The struggle for energy in podocytes leads to nephrotic syndrome

Podocytes are terminally differentiated post-mitotic cells similar to neurons, and their damage leads to nephrotic syndrome, which is characterized by massive proteinuria associated with generalized edema. A recent functional genetic approach has identified the pathological relevance of several mutated proteins in glomerular podocytes to the mechanism of proteinuria in hereditary nephrotic syndrome. In contrast, the pathophysiology of acquired primary nephrotic syndrome, including minimal change disease, is still largely unknown. We recently demonstrated the possible linkage of an energy-consuming process in glomerular podocytes to the mechanism of proteinuria. Puromycin aminonucleoside nephrosis, a rat model of minimal change disease, revealed the activation of the unfolded protein response (UPR) in glomerular podocytes to be a cause of proteinuria. The pretreatment of puromycin aminonucleoside rat podocytes and cultured podocytes with the mammalian target of rapamycin (mTOR) inhibitor everolimus further revealed that mTOR complex 1 consumed energy, which was followed by UPR activation. In this paper, we will review nutritional transporters to summarize the energy uptake process in podocytes and review the involvement of the UPR in the pathogenesis of glomerular diseases. We will also present additional data that reveal how mTOR complex 1 acts upstream of the UPR. Finally, we will discuss the potential significance of targeting the energy metabolism of podocytes to develop new therapeutic interventions for acquired nephrotic syndrome.     

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.     

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.     

Discovery of the congenital nephrotic syndrome gene discloses the structure of the mysterious molecular sieve of the kidney.

The molecular nature of the glomerular slit diaphragm, the site of renal ultrafiltration, has until recently remained a mystery. However, the identification of the gene affected in congenital nephrotic syndrome has revealed the presence of a novel protein, possibly specific for the slit diaphragm. This protein, which has been termed nephrin, is a transmembrane protein that probably forms the main building block of an isoporous zipper-like slit diaphragm filter structure. Defects in nephrin lead to abnormal or absent slit diaphragm leading to massive proteinuria and renal failure. The discovery of nephrin sheds new light on the glomerular filtration barrier, provides new insight into the pathomechanisms of proteinuria, and even opens up possibilities for the development of novel therapies for this common and severe kidney complication.     

Microspectrophotometric study of the sialomucin content of the glomerular filter in the nephrotic syndrome

A microspectrophotometric study of sialomucins in human glomerular filter of the kidney in some diseases was carried out. Sialomucins were detected in paraffine sections stained after Haim, and examined on scanning microspectrophotometer. Lipid nephrosis, secondary amyloidosis, membranous-proliferative glomerulonephritis with the nephrotic syndrome were accompanied by a significant decrease in the amount of sialic acid. No reduction of sialomucins occurred in subacute glomerulonephritis with the nephrotic syndrome. A connection between the selective proteinuria and a decrease of the amount of sialomucins in the glomerulus is suggested.     

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.     

Mitochondrial DNA deletion-dependent podocyte injuries in Mito-miceΔ, a murine model of mitochondrial disease

Focal segmental glomerulosclerosis (FSGS) is a major renal complication of human mitochondrial disease. However, its pathogenesis has not been fully explained. In this study, we focused on the glomerular injury of mito-miceΔ and investigated the pathogenesis of their renal involvement. We analyzed biochemical data and histology in mito-miceΔ. The proteinuria began to show in some mito-miceΔ with around 80% of mitochondrial DNA deletion, then proteinuria developed dependent with higher mitochondrial DNA deletion, more than 90% deletion. Mito-miceΔ with proteinuria histologically revealed FSGS. Immunohistochemistry demonstrated extensive distal tubular casts due to abundant glomerular proteinuria. Additionally, the loss of podocyte-related protein and podocyte’s number were found. Therefore, the podocyte injuries and its depletion had a temporal relationship with the development of proteinuria. This study suggested mitochondrial DNA deletion-dependent podocyte injuries as the pathogenesis of renal involvement in mito-miceΔ. The podocytes are the main target of mitochondrial dysfunction originated from the accumulation of mitochondrial DNA abnormality in the kidney.     

