Abstract

Thrombopoietin (TPO), a hematopoietic growth factor produced predominantly by the liver, is essential for thrombopoiesis. Prevailing theory posits that circulating TPO levels are maintained through its clearance by platelets and megakaryocytes via surface c-Mpl receptor internalization. Interestingly, we found a two- to threefold decrease in circulating TPO in GPIba2/2 mice compared with wild-type (WT) controls, which was consistent in GPIba-deficient human Bernard-Soulier syndrome (BSS) patients. We showed that lower TPO levels in GPIba-deficient conditions were not due to increased TPO clearance by GPIba2/2 platelets but rather to decreased hepatic TPO mRNA transcription and production. We found that WT, but not GPIba2/2, platelet transfusions rescued hepatic TPO mRNA and circulating TPO levels in GPIba2/2 mice. In vitro hepatocyte cocultures with platelets or GPIba-coupled beads further confirm the disruption of platelet-mediated hepatic TPO generation in the absence of GPIba. Treatment of GPIba2/2 platelets with neuraminidase caused significant desialylation; however, strikingly, desialylated GPIba2/2 platelets could not rescue impaired hepatic TPO production in vivo or in vitro, suggesting that GPIba, independent of platelet desialylation, is a prerequisite for hepatic TPO generation. Additionally, impaired hepatic TPO production was recapitulated in interleukin-4/GPIba–transgenic mice, as well as with antibodies targeting the extracellular portion of GPIba, demonstrating that the N terminus of GPIba is required for platelet-mediated hepatic TPO generation. These findings reveal a novel nonredundant regulatory role for platelets in hepatic TPO homeostasis, which improves our understanding of constitutive TPO regulation and has important implications in diseases related to GPIba, such as BSS and auto- and alloimmune-mediated thrombocytopenias. (Blood. 2018;132(6):622-634)