Microvilli-Mediated Transport and Internalization of Extracellularly Assembled Nanoparticles in Kidney Proximal Tubules.

Proximal tubules, with their unique microenvironment and specialized brush-border microvilli, actively reabsorb, metabolize, and process filtered molecular cargos, yet how they interact with renal-clearable nanomaterials after glomerular filtration remains poorly understood. Here, using reactive oxygen species (ROS)-responsive glutathione-coated Au11 nanoclusters (∼0.6 nm) as a model, we show that renal-filtered Au11 undergoes extracellular ROS-mediated biotransformation into 2-3 nm gold nanoparticles, followed by their assembly into large nanostructures (∼50-500 nm) within the proximal tubular lumen of healthy kidneys. Unexpectedly, high-resolution electron microscopy revealed that brush-border microvilli capture these large luminal nanoassemblies, transport them from microvillar tips to the microvillar base, and internalize them through apical pinching-off events. The internalized nanoassemblies subsequently traffic through apical vacuoles to lysosomes, where a second-stage biotransformation produces densely packed flower-like gold nanostructures that are ultimately re-eliminated into the tubular lumen through membrane extrusion. These findings demonstrate that extracellular ROS (∼10 μM) under physiological conditions could substantially transform renal-filtered nanomaterials and reveal an unrecognized function of proximal tubular microvilli in transporting and internalizing large luminal nanoassemblies.