Are podocytes passive or provocative in proteinuric glomerular pathology?

New insights into the immune functions of podocytes: the role of complement

Podocytes are differentiated epithelial cells which play an essential role to ensure a normal function of the glomerular filtration barrier (GFB). In addition to their adhesive properties in maintaining the integrity of the filtration barrier, they have other functions, such as synthesis of components of the glomerular basement membrane (GBM), production of vascular endothelial growth factor (VEGF), release of inflammatory proteins, and expression of complement components. They also participate in the glomerular crosstalk through multiple signalling pathways, including endothelin-1, VEGF, transforming growth factor β (TGFβ), bone morphogenetic protein 7 (BMP-7), latent transforming growth factor β-binding protein 1 (LTBP1), and extracellular vesicles.Growing literature suggests that podocytes share many properties of innate and adaptive immunity, supporting a multifunctional role ensuring a healthy glomerulus. As consequence, the "immune podocyte" dysfunction is thought to be involved in the pathogenesis of several glomerular diseases, referred to as "podocytopathies." Multiple factors like mechanical, oxidative, and/or immunologic stressors can induce cell injury. The complement system, as part of both innate and adaptive immunity, can also define podocyte damage by several mechanisms, such as reactive oxygen species (ROS) generation, cytokine production, and endoplasmic reticulum stress, ultimately affecting the integrity of the cytoskeleton, with subsequent podocyte detachment from the GBM and onset of proteinuria.Interestingly, podocytes are found to be both source and target of complement-mediated injury. Podocytes express complement proteins which contribute to local complement activation. At the same time, they rely on several protective mechanisms to escape this damage. Podocytes express complement factor H (CFH), one of the main regulators of the complement cascade, as well as membrane-bound complement regulators like CD46 or membrane cofactor protein (MCP), CD55 or decay-accelerating factor (DAF), and CD59 or defensin. Further mechanisms, like autophagy or actin-based endocytosis, are also involved to ensure podocyte homeostasis and protection against injury.This review will provide an overview of the immune functions of podocytes and their response to immune-mediated injury, focusing on the pathogenic link between complement and podocyte damage.

Protein convertase subtilisin/kexin type 9 biology in nephrotic syndrome: implications for use as therapy

Low-density lipoprotein cholesterol (LDL-C) levels almost constantly increased in patients with nephrotic syndrome (NS). Protein convertase subtilisin/kexin type 9 (PCSK9) [accelerates LDL-receptor (LDL-R) degradation] is overexpressed by liver cells in NS. Their levels, correlated inversely to LDL-R expression and directly to LDL-C, seem to play a central role in hypercholesterolaemia in NS. Hypersynthesis resulting from sterol regulatory element-binding protein dysfunction, hyperactivity induced by c-inhibitor of apoptosis protein expressed in response to stimulation by tumour necrosis factor-α produced by damaged podocytes and hypo-clearance are the main possible mechanisms. Increased LDL-C may damage all kidney cell populations (podocytes, mesangial and tubular cells) in a similar manner. Intracellular cholesterol accumulation produces oxidative stress, foam cell formation and apoptosis, all favoured by local inflammation. The cumulative effect of cellular lesions is worsened proteinuria and kidney function loss. Accordingly, NS patients should be considered high risk and treated by lowering LDL-C. However, there is still not enough evidence determining whether lipid-lowering agents are helpful in managing dyslipidaemia in NS. Based on good efficacy and safety proved in the general population, therapeutic modulation of PCSK9 via antibody therapy might be a reasonable solution. This article explores the established and forthcoming evidence implicating PCSK9 in LDL-C dysregulation in NS.

