HSPCs Build Their Bone Marrow Niche

Objectives:The hematopoietic stem cell (HSC) is a kind ofsomatic stem cells,which has been studied and clinically utilized for a long time. Because of the complex and dynamic molecular interaction between HSCs and their endogenous microenvironment, known as "niche", it is still quite important and difficult for us to find effective approachesformaintaining and expanding HSCs in vitro. In 1978, the regulation of HSC function by the stem cell niche was first postulated by Ray Schofield. With the constant development of technological and experimental approaches, the nature of bone marrow niche, the specific function of different niche components and the role of bone marrow niche involved in the regulation of hematopoiesis and in the development of hematopoietic disease have been determined gradually. Both endosteal niche and perivascular niche have been described to maintain the HSCs in bone marrow, but the origin of the niche components remains largely unknown. Here, we demonstrate that hematopoietic stem and progenitor cells (HSPCs), identified using genetic and functional markers pertinent to autophagy, are be the parental source of the bone marrow niche in postnatal mice. Specifically, this study is designed to (1) determine the effect of lethal irradiation for transplantation on both hematopoietic cells and non-hematopoietic niche cells; (2) investigate the origin of the niche components after HSPC transplantation; (3) examine the ability of HSPCs on the reconstruction of bone marrow niche.Methods:(1) The flow cytometry and CCK-8 assay were used to analyze the bone marrow cells from 9 Gy irradiated C57BL/6 mice at the indicated times to determine the effect of lethal irradiation for transplantation on both hematopoietic cells and non-hematopoietic niche cells in mice; (2) Wild-type (WT) and Atg7f/-f GFP-LC3 mice were irradiated, and transplantation was performed using 1×105 bone marrow LSK HSPCs from these mice. The flow cytometry and genotyping were performed to investigate the possible origin of the niche components after the LSK HSPC transplantation; (3) Micro-CT, flow cytometry, HE staining and immunohistochemistry of bone marrow section, and real-time RT-PCR were used to analyze the bone marrow microenvironment and niche cell composition in Atg7f/f;Vav-Cre mice, in which the HSC function is undermined due to the selective deletion of the autophagy-essential gene Atgl in the entire hematopoietic system; (4) competitive transplantation and non-competitive transplantation were performed to examine the ability of autophagy normal or defective HSPCs on the reconstruction of the bone marrow niche.Results:(1) Besides depletion of the hematopoietic cells, lethal irradiation (9 Gy) could also obliterated mesenchymal stem/stromal cells, CAR cells and endothelial cells in the bone marrow; (2) bone marrow stromal cells and endothelial cells in the host could be generated from the donor HSPCs; (3) HSPCs with autophagy defects led to altered bone marrow architecture and abnormal niche cell populations as well as dysfunction in Atg7f/f;Vav-Cre mice; (4) autophagy-impaired HSPCs were unable to reconstitute niche populations and restore hematopoiesis in the bone marrow of the irradiated hosts, whereas wild-type HSPCs were able to do so in the autophagy defective irradiated Atg7-/- Host.Conclusions:The postnatal HSPCs have the ability to build the bone marrow niche, which is autophagy-dependent.