Exfoliated human proximal tubular cells: a model of cystinosis and Fanconi syndrome

In Vitro and In Vivo Models to Study Nephropathic Cystinosis

The development over the past 50 years of a variety of cell lines and animal models has provided valuable tools to understand the pathophysiology of nephropathic cystinosis. Primary cultures from patient biopsies have been instrumental in determining the primary cause of cystine accumulation in the lysosomes. Immortalised cell lines have been established using different gene constructs and have revealed a wealth of knowledge concerning the molecular mechanisms that underlie cystinosis. More recently, the generation of induced pluripotent stem cells, kidney organoids and tubuloids have helped bridge the gap between in vitro and in vivo model systems. The development of genetically modified mice and rats have made it possible to explore the cystinotic phenotype in an in vivo setting. All of these models have helped shape our understanding of cystinosis and have led to the conclusion that cystine accumulation is not the only pathology that needs targeting in this multisystemic disease. This review provides an overview of the in vitro and in vivo models available to study cystinosis, how well they recapitulate the disease phenotype, and their limitations.

Assessment of chimerism in epithelial cancers in transplanted patients

Cancer is now the most severe complication in the long term in transplant recipients. As most solid-organ or hematopoietic stem-cell transplantations are allogeneic, chimerism studies can be performed on cancers occurring in recipients. We summarize here the different methods used to study chimerism in cancers developing in allogeneic-transplant recipients, analyze their respective advantages and report the main results obtained from these studies. Chimerism analyses of cancers in transplant recipients require methods suited to tissue samples. In the case of gender-mismatched transplantation, the XY chromosomes can be explored using fluorescent in situ hybridization on whole-tissue sections or Y-sequence-specific PCR after the laser microdissection of tumor cells. For cancers occurring after gender-matched transplantation, laser microdissection of tumor cells enables studies of microsatellite markers and high-resolution melting analysis of mitochondrial DNA on genes with marked polymorphism, provided these are different in the donor and the recipient. The results of different studies address the cancers that develop in both recipients and in transplants. The presence of chimeric cells in these two types of cancer implies an exchange of progenitor/stem-cells between transplant and recipient, and the plasticity of these progenitor/stem-cells contributes to epithelial cancers. The presence of chimeric cells in concomitant cancers and preneoplastic lesions implies that the oncogenesis of these cancers progresses through a multistep process.

Diagnostic tools for hypertension and salt sensitivity testing

PURPOSE OF REVIEW

One-third of the world's population has hypertension and it is responsible for almost 50% of deaths from stroke or coronary heart disease. These statistics do not distinguish salt-sensitive from salt-resistant hypertension or include normotensives who are salt-sensitive even though salt sensitivity, independent of blood pressure, is a risk factor for cardiovascular and other diseases, including cancer. This review describes new personalized diagnostic tools for salt sensitivity.

RECENT FINDINGS

The relationship between salt intake and cardiovascular risk is not linear, but rather fits a J-shaped curve relationship. Thus, a low-salt diet may not be beneficial to everyone and may paradoxically increase blood pressure in some individuals. Current surrogate markers of salt sensitivity are not adequately sensitive or specific. Tests in the urine that could be surrogate markers of salt sensitivity with a quick turn-around time include renal proximal tubule cells, exosomes, and microRNA shed in the urine.

SUMMARY

Accurate testing of salt sensitivity is not only laborious but also expensive, and with low patient compliance. Patients who have normal blood pressure but are salt-sensitive cannot be diagnosed in an office setting and there are no laboratory tests for salt sensitivity. Urinary surrogate markers for salt sensitivity are being developed.

Human de novo papillary renal-cell carcinomas in a kidney graft: evidence of recipient origin with adenoma-carcinoma sequence

Papillary renal-cell carcinoma (pRCC) is unusual for its occurrence in kidneys with chronic dysfunction, for its frequent multifocality and for its common association with papillary adenoma, a benign renal lesion morphologically indistinguishable from pRCC. Concomitant development of papillary adenoma and pRCC in five transplanted kidneys, where donor and recipient characteristics are well established, provided a unique opportunity for molecular studies of de novo pRCC carcinogenesis. We aimed to study this tumor type to determine whether or not the different papillary tumors have the same origin, and whether or not papillary adenomas are precursor lesions of pRCC. We performed XY-FISH in sex-mismatched kidney transplants, and polymorphic microsatellite DNA and high-resolution melting of mitochondrial DNA analyzes in all five patients on laser-microdissected tumor cells, then compared these molecular profiles to donor and recipient profiles. This study (i) identified the recipient origin of de novo papillary adenomas and pRCCs in a kidney transplant, (ii) demonstrated an identical origin for precursor cells of papillary adenomas and pRCCs and (iii) showed additional genetic alterations in pRCCs compared to papillary adenomas. This molecular approach of papillary tumors developed in transplanted kidney identified successive steps in carcinogenesis of human de novo papillary renal-cell carcinoma.

