Current concepts of the podocyte in nephrotic syndrome

Levamisole Modulation of Podocytes' Actin Cytoskeleton in Nephrotic Syndrome

Podocytes play a central role in glomerular diseases such as (idiopathic) nephrotic syndrome (iNS). Glucocorticoids are the gold standard therapy for iNS. Nevertheless, frequent relapses are common. In children with iNS, steroid-sparing agents are used to avoid prolonged steroid use and reduce steroid toxicity. Levamisole is one of these steroid-sparing drugs and although clinical effectiveness has been demonstrated, the molecular mechanisms of how levamisole exerts its beneficial effects remains poorly studied. Apart from immunomodulatory capacities, nonimmunological effects of levamisole on podocytes have also been suggested. We aimed to elaborate on the effects of levamisole on human podocytes in iNS. RNA sequencing data from a human podocyte cell line treated with levamisole showed that levamisole modulates the expression of various genes involved in actin cytoskeleton stabilization and remodeling. Functional experiments showed that podocytes exposed to puromycin aminonucleoside (PAN), lipopolysaccharides (LPS), and NS patient plasma resulted in significant actin cytoskeleton derangement, reduced cell motility, and impaired cellular adhesion when compared to controls, effects that could be restored by levamisole. Mechanistic studies revealed that levamisole exerts its beneficial effects on podocytes by signaling through the glucocorticoid receptor and by regulating the activity of Rho GTPases. In summary, our data show that levamisole exerts beneficial effects on podocytes by stabilizing the actin cytoskeleton in a glucocorticoid receptor-dependent manner.

Adeno-associated virus gene therapy prevents progression of kidney disease in genetic models of nephrotic syndrome

Gene therapy for kidney diseases has proven challenging. Adeno-associated virus (AAV) is used as a vector for gene therapy targeting other organs, with particular success demonstrated in monogenic diseases. We aimed to establish gene therapy for the kidney by targeting a monogenic disease of the kidney podocyte. The most common cause of childhood genetic nephrotic syndrome is mutations in the podocyte gene NPHS2, encoding podocin. We used AAV-based gene therapy to rescue this genetic defect in human and mouse models of disease. In vitro transduction studies identified the AAV-LK03 serotype as a highly efficient transducer of human podocytes. AAV-LK03-mediated transduction of podocin in mutant human podocytes resulted in functional rescue in vitro, and AAV 2/9-mediated gene transfer in both the inducible podocin knockout and knock-in mouse models resulted in successful amelioration of kidney disease. A prophylactic approach of AAV 2/9 gene transfer before induction of disease in conditional knockout mice demonstrated improvements in albuminuria, plasma creatinine, plasma urea, plasma cholesterol, histological changes, and long-term survival. A therapeutic approach of AAV 2/9 gene transfer 2 weeks after disease induction in proteinuric conditional knock-in mice demonstrated improvement in urinary albuminuria at days 42 and 56 after disease induction, with corresponding improvements in plasma albumin. Therefore, we have demonstrated successful AAV-mediated gene rescue in a monogenic renal disease and established the podocyte as a tractable target for gene therapy approaches.

ANGPTL3 impacts proteinuria and hyperlipidemia in primary nephrotic syndrome

Background

It is unclear why primary nephrotic syndrome (PNS) patients often have dyslipidemia. Recent studies have shown that angiopoietin-like protein 3 (ANGPTL3) is an important regulator of lipid metabolism. In this study, we explored how ANGPTL3 impacts dyslipidemia during PNS development.

Methods

We measured the serum levels of ANGPTL3 in PNS patients (n=196). Furthermore, the degree of proteinuria and lipid metabolism were examined in angptl3-overexpressing transgenic (angptl3-tg) mice at different ages. Moreover, in this study, we used the clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9) system to create angptl3-knockout (angptl3-/-) mice to investigate lipopolysaccharide (LPS)-induced nephrosis.