Glomerular Endothelial Cell Injury and Damage Precedes That of Podocytes in Adriamycin-Induced Nephropathy

The role of podocytes in the development and progression of glomerular disease has been extensively investigated in the past decade. However, the importance of glomerular endothelial cells in the pathogenesis of proteinuria and glomerulosclerosis has been largely ignored. Recent studies have demonstrated that endothelial nitric oxide synthatase (eNOS) deficiency exacerbates renal injury in anti-GBM and remnant kidney models and accelerates diabetic kidney damage. Increasing evidence also demonstrates the importance of the glomerular endothelium in preventing proteinuria. We hypothesize that endothelial dysfunction can initiate and promote the development and progression of glomerulopathy. Administration of adriamycin (ADR) to C57BL/6 mice, normally an ADR resistant strain, with an eNOS deficiency induced overt proteinuria, severe glomerulosclerosis, interstitial fibrosis and inflammation. We also examined glomerular endothelial cell and podocyte injury in ADR-induced nephropathy in Balb/c mice, an ADR susceptible strain, by immunostaining, TUNEL and Western blotting. Interestingly, down-regulation of eNOS and the appearance of apoptotic glomerular endothelial cells occurred as early as 24 hours after ADR injection, whilst synaptopodin, a functional podocyte marker, was reduced 7 days after ADR injection and coincided with a significant increase in the number of apoptotic podocytes. Furthermore, conditioned media from mouse microvascular endothelial cells over-expressing GFP-eNOS protected podocytes from TNF-α-induced loss of synaptopodin. In conclusion, our study demonstrated that endothelial dysfunction and damage precedes podocyte injury in ADR-induced nephropathy. Glomerular endothelial cells may protect podocytes from inflammatory insult. Understanding the role of glomerular endothelial dysfunction in the development of kidney disease will facilitate in the design of novel strategies to treat kidney disease.     

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.     

Humoral components of immunological response in nephrotic syndrome

Serum and urine were collected from 58 patients with nephrotic syndrome. Immunoglobulins (IgA, IgG and IgM), complement (C3) and transferrin levels were measured by single radial immunodiffusion. The extent of glomerular injury was estimated by determining the selectivity of proteinuria. The relationship between the severity of glomerular damage and serum concentrations of immunoglobulins and complement was assessed. Higher IgM and lower IgG serum concentrations were found in nephrotic patients than in normal controls (157 +/- 108 mg+ vs 127 +/- 38 mg% for IgM, 929 +/- 537 mg% for IgG). The difference was statistically significant (p less than 0.05 for IgM, p less than 0.001 for IgG). No correlation was present between the selectivity of proteinuria and serum levels of IgA, IgM, IgG or C3. The results indicate that abnormalities in humoral components of the immune system are present in nephrotic patients and are probably related to a basic immunological defect in the patients rather than to the severity of glomerular damage.     

Characteristics of mutant mice (ICGN) with spontaneous renal lesions: a new model for human nephrotic syndrome

Spontaneous nephrotic mice (ICGN mice), a new mutant strain of mouse from outbred ICR, were clinically, macroscopically, histologically and immunohistochemically studied to establish their value as a model for human nephrotic syndrome. Most of the affected mice developed proteinuria, hypoproteinaemia and hypercholesterolaemia, and some of them developed systemic oedema. A high concentration of blood urea nitrogen (BUN) and a low haematocrit value were also observed. The kidneys of severe cases showed a decrease in size and had a yellowish granular surface. These findings indicated that the mice were terminally affected by chronic renal insufficiency. Histopathology demonstrated glomerular lesions consisting of thickened basement membranes of the capillary loops with irregular spike-like protrusions and enlargement of the mesangium unaccompanied by cellular proliferation. The immunofluorescence technique revealed positive granular staining for IgA, IgG and IgM and to a lesser extent for C3 along the capillary loops in affected mice. The similarity between this spontaneous disease and human nephrotic syndrome caused by idiopathic glomerular lesions is discussed. ICGN mice may be a useful animal model for this human disease.     

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.     

Early glomerular filtration defect and severe renal disease in podocin-deficient mice

Podocytes are specialized epithelial cells covering the basement membrane of the glomerulus in the kidney. The molecular mechanisms underlying the role of podocytes in glomerular filtration are still largely unknown. We generated podocin-deficient (Nphs2-/-) mice to investigate the function of podocin, a protein expressed at the insertion of the slit diaphragm in podocytes and defective in a subset of patients with steroid-resistant nephrotic syndrome and focal and segmental glomerulosclerosis. Nphs2-/- mice developed proteinuria during the antenatal period and died a few days after birth from renal failure caused by massive mesangial sclerosis. Electron microscopy revealed the extensive fusion of podocyte foot processes and the lack of a slit diaphragm in the remaining foot process junctions. Using real-time PCR and immunolabeling, we showed that the expression of other slit diaphragm components was modified in Nphs2-/- kidneys: the expression of the nephrin gene was downregulated, whereas that of the ZO1 and CD2AP genes appeared to be upregulated. Interestingly, the progression of the renal disease, as well as the presence or absence of renal vascular lesions, depends on the genetic background. Our data demonstrate the crucial role of podocin in the establishment of the glomerular filtration barrier and provide a suitable model for mapping and identifying modifier genes involved in glomerular diseases caused by podocyte injuries.     

