Hippocampal long-term depression and depotentiation are defective in mice carrying a targeted disruption of the gene encoding the RI beta subunit of cAMP-dependent protein kinase

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.

Ca2+-stimulated ADCY1 and ADCY8 regulate distinct aspects of synaptic and cognitive flexibility

The type 1 and 8 adenylyl cyclase (ADCY1 and ADCY8) exclusively account for Ca2+-stimulated cyclic AMP (cAMP) production and regulate activity-dependent synaptic modification. In this study, we examined distinct forms of synaptic plasticity in the hippocampus of Adcy1-/- and Adcy8-/- mice. We found that, at the Schaffer collateral-CA1 synapses, while the Adcy8-/- mice displayed normal long-term potentiation (LTP) following various induction protocols with high-frequency stimulation (HFS), the Adcy1-/- mice showed protocol-dependent deficits in LTP. We also found that long-term depression (LTD) requires ADCY1 but not ADCY8. Interestingly, both Adcy1-/- and Adcy8-/- mice showed defective synaptic depotentiation (i.e., activity-dependent reversal of LTP); the deficits in Adcy8-/- mice were dependent on the induction protocol. Examination of spatial memory found that ADCY1 is required for the formation of both initial and reversal memory. ADCY8 is only required for reversal memory formation. These data demonstrate that ADCY1 and ADCY8 play distinct roles in regulating synaptic and cognitive flexibility that involves bidirectional modification of synaptic function.

Edmond Fischer's kinase legacy: History of the protein kinase inhibitor and protein kinase A

Although Fischer's extraordinary career came to focus mostly on the protein phosphatases, after his co-discovery of Phosphorylase Kinase with Ed Krebs he was clearly intrigued not only by cAMP-dependent protein kinase (PKA), but also by the heat-stable, high-affinity protein kinase inhibitor (PKI). PKI is an intrinsically disordered protein that contains at its N-terminus a pseudo-substrate motif that binds synergistically and with high-affinity to the PKA catalytic (C) subunit. The sequencing and characterization of this inhibitor peptide (IP20) were validated by the structure of the PKA C-subunit solved first as a binary complex with IP20 and then as a ternary complex with ATP and two magnesium ions. A second motif, nuclear export signal (NES), was later discovered in PKI. Both motifs correspond to amphipathic helices that convey high-affinity binding. The dynamic features of full-length PKI, recently captured by NMR, confirmed that the IP20 motif becomes dynamically and sequentially ordered only in the presence of the C-subunit. The type I PKA regulatory (R) subunits also contain a pseudo-substrate ATPMg2-dependent high-affinity inhibitor sequence. PKI and PKA, especially the Cβ subunit, are highly expressed in the brain, and PKI expression is also cell cycle-dependent. In addition, PKI is now linked to several cancers. The full biological importance of PKI and PKA signaling in the brain, and their importance in cancer thus remains to be elucidated.

Positional cloning of rat mutant genes reveals new functions of these genes

The laboratory rat (Rattus norvegicus) is a key model organism for biomedical research. Rats can be subjected to strict genetic and environmental controls. The rat's large body size is suitable for both surgical operations and repeated measurements of physiological parameters. These advantages have led to the development of numerous rat models for genetic diseases. Forward genetics is a proven approach for identifying the causative genes of these disease models but requires genome resources including genetic markers and genome sequences. Over the last few decades, rat genome resources have been developed and deposited in bioresource centers, which have enabled us to perform positional cloning in rats. To date, more than 100 disease-related genes have been identified by positional cloning. Since some disease models are more accessible in rats than mice, the identification of causative genes in these models has sometimes led to the discovery of novel functions of genes. As before, various mutant rats are also expected to be discovered and developed as disease models in the future. Thus, the forward genetics continues to be an important approach to find genes involved in disease phenotypes in rats. In this review, I provide an overview the development of rat genome resources and describe examples of positional cloning in rats in which novel gene functions have been identified.

Phenotypic characterization of seven individuals with Marbach-Schaaf neurodevelopmental syndrome

We present the phenotypes of seven previously unreported patients with Marbach-Schaaf neurodevelopmental syndrome, all carrying the same recurrent heterozygous missense variant c.1003C>T (p.Arg335Trp) in PRKAR1B. Clinical features of this cohort include global developmental delay and reduced sensitivity to pain, as well as behavioral anomalies. Only one of the seven patients reported here was formally diagnosed with autism spectrum disorder (ASD), while ASD-like features were described in others, overall indicating a lower prevalence of ASD in Marbach-Schaaf neurodevelopmental syndrome than previously assumed. The clinical spectrum of the current cohort is similar to that reported in the initial publication, delineating a complex developmental disorder with behavioral and neurologic features. PRKAR1B encodes the regulatory subunit R1β of the protein kinase A complex (PKA), and is expressed in the adult and embryonal central nervous system in humans. PKA is crucial to a plethora of cellular signaling pathways, and its composition of different regulatory and catalytic subunits is cell-type specific. We discuss potential molecular disease mechanisms underlying the patients' phenotypes with respect to the different known functions of PKA in neurons, and the phenotypes of existing R1β-deficient animal models.

Group I metabotropic glutamate receptors contribute to the antiepileptic effect of electrical stimulation in hippocampal CA1 pyramidal neurons

Low-frequency deep brain stimulation (LFS) inhibits neuronal hyperexcitability during epilepsy. Accordingly, the use of LFS as a treatment method for patients with drug-resistant epilepsy has been proposed. However, the LFS antiepileptic mechanisms are not fully understood. Here, the role of metabotropic glutamate receptors group I (mGluR I) in LFS inhibitory action on epileptiform activity (EA) was investigated. EA was induced by increasing the K⁺ concentration in artificial cerebrospinal fluid (ACSF) up to 12 mM in hippocampal slices of male Wistar rats. LFS (1 Hz, 900 pulses) was delivered to the bundles of Schaffer collaterals at the beginning of EA. The excitability of CA1 pyramidal neurons was assayed by intracellular whole-cell recording. Applying LFS reduced the firing frequency during EA and substantially moved the membrane potential toward repolarization after a high-K⁺ ACSF washout. In addition, LFS attenuated the EA-generated neuronal hyperexcitability. A blockade of both mGluR 1 and mGluR 5 prevented the inhibitory action of LFS on EA-generated neuronal hyperexcitability. Activation of mGluR I mimicked the LFS effects and had similar inhibitory action on excitability of CA1 pyramidal neurons following EA. However, mGluR I agonist's antiepileptic action was not as strong as LFS. The observed LFS effects were significantly attenuated in the presence of a PKC inhibitor. Altogether, the LFS' inhibitory action on neuronal hyperexcitability following EA relies, in part, on the activity of mGluR I and a PKC-related signaling pathway.

