Synthetic Studies on Labeled Nod1 Ligands for Cellular Imaging

Nod1 is known as an intracellular innate immune receptor (NLR family member) that recognizes bacterial cell wall component peptidoglycan (PGN), particularly the muropeptides containing γ-D-glutamyldiaminopimelic acid (γ-D-Glu-DAP; iE-DAP), from Gram-negative bacteria and some Gram-positive bacteria [1]. Nod1, then, regulates innate and adaptive immune responses by activating transcription factors including NF-κB. It has also been reported that the genetic variations in Nod1 are associated with susceptibility to allergic diseases such as athema. We have thus synthesized various fragment structures of PGN and analyzed the structure-activity relationships of their immunomodulation, including Nod1 activation. However, the detailed ligand’s recognition by Nod1 and the behavior in cells are not fully understood. In order to observe and understand the dynamics and the localization of the Nod1 ligand, we have synthesized fluorescence-labeled Nod1 ligands, based on our previously reported methods [2]. Because of the difficulties of introducing a linker to the ligands, we have tried several positions for the introduction site of the linker. The ligand structures for the labeling include iE-DAP, monosaccharide-tripeptide (MurNAc-L-Ala-D-Glu-meso-DAP), and disaccharide (MurNAc-GlcNAc) tripeptide (L-Ala-D-Glu-meso-DAP) structure. The synthesis of the labeled Nod1 ligands, evaluation of their Nod1 activity, and the cellular imaging will be discussed.