in vesicle endocytosis, which in turn, modulates neurotransmitter release and synaptic function.

Ubiquitin-dependent trafficking and turnover of ionotropic glutamate receptors

Changes in synaptic strength underlie the basis of learning and memory and are controlled, in part, by the insertion or removal of AMPA-type glutamate receptors at the postsynaptic membrane of excitatory synapses. Once internalized, these receptors may be recycled back to the plasma membrane by subunit-specific interactions with other proteins or by post-translational modifications such as phosphorylation. Alternatively, these receptors may be targeted for destruction by multiple degradation pathways in the cell. Ubiquitination, another post-translational modification, has recently emerged as a key signal that regulates the recycling and trafficking of glutamate receptors. In this review, we will discuss recent findings on the role of ubiquitination in the trafficking and turnover of ionotropic glutamate receptors and plasticity of excitatory synapses.

Acid-sensing ion channels: trafficking and pathophysiology

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are widely expressed in both the peripheral and central nervous systems. ASICs contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke, epileptic seizures and multiple sclerosis. Although much progress has been made in researching the structure-function relationship and pharmacology of ASICs, little is known about the trafficking of ASICs and its contribution to ASIC function. The recent identification of the mechanism of membrane insertion and endocytosis of ASIC1a highlights the emerging role of ASIC trafficking in regulating its pathophysiological functions. In this review, we summarize the recent advances and discuss future directions on this topic.

SUMOylation of synapsin Ia maintains synaptic vesicle availability and is reduced in an autism mutation

Synapsins are key components of the presynaptic neurotransmitter release machinery. Their main role is to cluster synaptic vesicles (SVs) to each other and anchor them to the actin cytoskeleton to establish the reserve vesicle pool, and then release them in response to appropriate membrane depolarization. Here we demonstrate that SUMOylation of synapsin Ia (SynIa) at K687 is necessary for SynIa function. Replacement of endogenous SynIa with a non-SUMOylatable mutant decreases the size of the releasable vesicle pool and impairs stimulated SV exocytosis. SUMOylation enhances SynIa association with SVs to promote the efficient reclustering of SynIa following neuronal stimulation and maintain its presynaptic localization. The A548T mutation in SynIa is strongly associated with autism and epilepsy and we show that it leads to defective SynIa SUMOylation. These results identify SUMOylation as a fundamental regulator of SynIa function and reveal a novel link between reduced SUMOylation of SynIa and neurological disorders.

Clathrin-independent trafficking of AMPA receptors

Membrane trafficking of AMPA receptors (AMPARs) is critical for neuronal function and plasticity. Although rapid forms of AMPAR internalization during long-term depression (LTD) require clathrin and dynamin, the mechanisms governing constitutive AMPAR turnover and internalization of AMPARs during slow homeostatic forms of synaptic plasticity remain unexplored. Here, we show that, in contrast to LTD, constitutive AMPAR internalization and homeostatic AMPAR downscaling in rat neurons do not require dynamin or clathrin function. Instead, constitutive AMPAR trafficking is blocked by a Rac1 inhibitor and is regulated by a dynamic nonstructural pool of F-actin. Our findings reveal a novel role for neuronal clathrin-independent endocytosis controlled by actin dynamics and suggest that the interplay between different modes of receptor endocytosis provides for segregation between distinct modes of neuronal plasticity.

Activity-dependent upregulation of presynaptic kainate receptors at immature CA3-CA1 synapses

