Diacylglycerol Enhances Hematopoietic Stem/Progenitor Cell Development and Engraftment Via MAP Kinase Activation

Hematopoietic stem and progenitor cells (HSPCs) are exposed to a variety of intrinsic and extrinsic factors regulating all processes needed during development, and for successful engraftment after transplantation. In order to decipher the molecular pathways that may promote engraftment of HSPCs after marrow transplantation, we performed a competitive transplantation screen using chemical genetics in zebrafish. Green fluorescent protein-labeled kidney marrow cells (equivalent to mammalian bone marrow cells) were treated ex vivo with single compounds of a chemical library of known biologically active compounds, and administered by retro-orbital venous injection to lethally irradiated recipient zebrafish. About 500 chemicals were screened. Untreated kidney marrow cells labeled with a red fluorescent protein were used as competitors. Imaging-based assessment of short-term engraftment demonstrated that 1,2-Didecanoylglycerol, a membrane permeable but non-physiologic analogue of diacylglycerol (DAG), significantly improved engraftment compared to competitor cells. Follow-up by FACS analysis showed a 3.5 fold increase of long-term repopulating units after DAG treatment. To interrogate whether DAG treatment not only affects HSPCs under transplant conditions, but also during normal embryonic development, we treated zebrafish embryos within the time window of HSC formation in the dorsal aorta. DAG treatment increased expression of the HSPC markers Runx1 and c-myb in the AGM (Aorto-Gonad-Mesonephros). Treatment after HSC specification also led to an upregulation of HSPC markers in the caudal hematopoietic tissue (equivalent to fetal liver in mammals). These data suggest that DAG affects not only HSC formation, but also migration and engraftment of HSPCs as hematopoiesis transitions from the AGM to the CHT during development. To determine whether HSPCs respond to DAG in a cell autonomous manner, and to identify the underlying molecular mechanism, we treated human CD34+ cells from umbilical cord blood with DAG and performed RNA-seq analysis. Ingenuity Pathway Analysis of the 395 differentially expressed genes (q-value < 0.05) implicated the MAP kinase pathway as an upstream regulator. Human Phosphokinase array analysis of treated CD34+ showed ERK 1/2 activation. DAG is known to activate Protein Kinase C (PKC) with subsequent Raf kinase phosphorylation, which has the potential to activate ERK. Co-treatment of CD34+ cells with DAG and the ERK inhibitor PD98059 blocked upregulation of downstream ERK-targets (e.g. AREG, CSF2, EGR1, HMOX, SERPINE1, DUSP4, DUSP6), whereas the PI3K family inhibitor LY294002 and the p38 MAP kinase inhibitor SB202190 did not alter the effect of DAG on expression of these genes. This demonstrates that DAG activates ERK and its downstream targets. Our competitive marrow transplantation-based chemical screen has led to the discovery of 1,2-Didecanoylglycerol as a novel modulator of HSPC development and engraftment after transplantation. This discovery may be of clinical relevance to marrow or cord blood hematopoietic stem cell transplantation.