CaMKII regulates the depalmitoylation and synaptic removal of the scaffold protein AKAP79/150 to mediate structural long-term depression

Palmitoylation in cardiovascular diseases: Molecular mechanism and therapeutic potential

Cardiovascular disease is one of the leading causes of mortality worldwide, and involves complex pathophysiological mechanisms that encompass various biological processes and molecular pathways. Post-translational modifications of proteins play crucial roles in the occurrence and progression of cardiovascular diseases, among which palmitoylation is particularly important. Various proteins associated with cardiovascular diseases can be palmitoylated to enhance the hydrophobicity of their molecular subdomains. This lipidation can significantly affect some pathophysiological processes, such as metabolism, inflammation by altering protein stability, localization, and signal transduction. In this review, we narratively summarize recent advances in the palmitoylation of proteins related to cardiovascular diseases and discuss its potential as a therapeutic target.

The neuroprotective γ-hydroxybutyrate analog 3-hydroxycyclopent-1-enecarboxylic acid does not directly affect CaMKIIα autophosphorylation at T286 or binding to GluN2B

The Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) mediates physiological long-term potentiation (LTP) of synaptic strength and pathological ischemic neuronal cell death. Both functions require CaMKII autophosphorylation at T286 (pT286) and binding to the NMDA-type glutamate receptor subunit GluN2B. The neuroprotection seen with 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) was thought to be mediated by impairing binding of the brain-specific CaMKIIα isozyme to GluN2B. However, we show that HOCPCA does not inhibit CaMKIIα enzymatic activity, pT286, cocondensation with GluN2B, or binding to GluN2B. Consistent with no effect on GluN2B binding in vitro or in HEK293 cells, HOCPCA also did not affect the CaMKIIα movement to excitatory synapses in hippocampal neurons in response to LTP stimuli. These findings leave the neuroprotective mechanism of HOCPCA unclear but explain why HOCPCA does not impair LTP. SIGNIFICANCE STATEMENT: This study found that the neuroprotective compound 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) does not directly interfere with Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) activity or GluN2B binding. Although this leaves the neuroprotective mechanism of HOCPCA unclear, it explains why HOCPCA does not impair long-term potentiation. Overall, this limits the use of HOCPCA as a tool compound to study CaMKII functions, but not its clinical potential.

Unveiling the significance of AKAP79/150 in the nervous system disorders: An emerging opportunity for future therapies?

A-kinase anchoring protein 79/150 (AKAP79/150) is a crucial scaffolding protein that positions various proteins at specific synaptic sites to modulate excitatory synaptic intensity. As our understanding of AKAP79/150's biology deepens, along with its significant role in the pathophysiology of various human disorders, there is growing evidence that reveals new opportunities for therapeutic interventions. In this review, we examine the fundamental structure and primary functions of AKAP79/150, emphasizing its pathophysiological mechanisms in different nervous system disorders, particularly inflammatory pain, epilepsy, depression, and Alzheimer's disease. We also discuss its potential therapeutic implications for patients suffering from these conditions.

Activation is only the beginning: mechanisms that tune kinase substrate specificity

Kinases are master coordinators of cellular processes, but to appropriately respond to the changing cellular environment, each kinase must recognize its substrates, target only those proteins on the correct amino acids, and in many cases, only phosphorylate a subset of potential substrates at any given time. Therefore, regulation of kinase substrate specificity is paramount to proper cellular function, and multiple mechanisms can be employed to achieve specificity. At the smallest scale, characteristics of the substrate such as its linear peptide motif and three-dimensional structure must be complementary to the substrate binding surface of the kinase. This surface is dynamically shaped by the activation loop and surrounding region of the substrate binding groove, which can adopt multiple conformations, often influenced by post-translational modifications. Domain-scale conformational changes can also occur, such as the interaction with pseudosubstrate domains or other regulatory domains in the kinase. Kinases may multimerize or form complexes with other proteins that influence their structure, function, and/or subcellular localization at different times and in response to different signals. This review will illustrate these mechanisms by examining recent work on four serine/threonine kinases: Aurora B, CaMKII, GSK3β, and CK1δ. We find that these mechanisms are often shared by this diverse set of kinases in diverse cellular contexts, so they may represent common strategies that cells use to regulate cell signaling, and it will be enlightening to continue to learn about the depth and robustness of kinase substrate specificity in additional systems.

