Biomimetic Randall's plaque as an in vitro model system for studying the role of acidic biopolymers in idiopathic stone formation

Osteogenic-Like Microenvironment of Renal Interstitium Induced by Osteomodulin Contributes to Randall's Plaque Formation

Calcium oxalate (CaOx) kidney stones are common and recurrent, lacking pharmacological prevention. Randall's plaques (RPs), calcium deposits in renal papillae, serve as niduses for some CaOx stones. This study explores the role of osteogenic-like cells in RP formation resembling ossification. CaP crystals deposit around renal tubules, interstitium, and blood vessels in RP tissues. Human renal interstitial fibroblasts (hRIFs) exhibit the highest osteogenic-like differentiation potential compared to chloride voltage-gated channel Ka positive tubular epithelial cells, aquaporin 2 positive collecting duct cells, and vascular endothelial cells, echoing the upregulated osteogenic markers primarily in hRIFs within RP tissues. Utilizing RNA-seq, osteomodulin (OMD) is found to be upregulated in hRIFs within RP tissues and hRIFs following osteogenic induction. Furthermore, OMD colocalizes with CaP crystals and calcium vesicles within RP tissues. OMD can enhance osteogenic-like differentiation of hRIFs in vitro and in vivo. Additionally, crystal deposits are attenuated in mice with Omd deletion in renal interstitial fibroblasts following CaOx nephrocalcinosis induction. Mechanically, a positive feedback loop of OMD/BMP2/BMPR1A/RUNX2/OMD drives hRIFs to adopt osteogenic-like fates, by which OMD induces osteogenic-like microenvironment of renal interstitium to participate in RP formation. We identify OMD upregulation as a pathological feature of RP, paving the way for preventing CaOx stones.

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.

Insight on the Role of Poly(acrylic acid) for Directing Calcium Phosphate Mineralization of Synthetic Polymer Bone Scaffolds

Bone is a complex tissue with robust mechanical and biological properties originating from its nanoscale composite structure. Although much research has been conducted on designing bioinspired artificial bone, the role of biological macromolecules such as noncollagenous proteins (NCPs) in influencing the formation of biominerals is not fully understood. In this work, we have designed nanofiber shish-kebab (NFSK) structures that can template mineral location by recruiting calcium cations from an ion-rich mineralization solution. Poly(acrylic acid) (PAA) is used as the NCP analogue to understand the role of polyelectrolytes in scaffold mineralization. We demonstrate that the addition of PAA in the mineralization solution suppresses the development of extrafibrillar minerals as well as slows down the accumulation and development of mineral phases within NFSKs. We probe the mechanism behind this effect by monitoring the free calcium ion concentration, investigating the PAA molecular weight effect, and conducting mineralization in membrane-partitioned solutions. Our results suggest the 2-fold effect of PAA as a solution stabilizer and physical barrier on the NFSK surface. This work could shed light on the understanding of the NCP effect in biomineralization.

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.

Natural inhibitors from earthworms for the crystallization of calcium oxalate monohydrate

Two proteins are proposed as CaOx nucleation and crystal growth regulators. The site-specific adsorption of inhibitors is confirmed from both macroscopic and microscopic perspectives.

Human kidney stones: a natural record of universal biomineralization

GeoBioMed - a new transdisciplinary approach that integrates the fields of geology, biology and medicine - reveals that kidney stones composed of calcium-rich minerals precipitate from a continuum of repeated events of crystallization, dissolution and recrystallization that result from the same fundamental natural processes that have governed billions of years of biomineralization on Earth. This contextual change in our understanding of renal stone formation opens fundamentally new avenues of human kidney stone investigation that include analyses of crystalline structure and stratigraphy, diagenetic phase transitions, and paragenetic sequences across broad length scales from hundreds of nanometres to centimetres (five Powers of 10). This paradigm shift has also enabled the development of a new kidney stone classification scheme according to thermodynamic energetics and crystalline architecture. Evidence suggests that ≥50% of the total volume of individual stones have undergone repeated in vivo dissolution and recrystallization. Amorphous calcium phosphate and hydroxyapatite spherules coalesce to form planar concentric zoning and sector zones that indicate disequilibrium precipitation. In addition, calcium oxalate dihydrate and calcium oxalate monohydrate crystal aggregates exhibit high-frequency organic-matter-rich and mineral-rich nanolayering that is orders of magnitude higher than layering observed in analogous coral reef, Roman aqueduct, cave, deep subsurface and hot-spring deposits. This higher frequency nanolayering represents the unique microenvironment of the kidney in which potent crystallization promoters and inhibitors are working in opposition. These GeoBioMed insights identify previously unexplored strategies for development and testing of new clinical therapies for the prevention and treatment of kidney stones.

