Primary cultures of glomerular parietal epithelial cells or podocytes with proven origin

The Role of Platelet-Derived Growth Factor in Focal Segmental Glomerulosclerosis

BACKGROUND

FSGS is the final common pathway to nephron loss in most forms of severe or progressive glomerular injury. Although podocyte injury initiates FSGS, parietal epithelial cells (PECs) are the main effectors. Because PDGF takes part in fibrotic processes, we hypothesized that the ligand PDGF-B and its receptor PDGFR- β participate in the origin and progression of FSGS.

METHODS

We challenged Thy1.1 transgenic mice, which express Thy1.1 in the podocytes, with anti-Thy1.1 antibody to study the progression of FSGS. We investigated the role of PDGF in FSGS using challenged Thy1.1 mice, 5/6 nephrectomized mice, Col4 -/- (Alport) mice, patient kidney biopsies, and primary murine PECs, and challenged Thy1.1 mice treated with neutralizing anti-PDGF-B antibody therapy.

RESULTS

The unchallenged Thy1.1 mice developed only mild spontaneous FSGS, whereas challenged mice developed progressive FSGS accompanied by a decline in kidney function. PEC activation, proliferation, and profibrotic phenotypic switch drove the FSGS. During disease, PDGF-B was upregulated in podocytes, whereas PDGFR- β was upregulated in PECs from both mice and patients with FSGS. Short- and long-term treatment with PDGF-B neutralizing antibody improved kidney function and reduced FSGS, PEC proliferation, and profibrotic activation. In vitro , stimulation of primary murine PECs with PDGF-B recapitulated in vivo findings with PEC activation and proliferation, which was inhibited by PDGF-B antibody or imatinib.

CONCLUSION

PDGF-B-PDGFR- β molecular crosstalk between podocytes and PECs drives glomerulosclerosis and the progression of FSGS.

PODCAST

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Novel therapeutic perspectives for crescentic glomerulonephritis through targeting parietal epithelial cell activation and proliferation

INTRODUCTION

Kidney injury is clinically classified as crescentic glomerulonephritis (CrGN) when ≥50% of the glomeruli in a biopsy sample contain crescentic lesions. However, current strategies, such as systemic immunosuppressive therapy and plasmapheresis for CrGN, are partially effective, and these drugs have considerable systemic side effects. Hence, targeted therapy to prevent glomerular crescent formation and expansion remains an unmet clinical need.

AREAS COVERED

Hyperproliferative parietal epithelial cells (PECs) are the main constituent cells of the glomerular crescent with cell-tracing evidence. Crescents obstruct the flow of primary urine, pressure the capillaries, and degenerate the affected nephrons. We reviewed the markers of PEC activation and proliferation, potential therapeutic effects of thrombin and thrombin receptor inhibitors, and how podocytes cross-talk with PECs. These experiments may help identify potential early specific targets for the prevention and treatment of glomerular crescentic injury.

EXPERT OPINION

Inhibiting PEC activation and proliferation in CrGN can alleviate glomerular crescent progression, which has been supported by preclinical studies with evidence of genetic deletion. Clarifying the outcome of PEC transformation to the podocyte phenotype and suppressing thrombin, thrombin receptors, and PEC hyperproliferation in early therapeutic strategies will be the research goals in the next ten years.

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA and synaptopodin appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery in response to microenvironmental stimuli and tested this hypothesis by developing an ex vivo culture system that allows control of the podocyte microenvironment. Results supported our hypothesis. SLSs in podocytes that migrated from isolated kidney glomeruli presented periodic synaptopodin-positive clusters that nucleated peripheral, foot process-like extensions. SLSs were mechanoresponsive to actomyosin inhibitors and substrate stiffness. Results suggest SLSs as mechanobiological mediators of podocyte recovery and as potential targets for therapeutic intervention.

Renal cell markers: lighthouses for managing renal diseases

Kidneys, one of the vital organs in our body, are responsible for maintaining whole body homeostasis. The complexity of renal function (e.g., filtration, reabsorption, fluid and electrolyte regulation, and urine production) demands diversity not only at the level of cell types but also in their overall distribution and structural framework within the kidney. To gain an in depth molecular-level understanding of the renal system, it is imperative to discern the components of kidney and the types of cells residing in each of the subregions. Recent developments in labeling, tracing, and imaging techniques have enabled us to mark, monitor, and identify these cells in vivo with high efficiency in a minimally invasive manner. In this review, we summarize different cell types, specific markers that are uniquely associated with those cell types, and their distribution in the kidney, which altogether make kidneys so special and different. Cellular sorting based on the presence of certain proteins on the cell surface allowed for the assignment of multiple markers for each cell type. However, different studies using different techniques have found contradictions in cell type-specific markers. Thus, the term "cell marker" might be imprecise and suboptimal, leading to uncertainty when interpreting the data. Therefore, we strongly believe that there is an unmet need to define the best cell markers for a cell type. Although the compendium of renal-selective marker proteins presented in this review is a resource that may be useful to researchers, we acknowledge that the list may not be necessarily exhaustive.

PodoSighter: A Cloud-Based Tool for Label-Free Podocyte Detection in Kidney Whole-Slide Images

BACKGROUND

Podocyte depletion precedes progressive glomerular damage in several kidney diseases. However, the current standard of visual detection and quantification of podocyte nuclei from brightfield microscopy images is laborious and imprecise.

METHODS

We have developed PodoSighter, an online cloud-based tool, to automatically identify and quantify podocyte nuclei from giga-pixel brightfield whole-slide images (WSIs) using deep learning. Ground-truth to train the tool used immunohistochemically or immunofluorescence-labeled images from a multi-institutional cohort of 122 histologic sections from mouse, rat, and human kidneys. To demonstrate the generalizability of our tool in investigating podocyte loss in clinically relevant samples, we tested it in rodent models of glomerular diseases, including diabetic kidney disease, crescentic GN, and dose-dependent direct podocyte toxicity and depletion, and in human biopsies from steroid-resistant nephrotic syndrome and from human autopsy tissues.

RESULTS

The optimal model yielded high sensitivity/specificity of 0.80/0.80, 0.81/0.86, and 0.80/0.91, in mouse, rat, and human images, respectively, from periodic acid-Schiff-stained WSIs. Furthermore, the podocyte nuclear morphometrics extracted using PodoSighter were informative in identifying diseased glomeruli. We have made PodoSighter freely available to the general public as turnkey plugins in a cloud-based web application for end users.

