Graduate Student Ryerson University Toronto , Ontario, Canada
Nikol Leshchyshyn (Ryerson University )| Laura Orofiamma (Ryerson University )| Christian Delos Santos (Ryerson University)| Sadia Rahmani (Ryerson University)| Costin Antonescu (Ryerson University)
Megalin is a transmembrane receptor protein that belongs to a family of proteins known as low-density lipoprotein receptors (LDLRs) that functions in reabsorption of various molecules from the urine in the proximal tubule epithelial cells (PTECs) of the kidney. Disruption of the regulation of Megalin is associated with various stages of renal disease. Megalin mediates reabsorption by functioning as a scavenger receptor: binding of molecules (cargo) within the kidney proximal tubule lumen to the ectodomain of Megalin followed by internalization of cargo-bound Megalin via clathrin-mediated endocytosis, the first stage in transcytosis of cargo molecules across the kidney epithelium. Kidney proximal tubule cells experience metabolic stress due to low oxygen availability. We have recently shown that metabolic stress resulting in activation of AMP-activated protein kinase (AMPK) regulates the endocytosis of a large number of cell surface proteins, including that of beta1-integrin, that has a similar requirement for dab2 for internalization as does Megalin. How AMPK may regulate Megalin internalization in kidney PTECs is not well understood. Megalin internalization occurs via recruitment of the receptor to clathrin-coated pits (CCPs), which are ~100 nm structures comprised of clathrin and other proteins at the cell surface. Upon assembly and receptor recruitment, CCPs mediate membrane budding and eventually scission, leading to an internalized clathrin-coated vesicle (CCV). Megalin recruitment to CCPs is mediated by the clathrin adaptor protein dab2 that binds to NPXY motif that is present on Megalin and other LDLR family proteins. Here, we examine how metabolic stress and AMPK regulate the formation of CCPs and the dab2-dependent recruitment of Megalin and related receptors for internalization. Using cells engineered to express fluorescently tagged clathrin and other proteins, together with total internal reflection fluorescence microscopy and automated image analysis, we uncovered that AMPK activation triggers a reduction in the size and rate of initiation of CCPs, but an increased efficiency of formation of clathrin-coated vesicles. In addition, AMPK activation resulted in a robust increase in the recruitment of dab2 to CCPs. We have complemented this approach with assays that monitor the cell surface membrane traffic of Megalin and related receptors such as integrins. Collectively, these studies reveal a novel signaling pathway that mediates the regulation of the clathrin internalization of several receptors by AMPK and metabolic stress. A better understanding of signals responsive to cell metabolism and metabolic stress in kidney PTECs and how these lead to remodeling of endocytic membrane traffic may reveal novel ways to treat certain forms of kidney diseases.