AKAP150 Palmitoylation Regulates Synaptic Incorporation of Ca2+-Permeable AMPA Receptors to Control LTP

Recruitment, regulation, and release: Control of signaling enzyme localization and function by reversible S-acylation

An ever-growing number of studies highlight the importance of S-acylation, a reversible protein-lipid modification, for diverse aspects of intracellular signaling. In this review, we summarize the current understanding of how S-acylation regulates perhaps the best-known class of signaling enzymes, protein kinases. We describe how S-acylation acts as a membrane targeting signal that localizes certain kinases to specific membranes, and how such membrane localization in turn facilitates the assembly of signaling hubs consisting of an S-acylated kinase's upstream activators and/or downstream targets. We further discuss recent findings that S-acylation can control additional aspects of the function of certain kinases, including their interactions and, surprisingly, their activity, and how such regulation might be exploited for potential therapeutic gain. We go on to describe the roles and regulation of de-S-acylases and how extracellular signals drive dynamic (de)S-acylation of certain kinases. We discuss how S-acylation has the potential to lead to "emergent properties" that alter the temporal profile and/or salience of intracellular signaling events. We close by giving examples of other S-acylation-dependent classes of signaling enzymes and by discussing how recent biological and technological advances should facilitate future studies into the functional roles of S-acylation-dependent signaling.

Roles of AMPA receptors in social behaviors

As a crucial player in excitatory synaptic transmission, AMPA receptors (AMPARs) contribute to the formation, regulation, and expression of social behaviors. AMPAR modifications have been associated with naturalistic social behaviors, such as aggression, sociability, and social memory, but are also noted in brain diseases featuring impaired social behavior. Understanding the role of AMPARs in social behaviors is timely to reveal therapeutic targets for treating social impairment in disorders, such as autism spectrum disorder and schizophrenia. In this review, we will discuss the contribution of the molecular composition, function, and plasticity of AMPARs to social behaviors. The impact of targeting AMPARs in treating brain disorders will also be discussed.

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.

Nano-organization of synaptic calcium signaling

Recent studies suggest an exquisite structural nano-organization within single synapses, where sites of evoked fusion - marked by clustering of synaptic vesicles, active zone proteins and voltage-gated calcium channels - are directly juxtaposed to postsynaptic receptor clusters within nanocolumns. This direct nanometer scale alignment between presynaptic fusion apparatus and postsynaptic receptors is thought to ensure the fidelity of synaptic signaling and possibly allow multiple distinct signals to occur without interference from each other within a single active zone. The functional specificity of this organization is made possible by the inherent nano-organization of calcium signals, where all the different calcium sources such as voltage-gated calcium channels, intracellular stores and store-operated calcium entry have dedicated local targets within their nanodomain to ensure precision of action. Here, we discuss synaptic nano-organization from the perspective of calcium signals, where some of the principal findings from early work in the 1980s continue to inspire current studies that exploit new genetic tools and super-resolution imaging technologies.

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.

Akap5 links synaptic dysfunction to neuroinflammatory signaling in a mouse model of infantile neuronal ceroid lipofuscinosis

Palmitoylation and depalmitoylation represent dichotomic processes by which a labile posttranslational lipid modification regulates protein trafficking and degradation. The depalmitoylating enzyme, palmitoyl-protein thioesterase 1 (PPT1), is associated with the devastating pediatric neurodegenerative condition, infantile neuronal ceroid lipofuscinosis (CLN1). CLN1 is characterized by the accumulation of autofluorescent lysosomal storage material (AFSM) in neurons and robust neuroinflammation. Converging lines of evidence suggest that in addition to cellular waste accumulation, the symptomology of CLN1 corresponds with disruption of synaptic processes. Indeed, loss of Ppt1 function in cortical neurons dysregulates the synaptic incorporation of the GluA1 AMPA receptor (AMPAR) subunit during a type of synaptic plasticity called synaptic scaling. However, the mechanisms causing this aberration are unknown. Here, we used the Ppt1-/- mouse model (both sexes) to further investigate how Ppt1 regulates synaptic plasticity and how its disruption affects downstream signaling pathways. To this end, we performed a palmitoyl-proteomic screen, which provoked the discovery that Akap5 is excessively palmitoylated at Ppt1-/- synapses. Extending our previous data, in vivo induction of synaptic scaling, which is regulated by Akap5, caused an excessive upregulation of GluA1 in Ppt1-/- mice. This synaptic change was associated with exacerbated disease pathology. Furthermore, the Akap5- and inflammation-associated transcriptional regulator, nuclear factor of activated T cells (NFAT), was sensitized in Ppt1-/- cortical neurons. Suppressing the upstream regulator of NFAT activation, calcineurin, with the FDA-approved therapeutic FK506 (Tacrolimus) modestly improved neuroinflammation in Ppt1-/- mice. These findings indicate that the absence of depalmitoylation stifles synaptic protein trafficking and contributes to neuroinflammation via an Akap5-associated mechanism.

Synaptic homeostasis transiently leverages Hebbian mechanisms for a multiphasic response to inactivity

Homeostatic regulation of synapses is vital for nervous system function and key to understanding a range of neurological conditions. Synaptic homeostasis is proposed to operate over hours to counteract the destabilizing influence of long-term potentiation (LTP) and long-term depression (LTD). The prevailing view holds that synaptic scaling is a slow first-order process that regulates postsynaptic glutamate receptors and fundamentally differs from LTP or LTD. Surprisingly, we find that the dynamics of scaling induced by neuronal inactivity are not exponential or monotonic, and the mechanism requires calcineurin and CaMKII, molecules dominant in LTD and LTP. Our quantitative model of these enzymes reconstructs the unexpected dynamics of homeostatic scaling and reveals how synapses can efficiently safeguard future capacity for synaptic plasticity. This mechanism of synaptic adaptation supports a broader set of homeostatic changes, including action potential autoregulation, and invites further inquiry into how such a mechanism varies in health and disease.