Results

Compared with that in the healthy group, the serum level of ANGPTL3 in the PNS group was significantly increased (32 (26.35-39.66) ng/ml vs. 70.44 (63.95-76.51) ng/ml, Z =-4.81, P < 0.001). There were significant correlations between the serum level of ANGPTL3 and the levels of cholesterol (r=0.34, P < 0.001), triglycerides (r= 0.25, P = 0.001) and low-density lipoprotein (r= 0.50, P < 0.001) in PNS patients. With increasing age, angptl3-tg mice exhibited increasingly severe hypertriglyceridemia and proteinuria. The pathological features of angptl3-tg mice included rich lipid droplet deposition in hepatocytes and diffuse podocyte effacement. Compared to wild-type mice, angptl3-/- mice showed significantly lower degrees of lipid dysfunction and proteinuria after stimulation with LPS. The effects of ANGPTL3 on nephrotic dyslipidemia were confirmed in cultured hepatocytes subjected to angptl3 knockdown or overexpression. Finally, significant alterations in lipoprotein lipase (LPL) levels were observed in liver tissues from Angptl3-/- and wild-type mice stimulated with LPS.

Conclusions

ANGPTL3 could be involved in the development of dyslipidemia, as well as proteinuria, during PNS pathogenesis. Inhibition of LPL expression may the mechanism by which ANGPTL3 induces hyperlipidemia in PNS.

Pediatric Nephrotic Syndrome: Pharmacologic and Nutrition Management

Nephrotic syndrome is a common kidney disease during childhood that is characterized by alterations in glomerular filtration and leads to protein, fluid, and nutrient loss in the urine. Most patients experience peripheral, gravity-dependent edema; however, serious cases exhibit anasarca and ascites. Many long-term complications of the disease exist due to the underlying pathology and the therapies used for treatment, including metabolic bone disease, micronutrient deficiencies, and hyperlipidemia. Pharmacologic and nutrition interventions are key to appropriate management. Fluid and sodium restriction in combination with corticosteroids, albumin, and diuretics are used to manage edema. Steroid-sparing therapies like alkylating agents and calcineurin inhibitors and dietary modification to eliminate dairy and gluten may be warranted in patients with frequent relapses or steroid-refractory disease. Nutrition clinicians should familiarize themselves with the nuances of treating this disease to optimize care for children with nephrotic syndrome.

TMT-based proteomic analysis reveals the effects of chloroquine on human podocytes

Chloroquine (CQ) is an antimalarial drug widely used in rheumatic, immunological and infectious diseases. CQ is also a well-known autophagy inhibitor. It slows the progression of renal injury in patients with rheumatology diseases. Long-term CQ treatment could also damage podocytes which are highly differentiated cells wrapping the glomerular capillary to maintain renal filtration. However, the related underlying mechanism remains unclear. The effects of CQ treatment on podocytes need to be elucidated. Our results showed that CQ diminished cell motility and disrupted actin cytoskeleton in human podocytes in vitro. Totally 210 up-regulated and 67 down-regulated differentially expressed proteins (DEPs) were identified after CQ treatment in podocytes by using tandem mass tag (TMT)-labeled quantitative proteomics analysis. Gene Ontology (GO) analysis revealed that proteins mainly functioned in cell motility, cell adhesion, localization of cells and response to external stimulus. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed that DEPs were predominantly associated with lysosome, cell adhesion molecules (CAMs) and cytokine-cytokine receptor interaction. Protein-protein interaction (PPI) analysis revealed that syndecan-4 was the core protein in regulating podocyte adhesion among differentially expressed CAMs. Moreover, activated RhoA, Cdc42 and Rac1 decreased after CQ treatment. Taken together, our findings suggested that CQ could alter the stability of podocyte cytoskeleton. Proteomic analysis revealed important molecules for understanding the effects of CQ on human podocytes.

CRISPR-Cas9-Mediated Correction of the G189R-PAX2 Mutation in Induced Pluripotent Stem Cells from a Patient with Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is defined by focal (involving few glomeruli) and segmental sclerosis of the glomerular tuft that manifests with nephrotic syndrome. Mutations in genes involved in the maintenance of structure and function of podocytes have been found in a minority of these patients. A family with adult-onset autosomal dominant FSGS was recently found to carry a new germline missense heterozygous mutation (p.G189R) in the octapeptide domain of the transcription factor PAX2. Here, we efficiently corrected this point mutation in patient-derived induced pluripotent stem cells (iPSCs) by means of CRISPR-Cas9-based homology-directed repair. The iPSC lines were differentiated into podocytes, which were tested for their motility. Editing the PAX2 p.G189R mutation restored podocyte motility, which was altered in podocytes derived from patient iPSCs.

