Implementing fluorescence anisotropy screening and crystallographic analysis to define PKA isoform-selective activation by cAMP analogs

Cyclic AMP (cAMP) is a ubiquitous second messenger that regulates many proteins, most notably cAMP-dependent protein kinase (PKA). PKA holoenzymes (comprised of two catalytic (C) and two regulatory (R) subunits) regulate a wide variety of cellular processes, and its functional diversity is amplified by the presence of four R-subunit isoforms, RIα, RIβ, RIIα, and RIIβ. Although these isoforms all respond to cAMP, they are functionally nonredundant and exhibit different biochemical properties. In order to understand the functional differences between these isoforms, we screened cAMP derivatives for their ability to selectively activate RI and RII PKA holoenzymes using a fluorescence anisotropy assay. Our results indicate that RIα holoenzymes are selectively activated by C8-substituted analogs and RIIβ holoenzymes by N6-substituted analogs, where HE33 is the most prominent RII activator. We also solved the crystal structures of both RIα and RIIβ bound to HE33. The RIIβ structure shows the bulky aliphatic substituent of HE33 is fully encompassed by a pocket comprising of hydrophobic residues. RIα lacks this hydrophobic lining in Domain A, and the side chains are displaced to accommodate the HE33 dipropyl groups. Comparison between cAMP-bound structures reveals that RIIβ, but not RIα, contains a cavity near the N6 site. This study suggests that the selective activation of RII over RI isoforms by N6 analogs is driven by the spatial and chemical constraints of Domain A and paves the way for the development of potent noncyclic nucleotide activators to specifically target PKA iso-holoenyzmes.

Cyr61 is regulated by cAMP-dependent protein kinase with serum levels correlating with prostate cancer aggressiveness

BACKGROUND

Cysteine-rich angiogenic inducer 61 (Cyr61) is an extracellular matrix protein involved in the transduction of growth factor and hormone signaling. Previously, we demonstrated that Cyr61 was highly expressed in prostate cancer (PCa) but that the expression levels were associated with a lower risk of PCa recurrence. In the present study, we demonstrate that serum Cyr61 is a potential biomarker that correlates with PCa aggressiveness. Furthermore, we also explore the potential mechanism underlying the changes in Cyr61 expression during PCa progression.

METHODS

Cyr61 concentrations in the medium from PCa cell lines and in serum samples obtained from PCa patients were measured by sandwich ELISA. Serum Cyr61 levels were correlated with disease characteristics and the association between Cyr61 expression changes by several types of stimulation or stress and cAMP/cAMP-dependent protein kinase (PKA) pathway were examined.

RESULTS

There was a positive correlation between Cyr61 levels in cell supernatants and mRNA expression in these cell lines. Serum Cyr61 levels were significantly higher in non-organ-confined PCa patients (116.3 ± 140.2 ng/ml) than in organ-confined PCa patients (79.7 ± 56.1 ng/ml) (P = 0.031). Cyr61 expression was up-regulated in response to both lysophosphatidic acid and androgen treatments which promoted PCa cell invasion. Serum starvation and phosphoinositide-3-kinase inhibition also resulted in Cyr61 up-regulation; however, they suppressed cell proliferation. Cyr61 up-regulation was correlated with an increase in cAMP and suppressed by PKA inhibition.

CONCLUSIONS

These findings suggest that Cyr61 expression in PCa is regulated by the cAMP/PKA pathway and that circulating Cyr61 levels are a potential serum-based biomarker for characterizing PCa.

