Assays for G-protein regulation of phospholipase C activity

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

Proton-sensing G protein-coupled receptor mobilizes calcium in human synovial cells

Lowered extracellular pH in a variety of tissues is associated with increased tissue destruction and initiation of inflammatory processes. Although the acid-sensing receptors described previously are ion channels, we describe a G protein-coupled proton-sensitive receptor that stimulates Ca(2+) release from intracellular stores in a tumor-derived synoviocyte cell line (SW982) and in primary cultures of human synovial cells from patients with inflammatory arthropathies. We established a link between proton-dependent receptor activation and intracellular Ca(2+) mobilization by demonstrating 1) dependence on the integrity of the intracellular Ca(2+) store, 2) independence from extracellular Ca(2+), and 3) proton-induced production of inositol phosphate and 4) by abolishing the effect with GTPase inhibitors. We propose that this G protein-coupled acid-sensing receptor linked to intracellular Ca(2+) mobilization in synoviocytes can contribute to downstream inflammatory and cellular proliferative processes in synovial fibroblasts. The acid-sensing receptor has distinct characteristics as a metabotropic G protein-coupled receptor on human synoviocytes in this emerging new class of receptors.

A dominant negative Galphas mutant that prevents thyroid-stimulating hormone receptor activation of cAMP production and inositol 1,4,5-trisphosphate turnover: competition by different G proteins for activation by a common receptor

A Ser to Asn mutation at position 54 of the alpha subunit of G(s) (designated N54-alpha(s)) was characterized after transient expression of it with various components of the receptor-adenylyl cyclase pathway in COS-1, COS-7, and HEK 293 cells. Previous studies of the N54-alpha(s) mutant revealed that it has a conditional dominant negative phenotype that prevents hormone-stimulated increases in cAMP without interfering with the regulation of basal cAMP levels (Cleator, J. H., Mehta, N. D., Kurtz, D. K., Hildebrandt, J. D. (1999) FEBS Lett. 243, 205-208). Experiments reported here were conducted to localize the mechanism of the dominant negative effect of the mutant. Competition studies conducted with activated alpha(s)* (Q212L) showed that the N54 mutant did not work down-stream by blocking the interaction of endogenous alpha(s) with adenylyl cyclase. The co-expression of wild type or N54-alpha(s) along with the thyroid-stimulating hormone (TSH) receptor and adenylyl cyclase isotypes differing with respect to betagamma stimulation (AC II or AC III) revealed that the phenotype of the mutant is not dependent upon the presence of adenylyl cyclase isoforms regulated by betagamma. These studies ruled out a downstream site of action of the mutant. To investigate an upstream site of action, N54-alpha(s) was co-expressed with either the TSH receptor that activates both alpha(s) and alpha(q) or with the alpha(1B)-adrenergic receptor that activates only alpha(q). N54-alpha(s) failed to inhibit alpha(1B)-adrenergic receptor stimulation of inositol 1,4,5-trisphosphate production but did inhibit TSH stimulation of inositol 1,4,5-trisphosphate. These results show that G(s) and G(q) compete for activation by the TSH receptor. They also indicate that the N54 protein has a dominant negative phenotype by blocking upstream receptor interactions with normal G proteins. This phenotype is different from that seen in analogous mutants of other G protein alpha subunits and suggests that either regulation or protein-protein interactions differ among G protein alpha subunits.

Role of dynamic interactions in effective signal transfer for Gbeta stimulation of phospholipase C-beta 2

Heterotrimeric G protein subunits regulate their effectors by protein-protein interactions. The regions involved in these direct interactions have either signal transfer or general binding functions (Buck, E., Li, J., Chen, Y., Weng, G., Scarlata, S., and Iyengar, R. (1999) Science 283, 1332-1335). Although key determinants of signal transfer regions for G protein subunits have been identified, the mechanisms of signal transfer are not fully understood. We have used a combinatorial peptide approach to analyze one Gbeta region, Gbeta86-105, involved in signal transfer to the effector phospholipase C (PLC)-beta2 to gain a more mechanistic understanding of Gbeta/PLC-beta2 signaling. Binding and functional studies with the combinatorial peptides on interaction with and stimulation/inhibition of phospholipase Cbeta2 indicate that binding affinity can be resolved from EC(50) for functional effects, such that peptides that have wild type binding affinities have 15- to 20-fold lower EC(50) values. Although more potent, these peptides display a much lower extent of maximal stimulation. These peptides synergize with Gbetagamma or peptides encoding the second Gbeta42-54 signal transfer region in maximally stimulating phospholipase C-beta2. Other combinatorial peptides from the Gbeta86-105 region that bind to PLC-beta2 by themselves submaximally stimulate and extensively inhibit Gbetagamma stimulation of PLC-beta2. The intrinsic stimulation function can be attributed to Arg-96 and Ser-97, the synergy function to Trp-99, and the binding affinity to Thr-87, Val-90, Pro-94, Arg-96, Ser-97, and Val-100. These results indicate that, even within signal transfer regions, residues involved in binding can be resolved from those involved in signal transfer and that signal transfer is likely to be achieved through dynamic rather than steady-state interactions.