A revised view of the role of CaMKII in learning and memory

The Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) plays a fundamental role in learning and possibly also in memory. However, current mechanistic models require fundamental revision. CaMKII autophosphorylation at Thr286 (pThr286) does not provide the molecular basis for long-term memory, as long believed. Instead, pThr286 mediates the signal processing required for induction of several distinct forms of synaptic plasticity, including Hebbian long-term potentiation and depression and non-Hebbian behavioral timescale synaptic plasticity. We discuss (i) the molecular computations by which CaMKII supports these diverse plasticity mechanisms, (ii) alternative CaMKII mechanisms that may contribute to the maintenance phase of LTP and (iii) the relationship of these mechanisms to behavioral learning and memory.

Mechanisms and functions of protein S-acylation

Over the past two decades, protein S-acylation (often referred to as S-palmitoylation) has emerged as an important regulator of vital signalling pathways. S-Acylation is a reversible post-translational modification that involves the attachment of a fatty acid to a protein. Maintenance of the equilibrium between protein S-acylation and deacylation has demonstrated profound effects on various cellular processes, including innate immunity, inflammation, glucose metabolism and fat metabolism, as well as on brain and heart function. This Review provides an overview of current understanding of S-acylation and deacylation enzymes, their spatiotemporal regulation by sophisticated multilayered mechanisms, and their influence on protein function, cellular processes and physiological pathways. Furthermore, we examine how disruptions in protein S-acylation are associated with a broad spectrum of diseases from cancer to autoinflammatory disorders and neurological conditions.

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.

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

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

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

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

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

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

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

Distinct synaptic pools of DAPK1 differentially regulate activity-dependent synaptic CaMKII accumulation

The death-associated protein kinase 1 (DAPK1) regulates the synaptic movement of the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII). Synaptic CaMKII accumulation is mediated via binding to the NMDA-receptor subunit GluN2B and is required for long-term potentiation (LTP). By contrast, long-term depression (LTD) instead requires specific suppression of this movement, which is mediated by competitive DAPK1 binding to GluN2B. We find here that DAPK1 localizes to synapses via two distinct mechanisms: basal localization requires F-actin, but retention of DAPK1 at synapses during LTD requires an additional binding mode, likely to GluN2B. While F-actin binding mediates DAPK1 enrichment at synapses, it is not sufficient to suppress synaptic CaMKII movement. However, it is a prerequisite that enables the additional LTD-specific binding mode of DAPK1, which in turn mediates suppression of the CaMKII movement. Thus, both modes of synaptic DAPK1 localization work together to regulate synaptic CaMKII localization and thereby synaptic plasticity.

Interaction between A-kinase anchoring protein 5 and protein kinase A mediates CaMKII/HDAC signaling to inhibit cardiomyocyte hypertrophy after hypoxic reoxygenation

We reported that A-kinase anchoring protein 5 (AKAP5) played a role in cardiomyocyte apoptosis after hypoxia-reoxygenation (H/R). The role of AKAP5 in cardiomyocyte hypertrophy has not been fully elucidated. Herein we investigated whether AKAP5 regulates cardiomyocyte hypertrophy through calcium/calmodulin-dependent protein kinase II (CaMKII). After H/R, deficiency of AKAP5 in H9C2 cardiomyocytes and neonatal rat cardiac myocytes activated CaMKII and stimulated cardiomyocyte hypertrophy. AKAP5 upregulation limited this. Low expression of AKAP5 increased CaMKII interaction with histone deacetylases 4/5 (HDAC4/5) and increased nuclear export of HDAC4/5. In addition, AKAP5 interactions with protein kinase A (PKA) and phospholamban (PLN) were diminished. Moreover, the phosphorylation of PLN was decreased, and intracellular calcium increased. Interference of this process with St-Ht31 increased CaMKII signaling, decreased PLN phosphorylation and promoted post-H/R cell hypertrophy. And PKA-anchoring deficient AKAP5ΔPKA could not attenuate hypoxia-reoxygenation-induced cardiomyocyte hypertrophy, but AKAP5 could. Altogether, AKAP5 downregulation exacerbated H/R-induced hypertrophy in cardiomyocytes. This was due to, in part, to less in AKAP5-PKA interaction and the accumulation of intracellular Ca²⁺ with a subsequent increase in CaMKII activity.

