A critical role for PSD-95/AKAP interactions in endocytosis of synaptic AMPA receptors

Regulation of depressive-like behaviours by palmitoylation: Role of AKAP150 in the basolateral amygdala

BACKGROUND AND PURPOSE

Protein palmitoylation is involved in learning and memory, and in emotional disorders. Yet, the underlying mechanisms in these processes remain unclear. Herein, we describe that A-kinase anchoring protein 150 (AKAP150) is essential and sufficient for depressive-like behaviours in mice via a palmitoylation-dependent mechanism.

EXPERIMENTAL APPROACH

Depressive-like behaviours in mice were induced by chronic restraint stress (CRS) and chronic unpredictable mild stress (CUMS). Palmitoylated proteins in the basolateral amygdala (BLA) were assessed by an acyl-biotin exchange assay. Genetic and pharmacological approaches were used to investigate the role of the DHHC2-mediated AKAP150 palmitoylation signalling pathway in depressive-like behaviours. Electrophysiological recording, western blotting and co-immunoprecipitation were performed to define the mechanistic pathway.

KEY RESULTS

Chronic stress successfully induced depressive-like behaviours in mice and enhanced AKAP150 palmitoylation in the BLA, and a palmitoylation inhibitor was enough to reverse these changes. Blocking the AKAP150-PKA interaction with the peptide Ht-31 abolished the CRS-induced AKAP150 palmitoylation signalling pathway. DHHC2 expression and palmitoylation levels were both increased after chronic stress. DHHC2 knockdown prevented CRS-induced depressive-like behaviours, as well as attenuating AKAP150 signalling and synaptic transmission in the BLA in CRS-treated mice.

CONCLUSION AND IMPLICATIONS

These results delineate that DHHC2 modulates chronic stress-induced depressive-like behaviours and synaptic transmission in the BLA via the AKAP150 palmitoylation signalling pathway, and this pathway may be considered as a promising novel therapeutic target for major depressive disorder.

Regulating neuronal excitability: The role of S-palmitoylation in NaV1.7 activity and voltage sensitivity

S-palmitoylation, a reversible lipid post-translational modification, regulates the functions of numerous proteins. Voltage-gated sodium channels (NaVs), pivotal in action potential generation and propagation within cardiac cells and sensory neurons, can be directly or indirectly modulated by S-palmitoylation, impacting channel trafficking and function. However, the role of S-palmitoylation in modulating NaV1.7, a significant contributor to pain pathophysiology, has remained unexplored. Here, we addressed this knowledge gap by investigating if S-palmitoylation influences NaV1.7 channel function. Acyl-biotin exchange assays demonstrated that heterologously expressed NaV1.7 channels are modified by S-palmitoylation. Blocking S-palmitoylation with 2-bromopalmitate resulted in reduced NaV1.7 current density and hyperpolarized steady-state inactivation. We identified two S-palmitoylation sites within NaV1.7, both located in the second intracellular loop, which regulated different properties of the channel. Specifically, S-palmitoylation of cysteine 1126 enhanced NaV1.7 current density, while S-palmitoylation of cysteine 1152 modulated voltage-dependent inactivation. Blocking S-palmitoylation altered excitability of rat dorsal root ganglion neurons. Lastly, in human sensory neurons, NaV1.7 undergoes S-palmitoylation, and the attenuation of this post-translational modification results in alterations in the voltage-dependence of activation, leading to decreased neuronal excitability. Our data show, for the first time, that S-palmitoylation affects NaV1.7 channels, exerting regulatory control over their activity and, consequently, impacting rodent and human sensory neuron excitability. These findings provide a foundation for future pharmacological studies, potentially uncovering novel therapeutic avenues in the modulation of S-palmitoylation for NaV1.7 channels.

AKAP150-anchored PKA regulates synaptic transmission and plasticity, neuronal excitability and CRF neuromodulation in the mouse lateral habenula

The scaffolding A-kinase anchoring protein 150 (AKAP150) is critically involved in kinase and phosphatase regulation of synaptic transmission/plasticity, and neuronal excitability. Emerging evidence also suggests that AKAP150 signaling may play a key role in brain's processing of rewarding/aversive experiences, however its role in the lateral habenula (LHb, as an important brain reward circuitry) is completely unknown. Using whole cell patch clamp recordings in LHb of male wildtype and ΔPKA knockin mice (with deficiency in AKAP-anchoring of PKA), here we show that the genetic disruption of PKA anchoring to AKAP150 significantly reduces AMPA receptor-mediated glutamatergic transmission and prevents the induction of presynaptic endocannabinoid-mediated long-term depression in LHb neurons. Moreover, ΔPKA mutation potentiates GABAA receptor-mediated inhibitory transmission while increasing LHb intrinsic excitability through suppression of medium afterhyperpolarizations. ΔPKA mutation-induced suppression of medium afterhyperpolarizations also blunts the synaptic and neuroexcitatory actions of the stress neuromodulator, corticotropin releasing factor (CRF), in mouse LHb. Altogether, our data suggest that AKAP150 complex signaling plays a critical role in regulation of AMPA and GABAA receptor synaptic strength, glutamatergic plasticity and CRF neuromodulation possibly through AMPA receptor and potassium channel trafficking and endocannabinoid signaling within the LHb.

Amyloid-β-induced dendritic spine elimination requires Ca2+-permeable AMPA receptors, AKAP-Calcineurin-NFAT signaling, and the NFAT target gene Mdm2

Alzheimer's Disease (AD) is associated with brain accumulation of synaptotoxic amyloid-β (Aβ) peptides produced by the proteolytic processing of amyloid precursor protein (APP). Cognitive impairments associated with AD correlate with dendritic spine and excitatory synapse loss, particularly within the hippocampus. In rodents, soluble Aβ oligomers impair hippocampus-dependent learning and memory, promote dendritic spine loss, inhibit NMDA-type glutamate receptor (NMDAR)-dependent long-term potentiation (LTP), and promote synaptic depression (LTD), at least in part through activation of the Ca2+-CaM-dependent phosphatase calcineurin (CaN). Yet, questions remain regarding Aβ-dependent postsynaptic CaN signaling specifically at the synapse to mediate its synaptotoxicity. Here, we use pharmacologic and genetic approaches to demonstrate a role for postsynaptic signaling via A kinase-anchoring protein 150 (AKAP150)-scaffolded CaN in mediating Aβ-induced dendritic spine loss in hippocampal neurons from rats and mice of both sexes. In particular, we found that Ca2+-permeable AMPA-type glutamate receptors (CP-AMPARs), which were previously shown to signal through AKAP-anchored CaN to promote both LTD and Aβ-dependent inhibition of LTP, are also required upstream of AKAP-CaN signaling to mediate spine loss via overexpression of APP containing multiple mutations linked to familial, early-onset AD and increased Aβ production. In addition, we found that the CaN-dependent nuclear factor of activated T-cells (NFAT) transcription factors are required downstream to promote Aβ-mediated dendritic spine loss. Finally, we identified the E3-ubiquitin ligase Mdm2, which was previously linked to LTD and developmental synapse elimination, as a downstream NFAT target gene upregulated by Aβ whose enzymatic activity is required for Aβ-mediated spine loss.Significance Statement Impaired hippocampal function and synapse loss are hallmarks of AD linked to Aβ oligomers. Aβ exposure acutely blocks hippocampal LTP and enhances LTD and chronically leads to dendritic spine synapse loss. In particular, Aβ hijacks normal plasticity mechanisms, biasing them toward synapse weakening/elimination, with previous studies broadly linking CaN phosphatase signaling to this synaptic dysfunction. However, we do not understand how Aβ engages signaling specifically at synapses. Here we elucidate a synapse-to-nucleus signaling pathway coordinated by the postsynaptic scaffold protein AKAP150 that is activated by Ca2+ influx through CP-AMPARs and transduced to nucleus by CaN-NFAT signaling to transcriptionally upregulate the E3-ubiquitin ligase Mdm2 that is required for Aβ-mediated spine loss. These findings identify Mdm2 as potential therapeutic target for AD.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.

AKAP150-anchored PKA regulation of synaptic transmission and plasticity, neuronal excitability and CRF neuromodulation in the lateral habenula

Numerous studies of hippocampal synaptic function in learning and memory have established the functional significance of the scaffolding A-kinase anchoring protein 150 (AKAP150) in kinase and phosphatase regulation of synaptic receptor and ion channel trafficking/function and hence synaptic transmission/plasticity, and neuronal excitability. Emerging evidence also suggests that AKAP150 signaling may play a critical role in brain's processing of rewarding/aversive experiences. Here we focused on an unexplored role of AKAP150 in the lateral habenula (LHb), a diencephalic brain region that integrates and relays negative reward signals from forebrain striatal and limbic structures to midbrain monoaminergic centers. LHb aberrant activity (specifically hyperactivity) is also linked to depression. Using whole cell patch clamp recordings in LHb of male wildtype (WT) and ΔPKA knockin mice (with deficiency in AKAP-anchoring of PKA), we found that the genetic disruption of PKA anchoring to AKAP150 significantly reduced AMPA receptor (AMPAR)-mediated glutamatergic transmission and prevented the induction of presynaptic endocannabinoid (eCB)-mediated long-term depression (LTD) in LHb neurons. Moreover, ΔPKA mutation potentiated GABAA receptor (GABAAR)-mediated inhibitory transmission postsynaptically while increasing LHb intrinsic neuronal excitability through suppression of medium afterhyperpolarizations (mAHPs). Given that LHb is a highly stress-responsive brain region, we further tested the effects of corticotropin releasing factor (CRF) stress neuromodulator on synaptic transmission and intrinsic excitability of LHb neurons in WT and ΔPKA mice. As in our earlier study in rat LHb, CRF significantly suppressed GABAergic transmission onto LHb neurons and increased intrinsic excitability by diminishing small-conductance potassium (SK) channel-mediated mAHPs. ΔPKA mutation-induced suppression of mAHPs also blunted the synaptic and neuroexcitatory actions of CRF in mouse LHb. Altogether, our data suggest that AKAP150 complex signaling plays a critical role in regulation of AMPAR and GABAAR synaptic strength, glutamatergic plasticity and CRF neuromodulation possibly through AMPAR and potassium channel trafficking and eCB signaling within the LHb.

