A High-Throughput Physiological Screen Reveals a Conserved Catecholamine-Dependent Axis Regulating Systemic Metabolic Flexibility

Metabolic flexibility, the capacity to switch between carbohydrate and lipid fuels, is fundamental to metabolic health, yet its genetic regulation has not been systematically explored in vivo . To address this gap, we developed a high-throughput physiological screen in mice using the respiratory exchange ratio range (ΔRER) as quantitative index of metabolic flexibility. Screening over 800 knockout lines identified Cyb561, a vesicular cytochrome required for catecholamine biosynthesis, as an unexpected regulator of systemic substrate utilisation.Mice lacking Cyb561 exhibited impaired lipolysis, blunted brown adipose thermogenesis, and delayed lipid clearance, while maintaining preserved glucose tolerance. Under high-fat feeding or at thermoneutrality (28°C), Cyb561 deficiency exacerbated insulin resistance, revealing metabolic vulnerability. Remarkably, despite impaired lipolysis and BAT dysfunction, Cyb561 knockout mice did not develop increased adiposity. Mechanistically, we demonstrate that catecholamines transcriptionally couple lipid uptake and release in white adipose tissue, thereby sustaining lipid turnover and maintaining systemic metabolic flexibility.Together, these findings establish catecholamine tone as a key determinant of whole-body fuel switching and demonstrate that high-throughput ΔRER phenotyping offers a powerful and sensitive approach for large-scale analysis of the genetic determinants of metabolic flexibility.