IGF-1-mediated osteoblastic niche expansion enhances long-term hematopoietic stem cell engraftment after murine bone marrow transplantation

Abstract

The efficiency of hematopoietic stem cell (HSC) engraftment after bone marrow (BM) transplantation depends largely on the capacity of the marrow microenvironment to accept the transplanted cells. While radioablation of BM damages osteoblastic stem cell niches, little is known about their restoration and mechanisms governing their receptivity to engraft transplanted HSCs. We previously reported rapid restoration and profound expansion of the marrow endosteal microenvironment in response to marrow radioablation. Here, we show that this reorganization represents proliferation of mature endosteal osteoblasts which seem to arise from a small subset of highproliferative, relatively radio-resistant endosteal cells. Multiple layers of osteoblasts form along the endosteal surface within 48 hours after total body irradiation, concomitant with a peak in marrow cytokine expression. This niche reorganization fosters homing of the transplanted hematopoietic cells to the host marrow space and engraftment of long-term-HSC. Inhibition of insulin-like growth factor (IGF)-1-receptor tyrosine kinase signaling abrogates endosteal osteoblast proliferation and donor HSC engraftment, suggesting that the cytokine IGF-1 is a crucial mediator of endosteal niche reorganization and consequently donor HSC engraftment. Further understanding of this novel mechanism of IGF-1-dependent osteoblastic niche expansion and HSC engraftment may yield clinical applications for improving engraftment efficiency after clinical HSC transplantation.

Continuous blockade of CXCR4 results in dramatic mobilization and expansion of hematopoietic stem and progenitor cells

“…That said, prolonged blockade of CXCR4 is, to the best of our knowledge, the first pharmacologic approach for in vivo expansion of HSPCs through their direct targeting described to date. For example, treatment with parathyroid hormone 47,48 and insulinlike growth factor 1 49 was associated with (and likely mechanistically caused by) major morphologic changes within BM, including increased (ectopic) formation of trabecular bone, whereas no apparent changes in BM stroma composition and/or structure were found after POL5551 infusion. Moreover, cytologic changes typically associated with G-CSF-induced mobilization (eg, depletion of bone lining osteolineage cells and macrophages) 24 were not detected in BM exposed to continuous infusion, which suggests limited effects within the BM niche compartment.…”

SDF-1 dynamically mediates megakaryocyte niche occupancy and thrombopoiesis at steady state and following radiation injury

“…39,46,[84][85][86] More relevant for chemotactic function, we identified a transient elevation of SDF-1 transcripts near the endosteum at day 2 following injury that directs MKs away from the vascular niche ( Figures 3C and 5A). Evidence pointing to a cell population that may contribute to altered SDF-1 distribution comes from the work of Dominici et al, 39 Olson et al, 47 and Caselli et al 87 who described an expansion of osteoblasts arising from a subset of radioresistant mesenchymal cells at the endosteal surface concomitant with MK relocation following lethal TBI. Mesenchymal stromal and progenitor cells are some of the highest producers of SDF-1 in the marrow at steady state 29,31,32 and are relatively radioresistant, 88 suggesting these populations may participate in marrow microenvironmental changes following TBI.…”

Identification of a murine CD45−F4/80lo HSC-derived marrow endosteal cell associated with donor stem cell engraftment

“…The vasculature was not clearly defined, and 40.70% 6 0.31% of the remaining megakaryocytes had migrated to the endosteal region, all consistent with our previous observations. 12,28,31 Histologic assessment of femurs from mice treated with DT and TBI revealed proliferative, but significantly flattened osteoblasts (cell thickness, 3.42 6 0.10 mm) in iDTR-expressing mice compared with similarly treated littermate controls (cell thickness, 8.45 6 0.31 mm; P , .0001; Figure 2A-B), suggesting that depletion of host hematopoietic lineages is associated with the regulation of osteoblast morphology as well as donor HSC engraftment. Interestingly, osteoblast thickness in iDTR mice treated with DT only (no TBI) was significantly decreased compared with that in WT controls treated with DT (cell thickness, 2.89 6 0.13 mm in iDTR mice vs 4.60 6 0.23 mm in controls; Figure 2B).…”

“…12,28 Briefly, femora and tibiae were fixed in either 10% neutral buffered formalin or 4% paraformaldehyde for 48 hours. Bones were then decalcified with either Regular Cal Immuno (BBC Biochemical, Mount Vernon, WA) for 5 to 7 days or 15% EDTA for 3 weeks before dehydration and paraffin embedding.…”

“…For assessment of osteoblast proliferation, the osteoblast score index was evaluated, as described by Caselli et al, 28 which represents the average number of endosteal lining cell layers per section. The mean of 5 sections was taken as the individual mouse score.…”