Podocyte-secreted angiopoietin-like-4 mediates proteinuria in glucocorticoid-sensitive nephrotic syndrome

The main manifestations of nephrotic syndrome include proteinuria, hypoalbuminemia, edema, hyperlipidemia and lipiduria. Common causes of nephrotic syndrome are diabetic nephropathy, minimal change disease (MCD), focal and segmental glomerulosclerosis (FSGS) and membranous nephropathy. Among the primary glomerular diseases, MCD is usually sensitive to glucocorticoid treatment, whereas the other diseases show variable responses. Despite the identification of key structural proteins in the glomerular capillary loop which may contribute to defects in ultrafiltration, many of the disease mechanisms of nephrotic syndrome remain unresolved. In this study, we show that the glomerular expression of angiopoietin-like-4 (Angptl4), a secreted glycoprotein, is glucocorticoid sensitive and is highly upregulated in the serum and in podocytes in experimental models of MCD and in the human disease. Podocyte-specific transgenic overexpression of Angptl4 (NPHS2-Angptl4) in rats induced nephrotic-range, and selective, proteinuria (over 500-fold increase in albuminuria), loss of glomerular basement membrane (GBM) charge and foot process effacement, whereas transgenic expression specifically in the adipose tissue (aP2-Angptl4) resulted in increased circulating Angptl4, but no proteinuria. Angptl4(-/-) mice that were injected with lipopolysaccharide (LPS) or nephritogenic antisera developed markedly less proteinuria than did control mice. Angptl4 secreted from podocytes in some forms of nephrotic syndrome lacks normal sialylation. When we fed the sialic acid precursor N-acetyl-D-mannosamine (ManNAc) to NPHS2-Angptl4 transgenic rats it increased the sialylation of Angptl4 and decreased albuminuria by more than 40%. These results suggest that podocyte-secreted Angptl4 has a key role in nephrotic syndrome.     

The cytoprotective role of autophagy in puromycin aminonucleoside treated human podocytes

Autophagy is a ubiquitous catabolic process involving degradation of damaged organelles and protein aggregates. It shows cytoprotective effects in many cell types and helps to maintain cell homeostasis. In many glomerular diseases, podocyte damage leads to the disruption of the renal filtration barrier and subsequent proteinuria. Puromycin aminonucleoside (PAN) which induces podocyte apoptosis in vitro and in vivo is widely used for studying the pathophysiology of glomerular diseases. It has been shown that PAN induces autophagy in podocytes. However, the relationship between autophagy and apoptosis in PAN treated human podocytes is not known and the role of PAN-induced autophagy in podocyte survival remains unclear. Here we demonstrate that PAN induced autophagy in human podocytes prior to apoptosis which was featured with the activation of mTOR complex 1 (mTORC1). When the PAN-induced autophagy was inhibited by 3-methyladenine (3-MA) or chloroquine (CQ), podocyte apoptosis increased significantly along with the elevation of active caspase-3. Under such circumstance, the podocyte cytoskeleton was also disrupted. Collectively, our results suggested that the induced autophagy may be an early adaptive cytoprotective mechanism for podocyte survival after PAN treatment.     

Trehalose,an mTOR Independent Autophagy Inducer,Alleviates Human Podocyte Injury after Puromycin Aminonucleoside Treatment

