MDP phosphorylation by NAGK is essential for NOD2 activation

Bacterial cell wall components provide various unique molecular structures that are detected by pattern recognition receptors (PRRs) of the innate immune system as non-self. Most bacterial species form a cell wall that consists of peptidoglycan, a polymeric structure comprising of alternating amino sugars. Muramyl dipeptide (MDP) is the minimal, biologically active molecular structure derived from peptidoglycan, and its immunogenicity is well documented in numerous studies. MDP is sensed by the cytosolic nucleotide-binding and nucleotide oligomerization domain-like receptor 2 (NOD2), which triggers a pro-inflammatory response upon engagement 1,2. Conducting a forward genetic screen to identify factors required for MDP detection, we discovered that N-acetylglucosamine kinase (NAGK) is essential for the immunostimulatory activity of MDP. NAGK, which has previously been identified to contribute to the hexosamine salvage pathway in humans, is broadly expressed and inducible in innate immune cells. Mechanistically, NAGK functions upstream of NOD2 in that it directly phosphorylates the N-acetylmuramic moiety of MDP at the hydroxyl group of its C6 position, yielding 6-O-phospho-MDP. NAGK-phosphorylated MDP constitutes an essential component of NOD2 dependent signal transduction. Altogether, these results unveil a previously unknown interconnection of amino sugar metabolism and innate immunity to bacterial cell walls.