Ca2+/calcineurin-dependent inactivation of neuronal L-type Ca2+ channels requires priming by AKAP-anchored protein kinase A

A CatSper-Uninvolved Mechanism to Induce Forward Sperm Motility in the Internal Fertilization

Sperm-specific cation channel (CatSper), sperm-specific Na + /H + exchanger (sNHE), and soluble adenylyl cyclase (sAC) are necessary in the signaling pathways to control sperm motility in many animals, whereas some animals have lost some or all of them. In the present study, we examined CatSper-uninvolved signaling for vigorous undulation of the undulating membrane that is attached to the sperm tail and gives thrust for forward motility in the internally fertilizing newt Cynops pyrrhogaster. Reverse-transcription PCR failed to detect sNHE in the newt sperm. However, the pH of sperm cytoplasm was raised under a high extracellular pH equivalent to that of egg jelly, where sperm motility is initiated by sperm motility-initiating substance (SMIS). Carbonic anhydrase XII/ XVI and SLC4A4/8 were suggested to be present in the sperm, and transported bicarbonates raised the intracellular pH. In egg jelly extract that contained SMIS, the anion transporter inhibitor DIDS weakened the undulation of the undulating membrane, while bicarbonates enhanced it. The cyclic AMP concentration was found to increase in sperm cytoplasm in the egg-jelly extract. An inhibitor of sAC (KH7) weakened the undulation of the undulating membrane, and dibutyryl cyclic AMP blocked the inhibitory effect. Inhibitor of transmembrane AC (DDA) limitedly affected the undulation. The undulation was weakened by an inhibitor of protein kinase A (H89), and by an inhibitor of transient receptor potential (TRP) channels (RN1747). Our results support the conclusions that the high pH of the egg jelly triggers a signaling pathway through sAC, PKA, and TRP channels, and coacts with SMIS to induce forward sperm motility.

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.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Neuronal ER-plasma membrane junctions couple excitation to Ca2+-activated PKA signaling

Junctions between the endoplasmic reticulum (ER) and the plasma membrane (PM) are specialized membrane contacts ubiquitous in eukaryotic cells. Concentration of intracellular signaling machinery near ER-PM junctions allows these domains to serve critical roles in lipid and Ca2+ signaling and homeostasis. Subcellular compartmentalization of protein kinase A (PKA) signaling also regulates essential cellular functions, however, no specific association between PKA and ER-PM junctional domains is known. Here, we show that in brain neurons type I PKA is directed to Kv2.1 channel-dependent ER-PM junctional domains via SPHKAP, a type I PKA-specific anchoring protein. SPHKAP association with type I PKA regulatory subunit RI and ER-resident VAP proteins results in the concentration of type I PKA between stacked ER cisternae associated with ER-PM junctions. This ER-associated PKA signalosome enables reciprocal regulation between PKA and Ca2+ signaling machinery to support Ca2+ influx and excitation-transcription coupling. These data reveal that neuronal ER-PM junctions support a receptor-independent form of PKA signaling driven by membrane depolarization and intracellular Ca2+, allowing conversion of information encoded in electrical signals into biochemical changes universally recognized throughout the cell.

Protein kinase A mediates modality-specific modulation of the mechanically-gated ion channel PIEZO2

Protein kinase A is a downstream effector of many inflammatory mediators that induce pain hypersensitivity by increasing the mechanosensitivity of nociceptive sensory afferent. Here we examine the molecular mechanism underlying protein kinase-A-dependent modulation of the mechanically-activated ion channel PIEZO2, which confers mechanosensitivity to many nociceptors. Using phosphorylation site prediction algorithms, we identified multiple putative and highly conserved PKA phosphorylation sites located on intracellular intrinsically disordered regions of PIEZO2. Site-directed mutagenesis and patch-clamp recordings showed that substitution of one or multiple putative PKA sites within a single intracellular domain does not alter PKA-induced PIEZO2 sensitization, whereas mutation of a combination of nine putative sites located on four different intracellular regions completely abolishes PKA-dependent PIEZO2 modulation, though it remains unclear whether all or just some of these nine sites are required. By demonstrating that PIEZO1 is not modulated by PKA, our data also reveals a previously unrecognized functional difference between PIEZO1 and PIEZO2. Moreover, by demonstrating that PKA only modulates PIEZO2 currents evoked by focal mechanical indentation of the cell, but not currents evoked by pressure-induced membrane stretch, we provide evidence suggesting that PIEZO2 is a polymodal mechanosensor that engages different protein domains for detecting different types of mechanical stimuli.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

The CaV1.2 G406R mutation decreases synaptic inhibition and alters L-type Ca2+ channel-dependent LTP at hippocampal synapses in a mouse model of Timothy Syndrome

Genetic alterations in autism spectrum disorders (ASD) frequently disrupt balance between synaptic excitation and inhibition and alter plasticity in the hippocampal CA1 region. Individuals with Timothy Syndrome (TS), a genetic disorder caused by CaV1.2 L-type Ca2+ channel (LTCC) gain-of function mutations, such as G406R, exhibit social deficits, repetitive behaviors, and cognitive impairments characteristic of ASD that are phenocopied in TS2-neo mice expressing G406R. Here, we characterized hippocampal CA1 synaptic function in male TS2-neo mice and found basal excitatory transmission was slightly increased and inhibitory transmission strongly decreased. We also found distinct impacts on two LTCC-dependent forms of long-term potentiation (LTP) synaptic plasticity that were not readily consistent with LTCC gain-of-function. LTP induced by high-frequency stimulation (HFS) was strongly impaired in TS2-neo mice, suggesting decreased LTCC function. Yet, CaV1.2 expression, basal phosphorylation, and current density were similar for WT and TS2-neo. However, this HFS-LTP also required GABAA receptor activity, and thus may be impaired in TS2-neo due to decreased inhibitory transmission. In contrast, LTP induced in WT mice by prolonged theta-train (PTT) stimulation in the presence of a β-adrenergic receptor agonist to increase CaV1.2 phosphorylation was partially induced in TS2-neo mice by PTT stimulation alone, consistent with increased LTCC function. Overall, our findings provide insights regarding how altered CaV1.2 channel function disrupts basal transmission and plasticity that could be relevant for neurobehavioral alterations in ASD.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca2+ channels activity through a differential mechanism

Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca²⁺ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca²⁺ channels in the absence as in the presence of extracellular Ca²⁺. Here, we investigate the mechanism underlying the modulation of Ca²⁺-independent Ca²⁺ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca²⁺, the Mn²⁺ has been used as a Ca²⁺ surrogate to determine the activity of Ca²⁺-independent Ca²⁺ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca²⁺-independent Ca²⁺ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La³⁺-sensitive Ca²⁺ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca²⁺ channels not sensitive to La³⁺, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca²⁺ channels by phosphorylation/de-phosphorylation-dependent mechanisms.

