Reduced podocin expression in minimal change disease and focal segmental glomerulosclerosis is related to the level of proteinuria

Background

Glomerular podocyte molecules are involved in the pathogenesis of congenital nephrotic syndrome. However, their role in primary nephrotic syndrome is not clear. This study investigated the expression of nephrin, podocin and synaptopodin in primary nephrotic syndrome.

Methods

Eighty-seven patients with primary nephrotic syndrome including minimal change disease (MCD), focal segmental glomerulosclerosis (FSGS), membranous nephropathy (MN) and membranoproliferative glomerulonephritis Type I (MPGN) were included in the study. Glomerular expression of nephrin, podocin and synaptopodin was studied in renal biopsies by immunofluorescence and immunohistochemistry. Correlation of expression with clinical and biochemical parameters was performed.

Results

The pattern of expression for all podocyte proteins in controls was uniform fine granular along the capillary walls towards the visceral epithelial cell aspect. Glomerular expression of nephrin was present in all renal biopsies and was similar to that in controls. Glomerular synaptopodin expression was seen in all MN and MPGN patients, while it was seen in 74 % (17/23) MCD and 93.5 % (29/31) FSGS. Reduced synaptopodin expression showed no correlation with clinical and biochemical factors. Podocin expression was present in 5/23 MCD (22 %), 3/31 FSGS (9.6 %), 13/17 MN (76.4 %) and 13/16 MPGN (81 %) patients. The reduced expression of podocin significantly correlated with the degree of proteinuria (p = 0.032). No correlation with age, gender and serum creatinine level was observed.

Conclusion

Reduction of glomerular podocin expression found in MCD and FSGS is related to the amount of proteinuria. Our findings suggest that alteration in podocyte phenotype may not be a primary event and may reflect the degree of podocyte injury in primary nephrotic syndrome.     

The relationship among nephrin, podocin, CD2AP, and alpha-actinin might not be a true 'interaction' in podocyte

The abnormality of a single podocyte molecule, caused by a single gene mutation, such as NPHS1, NPHS2, CD2AP, and ACTN4, can lead to the hereditary/congenital nephrotic syndromes (NS). Further studies suggested that more than one podocyte molecule were together involved in acquired or experimental NS. However, we do not know much on the relationship among these podocyte molecules, and the molecular response induced by the change of each podocyte protein to the remaining ones. We respectively knockdown the nephrin, podocin, CD2AP, or alpha-actinin-4 mRNA by using reconstructed RNA interference vector--psiRNA-hH1GFPzeo in mouse podocyte clone. The molecular behavior or response was revealed by the quantitative expression both at mRNA and protein levels with RT-PCR and Western blot, and by the molecular distribution detected with confocal microscopy. With nephrin knockdown, only CD2AP increased, whereas podocin showed no change. Contrarily, with podocin or CD2AP knockdown, nephrin decreased, while CD2AP or podocin increased. Nephrin, podocin, or CD2AP knockdown did not change the expression of alpha-actinin-4, whereas alpha-actinin-4 knockdown begetted the reduction of nephrin, and the increment of podocin and CD2AP. The redistributions of nephrin, podocin, and CD2AP were revealed around a predominant nuclear staining compared with the membrane surface staining in the control podocytes. Our data imply that the response between the four podocyte molecules is very complicated and evidently different. There is not always an interaction between podocyte molecules. The normal localization of podocyte molecules would depend on their normal expression quantity and the molecular reactions between them.     

Angiotensin II type 1 receptor blockade inhibits the development and progression of HIV-associated nephropathy in a mouse model

