Inhibition of Ca(2+)-dependent catecholamine release by myosin light chain kinase inhibitor, wortmannin, in adrenal chromaffin cells

Insulin stimulates translocation of human GLUT4 to the membrane in fat bodies of transgenic Drosophila melanogaster

The fruit fly Drosophila melanogaster is an excellent model system for studies of genes controlling development and disease. However, its applicability to physiological systems is less clear because of metabolic differences between insects and mammals. Insulin signaling has been studied in mammals because of relevance to diabetes and other diseases but there are many parallels between mammalian and insect pathways. For example, deletion of Drosophila Insulin-Like Peptides resulted in 'diabetic' flies with elevated circulating sugar levels. Whether this situation reflects failure of sugar uptake into peripheral tissues as seen in mammals is unclear and depends upon whether flies harbor the machinery to mount mammalian-like insulin-dependent sugar uptake responses. Here we asked whether Drosophila fat cells are competent to respond to insulin with mammalian-like regulated trafficking of sugar transporters. Transgenic Drosophila expressing human glucose transporter-4 (GLUT4), the sugar transporter expressed primarily in insulin-responsive tissues, were generated. After expression in fat bodies, GLUT4 intracellular trafficking and localization were monitored by confocal and total internal reflection fluorescence microscopy (TIRFM). We found that fat body cells responded to insulin with increased GLUT4 trafficking and translocation to the plasma membrane. While the amplitude of these responses was relatively weak in animals reared on a standard diet, it was greatly enhanced in animals reared on sugar-restricted diets, suggesting that flies fed standard diets are insulin resistant. Our findings demonstrate that flies are competent to mobilize translocation of sugar transporters to the cell surface in response to insulin. They suggest that Drosophila fat cells are primed for a response to insulin and that these pathways are down-regulated when animals are exposed to constant, high levels of sugar. Finally, these studies are the first to use TIRFM to monitor insulin-signaling pathways in Drosophila, demonstrating the utility of TIRFM of tagged sugar transporters to monitor signaling pathways in insects.

F-actin-myosin II inhibitors affect chromaffin granule plasma membrane distance and fusion kinetics by retraction of the cytoskeletal cortex

Chromaffin cell catecholamines are released when specialized secretory vesicles undergo exocytotic membrane fusion. Evidence indicates that vesicle supply and fusion are controlled by the activity of the cortical F-actin-myosin II network. To study in detail cell cortex and vesicle interactions, we use fluorescent labeling with GFP-lifeact and acidotropic dyes in confocal and evanescent wave microscopy. These techniques provide structural details and dynamic images of chromaffin granules caged in a complex cortical structure. Both the movement of cortical structures and granule motion appear to be linked, and this motion can be restricted by the myosin II-specific inhibitor, blebbistatin, and the F-actin stabilizer, jasplakinolide. These treatments also affect the position of the vesicles in relation to the plasma membrane, increasing the distance between them and the fusion sites. Consequently, we observed slower single vesicle fusion kinetics in treated cells after neutralization of acridine orange-loaded granules during exocytosis. Increasing the distance between the granules and the fusion sites appears to be linked to the retraction of the F-actin cytoskeleton when treated with jasplakinolide. Thus, F-actin-myosin II inhibitors appear to slow granule fusion kinetics by altering the position of vesicles after relaxation of the cortical network.

Direct stimulation of the transcellular and paracellular calcium transport in the rat cecum by prolactin

Prolactin (PRL) is reported to stimulate calcium absorption in the rat's small intestine. However, little is known regarding its effects on the cecum, a part of the large intestine with the highest rate of intestinal calcium transport. We demonstrated herein by quantitative real-time polymerase chain reaction and Western blot analysis that the cecum could be a target organ of PRL since cecal epithelial cells strongly expressed PRL receptors. In Ussing chamber experiments, PRL enhanced the transcellular cecal calcium absorption in a biphasic dose-response manner. PRL also increased the paracellular calcium permeability and passive calcium transport in the cecum, which could be explained by the PRL-induced decrease in transepithelial resistance and increase in cation selectivity of the cecal epithelium. PRL actions in the cecum were abolished by inhibitors of phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK), but not inhibitors of gene transcription and protein biosynthesis. In conclusion, PRL directly enhanced the transcellular and paracellular calcium transport in the rat cecum through the nongenomic signaling pathways involving PI3K, PKC, and ROCK.

1,25-dihydroxyvitamin D(3) rapidly stimulates the solvent drag-induced paracellular calcium transport in the duodenum of female rats

A calcium-regulating hormone 1alpha,25-dihydroxyvitamin D(3) (1,25-[OH](2)D(3)) has been known to rapidly stimulate the transcellular active calcium transport in the chick duodenum. However, its effects on the solvent drag-induced paracellular calcium transport, which normally contributes approximately 70% of the total active calcium transport, and the underlying mechanism were unknown. The present study aimed to investigate the rapid nongenomic actions of physiological concentrations of 1,25-(OH)(2)D(3), i.e., 1, 10, and 100 nmol/l, on the duodenal calcium absorption in female rats. Quantitative real-time PCR revealed strong expressions of the classical vitamin D receptor (VDR) and the membrane-associated rapid response steroid binding receptors (MARRS) in both small and large intestines. By using the Ussing chamber technique, we found that duodenal epithelia acutely exposed to 10 and 100 nmol/l 1,25-(OH)(2)D(3) rapidly increased the solvent drag-induced calcium transport, but not the transcellular calcium transport, in a dose-response manner. On the other hand, 3-day daily injections of 1,25-(OH)(2)D(3) enhanced the transcellular active duodenal calcium transport. The 1,25-(OH)(2)D(3)-stimulated solvent drag-induced transport was abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitors, 200 nmol/l wortmannin and 75 micromol/l LY294002, as well as PKC (1 micromol/l GF109203X) and MEK inhibitors (10 micromol/l U0126). Although 100 nmol/l 1,25-(OH)(2)D(3) did not alter the transepithelial mannitol flux, indicating no widening of the tight junction, it decreased the transepithelial resistance and increased both sodium and chloride permeability through the paracellular channel. We conclude that 1,25-(OH)(2)D(3) uses the nongenomic signaling pathways involving PI3K, PKC, and MEK to rapidly enhance the solvent drag-induced calcium transport, partly by altering the charge-selective property of the duodenal epithelium at least for the pathways involving PI3K and MEK.

