Cell cycle regulation and differentiation in the human podocyte lineage

Extracellular signal-regulated kinase is activated in podocytes from patients with diabetic nephropathy

In the past few decades, the global prevalence of diabetes has provided us with a warning about future chronic complications. Diabetic nephropathy (DN) is the main cause of end-stage kidney disease. Podocytes in the glomerulus play a critical role in regulating glomerular permeability, and podocyte injury is one of the main causes of DN. Extracellular signal-regulated kinase (ERK) is a member of the mitogen-activated protein kinase family that plays critical roles in intracellular signal transduction. In human patients with DN, phosphorylated ERK (pERK), the active form of ERK, is increased in the glomeruli. However, information on the expression of pERK, specifically in podocytes in DN, is limited. Meanwhile, high glucose induces ERK activation in immortalized podocyte cell lines, suggesting the involvement of podocytic ERK in DN. We performed an immunohistochemical study using Wilms' tumor-1 (WT-1) as a podocyte-specific marker to investigate whether podocytic pERK levels are increased in patients with DN. In the glomeruli of the DN group, we observed remarkable co-staining for WT-1 and pERK. In contrast, the glomeruli of the control group contained only a few pERK-positive podocytes. Statistical analyses revealed that, relative to healthy controls, patients with DN showed significantly increased pERK expression levels in cells that were positive for WT-1 (DN: 51.3 ± 13.1% vs. control: 7.3 ± 1.6%, p = 0.0158, t-test, n = 4 for each group). This suggests that ERK activation in podocytes is involved in the pathogenesis of DN.

Origins of cancer: ain't it just mature cells misbehaving?

A pervasive view is that undifferentiated stem cells are alone responsible for generating all other cells and are the origins of cancer. However, emerging evidence demonstrates fully differentiated cells are plastic, can be coaxed to proliferate, and also play essential roles in tissue maintenance, regeneration, and tumorigenesis. Here, we review the mechanisms governing how differentiated cells become cancer cells. First, we examine the unique characteristics of differentiated cell division, focusing on why differentiated cells are more susceptible than stem cells to accumulating mutations. Next, we investigate why the evolution of multicellularity in animals likely required plastic differentiated cells that maintain the capacity to return to the cell cycle and required the tumor suppressor p53. Finally, we examine an example of an evolutionarily conserved program for the plasticity of differentiated cells, paligenosis, which helps explain the origins of cancers that arise in adults. Altogether, we highlight new perspectives for understanding the development of cancer and new strategies for preventing carcinogenic cellular transformations from occurring.

The loss of profilin1 is catastrophic to podocytes

Profilin1 belongs to a family of small monomeric actin-binding proteins with diverse roles in fundamental actin-dependent cellular processes required for cell survival. Podocytes are postmitotic visceral epithelial cells critical for the structure and function of the kidney filtration barrier. There is emerging evidence that the actin-related mode of cell death known as mitotic catastrophe is an important pathway involved in podocyte loss. In this issue of the JCI, Tian, Pedigo, and colleagues demonstrate that profilin1 deficiency in podocytes triggered cell cycle reentry, resulting in abortive cytokinesis with a loss in ribosomal RNA processing that leads to podocyte loss and glomerulosclerosis. This study demonstrates the essential role of actin dynamics in mediating this fundamental mode of podocyte cell death.

Derivation of the Immortalized Cell Line UM51-PrePodo-hTERT and Its Responsiveness to Angiotensin II and Activation of the RAAS Pathway

Recent demographic studies predict there will be a considerable increase in the number of elderly people within the next few decades. Aging has been recognized as one of the main risk factors for the world's most prevalent diseases such as neurodegenerative disorders, cancer, cardiovascular disease, and metabolic diseases. During the process of aging, a gradual loss of tissue volume and organ function is observed, which is partially caused by replicative senescence. The capacity of cellular proliferation and replicative senescence is tightly regulated by their telomere length. When telomere length is critically shortened with progressive cell division, cells become proliferatively arrested, and DNA damage response and cellular senescence are triggered, whereupon the "Hayflick limit" is attained at this stage. Podocytes are a cell type found in the kidney glomerulus where they have major roles in blood filtration. Mature podocytes are terminal differentiated cells that are unable to undergo cell division in vivo. For this reason, the establishment of primary podocyte cell cultures has been very challenging. In our present study, we present the successful immortalization of a human podocyte progenitor cell line, of which the primary cells were isolated directly from the urine of a 51-year-old male. The immortalized cell line was cultured over the course of one year (~100 passages) with high proliferation capacity, endowed with contact inhibition and P53 expression. Furthermore, by immunofluorescence-based expression and quantitative real-time PCR for the podocyte markers CD2AP, LMX1B, NPHS1, SYNPO and WT1, we confirmed the differentiation capacity of the immortalized cells. Finally, we evaluated and confirmed the responsiveness of the immortalized cells on the main mediator angiotensin II (ANGII) of the renin-angiotensin system (RAAS). In conclusion, we have shown that it is possible to bypass cellular replicative senescence (Hayflick limit) by TERT-driven immortalization of human urine-derived pre-podocyte cells from a 51-year-old African male.

Expression of Pannexin 1 in the Human Kidney during Embryonal, Early Fetal and Postnatal Development and Its Prognostic Significance in Diabetic Nephropathy

Pannexins are transmembrane glycoproteins that constitute channels involved in purinergic signaling through ATP release from cells in various physiological and pathological processes. In this study, the distribution of Panx1 expression in different cell populations of healthy postnatal human kidneys and during human embryonic and early fetal development was investigated by double immunohistochemistry. In addition, the glomerular and tubular expression of Panx1 was examined in patients with type 2 diabetes mellitus (DM2) and the control group, and renal Panx1 expression was correlated with serum creatinine. In the 6th week of embryonic development (DW), Panx1 expression was found in mesonephric glomeruli and mesonephric tubules. At the transition from 6th to 7th DW, Panx1 immunoreactivity was found in the mesonephric tubules and mesonephric duct, as well as in the metanephric ureteric bud and ampullae. In the 7th DW, strong Panx1 immunoreactivity was observed in the developing ureteric bud in the metanephros, whereas no Panx1 immunoreactivity was found in the metanephric cup. In the 8th DW, Panx1 expression was also found in the ureteric bud of the metanephros, the renal vesicle and comma-shaped nephron, and the epithelial cells of Bowman’s capsule. Expression of Panx1 was found at an early stage in both the paramesonephric duct and the mesonephric duct and diminished toward the 8th DW. During the 6th–10th DW, colocalization of Panx1 with alpha smooth actin (aSMA) was found in developing blood vessels. In the postnatal kidney, strong Panx1 immunoreactivity was present in medullary and cortical collecting duct cells, renin-producing cells, and proximal tubules. Very weak Panx1 immunoreactivity was found in certain distal tubule cells and the thin descending limbs of the loop of Henle. Panx1 immunoreactivity was also found in nephrin-immunoreactive podocytes. Panx1 was not colocalized with aSMA immunoreactivity in the vessels of the postnatal human kidney, but it was present in the endothelium. A significant positive correlation was found between Panx1 expression in glomeruli and serum creatinine only in diabetic patients and was not found in the nondiabetic group. The spatiotemporal expression of Panx1 during the early stages of human kidney development supports its possible role in cellular differentiation, migration, and positioning in the developing human kidney. In addition, our data suggest that glomerular Panx1 expression is a potential indicator of worsening renal function in patients with type 2 diabetes.

