Platelet protein GPIbα plays an important role in regulating platelet-mediated production of thrombopoietin by the liver

Platelets lack a nucleus, and only live for about 10 days. They are produced by bigger cells in the bone marrow called megakaryocytes. Megakaryocytes release platelets into the blood stream continuously to ensure there are adequate numbers of platelets in circulation. On average, an adult produces about 100 billion platelets every day.

Production of platelets from megakaryocytes is regulated by a hormone called thrombopoietin (TPO), which is mostly produced by the liver. TPO circulates in the blood and binds to a receptor on the surface of platelets, called Mpl. Once bound, TPO is taken into the platelet and destroyed. It is thought that in this way, TPO’s control of platelet production can be fine-tuned in response to platelet counts. When platelet counts are high, there is less TPO available to stimulate platelet production from megakaryocytes. Conversely, if platelet counts drop, less TPO will be bound to platelets and destroyed, more TPO will be available, and megakaryocytes will produce more platelets. This inverse correlation between TPO levels circulating in the blood and platelet/megarkaryocyte mass is thought to be one of the major ways platelet production is regulated.

In this study, the researchers examined a platelet surface protein called GPIbα. This is part of a complex of proteins that is important in platelet adhesion and plug formation. People lacking GPIbα have a rare bleeding disorder called Bernard Soulier Syndrome, which is characterized by low platelet counts and abnormally large platelets. In mice lacking GPIbα, the researchers unexpectedly observed two to three times lower levels of TPO than in normal mice, leading them to hypothesize that GPIbα might affect TPO levels.

• In both mice and humans, lacking platelet GPIbα causes less circulating TPO.
• Although mice lacking GPIbα have larger platelets, there was no difference in amount of the Mpl receptor, or in the platelets’ ability to clear TPO. Therefore, the lower TPO levels seen in mice lacking GPIbα is not due to more clearance of TPO by platelets.
• Lower TPO levels in mice lacking GPIbα is due to impaired production of TPO by the liver. The livers of mice lacking GPIbα had about half the amount of TPO RNA compared to normal mice, indicating that their livers produce less TPO. Transfusing mice lacking GPIbα with normal platelets could “rescue” this by increasing levels of circulating TPO and levels of TPO RNA in the liver.
• Platelets lacking GPIbα do not bind well to liver cells, and do not stimulate the liver to produce TPO. Removing sugar residues from platelets of mice lacking GPIbα did not improve TPO production.
• The extracellular domain of GPIbα – the portion of the protein that extends out from the surface of the platelet – is necessary and sufficient for TPO production by the liver.
• Treating normal platelets with antibodies against GPIbα blocks TPO production by the liver.

This study was led by Heyu Ni, a Canadian Blood Services scientist and a professor in the departments of medicine, laboratory medicine and pathobiology, and physiology at the University of Toronto. Dr. Ni is the platform director for hematology, cancer, and immunologic diseases at St. Michael’s Hospital. The research team included members of Dr. Ni’s laboratory (Miao Xu, June Li, Miguel Antonio Dias Neves, Guangheng Zhu, Naadiya Carrim, Ruoying Yu), Canadian Blood Services colleagues (Donald R. Branch and Alan Lazarus), and collaborators from Toronto (Sahil Gupta, John Marshall, Ori Rotstein, John Freedman), China (Jun Peng, Ming Hou), Japan (Shinji Kunishima), and the United States (Jerry Ware, Zaverio M. Ruggeri).