Synaptic Incorporation of AMPA Receptors during LTP Is Controlled by a PKC Phosphorylation Site on GluR1

Fluorescence lifetime imaging of AMPA receptor endocytosis in living neurons: effects of Aβ and PP1

The relative amount of AMPA receptors expressed at the surface of neurons can be measured using superecliptic pHluorin (SEP) labeling at their N-terminus. However, the high signal variability resulting from protein overexpression in neurons and the low signal observed in intracellular vesicles make quantitative characterization of receptor trafficking difficult. Here, we establish a real-time live-cell assay of AMPAR trafficking based on fluorescence lifetime imaging (FLIM), which allows for simultaneous visualization of both surface and intracellular receptors. Using this assay, we found that elevating amyloid-beta (Aβ) levels leads to a strong increase in intracellular GluA1 and GluA2-containing receptors, indicating that Aβ triggers the endocytosis of these AMPARs. In APP/PS1 Alzheimer's disease model mouse neurons, FLIM revealed strikingly different AMPAR trafficking properties for GluA1- and GluA3-containing receptors, suggesting that chronic Aβ exposure triggered the loss of both surface and intracellular GluA3-containing receptors. Interestingly, overexpression of protein phosphatase 1 (PP1) also resulted in GluA1 endocytosis as well as depressed synaptic transmission, confirming the important role of phosphorylation in regulating AMPAR trafficking. This new approach allows for the quantitative measurement of extracellular pH, small changes in receptor trafficking, as well as simultaneous measurement of surface and internalized AMPARs in living neurons, and could therefore be applied to several different studies in the future.

Calcineurin regulates synaptic Ca2+-permeable AMPA receptors in hypothalamic presympathetic neurons via α2δ-1-mediated GluA1/GluA2 assembly

Hypertension is a major adverse effect of calcineurin inhibitors, such as tacrolimus (FK506) and cyclosporine, used clinically as immunosuppressants. Calcineurin inhibitor-induced hypertension (CIH) is linked to augmented sympathetic output from the hypothalamic paraventricular nucleus (PVN). GluA2-lacking, Ca2+-permeable AMPA receptors (CP-AMPARs) are a key feature of glutamatergic synaptic plasticity, yet their role in CIH remains elusive. Here, we found that systemic administration of FK506 in rats significantly increased serine phosphorylation of GluA1 and GluA2 in PVN synaptosomes. Strikingly, FK506 treatment reduced GluA1/GluA2 heteromers in both synaptosomes and endoplasmic reticulum-enriched fractions from the PVN. Blocking CP-AMPARs with IEM-1460 induced a larger reduction of AMPAR-mediated excitatory postsynaptic current (AMPAR-EPSC) amplitudes in retrogradely labelled, spinally projecting PVN neurons in FK506-treated rats than in vehicle-treated rats. Furthermore, FK506 treatment shifted the current-voltage relationship of AMPAR-EPSCs from linear to inward rectification in labelled PVN neurons. FK506 treatment profoundly enhanced physical interactions of α2δ-1 with GluA1 and GluA2 in the PVN. Inhibiting α2δ-1 with gabapentin, α2δ-1 genetic knockout, or disrupting α2δ-1-AMPAR interactions with an α2δ-1 C terminus peptide restored GluA1/GluA2 heteromers in the PVN and diminished inward rectification of AMPAR-EPSCs in labelled PVN neurons induced by FK506 treatment. Additionally, microinjection of IEM-1460 or α2δ-1 C terminus peptide into the PVN reduced renal sympathetic nerve discharges and arterial blood pressure elevated in FK506-treated rats but not in vehicle-treated rats. Thus, calcineurin in the hypothalamus constitutively regulates AMPAR subunit composition and phenotypes by controlling GluA1/GluA2 interactions with α2δ-1. Synaptic CP-AMPARs in PVN presympathetic neurons contribute to augmented sympathetic outflow in CIH. KEY POINTS: Systemic treatment with the calcineurin inhibitor increases serine phosphorylation of synaptic GluA1 and GluA2 in the PVN. Calcineurin inhibition enhances the prevalence of postsynaptic Ca2+-permeable AMPARs in PVN presympathetic neurons. Calcineurin inhibition potentiates α2δ-1 interactions with GluA1 and GluA2, disrupting intracellular assembly of GluA1/GluA2 heterotetramers in the PVN. Blocking Ca2+-permeable AMPARs or α2δ-1-AMPAR interactions in the PVN attenuates sympathetic outflow augmented by the calcineurin inhibitor.

Targeting metaplasticity mechanisms to promote sustained antidepressant actions

The discovery that subanesthetic doses of (R, S)-ketamine (ketamine) and (S)-ketamine (esketamine) rapidly induce antidepressant effects and promote sustained actions following drug clearance in depressed patients who are treatment-resistant to other therapies has resulted in a paradigm shift in the conceptualization of how rapidly and effectively depression can be treated. Consequently, the mechanism(s) that next generation antidepressants may engage to improve pathophysiology and resultant symptomology are being reconceptualized. Impaired excitatory glutamatergic synapses in mood-regulating circuits are likely a substantial contributor to the pathophysiology of depression. Metaplasticity is the process of regulating future capacity for plasticity by priming neurons with a stimulation that alters later neuronal plasticity responses. Accordingly, the development of treatment modalities that specifically modulate the duration, direction, or magnitude of glutamatergic synaptic plasticity events such as long-term potentiation (LTP), defined here as metaplastogens, may be an effective approach to reverse the pathophysiology underlying depression and improve depression symptoms. We review evidence that the initiating mechanisms of pharmacologically diverse rapid-acting antidepressants (i.e., ketamine mimetics) converge on consistent downstream molecular mediators that facilitate the expression/maintenance of increased synaptic strength and resultant persisting antidepressant effects. Specifically, while the initiating mechanisms of these therapies may differ (e.g., cell type-specificity, N-methyl-D-aspartate receptor (NMDAR) subtype-selective inhibition vs activation, metabotropic glutamate receptor 2/3 antagonism, AMPA receptor potentiation, 5-HT receptor-activating psychedelics, etc.), the sustained therapeutic mechanisms of putative rapid-acting antidepressants will be mediated, in part, by metaplastic effects that converge on consistent molecular mediators to enhance excitatory neurotransmission and altered capacity for synaptic plasticity. We conclude that the convergence of these therapeutic mechanisms provides the opportunity for metaplasticity processes to be harnessed as a druggable plasticity mechanism by next-generation therapeutics. Further, targeting metaplastic mechanisms presents therapeutic advantages including decreased dosing frequency and associated diminished adverse responses by eliminating the requirement for the drug to be continuously present.

