Comparison of the pathology of interstitial plaque in human ICSF stone patients to NHERF-1 and THP-null mice

Excretion of urine extracellular vesicles bearing markers of activated immune cells and calcium/phosphorus physiology differ between calcium kidney stone formers and non-stone formers

BACKGROUNDS

Previous studies have demonstrated that excretion of urinary extracellular vesicles (EVs) from different nephron segments differs between kidney stone formers and non-stone formers (NSFs), and could reflect pathogenic mechanisms of urinary stone disease. In this study we quantified selected populations of specific urinary EVs carrying protein markers of immune cells and calcium/phosphorus physiology in calcium oxalate stone formers (CSFs) compared to non-stone formers (NSFs).

METHODS

Biobanked urine samples from CSFs (n = 24) undergoing stone removal surgery and age- and sex- matched NSFs (n = 21) were studied. Urinary EVs carrying proteins related to renal calcium/phosphorus physiology (phosphorus transporters (PiT1 and PiT2), Klotho, and fibroblast growth factor 23 (FGF23); markers associated with EV generation (anoctamin-4 (ANO4) and Huntington interacting protein 1 (HIP1)), and markers shed from activated immune cells were quantified by standardized and published method of digital flow cytometry.

RESULTS

Urine excretion of calcium, oxalate, phosphorus, and calcium oxalate supersaturation (SS) were significantly higher in CSFs compared to NSFs (P < 0.05). Urinary excretion of EVs with markers of total leukocytes (CD45), neutrophils (CD15), macrophages (CD68), Klotho, FGF23, PiT1, PiT2, and ANO4 were each markedly lower in CSFs than NSFs (P < 0.05) whereas excretion of those with markers of monocytes (CD14), T-Lymphocytes (CD3), B-Lymphocytes (CD19), plasma cells (CD138 plus CD319 positive) were not different between the groups. Urinary excretion of EVs expressing PiT1 and PiT2 negatively (P < 0.05) correlated with urinary phosphorus excretion, whereas excretion of EVs expressing FGF23 negatively (P < 0.05) correlated with both urinary calcium and phosphorus excretion. Urinary EVs with markers of HIP1 and ANO4 correlated negatively (P < 0.05) with clinical stone events and basement membrane calcifications on papillary tip biopsies.

CONCLUSIONS

Urinary excretion of EVs derived from specific types of activated immune cells and EVs with proteins related to calcium/phosphorus regulation differed between CSFs and NSFs. Further validation of these and other populations of urinary EVs in larger cohort could identify biomarkers that elucidate novel pathogenic mechanisms of calcium stone formation in specific subsets of patients.

How do stones form? Is unification of theories on stone formation possible?

There are two basic pathways for formation of calcium based kidney stones. Most idiopathic calcium oxalate (CaOx) stones are formed in association with sub-epithelial plaques of calcium phosphate (CaP), known as Randall's plaques, on renal papillary surfaces. Crystal formation and retention within the terminal collecting ducts, the ducts of Bellini, leading to the formation of Randall's plugs, is the other pathway. Both pathways require supersaturation leading to crystallization, regulated by various crystallization modulators produced in response to changing urinary conditions. High supersaturation, as a result of a variety of genetic and environmental factors, leads to crystallization in the terminal collecting ducts, eventually plugging their openings into the renal pelvis. Stasis behind the plugs may lead to the formation of attached or unattached stones in the tubular lumen. Deposition of crystals on the plug surface facing the pelvic or tubular urine may result in stone formation on the Randall's plugs. Kidneys of idiopathic stone formers may be subjected to oxidative stress as a result of increased urinary excretion of calcium/oxalate/phosphate and/or decrease in the production of functional crystallization inhibitors or in relation to co-morbidities such as hypertension, atherosclerosis, or acute kidney injury. We have proposed that production of reactive oxygen species (ROS) causes dedifferentiation of epithelial/endothelial cells into osteoblast type cells and deposition of CaP in the basement membrane of renal tubules or vessels. Growth, aggregation and melding of CaP crystals leads to the formation of plaque which grows by further calcification of interstitial collagen and membranous vesicles. Plaque becomes exposed to pelvic urine once the covering papillary epithelium is breached. Surface layers of CaP are replaced by CaOx through direct transformation or demineralization of CaP and mineralization of CaOx. Alternatively, or in addition, CaOx crystals nucleate directly on the plaque surface. Stone growth may also depend upon supersaturation in the pelvic urine, triggering further nucleation, growth and aggregation.

