Novel bimodal effects of the G-protein tissue transglutaminase on adrenoreceptor signalling

Tissue transglutaminase (tTG) is a novel G-protein that previous studies showed can couple ligand-bound activated alpha(1B) adrenoreceptors to phospholipase C-delta, resulting in phosphoinositide (PI) hydrolysis. In human neuroblastoma SH-SY5Y cells we found that although endogenous tTG can facilitate alpha(1B) adrenoreceptor-stimulated PI hydrolysis, its contribution is minor compared with the classical heterotrimeric G-protein G(q/11). Further, we show that the alpha(1B) adrenoreceptor recruits tTG to the membrane and that this recruitment is enhanced by agonist occupancy of the receptor. In addition, the effects of tTG on signalling are bimodal. At low expression levels, tTG enhanced alpha(1B) adrenoreceptor-stimulated PI hydrolysis, whereas at higher expression levels tTG attenuated significantly this response. These findings are the first to demonstrate that a protein can both facilitate and attenuate receptor-stimulated PI hydrolysis.

Interactions of G(h)/transglutaminase with phospholipase Cdelta1 and with GTP

The inositol phosphate hydrolyzing activity of human phospholipase Cdelta1 (PLCdelta1) is markedly inhibited when the enzyme is coexpressed with the human heart G(h)/transglutaminase (TG) in human embryonic kidney cells. Because the cotransfection does not affect the amount of PLCdelta1 in the cells, the depression of phospholipase activity probably is a result of a direct interaction between the two proteins. An ELISA procedure was employed to document the associations of purified TG preparations from a variety of tissues (human red cells, rabbit lens, guinea pig liver) with PLCdelta1. Nucleotides (GTP > GDP > ATP > GMP = ADP, in order of decreasing efficiency) interfered with the formation of the PLCdelta1:TG complex. A conformational change in the TG partner, occurring with nucleotide binding, is thought to be responsible for dissociating the two proteins. The structural rearrangement produces a remarkable shift in the anodic mobility of TG in electrophoresis: TG(slow) + GTP -->/<-- [TG:GTP](fast). Altogether, our findings indicate that GTP controls PLCdelta1 activity by releasing this protein from an inhibitory association with G(h)/transglutaminase.

Coordinated regulation of radioadaptive response by protein kinase C and p38 mitogen-activated protein kinase

Eukaryotic cells are known to have an inducible or adaptive response that enhances radioresistance after a low priming dose of radiation. This radioadaptive response seems to present a novel cellular defense mechanism. However, its molecular processing and signaling mechanisms are largely unknown. Here, we studied the role of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) in the expression of radioadaptive response in cultured mouse cells. Protein immunoblot analysis using isoform-specific antibodies showed an immediate activation of PKC-alpha upon X-irradiation as indicated by a translocation from cytosol to membrane. A low priming dose caused a prolonged translocation, while a nonadaptive high dose dramatically downregulated the total PKC level. Low-dose X-rays also activated the p38 MAPK. The activation of p38 MAPK and resistance to chromosome aberration formation were blocked by SB203580, an inhibitor of p38 MAPK, and Calphostin C, an inhibitor of PKC. Furthermore, it was demonstrated that p38 MAPK was physically associated with delta1 isoform of phospholipase C (PLC-delta1), which hydrolyzed phosphatidylinositol bisphosphate into diacylglycerol, an activator of PKC, and that SB203580 also blocked the activation of PKC-alpha. These results indicate the presence of a novel mechanism for coordinated regulation of adaptive response to low-dose X-rays by a nexus of PKC-alpha/p38 MAPK/PLC-delta1 circuitry feedback signaling pathway with its breakage operated by downregulation of labile PKC-alpha at high doses or excess stimuli.