Activity-dependent mitochondrial ROS signaling regulates recruitment of glutamate receptors to synapses

Our understanding of mitochondrial signaling in the nervous system has been limited by the technical challenge of analyzing mitochondrial function in vivo. In the transparent genetic model Caenorhabditis elegans, we were able to manipulate and measure mitochondrial reactive oxygen species (mitoROS) signaling of individual mitochondria as well as neuronal activity of single neurons in vivo. Using this approach, we provide evidence supporting a novel role for mitoROS signaling in dendrites of excitatory glutamatergic C. elegans interneurons. Specifically, we show that following neuronal activity, dendritic mitochondria take up calcium (Ca2+) via the mitochondrial Ca2+ uniporter (MCU-1) that results in an upregulation of mitoROS production. We also observed that mitochondria are positioned in close proximity to synaptic clusters of GLR-1, the C. elegans ortholog of the AMPA subtype of glutamate receptors that mediate neuronal excitation. We show that synaptic recruitment of GLR-1 is upregulated when MCU-1 function is pharmacologically or genetically impaired but is downregulated by mitoROS signaling. Thus, signaling from postsynaptic mitochondria may regulate excitatory synapse function to maintain neuronal homeostasis by preventing excitotoxicity and energy depletion.

A Necessary Role for PKC-2 and TPA-1 in Olfactory Memory and Synaptic AMPAR Trafficking in Caenorhabditis elegans

Protein kinase C (PKC) functions are essential for synaptic plasticity, learning, and memory. However, the roles of specific members of the PKC family in synaptic function, learning, and memory are poorly understood. Here, we investigated the role of individual PKC homologs for synaptic plasticity in Caenorhabditis elegans and found a differential role for pkc-2 and tpa-1, but not pkc-1 and pkc-3 in associative olfactory learning and memory. More specifically we show that PKC-2 is essential for associative learning and TPA-1 for short-term associative memory (STAM). Using endogenous labeling and cell-specific rescues, we show that TPA-1 and PKC-2 are required in AVA for their functions. Previous studies demonstrated that olfactory learning and memory in C. elegans are tied to proper synaptic content and trafficking of AMPA-type ionotropic glutamate receptor homolog GLR-1 in the AVA command interneurons. Therefore, we quantified synaptic content, transport, and delivery of GLR-1 in AVA and showed that loss of pkc-2 and tpa-1 leads to decreased transport and delivery but only a subtle decrease in GLR-1 levels at synapses. AVA-specific expression of both PKC-2 and TPA-1 rescued these defects. Finally, genetic epistasis showed that PKC-2 and TPA-1 likely act in the same pathway to control GLR-1 transport and delivery, while regulating different aspects of olfactory learning and STAM. Thus, our data tie together cell-specific functions of 2 PKCs to neuronal and behavioral outcomes in C. elegans, enabling comparative approaches to understand the evolutionarily conserved role of PKC in synaptic plasticity, learning, and memory.

Sex difference in the effect of environmental enrichment on food restriction-induced persistence of cocaine conditioned place preference and mechanistic underpinnings

Psychosocial and environmental factors, including loss of natural reward, contribute to the risk of drug abuse. Reward loss has been modeled in animals by removal from social or sexual contact, transfer from enriched to impoverished housing, or restriction of food. We previously showed that food restriction increases the unconditioned rewarding effects of abused drugs and the conditioned incentive effects of drug-paired environments. Mechanistic studies provided evidence of decreased basal dopamine (DA) transmission, adaptive upregulation of signaling downstream of D1 DA receptor stimulation, synaptic upscaling and incorporation of calcium-permeable AMPA receptors (CP-AMPARs) in medium spiny neurons (MSNs) of nucleus accumbens (NAc). These findings align with the still evolving 'reward deficiency' hypothesis of drug abuse. The present study tested whether a compound natural reward that is known to increase DA utilization, environmental enrichment, would prevent the persistent expression of cocaine conditioned place preference (CPP) otherwise observed in food restricted rats, along with the mechanistic underpinnings. Because nearly all prior investigations of both food restriction and environmental enrichment effects on cocaine CPP were conducted in male rodents, both sexes were included in the present study. Results indicate that environmental enrichment curtailed the persistence of CPP expression, decreased signaling downstream of the D1R, and decreased the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) in NAc MSNs of food restricted male, but not female, rats. The failure of environmental enrichment to significantly decrease food restriction-induced synaptic insertion of CP-AMPARs, and how this may accord with previous pharmacological findings that blockade of CP-AMPARs reverses behavioral effects of food restriction is discussed. In addition, it is speculated that estrous cycle-dependent fluctuations in DA release, receptor density and MSN excitability may obscure the effect of increased DA signaling during environmental enrichment, thereby interfering with development of the cellular and behavioral effects that enrichment produced in males.

Molecular Mechanisms of AMPA Receptor Trafficking in the Nervous System

Synaptic plasticity enhances or reduces connections between neurons, affecting learning and memory. Postsynaptic AMPARs mediate greater than 90% of the rapid excitatory synaptic transmission in glutamatergic neurons. The number and subunit composition of AMPARs are fundamental to synaptic plasticity and the formation of entire neural networks. Accordingly, the insertion and functionalization of AMPARs at the postsynaptic membrane have become a core issue related to neural circuit formation and information processing in the central nervous system. In this review, we summarize current knowledge regarding the related mechanisms of AMPAR expression and trafficking. The proteins related to AMPAR trafficking are discussed in detail, including vesicle-related proteins, cytoskeletal proteins, synaptic proteins, and protein kinases. Furthermore, significant emphasis was placed on the pivotal role of the actin cytoskeleton, which spans throughout the entire transport process in AMPAR transport, indicating that the actin cytoskeleton may serve as a fundamental basis for AMPAR trafficking. Additionally, we summarize the proteases involved in AMPAR post-translational modifications. Moreover, we provide an overview of AMPAR transport and localization to the postsynaptic membrane. Understanding the assembly, trafficking, and dynamic synaptic expression mechanisms of AMPAR may provide valuable insights into the cognitive decline associated with neurodegenerative diseases.

Engineering memory with an extrinsically disordered kinase

Synaptic plasticity plays a crucial role in memory formation by regulating the communication between neurons. Although actin polymerization has been linked to synaptic plasticity and dendritic spine stability, the causal link between actin polymerization and memory encoding has not been identified yet. It is not clear whether actin polymerization and structural changes in dendritic spines are a driver or a consequence of learning and memory. Using an extrinsically disordered form of the protein kinase LIMK1, which rapidly and precisely acts on ADF/cofilin, a direct modifier of actin, we induced long-term enlargement of dendritic spines and enhancement of synaptic transmission in the hippocampus on command. The activation of extrinsically disordered LIMK1 in vivo improved memory encoding and slowed cognitive decline in aged mice exhibiting reduced cofilin phosphorylation. The engineered memory by an extrinsically disordered LIMK1 supports a direct causal link between actin-mediated synaptic transmission and memory.