Glomerular diseases are commonly characterized by podocyte injury including apoptosis, actin cytoskeleton rearrangement and detachment. However, the strategies for preventing podocyte damage remain insufficient. Recently autophagy has been regarded as a vital cytoprotective mechanism for keeping podocyte homeostasis. Thus, it is reasonable to utilize this mechanism to attenuate podocyte injury. Trehalose, a natural disaccharide, is an mTOR independent autophagy inducer. It is unclear whether trehalose alleviates podocyte injury. Therefore, we investigated the efficacy of trehalose in puromycin aminonucleoside (PAN)-treated podocytes which mimic cell damage in minimal change nephrotic syndrome in vitro. Human conditional immortalized podocytes were treated with trehalose with or without PAN. Autophagy was investigated by immunofluorescence staining for LC3 puncta and Western blotting for LC3, Atg5, p-AMPK, p-mTOR and its substrates. Podocyte apoptosis and necrosis were evaluated by flow cytometry and by measuring lactate dehydrogenase activity respectively. We also performed migration assay to examine podocyte recovery. It was shown that trehalose induced podocyte autophagy in an mTOR independent manner and without reactive oxygen species involvement. Podocyte apoptosis significantly decreased after trehalose treatment, while the inhibition of trehalose-induced autophagy abolished its protective effect. Additionally, the disrupted actin cytoskeleton of podocytes was partially reversed by trehalose, accompanying with less lamellipodias and diminished motility. These results suggested that trehalose induced autophagy in human podocytes and showed cytoprotective effects in PAN-treated podocytes.     

Nephrotic syndrome and subepithelial deposits in a mouse model of immune-mediated anti-podocyte glomerulonephritis

Subepithelial immune complex deposition in glomerular disease causes local inflammation and proteinuria by podocyte disruption. A rat model of membranous nephropathy, the passive Heymann nephritis, suggests that Abs against specific podocyte Ags cause subepithelial deposit formation and podocyte foot process disruption. In this study, we present a mouse model in which a polyclonal sheep anti-mouse podocyte Ab caused subepithelial immune complex formation. Mice developed a nephrotic syndrome with severe edema, proteinuria, hypoalbuminemia, and elevated cholesterol and triglycerides. Development of proteinuria was biphasic: an initial protein loss was followed by a second massive increase of protein loss beginning at approximately day 10. By histology, podocytes were swollen. Electron microscopy revealed 60-80% podocyte foot process effacement and subepithelial deposits, but no disruption of the glomerular basement membrane. Nephrin and synaptopodin staining was severely disrupted, and podocyte number was reduced in anti-podocyte serum-treated mice, indicating severe podocyte damage. Immunohistochemistry detected the injected anti-podocyte Ab exclusively along the glomerular filtration barrier. Immunoelectron microscopy localized the Ab to podocyte foot processes and the glomerular basement membrane. Similarly, immunohistochemistry localized mouse IgG to the subepithelial space. The third complement component (C3) was detected in a linear staining pattern along the glomerular basement membrane and in the mesangial hinge region. However, C3-deficient mice were not protected from podocyte damage, indicating a complement-independent mechanism. Twenty proteins were identified as possible Ags to the sheep anti-podocyte serum by mass spectrometry. Together, these data establish a reproducible model of immune-mediated podocyte injury in mice with subepithelial immune complex formation.     

Detection of urinary podocytes and nephrin as markers for children with glomerular diseases

The purpose of this study was to detect the urinary podocytes and its related protein, nephrin, in the urine of the children with glomerular disease in order to analyze the relationship of the clinical testing with the significance of the glomerular disease. A total of 65 children with nephrotic syndrome were selected for this study. The podocytes and nephrin were detected in the urinary sediment by indirect immunofluorescence, enzyme-linked immunosorbent assay, and Western blotting. The urinary podocytes and nephrin positive rates were 53.8% and 50.8%, respectively, in the children with glomerular disease. The serum total protein and albumin decreased in the podocyte-positive children, while the urine total protein at 24 h, urinary albumin/creatinine ratio, blood urea nitrogen, and serum creatinine were significantly elevated as compared to those of the podocyte-negative patients. Furthermore, the results were the same in the patients with positive nephrin as compared to that of the patients with negative nephrin. The podocyte number and nephrin level were significantly higher in the lupus nephritis group as compared to those of the other groups. Likewise, the podocyte number and nephrin level dramatically increased in the focal segmental glomerulosclerosis group as compared to those of the mesangial proliferative glomerulonephritis and minimal change disease groups. In addition, the podocyte numbers and nephrin expression were significantly higher in severe proteinuria group as compared to those of the mild proteinuria group. The urinary nephrin expression was positively related to podocyte and urinary albumin/creatinine ratio. We concluded that the detection of the urinary podocytes and nephrin could be taken as markers for children with glomerular disease, reflecting the type of the disease. Therefore, this can be used as a noninvasive method to evaluate the severity of the kidney disease in children.     