Role of Mesangial-Podocytic-Tubular Cross-Talk in IgA Nephropathy

IgA nephropathy (IgAN), a common primary glomerulonephritis worldwide, is associated with a substantial risk of progression to end-stage renal failure. The disease runs a highly variable clinical course with frequent involvement of tubulointerstitial damage. A subgroup of IgAN with proximal tubular epithelial cells (PTECs) and tubulointerstitial damage often is associated with rapid progression to end-stage renal failure. Human mesangial cell-derived mediators lead to podocyte and tubulointerstitial injury via mesangial-podocytic-tubular cross-talk. Although mesangial-podocytic communication plays a pathogenic role in podocytic injury, the implication of a podocyte-PTEC cross-talk pathway in the progression of tubulointerstitial injury in IgAN should not be underscored. We review the role of mesangial-podocytic-tubular cross-talk in the progression of IgAN. We discuss how podocytopathy in IgAN promotes subsequent PTEC dysfunction and whether tubulointerstitial injury affects the propagation of podocytic injury in IgAN. A thorough understanding of the cross-talk mechanisms among mesangial cells, podocytes, and PTECs may lead to better design of potential therapeutic options for IgAN.

Valproate hampers podocyte acquisition of immune phenotypes via intercepting the GSK3β facilitated NFkB activation

Glomerular podocytes are able to transdifferentiate under disease conditions, acquire de novo immune phenotypes and behave as immunocompetent cells, like phagocytes or antigen-presenting cells. Upon stimulation with lipopolysaccharide (LPS), a prototypical pathogen-associated molecular pattern, podocytes demonstrated de novo expression of a variety of NFkB-dependent immune molecules that are pivotal for immune response, including major histocompatibility complex (MHC) class II, costimulatory molecule CD80, lysosomal protease cathepsin L as well as CC chemokine ligand 2 and 5, ultimately resulting in podocyte dysfunction, characterized by cellular shrinkage, a spindle-like or asterlike cell shape and impairment of actin cytoskeleton integrity. The LPS-elicited podocyte phenotypic changes were concurrent with nuclear factor (NF) kB phosphorylation, which was associated with glycogen synthase kinase (GSK) 3β overactivity, marked by a diminished inhibitory phosphorylation of GSK3β. In contrast, valproate, an anticonvulsant and mood stabilizer, offset GSK3β overactivity in LPS-injured podocytes and mitigated NFkB activation and podocyte acquisition of immune phenotypes as well as the ensuing cytopathic changes, podocyte cytoskeleton disorganization and dysfunction. The protective effect of valproate was strikingly blunted in podocytes expressing the constitutively active GSK3β, suggesting an essential role of inhibitory phosphorylation of GSK3β. In vivo in LPS-injured mice, valproate therapy abolished GSK3β overactivity in glomeruli and attenuated podocyte injury and albuminuria, concomitant with a lessened NFkB activation and diminished induction of diverse podocytopathic immune molecules in podocytes and glomeruli. Taken together, valproate directly protects against podocyte injury and hampers podocyte acquisition of de novo immune phenotypes via intercepting the GSK3β facilitated NFkB activation.

The level of urinary semaphorin3A is associated with disease activity in patients with minimal change nephrotic syndrome

Semaphorin3A is a secreted protein known to be involved in organogenesis, immune responses and cancer. In the kidney, semaphorin3A is expressed in the glomerular podocytes, distal tubules and collecting tubules, and believed to play a role in the regulation of the kidney development and function. We examined the serum and urinary semaphorin3A levels in 72 patients with renal disease and 5 healthy volunteers. The patients had been diagnosed with thin basement membrane disease (n=4), minimal change nephrotic syndrome (MCNS; n=22), IgA nephritis (n=21), membranous nephropathy (n=16) and focal segmental glomerular sclerosis (n=9). The level of urinary semaphorin3A in MCNS patients tended to be relatively high among all disease groups. We also investigated the urinary semaphorin3A level in 7 patients with MCNS from disease onset to remission during the drug therapy. MCNS patients in pre-remission states had higher urinary semaphorin3A levels than those in post-remission states receiving immunosuppressive therapies. These results suggested that the urinary semaphorin3A level correlates with the MCNS activity. Semaphorin3A has the potential as a biomarker for MCNS to clarify the reactivity for therapy and may be useful in examining other glomerular diseases with proteinuria as well.