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.

A Novel CaMKII Inhibitory Peptide Blocks Relapse to Morphine Seeking by Influencing Synaptic Plasticity in the Nucleus Accumbens Shell

Drugs of abuse cause enduring functional disorders in the brain reward circuits, leading to cravings and compulsive behavior. Although people may rehabilitate by detoxification, there is a high risk of relapse. Therefore, it is crucial to illuminate the mechanisms of relapse and explore the therapeutic strategies for prevention. In this research, by using an animal model of morphine self-administration in rats and a whole-cell patch–clamp in brain slices, we found changes in synaptic plasticity in the nucleus accumbens (NAc) shell were involved in the relapse to morphine-seeking behavior. Compared to the controls, the amplitude of long-term depression (LTD) induced in the medium spiny neurons increased after morphine self-administration was established, recovered after the behavior was extinguished, and increased again during the relapse induced by morphine priming. Intravenous injection of MA, a new peptide obtained by modifying Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor “myr-AIP”, decreased CaMKII activity in the NAc shell and blocked the reinstatement of morphine-seeking behavior without influence on the locomotor activity. Moreover, LTD was absent in the NAc shell of the MA-pretreated rats, whereas it was robust in the saline controls in which morphine-seeking behavior was reinstated. These results indicate that CaMKII regulates morphine-seeking behavior through its involvement in the change of synaptic plasticity in the NAc shell during the relapse, and MA may be of great value in the clinical treatment of relapse to opioid seeking.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

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.

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of "counterpart to LTP" that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent "pruning" of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

GluN2B S1303 phosphorylation by CaMKII or DAPK1: no indication for involvement in ischemia or LTP

Binding of two different CaM kinases, CaMKII and DAPK1, to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B near S1303 has been implicated in excitotoxic/ischemic neuronal cell death. The GluN2BΔCaMKII mutation (L1298A, R1300Q) is neuroprotective but abolishes only CaMKII but not DAPK1 binding. However, both kinases can additionally phosphorylate GluN2B S1303. Thus, we here tested S1303 phosphorylation for possible contribution to neuronal cell death. The GluN2BΔCaMKII mutation completely abolished phosphorylation by CaMKII and DAPK1, suggesting that the mutation could mediate neuroprotection by disrupting phosphorylation. However, S1303 phosphorylation was not increased by excitotoxic insults in hippocampal slices or by global cerebral ischemia induced by cardiac arrest and cardiopulmonary resuscitation in vivo. In hippocampal cultures, S1303 phosphorylation was induced by chemical LTD but not LTP stimuli. These results indicate that the additional effect of the GluN2BΔCaMKII mutation on phosphorylation needs to be considered only in LTD but not in LTP or ischemia/excitotoxicity.

AKAP79 enables calcineurin to directly suppress protein kinase A activity

Interplay between the second messengers cAMP and Ca2+ is a hallmark of dynamic cellular processes. A common motif is the opposition of the Ca2+-sensitive phosphatase calcineurin and the major cAMP receptor, protein kinase A (PKA). Calcineurin dephosphorylates sites primed by PKA to bring about changes including synaptic long-term depression (LTD). AKAP79 supports signaling of this type by anchoring PKA and calcineurin in tandem. In this study, we discovered that AKAP79 increases the rate of calcineurin dephosphorylation of type II PKA regulatory subunits by an order of magnitude. Fluorescent PKA activity reporter assays, supported by kinetic modeling, show how AKAP79-enhanced calcineurin activity enables suppression of PKA without altering cAMP levels by increasing PKA catalytic subunit capture rate. Experiments with hippocampal neurons indicate that this mechanism contributes toward LTD. This non-canonical mode of PKA regulation may underlie many other cellular processes.