CONCLUSIONS

Our study demonstrates an automated computational approach to detect and quantify podocyte nuclei in standard histologically stained WSIs, facilitating podocyte research, and enabling possible future clinical applications.

The recruitment mechanisms and potential therapeutic targets of podocytes from parietal epithelial cells

Podocytes are differentiated postmitotic cells which cannot be replaced after podocyte injury. The mechanism of podocyte repopulation after injury has aroused wide concern. Parietal epithelial cells (PECs) are heterogeneous and only a specific subpopulation of PECs has the capacity to replace podocytes. Major progress has been achieved in recent years regarding the role and function of a subset of PECs which could transdifferentiate toward podocytes. Additionally, several factors, such as Notch, Wnt/ß-catenin, Wilms’ tumor-1, miR-193a and growth arrest-specific protein 1, have been shown to be involved in these processes. Finally, PECs serve as a potential therapeutic target in the conditions of podocyte loss. In this review, we discuss the latest observations and concepts about the recruitment of podocytes from PECs in glomerular diseases as well as newly identified mechanisms and the most recent treatments for this process.

Tripartite Separation of Glomerular Cell Types and Proteomes from Reporter-Free Mice

BACKGROUND

The glomerulus comprises podocytes, mesangial cells, and endothelial cells, which jointly determine glomerular filtration. Understanding this intricate functional unit beyond the transcriptome requires bulk isolation of these cell types for biochemical investigations. We developed a globally applicable tripartite isolation method for murine mesangial and endothelial cells and podocytes (timMEP).

METHODS

We separated glomerular cell types from wild-type or mT/mG mice via a novel FACS approach, and validated their purity. Cell type proteomes were compared between strains, ages, and sex. We applied timMEP to the podocyte-targeting, immunologic, THSD7A-associated, model of membranous nephropathy.

RESULTS

timMEP enabled protein-biochemical analyses of podocytes, mesangial cells, and endothelial cells derived from reporter-free mice, and allowed for the characterization of podocyte, endothelial, and mesangial proteomes of individual mice. We identified marker proteins for mesangial and endothelial proteins, and outlined protein-based, potential communication networks and phosphorylation patterns. The analysis detected cell type-specific proteome differences between mouse strains and alterations depending on sex, age, and transgene. After exposure to anti-THSD7A antibodies, timMEP resolved a fine-tuned initial stress response, chiefly in podocytes, that could not be detected by bulk glomerular analyses. The combination of proteomics with super-resolution imaging revealed a specific loss of slit diaphragm, but not of other foot process proteins, unraveling a protein-based mechanism of podocyte injury in this animal model.

CONCLUSION

timMEP enables glomerular cell type-resolved investigations at the transcriptional and protein-biochemical level in health and disease, while avoiding reporter-based artifacts, paving the way toward the comprehensive and systematic characterization of glomerular cell biology.

Establishment and characterization of a novel conditionally immortalized human parietal epithelial cell line

Parietal epithelial cells (PECs) are epithelial cells in the kidney, surrounding Bowman's space. When activated, PECs increase in cell volume, proliferate, migrate to the glomerular tuft and excrete extracellular matrix. Activated PECs are crucially involved in the formation of sclerotic lesions, seen in focal segmental glomerulosclerosis (FSGS). In FSGS, a number of glomeruli show segmental sclerotic lesions. Further disease progression will lead to increasing number of involved glomeruli and gradual destruction of the affected glomeruli. Although the involvement of PECs in FSGS has been acknowledged, little is known about the molecular processes driving PEC activation. To get more insights in this process, accurate in vivo and in vitro models are needed. Here, we describe the development and characterization of a novel conditionally immortalized human PEC (ciPEC) line. We demonstrated that ciPECs are differentiated when grown under growth-restrictive conditions and express important PEC-specific markers, while lacking podocyte and endothelial markers. In addition, ciPECs showed PEC-like morphology and responded to IL-1β treatment. We therefore conclude that we have successfully generated a novel PEC line, which can be used for future studies on the role of PECs in FSGS.

From Infancy to Fancy: A Glimpse into the Evolutionary Journey of Podocytes in Culture

Podocytes are critical components of the filtration barrier and responsible for maintaining healthy kidney function. An assault on podocytes is generally associated with progression of chronic glomerular diseases. Therefore, podocyte pathophysiology is a favorite research subject for nephrologists. Despite this, podocyte research has lagged because of the unavailability of techniques for culturing such specialized cells ex vivo in quantities that are adequate for mechanistic studies. In recent years, this problem was circumvented by the efforts of researchers, who successfully developed several in vitro podocyte cell culture model systems that paved the way for incredible discoveries in the field of nephrology. This review sets us on a journey that provides a comprehensive insight into the groundbreaking breakthroughs and novel technologic advances made in the field of podocyte cell culture so far, beginning from its inception, evolution, and progression. In this study, we also describe in detail the pros and cons of different models that are being used to culture podocytes. Our extensive and exhaustive deliberation on the status of podocyte cell culture will facilitate researchers to choose wisely an appropriate model for their own research to avoid potential pitfalls in the future.

Distinct differences between cultured podocytes and parietal epithelial cells of the Bowman's capsule

Phenotypic changes in culture hamper the identification and characterization of cultured podocytes and parietal epithelial cells of the Bowman's capsule (PECs). We have recently established culture conditions that restore podocytes to their differentiated phenotypes. We compared podocytes and PECs cultured under the same conditions to determine the unique characteristics of the two cell types. Performing this comparison under the same conditions accentuated these differences. Podocytes behaved like non-epithelial cells by extending cell processes even at confluence. By contrast, PECs behaved like typical epithelial cells by maintaining a polygonal appearance. Other differences were identified using immunostaining and RT-PCR; podocytes expressed high levels of podocyte-specific markers while PECs expressed high levels of PEC-specific markers. However, while podocytes expressed low levels of PEC markers, PECs expressed low levels of podocyte markers. Therefore, the identification of podocytes and PECs in culture requires the evaluation of respective cell markers and the expression of markers for other cell types.

