Proteomic Analysis of Urinary Extracellular Vesicles Reveals a Role for the Complement System in Medullary Sponge Kidney Disease.

Medullary sponge kidney (MSK) disease is a rare and neglected kidney condition often associated with nephrocalcinosis/nephrolithiasis and cystic anomalies in the precalyceal ducts. Little is known about the pathogenesis of this disease, so we addressed the knowledge gap using a proteomics approach. The protein content of microvesicles/exosomes isolated from urine of 15 MSK and 15 idiopathic calcium nephrolithiasis (ICN) patients was investigated by mass spectrometry, followed by weighted gene coexpression network analysis, support vector machine (SVM) learning, and partial least squares discriminant analysis (PLS-DA) to select the most discriminative proteins. Proteomic data were verified by ELISA. We identified 2998 proteins in total, 1764 (58.9%) of which were present in both vesicle types in both diseases. Among the MSK samples, only 65 (2.2%) and 137 (4.6%) proteins were exclusively found in the microvesicles and exosomes, respectively. Similarly, among the ICN samples, only 75 (2.5%) and 94 (3.1%) proteins were exclusively found in the microvesicles and exosomes, respectively. SVM learning and PLS-DA revealed a core panel of 20 proteins that distinguished extracellular vesicles representing each clinical condition with an accuracy of 100%. Among them, three exosome proteins involved in the lectin complement pathway maximized the discrimination between MSK and ICN: Ficolin 1, Mannan-binding lectin serine protease 2, and Complement component 4-binding protein ß. ELISA confirmed the proteomic results. Our data show that the complement pathway is involved in the MSK, revealing a new range of potential therapeutic targets and early diagnostic biomarkers.

Proteomic Analysis of Urinary Microvesicles and Exosomes in Medullary Sponge Kidney Disease and Autosomal Dominant Polycystic Kidney Disease.

BACKGROUND AND OBJECTIVES: Microvesicles and exosomes are involved in the pathogenesis of autosomal dominant polycystic kidney disease. However, it is unclear whether they also contribute to medullary sponge kidney, a sporadic kidney malformation featuring cysts, nephrocalcinosis, and recurrent kidney stones. We addressed this knowledge gap by comparative proteomic analysis. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: The protein content of microvesicles and exosomes isolated from the urine of 15 patients with medullary sponge kidney and 15 patients with autosomal dominant polycystic kidney disease was determined by mass spectrometry followed by weighted gene coexpression network analysis, support vector machine learning, and partial least squares discriminant analysis to compare the profiles and select the most discriminative proteins. The proteomic data were verified by ELISA. RESULTS: A total of 2950 proteins were isolated from microvesicles and exosomes, including 1579 (54%) identified in all samples but only 178 (6%) and 88 (3%) specific for medullary sponge kidney microvesicles and exosomes, and 183 (6%) and 98 (3%) specific for autosomal dominant polycystic kidney disease microvesicles and exosomes, respectively. The weighted gene coexpression network analysis revealed ten modules comprising proteins with similar expression profiles. Support vector machine learning and partial least squares discriminant analysis identified 34 proteins that were highly discriminative between the diseases. Among these, CD133 was upregulated in exosomes from autosomal dominant polycystic kidney disease and validated by ELISA. CONCLUSIONS: Our data indicate a different proteomic profile of urinary microvesicles and exosomes in patients with medullary sponge kidney compared with patients with autosomal dominant polycystic kidney disease. The urine proteomic profile of patients with autosomal dominant polycystic kidney disease was enriched of proteins involved in cell proliferation and matrix remodeling. Instead, proteins identified in patients with medullary sponge kidney were associated with parenchymal calcium deposition/nephrolithiasis and systemic metabolic derangements associated with stones formation and bone mineralization defects. PODCAST: This article contains a podcast at https://www.asn-online.org/media/podcast/CJASN/2019_04_24_CJASNPodcast_19_06_.mp3.

Reduced incidence of cardiovascular events in hyper-Lp(a) patients on lipoprotein apheresis. The G.I.L.A. (Gruppo Interdisciplinare Aferesi Lipoproteica) pilot study.