Modes of podocyte death in diabetic kidney disease: an update

Diabetic kidney disease (DKD) accounts for a large proportion of end-stage renal diseases that require renal replacement therapies including dialysis and transplantation. Therefore, it is critical to understand the occurrence and development of DKD. Podocytes are mainly injured during the development of DKD, ultimately leading to their extensive death and loss. In turn, the injury and death of glomerular podocytes are also the main culprits of DKD. This review introduces the characteristics of podocytes and summarizes the modes of their death in DKD, including apoptosis, autophagy, mitotic catastrophe (MC), anoikis, necroptosis, and pyroptosis. Apoptosis is characterized by nuclear condensation and the formation of apoptotic bodies, and it exerts a different effect from autophagy in mediating DKD-induced podocyte loss. MC mediates a faulty mitotic process while anoikis separates podocytes from the basement membrane. Moreover, pyroptosis activates inflammatory factors to aggravate podocyte injuries whilst necroptosis drives signaling cascades, such as receptor-interacting protein kinases 1 and 3 and mixed lineage kinase domain-like, ultimately promoting the death of podocytes. In conclusion, a thorough knowledge of the modes of podocyte death in DKD can help us understand the development of DKD and lay the foundation for strategies in DKD disease therapy.

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.

Cell Cycle Dysregulation and Renal Fibrosis

Precise regulation of cell cycle is essential for tissue homeostasis and development, while cell cycle dysregulation is associated with many human diseases including renal fibrosis, a common process of various chronic kidney diseases progressing to end-stage renal disease. Under normal physiological conditions, most of the renal cells are post-mitotic quiescent cells arrested in the G0 phase of cell cycle and renal cells turnover is very low. Injuries induced by toxins, hypoxia, and metabolic disorders can stimulate renal cells to enter the cell cycle, which is essential for kidney regeneration and renal function restoration. However, more severe or repeated injuries will lead to maladaptive repair, manifesting as cell cycle arrest or overproliferation of renal cells, both of which are closely related to renal fibrosis. Thus, cell cycle dysregulation of renal cells is a potential therapeutic target for the treatment of renal fibrosis. In this review, we focus on cell cycle regulation of renal cells in healthy and diseased kidney, discussing the role of cell cycle dysregulation of renal cells in renal fibrosis. Better understanding of the function of cell cycle dysregulation in renal fibrosis is essential for the development of therapeutics to halt renal fibrosis progression or promote regression.

AIM2 Suppresses Inflammation and Epithelial Cell Proliferation during Glomerulonephritis

Absent in melanoma-2 (AIM2) is an inflammasome-forming innate immune sensor for dsDNA but also exhibits inflammasome-independent functions such as restricting cellular proliferation. AIM2 is expressed in the kidney, but its localization and function are not fully characterized. In normal human glomeruli, AIM2 localized to podocytes. In patients with glomerulonephritis, AIM2 expression increased in CD44⁺-activated parietal epithelial cells within glomerular crescents. To explore AIM2 effects in glomerular disease, studies in Aim2 ^-/- mice were performed. Aim2^-/- glomeruli showed reduced expression of Wilm tumor gene-1 (WT1), WT1-driven podocyte genes, and increased proliferation in outgrowth assays. In a nephrotoxic serum (NTS)-induced glomerulonephritis model, Aim2^-/- (B6) mice exhibited more severe glomerular crescent formation, tubular injury, inflammation, and proteinuria compared with wild-type controls. Inflammasome activation markers were absent in both Aim2 ^-/- and wild-type kidneys, despite an increased inflammatory transcriptomic signature in Aim2 ^-/- mice. Aim2 ^-/- mice also demonstrated dysregulated cellular proliferation and an increase in CD44⁺ parietal epithelial cells during glomerulonephritis. The augmented inflammation and epithelial cell proliferation in Aim2 ^-/- (B6) mice was not due to genetic background, as Aim2 ^-/- (B6.129) mice demonstrated a similar phenotype during NTS glomerulonephritis. The AIM2-like receptor (ALR) locus was necessary for the inflammatory glomerulonephritis phenotype observed in Aim2 ^-/- mice, as NTS-treated ALR ^-/- mice displayed equal levels of injury as wild-type controls. Podocyte outgrowth from ALR ^-/- glomeruli was still increased, however, confirming that the ALR locus is dispensable for AIM2 effects on epithelial cell proliferation. These results identify a noncanonical role for AIM2 in suppressing inflammation and epithelial cell proliferation during glomerulonephritis.

Kidney organoid systems for studies of immune-mediated kidney diseases: challenges and opportunities

Acute and chronic kidney diseases are major contributors to morbidity and mortality in the global population. Many nephropathies are considered to be immune-mediated with dysregulated immune responses playing an important role in the pathogenesis. At present, targeted approaches for many kidney diseases are still lacking, as the underlying mechanisms remain insufficiently understood. With the recent development of organoids—a three-dimensional, multicellular culture system, which recapitulates important aspects of human tissues—new opportunities to investigate interactions between renal cells and immune cells in the pathogenesis of kidney diseases arise. To date, kidney organoid systems, which reflect the structure and closer resemble critical aspects of the organ, have been established. Here, we highlight the recent advances in the development of kidney organoid models, including pluripotent stem cell-derived kidney organoids and primary epithelial cell-based tubuloids. The employment and further required advances of current organoid models are discussed to investigate the role of the immune system in renal tissue development, regeneration, and inflammation to identify targets for the development of novel therapeutic approaches of immune-mediated kidney diseases.

