Mechanisms of luteinizing-hormone exocytosis in Staphylococcus aureus-alpha-toxin-permeabilized sheep gonadotropes

MLCK-independent phosphorylation of MLC20 and its regulation by MAP kinase pathway in human bladder smooth muscle cells

Myosins are a superfamily of actin-based molecular motor proteins, which hydrolyze ATP and generate various forms of eukaryotic motility and muscle contraction. Myosin light chain 20 (MLC20) is small ring around the neck region of heavy chain of myosins. Phosphorylation of MLC20 is thought to play a key role in regulation of smooth muscle contraction. Calcium- and calmodulin-dependent myosin light chain kinase (MLCK) is considered the primary regulator of MLC20 phosphorylation. However, several observations in smooth muscle contraction cannot be explained by the mode of phosphorylation. By performing a series of experiments in vitro and in vivo, we report here MLCK-independent MLC20 phosphorylation. Gene expression study reveals that expression of MLCK in smooth muscles is inconsistent with MLC20 phosphorylation at Ser19. None of inactivating calmodulin/MLCK, depriving of calcium and silencing MLCK expression by siRNA blocks effectively the phosphorylation of MLC20 at Ser19. In addition, by overexpressing active human MAP (mitogen-activated protein)-ERK kinase kinase-1 (MEKK1) and blocking its downstream messengers, we have demonstrated a new regulatory system of MLC phosphorylation via MEKK1, which downregulates Ser19 phosphorylation of MLC20 through its downstream molecules, p38, JNK, and ERK in human bladder smooth muscle cells.

Protein kinase C as a signal for exocytosis

We have studied signaling mechanisms that stimulate exocytosis and luteinizing hormone secretion in isolated male rat pituitary gonadotropes. As judged by reverse hemolytic plaque assays, phorbol-12-myristate-13-acetate (PMA) stimulates as many gonadotropes to secrete as does gonadotropin-releasing hormone (GnRH). However, PMA and GnRH use different signaling pathways. The secretagogue action of GnRH is not very sensitive to bisindolylmaleimide I, an inhibitor of protein kinase C, but is blocked by loading cells with a calcium chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. The secretagogue action of PMA is blocked by bisindolylmaleimide I and is not very sensitive to the intracellular calcium chelator. GnRH induces intracellular calcium elevations, whereas PMA does not. As judged by amperometric measurements of quantal catecholamine secretion from dopamine- or serotonin-loaded gonadotropes, the secretagogue action of PMA develops more slowly (in several minutes) than that of GnRH. We conclude that exocytosis of secretory vesicles can be stimulated independently either by calcium elevations or by activation of protein kinase C.

Irreversible activation of the gonadotropin-releasing hormone receptor by photoaffinity cross-linking: localization of attachment site to Cys residue in N-terminal segment

Photoaffinity cross-linking with [azidobenzoyl-d-Lys6]GnRH leads to irreversible activation of the gonadotropin-releasing hormone (GnRH) receptor. In order to localize the cross-linking site, the disulfide bridge structure was initially probed by mutagenesis. A consistent pattern of changes in the ability of GnRH to stimulate signal transduction after Ser substitutions of extracellularly located Cys residues indicated that Cys14 in the N-terminal domain is connected to Cys200 in the second extracellular loop, while Cys196 in this loop is connected to the highly conserved Cys114 at the extracellular end of transmembrane helix 3. Protein chemical analysis of radioactive fragments of cross-linked GnRH receptor following deglycosylation and enzymatic digest with endoproteinase Glu-C and trypsin before and after introduction or elimination of potential protease cleavage sites indicated that 125I[azidobenzoyl-d-Lys6]GnRH cross-links to a segment comprising residues 12-18 of the N-terminal domain. The existence of the Cys114-Cys196 bridge was directly confirmed as a labeled fragment, including that Cys114 was resolvable only under reducing conditions. The observation that the cross-linked N-terminal enzymatic fragments had identical apparent size under non-reducing conditions shows that the cross-linking reaction disconnected the disulfide bridge between Cys14 and Cys200 and indicates that Cys14 is probably the residue involved in cross-linking of the ligand. It is concluded that covalent tethering of GnRH through a photoreactive side chain located at position 6 in the middle of the peptide leads to continued activation of the receptor presumably through covalent binding to Cys14 in the N-terminal domain of the receptor.

