Mitotic cell cycle proteins increase in podocytes despite lack of proliferation

Transcriptomic profile of human iPSC-derived podocyte-like cells exposed to a panel of xenobiotics

Podocytes play a critical role in the formation and maintenance of the glomerular filtration barrier and injury to these cells can lead to a breakdown of the glomerular barrier causing permanent damage leading to progressive chronic kidney disease. Matured podocytes have little proliferative potential, which makes them critical cells from a health perspective, but also challenging cells to maintain in vitro. Differentiating podocyte-like cells from induced pluripotent stem cells (iPSC) provides a novel and continuous source of cells. Here, we investigated the effect of a 24-h exposure to eight compounds, including the known glomerular toxins doxorubicin and pamidronate, on transcriptomic alterations in iPSC derived podocytes. Doxorubicin (50 nM), pamidronate (50 μM), sodium arsenite (10 μM), and cyclosporine A (15 μM) had a strong impact on the transcriptome, gentamicin (450 μg/ml), lead chloride (15 μM) and valproic acid (500 μM) had a mild impact and busulfan (50 μM) exhibited no impact. Gene alterations and pathways analysis provided mechanistic insight for example, doxorubicin exposure affected the p53 pathway and dedifferentiation, pamidronate activated several pathways including HIF1alpha and sodium arsenite up-regulated oxidative stress and metal responses. The results demonstrate the applicability of iPSC derived podocytes for toxicological and mechanistic investigations.

Molecular Mechanisms of Kidney Injury and Repair

Chronic kidney disease (CKD) will become the fifth global cause of death by 2040, thus emphasizing the need to better understand the molecular mechanisms of damage and regeneration in the kidney. CKD predisposes to acute kidney injury (AKI) which, in turn, promotes CKD progression. This implies that CKD or the AKI-to-CKD transition are associated with dysfunctional kidney repair mechanisms. Current therapeutic options slow CKD progression but fail to treat or accelerate recovery from AKI and are unable to promote kidney regeneration. Unraveling the cellular and molecular mechanisms involved in kidney injury and repair, including the failure of this process, may provide novel biomarkers and therapeutic tools. We now review the contribution of different molecular and cellular events to the AKI-to-CKD transition, focusing on the role of macrophages in kidney injury, the different forms of regulated cell death and necroinflammation, cellular senescence and the senescence-associated secretory phenotype (SAPS), polyploidization, and podocyte injury and activation of parietal epithelial cells. Next, we discuss key contributors to repair of kidney injury and opportunities for their therapeutic manipulation, with a focus on resident renal progenitor cells, stem cells and their reparative secretome, certain macrophage subphenotypes within the M2 phenotype and senescent cell clearance.

MCC Regulator of WNT Signaling Pathway (MCC) Is a Podocyte Essential Gene

Podocytes are an integral part of the glomerular filtration barrier. Many genes are already known to be essential for podocyte survival, structure and function, but there are more podocyte essential genes to be identified. By single-cell RNA-seq of mouse podocytes, we detected the expression of gene encoding MCC regulator of WNT signaling pathway (MCC) in majority of the podocytes and speculated that MCC is essential for podocytes. We confirmed MCC expression in mouse podocytes and further showed its expression in human podocytes. To experimentally prove the essentiality of MCC for podocytes, we knocked down MCC in cultured podocytes and found marked morphological change of cell shape, cytoskeletal F-actin stress fiber disruption, increased apoptosis, and downregulation of podocyte essential genes, CD2AP and WT1, demonstrating that MCC is essential for podocytes. Since MCC has been implicated in cell cycle and β-catenin signaling, we examined the expression of cell cycle related genes and activity of β-catenin in the MCC knockdown podocytes, but did not find significant changes. To further explore the mechanism underlying the role of MCC in podocytes, we performed RNA-sequencing and bioinformatics analysis of MCC knockdown podocytes and found a significant enrichment of the regulated genes in lamellipodia formation. Consistently, we found that MCC is present in lamellipodia and MCC knockdown resulted in loss of lamellipodia in the cells. Lastly, we found that MCC was downregulated in podocytes treated with puromycin aminonucleosides and in glomeruli of diabetic mice and FSGS patients, implicating MCC is involved in the development of podocytopathy and proteinuria. In conclusion, MCC is potentially essential for podocytes and its downregulation may be involved in podocytopathy.

MAD2B-mediated cell cycle reentry of podocytes is involved in the pathogenesis of FSGS

Rationale: Focal segmental glomerulosclerosis (FSGS) is characterized by the dysfunction of “post-mitotic” podocytes. The reentry of podocytes in the cell cycle will ultimately result in cell death. Mitotic arrest deficient 2-like protein 2 (MAD2B), an inhibitor of anaphase-promoting complex (APC)/cyclosome, precisely controls the metaphase to anaphase transition and ordered cell cycle progression. However, the role of MAD2B in FSGS podocyte injury remains unknown.

Methods: To explore MAD2B function in podocyte cell cycle reentry, we used conditional mutant mice lacking MAD2B selectively in podocytes in ADR-induced FSGS murine model. Additionally, KU-55933, a specific inhibitor of ataxia-telangiectasia mutated (ATM) was utilized in vivo and in vitro to explore the role of ATM in regulating MAD2B.

Results: The expression of MAD2B in podocytes was dramatically increased in patients with FSGS and ADR-treated mice along with podocyte cell cycle reentry. Podocyte-specific knockout of MAD2B effectively attenuated proteinuria, podocyte injury, and prevented the aberrant cell cycle reentry. By bioinformatics analysis we revealed that ATM kinase is a key upstream regulator of MAD2B. Furthermore, inhibition of ATM kinase abolished MAD2B-driven cell cycle reentry and alleviated podocyte impairment in FSGS murine model. In vitro studies by site-directed mutagenesis and immunoprecipitation we revealed ATM phosphorylated MAD2B and consequently hampered the ubiquitination of MAD2B in a phosphorylation-dependent manner.

Conclusions: ATM kinase-MAD2B axis importantly contributes to the cell cycle reentry of podocytes, which is a novel pathogenic mechanism of FSGS, and may shed light on the development of its therapeutic approaches.

