Hematopoietic Stem Cells Depend upon Aggrephagy to Maintain Protein Homeostasis and Self-Renewal Activity

Abstract

Hematopoietic stem cells (HSCs) regenerate blood and immune cells throughout life. To preserve their health and longevity, HSCs are particularly dependent on maintaining protein homeostasis (proteostasis). HSCs maintain proteostasis partly by sustaining low protein synthesis rates that limit the biogenesis of misfolded proteins. Nevertheless, HSCs ultimately accumulate misfolded proteins that must be eliminated to preserve stem cell fitness. However, how HSCs purge misfolded proteins to maintain proteostasis is mostly unknown. We found that in contrast to most cell types that utilize the proteasome to degrade misfolded proteins, HSCs depend on aggrephagy, a selective form of autophagy, to maintain proteostasis in vivo. Young adult HSCs exhibit unusually high autophagic activity, and genetically disabling autophagy results in significant accumulation of protein aggregates and impaired HSC function. Furthermore, we determined that mouse and human HSCs preferentially express Bag3, a critical stress response gene that promotes aggrephagy and transport of misfolded proteins to aggresomes. Aggresomes are cytosolic inclusion bodies containing protein aggregates that typically form under stress conditions to help maintain proteostasis. Surprisingly, we found that the vast majority of HSCs contain aggresomes, even under steady state conditions in vivo. Conditional deletion of Bag3 causes HSCs to accumulate protein aggregates and impairs their self-renewal activity in vivo. Finally, we determined that old adult HSCs exhibit diminished autophagic flux and become increasingly reliant on the proteasome to degrade misfolded proteins. Overall, these studies demonstrate that protein degradation pathways are uniquely configured in HSCs to preserve proteostasis and fitness, and that disruptions in proteostasis network activity contribute to cellular and molecular changes in HSCs during aging.