Impaired urinary osteopontin excretion in Npt2a-/- mice

Mutations in the renal sodium-dependent phosphate cotransporters NPT2a and NPT2c have been reported in patients with renal stone disease and nephrocalcinosis. Oral phosphate supplementation is currently thought to reduce risk by reversing the hypercalciuria, but the exact mechanism remains unclear and the relative contribution of modifiers of mineralization such as osteopontin (Opn) to the formation of renal mineral deposits in renal phosphate wasting disorders has not been studied. We observed a marked decrease of renal gene expression and urinary excretion of Opn in Npt2a^-/- mice, a mouse model of these disorders, at baseline. Following supplementation with phosphate Opn gene expression was restored to wild-type levels in Npt2a^-/- mice; however, urine excretion of the protein remained low. To further investigate the role of Opn, we used a double-knockout strategy, which provides evidence that loss of Opn worsens the nephrocalcinosis and nephrolithiasis observed in these mice on a high-phosphate diet. These studies suggest that impaired Opn gene expression and urinary excretion in Npt2a^-/- mice may be an additional risk factor for nephrolithiasis, and normalizing urine Opn levels may improve the therapy of phosphaturic disorders.

Idiopathic hypercalciuria and formation of calcium renal stones

The most common presentation of nephrolithiasis is idiopathic calcium stones in patients without systemic disease. Most stones are primarily composed of calcium oxalate and form on a base of interstitial apatite deposits, known as Randall's plaque. By contrast some stones are composed largely of calcium phosphate, as either hydroxyapatite or brushite (calcium monohydrogen phosphate), and are usually accompanied by deposits of calcium phosphate in the Bellini ducts. These deposits result in local tissue damage and might serve as a site of mineral overgrowth. Stone formation is driven by supersaturation of urine with calcium oxalate and brushite. The level of supersaturation is related to fluid intake as well as to the levels of urinary citrate and calcium. Risk of stone formation is increased when urine citrate excretion is <400 mg per day, and treatment with potassium citrate has been used to prevent stones. Urine calcium levels >200 mg per day also increase stone risk and often result in negative calcium balance. Reduced renal calcium reabsorption has a role in idiopathic hypercalciuria. Low sodium diets and thiazide-type diuretics lower urine calcium levels and potentially reduce the risk of stone recurrence and bone disease.

Metabolic syndrome and nephrolithiasis

Problem

This review focuses on the association between the metabolic syndrome (MS) and nephrolithiasis.

Findings

Associations between nephrolithiasis and systemic diseases are recognized, including atherosclerosis, cardiovascular (CV) disease, hypertension (HNT), diabetes mellitus (DM)—composite risk factors grouped as the MS. Kidney stones incidence is increasing in this particularly high risk group. Those with stones are prone to the disease and those with the systemic disease are at risk for stone formation, with the highest incidence in persons with multiple traits of the MS. Pathophysiologic explanations for the increased stone risk related to MS are likely complex and dynamic.

Conclusions

Kidney stones disproportionately affect persons with some or all traits of MS. One unifying theory may be of a common systemic malfunction of inflammation and tissue damage as an underlying mechanism, but it is unlikely to be the only mechanistic explanation. Further research is needed to investigate this and other hypotheses that go beyond population based and urine physiochemical studies in order to elucidate the mechanisms behind the individual disease states themselves.

Do kidney stone formers have a kidney disease?

Nephrolithiasis is a highly prevalent disorder affecting approximately one in eleven people and is associated with multiple complications including hypertension, cardiovascular disease, and chronic kidney disease. Significant epidemiologic associations with chronic kidney disease and ESRD have been noted and are reviewed herein, but debate persists in the literature as to whether kidney stone formation is a pathogenic process contributing to kidney disease. Corroborating evidence supporting the presence of kidney disease in stone formers includes the variability of renal function by stone type, the positive association of stone size with renal dysfunction, the presence of markers of renal injury in the urine of even asymptomatic stone formers, and direct evidence of renal tissue injury on histopathology. Proposed pathogenic mechanisms include recurrent obstruction and comorbid conditions such as recurrent urinary tract infections and structural abnormalities. Recent work evaluating the renal histopathology of different groups of stone formers adds further granularity, suggesting variability in mechanisms of renal injury by stone type and confirming the pathogenic effects of crystal formation. Genetic abnormalities leading to stone formation including cystinuria and primary hyperoxaluria, among others, contribute to the burden of disease in the stone-forming population.

