Significance of early phenotypic change of glomerular podocytes detected by Pax2 in primary focal segmental glomerulosclerosis

Colony stimulating factor-1 receptor drives glomerular parietal epithelial cell activation in focal segmental glomerulosclerosis

Parietal epithelial cells (PECs) are kidney progenitor cells with similarities to a bone marrow stem cell niche. In focal segmental glomerulosclerosis (FSGS) PECs become activated and contribute to extracellular matrix deposition. Colony stimulating factor-1 (CSF-1), a hematopoietic growth factor, acts via its specific receptor, CSF-1R, and has been implicated in several glomerular diseases, although its role on PEC activation is unknown. Here, we found that CSF-1R was upregulated in PECs and podocytes in biopsies from patients with FSGS. Through in vitro studies, PECs were found to constitutively express CSF-1R. Incubation with CSF-1 induced CSF-1R upregulation and significant transcriptional regulation of genes involved in pathways associated with PEC activation. Specifically, CSF-1/CSF-1R activated the ERK1/2 signaling pathway and upregulated CD44 in PECs, while both ERK and CSF-1R inhibitors reduced CD44 expression. Functional studies showed that CSF-1 induced PEC proliferation and migration, while reducing the differentiation of PECs into podocytes. These results were validated in the Adriamycin-induced FSGS experimental mouse model. Importantly, treatment with either the CSF-1R-specific inhibitor GW2580 or Ki20227 provided a robust therapeutic effect. Thus, we provide evidence of the role of the CSF-1/CSF-1R pathway in PEC activation in FSGS, paving the way for future clinical studies investigating the therapeutic effect of CSF-1R inhibitors on patients with FSGS.

Mice with a Pax2 missense variant display impaired glomerular repair

PAX2 regulates kidney development, and its expression persists in parietal epithelial cells (PECs), potentially serving as a podocyte reserve. We hypothesized that mice with a Pax2 pathogenic missense variant (Pax2A220G/+) have impaired PEC-mediated podocyte regeneration. Embryonic wild-type mouse kidneys showed overlapping expression of PAX2/Wilms' tumor-1 (WT-1) until PEC and podocyte differentiation, reflecting a close lineage relationship. Embryonic and adult Pax2A220G/+ mice have reduced nephron number but demonstrated no glomerular disease under baseline conditions. Pax2A220G/+ mice compared with wild-type mice were more susceptible to glomerular disease after adriamycin (ADR)-induced podocyte injury, as demonstrated by worsened glomerular scarring, increased podocyte foot process effacement, and podocyte loss. There was a decrease in PAX2-expressing PECs in wild-type mice after adriamycin injury accompanied by the occurrence of PAX2/WT-1-coexpressing glomerular tuft cells. In contrast, Pax2A220G/+ mice showed no changes in the numbers of PAX2-expressing PECs after adriamycin injury, associated with fewer PAX2/WT-1-coexpressing glomerular tuft cells compared with injured wild-type mice. A subset of PAX2-expressing glomerular tuft cells after adriamycin injury was increased in Pax2A220G/+ mice, suggesting a pathological process given the worse outcomes observed in this group. Finally, Pax2A220G/+ mice have increased numbers of glomerular tuft cells expressing Ki-67 and cleaved caspase-3 compared with wild-type mice after adriamycin injury, consistent with maladaptive responses to podocyte loss. Collectively, our results suggest that decreased glomerular numbers in Pax2A220G/+ mice are likely compounded with the inability of their mutated PECs to regenerate podocyte loss, and together these two mechanisms drive the worsened focal segmental glomerular sclerosis phenotype in these mice.NEW & NOTEWORTHY Congenital anomalies of the kidney and urinary tract comprise some of the leading causes of kidney failure in children, but our previous study showed that one of its genetic causes, PAX2, is also associated with adult-onset focal segmental glomerular sclerosis. Using a clinically relevant model, our present study demonstrated that after podocyte injury, parietal epithelial cells expressing PAX2 are deployed into the glomerular tuft to assist in repair in wild-type mice, but this mechanism is impaired in Pax2A220G/+ mice.