Prolactin-stimulated transepithelial calcium transport in duodenum and Caco-2 monolayer are mediated by the phosphoinositide 3-kinase pathway

Prolactin (PRL) has been shown to stimulate intestinal calcium absorption but the mechanism was still unknown. This study aimed to investigate the mechanism and signaling pathway by which PRL enhanced calcium transport in the rat duodenum and Caco-2 monolayer. Both epithelia strongly expressed mRNAs and proteins of PRL receptors. Ussing chamber technique showed that the duodenal active calcium fluxes were increased by PRL in a dose-response manner with the maximal effective dose of 800 ng/ml. This response diminished after exposure to LY-294002, a phosphoinositide 3-kinase (PI3K) inhibitor. Caco-2 monolayer gave similar response to PRL with the maximal effective dose of 600 ng/ml. By nullifying the transepithelial potential difference, we showed that the voltage-dependent paracellular calcium transport did not contribute to the PRL-enhanced flux in Caco-2 monolayer. In contrast, the calcium gradient-dependent paracellular transport and calcium permeability were increased by PRL. Effects of PRL on Caco-2 monolayer were abolished by PI3K inhibitors (LY-294002 and wortmannin), but not by inhibitors of MEK (U-0126) or JAK2 (AG-490). To investigate whether the PRL-enhanced paracellular transport was linked to changes in the epithelial charge selectivity, the permeability ratio of sodium and chloride (P(Na)/P(Cl)) was determined. We found that PRL elevated the P(Na)/P(Cl) in both epithelia, and the effects were blocked by PI3K inhibitors. In conclusion, PRL directly and rapidly stimulated the active and passive calcium transport in the rat duodenum and Caco-2 monolayer via the nongenomic PI3K-signaling pathway. This PRL-enhanced paracellular calcium transport could have resulted from altered charge selectivity.

A role for Rho and Rac in secretagogue-induced amylase release by pancreatic acini

The actin cytoskeleton has long been implicated in protein secretion. We investigated whether Rho and Rac, known regulators of the cytoskeleton, are involved in amylase secretion by mouse pancreatic acini. Secretagogues, including cholecystokinin (CCK) and the acetylcholine analog carbachol, increased the amount of GTP-bound RhoA and Rac1 and induced translocation from cytosol to a membrane fraction. Immunocytochemistry revealed the translocation of Rho and Rac within the apical region of the cell. Expression by means of adenoviral vectors of dominant-negative Rho (RhoN19), dominant-negative Rac (RacN17), and Clostridium Botulinum C3 exotoxin, which ADP ribosylates and inactivates Rho, significantly inhibited amylase secretion by CCK and carbachol; inhibiting both Rho and Rac resulted in a greater reduction. This inhibitory effect of RhoN19 on CCK-induced amylase secretion was apparent in both the early and late phases of secretion, whereas RacN17 was more potent on the late phase of secretion. None of these three affected the basal Ca2+or the peak intracellular Ca2+concentration stimulated by CCK. Latrunculin, a marine toxin that sequesters actin monomers, time-dependently decreased the total amount of filamentous actin (F-actin) and dose-dependently decreased secretion by secretagogues without affecting Ca2+signaling. These data suggest that Rho and Rac are both involved in CCK-induced amylase release in pancreatic acinar cell possibly through an effect on the actin cytoskeleton.

New roles of myosin II during vesicle transport and fusion in chromaffin cells

Modified herpes virus (amplicons) were used to express myosin regulatory light chain (RLC) chimeras with green fluorescent protein (GFP) in cultured bovine chromaffin cells to study myosin II implication in secretion. After infection, RLC-GFP constructs were clearly identified in the cytoplasm and accumulated in the cortical region, forming a complex network that co-localized with cortical F-actin. Cells expressing wild type RLC-GFP maintained normal vesicle mobility, whereas cells expressing an unphosphorylatable form (T18A/S19A RLC-GFP) presented severe restrictions in granule movement as measured by individual tracking in dynamic confocal microscopy studies. Interestingly, the overexpression of this mutant form of RLC also affected the initial secretory burst elicited by either high K(+) or BaCl(2), as well as the secretion induced by fast release of calcium from caged compounds in individual cells. Moreover, T18A/S19A RLC-GFP-infected cells presented slower fusion kinetics of individual granules compared with controls as measured by analysis of amperometric spikes. Taken together, our results demonstrate the implication of myosin II in the transport of vesicles, and, surprisingly, in the final phases of exocytosis involving transitions affecting the activity of docked granules, and therefore uncovering a new role for this cytoskeletal element.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

