EGF-dependent translocation of green fluorescent protein-tagged PLC-gamma1 to the plasma membrane and endosomes

Inhibitory Receptor Trap: A Platform for Discovery of Inhibitory Receptors That Utilize Inositol Lipid and Phosphotyrosine Phosphatase Effectors

Among the areas of most impactful recent progress in immunology is the discovery of inhibitory receptors and the subsequent translation of this knowledge to the clinic. Although the original and canonical member of this family is FcγRIIB, more recent studies defined PD1 as an inhibitory receptor that constrains T cell immunity to tumors. These studies led to development of “checkpoint blockade” immunotherapies (CBT) for cancers in which PD1 interactions with its ligand are blocked. Unfortunately, although very effective in some patients, only a small proportion respond to this therapy. This suggests that additional as yet undescribed inhibitory receptors exist, which could be exploited. Here, we describe a new platform, termed inhibitory receptor trap (IRT), for discovery of members of this family. The approach takes advantage of the fact that many of the known inhibitory receptors mediate signaling by phospho-immunoreceptor tyrosine-based inhibition motif (ITIM) mediated recruitment of Src Homology 2 (SH2) domain-containing phosphatases including the SH2 domain-containing inositol phosphatase SHIP1 encoded by the INPP5D gene and the SH2 domain-containing phosphotyrosine phosphatases SHP1 and SHP2 encoded by the PTPN6 and PTPN11 genes respectively. Here, we describe the IRT discovery platform in which the SH2 domains of inhibitory phosphatases are used for affinity-based isolation and subsequent identification of candidate effectors via immunoblotting and high sensitivity liquid chromatography–mass spectrometry. These receptors may represent alternative targets that can be exploited for improved CBT. Salient observations from these studies include the following: SH2 domains derived from the respective phosphatases bind distinct sets of candidates from different cell types. Thus, cells of different identity and different activation states express partially distinct repertoires of up and downstream phosphatase effectors. Phosphorylated PD1 binds not only SHP2 but also SHIP1, thus the latter may be important in its inhibitory function. B cell antigen receptor signaling leads predominantly to CD79 mono-phosphorylation as indicated by much greater binding to LynSH2 than Syk(SH2)₂. This balance of ITAM mono- versus bi-phosphorylation likely tunes signaling by varying activation of inhibitory (Lyn) and stimulatory (Syk) pathways.

The FcεRI Signaling Cascade and Integrin Trafficking Converge at Patterned Ligand Surfaces

We examined the spatial targeting of early and downstream signaling mediated by the IgE receptor (FcεRI) in RBL mast cells utilizing surface-patterned 2,4 dinitrophenyl (DNP) ligands. Micron-sized features of DNP are presented as densely immobilized conjugates of bovine serum albumin (DNP-BSA) or mobile in a supported lipid bilayer (DNP-SLB). Although soluble anti-DNP IgE binds uniformly across features for both pattern types, IgE bound to FcεRI on cells shows distinctive distributions: uniform for DNP-SLB and edge-concentrated for DNP-BSA. These distributions of IgE-FcεRI propagate to the spatial recruitment of early signaling proteins, including spleen tyrosine kinase (Syk), linker for activation of T cells (LAT), and activated phospholipase C gamma 1 (PLCγ1), which all localize with engaged receptors. We found stimulated polymerization of F-actin is not required for Syk recruitment but is progressively involved in the recruitment of LAT and PLCγ1. We further found β1- and β3-integrins colocalize with IgE-FcεRI at patterned ligand surfaces as cells spread. This recruitment corresponds to directed exocytosis of recycling endosomes (REs) containing these integrins and their fibronectin ligand. Together, our results show targeting of signaling components, including integrins, to regions of clustered IgE-FcεRI in processes that depend on stimulated actin polymerization and outward trafficking of REs.

Spatially defined EGF receptor activation reveals an F-actin-dependent phospho-Erk signaling complex

We investigated the association of signaling proteins with epidermal growth factor (EGF) receptors (EGFR) using biotinylated EGF bound to streptavidin that is covalently coupled in an ordered array of micron-sized features on silicon surfaces. Using NIH-3T3 cells stably expressing EGFR, we observe concentration of fluorescently labeled receptors and stimulated tyrosine phosphorylation that are spatially confined to the regions of immobilized EGF and quantified by cross-correlation analysis. We observe recruitment of phosphorylated paxillin to activated EGFR at these patterned features, as well as β1-containing integrins that preferentially localize to more peripheral EGF features, as quantified by radial fluorescence analysis. In addition, we detect recruitment of EGFP-Ras, MEK, and phosphorylated Erk to patterned EGF in a process that depends on F-actin and phosphoinositides. These studies reveal and quantify the coformation of multiprotein EGFR signaling complexes at the plasma membrane in response to micropatterned growth factors.

