An organelle-extrusion-mediated self-renewal mechanism of proximal tubules

Proximal tubules energetically internalize and metabolize solutes filtered through glomeruli, yet throughout the lifespan they are constantly challenged by foreign substances and metabolic stress. These highly active cells reabsorb nearly two-thirds of the filtered load—including glucose, amino acids, electrolytes, and low–molecular weight proteins—making them one of the most energy-demanding segments of the nephron. In contrast to hepatocytes in the liver, which exhibit robust proliferative and regenerative capacity even after substantial injury, proximal tubule cells possess only limited regenerative potential. How these cells maintain long-term health and function in the absence of active division and regeneration remains an important unresolved question.

Using renal-clearable luminescent gold nanoparticles (AuNPs) as multimodality probes that can be filtered through the kidneys and readily detected by inductively coupled plasma mass spectrometry (ICP-MS) as well as optical and electron microscopies, we tracked their transport and interactions within the kidney down to the single-organelle level. This unique combination of renal clearance and high detection sensitivity enabled us to quantify nanoparticle biodistribution with ICP-MS, visualize their spatial dynamics in intact tissues by fluorescence imaging, and resolve their subcellular localization with electron microscopy. Through this integrated approach, we delineated how AuNPs traverse the glomerular filtration barrier, accumulate in proximal tubules, are internalized by tubular epithelial cells, metabolized within intracellular organelles, and ultimately re-eliminated back into the tubular lumen.

We found that renal clearable luminescent gold nanoparticles were efficiently taken up by the proximal tubules through clathrine-mediated endocytosis process. After internalization, these endocytosed 2–3 nm AuNPs  were rapidly and biochemically transformed into 200–300 nm nanoassemblies, followed by re-eliminated back into the tubular lumen through an previously unrecognized mechanism of PTECs. These PTECs were able to directly eject entire gold-containing lysosomes/endosomes along with other organelles (mitochondria, lysosomes without gold, smooth endoplasmic reticulum, apical vacuoles or even an entire nucleus in some rare cases) into the proximal tubular lumen to form ~5 µm extruded vesicles. This organelle-extrusion-mediated elimination of AuNPs represents a nanoparticle-elimination mechanism distinct from those membrane-fusion-mediated ones. However, this extrusion process is not activated by the AuNPs but by an intrinsic physiological ‘housekeeping’ function of normal PTs, used to remove unwanted substances and renew intracellular organelles without cell division.

This organelle-extrusion–mediated pathway represents a distinct mechanism of nanoparticle elimination, producing much larger vesicles (~5 µm) enriched with diverse intracellular organelles compared to conventional exocytosis. Beyond facilitating the clearance of non-biodegradable or biotransformed nanoparticles after uptake by proximal tubules, this process provides a fundamental explanation for how mitotically quiescent PTECs maintain homeostasis while performing their demanding functions. By ejecting entire organelles and waste, proximal tubules achieve continuous self-renewal without cell division, aligning with broader self-repair strategies observed across biological systems. These findings not only deepen our understanding of kidney physiology and nanoparticle transport but also open new avenues for exploring diagnostics and therapies for kidney disease.