Phosphorylation of the podocyte cytoskeleton in diabetic nephropathy

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Podocyte dysfunction is an early, key event in the pathogenesis of diabetic nephropathy. Podocytes rely on their cytoskeleton to maintain their structure and function while facing constant mechanical stress inside the glomerulus. a-Actinin 4 (ACTN4) is an essential crosslinker of the actin cytoskeleton; mutations in ACTN4 lead to human kidney disease. Recent evidence has identified an important phosphorylation event in ACTN4 at serine (S) 159. In phosphomimetic cellular and animal models, this phosphorylation of ACTN4 is associated with podocyte vulnerability under mechanical stress. Moreover, phosphorylation of wild type (WT) ACTN4 at S159 is stimulated by high glucose and associated with cytoskeletal derangements, similar to derangements associated with disease-causing mutant ACTN4. The long-term goal that this R03 application advances is to understand the cytoskeleton’s role in podocyte dysfunction underlying diabetic nephropathy. The overall objective of the current proposal is to elucidate the pathway by which high glucose leads to phosphorylation of ACTN4 as a potential mediator of podocyte vulnerability. The central hypothesis is that increased phosphorylation of ACTN4 is stimulated by high glucose and fosters podocyte vulnerability to mechanical stress. The rationale for this project is that finding new pathways leading to podocyte vulnerability could fill critical gaps in knowledge related to the pathogenesis of diabetic nephropathy. To attain the overall objective of this application, the following two specific aims will be pursued. Aim 1 will define the association between phosphorylation of ACTN4 and diabetic nephropathy in vivo. Wild type (WT) control and diabetic nephropathy kidney tissue will be obtained from mice, rats, and humans. Targeted mass spectrometry will be used to quantify ACTN4 phosphorylation across all samples. Aim 2 will determine the impact of high glucose-mediated ACTN4 phosphorylation in vitro. Microfluidic glomeruli-on-chips will be seeded with either human podocytes carrying WT ACTN4 or human podocytes carrying nonphosphorylatable S159A ACTN4. These glomeruli-on-chips will be exposed to culture media containing high glucose while subjected to mechanical stretch and shear stress. The proposed research is innovative since it employs the latest, state-of-the art techniques to study phosphorylation of the cytoskeleton and podocyte vulnerability. The proposed research is significant because it will define a novel pathway by which high glucose mediates podocyte dysfunction through phosphorylation of the podocyte cytoskeleton. Demonstrating that (1) this pathway is upregulated in in vivo models of diabetic nephropathy and that (2) this pathway contributes to podocyte vulnerability under mechanical stress will provide strong justification to further study phosphorylation of ACTN4 as a mechanism underlying the onset and progression of diabetic nephropathy. Data from this R03 will support an R01 to identify the kinases and/or phosphatases that regulate phosphorylation of ACTN4. This future work will not only provide new mechanistic insights into podocyte dysfunction involved in diabetic nephropathy, but also potential targets for novel treatments that mitigate podocyte dysfunction.