Spatio-temporally precise activation of engineered receptor tyrosine kinases by light

Receptor tyrosine kinases (RTKs) are a large family of cell surface receptors that sense growth factors and hormones and regulate a variety of cell behaviours in health and disease. Contactless activation of RTKs with spatial and temporal precision is currently not feasible. Here, we generated RTKs that are insensitive to endogenous ligands but can be selectively activated by low-intensity blue light. We screened light-oxygen-voltage (LOV)-sensing domains for their ability to activate RTKs by light-activated dimerization. Incorporation of LOV domains found in aureochrome photoreceptors of stramenopiles resulted in robust activation of the fibroblast growth factor receptor 1 (FGFR1), epidermal growth factor receptor (EGFR) and rearranged during transfection (RET). In human cancer and endothelial cells, light induced cellular signalling with spatial and temporal precision. Furthermore, light faithfully mimicked complex mitogenic and morphogenic cell behaviour induced by growth factors. RTKs under optical control (Opto-RTKs) provide a powerful optogenetic approach to actuate cellular signals and manipulate cell behaviour.

Migfilin protein promotes migration and invasion in human glioma through epidermal growth factor receptor-mediated phospholipase C-γ and STAT3 protein signaling pathways

Migfilin is critical for cell shape and motile regulation. However, its pathological role in glioma is unknown. Using an immunohistochemical staining assay, we demonstrate that there is a significant correlation between expression of Migfilin and pathological tumor grade in 217 clinical glioma samples. High Migfilin expression is associated with poor prognosis for patients with glioma. Investigation of the molecular mechanism shows that Migfilin promotes migration and invasion in glioma cells. Moreover, Migfilin positively modulates the expression and activity of epidermal growth factor receptor, and Migfilin-mediated migration and invasion depend on epidermal growth factor receptor-induced PLC-γ and STAT3-signaling pathways. Our results may provide significant clinical application, including use of Migfilin as a molecular marker in glioma for early diagnosis and as an indicator of prognosis.

EGFR-PLCgamma1 signaling mediates high glucose-induced PKCbeta1-Akt activation and collagen I upregulation in mesangial cells

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. We have recently shown that epidermal growth factor receptor (EGFR) transactivation mediates high glucose (HG)-induced collagen I upregulation through PI3K-PKCbeta1-Akt signaling in mesangial cells (MC). Phospholipase Cgamma1 (PLCgamma1) interacts with activated growth factor receptors and activates classic PKC isoforms. We thus studied its role in HG-induced collagen I upregulation in MC. Primary rat MC were treated with HG (30 mM) or mannitol as osmotic control. Protein kinase activation was assessed by Western blotting and collagen I upregulation by Northern blotting. Diabetes was induced in rats by streptozotocin. HG treatment for 1 h led to PLCgamma1 membrane translocation and Y783 phosphorylation, both indicative of its activation. Mannitol was without effect. PLCgamma1 Y783 phosphorylation was also seen in cortex and glomeruli of diabetic rats. HG induced a physical association between EGFR and PLCgamma1 as identified by coimmunoprecipitation. PLCgamma1 activation required EGFR kinase activity since it was prevented by the EGFR inhibitor AG1478 or overexpression of kinase-inactive EGFR (K721A). Phosphoinositide-3-OH kinase inhibition also prevented PLCgamma1 activation. HG-induced Akt S473 phosphorylation, effected by PKCbeta1, was inhibited by the PLCgamma inhibitor U73122. PLCgamma1 inhibition or downregulation by small interference RNA also prevented HG-induced collagen I upregulation. Our results indicate that EGFR-PLCgamma1 signaling mediates HG-induced PKCbeta1-Akt activation and subsequent collagen I upregulation in MC. Inhibition of EGFR or PLCgamma1 may provide attractive therapeutic targets for the treatment of diabetic nephropathy.

Ras/MAPK signaling from endomembranes

Signal transduction along the Ras/MAPK pathway has been generally thought to take place at the plasma membrane. It is now evident that the plasma membrane is not the only platform capable of Ras/MAPK signal induction. Fusion of Ras with green fluorescent protein and the development of genetically encoded fluorescent probes for Ras activation have revealed signaling events on a variety of intracellular membranes including endosomes, the Golgi apparatus and the endoplasmic reticulum. Thus, the Ras/MAPK pathway is spatially compartmentalized within cells and this may afford greater complexity of signal output.

