The influence of deletion mutations on phospholipase C-gamma 1 activity

Dynamics of allosteric regulation of the phospholipase C-γ isozymes upon recruitment to membranes

Numerous receptor tyrosine kinases and immune receptors activate phospholipase C-γ (PLC-γ) isozymes at membranes to control diverse cellular processes including phagocytosis, migration, proliferation, and differentiation. The molecular details of this process are not well understood. Using hydrogen-deuterium exchange mass spectrometry, we show that PLC-γ1 is relatively inert to lipid vesicles that contain its substrate, phosphatidylinositol 4,5-bisphosphate (PIP₂), unless first bound to the kinase domain of the fibroblast growth factor receptor (FGFR1). Exchange occurs throughout PLC-γ1 and is exaggerated in PLC-γ1 containing an oncogenic substitution (D1165H) that allosterically activates the lipase. These data support a model whereby initial complex formation shifts the conformational equilibrium of PLC-γ1 to favor activation. This receptor-induced priming of PLC-γ1 also explains the capacity of a kinase-inactive fragment of FGFR1 to modestly enhance the lipase activity of PLC-γ1 operating on lipid vesicles but not a soluble analog of PIP₂ and highlights potential cooperativity between receptor engagement and membrane proximity. Priming is expected to be greatly enhanced for receptors embedded in membranes and nearly universal for the myriad of receptors and co-receptors that bind the PLC-γ isozymes.

Mammalian phospholipase C

Phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). DAG and IP(3) each control diverse cellular processes and are also substrates for synthesis of other important signaling molecules. PLC is thus central to many important interlocking regulatory networks. Mammals express six families of PLCs, each with both unique and overlapping controls over expression and subcellular distribution. Each PLC also responds acutely to its own spectrum of activators that includes heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca(2+), and phospholipids. Mammalian PLCs are autoinhibited by a region in the catalytic TIM barrel domain that is the target of much of their acute regulation. In combination, the PLCs act as a signaling nexus that integrates numerous signaling inputs, critically governs PIP(2) levels, and regulates production of important second messengers to determine cell behavior over the millisecond to hour timescale.

bFGF and PDGF-BB have a synergistic effect on the proliferation, migration and VEGF release of endothelial progenitor cells

We have investigated the synergistic effects of bFGF (basic fibroblast growth factor) and PDGF-BB (platelet-derived growth factor-BB) on the proliferation, migration and VEGF (vascular endothelial growth factor) release of EPCs (endothelial progenitor cells). The proliferation of EPCs was assayed by MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium]. EPCs migration was detected using the Transwell system. Real-time PCR was used to assess the transcription of PDGFRβ mRNA. PLC-γ (phospholipase C gamma) expression and VEGF release were analysed by Western blot and ELISA. bFGF and PDGF-BB could, respectively, or synergistically, promote the proliferation and migration of EPCs, and these effects of bFGF and PDGF-BB were implemented by enhancing PDGFRβ mRNA, PLC-γ and VEGF expression, while inhibitor of PDGF receptor kinase (AG1296) and the selective PLC inhibitor (U73122) could block these effects of bFGF and PDGF-BB. In the meantime, we proved that the amplification by bFGF and PDGF-BB-stimulated PDGFRβ mRNA, PLC-γ and VEGF expression was abrogated by anti-bFGF antibody, AG1296 and U73122. These results strongly suggest that the proliferation and migration of EPCs may depend on bFGF and/or PDGF-BB by PDGFRβ/PLC-γ signalling pathway, and bFGF and/or PDGF-BB stimulate VEGF release at a point downstream from PDGFRβ/PLC-γ in EPCs.

