Immunocytochemical localization of the neural-specific regulatory subunit of the type II cyclic AMP-dependent protein kinase to postsynaptic structures in the rat brain

Elucidating biological risk factors in suicide: role of protein kinase A

Suicide is a major public health concern. Although there have been several studies of suicidal behavior that focused on the roles of psychosocial and sociocultural factors, these factors are of too little predictive value to be clinically useful. Therefore, research on the biological perspective of suicide has gained a stronghold and appears to provide a promising approach to identify biological risk factors associated with suicidal behavior. Recent studies demonstrate that an alteration in synaptic and structural plasticity is key to affective illnesses and suicide. Signal transduction molecules play an important role in such plastic events. Protein kinase A (PKA) is a crucial enzyme in the adenylyl cyclase signal transduction pathway and is involved in regulating gene transcription, cell survival, and plasticity. In this review, we critically and comprehensively discuss the role of PKA in suicidal behavior. Because stress is an important component of suicide, we also discuss whether stress affects PKA and how this may be associated with suicidal behavior. In addition, we also discuss the functional significance of the findings regarding PKA by describing the role of important PKA substrates (i.e., Rap1, cyclic adenosine monophosphate response element binding protein, and target gene brain-derived neurotrophic factor). These studies suggest the interesting possibility that PKA and related signaling molecules may serve as important neurobiological factors in suicide and may be relevant in target-specific therapeutic interventions for these disorders.

Mutations in AKAP5 disrupt dendritic signaling complexes and lead to electrophysiological and behavioral phenotypes in mice

AKAP5 (also referred to as AKAP150 in rodents and AKAP79 in humans) is a scaffolding protein that is highly expressed in neurons and targets a variety of signaling molecules to dendritic membranes. AKAP5 interacts with PKA holoenzymes containing RIIα or RIIβ as well as calcineurin (PP2B), PKC, calmodulin, adenylyl cyclase type V/VI, L-type calcium channels, and β-adrenergic receptors. AKAP5 has also been shown to interact with members of the MAGUK family of PSD-scaffolding proteins including PSD95 and SAP97 and target signaling molecules to receptors and ion channels in the postsynaptic density (PSD). We created two lines of AKAP5 mutant mice: a knockout of AKAP5 (KO) and a mutant that lacks the PKA binding domain of AKAP5 (D36). We find that PKA is delocalized in both the hippocampus and striatum of KO and D36 mice indicating that other neural AKAPs cannot compensate for the loss of PKA binding to AKAP5. In AKAP5 mutant mice, a significant fraction of PKA becomes localized to dendritic shafts and this correlates with increased binding to microtubule associated protein-2 (MAP2). Electrophysiological and behavioral analysis demonstrated more severe deficits in both synaptic plasticity and operant learning in the D36 mice compared with the complete KO animals. Our results indicate that the targeting of calcineurin or other binding partners of AKAP5 in the absence of the balancing kinase, PKA, leads to a disruption of synaptic plasticity and results in learning and memory defects.

Adenylyl cyclase-cyclicAMP signaling in mood disorders: role of the crucial phosphorylating enzyme protein kinase A

Mood disorders are among the most prevalent and recurrent forms of psychiatric illnesses. In the last decade, there has been increased understanding of the biological basis of mood disorders. In fact, novel mechanistic concepts of the neurobiology of unipolar and bipolar disorders are evolving based on recent pre-clinical and clinical studies, most of which now focus on the role of signal transduction mechanisms in these psychiatric illnesses. Particular investigative emphasis has been given to the role of phosphorylating enzymes, which are crucial in regulating gene expression and neuronal and synaptic plasticity. Among the most important phosphorylating enzyme is protein kinase A (PKA), a component of adenylyl cyclase-cyclic adenosine monophosphate (AC-cAMP) signaling system. In this review, we critically and comprehensively discuss the role of various components of AC-cAMP signaling in mood disorders, with a special focus on PKA, because of the interesting observation that have been made about its involvement in unipolar and bipolar disorders. We also discuss the functional significance of the findings regarding PKA by discussing the role of important PKA substrates, namely, Rap-1, cyclicAMP-response element binding protein, and brain-derived neurotrophic factor. These studies suggest the interesting possibility that PKA and related signaling molecules may serve as important neurobiological factors in mood disorders and may be relevant in target-specific therapeutic interventions for these disorders.

Regional differences in hippocampal PKA immunoreactivity after training and reversal training in a spatial Y-maze task