A Biomimetic Electrospun Membrane Supports the Differentiation and Maturation of Kidney Epithelium from Human Stem Cells

Podocytes derived from human induced pluripotent stem (hiPS) cells are enabling studies of kidney development and disease. However, many of these studies are carried out in traditional tissue culture plates that do not accurately recapitulate the molecular and mechanical features necessary for modeling tissue- and organ-level functionalities. Overcoming these limitations requires the design and application of tunable biomaterial scaffolds. Silk fibroin is an attractive biomaterial due to its biocompatibility and versatility, which include its ability to form hydrogels, sponge-like scaffolds, and electrospun fibers and membranes appropriate for tissue engineering and biomedical applications. In this study, we show that hiPS cells can be differentiated into post-mitotic kidney glomerular podocytes on electrospun silk fibroin membranes functionalized with laminin. The resulting podocytes remain viable and express high levels of podocyte-specific markers consistent with the mature cellular phenotype. The resulting podocytes were propagated for at least two weeks, enabling secondary cell-based applications and analyses. This study demonstrates for the first time that electrospun silk fibroin membrane can serve as a supportive biocompatible platform for human podocyte differentiation and propagation. We anticipate that the results of this study will pave the way for the use of electrospun membranes and other biomimetic scaffolds for kidney tissue engineering, including the development of co-culture systems and organs-on-chips microphysiological devices.

PAX2 Mutation-Related Oligomeganephronia in a Young Adult Patient

Oligomeganephronic hypoplasia, commonly referred to as oligomeganephronia (OMN), is a rare pediatric disorder characterized by small kidneys. Histologically a paucity of nephrons is observed which show compensatory enlargement. Hyperfiltration injury leads to end-stage kidney disease. Here we report a 23-year-old Caucasian female patient who presented with a 7-year history of nonnephrotic proteinuria, slow worsening of renal function, normal-sized kidneys, normal blood pressure, healthy weight, and normoglycemia. Evaluation of a kidney biopsy specimen revealed sparsely distributed and markedly enlarged glomeruli (glomerular density 0.63/mm², glomerular diameter 268 µm), focal segmental glomerulosclerosis (FSGS), and 70% effacement of the foot processes. The glomerular basement membrane was normal (mean thickness 285 nm). The genetic analysis of 19 genes known to cause FSGS identified a heterozygous de novo nonsense mutation of PAX2 in exon 4 (NM_003990.3:c.430C>T and NP_003981.2:p.Gln144Ter). Clinical investigations ruled out optic nerve coloboma, hearing loss, and vesicoureteral reflux. Magnetic resonance imaging of the urogenital tract found the uterus to be bicornuate. Based on these data, OMN in nonhypoplastic kidneys and adaptive FSGS related to PAX2 mutation was diagnosed. Her kidney function worsened during the 30-month follow-up (last visit: eGFR-EPI 32 mL/min/1.73 m²) despite angiotensin-converting enzyme inhibitor treatment. To our best knowledge, our patient is the seventh in the English-language literature with a biopsy diagnosis of OMN in an adult, the first observed with normal-sized kidneys, and the first in whom a specific etiologic genetic diagnosis was established. Nonsense PAX2 mutations between the paired domain and the octapeptide domain appear to manifest in renal-limited phenotype.

Prolonged podocyte depletion in larval zebrafish resembles mammalian focal and segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) is a histological pattern frequently found in patients with nephrotic syndrome that often progress to end-stage kidney disease. The initial step in development of this histologically defined entity is injury and ultimately depletion of podocytes, highly arborized interdigitating cells on the glomerular capillaries with important function for the glomerular filtration barrier. Since there are still no causal therapeutic options, animal models are needed to develop new treatment strategies. Here, we present an FSGS-like model in zebrafish larvae, an eligible vertebrate model for kidney research. In a transgenic zebrafish strain, podocytes were depleted, and the glomerular response was investigated by histological and morphometrical analysis combined with immunofluorescence staining and ultrastructural analysis by transmission electron microscopy. By intravenous injection of fluorescent high-molecular weight dextran, we confirmed leakage of the size selective filtration barrier. Additionally, we observed severe podocyte foot process effacement of remaining podocytes, activation of proximal tubule-like parietal epithelial cells identified by ultrastructural cytomorphology, and expression of proximal tubule markers. These activated cells deposited extracellular matrix on the glomerular tuft which are all hallmarks of FSGS. Our findings indicate that glomerular response to podocyte depletion in larval zebrafish resembles human FSGS in several important characteristics. Therefore, this model will help to investigate the disease development and the effects of potential drugs in a living organism.

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.