Aß Pathology and Neuron-Glia Interactions: A Synaptocentric View

Alzheimer's disease (AD) causes the majority of dementia cases worldwide. Early pathological hallmarks include the accumulation of amyloid-ß (Aß) and activation of both astrocytes and microglia. Neurons form the building blocks of the central nervous system, and astrocytes and microglia provide essential input for its healthy functioning. Their function integrates at the level of the synapse, which is therefore sometimes referred to as the "quad-partite synapse". Increasing evidence puts AD forward as a disease of the synapse, where pre- and postsynaptic processes, as well as astrocyte and microglia functioning progressively deteriorate. Here, we aim to review the current knowledge on how Aß accumulation functionally affects the individual components of the quad-partite synapse. We highlight a selection of processes that are essential to the healthy functioning of the neuronal synapse, including presynaptic neurotransmitter release and postsynaptic receptor functioning. We further discuss how Aß affects the astrocyte's capacity to recycle neurotransmitters, release gliotransmitters, and maintain ion homeostasis. We additionally review literature on how Aß changes the immunoprotective function of microglia during AD progression and conclude by summarizing our main findings and highlighting the challenges in current studies, as well as the need for further research.

The role of post-translational modifications in synaptic AMPA receptor activity

AMPA-type receptors for the neurotransmitter glutamate are very dynamic entities, and changes in their synaptic abundance underlie different forms of synaptic plasticity, including long-term synaptic potentiation (LTP), long-term depression (LTD) and homeostatic scaling. The different AMPA receptor subunits (GluA1-GluA4) share a common modular structure and membrane topology, and their intracellular C-terminus tail is responsible for the interaction with intracellular proteins important in receptor trafficking. The latter sequence differs between subunits and contains most sites for post-translational modifications of the receptors, including phosphorylation, O-GlcNAcylation, ubiquitination, acetylation, palmitoylation and nitrosylation, which affect differentially the various subunits. Considering that each single subunit may undergo modifications in multiple sites, and that AMPA receptors may be formed by the assembly of different subunits, this creates multiple layers of regulation of the receptors with impact in synaptic function and plasticity. This review discusses the diversity of mechanisms involved in the post-translational modification of AMPA receptor subunits, and their impact on the subcellular distribution and synaptic activity of the receptors.

Restoring glutamate receptor signaling in pancreatic alpha cells rescues glucagon responses in type 1 diabetes

Glucagon secretion from pancreatic alpha cells is crucial to prevent hypoglycemia. People with type 1 diabetes lose this glucoregulatory mechanism and are susceptible to dangerous hypoglycemia for reasons still unclear. Here we determine that alpha cells in living pancreas slices from donors with type 1 diabetes do not mount an adequate glucagon response and cannot activate the positive autocrine feedback mediated by AMPA/kainate glutamate receptors. This feedback is required to elicit full glucagon responses in the healthy state. Reactivating residual AMPA/kainate receptor function with positive allosteric modulators restores glucagon secretion in human slices from donors with type 1 diabetes as well as glucose counterregulation in non-obese diabetic mice. Our study thus identifies a defect in autocrine signaling that contributes to alpha cell failure. The use of positive allosteric modulators of AMPA/kainate receptors overcomes this deficiency and prevents hypoglycemia, an effect that could be used to improve the management of diabetes.

Enhanced activity of Alzheimer disease-associated variant of protein kinase Cα drives cognitive decline in a mouse model

Exquisitely tuned activity of protein kinase C (PKC) isozymes is essential to maintaining cellular homeostasis. Whereas loss-of-function mutations are generally associated with cancer, gain-of-function variants in one isozyme, PKCα, are associated with Alzheimer's disease (AD). Here we show that the enhanced activity of one variant, PKCα M489V, is sufficient to rewire the brain phosphoproteome, drive synaptic degeneration, and impair cognition in a mouse model. This variant causes a modest 30% increase in catalytic activity without altering on/off activation dynamics or stability, underscoring that enhanced catalytic activity is sufficient to drive the biochemical, cellular, and ultimately cognitive effects observed. Analysis of hippocampal neurons from PKCα M489V mice reveals enhanced amyloid-β-induced synaptic depression and reduced spine density compared to wild-type mice. Behavioral studies reveal that this mutation alone is sufficient to impair cognition, and, when coupled to a mouse model of AD, further accelerates cognitive decline. The druggability of protein kinases positions PKCα as a promising therapeutic target in AD.

The Mfn1-βIIPKC Interaction Regulates Mitochondrial Dysfunction via Sirt3 Following Experimental Subarachnoid Hemorrhage

Neuronal injury following subarachnoid hemorrhage (SAH) has been shown to be associated with mitochondrial dysfunction and oxidative stress. βIIPKC, a subtype of protein kinase C (PKC), accumulates on the mitochondrial outer membrane and phosphorylates mitofusin 1 (Mfn1) at serine 86. Here, we investigated the role of Mfn1-βIIPKC interaction in brain damage and neurological function in both in vivo and in vitro experimental SAH models. The expression of βIIPKC protein and the interaction of Mfn1-βIIPKC were found to be increased after OxyHb treatment in primary cultured cortical neurons and were also observed in the brain following SAH in rats. Treatment with the βIIPKC inhibitor βIIV5-3 or SAMβA, a peptide that selectively antagonizes Mfn1-βIIPKC association, significantly attenuated the OxyHb-induced neuronal injury and apoptosis. These protective effects were accompanied by inhibited mitochondrial dysfunction and preserved mitochondrial biogenesis. The results of western blot showed that βIIV5-3 or SAMβA markedly increased the expression of Sirt3 and enhanced the activities of its downstream mitochondrial antioxidant enzymes in OxyHb-treated neurons. Knockdown of Sirt3 via specific targeted small interfering RNA (siRNA) partially prevented the βIIV5-3- or SAMβA-induced protection and antioxidative effects. In addition, treatment with βIIV5-3 or SAMβA in vivo was found to obviously reduce brain edema, alleviate neuroinflammation, and preserve neurological function after experimental SAH in rats. In congruent with in vitro data, the protection induced by βIIV5-3 or SAMβA was reduced by Sirt3 knockdown in vivo. In summary, our present results showed that blocking Mfn1-βIIPKC interaction protects against brain damage and mitochondrial dysfunction via Sirt3 following experimental SAH.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

NMDA and AMPA receptor physiology and role in visceral hypersensitivity: a review

N-methyl-d-aspartate receptors (NMDARs) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are excitatory neurotransmission receptors of the central nervous system and play vital roles in synaptic plasticity. Although not fully elucidated, visceral hypersensitivity is one of the most well-characterized pathophysiologic abnormalities of functional gastrointestinal diseases and appears to be associated with increased synaptic plasticity. In this study, we review the updated findings on the physiology of NMDARs and AMPARs and their relation to visceral hypersensitivity, which propose directions for future research in this field with evolving importance.

