Iron metabolism at the host pathogen interface: lipocalin 2 and the pathogen-associated iroA gene cluster

Abstract

The host innate immune defense protein lipocalin 2 binds bacterial enterobactin siderophores to limit bacterial iron acquisition. To counteract this host defense mechanism bacteria have acquired the iroA gene cluster, which encodes enzymatic machinery and transporters that revitalize enterobactin in the form of salmochelin. The iroB enzyme introduces glucosyl residues at the C5 site on 2,3-dihydroxybenzoylserine moieties of enterobactin and thereby prevents lipocalin 2 binding. Additional strategies to evade lipocalin 2 have evolved in other bacteria, such as Mycobacteria tuberculosis and Bacillus anthracis. Targeting these specialized bacterial evasion strategy may provide a mechanism to reinvigorate lipocalin 2 in defense against specific pathogens.

Figure 1. Competition for iron at the host pathogen interface

Bacteria synthesize enterobactin siderophores from chorismate using genes encoded by the ent operon. Enterobactin is transported out of the inner membrane via EntS, shuttled through the periplasmic space via an unknown mechanism, and secreted through the outer membrane protein TolC. Enterobactin can be glucosylated by IroB to generate salmochelin. The transport of salmochelin is still under investigation, and the functional role of IroC is disputed. The IroC protein has been implicated as a inner membrane exporter (Baumler et al., 1996) and an inner membrane importer (Zhu, Valdebenito, Winkelmann, & Hantke, 2005). We have illustrated the former model where salmochelin is exported through the inner membrane protein IroC, and then shuttled across the periplasmic space and secreted through the outer membrane via an unknown mechanism. Both siderophores bind iron and are transported into the cells by their respective transporters (FepA for enterobactin and IroN for salmochelin) in a TonB complex-dependent manner. An uncharacterized mechanism (possibly involving IroC) shuttles the salmochelin when bound to iron through the periplasm and across the inner membrane. FepB shuttles enterobactin through the periplasm and the FepD/FepG/FepC complex transports enterobactin across the inner membrane into the cell cytoplasm. Once in the bacterial cell cytoplasm, the siderophores are degraded to release iron. Host cells, such as macrophages, sense the presence of bacteria via their innate immune receptors (Toll-like receptors, TLR, and NOD- like receptors, NLR), and induce the expression and secretion of lipocalin 2 (Lcn2). Neutrophils constitutively express lipocalin 2 in their granules and release their granule contents upon activation. The secreted lipocalin 2 binds enterobactin and efficiently competes with FepA to prevent enterobactin-mediated iron acquisition. Salmochelin does not bind lipocalin 2 and circumvents this host defense mechanism. Other signals, such as ischemic or toxic injury, can induce lipocalin 2 expression via unknown mechanisms, presumably to fortify host defense in this state of increased vulnerability.