Modulation of apolipoprotein L1-microRNA-193a axis prevents podocyte dedifferentiation in high-glucose milieu

The loss of podocyte (PD) molecular phenotype is an important feature of diabetic podocytopathy. We hypothesized that high glucose (HG) induces dedifferentiation in differentiated podocytes (DPDs) through alterations in the apolipoprotein (APO) L1-microRNA (miR) 193a axis. HG-induced DPD dedifferentiation manifested in the form of downregulation of Wilms' tumor 1 (WT1) and upregulation of paired box 2 (PAX2) expression. WT1-silenced DPDs displayed enhanced expression of PAX2. Immunoprecipitation of DPD cellular lysates with anti-WT1 antibody revealed formation of WT1 repressor complexes containing Polycomb group proteins, enhancer of zeste homolog 2, menin, and DNA methyltransferase (DNMT1), whereas silencing of either WT1 or DNMT1 disrupted this complex with enhanced expression of PAX2. HG-induced DPD dedifferentiation was associated with a higher expression of miR193a, whereas inhibition of miR193a prevented DPD dedifferentiation in HG milieu. HG downregulated DPD expression of APOL1. miR193a-overexpressing DPDs displayed downregulation of APOL1 and enhanced expression of dedifferentiating markers; conversely, silencing of miR193a enhanced the expression of APOL1 and preserved DPD phenotype. Moreover, stably APOL1G0-overexpressing DPDs displayed the enhanced expression of WT1 but attenuated expression of miR193a; nonetheless, silencing of APOL1 reversed these effects. Since silencing of APOL1 enhanced miR193a expression as well as dedifferentiation in DPDs, it appears that downregulation of APOL1 contributed to dedifferentiation of DPDs through enhanced miR193a expression in HG milieu. Vitamin D receptor agonist downregulated miR193a, upregulated APOL1 expression, and prevented dedifferentiation of DPDs in HG milieu. These findings suggest that modulation of the APOL1-miR193a axis carries a potential to preserve DPD molecular phenotype in HG milieu.

Association of PAX2 and Other Gene Mutations with the Clinical Manifestations of Renal Coloboma Syndrome

BACKGROUND

Renal coloboma syndrome (RCS) is characterized by renal anomalies and optic nerve colobomas. PAX2 mutations contribute to RCS. However, approximately half of the patients with RCS have no mutation in PAX2 gene.

METHODS

To investigate the incidence and effects of mutations of PAX2 and 25 candidate genes, patient genes were screened using next-generation sequence analysis, and candidate mutations were confirmed using Sanger sequencing. The correlation between mutations and clinical manifestation was evaluated.

RESULT

Thirty patients, including 26 patients (two families of five and two, 19 sporadic cases) with RCS, and 4 optic nerve coloboma only control cases were evaluated in the present study. Six PAX2 mutations in 21 probands [28%; two in family cohorts (n = 5 and n = 2) and in 4 out of 19 patients with sporadic disease] including four novel mutations were confirmed using Sanger sequencing. Moreover, four other sequence variants (CHD7, SALL4, KIF26B, and SIX4) were also confirmed, including a potentially pathogenic novel KIF26B mutation. Kidney function and proteinuria were more severe in patients with PAX2 mutations than in those without the mutation. Moreover, the coloboma score was significantly higher in patients with PAX2 gene mutations. Three out of five patients with PAX2 mutations had focal segmental glomerulosclerosis (FSGS) diagnosed from kidney biopsies.

CONCLUSION

The results of this study identify several new mutations of PAX2, and sequence variants in four additional genes, including a novel potentially pathogenic mutation in KIF26B, which may play a role in the pathogenesis of RCS.

Influence of bushenhuoxue on podocytes of focal segmental glomerulosclerosis mice

OBJECTIVE

To observe the effects and mechanisms of Bushenhuoxue on desmin and nephrin expression in mice podocytes, and to investigate its effects on wt1 expression in Wilms' tumor.

METHODS

Adriamycin (ADR) was used to induce focal segmental glomerulous sclerosis (FSGS) in mice. Bushenhuoxue was used to treat FSGS for 6 weeks. We measured body mass and right renal mass, and determined serum albumin (ALB) levels, protein content in urine, and urinary protein and albumin creatinine ratio (UACR). Changes in renal tissue morphology were evaluated by microscopy. wt1 and nephrin expression in podocytes were detected using immunofluorescence. Expression levels of desmin, wt1 and nephrin mRNAs in renal tissue were determined using reverse transcription polymerase chain reaction assays.

