Effects of Perfusion Pressures on Podocyte Loss in the Isolated Perfused Mouse Kidney.

BACKGROUND/AIMS: Podocytes are lost in most glomerular diseases, leading to glomerulosclerosis and progressive kidney disease. It is generally assumed, that podocytes are exposed to the filtration flow and thus to significant shear forces driving their detachment from the glomerular basement membrane (GBM). In this context, foot process effacement has been proposed as potential adaptive response to increase adhesion of podocytes to the GBM. METHODS: We have tested these hypotheses using optical clearing and high-resolution 3-dimensional morphometric analysis in the isolated perfused murine kidney. We investigated the dynamics of podocyte detachment at different perfusion pressures (50, 300 and more than 450 mmHg) in healthy young or old mice (20 vs. 71 weeks of age), or mice injected with anti-GBM serum to induce global foot process effacement. RESULTS: Results show that healthy podocytes in young mice are tightly attached onto the GBM and even supramaximal pressures did not cause significant detachment. Compared to young mice, in aged mice and mice with anti-GBM nephritis and foot process effacement, gradual progressive loss of podocytes had occurred already before perfusion. High perfusion pressures resulted in a relatively minor additional loss of podocytes in aged mice. In mice with anti-GBM nephritis significant additional podocyte loss occurred at this early time point when increasing perfusion pressures to 300 mmHg or higher. CONCLUSION: This work provides the first experimental evidence that podocytes are extraordinarily resistant to acutely increased perfusion pressures in an ex vivo isolated kidney perfusion model. Only in glomerular disease, significant numbers of injured podocytes detached following acute increases in perfusion pressure.

The Inability of Podocytes to Proliferate: Cause, Consequences, and Origin.

This study presents a theoretical analysis of the problems related to the inability of podocytes to proliferate. The basis of these problems is the very high rate of glomerular filtration. Podocytes do not in general die by apoptosis or necrosis but are lost by detachment from the glomerular basement membrane (GBM) as viable cells. Podocytes situated on the outside of the filtration barrier and attached to the GBM only by their foot processes are permanently exposed to the flow dynamic forces of the high filtration rate tending to detach them from the GBM. The major challenge seems to consist of the high shear stresses on the foot processes within the filtration slits due to filtrate flow. Healthy podocytes are able to resist this challenge, injured podocytes are not, and may undergo foot process detachment, leading to a gap in the podocyte cover of the GBM. This represents a mortal event. Like a dam break, such a leak cannot be repaired. The ongoing exposure to filtrate flow prevents any attempt to close the gap, thus preventing any regeneration including cell proliferation. An improvement of this precarious situation consists of healing by scarring that may involve only one lobule of the glomerulus, permitting the remaining lobules to maintain filtration. An answer to the question of which waste product requires such a high filtration rate for its excretion may be in the huge quantity of circulating peptides, a problem that dates far back in evolution.

Towards an in vitro model of the glomerular barrier unit with an innovative bioassembly method.

BACKGROUND: The development of an artificial glomerular unit may be pivotal for renal pathophysiology studies at a multicellular scale. Using a tissue engineering approach, we aimed to reproduce in part the specific glomerular barrier architecture by manufacturing a glomerular microfibre (Mf). METHODS: Immortalized human glomerular cell lines of endothelial cells (GEnCs) and podocytes were used. Cells and a three-dimensional (3D) matrix were characterized by immunofluorescence with confocal analysis, Western blot and polymerase chain reaction. Optical and electron microscopy were used to study Mf and cell shapes. We also analysed cell viability and cell metabolism within the 3D construct at 14 days. RESULTS: Using the Mf manufacturing method, we repeatedly obtained a cellularized Mf sorting human glomerular cells in 3D. Around a central structure made of collagen I, we obtained an internal layer composed of GEnC, a newly formed glomerular basement membrane rich in α5 collagen IV and an external layer of podocytes. The cell concentration, optimal seeding time and role of physical stresses were modulated to obtain the Mf. Cell viability and expression of specific proteins (nephrin, synaptopodin, vascular endothelial growth factor receptor 2 (VEGFR2) and von Willebrandt factor (vWF)) were maintained for 19 days in the Mf system. Mf ultrastructure, observed with EM, had similarities with the human glomerular barrier. CONCLUSION: In summary, with our 3D bio-engineered glomerular fibre, GEnC and podocytes produced a glomerular basement membrane. In the future, this glomerular Mf will allow us to study cell interactions in a 3D system and increase our knowledge of glomerular pathophysiology.

