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

NEF-Induced HIV-Associated Nephropathy Through HCK/LYN Tyrosine Kinases

HIV-1-associated nephropathy (HIVAN) is a severe complication of HIV-1 infection. To gain insight into the pathogenesis of kidney disease in the setting of HIV, a transgenic (Tg) mouse model [CD4C/HIV-negative regulator factor (Nef)] was used in which HIV-1 nef expression is under control of regulatory sequences (CD4C) of the human CD4 gene, thus allowing expression in target cells of the virus. These Tg mice develop a collapsing focal segmental glomerulosclerosis associated with microcystic dilatation, similar to human HIVAN. To identify kidney cells permissive to the CD4C promoter, CD4C reporter Tg lines were used. They showed preferential expression in glomeruli, mainly in mesangial cells. Breeding CD4C/HIV Tg mice on 10 different mouse backgrounds showed that HIVAN was modulated by host genetic factors. Studies of gene-deficient Tg mice revealed that the presence of B and T cells and that of several genes was dispensable for the development of HIVAN: those involved in apoptosis (Trp53, Tnfsf10, Tnf, Tnfrsf1b, and Bax), in immune cell recruitment (Ccl3, Ccl2, Ccr2, Ccr5, and Cx3cr1), in nitric oxide (NO) formation (Nos3 and Nos2), or in cell signaling (Fyn, Lck, and Hck/Fgr). However, deletion of Src partially and that of Hck/Lyn largely abrogated its development. These data suggest that Nef expression in mesangial cells through hematopoietic cell kinase (Hck)/Lck/Yes novel tyrosine kinase (Lyn) represents important cellular and molecular events for the development of HIVAN in these Tg mice.

Differences in Immunohistochemical and Ultrastructural Features between Podocytes and Parietal Epithelial Cells (PECs) Are Observed in Developing, Healthy Postnatal, and Pathologically Changed Human Kidneys

During human kidney development, cells of the proximal nephron gradually differentiate into podocytes and parietal epithelial cells (PECs). Podocytes are terminally differentiated cells that play a key role in both normal and pathological kidney function. Therefore, the potential of podocytes to regenerate or be replaced by other cell populations (PECs) is of great interest for the possible treatment of kidney diseases. In the present study, we analyzed the proliferation and differentiation capabilities of podocytes and PECs, changes in the expression pattern of nestin, and several early proteins including WNT4, Notch2, and Snail, as well as Ki-67, in tissues of developing, postnatal, and pathologically changed human kidneys by using immunohistochemistry and electron microscopy. Developing PECs showed a higher proliferation rate than podocytes, whereas nestin expression characterized only podocytes and pathologically changed kidneys. In the developing kidneys, WNT4 and Notch2 expression increased moderately in podocytes and strongly in PECs, whereas Snail increased only in PECs in the later fetal period. During human kidney development, WNT4, Notch2, and Snail are involved in early nephrogenesis control. In kidneys affected by congenital nephrotic syndrome of the Finnish type (CNF) and focal segmental glomerulosclerosis (FSGS), WNT4 decreased in both cell populations, whereas Notch2 decreased in FSGS. In contrast, Snail increased both in CNF and FSGS, whereas Notch2 increased only in CNF. Electron microscopy revealed cytoplasmic processes spanning the urinary space between the podocytes and PECs in developing and healthy postnatal kidneys, whereas the CNF and FSGS kidneys were characterized by numerous cellular bridges containing cells with strong expression of nestin and all analyzed proteins. Our results indicate that the mechanisms of gene control in nephrogenesis are reactivated under pathological conditions. These mechanisms could have a role in restoring glomerular integrity by potentially inducing the regeneration of podocytes from PECs.

