Understanding how cells respond to ligands in complex physiological and pathological contexts in vivo requires a knowledge of the dynamics of signalling in addition to an understanding of the molecular wiring of the pathway. We have addressed this by using a combined experimental and computational modelling approach to study TGF-β superfamily signalling dynamics.
In particular, we have dissected the mechanisms that determine cellular responsiveness to acute and chronic signals, as well as to repeated ligand stimulations. We have shown that stimulation of cells with TGF-β produces a transient response that attenuates over time, resulting in desensitised cells that are refractory to further acute stimulation.
This loss of signalling competence depends on ligand binding, but not on receptor activity, and is only reverted after the ligand has been depleted. Using surface receptor biotinylation experiments we have shown that TGF-β binding triggers the rapid depletion from the cell surface of signalling-competent receptors, with the type I and type II receptors exhibiting different degradation and trafficking kinetics.
In collaboration with Bernhard Schmierer (Karolinska Institute, Stockholm) we have gone on to generate a computational model of TGF-β signal transduction from the membrane to the nucleus that incorporates this mechanism. We have used the model to predict which parameters are critical determinants of the dynamics of TGF-β signalling, and find that the rate of receptor turnover and the ratio between ligand-induced and constitutive receptor degradation are crucial, but not the initial levels of surface receptors.
The model also predicts that autocrine signalling, as demonstrated in many tumours, would severely compromise TGF-β responses, and we have been able to confirm this experimentally using a panel of tumour cell lines (Vizán et al., 2013; Science Signalling. 6:ra106).
We have now extended this work to other TGF-β superfamily members and have performed several whole genome high throughput screens to find new regulators of TGF-β signalling dynamics.