Bildungsmechanismen von Calciumphosphat-Plaques und anhaftenden Calciumoxalat-Nierensteinen
Reproduktionsmedizin, Urologie
Zusammenfassung der Projektergebnisse
Calcium oxalate (CaOx) kidney stones are a major human health problem, the causes and pathogenesis of which are not entirely clear. After successful stone removal, there is a high risk of recurrent CaOx stones. For most CaOx stones, a precondition is the formation of calcium phosphate (CaP) precipitates in extracellular tissue of the kidney. These calcifications, called Randall’s plaque (RP), presumably form during urine production in the tubular system of the kidney, when ions become reabsorbed and diffuse through the body fluid of the surrounding tissue. This interstitial fluid of the innermost part of the renal medulla per se holds a high load of solutes in order to facilitate urine concentration by reabsorption of water. We proposed that local increases in pH and of calcium and phosphate ion concentrations may induce critical CaP supersaturation of the body fluid. As a consequence, CaP minerals can form inside the porous tissue, leading to lesions in the epithelium of the renal pelvis, where they come in contact with concentrated urine. Subsequently, CaOx stones form from urine by crystal growth at these RP surfaces. However, the conditions for RP formation were not well known and the role of RP in the formation of CaOx stones was not entirely clear. Therefore, model experiments based on ion diffusion from simulated body fluids (SBF) in gelatin hydrogels were performed to simulate the formation of CaP plaques. Samples were freeze-dried and characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffractometry. Unexpectedly, in-situ monitoring the precipitation processes with hyperspectral confocal Raman spectroscopy was impeded by too low signal intensities due to insufficient ion concentrations and particle distribution densities. Experimental results demonstrated that body fluid with a high solute load, being regularly expected in innermost medullary, is in a metastable state which, however, easily tips towards CaP precipitation if Ca2+ and/or phosphate concentrations increase. In addition to Ca2+ and phosphate concentrations the process of precipitation is found to be influenced by matrix porosity and the presence of dissolved urea. Smaller pores retard the start of precipitation. Experimental precipitates initially formed as spherical nanoparticles consisting of amorphous CaP (ACP), which were transformed at higher Ca2+ and phosphate concentrations to hydroxylapatite (HA) particles of low crystallinity. The transformation took place as a dissolution-reprecipitation process. Dissolved urea in this system impedes this mineral phase transition. In the kidney system, stabilization of the much more soluble ACP could retain the chance of redissolving plaque in its early stage during temporary milder conditions. It is worth noting that CaP particles were always nucleated heterogeneously inside or on the gelatin pore walls, suggesting a role of the molecular structure of gelatin in the precipitation process. Structural similarities of experimental CaP plaques with pathological RP are expected to provide insights about the conditions of RP formation. Precipitation experiments with oxalate-containing simulated urine (SU) on fresh experimental gelatin-CaP plaque demonstrated that CaOx can form crystal aggregates on the plaque surface with Ca2+ ions from the SBF. Dissolution of CaP as a source for Ca2+ was not necessarily required. It may not only be the urine-exposed CaP surface of RP that induces the formation of CaOx stones, but Ca2+-containing body fluid that seeps into the urine at RP-caused lesions in the epithelium may represent the actual trigger for initial CaOx precipitates. This initiating mechanism would be particularly relevant for the initial formation of kidney stones and it would in addition promote their recurrence as well. The presented model experiments help understanding of the formation of RP and CaOx kidney stones and may represent a basic contribution to future developments towards preventive medical treatments.
Projektbezogene Publikationen (Auswahl)
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Precipitation of renal calcium phosphate plaques and their relevance for the growth of calcium oxalate kidney stones. 15th International Symposium on Biomineralization, Munich, Germany, 09–13 September 2019
Sethmann, I., Kleebe, H.-J., Geisler-Wierwille, T., Wendt-Nordahl, G., Knoll, T.