Phospholipase C-η1 is activated by intracellular Ca(2+) mobilization and enhances GPCRs/PLC/Ca(2+) signaling

Phospholipase C-η1 (PLC-η1) is the most recently identified PLC isotype and is primarily expressed in nerve tissue. However, its functional role is unclear. In the present study, we report for the first time that PLC-η1 acts as a signal amplifier in G protein-coupled receptor (GPCR)-mediated PLC and Ca(2+) signaling. Short-hairpin RNA (shRNA)-mediated knockdown of endogenous PLC-η1 reduced lysophosphatidic acid (LPA)-, bradykinin (BK)-, and PACAP-induced PLC activity in mouse neuroblastoma Neuro2A (N2A) cells, indicating that PLC-η1 participates in GPCR-mediated PLC activation. Interestingly, ionomycin-induced PLC activity was significantly decreased by PLC-η1, but not PLC-η2, knockdown. In addition, we found that intracellular Ca(2+) source is enough for PLC-η1 activation. Furthermore, the IP(3) receptor inhibitor, 2-APB, inhibited LPA-induced PLC activity in control N2A cells, whereas this effect was not observed in PLC-η1 knockdown N2A cells, suggesting a pivotal role of intracellular Ca(2+) mobilization in PLC-η1 activation. Finally, we found that LPA-induced ERK1/2 phosphorylation and expression of the downstream target gene, krox-24, were significantly decreased by PLC-η1 knockdown, and these knockdown effects were abolished by 2-APB. Taken together, our results strongly suggest that PLC-η1 is activated via intracellular Ca(2+) mobilization from the ER, and therefore amplifies GPCR-mediated signaling.

Mechanism of phosphorylation-induced activation of phospholipase C-gamma isozymes

The lipase activity of most phospholipases C (PLCs) is basally repressed by a highly degenerate and mostly disordered X/Y linker inserted within the catalytic domain. Release of this auto-inhibition is driven by electrostatic repulsion between the plasma membrane and the electronegative X/Y linker. In contrast, PLC-γ isozymes (PLC-γ1 and -γ2) are structurally distinct from other PLCs because multiple domains are present in their X/Y linker. Moreover, although many tyrosine kinases directly phosphorylate PLC-γ isozymes to enhance their lipase activity, the underlying molecular mechanism of this activation remains unclear. Here we define the mechanism for the unique regulation of PLC-γ isozymes by their X/Y linker. Specifically, we identify the C-terminal SH2 domain within the X/Y linker as the critical determinant for auto-inhibition. Tyrosine phosphorylation of the X/Y linker mediates high affinity intramolecular interaction with the C-terminal SH2 domain that is coupled to a large conformational rearrangement and release of auto-inhibition. Consequently, PLC-γ isozymes link phosphorylation to phospholipase activation by elaborating upon primordial regulatory mechanisms found in other PLCs.

rac regulates its effector phospholipase Cgamma2 through interaction with a split pleckstrin homology domain

Several isoforms of phospholipase C (PLC) are regulated through interactions with Ras superfamily GTPases, including Rac proteins. Interestingly, of two closely related PLCgamma isoforms, only PLCgamma(2) has previously been shown to be activated by Rac. Here, we explore the molecular basis of this interaction as well as the structural properties of PLCgamma(2) required for activation. Based on reconstitution experiments with isolated PLCgamma variants and Rac2, we show that an unusual pleckstrin homology (PH) domain, designated as the split PH domain (spPH), is both necessary and sufficient to effect activation of PLCgamma(2) by Rac2. We also demonstrate that Rac2 directly binds to PLCgamma(2) as well as to the isolated spPH of this isoform. Furthermore, through the use of NMR spectroscopy and mutational analysis, we determine the structure of spPH, define the structural features of spPH required for Rac interaction, and identify critical amino acid residues at the interaction interface. We further discuss parallels and differences between PLCgamma(1) and PLCgamma(2) and the implications of our findings for their respective signaling roles.

General and versatile autoinhibition of PLC isozymes

Phospholipase C (PLC) isozymes are directly activated by heterotrimeric G proteins and Ras-like GTPases to hydrolyze phosphatidylinositol 4,5-bisphosphate into the second messengers diacylglycerol and inositol 1,4,5-trisphosphate. Although PLCs play central roles in myriad signaling cascades, the molecular details of their activation remain poorly understood. As described here, the crystal structure of PLC-beta2 illustrates occlusion of the active site by a loop separating the two halves of the catalytic TIM barrel. Removal of this insertion constitutively activates PLC-beta2 without ablating its capacity to be further stimulated by classical G protein modulators. Similar regulation occurs in other PLC members, and a general mechanism of interfacial activation at membranes is presented that provides a unifying framework for PLC activation by diverse stimuli.

