Nanouric acid or nanocalcium phosphate as central nidus to induce calcium oxalate stone formation: a high-resolution transmission electron microscopy study on urinary nanocrystallites

Size-Dependent Cytotoxicity, Adhesion, and Endocytosis of Micro-/Nano-hydroxyapatite Crystals in HK-2 Cells

Nano-hydroxyapatite (nano-HAP) is often used as a crystal nest to induce calcium oxalate (CaOx) kidney stone formation, but the mechanism of interaction between HAP crystals of different properties and renal tubular epithelial cells remains unclear. In this study, the adhesion and endocytosis of HAP crystals with sizes of 40 nm, 70 nm, 1 μm, and 2 μm (HAP-40 nm, HAP-70 nm, HAP-1 μm, and HAP-2 μm, respectively) to human renal proximal tubular epithelial cells (HK-2) were comparatively studied. The results showed that HAP crystals of all sizes promoted the expression of osteopontin and hyaluronic acid on the cell surface, destroyed the integrity of the lysosomes, and induced the apoptosis and necrosis of cells. Nano-HAP crystals had a higher specific surface area, a smaller contact angle, a higher surface energy, and a lower Zeta potential than those of micro-HAP. Therefore, the abilities of HK-2 cells to adhere to and endocytose nano-HAP crystals were greater than their abilities to do the same for micro-HAP crystals. The order of the endocytosed crystals was as follows: HAP-40 nm > HAP-70 nm > HAP-1 μm > HAP-2 μm. The endocytosed HAP crystals entered the lysosomes. The more crystal endocytosis and adhesion there is, the more toxic it is to HK-2 cells. The results of this study showed that nanosized HAP crystals greatly promoted the formation of kidney stones than micrometer-sized HAP crystals.

In focus: perplexing increase of urinary stone disease in children, adolescent and young adult women and its economic impact

Background

Urinary stone disease (USD) historically has affected older men, but studies suggest recent increases in women, leading to a near identical sex incidence ratio. USD incidence has doubled every 10 years, with disproportionate increases amongst children, adolescent, and young adult (AYA) women. USD stone composition in women is frequently apatite (calcium phosphate), which forms in a higher urine pH, low urinary citrate, and an abundance of urinary uric acid, while men produce more calcium oxalate stones. The reasons for this epidemiological trend are unknown.

Methods

This perspective presents the extent of USD with data from a Canadian Province and a North American institution, explanations for these findings and offers potential solutions to decrease this trend. We describe the economic impact of USD.

Findings

There was a significant increase of 46% in overall surgical interventions for USD in Ontario. The incidence rose from 47.0/100,000 in 2002 to 68.7/100,000 population in 2016. In a single United States institution, the overall USD annual unique patient count rose from 10,612 to 17,706 from 2015 to 2019, and the proportion of women with USD was much higher than expected. In the 10-17-year-old patients, 50.1% were girls; with 57.5% in the 18-34 age group and 53.6% in the 35-44 age group. The roles of obesity, diet, hormones, environmental factors, infections, and antibiotics, as well as the economic impact, are discussed.

Interpretation

We confirm the significant increase in USD among women. We offer potential explanations for this sex disparity, including microbiological and pathophysiological aspects. We also outline innovative solutions - that may require steps beyond typical preventive and treatment recommendations.

Recent advances on the mechanisms of kidney stone formation (Review)

Kidney stone disease is one of the oldest diseases known to medicine; however, the mechanisms of stone formation and development remain largely unclear. Over the past decades, a variety of theories and strategies have been developed and utilized in the surgical management of kidney stones, as a result of recent technological advances. Observations from the authors and other research groups suggest that there are five entirely different main mechanisms for kidney stone formation. Urinary supersaturation and crystallization are the driving force for intrarenal crystal precipitation. Randall's plaques are recognized as the origin of calcium oxalate stone formation. Sex hormones may be key players in the development of nephrolithiasis and may thus be potential targets for new drugs to suppress kidney stone formation. The microbiome, including urease-producing bacteria, nanobacteria and intestinal microbiota, is likely to have a profound effect on urological health, both positive and negative, owing to its metabolic output and other contributions. Lastly, the immune response, and particularly macrophage differentiation, play crucial roles in renal calcium oxalate crystal formation. In the present study, the current knowledge for each of these five aspects of kidney stone formation is reviewed. This knowledge may be used to explore novel research opportunities and improve the understanding of the initiation and development of kidney stones for urologists, nephrologists and primary care.

Rare non-papillary lithiasis of calcium oxalate monohydrate generated on a central core of potassium urate

This report describes a patient who developed a spheroidal calculus with a central part composed of potassium urate, surrounded by a continuous layer of calcium oxalate monohydrate with crystals of calcium oxalate dihydrate on the surface. The mechanism of calculus development is also suggested.

Size-dependent cellular uptake mechanism and cytotoxicity toward calcium oxalate on Vero cells

Urinary crystals with various sizes are present in healthy individuals and patients with kidney stone; however, the cellular uptake mechanism of calcium oxalate of various sizes has not been elucidated. This study aims to compare the internalization of nano-/micron-sized (50 nm, 100 nm, and 1 μm) calcium oxalate monohydrate (COM) and dihydrate (COD) crystals in African green monkey renal epithelial (Vero) cells. The internalization and adhesion of COM and COD crystals to Vero cells were enhanced with decreasing crystal size. Cell death rate was positively related to the amount of adhered and internalized crystals and exhibited higher correlation with internalization than that with adhesion. Vero cells mainly internalized nano-sized COM and COD crystals through clathrin-mediated pathways as well as micron-sized crystals through macropinocytosis. The internalized COM and COD crystals were distributed in the lysosomes and destroyed lysosomal integrity to some extent. The results of this study indicated that the size of crystal affected cellular uptake mechanism, and may provide an enlightenment for finding potential inhibitors of crystal uptake, thereby decreasing cell injury and the occurrence of kidney stones.

Inhibition of urinary macromolecule heparin on aggregation of nano-COM and nano-COD crystals

Purpose: This research aims to study the influences of heparin (HP) on the aggregation of nano calcium oxalate monohydrate (COM) and nano calcium oxalate dihydrate (COD) with mean diameter of about 50 nm. Method: The influences of different concentrations of HP on the mean diameter and Zeta potential of nano COM and nano COD were investigated using a nanoparticle size Zeta potential analyzer. Results: HP could be adsorbed on the surface of nano COM and nano COD crystals, leading to an increase in the absolute value of Zeta potential on the crystals and an increase in the electrostatic repulsion force between crystals. Consequently, the aggregation of the crystals is reduced and the stability of the system is improved. The strong adsorption ability of HP was closely related to the -OSO₃⁻ and -COO⁻ groups contained in the HP molecules. X-ray photoelectron spectroscopy confirmed the coordination of HP with Ca²⁺ ions of COM and COD crystals. Conclusion: HP could inhibit the aggregation of nano COM and nano COD crystals and increase their stability in aqueous solution, which is conducive in inhibiting the formation of calcium oxalate stones.