Yes-associated protein regulates podocyte cell cycle re-entry and dedifferentiation in adriamycin-induced nephropathy

Podocytes are terminally differentiated cells with little proliferative capacity. The high expression levels of cell cycle inhibitory proteins, including p21, p27, and p57, play an important role in maintaining the low level of proliferation of mature podocytes. In the present study, we aimed to explore the role of yes-associated protein (YAP) signalling in adriamycin-induced podocyte re-entry into the cell cycle and dedifferentiation. Proliferating cell nuclear antigen (PCNA)-, cyclin-dependent kinase 4 (CDK4)-, and Cyclin D1-positive podocytes were found in mice with adriamycin-induced nephropathy. In vitro, adriamycin administration increased the percentage of cells in S phase and the upregulation of mesenchymal-related marker proteins. CDK4 and cyclin D1 were significantly up-regulated after incubation with adriamycin. Overexpression of YAP in podocytes promoted their entry into the cell cycle; up-regulated cyclin D1, desmin, and snail2 expression and down-regulated Wilms’ tumour 1 (WT1) and nephrin production. Recombinant murine FGF-basic induced podocytes to re-enter the cell cycle, inhibited WT1 and nephrin, and increased desmin and snail2 expression. Pretreating podocytes with verteporfin, an inhibitor of YAP/ TEA domain transcription factor (TEAD), decreased the adriamycin-induced overexpression of cyclin D1 and reduced the ratio of S-phase podocytes. This result was further verified by knocking down YAP expression using RNA interference. In conclusion, adriamycin induced podocytes to re-enter the cell cycle via upregulation of CDK4 and cyclin D1 expression, which was at least partly mediated by YAP signalling. Re-entry into the cell cycle induced the over-expression of mesenchymal markers in podocytes.

Krüppel-like factor 4 is a negative regulator of STAT3-induced glomerular epithelial cell proliferation

Pathologic glomerular epithelial cell (GEC) hyperplasia is characteristic of both rapidly progressive glomerulonephritis (RPGN) and subtypes of focal segmental glomerulosclerosis (FSGS). Although initial podocyte injury resulting in activation of STAT3 signals GEC proliferation in both diseases, mechanisms regulating this are unknown. Here, we show that the loss of Krüppel-like factor 4 (KLF4), a zinc-finger transcription factor, enhances GEC proliferation in both RPGN and FSGS due to dysregulated STAT3 signaling. We observed that podocyte-specific knockdown of Klf4 (C57BL/6J) increased STAT3 signaling and exacerbated crescent formation after nephrotoxic serum treatment. Interestingly, podocyte-specific knockdown of Klf4 in the FVB/N background alone was sufficient to activate STAT3 signaling, resulting in FSGS with extracapillary proliferation, as well as renal failure and reduced survival. In cultured podocytes, loss of KLF4 resulted in STAT3 activation and cell-cycle reentry, leading to mitotic catastrophe. This triggered IL-6 release into the supernatant, which activated STAT3 signaling in parietal epithelial cells. Conversely, either restoration of KLF4 expression or inhibition of STAT3 signaling improved survival in KLF4-knockdown podocytes. Finally, human kidney biopsy specimens with RPGN exhibited reduced KLF4 expression with a concomitant increase in phospho-STAT3 expression as compared with controls. Collectively, these results suggest the essential role of KLF4/STAT3 signaling in podocyte injury and its regulation of aberrant GEC proliferation.

Decreased DACH1 expression in glomerulopathy is associated with disease progression and severity

Cell fate determination factor dachshund1 (DACH1) is a chromosome-associated protein that regulates cellular differentiation throughout development. Recent genome-wide association studies have show that missense mutation in DACH1 leads to hereditary renal hypodysplasia. Renal DACH1 expression can be used to estimate glomerular filtration rate (eGFR). We firstly characterized the function of DACH1 in normal and diseased renal tissue using immunohistochemistry to assess DACH1 in human renal biopsy specimens from 40 immunoglobulin A nephropathy (IgAN) patients, 20 idiopathic membranous nephropathy (IMN) patients, and 15 minimal change disease (MCD) patients. We found that DACH1 expression was decreased in the nephropathy group relative to healthy controls. DACH1 staining in the glomerulus correlated positively with eGFR (r = 0.41, p < 0.001) but negatively with serum creatinine (r = −0.37, p < 0.01). In vitro, DACH1 overexpression in human podocytes or HK2 cells decreased expression of cyclin D1, but increased expression of p21 and p53, which suggested that DACH1 overexpression in human podocytes or HK2 cells increased the G1/S phase or G2/M cell arrest. Together, These findings indicate that DACH1 expression is decreased in glomerulopathy imply a potential role for DACH1 in the this development of human chornic glomerulopathy. These data suggest that DACH1 is a potential a marker of disease progression and severity for glomerular diseases.

MDM2 is implicated in high-glucose-induced podocyte mitotic catastrophe via Notch1 signalling

Podocyte injury and depletion are essential events involved in the pathogenesis of diabetic nephropathy (DN). As a terminally differentiated cell, podocyte is restricted in ‘post‐mitosis’ state and unable to regenerate. Re‐entering mitotic phase will cause podocyte disastrous death which is defined as mitotic catastrophe (MC). Murine double minute 2 (MDM2), a cell cycle regulator, is widely expressed in renal resident cells including podocytes. Here, we explore whether MDM2 is involved in podocyte MC during hyperglycaemia. We found aberrant mitotic podocytes with multi‐nucleation in DN patients. In vitro, cultured podocytes treated by high glucose (HG) also showed an up‐regulation of mitotic markers and abnormal mitotic status, accompanied by elevated expression of MDM2. HG exposure forced podocytes to enter into S phase and bypass G2/M checkpoint with enhanced expression of Ki67, cyclin B1, Aurora B and p‐H3. Genetic deletion of MDM2 partly reversed HG‐induced mitotic phase re‐entering of podocytes. Moreover, HG‐induced podocyte injury was alleviated by MDM2 knocking down but not by nutlin‐3a, an inhibitor of MDM2‐p53 interaction. Interestingly, knocking down MDM2 or MDM2 overexpression showed inhibition or activation of Notch1 signalling, respectively. In addition, genetic silencing of Notch1 prevented HG‐mediated podocyte MC. In conclusion, high glucose up‐regulates MDM2 expression and leads to podocyte MC. Notch1 signalling is an essential downstream pathway of MDM2 in mediating HG‐induced MC in podocytes.

