Development of a two-stage in vitro model system to investigate the mineralization mechanisms involved in idiopathic stone formation: stage 1-biomimetic Randall's plaque using decellularized porcine kidneys

Osteopontin stabilization and collagen containment slows amorphous calcium phosphate transformation during human aortic valve leaflet calcification

Calcification of aortic valve leaflets is a growing mortality threat for the 18 million human lives claimed globally each year by heart disease. Extensive research has focused on the cellular and molecular pathophysiology associated with calcification, yet the detailed composition, structure, distribution and etiological history of mineral deposition remains unknown. Here transdisciplinary geology, biology and medicine (GeoBioMed) approaches prove that leaflet calcification is driven by amorphous calcium phosphate (ACP), ACP at the threshold of transformation toward hydroxyapatite (HAP) and cholesterol biomineralization. A paragenetic sequence of events is observed that includes: (1) original formation of unaltered leaflet tissues: (2) individual and coalescing 100's nm- to 1 μm-scale ACP spherules and cholesterol crystals biomineralizing collagen fibers and smooth muscle cell myofilaments; (3) osteopontin coatings that stabilize ACP and collagen containment of nodules preventing exposure to the solution chemistry and water content of pumping blood, which combine to slow transformation to HAP; (4) mm-scale nodule growth via ACP spherule coalescence, diagenetic incorporation of altered collagen and aggregation with other ACP nodules; and (5) leaflet diastole and systole flexure causing nodules to twist, fold their encasing collagen fibers and increase stiffness. These in vivo mechanisms combine to slow leaflet calcification and establish previously unexplored hypotheses for testing novel drug therapies and clinical interventions as viable alternatives to current reliance on surgical/percutaneous valve implants.

Understanding formation processes of calcareous nephrolithiasis in renal interstitium and tubule lumen

Kidney stone, one of the oldest known diseases, has plagued humans for centuries, consistently imposing a heavy burden on patients and healthcare systems worldwide due to their high incidence and recurrence rates. Advancements in endoscopy, imaging, genetics, molecular biology and bioinformatics have led to a deeper and more comprehensive understanding of the mechanism behind nephrolithiasis. Kidney stone formation is a complex, multi-step and long-term process involving the transformation of stone-forming salts from free ions into asymptomatic or symptomatic stones influenced by physical, chemical and biological factors. Among the various types of kidney stones observed in clinical practice, calcareous nephrolithiasis is currently the most common and exhibits the most intricate formation mechanism. Extensive research suggests that calcareous nephrolithiasis primarily originates from interstitial subepithelial calcified plaques and/or calcified blockages in the openings of collecting ducts. These calcified plaques and blockages eventually come into contact with urine in the renal pelvis, serving as a nidus for crystal formation and subsequent stone growth. Both pathways of stone formation share similar mechanisms, such as the drive of abnormal urine composition, involvement of oxidative stress and inflammation, and an imbalance of stone inhibitors and promoters. However, they also possess unique characteristics. Hence, this review aims to provide detailed description and present recent discoveries regarding the formation processes of calcareous nephrolithiasis from two distinct birthplaces: renal interstitium and tubule lumen.

Confining calcium oxalate crystal growth in a carbonated apatite-coated microfluidic channel to better understand the role of Randall's plaque in kidney stone formation