The differential regulation of steroidogenic acute regulatory protein-mediated steroidogenesis by type I and type II PKA in MA-10 cells

Following tropic hormone challenge, steroidogenic tissues utilize PKA to phosphorylate unique subsets of proteins necessary to facilitate steroidogenesis. This includes the PKA-dependent expression and activation of the steroidogenic acute regulatory protein (STAR), which mediates the rate-limiting step of steroidogenesis by inducing the transfer of cholesterol from the outer to the inner mitochondrial membrane. Since both type I and type II PKA are present in steroidogenic tissues, we have utilized cAMP analog pairs that preferentially activate each PKA subtype in order to examine their impact on STAR synthesis and activity. In MA-10 mouse Leydig tumor cells Star gene expression is more dependent upon type I PKA, while the post-transcriptional regulation of STAR appears subject to type II PKA. These experiments delineate the discrete effects that type I and type II PKA exert on STAR-mediated steroidogenesis, and suggest complimentary roles for each subtype in coordinating steroidogenesis.

Site-specific effects of tau phosphorylation on its microtubule assembly activity and self-aggregation

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in brains with Alzheimer's disease. The phosphorylation sites of tau are mainly localized in the proline-rich (residues 172-251) and C-terminal tail (residues 368-441) regions, which flank the microtubule-binding repeats. Here, we investigated the effects of tau phosphorylation at these distinct sites/regions on its activity of stimulating microtubule assembly and its self-aggregation. We found that tau phosphorylation at the proline-rich region by dual-specificity tyrosine-phosphorylated and -regulated kinase 1A inhibited its microtubule assembly activity moderately and promoted its self-aggregation slightly. Tau phosphorylation at the C-terminal tail region by glycogen synthase kinase-3beta increased its activity and promoted its self-aggregation markedly. Tau phosphorylation at both regions plus the microtubule-binding region by cAMP-dependent protein kinase diminished its activity (approximately 70% inhibition) and disrupted microtubules. These studies reveal the differential regulation of tau's biological activity and self-aggregation by phosphorylation at various sites/regions.

PKA modulates GSK-3beta- and cdk5-catalyzed phosphorylation of tau in site- and kinase-specific manners

Phosphorylation of tau protein is regulated by several kinases, especially glycogen synthase kinase 3beta (GSK-3beta), cyclin-dependent protein kinase 5 (cdk5) and cAMP-dependent protein kinase (PKA). Phosphorylation of tau by PKA primes it for phosphorylation by GSK-3beta, but the site-specific modulation of GSK-3beta-catalyzed tau phosphorylation by the prephosphorylation has not been well investigated. Here, we found that prephosphorylation by PKA promotes GSK-3beta-catalyzed tau phosphorylation at Thr181, Ser199, Ser202, Thr205, Thr217, Thr231, Ser396 and Ser422, but inhibits its phosphorylation at Thr212 and Ser404. In contrast, the prephosphorylation had no significant effect on its subsequent phosphorylation by cdk5 at Thr181, Ser199, Thr205, Thr231 and Ser422; inhibited it at Ser202, Thr212, Thr217 and Ser404; and slightly promoted it at Ser396. These studies reveal the nature of the inter-regulation of tau phosphorylation by the three major tau kinases.

Fluorescent indicators of cAMP and Epac activation reveal differential dynamics of cAMP signaling within discrete subcellular compartments

Second messenger cAMP regulates many cellular functions through its effectors, such as cAMP-dependent protein kinase (PKA) and Epac (exchange proteins directly activated by cAMP). Spatial and temporal control of cAMP signaling is crucial to differential regulation of cellular targets involved in various signaling cascades. To investigate the compartmentalized cAMP signaling, we constructed fluorescent indicators that report intracellular cAMP dynamics and Epac activation by sandwiching the full-length Epac1 between cyan and yellow mutants of GFP. Elevations of cAMP decreased FRET and increased the ratio of cyan-to-yellow emissions by 10-30% in living mammalian cells. This response can be reversed by removing cAMP-elevating agents and abolished by mutating the critical residue responsible for cAMP binding. Targeting of the reporter to the plasma membrane, where cAMP is produced in response to the activation of beta-adrenergic receptor, revealed a faster cAMP response at the membrane than in the cytoplasm and mitochondria. Simultaneous imaging with targeted cAMP indicator and PKA activity reporter allowed the detection of a much delayed PKA response in the nucleus after the rapid accumulation of cAMP at the plasma membrane of the same cell, despite the immediate presence of a pool of cAMP in the nucleus. Thus, cAMP dynamics and the activation of its effectors are precisely controlled spatiotemporally in vivo.

