Methotrexate suppresses psoriatic skin inflammation by inhibiting muropeptide transporter SLC46A2 activity.

Cytosolic innate immune sensing is critical for protecting barrier tissues. NOD1 and NOD2 are cytosolic sensors of small peptidoglycan fragments (muropeptides) derived from the bacterial cell wall. These muropeptides enter cells, especially epithelial cells, through unclear mechanisms. We previously implicated SLC46 transporters in muropeptide transport in Drosophila immunity. Here, we focused on Slc46a2, which was highly expressed in mammalian epidermal keratinocytes, and showed that it was critical for the delivery of diaminopimelic acid (DAP)-muropeptides and activation of NOD1 in keratinocytes, whereas the related transporter Slc46a3 was critical for delivering the NOD2 ligand MDP to keratinocytes. In a mouse model, Slc46a2 and Nod1 deficiency strongly suppressed psoriatic inflammation, whereas methotrexate, a commonly used psoriasis therapeutic, inhibited Slc46a2-dependent transport of DAP-muropeptides. Collectively, these studies define SLC46A2 as a transporter of NOD1-activating muropeptides, with critical roles in the skin barrier, and identify this transporter as an important target for anti-inflammatory intervention.

Synthesis and validation of click-modified NOD1/2 agonists.

NOD1 and NOD2 sense small bacterial peptidoglycan fragments, often called muropeptides, that access the cytosol. These muropeptides include iE-DAP and MDP, the minimal agonists for NOD1 and NOD2, respectively. Here, we synthesized and validated alkyne-modified muropeptides, iE-DAP-Alk and MDP-Alk, for use in click-chemistry reactions. While it has long been known that many cell types respond to extracellular exposure to muropeptides, it is unclear how these innate immune activators access their cytosolic innate immune receptors, NOD1 and NOD2. The subcellular trafficking and transport mechanisms by which muropeptides access these cytosolic innate immune receptors are a major gap in our understanding of these critical host responses. The click-chemistry-enabled agonists developed here will be particularly powerful to decipher the underlying cell biology and biochemistry of NOD1 and NOD2 innate immune sensing.