Effective prevention of recurrent kidney stone disease requires the understanding of the mechanisms of its formation. Numerous in vivo observations have demonstrated that a large number of pathological calcium oxalate kidney stones develop on an apatitic calcium phosphate deposit, known as Randall's plaque. In an attempt to understand the role of the inorganic hydroxyapatite phase in the formation and habits of calcium oxalates, we confined their growth under dynamic physicochemical and flow conditions in a reversible microfluidic channel coated with hydroxyapatite. Using multi-scale characterization techniques including scanning electron and Raman microscopy, we showed the successful formation of carbonated hydroxyapatite as found in Randall's plaque. This was possible due to a new two-step flow seed-mediated growth strategy which allowed us to coat the channel with carbonated hydroxyapatite. Precipitation of calcium oxalates under laminar flow from supersaturated solutions of oxalate and calcium ions showed that the formation of crystals is a substrate and time dependent complex process where diffusion of oxalate ions to the surface of carbonated hydroxyapatite and the solubility of the latter are among the most important steps for the formation of calcium oxalate crystals. Indeed when an oxalate solution was flushed for 24 h, dissolution of the apatite layer and formation of calcium carbonate calcite crystals occurred which seems to promote calcium oxalate crystal formation. Such a growth route has never been observed in vivo in the context of kidney stones. Under our experimental conditions, our results do not show any direct promoting role of carbonated hydroxyapatite in the formation of calcium oxalate crystals, consolidating therefore the important role that macromolecules can play in the process of nucleation and growth of calcium oxalate crystals on Randall's plaque.

Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Liquid-Liquid Phase Separation in Nucleation Process of Biomineralization

Biomineralization is a typical interdisciplinary subject attracting biologists, chemists, and geologists to figure out its potential mechanism. A mounting number of studies have revealed that the classical nucleation theory is not suitable for all nucleation process of biominerals, and phase-separated structures such as polymer-induced liquid precursors (PILPs) play essential roles in the non-classical nucleation processes. These structures are able to play diverse roles biologically or pathologically, and could also give inspiring clues to bionic applications. However, a lot of confusion and dispute occurred due to the intricacy and interdisciplinary nature of liquid precursors. Researchers in different fields may have different opinions because the terminology and current state of understanding is not common knowledge. As a result, our team reviewed the most recent articles focusing on the nucleation processes of various biominerals to clarify the state-of-the-art understanding of some essential concepts and guide the newcomers to enter this intricate but charming field.

Multimodal imaging reveals a unique autofluorescence signature of Randall's plaque

Kidney stones frequently develop as an overgrowth on Randall's plaque (RP) which is formed in the papillary interstitium. The organic composition of RP is distinct from stone matrix in that RP contains fibrillar collagen; RP in tissue has also been shown to have two proteins that are also found in stones, but otherwise the molecular constituents of RP are unstudied. We hypothesized that RP contains unique organic molecules that can be differentiated from the stone overgrowth by fluorescence. To test this, we used micro-CT-guided polishing to expose the interior of kidney stones for multimodal imaging with multiphoton, confocal and infrared microscopy. We detected a blue autofluorescence signature unique to RP, the specificity of which was also confirmed in papillary tissue from patients with stone disease. High-resolution mineral mapping of the stone also showed a transition from the apatite within RP to the calcium oxalate in the overgrowth, demonstrating the molecular and spatial transition from the tissue to the urine. This work provides a systematic and practical approach to uncover specific fluorescence signatures which correlate with mineral type, verifies previous observations regarding mineral overgrowth onto RP and identifies a novel autofluorescence signature of RP demonstrating RP's unique molecular composition.

Structural Biology of Calcium Phosphate Nanoclusters Sequestered by Phosphoproteins

Biofluids that contain stable calcium phosphate nanoclusters sequestered by phosphopeptides make it possible for soft and hard tissues to co-exist in the same organism with relative ease. The stability diagram of a solution of nanocluster complexes shows how the minimum concentration of phosphopeptide needed for stability increases with pH. In the stable region, amorphous calcium phosphate cannot precipitate. Nevertheless, if the solution is brought into contact with hydroxyapatite, the crystalline phase will grow at the expense of the nanocluster complexes. The physico-chemical principles governing the formation, composition, size, structure, and stability of the complexes are described. Examples are given of complexes formed by casein, osteopontin, and recombinant phosphopeptides. Application of these principles and properties to blood serum, milk, urine, and resting saliva is described to show that under physiological conditions they are in the stable region of their stability diagram and so cannot cause soft tissue calcification. Stimulated saliva, however, is in the metastable region, consistent with its role in tooth remineralization. Destabilization of biofluids, with consequential ill-effects, can occur when there is a failure of homeostasis, such as an increase in pH without a balancing increase in the concentration of sequestering phosphopeptides.