Presynaptic kainate-type glutamate receptors (KARs) regulate glutamate release probability and short-term plasticity in various areas of the brain. Here we show that long-term depression (LTD) in the area CA1 of neonatal rodent hippocampus is associated with an upregulation of tonic inhibitory KAR activity, which contributes to synaptic depression and causes a pronounced increase in short-term facilitation of transmission. This increased KAR function was mediated by high-affinity receptors and required activation of NMDA receptors, nitric oxide (NO) synthetase, and postsynaptic calcium signaling. In contrast, KAR activity was irreversibly downregulated in response to induction of long-term potentiation in a manner that depended on activation of the TrkB-receptor of BDNF. Both tonic KAR activity and its plasticity were restricted to early stages of synapse development and were lost in parallel with maturation of the network due to ongoing BDNF-TrkB signaling. These data show that presynaptic KARs are targets for activity-dependent modulation via diffusible messengers NO and BDNF, which enhance and depress tonic KAR activity at immature synapses, respectively. The plasticity of presynaptic KARs in the developing network allows nascent synapses to shape their response to incoming activity. In particular, upregulation of KAR function after LTD allows the synapse to preferentially pass high-frequency afferent activity. This can provide a potential rescue from synapse elimination by uncorrelated activity and also increase the computational dynamics of the developing CA3-CA1 circuitry.

Tyrosine phosphorylation of GluK2 up-regulates kainate receptor-mediated responses and downstream signaling after brain ischemia

Although kainate receptors play important roles in ischemic stroke, the molecular mechanisms underlying postischemic regulation of kainate receptors remain unclear. In this study we demonstrate that Src family kinases contribute to the potentiation of kainate receptor function. Brain ischemia and reperfusion induce rapid and sustained phosphorylation of the kainate receptor subunit GluK2 by Src in the rat hippocampus, implicating a critical role for Src-mediated GluK2 phosphorylation in ischemic brain injury. The NMDA and kainate receptors are involved in the tyrosine phosphorylation of GluK2. GluK2 binds to Src, and the tyrosine residue at position 590 (Y590) on GluK2 is a major site of phosphorylation by Src kinases. GluK2 phosphorylation at Y590 is responsible for increases in whole-cell currents and calcium influx in response to transient kainate stimulation. In addition, GluK2 phosphorylation at Y590 facilitates the endocytosis of GluK2 subunits, and the activation of JNK3 and its substrate c-Jun after long-term kainate treatment. Thus, Src phosphorylation of GluK2 plays an important role in the opening of kainate receptor channels and downstream proapoptosis signaling after brain ischemia. The present study reveals an additional mechanism for the regulation of GluK2-containing kainate receptors by Src family kinases, which may be of pathological significance in ischemic stroke.

Kainate receptor activation induces glycine receptor endocytosis through PKC deSUMOylation

Surface expression and regulated endocytosis of glycine receptors (GlyRs) play a critical function in balancing neuronal excitability. SUMOylation (SUMO modification) is of critical importance for maintaining neuronal function in the central nervous system. Here we show that activation of kainate receptors (KARs) causes GlyR endocytosis in a calcium- and protein kinase C (PKC)-dependent manner, leading to reduced GlyR-mediated synaptic activity in cultured spinal cord neurons and the superficial dorsal horn of rat spinal cord slices. This effect requires SUMO1/sentrin-specific peptidase 1 (SENP1)-mediated deSUMOylation of PKC, indicating that the crosstalk between KARs and GlyRs relies on the SUMOylation status of PKC. SENP1-mediated deSUMOylation of PKC is involved in the kainate-induced GlyR endocytosis and thus plays an important role in the anti-homeostatic regulation between excitatory and inhibitory ligand-gated ion channels. Altogether, we have identified a SUMOylation-dependent regulatory pathway for GlyR endocytosis, which may have important physiological implications for proper neuronal excitability.

Maintenance of proper membrane excitability is vital to neuronal function and in several neuronal types this relies on a balance between receptor-mediated excitation and inhibition. Here the authors report a crosstalk between excitatory kainate receptors and inhibitory glycine receptors that relies on the SUMOylation status of PKC.