Group 1 innate lymphoid cells are involved in the progression of experimental anti-glomerular basement membrane glomerulonephritis and are regulated by peroxisome proliferator-activated receptor α.

Innate lymphoid cells play an important role in the early effector cytokine-mediated response. In Wistar Kyoto rats, CD8+ non-T lymphocytes (CD8+Lym) infiltrate into glomeruli during the development of anti-glomerular basement membrane (anti-GBM) glomerulonephritis. Here, we examined the profiles and roles of CD8+Lym in anti-GBM glomerulonephritis. The regulation of CD8+Lym by peroxisome proliferator-activated receptor (PPAR)-α in anti-GBM glomerulonephritis was also evaluated. Glomerular infiltrating CD8+Lym were lineage-negative cells that showed markedly high expression of IFN-γ and T-bet mRNAs but not Eomes, indicating these cells are group 1 innate lymphoid cells. In anti-GBM glomerulonephritis, the glomerular mRNAs of innate lymphoid cell-related cytokines (IFN-γ and TNF-α) and chemokines (CXCL9, CXCL10, and CXCL11) are significantly increased. Treatment with a PPARα agonist ameliorated renal injury, with reduced expression of these mRNAs. In vitro, enhanced IFN-γ production from innate lymphoid cells upon IL-12 and IL-18 stimulation was reduced by the PPARα agonist. Moreover, CXCL9 mRNA in glomerular endothelial cells and CXCL9, CXCL10, and CXCL11 mRNAs in podocytes and macrophages were upregulated by IFN-γ, whereas the PPARα agonist downregulated their expression. We also detected the infiltration of innate lymphoid cells into glomeruli in human anti-GBM glomerulonephritis. Thus, innate lymphoid cells are involved in the progression of anti-GBM glomerulonephritis and regulated directly or indirectly by PPARα. Our findings suggest that innate lymphoid cells could serve as novel therapeutic targets for anti-GBM glomerulonephritis.

Properdin binds independent of complement activation in an in vivo model of anti-glomerular basement membrane disease.

Properdin is the only known positive regulator of complement activation by stabilizing the alternative pathway convertase through C3 binding, thus prolonging its half-life. Recent in vitro studies suggest that properdin may act as a specific pattern recognition molecule. To better understand the role of properdin in vivo, we used an experimental model of acute anti-glomerular basement membrane disease with wild-type, C3- and properdin knockout mice. The model exhibited severe proteinuria, acute neutrophil infiltration and activation, classical and alternative pathway activation, and progressive glomerular deposition of properdin, C3 and C9. Although the acute renal injury was likely due to acute neutrophil activation, we found properdin deposition in C3-knockout mice that was not associated with IgG. Thus, properdin may deposit in injured tissues in vivo independent of its main ligand C3.

Unexpected kidney-restricted role for IL-17 receptor signaling in defense against systemic Candida albicans infection.

Kidney injury is a frequent outcome in patients with disseminated Candida albicans fungal infections. IL-17 receptor (IL-17R) signaling is critical for renal protection against disseminated candidiasis, but the identity and function of IL-17-responsive cells in mediating renal defense remains an active area of debate. Using BM chimeras, we found that IL-17R signaling is required only in nonhematopoietic cells for immunity to systemic C. albicans infection. Since renal tubular epithelial cells (RTEC) are highly responsive to IL-17 in vitro, we hypothesized that RTEC might be the dominant target of IL-17 activity in the infected kidney. We generated mice with a conditional deletion of IL-17 receptor A (Il17ra) in RTEC (Il17raΔRTEC). Strikingly, Il17raΔRTEC mice showed enhanced kidney damage and early mortality following systemic infection, very similar to Il17ra-/- animals. Increased susceptibility to candidiasis in Il17raΔRTEC mice was associated with diminished activation of the renal protective Kallikrein-kinin system (KKS), resulting in reduced apoptosis of kidney-resident cells during hyphal invasion. Moreover, protection was restored by treatment with bradykinin, the major end-product of KKS activation, which was mediated dominantly via bradykinin receptor b1. These data show that IL-17R signaling in RTEC is necessary and likely sufficient for IL-17-mediated renal defense against fatal systemic C. albicans infection.