In situ flow cell platform for examining calcium oxalate and calcium phosphate crystallization on films of basement membrane extract in the presence of urinary 'inhibitors'

A significant portion of the population suffers from idipoathic calcium oxalate (CaOx) kidney stones, and current clinical treatments of stones have limited lasting success with a high rate of patients suffering from reoccurring stones. Understanding the role of physiologically relevant urinary species on the formation, aggregation, and growth of CaOx crystals can allow for better understanding of this complex biomineralization process and lead to more effective clinical treatments. Our prior work has focused on developing a two-stage model system, where the first stage emulates the formation of Randall's plaque, and the second stage examines the influence of the plaque on overgrowth of CaOx into a stone. Herein, we report on the development of an easy-to-use flow-cell platform that utilizes basement membrane extract (BME) as a biologically relevant crystallization substrate to study the influence of urinary 'inhibitors' on the in situ formation and growth of CaOx on BME under flow conditions. Magnesium, citrate, and osteopontin were studied because of their known ability to inhibit CaOx formation, but their influence also led to interesting modifications to the terminal crystal habit. Magnesium had little to no effect on the CaOx crystallization, but both citrate and osteopontin resulted in significant changes to the crystallization kinetics and the terminal crystal habits. Triply inhibited artificial urine solutions resulted in CaOx monohydrate formations that resembled physiological stones, and the in situ platform allowed for morphogenesis to be dynamically monitored. The BME was also used in a two-stage model system to first grow CaP that mimicked Randall's plaques, whereby the impact of the CaP crystallizing surface on CaOx formation could be studied. It was found that the CaP surface did not result in any significant changes in CaOx crystal formation or growth indicating that the urinary inhibitors and the basement membrane substrate were the dominant factors in modulating CaOx crystallization. It was also found that the basement membrane surface promoted the attachment and/or nucleation and growth of both CaOx and CaP crystals compared to bare glass surfaces, thereby enabling easy study of the urinary inhibitors. The work presented here has elucidated the terminal growth habit of different COM structures and has provided an easy to use platform that can be widely adopted by the kidney stone and other crystallization communities.

Development of a two-stage model system to investigate the mineralization mechanisms involved in idiopathic stone formation: stage 2 in vivo studies of stone growth on biomimetic Randall's plaque

Idiopathic stone formers often form calcium oxalate (CaOx) stones that are attached to calcium phosphate (CaP) deposits in the renal tissue, known as Randall's plaques (RP). Plaques are suggested to originate in the renal tubular basement membrane and spread into the interstitial regions where collagen fibrils and vesicles become mineralized; if the epithelium is breached, the RP becomes overgrown with CaOx upon exposure to urine. We have developed a two-stage model system of CaP-CaOx composite stones, consisting of Stage (1) CaP mineralized plaque, followed by Stage (2) CaOx overgrowth into a stone. In our first paper in this series (Stage 1), osteopontin (and polyaspartate) were found to induce a non-classical mineralization of porcine kidney tissues, producing features that resemble RP. For the Stage 2 studies presented here, biomimetic RPs from Stage 1 were implanted into the bladders of rats. Hyperoxaluria was induced with ethylene glycol for comparison to controls (water). After 4 weeks, rats were sacrificed and the implants were analyzed using electron microscopy and X-ray microanalyses. Differences in crystal phase and morphologies based upon the macromolecules present in the biomimetic plaques suggest that the plaques have the capacity to modulate the crystallization reactions. As expected, mineral overgrowths on the implants switched from CaP (water) to CaOx (hyperoxaluric). The CaOx crystals were aggregated and mixed with organic material from the biomimetic RP, along with some amorphous and spherulitic CaOx near the "stone" surfaces, which seemed to have become compact and organized towards the periphery. This system was successful at inducing "stones" more similar to human idiopathic kidney stones than other published models.