Advanced glomerulosclerosis is reversible in nephrotic mice

Advanced glomerulosclerosis, a common hallmark of chronic renal diseases (CRD) is believed to be irreversible, and it is thought that glomerular hyperfiltration and hypertrophy may participate in its pathogenesis. We demonstrate here that glomerulosclerosis is "reversible" in an animal model. We used nephrotic ICGN (nep/nep) mice which showed a rapid progression of glomerulosclerosis, accompanied by histological findings for glomerular hyperfiltration. It is known that ureter ligation reduces glomerular filtration in ligated kidneys. When ureter ligation was applied to our model, glomerulosclerosis (characterized by myofibroblast hyperplasia and over-accumulated matrix protein) weakened in conjunction with suppressed glomerular hypertrophy. During this process, glomerular myofibroblasts showed apoptotic cell death after unilateral ureter ligation (UUO) treatment. Our results suggest that inhibition of glomerular filtration in sclerotic tufts may cause glomerular remodeling through the modulation of molecular and cellular sclerogenesis.     

SIRPα interacts with nephrin at the podocyte slit diaphragm

The slit diaphragm (SD) is an intercellular junction between renal glomerular epithelial cells (podocytes) that is essential for permselectivity in glomerular ultrafiltration. The SD components, nephrin and Neph1, assemble a signaling complex in a tyrosine phosphorylation dependent manner, and regulate the unique actin cytoskeleton of podocytes. Mutations in the NPHS1 gene that encodes nephrin cause congenital nephrotic syndrome (CNS), which is characterized by the loss of the SD and massive proteinuria. Recently, we have identified the expression of the transmembrane glycoprotein signal regulatory protein α (SIRPα) at the SD. In the present study, we analyzed the expression of SIRPα in developing kidneys, in kidneys from CNS patients and in proteinuric rat models. The possibility that SIRPα interacts with known SD proteins was also investigated. SIRPα was concentrated at the SD junction during the maturation of intercellular junctions. In the glomeruli of CNS patients carrying mutations in NPHS1, where SD formation is disrupted, the expression of SIRPα as well as Neph1 and nephrin was significantly decreased, indicating that SIRPα is closely associated with the nephrin complex. Indeed, SIRPα formed hetero-oligomers with nephrin in cultured cells and in glomeruli. Furthermore, the cytoplasmic domain of SIRPα was highly phosphorylated in normal glomeruli, and its phosphorylation was dramatically decreased upon podocyte injury in?vivo. Thus, SIRPα interacts with nephrin at the SD, and its phosphorylation is dynamically regulated in proteinuric states. Our data provide new molecular insights into the phosphorylation events triggered by podocyte injury. Structured digital abstract ? Sirp-alpha?physically interacts?with?Nephrin?by?anti bait coimmunoprecipitation?(View interaction) ? Sirp-alpha?physically interacts?with?Nephrin?by?anti tag coimmunoprecipitation?(View interaction).     

Kidney Disease and Pregnancy

The course of 41 pregnancies was studied in 25 patients with renal disease. Histological classification of the disease in each patient was made by renal biopsy. The analysis suggests that the course of pregnancy in women with coexistent renal disease correlates with the underlying renal lesion and probably with the glomerular filtration rate and blood pressure. In patients with heavy proteinuria there is increased oedema formation, making the nephrotic syndrome a frequent complication.     

基于网络药理学探讨“黄芪-白术-熟地黄”组方防治肾病综合征的作用机制

本研究旨在通过网络药理学方法和分子对接技术探讨黄芪-白术-熟地黄组方(HBS)治疗肾病综合征的作用机制.通过多个数据库获取肾病综合征基因并进行功能模块分解,找出肾病综合征基因参与的主要生物学过程.通过文献以及数据库查找HBS活性成分和基因靶点,筛选出HBS治疗肾病综合征的有效靶点.通过有效靶点的KEGG和GO富集分析,...