Immunological distinction between guanosine 3':5'-monophosphate-dependent and adenosine 3':5'-monophosphate-dependent protein kinases

Phosphorylation-Dependent Differences in CXCR4-LASP1-AKT1 Interaction between Breast Cancer and Chronic Myeloid Leukemia

The serine/threonine protein kinase AKT1 is a downstream target of the chemokine receptor 4 (CXCR4), and both proteins play a central role in the modulation of diverse cellular processes, including proliferation and cell survival. While in chronic myeloid leukemia (CML) the CXCR4 is downregulated, thereby promoting the mobilization of progenitor cells into blood, the receptor is highly expressed in breast cancer cells, favoring the migratory capacity of these cells. Recently, the LIM and SH3 domain protein 1 (LASP1) has been described as a novel CXCR4 binding partner and as a promoter of the PI3K/AKT pathway. In this study, we uncovered a direct binding of LASP1, phosphorylated at S146, to both CXCR4 and AKT1, as shown by immunoprecipitation assays, pull-down experiments, and immunohistochemistry data. In contrast, phosphorylation of LASP1 at Y171 abrogated these interactions, suggesting that both LASP1 phospho-forms interact. Finally, findings demonstrating different phosphorylation patterns of LASP1 in breast cancer and chronic myeloid leukemia may have implications for CXCR4 function and tyrosine kinase inhibitor treatment.

Oxidant sensor in the cGMP-binding pocket of PKGIα regulates nitroxyl-mediated kinase activity

Despite the mechanisms for endogenous nitroxyl (HNO) production and action being incompletely understood, pharmacological donors show broad therapeutic promise and are in clinical trials. Mass spectrometry and site-directed mutagenesis showed that chemically distinct HNO donors 1-nitrosocyclohexyl acetate or Angeli’s salt induced disulfides within cGMP-dependent protein kinase I-alpha (PKGIα), an interdisulfide between Cys42 of the two identical subunits of the kinase and a previously unobserved intradisulfide between Cys117 and Cys195 in the high affinity cGMP-binding site. Kinase activity was monitored in cells transfected with wildtype (WT), Cys42Ser or Cys117/195Ser PKGIα that cannot form the inter- or intradisulfide, respectively. HNO enhanced WT kinase activity, an effect significantly attenuated in inter- or intradisulfide-deficient PKGIα. To investigate whether the intradisulfide modulates cGMP binding, real-time imaging was performed in vascular smooth muscle cells expressing a FRET-biosensor comprising the cGMP-binding sites of PKGIα. HNO induced FRET changes similar to those elicited by an increase of cGMP, suggesting that intradisulfide formation is associated with activation of PKGIα. Intradisulfide formation in PKGIα correlated with enhanced HNO-mediated vasorelaxation in mesenteric arteries in vitro and arteriolar dilation in vivo in mice. HNO induces intradisulfide formation in PKGIα, inducing the same effect as cGMP binding, namely kinase activation and thus vasorelaxation.

Protein kinase A activation by the anti-cancer drugs ABT-737 and thymoquinone is caspase-3-dependent and correlates with platelet inhibition and apoptosis

Chemotherapy-induced thrombocytopenia is a common bleeding risk in cancer patients and limits chemotherapy dose and frequency. Recent data from mouse and human platelets revealed that activation of protein kinase A/G (PKA/PKG) not only inhibited thrombin/convulxin-induced platelet activation but also prevented the platelet pro-coagulant state. Here we investigated whether or not PKA/PKG activation could attenuate caspase-dependent apoptosis induced by the anti-cancer drugs ABT-737 (the precursor of navitoclax) and thymoquinone (TQ), thereby potentially limiting chemotherapy-induced thrombocytopenia. This is particularly relevant as activation of cyclic nucleotide signalling in combination chemotherapy is an emerging strategy in cancer treatment. However, PKA/PKG-activation, as monitored by phosphorylation of Vasodilator-stimulated phosphoprotein (VASP), did not block caspase-3-dependent platelet apoptosis induced by the compounds. In contrast, both substances induced PKA activation themselves and PKA activation correlated with platelet inhibition and apoptosis. Surprisingly, ABT-737- and TQ-induced VASP-phosphorylation was independent of cAMP levels and neither cyclases nor phosphatases were affected by the drugs. In contrast, however, ABT-737- and TQ-induced PKA activation was blocked by caspase-3 inhibitors. In conclusion, we show that ABT-737 and TQ activate PKA in a caspase-3-dependent manner, which correlates with platelet inhibition and apoptosis and therefore potentially contributes to the bleeding risk in chemotherapy patients.

The oligopeptide DT-2 is a specific PKG I inhibitor only in vitro, not in living cells

BACKGROUND AND PURPOSE

cGMP is involved in the regulation of many cellular processes including cardiac and smooth muscle contractility, aldosterone synthesis and inhibition of platelet activation. Intracellular effects cGMP are mediated by cGMP-dependent PKs, cGMP-regulated PDEs and cGMP-gated ion channels. PKG inhibitors are widely used to discriminate PKG-specific effects. They can be divided into cyclic nucleotide-binding site inhibitors such as Rp-phosphorothioate analogues (Rp-cGMPS), ATP-binding site inhibitors such as KT5823, and substrate binding site inhibitors represented by the recently described DT-oligopeptides. As it has been shown that Rp-cGMPS and KT5823 have numerous non-specific effects, we analysed the pharmacological properties of the oligopeptide (D)-DT-2 described as a highly specific, membrane-permeable, PKG inhibitor.

EXPERIMENTAL APPROACH

Specificity and potency of (D)-DT-2 to inhibit PKG activity was evaluated using biochemical assays in vitro and by substrate phosphorylation analysis in various cell types including human platelets, rat mesangial cells and rat neonatal cardiomyocytes.

KEY RESULTS

Despite potent inhibition of PKGI in vitro, (D)-DT-2 lost specificity for PKG in cell homogenates and particularly in living cells, as demonstrated by phosphorylation of different substrates. Instead, (D)-DT-2 modulated activity of other kinases including ERK, p38, PKB and PKC, thereby inducing unpredicted and often opposing functional effects.

CONCLUSIONS AND IMPLICATIONS

We conclude that DT-oligopeptides, as other inhibitors, cannot be used to specifically inhibit PKG in intact cells. Therefore, no specific pharmacological PKG inhibitors are available, and reliable studies of PKG signalling can only be made by using RNA knockdown or genetic deletion methods.

Phosphorylation of mouse LASP-1 on threonine 156 by cAMP- and cGMP-dependent protein kinase

LIM and SH3 domain protein (LASP-1) is a specific focal adhesion protein involved in cell migration. Overlay studies demonstrate that LASP-1 directly binds to the proline-rich domains of zyxin, lipoma preferred partner (LPP), and vasodilator-stimulated phosphoprotein (VASP), with zyxin being the most prominent interacting partner. Despite the LIM/zinc-finger domain, hypothesized to be involved in homodimerization, LASP-1 exists as a monomer. In vitro phosphorylation of recombinant mouse LASP-1 by cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) occurs at serine 61, serine 99, and threonine 156 whereas in intact cells mouse LASP-1 is phosphorylated only at threonine 156. This site is different from the known in vivo phosphorylation sites in human (serine 146) and rabbit (serine 99 and serine 146). Nevertheless, immunofluorescence of LASP-1 in human and mouse mesangial cells revealed no difference in subcellular distribution. Exposure of the cells to forskolin induced a translocation of both, human and mouse LASP-1, from the focal contacts to the cell interior without affecting F-actin structure. Immunoblotting of LASP-1 in various mouse and human tissues detected a similar prominent expression in non-muscle tissue. Altogether, our data suggest so far no functional differences between human and mouse LASP-1.