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

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

Recurrent Glomerulonephritis after Renal Transplantation: The Clinical Problem

Glomerulonephritis (GN) continues to be one of the main causes of end-stage kidney disease (ESKD) with an incidence rating from 10.5% to 38.2%. Therefore, recurrent GN, previously considered to be a minor contributor to graft loss, is the third most common cause of graft failure 10 years after renal transplantation. However, the incidence, pathogenesis, and natural course of recurrences are still not completely understood. This review focuses on the most frequent diseases that recur after renal transplantation, analyzing rate of recurrence, epidemiology and risk factors, pathogenesis and bimolecular mechanisms, clinical presentation, diagnosis, and therapy, taking into consideration the limited data available in the literature. First of all, the risk for recurrence depends on the type of glomerulonephritis. For example, recipient patients with anti-glomerular basement membrane (GBM) disease present recurrence rarely, but often exhibit rapid graft loss. On the other hand, recipient patients with C3 glomerulonephritis present recurrence in more than 50% of cases, although the disease is generally slowly progressive. It should not be forgotten that every condition that can lead to chronic graft dysfunction should be considered in the differential diagnosis of recurrence. Therefore, a complete workup of renal biopsy, including light, immunofluorescence and electron microscopy study, is essential to provide the diagnosis, excluding alternative diagnosis that may require different treatment. We will examine in detail the biomolecular mechanisms of both native and transplanted kidney diseases, monitoring the risk of recurrence and optimizing the available treatment options.

Differential expression of parietal epithelial cell and podocyte extracellular matrix proteins in focal segmental glomerulosclerosis and diabetic nephropathy

In healthy glomeruli, parietal epithelial cell (PEC)-derived extracellular matrix (ECM) proteins include laminin-β1, perlecan, and collagen type IV-α2 and podocyte-specific ECM proteins include laminin-β2, agrin, and collagen type IV-α4. This study aimed to define individual ECM protein isoform expression by PECs in both experimental and human focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy (DN) and to determine if changes were CD44 dependent. In experimental FSGS induced with a cytotoxic podocyte antibody and in the BTBR ob/ob mouse model of DN, PEC-derived protein staining was significantly increased in PECs. Dual staining also showed de novo expression of the podocyte-specific ECM proteins laminin-β2 and agrin in PECs. Similar findings were observed in biopsies from patients with FSGS and DN. Increases in individual ECM proteins colocalized with CD44 in PECs in disease. To determine the role of CD44, FSGS was induced in CD44-/- and CD44+/+ mice. PEC staining for perlecan, collagen type IV-α2, laminin-β2, and agrin were significantly lower in diseased CD44-/- mice compared with diseased CD44+/+ mice. These results show that in experimental and human FSGS and DN, PECs typically in an activated state, produce both PEC-derived and podocyte-specific ECM protein isoforms, and that the majority of these changes were dependent on CD44.

Compound effects of aging and experimental FSGS on glomerular epithelial cells

Advanced age portends a poorer prognosis in FSGS. To understand the impact of age on glomerular podocytes and parietal epithelial cells (PECs), experimental FSGS was induced in 3m-old mice (20-year old human age) and 27m-old mice (78-year old human age) by abruptly depleting podocytes with a cytopathic anti-podocyte antibody. Despite similar binding of the disease-inducing antibody, podocyte density was lower in aged FSGS mice compared to young FSGS mice. Activated PEC density was higher in aged versus young FSGS mice, as was the percentage of total activated PECs. Additionally, the percentage of glomeruli containing PECs with evidence of phosphorylated ERK and EMT was higher in aged FSGS mice. Extracellular matrix, measured by collagen IV and silver staining, was higher in aged FSGS mice along Bowman's capsule. However, collagen IV accumulation in the glomerular tufts alone and in glomeruli with both tuft and Bowman's capsule accumulation were similar in young FSGS and aged FSGS mice. Thus, the major difference in collagen IV staining in FSGS was along Bowman's capsule in aged mice. The significant differences in podocytes, PECs and extracellular matrixaccumulation between young mice and old mice with FSGS might explain the differences in outcomes in FSGS based on age.