Calcium-permeable AMPA and kainate receptors of GABAergic neurons

This Commentary presents a brief discussion of the action of glutamate calcium permeable receptors present with neurons on the release of the neurotransmitter gamma-aminobutyric acid (GABA). In particular, Glutamate sensitive Kainic Acid Receptors (KARs) and α-Amino-3-hydroxy-5-Methyl-4-isoxazole Propionic Acid Receptor (AMPARs) are Na+ channels that typically cause neuronal cells to depolarize and release GABA. Some of these receptors are also permeable to Ca2+ and are hence involved in the calcium-dependent release of GABA neurotransmitters. Calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs) are predominantly located in GABAergic neurons in the mature brain and their primary role is to regulate GABA release. AMPARs which do not contain the GluA2 subunit are mainly localized in the postsynaptic membrane. CP-KAR receptors are located mainly in the presynapse. GABAergic neurons expressing CP-KARs and CP-AMPARs respond to excitation earlier and faster, suppressing hyperexcitation of other neurons by the advanced GABA release due to an early rapid [Ca2+]i increase. CP-AMPARs have demonstrated a more pronounced impact on plasticity compared to NMDARs because of their capacity to elevate intracellular Ca2+ levels independently of voltage. GABAergic neurons that express CP-AMPARs contribute to the disinhibition of glutamatergic neurons by suppressing GABAergic neurons that express CP-KARs. Hence, the presence of glutamate CP-KARs and CP-AMPARs is crucial in governing hyperexcitation and synaptic plasticity in GABAergic neurons.

The role of s-palmitoylation in neurological diseases: implication for zDHHC family

S-palmitoylation is a reversible posttranslational modification, and the palmitoylation reaction in human-derived cells is mediated by the zDHHC family, which is composed of S-acyltransferase enzymes that possess the DHHC (Asp-His-His-Cys) structural domain. zDHHC proteins form an autoacylation intermediate, which then attaches the fatty acid to cysteine a residue in the target protein. zDHHC proteins sublocalize in different neuronal structures and exert dif-ferential effects on neurons. In humans, many zDHHC proteins are closely related to human neu-rological disor-ders. This review focuses on a variety of neurological disorders, such as AD (Alz-heimer's disease), HD (Huntington's disease), SCZ (schizophrenia), XLID (X-linked intellectual disability), attention deficit hyperactivity disorder and glioma. In this paper, we will discuss and summarize the research progress regarding the role of zDHHC proteins in these neu-rological disorders.

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

AKAP150 from nucleus accumbens dopamine D1 and D2 receptor-expressing medium spiny neurons regulates morphine withdrawal

Dopamine D1 receptor-expressing medium spiny neurons (D1R-MSNs) and dopamine D2 receptor-expressing MSNs (D2R-MSNs) in nucleus accumbens (NAc) have been demonstrated to show different effects on reward and memory of abstinence. A-kinase anchoring protein 150 (AKAP150) expression in NAc is significantly upregulated and contributes to the morphine withdrawal behavior. However, the underlying mechanism of AKAP150 under opioid withdrawal remains unclear. In this study, AKAP150 expression in NAc is upregulated in naloxone-precipitated morphine withdrawal model, and knockdown of AKAP150 alleviates morphine withdrawal somatic signs and improves the performance of conditioned place aversion (CPA) test. AKAP150 in NAc D1R-MSNs is related to modulation of the performance of morphine withdrawal CPA test, while AKAP150 in NAc D2R-MSNs is relevant to the severity of somatic responses. Our results suggest that AKAP150 from D1R-MSNs or D2R-MSNs in NAc contributes to the developmental process of morphine withdrawal but plays different roles in aspects of behavior or psychology.

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.

A Ventromedial Prefrontal-to-Lateral Entorhinal Cortex Pathway Modulates the Gain of Behavioral Responding During Threat

BACKGROUND

The ability to correctly associate cues and contexts with threat is critical for survival, and the inability to do so can result in threat-related disorders such as posttraumatic stress disorder. The prefrontal cortex (PFC) and hippocampus are well known to play critical roles in cued and contextual threat memory processing. However, the circuits that mediate prefrontal-hippocampal modulation of context discrimination during cued threat processing are less understood. Here, we demonstrate the role of a previously unexplored projection from the ventromedial region of PFC (vmPFC) to the lateral entorhinal cortex (LEC) in modulating the gain of behavior in response to contextual information during threat retrieval and encoding.

METHODS

We used optogenetics followed by in vivo calcium imaging in male C57/B6J mice to manipulate and monitor vmPFC-LEC activity in response to threat-associated cues in different contexts. We then investigated the inputs to, and outputs from, vmPFC-LEC cells using Rabies tracing and channelrhodopsin-assisted electrophysiology.

RESULTS

vmPFC-LEC cells flexibly and bidirectionally shaped behavior during threat expression, shaping sensitivity to contextual information to increase or decrease the gain of behavioral output in response to a threatening or neutral context, respectively.

CONCLUSIONS

Glutamatergic vmPFC-LEC cells are key players in behavioral gain control in response to contextual information during threat processing and may provide a future target for intervention in threat-based disorders.