Src homology domains in phospholipase C-gamma1 mediate its anti-apoptotic action through regulating the enzymatic activity

Phospholipase-gamma1 (PLC-gamma1) prevents programmed cell death, for which the enzymatic activity has been implicated. However, the biological function of Src homology (SH) domains of PLC-gamma1 in promoting cell survival remains elusive. Here, we showed that deletion of the N-SH2 domain or both N-SH2 and C-SH2 domains, but not the SH3 domain, abolished the anti-apoptotic activity of PLC-gamma1. Surprisingly, removal of the whole SH domain inhibited apoptosis. The lipase-inactive PLC-gamma1 mutant (LIM) failed to suppress apoptosis. Moreover, the phospholipase activity in SH3- or whole SH domain-deleted cells was comparable to that of wild-type cells. By contrast, the enzymatic activity was substantially ablated in SH2 domain-deleted or LIM cells. A pharmacological inhibitor of PLC-gamma1 robustly diminished the anti-apoptotic action in wild-type, SH3- or whole SH domain-deleted cells, whereas pretreatment of SH2 domain-deleted or LIM cells with agents activating PKC and calcium mobilization markedly promoted cell survival. These results indicate that SH domains in PLC-gamma1 might mediate its anti-apoptotic action by regulating the enzymatic activity.

Activation of phospholipase Cgamma2 by tyrosine phosphorylation

Phospholipase Cgamma2 (PLCgamma2) has been implicated in collagen-induced signal transduction in platelets and antigen-dependent signaling in B-lymphocytes. It has been suggested that tyrosine kinases activate PLCgamma2. We expressed the full-length cDNA for human PLCgamma2 in bacteria and purified the recombinant enzyme. The recombinant enzyme was Ca(2+)-dependent with optimal activity in the range of 1 to 10 microM Ca(2+). In vitro phosphorylation experiments with recombinant PLCgamma2 and recombinant Lck, Fyn, and Lyn tyrosine kinases showed that phosphorylation of PLCgamma2 led to activation of the recombinant enzyme. Using site-directed mutagenesis, we investigated the role of specific tyrosine residues in activation of PLCgamma2. A mutant form of PLCgamma2, in which all three tyrosines at positions 743, 753, and 759 in the SH2-SH3 linker region were replaced by phenylalanines, exhibited decreased Lck-induced phosphorylation and completely abolished the Lck-dependent activation of PLCgamma2. Individual mutations of these tyrosine residues demonstrated that tyrosines 753 and 759, but not 743, were responsible for Lck-induced activation of PLCgamma2. To confirm these results, we procured a phosphospecific antibody to a peptide containing phosphorylated tyrosines corresponding to residues 753 and 759. This antibody recognized phosphorylated wild-type PLCgamma2 on Western blots but did not interact with unphosphorylated PLCgamma2 or with PLCgamma2 containing mutated tyrosine residues at 753 and 759. Using this antibody, we showed in intact platelets that collagen, a PLCgamma2-dependent agonist, induces phosphorylation of PLCgamma2 at Y753 and Y759. These studies demonstrate the importance of these two tyrosine residues in regulating the activity of PLCgamma2.