Connective tissue growth factor modulates podocyte actin cytoskeleton and extracellular matrix synthesis and is induced in podocytes upon injury

Structural changes of podocytes and retraction of their foot processes are a critical factor in the pathogenesis of minimal change nephritis and glomerulosclerosis. Here we tested, if connective tissue growth factor (CTGF) is involved in podocyte injury during acute and chronic puromycin aminonucleoside nephrosis (PAN) as animal models of minimal change nephritis, and focal segmental glomerulosclerosis, respectively. Rats were treated once (acute PAN) or for 13 weeks (chronic PAN). In both experimental conditions, CTGF and its mRNA were found to be highly upregulated in podocytes. The upregulation correlated with onset and duration of proteinuria in acute PAN, and glomerulosclerosis and high expression of glomerular fibronectin, and collagens I, III, and IV in chronic PAN. In vitro, treatment of podocytes with recombinant CTGF increased amount and density of actin stress fibers, the expression of actin-associated molecules such as podocalyxin, synaptopodin, ezrin, and actinin-4, and activation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK). Moreover, we observed increased podocyte expression of mRNA for transforming growth factor (TGF)-β2, TGF-β receptor II, fibronectin, and collagens I, III, and IV. Treatment of cultured podocytes with puromycin aminonucleoside resulted in loss of actin stress fibers and cell death, effects that were partially prevented when CTGF was added to the culture medium. Depletion of CTGF mRNA in cultured podocytes by RNA interference reduced both the number of actin stress fibers and the expression of actin-associated molecules. We propose that the expression of CTGF is acutely upregulated in podocytes as part of a cellular attempt to repair structural changes of the actin cytoskeleton. When the damaging effects on podocyte structure and function persist chronically, continuous CTGF expression in podocytes is a critical factor that promotes progressive accumulation of glomerular extracellular matrix and glomerulosclerosis.     

New insights into mechanisms of immune glomerular injury.

Although glomerular disease remains the most common cause of end-stage renal disease worldwide, major advances have been made recently in understanding the cellular and molecular mechanisms that mediate these disorders. The nephrotic syndrome in noninflammatory lesions such as minimal change or focal sclerosis and membranous nephropathy results from disorders of the glomerular epithelial cell that can be simulated in animal models by antibodies to various epithelial cell membrane epitopes. Clarification of how these antibodies affect epithelial cells to induce a loss of glomerular barrier function should substantially improve understanding of the pathogenesis of minimal change or focal sclerosis. In membranous nephropathy, proteinuria is mediated primarily by the C5b-9 complex through similar mechanisms that also involve glomerular epithelial cells as targets. Inflammatory glomerular lesions are induced by circulating inflammatory cells or proliferating resident glomerular cells. Understanding of how these cells induce tissue injury has also evolved considerably over the past decade. Neutrophil-induced disease involves leukocyte adhesion molecules in regulating neutrophil localization; proteases, oxidants, and myeloperoxidase in mediating injury; and platelets in augmenting these processes. The activated mesangial cell exhibits altered phenotype and proliferation with the release of oxidants and proteases. Mesangial cell proliferation may be initiated by basic fibroblast growth factor and is maintained by an autocrine mechanism involving platelet-derived growth factor. Transforming growth factor beta is important in the subsequent development of sclerosis.     