Cytoskeletal elements and intracellular transport

Recent advances in the understanding of the functions of various components of the cytoskeleton indicate that, besides serving a structural role, the cytoskeletal elements may regulate the transport of several proteins in the cell. Studies reveal that there are co-operative interactions between the actin and microtubule cytoskeletons including functional overlap in the transport influenced by different motor families. Multiple motors are probably involved in the control of the dynamics of many proteins and intriguing hints about how these motors are co-ordinated are appearing. It has been shown that some of the intermediate elements also participate in selected intracellular transport mechanisms. In view of the author's preoccupation with the steroid receptor systems, special attention has been given to the role of the cytoskeletal elements, particularly actin, in the intracellular transport of steroid receptors and receptor-related proteins.

A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

Compartmentalized Ras/MAPK signaling

Signal transduction down the Ras/MAPK pathway, including that critical to T cell activation, proliferation, and differentiation, has been generally considered to occur at the plasma membrane. It is now clear that the plasma membrane does not represent the only platform for Ras/MAPK signaling. Moreover, the plasma membrane itself is no longer considered a uniform structure but rather a patchwork of microdomains that can compartmentalize signaling. Signaling on internal membranes was first recognized on endosomes. Genetically encoded fluorescent probes for signaling events such as GTP/GDP exchange on Ras have revealed signaling on a variety of intracellular membranes, including the Golgi apparatus. In fibroblasts, Ras is activated on the plasma membrane and Golgi with distinct kinetics. The pathway by which Golgi-associated Ras becomes activated involves PLCgamma and RasGRP1 and may also require retrograde trafficking of Ras from the plasma membrane to the Golgi as a consequence of depalmitoylation. Thus, the Ras/MAPK pathway represents a clear example of compartmentalized signaling.

Targeting of protein kinase C-epsilon during Fcgamma receptor-dependent phagocytosis requires the epsilonC1B domain and phospholipase C-gamma1

Protein kinase C-epsilon (PKC-epsilon) translocates to phagosomes and promotes uptake of IgG-opsonized targets. To identify the regions responsible for this concentration, green fluorescent protein (GFP)-protein kinase C-epsilon mutants were tracked during phagocytosis and in response to exogenous lipids. Deletion of the diacylglycerol (DAG)-binding epsilonC1 and epsilonC1B domains, or the epsilonC1B point mutant epsilonC259G, decreased accumulation at phagosomes and membrane translocation in response to exogenous DAG. Quantitation of GFP revealed that epsilonC259G, epsilonC1, and epsilonC1B accumulation at phagosomes was significantly less than that of intact PKC-epsilon. Also, the DAG antagonist 1-hexadecyl-2-acetyl glycerol (EI-150) blocked PKC-epsilon translocation. Thus, DAG binding to epsilonC1B is necessary for PKC-epsilon translocation. The role of phospholipase D (PLD), phosphatidylinositol-specific phospholipase C (PI-PLC)-gamma1, and PI-PLC-gamma2 in PKC-epsilon accumulation was assessed. Although GFP-PLD2 localized to phagosomes and enhanced phagocytosis, PLD inhibition did not alter target ingestion or PKC-epsilon localization. In contrast, the PI-PLC inhibitor U73122 decreased both phagocytosis and PKC-epsilon accumulation. Although expression of PI-PLC-gamma2 is higher than that of PI-PLC-gamma1, PI-PLC-gamma1 but not PI-PLC-gamma2 consistently concentrated at phagosomes. Macrophages from PI-PLC-gamma2-/- mice were similar to wild-type macrophages in their rate and extent of phagocytosis, their accumulation of PKC-epsilon at the phagosome, and their sensitivity to U73122. This implicates PI-PLC-gamma1 as the enzyme that supports PKC-epsilon localization and phagocytosis. That PI-PLC-gamma1 was transiently tyrosine phosphorylated in nascent phagosomes is consistent with this conclusion. Together, these results support a model in which PI-PLC-gamma1 provides DAG that binds to epsilonC1B, facilitating PKC-epsilon localization to phagosomes for efficient IgG-mediated phagocytosis.