Nitric oxide/cGMP-mediated protein kinase A activation in the antennal lobes plays an important role in appetitive reflex habituation in the honeybee

Habituation, a form of non-associative learning, is observed throughout the animal kingdom. However, in contrast to associative learning, little is known about the underlying molecular mechanisms. Using the appetitive proboscis extension reflex in honeybees, we show that the cAMP-dependent protein kinase A (PKA) in the antennal lobe (AL) is implicated in the graded decline of behavioral response during habituation. Repeated stimulation leads to a slow and gradual increase in PKA activity superimposed on a fast transient PKA activation induced by each stimulus. These temporally distinct components of PKA activation are pharmacologically dissectible and are restricted to the AL on the stimulated side. Whereas the transient PKA activation induced by each stimulus requires monoaminergic transmission, the slow component of PKA activation is mediated by the nitric oxide (NO)/cGMP system. Local manipulation of the slow component of PKA activation in single ALs specifically interferes with the dynamic of habituation on the corresponding side. Our results provide strong evidence that NO/cGMP-mediated PKA activation in each AL contributes to temporal signal integration during habituation. Dishabituation by a sensory stimulus or spontaneous recovery from habituation does not require the PKA cascade. This provides evidence that the mechanisms underlying dishabituation and spontaneous recovery differ from those underlying temporal signal integration during habituation of the proboscis extension response.

Modulation of Drosophila slowpoke calcium-dependent potassium channel activity by bound protein kinase a catalytic subunit

Drosophila Slowpoke (dSlo) calcium-dependent potassium channels bind directly to the catalytic subunit of cAMP-dependent protein kinase (PKAc). We demonstrate here that coexpression of PKAc with dSlo in mammalian cells results in a dramatic decrease of dSlo channel activity. This modulation requires catalytically active PKAc but is not mediated by phosphorylation of S942, the only PKA consensus site in the dSlo C-terminal domain. dSlo binds to free PKAc but not to the PKA holoenzyme that includes regulatory subunits and is inactive. Activators of endogenous PKA that stimulate dSlo phosphorylation, but do not produce detectable PKAc binding to dSlo, do not modulate channel function. Furthermore, the catalytically inactive PKAc mutant does bind to dSlo but does not modulate channel activity. These results are consistent with the hypothesis that both binding of active PKAc to dSlo and phosphorylation of dSlo or some other protein are necessary for channel modulation.

Cyclic AMP-dependent protein kinase phosphorylates group III metabotropic glutamate receptors and inhibits their function as presynaptic receptors

Recent evidence suggests that the functions of presynaptic metabotropic glutamate receptors (mGluRs) are tightly regulated by protein kinases. We previously reported that cAMP-dependent protein kinase (PKA) directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail region of the receptor, and that phosphorylation of this site inhibits coupling of mGluR2 to GTP-binding proteins. This may be the mechanism by which the adenylyl cyclase activator forskolin inhibits presynaptic mGluR2 function at the medial perforant path-dentate gyrus synapse. We now report that PKA also directly phosphorylates several group III mGluRs (mGluR4a, mGluR7a, and mGluR8a), as well as mGluR3 at single conserved serine residues on their C-terminal tails. Furthermore, activation of PKA by forskolin inhibits group III mGluR-mediated responses at glutamatergic synapses in the hippocampus. Interestingly, beta-adrenergic receptor activation was found to mimic the inhibitory effect of forskolin on both group II and III mGluRs. These data suggest that a common PKA-dependent mechanism may be involved in regulating the function of multiple presynaptic group II and group III mGluRs. Such regulation is not limited to the pharmacological activation of adenylyl cyclase but can also be elicited by the stimulation of endogenous G(s)-coupled receptors, such as beta-adrenergic receptors.

