Genetics and phenotypic heterogeneity of Dent disease: the dark side of the moon

Dent disease is a rare genetic proximal tubulopathy which is under-recognized. Its phenotypic heterogeneity has led to several different classifications of the same disorder, but it is now widely accepted that the triad of symptoms low-molecular-weight proteinuria, hypercalciuria and nephrocalcinosis/nephrolithiasis are pathognomonic of Dent disease. Although mutations on the CLCN5 and OCRL genes are known to cause Dent disease, no such mutations are found in about 25-35% of cases, making diagnosis more challenging. This review outlines current knowledge regarding Dent disease from another perspective. Starting from the history of Dent disease, and reviewing the clinical details of patients with and without a genetic characterization, we discuss the phenotypic and genetic heterogeneity that typifies this disease. We focus particularly on all those confounding clinical signs and symptoms that can lead to a misdiagnosis. We also try to shed light on a concealed aspect of Dent disease. Although it is a proximal tubulopathy, its misdiagnosis may lead to patients undergoing kidney biopsy. In fact, some individuals with Dent disease have high-grade proteinuria, with or without hematuria, as in the clinical setting of glomerulopathy, or chronic kidney disease of uncertain origin. Although glomerular damage is frequently documented in Dent disease patients' biopsies, there is currently no reliable evidence of renal biopsy being of either diagnostic or prognostic value. We review published histopathology reports of tubular and glomerular damage in these patients, and discuss current knowledge regarding the role of CLCN5 and OCRL genes in glomerular function

Albumin uptake in human podocytes: a possible role for the cubilin-amnionless (CUBAM) complex

Abstract Albumin re-uptake is a receptor-mediated pathway located in renal proximal tubuli. There is increasing evidence of glomerular protein handling by podocytes, but little is known about the mechanism behind this process. In this study, we found that human podocytes in vitro are committed to internalizing albumin through a receptor-mediated mechanism even after exposure to low doses of albumin. We show that these cells express cubilin, megalin, ClC-5, amnionless and Dab2, which are partners in the tubular machinery. Exposing human podocytes to albumin overload prompted an increase in CUBILIN, AMNIONLESS and CLCN5 gene expression. Inhibiting cubilin led to a reduction in albumin uptake, highlighting its importance in this mechanism. We demonstrated that human podocytes are committed to performing endocytosis via a receptor-mediated mechanism even in the presence of low doses of albumin. We also disclosed that protein overload first acts on the expression of the cubilin-amnionless (CUBAM) complex in these cells, then involves the ClC-5 channel, providing the first evidence for a possible role of the CUBAM complex in albumin endocytosis in human podocytes

Human proximal tubular cells can form calcium phosphate deposits in osteogenic culture: role of cell death and osteoblast-like transdifferentiation

Abstract Nephrocalcinosis is a clinicopathological entity characterized by microscopic calcium crystals in the renal parenchyma, within the tubular lumen or in the interstitium. Crystal binding to tubular cells may be the cause underlying nephrocalcinosis and nephrolithiasis. Pathological circumstances, such as acute cortical necrosis, may induce healthy cells to acquire a crystal-binding phenotype. The present study aimed to investigate whether human renal proximal tubular cells (HK-2 cells) can form calcium phosphate deposits under osteogenic conditions, and whether apoptosis and/or osteogenic-like processes are involved in cell calcification. HK-2 cells were cultured in standard or osteogenic medium for 1, 5, and 15 days. Von Kossa staining and ESEM were used to analyze crystal deposition. Apoptosis was investigated, analyzing caspase activation by in-cell Western assay, membrane translocation of phosphotidylserine by annexin V-FITC/propidium iodide staining, and DNA fragmentation by TUNEL assay. qRT/PCR, immunolabeling and cytochemistry were performed to assess osteogenic activation (Runx2, Osteonectin, Osteopontin and ALP), and early genes of apoptosis (BAX, Bcl-2). HK-2 cell mineralization was successfully induced on adding osteogenic medium. Calcium phosphate deposition increased in a time-dependent manner, and calcified cell aggregates exhibited characteristic signs of apoptosis. At 15 days, calcifying HK-2 cells revealed osteogenic markers, such as Runx2, ALP, osteonectin and osteopontin. Monitoring the processes at 1, 5, and 15 days showed apoptosis starting already after 5 days of osteogenic induction, when the first small calcium phosphate crystals began to appear on areas where cell aggregates were in apoptotic conditions. The cell death process proved caspase-dependent. The importance of apoptosis was reinforced by the time-dependent increase in BAX expression, starting from day 1. These findings strongly support the hypothesis that apoptosis triggered HK-2 calcification even before any calcium phosphate crystal deposition or acquisition of an osteogenic phenotype

