Modelling of signalling via G-protein coupled receptors: pathway-dependent agonist potency and efficacy

Tracheal relaxation through calcium channel blockade of Achillea millefolium hexanic extract and its main bioactive compounds

ETHNOPHARMACOLOGICAL IMPORTANCE

Achillea millefolium L. (Asteraceae) is used for the treatment of respiratory diseases, diabetes, and hypertension.

AIM

to explore its tracheal relaxant properties and clarify its functional mechanism of action on smooth muscle cells, which allow us to propose it as a potential anti-asthmatic drug.

MATERIAL AND METHODS

organic and hydro-alcoholic extracts from A. millefolium were obtained by macerations, then their relaxing effect on ex vivo isolated rat trachea rings was determined. Most active extract (hexanic extract, EHAm) was studied to determine its functional mechanism of action using synergic, antagonist and inhibitor agents related with the contraction/relaxation process of the smooth muscle. Also, EHAm was subjected to bio-guided fractionation by open-column chromatography (on silica gel) using cyclohexane-EtOAc (80:20) in an isocratic way to isolate main bioactive compounds.

RESULTS

organic and hydro-alcoholic extracts showed relaxant effect in a concentration-response dependent manner, being EHAm the most active. The functional mechanism of action indicates that EHAm induced a non-competitive antagonism to the muscarinic receptors ; in addition, the NO/cGMP pathway is involved in the relaxation process of the tracheal smooth muscle. However, the most important mechanism of action showed by EHAm was related with the calcium channel blockade influx into the smooth muscle cells. On the other hand, epimeric sesquiterpene lactones leucodin (1) and achillin (2) were isolated and purified, which are responsible for the observed smooth muscle relaxant activity of the extract.

CONCLUSION

hexanic extract of A. millefollium induced a significant relaxant effect on tracheal rat rings by calcium channel blockade and NO release.

Modelling and mathematical analysis of the M$_{2}$ receptor-dependent joint signalling and secondary messenger network in CHO cells

The muscarinic M$_{2}$ receptor is a prominent member of the GPCR family and strongly involved in heart diseases. Recently published experimental work explored the cellular response to iperoxo-induced M$_{2}$ receptor stimulation in Chinese hamster ovary (CHO) cells. To better understand these responses, we modelled and analysed the muscarinic M$_{2}$ receptor-dependent signalling pathway combined with relevant secondary messenger molecules using mass action. In our literature-based joint signalling and secondary messenger model, all binding and phosphorylation events are explicitly taken into account in order to enable subsequent stoichiometric matrix analysis. We propose constraint flux sampling (CFS) as a method to characterize the expected shift of the steady state reaction flux distribution due to the known amount of cAMP production and PDE4 activation. CFS correctly predicts an experimentally observable influence on the cytoskeleton structure (marked by actin and tubulin) and in consequence a change of the optical density of cells. In a second step, we use CFS to simulate the effect of knock-out experiments within our biological system, and thus to rank the influence of individual molecules on the observed change of the optical cell density. In particular, we confirm the relevance of the protein RGS14, which is supported by current literature. A combination of CFS with Elementary Flux Mode analysis enabled us to determine the possible underlying mechanism. Our analysis suggests that mathematical tools developed for metabolic network analysis can also be applied to mixed secondary messenger and signalling models. This could be very helpful to perform model checking with little effort and to generate hypotheses for further research if parameters are not known.

Modelling and simulation of biased agonism dynamics at a G protein-coupled receptor

Theoretical models of G protein-coupled receptor (GPCR) concentration-response relationships often assume an agonist producing a single functional response via a single active state of the receptor. These models have largely been analysed assuming steady-state conditions. There is now much experimental evidence to suggest that many GPCRs can exist in multiple receptor conformations and elicit numerous functional responses, with ligands having the potential to activate different signalling pathways to varying extents-a concept referred to as biased agonism, functional selectivity or pluri-dimensional efficacy. Moreover, recent experimental results indicate a clear possibility for time-dependent bias, whereby an agonist's bias with respect to different pathways may vary dynamically. Efforts towards understanding the implications of temporal bias by characterising and quantifying ligand effects on multiple pathways will clearly be aided by extending current equilibrium binding and biased activation models to include G protein activation dynamics. Here, we present a new model of time-dependent biased agonism, based on ordinary differential equations for multiple cubic ternary complex activation models with G protein cycle dynamics. This model allows simulation and analysis of multi-pathway activation bias dynamics at a single receptor for the first time, at the level of active G protein (αGTP), towards the analysis of dynamic functional responses. The model is generally applicable to systems with NG G proteins and N* active receptor states. Numerical simulations for NG=N*=2 reveal new insights into the effects of system parameters (including cooperativities, and ligand and receptor concentrations) on bias dynamics, highlighting new phenomena including the dynamic inter-conversion of bias direction. Further, we fit this model to 'wet' experimental data for two competing G proteins (Gi and Gs) that become activated upon stimulation of the adenosine A1 receptor with adenosine derivative compounds. Finally, we show that our model can qualitatively describe the temporal dynamics of this competing G protein activation.