IL-6/STAT3 signaling activation exacerbates high fructose-induced podocyte hypertrophy by ketohexokinase-A-mediated tristetraprolin down-regulation

Glomerular hypertrophy is a crucial factor of severe podocyte damage and proteinuria. Our previous study showed that high fructose induced podocyte injury. The current study aimed to explore a novel molecular mechanism underlying podocyte hypertrophy induced by high fructose. Here we demonstrated for the first time that high fructose significantly initiated the hypertrophy in rat glomeruli and differentiated human podocytes (HPCs). Consistently, it induced inflammatory response with the down-regulation of anti-inflammatory factor zinc-finger protein tristetraprolin (TTP) and the activation of interleukin-6 (IL-6)/signal transducer and activator of transcription 3 (STAT3) signaling in these animal and cell models. Subsequently, high-expression of microRNA-92a-3p (miR-92a-3p) and its target protein cyclin-dependent kinase inhibitor p57 (P57) down-regulation, representing abnormal proliferation and apoptosis, were observed in vivo and in vitro. Moreover, high fructose increased ketohexokinase-A (KHK-A) expression in rat glomeruli and differentiated HPCs. Exogenous IL-6 stimulation up-regulated IL-6/STAT3 signaling and miR-92a-3p, reduced P57 expression and promoted podocyte proliferation, apoptosis and hypertrophy in vitro. The data from anti-inflammatory agent maslinic acid treatment or TTP siRNA transfection showed that high fructose may decrease TTP to activate IL-6/STAT3 signaling in podocyte overproliferation and apoptosis, causing podocyte hypertrophy. Whereas, KHK-A siRNA transfection remarkably restored high fructose-induced TTP down-regulation, IL-6/STAT3 signaling activation, podocyte overproliferation, apoptosis and hypertrophy in differentiated HPCs. Taken together, these results suggested that high fructose possibly increased KHK-A expression to down-regulate TTP, subsequently activated IL-6/STAT3 signaling to interfere with podocyte proliferation and apoptosis by up-regulating miR-92a-3p to suppress P57 expression, causing podocyte hypertrophy. Therefore, the inactivation of IL-6/STAT3 to relieve podocyte hypertrophy mediated by inhibiting KHK-A to increase TTP may be a novel strategy for high fructose diet-associated podocyte injury and proteinuria.

Growth hormone induces mitotic catastrophe of glomerular podocytes and contributes to proteinuria

Glomerular podocytes are integral members of the glomerular filtration barrier in the kidney and are crucial for glomerular permselectivity. These highly differentiated cells are vulnerable to an array of noxious stimuli that prevail in several glomerular diseases. Elevated circulating growth hormone (GH) levels are associated with podocyte injury and proteinuria in diabetes. However, the precise mechanism(s) by which excess GH elicits podocytopathy remains to be elucidated. Previous studies have shown that podocytes express GH receptor (GHR) and induce Notch signaling when exposed to GH. In the present study, we demonstrated that GH induces TGF-β1 signaling and provokes cell cycle reentry of otherwise quiescent podocytes. Though differentiated podocytes reenter the cell cycle in response to GH and TGF-β1, they cannot accomplish cytokinesis, despite karyokinesis. Owing to this aberrant cell cycle event, GH- or TGF-β1-treated cells remain binucleated and undergo mitotic catastrophe. Importantly, inhibition of JAK2, TGFBR1 (TGF-β receptor 1), or Notch prevented cell cycle reentry of podocytes and protected them from mitotic catastrophe associated with cell death. Inhibition of Notch activation prevents GH-dependent podocyte injury and proteinuria. Similarly, attenuation of GHR expression abated Notch activation in podocytes. Kidney biopsy sections from patients with diabetic nephropathy (DN) show activation of Notch signaling and binucleated podocytes. These data indicate that excess GH induced TGF-β1-dependent Notch1 signaling contributes to the mitotic catastrophe of podocytes. This study highlights the role of aberrant GH signaling in podocytopathy and the potential application of TGF-β1 or Notch inhibitors, as a therapeutic agent for DN.

Biphasic MIF and SDF1 expression during podocyte injury promote CD44-mediated glomerular parietal cell migration in focal segmental glomerulosclerosis

Glomerular parietal epithelial cell (PEC) activation, as revealed by de novo expression of CD44 and cell migration toward the injured filtration barrier, is a hallmark of podocyte injury-driven focal segmental glomerulosclerosis (FSGS). However, the signaling pathway that mediates activation of PECs in response to podocyte injury is unknown. The present study focused on CD44 signaling, particularly the roles of two CD44-related chemokines, migration inhibitory factor (MIF) and stromal cell-derived factor 1 (SDF1), and their common receptor, chemokine (C-X-C motif) receptor 4 (CXCR4), in the NEP25/LMB2 mouse podocyte-toxin model of FSGS. In the early phase of the disease, CD44-positive PECs were locally evident on the opposite side of the intact glomerular tuft and subsequently increased in the vicinity of synechiae with podocyte loss. Expression of MIF and SDF1 was first increased in injured podocytes and subsequently transferred to activated PECs expressing CD44 and CXCR4. In an immortalized mouse PEC (mPEC) line, recombinant MIF and SDF1 (rMIF and rSDF1, respectively) individually increased CD44 and CXCR4 mRNA and protein levels. rMIF and rSDF1 stimulated endogenous MIF and SDF1 production. rMIF- and rSDF1-induced mPEC migration was suppressed by CD44 siRNA. However, MIF and SDF1 inhibitors failed to show any impact on proteinuria, podocyte number, and CD44 expression in NEP25/LMB2 mice. Our data suggest that injured podocytes upregulate MIF and SDF1 that stimulate CD44 expression and CD44-mediated migration, which is enhanced by endogenous MIF and SDF1 in PECs. This biphasic expression pattern of the chemokine-CD44 axis in podocytes and PECs may be a novel mechanism of "podocyte-PEC cross-talk" signaling underlying podocyte injury-driven FSGS.

Mice lacking core 1-derived O-glycan in podocytes develop transient proteinuria, resulting in focal segmental glomerulosclerosis

The glomerular filtration barrier is composed of podocytes, glomerular basement membrane, and endothelial cells. Disruption of these structures causes several glomerular injuries, such as focal segmental glomerulosclerosis (FSGS). The surface of podocyte apical membranes is coated by negatively charged sialic acids on core 1-derived mucin-type O-glycans. Here, we aimed to investigate the physiological role of core 1-derived O-glycans in the podocytes using adult mice lacking podocyte-specific core 1-derived O-glycans (iPod-Cos). iPod-Cos mice exhibited early and transient proteinuria with foot process effacements and developed typical FSGS-like disease symptoms. To identify the key molecules responsible for the FSGS-like phenotype, we focused on podocalyxin and podoplanin, which possess mucin-type O-glycans. Expression and localization of podocalyxin did not change in iPod-Cos glomeruli. Besides, western blot analysis revealed significantly lower levels of intact podocalyxin in isolated glomeruli of iPod-Cos mice, and high levels of processed forms in iPod-Cos glomeruli, as compared to that in control glomeruli. Conversely, podoplanin mRNA, and protein levels were lower in iPod-Cos mice than in control mice. These results demonstrated that core 1-derived O-glycan on podocytes is required for normal glomerular filtration and may contribute to the stable expression of podocalyxin and podoplanin.

Integrated Functional Genomic Analysis Enables Annotation of Kidney Genome-Wide Association Study Loci

BACKGROUND

Linking genetic risk loci identified by genome-wide association studies (GWAS) to their causal genes remains a major challenge. Disease-associated genetic variants are concentrated in regions containing regulatory DNA elements, such as promoters and enhancers. Although researchers have previously published DNA maps of these regulatory regions for kidney tubule cells and glomerular endothelial cells, maps for podocytes and mesangial cells have not been available.