BACKGROUND: Lipoprotein apheresis (LA) is the elective therapy for homozygous and other forms of Familial Hypercholesterolemia, Familial Combined Hypercholesterolemia, resistant/intolerant to lipid lowering drugs, and hyper-lipoproteinemia(a). Lipoprotein(a) [Lp(a)] has been classified as the most prevalent genetic risk factor for coronary artery disease and aortic valve stenosis. AIM: Our multicenter retrospective study has the aim to analyze the incidence of adverse cardiovascular events (ACVE) before and during the LA treatment, in subjects with elevated level of Lp(a) (>60 mg/dL) [hyper-Lp(a)] and chronic ischemic heart disease. METHODS: We collected data of 23 patients (mean age 63 ± 9 years, male 77%; from hospital of Pisa 11/23, Pistoia 7/23, Verona 2/23, Padova 2/23 and Ferrara 1/23), with hyper-Lp(a), pre-apheresis LDL-cholesterol <100 mg/dL, cardiovascular disease, on maximally tolerated lipid lowering therapy and LA treatment (median 7 years, interquartile range 3-9 years). The LA treatment was performed by heparin-induced LDL precipitation apheresis (16/23), dextran-sulphate (4/23), cascade filtration (2/23) and immunoadsorption (1/23). The time lapse between first cardiovascular event and beginning of apheresis was 6 years (interquartile range 1-12 years). RESULTS: The recorded ACVE, before and after the LA treatment inception, were 40 and 10 respectively (p < 0.05), notably, the AVCE rates/year were 0.43 and 0.11 respectively (p < 0.05) with a 74% reduction of event occurrence. CONCLUSIONS: Our data confirm long-term efficacy and positive impact of LA on morbidity in patients with hyper-Lp(a) and chronic ischemic heart disease on maximally tolerated lipid lowering therapy.

Proteomic-based research strategy identified laminin subunit alpha 2 as a potential urinary-specific biomarker for the medullary sponge kidney disease.

Medullary sponge kidney (MSK) disease, a rare kidney malformation featuring recurrent renal stones and nephrocalcinosis, continues to be diagnosed using expensive and time-consuming clinical/instrumental tests (mainly urography). Currently, no molecular diagnostic biomarkers are available. To identify such we employed a proteomic-based research strategy utilizing urine from 22 patients with MSK and 22 patients affected by idiopathic calcium nephrolithiasis (ICN) as controls. Notably, two patients with ICN presented cysts. In the discovery phase, the urine of 11 MSK and 10 controls, were randomly selected, processed, and analyzed by mass spectrometry. Subsequently, several statistical algorithms were undertaken to select the most discriminative proteins between the two study groups. ELISA, performed on the entire patients' cohort, was used to validate the proteomic results. After an initial statistical analysis, 249 and 396 proteins were identified exclusive for ICN and MSK, respectively. A Volcano plot and ROC analysis, performed to restrict the number of MSK-associated proteins, indicated that 328 and 44 proteins, respectively, were specific for MSK. Interestingly, 119 proteins were found to differentiate patients with cysts (all patients with MSK and the two ICN with renal cysts) from ICN without cysts. Eventually, 16 proteins were found to be common to three statistical methods with laminin subunit alpha 2 (LAMA-2) reaching the higher rank by a Support Vector Machine, a binary classification/prediction scheme. ELISA for LAMA-2 validated proteomic results. Thus, using high-throughput technology, our study identified a candidate MSK biomarker possibly employable in future for the early diagnosis of this disease.

New non-renal congenital disorders associated with medullary sponge kidney (MSK) support the pathogenic role of GDNF and point to the diagnosis of MSK in recurrent stone formers.

Medullary sponge kidney (MSK) is a congenital renal disorder. Its association with several developmental abnormalities in other organs hints at the likelihood of some shared step(s) in the embryogenesis of the kidney and other organs. It has been suggested that the REarranged during Transfection (RET) proto-oncogene and the Glial cell line-Derived Neurotrophic Factor (GDNF) gene are defective in patients with MSK, and both RET and GDNF are known to have a role in the development of the central nervous system, heart, and craniofacial skeleton. Among a cohort of 143 MSK patients being followed up for nephrolithiasis and chronic kidney disease at our institution, we found six with one or more associated non-renal anomalies: one patient probably has congenital hemihyperplasia and hypertrophic cardiomyopathy with adipose metaplasia and mitral valve prolapse; one has Marfan syndrome; and the other four have novel associations between MSK and nerve and skeleton abnormalities described here for the first time. The discovery of disorders involving the central nervous system, cardiovascular system and craniofacial skeleton in MSK patients supports the hypothesis of a genetic alteration on the RET-GDNF axis having a pivotal role in the pathogenesis of MSK, in a subset of patients at least. MSK seems more and more to be a systemic disease, and the identification of extrarenal developmental defects could be important in arousing the suspicion of MSK in recurrent stone formers.