Accelerated podocyte detachment and progressive podocyte loss from glomeruli with age in Alport Syndrome.

Podocyte depletion is a common mechanism driving progression in glomerular diseases. Alport Syndrome glomerulopathy, caused by defective α3α4α5 (IV) collagen heterotrimer production by podocytes, is associated with an increased rate of podocyte detachment detectable in urine and reduced glomerular podocyte number suggesting that defective podocyte adherence to the glomerular basement membrane might play a role in driving progression. Here a genetically phenotyped Alport Syndrome cohort of 95 individuals [urine study] and 41 archived biopsies [biopsy study] were used to test this hypothesis. Podocyte detachment rate (measured by podocin mRNA in urine pellets expressed either per creatinine or 24-hour excretion) was significantly increased 11-fold above control, and prior to a detectably increased proteinuria or microalbuminuria. In parallel, Alport Syndrome glomeruli lose an average 26 podocytes per year versus control glomeruli that lose 2.3 podocytes per year, an 11-fold difference corresponding to the increased urine podocyte detachment rate. Podocyte number per glomerulus in Alport Syndrome biopsies is projected to be normal at birth (558/glomerulus) but accelerated podocyte loss was projected to cause end-stage kidney disease by about 22 years. Biopsy data from two independent cohorts showed a similar estimated glomerular podocyte loss rate comparable to the measured 11-fold increase in podocyte detachment rate. Reduction in podocyte number and density in biopsies correlated with proteinuria, glomerulosclerosis, and reduced renal function. Thus, the podocyte detachment rate appears to be increased from birth in Alport Syndrome, drives the progression process, and could potentially help predict time to end-stage kidney disease and response to treatment.

Podocyte-specific JAK2 overexpression worsens diabetic kidney disease in mice.

Activation of JAK-STAT signaling has been implicated in the pathogenesis of diabetic kidney disease. An increased expression of JAK-STAT genes was found in kidney glomerular cells, including podocytes, in patients with early diabetic kidney disease. However, it is not known whether increased expression of JAK or STAT isoforms in glomerular cells can lead to worsening nephropathy in the setting of diabetes. Therefore, we overexpressed JAK2 mRNA specifically in glomerular podocytes of 129S6 mice to determine whether this change alone could worsen diabetic kidney disease. A 2-3 fold increase in glomerular JAK2 expression, an increase similar to that found in humans with early diabetic kidney disease, led to substantial and statistically significant increases in albuminuria, mesangial expansion, glomerulosclerosis, glomerular fibronectin accumulation, and glomerular basement membrane thickening, and a significant reduction in podocyte density in diabetic mice. Treatment with a specific JAK1/2 inhibitor for 2 weeks partly reversed the major phenotypic changes of diabetic kidney disease and specifically normalized expression of a number of downstream STAT3-dependent genes implicated in diabetic kidney disease progression. Thus, moderate increases in podocyte JAK2 expression at levels similar to those in patients with early diabetic kidney disease can lead directly to phenotypic and other alterations of progressive diabetic glomerulopathy. Hence, inhibition of these changes by treatment with a JAK1/2 inhibitor suggests that such treatment may help retard progression of early diabetic kidney disease in patients.

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.

[The role of podocyte injury in chronic kidney disease].

It has recently become clear that initial glomerular injury affects glomerular visceral epithelial cells (also called as podocytes) as important target cells for progression of chronic kidney disease (CKD) and end-stage kidney disease. Podocytes are injured in many human kidney diseases including minimal change disease, focal segmental glomerulosclerosis, diabetic nephropathy, membranous nephropathy and lupus nephritis. Podocytes are highly specialized epithelial cells that cover the outer layer of the glomerular basement membrane (GBM). Podocytes serve as the final barrier to urinary protein loss through the special formation and maintenance of foot-processes and an interposed slit-diaphragm. Chronic podocyte injury may cause podocyte detachment from the GBM, which leads to glomerulosclerosis. The elucidation of podocyte biology during the last 15 years has significantly improved our understanding of the pathophysiologic processes of proteinuria and glomerulosclerosis. In this review, we highlight some of new data including our recent findings for translating podocyte biology into new examinations and therapies for podocyte injury.

Podocytes are nonhematopoietic professional antigen-presenting cells.