Cre recombinase toxicity in podocytes: a novel genetic model for FSGS in adolescent mice

Here, we show that inducible overexpression of Cre recombinase in glomerular podocytes but not in parietal epithelial cells may trigger focal segmental glomerulosclerosis (FSGS) in juvenile transgenic homocygous Pod-rtTA/LC1 mice. Administration of doxycycline shortly after birth, but not at any other time point later in life, resulted in podocyte injury and development of classical FSGS lesions in these mice. Sclerotic lesions were formed as soon as 3 wk of age, and FSGS progressed with low variability until 13 wk of age. In addition, our experiments identified Cre toxicity as a potentially relevant limitation for studies in podocytes of transgenic animals. In summary, our study establishes a novel genetic model for FSGS in mice, which exhibits low variability and manifests already at a young age.

mTOR-mediated podocyte hypertrophy regulates glomerular integrity in mice and humans

The cellular origins of glomerulosclerosis involve activation of parietal epithelial cells (PECs) and progressive podocyte depletion. While mammalian target of rapamycin-mediated (mTOR-mediated) podocyte hypertrophy is recognized as an important signaling pathway in the context of glomerular disease, the role of podocyte hypertrophy as a compensatory mechanism preventing PEC activation and glomerulosclerosis remains poorly understood. In this study, we show that glomerular mTOR and PEC activation-related genes were both upregulated and intercorrelated in biopsies from patients with focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, suggesting both compensatory and pathological roles. Advanced morphometric analyses in murine and human tissues identified podocyte hypertrophy as a compensatory mechanism aiming to regulate glomerular functional integrity in response to somatic growth, podocyte depletion, and even glomerulosclerosis - all of this in the absence of detectable podocyte regeneration. In mice, pharmacological inhibition of mTOR signaling during acute podocyte loss impaired hypertrophy of remaining podocytes, resulting in unexpected albuminuria, PEC activation, and glomerulosclerosis. Exacerbated and persistent podocyte hypertrophy enabled a vicious cycle of podocyte loss and PEC activation, suggesting a limit to its beneficial effects. In summary, our data highlight a critical protective role of mTOR-mediated podocyte hypertrophy following podocyte loss in order to preserve glomerular integrity, preventing PEC activation and glomerulosclerosis.

Dual lineage tracing shows that glomerular parietal epithelial cells can transdifferentiate toward the adult podocyte fate

Podocytes are differentiated post-mitotic cells that cannot replace themselves after injury. Glomerular parietal epithelial cells are proposed to be podocyte progenitors. To test whether a subset of parietal epithelial cells transdifferentiate to a podocyte fate, dual reporter PEC-rtTA|LC1|tdTomato|Nphs1-FLPo|FRT-EGFP mice, named PEC-PODO, were generated. Doxycycline administration permanently labeled parietal epithelial cells with tdTomato reporter (red), and upon doxycycline removal, the parietal epithelial cells (PECs) cannot label further. Despite the presence or absence of doxycycline, podocytes cannot label with tdTomato, but are constitutively labeled with an enhanced green fluorescent protein (EGFP) reporter (green). Only activation of the Nphs1-FLPo transgene by labeled parietal epithelial cells can generate a yellow color. At day 28 of experimental focal segmental glomerulosclerosis, podocyte density was 20% lower in 20% of glomeruli. At day 56 of experimental focal segmental glomerulosclerosis, podocyte density was 18% lower in 17% of glomeruli. TdTomato+ parietal epithelial cells were restricted to Bowman's capsule in healthy mice. However, by days 28 and 56 of experimental disease, two-thirds of tdTomato+ parietal epithelial cells within glomerular tufts were yellow in color. These cells co-expressed the podocyte markers podocin, nephrin, p57 and VEGF164, but not markers of endothelial (ERG) or mesangial (Perlecan) cells. Expansion microscopy showed primary, secondary and minor processes in tdTomato+EGFP+ cells in glomerular tufts. Thus, our studies provide strong evidence that parietal epithelial cells serve as a source of new podocytes in adult mice.

The tetraspanin CD9 controls migration and proliferation of parietal epithelial cells and glomerular disease progression

The mechanisms driving the development of extracapillary lesions in focal segmental glomerulosclerosis (FSGS) and crescentic glomerulonephritis (CGN) remain poorly understood. A key question is how parietal epithelial cells (PECs) invade glomerular capillaries, thereby promoting injury and kidney failure. Here we show that expression of the tetraspanin CD9 increases markedly in PECs in mouse models of CGN and FSGS, and in kidneys from individuals diagnosed with these diseases. Cd9 gene targeting in PECs prevents glomerular damage in CGN and FSGS mouse models. Mechanistically, CD9 deficiency prevents the oriented migration of PECs into the glomerular tuft and their acquisition of CD44 and β1 integrin expression. These findings highlight a critical role for de novo expression of CD9 as a common pathogenic switch driving the PEC phenotype in CGN and FSGS, while offering a potential therapeutic avenue to treat these conditions.

In both focal segmental glomerulosclerosis (FSGS) and crescentic glomerulonephritis (CGN), kidney injury is characterised by the invasion of glomerular tufts by parietal epithelial cells (PECs). Here Lazareth et al. identify the tetraspanin CD9 as a key regulator of PEC migration, and find its upregulation in FSGS and CGN contributes to kidney injury in both diseases.

Albumin induces CD44 expression in glomerular parietal epithelial cells by activating extracellular signal-regulated kinase 1/2 pathway

De novo expression of CD44 in glomerular parietal epithelial cells (PECs) leads to a prosclerotic and migratory PEC phenotype in glomerulosclerosis. However, the regulatory mechanisms underlying CD44 expression by activated PECs remain largely unknown. This study was performed to examine the mediators responsible for CD44 induction in glomerular PECs in association with diabetes. CD44 expression and localization were evaluated in the glomeruli of Zucker diabetic rat kidneys and primary cultured PECs upon albumin stimulation. Real-time polymerase chain reaction confirmed an albuminuria-associated upregulation of the CD44 gene in the glomeruli of diabetic rats. Immunostaining analysis of diabetic kidneys further revealed an increase in CD44 in hypertrophic PECs, which often contain albumin-positive vesicles. Losartan treatment significantly attenuated albuminuria and lowered CD44 protein levels in the diabetic kidneys. In primary cultured rat PECs, rat serum albumin (0.25-1 mg/ml) caused a dose-dependent upregulation of CD44, claudin-1, and megalin protein expression, which was accompanied by an activation of extracellular signal-regulated kinase1/2 (ERK1/2) signaling. Albumin-induced CD44 and claudin-1 expression were greatly suppressed in the presence of the ERK1/2 inhibitor, U0126. In addition, knockdown of megalin by small interfering RNA interference in PECs resulted in a significant reduction of albumin-induced CD44 and claudin-1 proteins. Taken together, our results demonstrate that albumin induces CD44 expression by PECs via the activation of the ERK signaling pathway, which is partially mediated by endocytic receptor megalin.