Phospholipase C-delta1 is activated by capacitative calcium entry that follows phospholipase C-beta activation upon bradykinin stimulation

To characterize the regulatory mechanism of phospholipase C-delta1 (PLC-delta1) in the bradykinin (BK) receptor-mediated signaling pathway, we used a clone of PC12 cells, which stably overexpress PLC-delta1 (PC12-D1). Stimulation with BK induced a significantly higher Ca(2+) elevation and inositol 1,4,5-trisphosphate (IP(3)) production with a much lower half-maximal effective concentration (EC(50)) of BK in PC12-D1 cells than in wild type (PC12-W) or vector-transfected (PC12-V) cells. However, BK-induced intracellular Ca(2+) release and IP(3) generation was similar between PC12-V and PC12-D1 cells in the absence of extracellular Ca(2+), suggesting that the availability of extracellular Ca(2+) is essential to the activation of PLC-delta1. When PC12-D1 cells were treated with agents that induce Ca(2+) influx, more IP(3) was produced, suggesting that the Ca(2+) entry induces IP(3) production in PC12-D1 cells. Furthermore, the additional IP(3) production after BK-induced capacitative calcium entry was detected in PC12-D1 cells, suggesting that PLC-delta1 is mainly activated by capacitative calcium entry. When cells were stimulated with BK in the presence of extracellular Ca(2+), [(3)H]norepinephrine secretion was much greater from PC12-D1 cells than from PC12-V cells. Our results suggest that PLC-delta1 is activated by capacitative calcium entry following the activation of PLC-beta, additively inducing IP(3) production and Ca(2+) rise in BK-stimulated PC12 cells.

Molecular cloning and expression analysis of a mouse phospholipase C-delta1

We describe here the molecular cloning and expression analysis of mouse PLC-delta1 (mPLC-delta1), a key enzyme in cell signal transduction. A mouse brain cDNA library was screened in order to isolate the mPLC-delta1 cDNA. The mPLC-delta1 cDNA was 2660 bp in length. The predicted open reading frame encodes a protein of 756 amino acids with an estimated molecular mass of 85 kDa. The deduced amino acid sequence exhibits 96.9% and 92.7% identity with the sequence of rat and human PLC-delta1, respectively. The mPLC-delta1 mRNA was highly expressed in brain, heart, lung, and testis. We found that transcripts of mPLC-delta1 are present in almost all regions of mouse brain examined, implying that the enzyme may play a role in some fundamental cellular process in brain. In male reproductive tract, mPLC-delta1 mRNA was widely expressed in the epididymis as well as in the testis. In situ hybridization studies indicate that distribution of mPLC-delta1 mRNA in mouse testis is discrete and unique. The expression of mPLC-delta1 mRNA was defined in the periphery of each seminiferous tubule, especially in spermatogonia, which might imply that mPLC-delta1 plays a role in proliferation of spermatogonia. To the best our knowledge, this is the first report to demonstrate the high expression of mPLC-delta1 mRNA in spermatogonia of testis. Taken together, these results suggest that mPLC-delta1 may carry out fundamental roles in almost all of mouse tissues, especially in brain and specific roles in testis.

Repeated administration of dexamethasone increases phosphoinositide-specific phospholipase C activity and mRNA and protein expression of the phospholipase C beta 1 isozyme in rat brain

Altered hypothalamic-pituitary-adrenal (HPA) function has been shown to be associated with changes in mood and behavior. The enzyme phosphoinositide-specific phospholipase C (PI-PLC), an important component of the PI signal transduction system, plays a major role in mediating various physiological functions. In the present study, we investigated the effects of a single dose and of repeated administration (0.5 or 1.0 mg/kg for 10 days) of dexamethasone (DEX), a synthetic glucocorticoid, on PI-PLC activity and on expression of PLC isozymes (beta1, delta1, and gamma1) in rat brain. Repeated administration of DEX (1.0 mg/kg) caused a significant increase in PI-PLC activity and in protein expression of the PLC beta1 isozyme in both membrane and cytosol fractions of cortex and hippocampus; however, the repeated administration of a smaller dose of DEX (0.5 mg/kg) caused these changes only in hippocampus but not in cortex. The increase in PLC beta1 protein was associated with an increase in its mRNA level, as measured by competitive RT-PCR. A single administration of DEX (0.5 or 1.0 mg/kg) to rats had no significant effects on PI-PLC activity or on the protein expression of PLC isozymes. These results suggest that DEX up-regulates PI-PLC in rat brain, which presumably is due to a selective increase in expression of the PLC beta1 isozyme, and that these changes in PI-PLC may be related to HPA axis-mediated changes in mood and behavior.