HIV-associated nephropathy (HIVAN) is characterized by a collapsed glomerular capillary tuft with hyperplasia and hypertrophy of podocytes. Recently generated were conditional transgenic mice (podocin/Vpr) that express one of the HIV-1 accessory genes, vpr, selectively in podocytes using podocin promoter and Tet-on system. These transgenic mice developed renal injury similar to HIVAN when treated with doxycycline for 8 to 12 wk. This study demonstrated that nephron reduction by heminephrectomy markedly enhanced phenotypic changes of podocytes and led to severe FSGS within 4 wk. Nephrotic-range proteinuria was observed already at 2 wk, together with dedifferentiation and dysregulation of podocytes, indicated by decreased expression of nephrin, synaptopodin, and Wilms' tumor 1 protein and increased expression of Ki-67. The acceleration of phenotypic changes of podocytes, proteinuria, and subsequent glomerulosclerosis by heminephrectomy was almost completely inhibited by angiotensin II type 1 receptor (AT1R) blocker olmesartan. In contrast, the renoprotective effect of the calcium channel antagonist azelnidipine was minimal, although it lowered systemic BP to the same level as olmesartan, demonstrating that the inhibitory effect of AT1R blocker was independent of systemic BP. Olmesartan also reduced proteinuria and prevented glomerulosclerosis even by the delayed treatment, which was initiated after the podocyte injury appeared. These data suggest that nephron reduction exaggerates podocyte injury and subsequent glomerulosclerosis, possibly through glomerular hypertension, in the mouse model of HIVAN. AT1R blockade could be beneficial in the treatment of HIVAN by ameliorating podocyte injury by avoiding the vicious cycle of nephron reduction and glomerular hypertension.     

Plasma from a case of recurrent idiopathic FSGS perturbs non-muscle myosin IIA (MYH9 protein) in human podocytes

The MYH9 gene encodes a non-muscle myosin IIA heavy chain (NMMHC-IIA) expressed in podocytes. Heterozygous MYH9 mutations cause a set of overlapping syndromes characterized by variable degrees of deafness, morphologic abnormalities of platelets and focal segmental glomerulosclerosis (FSGS) with progressive renal dysfunction. Similar glomerular lesions are seen in a variety of nephropathies, including an idiopathic form of FSGS in children which recurs in renal allografts, implying a circulating factor that affects glomerular podocyte biology. It is unknown whether NMMHC-IIA is perturbed in the idiopathic form of FSGS. We describe a pediatric patient with typical idiopathic FSGS, in whom proteinuria recurred within hours of deceased donor renal transplantation but who responded to plasmapheresis. We demonstrate in vitro that plasmapheresis effluent from our patient rapidly decreased cultured podocyte levels of the phosphorylated myosin light chain (MLC) that mediates NMMHC-IIA binding to actin and induced dispersion of NMMHC-IIA from its usual position along actin stress fibers. FSGS plasma also caused dispersion of slit diaphragm proteins (nephrin and podocin) and vinculin-positive focal adhesion complexes. Our observations suggest that the putative circulating factor in idiopathic FSGS disrupts normal NMMHC-IIA function in podocytes and might contribute to the pathogenesis of recurrent FSGS in other children.     

EP1 receptor mediates Adriamycin-induced podocyte injury through activating p38 MAPK

Objective To investigate the effect of prostaglandin E2 receptor 1 (EP1) on Adriamycin (ADR)-induced glomerular podocytes injury and its possible mechanism. Methods (1) In vivo experiments: 6-8 weeks old male Balb/c mice were randomly divided into four groups: Control group; ADR group; EP1 agonist 17-phenyl PGE2+ADR group; EP1 antagonist SC-19220+ADR group. The mouse model of nephrotic syndrome was induced by injection of ADR (10 mg/kg) into tail vein, and then EP1 agonist (1 μg/g) and antagonist (25 μg/g) were administered respectively. Six weeks later, all mice were sacrificed and urine, blood and kidney tissues were collected. Detecting urine protein, blood chemistry, changes of renal pathology and podocyte-related proteins, electron microscopy changes of podocytes. (2) In vitro experiments: Podocytes were cultured in vitro and divided into different groups: Control group; ADR group (0.2 μmol/L); EP1 agonist (0.1, 1, 10 μmol/L)+ADR (0.2 μmol/L) group; antagonist (0.1, 0.5, 1 μmol/L)+ADR (0.2 μmol/L) group. The proliferation of podocytes was measured by CCK-8. Expression of PGE2 in podocytes was detected by ELISA. Indirect immunofluorescence was used to determine the localization of podocyte-related proteins nephrin, podocin and CD2AP. Expression of nephrin, podocin, CD2AP, COX2 in podocytes was detected by Western blotting and Real-time quantitative PCR. p38 MAPK or phospho-p38 MAPK was measured by Western blotting as well. Flow cytometry was used to detect cell apoptosis. Results (1) In vivo experiments: Compared with control group, obvious proteinuria, blood biochemical changes and renal pathological changes were observed in ADR group, proteinuria, blood biochemical and renal pathological changes were more serious in mice dealt with agonist, while antagonist could reduce ADR-induced injury (all P＜0.05). Results of immunohistochemistry showed that the expression of podocyte-related proteins nephrin, podocin and CD2AP in ADR group were significantly lower than those in control group, and EP1 agonist could further inhibit expression of these proteins, while antagonist could reverse this inhibitory effects (P＜0.05). Electron microscopic results showed that mice in ADR group appeared foot enlargement and fusion, and the agonist group further aggravated the injury, while antagonist intervention could inhibit the injury of podocytes. (2) In vitro experiments: Compared with control group, expression of PGE2 and COX2 were increased; mRNA and protein expression of nephrin, podocin, CD2AP were decreased, p38 MAPK activity and podocytes apoptosis were increased in ADR group (P＜0.05); Agonist could aggravate podocytes damage (P＜0.05), while Antagonist could down-regulate the expression of PGE2 and COX2, promote the expression of nephrin, podocin and CD2AP, and inhibit the activity of p38 MAPK and podocytes apoptosis (P＜0.05). The addition of p38 MAPK inhibitor(10 μmol/L) could reduce the inhibitory effect of EP1 agonist on the expression of podocyte-related proteins nephrin, podocin and CD2AP (P＜0.05). Conclusions EP1 receptor may activate the p38 MAPK signaling pathway to inhibit podocytes-related proteins nephrin, podocin and CD2AP, as well as mediate the ADR induced podocyte injury. Inhibition of EP1 receptor however have a protective effect.     