Calcineurin Participation in Hebbian and Homeostatic Plasticity Associated With Extinction

In nature, animals need to adapt to constant changes in their environment. Learning and memory are cognitive capabilities that allow this to happen. Extinction, the reduction of a certain behavior or learning previously established, refers to a very particular and interesting type of learning that has been the basis of a series of therapies to diminish non-adaptive behaviors. In recent years, the exploration of the cellular and molecular mechanisms underlying this type of learning has received increasing attention. Hebbian plasticity (the activity-dependent modification of the strength or efficacy of synaptic transmission), and homeostatic plasticity (the homeostatic regulation of plasticity) constitute processes intimately associated with memory formation and maintenance. Particularly, long-term depression (LTD) has been proposed as the underlying mechanism of extinction, while the protein phosphatase calcineurin (CaN) has been widely related to both the extinction process and LTD. In this review, we focus on the available evidence that sustains CaN modulation of LTD and its association with extinction. Beyond the classic view, we also examine the interconnection among extinction, Hebbian and homeostatic plasticity, as well as emergent evidence of the participation of kinases and long-term potentiation (LTP) on extinction learning, highlighting the importance of the balance between kinases and phosphatases in the expression of extinction. Finally, we also integrate data that shows the association between extinction and less-studied phenomena, such as synaptic silencing and engram formation that open new perspectives in the field.

Androgen-dependent miR-125a-5p targets LYPLA1 and regulates global protein palmitoylation level in late-onset hypogonadism males

Late-onset hypogonadism (LOH) is defined as a clinical and biochemical syndrome with multiple symptoms caused by testosterone deficiency in aging males. An in-depth exploration of the molecular mechanism underlying LOH development is insufficient. We previously identified miR-125a-5p as a dysregulated microRNA in LOH patients and potential diagnostic biomarker for LOH. The present study demonstrated that plasma miR-125a-5p was upregulated after testosterone supplementation in both LOH patients and castrated mice, and positively associated with the testosterone concentrations, suggesting direct regulation of miR-125a-5p expression by testosterone. Androgen response element in the promoter of miR-125a-5p was subsequently identified. Target gene screening and confirmation verified that LYPLA1, encoding acyl-protein thioesterase 1 which catalyzed protein depalmitoylation process, was a target gene of miR-125a-5p. Furthermore, in cells cultured with testosterone deprivation and organs from castrated mice, testosterone deficiency led to decreased global protein palmitoylation level. In aging males, global protein palmitoylation in peripheral blood showed a notable decline in LOH patients contrast to the normal elderly males. And the palmitoylation level was positively correlative with serum testosterone concentrations. Our results suggested that testosterone could regulate global palmitoylation level through miR-125a-5p/LYPLA1 signaling pathway. Given that protein palmitoylation is pivotal for protein function and constitutes the pathogenesis of various diseases, testosterone/miR-125a-5p/LYPLA1 may contribute to the molecular mechanism underlying multiple symptoms caused by testosterone deficiency in LOH patients, and aberrant global palmitoylation could be a potential biomarker for LOH.

Young DAPK1 knockout mice have altered presynaptic function

The death-associated protein kinase 1 (DAPK1) has recently been shown to have a physiological function in long-term depression (LTD) of glutamatergic synapses: acute inhibition of DAPK1 blocked the LTD that is normally seen at the hippocampal CA1 synapse in young mice, and a pharmacogenetic combination approach showed that this specifically required DAPK1-mediated suppression of postsynaptic Ca²⁺/calmodulin-dependent protein kinase II binding to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B during LTD stimuli. Surprisingly, we found here that genetic deletion of DAPK1 (in DAPK1^-/- mice) did not reduce LTD. Paired pulse facilitation experiments indicated a presynaptic compensation mechanism: in contrast to wild-type mice, LTD stimuli in DAPK1^-/- mice decreased presynaptic release probability. Basal synaptic strength was normal in young DAPK1^-/- mice, but basal glutamate release probability was reduced, an effect that normalized with maturation.NEW & NOTEWORTHY Young death-associated protein kinase 1 (DAPK1) knockout mice have reduced basal glutamate release probability, an effect that normalized with maturation. This provided a compensatory mechanism that may have prevented a reduction of long-term depression in the young DAPK1 knockout mice.