Vascular Calcification and Stone Disease: A New Look towards the Mechanism

Calcium phosphate (CaP) crystals are formed in pathological calcification as well as during stone formation. Although there are several theories as to how these crystals can develop through the combined interactions of biochemical and biophysical factors, the exact mechanism of such mineralization is largely unknown. Based on the published scientific literature, we found that common factors can link the initial stages of stone formation and calcification in anatomically distal tissues and organs. For example, changes to the spatiotemporal conditions of the fluid flow in tubular structures may provide initial condition(s) for CaP crystal generation needed for stone formation. Additionally, recent evidence has provided a meaningful association between the active participation of proteins and transcription factors found in the bone forming (ossification) mechanism that are also involved in the early stages of kidney stone formation and arterial calcification. Our review will focus on three topics of discussion (physiological influences-calcium and phosphate concentration-and similarities to ossification, or bone formation) that may elucidate some commonality in the mechanisms of stone formation and calcification, and pave the way towards opening new avenues for further research.

Unified theory on the pathogenesis of Randall's plaques and plugs

Kidney stones develop attached to sub-epithelial plaques of calcium phosphate (CaP) crystals (termed Randall's plaque) and/or form as a result of occlusion of the openings of the Ducts of Bellini by stone-forming crystals (Randall's plugs). These plaques and plugs eventually extrude into the urinary space, acting as a nidus for crystal overgrowth and stone formation. To better understand these regulatory mechanisms and the pathophysiology of idiopathic calcium stone disease, this review provides in-depth descriptions of the morphology and potential origins of these plaques and plugs, summarizes existing animal models of renal papillary interstitial deposits, and describes factors that are believed to regulate plaque formation and calcium overgrowth. Based on evidence provided within this review and from the vascular calcification literature, we propose a "unified" theory of plaque formation-one similar to pathological biomineralization observed elsewhere in the body. Abnormal urinary conditions (hypercalciuria, hyperoxaluria, and hypocitraturia), renal stress or trauma, and perhaps even the normal aging process lead to transformation of renal epithelial cells into an osteoblastic phenotype. With this de-differentiation comes an increased production of bone-specific proteins (i.e., osteopontin), a reduction in crystallization inhibitors (such as fetuin and matrix Gla protein), and creation of matrix vesicles, which support nucleation of CaP crystals. These small deposits promote aggregation and calcification of surrounding collagen. Mineralization continues by calcification of membranous cellular degradation products and other fibers until the plaque reaches the papillary epithelium. Through the activity of matrix metalloproteinases or perhaps by brute physical force produced by the large sub-epithelial crystalline mass, the surface is breached and further stone growth occurs by organic matrix-associated nucleation of CaOx or by the transformation of the outer layer of CaP crystals into CaOx crystals. Should this theory hold true, developing an understanding of the cellular mechanisms involved in progression of a small, basic interstitial plaque to that of an expanding, penetrating plaque could assist in the development of new therapies for stone prevention.

Mechanisms of human kidney stone formation

The precise mechanisms of kidney stone formation and growth are not completely known, even though human stone disease appears to be one of the oldest diseases known to medicine. With the advent of the new digital endoscope and detailed renal physiological studies performed on well phenotyped stone formers, substantial advances have been made in our knowledge of the pathogenesis of the most common type of stone former, the idiopathic calcium oxalate stone former as well as nine other stone forming groups. The observations from our group on human stone formers and those of others on model systems have suggested four entirely different pathways for kidney stone formation. Calcium oxalate stone growth over sites of Randall's plaque appear to be the primary mode of stone formation for those patients with hypercalciuria. Overgrowths off the ends of Bellini duct plugs have been noted in most stone phenotypes, do they result in a clinical stone? Micro-lith formation does occur within the lumens of dilated inner medullary collecting ducts of cystinuric stone formers and appear to be confined to this space. Lastly, cystinuric stone formers also have numerous small, oval, smooth yellow appearing calyceal stones suggestive of formation in free solution. The scientific basis for each of these four modes of stone formation are reviewed and used to explore novel research opportunities.