Ion channels in mesangial cells: function, malfunction, or fiction

Ion channels in glomerular mesangial cells from humans, rats, and mice have been studied by electrophysiological, molecular, and gene-knockout methods. Two channels, a large, Ca(2+)-activated K(+) channel (BK) and a store-operated Ca(2+) channel (SOCC), can be defined with respect to molecular structure and function. Human BK, comprised of a pore-forming alpha-subunit and an accessory beta1-subunit, operate as Ca(2+)-sensing feedback modulators of contractile tone. SOCC have also been characterized in a mouse cell line; they are comprised of molecules belonging to the transient receptor potential subfamily.

An electrostatic engine model for autoinhibition and activation of the epidermal growth factor receptor (EGFR/ErbB) family

We propose a new mechanism to explain autoinhibition of the epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases based on a structural model that postulates both their juxtamembrane and protein tyrosine kinase domains bind electrostatically to acidic lipids in the plasma membrane, restricting access of the kinase domain to substrate tyrosines. Ligand-induced dimerization promotes partial trans autophosphorylation of ErbB1, leading to a rapid rise in intracellular [Ca²⁺] that can activate calmodulin. We postulate the Ca²⁺/calmodulin complex binds rapidly to residues 645–660 of the juxtamembrane domain, reversing its net charge from +8 to −8 and repelling it from the negatively charged inner leaflet of the membrane. The repulsion has two consequences: it releases electrostatically sequestered phosphatidylinositol 4,5-bisphosphate (PIP₂), and it disengages the kinase domain from the membrane, allowing it to become fully active and phosphorylate an adjacent ErbB molecule or other substrate. We tested various aspects of the model by measuring ErbB juxtamembrane peptide binding to phospholipid vesicles using both a centrifugation assay and fluorescence correlation spectroscopy; analyzing the kinetics of interactions between ErbB peptides, membranes, and Ca²⁺/calmodulin using fluorescence stop flow; assessing ErbB1 activation in Cos1 cells; measuring fluorescence resonance energy transfer between ErbB peptides and PIP₂; and making theoretical electrostatic calculations on atomic models of membranes and ErbB juxtamembrane and kinase domains.

Growth factor receptor binding protein 2-mediated recruitment of the RING domain of Cbl to the epidermal growth factor receptor is essential and sufficient to support receptor endocytosis

Knockdown of growth factor receptor binding protein 2 (Grb2) by RNA interference strongly inhibits clathrin-mediated endocytosis of the epidermal growth factor receptor (EGFR). To gain insights into the function of Grb2 in EGFR endocytosis, we have generated cell lines in which endogenous Grb2 was replaced by yellow fluorescent protein (YFP)-tagged Grb2 expressed at the physiological level. In these cells, Grb2-YFP fully reversed the inhibitory effect of Grb2 knockdown on EGFR endocytosis and, moreover, trafficked together with EGFR during endocytosis. Overexpression of Grb2-binding protein c-Cbl did not restore endocytosis in Grb2-depleted cells. However, EGFR endocytosis was rescued in Grb2-depleted cells by chimeric proteins consisting of the Src homology (SH) 2 domain of Grb2 fused to c-Cbl. The "knockdown and rescue" analysis revealed that the expression of Cbl-Grb2/SH2 fusions containing RING finger domain of Cbl restores normal ubiquitylation and internalization of the EGFR in the absence of Grb2, consistent with the important role of the RING domain in EGFR endocytosis. In contrast, the carboxy-terminal domain of Cbl, when attached to Grb2 SH2 domain, had 4 times smaller endocytosis-rescue effect compared with the RING-containing chimeras. Together, the data suggest that the interaction of Cbl carboxy terminus with CIN85 has a minor and a redundant role in EGFR internalization. We concluded that Grb2-mediated recruitment of the functional RING domain of Cbl to the EGFR is essential and sufficient to support receptor endocytosis.

Interdependent assembly of specific regulatory lipids and membrane fusion proteins into the vertex ring domain of docked vacuoles

Membrane microdomains are assembled by lipid partitioning (e.g., rafts) or by protein–protein interactions (e.g., coated vesicles). During docking, yeast vacuoles assemble “vertex” ring-shaped microdomains around the periphery of their apposed membranes. Vertices are selectively enriched in the Rab GTPase Ypt7p, the homotypic fusion and vacuole protein sorting complex (HOPS)–VpsC Rab effector complex, SNAREs, and actin. Membrane fusion initiates at vertex microdomains. We now find that the “regulatory lipids” ergosterol, diacylglycerol and 3- and 4-phosphoinositides accumulate at vertices in a mutually interdependent manner. Regulatory lipids are also required for the vertex enrichment of SNAREs, Ypt7p, and HOPS. Conversely, SNAREs and actin regulate phosphatidylinositol 3-phosphate vertex enrichment. Though the PX domain of the SNARE Vam7p has direct affinity for only 3-phosphoinositides, all the regulatory lipids which are needed for vertex assembly affect Vam7p association with vacuoles. Thus, the assembly of the vacuole vertex ring microdomain arises from interdependent lipid and protein partitioning and binding rather than either lipid partitioning or protein interactions alone.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling

Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.

Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglycerol-dependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav- and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-gamma, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.

Regulation of EGF-induced phospholipase C-gamma1 translocation and activation by its SH2 and PH domains

Translocation of phospholipase C-gamma1 is essential for its function in response to growth factors. However, in spite of recent progress, the phospholipase C-gamma1 translocation pattern and the molecular mechanism of the translocation are far from fully understood. Contradictory results were reported as to which domain, PH or SH2, controls the epidermal growth factor-induced translocation of phospholipase C-gamma1. In this communication, we studied epidermal growth factor-induced translocation of phospholipase C-gamma1 by using comprehensive approaches including biochemistry, indirect fluorescence and live fluorescence imaging. We provided original evidence demonstrating that: (i) endogenous phospholipase C-gamma1, similar to YFP-tagged phospholipase C-gamma1, translocated to endosomes following its initial translocation from cytosol to the plasma membrane in response to epidermal growth factor; (ii) phospholipase C-gamma1 remained phosphorylated in endosomes, but phospholipase C-gamma1 activity is not required for its translocation, which suggests a signaling role for phospholipase C-gamma1 in endosomes; (iii) the PH domain was not required for the initial translocation of phospholipase C-gamma1 from cytosol to the plasma membrane, but it stabilizes phospholipase C-gamma1 in the membrane at a later time; (iv) the function of the phospholipase C-gamma1 PH domain in stabilizing phospholipase C-gamma1 membrane association is very important in maintaining the activity of phospholipase C-gamma1; and (v) the role of the PH domain in phospholipase C-gamma1 membrane association and activation is dependent on PI3K activity. We conclude that the phospholipase C-gamma1 SH2 and PH domains coordinate to determine epidermal growth factor-induced translocation and activation of phospholipase C-gamma1.

Requirements for distinct steps of phospholipase Cgamma2 regulation, membrane-raft-dependent targeting and subsequent enzyme activation in B-cell signalling

Studies of PLCgamma (phospholipase Cgamma) have identified a number of regulatory components required for signalling; however, molecular mechanisms and the relationship between events leading to translocation and an increase of substrate hydrolysis have not been well defined. The addition of a membrane-targeting tag to many signal transducers results in constitutive activation, suggesting that these processes could be closely linked and difficult to dissect. The present study of PLCgamma2 regulation by cross-linking of the BCR (B-cell antigen receptor) or H2O2 stress in DT40 B-cells, demonstrated that the membrane targeting is a separate step from further changes that result in enzyme activation and substrate hydrolysis. Furthermore, we have defined the roles of different domains of PLCgamma2 and, using a panel of cell lines deficient in components linked to PLCgamma2 regulation, the involvement of signalling molecules with respect to each of the steps. We have found that only the lipid-raft-targeted Lyn-PLCgamma2 construct, unlike non-specific membrane targeting, overcame the requirement for the adapter protein BLNK (B-cell linker). The stable expression of Lyn-PLCgamma2 was not accompanied by an increase in substrate hydrolysis in resting cells, which followed stimulation and specifically required the presence and/or activation of Syk, Btk, phosphoinositide 3-kinase but not BLNK, as established using deficient cell lines or specific inhibitors. Based on mutational analysis of the specific tyrosine residues [Tyr753-->Phe (Y753F)/Y759F] and SH2 (Src homology 2) domains (R564A/R672A) in the context of Lyn-PLCgamma2, we found that Tyr753/Tyr759 were essential, whereas the PLCgamma2 SH2 domains did not have an important role in the transient activation of Lyn-PLCgamma2 but may serve to stabilize an activated form in sustained activation.

