A novel role for endothelial tetrahydrobiopterin in mitochondrial redox balance

The redox co-factor tetrahydrobiopterin (BH₄) regulates nitric oxide (NO) and reactive oxygen species (ROS) production by endothelial NOS (eNOS) and is an important redox-dependent signalling molecule in the endothelium. Loss of endothelial BH₄ is observed in cardiovascular disease (CVD) states and results in decreased NO and increased superoxide (O₂^-) generation via eNOS uncoupling. Genetic mouse models of augmented endothelial BH₄ synthesis have shown proof of concept that endothelial BH₄ can alter CVD pathogenesis. However, clinical trials of BH₄ therapy in vascular disease have been limited by systemic oxidation, highlighting the need to explore the wider roles of BH₄ to find novel therapeutic targets. In this study, we aimed to elucidate the effects of BH₄ deficiency on mitochondrial function and bioenergetics using targeted knockdown of the BH₄ synthetic enzyme, GTP Cyclohydrolase I (GTPCH). Knockdown of GTPCH by >90% led to marked loss of cellular BH₄ and a striking induction of O₂^- generation in the mitochondria of murine endothelial cells. This effect was likewise observed in BH₄-depleted fibroblasts devoid of NOS, indicating a novel NOS-independent role for BH₄ in mitochondrial redox signalling. Moreover, this BH₄-dependent, mitochondria-derived ROS further oxidised mitochondrial BH₄, concomitant with changes in the thioredoxin and glutathione antioxidant pathways. These changes were accompanied by a modest increase in mitochondrial size, mildly attenuated basal respiratory function, and marked changes in the mitochondrial proteome and cellular metabolome, including the accumulation of the TCA intermediate succinate. Taken together, these data reveal a novel NOS-independent role for BH₄ in the regulation of mitochondrial redox signalling and bioenergetic metabolism.