3002 – CIRCADIAN CUES REPROGRAM METABOLIC PATHWAYS IN LONG-TERM REPOPULATING HEMATOPOIETIC STEM CELLS

Hematopoietic stem and progenitor cells (HSPCs) are primarily maintained in an undifferentiated, and quiescent state in the mouse bone marrow (BM). We have recently found that circadian cues regulate daily HSPC fate decisions. Daily light and darkness cues induce a fraction of BM HSPCs to differentiate and egress from the BM in daylight to replenish the circulation with mature blood and immune cells. While darkness metabolically elevates their self-renewal to maintain the BM reservoir of undifferentiated HSPC. Previously, we have reported higher competitive long-term HSC (LT-HSC) repopulation at night compared with morning. The dynamic switch between HSPC quiescence and activation involves transient morning increase in their ROS levels. By applying single stem cell RNA sequencing and flow cytometry with functional HSPC assays, we report night LT-HSCs to have lower mitochondrial membrane potential, previously associated with increased LT-HSC competitive repopulation potential. Darkness cues are associated with enhanced hypoxia and enhanced transcriptional expression of glycolytic pathway genes. We also found both increased expression of the glucose transporter, Glut1 and higher glucose uptake at night in LT-HSCs, pointing to Glut1 as a metabolic mediator of LT-HSC function under circadian regulation. Mechanistically, we found higher levels of the master metabolic transcriptional coactivator, PGC1a at night, known to regulate the Glut1 transcription. These metabolic changes are accompanied with higher LT-HSC mitochondrial ROS and calcium levels at night and an increase in the fatty acid oxidation pathway occurring within the mitochondrial matrix. Moreover, we observed an increase in expression of mitophagy-related genes at night. Thus, a balance between metabolic regulation and mitochondrial dynamics influences LT-HSC function under light and darkness control.