It is suggested that the hippocampus functions as a comparator by making a comparison between the internal representation and actual sensory information from the environment (for instance, comparing a previously learned location of a food reward with an actual novel location of a food reward in a Y-maze). However, it remains unclear to what extent the various hippocampal regions contribute to this comparator function. One of the proteins known to be crucially involved in the formation of hippocampus-dependent long-term memory is the adenosine 3',5' cyclic monophosphate dependent protein kinase (PKA). Here, we examined region-specific changes in immunoreactivity (ir) of the regulatory IIalpha,beta subunits of PKA (PKA RIIalpha,beta-ir) in the hippocampus during various stages of spatial learning in a Y-maze reference task. Thereafter, we compared changes in hippocampal PKA RIIalpha,beta-ir induced by training and reversal training in which the food reward was relocated to the previously unrewarded arm. We show that: (1) There was a clear correlation between behavioral performance and elevated PKA RIIalpha,beta-ir during the acquisition phase of both training and reversal training in area CA3 and dentate gyrus (DG), (2) PKA RIIalpha,beta-ir was similarly enhanced in area CA1 during the acquisition phase of reversal training, but did not correlate with behavioral performance, (3) PKA RIIalpha,beta-ir did not change during training or reversal training in the subiculum (SUB), (4) No changes in PKA RIIalpha,beta protein levels were found using Western blotting, and (5) AMPA receptor phosphorylation at serine 845 (S845p; the PKA site on the glutamate receptor 1 subunit (GluR1)), was enhanced selectively during the acquisition phase of reversal training. These findings reveal that training and reversal training induce region-specific changes in hippocampal PKA RIIalpha,beta-ir and suggest a differential involvement of hippocampal subregions in match-mismatch detection in case of Y-maze reference learning. Alterations in AMPA receptor regulation at the S845 site seems specifically related to the novelty detector function of the hippocampus important for match-mismatch detection.

Barrel map development relies on protein kinase A regulatory subunit II beta-mediated cAMP signaling

The cellular and molecular mechanisms mediating the activity-dependent development of brain circuitry are still incompletely understood. Here, we examine the role of cAMP-dependent protein kinase [protein kinase A (PKA)] signaling in cortical development and plasticity, focusing on its role in thalamocortical synapse and barrel map development. We provide direct evidence that PKA activity mediates barrel map formation using knock-out mice that lack type IIbeta regulatory subunits of PKA (PKARIIbeta). We show that PKARIIbeta-mediated PKA function is required for proper dendritogenesis and the organization of cortical layer IV neurons into barrels, but not for the development and plasticity of thalamocortical afferent clustering into a barrel pattern. We localize PKARIIbeta function to postsynaptic processes in barrel cortex and show that postsynaptic PKA targets, but not presynaptic PKA targets, have decreased phosphorylation in pkar2b knock-out (PKARIIbeta(-/-)) mice. We also show that long-term potentiation at TC synapses and the associated developmental increase in AMPA receptor function at these synapses, which normally occurs as barrels form, is absent in PKARIIbeta(-/-) mice. Together, these experiments support an activity-dependent model for barrel map development in which the selective addition and elimination of thalamocortical synapses based on Hebbian mechanisms for synapse formation is mediated by a cAMP/PKA-dependent pathway that relies on PKARIIbeta function.

Requirement for the RIIbeta isoform of PKA, but not calcium-stimulated adenylyl cyclase, in visual cortical plasticity

The cAMP-dependent protein kinase (PKA) signaling pathway plays a key role in visual cortical plasticity. Inhibitors that block activation of all PKA regulatory subunits (RIalpha,RIbeta, RIIalpha, RIIbeta) abolish long-term potentiation (LTP) and long-term depression (LTD) in vitro and ocular dominance plasticity (ODP) in vivo. The details of this signaling cascade, however, including the source of PKA signals and which PKA subunits are involved, are unknown. To investigate these issues we have examined LTP, LTD, and ODP in knock-out mice lacking either the two cortically expressed Ca2+-stimulated adenylyl cyclases (AC1 and AC8) or the predominant neocortical subunit of PKA (RIIbeta). Here we show that plasticity remains intact in AC1/AC8-/- mice, whereas ODP and LTD, but not LTP, are absent in RIIbeta-/- mice. We conclude that (1) plasticity in the visual cortex does not require the activity of known Ca2+-stimulated adenylyl cyclases, (2) the PKA dependence of ODP and LTD, but not LTP, is mediated by RIIbeta-PKA, and (3) multiple isoforms of PKA contribute to LTD.

Altered protein kinase a in brain of learned helpless rats: effects of acute and repeated stress

BACKGROUND

Stress-induced learned helplessness (LH) in animals serves as a model of behavioral depression and some aspects of posttraumatic stress disorder. We examined whether LH behavior is associated with alterations in protein kinase A (PKA), a critical phosphorylating enzyme, how long these alterations persist after inescapable shock (IS), and whether repetition of IS prolongs the duration of LH behavior and changes in PKA.

METHODS

Rats were exposed to IS either on day 1 or twice, on day 1 and day 7. Rats were tested for escape latency on days 2 and 4 after day 1 IS or days 2, 8, and 14 after day 1 and day 7 IS. [(3)H]cAMP (cyclic adenosine monophosphate) binding, catalytic activity and expression of PKA subunits were determined in frontal cortex and hippocampus.

RESULTS

Higher escape latencies were observed in rats tested on day 2 after single IS and on day 14 after repeated IS. Concurrently, reduced [(3)H]cAMP binding, PKA activity, and expression of selective PKA RIIbeta and Calpha and Cbeta subunits were observed in the brains of these rats.

CONCLUSIONS

Repeated IS prolongs the duration of LH behavior, and LH behavior is associated with reductions in apparent activity and expression of PKA. These reductions in PKA may be critical in the pathophysiology of depression and other stress-related disorders.