Variants in PRKAR1B cause a neurodevelopmental disorder with autism spectrum disorder, apraxia, and insensitivity to pain

PURPOSE

We characterize the clinical and molecular phenotypes of six unrelated individuals with intellectual disability and autism spectrum disorder who carry heterozygous missense variants of the PRKAR1B gene, which encodes the R1β subunit of the cyclic AMP-dependent protein kinase A (PKA).

METHODS

Variants of PRKAR1B were identified by single- or trio-exome analysis. We contacted the families and physicians of the six individuals to collect phenotypic information, performed in vitro analyses of the identified PRKAR1B-variants, and investigated PRKAR1B expression during embryonic development.

RESULTS

Recent studies of large patient cohorts with neurodevelopmental disorders found significant enrichment of de novo missense variants in PRKAR1B. In our cohort, de novo origin of the PRKAR1B variants could be confirmed in five of six individuals, and four carried the same heterozygous de novo variant c.1003C>T (p.Arg335Trp; NM_001164760). Global developmental delay, autism spectrum disorder, and apraxia/dyspraxia have been reported in all six, and reduced pain sensitivity was found in three individuals carrying the c.1003C>T variant. PRKAR1B expression in the brain was demonstrated during human embryonal development. Additionally, in vitro analyses revealed altered basal PKA activity in cells transfected with variant-harboring PRKAR1B expression constructs.

CONCLUSION

Our study provides strong evidence for a PRKAR1B-related neurodevelopmental disorder.

Deficiency of the RIβ subunit of protein kinase A causes body tremor and impaired fear conditioning memory in rats

The RIβ subunit of cAMP-dependent protein kinase (PKA), encoded by Prkar1b, is a neuronal isoform of the type I regulatory subunit of PKA. Mice lacking the RIβ subunit exhibit normal long-term potentiation (LTP) in the Schaffer collateral pathway of the hippocampus and normal behavior in the open-field and fear conditioning tests. Here, we combined genetic, electrophysiological, and behavioral approaches to demonstrate that the RIβ subunit was involved in body tremor, LTP in the Schaffer collateral pathway, and fear conditioning memory in rats. Genetic analysis of WTC-furue, a mutant strain with spontaneous tremors, revealed a deletion in the Prkar1b gene of the WTC-furue genome. Prkar1b-deficient rats created by the CRISPR/Cas9 system exhibited body tremor. Hippocampal slices from mutant rats showed deficient LTP in the Schaffer collateral-CA1 synapse. Mutant rats also exhibited decreased freezing time following contextual and cued fear conditioning, as well as increased exploratory behavior in the open field. These findings indicate the roles of the RIβ subunit in tremor pathogenesis and contextual and cued fear memory, and suggest that the hippocampal and amygdala roles of this subunit differ between mice and rats and that rats are therefore beneficial for exploring RIβ function.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Graphene Oxide Ameliorates the Cognitive Impairment Through Inhibiting PI3K/Akt/mTOR Pathway to Induce Autophagy in AD Mouse Model

Alzheimer's disease (AD) is a neurodegenerative disease of the central nervous system characterised by cognitive impairment. Its major pathological feature is the deposition of β-amyloid (Aβ) peptide, which triggers a series of pathological cascades. Autophagy is a main pathway to eliminate abnormal aggregated proteins, and increasing autophagy represents a plausible treatment strategy against relative overproduction of neurotoxic Aβ. Graphene oxide (GO) is an emerging carbon-based nanomaterial. As a derivative of graphene with neuroprotective effects, it can effectively increase the clearance of abnormally aggregated protein. In this article, we investigated the protective function of GO in an AD mouse model. GO (30 mg/kg, intraperitoneal) was administered for 2 weeks. The results of the Morris water maze test and the novel object recognition test suggested that GO ameliorated learning and memory impairments in 5xFAD mice. The long-term potentiation and depotentiation from the perforant path to the dentate gyrus in the hippocampus were increased with GO treatment in 5xFAD mice. Furthermore, GO upregulated the expression of synapse-related proteins and increased the cell density in the hippocampus. Our results showed that GO up-regulated LC3II/LC3I and Beclin-1 and decreased p62 protein levels in 5xFAD mice. In addition, GO downregulated the PI3K/Akt/mTOR signalling pathway to induce autophagy. These results have revealed the protective potential of GO in AD.

Optogenetic Manipulation of Postsynaptic cAMP Using a Novel Transgenic Mouse Line Enables Synaptic Plasticity and Enhances Depolarization Following Tetanic Stimulation in the Hippocampal Dentate Gyrus

cAMP is a positive regulator tightly involved in certain types of synaptic plasticity and related memory functions. However, its spatiotemporal roles at the synaptic and neural circuit levels remain elusive. Using a combination of a cAMP optogenetics approach and voltage-sensitive dye (VSD) imaging with electrophysiological recording, we define a novel capacity of postsynaptic cAMP in enabling dentate gyrus long-term potentiation (LTP) and depolarization in acutely prepared murine hippocampal slices. To manipulate cAMP levels at medial perforant path to granule neuron (MPP-DG) synapses by light, we generated transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC) in DG granule neurons. Using these Tg(CMV-Camk2a-RFP/bPAC)3Koka mice, we recorded field excitatory postsynaptic potentials (fEPSPs) from MPP-DG synapses and found that photoactivation of PAC during tetanic stimulation enabled synaptic potentiation that persisted for at least 30 min. This form of LTP was induced without the need for GABA receptor blockade that is typically required for inducing DG plasticity. The paired-pulse ratio (PPR) remained unchanged, indicating the cAMP-dependent LTP was likely postsynaptic. By employing fast fluorescent voltage-sensitive dye (VSD: di-4-ANEPPS) and fluorescence imaging, we found that photoactivation of the PAC actuator enhanced the intensity and extent of dentate gyrus depolarization triggered following tetanic stimulation. These results demonstrate that the elevation of cAMP in granule neurons is capable of rapidly enhancing synaptic strength and neuronal depolarization. The powerful actions of cAMP are consistent with this second messenger having a critical role in the regulation of synaptic function.

