Update in podocyte biology: putting one's best foot forward

Erzhi Formula Extracts Reverse Renal Injury in Diabetic Nephropathy Rats by Protecting the Renal Podocytes

Podocytes injury was a crucial factor resulting in diabetic nephropathy (DN). Erzhi formula extract (EZF) was a clinical effective Chinese medicine on DN, but its mechanism was unclear. In this study, the main compounds of EZF and their pharmacokinetics in rat were detected by HPLC-MS/MS. And then, blood glucose, urine protein, renal index, renal microstructural (HE/PAS staining), inflammatory factors (IL-β, TNF-α, IL-6), and protein/mRNA expression related to the function of podocyte (CD2AP and Podocin) in DN rats were investigated after the oral administration of EZF. The concentrations of specnuezhenide and wedelolactone in rat kidney were 7.19 and 0.057 mg/kg, respectively. The T_max of specnuezhenide and wedelolactone were 2.0 and 1.50 h, respectively. Their C_max were, respectively, 30.24 ± 2.68 and 6.39 ± 0.05 μg/L. Their AUC_(0-∞) were 123.30 ± 2.68 and 16.56 ± 0.98 μg/L⁎h, respectively. Compared with the model group, the blood glucose and the 24-hour urinary protein were significantly decreased (P < 0.05) after 16 weeks' treatment of EZF. The expressions of Podocin and CD2AP protein/mRNA were increased (P < 0. 05). The deteriorate of glomerular morphology was alleviated under the treatment of EZF. EZF prominently decreased the levels of inflammatory factors (P < 0.05). MDA was significantly decreased (P < 0.05) with the significant increase of SOD activity (P < 0.05) in EZF groups. All the results proved that EZF repaired glomerular mesangial matrix, protected renal tubule, and improved renal function in DN rats by upregulating the expression of Podocin and CD2AP protein/mRNA in podocytes.

Combination with miR-124a improves the protective action of BMSCs in rescuing injured rat podocytes from abnormal apoptosis and autophagy

This in vitro study was performed to identify the role of miR-124a in bone marrow stromal stem cells (BMSCs) therapy for H₂ O₂ -induced rat podocyte injury, and determine whether combination treatment with miR-124a could improve the protective effect of BMSCs. Cell viability of podocytes was detected by CCK-8 assay. Detection of ROS level, apoptotic rate, and autophagy rate was carried out using flow cytometry assays. Oxidative stress parameters were analyzed using the ELISA assays. MiR-124a and mRNA levels were determined using real-time PCR. Protein expression was detected using Western blotting. Our study revealed a pivotal role of miR-124a in the protective action of BMSCs on podocyte injury driven by oxidative stress. BMSCs could rescue injured podocytes from aberrant apoptosis and autophagy by regulating cleaved caspase-3, Bax, Bcl-2, LC3-II/I, and p62. Suppression of the PI3 K/Akt/mTOR signaling pathway is likely one of the main mechanisms underlying the protective action of BMSCs transfected with miR-124a. Our study revealed that miR-124a further improves the protective effect of BMSCs in injured podocytes. Thus, the combination of BMSCs and microRNAs could be a beneficial treatment for renal diseases in the near future.

Lupus Nephritis IgG Induction of Calcium/Calmodulin-Dependent Protein Kinase IV Expression in Podocytes and Alteration of Their Function

OBJECTIVE

Kidney podocytes and their slit diaphragms prevent urinary protein loss. T cells from patients with systemic lupus erythematosus display increased expression of calcium/calmodulin-dependent protein kinase IV (CaMKIV). The present study was undertaken to investigate the role of CaMKIV in podocyte function in lupus nephritis (LN).

METHODS

We treated kidney podocytes with IgG derived from healthy individuals or patients with LN and then analyzed gene expression using a DNA microarray. The localization of IgG in podocytes was analyzed by immunofluorescence staining, with or without silencing of neonatal Fc receptor (FcRn). In addition, we silenced CAMK4 in podocytes and analyzed the expression of selected genes. We also examined the expression of CD86 in kidney podocytes from MRL/lpr, MRL/lpr.camkiv(-/-), and MRL/MPJ mice by in situ hybridization.

