A method for building proteins that activate only in the presence of nitric oxide (NO) — a signaling molecule produced at elevated levels at sites of inflammation, infection, and tumor growth — offers a potential route to therapeutics and diagnostics that respond selectively to disease microenvironments, according to a study published in Nature by researchers at Peking University.
The central finding, led by corresponding author Tao Liu at Peking University's School of Pharmaceutical Sciences, is that synthetic non-canonical amino acids bearing NO-sensitive chemical cages can be genetically encoded into proteins, enabling their function to be held in an inactive state until NO triggers removal of the cage. The approach, which the researchers call NOCAGE, was demonstrated across a range of protein classes in preclinical models, including antibodies, cytokines, and bacterial toxins.
How the system works
Most protein engineering strategies for conditional activation rely on physical stimuli such as light, or on enzymatic cleavage that can occur in off-target tissues. NO offers a different kind of selectivity because its overproduction is tightly associated with pathological states — macrophage activation, hypoxic tumor cores, and inflamed tissue — rather than normal physiology. The NOCAGE system works by incorporating a chemically modified amino acid at a site within the protein that is essential for its activity. The modification blocks function until NO reacts with the cage group, releasing the unmodified amino acid and regenerating the native glutamate side chain.
The team used an expanded genetic code approach to insert these caged residues site-specifically, meaning the activation site can in principle be tuned to any protein whose functional residues are structurally characterized.
Preclinical evidence
In cell-based and mouse experiments, NOCAGE-modified proteins remained largely inactive under normal physiological conditions and recovered function selectively in high-NO environments. Applied to a cytokine with known immunostimulatory activity, the caged version showed attenuated activity in healthy tissue contexts but restored signaling in inflamed microenvironments in the mouse models tested. A NOCAGE-modified antibody demonstrated conditional target engagement in vitro. The researchers also showed the approach extended to bacterial toxins (Pseudomonas Exotoxin A) and viral capsids (AAV), suggesting breadth across mechanistic classes.