Critical role of cAMP-dependent protein kinase anchoring to the L-type calcium channel Cav1.2 via A-kinase anchor protein 150 in neurons

The cAMP-dependent protein kinase (PKA) regulates a wide array of cellular functions. In brain and heart PKA increases the activity of the L-type Ca2+ channel Cav1.2 in response to beta-adrenergic stimulation. Cav1.2 forms a complex with the beta2-adrenergic receptor, the trimeric GS protein, adenylyl cyclase, and PKA wherein highly localized signaling occurs [Davare, M. A., Avdonin, V., Hall, D. D., Peden, E. M., Burette, A., Weinberg, R. J., Horne, M. C., Hoshi, T., and Hell, J. W. (2001) Science 293, 98-101]. PKA primarily phosphorylates Cav1.2 on serine 1928 of the central, pore-forming alpha11.2 subunit. Here we demonstrate that the A-kinase anchor protein 150 (AKAP150) is critical for PKA-mediated regulation of Cav1.2 in the brain. AKAP150 and MAP2B specifically co-immunoprecipitate with Cav1.2 from rat brain. Recombinant AKAP75, the bovine homologue to rat AKAP150, binds directly to three different sites of alpha11.2. MAP2B from rat brain also interacts with these same sites in pull-down assays. Gene disruption of AKAP150 in mice dramatically reduces co-immunoprecipitation of PKA with Cav1.2 and prevents phosphorylation of serine 1928 upon beta-adrenergic stimulation in vivo. These results demonstrate the physiological relevance of PKA anchoring by AKAPs in general and AKAP150 specifically in the regulation of Cav1.2 in vivo.     

Occurrence of A-kinase anchor protein and associated cAMP-dependent protein kinase in the inner compartment of mammalian mitochondria

Evidence showing the existence in the inner compartment of rat-heart mitochondria of AKAP121 and associated PKA is presented. Immunoblotting analysis and trypsin digestion pattern show that 90% or more of mitochondrial C-PKA, R-PKA and AKAP121 is localized in the inner mitochondrial compartment, when prepared both from isolated mitochondria or cardiomyocyte cultures. This localization is verified by measurement of the specific catalytic activity of PKA, radiolabelling of R-PKA by (32)P-phosphorylated C-PKA and of AKAP by (32)P-phosphorylated R-PKA and electron microscopy of mitochondria exposed to gold-conjugated AKAP121 antibody.     

Phosphorylation of presynaptic and postsynaptic calcium channels by cAMP-dependent protein kinase in hippocampal neurons.

Phosphorylation by cAMP-dependent protein kinase (PKA) and other second messenger-activated protein kinases modulates the activity of a variety of effector proteins including ion channels. Anti-peptide antibodies specific for the alpha 1 subunits of the class B, C or E calcium channels from rat brain specifically recognize a pair of polypeptides of 220 and 240 kDa, 200 and 220 kDa, and 240 and 250 kDa, respectively, in hippocampal slices in vitro. These calcium channels are localized predominantly on presynaptic and dendritic, somatic and dendritic, and somatic sites, respectively, in hippocampal neurons. Both size forms of alpha 1B and alpha 1E and the full-length form of alpha 1C are phosphorylated by PKA after solubilization and immunoprecipitation. Stimulation of PKA in intact hippocampal slices also induced phosphorylation of 25-50% of the PKA sites on class B N-type calcium channels, class C L-type calcium channels and class E calcium channels, as assessed by a back-phosphorylation method. Tetraethylammonium ion (TEA), which causes neuronal depolarization and promotes repetitive action potentials and neurotransmitter release by blocking potassium channels, also stimulated phosphorylation of class B, C and E alpha 1 subunits, suggesting that these three classes of channels are phosphorylated by PKA in response to endogenous electrical activity in the hippocampus. Regulation of calcium influx through these calcium channels by PKA may influence calcium-dependent processes within hippocampal neurons, including neurotransmitter release, calcium-activated enzymes and gene expression.     

