Changes in the components of a nuclear inositide cycle during differentiation in murine erythroleukaemia cells

Differentiation of murine erythroleukaemia cells with the chemical agent DMSO leads to a cessation of proliferation and the production of a number of erythrocyte markers such as haemoglobin. We have previously demonstrated that activation of proliferation leads to an increase in the production of nuclear diacylglycerol (DAG). Here we demonstrate that differentiation leads to a decrease in the levels of nuclear DAG and the activity of the nuclear-associated phosphoinositidase C (PIC). The change in activity appears to be due to a decrease in the mass levels of the beta 1 isoform, as demonstrated by the use of isoform-specific antibodies. Moreover, the changes correlate with the cessation of proliferation and an increase in the number of cells in G1 phase of the cell cycle, rather than with the number of cells which have differentiated. Indeed, although treatment of the cells with phorbol 12-myristate 13-acetate (PMA) inhibits the differentiation programme as assessed by haemoglobin staining, it does not inhibit the number of cells blocking in G1 of the cell cycle or the changes in nuclear DAG or PIC activity. The possible involvement of this nuclear inositide cycle during progression through the cell cycle is discussed.

Specific and high-affinity binding of inositol phosphates to an isolated pleckstrin homology domain

Pleckstrin homology (PH) domains are found in many signaling molecules and are thought to be involved in specific intermolecular interactions. Their binding to several proteins and to membranes containing 1-alpha-phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been reported. A region that includes the PH domain has also been implicated in binding of phospholipase C-delta 1 (PLC-delta 1) to both PtdIns(4,5)P2 and D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] [Cifuentes, M. E., Delaney, T. & Rebecchi, M. J. (1994) J. Biol. Chem. 269, 1945-1948]. We report herein that the isolated PH domain from PLC-delta 1 binds to both PtdIns(4,5)P2 and Ins(1,4,5)P3 with high affinity and shows the same binding specificity seen by others with whole PLC-delta 1. Thus the PH domain is functionally and structurally modular. These results demonstrate stereo-specific high-affinity binding by an isolated PH domain and further support a functional role for PH domains in the regulation of PLC isoforms. Other PH domains did not bind strongly to the compounds tested, suggesting that inositol phosphates and phospholipids are not likely physiological ligands for all PH domains. Nonetheless, since all PH-domain-containing proteins are associated with membrane surfaces, several PH domains bind to specific sites on membranes, and PH domains appear to be electrostatically polarized, a possible general role for PH domains in membrane association is suggested.

Activation of phosphatidylinositol bisphosphate signal transduction pathway after experimental brain injury: a lipid study

Regional levels of phosphatidylinositol 4,5-bisphosphate (PIP2), diacylglycerol (DG) and free fatty acids (FFA), involved in the signal transduction pathway of the excitatory neurotransmitter system, were measured after lateral fluid percussion (FP) brain injury in rats. At 5 min postinjury, tissue PIP2 concentrations were significantly reduced in the cortices and hippocampi of both ipsilateral and contralateral hemispheres. Only levels of stearic and arachidonic acids were substantially decreased in PIP2 in these regions of the brain. At the same time after injury, both DG and FFA were significantly increased in the cortices and hippocampi of both hemispheres. As was true for PIP2, only levels of stearic and arachidonic acids markedly changed in both DG and FFA in these regions of the brain. At 20 min postinjury, a significant decrease in PIP2 concentration and significant increases in levels of DG and FFA were observed only in the injured left cortex. In addition to the increases in stearic and arachidonic acids in FFA, increased amounts of palmitic and oleic acids were also found in the injured left cortex at 20 min after injury. These results suggest that the PIP2 signal transduction pathway is activated in the cortex and hippocampus at the onset of lateral FP brain injury and that the enhanced phospholipase C-catalyzed phosphodiestric breakdown of PIP2 is a major mechanism of liberation of FFA in these sites immediately after such injury.

Differential targeting of protein kinase C and CaM kinase II signalings to vimentin

Hydrolysis of inositol phospholipids by receptor stimulation activates two separate signaling pathways, one leading to the activation of protein kinase C (C kinase) via formation of diacylglycerol. The other is the inositol trisphosphate (IP3)/Ca2+ pathway and a major downstream kinase which is activated is Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). To examine signaling pathways of C kinase and CaM kinase II to the cytoskeletal protein vimentin, we prepared monoclonal antibodies YT33 and MO82 which recognize the phosphorylation state of vimentin by C kinase and by CaM kinase II, respectively. Ectopic expression of constitutively active C kinase or CaM kinase II in primary cultured astrocytes by microinjection of the corresponding expression vectors induced phosphorylation of vimentin at each specific phosphorylation site, followed by reorganization of vimentin filament networks. In contrast, simultaneous activation of C kinase and CaM kinase II by inositol phospholipid hydrolysis with receptor stimulation led to an exclusive phosphorylation of vimentin at the CaM kinase II site, not at the site of C kinase. These results indicate that the intracellular targeting of C kinase and CaM kinase II signalings to vimentin is regulated separately, under physiological conditions.