Genetic manipulation of cyclic nucleotide signaling during hippocampal neuroplasticity and memory formation

Decades of research have underscored the importance of cyclic nucleotide signaling in memory formation and synaptic plasticity. In recent years, several new genetic techniques have expanded the neuroscience toolbox, allowing researchers to measure and modulate cyclic nucleotide gradients with high spatiotemporal resolution. Here, we will provide an overview of studies using genetic approaches to interrogate the role cyclic nucleotide signaling plays in hippocampus-dependent memory processes and synaptic plasticity. Particular attention is given to genetic techniques that measure real-time changes in cyclic nucleotide levels as well as newly-developed genetic strategies to transiently manipulate cyclic nucleotide signaling in a subcellular compartment-specific manner with high temporal resolution.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Legumain acts on neuroinflammatory to affect CUS-induced cognitive impairment

Cognitive impairment has been widely recognized as a central feature of depression. Legumain, a lysosomal cysteine protease, plays an important role in cancer, atherosclerosis, inflammation and other pathological conditions. Meanwhile, it has been reported that the activation of legumain aggravates the cognitive impairment in neurodegenerative diseases. In this study, we explored the role of legumain in cognitive impairment of stressed mice. Legumain knockout (legumain KO) and wildtype (WT) mice were divided into four groups: control group, chronic mild unpredictable stressed (CUS) group, legumain KO group and legumain KO + CUS group. Our results demonstrated that CUS (4 weeks) induced cognitive impairment in mice effectively based on Morris water maze (MWM) test and novel object recognition (NOR) test and decreased the synaptic plasticity. Additionally, CUS exposure significantly decreased the expression of hippocampal synapse related proteins and the cell density in the DG region, accompanied by increasing the expression of hippocampal inflammatory cytokines and promoting the activation of microglia in the hippocampus. Legumain KO distinctly restored the CUS-induced negative effects on the indicators mentioned above. In conclusion, our results suggested that legumain may be an effective therapeutic target for cognitive impairment as was seen within the CUS model and legumain KO reduced the level of neuroinflammation, thereby improving the hippocampal synaptic plasticity and cognitive impairment of stressed mice.

Two PKA RIα holoenzyme states define ATP as an isoform-specific orthosteric inhibitor that competes with the allosteric activator, cAMP

Protein kinase A (PKA) holoenzyme, comprised of a cAMP-binding regulatory (R)-subunit dimer and 2 catalytic (C)-subunits, is the master switch for cAMP-mediated signaling. Of the 4 R-subunits (RIα, RIβ, RIIα, RIIβ), RIα is most essential for regulating PKA activity in cells. Our 2 RIα2C2 holoenzyme states, which show different conformations with and without ATP, reveal how ATP/Mg2+ functions as a negative orthosteric modulator. Biochemical studies demonstrate how the removal of ATP primes the holoenzyme for cAMP-mediated activation. The opposing competition between ATP/cAMP is unique to RIα. In RIIβ, ATP serves as a substrate and facilitates cAMP-activation. The isoform-specific RI-holoenzyme dimer interface mediated by N3A-N3A' motifs defines multidomain cross-talk and an allosteric network that creates competing roles for ATP and cAMP. Comparisons to the RIIβ holoenzyme demonstrate isoform-specific holoenzyme interfaces and highlights distinct allosteric mechanisms for activation in addition to the structural diversity of the isoforms.

Fast changes of NMDA and AMPA receptor activity under acute hyperammonemia in vitro

It was established in experiments on cell cultures of neurons and astrocytes that ammonium ions at concentrations of 4-8 mM cause hyperexcitation of the neuronal network, as a result of which there is a disturbance of calcium homeostasis, which can lead to the death of neurons. In the present study, we investigated the effect of toxic doses of ammonium (8 mM NH₄Cl) on the activity of NMDA and AMPA receptors and the role of these receptors in spontaneous synchronous activity (SSA). In a control experiment in the absence of NH₄Cl, SSA is not suppressed by NMDA receptor inhibitors, but is suppressed by AMPA receptor antagonists. In the presence of toxic doses of NH₄Cl, SSA is completely inhibited by NMDA receptor inhibitors in 63% of neurons and by AMPA receptor inhibitors in 33% of neurons. After short-term applications of toxic doses of ammonium, the amplitude of the Ca²⁺ response to 10 μM NMDA increases, and decreases in response to 500 nM FW (agonist of AMPA receptors). NMDA receptor blocker MK-801 (20 μM), competitive antagonist D-AP5 (10 μM) and competitive AMPA receptor antagonist NBQX (2 μM) abolished the activating ammonium mediated effect on the NMDA receptors while only MK-801, but not NBQX, abolished the inhibiting ammonium mediated effect on AMPA receptors. These data indicate that under acute hyperammonemia, the activity of NMDA receptors increases, while the activity of AMPA receptors decreases. This phenomenon could explain such a wide range of toxic effects of ammonium ions mediated by NMDA receptors.

The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit

Assembly of multi enzyme complexes at subcellular localizations by anchoring- and scaffolding proteins represents a pivotal mechanism for achieving spatiotemporal regulation of cellular signaling after hormone receptor targeting [for review, see (1)]. In the 3' 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) signaling pathway it is generally accepted that specificity is secured at several levels. This includes at the first level stimulation of receptors coupled to heterotrimeric G proteins which through stimulation of adenylyl cyclase (AC) forms the second messenger cAMP. Cyclic AMP has several receptors including PKA. PKA is a tetrameric holoenzyme consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. The R subunit is the receptor for cAMP and compartmentalizes cAMP signals through binding to cell and tissue-specifically expressed A kinase anchoring proteins (AKAPs). The current dogma tells that in the presence of cAMP, PKA dissociates into an R subunit dimer and two C subunits which are free to phosphorylate relevant substrates in the cytosol and nucleus. The release of the C subunit has raised the question how specificity of the cAMP and PKA signaling pathway is maintained when the C subunit no longer is attached to the R subunit-AKAP complex. An increasing body of evidence points toward a regulatory role of the cAMP and PKA signaling pathway by targeting the C subunits to various C subunit binding proteins in the cytosol and nucleus. Moreover, recent identification of isoform specific amino acid sequences, motifs and three dimensional structures have together provided new insight into how PKA at the level of the C subunit may act in a highly isoform-specific fashion. Here we discuss recent understanding of specificity of the cAMP and PKA signaling pathway based on C subunit subcellular targeting as well as evolution of the C subunit structure that may contribute to the dynamic regulation of C subunit activity.

Switching Cyclic Nucleotide-Selective Activation of Cyclic Adenosine Monophosphate-Dependent Protein Kinase Holoenzyme Reveals Distinct Roles of Tandem Cyclic Nucleotide-Binding Domains

The cyclic adenosine monophosphate (cAMP)- and cyclic guanosine monophosphate (cGMP)-dependent protein kinases (PKA and PKG) are key effectors of cyclic nucleotide signaling. Both share structural features that include tandem cyclic nucleotide-binding (CNB) domains, CNB-A and CNB-B, yet their functions are separated through preferential activation by either cAMP or cGMP. Based on structural studies and modeling, key CNB contact residues have been identified for both kinases. In this study, we explored the requirements for conversion of PKA activation from cAMP-dependent to cGMP-dependent. The consequences of the residue substitutions T192R/A212T within CNB-A or G316R/A336T within CNB-B of PKA-RIα on cyclic nucleotide binding and holoenzyme activation were assessed in vitro using purified recombinant proteins, and ex vivo using RIα-deficient mouse embryonic fibroblasts genetically reconstituted with wild-type or mutant PKA-RIα. In vitro, a loss of binding and activation selectivity was observed when residues in either one of the CNB domains were mutated, while mutations in both CNB domains resulted in a complete switch of selectivity from cAMP to cGMP. The switch in selectivity was also recapitulated ex vivo, confirming their functional roles in cells. Our results highlight the importance of key cyclic nucleotide contacts within each CNB domain and suggest that these domains may have evolved from an ancestral gene product to yield two distinct cyclic nucleotide-dependent protein kinases.