Stimulation of arterial smooth muscle L-type calcium channels by hydrogen peroxide requires protein kinase C

Changes in intracellular calcium regulate countless biological processes. In arterial smooth muscle, voltage-dependent L-type calcium channels are major conduits for calcium entry with the primary function being determination of arterial diameter. Similarly, changes in intracellular redox status, either discrete controlled changes or global pathological perturbations, are also critical determinants of cell function. We recently reported that in arterial smooth muscle cells, local generation of hydrogen peroxide leads to colocalized calcium entry through L-type calcium channels. Here we extend our investigation into mechanisms linking hydrogen peroxide to calcium influx through L-type calcium channels by focusing on the role of protein kinase C (PKC). Our data indicate that stimulation of L-type calcium channels by hydrogen peroxide requires oxidant-dependent increases in PKC catalytic activity. This effect is independent of classical cofactor-dependent activation of PKC by diacylglycerol. These data provide additional experimental evidence supporting the concept of oxidative stimulation of L-type calcium channels.     

An endogenous membrane-bound protein inhibits the phosphorylation of the L-type calcium channel by the cAMP-dependent protein kinase.

An endogenous protein inhibits the PKA-phosphorylation of the DHP-binding calcium channel complex in vitro. The inhibitory activity could be reduced by a treatment with detergents or dithiothreitol. Further purification separates the inhibitory activity from the dihydropyridine-binding calcium channel complex. Both activities are localized in the plasma membrane indicating that this protein kinase-inhibitor could interfere with the phosphorylation of the calcium channel by the cAMP-dependent protein kinase. The inhibitory activity may therefore take part in the regulation of the calcium channel current.     

cAMP-dependent protein kinase and Ca2+ influx through L-type voltage-gated calcium channels mediate Raf-independent activation of extracellular regulated kinase in response to glucagon-like peptide-1 in pancreatic beta-cells

Glucagon like peptide-1 (GLP1) is a G(s)-coupled receptor agonist that exerts multiple effects on pancreatic beta-cells, including the stimulation of insulin gene expression and secretion. In this report, we show that treatment of the mouse pancreatic beta-cell line MIN6 with GLP1 leads to the glucose-dependent activation of Erk. These effects are mimicked by forskolin, a direct activator of adenylate cyclase, and blocked by H89, an inhibitor of cAMP-dependent protein kinase. Additionally, we provide evidence that GLP1-stimulated activation of Erk requires an influx of calcium through L-type voltage-gated calcium channels and the activation of calcium/calmodulin-dependent protein kinase II. GLP1-stimulated activation of Erk is blocked by inhibitors of MEK, but GLP1 does not induce the activation of A-Raf, B-Raf, C-Raf, or Ras. Additionally, dominant negative forms of Ras(N17) and Rap1(N17) fail to block GLP1-stimulated activation of Erk. In conclusion, our results indicate that, in the presence of stimulatory concentrations of glucose, GLP1 stimulates the activation of Erk through a mechanism dependent on MEK but independent of both Raf and Ras. This requires 1) the activation of cAMP-dependent protein kinase, 2) an influx of extracellular Ca(2+) through L-type voltage-gated calcium channels, and 3) the activation of CaM kinase II.     

Calcineurin activation by slow calcium release from intracellular stores suppresses protein kinase C regulation of L-type calcium channels in L6 cells

L-type Ca²⁺ channel activity was assayed in L6 cells as the rate of nifedipine-sensitive Ba²⁺ influx in a depolarizing medium. In the absence of extracellular Ca²⁺, activation of protein kinase C (PKC) with phorbol-12-myristate-13-acetate (PMA) or thymeleatoxin (TMX) inhibited Ba²⁺ influx by 38%. Thapsigargin (Tg), a selective inhibitor of the Ca²⁺-ATPase in the sarcoplasmic reticulum, evoked a rise in the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in a Ca²⁺-free medium from 30 to 80 nM. This [Ca²⁺]_i increase declined slowly, giving rise to a modest elevation of [Ca²⁺]_i that persisted for >5 min. The inhibitory effects of PMA and TMX on channel activity were abolished when tested in Tg-treated cells in a Ca²⁺-free medium. However, when the Ca²⁺ ionophore, ionomycin, was applied with Tg, PMA and TMX retained their inhibitory effect on L-type Ca²⁺ channel activity, suggesting that a lower amplitude and prolonged release of Ca²⁺ stores is necessary for abrogating PKC-mediated inhibition of LCC. Cyclosporin A (5 μM) and ascomycin (5 μM), inhibitors of the Ca²⁺/calmodulin-dependent protein phosphatase, calcineurin, fully restored the inhibitory effect of PMA and TMX on channel activity. Addition of 1 mM CaCl₂ to the Tg-treated cells increased [Ca²⁺]_i to 165 nM and also restored the inhibitory effects of PMA and TMX. These results indicate that a small, relatively prolonged [Ca²⁺]_i increase elicited by passive depletion of internal Ca²⁺ stores led to activation of calcineurin, giving rise to an increase in protein phosphatase activity that counteracted the inhibitory effects of PKC on channel activity. A larger increase in [Ca²⁺]_i via store-dependent Ca²⁺ entry enhanced the activity of PKC sufficiently to overcome the protein phosphatase activity of calcineurin. This study is the first to demonstrate that the regulation of L-type Ca²⁺ channels in a myocyte model involves a balance between the differential Ca²⁺ sensitivities and opposing actions of PKC and calcineurin.     