Light adaptation of bovine retinas in situ stimulates phosphatidylinositol synthesis in rod outer segments in vitro

Light-stimulated phosphoinositide turnover has been reported in both vertebrate retina and isolated rod outer segments (ROS). In the current investigation, we examined the incorporation of [3H]-inositol in vitro in bovine ROS isolated from dark adapted (DROS) or bleached (BROS) retinas. Incorporation of [3H]-inositol into phosphoinositides in BROS was 3-5 fold higher than in DROS. The majority (approximately 90%) of [3H]-inositol was found in phosphatidylinositol (PI), whereas phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) accounted for 7-8% of the label. The enhanced labelling of PI was only observed when bovine retinas were light-adapted prior to ROS preparation, suggesting the requirement for an intact photoreceptor for the observed effect. Our data strongly suggest that bleaching of bovine retina in situ stimulates PI synthesis in isolated ROS in vitro.

Early signalling mechanism in colonic epithelial cell response to gastrin

The hormone gastrin exerts a growth-promoting effect on gastrointestinal cells. The molecular mechanisms by which colonic epithelial cells respond to gastrin are still poorly understood. In this study, we demonstrate a novel feature of the action of gastrin on normal colonic cells, namely the rapid phosphorylation on tyrosine of phospholipase C gamma 1 (PLC gamma 1). Tyrosine phosphorylation of PLC gamma 1, elicited by gastrin, was transient, concentration-dependent, and was abrogated by pretreating the colonic cells with the gastrin-receptor antagonist proglumide, the tyrosine kinase inhibitor genistein, and by removal of the tyrosine phosphatase inhibitor orthovanadate from the isolation buffer. Tyrosine phosphorylation of PLC gamma 1 correlated with the time- and concentration-dependent decrease in the mass of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) and the increase in the epithelial concentration of inositol 1,4,5-trisphosphate (IP3). Likewise, the stimulated increase in IP3 was also prevented by proglumide and genistein. Gastrin induced a definite but transient increase in the intracellular concentration of free Ca2+ [Ca2+]i, and increased membrane-translocation of immunoreactive alpha- and beta-protein kinase C. The data thus indicate that gastrin elicits at least one signalling cascade, through rapid tyrosine phosphorylation of PLC gamma 1, leading to the activation of a PIP2-specific PLC pathway.

Decreased m3-muscarinic and alpha 1-adrenergic receptor stimulation of PIP2 hydrolysis in parotid gland membranes from aged rats: defect in activation of G alpha q/11

m3-Muscarinic cholinergic receptor (m3-AChR) and alpha 1-adrenergic receptor (alpha 1-AR) stimulation of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis (by a PIP2-specific phospholipase C, PLC) in rat parotid gland membranes is mediated via activation of alpha subunits of the Gq/11 family of G-proteins. This study examines m3-AChR and alpha 1-AR stimulation of PIP2 hydrolysis in membranes isolated from parotid glands of old (24 months) and young (3 months) rats (old and young rat membranes). Old rat membranes exhibited reduced stimulation of PIP2 hydrolysis in response to the addition of guanosine-5'-O-(3-thiotrisphosphate) (GTP gamma S) alone or GTP gamma S plus either carbachol (m3-AChR agonist) or epinephrine (alpha 1-AR agonist). This reduction in receptor-stimulated PIP2 hydrolysis was not due to a decrease in PLC activity per se since cholate-solubilized PLC activity was similar in old and young rat membranes. Additionally, these membranes exhibited comparable, immunologically detectable, levels of PLC beta 3, G alpha q/11, and G beta. In the presence of 10 microM AlCl3 and 10 mM NaF, stimulation of PIP2 hydrolysis in both old and young rat membranes was similar. Preincubation of membranes from old rats with GTP gamma S induced a time-dependent increase in the rate of PIP2 hydrolysis and, with 20 min preincubation, the rates of hydrolysis in old and young rat membranes were not statistically different. In aggregate, these data indicate that there is a defect in the activation of G alpha q/11 in parotid gland membranes from old rats.

