Shuta Ishibe MD
Assistant Professor of Medicine (Nephrology)
Podocytes; Glomerular Filtration Barrier
Analyzing the role of podocyte cell-matrix interactions
Analyzing the role of podocyte endocytosis
Our laboratory is interested in defining the mechanism of proteinuria by studying podocytes, which are specialized cells that line the capillary loops and play a key role in maintenance of the glomerular filtration barrier. We have identified a network of proteins that bind directly or indirectly to proteins that human genetic studies have implicated to be causal for nephrotic syndrome. Through mice genetics, loss of specific proteins in this network has demonstrated severe proteinuria, and podocyte foot process effacement. By utilizing fluorescently tagged proteins,we have visualized that these proteins lie at the interface of endocytosis and the actin cytoskeleton, and are implicated in controlling the formation and maintenance of the glomerular filtration barrier.
Extensive Research Description
My research in Dr. Lloyd Cantley’s laboratory (Professor of Internal Medicine at Yale University) had focused on hepatocyte growth factor (HGF), a secreted protein that plays important roles in epithelial morphogenesis, mitogenesis, and motogenesis. We made the novel discovery that paxillin functions as a scaffolding protein that regulates the HGF-dependent activation of ERK at focal adhesions. In a follow-up study, we demonstrated that paxillin itself is a substrate to ERK-dependent phosphorylation, which recruits FAK and Rac and is necessary for HGF-dependent tubulogenesis. These findings were published as two first-authored papers in Molecular Cell and are important because they elucidate the mechanism by which HGF regulates focal adhesion turnover, which is essential for cell spreading and morphogenesis. More generally, they explain how compartmentalization of MAP kinase signaling is achieved through local activation on scaffolding proteins, such as paxillin. In a third study we observed that during in vitro tubulogenesis, the cells located in the middle of the tubules appeared well differentiated whereas cells located at the ends had a mesenchymal appearance. We discovered that cell confluency modulates the responsiveness to HGF through its effects on the activation of Akt and beta-catenin. These findings were published in MCB and are relevant to the mechanism of tubular regeneration after acute kidney injury, which we hypothesize involves HGF-stimulated cell spreading, proliferation, and dedifferentiation
After starting my own laboratory, we have focused on podocytes, the specialized epithelial cells that maintain the glomerular filtration barrier, and are critical targets in proteinuric disorders such as diabetic nephropathy. My current research in podocyte biology is concentrated in two areas.
One interest lies in the further understanding of cell-matrix biology in his study of podocytes. When podocytes are injured, the foot processes undergo a term known as effacement, where the cells collapse losing cell-cell junction proteins resulting in proteinuria. A mechanisms of podocytes undergoing foot process effacement likely requires cell spreading and motility which requires turnover of focal adhesion proteins, which acts as a“glue” to keep cells attached to its matrix. To further address this hypothesis, a key focal adhesion protein, FAK, was conditionally deleted in podocytes taking advantage of mice genetics. Generation of these mice resulted in resistance to proteinuria induced by various injury models. Moreover, as FDA approved FAK inhibitors are available, through collaboration with Novartis Pharmaceuticals, treatment of mice with FAK inhibitors during injury significantly reduced proteinuria and histological kidney injury.
My second interest in podocyte biology is to evaluate the importance of various interacting partners of known human disease causing mutations that result in nephrotic syndrome. Currently, we have identified a network of proteins that bind directly or indirectly to proteins that human genetic studies have implicated to be causal for nephrotic syndrome. Through mice genetics, loss of specific proteins in this network has demonstrated severe proteinuria, and podocyte foot process effacement. Through collaboration with Dr. Pietro De Camilli, we have found that these network of proteins lie at the interface of endocytosis and the actin cytoskeleton and now are in the process of expanding our understanding by determining critical factors that are being endocytosed in the podocytes. Hopefully these fundamental findings will lead to therapeutic targets in the treatment of nephrotic syndrome in the future.
The future directions of my lab are to further understand cell-matrix biology and endocytosis in podocytes by using mice genetic models of disease. Moreover, we are in the process of obtaining human samples to discover novel disease mutations, which may lead to further understanding of disease processes leading to nephrotic syndrome.