AKAP signaling complexes in regulation of excitatory synaptic plasticity

Subchronic oral administration of Benzo[a]pyrene impairs motor and cognitive behavior and modulates S100B levels and MAPKs in rats

Benzo[a]pyrene (BaP) is an environmental contaminant produced during incomplete combustion of organic material that is well known as a mutagenic and carcinogenic toxin. There are few studies addressing the molecular and cellular basis of behavioural alterations related to BaP exposure. The aim of this study was to evaluate the effect of subchronic oral administration of BaP on behavioral and neurochemical parameters. Wistar male rats received BaP (2 mg/kg) or corn oil (control), once a day for 28 days (n = 12/group). Spontaneous locomotor activity and short- and long-term memories were evaluated. Glial fibrillary acid protein and S100B content in the hippocampus, serum and CSF were measured using ELISA and total and phosphorylated forms of mitogen activated protein kinases (MAPKs) named extracellular signal-regulated kinases 1 and 2, p38(MAPK) and c-Jun amino-terminal kinases 1 and 2, in the hippocampus, were evaluated by western blotting. BaP induced a significant increase on locomotor activity and a decrease in short-term memory. S100B content was increased significantly in cerebrospinal fluid. BaP induced a decrease on ERK2 phosphorylation in the hippocampus. Thus, BaP subchronic treatment induces an astroglial response and impairs both motor and cognitive behavior, with parallel inhibition of ERK2, a signaling enzyme involved in the hippocampal neuroplasticity. All these effects suggest that BaP neurotoxicity is a concern for environmental pollution.

Early stress evokes temporally distinct consequences on the hippocampal transcriptome, anxiety and cognitive behaviour

The early stress of maternal separation (ES) exerts long-lasting effects on cognition and anxiety. Recent evidence indicates enhanced hippocampus-dependent spatial learning in young adult ES animals, which shifts towards a decline in long-term memory in middle-aged life. Further, we find that ES animals exhibit enhanced anxiety in young adulthood that does not persist into middle-aged life. Here, we demonstrate unique, predominantly non-overlapping, hippocampal transcriptomes in young adult and middle-aged ES animals that accompany the temporally-specific behavioural consequences. Strikingly, the extent of gene dysregulation in middle-aged ES animals was substantially higher than in young adulthood. Functional analysis revealed distinct biological processes enriched at the two ages, highlighting the temporal shift in ES-evoked gene regulation. Our results suggest that ES history interacts with aging to exacerbate age-associated transcriptional changes and cognitive decline. qPCR profiling of histone deacetylases (Hdacs) and histone methyltransferases (HMTs) revealed an age-dependent, opposing regulation with decreased expression noted in young adult ES animals (Hdac 2, 7, 8, 9 and Suv39h1) and enhanced levels in middle-aged life (Hdac 2, 6, 8 and Suv39h1). While altered expression of histone modifying enzymes did not translate into global histone acetylation or methylation changes, we noted differential enrichment of histone acetylation and methylation modifications at the promoters of multiple genes regulated in the hippocampi of young adult and middle-aged ES animals. Our results highlight the differential molecular and behavioural consequences of ES across a life-span, and suggest a possible role for epigenetic mechanisms in contributing to the temporally-specific transcriptional changes following ES.

Genetically encoded fluorescent biosensors for live-cell visualization of protein phosphorylation

Fluorescence-based, genetically encodable biosensors are widely used tools for real-time analysis of biological processes. Over the last few decades, the number of available genetically encodable biosensors and the types of processes they can monitor have increased rapidly. Here, we aim to introduce the reader to general principles and practices in biosensor development and highlight ways in which biosensors can be used to illuminate outstanding questions of biological function. Specifically, we focus on sensors developed for monitoring kinase activity and use them to illustrate some common considerations for biosensor design. We describe several uses to which kinase and second-messenger biosensors have been put, and conclude with considerations for the use of biosensors once they are developed. Overall, as fluorescence-based biosensors continue to diversify and improve, we expect them to continue to be widely used as reliable and fruitful tools for gaining deeper insights into cellular and organismal function.

Direct association of the unique C-terminal tail of transmembrane AMPA receptor regulatory protein γ-8 with calcineurin

Transmembrane α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor regulatory proteins (TARPs) are auxiliary subunits that regulate AMPA receptor trafficking to the plasma membrane and localization to postsynaptic sites. The classical TARP family consists of four members: stargazin/γ-2, γ-3, γ-4 and γ-8. The TARP γ-8 isoform, which is highly expressed in the hippocampus, has a unique, long C-terminal domain with five distinct regions: two glycine-rich regions, a serine/arginine-rich region, a proline/alanine (P/A) rich region, and a PSD-95/Dlg/ZO-1 (PDZ) binding motif. We performed mass spectrometry and immunoprecipitation assays to identify specific binding partners for the γ-8 C-terminal tail and found that γ-8, but not stargazin/γ-2, co-immunoprecipitated with calcineurin/PP2B, a Ca(2+) /calmodulin-dependent Ser/Thr phosphatase. Co-immunoprecipitation and immunoblot analyses of lysates from COS-7 cells co-transfected with calcineurin and either wild type or chimeric γ-8 revealed that a section of the C-terminal tail (residues 356-421) can bind calcineurin. Futhermore, γ-8 lacking the P/A-rich region (residues 383-399) did not bind to calcineurin. In addition, the GST-γ-8 C-terminal tail (residues 353-414) fusion protein containing the P/A-rich region bound to purified calcineurin in a Ca(2+) /calmodulin-dependent manner, whereas GST-γ-8 with a deletion of the P/A-rich region did not. Peptide competition assays demonstrated that γ-8 may interact with the hydrophobic pocket defined by β-sheet 14 and/or adjacent regions of the catalytic A subunit of calcineurin. These results indicate that the γ-8 P/A-rich region is essential for binding calcineurin, suggesting that the γ-8/calcineurin complex may regulate AMPA receptor phosphorylation and trafficking.