Unfractionated bone marrow cells attenuate paraquat-induced glomerular injury and acute renal failure by modulating the inflammatory response

The aim of this study was to evaluate the efficacy of unfractionated bone marrow cells (BMCs) in attenuating acute kidney injury (AKI) induced by paraquat (PQ) in a mouse model. PQ (55 mg/kg BW) was intraperitoneally injected into C57BL/6 female mice to induce AKI, including renal function failure, glomerular damage and renal tubule injury. Glomerular podocytes were the first target damaged by PQ, which led to glomerular injury. Upon immunofluorescence staining, podocytes depletion was validated and accompanied by increased urinary podocin levels, measured on days 1 and 6. A total of 5.4 × 10(6) BMCs obtained from the same strain of male mice were injected into AKI mice through the tail vein at 3, 24, and 48 hours after PQ administration. As a result, renal function increased, tubular and glomerular injury were ameliorated, podocytes loss improved, and recipient mortality decreased. In addition, BMCs co-treatment decreased the extent of neutrophil infiltration and modulated the inflammatory response by shifting from pro-inflammatory Th1 to an anti-inflammatory Th2 profile, where IL-1β, TNF-α, IL-6 and IFN-γ levels declined and IL-10 and IL-4 levels increased. The present study provides a platform to investigate PQ-induced AKI and repeated BMCs injection represents an efficient therapeutic strategy.

Effects of ROS-relative NF-κB signaling on high glucose-induced TLR4 and MCP-1 expression in podocyte injury

High glucose (HG) induced inflammation is central to progression in diabetic nephropathy (DN). Recent studies have suggested that nuclear factor-kappa B (NF-κB) signaling activation is associated with DN, and podocyte damage may be involved in orchestrating these effects. Therefore, the aim of this study was to investigate the effects of NF-κB signaling on podocytes under HG conditions. The effects of HG and NF-κB signaling on podocytes were assessed by CCK-8 assay, cellular NF-κB translocation assay, measurement of reactive oxygen species (ROS) and Western blot analysis. We found that HG reduced cell viability, activated NF-κB signaling and up-regulated toll-like receptor 4 (TLR4) and monocyte chemoattractant protein-1 (MCP-1). In these cells, NF-κB inhibition with ammonium pyrrolidinethiocarbamate (PDTC) resulted in effectively constraining TLR4 and MCP-1 up-regulation, indicating that protective effects associated with the inhibition of NF-κB were linked to TLR4 and MCP-1 down-regulation in podocytes. Furthermore, HG significantly increased the production of intracellular ROS. Pretreatment with N-acetyl-l-cysteine (NAC) significantly inhibited intracellular ROS generation and increased cell viability, accompanied by a significant NF-κB inhibition and suppression of TLR4 and inflammatory cytokine MCP-1 expression. Collectively, our novel data suggest that HG induces the over-experssion of TLR-4 and MCP-1 through a NF-κB-dependent signaling. NF-κB-mediated increased inflammation is possibly via ROS and contributes to the cell injury. These results may provide potential therapeutic target for diabetic nephropathy in the future.

Crosstalk between protective autophagy and NF-κB signal in high glucose-induced podocytes