Cell cycle re-entry sensitizes podocytes to injury induced death

Podocytes are terminally differentiated renal cells, lacking the ability to regenerate by proliferation. However, during renal injury, podocytes re-enter into the cell cycle but fail to divide. Earlier studies suggested that re-entry into cell cycle results in loss of podocytes, but a direct evidence for this is lacking. Therefore, we established an in vitro model to test the consequences of re-entry into the cell cycle on podocyte survival. A mouse immortalized podocyte cell line was differentiated to non-permissive podocytes and stimulated with e.g. growth factors. Stimulated cells were analyzed for mRNA-expression or stained for cell cycle analysis using flow cytometry and immunocytofluorescence microscopy. After stimulation to re-entry into cell cycle, podocytes were stressed with puromycin aminonucleoside (PAN) and analyzed for survival. During permissive stage more than 40% of immortalized podocytes were in the S-phase. In contrast, S-phase in non-permissive differentiated podocytes was reduced to 5%. Treatment with b-FGF dose dependently induced re-entry into cell cycle increasing the number of podocytes in the S-phase to 10.7% at an optimal bFGF dosage of 10 ng/ml. Forty eight hours after stimulation with bFGF the number of bi-nucleated podocytes significantly increased. A secondary injury stimulus significantly reduced podocyte survival preferentially in bi-nucleated podocytes In conclusion, stimulation of podocytes using bFGF was able to induce re-entry of podocytes into the cell cycle and to sensitize the cells for cell death by secondary injuries. Therefore, this model is appropriate for testing new podocyte protective substances that can be used for therapy.

Progenitor cells and podocyte regeneration

The very limited ability of adult podocytes to proliferate in vivo is clinically significant because podocytes form a vascular barrier that is functionally critical to the nephron, podocyte hypoplasia is a characteristic of disease, and inadequate regeneration of podocytes is a major cause of persistent podocyte hypoplasia. Excessive podocyte loss or inadequate replacement leads to glomerulosclerosis in many progressive kidney diseases. Thus, restoration of podocyte cell density almost certainly is reliant on regeneration by podocyte progenitors. However, such putative progenitors have remained elusive until recently. In this review, we describe the developmental processes leading to podocyte and parietal epithelial cell (PEC) formation during glomerulogenesis. We compare evidence that in normal human kidneys PECs expressing progenitor markers CD133 and CD24 can differentiate into podocytes in vitro and in vivo, with evidence from animal models suggesting a more limited role of the PEC's capacity to serve as a podocyte progenitor in adults. We highlight tantalizing new evidence that specialized vascular wall cells of afferent arterioles, including those that produce renin in healthy kidney, provide a novel local progenitor source of new PECs and podocytes in response to podocyte hypoplasia in the adult, and draw comparisons with glomerulogenesis.

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.

Stem cells: potential and challenges for kidney repair

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

New insights into the pathology of podocyte loss: mitotic catastrophe

Podocytes represent an essential component of the kidney's glomerular filtration barrier. They stay attached to the glomerular basement membrane via integrin interactions that support the capillary wall to withstand the pulsating filtration pressure. Podocyte structure is maintained by a dynamic actin cytoskeleton. Terminal differentiation is coupled with permanent exit from the cell cycle and arrest in a postmitotic state. Postmitotic podocytes do not have an infinite life span; in fact, physiologic loss in the urine is documented. Proteinuria and other injuries accelerate podocyte loss or induce death. Mature podocytes are unable to replicate and maintain their actin cytoskeleton simultaneously. By the end of mitosis, cytoskeletal actin forms part of the contractile ring, rendering a round shape to podocytes. Therefore, when podocyte mitosis is attempted, it may lead to aberrant mitosis (ie, mitotic catastrophe). Mitotic catastrophe implies that mitotic podocytes eventually detach or die; this is a previously unrecognized form of podocyte loss and a compensatory mechanism for podocyte hypertrophy that relies on post-G1-phase cell cycle arrest. In contrast, local podocyte progenitors (parietal epithelial cells) exhibit a simple actin cytoskeleton structure and can easily undergo mitosis, supporting podocyte regeneration. In this review we provide an appraisal of the in situ pathology of mitotic catastrophe compared with other proposed types of podocyte death and put experimental and renal biopsy data in a unified perspective.

Podocyte loss involves MDM2-driven mitotic catastrophe

Podocyte apoptosis as a pathway of podocyte loss is often suspected but rarely detected. To study podocyte apoptosis versus inflammatory forms of podocyte death in vivo, we targeted murine double minute (MDM)-2 for three reasons. First, MDM2 inhibits p53-dependent apoptosis; second, MDM2 facilitates NF-κB signalling; and third, podocytes show strong MDM2 expression. We hypothesized that blocking MDM2 during glomerular injury may trigger p53-mediated podocyte apoptosis, proteinuria, and glomerulosclerosis. Unexpectedly, MDM2 blockade in early adriamycin nephropathy of Balb/c mice had the opposite effect and reduced intra-renal cytokine and chemokine expression, glomerular macrophage and T-cell counts, and plasma creatinine and blood urea nitrogen levels. In cultured podocytes exposed to adriamycin, MDM2 blockade did not trigger podocyte death but induced G2/M arrest to prevent aberrant nuclear divisions and detachment of dying aneuploid podocytes, a feature of mitotic catastrophe in vitro and in vivo. Consistent with these observations, 12 of 164 consecutive human renal biopsies revealed features of podocyte mitotic catastrophe but only in glomerular disorders with proteinuria. Furthermore, delayed MDM2 blockade reduced plasma creatinine levels, blood urea nitrogen, tubular atrophy, interstitial leukocyte numbers, and cytokine expression as well as interstitial fibrosis. Together, MDM2-mediated mitotic catastrophe is a previously unrecognized variant of podocyte loss where MDM2 forces podocytes to complete the cell cycle, which in the absence of cytokinesis leads to podocyte aneuploidy, mitotic catastrophe, and loss by detachment. MDM2 blockade with nutlin-3a could be a novel therapeutic strategy to prevent renal inflammation, podocyte loss, glomerulosclerosis, proteinuria, and progressive kidney disease.

