Elise Loppinet

All Publications

• LRP-1 links post-translational modifications to efficient presentation of celiac disease-specific Tcell antigens. Cell chemical biology Loppinet, E., Besser, H. A., Sewa, A. S., Yang, F., Jabri, B., Khosla, C. 2022

  Abstract

  Celiac disease (CeD) is an autoimmune disorder in which gluten-derived antigens trigger inflammation. Antigenic peptides must undergo site-specific deamidation to be presentable to CD4+ Tcells in an HLA-DQ2 or -DQ8 restricted manner. While the biochemical basis for this post-translational modification is understood, its localization in the patient's intestine remains unknown. Here, we describe a mechanism by which gluten peptides undergo deamidation and concentration in the lysosomes of antigen-presenting cells, explaining how the concentration of gluten peptides necessary to elicit an inflammatory response in CeD patients is achieved. A ternary complex forms between a gluten peptide, transglutaminase-2 (TG2), and ubiquitous plasma protein alpha2-macroglobulin, and is endocytosed by LRP-1. The covalent TG2-peptide adduct undergoes endolysosomal decoupling, yielding the expected deamidated epitope. Our findings invoke a pathogenic role for dendritic cells and/or macrophages in CeD and implicate TG2 in the lysosomal clearance of unwanted self and foreign extracellular proteins.

  View details for DOI 10.1016/j.chembiol.2022.12.002

  View details for PubMedID 36608691

• An Unusual "OR" Gate for Allosteric Regulation of Mammalian Transglutaminase 2 in the Extracellular Matrix JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Melkonian, A., Loppinet, E., Martin, R., Porteus, M., Khosla, C. 2021; 143 (28): 10537-10540

  Abstract

  Transglutaminase 2 (TG2) is a highly expressed mammalian enzyme whose biological function is unclear, although its catalytic activity in the small intestine appears necessary for celiac disease (CeD) pathogenesis. While TG2 activity is reversibly regulated by multiple allosteric mechanisms, their roles under fluctuating physiological conditions are not well understood. Here, we demonstrate that extracellular TG2 activity is competitively controlled by the mutually exclusive binding of a high-affinity Ca2+ ion or the formation of a strained disulfide bond. Binding of Ca2+ at the high-affinity site does not activate TG2 per se, but it protects against oxidative enzyme deactivation while preserving the ability of Ca2+ ions to occupy weaker binding sites capable of allosteric TG2 activation. In contrast, disulfide bond formation competitively occludes the high-affinity Ca2+ site while resulting in complete TG2 inactivation. Because both outcomes are comparably favorable under typical extracellular conditions, subtle changes in the availability of redox catalysts or promoters in the extracellular matrix can dramatically alter steady-state TG2 activity. Thus, TG2 harbors a molecular "OR" gate that determines its catalytic fate upon export from cells.

  View details for DOI 10.1021/jacs.1c04616

  View details for Web of Science ID 000677544800008

  View details for PubMedID 34232639