Trafficking of kainate receptors

Ionotropic glutamate receptors (iGluRs) mediate the vast majority of excitatory neurotransmission in the central nervous system of vertebrates. In the protein family of iGluRs, kainate receptors (KARs) comprise the probably least well understood receptor class. Although KARs act as key players in the regulation of synaptic network activity, many properties and functions of these proteins remain elusive until now. Especially the precise pre-, extra-, and postsynaptic localization of KARs plays a critical role for neuronal function, as an unbalanced localization of KARs would ultimately lead to dysregulated neuronal excitability. Recently, important advances in the understanding of the regulation of surface expression, function, and agonist-dependent endocytosis of KARs have been achieved. Post-translational modifications like PKC-mediated phosphorylation and SUMOylation have been reported to critically influence surface expression and endocytosis, while newly discovered auxiliary proteins were shown to shape the functional properties of KARs.

Differential regulation of GABAB receptor trafficking by different modes of N-methyl-D-aspartate (NMDA) receptor signaling

Inhibitory GABAB receptors (GABABRs) can down-regulate most excitatory synapses in the CNS by reducing postsynaptic excitability. Functional GABABRs are heterodimers of GABAB1 and GABAB2 subunits and here we show that the trafficking and surface expression of GABABRs is differentially regulated by synaptic or pathophysiological activation of NMDA receptors (NMDARs). Activation of synaptic NMDARs using a chemLTP protocol increases GABABR recycling and surface expression. In contrast, excitotoxic global activation of synaptic and extrasynaptic NMDARs by bath application of NMDA causes the loss of surface GABABRs. Intriguingly, exposing neurons to extreme metabolic stress using oxygen/glucose deprivation (OGD) increases GABAB1 but decreases GABAB2 surface expression. The increase in surface GABAB1 involves enhanced recycling and is blocked by the NMDAR antagonist AP5. The decrease in surface GABAB2 is also blocked by AP5 and by inhibiting degradation pathways. These results indicate that NMDAR activity is critical in GABABR trafficking and function and that the individual subunits can be separately controlled to regulate neuronal responsiveness and survival.

RIM1α SUMOylation is required for fast synaptic vesicle exocytosis

The rapid, activity-dependent quantal presynaptic release of neurotransmitter is vital for brain function. The complex process of vesicle priming, fusion, and retrieval is very precisely controlled and requires the spatiotemporal coordination of multiple protein-protein interactions. Here, we show that posttranslational modification of the active zone protein Rab3-interacting molecule 1α (RIM1α) by the small ubiquitin-like modifier 1 (SUMO-1) functions as a molecular switch to direct these interactions and is essential for fast synaptic vesicle exocytosis. RIM1α SUMOylation at lysine residue K502 facilitates the clustering of Ca_V2.1 calcium channels and enhances the Ca²⁺ influx necessary for vesicular release, whereas non-SUMOylated RIM1α participates in the docking/priming of synaptic vesicles and maintenance of active zone structure. These results demonstrate that SUMOylation of RIM1α is a key determinant of rapid, synchronous neurotransmitter release, and the SUMO-mediated “switching” of RIM1α between binding proteins provides insight into the mechanisms underpinning synaptic function and dysfunction.

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RIM1α is SUMOylated at K502

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SUMOylation of RIM1α is required for its PDZ-dependent binding to Ca_V2.1

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Preventing RIM1α SUMOylation reduces Ca_V2.1 clustering and Ca²⁺ entry

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RIM1α SUMOylation is necessary for fast synaptic vesicle exocytosis

Modulation of GluK2a subunit-containing kainate receptors by 14-3-3 proteins

Kainate receptors (KARs) are one of the ionotropic glutamate receptors that mediate excitatory postsynaptic currents (EPSCs) with characteristically slow kinetics. Although mechanisms for the slow kinetics of KAR-EPSCs are not totally understood, recent evidence has implicated a regulatory role of KAR-associated proteins. Here, we report that decay kinetics of GluK2a-containing receptors is modulated by closely associated 14-3-3 proteins. 14-3-3 binding requires PKC-dependent phosphorylation of serine residues localized in the carboxyl tail of the GluK2a subunit. In transfected cells, 14-3-3 binding to GluK2a slows desensitization kinetics of both homomeric GluK2a and heteromeric GluK2a/GluK5 receptors. Moreover, KAR-EPSCs at mossy fiber-CA3 synapses decay significantly faster in the 14-3-3 functional knock-out mice. Collectively, these results demonstrate that 14-3-3 proteins are an important regulator of GluK2a-containing KARs and may contribute to the slow decay kinetics of native KAR-EPSCs.