Reduced excitatory neurotransmission in the hippocampus after inflammation and sevoflurane anaesthesia

Background

Inflammation and general anaesthesia likely contribute to perioperative neurocognitive disorders, possibly by causing a neuronal imbalance of excitation and inhibition. We showed previously that treatment with lipopolysaccharide (LPS) and sevoflurane causes a sustained increase in a tonic inhibitory conductance in the hippocampus; however, whether excitatory neurotransmission is also altered remains unknown. The goal of this study was to examine excitatory synaptic currents in the hippocampus after treatment with LPS and sevoflurane. Synaptic plasticity in the hippocampus, a cellular correlate of learning and memory, was also studied.

Methods

Mice were injected with vehicle or LPS (1 mg kg-1 i.p.), and after 24 h they were then exposed to vehicle or sevoflurane (2.3%; 2 h). Hippocampal slices were prepared 48 h later. Excitatory synaptic currents were recorded from pyramidal neurones. Long-term potentiation (LTP) and long-term depression (LTD) were studied in the Schaffer collateral-cornu ammonis 1 pathway.

Results

The amplitude of miniature excitatory postsynaptic currents (EPSCs) was reduced after LPS+sevoflurane (P<0.001), whereas that of spontaneous EPSCs was unaltered, as evidenced by cumulative distribution plots. The frequency, area, and kinetics of both miniature and spontaneous EPSCs were unchanged, as were LTP and LTD.

Conclusions

The reduced amplitude of miniature EPSCs, coupled with the previously reported increase in tonic inhibition, indicates that the combination of LPS and sevoflurane markedly disrupts the balance of excitation and inhibition. Restoring this balance by pharmacologically enhancing excitatory neurotransmission and inhibiting the tonic current may represent an effective therapeutic option for perioperative neurocognitive disorders.

Activity-dependent post-translational regulation of palmitoylating and depalmitoylating enzymes in the hippocampus

Activity-induced changes in protein palmitoylation can regulate the plasticity of synaptic connections, critically impacting learning and memory. Palmitoylation is a reversible post-translational modification regulated by both palmitoyl-acyl transferases that mediate palmitoylation and palmitoyl thioesterases that depalmitoylate proteins. However, it is not clear how fluctuations in synaptic activity can mediate the dynamic palmitoylation of neuronal proteins. Using primary hippocampal cultures, we demonstrate that synaptic activity does not impact the transcription of palmitoylating and depalmitoylating enzymes, changes in thioesterase activity, or post-translational modification of the depalmitoylating enzymes of the ABHD17 family and APT2 (also known as LYPLA2). In contrast, synaptic activity does mediate post-translational modification of the palmitoylating enzymes ZDHHC2, ZDHHC5 and ZDHHC9 (but not ZDHHC8) to influence protein-protein interactions, enzyme stability and enzyme function. Post-translational modifications of the ZDHHC enzymes were also observed in the hippocampus following fear conditioning. Taken together, our findings demonstrate that signaling events activated by synaptic activity largely impact activity of the ZDHHC family of palmitoyl-acyl transferases with less influence on the activity of palmitoyl thioesterases.

A novel approach for vital visualization and studying of neurons containing Ca2+ -permeable AMPA receptors

Calcium-permeable AMPA receptors (CP-AMPARs) play a pivotal role in brain functioning in health and disease. They are involved in synaptic plasticity, synaptogenesis, and neuronal circuits development. However, the functions of neurons expressing CP-AMPARs and their role in the modulation of network activity remain elusive since reliable and accurate visualization methods are absent. Here we developed an approach allowing the vital identification of neurons containing CP-AMPARs. The proposed method relies on evaluating Ca²⁺ influx in neurons during activation of AMPARs in the presence of NMDAR and KAR antagonists, and blockers of voltage-gated Ca²⁺ channels. Using this method, we studied the properties of CP-AMPARs-containing neurons. We showed that the overwhelming majority of neurons containing CP-AMPARs are GABAergic, and they are distinguished by higher amplitudes of the calcium responses to applications of the agonists. Furthermore, about 30% of CP-AMPARs-containing neurons demonstrate the presence of GluK1-containing KARs. Although CP-AMPARs-containing neurons are characterized by more significant Ca²⁺ influx during the activation of AMPARs than other neurons, AMPAR-mediated Na⁺ influx is similar in these two groups. We revealed that neurons containing CP-AMPARs demonstrate weak GABA(A)R-mediated inhibition because of the low percentage of GABAergic synapses on the soma of these cells. However, our data show that weak GABA(A)R-mediated inhibition is inherent to all GABAergic neurons in the culture and cannot be considered a unique feature of CP-AMPARs-containing neurons. We believe that the suggested approach will help to understand the role of CP-AMPARs in the mammalian nervous system in more detail.

S-Palmitoylation of Synaptic Proteins in Neuronal Plasticity in Normal and Pathological Brains

Protein lipidation is a common post-translational modification of proteins that plays an important role in human physiology and pathology. One form of protein lipidation, S-palmitoylation, involves the addition of a 16-carbon fatty acid (palmitate) onto proteins. This reversible modification may affect the regulation of protein trafficking and stability in membranes. From multiple recent experimental studies, a picture emerges whereby protein S-palmitoylation is a ubiquitous yet discrete molecular switch enabling the expansion of protein functions and subcellular localization in minutes to hours. Neural tissue is particularly rich in proteins that are regulated by S-palmitoylation. A surge of novel methods of detection of protein lipidation at high resolution allowed us to get better insights into the roles of protein palmitoylation in brain physiology and pathophysiology. In this review, we specifically discuss experimental work devoted to understanding the impact of protein palmitoylation on functional changes in the excitatory and inhibitory synapses associated with neuronal activity and neuronal plasticity. The accumulated evidence also implies a crucial role of S-palmitoylation in learning and memory, and brain disorders associated with impaired cognitive functions.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

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.