Podocytes are essential to the structure and function of the glomerular filtration barrier; however, they also exhibit increased expression of MHC class II molecules under inflammatory conditions, and they remove Ig and immune complexes from the glomerular basement membrane (GBM). This finding suggests that podocytes may act as antigen-presenting cells, taking up and processing antigens to initiate specific T cell responses, similar to professional hematopoietic cells such as dendritic cells or macrophages. Here, MHC-antigen complexes expressed exclusively on podocytes of transgenic mice were sufficient to activate CD8+ T cells in vivo. In addition, deleting MHC class II exclusively on podocytes prevented the induction of experimental anti-GBM nephritis. Podocytes ingested soluble and particulate antigens, activated CD4+ T cells, and crosspresented exogenous antigen on MHC class I molecules to CD8+ T cells. In conclusion, podocytes participate in the antigen-specific activation of adaptive immune responses, providing a potential target for immunotherapies of inflammatory kidney diseases and transplant rejection.

The podocyte's response to stress: the enigma of foot process effacement.

Progressive loss of podocytes is the most frequent cause accounting for end-stage renal failure. Podocytes are complex, terminally differentiated cells incapable of replicating. Thus lost podocytes cannot be replaced by proliferation of neighboring undamaged cells. Moreover, podocytes occupy a unique position as epithelial cells, adhering to the glomerular basement membrane (GBM) only by their processes, whereas their cell bodies float within the filtrate in Bowman's space. This exposes podocytes to the danger of being lost by detachment as viable cells from the GBM. Indeed, podocytes are continually excreted as viable cells in the urine, and the rate of excretion dramatically increases in glomerular diseases. Given this situation, it is likely that evolution has developed particular mechanisms whereby podocytes resist cell detachment. Podocytes respond to stress and injury by undergoing tremendous changes in shape. Foot process effacement is the most prominent and, yet in some ways, the most enigmatic of those changes. This review summarizes the various structural responses of podocytes to injury, focusing on foot process effacement and detachment. We raise the hypothesis that foot process effacement represents a protective response of podocytes to escape detachment from the GBM.

Role of the podocyte (and glomerular endothelium) in building the GBM.

This article summarizes the basic cellular and extracellular events in the development of the glomerulus and assembly of the glomerular basement membrane (GBM), paying special attention to laminin (LM) and type IV collagen. Cellular receptors for GBM proteins, including the integrins, dystroglycan, and discoidin domain receptor 1 also are discussed. Evidence is reviewed showing that the laminin isoform present in the earliest GBM, LM-111, and final isoform found in the mature GBM, LM-521, are each derived from both endothelial cells and podocytes. Although the early collagen α1α2α1(IV) similarly derives from endothelial cells and podocytes, collagen α3α4α5(IV) found in fully mature GBM is a product solely of podocytes. Genetic diseases affecting laminin and type IV collagen synthesis also are presented, with an emphasis on mutations to LAMB2 (Pierson syndrome) and COL4A3, COL4A4, and COL4A5 (Alport syndrome), and their experimental mouse models. Stress is placed on the assembly of a compositionally correct GBM for the acquisition and maintenance of glomerular barrier properties.

Mechanisms and consequences of TGF-ß overexpression by podocytes in progressive podocyte disease.

In patients with progressive podocyte disease, such as focal segmental glomerulosclerosis (FSGS) and membranous nephropathy, upregulation of transforming growth factor-ß (TGF-ß) is observed in podocytes. Mechanical pressure or biomechanical strain in podocytopathies may cause overexpression of TGF-ß and angiotensin II (Ang II). Oxidative stress induced by Ang II may activate the latent TGF-ß, which then activates Smads and Ras/extracellular signal-regulated kinase (ERK) signaling pathways in podocytes. Enhanced TGF-ß activity in podocytes may lead to thickening of the glomerular basement membrane (GBM) by overproduction of GBM proteins and impaired GBM degradation in podocyte disease. It may also lead to podocyte apoptosis and detachment from the GBM, and epithelial-mesenchymal transition (EMT) of podocytes, initiating the development of glomerulosclerosis. Furthermore, activated TGF-ß/Smad signaling by podocytes may induce connective tissue growth factor and vascular endothelial growth factor overexpression, which could act as a paracrine effector mechanism on mesangial cells to stimulate mesangial matrix synthesis. In proliferative podocytopathies, such as cellular or collapsing FSGS, TGF-ß-induced ERK activation may play a role in podocyte proliferation, possibly via TGF-ß-induced EMT of podocytes. Collectively, these data bring new mechanistic insights into our understanding of the TGF-ß overexpression by podocytes in progressive podocyte disease.

