Project Summary (Abstract) Urinary tract infections (UTI) are one of the most common bacterial infections and are caused predominantly by uropathogenic Escherichia coli (UPEC). While broad-spectrum antimicrobials have been the drug of choice to treat UTIs, the emergence of antimicrobial drug resistance and related recurrent UTI suggests that this is not an effective approach to treat all UTIs. Hence, further understanding of the intrinsic host defense mechanisms during bladder and kidney infection should provide new targets for selective, antimicrobial-sparing therapeutic approaches. There is growing evidence that mucin 1 (MUC1) is a crucial element in the host defense against infection. MUC1 is a heterodimeric transmembrane glycoprotein expressed on the apical surface of polarized epithelial cells in several tissues including the bladder and kidney. The two subunits remain non-covalently associated but the larger mucin-like subunit is cleaved and shed into the lumen, while the cytoplasmic domain of the smaller subunit is involved in signal transduction through interactions with numerous kinases and transactivation of transcription factors after trafficking to the nucleus. It is well established that MUC1 protects the gastrointestinal and respiratory tracts by acting as a decoy receptor for bacterial adhesins, thereby limiting infection and colonization, and more importantly plays a significant role as a modulator of pathogen-induced inflammation. However, the role of MUC1 in host response and cellular signaling during UTI has not been investigated. We hypothesize that MUC1, a transmembrane glycoprotein, plays a central role during UTI by acting as a host receptor for bacterial pathogens and modulating host responses to microbial infection. Using purified MUC1, cultured cells with different MUC1 expression levels and MUC1 glycosylation patterns, and newly generated bladder- and kidney- specific KO mice (Uroth-Muc1-/- and Kd-Muc1-/-, respectively) global Muc1-/- mice and littermate controls in our murine model of experimental UTI with different bacterial strains, we will (i) characterize UPEC binding to MUC1 and determine whether MUC1 ECD acts as a cellular receptor (uroplakin model) or rather like a decoy receptor (uromodulin model) for UPEC, (ii) verify whether UPEC interactions with MUC1 result in the enhancement or the suppression of bacterial infection, (iii) determine UT epithelial cells demonstrate MUC1 dependent responses to UPEC challenge, and finally (iv) assess MUC1-mediated host signaling and transcriptional activities while dissecting the role of the two MUC1 domains (ECD vs CD) upon UPEC challenge. This a new direction in my research focus from the role of MUC1 in ischemic injury in the kidney to urinary tract host defense against bacterial infection will provide crucial insights into UTI pathogenesis and will identify novel alternative targets to treat UTI and prevent its recurrence.