Cells of renin lineage are progenitors of podocytes and parietal epithelial cells in experimental glomerular disease

Glomerular injury leads to podocyte loss, a process directly underlying progressive glomerular scarring and decline of kidney function. The inherent repair process is limited by the inability of podocytes to regenerate. Cells of renin lineage residing alongside glomerular capillaries are reported to have progenitor capacity. We investigated whether cells of renin lineage can repopulate the glomerulus after podocyte injury and serve as glomerular epithelial cell progenitors. Kidney cells expressing renin were genetically fate-mapped in adult Ren1cCreER×Rs-tdTomato-R, Ren1cCre×Rs-ZsGreen-R, and Ren1dCre×Z/EG reporter mice. Podocyte depletion was induced in all three cell-specific reporter mice by cytotoxic anti-podocyte antibodies. After a decrease in podocyte number, a significant increase in the number of labeled cells of renin lineage was observed in glomeruli in a focal distribution along Bowman's capsule, within the glomerular tuft, or in both locations. A subset of cells lining Bowman's capsule activated expression of the glomerular parietal epithelial cell markers paired box protein PAX2 and claudin-1. A subset of labeled cells within the glomerular tuft expressed the podocyte markers Wilms tumor protein 1, nephrin, podocin, and synaptopodin. Neither renin mRNA nor renin protein was detected de novo in diseased glomeruli. These findings provide initial evidence that cells of renin lineage may enhance glomerular regeneration by serving as progenitors for glomerular epithelial cells in glomerular disease characterized by podocyte depletion.

The antiviral cytokines IFN-α and IFN-β modulate parietal epithelial cells and promote podocyte loss: implications for IFN toxicity, viral glomerulonephritis, and glomerular regeneration

Interferon (IFN)-α and IFN-β are the central regulators of antiviral immunity but little is known about their roles in viral glomerulonephritis (eg, HIV nephropathy). We hypothesized that IFN-α and IFN-β would trigger local inflammation and podocyte loss. We found that both IFNs consistently activated human and mouse podocytes and parietal epithelial cells to express numerous IFN-stimulated genes. However, only IFN-β significantly induced podocyte death and increased the permeability of podocyte monolayers. In contrast, only IFN-α caused cell-cycle arrest and inhibited the migration of parietal epithelial cells. Both IFNs suppressed renal progenitor differentiation into mature podocytes. In Adriamycin nephropathy, injections with either IFN-α or IFN-β aggravated proteinuria, macrophage influx, and glomerulosclerosis. A detailed analysis showed that only IFN-β induced podocyte mitosis. This did not, however, lead to proliferation, but was associated with podocyte loss via podocyte detachment and/or mitotic podocyte death (mitotic catastrophe). We did not detect TUNEL-positive podocytes. Thus, IFN-α and IFN-β have both common and differential effects on podocytes and parietal epithelial cells, which together promote glomerulosclerosis by enhancing podocyte loss while suppressing podocyte regeneration from local progenitors.

How many ways can a podocyte die?

Podocytes are highly specialized epithelial cells that line the urinary surface of the glomerular capillary tuft. To maintain kidney filtration, podocytes oppose the high intraglomerular hydrostatic pressure, form a molecular sieve, secrete soluble factors to regulate other glomerular cell types, and provide synthesis and maintenance of the glomerular basement membrane. Impairment of any of these functions after podocyte injury results in proteinuria and possibly renal failure. Loss of glomerular podocytes is a key feature for the progression of renal diseases, and detached podocytes can be retrieved in the urine of patients with progressive glomerular diseases. Thus, the concept of podocyte loss as a hallmark of progressive glomerular disease has been widely accepted. However, the nature of events that promote podocyte detachment and whether detachment is preceded by any kind of podocyte cell death, such as apoptosis, necroptosis, or necrosis, still remains unclear and is discussed in this review.

The complement cascade in kidney disease: from sideline to center stage

Activation of the complement pathway is implicated in the pathogenesis of many kidney diseases. The pathologic and clinical features of these diseases are determined in part by the mechanism and location of complement activation within the kidney parenchyma. This review describes the physiology, action, and control of the complement cascade and explains the role of complement overactivation and dysregulation in kidney disease. There have been recent advances in the understanding of the effects of upregulation of the complement cascade after kidney transplantation. Complement plays an important role in initiating and propagating damage to transplanted kidneys in ischemia-reperfusion injury, antibody-mediated rejection, and cell-mediated rejection. Complement-targeting therapies presently are in development, and the first direct complement medication for kidney disease was licensed in 2011. The potential therapeutic targets for anticomplement drugs in kidney disease are described. Clinical and experimental studies are ongoing to identify further roles for complement-targeting therapy.

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.

Cyclin D1 expression in podocytes: regulated by mitogens in collaboration with integrin-extracellular matrix interaction through extracellular signal-regulated kinase

Cyclin D1 plays significant roles in cell cycle entry and migration. We have documented that both integrin α3β1 expressions and the number of podocytes were reduced in focal segmental glomerulosclerosis. We wondered whether integrin-extracellular matrix (ECM) interaction was involved in the regulation of cyclin D1 expression, and the possible signaling pathways in mitogen-stimulating podocytes. Cultured podocytes were divided into serum (mitogens/growth factors)-starved and serum-stimulated groups. Reverse transcription polymerase chain reaction was used to detect cyclin D1 mRNA, and Western blot analysis was used to measure protein concentrations of cyclin D1 and extracellular signal-regulated kinase (ERK) activation (p-ERK/ERK). The integrin-ECM interaction was blocked by anti-β1-integrin monoclonal antibody or RGDS (Arg-Gly-Asp-Ser). The MEK inhibitor, U0126, was used to inhibit ERK activation. The results showed that there was little cyclin D1 protein in serum-starved groups, but it was abundant in serum-stimulated groups. Both cyclin D1 mRNA and protein levels were reduced in serum-stimulated podocytes after blocking integrin-ECM interaction. ERK activation in serum-stimulated podocytes was significantly decreased after blocking integrin-ECM interaction. Cyclin D1 mRNA and protein concentrations in serum-stimulated podocytes were reduced after blocking ERK activation by U0126. We demonstrate that integrin-ECM interaction collaborates with mitogens to activate ERK/mitogen-activated protein kinase pathways which are essential for cyclin D1 expression in podocytes.