Nephrotic syndrome in a dish: recent developments in modeling in vitro

Nephrotic syndrome is a heterogeneous disease, and one of the most frequent glomerular disorders among children. Depending on the etiology, it may result in end-stage renal disease and the need for renal replacement therapy. A dysfunctional glomerular filtration barrier, comprising of endothelial cells, the glomerular basement membrane and podocytes, characterizes nephrotic syndrome. Podocytes are often the primary target cells in the pathogenesis, in which not only the podocyte function but also their crosstalk with other glomerular cell types can be disturbed due to a myriad of factors. The pathophysiology of nephrotic syndrome is highly complex and studying molecular mechanisms in vitro requires state-of-the-art cell-based models resembling the in vivo situation and preferably a fully functional glomerular filtration barrier. Current advances in stem cell biology and microfluidic platforms have heralded a new era of three-dimensional (3D) cultures that might have the potential to recapitulate the glomerular filtration barrier in vitro. Here, we highlight the molecular basis of nephrotic syndrome and discuss requirements to accurately study nephrotic syndrome in vitro, including an overview of specific podocyte markers, cutting-edge stem cell organoids, and the implementation of microfluidic platforms. The development of (patho) physiologically relevant glomerular models will accelerate the identification of molecular targets involved in nephrotic syndrome and may be the harbinger of a new era of therapeutic avenues.

Treatment of steroid-resistant nephrotic syndrome in the genomic era

The pathogenesis of steroid-resistant nephrotic syndrome (SRNS) is not completely known. Recent advances in genomics have elucidated some of the molecular mechanisms and pathophysiology of the disease. More than 50 monogenic causes of SRNS have been identified; however, these genes are responsible for only a small fraction of SRNS in outbred populations. There are currently no guidelines for genetic testing in SRNS, but evidence from the literature suggests that testing should be guided by the genetic architecture of the disease in the population. Notably, most genetic forms of SRNS do not respond to current immunosuppressive therapies; however, a small subset of patients with monogenic SRNS will achieve partial or complete remission with specific immunomodulatory agents, presumably due to non-immunosuppressive effects of these agents. We suggest a pragmatic approach to the therapy of genetic SRNS, as there is no evidence-based algorithm for the management of the disease.

Reversal of endothelial dysfunction post-immunosuppressive therapy in adult-onset podocytopathy and primary membranous nephropathy

BACKGROUND AND AIMS

The effect of nephrotic syndrome (NS) and its treatment on endothelial dysfunction is not evident. This study assessed endothelial dysfunction in adult-onset NS and its impact of immunosuppressive therapy.

METHODS

Newly diagnosed patients with adult-onset NS (podocytopathy and primary membranous nephropathy (PMN)) and normal renal function were enrolled. Flow mediated vasodilatation (FMD) assessed endothelial function and CD4⁺CD28null T cells, E-selectin and pulse wave velocities (PWV) were measured at baseline and after treatment to characterize this further. Monitoring included monthly proteinuria, serum albumin, creatinine and lipid profile at baseline and post-treatment. The healthy control (HC) included 25 voluntary kidney donors who were assessed for markers of endothelial dysfunction.

RESULTS

Fifty participants with new-onset NS were studied. Amongst the NS group, 26 (52%) patients had PMN, while the remaining 24 (48%) had podocytopathy. Twenty-one (88%) patients in the podocytopathy and 18 (69%) patients in the PMN cohort were in either complete or partial remission at the end of 8 months. FMD at baseline in NS patients was significantly lower as compared to HC (p = 0.002) while PWV (p = 0.007), E-selectin (p < 0.001) and CD4⁺CD28null T cells (p = 0.003) were significantly higher as compared with HC. Following treatment with immunosuppressive medication, FMD increased from 3 to 8% (p < 0.001). PWV also improved from a baseline of 7.70 to 6.65 m/s (p = 0.001). At the end of 8 months, E-selectin decreased significantly from 127 to 82 ng/ml (p = 0.002) while the CD4⁺CD28null T cell population reduced from 5.20 to 3.70% (p = 0.032) of total CD4⁺ cells. In the PMN cohort, despite significant reduction, E-selectin and CD4⁺CD28null T cells at follow-up remained higher than in healthy controls.