Local Ca2+ release from internal stores controls exocytosis in pituitary gonadotrophs

Exocytosis and the cell-averaged cytosolic [Ca2+], [Ca2+]i, were tracked in single gonadotrophs. Cells released 100 granules/s at 1 microM = [Ca2+]i when gonadotropin-releasing hormone (GnRH) activated IP3-mediated Ca2+ release from internal stores, but only 1 granule/s when [Ca2+]i was raised uniformly to 1 microM by other means. Strong exocytosis was then seen only at higher [Ca2+]i (half-maximal at 16 microM). Parallel second messengers did not contribute to GnRH-induced exocytosis, because IP3 alone was as effective as GnRH, and because even GnRH failed to trigger rapid exocytosis when the [Ca2+]i rise was blunted by EGTA. When [Ca2+]i was released from stores, exocytosis depended on [Ca2+]i rising rapidly, as if governed by Ca2+ flux into the cytosol. We suggest that IP3 releases Ca2+ selectively from subsurface cisternae, raising [Ca2+] near exocytic sites 5-fold above the cell average.

Modulation of Ca2+ oscillation and apamin-sensitive, Ca2+-activated K+ current in rat gonadotropes

In rat pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) stimulates rhythmic release of Ca2+ from stores sensitive to inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which in turn induces an oscillatory activation of apamin-sensitive Ca2+-activated K+ current, IK(Ca). Since GnRH also activates protein kinase C (PKC), we investigate the action of PKC while simultaneously measuring intracellular Ca2+ concentration ([Ca2+]i) and IK(Ca). Stimulation of PKC by application of phorbol 12-myristate 13-acetate (PMA) did not affect basal [Ca2+]i. However, PMA or phorbol 12,13-dibutyrate (PdBu), but not the inactive 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), reduced the frequency of GnRH-induced [Ca2+]i oscillation and augmented the IK(Ca) induced by any given level of [Ca2+]i. The slowing of oscillations and the enhancement of IK(Ca) were mimicked by synthetic diacylglycerol (1,2-dioctanoyl-sn-glycerol) and could be induced during ongoing oscillations that had been initiated irreversibly in cells loaded with guanosine 5'-O-(3-thiotriphosphate) (GTP-[gammaS]). In contrast, when oscillations were initiated by loading cells with Ins(1,4,5)P3, phorbol esters enhanced IK(Ca) without affecting the frequency of oscillation. The protein kinase inhibitor, staurosporine, reduced IK(Ca) without affecting [Ca2+]i and partially reversed the phorbol-ester-induced slowing of oscillation. Therefore, activation of PKC has two rapid effects on gonadotropes. It slows [Ca2+]i oscillations probably by actions on phospholipase C, and it enhances IK(Ca) probably by a direct action on the channels.

Simultaneous measurements of exocytosis and intracellular calcium concentration with fluorescent indicators in single pituitary gonadotropes