Podocyte damage resulting in podocyturia: a potential diagnostic marker to assess glomerular disease activity

A decrease in podocyte number contributes to the development of glomerulosclerosis in most forms of glomerular disease [1, 2, 3, 4, 5]. Traditionally, it has been argued that this decrease may be caused by the inability of podocytes to proliferate and replace those lost following immune, metabolic, toxic or hemodynamic injury. These data contrast with recent studies showing that podocytes are able to enter the cell cycle after injury, to progress through the different phases of the cell cycle and even enter mitosis. However, experimental and human data suggest that entry of podocytes into the cell cycle may result in reduced adhesion to the glomerular basement membrane with subsequent loss of podocytes into the urine and excretion of both viable and apoptotic podocytes. Viable urinary podocytes can be cultivated ex vivo for up to 2-3 weeks and in experimental models precede the onset of proteinuria. More importantly, podocyturia can decrease despite persistent proteinuria. The latter observation suggests that podocyturia may serve as the first non-invasive marker of 'active' glomerular damage and might thus drive therapeutic interventions in the future. However, at present technical issues still prevent a broad clinical application of podocyturia detection in clinical practice.

Lessons from studies on focal segmental glomerulosclerosis: an important role for parietal epithelial cells?

Glomerular diseases are caused by multiple mechanisms. Progressive glomerular injury is characterized by the development of segmental or global glomerulosclerosis independent of the nature of the underlying renal disease. Most studies on glomerular disease focus on the constituents of the filtration barrier (podocytes, glomerular basement membrane (GBM), endothelial cells) or the mesangial cells. Little attention is given to the epithelial cells lining Bowman's capsule, the so called parietal epithelial cells (PECs). This 'lack of attention' is partly explained by the presumed 'passive' function of PECs, which are large, flattened cells that cover Bowman's capsule in a single cell layer and form a barrier between the ultrafiltrate and the periglomerular interstitium, in normal glomerular physiology. A more important reason has been the lack of an established primary role for the parietal epithelium in glomerular diseases. However, in recent years, several studies have demonstrated that PECs are involved in extracapillary proliferation. In addition, PECs can become highly active, proliferating cells, expressing many growth factors, chemokines, cytokines, and their receptors. It was recently demonstrated that PECs also play a part in the development of focal segmental glomerulosclerosis (FSGS). This review summarises current knowledge of the PEC, with emphasis on the role of PECs in the development of FSGS.

The podocyte's response to injury: role in proteinuria and glomerulosclerosis

The terminally differentiated podocyte, also called glomerular visceral epithelial cell, are highly specialized cells. They function as a critical size and charge barrier to prevent proteinuria. Podocytes are injured in diabetic and non-diabetic renal diseases. The clinical signature of podocyte injury is proteinuria, with or without loss of renal function owing to glomerulosclerosis. There is an exciting and expanding literature showing that hereditary, congenital, or acquired abnormalities in the molecular anatomy of podocytes leads to proteinuria, and at times, glomerulosclerosis. The change in podocyte shape, called effacement, is not simply a passive process following injury, but is owing to a complex interplay of proteins that comprise the molecular anatomy of the different protein domains of podocytes. These will be discussed in this review. Recent studies have also highlighted that a reduction in podocyte number directly causes proteinuria and glomerulosclerosis. This is owing to several factors, including the relative inability for these cells to proliferate, detachment, and apoptosis. The mechanisms of these events are being elucidated, and are discussed in this review. It is the hope that by delineating the events following injury to podocytes, therapies might be developed to reduce the burden of proteinuric renal diseases.

Role of calcium-phosphorous disorders in the progression of renal failure

Among factors related to disturbed calcium-phosphate metabolism in chronic kidney disease, the following must be mainly considered as potential culprits in the progression of renal dysfunction: hyperphosphatemia, hyperparathyroidism, lack of active vitamin D, and possibly excess of the phosphaturic hormone FGF 23. Early experimental work suggested a parathyroid hormone (PTH)-independent beneficial role of phosphate restriction on progression in rats (animals with physiologic hyperphosphatemia), so that the generalization of the data is uncertain. Recent observational studies also found a correlation between S-phosphate and progression, but it remains uncertain whether the relationship is causal. There is very little direct experimental or clinical evidence for a role of PTH in accelerating progression, although the PTH1 receptor is expressed in podocytes and PTH affects podocyte function (i.e., Kf). It is undoubtedly a candidate that requires more sophisticated investigation. Recently, it has been shown that progression is significantly attenuated by calcimimetics (and equally by parathyroidectomy), but it is currently impossible to exclude a confounding effect of lower blood pressure values. The most solid evidence for an impact on progression exists for active vitamin D. In the past, it was widely assumed that vitamin D was "nephrotoxic." In retrospect, nephrotoxicity was the result of hypercalcemia. Recent evidence is overwhelming that 1,25(OH)2D3 and its analogues attenuate progression in noninflammatory and inflammatory models of chronic kidney disease. The main target cells identified so far are podocytes and mesangial cells. It is currently unknown whether the novel phosphaturic hormones have an impact on progression.

Viable podocytes detach in experimental diabetic nephropathy: potential mechanism underlying glomerulosclerosis

BACKGROUND

A decrease in podocyte number contributes to the development of glomerulosclerosis in diabetic nephropathy. Although podocytes have been detected in the urine in certain glomerular diseases, their viability is poorly understood.

METHODS

Diabetes was induced in rats with streptozotocin. Urine was collected from control rats (given citrate), and rats with diabetic nephropathy, and cells obtained by centrifugation were resuspended in tissue culture media, and seeded onto collagen-coated tissue culture plates. Cells were grown under standard cell culture conditions ex vivo. Cell number was measured, the cell type in the urine was identified by immunostaining with specific antibodies, and morphology was assessed by light and electron microscopy.

