Abstract A major function of cells linking the proximal tubule (PT) of the kidney is to recover proteins that escape the glomerular filtration barrier to maintain a protein-free urine. Apical endocytosis in PT cells is acutely modulated by changes in fluid shear stress (FSS), presumably to enable efficient protein uptake over normal variations in glomerular filtration rate (GFR). The large multiligand receptors megalin and cubilin/amnionless (CUBAM) are expressed at the apical surface of PT cells and mediate the internalization of >50 different plasma proteins. Despite the critical role of the PT in reclaiming proteins from the ultrafiltrate, we know surprisingly little about how the cells lining this segment accommodate variations in filtered load to maintain a protein free urine. Fundamental issues, including how the robust PT apical endocytic pathway is developed and maintained, the individual role of megalin and CUBAM receptors in albumin uptake, how PT cells respond to changes in albumin concentration or tubular flow rate, and the fate of internalized albumin remain controversial. We have developed a new cell culture model that recapitulates morphological and functional features of PT cells in vivo necessary for efficient and rapidly modulated apical endocytosis of albumin. We will use these cells in conjunction with studies in mouse models to address the following questions about how PT cells respond to normal and pathologic variations in flow and filtered protein load: 1) How do PT cells develop and maintain a high capacity apical endocytic pathway? 2) How do megalin and CUBAM contribute to albumin uptake under normal and nephrotic conditions? 3) How does the PT respond endocytically to acute changes in GFR? and 4) How does endocytosis contribute to cytotoxic responses of PT cells during albumin overload?