Podocyte proteins in Galloway-Mowat syndrome

Galloway-Mowat syndrome is an autosomal recessive disorder characterized by early onset nephrotic syndrome and central nervous system anomalies. Mutations in podocyte proteins, such as nephrin, α-actinin 4, and podocin, are associated with proteinuria and nephrotic syndrome. The genetic defect in Galloway-Mowat syndrome is as yet unknown. We postulated that in Galloway-Mowat syndrome the mutation would be in a protein that is expressed both in podocytes and neurons, such as synaptopodin, GLEPP1, or nephrin. We therefore analyzed kidney tissue from normal children (n=3), children with congenital nephrotic syndrome of the Finnish type (CNF, n=3), minimal change disease (MCD, n=3), focal segmental glomerulosclerosis (FSGS, n=3), and Galloway-Mowat syndrome (n=4) by immunohistochemistry for expression of synaptopodin, GLEPP1, intracellular domain of nephrin (nephrin-I), and extracellular domain of nephrin (nephrin-E). Synaptopodin, GLEPP1, and nephrin were strongly expressed in normal kidney tissue. Nephrin was absent, and synaptopodin and GLEPP1 expression were decreased in CNF. The expression of all three proteins was reduced in MCD and FSGS; the decrease in expression being more marked in FSGS. Synaptopodin, GLEPP1, and nephrin expression was present, although reduced in Galloway-Mowat syndrome. We conclude that the reduced expression of synaptopodin, GLEPP1, and nephrin in Galloway- Mowat syndrome is a secondary phenomenon related to the proteinuria, and hence synaptopodin, GLEPP1, and nephrin are probably not the proteins mutated in Galloway-Mowat syndrome. Received: 27 April 2001 / Revised: 15 June 2001 / Accepted: 18 June 2001     

Bradykinin receptor 1 activation exacerbates experimental focal and segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) is one of the most important causes of end-stage renal failure. The bradykinin B1 receptor has been associated with tissue inflammation and renal fibrosis. To test for a role of the bradykinin B1 receptor in podocyte injury, we pharmacologically modulated its activity at different time points in an adriamycin-induced mouse model of FSGS. Estimated albuminuria and urinary protein to creatinine ratios correlated with podocytopathy. Adriamycin injection led to loss of body weight, proteinuria, and upregulation of B1 receptor mRNA. Early treatment with a B1 antagonist reduced albuminuria and glomerulosclerosis, and inhibited the adriamycin-induced downregulation of podocin, nephrin, and α-actinin-4 expression. Moreover, delayed treatment with antagonist also induced podocyte protection. Conversely, a B1 agonist aggravated renal dysfunction and even further suppressed the levels of podocyte-related molecules. Thus, we propose that kinin has a crucial role in the pathogenesis of FSGS operating through bradykinin B1 receptor signaling.     