RESULTS

Within 24 h, cells obtained from the urine of diabetic rats attached to tissue culture plates ex vivo. Cells were not detected in the urine from control rats. All cells from diabetic rats stained positive for the podocyte-specific proteins synaptopodin, nephrin, podocin and Glepp-1 and negative for mesangial (OX-7), tubular (Tamm-Horsfall protein) and endothelial (RECA) cell antigens. The cell number increased daily, which is consistent with cell growth ex vivo.

CONCLUSIONS

Rats with diabetic nephropathy shed podocytes into the urine that attach and grow ex vivo. These results are consistent with the detachment of viable podocytes in diabetes and add new perspectives into our understanding of development of glomerulosclerosis in diabetes mellitus.

Experimental membranous nephropathy redux

Membranous nephropathy (MN) is a common cause of nephrotic syndrome in adults. Active and passive Heymann nephritis (HN) in rats are valuable experimental models because their features so closely resemble human MN. In HN, subepithelial immune deposits form in situ as a result of circulating antibodies. Complement activation leads to assembly of C5b-9 on glomerular epithelial cell (GEC) plasma membranes and is essential for sublethal GEC injury and the onset of proteinuria. This review revisits HN and focuses on areas of substantial progress in recent years. The response of the GEC to sublethal C5b-9 attack is not simply due to disruption of the plasma membrane but is due to the activation of specific signaling pathways. These include activation of protein kinases, phospholipases, cyclooxygenases, transcription factors, growth factors, NADPH oxidase, stress proteins, proteinases, and others. Ultimately, these signals impact on cell metabolic pathways and the structure/function of lipids and key proteins in the cytoskeleton and slit-diaphragm. Some signals affect GEC adversely. Thus C5b-9 induces partial dissolution of the actin cytoskeleton. There is a decline in nephrin expression, reduction in F-actin-bound nephrin, and loss of slit-diaphragm integrity. Other signals, such as endoplasmic reticulum stress, may limit complement-induced injury, or promote recovery. The extent of complement activation and GEC injury is dependent, in part, on complement-regulatory proteins, which act at early or late steps within the complement cascade. Identification of key steps in complement activation, the cellular signaling pathways, and the targets will facilitate therapeutic intervention in reversing GEC injury in human MN.

R-roscovitine (CYC202) alleviates renal cell proliferation in nephritis without aggravating podocyte injury

BACKGROUND

Cyclin-dependent kinase (CDK) inhibition is a new therapeutic approach to proliferative glomerulonephritides. CDK2 is required for G(1)/S transition and DNA synthesis and is inhibited by CYC202 (R-roscovitine). Since podocytes express CDK2 in nephritis and since loss of podocytes contributes to glomerulosclerosis, the rationale of the present study was to test whether CDK2 inhibition is safe in instances of podocyte injury.

METHODS

Rats with passive Heymann nephritis, a model of membranous glomerulonephritis, were treated (day 3 to 30) with vehicle, low (25 mg/kg/day), or high (50 mg/kg/day) doses of CYC202.

RESULTS

On day 27, blood pressure was normal in nephritic controls and was dose-dependently reduced by CYC202. Urinary albumin excretion did not differ between the groups on days 9, 16, 23, and 30. To investigate podocyte injury, we assessed the glomerular de novo expression of desmin, which was markedly up-regulated in almost all passive Heymann nephritis glomeruli but was not significantly different between the three groups. No tubulointerstitial de novo expression of desmin or alpha-smooth muscle actin (alpha-SMA), or tubulointerstitial monocyte/macrophage infiltration was noted in any group. Biologic activity of CYC202 was evident in the form of a dose-dependent decrease in the number of glomerular and tubulointerstitial mitotic figures as compared to vehicle alone. Glomerular immunostaining for cyclin D1, a marker for G(0) to G(1) transition, was significantly decreased in CYC202 treated groups at day 9.

CONCLUSION

Whereas inhibition of CDKs by CYC202 reduced intrarenal cell proliferation in passive Heymann nephritis it did not aggravate podocyte damage, suggesting that this novel therapeutic approach is safe in renal diseases characterized by podocyte injury.

Functional consequences of integrin-linked kinase activation in podocyte damage

BACKGROUND

The delicate foot process architecture of glomerular podocytes critically depends on integrin mediated cell-glomerular basement membrane (GBM) interaction. Integrin signaling via the integrin-linked kinase (ILK) is activated in podocyte damage and associated with considerable podocyte phenotype alterations. ILK has been shown to regulate cell fate via nuclear interaction of beta-catenin with lymphoid enhancer factor (LEF-1) transcription factors. The aim of this study was to elucidate the molecular mechanisms of ILK dependant phenotype regulation in podocytes.

METHODS

ILK function was evaluated in conditionally immortalized murine glomerular epithelial cells using overexpression of ILK and a small molecule ILK inhibitor in puromycin/adriamycin-induced podocyte damage in vitro and in vivo.

RESULTS

Kinase active, but not mutant ILK induced translocation of beta-catenin to the cell nucleus, de novo expression of LEF-1, and nuclear colocalization of beta-catenin and LEF-1. The role of ILK signaling in podocyte damage was evaluated using puromycin, an agent known to cause selective proteinuria and to increase ILK activity. The small molecular ILK inhibitor MC-5 blocked puromycin-induced nuclear translocation of beta-catenin, podocyte detachment, cell proliferation, and repression of the slit membrane molecules P-cadherin and CD2ap. In vivo activation of the beta-catenin pathway could be shown by nuclear colocalization of beta-catenin with WT-1 in adriamycin nephropathy.

CONCLUSION

ILK regulates podocyte cell matrix interaction, proliferation, and slit membrane gene expression in podocyte damage. As this pathway is amendable to pharmacologic intervention, further detailed studies of in vivo ILK function in glomerular disease appear justified.