Protein kinase A in postmortem brain of depressed suicide victims: altered expression of specific regulatory and catalytic subunits

BACKGROUND

We recently reported reduced [3H]cyclic adenosine monophosphate binding and catalytic activity of protein kinase A in prefrontal cortex of depressed suicide victims. Here we examined the molecular basis of these alterations and whether these findings can be replicated in another cohort.

METHODS

Prefrontal cortex from depressed suicide victims and nonpsychiatric controls were obtained from the Lenhossek Human Brain Program, Budapest and the Maryland Brain Collection Program. [3H]cyclic adenosine monophosphate binding and protein kinase A activity were determined by radioligand binding and enzymatic assay, respectively. Expression of catalytic and regulatory subunits was determined by quantitative reverse transcription polymerase chain reaction and Western blot, respectively.

RESULTS

[3H]cyclic adenosine monophosphate binding and total and endogenous protein kinase A activity were significantly decreased in membrane and cytosol fractions of prefrontal cortex of depressed suicide victims from the Budapest cohort, with a similar magnitude (33%-40% reduction) as reported for the Maryland cohort. In both cohorts, selective reduction (36%-41%) in mRNA and protein expression of the regulatory RIIbeta and the catalytic Cbeta was observed.

CONCLUSIONS

Our results suggest abnormalities in [3H]cyclic adenosine monophosphate binding and catalytic activity kinase A in brain of depressed suicide victims, which could be due to reduced expression of RIIbeta and Cbeta. These abnormalities in PKA may be critical in the pathophysiology of depression.

The role of PKA, CaMKII, and PKC in avoidance conditioning: permissive or instructive?

This article explores the causal and correlative relationships between kinases and learning and memory. Specifically, the contributions of three kinases-protein kinase A (PKA), calcium calmodulin-dependent kinase II (CaMKII), and protein kinase C (PKC)-are assessed during the consolidation phase of avoidance conditioning. The following sources of evidence are considered: inhibitor data, activity monitoring, and transgenic studies. An exhaustive effort is made to address several issues regarding the participation of these kinases in (a) posttraining timing and magnitude, (b) location across many brain regions, and (c) the use of multiple pharmacological agents and assays. In addition, this article attempts to integrate the behavioral data with the purported role of kinases in long-term potentiation (LTP).

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Inhibition of cyclic AMP-dependent protein kinase in the acute phase of focal cerebral ischemia in the rat

Binding of cyclic AMP to the regulatory subunit of cyclic AMP-dependent protein kinase is an essential step in cyclic AMP-mediated intracellular signal transduction. In the present study, the binding capacity of cyclic AMP-dependent protein kinase for cyclic AMP was examined by autoradiography with local cerebral blood flow in focal cerebral ischemia in the rat, which was induced by occlusion of the middle cerebral artery using the intraluminal suture method. The binding capacity of cyclic AMP-dependent protein kinase and local cerebral blood flow were assessed by the in vitro [3H]cyclic AMP binding and the [14C]iodoantipyrine methods, respectively. At 3 h of occlusion, a significant reduction in the binding of cyclic AMP-dependent protein kinase to cyclic AMP was already noted in the lateral region of the caudate-putamen and the parietal cortex. Between three and five hours of occlusion, the area with reduced cyclic AMP binding was significantly expanded to the peri-ischemic regions including the frontal cortex and the medial region of the caudate-putamen. The threshold in local cerebral blood flow for reduced cyclic AMP binding was clearly noted at 5 h of ischemia, and was 45 ml/100 g per min in the cerebral cortices, and 38 ml/100 g per min in the caudate-putamen, respectively. No threshold was noted at 3 h of ischemia, since cyclic AMP binding showed a large variation ranging from reduced to normal values even when local cerebral blood flow was below 20 ml/100 g per min. Recirculation for 3.5 h following 1.5 h of ischemia restored the normal cyclic AMP binding in the cerebral cortices, but failed to normalize cyclic AMP binding in the caudate-putamen despite good recovery of local cerebral blood flow. Western blot analysis suggested that this reduction in cyclic AMP binding was not due to loss or degradation of the subunit protein of cyclic AMP-dependent protein kinase, and may therefore have resulted from conformational changes in the protein. A significant increase in cyclic AMP binding was noted after recirculation in the non-ischemic regions such as the frontal and the cingulate cortices on the occluded side and in the contralateral cortices. These data indicate that cyclic AMP-mediated signal transduction in the brain tissue may be very susceptible to ischemic stress, and the region of disrupted signal transduction may expand progressively from the ischemic core to peri-ischemic regions in the acute phase of ischemia. Such impairment of signal transduction may not be restored in the caudate-putamen even when cerebral circulation is fully recovered after short-term ischemia, suggesting that a regional vulnerability to ischemic stress may also exist in cyclic AMP-mediated signal transduction. A significant increase in cyclic AMP binding after recirculation in regions outside of ischemic area may be closely related with the protective mechanisms of brain tissue, since cyclic AMP has been reported to exert various neuroprotective actions.

