LMAN1 serves as a cargo receptor for thrombopoietin

Area of Science:

• Hematology
• Cell Biology
• Molecular Biology

Background:

• Thrombopoietin (TPO) regulates platelet production by binding its receptor on megakaryocytes (MKs).
• The secretion mechanism of TPO from hepatocytes is not well understood.
• Lectin mannose-binding 1 (LMAN1) and multiple coagulation factor deficiency 2 (MCFD2) complex facilitates cargo protein secretion via COPII vesicles.

Purpose of the Study:

• To investigate the role of LMAN1 in TPO secretion and its impact on platelet production.
• To determine if LMAN1-mediated TPO secretion is dependent on MCFD2.

Main Methods:

• Utilized LMAN1-deficient and MCFD2-deleted mouse models.
• Analyzed peripheral blood for platelet counts, size, and morphology.
• Assessed bone marrow for MKs and progenitors.
• Performed hepatocyte-specific and hematopoietic Lman1 deletion studies.
• Measured plasma TPO levels and Tpo mRNA in livers.
• Conducted coimmunoprecipitation assays and analyzed intracellular protein accumulation in cell lines.

Main Results:

• LMAN1-deficient mice showed mild thrombocytopenia, while MCFD2-deleted mice had normal platelet counts.
• Platelet size and granule morphology were normal in LMAN1-deficient mice.
• Hepatocyte-specific Lman1 deletion, but not hematopoietic deletion, caused thrombocytopenia.
• Plasma TPO levels were reduced in LMAN1-deficient mice despite normal Tpo mRNA.
• TPO interacted with LMAN1, and TPO accumulated intracellularly in LMAN1-deleted cells.

Conclusions:

• Hepatocytes are the primary source of TPO production in vivo.
• LMAN1 acts as an endoplasmic reticulum cargo receptor essential for efficient TPO secretion.
• TPO secretion is LMAN1-dependent but independent of MCFD2, representing a novel pathway.