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.

Depolarization induces nociceptor sensitization by CaV1.2-mediated PKA-II activation

Depolarization drives neuronal plasticity. However, whether depolarization drives sensitization of peripheral nociceptive neurons remains elusive. By high-content screening (HCS) microscopy, we revealed that depolarization of cultured sensory neurons rapidly activates protein kinase A type II (PKA-II) in nociceptors by calcium influx through Ca_V1.2 channels. This effect was modulated by calpains but insensitive to inhibitors of cAMP formation, including opioids. In turn, PKA-II phosphorylated Ser1928 in the distal C terminus of Ca_V1.2, thereby increasing channel gating, whereas dephosphorylation of Ser1928 involved the phosphatase calcineurin. Patch-clamp and behavioral experiments confirmed that depolarization leads to calcium- and PKA-dependent sensitization of calcium currents ex vivo and local peripheral hyperalgesia in the skin in vivo. Our data suggest a local activity-driven feed-forward mechanism that selectively translates strong depolarization into further activity and thereby facilitates hypersensitivity of nociceptor terminals by a mechanism inaccessible to opioids.

Dependence of sperm structural and functional integrity on testicular calcineurin isoform PPP3R2 expression

After leaving the testis, mammalian sperm undergo a sequential maturation process in the epididymis followed by capacitation during their movement through the female reproductive tract. These phenotypic changes are associated with modification of protein phosphorylation and membrane remodeling, which is requisite for sperm to acquire forward motility and induce fertilization. However, the molecular mechanisms underlying sperm maturation and capacitation are still not fully understood. Herein, we show that PPP3R2, a testis-specific regulatory subunit of protein phosphatase 3 (an isoform of calcineurin in the testis), is essential for sperm maturation and capacitation. Knockout of Ppp3r2 in mice leads to male sterility due to sperm motility impairment and morphological defects. One very noteworthy change includes increases in sperm membrane stiffness. Moreover, PPP3R2 regulates sperm maturation and capacitation via (i) modulation of membrane diffusion barrier function at the annulus and (ii) facilitation of cholesterol efflux during sperm capacitation. Taken together, PPP3R2 plays a critical role in modulating cholesterol efflux and mediating the dynamic control of membrane remodeling during sperm maturation and capacitation.

Selective targeting of peripheral cannabinoid receptors prevents behavioral symptoms and sensitization of trigeminal neurons in mouse models of migraine and medication overuse headache

ABSTRACT

Migraine affects ∼15% of the world's population greatly diminishing their quality of life. Current preventative treatments are effective in only a subset of migraine patients, and although cannabinoids seem beneficial in alleviating migraine symptoms, central nervous system side effects limit their widespread use. We developed peripherally restricted cannabinoids (PRCBs) that relieve chronic pain symptoms of cancer and neuropathies, without appreciable central nervous system side effects or tolerance development. Here, we determined PRCB effectiveness in alleviating hypersensitivity symptoms in mouse models of migraine and medication overuse headache. Long-term glyceryl trinitrate (GTN, 10 mg/kg) administration led to increased sensitivity to mechanical stimuli and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment, but not posttreatment, prevented behavioral and biochemical correlates of GTN-induced sensitization. Low pH-activated and allyl isothiocyanate-activated currents in acutely isolated trigeminal neurons were reversibly attenuated by PRCB application. Long-term GTN treatment significantly enhanced these currents. Long-term sumatriptan treatment also led to the development of allodynia to mechanical and cold stimuli that was slowly reversible after sumatriptan discontinuation. Subsequent challenge with a previously ineffective low-dose GTN (0.1-0.3 mg/kg) revealed latent behavioral sensitization and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment prevented all behavioral and biochemical correlates of allodynia and latent sensitization. Importantly, long-term PRCB treatment alone did not produce any behavioral or biochemical signs of sensitization. These data validate peripheral cannabinoid receptors as potential therapeutic targets in migraine and medication overuse headache.

Mechanisms of endocannabinoid control of synaptic plasticity

The endogenous cannabinoid transmitter system regulates synaptic transmission throughout the nervous system. Unlike conventional transmitters, specific stimuli induce synthesis of endocannabinoids (eCBs) in the postsynaptic neuron, and these travel backwards to modulate presynaptic inputs. In doing so, eCBs can induce short-term changes in synaptic strength and longer-term plasticity. While this eCB regulation is near ubiquitous, it displays major regional and synapse specific variations with different synapse specific forms of short-versus long-term plasticity throughout the brain. These differences are due to the plethora of pre- and postsynaptic mechanisms which have been implicated in eCB signalling, the intricacies of which are only just being realised. In this review, we shall describe the current understanding and highlight new advances in this area, with a focus on the retrograde action of eCBs at CB1 receptors (CB₁Rs).

Compartmentalized Signaling in Aging and Neurodegeneration

The cyclic AMP (cAMP) signalling cascade is necessary for cell homeostasis and plays important roles in many processes. This is particularly relevant during ageing and age-related diseases, where drastic changes, generally decreases, in cAMP levels have been associated with the progressive decline in overall cell function and, eventually, the loss of cellular integrity. The functional relevance of reduced cAMP is clearly supported by the finding that increases in cAMP levels can reverse some of the effects of ageing. Nevertheless, despite these observations, the molecular mechanisms underlying the dysregulation of cAMP signalling in ageing are not well understood. Compartmentalization is widely accepted as the modality through which cAMP achieves its functional specificity; therefore, it is important to understand whether and how this mechanism is affected during ageing and to define which is its contribution to this process. Several animal models demonstrate the importance of specific cAMP signalling components in ageing, however, how age-related changes in each of these elements affect the compartmentalization of the cAMP pathway is largely unknown. In this review, we explore the connection of single components of the cAMP signalling cascade to ageing and age-related diseases whilst elaborating the literature in the context of cAMP signalling compartmentalization.

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.