Long-term potentiation induced by theta frequency stimulation is regulated by a protein phosphatase-1-operated gate

Long-term potentiation (LTP) can be induced in the Schaffer collateral-->CA1 synapse of hippocampus by stimulation in the theta frequency range (5-12 Hz), an effect that depends on activation of the cAMP pathway. We investigated the mechanisms of the cAMP contribution to this form of LTP in the rat hippocampal slice preparation. theta pulse stimulation (TPS; 150 stimuli at 10 Hz) by itself did not induce LTP, but the addition of either the beta-adrenergic agonist isoproterenol or the cAMP analog 8-bromo-cAMP (8-Br-cAMP) enabled TPS-induced LTP. The isoproterenol effect was blocked by postsynaptic inhibition of cAMP-dependent protein kinase. Several lines of evidence indicated that cAMP enabled LTP by blocking postsynaptic protein phosphatase-1 (PP1). Activators of the cAMP pathway reduced PP1 activity in the CA1 region and increased the active form of inhibitor-1, an endogenous inhibitor of PP1. Postsynaptic injection of activated inhibitor-1 mimicked the LTP-enabling effect of cAMP pathway stimulation. TPS evoked complex spiking when isoproterenol was present. However, complex spiking was not sufficient to enable TPS-induced LTP, which additionally required the inhibition of postsynaptic PP1. PP1 inhibition seems to promote the activation of Ca(2+)/calmodulin-dependent protein kinase (CaMKII), because (1) a CaMKII inhibitor blocked the induction of LTP by TPS paired with either isoproterenol or activated inhibitor-1 and (2) CaMKII in area CA1 was activated by the combination of TPS and 8-Br-cAMP but not by either stimulus alone. These results indicate that the cAMP pathway enables TPS-induced LTP by inhibiting PP1, thereby enhancing Ca(2+)-independent CaMKII activity.

Inhibition of the cAMP pathway decreases early long-term potentiation at CA1 hippocampal synapses

Long-term potentiation (LTP) has several different phases, and there is general agreement that the late phase of LTP requires the activation of adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). In contrast, several studies indicate that the early LTP is not affected by interfering with the cAMP pathway. We have further tested the role of the cAMP pathway in early LTP using several types of inhibitors. Bath application of the PKA inhibitor H89 suppressed the early LTP induced by a single tetanus. Similarly, the LTP induced by a pairing protocol was decreased by postsynaptic intracellular perfusion of the peptide PKA inhibitor PKI(6-22) amide. The decrease of LTP produced by these inhibitors was evident immediately after induction. These results indicate that PKA is important in early LTP, that its locus of action is postsynaptic, and that it does not act merely by enhancing the depolarization required for LTP induction. The failure of some other inhibitors of the cAMP pathway to affect the early phase of LTP might be attributable to the saturation of some step in the cAMP pathway during a tetanus. In agreement with this hypothesis we found that application of the AC inhibitor SQ 22536 by itself did not affect the early phase of LTP, but did produce a reduction if the cAMP pathway was already attenuated by the PKA inhibitor H89. Our analysis of the results of genetic modifications of the cAMP pathway, especially the work on AC knock-outs, indicates that the genetic data are generally consistent with the pharmacological results showing the importance of this pathway in early LTP.