CaMKII holoenzyme mechanisms that govern the LTP versus LTD decision

Higher brain functions are thought to require synaptic frequency decoding that can lead to long-term potentiation (LTP) or depression (LTD). We show that the LTP versus LTD decision is determined by complex cross-regulation of T286 and T305/306 autophosphorylation within the 12meric CaMKII holoenzyme, which enabled molecular computation of stimulus frequency, amplitude, and duration. Both LTP and LTD require T286 phosphorylation, but T305/306 phosphorylation selectively promoted LTD. In response to excitatory LTP versus LTD stimuli, the differential T305/306 phosphorylation directed CaMKII movement to either excitatory or inhibitory synapses, thereby coordinating plasticity at both synapse types. Fast T305/306 phosphorylation required prior T286 phosphorylation and then curbed CaMKII activity by two mechanisms: (i) a cis-subunit reaction reduced both Ca²⁺ stimulation and autonomous activity and (ii) a trans-subunit reaction enabled complete activity shutdown and feed-forward inhibition of further T286 phosphorylation. These are fundamental additions to the long-studied CaMKII regulation and function in neuronal plasticity.

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.

Non-ionotropic NMDA receptor signaling gates bidirectional structural plasticity of dendritic spines

Experience-dependent refinement of neuronal connections is critically important for brain development and learning. Here, we show that ion-flow-independent NMDA receptor (NMDAR) signaling is required for the long-term dendritic spine growth that is a vital component of brain circuit plasticity. We find that inhibition of p38 mitogen-activated protein kinase (p38 MAPK), which is downstream of non-ionotropic NMDAR signaling in long-term depression (LTD) and spine shrinkage, blocks long-term potentiation (LTP)-induced spine growth but not LTP. We hypothesize that non-ionotropic NMDAR signaling drives the cytoskeletal changes that support bidirectional spine structural plasticity. Indeed, we find that key signaling components downstream of non-ionotropic NMDAR function in LTD-induced spine shrinkage are also necessary for LTP-induced spine growth. Furthermore, NMDAR conformational signaling with coincident Ca²⁺ influx is sufficient to drive CaMKII-dependent long-term spine growth, even when Ca²⁺ is artificially driven through voltage-gated Ca²⁺ channels. Our results support a model in which non-ionotropic NMDAR signaling gates the bidirectional spine structural changes vital for brain plasticity.

Structural plasticity of dendritic spines is a critical step in the remodeling of brain circuits during learning. Stein et al. demonstrate a vital role for ion-flux-independent NMDAR signaling in plasticity-associated dendritic spine growth, supporting a model in which non-ionotropic NMDAR signaling primes the spine actin cytoskeleton for bidirectional structural plasticity.

Molecular changes in glaucomatous trabecular meshwork. Correlations with retinal ganglion cell death and novel strategies for neuroprotection

Glaucoma is a chronic neurodegenerative disease characterized by retinal ganglion cell loss. Although significant advances in ophthalmologic knowledge and practice have been made, some glaucoma mechanisms are not yet understood, therefore, up to now there is no effective treatment able to ensure healing. Indeed, either pharmacological or surgical approaches to this disease aim in lowering intraocular pressure, which is considered the only modifiable risk factor. However, it is well known that several factors and metabolites are equally (if not more) involved in glaucoma. Oxidative stress, for instance, plays a pivotal role in both glaucoma onset and progression because it is responsible for the trabecular meshwork cell damage and, consequently, for intraocular pressure increase as well as for glaucomatous damage cascade. This review at first shows accurately the molecular-derived dysfunctions in antioxidant system and in mitochondria homeostasis which due to both oxidative stress and aging, lead to a chronic inflammation state, the trabecular meshwork damage as well as the glaucoma neurodegeneration. Therefore, the main molecular events triggered by oxidative stress up to the proapoptotic signals that promote the ganglion cell death have been highlighted. The second part of this review, instead, describes some of neuroprotective agents such as polyphenols or polyunsaturated fatty acids as possible therapeutic source against the propagation of glaucomatous damage.

Simultaneous Live Imaging of Multiple Endogenous Proteins Reveals a Mechanism for Alzheimer's-Related Plasticity Impairment

CaMKIIα is a central mediator of bidirectional synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). To study how CaMKIIα movement during plasticity is affected by soluble amyloid-β peptide oligomers (Aβ), we used FingR intrabodies to simultaneously image endogenous CaMKIIα and markers for excitatory versus inhibitory synapses in live neurons. Aβ blocks LTP-stimulus-induced CaMKIIα accumulation at excitatory synapses. This block requires CaMKII activity, is dose and time dependent, and also occurs at synapses without detectable Aβ; it is specific to LTP, as CaMKIIα accumulation at inhibitory synapses during LTD is not reduced. As CaMKII movement to excitatory synapses is required for normal LTP, its impairment can mechanistically explain Aβ-induced impairment of LTP. CaMKII movement during LTP requires binding to the NMDA receptor, and Aβ induces internalization of NMDA receptors. However, surprisingly, this internalization does not cause the block in CaMKIIα movement and is observed for extrasynaptic, but not synaptic, NMDA receptors.

