Many proteins have been proposed to act as surrogate markers of organ damage, yet for many candidates the essential characteristics which link the protein to the injured organ have not yet been described. We generated an NGAL-reporter mouse by inserting a di-fusion reporter gene, Luciferase2(Luc2)/mCherry(mC) into the Ngal locus. The Ngal-Luc2/mC reporter accurately recapitulated the endogenous message and illuminated injuries in vivo in real-time. In the kidney, Ngal-Luc2/mC imaging showed a sensitive, rapid, dose-dependent, reversible, and organ and cellular specific relationship with tubular stress, which quantitatively paralleled urinary Ngal (uNgal). Unexpectedly, specific cells of the distal nephron were the source of uNgal. Cells isolated from Ngal-Luc2/mC mice could also track both the onset and the resolution of the injury, and monitor the actions of NF-κB inhibitors and antibiotics in the case of infection. Accordingly, the imaging of Ngal-Luc2/mC mice and cells identified injurious and reparative agents which effect kidney damage.

The lipocalins are secreted proteins that bind small organic molecules. Scn-Ngal [known as Neutrophil Gelatinase Associated Lipocalin, Siderocalin, Lipocalin 2] sequesters bacterial iron chelators, called siderophores, and consequently blocks bacterial growth. However, Scn-Ngal is also prominently expressed in aseptic diseases, implying that it binds additional ligands and serves additional functions. Using chemical screens, crystallography, and fluorescence methods, we report that Scn-Ngal binds iron together with a small metabolic product called catechol. The formation of the complex blocked the reactivity of iron and permitted its transport once introduced into circulation in vivo. Scn-Ngal then recycled its iron in endosomes by a pH sensitive mechanism. Since catechols derive from bacterial and mammalian metabolism of dietary compounds, the Scn-Ngal:catechol:iron complex represents an unforeseen microbial-host interaction, which mimics Scn-Ngal:siderophore interactions, but instead traffics iron in aseptic tissues. These results identify an endogenous siderophore, which may link the disparate roles of Scn-Ngal in different diseases.

The arginine catabolic mobile element (ACME) is the largest genomic region distinguishing epidemic USA300 strains of methicillin-resistant Staphylococcus aureus (MRSA) from other S. aureus strains. However, the functional relevance of ACME to infection and disease has remained unclear. Using phylogenetic analysis, we have shown that the modular segments of ACME were assembled into a single genetic locus in Staphylococcus epidermidis and then horizontally transferred to the common ancestor of USA300 strains in an extremely recent event. Acquisition of one ACME gene, speG, allowed USA300 strains to withstand levels of polyamines (e.g., spermidine) produced in skin that are toxic to other closely related S. aureus strains. speG-mediated polyamine tolerance also enhanced biofilm formation, adherence to fibrinogen/fibronectin, and resistance to antibiotic and keratinocyte-mediated killing. We suggest that these properties gave USA300 a major selective advantage during skin infection and colonization, contributing to the extraordinary evolutionary success of this clone.

Lactobacillus iners is a common constituent of the human vaginal microbiota. This species was only recently characterized due to its fastidious growth requirements and has been hypothesized to play a role in the pathogenesis of bacterial vaginosis. Here we present the identification and molecular characterization of a protein toxin produced by L. iners. The L. iners genome encodes an open reading frame with significant primary sequence similarity to intermedilysin (ILY; 69.2% similarity) and vaginolysin (VLY; 68.4% similarity), the cholesterol-dependent cytolysins from Streptococcus intermedius and Gardnerella vaginalis, respectively. Clinical isolates of L. iners produce this protein, inerolysin (INY), during growth in vitro, as assessed by Western analysis. INY is a pore-forming toxin that is activated by reducing agents and inhibited by excess cholesterol. It is active across a pH range of 4.5 to 6.0 but is inactive at pH 7.4. At sublytic concentrations, INY activates p38 mitogen-activated protein kinase and allows entry of fluorescent phalloidin into the cytoplasm of epithelial cells. Unlike VLY and ILY, which are human specific, INY is active against cells from a broad range of species. INY represents a new target for studies directed at understanding the role of L. iners in states of health and disease at the vaginal mucosal surface.The cholesterol-dependent cytolysins (CDCs) are a family of protein toxins produced by a wide range of Gram-positive bacteria. CDCs share several characteristics, including a fourdomain structure, a requirement for membrane cholesterol for efficient activity, and an ability to form large pores in host cells, exceeding 150 Å in diameter (38). In general, soluble CDC monomers are secreted into the extracellular environment and bind to target cell membranes through direct recognition of cholesterol or, in the cases of the human-specific toxins vaginolysin (VLY) from Gardnerella vaginalis and intermedilysin (ILY) from Streptococcus intermedius, via recognition of human CD59 on the target cell surface (12,14,21). Following membrane association, CDCs oligomerize to form a prepore structure, a process that is dependent upon the availability of cholesterol (22)(23)(24)34). In many cases, CDCs are required for virulence for their cognate organisms, and rather than acting solely as cytolytic toxins, CDCs may have more sophisticated roles in disease pathogenesis (15,18,30). Understanding CDC evolution and host specificity is of considerable interest and has been limited by incomplete knowledge of the diversity of the CDC family. In particular, characterization of cytolysins most closely related to those in which host specificity has evolved may provide additional insights into the mechanism and effects of such restriction.Lactobacillus iners is a relatively recently recognized member of the human vaginal microbiota (9, 33, 42) that was initially overlooked because of its inability to grow on de ManRogosa-Sharpe agar, which is normally used to isolate vaginal lactobacilli. In healthy women...

α-Intercalated cells (A-ICs) within the collecting duct of the kidney are critical for acid-base homeostasis. Here, we have shown that A-ICs also serve as both sentinels and effectors in the defense against urinary infections. In a murine urinary tract infection model, A-ICs bound uropathogenic E. coli and responded by acidifying the urine and secreting the bacteriostatic protein lipocalin 2 (LCN2; also known as NGAL). A-IC-dependent LCN2 secretion required TLR4, as mice expressing an LPS-insensitive form of TLR4 expressed reduced levels of LCN2. The presence of LCN2 in urine was both necessary and sufficient to control the urinary tract infection through iron sequestration, even in the harsh condition of urine acidification. In mice lacking A-ICs, both urinary LCN2 and urinary acidification were reduced, and consequently bacterial clearance was limited. Together these results indicate that A-ICs, which are known to regulate acid-base metabolism, are also critical for urinary defense against pathogenic bacteria. They respond to both cystitis and pyelonephritis by delivering bacteriostatic chemical agents to the lower urinary system.