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

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

Urinary podocyte loss is a more specific marker of ongoing glomerular damage than proteinuria

Podocyte loss contributes to the development of glomerulosclerosis. Although podocyte detachment has been recognized as a new mechanism of podocyte loss in glomerular diseases, its time course and relationship to disease activity are not known. Urinary excretion of viable podocytes was quantified in two models of transient glomerular injury, i.e., rats with puromycin aminonucleoside-induced nephrosis (PAN) and mesangioproliferative nephropathy (anti-Thy 1.1 nephritis model), as well as in a model of continuous glomerular injury, i.e., hypertensive nephropathy (5/6-nephrectomy model), and in aging rats. The number of glomerular Wilm's tumor (WT)-1-positive podocytes and the glomerular expression of cell-cycle proteins in vivo were assessed. Urinary podocyte loss occurred in both primary (PAN) and secondary (anti-Thy 1.1 nephritis) in parallel to the onset of proteinuria. However, subsequently proteinuria persisted despite remission of podocyturia. In continuous glomerular injury, i.e., after 5/6-nephrectomy, podocyturia paralleled the course of proteinuria and of systemic hypertension, whereas no podocyturia became detectable during normal aging (up to 12 mo). Despite podocyte detachment of varying degrees, no decrease in glomerular podocyte counts (i.e., WT-1 positive nuclei) was noted in either disease model. Podocyturia in the PAN and anti-Thy 1.1 nephritis model was preceded by entry of glomerular podocytes into the cell cycle, i.e., cyclin D1, cdc2, and/or proliferating cell nuclear antigen (PCNA) expression. Podocyturia is a widespread phenomenon in glomerular disease and not simply a reflection of proteinuria because it is limited to phases of ongoing glomerular injury. The data suggest that podocyturia may become a more sensitive means to assess the activity of glomerular damage than proteinuria.

Cellular Response to Injury in Membranous Nephropathy

The pathogenesis of membranous nephropathy (MN) involves in situ formation of subepithelial immune deposits that produce glomerular injury by damaging and/or activating podocytes through complement-dependent processes. C5b-9 formation and insertion into podocyte cell membranes causes glomerular injury in MN. C5b-9 in sublytic quantities stimulates podocytes to produce proteases, oxidants, prostanoids, extracellular matrix components, and cytokines including TGF-beta. C5b-9 also causes alterations of the cytoskeleton that lead to abnormal distribution of slit diaphragm protein and detachment of viable podocytes that are shed into Bowman's space. These events result in disruption of the functional integrity of the glomerular basement membrane and the protein filtration barrier of podocytes with subsequent development of massive proteinuria. Complement components in proteinuric urine also induce tubular epithelial cell injury and mediate progressive interstitial disease in MN. Measurements of urinary C5b-9 or podocyte excretion in the urine may be useful in the diagnosis of MN and as measures of disease activity and response to therapy. Recent studies of cell-cycle proteins and DNA damage in podocytes have clarified why podocytes fail to proliferate in response to C5b-9-mediated injury and podocyte loss in MN, resulting in the development of glomerular sclerosis and renal failure. Improved understanding of the role of complement in the pathogenesis of MN and of the cellular response to C5b-9 attack creates several new opportunities for therapeutic intervention that may benefit patients with MN in the future.

A 20-year history of childhood HIV-associated nephropathy

In 1984, physicians in New York and Miami reported HIV-infected adult patients with heavy proteinuria and rapid progression to end-stage renal disease. These patients showed large edematous kidneys with a combination of focal segmental glomerulosclerosis (FSGS) and tubulointerstitial lesions. This renal syndrome, named HIV-associated nephropathy (HIVAN), was found predominantly in African Americans. Subsequent studies confirmed the presence of HIVAN in children, who frequently develop nephrotic syndrome in association with FSGS and/or mesangial hyperplasia with microcystic tubular dilatation. Since then, substantial progress has been made in our understanding of the etiology and pathogenesis of HIVAN. This article reviews 20 years of research into the pathogenesis of HIVAN and discusses how these concepts could be applied to the treatment of children with HIVAN. HIV-1 infection plays a direct role in the pathogenesis of childhood HIVAN, at least partially by affecting the growth and differentiation of glomerular and tubular epithelial cells and enhancing the renal recruitment of infiltrating mononuclear cells and cytokines. An up-regulation of renal heparan sulfate proteoglycans seems to play a relevant role in this process, by increasing the recruitment of heparin-binding growth factors (i.e., FGF-2), chemokines, HIV-infected cells, and viral proteins (i.e., gp120, Tat). These changes enhance the infectivity of HIV-1 in the kidney and induce injury and proliferation of intrinsic renal cells. Highly active anti-retroviral therapy (HAART) appears to be the most promising treatment to prevent the progression of childhood HIVAN. Hopefully, in the near future, better education, prevention, and treatment programs will lead to the eradication of this fatal childhood disease.

Cell-cycle mechanisms involved in podocyte proliferation in cellular lesion of focal segmental glomerulosclerosis

BACKGROUND

Podocyte injury may induce podocyte proliferation, which results in glomerular scarring. The cellular lesion, seen in some patients with primary focal segmental glomerulosclerosis (FSGS), is characterized by proliferation of cells covering the sclerotic or collapsed glomerular tufts. Cell-cycle mechanisms by which podocyte proliferation occurs in the cellular lesion of FSGS are unclear.

METHODS

We examined expression patterns of cyclin D1; cyclin E; cyclin A; cyclin B1; cyclin-dependent kinase (CDK)2; CDK4; such CDK inhibitors as p21WAF1/CIP1 (p21), p27kip1 (p27), and p57kip2 (p57); and Wilms' tumor protein-1 (WT-1) in 12 renal biopsy specimens with the cellular lesion of FSGS and 6 renal biopsy specimens with no detectable abnormalities by immunohistochemistry and immunoelectron microscopy. Messenger RNA (mRNA) expression patterns of cyclin D1, cyclin E, p21, p27, and p57 were evaluated further by in situ hybridization.

RESULTS

In controls, immunostaining for cyclin A, cyclin B1, CDK2, CDK4, and p21 was almost negligible, but positive signals for cyclin D1, cyclin E, p27, and p57 were observed in glomerular epithelial cells (GECs). In the cellular lesion of FSGS, positive signals for cyclin E, cyclin A, cyclin B1, CDK2, and p21 were present in GEC nuclei, in which WT-1, p27, p57, and cyclin D1 were undetected. Immunoelectron microscopy showed that cyclin E-, CDK2-, and p21-specific gold particles were increased significantly in GEC nuclei in the cellular lesion in which cyclin D- and p57-specific particles were absent compared with controls. An in situ hybridization study showed specific signals of cyclin D1, cyclin E, p21, p27, and p57 mRNA in GECs forming the cellular lesion of FSGS.