AKAPS act in a two-step mechanism of memory acquisition

Defining the molecular and neuronal basis of associative memories is based upon behavioral preparations that yield high performance due to selection of salient stimuli, strong reinforcement, and repeated conditioning trials. One of those preparations is the Drosophila aversive olfactory conditioning procedure where animals initiate multiple memory components after experience of a single cycle training procedure. Here, we explored the analysis of acquisition dynamics as a means to define memory components and revealed strong correlations between particular chronologies of shock impact and number experienced during the associative training situation and subsequent performance of conditioned avoidance. Analyzing acquisition dynamics in Drosophila memory mutants revealed that rutabaga (rut)-dependent cAMP signals couple in a divergent fashion for support of different memory components. In case of anesthesia-sensitive memory (ASM) we identified a characteristic two-step mechanism that links rut-AC1 to A-kinase anchoring proteins (AKAP)-sequestered protein kinase A at the level of Kenyon cells, a recognized center of olfactory learning within the fly brain. We propose that integration of rut-derived cAMP signals at level of AKAPs might serve as counting register that accounts for the two-step mechanism of ASM acquisition.

Subunit-specific trafficking mechanisms regulating the synaptic expression of Ca(2+)-permeable AMPA receptors

AMPA receptors are the main excitatory neurotransmitter receptor in the brain, and hence regulating the number or properties of synaptic AMPA receptors brings about critical changes in synaptic transmission. Synaptic plasticity is thought to underlie learning and memory, and can be brought about by decreasing or increasing the number of AMPA receptors localised to synaptic sites by precisely regulating AMPA receptor trafficking. AMPA receptors are tetrameric assemblies of subunits GluA1-4, and the vast majority are GluA1/2 and GluA2/3 heteromers. The inclusion of GluA2 subunit is critical because it renders the AMPA receptor channel impermeable to Ca(2+) ions. The vast majority of synaptic AMPA receptors in the brain contain GluA2, but relatively recent discoveries indicate that an increasing number of specific forms of synaptic plasticity involve not only an alteration of the number of synaptic AMPA receptors, but also changes to their GluA2 content. The resulting change in AMPA receptor Ca(2+) permeability clearly has profound consequences for synaptic transmission and intracellular signalling events. The subunit-specific trafficking mechanisms that cause such changes represent an emerging field of research with implications for an increasing number of physiological or pathological situations, and are the topic of this review.

Cell biology in neuroscience: the interplay between Hebbian and homeostatic synaptic plasticity

Synaptic plasticity, a change in the efficacy of synaptic signaling, is a key property of synaptic communication that is vital to many brain functions. Hebbian forms of long-lasting synaptic plasticity-long-term potentiation (LTP) and long-term depression (LTD)-have been well studied and are considered to be the cellular basis for particular types of memory. Recently, homeostatic synaptic plasticity, a compensatory form of synaptic strength change, has attracted attention as a cellular mechanism that counteracts changes brought about by LTP and LTD to help stabilize neuronal network activity. New findings on the cellular mechanisms and molecular players of the two forms of plasticity are uncovering the interplay between them in individual neurons.

Scaffold state switching amplifies, accelerates, and insulates protein kinase C signaling

Scaffold proteins localize two or more signaling enzymes in close proximity to their downstream effectors. A-kinase-anchoring proteins (AKAPs) are a canonical family of scaffold proteins known to bind protein kinase A (PKA) and other enzymes. Several AKAPs have been shown to accelerate, amplify, and specify signal transduction to dynamically regulate numerous cellular processes. However, there is little theory available to mechanistically explain how signaling on protein scaffolds differs from solution biochemistry. In our present study, we propose a novel kinetic mechanism for enzymatic reactions on protein scaffolds to explain these phenomena, wherein the enzyme-substrate-scaffold complex undergoes stochastic state switching to reach an active state. This model predicted anchored enzymatic reactions to be accelerated, amplified, and insulated from inhibition compared with those occurring in solution. We exploited a direct interaction between protein kinase C (PKC) and AKAP7α as a model to validate these predictions experimentally. Using a genetically encoded PKC activity reporter, we found that both the strength and speed of substrate phosphorylation were enhanced by AKAP7α. PKC tethered to AKAP7α was less susceptible to inhibition from the ATP-competitive inhibitor Gö6976 and the substrate-competitive inhibitor PKC 20-28, but not the activation-competitive inhibitor calphostin C. Model predictions and experimental validation demonstrated that insulation is a general property of scaffold tethering. Sensitivity analysis indicated that these findings may be applicable to many other scaffolds as well. Collectively, our findings provide theoretical and experimental evidence that scaffold proteins can amplify, accelerate, and insulate signal transduction.

AKAP signaling complexes: pointing towards the next generation of therapeutic targets?

A-kinase anchoring proteins (AKAPs) streamline signal transduction by localizing signaling enzymes with their substrates. Great strides have been made in elucidating the role of these macromolecular signaling complexes as new binding partners and novel AKAPs are continually being uncovered. The mechanics and dynamics of these multi-enzyme assemblies suggest that AKAP complexes are viable targets for therapeutic intervention. This review will highlight recent advances in AKAP research focusing on local signaling events that are perturbed in disease.