Activation of PRK1 by phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. A comparison with protein kinase C isotypes

As potential targets for polyphosphoinositides, activation of protein kinase C (PKC) isotypes (beta 1, epsilon, zeta, nu) and a member of the PKC-related kinase (PRK) family, PRK1, has been compared in vitro. PRK1 is shown to be activated by both phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2) as well as phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3) either as pure sonicated lipids or in detergent mixed micelles. When presented as sonicated lipids, PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were equipotent in activating PRK1, and, furthermore, sonicated phosphatidylinositol (PtdIns) and phosphatidylserine (PtdSer) were equally effective. In detergent mixed micelles, PtdIns-4,5-P2 and PtdIns-3,4,5-P3 also showed a similar potency, but PtdIns and PtdSer were 10-fold less effective in this assay. Similarly, PKC-beta 1, -epsilon, and -nu were all activated by PtdIns-4,5-P2 and PtdIns-3,4,5-P3 in detergent mixed micelles. The activation constants for PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were essentially the same for all the kinases tested, implying no specificity in this in vitro analysis. Consistent with this conclusion, the effects of PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were found to be inhibited at 10 mM Mg2+ and mimicked by high concentrations of inositol hexaphosphate and inositol hexasulfate. The similar responses of these two classes of lipid-activated protein kinase to these phosphoinositides are discussed in light of their potential roles as second messengers.

Characterization and partial purification of phospholipase D from human placenta

We report the existence in the human placenta of a phosphatidylcholine-hydrolyzing phospholipase D (PLD) activity, which has been characterized and partially purified. Triton X-100 effectively solubilized PLD from the particulate fraction of human placenta in a dose-dependent manner. However, Triton X-100 caused decreasing enzyme activities. Maximum transphosphatidylation was obtained with 2% ethanol. The enzyme was found to have a pH optimum of 7.0-7.5 and an apparent Km of 33 mol% (or 0.8 mM). Ca2+ and Mg2+ was not required for the enzyme activity. Addition of phosphatidyl-4,5-bisphosphate, but not phosphatidylethanolamine, to the substrate mixture gave rise to a pronounced dose-dependent increase in PLD activity (EC50 = 0.3 mol%), suggesting a regulatory role of this phospholipid in PLD action. The enzyme was inhibited by sodium oleate when partly or fully substituting for octylglucoside in the substrate mixture. The PLD activity was enriched 15-fold by solubilization and purification on a DEAE-Sepharose column. N-Ethylmaleimide (10 mM) markedly inhibited the purified enzyme, indicating the presence of free thiol groups on PLD. Sphingosine (20 microM) and (+/-) propranolol (53 microM) had no direct effect on PLD activity. The present results form the basis for further purification of a PLD from human tissue.

Localization of a binding site for phosphatidylinositol 4,5-bisphosphate on human profilin

Profilin is a small 12-15-kDa actin-binding protein, which in eukaryotic organisms is ubiquitous and necessary for normal cell growth and function. Although profilin's interactions with its three known ligands (actin monomers, phosphatidylinositol 4,5-bisphosphate (PIP2), and poly-L-proline (PLP)) have been well characterized in vitro, its precise role in cells remains largely unknown. By binding to clusters of PIP2, profilin is able to inhibit the hydrolysis of PIP2 by phospholipase C gamma 1 (PLC gamma 1). This ability is the result of profilin's affinity for PIP2, but the specific residues of profilin's amino acid sequence involved in the binding of PIP2 are not known. Using site-directed mutagenesis, we sought to localize regions of profilin important for this interaction by generating the following mutants of human profilin (named according to the wild-type amino acid altered, its position, and the amino acid substituted in its place): Y6F, D8A, L10R, K25Q, K53I, R74L, R88L, R88L/K90E, H119D, G121D, and K125Q. With the exception of L10R, all of the mutants were successfully expressed in Escherichia coli and purified by affinity chromatography on PLP-Sepharose. Only Y6F and K25Q demonstrated moderately less stringent binding to PLP, indicating that most of the mutations did not induce marked alterations of profilin's structure. When tested for their relative abilities to inhibit the hydrolysis of PIP2 by PLC gamma 1, most of the mutants were indistinguishable from wild-type profilin. Exceptions included D8A, which demonstrated increased inhibition of PLC gamma 1, and R88L, which demonstrated decreased inhibition of PLC gamma 1. To assess the importance of the region surrounding residue 88 of human profilin, three synthetic decapeptides selected to correspond to non-overlapping stretches of the human profilin sequence were tested for their abilities to inhibit PLC gamma 1. We found that only te decapeptide that matched the peptide stretch centered around residue 88 was able to inhibit PLC gamma 1 activity substantially and was able to do so at nearly wild-type profilin levels. Taken together with the finding that mutating residue 88 resulted in decreased inhibition of PLC gamma 1 activity, these data provide strong evidence that this region of human profilin represents an important binding site for PIP2.