A linear relationship between the ex-vivo sodium mediated expression of two sodium regulatory pathways as a surrogate marker of salt sensitivity of blood pressure in exfoliated human renal proximal tubule cells: the virtual renal biopsy

BACKGROUND

Salt sensitivity (SS) of blood pressure (BP) affects 25% of adults, shares comorbidity with hypertension, and has no convenient diagnostic test. We tested the hypothesis that urine-derived exfoliated renal proximal tubule cells (RPTCs) could diagnose the degree of an individual's SS of BP.

METHODS

Subjects were selected who had their SS of BP determined 5 y prior to this study (salt-sensitive: ≥7 mm Hg increase in mean arterial pressure (MAP) following transition from a random weekly diet of low (10 mmol/day) to high (300 mmol/day) sodium (Na(+)) intake, N=4; inverse salt-sensitive (ISS): ≥7 mm Hg increase in MAP transitioning from a high to low Na(+) diet, N=3, and salt-resistant (SR): <7 mm Hg change in MAP transitioned on either diet, N=5). RPTC responses to 2 independent Na(+) transport pathways were measured.

RESULTS

There was a negative correlation between the degree of SS and dopamine-1 receptor (D1R) plasma membrane recruitment (y=-0.0107x+0.68 relative fluorescent units (RFU), R(2)=0.88, N=12, P<0.0001) and angiotensin II-stimulated intracellular Ca(++) (y=-0.0016x+0.0336, R(2)=0.7112, P<0.001, N=10) concentration over baseline.

CONCLUSIONS

Isolating RPTCs from urine provides a personalized cell-based diagnostic test of SS index that offers advantages over a 2-week controlled diet with respect to cost and patient compliance. Furthermore, the linear relationship between the change in MAP and response to 2 Na(+) regulatory pathways suggests that an individual's RPTC response to intracellular Na(+) is personalized and predictive.

FXYD2 and Na,K-ATPase expression in isolated human proximal tubular cells: disturbed upregulation on renal hypomagnesemia?

Autosomal dominant renal hypomagnesemia (OMIM 154020), associated with hypocalciuria, has been linked to a 121G to A mutation in the FXYD2 gene. To gain insight into the molecular mechanisms linking this mutation to the clinical phenotype, we studied isolated proximal tubular cells from urine of a patient and a healthy subject. Cells were immortalized and used to assess the effects of hypertonicity-induced overexpression of FXYD2 on amount, activity and apparent affinities for Na⁺, K⁺ and ATP of Na,K-ATPase. Both cell lines expressed mRNA for FXYD2a and FXYD2b, and patient cells contained both the wild-type and mutated codons. FXYD2 protein expression was lower in patient cells and could be increased in both cell lines upon culturing in hyperosmotic medium but to a lesser extent in patient cells. Similarly, hyperosmotic culturing increased Na,K-ATPase protein expression and ATP hydrolyzing activity but, again, to a lesser extent in patient cells. Apparent affinities of Na,K-ATPase for Na⁺, K⁺ and ATP did not differ between patient and control cells or after hyperosmotic induction. We conclude that human proximal tubular cells respond to a hyperosmotic challenge with an increase in FXYD2 and Na,K-ATPase protein expression, though to a smaller absolute extent in patient cells.

Primary cultures of renal proximal tubule cells derived from individuals with primary hyperoxaluria

The primary hyperoxalurias, PH1 and PH2, are inherited disorders caused by deficiencies of alanine:glyoxylate aminotransferase and glyoxylate reductase, respectively. Mutations in either of these enzymes leads to endogenous oxalate overproduction primarily in the liver, but most pathological effects are exhibited in the kidney ultimately leading to end-stage renal failure and systemic oxalosis. To provide a non-invasive means of accessing kidney cells from individuals with primary hyperoxaluria, we have derived primary cultures of renal proximal tubule cells from the urine of these patients. The cells stain positively for the epithelial markers pan-cytokeratin and zonula occludens 1 and the proximal tubule marker gamma-glutamyl transpeptidase. Mutation analysis confirmed that the cultured cells had the same genotype as the leucocytes of the patients and also expressed glyoxylate reductase at the mRNA level, illustrating their potential value as a source of renal material from these individuals.

Mitochondrial complex V expression and activity in cystinotic fibroblasts

Alterations in ATP metabolism have been proposed to be involved in the pathogenesis of cystinosis, the most common form of inherited Fanconi syndrome. A recent study showed normal activity of respiratory chain complexes I-IV with decreased ATP levels in cystinotic fibroblasts. Here, we show normal complex V expression and activity in mitochondria of cystinotic fibroblasts. This indicates that alterations in mitochondrial oxidative phosphorylation enzymes are not responsible for ATP decrease in cystinotic fibroblasts.