Ser67-phosphorylated inhibitor 1 is a potent protein phosphatase 1 inhibitor

Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), a major eukaryotic Ser/Thr phosphatase. Nonphosphorylated I-1 is inactive, whereas phosphorylated I-1 is a potent PP1 inhibitor. I-1 is phosphorylated in vivo on Thr(35) and Ser(67). Thr(35) is phosphorylated by cAMP-dependent protein kinase (A kinase), and Thr(35)-phosphorylated I-1 inhibits PP1. Until now the kinase that phosphorylates Ser(67) had not been identified and the physiological role of Ser(67) phosphorylation was unknown. In this study we detected a high level of kinase activity in brain extract when a glutathione S-transferase (GST) fusion I-1 mutant containing an Ala substituted for Thr(35) [GST-I-1(T35A)] was used as the substrate. GST-I-1(T35A) kinase and neuronal cdc2-like protein kinase (NCLK) in the brain extract could not be separated from each other by a series of sequential chromatographies. GST-I-1(T35A) kinase immunoprecipitated with anti-NCLK antibody from kinase-active column fractions. Purified NCLK-phosphorylated GST-I-1(T35A) and I-1 (0.7 mole of phosphate per mole of I-1). HPLC phosphopeptide mapping, amino acid sequencing, and site-directed mutagenesis determined that NCLK phosphorylates Ser(67) of I-1. NCLK-phosphorylated I-1 and I-1(T35A) inhibited PP1 with IC(50) values approximately 9.5 and 13. 8 nM, respectively. When compared, A kinase-phosphorylated I-1 was only approximately 1.2 times more inhibitory than NCLK-phosphorylated I-1. Our data indicate that NCLK is a potential in vivo I-1 kinase and that Thr(35) and Ser(67) phosphorylation independently activate I-1.

Reversible downregulation of protein kinase A during olfactory learning using antisense technique impairs long-term memory formation in the honeybee, Apis mellifera

In this study, we examined the role of cAMP-dependent protein kinase (PKA) in associative olfactory learning of the honeybee, Apis mellifera. In the bee, specific interference with molecules to clarify their role in a certain behavior is difficult, because genetic approaches, such as mutants or transgenic animals, are not feasible at the moment. As a new approach in insects in vivo, we report the use of short antisense oligonucleotides. We show that phosphorothioate-modified oligodeoxynucleotides complementary to the mRNA of a catalytic subunit of PKA directly injected into the bee brain cause a reversible and specific downregulation of both the amount of the catalytic subunit and of PKA activity by 10-15%. The amounts of the regulatory subunit of PKA, as well as PKC, are not affected. The slight "knockdown" of PKA activity during the training procedure, a classical olfactory conditioning of the proboscis extension reflex, neither affects acquisition nor memory retention 3 or 6 hr after training. However, it causes an impairment of long-term memory retention 24 hr after training. Downregulation of PKA 3 hr after training has no detectable effect on memory formation. We conclude that PKA contributes to the induction of a long-term memory 24 hr after training when activated during learning. Second, we demonstrate that the antisense technique is feasible in honeybees in vivo and provides a new and powerful tool for the study of the molecular basis of learning and memory formation in insects.

Voltage-dependent neuromodulation of Na+ channels by D1-like dopamine receptors in rat hippocampal neurons

Activation of D1-like dopamine (DA) receptors reduces peak Na+ current in acutely isolated hippocampal neurons through phosphorylation of the alpha subunit of the Na+ channel by cAMP-dependent protein kinase (PKA). Here we report that neuromodulation of Na+ currents by DA receptors via PKA is voltage-dependent in the range of -110 to -70 mV and is also sensitive to concurrent activation of protein kinase C (PKC). Depolarization enhanced the ability of D1-like DA receptors to reduce peak Na+ currents via the PKA pathway. Similar voltage-dependent modulation was observed when PKA was activated directly with the membrane-permeant PKA activator DCl-cBIMPS (cBIMPS; 20 microM), indicating that the membrane potential dependence occurs downstream of PKA. PKA activation caused only a small (-2.9 mV) shift in the voltage dependence of steady-state inactivation and had no effect on slow inactivation or on the rates of entry into the fast or slow inactivated states, suggesting that another mechanism is responsible for coupling of membrane potential changes to PKA modulation. Activation of PKC with a low concentration of the membrane-permeant diacylglycerol analog oleylacetyl glycerol also potentiated modulation by SKF 81297 or cBIMPS, and these effects were most striking at hyperpolarized membrane potentials where PKA modulation was not stimulated by membrane depolarization. Thus, activation of D1-like DA receptors causes a strong reduction in Na+ current via the PKA pathway, but it is effective primarily when it is combined with depolarization or activation of PKC. The convergence of these three distinct signaling modalities on the Na+ channel provides an intriguing mechanism for integration of information from multiple signaling pathways in the hippocampus and CNS.

