Life science
Medical & healthcare, Drug development

Niche cells essential for hematopoietic stem cell maintenance andhematopoiesis and adipo-osteogenesis in bone marrow
Graduate School of Frontier Biosciences/Graduate School of Medicine/Immunology Frontier Research Center
Distinguished Professor Takashi Nagasawa

https://researchmap.jp/read0094728?lang=en

Associated Professor Yoshiki Omatsu		

https://researchmap.jp/yskomt?lang=en

Life science

Abstract
Special microenvironments known as niches are essential forthe maintenance of hematopoietic stem cells (HSCs) and lympho-hematopoiesis within bone marrow. We found that CAR cells
are the major producer of CXCL12 and SCF and the major cellularcomponents of niches for HSCs and immune cells, that CAR cells
are mesenchymal stem cells, which give rise to adipocytes andosteoblasts, and that the transcription factors, Foxc1 and Ebf3 are
preferentially expressed in CAR cells and play a critical role in theformation and maintenance of niches for HSCs and immune cells,
inhibiting differentiation of CAR cells into adipocytes and osteoblasts, respectively. Furthermore, we found that CAR cells require
the transcription factors Runx1 or Runx2 to prevent their fibrotic
conversion and maintain HSCs and hematopoiesis in adults. Recently, we identified the human counterpart of CAR cells, enabling
the evaluation of their alterations in various hematological disorders
by flow cytometric and histological analyses.

▍ Background & Results
Special microenvironments known as niches are essential for themaintenance of hematopoietic stem cells (HSCs), which give riseto all blood cells, within bone marrow. We identified a population offibroblastic reticular cells expressing CXCL12 at high levels, termedCXCL12-abundant reticular (CAR) cells within murine bone marrowand found that CAR cells are the major producer of CXCL12 and SCF,
and the major cellular components of niches for HSCs and immunecells. In addition, we determined the nature of CAR cells, showingthat CAR cells are mesenchymal stem cells, which give rise to adipocytes and osteoblasts and that transcription factors, Foxc1 and Ebf3
are preferentially expressed in CAR cells and play a critical role in
the formation and maintenance of niches for HSCs and immune cells,
inhibiting differentiation of CAR cells into adipocytes and osteoblasts,respectively. Recently, we found that CAR cells require the transcription factors Runx1 or Runx2 to prevent their fibrotic conversion andmaintain HSCs and hematopoiesis in adults. Here, we showed thepresence of cells expressing much higher CXCL12 than other cells
in human adult bone marrow. CXCL12hi cells express the receptorof leptin (LepR) and most CXCL12hi cells or LepR+ cells expressedhigh levels of SCF, FOXC1, and EBF3 and contained cells, which had
the potential to differentiate into adipocytes and osteoblasts. Histologically, the nuclei of CXCL12hi cells were identified and quantified
by EBF3 expression in fixed marrow sections. CXCL12hi cells sorted
from residual bone marrow aspirates of chronic myeloid leukemia patients expressed reduced levels of CXCL12, SCF, FOXC1, and EBF3
in correlation with increased leukemic burden. Together, the humancounterpart of CAR cells are major components of non-hematopoietic
and non-endothelial cells in the bone marrow in humans.

▍ Significance of the research and Future perspective
We identified the human counterpart of CAR cells, enabling theevaluation of their alterations in various hematological disorders byflow cytometric and histological analyses. For clinical application,
our results raise the possibility that activators and/or inhibitors of
CAR cell functions might be applied to novel non-cell-autonomous
therapies targeting the niches for HSCs and immune cells in hematological disorders and infection.

Figure 1. CAR cells are the major cellular component of HSC niches.
Foxc1 and Ebf3 are preferentially expressed in CAR cells and play a critical role in theformation and maintenance of niches for HSCs, inhibiting differentiation of CAR cells into
adipocytes and osteoblasts, respectively. CAR cells require the transcription factors Runx1 orRunx2 to prevent their fibrotic conversion and maintain HSCs and hematopoiesis in adults.

Figure 2. CAR cells are present in human bone marrow.
The human counterpart of CAR cells express LepR and are major components ofnon-hematopoietic and non-endothelial cells in the bone marrow in humans.

Figure 3. Human CAR cells can be observed in fixed marrow sections.
Histologically, the nuclei of CXCL12hi cells were identified and quantified by EBF3 expression infixed marrow sections. CAR cells expressing CD271 (green) and EBF3 (red) have long processes.

P a t e n t
Omatsu, Yoshiki; Aiba, Shota; Nagasawa, Takashi et al. Runx1 and Runx2 inhibit fibrotic conversion of cellular niches for hematopoietic stem cells. NatCommun. 2022;13(1), 2654. doi: 10.1038/s41467-022-30266-yAoki, Kazunari, Kurashige, Masako; Nagasawa, Takashi et al. Identification of CXCL12-abundant reticular cells in human adult bone marrow. Br. J.
Haematol. 2021 May;193(3), 659-668. doi: 10.1111/bjh.17396
Seike, Masanari; Omatsu, Yoshiki; Nagasawa, Takashi et al. Stem cell niche-specific Ebf3 maintains the bone marrow cavity. Genes Dev. 2018, 32(5-6),
359-372. doi: 10.1101/gad.311068.117
Tr e a t i s e
Omatsu, Yoshiki; Seike, Masanari; Nagasawa, Takashi et al. Foxc1 is a critical regulator of haematopoietic stem/progenitor cell niche formation. Nature2014, 508, 536-540. doi: 10.1038/nature13071
Omatsu, Yoshiki; Sugiyama, Tatsuki; Nagasawa, Takashi et al. The essential functions of adipo-osteogenic progenitors as the hematopoietic stem andprogenitor cell niche. Immunity 2010, 33, 387-399. doi: 10.1016/j.immuni.2010.08.017
Sugiyama, Tatsuki; Kohara, Hiroshi; Nagasawa, Takashi et al. Maintenance of the hematopoietic stem cell pool by CXCL12-CXCR4 chemokine signaling inbone marrow stromal cell niches. Immunity 2006, 25, 977-988. doi: 10.1016/j.immuni.2006.10.016
URLK e y w o r d hematopoietic stm cell, niche, bone marrow