RESULTS

We found that exposure of podocytes to IgG resulted in entry of IgG into the cytoplasm. IgG entered podocytes via the FcRn because less IgG was found in the cytoplasm of podocytes treated with FcRn small interfering RNA. DNA microarray studies of podocytes exposed to LN-derived IgG revealed up-regulation of genes related to the activation of immune cells or podocyte damage. Interestingly, CD86 expression decreased after silencing CAMK4 in podocytes. Also, in situ hybridization experiments showed that the expression of CD86 was reduced in podocytes from MRL/lpr.camkiv(-/-) mice.

CONCLUSION

LN-derived IgG enters podocytes and up-regulates CAMK4, which is followed by increased expression of genes known to be linked to podocyte damage and T cell activation. Targeted inhibition of CAMK4 in podocytes may prove to be clinically useful in patients with LN.

Proteomics analysis of the non-muscle myosin heavy chain IIa-enriched actin-myosin complex reveals multiple functions within the podocyte

MYH9 encodes non-muscle myosin heavy chain IIA (NMMHCIIA), the predominant force-generating ATPase in non-muscle cells. Several lines of evidence implicate a role for MYH9 in podocytopathies. However, NMMHCIIA‘s function in podocytes remains unknown. To better understand this function, we performed immuno-precipitation followed by mass-spectrometry proteomics to identify proteins interacting with the NMMHCIIA-enriched actin-myosin complexes. Computational analyses revealed that these proteins belong to functional networks including regulators of cytoskeletal organization, metabolism and networks regulated by the HIV-1 gene nef. We further characterized the subcellular localization of NMMHCIIA within podocytes in vivo, and found it to be present within the podocyte major foot processes. Finally, we tested the effect of loss of MYH9 expression in podocytes in vitro, and found that it was necessary for cytoskeletal organization. Our results provide the first survey of NMMHCIIA-enriched actin-myosin-interacting proteins within the podocyte, demonstrating the important role of NMMHCIIA in organizing the elaborate cytoskeleton structure of podocytes. Our characterization of NMMHCIIA’s functions goes beyond the podocyte, providing important insights into its general molecular role.

Effect of Cordyceps sinensis and Tripterygium wilfordii polyglycosidium on podocytes in rats with diabetic nephropathy

The aim of the present study was to investigate the effects of Cordyceps sinensis (CS) and Tripterygium wilfordii polyglycosidium (TWP) on podocytes in rats with diabetic nephropathy (DN). DN rat models were established and divided randomly into normal control (group A), DN (group B), CS (group C), TWP (group D) and CS and TWP groups (group E). After 12 weeks, levels of 24-h urinary protein, blood urea nitrogen (BUN), serum creatinine (SCR), white blood cells, blood glucose (GLU), aspartate aminotransferase, alanine aminotransferase and kidney weight (KW)/body weight (BW) were determined. Renal pathological changes were evaluated using hematoxylin and eosin staining, whereas the structural changes in the podocytes were observed under a transmission electron microscope. The expression levels of nephrin and podocin were evaluated using immunofluorescence staining. Compared with group A, the SCR and BUN levels in group B were higher (P<0.05) and the GLU, KW/BW and the 24-h urine protein were markedly higher (P<0.01). Moreover, incidences of glomerular disorders, chronic tubulointerstitial damage and glomerular podocyte lesions in groups B, C, D and E were observed, compared with group A. The high cortical expression of nephrin and podocin protein decreased. Compared with group B, the KW/BW and 24-h urinary protein level in groups C, D and E were lower (P<0.01). The glomeruli, tubules and podocytes exhibited pathomorphological improvements and the nephrin and podocin protein expression levels were higher in the nephridial tissue. A decrease in KW/BW and the 24-h urinary protein level, as well as improvements in glomerular disorder, chronic tubulointerstitial damage and glomerular podocyte lesions, were observed in groups C, D and E. Therefore, the results demonstrated that CS and TWP exhibited a protective effect on the podocytes of rats with DN. Moreover, CS combined with TWP increased this protective effect.

A compendium of urinary biomarkers indicative of glomerular podocytopathy

It is well known that glomerular podocyte injury and loss are present in numerous nephropathies and that the pathophysiologic consecution of disease hinges upon the fate of the podocyte. While multiple factors play a hand in glomerulopathy progression, basic logic lends that if one monitors the podocyte's status, that may reflect the status of disease. Recent investigations have focused on what one can elucidate from the noninvasive collection of urine, and have proven that certain, specific biomarkers of podocytes can be readily identified via varying techniques. This paper has brought together all described urinary biomarkers of podocyte injury and is made to provide a concise summary of their utility and testing in laboratory and clinical theatres. While promising in the potential that they hold as tools for clinicians and investigators, the described biomarkers require further comprehensive vetting in the form of larger clinical trials and studies that would give their value true weight. These urinary biomarkers are put forth as novel indicators of glomerular disease presence, disease progression, and therapeutic efficacy that in some cases may be more advantageous than the established parameters/measures currently used in practice.