Molecular Mechanisms of Non-ionotropic NMDA Receptor Signaling in Dendritic Spine Shrinkage

Structural plasticity of dendritic spines is a key component of the refinement of synaptic connections during learning. Recent studies highlight a novel role for the NMDA receptor (NMDAR), independent of ion flow, in driving spine shrinkage and LTD. Yet little is known about the molecular mechanisms that link conformational changes in the NMDAR to changes in spine size and synaptic strength. Here, using two-photon glutamate uncaging to induce plasticity at individual dendritic spines on hippocampal CA1 neurons from mice and rats of both sexes, we demonstrate that p38 MAPK is generally required downstream of non-ionotropic NMDAR signaling to drive both spine shrinkage and LTD. In a series of pharmacological and molecular genetic experiments, we identify key components of the non-ionotropic NMDAR signaling pathway driving dendritic spine shrinkage, including the interaction between NOS1AP (nitric oxide synthase 1 adaptor protein) and neuronal nitric oxide synthase (nNOS), nNOS enzymatic activity, activation of MK2 (MAPK-activated protein kinase 2) and cofilin, and signaling through CaMKII. Our results represent a large step forward in delineating the molecular mechanisms of non-ionotropic NMDAR signaling that can drive shrinkage and elimination of dendritic spines during synaptic plasticity.SIGNIFICANCE STATEMENT Signaling through the NMDA receptor (NMDAR) is vitally important for the synaptic plasticity that underlies learning. Recent studies highlight a novel role for the NMDAR, independent of ion flow, in driving synaptic weakening and dendritic spine shrinkage during synaptic plasticity. Here, we delineate several key components of the molecular pathway that links conformational signaling through the NMDAR to dendritic spine shrinkage during synaptic plasticity.

Proteasomal-Mediated Degradation of AKAP150 Accompanies AMPAR Endocytosis during cLTD

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

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

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

S-Palmitoylation of the sodium channel Nav1.6 regulates its activity and neuronal excitability

S-Palmitoylation is a reversible post-translational lipid modification that dynamically regulates protein functions. Voltage-gated sodium channels are subjected to S-palmitoylation and exhibit altered functions in different S-palmitoylation states. Our aim was to investigate whether and how S-palmitoylation regulates Nav1.6 channel function and to identify S-palmitoylation sites that can potentially be pharmacologically targeted. Acyl-biotin exchange assay showed that Nav1.6 is modified by S-palmitoylation in the mouse brain and in a Nav1.6 stable HEK 293 cell line. Using whole-cell voltage clamp, we discovered that enhancing S-palmitoylation with palmitic acid increases Nav1.6 current, whereas blocking S-palmitoylation with 2-bromopalmitate reduces Nav1.6 current and shifts the steady-state inactivation in the hyperpolarizing direction. Three S-palmitoylation sites (Cys¹¹⁶⁹, Cys¹¹⁷⁰, and Cys¹⁹⁷⁸) were identified. These sites differentially modulate distinct Nav1.6 properties. Interestingly, Cys¹⁹⁷⁸ is exclusive to Nav1.6 among all Nav isoforms and is evolutionally conserved in Nav1.6 among most species. Cys¹⁹⁷⁸ S-palmitoylation regulates current amplitude uniquely in Nav1.6. Furthermore, we showed that eliminating S-palmitoylation at specific sites alters Nav1.6-mediated excitability in dorsal root ganglion neurons. Therefore, our study reveals S-palmitoylation as a potential isoform-specific mechanism to modulate Nav activity and neuronal excitability in physiological and diseased conditions.

Low expression of A-kinase anchor protein 5 predicts poor prognosis in non-mucin producing stomach adenocarcinoma based on TCGA data

BACKGROUND

In the past, there were not a lot of studies on how A-kinase anchor protein 5 (AKAP5) involving in the pathogenesis and prognosis of non-mucin producing stomach adenocarcinoma (NMSA). Therefore, we studied the relationship between AKAP5 and the prognosis of NMSA and its possible mechanisms using publicly available data from The Cancer Genome Atlas (TCGA).

