Loss of Cpt1a Results in Elevated Mitochondrial Reactive Oxygen Species from Glucose-Fueled Mitochondrial Oxidative Phosphorylation and Defective Hematopoietic Stem Cells

Background:

Hematopoietic stem cells (HSCs) rely on self-renewal to sustain stem cell potential and undergo differentiation to generate mature blood cells. Mitochondrial fatty acid β-oxidation (FAO) is essential for HSC maintenance. Carnitine palmitoyl transferase 1a (Cpt1a), a key enzyme in FAO, is located in the mitochondrial outer membrane and facilitates the transport of fatty acids into mitochondria, where they are broken down via the FAO pathway, thereby leading to production of ATP. Specifically, the CPT1A-mediated FAO pathway generates NADH, which can be used later by mitochondrial respiratory chain complexes to produce ATP through OXPHOS. Further, acetyl-CoA, an end product of FAO, can enter the tricarboxylic acid cycle (TCA), wherein both NADH and ATP are also generated. Recent studies have highlighted the significant role of mitochondrial metabolism in regulating both the self-renewal and differentiation capacities of HSCs. It is generally believed that a metabolic transition of HSCs from glycolysis to OXPHOS, and the increase of reactive oxygen species (ROS), drives the exit of HSCs from quiescence and initiates their subsequent differentiation. However, the precise role of Cpt1a and FAO in HSCs, and of mitochondrial metabolism more generally, remains elusive.

Results:

Cpt1a is highly enriched in HSCs. In this study, utilizing a Cpt1a hematopoietic specific conditional knock-out (Cpt1a^Δ/Δ) mouse model, we found that loss of Cpt1a leads to HSC defects, including loss of HSC quiescence and self-renewal, and increased differentiation. Cpt1a^Δ/Δ HSCs have compromised reconstitution and self-renewal capacities. Genotoxic stresses, such as 5-FU and irradiation, downregulate Cpt1a, which restricts glucose-fueled mitochondrial oxidative phosphorylation and mitochondrial ROS production and maintains hematopoietic stem cells. Increased glucose-fueled mitochondrial function in Cpt1a^Δ/Δ HSCs. Mechanistically, we found that loss of Cpt1a results in elevated levels of mitochondrial respiratory chain complex components and their activities, as well as increased ATP production, and accumulation of mitochondrial reactive oxygen species (mitoROS) in HSCs. Our data suggests hyperactivation of mitochondria and metabolic rewiring via upregulated glucose-fueled oxidative phosphorylation (OXPHOS).

Conclusion:

In summary, our findings illuminate the intricate connections among genotoxic stresses, Cpt1a, mitochondria and HSCs. Further, Cpt1a emerges as a pivotal player in maintaining HSC homeostasis by modulating mitochondrial function. Finally, our results suggest the possibility that Cpt1a activity and pathway may serve as a potential target for addressing HSC exhaustion and may offer a novel avenue to sustain functional HSCs under various genotoxic stresses and prevent both inherited and acquired bone marrow failures.

Keywords: Cpt1a, LT-HSCs, mitochondrial FAO, glucose-fueled OXPHOS, ROS