The Discovery and Characterization of the BMP4-mediated Stress Erythroid Response to Acute Anemia
Open Access
- Author:
- Perry, John Mark
- Graduate Program:
- Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 25, 2005
- Committee Members:
- Robert Paulson, Committee Chair/Co-Chair
Pamela Hankey Giblin, Committee Member
Ross Cameron Hardison, Committee Member
Andrew Thomas Henderson, Committee Member
Pamela J Mitchell, Committee Member - Keywords:
- stress erythropoiesis
erythropoisis
acute anemia
BMP4 - Abstract:
- Acute anemia results in the rapid mobilization and differentiation of erythroid progenitors in the adult spleen. flexed-tail (f) mice exhibit normal steady state erythropoiesis but are unable to rapidly respond to acute erythropoietic stress. Analysis of the f locus shows that the mechanisms that regulate expansive erythropoiesis are distinct from the mechanisms that regulate steady state erythropoiesis. We show that f/f mice have a mutation in Madh5, which impairs signaling downstream of the BMP4 receptor. Further analysis of mutant and control mice demonstrates that during the recovery from acute anemia, BMP4 expression is induced in the spleen, which drives the differentiation of a BMP4 responsive progenitor into an Epo responsive stress BFU-E. Stress BFU-E rapidly form large colonies in the presence of Epo alone, which distinguishes them from steady state bone marrow BFU-E. These findings suggest a new model where stress BFU-E in the spleen are poised to respond to changes in the microenvironment induced by acute anemia. Analysis of the stress erythroid response has also demonstrated the importance of SCF/Kit signaling, which we show to be necessary for the normal stress BFU-E response. BMP4 responsive cells express Kit on their surface and previous work in the field has shown that mice mutant for the Kit receptor (W/WV) are slow to recover from acute anemia. Although W/WV mice are chronically anemic, they do not constitutively express BMP4; however, they rapidly up-regulate BMP4 expression in response to phenylhydrazine treatment. Despite rapid induction of BMP4, the expansion of stress BFU-E is significantly delayed. Unlike control mice, W/WV mice do not maintain a pool of BMP4 responsive cells in the spleen. In vitro analysis reveals that BMP4 and SCF play distinct roles in the response to acute anemia. BMP4 treatment increased the number of stress BFU-E, while SCF affected the size of the stress BFU-E-derived colonies. These results demonstrate that, while BMP4 regulates the differentiation of stress BFU-E, SCF/Kit signaling regulates the expansion of stress BFU-E progeny. Furthermore, we found that hypoxia increases both colony number and size, and hypoxia synergizes with SCF in order to further increase colony number. In vitro treatment of spleen cells with hypoxia, SCF and BMP4 recapitulate the stress BFU-E expansion observed in vivo in response to acute anemia. Further characterization of the stress erythroid response demonstrates that BMP4 responsive cells and stress BFU-E do not self-renew in the spleen but must be replenished by bone marrow progenitors. Bone marrow-derived progenitors are found to give rise to BMP4 responsive cells and stress BFU-E upon exposure to the splenic microenvironment. Desert hedgehog is highly expressed in the spleen and is a potential candidate signal for converting bone marrow progenitors into BMP4 responsive cells. We show that exposure of bone marrow cells to Sonic hedgehog protein induces BMP4 expression as well as the expansion of stress BFU-E. Also, unlike controls, Smoothened mutant bone marrow cells, which are unable to transmit hedgehog signals, are incapable of giving rise to BMP4 responsive cells in the spleen. Together, our data demonstrate that certain bone marrow progenitors differentiate into BMP4 responsive cells upon exposure to hedgehog signals in the spleen and serve to replenish BMP4 responsive cells which have been exhausted during the recovery from acute anemia. The discovery and characterization of stress BFU-E provides a new model for the response to acute anemia. Induction of BMP4 in the spleen following acute anemia results in the differentiation of BMP4 responsive cells into stress BFU-E. Hypoxia and SCF serve to increase the number and size of stress BFU-E and together with BMP4 are responsible for the stress BFU-E response to acute anemia. This stress BFU-E response is then replenished by bone marrow progenitors which become BMP4 responsive cells upon exposure to hedgehog signals in the spleen.