The human body produces and removes 1011 platelets daily to maintain a normal steady-state platelet count, and the level of production can be greatly increased under conditions of platelet destruction. We provided the first evidence that survival of platelets is intimately tied to surface glycans and have shown that platelets with impaired Siaa2-3Galb1-4GlcNAc (LacNAc) structures are removed in the liver by Kupffer cells and hepatocytes. Here, we continue to investigative this path and will dissect how targeting to hepatocytes or Kupffer cells is regulated.
Aim 1 proposes to define the differential roles of hepatic and macrophage galactose- binding receptors in terminating platelet circulation and will dissect the contribution of O- and N-linked glycan recognition by each of these receptors. We will survey platelet glycoproteins from mice deficient for the hepatic asialoglycoprotein receptor2 (Asgr2), the macrophage galactose lectin (MGL), or both to define the glycan- bearing platelet ligands for the Asgr and MGL, as these platelets carry increased levels of desialylated proteins because of diminished clearance. Further, use of mice deficient in sialyltransferase 3 (ST3) Gal-I or Gal-IV that normally sialylate N-linked or O-linked glycans will decipher the relative roles o desialylated O- and N-linked glycans as ligands for these galactose receptors. Characterization of the desialylated targets will reveal which are normally modified in circulating blood. This data will be compared to those found in stored blood. Lastly, we will establish if desialylation of O- o N-linked glycans promotes GPIba release by TACE. Our work further suggests the existence of novel and continuous crosstalk between platelets, liver cells and bone marrow. Liver regeneration depends on the delivery of factors to hepatocytes by platelets and on the expression of the Asgr1/2.
Aim 2 will address the role of O- and N-linked asialoglycans in platelet ingestion by hepatocytes and determine the functional consequences focusing on how they influence hepatocyte homeostasis as well as remodeling following liver injury. In addition we will begin to investigate how impaired glycan-mediated platelet uptake by hepatocytes leads to bone marrow abnormalities. We will carefully determine the proportion of hematopoietic stem cells (HSCs) and osteoblastic niche cells and stromal cells in Asgr2-/-, ST3GalIV-/- and WT bone marrows (BMs). The hypothesis that systemic signals altered in the absence of Asgr1/2 regulate BM homeostasis will be solidified, and we will begin to identify these systemic factors.
The only current solution to thrombocytopenia is platelet transfusion and procedures to obtain and store platelets lead to damage that targets many for removal. The understanding of the underlying mechanisms of platelet removal is crucial for development of novel concepts to improve platelet transfusion as treatment of thrombocytopenia.
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