Our group is working on the protease-activated receptors (PARs), which play a major role in orchestrating the interactions between coagulation and inflammation. We aim to gain a better understanding of the molecular mechanisms involved in the regulation of PAR activation, which may lead to novel therapeutic options for treating PAR-driven inflammatory and malignant diseases. PAR1, PAR3 and PAR4 are major regulators of platelet activation and vascular barrier function while PAR2 plays a major role in the regulation of extravascular inflammation and cancer.
Proteolytic activation of protease-activated receptors (PARs)
PARs belong to the receptor family of seven transmembrane G-protein coupled receptors (GPCR), however are unique in their lack of physiologically soluble ligands. Proteases, soluble or cell membrane associated (bound to co-receptors or specific membrane compartments) cleave off specific N-terminal sequences of PARs, resulting in the exposure of new N-terminal sequences that serve as tethered activation ligands. The tethered ligands bind to a conserved region on PARs extracellular loop 2, this initiates conformational changes and alters affinity towards intracellular G proteins. The cleavage derived tethered ligands alternatively lead to the transactivation of receptors such as co-localized PARs, ion channels, and toll-like receptors.
PARs are specifically cleaved and irreversibly activated by various endogenous as well as exogenous proteases originating from bacteria, amoeba, plants, fungi, insects or reptiles.
Heuberger and Schüpbach Thromb J. 2019
Project 1: Development of a novel tool for the regulation of PAR1-dependent vascular inflammation
PAR1 is the major thrombin receptor regulating platelet activation. On most other cells such as endothelial cells, PAR1 exerts a dual role. PAR1 activation by thrombin mediates inflammation while the activation by the anticoagulant protease activated protein C (aPC) induces anti-inflammatory and cytoprotective effects (Schüpbach et al. Blood. 2008, Schüpbach et al. J Thromb Haemost. 2012). The dual function of PAR1 regulates the vascular barrier function and is crucial in pathological conditions such as chronic or acute vascular inflammation. So far, a few PAR1 modulators as vorapaxar or the pepducin PZ-128 have been tested in clinical studies. However, all existing PAR1-modulators lacked sufficient specificity. Despite abundant research conducted, the understanding of the complex mechanisms of PAR1 activation remains incomplete and therapeutic approaches are still lacking.
To regulate the vascular barrier function, we engineered a new PAR1-agonist, a scFv-construct carrying the PAR1 N-terminal sequence corresponding to the thrombin- and aPC-cleaved PAR1 tethered ligand. We examine whether this agonist, similar to the PAR1-activating protease thrombin or aPC respectively, induces the activation of the pro-inflammatory or anti-inflammatory signaling. Next, we aim to investigate if the PAR1-agonist enhances or protects barrier disruption of the endothelium. Therefore, we analyze the secretion of pro- or anti-inflammatory mediators as cytokines and we perform impedance measurements on endothelial cells expressing PAR1 in presence or absence of our new PAR1-agonist.
Project 2: Regulation of thrombin-induced PAR2 activation in pulmonary inflammation
The role of PAR2 seems to be very important in extravascular inflammatory conditions as rheumatic diseases, COPD or ARDS. In this project, we focus on the role of PAR2 on chronic and acute diseases of the airway. Our recently published paper (Heuberger et al. Thromb Res. 2019) provides novel evidence that PAR2 is cleaved by thrombin in presence of the co-factor thrombomodulin and thus induces pro-inflammatory signaling of alveolar type II epithelial cells. Furthermore, we showed that PAR2 sustains the thrombin-induced inflammation over a longer period and thus, we suggest that this sustained PAR2 activation might explain the mechanism of thrombin-induced chronic inflammation.
Based on our novel findings, we are interested on the effects of the PAR2 activation by thrombomodulin-bound thrombin on the alveolar barrier. Therefore, we analyze the effects of PAR2 activation on the barrier integrity in different in vitro lung models. In addition, we aim to clarify the role of PAR2 on the skin and intestinal barrier to promote the development of an efficient PAR2-dependent anti-inflammatory agent.
Project 3: Effects of bacterial proteases cleaving PARs
Commensal as well as pathogenic bacteria secret various proteases that cleave PARs and act as inflammatory modulators. In this project, we test various bacterial proteases that can activate PARs, and thus allow bacteria to penetrate host barriers, or inactivate PARs to prevent inflammatory signaling by the host. Blood infections by certain strains of Streptococcus pyogenes or Staphylococcus aureus still causes millions of death worldwide.
In a recent project, we observed that the streptococcal protease speB interferes with the platelet activation by the inactivation of PAR1 (Ender et al. PLoS One. 2013). Beside streptococcal proteases, we test the cleavage and activation of all PAR1 and PAR2 by staphylococcal proteases and the effect of vascular inflammation and barrier integrity.
Project 4: Role of co-factor-dependent PAR4 activation on platelet activation and vascular inflammation
There is rising evidence that PAR4 is crucial in both, coagulation and inflammation, and thus further research is needed on PAR4. PAR4 is a major thrombin receptor for platelet activation. However, in comparison to PAR1, PAR4 is known to be activated only by very high thrombin concentrations.
For a better understanding of PAR4 activation by thrombin, we test various concentrations for PAR4 cleavage and investigate whether low concentrations of thrombin cleave PAR4 more efficient in presence of thrombin-binding co-factors. We examine, whether the thrombin cleavage of PAR4 results in inflammation of the vascular endothelium and in the disruption of the vascular barrier. Beside inflammation, we are interested in the anti-coagulatory effects of the co-factor-dependent thrombin activation of PAR4.
Schüpbach et al. Blood. 2008 Mar 1;111(5):2667-73.doi: 10.1182/blood-2007-09-113076
Schüpbach et al. J Thromb Haemost. 2012 Aug;10(8):1675-84. doi: 10.1111/j.1538-7836.2012.04825.x.
Ender et al. PLoS One. 2013 Nov 22;8(11):e81298. doi: 10.1371/journal.pone.0081298
Heuberger and Schüpbach Thromb J. 2019 Mar 29;17:4. doi: 10.1186/s12959-019-0194-8.
Heuberger et al. Thromb Res. 2019 May;177:91-101. doi: 10.1016/j.thromres.2019.02.032.
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