Bacteria-derived peptidoglycan triggers a non-canonical NF-κB dependent response in Drosophila gustatory neurons

Probing the external world is essential for eukaryotes to distinguish beneficial from pathogenic microorganisms. If it is clear that the main part of this task falls to the immune cells, recent work shows that neurons can also detect microbes, although the molecules and mechanisms involved are less characterized. In Drosophila, detection of bacteria-derived peptidoglycan by pattern recognition receptors of the PGRP family expressed in immune cells, triggers NF-κB/IMD dependent signaling. We show here that one PGRP protein, called PGRP-LB, is expressed in some proboscis’s bitter gustatory neurons. In vivo calcium imaging in female flies reveals that the PGRP/IMD pathway is cell-autonomously required in these neurons to transduce the peptidoglycan signal. We finally show that NF-κB/IMD pathway activation in bitter-sensing gustatory neurons influences fly behavior. This demonstrates that a major immune response elicitor and signaling module are required in the peripheral nervous system to sense the presence of bacteria in the environment. SIGNIFICANCE STATEMENT: In addition to the classical immune response, eukaryotes rely on neuronally-controlled mechanisms to detect microbes and engage in adapted behaviors. However, the mechanisms of microbe detection by the nervous system are poorly understood. Using genetic analysis and calcium imaging, we demonstrate here that bacteria-derived peptidoglycan can activate bitter gustatory neurons. We further show that this response is mediated by the PGRP-LC membrane receptor and downstream components of a non-canonical NF-κB signaling cascade. Activation of this signaling cascade triggers behavior changes. These data demonstrate that bitter-sensing neurons and immune cells share a common detection and signaling module to either trigger the production of antibacterial effectors or to modulate the behavior of flies that are in contact with bacteria. Since PGN detection doesn’t mobilize the known gustatory receptors, it also demonstrates that taste perception is much more complex than anticipated