[Cystinic nephrolythiasis: clinical experience and new diagnostic and therapeutic perspectives]. / Nefrolitiasi cistinica: dall'esperienza clinica alle nuove prospettive diagnostiche e terapeutiche.

Cystinuria is an inherited autosomal recessive disease with a prevalence 1:7000 and typical age of onset in the second decade of life. This nephrolithiasis is not always well known and well studied and for this reason it is often underdiagnosed. Cystinuria is characterized by increased urinary excretion of cystine and dibasic amino acids (lysine, ornithine, arginine) caused by defective transport of these amino acids across the luminal membrane of proximal tubule and small intestine cells. Two mutated genes responsible of this tubular defect are SLC3A1 on chromosome 2 and SLC7A9 on chromosome 19. Clinical manifestations of cystinuria are essentially those related to stones formation and their movement across the urinary tract, like flank pain/abdomen pain and hematuria, as occurred in other nephrolithiasis types. Diagnosis is based on biochemical urine analysis, stone analysis and imaging. Genetic study of this disease may be a new and stimulating approach to better understand the defects and identify new therapeutic targets. A wider knowledge and a more detailed approach to cystinuria may help to ameliorate patients quality of life, to prevent recurrences and complications and to develop more specific and adequate treatments.

Spontaneous calcification process in primary renal cells from a medullary sponge kidney patient harbouring a GDNF mutation.

Medullary nephrocalcinosis is a hallmark of medullary sponge kidney (MSK). We had the opportunity to study a spontaneous calcification process in vitro by utilizing the renal cells of a patient with MSK who was heterozygous for the c.-27 + 18G>A variant in the GDNF gene encoding glial cell-derived neurotrophic factor. The cells were obtained by collagenase digestion of papillary tissues from the MSK patient and from two patients who had no MSK or nephrocalcinosis. These cells were typed by immunocytochemistry, and the presence of mineral deposits was studied using von Kossa staining, scanning electron microscopy analysis and an ALP assay. Osteoblastic lineage markers were studied using immunocytochemistry and RT-PCR. Staminality markers were also analysed using flow cytometry, magnetic cell separation technology, immunocytochemistry and RT-PCR. Starting from p2, MSK and control cells formed nodules with a behaviour similar to that of calcifying pericytes; however, Ca2PO4 was only found in the MSK cultures. The MSK cells had morphologies and immunophenotypes resembling those of pericytes or stromal stem cells and were positive for vimentin, ZO1, αSMA and CD146. In addition, the MSK cells expressed osteocalcin and osteonectin, indicating an osteoblast-like phenotype. In contrast to the control cells, GDNF was down-regulated in the MSK cells. Stable GDNF knockdown was established in the HK2 cell line and was found to promote Ca2PO4 deposition when the cells were incubated with calcifying medium by regulating the osteonectin/osteopontin ratio in favour of osteonectin. Our data indicate that the human papilla may be a perivascular niche in which pericyte/stromal-like cells can undergo osteogenic differentiation under particular conditions and suggest that GDNF down-regulation may have influenced the observed phenomenon.

The relationship between calcium kidney stones, arterial stiffness and bone density: unraveling the stone-bone-vessel liaison.

