Blood platelets play an essential role in stopping bleeding but are also directly responsible for arterial thrombosis. Platelet activation is accompanied by the secretion of the contents of their granules alpha and granules dense. This step is all the more important as these storage organelles contain proteins and agonists that play a role in both platelet activation and coagulation, but also in angiogenesis and inflammation. In addition, pathologies resulting from abnormal alpha and / or dense granules are associated with high bleeding risk.
Despite the importance of platelet granules in different physiological processes, little is known about their biogenesis. Platelet granules belong to a group of organelles related to lysosomes (LRO for Lysosome Related Organelles), which derive from the endocytic pathway. In addition to platelet granules, this group includes, among others, melanosomes, pigmentary organelles of melanocytes of the skin and ocular cells, and other organelles that regulate lung plasticity or immunity. Some LROs are malformed in Hermansky-Pudlak (HPS) syndromes, a group of rare diseases characterized by partial albinism, excessive bleeding and often lethal pulmonary fibrosis.
The molecular bases by which the molecules are selectively sorted and directed to the different cellular compartments in the megakaryocyte are not clearly established. The project will focus on the study of a group of proteins playing a role of “molecular switch”, Rab proteins. After activation, these proteins are able to bind a variety of effectors that play a role in membrane fusion or fission, or transport along the cytoskeleton. In particular, the Rab32 and Rab38 proteins play a key and partially redundant role in the maturation of melanosomes, the pigmentary organelles of melanin-synthesizing cells, by making it possible to bind AP-3 molecular adapter proteins. These proteins also play a role in the biogenesis of dense granules, but the data are less clear and rely solely on tumor lines in vitro.
The project will aim to determine 1) the respective localization of Rab32 and Rab38 proteins in the organelles of the megakaryocytes native to the marrow; 2) their respective roles during the various stages of platelet granule biogenesis and / or granule fusion; 3) the effector partners of these proteins; 4) the possible repercussions of the absence of one or both proteins on the platelet functions. The work will be based on the study of mouse models of HPS diseases (knockout mice for Rab32, Rab38 as well as double knockout proteins) by in vivo, in situ, ex vivo and in vitro approaches.
This work will make it possible to understand one of the essential mechanisms for the functionality of future platelets, with the possibility of targeting by genetic manipulation the secretory function to fight against thromboses or haemorrhages, or to use granules to convey molecules for therapeutic purposes. From a clinical point of view, this work will only test the possibility that many patients with platelet disease of unknown etiology may have mutations in the molecules involved in the biogenesis or fusion of granules. More generally, this work will improve knowledge in cell biology concerning the transport and distribution of organelles in eukaryotic cells.

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