Role of lipid packing in the activity of phospholipase C-delta1 as determined by hydrostatic pressure measurements

Previous studies with phospholipid monolayers revealed a large decrease in the activity of phosphoinositide-specific phospholipase C-delta(1) (PLC-delta(1)) which catalyses the hydrolysis of PtdIns(4, 5)P(2) as lateral pressure is applied to the membrane. If stress on the membrane is the sole inhibitor of PLC-delta(1) activity, the enzyme must penetrate the membrane surface to engage its substrate. To test the effect on PLC-delta(1) activity of lipid packing in the absence of a directional stress, we examined the effects of increasing hydrostatic pressure on enzymic activity. We find that, in contrast with monolayer studies, increasing lipid packing by hydrostatic pressure does not affect membrane binding and increases enzymic activity by 90% in going from atmospheric pressure to 10(8) Pa (approx. 1000 atm). The increase in activity could be accounted for mainly by electrostriction of water around the multiply-charged product. Our results show that when there is no net stress on the monolayer, lipid packing does not alter PLC-delta(1) activity, possibly because penetration of the enzyme into the membrane surface is shallow. We suggest that, in biological membranes, the activity of this and possibly other interfacial proteins is independent of headgroup packing.

Morphological changes and detachment of adherent cells induced by p122, a GTPase-activating protein for Rho

We recently cloned a novel signaling molecule, p122, that shows a GTPase-activating activity specific for Rho and the ability to enhance the phosphatidylinositol 4,5-bisphosphate-hydrolyzing activity of phospholipase C delta1 in vitro. Here we analyzed the in vivo function of p122. Microinjection of the GTPase-activating domain of p122 suppressed the formation of stress fibers and focal adhesions induced by lysophosphatidic acid, suggesting a GTPase-activating activity for Rho as in in vitro. Transfection of p122 also induced the disassembly of stress fibers and the morphological rounding of various adherent cells. Analyses using deletion and point mutants demonstrated that the GTPase-activating domain of p122 is responsible for the morphological changes and detachment and that arginine residues at positions 668 and 710 and a lysine residue at position 706 in the GTPase-activating domain are essential. Using Fluo-3-based Ca2+ microscopy, we found that p122 evoked a rapid elevation of intracellular Ca2+ levels, suggesting that p122 stimulates the phosphatidylinositol 4, 5-bisphosphate-hydrolyzing activity of phospholipase C delta1. These results demonstrate that p122 synergistically functions as a GTPase-activating protein specific for Rho and an activator of phospholipase C delta1 in vivo and induces morphological changes and detachment through cytoskeletal reorganization.

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Spatiotemporal dynamics of inositol 1,4,5-trisphosphate that underlies complex Ca2+ mobilization patterns

Inositol 1,4,5-trisphosphate (IP3) is a second messenger that elicits complex spatiotemporal patterns of calcium ion (Ca2+) mobilization and has essential roles in the regulation of many cellular functions. In Madin-Darby canine kidney epithelial cells, green fluorescent protein-tagged pleckstrin homology domain translocated from the plasma membrane to the cytoplasm in response to increased concentration of IP3. The detection of translocation enabled monitoring of IP3 concentration changes within single cells and revealed spatiotemporal dynamics in the concentration of IP3 synchronous with Ca2+ oscillations and intracellular and intercellular IP3 waves that accompanied Ca2+ waves. Such changes in IP3 concentration may be fundamental to Ca2+ signaling.

