NOVEL SIGNALS IN STRESS ERYTHROPOIESIS: COORDINATED CONTROL OF WNT, PGE2 AND PERK SIGNALING PATHWAYS REGULATES EXPANSION AND DIFFERENTIATION OF STRESS ERYTHROID PROGENITORS
Open Access
- Author:
- Chen, Yuanting
- Graduate Program:
- Molecular, Cellular and Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 26, 2020
- Committee Members:
- Robert Paulson, Dissertation Advisor/Co-Advisor
Robert Paulson, Committee Chair/Co-Chair
Kumble Sandeep Prabhu, Committee Member
Zhi-Chun Lai, Committee Member
Xiaojun Lance Lian, Outside Member
Wendy Hanna-Rose, Committee Member
Melissa Rolls, Program Head/Chair - Keywords:
- Stress erythropoiesis
Wnt
PGE2
PERK
mTORC1 - Abstract:
- In response to tissue hypoxia caused by anemia, hemorrhage or infection, the process termed stress erythropoiesis restores tissue oxygenation by rapidly producing large numbers of erythrocytes. In contrast to steady state erythropoiesis, stress erythropoiesis utilizes specialized progenitor cells and distinct set of signals to generate large numbers of erythrocytes. During the early stage of stress erythropoiesis, the immature stress erythroid progenitors (SEPs) rapidly expand without differentiation. The amplifying SEPs maintain their stem cell phenotype until the appearance of transition signals. Upon receiving these signals, SEPs undergo rapid differentiation and give rise to a wave of stress erythroid burst-forming units (BFU-Es), which further develop into mature erythrocytes. The development of SEPs also relies on the regulation derived from the microenvironment. Erythroblastic islands (EBIs) made up of macrophage/monocytes in close contact with developing erythroblasts have been shown to be a necessary niche in erythropoiesis. Our previous work found that SEPs mature in a specific EBI niche. In the stress erythroid niche, monocytes are recruited to the spleen and mature in concert with developing SEPs. Erythropoietin (Epo) is the key transition signal in stress erythropoiesis. In chapter 2, we discussed the microenvironmental change induced by Epo in regulating SEP differentiation. We found that rather than directly mediating SEPs, Epo signaling acts on EBI macrophages to promote the SEP differentiation by mediating a series of changes in the microenvironment. During the amplifying stage, EBI macrophages generate canonical Wnt ligands, which promote the proliferation of SEPs by targeting β-catenin-dependent gene transcription. Epo induces the transition from proliferating microenvironment to differentiating microenvironment. Two prostaglandins, delta-12-prostaglandin J2 (Δ12-PGJ2) and prostaglandin E2 (PGE2), are produced by EBI macrophages in response to Epo. Δ12-PGJ2 activates the peroxisome proliferator-activated receptor gamma (PPARγ), which represses the Wnt expression in macrophages. As another effector of Epo, PGE2 drives the differentiation of SEPs simultaneously. As a result, proliferation and differentiation of SEPs become very efficient because of Epo-orchestrated microenvironmental signals. In chapter 3, we further investigated the mechanism of the PGE2-mediated SEP differentiation. We demonstrated that the increase in PGE2 activates the protein kinase RNA-like endoplasmic reticulum kinase (PERK) signaling pathway by regulating intracellular Ca2+ flux. PERK belongs to the integrated stress response (ISR) kinase family. We found that the mutation or inhibition of PERK activity causes severe defects in stress erythropoiesis. Perturbation of PERK blocks the transition from amplifying progenitors to differentiating stress BFU-Es in both mouse and human cell cultures. The activation of PERK induces translation of the activating transcription factor 4 (ATF4), which increases the intracellular concentration of amino acids by promoting the expression of amino acid transporter genes. As a microenvironment sensor, the mammalian target of rapamycin complex 1 (mTORC1) is activated in response to the amino acid influx. Consequently, mTORC1 promotes global protein translation in SEPs. Such high rate of protein translation enables the SEP differentiation to generate mature erythrocytes, which eventually alleviates the anemic stress. In conclusion, the data presented in this thesis identify a key regulatory point in stress erythropoiesis. Unlike the constant production of steady state erythropoiesis, stress erythropoiesis generates a bolus of new erythrocytes, which is designed to maintain homeostasis until steady state erythropoiesis can resume. The initial expansion of immature SEPs represents the first and essential step. There will be inadequate erythrocytes if early progenitors are not efficiently proliferated. Therefore, precise regulation of the expansion and transition of SEPs is vital in stress erythropoiesis. Here we showed that Epo-mediated signal changes in the EBI macrophages promote the transition from proliferating SEPs to differentiating SEPs. These signals also increase the protein translation in SEPs, allowing the final differentiation of SEPs into mature erythrocytes.