Directed Differentiation of Human Pluripotent Stem Cells to Podocytes under Defined Conditions

A major cause of chronic kidney disease (CKD) is glomerular disease, which can be attributed to a spectrum of podocyte disorders. Podocytes are non-proliferative, terminally differentiated cells. Thus, the limited supply of primary podocytes impedes CKD research. Differentiation of human pluripotent stem cells (hPSCs) into podocytes has the potential to produce podocytes for disease modeling, drug screening, and cell therapies. In the podocyte differentiation process described here, hPSCs are first induced to primitive streak-like cells by activating canonical Wnt signaling. Next, these cells progress to mesoderm precursors, proliferative nephron progenitors, and eventually become mature podocytes by culturing in a serum-free medium. Podocytes generated via this protocol adopt podocyte morphology, express canonical podocyte markers, and exhibit podocyte phenotypes, including albumin uptake and TGF-β1 triggered cell death. This study provides a simple, defined strategy to generate podocytes for in vitro modeling of podocyte development and disease or for cell therapies.

Induced Pluripotent Stem Cell-Derived Podocyte-Like Cells as Models for Assessing Mechanisms Underlying Heritable Disease Phenotype: Initial Studies Using Two Alport Syndrome Patient Lines Indicate Impaired Potassium Channel Activity

Renal podocyte survival depends upon the dynamic regulation of a complex cell architecture that links the glomerular basement membrane to integrins, ion channels, and receptors. Alport syndrome is a heritable chronic kidney disease where mutations in α3, α4, or α5 collagen genes promote podocyte death. In rodent models of renal failure, activation of the calcium-sensing receptor (CaSR) can protect podocytes from stress-related death. In this study, we assessed CaSR function in podocyte-like cells derived from induced-pluripotent stem cells from two patients with Alport Syndrome (AS1 & AS2) and a renal disease free individual [normal human mesangial cell (NHMC)], as well as a human immortalized podocyte-like (HIP) cell line. Extracellular calcium elicited concentration-dependent elevations of intracellular calcium in all podocyte-like cells. NHMC and HIP, but not AS1 or AS2 podocyte-like cells, also showed acute reductions in intracellular calcium prior to elevation. In NHMC podocyte-like cells this acute reduction was blocked by the large-conductance potassium channel (KCNMA1) inhibitors iberiotoxin (10 nM) and tetraethylammonium (5 mM), as well as the focal adhesion kinase inhibitor PF562271 (N-methyl-N-(3-((2-(2-oxo-2,3-dihydro-1H-indol-5-ylamino)-5-trifluoromethyl-pyrimidin-4-ylamino)-methyl)-pyridin-2-yl)-methanesulfonamide, 10 nM). Quantitative polymerase chain reaction (qPCR) and immunolabeling showed the presence of KCNMA1 transcript and protein in all podocyte-like cells tested. Cultivation of AS1 podocytes on decellularized plates of NHMC podocyte-like cells partially restored acute reductions in intracellular calcium in response to extracellular calcium. We conclude that the AS patient-derived podocyte-like cells used in this study showed dysfunctional integrin signaling and potassium channel function, which may contribute to podocyte death seen in Alport syndrome.

Selective pharmacological inhibition of DDR1 prevents experimentally-induced glomerulonephritis in prevention and therapeutic regime

Background

Discoidin domain receptor 1 (DDR1) is a collagen-activated receptor tyrosine kinase extensively implicated in diseases such as cancer, atherosclerosis and fibrosis. Multiple preclinical studies, performed using either a gene deletion or a gene silencing approaches, have shown this receptor being a major driver target of fibrosis and glomerulosclerosis.

Methods

The present study investigated the role and relevance of DDR1 in human crescentic glomerulonephritis (GN). Detailed DDR1 expression was first characterized in detail in human GN biopsies using a novel selective anti-DDR1 antibody using immunohistochemistry. Subsequently the protective role of DDR1 was investigated using a highly selective, novel, small molecule inhibitor in a nephrotoxic serum (NTS) GN model in a prophylactic regime and in the NEP25 GN mouse model using a therapeutic intervention regime.

Results

DDR1 expression was shown to be mainly limited to renal epithelium. In humans, DDR1 is highly induced in injured podocytes, in bridging cells expressing both parietal epithelial cell (PEC) and podocyte markers and in a subset of PECs forming the cellular crescents in human GN. Pharmacological inhibition of DDR1 in NTS improved both renal function and histological parameters. These results, obtained using a prophylactic regime, were confirmed in the NEP25 GN mouse model using a therapeutic intervention regime. Gene expression analysis of NTS showed that pharmacological blockade of DDR1 specifically reverted fibrotic and inflammatory gene networks and modulated expression of the glomerular cell gene signature, further validating DDR1 as a major mediator of cell fate in podocytes and PECs.

Conclusions

Together, these results suggest that DDR1 inhibition might be an attractive and promising pharmacological intervention for the treatment of GN, predominantly by targeting the renal epithelium.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1524-5) contains supplementary material, which is available to authorized users.

The transcription factor Dach1 is essential for podocyte function

Dedifferentiation and loss of podocytes are the major cause of chronic kidney disease. Dach1, a transcription factor that is essential for cell fate, was found in genome‐wide association studies to be associated with the glomerular filtration rate. We found that podocytes express high levels of Dach1 in vivo and to a much lower extent in vitro. Parietal epithelial cells (PECs) that are still under debate to be a type of progenitor cell for podocytes expressed Dach1 only at low levels. The transfection of PECs with a plasmid encoding for Dach1 induced the expression of synaptopodin, a podocyte‐specific protein, demonstrated by immunocytochemistry and Western blot. Furthermore, synaptopodin was located along actin fibres in a punctate pattern in Dach1‐expressing PECs comparable with differentiated podocytes. Moreover, dedifferentiating podocytes of isolated glomeruli showed a significant reduction in the expression of Dach1 together with synaptopodin after 9 days in cell culture. To study the role of Dach1 in vivo, we used the zebrafish larva as an animal model. Knockdown of the zebrafish ortholog Dachd by morpholino injection into fertilized eggs resulted in a severe renal phenotype. The glomeruli of the zebrafish larvae showed morphological changes of the glomerulus accompanied by down‐regulation of nephrin and leakage of the filtration barrier. Interestingly, glomeruli of biopsies from patients suffering from diabetic nephropathy showed also a significant reduction of Dach1 and synaptopodin in contrast to control biopsies. Taken together, Dach1 is a transcription factor that is important for podocyte differentiation and proper kidney function.

