Platelets are small anucleate blood cells that play a central role in thrombosis and haemostasis.  They have also been implicated in inflammation, immunity, vessel development and cancer.  Vital to their role in these pathophysiological processes is the number and reactivity of platelets in the circulation.  Too few or a high threshold of activation can result in excessive bleeding following vessel injury, whereas elevated platelets counts and a low threshold of activation predisposes to thrombosis.  Although much is known about the development of platelet progenitors, called megakaryocytes, at sites of haematopoiesis, regulated primarily by the cytokine thrombopoietin, relatively little is known about the intrinsic and extrinsic cues regulating proplatelet formation and eventual platelet release into the circulation.

Recent findings from our group have established the inhibitory immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor G6b-B as a critical regulator of platelet homeostasis in mice and humans.  Structurally, G6b-B consists of a single extracellular immunoglobulin-variable domain, a transmembrane domain and a cytoplasmic tail containing an ITIM and an immunoreceptor tyrosine-based switch motif (ITSM), tyrosine phosphorylation of which provides a high affinity docking site for the Src homology 2 (SH2) domain-containing protein-tyrosine phosphatases (PTP) Shp1 and Shp2.  Null and loss-of-function mutations in G6b-B results in severe macrothrombcytopenia, mild-to-moderate bleeding, megakaryocyte clusters and focal myelofibrosis in the bone marrow and spleen in mice and humans.  A more severe phenotype is observed in mice lacking Shp1 and Shp2 in the megakaryocyte lineage, demonstrating that Shp1 and Shp2 play important roles in regulating megakaryocyte development that go beyond transmitting inhibitory signals downstream of G6b-B.

We recently discovered that G6b-B binds and is regulated by the heparan sulfate side-chains of the extracellular matrix protein perlecan.  It also binds single chain heparin and heparan sulfate with high affinity, and multivalent heparin, called anti-coagulant anti-platelet (APAC) heparin, has a robust inhibitory effect on platelet activation that is mediated by G6b-B and downstream Shp1 and Shp2.  We have also generated a panel of monoclonal antibodies to the ectodomain of human G6b-B with unique binding properties and biological effects that mimic and antagonize ligand engagement.

In this PhD project, the student will use a combination of mouse models, primary human megakaryocytes and pharmacological-based approaches to study the effects of heparan sulfate-derivatives and monoclonal antibodies targeting G6b-B to investigate how this receptor regulates the number and reactivity of platelets in the circulation.  Findings from this project will provide important new insights into the molecular mechanism controlling platelet production, and provide proof-of-concept for targeting G6b-B in human myeloproliferative disorders and platelet production in vitro for therapeutic application.

Directeur de thèse : Yotis senis / Catherine Strassel

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