We hypothesized that megakaryocyte (MK) phosphoinositide signaling mediated by phosphatidylinositol transfer proteins (PITPs) contributes to hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) regulation. Conditional knockout mice lacking PITPs specifically in MKs and platelets (pitpa2/2 and pitpa2/2/b2/2) bone marrow (BM) manifested decreased numbers of HSCs, MK-erythrocyte progenitors, and cycling HPCs. Further, pitpa2/2/b2/2 BM had significantly reduced engrafting capability in competitive transplantation and limiting dilution analysis. Conditioned media (CM) from cultured pitpa2/2 and pitpa2/2/b2/2 BM MKs contained higher levels of transforming growth factor b1 (TGF-b1) and interleukin-4 (IL-4), among other myelosuppressive cytokines, than wild-type BM MKs. Correspondingly, BM flush fluid from pitpa2/2 and pitpa2/2/b2/2 mice had higher concentrations of TGF-b1. CM from pitpa2/2 and pitpa2/2/b2/2 MKs significantly suppressed HPC colony formation, which was completely extinguished in vitro by neutralizing anti–TGF-b antibody, and treatment of pitpa2/2/b2/2 mice in vivo with anti–TGF-b antibodies completely reverted their defects in BM HSC and HPC numbers. TGF-b and IL-4 synergized to inhibit HPC colony formation in vitro. Electron microscopy analysis of pitpa2/2/b2/2 MKs revealed ultrastructural defects with depleted a-granules and large, misshaped multivesicular bodies. Von Willebrand factor and thrombospondin-1, like TGF-b, are stored in MK a-granules and were also elevated in CM of cultured pitpa2/2/b2/2 MKs. Altogether, these data show that ablating PITPs in MKs indirectly dysregulates hematopoiesis in the BM by disrupting a-granule physiology and secretion of TGF-b1.
ASJC Scopus subject areas
- Cell Biology