RESULTS

Protein levels in urine and UACR were significantly increased in FSGS model mice compared with Bushenhuoxue-treated and control mice. Body mass and ALB levels were decreased in FSGS mice compared with control and Bushenhuoxue-treated mice. Expression of the wt1 protein was observed in control mice. Compared with controls, wt1 expression levels were reduced in Bushenhuoxue-treated mice, and to a greater extent in FSGS mice. Nephrin protein expression was widespread in FSGS mice, and significantly reduced in control and Bushenhuoxue mice. Expression levels of wt1 and nephrin mRNAs in FSGS mice were lower compared with those in control and Bushenhuoxue-treated mice. Desmin mRNA levels in FSGS mice were reduced compared with those in control and Bushenhuoxue-treated mice.

CONCLUSION

Bushenhuoxue ameliorated albuminuria in FSGS mice; this was possibly related to the up-regulation of wt1 and nephrin, and down-regulation of desmin.

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.

The pathogenesis of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a histologic pattern of injury on kidney biopsy that can arise from a diverse range of causes and mechanisms. Although primary and secondary forms are described based on the underlying cause, there are many common factors that underlie the development of this segmental injury. In this review, we will describe the currently accepted model for the pathogenesis of classic FSGS and review the data supporting this model. Although the podocyte is considered the major target of injury in FSGS, we will also highlight the contributions of other resident glomerular cells in the development of FSGS.

Balance between the two kinin receptors in the progression of experimental focal and segmental glomerulosclerosis in mice

Focal and segmental glomerulosclerosis (FSGS) is one of the most important renal diseases related to end-stage renal failure. Bradykinin has been implicated in the pathogenesis of renal inflammation, whereas the role of its receptor 2 (B2RBK; also known as BDKRB2) in FSGS has not been studied. FSGS was induced in wild-type and B2RBK-knockout mice by a single intravenous injection of Adriamycin (ADM). In order to further modulate the kinin receptors, the animals were also treated with the B2RBK antagonist HOE-140 and the B1RBK antagonist DALBK. Here, we show that the blockage of B2RBK with HOE-140 protects mice from the development of FSGS, including podocyte foot process effacement and the re-establishment of slit-diaphragm-related proteins. However, B2RBK-knockout mice were not protected from FSGS. These opposite results were due to B1RBK expression. B1RBK was upregulated after the injection of ADM and this upregulation was exacerbated in B2RBK-knockout animals. Furthermore, treatment with HOE-140 downregulated the B1RBK receptor. The blockage of B1RBK in B2RBK-knockout animals promoted FSGS regression, with a less-inflammatory phenotype. These results indicate a deleterious role of both kinin receptors in an FSGS model and suggest a possible cross-talk between them in the progression of disease.

Renal progenitors and childhood: from development to disorders

Nephropathies arise from conditions that alter nephron development or trigger nephron damage during neonatal, juvenile, and adult stages of life. Much evidence suggests that a key role in maintaining kidney integrity, homeostasis, and regenerative capacity is played by a population of progenitor cells resident in the organ. Although the primary goals in the field of renal progenitor cells are understanding their ability to regenerate nephrons and to restore damaged kidney function, the discovery of these cells could also be used to elucidate the molecular and pathophysiological basis of kidney diseases. As a result, once the identification of a subset of progenitor cells capable of kidney regeneration has been obtained, the increasing knowledge about their characteristics and about the mechanisms of renal development had pointed out the possibility of understanding the molecular basis of kidney diseases, so that, nowadays, some renal disorders could also be related to renal progenitor dysfunction. In this review, we summarize the evidence on the existence of renal progenitors in fetal and adult kidneys and discuss their role in physiology as well as in the pathogenesis of renal disorders with a particular focus on childhood age.

Podocyte dedifferentiation: a specialized process for a specialized cell

The podocyte is one of the two cell types that contribute to the formation of the glomerular filtration barrier (GFB). It is a highly specialized cell with a unique structure. The key feature of the podocyte is its foot processes that regularly interdigitate. A structure known as the slit diaphragm can be found bridging the interdigitations. This molecular sieve comprises the final layer of the GFB. It is well accepted that the podocyte is the target cell in the pathogenesis of nephrotic syndrome. In nephrotic syndrome, the GFB no longer restricts the passage of macromolecules and protein is lost into the urine. A number of phenotypic and morphological changes are seen in the diseased podocyte and in the literature these have been described as an epithelial-mesenchymal transition (EMT). However, there is a growing appreciation that this term does not accurately describe the changes that are seen. Definitions of type-2 EMT are based on typical epithelial cells. While the podocyte is known as a visceral epithelial cell, it is not a typical epithelial cell. Moreover, podocytes have several features that are more consistent with mesenchymal cells. Therefore, we suggest that the term podocyte disease transformation is more appropriate.