Synaptic NMDA receptor stimulation activates PP1 by inhibiting its phosphorylation by Cdk5

Synaptic stimulation promotes proteasome-dependent degradation of p35, inactivation of Cdk5, and decreased phosphorylation of PP1, allowing PP1 to act in the induction of long-term depression.

The serine/threonine protein phosphatase protein phosphatase 1 (PP1) is known to play an important role in learning and memory by mediating local and downstream aspects of synaptic signaling, but how PP1 activity is controlled in different forms of synaptic plasticity remains unknown. We find that synaptic N-methyl-d-aspartate (NMDA) receptor stimulation in neurons leads to activation of PP1 through a mechanism involving inhibitory phosphorylation at Thr320 by Cdk5. Synaptic stimulation led to proteasome-dependent degradation of the Cdk5 regulator p35, inactivation of Cdk5, and increased auto-dephosphorylation of Thr320 of PP1. We also found that neither inhibitor-1 nor calcineurin were involved in the control of PP1 activity in response to synaptic NMDA receptor stimulation. Rather, the PP1 regulatory protein, inhibitor-2, formed a complex with PP1 that was controlled by synaptic stimulation. Finally, we found that inhibitor-2 was critical for the induction of long-term depression in primary neurons. Our work fills a major gap regarding the regulation of PP1 in synaptic plasticity.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Mapping the binding site of TRPV1 on AKAP79: implications for inflammatory hyperalgesia

Inflammation causes hyperalgesia, an enhanced sensitivity to noxious stimuli. Transient receptor potential vanilloid 1 (TRPV1), a thermo-TRP ion channel activated by painful levels of heat, is an important contributor because hyperalgesia is reduced when TRPV1 is either genetically deleted or pharmacologically blocked. Inflammatory mediators such as prostaglandin-E2 or bradykinin cause hyperalgesia by activating cellular kinases that phosphorylate TRPV1, a process that has recently been shown to rely on a scaffolding protein, AKAP79, to target the kinases to TRPV1. Here we use Förster resonance energy transfer, immunoprecipitation, and TRPV1 membrane trafficking experiments to identify a key region on AKAP79, between amino acids 326-336, which is responsible for its interaction with TRPV1. A peptide identical to this domain inhibited sensitization of TRPV1 in vitro, and when covalently linked to a TAT peptide to promote uptake across the cell membrane the peptide inhibited in vivo inflammatory hyperalgesia in mice. Critically, it did so without affecting pain thresholds in the absence of inflammation. These results suggest that antagonizing the TRPV1-AKAP79 interaction will be a useful strategy for inhibiting inflammatory hyperalgesia.

Distinct set of kinases induced after retrieval of spatial memory discriminate memory modulation processes in the mouse hippocampus

Protein phosphorylation and dephosphorylation events play a key role in memory formation and various protein kinases and phosphatases have been firmly associated with memory performance. Here, we determined expression changes of protein kinases and phosphatases following retrieval of spatial memory in CD1 mice in a Morris Water Maze task, using antibody microarrays and confirmatory Western blot. Comparing changes following single and consecutive retrieval, we identified stably and differentially expressed kinases, some of which have never been implicated before in memory functions. On the basis of these findings we define a small signaling network associated with spatial memory retrieval. Moreover, we describe differential regulation and correlation of expression levels with behavioral performance of polo-like kinase 1. Together with its recently observed genetic association to autism-spectrum disorders our data suggest a role of this kinase in balancing preservation and flexibility of learned behavior.

A role for hypocretin/orexin receptor-1 in cue-induced reinstatement of nicotine-seeking behavior

Hypocretin/orexin signaling is critically involved in relapse to drug-seeking behaviors. In this study, we investigated the involvement of the hypocretin system in the reinstatement of nicotine-seeking behavior induced by nicotine-associated cues. Pretreatment with the hypocretin receptor-1 antagonist SB334867, but not with the hypocretin receptor-2 antagonist TCSOX229, attenuated cue-induced reinstatement of nicotine-seeking, which was associated with an activation of hypocretin neurons of the lateral and perifornical hypothalamic areas. In addition, relapse to nicotine-seeking increased the phosphorylation levels of GluR2-Ser880, NR1-Ser890, and p38 MAPK in the nucleus accumbens (NAc), but not in the prefrontal cortex. Notably, phosphorylation levels of NR1-Ser890 and p38 MAPK, but not GluR2-Ser880, were dependent on hypocretin receptor-1 activation. The intra-accumbens infusion of the protein kinase C (PKC) inhibitor NPC-15437 reduced nicotine-seeking behavior elicited by drug-paired cues consistent with the PKC-dependent phosphorylations of GluR2-Ser880 and NR1-Ser890. SB334867 failed to modify cue-induced reinstatement of food-seeking, which did not produce any biochemical changes in the NAc. These data identify hypocretin receptor-1 and PKC signaling as potential targets for the treatment of relapse to nicotine-seeking induced by nicotine-associated cues.