Tenuifolin ameliorates chronic restraint stress-induced cognitive impairment in C57BL/6J mice

The general consensus is that stress affects the central nervous system and can lead to cognitive problems. The root of Polygala tenuifolia (P. tenuifolia) is a well-known traditional Chinese medicine used for improving brain function. Tenuifolin (TEN) is the major constituent of P. tenuifolia and has a promising neuroprotective property. The purpose of this study was to investigate the alleviating effect of TEN on cognitive impairment induced by chronic restraint stress (CRS) and its mechanism. Our results showed that CRS exposure resulted in impaired cognitive performance in C57BL/6J mice, as indicated by decreased responses in Y-maze, novel objects recognition, and step-through passive avoidance tests. TEN treated daily orally (10 and 20 mg/kg) for 30 days reversed these behavior changes. Meanwhile, TEN could significantly regulate interleukin (IL)-6 and IL-10 levels in the hippocampus. TEN inhibited the toll-like receptor 4/nuclear factor-kappa B-mediated inflammation, as well as adrenocorticotropic hormone and corticosterone levels in serum. Most importantly, we found that TEN also upregulated the expressions of brain-derived neurotrophic factor, tropomyosin kinase B, glucocorticoid receptor, glutamate receptor 1, and synapse-associated proteins. Collectively, these data suggest that TEN has a potential improvement effect on memory loss caused by CRS.

Spinal AMPA receptors: Amenable players in central sensitization for chronic pain therapy?

The activity-dependent trafficking of AMPA receptors (AMPAR) mediates synaptic strength and plasticity, while the perturbed trafficking of the receptors of different subunit compositions has been linked to memory impairment and to causing neuropathology. In the spinal cord, nociceptive-induced changes in AMPAR trafficking determine the central sensitization of the dorsal horn (DH): changes in AMPAR subunit composition compromise the balance between synaptic excitation and inhibition, rendering interneurons hyperexcitable to afferent inputs, and promoting Ca2+ influx into the DH neurons, thereby amplifying neuronal hyperexcitability. The DH circuits become over-excitable and carry out aberrant sensory processing; this causes an increase in pain sensation in central sensory pathways, giving rise to chronic pain syndrome. Current knowledge of the contribution of spinal AMPAR to the cellular mechanisms relating to chronic pain provides opportunities for developing target-based therapies for chronic pain intervention.

Analysis and Construction of a Molecular Diagnosis Model of Drug-Resistant Epilepsy Based on Bioinformatics

Background: Epilepsy is a complex chronic disease of the nervous system which influences the health of approximately 70 million patients worldwide. In the past few decades, despite the development of novel antiepileptic drugs, around one-third of patients with epilepsy have developed drug-resistant epilepsy. We performed a bioinformatic analysis to explore the underlying diagnostic markers and mechanisms of drug-resistant epilepsy.

Methods: Weighted correlation network analysis (WGCNA) was applied to genes in epilepsy samples downloaded from the Gene Expression Omnibus database to determine key modules. The least absolute shrinkage and selection operator (LASSO) regression and support vector machine-recursive feature elimination (SVM-RFE) algorithms were used to screen the genes resistant to carbamazepine, phenytoin, and valproate, and sensitivity of the three-class classification SVM model was verified through the receiver operator characteristic (ROC) curve. A protein–protein interaction (PPI) network was utilized to analyze the protein interaction relationship. Finally, ingenuity pathway analysis (IPA) was adopted to conduct disease and function pathway and network analysis.

Results: Through WGCNA, 72 genes stood out from the key modules related to drug resistance and were identified as candidate resistance genes. Intersection analysis of the results of the LASSO and SVM-RFE algorithms selected 11, 4, and 5 drug-resistant genes for carbamazepine, phenytoin, and valproate, respectively. Subsequent union analysis obtained 17 hub resistance genes to construct a three-class classification SVM model. ROC showed that the model could accurately predict patient resistance. Expression of 17 hub resistance genes in healthy subjects and patients was significantly different. The PPI showed that there are six resistance genes (CD247, CTSW, IL2RB, MATK, NKG7, and PRF1) that may play a central role in the resistance of epilepsy patients. Finally, IPA revealed that resistance genes (PRKCH and S1PR5) were involved in “CREB signaling in Neurons.”

Conclusion: We obtained a three-class SVM model that can accurately predict the drug resistance of patients with epilepsy, which provides a new theoretical basis for research and treatment in the field of drug-resistant epilepsy. Moreover, resistance genes PRKCH and S1PR5 may cooperate with other resistance genes to exhibit resistance effects by regulation of the cAMP-response element-binding protein (CREB) signaling pathway.

Protein kinase D promotes activity-dependent AMPA receptor endocytosis in hippocampal neurons

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors (AMPARs) mediate the majority of fast excitatory neurotransmission in the brain. The continuous trafficking of AMPARs into and out of synapses is a core feature of synaptic plasticity, which is considered as the cellular basis of learning and memory. The molecular mechanisms underlying the postsynaptic AMPAR trafficking, however, are still not fully understood. In this work, we demonstrate that the protein kinase D (PKD) family promotes basal and activity-induced AMPAR endocytosis in primary hippocampal neurons. Pharmacological inhibition of PKD increased synaptic levels of GluA1-containing AMPARs, slowed down their endocytic trafficking and increased neuronal network activity. By contrast, ectopic expression of constitutive active PKD decreased the synaptic level of AMPARs, while increasing their colocalization with early endosomes. Our results thus establish an important role for PKD in the regulation of postsynaptic AMPAR trafficking during synaptic plasticity.

Subunit-dependent and subunit-independent rules of AMPA receptor trafficking during chemical long-term depression in hippocampal neurons

Long-term potentiation (LTP) and long-term depression (LTD) of excitatory neurotransmission are believed to be the neuronal basis of learning and memory. Both processes are primarily mediated by neuronal activity-induced transport of postsynaptic AMPA-type glutamate receptors (AMPARs). While AMPAR subunits and their specific phosphorylation sites mediate differential AMPAR trafficking, LTP and LTD could also occur in a subunit-independent manner. Thus, it remains unclear whether and how certain AMPAR subunits with phosphorylation sites are preferentially recruited to or removed from synapses during LTP and LTD. Using immunoblot and immunocytochemical analysis, we show that phosphomimetic mutations of the membrane-proximal region (MPR) in GluA1 AMPAR subunits affect the subunit-dependent endosomal transport of AMPARs during chemical LTD. AP-2 and AP-3, adaptor protein complexes necessary for clathrin-mediated endocytosis and late endosomal/lysosomal trafficking, respectively, are reported to be recruited to AMPARs by binding to the AMPAR auxiliary subunit, stargazin (STG), in an AMPAR subunit-independent manner. However, the association of AP-3, but not AP-2, with STG was indirectly inhibited by the phosphomimetic mutation in the MPR of GluA1. Thus, although AMPARs containing the phosphomimetic mutation at the MPR of GluA1 were endocytosed by a chemical LTD-inducing stimulus, they were quickly recycled back to the cell surface in hippocampal neurons. These results could explain how the phosphorylation status of GluA1-MPR plays a dominant role in subunit-independent STG-mediated AMPAR trafficking during LTD.