DHHC2 regulates fear memory formation, LTP, and AKAP150 signaling in the hippocampus

Palmitoyl acyltransferases (PATs) have been suggested to be involved in learning and memory. However, the underlying mechanisms have not yet been fully elucidated. Here, we found that the activity of DHHC2 was upregulated in the hippocampus after fear conditioning, and DHHC2 knockdown impaired fear induced memory and long-term potentiation (LTP). Additionally, the activity of DHHC2 and its synaptic expression were increased after high frequency stimulation (HFS) or glycine treatment. Importantly, fear learning selectively augmented the palmitoylation level of AKAP150, not PSD-95, and this effect was abolished by DHHC2 knockdown. Furthermore, 2-bromopalmitic acid (2-BP), a palmitoylation inhibitor, attenuated the increased palmitoylation level of AKAP150 and the interaction between AKAP150 and PSD-95 induced by HFS. Lastly, DHHC2 knockdown reduced the phosphorylation level of GluA1 at Ser845, and also induced an impairment of LTP in the hippocampus. Our results suggest that DHHC2 plays a critical role in regulating fear memory via AKAP150 signaling.

Palmitoylation-dependent control of JAK1 kinase signaling governs responses to neuropoietic cytokines and survival in DRG neurons

Janus Kinase-1 (JAK1) plays key roles during neurodevelopment and following neuronal injury, while activatory JAK1 mutations are linked to leukemia. In mice, Jak1 genetic deletion results in perinatal lethality, suggesting non-redundant roles and/or regulation of JAK1 for which other JAKs cannot compensate. Proteomic studies reveal that JAK1 is more likely palmitoylated compared to other JAKs, implicating palmitoylation as a possible JAK1-specific regulatory mechanism. However, the importance of palmitoylation for JAK1 signaling has not been addressed. Here, we report that JAK1 is palmitoylated in transfected HEK293T cells and endogenously in cultured Dorsal Root Ganglion (DRG) neurons. We further use comprehensive screening in transfected non-neuronal cells and shRNA-mediated knockdown in DRG neurons to identify the related enzymes ZDHHC3 and ZDHHC7 as dominant protein acyltransferases (PATs) for JAK1. Surprisingly, we found palmitoylation minimally affects JAK1 localization in neurons, but is critical for JAK1's kinase activity in cells and even in vitro. We propose this requirement is likely because palmitoylation facilitates transphosphorylation of key sites in JAK1's activation loop, a possibility consistent with structural models of JAK1. Importantly, we demonstrate a leukemia-associated JAK1 mutation overrides the palmitoylation-dependence of JAK1 activity, potentially explaining why this mutation is oncogenic. Finally, we show that JAK1 palmitoylation is important for neuropoietic cytokine-dependent signaling and neuronal survival and that combined Zdhhc3/7 loss phenocopies loss of palmitoyl-JAK1. These findings provide new insights into the control of JAK signaling in both physiological and pathological contexts.

The Expression of Epac2 and GluA3 in an Alzheimer's Disease Experimental Model and Postmortem Patient Samples

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative diseases, characterized by amyloid beta (Aβ) and hyperphosphorylated tau accumulation in the brain. Recent studies indicated that memory retrieval, rather than memory formation, was impaired in the early stage of AD. Our previous study reported that pharmacological activation of hippocampal Epac2 promoted memory retrieval in C57BL/6J mice. A recent study suggested that pharmacological inhibition of Epac2 prevented synaptic potentiation mediated by GluA3-containing AMPARs. In this study, we aimed to investigate proteins associated with Epac2-mediated memory in hippocampal postmortem samples of AD patients and healthy controls compared with the experimental AD model J20 and wild-type mice. Epac2 and phospho-Akt were downregulated in AD patients and J20 mice, while Epac1 and phospho-ERK1/2 were not altered. GluA3 was reduced in J20 mice and tended to decrease in AD patients. PSD95 tended to decrease in AD patients and J20. Interestingly, AKAP5 was increased in AD patients but not in J20 mice, implicating its role in tau phosphorylation. Our study points to the downregulation of hippocampal expression of proteins associated with Epac2 in AD.

A whole transcriptome profiling analysis for antidepressant mechanism of Xiaoyaosan mediated synapse loss via BDNF/trkB/PI3K signal axis in CUMS rats

BACKGROUND

Depression is a neuropsychiatric disease resulting from deteriorations of molecular networks and synaptic injury induced by stress. Traditional Chinese formula Xiaoyaosan (XYS) exert antidepressant effect, which was demonstrated by a great many of clinical and basic investigation. However, the exact mechanism of XYS has not yet been fully elucidated.

METHODS

In this study, chronic unpredictable mild stress (CUMS) rats were used as a model of depression. Behavioral test and HE staining were used to detect the anti-depressant effects of XYS. Furthermore, whole transcriptome sequencing was employed to establish the microRNA (miRNA), long non-coding RNA (lncRNA), circular RNA (circRNA), and mRNA profiles. The biological functions and potential mechanisms of XYS for depression were gathered from the GO and KEGG pathway. Then, constructed the competing endogenous RNA (ceRNA) networks to illustrate the regulatory relationship between non-coding RNA (ncRNA) and mRNA. Additionally, longest dendrite length, total length of dendrites, number of intersections, and density of dendritic spines were detected by Golgi staining. MAP2, PSD-95, SYN were detected by immunofluorescence respectively. BDNF, TrkB, p-TrkB, PI3K, Akt, p-Akt were measured by Western Blotting.

RESULTS

The results showed that XYS could increase the locomotor activity and sugar preference, decreased swimming immobility time as well as attenuate hippocampal pathological damage. A total of 753 differentially expressed lncRNAs (DElncRNAs), 28 circRNAs (DEcircRNAs), 101 miRNAs (DEmiRNAs), and 477 mRNAs (DEmRNAs) were identified after the treatment of XYS in whole transcriptome sequencing analysis. Enrichment results revealed that XYS could regulate multiple aspects of depression through different synapse or synaptic associated signal, such as neurotrophin signaling and PI3K/Akt signaling pathways. Then, vivo experiments indicated that XYS could promote length, density, intersections of synapses and also increase the expression of MAP2 in hippocampal CA1, CA3 regions. Meanwhile, XYS could increase the expression of PSD-95, SYN in the CA1, CA3 regions of hippocampal by regulating the BDNF/trkB/PI3K signal axis.

CONCLUSION

The possible mechanism on synapse of XYS in depression was successfully predicted. BDNF/trkB/PI3K signal axis were the potential mechanism of XYS on synapse loss for its antidepressant. Collectively, our results provided novel information about the molecular basis of XYS in treating depression.

A vital role for PICK1 in the differential regulation of metabotropic glutamate receptor internalization and synaptic AMPA receptor endocytosis

Group I metabotropic glutamate receptors (mGluRs) play important roles in many neuronal processes and are believed to be involved in synaptic plasticity underlying the encoding of experience, including classic paradigms of learning and memory. These receptors have also been implicated in various neurodevelopmental disorders, such as Fragile X syndrome and autism. Internalization and recycling of these receptors in the neuron are important mechanisms to regulate the activity of the receptor and control the precise spatio-temporal localization of these receptors. Applying a "molecular replacement" approach in hippocampal neurons derived from mice, we demonstrate a critical role for protein interacting with C kinase 1 (PICK1) in regulating the agonist-induced internalization of mGluR1. We show that PICK1 specifically regulates the internalization of mGluR1 but it does not play any role in the internalization of the other member of group I mGluR family, mGluR5. Various regions of PICK1 viz., the N-terminal acidic motif, PDZ domain and BAR domain play important roles in the agonist-mediated internalization of mGluR1. Finally, we demonstrate that PICK1-mediated internalization of mGluR1 is critical for the resensitization of the receptor. Upon knockdown of endogenous PICK1, mGluR1s stayed on the cell membrane as inactive receptors, incapable of triggering the MAP-kinase signaling. They also could not induce AMPAR endocytosis, a cellular correlate for mGluR-dependent synaptic plasticity. Thus, this study unravels a novel role for PICK1 in the agonist-mediated internalization of mGluR1 and mGluR1-mediated AMPAR endocytosis that might contribute to the function of mGluR1 in neuropsychiatric disorders.