Nephrotic plasma alters slit diaphragm-dependent signaling and translocates nephrin, Podocin, and CD2 associated protein in cultured human podocytes

Podocytes are critical in maintaining the filtration barrier of the glomerulus and are dependent on the slit diaphragm (SD) proteins nephrin, podocin, and CD2-associated protein (CD2AP) to function optimally. The effects of normal human plasma and nephrotic plasma on podocytes were tested, focusing particularly on the SD complex. With the use of a conditionally immortalized human podocyte cell line, it first was shown that exposure to normal and non-nephrotic human plasma leads to a concentration of nephrin, podocin, CD2AP, and actin at the cell surface. Next, the effects of plasma from patients with nephrotic conditions to non-nephrotic conditions were compared. When exposed to all nephrotic plasma samples (and a non-human serum control), nephrin podocin and CD2AP assumed a cytoplasmic distribution; nephrin and synaptopodin were selectively downregulated, and the relocation of nephrin induced by nephrotic plasma could be rescued back to the plasma membrane by co-incubation with non-nephrotic plasma. Furthermore, intracellular calcium signaling was altered by nephrotic plasma, which was mediated by tyrosine kinase phosphorylation. With the use of nephrin mutant human cell lines, it was shown that this signaling and translocation response to normal plasma is nephrin dependent. This work demonstrates that nephrotic plasma seems to be deficient in factors that act via the podocyte SD complex, which are essential in maintaining its physiologic function.     

Essential role of integrin-linked kinase in podocyte biology: Bridging the integrin and slit diaphragm signaling

Integrin-linked kinase (ILK) has been implicated in the pathogenesis of proteinuria and congenital nephrotic syndrome. However, the function of ILK in glomerular podocyte in a physiologic setting remains unknown. In this study, a mouse model was generated in which ILK gene was selectively disrupted in podocytes by using the Cre-LoxP system. Podocyte-specific ablation of ILK resulted in heavy albuminuria, glomerulosclerosis, and kidney failure, which led to animal death beginning at 10 wk of age. Podocyte detachment and apoptosis were not observed at 4 wk of age, when albuminuria became prominent, indicating that they are not the initial cause of proteinuria. Electron microscopy revealed an early foot process effacement, as well as morphologic abnormality, in ILK-deficient podocytes. ILK deficiency caused an aberrant distribution of nephrin and alpha-actinin-4 in podocytes, whereas the localization of podocin and synaptopodin remained relatively intact. Co-immunoprecipitation demonstrated that ILK physically interacted with nephrin to form a ternary complex, and alpha-actinin-4 participated in ILK/nephrin complex formation. Therefore, ILK plays an essential role in specifying nephrin and alpha-actinin-4 distribution and in maintaining the slit diaphragm integrity and podocyte architecture. These results also illustrate that the integrin and slit diaphragm signals in podocytes are intrinsically coupled through an ILK-dependent mechanism.     

通过基因敲低探讨多个足细胞分子作用及分子间反应

目的 研究足细胞裂孔隔膜(SD)复合体分子nephrin､podocin和CD2AP,以及足细胞骨架蛋白α-辅肌动蛋白(actinin)-4的作用及分子间反应｡方法 针对nephrin､ podocin､CD2AP和α-actinin-4 mRNA序列分别设计并构建2个特异RNA干扰质粒-psiRNA-hH1GFPzeo,分别导入小鼠足细胞系MPC5以 “敲低”其表达｡免疫荧光染色观察其分布方式｡半定量RT-PCR和免疫蛋白印迹检测其mRNA和蛋白表达｡ 结果 (1) podocin敲低组(siPod966和siPod54):未检测到podocin及nephrin mRNA,其蛋白分别下降了92%､79%及82%､67%｡而CD2AP mRNA和蛋白分别增加了62%､42%及71%､46%｡α-actinin-4无变化｡(2)nephrin敲低组(siNep492):未检测到nephrin mRNA和蛋白｡而CD2AP mRNA和蛋白分别增加了35%､48%｡Podocin和α-actinin-4无变化｡(3)CD2AP敲低组(siCda744和siCda21):未检测到CD2AP mRNA,其蛋白分别下降了92%和83%｡Nephrin mRNA和蛋白分别下降了60%､48%及76%､72%;而podocin mRNA和蛋白分别增加了38%､22%及56%､44%｡Α-actinin-4无变化｡(4)α-actinin-4敲低组(siAct1790和siAct319):α-actinin-4和nephrin 的mRNA分别下降了69%､58%及64%､49%;蛋白分别下降了81%和55%以及71%､64%｡而podocin以及CD2AP mRNA分别增加了50%､34%及45%､28%;蛋白分别增加了64%､46%及65%､42%｡(5)敲低nephrin､podocin和CD2AP后,这些表达量降低的分子的分布发生了明显改变,即以核周为主;而相应分子敲低后引起的podocin和CD2AP表达增加,其分布亦主要以核周染色增强为主｡α-actinin-4即使表达降低,分布亦无变化,仍呈细丝状分布于胞质及足细胞伸出的突起中｡结论 (1)在SD复合体分子中,nephrin可能具有相对独立的作用｡(2) α-actinin-4对nephrin､podocin和CD2AP有直接或间接的作用｡(3)足细胞分子间的作用和联系不总是“一致的”,可能是“单向的”､也可能是“双向的”｡(4)nephrin､podocin､CD2AP和α-actinin-4在足细胞的分布有赖于其表达量的正常及正常的分子间反应｡     