Phosphorylation of FMRP and alterations of FMRP complex underlie enhanced mLTD in adult rats triggered by early life seizures

Outside of Fragile X syndrome (FXS), the role of Fragile-X Mental Retardation Protein (FMRP) in mediating neuropsychological abnormalities is not clear. FMRP, p70-S6 kinase (S6K) and protein phosphatase 2A (PP2A) are thought to cooperate as a dynamic signaling complex. In our prior work, adult rats have enhanced CA1 hippocampal long-term depression (LTD) following an early life seizure (ELS). We now show that mGluR-mediated LTD (mLTD) is specifically enhanced following ELS, similar to FMRP knock-outs. Total FMRP expression is unchanged but S6K is hyperphosphorylated, consistent with S6K overactivation. We postulated that either disruption of the FMRP-S6K-PP2A complex and/or removal of this complex from synapses could explain our findings. Using subcellular fractionation, we were surprised to find that concentrations of FMRP and PP2A were undisturbed in the synaptosomal compartment but reduced in parallel in the cytosolic compartment. Following ELS FMRP phosphorylation was reduced in the cytosolic compartment and increased in the synaptic compartment, in parallel with the compartmentalization of S6K activation. Furthermore, FMRP and PP2A remain bound following ELS. In contrast, the interaction of S6K with FMRP is reduced by ELS. Blockade of PP2A results in enhanced mLTD; this is occluded by ELS. This suggests a critical role for the location and function of the FMRP-S6K-PP2A signaling complex in limiting the amount of mLTD. Specifically, non-synaptic targeting and the function of the complex may influence the "set-point" for regulating mLTD. Consistent with this, striatal-enriched protein tyrosine phosphatase (STEP), an FMRP "target" which regulates mLTD expression, is specifically increased in the synaptosomal compartment following ELS. Further, we provide behavioral data to suggest that FMRP complex dysfunction may underlie altered socialization, a symptom associated and observed in other rodent models of autism, including FXS.

Early antidepressant efficacy modulation by glutamatergic gene variants in the STAR*D

The glutamatergic system has been suggested as a modulator of rapid antidepressant response. Thus, 44 glutamatergic genes were selected according to the literature and investigated in 1541 major depressive patients from the STAR*D genome wide dataset. Outcomes of interest were early response (2nd week) and late response (from the 4th to the 14th week) compared to non-response and stability of response through the STAR*D level 1. A complete agglomerative clustering, based on pairwise identity-by-state (IBS) matrix, was applied in order to control for genetic admixture. A chi-square test was employed as exploratory analysis and a logistic regression was employed to corroborate SNPs associated to the outcomes at p<0.001. Covariates were selected accordingly to their impact on phenotypes. A Bonferroni correction was applied. PLINK served for the analysis. About 1995 SNPs were available after quality control. Our results suggested that the rs1083801 within the GRM7 (glutamate receptor, metabotropic 7) gene was associated to early response under a recessive model (GG genotype observed in 14.34% of early responders vs 5.25% of late responders, OR=0.33, 95% CI=0.21-0.54, p=6.41e-06. GG genotype observed in 5.34% of non-responders, OR=0.33, 95% CI=0.20-0.56, p=4.07e-05). The result was confirmed in the white non-Hispanic group (GG genotype observed in 17.46% of early responders vs 5.81% of the rest of the sample, OR=0.29, 95% CI=0.18-0.46, p=2.04e-07). No marker predicted the stability of response. Glutamatergic genes may be useful markers of early antidepressant efficacy. This result may be relevant in further understanding the pathophysiology of the drug induced antidepressant effect.

Disrupting sensitization of transient receptor potential vanilloid subtype 1 inhibits inflammatory hyperalgesia

Transient receptor potential vanilloid subtype 1 (TRPV1) is a heat-sensitive ion channel that plays a key role in enhanced pain sensation after inflammation, but directly blocking TRPV1 causes hyperthermia and decreased sensitivity to painful levels of heat in animals and humans. Here we explore an alternative analgesic strategy in which the modulation of TRPV1 is inhibited by antagonizing the interaction between TRPV1 and A kinase anchoring protein 79 (AKAP79), a scaffolding protein essential for positioning serine-threonine kinases adjacent to target phosphorylation sites. We first defined key residues in the domain in TRPV1 that interacts with AKAP79, and we then used this information to construct short peptides capable of preventing TRPV1-AKAP79 interaction. An effective peptide, when coupled to a TAT sequence conferring cell permeability, was found to be analgesic in three mouse models of inflammatory hyperalgesia. These results demonstrate the potential value of interfering with the interaction between TRPV1 and AKAP79 as a novel analgesic strategy.

Increased density of AKAP5-expressing neurons in the anterior cingulate cortex of subjects with bipolar disorder

Brain anatomical abnormalities as well as cognitive and emotional processing deficits have been reported for the prefrontal cortex in bipolar disorder, which are in part attributable to cellular and laminar abnormalities in postsynaptic protein expression. A kinase anchoring protein (AKAP) 5/79 plays a key role in postsynaptic signalling of excitatory synapses. We aimed to reveal if the cellular expression of AKAP5/79 protein is altered in the anterior cingulate cortex and the dorsolateral prefrontal cortex in bipolar disorder. Ten subjects with bipolar disorder and ten control cases were investigated by use of immunohistochemical and morphometric techniques. Compared with controls in subjects with bipolar disorder, the numerical density of AKAP5-expressing neurons was significantly increased in the left (p = 0.002) and right (p = 0.008) anterior cingulate cortex. Layer-specific counting revealed that left side layers II (p = 0.000), III (p = 0.001) and V (p = 0.005) as well as right side layers III (p = 0.007), IV (p = 0.007) and V (p = 0.004) had significantly increased AKAP5-positive cell densities in bipolar disorder. In contrast, no statistically significant differences were found for the dorsolateral prefrontal cortex. However, we observed a more intense intraneuronal immunostaining in both prefrontal areas in bipolar disorder patients. Elevated cell numbers and increased intracellular expression of AKAP, together with the altered expression patterns of most intracellular interaction partners of this protein in bipolar disorder as known from the literature, might point to disease-related abnormalities of the AKAP-associated signalosome in prefrontal cortex neurons.