Myristoylation-dependent palmitoylation of cyclin Y modulates long-term potentiation and spatial learning

Many psychiatric disorders accompany deficits in cognitive functions and synaptic plasticity, and abnormal lipid modifications of neuronal proteins are associated with their pathophysiology. Lipid modifications, including palmitoylation and myristoylation, play crucial roles in the subcellular localization and trafficking of proteins. Cyclin Y (CCNY), enriched in the postsynaptic compartment, acts as an inhibitory modulator of functional and structural long-term potentiation (LTP) in the hippocampal neurons. However, cellular and molecular mechanisms underlying CCNY-mediated inhibitory functions in the synapse remain largely unknown. Here, we report that myristoylation located CCNY to the trans-Golgi network (TGN), and subsequent palmitoylation directed the myristoylated CCNY from the TGN to the synaptic cell surface. This myristoylation-dependent palmitoylation of CCNY was required for the inhibitory role of CCNY in excitatory synaptic transmission, activity-induced dynamics of AMPA receptors and PSD-95, LTP, and spatial learning. Furthermore, spatial learning significantly reduced palmitoyl- and myristoyl-CCNY levels, indicating that spatial learning lowers the synaptic abundance of CCNY. Our findings provide mechanistic insight into how CCNY is clustered adjacent to postsynaptic sites where it could play its inhibitory roles in synaptic plasticity and spatial learning.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

Contribution of Membrane Lipids to Postsynaptic Protein Organization

The precise subsynaptic organization of proteins at the postsynaptic membrane controls synaptic transmission. In particular, postsynaptic receptor complexes are concentrated in distinct membrane nanodomains to optimize synaptic signaling. However, despite the clear functional relevance of subsynaptic receptor organization to synaptic transmission and plasticity, the mechanisms that underlie the nanoscale organization of the postsynaptic membrane remain elusive. Over the last decades, the field has predominantly focused on the role of protein-protein interactions in receptor trafficking and positioning in the synaptic membrane. In contrast, the contribution of lipids, the principal constituents of the membrane, to receptor positioning at the synapse remains poorly understood. Nevertheless, there is compelling evidence that the synaptic membrane is enriched in specific lipid species and that deregulation of lipid homeostasis in neurons severely affects synaptic functioning. In this review we focus on how lipids are organized at the synaptic membrane, with special emphasis on how current models of membrane organization could contribute to protein distribution at the synapse and synaptic transmission. Finally, we will present an outlook on how novel technical developments could be applied to study the dynamic interplay between lipids and proteins at the postsynaptic membrane.

A sticky situation: regulation and function of protein palmitoylation with a spotlight on the axon and axon initial segment

In neurons, the axon and axon initial segment (AIS) are critical structures for action potential initiation and propagation. Their formation and function rely on tight compartmentalisation, a process where specific proteins are trafficked to and retained at distinct subcellular locations. One mechanism which regulates protein trafficking and association with lipid membranes is the modification of protein cysteine residues with the 16-carbon palmitic acid, known as S-acylation or palmitoylation. Palmitoylation, akin to phosphorylation, is reversible, with palmitate cycling being mediated by substrate-specific enzymes. Palmitoylation is well-known to be highly prevalent among neuronal proteins and is well studied in the context of the synapse. Comparatively, how palmitoylation regulates trafficking and clustering of axonal and AIS proteins remains less understood. This review provides an overview of the current understanding of the biochemical regulation of palmitoylation, its involvement in various neurological diseases, and the most up-to-date perspective on axonal palmitoylation. Through a palmitoylation analysis of the AIS proteome, we also report that an overwhelming proportion of AIS proteins are likely palmitoylated. Overall, our review and analysis confirm a central role for palmitoylation in the formation and function of the axon and AIS and provide a resource for further exploration of palmitoylation-dependent protein targeting to and function at the AIS.

Vezatin regulates seizures by controlling AMPAR-mediated synaptic activity

Although many studies have explored the mechanism of epilepsy, it remains unclear and deserves further investigation. Vezatin has been reported to be a synaptic regulatory protein involved in regulating neuronal synaptic transmission (NST). However, the role of vezatin in epilepsy remains unknown. Therefore, the aims of this study are to investigate the underlying roles of vezatin in epilepsy. In this study, vezatin expression was increased in hippocampal tissues from pilocarpine (PILO)-induced epileptic mice and a Mg2+-free medium-induced in vitro seizure-like model. Vezatin knockdown suppressed seizure activity in PILO-induced epileptic mice. Mechanistically, vezatin knockdown suppressed AMPAR-mediated synaptic events in epileptic mice and downregulated the surface expression of the AMPAR GluA1 subunit (GluA1). Interestingly, vezatin knockdown decreased the phosphorylation of GluA1 at serine 845 and reduced protein kinase A (PKA) phosphorylation; when PKA phosphorylation was suppressed by H-89 (a selective inhibitor of PKA phosphorylation) in vitro, the effects of vezatin knockdown on reducing the phosphorylation of GluA1 at serine 845 and the surface expression of GluA1 were blocked. Finally, we investigated the pattern of vezatin in brain tissues from patients with temporal lobe epilepsy (TLE), and we found that vezatin expression was also increased in patients with TLE. In summary, the vezatin expression pattern is abnormal in individuals with epilepsy, and vezatin regulates seizure activity by affecting AMPAR-mediated NST and the surface expression of GluA1, which is involved in PKA-mediated phosphorylation of GluA1 at serine 845, indicating that vezatin-mediated regulation of epileptic seizures represents a novel target for epilepsy.