Understanding the Pathophysiology of Nephrocalcinosis

Many in vitro and in vivo studies on the mechanisms underlying calcium nephrolithiasis have provided evidence of a frequently associated condition, i.e., a microscopic renal crystal deposition that can occur within the tubular lumen (intratubular nephrocalcinosis) or in the interstitium (interstitial nephrocalcinosis). Medullary nephrocalcinosis is the typical pattern seen in 98% of cases of human nephrocalcinosis, with calcification clustering around each renal pyramid. It is common in patients with metabolic conditions that predispose them to renal calcium stones. Cortical nephrocalcinosis is rare and usually results from severe destructive disease of the cortex. It has been described in chronic glomerulonephritis, but often in association with another factor, such as an increased calcium ingestion, acute cortical necrosis, chronic pyelonephritis or trauma. The most accredited hypothesis to explain the onset of interstitial nephrocalcinosis is purely physicochemical, relating to spontaneous Ca2PO4 crystallization in the interstitium due to oversaturation of Ca2PO4salts in this milieu. The theory that nephrocalcinosis is a process driven by osteogenic cells was first proposed by our group. We review nephrocalcinosis in terms of its definition, genetic associations, and putative mechanisms, pointing out how much evidence in the literature suggests that it may have some features in common with, and pathogenic links to vascular calcification

Profiling Insulin Like Factor 3 (INSL3) Signaling in Human Osteoblasts

Abstract BACKGROUND: Young men with mutations in the gene for the INSL3 receptor (Relaxin family peptide 2, RXFP2) are at risk of reduced bone mass and osteoporosis. Consistent with the human phenotype, bone analyses of Rxfp2(-/-) mice showed decreased bone volume, alterations of the trabecular bone, reduced mineralizing surface, bone formation, and osteoclast surface. The aim of this study was to elucidate the INSL3/RXFP2 signaling pathways and targets in human osteoblasts. METHODOLOGY/PRINCIPAL FINDINGS: Alkaline phosphatase (ALP) production, protein phosphorylation, intracellular calcium, gene expression, and mineralization studies have been performed. INSL3 induced a significant increase in ALP production, and Western blot and ELISA analyses of multiple intracellular signaling pathway molecules and their phosphorylation status revealed that the MAPK was the major pathway influenced by INSL3, whereas it does not modify intracellular calcium concentration. Quantitative Real Time PCR and Western blotting showed that INSL3 regulates the expression of different osteoblast markers. Alizarin red-S staining confirmed that INSL3-stimulated osteoblasts are fully differentiated and able to mineralize the extracellular matrix. CONCLUSIONS/SIGNIFICANCE: Together with previous findings, this study demonstrates that the INSL3/RXFP2 system is involved in bone metabolism by acting on the MAPK cascade and stimulating transcription of important genes of osteoblast maturation/differentiation and osteoclastogenesis

Protein Uptake at Glomerular Level: Possible Involvement of an Endocytic Machinery in Cell Culture and in Patients with Lupus Nephritis