Previously, we established a method for the estimation of exocytosis in single gonadotropes using an impermeable fluorescent membrane probe, TMA-DPH. In this study, we have developed a method for the simultaneous measurement of exocytosis and intracellular free Ca2+ concentration ([Ca2+]i) by double-labeling with TMA-DPH and the intracellular Ca2+ probe, Fura-2/AM, using a fluorescence microscope with a 3-wavelength excitation and 2-wavelength emission system. We, therefore, clarified the relationship between spontaneous [Ca2+]i oscillation or gonadotropin releasing hormone (GnRH)-induced intracellular Ca2+ mobilization and exocytosis in gonadotropes. Under resting conditions, some gonadotropes showed various types of spontaneous [Ca2+]i oscillations, while others did not, but all showed basal exocytosis. Each [Ca2+]i peak oscillation did not cause Ca(2+)-regulated exocytosis, and even complete blockage of the [Ca2+]i increase by the intracellular Ca2+ chelator BAPTA/AM had no effect on basal exocytosis. Both GnRH-induced intracellular Ca2+ mobilization and regulated exocytosis showed a similar pattern of peaks and plateaus. Blockage of the [Ca2+]i increase by BAPTA/AM almost completely inhibited the GnRH-stimulated exocytosis. These results show that spontaneous [Ca2+]i oscillations under resting conditions are not linked to regulated or basal exocytosis, and that intracellular Ca2+ mobilization is essential for GnRH-stimulated exocytosis.

Absence of rapid desensitization of the mouse gonadotropin-releasing hormone receptor

Desensitization of gonadotropin release by the pituitary gland in response to gonadotropin-releasing hormone (GnRH) agonists has clinical applications in the treatment of gonadal-hormone-dependent disorders. We therefore investigated possible desensitization of inositol phosphate (IP) responses of GNRH receptors. No short-term homologous desensitization of the IP response to GnRH was observed in either alpha T3 gonadotrope cells line or GH3 cells transfected with GnRH receptor cDNA. The absence of homologous desensitization is unusual among G-protein-coupled receptors, and may be due to the absence of a C-terminal cytoplasmic tail, a unique feature of the GnRH receptor. Several potential protein kinase C phosphorylation sites which might mediate heterologous desensitization are present on the GnRH receptor. In both alpha T3 cells and GnRH-receptor-transfected Cos-1 cells, activation of protein kinase C by pretreatment with phorbol ester caused a 35-53% decrease in the IP response to GnRH. However, phorbol ester also inhibited guanosine 5'-[gamma-thio]triphosphate-stimulated IP production in permeabilized Cos-1 cells, suggesting that this inhibition is mediated at a post-receptor site.

Inhibition of pituitary hormone exocytosis by a synthetic peptide related to the rab effector domain

GTP-binding proteins of the rab family are believed to function at several steps in intracellular vesicular transport. We examined the effects of a rab-related peptide in permeabilized pituitary cells, in which exocytosis can be triggered by distinct Ca(2+)-dependent or Ca(2+)-independent pathways. We report that a synthetic peptide of 18 amino acids related to the rab effector domain, rab3AL (30-47) inhibited luteinizing hormone (LH) and growth hormone (GH) exocytosis triggered by either pathway. Ca(2+)-stimulated LH and GH release were inhibited by more than 80% and 50%, respectively, by 100 microM peptide. The peptide (100 microM) also inhibited LH and GH exocytosis stimulated by phorbol myristate acetate plus cAMP by more than 45% and 80%, respectively. The effect was sequence-specific since a second peptide, lacking the first 3 amino acids but otherwise identical failed to inhibit exocytosis. These results suggest that a protein of the rab family is involved in regulated pituitary hormone exocytosis, and they identify 3 amino acids of the putative rab effector domain which may be functionally important in exocytosis.

GTP gamma S stimulates exocytosis in patch-clamped rat melanotrophs

We investigated the molecular mechanisms regulating exocytosis in patch-clamped melanotrophs by measuring the membrane capacitance. Ca(2+)-dependent exocytosis could be induced by membrane depolarization or by including solutions containing 2 microM free Ca2+ in the patch pipette. Ca(2+)-dependent exocytosis was inhibited by GDP beta S, suggesting involvement of a GTP-binding protein. The hydrolysis-resistant GTP analogs, GTP gamma S and GppNHp, were able to stimulate exocytosis at low free Ca2+ concentrations. The stimulatory response to GTP gamma S was abolished by both GDP beta S and GTP. The latter suggests that a sustained activation of a GTP-binding protein is necessary for exocytosis. This behavior is similar to the stimulation of exocytosis by guanine nucleotides in mast cells and other nonexcitable cells and suggests a common regulatory mechanism.