Adaptations in AMPA receptor transmission in the nucleus accumbens contributing to incubation of cocaine craving

Cue-induced cocaine craving in rodents intensifies or "incubates" during the first months of withdrawal from long access cocaine self-administration. This incubation phenomenon is relevant to human users who achieve abstinence but exhibit persistent vulnerability to cue-induced relapse. It is well established that incubation of cocaine craving involves complex neuronal circuits. Here we will focus on neuroadaptations in the nucleus accumbens (NAc), a region of convergence for pathways that control cocaine seeking. A key adaptation is a delayed (~3-4 weeks) accumulation of Ca(2+)-permeable AMPAR receptors (CP-AMPARs) in synapses on medium spiny neurons (MSN) of the NAc. These CP-AMPARs mediate the expression of incubation after prolonged withdrawal, although different mechanisms must be responsible during the first weeks of withdrawal, prior to CP-AMPAR accumulation. The cascade of events leading to CP-AMPAR accumulation is still unclear. However, several candidate mechanisms have been identified. First, mGluR1 has been shown to negatively regulate CP-AMPAR levels in NAc synapses, and it is possible that a withdrawal-dependent decrease in this effect may help explain CP-AMPAR accumulation during incubation. Second, an increase in phosphorylation of GluA1 subunits (at the protein kinase A site) within extrasynaptic homomeric GluA1 receptors (CP-AMPARs) may promote their synaptic insertion and oppose their removal. Finally, elevation of brain-derived neurotrophic factor (BDNF) levels in the NAc may contribute to maintenance of incubation after months of withdrawal, although incubation-related increases in BDNF accumulation do not account for CP-AMPAR accumulation. Receptors and pathways that negatively regulate incubation, such as mGluR1, are promising targets for the development of therapeutic strategies to help recovering addicts maintain abstinence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Calcium, calpain, and calcineurin in low-frequency depression of transmitter release

Low-frequency depression (LFD) of transmitter release occurs at phasic synapses with stimulation at 0.2 Hz in both isolated crayfish (Procambarus clarkii) neuromuscular junction (NMJ) preparations and in intact animals. LFD is regulated by presynaptic activity of the Ca(2+)-dependent phosphatase calcineurin (Silverman-Gavrila and Charlton, 2009). Since the fast Ca(2+) chelator BAPTA-AM inhibits LFD but the slow chelator EGTA-AM does not, the Ca(2+) sensor for LFD may be close to a Ca(2+) source at active zones. Calcineurin can be activated by the Ca(2+)-activated protease calpain, and immunostaining showed that both proteins are present at nerve terminals. Three calpain inhibitors, calpain inhibitor I, MDL-28170, and PD150606, but not the control compound PD145305, inhibit LFD both in the intact animal as shown by electromyograms and by intracellular recordings at neuromuscular junctions. Analysis of mini-EPSPs indicated that these inhibitors had minimal postsynaptic effects. Proteolytic activity in CNS extract, detected by a fluorescent calpain substrate, was modulated by Ca(2+) and calpain inhibitors. Western blot analysis of CNS extract showed that proteolysis of calcineurin to a fragment consistent with the constitutively active form required Ca(2+) and was blocked by calpain inhibitors. Inhibition of LFD by calpain inhibition blocks the reduction in phosphoactin and the depolymerization of tubulin that normally occurs in LFD, probably by blocking the dephosphorylation of cytoskeletal proteins by calcineurin. In contrast, high-frequency depression does not involve protein phosphorylation- or calpain-dependent mechanisms. LFD may involve a specific pathway in which local Ca(2+) signaling activates presynaptic calpain and calcineurin at active zones and causes changes of tubulin cytoskeleton.

A-kinase anchoring protein-calcineurin signaling in long-term depression of GABAergic synapses

The postsynaptic scaffolding A-kinase anchoring protein 79/150 (AKAP79/150) signaling complex regulates excitatory synaptic transmission and strength through tethering protein kinase A (PKA), PKC, and calcineurin (CaN) to the postsynaptic densities of neurons (Sanderson and Dell'Acqua, 2011), but its role in inhibitory synaptic transmission and plasticity is unknown. Using immunofluorescence and whole-cell patch-clamp recording in rat midbrain slices, we show that activation of postsynaptic D(2)-like family of dopamine (DA) receptor in the ventral tegmental area (VTA) induces long-term depression (LTD) of GABAergic synapses on DA neurons through an inositol triphosphate receptor-mediated local rise in postsynaptic Ca(2+) and CaN activation accompanied by PKA inhibition, which requires AKAP150 as a bridging signaling molecule. Our data also illuminate a requirement for a clathrin-mediated internalization of GABA(A) receptors in expression of LTD(GABA). Moreover, disruption of AKAP-PKA anchoring does not affect glutamatergic synapses onto DA neurons, suggesting that the PKA-AKAP-CaN complex is uniquely situated at GABA(A) receptor synapses in VTA DA neurons to regulate plasticity associated with GABA(A) receptors. Drug-induced modulation of GABAergic plasticity in the VTA through such novel signaling mechanisms has the potential to persistently alter the output of individual DA neurons and of the VTA, which may contribute to the reinforcing or addictive properties of drugs of abuse.

