Differential inducibility of transmembrane peptidoglycan recognition proteins (PGRPs) by bacterial challenges in the earthworm, Eisenia andrei

•Five new Ean-PGRPLs were characterized as long-type PGRPs in earthworms.•Expression of Ean-PGRPL mRNAs is specifically induced by Gram-negative bacteria.•All Ean-PGRPLs are anticipated to interact with DAP-type PGN preferentially.•Ean-PGRPLs may act as PRR, linking intracellular signaling to Gram-negative bacteria. Peptidoglycan recognition proteins (PGRPs) and Toll-like receptors (TLRs) are highly conserved pattern recognition receptors (PRRs). Earthworms possess genes encoding TLRs that specifically respond to Gram-positive bacteria. In addition, several PGRPs have been recently identified, which are predicted to exhibit amidase activity but lack receptor function. In lophotrochozoans, a membrane-bound PRR responsible for detecting Gram-negative bacteria remains unidentified. This study reveals several novel transmembrane peptidoglycan recognition proteins (Ean-PGRPLs) in earthworms, whose mRNA expression increases in response to Gram-negative but not Gram-positive bacteria. This indicates that Ean-PGRPLs may serve as a PRR associated with intracellular signaling for Gram-negative bacteria.