The role of myosin in vesicle transport during bovine chromaffin cell secretion

Bovine adrenomedullary cells in culture have been used to study the role of myosin in vesicle transport during exocytosis. Amperometric determination of calcium-dependent catecholamine release from individual digitonin-permeabilized cells treated with 3 microM wortmannin or 20 mM 2,3-butanedione monoxime (BDM) and stimulated by continuous as well as repetitive calcium pulses showed alteration of slow phases of secretion when compared with control untreated cells. The specificity of these drugs for myosin inhibition was further supported by the use of peptide-18, a potent peptide affecting myosin light-chain kinase activity. These results were supported also by studying the impact of these myosin inhibitors on chromaffin granule mobility using direct visualization by dynamic confocal microscopy. Wortmannin and BDM affect drastically vesicle transport throughout the cell cytoplasm, including the region beneath the plasma membrane. Immunocytochemical studies demonstrate the presence of myosin types II and V in the cell periphery. The capability of antibodies to myosin II in abrogating the secretory response from populations of digitonin-permeabilized cells compared with the modest effect caused by anti-myosin V suggests that myosin II plays a fundamental role in the active transport of vesicles occurring in the sub-plasmalemmal area during chromaffin cell secretory activity.

Differential regulation of pituitary hormone secretion and gene expression by thyrotropin-releasing hormone. A role for mitogen-activated protein kinase signaling cascade in rat pituitary GH3 cells

We examined the possible involvement of mitogen-activated protein (MAP) kinase activation in the secretory process and gene expression of prolactin and growth hormone. Thyrotropin-releasing hormone (TRH) rapidly stimulated the secretion of both prolactin and growth hormone from GH3 cells. Secretion induced by TRH was not inhibited by 50 microM PD098059, but was completely inhibited by 1 microM wortmannin and 10 microM KN93, suggesting that MAP kinase does not mediate the secretory process. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin, whereas expression of growth hormone mRNA was largely attenuated. The increase in prolactin mRNA stimulated by TRH was inhibited by addition of PD098059, and the decrease in growth hormone mRNA was also inhibited by PD098059. Transfection of the cells with a pFC-MEKK vector (a constitutively active MAP kinase kinase kinase), significantly increased the synthesis of prolactin and decreased the synthesis of growth hormone. These data taken together indicate that MAP kinase mediates TRH-induced regulation of prolactin and growth hormone gene expression. Reporter gene assays showed that prolactin promoter activity was increased by TRH and was completely inhibited by addition of PD098059, but that the promoter activity of growth hormone was unchanged by TRH. These results suggest that TRH stimulates both prolactin and growth hormone secretion, but that the gene expressions of prolactin and growth hormone are differentially regulated by TRH and are mediated by different mechanisms.

Myosin XVA expression in the pituitary and in other neuroendocrine tissues and tumors

The myosin superfamily of molecular motor proteins includes conventional myosins and several classes of unconventional myosins. Recent studies have characterized the human and mouse unconventional myosin XVA, which has a role in the formation and/or maintenance of the unique actin-rich structures of inner ear sensory hair cells. Myosin XVA is also highly expressed in human anterior pituitary cells. In this study we examined the distribution of myosin XVA protein and mRNA in normal and neoplastic human pituitaries and other neuroendocrine cells and tumors. Myosin XVA was expressed in all types of normal anterior pituitary cells and pituitary tumors and in other neuroendocrine cells and tumors including those of the adrenal medulla, parathyroid, and pancreatic islets. Most nonneuroendocrine tissues examined including liver cells were negative for myosin XVA protein and mRNA, although the distal and proximal tubules of normal kidneys showed moderate immunoreactivity for myosin XVA. Ultrastructural immunohistochemistry localized myosin XVA in association with secretory granules of human anterior pituitary cells and human pituitary tumors. These data suggest that in neuroendocrine cells myosin XVA may have a role in secretory granule movement and/or secretion.

Mechanism by which wortmannin and LY294002 inhibit catecholamine secretion in the rat adrenal medullary cells

The effects of wortmannin and LY294002, inhibitors of PI(3)-kinase, in secretagogue-stimulated rat adrenal chromaffin cells loaded with Calcium Green-1 were studied by simultaneously measuring changes in the fluorescence intensity of the indicator (Ca-response) and in the release of catecholamine (secretory response). Before application of these agents, the profile of the secretory response evoked by a 10-min stimulation with 30 mM K(+)] was approximated by the k th (2.6 on average) power of that of the Ca-response. Both agents dose-dependently inhibited the high-K(+)-elicited Ca-response and secretory response in a similar mode to which the k th power relation was preserved despite the occurrence of profound changes in the shapes and sizes of these two responses. The L-type Ca(2+)-channel blocker PN200-110 inhibited the high-K(+)-evoked responses in a similar fashion. Thus, it is likely that wortmannin and LY294002 inhibit high-K(+)-evoked CA secretion by inhibiting a Ca(2+)-influx through voltage-dependent Ca(2+)channels. Although regulation of L-type Ca(2+)channel activity via PI(3)-kinase has been reported in vascular myocytes, this possibility may be limited in the present case since the doses of LY294002 and wortmannin used to inhibit the secretory response are much higher than IC(50)'s for inhibition of PI(3)-kinase with these agents. Compared with the high-K(+)-elicited responses, muscarine-evoked Ca-responses and secretory responses were more strongly inhibited by wortmannin, but less affected by LY294002. The differential effects suggest that the inhibition of the muscarine-evoked secretion by these agents i s not associated with the inhibition of PI(3)-kinase.