Impaired synaptic transmission in dorsal dentate gyrus increases impulsive alcohol seeking

Both human and animal studies indicate that the dentate gyrus (DG) of the hippocampus is highly exploited by drug and alcohol abuse. Yet, it is poorly understood how DG dysfunction affects addiction-related behaviors. Here, we used an animal model of alcohol use disorder (AUD) in automated IntelliCages and performed local genetic manipulation to investigate how synaptic transmission in the dorsal DG (dDG) affects alcohol-related behaviors. We show that a cue light induces potentiation-like plasticity of dDG synapses in alcohol-naive mice. This process is impaired in mice trained to drink alcohol. Acamprosate (ACA), a drug that reduces alcohol relapse, rescues the impairment of dDG synaptic transmission in alcohol mice. A molecular manipulation that reduces dDG synaptic AMPAR and NMDAR levels increases impulsive alcohol seeking during cue relapse (CR) in alcohol mice but does not affect alcohol reward, motivation or craving. These findings suggest that hindered dDG synaptic transmission specifically underlies impulsive alcohol seeking induced by alcohol cues, a core symptom of AUD.

Crosstalk between biochemical signalling network architecture and trafficking governs AMPAR dynamics in synaptic plasticity

Synaptic plasticity involves modification of both biochemical and structural components of neurons. Many studies have revealed that the change in the number density of the glutamatergic receptor AMPAR at the synapse is proportional to synaptic weight update; an increase in AMPAR corresponds to strengthening of synapses while a decrease in AMPAR density weakens synaptic connections. The dynamics of AMPAR are thought to be regulated by upstream signalling, primarily the calcium-CaMKII pathway, trafficking to and from the synapse, and influx from extrasynaptic sources. Previous work in the field of deterministic modelling of CaMKII dynamics has assumed bistable kinetics, while experiments and rule-based modelling have revealed that CaMKII dynamics can be either monostable or ultrasensitive. This raises the following question: how does the choice of model assumptions involving CaMKII dynamics influence AMPAR dynamics at the synapse? To answer this question, we have developed a set of models using compartmental ordinary differential equations to systematically investigate contributions of different signalling and trafficking variations, along with their coupled effects, on AMPAR dynamics at the synaptic site. We find that the properties of the model including network architecture describing different stability features of CaMKII and parameters that capture the endocytosis and exocytosis of AMPAR significantly affect the integration of fast upstream species by slower downstream species. Furthermore, we predict that the model outcome, as determined by bound AMPAR at the synaptic site, depends on (1) the choice of signalling model (bistable CaMKII or monostable CaMKII dynamics), (2) trafficking versus influx contributions and (3) frequency of stimulus. KEY POINTS: The density of AMPA receptors (AMPARs) at the postsynaptic density of the synapse provides a readout of synaptic plasticity, which involves crosstalk between complex biochemical signalling networks including CaMKII dynamics and trafficking pathways including exocytosis and endocytosis. Here we build a model that integrates CaMKII dynamics and AMPAR trafficking to explore this crosstalk. We compare different models of CaMKII that result in monostable or bistable kinetics and their impact on AMPAR dynamics. Our results show that AMPAR density depends on the coupling between aspects of biochemical signalling and trafficking. Specifically, assumptions regarding CaMKII dynamics and its stability features can alter AMPAR density at the synapse. Our model also predicts that the kinetics of trafficking versus influx of AMPAR from the extrasynaptic space can further impact AMPAR density. Thus, the contributions of both signalling and trafficking should be considered in computational models.

Transsynaptic Signaling of Ephs in Synaptic Development, Plasticity, and Disease

Synapse formation between neurons is critical for proper circuit and brain function. Prior to activity-dependent refinement of connections between neurons, activity-independent cues regulate the contact and recognition of potential synaptic partners. Formation of a synapse results in molecular recognition events that initiate the process of synaptogenesis. Synaptogenesis requires contact between axon and dendrite, selection of correct and rejection of incorrect partners, and recruitment of appropriate pre- and postsynaptic proteins needed for the establishment of functional synaptic contact. Key regulators of these events are families of transsynaptic proteins, where one protein is found on the presynaptic neuron and the other is found on the postsynaptic neuron. Of these families, the EphBs and ephrin-Bs are required during each phase of synaptic development from target selection, recruitment of synaptic proteins, and formation of spines to regulation of synaptic plasticity at glutamatergic spine synapses in the mature brain. These roles also place EphBs and ephrin-Bs as important regulators of human neurological diseases. This review will focus on the role of EphBs and ephrin-Bs at synapses.

A-kinase anchoring protein 79/150 coordinates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor sensitization in sensory neurons

Changes in sensory afferent activity contribute to the transition from acute to chronic pain. However, it is unlikely that a single sensory receptor is entirely responsible for persistent pain. It is more probable that extended changes to multiple receptor proteins expressed by afferent neurons support persistent pain. A-Kinase Anchoring Protein 79/150 (AKAP) is an intracellular scaffolding protein expressed in sensory neurons that spatially and temporally coordinates signaling events. Since AKAP scaffolds biochemical modifications of multiple TRP receptors linked to pain phenotypes, we probed for other ionotropic receptors that may be mediated by AKAP and contribute to persistent pain. Here, we identify a role for AKAP modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor (AMPA-R) functionality in sensory neurons. Pharmacological manipulation of distinct AMPA-R subunits significantly reduces persistent mechanical hypersensitivity observed during hyperalgesic priming. Stimulation of both protein kinases C and A (PKC, PKA, respectively) modulate AMPA-R subunit GluR1 and GluR2 phosphorylation and surface expression in an AKAP-dependent manner in primary cultures of DRG neurons. Furthermore, AKAP knock out reduces sensitized AMPA-R responsivity in DRG neurons. Collectively, these data indicate that AKAP scaffolds AMPA-R subunit organization in DRG neurons that may contribute to the transition from acute-to-chronic pain.

Epigenetic signature in neural plasticity: the journey so far and journey ahead

Neural plasticity is a remarkable characteristic of the brain which allows neurons to rewire their structure in response to internal and external stimuli. Many external stimuli collectively referred to as 'epigenetic factors' strongly influence structural and functional reorganization of the brain, thereby acting as a potential driver of neural plasticity. DNA methylation and demethylation, histone acetylation, and deacetylation are some of the frontline epigenetic mechanisms behind neural plasticity. Epigenetic signature molecules (mostly proteins) play a pivotal role in epigenetic reprogramming. Though neuro-epigenetics is an incredibly important field of emerging research, the critical role of signature proteins associated with epigenetic alteration and their involvement in neural plasticity needs further attention. This study gives an integrated and systematic overview of the current state of knowledge with a clear idea of types of neural plasticity and the context-dependent role of epigenetic signature molecules and their modulation by some natural bioactive compounds.

Development of single-molecule ubiquitination mediated fluorescence complementation to visualize protein ubiquitination dynamics in dendrites

The ubiquitination/proteasome system is important for the spatiotemporal control of protein synthesis and degradation at synapses, while dysregulation may underlie autism spectrum disorders (ASDs). However, methods allowing direct visualization of the subcellular localization and temporal dynamics of protein ubiquitination are lacking. Here we report the development of Single-Molecule Ubiquitin Mediated Fluorescence Complementation (SM-UbFC) as a method to visualize and quantify the dynamics of protein ubiquitination in dendrites of live neurons in culture. Using SM-UbFC, we demonstrate that the rate of PSD-95 ubiquitination is elevated in dendrites of FMR1 KO neurons compared with wild-type controls. We further demonstrate the rapid ubiquitination of the fragile X messenger ribonucleoprotein, FMRP, and the AMPA receptor subunit, GluA1, which are known to be key events in the regulation of synaptic protein synthesis and plasticity. SM-UbFC will be useful for future studies on the regulation of synaptic protein homeostasis.

Palmitoylation of A-kinase anchoring protein 79/150 modulates its nanoscale organization, trafficking, and mobility in postsynaptic spines

A-kinase anchoring protein 79-human/150-rodent (AKAP79/150) organizes signaling proteins to control synaptic plasticity. AKAP79/150 associates with the plasma membrane and endosomes through its N-terminal domain that contains three polybasic regions and two Cys residues that are reversibly palmitoylated. Mutations abolishing palmitoylation (AKAP79/150 CS) reduce its endosomal localization and association with the postsynaptic density (PSD). Here we combined advanced light and electron microscopy (EM) to characterize the effects of AKAP79/150 palmitoylation on its postsynaptic nanoscale organization, trafficking, and mobility in hippocampal neurons. Immunogold EM revealed prominent extrasynaptic membrane AKAP150 labeling with less labeling at the PSD. The label was at greater distances from the spine membrane for AKAP150 CS than WT in the PSD but not in extra-synaptic locations. Immunogold EM of GFP-tagged AKAP79 WT showed that AKAP79 adopts a vertical, extended conformation at the PSD with its N-terminus at the membrane, in contrast to extrasynaptic locations where it adopts a compact or open configurations of its N- and C-termini with parallel orientation to the membrane. In contrast, GFP-tagged AKAP79 CS was displaced from the PSD coincident with disruption of its vertical orientation, while proximity and orientation with respect to the extra-synaptic membrane was less impacted. Single-molecule localization microscopy (SMLM) revealed a heterogeneous distribution of AKAP150 with distinct high-density, nano-scale regions (HDRs) overlapping the PSD but more prominently located in the extrasynaptic membrane for WT and the CS mutant. Thick section scanning transmission electron microscopy (STEM) tomography revealed AKAP150 immunogold clusters similar in size to HDRs seen by SMLM and more AKAP150 labeled endosomes in spines for WT than for CS, consistent with the requirement for AKAP palmitoylation in endosomal trafficking. Hidden Markov modeling of single molecule tracking data revealed a bound/immobile fraction and two mobile fractions for AKAP79 in spines, with the CS mutant having shorter dwell times and faster transition rates between states than WT, suggesting that palmitoylation stabilizes individual AKAP molecules in various spine subpopulations. These data demonstrate that palmitoylation fine tunes the nanoscale localization, mobility, and trafficking of AKAP79/150 in dendritic spines, which might have profound effects on its regulation of synaptic plasticity.