β-Amyloid disruption of LTP/LTD balance is mediated by AKAP150-anchored PKA and Calcineurin regulation of Ca2+-permeable AMPA receptors

Regulated insertion and removal of postsynaptic AMPA glutamate receptors (AMPARs) mediates hippocampal long-term potentiation (LTP) and long-term depression (LTD) synaptic plasticity underlying learning and memory. In Alzheimer’s disease β-amyloid (Aβ) oligomers may impair learning and memory by altering AMPAR trafficking and LTP/LTD balance. Importantly, Ca²⁺-permeable AMPARs (CP-AMPARs) assembled from GluA1 subunits are excluded from hippocampal synapses basally but can be recruited rapidly during LTP and LTD to modify synaptic strength and signaling. By employing mouse knockin mutations that disrupt anchoring of the kinase PKA or phosphatase Calcineurin (CaN) to the postsynaptic scaffold protein AKAP150, we find that local AKAP-PKA signaling is required for CP-AMPAR recruitment, which can facilitate LTP but also, paradoxically, prime synapses for Aβ impairment of LTP mediated by local AKAP-CaN LTD signaling that promotes subsequent CP-AMPAR removal. These findings highlight the importance of PKA/CaN signaling balance and CP-AMPARs in normal plasticity and aberrant plasticity linked to disease.

In Alzheimer’s disease, Aβ oligomers disrupt hippocampal neuronal plasticity and cognition. Sanderson et al. show how the postsynaptic scaffold protein AKAP150 coordinates PKA and Calcineurin regulation of Ca²⁺-permeable AMPA-type glutamate receptors to mediate disruption of synaptic plasticity by Aβ oligomers.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

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.

Interaction and Subcellular Association of PRRT1/SynDIG4 With AMPA Receptors

AMPA receptors (AMPAR) are organized into supramolecular complexes in association with other membrane proteins that provide exquisite regulation of their biophysical properties and subcellular trafficking. Proline-rich transmembrane protein 1 (PRRT1), also named as SynDIG4, is a component of native AMPAR complexes in multiple brain regions. Deletion of PRRT1 leads to altered surface levels and phosphorylation status of AMPARs, as well as impaired forms of synaptic plasticity. Here, we have investigated the mechanisms underlying the observed regulation of AMPARs by investigating the interaction properties and subcellular localization of PRRT1. Our results show that PRRT1 can interact physically with all AMPAR subunits GluA1-GluA4. We decipher the membrane topology of PRRT1 to find that contrary to the predicted dual membrane pass, only the second hydrophobic segment spans the membrane completely, and is involved in mediating the interaction with AMPARs. We also report a physical interaction of PRRT1 with phosphatase PP2B that dephosphorylates AMPARs during synaptic plasticity. Our co-localization analysis in primary neuronal cultures identifies that PRRT1 associates with AMPARs extrasynaptically where it localizes to early and recycling endosomes as well as to the plasma membrane. These findings advance the understanding of the mechanisms by which PRRT1 regulates AMPARs under basal conditions and during synaptic plasticity.

GluA1-homomeric AMPA receptor in synaptic plasticity and neurological diseases

Synaptic transmission is one of the fundamental processes that all brain functions are based on. Changes in the strength of synaptic transmission among neurons are crucial for information processing in the central nervous system. The α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid subtype of ionotropic glutamate receptors (AMPARs) mediate the majority of the fast excitatory synaptic transmission in the mammalian brain. Rapid trafficking of AMPARs in and out of the postsynaptic membrane is proposed to be a major mechanism for synaptic plasticity, and learning and memory. Defects in the regulated AMPAR trafficking have been shown to be involved in the pathogenesis of certain psychiatric and neurodegenerative diseases. Studies accumulated in the past 30 years have provided a detailed molecular insight on how the trafficking of AMPARs is modulated in a subunit-specific manner. In particular, emerging evidence supports that the regulated expression and trafficking of Ca²⁺-permeable, GluA1-homomeric subtype of AMPARs mediates diverse types of synaptic plasticity, thereby playing critical roles in brain function and dysfunction. In this review, we will discuss the current knowledge of AMPAR subunit-specific trafficking, with a particular emphasis on the involvement of GluA1-homomeric receptor trafficking in synaptic plasticity and brain disorders.

Secreted Amyloid Precursor Protein-Alpha Enhances LTP Through the Synthesis and Trafficking of Ca2+-Permeable AMPA Receptors

Regulation of AMPA receptor expression by neuronal activity and neuromodulators is critical to the expression of both long-term potentiation (LTP) and memory. In particular, Ca²⁺-permeable AMPARs (CP-AMPAR) play a unique role in these processes due to their transient, activity-regulated expression at synapses. Secreted amyloid precursor protein-alpha (sAPPα), a metabolite of the parent amyloid precursor protein (APP) has been previously shown to enhance hippocampal LTP as well as memory formation in both normal animals and in Alzheimer’s disease models. In earlier work we showed that sAPPα promotes trafficking of GluA1-containing AMPARs to the cell surface and specifically enhances synthesis of GluA1. To date it is not known whether de novo synthesized GluA1 form CP-AMPARs or how they contribute to sAPPα-mediated plasticity. Here, using fluorescent non-canonical amino acid tagging–proximity ligation assay (FUNCAT-PLA), we show that brief treatment of primary rat hippocampal neurons with sAPPα (1 nM, 30 min) rapidly enhanced the cell-surface expression of de novo GluA1 homomers and reduced levels of de novo GluA2, as well as extant GluA2/3-AMPARs. The de novo GluA1-containing AMPARs were localized to extrasynaptic sites and later internalized by sAPPα-driven expression of the activity-regulated cytoskeletal-associated protein, Arc. Interestingly, longer exposure to sAPPα increased synaptic levels of GluA1/2 AMPARs. Moreover, the sAPPα-mediated enhancement of LTP in area CA1 of acute hippocampal slices was dependent on CP-AMPARs. Together, these findings show that sAPPα engages mechanisms which specifically enhance the synthesis and cell-surface expression of GluA1 homomers, underpinning the sAPPα-driven enhancement of synaptic plasticity in the hippocampus.