Actin binding of human LIM and SH3 protein is regulated by cGMP- and cAMP-dependent protein kinase phosphorylation on serine 146

Various drugs that elevate cGMP levels and activate cGMP-dependent protein kinase (cGK) inhibit agonist-induced platelet activation. In the present study we identified the LIM and SH3 domain protein (LASP) that was recently cloned from human breast cancer cells (Tomasetto, C., Regnier, C., Moog-Lutz, C., Mattei, M. G., Chenard, M. P., Liderau, R., Basset, P., and Rio, M. C. (1995) Genomics 28, 367-376) as a novel substrate of cGK in human platelets. Recombinant human LASP was phosphorylated by cGMP- and cAMP-dependent protein kinase (cAK) in vitro. Cotransfection of PtK-2 cells with LASP and cGK confirmed phosphorylation of LASP in vivo. Studies with human LASP mutants identified serine 146 as a specific phosphorylation site for cGK and cAK in vivo. LASP is an actin-binding protein, and the phospho-LASP-mimicking mutant S146D showed reduced binding affinity for F-actin in cosedimentation experiments. Immunofluorescence of transfected PtK2 cells demonstrated the localization of LASP in the tips of cell membrane extensions and at cell-cell contacts. Expression of the human LASP mutant S146D resulted in nearly complete relocalization to the cytosol and reduced migration of the cells. Taken together, these data suggest that phosphorylation of LASP by cGK and cAK may be involved in cytoskeletal organization and cell motility.

A cGMP-dependent protein kinase assay for high throughput screening based on time-resolved fluorescence resonance energy transfer

Activation of cyclic GMP-dependent protein kinase (cGK) is an important event in the regulation of blood pressure and platelet function. Upstream signals are the generation of nitric oxide (NO) by NO synthases and the subsequent rise in cyclic GMP levels mediated by NO-dependent guanylyl cyclases (GCs). The identification of new cGK activators by high throughput screening (HTS) may lead to the development of a novel class of therapeutics for the treatment of cardiovascular diseases. Therefore, a homogeneous, nonradioactive assay for cGK activity was developed using a biotinylated peptide derived from vasodilator-stimulated phosphoprotein (VASP), a well-characterized natural cGK substrate. The phosphorylated peptide could be detected by a VASP-specific monoclonal phosphoserine antibody and a fluorescent detection system consisting of a europium-labeled secondary antibody and allophycocyanin (APC)-labeled streptavidin. Fluorescence resonance energy transfer (FRET) from europium to APC was detected in a time-resolved fashion (TR-FRET). Activation and inhibition constants for known substances determined by this new fluorescence-based assay correlated well with published results obtained by conventional radioactive cGK activity assays. The assay proved to be sensitive, robust, highly specific for cGK, and suitable for HTS in 96- and 384-well formats. This assay is applicable to purified enzymes as well as to complex samples such as human platelet extracts.

Site-specific phosphorylation and point mutations of telokin modulate its Ca2+-desensitizing effect in smooth muscle

Forskolin and 8-bromoguanosine 3'-5'-cyclic monophosphate (8-Br-cGMP) induce phosphorylation of Ser-13 of telokin and relaxation of smooth muscle at constant calcium. Comparison with the effect of wild type with aspartate (D; to mimic phosphorylation) and alanine (A; non-phosphorylatable) mutants of telokin showed that the S13D mutant was more effective than wild type in relaxing smooth muscle at constant calcium. The efficacy of the Ser-13A, S12A, and S12D mutants was not significantly different from that of wild-type telokin. The effect of neither S13D nor Ser-13A was affected by 8-Br-cGMP, whereas the effect of wild type, S12A, and S12D was enhanced by 8-Br-cGMP, indicating the specificity of Ser-13 charge modification. Mutation of Ser-19 (a mitogen-activated protein kinase site) showed the S19A to be more effective than, and S19D to be not different from, wild-type telokin. The effect of both mutants was slightly enhanced by 8-Br-cGMP. A truncated (residues 1-142) form lacking the acidic C terminus had the same relaxant effect as wild-type telokin, whereas the C-terminal peptide (residues 142-155) had no effect. We conclude that site-specific modification of the N terminus modulates the Ca2+ -desensitizing effect of telokin on force.

Heat shock protein 27 is a substrate of cGMP-dependent protein kinase in intact human platelets: phosphorylation-induced actin polymerization caused by HSP27 mutants

Phosphorylation of heat shock protein 27 (Hsp27) in human platelets by mitogen-activated protein kinase-activated protein kinase (MAPKAP) 2 is associated with signaling events involved in platelet aggregation and regulation of microfilament organization. We now show that Hsp27 is also phosphorylated by cGMP-dependent protein kinase (cGK), a signaling system important for the inhibition of platelet aggregation. Stimulation of washed platelets with 8-para-chlorophenylthio-cGMP, a cGK specific activator, resulted in a time-dependent phosphorylation of Hsp27. This is supported by the ability of cGK to phosphorylate Hsp27 in vitro to an extent comparable with the cGK-mediated phosphorylation of its established substrate vasodilator-stimulated phosphoprotein. Studies with Hsp27 mutants identified threonine 143 as a yet uncharacterized phosphorylation site in Hsp27 specifically targeted by cGK. To test the hypothesis that cGK could inhibit platelet aggregation by phosphorylating Hsp27 and interfering with the MAPKAP kinase phosphorylation of Hsp27, the known MAPKAP kinase 2-phosphorylation sites (Ser15, Ser78, and Ser82) as well as Thr143 were replaced by negatively charged amino acids, which are considered to mimic phosphate groups, and tested in actin polymerization experiments. Mimicry at the MAPKAP kinase 2 phosphorylation sites led to mutants with a stimulating effect on actin polymerization. Mutation of the cGK-specific site Thr143 alone had no effect on actin polymerization, but in the MAPKAP kinase 2 phosphorylation-mimicking mutant, this mutation reduced the stimulation of actin polymerization significantly. These data suggest that phosphorylation of Hsp27 and Hsp27-dependent regulation of actin microfilaments contribute to the inhibitory effects of cGK on platelet function.

KT5823 inhibits cGMP-dependent protein kinase activity in vitro but not in intact human platelets and rat mesangial cells

Many signal transduction pathways are mediated by the second messengers cGMP and cAMP, cGMP- and cAMP-dependent protein kinases (cGK and PKA), phosphodiesterases, and ion channels. To distinguish among the different cGMP effectors, inhibitors of cGK and PKA have been developed including the K-252 compound KT5823 and the isoquinolinesulfonamide H89. KT5823, an in vitro inhibitor of cGK, has also been used in numerous studies with intact cells to implicate or rule out the involvement of this protein kinase in a given cellular response. However, the efficacy and specificity of KT5823 as cGK inhibitor in intact cells or tissues have never been demonstrated. Here, we analyzed the effects of both KT5823 and H89 on cyclic-nucleotide-mediated phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in intact human platelets and rat mesangial cells. These two cell types both express high levels of cGK. KT5823 inhibited purified cGK. However, with both intact human platelets and rat mesangial cells, KT5823 failed to inhibit cGK-mediated serine 157 and serine 239 phosphorylation of VASP induced by nitric oxide, atrial natriuretic peptide, or the membrane-permeant cGMP analog, 8-pCPT-cGMP. KT5823 enhanced 8-pCPT-cGMP-stimulated VASP phosphorylation in platelets and did not inhibit forskolin-stimulated VASP phosphorylation in either platelets or mesangial cells. In contrast H89, an inhibitor of both PKA and cGK, clearly inhibited 8-pCPT-cGMP and forskolin-stimulated VASP phosphorylation in the two cell types. The data indicate that KT5823 inhibits purified cGK but does not affect a cGK-mediated response in the two different cell types expressing cGK I. These observations indicate that data that interpret the effects of KT5823 in intact cells as the major or only criteria supporting the involvement of cGK clearly need to be reconsidered.