Flavonoid fisetin reverses impaired hippocampal synaptic plasticity and cognitive function by regulating the function of AMPARs in a male rat model of schizophrenia

Cognitive deficits are the core feature of schizophrenia and effective treatment strategies are still missing. Previous studies have reported that fisetin promotes long-term potentiation (LTP) and cognitive function in normal rodents and other model animals of neurological diseases. The aim of this study was to assess the effect of fisetin on synaptic plasticity and cognitive deficits caused by a brief disruption of N-methyl-D-aspartate receptors (NMDARs) with dizocilpine (MK-801) during early development in rats. The cognitive performance was examined by the Morris water maze task and a fear conditioning test. Hippocampal synaptic plasticity was investigated by field potential recording. The expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) and cognition-related proteins was measured by western blotting. We found that intraperitoneal administration of fisetin rescued hippocampus-dependent spatial and contextual fear memory in MK-801 rats. In parallel with these behavioral results, fisetin treatment in MK-801 rats reversed the impairment of hippocampal LTP. At the molecular level, fisetin treatment selectively increased the phosphorylation and surface expression of AMPA receptor subunit 1 (GluA1) in MK-801-treated rats. Moreover, fisetin restored the phosphorylation levels of calcium-calmodulin-dependent kinaseII (CaMKII), cAMP response element-binding protein (CREB), and the extracellular signal-regulated kinase (ERK1/2) in MK-801-treated rats. Collectively, our findings demonstrate that fisetin treatment can reverse the deficits of hippocampal synaptic plasticity and memory in a male rat model of schizophrenia by restoring the phosphorylation and surface expression of AMPAR GluA1 subunit, suggesting fisetin as a promising therapeutic candidate for schizophrenia-associated cognitive deficits.

RGS14 Regulation of Post-Synaptic Signaling and Spine Plasticity in Brain

The regulator of G-protein signaling 14 (RGS14) is a multifunctional signaling protein that regulates post synaptic plasticity in neurons. RGS14 is expressed in the brain regions essential for learning, memory, emotion, and stimulus-induced behaviors, including the basal ganglia, limbic system, and cortex. Behaviorally, RGS14 regulates spatial and object memory, female-specific responses to cued fear conditioning, and environmental- and psychostimulant-induced locomotion. At the cellular level, RGS14 acts as a scaffolding protein that integrates G protein, Ras/ERK, and calcium/calmodulin signaling pathways essential for spine plasticity and cell signaling, allowing RGS14 to naturally suppress long-term potentiation (LTP) and structural plasticity in hippocampal area CA2 pyramidal cells. Recent proteomics findings indicate that RGS14 also engages the actomyosin system in the brain, perhaps to impact spine morphogenesis. Of note, RGS14 is also a nucleocytoplasmic shuttling protein, where its role in the nucleus remains uncertain. Balanced nuclear import/export and dendritic spine localization are likely essential for RGS14 neuronal functions as a regulator of synaptic plasticity. Supporting this idea, human genetic variants disrupting RGS14 localization also disrupt RGS14’s effects on plasticity. This review will focus on the known and unexplored roles of RGS14 in cell signaling, physiology, disease and behavior.

PPM1F in hippocampal dentate gyrus regulates the depression-related behaviors by modulating neuronal excitability

Major depressive disorder (MDD) is a common, serious, debilitating mental illness. Protein phosphatase Mg²⁺/Mn²⁺-dependent 1F (PPM1F), a serine/threonine phosphatase, has been reported to have multiple biological and cellular functions. However, the effects of PPM1F and its neuronal substrates on depressive behaviors remain largely unknown. Here, we showed that PPM1F is widely distributed in the hippocampus, and chronic unpredictable stress (CUS) can induce increased expression of PPM1F in the hippocampus, which was correlated with depression-associated behaviors. Overexpression of PPM1F mediated by adeno-associated virus (AAV) in the dentate gyrus (DG) produced depression-related behaviors and enhanced susceptibility to subthreshold CUS (SCUS) in both male and female mice, while, knockout of PPM1F in DG produced antidepressant phonotypes under stress conditions. Whole-cell patch-clamp recordings demonstrated that overexpression of PPM1F increased the neuronal excitability of the granule cells in the DG. Consistent with neuronal hyperexcitability, overexpression of PPM1F regulated the expression of certain ion channel genes and induced decreased phosphorylation of Ca2+/calmodulin-dependent protein kinase II (CAMKII) and Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) in hippocampus. These results suggest that PPM1F in the DG regulates depression-related behaviors by modulating neuronal excitability, which might be an important pathological gene for depression or other mental diseases.

Involvement of PKCβI-SERT activity in stress vulnerability of mice exposed to twice-swim stress

Stress vulnerability and pathogenic mechanisms in stress-related disorders are strongly associated with the functions of serotonin transporter (SERT). SERT phosphorylation induces a reduction of the serotonin (5-HT, 5-hydroxytryptamine) transport properties, its phosphorylation regulated by protein kinase C (PKC). However, the functional relationship between regulated SERT activity by PKC and stress vulnerability remains unclear. Here, we investigated whether the functional regulation of SERT by PKC was involved in stress vulnerability using mice exposed to twice-swim stress that exhibited the impairment of social behaviors. The mild-swim stress (6 min) given just before the social interaction test did not affect the social behaviors of mice. However, mice exposed to strong-swim stress (15 min) became vulnerable to the mild-swim stress, and subsequent social behaviors were impaired. Chelerythrine, a PKC inhibitor, exacerbated decreased sociality in mice exposed to acute mild-swim stress. Phorbol 12-myristate 13-acetate (PMA), a PKC activator, ameliorated the impairment of social behaviors in mice exposed to twice-swim stress. Phosphorylated PKCβI or SERT and 5-HT levels were decreased in the prefrontal cortex of twice-stressed mice. These decreases were attenuated by PMA. Our findings demonstrate that mice exposed to twice-swim stress developed stress vulnerability, which may be involved in the regulation of SERT phosphorylation and 5-HT levels accompanying PKCβI activity.

Dorsolateral striatum dopamine-dependent cocaine seeking is resistant to pavlovian cue extinction in male and female rats

Cue exposure therapy (CET) reduces craving induced by drug-associated cues in individuals with substance use disorders. A preclinical model of CET, cue extinction, similarly reduces cue-induced cocaine seeking in rodent self-administration models; however, those models may not capture the habitual or compulsive aspects of drug use. Thus, the effectiveness of cue extinction was tested in male and female rats trained to self-administer cocaine using second-order (SO) or fixed-ratio (FR) schedules of reinforcement to facilitate dorsolateral striatum (DLS) dopamine-dependent or -independent response strategies, respectively. Cue extinction significantly reduced drug seeking in FR-trained rats, replicating prior results, but was ineffective in SO-trained rats. SO-trained rats also showed increased indices of glutamate signaling in the DLS relative to FR-trained rats, despite comparable levels of cocaine intake. Furthermore, in SO-trained rats, antagonism of AMPA receptors in the DLS rescued the efficacy of cue extinction to reduce drug seeking. Finally, the effectiveness of cue extinction was also revealed in SO-trained rats when they were taught to perform a new, non-habitual response for cocaine cue presentation. Overall, our findings indicate that habit-like drug seeking leads to plasticity in the DLS and behavior that is resistant to cue extinction, but that the effects of cue extinction are restored when DLS glutamatergic signaling is blocked. These results have implications for the effectiveness of clinical cue exposure therapy.