BACKGROUND AND OBJECTIVES: Kidney stone disease is associated with a higher incidence of cardio-vascular (CV) events for still unclear reasons. Reduced bone density is also a frequent finding in calcium kidney stones. The association of reduced bone density with increased vascular stiffness and calcification has been discovered in a number of conditions. We investigated the hypothesis that patients with calcium kidney stones have increased arterial stiffness, which would be associated with reduced bone density and higher CV risk. DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS: We compared measures of arterial stiffness [carotid-radial pulse-wave velocity (CR-PWV), carotid-femoral pulse-wave velocity (CF-PWV) and augmentation index (AI)] and of bone density (T-scores determined at lumbar spine, neck and hip) among 42 idiopathic calcium stone formers compared with 42 age- and sex-matched healthy volunteers. RESULTS: Stone formers had higher values of CR-PWV, CF-PWV and AI, and lower values of all T-scores. Furthermore, the prevalence of abnormal arterial stiffness and reduced bone density was significantly higher among stone formers. Statistical adjustment for age, sex, body mass index and other covariates did not change the results. CONCLUSIONS: Our study confirms that stone formers have increased arterial stiffness and reduced bone density. Abnormal arterial stiffness appears to be independent of reduced bone density and may explain the higher CV risk observed in stone formers.

Decreased transcriptional activity of calcium-sensing receptor gene promoter 1 is associated with calcium nephrolithiasis.

BACKGROUND: CaSR gene is a candidate for calcium nephrolithiasis. Single-nucleotide polymorphisms (SNPs) encompassing its regulatory region were associated with calcium nephrolithiasis. AIMS: We tested SNPs in the CaSR gene regulatory region associated with calcium nephrolithiasis and their effects in kidney. SUBJECTS AND METHODS: One hundred sixty-seven idiopathic calcium stone formers and 214 healthy controls were genotyped for four CaSR gene SNPs identified by bioinformatics analysis as modifying transcription factor binding sites. Strontium excretion after an oral load was tested in 55 stone formers. Transcriptional activity induced by variant alleles at CaSR gene promoters was compared by luciferase reporter gene assay in HEK-293 and HKC-8 cells. CaSR and claudin-14 mRNA levels were measured by real-time PCR in 107 normal kidney medulla samples and compared in patients with different CaSR genotype. RESULTS: Only rs6776158 (A>G), located in the promoter 1, was associated with nephrolithiasis. Its minor G allele was more frequent in stone formers than controls (37.8% vs 26.4%, P = .001). A reduced strontium excretion was observed in GG homozygous stone formers. Luciferase fluorescent activity was lower in cells transfected with the promoter 1 including G allele at rs6776158 than cells transfected with the A allele. CaSR mRNA levels were lower in kidney medulla samples from homozygous carriers for the G allele at rs6776158 than carriers for the A allele. Claudin-14 mRNA levels were also lower in GG homozygous subjects. CONCLUSIONS: Minor allele at rs6776158 may predispose to calcium stones by decreasing transcriptional activity of the CaSR gene promoter 1 and CaSR expression in kidney tubules.

Medullary sponge kidney.

PURPOSE OF REVIEW: After it was first described in 1939, medullary sponge kidney (MSK) received relatively little attention. This was because it was believed to have a low prevalence and because it was considered a benign condition. Studies in recent years have been changing these convictions however, hence the present review. RECENT FINDINGS: Insight has been obtained on the genetic basis of this disease, supporting the hypothesis that MSK is due to a disruption at the 'ureteric bud-metanephric mesenchyme' interface. This explains why so many tubular defects coexist in this disease, and particularly a distal tubular acidification defect of which the highly prevalent metabolic bone disease is one very important consequence. In addition to the typical clinical phenotype of recurrent stone disease, other clinical profiles have now been recognized, that is, an indolent, almost asymptomatic MSK, and a rare form characterized by intractable, excruciating pain. SUMMARY: Findings suggest the need for a more comprehensive clinical characterization of MSK patients. The genetic grounds for the condition warrant further investigation, and reliable methods are needed to diagnose MSK.

Familial clustering of medullary sponge kidney is autosomal dominant with reduced penetrance and variable expressivity.

Medullary sponge kidney (MSK) is a renal malformation typically associated with nephrocalcinosis and recurrent calcium nephrolithiasis. Approximately 12% of recurrent stone formers have MSK, which is generally considered a sporadic disorder. Since its discovery, three pedigrees have been described in which an apparently autosomal dominant inheritance was suggested. Here, family members of 50 patients with MSK were systematically investigated by means of interviews, renal imaging, and biochemical studies in an effort to establish whether MSK is an inheritable disorder. Twenty-seven MSK probands had 59 first- and second-degree relatives of both genders with MSK in all generations. There were progressively lower mean levels of serum calcium, urinary sodium, pH, and volume, combined with higher serum phosphate and potassium from probands to relatives with bilateral, to those with unilateral, and to those unaffected by MSK. This suggests that most affected relatives have a milder form of MSK than the probands, which would explain why they had not been so diagnosed. Thus, our study provides strong evidence that familial clustering of MSK is common, and has an autosomal dominant inheritance, a reduced penetrance, and variable expressivity.