Effects of wortmannin upon the late stages of the secretory pathway of AtT-20 cells

Heterotrimeric GTP-binding (G) proteins, termed Ge, have a role in the late stages of the adrenocorticotrophin (ACTH) secretory pathway in the mouse AtT-20/D16-16 anterior pituitary tumour cell line. The wortmannin sensitivity of Ge-controlled mechanisms in AtT-20 cells was investigated to provide information on the possible mechanisms linking Ge with secretion. Permeabilised cells exposed to calcium ions (10(-9) to 10(-3) M), guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (10(-8) to 10(-4) M) and mastoparan (10(-8) to 10(-5) M) demonstrated a significant and concentration-dependent stimulation of ACTH secretion from non-stimulated levels for all three agents. Coincubation with wortmannin (10(-5) M) significantly inhibited both calcium-independent and -stimulated secretion. The effect of wortmannin was concentration-dependent being maximal at 10(-6) M. The study shows that wortmannin inhibits both calcium-independent and -stimulated secretion from permeabilised AtT-20 cells indicating a role for phosphatidylinositol-3 kinase in determining the size of the readily releasable pool of ACTH and/or in mediating calcium/Ge-evoked secretion from this pool.

Light-evoked recovery from wortmannin-induced inhibition of catecholamine secretion and synaptic transmission

Wortmannin (WT) is known to inhibit catecholamine (CA) secretion in chromaffin cells. This effect was found to be sensitive to UV light in experiments designed to perform simultaneous monitoring of changes in [Ca2+]i and CA secretion in perfused rat adrenal medullas. When the change in [Ca2+]i was measured using calcium green-1 (490 nm excitation), a 35-min treatment with 10 microM WT caused a 69% inhibition of CA secretion evoked by excess (30 mM) extracellular K+ and a moderate inhibition of the [Ca2+]i response. In contrast, the same treatment of fura-2-loaded cells with WT caused only an 11% inhibition of the high-K+-evoked secretion and no significant attenuation of the [Ca2+]i response. However, during interruption of fluorometry with fura-2, the inhibitory effect of WT developed at a rate similar to that exhibited in calcium green-1-loaded cells. The WT-induced inhibition of high-K+- or bradykinin-evoked secretory responses, which was otherwise irreversible, was reversed by exposing WT-treated chromaffin cells to 380-nm light. When WT was reapplied to the cells of which the secretory ability had been restored by light irradiation, the secretory response was inhibited with a time course similar to that shown during the initial treatment with WT. The photosensitive effect of WT was also demonstrated using bullfrog sympathetic ganglia in which WT-induced inhibition of synaptic transmission was reversed by irradiation with 380-nm light. These results suggest that UV light removes the inhibitory effects of WT by disrupting the covalent bond formed between WT and a target molecule which remains to be determined, although myosin light chain kinase has been reported as the target molecule in both cases examined in this study.

Mitogen-activated protein kinase activation by stimulation with thyrotropin-releasing hormone in rat pituitary GH3 cells

We examined whether mitogen-activated protein (MAP) kinase is activated by thyrotropin-releasing hormone (TRH) in GH3 cells, and whether MAP kinase activation is involved in secretion of prolactin from these cells. Protein kinase inhibitors--such as PD098059, calphostin C, and genistein--and removal of extracellular Ca2+ inhibited MAP kinase activation by TRH. A cAMP analogue activated MAP kinase in these cells. Effects of cAMP on MAP kinase activation were inhibited by PD098059. TRH-induced prolactin secretion was not inhibited by levels of PD098059 sufficient to i activation but was inhibited by wortmannin (1 microM) and KN93. Treatment of GH3 cells with either TRH or cAMP significantly inhibited DNA synthesis and induced morphological changes. The effects stimulated by TRH were reversed by PD098059 treatment, but the same effects stimulated by cAMP were not. Treatment of GH3 cells with TRH for 48 h significantly increased the prolactin content in GH3 cells and decreased growth hormone content. The increase in prolactin was completely abolished by PD098059, but the decrease in growth hormone was not. These results suggest that TRH-induced MAP kinase activation is involved in prolactin synthesis and differentiation of GH3 cells, but not in prolactin secretion.

Myosin light chain kinase inhibitors and calmodulin antagonist inhibit Ca(2+)- and ATP-dependent catecholamine secretion from bovine adrenal chromaffin cells

We have used stage-specific assays for ATP-dependent priming and for Ca(2+)-activated triggering in the absence of ATP to examine the effects of myosin light chain kinase (MLCK) inhibitors, ML-9 and ML-7, and calmodulin antagonists, W-7 and trifluoperazine (TFP), on regulated exocytosis in beta-escin-permeabilized bovine adrenal chromaffin cells. Ca2+ (0.1-30 microM) induced a significantly greater secretion of catecholamines in the presence of MgATP (2 mM) than in the absence of MgATP. ML-9 (30 and 100 microM), ML-7 (30 and 100 microM), W-7 (30 and 100 microM) and TFP (10 and 30 microM) inhibited the Ca(2+)-induced catecholamine secretion in the presence of MgATP, but did not affect the catecholamine response to Ca2+ in the absence of MgATP. In intact cells all these compounds inhibited catecholamine secretion in responses to acetylcholine (100 microM) and high K+ (40 mM). The results obtained in permeabilized cells suggest that the calmodulin-MLCK system plays an essential role in the ATP-requiring priming stage but not in the Ca2(+)-triggered fusion step in the exocytotic process in bovine adrenal chromaffin cells.