Despite a great deal of recent studies focused on the pivotal role of autophagy in maintaining podocyte energy homeostasis, the mechanisms of autophagy in regulating transcriptional factors under high glucose (HG) condition are not fully understood. Here, we evaluated the effect of HG on nuclear factor-kappa B (NF-κB) signaling and autophagic process. The results showed that HG promoted autophagy in podocytes. Bafilomycin A1 (Baf A1) further enhanced this effect, but 3-methyadenine (3-MA) inhibited it. The proautophagic effects of HG manifested in the form of enhanced podocyte expression of light chain 3 (LC3)-II. In these cells, blockade of NF-κB signal by ammonium pyrrolidinethiocarbamate constrained in effectively reducing LC3-II up-regulation and increasing podocyte apoptosis. Furthermore, the autophagy inhibitors, such as Baf A1 and 3-MA, significantly enhanced HG-induced NF-κB activation and increased apoptosis. Thus, we conclude that the accumulation of autophagosomes results from enhancement of the autophagic flux, but not the blockage of autophagosome-lysosome fusion by HG. We also prove that HG-induced apoptosis, autophagy, and NF-κB signal are in a close crosstalk through a yet undetermined mechanism in podocytes.

LPS and PAN-induced podocyte injury in an in vitro model of minimal change disease: changes in TLR profile

Minimal change disease (MCD), the most common idiopathic nephrotic syndrome in children, is characterized by proteinuria and loss of glomerular visceral epithelial cell (podocyte) ultrastructure. Lipopolysaccharide (LPS) and puromycin aminonucleoside (PAN) are used to study podocyte injury in models of MCD in vivo and in vitro. We hypothesized that LPS and PAN influence components of the innate immune system in podocytes such as the Toll-Like Receptor (TLRs), TLR adapter molecules, and associated cytokines. Our results show that cultured human podocytes constitutively express TLRs 1-6 and TLR-10, but not TLRs 7-9. LPS (25 μg/ml) or PAN (60 μg/ml) caused comparable derangement of the actin cytoskeleton in podocytes. Quantitative RT-PCR analysis show that LPS differentially up-regulated the expression of genes for TLRs (1 > 4 ≥ 2 > 3 > 6 > 5), the adapter molecule, MyD88, and transcription factor NF-κB within one hour. LPS also caused increased levels of IL-6, IL-8 and MCP1 without exerting any effect on TNF-α, IFN-α or TGF-β1 at 24 h. Immunofluorescence intensity analysis of confocal microscopy images showed that LPS induced a significant increase in nuclear translocation of NF-κB by 6 h. In contrast, PAN-induced only small changes in the expression of TLRs 2-6 that included a persistent increase in TLRs 2 and 5, a transient increase in TLR-4, and a gradual increase in TLRs 3 and 6 between 1 and 6 h. Correspondingly, it did not alter pro-inflammatory cytokine levels in podocytes. However, PAN induced a low but significant increase in NF-κB nuclear translocation within one hour that remained unchanged up to 6 h. In summary, these novel findings show that LPS, a known TLR-4 ligand, induced the gene expression of multiple TLRs with maximum effect on the expression of TLR-1 suggesting a loss of receptor selectivity and induction of receptor interactions in podocytes. A comparable derangement of the podocyte cytoskeleton and significant increase in the nuclear translocation of NF-κB by PAN suggest that disparate but complementary mechanisms may contribute to the development of podocytopathy in MCD.

TNFR2 interposes the proliferative and NF-κB-mediated inflammatory response by podocytes to TNF-α

The development of proliferative podocytopathies has been linked to ligation of tumor necrosis factor receptor 2 (TNFR2) expressed on the renal parenchyma; however, the TNFR2-positive cells within the kidney responsible for podocyte injury are unknown. We detected de novo expression of TNFR2 on podocytes before hyperplastic injury in crescentic glomerulonephritis of mice with nephrotoxic nephritis, and in collapsing glomerulopathy of Tg26(HIV/nl) mice, kd/kd mice, and human beings. We further found that serum levels of soluble TNF-α and TNFR2 correlated significantly with renal injury in Tg26(HIV/nl) mice. Thus, we asked whether ligand binding of TNFR2 on podocytes ex vivo precipitates the characteristic proliferative and pro-inflammatory diseased podocyte phenotypes. Soluble TNF-α activated NF-κB and dose-dependently induced podocyte proliferation, marked by the expression of the podocyte G(1) cyclin and NF-κB target gene, cyclin D1. Microarray gene and chemokine protein expression profiling showed a marked pro-inflammatory NF-κB signature, and activated podocytes secreting CCL2- and CCL5-induced macrophage migration in transwell assays. Neutralization of TNFR2 on podocytes with blocking antibodies abrogated NF-κB activation and the induction of cyclin D1 by TNF-α, and identified TNFR2 as the primary receptor that induced IκBα degradation, the initiating event in NF-κB activation. These results suggest that TNFR2 expressed on podocytes and its canonical NF-κB signaling may directly interpose the compound pathogenic responses by podocytes to TNF-α, in the absence of other TNFR2-positive renal cell types in proliferative podocytopathies.

