In Vivo Measurement of Redox-Regulated TG2 Activity

: Transglutaminase 2 (TG2) is a ubiquitous mammalian enzyme that is implicated in a variety of physiological processes and human diseases. Normally, extracellular TG2 is catalytically dormant due to formation of an allosteric disulphide bond between Cys370 and 371 of the enzyme. In this protocol, we describe a method to reduce this disulphide bond in living mice and to monitor the resulting in vivo TG2 activity. Briefly, exogenous thioredoxin-1 protein (TRX) is prepared and administered as a specific, physiologically relevant reductant of the Cys370-371 disulphide along with the small molecule 5-biotinamidopentylamine (5-BP) as a TG2 activity probe. Tissue cryosections are then analyzed by immunohistochemistry to ascertain the extent of 5-BP incorporation, which serves as a record of the redox state of TG2 in vivo. This protocol focuses on the modulation and measurement of TG2 in the small intestine, but we encourage investigators to evaluate it in their organ(s) of interest.