ClC-5 with megalin (LRP2), cubilin, Disabled 2 (Dab2), and Amnionless (AMN), is part of the molecular complex involved at proximal tubular level in the endocytic re-uptake of low-molecular-weight proteins and albumin. ClC-5, megalin and cubilin expression in podocytes of human renal biopsies was already reported. Moreover, it was demonstrated that podocytes are able to internalize albumin through an endocytic process. It is reasonable to assume that there should be an involvement of this system in protein uptake mediated by podocytes. A disruption of this system can lead to proteinuria which is one of the first manifestation of kidney disease in Systemic Lupus Erythematous (SLE). Aims of this study were to explore the presence of the tubular endocytic machinery components also in human podocytes in vitro and to evaluate whether albumin modulates this system. Moreover, we wanted to verify and quantify the expression of ClC-5, megalin and cubilin in both glomerular and tubular compartments in renal biopsies of patients presenting Lupus nephritis and observing the presence of a relationship with clinical data. To verify the presence of an uptake mechanism in human podocytes in vitro, we performed time lapse experiments with a low dose of FITC albumin (10 µg/ml). We observed albumin internalization starting from 2 to 15 hours. To evaluate the uptake kinetic, we stimulated podocytes at different time (30 min and 2 hours) and doses (10 µg/ml, 100 µg/ml and 1 mg/ml) at 37°C and 4°C. We observed a significant dose dependent increase in fluorescence vs controls after 2 hours’ stimulation with a typical receptor-mediated kinetic since it is inhibited at 4 °C. Moreover, we disclosed the presence of ClC-5, Dab2 and AMN beyond megalin and cubilin, in this in vitro system using immunohistochemistry (IHC) and immunofluorescence (IF) techniques, highlighting a co-localization between albumin and both receptors. To analyze whether the proteinuric environment modulates CLCN5, LRP2, CUBN, DAB2 and AMN expression, we stimulated human podocytes with increasing concentrations of BSA (range 10 µg/ml - 30 mg/ml) and we evaluated the mRNA expression at different time points (2, 4, 8, 24, 48 and 72 hours). Using Real Time PCR we observed a significant time and dose dependent increase in CLCN5, CUBN and AMN expression and an up-regulation of DAB2 only at 24 hours. We collected 23 SLE renal biopsies, 6 control biopsies and 1 case of Minimal Change Disease. As clinical parameters we considered proteinuria and pharmacological therapy. IHC and IF were used to analyze ClC-5, megalin and cubilin protein expression in serial sections. Morphometric quantification revealed a direct correlation between tubular and glomerular expression of all molecules in SLE patients, highlighting a relationship between glomerular and tubular compartments independently from proteinuria levels. Furthermore, preliminary data on patients without ACEi/ARB and immunosuppressive drugs disclosed a positive trend among these molecules at glomerular level. Interestingly, we revealed megalin and cubilin expression in hypertrophic PECs of some SLE patients and characterization experiments identified a subpopulation with an intermediate phenotype between mature and progenitor cells. In conclusion, for the first time we demonstrated that human podocytes are naturally committed to perform albumin endocytosis via a receptor-mediated mechanism. Moreover, protein overload upregulates CUBN, AMN and CLCN5 in these cells. Functional studies regarding the role of cubilin in albumin uptake underlined its participation in this mechanism even if this is not the only pathway involved. Further studies will be necessary to analyze which may be the partner(s) of the CUBAM complex in this mechanism. For the first time we demonstrated the presence of ClC-5, megalin and cubilin in glomeruli of patients with SLE and MCD in addition to controls confirming in vitro data. Furthermore, in SLE biopsies we highlighted a strong correlation between the two renal compartments in the expression of the protein uptake system, supporting the idea of a partnership between tubular and glomerular cells in albumin uptake via the same mechanism of internalization. In addition, preliminary data on patients without ACEi/ARB and immunosuppressive drugs lead us to suppose that the pharmacological therapy could affect the expression of this system, in particular at glomerular level. The differences observed from in vivo and in vitro data, especially on megalin expression, underline the involvement of other glomerular cell types in addition to podocytes in protein uptake. Finally, megalin and cubilin expression in PECs of SLE patients is a real interesting but complex data, since characterization experiments identified a subpopulation with an intermediate phenotype between mature and progenitor cells. Further studies will be performed to better characterize the role of these double-positive cells and their correlation with clinical data and/or disease progression.ClC-5, megalina (LRP2), cubilina, Disabled 2 (Dab2) ed Amnionless (AMN) fanno parte del complesso molecolare coinvolto a livello del tubulo prossimale nel recupero delle proteine a basso peso molecolare e dell’albumina mediante endocitosi. È già stata riportata la presenza di ClC-5, megalina e cubilina a livello dei podociti in biopsie renali umane. Inoltre è stato dimostrato che i podociti sono