The regulation of thrombopoietin levels

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Project Summary Thrombopoietin (TPO), a plasma glycoprotein made in hepatocytes, binds to its cell surface receptor MPL expressed on megakaryocytes and megakaryocyte progenitors, promoting cell proliferation and maturation. In addition to its role in megakaryocyte development and platelet production, TPO also plays a critical role in hematopoietic stem cell survival and maintenance. In humans, gain-of-function mutations in TPO result in autosomal dominant thrombocytosis (high platelet counts), while loss-of-function mutations in TPO (or in its receptor MPL) result in congenital amegakaryocytic thrombocytopenia, a disease characterized by low platelet counts and absence of bone marrow megakaryocytes at birth with subsequent bone marrow aplasia and failure. The transcriptional regulation and plasma clearance of TPO have been well studied. In contrast, the mechanism by which TPO is secreted from hepatocytes and by which TPO is degraded intracellularly remain unknown. We have shown in preliminary results that mice deficient in LMAN1 exhibit thrombocytopenia and that the thrombocytopenia is recapitulated in mice with hepatocyte-specific LMAN1 deletion but not in mice with hematopoietic-specific LMAN1 deletion. We have additionally shown that the plasma TPO level is low in LMAN1 deficient mice (~50% of normal). Taken together with LMAN1’s known function as an endoplasmic reticulum cargo receptor, these results strongly suggest that the thrombocytopenia observed in LMAN1 deficient mice is due to impaired secretion of TPO from hepatocytes. Consistent with these findings in mice, we have shown that LMAN1 deficiency results in intracellular accumulation of TPO in human cells. Therefore, the role of LMAN1 in regulating TPO appears to be conserved in mice and humans. In this proposal, we aim to define the role of LMAN1 in regulating the plasma TPO levels in mice and to dissect the mechanism by which LMAN1 regulates TPO secretion, both under a normal physiological state and in the setting of enhanced TPO production. In additional preliminary results, we have performed an unbiased genome-scale CRISPR knock-out screen to identify novel regulators of the intracellular TPO level. This screen, performed in biological triplicates, followed by validation experiments, demonstrated that deletion of UBE3C results in intracellular accumulation of TPO but no intracellular accumulation of any of the 3 control secretory proteins tested. UBE3C is an E3 ubiquitin ligase, suggesting that TPO is a substrate for UBE3C and that in the absence of UBE3C, TPO ubiquitination is impaired, resulting in reduced degradation. Therefore, an additional aim of this proposal is to define the role of UBE3C in regulating the intracellular TPO level, both under steady state TPO production and in a state of high TPO production. This proposal has important implications for understanding the mechanisms of TPO regulation at the level of its intracellular trafficking/secretion and at the level of its intracellular degradation. Knowledge gained from this proposal may lay the foundation for the development of novel strategies to therapeutically regulate the plasma TPO level.