Regulation of human endothelial cell focal adhesion sites and migration by cGMP-dependent protein kinase I

cGMP-dependent protein kinase type I (cGK I), a major constituent of the atrial natriuretic peptide (ANP)/nitric oxide/cGMP signal transduction pathway, phosphorylates the vasodilator-stimulated phosphoprotein (VASP), a member of the Ena/VASP family of proteins involved in regulation of the actin cytoskeleton. Here we demonstrate that stimulation of human umbilical vein endothelial cells (HUVECs) by both ANP and 8-(4-chlorophenylthio)guanosine 3':5'-monophosphate (8-pCPT-cGMP) activates transfected cGK I and causes detachment of VASP and its known binding partner (zyxin) from focal adhesions in >60% of cells after 30 min. The ANP effects, but not the 8-pCPT-cGMP effects, reversed after 3 h of treatment. In contrast, a catalytically inactive cGK Ibeta mutant (cGK Ibeta-K405A) was incapable of mediating these effects. VASP mutated (Ser/Thr to Ala) at all three of its established phosphorylation sites (vesicular stomatitis virus-tagged VASP-AAA mutant) was not phosphorylated by cGK I and was resistant to detaching from HUVEC focal adhesions in response to 8-pCPT-cGMP. Furthermore, activation of cGK I, but not of mutant cGK Ibeta-K405A, caused a 1.5-2-fold inhibition of HUVEC migration, a dynamic process highly dependent on focal adhesion formation and disassembly. These results indicate that cGK I phosphorylation of VASP results in loss of VASP and zyxin from focal adhesions, a response that could contribute to cGK alteration of cytoskeleton-regulated processes such as cell migration.

Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent protein kinases

Endothelial nitric-oxide synthase (NOS-III) is defined as being strictly dependent on Ca(2+)/calmodulin (CaM) for activity, although NO release from endothelial cells has been reported to also occur at intracellular free Ca(2+) levels that are substimulatory for the purified enzyme. We demonstrate here that NOS-III, but neither NOS-I nor -II, is rapidly and strongly activated and phosphorylated on both Ser and Thr in the presence of cGMP-dependent protein kinase II (cGK II) and the catalytic subunit of cAMP-dependent protein kinase (cAK) in vitro. Phosphopeptide analysis by mass spectrometry identified Ser(1177), as well as Ser(633) which is situated in a recently defined CaM autoinhibitory domain within the flavin-binding region of human NOS-III. Phosphoamino acid analysis identified a putative phosphorylation site at Thr(495) in the CaM-binding domain. Importantly, both cAK and cGK phosphorylation of NOS-III in vitro caused a highly reproducible partial (10-20%) NOS-III activation which was independent of Ca(2+)/CaM, and as much as a 4-fold increase in V(max) in the presence of Ca(2+)/CaM. cAK stimulation in intact endothelial cells also increased both Ca(2+/)CaM-independent and -dependent activation of NOS-III. These data collectively provide new evidence for cAK and cGK stimulation of both Ca(2+)/CaM-independent and -dependent NOS-III activity, and suggest possible cross-talk between the NO and prostaglandin I(2) pathways and a positive feedback mechanism for NO/cGMP signaling.

Molecular identification of human G-substrate, a possible downstream component of the cGMP-dependent protein kinase cascade in cerebellar Purkinje cells

G-substrate, an endogenous substrate for cGMP-dependent protein kinase, exists almost exclusively in cerebellar Purkinje cells, where it is possibly involved in the induction of long-term depression. A G-substrate cDNA was identified by screening expressed sequence tag databases from a human brain library. The deduced amino acid sequence of human G-substrate contained two putative phosphorylation sites (Thr-68 and Thr-119) with amino acid sequences [KPRRKDT(p)PALH] that were identical to those reported for rabbit G-substrate. G-substrate mRNA was expressed almost exclusively in the cerebellum as a single transcript. The human G-substrate gene was mapped to human chromosome 7p15 by radiation hybrid panel analysis. In vitro translation products of the cDNA showed an apparent molecular mass of 24 kDa on SDS/PAGE which was close to that of purified rabbit G-substrate (23 kDa). Bacterially expressed human G-substrate is a heat-stable and acid-soluble protein that cross-reacts with antibodies raised against rabbit G-substrate. Recombinant human G-substrate was phosphorylated efficiently by cGMP-dependent protein kinase exclusively at Thr residues, and it was recognized by antibodies specific for rabbit phospho-G-substrate. The amino acid sequences surrounding the sites of phosphorylation in G-substrate are related to those around Thr-34 and Thr-35 of the dopamine- and cAMP-regulated phosphoprotein DARPP-32 and inhibitor-1, respectively, two potent inhibitors of protein phosphatase 1. However, purified G-substrate phosphorylated by cGMP-dependent protein kinase inhibited protein phosphatase 2A more effectively than protein phosphatase 1, suggesting a distinct role as a protein phosphatase inhibitor.

Analysis and regulation of vasodilator-stimulated phosphoprotein serine 239 phosphorylation in vitro and in intact cells using a phosphospecific monoclonal antibody

The development and functional analysis of a monoclonal antibody (16C2) are reported; the antibody recognizes vasodilator-stimulated phosphoprotein (VASP; an established substrate of both cAMP- and cGMP-dependent protein kinase) only when serine 239 is phosphorylated. VASP serine 239 represents one of the best characterized cGMP-dependent protein kinase phosphorylation sites in vitro and in intact cells. Experiments with purified, recombinant human VASP and various VASP constructs with mutated phosphorylation sites (S157A, S239A, T278A) and experiments with intact cells (human/rat platelets and other cells) treated with cyclic nucleotide-elevating agents demonstrated the specificity of the monoclonal antibody 16C2. Quantitative analysis of the VASP shift from 46 to 50 kDa (indicating VASP serine 157 phosphorylation) and the appearance of VASP detected by the 16C2 monoclonal antibody (VASP serine 239 phosphorylation) in human platelets stimulated by selective protein kinase activators confirmed that serine 239 is the VASP phosphorylation site preferred by cGMP-dependent protein kinase in intact cells. Immunofluorescence experiments with human platelets treated with cGMP analogs showed that the 16C2 monoclonal antibody also detects VASP serine 239 phosphorylation in situ at established intracellular localization sites. Analysis of VASP serine 239 phosphorylation by the 16C2 antibody appears to be the best method presently available to measure cGMP-dependent protein kinase activation in intact cells. Also, the 16C2 antibody promises to be an excellent tool for the evaluation of VASP function in intact cells.