Cav1.2 Activity and Downstream Signaling Pathways in the Hippocampus of An Animal Model of Depression

Functional and morphological modifications in the brain caused by major mood disorders involve many brain areas, including the hippocampus, leading to cognitive and mood alterations. Ca_v1.2 channel expression has been found to increase in animals with depressive-like behaviors. Calcium influx through these channels is associated with changes in excitation-transcriptional coupling by several intracellular signal pathways that are regulated by its C-terminus region. However, which of these signaling pathways is activated during the development of depressive-like behaviors is not known. Here, we evaluate the phosphorylation and expression levels of crucial kinases and transcription factors at the hippocampus of rats after 21 days of chronic restraint stress. Our results show that rats subjected to CRS protocol achieve less body weight, have heavier adrenal glands, and exhibit depression-like behaviors such as anhedonia, behavioral despair and decreased social interaction. Ca_v1.2 mRNA and protein expression levels, plus l-type calcium current amplitude, are also increased in treated rats when compared with control animals. Out of the three main signaling pathways activated by l-type currents, we only observed an increment of CaM-NFAT axis activity with the concomitant increment in Fas ligand expression. Thus, our results suggest that CRS activates specific pathways, and the increased expression of Ca_v1.2 could lead to neuronal death in the hippocampus.

Drugs That Regulate Local Cell Signaling: AKAP Targeting as a Therapeutic Option

Cells respond to environmental cues by mobilizing signal transduction cascades that engage protein kinases and phosphoprotein phosphatases. Correct organization of these enzymes in space and time enables the efficient and precise transmission of chemical signals. The cyclic AMP-dependent protein kinase A is compartmentalized through its association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of multivalent scaffolds that constrain signaling enzymes and effectors at subcellular locations to drive essential physiological events. More recently, it has been recognized that defective signaling in certain endocrine disorders and cancers proceeds through pathological AKAP complexes. Consequently, pharmacologically targeting these macromolecular complexes unlocks new therapeutic opportunities for a growing number of clinical indications. This review highlights recent findings on AKAP signaling in disease, particularly in certain cancers, and offers an overview of peptides and small molecules that locally regulate AKAP-binding partners.

Aberrant expression of S-SCAM causes the loss of GABAergic synapses in hippocampal neurons

The duplication and deletion mutations of the S-SCAM/MAGI-2 gene are associated with schizophrenia and infantile spasms, respectively. S-SCAM is a unique synaptic scaffolding protein that localizes to both excitatory and GABAergic synapses. However, consequences of aberrant S-SCAM expression on GABAergic synapses is little studied. Here we report the effect of S-SCAM knockdown and overexpression on GABAergic synapses. S-SCAM knockdown in cultured hippocampal neurons caused a drastic loss of both pre- and post-synaptic components of GABAergic synapses, indicating its essential role in GABAergic synapse formation and maintenance. Surprisingly, S-SCAM overexpression also attenuated GABAergic synapses, but the effect is mediated by the loss of postsynaptic GABA_A receptors, gephyrin, and neuroligin 2 and does not involve presynaptic component vesicular GABA transporters. Overexpression studies using S-SCAM mutants with various domain deletions indicated that GABAergic synapse loss correlates with their ability to increase excitatory synaptic function. Consistently, AMPA receptor antagonist CNQX or calcineurin inhibitor FK506 abolished the S-SCAM overexpression-induced loss of GABA_A receptors, supporting that GABAergic synapse loss by S-SCAM overexpression is due to the activity-induced dispersal of synaptic GABA_A receptors. These results suggest that abnormal S-SCAM protein levels disrupt excitation/inhibition balance in neurons, which may explain the pathogenic nature of S-SCAM copy number variations.

β2 Adrenergic Receptor Complexes with the L-Type Ca2+ Channel CaV1.2 and AMPA-Type Glutamate Receptors: Paradigms for Pharmacological Targeting of Protein Interactions

Formation of signaling complexes is crucial for the orchestration of fast, efficient, and specific signal transduction. Pharmacological disruption of defined signaling complexes has the potential for specific intervention in selected regulatory pathways without affecting organism-wide disruption of parallel pathways. Signaling by epinephrine and norepinephrine through α and β adrenergic receptors acts on many signaling pathways in many cell types. Here, we initially provide an overview of the signaling complexes formed between the paradigmatic β₂ adrenergic receptor and two of its most important targets, the L-type Ca²⁺ channel Ca_V1.2 and the AMPA-type glutamate receptor. Importantly, both complexes contain the trimeric G_s protein, adenylyl cyclase, and the cAMP-dependent protein kinase, PKA. We then discuss the functional implications of the formation of these complexes, how those complexes can be specifically disrupted, and how such disruption could be utilized in the pharmacological treatment of disease.

Calcium/calmodulin-stimulated adenylyl cyclases 1 and 8 regulate reward-related brain activity and ethanol consumption

Evidence suggests a predictive link between elevated basal activity within reward-related networks (e.g., cortico-basal ganglia-thalamic networks) and vulnerability for alcoholism. Both calcium channel function and cyclic adenosine monophosphate (cAMP)/protein kinase A-mediated signaling are critical modulators of reward neurocircuitry and reward-related behaviors. Calcium/calmodulin-stimulated adenylyl cyclases (AC) 1 and 8 are sensitive to activity-dependent increases in intracellular calcium and catalyze cAMP production. Therefore, we hypothesized AC1 and 8 regulate brain activity in reward regions of the cortico-basal ganglia-thalamic circuit and that this regulatory influence predicts voluntary ethanol drinking responses. This hypothesis was evaluated by manganese-enhanced magnetic resonance imaging and chronic, intermittent ethanol access procedures. Ethanol-naïve mice with genetic deletion of both AC1 and 8 (DKO mice) exhibited bilateral reductions in baseline activity within cortico-basal ganglia-thalamic regions associated with reward processing compared to wild-type controls (WT, C57BL/6 mice). Significant activity changes were not evident in regions either outside of the cortico-basal ganglia-thalamic network or within the network that are not associated with reward processing. Parallel studies demonstrated that reward network hypoactivity in DKO mice predicted a significant attenuation in consumption and preference levels to escalating ethanol concentrations (12, 20 and 30%) compared to WT mice, an effect that was maintained over extended access (14 sessions) to 20% ethanol. Summarizing, these data support a contribution of AC1 and 8 in cortico-basal ganglia-thalamic activity and the predictive value of this regulatory influence on ethanol drinking behavior, which merits the future evaluation of calcium-stimulated ACs in the neural processes that engender vulnerability to maladaptive alcohol drinking.