AKAP79/150 coordinates leptin-induced PKA signaling to regulate KATP channel trafficking in pancreatic β-cells

The adipocyte hormone leptin regulates glucose homeostasis both centrally and peripherally. A key peripheral target is the pancreatic β-cell, which secretes insulin upon glucose stimulation. Leptin is known to suppress glucose-stimulated insulin secretion by promoting trafficking of K_ATP channels to the β-cell surface, which increases K⁺ conductance and causes β-cell hyperpolarization. We have previously shown that leptin-induced K_ATP channel trafficking requires protein kinase A (PKA)-dependent actin remodeling. However, whether PKA is a downstream effector of leptin signaling or PKA plays a permissive role is unknown. Using FRET-based reporters of PKA activity, we show that leptin increases PKA activity at the cell membrane and that this effect is dependent on N-methyl-D-aspartate receptors, CaMKKβ, and AMPK, which are known to be involved in the leptin signaling pathway. Genetic knockdown and rescue experiments reveal that the increased PKA activity upon leptin stimulation requires the membrane-targeted PKA-anchoring protein AKAP79/150, indicating that PKA activated by leptin is anchored to AKAP79/150. Interestingly, disrupting protein phosphatase 2B (PP2B) anchoring to AKAP79/150, known to elevate basal PKA signaling, leads to increased surface K_ATP channels even in the absence of leptin stimulation. Our findings uncover a novel role of AKAP79/150 in coordinating leptin and PKA signaling to regulate K_ATP channel trafficking in β-cells, hence insulin secretion. The study further advances our knowledge of the downstream signaling events that may be targeted to restore insulin secretion regulation in β-cells defective in leptin signaling, such as those from obese individuals with type 2 diabetes.

Super-resolution imaging reveals extrastriatal synaptic dysfunction in presymptomatic Huntington disease mice

Synaptic structure and function are compromised prior to cell death and symptom onset in a variety of neurodegenerative diseases. In Huntington disease (HD), a CAG repeat expansion in the gene encoding the huntingtin protein results in a presymptomatic stage that typically spans multiple decades and is followed by striking degeneration of striatal tissue and the progression of debilitating motor symptoms. Many lines of evidence demonstrate that the HD presymptomatic window is associated with injurious effects to striatal synapses, many of which appear to be prerequisites to subsequent cell death. While the striatum is the most vulnerable region in the HD brain, it is widely recognized that HD is a brain-wide disease, affecting numerous extrastriatal regions that contribute to debilitating non-motor symptoms including cognitive dysfunction. Currently, we have a poor understanding of the synaptic integrity, or lack thereof, in extrastriatal regions in the presymptomatic HD brain. If early therapeutic intervention seeks to maintain healthy synaptic function, it is important to understand early HD-associated synaptopathy at a brain-wide, rather than striatal-exclusive, level. Here, we focused on the hippocampus as this structure is generally thought to be affected only in manifest HD despite the subtle cognitive deficits known to emerge in prodromal HD. We used super-resolution microscopy and multi-electrode array electrophysiology as sensitive measures of excitatory synapse structure and function, respectively, in the hippocampus of presymptomatic heterozygous HD mice (Q175FDN model). We found clear evidence for enhanced AMPA receptor subunit clustering and hyperexcitability well before the onset of a detectable HD-like behavioral phenotype. In addition, activity-dependent re-organization of synaptic protein nanostructure, and functional measures of synaptic plasticity were impaired in presymptomatic HD mice. These data demonstrate that synaptic abnormalities in the presymptomatic HD brain are not exclusive to the striatum, and highlight the need to better understand the region-dependent complexities of early synaptopathy in the HD brain.

Oxidative Stress Underlies the Ischemia/Reperfusion-Induced Internalization and Degradation of AMPA Receptors

Stroke is the fifth leading cause of death annually in the United States. Ischemic stroke occurs when a blood vessel supplying the brain is occluded. The hippocampus is particularly susceptible to AMPA receptor-mediated delayed neuronal death as a result of ischemic/reperfusion injury. AMPA receptors composed of a GluA2 subunit are impermeable to calcium due to a post-transcriptional modification in the channel pore of the GluA2 subunit. GluA2 undergoes internalization and is subsequently degraded following ischemia/reperfusion. The subsequent increase in the expression of GluA2-lacking, Ca2+-permeable AMPARs results in excitotoxicity and eventually delayed neuronal death. Following ischemia/reperfusion, there is increased production of superoxide radicals. This study describes how the internalization and degradation of GluA1 and GluA2 AMPAR subunits following ischemia/reperfusion is mediated through an oxidative stress signaling cascade. U251-MG cells were transiently transfected with fluorescently tagged GluA1 and GluA2, and different Rab proteins to observe AMPAR endocytic trafficking following oxygen glucose-deprivation/reperfusion (OGD/R), an in vitro model for ischemia/reperfusion. Pretreatment with Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin (MnTMPyP), a superoxide dismutase mimetic, ameliorated the OGD/R-induced, but not agonist-induced, internalization and degradation of GluA1 and GluA2 AMPAR subunits. Specifically, MnTMPyP prevented the increased colocalization of GluA1 and GluA2 with Rab5, an early endosomal marker, and with Rab7, a late endosomal marker, but did not affect the colocalization of GluA1 with Rab11, a marker for recycling endosomes. These data indicate that oxidative stress may play a vital role in AMPAR-mediated cell death following ischemic/reperfusion injury.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

