The structure of NAD+ consuming protein Acinetobacter baumannii TIR domain shows unique kinetics and conformations.

Toll-like and interleukin-1/18 receptor/resistance (TIR) domain-containing proteins function as important signaling and immune regulatory molecules. TIR domain-containing proteins identified in eukaryotic and prokaryotic species also exhibit NAD+ hydrolase activity in select bacteria, plants, and mammalian cells. We report the crystal structure of the Acinetobacter baumannii TIR domain protein (AbTir-TIR) with confirmed NAD+ hydrolysis and map the conformational effects of its interaction with NAD+ using hydrogen-deuterium exchange-mass spectrometry. NAD+ results in mild decreases in deuterium uptake at the dimeric interface. In addition, AbTir-TIR exhibits EX1 kinetics indicative of large cooperative conformational changes, which are slowed down upon substrate binding. Additionally, we have developed label-free imaging using the minimally invasive spectroscopic method 2-photon excitation with fluorescence lifetime imaging, which shows differences in bacteria expressing native and mutant NAD+ hydrolase-inactivated AbTir-TIRE208A protein. Our observations are consistent with substrate-induced conformational changes reported in other TIR model systems with NAD+ hydrolase activity. These studies provide further insight into bacterial TIR protein mechanisms and their varying roles in biology.

Lipopolysaccharide induction of autophagy is associated with enhanced bactericidal activity in Dictyostelium discoideum.

Innate immune cells respond to microbial invaders using pattern recognition receptors that detect conserved microbial patterns. Among the cellular processes stimulated downstream of pattern recognition machinery is the initiation of autophagy, which plays protective roles against intracellular microbes. We have shown recently that Dictyostelium discoideum, which takes up bacteria for nutritive purposes, may employ pattern recognition machinery to respond to bacterial prey, as D. discoideum cells upregulate bactericidal activity upon stimulation by lipopolysaccharide (LPS). Here we extend these findings, showing that LPS treatment leads to induction of autophagosomal maturation in cells responding to the bacteria Staphylococcus aureus. Cells treated with the autophagy-inducing drug rapamycin clear internalized bacteria at an accelerated rate, while LPS-enhanced clearance of bacteria is reduced in cells deficient for the autophagy-related genes atg1 and atg9. These findings link microbial pattern recognition with autophagy in the social amoeba D. discoideum.

Cobbling Together the Myddosome.

An organism's ability to recognize and respond quickly and appropriately to pathogenic stimuli is a fundamental aspect of innate immunity. Harnessing the dynamic nature of fluorescent microscopy and the resolution of cryo-electron microscopy, Moncrieffe et al. (2020) characterize MyD88-only filaments and provide insight into the mechanisms underlying innate immune signaling.

Bacterial discrimination: Dictyostelium's discerning taste.

New research indicates that the social amoeba Dictyostelium discoideum recognizes distinctions between Gram(-) and Gram(+) bacterial prey and responds discriminately to these two groups of bacteria. These findings may lend insight to the origins of microbial pattern recognition in innate immunity.

Lipopolysaccharide enhances bactericidal activity in Dictyostelium discoideum cells.

Innate immune cells respond to invading microbes upon detection of pathogen-associated molecular patterns (PAMPS). PAMP-recognition machinery is evolutionarily conserved, allowing for characterization in model organisms. The model organism Dictyostelium discoideum can exist as single-celled amoebae, which phagocytize bacteria for nutrients. Although D. discoideum is used extensively to study phagocytosis, it has not been determined if D. discoideum detects bacterial PAMPs using pattern-recognition machinery. Here we show that D. discoideum mounts responses against the bacterial cell wall PAMP, lipopolysaccharide (LPS). Upon treatment with LPS or its active component Lipid A, D. discoideum cells more efficiently clear phagocytized bacteria. LPS-enhanced bactericidal activity appears dependent both on MAPK signaling pathways as well as on the D. discoideum toll/interleukin-1 receptor domain-containing protein, TirA. These findings indicate that pattern-recognition machinery required to detect and respond to bacterial PAMPs may be conserved in D. discoideum.