Postsynaptic kainate receptor recycling and surface expression are regulated by metabotropic autoreceptor signalling

Kainate receptors (KARs) play fundamentally important roles in controlling synaptic function and regulating neuronal excitability. Postsynaptic KARs contribute to excitatory neurotransmission but the molecular mechanisms underlying their activity-dependent surface expression are not well understood. Strong activation of KARs in cultured hippocampal neurons leads to the downregulation of postsynaptic KARs via endocytosis and degradation. In contrast, low-level activation augments postsynaptic KAR surface expression. Here, we show that this increase in KARs is due to enhanced recycling via the recruitment of Rab11-dependent, transferrin-positive endosomes into spines. Dominant-negative Rab11 or the recycling inhibitor primaquine prevents the kainate-evoked increase in surface KARs. Moreover, we show that the increase in surface expression is mediated via a metabotropic KAR signalling pathway, which is blocked by the protein kinase C inhibitor chelerythrine, the calcium chelator BAPTA and the G-protein inhibitor pertussis toxin. Thus, we report a previously uncharacterized positive feedback system that increases postsynaptic KARs in response to low- or moderate-level agonist activation and can provide additional flexibility to synaptic regulation.

Molecular mechanism of constitutive endocytosis of Acid-sensing ion channel 1a and its protective function in acidosis-induced neuronal death

Acid-sensing ion channels (ASICs) are proton-gated cation channels widely expressed in the peripheral and CNSs, which critically contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke and epileptic seizures. However, the trafficking mechanisms of ASICs and the related proteins remain largely unknown. Here, we demonstrate that ASIC1a, the main ASIC subunit in the brain, undergoes constitutive endocytosis in a clathrin- and dynamin-dependent manner in both mouse cortical neurons and heterologous cell cultures. The endocytosis of ASIC1a was inhibited by either the small molecular inhibitor tyrphostin A23 or knockdown of the core subunit of adaptor protein 2 (AP2) μ2 using RNA interference, supporting a clathrin-dependent endocytosis of ASIC1a. In addition, the internalization of ASIC1a was blocked by dominant-negative dynamin1 mutation K44A and the small molecular inhibitor dynasore, suggesting that it is also dynamin-dependent. We show that the membrane-proximal residues (465)LCRRG(469) at the cytoplasmic C terminus of ASIC1a are critical for interaction with the endogenous adaptor protein complex and inhibition of ASIC1a internalization strongly exacerbated acidosis-induced death of cortical neurons from wild-type but not ASIC1a knock-out mice. Together, these results reveal the molecular mechanism of ASIC1a internalization and suggest the importance of endocytic pathway in functional regulation of ASIC1a channels as well as neuronal damages mediated by these channels during neurodegeneration.

Kainate receptor post-translational modifications differentially regulate association with 4.1N to control activity-dependent receptor endocytosis

Kainate receptors exhibit a highly compartmentalized distribution within the brain; however, the molecular and cellular mechanisms that coordinate their expression at neuronal sites of action are poorly characterized. Here we report that the GluK1 and GluK2 kainate receptor subunits interact with the spectrin-actin binding scaffolding protein 4.1N through a membrane-proximal domain in the C-terminal tail. We found that this interaction is important for the forward trafficking of GluK2a receptors, their distribution in the neuronal plasma membrane, and regulation of receptor endocytosis. The association between GluK2a receptors and 4.1N was regulated by both palmitoylation and protein kinase C (PKC) phosphorylation of the receptor subunit. Palmitoylation of the GluK2a subunit promoted 4.1N association, and palmitoylation-deficient receptors exhibited reduced neuronal surface expression and compromised endocytosis. Conversely, PKC activation decreased 4.1N interaction with GluK2/3-containing kainate receptors in acute brain slices, an effect that was reversed after inhibition of PKC. Our data and previous studies therefore demonstrate that these two post-translational modifications have opposing effects on 4.1N association with GluK2 kainate and GluA1 AMPA receptors. The convergence of the signaling pathways regulating 4.1N protein association could thus result in the selective removal of AMPA receptors from the plasma membrane while simultaneously promoting the insertion and stabilization of kainate receptors, which may be important for tuning neuronal excitability and synaptic plasticity.