METHODS

We generated regulatory DNA maps (DNase-seq) and paired gene expression profiles (RNA-seq) from primary outgrowth cultures of human glomeruli that were composed mainly of podocytes and mesangial cells. We generated similar datasets from renal cortex cultures, to compare with those of the glomerular cultures. Because regulatory DNA elements can act on target genes across large genomic distances, we also generated a chromatin conformation map from freshly isolated human glomeruli.

RESULTS

We identified thousands of unique regulatory DNA elements, many located close to transcription factor genes, which the glomerular and cortex samples expressed at different levels. We found that genetic variants associated with kidney diseases (GWAS) and kidney expression quantitative trait loci were enriched in regulatory DNA regions. By combining GWAS, epigenomic, and chromatin conformation data, we functionally annotated 46 kidney disease genes.

CONCLUSIONS

We demonstrate a powerful approach to functionally connect kidney disease-/trait-associated loci to their target genes by leveraging unique regulatory DNA maps and integrated epigenomic and genetic analysis. This process can be applied to other kidney cell types and will enhance our understanding of genome regulation and its effects on gene expression in kidney disease.

Glomerulogenesis and the role of endothelium

PURPOSE OF REVIEW

Earlier works of the glomerulogenesis described morphological steps and protein expression during in-vivo and in-vitro kidney development. Recent technologies using cell-specific or conditional knock-out mice for several factors provide important knowledge about cross-talk signaling among resident cells as local events. Based on the recent advancement, this review revisits comprehensive morphological development of the glomerulus.

RECENT FINDINGS

Interactions of presumptive podocyte vascular endothelial growth factor with vascular endothelial growth factor-2 on angioblasts initiate glomerular vascularization. In induced pluripotent stem cells or organoid-derived nephron formation, the lack of endothelium and mesangial cells under differentiated podocytes suggests the presence of another unknown mechanism for glomerular neovascularization. Mesangial cell migration is prerequisite for glomerular looping by interaction of endothelial platelet-derived grothe factor beta and mesangial platelet-derived growth factor receptor beta and requires the coreceptor neuropilin1. Development of the filtration barrier is promoted by cross-talk among resident cells and may need shear stress. The components of the glomerular basement membrane change during glomerulogenesis, and endothelium and podocytes produce laminin and type IV collagen α1 and α2, whereas type IV collagen α3, α4, α5 is derived only from podocytes.

SUMMARY

Glomerulogenesis progresses by dynamic cellular migration/differentiation induced by cross-talk signaling in resident cells. Glomerular vasculogenesis and subsequent capillary development provide insight into glomerular regeneration and remodeling for medical application.

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.

Live or Let Die: Is There any Cell Death in Podocytes?

Ultimately, the common final pathway of any glomerular disease is podocyte effacement, podocyte loss, and, eventually, glomerular scarring. There has been a long-standing debate on the underlying mechanisms for podocyte depletion, ranging from necrosis and apoptosis to detachment of viable cells from the glomerular basement membrane. However, this debate still continues because additional pathways of programmed cell death have been reported in recent years. Interestingly, viable podocytes can be isolated out of the urine of proteinuric patients easily, emphasizing the importance of podocyte detachment in glomerular diseases. In contrast, detection of apoptosis and other pathways of programmed cell death in podocytes is technically challenging. In fact, we still are lacking direct evidence showing, for example, the presence of apoptotic bodies in podocytes, leaving the question unanswered as to whether podocytes undergo mechanisms of programmed cell death. However, understanding the mechanisms leading to podocyte depletion is of particular interest because future therapeutic strategies might interfere with these to prevent glomerular scarring. In this review, we summarize our current knowledge on podocyte cell death, the different molecular pathways and experimental approaches to study these, and, finally, focus on the mechanisms that prevent the onset of programmed cell death.

Genetic and pharmacological inhibition of microRNA-92a maintains podocyte cell cycle quiescence and limits crescentic glomerulonephritis

Crescentic rapidly progressive glomerulonephritis (RPGN) represents the most aggressive form of acquired glomerular disease. While most therapeutic approaches involve potentially toxic immunosuppressive strategies, the pathophysiology remains incompletely understood. Podocytes are glomerular epithelial cells that are normally growth-arrested because of the expression of cyclin-dependent kinase (CDK) inhibitors. An exception is in RPGN where podocytes undergo a deregulation of their differentiated phenotype and proliferate. Here we demonstrate that microRNA-92a (miR-92a) is enriched in podocytes of patients and mice with RPGN. The CDK inhibitor p57^Kip2 is a major target of miR-92a that constitutively safeguards podocyte cell cycle quiescence. Podocyte-specific deletion of miR-92a in mice de-repressed the expression of p57^Kip2 and prevented glomerular injury in RPGN. Administration of an anti-miR-92a after disease initiation prevented albuminuria and kidney failure, indicating miR-92a inhibition as a potential therapeutic strategy for RPGN. We demonstrate that miRNA induction in epithelial cells can break glomerular tolerance to immune injury.

Crescentic rapidly progressive glomerulonephritis is a severe form of glomerula disease characterized by podocyte proliferation and migration. Here Henique et al. demonstrate that inhibition of miRNA-92a prevents kidney failure by promoting the expression of CDK inhibitor p57^Kip2 that regulates podocyte cell cycle.

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.

The zebrafish kidney mutant zeppelin reveals that brca2/fancd1 is essential for pronephros development

The zebrafish kidney is conserved with other vertebrates, making it an excellent genetic model to study renal development. The kidney collects metabolic waste using a blood filter with specialized epithelial cells known as podocytes. Podocyte formation is poorly understood but relevant to many kidney diseases, as podocyte injury leads to progressive scarring and organ failure. zeppelin (zep) was isolated in a forward screen for kidney mutants and identified as a homozygous recessive lethal allele that causes reduced podocyte numbers, deficient filtration, and fluid imbalance. Interestingly, zep mutants had a larger interrenal gland, the teleostean counterpart of the mammalian adrenal gland, which suggested a fate switch with the related podocyte lineage since cell proliferation and cell death were unchanged within the shared progenitor field from which these two identities arise. Cloning of zep by whole genome sequencing (WGS) identified a splicing mutation in breast cancer 2, early onset (brca2)/fancd1, which was confirmed by sequencing of individual fish. Several independent brca2 morpholinos (MOs) phenocopied zep, causing edema, reduced podocyte number, and increased interrenal cell number. Complementation analysis between zep and brca2^ZM_00057434 -/- zebrafish, which have an insertional mutation, revealed that the interrenal lineage was expanded. Importantly, overexpression of brca2 rescued podocyte formation in zep mutants, providing critical evidence that the brca2 lesion encoded by zep specifically disrupts the balance of nephrogenesis. Taken together, these data suggest for the first time that brca2/fancd1 is essential for vertebrate kidney ontogeny. Thus, our findings impart novel insights into the genetic components that impact renal development, and because BRCA2/FANCD1 mutations in humans cause Fanconi anemia and several common cancers, this work has identified a new zebrafish model to further study brca2/fancd1 in disease.