Synthetic segments of the mammalian beta AR are preferentially recognized by cAMP-dependent protein kinase and protein kinase C

Desensitization of the beta-adrenergic receptor has been correlated in some cell systems with receptor phosphorylation. Various kinases have been implicated in these phosphorylation processes, including both cAMP-dependent protein kinase and protein kinase C. In the present study, we have utilized the protein sequence information obtained from the cloning of the mammalian beta-adrenergic receptor to prepare synthetic peptides corresponding to regions of the receptor which would be predicted to act as possible substrates for these kinases in vivo. Two of these receptor-derived peptides were found to serve as substrates for these protein kinases. A peptide corresponding to amino acids 257-264 of the beta-receptor is the preferred substrate for the cAMP-dependent protein kinase, while protein kinase C showed a marked preference for phosphorylation of a peptide corresponding to residues 341-351 of the beta-adrenergic receptor.     

Functional regulation of L-type calcium channels via protein kinase A-mediated phosphorylation of the beta(2) subunit

Activation of protein kinase A (PKA) through the beta-adrenergic receptor pathway is crucial for the positive regulation of cardiac L-type currents; however it is still unclear which phosphorylation events cause the robust regulation of channel function. In order to study whether or not the recently identified PKA phosphorylation sites on the beta(2) subunit are of functional significance, we coexpressed wild-type (WT) or mutant beta(2) subunits in tsA-201 cells together with an alpha(1C) subunit, alpha(1C)Delta1905, that lacked the C-terminal 265 amino acids, including the only identified PKA site at Ser-1928. This truncated alpha(1C) subunit was similar to the truncated alpha(1C) subunit isolated from cardiac tissue not only in size ( approximately 190 kDa), but also with respect to its failure to serve as a PKA substrate. In cells transfected with the WT beta(2) subunit, voltage-activated Ba(2+) currents were significantly increased when purified PKA was included in the patch pipette. Furthermore, mutations of Ser-478 and Ser-479 to Ala, but not Ser-459 to Ala, on the beta(2) subunit, completely abolished the PKA-induced increase of currents. The data indicate that the PKA-mediated stimulation of cardiac L-type Ca(2+) currents may be at least partially caused by phosphorylation of the beta(2) subunit at Ser-478 and Ser-479.     

Identification of the sites phosphorylated by cyclic AMP-dependent protein kinase on the beta 2 subunit of L-type voltage-dependent calcium channels.

Voltage-dependent L-type calcium (Ca) channels are heteromultimeric proteins that are regulated through phosphorylation by cAMP-dependent protein kinase (PKA). We demonstrated that the beta 2 subunit was a substrate for PKA in intact cardiac myocytes through back-phosphorylation experiments. In addition, a heterologously expressed rat beta 2a subunit was phosphorylated at two sites in vitro by purified PKA. This beta 2a subunit contains two potential consensus sites for PKA-mediated phosphorylation at Thr164 and Ser591. However, upon mutation of both of these residues to alanines, the beta 2a subunit remained a good substrate for PKA. The actual sites of phosphorylation on the beta 2a subunit were identified by phosphopeptide mapping and microsequencing. Phosphopeptide maps of a bacterially expressed beta 2a subunit demonstrated that this subunit was phosphorylated similarly to the beta 2 subunit isolated from heart tissue and that the phosphorylation sites were contained in the unique C-terminal region. Microsequencing identified three serine residues, each of which conformed to loose consensus sites for PKA-mediated phosphorylation. Mutation of these residues to alanines resulted in the loss of the PKA-mediated phosphorylation of the beta 2a subunit. The results suggest that phosphorylation of the beta 2a subunit by PKA occurs at three loose consensus sites for PKA in the C-terminus and not at either of the two strong consensus sites for PKA. The results also highlight the danger of assuming that consensus sites represent actual sites of phosphorylation. The actual sites of PKA-mediated phosphorylation are conserved in most beta 2 subunit isoforms and thus represent potential sites for regulation of channel activity. The sites phosphorylated by PKA are not substrates for protein kinase C (PKC), as the mutated beta 2 subunits lacking PKA sites remained good substrates for PKC.     