METHODS

RNA high-throughput sequencing and clinicopathologic data of NMSA were downloaded from the TCGA. Clinical pathologic features associated with AKAP5 expression were analyzed using the chi-square and Fisher exact tests. The relationship between the overall survival (OS) and AKAP5 expression was analyzed by the Kaplan-Meier method and the Cox regression analysis. GSEA analysis was performed using the TCGA dataset.

RESULTS

Our results indicated that the AKAP5 expression was increased in NMSA (all tumor vs. adjacent mucosa). Also, histologic grade, clinical stage, N classification, and survival status were significantly correlated with AKAP5 expression. Kaplan-Meier curves showed that low AKAP5 expression was associated with a poor OS among the NMSA patients (P=5.003e-05), and in the clinical stage III and IV (P=4.646e-05), TNM stage T3 (P=0.016), T4 (P=0.001), N2 (P=0.012), N3 (P=0.003), M0 (P=3.911e-05), and histological grade G3 (P=1.658e-04) subgroups. Cox regression analysis showed that reduced AKAP5 expression in NMSA is associated with age (HR =1.03, P=0.007), stage (HR =1.84 for stage I, II vs. stage III, IV, P=0.002) and M classification (HR =1.8 for M0 vs. M1, P=0.010). Gene sets related to cholesterol homeostasis, glycolysis, estrogen response late, adipogenesis, estrogen response early, notch signaling, and peroxisome were differentially enriched with the low AKAP5 expression phenotype.

CONCLUSIONS

Low expression of AKAP5 may be a potential molecular marker for predicting poor prognosis of NMSA. Besides, cholesterol homeostasis, glycolysis, estrogen response, adipogenesis, notch signaling, and peroxisome may be the key pathways regulated by AKAP5 in NMSA. It also suggested that AKAP5 might potentially have biological functions in the development of stomach adenocarcinoma.

CaM Kinase: Still Inspiring at 40

The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) was touted as a memory molecule, even before its involvement in long-term potentiation (LTP) was shown. The enzyme has not disappointed, with subsequent demonstrations of remarkable structural and regulatory properties. Its neuronal functions now extend to long-term depression (LTD), and last year saw the first direct evidence for memory storage by CaMKII. Although CaMKII may have taken the spotlight, it is a member of a large family of diverse and interesting CaM kinases. Our aim is to place CaMKII in context of the other CaM kinases and then review certain aspects of this kinase that are of current interest.

Loss of MsrB1 perturbs spatial learning and long-term potentiation/long-term depression in mice

MsrB1 belongs to the methionine sulfoxide reductase family, it is also known as selenoprotein R for the sake of possessing a selenocysteine residue. It has been reported that MsrB1 could interact with actin, TRPM6, clusterin, and amyloid-beta in vitro. Thus, we presumed that MsrB1 may play an important role in central nervous system. To examine whether MsrB1 knockout has any effects on brain development or learning behavior, we carried out histological study on brains of MsrB1 deficient mice, and further tested spatial learning ability and long-term synaptic plasticity of these mice by using Morris water maze and electrophysiological methods. It was observed that loss of MsrB1 did not perturb the overall development of central nervous system except for the astrogliosis in hippocampus, however, it led mice to be incapable in spatial learning and severe impairments in LTP/LTD expression in CA1 of brain slices, along with the down-regulation of the synaptic proteins including PSD95, SYP, GluN2A and GluN2B, as well as the dramatic decrease of CaMKIIs phosphorylation at 286(287) compared with wild type mice. Taken together, these results suggest that MsrB1 is essential for mice spatial learning and LTP/LTD induction, and the MsrB1 related redox homeostasis may be involved in regulating the phosphorylation of CaMKIIs.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

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.