The complex of phosphatidylinositol 4,5-bisphosphate and calcium ions is not responsible for Ca2+-induced loss of phospholipid asymmetry in the human erythrocyte: a study in Scott syndrome, a disorder of calcium-induced phospholipid scrambling

Elevation of cytoplasmic Ca2+ levels in human erythrocytes induces a progressive loss of membrane phospholipid asymmetry, a process that is impaired in erythrocytes from a patient with Scott syndrome. We show here that porcine erythrocytes are similarly incapable of Ca2+-induced redistribution of membrane phospholipids. Because a complex of phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ has been proposed as the mediator of enhanced transbilayer movement of lipids (J Biol Chem 269:6347,1994), these cell systems offer a unique opportunity for testing this mechanism. Analysis of both total PIP2 content and the metabolic-resistant pool of PIP2 that remains after incubation with Ca2+ ionophore showed no appreciable differences between normal and Scott erythrocytes. Moreover, porcine erythrocytes were found to have slightly higher levels of both total and metabolic-resistant PIP2 in comparison with normal human erythrocytes. Although loading of normal erythrocytes with exogenously added PIP2 gave rise to a Ca2+-induced increase in prothrombinase activity and apparent transbilayer movement of nitrobenzoxadiazolyl (NBD)-phospholipids, these PIP2-loaded cells were also found to undergo progressive Ca2+-dependent cell lysis, which seriously hampers interpretation of these data. Moreover, loading Scott cells with PIP2 did not abolish their impaired lipid scrambling, even in the presence of a Ca2+-ionophore. Finally, artificial lipid vesicles containing no PIP2 or 1 mole percent of PIP2 were indistinguishable with respect to transbilayer movement of NBD-phosphatidylcholine in the presence of Ca2+. Our findings suggest that Ca2+-induced redistribution of membrane phospholipids cannot simply be attributed to the steady-state concentration of PIP2, and imply that such lipid movement is regulated by other cellular processes.

Intracellular signalling. New directions for phosphatidylinositol transfer

Newly revealed properties of phosphatidylinositol transfer protein help to explain the cellular targeting of lipids involved in signal transduction, and indicate that inositol lipids play a part in directing membrane traffic.

Histamine-evoked chromaffin cell scinderin redistribution, F-actin disassembly, and secretion: in the absence of cortical F-actin disassembly, an increase in intracellular Ca2+ fails to trigger exocytosis

Histamine is a known chromaffin cell secretagogue that induces Ca(2+) -dependent release of catecholamines. However, conflicting evidence exists as to the source of Ca2+ utilized in histamine-evoked secretion. Here we report that histamine-H1 receptor activation induces redistribution of scinderin, a Ca(2+)-dependent F-actin severing protein, cortical F-actin disassembly, and catecholamine release. Histamine evoked similar patterns of distribution of scinderin and filamentous actin. The rapid responses to histamine occurred in the absence of extracellular Ca2+ and were triggered by release of Ca2+ from intracellular stores. The trigger for the release of Ca2+ was inositol 1,4,5-trisphosphate because U-73122, a phospholipase C inhibitor, but not its inactive isomer (U-73343), inhibited the increases in IP3 and intracellular Ca2+ levels, scinderin redistribution, cortical F-actin disassembly, and catecholamine release in response to histamine. Thapsigargin, an agent known to mobilize intracellular Ca2+, blocked the rise in intracellular Ca2+ concentration, scinderin redistribution, F-actin disassembly, and catecholamine secretion in response to histamine. Calphostin C and chelerythrine, two inhibitors of protein kinase C, blocked all responses to histamine with the exception of the release of Ca2+ from intracellular stores. This suggests that protein kinase C is involved in histamine-induced responses. The results also show that in the absence of F-actin disassembly, rises in intracellular Ca2+ concentration are not by themselves capable of triggering catecholamine release.

Distinct effects of alpha-SNAP, 14-3-3 proteins, and calmodulin on priming and triggering of regulated exocytosis

We have used stage-specific assays for MgATP-dependent priming and for Ca(2+)-activated triggering in the absence of free MgATP to examine the effects of alpha-SNAP, 14-3-3 proteins and calmodulin on regulated exocytosis in permeabilized adrenal chromaffin cells. All three proteins lead to a Ca(2+)-dependent increase in catecholamine secretion. Both alpha-SNAP and 14-3-3 proteins stimulated in a priming but not in a triggering assay. In contrast, calmodulin was stimulatory in triggering but not priming. The effects of alpha-SNAP and 14-3-3 proteins were likely to be due to distinct mechanisms of action since they differed in Ca(2+)-dependency, time course and extent of stimulation and their effects were additive. alpha-SNAP and 14-3-3 proteins did not appear to exert their priming action through changes in synthesis of phosphatidylinositol (4,5) bisphosphate. The data show that these three proteins have distinct stage-specific actions on exocytosis and indicate that alpha-SNAP acts in an early MgATP-requiring stage and not in the late Ca(2+)-triggered steps immediately prior to membrane fusion as previously suggested.