Stem cells: potential and challenges for kidney repair

Renal damage resulting from acute and chronic kidney injury poses an important problem to public health. Currently, patients with end-stage renal disease rely solely on kidney transplantation or dialysis for survival. Emerging therapies aiming to prevent and reverse kidney damage are thus in urgent need. Although the kidney was initially thought to lack the capacity for self-repair, several studies have indicated that this might not be the case; progenitor and stem cells appear to play important roles in kidney repair under various pathological conditions. In this review, we summarize recent findings on the role of progenitor/stem cells on kidney repair as well as discuss their potential as a therapeutic approach for kidney diseases.

Molecular genetic analysis of podocyte genes in focal segmental glomerulosclerosis--a review

This review deals with podocyte proteins that play a significant role in the structure and function of the glomerular filter. Genetic linkage studies has identified several genes involved in the development of nephrotic syndrome and contributed to the understanding of the pathophysiology of glomerular proteinuria and/or focal segmental glomerulosclerosis. Here, we describe already well-characterized genetic diseases due to mutations in nephrin, podocin, CD2AP, alpha-actinin-4, WT1, and laminin beta2 chain, as well as more recently identified genetic abnormalities in TRPC6, phospholipase C epsilon, and the proteins encoded by the mitochondrial genome. In addition, the role of the proteins which have shown to be important for the structure and functions by gene knockout studies in mice, are also discussed. Furthermore, some rare syndromes with glomerular involvement, in which molecular defects have been recently identified, are briefly described. In summary, this review updates the current knowledge of genetic causes of congenital and childhood nephrotic syndrome and provides new insights into mechanisms of glomerular dysfunction.

CRIM1 is localized to the podocyte filtration slit diaphragm of the adult human kidney

Background. CRIM1 is a plasma membrane bound protein containing six cysteine-rich repeats (CRR). Through these, CRIM1 has been shown to interact with a subgroup of the TGF-β superfamily, the bone morphogenic proteins (BMP) isoforms 2, 4 and 7. The probable action is to modulate the signalling properties of these factors. CRIM1 has also been shown to regulate the release of VEGFA by podocytes during renal organogenesis. Knock-out studies in mice have shown that CRIM1 is critically involved in the development of the central nervous system, eye and kidney. Replacement of CRIM1 with a defective version leads to renal dysgenesis and perinatal death. We have analysed the distribution of CRIM1 in adult human renal tissue.

Methods. To this end, we have used immunofluorescence, immunohistochemistry and immunoelectron microscopy. We performed western blotting for the CRIM1 protein, using lysates from isolated glomerular podocytes and human renal tissue homogenate. By using quantitative PCR, we compared the CRIM1 mRNA levels in podocytes, human renal tissue homogenate, primary human renal proximal tubular epithelial cells and primary human pulmonary artery smooth muscle cells.

Results. The results show that in the human adult kidney, CRIM1 is mainly expressed in the glomerular podocytes and is associated with the insertional region of the filtration slit diaphragm (SD) of the podocyte pedicles.

Conclusions. CRIM1 is a protein that should be added to the list of proteins associated with the podocyte filtration SD and with the probable action of modulating BMP and VEGFA signalling.

The role of the type I insulin-like growth factor receptor (IGF-IR) in glomerular integrity

Insulin-like growth factors (IGFs) have been implicated in normal mammalian kidney development. To confirm a role for the IGF system in podocyte and glomerular integrity, we generated a transgenic mouse that expresses a dominant-negative type 1 IGF receptor (IGF-IR) and determined the structural and functional consequences. Using a 4.25kb fragment of the murine nephrin promoter, the dominant-negative construct was expressed exclusively in the kidney, confirmed by Southern blot and RT-PCR analysis. IGF-Ir486(FLAGstop) protein localized specifically to the glomerular podocyte based on FLAG immunohistochemistry and on co-localization with nephrin and podocin. Wild type and transgenic glomeruli expressed both the alpha- and beta-subunits of the endogenous IGF-IR, with normal expression of both nephrin and podocin. Although the animals were viable and phenotypically normal, histological analysis of the kidneys revealed abnormal and small glomeruli with dilated glomerular capillaries and condensed podocyte nuclei, while ultra-structural examination revealed diffuse but segmental podocyte foot process broadening, fusion, and effacement. Explanted glomeruli from transgenic animals demonstrated a significant inhibition of podocyte cell outgrowth when compared to controls. These studies suggest that IGF signaling is essential for maintaining the integrity of the podocyte and that alterations of IGF signaling may play a role in progressive glomerular disease.