The direction and role of phenotypic transition between podocytes and parietal epithelial cells in focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a podocyte disease. Among the various histologies of FSGS, active epithelial changes, hyperplasia, as typically seen in the collapsing variant, indicates disease progression. Using a podocyte-specific injury model of FSGS carrying a genetic podocyte tag combined with double immunostaining by different sets of podocytes and parietal epithelial cell (PEC) markers [nestin/Pax8, Wilms' tumor-1 (WT1)/claudin1, and podocalyxin/Pax2], we investigated the direction of epithelial phenotypic transition and its role in FSGS. FSGS mice showed progressive proteinuria and renal dysfunction often accompanied by epithelial hyperplasia, wherein 5-bromo-4-chloro-3-indoyl β-d-galactoside (X-gal)-positive podocyte-tagged cells were markedly decreased. The average numbers of double-positive cells in all sets of markers were significantly increased in the FSGS mice compared with the controls. In addition, the average numbers of double-positive cells for X-gal/Pax8, nestin/Pax8 and podocalyxin/Pax2 staining in the FSGS mice were comparable, whereas those of WT1/claudin1 were significantly increased. When we divided glomeruli from FSGS mice into those with FSGS lesions and those without, double-positive cells tended to be more closely associated with glomeruli without FSGS lesions compared with those with FSGS lesions. Moreover, the majority of double-positive cells appeared to be isolated and very rarely associated with FSGS lesions (1/1,997 glomeruli). This study is the first to show the incidence and localization of epithelial cells with phenotypical changes in FSGS using a genetic tag. The results suggest that the major direction of epithelial phenotypic transition in cellular FSGS is from podocytes to PECs and that these cells were less represented in the active lesions of FSGS.

Glomerular proteinuria: a complex interplay between unique players

Protein leak in the urine is a harbinger of disruption of the glomerular filtration barrier. It also correlates with disease progression and development of ESRD. At present, therapies are aimed at decreasing proteinuria to decrease further damage to the filter and as a marker of remission. Understanding the mechanism of molecular events that lead to protein leak is vital to developing new therapeutic interventions. There has been tremendous progress over the last decade in identifying gene defects which result in hereditary proteinuric defects. This has led to identifying pathways by which these genes regulate the structure and function of the components of the filtration barrier, namely the podocytes, mesangial cells, endothelial cells, and the basement membrane. Using gene knockout mouse models, a role of tubular cells in regulating proteinuria is also emerging. In this review, we have attempted to present some of the prevailing understanding of the underlying mechanisms and physiology of proteinuria.

Focal segmental glomerulosclerosis plays a major role in the progression of IgA nephropathy. II. Light microscopic and clinical studies

It is well known that lesions morphologically identical with focal segmental glomerulosclerosis (FSGS) may appear in IgA nephropathy (IgAN). Capsular adhesions without underlying abnormalities in the tuft, often the first sign of FSGS, are frequent in IgAN. In this retrospective study, a new cohort of 128 adult patients with IgAN was used to validate the new Oxford classification system of IgAN, and shown to have highly significant associations with clinical and outcome parameters. We then used these patients to determine the extent to which IgAN could be accounted for in terms of FSGS. Some form of lesion consistent with FSGS, notably hyalinosis and collapsing glomerulopathy, was found in 101 of these patients. No glomerular lesions were found in 16 patients, and 11 had mild lesions not definable as FSGS. Those with FSGS had significantly worse renal survival at 80 months than those without. Comparison of pure forms of FSGS (excluding collapsing glomerulopathy) with cases of FSGS having other glomerular lesions (mesangial hyperplasia, endocapillary hypercellularity, glomerular necroses, extracapillary proliferation) revealed that those with FSGS and other superimposed lesions did significantly worse than cases of pure FSGS at 80 months following diagnosis. Importantly, patients with pure FSGS had relatively poor survival even without other superimposed glomerular abnormalities. Thus, the majority of cases of IgAN can be interpreted as representing one or another variant of FSGS. Hence, interpreting IgAN in terms of FSGS emphasizes the role that podocyte lesions may play in the pathogenesis and progression of this disease.