Medullary sponge kidney: state of the art.

Medullary sponge kidney (MSK) is a kidney malformation that generally manifests with nephrocalcinosis and recurrent renal stones; other signs may be renal acidification and concentration defects, and pre-calyceal duct ectasias. MSK is generally considered a sporadic disorder, but an apparently autosomal dominant inheritance has also been observed. As MSK reveals abnormalities in both the lower and the upper nephron and is often associated with urinary tract developmental anomalies, its pathogenesis should probably be sought in one of the numerous steps characterizing renal morphogenesis. Given the key role of the GDNF-RET interaction in kidney and urinary tract development and nephrogenesis, anomalies in these molecules are reasonable candidates for explaining a disorder such as MSK. As a matter of fact, we detected two, hitherto unknown, rare variants of the GDNF gene in MSK patients. We surmise that a defective distal acidification has a central role in MSK and is followed by a chain of events including defective bone mineralization, hypercalciuria, hypocitraturia and stone formation.

Long-term treatment with potassium citrate and renal stones in medullary sponge kidney.

BACKGROUND AND OBJECTIVES: Medullary sponge kidney (MSK) is a renal malformation typically associated with nephrocalcinosis and recurrent calcium stones. Incomplete distal renal tubular acidosis, hypocitraturia, and hypercalciuria are common. For stone prevention, patients with MSK generally receive the standard "stone clinic" recommendations and often receive potassium citrate (KC). However, the effect on stone recurrence of citrate treatment in these patients has never been studied. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: The issue was retrospectively analyzed on an outpatient basis in 97 patients with a radiologic diagnosis of MSK: 65 had at least one stone risk factor (SRF; hypercalciuria, hypocitraturia, hyperuricosuria, hyperoxaluria) and received KC [29 +/- 8 (SD) mEq/d]; 10 patients with SRF and 22 without received only general stone clinic suggestions. Follow-up was 78 +/- 13, 72 +/- 15, and 83 +/- 14 months, respectively. The 24-hour urinary excretion of calcium, oxalate, uric acid, citrate, and morning urine pH were investigated at baseline and at the end of follow-up. RESULTS: Parallel to a significant rise in urinary citrate and decreased urinary calcium (all P < 0.001), KC led to a dramatic reduction in the stone event rate (from 0.58 to 0.10 stones/yr per patient). The existence of a group of patients with MSK, those without SRF, with a very low stone rate and no SRF was recognized. CONCLUSIONS: Treatment with KC is effective in preventing renal stones in the typical patient with MSK. It seems that two clinical phenotypes among patients showing typical MSK features during radiologic study exist.

Identification of GDNF gene sequence variations in patients with medullary sponge kidney disease.

BACKGROUND AND OBJECTIVES: Medullary sponge kidney (MSK) is a rare nephropathy characterized by cystic anomalies of precalyceal ducts, nephrocalcinosis, renal stones, and tubule dysfunctions. Its association with various malformations and cases of familial aggregation supports the conviction that genetic factors are involved, but no genetic studies have been conducted to date. It is hypothesized that MSK is due to a disruption at the "ureteric bud/metanephric blastema" interface caused by critical developmental genes functioning abnormally. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Fifty-five apparently sporadic MSK patients were analyzed by direct DNA sequencing of all exons and exon-intron boundaries of glial cell-derived neurotrophic factor (GDNF) gene and rearranged during transfection (RET) gene, which have a leading role in renal development. RESULTS: Two novel variants were found in heterozygosity in the MSK case population: GDNF{ENST00000344622}:c.-45G>C and c.-27+18G>A in a putative binding domain for paired-box 2 transcription factor. As a whole, eight patients showed these variations: four patients carried the c.[-45G>C; -27+18G>A] complex allele, and the others had the c.-27+18G>A alone. A case-control study revealed that these two alleles were significantly associated with MSK. Five of the eight cases were found to be familial, and the allele variants cosegregated with the disease in a seemingly dominant pattern of inheritance. Patients revealed no mutations in the RET gene. CONCLUSIONS: This is the first report identifying GDNF gene sequence variations in patients with MSK and suggesting a role for this gene in the pathogenesis of some cases of the disease.