The activation of extracellular signal-regulated kinase is responsible for podocyte injury

Podocyte and its slit diaphragm play an important role in maintaining normal glomerular filtration barrier function and structure. Podocyte apoptosis and slit diaphragm injury leads to proteinuria and glomerulosclerosis. However, the molecular mechanism of podocyte injury remains poorly understood. The family of mitogen-activated protein kinases including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase, and p38 signal pathways, are implicated in the progression of various glomerulopathies. However, the role of the activated signal pathway(s) in podocyte injury is elusive. This study examined phosphorylation of ERK in rat puromycin aminonucleoside (PAN) nephropathy as well as conditionally immortalized mouse podocyte treated with PAN in vitro. The effect of treatment with U0126, an inhibitor of ERK, was also investigated. In PAN nephropathy, the phosphorylation of ERK was marked. In podocyte injury, the marked and sustained activation of ERK pathway was also observed before the appearance of significant podocyte apoptosis. Pretreatment with U0126 to podocyte completely inhibited ERK activation, with complete suppression podocyte apoptosis and ameliorated nephrin protein expression along with the phosphorylation of nephrin in podocyte injury. In cultured podocyte, PAN induced actin recorganition, and U0126 inhibited such change. However, U0126 did not recovery the phosphorylation change of neph1 in podocyte injury. We concluded that the sustained activation of ERK along with the phosphorylation of neph1 might be necessary for podocyte injury. The study here suggested that ERK might become a potential target for therapeutic intervention to prevent podocytes from injury which will result in proteinuria.     

Interaction with podocin facilitates nephrin signaling

Mutations of NPHS1 or NPHS2, the genes encoding for the glomerular podocyte proteins nephrin and podocin, cause steroid-resistant proteinuria. In addition, mice lacking CD2-associated protein (CD2AP) develop a nephrotic syndrome that resembles NPHS mutations suggesting that all three proteins are essential for the integrity of glomerular podocytes. Although the precise glomerular function of either protein remains unknown, it has been suggested that nephrin forms zipper-like interactions to maintain the structure of podocyte foot processes. We demonstrate now that nephrin is a signaling molecule, which stimulates mitogen-activated protein kinases. Nephrin-induced signaling is greatly enhanced by podocin, which binds to the cytoplasmic tail of nephrin. Mutational analysis suggests that abnormal or inefficient signaling through the nephrin-podocin complex contributes to the development of podocyte dysfunction and proteinuria.     

Angiopoietin-like protein 3 modulates barrier properties of human glomerular endothelial cells through a possible signaling pathway involving phosphatidylinositol-3 kinase/protein kinase B and integrin alphaVbeta3

Podocytes can influence glomerular endothelial cell (GEnC) barrier properties and take part in the development of proteinuria by some molecules. Angiopoietin-like protein 3 (Angptl3), secreted by podocytes, is a member of the angiopoietin-like protein family that has important biological functions in endothelial cells. In our previous studies, we showed that mRNA expression of Angptl3 increased significantly in kidneys of children with minimal change nephrotic syndrome. And the mRNA level of Angptl3 was increased in the glomerulus of adriamycin rats with the development of proteinuria. It was also found that Angptl3 was expressed in the cytoplasm of cultured podocytes. Thus, Angptl3 might influence the biological functions of GEnCs in a paracrine manner. In this study, we found that Angptl3 could increase the permeability of GEnCs and increase the level of protein kinase B phosphorylation in cultured GEnCs in vitro. LY294002, a phosphatidylinositol-3 kinase inhibitor, could prevent the increase of permeability of GEnCs induced by Angptl3. Our results also indicated that the integrin αVβ3 antibody (LM609) could block the Angptl3-induced protein kinase B phosphorylation.     