Directional motility induced by epidermal growth factor requires Cdc42

Cell motility is actuated by a host of intracellular signaling cascades that result in movement of the cell in one direction, even without an external gradient. Phospholipase C-gamma (PLCgamma) has been shown to be important for growth factor-induced lamellipodial protrusion at the front of the cell while Cdc42 has been implicated in both filopodium formation at the leading edge and control of polarity of migrating cells. We asked whether these asymmetries in effector molecules may be linked. When we overexpressed either constitutively active, dominant negative, or GFP-tagged Cdc42, wild-type NR6 fibroblasts lost directionality, as expected. On epidermal growth factor (EGF) exposure these cells produced multiple, transient protrusions in every direction; these extensions failed to result in productive motility. GFP-tagged Cdc42 appeared transiently at edges of newly formed protrusions in EGF-stimulated cells while they moved haphazardly. While PLCgamma is distributed throughout the cell, the ratio of active, tyrosyl-phosphorylated PLCgamma was increased at the leading edge, where phosphatidylinositol (4,5)-bisphosphate (PIP(2)) hydrolysis is concentrated. This co-localization of activities may be due to Cdc42 directing PLCgamma to the cell front, as PLCgamma associated with Cdc42 in an EGF-dependent manner. We conclude that Cdc42 controls cell polarity, likely in part, through its binding to active PLCgamma.

Endothelin-1 activates mesangial cell ERK1/2 via EGF-receptor transactivation and caveolin-1 interaction

Endothelin-1 (ET-1) stimulates glomerular mesangial cell proliferation and extracellular matrix protein transcription through an ERK1/2-dependent pathway. In this study, we determined whether ET-1 activation of glomerular mesangial cell ERK1/2 is mediated through EGF receptor (EGF-R) transactivation and whether intact caveolae are required. We showed that ET-1 stimulated tyrosine phosphorylation of the EGF-R in primary cultured, growth-arrested rat mesangial cells. In response to ET-1, ERK1/2 phosphorylation was increased by 27 +/- 1-fold and attenuated by AG-1478, a specific EGF-R inhibitor, to 9 +/- 1-fold. Moreover, filipin III and beta-cyclodextrin, two cholesterol-depleting drugs known to disrupt caveolae, significantly reduced ET-1-induced phosphorylation of ERK1/2. In addition, preincubation of mesangial cells with a myristoylated peptide that binds to the caveolin-1 scaffolding domain diminished ET-1 activation of ERK1/2. ET-1 caused interaction of caveolin-1 with phosphorylated ERK1/2 identified by coimmunoprecipitation. Activation of ERK1/2 and its interaction with caveolin-1 were reduced by AG-1478, beta-cyclodextrin, or inhibition of PKC. Phosphorylated ERK1/2 localized at focal adhesion complexes along with phospho-caveolin-1, suggesting specific sites of compartmentalization of these signaling molecules. Hence, ET-1 activates mesangial cell ERK1/2 predominantly through a pathway involving EGF-R transactivation, leading to a mechanism involving attachment to caveolin-1, presumably in caveolae.

Calcium mobilization evoked by hepatocellular swelling is linked to activation of phospholipase Cgamma

Recovery from swelling of hepatocytes and selected other epithelia is triggered by intracellular Ca(2+) release from the endoplasmic reticulum, which leads to fluid and electrolyte efflux through volume-sensitive K(+) and Cl(-) channels. The aim of this study was to determine the mechanisms responsible for swelling-mediated hepatocellular Ca(2+) mobilization. Swelling of HTC rat hepatoma cells, evoked by exposure to hypotonic medium, elicited transient increases in intracellular levels of inositol 1,4,5-trisphosphate (IP(3)) and cytosolic [Ca(2+)]. The latter was attenuated by inhibition of phospholipase C (PLC) with and by IP(3) receptor blockade with 2-aminoethoxydiphenyl borate, but it was unaffected by ryanodine, an inhibitor of intracellular Ca(2+)-induced Ca(2+) release channels. Hypotonic swelling was associated with a transient increase in tyrosine phosphorylation of PLCgamma, with kinetics that paralleled the increases in intracellular IP(3) levels and cytosolic [Ca(2+)]. Confocal imaging of HTC cells exposed to hypotonic medium revealed a swelling-induced association of tyrosine-phosphorylated PLCgamma with the plasma membrane. These findings suggest that activation of PLCgamma by hepatocellular swelling leads to the generation of IP(3) and stimulates discharge of Ca(2+) from the endoplasmic reticulum via activation of IP(3) receptors. By extension, these data support the concept that tyrosine phosphorylation of PLCgamma represents a critical step in adaptive responses to hepatocellular swelling.

Signal transduction and endocytosis: close encounters of many kinds

Binding of hormones, growth factors and other cell modulators to cell-surface receptors triggers a complex array of signal-transduction events. The activation of many receptors also accelerates their endocytosis. Endocytic transport is important in regulating signal transduction and in mediating the formation of specialized signalling complexes. Conversely, signal-transduction events modulate specific components of the endocytic machinery. Recent studies of protein tyrosine kinases and G-protein-coupled receptors have shed new light on the mechanisms and functional consequences of this bidirectional interplay between signalling and membrane-transport networks.

