Canonical Signaling By TGF Family Members in Mesenchymal Stromal Cells Is Dispensable for Hematopoietic Niche Maintenance Under Basal and Stress Conditions

The bone marrow contains a complex population of stromal and hematopoietic cells that together generate a unique microenvironment, or niche, to support hematopoiesis. Mesenchymal stromal cells are an important component of the bone marrow hematopoietic niche and include CXCL12-abundant reticular (CAR) cells, adipocytes, osteolineage cells, and arteriolar pericytes, all of which have been implicated in hematopoietic stem/progenitor cell (HSPC) maintenance. There also is evidence that adaptive changes in bone marrow stromal cells contributes to recovery from myelosuppresive therapy and the development of certain hematopoietic malignancies. However, the signals that contribute to the development, maintenance, and stress response of bone marrow mesenchymal stromal cells are poorly understood. Here, we test the hypothesis that cytokines of the transforming growth factor superfamily, which include bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs), and activins/inhibins, provide signals to mesenchymal stromal cells that contribute to basal and stress hematopoiesis responses.

To test this hypothesis, we abrogated canonical TGF family signaling in mesenchymal stem/progenitor cells by deleting Smad4 using a doxycycline-repressible Osterix-Cre transgene (Osx-Cre), which targets all mesenchymal stromal cells in the bone marrow. We first performed lineage-tracing studies using Osx-Cre Smad4^fl/fl Ai9 mice to show that activation of Osx-Cre at birth (by removal of doxycycline) results in the efficient targeting of bone marrow mesenchymal stromal cells. Moreover, we show that Smad4 mRNA expression is essentially undetectable in sorted mesenchymal stromal cells sorted from the bone marrow of these mice. Basal hematopoiesis and bone marrow stromal cells were analyzed in 6-8 week old Osx-Cre Smad4^fl/fl mice. No alterations in the number or spatial organization of CAR cells, osteoblasts, or adipocytes was observed, and expression of key niche factors, including Scf, Cxcl12, and Spp1 was normal. Basal hematopoiesis, including the number of phenotypic HSCs in bone marrow and spleen, also was normal. Recent studies have shown that inhibition of activin signaling by treating with an activin receptor 2 alpha (ACVR2a) ligand trap stimulates erythropoiesis. Although ACVR2a signaling in erythroid progenitors contributes to this effect, two groups showed that inhibition of ACVR2a signaling in bone marrow stromal cells also stimulates erythropoiesis. Thus, we next characterized basal and stress erythropoiesis in Osx-Cre Smad4^fl/fl mice. The frequency of phenotypic erythroid progenitors in bone marrow and spleen was similar to control mice. The stress erythropoiesis response was assessed after induction of acute hemolytic anemia by phenylhydrazine treatment. Both the magnitude of anemia and kinetics of erythroid recovery were similar to control mice. Myelosuppressive therapy induces marked alterations in the bone marrow microenvironment that includes an expansion of osteolineage cells and adipocytes, which have been linked to hematopoietic recovery. Thus, we next characterized stress hematopoiesis in Osx-Cre Smad4^fl/fl mice in response to 5-fluorouracil (5-FU) treatment. Compared to control mice, the magnitude and duration of neutropenia following 5-FU were similar. Moreover, mouse survival after repeated weekly doses of 5-FU was comparable to control mice. HSPC mobilization by G-CSF is due, in large part, by downregulation of CXCL12 expression in bone marrow mesenchymal stromal cells. A prior study suggested that SMAD signaling negatively regulates CXCL12 expression in stromal cells. Consistent with this finding, we show that treatment of cultured bone marrow derived MSCs with TGF-b1 for 48 hours results in a significant (3.3-fold, P<0.0001) decrease in CXCL12 mRNA expression. Thus, in the final experiments, we characterized G-CSF induced HSPC mobilization in Osx-Cre, Smad4^fl/fl or Osx-Cre, Tgfbr2^fl/fl mice. HSPC mobilization, as quantified by CFU-C and Kit⁺ Sca⁺ lineage^- (KSL) cell number in blood or spleen after 5 days of G-CSF treatment was comparable to control mice. Collectively, these data suggest the TGF family member signaling in mesenchymal stromal cells is dispensable for hematopoietic niche maintenance under basal and stress conditions.