Involvement of pertussis toxin-sensitive and -insensitive GTP-binding proteins in luteinizing hormone exocytosis distal to second messenger generation

Inhibition of luteinizing hormone (LH) exocytosis by guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) in permeabilized pituitary cells has indicated the involvement of one or more GTP-binding proteins in the exocytotic mechanism distal to second messenger generation. We now report that two inhibitory sites of action of GTP gamma S can be distinguished by their dependence on GTP gamma S concentration and their sensitivity to pertussis toxin. Ca(2+)-stimulated exocytosis was half-maximally inhibited by 6.8 microM GTP gamma S, a six-fold higher concentration than that required for inhibition of exocytosis stimulated by phorbol ester plus cAMP. In addition, GTP gamma S inhibition of Ca(2+)-stimulated exocytosis was insensitive to pertussis toxin, in contrast to the inhibition of exocytosis stimulated by phorbol ester plus cAMP, which was abolished by pretreatment with pertussis toxin. These results indicate that at least two stimulus-specific GTP-binding proteins are involved in regulating LH exocytosis distal to second messenger generation.

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of luteinizing-hormone exocytosis by guanosine 5'-[gamma-thio]triphosphate reveals involvement of a GTP-binding protein distal to second-messenger generation

Dual inhibitory and stimulatory actions of guanine nucleotides on luteinizing-hormone (LH) exocytosis were observed in primary sheep gonadotropes permeabilized with staphylococcal alpha-toxin. At resting cytosolic [Ca2+]free (pCa 7), 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) stimulated rapid LH exocytosis, which was maximal between 5 and 10 min. GTP[S] and p[NH]ppG had similar potencies (50% of maximum effect at 20-50 microM), but the effect of p[NH]ppG was more prolonged. Experiments carried out in the presence of saturating concentrations of phorbol 12-myristate 13-acetate (PMA), or in PMA-desensitized cells, suggested that stimulation by p[NH]ppG is mediated by a mechanism additional to protein kinase C (PKC) activation. Furthermore, p[NH]ppG stimulated LH exocytosis in the presence of saturating cyclic AMP (cAMP) concentrations, although its effect was less than additive. However, when both PMA and cAMP were present, p[NH]ppG did not stimulate a further increase in the rate of LH exocytosis. In contrast, pretreatment of cells with GTP[S] at low [Ca2+]free markedly inhibited subsequent responses to Ca2+, cAMP, PMA, and cAMP plus PMA. This inhibitory effect required lower GTP[S] concentrations than the stimulatory effect (50% inhibition at 1-10 microM), and was not observed with p[NH]ppG. A similar inhibition was observed with adenosine 5'-[gamma-thio]triphosphate, probably by its conversion into GTP[S]. These results suggest that the stimulatory actions of guanine nucleotides can be accounted for by the combined activation of PKC and generation of cAMP, resulting from activation of conventional signal-transducing GTP-binding proteins. The inhibitory effect of GTP[S] can be clearly distinguished and indicates the involvement of a distinct GTP-binding protein in exocytosis at a site distal to second-messenger generation.

Cyclic AMP stimulates luteinizing-hormone (lutropin) exocytosis in permeabilized sheep anterior-pituitary cells. Synergism with protein kinase C and calcium