Amyloid precursor protein processing is stimulated by metabotropic glutamate receptors

Stimulation of muscarinic m1 or m3 receptors can, by generating diacylglycerol and activating protein kinase C, accelerate the breakdown of the amyloid precursor protein (APP) to form soluble, nonamyloidogenic derivatives (APPs), as previously shown. This relationship has been demonstrated in human glioma and neuroblastoma cells, as well as in transfected human embryonic kidney 293 cells and PC-12 cells. We now provide evidence that stimulation of metabotropic glutamate receptors (mGluRs), which also are coupled to phosphatidylinositol 4,5-bisphosphate hydrolysis, similarly accelerates processing of APP into nonamyloidogenic APPs. This process is demonstrated both in hippocampal neurons derived from fetal rats and in human embryonic kidney 293 cells transfected with cDNA expression constructs encoding the mGluR 1 alpha subtype. In hippocampal neurons, both an mGluR antagonist, L-(+)-2-amino-3-phosphonopropionic acid, and an inhibitor of protein kinase C, GF 109203X, blocked the APPs release evoked by glutamate receptor stimulation. Ionotropic glutamate agonists, N-methyl-D-aspartate or S(-)-5-fluorowillardiine, failed to affect APPs release. These data show that selective mGluR agonists that initiate signal-transduction events can regulate APP processing in bona fide primary neurons and transfected cells. As glutamatergic neurons in the cortex and hippocampus are damaged in Alzheimer disease, amyloid production in these regions may be enhanced by deficits in glutamatergic neurotransmission.

Structural characterization of the interaction between a pleckstrin homology domain and phosphatidylinositol 4,5-bisphosphate

The pleckstrin homology (PH) domain is a protein module of approximately 100 amino acids that is found in several proteins involved in signal transduction [for a recent review, see Gibson et al. (1994) Trends Biochem. Sci. 19, 349-353]. Although the specific function of the PH domain has not yet been elucidated, many of the proteins which contain this domain associate with phospholipid membranes, and PH domains have been shown to bind to phosphatidylinositol 4,5-bisphosphate (PIP2) [Harlan et al. (1994) Nature 371, 168-170] and the beta gamma subunits of G-proteins [Touhara et al. (1994) J. Biol. Chem. 269, 10217-10220]. We have postulated that pleckstrin homology domains may be important for the translocation of proteins to the membrane by an interaction with the negatively charged head group of phospholipids. Here we show the importance of three conserved lysine residues for binding to PIP2 by site-directed mutagenesis. These results should aid future site-directed mutagenesis studies in probing the function of PIP2-PH domain interactions in the various proteins containing this module. In addition, we examine the specificity of this binding and illustrate the importance of charge--charge interactions in PIP2-PH domain complex formation from binding experiments involving PIP2 analogs.

Arachidonic acid inhibits hCG-stimulated progesterone production by corpora lutea of primates: potential mechanism of action

Arachidonic acid (AA) is a precursor of metabolites known to affect the corpus luteum (CL) in many species, including primates. We have shown that some of these products (prostaglandins F2 alpha and E2) inhibit pro-gesterone (P4) production and activate the phosphatidylinositol (PI) pathway in CL of rhesus monkeys. A direct role of AA in luteal function has also been suggested. The current experiments were designed to investigate the effect of AA on P4 synthesis and to examine the ability of AA to activate the PI pathway in CL of rhesus monkeys. Basal and hCG-stimulated P4 production by luteal cells collected during the midluteal phase was measured after treatment with AA (1, 5, and 10 microM) or linoleic acid (1, 5, and 10 microM). Dispersed cells (50,000/tube) were incubated at 37 degrees C for 2 h. AA elicited a dose-dependent decrease in hCG-stimulated, but not in basal, P4 production. hCG-stimulated P4 production was reduced (P < 0.01) at AA doses of 5 microM (12.1 +/- 1.5 ng/mL) and 10 microM (8.6 +/- 1.8 mg/mL) to hCG alone (18 +/- 1.6 ng/mL). There was no significant effect of 1 microM AA (15.2 +/- 1.6). Response to linoleic acid was dissimilar and was not dose-dependent. Viability of cells was not affected by any treatment. Indomethacin, a prostaglandin synthesis inhibitor, and nordihydroguaiaretic acid, an inhibitor of lipoxygenase, did not interfere with the inhibitory effect of AA. Activation of the PI pathway was assessed by monitoring the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) to inositol phosphates and by monitoring increases in intracellular free calcium concentrations ([Ca2+]i) in individual cells. Moreover, the ability of AA to activate protein kinase C (PKC) in luteal cells was measured using a [3H]phorbol dibutyrate (PDBu) binding assay. AA did not alter PIP2 hydrolysis or [Ca2+]i, however, AA (10 microM) increased specific binding of [3H]PDBu to luteal cells (P < 0.05). We conclude that AA inhibits hCG-stimulated P4 production by primate luteal cells. AA exerts this action without being converted to prostaglandins or leukotrienes. This inhibition may be mediated through the activation of PKC. These results suggest a possible role for AA in the regulation of luteal function in primates, and that PKC-activation by AA may promote its effects.