NH2-Terminal targeting motifs direct dual specificity A-kinase-anchoring protein 1 (D-AKAP1) to either mitochondria or endoplasmic reticulum

Subcellular localization directed by specific targeting motifs is an emerging theme for regulating signal transduction pathways. For cAMP-dependent protein kinase (PKA), this is achieved primarily by its association with A-kinase-anchoring proteins (AKAPs). Dual specificity AKAP1, (D-AKAP1) binds to both type I and type II regulatory subunits and has two NH2-terminal (N0 and N1) and two COOH-terminal (C1 and C2) splice variants (. J. Biol. Chem. 272:8057). Here we report that the splice variants of D-AKAP1 are expressed in a tissue-specific manner with the NH2-terminal motifs serving as switches to localize D-AKAP1 at different sites. Northern blots showed that the N1 splice is expressed primarily in liver, while the C1 splice is predominant in testis. The C2 splice shows a general expression pattern. Microinjecting expression constructs of D-AKAP1(N0) epitope-tagged at either the NH2 or the COOH terminus showed their localization to the mitochondria based on immunocytochemistry. Deletion of N0(1-30) abolished mitochondrial targeting while N0(1-30)-GFP localized to mitochondria. Residues 1-30 of N0 are therefore necessary and sufficient for mitochondria targeting. Addition of the 33 residues of N1 targets D-AKAP1 to the ER and residues 1-63 fused to GFP are necessary and sufficient for ER targeting. Residues 14-33 of N1 are especially important for targeting to ER; however, residues 1-33 alone fused to GFP gave a diffuse distribution. N1(14-33) thus serves two functions: (a) it suppresses the mitochondrial-targeting motif located within residues 1-30 of N0 and (b) it exposes an ER-targeting motif that is at least partially contained within the N0(1-30) motif. This represents the first example of a differentially targeted AKAP and adds an additional level of complexity to the PKA signaling network.

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.

D1/D5 dopamine receptors inhibit depotentiation at CA1 synapses via cAMP-dependent mechanism

Recent work has shown that D1/D5 dopamine receptors can enhance long-term potentiation (LTP). We investigated whether D1/D5 receptors also affect depotentiation, the reversal of LTP by low-frequency stimulation. D1/D5 agonists greatly reduced depotentiation, an effect that was inhibited by a D1/D5 antagonist. The D1/D5 effect appears to be mediated by adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA), because it was mimicked by the AC activator forskolin and was inhibited by the AC and PKA inhibitors. In vivo studies show that dopamine is released when a reward occurs. Our results raise the possibility that the memory of events before reward might be retained selectively, because dopamine blocks their erasure.

Involvement of presynaptic and postsynaptic mechanisms in a cellular analog of classical conditioning at Aplysia sensory-motor neuron synapses in isolated cell culture

Temporal pairing of presynaptic activity and serotonin produces enhanced facilitation at Aplysia sensory-motor neuron synapses (pairing-specific facilitation), which may contribute to classical conditioning of the gill and siphon withdrawal reflex. This cellular analog of conditioning is thought to involve Ca2+ priming of the cAMP pathway in the sensory neurons. Consistent with that idea, we have found that pairing-specific facilitation by serotonin is greatly reduced by presynaptic injection of a slow Ca2+ chelator or a specific inhibitor of cAMP-dependent protein kinase and is accompanied by a transient increase in the frequency but by no change in the amplitude of spontaneous, miniature EPSPs. However, like post-tetanic potentiation (PTP) and long-term potentiation (LTP) at these synapses, pairing-specific facilitation is also greatly reduced by postsynaptic injection of a rapid Ca2+ chelator or by postsynaptic hyperpolarization during training, although postsynaptic hyperpolarization has no effect on the increase in frequency or on the amplitude of spontaneous EPSPs. These results suggest that pairing-specific facilitation by serotonin involves Hebbian postsynaptic as well as non-Hebbian presynaptic components that interact in some way, perhaps via retrograde signaling, to specifically enhance evoked, synchronized release of transmitter.

