Chronic kidney disease (CKD), like many complex disease phenotypes, reflects dependent interactions between genetic susceptibilities and environmental factors. APOL1 variants explain much of the excess risk of advanced non-diabetic CKD in patients of African ancestry. However, only some patients with risk genotypes develop kidney disease, suggesting that genetic epistasis or environmental stress is required to trigger variant APOL1-dependent kidney injury. To investigate how APOL1 variants result in CKD, we have focused on HIV- associated nephropathy (HIVAN), the disease most robustly associated with APOL1 risk haplotypes and clearly dependent on an environmental factor, HIV-1 infection. Our preliminary data demonstrate APOL1 is expressed in the podocyte, and APOL1 overexpression activates autophagy. Although mTor-dependent suppression of core autophagy may cause diabetic glomerulopathy, a renal pathology not associated with APOL1 variants, we provide evidence that the genetic association of APOL1 with CKD results from APOL1- dependent regulation of distinct, selective autophagy pathways, which result in the destruction of pathogens. We hypothesize that APOL1 is an autophagic adaptor that tethers a docking SNARE (VAMP8) displayed on lysosomes to the autophagosomal protein LC3-II. The tethering function of APOL1 is activated by binding a triggering molecule, the HIV protein Nef, and results in the selective degradation of autophagosomes carrying HIV viral particles and proteins. Variant APOL1 proteins are functionally defective, permitting HIV to persist in the infected cell and continue synthesis of viral proteins including Nef. Nef blocks autophagic elimination of the virus and is critical for development of HIVAN. Our proposed experiments address three unanswered and novel questions regarding APOL1 function in CKD. Is APOL1 synthesized in situ in kidney and what is its subcellular home? Does circulating or renal-expressed variant APOL1 mediate kidney disease? Does dysregulation of autophagy pathways activated in response to specific environmental stresses result in kidney disease in patients with APOL1 risk genotypes? These questions are addressed with the following Specific Aims: 1. Determine APOL1 expression and intracellular location in normal kidney, and determine if APOL1 localization varies with risk genotype and/or disease diagnosis;2. Generate in vivo models for the study of APOL1 function;3. Characterize protein interactions between APOL1 with VAMP8 and the HIV protein Nef and examine normal and variant APOL1 regulation of autophagy in cell lines and cultured podocytes. These studies can result in novel, mechanism-based therapies, improve health disparities in African American patients and identify novel mechanisms of human CKD.
Approximately 26 million Americans have evidence for chronic kidney disease (CKD) and more than 570,000 patients have kidney failure (end-stage renal disease [ESRD]). Health care for ESRD patients, who comprise less than 1% of Medicare recipients, accounts for 7% of Medicare expenditures, approximately $30 billion. ESRD patients are at risk for accelerated cardiovascular disease and early death. African American CKD patients in the United States are four times more likely than white patients to progress to ESRD, and a single genetic susceptibility locus (APOL1) explains much of this increased risk. This project's goal is to define biological reasons for the association of APOL1 with CKD in patients with African ancestry in order to improve health disparities in this high-risk population.
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