PKA-Dependent Membrane Surface Recruitment of CI-AMPARs Is Crucial for BCP-Mediated Protection Against Post-acute Ischemic Stroke Cognitive Impairment

Post-acute ischemic stroke cognitive impairment frequently occurs and seriously affects patients daily activities. Recruitment of GluA2-containing Ca²⁺-impermeable AMPA receptors (CI-AMPARs) to hippocampal synaptic membrane surfaces was shown to trigger synaptic plasticity. Currently, the effect of CI-AMPAR trafficking on acute ischemic stroke remains poorly understood. β-Caryophyllene (BCP) has been shown to ameliorate cognitive impairment. However, the mechanism has not been characterized. In this study, a 60-min temporary middle cerebral artery occlusion (MCAO) model was established to simulate the pathology of acute ischemic stroke. BCP reduced neurologic deficits, cerebral infarct volume, and pathological damage in MCAO mice and caused CI-AMPARs to translocate to synaptic membranes in the hippocampus; surface expression of CI-AMPARs was also decreased in MCAO mice. Furthermore, this study also showed that BCP treatment significantly activated the cAMP/PKA pathway, which is consistent with the synaptic membrane expression of CI-AMPARs. To better understand the underlying mechanisms, the PKA inhibitor H-89 was used to study the role of BCP in MCAO mice. Interestingly, H-89 treatment significantly disrupted the BCP-mediated facilitation of CI-AMPAR translocation to the synaptic membrane surface and substantially attenuated BCP-induced protection against acute ischemic stroke. Additionally, inhibition the cAMP/PKA pathway not only reduced BCP-induced inhibition of AMPAR-mediated excitatory postsynaptic currents in the hippocampal CA1 region but also decreased the effect of BCP-mediated protection against post-acute ischemic stroke cognitive impairment. Taken together, these data indicate that PKA-dependent synaptic membrane surface recruitment of CI-AMPARs is crucial for the neuroprotective effect of BCP against acute ischemic stroke and protection against post-acute ischemic stroke cognitive impairment.

Spatially compartmentalized phase regulation of a Ca2+-cAMP-PKA oscillatory circuit

Signaling networks are spatiotemporally organized to sense diverse inputs, process information, and carry out specific cellular tasks. In β cells, Ca2+, cyclic adenosine monophosphate (cAMP), and Protein Kinase A (PKA) exist in an oscillatory circuit characterized by a high degree of feedback. Here, we describe a mode of regulation within this circuit involving a spatial dependence of the relative phase between cAMP, PKA, and Ca2+. We show that in mouse MIN6 β cells, nanodomain clustering of Ca2+-sensitive adenylyl cyclases (ACs) drives oscillations of local cAMP levels to be precisely in-phase with Ca2+ oscillations, whereas Ca2+-sensitive phosphodiesterases maintain out-of-phase oscillations outside of the nanodomain. Disruption of this precise phase relationship perturbs Ca2+ oscillations, suggesting the relative phase within an oscillatory circuit can encode specific functional information. This work unveils a novel mechanism of cAMP compartmentation utilized for localized tuning of an oscillatory circuit and has broad implications for the spatiotemporal regulation of signaling networks.

Developmental onset of enduring long-term potentiation in mouse hippocampus

Analysis of long-term potentiation (LTP) provides a powerful window into cellular mechanisms of learning and memory. Prior work shows late LTP (L-LTP), lasting >3 hr, occurs abruptly at postnatal day 12 (P12) in the stratum radiatum of rat hippocampal area CA1. The goal here was to determine the developmental profile of synaptic plasticity leading to L-LTP in the mouse hippocampus. Two mouse strains and two mutations known to affect synaptic plasticity were chosen: C57BL/6J and Fmr1^-/y on the C57BL/6J background, and 129SVE and Hevin^-/- (Sparcl1^-/- ) on the 129SVE background. Like rats, hippocampal slices from all of the mice showed test pulse-induced depression early during development that was gradually resolved with maturation by 5 weeks. All the mouse strains showed a gradual progression between P10-P35 in the expression of short-term potentiation (STP), lasting ≤1 hr. In the 129SVE mice, L-LTP onset (>25% of slices) occurred by 3 weeks, reliable L-LTP (>50% slices) was achieved by 4 weeks, and Hevin^-/- advanced this profile by 1 week. In the C57BL/6J mice, L-LTP onset occurred significantly later, over 3-4 weeks, and reliability was not achieved until 5 weeks. Although some of the Fmr1^-/y mice showed L-LTP before 3 weeks, reliable L-LTP also was not achieved until 5 weeks. L-LTP onset was not advanced in any of the mouse genotypes by multiple bouts of theta-burst stimulation at 90 or 180 min intervals. These findings show important species differences in the onset of STP and L-LTP, which occur at the same age in rats but are sequentially acquired in mice.

Nanoscale Subsynaptic Domains Underlie the Organization of the Inhibitory Synapse

Inhibitory synapses mediate the majority of synaptic inhibition in the brain, thereby controlling neuronal excitability, firing, and plasticity. Although essential for neuronal function, the central question of how these synapses are organized at the subsynaptic level remains unanswered. Here, we use three-dimensional (3D) super-resolution microscopy to image key components of the inhibitory postsynaptic domain and presynaptic terminal, revealing that inhibitory synapses are organized into nanoscale subsynaptic domains (SSDs) of the gephyrin scaffold, GABA_ARs and the active-zone protein Rab3-interacting molecule (RIM). Gephyrin SSDs cluster GABA_AR SSDs, demonstrating nanoscale architectural interdependence between scaffold and receptor. GABA_AR SSDs strongly associate with active-zone RIM SSDs, indicating an important role for GABA_AR nanoscale organization near sites of GABA release. Finally, we find that in response to elevated activity, synapse growth is mediated by an increase in the number of postsynaptic SSDs, suggesting a modular mechanism for increasing inhibitory synaptic strength.