Caldesmon phosphorylation in intact human platelets by cAMP-dependent protein kinase and protein kinase C

Caldesmon is a calmodulin- and actin-binding protein present in both smooth and non-muscle tissue. The present study demonstrates that platelet caldesmon is a substrate for cAMP-dependent protein kinase (protein kinase A). Purified platelet caldesmon has an apparent molecular mass of 82 kDa on sodium dodecyl sulfate-polyacrylamide gels and can be phosphorylated in vitro by the catalytic subunit of protein kinase A to a level of 2 mol of phosphate/mol of caldesmon. Phosphorylation of caldesmon by protein kinase A results in a shift in the apparent molecular mass of the protein to 86 kDa. When caldesmon was immunoprecipitated from intact platelets treated with prostacyclin (PGI2) the same shift in apparent molecular mass of caldesmon was observed. Comparison of two-dimensional tryptic phosphopeptide maps of caldesmon phosphorylated in vitro by protein kinase A with caldesmon immunoprecipitated from intact platelets verified that protein kinase A was responsible for the observed increase in caldesmon phosphorylation in PGI2-treated platelets. The present study demonstrates that although caldesmon is basally phosphorylated in the intact platelet, activation of protein kinase A by PGI2 results in the significant incorporation of phosphate into two new sites. In addition, the effects of phorbol ester, collagen, and thrombin on caldesmon phosphorylation were also examined. Although phorbol ester treatment results in a significant increase in caldesmon phosphorylation apparently by protein kinase C, treatment of intact platelets with thrombin or collagen does not result in an increase in caldesmon phosphorylation.     

Inhibition of N- and L-type calcium channels by muscarinic receptor activation in rat sympathetic neurons.

Modulation of N- and L-type Ca2+ channels by oxotremorine-M (oxo-M) acting on muscarinic receptors and norepinephrine (NE) acting on alpha-adrenergic receptors was studied in superior cervical ganglion neurons. Oxo-M depresses dihydropyridine-augmented tail currents in whole-cell recordings, whereas NE does not. This modulation of L-type Ca2+ channels by oxo-M is abolished by adding 20 mM BAPTA to the pipette solution. Oxo-M, acting via a diffusible messenger, reduces the probability of opening of single N- and L-type channels recorded in cell-attached patches. We conclude that a diffusible messenger signaling pathway activated by oxo-M inhibits both N- and L-type Ca2+ channels, whereas a membrane-delimited pathway activated by oxo-M and NE inhibits only N-type Ca2+ channels.     

Comparison of phosphorylation sites in protamines between protein kinase C and cAMP-dependent protein kinase

Phosphorylation sites of protamines by protein kinase C and cAMP-dependent protein kinase (protein kinase A) were studied. Using clupeine Y1 as a substrate, protein kinase C phosphorylates both Ser and Thr residues, whereas protein kinase A phosphorylates only Ser residue(s). Protein kinase C phosphorylates all Ser and Thr residues of clupeine Y2 and Z, however protein kinase A phosphorylates mainly Ser9 and slightly Thr5 in clupeine Y2 and Ser6 and Ser10 in clupeine Z. These results suggest that protein kinase C recognizes more sites than those of protein kinase A and may participate in protamine phosphorylation in vivo.     