CONCLUSION

Immunosuppressive treatment contributes substantially to the improvement of endothelial dysfunction present at baseline in NS patients. Persistent subtle endothelial dysfunction remains in the sub-group of patients with PMN.

Engineering renal epithelial cells: programming and directed differentiation towards glomerular podocyte's progenitor and mature podocyte

Current knowledge of normal developmental physiology and identification of specific cell types of the kidney at molecular levels enables us to generate various cells of the kidney. The generation of renal specialized cells in vitro with its correct molecular and functional implications is the urgent need for cellular therapy in chronic kidney diseases and for organ formation. Glomerular podocytes are one of the major renal cells lose its functionality to maintain glomerular blood filtration function. In vitro, many inductions or reprogramming methods have been established for podocytes development. In these methods transcription factors, small molecules, and growth factors play the major role to remodel stem cells into podocyte progenitors and towards mature podocytes. Micro ribonucleic acids (miRNAs) have been utilizing as another strategy to generate podocyte. In this review, current protocols for in vitro glomerular podocyte differentiation have summarized emphasizing programming methods, signaling modulation, and cytoskeletal changes. Novel ideas are also pointed out, which are required for efficient optimal glomerular podocyte generation and their functional characterization in vitro with nanoarchitecture impression of the glomerular basement membrane.

Risedronate Attenuates Podocyte Injury in Phosphate Transporter-Overexpressing Rats

Osteoporosis patients with chronic kidney disease (CKD) are becoming common in our superaging society. Renal dysfunction causes phosphorus accumulation in the circulating plasma and leads to the development of CKD-mineral bone disorder (MBD). We have previously reported that type III Pi transporter-overexpressing transgenic (Pit-1 TG) rats manifest phosphate (Pi)-dependent podocyte injury. In the present study, we explored the effect of risedronate on Pi-induced podocyte injury in vivo. Pit-1 TG rats and wild-type rats at 5 weeks old were divided into a risedronate-treated group and an untreated group. We subcutaneously administered 5 μg/kg body weight of risedronate or saline twice a week during the experimental period. Risedronate did not alter serum creatinine levels at 5, 8, and 12 weeks of age. However, electron microscopy images showed that thickening of the glomerular basement membrane was improved in the risedronate treatment group. Furthermore, immunostaining for podocyte injury markers revealed that both desmin- and connexin43-positive areas were smaller in the risedronate-treated group than in the untreated group, suggesting that bisphosphonates could rescue Pi-induced podocyte injury. In conclusion, our findings suggest that risedronate could maintain glomerular barrier function by rescuing Pi-induced podocyte injury.

Curvature facilitates podocyte culture in a biomimetic platform

Most kidney diseases begin with abnormalities in glomerular podocytes, motivating the need for podocyte models to study pathophysiological mechanisms and new treatment options. However, podocytes cultured in vitro face a limited ability to maintain appreciable extents of differentiation hallmarks, raising concerns over the relevance of study results. Many key properties such as nephrin expression and morphology reach plateaus that are far from the in vivo levels. Here, we demonstrate that a biomimetic topography, consisting of microhemispheres arrayed over the cell culture substrate, promotes podocyte differentiation in vitro. We define new methods for fabricating microscale curvature on various substrates, including a thin porous membrane. By growing podocytes on our topographic substrates, we found that these biophysical cues augmented nephrin gene expression, supported full-size nephrin protein expression, encouraged structural arrangement of F-actin and nephrin within the cell, and promoted process formation and even interdigitation compared to the flat substrates. Furthermore, the topography facilitated nephrin localization on curved structures while nuclei lay in the valleys between them. The improved differentiation was also evidenced by tracking barrier function to albumin over time using our custom topomembranes. Overall, our work presents accessible methods for incorporating microcurvature on various common substrates, and demonstrates the importance of biophysical stimulation in supporting higher-fidelity podocyte cultivation in vitro.

Eucalyptol Inhibits Advanced Glycation End Products-Induced Disruption of Podocyte Slit Junctions by Suppressing Rage-Erk-C-Myc Signaling Pathway

SCOPE

The maintenance of interpodocyte slit diaphragm is critical in the sieving function of glomerular filtration barrier. Eucalyptol is a natural constituent in aromatic plants with antioxidant properties. This study investigates whether and how eucalyptol inhibits podocyte slit diaphragm malfunction in glucose-exposed podocytes and diabetic mouse kidneys.

