Nitric oxide regulates metabolism in murine stress erythroid progenitors to promote recovery during inflammatory anemia.

Inflammation skews bone marrow hematopoiesis increasing the production of myeloid effector cells at the expense of steady-state erythropoiesis. A compensatory stress erythropoiesis response is induced to maintain homeostasis until inflammation is resolved. In contrast to steady-state erythroid progenitors, stress erythroid progenitors (SEPs) utilize signals induced by inflammatory stimuli. However, the mechanistic basis for this is not clear. Here we reveal a nitric oxide (NO)-dependent regulatory network underlying two stages of stress erythropoiesis, namely proliferation, and the transition to differentiation. In the proliferative stage, immature SEPs and cells in the niche increased expression of inducible nitric oxide synthase ( Nos2 or iNOS ) to generate NO. Increased NO rewires SEP metabolism to increase anabolic pathways, which drive the biosynthesis of nucleotides, amino acids and other intermediates needed for cell division. This NO-dependent metabolism promotes cell proliferation while also inhibiting erythroid differentiation leading to the amplification of a large population of non-committed progenitors. The transition of these progenitors to differentiation is mediated by the activation of nuclear factor erythroid 2-related factor 2 (Nfe2l2 or Nrf2). Nrf2 acts as an anti-inflammatory regulator that decreases NO production, which removes the NO-dependent erythroid inhibition and allows for differentiation. These data provide a paradigm for how alterations in metabolism allow inflammatory signals to amplify immature progenitors prior to differentiation. Key points: Nitric-oxide (NO) dependent signaling favors an anabolic metabolism that promotes proliferation and inhibits differentiation.Activation of Nfe2l2 (Nrf2) decreases NO production allowing erythroid differentiation.

Metabolic regulation of stress erythropoiesis, outstanding questions, and possible paradigms.

Steady state erythropoiesis produces new erythrocytes at a constant rate to replace the senescent cells that are removed by macrophages in the liver and spleen. However, infection and tissue damage disrupt the production of erythrocytes by steady state erythropoiesis. During these times, stress erythropoiesis is induced to compensate for the loss of erythroid output. The strategy of stress erythropoiesis is different than steady state erythropoiesis. Stress erythropoiesis generates a wave of new erythrocytes to maintain homeostasis until steady state conditions are resumed. Stress erythropoiesis relies on the rapid proliferation of immature progenitor cells that do not differentiate until the increase in serum Erythropoietin (Epo) promotes the transition to committed progenitors that enables their synchronous differentiation. Emerging evidence has revealed a central role for cell metabolism in regulating the proliferation and differentiation of stress erythroid progenitors. During the initial expansion stage, the immature progenitors are supported by extensive metabolic changes which are designed to direct the use of glucose and glutamine to increase the biosynthesis of macromolecules necessary for cell growth and division. At the same time, these metabolic changes act to suppress the expression of genes involved in erythroid differentiation. In the subsequent transition stage, changes in niche signals alter progenitor metabolism which in turn removes the inhibition of erythroid differentiation generating a bolus of new erythrocytes to alleviate anemia. This review summarizes what is known about the metabolic regulation of stress erythropoiesis and discusses potential mechanisms for metabolic regulation of proliferation and differentiation.

Epo receptor signaling in macrophages alters the splenic niche to promote erythroid differentiation.

Anemic stress induces stress erythropoiesis, which rapidly generates new erythrocytes to restore tissue oxygenation. Stress erythropoiesis is best understood in mice where it is extramedullary and occurs primarily in the spleen. However, both human and mouse stress erythropoiesis use signals and progenitor cells that are distinct from steady-state erythropoiesis. Immature stress erythroid progenitors (SEPs) are derived from short-term hematopoietic stem cells. Although the SEPs are capable of self-renewal, they are erythroid restricted. Inflammation and anemic stress induce the rapid proliferation of SEPs, but they do not differentiate until serum erythropoietin (Epo) levels increase. Here we show that rather than directly regulating SEPs, Epo promotes this transition from proliferation to differentiation by acting on macrophages in the splenic niche. During the proliferative stage, macrophages produce canonical Wnt ligands that promote proliferation and inhibit differentiation. Epo/Stat5-dependent signaling induces the production of bioactive lipid mediators in macrophages. Increased production of prostaglandin J2 (PGJ2) activates peroxisome proliferator-activated receptor γ (PPARγ)-dependent repression of Wnt expression, whereas increased production of prostaglandin E2 (PGE2) promotes the differentiation of SEPs.

Stress Erythropoiesis is a Key Inflammatory Response.

Bone marrow medullary erythropoiesis is primarily homeostatic. It produces new erythrocytes at a constant rate, which is balanced by the turnover of senescent erythrocytes by macrophages in the spleen. Despite the enormous capacity of the bone marrow to produce erythrocytes, there are times when it is unable to keep pace with erythroid demand. At these times stress erythropoiesis predominates. Stress erythropoiesis generates a large bolus of new erythrocytes to maintain homeostasis until steady state erythropoiesis can resume. In this review, we outline the mechanistic differences between stress erythropoiesis and steady state erythropoiesis and show that their responses to inflammation are complementary. We propose a new hypothesis that stress erythropoiesis is induced by inflammation and plays a key role in maintaining erythroid homeostasis during inflammatory responses.

Gdf15 regulates murine stress erythroid progenitor proliferation and the development of the stress erythropoiesis niche.

Anemic stress induces the proliferation of stress erythroid progenitors in the murine spleen that subsequently differentiate to generate erythrocytes to maintain homeostasis. This process relies on the interaction between stress erythroid progenitors and the signals generated in the splenic erythroid niche. In this study, we demonstrate that although growth-differentiation factor 15 (Gdf15) is not required for steady-state erythropoiesis, it plays an essential role in stress erythropoiesis. Gdf15 acts at 2 levels. In the splenic niche, Gdf15-/- mice exhibit defects in the monocyte-derived expansion of the splenic niche, resulting in impaired proliferation of stress erythroid progenitors and production of stress burst forming unit-erythroid cells. Furthermore, Gdf15 signaling maintains the hypoxia-dependent expression of the niche signal, Bmp4, whereas in stress erythroid progenitors, Gdf15 signaling regulates the expression of metabolic enzymes, which contribute to the rapid proliferation of stress erythroid progenitors. Thus, Gdf15 functions as a comprehensive regulator that coordinates the stress erythroid microenvironment with the metabolic status of progenitors to promote stress erythropoiesis.

Molecular and functional characterization of peptidoglycan-recognition protein SC2 (PGRP-SC2) from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae.

PGRP-SC2, the member of PGRP family, plays an important role in regulation of innate immune response. In this paper, a PGRP-SC2 gene of Nile tilapia, Oreochromis niloticus (designated as On-PGRP-SC2) was cloned and its expression pattern under the infection of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for amidase activity were detected in the deduced amino acid sequence of On-PGRP-SC2. In healthy tilapia, the On-PGRP-SC2 transcripts could be detected in all the examined tissues, with the most abundant expression in the muscle. When infected with S. agalactiae, there was a clear time-dependent expression pattern of On-PGRP-SC2 in the spleen, head kidney and brain. The assays for the amidase activity suggested that recombinant On-PGRP-SC2 protein had a Zn(2+)-dependent PGN-degrading activity. Moreover, our works showed that recombinant On-PGRP-SC2 protein could significantly reduce bacterial load in target organs attacked by S. agalactiae. These findings indicated that On-PGRP-SC2 may play important roles in the immune response to S. agalactiae in Nile tilapia.