Reducing mTOR augments parietal epithelial cell density in a model of acute podocyte depletion and in aged kidneys

Parietal epithelial cell (PEC) response to glomerular injury may underlie a common pathway driving fibrogenesis following podocyte loss that typifies several glomerular disorders. Although the mammalian target of rapamycin (mTOR) pathway is important in cell homeostasis, little is known of the biological role or impact of reducing mTOR activity on PEC response following podocyte depletion, nor in the aging kidney. The purpose of these studies was to determine the impact on PECs of reducing mTOR activity following abrupt experimental depletion in podocyte number, as well as in a model of chronic podocyte loss and sclerosis associated with aging. Podocyte depletion was induced by an anti-podocyte antibody and rapamycin started at day 5 until death at day 14 Reducing mTOR did not lead to a greater reduction in podocyte density, despite greater glomerulosclerosis. However, mTOR inhibition lead to an increase in PEC density and PEC-derived crescent formation. Additionally, markers of epithelial-to-mesenchymal transition (platelet-derived growth factor receptor-β, α-smooth muscle actin, Notch-3) and PEC activation (CD44, collagen IV) were further increased by mTOR reduction. Aged mice treated with rapamycin for 1, 2, and 10 wk before death at 26.5 mo (≈75-yr-old human age) had increased the number of glomeruli with a crescentic appearance. mTOR inhibition at either a high or low level lead to changes in PEC phenotype, indicating PEC morphology is sensitive to changes mediated by global mTOR inhibition.

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

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

Of mice and men: divergence of gene expression patterns in kidney

Since the development of methods for homologous gene recombination, mouse models have played a central role in research in renal pathophysiology. However, many published and unpublished results show that mice with genetic changes mimicking human pathogenic mutations do not display the human phenotype. These functional differences may stem from differences in gene expression between mouse and human kidneys. However, large scale comparison of gene expression networks revealed conservation of gene expression among a large panel of human and mouse tissues including kidneys. Because renal functions result from the spatial integration of elementary processes originating in the glomerulus and the successive segments constituting the nephron, we hypothesized that differences in gene expression profiles along the human and mouse nephron might account for different behaviors. Analysis of SAGE libraries generated from the glomerulus and seven anatomically defined nephron segments from human and mouse kidneys allowed us to identify 4644 pairs of gene orthologs expressed in either one or both species. Quantitative analysis shows that many transcripts are present at different levels in the two species. It also shows poor conservation of gene expression profiles, with less than 10% of the 4644 gene orthologs displaying a higher conservation of expression profiles than the neutral expectation (p<0.05). Accordingly, hierarchical clustering reveals a higher degree of conservation of gene expression patterns between functionally unrelated kidney structures within a given species than between cognate structures from the two species. Similar findings were obtained for sub-groups of genes with either kidney-specific or housekeeping functions. Conservation of gene expression at the scale of the whole organ and divergence at the level of its constituting sub-structures likely account for the fact that although kidneys assume the same global function in the two species, many mouse “models” of human pathologies do not display the expected phenotype.

Focal segmental glomerulosclerosis after renal transplantation

Focal segmental glomerulosclerosis (FSGS) is a clinicopathologic syndrome of proteinuria, usually of nephrotic range, associated with focal and segmental sclerotic glomerular lesions. Therefore, FSGS is diagnosed by clinical features and histopathological examination of renal biopsy. The natural history of the condition varies, and although it may respond to treatment, FSGS is an important disease in the etiology of end-stage renal disease (ESRD). Furthermore, after kidney transplantation, approximately 30% of patients with FSGS develop recurrent FSGS. The risk factors for recurrence of FSGS include childhood onset and age <15 yr, rapid progression of the primary FSGS to ESRD, recurrence of FSGS in a previous allograft, diffuse mesangial hypercellularity in the native kidney, collapsing FSGS, and podocin gene mutation. In addition, after kidney transplantation, de novo FSGS also develops in approximately 10-20% of allografts, associated with a complication of hyperfiltration injury, chronic transplant glomerulopathy, and calcineurin inhibitor toxicity. FSGS is considered a podocyte disease, and the pathology is characterized by segmental FSGS lesion with glomerular epithelial hypercellularity. The pathological diagnosis of FSGS is based on the 2004 Columbia classification system. In the present minireview, we discuss the pathology of recurrence and de novo FSGS after kidney transplantation.

