Chronic kidney disease (CKD) is a national health problem that affects ~ 39 million Americans. Progression of chronic kidney disease to end stage kidney disease is associated with considerable cardiovascular morbidity and mortality. Albuminuria is one of the key features of chronic kidney disease and a marker of CKD progression. The mechanisms of albumin excretion in the urine are poorly understood. In normal subjects, resistance to albumin excretion in the urine is mediated by an intact glomerular filtration barrier which is comprised of glomerular endothelial cells, the glomerular basement membrane, podocyte foot processes and the sub-podocyte space. In chronic kidney disease and other proteinuric states these barriers are disturbed allowing significant quantities of albumin to escape into the urine. Podocytes can take up albumin but the mechanisms of endocytosis in podocytes are not known. Our overall hypothesis is that healthy podocytes endocytose albumin in a polarized and receptor-mediated manner and that in albuminuric states the uptake and disposal of albumin is overwhelmed, leading to toxicity to the podocyte and increased podocyte death. In Specific Aim 1 we will examine albumin endocytosis and degradation in healthy podocytes. Specifically, we will test the hypothesis that albumin endocytosis occurs at the basal cell membrane and is mediated by megalin and that albumin degradation occurs in the lysosome. In Specific Aim 2 we will examine albumin handling in podocytes in the nephrotic state. We hypothesize that in heavy albuminuria albumin uptake is disordered and albumin degradation is impaired. Specifically, we will examine whether in nephrotic states albumin uptake is no longer confined to the basal membrane and whether albumin overload leads to activation of lysosomal enzymes and disruption of lysosomal integrity. To test our hypotheses we will use a combination of molecular biology and advanced imaging techniques in cultured human podocytes and will extend our findings to a mouse model of nephrotic syndrome. Our proposal is novel in that it proposes a mechanistic link between impaired albumin handling and podocyte death. Increased podocyte loss due to albumin toxicity may be an important factor in kidney disease progression since podocytes are terminally differentiated cells with low regenerative capacity and podocyte loss is strongly correlated with progressive kidney failure. In addition, this proposal will lay the foundation for an R01 submission. Determination of the mechanisms of albumin uptake and disposal in normal podocytes will allow comparison of these mechanisms in APOL1 risk allele podocytes. We hypothesize that albumin acts as a second hit in APOL1 mutant podocytes that have a genetic predisposition to accelerated loss. In addition, identification of the pathways necessary for albumin uptake may allow for future intervention studies that limit albumin uptake by podocytes in heavy albuminuria, thereby decreasing podocyte death and slowing kidney disease progression.