Inflammatory regulation of hematopoiesis in the normal and hyperglycemic vertebrate embryo

Abstract

Hematopoietic stem cells (HSCs) arise in the embryonic aorta through a Runx1-dependent process of endothelial-to-hematopoietic transition. Once formed, HSCs colonize secondary niches, where they proliferate and differentiate to maintain lifelong hematopoiesis. Although many intrinsic factors regulating HSCs have been revealed, there is an emerging role for cell-extrinsic regulation of HSC formation and long-term function. We previously reported that acute exposure to excess glucose accelerated the onset and magnitude of HSC formation in the zebrafish embryo. Additionally, we, and others, recently demonstrated that pro-inflammatory cytokines stimulate HSC production across vertebrates. Here, we explored the hypothesis that hyperglycemia augments inflammatory signaling during HSC formation to influence output and cell fate. In support of this possibility, exposure to elevated glucose levels during HSC specification (12-36 hours post fertilization (hpf)) increased expression of several pro-inflammatory cytokines and receptors; morpholino-mediated knockdown of ifng and il1b each blunted the inductive effects of glucose on runx1 expression. As Il1β processing is regulated by inflammasome activity, we treated embryos with inflammasome activators, and found that they also increased runx1/cmyb expression and numbers of functional HSCs, while loss of the inflammasome adapter protein pycard reduced HSC numbers. We next examined the effects of prolonged glucose exposure, and found that treatment from 24-120hpf robustly increased expression of pro-inflammatory cytokines. Interestingly, glucose exposure during maturation and seeding of secondary organs (72-120hpf) increased mpo expression and Mpo+ myeloid cells enumerated by flow cytometry; conversely, rag1 expression was reduced in the thymus, suggesting altered HSC function or maintenance. Investigation of the mechanisms linking prolonged hyperglycemia and inflammation implicates both inflammasome activation and insulin signaling in promoting lineage skewing. As children of diabetic mothers exhibit a higher risk of leukemia, understanding the consequences of metabolic alterations on HSCs is of clinical interest.