CaMKII-dependent phosphorylation of GluK5 mediates plasticity of kainate receptors

Calmodulin-dependent kinase II (CaMKII) is key for long-term potentiation of synaptic AMPA receptors. Whether CaMKII is involved in activity-dependent plasticity of other ionotropic glutamate receptors is unknown. We show that repeated pairing of pre- and postsynaptic stimulation at hippocampal mossy fibre synapses induces long-term depression of kainate receptor (KAR)-mediated responses, which depends on Ca(2+) influx, activation of CaMKII, and on the GluK5 subunit of KARs. CaMKII phosphorylation of three residues in the C-terminal domain of GluK5 subunit markedly increases lateral mobility of KARs, possibly by decreasing the binding of GluK5 to PSD-95. CaMKII activation also promotes surface expression of KARs at extrasynaptic sites, but concomitantly decreases its synaptic content. Using a molecular replacement strategy, we demonstrate that the direct phosphorylation of GluK5 by CaMKII is necessary for KAR-LTD. We propose that CaMKII-dependent phosphorylation of GluK5 is responsible for synaptic depression by untrapping of KARs from the PSD and increased diffusion away from synaptic sites.

Activity-dependent regulation of the sumoylation machinery in rat hippocampal neurons

BACKGROUND INFORMATION

Sumoylation is a key post-translational modification by which the Small Ubiquitin-like MOdifier (SUMO) polypeptide is covalently attached to specific lysine residues of substrate proteins through a specific enzymatic pathway. Although sumoylation participates in the regulation of nuclear homeostasis, the sumoylation machinery is also expressed outside of the nucleus where little is still known regarding its non-nuclear functions, particularly in the Central Nervous System (CNS). We recently reported that the sumoylation process is developmentally regulated in the rat CNS.

RESULTS

Here, we demonstrate that there is an activity-dependent redistribution of endogenous sumoylation enzymes in hippocampal neurons. By performing biochemical and immunocytochemical experiments on primary cultures of rat hippocampal neurons, we show that sumoylation and desumoylation enzymes are differentially redistributed in and out of synapses upon neuronal stimulation. This enzymatic redistribution in response to a neuronal depolarisation results in the transient decrease of sumoylated protein substrates at synapses.

CONCLUSIONS

Taken together, our data identify an activity-dependent regulation of the sumoylation machinery in neurons that directly impacts on synaptic sumoylation levels. This process may provide a mechanism for neurons to adapt their physiological responses to changes occurring during neuronal activation.

Modification and movement: Phosphorylation and SUMOylation regulate endocytosis of GluK2-containing kainate receptors

Kainate receptors (KARs) are tetrameric glutamate-gated ion channels composed of combinations of the subunits GluK1-5. Depending on their precise localization and subunit composition, KARs can regulate neurotransmitter release, synaptic function and neuronal excitability. Because of these diverse roles, the regulated and precisely targeted trafficking of KARs is of crucial importance to neuronal function. We previously reported that the KAR subunit GluK2 is post-translationally modified by attachment of Small Ubiquitin-like Modifier 1 (SUMO-1) and that SUMOylation is required for agonist-dependent endocytosis of GluK2. We recently extended these findings to demonstrate that agonist activation leads to PKC-mediated phosphorylation of GluK2 at serine 868, which directly enhances GluK2 SUMOylation and, in turn, leads to endocytosis of the receptor. These new data demonstrate the importance of interplay between two post-translational modifications in orchestrating the temporal and spatial regulation of kainate receptor trafficking.