EGF-dependent association of phospholipase C-gamma1 with c-Cbl

The structure of phospholipase Cgamma1 (PLC-gamma1) contains two SH2 domains and one SH3 domain. While the function of the SH2 domains in PLC-gamma1 are well described, to date no growth factor-dependent function for the SH3 domain has been presented. To assess SH3 domain function in the context of the full-length PLC-gamma1, this domain was deleted and the mutant was stably expressed in Plcg1 null mouse embryonic fibroblasts. Following EGF treatment of cells, the PLC-gamma1DeltaSH3 mutant displayed the same increased level of tyrosine phosphorylation and association with EGF receptor as wild-type PLC-gamma1. Also, the SH3 mutant demonstrated membrane translocation and mediated the mobilization of intracellular Ca(2+) in response to EGF. c-Cbl is shown to associate with tyrosine phosphorylated PLC-gamma1 in an EGF-dependent manner, but no association was detected with the PLC-gamma1DeltaSH3 mutant. Interestingly, PDGF, which also tyrosine phosphorylates PLC-gamma1, failed to induce c-Cbl association with PLC-gamma1 and also provoked no c-Cbl tyrosine phosphorylation. This suggests that c-Cbl tyrosine phosphorylation is necessary for its interaction with PLC-gamma1. Evidence of a direct association of c-Cbl with PLC-gamma1 was provided by pull-down and overlay experiments, using glutathione S-transferase fusion proteins that contain the SH3 domain of PLC-gamma1. The data, therefore, show an EGF-inducible direct association of PLC-gamma1 with c-Cbl in vivo that is mediated by the SH3 domain of PLC-gamma1.

Hypercholesterolemia promotes a CD36-dependent and endothelial nitric-oxide synthase-mediated vascular dysfunction

Numerous studies have implicated either the presence or absence of CD36 in the development of hypertension. In addition, hypercholesterolemia is associated with the loss of nitric oxide-induced vasodilation and the subsequent increase in blood pressure. In the current study, we tested the hypothesis that diet-induced hypercholesterolemia promotes the disruption of agonist-stimulated nitric oxide generation and vasodilation in a CD36-dependent manner. To test this, C57BL/6, apoE null, CD36 null, and apoE/CD36 null mice were maintained on chow or high fat diets. In contrast to apoE null mice fed a chow diet, apoE null mice fed a high fat diet did not respond to acetylcholine with a decrease in blood pressure. Caveolae isolated from in vivo vessels did not contain endothelial nitric-oxide synthase and were depleted of cholesterol. Age-matched apoE/CD36 null mice fed a chow or high fat diet responded to acetylcholine with a decrease in blood pressure. The mechanism underlying the vascular dysfunction was reversible because vessels isolated from apoE null high fat-fed mice regained responsiveness to acetylcholine when incubated with plasma obtained from chow-fed mice. Further analysis demonstrated that the plasma low density lipoprotein fraction was responsible for depleting caveolae of cholesterol, removing endothelial nitric-oxide synthase from caveolae, and preventing nitric oxide production. In addition, the pharmacological removal of caveola cholesterol with cyclodextrin mimicked the effects caused by the low density lipoprotein fraction. We conclude that the ablation of CD36 prevented the negative impact of hypercholesterolemia on agonist-stimulated nitric oxide-mediated vasodilation in apoE null mice. These studies provide a direct link between CD36 and the early events that underlie hypercholesterolemia-mediated hypertension and mechanistic linkages between CD36 function, nitric-oxide synthase activation, caveolae integrity, and blood pressure regulation.

PAK1 kinase is required for CXCL1-induced chemotaxis

The CXC subfamily of chemokines plays an important role in diverse processes, including inflammation, wound healing, growth regulation, angiogenesis, and tumorigenesis. The CXC chemokine CXCL1, or MGSA/GROalpha, is traditionally considered to be responsible for attracting leukocytes into sites of inflammation. To better understand the molecular mechanisms by which CXCL1 induces CXCR2-mediated chemotaxis, the signal transduction components involved in CXCL1-induced chemotaxis were examined. It is shown here that CXCL1 induces cdc42 and PAK1 activation in CXCR2-expressing HEK293 cells. Activation of the cdc42-PAK1 cascade is required for CXCL1-induced chemotaxis but not for CXCL1-induced intracellular Ca2+ mobilization. Moreover, CXCL1 activation of PAK1 is independent of ERK1/2 activation, a conclusion based on the observations that the inhibition of MEK-ERK activation by expression of dominant negative ERK or by the MEK inhibitor, PD98059, has no effect on CXCL1-induced PAK1 activation or CXCL1-induced chemotaxis.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