Rac homologues and compartmentalized phosphatidylinositol 4, 5-bisphosphate act in a common pathway to regulate polar pollen tube growth

Pollen tube cells elongate based on actin- dependent targeted secretion at the tip. Rho family small GTPases have been implicated in the regulation of related processes in animal and yeast cells. We have functionally characterized Rac type Rho family proteins that are expressed in growing pollen tubes. Expression of dominant negative Rac inhibited pollen tube elongation, whereas expression of constitutive active Rac induced depolarized growth. Pollen tube Rac was found to accumulate at the tip plasma membrane and to physically associate with a phosphatidylinositol monophosphate kinase (PtdIns P-K) activity. Phosphatidylinositol 4, 5-bisphosphate (PtdIns 4, 5-P2), the product of PtdIns P-Ks, showed a similar intracellular localization as Rac. Expression of the pleckstrin homology (PH)-domain of phospholipase C (PLC)-delta1, which binds specifically to PtdIns 4, 5-P2, inhibited pollen tube elongation. These results indicate that Rac and PtdIns 4, 5-P2 act in a common pathway to control polar pollen tube growth and provide direct evidence for a function of PtdIns 4, 5-P2 compartmentalization in the regulation of this process.

Alpha 1B-adrenoceptor interacts with multiple sites of transglutaminase II: characteristics of the interaction in binding and activation

We previously reported that a novel GTP binding protein (G alpha h) is tissue type transglutaminase (TGII) and transmits the alpha 1B-adrenoceptor (AR) signal to phospholipase C (PLC) through its GTPase function. We have also shown that PLC-delta 1 is the effector in TGII-mediated signaling. In this study, interaction sites on TGII for the alpha 1B-AR were identified using a peptide approach and site-directed mutagenesis, including in vivo reconstitution of TGIIs with the alpha 1B-AR and PLC-delta 1. To identify the interaction sites, 11 synthetic peptides covering approximately 132 amino acid residues of the C-terminal domain of TGII were tested. The studies with the peptides revealed that three peptides, L547-I561, R564-D581, and Q633-E646, disrupted formation of an alpha 1-agonist-alpha 1B-AR-TGII complex and blocked alpha 1B-AR-mediated TGase inhibition in a dose-dependent manner, indicating that these peptide regions are involved in recognition and activation of TGII by the alpha 1B-AR. These three regions were further evaluated with full-length TGIIs by constructing and coexpressing each site-directed mutant with the alpha 1B-AR and PLC-delta 1 in COS-1 cells. Supporting the findings with these peptides, these TGII mutants lost 56-82% the receptor binding ability and reduced by 29-68% the level of alpha 1B-AR-mediated IP3 production via PLC-delta 1 as compared to those with wild-type TGII. The results also revealed that the regions of R564-D581 and Q633-E646 were the high-affinity binding sites of TGII for the receptor and critical for the activation of TGII by the receptor. Taken together, the studies demonstrate that multiple regions of TGII interact with the alpha 1B-AR and that the alpha 1B-AR stimulates PLC-delta 1 via TGII.

Identification of PLCgamma-dependent and -independent events during fertilization of sea urchin eggs