Regulation of Mct1 by cAMP-dependent internalization in rat brain endothelial cells

In the cerebrovascular endothelium, monocarboxylic acid transporter 1 (Mct1) controls blood-brain transport of short chain monocarboxylic and keto acids, including pyruvate and lactate, to support brain energy metabolism. Mct1 function is acutely decreased in rat brain cerebrovascular endothelial cells by β-adrenergic signaling through cyclic adenosine monophosphate (cAMP); however, the mechanism for this acute reduction in transport capacity is unknown. In this report, we demonstrate that cAMP induces the dephosphorylation and internalization of Mct1 from the plasma membrane into caveolae and early endosomes in the RBE4 rat brain cerebrovascular endothelial cell line. Additionally, we provide evidence that Mct1 constitutively cycles through clathrin vesicles and recycling endosomes in a pathway that is not dependent upon cAMP signaling in these cells. Our results are important because they show for the first time the regulated and unregulated vesicular trafficking of Mct1 in cerebrovascular endothelial cells; processes which have significance for better understanding normal brain energy metabolism, and the etiology and potential therapeutic approaches to treating brain diseases, such as stroke, in which lactic acidosis is a key component.

SUMOylation and phosphorylation of GluK2 regulate kainate receptor trafficking and synaptic plasticity

Phosphorylation or SUMOylation of the kainate receptor (KAR) subunit GluK2 have both individually been shown to regulate KAR surface expression. However, it is unknown whether phosphorylation and SUMOylation of GluK2 are important for activity-dependent KAR synaptic plasticity. We found that protein kinase C–mediated phosphorylation of GluK2 at serine 868 promotes GluK2 SUMOylation at lysine 886 and that both of these events are necessary for the internalization of GluK2-containing KARs that occurs during long-term depression of KAR-mediated synaptic transmission at rat hippocampal mossy fiber synapses. Conversely, phosphorylation of GluK2 at serine 868 in the absence of SUMOylation led to an increase in KAR surface expression by facilitating receptor recycling between endosomal compartments and the plasma membrane. Our results suggest a role for the dynamic control of synaptic SUMOylation in the regulation of KAR synaptic transmission and plasticity.

Regulation of Neuronal Protein Trafficking and Translocation by SUMOylation

Post-translational modifications of proteins are essential for cell function. Covalent modification by SUMO (small ubiquitin-like modifier) plays a role in multiple cell processes, including transcriptional regulation, DNA damage repair, protein localization and trafficking. Factors affecting protein localization and trafficking are particularly crucial in neurons because of their polarization, morphological complexity and functional specialization. SUMOylation has emerged as a major mediator of intranuclear and nucleo-cytoplasmic translocations of proteins involved in critical pathways such as circadian rhythm, apoptosis and protein degradation. In addition, SUMO-regulated re-localization of extranuclear proteins is required to sustain neuronal excitability and synaptic transmission. Thus, SUMOylation is a key arbiter of neuronal viability and function. Here, we provide an overview of recent advances in our understanding of regulation of neuronal protein localization and translocation by SUMO and highlight exciting areas of ongoing research.

SUMOylation of the kainate receptor subunit GluK2 contributes to the activation of the MLK3-JNK3 pathway following kainate stimulation

Protein SUMOylation has been implicated in the pathogenesis of ischemic stroke. However, the underlying mechanisms remain unclear. Here, we found that global brain ischemia evokes a sustained elevation of GluK2 SUMOylation in the rat hippocampal CA1 region. Over-expression of wild-type GluK2, but not SUMOylation-deficient mutant, significantly increased the activity of MLK3 and JNK3 after kainate stimulation. SUMOylation deficiency attenuated the kainate-stimulated interaction between MLK3 and GluK2. In addition, inhibition of kainate-evoked GluK2 endocytosis decreased the activation of MLK3-JNK3 signaling and the binding of MLK3-GluK2 in cultured cortical neurons. These results suggest that the internalization of GluK2 following SUMO modification promotes its binding with MLK3, thereby activating the MLK3-JNK3 pathway, which may be responsible for ischemic neuronal cell death.

