The hematopoietic stem cells supportive megakaryocytes as shapers of the bone marrow niche.

The bone marrow (BM) niche plays a pivotal role in regulating the fate of hematopoietic stem cells (HSCs), and its integrity changes significantly during aging and in rare hematological disease, as in myelofibrosis (MF). In this study, we investigated how the localization and dynamics of HSCs are influenced under physiological and pathological conditions by a newly identified by HSC-supportive megakaryocytes (MKs) subpopulation. Using huCD34tTA/TetO-H2BGFP reporter mice, we analyzed HSCs distribution within the BM and quantified nuclear green fluorescent protein (GFP) intensity to assess the repopulating potential of aged controls and mutated Gata1low mice for MF. In the control group of aged mice, cells with high levels of GFP are clustered, and adjacent to cells morphologically identifiable as supportive MKs. These clusters displayed homogeneous GFP intensity, indicating that HSCs with similar functional properties tend to co-localize in proximity to supportive MKs. By contrast, in aged huCD34/TET/Gata1low mice, GFP cells were predominantly isolated and showed reduced fluorescence intensity. Although the frequency of MKs with a supportive phenotype was increased in MF mice, analyses of GFP revealed that the ability of these MKs to maintain the HSCs in their niche was significantly impaired. Our results provide new insights on the maladaptive remodeling of the BM niche. They highlight the supportive role of MKs as potential key regulators of HSCs homeostasis. Despite their numerical expansion in MF, these cells are functionally compromised, thereby contributing to altered HSCs localization, mobilization, and to hematopoietic failure.