Unveiling Tryptophan Dynamics and Functions Across Model Organisms via Quantitative Imaging

Tryptophan is an essential amino acid involved in many cellular processes in vertebrates. Systemic and quantitative measurement of tryptophan is crucial for evaluating its essential role as a precursor of serotonin and kynurenine, key neuromodulators affecting neural and immune functions. We utilized a robust and highly responsive ratiometric indicator for tryptophan (GRIT) to quantitatively measure tryptophan dynamics in bacteria, mitochondria of mammalian cell cultures, and human serum. At the cellular scale, these analyses uncovered differences in tryptophan dynamics across cell types and organelles. At the whole-organism scale, we revealed that inflammation-induced tryptophan concentration increases in zebrafish brain led to elevated tryptophan metabolites serotonin and kynurenine levels, which is associated with the prolonged sleep duration. The reduction of zebrafish plasma tryptophan was mirrored in patients with inflammation symptoms and could serve as a biochemical marker of inflammation. In summary, this study introduces GRIT as a powerful method for studying tryptophan metabolism and functions across scales and species and suggests that tryptophan metabolic processes link the immune response and animal behavior. ### Competing Interest Statement The authors have declared no competing interest.