Wolf-Hirschhorn syndrome candidate 1-like 1 epigenetically regulates nephrin gene expression

Altered expression of nephrin underlies the pathophysiology of proteinuria in both congenital and acquired nephrotic syndrome. However, the epigenetic mechanisms of nephrin gene regulation remain elusive. Here, we show that Wolf-Hirschhorn syndrome candidate 1-like 1 long form (WHSC1L1-L) is a novel epigenetic modifier of nephrin gene regulation. WHSC1L1-L was associated with histone H3K4 and H3K36 in human embryonic kidney cells. WHSC1L1-L gene was expressed in the podocytes, and functional protein product was detected in these cells. WHSC1L1-L was found to bind nephrin but not other podocyte-specific gene promoters, leading to its inhibition/suppression, abrogating the stimulatory effect of WT1 and NF-κB. Gene knockdown of WHSC1L1-L in primary cultured podocytes accelerated the transcription of nephrin but not CD2AP. An in vivo zebrafish study involving the injection of Whsc1l1 mRNA into embryos demonstrated an apparent reduction of nephrin mRNA but not podocin and CD2AP mRNA. Immunohistochemistry showed that both WHSC1L1-L and nephrin emerged at the S-shaped body stage in glomeruli. Immunofluorescence and confocal microscopy displayed WHSC1L1 to colocalize with trimethylated H3K4 in the glomerular podocytes. Chromatin immunoprecipitation assay revealed the reduction of the association of trimethylated H3K4 at the nephrin promoter regions. Finally, nephrin mRNA was upregulated in the glomerulus at the early proteinuric stage of mouse nephrosis, which was associated with the reduction of WHSC1L1. In conclusion, our results demonstrate that WHSC1L1-L acts as a histone methyltransferase in podocytes and regulates nephrin gene expression, which may in turn contribute to the integrity of the slit diaphragm of the glomerular filtration barrier.

Investigations of Glucocorticoid Action in GN

For several decades, glucocorticoids have been used empirically to treat rapid progressive GN. It is commonly assumed that glucocorticoids act primarily by dampening the immune response, but the mechanisms remain incompletely understood. In this study, we inactivated the glucocorticoid receptor (GR) specifically in kidney epithelial cells using Pax8-Cre/GR^fl/fl mice. Pax8-Cre/GR^fl/fl mice did not exhibit an overt spontaneous phenotype. In mice treated with nephrotoxic serum to induce crescentic nephritis (rapidly progressive GN), this genetic inactivation of the GR in kidney epithelial cells exerted renal benefits, including inhibition of albuminuria and cellular crescent formation, similar to the renal benefits observed with high-dose prednisolone in control mice. However, genetic inactivation of the GR in kidney epithelial cells did not induce the immunosuppressive effects observed with prednisolone. In vitro, prednisolone and the pharmacologic GR antagonist mifepristone each acted directly on primary cultures of parietal epithelial cells, inhibiting cellular outgrowth and proliferation. In wild-type mice, pharmacologic treatment with the GR antagonist mifepristone also attenuated disease as effectively as high-dose prednisolone without the systemic immunosuppressive effects. Collectively, these data show that glucocorticoids act directly on activated glomerular parietal epithelial cells in crescentic nephritis. Furthermore, we identified a novel therapeutic approach in crescentic nephritis, that of glucocorticoid antagonism, which was at least as effective as high-dose prednisolone with potentially fewer adverse effects.

Isolation and Primary Culture of Murine Podocytes with Proven Origin

Genetic studies on hereditary kidney diseases and in vivo experimental model studies have revealed a critical role for the podocyte in glomerular function and disease. Primary podocyte cultures as well as immortalized podocyte cell lines have been used extensively to study podocyte function. Although, primary cells often more closely resemble the in vivo cells, they may have only a finite replicative life span before they reach senescence. Therefore, the success of studies using primary cell cultures depends on standardized isolation and culture protocols that allow reproducible generation of stable primary cultures.This chapter describes the isolation of primary podocytes with a proven origin using the novel technology of cell-specific genetic tagging. Podocytes are isolated from glomeruli from a podocyte-specific transgenic reporter mouse. The podocyte-specific reporter gene beta-galactosidase is used to identify and specifically isolate the labeled podocytes from other glomerular cells by FACS.

Novel role of Vav1-Rac1 pathway in actin cytoskeleton regulation in interleukin-13-induced minimal change-like nephropathy

Our established interleukin-13 (IL-13) overexpression rat model of minimal change-like nephropathy provided a platform to study the molecular signalling pathways in T-helper 2 (Th2) cytokine associated minimal change nephrotic syndrome (MCNS). We hypothesized that IL-13 may act directly on podocytes, causing podocyte foot process effacement and hence proteinuria in our rat model of minimal change-like nephropathy. The present study aimed firstly to delineate the glomerular 'gene signature' associated with IL-13-mediated dysregulation of podocyte-related proteins, and subsequently to investigate the role of the differentially regulated genes (DEGs) in IL-13-mediated podocyte injury. Glomerular transcriptional profile of IL-13-overexpressed rats showed characteristic features of podocyte injury with 87% of podocyte-related genes being significantly down-regulated. Gene expression of Vav1 was shown to be highly up-regulated in the glomeruli of IL-13-overexpressed rats and pathway analysis of the DEGs suggested a possible novel role of Vav1 in podocyte cytoskeleton remodelling. Immunofluorescence examination demonstrated glomerular expression of Vav1 in rats which co-localized with synaptopodin, confirming podocyte expression. However, positive staining for the phosphorylated form of Vav1 (p-Vav1) was only seen in IL-13-overexpressed rats. Moreover, in vitro IL-13 stimulation of human podocytes resulted in phosphorylation of Vav1. This was associated with Rac1 activation and actin cytoskeleton rearrangement, which was abrogated in Vav1 knockdown podocytes. In conclusion, we have demonstrated the role of Vav1-Rac1 pathway characterized by phosphorylation of Vav1, activation of Rac1 and the subsequent actin cytoskeleton rearrangement in IL-13-induced podocyte injury, possibly explaining the podocyte foot process effacement seen in our IL-13 overexpression rat model.