Phosphorylation of phospholamban correlates with relaxation of coronary artery induced by nitric oxide, adenosine, and prostacyclin in the pig

The intracellular mechanisms underlying the action of the endogenous vasodilators such as NO/EDRF, adenosine, and prostacyclin acting through cGMP and cAMP, respectively, are not well understood. One important action of cyclic nucleotides in smooth muscle relaxation is to lower the cytosolic Ca2+ concentration by enhanced sequestration into the sarcoplasmic reticulum. The present study was undertaken to elucidate the potential role of phosphorylation of phospholamban, the regulator of sarcoplasmic reticulum Ca2+ pump, for the control of coronary vascular tone by NO/EDRF, adenosine, and prostacyclin. Phospholamban was identified in pig coronary artery preparations by immunofluorescence microscopy, Western blotting and in vitro phosphorylation. Segments of pig coronary artery, with either intact or denuded endothelium, were precontracted with prostaglandin F2alpha (PGF2alpha). In endothelium-denuded preparations 3-morpholinosydnonimine (SIN-1), 5'-N-ethylcarboxiamidoadenosine (NECA), and iloprost (ILO) caused both relaxation and phospholamban phosphorylation with the potency: SIN-1 > NECA > ILO. The regulatory myosin light chain was significantly dephosphorylated only by SIN-1. In endothelium-intact pig coronary artery, L-NAME caused additional vasoconstriction and a decrease in phospholamban phosphorylation, while phosphorylation of myosin light chain remained unchanged. An inverse relationship between phospholamban phosphorylation and vessel tone was obtained. Our findings demonstrate significant phospholamban phosphorylation during coronary artery relaxation evoked by NO, prostacyclin, and adenosine receptor activation. Because of the close correlation between phosphorylation of phospholamban and vessel relaxation, we propose that phospholamban phosphorylation is an important mechanism by which endogenous vasodilators, especially endothelial NO/EDRF, control coronary vascular smooth muscle tone.

Endogenous type II cGMP-dependent protein kinase exists as a dimer in membranes and can Be functionally distinguished from the type I isoforms

In mammalian tissues two types of cGMP-dependent protein kinase (cGK) have been identified. In contrast to the dimeric cGK I, cGK II purified from pig intestine was shown previously to behave as a monomer. However, recombinant rat cGK II was found to have hydrodynamic parameters indicative of a homodimer. Chemical cross-linking studies showed that pig cGK II in intestinal membranes has a dimeric structure as well. However, after purification, cGK II was found to be partly proteolyzed into C-terminal monomeric fragments. Phosphorylation studies in rat intestinal brush borders revealed that the potency of cGMP analogs to stimulate or inhibit native cGK II in vitro (i.e. 8-(4-chlorophenylthio)-cGMP > cGMP > beta-phenyl-1,N2-etheno-8-bromo-cGMP > beta-phenyl-1,N2-etheno-cGMP and Rp-8-(4-chlorophenylthio)-cGMPs > Rp-beta-phenyl-1, N2-etheno-8-bromo-cGMPs, respectively) correlated well with their potency to stimulate or inhibit cGK II-mediated Cl- secretion across intestinal epithelium but differed strikingly from their potency to affect cGK I activity. These data show that the N terminus of cGK II is involved in dimerization and that endogenous cGK II displays a distinct activation/inhibition profile with respect to cGMP analogs, which permits a pharmacological dissection between cGK II- and cGK I-mediated physiological processes.

Site-specific phosphorylation of a phospholamban peptide by cyclic nucleotide- and Ca2+/calmodulin-dependent protein kinases of cardiac sarcoplasmic reticulum

Phospholamban (PLB), the regulator of the cardiac sarcoplasmic reticulum (SR) Ca2+ pump is specifically phosphorylated at Ser16 and Thr17 by cAMP-dependent protein kinase (PKA) and Ca2+/calmodulin-dependent protein kinase (CaMK), respectively. The regulation of this dual-site phosphorylation of amino acid residues in direct proximity is only poorly understood. In order to study the site-specific phosphorylation of PLB, we used a synthetic peptide (PLB-24) corresponding to the cytosolic part of the PLB monomer with the phosphorylation sites as a model substrate. PLB-24 possesses substrate properties as the native PLB as demonstrated by phosphorylation with exogenous, purified PKA, cGMP-dependent protein kinase (PKG) and a type II CaMK (CaMKII). In isolated vesicles of cardiac SR there was a rapid phosphorylation of the peptide by the endogenous PKA (SR-PKA) and CaMK (SR-CaMK), but not under conditions that activate PKG. Both SR-PKA and SR-CaMK incorporated the same amount of 32P into PLB-24, 0.60 +/- 0.01 nmol 32P/mg SR protein and 0.61 +/- 0.03 nmol 32P/mg SR protein, respectively. Phosphorylation by SR-PKA was abolished by the specific PKA inhibitor (IC50 = 0.2 microM), whereas SR-CaMK phosphorylation was inhibited by calmidazolium (IC50 = 1.6 microM) and a CaMKII-specific inhibitor peptide (IC50 = 2.5 microM). Phosphorylation by SR-PKA was exclusively at Ser, whereas SR-CaMK phosphorylated only Thr. After simultaneous activation of both SR-kinases 32P incorporation into PLB-24 was additive and occurred at Ser as well as at Thr. Sequential activation of SR-PKA and SR-CaMK also caused the additive phosphorylation of PLB-24 independently of which kinase was activated first. Thus, at the monomeric level of PLB the respective phosphorylation site appears to be accessible to its related SR protein kinase in vitro even when the adjacent site is phosphorylated.

Inhibition of cyclic GMP-dependent protein kinase-mediated effects by (Rp)-8-bromo-PET-cyclic GMPS

1. The modulation of the guanosine 3':5'-cyclic monophosphate (cyclic GMP)- and adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase activities by the diastereomers of 8-bromo-beta phenyl-1, N2-ethenoguanosine 3':5'-cyclic monophosphorothioate, ((Rp)- and (Sp)-8-bromo-PET-cyclic GMPS) was investigated by use of purified protein kinases. In addition, the effects of (Rp)-8-bromo-PET-cyclic GMPS on protein phosphorylation in intact human platelets and on [3H]-noradrenaline release and neurogenic vasoconstriction in electrical field stimulated rat tail arteries were also studied. 2. Kinetic analysis with purified cyclic GMP-dependent protein kinase (PKG) type I alpha and I beta, which are expressed in the rat tail artery, revealed that (Rp)-8-bromo-PET-cyclic GMPS is a competitive inhibitor with an apparent Ki of 0.03 microM. The activation of purified cyclic AMP-dependent protein kinase (PKA) type II was antagonized with an apparent Ki of 10 microM. 3. In human platelets, (Rp)-8-bromo-PET-cyclic GMPS (0.1 mM) antagonized the activation of the PKG by the selective activator 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphate (8-pCPT-cyclic GMP; 0.2 mM) without affecting the activation of PKA by (Sp)-5, 6-dichloro-1-beta-D-ribofurano-sylbenzimidazole- 3':5'-cyclic monophosphorothioate ((Sp)-5,6-DCl-cyclic BiMPS; 0.1 mM). 4. (Rp)-8-bromo-PET-cyclic GMPS was not hydrolysed by the cyclic GMP specific phosphodiesterase (PDE) type V from bovine aorta but potently inhibited this PDE. 5. The corresponding sulphur free cyclic nucleotide of the two studied phosphorothioate derivatives, 8-bromo-beta-phenyl-1, N2-ethenoguanosine-3':5'-cyclic monophosphate (8-bromo-PET-cyclic GMP), had no effect on electrically-induced [3H]-noradrenaline release but concentration-dependently decreased the stimulation-induced vasoconstriction. (Rp)-8-bromo-PET-cyclic GMPS (3 microM) shifted the vasoconstriction response to the right without affecting stimulation evoked tritium overflow. 6. The NO donor, 3-morpholinosydnonimine (SIN-1) relaxed rat tail arteries precontracted with phenylephrine (1 microM). The SIN-1 concentration-relaxation curve was shifted in a parallel manner to the right by (Rp)-8-bromo-PET-cyclic GMPS, suggesting that the relaxation was mediated by a cyclic GMP/PKG-dependent mechanism. 7. The [3H]-noradrenaline release-enhancing effect and stimulation-induced decrease in vasoconstriction of forskolin were unaffected by (Rp)-8-bromo-PET-cyclic GMPS. Moreover, the forskolin concentration-relaxation curve was not changed in the presence of the PKG inhibitor, suggesting a high selectivity in intact cells for PKG- over PKA-mediated effects. 8. The results obtained indicate that (Rp)-8-bromo-PET-cyclic GMPS presently is the most potent and selective inhibitor of PKG and is helpful in distinguishing between cyclic GMP and cyclic AMP messenger pathways activation. Therefore, this phosphorothioate stereomer may be a useful tool for studying the role of cyclic GMP in vitro.