Focal segmental glomerulosclerosis plays a major role in the progression of IgA nephropathy. I. Immunohistochemical studies

IgA nephropathy (IgAN) often shows lesions morphologically identical with those of focal segmental glomerulosclerosis (FSGS). In order to determine the possible role of FSGS in IgAN lesions, we measured glomerular capsular adhesions, often the first step toward FSGS, in biopsies from 127 patients with IgAN, 100 with lupus nephritis, and 26 with primary FSGS. Capsular adhesions with no lesions in the underlying tuft, consistent with podocyte abnormality or loss, were found regularly in FSGS and IgAN, but infrequently in lupus. Fifteen biopsies of patients with IgAN were studied immunohistochemically using markers for podocytes, Bowman's parietal epithelial cells, proliferating cells, and macrophages. Cytokeratins CK-8 and C2562 differentiated normal podocytes (negative) from parietal epithelial cells (variably positive). There was focal loss of the podocyte markers synaptopodin, glomerular epithelial protein 1 (GLEPP-1), nephrin, and vascular endothelial growth factor (VEGF), particularly at sites of capsular adhesions in otherwise histologically normal glomeruli. Cells displaying the parietal epithelial cell markers PAX2 (paired box gene 2) and the cytokeratins were also positive for the proliferating cell marker, proliferating cell nuclear antigen. These cells gathered at sites of adhesion, and in response to active lesions in the tuft, grew inward along the adhesion onto the tuft, forming a monolayer positive for parietal markers and the podocyte marker Wilms tumor protein-1 (WT-1). These cells deposited a layer of collagen over the sclerosing tuft. Thus, all biopsies of patients with IgAN had changes basically identical to those classically described in FSGS. Hence, our study strongly suggests that podocytopathy of a type similar to that in primary FSGS occurs frequently in IgAN.

Tracing the origin of glomerular extracapillary lesions from parietal epithelial cells

Cellular lesions form in Bowman's space in both crescentic glomerulonephritis and collapsing glomerulopathy. The pathomechanism and origin of the proliferating cells in these lesions are unknown. In this study, we examined proliferating cells by lineage tracing of either podocytes or parietal epithelial cells (PECs) in the nephrotoxic nephritis model of inflammatory crescentic glomerulonephritis. In addition, we traced the fate of genetically labeled PECs in the Thy-1.1 transgenic mouse model of collapsing glomerulopathy. In both models, cellular bridges composed of PECs were observed between Bowman's capsule and the glomerular tuft. Genetically labeled PECs also populated larger, more advanced cellular lesions. In these lesions, we detected de novo expression of CD44 in activated PECs. In contrast, we rarely identified genetically labeled podocytes within the cellular lesions of crescentic glomerulonephritis. In conclusion, PECs constitute the majority of cells that compose early extracapillary proliferative lesions in both crescentic glomerulonephritis and collapsing glomerulopathy, suggesting similar pathomechanisms in both diseases.

Nephrin deficiency activates NF-kappaB and promotes glomerular injury

Increasing evidence implicates activation of NF-kappaB in a variety of glomerular diseases, but the mechanisms involved are unknown. Here, upregulation of NF-kappaB in the podocytes of transgenic mice resulted in glomerulosclerosis and proteinuria. Absence of the podocyte protein nephrin resulted in NF-kappaB activation, suggesting that nephrin negatively regulates the NF-kappaB pathway. Signal transduction assays supported a functional relationship between nephrin and NF-kappaB and suggested the involvement of atypical protein kinase C (aPKCzeta/lambda/iota) as an intermediary. We propose that disruption of the slit diaphragm leads to activation of NF-kappaB; subsequent upregulation of NF-kappaB-driven genes results in glomerular damage mediated by NF-kappaB-dependent pathways. In summary, nephrin may normally limit NF-kappaB activity in the podocyte, suggesting a mechanism by which it might discourage the evolution of glomerular disease.

Genetic podocyte lineage reveals progressive podocytopenia with parietal cell hyperplasia in a murine model of cellular/collapsing focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a progressive renal disease, and the glomerular visceral cell hyperplasia typically observed in cellular/collapsing FSGS is an important pathological factor in disease progression. However, the cellular features that promote FSGS currently remain obscure. To determine both the origin and phenotypic alterations in hyperplastic cells in cellular/collapsing FSGS, the present study used a previously described FSGS model in p21-deficient mice with visceral cell hyperplasia and identified the podocyte lineage by genetic tagging. The p21-deficient mice with nephropathy showed significantly higher urinary protein levels, extracapillary hyperplastic indices on day 5, and glomerular sclerosis indices on day 14 than wild-type controls. X-gal staining and immunohistochemistry for podocyte and parietal epithelial cell (PEC) markers revealed progressive podocytopenia with capillary collapse accompanied by PEC hyperplasia leading to FSGS. In our investigation, non-tagged cells expressed neither WT1 nor nestin. Ki-67, a proliferation marker, was rarely associated with podocytes but was expressed at high levels in PECs. Both terminal deoxynucleotidyl transferase dUTP nick-end labeling staining and electron microscopy failed to show evidence of significant podocyte apoptosis on days 5 and 14. These findings suggest that extensive podocyte loss and simultaneous PEC hyperplasia is an actual pathology that may contribute to the progression of cellular/collapsing FSGS in this mouse model. Additionally, this is the first study to demonstrate the regulatory role of p21 in the PEC cell cycle.

