Acute kidney injury (AKI) is a major determinant of mortality and morbidity in hospitalized veterans. Understanding the pathophysiology of AKI is essential for the development of therapy. This application proposes to study the role of Tamm-Horsfall protein (THP) in AKI. This protein is expressed exclusively in the kidney by cells of the thick ascending limbs (TAL) of Henle. In the last funding cycle, we uncovered a key role of THP in mediating a protective tubular cross-talk in AKI. We showed that THP regulates inflammatory signaling and injury to S3 segments, which are neighboring tubules to TAL in the kidney outer medulla. The immediate goal of the current proposal is to study the importance of the interaction of THP with S3 segments in regulating neutrophil infiltration in AKI. These studies will form the basis to transition into therapeutic applications. Indeed, the PI's long term goal is to understand how the role of THP can be modulated to treat veterans who develop AKI. The central hypothesis is that Tamm-Horsfall protein regulates the inflammatory response triggered by ischemic injury in the outer medulla; an effect that requires an interaction of THP with the surrounding tubular elements. This hypothesis has been formulated on the basis of strong preliminary and published data. The following specific aims will be used to investigate this hypothesis. Aim 1 will investigate the role of THP as a key regulator of neutrophil infiltration in the kidney after AKI. Aim 2 will study specific inflammatory pathways regulated by THP in the neighboring epithelium during kidney injury Aim 2 will investigate therapeutic modulation of THP to inhibit neutrophil influx and promote recovery after AKI This research will involve the use of THP knockout and wild type mice. The animal model used for AKI is renal ischemia-reperfusion injury achieved through renal pedicle clamping. Other techniques used in this work include: Immunofluorescence laser micro-dissection of specific tubular sections, Immunofluoresence confocal and intravital microscopy, flow cytometry, real time-PCR, western blot, various forms of chromatography and mass spectrometry. This research is innovative, because it will uncover novel regulatory functions for THP that may be relevant not only in kidney injury but also in other forms of acute or chronic renal disease. Future strategies that enhance the role of THP in the kidney will be of important therapeutic significance.