The origin of nephrocalcinosis, Randall's plaque and renal stones: a cell biology viewpoint.

Medullary nephrocalcinosis is a rare condition typically observed in metabolic conditions prone to renal calcium stones. Randall's plaques are very frequently observed in the common idiopathic calcium-oxalate nephrolithiasis. These plaques are apatite mineral structures, and we propose they also are an example of nephrocalcinosis. While these calcium deposits are generally considered to be the consequence of purely physico-chemical phenomena, we advance the hypothesis that they form because of a true ectopic biomineralization in the renal tissue. Henle's loop epithelial cells, or pericyte-like interstitial cells, or papillary stem-cells differentiating along a bone lineage could be involved.

Bone disease in medullary sponge kidney and effect of potassium citrate treatment.

BACKGROUND AND OBJECTIVES: In medullary sponge kidney (MSK)-a common malformative renal condition in patients with calcium nephrolithiasis-hypercalciuria, incomplete distal renal tubular acidosis, and hypocitraturia are common. Clinical conditions with concomitant hypercalciuria and/or incomplete distal renal tubular acidosis are almost invariably associated with bone disease, making osteopathy highly likely in MSK, too. Patients with MSK have never been investigated for osteopathy; neither has the potential effect of potassium citrate administration (CA) on their urinary metabolic risk factors and on bone mineralization. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: These issues were retrospectively analyzed in 75 patients with MSK and primary stone risk factor (PSRF; hypercalciuria, hypocitraturia, hyperuricosuria, and/or hyperoxaluria) on an outpatient basis; 65 received CA (2.9 +/- 0.8 g/d), whereas 10 received only general "stone clinic" suggestions. The 24-h urinary excretion of calcium, phosphate, oxalate, uric acid, and citrate; morning urine pH; serum biochemistry; and bone mineral density were investigated at baseline and at the end of follow-up (78 +/- 13 and 72 +/- 15 mo in groups A and B, respectively). RESULTS: CA led to a significant rise in urinary pH and citrate and decreased urinary calcium and phosphate (all P < 0.001). Patients with MSK and PSRF had reduced bone density. Bone density improved significantly in the group that was treated with oral CA. CONCLUSIONS: Bone disease is very frequent in patients with MSK and concomitant PSRF. Long-term CA improves bone density. The concurrent effects of treatment on PSRF suggest that the subtle acidosis plays a pivotal role in bone disease and hypercalciuria in patients with MSK.

Unusual renal presentation of Fabry disease in a female patient.

BACKGROUND: A 29-year-old white woman with a family history of Fabry disease was referred to a nephrology clinic with hypertension and nephropathy. Her renal function was below normal (serum creatinine level 141 micromol/l; estimated glomerular filtration rate 41 ml/min/1.73 m2) with no proteinuria or albuminuria. INVESTIGATIONS: Medical history, physical examination, leukocyte alpha-galactosidase A assay, laboratory tests (for antinuclear antibodies, antineutrophil cytoplasmic antibodies, lupus anticoagulant, anticardiolipin antibodies, complement and cryoglobulin), ophthalmological examination, echocardiography, brain magnetic resonance angiography, renal ultrasonography, renal color echo-Doppler scan, renal magnetic resonance angiography, renal angiography and renal biopsy. DIAGNOSIS: Diffuse sclero-atrophic renal tissue changes and widespread renal arterio-arteriolosclerotic changes secondary to Fabry disease. TREATMENT: Angiotensin-converting-enzyme inhibitors and maintenance treatment with agalsidase-beta, 1 mg/kg body weight, every 2 weeks.

Pathogenesis of nephrolithiasis: recent insight from cell biology and renal pathology.

Randall's plaques are very common in idiopathic calcium-oxalate nephrolithiasis. These papillary plaques have an apatite mineral structure. While these calcium deposits are generally assumed to be secondary to a purely physico-chemical phenomenon, we advance the hypothesis that they form due to a truly ectopic biomineralization in the renal tissue, and that Henle's loop epithelial cells, or pericyte-like interstitial cells, or papillary stem cells differentiating along a bone lineage might be involved.