Drug-induced Kidney Disease – Pathology and Current Concepts

The kidneys can be damaged by a large number of therapeutic agents. The aim of this article is to discuss the pathological features of drug-induced renal disease as diagnosed by kidney biopsy. The literature is reviewed and cases seen by the authors that have a known drug association are analysed. Mechanisms of injury are varied and all renal structures may be affected. The tubulointerstitial compartment is most frequently involved, but glomerular and vascular lesions are seen in a significant proportion of cases. Key words: Drug, Kidney, Nephrotoxicity, Pathology

TGF-beta1 and integrin synergistically facilitate the differentiation of rat podocytes by increasing alpha-smooth muscle actin expression

Phenotypic changes can be found in certain glomerular diseases, and the cell origin is not defined. This study was designed to identify whether podocytes can differentiate by the expression of alpha-smooth muscle actin (alpha-SMA), under the effects of TGF-beta(1) (transforming growth factor-beta(1)) and integrin. Western and Northern blot analyses were performed to identify the protein and mRNA (messenger ribonucleic acid) expression of alpha-SMA. The number of podocytes, which express alpha-SMA, was measured by immunocytochemical staining. The results showed that TGF-beta(1) dose-dependently increased alpha-SMA protein and mRNA expression at 4 and 2 days, respectively. TGF-beta(1) also dose-dependently increased the alpha-SMA staining of podocytes. The alpha-SMA-positive podocytes showed front-end and back-end polarity. The integrinalpha3beta(1) antagonists, anti-integrinbeta(1) monoclonal antibody and Gly-Arg-Gly-Asp (GRGD), decreased the expression of alpha-SMA protein and the percentage of alpha-SMA positive cells stimulated by TGF-beta(1) (both P < 0.01). The addition of calphostin [inhibitor of protein kinase C (PKC)] and genistein [inhibitor of focal adhesion kinase (FAK)] also decreased the expression of alpha-SMA protein and the percentage of alpha-SMA positive cells stimulated by TGF-beta(1) (both P < 0.01). In conclusion, this study indicated that TGF-beta(1) may act synergistically with integrins, through activation of PKC and FAK, to induce the phenotypic changes of rat podocytes with increasing alpha-SMA expression.

Role of PAX2 gene polymorphisms in Henoch-Schonlein purpura nephritis

OBJECTIVE

To investigate the distribution of polymorphisms in the PAX2 gene in children with Henoch-Schonlein purpura with and without nephritis (HSPN and HSP, respectively), with particular attention to the relationship between PAX2 gene polymorphisms and the development of kidney pathology.

METHODS

Genomic DNA was extracted from the peripheral leukocytes of 39 HSPN patients, 23 HSP patients without nephritis and 100 normal children, and three known single nucleotide polymorphisms (SNP), including 1410C>T, 1521A>C and 1544C>T in exon 8 and exon 9 of the PAX2 gene were studied as the candidate polymorphisms. The above two exons were amplified, the polymerase chain reaction (PCR) products were detected by denatured high-pressure liquid chromatography and direct DNA sequencing was performed for sequences with abnormal elution peaks.

RESULTS

In all samples confirmed by direct sequencing, we identified two SNP, which present as complete linkage haplotype 1410C>T + 1521A>C, in exon 8. We did not identify any SNP in exon 9. The frequency of the PAX2 heterozygous genotype 1410CT/1521AC in the HSPN group (28.20%) was significantly higher than in the HSP without HSPN group (4.35%) or in the control group (12.00%) (P < 0.05). The odds ratio (OR) values for HSPN and HSP were 6.05 and 2.62, respectively, and the 95% confidence intervals (CI) were 1.23-29.78 and 1.09-6.30, respectively. However, no differences in the frequency distribution was found between the HSP without nephritis and normal groups. Furthermore, there was no significant correlation between the polymorphism and clinical manifestation or kidney pathology in the HSPN group (P > 0.05).