足细胞分子高表达致阿霉素肾病大鼠发生蛋白尿

目的 动态观察足细胞裂孔隔膜复合体分子nephrin、podocin、CD2AP及α-actinin-4在阿霉素肾病大鼠蛋白尿发生发展中的表达变化，探讨其在蛋白尿发生发展中的分子行为及其机制。方法 尾静脉注射阿霉素建立阿霉素肾病大鼠模型，3、7、14、28 d每组处死6只大鼠留取肾脏标本。应用间接免疫荧光染色、实时定量PCR、Western 印迹分别检测各个时间点nephrin、podocin、CD2AP、α-actinin-4的分布、mRNA和蛋白表达量的变化。结果 (1)14 d肾病组大鼠24 h尿蛋白显著高于对照组(P < 0.01)，增高持续到28 d。(2)透射电镜显示14 d肾病组足突不同程度变宽，28 d足突弥漫性融合。(3)与对照组相比，从第7天开始肾病组nephrin和podocin染色从沿肾小球毛细血管袢线样分布向不连续粗颗粒样的分布模式转变；CD2AP节段染色增强区逐渐扩大，有的区域呈斑片状和连续线状增强；α-actinin-4从沿肾小球毛细血管袢均匀的点线样分布向不均匀粗颗粒的分布转变，而且随时间进展这种转变逐渐加重。(4)与对照组相比，肾病组于7 d时nephrin mRNA表达显著增高(P < 0.01)；podocin mRNA表达14 d时显著增高(P < 0.05)，直至28 d(P < 0.05)；CD2AP mRNA表达28 d时显著增高(P < 0.05)。(5)与对照组相比，肾病组nephrin蛋白表达28 d时显著增高(P < 0.05)；podocin蛋白表达于7 d时显著增高(P < 0.05)，而28 d时又显著降低(P < 0.05)；CD2AP蛋白表达于14 d时显著增高(P < 0.05)，直至28 d (P < 0.05)。(6)α-actinin-4 mRNA与蛋白表达在实验过程中未出现明显变化。结论 nephrin、podocin和CD2AP的表达增加及分布异常是导致阿霉素肾病大鼠蛋白尿发生发展的分子机制，而分子表达的增加是足细胞抵抗损伤的一种代偿反应。     

Amiloride off-target effect inhibits podocyte urokinase receptor expression and reduces proteinuria

The urokinase receptor (uPAR) and its soluble form play a key role in the pathogenesis of focal segmental glomerulosclerosis (FSGS). The modification of uPAR pathological actions on podocytes will become an important task for the development of improved nephroprotective therapeutics. Here we show that podocyte uPAR expression can be reduced using amiloride. Amiloride has a significant role in the reduction of podocyte cell motility in vitro and proteinuria in mice. Amiloride inhibited the induction of uPAR protein and PLAUR messenger RNA (encoding uPAR) and with that it reduced uPAR-mediated β3 integrin activation in lipopolysaccharide (LPS)-treated podocytes. Transwell migration assay and wound healing assay showed that directed and random podocyte motility of LPS-treated podocytes were increased and substantially reduced by amiloride. The off-target effect of amiloride was independent of its function as epithelial sodium channel blocker and different from triamterene. Amiloride was also effective in the LPS mouse model of transient proteinuria (LPS mice) and in the 5/6 nephrectomy rat FSGS model (NTX) by significantly inhibiting podocyte uPAR induction, reducing proteinuria. In addition, amiloride attenuated glomerulosclerosis, as determined by glomerulosclerotic index. Thus, our observations show that amiloride inhibits podocyte uPAR induction and reduces proteinuria in NTX rats and LPS mice. Given the pathological relevance of the uPAR-β3 integrin signaling axis in FSGS, amiloride may be utilized in patients with FSGS.     