Glomerular endothelial glycocalyx constitutes a barrier to protein permeability

Glycocalyx, composed of glycoproteins including proteoglycans, coats the luminal surface of the glomerular capillaries. Human heparanase degrades heparan sulphate glycosaminoglycans and is up-regulated in proteinuric states. In this study, we analyze the structure of the human glomerular endothelial cell glycocalyx in vitro and examine its functional relevance, especially after treatment with human heparanase. Electron microscopy of conditionally immortalized glomerular endothelial cells revealed a 200-nm thick glycocalyx over the plasma membrane, which was also demonstrated by confocal microscopy. Neuraminidase treatment removed the majority of glycocalyx, reduced trans-endothelial electrical resistance by 59%, and increased albumin flux by 207%. Heparinase III and human heparanase specifically cleaved heparan sulphate: this caused no change in trans-endothelial electrical resistance, but increased the albumin passage across the monolayers by 40% and 39%, respectively. Therefore, we have characterized the glomerular endothelial cell glycocalyx and have shown that it contributes to the barrier to flux of albumin across the cell layer. These results suggest an important role for this glycocalyx in the restriction of glomerular protein passage in vivo and suggest ways in which human heparanase levels may be linked to proteinuria in clinical disease.

Shigatoxin-induced endothelin-1 expression in cultured podocytes autocrinally mediates actin remodeling

Shigatoxin (Stx) is the offending agent of post-diarrheal hemolytic uremic syndrome, characterized by glomerular ischemic changes preceding microvascular thrombosis. Because podocytes are highly sensitive to Stx cytotoxicity and represent a source of vasoactive molecules, we studied whether Stx-2 modulated the production of endothelin-1 (ET-1), taken as candidate mediator of podocyte dysfunction. Stx-2 enhanced ET-1 mRNA and protein expression via activation of nuclear factor kappaB (NF-kappaB) and activator protein-1 (Ap-1) to the extent that transfection with the dominant-negative mutant of IkappaB-kinase 2 or with Ap-1 decoy oligodeoxynucleotides reduced ET-1 mRNA levels. We propose a role for p38 and p42/44 mitogen-activated protein kinases (MAPKs) in mediating NF-kappaB-dependent gene transcription induced by Stx-2, based on data that Stx-2 phosphorylated p38 and p42/44 MAPKs and that MAPK inhibitors reduced transcription of NF-kappaB promoter/luciferase reporter gene construct induced by Stx-2. Stx-2 caused F-actin redistribution and intercellular gaps via production of ET-1 acting on ETA receptor, because cytoskeleton changes were prevented by ETA receptor blockade. Exogenous ET-1 induced cytoskeleton rearrangement and intercellular gaps via phosphatidylinositol-3 kinase and Rho-kinase pathway and increased protein permeability across the podocyte monolayer. These data suggest that the podocyte is a target of Stx, a novel stimulus for the synthesis of ET-1, which may control cytoskeleton remodeling and glomerular permeability in an autocrine fashion.

TGF-β1 Regulates the PINCH-1–Integrin-Linked Kinase–α-Parvin Complex in Glomerular Cells

Glomerular damage is a major cause of renal failure. Recent studies suggest that a ternary protein complex that consists of PINCH-1, integrin-linked kinase, and alpha-parvin, cytoplasmic components of cell-extracellular matrix adhesions, plays pivotal roles in regulation of glomerular cell behavior. It is reported here that TGF-beta1, a key factor in the progression of glomerular failure, regulates the PINCH-1-integrin-linked kinase-alpha-parvin (PIP) complex formation in glomerular podocytes and mesangial cells. Treatment of podocytes with TGF-beta1 inhibited the PIP complex formation. Forced disruption of the PIP complex in podocytes activated p38 mitogen-activated protein kinase and promoted apoptosis. Importantly, inhibition of p38 mitogen-activated protein kinase, either with a chemical p38 inhibitor (SB202190) or with a dominant negative form of p38alpha, alleviates podocyte apoptosis that is induced by the disruption of the PIP complex. In contrast to an inhibitory role in podocytes, TGF-beta1 promotes the PIP complex formation in mesangial cells. Thus, TGF-beta1 regulates the PIP complex in a cell type-dependent manner. Because the PIP complex promotes glomerular mesangial matrix deposition and protects podocytes from apoptosis, the TGF-beta1-induced up- and downregulation of the PIP complex likely contribute to the pleiotropic effects of TGF-beta1 on different glomerular cell types and hence the progression of glomerular failure.