At fertilization, sea urchin eggs undergo a series of activation events, including a Ca2+ action potential, Ca2+ release from the endoplasmic reticulum, an increase in intracellular pH, sperm pronuclear formation, MAP kinase dephosphorylation, and DNA synthesis. To examine which of these events might be initiated by activation of phospholipase Cgamma (PLCgamma), which produces the second messengers inositol trisphosphate (IP3) and diacylglycerol, we used recombinant SH2 domains of PLCgamma as specific inhibitors. Sea urchin eggs were co-injected with a GST fusion protein composed of the two tandem SH2 domains of bovine PLCgamma and (1) Ca2+ green dextran to monitor intracellular free Ca2+, (2) BCECF dextran to monitor intracellular pH, (3) Oregon Green dUTP to monitor DNA synthesis, or (4) fluorescein 70-kDa dextran to monitor nuclear envelope formation. Microinjection of the tandem SH2 domains of PLCgamma produced a concentration-dependent inhibition of Ca2+ release and also inhibited cortical granule exocytosis, cytoplasmic alkalinization, MAP kinase dephosphorylation, DNA synthesis, and cleavage after fertilization. However, the Ca2+ action potential, sperm entry, and sperm pronuclear formation were not prevented by injection of the PLCgammaSH2 domain protein. Microinjection of a control protein, the tandem SH2 domains of the phosphatase SHP2, had no effect on Ca2+ release, cortical granule exocytosis, DNA synthesis, or cleavage. Specificity of the inhibitory action of the PLCgammaSH2 domains was further indicated by the finding that microinjection of PLCgammaSH2 domains that had been point mutated at a critical arginine did not inhibit Ca release at fertilization. Additionally, Ca2+ release in response to microinjection of IP3, cholera toxin, cADP ribose, or cGMP was not inhibited by the PLCgammaSH2 fusion protein. These results indicate that PLCgamma plays a key role in several fertilization events in sea urchin eggs, including Ca2+ release and DNA synthesis, but that the action potential, sperm entry, and male pronuclear formation can occur in the absence of PLCgamma activation or Ca2+ increase.

Differential association of the pleckstrin homology domains of phospholipases C-beta 1, C-beta 2, and C-delta 1 with lipid bilayers and the beta gamma subunits of heterotrimeric G proteins

Pleckstrin homology (PH) domains are recognized in more than 100 different proteins, including mammalian phosphoinositide-specific phospholipase C (PLC) isozymes (isotypes beta, gamma, and delta). These structural motifs are thought to function as tethering devices linking their host proteins to membranes containing phosphoinositides or beta gamma subunits of heterotrimeric GTP binding (G) proteins. Although the PH domains of PLC-delta and PLC-gamma have been studied, the comparable domains of the beta isotypes have not. Here, we have measured the affinities of the isolated PH domains of PLC-beta 1 and -beta 2 (PH-beta 1 and PH-beta 2, respectively) for lipid bilayers and G-beta gamma subunits. Like the intact enzymes, these PH domains bind to membrane surfaces composed of zwitterionic phosphatidylcholine with moderate affinity. Inclusion of the anionic lipid phosphatidylserine or phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and inclusion of G-beta gamma subunits had little affect on their membrane affinity. In contrast, binding of PLC-delta 1 or its PH domain was highly dependent on PI(4,5)P2. We also determined whether these domains laterally associate with G-beta gamma subunits bound to membrane surfaces using fluorescence resonance energy transfer. Affinities for G-beta gamma were in the following order: PH-beta 2 >/= PH-beta 1 > PH-delta 1; the affinities of the native enzyme were as follows: PLC-beta 2 >> PLC-delta 1 > PLC-beta 1. Thus, the PH domain of PLC-beta 1 interacts with G-beta gamma in isolation, but not in the context of the native enzyme. By contrast, docking of the PH domain of PLC-beta2 with G-beta gamma is comparable to that of the full-length protein and may play a key role in G-beta gamma recognition.

A novel phospholipase C delta4 (PLCdelta4) splice variant as a negative regulator of PLC