Expression, purification, and characterization of the cGMP-dependent protein kinases I beta and II using the baculovirus system

Detailed studies of differences in distinct cGMP kinase isoforms are highly dependent on expression of large amounts of these enzyme isoforms that are not easily purified by conventional methods. Here cGMP-dependent protein kinases, the type I beta soluble form from human placenta, and the type II membrane-associated form from rat intestine, were each expressed in a baculovirus/Sf9 cell system and purified in milligram amounts by affinity chromatography. The expressed recombinant proteins displayed characteristics like those of their native counterparts. cGK I beta was expressed as a 76 kDa protein predominantly found in the cytosol fraction, whereas cGK II was expressed as an 86 kDa protein predominantly associated with the membrane fraction. The apparent Ka and Vmax of cGMP for activation of cGK I beta were 0.5 microM and 3.4 mumol/min/mg, and for cGK II were 0.04 microM and 1.8 mumol/min/mg.

Isotype-specific activation of cystic fibrosis transmembrane conductance regulator-chloride channels by cGMP-dependent protein kinase II

Type II cGMP-dependent protein kinase (cGKII) isolated from pig intestinal brush borders and type I alpha cGK (cGKI) purified from bovine lung were compared for their ability to activate the cystic fibrosis transmembrane conductance regulator (CFTR)-Cl- channel in excised, inside-out membrane patches from NIH-3T3 fibroblasts and from a rat intestinal cell line (IEC-CF7) stably expressing recombinant CFTR. In both cell models, in the presence of cGMP and ATP, cGKII was found to mimic the effect of the catalytic subunit of cAMP-dependent protein kinase (cAK) on opening CFTR-Cl-channels, albeit with different kinetics (2-3-min lag time, reduced rate of activation). By contrast, cGKI or a monomeric cGKI catalytic fragment was incapable of opening CFTR-Cl- channels and also failed to potentiate cGKII activation of the channels. The cAK activation but not the cGKII activation was blocked by a cAK inhibitor peptide. The slow activation by cGKII could not be ascribed to counteracting protein phosphatases, since neither calyculin A, a potent inhibitor of phosphatase 1 and 2A, nor ATP gamma S (adenosine 5'-O-(thiotriphosphate)), producing stable thiophosphorylation, was able to enhance the activation kinetics. Channels preactivated by cGKII closed instantaneously upon removal of ATP and kinase but reopened in the presence of ATP alone. Paradoxically, immunoprecipitated CFTR or CF-2, a cloned R domain fragment of CFTR (amino acids 645-835) could be phosphorylated to a similar extent with only minor kinetic differences by both isotypes of cGK. Phosphopeptide maps of CF-2 and CFTR, however, revealed very subtle differences in site-specificity between the cGK isoforms. These results indicate that cGKII, in contrast to cGKI alpha, is a potential activator of chloride transport in CFTR-expressing cell types.

Expression of cGMP-dependent protein kinase I and phosphorylation of its substrate, vasodilator-stimulated phosphoprotein, in human endothelial cells of different origin

Previous studies demonstrated that the thrombin-induced permeability of endothelial cell monolayers is reduced by the elevation of cGMP. In the present study, the presence of cGMP-dependent protein kinase (cGMP-PK) immunoreactivity and activity in various types of human endothelial cells (ECs) and the role of cGMP-PK in the reduction of thrombin-induced endothelial permeability was investigated. cGMP-PK type I was demonstrated in freshly isolated ECs from human aorta and iliac artery as well as in cultured ECs from human aorta, iliac vein, and foreskin microvessels. Addition of the selective cGMP-PK activator 8-(4-chlorophenylthio)-cGMP (8-pCPT-cGMP) to these ECs caused phosphorylation of the vasodilator-stimulated phosphoprotein (VASP), an established cGMP-PK substrate, which is localized at cell-cell contact sites of confluent ECs. cGMP-PK type I expression decreased during serial passage of ECs, which correlated with a diminished ability of 8-pCPT-cGMP to induce VASP phosphorylation. Preincubation of aorta and microvascular EC monolayers with 8-pCPT-cGMP caused a 50% reduction of the thrombin-stimulated permeability, as determined by measuring the peroxidase passage through EC monolayers on porous filters. Furthermore, the thrombin-induced rise in cytoplasmic [Ca2+]i was strongly attenuated by the cGMP-PK activator in fura 2-loaded aorta ECs. In contrast, cGMP-PK could not be demonstrated in freshly isolated and cultured human umbilical vein ECs. Incubation of umbilical vein ECs with 8-pCPT-cGMP did not cause VASP phosphorylation and had no effect on the thrombin-induced increases in cytoplasmic Ca2+ and endothelial permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

(Rp)-8-pCPT-cGMPS, a novel cGMP-dependent protein kinase inhibitor

In the present study, the inhibitory effect of the cGMP analog (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate ((Rp)-8-pCPT-cGMPS) on the cGMP-dependent protein kinase-mediated protein phosphorylation in intact human platelets was investigated. In vitro phosphorylation experiments with the substrate kemptide demonstrated an inhibition of the cGMP-dependent protein kinase by (Rp)-8-pCPT-cGMPS with a Ki of 0.5 microM. In intact human platelets, (Rp)-8-pCPT-cGMPS antagonized the activation of the cGMP-dependent protein kinase by 8-pCPT-cGMP without affecting cAMP-dependent protein kinase or cGMP-regulated phosphodiesterases. The data obtained suggest that (Rp)-8-pCPT-cGMPS may be a useful tool for studying the role of cGMP in vitro and in intact cells.

Inhibition of cyclin-dependent kinases by purine analogues

While testing purines related to the non-specific protein kinase inhibitors N6-dimethylaminopurine and N6-(delta 2-isopentenyl)adenine as potential inhibitors of the p34cdc2/cyclin B kinase, we discovered a compound with high specificity, 2-(2-hydroxyethylamino)-6- benzylamino-9-methylpurine (olomoucine). Kinetic analysis of kinase inhibition reveals that olomoucine behaves as a competitive inhibitor for ATP and as a non-competitive inhibitor for histone H1 (linear inhibition for both substrates). The kinase specificity of this inhibition was investigated for 35 highly purified kinases (including p34cdk4/cyclin D1, p40cdk6/cyclin D3, cAMP-dependent and cGMP-dependent kinases, eight protein kinase C isoforms, calmodulin-dependent kinase II, myosin light-chain kinase, mitogen-activated S6 kinase, casein kinase 2, double-stranded RNA-activated protein kinase, AMP-stimulated kinase, eight tyrosine kinases). Most kinases are not significantly inhibited. Only the cell-cycle regulating p34cdc2/cyclin B, p33cdk2/cyclin A and p33cdk2/cyclin E kinases, the brain p33cdk5/p35 kinase and the ERK1/MAP-kinase (and its starfish homologue p44mpk) are substantially inhibited by olomoucine (IC50 values are 7, 7, 7, 3 and 25 microM, respectively). The cdk4/cyclin D1 and cdk6/cyclin D3 kinases are not significantly sensitive to olomoucine (IC50 values greater than 1 mM and 150 microM, respectively). N6-(delta 2-Isopentenyl)adenine is confirmed as a general kinase inhibitor with IC50 values of 50-100 microM for many kinases. The purine specificity of cyclin-dependent kinase inhibition was investigated: among 81 purine derivatives tested, only C2, N6 and N9-substituted purines exert a strong inhibitory effect on the p34cdc2/cyclin B kinase. An essentially similar sensitivity to this olomoucine family of compounds was observed for the brain-specific cdk5/p35 kinase. Structure/activity relationship studies allow speculation on the interactions of olomoucine and its analogues with the kinase catalytic subunit. Olomoucine inhibits in vitro M-phase-promoting factor activity in metaphase-arrested Xenopus egg extracts, inhibits in vitro DNA synthesis in Xenopus interphase egg extracts and inhibits the licensing factor, an essential replication factor ensuring that DNA is replicated only once in each cell cycle. Olomoucine inhibits the starfish oocyte G2/M transition in vivo. Through its unique selectivity olomoucine provides an anti-mitotic reagent that may preferentially inhibit certain steps of the cell cycle.