cAMP-dependent protein kinase phosphorylations on tau in Alzheimer's disease

To elucidate the role cAMP-dependent protein kinase (PKA) phosphorylations on tau play in Alzheimer's disease, we have generated highly specific monoclonal antibodies, CP-3 and PG-5, which recognize the PKA-dependent phosphorylations of ser214 and ser409 in tau respectively. The present study demonstrates by immunohistochemical analysis, CP-3 and PG-5 immunoreactivity with neurofibrillary pathology in both early and advanced Alzheimer's disease, but not in normal brain tissue and demonstrates that cAMP-dependent protein kinase phosphorylations on tau precede or are coincident with the initial appearance of filamentous aggregates of tau. Studies using heat-stable preparations demonstrate that neither site appears to be phosphorylated to any appreciable extent in normal rodent or human brain. Further analysis demonstrates that the beta catalytic subunit of PKA (Cbeta), the beta II regulatory subunit of PKA (RIIbeta), and the 79 kDa A-kinase-anchoring-protein (AKAP79), are tightly associated with the neurofibrillary pathology, positioning cAMP-dependent protein kinase to participate directly in the pathological hyperphosphorylation of tau seen in Alzheimer's disease.

Distribution of adenylate cyclase in the brain of Apteronotus leptorhynchus as revealed by forskolin binding

An antibody directed against an isoform of the rat regulatory subunit of protein kinase A and brain dissection was used for immunoblot analysis of this protein in various brain regions of Apteronotus leptorhynchus. Western blots revealed that the antibody labeled a band of the expected molecular mass (approximately 53 kDa) for this enzyme in mammalian cortex and electric fish brain, suggesting that this protein is also found in fish brains. The 53-kDa band was enriched in fish forebrain. [3H]Forskolin binding was used as a marker for the distribution of adenylate cyclase. [3H]Forskolin binding was nearly completely displaced by excess cold forskolin; specific [3H]forskolin binding sites were heterogenously distributed with relatively high densities in some gray matter regions and low densities in fiber tracts. A high density of [3H]forskolin binding sites was found in the dorsal forebrain with lower densities in most ventral forebrain nuclei. Moderate binding densities were observed in the preoptic and hypothalamic areas with the exception of the nucleus tuberis anterior, which had high levels. The thalamus and midbrain had low levels of binding. The cerebellar molecular layer had dense binding, in contrast to the granule cell layer where binding was low. In the electrosensory lateral line lobe (ELL), there was moderate binding in the dorsal and ventral molecular layers, which contain feedback inputs; the cellular layers of the ELL had low binding densities.

Defective motor behavior and neural gene expression in RIIbeta-protein kinase A mutant mice

Motor behavior is modulated by dopamine-responsive neurons in the striatum, where dopaminergic signaling uses G-protein-coupled pathways, including those that result in the activation of cAMP-dependent protein kinase (PKA). The RIIbeta isoform of PKA is highly enriched in the striatum, and targeted disruption of the RIIbeta gene in mice leads to a dramatic reduction in total PKA activity in this region. Although the mutant mice show typical locomotor responses after acute administration of dopaminergic drugs, they display abnormalities in two experience-dependent locomotor behaviors: training on the rotarod task and locomotor sensitization to amphetamine. In addition, amphetamine induction of fos is absent, and the basal expression of dynorphin mRNA is reduced in the striatum. These results demonstrate that motor learning and the regulation of neuronal gene expression require RIIbeta PKA, whereas the acute locomotor effects of dopaminergic drugs are relatively unaffected by this PKA deficiency.

PKA isoforms, neural pathways, and behaviour: making the connection

In mammals, the cAMP-dependent protein kinase (PKA) family of enzymes is assembled from the products of four regulatory and two catalytic subunit genes, all of which are expressed in neurons. Specific isoforms of PKA display differences in biochemical properties and subcellular localization, but it has been difficult to ascribe specific physiological functions to any given isoform. The recent development of gene knockout and transgenic mouse models has allowed for a more integrated examination of the in vivo roles of specific PKA isoforms in gene expression, synaptic plasticity, and behaviour.

Flow threshold for reduction of cyclic AMP binding in the hippocampus CA1 and other brain regions during stroke development in gerbils

The flow threshold for alterations of the in vitro [3H]cyclic AMP (cAMP) binding, an indicator of the total amount of particulate cAMP-dependent protein kinase, was evaluated in the gerbil brain after 30 min, 2 h, and 6 h of unilateral common carotid artery occlusion, respectively. The autoradiographic method developed in our laboratory enabled us to measure the [3H]cAMP binding and local CBF in each region of the same brain. The ischemic flow thresholds for reduction of the cAMP binding in the hippocampus CA1 were 18, 34, and 49 ml 100 g-1 min-1 after 30-min, 2-h, and 6-h ischemia, respectively. These values were higher than those in other regions such as the hippocampus CA, and temporal cerebral cortex in each duration of ischemia. These findings indicate that (a) the ischemic flow threshold for perturbation of the cAMP system may be higher in the hippocampus CA1 than in other brain regions, suggesting that the hippocampus CA1 could be especially vulnerable to acute ischemic stress; and (b) the level of the aforementioned threshold may increase progressively during the time course of ischemia in particular regions such as the hippocampus CA1 and CA3, suggesting that the duration of ischemia exerts a definite influence on the viability of the ischemic neuronal cells in these regions.