METHODS AND RESULTS

Podocytes were incubated in media containing 33 mm glucose with 1-20 μm eucalyptol. The in vivo model employed db/db mice orally administrated with 10 mg kg^-1 eucalyptol. Nontoxic eucalyptol enhanced podocyte expression of nephrin, podocin, FAT-1, CD2AP, and α-actinin-4 diminished by glucose. Oral administration of eucalyptol augmented the induction of the slit diaphragm proteins, α-actinin-4, and integrin β1 in diabetic kidneys, and ameliorated glomerular fibrosis and foot process effacement. Eucalyptol counteracted the receptor of advanced glycation end products (RAGE) induction in podocytes with glucose or AGE-BSA, and elevated the reduction of the slit diaphragm proteins by AGE-BSA. Eucalyptol attenuated the RAGE induction and AGE accumulation in diabetic kidneys. The blockade of ERK-c-Myc signaling enhanced the nephrin and CD2AP expression downregulated in AGE-exposed podocytes. These results indicate that eucalyptol blocked glucose-induced AGE-RAGE axis and podocyte injury through disturbing RAGE-ERK-c-Myc signaling.

CONCLUSION

Eucalyptol may be a potent agent antagonizing diabetes-associated malformation of interpodocyte slit junction and podocyte actin cytoskeleton.

Nephrin-Binding Ephrin-B1 at the Slit Diaphragm Controls Podocyte Function through the JNK Pathway

Background B-type ephrins are membrane-bound proteins that maintain tissue function in several organs. We previously reported that ephrin-B1 is localized at the slit diaphragm of glomerular podocytes. However, the function of ephrin-B1 at this location is unclear.Methods We analyzed the phenotype of podocyte-specific ephrin-B1 knockout mice and assessed the molecular association of ephrin-B1 and nephrin, a key molecule of the slit diaphragm, in HEK293 cells and rats with anti-nephrin antibody-induced nephropathy.Results Compared with controls, ephrin-B1 conditional knockout mice displayed altered podocyte morphology, disarrangement of the slit diaphragm molecules, and proteinuria. Ephrin-B1 expressed in HEK293 cells immunoprecipitated with nephrin, which required the basal regions of the extracellular domains of both proteins. Treatment of cells with an anti-nephrin antibody promoted the phosphorylation of nephrin and ephrin-B1. However, phosphorylation of ephrin-B1 did not occur in cells expressing a mutant nephrin lacking the ephrin-B1 binding site or in cells treated with an Src kinase inhibitor. The phosphorylation of ephrin-B1 enhanced the phosphorylation of nephrin and promoted the phosphorylation of c-Jun N-terminal kinase (JNK), which was required for ephrin-B1-promoted cell motility in wound-healing assays. Notably, phosphorylated JNK was detected in the glomeruli of control mice but not ephrin-B1 conditional knockout mice. In rats, the phosphorylation of ephrin-B1, JNK, and nephrin occurred in the early phase (24 hours) of anti-nephrin antibody-induced nephropathy.Conclusions Through interactions with nephrin, ephrin-B1 maintains the structure and barrier function of the slit diaphragm. Moreover, phosphorylation of ephrin-B1 and, consequently, JNK are involved in the development of podocyte injury.

Dyslipidaemia in nephrotic syndrome: mechanisms and treatment

Nephrotic syndrome is a highly prevalent disease that is associated with high morbidity despite notable advances in its treatment. Many of the complications of nephrotic syndrome, including the increased risk of atherosclerosis and thromboembolism, can be linked to dysregulated lipid metabolism and dyslipidaemia. These abnormalities include elevated plasma levels of cholesterol, triglycerides and the apolipoprotein B-containing lipoproteins VLDL and IDL; decreased lipoprotein lipase activity in the endothelium, muscle and adipose tissues; decreased hepatic lipase activity; and increased levels of the enzyme PCSK9. In addition, there is an increase in the plasma levels of immature HDL particles and reduced cholesterol efflux. Studies from the past few years have markedly improved our understanding of the molecular pathogenesis of nephrotic syndrome-associated dyslipidaemia, and also heightened our awareness of the associated exacerbated risks of cardiovascular complications, progressive kidney disease and thromboembolism. Despite the absence of clear guidelines regarding treatment, various strategies are being increasingly utilized, including statins, bile acid sequestrants, fibrates, nicotinic acid and ezetimibe, as well as lipid apheresis, which seem to also induce partial or complete clinical remission of nephrotic syndrome in a substantial percentage of patients. Future potential treatments will likely also include inhibition of PCSK9 using recently-developed anti-PCSK9 monoclonal antibodies and small inhibitory RNAs, as well as targeting newly identified molecular regulators of lipid metabolism that are dysregulated in nephrotic syndrome.