复方积雪草防治局灶节段性肾小球硬化模型大鼠足细胞损伤的实验研究

目的:探讨复方积雪草对局灶节段性肾小球硬化(focal segmental glomerulo sclerosis,FSGS)模型大鼠足细胞裂孔膜蛋白nephrin、podocin表达的影响,阐述其延缓肾小球硬化的部分分子生物学机制。方法:采用左肾摘除+阿霉素重复静脉注射方法建立FSGS模型。复方组分别予高中低剂量复方积雪草膏剂灌胃;对照组予苯那普利混悬液;于第8周末留取24h尿蛋白定量,检测血肌酐(Scr)、尿素氮(BUN)、胆固醇(TC)、三酰甘油(TG)和血浆蛋白(Alb);留取右肾标本,光镜标本行HE染色;用免疫组化法观察裂孔膜蛋白nephrin和podocin表达;RT-PCR法检测肾组织nephrin和podocinmRNA表达。结果:各复方组24h尿蛋白定量及血Scr、BUN、TC、TG较模型组有显著改善,且与西药组疗效相似。RT-PCR及免疫组化结果显示模型组足细胞裂孔膜蛋白表达较正常组明显降低(P<0.05)。而各治疗组表达较模型组均有明显增加(P<0.05),且与正常组比较差异无统计学意义(P>0.05)。结论:复方积雪草能通过上调模型大鼠肾小球内nephrin和podocin分子表达,减轻足细胞损伤,延缓肾小球硬化。     

Notch promotes dynamin-dependent endocytosis of nephrin

Notch signaling in podocytes causes proteinuria and glomerulosclerosis in humans and rodents, but the underlying mechanism remains unknown. Here, we analyzed morphologic, molecular, and cellular events before the onset of proteinuria in newborn transgenic mice that express activated Notch in podocytes. Immunohistochemistry revealed a loss of the slit diaphragm protein nephrin exclusively in podocytes expressing activated Notch. Podocyte-specific deletion of Rbpj, which is essential for canonical Notch signaling, prevented this loss of nephrin. Overexpression of activated Notch decreased cell surface nephrin and increased cytoplasmic nephrin in transfected HEK293T cells; pharmacologic inhibition of dynamin, but not depletion of cholesterol, blocked these effects on nephrin, suggesting that Notch promotes dynamin-dependent, raft-independent endocytosis of nephrin. Supporting an association between Notch signaling and nephrin trafficking, electron microscopy revealed shortened podocyte foot processes and fewer slit diaphragms among the transgenic mice compared with controls. These data suggest that Notch signaling induces endocytosis of nephrin, thereby triggering the onset of proteinuria.     

The role of podocytes in glomerular pathobiology

Podocytes are unique cells with a complex cellular organization. With respect to their cytoarchitecture, podocytes may be divided into three structurally and functionally different segments: cell body, major processes, and foot processes (FPs). The FPs of neighboring podocytes regularly interdigitate, leaving between them the filtration slits that are bridged by an extracellular structure, known as the slit diaphragm (SD). Podocytes cover the outer aspect of the glomerular basement membrane (GBM). They therefore form the final barrier to protein loss, which explains why podocyte injury is typically associated with marked proteinuria. Chronic podocyte injury may lead to podocyte detachment from the GBM. Our knowledge of the molecular structure of the SD has been remarkably improved in the past few years. Several molecules, including nephrin, CD2AP, FAT, ZO-1, P-cadherin, Podocin, and Neph 1-3 have all been shown to be associated with the SD complex, and some of these molecules are critical for its integrity. Podocytes are injured in many forms of human and experimental glomerular disease. The early events are characterized either by alterations in the molecular composition of the SD without visible changes in morphology or, more obviously, by a reorganization of FP structure with the fusion of filtration slits and the apical displacement of the SD. Based on recent insights into the molecular pathology of podocyte injury, at least four major causes have been identified that lead to the uniform reaction of FP effacement and proteinuria: (1) interference with the SD complex and its lipid rafts; (2) direct interference with the actin cytoskeleton; (3) interference with the GBM or with podocyte-GBM interaction; and (4) interference with the negative surface charge of podocytes. There is also evidence, in focal segmental glomerular sclerosis (FSGS) and in idiopathic nephrotic syndrome in humans and rats, that podocyte damage may be caused by circulating albuminuric factors. Ongoing studies in many laboratories are aiming at an understanding of the dynamic relationship between SD proteins, the actin cytoskeleton, and the dynamics of FP structure in nephrotic syndrome and FSGS. These studies should provide us with a better understanding of the biological mechanism underlying the podocyte response to injury. Such studies will potentially translate into more refined treatment and the prevention of proteinuria and progressive glomerular disease.     