It has been reported that there are two alternatively spliced variants of phospholipase C-delta4 (PLCdelta4), termed ALT I and II, that contain an additional 32 and 14 amino acids in their respective sequences in the linker region between the catalytic X and Y domains (Lee, S. B., and Rhee, S. G. (1996) J. Biol. Chem. 271, 25-31). We report here the isolation and characterization of a novel alternative splicing isoform of PLCdelta4, termed ALT III, as a negative regulator of PLC. In ALT III, alternative splicing occurred in the catalytic X domain, i.e. 63 amino acids (residues 424-486) containing the C-terminal of the X domain and linker region were substituted for 32 amino acids corresponding to the insert sequence of ALT I. Although the expression level of ALT III was found to be much lower in most tissues and cells compared with that of PLCdelta4, it was significantly higher in some neural cells, such as NIE-115 cells and p19 cells differentiated to neural cells by retinoic acid. Interestingly, recombinant ALT III protein did not retain enzymatic activity, and the activity of PLCdelta4 overexpressed in COS7 cells was markedly decreased by the co-expression of ALT III but not by ALT I or II. Moreover, N-terminal pleckstrin homology domain (PH domain) of ALT III alone could inhibit the increase of inositol-1,4, 5-trisphosphate levels in PLCdelta4-overexpressing NIH3T3 cells, whereas a PH domain deletion mutant could not, indicating that the PH domain is necessary and sufficient for its inhibitory effect. The ALT III PH domain specifically bound to phosphatidylinositol (PtdIns)-4,5-P2 and PtdIns-3,4,5-P3 but not PtdIns, PtdIns-4-P, or inositol phosphates, and the mutant R36G, which retained only weak affinity for PtdIns-4,5-P2, could not inhibit the activity of PLCdelta4. These results indicate that PtdIns-4,5-P2 binding to PH domain is essential for the inhibitory effect of ALT III. ALT III also inhibited PLCdelta1 activity and partially suppressed PLCgamma1 activity, but not PLCbeta1 in vitro; it did inhibit all types of isozymes tested in vivo. Taken together, our results indicate that ALT III is a negative regulator of PLC that is most effective against the PLC delta-type isozymes, and its PH domain is essential for its function.

Real-time visualization of PH domain-dependent translocation of phospholipase C-delta1 in renal epithelial cells (MDCK): response to hypo-osmotic stress

Green fluorescent protein (GFP)-tagged phospholipase C (PLC)-delta1 and its mutants were expressed in Madin-Darby canine kidney (MDCK) cells. GFP-PLC-delta1 or the GFP-tagged pleckstrin homology (PH) domain of PLC-delta1 itself was found to be predominantly localized at the plasma membrane. The DeltaPH mutant or a site-directed mutant containing a PH domain which does not bind inositol 1,4, 5-trisphosphate and cannot hydrolyze phosphatidylinositol 4, 5-bisphosphate in vitro was seen only in the cytosol. In living MDCK cells hypo-osmotic stress caused a rapid dissociation of GFP-PLC-delta1 from the plasma membrane, which coincided with phosphoinositide breakdown. A PLC inhibitor, U73122, blocked this translocation, but depletion of extracellular Ca2+ had no effect. The translocation was reversed by replacement with an iso-osmotic buffer. Our results demonstrate that the PH domain plays a critical role in the membrane targeting of PLC-delta1 and that the intracellular distribution of the enzyme is regulated by osmotic stress-driven phosphoinositide turnover.

Enhanced phosphoinositide hydrolysis via overexpression of phospholipase C beta1 or delta1 inhibits stimulus-induced insulin release in insulinoma MIN6 cells

To study the effects of enhanced phosphoinositide hydrolysis on insulin secretion, phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) or PLCdelta1 was overexpressed in insulinoma MIN6 cells via adenoviral vectors. Inositol phosphate production stimulated by NaF (with AlCl3) in PLCbeta1-overexpressing cells and that stimulated by KCl or glucose in both PLCbeta1- and PLCdelta1-overexpressing cells were greater than that in control cells. In addition, reduced phosphatidylinositol-4,5-bisphosphate levels were observed in these cells stimulated by NaF or KCl. The greater phosphoinositide hydrolysis was accompanied by 25-45% inhibition of insulin secretion. These data suggest that excessive phosphoinositide hydrolysis inhibits secretagogue-induced insulin release in MIN6 cells.