Transient appearance of Ca2+ -permeable AMPA receptors is crucial for the production of repetitive LTP-induced synaptic enhancement (RISE) in cultured hippocampal slices

We have previously shown that repetitive induction of long-term potentiation (LTP) by glutamate (100 μM, 3 min, three times at 24-hr intervals) provoked long-lasting synaptic enhancement accompanied by synaptogenesis in rat hippocampal slice cultures, a phenomenon termed RISE (repetitive LTP-induced synaptic enhancement). Here, we examined the role of Ca²⁺ -permeable (CP) AMPA receptors (AMPARs) in the establishment of RISE. We first found a component sensitive to the Joro-spider toxin (JSTX), a blocker of CP-AMPARs, in a field EPSP recorded from CA3-CA1 synapses at 2-3 days after stimulation, but this component was not found for 9-10 days. We also observed that rectification of AMPAR-mediated current appeared only 2-3 days after stimulation, using a whole-cell patch clamp recording from CA1 pyramidal neurons. These findings indicate that CP-AMPAR is transiently expressed in the developing phase of RISE. The blockade of CP-AMPARs by JSTX for 24 hr at this developing phase inhibited RISE establishment, accompanied by the loss of small synapses at the ultrastructural level. These results suggest that transiently induced CP-AMPARs play a critical role in synaptogenesis in the developing phase of long-lasting hippocampal synaptic plasticity, RISE.

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.

Acyl-PEGyl Exchange Gel Shift Assay for Quantitative Determination of Palmitoylation of Brain Membrane Proteins

Activity-dependent alterations in the levels of synaptic AMPA receptors (AMPARs) within the postsynaptic density (PSD) is thought to represent a cellular mechanism for learning and memory. Palmitoylation regulates localization and function of many synaptic proteins including AMPA-Rs, auxiliary factors and synaptic scaffolds in an activity-dependent manner. We identified the synapse differentiation induced gene (SynDIG) family of four genes (SynDIG1-4) encoding brain-specific transmembrane proteins that associate with AMPARs and regulate synapse strength. SynDIG1 is palmitoylated at two cysteine residues located at positions 191 and 192 in the juxta-transmembrane region important for activity-dependent excitatory synapse development. Here, we describe an innovative biochemical approach, the acyl-PEGyl exchange gel shift (APEGS) assay, to investigate the palmitoylation state of any protein of interest and demonstrate its utility with the SynDIG family of proteins in mouse brain lysates.

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).

PRRT1 regulates basal and plasticity-induced AMPA receptor trafficking

AMPA-type glutamate receptors (AMPAR) are one of the principal mediators of fast excitatory synaptic transmission in the brain. These receptors associate with multiple integral membrane proteins which influence their trafficking and channel properties. Proline-rich transmembrane protein 1 (PRRT1) is a membrane protein and an understudied component of native AMPAR complexes. In order to understand the regulation of AMPARs by PRRT1, we have performed electrophysiological and biochemical investigations on acute hippocampal slices derived from PRRT1 knockout mice. Our results show that PRRT1 controls the levels of AMPARs at the cell surface, though it is dispensable for synaptic transmission. PRRT1 has differential effects on the stability of AMPAR GluA1 subunit phosphorylated at S845 and at S831, two residues at which the phosphorylation status has major influences on receptor trafficking. Furthermore, PRRT1 is required for NMDA receptor-dependent long-term depression (LTD) and proper NMDA-induced AMPAR trafficking. These findings position PRRT1 as an important regulator of AMPAR stabilization and trafficking in different subcellular pools under basal conditions and during synaptic plasticity.

Palmitoylation-mediated synaptic regulation of AMPA receptor trafficking and function

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a major glutamate-gated ion channel in the brain and is important for synaptic transmission, synaptic plasticity, and learning. Palmitoylation, a post-translational modification, is a critical process regulating AMPAR trafficking, synaptic function and plasticity, and learning and memory in health and diseases. In this review, we discuss current knowledge on the palmitoylation-dependent regulation of AMPAR trafficking and functions. We focus on the palmitoylation of AMPARs and other synaptic proteins that directly or indirectly interact with AMPARs, including postsynaptic density 95, glutamate receptor-interacting protein/AMPAR-binding protein, A-kinase anchoring protein 79/150, and protein interacting with C kinase 1. Finally, we discuss what future studies should address in the field of palmitoylation-dependent AMPAR trafficking and function with regard to physiology and neurodegenerative diseases.

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.

Role of Palmitoylation of Postsynaptic Proteins in Promoting Synaptic Plasticity

Many postsynaptic proteins undergo palmitoylation, the reversible attachment of the fatty acid palmitate to cysteine residues, which influences trafficking, localization, and protein interaction dynamics. Both palmitoylation by palmitoyl acyl transferases (PAT) and depalmitoylation by palmitoyl-protein thioesterases (PPT) is regulated in an activity-dependent, localized fashion. Recently, palmitoylation has received attention for its pivotal contribution to various forms of synaptic plasticity, the dynamic modulation of synaptic strength in response to neuronal activity. For instance, palmitoylation and depalmitoylation of the central postsynaptic scaffold protein postsynaptic density-95 (PSD-95) is important for synaptic plasticity. Here, we provide a comprehensive review of studies linking palmitoylation of postsynaptic proteins to synaptic plasticity.