Isoform-specific subcellular localization and function of protein kinase A identified by mosaic imaging of mouse brain

Protein kinase A (PKA) plays critical roles in neuronal function that are mediated by different regulatory (R) subunits. Deficiency in either the RIβ or the RIIβ subunit results in distinct neuronal phenotypes. Although RIβ contributes to synaptic plasticity, it is the least studied isoform. Using isoform-specific antibodies, we generated high-resolution large-scale immunohistochemical mosaic images of mouse brain that provided global views of several brain regions, including the hippocampus and cerebellum. The isoforms concentrate in discrete brain regions, and we were able to zoom-in to show distinct patterns of subcellular localization. RIβ is enriched in dendrites and co-localizes with MAP2, whereas RIIβ is concentrated in axons. Using correlated light and electron microscopy, we confirmed the mitochondrial and nuclear localization of RIβ in cultured neurons. To show the functional significance of nuclear localization, we demonstrated that downregulation of RIβ, but not of RIIβ, decreased CREB phosphorylation. Our study reveals how PKA isoform specificity is defined by precise localization.

DOI:http://dx.doi.org/10.7554/eLife.17681.001

Genealogical correspondence of a forebrain centre implies an executive brain in the protostome-deuterostome bilaterian ancestor

Orthologous genes involved in the formation of proteins associated with memory acquisition are similarly expressed in forebrain centres that exhibit similar cognitive properties. These proteins include cAMP-dependent protein kinase A catalytic subunit (PKA-Cα) and phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (pCaMKII), both required for long-term memory formation which is enriched in rodent hippocampus and insect mushroom bodies, both implicated in allocentric memory and both possessing corresponding neuronal architectures. Antibodies against these proteins resolve forebrain centres, or their equivalents, having the same ground pattern of neuronal organization in species across five phyla. The ground pattern is defined by olfactory or chemosensory afferents supplying systems of parallel fibres of intrinsic neurons intersected by orthogonal domains of afferent and efferent arborizations with local interneurons providing feedback loops. The totality of shared characters implies a deep origin in the protostome-deuterostome bilaterian ancestor of elements of a learning and memory circuit. Proxies for such an ancestral taxon are simple extant bilaterians, particularly acoels that express PKA-Cα and pCaMKII in discrete anterior domains that can be properly referred to as brains.

Autophagy ameliorates cognitive impairment through activation of PVT1 and apoptosis in diabetes mice

The underlying mechanisms of cognitive impairment in diabetes remain incompletely characterized. Here we show that the autophagic inhibition by 3-methyladenine (3-MA) aggravates cognitive impairment in streptozotocin-induced diabetic mice, including exacerbation of anxiety-like behaviors and aggravation in spatial learning and memory, especially the spatial reversal memory. Further neuronal function identification confirmed that both long term potentiation (LTP) and depotentiation (DPT) were exacerbated by autophagic inhibition in diabetic mice, which indicating impairment of synaptic plasticity. However, no significant change of pair-pulse facilitation (PPF) was recorded in diabetic mice with autophagic suppression compared with the diabetic mice, which indicated that presynaptic function was not affected by autophagic inhibition in diabetes. Subsequent hippocampal neuronal cell death analysis showed that the apoptotic cell death, but not the regulated necrosis, significantly increased in autophagic suppression of diabetic mice. Finally, molecular mechanism that may lead to cell death was identified. The long non-coding RNA PVT1 (plasmacytoma variant translocation 1) expression was analyzed, and data revealed that PVT1 was decreased significantly by 3-MA in diabetes. These findings show that PVT1-mediated autophagy may protect hippocampal neurons from impairment of synaptic plasticity and apoptosis, and then ameliorates cognitive impairment in diabetes. These intriguing findings will help pave the way for exciting functional studies of autophagy in cognitive impairment and diabetes that may alter the existing paradigms.

Molecular Genetic Strategies in the Study of Corticohippocampal Circuits

The first reproductively viable genetically modified mice were created in 1982 by Richard Palmiter and Ralph Brinster (Palmiter RD, Brinster RL, Hammer RE, Trumbauer ME, Rosenfeld MG, Birnberg NC, Evans RM. 1982. Dramatic growth of mice that develop from eggs microinjected with metallothionein-growth hormone fusion genes. Nature 300: 611-615). In the subsequent 30 plus years, numerous ground-breaking technical advancements in genetic manipulation have paved the way for improved spatially and temporally targeted research. Molecular genetic studies have been especially useful for probing the molecules and circuits underlying how organisms learn and remember—one of the most interesting and intensively investigated questions in neuroscience research. Here, we discuss selected genetic tools, focusing on corticohippocampal circuits and their implications for understanding learning and memory.

Discovery of Allostery in PKA Signaling

cAMP-dependent protein kinase (PKA) was the second protein kinase to be discovered and the PKA catalytic (C) subunit serves as a prototype for the large protein kinase superfamily that contains over 500 gene products. The protein kinases regulate much of biology in eukaryotic cells and they are now also a major therapeutic target. Although PKA was discovered nearly 50 years ago and the subsequent discovery of the regulatory subunits that bind cAMP and release the catalytic activity from the holoenzyme followed quickly. Thus in PKA we see the convergence of two major signaling mechanisms - protein phosphorylation and second messenger signaling through cAMP. Crystallography provides a foundation for understanding function, and the structure of the isolated regulatory (R) and C-subunits have been extremely informative. Yet it is the R₂C₂ holoenzyme that predominates in cells, and one can only appreciate the allosteric features of PKA signaling by seeing the full length protein. The symmetry and the quaternary constraints that one R:C hetero-dimer exerts on the other in the holoenzyme simply are not present in the isolated subunits or even in the R:C hetero-dimer.

Molecular and genetic determinants of the NMDA receptor for superior learning and memory functions

The opening-duration of the NMDA receptors implements Hebb's synaptic coincidence-detection and is long thought to be the rate-limiting factor underlying superior memory. Here, we investigate the molecular and genetic determinants of the NMDA receptors by testing the “synaptic coincidence-detection time-duration” hypothesis vs. “GluN2B intracellular signaling domain” hypothesis. Accordingly, we generated a series of GluN2A, GluN2B, and GluN2D chimeric subunit transgenic mice in which C-terminal intracellular domains were systematically swapped and overexpressed in the forebrain excitatory neurons. The data presented in the present study supports the second hypothesis, the “GluN2B intracellular signaling domain” hypothesis. Surprisingly, we found that the voltage-gated channel opening-durations through either GluN2A or GluN2B are sufficient and their temporal differences are marginal. In contrast, the C-terminal intracellular domain of the GluN2B subunit is necessary and sufficient for superior performances in long-term novel object recognition and cued fear memories and superior flexibility in fear extinction. Intriguingly, memory enhancement correlates with enhanced long-term potentiation in the 10–100 Hz range while requiring intact long-term depression capacity at the 1–5 Hz range.