A phosphatidylinositol 4,5-bisphosphate-sensitive casein kinase I alpha associates with synaptic vesicles and phosphorylates a subset of vesicle proteins

In interphase cells, alpha-casein kinase I (alpha-CKI) is found associated with cytosolic vesicular structures, the centrosome, and within the nucleus. To identify the specific vesicular structures with which alpha-CKI is associated, established cell lines and primary rat neurons were immunofluorescently labeled with an antibody raised to alpha-CKI. In nonneuronal cells, alpha-CKI colocalizes with vesicular structures which align with microtubules and are partially coincident with both Golgi and endoplasmic reticulum markers. In neurons, alpha-CKI colocalizes with synaptic vesicle markers. When synaptic vesicles were purified from rat brain, they were highly enriched in a CKI, based on activity and immunoreactivity. The synaptic vesicle-associated CKI is an extrinsic kinase and was eluted from synaptic vesicles and purified. This purified CKI has properties most similar to alpha-CKI. When the activities of casein kinase I or II were specifically inhibited on isolated synaptic vesicles, CKI was shown to phosphorylate a specific subset of vesicle proteins, one of which was identified as the synaptic vesicle-specific protein SV2. As with alpha-CKI, the synaptic vesicle CKI is inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2). However, synthesis of PIP2 was detected only in plasma membrane-containing fractions. Therefore, PIP2 may spatially regulate CKI. Since PIP2 synthesis is required for secretion, this inhibition of CKI may be important for the regulation of secretion.

Interferon beta mediated intracellular signalling traffic in human lymphocytes

The early molecular mechanisms activated by the treatment of human lymphocytes with human interferon beta have been studied. These identify an early increase with respect to control, in diacylglycerol (DG) levels as response to interferon treatment. Such a DG production was derived from the rapid and sequential activation of phosphoinositide specific phospholipase C and phospholipase D pathway. This suggests that a synergistic involvement of phosphatidylinositol-bis-phosphate (PIP2) hydrolysis and phosphatidylcholine (PC) breakdown provide early molecular events upon the interaction between interferon beta and its cell surface receptors. This finally leads to the slowing down of cell growth.

Mutational analysis of the pleckstrin homology domain of the beta-adrenergic receptor kinase. Differential effects on G beta gamma and phosphatidylinositol 4,5-bisphosphate binding

The beta gamma subunits of heterotrimeric G proteins (G beta gamma) play a variety of roles in cellular signaling, one of which is membrane targeting of the beta-adrenergic receptor kinase (beta ARK). This is accomplished via a physical interaction of G beta gamma and a domain within the carboxyl terminus of beta ARK which overlaps with a pleckstrin homology (PH) domain. The PH domain of beta ARK not only binds G beta gamma but also interacts with phosphatidylinositol 4,5-bisphosphate (PIP2). Based on previous mapping of the G beta gamma binding region of beta ARK, and conserved residues within the PH domain, we have constructed a series of mutants in the carboxyl terminus of beta ARK in order to determine important residues involved in G beta gamma and PIP2 binding. To examine the effects of mutations on G beta gamma binding, we employed three different methodologies: direct G beta gamma binding to GST fusion proteins; the ability of GST fusion proteins to inhibit G beta gamma-mediated beta ARK translocation to rhodopsin-enriched rod outer segments; and the ability of mutant peptides expressed in cells to inhibit G beta gamma-mediated inositol phosphate accumulation. Direct PIP2 binding was also assessed on mutant GST fusion proteins. Ala residue insertion following Trp643 completely abolished the ability of beta ARK to bind G beta gamma, suggesting that a proper alpha-helical conformation is necessary for the G beta gamma.beta ARK interaction. In contrast, this insertional mutation had no effect on PIP2 binding. Both G beta gamma binding and PIP2 binding were abolished following Ala replacement of Trp643, suggesting that this conserved residue within the last subdomain of the PH domain is crucial for both interactions. Other mutations also produced differential effects on the physical interactions of the beta ARK carboxyl terminus with G beta gamma and PIP2. These results suggest that the last PH subdomain and its neighboring sequences within the carboxyl terminus of beta ARK, including Trp643, Leu647, and residues Lys663-Arg669, are critical for G beta gamma binding while Trp643 and residues Asp635-Glu639 are important for the PH domain to form the correct structure for binding to PIP2.

