Our goal is to analyze the molecular mechanisms that underlie the formation of tubulointerstitial (TI) fibrosis, to devise more effective therapies to prevent its progression. We study the collagen receptor integrins 11 and showed that it downregulates collagen synthesis, and its loss leads to increased glomerular fibrosis following injury. Integrin 11 plays an anti-fibrotic action by negatively regulating the phosphorylation state of pro-fibrotic growth factor receptors via activation of the tyrosine phosphatase TCPTP. We have also found that integrin 11 is a negative regulator of TI fibrosis, as integrin 1KO mice show increased unilateral ureteral obstruction-induced fibrosis. Moreover, integrin 1KO collecting duct (CD) cells have increased activation of TGF- receptor (TR)-dependent pro-fibrotic signaling, such as phosphorylated Smad3 and collagen levels. TGF- exerts its functions via activation of the serine/threonine kinases TRI and TRII. Binding of TGF- to TRII leads to phosphorylation of TRI and subsequent activation of the major pro-fibrotic mediator Smad3. Interestingly, the cytoplasmic tail of TRII can also be phosphorylated on tyrosine residues. However, whether these tyrosines play any physiological or pathological role in renal cells is unknown. The novelty of this proposal is that integrin 1KO CD cells show increased basal levels of tyrosine phosphorylated TRII. This result suggests that, in renal cells, integrin 11 crosstalks with TRII and it might prevent its pro-fibrotic action by downregulating its tyrosine phosphorylation levels. Interestingly, inhibition of TCPTP in CD cells leads to increased Smad3 activation and collagen synthesis. This result, together with the finding that 3 tyrosines in the TRII tail can be potential substrates of TCPTP, forms the hypothesis that integrin 11 negatively regulates TRII tyrosine phosphorylation via activation of TCPTP. Thus, we propose that integrin 11/TCPTP-mediated dephosphorylation of TRII represents an important, but previously undescribed mechanism to selectively reduce TRII activation and consequent progression of fibrosis. To test this hypothesis: Aim 1 will analyze the role of TRII-mediated pro-fibrotic signaling in TI injury in the integrin 1KO mice. We will cross integrin 1KO mice with global null or floxed Smad3 mice to determine if preventing TR/Smad3 axis in the collecting system is sufficient to ameliorate TI fibrosis in the 1KO mice. We will then determine if in vivo activation of TCPTP is beneficial in the setting of TI injury by counteracting TR-mediated pro-fibrotic action. Aim 2 will determine the mechanisms whereby integrin 11 negatively regulates TRII. We will use in vitro approaches to analyze i) if TCPTP directly binds and dephosphorylates TRII; ii) if tyrosine residues are important to control TRII-mediated Smad3 activation and collagen synthesis; and iii) which tyrosine(s) controls TRII-mediated functions. This study will lead to the identification of a novel crosstalk between integrin 11 an TRII and, most importantly, a novel mechanism whereby TRII-mediated pro-fibrotic signaling can be negatively modulated.