Partial reconstruction of the adult Lurcher cerebellar circuitry by neural grafting

Solid cerebellar grafts, taken from normal mouse embryos (gestational day 12-14), were transplanted into the cerebellum of adult Lurcher mice. The degree of Purkinje cell replacement was analysed one to three months after transplantation by means of immunocytochemistry (antibodies against calbindin, cGMP-dependent protein kinase and neurofilament proteins) and electron microscopy. Grafted Purkinje cells succeed in moving out of the graft and migrate into the host cerebellar cortex. They are present next to the graft in the granule cell and molecular layers, and far from the graft remnant, only in the molecular layer, indicating that, although both layers subserve Purkinje cell migration, the molecular layer is the ultimate target. In the host molecular layer, axons of transplanted Purkinje cells form thick bundles running in the frontal plane over long distances. Most of them terminate in the upper granule cell layer by enlarged bulbs resembling collapsed growth cones. Axons reaching their normal targets (the neurons of the deep cerebellar nuclei) are observed only in cases where the granule cell layer is disrupted and/or grafted Purkinje cells remain in the white matter. The projection is massive only from grafts lying in the close vicinity of the target neurons. Electron-microscopic analysis of grafted Purkinje cells populating the host cerebellar cortex reveals that their synaptic investment is abnormal. In the molecular layer, where the normal inputs are reduced, the compartmentation in proximal and distal dendritic segments is severely affected, climbing fibre synapses only form on a minority of grafted cells and "pinceau" formations are absent. In the granule cell layer, the synaptic investment is similar to that of Purkinje cells in agranular cerebellum, and even heterelogous synapses with mossy fibres have been observed. These results, compared to those previously obtained with grafting experiments in Purkinje cell degeneration mutant mouse, allow us to conclude that: (i) the Purkinje cell-deficient molecular layer of the host, despite its severe atrophy and reactive gliosis, still exerts a positive neurotropism specific for grafted Purkinje cells; (ii) the unlesioned host granule cell layer underlying the molecular layer containing grafted Purkinje cells, even if almost depleted of granule cells, remains an obstacle for the re-establishment of a corticonuclear projection; and (iii) the degree of synaptic integration of grafted Purkinje cells is directly related to the nearby presence of available host axon terminals. Hence, owing to the atrophy of the Lurcher cerebellum, the postgrafting restoration of the cerebellar cortical circuit is much less complete in this mutant.

Plasticity of GABAergic terminals in Deiters' nucleus of weaver mutant and normal mice: a quantitative light microscopic study

This study reports on the developmental changes in size and the average density of GABAergic axonal boutons bordering on the somata of large neurons in the dorsal part of the lateral vestibular nucleus (Deiters' nucleus) in normal and mutant mice. Weaver mutants, PCD mutants and the corresponding wild types were used to test for size alterations and differences in the number of GABA-immunopositive terminals. Hemicerebellectomized animals were examined in addition. Quantification of bouton profile size was performed from 30-microns-thick vibratome and 0.5-micron Araldite-embedded semi-thin sections immunoreacted for GABA from 7 days postnatally up to an age of 9 months. Terminal density was determined at the 5-6 month stage from semi-thin sections only. Morphometric analysis over the lifetime of normal animals (B6CBA) revealed a progressive increase in the size of bouton profiles, which peaked at 5-6 months and reached sizes of 2-3 microns2. In weaver mutants a parallel development in terminal size was found to be present, but the size of the largest terminals exceeded those of the controls by 75-100%, reaching 3-6 microns2 with the same time course. PCD mutants, with an almost total absence of Purkinje cells had, on the contrary, small bouton profiles that reached a maximum of only 2 microns2. The hemicerebellectomized animals responded with decreased bouton profile size ipsilaterally. The terminal numbers per unit membrane length were surprisingly similar in wild types and weaver mutants, despite a reduction in Purkinje cells of almost 50% in the weaver anterior lobe.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the functional role of cGMP-dependent protein kinase in intact human platelets using a specific activator 8-para-chlorophenylthio-cGMP

8-(p-Chlorophenylthio)-cGMP (8-pCPT-cGMP) and 8-bromo-cGMP were compared with respect to their chemical and biological properties in order to evaluate their potential as selective activators of cGMP-dependent protein kinase (cGMP-PK; EC 2.7.1.37) in intact human platelets. 8-pCPT-cGMP, 8-Br-cGMP and cGMP were shown to be potent and selective activators of purified bovine lung cGMP-PK and of cGMP-PK present in human platelet membranes when compared with the activation of cAMP-dependent protein kinase (cAMP-PK; EC 2.7.1.37). 8-pCPT-cGMP was not hydrolysed by the purified cGMP-stimulated phosphodiesterase (cGS-PDE), cGMP-inhibited phosphodiesterase (cGI-PDE) and Ca(2+)-calmodulin-dependent phosphodiesterase (CaM-PDE), whereas cGMP and, to a lesser extent, 8-Br-cGMP were hydrolysed by all three types of 3',5' cyclic nucleotide phosphodiesterases (EC 3.1.4.17) examined. Also, 8-pCPT-cGMP was not hydrolysed by a human platelet homogenate which contains a high level of the cGMP-specific cGMP-binding phosphodiesterase (cGB-PDE). Additionally, 8-pCPT-cGMP did not activate the cGS-PDE or inhibit the cGI-PDE, whereas half-maximal inhibition of cGI-PDE occurred at 8 microM 8-Br-cGMP. The apparent lipophilicity of 8-pCPT-cGMP was higher than that of 8-Br-cGMP. Extracellular application of 8-pCPT-cGMP to intact human platelets reproduced the pattern of protein phosphorylation induced by sodium nitroprusside (SNP), a cGMP-elevating inhibitor of platelet activation. Quantitatively, 8-pCPT-cGMP was more effective than 8-Br-cGMP in inducing phosphorylation of the 46/50 kDa vasodilator-stimulated phosphoprotein, a major substrate of cGMP-PK in intact platelets. As observed with SNP, pretreatment of human platelets with 8-pCPT-cGMP prevented the aggregation induced by thrombin. The results suggest that 8-pCPT-cGMP is a very potent and selective activator of cGMP-PK in cell extracts and in intact human platelets and, in this respect, is superior to 8-Br-cGMP and other cGMP analogs used for intact cell studies. The data also suggest that inhibition of platelet activation in intact human platelets by nitrovasodilators is mediated by cGMP-PK.