A p57 conditional mutant allele that allows tracking of p57-expressing cells

p57^Kip2 (p57) is a maternally expressed imprinted gene regulating growth arrest which belongs to the CIP/KIP family of cyclin-dependent kinase inhibitors. While initially identified as a cell cycle arrest protein through inhibition of cyclin and cyclin-dependent kinase complexes, p57 activity has also been linked to differentiation, apoptosis, and senescence. In addition, p57 has recently been shown to be involved in tumorigenesis and cell fate decisions in stem cells. Yet, p57 function in adult tissues remains poorly characterized due to the perinatal lethality of p57 knock-out mice. To analyze p57 tissue-specific activity, we generated a conditional mouse line (p57^FL-ILZ/+ ) by flanking the coding exons 2-3 by LoxP sites. To track p57-expressing or mutant cells, the p57^FL-ILZ allele also contains an IRES-linked β-galactosidase reporter inserted in the 3' UTR of the gene. Here, we show that the β-galactosidase reporter expression pattern recapitulates p57 tissue specificity during development and in postnatal mice. Furthermore, we crossed the p57^FL-ILZ/+ mice with PGK-Cre mice to generate p57^cKO-ILZ/+ animals with ubiquitous loss of p57. p57^cKO-ILZ/+ mice display developmental phenotypes analogous to previously described p57 knock-outs. Thus, p57^FL-ILZ/+ is a new genetic tool allowing expression and functional conditional analyses of p57.

Nephron, Wilms' tumor-1 (WT1), and synaptopodin expression in developing podocytes of mice

Newborn mouse glomeruli are still immature with a morphological feature of an early capillary loop stage, but infant mice do not manifest proteinuria. Little is known about the molecular mechanism whereby infant mice are resistant to proteinuria. Nephrin and synaptopodin are crucial for slit diaphragm and foot process (FP) formation for avoiding proteinuria. Nephrin tyrosine phosphorylation means a transient biological signaling required for FP repair or extension during nephrotic disease. Using an immunohistochemical technique, we examined the natural course of nephrin, Wilms’ tumor-1 (WT1) and synaptopodin at 16.5 days of embryonic age (E16.5d) and E19.5d, 7 days of post-neonatal age (P7d) and P42d during renal development of mice. As a result, nephrin and synaptopodin were detected at E19.5d in S-shaped bodies. WT1, a transcriptional factor for nephrin, was detected in nucleus in podocyte-like cells in all stages. Nephrin tyrosine phosphorylation was evident in glomeruli at P7d, and this was associated with an early-stage of FP extension. Inversely, nephrin phosphorylation became faint at P42d, along with maturated FP. Based on the present results, we suggest the sequential molecular mechanism to protect growing mice from proteinuria: (i) WT1-induced nephrin production by podocytes in S-shaped bodies at E19.5d; (ii) Synchronized induction of synaptopodin at the same period; and (iii) FP extension is initiated at a milk-suckling stage under a nephrin tyrosine-phosphorylated condition, while it is arrested at an adult stage, associated with a loss of nephrin-based signaling.

Podocyte injury and its consequences

Podocytes maintain the glomerular filtration barrier, and the stability of this barrier depends on their highly differentiated postmitotic phenotype, which also defines the particular vulnerability of the glomerulus. Recent podocyte biology and gene disruption studies in vivo indicate a causal relationship between abnormalities of single podocyte molecules and proteinuria and glomerulosclerosis. Podocytes live under various stresses and pathological stimuli. They adapt to maintain homeostasis, but excessive stress leads to maladaptation with complex biological changes including loss of integrity and dysregulation of cellular metabolism. Podocyte injury causes proteinuria and detachment from the glomerular basement membrane. In addition to "sick" podocytes and their detachment, our understanding of glomerular responses following podocyte loss needs to address the pathways from podocyte injury to sclerosis. Studies have found a variety of glomerular responses to podocyte dysfunction in vivo, such as disruption of podocyte-endothelial cross talk and activation of podocyte-parietal cell interactions, all of which help us to understand the complex scenario of podocyte injury and its consequences. This review focuses on the cellular aspects of podocyte dysfunction and the adaptive or maladaptive glomerular responses to podocyte injury that lead to its major consequence, glomerulosclerosis.

Biallelic Mutations in Nuclear Pore Complex Subunit NUP107 Cause Early-Childhood-Onset Steroid-Resistant Nephrotic Syndrome

The nuclear pore complex (NPC) is a huge protein complex embedded in the nuclear envelope. It has central functions in nucleocytoplasmic transport, nuclear framework, and gene regulation. Nucleoporin 107 kDa (NUP107) is a component of the NPC central scaffold and is an essential protein in all eukaryotic cells. Here, we report on biallelic NUP107 mutations in nine affected individuals who are from five unrelated families and show early-onset steroid-resistant nephrotic syndrome (SRNS). These individuals have pathologically focal segmental glomerulosclerosis, a condition that leads to end-stage renal disease with high frequency. NUP107 is ubiquitously expressed, including in glomerular podocytes. Three of four NUP107 mutations detected in the affected individuals hamper NUP107 binding to NUP133 (nucleoporin 133 kDa) and NUP107 incorporation into NPCs in vitro. Zebrafish with nup107 knockdown generated by morpholino oligonucleotides displayed hypoplastic glomerulus structures and abnormal podocyte foot processes, thereby mimicking the pathological changes seen in the kidneys of the SRNS individuals with NUP107 mutations. Considering the unique properties of the podocyte (highly differentiated foot-process architecture and slit membrane and the inability to regenerate), we propose a "podocyte-injury model" as the pathomechanism for SRNS due to biallelic NUP107 mutations.

Knockdown of Tmem234 in zebrafish results in proteinuria

Podocytes are highly specialized epithelial cells located at the outer aspects of the glomerular capillary tuft and critical components of the kidney filtration barrier. To maintain their unique features, podocytes express a number of proteins that are only sparsely found elsewhere in the body. In this study, we have identified four (Tmem234, Znf185, Lrrc49, and Slfn5) new highly podocyte-enriched proteins. The proteins are strongly expressed by podocytes, while other parts of the kidney show only weak or no expression. Tmem234, Slfn5, and Lrrc49 are located in foot processes, whereas Znf185 is found in both foot and major processes. Expressional studies in developing kidneys show that these proteins are first expressed at the capillary stage glomerulus, the same stage when the formation of major and foot processes begins. We identified zebrafish orthologs for Tmem234 and Znf185 genes and knocked down their expression using morpholino technology. Studies in zebrafish larvae indicate that Tmem234 is essential for the organization and functional integrity of the pronephric glomerulus filtration barrier, as inactivation of Tmem234 expression results in foot process effacement and proteinuria. In summary, we have identified four novel highly podocyte-enriched proteins and show that one of them, Tmem234, is essential for the normal filtration barrier in the zebrafish pronephric glomerulus. Identification of new molecular components of the kidney filtration barrier opens up possibilities to study their role in glomerulus biology and diseases.

