Reactive oxygen species (ROS), produced during various electron transfer reactions in vivo, are generally considered to be deleterious to cells. In the mammalian haematopoietic system, haematopoietic stem cells contain low levels of ROS. However, unexpectedly, the common myeloid progenitors (CMPs) produce significantly increased levels of ROS(2). The functional significance of this difference in ROS level in the two progenitor types remains unresolved. Here we show that Drosophila multipotent haematopoietic progenitors, which are largely akin to the mammalian myeloid progenitors, display increased levels of ROS under in vivo physiological conditions, which are downregulated on differentiation. Scavenging the ROS from these haematopoietic progenitors by using in vivo genetic tools retards their differentiation into mature blood cells. Conversely, increasing the haematopoietic progenitor ROS beyond their basal level triggers precocious differentiation into all three mature blood cell types found in Drosophila, through a signalling pathway that involves JNK and FoxO activation as well as Polycomb downregulation. We conclude that the developmentally regulated, moderately high ROS level in the progenitor population sensitizes them to differentiation, and establishes a signalling role for ROS in the regulation of haematopoietic cell fate. Our results lead to a model that could be extended to reveal a probable signalling role for ROS in the differentiation of CMPs in mammalian haematopoietic development and oxidative stress response.

Respiratory complex I is a proton-pumping oxidoreductase key to bioenergetic metabolism. Biochemical studies have found a divide in the behavior of complex I in metazoans that aligns with the evolutionary split between Protostomia and Deuterostomia. Complex I from Deuterostomia including mammals can adopt a biochemically defined off-pathway 'deactive' state, whereas complex I from Protostomia cannot. The presence of off-pathway states complicates the interpretation of structural results and has led to considerable mechanistic debate. Here, we report the structure of mitochondrial complex I from the thoracic muscles of the model protostome Drosophila melanogaster. We show that although D. melanogaster complex I (Dm-CI) does not have a NEM-sensitive deactive state, it does show slow activation kinetics indicative of an off-pathway resting state. The resting-state structure of Dm-CI from the thoracic muscle reveals multiple conformations. We identify a helix-locked state in which an N-terminal α-helix on the NDUFS4 subunit wedges between the peripheral and membrane arms. Comparison of the Dm-CI structure and conformational states to those observed in bacteria, yeast, and mammals provides insight into the roles of subunits across organisms, explains why the Dm-CI off-pathway resting state is NEM insensitive, and raises questions regarding current mechanistic models of complex I turnover.

Objective

Diabetes is characterized by pancreatic β-cell dedifferentiation. Dedifferentiating β cells inappropriately metabolize lipids over carbohydrates and exhibit impaired mitochondrial oxidative phosphorylation. However, the mechanism linking the β-cell's response to an adverse metabolic environment with impaired mitochondrial function remains unclear.

Methods

Here we report that the oxidoreductase cytochrome b5 reductase 3 (Cyb5r3) links FoxO1 signaling to β-cell stimulus/secretion coupling by regulating mitochondrial function, reactive oxygen species generation, and nicotinamide actin dysfunction (NAD)/reduced nicotinamide actin dysfunction (NADH) ratios.

Results

The expression of Cyb5r3 is decreased in FoxO1-deficient β cells. Mice with β-cell-specific deletion of Cyb5r3 have impaired insulin secretion, resulting in glucose intolerance and diet-induced hyperglycemia. Cyb5r3-deficient β cells have a blunted respiratory response to glucose and display extensive mitochondrial and secretory granule abnormalities, consistent with altered differentiation. Moreover, FoxO1 is unable to maintain expression of key differentiation markers in Cyb5r3-deficient β cells, suggesting that Cyb5r3 is required for FoxO1-dependent lineage stability.

Conclusions

The findings highlight a pathway linking FoxO1 to mitochondrial dysfunction that can mediate β-cell failure.