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

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, aSMA 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

Polyunsaturated fatty acids and renal fibrosis:pathophysiologic link and potential clinical implications

The role of polyunsaturated fatty acids in renal fibrosis. Several studies suggest a close relationship between polyunsaturated fatty acids (PUFA) and renal inflammation and fibrosis, which are crucial stages in chronic kidney disease (CKD). Beneficial effects of n-3 PUFA on the course of experimental and human nephropathies have been reported. PUFA can ameliorate chronic, progressive renal injury beyond the simple reduction of serum lipid levels. These pleiotropic effects of PUFA are due to their properties of interfering with the synthesis of a variety of inflammatory factors and events, through effects related both to the modulation of the balance of n-6 and n-3-derived eicosanoids and to direct action on the cellular production of the major cytokine mediators of inflammation and on endothelium function. The mechanisms by which PUFA can favorably interfere with some stages in renal fibrosis processes, such as mesangial cell activation and proliferation and extracellular matrix protein synthesis, include the regulation of some pro-inflammatory cytokine production, renin and nitric oxide (NO) systems and peroxisome proliferator-activated receptor gene expression. An optimal n-6/n-3 PUFA ratio dietary intake could offer new therapeutic strategies aimed at interrupting the irreversible process of renal fibrosis and ameliorating chronic renal injury. However, further experimental, epidemiological and clinical investigations are needed to confirm the role of PUFA in the renal fibrosis pathway and the natural history of chronic nephropathies

SP078CLC5 AND MEGALIN/CUBILIN COMPLEX IN HUMAN MESANGIAL CELLS: A POSSIBLE ROLE IN PROTEIN UPTAKE

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EPA and DHA suppress AngII- and arachidonic acid-induced expression of profibrotic genes in human mesangial cells.

BACKGROUND: There is some evidence suggesting a close relationship between polyunsaturated fatty acids (PUFAs) and renal inflammation and fibrosis, which are crucial stages in chronic kidney disease. METHODS: To verify the role of PUFAs in renal fibrosis processes, we investigated the effects of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (AA) on the gene expression of TGFbeta, fibronectin (FN), connective tissue growth factor (CTGF) and type IV collagen (COLIV) in human mesangial cells, in the absence or presence of angiotensin II (AngII), using reverse transcriptase PCR. RESULTS: The addition of AA to mesangial cell cultures induced a significant up-regulation of TGFbeta, FN, CTGF and COLIV expression, similar to that induced by AngII, while EPA and DHA had no stimulatory effects. The coincubation of cells with AngII and AA potentiated AngII-induced gene expression; on the contrary, the coexposure of cells to EPA or DHA suppressed the AngII- and AA-induced up-regulation of TGFbeta, FN, CTGF and COLIV. CONCLUSION: We conclude that the PUFAs have different effects, dependent on their chemical structure, on the AngII-TGFbeta system, a major regulator of the renal fibrotic process. Our in vitro results may provide new therapeutic options toward interrupting the irreversible process of renal fibrosis and ameliorating chronic renal injury

Effects of unsaturated free fatty acids on adhesion and on gene expression of extracellular matrix macromolecules in human osteoblast-like cultures

To investigate the possible role for unsaturated free fatty acids in osteoblast adhesion, the effects of two polyunsaturated fatty acids (PUFAs), arachidonic (AA) and eicosapentaenoic (EPA) acids, and of one monounsaturated fatty acid, oleic acid (OA), on adhesion to the substrate and on gene expression of three extracellular matrix macromolecules were investigated in an in vitro model system--cultured osteoblast-like human cells. AA, but neither EPA nor OA, diminished bone cell adhesion, whereas both EPA and OA, but not AA, increased gene expression of type I collagen and fibronectin via a transforming growth factor-beta-independent mechanism. These results extend previous evidence for unsaturated fatty acids in bone cell metabolism