Differential effects of wortmannin on the release of substance P and amino acids from the isolated spinal cord of the neonatal rat

1. Effects of wortmannin, an inhibitor of myosin light chain kinase, on the release of substance P and amino acids, GABA and glutamate, were investigated in the isolated spinal cord preparation of the neonatal rat. 2. Wortmannin at 0.5 - 10 microM depressed the release of substance P evoked by high-K+ (90 mM) medium from the spinal cord (IC50 = 1.1 microM). Wortmannin also depressed the high-K+ (70 mM)-evoked release of substance P from cultured dorsal root ganglion neurons of neonatal rats. In contrast, the high-K+ (90 mM)-evoked release of GABA and glutamate from the spinal cord was not affected by wortmannin (0.1 - 10 microM). 3. Upon stimulation of a dorsal root, a monosynaptic reflex and a subsequent slow ventral root depolarization were evoked in the ipsilateral ventral root of the same segment in the isolated spinal cord preparation. The magnitude of the slow ventral root depolarization was depressed gradually to about 70% of the control during the course of 30 min under wortmannin (1 microM). In contrast, the monosynaptic reflex was unaffected by wortmannin. 4. Immunofluorescent staining revealed that immunoreactivities of substance P and myosin II were colocalized at presynaptic terminals in the dorsal horn of the neonatal rat spinal cord. 5. The present results suggest that myosin phosphorylation by myosin light chain kinase may play a crucial role in the release of substance P, but not in the release of GABA and glutamate in the neonatal rat spinal cord. This may reflect a difference in the exocytic mechanisms of substance P-containing large dense core vesicles and amino acid-containing small clear vesicles.

Potassium current development and its linkage to membrane expansion during growth of cultured embryonic mouse hippocampal neurons: sensitivity to inhibitors of phosphatidylinositol 3-kinase and other protein kinases

Hippocampal pyramidal neurons express three major voltage-dependent potassium currents, IA, ID, and IK. During hippocampal development, IA, the rapidly activating and inactivating transient potassium current, is detected soon after pyramidal neurons can be morphologically identified. Appearance of IA in developing pyramidal neurons is dependent on contact with cocultured astroglial cells; cultured pyramidal neurons not in contact with astroglial cells have reduced membrane area and IA (Wu and Barish, 1994). We have examined intracellular signaling pathways that could contribute to the regulation of IA development by probing developing pyramidal neurons with kinase inhibitors. We observed that exposure to LY294002 or wortmannin, inhibitors of phosphatidylinositol (PI) 3-kinase, reduced somatic cross-sectional area, neurite outgrowth, whole-cell capacitance, IA amplitude and density (amplitude normalized to membrane area), and immunoreactivity for Kv4.2 and/or Kv4.3 (potassium channel subunits likely to be present in the channels carrying IA). In contrast, exposure to ML-9 or KN-62, inhibitors of myosin light chain kinase or Ca2+-calmodulin-dependent protein kinase II (CaMKII), reduced membrane area and IA amplitude but did not affect IA density or Kv4. 2/3 immunoreactivity to the same extent as inhibitors of PI 3-kinase. Unexpectedly, exposure to bisindolymaleimide I or calphostin C, inhibitors of protein kinase C (PKC), did not affect membrane area or potassium current development. Our data suggest that PI 3-kinases regulate both A-type potassium channel synthesis and plasmalemmal insertion of vesicles bearing these potassium channels. CaMKII appears to regulate fusion of channel-bearing vesicles with the plasmalemma and myosin light chain kinase to regulate centripetal transport of channel-bearing vesicles from the Golgi. We further suggest that astroglial cells exert their influence on pyramidal neuron development through activation of PI 3-kinases.

Myosin light-chain phosphorylation controls insulin secretion at a proximal step in the secretory cascade

The aim of this study was to investigate how insulin secretion is controlled by phosphorylation of the myosin light chain (MLC). Ca2+-evoked insulin release from pancreatic islets permeabilized with streptolysin O was inhibited by different monoclonal antibodies against myosin light-chain kinase (MLCK) to an extent parallel to their inhibition of purified MLCK. Anti-MLCK antibody also inhibited insulin release caused by the stable GTP analog guanosine 5'-O-(3-thiodiphosphate), even at a substimulatory concentration (0.1 microM) of Ca2+. Free Ca2+ increased MLC peptide phosphorylation by beta-cell extracts in vitro. In contrast to the phosphorylation by purified MLCK or by calmodulin (CaM) kinase II, the activity partially remained with the beta-cell under nonstimulatory Ca2+ (0.1 microM) conditions. The MLCK inhibitor ML-9 inhibited the activity in the beta-cell with both substimulatory and stimulatory Ca2+, whereas KN-62, an inhibitor of CaM kinase II, only exerted an influence in the latter case. ML-9 decreased intracellular granule movement in MIN6 cells under basal and acetylcholine-stimulated conditions. We propose that MLC phosphorylation may modulate translocation of secretory granules, resulting in enhanced insulin secretion.

Wortmannin-sensitive and -insensitive steps in calcium-controlled exocytosis in pituitary gonadotrophs: evidence that myosin light chain kinase mediates calcium-dependent and wortmannin-sensitive gonadotropin secretion