Modular design of Gbeta as the basis for reversible specificity in effector stimulation

The G protein Gbetagamma subunit complex stimulates effectors by direct interactions utilizing extensive Gbeta regions over the surface of its propeller structure that faces the Galpha subunit. Our previous experiments have shown the resolved functions of signal transfer and general binding for Gbeta regions involved in stimulation of the effector phospholipase C-beta2, PLC-beta2, within the region Gbeta-(86-135), which comprises three beta strands arranged in a structurally contiguous fashion (Buck, E., Li, J., Chen, Y., Weng, G., Sacarlata, S., and Iyengar, R. (1999) Science 283, 1332-1335). This raises an important question as to why mutagenesis studies indicate that an extensive set of sites all over the Gbeta propeller structure and outside the 86-135 region are involved in Gbeta regulation of PLC-beta2. Using peptides to define functions of these Gbeta regions, we find that Gbeta signaling to PLC-beta2 relies on a collection of modular signal transfer and general binding units, each with lower apparent affinity relative to Gbetagamma-PLC interactions. Gbeta-(42-54) functions as a signal transfer region, Gbeta-(228-249) and Gbeta-(321-340) function in general binding, and Gbeta-(64-84) and Gbeta-(300-313) seem to play a structural role rather than a direct contact with the effector. A substitution within the Gbeta-(42-54) signal transfer region that increases the K(act) of this peptide for PLC-beta2 is accompanied by an increase in the observed maximal extent of signal transfer. We conclude that the lower K(act) for individual signal transfer regions may result in a decrease in the maximal effect of signal transfer. The spatial resolution of the signal transfer and general binding regions over a wide surface of Gbeta allow geometrical constraints to achieve specificity even with relatively low affinity interactions.

Monitoring Inositol-Specific Phospholipase C Activity Using a Phospholipid FlashPlate(R)

Inositol-specific phospholipase Cs(PLCs) are a group of enzymes involved in the signal transduction pathway of many plasma membrane receptor mediated events. We developed a modified solid surface to capture [(3)H] PIP(2) onto the Basic FlashPlate(R) in order to monitor PLC activity. Our results clearly demonstrate the utility of [(3)H] PIP(2)-Coated Phospholipid FlashPlate(R) microtiter plates for assessing PLC activity for HTS of receptor-coupled functional assays. The results show that PLC activity can be measured easily from a variety of sources including purified recombinant enzyme preparations, crude HL60 cell lysates and permeabilized A431 human carcinoma cells. Moreover, this format provides a surface comparable to that used for classical solution based radiolabeled mixed phospholipid micelle studies and illustrates the feasibility of this assay for measuring PLC activation in a variety of different drug screening assays.

Resolution of a signal transfer region from a general binding domain in gbeta for stimulation of phospholipase C-beta2

Signaling by guanine nucleotide-binding proteins (G proteins) involves sequential protein-protein interactions. G protein-betagamma subunit (Gbetagamma) interactions with phospholipase C-beta2 (PLC-beta2) were studied to determine if all Gbeta contacts are required for signaling. A peptide encoding Gbeta amino acid residues 86 to 105 stimulated PLC-beta2. Six residues (96 to 101) within this sequence could transfer signals and thus constitute a core signal transfer region. Another peptide, encoding Gbeta amino acid residues 115 to 135, did not substantially stimulate PLC-beta2 by itself but inhibited Gbetagamma stimulation, indicating that residues 115 to 135 constitute a general binding domain. Resolution of signal transfer regions from general binding domains indicates that all protein-protein contacts are not required for signal transfer and that it may be feasible to synthesize agonists and antagonists that regulate intracellular signal flow.

A region of adenylyl cyclase 2 critical for regulation by G protein beta gamma subunits

Receptor-mediated activation of heterotrimeric guanine nucleotide-binding proteins (G proteins) results in the dissociation of alpha from beta gamma subunits, thereby allowing both to regulate effectors. Little is known about the regions of effectors required for recognition of G beta gamma. A peptide encoding residues 956 to 982 of adenylyl cyclase 2 specifically blocked G beta gamma stimulation of adenylyl cyclase 2, phospholipase C-beta 3, potassium channels, and beta-adrenergic receptor kinase as well as inhibition of calmodulin-stimulated adenylyl cyclases, but had no effect on interactions between G beta gamma and G alpha o. Substitutions in this peptide identified a functionally important motif, Gln-X-X-Glu-Arg, that is also conserved in regions of potassium channels and beta-adrenergic receptor kinases that participate in G beta gamma interactions. Thus, the region defined by residues 956 to 982 of adenylyl cyclase 2 may contain determinants important for receiving signals from G beta gamma.