PRKAR1B mutation associated with a new neurodegenerative disorder with unique pathology

Pathological accumulation of intermediate filaments can be observed in neurodegenerative disorders, such as Alzheimer's disease, frontotemporal dementia and Parkinson's disease, and is also characteristic of neuronal intermediate filament inclusion disease. Intermediate filaments type IV include three neurofilament proteins (light, medium and heavy molecular weight neurofilament subunits) and α-internexin. The phosphorylation of intermediate filament proteins contributes to axonal growth, and is regulated by protein kinase A. Here we describe a family with a novel late-onset neurodegenerative disorder presenting with dementia and/or parkinsonism in 12 affected individuals. The disorder is characterized by a unique neuropathological phenotype displaying abundant neuronal inclusions by haematoxylin and eosin staining throughout the brain with immunoreactivity for intermediate filaments. Combining linkage analysis, exome sequencing and proteomics analysis, we identified a heterozygous c.149T>G (p.Leu50Arg) missense mutation in the gene encoding the protein kinase A type I-beta regulatory subunit (PRKAR1B). The pathogenicity of the mutation is supported by segregation in the family, absence in variant databases, and the specific accumulation of PRKAR1B in the inclusions in our cases associated with a specific biochemical pattern of PRKAR1B. Screening of PRKAR1B in 138 patients with Parkinson's disease and 56 patients with frontotemporal dementia did not identify additional novel pathogenic mutations. Our findings link a pathogenic PRKAR1B mutation to a novel hereditary neurodegenerative disorder and suggest an altered protein kinase A function through a reduced binding of the regulatory subunit to the A-kinase anchoring protein and the catalytic subunit of protein kinase A, which might result in subcellular dislocalization of the catalytic subunit and hyperphosphorylation of intermediate filaments.

Role of inhibitory autophosphorylation of calcium/calmodulin-dependent kinase II (αCAMKII) in persistent (>24 h) hippocampal LTP and in LTD facilitated by novel object-place learning and recognition in mice

Experience-dependent synaptic plasticity is widely expressed in the mammalian brain and is believed to underlie memory formation. Persistent forms of synaptic plasticity in the hippocampus, such as long-term potentiation (LTP) and long-term depression (LTD) are particularly of interest, as evidence is accumulating that they are expressed as a consequence of, or at the very least in association with, hippocampus-dependent novel learning events. Learning-facilitated plasticity describes the property of hippocampal synapses to express persistent synaptic plasticity when novel spatial learning is combined with afferent stimulation that is subthreshold for induction of changes in synaptic strength. In mice it occurs following novel object recognition and novel object-place recognition. Calmodulin-dependent kinase II (CAMKII) is strongly expressed in synapses and has been shown to be required for hippocampal LTP in vitro and for spatial learning in the water maze. Here, we show that in mice that undergo persistent inhibitory autophosphorylation of αCAMKII, object-place learning is intact. Furthermore, these animals demonstrate a higher threshold for induction of persistent (>24 h) hippocampal LTP in the hippocampal CA1 region during unrestrained behaviour. The transgenic mice also express short-term depression in response to afferent stimulation frequencies that are ineffective in controls. Furthermore, they express stronger LTD in response to novel learning of spatial configurations compared to controls. These findings support that modulation of αCAMKII activity via autophosphorylation at the Thr305/306 site comprises a key mechanism for the maintenance of synaptic plasticity within a dynamic range. They also indicate that a functional differentiation occurs in the way spatial information is encoded: whereas LTP is likely to be critically involved in the encoding of space per se, LTD appears to play a special role in the encoding of the content or features of space.

Lack of the presynaptic RhoGAP protein oligophrenin1 leads to cognitive disabilities through dysregulation of the cAMP/PKA signalling pathway

Loss-of-function mutations in the gene encoding for the RhoGAP protein of oligophrenin-1 (OPHN1) lead to cognitive disabilities (CDs) in humans, yet the underlying mechanisms are not known. Here, we show that in mice constitutive lack of Ophn1 is associated with dysregulation of the cyclic adenosine monophosphate/phosphate kinase A (cAMP/PKA) signalling pathway in a brain-area-specific manner. Consistent with a key role of cAMP/PKA signalling in regulating presynaptic function and plasticity, we found that PKA-dependent presynaptic plasticity was completely abolished in affected brain regions, including hippocampus and amygdala. At the behavioural level, lack of OPHN1 resulted in hippocampus- and amygdala-related learning disabilities which could be fully rescued by the ROCK/PKA kinase inhibitor fasudil. Together, our data identify OPHN1 as a key regulator of presynaptic function and suggest that, in addition to reported postsynaptic deficits, loss of presynaptic plasticity contributes to the pathophysiology of CDs.

Hippocampal long-term depression in freely behaving mice requires the activation of beta-adrenergic receptors

In the intact mouse hippocampus patterned afferent stimulation does not lead to long-term depression (LTD) at Schaffer collateral (Sc)-CA1 synapses, but the same synapses express robust LTD (<24 h) if test-pulse or patterned afferent experience is coupled with novel spatial learning. This suggests that the failure of sole afferent stimulation to elicit LTD relates to the absence of neuromodulatory input related to increased arousal or novelty during learning. Locus coeruleus (LC) firing increases during novel experience, and in rats patterned stimulation of the LC together with test-pulse stimulation of Sc-CA1 synapses leads to robust LTD in vivo. This effect is mediated by beta-adrenergic receptors. Here, we explored if activation of beta-adrenergic receptors supports the expression of LTD in freely behaving mice. We also explored if beta-adrenergic receptors contribute to endogenous LTD that is expressed following spatial learning. Patterned stimulation of Sc-CA1 synapses at 3 Hz (200 pulses) resulted in short-term depression (STD). Pretreatment with isoproterenol, an agonist of beta-adrenergic receptors, resulted in robust LTD (<24 h). Test-pulse stimulation under control conditions elicited field potentials that were stable for the 24-h monitoring period. Coupling of test-pulses with a novel spatial object recognition task resulted in robust endogenous LTD (<24 h). Pretreatment with propranolol, a beta-adrenergic receptor antagonist, completely prevented endogenous LTD that was enabled by learning and prevented object recognition learning itself. These data indicate that the absence of LTD in freely behaving mice, under standard recording conditions, does not reflect an inability of mice to express LTD, rather it is due to the absence of a noradrenalin tonus. Our data also support that spatial object recognition requires beta-adrenergic receptor activation. Furthermore, LTD that is enabled by novel spatial learning critically depends on activation of beta-adrenergic receptors that are presumably activated by noradrenalin released by the LC in response to the novel experience.

