The effect of intracrystalline and surface‐bound osteopontin on the attachment of calcium oxalate dihydrate crystals to Madin‐Darby canine kidney (MDCK) cells in ultrafiltered human urine
Authors: Thurgood Lauren A. , Sørensen Esben S. , Ryall Rosemary L. 
Published: September 2011

What's known on the subject? and What does the study add?

Osteopontin has been shown to both promote and inhibit the attachment of calcium oxalate monohydrate crystals to cultured renal epithelial cells in aqueous media. In this study we examined the role of OPN on the growth and attachment of calcium oxalate dihydrate crystals, to which it more readily associates, to renal epithelial cells using urine as the binding medium. We showed that OPN inhibits the growth of COD crystals and that it reduces the binding of COD crystals to cells.

OBJECTIVE

To determine the effects of intracrystalline (IC), surface‐bound (SB) and combined IC + SB osteopontin (OPN) on the binding of urinary calcium oxalate dihydrate (COD) crystals to Madin‐Darby canine kidney (MDCK‐II) cells in ultrafiltered (UF) human urine.

MATERIALS AND METHODS

14C‐oxalic acid‐labelled urinary COD crystals containing IC OPN were generated in pooled UF human urine containing human milk OPN at concentrations of 0, 1.0 and 5.0 mg/L. Additional labelled crystals were nucleated from a separate sample of the same pooled UF urine, to which were later added the same amounts of protein to produce crystals with SB OPN. COD crystals with IC+SB OPN were prepared using a combination of both techniques. Control crystals were prepared in the absence of OPN.

Crystals were incubated with MDCK‐II cells for up to 180 min in UF urine adjusted to 8 mm Ca2+.

Binding values for individual concentrations at specific time points and overall differences between binding curves were compared using the Mann‐Whitney U‐test.

Crystal morphology and attachment to the cells were confirmed using field emission scanning electron microscopy (FESEM).

RESULTS

The sizes of crystals precipitated from UF urine in the presence of 0, 1 and 5 mg/L OPN were 21.9 µm, 19.3 µm and 16.5 µm, indicating that OPN had inhibited crystal growth in a dose‐dependent fashion.

Binding curves for control crystals were smooth, while those of the IC and IC+SB COD crystals associated with 1 and 5 mg/L OPN were bimodal, as were those of the 1 mg/L SB crystals. This suggests that OPN induces or potentiates a transient response that enables MDCK‐II cells to release COD crystals after they have attached.

Although OPN generally reduced the binding of urinary COD crystals to MDCK‐II cells, at times it also appeared to mediate adhesion. It is possible therefore that OPN can reduce or increase crystal binding, and that our data represent the net effect of its opposing inhibitory or promotory properties.

CONCLUSIONS

In UF urine, OPN inhibits the growth of COD crystals and reduces the binding of urinary COD crystals to MDCK‐II cells, regardless of whether it is IC, SB, or IC+SB.

Future studies aimed at clarifying the effects of OPN, or indeed any urinary component, on crystal–cell interaction, should use crystals precipitated from urine and be performed under urinary conditions.