CONCLUSION

Our results suggest that damaged podocytes may inhibit p27 and p57 protein expression, but activate a cyclin D1-independent cell-cycle mechanism and mitotic cell cyclins to promote GEC proliferation in the cellular lesion of FSGS.

1,25-Dihydroxyvitamin D3 decreases podocyte loss and podocyte hypertrophy in the subtotally nephrectomized rat

1,25(OH)(2)D(3) has antiproliferative effects and promotes cell differentiation. This consideration has provided the rationale for studies in subtotally nephrectomized rats showing that 1,25(OH)(2)D(3) interfered with glomerulosclerosis. The cellular mechanisms involved have remained obscure, however. It was the purpose of the present study to assess glomerular structure and cellular composition in subtotally nephrectomized (SNX) rats treated with nonpharmacological doses of 1,25(OH)(2)D(3). Male Sprague-Dawley rats were sham operated (sham) or underwent SNX under general anesthesia and received either solvent or 1,25(OH)(2)D(3) (3 ng.100 g body wt(-1).day(-1) sc). Blood pressure (BP) and albuminuria were measured. After 16 wk, the remnant renal tissue was perfusion fixed and morphometric and stereological measurements were carried out. The expression of proliferating cellular antigen (PCNA), cyclin-dependent kinase inhibitor p27, Wilms tumor gene (WT1), and desmin, a marker of early podocyte damage, was investigated by immunohistology. BP, serum creatinine, and urinary albumin excretion were significantly higher in SNX than in sham rats. Albuminuria was significantly lower in SNX+1,25(OH)(2)D(3) compared with SNX+solvent rats. Mean glomerular tuft volume was significantly higher in SNX+solvent (2.69 +/- 0.21 gx 10(6) microm(3)) than in sham rats (1.44 +/- 0.17 and 1.28 +/- 0.14 x 10(6) microm(3)); it was significantly (P < 0.05) lower in SNX+1,25(OH)(2)D(3) rats (1.81 +/- 0.16 x 10(6) microm(3)). The main finding was a significantly higher number of podocytes in SNX+1,25(OH)(2)D(3) (88 +/- 9) and sham (98 +/- 17) compared with SNX+solvent rats (81 +/- 8.7). In parallel, the increase in podocyte volume in SNX+solvent rats was abrogated by treatment with 1,25(OH)(2)D(3), and immunohistochemistry revealed less expression of desmin, PCNA, and p27, suggesting less podocyte injury and activation of the cyclin cascade. This study identifies the podocyte as an important target cell for the renoprotective action of 1,25(OH)(2)D(3). This notion is suggested by less evidence of podocyte injury, decreased podocytes loss, and abrogation of podocyte hypertrophy, findings that may also explain less pronounced albuminuria and glomerulosclerosis.

Podocytes that detach in experimental membranous nephropathy are viable

BACKGROUND

Podocyte loss contributes to the development of glomerulosclerosis. Although podocytes have been detected in the urine in certain glomerular diseases, the viability of detached cells is not known.

METHODS

Urine was collected from rats with experimental membranous nephropathy [passive Heymann nephritis (PHN) model], centrifuged, and following resuspension in tissue culture media, cells were seeded onto collagen-coated tissue culture plates. Cells were grown under typical cell culture conditions. Cell number was measured, the cell type was identified by immunostaining with specific antibodies, and cell morphology was assessed by light and electron microscopy.

RESULTS

Cells obtained in the urine from PHN rats were positive for synaptopodin, nephrin, podocin, WT-1, and GLEPP1 (podocyte-specific antigens). When grown ex vivo under cell culture conditions, cells obtained in the urine from PHN rats adhered to tissue culture plates, and expressed podocyte-specific proteins at the mRNA [reverse transcription-polymerase chain reaction (RT-PCR)] and protein (immunostaining) level. Cells did not stain with antibodies to mesangial (OX-7), tubular (Tamm-Horsfall protein) and endothelial (RECA) cells. Electron microscopy showed the presence of foot processes, and podocytes from PHN rats stained positive for C5b-9. Although podocyte number increased transiently during the first 5 days ex vivo, apoptosis increased significantly thereafter, reducing overall cell number.

CONCLUSION

Rats with experimental membranous nephropathy shed podocytes into the urine that attach to tissue culture plates ex-vivo, and proliferate. These results suggest that detached podocytes are viable. These results add new perspectives into our understanding of podocyte loss in the development of glomerulosclerosis.

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

PURPOSE OF REVIEW

The rapidly developing field of podocyte cell biology is reviewed, focusing on papers published in the last 12 months.

RECENT FINDINGS

Four areas of particular progress can be discerned. First, podocytes proliferate during early metanephric development, are quiescent after the capillary loop stage, and re-enter the cell cycle only in the disease group termed collapsing glomerulopathy. We have learned that control of the podocyte cell cycle involves both expression of cell-cycle regulating proteins and the process of cytokinesis. Second, the podocyte slit diaphragm is the final component of the filtration barrier. The structure and maintenance of the slit diaphragm has been a major focus of research activity, and a multiplicity of relevant molecular interactions have been defined. Significant advances have been made in understanding the complex and interacting role of nephrin and podocin mutations in the genesis of clinical glomerular disease. Third, several proteins essential to controlling discrete podocyte transcriptional programs have been defined. Finally, conditionally-immortalized podocyte cell lines, derived from mouse and human tissue, have proven their worth as models to advance investigations of podocyte biology.

SUMMARY

Podocyte injury occurs as a consequence of genetic mutation, immunological injury, viral infection, or abnormal hemodynamic forces within the glomerulus. As we understand more about the podocyte proteome and cell biology, we gain an increasingly detailed molecular understanding of podocyte structure and function. In this drama we have many molecular players and increasing stretches of molecular dialogue, but the script remains largely to be deciphered. Nevertheless, we do understand the consequences that arise when the podocyte cannot put its best foot (processes) forward.