Phosphatidylinositol-4,5-bisphosphate is required for endocytic coated vesicle formation

Receptor-mediated endocytosis via clathrin-coated vesicles has been extensively studied and, while many of the protein players have been identified, much remains unknown about the regulation of coat assembly and the mechanisms that drive vesicle formation [1]. Some components of the endocytic machinery interact with inositol polyphosphates and inositol lipids in vitro, implying a role for phosphatidylinositols in vivo [2] [3]. Specifically, the adaptor protein complex AP2 binds phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2), PtdIns(3)P, PtdIns(3,4,5)P3 and inositol phosphates. Phosphatidylinositol binding regulates AP2 self-assembly and the interactions of AP2 complexes with clathrin and with peptides containing endocytic motifs [4] [5]. The GTPase dynamin contains a pleckstrin homology (PH) domain that binds PtdIns(4,5)P2 and PtdIns(3,4,5)P3 to regulate GTPase activity in vitro [6] [7]. However, no direct evidence for the involvement of phosphatidylinositols in clathrin-mediated endocytosis exists to date. Using well-characterized PH domains as high affinity and high specificity probes in combination with a perforated cell assay that reconstitutes coated vesicle formation, we provide the first direct evidence that PtdIns(4,5)P2 is required for both early and late events in endocytic coated vesicle formation.

Specificity and promiscuity in phosphoinositide binding by pleckstrin homology domains

Pleckstrin homology (PH) domains are small protein modules involved in recruitment of signaling molecules to cellular membranes, in some cases by binding specific phosphoinositides. We describe use of a convenient "dot-blot" approach to screen 10 different PH domains for those that recognize particular phosphoinositides. Each PH domain bound phosphoinositides in the assay, but only two (from phospholipase C-delta1 and Grp1) showed clear specificity for a single species. Using soluble inositol phosphates, we show that the Grp1 PH domain (originally cloned on the basis of its phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) binding) binds specifically to D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) (the PtdIns(3,4,5)P3 headgroup) with KD = 27.3 nM, but binds D-myo-inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) or D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) over 80-fold more weakly. We show that this specificity allows localization of the Grp1 PH domain to the plasma membrane of mammalian cells only when phosphatidylinositol 3-kinase (PI 3-K) is activated. The presence of three adjacent equatorial phosphate groups was critical for inositol phosphate binding by the Grp1 PH domain. By contrast, another PH domain capable of PI 3-K-dependent membrane recruitment (encoded by EST684797) does not distinguish Ins(1,3,4)P3 from Ins(1,3,4,5)P3 (binding both with very high affinity), despite selecting strongly against Ins(1,4,5)P3. The remaining PH domains tested appear significantly less specific for particular phosphoinositides. Together with data presented in the literature, our results suggest that many PH domains bind similarly to multiple phosphoinositides (and in some cases phosphatidylserine), and are likely to be regulated in vivo by the most abundant species to which they bind. Thus, using the same simple approach to study several PH domains simultaneously, our studies suggest that highly specific phosphoinositide binding is a characteristic of relatively few cases.

Lipid binding ridge on loops 2 and 3 of the C2A domain of synaptotagmin I as revealed by NMR spectroscopy

The C2A domain of synaptotagmin I, which binds Ca2+ and anionic phospholipids, serves as a Ca2+ sensor during excitation-secretion coupling. We have used multidimensional NMR to locate the region of C2A from rat synaptotagmin I that interacts, in the presence of Ca2+, with phosphatidylserine. Untagged, recombinant C2A was double-labeled with 13C and 15N, and triple-resonance NMR data were collected from C2A samples containing either Ca2+ alone or Ca2+ plus 6:0 phosphatidylserine. Phospholipid binding led to changes in chemical shifts of backbone atoms in residues Arg233 and Phe234 of loop 3 (a loop that also binds Ca2+) and His198, Val205, and Phe206 of loop 2. These residues lie along a straight line on a surface ridge of the C2A domain. The only other residue that exhibited appreciable chemical shift changes upon adding lipid was His254; however, because His254 is located on the other side of the molecule from the phospholipid docking site defined by the other residues, its shifts may result from nonspecific interactions. The results show that the "docking ridge" responsible for Ca2+-dependent membrane association is localized on the opposite side of the C2A domain from the transmembrane and C2B domains of synaptotagmin.