The Requirement of the C-Terminal Domain of GluA1 in Different Forms of Long-Term Potentiation in the Hippocampus Is Age-Dependent

Long-term potentiation (LTP) at glutamatergic synapses is an extensively studied form of long-lasting synaptic plasticity widely regarded as the cellular basis for learning and memory. At the CA1 synapse, there are multiple forms of LTP with distinct properties. Although AMPA glutamate receptors (AMPARs) are a key target of LTP expression, whether they are required in all forms of LTP remains unclear. To address this question, we have used our recently developed mouse line, GluA1^C2KI, where the c-terminal domain (CTD) of the endogenous GluA1 is replaced by that of GluA2. Unlike traditional GluA1 global or conditional KO mice, GluA1^C2KI mice have no changes in basal AMPAR properties or synaptic transmission allowing a better assessment of GluA1 in synaptic plasticity. We previously showed that these mice are impaired in LTP induced by high-frequency stimulation (HFS-LTP), but whether other forms of LTP are also affected in these mice is unknown. In this study, we compared various forms of LTP at CA1 synapses between GluA1^C2KI and wild-type littermates by using several induction protocols. We show that HFS-LTP is impaired in both juvenile and adult GluA1^C2KI mice. The LTP induced by theta-burst stimulation (TBS-LTP) is also abolished in juvenile GluA1^C2KI mice. Interestingly, TBS-LTP can still be induced in adult GluA1^C2KI mice, but its mechanisms are altered becoming more sensitive to protein synthesis and the extracellular signal-regulated kinase (ERK) inhibitors compared to wild type (WT) control. The GluA1^C2KI mice are also differentially altered in several forms of LTP induced under whole-cell recording paradigms. These results indicate that the CTD of GluA1 is differentially involved in different forms of LTP at CA1 synapse highlighting the complexity and adaptative potential of LTP expression mechanisms in the hippocampus.

Subunit-Specific Augmentation of AMPA Receptor Ubiquitination by Phorbol Ester

Excitatory neurotransmission relies on the precise targeting of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors to the neuronal plasma membrane. Activity-dependent ubiquitination of AMPA receptor (AMPAR) subunits sorts internalised receptors to late endosomes for degradation, which ultimately determines the number of AMPARs on neuronal membrane. Our recent study has demonstrated a functional cross-talk between the phosphorylation and ubiquitination of the GluA1 subunit in mammalian central neurons. However, the existence of such a cross modulation for the GluA2 subunit remains unknown. Here, we have shown that bicuculline induced GluA2 ubiquitination on the same lysine residues (Lys-870 and Lys-882) in the C-terminal as those elicited by the AMPA treatment. Interestingly, bicuculline-induced ubiquitination was markedly enhanced by the phospho-mimetic GluA2 S880E mutant. Pharmacological activation of protein kinase C (PKC) by phorbol ester, which mediates the phosphorylation of GluA2 at Ser-880, augmented bicuculline-induced ubiquitination of GluA2 in cultured neurons. This effect was specific for the GluA2 subunit because phorbol ester did not alter the level of GluA1 ubiquitination. However, phorbol ester-induced enhancement of GluA2 ubiquitination did not require Ser-880 phosphorylation. This suggests that pseudo-phosphorylation of Ser-880 is sufficient but is not necessary for the augmentation of bicuculline-induced GluA2 ubiquitination. Collectively, these data provide the first demonstration of subunit-specific modulation of AMPAR ubiquitination by the PKC-dependent signalling pathway in mammalian central neurons.

Reactive Oxygen Species Modulate Activity-Dependent AMPA Receptor Transport in C. elegans

The AMPA subtype of synaptic glutamate receptors (AMPARs) plays an essential role in cognition. Their function, numbers, and change at synapses during synaptic plasticity are tightly regulated by neuronal activity. Although we know that long-distance transport of AMPARs is essential for this regulation, we do not understand the associated regulatory mechanisms of it. Neuronal transmission is a metabolically demanding process in which ATP consumption and production are tightly coupled and regulated. Aerobic ATP synthesis unavoidably produces reactive oxygen species (ROS), such as hydrogen peroxide, which are known modulators of calcium signaling. Although a role for calcium signaling in AMPAR transport has been described, there is little understanding of the mechanisms involved and no known link to physiological ROS signaling. Here, using real-time in vivo imaging of AMPAR transport in the intact C. elegans nervous system, we demonstrate that long-distance synaptic AMPAR transport is bidirectionally regulated by calcium influx and activation of calcium/calmodulin-dependent protein kinase II. Quantification of in vivo calcium dynamics revealed that modest, physiological increases in ROS decrease calcium transients in C. elegans glutamatergic neurons. By combining genetic and pharmacological manipulation of ROS levels and calcium influx, we reveal a mechanism in which physiological increases in ROS cause a decrease in synaptic AMPAR transport and delivery by modulating activity-dependent calcium signaling. Together, our results identify a novel role for oxidant signaling in the regulation of synaptic AMPAR transport and delivery, which in turn could be critical for coupling the metabolic demands of neuronal activity with excitatory neurotransmission.SIGNIFICANCE STATEMENT Synaptic AMPARs are critical for excitatory synaptic transmission. The disruption of their synaptic localization and numbers is associated with numerous psychiatric, neurologic, and neurodegenerative conditions. However, very little is known about the regulatory mechanisms controlling transport and delivery of AMPAR to synapses. Here, we describe a novel physiological signaling mechanism in which ROS, such as hydrogen peroxide, modulate AMPAR transport by modifying activity-dependent calcium signaling. Our findings provide the first evidence in support of a mechanistic link between physiological ROS signaling, AMPAR transport, localization, and excitatory transmission. This is of fundamental and clinical significance since dysregulation of intracellular calcium and ROS signaling is implicated in aging and the pathogenesis of several neurodegenerative disorders, including Alzheimer's and Parkinson's disease.

