Identification of an in vitro interaction between an insect immune suppressor protein (CrV2) and G alpha proteins.

The protein CrV2 is encoded by a polydnavirus integrated into the genome of the endoparasitoid Cotesia rubecula (Hymenoptera:Braconidae:Microgastrinae) and is expressed in host larvae with other gene products of the polydnavirus to allow successful development of the parasitoid. CrV2 expression has previously been associated with immune suppression, although the molecular basis for this was not known. Here, we have used time-resolved Förster resonance energy transfer (TR-FRET) to demonstrate high affinity binding of CrV2 to Gα subunits (but not the Gßγ dimer) of heterotrimeric G-proteins. Signals up to 5-fold above background were generated, and an apparent dissociation constant of 6.2 nm was calculated. Protease treatment abolished the TR-FRET signal, and the presence of unlabeled CrV2 or Gα proteins also reduced the TR-FRET signal. The activation state of the Gα subunit was altered with aluminum fluoride, and this decreased the affinity of the interaction with CrV2. It was also demonstrated that CrV2 preferentially bound to Drosophila Gα(o) compared with rat Gα(i1). In addition, three CrV2 homologs were detected in sequences derived from polydnaviruses from Cotesia plutellae and Cotesia congregata (including the immune-related early expressed transcript, EP2). These data suggest a potential mode-of-action of immune suppressors not previously reported, which in addition to furthering our understanding of insect immunity may have practical benefits such as facilitating development of novel controls for pest insect species.

GPCR expression using baculovirus-infected Sf9 cells.

Expression of proteins in insect cells using recombinant baculoviruses has gained wide use in the G protein-coupled receptor (GPCR) community. This expression system produces high yields of functional receptor, is able to perform post-translational modifications, and is readily adaptable to large-scale culture. Here, we describe the generic methods for expressing a GPCR using baculovirus-infected insect cells, including the maintenance of insect cell culture. Data are presented for polyhedrin promoter-driven expression of a C-terminal 6 x histidine-tagged mammalian M(2) muscarinic receptor in Sf9 cells. Results demonstrate that expressed receptor could be detected and quantified using radiolabeled ligand binding, that expression was maximal at approximately 72 h post-infection, and that expression levels could be altered by addition of various ligands to cultures of infected insect cells.

[35S]GTPgammaS binding in G protein-coupled receptor assays.

The [(35)S]GTPgammaS binding assay to measure G protein activation following agonist binding to G protein-coupled receptors (GPCRs) remains a powerful molecular technique to substantiate traditional pharmacological values of potency, efficacy, and affinity. The method described uses membrane preparations of the alpha(2A)-adrenergic receptor and purified G protein subunits expressed in Sf9 cells, reconstituted into a functional signaling system. This technology is generic and could be used with other GPCRs to demonstrate initial signaling events following receptor activation. Agonist-stimulated [(35)S]GTPgammaS binding is measured in a 96-well plate format using scintillation counting.

Measurement of heterotrimeric G-protein and regulators of G-protein signaling interactions by time-resolved fluorescence resonance energy transfer.

G-protein-coupled receptors transduce their signals through G-protein subunits which in turn are subject to modulation by other intracellular proteins such as the regulators of G-protein signaling (RGS) proteins. We have developed a cell-free, homogeneous (mix and read format), time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor heterotrimeric G-protein subunit interactions and the interaction of the G alpha subunit with RGS4. The assay uses a FRET pair consisting of a terbium cryptate chelate donor spectrally matched to an Alexa546 fluor acceptor, each of which is conjugated to separate protein binding partners, these being G alpha(i1):beta4gamma2 or G alpha(i1):RGS4. Under conditions favoring specific binding between labeled partners, high-affinity interactions were observed as a rapid increase (>fivefold) in the FRET signal. The specificity of these interactions was demonstrated using denaturing or competitive conditions which caused significant reductions in fluorescence (50-85%) indicating that labeled proteins were no longer in close proximity. We also report differential binding effects as a result of altered activation state of the G alpha(i1) protein. This assay confirms that interactions between G-protein subunits and RGS4 can be measured using TR-FRET in a cell- and receptor-free environment.

Dietary fish oil alters cardiomyocyte Ca2+ dynamics and antioxidant status.