Podocyte migration during nephrotic syndrome requires a coordinated interplay between cathepsin L and alpha3 integrin

Podocyte foot process effacement and disruption of the slit diaphragm are typically associated with glomerular proteinuria and can be induced in rats by the injection of puromycin aminonucleoside. Here, we show that the induction of puromycin aminonucleoside nephrosis involves podocyte migration conducted by a coordinated interplay between the cysteine protease cathepsin L and alpha(3) integrin. Puromycin aminonucleoside treatment up-regulates cathepsin L expression in podocytes in vivo as well as expression and enzymatic activity of cathepsin L in podocytes in vitro. Isolated podocytes from mice lacking cathepsin L are protected from cell puromycin aminonucleoside-induced cell detachment. The functional significance of cathepsin L expression was underscored by the observation that puromycin aminonucleoside-induced cell migration was slowed down in cathepsin L-deficient podocytes and by the preservation of cell-cell contacts and expression of vital slit diaphragm protein CD2AP. Cathepsin L expression and activity were induced in podocytes lacking alpha(3) integrin. Similarly, acute functional inhibition of alpha(3) integrin in wild type podocytes with a blocking antibody increased the expression of cathepsin L activity. Down-regulation of alpha(3) integrin protected against puromycin aminonucleoside-induced podocyte detachment. In summary, these data establish that podocyte foot process effacement is a migratory event involving a novel interplay between cathepsin L and alpha(3) integrin.     

Intrinsically disordered regions mediate macromolecular assembly of the Slit diaphragm proteins associated with Nephrotic syndrome

The glomerular filtration barrier (GFB) of the kidney plays an instrumental role in preventing the excretion of large molecules and ensuring the formation of ultra-filtrated urine. Podocytes are essential components of GFB that provide epithelial coverage to the fine glomerular capillaries. Slit-diaphragm (SD) that forms the sole contact between adjacent foot-processes of the podocytes consists of multimeric protein assemblies. SD serves as a molecular sieve and confers size and charge-selective barrier. Nephrin, podocin, TRPC6, and CD2AP are some of the key proteins that constitute the SD. Mutations in these proteins are implicated in nephrotic syndrome and congenital nephropathies which are characterised by heavy proteinuria. The mechanism of how mutations in these proteins predispose to proteinuria is not known. Furthermore, the structural details of proteins that constitute SD are largely unknown. In this study, we built models for nephrin, CD2AP, podocin, and TRPC6 followed by docking and molecular dynamics simulations of the complex of these proteins. We speculate that the interfacial residues of SD proteins form a macromolecular complex through intrinsically disordered regions thereby conferring architectural stability to the SD, which is critical for glomerular permselectivity.     

AMPK signalling: Implications for podocyte biology in diabetic nephropathy

Diabetic nephropathy is a major long‐term complication of diabetes mellitus and one of the most common causes of end‐stage renal disease. Thickening of the glomerular basement membrane, glomerular cell hypertrophy and podocyte loss are among the main pathological changes that occur during diabetic nephropathy, resulting in proteinuria. Injury to podocytes, which are a crucial component of the glomerular filtration barrier, seems to play a key role in the development of diabetic nephropathy. Recent studies have suggested that dysregulation of AMP‐activated kinase protein, which is an essential cellular energy sensor, may play a fundamental role in this process. The purpose of this review is to highlight the molecular mechanisms associated with AMP‐activated protein kinase (AMPK) in podocytes that are involved in the pathogenesis of diabetic nephropathy.     

Nephrin phosphorylation regulates podocyte adhesion through the PINCH-1-ILK-α-parvin complex

Nephrin, a structural molecule, is also a signaling molecule after phosphorylation. Inhibition of nephrin phosphorylation is correlated with podocyte injury. The PINCH-1-ILK-α-parvin (PIP) complex plays a crucial role in cell adhesion and cytoskeleton formation. We hypothesized that nephrin phosphorylation influenced cytoskeleton and cell adhesion in podocytes by regulating the PIP complex. The nephrin phosphorylation, PIP complex formation, and F-actin in Wistar rats intraperitoneally injected with puromycin aminonucleoside were gradually decreased but increased with time, coinciding with the recovery from glomerular/podocyte injury and proteinuria. In cultured podocytes, PIP complex knockdown resulted in cytoskeleton reorganization and decreased cell adhesion and spreading. Nephrin and its phosphorylation were unaffected after PIP complex knockdown. Furthermore, inhibition of nephrin phosphorylation suppressed PIP complex expression, disorganized podocyte cytoskeleton, and decreased cell adhesion and spreading. These findings indicate that alterations in nephrin phosphorylation disorganize podocyte cytoskeleton and decrease cell adhesion through a PIP complex-dependent mechanism. [BMB Reports 2013; 46(4): 230-235]     