PLC-gamma1 enzyme activity is required for insulin-induced DNA synthesis

Previously, we had shown that inhibition of PLC activity impaired the ability of insulin to activate ERK in 3T3-L1 adipocytes. In this study, we confirmed that the insulin receptor and PLC-gamma1 are physically associated in hIRcB fibroblasts, insulin stimulates PLC-gamma1 enzyme activity, and inhibition of PLC activity impairs activation of ERK. We subsequently investigated whether PLC-gamma1 is required for insulin-stimulated mitogenesis. First, inhibition of PLC activity using U73122 impairs the ability of insulin to stimulate DNA synthesis. Second, disruption of the interaction of the insulin receptor with PLC-gamma1 by microinjection of SH2 domains derived from PLC-gamma1 or Grb2 but not Shc similarly blocks insulin-induced DNA synthesis. Third, microinjection of neutralizing antibodies to PLC-gamma1 blocks DNA synthesis, but nonneutralizing antibodies do not. The blockade in all three cases is rescued by synthetic diacylglycerols but not by inositol-1,4,5-trisphosphate, indicating a requirement for PLC enzyme activity. These experimental data point to a requirement for PLC-gamma1 in insulin-stimulated mitogenesis in hIRcB cells.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.

Identification of a phospholipase C-gamma1 (PLC-gamma1) SH3 domain-binding site in SLP-76 required for T-cell receptor-mediated activation of PLC-gamma1 and NFAT

SLP-76 is an adapter protein required for T-cell receptor (TCR) signaling. In particular, TCR-induced tyrosine phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1), and the resultant TCR-inducible gene expression, depend on SLP-76. Nonetheless, the mechanisms by which SLP-76 mediates PLC-gamma1 activation are not well understood. We now demonstrate that SLP-76 directly interacts with the Src homology 3 (SH3) domain of PLC-gamma1. Structure-function analysis of SLP-76 revealed that each of the previously defined protein-protein interaction domains can be individually deleted without completely disrupting SLP-76 function. Additional deletion mutations revealed a new, 67-amino-acid functional domain within the proline-rich region of SLP-76, which we have termed the P-1 domain. The P-1 domain mediates a constitutive interaction of SLP-76 with the SH3 domain of PLC-gamma1 and is required for TCR-mediated activation of Erk, PLC-gamma1, and NFAT (nuclear factor of activated T cells). The adjacent Gads-binding domain of SLP-76, also within the proline-rich region, mediates inducible recruitment of SLP-76 to a PLC-gamma1-containing complex via the recruitment of both PLC-gamma1 and Gads to another cell-type-specific adapter, LAT. Thus, TCR-induced activation of PLC-gamma1 entails the binding of PLC-gamma1 to both LAT and SLP-76, a finding that may underlie the requirement for both LAT and SLP-76 to mediate the optimal activation of PLC-gamma1.

Pleiotropic contributions of phospholipase C-gamma1 (PLC-gamma1) to T-cell antigen receptor-mediated signaling: reconstitution studies of a PLC-gamma1-deficient Jurkat T-cell line

Phospholipase C-gamma1 (PLC-gamma1) plays a crucial role in the coupling of T-cell antigen receptor (TCR) ligation to interleukin-2 (IL-2) gene expression in activated T lymphocytes. In this study, we have isolated and characterized two novel, PLC-gamma1-deficient sublines derived from the Jurkat T-leukemic cell line. The P98 subline displays a >90% reduction in PLC-gamma1 expression, while the J.gamma1 subline contains no detectable PLC-gamma1 protein. The lack of PLC-gamma1 expression in J.gamma1 cells caused profound defects in TCR-dependent Ca(2+) mobilization and NFAT activation. In contrast, both of these responses occurred at normal levels in PLC-gamma1-deficient P98 cells. Unexpectedly, the P98 cells displayed significant and selective defects in the activation of both the composite CD28 response element (RE/AP) and the full-length IL-2 promoter following costimulation with anti-TCR antibodies and phorbol ester. These transcriptional defects were reversed by transfection of P98 cells with a wild-type PLC-gamma1 expression vector but not by expression of mutated PLC-gamma1 constructs that lacked a functional, carboxyl-terminal SH2 [SH2(C)] domain or the major Tyr(783) phosphorylation site. On the other hand, the amino-terminal SH2 [SH2(N)] domain was not essential for reconstitution of RE/AP- or IL-2 promoter-dependent transcription but was required for the association of PLC-gamma1 with LAT, as well as the tyrosine phosphorylation of PLC-gamma1 itself, in activated P98 cells. These studies demonstrate that the PLC-gamma1 SH2(N) and SH2(C) domains play functionally distinct roles during TCR-mediated signaling and identify a non-Ca(2+)-related signaling function linked to the SH2(C) domain, which couples TCR plus phorbol ester-CD28 costimulation to the activation of the IL-2 promoter in T lymphocytes.