The targeted podocyte

The podocyte plays a key role both in maintenance of the glomerular filtration barrier and in glomerular structural integrity. Podocyte injury and loss contribute to proteinuria and progressive sclerosis. Inhibitors of mammalian target of rapamycin (mTOR) have variably decreased or caused proteinuria and sclerosis in human disease and experimental settings. In this issue of the JCI, two interesting studies of podocyte-specific manipulation of the mTOR system shed light on the complexity of this pathway in the podocyte.

The relevance of periglomerular fibrosis in the evaluation of routine needle core renal biopsies

CONTEXT

Renal interstitial fibrosis and, to a lesser extent, sclerotic glomeruli correlate with poor renal function. However, not all nonfunctional glomeruli are sclerotic. Many or most glomeruli with periglomerular fibrosis, while retaining blood flow, probably do not filter; therefore, they may not contribute to renal function.

OBJECTIVE

To examine the relationship of periglomerular fibrosis and the sum of globally sclerotic glomeruli and glomeruli with periglomerular fibrosis (GSG+PF) with interstitial fibrosis and renal function.

DESIGN

Native kidney biopsies from 177 patients with chronic renal injury were assessed for interstitial fibrosis, glomerular sclerosis, and GSG+PF. Renal biopsies with active or acute lesions were not included. The percentage of globally sclerotic glomeruli and GSG+PF was correlated with the degree of interstitial fibrosis and serum creatinine levels.

RESULTS

The percentage of GSG+PF correlates better with the degree of interstitial fibrosis and renal function than does the percentage of globally sclerotic glomeruli alone. This appears particularly true in chronic renal diseases of patients without diabetes. The number of globally sclerotic glomeruli correlates better with interstitial fibrosis and renal function than does the sum of globally and segmentally sclerotic glomeruli.

CONCLUSIONS

The percentage of GSG+PF in a renal biopsy specimen provides a better estimate of chronic renal injury than does the percentage of sclerotic glomeruli alone, probably because many or most glomeruli with periglomerular fibrosis are nonfunctional. Therefore, we recommend that the number of glomeruli with periglomerular fibrosis also be provided in the renal biopsy report.

Etiopathology of chronic tubular, glomerular and renovascular nephropathies: clinical implications

Chronic kidney disease (CKD) comprises a group of pathologies in which the renal excretory function is chronically compromised. Most, but not all, forms of CKD are progressive and irreversible, pathological syndromes that start silently (i.e. no functional alterations are evident), continue through renal dysfunction and ends up in renal failure. At this point, kidney transplant or dialysis (renal replacement therapy, RRT) becomes necessary to prevent death derived from the inability of the kidneys to cleanse the blood and achieve hydroelectrolytic balance. Worldwide, nearly 1.5 million people need RRT, and the incidence of CKD has increased significantly over the last decades. Diabetes and hypertension are among the leading causes of end stage renal disease, although autoimmunity, renal atherosclerosis, certain infections, drugs and toxins, obstruction of the urinary tract, genetic alterations, and other insults may initiate the disease by damaging the glomerular, tubular, vascular or interstitial compartments of the kidneys. In all cases, CKD eventually compromises all these structures and gives rise to a similar phenotype regardless of etiology. This review describes with an integrative approach the pathophysiological process of tubulointerstitial, glomerular and renovascular diseases, and makes emphasis on the key cellular and molecular events involved. It further analyses the key mechanisms leading to a merging phenotype and pathophysiological scenario as etiologically distinct diseases progress. Finally clinical implications and future experimental and therapeutic perspectives are discussed.