In cultured rat pituitary cells, increases in the cytosolic calcium concentration ([Ca2+]i) and LH release are induced by activation of GnRH receptors as well as by nonreceptor-mediated stimuli. Treatment of pituitary cells with the myosin light chain kinase (MLCK) inhibitor, wortmannin, attenuated GnRH-induced LH release. Wortmannin also reduced the LH responses to nonreceptor-mediated elevation of [Ca2+]i by ionomycin and activation of voltage-sensitive Ca2+ channels by Bay K 8644 or high K+, as well as Ca2+-induced LH release in permeabilized pituitary cells. The [Ca2+]i responses to these stimuli were unaltered in wortmannin-treated pituitary cells, indicating that this compound inhibits a Ca2+-dependent step in exocytosis without affecting Ca2+ signaling. In perifused pituitary cells, the GnRH-induced early spike phase of LH release was not affected by wortmannin, whereas the subsequent plateau phase was almost completely inhibited. No significant changes in GnRH-induced phospholipase D activity and diacylglycerol production were observed in wortmannin-treated pituitary cells during the sustained phase of agonist stimulation. Wortmannin also had no effect on LH responses to the protein kinase C activator, phorbol 12-myristate 13-acetate, further indicating that the attenuation of agonist-induced LH release is not related to inhibition of the diacylglycerol/protein kinase C pathway. In addition, agonist-induced LH release was attenuated by two other MLCK inhibitors, MS-347a and KT5926. These data suggest that MLCK mediates the downstream effects of Ca2+ on exocytosis, an action supported by the finding of wortmannin-sensitive phosphorylation of a 20-kDa protein in pituitary cells and alphaT3-1 gonadotrophs treated with GnRH, K+, and Bay K 8644. This protein was coprecipitated from pituitary extracts with a specific antibody to nonmuscle myosin IIB and comigrated with 20-kDa smooth muscle myosin light chain on SDS-PAGE. These results demonstrate that Ca2+ controls exocytosis through an initial wortmannin-insensitive step and a sustained wortmannin-sensitive step and suggest that the latter event in the cascade of cellular responses is dependent on phosphorylation of nonmuscle myosin IIB light chain by MLCK.

Inhibition of renin secretion by Ca2+ through activation of myosin light chain kinase

This study sought to identify specific enzyme(s) involved in the biochemical cascade of inhibition of renin secretion through Ca(2+)-calmodulin mediation with the use of inhibitors of protein kinase and phosphatases. Inhibition of renin secretion mediated by Ca(2+)-calmodulin was induced by incubating rat renal cortical slices in K(+)-rich depolarizing medium, producing > 50% inhibition. This inhibition was completely blocked by the calmodulin antagonist calmidazolium. The inhibitor of protein kinase with broad specificity, K-252a, blocked the inhibition of renin secretion. Neither KN-62, a specific inhibitor of Ca(2+)-calmodulin-dependent protein kinase II (CaMK II), nor specific inhibitors of protein phosphatase 2B (PP2B), cyclosporin A and FK-506, blocked the inhibition. On the other hand, all four known inhibitors specific for myosin light chain kinase (MLCK), with different chemical structures and mechanisms of inhibition (ML-9, ML-7, KT-5926 and wortmannin), almost completely protected renin secretion against the inhibition by Ca2+. In particular, ML-9 reversively protected > 77% secretion against the inhibition both in K(+)-rich medium alone and in combination with the calcium ionophore A-23187 in a concentration-dependent manner. Together, these findings from our present study provide the first evidence, albeit indirect in nature, for the possibility that activation of Ca(2+)-calmodulin-dependent MLCK at the downstream of Ca2+ influx into juxtaglomerular (JG) cells leads to phosphorylation of 20-kDa regulatory myosin light chain (MLC20). Through interaction with actin, the phosphorylated MLC20 may play an important role in the inhibitory stimulus-secretion coupling of renin.

Effects of wortmannin, a novel myosin light-chain kinase inhibitor, on bile canalicular contraction in vitro and in vivo

BACKGROUND

The cytoskeletal system is believed to play an important role in normal bile formation. The effects of wortmannin, a new myosin light-chain kinase inhibitor, on bile canalicular contraction and bile flow have been observed.

METHODS

The bile canalicular contraction of cultured hepatocyte doublets was investigated, using an image analyzer with a phase contrast microscope, and the intracellular Ca2+ concentration was measured, using microscopic fluorometry. We also investigated bile flow by in vivo intraportal infusion of the drug in rats.

RESULTS

Treatment with wortmannin inhibited norepinephrine-induced canalicular contraction and caused a decrease in bile flow without changing systematic and portal blood pressure. Morphologic examination of the electron microscopic study showed that most bile canaliculi were dilated, with loss of microvilli, but no other apparent damage was seen in parenchymal hepatocytes.

CONCLUSIONS

These data suggest that the integrity of the phosphorylation system of myosin is essential for normal bile flow.

Effects of wortmannin on glucose uptake and protein kinase C activity in rat adipocytes

Wortmannin is known to be an inhibitor of myosin light chain kinase and phosphatidylinositol 3-kinase (PI 3-kinase) (J. Biol. Chem. 268, 25846, 1993). We studied the effects of wortmannin on insulin- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced glucose uptake, purified PKC activity and in vitro 80 kDa protein phosphorylation to elucidate the relationship between insulin-induced PI 3-kinase and PKC activations. Pretreatment with 10(-12)-10(-6) M wortmannin for 60 min resulted in a dose-responsive reduction of 10 nM insulin-stimulated glucose uptake in rat adipocytes. Pretreatment with 10(-6) M wortmannin resulted in 80% and 20% decreases of glucose uptake stimulated by insulin and TPA, respectively. Partially purified rat brain PKC activity and 80 kDa protein in vitro phosphorylation of rat adipocyte cytosol by addition of Ca2+ and phospholipid were dose-dependently decreased by 10(-8)-10(-6) M wortmannin; 20% decrease of PKC activity and 50% decrease of 80 kDa protein phosphorylation by 10(-6) M wortmannin were observed. These results suggest that wortmannin has a potent inhibitory effect on PI 3-kinase and a weak inhibitory effect on PKC activity, and both effects cause a significant inhibition of insulin-stimulated glucose uptake in rat adipocytes.