AMPA Receptor Function in Hypothalamic Synapses

AMPA receptors (AMPARs) are critical for mediating glutamatergic synaptic transmission and plasticity, thus playing a major role in the molecular machinery underlying cellular substrates of memory and learning. Their expression pattern, transport and regulatory mechanisms have been extensively studied in the hippocampus, but their functional properties in other brain regions remain poorly understood. Interestingly, electrophysiological and molecular evidence has confirmed a prominent role of AMPARs in the regulation of hypothalamic function. This review summarizes the existing evidence on AMPAR-mediated transmission in the hypothalamus, where they are believed to orchestrate the role of glutamatergic transmission in autonomous, neuroendocrine function, body homeostasis, and social behavior.

N-Cadherin Regulates GluA1-Mediated Depressive-Like Behavior in Adolescent Female Rat Offspring following Prenatal Stress

BACKGROUND

The incidence of depression is twice higher in women than in men, and gender differences in the prevalence rates first emerge around puberty. Prenatal stress (PS) induces gender-dependent depressive-like behavior in adolescent offspring, but the neuro-physiological mechanisms remain unclear. Our study aimed to investigate the possible neuro-physiological mechanisms of gender-dependent depressive-like behavior in PS adolescent offspring and further explored the possibility of treating depression in adolescent female rats.

METHODS

The pregnant rats were exposed to restraint stress in the third trimester for 7 days. The depressive-like behavior and the expression of N-cadherin and AMPARs in the hippocampus of adolescent offspring rats were assessed. 10 mg/kg AMPAR antagonist CNQX and 10 mg/kg N-cadherin antagonist ADH-1 were intraperitoneally injected into female adolescent offspring, respectively; 0.2 µg AMPAR agonist CX546 was administered to the dentate gyrus of male adolescent offspring to determine the role of N-cadherin-AMPARs in depressive-like behavior of the offspring following PS.

RESULTS

We found that PS increased N-cadherin expression, which upregulated GluA1 expression in the dentate gyrus, mediating depressive-like behavior in adolescent female rat offspring by reducing PSD-95. In addition, ADH-1 and CNQX improved depressive-like behavior in adolescent female offspring following PS. Furthermore, injection of the CX546 into the dentate gyrus induced depressive-like behavior in PS male offspring.

CONCLUSION

The gender-dependent expression of N-cadherin-GluA1 pathway in adolescent offspring in the dentate gyrus was the key factor in gender differences of depressive-like behavior following PS.

AKAP150 and its Palmitoylation Contributed to Pain Hypersensitivity Via Facilitating Synaptic Incorporation of GluA1-Containing AMPA Receptor in Spinal Dorsal Horn

The A-kinase anchoring protein 150 (AKAP150) organizes kinases and phosphatases to regulate AMPA receptors (AMPARs) that are pivotal for synaptic plasticity. AKAP150 itself undergoes S-palmitoylation. However, the roles of AKAP150 and its palmitoylation in spinal nociceptive processing remain unknown. In this study, we found that intraplantar injection of complete Freund's adjuvant (CFA) significantly increased the synaptic expression of AKAP150 and caused a reorganization of AKAP150 signaling complex in spinal dorsal horn. Knockdown of AKAP150 or interruption of its interactions with kinases effectively suppressed the CFA-induced synaptic expression of GluA1 subunit of AMPARs. Our data also showed that an upregulation of AKAP150 palmitoylation was involved in the synaptic redistribution of AKAP150. Inhibition of AKAP150 palmitoylation by expression of palmitoylation-defective mutant AKAP150 (C36, 123S) effectively repressed the CFA-induced phosphorylation and synaptic expression of GluA1 subunit, meanwhile, attenuated the development of mechanical allodynia and thermal hyperalgesia. Furthermore, we found that an increased expression of palmitoyl acyltransferase ZDHHC2 contributed to the upregulation of AKAP150 palmitoylation and GluA1 accumulation in inflamed mouse. These data indicated that AKAP150 and its palmitoylation were involved in AMPA receptor-dependent modification of nociceptive transmission, and the manipulations of AKAP150 signaling complex and palmitoylation might serve as potential therapeutic strategies for persistent pain after inflammation.

A kinesin 1-protrudin complex mediates AMPA receptor synaptic removal during long-term depression

The synaptic removal of AMPA-type glutamate receptors (AMPARs) is a core mechanism for hippocampal long-term depression (LTD). In this study, we address the role of microtubule-dependent transport of AMPARs as a driver for vesicular trafficking and sorting during LTD. Here, we show that the kinesin-1 motor KIF5A/C is strictly required for LTD expression in CA3-to-CA1 hippocampal synapses. Specifically, we find that KIF5 is required for an efficient internalization of AMPARs after NMDA receptor activation. We show that the KIF5/AMPAR complex is assembled in an activity-dependent manner and associates with microsomal membranes upon LTD induction. This interaction is facilitated by the vesicular adaptor protrudin, which is also required for LTD expression. We propose that protrudin links KIF5-dependent transport to endosomal sorting, preventing AMPAR recycling to synapses after LTD induction. Therefore, this work identifies an activity-dependent molecular motor and the vesicular adaptor protein that executes AMPAR synaptic removal during LTD.

Neuronal activity recruits the CRTC1/CREB axis to drive transcription-dependent autophagy for maintaining late-phase LTD

Cellular resources must be reorganized for long-term synaptic plasticity during brain information processing, in which coordinated gene transcription and protein turnover are required. However, the mechanism underlying this process remains elusive. Here, we report that activating N-methyl-d-aspartate receptors (NMDARs) induce transcription-dependent autophagy for synaptic turnover and late-phase long-term synaptic depression (L-LTD), which invokes cytoplasm-to-nucleus signaling mechanisms known to be required for late-phase long-term synaptic potentiation (L-LTP). Mechanistically, LTD-inducing stimuli specifically dephosphorylate CRTC1 (CREB-regulated transcription coactivator 1) at Ser-151 and are advantaged in recruiting CRTC1 from cytoplasm to the nucleus, where it competes with FXR (fed-state sensing nuclear receptor) for binding to CREB (cAMP response element-binding protein) and drives autophagy gene expression. Disrupting synergistic actions of CREB and CRTC1 (two essential L-LTP transcription factors) impairs transcription-dependent autophagy induction and prevents NMDAR-dependent L-LTD, which can be rescued by constitutively inducing mechanistic target of rapamycin (mTOR)-dependent autophagy. Together, these findings uncover mechanistic commonalities between L-LTP and L-LTD, suggesting that synaptic activity can tune excitation-transcription coupling for distinct long-lasting synaptic remodeling.

Homeostatic synaptic scaling establishes the specificity of an associative memory

Correlation-based (Hebbian) forms of synaptic plasticity are crucial for the initial encoding of associative memories but likely insufficient to enable the stable storage of multiple specific memories within neural circuits. Theoretical studies have suggested that homeostatic synaptic normalization rules provide an essential countervailing force that can stabilize and expand memory storage capacity. Although such homeostatic mechanisms have been identified and studied for decades, experimental evidence that they play an important role in associative memory is lacking. Here, we show that synaptic scaling, a widely studied form of homeostatic synaptic plasticity that globally renormalizes synaptic strengths, is dispensable for initial associative memory formation but crucial for the establishment of memory specificity. We used conditioned taste aversion (CTA) learning, a form of associative learning that relies on Hebbian mechanisms within gustatory cortex (GC), to show that animals conditioned to avoid saccharin initially generalized this aversion to other novel tastants. Specificity of the aversion to saccharin emerged slowly over a time course of many hours and was associated with synaptic scaling down of excitatory synapses onto conditioning-active neuronal ensembles within gustatory cortex. Blocking synaptic scaling down in the gustatory cortex enhanced the persistence of synaptic strength increases induced by conditioning and prolonged the duration of memory generalization. Taken together, these findings demonstrate that synaptic scaling is crucial for sculpting the specificity of an associative memory and suggest that the relative strengths of Hebbian and homeostatic plasticity can modulate the balance between stable memory formation and memory generalization.