Atlas of gene expression in the mouse kidney: new features of glomerular parietal cells

To gain molecular insight into kidney function, we performed a high-resolution quantitative analysis of gene expression in glomeruli and nine different nephron segments dissected from mouse kidney using Serial Analysis of Gene Expression (SAGE). We also developed dedicated bioinformatics tools and databases to annotate mRNA tags as transcripts. Over 800,000 mRNA SAGE tags were sequenced corresponding to >20,000 different mRNA tags present at least twice in at least one library. Hierarchical clustering analysis of tags demonstrated similarities between the three anatomical subsegments of the proximal tubule, between the cortical and medullary segments of the thick ascending limb of Henle's loop, and between the three segments constituting the aldosterone-sensitive distal nephron segments, whereas the glomerulus and distal convoluted tubule clusterized independently. We also identified highly specific mRNA markers of each subgroup of nephron segments and of most nephron segments. Tag annotation also identified numbers of putative antisense mRNAs. This database constitutes a reference resource in which the quantitative expression of a given gene can be compared with that of other genes in the same nephron segment, or between different segments of the nephron. To illustrate possible applications of this database, we performed a deeper analysis of the glomerulus transcriptome that unexpectedly revealed expression of several ion and water carriers; within the glomerulus, they were found to be preferentially expressed in the parietal sheet. It also revealed the major role of the zinc finger transcription factor Wt1 in the specificity of gene expression in the glomerulus. Finally, functional annotation of glomerulus-specific transcripts suggested a high proliferation activity of glomerular cells. Immunolabeling for PCNA confirmed a high percentage of proliferating cells in the glomerulus parietal sheet.

A molecular profile of focal segmental glomerulosclerosis from formalin-fixed, paraffin-embedded tissue

Focal segmental glomerulosclerosis (FSGS) is a common form of idiopathic nephrotic syndrome defined by the characteristic lesions of focal glomerular sclerosis and foot process effacement; however, its etiology and pathogenesis are unknown. We used mRNA isolated from laser-captured glomeruli from archived formalin-fixed, paraffin-embedded renal biopsies, until recently considered an unsuitable source of mRNA for microarray analysis, to investigate the glomerular gene expression profiles of patients with primary classic FSGS, collapsing FSGS (COLL), minimal change disease (MCD), and normal controls (Normal). Amplified mRNA was hybridized to an Affymetrix Human X3P array. Unsupervised (unbiased) hierarchical clustering revealed two distinct clusters delineating FSGS and COLL from Normal and MCD. Class comparison analysis of FSGS + COLL combined versus Normal + MCD revealed 316 significantly differentially regulated genes (134 up-regulated, 182 down-regulated). Among the differentially regulated genes were those known to be part of the slit diaphragm junctional complex and those previously described in the dysregulated podocyte phenotype. Analysis based on Gene Ontology categories revealed overrepresented biological processes of development, differentiation and morphogenesis, cell motility and migration, cytoskeleton organization, and signal transduction. Transcription factors associated with developmental processes were heavily overrepresented, indicating the importance of reactivation of developmental programs in the pathogenesis of FSGS. Our findings reveal novel insights into the molecular pathogenesis of glomerular injury and structural degeneration in FSGS.

Glomerular epithelial stem cells: the good, the bad, and the ugly

Global glomerulosclerosis with loss of podocytes in humans is typical of end-stage renal pathology. Although mature podocytes are highly differentiated and nondividing, converging evidence from experimental and clinical data suggests adult stem cells within Bowman's capsule can rescue some of this loss. Glomerular epithelial stem cells generate podocytes during kidney growth and regenerate podocytes after injury, thus explaining why various glomerular disorders undergo remission occasionally. This regenerative process, however, is often inadequate because of inefficient proliferative responses by glomerular epithelial stem cells with aging or in the setting of focal segmental glomerulosclerosis. Alternatively, an excessive proliferative response by glomerular epithelial stem cells after podocyte injury can generate new lesions such as extracapillary crescentic glomerulonephritis, collapsing glomerulopathy and tip lesions. Better understanding of the mechanisms that regulate growth and differentiation of glomerular epithelial stem cells may provide new clues for prevention and treatment of glomerulosclerosis.