Integrity of myosin light chain kinase is essential for Ito cell contraction

Hepatic sinusoidal Ito cells (fat storing cells) are believed to play a regulatory role on hepatic sinusoidal blood flow through their contraction. The detailed mechanism of contraction of Ito cells, however, is still unknown. The present study was undertaken to clarify the effect of new myosin light chain kinase inhibitor, wortmannin, on Ito cell contraction. Ito cells prepared from rat liver were cultured for 4 days before the study. The contraction of Ito cells, which was monitored and analysed by time-lapse video tape recording, was triggered by addition of endothelin-1. Wortmannin pretreatment for 1 h inhibited endothelin-induced Ito cell contraction dose-dependently. Therefore, the integrity of the actomyosin system is essential for Ito cell contraction and normal sinusoidal blood flow.

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

Molecular basis of intracistronic complementation in the Passover locus of Drosophila

The only demonstrated mechanism for intracistronic genetic complementation requires physical interaction of protein subunits to create a functional molecule. We demonstrate another and perhaps quite general mechanism utilizing proteins with unique and shared domains. The Drosophila neural mutant Passover (Pas) disrupts specific synaptic connections. Alleles of a lethal complementation group exhibit a complex pattern of complementation with Pas alleles. Whereas all heterozygotes between these lethal alleles and Pas are viable, only some alleles complement the neural defect of Pas. Lethal and neural functions are separately encoded by two proteins that have distinct N-terminal domains and a common C-terminal portion. Neural-specific and lethal-specific mutations map to unique exons, while neural-lethal mutations map to shared exons. Combinations of lethal and neural alleles result in production of both proteins and demonstrate intracistronic complementation.

Myosin-actin interaction plays an important role in human immunodeficiency virus type 1 release from host cells

We examined the potential role of myosin and actin in the release of human immunodeficiency virus type 1 (HIV-1) from infected cells. Wortmannin (100 nM to 5 microM), an effective inhibitor of myosin light chain kinase, blocked the release of HIV-1 from infected T-lymphoblastoid and monocytoid cells in a concentration-dependent manner. Cytochalasin D, a reagent that disrupts the equilibrium between monomeric and polymeric actin, also partially inhibited the release of HIV-1 from the infected cells. At the budding stage, myosin and HIV-1 protein were detected in the same areas on the plasma membrane by using dual-label immunofluorescence microscopy and immunoelectron microscopy. In the presence of 5 microM wortmannin, viral components were observed on the plasma membrane by using immunofluorescence microscopy and electron microscopy, implying that wortmannin did not disturb the transport of viral proteins to the plasma membrane but rather inhibited budding.

ATP release evoked by isoprenaline from adrenergic nerves of guinea pig atrium

Mode and site of release of ATP evoked by isoprenaline were evaluated in the electrically driven left atrial segment of guinea pig. The peak release of ATP 5 min after 1 microM isoprenaline was inhibited by 1 microM propranolol and 1 microM butoxamine, but not by 1 microM atenolol, showing that the ATP release is due to stimulation of the presynaptic beta 2-adrenoceptor by isoprenaline. The maximum ATP release was markedly reduced by Ca2+/calmodulin antagonists, W-7 and trifluoperazine, and by a mitotic inhibitor, vinblastine. Further, the release was similarly inhibited by myosin light chain kinase inhibitors, ML-7 and wortmannin. Nifedipine, a Ca(2+)-channel blocker, decreased the release of ATP evoked by isoprenaline. By contrast, Bay K 8644, a Ca(2+)-channel opener, tended to enhance the ATP release. These findings suggest that isoprenaline produces ATP release from adrenergic nerve terminals of atrium, implying that ATP serves as a co-transmitter.

Role of myosin in neurotransmitter release: functional studies at synapses formed in culture

To determine the functional role of presynaptic proteins in the neurotransmitter release, I have employed cholinergic synapses formed between superior cervical ganglion neurons in culture. These synapses expressed proteins characteristic of mature synapses: immunofluorescence staining showed the presence of synaptophysin, synaptotagmin, VAMP/synaptobrevin-2, syntaxin and neurexin. The function of these proteins seems to be similar to that of mature synapses because botulinum neurotoxins A, E and C1 inhibited neurotransmitter release evoked by presynaptic action potentials. With this preparation, I have obtained evidence supporting roles for myosin II and myosin light chain kinase in neurotransmitter secretion. Acetylcholine release was inhibited by introduction of antibody against myosin II or inhibitors of myosin light chain kinase. This evidence suggests a model in which myosin light chain kinase phosphorylates myosin, and the resultant change in actin-myosin interactions is involved in some steps of transmitter release.

Effect of disruption of actin filaments by Clostridium botulinum C2 toxin on insulin secretion in HIT-T15 cells and pancreatic islets