Coordinated traffic of Grb2 and Ras during epidermal growth factor receptor endocytosis visualized in living cells

Activation of the epidermal growth factor receptor (EGFR) triggers multiple signaling pathways and rapid endocytosis of the epidermal growth factor (EGF)-receptor complexes. To directly visualize the compartmentalization of molecules involved in the major signaling cascade, activation of Ras GTPase, we constructed fusions of Grb2, Shc, H-Ras, and K-Ras with enhanced cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP), and used live-cell fluorescence imaging microscopy combined with the fluorescence resonance energy transfer (FRET) technique. Stimulation of cells by EGF resulted in the accumulation of large pools of Grb2-CFP and YFP-Shc in endosomes, where these two adaptor proteins formed a complex with EGFR. H-Ras and K-Ras fusion proteins were found at the plasma membrane, particularly in ruffles and lamellipodia, and also in endosomes independently of GTP/GDP loading and EGF stimulation. The relative amount of endosomal H-Ras was higher than that of K-Ras, whereas K-Ras predominated at the plasma membrane. On application of EGF, Grb2, and Ras converge in the same endosomes through the fusion of endosomes containing either Grb2 or Ras or through the joint internalization of two proteins from the plasma membrane. To examine the localization of the GTP-bound form of Ras, we used a FRET assay that exploits the specific interaction of GTP-bound CFP-Ras with the YFP-fused Ras binding domain of c-Raf. FRET microscopy revealed that GTP-bound Ras is located at the plasma membrane, mainly in ruffles and at the cell edges, as well as in endosomes containing EGFR. These data point to the potential for endosomes to serve as sites of generation for persistent signaling through Ras.

Heterologous desensitization of EGF receptors and PDGF receptors by sequestration in caveolae

Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) receptors have been reported to signal via caveolin-containing membranes called caveolae. In contrast, others report that EGF and PDGF receptors are exclusively associated with caveolin-devoid membranes called rafts. Our subcellular fractionation and coimmunoprecipitation studies demonstrate that, in the absence of ligand, EGF and PDGF receptors are associated with rafts. However, in the presence of ligand, EGF and PDGF receptors transiently associate with caveolae. Surprisingly, pretreatment of cells with EGF prevents PDGF-dependent phosphorylation of PDGF receptors and extracellular signal-regulated kinase (ERK) 1/2 kinase activation. Furthermore, cells pretreated with PDGF prevent EGF-dependent phosphorylation of EGF receptors and ERK1/2 kinase activation. Radioligand binding studies demonstrate that incubation of cells with EGF or PDGF causes both EGF and PDGF receptors to be reversibly sequestered from the extracellular space. Experiments with methyl-beta-cyclodextrin, filipin, and antisense caveolin-1 demonstrate that sequestration of the receptors is dependent on cholesterol and caveolin-1. We conclude that ligand-induced stimulation of EGF or PDGF receptors can cause the heterologous desensitization of the other receptor by sequestration in cholesterol-rich, caveolin-containing membranes or caveolae.

Active EGF receptors have limited access to PtdIns(4,5)P2 in endosomes: implications for phospholipase C and PI 3-kinase signaling

Although prolonged cell signaling is attenuated by internalization and downregulation of active receptors, it is now appreciated that many receptors continue to signal in intracellular compartments. Employing enhanced green fluorescent protein fusion probes, we have investigated the hypothesis that multiple signaling pathways are affected by the differential trafficking of membrane substrates such as PtdIns(4,5)P2. A phosphotyrosine-specific probe, but not a PtdIns(4,5)P2-specific probe, colocalized with internalized EGF as well as transferrin in EGF-stimulated living cells expressing autophosphorylation-competent EGF receptors. Neither probe colocalized with transferrin in the absence of EGF, demonstrating that the reduced level of accessible PtdIns(4,5)P2 in endosomes is constitutive. Finally, a PtdIns(3,4,5)P3-specific probe, which monitors phosphorylation of PtdIns(4,5)P2 by phosphoinositide 3-kinases, was recruited to the plasma membrane but not to EGF- or transferrin-containing endosomes in response to EGF stimulation. These results suggest that while many internalized receptors continue to engage intracellular enzymes, the phospholipase C and phosphoinositide 3-kinase signaling pathways are abrogated by the constitutive lack of accessible PtdIns(4,5)P2 in endosomes.