Renal progenitor cells contribute to hyperplastic lesions of podocytopathies and crescentic glomerulonephritis

Glomerular injury can involve excessive proliferation of glomerular epithelial cells, resulting in crescent formation and obliteration of Bowman's space. The origin of these hyperplastic epithelial cells in different glomerular disorders is controversial. Renal progenitors localized to the inner surface of Bowman's capsule can regenerate podocytes, but whether dysregulated proliferation of these progenitors contributes to crescent formation is unknown. In this study, we used confocal microscopy, laser capture microdissection, and real-time quantitative reverse transcriptase-PCR to demonstrate that hypercellular lesions of different podocytopathies and crescentic glomerulonephritis consist of three distinct populations: CD133(+)CD24(+)podocalyxin (PDX)(-)nestin(-) renal progenitors, CD133(+)CD24(+)PDX(+)nestin(+) transitional cells, and CD133(-)CD24(-)PDX(+)nestin(+) differentiated podocytes. In addition, TGF-beta induced CD133(+)CD24(+) progenitors to produce extracellular matrix, and these were the only cells to express the proliferation marker Ki67. Taken together, these results suggest that glomerular hyperplastic lesions derive from the proliferation of renal progenitors at different stages of their differentiation toward mature podocytes, providing an explanation for the pathogenesis of hyperplastic lesions in podocytopathies and crescentic glomerulonephritis.

The enigmatic parietal epithelial cell is finally getting noticed: a review

Although the normal glomerulus comprises four resident cell types, least is known about the parietal epithelial cells (PECs). This comprehensive review addresses the cellular origin of PECs, discusses the normal structure and protein makeup of PECs, describes PEC function, and defines the responses to injury in disease and how these events lead to clinical events. The data show that PECs have unique properties and that new functions are being recognized such as their role in differentiating into podocytes during disease.

Regeneration of glomerular podocytes by human renal progenitors

Depletion of podocytes, common to glomerular diseases in general, plays a role in the pathogenesis of glomerulosclerosis. Whether podocyte injury in adulthood can be repaired has not been established. Here, we demonstrate that in the adult human kidney, CD133+CD24+ cells consist of a hierarchical population of progenitors that are arranged in a precise sequence within Bowman's capsule and exhibit heterogeneous potential for differentiation and regeneration. Cells localized to the urinary pole that expressed CD133 and CD24, but not podocyte markers (CD133+CD24+PDX- cells), could regenerate both tubular cells and podocytes. In contrast, cells localized between the urinary pole and vascular pole that expressed both progenitor and podocytes markers (CD133+CD24+PDX+) could regenerate only podocytes. Finally, cells localized to the vascular pole did not exhibit progenitor markers, but displayed phenotypic features of differentiated podocytes (CD133-CD24-PDX+ cells). Injection of CD133+CD24+PDX- cells, but not CD133+CD24+PDX+ or CD133-CD24- cells, into mice with adriamycin-induced nephropathy reduced proteinuria and improved chronic glomerular damage, suggesting that CD133+CD24+PDX- cells could potentially treat glomerular disorders characterized by podocyte injury, proteinuria, and progressive glomerulosclerosis.

Glomerular epithelial cells in the urine: what has to be done to make them worthwhile?

The significance of the native urine sediment in the differential of glomerular diseases needs no further comment. However, the question arises whether it could be useful to develop a more specific diagnostic approach to identify the origin of renal epithelial cells that can be detected in the urine sediments as well. Especially the detection of podocytes in the urine could be a valuable noninvasive method to get information about the disease activity or disease type and could be used as a follow-up after a biopsy in an outpatient setting. So far, there are only a few studies that analyzed the clinical relevance of renal epithelial cells in the urine systematically or prospectively. The reason for this could be the nature of the material since it will remain unclear whether detachment and changes in the urine milieu have a direct effect on the expression of marker proteins on the detected cells. Dedifferentiation or transdifferentiation of cells that goes along with changed marker expression is certainly also part of the underlying disease process. This review summarizes the available information on marker proteins that have been successfully used in the diagnostic of "podocytes" in the urine. Furthermore, it gives an overview of marker expression on podocytes in situ in development and disease and examines the role of glomerular epithelial shedding in the urine at the interface of basic science and clinical medicine.