The n-3 polyunsaturated fatty acids (PUFAs) found in fish oil (FO) have been shown to protect against reperfusion arrhythmias, a manifestation of reperfusion injury, which is believed to be induced by the formation of reactive oxygen species (ROS) and intracellular calcium (Ca2+) overload. Adult rats fed a diet supplemented with 10% FO had a higher proportion of myocardial n-3 PUFAs and increased expression of antioxidant enzymes compared with the saturated fat (SF)-supplemented group. Addition of hydrogen peroxide (H2O2) to cardiomyocytes isolated from rats in the SF-supplemented group increased the proportions of cardiomyocytes contracting in an asynchronous manner, increased the rate of Ca2+ influx, and increased the diastolic and systolic [Ca2+]i compared with the FO group. H2O2 exposure increased the membrane fluidity of cardiomyocytes from the FO group. These results demonstrate that dietary FO supplementation is associated with a reduction in the susceptibility of myocytes to ROS-induced injury and this may be related to membrane incorporation of n-3 PUFAs, increased antioxidant defenses, changes in cardiomyocyte membrane fluidity, and the ability to prevent rises in cellular Ca2+ in response to ROS.

GPCR-induced dissociation of G-protein subunits in early stage signal transduction.

G-protein coupled receptors (GPCRs) form a ternary complex of agonist, receptor and G-proteins during primary signal transduction at the cell membrane. Downstream signalling is thought to be preceded by the process of dissociation of Galpha and Gbetagamma subunits, thus exposing new surfaces to interact with downstream effectors. We demonstrate here for the first time, the dissociation of heterotrimeric G-protein subunits (i.e., Galpha and Gbetagamma) following agonist-induced GPCR (alpha(2A)-adrenergic receptor; alpha(2A)-AR) activation in a cell-free assay system. alpha(2A)-AR membranes were reconstituted with the G-proteins (+/-hexahistidine-tagged) Galpha(i1) and Gbeta1gamma2 and functional signalling was determined following activation of the reconstituted receptor:G-protein complex with the potent agonist UK-14304, and [35S]GTPgammaS. In the presence of Ni(2+)-coated agarose beads, the activated his-tagged Galpha(i1)his-[35S]GTPgammaS complex was captured on the Ni(2+)-presenting surface. When his-tagged Gbeta1gamma2 (Gbeta1gamma2his) was used with Galpha(i1), the [35S]GTPgammaS-bound Galpha(i1) was not present on the Ni(2+)-coated beads, but rather, it was separated from the beta1gamma2(his)-beads, demonstrating receptor-induced dissociation of Galpha and Gbetagamma subunits. Treatment of the reconstituted alpha(2A)-AR membranes containing Gbeta1gamma2his:Galpha(i1) with imidazole confirmed the specificity for the Ni2+:G-protein surface dissociation of Galpha(i1) from Gbeta1gamma2his. These data demonstrate for the first time, the complete dissociation of the G-protein subunits and extend observations on the role of G-proteins in the assembly and disassembly of the ternary complex in the primary events of GPCR signalling.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches.

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Dietary fish oil increases acetylcholine- and eicosanoid-induced contractility of isolated rat ileum.

The long-chain (n-3) polyunsaturated fatty acids (PUFA) have been reported to exhibit health benefits and healing properties for the gastrointestinal tract. The aim of this study was to investigate the effects of dietary fish oil supplementation on the in vitro contractility of gut tissue. Rats (9 wk old) were fed synthetic diets supplemented with 170 g/kg Sunola oil (SO; 850 g/kg as oleic acid [18:1(n-9)]) or with 100 g/kg of the SO replaced by saturated animal fat (SF) or fish oil (FO) for 4 wk. In the colon, there was no difference in the sensitivity (50% effective concentration) or the maximal contraction among the three dietary groups induced by acetylcholine or 8-iso-prostaglandin (PG)E(2) with the rat colon being relatively insensitive to the thromboxane mimetic U-46619. However, in the ileum, the FO group had greater maximal contractions induced by acetylcholine and 8-iso-PGE(2) compared with the SO and SF groups (P < 0.05), and greater maximal contractions induced by PGE(2), PGF(2alpha) and U-46619 compared with the SF group (P < 0.05). FO feeding increased the incorporation of (n-3) PUFA (eicosapentaenoic [20:5(n-3)], docosapentaenoic [22:5(n-3)] and docosahexaenoic acids [22:6(n-3) primarily at the expense of (n-6) PUFA (linoleic [18:2(n-6)] and arachidonic acids [20:4(n-6)]) in the ileum and colon phospholipid fatty acids (P < 0.05). The FO group had a lower cecal digesta pH (P < 0.001) and a greater butyrate concentration than the SF group (P < 0.05). These results suggest that dietary (n-3) PUFA may modulate the contractility of the small intestine.