Interstitial calcinosis in renal papillae of genetically engineered mouse models: relation to Randall's plaques

Genetically engineered mouse models (GEMMs) have been highly instrumental in elucidating gene functions and molecular pathogenesis of human diseases, although their use in studying kidney stone formation or nephrolithiasis remains relatively limited. This review intends to provide an overview of several knockout mouse models that develop interstitial calcinosis in the renal papillae. Included herein are mice deficient for Tamm-Horsfall protein (THP; also named uromodulin), osteopontin (OPN), both THP and OPN, Na(+)-phosphate cotransporter Type II (Npt2a) and Na(+)/H(+) exchanger regulatory factor (NHERF-1). The baseline information of each protein is summarized, along with key morphological features of the interstitial calcium deposits in mice lacking these proteins. Attempts are made to correlate the papillary interstitial deposits found in GEMMs with Randall's plaques, the latter considered precursors of idiopathic calcium stones in patients. The pathophysiology that underlies the renal calcinosis in the knockout mice is also discussed wherever information is available. Not all the knockout models are allocated equal space because some are more extensively characterized than others. Despite the inroads already made, the exact physiological underpinning, origin, evolution and fate of the papillary interstitial calcinosis in the GEMMs remain incompletely defined. Greater investigative efforts are warranted to pin down the precise role of the papillary interstitial calcinosis in nephrolithiasis using the existing models. Additionally, more sophisticated, second-generation GEMMs that allow gene inactivation in a time-controlled manner and "compound mice" that bear several genetic alterations are urgently needed, in light of mounting evidence that nephrolithiasis is a multifactorial, multi-stage and polygenic disease.

Distinguishing characteristics of idiopathic calcium oxalate kidney stone formers with low amounts of Randall's plaque

BACKGROUND

Overgrowth of calcium oxalate on Randall's plaque is a mechanism of stone formation among idiopathic calcium oxalate stone-formers (ICSFs). It is less clear how stones form when there is little or no plaque.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Participants were a consecutive cohort of ICSFs who underwent percutaneous nephroscopic papillary mapping in the kidney or kidneys containing symptomatic stones and a papillary tip biopsy from a representative calyx during a stone removal procedure between 2009 and 2013. The distribution of Randall's plaque coverage was analyzed and used to divide ICSFs into those with a high (≥5%; mean, 10.5%; n=10) versus low (<5%; mean, 1.5%; n=32) amount of plaque coverage per papilla. Demographic and laboratory features were compared between these two groups.

RESULTS

Low-plaque stone formers tended to be obese (50% versus 10%; P=0.03) and have a history of urinary tract infection (34% versus 0%; P=0.04). They were less likely to have multiple prior stone events (22% versus 80%; P=0.002) and had a lower mean 24-hour urine calcium excretion (187±86 mg versus 291±99 mg; P<0.01). Morphologically, stones from patients with low amounts of plaque lacked a calcium phosphate core by microcomputed tomography. Papillary biopsies from low plaque stone-formers revealed less interstitial and basement membrane punctate crystallization.

CONCLUSIONS

These findings suggest that other pathways independent of Randall's plaque may contribute to stone pathogenesis among a subgroup of ICSFs who harbor low amounts of plaque.

On the origin of calcium oxalate monohydrate papillary renal stones

Calcium oxalate monohydrate (COM) papillary calculi can be initiated by subepithelial calcification of the renal papillae. Hydroxyapatite disruption of the papillary epithelial layer can become the nidus of a COM papillary calculus. This study evaluated the causes of papillary tissue calcifications in 60 patients with calcium oxalate lithiasis, 30 with COM papillary and 30 with calcium oxalate dihydrate (COD) calculi. Urinary redox potential was higher in the COM than the COD group, suggesting that the former is more deficient in antioxidants due to increased oxidative stress. Urinary calcium was significantly higher in the COD group, whereas urinary oxalate was significantly higher in the COM group, suggesting a greater degree of oxidative injury of renal cells. Evaluations of their diets showed that both groups consumed low amounts of phytate-rich products. Of chronic diseases possibly associated with urolithiasis, only the prevalence of gastroduodenal ulcer differed significantly, being higher in the COM group and suggesting that epithelial lesions are common to gastroduodenal ulcers and COM papillary renal stones. Occupational exposure to cytotoxic products occurred in 47 % of the COM and 27 % of the COD group, but this difference was not statistically significant. These findings indicate that oxidative stress is associated with injury to papillary tissue and that this is the origin of intrapapillary calcifications. The continuation of this process is due to modulators and/or deficiencies in inhibitors of crystallization. Identifying and eliminating the causes of injury may prevent recurrent episodes in patients with papillary COM calculi.

