8709 FABP4+ Bone Marrow Endothelial Cells Support Hematopoietic Stem Cell Niches And Osteogenesis In Response To Vascular Insufficiency

Abstract Disclosure: Y. Kim: None. M. Song: None. H. Sun: None. S. Cho: None. Background: Numerous non-communicable and aging-related diseases are linked to systemic vascular insufficiency. The consequences of vascular insufficiency in bone marrow extend to compromising osteogenesis and hematopoiesis, with a more pronounced impact on aging individuals compared to younger counterparts affected by these conditions. Recent advances in scRNA-seq have shed light on the cellular taxonomy in bone marrow; however, a comprehensive understanding of cellular complexity requires further elucidation, and there are limitations in delineating impaired osteogenesis and hematopoiesis resulting from vascular insufficiency. In this study, we employed a combined approach of scRNA-seq and intensive ex vivo investigations to elucidate the regeneration process of bone marrow microenvironment following vascular insufficiency. Method Female C57BL/6 mice at different ages (6-wkks and 14 weeks) were subjected to sham (sham group) or right femoral artery ligation surgery (ischemia group). Reperfusion duration was quantified by color laser doppler. Osteogenesis was assessed by H&E staining, DXA and microCT scan. Flow cytometry was employed to count the number of type H endothelial cell (THEC, Endomucinhi). Bone marrow regeneration capacity was examined in vitro by treating with conditioned medium (CM) of THEC and in vivo via THEC injection. scRNA-seq was performed on the bone marrow of both sham and ischemia group 7 days post-ischemia. Result In young mice, reperfusion occurred within 7 days and osteogenesis was not different between sham and ischemia group. However, in old mice, reperfusion extended to 6 weeks, and accompanied by a reduction in osteogenesis, which did not recover until 12 weeks post-ischemia compared to sham group. The percentage of THEC increased after ischemia in young mice from 1.01% to 3.12 % (n=10, p<0.05), while those were comparable between sham and ischemia group in old mice (from 0.66% to 1.02%, n = 10, p=0.56). in MC3T3-E1 preosteoblast cell line, treatment with THEC-CM upregulated gene expressions associated with bone formation such as Alpl, Bglap, and Col1a1. In vivo THEC injection in old mice following ischemia surgery resulted in a reduction in reperfusion duration and an improvement in osteogenesis. scRNA-seq of bone marrow identified a significant increase of bone marrow stromal cells (BMSCs) and Cxcl12-abundant reticular (CAR) cells 7-days post-ischemia, which provides favorable microenvironment for dormant hematopoietic stem cells (HSCs). The number of FABP4+ THEC synchronized with the increase in BMSCs and CARs. In addition, FABP4+ THEC expressed Col1a1 and Col1a2, associated with osteogenesis, along with Cxcl12 and Foxc1, linked to HSC niches. Conclusion: FABP4+ THECs contribute to favorable microenvironment for HSCs and play a pivotal role in angiogenesis-osteogenesis coupling following vascular insufficiency. Presentation: 6/1/2024