Inhibition of monoamine oxidase attenuates social defeat-induced memory impairment in goldfish, (Carassius auratus): A possible involvement of synaptic proteins and BDNF

Social defeat (SD) has been implicated in different modulatory effects of physiology and behaviour including learning and memory. We designed an experiment to test the functional role of monoamine oxidase (MAO) in regulation of synaptic transmission, synaptic plasticity and memory in goldfish Carassius auratus. To test this, individuals were divided into three groups: (i) control; (ii) social defeat (SD) group (individuals were subjected to social defeat for 10 min by Pseudotropheus demasoni) and (iii) SD + MAO inhibitor pre-treated group. All experimental groups were subjected to spatial learning and then memory. Our results suggest that SD affects a spatial learning and memory, whereas SD exerts no influence on MAOI pre-treated group. In addition, we noted that the expression of monoamine oxidase-A (MAO-A) was up-regulated and level of serotonin (5-hydroxytryptamine; 5-HT), expression of serotonin transporter (SERT), synaptophysin (SYP), synaptotagmin -1 (SYT-1), N-methyl-D-asparate (NMDA) receptors subunits (NR2A and NR2B), postsynaptic density-95 (PSD-95) and brain-derived neurotrophic factor (BDNF) were reduced by SD, while MAOIs pretreatment protects the effect of SD. Taken together, our results suggest that MAO is an essential component in the serotonergic system that finely tunes the level of 5-HT, which further regulates the molecules involving in synaptic transmission, synaptic plasticity and memory.

Photobiomodulation suppresses JNK3 by activation of ERK/MKP7 to attenuate AMPA receptor endocytosis in Alzheimer's disease

Alzheimer's disease (AD), a severe age‐related neurodegenerative disorder, lacks effective therapeutic methods at present. Physical approaches such as gamma frequency light flicker that can effectively reduce amyloid load have been reported recently. Our previous research showed that a physical method named photobiomodulation (PBM) therapy rescues Aβ‐induced dendritic atrophy in vitro. However, it remains to be further investigated the mechanism by which PBM affects AD‐related multiple pathological features to improve learning and memory deficits. Here, we found that PBM attenuated Aβ‐induced synaptic dysfunction and neuronal death through MKP7‐dependent suppression of JNK3, a brain‐specific JNK isoform related to neurodegeneration. The results showed PBM‐attenuated amyloid load, AMPA receptor endocytosis, dendrite injury, and inflammatory responses, thereby rescuing memory deficits in APP/PS1 mice. We noted JNK3 phosphorylation was dramatically decreased after PBM treatment in vivo and in vitro. Mechanistically, PBM activated ERK, which subsequently phosphorylated and stabilized MKP7, resulting in JNK3 inactivation. Furthermore, activation of ERK/MKP7 signaling by PBM increased the level of AMPA receptor subunit GluR 1 phosphorylation and attenuated AMPA receptor endocytosis in an AD pathological model. Collectively, these data demonstrated that PBM has potential therapeutic value in reducing multiple pathological features associated with AD, which is achieved by regulating JNK3, thus providing a noninvasive, and drug‐free therapeutic strategy to impede AD progression.

Standardized Bacopa monnieri Extract Ameliorates Learning and Memory Impairments through Synaptic Protein, Neurogranin, Pro-and Mature BDNF Signaling, and HPA Axis in Prenatally Stressed Rat Offspring

Prenatal stress (PNS) influences offspring neurodevelopment, inducing anxiety-like behavior and memory deficits. We investigated whether pretreatment of Bacopa monnieri extract (CDRI-08/BME) ameliorates PNS-induced changes in signaling molecules, and changes in the behavior of Wistar rat offspring. Pregnant rats were randomly assigned into control (CON)/prenatal stress (PNS)/PNS and exposed to BME treatment (PNS + BME). Dams were exposed to stress by placing them in a social defeat cage, where they observed social defeat from gestational day (GD)-16–18. Pregnant rats in the PNS + BME group were given BME treatment from GD-10 to their offspring’s postnatal day (PND)-23, and to their offspring from PND-15 to -30. PNS led to anxiety-like behavior; impaired memory; increased the level of corticosterone (CORT), adrenocorticotropic hormone, glucocorticoid receptor, pro-apoptotic Casepase-3, and 5-HT_2C receptor; decreased anti-apoptotic Bcl-2, synaptic proteins (synaptophysin, synaptotagmin-1), 5-HT_1A, receptor, phosphorylation of calmodulin-dependent protein kinase II/neurogranin, N-methyl-D-aspartate receptors (2A,2B), postsynaptic density protein 95; and conversion of pro and mature brain derived neurotropic factor in their offspring. The antioxidant property of BME possibly inhibiting the PNS-induced changes in observed molecules, anxiety-like behavior, and memory deficits. The observed results suggest that pretreatment of BME could be an effective coping strategy to prevent PNS-induced behavioral impairments in their offspring.

The post-synaptic scaffolding protein tamalin regulates ligand-mediated trafficking of metabotropic glutamate receptors

Group I metabotropic glutamate receptors (mGluRs) play important roles in various neuronal functions and have also been implicated in multiple neuropsychiatric disorders like fragile X syndrome, autism, and others. mGluR trafficking not only plays important roles in controlling the spatiotemporal localization of these receptors in the cell but also regulates the activity of these receptors. Despite this obvious significance, the cellular machineries that control the trafficking of group I metabotropic glutamate receptors in the central nervous system have not been studied in detail. The post-synaptic scaffolding protein tamalin has been shown to interact with group I mGluRs and also with many other proteins involved in protein trafficking in neurons. Using a molecular replacement approach in mouse hippocampal neurons, we show here that tamalin plays a critical role in the ligand-dependent internalization of mGluR1 and mGluR5, members of the group I mGluR family. Specifically, knockdown of endogenous tamalin inhibited the ligand-dependent internalization of these two receptors. Both N-terminal and C-terminal regions of tamalin played critical roles in mGluR1 endocytosis. Furthermore, we found that tamalin regulates mGluR1 internalization by interacting with S-SCAM, a protein that has been implicated in vesicular trafficking. Finally, we demonstrate that tamalin plays a critical role in mGluR-mediated internalization of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, a process believed to be the cellular correlate for mGluR-dependent synaptic plasticity. Taken together, these findings reveal a mechanistic role of tamalin in the trafficking of group I mGluRs and suggest its physiological implications in the brain.

Proteasomal-Mediated Degradation of AKAP150 Accompanies AMPAR Endocytosis during cLTD

The number and function of synaptic AMPA receptors (AMPARs) tightly regulates excitatory synaptic transmission. Current evidence suggests that AMPARs are inserted into the postsynaptic membrane during long-term potentiation (LTP) and are removed from the membrane during long-term depression (LTD). Dephosphorylation of GluA1 at Ser-845 and enhanced endocytosis are critical events in the modulation of LTD. Moreover, changes in scaffold proteins from the postsynaptic density (PSD) could be also related to AMPAR regulation in LTD. In the present study we analyzed the effect of chemical LTD (cLTD) on A-kinase anchoring protein (AKAP)150 and AMPARs levels in mouse-cultured neurons. We show that cLTD induces AKAP150 protein degradation via proteasome, coinciding with GluA1 dephosphorylation at Ser-845 and endocytosis of GluA1-containing AMPARs. Pharmacological inhibition of proteasome activity, but not phosphatase calcineurin (CaN), reverted cLTD-induced AKAP150 protein degradation. Importantly, AKAP150 silencing induced dephosphorylation of GluA1 Ser-845 and GluA1-AMPARs endocytosis while AKAP150 overexpression blocked cLTD-mediated GluA1-AMPARs endocytosis. Our results provide direct evidence that cLTD-induced AKAP150 degradation by the proteasome contributes to synaptic AMPARs endocytosis.

Phosphorylation-Dependent Regulation of Ca2+-Permeable AMPA Receptors During Hippocampal Synaptic Plasticity

Experience-dependent learning and memory require multiple forms of plasticity at hippocampal and cortical synapses that are regulated by N-methyl-D-aspartate receptors (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors (NMDAR, AMPAR). These plasticity mechanisms include long-term potentiation (LTP) and depression (LTD), which are Hebbian input-specific mechanisms that rapidly increase or decrease AMPAR synaptic strength at specific inputs, and homeostatic plasticity that globally scales-up or -down AMPAR synaptic strength across many or even all inputs. Frequently, these changes in synaptic strength are also accompanied by a change in the subunit composition of AMPARs at the synapse due to the trafficking to and from the synapse of receptors lacking GluA2 subunits. These GluA2-lacking receptors are most often GluA1 homomeric receptors that exhibit higher single-channel conductance and are Ca2+-permeable (CP-AMPAR). This review article will focus on the role of protein phosphorylation in regulation of GluA1 CP-AMPAR recruitment and removal from hippocampal synapses during synaptic plasticity with an emphasis on the crucial role of local signaling by the cAMP-dependent protein kinase (PKA) and the Ca2+calmodulin-dependent protein phosphatase 2B/calcineurin (CaN) that is coordinated by the postsynaptic scaffold protein A-kinase anchoring protein 79/150 (AKAP79/150).

Protein kinase A facilitates relaxation of mouse ileum via phosphorylation of neuronal nitric oxide synthase

BACKGROUND AND PURPOSE

The enteric neurotransmitter nitric oxide (NO) regulates gastrointestinal motility by relaxing smooth muscle. Pharmacological cAMP induction also relaxes gastrointestinal smooth muscle, but it is uncertain whether cAMP augments or suppresses enteric NO signalling. In other organ systems, cAMP can increase neuronal NO production by stimulating protein kinase A (PKA) to phosphorylate neuronal NOS (nNOS) Serine-1412 (S1412). We hypothesized that cAMP also increases nNOS S1412 phosphorylation by PKA in enteric neurons to augment nitrergic relaxation of mouse ileum.

EXPERIMENTAL APPROACH

We measured contractile force and nNOS S1412 phosphorylation in ileal rings suspended in an organ bath. We used forskolin to induce cAMP-dependent relaxation of wild type, nNOS^S1412A knock-in and nNOSα-null ileal rings in the presence or absence of PKA, protein kinase B (Akt) and NOS inhibitors.