in grado di internalizzare l’albumina attraverso un meccanismo di endocitosi. Il mancato funzionamento di questo sistema può portare a proteinuria, che è una delle prime manifestazioni del coinvolgimento renale nel Lupus Eritematoso Sistemico (LES). È quindi ragionevole supporre che vi possa essere un coinvolgimento di questo sistema nel meccanismo di uptake delle proteine da parte dei podociti. Gli scopi di questo studio sono stati di esplorare la presenza dei componenti del sistema tubulare di endocitosi delle proteine in podociti umani in coltura e di valutare se e come l’albumina ne modulasse l’espressione. Inoltre, si è voluto indagare l’espressione di ClC-5, megalina e cubilina sia a livello glomerulare che tubulare in biopsie renali di pazienti con nefrite lupica, valutando una possibile relazione con i dati clinici. Abbiamo verificato la presenza di un meccanismo di uptake in podociti umani in coltura attraverso esperimenti di time-lapse con basse dosi di FITC-BSA (10 µg/ml) ed abbiamo osservato l’inizio del processo di internalizzazione in un periodo di tempo variabile dalle 2 alle 15 ore. Per caratterizzare il tipo di cinetica di uptake della FITC-BSA, i podociti sono stati stimolati a differenti tempi (30 min e 2 ore) e dosi (10 µg/ml, 100 µg/ml and 1 mg/ml) mantenendo la coltura a 37°C o 4°C. Si è osservato un aumento significativo della fluorescenza dose-dipendente rispetto al controllo dopo 2 ore dalla stimolazione con una tipica cinetica di internalizzazione recettore-mediata poiché veniva inibita a 4°C. Abbiamo osservato la presenza di ClC-5, Dab2 e AMN oltre a quella di megalina e cubilina in podociti umani in coltura in condizioni basali mediante tecniche di immunoistochimica (IHC) ed immunofluorescenza (IF) ed abbiamo dimostrato la co-localizzazione dei due recettori con l’albumina fluorescente. Per valutare se l’ambiente proteinurico fosse in grado di modulare l’espressione di CLCN5, LRP2, CUBN, DAB2 ed AMN, i podociti umani sono stati stimolati con concentrazioni crescenti di BSA (range 10 µg/ml - 30 mg/ml) e l’espressione dell’RNA messaggero è stata valutata a tempi diversi (2, 4, 8, 24, 48 and 72 hours). Mediante analisi in Real Time PCR, abbiamo osservato un aumento significativo tempo e dose-dipendente di CLCN5, CUBN ed AMN ed un aumento di DAB2 solamente alle 24 ore. Abbiamo raccolto 23 biopsie renali di pazienti con LES, 6 biopsie di controllo ed un caso di Minimal Change Disease. Come parametri clinici abbiamo considerato la proteinuria e la terapia farmacologica. Mediante IHC ed IF abbiamo analizzato l’espressione proteica di ClC-5, megalina e cubilina in sezioni seriali. La quantificazione eseguita mediante analisi morfometrica ha rivelato una correlazione diretta dell’espressione di tutte le molecole in analisi tra il compartimento tubulare e glomerulare, evidenziando una stretta relazione tra i due compartimenti indipendentemente dai livelli di proteinuria. Inoltre, dati preliminari su pazienti privi di terapia farmacologica (ACEi/ARB o immunosoppressivi) hanno mostrato un trend positivo tra l’espressione di queste molecole a livello glomerulare. Curiosamente, abbiamo evidenziato l’espressione di megalina e cubilina in cellule parietali della capsula (PECs) con morfologia ipertrofica in alcuni pazienti LES che, mediante esperimenti di caratterizzazione, abbiamo identificato come una nuova sottopopolazione con un fenotipo intermedio tra cellule mature e progenitrici. Concludendo, per la prima volta abbiamo dimostrato che i podociti umani sono naturalmente predisposti ad effettuare l’endocitosi dell’albumina attraverso un meccanismo recettore-mediato. Inoltre, l’overload proteico è in grado di aumentare l’espressione di CLCN5, CUBN ed AMN in queste cellule. Studi funzionali per dimostrare il ruolo di cubilina nel processo di uptake dell’albumina hanno sottolineato la sua partecipazione in questo meccanismo anche se, verosimilmente, non è l’unico pathway coinvolto. Ulteriori studi saranno necessari per analizzare quali altre molecole possano essere chiamate in causa in questo meccanismo. Per la prima volta abbiamo dimostrato la presenza di ClC-5, megalina e cubilina in glomeruli di pazienti con LES, MCD e controlli, confermando i dati in vitro. Inoltre, nelle biopsie dei pazienti LES abbiamo evidenziato una stretta relazione tra i due compartimenti renali nell’espressione dei componenti di questo sistema, supportando l’idea di una partnership tra cellule tubulari e glomerulari nell’uptake dell’albumina attraverso lo stesso meccanismo di internalizzazione. In aggiunta, dati preliminari ottenuti da pazienti privi di terapia con ACEi/ARB o immunosoppressivi ci ha fatto supporre che il trattamento farmacologico possa influire sull’espressione di questo sistema a livello glomerulare. Le differenze osservate tra lo studio in vivo e quello in vitro, in particolare riguardo l’espressione di megalina, suggeriscono il coinvolgimento di altre cellule del glomerulo oltre ai podociti. Infine, l’espressione di megalina e cubilina nelle PECs dei pazienti LES è un dato molto interessante ma complesso, poiché gli esperimenti di caratterizzazione hanno identificato una sottopopolazione con un fenotipo intermedio tra cellule mature e progenitrici. Ulteriori studi dovranno essere condotti per meglio caratterizzare il ruolo di queste cellule con doppia positività e la loro correlazione con i dati clinici o di progressione della malattia