Anne Angelillo-Scherrer, MD, Associate Professor Anne.Angelillo-Scherrer@chuv.ch

Research Topics

Our laboratory is interested in signal transduction molecules mediating cellular interactions during important processes such as erythropoiesis, haemostasis and thrombosis, and response to inflammation. We employ several experimental models, most of them in the mouse, which exploit disease models, cell biology, biochemistry and molecular biology.
Over the last years, we have focused in particular on the Tyro3 family of receptor tyrosine kinases and their ligands growth arrest-specific gene 6 (Gas6) and protein S. We have investigated the role of these proteins in erythropoiesis and anemia, haemostasis and thrombosis, and response to inflammation by analyzing knock-out and transgenic mice models.
Gas6 is a secreted vitamin K-dependent protein that can interact with phospholipid membranes by its -carboxyglutamic acid-containing tail and binds to the receptor tyrosine kinases Tyro 3, Axl and Mer by its carboxy-terminal globular domain. It has been implicated in reversible cell growth arrest, survival, proliferation and cell adhesion. Its protein domain organization is similar to protein S, a cofactor for activated protein C. Each of the Gas6 receptors has an extracellular ligand-binding domain and a cytoplasmic tyrosine kinase domain, the activity of which is enhanced by Gas6, leading to intracellular signaling. Gas6 receptors are not only expressed as transmembrane protein, but also cleaved in the extracellular domain to generate soluble receptors. The function of the soluble receptors could be that of Gas6 trapping, thereby inhibiting the interaction of Gas6 with cell-associated receptors. They could also be involved in binding to the cell-associated receptors themselves.

Role of GAS6 in anemia of chronic disease
Anemia of chronic disease is the second most prevalent anemia after iron deficiency anemia. Anemia results from insufficient production, excessive destruction or loss of red blood cells. By limiting the capacity of the blood to carry oxygen, anemia may lead to multiple organs malfunction. Anemia of chronic disease is associated with numerous disorders promoting inflammation such as rheumatoid arthritis and cancer. Correction of anemia in these patients reduces their morbidity and therefore improves their quality of life.
Gas6 is a cell survival factor that amplifies erythropoietin response during response to anemia and a down-regulator of inflammation. In addition, Gas6 insures normal iron levels in blood. Iron is necessary for the generation of new red blood cells in response to anemia but also to replace continuously old red blood cells by new red blood cells. In anemia of chronic disease, iron is diverted from blood into storage tissues (bone marrow, liver, spleen), reducing considerably its availability for the generation of new red blood cells. We therefore hypothesize that Gas6 deficiency favours anemia of chronic disease by inducing a resistance to erythropoietin, creating a proinflammatory environment and impairing iron metabolism.
Consequently, we plan to investigate the role of Gas6 in iron metabolism and its potential use to treat anemia of chronic disease in experimental models. We will also determine whether Gas6 levels in blood might have a diagnostic value for anemia of chronic disease.

Role of GAS6 and proteins pathways in thrombosis and hemostasis
Gas6 and its receptor tyrosine kinases Tyro3, Axl and Mer are present in platelets. We have shown that Gas6 deficient mice are protected against thrombosis, but do not suffer spontaneous bleeding. The antithrombotic mechanism of Gas6 deficiency is at least partly due to platelet dysfunction. More recently we have reported that loss of any one of the Gas6 receptors (Gas6-R), i.e. Tyro3, Axl or Mer, or delivery of a soluble extracellular domain of Axl which traps Gas6, protects mice against life-threatening thrombosis. Loss of a Gas6-R does not prevent initial platelet aggregation, but impairs subsequent stabilization of platelet aggregates, at least in part by reducing “outside-in” signaling and platelet granule secretion. Gas6, through its receptors, activates PI3K and Akt and stimulates tyrosine phosphorylation of the ₃ integrin, thereby amplifying “outside-in” signaling via _IIb3.  Blocking the Gas6-R – _IIb3 integrin crosstalk might be a novel approach for reducing thrombosis.
As the lack of only one Gas6-R is sufficient to induce a platelet dysfunction, we assumed that Gas6-Rs are not redundant in platelets. We thus investigated whether Gas6-Rs form heterodimers. We did no succeed in demonstrating the formation of Gas6-Rs heterodimers in platelets, erythroblasts and endothelial cells. We will then investigate whether Gas6-Rs form a complex.
We intercrossed mice lacking Tyro3, Axl and Mer to obtain double- and triple-knock-out to study the platelet function of mice with only one or none Gas6-R.
In order to better understand the role of protein S in hemostasis, mice totally deficient in mPROS1 will be generated by homologous recombination of the targeting vector. We will generate mice deficient in mPROS1 only in platelets by intercross with platelet factor 4 (PF4) Cre mice (provided by Prof. Radek Skoda, University of Basel, Switzerland). Coagulation, platelet function and thrombus formation will be studied in these mice.

GAS6 blunts systemic inflammatory response syndrome
Sepsis is defined as the systemic inflammatory response syndrome (SIRS) to infection. We found that Gas6 raises in plasma of healthy volunteers in response to endotoxin challenge. In septic patients, high Gas6 levels are associated with fatal outcome. In mice, high Gas6 plasma titer in experimental endotoxemia and microbial peritonitis models correlates with mortality. However, Gas6^-/- mice survival in these models is reduced compared to Gas6^+/+, at least partly because increased plasma levels of numerous cytokines provoke SIRS in Gas6^-/- mice. In vitro, Gas6 secreted by monocytic cells in response to endotoxin blunts further monocytic cells activation by activating Akt and restraining p38 and NFB activation thereby dampening cytokines release. Thus, during endotoxemia, Gas6^-/- mice phenotype resembles that of mice which have undergone PI3K inhibition, pointing to Gas6 as a major modulator of innate immunity.

Prominent role for the gap junction protein connexin37 in hemostasis and thrombosis
Formation of platelet plug initiates hemostasis at sites of vascular injury, and triggers pathological thrombosis in ischemic tissue disease. Although various crucial molecules for platelet function have been identified in recent years, the mechanisms of inter- and intra-cellular signaling leading to the formation of a stable platelet plug are still poorly understood. Connexins form gap junctions, clusters of intercellular channels that are known to synchronize responses in multi-cellular organisms through the direct exchange of ions, small metabolites and other second messenger molecules between adjacent cells. The expression of the gap junction protein connexin37 (Cx37) in mouse and human platelets was studied in collaboration with Professor Brenda Kwak and the Dr Pierre Fontana at the University of Geneva (Switzerland). We observed functional gap junction communication between platelets during platelet aggregation in vitro. Our results demonstrate that Cx37 plays a crucial role in the physiological and pathological signaling in platelets leading to hemostasis and thrombosis. We hypothesize that gap junctions synchronize responses in platelets brought in close contact during their activation, likely by transmitting intracellular signals to neighboring cells.

Dernière modification le 08.04.2013 - Impressum - Informations juridiques