A mathematical Model of Signaling in Podocyte Foot Processes

Abstract

The first stage of blood filtration occurs in the glomerulus, where water and other small sized molecules are freely filtered into the urinary space while albumin and larger proteins are retained in the blood capillaries. Maintenance of the size-selective glomerular filtration barrier is regulated by highly differentiated cells, podocytes, their cell-cell interactions in the slit diaphragm and the cell-GBM (glomerular basement membrane) interaction of the podocyte foot processes. Mutations of the nephrin gene (NHPHS1) triggers actin reorganization, loss of the podocyte functional morphology and massive proteinuria. The glomerular tissue is challenging to study in vitro, because podocytes from isolated glomeruli undergo de-differentiation within hours, while cultures of stabilized cell lines never complete differentiation. Therefore, to provide insight into how the integrity of the filtration barrier is dynamically maintained, we have developed a mathematical model of the podocyte that preserves the spatial organization found in the intact glomerulus, focusing on the nephrin pathway. Nephrin has four binding sites phosphorylated by the same mechanism (nephrin clustering and Fyn phosphorylation) that can trigger the formation of actin branches via Arp2/3. Importantly, Nck (a scaffold that coordinates F-actin nucleation), PLCγ1 (an enzyme that hydrolyzes PIP2) and podocin (important for nephrin localization) all compete for one of the phosphorylation sites on nephrin. The model includes complex formation around nephrin and several proteins involved in actin cytoskeleton remodeling, such as PI3K and Fyn. We use rule-based modeling to account for multiple complexes and phosphoforms and investigate the signaling on the level of domain-domain interactions. The model is associated with the spatial organization of the foot processes, incorporating localization effects. (supported by NIH grant TRO1DK087650)