Developmental regulation and spatiotemporal redistribution of the sumoylation machinery in the rat central nervous system

Background

Small Ubiquitin-like MOdifier protein (SUMO) is a key regulator of nuclear functions but little is known regarding the role of the post-translational modification sumoylation outside of the nucleus, particularly in the Central Nervous System (CNS).

Methodology/Principal Findings

Here, we report that the expression levels of SUMO-modified substrates as well as the components of the sumoylation machinery are temporally and spatially regulated in the developing rat brain. Interestingly, while the overall sumoylation is decreasing during brain development, there are progressively more SUMO substrates localized at synapses. This increase is correlated with a differential redistribution of the sumoylation machinery into dendritic spines during neuronal maturation.

Conclusions/Significance

Overall, our data clearly demonstrate that the sumoylation process is developmentally regulated in the brain with high levels of nuclear sumoylation early in the development suggesting a role for this post-translational modification during the synaptogenesis period and a redistribution of the SUMO system towards dendritic spines at a later developmental stage to modulate synaptic protein function.

Agonist-induced PKC phosphorylation regulates GluK2 SUMOylation and kainate receptor endocytosis

The surface expression and regulated endocytosis of kainate (KA) receptors (KARs) plays a critical role in neuronal function. PKC can modulate KAR trafficking, but the sites of action and molecular consequences have not been fully characterized. Small ubiquitin-like modifier (SUMO) modification of the KAR subunit GluK2 mediates agonist-evoked internalization, but how KAR activation leads to GluK2 SUMOylation is unclear. Here we show that KA stimulation causes rapid phosphorylation of GluK2 by PKC, and that PKC activation increases GluK2 SUMOylation both in vitro and in neurons. The intracellular C-terminal domain of GluK2 contains two predicted PKC phosphorylation sites, S846 and S868, both of which are phosphorylated in response to KA. Phosphomimetic mutagenesis of S868 increased GluK2 SUMOylation, and mutation of S868 to a nonphosphorylatable alanine prevented KA-induced SUMOylation and endocytosis in neurons. Infusion of SUMO-1 dramatically reduced KAR-mediated currents in HEK293 cells expressing WT GluK2 or nonphosphorylatable S846A mutant, but had no effect on currents mediated by the S868A mutant. These data demonstrate that agonist activation of GluK2 promotes PKC-dependent phosphorylation of S846 and S868, but that only S868 phosphorylation is required to enhance GluK2 SUMOylation and promote endocytosis. Thus, direct phosphorylation by PKC and GluK2 SUMOylation are intimately linked in regulating the surface expression and function of GluK2-containing KARs.

Relative input strength rapidly regulates dendritic structure of chick auditory brainstem neurons

Competition between presynaptic inputs has been suggested to shape dendritic form. This hypothesis can be directly tested on bitufted, auditory neurons in chicken nucleus laminaris (NL). Each NL neuron contains two relatively symmetrical dendritic arbors; the dorsal dendrites receive excitatory glutamatergic input from the ipsilateral ear, and the ventral dendrites receive corresponding input from the contralateral ear. To assess the effect of relative synaptic strength on NL dendrites, we used single-cell electroporation; electrophysiology; and live, two-photon laser scanning microscopy to manipulate both the amount and the balance of synaptic input to the two matching sets of dendrites. With simultaneous activation, both sets of dendrites changed together, either growing or retracting over the imaging period. In contrast, stimulation of only one set of dendrites (either dorsal or ventral) resulted in the unstimulated dendrites losing total dendritic branch length, whereas the stimulated dendrites exhibited a tendency to grow. In this system, balanced input leads to balanced changes in the two sets of dendrites, but imbalanced input results in differential changes. Time-lapse imaging revealed that NL dendrites respond to differential stimulation by first decreasing the size of their unstimulated dendrites and then increasing the size of their stimulated dendrites. This result suggests that the relative activity of presynaptic neurons dynamically controls dendritic structure in NL and that dendritic real estate can rapidly be shifted from inactive inputs to active inputs.