The long-term objective of this proposal is to define the cellular and molecular mediators crucial for osteoblastic control of HSCs. We previously established that parathyroid hormone (PTH) activates osteoblastic cells to increase hematopoietic stem cell (HSC) numbers and that PTH improves HSC survival after radiation injury. These results give us a model to define a novel therapeutic approach to increase HSCs following iatrogenic or toxic injury to the bone marrow by stimulating osteoblastic cells. However, the specific osteoblastic cell subsets and the key osteoblastic-dependent molecular events regulating HSCs are unknown. Using pharmacologic and genetic models, we have identified Notch signaling as a potential mediator of PTH-dependent HSC regulation. Notch activation requires direct interaction of cell-bound ligands with receptors on neighboring cells. We demonstrated that 1) PTH or activation of its receptor stimulate the Notch ligand Jagged1 (Jag1) in osteoblastic cells;2) in mice with constitutively active PTH receptors in osteoblastic cells, HSCs have increased Notch activation;3) the PTH-dependent HSC increase is blocked by inhibition of ?-secretase activity, which is required for Notch activation. Our preliminary studies now demonstrate that expression of Jag1 in osteoblastic cells is required for the PTH-dependent HSC expansion. Together, these data suggest that PTH expands HSC through osteoblastic expression of Jag1, which then activates Notch signaling in neighboring bone marrow cells. Based on our data, we hypothesize that HSC expansion by osteoblasts requires Jag1-initiated Notch activation in the bone marrow microenvironment. To test this hypothesis, in Aim1 we will define the osteoblastic cell subset in which Jag1 is necessary and sufficient to mediate HSC expansion.
In Aim2, we will identify the cell population (HSC, osteoblastic cells and/or other components of the bone marrow) in which Notch activation is required to achieve osteoblastic-dependent HSC expansion. Finally in Aim3 we will determine the contribution of Notch signaling to the myeloprotective effects of PTH, a clinical scenario in which HSC niche manipulation could be a novel strategy to reduce morbidity and mortality. We have already established and fully characterized in vivo models in which microenvironmental signals increase HSCs. Now that osteoblastic Jag1 has been identified as a key element of PTH-dependent HSC expansion, we have the unprecedented opportunity of defining the cellular and molecular components of the HSC niche using the in vivo strategies proposed here. Completion of our experimental aims will thus define novel therapeutic targets for HSC manipulation in the bone marrow microenvironment, which can be exploited to improve survival after bone marrow injury.

Public Health Relevance

In this proposal, we study the regulation of hematopoietic stem cells (HSC) by their bone marrow microenvironment. Since HSC give rise to all blood cells, these regulatory mechanisms could be therapeutically exploited to increase HSC in specific situations of blood cell injury or deficiency.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Research Project (R01)
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Skeletal Biology Structure and Regeneration Study Section (SBSR)
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Bishop, Terry Rogers
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University of Rochester
Internal Medicine/Medicine
Schools of Dentistry
United States
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Lawal, Rialnat A; Zhou, Xichao; Batey, Kaylind et al. (2017) The Notch Ligand Jagged1 Regulates the Osteoblastic Lineage by Maintaining the Osteoprogenitor Pool. J Bone Miner Res 32:1320-1331
Balderman, Sophia R; Li, Allison J; Hoffman, Corey M et al. (2016) Targeting of the bone marrow microenvironment improves outcome in a murine model of myelodysplastic syndrome. Blood 127:616-25
Evans, Andrew G; Calvi, Laura M (2015) Notch signaling in the malignant bone marrow microenvironment: implications for a niche-based model of oncogenesis. Ann N Y Acad Sci 1335:63-77
Calvi, Laura M; Link, Daniel C (2015) The hematopoietic stem cell niche in homeostasis and disease. Blood 126:2443-51
Balderman, Sophia R; Calvi, Laura M (2014) Biology of BM failure syndromes: role of microenvironment and niches. Hematology Am Soc Hematol Educ Program 2014:71-6
Calvi, Laura M; Link, Daniel C (2014) Cellular complexity of the bone marrow hematopoietic stem cell niche. Calcif Tissue Int 94:112-24
Hoffman, Corey M; Calvi, Laura M (2014) Minireview: complexity of hematopoietic stem cell regulation in the bone marrow microenvironment. Mol Endocrinol 28:1592-601
Frisch, Benjamin J; Calvi, Laura M (2014) Osteoblasts as leukemia-initiating cells. Bonekey Rep 3:572
Frisch, Benjamin J; Calvi, Laura M (2014) Hematopoietic stem cell cultures and assays. Methods Mol Biol 1130:315-324
Porter, Rebecca L; Georger, Mary A; Bromberg, Olga et al. (2013) Prostaglandin E2 increases hematopoietic stem cell survival and accelerates hematopoietic recovery after radiation injury. Stem Cells 31:372-83

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