Protein Homeostasis Is Dynamically Regulated throughout Hematopoietic Stem Cell Ontogeny

Abstract

Hematopoietic stem cells (HSCs) are required for establishing hematopoiesis during development and for maintaining lifelong regeneration of blood cells. As demands for blood cell production change throughout ontogeny, there is a commensurate change in HSC biology. Fetal, neonatal, and adult HSCs exhibit differences in gene expression, developmental potential, and self-renewal activity. Thus, fetal, neonatal, and adult HSCs are, at least functionally, distinct populations. We recently found that fetal HSCs exhibit substantially higher rates of protein synthesis as compared to adult HSCs, which depend on unusually low protein synthesis rates to limit the biogenesis of misfolded and unfolded proteins to preserve protein homeostasis (proteostasis) and fitness. This raised questions of whether fetal HSCs experience significant proteotoxic stress in vivo and how proteostasis is regulated throughout HSC development. Remarkably, we found that fetal HSCs restrict accumulation of unfolded proteins to levels similar to those observed in adult HSCs, despite having nearly 7-fold higher protein synthesis rates. Surprisingly, however, we determined that unfolded protein abundance spikes dramatically within HSCs at birth. This transient burst of proteotoxic stress is followed by rapid restoration of proteostasis in neonates, as unfolded protein abundance declines precipitously, ultimately reaching unusually low levels by P14 before moderately rising again in adulthood. In contrast to the dynamic oscillation of unfolded protein abundance, protein synthesis rates progressively decline throughout HSC ontogeny. Fetal HSCs synthesize significantly more protein per hour than HSCs from newborn and postnatal mice, which in turn have nearly 3-fold and 2-fold higher protein synthesis rates than adult HSCs, respectively. Strikingly, however, postnatal declines in HSC protein synthesis occur gradually and non-uniformly. While fetal and adult HSCs have narrow distributions of protein synthesis rates, neonatal HSCs exhibit significant heterogeneity, suggesting an uncoordinated transition to achieving low protein synthesis rates and proteostasis in adulthood. Taken together, these findings indicate that HSC ontogeny is associated with rapid, dynamic, and robust changes in proteostasis activity, and that fetal, neonatal, and adult HSCs utilize distinct mechanisms to regulate proteostasis in vivo. The regulation of proteostasis is thus a key distinguishing feature of HSC ontogeny that could provide a framework to uncover how pathogenic changes disrupt hematopoietic development and function in an age-specific manner. Furthermore, harnessing developmental proteostasis mechanisms could open new avenues to restore HSC fitness in response to various stressors and blood disorders that emerge throughout life.