CONCLUSION

The 1410CT/1521AC PAX2 genotype does not increase susceptibility for HSP, but is likely to increase the susceptibility of kidney involvement, resulting in a HSPN diagnosis.

The parietal epithelial cell is crucially involved in human idiopathic focal segmental glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is one of the most common patterns of glomerular injury encountered in human renal biopsies. Epithelial hyperplasia, which can be prominent in FSGS, has been attributed to dedifferentiation and proliferation of podocytes. Based on observations in a mouse model of FSGS, we pointed to the role of parietal epithelial cells (PECs). In the present study we investigated the relative role of PECs and podocytes in human idiopathic FSGS.

METHODS

We performed a detailed study of lesions from a patient with recurrent idiopathic FSGS by serial sectioning, marker analysis and three-dimensional reconstruction of glomeruli. We have studied the expression of markers for podocytes, PECs, mesangial cells, endothelium, and myofibroblasts. We also looked at proliferation and composition of the deposited extracellular matrix (ECM).

RESULTS

We found that proliferating epithelial cells in FSGS lesions are negative for podocyte and macrophage markers, but stain for PEC markers. The composition of the matrix deposited by these cells is identical to Bowman's capsule.

CONCLUSION

Our study demonstrates that PECs are crucially involved in the pathogenesis of FSGS lesions.

A WT1 co-regulator controls podocyte phenotype by shuttling between adhesion structures and nucleus

Glomerular podocyte differentiation state is critical for filtration barrier function and is regulated by WT1, a zinc finger transcription factor. A yeast two-hybrid assay identified a novel, WT1-interacting protein (WTIP) that maps to human chromosome 19q13.1, a region with genes linked to familial focal segmental glomerulosclerosis. The domain structure of WTIP is similar to the zyxin subfamily of cytosolic LIM domain-containing proteins, which contain three carboxyl-terminal LIM protein-protein interaction domains and a proline-rich, pre-LIM region with a nuclear export signal. Other LIM domain-containing proteins (zyxin and mouse muscle LIM protein) did not interact with WT1 in two-hybrid assays, and WTIP did not interact with an unrelated transcription factor, LMX1B. WTIP mRNA was detected in cultured podocytes and was developmentally regulated, with expression peaking in mouse kidney at embryonic day 15-16 (E15-E16) in kidney but persisting into adulthood. In situ hybridization demonstrated WTIP expression in developing E15 glomeruli and in cultured podocytes. The partial WTIP clone, which interacted with WTIP in the two-hybrid assay, co-localized with WT1 in nuclei, co-precipitated with WT1, and inhibited WT1-dependent transcriptional activation of the amphiregulin promoter. In contrast, full-length WTIP was excluded from cell nuclei, but after the addition of leptomycin B, an inhibitor of Crm1-mediated nuclear export, it accumulated in the nucleus and co-precipitated with WT1 in whole cell lysates. Epitope-tagged WTIP co-localized with the adaptor protein CD2AP (CMS) in podocyte actin spots and with Mena at cell-cell junctions. We propose that WTIP monitors slit diaphragm protein assembly as part of a multiple protein complex, linking this specialized adhesion junction to the actin cytoskeleton, and shuttles into the nucleus after podocyte injury, providing a mechanism whereby changes in slit diaphragm structure modulate gene expression.

Expression of nephrin, podocin, alpha-actinin, and WT1 in children with nephrotic syndrome

Recently, nephrin, podocin, alpha-actinin, and WT1, which are located at the slit diaphragm and expressed by the podocyte, were found to be causative in congenital/familial nephrotic syndrome (NS), but their role in acquired NS remains unclear. We studied their expression in NS with the aim of disclosing their possible role in the development of proteinuria. Immunofluorescence, confocal microscopy, and image analysis were used to study the expression and the distribution in 19 children with primary NS, 9 with isolated hematuria, and 9 controls. All the children with NS presented with heavy proteinuria and foot process effacement was identified by electron microscopy. No proteinuria and foot process effacement was seen in the group with hematuria. A dramatic decrease of podocin expression was found in NS (86.66+/-22.74) compared with control groups ( P=0.014). Furthermore, we also found the pattern of distribution of nephrin, podocin, and alpha-actinin changed in children with NS. In conclusion, a dramatic decrease of podocin expression and abnormal distribution of nephrin, podocin, and alpha-actinin were found in children with NS. No differences were found in children with isolated hematuria, suggesting involvement of these molecules in the development of proteinuria in primary NS.

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.