Cyclic diguanylate monophosphate directly binds to human siderocalin and inhibits its antibacterial activity

Abstract

Cyclic diguanylate monophosphate (c-di-GMP) is a well-conserved second messenger in bacteria. During infection, the innate immune system can also sense c-di-GMP; however, whether bacterial pathogens utilize c-di-GMP as a weapon to fight against host defense for survival and possible mechanisms underlying this process remain poorly understood. Siderocalin (LCN2) is a key antibacterial component of the innate immune system and sequesters bacterial siderophores to prevent acquisition of iron. Here we show that c-di-GMP can directly target the human LCN2 protein to inhibit its antibacterial activity. We demonstrate that c-di-GMP specifically binds to LCN2. In addition, c-di-GMP can compete with bacterial ferric siderophores to bind LCN2. Furthermore, c-di-GMP can significantly reduce LCN2-mediated inhibition on the in vitro growth of Escherichia coli. Thus, LCN2 acts as a c-di-GMP receptor. Our findings provide insight into the mechanism by which bacteria utilize c-di-GMP to interfere with the innate immune system for survival.

A Pseudomonas T6SS effector recruits PQS-containing outer membrane vesicles for iron acquisition

“…This arm race drives bacterial pathogens to evolve sophisticated mechanisms to scavenge this essential nutrient from the environment, including their hosts. Commonly used strategies include: the synthesis and secretion of siderophores, molecules that bind iron with high affinity in extracellular milieu and re-enter cells via specific transporters56; the use of receptors on cell surface to recognize and import haeme and haemoprotein, which after being shuttled across the cell wall, are degraded by a haeme oxygenase to release iron7; expression of proteins that directly target host iron-binding proteins, such as transferrin and lactoferrin89; and interference with the function of host proteins involved in iron sequestration10. It is important to note that a bacterial pathogen often utilize combinations of these strategies to effectively compete for iron to maximize their proliferation under specific conditions.…”

Cyclic Dimeric Guanosine Monophosphate: Activation and Inhibition of Innate Immune Response

“…Our in vitro assay shows that c-di-GMP can rescue the growth of Escherichia coli and M. tuberculosis H37Ra from LCN2-mediated inhibition [24]. Compared with the secretion of LCN2-resistant siderophores, this result represents a novel molecular mechanism of bacterial response to LCN2-mediated iron restriction by the cooperation of c-di-GMP and bacterial siderophores (Fig.…”

“…To date, three direct immune proteins targeted by c-di-GMP, namely, STING [18], DDX41 [22], and LCN2 [24], have been reported. However, the interaction between c-di-GMP with the host immune proteins leads to two intuitively opposite consequences for bacteria.…”

“…Recently, by using liquid chromatography and mass spectrometry, we determined the concentrations of c-di-GMP in bacterial culture supernatants to be 1.042 ± 0.12 mM for E. coli cells and 0.568 ± 0.06 mM for M. tuberculosis [24]. In addition, c-di-GMP was found to regulate type III and type VI secretion systems in Pseudomonas aeruginosa [39, 40].…”

“…Subsequent analysis indicated that c-di-GMP plays a novel role in the iron seesaw battle between bacteria and host [24]. …”