Localization of phospholipase C delta3 in the cell and regulation of its activity by phospholipids and calcium

The localization of phospholipase C delta3 (PLC delta3) in the cell and its regulatory properties has been investigated. Western blotting showed that human platelet PLC delta3 is located in the membrane and cytosolic fraction. The enzyme amount in the cytosolic fraction was significantly lower than that in the membrane fraction. In rat liver, PLC delta3 was present in both the membrane and cytosolic fraction and was absent in nuclei. Examination of the effects of phospholipids on PLC delta3 revealed that this enzyme is inhibited by phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho). Similar inhibition was observed in the presence of sphingomyelin and phosphatidylserine (PtdSer). This is in contrast to PLC delta1, which is activated by PtdCho and PtdEtn. In a detergent assay, PLC delta1 is activated by spermine and sphingosine, whereas PLC delta3 was inhibited by both these compounds at concentrations that maximally stimulated PLC delta1. A deletion mutant of PLC delta3, lacking the entire pleckstrin homology (PH) domain (residues 1-137), was fully active in the detergent assay, and it was inhibited by spermine, sphingosine and phospholipids to the same extent as the native enzyme. PLC delta3 activation required calcium ions. The relationship between the Ca2+ concentration and enzymatic activity was almost identical for the deletion mutant and the native enzyme. However, in the liposome assay, PLC delta3 was less sensitive to Ca2+ stimulation. This is in contrast to PLC delta1, which is equally sensitive to Ca2+ stimulation in both the detergent and liposome assays. We conclude that Ca2+ is necessary to induce specific conformational changes of PLC delta3, which leads to a productive orientation of the catalytic domain relative to the membrane. The regulatory properties of PLC delta3 described in this report suggest that PLC delta3 has a relatively low activity in cellular conditions that fully activate PLC delta1.

Signaling events activated by angiotensin II receptors: what goes before and after the calcium signals

Angiotensin II (Ang II) receptors of the AT1 subtype are coupled to heterotrimeric G nucleotide-binding proteins, G(q/11), to activate phospholipase C-beta isoforms with production of inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol. The resultant release of intracellular Ca2+ and increased Ca2+ influx are major determinants of several acute cellular responses initiated by Ang II, including secretion of aldosterone from the adrenal cortex and smooth muscle contraction. However, cellular events related to more prolonged effects of Ang II, such as hypertrophic and hyperplastic responses, are triggered by intracellular signaling cascades that are less dependent on Ca2+ signals. The Ang II-induced activation of Raf-1 kinase, p42 MAP-kinase and c-fos expression in response to Ang II in adrenal glomerulosa cells does not require Ca2+ influx. Moreover, the dose-response relationships for Raf-1 activation, MAP-kinase activation and mitogenesis show significantly higher sensitivity to Ang II than the InsP3, Ca2+-release and aldosterone secretory responses. The sensitivities of both Raf-1 kinase and MAP-kinase stimulation by Ang II to the inhibitors of phosphoinositide kinases, wortmannin and LY 294002, suggest that inositol phospholipids may play a role in these activation events unrelated to their role in Ca2+ signaling. To investigate the changes of various inositides after stimulation at the single cell level, fluorescent probes were developed in which pleckstrin homology domains with distinct binding specificities to inositol phospholipids were fused to the green fluorescent protein and expressed in NIH 3T3 cells. The use of these probes revealed heterogeneity of the inositol lipid pools and their complex relationship to Ca2+ signals. The use of these tools will help to further clarify the complex role of these lipids in initiating Ca2+-dependent and -independent signaling responses.