Sheep anterior-pituitary cells permeabilized with Staphylococcus aureus alpha-toxin were used to investigate the role of cyclic AMP (cAMP) in exocytosis of luteinizing hormone (lutropin, LH) under conditions where the intracellular free Ca2+ concentration ([Ca2+]free) is clamped by Ca2+ buffers. At resting [Ca2+]free (pCa 7), cAMP rapidly stimulated LH exocytosis (within 5 min) and continued to stimulate exocytosis for at least 30 min. When cAMP breakdown was inhibited by 3-isobutyl-1-methylxanthine (IBMX), the concentration giving half-maximal response (EC50) for cAMP-stimulated exocytosis was 10 microM. cAMP-stimulated exocytosis required millimolar concentrations of MgATP, as has been found with Ca2(+)- and phorbol-ester-stimulated LH exocytosis. cAMP caused a modest enhancement of Ca2(+)-stimulated LH exocytosis by decreasing in the EC50 for Ca2+ from pCa 5.6 to pCa 5.9, but had little effect on the maximal LH response to Ca2+. Activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) dramatically enhanced cAMP-stimulated LH exocytosis by both increasing the maximal effect 5-7-fold and decreasing the EC50 for cAMP to 3 microM. This synergism between cAMP and PMA was further augmented by increasing the [Ca2+]free. Gonadotropin-releasing hormone (gonadoliberin, GnRH) stimulated cAMP production in intact pituitary cells. Since GnRH stimulation is reported to activate PKC and increase the intracellular [Ca2+]free, our results suggest that a synergistic interaction of the cAMP, PKC and Ca2+ second-messenger systems is of importance in the mechanism of GnRH-stimulated LH exocytosis.

Staurosporine enhances gonadotrophin-releasing hormone-stimulated luteinizing hormone secretion

In intact sheep gonadotropes, the protein kinase inhibitor, staurosporine, inhibited the stimulatory effect of phorbol 12-myristate 13-acetate (PMA) on luteinizing hormone (LH) secretion. Under the same conditions staurosporine enhanced gonadotrophin-releasing hormone (GnRH)-stimulated LH exocytosis without altering the EC50 of GnRH and without affecting basal LH exocytosis. These results suggest that PKC does not play a major role in mediating acute GnRH-stimulated LH exocytosis. Furthermore, they demonstrate that staurosporine enhances GnRH stimulus-secretion coupling. Both extracellular Ca2(+)-dependent and Ca2(+)-independent components of GnRH-stimulated LH secretion were enhanced by the drug. Staurosporine had no effect on GnRH stimulation of cAMP and inositol phosphate synthesis. In permeabilized cells staurosporine did not enhance Ca2(+)- and cAMP-stimulated LH exocytosis. Based on these results we hypothesize that staurosporine inhibits a protein kinase which is activated by GnRH and which negatively modulates GnRH stimulus-secretion coupling.

Calcium stimulates luteinizing-hormone (lutropin) exocytosis by a mechanism independent of protein kinase C

Using permeabilized gonadotropes, we examined whether Ca2(+)-stimulated luteinizing-hormone (LH) exocytosis is mediated by the Ca2(+)-activated phospholipid-dependent protein kinase (protein kinase C). In the presence of high [Ca2+]free (pCa 5), alpha-toxin-permeabilized sheep gonadotropes secrete a burst of LH and then become refractory to maintained high [Ca2+]free. The protein kinase C activator phorbol myristate acetate (PMA) is able to stimulate further LH release from cells made refractory to high [Ca2+]free, suggesting that Ca2+ does not stimulate LH release by activating protein kinase C. Staurosporine, a protein kinase C inhibitor, inhibited PMA-stimulated (50% inhibition at 20 nM), but not Ca2(+)-stimulated, LH exocytosis. In cells desensitized to PMA by prolonged exposure to a high PMA concentration, Ca2(+)-stimulated LH exocytosis (when corrected for depletion of total cellular LH) was not inhibited. Ba2+ was able to stimulate LH exocytosis to a maximal extent similar to Ca2+, although higher Ba2+ concentrations were necessary. Ba2+ and Ca2+ stimulated LH exocytosis with a similar time course, and both were inhibitory at high concentrations. Furthermore, cells made refractory to Ca2+ were also refractory to Ba2+. These data strongly suggest that Ba2+ and Ca2+ act through the same mechanism. Since Ba2+ is a poor activator of protein kinase C, these findings are additional evidence against a major role for protein kinase C in mediating Ca2(+)-stimulated LH exocytosis.