Kinetic and mechanistic differences in the interactions between caldendrin and calmodulin with AKAP79 suggest different roles in synaptic function

The kinetic and mechanistic details of the interaction between caldendrin, calmodulin and the B-domain of AKAP79 were determined using a biosensor-based approach. Caldendrin was found to compete with calmodulin for binding at AKAP79, indicating overlapping binding sites. Although the AKAP79 affinities were similar for caldendrin (K(D) = 20 nM) and calmodulin (K(D) = 30 nM), their interaction characteristics were different. The calmodulin interaction was well described by a reversible one-step model, but was only detected in the presence of Ca(2+). Caldendrin interacted with a higher level of complexity, deduced to be an induced fit mechanism with a slow relaxation back to the initial encounter complex. It interacted with AKAP79 also in the absence of Ca(2+), but with different kinetic rate constants. The data are consistent with a similar initial Ca(2+)-dependent binding step for the two proteins. For caldendrin, a second Ca(2+)-independent rearrangement step follows, resulting in a stable complex. The study shows the importance of establishing the mechanism and kinetics of protein-protein interactions and that minor differences in the interaction of two homologous proteins can have major implications in their functional characteristics. These results are important for the further elucidation of the roles of caldendrin and calmodulin in synaptic function.

AKAP150-anchored calcineurin regulates synaptic plasticity by limiting synaptic incorporation of Ca2+-permeable AMPA receptors

AMPA receptors (AMPARs) are tetrameric ion channels assembled from GluA1-GluA4 subunits that mediate the majority of fast excitatory synaptic transmission in the brain. In the hippocampus, most synaptic AMPARs are composed of GluA1/2 or GluA2/3 with the GluA2 subunit preventing Ca(2+) influx. However, a small number of Ca(2+)-permeable GluA1 homomeric receptors reside in extrasynaptic locations where they can be rapidly recruited to synapses during synaptic plasticity. Phosphorylation of GluA1 S845 by the cAMP-dependent protein kinase (PKA) primes extrasynaptic receptors for synaptic insertion in response to NMDA receptor Ca(2+) signaling during long-term potentiation (LTP), while phosphatases dephosphorylate S845 and remove synaptic and extrasynaptic GluA1 during long-term depression (LTD). PKA and the Ca(2+)-activated phosphatase calcineurin (CaN) are targeted to GluA1 through binding to A-kinase anchoring protein 150 (AKAP150) in a complex with PSD-95, but we do not understand how the opposing activities of these enzymes are balanced to control plasticity. Here, we generated AKAP150ΔPIX knock-in mice to selectively disrupt CaN anchoring in vivo. We found that AKAP150ΔPIX mice lack LTD but express enhanced LTP at CA1 synapses. Accordingly, basal GluA1 S845 phosphorylation is elevated in AKAP150ΔPIX hippocampus, and LTD-induced dephosphorylation and removal of GluA1, AKAP150, and PSD-95 from synapses are impaired. In addition, basal synaptic activity of GluA2-lacking AMPARs is increased in AKAP150ΔPIX mice and pharmacologic antagonism of these receptors restores normal LTD and inhibits the enhanced LTP. Thus, AKAP150-anchored CaN opposes PKA phosphorylation of GluA1 to restrict synaptic incorporation of Ca(2+)-permeable AMPARs both basally and during LTP and LTD.

Scaffolding by A-kinase anchoring protein enhances functional coupling between adenylyl cyclase and TRPV1 channel

Scaffolding proteins often bring kinases together with their substrates to facilitate cell signaling. This arrangement is critical for the phosphorylation and regulation of the transient receptor potential vanilloid 1 (TRPV1) channel, a key target of inflammatory mediators such as prostaglandins. The protein kinase A anchoring protein AKAP79/150 organizes a multiprotein complex to position protein kinase A (PKA) and protein kinase C (PKC) in the immediate proximity of TRPV1 channels to enhance phosphorylation efficiency. This arrangement suggests that regulators upstream of the kinases must also be present in the signalosome. Here, we show that AKAP79/150 facilitates a complex containing TPRV1 and adenylyl cyclase (AC). The anchoring of AC to this complex generates local pools of cAMP, shifting the concentration of forskolin required to attenuate capsaicin-dependent TRPV1 desensitization by ∼100-fold. Anchoring of AC to the complex also sensitizes the channel to activation by β-adrenergic receptor agonists. Significant AC activity is found associated with TRPV1 in dorsal root ganglia. The dissociation of AC from an AKAP150-TRPV1 complex in dorsal root ganglia neurons abolishes sensitization of TRPV1 induced by forskolin and prostaglandin E(2). Thus, the direct anchoring of both PKA and AC to TRPV1 by AKAP79/150 facilitates the response to inflammatory mediators and may be critical in the pathogenesis of thermal hyperalgesia.

Creating order from chaos: cellular regulation by kinase anchoring

Second messenger responses rely on where and when the enzymes that propagate these signals become active. Spatial and temporal organization of certain signaling enzymes is controlled in part by A-kinase anchoring proteins (AKAPs). This family of regulatory proteins was originally classified on the basis of their ability to compartmentalize the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (also known as protein kinase A, or PKA). However, it is now recognized that AKAPs position G protein-coupled receptors, adenylyl cyclases, G proteins, and their effector proteins in relation to protein kinases and signal termination enzymes such as phosphodiesterases and protein phosphatases. This arrangement offers a simple and efficient means to limit the scope, duration, and directional flow of information to sites deep within the cell. This review focuses on the pros and cons of reagents that define the biological role of kinase anchoring inside cells and discusses recent advances in our understanding of anchored second messenger signaling in the cardiovascular and immune systems.