Calcium Channels, Synaptic Plasticity, and Neuropsychiatric Disease

Voltage-gated calcium channels couple depolarization of the cell-surface membrane to entry of calcium, which triggers secretion, contraction, neurotransmission, gene expression, and other physiological responses. They are encoded by ten genes, which generate three voltage-gated calcium channel subfamilies: Ca_V1; Ca_V2; and Ca_V3. At synapses, Ca_V2 channels form large signaling complexes in the presynaptic nerve terminal, which are responsible for the calcium entry that triggers neurotransmitter release and short-term presynaptic plasticity. Ca_V1 channels form signaling complexes in postsynaptic dendrites and dendritic spines, where their calcium entry induces long-term potentiation. These calcium channels are the targets of mutations and polymorphisms that alter their function and/or regulation and cause neuropsychiatric diseases, including migraine headache, cerebellar ataxia, autism, schizophrenia, bipolar disorder, and depression. This article reviews the molecular properties of calcium channels, considers their multiple roles in synaptic plasticity, and discusses their potential involvement in this wide range of neuropsychiatric diseases.

Synaptic crosstalk conferred by a zone of differentially regulated Ca2+ signaling in the dendritic shaft adjoining a potentiated spine

Patterns of postsynaptic activity that induce long-term potentiation of fast excitatory transmission at glutamatergic synapses between hippocampal neurons cause enlargement of the dendritic spine and promote growth in spine endoplasmic reticulum (ER) content. Such postsynaptic activity patterns also impact Ca2+ signaling in the adjoining dendritic shaft, in a zone centered on the spine-shaft junction and extending ∼10-20 µm in either direction along the shaft. Comparing this specialized zone in the shaft with the dendrite in general, plasticity-inducing stimulation of a single spine causes more profound depletion of Ca2+ stores in the ER, a greater degree of interaction between stromal interaction molecule 1 (STIM1) and L-type Ca2+ channels, and thus stronger STIM1 inhibition of these channels. Here we show that the length of this zone along the dendritic axis can be approximately doubled through the neuromodulatory action of β-adrenergic receptors (βARs). The mechanism of βAR enlargement of the zone arises from protein kinase A-mediated enhancement of L-type Ca2+ current, which in turn lowers [Ca2+]ER through ryanodine receptor-dependent Ca2+-induced Ca2+ release and activates STIM1 feedback inhibition of L-type Ca2+ channels. An important function of this dendritic zone is to support crosstalk between spines along its length such that spines neighboring a strongly stimulated spine are enabled to undergo structural plasticity in response to stimulation that would otherwise be subthreshold for spine structural plasticity. This form of crosstalk requires L-type Ca2+ channel current to activate STIM1, and βAR activity extends the range along the shaft over which such spine-to-spine communication can occur.

AKAP79/150 recruits the transcription factor NFAT to regulate signaling to the nucleus by neuronal L-type Ca2+ channels

In neurons, regulation of activity-dependent transcription by the nuclear factor of activated T-cells (NFAT) depends upon Ca²⁺ influx through voltage-gated L-type calcium channels (LTCC) and NFAT translocation to the nucleus following its dephosphorylation by the Ca²⁺-dependent phosphatase calcineurin (CaN). CaN is recruited to the channel by A-kinase anchoring protein (AKAP) 79/150, which binds to the LTCC C-terminus via a modified leucine-zipper (LZ) interaction. Here we sought to gain new insights into how LTCCs and signaling to NFAT are regulated by this LZ interaction. RNA interference–mediated knockdown of endogenous AKAP150 and replacement with human AKAP79 lacking its C-terminal LZ domain resulted in loss of depolarization-stimulated NFAT signaling in rat hippocampal neurons. However, the LZ mutation had little impact on the AKAP–LTCC interaction or LTCC function, as measured by Förster resonance energy transfer, Ca²⁺ imaging, and electrophysiological recordings. AKAP79 and NFAT coimmunoprecipitated when coexpressed in heterologous cells, and the LZ mutation disrupted this association. Critically, measurements of NFAT mobility in neurons employing fluorescence recovery after photobleaching and fluorescence correlation spectroscopy provided further evidence for an AKAP79 LZ interaction with NFAT. These findings suggest that the AKAP79/150 LZ motif functions to recruit NFAT to the LTCC signaling complex to promote its activation by AKAP-anchored calcineurin.

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.

Modulator-Gated, SUMOylation-Mediated, Activity-Dependent Regulation of Ionic Current Densities Contributes to Short-Term Activity Homeostasis

Neurons operate within defined activity limits, and feedback control mechanisms dynamically tune ionic currents to maintain this optimal range. This study describes a novel, rapid feedback mechanism that uses SUMOylation to continuously adjust ionic current densities according to changes in activity. Small ubiquitin-like modifier (SUMO) is a peptide that can be post-translationally conjugated to ion channels to influence their surface expression and biophysical properties. Neuronal activity can regulate the extent of protein SUMOylation. This study on the single, unambiguously identifiable lateral pyloric neuron (LP), a component of the pyloric network in the stomatogastric nervous system of male and female spiny lobsters (Panulirus interruptus), focused on dynamic SUMOylation in the context of activity homeostasis. There were four major findings: First, neuronal activity adjusted the balance between SUMO conjugation and deconjugation to continuously and bidirectionally fine-tune the densities of two opposing conductances: the hyperpolarization activated current (I_h) and the transient potassium current (I_A). Second, tonic 5 nm dopamine (DA) gated activity-dependent SUMOylation to permit and prevent activity-dependent regulation of I_h and I_A, respectively. Third, DA-gated, activity-dependent SUMOylation contributed to a feedback mechanism that restored the timing and duration of LP activity during prolonged modulation by 5 μm DA, which initially altered these and other activity features. Fourth, DA modulatory and metamoduatory (gating) effects were tailored to simultaneously alter and stabilize neuronal output. Our findings suggest that modulatory tone may select a subset of rapid activity-dependent mechanisms from a larger menu to achieve homeostasis under varying conditions.SIGNIFICANCE STATEMENT Post-translational SUMOylation of ion channel subunits controls their interactions. When subunit SUMOylation is dysregulated, conductance densities mediated by the channels are distorted, leading to nervous system disorders, such as seizures and chronic pain. Regulation of ion channel SUMOylation is poorly understood. This study demonstrated that neuronal activity can regulate SUMOylation to reconfigure ionic current densities over minutes, and this regulation was gated by tonic nanomolar dopamine. Dynamic SUMOylation was necessary to maintain specific aspects of neuronal output while the neuron was being modulated by high (5 μm) concentrations of dopamine, suggesting that the gating function may ensure neuronal homeostasis during extrinsic modulation of a circuit.