Cell cycle control in the kidney

Proper control of the cell cycle is mandatory during homeostasis and disease. The balance of p53 and MDM2 integrates numerous signalling pathways to regulate the cell cycle, which is executed by multiple proteins including the cyclins, cyclin kinases and cyclin kinase inhibitors. Mutations or environmental factors that affect cell cycle control can lead to inappropriate hyperplasia or cancer as well as to cell loss and tissue atrophy. Normal kidney function is maintained largely by post-mitotic quiescent cells in the G0 phase with a low turnover. Early cell cycle activation during kidney injury contributes to cell death via mitotic catastrophe, i.e. death via mitosis, e.g. of cell with significant DNA damage. At later stages, cell cycle entry supports tissue regeneration and functional reconstitution via cell hypertrophy and/or cell proliferation. It is of note that so-called proliferation markers such as Ki67, PCNA or BrdU identify only cell cycle entry without telling whether this results in cell hypertrophy, cell division or mitotic catastrophe. With this in mind, some established concepts on kidney injury and regeneration are to be re-evaluated. Here, we discuss the components and functional roles of p53/MDM2-mediated cell cycle regulation in kidney homeostasis and disease.

Causes and pathogenesis of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) describes both a common lesion in progressive kidney disease, and a disease characterized by marked proteinuria and podocyte injury. The initial injuries vary widely. Monogenetic forms of FSGS are largely due to alterations in structural genes of the podocyte, many of which result in early onset of disease. Genetic risk alleles in apolipoprotein L1 are especially prevalent in African Americans, and are linked not only to adult-onset FSGS but also to progression of some other kidney diseases. The recurrence of FSGS in some transplant recipients whose end-stage renal disease was caused by FSGS points to circulating factors in disease pathogenesis, which remain incompletely understood. In addition, infection, drug use, and secondary maladaptive responses after loss of nephrons from any cause may also cause FSGS. Varying phenotypes of the sclerosis are also manifest, with varying prognosis. The so-called tip lesion has the best prognosis, whereas the collapsing type of FSGS has the worst prognosis. New insights into glomerular cell injury response and repair may pave the way for possible therapeutic strategies.

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 energy metabolism and glomerular diseases

Mitochondria are crucial organelles that produce and deliver adenosine triphosphate (ATP), by which all cellular processes are driven. Although the mechanisms that control mitochondrial biogenesis, function and dynamics are complex process and vary among different cell types, recent studies provided many new discoveries in this field. Podocyte injury is a crucial step in the development of a large number of glomerular diseases. Glomerular podocytes are unique cells with complex foot processes that cover the outer layer of the glomerular basement membrane, and are the principle cells composing filtration barriers of glomerular capillaries. Little is known on the modalities and the regulation of podocyte's energetics as well as the type of energy substrate primarily used for their activity, recent studies revealed that dysfunction of energy transduction in podocytes may underlie the podocyte injury associated with numerous glomerular diseases. We herein review and discuss the importance of a fine regulation of energy metabolism in podocytes for maintaining their cellular structure and related kidney function. In the future, understanding these mechanisms will open up new areas of treatment for glomerular diseases.

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.

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.

Roles of the Skp2/p27 axis in the progression of chronic nephropathy

S-phase kinase-associated protein 2 (Skp2) is an F-box protein component of the Skp/Cullin/F-box-type E3 ubiquitin ligase that targets several cell cycle regulatory proteins for degradation through the ubiquitin-dependent pathway. Skp2-mediated degradation of p27, a cyclin-dependent kinase inhibitor, is involved in cell cycle regulation. Tubular epithelial cell proliferation is a characteristic feature of renal damage that is apparent in the early stages of nephropathy. The p27 level is associated with the progression of renal injury, and increased Skp2 expression in progressive nephropathy is implicated in decreases of p27 expression. In Skp2^−/− mice, renal damage caused by unilateral ureteral obstruction (UUO) was ameliorated by p27 accumulation, mainly in tubular epithelial cells. However, the amelioration of UUO-induced renal injury in Skp2^−/− mice was prevented by p27 deficiency in Skp2^−/−/p27^−/− mice. These results suggest that the Skp2-mediated reduction in p27 is a pathogenic activity that occurs during the progression of nephropathy. Here, we discuss the roles of the Skp2/p27 axis and/or related signaling pathways/components in the progression of chronic nephropathy.

Reversible cell-cycle entry in adult kidney podocytes through regulated control of telomerase and Wnt signaling

Mechanisms of epithelial cell renewal remain poorly understood in the mammalian kidney, particularly in the glomerulus, a site of cellular damage in chronic kidney disease. Within the glomerulus, podocytes--differentiated epithelial cells crucial for filtration--are thought to lack substantial capacity for regeneration. Here we show that podocytes rapidly lose differentiation markers and enter the cell cycle in adult mice in which the telomerase protein component TERT is conditionally expressed. Transgenic TERT expression in mice induces marked upregulation of Wnt signaling and disrupts glomerular structure, resulting in a collapsing glomerulopathy resembling those in human disease, including HIV-associated nephropathy (HIVAN). Human and mouse HIVAN kidneys show increased expression of TERT and activation of Wnt signaling, indicating that these are general features of collapsing glomerulopathies. Silencing transgenic TERT expression or inhibiting Wnt signaling through systemic expression of the Wnt inhibitor Dkk1 in either TERT transgenic mice or in a mouse model of HIVAN results in marked normalization of podocytes, including rapid cell-cycle exit, re-expression of differentiation markers and improved filtration barrier function. These data reveal an unexpected capacity of podocytes to reversibly enter the cell cycle, suggest that podocyte renewal may contribute to glomerular homeostasis and implicate the telomerase and Wnt-β-catenin pathways in podocyte proliferation and disease.

Glcci1 deficiency leads to proteinuria

Unbiased transcriptome profiling and functional genomics approaches identified glucocorticoid-induced transcript 1 (GLCCI1) as being a transcript highly specific for the glomerulus, but its role in glomerular development and disease is unknown. Here, we report that mouse glomeruli express far greater amounts of Glcci1 protein compared with the rest of the kidney. RT-PCR and Western blotting demonstrated that mouse glomerular Glcci1 is approximately 60 kD and localizes to the cytoplasm of podocytes in mature glomeruli. In the fetal kidney, intense Glcci1 expression occurs at the capillary-loop stage of glomerular development. Using gene knockdown in zebrafish with morpholinos, morphants lacking Glcci1 function had collapsed glomeruli with foot-process effacement. Permeability studies of the glomerular filtration barrier in these zebrafish morphants demonstrated a disruption of the selective glomerular permeability filter. Taken together, these data suggest that Glcci1 promotes the normal development and maintenance of podocyte structure and function.