Cyclin I Protects Podocytes from Apoptosis

The limited regenerative capacity of the glomerular podocyte following injury underlies the development of glomerulosclerosis and progressive renal failure in a diverse range of kidney diseases. We discovered that, in the kidney, cyclin I is uniquely expressed in the glomerular podocyte, and have constructed cyclin I knock-out mice to explore the biological function of cyclin I in these cells. Cyclin I knock-out (-/-) podocytes showed an increased susceptibility to apoptosis both in vitro and in vivo. Following induction of experimental glomerulonephritis, podocyte apoptosis was increased 4-fold in the cyclin I -/- mice, which was associated with dramatically decreased renal function. Our previous data showed that the Cdk inhibitor p21(Cip1/Waf1) protects podocytes from certain apoptotic stimuli. In cultured cyclin I -/- podocytes, the level of p21(Cip1/Waf1) was lower at base line, had a shorter half-life, and declined more rapidly in response to apoptotic stimuli than in wild-type cells. Enforced expression of p21(Cip1/Waf1) reversed the susceptibility of cyclin I -/- podocytes to apoptosis. Cyclin I protects podocytes from apoptosis, and we provide preliminary data to suggest that this is mediated by stabilization of p21(Cip1/Waf1).

Molecular Mechanisms and Therapeutic Strategies of Chronic Renal Injury: Role of Rho-Kinase in the Development of Renal Injury

Rho/Rho-kinase plays an important role not only in the vasoconstrictor mechanism but also in cellular morphology, motility, adhesion, and proliferation. This pathway also serves to modulate the structure and function of various kidney cells including tubular epithelial cells, mesangial cells, and podocytes. The inhibition of the Rho/Rho-kinase pathway elicits marked increases in renal blood flow in vivo and dilates both afferent and efferent arterioles preconstricted by angiotensin II in vitro. In renal injury, intrarenal angiotensin II is reported to be activated, which subsequently would upregulate the Rho-kinase pathway. A selective Rho-kinase inhibitor, fasudil, has recently been shown to improve renal damage resulting from hypertensive glomerulosclerosis, unilateral ureteral obstruction (for interstitial renal fibrosis) and subtotal nephrectomy. Of interest, fasudil upregulated the expression of p27(kip1), a cyclin-dependent kinase inhibitor, and increased the p27(kip1) immuno-positive cells in both glomeruli and tubulointerstitium with the use of immunohistochemistry. Collectively, the Rho-kinase pathway is involved in the pathogenesis of renal injury. Clinical application of this type of therapy however awaits further investigations.

Redox dependence of glomerular epithelial cell hypertrophy in response to glucose

Podocytes or glomerular epithelial cells (GECs) are important targets of the diabetic microenvironment. Podocyte foot process effacement and widening, loss of GECs and hypertrophy are pathological features of this disease. ANG II and oxidative stress are key mediators of renal hypertrophy in diabetes. The cellular mechanisms responsible for GEC hypertrophy in diabetes are incompletely characterized. We investigated the effect of high glucose on protein synthesis and GEC hypertrophy. Exposure of GECs to high glucose dose dependently stimulated [(3)H]leucine incorporation, but not [(3)H]thymidine incorporation. High glucose resulted in the activation of ERK1/2 and Akt/PKB. ERK1/2 pathway inhibitor or the dominant negative mutant of Akt/PKB inhibited high glucose-induced protein synthesis. High glucose elicited a rapid generation of reactive oxygen species (ROS). The stimulatory effect of high glucose on ROS production, ERK1/2, and Akt/PKB activation was prevented by the antioxidants catalase, diphenylene iodonium, and N-acetylcysteine. Exposure of the cells to hydrogen peroxide mimicked the effects of high glucose. In addition, ANG II resulted in the activation of ERK1/2 and Akt/PKB and GEC hypertrophy. Moreover, high glucose and ANG II exhibited additive effects on ERK1/2 and Akt/PKB activation as well as protein synthesis. These additive responses were abolished by treatment of the cells with the antioxidants. These data demonstrate that high glucose stimulates GEC hypertrophy through a ROS-dependent activation of ERK1/2 and Akt/PKB. Enhanced ROS generation accounts for the additive effects of high glucose and ANG II, suggesting that this signaling cascade contributes to GEC injury in diabetes.