Calcineurin and Its Role in Synaptic Transmission

Calcineurin (CaN) is a serine/threonine phosphatase widely expressed in different cell types and structures including neurons and synapses. The most studied role of CaN is its involvement in the functioning of postsynaptic structures of central synapses. The role of CaN in the presynaptic structures of central and peripheral synapses is less understood, although it has generated a considerable interest and is a subject of a growing number of studies. The regulatory role of CaN in synaptic vesicle endocytosis in the synapse terminals is actively studied. In recent years, new targets of CaN have been identified and its role in the regulation of enzymes and neurotransmitter secretion in peripheral neuromuscular junctions has been revealed. CaN is the only phosphatase that requires calcium and calmodulin for activation. In this review, we present details of CaN molecular structure and give a detailed description of possible mechanisms of CaN activation involving calcium, enzymes, and endogenous and exogenous inhibitors. Known and newly discovered CaN targets at pre- and postsynaptic levels are described. CaN activity in synaptic structures is discussed in terms of functional involvement of this phosphatase in synaptic transmission and neurotransmitter release.

A Friend or Foe: Calcineurin across the Gamut of Neurological Disorders

The serine/threonine phosphatase calcineurin (CaN) is a unique but confounding calcium/calmodulin-mediated enzyme. CaN has shown to play essential roles from regulating calcium homeostasis to being an intricate part of learning and memory formation. Neurological disorders, despite differing in their etiology, share similar pathological outcomes, such as mitochondrial dysfunction and apoptotic signaling brought about by excitotoxic elements. CaN, being deeply integrated in vital neuronal functions, may be implicated in various neurological disorders. Understanding the enzyme and its physiological niche in the nervous system is vital in uncovering its roles in the spectrum of brain disorders. By reviewing the crosstalk in different neurological pathologies, a possible grasp of CaN's complex signaling may lead to forming better neurotherapy. This Outlook attempts to explore the various neuronal functions of CaN and investigate its pervasive role through the gamut of neurological disorders.

PKA phosphorylation of Cav1.2 channel modulates the interaction of calmodulin with the C terminal tail of the channel

Activity of cardiac Cav1.2 channels is enhanced by cyclic AMP-PKA signaling. In this study, we studied the effects of PKA phosphorylation on the binding of calmodulin to the fragment peptide of the proximal C-terminal tail of α1C subunit (CT1, a.a. 1509-1789 of guinea-pig). In the pull-down assay, in vitro PKA phosphorylation significantly decreased calmodulin binding to CT1 (61%) at high [Ca²⁺]. The phosphoresistant (CT1_SA) and phosphomimetic (CT1_SD) CT1 mutants, in which three PKA sites (Ser1574, 1626, 1699) were mutated to Ala and Asp, respectively, bound with calmodulin with 99% and 65% amount, respectively, compared to that of wild-type CT1. In contrast, at low [Ca²⁺], calmodulin-binding to CT1_SD was higher (33-35%) than that to CT1_SA. The distal C-terminal region of α1C (CT3, a.a. 1942-2169) is known to interact with CT1 and inhibit channel activity. CT3 bound to CT1_SD was also significantly less than that to CT1_SA. In inside-out patch, PKA catalytic subunit (PKAc) facilitated Ca²⁺ channel activity at both high and low Ca²⁺ condition. Altogether, these results support the hypothesis that PKA phosphorylation may enhance channel activity and attenuate the Ca²⁺-dependent inactivation, at least partially, by modulating calmodulin-CT1 interaction both directly and indirectly via CT3-CT1 interaction.

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

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

Layer- and Cell Type-Specific Modulation of Excitatory Neuronal Activity in the Neocortex

From an anatomical point of view the neocortex is subdivided into up to six layers depending on the cortical area. This subdivision has been described already by Meynert and Brodmann in the late 19/early 20. century and is mainly based on cytoarchitectonic features such as the size and location of the pyramidal cell bodies. Hence, cortical lamination is originally an anatomical concept based on the distribution of excitatory neuron. However, it has become apparent in recent years that apart from the layer-specific differences in morphological features, many functional properties of neurons are also dependent on cortical layer or cell type. Such functional differences include changes in neuronal excitability and synaptic activity by neuromodulatory transmitters. Many of these neuromodulators are released from axonal afferents from subcortical brain regions while others are released intrinsically. In this review we aim to describe layer- and cell-type specific differences in the effects of neuromodulator receptors in excitatory neurons in layers 2–6 of different cortical areas. We will focus on the neuromodulator systems using adenosine, acetylcholine, dopamine, and orexin/hypocretin as examples because these neuromodulator systems show important differences in receptor type and distribution, mode of release and functional mechanisms and effects. We try to summarize how layer- and cell type-specific neuromodulation may affect synaptic signaling in cortical microcircuits.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

Phosphorylation of Ser1928 mediates the enhanced activity of the L-type Ca2+ channel Cav1.2 by the β2-adrenergic receptor in neurons

The L-type Ca²⁺ channel Ca_v1.2 controls multiple functions throughout the body including heart rate and neuronal excitability. It is a key mediator of fight-or-flight stress responses triggered by a signaling pathway involving β-adrenergic receptors (βARs), cyclic adenosine monophosphate (cAMP), and protein kinase A (PKA). PKA readily phosphorylates Ser¹⁹²⁸ in Ca_v1.2 in vitro and in vivo, including in rodents and humans. However, S1928A knock-in (KI) mice have normal PKA-mediated L-type channel regulation in the heart, indicating that Ser¹⁹²⁸ is not required for regulation of cardiac Ca_v1.2 by PKA in this tissue. We report that augmentation of L-type currents by PKA in neurons was absent in S1928A KI mice. Furthermore, S1928A KI mice failed to induce long-term potentiation in response to prolonged theta-tetanus (PTT-LTP), a form of synaptic plasticity that requires Ca_v1.2 and enhancement of its activity by the β₂-adrenergic receptor (β₂AR)-cAMP-PKA cascade. Thus, there is an unexpected dichotomy in the control of Ca_v1.2 by PKA in cardiomyocytes and hippocampal neurons.