The lymphotoxin β receptor is a potential therapeutic target in renal inflammation

Accumulation of inflammatory cells in different renal compartments is a hallmark of progressive kidney diseases including glomerulonephritis (GN). Lymphotoxin β receptor (LTβR) signaling is crucial for the formation of lymphoid tissue, and inhibition of LTβR signaling has ameliorated several non-renal inflammatory models. Therefore, we tested whether LTβR signaling could also have a role in renal injury. Renal biopsies from patients with GN were found to express both LTα and LTβ ligands, as well as LTβR. The LTβR protein and mRNA were localized to tubular epithelial cells, parietal epithelial cells, crescents, and cells of the glomerular tuft, whereas LTβ was found on lymphocytes and tubular epithelial cells. Human tubular epithelial cells, mesangial cells, and mouse parietal epithelial cells expressed both LTα and LTβ mRNA upon stimulation with TNF in vitro. Several chemokine mRNAs and proteins were expressed in response to LTβR signaling. Importantly, in a murine lupus model, LTβR blockade improved renal function without the reduction of serum autoantibody titers or glomerular immune complex deposition. Thus, a preclinical mouse model and human studies strongly suggest that LTβR signaling is involved in renal injury and may be a suitable therapeutic target in renal diseases.

Glomerular epithelial CD44 expression and segmental sclerosis in IgA nephropathy

BACKGROUND

CD44 is a marker of activated parietal epithelial cells (PECs), and is expressed in glomerular visceral epithelial cells (VECs) during development of segmental sclerosis. We explored the significance of glomerular epithelial CD44 expression in relation to segmental sclerosis in patients with mild IgA nephropathy (IgAN).

METHODS

A total of 126 cases of IgAN were divided into three groups based on glomerular morphology: normal (group A, n = 30), mild mesangial proliferation without segmental sclerosis or synechia (SS) (group B, n = 31), or mild mesangial proliferation with SS (group C, n = 65). The number of CD44-expressing PECs and VECs was counted in each glomerulus and expressed as the mean number per case.

RESULTS

CD44 staining was positive in VECs in 59.5 %, in PECs in 79.4 % and in both cell types in 56.3 % of cases. The number of CD44⁺ PECs or VECs was significantly higher in group C than in groups A or B. Cases with >1 CD44⁺ cell (PECs and VECs) per glomerulus were associated with increased urine protein/creatinine ratio (UPCr) at last follow-up. The presence of >1 CD44⁺ VEC/glomerulus was associated with increased UPCr and serum creatinine levels, and decreased estimated glomerular filtration rate (eGFR) even in the absence of SS at the time of biopsy.

CONCLUSION

CD44 was expressed in PECs and VECs in association with SS in IgAN. Increased CD44 expression in VECs is a sign of active glomerular injury and dysfunction in these patients.

Macrophage Migration Inhibitory Factor Mediates Proliferative GN via CD74

Pathologic proliferation of mesangial and parietal epithelial cells (PECs) is a hallmark of various glomerulonephritides. Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine that mediates inflammation by engagement of a receptor complex involving the components CD74, CD44, CXCR2, and CXCR4. The proliferative effects of MIF may involve CD74 together with the coreceptor and PEC activation marker CD44. Herein, we analyzed the effects of local glomerular MIF/CD74/CD44 signaling in proliferative glomerulonephritides. MIF, CD74, and CD44 were upregulated in the glomeruli of patients and mice with proliferative glomerulonephritides. During disease, CD74 and CD44 were expressed de novo in PECs and colocalized in both PECs and mesangial cells. Stress stimuli induced MIF secretion from glomerular cells in vitro and in vivo, in particular from podocytes, and MIF stimulation induced proliferation of PECs and mesangial cells via CD74. In murine crescentic GN, Mif-deficient mice were almost completely protected from glomerular injury, the development of cellular crescents, and the activation and proliferation of PECs and mesangial cells, whereas wild-type mice were not. Bone marrow reconstitution studies showed that deficiency of both nonmyeloid and bone marrow-derived Mif reduced glomerular cell proliferation and injury. In contrast to wild-type mice, Cd74-deficient mice also were protected from glomerular injury and ensuing activation and proliferation of PECs and mesangial cells. Our data suggest a novel molecular mechanism and glomerular cell crosstalk by which local upregulation of MIF and its receptor complex CD74/CD44 mediate glomerular injury and pathologic proliferation in GN.

Schip1 is a novel podocyte foot process protein that mediates actin cytoskeleton rearrangements and forms a complex with Nherf2 and ezrin

Background

Podocyte foot process effacement accompanied by actin cytoskeleton rearrangements is a cardinal feature of many progressive human proteinuric diseases.

Results

By microarray profiling of mouse glomerulus, SCHIP1 emerged as one of the most highly enriched transcripts. We detected Schip1 protein in the kidney glomerulus, specifically in podocytes foot processes. Functionally, Schip1 inactivation in zebrafish by morpholino knock-down results in foot process disorganization and podocyte loss leading to proteinuria. In cultured podocytes Schip1 localizes to cortical actin-rich regions of lamellipodia, where it forms a complex with Nherf2 and ezrin, proteins known to participate in actin remodeling stimulated by PDGFβ signaling. Mechanistically, overexpression of Schip1 in vitro causes accumulation of cortical F-actin with dissolution of transversal stress fibers and promotes cell migration in response to PDGF-BB stimulation. Upon actin disassembly by latrunculin A treatment, Schip1 remains associated with the residual F-actin-containing structures, suggesting a functional connection with actin cytoskeleton possibly via its interaction partners. A similar assay with cytochalasin D points to stabilization of cortical actin cytoskeleton in Schip1 overexpressing cells by attenuation of actin depolymerisation.

Conclusions

Schip1 is a novel glomerular protein predominantly expressed in podocytes, necessary for the zebrafish pronephros development and function. Schip1 associates with the cortical actin cytoskeleton network and modulates its dynamics in response to PDGF signaling via interaction with the Nherf2/ezrin complex. Its implication in proteinuric diseases remains to be further investigated.

Glomerular parietal epithelial cells in kidney physiology, pathology, and repair

PURPOSE OF REVIEW

We have summarized recently published glomerular parietal epithelial cell (PEC) research, focusing on their roles in glomerular development and physiology, and in certain glomerular diseases. The rationale is that PECs have been largely ignored until the recent availability of cell lineage tracing studies, human and murine PEC culture systems, and potential therapeutic interventions of PECs.

RECENT FINDINGS

Several new paradigms involving PECs have emerged demonstrating their significant contribution to glomerular physiology and numerous glomerular diseases. A subset of PECs serving as podocyte progenitors have been identified in normal human glomeruli. They provide a source for podocytes in adolescent mice, and their numbers increase in states of podocyte depletion. PEC progenitor number is increased by retinoids and angiotensin-converting enzyme inhibition. However, dysregulated growth of PEC progenitors leads to pseudo-crescent and crescent formation. In focal segmental glomerulosclerosis, considered a podocyte disease, activated PECs increase extracellular matrix production, which leads to synechial attachment and, when they move to the glomerular tuft, to segmental glomerulosclerosis. Finally, PECs might be adversely affected in proteinuric states by undergoing apoptosis.