Concentration and regulation of cyclic nucleotides, cyclic-nucleotide-dependent protein kinases and one of their major substrates in human platelets. Estimating the rate of cAMP-regulated and cGMP-regulated protein phosphorylation in intact cells

Vasodilators capable of elevating cAMP or cGMP inhibit the activation of human platelets and stimulate the phosphorylation of a 46-kDa protein (vasodilator-stimulated phosphoprotein, VASP) mediated by cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG). The availability of purified proteins and specific antisera against VASP, PKG and the catalytic subunit of PKA enabled us to measure and estimate the concentration of these regulatory proteins in intact human platelets. In addition, the rate of PKA- and PKG-mediated VASP phosphorylation in intact human platelets was estimated. For these calculations, a homogeneous population of human platelets and a homogeneous intracellular distribution of proteins and second messengers was assumed. Unstimulated washed human platelets contain 4.4 microM cAMP and 3.1 microM catalytic subunit of PKA, which is equivalent to 6.2 microM cAMP-binding sites due to PKA. Unstimulated washed human platelets also contain 0.4 microM cGMP and 7.3 microM PKG monomer, equivalent to 14.6 microM cGMP-binding sites due to the PKG. The intracellular concentration of VASP in platelets was estimated to be 25 microM. Treatment of washed human platelets with 10 microM (or 10 mM) prostaglandin E1 (PGE1) elevated the intracellular cAMP concentration to 27 microM (10 microM with 10 nM PGE1) within 30 s, accompanied by a rapid, up to 55% (35%), conversion of VASP from the dephosphorylated form (46-kDa protein) to the phosphorylated form (50-kDa protein). Treatment of washed human platelets with 100 microM (or 1 microM) sodium nitroprusside elevated the platelet cGMP level to 4 microM (0.9 microM with 1 microM sodium nitroprusside) within 2 min, accompanied by a less-rapid VASP phosphorylation of 45% (27% with 1 microM sodium nitroprusside). PGE1 and sodium nitroprusside had no significant effect on human platelet cGMP or cAMP levels, respectively. The results suggest for human platelets that relatively small increase in cAMP levels are required for activation of most of PKA, whereas even several-fold increases in platelet cGMP levels are capable of stimulating only a small fraction of total PKG. This interpretation was also supported by phosphorylation experiments with purified VASP, PKG and catalytic subunit of PKA. The results also support the hypothesis that in human platelets both cAMP/PKA- and cGMP/PKG-regulated VASP phosphorylation are components of an efficient and sensitive signal-transduction pathway, most likely involved in the inhibition of platelet activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein phosphorylation regulated by cyclic nucleotide-dependent protein kinases in cell extracts and in intact human lymphocytes

A specific 46,000/50,000 molecular weight protein substrate for both cAMP-dependent protein kinase (cAK) and cGMP-dependent protein kinase (cGK) extensively characterized and purified from human platelets was found to be present also in human T-lymphocytes, B-lymphocytes and other cells and tumour cell lines. This protein termed vasodilator-stimulated phosphoprotein (VASP) was present in cytosol and membranes of lymphocytes. Addition of exogenous purified cAK or cGK to lymphocyte cytosol or membranes converted 80-90% of VASP to its phosphoform. Endogenous VASP phosphorylation in both cytosol and membranes was stimulated by the addition of cAMP but not by cGMP. With intact lymphocytes, prostaglandin E1 (PGE1) and prostaglandin E2 (PGE2) induced an increase of cAMP and converted 70% of VASP to its phosphoform. In contrast, an increase of cGMP was not associated with VASP phosphorylation although cGK was detected in lymphocytes. These data support the hypothesis that VASP phosphorylation may be an important component of cAMP-mediated regulation of lymphocyte function.

Analysis, purification and properties of a 50,000-dalton membrane-associated phosphoprotein from human platelets

Recently, the development of a monospecific antiserum against a 46,000/50,000-dalton membrane protein from human platelets which was stoichiometrically and reversibly phosphorylated in intact human platelets in response to vasodilators was reported. Using this antiserum, the subcellular distribution and the purification of this vasodilator-stimulated phosphoprotein (VASP) from human platelets has now been analysed. The VASP of human platelets is primarily a membrane-associated protein and can be purified to apparent homogeneity by salt extraction and sequential ion-exchange and dye-ligand chromatography with a purification factor of 1200 and a yield of 13%. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis under reducing and non-reducing conditions indicated that purified monomers of this VASP are linked by interchain disulphide bonding.

Purification of a vasodilator-regulated phosphoprotein from human platelets

Cyclic-nucleotide-elevating vasodilators such as prostaglandin E1, prostacyclin, sodium nitroprusside and endothelium-derived relaxing factor inhibit both contraction of vascular smooth muscle cells and the aggregation of platelets at an early step of the activation cascade. Previous studies from this laboratory [Waldmann, R., Nieberding, M. and Walter, U. (1987) Eur. J. Biochem. 167, 441-448) established that in human platelets cyclic-nucleotide-elevating vasodilators stimulated a pattern of protein phosphorylation which was mediated by both cAMP- and cGMP-dependent protein kinases. Of particular interest was a membrane-bound 50-kDa protein whose phosphorylation was increased both by cAMP- and cGMP-elevating vasodilators in intact platelets and by endogenous cAMP- and cGMP-dependent protein kinase in platelet membranes. Since the molecular mechanism of action of cyclic-nucleotide-elevating vasodilators is unknown, this 50-kDa phosphoprotein from human platelets was purified to apparent homogeneity by salt extraction, anion, cation and dye-ligand chromatography. The purified protein migrated as a 46-kDa protein in SDS/PAGE, was an excellent substrate for both cAMP- and cGMP-dependent protein kinases and migrated in SDS/PAGE as a 50-kDa protein after phosphorylation by these protein kinases. Analysis by limited proteolysis, tryptic fingerprinting and of phosphoamino acids established that the purified protein is identical with the 50-kDa protein phosphorylated by both cAMP- and cGMP-dependent protein kinases in platelet membranes and in response to cAMP- and cGMP-elevating vasodilators with intact platelets. Evidence is presented that the purified protein contains at least two phosphorylation sites, each of which is preferentially phosphorylated by either cAMP- or cGMP-dependent protein kinase. The availability of this vasodilator-regulated phosphoprotein as a purified protein should now allow new approaches for investigating the function of this protein and its possible role in the mechanism of action of cyclic-nucleotide-elevating vasodilators.

Insulin-induced S6 kinase activation in HeLa cells and its reversal by hyperthermic stress

Insulin treatment of HeLa S3 cells activates an S6-phosphorylating protein kinase. Although this enzyme has chromatographic properties resembling those of described proteolytic fragments of other protein kinases, namely protein kinase C, protease-activated kinase II and histone-4 protein kinase, and although insulin has been proposed by others to cause S6 phosphorylation via proteolytic protein kinase activation, the insulin-induced increase in S6-kinase activity described here is probably not due to proteolysis. Rather, the activity indicates the existence, in HeLa cells, of an interconvertible S6 kinase, since the insulin-induced activity increase was rapidly reversed under hyperthermic stress, and since this effect of hyperthermia was itself reversible. The S6-kinase activities from serum- and from insulin-stimulated HeLa cells resemble each other closely and are likely to represent the same enzyme. The enzyme may therefore mediate both signals delivered by mitogens and the insulin signal. Analysed at an in vitro transfer of 1 mol phosphate/mol S6, this S6 kinase activity does not phosphorylate the (principal) S6 site recognized by the cAMP-dependent protein kinase.