Decrease of high voltage Ca2+ currents in the dentate gyrus granule cells by entorhinal amyloidopathy is reversed by calcium channel blockade

In the Alzheimer's disease (AD), entorhinal-hippocampal circuit is one of the earliest affected networks. There are some evidences indicating abnormal neuronal excitability and impaired synaptic plasticity in the dentate gyrus (DG) of AD animal model. However, the underlying mechanism leading to DG dysfunction particularly in the early phase of AD is not known. Since calcium dyshomeostasis has a critical role in the etiology of AD, it is possible that this phenomenon precedes electrophysiological alteration in the DG. Here, the effect of the amyloid pathogenesis in the entorhinal cortex (EC) on high activated Ca²⁺ currents in the DG granule cells was investigated. One week after bilaterally injection of amyloid beta (Aβ) 1-42 into the EC, Ca²⁺ currents in the DG granule cells were assessed by whole cell patch clamp. Voltage clamp recording showed the amplitude of high voltage calcium currents in the DG granule cells was decreased following EC amyloidopathy. However, the Ca²⁺ current decay was slower than control. Double-pulse recording revealed that Ca²⁺-dependent inactivation of calcium current (CDI) was more pronounced in the EC-Aβ group compared to the control group. However, chronic treatment by calcium channel blocker (CCBs), isradipine or nimodipine, reverse the Ca²⁺ currents toward the control level. On the other hand, there was no significant difference in the calbindin level in the DG of different groups. In conclusion, our results suggest that Aβ in the EC independent of calbindin level triggers a decreased Ca²⁺ currents along with increased CDI in the DG granule cells which may lead to further electrophysiological alterations in these cells, and treatment by CCBs could preserve normal calcium current and may ultimately normal function against the Aβ toxicity.

Loss of β-adrenergic-stimulated phosphorylation of CaV1.2 channels on Ser1700 leads to heart failure

L-type Ca²⁺ currents conducted by voltage-gated calcium channel 1.2 (Ca_V1.2) initiate excitation-contraction coupling in the heart, and altered expression of Ca_V1.2 causes heart failure in mice. Here we show unexpectedly that reducing β-adrenergic regulation of Ca_V1.2 channels by mutation of a single PKA site, Ser1700, in the proximal C-terminal domain causes reduced contractile function, cardiac hypertrophy, and heart failure without changes in expression, localization, or function of the Ca_V1.2 protein in the mutant mice (SA mice). These deficits were aggravated with aging. Dual mutation of Ser1700 and a nearby casein-kinase II site (Thr1704) caused accelerated hypertrophy, heart failure, and death in mice with these mutations (STAA mice). Cardiac hypertrophy was increased by voluntary exercise and by persistent β-adrenergic stimulation. PKA expression was increased, and PKA sites Ser2808 in ryanodine receptor type-2, Ser16 in phospholamban, and Ser23/24 in troponin-I were hyperphosphorylated in SA mice, whereas phosphorylation of substrates for calcium/calmodulin-dependent protein kinase II was unchanged. The Ca²⁺ pool in the sarcoplasmic reticulum was increased, the activity of calcineurin was elevated, and calcineurin inhibitors improved contractility and ameliorated cardiac hypertrophy. Cardio-specific expression of the SA mutation also caused reduced contractility and hypertrophy. These results suggest engagement of compensatory mechanisms, which initially may enhance the contractility of individual myocytes but eventually contribute to an increased sensitivity to cardiovascular stress and to heart failure in vivo. Our results demonstrate that normal regulation of Ca_V1.2 channels by phosphorylation of Ser1700 in cardiomyocytes is required for cardiovascular homeostasis and normal physiological regulation in vivo.

Clustering and Functional Coupling of Diverse Ion Channels and Signaling Proteins Revealed by Super-resolution STORM Microscopy in Neurons

The fidelity of neuronal signaling requires organization of signaling molecules into macromolecular complexes, whose components are in intimate proximity. The intrinsic diffraction limit of light makes visualization of individual signaling complexes using visible light extremely difficult. However, using super-resolution stochastic optical reconstruction microscopy (STORM), we observed intimate association of individual molecules within signaling complexes containing ion channels (M-type K⁺, L-type Ca²⁺, or TRPV1 channels) and G protein-coupled receptors coupled by the scaffolding protein A-kinase-anchoring protein (AKAP)79/150. Some channels assembled as multi-channel supercomplexes. Surprisingly, we identified novel layers of interplay within macromolecular complexes containing diverse channel types at the single-complex level in sensory neurons, dependent on AKAP79/150. Electrophysiological studies revealed that such ion channels are functionally coupled as well. Our findings illustrate the novel role of AKAP79/150 as a molecular coupler of different channels that conveys crosstalk between channel activities within single microdomains in tuning the physiological response of neurons.

The Emerging Roles of the Calcineurin-Nuclear Factor of Activated T-Lymphocytes Pathway in Nervous System Functions and Diseases

The ongoing epidemics of metabolic diseases and increase in the older population have increased the incidences of neurodegenerative diseases. Evidence from murine and cell line models has implicated calcineurin-nuclear factor of activated T-lymphocytes (NFAT) signaling pathway, a Ca²⁺/calmodulin-dependent major proinflammatory pathway, in the pathogenesis of these diseases. Neurotoxins such as amyloid-β, tau protein, and α-synuclein trigger abnormal calcineurin/NFAT signaling activities. Additionally increased activities of endogenous regulators of calcineurin like plasma membrane Ca²⁺-ATPase (PMCA) and regulator of calcineurin 1 (RCAN1) also cause neuronal and glial loss and related functional alterations, in neurodegenerative diseases, psychotic disorders, epilepsy, and traumatic brain and spinal cord injuries. Treatment with calcineurin/NFAT inhibitors induces some degree of neuroprotection and decreased reactive gliosis in the central and peripheral nervous system. In this paper, we summarize and discuss the current understanding of the roles of calcineurin/NFAT signaling in physiology and pathologies of the adult and developing nervous system, with an emphasis on recent reports and cutting-edge findings. Calcineurin/NFAT signaling is known for its critical roles in the developing and adult nervous system. Its role in physiological and pathological processes is still controversial. However, available data suggest that its beneficial and detrimental effects are context-dependent. In view of recent reports calcineurin/NFAT signaling is likely to serve as a potential therapeutic target for neurodegenerative diseases and conditions. This review further highlights the need to characterize better all factors determining the outcome of calcineurin/NFAT signaling in diseases and the downstream targets mediating the beneficial and detrimental effects.

Phosphorylation of Cav1.2 on S1928 uncouples the L-type Ca2+ channel from the β2 adrenergic receptor

Agonist-triggered downregulation of β-adrenergic receptors (ARs) constitutes vital negative feedback to prevent cellular overexcitation. Here, we report a novel downregulation of β2AR signaling highly specific for Cav1.2. We find that β2-AR binding to Cav1.2 residues 1923-1942 is required for β-adrenergic regulation of Cav1.2. Despite the prominence of PKA-mediated phosphorylation of Cav1.2 S1928 within the newly identified β2AR binding site, its physiological function has so far escaped identification. We show that phosphorylation of S1928 displaces the β2AR from Cav1.2 upon β-adrenergic stimulation rendering Cav1.2 refractory for several minutes from further β-adrenergic stimulation. This effect is lost in S1928A knock-in mice. Although AMPARs are clustered at postsynaptic sites like Cav1.2, β2AR association with and regulation of AMPARs do not show such dissociation. Accordingly, displacement of the β2AR from Cav1.2 is a uniquely specific desensitization mechanism of Cav1.2 regulation by highly localized β2AR/cAMP/PKA/S1928 signaling. The physiological implications of this mechanism are underscored by our finding that LTP induced by prolonged theta tetanus (PTT-LTP) depends on Cav1.2 and its regulation by channel-associated β2AR.