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.

Prevention of Calcium Nephrolithiasis: The Influence of Diuresis on Calcium Oxalate Crystallization in Urine

A high fluid intake is still the most evidence-based measure for the prevention of idiopathic stone disease. The recommendation of current guidelines on urolithiasis to increase diuresis to 2-2.5 L/day is mainly based on a single clinical study. The present paper shows the influence of diuresis on calcium oxalate (CaOx) crystallization and especially aggregation (AGN) which can explain the initial development of Ca stones on papillary calcifications as well as stone growth in the renal pelvic system. Diuresis determines the urinary transit time (UT) through the kidney and together with the afflux of Ca and Ox the state of urinary saturation with respect to CaOx being the most frequent stone mineral. High supersaturation inducing crystallization during UT and a high urinary ion concentration interfering with the inhibition of crystal AGN by urinary macromolecules seem to be critical parameters for stone formation. Using data from the literature the influence of diuresis on these parameters is evaluated for short-term recurrent stone formers (RSF), idiopathic stone patients, and healthy controls, the latter two collectives with and without excessive oxalate ingestion. This investigation suggests that a diuresis of 2 L/day may protect from stone formation even after dietary Ox excesses and in RSF. However, in RSF with a continuously high Ca and Ox afflux into urine a permanent high diuresis is required which is difficult to sustain over 24 hours.

The potential effects of dietary food and beverage intakes on the risk of kidney stone formation

ABSTRACT Objective To determine the effect of nutritional habits on kidney stone formation and recurrence. Methods This study was conducted on 44 healthy individuals and 44 patients diagnosed with nephrolithiasis and aging between 20 and 65 years. Participants shared their salt consumption habits, daily fluid consumption amounts and general information about themselves in a questionnaire form. In addition, food and beverage consumption frequencies of participants were recorded through a food frequency questionnaire. Results Salt consumption frequencies of patients are higher than that of healthy individuals in both genders (p<0.05). It was found out that male individuals in the patient group salt dishes without tasting more frequently (p<0.05). Daily total water consumption of both genders in patient group is lower than that of healthy individuals (p<0.05). Meat consumption of male patients (51.6±31.35g/day) was found to be higher than that of healthy group (34.1±22.58g/day) (p<0.05). Additionally, individuals in the patient group consume less stinging nettle, corn, plum, loquat, orange juice and lemonade than healthy individuals (p<0.05). Conclusion Results of the study showed that total fluid intake, salt consumption habits, and vegetable, fruit and beverage consumption may be correlated with stone formation risk and nutrition habits may affect stone recurrence.

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.

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

Idiopathic calcium oxalate (CaOx) stone formers form stones that are commonly 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, where they exhibit a morphology of concentrically laminated apatitic spherules, while in the interstitial regions, the collagen fibrils and vesicles become mineralized. We hypothesize that these minerals might form by non-classical crystallization mechanisms, such as via amorphous precursors, some of which might originate from a polymer-induced liquid-precursor (PILP) process. Thus, our goal is to identify mineralogical 'signatures' of various stone formation mechanisms. To do this for idiopathic CaOx stones, we are developing 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. For the studies presented here, decellularized porcine kidneys were mineralized with CaP using polyaspartic acid or the protein osteopontin (OPN) to induce the PILP process and create biomimetic RP. Analysis of the PILP-mineralized tissues shows features that resemble the native plaques, including mineral spherules and collagen with intrafibrillar mineral. In contrast, the classical crystallization produced large apatitic spherulites, which is a very different morphology, but one which is also found in some stones. An alternative hypothesis regarding Randall's plaque, and if or when it becomes pathological, is discussed.