Olaia Naveiras

In the BM, osteoblasts and endothelium constitute functional niches providing positive or negative signals for HSC self-renewal. In addition to hematopoietic cells, endothelial cells, and osteoblasts, adult BM contains numerous adipocytes. Interestingly, the number of adipocytes correlates inversely with the gross hematopoietic activity of the marrow. Whether adipocytes have a direct effect on hematopoietic progenitors or whether they act as mere space-fillers in this context remains unclear. To determine the potential role of bone marrow adipocytes in hematopoiesis, we induced bone marrow-derived OP9 mesenchymal cells to differentiate into either osteoblastic or adipocytic stroma, and then tested their capacity to serve as surrogate HSC niches during in vitro hematopoietic culture in the absence of exogenous growth factors. We found that the presence of bone marrow-derived adipocytes suppresses the expansion of short-term hematopoietic progenitors by at least two fold as measured by the number of CD45+ cells expanded, the number of colony forming units and the competitive repopulation during the first two months post-transplant. As an in vivo correlate, we compared the hematopoietic activity within the bone marrow of the adipocyte-poor thoracic vertebrae and the adipocyte rich proximal tail vertebrae, and found that the percentage of HSCs, CMPs, GMPs and MEPs was decreased by two fold as determined phenotypically by FACS and functionally by competitive repopulation. Mechanistically, oligonuclotide expression microarrays and conditioned media experiments suggested that the inhibition of the progenitor compartment in adipocytic-rich environments is due to the loss of supportive membrane-bound and soluble factors (Jagged, N-cadherin, SCF, Angiopoietin-1 and BMPs) in addition to the presence of an active inhibitor. Finally, we found bone marrow adipocytes to accumulate in great numbers upon bone marrow ablation, a process that is hindered in genetically adipocyte-deficient mice. Since early BM transplant survival depends on the rapid accumulation of short term hematopoietic progenitors, we were interested to know if the absence of adipocytes in this context would foster faster recovery in lethally irradiated mice. As predicted, leukocyte counts on the third week post-transplant were 3-5 times higher on the recovering fatless mice. Accordingly, the bone marrow from fatless mice contained double the amount of CFUs, HSCs, CMPs, GMPs and MEPs on day 17 post-transplant than their wildtype littermates. We therefore conclude that, as seen in vitro, the presence of adipocytes in the recovering HSC niche is detrimental to the rapid hematopoietic expansion required to reconstitute blood production. We are currently examining the pharmacologic modulation of adipocyte formation for its effects in BM transplantation.



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			Olaia Naveiras, M.D., Ph.D. Experimental Pathology Harvard Medical School, Boston, Massachusetts - 2008 Post-Doctoral Fellow
			I am interested in how the environment provides signalling cues to determine the hematopoietic stem cell (HSC) fate. During development, HSCs encounter at least four different niches in the four different sites where they originate: the yolk sac, the aorta-gonada-mesonephros (AGM), the fetal liver, and the bone marrow. the Calvi et al. and Zhang et al. have provided strong evidence in 2003 that the osteoblast provides the niche for HSCs on the bone marrow. However, the molecules responsible for the osteoblast's ability to maintain HSCs uncommitted are still to be confirmed. N-cadherin, Notch1/Jagged2, Tie2/Angiopoietin-1, Frizzled/Wnt, and BMPR-BMP4 molecular pairs from the osteoblast to the HSC have been implicated (see figure). Our ability to produce HSCs in vitro from mouse embryonic stem cells in the context of bone marrow-derived OP9 supportive stroma provides us with an in vitro surrogate assay to study both the phenotype of the ES-derived mouse HSC and the molecular interactions necessary for the stroma to induce HSC's asymmetric divisions.
Articles in PubMed



	In the BM, osteoblasts and endothelium constitute functional niches providing positive or negative signals for HSC self-renewal. In addition to hematopoietic cells, endothelial cells, and osteoblasts, adult BM contains numerous adipocytes. Interestingly, the number of adipocytes correlates inversely with the gross hematopoietic activity of the marrow. Whether adipocytes have a direct effect on hematopoietic progenitors or whether they act as mere space-fillers in this context remains unclear. To determine the potential role of bone marrow adipocytes in hematopoiesis, we induced bone marrow-derived OP9 mesenchymal cells to differentiate into either osteoblastic or adipocytic stroma, and then tested their capacity to serve as surrogate HSC niches during in vitro hematopoietic culture in the absence of exogenous growth factors. We found that the presence of bone marrow-derived adipocytes suppresses the expansion of short-term hematopoietic progenitors by at least two fold as measured by the number of CD45+ cells expanded, the number of colony forming units and the competitive repopulation during the first two months post-transplant. As an in vivo correlate, we compared the hematopoietic activity within the bone marrow of the adipocyte-poor thoracic vertebrae and the adipocyte rich proximal tail vertebrae, and found that the percentage of HSCs, CMPs, GMPs and MEPs was decreased by two fold as determined phenotypically by FACS and functionally by competitive repopulation. Mechanistically, oligonuclotide expression microarrays and conditioned media experiments suggested that the inhibition of the progenitor compartment in adipocytic-rich environments is due to the loss of supportive membrane-bound and soluble factors (Jagged, N-cadherin, SCF, Angiopoietin-1 and BMPs) in addition to the presence of an active inhibitor. Finally, we found bone marrow adipocytes to accumulate in great numbers upon bone marrow ablation, a process that is hindered in genetically adipocyte-deficient mice. Since early BM transplant survival depends on the rapid accumulation of short term hematopoietic progenitors, we were interested to know if the absence of adipocytes in this context would foster faster recovery in lethally irradiated mice. As predicted, leukocyte counts on the third week post-transplant were 3-5 times higher on the recovering fatless mice. Accordingly, the bone marrow from fatless mice contained double the amount of CFUs, HSCs, CMPs, GMPs and MEPs on day 17 post-transplant than their wildtype littermates. We therefore conclude that, as seen in vitro, the presence of adipocytes in the recovering HSC niche is detrimental to the rapid hematopoietic expansion required to reconstitute blood production. We are currently examining the pharmacologic modulation of adipocyte formation for its effects in BM transplantation.