GABA produced by multiple bone marrow niche cell types regulate hematopoietic stem and progenitor cells
Hematopoietic stem and progenitor cells (HSPCs) maintain homeostasis of the blood system by balancing proliferation and differentiation. Many extrinsic signals in the bone marrow (BM) microenvironment that regulate this balance are still unknown. We report gamma aminobutyric acid (GABA) metabolite produced in the BM as a  regulator of HSPCs . Deletion of the glutamate decarboxylase enzymes (Gad1/2) that produce GABA in either B lineages or BM endothelial cells (BMECs) leads to a moderate reduction in BM GABA levels and HSPC number, suggesting both cell types are GABA sources. However, simultaneous blockade of GABA production from both hematopoietic cells and BMECs resulted in a greater reduction of both GABA levels and HSPC numbers. Lower GABA levels in the BM altered the gene expression profile of HSPCs, with expression reduced for proliferation-associated genes and increased for B lineage genes . Our findings suggest GABA from multiple sources coordinates to regulate HSPC activity. Overall design: RNA-seq profiling of HSPCs from 8-12 week-old wild-type or Cd19-cre+;Gad1/2fl/fl mice, Cdh5-creERT2+;Gad1/2fl/fl mice and Tie2-cre+;Gad1/2fl/fl mice (triplicates for each group) .