Marker expression, behaviors, and responses vary in different lines of conditionally immortalized cultured podocytes

The state-of-the-art cultured podocyte is conditionally immortalized by expression of a temperature-sensitive mutant of the SV40 large-T antigen. These cultures proliferate at 33°C and differentiate at 37°C into arborized cells that more closely resemble in vivo podocytes. However, the degree of resemblance remains controversial. In this study, several parameters were measured in podocyte cell lines derived from mouse (JR, KE), human (MS), and rat (HK). In all lines, the quantities of NEPH1 and podocin proteins and NEPH1 and SYNPO mRNAs were comparable to glomeruli, while synaptopodin and nephrin proteins and NPHS1 and NPHS2 mRNAs were <5% of glomerular levels. Expression of Wilms' tumor-1 (WT1) mRNA in mouse lines was comparable to glomeruli, but rat and human lines expressed little WT1. Undifferentiated human and mouse lines had similar proliferation rates that decreased after differentiation, while the rate in rat cells remained constant. The motility of different lines varied as measured by both general motility and wound-healing assays. The toxicity of puromycin aminonucleoside was MS ∼ JR >> KE, and of doxorubicin was JR ∼ KE > MS, while HK cells were almost unaffected. Process formation was largely a result of contractile action after formation of lamellipodia. These findings demonstrate dramatic differences in marker expression, response to toxins, and motility between lines of podocytes from different species and even between similarly-derived mouse lines.

Diminished met signaling in podocytes contributes to the development of podocytopenia in transplant glomerulopathy

Transplant glomerulopathy (TxG) can show secondary focal and segmental glomerulosclerosis (FSGS). FSGS in native kidneys is caused by podocytopenia. This study examines podocytopenia and the role of decreased paracrine Met activation on podocytes by decreased glomerular hepatocyte growth factor (HGF) levels in the development of podocytopenia in TxG. Podocytes were counted in 10 zero-hour biopsies and 10 specimens each with and without TxG. HGF/Met was examined with immunostains and quantitative RT-PCR in a set of three consecutive biopsies from 10 patients with TxG, including the diagnostic biopsy (DiagnBx) and the two previous biopsies (1stPrevBx and 2ndPrevBx). Antiapoptotic effects of HGF on podocytes were examined in vitro. Mean podocyte numbers per glomerulus were lower and glomerular volume higher in TxG. Fewer of the two preceding biopsies of the patients than of the controls contained phospho-Met(Tyr1349)-positive podocytes (2 of 8 versus 7 of 7, P = 0.0070; 4 of 9 versus 9 of 9, P = 0.0294). Glomerular HGF mRNA levels were lower in the 1stPrevBx of the patients (0.049 ± 0.083 versus 0.284 ± 0.331; P = 0.0155). In vitro, HGF stimulation of podocytes resulted in antiapoptotic phosphorylation of AKT and extracellular signal-regulated kinase (ERK) and induction of X-linked inhibitor of apoptosis protein (XIAP). Decreased antiapoptotic Met signaling in podocytes, probably due to decreased HGF secretion by glomerular epithelial cells, could contribute to podocyte loss and FSGS in TxG.

Bone marrow-derived progenitor cells do not contribute to podocyte turnover in the puromycin aminoglycoside and renal ablation models in rats

A key event in the progression of glomerular disease is podocyte loss that leads to focal and segmental glomerulosclerosis (FSGS). Because adult podocytes are postmitotic cells, podocyte replacement by bone marrow-derived progenitors could prevent podocytopenia and FSGS. This study uses double immunofluorescence for Wilms' tumor-1 and enhanced green fluorescent protein (eGFP) to examine whether an eGFP-positive bone marrow transplant can replace podocytes under normal circumstances and in 3 different rat models of FSGS: puromycin aminoglycoside nephropathy, subtotal nephrectomy, and uninephrectomy. Bone marrow engraftment was successful, with more than 70% eGFP-positive cells and virtually normal histologic findings. No bone marrow transplant-derived podocytes were found in four control rats after transplantation, in nine rats at up to 10 weeks after puromycin aminoglycoside nephropathy induction, in three rats 23 days after subtotal nephrectomy, and in six rats up to 21 days after uninephrectomy. A total of 2200 glomeruli with 14,474 podocytes were evaluated in all groups. Thus, podocyte replacement by bone marrow-derived cells does not contribute to podocyte turnover in rats, even in models of podocyte damage. This is in contrast to previous studies in mice, in which bone marrow-derived podocytes were found. Further studies will address this discrepancy, which could be explained by species differences or by predominant podocyte regeneration from a parietal epithelial cell niche.

Ontogeny of the kidney and renal developmental markers in the rhesus monkey (Macaca mulatta)

Nonhuman primates share many developmental similarities with humans, thus they provide an important preclinical model for understanding the ontogeny of biomarkers of kidney development and assessing new cell-based therapies to treat human disease. To identify morphological and developmental changes in protein and RNA expression patterns during nephrogenesis, immunohistochemistry and quantitative real-time PCR were used to assess temporal and spatial expression of WT1, Pax2, Nestin, Synaptopodin, alpha-smooth muscle actin (α-SMA), CD31, vascular endothelial growth factor (VEGF), and Gremlin. Pax2 was expressed in the condensed mesenchyme surrounding the ureteric bud and in the early renal vesicle. WT1 and Nestin were diffusely expressed in the metanephric mesenchyme, and expression increased as the Pax2-positive condensed mesenchyme differentiated. The inner cleft of the tail of the S-shaped body contained the podocyte progenitors (visceral epithelium) that were shown to express Pax2, Nestin, and WT1 in the early second trimester. With maturation of the kidney, Pax2 expression diminished in these structures, but was retained in cells of the parietal epithelium, and as WT1 expression was upregulated. Mature podocytes expressing WT1, Nestin, and Synaptopodin were observed from the mid-third trimester through adulthood. The developing glomerulus was positive for α-SMA (vascular smooth muscle) and Gremlin (mesangial cells), CD31 (glomerular endothelium), and VEGF (endothelium), and showed loss of expression of these markers as glomerular maturation was completed. These data form the basis for understanding nephrogenesis in the rhesus monkey and will be useful in translational studies that focus on embryonic stem and other progenitor cell populations for renal tissue engineering and repair.

Detection of Bcl-2 mRNA and its product in the glomerular podocytes of the normal rat kidney

Podocyte apoptosis underlies podocytepenia leading to glomerulosclerosis. An apoptosis inhibitory protein Bcl-2 is expressed in the podocytes in the early stage of nephrogenesis and downregulated in the maturing stage of human fetal kidneys. Recent studies reported changed localization and expression of Bcl-2 in the renal glomeruli under the pathologic conditions. This study aimed to confirm in situ localization of Bcl-2 mRNA and its product in the glomeruli, and to demonstrate the local expression of Bcl-2 mRNA in normal rat glomeruli. Paraffin sections of the kidneys from normal male Wistar rats were immunostained by anti-Bcl-2 monoclonal antibody. The localization of Bcl-2 mRNA in the glomeruli was evaluated by in situ hybridization. The glomeruli were dissected from frozen sections of the kidneys with the laser microdissection (LMD) system. Total RNA extracted from 10, 100 or 200 dissected glomeruli was used for reverse-transcription polymerase chain reaction (RT-PCR) and real-time PCR. Bcl-2 mRNA and its product were detected in the podocytes but barely in the mesangial cells. In RT-PCR, the specific-sized bands of Bcl-2 from 100 or 200 dissected glomeruli were clearly observed. Real-time PCR for Bcl-2 showed that cDNA from 100 or 200 dissected glomeruli became amplified at 36 or 33cycles, respectively. Bcl-2 is expressed in the glomerular podocytes of the normal rat kidney and quantitative analysis of Bcl-2 mRNA in the renal glomeruli is possible using the LMD technique.