Dopaminergic modulation of sodium current in hippocampal neurons via cAMP-dependent phosphorylation of specific sites in the sodium channel alpha subunit

Phosphorylation of brain Na+ channel alpha subunits by cAMP-dependent protein kinase (PKA) decreases peak Na+ current in cultured brain neurons and in mammalian cells and Xenopus oocytes expressing cloned brain Na+ channels. We have studied PKA regulation of Na+ channel function by activation of D1-like dopamine receptors in acutely isolated hippocampal neurons using whole-cell voltage-clamp recording techniques. The D1 agonist SKF 81297 reversibly reduced peak Na+ current in a concentration-dependent manner. No changes in the voltage dependence or kinetics of activation or inactivation were observed. This effect was mediated by PKA, as it was mimicked by application of the PKA activator Sp-5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-monophosphorothioate(cBIMPS) and was inhibited by the specific PKA inhibitor peptide PKAI5-24. cBIMPS had similar effects on type IIA brain Na+ channel alpha subunits expressed in tsA-201 cells, but no effect was observed on a mutant Na+ channel alpha subunit in which serine residues in five PKA phosphorylation sites in the intracellular loop connecting domains I and II (LI-II) had been replaced by alanine. A single mutation, S573A, similarly eliminated cBIMPS modulation. Thus, activation of D1-like dopamine receptors results in PKA-dependent phosphorylation of specific sites in LI-II of the Na+ channel alpha subunit, causing a reduction in Na+ current. Such modulation is expected to exert a profound influence on overall neuronal excitability. Dopaminergic input to the hippocampus from the mesocorticolimbic system may exert this influence in vivo.

Modulation of the cloned skeletal muscle L-type Ca2+ channel by anchored cAMP-dependent protein kinase

Ca2+ influx through skeletal muscle Ca2+ channels and the force of contraction are increased in response to beta-adrenergic stimulation and high-frequency electrical stimulation. These effects are thought to be mediated by cAMP-dependent phosphorylation of the skeletal muscle Ca2+ channel. Modulation of the cloned skeletal muscle Ca2+ channel by cAMP-dependent phosphorylation and by depolarizing prepulses was reconstituted by transient expression in tsA-201 cells and compared to modulation of the native skeletal muscle Ca2+ channel as expressed in mouse 129CB3 skeletal muscle cells. The heterologously expressed Ca2+ channel consisting of alpha1, alpha2delta, and beta subunits gave currents that were similar in time course, current density, and dihydropyridine sensitivity to the native Ca2+ channel. cAMP-dependent protein kinase (PKA) stimulation by Sp-5,6-DCl-cBIMPS (cBIMPS) increased currents through both native and expressed channels two- to fourfold. Tail currents after depolarizations to potentials between -20 and +80 mV increased in amplitude and decayed more slowly as either the duration or potential of the depolarization was increased. The time- and voltage-dependent slowing of channel deactivation required the activity of PKA, because it was enhanced by cBIMPS and reduced or eliminated by the peptide PKA inhibitor PKI (5-24) amide. This voltage-dependent modulation of the cloned skeletal muscle Ca2+ channel by PKA also required anchoring of PKA by A-Kinase Anchoring Proteins because it was blocked by peptide Ht 31, which disrupts such anchoring. The results show that the skeletal muscle Ca2+ channel expressed in heterologous cells is modulated by PKA at rest and during depolarization and that this modulation requires anchored protein kinase, as it does in native skeletal muscle cells.