Modelling of the activation of G-protein coupled receptors: drug free constitutive receptor activity

G-protein coupled receptors (GPCRs) form a crucial component of approximately 80% of hormone pathways. In this paper, the most popular mechanism for activation of GPCRs-the shuttling mechanism-is modelled mathematically. An asymptotic analysis of this model clarifies the dynamics of the system in the absence of drug, in particular which reactions dominate during the different timescales. Equilibrium analysis of the model demonstrates the model's ability to predict constitutive receptor activity.

A simple reaction kinetic model of rapid (G protein dependent) and slow (beta-Arrestin dependent) transmission

In this paper the qualitative dynamic behavior of reaction kinetic models of G protein signaling is examined. A simplified basic G protein signaling structure is defined, which is extended to be able to take the effect of slow transmission, RGS mediated feedback regulation and ERK-phosphatase mediated feedback regulation into account. The resulting model gives rise to an acceptable qualitative approximation of the G protein dependent and independent ERK activation dynamics that is in good agreement with the experimentally observed behavior.

Caveolin-1 triggers T-cell activation via CD26 in association with CARMA1

CD26 is a widely distributed 110-kDa cell surface glycoprotein with an important role in T-cell costimulation. We demonstrated previously that CD26 binds to caveolin-1 in antigen-presenting cells, and following exogenous CD26 stimulation, Tollip and IRAK-1 disengage from caveolin-1 in antigen-presenting cells. IRAK-1 is then subsequently phosphorylated to up-regulate CD86 expression, resulting in subsequent T-cell proliferation. However, it is unclear whether caveolin-1 is a costimulatory ligand for CD26 in T-cells. Using soluble caveolin-1-Fc fusion protein, we now show that caveolin-1 is the costimulatory ligand for CD26, and that ligation of CD26 by caveolin-1 induces T-cell proliferation and NF-kappaB activation in a T-cell receptor/CD3-dependent manner. We also demonstrated that the cytoplasmic tail of CD26 interacts with CARMA1 in T-cells, resulting in signaling events that lead to NF-kappaB activation. Ligation of CD26 by caveolin-1 recruits a complex consisting of CD26, CARMA1, Bcl10, and IkappaB kinase to lipid rafts. Taken together, our findings provide novel insights into the regulation of T-cell costimulation via the CD26 molecule.

Modelling the effect of caveolae on G-protein activation

In this paper, we study the effects on G-protein activation of a non-uniform distribution of signalling components. The spatial heterogeneity is attributed to caveolae, a specific membrane microdomain which has been observed to redistribute and concentrate signalling molecules. Diffusive coagulation-fragmentation equations are used to describe the aggregation of caveolin homo-oligomers and the subsequent formation of caveolae. A system of reaction-diffusion equation is thus formulated and, in order to describe the restrictions imposed by caveolae on the movement of receptors and G-protein, a segregation coefficient is introduced which serves to regulate the preference of the species to segregate according to the concentration of caveolae. The results demonstrate that the heterogeneous distribution of the signalling components and the efficiency of G-protein activation can vary significantly, depending on the concentration of caveolae.

CD28 induces immunostimulatory signals in dendritic cells via CD80 and CD86

Bidirectional signaling along the B7-CTLA-4 coreceptor pathway enables reciprocal conditioning of T cells and dendritic cells. Although T cells can instruct dendritic cells to manifest tolerogenic properties after CTLA-4 engagement of B7, such a B7-mediated signaling is not known to occur in response to CD28. Here we show that mouse dendritic cells were induced by soluble CD28 to express interleukin 6 and interferon-gamma. Production of interleukin 6 required B7-1 (CD80), B7-2 (CD86) and p38 mitogen-activated protein kinase and prevented interferon-gamma-driven expression of immunosuppressive tryptophan catabolism. In vivo, an adjuvant activity of soluble CD28 was demonstrated as enhanced T cell-mediated immunity to tumor and self peptides and protection against microbial and tumor challenge. Thus, different ligands of B7 can signal dendritic cells to express functionally distinct effector responses.