Impaired hippocampal plasticity in mice lacking the Cbeta1 catalytic subunit of cAMP-dependent protein kinase

Neural pathways within the hippocampus undergo use-dependent changes in synaptic efficacy, and these changes are mediated by a number of signaling mechanisms, including cAMP-dependent protein kinase (PKA). The PKA holoenzyme is composed of regulatory and catalytic (C) subunits, both of which exist as multiple isoforms. There are two C subunit genes in mice, Calpha and Cbeta, and the Cbeta gene gives rise to several splice variants that are specifically expressed in discrete regions of the brain. We have used homologous recombination in embryonic stem cells to introduce an inactivating mutation into the mouse Cbeta gene, specifically targeting the Cbeta1-subunit isoform. Homozygous mutants showed normal viability and no obvious pathological defects, despite a complete lack of Cbeta1. The mice were analyzed in electrophysiological paradigms to test the role of this isoform in long-term modulation of synaptic transmission in the Schaffer collateral-CA1 pathway of the hippocampus. A high-frequency stimulus produced potentiation in both wild-type and Cbeta1-/- mice, but the mutants were unable to maintain the potentiated response, resulting in a late phase of long-term potentiation that was only 30% of controls. Paired pulse facilitation was unaffected in the mutant mice. Low-frequency stimulation produced long-term depression and depotentiation in wild-type mice but failed to produce lasting synaptic depression in the Cbeta1 -/- mutants. These data provide direct genetic evidence that PKA, and more specifically the Cbeta1 isoform, is required for long-term depression and depotentiation, as well as the late phase of long-term potentiation in the Schaffer collateral-CA1 pathway.

Immunohistochemical localization of adenylyl cyclase in rat brain indicates a highly selective concentration at synapses

Only three isoforms of adenylyl cyclase (EC 4.6.1.1) mRNAs (AC1, -2, and -5) are expressed at high levels in rat brain. AC1 occurs predominantly in hippocampus and cerebellum, AC5 is restricted to the basal ganglia, whereas AC2 is more widely expressed, but at much lower levels. The distribution and abundance of adenylyl cyclase protein were examined by immunohistochemistry with an antiserum that recognizes a peptide sequence shared by all known mammalian adenylyl cyclase isoforms. The immunoreactivity in striatum and hippocampus could be readily interpreted within the context of previous in situ hybridization studies. However, extending the information that could be gathered by comparisons with in situ hybridization analysis, it was apparent that staining was confined to the neuropil--corresponding to immunoreactive dendrites and axon terminals. Electron microscopy indicated a remarkably selective subcellular distribution of adenylyl cyclase protein. In the CA1 area of the hippocampus, the densest immunoreactivity was seen in postsynaptic densities in dendritic spine heads. Labeled presynaptic axon terminals were also observed, indicating the participation of adenylyl cyclase in the regulation of neurotransmitter release. The selective concentration of adenylyl cyclases at synaptic sites provides morphological data for understanding the pre- and postsynaptic roles of adenylyl cyclase in discrete neuronal circuits in rat brain. The apparent clustering of adenylyl cyclases, coupled with other data that suggest higher-order associations of regulatory elements including G proteins, N-methyl-D-aspartate receptors, and cAMP-dependent protein kinases, suggests not only that the primary structural information has been encoded to render the cAMP system responsive to the Ca(2+)-signaling system but also that higher-order strictures are in place to ensure that Ca2+ signals are economically delivered and propagated.

Patterns of cyclic AMP-dependent protein kinase gene expression during ontogeny of the murine palate

Normal growth and differentiation of embryonic palatal tissue depends on regulated levels of intracellular cAMP. Cyclic AMP-dependent protein kinases (PKA) act to mediate the biological activities of cAMP. PKA isozyme protein profiles demonstrate a clear pattern of temporal alterations in embryonic palatal tissue during its development. In order to ascertain the molecular basis for changing PKA isozyme profiles during palatal ontogeny, the spatial and temporal expression of mRNAs for regulatory (RI alpha, RII alpha, and RII beta) and catalytic (C alpha) subunits of PKA was examined. RNA extracted from murine embryonic palatal tissue (days 12-14 of gestation) was examined by Northern blot analysis. Significant levels of constitutively expressed RI alpha and C alpha mRNA were seen on all days of gestation examined. RI alpha transcripts were substantially less abundant in palate mesenchymal cells in vitro than in palatal tissue in vivo. Levels of RII alpha and RII beta mRNA were highest on gestational day (GD) 12, a period characterized by pronounced palatal tissue growth. In addition, patterns of tissue distribution of RII beta, not previously described, were examined in the developing embryonic palate. A dramatic developmental shift in tissue distribution of RII beta was seen. The isozyme was evenly distributed between palatal epithelial and mesenchymal cells on GD 12 but by GD 14, RII beta was predominantly localized to palatal epithelial cells. Direct activation of adenylate cyclase with forskolin in murine embryonic palate mesenchymal (MEPM) cells resulted in an increase in RII alpha mRNA levels but had no effect on steady state levels of RII beta or C alpha mRNA. In addition, elevation of intracellular levels of cAMP resulted in a shift in the transcriptional profile of RI alpha mRNAs. Results of this study document specific patterns of expression for the genes encoding the various cAMP-dependent protein kinase regulatory and C alpha subunits in murine embryonic palatal tissue. In addition, we have demonstrated adaptational changes of this kinase in MEPM cells in response to conditions of increased intracellular levels of cAMP.