To examine their role in insulin secretion, actin filaments (AFs) were disrupted by Clostridium botulinum C2 toxin that ADP-ribosylates G-actin. Ribosylation also prevents polymerization of G-actin to F-actin and inhibits AF assembly by capping the fast-growing end of F-actin. Pretreatment of HIT-T15 cells with the toxin inhibited stimulated insulin secretion in a time- and dose-dependent manner. The toxin did not affect cellular insulin content or nonstimulated secretion. In static incubation, toxin treatment caused 45-50% inhibition of secretion induced by nutrients alone (10 mM glucose + 5 mM glutamine + 5 mM leucine) or combined with bombesin (phospholipase C-activator) and 20% reduction of that potentiated by forskolin (stimulator of adenylyl cyclase). In perifusion, the stimulated secretion during the first phase was marginally diminished, whereas the second phase was inhibited by approximately 80%. Pretreatment of HIT cells with wartmannin, a myosin light chain kinase inhibitor, caused a similar pattern of inhibition of the biphasic insulin release as C2 toxin. Nutrient metabolism and bombesin-evoked rise in cytosolic free Ca2+ were not affected by C2 toxin, indicating that nutrient recognition and the coupling between receptor activation and second messenger generation was not changed. In the toxin-treated cells, the AF web beneath the plasma membrane and the diffuse cytoplasmic F-actin fibers disappeared, as shown both by staining with an antibody against G- and F-actin and by staining F-actin with fluorescent phallacidin. C2 toxin dose-dependently reduced cellular F-actin content. Stimulation of insulin secretion was not associated with changes in F-actin content and organization. Treatment of cells with cytochalasin E and B, which shorten AFs, inhibited the stimulated insulin release by 30-50% although differing in their effects on F-actin content. In contrast to HIT-T15 cells, insulin secretion was potentiated in isolated rat islets after disruption of microfilaments with C2 toxin, most notably during the first phase. This effect was, however, diminished, and the second phase became slightly inhibited when the islets were degranulated. These results indicate an important role for AFs in insulin secretion. In the poorly granulated HIT-T15 cells actin-myosin interactions may participate in the recruitment of secretory granules to the releasable pool. In native islet beta-cells the predominant function of AFs appears to be the limitation of the access of granules to the plasma membrane.

Myosin II is involved in transmitter release at synapses formed between rat sympathetic neurons in culture

The presynaptic function of myosin II was studied at cholinergic synapses formed between rat superior cervical ganglion neurons in culture. Immunofluorescent staining showed that myosin II was colocalized with synaptophysin at the presynaptic nerve terminals. Antimyosin II antibody introduced into presynaptic neurons inhibited synaptic transmission. Transmission was also inhibited in a dose-dependent manner by two inhibitors of myosin light chain kinase: a peptide, SM-1, and an organic inhibitor, wortmannin. The inhibition produced by these agents was dependent on presynaptic activity. Extracellularly applied wortmannin also blocked synaptic transmission, but its effects were slower in onset. Wortmannin also decreased postsynaptic potentials and post-tetanic potentiation in intact superior cervical ganglia. These results suggest a model in which myosin light chain kinase phosphorylates myosin, and the resultant change in actin-myosin interactions is involved in neurotransmitter release.

Analysis of the mechanism for acetylcholine release at the synapse formed between rat sympathetic neurons in culture

Superior cervical ganglion neurons (SCGNs) were isolated from 7-day-old rat SCG and cultured in MEM containing horse serum, fetal calf serum, and nerve growth factor. In this culture condition, it is well known that the SCGNs form cholinergic synapse. In 3-4 weeks cultured neurons, immunofluorescent staining for synaptophysin, a small synaptic vesicle associated protein, showed the presence of synaptophysin as small dots on the surface of the soma. Postsynaptic potentials could be recorded in 50-80% of the neurons responding to evoked action potentials elicited in neighboring neurons. Because of its relatively large cell size and the short distance to the terminal, this synapse is a useful model for studying the mechanisms of acetylcholine (ACh) release by introducing substances such as antibodies or selective inhibitors into the presynaptic neuron by means of the whole-cell clamp technique. In this model synapse we tested the possible role of myosin in ACh release. The distribution of myosin was studied by the immunofluorescent staining technique. Myosin was recognized by the anti-myosin II IgG at the same synaptic terminals that showed the presence of synaptophysin with its antibody. The functional blockade of myosin by the antibody itself, and that of myosin light chain kinase (MLCK) by a pseudosubstrate inhibitor of MLCK, SM-1, or by a selective inhibitor of MLCK, wortmannin, induced depression of synaptic transmission in a dose-dependent manner. These indicate that phosphorylation of myosin by MLCK may be necessary for ACh release mechanisms.

Myosin light chain kinase occurs in bullfrog sympathetic neurons and may modulate voltage-dependent potassium currents

A polyclonal antibody against myosin light chain kinase (MLCK) of chicken gizzard recognized a 130 kd peptide of bullfrog sympathetic ganglia as MLCK. MLCK immunoreactivity was confined to the neuronal cell body. A synthetic peptide corresponding to an inhibitory domain of MLCK (Ala783-Gly804) was applied intracellularly to isolated sympathetic neurons during whole-cell recordings of ionic currents. The peptide inhibitor reversibly decreased M-type potassium current (IM) while not affecting A-type of delayed rectifier-type potassium currents. Intracellular application of an active fragment of MLCK enhanced IM, whereas application of an inactive MLCK fragment did not. The results suggest that IM can be modulated by MLCK-catalyzed phosphorylation.

Enhancing effect of wortmannin on muscarinic stimulation of phospholipase D in rat pheochromocytoma PC12 cells

Wortmannin, a specific inhibitor of myosin light chain kinase (MLCK), enhanced carbachol-induced formation of [3H]phosphatidylethanol ([3H]PEt), a marker of phospholipase D (PLD) activity, in [3H]palmitic acid-labeled PC12 cells. The apparent EC50 value was 1.5 microM, and the effect was maximal at 3 microM and slightly attenuated at higher concentration. Wortmannin alone had no significant effect on [3H]PEt formation. The enhancing effect of wortmannin was observed at the initial increasing phase of [3H]PEt formation but not at the subsequent plateau phase. Wortmannin enhanced also phorbol ester-induced PLD activation. Although the precise mechanism remains to be clarified, these results suggest that MLCK may be involved in PLD regulation in PC12 cells.