SUMMARY

PECs play a critical role in glomerular repair through their progenitor function, but under certain circumstances paradoxically contribute to deterioration by augmenting scarring and crescent formation.

Urine-derived stem cells: A novel and versatile progenitor source for cell-based therapy and regenerative medicine

Engineered functional organs or tissues, created with autologous somatic cells and seeded on biodegradable or hydrogel scaffolds, have been developed for use in individuals with tissue damage suffered from congenital disorders, infection, irradiation, or cancer. However, in those patients, abnormal cells obtained by biopsy from the compromised tissue could potentially contaminate the engineered tissues. Thus, an alternative cell source for construction of the neo-organ or functional recovery of the injured or diseased tissues would be useful. Recently, we have found stem cells existing in the urine. These cells are highly expandable, and have self-renewal capacity, paracrine properties, and multi-differentiation potential. As a novel cell source, urine-derived stem cells (USCs) provide advantages for cell therapy and tissue engineering applications in regeneration of various tissues, particularly in the genitourinary tract, because they originate from the urinary tract system. Importantly, USCs can be obtained via a non-invasive, simple, and low-cost approach and induced with high efficiency to differentiate into three dermal cell lineages.

Transcriptional landscape of glomerular parietal epithelial cells

Very little is known about the function of glomerular parietal epithelial cells (PECs). In this study, we performed genome-wide expression analysis on PEC-enriched capsulated vs. PEC-deprived decapsulated rat glomeruli to determine the transcriptional state of PECs under normal conditions. We identified hundreds of differentially expressed genes that mapped to distinct biologic modules including development, tight junction, ion transport, and metabolic processes. Since developmental programs were highly enriched in PECs, we characterized several of their candidate members at the protein level. Collectively, our findings confirm that PECs are multifaceted cells and help define their diverse functional repertoire.

The terminator mouse: salvation for primary cell culture

The Terminator had to come back from the future already several times in an effort to bring salvation to mankind. In the present issue of Kidney International, Guo et al. brought us a novel transgenic mouse model: the terminator mouse. This highly elegant mouse may facilitate significantly the derivation of primary cultures of a specific cell type from a tissue containing multiple cell populations.

Gp130-dependent signaling in the podocyte

Renal inflammation, in particular glomerular, is often characterized by increased IL-6 levels. The in vivo relevance of IL-6 signaling in glomerular podocytes, which play central roles in most glomerular diseases, is unknown. Here, we show that in normal mice, podocytes express gp130, the common signal-transducing receptor subunit of the IL-6 family of cytokines. Following systemic IL-6 or LPS injection in mice, podocyte IL-6 signaling was evidenced by downstream STAT3 phosphorylation. Next, we generated mice deficient for gp130 in podocytes. Expectedly, these mice exhibited abrogated IL-6 downstream signaling in podocytes. At the age of 40 wk, they did not show spontaneous renal pathology or abnormal renal function. The mice were then challenged using two LPS injury models as well as nephrotoxic serum to induce crescentic nephritis. Under all conditions, circulating IL-6 levels increased markedly and the mice developed the pathological hallmarks of the corresponding injury models such as proteinuria and development of glomerular crescents, respectively. However, despite the capacity of normal podocytes to transduce IL-6 family signals downstream, there were no significant differences between mice bearing the podocyte-specific gp130 deletion and their control littermates in any of these models. In conclusion, under the different conditions tested, gp130 signaling was not a critical component of the (patho-)biology of the podocyte in vivo.

Hyperglycemia decreases the expression of ATP synthase β subunit and enolase 2 in glomerular epithelial cells

Glomerular epithelial cells (GECs) are known to play a key role in maintaining the structure and function of the glomerulus. GEC injury induced by hyperglycemia is present in early-stage diabetic nephropathy (DN), which is the most common cause of renal failure. In an attempt to identify target proteins involved in the pathogenesis of GEC injury at early DN, we performed the proteomic analysis using primary cultures of GECs, prepared from the dissected rat glomeruli. The protein expression profiles in the two-dimensional electrophoresis gels were compared between GECs treated for three days with normal glucose (5 mM) and those with high glucose (30 mM) concentrations. These concentrations correspond to blood glucose concentrations under normoglycemia and hyperglycemia, respectively. Proteins with differential expression levels were identified using ESI-Q-TOF tandem mass spectrometry. The primary GECs cultured in hyperglycemic conditions showed cellular hypertrophy and increased production of reactive oxygen species, both of which reflect the GEC injury. Our proteomic analysis identified eight proteins with differential expression profiles, depending on glucose concentrations. Among them, we selected ATP synthase β subunit and enolase 2 that are related to energy metabolism and are down-regulated under hyperglycemia, and confirmed that hyperglycemia decreased the expression levels of ATP synthase β subunit and enolase 2 proteins by western blotting analysis. Hyperglycemia may impair mitochondrial function and alter glycolysis in GECs by down-regulating the expression of ATP synthase β subunit and enolase 2. The present study may provide a better understanding of the pathogenic mechanisms of GEC injury in early DN.

The emergence of the glomerular parietal epithelial cell

Glomerular diseases are the leading causes of chronic and end-stage kidney disease. In the 1980s and 1990s, attention was focused on the biology and role of glomerular endothelial and mesangial cells. For the past two decades, seminal discoveries have been made in podocyte biology in health and disease. More recently, the glomerular parietal epithelial cell (PEC)-the fourth resident glomerular cell type-has been under active study, leading to a better understanding and definition of how these cells behave normally, and their potential roles in glomerular disease. Accordingly, this Review will focus on our current knowledge of PECs, in both health and disease. We discuss model systems to study PECs, how PECs might contribute to glomerulosclerosis, crescent and pseudocrescent formation and how PECs handle filtered albumin. These events have consequences on PEC structure and function, and PECs have potential roles as stem or progenitor cells for podocytes in glomerular regeneration, which will also be described.