In response to protein load podocytes reorganize cytoskeleton and modulate endothelin-1 gene: implication for permselective dysfunction of chronic nephropathies

Effacement of podocyte foot processes occurs in many proteinuric nephropathies and is accompanied by rearrangement of the actin cytoskeleton. Here, we studied whether protein overload affects intracellular pathways, leading to cytoskeletal architecture changes and ultimately to podocyte dysfunction. Mouse podocytes bound and endocytosed both albumin and IgG via receptor-specific mechanisms. Protein overload caused redistribution of F-actin fibers instrumental to up-regulation of the prepro-endothelin (ET)-1 gene and production of the corresponding peptide. Increased DNA-binding activity for nuclear factor (NF)-kappaB and Ap-1 nuclear proteins was measured in nuclear extracts of podocytes exposed to excess proteins. Both Y27632, which inhibits Rho kinase-dependent stress fiber formation, and jasplakinolide, an F-actin stabilizer, decreased NF-kappaB and Ap-1 activity and reduced ET-1 expression. This suggested a role for the cytoskeleton, through activated Rho, in the regulation of the ET-1 peptide. Focal adhesion kinase (FAK), an integrin-associated nonreceptor tyrosine kinase, was phosphorylated by albumin treatment via Rho kinase-triggered actin reorganization. FAK activation led to NF-kappaB- and Ap-1-dependent ET-1 expression. These data suggest that reorganization of the actin cytoskeletal network in response to protein load is implicated in modulation of the ET-1 gene via Rho kinase-dependent FAK activation of NF-kappaB and Ap-1 in differentiated podocytes. Increased ET-1 generation might alter glomerular permselectivity and amplify the noxious effect of protein overload on dysfunctional podocytes.

Formation and phosphorylation of the PINCH-1-integrin linked kinase-alpha-parvin complex are important for regulation of renal glomerular podocyte adhesion, architecture, and survival

Alterations in the cellular architecture, adhesion, and/or loss of glomerular podocytes are causal factors in the development of proteinuria and the progression to end-stage renal failure. With the use of an inducible podocyte differentiation system, it was found that the cellular levels of PINCH-1, integrin linked kinase (ILK), and alpha-parvin, cytoplasmic components of cell-extracellular matrix adhesions, were significantly increased during podocyte differentiation. Concomitantly, an increased amount of the PINCH-1-ILK-alpha-parvin complex was detected in the differentiated, foot process-containing podocytes. Overexpression of the PINCH-1-binding ankyrin repeat domain of ILK but not that of a PINCH-1-binding defective mutant form of the ankyrin domain effectively inhibited the formation of the PINCH-1-ILK-alpha-parvin complex. Disruption of the PINCH-1-ILK-alpha-parvin complex significantly reduced the podocyte-matrix adhesion and foot process formation. Furthermore, a marked increase of apoptosis in the podocytes in which the assembly of the PINCH-1-ILK-alpha-parvin complex was compromised was detected. Inhibition of ILK with a small compound inhibitor also altered podocyte cytoskeleton and increased apoptosis. Finally, it is shown that alpha-parvin is phosphorylated in podocytes. Mutations at the alpha-parvin N-terminal proline-directed serine phosphorylation sites reduced its complex formation with ILK and resulted in defects in podocyte adhesion, architecture, and survival. These results provide important evidence for a crucial role of the PINCH-1-ILK-alpha-parvin complex in the control of podocyte adhesion, morphology, and survival.

Protective effect of 20-hydroxyeicosatetraenoic acid (20-HETE) on glomerular protein permeability barrier

BACKGROUND

Proteinuria is a significant problem in medicine today, although glomerular events underlying it are unknown. Products of cytochrome P450 (CYP450) pathway of arachidonic acid metabolism are increasingly recognized as playing major roles in renal function. We used in vitro albumin permeability (P(alb)) as a measure of injury and puromycin aminonucleoside (PAN) as an injurious agent to test the hypothesis that 20-hydroxyeicosatetraenoic acid (20-HETE) protects the glomerular filtration barrier from increased P(alb).