Thrombin-Induced Podocyte Injury Is Protease-Activated Receptor Dependent

Nephrotic syndrome is characterized by massive proteinuria and injury of specialized glomerular epithelial cells called podocytes. Studies have shown that, whereas low-concentration thrombin may be cytoprotective, higher thrombin concentrations may contribute to podocyte injury. We and others have demonstrated that ex vivo plasma thrombin generation is enhanced during nephrosis, suggesting that thrombin may contribute to nephrotic progression. Moreover, nonspecific thrombin inhibition has been shown to decrease proteinuria in nephrotic animal models. We thus hypothesized that thrombin contributes to podocyte injury in a protease-activated receptor-specific manner during nephrosis. Here, we show that specific inhibition of thrombin with hirudin reduced proteinuria in two rat nephrosis models, and thrombin colocalized with a podocyte-specific marker in rat glomeruli. Furthermore, flow cytometry immunophenotyping revealed that rat podocytes express the protease-activated receptor family of coagulation receptors in vivo High-concentration thrombin directly injured conditionally immortalized human and rat podocytes. Using receptor-blocking antibodies and activation peptides, we determined that thrombin-mediated injury depended upon interactions between protease-activated receptor 3 and protease-activated receptor 4 in human podocytes, and between protease-activated receptor 1 and protease-activated receptor 4 in rat podocytes. Proximity ligation and coimmunoprecipitation assays confirmed thrombin-dependent interactions between human protease-activated receptor 3 and protease-activated receptor 4, and between rat protease-activated receptor 1 and protease-activated receptor 4 in cultured podocytes. Collectively, these data implicate thrombinuria as a contributor to podocyte injury during nephrosis, and suggest that thrombin and/or podocyte-expressed thrombin receptors may be novel therapeutic targets for nephrotic syndrome.

Montelukast improves the changes of cytoskeletal and adaptor proteins of human podocytes by interleukin-13

OBJECTIVE AND DESIGN

Interleukin-13 (IL-13) has recently been reported to be a potential cytokine in the pathogenesis of minimal-change nephrotic syndrome (MCNS). However, the mechanistic insights associated with podocyte dysfunction mediated by IL-13-induced changes in various slit diaphragm (SD) and cytoskeletal molecules have not yet been shown in cultured human podocytes in vitro.

MATERIALS

Human conditionally immortalized podocytes were used.

TREATMENT

Podocytes were incubated with various concentrations of IL-13 during the indicated time periods (6, 12, and 24 h) and montelukast was administered with the dose of 0.1 μg.

RESULTS

Treatment of IL-13 resulted in a progressive decrease in distinct processes or projections of the human podocytes and high dose of IL-13 increased podocyte permeability in vitro at 6 h. IL-13 had a substantial impact on the redistribution and rearrangement of zonula occludens (ZO)-1, synaptopodin, α-actinin, CD2-associated protein (CD2AP) in podocytes and disrupted the cytoskeletal connections in a concentration-dependent manner on confocal microscopy. IL-13 also down-modulated ZO-1, synaptopodin, α-actinin, CD2AP, and p130Cas at protein levels and upregulated β-catenin and B7-1 in podocytes. Furthermore, we demonstrated that down-modulated changes in various SD and cytoskeletal structures of human podocytes induced by IL-13 was significantly restored after treatment with montelukast with upregulation of B7-1.

CONCLUSION

Our results suggest that targeting IL-13 may be one of the important cytokines in the pathogenesis of MCNS and targeting IL-13 could be one of the potential therapeutic strategies in MCNS.

