HSPCs express GABA B receptor Gabbr1 and B-cell lineages express glutamate decarboxylase Gad1. (A) RT-qPCR of relative Gabbr1 expression in sorted HSPC populations. (B) Published single-cell RNA-seq data detected Gabbr1 in a subset of HSPCs.²²  (C) Representative flow cytometry data and bar graph showing ∼12% of Lin⁻Sca1⁺cKit⁺ (LSK) progenitors, and ∼15% of HSCs (LSK/CD48⁻/CD150⁺) express cell-surface Gabbr1 receptor. (D) RT-qPCR analysis of Gad1 expression in sorted phenotypic B cells (B220⁺), CD11b⁺ cells, osteoblastic cells (OBCs), and BM endothelial cells (BECs) (n = 3, from 2 independent experiments). (E) Representative intracellular flow cytometry data and bar graph showing that 77.7% of B220⁺/CD93⁻ cells express Gad1, compared with CD11b⁺ cells that express almost none (n = 3, from 2 independent experiments). ***P < .001. (F) Immunofluorescent antibody staining (scale bar, 20 μm) of sternum BM showing that Gad1 (green) is expressed in B220⁺ (red) B cells (arrowheads point to cells that express both markers). Gad1 is not expressed in Cd11b⁺/Gr1⁺ myeloid cells (representative of n = 9 sternum regions from n = 4 mice) (see also supplemental Figure 1). CMP, common myeloid progenitor; DAPI, 4′,6-diamidino-2-phenylindole; GMP, granulocyte-macrophage progenitor; LT-HSC, long-term HSC; MEP, megakaryocyte-erythroid progenitor; MPP, multipotent progenitor; N.D., not determined; Prog, progenitor.