Association of protein kinase A and protein phosphatase 2B with a common anchoring protein

Specificity of protein kinases and phosphatases may be achieved through compartmentalization with preferred substrates. In neurons, adenosine 3', 5'-monophosphate (cAMP)-dependent protein kinase (PKA) is localized at postsynaptic densities by association of its regulatory subunit with an A kinase anchor protein, AKAP79. Interaction cloning experiments demonstrated that AKAP79 also binds protein phosphatase 2B, or calcineurin (CaN). A ternary complex of PKA, AKAP, and CaN was isolated from bovine brain, and colocalization of the kinase and the phosphatase was established in neurites of cultured hippocampal neurons. The putative CaN-binding domain of AKAP79 is similar to that of the immunophilin FKBP-12, and AKAP79 inhibited CaN phosphatase activity. These results suggest that both PKA and CaN are targeted to subcellular sites by association with a common anchor protein and thereby regulate the phosphorylation state of key neuronal substrates.

cAMP signaling in neurons: patterns of neuronal expression and intracellular localization for a novel protein, AKAP 150, that anchors the regulatory subunit of cAMP-dependent protein kinase II beta

In mammalian brain, physiological signals carried by cyclic AMP (cAMP) seem to be targeted to effector sites via the tethering of cAMP-dependent protein kinase II beta (PKAII beta) to intracellular structures. Recently characterized A kinase anchor proteins (AKAPs) are probable mediators of the sequestration of PKAII beta because they contain a high-affinity binding site for the regulatory subunit (RII beta) of the kinase and a distinct intracellular targeting domain. To establish a cellular basis for this targeting mechanism, we have employed immunocytochemistry to 1) identify the types of neurons that are enriched in AKAPs, 2) determine the primary intracellular location of the anchor protein, and 3) demonstrate that an AKAP and RII beta are coenriched and colocalized in neurons that utilize the adenylate cyclase-cyclic AMP-dependent protein kinase (PKA) signaling pathway. Antibodies directed against rat brain AKAP 150 were used to elucidate the regional, cellular and intracellular distribution of a prototypic anchor protein in the CNS. AKAP 150 is abundant in Purkinje cells and in neurons of the olfactory bulb, basal ganglia, cerebral cortex, and other forebrain regions. In contrast, little AKAP 150 is detected in neurons of the thalamus, hypothalamus, midbrain, and hindbrain. A high proportion of total AKAP 150 is concentrated in primary branches of dendrites, where it is associated with microtubules. We also discovered that the patterns of accumulation and localization of RII beta (and PKAII beta) in brain are similar to those of AKAP 150. The results suggest that bifunctional AKAP 150 tethers PKAII beta to the dendritic cytoskeleton, thereby creating a discrete target site for the reception and propagation of signals carried by cAMP.

The paradoxical effect of NMDA receptor stimulation on electrical activity of the sensorimotor cortex in freely behaving rats: Analysis by combined EEG-intracerebral microdialysis

This study was designed to determine the effects of N-methyl-D-aspartate (NMDA) receptor stimulation on the electrical activity of neocortex in freely behaving rats. Electroencephalogram (EEG) recording and intracerebral microdialysis were conducted simultaneously in the same site of the sensorimotor cortex, where the basal extracellular concentrations of aspartate and glutamate were 2.1 +/- 0.7 microM and 11.5 +/- 2.4 microM, respectively. Microdialysis with NMDA solutions (ranging from 10.0 microM to 10.0 mM) reduced the amplitude of the EEG activity and decreased the power of all frequency bands, with a virtual elimination of the high frequency waves, in a dose-dependent manner. These EEG changes were reversed after washing out the drug from the microdialysis fluid, and could be effectively antagonized with the competitive NMDA receptor antagonist DL-2-amino-5-phosphonovalerate. Remarkably, the NMDA actions were not associated with epileptiform behavioral or electrographic events. Control studies demonstrated that in the same experimental conditions, cholinergic receptor agonist carbachol caused seizures, and microdialysis with NMDA in the hippocampus readily induced epileptiform spikes. Our study shows that NMDA receptor stimulation in the rat sensorimotor cortex, although excitatory at synaptic level, can depress the local EEG activity. This may indicate that the NMDA receptor-mediated signals are processed by the neocortical network in a different way than by many other brain circuitries including hippocampus.

The postsynaptic density: constituent and associated proteins characterized by electrophoresis, immunoblotting, and peptide sequencing