Regulation of different human NFAT isoforms by neuronal activity

Nuclear factor of activated T-cells (NFAT) is a family of transcription factors comprising four calcium-regulated members: NFATc1, NFATc2, NFATc3, and NFATc4. Upon activation by the calcium-dependent phosphatase calcineurin (CaN), NFATs translocate from cytosol to the nucleus and regulate their target genes, which in the nervous system are involved in axon growth, synaptic plasticity, and neuronal survival. We have shown previously that there are a number of different splice variants of NFAT genes expressed in the brain. Here, we studied the subcellular localizations and transactivation capacities of alternative human NFAT isoforms in rat primary cortical or hippocampal neurons in response to membrane depolarization and compared the induced transactivation levels in neurons to those obtained from HEK293 cells in response to calcium signaling. We confirm that in neurons the translocation to the nucleus of all NFAT isoforms is reliant on the activity of CaN. However, our results suggest that both the regulation of subcellular localization and transcriptional activity of NFAT proteins in neurons is isoform specific. We show that in primary hippocampal neurons NFATc2 isoforms have very fast translocation kinetics, whereas NFATc4 isoforms translocate relatively slowly to the nucleus. Moreover, we demonstrate that the strongest transcriptional activators in HEK293 cells are NFATc1 and NFATc3, but in neurons NFATc3 and NFATc4 lead to the highest induction, and NFATc2 and NFATc1 display isoform-specific transcription activation capacities. Altogether, our results indicate that the effects of calcium signaling on the action of NFAT proteins are isoform-specific and can differ between cell types. We show that the effects of calcium signaling on the action of NFAT proteins are isoform-specific and differ between cell types. Although nuclear localization of all NFAT isoforms in neurons requires calcineurin, the subcellular distributions, neuronal activity-induced nuclear translocation extent and kinetics, and transcription activation capacities of alternative NFAT proteins vary.

Involvement of calcium/calmodulin-dependent protein kinase II in methamphetamine-induced neural damage

Methamphetamine (METH), an illicit drug, is widely abused in many parts of the world. Mounting evidence shows that METH exposure contributes to neurotoxicity, particularly for the monoaminergic neurons. However, to date, only a few studies have tried to unravel the mechanisms involved in METH-induced non-monoaminergic neural damage. Therefore, in the present study, we tried to explore the mechanisms for METH-induced neural damage in cortical neurons. Our results showed that METH significantly increased intracellular [Ca(2) (+) ]i in Ca(2) (+) -containing solution rather than Ca(2) (+) -free solution. Moreover, METH also upregulated calmodulin (CaM) expression and activated CaM-dependent protein kinase II (CaMKII). Significantly, METH-induced neural damage can be partially retarded by CaM antagonist W7 as well as CaMKII blocker KN93. In addition, L-type Ca(2) (+) channel was also proved to be involved in METH-induced cell damage, as nifedipine, the L-type Ca(2) (+) channel-specific inhibitor, markedly attenuated METH-induced neural damage. Collectively, our results suggest that Ca(2) (+) -CaM-CaMKII is involved in METH-mediated neurotoxicity, and it might suggest a potential target for the development of therapeutic strategies for METH abuse. Copyright © 2016 John Wiley & Sons, Ltd.

Coordination of Protein Phosphorylation and Dephosphorylation in Synaptic Plasticity

A central theme in nervous system function is equilibrium: synaptic strengths wax and wane, neuronal firing rates adjust up and down, and neural circuits balance excitation with inhibition. This push/pull regulatory theme carries through to the molecular level at excitatory synapses, where protein function is controlled through phosphorylation and dephosphorylation by kinases and phosphatases. However, these opposing enzymatic activities are only part of the equation as scaffolding interactions and assembly of multi-protein complexes are further required for efficient, localized synaptic signaling. This review will focus on coordination of postsynaptic serine/threonine kinase and phosphatase signaling by scaffold proteins during synaptic plasticity.

AKAP5 keeps L-type channels and NFAT on their toes

In this issue of Cell Reports, Murphy et al. and Dittmer et al. present exciting new insight into the regulation of Ca2+ influx via the L-type Ca2+ channel Cav1.2 and how increased Ca2+ influx translates into localized activation of the nuclear transcription factor NFAT upon depolarization in neurons.

Mechanisms and dynamics of AKAP79/150-orchestrated multi-protein signalling complexes in brain and peripheral nerve

A-kinase anchoring proteins (AKAPs) have emerged as a converging point of diverse signals to achieve spatiotemporal resolution of directed cellular regulation. With the extensive studies of AKAP79/150 in regulation of ion channel activity, the major questions to be posed centre on the mechanism and functional role of synergistic regulation of ion channels by such signalling proteins. In this review, we summarize recent discoveries of AKAP79/150-mediated modulation of voltage-gated neuronal M-type (KCNQ, Kv7) K(+) channels and L-type CaV 1 Ca(2+) channels, on both short- and longer-term time scales, highlighting the dynamics of the macromolecular signalling complexes in brain and peripheral nerve We also discuss several models for the possible mechanisms of these multi-protein assemblies and how they serve the agenda of the neurons in which they occur.

