Gold nanoparticles as physiological markers of urine internalization into urothelial cells in vivo

Samo Hudoklin,¹ Daša Zupancic,¹ Darko Makovec,² Mateja Erdani Kreft,¹ Rok Romih<sup>1

1</sup>Institute of Cell Biology, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia; ²Department for Materials Synthesis, Jozef Stefan Institute, Ljubljana, Slovenia

Background: Urothelial bladder is the reservoir of urine and the urothelium minimizes the exchange of urine constituents with this tissue. Our aim was to test 1.9 nm biocompatible gold nanoparticles as a novel marker of internalization into the urothelial cells under physiological conditions in vivo.
Methods: We compared normal and neoplastic mice urothelium. Neoplastic lesions were induced by 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in drinking water for 10 weeks. Nanoparticles, intravenously injected into normal and BBN-treated mice, were filtered through the kidneys and became constituents of the urine within 90 minutes after injection.
Results: Gold nanoparticles were densely accumulated in the urine, while their internalization into urothelial cells depended on the cell differentiation stage. In the terminally differentiated superficial urothelial cells of normal animals, nanoparticles were occasionally found in the endosomes, but not in the fusiform vesicles. Regions of exfoliated cells were occasionally found in the normal urothelium. Superficial urothelial cells located next to exfoliated regions contained gold nanoparticles in the endosomes and in the cytosol beneath the apical plasma membrane. The urothelium of BBN-treated animals developed flat hyperplasia with moderate dysplasia. The superficial cells of BBN-treated animals were partially differentiated as demonstrated by the lack of fusiform vesicles. These cells contained the gold nanoparticles distributed in the endosomes and throughout their cytosol.
Conclusion: Gold nanoparticles are a valuable marker to study urine internalization into urothelial cells in vivo. Moreover, they can be used as a sensitive marker of differentiation and functionality of urothelial cells.

Keywords: urinary bladder, urothelial plaques, membrane internalization, gold nanoparticles, cancer models, electron microscopy