The proteins of the postsynaptic density (PSD) fraction of cerebral cortex were resolved by two-dimensional electrophoresis (2DE) and more than 30 proteins identified by characteristic 2DE mobility, immunoblotting with specific antibodies, and N-terminal and peptide sequencing. The PSD fraction is enriched for spectrin, actin, tublin and microtubule associated protein II, myosin, enzymes of glycolysis, creatine kinase, elongation factor 1 alpha, and receptor protein. The three neurofilament proteins are detected but a 58-kDa protein is prominent and is, by peptide sequencing, the bovine homolog of the recently cloned 66-kDa neurofilament protein; in contrast to the latter, however, it is enriched in cerebrum compared with spinal cord. A 68-kDa protein is identified as a member of the hsp70/BiP family of proteins. A protein, designated dynamin, indicating its putative role as a microtubule motor, is identified as a major protein, is found, however, greatly enriched in the particulate fraction, and is significantly denaturant and detergent insoluble. A protein designated N-ethylmaleimide-sensitive factor is also detected. Thus, two proteins implicated in vesicular transport are present in the PSD fraction. Seven polyclonal antibodies were produced to 2DE separated and electroeluted proteins of the PSD and were identified by peptide sequence analysis and 2DE profile as the hsp70/BiP homologous protein, the novel neurofilament protein synapsin IIa, pyruvate kinase, dynamin, aconitase and an unknown contaminating protein, and a 115-kDa protein that by subcellular fractionation and immunoblotting is a diagnostic PSD molecule. In addition, peptide sequences are obtained for four additional higher molecular weight proteins of the PSD that are not related at the level of primary structure to any known proteins.

Dibutyryl cyclic AMP has epileptogenic potential in the hippocampus of freely behaving rats: a combined EEG-intracerebral microdialysis study

The effects of dibutyryl cyclic AMP were studied with the combined EEG-intracerebral microdialysis technique in the hippocampus of freely behaving rats. It was found that intrahippocampal microdialysis with this drug produced epileptiform EEG events associated with limbic type behavioral seizures. The dibutyryl cyclic AMP-induced seizures developed with a long latency, and persisted for a prolonged period even after the removal of the drug from the microdialysis fluid. Similar EEG or behavioral manifestations did not occur during intrahippocampal microdialysis with artificial cerebrospinal fluid or ATP solutions. These data suggest that in the hippocampus, in vivo, the cyclic AMP second messenger system may be involved in potentially epileptogenic excitatory processes.

The combined EEG-intracerebral microdialysis technique: a new tool for neuropharmacological studies on freely behaving animals

In this study we combined EEG and intracerebral microdialysis techniques in freely behaving rats. Various drugs were delivered into the hippocampus and cerebral cortex by means of microdialysis and, simultaneously, the EEG activity of the dialyzed area was monitored. The microdialysis procedure itself, when artificial cerebrospinal fluid was perfused, did not change the normal hippocampal or cortical EEG pattern. Drug inclusions into the microdialysis fluid, however, caused marked changes in the electrical activity of the dialyzed sites. In this report we present the following examples: (1) the dose-dependent spike-provoking effect of NMDA in hippocampus, (2) the potentiation of this NMDA effect in hippocampus by dibutyryl cyclic AMP, and (3) the EEG depressant effect of high concentration of K+ in the cerebral cortex. The artificial cerebrospinal fluid and drug solutions were alternated in the microdialysis system with a 2-way valve placed outside the test chamber. As a consequence, the drugs were delivered into the brain without interrupting the ongoing behavior, including sleep, of the examined animals. This study shows that the combined EEG-intracerebral microdialysis technique is a useful tool, with many unique advantages, for in vivo neuropharmacological studies.

Regional localization of the regulatory subunit (RII beta) of the type II cAMP-dependent protein kinase in human brain

The distribution of the regulatory (RII beta) subunits of type II cAMP-dependent protein kinase in cortical and subcortical areas was examined in human control and Alzheimer's disease (AD) brains. Four monoclonal antibodies generated against bovine brain RII, which cross-reacted with human brain RII beta, detected RII-immunoreactivity in pyramidal neurons of the hippocampus and frontal, occipital, parietal and superior temporal cortices and in non-pyramidal neurons of the amygdala and putamen. RII beta immunoreactivity was localized to neuronal perikarya, proximal dendrites and cell processes. With the exception of rare processes in the ventroposterior lateral nucleus, RII-immunoreactivity was not seen in the thalamus. Other areas lacking RII-immunoreactivity included the midbrain, caudate nucleus and globus pallidus. RII-immunoreactivity was not detected in endothelia or glia. Except for the neocortex, the distribution of RII beta immunoreactivity was the same in AD and non-demented control brains; however, cell bodies and their processes stained more intensely and uniformly in the neocortical regions of non-demented controls compared to AD. In the neocortex of AD, RII beta immunoreactivity was substantially decreased in the superior temporal and occipital cortices, but not in the frontal cortex. Our data suggest that RII subunits are regionally distributed in the human brain. RII-immunoreactivity was decreased in some regions of neocortex in AD, but it did not preferentially colocalize with neurofibrillary tangles (NFT), senile plaques, or neuropil threads.

Regional variations in particulate cyclic AMP dependent-protein kinase binding activity in the gerbil hippocampus following transient forebrain ischemia by [3H]cyclic AMP binding

Changes in the binding of [3H]cyclic AMP as an indicator of particulate cyclic AMP-dependent protein kinase (AMP-DPK) binding activity following transient forebrain ischemia were studied in the gerbil using in vitro autoradiography. [3H]Cyclic AMP binding in the strata pyramidale and lacunosum-moleculare of the hippocampal CA1, the stratum pyramidale of the CA3, and the dentate gyrus decreased transiently in the early postischemic phase but then recovered. However, [3H]cyclic AMP binding in the strata pyramidale and radiatum of the CA1, the granular layer of the dentate gyrus, and the upper layer of the cortex decreased again 7 days after ischemia. In the CA4 subfield and the lower layer of the cortex, the binding showed no significant alterations after ischemia. Administration of pentobarbital prior to the induction of ischemia prevented the decrease in [3H]cyclic AMP binding in the CA1 subfield 6 h and 7 days after ischemia, and showed protective effects against neuronal death of the CA1 pyramidal cells 7 days after ischemia. These results indicate that marked alteration of intracellular signal transduction precedes neuronal damage in the hippocampal CA1 subfield. Furthermore, postischemic reduction of [3H]cyclic AMP binding in the histologically intact cerebral cortex, CA3, and dentate gyrus may be the reflection of cellular dysfunction after energy failure.