The regenerative potential of parietal epithelial cells in adult mice

Previously, we showed that some podocytes in juvenile mice are recruited from cells lining Bowman's capsule, suggesting that parietal epithelial cells (PECs) are a progenitor cell population for podocytes. To investigate whether PECs also replenish podocytes in adult mice, PECs were genetically labeled in an irreversible fashion in 5-week-old mice. No significant increase in labeled podocytes was observed, even after 18 months. To accelerate a potential regenerative mechanism, progressive glomerular hypertrophy was induced by progressive partial nephrectomies. Again, no significant podocyte replenishment was observed. Rather, labeled PECs exclusively invaded segments of the tuft affected by glomerulosclerosis, consistent with our previous findings. We next reassessed PEC recruitment in juvenile mice using a different reporter mouse and confirmed significant recruitment of labeled PECs onto the glomerular tuft. Moreover, some labeled cells on Bowman's capsule expressed podocyte markers, and cells on Bowman's capsule were also directly labeled in juvenile podocyte-specific Pod-rtTA transgenic mice. In 6-week-old mice, however, cells on Bowman's capsule no longer expressed podocyte-specific markers. Similarly, in human kidneys, some cells on Bowman's capsule expressed the podocyte marker synaptopodin from 2 weeks to 2 years of age but not at 7 years of age. In summary, podocyte regeneration from PECs could not be detected in aging mice or models of glomerular hypertrophy. We propose that a small fraction of committed podocytes reside on Bowman's capsule close to the vascular stalk and are recruited onto the glomerular tuft during infancy to adolescence in mice and humans.

Angiotensin II blockade and renal protection

Current national guidelines have recommended the use of renin-angiotensin system inhibitors, including angiotensin II type 1 receptor blockers (ARBs), in preference to other antihypertensive agents for treating hypertensive patients with chronic kidney disease. However, the mechanisms underlying the renoprotective effects of ARBs are multiple and complex. Blood pressure reduction by systemic vasodilation with an ARB contributes to its beneficial effects in treating kidney disease. Furthermore, ARB-induced renal vasodilation results in an increase in renal blood flow, leading to improvement of renal ischemia and hypoxia. ARBs are also effective in reducing urinary albumin excretion through a reduction in intraglomerular pressure and the protection of glomerular endothelium and/or podocyte injuries. In addition to blocking angiotensin II-induced renal cell and tissue injuries, ARBs can decrease intrarenal angiotensin II levels by reducing proximal tubular angiotensinogen and production of collecting duct renin, as well as angiotensin II accumulation in the kidney. In this review, we will briefly summarize our current understanding of the pharmacological effects of an ARB in the kidney. We will also discuss the possible mechanisms responsible for the renoprotective effects of ARBs on type 2 diabetic nephropathy.

Albumin is recycled from the primary urine by tubular transcytosis

Under physiologic conditions, significant amounts of plasma protein pass the renal filter and are reabsorbed by proximal tubular cells, but it is not clear whether the endocytosed protein, particularly albumin, is degraded in lysosomes or returned to the circulatory system intact. To resolve this question, a transgenic mouse with podocyte-specific expression of doxycycline-inducible tagged murine albumin was developed. To assess potential glomerular backfiltration, two types of albumin with different charges were expressed. On administration of doxycycline, podocytes expressed either of the two types of transgenic albumin, which were secreted into the primary filtrate and reabsorbed by proximal tubular cells, resulting in serum accumulation. Renal transplantation experiments confirmed that extrarenal transcription of transgenic albumin was unlikely to account for these results. Genetic deletion of the neonatal Fc receptor (FcRn), which rescues albumin and IgG from lysosomal degradation, abolished transcytosis of both types of transgenic albumin and IgG in proximal tubular cells. In summary, we provide evidence of a transcytosis within the kidney tubular system that protects albumin and IgG from lysosomal degradation, allowing these proteins to be recycled intact.

The Terminator mouse is a diphtheria toxin-receptor knock-in mouse strain for rapid and efficient enrichment of desired cell lineages

Biomedical research often requires primary cultures of specific cell types, which are challenging to obtain at high purity in a reproducible fashion. Here we engineered the murine Rosa26 locus by introducing the diphtheria toxin receptor flanked by loxP sites. The resultant strain was nicknamed the Terminator mouse. This approach results in diphtheria toxin receptor expression in all non-Cre expressing cell types, making these cells susceptible to diphtheria toxin exposure. In primary cultures of kidney cells derived from the Terminator mouse, over 99.99% of cells were dead within 72 hours of diphtheria toxin treatment. After crossing the Terminator with the Podocin-Cre (podocyte specific) mouse or the Ggt-Cre (proximal tubule specific) mouse, diphtheria toxin treatment killed non-Cre expressing cells but spared podocytes and proximal tubule cells, respectively, enriching the primary cultures to over 99% purity based on both Western blotting and immunostaining of marker proteins. Thus, the Terminator mouse can be a useful tool to selectively and reproducibly obtain even low abundant cell types at high quantity and purity.

PodNet, a protein-protein interaction network of the podocyte

Interactions between proteins crucially determine cellular structure and function. Differential analysis of the interactome may help elucidate molecular mechanisms during disease development; however, this analysis necessitates mapping of expression data on protein-protein interaction networks. These networks do not exist for the podocyte; therefore, we built PodNet, a literature-based mouse podocyte network in Cytoscape format. Using database protein-protein interactions, we expanded PodNet to XPodNet with enhanced connectivity. In order to test the performance of XPodNet in differential interactome analysis, we examined podocyte developmental differentiation and the effect of cell culture. Transcriptomes of podocytes in 10 different states were mapped on XPodNet and analyzed with the Cytoscape plugin ExprEssence, based on the law of mass action. Interactions between slit diaphragm proteins are most significantly upregulated during podocyte development and most significantly downregulated in culture. On the other hand, our analysis revealed that interactions lost during podocyte differentiation are not regained in culture, suggesting a loss rather than a reversal of differentiation for podocytes in culture. Thus, we have developed PodNet as a valuable tool for differential interactome analysis in podocytes, and we have identified established and unexplored regulated interactions in developing and cultured podocytes.

Molecular fingerprinting of the podocyte reveals novel gene and protein regulatory networks

A thorough characterization of the transcriptome and proteome of endogenous podocytes has been hampered by low cell yields during isolation. Here we describe a double fluorescent reporter mouse model combined with an optimized bead perfusion protocol and efficient single cell dissociation to yield more than 500,000 podocytes per mouse allowing for global, unbiased downstream applications. Combining mRNA and miRNA transcriptional profiling with quantitative proteomic analyses revealed programs of highly specific gene regulation tightly controlling cytoskeleton, cell differentiation, endosomal transport, and peroxisome function in podocytes. Strikingly, the analyses further predict that these podocyte-specific gene regulatory networks are accompanied by alternative splicing of respective genes. Thus, our 'omics' approach will facilitate the discovery and integration of novel gene, protein, and organelle regulatory networks that deepen our systematic understanding of podocyte biology.