Spinal microglia-neuron interactions in chronic pain

Current deficiency in our understanding of acute-to-chronic pain transition remains a hurdle for developing effective treatments against chronic pain. Whereas neurocentric mechanisms alone are insufficient to provide satisfactory explanation for such transition, neuro-immune crosstalk has attracted attention in recent pain research. In contrast to brain microglia, spinal microglia are activated immediately in various pain states. The fast-responsive enrichment and activation of spinal microglia among different pain conditions have highlighted the crucial role of neuroinflammation caused by microglia-neuron crosstalk in pain initiation. Recent studies have revealed spinal microglia-neuron interactions are also involved in chronic pain maintenance, albeit, with different anatomic distribution, cellular and molecular mechanisms, and biologic functions. Delineating the exact temporal discrepancies of spinal microglia distribution and functions along acute-to-chronic pain transition may provide additional mechanistic insights for drug development to prevent deterioration of acute pain into the chronic state. This narrative review summerizes the longitudinal alterations of spinal microglia-neuron interactions in the initiation of pain hypersensitivity, acute-to-chronic pain progression, and chronic pain maintenance, followed by an overview of current clinical translation of preclinical studies on spinal microglia. This review highlights the crucial role of the interaction between spinal microglia and neighboring neurons in the initiation and maintenance of pain hypersensitivity, in relation to the release of cytokines, chemokines, and neuroactive substances, as well as the modulation of synaptic plasticity. Further exploration of the uncharted functions of spinal microglia-neuron crosstalk may lead to the design of novel drugs for preventing acute-to-chronic pain transition.

The physiological modulation by intracellular kinases of hippocampal γ-oscillation in vitro

Hippocampal network oscillations at gamma frequency band (γ-oscillation, 20-80 Hz) are synchronized synaptic activities generated by the interactions between the excitatory and inhibitory interneurons and are associated with higher brain function such as learning and memory. Despite extensive studies about the modulation of intracellular kinases on synaptic transmission and plasticity, little is known about the effects of these kinases on γ-oscillations. In this study, we examined the effects of several critical intracellular kinases such as cyclic AMP-dependent protein kinase (PKA), protein kinase B (PKB)/Akt, protein kinase C (PKC), extracellular-regulated protein kinases (ERK) and AMP-activated protein kinase (AMPK), known to regulate synaptic transmission, on hippocampal γ-oscillations in vitro. We found that AMPK inhibitor but not PKA, PKC, or ERK inhibitor, strongly enhanced the power of γ-oscillation (γ-power) and that Akt inhibitor weakly increased γ-power. Western blot analysis confirmed that AMPK inhibitor reduced the expression of p-AMPK but not total AMPK. By using the slice whole cell voltage-clamp technique, we found that AMPK inhibitor increased the frequency but not amplitude of spontaneous inhibitory postsynaptic currents (sIPSC) and had no effect on either frequency or amplitude of spontaneous excitatory postsynaptic currents (sEPSC). Therefore, AMPK activation negatively modulates hippocampal γ-oscillation via modulation of the inhibitory neurons.

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.

Molecular Mechanisms in Hippocampus Involved on Object Recognition Memory Consolidation and Reconsolidation

Acquired information is stabilized into long-term memory through a process known as consolidation. Though, after consolidation, when stored information is retrieved they can be again susceptible, allowing modification, updating and strengthening and to be re-stabilized they need a new process referred to as memory reconsolidation. However, the molecular mechanisms of recognition memory consolidation and reconsolidation are not fully understood. Also, considering that the study of the link between synaptic proteins is key to understanding of memory processes, we investigated, in male Wistar rats, molecular mechanisms in the hippocampus involved on object recognition memory (ORM) consolidation and reconsolidation. We verified that the blockade of AMPA receptors (AMPAr) and L-VDCCs calcium channels impaired ORM consolidation and reconsolidation when administered into CA1 immediately after sample phase or reactivation phase and that these impairments were blocked by the administration of AMPAr agonist and of neurotrophin BDNF. Also, the blockade of CaMKII impaired ORM consolidation when administered 3 h after sample phase but had no effect on ORM reconsolidation and its effect was blocked by the administration of BDNF, but not of AMPAr agonist. So, this study provides new evidence of the molecular mechanisms involved on the consolidation and reconsolidation of ORM, demonstrating that AMPAr and L-VDCCs are necessary for the consolidation and reconsolidation of ORM while CaMKII is necessary only for the consolidation and also that there is a link between BDNF and AMPAr, L-VDCCs and CaMKII as well as a link between AMPAr and L-VDCCs on ORM consolidation and reconsolidation.

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).

Single Synapse LTP: A Matter of Context?

The most commonly studied form of synaptic plasticity is long-term potentiation (LTP). Over the last 15 years, it has been possible to induce structural and functional LTP in dendritic spines using two-photon glutamate uncaging, allowing for studying the signaling mechanisms of LTP with single synapse resolution. In this review, we compare different stimulation methods to induce single synapse LTP and discuss how LTP is expressed. We summarize the underlying signaling mechanisms that have been studied with high spatiotemporal resolution. Finally, we discuss how LTP in a single synapse can be affected by excitatory and inhibitory synapses nearby. We argue that single synapse LTP is highly dependent on context: the choice of induction method, the history of the dendritic spine and the dendritic vicinity crucially affect signaling pathways and expression of single synapse LTP.

Reduction of spike generation frequency by cooling in brain slices from rats and from patients with epilepsy

This study aimed to understand the mechanism by which brain cooling terminates epileptic discharge. Cortical slices were prepared from rat brains (n = 19) and samples from patients with intractable epilepsy that had undergone temporal lobectomy (n = 7). We performed whole cell current clamp recordings at approximately physiological brain temperature (35℃) and at cooler temperatures (25℃ and 15℃). The firing threshold in human neurons was lower at 25℃ (-32.6 mV) than at 35℃ (-27.0 mV). The resting potential and spike frequency were similar at 25℃ and 35℃. Cooling from 25℃ to 15℃ did not change the firing threshold, but the resting potential increased from -65.5 to -54.0 mV and the waveform broadened from 1.85 to 6.55 ms, due to delayed repolarization. These changes enhanced the initial spike appearance and reduced spike frequency; moreover, spike frequency was insensitive to increased levels of current injections. Similar results were obtained in rat brain studies. We concluded that the reduction in spike frequency at 15℃, due to delayed repolarization, might be a key mechanism by which brain cooling terminates epileptic discharge. On the other hand, spike frequency was not influenced by the reduced firing threshold or the elevated resting potential caused by cooling.

Self-organization of modular network architecture by activity-dependent neuronal migration and outgrowth

The spatial distribution of neurons and activity-dependent neurite outgrowth shape long-range interaction, recurrent local connectivity and the modularity in neuronal networks. We investigated how this mesoscale architecture develops by interaction of neurite outgrowth, cell migration and activity in cultured networks of rat cortical neurons and show that simple rules can explain variations of network modularity. In contrast to theoretical studies on activity-dependent outgrowth but consistent with predictions for modular networks, spontaneous activity and the rate of synchronized bursts increased with clustering, whereas peak firing rates in bursts increased in highly interconnected homogeneous networks. As Ca²⁺ influx increased exponentially with increasing network recruitment during bursts, its modulation was highly correlated to peak firing rates. During network maturation, long-term estimates of Ca²⁺ influx showed convergence, even for highly different mesoscale architectures, neurite extent, connectivity, modularity and average activity levels, indicating homeostatic regulation towards a common set-point of Ca²⁺ influx.