METHODS

We determined P(alb) in the following experimental groups: (1) isolated rat glomeruli incubated with PAN (5 microg/mL) for 5, 15, 30 or 60 minutes; (2) isolated glomeruli preincubated with 20-HETE (1.0 nmol/L to 100 nmol/L) for 15 minutes followed by additional incubation with PAN (5 microg/mL) for 15 minutes; (3) isolated glomeruli from rats treated with the CYP450 4A inducer clofibrate, and incubated with PAN (5 microg/mL) for 15 minutes; and (4) appropriate controls for each group. CYP450 4A levels were measured in glomeruli isolated from rats treated with clofibrate or vehicle.

RESULTS

PAN increased P(alb) of isolated glomeruli as early as 5 minutes (P(alb) 0.33 +/- 0.21, P < 0.05 vs. control). Maximal effect occurred at 30 minutes (P(alb) 0.75 +/- 0.16, P < 0.001 vs. control). Inclusion of 20-HETE (100 nmol/L) blocked the increased P(alb) caused by PAN (P(alb) 0.05 +/- 0.13). Likewise, glomeruli isolated from rats treated with clofibrate were protected from PAN-induced increase in P(alb) (P(alb) 0.19 +/- 0.03). Treatment with clofibrate significantly increased glomerular CYP450 4A expression.

CONCLUSION

PAN directly and immediately affects the glomerular permeability barrier. Furthermore, exogenous 20-HETE or clofibrate treatment protects glomeruli from increased P(alb) caused by PAN. Relative lack of 20-HETE may be a general characteristic of proteinuric states. Conversely, measures used to treat and/or prevent proteinuria may act to restore or increase glomerular 20-HETE levels.

Molecular basis of proteinuria

The glomerular filtration barrier is composed of endothelial cells, basement membrane, and podocytes. In recent years, remarkable progress has been made in our understanding of the molecular structure of the filtration barrier and its relation to the effectiveness of the barrier function. The glomerular basement membrane is composed of a multitude of proteins, including collagen IV, heparan sulfate proteoglycans, and laminin, among others. The slit diaphragm, which is seen as a membrane covering the space between adjacent foot processes close to the basement membrane, is an extremely important structure with a crucial role in permselectivity of the filtration barrier. Its composition is now understood to consist primarily of a unique protein called nephrin. Mutations in the gene-encoding nephrin are known to result in the Finnish type of nephrotic syndrome. The exact mechanism by which nephrin controls permselectivity is not yet clear, but it is known to interact with several podocyte proteins including CD2AP, podocin, and alpha-actinin-4. Abnormalities of any of these proteins may result in proteinuria. The role of nephrin and its associated proteins in the pathogenesis of common acquired glomerulopathies in humans is still under investigation. Normal function of podocyte also depends upon maintaining a fully mature and terminally differentiated phenotype. A host of transcription factors, especially WT1 and PAX2, play a significant role in modulating podocyte function.

Process formation of the renal glomerular podocyte: is there common molecular machinery for processes of podocytes and neurons?

The renal glomerular podocyte exhibits a highly arborized morphology. In comparison with the neuron, which is the best studied process-bearing cell, the podocyte major processes share many cell biological characteristics with neuronal dendrites. Both podocytes and neurons develop microtubule-based thick processes with branching morphology and both have thin actin-based projections (i.e. podocyte foot processes and dendritic spines). Formation of podocyte processes and neuronal dendrites depends on the assembly of microtubules. Because the assembly of microtubules is regulated by phosphorylation of microtubule-associated proteins, inhibition of protein phosphatases abolishes and inhibition of protein kinases promotes process formation. Podocytes and dendrites also share the machinery of intracellular traffic of membranous vesicles, as well as cytoskeletal elements, which is indispensable for the elongation of these processes. Furthermore, these two cell types share expression of various molecules working for signal transduction, transmembranous transport and intercellular contacts. Such common gene expression implies a similar transcriptional regulation in these cells. Concerning the formation of podocyte foot processes and dendritic branches, actin filaments are thought to play a central role in orchestrating the function of various molecules and the regulation of actin assembly is necessary to establish and maintain such sophisticated cellular architecture. The molecular mechanism of foot process formation seems to include Rho family small GTP-binding proteins, which are known to be responsible for the establishment of dendritic branching morphology.