Phosphatidylinositol-specific phospholipase C-gamma1 undergoes pH-induced activation and conformational change

Phospholipase C-gamma1 displayed sigmoidal kinetics with a S(0.5) value of 0.17 mole fraction PIP(2) when assayed at pH 6.8 using detergent:lipid mixed micelles. The pH optimum for hydrolysis of phosphatidylinositol 4,5-bisphosphate by phospholipase C-gamma1 was dependent on the mole fraction of substrate in the micelle. The pH optimum was 5.5 when the enzyme was assayed below the S(0.5). The pH optima shifted to a pH range of 6.0-6.3 when the enzyme was assayed above the S(0.5). The kinetic parameters for phospholipase C-gamma1 assayed at various pH values from pH 7.0 to 5.0 yielded similar n values (n=4), but the constant, K', decreased from 1x10(-2) (mole fraction)(2) at pH 7.0 to 1x10(-5) (mole fraction)(2) at pH 5.0. Maximum enzyme specificity occurred at pH values below pH 6.0 as determined by the plot of logk(cat)/S(0.5) versus pH. Intrinsic fluorescence spectroscopy revealed that at a pH value above 7.0 or below 6.3, tryptophan quenching occurred. Fluorescence quenching experiments performed with acrylamide determined phospholipase C-gamma1 incubated at pH 5.0 had a larger collisional quenching constant than enzyme incubated at pH 7.0. Lowering the pH to 5.0 apparently resulted in interior tryptophans becoming more solvent accessible. These data suggest that pH may activate phospholipase C-gamma1 by disrupting ionizable groups leading to a conformational change.

Effect of limited proteolysis on phospholipase C-gamma1 kinetics

Phospholipase C-gamma1 is a tightly regulated, multidomain protein that generates the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. Kinetic analysis reveals that phospholipase C-gamma1 displays apparent allosteric behavior. A previous study determined that proteolytic cleavage of the SH domain region of phospholipase C-gamma1 yields an activated form of the enzyme (A. W. Fernald, G. A. Jones, and G. Carpenter Biochem. J. 302, 508, 1994). In this study, we show that activation of phospholipase C-gamma1 by proteolysis decreases both the cooperativity and the half-maximal value of the enzyme for substrate. Kinetic analysis revealed that the mole fraction of phosphatidylinositol 4,5-bisphosphate (PIP(2)) that resulted in half-maximal PIP(2) hydrolysis (S(0.5)) was lower for proteolyzed than uncleaved phospholipase C-gamma1 (0.08 mole fraction vs 0.18 mole fraction of PIP(2)). The cooperativity index was lower for proteolyzed than full-length phospholipase C-gamma1 (n = 2.5 vs n = 4). Kinetic analysis also revealed that the estimated dissociation constant was lower for phospholipase C-gamma1 that had been subjected to proteolysis (0.1 mM vs 1.0 mM PIP(2) for cleaved vs uncleaved phospholipase C-gamma1, respectively). It was previously hypothesized that activation of phospholipase C-gamma1 requires a conformational change that results in increased accessibility of substrate to the active site and that the SH domain of the enzyme is involved in the activation event. These experiments support the hypothesis that a portion of the protein covers the active site, allosterically inhibiting the enzyme, and that the removal of this "lid" domain activates the enzyme.

Mammalian phosphoinositide-specific phospholipase C

Mammalian phosphoinositide-specific phospholipases C (PI-PLCs) are involved in most receptor-mediated signal transduction pathways. The mammalian isozymes employ a modular arrangement of domains to achieve a regulated production of two key second messengers. The roles of the PH, EF hand, C2, SH2 and SH3 modules in regulation of these enzymes and in interactions with membranes and other proteins is becoming apparent from recent structural and functional studies.