Depletion of microenvironmental syndecan-2 impairs hematopoietic stem cell self-renewal and cytokine responses

Syndecan-2 is a heparan sulfate proteoglycan highly enriched on murine bone marrow hematopoietic stem cells (HSCs) compared to terminally differentiated hematopoietic cells. Syndecan-2 binds growth factors via its heparan sulfate glycosaminoglycan chains to coordinate cell signaling. Knockdown of syndecan-2 reduces HSC self-renewal ability and promotes cell cycling via Cdkn1c . In this study, we analyzed the function of syndecan-2 expressed by bone marrow niche cells in hematopoiesis and HSC self-renewal. We determined that syndecan-2 is highly expressed by bone marrow mesenchymal stromal cells and moderately expressed by endothelial cells. To test the function of niche-expressed syndecan-2 in hematopoiesis, we generated transgenic mice depleted of Sdc2 in Lepr- targeted mesenchymal stromal cells ( Sdc2 ^ΔMSC mice) or Cdh5 -targeted endothelial cells ( Sdc2 ^ΔEC mice). Loss of syndecan-2 from endothelial or mesenchymal stromal cells did not change bone marrow HSC frequencies or numbers. However, depletion of syndecan-2 from Lepr -targeted mesenchymal stromal cells, but not Cdh5 -targeted endothelial cells, diminishes HSC self-renewal ability analyzed by competitive transplants into lethally irradiated mice. Ex vivo studies further show that HSCs co-cultured with HS-5 stromal cells depleted of SDC2 exhaust more rapidly than HSCs cultured with control HS-5 cells. Single-cell RNA sequencing analyses reveal that the depletion of Sdc2 from mesenchymal stromal cells significantly remodels the HSC transcriptome by enriching for pathways associated with excessive growth factor signaling. Together, our findings suggest that HSC self-renewal is supported by cell-extrinsic mechanisms enacted by syndecan-2 from the MSC niche, highlighting the importance of the niche proteoglycome in HSC functions.