Organization of the pronephric filtration apparatus in zebrafish requires Nephrin, Podocin and the FERM domain protein Mosaic eyes

Podocytes are specialized cells of the kidney that form the blood filtration barrier in the kidney glomerulus. The barrier function of podocytes depends upon the development of specialized cell-cell adhesion complexes called slit-diaphragms that form between podocyte foot processes surrounding glomerular blood vessels. Failure of the slit-diaphragm to form results in leakage of high molecular weight proteins into the blood filtrate and urine, a condition called proteinuria. In this work, we test whether the zebrafish pronephros can be used as an assay system for the development of glomerular function with the goal of identifying novel components of the slit-diaphragm. We first characterized the function of the zebrafish homolog of Nephrin, the disease gene associated with the congenital nephritic syndrome of the Finnish type, and Podocin, the gene mutated in autosomal recessive steroid-resistant nephrotic syndrome. Zebrafish nephrin and podocin were specifically expressed in pronephric podocytes and required for the development of pronephric podocyte cell structure. Ultrastructurally, disruption of nephrin or podocin expression resulted in a loss of slit-diaphragms at 72 and 96 h post-fertilization and failure to form normal podocyte foot processes. We also find that expression of the band 4.1/FERM domain gene mosaic eyes in podocytes is required for proper formation of slit-diaphragm cell-cell junctions. A functional assay of glomerular filtration barrier revealed that absence of normal nephrin, podocin or mosaic eyes expression results in loss of glomerular filtration discrimination and aberrant passage of high molecular weight substances into the glomerular filtrate.     

Congenital nephrotic syndromes

Many acquired and familial renal diseases in man lead to kidney dysfunction and nephrotic syndrome. These diseases share a common pathological fate in the form of glomerular dysfunction and proteinuria. Classification of the disease is difficult because the onset of pathological appearance in congenital nephrotic syndrome (CNS) varies considerably. Recently, classification has been aided by applying molecular genetics to identify genes involved in the pathogenesis of proteinuria. Light has also been shed on the biology and mechanisms of glomerular filtration and the molecular pathogenesis of CNS.     

Requirement for class II phosphoinositide 3-kinase C2alpha in maintenance of glomerular structure and function

An early lesion in many kidney diseases is damage to podocytes, which are critical components of the glomerular filtration barrier. A number of proteins are essential for podocyte filtration function, but the signaling events contributing to development of nephrotic syndrome are not well defined. Here we show that class II phosphoinositide 3-kinase C2α (PI3KC2α) is expressed in podocytes and plays a critical role in maintaining normal renal homeostasis. PI3KC2α-deficient mice developed chronic renal failure and exhibited a range of kidney lesions, including glomerular crescent formation and renal tubule defects in early disease, which progressed to diffuse mesangial sclerosis, with reduced podocytes, widespread effacement of foot processes, and modest proteinuria. These findings were associated with altered expression of nephrin, synaptopodin, WT-1, and desmin, indicating that PI3KC2α deficiency specifically impacts podocyte morphology and function. Deposition of glomerular IgA was observed in knockout mice; importantly, however, the development of severe glomerulonephropathy preceded IgA production, indicating that nephropathy was not directly IgA mediated. PI3KC2α deficiency did not affect immune responses, and bone marrow transplantation studies also indicated that the glomerulonephropathy was not the direct consequence of an immune-mediated disease. Thus, PI3KC2α is critical for maintenance of normal glomerular structure and function by supporting normal podocyte function.