Angiopoietin-like protein 3 regulates the motility and permeability of podocytes by altering nephrin expression in vitro

It is well known that podocyte injury plays a vital role in massive proteinuria. The increase of podocyte motility results in podocyte foot process (FP) effacement, a typical form of podocyte injury. Our previous studies demonstrated that glomerular podocytes can express angiopoietin-like protein 3 (ANGPTL3) and that the increase of ANGPTL3 in dysfunctional glomerulus is correlated with podocyte FP effacement. Little is known, however, about the role of ANGPTL3 in podocyte injury. In this study, we investigated ANGPTL3's effect on the motility and permeability of podocytes and on the expression of nephrin, a key molecule in podocytes. By scrape-wound and transwell migration assay, we found that ANGPTL3 over-expression significantly increased podocyte motility, whereas after ANGPTL3 knockdown by RNA interference, motility remained the same as that of the control group. Adriamycin (ADR) treatment significantly promoted podocyte motility. However, the same dose of ADR treatment could not promote motility after the knockdown of ANGPTL3. In addition, we assayed the diffusion of FITC-BSA across the podocytes' monolayer to investigate whether ANGPTL3 could promote protein loss by means of an increase in podocyte motility. The results showed that the changes in the FITC-BSA permeability of the podocytes corresponded to changes in motility. Furthermore, we found that ANGPTL3 over-expression dramatically increased the expression of nephrin but that the up-regulation of nephrin induced by ADR was significantly inhibited when ANGPTL3 was diminished by RNAi. In conclusion, we found ANGPTL3 to be capable of regulating the motility and permeability of podocytes and that the mechanism of ANGPTL3's regulation could be associated with the altered expression of nephrin.

alpha- and beta-Adducin polymorphisms affect podocyte proteins and proteinuria in rodents and decline of renal function in human IgA nephropathy

Adducins are cytoskeletal actin-binding proteins (α, β, γ) that function as heterodimers and heterotetramers and are encoded by distinct genes. Experimental and clinical evidence implicates α- and β-adducin variants in hypertension and renal dysfunction. Here, we have addressed the role of α- and β-adducin on glomerular function and disease using β-adducin null mice, congenic substrains for α- and β-adducin from the Milan hypertensive (MHS) and Milan normotensive (MNS) rats and patients with IgA nephropathy. Targeted deletion of β-adducin in mice reduced urinary protein excretion, preceded by an increase of podocyte protein expression (phospho-nephrin, synaptopodin, α-actinin, ZO-1, Fyn). The introgression of polymorphic MHS β-adducin locus into MNS (Add2, 529R) rats was associated with an early reduction of podocyte protein expression (nephrin, synaptopodin, α-actinin, ZO-1, podocin, Fyn), followed by severe glomerular and interstitial lesions and increased urinary protein excretion. These alterations were markedly attenuated when the polymorphic MHS α-adducin locus was also present (Add1, 316Y). In patients with IgA nephropathy, the rate of decline of renal function over time was associated to polymorphic β-adducin (ADD2, 1797T, rs4984) with a significant interaction with α-adducin (ADD1, 460W, rs4961). These findings suggest that adducin genetic variants participate in the development of glomerular lesions by modulating the expression of specific podocyte proteins.