A biomimetic gelatin-based platform elicits a pro-differentiation effect on podocytes through mechanotransduction

Using a gelatin microbial transglutaminase (gelatin-mTG) cell culture platform tuned to exhibit stiffness spanning that of healthy and diseased glomeruli, we demonstrate that kidney podocytes show marked stiffness sensitivity. Podocyte-specific markers that are critical in the formation of the renal filtration barrier are found to be regulated in association with stiffness-mediated cellular behaviors. While podocytes typically de-differentiate in culture and show diminished physiological function in nephropathies characterized by altered tissue stiffness, we show that gelatin-mTG substrates with Young’s modulus near that of healthy glomeruli elicit a pro-differentiation and maturation response in podocytes better than substrates either softer or stiffer. The pro-differentiation phenotype is characterized by upregulation of gene and protein expression associated with podocyte function, which is observed for podocytes cultured on gelatin-mTG gels of physiological stiffness independent of extracellular matrix coating type and density. Signaling pathways involved in stiffness-mediated podocyte behaviors are identified, revealing the interdependence of podocyte mechanotransduction and maintenance of their physiological function. This study also highlights the utility of the gelatin-mTG platform as an in vitro system with tunable stiffness over a range relevant for recapitulating mechanical properties of soft tissues, suggesting its potential impact on a wide range of research in cellular biophysics.

RNA sequencing analysis of human podocytes reveals glucocorticoid regulated gene networks targeting non-immune pathways

Glucocorticoids are steroids that reduce inflammation and are used as immunosuppressive drugs for many diseases. They are also the mainstay for the treatment of minimal change nephropathy (MCN), which is characterised by an absence of inflammation. Their mechanisms of action remain elusive. Evidence suggests that immunomodulatory drugs can directly act on glomerular epithelial cells or ‘podocytes’, the cell type which is the main target of injury in MCN. To understand the nature of glucocorticoid effects on non-immune cell functions, we generated RNA sequencing data from human podocyte cell lines and identified the genes that are significantly regulated in dexamethasone-treated podocytes compared to vehicle-treated cells. The upregulated genes are of functional relevance to cytoskeleton-related processes, whereas the downregulated genes mostly encode pro-inflammatory cytokines and growth factors. We observed a tendency for dexamethasone-upregulated genes to be downregulated in MCN patients. Integrative analysis revealed gene networks composed of critical signaling pathways that are likely targeted by dexamethasone in podocytes.

Dexamethasone-dependent modulation of cyclic GMP synthesis in podocytes

Podocytes may be direct target for glucocorticoid therapy in glomerular proteinuric disease. Permeability of podocytes largely depends on their capacity to migrate which involves the contractile apparatus in their foot processes. In this study, we examined the effect of synthetic glucocorticoid dexamethasone (DEX) on the ability of podocytes to produce cyclic guanosine monophosphate (cGMP) in the presence of vasoactive factors, atrial natriuretic peptide (ANP), nitric oxide (NO), and angiotensin II (Ang II). We investigated also the effects of cGMP and DEX on podocyte motility. Primary rat podocytes and immortalized mouse podocytes were pretreated with 1 µM DEX for 4 or 24 h. Glomerular hypertension was mimicked by subjecting the cells to mechanical stress. Total and subcellular cGMP levels were determined in podocytes incubated with 0.1 µM ANP, 1 µM S-nitroso-N-acetyl penicillamine (SNAP), and 1 µM Ang II. Cell motility was estimated by a wound-healing assay. The ANP-dependent production of cGMP increased after 4 h exposition to DEX, but was attenuated after 24 h. Adversely, a 24-h pretreatment with DEX augmented the NO-dependent cGMP synthesis. Ang II suppressed the ANP-dependent cGMP production and the effect was enhanced by DEX in mechanical stress conditions. Mechanical stress reduced total cGMP production in the presence of all stimulators, whereas extracellular to total cGMP ratio increased. 8-Br cGMP enhanced podocyte migration which was accompanied by F-actin disassembly. In the presence of DEX these effects were prevented. We conclude that DEX modulates the production of cGMP in podocytes stimulated with vasoactive factors such as Ang II, ANP, and NO, and the effect is time-dependent. cGMP increases podocyte motility, which is prevented by DEX. This mechanism may account for the antiproteinuric effect of glucocorticoids.