The spectrum of focal segmental glomerulosclerosis: new insights

PURPOSE OF REVIEW

Focal segmental glomerulosclerosis (FSGS) is a disease with diverse histologic patterns and etiologic associations. Genetic, toxic, infectious and inflammatory mediators have been identified. This review will focus on new evidence supporting the potential mechanistic basis underlying the histologic variants and their clinical relevance.

RECENT FINDINGS

Evidence from animal models and in-vitro studies suggests that injury inherent within or directed to the podocyte is a central pathogenetic factor. Disruption of signaling from any of the podocyte's specialized membrane domains, including slit diaphragm, apical and basal membranes, or originating at the level of the actin cytoskeleton, may promote the characteristic response of foot process effacement. Irreversible podocyte stress leading to podocyte depletion through apoptosis or detachment is a critical mechanism in most forms of FSGS. In the collapsing variant, podocyte dysregulation leads to podocyte dedifferentiation and glomerular epithelial cell proliferation.

SUMMARY

Translation studies in humans and new evidence from animal models have provided mechanistic insights into the diverse phenotypes of FSGS.

Familial focal segmental glomerulosclerosis associated with an ACTN4 mutation and paternal germline mosaicism

Mutations in the ACTN4 gene cause focal segmental glomerulosclerosis (FSGS), which shows autosomal dominant inheritance (Online Mendelian Inheritance in Man No. 603278, FSGS1). Most patients with a diagnosis of FSGS1 show a mild to moderate degree of proteinuria during adolescence or later, and some patients gradually progress to end-stage renal disease. Here, we report a familial case of FSGS1 in which 2 affected siblings showed unusual clinical, pathological, and genetic features. Both patients presented with full-blown rapidly progressing nephrotic syndrome in early childhood. Renal pathological findings were of an FSGS collapsing variant and FSGS not otherwise specified. A novel ACTN4 mutation, p.Ser262Phe, was detected in the patients, and their father was found to have a germline mosaicism for the mutation. In addition, these siblings also had a heterozygous p.Thr5Met substitution in NPHS1, which encodes nephrin, although the functional significance of this substitution is unclear. This is the third clinical report of FSGS1 and the first case report of germline mosaicism confirmed in patients with hereditary podocyte disorders. FSGS1 may have widely variable clinical and pathological phenotypes and therefore should be considered in young children with full-blown and rapidly progressing nephrotic syndrome. The possibility of germline mosaicism makes interpretation of molecular diagnoses and genetic counseling more difficult.

Collapsing glomerulopathy: an inflammatory podocytopathy?

PURPOSE OF REVIEW

Collapsing glomerulopathy is a relatively new and debated podocytopathy. Among several conjectures, inflammatory injury orchestrated by podocytes is emerging to explain the pathogenesis of collapsing glomerulopathy. Here, we briefly summarize recent studies in support of this novel and intriguing hypothesis.

RECENT FINDINGS

Immunohistochemical analyses of markers conventionally used to demarcate podocytes apart from parietal epithelium identified the parietal podocyte. MafB-deficient mice exhibited abnormal podocyte and macrophage differentiation, suggesting ancestral and functional overlap. These apparent developmental anomalies were detected in studies showing an admixture of hyperplastic podocytes with macrophage epitopes and hyperplastic parietal epithelium in pseudocrescents and in true crescents. Experimental antibody-mediated injury of podocytes could trigger capillary collapse and pseudocrescent formation marked by recruitment of epithelial cells from Bowman's capsule. In contrast, experimental stabilization of hypoxia-inducible factors within podocytes--a known inflammatory response by macrophages--could trigger podocyte proliferation and the formation of true necrotizing crescents.

SUMMARY

Preliminary evidence suggests that visceral and parietal podocytes may become macrophage-like inflammatory mediators of proliferative epithelial injury within the glomerulus. This may manifest as collapsing glomerulopathy or crescentic glomerulonephritis--lesions that appear to be anatomically and pathogenically linked.