Transglutaminase 2 (TG2) is a widely expressed enzyme that catalyzes the modification of glutamine residues in peptide and protein substrates. Prior work has shown that TG2 site-specifically deamidates glutamine residues in gluten peptides, a chemical change that renders these peptides significantly more immunogenic in celiac disease (CeD). A mechanistic understanding of TG2 catalysis allowed us to develop a set of novel gluten peptidomimetic inhibitors and probes to study TG2 chemistry and enzymology. Application of these TG2-specific probes in vitro and in vivo has yielded novel insights into celiac disease biology and immunology.
Biochemical studies with these molecules enabled structural characterization of a unique, catalytically active conformation of TG2. This structure captures TG2 in complex with both a gluten peptidomimetic and calcium, representing the most physiologically relevant TG2 structure solved to date. Additionally, in vitro studies with these inhibitors and probes revealed a novel endocytic pathway by which gluten peptides are simultaneously concentrated and deamidated in the endolysosomal system. In this mechanism, TG2-gluten peptide complexes are recognized and internalized by LDL receptor-related protein 1 (LRP1). This pathway enables more potent antigen internalization and presentation by immune cells in CeD. Finally, in vivo studies with these probes identified a dendritic cell population known to play an essential role in oral tolerance to dietary antigens. Overall, this work provides an upgraded toolkit to study the TG2-LRP1 pathway as it relates not only to celiac disease, but also to the diverse physiologic and pathologic processes in which it has been implicated.