Tonic 5nM DA stabilizes neuronal output by enabling bidirectional activity-dependent regulation of the hyperpolarization activated current via PKA and calcineurin

Volume transmission results in phasic and tonic modulatory signals. The actions of tonic dopamine (DA) at type 1 DA receptors (D1Rs) are largely undefined. Here we show that tonic 5nM DA acts at D1Rs to stabilize neuronal output over minutes by enabling activity-dependent regulation of the hyperpolarization activated current (I _h). In the presence but not absence of 5nM DA, I _h maximal conductance (G _max) was adjusted according to changes in slow wave activity in order to maintain spike timing. Our study on the lateral pyloric neuron (LP), which undergoes rhythmic oscillations in membrane potential with depolarized plateaus, demonstrated that incremental, bi-directional changes in plateau duration produced corresponding alterations in LP I _hG _max when preparations were superfused with saline containing 5nM DA. However, when preparations were superfused with saline alone there was no linear correlation between LP I _hG_max and duty cycle. Thus, tonic nM DA modulated the capacity for activity to modulate LP I _h G _max; this exemplifies metamodulation (modulation of modulation). Pretreatment with the Ca2+-chelator, BAPTA, or the specific PKA inhibitor, PKI, prevented all changes in LP I _h in 5nM DA. Calcineurin inhibitors blocked activity-dependent changes enabled by DA and revealed a PKA-mediated, activity-independent enhancement of LP I _hG _max. These data suggested that tonic 5nM DA produced two simultaneous, PKA-dependent effects: a direct increase in LP I _h G _max and a priming event that permitted calcineurin regulation of LP I _h. The latter produced graded reductions in LP I _hG _max with increasing duty cycles. We also demonstrated that this metamodulation preserved the timing of LP’s first spike when network output was perturbed with bath-applied 4AP. In sum, 5nM DA permits slow wave activity to provide feedback that maintains spike timing, suggesting that one function of low-level, tonic modulation is to stabilize specific features of a dynamic output.

Calcium-dependent inactivation of calcium channels in the medial striatum increases at eye opening

Influx of calcium through voltage-gated calcium channels (VGCCs) is essential for striatal function and plasticity. VGCCs expressed in striatal neurons have varying kinetics, voltage dependences, and densities resulting in heterogeneous subcellular calcium dynamics. One factor that determines the calcium dynamics in striatal medium spiny neurons is inactivation of VGCCs. Aside from voltage-dependent inactivation, VGCCs undergo calcium-dependent inactivation (CDI): inactivating in response to an influx of calcium. CDI is a negative feedback control mechanism; however, its contribution to striatal neuron function is unknown. Furthermore, although the density of VGCC expression changes with development, it is unclear whether CDI changes with development. Because calcium influx through L-type calcium channels is required for striatal synaptic depression, a change in CDI could contribute to age-dependent changes in striatal synaptic plasticity. Here we use whole cell voltage clamp to characterize CDI over developmental stages and across striatal regions. We find that CDI increases at the age of eye opening in the medial striatum but not the lateral striatum. The developmental increase in CDI mostly involves L-type channels, although calcium influx through non-L-type channels contributes to the CDI in both age groups. Agents that enhance protein kinase A (PKA) phosphorylation of calcium channels reduce the magnitude of CDI after eye opening, suggesting that the developmental increase in CDI may be related to a reduction in the phosphorylation state of the L-type calcium channel. These results are the first to show that modifications in striatal neuron properties correlate with changes to sensory input.

More than a pore: ion channel signaling complexes

Voltage- and ligand-gated ion channels form the molecular basis of cellular excitability. With >400 members and accounting for ∼1.5% of the human genome, ion channels are some of the most well studied of all proteins in heterologous expression systems. Yet, ion channels often exhibit unexpected properties in vivo because of their interaction with a variety of signaling/scaffolding proteins. Such interactions can influence the function and localization of ion channels, as well as their coupling to intracellular second messengers and pathways, thus increasing the signaling potential of these ion channels in neurons. Moreover, functions have been ascribed to ion channels that are largely independent of their ion-conducting roles. Molecular and functional dissection of the ion channel proteome/interactome has yielded new insights into the composition of ion channel complexes and how their dysregulation leads to human disease.

A CACNA1C variant associated with reduced voltage-dependent inactivation, increased CaV1.2 channel window current, and arrhythmogenesis

Mutations in CACNA1C that increase current through the Ca_V1.2 L-type Ca²⁺ channel underlie rare forms of long QT syndrome (LQTS), and Timothy syndrome (TS). We identified a variant in CACNA1C in a male child of Filipino descent with arrhythmias and extracardiac features by candidate gene sequencing and performed functional expression studies to electrophysiologically characterize the effects of the variant on Ca_V1.2 channels. As a baby, the subject developed seizures and displayed developmental delays at 30 months of age. At age 5 years, he displayed a QTc of 520 ms and experienced recurrent VT. Physical exam at 17 years of age was notable for microcephaly, short stature, lower extremity weakness and atrophy with hyperreflexia, spastic diplegia, multiple dental caries and episodes of rhabdomyolysis. Candidate gene sequencing identified a G>C transversion at position 5731 of CACNA1C (rs374528680) predicting a glycine>arginine substitution at residue 1911 (p.G1911R) of Ca_V1.2. The allele frequency of this variant is 0.01 in Malays, but absent in 984 Caucasian alleles and in the 1000 genomes project. In electrophysiological analyses, the variant decreased voltage-dependent inactivation, thus causing a gain of function of Ca_V1.2. We also observed a negative shift of V_1/2 of activation and positive shift of V_1/2 of channel inactivation, resulting in an increase of the window current. Together, these suggest a gain-of-function effect on Ca_V1.2 and suggest increased susceptibility for arrhythmias in certain clinical settings. The p.G1911R variant was also identified in a case of sudden unexplained infant death (SUID), for which an increasing number of clinical observations have demonstrated can be associated with arrhythmogenic mutations in cardiac ion channels. In summary, the combined effects of the CACNA1C variant to diminish voltage-dependent inactivation of Ca_V1.2 and increase window current expand our appreciation of mechanisms by which a gain of function of Ca_V1.2 can contribute to QT prolongation.

AKAP-anchored PKA maintains neuronal L-type calcium channel activity and NFAT transcriptional signaling

L-type voltage-gated Ca2+ channels (LTCC) couple neuronal excitation to gene transcription. LTCC activity is elevated by the cyclic AMP (cAMP)-dependent protein kinase (PKA) and depressed by the Ca2+-dependent phosphatase calcineurin (CaN), and both enzymes are localized to the channel by A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 anchoring of CaN also promotes LTCC activation of transcription through dephosphorylation of the nuclear factor of activated T cells (NFAT). We report here that the basal activity of AKAP79/150-anchored PKA maintains neuronal LTCC coupling to CaN-NFAT signaling by preserving LTCC phosphorylation in opposition to anchored CaN. Genetic disruption of AKAP-PKA anchoring promoted redistribution of the kinase out of postsynaptic dendritic spines, profound decreases in LTCC phosphorylation and Ca2+ influx, and impaired NFAT movement to the nucleus and activation of transcription. Thus, LTCC-NFAT transcriptional signaling in neurons requires precise organization and balancing of PKA and CaN activities in the channel nanoenvironment, which is only made possible by AKAP79/150 scaffolding.