Mice overexpressing type 1 adenylyl cyclase show enhanced spatial memory flexibility in the absence of intact synaptic long-term depression

There is significant interest in understanding the contribution of intracellular signaling and synaptic substrates to memory flexibility, which involves new learning and suppression of obsolete memory. Here, we report that enhancement of Ca²⁺-stimulated cAMP signaling by overexpressing type 1 adenylyl cyclase (AC1) facilitated long-term potentiation (LTP) but impaired long-term depression (LTD) at the hippocampal Shaffer collateral-CA1 synapses. However, following the induction of LTP, low-frequency stimulation caused comparable synaptic depotentiation in both wild type and AC1 transgenic (AC1 TG) mice. Although previous studies have suggested the function of LTD in spatial memory flexibility, AC1 TG mice showed not only better initial learning in the Morris water maze, but also faster acquisition and increased ratio of new memory formation to old memory retention during the reversal platform training. In the memory extinction test, which requires suppression of old memory without involving the acquisition of the new platform information, AC1 TG and wild type mice showed comparable performance. Our results demonstrate new functions of Ca²⁺-stimulated AC1, and also suggest that certain aspects of hippocampus-dependent behavioral flexibility may not require intact LTD.

cAMP/PKA signaling defects in tumors: genetics and tissue-specific pluripotential cell-derived lesions in human and mouse

In the last few years, bench and clinical studies led to significant new insight into how cyclic adenosine monophosphate (cAMP) signaling, the molecular pathway that had been identified in the early 2000s as the one involved in most benign cortisol-producing adrenal hyperplasias, affects adrenocortical growth and development, as well as tumor formation. A major discovery was the identification of tissue-specific pluripotential cells (TSPCs) as the culprit behind tumor formation not only in the adrenal, but also in bone. Discoveries in animal studies complemented a number of clinical observations in patients. Gene identification continued in parallel with mouse and other studies on the cAMP signaling and other pathways.

The role of phosphodiesterases in hippocampal synaptic plasticity

Phosphodiesterases (PDEs) degrade cyclic nucleotides, signalling molecules that play important roles in synaptic plasticity and memory. Inhibition of PDEs may therefore enhance synaptic plasticity and memory as a result of elevated levels of these signalling molecules, and this has led to interest in PDE inhibitors as cognitive enhancers. The development of new mouse models in which PDE subtypes have been selectively knocked out and increasing selectivity of PDE antagonists means that this field is currently expanding. Roles for PDE2, 4, 5 and 9 in synaptic plasticity have so far been demonstrated and we review these studies here in the context of cyclic nucleotide signalling more generally. The role of other PDE families in synaptic plasticity has not yet been investigated, and this area promises to advance our understanding of cyclic nucleotide signalling in synaptic plasticity in the future. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Synaptic depression in the CA1 region of freely behaving mice is highly dependent on afferent stimulation parameters

Persistent synaptic plasticity has been subjected to intense study in the decades since it was first described. Occurring in the form of long-term potentiation (LTP) and long-term depression (LTD), it shares many cellular and molecular properties with hippocampus-dependent forms of persistent memory. Recent reports of both LTP and LTD occurring endogenously under specific learning conditions provide further support that these forms of synaptic plasticity may comprise the cellular correlates of memory. Most studies of synaptic plasticity are performed using in vitro or in vivo preparations where patterned electrical stimulation of afferent fibers is implemented to induce changes in synaptic strength. This strategy has proven very effective in inducing LTP, even under in vivo conditions. LTD in vivo has proven more elusive: although LTD occurs endogenously under specific learning conditions in both rats and mice, its induction has not been successfully demonstrated with afferent electrical stimulation alone. In this study we screened a large spectrum of protocols that are known to induce LTD either in hippocampal slices or in the intact rat hippocampus, to clarify if LTD can be induced by sole afferent stimulation in the mouse CA1 region in vivo. Low frequency stimulation at 1, 2, 3, 5, 7, or 10 Hz given in the range of 100 through 1800 pulses produced, at best, short-term depression (STD) that lasted for up to 60 min. Varying the administration pattern of the stimuli (e.g., 900 pulses given twice at 5 min intervals), or changing the stimulation intensity did not improve the persistency of synaptic depression. LTD that lasts for at least 24 h occurs under learning conditions in mice. We conclude that a coincidence of factors, such as afferent activity together with neuromodulatory inputs, play a decisive role in the enablement of LTD under more naturalistic (e.g., learning) conditions.

Increased NR2A:NR2B ratio compresses long-term depression range and constrains long-term memory

The NR2A:NR2B subunit ratio of the NMDA receptors is widely known to increase in the brain from postnatal development to sexual maturity and to aging, yet its impact on memory function remains speculative. We have generated forebrain-specific NR2A overexpression transgenic mice and show that these mice had normal basic behaviors and short-term memory, but exhibited broad long-term memory deficits as revealed by several behavioral paradigms. Surprisingly, increased NR2A expression did not affect 1-Hz-induced long-term depression (LTD) or 100 Hz-induced long-term potentiation (LTP) in the CA1 region of the hippocampus, but selectively abolished LTD responses in the 3–5 Hz frequency range. Our results demonstrate that the increased NR2A:NR2B ratio is a critical genetic factor in constraining long-term memory in the adult brain. We postulate that LTD-like process underlies post-learning information sculpting, a novel and essential consolidation step in transforming new information into long-term memory.

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.

Plasticity of metabotropic glutamate receptor-dependent long-term depression in the anterior cingulate cortex after amputation

Long-term depression (LTD) is a key form of synaptic plasticity important in learning and information storage in the brain. It has been studied in various cortical regions, including the anterior cingulate cortex (ACC). ACC is a crucial cortical region involved in such emotion-related physiological and pathological conditions as fear memory and chronic pain. In the present study, we used a multielectrode array system to map cingulate LTD in a spatiotemporal manner within the ACC. We found that low-frequency stimulation (1 Hz, 15 min) applied onto deep layer V induced LTD in layers II/III and layers V/VI. Cingulate LTD requires activation of metabotropic glutamate receptors (mGluRs), while L-type voltage-gated calcium channels and NMDA receptors also contribute to its induction. Peripheral amputation of the distal tail impaired ACC LTD, an effect that persisted for at least 2 weeks. The loss of LTD was rescued by priming ACC slices with activation of mGluR1 receptors by coapplying (RS)-3,5-dihydroxyphenylglycine and MPEP, a form of metaplasticity that involved the activation of protein kinase C. Our results provide in vitro evidence of the spatiotemporal properties of ACC LTD in adult mice. We demonstrate that tail amputation causes LTD impairment within the ACC circuit and that this can be rescued by activation of mGluR1.