Contrasting histopathology and crystal deposits in kidneys of idiopathic stone formers who produce hydroxy apatite, brushite, or calcium oxalate stones

Our previous work has shown that stone formers who form calcium phosphate (CaP) stones that contain any brushite (BRSF) have a distinctive renal histopathology and surgical anatomy when compared with idiopathic calcium oxalate stone formers (ICSF). Here we report on another group of idiopathic CaP stone formers, those forming stone containing primarily hydroxyapatite, in order to clarify in what ways their pathology differs from BRSF and ICSF. Eleven hydroxyapatite stone formers (HASF) (2 males, 9 females) were studied using intra-operative digital photography and biopsy of papillary and cortical regions to measure tissue changes associated with stone formation. Our main finding is that HASF and BRSF differ significantly from each other and that both differ greatly from ICSF. Both BRSF and ICSF patients have significant levels of Randall's plaque compared with HASF. Intra-tubular deposit number is greater in HASF than BRSF and nonexistent in ICSF while deposit size is smaller in HASF than BRSF. Cortical pathology is distinctly greater in BRSF than HASF. Four attached stones were observed in HASF, three in 25 BRSF and 5-10 per ICSF patient. HASF and BRSF differ clinically in that both have higher average urine pH, supersaturation of CaP, and calcium excretion than ICSF. Our work suggests that HASF and BRSF are two distinct and separate diseases and both differ greatly from ICSF.

Molecular mechanisms of crystal-related kidney inflammation and injury. Implications for cholesterol embolism, crystalline nephropathies and kidney stone disease

Crystals are particles of endogenous inorganic or organic composition that can trigger kidney injury when deposited or formed inside the kidney. While decades of research have focused on the molecular mechanisms of solute supersaturation and crystal formation, the pathomechanisms of crystal-induced renal inflammation remain largely unknown. The recent discovery of the intracellular NLRP3 inflammasome as a pattern recognition platform that translates crystal uptake into innate immune activation via secretion of IL-1β and IL-18 revised the pathogenesis of gout, silicosis, asbestosis, atherosclerosis and other crystal-related disorders. As a proof of concept, the NLRP3 inflammasome was now shown to trigger inflammation and acute kidney injury (AKI) in oxalate nephropathy. It seems likely that this and potentially other innate immunity mechanisms drive crystalline nephropathies (CNs) that are associated with crystals of calcium phosphate, uric acid, cysteine, adenine, certain drugs or contrast media, and potentially of myoglobin during rhabdomyolysis and of light chains in myeloma. Here, we discuss the proven and potential mechanisms of renal inflammation and kidney injury in crystal-related kidney disorders. In addition, we list topics for further research in that field. This perspective may also provide novel therapeutic options that can help to avoid progressive tissue remodeling and chronic kidney disease in patients with kidney stone disease or other CNs.

Renal papillary calcification and the development of calcium oxalate monohydrate papillary renal calculi: a case series study

BACKGROUND

The objective of this study is to determine in a case series (four patients) how calcified deposits in renal papillae are associated with the development of calcium oxalate monohydrate (COM) papillary calculi.

METHODS

From the recently collected papillary calculi, we evaluated retrospectively patients, subjected to retrograde ureteroscopy, with COM papillary lithiasis.

RESULTS

The COM papillary calculi were found to result from subepithelial injury. Many of these lesions underwent calcification by hydroxyapatite (HAP), with calculus morphology and the amount of HAP in the concave zone dependent on the location of the calcified injury. Most of these HAP deposits grew, eroding the epithelium covering the renal papillae, coming into contact with urine and starting the development of COM calculi. Subepithelial HAP plaques may alter the epithelium covering the papillae, resulting in the deposit of COM crystals directly onto the epithelium. Tissue calcification depends on a pre-existing injury, the continuation of this process is due to modulators and/or crystallization inhibitors deficiency.

CONCLUSIONS

Since calculus morphology and the amount of detected HAP are dependent on the location and widespread of calcified injury, all types of papillary COM calculi can be found in the same patient. All patients had subepithelial calcifications, with fewer papillary calculi, demonstrating that some subepithelial calcifications did not further evolve and were reabsorbed. A high number of subepithelial calcifications increases the likelihood that some will be transformed into COM papillary calculi.