Mechanistic Understanding of the Phosphorylation-Induced Conformational Rigidity at the AMPA Receptor C-terminal Domain

Phosphorylation at the intracellular C-terminal domain (CTD) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors induces conformational rigidity. Such intracellular alterations to the AMPA receptor influence its functional responses, which are involved in multiple synaptic processes and neuronal signaling. The structure of the CTD still remains unresolved, which poses challenges toward providing a mechanism for the process of phosphorylation and deciphering the role of each phosphorylation step in causing the resultant conformational behavior. Herein, we utilize smFRET spectroscopy to understand the mechanism of phosphorylation, with the help of strategic point mutations that mimic phosphorylation. Our results reveal that first, phosphorylation at three target sites (S818, S831, and T840) is necessary for the change in the secondary structure of the existing disordered native sequence. Also, the results suggest that the formation of the tertiary structure through electrostatic interaction involving one specific phosphorylation site (S831) stabilizes the structure and renders conformational rigidity.

The AMPA Receptor Code of Synaptic Plasticity

Changes in the properties and postsynaptic abundance of AMPA-type glutamate receptors (AMPARs) are major mechanisms underlying various forms of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), and homeostatic scaling. The function and the trafficking of AMPARs to and from synapses is modulated by specific AMPAR GluA1-GluA4 subunits, subunit-specific protein interactors, auxiliary subunits, and posttranslational modifications. Layers of regulation are added to AMPAR tetramers through these different interactions and modifications, increasing the computational power of synapses. Here we review the reliance of synaptic plasticity on AMPAR variants and propose "the AMPAR code" as a conceptual framework. The AMPAR code suggests that AMPAR variants will be predictive of the types and extent of synaptic plasticity that can occur and that a hierarchy exists such that certain AMPARs will be disproportionally recruited to synapses during LTP/homeostatic scaling up, or removed during LTD/homeostatic scaling down.

Expression Changes of NMDA and AMPA Receptor Subunits in the Hippocampus in rats with Diabetes Induced by Streptozotocin Coupled with Memory Impairment

Cognitive impairment in diabetes (CID) is a severe chronic complication of diabetes mellitus (DM). It has been hypothesized that diabetes can lead to cognitive dysfunction due to expression changes of excitatory neurotransmission mediated by N-methyl-D-aspartate receptors (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR); however, the pathogenesis involved in this has not been fully understood, especially at early phase of DM. Here, we sought to determine the cognitive changes and aim to correlate this with the expression changes of NMDAR and AMPAR of glutamate signaling pathways in the rat hippocampus from early phase of DM and in the course of the disease progression. By Western blot analysis and immunofluorescence labeling, the hippocampus in diabetic rats showed a significant increase in protein expression NMDAR subunits NR1, NR2A and NR2B and AMPAR subunit GluR1. Along with this, behavioral test by Morris water maze showed a significant decline in their performance when compared with the control rats. It is suggested that NR1, NR2A, NR2B and GluR1are involved in learning and memory and that their expression alterations maybe correlated with the occurrence and development of CID in diabetic rats induced by streptozotocin.

Preferential generation of Ca2+-permeable AMPA receptors by AKAP79-anchored protein kinase C proceeds via GluA1 subunit phosphorylation at Ser-831

AMPA-type glutamate receptors (AMPARs) mediate fast excitatory neurotransmission in the mammalian central nervous system. Preferential AMPAR subunit assembly favors heteromeric GluA1/GluA2 complexes. The presence of the GluA2 subunit generates Ca²⁺-impermeable (CI) AMPARs that have linear current-voltage (I-V) relationships. However, diverse forms of synaptic plasticity and pathophysiological conditions are associated with shifts from CI to inwardly rectifying, GluA2-lacking, Ca²⁺-permeable (CP) AMPARs on time scales ranging from minutes to days. These shifts have been linked to GluA1 phosphorylation at Ser-845, a protein kinase A (PKA)-targeted site within its intracellular C-terminal tail, often in conjunction with protein kinase A anchoring protein 79 (AKAP79; AKAP150 in rodents), which targets PKA to GluA1. However, AKAP79 may impact GluA1 phosphorylation at other sites by interacting with other signaling enzymes. Here, we evaluated the ability of AKAP79, its signaling components, and GluA1 phosphorylation sites to induce CP-AMPARs under conditions in which CI-AMPARs normally predominate. We found that GluA1 phosphorylation at Ser-831 is sufficient for the appearance of CP-AMPARs and that AKAP79-anchored protein kinase C (PKC) primarily drives the appearance of these receptors via this site. In contrast, other AKAP79-signaling components and C-terminal tail GluA1 phosphorylation sites exhibited a permissive role, limiting the extent to which AKAP79 promotes CP-AMPARs. This may reflect the need for these sites to undergo active phosphorylation/dephosphorylation cycles that control their residency within distinct subcellular compartments. These findings suggest that AKAP79, by orchestrating phosphorylation, represents a key to a GluA1 phosphorylation passcode, which allows the GluA1 subunit to escape GluA2 dominance and promote the appearance of CP-AMPARs.

Phosphorylation Induces Conformational Rigidity at the C-Terminal Domain of AMPA Receptors

The intracellular C-terminal domain (CTD) of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor undergoes phosphorylation at specific locations during long-term potentiation. This modification enhances conductance through the AMPA receptor ion channel and thus potentially plays a crucial role in modulating receptor trafficking and signaling. However, because the CTD structure is largely unresolved, it is difficult to establish if phosphorylation induces conformational changes that might play a role in enhancing channel conductance. Herein, we utilize single-molecule Förster resonance energy transfer (smFRET) spectroscopy to probe the conformational changes of a section of the AMPA receptor CTD, under the conditions of point-mutated phosphomimicry. Multiple analysis algorithms fail to identify stable conformational states within the smFRET distributions, consistent with a lack of well-defined secondary structure. Instead, our results show that phosphomimicry induces conformational rigidity to the CTD, and such rigidity is electrostatically tunable.

PKCε Inhibits Neuronal Dendritic Spine Development through Dual Phosphorylation of Ephexin5

Protein kinase C (PKC)-dependent mechanisms promote synaptic function in the mature brain. However, the roles of PKC signaling during synapse development remain largely unknown. Investigating each brain-enriched PKC isoform in early neuronal development, we show that PKCε acutely and specifically reduces the number of dendritic spines, sites of eventual synapse formation on developing dendrites. This PKCε-mediated spine suppression is temporally restricted to immature neurons and mediated through the phosphorylation and activation of Ephexin5, a RhoA guanine nucleotide exchange factor (GEF) and inhibitor of hippocampal synapse formation. Our data suggest that PKCε acts as an early developmental inhibitor of dendritic spine formation, in contrast to its emerging pro-synaptic roles in mature brain function. Moreover, we identify a substrate of PKCε, Ephexin5, whose early-elevated expression in developing neurons may in part explain the mechanism by which PKCε plays seemingly opposing roles that depend on neuronal maturity.