Characterization and Regional Distribution of a Class of Synapses with Highly Concentrated cAMP Binding Sites in the Rat Brain

A class of putative synaptic terminals with concentrated cAMP binding sites are labelled in unfixed sections of rat brain by means of the ligand 8-thioacetamido fluorescein cAMP (SAF-cAMP), a fluorescent analogue of cAMP. The labelled terminals appear as sharply delimited bouton-like structures in close proximity but external to the cell body of neurons. The SAF-cAMP binding, measured at equilibrium in competition with other nucleotides, indicates that the binding site recognizes the cAMP moiety of SAF-cAMP. In the labelled terminals of the frontal cortex the concentration of SAF-cAMP binding sites is estimated to be in the millimolar range (at least 2.1 +/- 1.0 mM). In a brain homogenate, labelled terminals are visualized only in the membrane fraction enriched in synaptosomes. The cAMP binding activity of the synaptosomes is insoluble in high and in low ionic strength solution and is only partially solubilized by detergents, suggesting that the binding sites are intrinsic membrane proteins and/or proteins associated with the cytoskeleton. There is the possibility that SAF-cAMP labels new cAMP binding sites highly concentrated in a class of synaptic terminals. SAF-cAMP labelling is prominent in well defined regions of the rat brain: (i) the frontal and entorhinal areas of the cortex; (ii) the field CA1 of the hippocampus; (iii) the olfactory system; (iv) the medial nuclei of the thalamus; (v) the parabrachial nuclei and other less defined regions of the reticular substance; (vi) the substantia gelatinosa of Rolando in the spinal cord; and (vii) the neo- and paleocerebellum in the Purkinje cell layer, the archicerebellum in the granular cell layer. SAF-cAMP labelling is absent in specific motor and sensory structures, with the exception of the olfactory system. It is proposed that SAF-cAMP binding sites single out a new type of synaptic terminals involved in complex nervous functions.

Autoradiographic localization of binding sites for second messengers on neurones and astrocytes of cultured rat cerebellum

Using autoradiographic techniques, studies on the cellular localization of binding sites for second messengers in explant cultures of rat cerebellum have been performed. Binding of [3H]inositoltrisphosphate ([3H]IP3), [3H]phorbol dibutyrate ([3H]PDBu) occurred to Purkinje cells whereas binding sites for [3H]forskolin were mainly confined to granule cells and outgrowing nerve fibres. A great number of astrocytes was also labelled by all three radio-ligands. Binding of [3H]IP3, [3H]PDBu and [3H]forskolin was displaced by unlabelled compounds, suggesting specific binding. Our observations provide strong evidence that in addition to neurones, astrocytes also express binding sites for second messengers.

Electron microscopic immunocytochemical evidence that the calmodulin-dependent cyclic nucleotide phosphodiesterase is localized predominantly at postsynaptic sites in the rat brain

The calmodulin-dependent cyclic nucleotide phosphodiesterase represents an important junction between the Ca2+ and the cyclic AMP/cyclic GMP second messenger systems. In brain it is a major cyclic nucleotide-degrading activity and is selectively expressed in the soma and dendrites of regional output neurons [Kincaid et al. (1987) Proc. natn. Acad. Sci. U.S.A. 84, 1118-1122]. In this study the subcellular localization of this enzyme in cerebral cortex, hippocampus and inferior colliculus of rat brain was analysed by electron microscopic immunocytochemical methods using affinity-purified antibodies. The immunoreactivity was found exclusively within neurons whereas glial cells were unstained; preabsorption of antibody with phosphodiesterase eliminated this reactivity, demonstrating the specificity of immunostaining. In the neuronal cell bodies, deposits of immunoreaction product occurred as sparse patches in the cytoplasm and were often associated with organelles such as mitochondria, Golgi-complex and endoplasmic reticulum; nuclei, however, were free from immunoreaction product. In the neuronal processes immunoreactivity was found within dendrites and dendritic spines, whereas the myelinated axons and axon terminals were immunonegative. The postsynaptic densities of asymmetric synapses were associated with especially high concentrations of immunoreaction product. However, the immunopositive synaptic profiles appeared to be quite selective, comprising only a small percentage of the total number of synapses in the neuropil. Our results indicate that the calmodulin-dependent cyclic nucleotide phosphodiesterase is concentrated at postsynaptic sites in specific classes of neurons. This finding supports other morphological evidence indicating a primary role for cyclic nucleotide action in postsynaptic and not presynaptic structures. Furthermore, since this enzyme is regulated by Ca2+, this interface between second messenger systems seems to play a significant role in the postsynaptic integration of Ca(2+)-mediated neuronal inputs.