Assessment and Risk Prediction of Chronic Kidney Disease and Kidney Fibrosis Using Non-Invasive Biomarkers

Effective management of chronic kidney disease (CKD), a major health problem worldwide, requires accurate and timely diagnosis, prognosis of progression, assessment of therapeutic efficacy, and, ideally, prediction of drug response. Multiple biomarkers and algorithms for evaluating specific aspects of CKD have been proposed in the literature, many of which are based on a small number of samples. Based on the evidence presented in relevant studies, a comprehensive overview of the different biomarkers applicable for clinical implementation is lacking. This review aims to compile information on the non-invasive diagnostic, prognostic, and predictive biomarkers currently available for the management of CKD and provide guidance on the application of these biomarkers. We specifically focus on biomarkers that have demonstrated added value in prospective studies or those based on prospectively collected samples including at least 100 subjects. Published data demonstrate that several valid non-invasive biomarkers of potential value in the management of CKD are currently available.

Deficiency of Complement C3a and C5a receptors Does Not Prevent Angiotensin II-Induced Hypertension and Hypertensive End-Organ Damage

BACKGROUND

Complement may drive the pathology of hypertension through effects on innate and adaptive immune responses. Recently an injurious role for the anaphylatoxin receptors C3aR (complement component 3a receptor) and C5aR1 (complement component 5a receptor) in the development of hypertension was shown through downregulation of Foxp3+ (forkhead box protein 3) regulatory T cells. Here, we deepen our understanding of the therapeutic potential of targeting both receptors in hypertension.

METHODS

Data from the European Renal cDNA Bank, single cell sequencing and immunohistochemistry were examined in hypertensive patients. The effect of C3aR or C3aR/C5aR1 double deficiency was assessed in two models of Ang II (angiotensin II)-induced hypertension in knockout mice.

RESULTS

We found increased expression of C3aR, C5aR1 and Foxp3 cells in kidney biopsies of patients with hypertensive nephropathy. Expression of both receptors was mainly found in myeloid cells. No differences in blood pressure, renal injury (albuminuria, glomerular filtration rate, glomerular and tubulointerstitial injury, inflammation) or cardiac injury (cardiac fibrosis, heart weight, gene expression) between control and mutant mice was discerned in C3aR-/- as well as C3aR/C5aR1-/- double knockout mice. The number of renal Tregs was not decreased in Ang II as well as in DOCA salt induced hypertension.

CONCLUSIONS

Hypertensive nephropathy in mice and men is characterized by an increase of renal regulatory T cells and enhanced expression of anaphylatoxin receptors. Our investigations do not corroborate a role for C3aR/C5aR1 axis in Ang II-induced hypertension hence challenging the concept of anaphylatoxin receptor targeting in the treatment of hypertensive disease.

Complement component C3 as a new target to lower albuminuria in hypertensive kidney disease

BACKGROUND AND PURPOSE

Complement activation may drive hypertension through its effects on immunity and tissue integrity.

EXPERIMENTAL APPROACH

We examined expression of C3, the central protein of the complement cascade, in hypertension.

KEY RESULTS

Increased C3 expression was found in kidney biopsies and micro-dissected glomeruli of patients with hypertensive nephropathy. Renal single cell RNA sequence data from normotensive and hypertensive patients confirmed expression of C3 in different cellular compartments of the kidney. In angiotensin II (Ang II) induced hypertension renal C3 expression was up-regulated. C3-/- mice revealed a significant lower albuminuria in the early phase of hypertension. However, no difference was found for blood pressure, renal injury (histology, glomerular filtration rate, inflammation) and cardiac injury (fibrosis, weight, gene expression) between C3-/- and wildtype mice after Ang II infusion. Also, in deoxycorticosterone acetate (DOCA) salt hypertension, a significantly lower albuminuria was found in the first weeks of hypertension in C3 deficient mice but no significant difference in renal and cardiac injury. Down-regulation of C3 by C3 targeting GalNAc (n-acetylgalactosamine) small interfering RNA (siRNA) conjugate decreased C3 in the liver by 96% and lowered albuminuria in the early phase but showed no effect on blood pressure and end-organ damage. Inhibition of complement C5 by siRNA showed no effect on albuminuria.

CONCLUSION AND IMPLICATIONS

Increased C3 expression is found in the kidneys of hypertensive mice and men. Genetic and therapeutic knockdown of C3 improved albuminuria in the early phase of hypertension but did not ameliorate arterial blood pressure nor renal and cardiac injury.

