iGLuc: a luciferase-based inflammasome and protease activity reporter

A reporter of caspase-1 activity based on Gaussia luciferase—iGLuc—reveals inflammasome activation in mouse cells and in vivo. iGLuc forms nonluminescent aggregates in cells that can be cleaved by specific proteases and, consequently, lead to luciferase activity. The design principle is applied to build reporters for several other proteases. Measurement of protease activity in living cells or organisms remains a challenging task. We here present a transgene-encoded biosensor that reports the proteolytic activity of caspase-1 in the course of inflammasome activation and that of other proteases in a highly sensitive and specific manner. This protease reporter is based on the biological activity of a pro–interleukin (IL)-1β–Gaussia luciferase (iGLuc) fusion construct, in which pro–IL-1β–dependent formation of protein aggregates renders GLuc enzyme inactive. Cleavage leads to monomerization of this biosensor protein, resulting in a strong gain in luciferase activity. Exchange of the canonical caspase-1 cleavage site in this reporter construct allows the generation of protease biosensors with additional specificities. The high sensitivity, signal-to-background ratio and specificity of the iGLuc system renders it a useful tool to study proteolytic events in mouse and human cells at high throughput and to monitor protease activity in mice in vivo.