Deletion of von Hippel-Lindau in glomerular podocytes results in glomerular basement membrane thickening, ectopic subepithelial deposition of collagen {alpha}1{alpha}2{alpha}1(IV), expression of neuroglobin, and proteinuria.

Vascular endothelial growth factor, which is critical for blood vessel formation, is regulated by hypoxia inducible transcription factors (HIFs). A component of the E3 ubiquitin ligase complex, von Hippel-Lindau (VHL) facilitates oxygen-dependent polyubiquitination and proteasomal degradation of HIFalpha subunits. Hypothesizing that deletion of podocyte VHL would result in HIFalpha hyperstabilization, we crossed podocin promoter-Cre transgenic mice, which express Cre recombinase in podocytes beginning at the capillary loop stage of glomerular development, with floxed VHL mice. Vascular patterning and glomerular development appeared unaltered in progeny lacking podocyte VHL. However, urinalysis showed increased albumin excretion by 4 weeks when compared with wild-type littermates with several sever cases (>1000 microg/ml). Many glomerular ultrastructural changes were seen in mutants, including focal subendothelial delamination and widespread podocyte foot process broadening, and glomerular basement membranes (GBMs) were significantly thicker in 16-week-old mutants compared with controls. Moreover, immunoelectron microscopy showed ectopic deposition of collagen alpha1alpha2alpha1(IV) in GBM humps beneath podocytes. Significant increases in the number of Ki-67-positive mesangial cells were also found, but glomerular WT1 expression was significantly decreased, signifying podocyte death and/or de-differentiation. Indeed, expression profiling of mutant glomeruli suggested a negative regulatory feedback loop involving the HIFalpha prolyl hydroxylase, Egln3. In addition, the brain oxygen-binding protein, Neuroglobin, was induced in mutant podocytes. We conclude that podocyte VHL is required for normal maintenance of podocytes, GBM composition and ultrastructure, and glomerular barrier properties.

Mechanical forces and TGFbeta1 reduce podocyte adhesion through alpha3beta1 integrin downregulation.

BACKGROUND: Podocyturia is a marker of diabetic nephr- opathy, a possible determinant of its progression and a powerful risk factor for cardiovascular disease. A reduction in podocyte adhesion to the glomerular basement membrane (GBM) via downregulation of alpha3beta1 integrin expression, the main podocyte anchoring dimer to the GBM, may represent one of the mechanisms of podocyturia in glomerular disease. This study investigated the role of mechanical forces and transforming growth factor beta1 (TGFbeta1) in podocyte adhesion and integrin expression. METHODS: Conditionally immortalized murine podocytes were exposed to mechanical stretch and/or TGFbeta1 for 48 h. Podocyte adhesion, apoptosis and alpha3beta1 integrin expression were assessed. RESULTS: Stretch and TGFbeta1 significantly reduced podocyte adhesion and alpha3beta1 integrin expression, events paralleled by increased apoptosis. Blockade of beta1 integrin, with a specific antibody, demonstrated a reduced podocyte adhesion indicating that beta1 integrin downregulation was required for the loss of podocyte adhesion. This was linked to an increase in podocyte apoptosis. The role of apoptosis in podocyte adhesion was further investigated using caspase-3 inhibitors. Podocyte apoptosis inhibition did not affect stretch- and TGFbeta1-mediated integrin downregulation and the loss of podocyte adhesion, suggesting that alpha3beta1 integrin downregulation is sufficient to alter cell adhesion. Although stretch significantly increased podocyte TGFbeta type I, II and III receptors but not podocyte TGFbeta1 secretion, the combination of stretch and TGFbeta1 did not show any additive or synergistic effects on podocyte adhesion and alpha3beta1 integrin expression. CONCLUSIONS: These results suggest that downregulation of alpha3beta1 integrin expression, by mechanical forces or TGFbeta1, is per se sufficient to reduce podocyte adhesion. Apoptosis may represent a parallel important determinant of the podocyte loss from the GBM.