Palmitoylation of A-kinase anchoring protein 79/150 regulates dendritic endosomal targeting and synaptic plasticity mechanisms

NMDA receptor-dependent long-term potentiation (LTP) and depression (LTD) are forms of synaptic plasticity underlying learning and memory that are expressed through increases and decreases, respectively, in dendritic spine size and AMPA receptor (AMPAR) phosphorylation and postsynaptic localization. The A-kinase anchoring protein 79/150 (AKAP79/150) signaling scaffold regulates AMPAR phosphorylation, channel activity, and endosomal trafficking associated with LTP and LTD. AKAP79/150 is targeted to dendritic spine plasma membranes by an N-terminal polybasic domain that binds phosphoinositide lipids, F-actin, and cadherin cell adhesion molecules. However, we do not understand how regulation of AKAP targeting controls AMPAR endosomal trafficking. Here, we report that palmitoylation of the AKAP N-terminal polybasic domain targets it to postsynaptic lipid rafts and dendritic recycling endosomes. AKAP palmitoylation was regulated by seizure activity in vivo and LTP/LTD plasticity-inducing stimuli in cultured rat hippocampal neurons. With chemical LTP induction, we observed AKAP79 dendritic spine recruitment that required palmityolation and Rab11-regulated endosome recycling coincident with spine enlargement and AMPAR surface delivery. Importantly, a palmitoylation-deficient AKAP79 mutant impaired regulation of spine size, endosome recycling, AMPAR trafficking, and synaptic potentiation. These findings emphasize the emerging importance of palmitoylation in controlling synaptic function and reveal novel roles for the AKAP79/150 signaling complex in dendritic endosomes.

AKAP79/150 interacts with the neuronal calcium-binding protein caldendrin

The A kinase-anchoring protein AKAP79/150 is a postsynaptic scaffold molecule and a key regulator of signaling events. At the postsynapse it coordinates phosphorylation and dephosphorylation of receptors via anchoring kinases and phosphatases near their substrates. Interactions between AKAP79 and two Ca(2+) -binding proteins caldendrin and calmodulin have been investigated here. Calmodulin is a known interaction partner of AKAP79/150 that has been shown to regulate activity of the kinase PKC in a Ca(2+) -dependent manner. Pull-down experiments and surface plasmon resonance biosensor analyses have been used here to demonstrate that AKAP79 can also interact with caldendrin, a neuronal calcium-binding protein implicated in regulation of Ca(2+) -influx and release. We demonstrate that calmodulin and caldendrin compete for a partially overlapping binding site on AKAP79 and that their binding is differentially dependent on calcium. Therefore, this competition is regulated by calcium levels. Moreover, both proteins have different binding characteristics suggesting that the two proteins might play complementary roles. The postsynaptic enrichment, the complex binding mechanism, and the competition with calmodulin, makes caldendrin an interesting novel player in the signaling toolkit of the AKAP interactome.

Ethanol up-regulates nucleus accumbens neuronal activity dependent pentraxin (Narp): implications for alcohol-induced behavioral plasticity

Neuronal activity dependent pentraxin (Narp) interacts with α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors to facilitate excitatory synapse formation by aggregating them at established synapses. Alcohol is well-characterized to influence central glutamatergic transmission, including AMPA receptor function. Herein, we examined the influence of injected and ingested alcohol upon Narp protein expression, as well as basal Narp expression in mouse lines selectively bred for high blood alcohol concentrations under limited access conditions. Alcohol up-regulated accumbens Narp levels, concomitant with increases in levels of the GluR1 AMPA receptor subunit. However, accumbens Narp or GluR1 levels did not vary as a function of selectively bred genotype. We next employed a Narp knock-out (KO) strategy to begin to understand the behavioral relevance of alcohol-induced changes in protein expression in several assays of alcohol reward. Compared to wild-type mice, Narp KO animals: fail to escalate daily intake of high alcohol concentrations under free-access conditions; shift their preference away from high alcohol concentrations with repeated alcohol experience; exhibit a conditioned place-aversion in response to the repeated pairing of 3 g/kg alcohol with a distinct environment and fail to exhibit alcohol-induced locomotor hyperactivity following repeated alcohol treatment. Narp deletion did not influence the daily intake of either food or water, nor did it alter any aspect of spontaneous or alcohol-induced motor activity, including the development of tolerance to its motor-impairing effects with repeated treatment. Taken together, these data indicate that Narp induction, and presumably subsequent aggregation of AMPA receptors, may be important for neuroplasticity within limbic subcircuits mediating or maintaining the rewarding properties of alcohol.

