ABSTRACT Chronic kidney disease (CKD) affects an estimated 37 million people in the United States. CKD progression involves activation of inflammatory and fibrotic responses leading to irreversible damage and loss of kidney function. The (pro)renin receptor (PRR) is implicated in the pathogenesis of CKD and can exist as the full length form, bound to cell membrane or be cleaved to generate a soluble PRR (sPRR) and M8.9 fragments. Although the function of the full-length PRR both at a molecular and system level has been studied to some extent, the pathophysiologic role of sPRR in CKD is unknown. This is especially important, since elevated plasma sPRR levels have been described in patients with CKD and correlates with the stage of CKD. We recently developed a novel mouse model with absence of sPRR using CRISPR-Cas9 directed mutagenesis of the PRR cleavage site. Preliminary analyses show mutant sPRR mice have reduced renal injury, inflammation and fibrosis compared to control mice and may involve inflammatory signaling and oxidative phosphorylation pathways. The following specific aims will be addressed: 1. Investigate the pathophysiological role of sPRR in kidney disease. CKD will be induced in control and mutant sPRR mice using adenine diet or unilateral ureteral obstruction. Renal function, tubular injury, inflammation and fibrosis will be examined in conjunction with targeted comparative transcriptomics to identify active signaling pathways. 2. Investigate the cellular mechanisms by which sPRR modulates kidney injury. sPRR regulation of inflammatory signaling pathways and oxidative phosphorylation and mitochondrial function will be examined in primary proximal tubule cell culture from control and mutant sPRR mice in presence of adenine or TGF-. Recombinant sPRR will be added to control and mutant cells lacking sPRR to examine if restoring sPRR levels reverses the renoprotective effects. 3. Investigate the molecular interaction partners of sPRR. How sPRR mediates intracellular cell signaling will be examine by identifying protein-protein interactors through structure- guided affinity purification and mass-spectrometry and enzyme catalyzed proximity labeling in HEK293 cells. Mass spectrometry and proteomics analyses will delineate sPRR protein interaction and signaling under physiological conditions and in kidney disease. This proposal examines a novel modulator of kidney injury and fibrosis and will delineate the mechanisms involved in mediating these effects. The integrative approach used herein will identify systemic and molecular effects of sPRR in fibrosis and may help in the development of a new therapeutic approach for CKD.