Cellular basis of Albuminuria

The kidney cleans the blood via a filtration process and in kidney disease this process is disrupted, allowing proteins to escape into the urine (albuminuria). Over time this disease results in the destruction of the kidney resulting in the patient requiring dialysis and ultimately a kidney transplant. In diabetes, damage to the kidney filtration barrier (diabetic nephropathy) is a major complication and as a result of the current diabetes epidemic it is now the most common cause of kidney failure in the Western world. Although the link between diabetes and kidney failure is not understood, the progression of diabetic nephropathy follows a predictable clinical course. Initially, the kidneys leak small amounts of protein, and as the kidneys deteriorate further this amount increases. At present our understanding of how the kidney?s filters work and how diabetes causes damage is poor. The kidney?s filters consist of two cell types called podocytes and endothelial cells which interact together to form the filter. We can grow both cell types within our laboratory and the aim of our project is to understand how these cells interact and communicate with each other to form the filter in a healthy individual, and how this healthy state is disrupted during diabetes. Also we are trying to grow these cells together so that we can simulate the kidney?s filters in the laboratory and understand the filtration function of the kidney and how this is damaged in renal disease. We are studying two novel forms of treatment that can reduce albuminuria: learning how they achieve this will help us to design and develop new therapeutic approaches to deal with this problem.

Clinical importance of albuminuria is widely accepted, being a cardinal sign of kidney disease and a marker of cardiovascular risk in both diabetic and non-diabetic populations. However, cellular mechanisms underlying albuminuria are still unclear. The renal glomerular capillary wall allows passage of water and small molecules but in healthy individuals prevents passage of albumin and other key proteins, functioning as a complex biological sieve. It has a unique 3-layer structure: inner glomerular endothelium with glycocalyx and fenestrations, underlying glomerular basement membrane, and outer (urinary side) podocytes with characteristic foot processes linked by slit diaphragms. Mutations in genes encoding podocyte-specific proteins expressed at slit diaphragms (e.g. nephrin, podocin) cause congenital nephrotic syndrome; disturbances of slit diaphragms may be important in diabetic nephropathy and other acquired diseases, as may dysfunction of GBM and/or glomerular endothelium. In work currently funded by MRC we have utilised our unique human glomerular endothelial and podocyte cell lines to study phenotype and function of these two cells plus interactions between them. We now propose to extend our mechanistic understanding of the cellular responses, especially intracellular signalling pathways involved, increase sophistication of our in vitro model systems, continue our in vivo validation work in rodents (mostly in collaborative work that is separately funded) and move towards translational application of our results into anti-proteinuric therapies. In particular we will focus on (1) insulin and insulin-like growth factor-induced signalling and the effects on these of culture conditions designed to mimic diabetes mellitus, using mutant and reconstituted cell lines to analyse the role of nephrin and podocin in these responses; (2) engineering the glomerular capillary wall in vitro including studies of glomerular endothelial glycocalyx and fenestrations; and (3) study of modes of action of the anti-proteinuric agents sulodexide and interferon Beta. Via our unique cellular resources, experience and skills accumulated in recent years and a network of international collaborations allowing in vivo validation work, our results will move us from cell-based studies towards therapeutic targets in an area of major clinical significance.