Stromal STAT5-Mediated Trophic Activity Regulates Hematopoietic Multipotent Progenitor Niche Factors

Signal transducer and activator of transcription 5 (STAT5a and STAT5b isoforms) are latent transcription factors that are intrinsically critical for normal hematopoiesis but are also expressed in bone marrow stromal cells. However, despite known STAT5a (prolactin) and STAT5b (growth hormone) functions in breast and liver tissue respectively, hematopoiesis-supporting stromal functions have not been explored. Here, floxed STAT5ab double knockout (KO) was generated with a variety of hematopoietic and stromal Cre-expressing transgenic mouse strains to test whether STAT5a or STAT5b play a role in extrinsic regulation of hematopoiesis. Pan-hematopoietic deletion with Vav1-Cre was the positive control for loss of multipotent hematopoietic function but surprisingly Vav1-Cre also deleted STAT5ab in CD45neg cells, dysregulated stromal niche factor mRNA expression, and altered MSC osteo/chondro-lineage differentiation priming. Single cell transcriptome analysis of bone marrow from wild-type or Vav1-Cre KO mice long-term repopulating hematopoietic stem cells (LT-HSC) showed upregulated oxidative phosphorylation (P<10^-10) and protein translation (P<10^-39) genes in LT-HSCs and throughout myeloid-primed clusters, while in contrast common lymphoid progenitor-like cells were completely eliminated. Similarly, like Vav1+ cells and based on scRNAseq analysis, Nes+ cells were also detected in both CD45neg and CD45+ cell clusters. Deletion of STAT5ab with Nes-Cre reduced the number of LT-HSCs (30% reduction compared to Nes-Cre control mice, P=0.02, N=4) and caused hematopoietic repopulating defects. To follow up on these promiscuous Cre promoter deletions in CD45+ and CD45neg bone marrow cell populations, additional stroma-specific Cre strains (LepR, Osx, Ocn, Prrx1, Nes-CreERT2) as well as endothelial-specific (Cdh5) strains were generated lacking STAT5ab. Viable mice were assessed for peripheral blood hematology, competitive repopulation, as well as hematopoietic stem/progenitor subsets (LT-HSC, MPP1, MPP2, MPP3, MPP4, KLS) using well-accepted flow cytometry antibody panels. Deletion of STAT5 in stromal cells with LepR-Cre significantly reduced MPP4 (35% reduction, P=0.025, N=4). Overall, extrinsic stromal (but not endothelial) cells were deficient in supporting multipotent and lineage-committed hematopoietic progenitors. However, peripheral blood hematology was rescued through homeostatic production. Gene regulation profiles resembled that of cytokine-starved MPPs. Functional support for niche-supporting activity was also assessed using STAT5ab KO MSCs in vitro. With Lepr-Cre mediated STAT5ab deletion, bone marrow niche factor mRNAs were downregulated (Cxcl12, Angpt1, Igfbp4, Igf1, Il34) and support of multipotent hematopoietic cells was significantly impaired. ELISA assay showed that SDF-1a was reduced 3-fold (P<0.001, N=5) and IGF-1 was reduced 2-fold (P<0.01, N=5) compared to wild type MSCs. Inversely, mRNAs for Kitl and Grem1 were increased in the same stromal cells, consistent with loss of multipotency and mixed HSC vs. MPP stromal support activities. Furthermore, novel scRNAseq analysis on simultaneous bone marrow hematopoietic and stromal cells compared wild-type and KO mice. Differentially expressed gene sets with Maximum Mean Discrepancy (MMD) co-expression revealed a key central role for coordinated STAT5ab/Cish balance as a biomarker of the cellular progenitor activity state (metabolic, protein translation, survival) associated with multipotency, comparing KO to wild-type cells. Importantly, Cish was strongly co-expressed in cells primed for lineage-specific differentiation (MSC - osteo/chondrogenic-primed; HSC - myeloid-primed) with STAT5ab co-expressed genes being associated with multipotent potential. Therefore, novel STAT5ab/Cish-mediated gene regulation programs operate within the bone marrow microenvironment to mediate cytokine production and responsiveness. This work highlights the importance of examining heterotypic cell types within bone marrow to elucidate functional cross-talk underlying hematopoietic homeostasis and defines conserved STAT5ab-mediated gene signatures as potential biomarkers to test in human hematopoietic and immune cell disorders.

No relevant conflicts of interest to declare.