Assembly of allosteric macromolecular switches: lessons from PKA

Protein kinases are dynamic molecular switches that have evolved to be only transiently activated. Kinase activity is embedded within a conserved kinase core, which is typically regulated by associated domains, linkers and interacting proteins. Moreover, protein kinases are often tethered to large macromolecular complexes to provide tighter spatiotemporal control. Thus, structural characterization of kinase domains alone is insufficient to explain protein kinase function and regulation in vivo. Recent progress in structural characterization of cyclic AMP-dependent protein kinase (PKA) exemplifies how our knowledge of kinase signalling has evolved by shifting the focus of structural studies from single kinase subunits to macromolecular complexes.

Localization and quaternary structure of the PKA RIβ holoenzyme

Specificity for signaling by cAMP-dependent protein kinase (PKA) is achieved by both targeting and isoform diversity. The inactive PKA holoenzyme has two catalytic (C) subunits and a regulatory (R) subunit dimer (R(2):C(2)). Although the RIα, RIIα, and RIIβ isoforms are well studied, little is known about RIβ. We show here that RIβ is enriched selectively in mitochondria and hypothesized that its unique biological importance and functional nonredundancy will correlate with its structure. Small-angle X-ray scattering showed that the overall shape of RIβ(2):C(2) is different from its closest homolog, RIα(2):C(2). The full-length RIβ(2):C(2) crystal structure allows us to visualize all the domains of the PKA holoenzyme complex and shows how isoform-specific assembly of holoenzyme complexes can create distinct quaternary structures even though the R(1):C(1) heterodimers are similar in all isoforms. The creation of discrete isoform-specific PKA holoenzyme signaling "foci" paves the way for exploring further biological roles of PKA RIβ and establishes a paradigm for PKA signaling.

Bidirectional synaptic plasticity and spatial memory flexibility require Ca2+-stimulated adenylyl cyclases

When certain memory becomes obsolete, effective suppression of the previously established memory is essential for animals to adapt to the changing environment. At the cellular level, reversal of synaptic potentiation may be important for neurons to acquire new information and to prevent synaptic saturation. Here, we investigated the function of Ca(2+)-stimulated cAMP signaling in the regulation of bidirectional synaptic plasticity and spatial memory formation in double knock-out mice (DKO) lacking both type 1 and 8 adenylyl cyclases (ACs). In anesthetized animals, the DKO mutants showed defective long-term potentiation (LTP) after a single high-frequency stimulation (HFS) or two spaced HFSs at 100 Hz. However, DKO mice showed normal LTP after a single HFS at 200 Hz or two compressed HFSs at 100 Hz. Interestingly, reversal of synaptic potentiation as well as de novo synaptic depression was impaired in DKO mice. In the Morris water maze, DKO mice showed defective acquisition and memory retention, although the deficits could be attenuated by overtraining or compressed trainings with a shorter intertrial interval. In the reversal platform test, DKO animals were impaired in both relearning and old memory suppression. Furthermore, the extinction of the old spatial memory was not efficient in DKO mice. These data demonstrate that Ca(2+)-stimulated AC activity is important not only for LTP and spatial memory formation but also for the suppression of both previously established synaptic potentiation and old spatial memory.

Aggregates of cAMP-dependent kinase isoforms characterize different areas in the developing central nervous system of the chicken, Gallus gallus

The intracellular second messenger adenosine 3',5'-cyclic monophosphate (cAMP) acts mainly through cAMP-dependent protein kinases (PKA). In mammals and reptiles, the PKA regulatory isoforms (RI and RII) are differentially distributed among the various brain areas and cell types, according to the age of the animal. Since PKA distribution may be an additional marker for homologous areas, PKA regulatory subunit types RI and RII were examined in the chicken brain, a species not yet investigated. Chicken brains were examined from prehatching to adult age, by means of immunohistochemistry and biochemical characterization. Most PKA regulatory subunits were segregated in discrete non-soluble clusters that contained either RI or RII. While RII aggregates were present also in non-neuronal cells, RI aggregates were detected only in neurons of some brain areas that are mainly related to the telencephalon. They appeared later than RII aggregates; their presence and location varied during development. RI aggregates were detected first in the olfactory bulb, around embryonic day 14; within 3 days they appeared in the hyperpallium and nidopallium, where the most intense labeling was observed in the perihatching period. Fainter RI aggregates persisted up to 3 years in the olfactory bulb and nidopallium caudale. Less intense RI aggregates were present for a shorter time, from 2 weeks to 3 months, in the septal nuclei, thalamic medial nuclei, periventricular hypothalamus, optic tectum periventricular area, brainstem reticular formation and spinal cord substantia gelatinosa. RI aggregates were not detected in many brain areas including the arcopallium, striatum and cranial nerve nuclei. RII distribution showed less variation during development. From embryonic day 12, some insoluble RII aggregates were detected in the brain; however, only minor modifications were observed in positive structures once they started to harbor insoluble RII aggregates. The present results suggest that the distribution of PKA aggregates may assist in characterizing phylogenetically homologous structures of the vertebrate central nervous system and may also unravel biochemical differences among areas considered homologous.

Isoform-specific targeting of PKA to multivesicular bodies

PKA RIα subunit is localized to MVBs by the A-kinase–anchoring protein AKAP11 when disassociated from the PKA catalytic subunit.

Although RII protein kinase A (PKA) regulatory subunits are constitutively localized to discrete cellular compartments through binding to A-kinase–anchoring proteins (AKAPs), RI subunits are primarily diffuse in the cytoplasm. In this paper, we report a novel AKAP-dependent localization of RIα to distinct organelles, specifically, multivesicular bodies (MVBs). This localization depends on binding to AKAP11, which binds tightly to free RIα or RIα in complex with catalytic subunit (holoenzyme). However, recruitment to MVBs occurs only with the release of PKA catalytic subunit (PKAc). This recruitment is reversed by reassociation with PKAc, and it is disrupted by the presence of AKAP peptides, mutations in the RIα AKAP-binding site, or knockdown of AKAP11. Cyclic adenosine monophosphate binding not only unleashes active PKAc but also leads to the targeting of AKAP11:RIα to MVBs. Therefore, we show that the RIα holoenzyme is part of a signaling complex with AKAP11, in which AKAP11 may direct RIα functionality after disassociation from PKAc. This model defines a new paradigm for PKA signaling.

Transient gastric irritation in the neonatal rats leads to changes in hypothalamic CRF expression, depression- and anxiety-like behavior as adults

AIMS

A disturbance of the brain-gut axis is a prominent feature in functional bowel disorders (such as irritable bowel syndrome and functional dyspepsia) and psychological abnormalities are often implicated in their pathogenesis. We hypothesized that psychological morbidity in these conditions may result from gastrointestinal problems, rather than causing them.