Selective impairment of gene expression and assembly of nephrin in human diabetic nephropathy

BACKGROUND: Recent disclosure of podocyte proteins has unraveled previously rather mysterious mechanisms that govern glomerular perm-selectivity in health and disease. Here we addressed the role of nephrin, CD2-associated protein (CD2AP), and podocin together with the integrity of the slit diaphragm in the pathogenesis of proteinuria of patients with diabetes and nephropathy. METHODS: Nephrin mRNA and protein expression were evaluated in parallel in adult diabetic patients by in situ hybridization and immunohistochemistry. For comparison, nondiabetic patients with minimal change nephrosis and normal control patients were evaluated. CD2AP and podocin expression by immunohistochemistry was also assessed. The filtration slit was analyzed by morphometry and transmission electron microscopy. RESULTS: Extracellular nephrin mRNA and protein were markedly reduced in diabetic patients. No changes were found in patients with minimal change versus controls. CD2AP and podocin were comparable in all subjects. Ultrastructural analysis showed in diabetic patients a remarkable reduction in the percentage of electron dense slit diaphragms, despite a frequency of the filtration slits comparable to control patients. CONCLUSION: Down-regulation of nephrin and loss of the electron dense structure of slit diaphragm indicate a novel mechanism accounting for proteinuria in diabetic nephropathy. To the extent that glomerular protein trafficking contributes to renal disease progression, our findings may have clinical relevance. Reduction of nephrin in the context of normal expression of CD2AP and podocin can be taken reasonably as a specific marker of renal disease in diabetes. Therapies targeted at correcting podocyte nephrin might be of value for diabetic medicine.     

血管紧张素Ⅱ灌注诱导nephrin表达改变与足细胞凋亡

目的 研究血管紧张素Ⅱ（AngⅡ）灌注对大鼠足细胞裂隙膜分子nephrin表达及足细胞凋亡的影响，以及探讨AngⅡ引起蛋白尿及肾小球硬化的机制。方法 36只雄性Sprague Dawley大鼠分为AngⅡ灌注组(400 ng&#8226;kg-1&#8226;min-1)、生理盐水灌注组和正常对照组，测定28 d内大鼠血压及尿蛋白。分别于14、28 d处死动物取肾，观察组织学改变，并用免疫荧光、免疫电镜检测nephrin分布。RT-PCR及Western印迹法分别检测nephrin mRNA及蛋白表达。TUNEL法检测足细胞凋亡。结果 (1) AngⅡ灌注组大鼠血压升高，14 d达峰值并维持该水平至28 d；AngⅡ灌注7 d即出现蛋白尿，并持续增加。(2) AngⅡ灌注14 d时，足细胞裂隙膜变窄；灌注28 d时，足突增宽及节段性融合，部分足细胞有凋亡小体形成，少数肾小球出现节段性硬化。TUNEL法检测发现足细胞凋亡[（2.7±1.6）个/肾小球切面]，凋亡数与蛋白尿量呈正相关（r = 0.86，P < 0.01）。(3) AngⅡ灌注14 d时，肾皮质nephrin mRNA及蛋白表达上调（P < 0.05）。nephrin由正常的沿毛细血管袢线状分布向粗颗粒、团块状分布模式转变。AngⅡ灌注28 d时，肾皮质nephrin mRNA及蛋白表达下降（P < 0.05），且nephrin蛋白表达与足细胞凋亡数呈负相关（r = －0.63，P < 0.01）。 结论 AngⅡ灌注诱导的nephrin表达及分布改变可能导致了足细胞凋亡及肾小球硬化的发生与发展。