Inhibition of creatine kinase activity by cystine in the kidney of young rats

Nephropathic cystinosis is a lethal genetic disease caused by a lysosomal transport disorder leading to intralysosomal cystine accumulation in all tissues. Cystinosis is the most common inherited cause of Fanconi syndrome, but the mechanisms by which cystine causes tissue damage are not fully understood. Thiol-containing enzymes are critical for renal energy metabolism and may be altered by disulfides like cystine. Therefore, in the present study our main objective was to investigate the in vivo and in vitro effects of cystine on creatine kinase, which contains critical thiol groups in its structure, in the kidney of young Wistar rats. We observed that cystine inhibited in vivo and in vitro the enzyme activity and that this inhibition was prevented by cysteamine and glutathione. The results suggest oxidation of essential sulfhydryl groups necessary for creatine kinase function by cystine. Considering that creatine kinase and other thiol-containing enzymes are crucial for renal energy metabolism, and programmed cell death occurs in situations of energy deficiency, the enzyme inhibition caused by cystine released from lysosomes might be a mechanism of tissue damage in patients with cystinosis.

Glutathione depletion and increased apoptosis rate in human cystinotic proximal tubular cells

We have determined levels of glutathione (GSH), ATP, mitochondrial complex activity and apoptosis rate in proximal tubular cells (PTCs) exfoliated from urine in cystinotic (n=9) and control (n=9) children. Intracellular GSH was significantly depleted in cystinotic PTCs compared with controls (6.8 nmol GSH/mg protein vs 11.8 nmol GSH/mg protein; P<0.001), but there were no significant differences in mitochondrial complex activities or ATP levels under basal conditions. Cystinotic PTCs showed significantly increased apoptosis rate. After PTCs had been stressed by hypoxia, there was further depletion of GSH in cystinotic and control PTCs (2.4 nmol GSH/mg protein vs 7.2 nmol GSH/mg protein; P<0.001). Hypoxic stress led to increased complex I and complex IV activities in control but not in cystinotic PTCs. ATP levels were significantly reduced in cystinotic PTCs after hypoxic stress (12.2 nmol/mg protein vs 26.9 nmol/mg protein; P<0.001). GSH depletion occurs in this in vitro model of cystinotic PTCs, is exaggerated by hypoxic stress and may contribute to reduced ATP and failure to increase complex I/IV activities. Apoptotic rate is also increased, and these mechanisms may contribute to cellular dysfunction in cultured, human cystinotic PTCs.

Decreased intracellular ATP content and intact mitochondrial energy generating capacity in human cystinotic fibroblasts

Cystinosis is an autosomal recessive lysosomal storage disorder caused by a defect in the lysosomal cystine carrier cystinosin. Cystinosis is the most common cause of inherited Fanconi syndrome leading to renal failure, in which the pathogenesis is still enigmatic. Based on studies of proximal tubules loaded with cystine dimethyl ester (CDME), altered mitochondrial adenosine triphosphate (ATP) production was proposed to be an underlying pathologic mechanism. Thus far, however, experimental evidence supporting this hypothesis in humans is lacking. In this study, energy metabolism was extensively investigated in primary fibroblasts derived from eight healthy subjects and eight patients with cystinosis. Patient's fibroblasts accumulated marked amounts of cystine and displayed a significant decrease in intracellular ATP content. Remarkably, overall energy-generating capacity, activity of respiratory chain complexes, ouabain-dependent rubidium uptake reflecting Na,K-ATPase activity, and bradykinin-stimulated mitochondrial ATP production were all normal in these cells. In conclusion, the data presented demonstrate that mitochondrial energy-generating capacity and Na,K-ATPase activity are intact in cultured cystinotic fibroblasts, thus questioning the idea of altered mitochondrial ATP synthesis as a keystone for the pathogenesis of cystinosis.

Elevated oxidized glutathione in cystinotic proximal tubular epithelial cells

Cystinosis, the most frequent cause of inborn Fanconi syndrome, is characterized by the lysosomal cystine accumulation, caused by mutations in the CTNS gene. To elucidate the pathogenesis of cystinosis, we cultured proximal tubular cells from urine of cystinotic patients (n = 9) and healthy controls (n = 9), followed by immortalization with human papilloma virus (HPV E6/E7). Obtained cell lines displayed basolateral polarization, alkaline phosphatase activity, and presence of aminopeptidase N (CD-13) and megalin, confirming their proximal tubular origin. Cystinotic cell lines exhibited elevated cystine levels (0.86 +/- 0.95 nmol/mg versus 0.09 +/- 0.01 nmol/mg protein in controls, p = 0.03). Oxidized glutathione was elevated in cystinotic cells (1.16 +/- 0.83 nmol/mg versus 0.29 +/- 0.18 nmol/mg protein, p = 0.04), while total glutathione, free cysteine, and ATP contents were normal in these cells. In conclusion, elevated oxidized glutathione in cystinotic proximal tubular epithelial cell lines suggests increased oxidative stress, which may contribute to tubular dysfunction in cystinosis.