KEY RESULTS

Forskolin stimulated phosphorylation of nNOS S1412 in mouse ileum. Forskolin relaxed nNOSα-null and nNOS^S1412A ileal rings less than wild-type ileal rings. PKA inhibition blocked forskolin-induced nNOS phosphorylation and attenuated relaxation of wild type but not nNOS^S1412A ileum. Akt inhibition did not alter nNOS phosphorylation with forskolin but did attenuate relaxation of wild type and nNOS^S1412A . NOS inhibition with L-NAME eliminated the effects of PKA and Akt inhibitors on relaxation.

CONCLUSION AND IMPLICATIONS

PKA phosphorylation of nNOS S1412 augments forskolin-induced nitrergic ileal relaxation. The relationship between cAMP/PKA and NO is therefore synergistic in enteric nitrergic neurons. Because NO regulates gut motility, selective modulation of enteric neuronal cAMP synthesis may be useful for the treatment of gastrointestinal motility disorders.

Multiple Domains in the Kv7.3 C-Terminus Can Regulate Localization to the Axon Initial Segment

The voltage-gated Kv7.2/Kv7.3 potassium channel is a critical regulator of neuronal excitability. It is strategically positioned at the axon initial segment (AIS) of neurons, where it effectively inhibits repetitive action potential firing. While the selective accumulation of Kv7.2/Kv7.3 channels at the AIS requires binding to the adaptor protein ankyrin G, it is currently unknown if additional molecular mechanisms contribute to the localization and fine-tuning of channel numbers at the AIS. Here, we utilized a chimeric approach to pinpoint regions within the Kv7.3 C-terminal tail with an impact upon AIS localization. This strategy identified two domains with opposing effects upon the AIS localization of Kv7.3 chimeras expressed in cultured hippocampal neurons. While a membrane proximal domain reduced AIS localization of Kv7.3 chimeras, helix D increased and stabilized chimera AIS localization. None of the identified domains were required for AIS localization. However, the domains modulated the relative efficiency of the localization raising the possibility that the two domains contribute to the regulation of Kv7 channel numbers and nanoscale organization at the AIS.

BRAG1/IQSEC2 as a regulator of small GTPase-dependent trafficking

Precise trafficking events, such as those that underlie synaptic transmission and plasticity, require complex regulation. G-protein signaling plays an essential role in the regulation of membrane and protein trafficking. However, it is not well understood how small GTPases and their regulatory proteins coordinate such specific events. Our recent publication focused on a highly abundant synaptic GEF, BRAG1, whose physiologic relevance was unknown. We find that BRAG1s GEF activity is required for activity-dependent trafficking of AMPARs. Moreover, BRAG1 bidirectionally regulates synaptic transmission in a manner independent of this activity. In addition to the GEF domain, BRAG1 contains several functional domains whose roles are not yet understood but may mediate protein-protein interactions and regulatory effects necessary for its role in regulation of AMPAR trafficking. In this commentary, we explore the potential for BRAG1 to provide specificity of small GTPase signaling, coordinating activity-dependent activation of small GTPase activity with signaling and scaffolding molecules involved in trafficking through its GEF activity and other functional domains.

A-Kinase Anchoring Protein 150 and Protein Kinase A Complex in the Basolateral Amygdala Contributes to Depressive-like Behaviors Induced by Chronic Restraint Stress

BACKGROUND

The basolateral amygdala (BLA) has been widely implicated in the pathophysiology of major depressive disorder. A-kinase anchoring protein 150 (AKAP150) directs kinases and phosphatases to synaptic glutamate receptors, controlling synaptic transmission and plasticity. However, the role of the AKAP150 in the BLA in major depressive disorder remains poorly understood.

METHODS

Depressive-like behaviors in C57BL/6J mice were developed by chronic restraint stress (CRS). Mice received either intra-BLA injection of lentivirus-expressing Akap5 short hairpin RNA or Ht-31, a peptide to disrupt the interaction of AKAP150 and protein kinase A (PKA), followed by depressive-like behavioral tests. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate receptor (AMPAR)-mediated miniature excitatory postsynaptic currents were recorded by whole-cell patch-clamp techniques.

RESULTS

Chronic stress exposure induced depressive-like behaviors, which were accompanied by an increase in total and synaptic AKAP150 expression in the BLA. Accordingly, CRS facilitated the association of AKAP150 with PKA, but not of calcineurin in the BLA. Intra-BLA infusion of lentivirus-expressing Akap5 short hairpin RNA or Ht-31 prevented depressive-like behaviors and normalized phosphorylation of serine 845 and surface expression of AMPAR subunit 1 (GluA1) in the BLA of CRS mice. Finally, blockage of AKAP150-PKA complex signaling rescued the changes in AMPAR-mediated miniature excitatory postsynaptic currents in depressive-like mice.

CONCLUSIONS

These results suggest that AKAP150-PKA directly modulates BLA neuronal synaptic strength, and that AKAP150-PKA-GluA1 streamline signaling complex is responsible for CRS-induced disruption of synaptic AMPAR-mediated transmission and depressive-like behaviors in mice.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

Contextual fear memory modulates PSD95 phosphorylation, AMPAr subunits, PKMζ and PI3K differentially between adult and juvenile rats

It is well known that young organisms do not maintain memories as long as adults, but the mechanisms for this ontogenetic difference are undetermined. Previous work has revealed that the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAr) subunits are trafficked into the synaptic membrane following memory retrieval in adults. Additionally, phosphorylated PSD-95-pS295 promotes AMPAr stabilization at the synapse. We investigated these plasticity related proteins as potential mediators in the differential contextual stress memory retrieval capabilities observed between adult and juvenile rats. Rats were assigned to either pedestal stress (1 h) or no stress control (home cage). Each animal was placed alone in an open field for 5 min at the base of a 6 × 6 sq inch pedestal (4ft high). Stress subjects were then placed on this pedestal for 1hr and control subjects were placed in their home cage following initial exploration. Each animal was returned to the open field for 5 min either 1d or 7d following initial exposure. Freezing postures were quantified during the memory retrieval test. The 1d test shows adult (P90) and juvenile (P26) stressed rats increase their freezing time compared to controls. However, the 7d memory retrieval test shows P90 stress rats but not P26 stress rats freeze while in the fear context. Twenty minutes after the memory retrieval test, hippocampi and amygdala were micro-dissected and prepared for western blot analysis. Our results show that 1d fear memory retrieval induced an upregulation of PSD-95 and pS295 in the adult amygdala but not in the juvenile. However, the juvenile animals upregulated PKMζ, PI3K and GluA2/3, GluA1-S845 in the dorsal hippocampus (DH), but the adults did not. Following the 7d memory retrieval test, adults upregulated GluA2 in the amygdala but not the juveniles. In the DH, adults increased PSD-95 and pS295 but not the juveniles. The adults appear to preferentially increase amygdala-driven processing at 1d and increase DH-driven context specific processing at 7d. These data identify molecular processes that may underlie the reduced fear-memory retrieval capability of juveniles. Together these data provide a potential molecular target that could be beneficial in treatment of anxiety disorders and PTSD.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

Synaptic loss in schizophrenia: a meta-analysis and systematic review of synaptic protein and mRNA measures

Although synaptic loss is thought to be core to the pathophysiology of schizophrenia, the nature, consistency and magnitude of synaptic protein and mRNA changes has not been systematically appraised. Our objective was thus to systematically review and meta-analyse findings. The entire PubMed database was searched for studies from inception date to the 1st of July 2017. We selected case-control postmortem studies in schizophrenia quantifying synaptic protein or mRNA levels in brain tissue. The difference in protein and mRNA levels between cases and controls was extracted and meta-analysis conducted. Among the results, we found a significant reduction in synaptophysin in schizophrenia in the hippocampus (effect size: -0.65, p < 0.01), frontal (effect size: -0.36, p = 0.04), and cingulate cortices (effect size: -0.54, p = 0.02), but no significant changes for synaptophysin in occipital and temporal cortices, and no changes for SNAP-25, PSD-95, VAMP, and syntaxin in frontal cortex. There were insufficient studies for meta-analysis of complexins, synapsins, rab3A and synaptotagmin and mRNA measures. Findings are summarised for these, which generally show reductions in SNAP-25, PSD-95, synapsin and rab3A protein levels in the hippocampus but inconsistency in other regions. Our findings of moderate-large reductions in synaptophysin in hippocampus and frontal cortical regions, and a tendency for reductions in other pre- and postsynaptic proteins in the hippocampus are consistent with models that implicate synaptic loss in schizophrenia. However, they also identify potential differences between regions and proteins, suggesting synaptic loss is not uniform in nature or extent.

