Acinetobacter baumannii is an important nosocomial pathogen that causes a range of diseases, including respiratory and urinary tract infections, meningitis, endocarditis, wound infections, and bacteremia. In fact, A. baumannii is now responsible for up to 20% of all intensive care unit infections in some regions of the world with pneumonia being the most common presentation. The clinical significance of A. baumannii has been propelled by this organism's rapid acquisition of resistance to virtually all antibiotics. The identification of novel targets for therapeutic intervention is critical to our ability to protect the public health from this emerging infectious threat. One promising potential area of therapeutic development involves targeting bacterial access to nutrient metal or metal handling. This strategy is based on the fact that all bacterial pathogens require nutrient metal in order to colonize their hosts, and alterations in dietary metal levels profoundly affect susceptibility to infection. The host protein calprotectin (CP) is one of the most important contributors to immune-mediated metal restriction and CP protects against infection through the chelation of nutrient zinc (Zn) and manganese (Mn). In this application, we describe our use of CP as a probe to uncover a genetic locus within A. baumannii that is involved in survival during conditions of CP-dependent Zn starvation. This locus encodes a member of the conserved COG0523 family of GTPases that we have named Zur-induced GTPase A (ZigA), and a D-alanine D-alanine carboxypeptidase that we have named Zn-regulated lipoprotein A (ZrlA). We have discovered that ZigA is a metallochaperone that provides Zn to client proteins and is required for the liberation of a biovavailable Zn pool during conditions of Zn starvation. This finding establishes ZigA as the first example of a Zn metallochaperone in nature. In addition, our model predicts that ZrlA is required to maintain cell wall architecture during Zn stress. Based on these fundamental discoveries, we hypothesize that upon Zn starvation, A. baumannii mobilizes Zn to critical Zn-requiring client proteins, while also activating the expression of a carboxypeptidase involved in peptidoglycan remodeling that substitutes for the loss of activity of a Zn-requiring paralog. To test this central hypothesis, we propose a series of experiments aimed at understanding the mechanism and pathophysiological consequence of the A. baumannii response to dietary and host-imposed Zn deprivation during the pathogenesis of pneumonia. In these studies, we will (i) define the target of the ZigA Zn-metallochaperone during conditions of CP- imposed Zn restriction, (ii) elucidate the contribution of ZrlA to peptidoglycan remodeling during conditions of Zn deprivation, and (iii) determine the importance of Zn distribution during the pathogenesis of A. baumannii pneumonia. These results will provide fundamental insight into how A. baumannii responds to nutrient deprivation in the vertebrate host, and lay the foundation for the creation of peptide therapeutics based on a CP scaffold that inhibit microbial growth through nutrient metal chelation.
This proposal will define the strategies employed by the important human pathogen Acinetobacter baumannii to combat calprotectin-dependent zinc starvation during the pathogenesis of pneumonia. All microbial pathogens require zinc acquisition and metabolism to colonize the vertebrate host; therefore these studies will be broadly applicable to numerous infectious diseases. Moreover, this work has the potential to uncover new principles in metal biology that may lay the groundwork for the design of intervention measures to target these emerging infectious threats.
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