FGF signaling facilitates postinjury recovery of mouse hematopoietic system

Abstract

Previous studies have shown that fibroblast growth factor (FGF) signaling promotes hematopoietic stem and progenitor cell (HSPC) expansion in vitro. However, it is unknown whether FGF promotes HSPC expansion in vivo. Here we examined FGF receptor 1 (FGFR1) expression and investigated its in vivo function in HSPCs. Conditional knockout (CKO) of Fgfr1 did not affect phenotypical number of HSPCs and homeostatic hematopoiesis, but led to a reduced engraftment only in the secondary transplantation. When treated with 5-fluorouracil (5FU), the Fgfr1 CKO mice showed defects in both proliferation and subsequent mobilization of HSPCs. We identified megakaryocytes (Mks) as a major resource for FGF production, and further discovered a novel mechanism by which Mks underwent FGF-FGFR signaling dependent expansion to accelerate rapid FGF production under stress. Within HSPCs, we observed an up-regulation of nuclear factor B and CXCR4, a receptor for the chemoattractant SDF-1, in response to bone marrow damage only in control but not in Fgfr1 CKO model, accounting for the corresponding defects in proliferation and migration of HSPCs. This study provides the first in vivo evidence that FGF signaling facilitates postinjury recovery of the mouse hematopoietic system by promoting proliferation and facilitating mobilization of HSPCs. (Blood. 2012; 120(9):1831-1842) IntroductionFibroblast growth factors (FGFs) are a large group of secreted molecules that regulate cell migration, proliferation, and differentiation in both embryonic and adult development. 1,2 FGFs mediate their cellular responses by binding to and activating a family of 4 receptor tyrosine kinases designated as the FGF-receptors FGFR1 through FGFR4, which display different ligand-binding characteristics and biologic functions. 3 FGF signaling is important for hematopoietic developmental regulation, 4,5 and FGFR1 was shown to be preferentially expressed in adult hematopoietic stem and progenitor cells (HSPCs). 6 Although FGF ligands support HSPC expansion in vitro, 7,8 the role of FGF signaling via FGFR1 in vivo has not been elucidated.Treatment with chemotherapeutic drugs, such as cyclophosphamide and 5-fluorouracil (5FU), 9,10 induces a multistep bone marrow (BM) stress response: (1) actively cycling cells are eliminated, including cycling HSPCs 9,10 ; (2) surviving quiescent longterm hematopoietic stem cells (LT-HSCs) are subsequently activated to expand; (3) some expanded HSCs give rise to short-term HSCs (ST-HSCs) for further proliferation; and (4) some HSPCs egress from BM to the blood circulation and extramedullary sites, such as spleen (ie, mobilization), to further proliferate and differentiate. [11][12][13] In homeostatic hematopoiesis, HSPCs are primarily localized within BM where they associate with niches that regulate their activity. [14][15][16][17][18] Although a small percentage of HSPCs routinely circulate from BM to peripheral blood (PB) and home back to BM,19,20 the number of HSPCs that migrate from BM can be markedly increased by certain stimuli dur...

Endothelial cell FGF signaling is required for injury response but not for vascular homeostasis

“…The Flk1-Cre and Tie2-Cre deleter alleles used in the current study, which are known to target the hematopoietic lineage (19,48), allowed us to test the requirement for hematopoietic cell FGFR1/2 signaling during development, homeostasis, and response to injury. Consistent with Zhao et al (22), we found that inactivation of FGFR1/2 signaling in the hematopoietic lineage does not adversely affect hematopoietic development or homeostasis under normal physiological conditions. We also observed that leukocyte infiltration in DCKO and DFF mice was similar following cutaneous wounding.…”

“…Furthermore, Zhao et al (22) showed that HSPC mobilization was impaired in mice lacking FGFR1 in the hematopoietic lineage suggesting impaired migration. The Flk1-Cre and Tie2-Cre deleter alleles used in the current study, which are known to target the hematopoietic lineage (19,48), allowed us to test the requirement for hematopoietic cell FGFR1/2 signaling during development, homeostasis, and response to injury.…”

“…Recent studies by Zhao et al (22) have shown that FGFR1 signaling is required for hematopoietic stem and progenitor cell (HSPC) engraftment following secondary adoptive transfer, but not for developmental or homeostatic hematopoiesis. Furthermore, Zhao et al (22) showed that HSPC mobilization was impaired in mice lacking FGFR1 in the hematopoietic lineage suggesting impaired migration.…”

Regulation of hematopoietic stem cells in the niche

“…This can be explained by the inactive FGF signals during homeostasis with low levels of FGFs in BM and FGFRs in HSCs; however, both FGF ligands in BM and FGF receptors were boosted under stress to turn on FGF signals for promoting HSC proliferation through activation of AKT and NF-B mechanisms. Interestingly, FGF signal was also shown to play a role in mobilizing HSCs to spleen for extramedullary hematopoiesis through regulating CXCR4-CXCL12 pathway that further facilitated HSC regeneration under stress [24,58,59].…”

Role of osteoclasts in regulating hematopoietic stem and progenitor cells

“…Since hematopoietic stem cells (HSCs) are maintained in a quiescent state, HSCs are resistant to 5-FU induced cell death. Thus, serial 5-FU injection has been utilized to evaluate HSC cell function and maintenance in vivo [71][72][73][74] . We hypothesized that osteopetrotic mice show a reduced HSC pool and function due to lack of BM cavities and niches.…”