Nutrient limitation affects presynaptic structures through dissociable Bassoon autophagic degradation and impaired vesicle release

Acute mismatch between metabolic requirements of neurons and nutrients/growth factors availability characterizes several neurological conditions such as traumatic brain injury, stroke and hypoglycemia. Although the effects of this mismatch have been investigated at cell biological level, the effects on synaptic structure and function are less clear. Since synaptic activity is the most energy-demanding neuronal function and it is directly linked to neuronal networks functionality, we have explored whether nutrient limitation (NL) affects the ultrastructure, function and composition of pre and postsynaptic terminals. We show that upon NL, presynaptic terminals show disorganized vesicle pools and reduced levels of the active zone protein Bassoon (but not of Piccolo). Moreover, NL triggers an impaired vesicle release, which is reversed by re-administration of glucose but not by the blockade of autophagic or proteasomal protein degradation. This reveals a dissociable correlation between presynaptic architecture and vesicle release, since restoring vesicle fusion does not necessarily depend from the rescue of Bassoon levels. Thus, our data show that the presynaptic compartment is highly sensitive to NL and the rescue of presynaptic function requires re-establishment of the metabolic supply rather than preventing local protein degradation.

Postsynaptic localization and regulation of AMPA receptors and Cav1.2 by β2 adrenergic receptor/PKA and Ca2+/CaMKII signaling

The synapse transmits, processes, and stores data within its tiny space. Effective and specific signaling requires precise alignment of the relevant components. This review examines current insights into mechanisms of AMPAR and NMDAR localization by PSD-95 and their spatial distribution at postsynaptic sites to illuminate the structural and functional framework of postsynaptic signaling. It subsequently delineates how β2 adrenergic receptor (β2 AR) signaling via adenylyl cyclase and the cAMP-dependent protein kinase PKA is organized within nanodomains. Here, we discuss targeting of β2 AR, adenylyl cyclase, and PKA to defined signaling complexes at postsynaptic sites, i.e., AMPARs and the L-type Ca2+ channel Cav1.2, and other subcellular surface localizations, the role of A kinase anchor proteins, the physiological relevance of the spatial restriction of corresponding signaling, and their interplay with signal transduction by the Ca2+- and calmodulin-dependent kinase CaMKII How localized and specific signaling by cAMP occurs is a central cellular question. The dendritic spine constitutes an ideal paradigm for elucidating the dimensions of spatially restricted signaling because of their small size and defined protein composition.

Neuronal calcineurin transcriptional targets parallel changes observed in Alzheimer disease brain

Synaptic dysfunction and loss are core pathological features in Alzheimer disease (AD). In the vicinity of amyloid-β plaques in animal models, synaptic toxicity occurs and is associated with chronic activation of the phosphatase calcineurin (CN). Indeed, pharmacological inhibition of CN blocks amyloid-β synaptotoxicity. We therefore hypothesized that CN-mediated transcriptional changes may contribute to AD neuropathology and tested this by examining the impact of CN over-expression on neuronal gene expression in vivo. We found dramatic transcriptional down-regulation, especially of synaptic mRNAs, in neurons chronically exposed to CN activation. Importantly, the transcriptional profile parallels the changes in human AD tissue. Bioinformatics analyses suggest that both nuclear factor of activated T cells and numerous microRNAs may all be impacted by CN, and parallel findings are observed in AD. These data and analyses support the hypothesis that at least part of the synaptic failure characterizing AD may result from aberrant CN activation leading to down-regulation of synaptic genes, potentially via activation of specific transcription factors and expression of repressive microRNAs.

OPEN PRACTICES

Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/ Read the Editorial Highlight for this article on page 8.

A Critical Role for Sorting Nexin 1 in the Trafficking of Metabotropic Glutamate Receptors

Group I metabotropic glutamate receptors (mGluRs) function as modulators of neuronal physiology and they have also been implicated in various neuropsychiatric disorders. Trafficking of mGluRs plays important roles in controlling the precise localization of these receptors at specific region of the cell, as well as it regulates the activity of these receptors. Despite this obvious significance, we know very little about the cellular machineries that control the trafficking of these receptors in the CNS. Sorting nexin 1 (SNX1) has been shown to regulate the endosomal sorting of few cell surface receptors either to lysosomes where they are downregulated or back to the cell surface. Using "molecular replacement" approach in hippocampal neurons derived from mice of both sexes, we show here that SNX1 plays critical role in the trafficking of mGluR1, a member of the group I mGluR family. Overexpression of dominant-negative SNX1 or knockdown of endogenous SNX1 resulted in the rapid recycling of the receptor. Importantly, recycling via the rapid recycling route, did not allow the resensitization of the receptors. Our data suggest that both, N-terminal and C-terminal region of SNX1 play critical role in the normal trafficking of the receptor. In addition, we also show here that SNX1 regulates the trafficking of mGluR1 through the interaction with Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate), a protein that has been implicated in both signaling and vesicular trafficking. Thus, these studies reveal a mechanistic role of SNX1 in the trafficking of group I mGluRs and its physiological implications.SIGNIFICANCE STATEMENT Group I mGluRs are activated by the neurotransmitter glutamate in the CNS, and play various important roles in the brain. Similar to many other receptors, trafficking plays crucial roles in controlling the precise localization as well as activity of these receptors. Despite this obvious significance very little is known about the cellular machineries that control the trafficking of these receptors. We demonstrate here, that SNX1 plays a critical role in the trafficking of mGluR1, a member of the group I mGluR family. SNX1-mediated trafficking is critical for the resensitization of the receptor. SNX1 controls the trafficking of the receptor through the interaction with another protein, Hrs. The results suggest a role for SNX1 in the regulation of group I mGluRs.

A-Kinase Anchoring Protein 150 (AKAP150) Promotes Cocaine Reinstatement by Increasing AMPA Receptor Transmission in the Accumbens Shell

Previous work indicated that activation of D1-like dopamine receptors (D1DRs) in the nucleus accumbens shell promoted cocaine seeking through a process involving the activation of PKA and GluA1-containing AMPA receptors (AMPARs). A-kinase anchoring proteins (AKAPs) localize PKA to AMPARs leading to enhanced phosphorylation of GluA1. AKAP150, the most well-characterized isoform, plays an important role in several forms of neuronal plasticity. However, its involvement in drug addiction has been minimally explored. Here we examine the role of AKAP150 in cocaine reinstatement, an animal model of relapse. We show that blockade of PKA binding to AKAPs in the nucleus accumbens shell of Sprague-Dawley rats attenuates reinstatement induced by either cocaine or a D1DR agonist. Moreover, this effect is specific to AKAP150, as viral overexpression of a PKA-binding deficient mutant of AKAP150 also impairs cocaine reinstatement. This viral-mediated attenuation of cocaine reinstatement was accompanied by decreased phosphorylation of GluA1-containing AMPARs and attenuated AMPAR eEPSCs. Collectively, these results suggest that AKAP150 facilitates the reinstatement of cocaine-seeking behavior by amplifying D1DR/PKA-dependent AMPA transmission in the nucleus accumbens.

Control of Homeostatic Synaptic Plasticity by AKAP-Anchored Kinase and Phosphatase Regulation of Ca2+-Permeable AMPA Receptors

Neuronal information processing requires multiple forms of synaptic plasticity mediated by NMDARs and AMPA-type glutamate receptors (AMPARs). These plasticity mechanisms include long-term potentiation (LTP) and long-term depression (LTD), which are Hebbian, homosynaptic mechanisms locally regulating synaptic strength of specific inputs, and homeostatic synaptic scaling, which is a heterosynaptic mechanism globally regulating synaptic strength across all inputs. In many cases, LTP and homeostatic scaling regulate AMPAR subunit composition to increase synaptic strength via incorporation of Ca²⁺-permeable receptors (CP-AMPAR) containing GluA1, but lacking GluA2, subunits. Previous work by our group and others demonstrated that anchoring of the kinase PKA and the phosphatase calcineurin (CaN) to A-kinase anchoring protein (AKAP) 150 play opposing roles in regulation of GluA1 Ser845 phosphorylation and CP-AMPAR synaptic incorporation during hippocampal LTP and LTD. Here, using both male and female knock-in mice that are deficient in PKA or CaN anchoring, we show that AKAP150-anchored PKA and CaN also play novel roles in controlling CP-AMPAR synaptic incorporation during homeostatic plasticity in hippocampal neurons. We found that genetic disruption of AKAP-PKA anchoring prevented increases in Ser845 phosphorylation and CP-AMPAR synaptic recruitment during rapid homeostatic synaptic scaling-up induced by combined blockade of action potential firing and NMDAR activity. In contrast, genetic disruption of AKAP-CaN anchoring resulted in basal increases in Ser845 phosphorylation and CP-AMPAR synaptic activity that blocked subsequent scaling-up by preventing additional CP-AMPAR recruitment. Thus, the balanced, opposing phospho-regulation provided by AKAP-anchored PKA and CaN is essential for control of both Hebbian and homeostatic plasticity mechanisms that require CP-AMPARs.SIGNIFICANCE STATEMENT Neuronal circuit function is shaped by multiple forms of activity-dependent plasticity that control excitatory synaptic strength, including LTP/LTD that adjusts strength of individual synapses and homeostatic plasticity that adjusts overall strength of all synapses. Mechanisms controlling LTP/LTD and homeostatic plasticity were originally thought to be distinct; however, recent studies suggest that CP-AMPAR phosphorylation regulation is important during both LTP/LTD and homeostatic plasticity. Here we show that CP-AMPAR regulation by the kinase PKA and phosphatase CaN coanchored to the scaffold protein AKAP150, a mechanism previously implicated in LTP/LTD, is also crucial for controlling synaptic strength during homeostatic plasticity. These novel findings significantly expand our understanding of homeostatic plasticity mechanisms and further emphasize how intertwined they are with LTP and LTD.