Phosphatidylinositol transfer protein dictates the rate of inositol trisphosphate production by promoting the synthesis of PIP2

BACKGROUND

Phosphatidylinositol transfer protein (PI-TP), which has the ability to transfer phosphatidylinositol (PI) from one membrane compartment to another, is required in the inositol lipid signalling pathway through phospholipase C-beta (PLC-beta) that is regulated by GTP-binding protein(s) in response to extracellular signals. Here, we test the hypothesis that the principal role of PI-TP is to couple sites of lipid hydrolysis to sites of synthesis, and so to replenish depleted substrate for PLC-beta.

RESULTS

We have designed an experimental protocol that takes advantage of the different rates of release of endogenous PI-TP and PLC-beta from HL60 cells permeabilized with streptolysin O. We have examined the kinetics of stimulated inositol lipid hydrolysis in cells depleted of PI-TP, but not of endogenous PLC-beta, in the presence and absence of exogenous PI-TP. Linear time-courses were observed in the absence of any added protein, and the rate was accelerated by PI-TP using either guanosine 5'[gamma-thio]-triphosphate (GTP gamma S) or the receptor-directed agonist fMetLeuPhe as activators. In addition, depletion from the cells of both PI-TP and PLC-beta isoforms by extended permeabilization (40 minutes) allowed us to control the levels of PLC-beta present in the cells. Once again, PI-TP increased the rates of reactions. To identify whether the role of PI-TP was to make available the substrate phosphatidylinositol bisphosphate (PIP2) for the PLC, we examined the synthesis of PIP2 in cells depleted of PI-TP. We found that PI-TP was essential for the synthesis of PIP2.

CONCLUSIONS

The predicted function of PI-TP in inositol lipid signalling is the provision of substrate for PLC-beta from intracellular sites where PI is synthesized. We propose that PI-TP is in fact a co-factor in inositol lipid signalling and acts by interacting with the inositol lipid kinases. We hypothesize that the preferred substrate for PLC-beta is not the lipid that is resident in the membrane but that provided through PI-TP.

T cell receptor-mediated Ca2+ signaling: release and influx are independent events linked to different Ca2+ entry pathways in the plasma membrane

In this study, we showed that cross-linking CD3 molecules on the T cell surface resulted in Ca2+ release from the intracellular stores followed by a sustained Ca2+ influx. Inhibition of release with TMB-8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, stimulated Ca2+ entry into the cells by a mechanism other than emptying Ca2+ stores. In addition, Ca2+ entry into the Ca(2+)-depleted cells was stimulated by low basal level of cytosolic Ca2+, not by the emptying of intracellular Ca2+ stores. Both the Ca2+ release and influx were dependent on high and low concentrations of extracellular Ca2+. At low concentrations, Mn2+ entered the cell through the Ca2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca2+ release was inhibited by low concentrations of Ni2+, La3+, and EGTA, while Ca2+ influx was inhibited by high concentrations. Thus, in T cells Ca2+ influx occurs independently of IP3-dependent Ca2+ release. However, some other PIP2 hydrolysis-dependent event was involved in prolonged activation of Ca2+ influx. Extracellular Ca2+ influenced Ca2+ release and influx through the action of two plasma membrane Ca2+ entry pathways with different pharmacological and biochemical properties.