Proximal trajectory of the brachium conjunctivum in rat fetuses and its early association with the parabrachial nucleus. A study combining in vitro HRP anterograde axonal tracing and immunocytochemistry

The proximal course of the developing brachium conjunctivum (BC) in the rat described from embryonic day 16 (E16) to one day postnatal (P1). Axons of the cerebellar deep nuclear neurons entering this bundle were identified by anterograde axonal tracing after in vitro horseradish peroxidase (HRP) injections in the cerebellar plate. At all ages, the main ascending limb of the BC can be followed from its emergence, dorsal to the cerebellar plate where it assumes an almost vertical course, up to its decussation. Close to the ventricle at E16, the decussating fibers are progressively displaced ventrally probably because of the fusion, on the midline, of bilaterally produced raphe neurons. In E16 and E17 embryos, labeled BC fibers extend beyond the decussation in the caudal part of the red nucleus. Decussating BC axons, in some E16 early embryos, end with large and complicated growth cones, as described previously in 'decision regions' for chick embryo motoneurons. Growth cones were never observed in this region in older embryos. In addition to the main ascending limb of the BC, we also traced its ipsilateral descending limb and the cerebello-olivary projections. In parallel, the development of a nucleus immunoreactive for the vitamin D-dependent calcium-binding protein (CaBP) is reported. By E16, its neurons migrate rostrally and settle in the region where the BC is demonstrated by tracing experiments. At E17 and thereafter this isthmic nucleus is composed of a shell of CaBP-immunoreactive neurons ensheathing an immunonegative cylinder. Between E17 and birth, in spite of the profound modifications of the isthmic region, this CaBP-immunoreactive nucleus remains in close proximity to the BC. This nucleus is identified as the marginal nucleus of the BC or parabrachial nucleus, by double-labeling experiments combining the visualization of the retrogradely labeled axons and neurons of the deep cerebellar nuclei inside the CaBP immunofluorescently labeled parabrachial nucleus. Subsequently the deep cerebellar neurons translocate caudoventrally moving away from the parabrachial nucleus inside which their axons become visible. This pattern of migration could indicate that a few neurons of the deep nuclei remain ectopic, wedged between the restiform body and the BC while receiving an appropriate Purkinje cell (PC) projection.

Cell-specific localization of the alpha-subunit of calcium/calmodulin-dependent protein kinase II in Purkinje cells in rodent cerebellum

Brain calcium/calmodulin-dependent protein kinase type II, a multimeric 600-650 kDa enzyme composed of alpha- (50 kDa) and beta/beta' (60 and 58 kDa) subunits, may be formed by alpha- and beta-subunits combining in variable proportions in different types of neurons. This study presents evidence, using cerebella from mutant mice, that the alpha-subunit displays a restricted localization in the rodent cerebellum, being detectable only in Purkinje cells. Immunocytochemical analysis of normal rat cerebellum with an antibody selective for the alpha-subunit confirmed that this subunit was detectable only in Purkinje cells. In contrast, the beta/beta'-subunits appeared to be present in all types of cerebellar mutants examined. These results indicate that different cells of the cerebellum express distinct isozymic forms of the multifunctional calcium/calmodulin-dependent protein kinase type II. It appears that Purkinje cells primarily contain an isoenzyme formed by both alpha- and beta/beta'-subunits, and that non-Purkinje cells contain an isoenzyme formed primarily by beta/beta'-subunits.

Skeletal muscle sarcolemma proteins as targets for adenosine 3':5'-monophosphate-dependent and calcium-dependent protein kinases

The present study documents the existence in rat skeletal muscle plasma membrane (sarcolemma) of a distinct set of proteins, most of which represent unknown protein species, which can be phosphorylated in vitro by addition of cAMP-dependent or calcium-dependent protein kinases. Under the experimental conditions used, cAMP-regulated protein phosphorylation appeared to be the most important phosphorylation system in these membranes, followed by the calcium/phospholipid-regulated, and, with only a few substrates detected, the calcium/calmodulin-regulated systems. No specific substrate for cGMP-dependent protein kinase was found. In contrast, calcium/calmodulin-regulated protein phosphorylation was the most important in the sarcoplasmic reticulum fraction. Most of the cAMP-regulated and calcium/phospholipid-regulated sarcolemma phosphoproteins appeared to be intrinsic membrane proteins, at least three of which appeared to be phosphorylated by both these protein kinases. These phosphoproteins may represent membrane targets for multiple hormone or transmitter actions in skeletal muscle cells. Our results, therefore, suggest that protein phosphorylation systems, particularly those regulated by cAMP or calcium/phospholipid, may be more important in the regulation of sarcolemma function than hitherto believed.

Phosphorylation of acid-soluble chromatin proteins from tissues of different species by purified cyclic GMP-dependent protein kinase

1. Phosphorylation of acid-soluble chromatin proteins from thymus or liver of calf, rabbit, pig, rat, rooster and trout by purified cyclic GMP-dependent protein kinase was studied in vitro using acetic acid-urea slab gel electrophoresis and autoradiography. 2. HMG 14, histone H1 and an unknown band representing probably a proteolytic fragment of histone H1 were phosphorylated in all mammals studied. 3. In avian liver, HMG 14 showed no phosphorylation and histone H1 was replaced by a H1(0)/H5-like heavily phosphorylated protein. 4. The only 32P-acceptor in trout liver apparently belongs to the C/D-family of acid-soluble chromatin proteins. H6-protein was not phosphorylated.

Acetylcholine reduces net outward currents measured in vivo with single electrode voltage clamp techniques in neurons of the motor cortex of cats

Effects of acetylcholine (ACh) on membrane currents of cells of the motor cortex were measured directly, in vivo, in awake cats using single electrode voltage clamp (SEVC) techniques. Extracellular applications (90-95 nA) of 2 M ACh for periods of 30 s or less produced significant decreases in net outward currents elicited by depolarizing commands whereas applications of saline did not. Reductions of net outward currents were also obtained after intracellular pressure injections of cyclic guanosine monophosphate (GMP)-dependent protein kinase (cGPK) mixed with 10 microM cyclic GMP.

The cyclic nucleotide-dependent phosphorylation of aortic smooth muscle membrane proteins

Membrane proteins of Mr 240,000, 130,000, and 85,000 (GS-proteins) were rapidly and selectively phosphorylated in particulate fractions of rabbit aortic smooth muscle in the presence of [Mg-32P]ATP and low concentrations of cGMP (Ka = 0.01 microM) or cAMP (Ka = 0.2 microM). The effects of both cyclic nucleotides in this preparation were mediated entirely by an endogenous, membrane-bound form of cGMP-dependent protein kinase (G-kinase). The GS-proteins were also phosphorylated by the soluble form of G-kinase purified from bovine lung; this effect was most evident following removal of endogenous G-kinase from the membranes using Na2CO3 and high salt washes. The membrane-bound and cytosolic forms of G-kinase phosphorylated the Mr 130,000 GS-protein with the same specificity as determined by two-dimensional peptide mapping. Despite this functional homology between the two forms of G-kinase, only the particulate enzyme appears to play a role in phosphorylating the GS-proteins. Although little endogenous cAMP-dependent protein kinase (A-kinase) activity was detected in washed aortic smooth muscle membranes, the GS-proteins could be phosphorylated when purified A-kinase catalytic subunit was added to this preparation. Peptide mapping of the Mr 130,000 GS-protein indicated that A-kinase phosphorylated a subset of the same peptides labeled by the two forms of G-kinase. The endogenous A-kinase of rabbit aortic smooth muscle homogenates was also found to phosphorylate the GS-proteins. Since the intracellular concentrations of cGMP or cAMP can be selectively elevated by different stimuli, these results suggest several possible mechanisms by which the phosphorylation state of the GS-proteins may be regulated by cyclic nucleotides: activation of the membrane-bound G-kinase by cGMP or cAMP; and activation of cytosolic A-kinase by cAMP.