Sevoflurane Acts on Ubiquitination-Proteasome Pathway to Reduce Postsynaptic Density 95 Protein Levels in Young Mice

BACKGROUND

Children with multiple exposures to anesthesia and surgery may have an increased risk of developing cognitive impairment. Sevoflurane, a commonly used anesthetic in children, has been reported to decrease levels of postsynaptic density 95 protein. However, the upstream mechanisms and downstream consequences of the sevoflurane-induced reduction in postsynaptic density 95 protein levels remains largely unknown. We therefore set out to assess whether sevoflurane acts on ubiquitination-proteasome pathway to facilitate postsynaptic density 95 protein degradation.

METHODS

Six-day-old wild-type mice received anesthesia with 3% sevoflurane 2 h daily for 3 days starting on postnatal day 6. We determined the effects of the sevoflurane anesthesia on mRNA, protein and ubiquitinated levels of postsynaptic density 95 protein in neurons, and synaptosomes and hippocampus of young mice. Cognitive function in the mice was determined at postnatal day 31 by using a Morris water maze. Proteasome inhibitor MG132 and E3 ligase mouse double mutant 2 homolog inhibitor Nutlin-3 were used for the interaction studies.

RESULTS

The sevoflurane anesthesia decreased protein, but not mRNA, levels of postsynaptic density 95, and reduced ubiquitinated postsynaptic density 95 protein levels in neurons, synaptosomes, and hippocampus of young mice. Both MG132 and Nutlin-3 blocked these sevoflurane-induced effects. Sevoflurane promoted the interaction of mouse double mutant 2 homolog and postsynaptic density 95 protein in neurons. Finally, MG132 and Nutlin-3 ameliorated the sevoflurane-induced cognitive impairment in the mice.

CONCLUSIONS

These data suggest that sevoflurane acts on the ubiquitination-proteasome pathway to facilitate postsynaptic density 95 protein degradation, which then decreases postsynaptic density 95 protein levels, leading to cognitive impairment in young mice. These studies would further promote the mechanistic investigation of anesthesia neurotoxicity in the developing brain.

Proteasome-independent polyubiquitin linkage regulates synapse scaffolding, efficacy, and plasticity

Ubiquitination-directed proteasomal degradation of synaptic proteins, presumably mediated by lysine 48 (K48) of ubiquitin, is a key mechanism in synapse and neural circuit remodeling. However, more than half of polyubiquitin (polyUb) species in the mammalian brain are estimated to be non-K48; among them, the most abundant is Lys 63 (K63)-linked polyUb chains that do not tag substrates for degradation but rather modify their properties and activity. Virtually nothing is known about the role of these nonproteolytic polyUb chains at the synapse. Here we report that K63-polyUb chains play a significant role in postsynaptic protein scaffolding and synaptic strength and plasticity. We found that the postsynaptic scaffold PSD-95 (postsynaptic density protein 95) undergoes K63 polyubiquitination, which markedly modifies PSD-95's scaffolding potentials, enables its synaptic targeting, and promotes synapse maturation and efficacy. TNF receptor-associated factor 6 (TRAF6) is identified as a direct E3 ligase for PSD-95, which, together with the E2 complex Ubc13/Uev1a, assembles K63-chains on PSD-95. In contrast, CYLD (cylindromatosis tumor-suppressor protein), a K63-specific deubiquitinase enriched in postsynaptic densities, cleaves K63-chains from PSD-95. We found that neuronal activity exerts potent control of global and synaptic K63-polyUb levels and, through NMDA receptors, drives rapid, CYLD-mediated PSD-95 deubiquitination, mobilizing and depleting PSD-95 from synapses. Silencing CYLD in hippocampal neurons abolishes NMDA-induced chemical long-term depression. Our results unveil a previously unsuspected role for nonproteolytic polyUb chains in the synapse and illustrate a mechanism by which a PSD-associated K63-linkage-specific ubiquitin machinery acts on a major postsynaptic scaffold to regulate synapse organization, function, and plasticity.

Postsynaptic density 95 (PSD-95) serine 561 phosphorylation regulates a conformational switch and bidirectional dendritic spine structural plasticity

Postsynaptic density 95 (PSD-95) is a major synaptic scaffolding protein that plays a key role in bidirectional synaptic plasticity, which is a process important for learning and memory. It is known that PSD-95 shows increased dynamics upon induction of plasticity. However, the underlying structural and functional changes in PSD-95 that mediate its role in plasticity remain unclear. Here we show that phosphorylation of PSD-95 at Ser-561 in its guanylate kinase (GK) domain, which is mediated by the partitioning-defective 1 (Par1) kinases, regulates a conformational switch and is important for bidirectional plasticity. Using a fluorescence resonance energy transfer (FRET) biosensor, we show that a phosphomimetic mutation of Ser-561 promotes an intramolecular interaction between GK and the nearby Src homology 3 (SH3) domain, leading to a closed conformation, whereas a non-phosphorylatable S561A mutation or inhibition of Par1 kinase activity decreases SH3-GK interaction, causing PSD-95 to adopt an open conformation. In addition, S561A mutation facilitates the interaction between PSD-95 and its binding partners. Fluorescence recovery after photobleaching imaging reveals that the S561A mutant shows increased stability, whereas the phosphomimetic S561D mutation increases PSD-95 dynamics at the synapse. Moreover, molecular replacement of endogenous PSD-95 with the S561A mutant blocks dendritic spine structural plasticity during chemical long-term potentiation and long-term depression. Endogenous Ser-561 phosphorylation is induced by synaptic NMDA receptor activation, and the SH3-GK domains exhibit a Ser-561 phosphorylation-dependent switch to a closed conformation during synaptic plasticity. Our results provide novel mechanistic insight into the regulation of PSD-95 in dendritic spine structural plasticity through phosphorylation-mediated regulation of protein dynamics and conformation.

The Retromer Supports AMPA Receptor Trafficking During LTP

Alterations in the function of the retromer, a multisubunit protein complex that plays a specialized role in endosomal sorting, have been linked to Alzheimer's and Parkinson's diseases, yet little is known about the retromer's role in the mature brain. Using in vivo knockdown of the critical retromer component VPS35, we demonstrate a specific role for this endosomal sorting complex in the trafficking of AMPA receptors during NMDA-receptor-dependent LTP at mature hippocampal synapses. The impairment of LTP due to VPS35 knockdown was mechanistically independent of any role of the retromer in the production of Aβ from APP. Finally, we find surprising differences between Alzheimer's- and Parkinson's-disease-linked VPS35 mutations in supporting this pathway. These findings demonstrate a key role for the retromer in LTP and provide insights into how retromer malfunction in the mature brain may contribute to symptoms of common neurodegenerative diseases. VIDEO ABSTRACT.

Identification of the Role of miR-142-5p in Alzheimer's Disease by Comparative Bioinformatics and Cellular Analysis

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by the formation of amyloid beta (Aβ) or tau protein aggregates, the hallmark of cognitive decline. MicroRNAs (miRNAs) have emerged as critical factors in neurogenesis and synaptic functions in the central nervous system (CNS). Recent studies have reported alterations in miRNA expression in patients with AD. However, miRNAs associated with AD varied with patient groups or experimental models, suggesting the need for a comparative study to identify miRNAs commonly dysregulated in diverse AD models. Here, we investigated the miRNAs that show dysregulated expression in two different human AD groups and mouse and cellular AD models. After selection of commonly dysregulated miRNAs in these groups, we investigated the pathophysiological significance of miR-142-5p in SH-SY5Y neuronal cells. We found that miR-142-5p was increased upon treatment with Aβ peptide 1-42 (Aβ₄₂). Inhibition of miR-142-5p rescued the Aβ₄₂-mediated synaptic dysfunctions, as indicated by the expression of postsynaptic density protein 95 (PSD-95). Among genes with decreased expression in Aβ₄₂-treated SH-SY5Y cells, the predicted miR-142-5p target genes were significantly related with neuronal function and synapse plasticity. These findings suggest that dysregulation in miR-142-5p expression contributes the pathogenesis of AD by triggering synaptic dysfunction associated with Aβ₄₂-mediated pathophysiology.

Calpain-1 deletion impairs mGluR-dependent LTD and fear memory extinction

Recent studies indicate that calpain-1 is required for the induction of long-term potentiation (LTP) elicited by theta-burst stimulation in field CA1 of hippocampus. Here we determined the contribution of calpain-1 in another type of synaptic plasticity, the long-term depression (LTD) elicited by activation of type-I metabotropic glutamate receptors (mGluR-LTD). mGluR-LTD was associated with calpain-1 activation following T-type calcium channel opening, and resulted in the truncation of a regulatory subunit of PP2A, B56α. This signaling pathway was required for both the early and late phase of Arc translation during mGluR-LTD, through a mechanism involving mTOR and ribosomal protein S6 activation. In contrast, in hippocampal slices from calpain-1 knock-out (KO) mice, application of the mGluR agonist, DHPG, did not result in B56α truncation, increased Arc synthesis and reduced levels of membrane GluA1-containing AMPA receptors. Consistently, mGluR-LTD was impaired in calpain-1 KO mice, and the impairment could be rescued by phosphatase inhibitors, which also restored Arc translation in response to DHPG. Furthermore, calpain-1 KO mice exhibited impairment in fear memory extinction to tone presentation. These results indicate that calpain-1 plays a critical role in mGluR-LTD and is involved in many forms of synaptic plasticity and learning and memory.