Role of the retinoic acid receptor-α in HIV-associated nephropathy

All-trans retinoic acid protects against the development of HIV-associated nephropathy (HIVAN) in HIV-1 transgenic mice (Tg26). In vitro, all-trans retinoic acid inhibits HIV-induced podocyte proliferation and restores podocyte differentiation markers by activating its receptor-α (RARα). Here, we report that Am580, a water-soluble RARα-specific agonist, attenuated proteinuria, glomerosclerosis, and podocyte proliferation, and restored podocyte differentiation markers in kidneys of Tg26 mice. Furthermore, RARα-/- Tg26 mice developed more severe kidney and podocyte injury than did RARα+/- Tg26 mice. Am580 failed to ameliorate kidney injury in RARα-/- Tg26 mice, confirming our hypothesis that Am580 acts through RARα. Although the expression of RARα-target genes was suppressed in the kidneys of Tg26 mice and of patients with HIVAN, the expression of RARα in the kidney was not different between patients with HIVAN and minimal change disease. However, the tissue levels of retinoic acid were reduced in the kidney cortex and isolated glomeruli of Tg26 mice. Consistent with this, the expression of two key enzymes in the retinoic acid synthetic pathway, retinol dehydrogenase type 1 and 9, and the overall enzymatic activity for retinoic acid synthesis were significantly reduced in the glomeruli of Tg26 mice. Thus, a defect in the endogenous synthesis of retinoic acid contributes to loss of the protection by retinoic acid in HIVAN. Hence, RARα agonists may be potential agents for the treatment of HIVAN.

Activation of Notch signaling pathway in HIV-associated nephropathy

OBJECTIVE

HIV-associated nephropathy (HIVAN) is characterized by the development of glomerulosclerosis and is associated with glomerular epithelial cell proliferation. It has recently been shown that activation of the Notch signaling pathway in podocytes results in glomerulosclerosis and podocyte proliferation. To determine whether Notch signaling is involved in renal disorder associated with HIVAN, we evaluated the expression of Notch receptors in HIVAN.

DESIGN

We evaluated the expression of the Notch signaling pathway using an HIV-transgenic (HIV-Tg) rat model of HIVAN, and biopsy samples from HIVAN and normal controls.

METHODS

Paraffin sections and kidney lysates were used for immunohistochemistry, immunofluorescence and western blot analysis.

RESULTS

A collapsing variant of glomerulosclerosis and focal segmental sclerosis was observed in HIV-Tg rats. Glomeruli of HIV-Tg rats demonstrated activation of Notch1 and Notch4, as determined by the presence of the intracellular domains. In addition, we observed increased expression of the Notch target protein, hairy enhancer of split homolog-1 in glomeruli of these animals. The expression of the Groucho homolog transducin-like enhancer protein 4, a Notch effector protein, and the homeodomain protein cut homeobox 1 were also significantly increased in glomeruli of HIV-Tg rats, and this was associated with decreased expression of the cyclin kinase inhibitor p27. Intriguingly, renal biopsy samples from HIVAN patients also showed upregulation of cleaved Notch1 and Notch4 in the glomeruli compared with the expression in normal kidneys.

CONCLUSION

Our results demonstrate activation of Notch signaling pathway in HIVAN, thereby underscoring its role in disease pathogenesis.

Podocyte number predicts progression of proteinuria in IgA nephropathy

Podocyte injury is a feature of glomerulopathies associated with proteinuria, which in turn has been used as a clinical prognostic factor for glomerular diseases. The goal of this study is to investigate the relationship between podocyte injury found in biopsied renal tissue and change of proteinuria in IgA nephropathy (IgAN). In all, 35 patients with biopsy-proven IgAN and proteinuria (>1.0 g per 24 h) were enrolled in the IgAN group, while 8 patients with excision of renal harmatoma or carcinoma served as kidney controls (Control). Immunohistochemistry was applied to detect the expression of nestin, cell-cycle regulatory protein p27, as well as complement C5b-9 and complement receptor 1 (CR1). Podocyte foot process width (FPW) and podocyte population in renal biopsied samples were measured by morphometric analysis. On the basis of the podocyte density (Nv), the IgAN patients were divided into podocytopenic group (n=17, Nv<57.10 /microm(3) x 10(6)) and normopodocytic group (n=18, Nv> or =57.10 /microm(3) x 10(6)). Changes of proteinuria were followed for 18 months after biopsy. Compared with the Control, IgAN glomeruli had reduced podocyte expression of p27 and nestin along with decreased podocyte number. IgAN glomeruli also showed activation of C5b-9 in mesangial and subepithelial areas with decreased CR1 expression in podocytes. The C5b-9 positivity was inversely correlated with the number of WT-1-positive podocytes. Although the magnitude of proteinuria at biopsy correlated with podocyte FPW (P<0.05), the change in the amount of proteinuria expressed as proteinuria progression rate significantly correlated with the podocyte density. Thus, the normopodocytic group showed significantly lower proteinuria progression rate than the podocytopenic group regardless the comparable clinical features at biopsy and treatment regimen between the two groups. The results of this study indicate that, in IgAN, podocyte injury is involved in development of proteinuria and loss of podocytes predicts progression of the proteinuria. Complement activation may contribute to podocyte damage in IgAN.

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.

HIV-associated nephropathy: clinical presentation, pathology, and epidemiology in the era of antiretroviral therapy

The classic kidney disease of human immunodeficiency virus (HIV) infection, HIV-associated nephropathy, is characterized by progressive acute renal failure, often accompanied by proteinuria and ultrasound findings of enlarged, echogenic kidneys. Definitive diagnosis requires kidney biopsy, which shows collapsing focal segmental glomerulosclerosis with associated microcystic tubular dilatation and interstitial inflammation. Podocyte proliferation is a hallmark of HIV-associated nephropathy, although this classic pathology is observed less frequently in antiretroviral-treated patients. The pathogenesis of HIV-associated nephropathy involves direct HIV infection of renal epithelial cells, and the widespread introduction of combination antiretroviral therapy has had a significant impact on the natural history and epidemiology of this unique disease. These observations have established antiretroviral therapy as the cornerstone of treatment for HIV-associated nephropathy in the absence of prospective clinical trials. Adjunctive therapy for HIV-associated nephropathy includes angiotensin-converting enzyme inhibitors or angiotensin-receptor blockers, as well as corticosteroids in selected patients with significant interstitial inflammation or rapid progression.