Renal proximal tubule epithelial cells (PTCs) are vital to the function of the kidney and are also integral to the pathology of the injured kidney. They are the most sensitive cell type in the kidney to ischemia and nephrotoxic insults. PTCs are also involved in the signaling pathways governing inflammation, such as secretion of cytokines and presentation of antigens to MHC II, and resolution following injury. We have identified Kim-1 as the most highly upregulated protein in injured PTCs. While Kim-1 has been shown to be a promising diagnostic tool for detecting kidney injury, the pathobiological function of this protein and its role in kidney injury are not known. Our preliminary data suggest that Kim-1-mediated phagocytosis induces autophagy and cytoprotection in PTCs following acute kidney injury. Prolonged expression of Kim-1, however, leads to inflammation and tubule damage, mimicking chronic kidney disease. Furthermore, we have found that Kim-1-mediated phagocytosis and autophagy induction leads to antigen presentation, and depending on the ligands and duration of Kim-1 expression, the outcome could be pro or anti-inflammatory. Therefore, I propose that Kim-1 mediated phagocytosis of apoptotic cells in AKI induces autophagy, which leads to immune modulation through MHC presentation. In AKI, the consequences of autophagy are cytoprotective while in chronic states where the PT lumen contains other Kim-1 ligands, such as oxidized lipids, and fewer apoptotic cells, Kim-1 mediated uptake of these ligands induces a pro-inflammatory, profibrotic response. This is a multidisciplinary study that will build on my knowledge and research skills in cell biology and extend my training into novel areas such as immunology and mouse genetics. To address this hypothesis we will first examine if the protective effect of Kim-1 in AKI is due to its autophagy induction. We will test if Kim- 1-induced autophagy is protective against various insults. Then we will examine if mice expressing a Kim-1 phaogocytosis deficient mouse has an altered autophagic response. To understand how Kim-1 regulates autophagy, the role of Kim-1 phosphorylation and its interaction with GABARAP (an LC3 family member) in autophagy induction. Second, we will determine if Kim-1-mediated endocytosis contributes to chronic injury. Unlike phagocytosis of apoptotic cells, endocytosis of ligands such as ox-LDL, leads to mitochondrial fragmentation and caspase activation. We will test if transgenic overexpression of wt Kim-1 or Kim-1 mutant which can take up ox-LDL but not apoptotic cells leads to greater injury than overexpression of a mutant which takes up neither apoptotic cells or ox-LDL. We will then test if the mitochondrial fragmentation observed following Kim-1-mediated uptake of ox-LDL is due to activation of the mitochondrial fission pathway and whether inhibiting mitochondrial fragmentation prevents cellular injury induced by Kim-1-mediated endocytosis of ox-LDL. Finally, we will examine if Kim-1-induced autophagy modulates the PTC immune response through MHC presentation. We will examine the role of MHC II presentation in AKI and CKD through conditional knockout of MHC II. We will then determine if mice expressing Kim-1 phagocytosis deficient mutant have an altered immune response and autoantibody production. As proximal tubule cells also express co-stimulatory and co-inhibitory factors, we will test if Kim-1 induced autophagy modulates the expression of these factors. The findings from this study will shed light on the role of Kim-1 in acute and chronic kidney injury, as well as highlight the potential therapeutic benefits of modulating Kim-1 function in kidney injury.
Kidney injury continues to have unacceptably high morbidity and mortality with few therapeutic options. Kidney proximal tubule cells are the most sensitive cells in the kidney to ischemic and nephrotoxic insults. This study will highlight the role of Kim-, the most upregulated protein in the injured proximal tubule, in kidney injury and illustrate the therapeutic potential of modulating Kim-1 in acute and chronic kidney injury.
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|Freedman, Benjamin S; Brooks, Craig R; Lam, Albert Q et al. (2015) Modelling kidney disease with CRISPR-mutant kidney organoids derived from human pluripotent epiblast spheroids. Nat Commun 6:8715|
|Yang, Li; Brooks, Craig R; Xiao, Sheng et al. (2015) KIM-1-mediated phagocytosis reduces acute injury to the kidney. J Clin Invest 125:1620-36|
|Xiao, Sheng; Brooks, Craig R; Zhu, Chen et al. (2012) Defect in regulatory B-cell function and development of systemic autoimmunity in T-cell Ig mucin 1 (Tim-1) mucin domain-mutant mice. Proc Natl Acad Sci U S A 109:12105-10|