A genetically encoded sensor for visualizing leukotriene B4 gradients in vivo

Leukotriene B(4) (LTB(4)) is a potent lipid chemoattractant driving inflammatory responses during host defense, allergy, autoimmune and metabolic diseases. Gradients of LTB(4) orchestrate leukocyte recruitment and swarming to sites of tissue damage and infection. How LTB(4) gradients form and spread in live tissues to regulate these processes remains largely elusive due to the lack of suitable tools for monitoring LTB(4) levels in vivo. Here, we develop GEM-LTB(4), a genetically encoded green fluorescent LTB(4) biosensor based on the human G-protein-coupled receptor BLT1. GEM-LTB(4) shows high sensitivity, specificity and a robust fluorescence increase in response to LTB(4) without affecting downstream signaling pathways. We use GEM-LTB(4) to measure ex vivo LTB(4) production of murine neutrophils. Transgenic expression of GEM-LTB(4) in zebrafish allows the real-time visualization of both exogenously applied and endogenously produced LTB(4) gradients. GEM-LTB(4) thus serves as a broadly applicable tool for analyzing LTB(4) dynamics in various experimental systems and model organisms.