Phosphatidylinositol 4,5-bisphosphate hydrolysis accompanies T cell receptor-induced apoptosis of murine thymocytes within the thymus

Regulation of the development of thymocytes into mature T cells within the thymus is now known to involve antigen-induced deletion, by apoptosis, of potentially autoreactive thymocytes, and it can be mimicked either by stimulating the T cell receptor (TcR) complex by monoclonal antibody (mAb) or by ionophore-induced elevation of cytosolic [Ca2+]. To identify signaling pathways employed by the TcR complex of immature thymocytes, we examined the effects of anti-CD3 and anti-TcR beta constant (c) region mAb, staphylococcal enterotoxin B (SEB) and pharmacological agents on the generation of inositol phosphates through hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] both in cultured fetal mouse thymic lobes and in the CD4+CD8+ immature thymocyte cell line, TM10G. Stimulation of the TcR complex with anti-CD3 mAb provoked an accumulation of inositol phosphates diagnostic of the occurrence of receptor-stimulated phosphoinositidase C (PLC) activation. Anti-TcRC beta mAb and SEB provoked smaller but similar responses. The PLC activation evoked by anti-CD3 mAb was suppressed by inhibitors of receptor tyrosine kinases and was unmodified by protein kinase C activation or elevation of cytosolic [Ca2+]. It thus appears that apoptosis triggered by TcR stimulation is associated with PLC activation by a receptor-regulated tyrosine kinase. Treatment of thymic lobes or TM10G cells with fluoroaluminate provoked apoptosis of a wider range of thymocyte subtypes and such stimulation also provoked an accumulation of inositol phosphates. The responses to fluoroaluminate were not prevented by inhibitors of tyrosine kinases, suggesting that unidentified GTP-binding proteins which couple to PLC activation may also be capable of initiating apoptosis by a route independent of the TcR. These results, when considered alongside previous studies of mature T cells, indicate that stimulation of immature thymocytes or of mature T cells through their TcR complex activates the PLC-catalyzed PtdIns(4,5)P2 hydrolysis signaling pathway, and thus that this signaling pathway may be implicated both in provoking apoptosis in immature T cells and in initiating proliferation in mature T cells.

Differential effects of spermine on phosphatidylinositol 3-kinase and phosphatidylinositol phosphate 5-kinase

The metabolism of phosphoinositides plays an important role in the signal transduction pathways. We report here that naturally occurring polyamines affect the activities of phosphatidylinositol (PI) 3-kinase and PI 4-phosphate (PIP) 5-kinase differently. While polyamines inhibited the PI 3-kinase activity, they stimulated the activity of PIP 5-kinase in the order of spermine > spermidine > putrescine. Spermine inhibited the PI 3-kinase activity in a concentration-dependent manner with an IC50 of 100 microM. On the other hand, spermine (5 mM) stimulated the activity of PIP 5-kinase 2-3 fold. Kinetic studies of spermine-mediated inhibition of PI 3-kinase revealed that it was noncompetitive with respect to ATP. The effect of Mg2+ and PIP2 concentration on kinase activity was sigmoidal, with spermine inhibiting PI 3-kinase activity at all PIP2 concentrations. While 1 mM calcium stimulated PI 3-kinase activity at submaximal concentrations of Mg2+ (1.25 mM), inhibition was observed at optimal concentration of Mg2+ (2 mM). We propose that spermine may modulate the cellular signal by virtue of its differential effects on phosphoinositide kinases.

Effect of carbachol on phospholipase C-mediated phosphatidylinositol 4,5-bisphosphate hydrolysis, and its modulation by isoproterenol in rabbit corneal epithelial cells

The effects of carbachol (CCh) on phospholipase C(PLC)-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and its modulation by isoproterenol were investigated in SV40-adenovirus transformed rabbit corneal epithelial cells (RCEC). When examined under light microscope, these cells exhibited a cobblestone-like appearance typical of the corneal epithelial cells grown in primary culture. Addition of CCh (0.1 mM) for 30 min to RCEC, prelabeled with 32Pi, decreased the radioactivity in phosphatidylinositol 4-phosphate and PIP2 by 15 and 27%, respectively, and concomitantly increased the radioactivity in phosphatidylinositol and phosphatidic acid by 14 and 38%, respectively. When the concentration of CCh was increased to 1 mM, the changes in radioactivity were even more pronounced. Addition of CCh (0.1 mM) to the cells, prelabeled with myo[3H]inositol, increased the accumulation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3) by 115%, indicating stimulation of PLC-mediated PIP2 hydrolysis. Similar increases were also observed in [3H]InsP1 and [3H]InsP2. The effects of CCh on inositol phosphate accumulation were time- and dose-dependent, and were inhibited by atropine (10 microM), suggesting that the observed effects of CCh were mediated by activation of muscarinic cholinergic receptors. The effects of CCh were antagonized more potently by 4-diphenylacetoxy N-methyl-piperidine than by pirenzepine, indicating that the muscarinic receptors involved in PLC activation are probably of M3 type. By Western immunoblotting analysis with various anti-PLC antibodies, the RCEC were shown to contain PLC gamma 1 and PLC delta 1 in the soluble fraction and PLC beta 1 in the microsomal fraction. Addition of isoproterenol to RCEC, increased cAMP both in a time- and dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)