The KN-93 Molecule Inhibits Calcium/Calmodulin-Dependent Protein Kinase II (CaMKII) Activity by Binding to Ca2+/CaM

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a multifunctional serine/threonine protein kinase that transmits calcium signals in various cellular processes. CaMKII is activated by calcium-bound calmodulin (Ca²⁺/CaM) through a direct binding mechanism involving a regulatory C-terminal α-helix in CaMKII. The Ca²⁺/CaM binding triggers transphosphorylation of critical threonine residues proximal to the CaM-binding site leading to the autoactivated state of CaMKII. The demonstration of its critical roles in pathophysiological processes has elevated CaMKII to a key target in the management of numerous diseases. The molecule KN-93 is the most widely used inhibitor for studying the cellular and in vivo functions of CaMKII. It is widely believed that KN-93 binds directly to CaMKII, thus preventing kinase activation by competing with Ca²⁺/CaM. Herein, we employed surface plasmon resonance, NMR, and isothermal titration calorimetry to characterize this presumed interaction. Our results revealed that KN-93 binds directly to Ca²⁺/CaM and not to CaMKII. This binding would disrupt the ability of Ca²⁺/CaM to interact with CaMKII, effectively inhibiting CaMKII activation. Our findings also indicated that KN-93 can specifically compete with a CaMKIIδ-derived peptide for binding to Ca²⁺/CaM. As indicated by the surface plasmon resonance and isothermal titration calorimetry data, apparently at least two KN-93 molecules can bind to Ca²⁺/CaM. Our findings provide new insight into how in vitro and in vivo data obtained with KN-93 should be interpreted. They further suggest that other Ca²⁺/CaM-dependent, non-CaMKII activities should be considered in KN-93-based mechanism-of-action studies and drug discovery efforts.

Mechanisms of AMPA Receptor Endosomal Sorting

The regulation of synaptic AMPA receptors (AMPARs) is critical for excitatory synaptic transmission, synaptic plasticity and the consequent formation of neural circuits during brain development and their modification during learning and memory processes. The number of synaptic AMPARs is regulated through endocytosis, exocytosis and endosomal sorting that results in recycling back to the plasma membrane or degradation in the lysosome. Hence, endo-lysosomal sorting is vitally important in maintaining AMPAR expression at the synapse, and the dynamic regulation of these trafficking events is a key component of synaptic plasticity. A reduction in synaptic strength such as in long-term depression (LTD) involves AMPAR sorting to lysosomes to reduce synaptic AMPAR number, whereas long-term potentiation (LTP) involves an increase in AMPAR recycling to increase the number of AMPARs at synapses. Here, we review our current understanding of the endosomal trafficking routes taken by AMPARs, and the mechanisms involved in AMPAR endosomal sorting, focussing on the numerous AMPAR associated proteins that have been implicated in this complex process. We also discuss how these events are dysregulated in brain disorders.

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

Ca2+-permeable AMPA-type glutamate receptors (CP-AMPARs) containing GluA1 but lacking GluA2 subunits contribute to multiple forms of synaptic plasticity, including long-term potentiation (LTP), but mechanisms regulating CP-AMPARs are poorly understood. A-kinase anchoring protein (AKAP) 150 scaffolds kinases and phosphatases to regulate GluA1 phosphorylation and trafficking, and trafficking of AKAP150 itself is modulated by palmitoylation on two Cys residues. Here, we developed a palmitoylation-deficient knockin mouse to show that AKAP150 palmitoylation regulates CP-AMPAR incorporation at hippocampal synapses. Using biochemical, super-resolution imaging, and electrophysiological approaches, we found that palmitoylation promotes AKAP150 localization to recycling endosomes and the postsynaptic density (PSD) to limit CP-AMPAR basal synaptic incorporation. In addition, we found that AKAP150 palmitoylation is required for LTP induced by weaker stimulation that recruits CP-AMPARs to synapses but not stronger stimulation that recruits GluA2-containing AMPARs. Thus, AKAP150 palmitoylation controls its subcellular localization to maintain proper basal and activity-dependent regulation of synaptic AMPAR subunit composition.

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

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

Lipids and phosphates at odds in synaptic depression

Long-term depression (LTD) is a reduction in the efficacy of neuronal synapses, but the molecular basis of LTD signaling and how these signals lead to phenotypic outcomes, such as the shrinkage of synaptic regions, is not clear. In a new report, Woolfrey et al use chemically-induced LTD and a multitude of in vitro biochemical assays to provide evidence that synaptic removal of the scaffolding protein AKAP79/150 promotes LTD-induced spine shrinkage. The further identification of CaMKII, a kinase primarily associated with long-term potentiation (LTP), as a requirement for AKAP79/150 removal, uncovers unexpected interplay between different post-translational modifications and points to a new model of LTD.