L-type calcium channels and cytochrome b5 reductase are components of protein complexes tightly associated with lipid rafts microdomains of the neuronal plasma membrane

The presence of cytosolic calcium microcompartments in neurons is well established. L-type voltage calcium channels play a leading role in the rise of cytosolic calcium in the neuronal soma and are sensitive to redox modulation. In a recent work [Samhan-Arias, A.K., García-Bereguiaín, M.A., Martín-Romero, F.J. and Gutiérrez-Merino, C. (2009) Mol. and Cell. Neurosci. 40, 14–26], we have shown that cytochrome b₅ reductase, whose deregulation leads to an overshot of superoxide anion production at the neuronal plasma membrane that triggers apoptosis in primary cultures of cerebellar granule neurons in culture, forms a large mesh of redox centres associated with lipid rafts in these neurons. In this work, we have implemented the use of fluorescent antibodies as reagents for quantitative Förster resonance energy transfer measurements and analysis using fluorescence microscopy images of cerebellar granule neurons in culture. The results of this study show that L-type voltage-operated calcium channels are also enriched in lipid rafts associated protein microdomains at a distance between 10 and 100 nm from cytochrome b₅ reductase. The methodological improvements done in this work can be also valuable for the study of proteins compartmentalization within other subcellular microdomains in any cell type in culture.     

A single step purification for recombinant proteins. Characterization of a microtubule associated protein (MAP 2) fragment which associates with the type II cAMP-dependent protein kinase.

A 167 base pair DNA cassette has been constructed to facilitate the detection and purification of recombinant proteins. This cassette, kfc, encodes three distinct peptide units: a phosphorylation site for the cAMP-dependent protein kinase (PKA), called kemptide, a factor Xa cleavage site, and a calmodulin-binding peptide. Expressed kfc fusion proteins can be purified from bacterial lysates in one step by affinity chromatography on calmodulin-agarose using EGTA as eluant. As a test of this system, we describe the expression, purification and characterization of the PKA binding domain of the microtubule associated protein (MAP 2).     

Phosphorylation of keratin intermediate filaments by protein kinase C, by calmodulin-dependent protein kinase and by cAMP-dependent protein kinase.

Keratins, constituent proteins of intermediate filaments of epithelial cells, are phosphoproteins containing phosphoserine and phosphothreonine. We examined the in vitro phosphorylation of keratin filaments by cAMP-dependent protein kinase, protein kinase C and Ca2+/calmodulin-dependent protein kinase II. When rat liver keratin filaments reconstituted by type I keratin 18 (molecular mass 47 kDa; acidic type) and type II keratin 8 (molecular mass 55 kDa; basic type) in a 1:1 ratio were used as substrates, all the protein kinases phosphorylated both of the constituent proteins to a significant rate and extent, and disassembly of the keratin filament structure occurred. Kinetic analysis suggested that all these protein kinases preferentially phosphorylate keratin 8, compared to keratin 18. The amino acid residues of keratins 8 and 18 phosphorylated by cAMP-dependent protein kinase or protein kinase C were almost exclusively serine, while those phosphorylated by Ca2+/calmodulin-dependent protein kinase II were serine and threonine. Peptide mapping analysis indicated that these protein kinases phosphorylate keratins 8 and 18 in a different manner. These observations gave the way for in vivo studies of the role of phosphorylation in the reorganization of keratin filaments.     

A glutathione deficit alters dopamine modulation of L-type calcium channels via D2 and ryanodine receptors in neurons

Synthesis of glutathione, a major redox regulator, is compromised in schizophrenia. We postulated that the resulting glutathione deficit via its effect on redox-sensitive proteins could contribute to dysfunction of some neurotransmitter systems in schizophrenia. We investigated whether a glutathione deficit, induced by a blocker of glutathione synthesis, L-buthionine-(S,R)-sulfoximine, affects intracellular pathways implicated in dopamine signaling in neurons, namely dopamine modulation of calcium responses to NMDA. Such a glutathione deficit changed the modulation of responses by dopamine, from enhanced responses in control neurons (likely via D1-type receptors) to decreased responses in low-glutathione neurons (via D2-type receptors). This difference in dopamine modulation was due to a different modulation of L-type calcium channels activated during NMDA stimulation: dopamine enhanced function of these channels in control neurons but decreased it in low-glutathione neurons. The effect of a glutathione deficit on dopamine signaling was dependent on the redox-sensitive ryanodine receptors (RyRs), whose function was enhanced in low-glutathione neurons. This suggests that enhanced RyRs in low-glutathione neurons strengthens intracellular calcium-dependent pathways following activation of D2-type receptors and causes a decrease in function of L-type channels. This represents a mechanism by which dopaminergic systems could be dysfunctional under conditions of impaired glutathione synthesis as in schizophrenia.