A-kinase anchoring proteins regulate compartmentalized cAMP signaling in airway smooth muscle

A-kinase anchoring proteins (AKAPs) have emerged as important regulatory molecules that can compartmentalize cAMP signaling transduced by β2-adrenergic receptors (β(2)ARs); such compartmentalization ensures speed and fidelity of cAMP signaling and effects on cell function. This study aimed to assess the role of AKAPs in regulating global and compartmentalized β(2)AR signaling in human airway smooth muscle (ASM). Transcriptome and proteomic analyses were used to characterize AKAP expression in ASM. Stable expression or injection of peptides AKAP-IS or Ht31 was used to disrupt AKAP-PKA interactions, and global and compartmentalized cAMP accumulation stimulated by β-agonist was assessed by radioimmunoassay and membrane-delineated flow through cyclic nucleotide-gated channels, respectively. ASM expresses multiple AKAP family members, with gravin and ezrin among the most readily detected. AKAP-PKA disruption had minimal effects on whole-cell cAMP accumulation stimulated by β-agonist (EC(50) and B(max)) concentrations, but significantly increased the duration of plasma membrane-delineated cAMP (τ=251±51 s for scrambled peptide control vs. 399±79 s for Ht31). Direct PKA inhibition eliminated decay of membrane-delineated cAMP levels. AKAPs coordinate compartmentalized cAMP signaling in ASM cells by regulating multiple elements of β(2)AR-mediated cAMP accumulation, thereby representing a novel target for manipulating β(2)AR signaling and function in ASM.

Functional consequences of mutations in postsynaptic scaffolding proteins and relevance to psychiatric disorders

Functional studies on postsynaptic scaffolding proteins at excitatory synapses have revealed a plethora of important roles for synaptic structure and function. In addition, a convergence of recent in vivo functional evidence together with human genetics data strongly suggest that mutations in a variety of these postsynaptic scaffolding proteins may contribute to the etiology of diverse human psychiatric disorders such as schizophrenia, autism spectrum disorders, and obsessive-compulsive spectrum disorders. Here we review the most recent evidence for several key postsynaptic scaffolding protein families and explore how mouse genetics and human genetics have intersected to advance our knowledge concerning the contributions of these important players to complex brain function and dysfunction.

Signaling in dendritic spines and spine microdomains

The specialized morphology of dendritic spines creates an isolated compartment that allows for localized biochemical signaling. Recent studies have revealed complexity in the function of the spine head as a signaling domain and indicate that (1) the spine is functionally subdivided into multiple independent microdomains and (2) not all biochemical signals are equally compartmentalized within the spine. Here we review these findings as well as the developments in fluorescence microscopy that are making possible direct monitoring of signaling within spines and, in the future, within sub-spine microdomains.

Mechanisms of CaMKII action in long-term potentiation

Long-term potentiation (LTP) of synaptic strength occurs during learning and can last for long periods, making it a probable mechanism for memory storage. LTP induction results in calcium entry, which activates calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII subsequently translocates to the synapse, where it binds to NMDA-type glutamate receptors and produces potentiation by phosphorylating principal and auxiliary subunits of AMPA-type glutamate receptors. These processes are all localized to stimulated spines and account for the synapse-specificity of LTP. In the later stages of LTP, CaMKII has a structural role in enlarging and strengthening the synapse.

Protein phosphatases and Alzheimer's disease

Alzheimer's Disease (AD) is characterized by progressive loss of cognitive function, linked to marked neuronal loss. Pathological hallmarks of the disease are the accumulation of the amyloid-β (Aβ) peptide in the form of amyloid plaques and the intracellular formation of neurofibrillary tangles (NFTs). Accumulating evidence supports a key role for protein phosphorylation in both the normal and pathological actions of Aβ as well as the formation of NFTs. NFTs contain hyperphosphorylated forms of the microtubule-binding protein tau, and phosphorylation of tau by several different kinases leads to its aggregation. The protein kinases involved in the generation and/or actions of tau or Aβ are viable drug targets to prevent or alleviate AD pathology. However, it has also been recognized that the protein phosphatases that reverse the actions of these protein kinases are equally important. Here, we review recent advances in our understanding of serine/threonine and tyrosine protein phosphatases in the pathology of AD.

cAMP: novel concepts in compartmentalised signalling

Cyclic adenosine 3,'5'-monophosphate (cAMP) is the archetypal second messenger produced at the membrane by adenylyl cyclase following activation of many different G protein-coupled receptor (GPCR) types. Although discovered over fifty years ago, the notion that cAMP responses were compartmentalised was born in the 1980s. Since then, modern molecular techniques have facilitated visualisation of cellular cAMP dynamics in real time and helped us to understand how a single, ubiquitous second messenger can direct receptor-specific functions in cells. The aim of this review is to highlight emerging ideas in the cAMP field that are currently developing the concept of compartmentalised cAMP signalling systems.

Diverse roles for auxiliary subunits in phosphorylation-dependent regulation of mammalian brain voltage-gated potassium channels

Voltage-gated ion channels are a diverse family of signaling proteins that mediate rapid electrical signaling events. Among these, voltage-gated potassium or Kv channels are the most diverse partly due to the large number of principal (or α) subunits and auxiliary subunits that can assemble in different combinations to generate Kv channel complexes with distinct structures and functions. The diversity of Kv channels underlies much of the variability in the active properties between different mammalian central neurons and the dynamic changes that lead to experience-dependent plasticity in intrinsic excitability. Recent studies have revealed that Kv channel α subunits and auxiliary subunits are extensively phosphorylated, contributing to additional structural and functional diversity. Here, we highlight recent studies that show that auxiliary subunits exert some of their profound effects on dendritic Kv4 and axonal Kv1 channels through phosphorylation-dependent mechanisms, either due to phosphorylation on the auxiliary subunit itself or by influencing the extent and/or impact of α subunit phosphorylation. The complex effects of auxiliary subunits and phosphorylation provide a potent mechanism to generate additional diversity in the structure and function of Kv4 and Kv1 channels, as well as allowing for dynamic reversible regulation of these important ion channels.