METHODS

Functional dyspepsia was induced by neonatal gastric irritation in male rats. 10-day old male Sprague-Dawley rats received 0.1% iodoacetamide (IA) or vehicle by oral gavage for 6 days. At 8-10 weeks of age, rats were tested with sucrose preference and forced-swimming tests to examine depression-like behavior. Elevated plus maze, open field and light-dark box tests were used to test anxiety-like behaviors. ACTH and corticosterone responses to a minor stressor, saline injection, and hypothalamic CRF expression were also measured.

RESULTS

Behavioral tests revealed changes of anxiety- and depression-like behaviors in IA-treated, but not control rats. As compared with controls, hypothalamic and amygdaloid CRF immunoreactivity, basal levels of plasma corticosterone and stress-induced ACTH were significantly higher in IA-treated rats. Gastric sensory ablation with resiniferatoxin had no effect on behaviors but treatment with CRF type 1 receptor antagonist, antalarmin, reversed the depression-like behavior in IA-treated rats

CONCLUSIONS

The present results suggest that transient gastric irritation in the neonatal period can induce a long lasting increase in depression- and anxiety-like behaviors, increased expression of CRF in the hypothalamus, and an increased sensitivity of HPA axis to stress. The depression-like behavior may be mediated by the CRF1 receptor. These findings have significant implications for the pathogenesis of psychological co-morbidity in patients with functional bowel disorders.

Long-term depression in the CNS

Long-term depression (LTD) in the CNS has been the subject of intense investigation as a process that may be involved in learning and memory and in various pathological conditions. Several mechanistically distinct forms of this type of synaptic plasticity have been identified and their molecular mechanisms are starting to be unravelled. Most studies have focused on forms of LTD that are triggered by synaptic activation of either NMDARs (N-methyl-D-aspartate receptors) or metabotropic glutamate receptors (mGluRs). Converging evidence supports a crucial role of LTD in some types of learning and memory and in situations in which cognitive demands require a flexible response. In addition, LTD may underlie the cognitive effects of acute stress, the addictive potential of some drugs of abuse and the elimination of synapses in neurodegenerative diseases.

Cocaine- and amphetamine-regulated transcript (CART) peptide activates ERK pathways via NMDA receptors in rat spinal cord dorsal horn in an age-dependent manner

Activation of extracellular signal-regulated kinase (ERK) cascade in the spinal cord dorsal horn may contribute to pain hypersensitivity. Our recent study showed that cocaine- and amphetamine-regulated transcript peptide fragment 55-102 (CARTp) increased the levels of phosphoserine 896 and phosphoserine 897 on the N-methyl-d-aspartate (NMDA) receptor NR1 subunit (pNR1-ser896 and pNR1-ser897) via protein kinase A (PKA) and protein kinase C (PKC) signaling pathways leading to increases in NMDA receptor function in spinal cord dorsal horn neurons. Because NMDA receptor, PKC, and PKA signaling pathways may participate in ERK activation, we examined the effects of CARTp on ERK activation in spinal cord dorsal horn neurons in vitro. Western blot analysis showed a significant increase in the level of phosphorylated (activated) ERK (pERK) in the dorsal part of the spinal cord slices after incubation of the slices with CARTp (300nM). Co-administration of CARTp with an NMDA receptor antagonist, MK801 or AP5, or an ERK inhibitor PD98059 blocked the increase in the level of pERK. Interestingly, the increase in the level of pERK by CARTp was observed in postnatal week 3 (W3) and postnatal week 4 (W4), but not in postnatal week 2 (W2) rats. The age-related responses were also noted by CARTp-induced increases in the levels of pNR1-ser896 and pNR1-ser897. In the in vitro electrophysiological study, CARTp increased the amplitude of NMDA-mediated depolarizations in spinal substantia gelatinosa neurons of W3 and W4 rats, but not W2 rats. The results suggest that CARTp activation of ERK signals via the NMDA receptor in the spinal cord dorsal horn was age-dependent.

Bidirectional regulation of hippocampal long-term synaptic plasticity and its influence on opposing forms of memory

Reference memory characterizes the long-term storage of information acquired through numerous trials. In contrast, working memory represents the short-term acquisition of trial-unique information. A number of studies in the rodent hippocampus have focused on the contribution of long-term synaptic potentiation (LTP) to long-term reference memory. In contrast, little is known about the synaptic plasticity correlates of hippocampal-based components of working memory. Here, we described a mouse with selective expression of a dominant-negative mutant of the regulatory subunit of protein kinase A (PKA) only in two regions of the hippocampus, the dentate gyrus and area CA1. This mouse showed a deficit in several forms of LTP in both hippocampal subregions and a lowered threshold for the consolidation of long-term synaptic depression (LTD). When trained with one trial per day in a water maze task, mutant mice displayed a deficit in consolidation of long-term memory. In contrast, these mice proved to be more flexible after a transfer test and also showed a delay-dependent increased performance in working memory, when repetitive information (proactive interference) was presented. We suggest that through its bidirectional control over synaptic plasticity PKA can regulate opposing forms of memory. The defect in L-LTP disrupts long-term memory consolidation. The persistence of LTD may allow acquisition of new information by restricting the body of previously stored information and suppressing interference.

Genetic evidence for a role for protein kinase A in the maintenance of sleep and thalamocortical oscillations

STUDY OBJECTIVES

Genetic manipulation of cAMP-dependent protein kinase A (PKA) in Drosophila has implicated an important role for PKA in sleeplwake state regulation. Here, we characterize the role of this signaling pathway in the regulation of sleep using electroencephalographic (EEG) and electromyographic (EMG) recordings in R(AB) transgenic mice that express a dominant negative form of the regulatory subunit of PKA in neurons within cortex and hippocampus. Previous studies have revealed that these mutant mice have reduced PKA activity that results in the impairment of hippocampus-dependent long-term memory and long-lasting forms of hippocampal synaptic plasticity.

DESIGN

PKA assays, in situ hybridization, immunoblots, and sleep studies were performed in R(AB) transgenic mice and wild-type control mice.

MEASUREMENTS AND RESULTS

We have found that R(AB) transgenic mice have reduced PKA activity within cortex and reduced Ser845 phosphorylation of the glutamate receptor subunit GluR1. R(AB) transgenic mice exhibit non-rapid eye movement (NREM) sleep fragmentation and increased amounts of rapid eye movement (REM) sleep relative to wild-type mice. Further, R(AB) transgenic mice have more delta power but less sigma power during NREM sleep relative to wild-type mice. After sleep deprivation, the amounts of NREM and REM sleep were comparable between wild-type and R(AB) transgenic mice. However, the homeostatic rebound of sigma power in R(AB) transgenic mice was reduced.

CONCLUSIONS

Alterations in cortical synaptic receptors, impairments in sleep continuity, and alterations in sleep oscillations in R(AB) mice imply that PKA is involved not only in synaptic plasticity and memory storage but also in the regulation of sleep/wake states.