An integrated organoid omics map extends modeling potential of kidney disease

Kidney organoids are a promising model to study kidney disease, but their use is constrained by limited knowledge of their functional protein expression profile. Here, we define the organoid proteome and transcriptome trajectories over culture duration and upon exposure to TNFα, a cytokine stressor. Older organoids increase deposition of extracellular matrix but decrease expression of glomerular proteins. Single cell transcriptome integration reveals that most proteome changes localize to podocytes, tubular and stromal cells. TNFα treatment of organoids results in 322 differentially expressed proteins, including cytokines and complement components. Transcript expression of these 322 proteins is significantly higher in individuals with poorer clinical outcomes in proteinuric kidney disease. Key TNFα-associated protein (C3 and VCAM1) expression is increased in both human tubular and organoid kidney cell populations, highlighting the potential for organoids to advance biomarker development. By integrating kidney organoid omic layers, incorporating a disease-relevant cytokine stressor and comparing with human data, we provide crucial evidence for the functional relevance of the kidney organoid model to human kidney disease.

Cold Shock Domain Protein DbpA Orchestrates Tubular Cell Damage and Interstitial Fibrosis in Inflammatory Kidney Disease

DNA-binding protein A (DbpA) belongs to the Y-box family of cold shock domain proteins that exert transcriptional and translational activities in the cell via their ability to bind and regulate mRNA. To investigate the role of DbpA in kidney disease, we utilized the murine unilateral ureter obstruction (UUO) model, which recapitulates many features of obstructive nephropathy seen in humans. We observed that DbpA protein expression is induced within the renal interstitium following disease induction. Compared with wild-type animals, obstructed kidneys from Ybx3-deficient mice are protected from tissue injury, with a significant reduction in the number of infiltrating immune cells as well as in extracellular matrix deposition. RNAseq data from UUO kidneys show that Ybx3 is expressed by activated fibroblasts, which reside within the renal interstitium. Our data support a role for DbpA in orchestrating renal fibrosis and suggest that strategies targeting DbpA may be a therapeutic option to slow disease progression.

Down-regulation of human long non-coding RNA LINC01187 is associated with nephropathies

Chronic kidney diseases affect a substantial percentage of the adult population worldwide. This observation emphasizes the need for novel insights into the molecular mechanisms that control the onset and progression of renal diseases. Recent advances in genomics have uncovered a previously unanticipated link between the non-coding genome and human kidney diseases. Here we screened and analysed long non-coding RNAs (lncRNAs) previously identified in mouse kidneys by genome-wide transcriptomic analysis, for conservation in humans and differential expression in renal tissue from healthy and diseased individuals. Our data suggest that LINC01187 is strongly down-regulated in human kidney tissues of patients with diabetic nephropathy and rapidly progressive glomerulonephritis, as well as in murine models of kidney diseases, including unilateral ureteral obstruction, nephrotoxic serum-induced glomerulonephritis and ischemia/reperfusion. Interestingly, LINC01187 overexpression in human kidney cells in vitro inhibits cell death indicating an anti-apoptotic function. Collectively, these data suggest a negative association of LINC01187 expression with renal diseases implying a potential protective role.

CD4+ T cells produce GM-CSF and drive immune-mediated glomerular disease by licensing monocyte-derived cells to produce MMP12

GM-CSF in glomerulonephritisDespite glomerulonephritis being an immune-mediated disease, the contributions of individual immune cell types are not clear. To address this gap in knowledge, Paust et al. characterized pathological immune cells in samples from patients with glomerulonephritis and in samples from mice with the disease. The authors found that CD4+ T cells producing granulocyte-macrophage colony-stimulating factor (GM-CSF) licensed monocytes to promote disease by producing matrix metalloproteinase 12 and disrupting the glomerular basement membrane. Targeting GM-CSF to inhibit this axis reduced disease severity in mice, implicating this cytokine as a potential therapeutic target for patients with glomerulonephritis. -CM.

Single-cell transcriptomics reveals a mechanosensitive injury signaling pathway in early diabetic nephropathy

BACKGROUND

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease, and histopathologic glomerular lesions are among the earliest structural alterations of DN. However, the signaling pathways that initiate these glomerular alterations are incompletely understood.

METHODS

To delineate the cellular and molecular basis for DN initiation, we performed single-cell and bulk RNA sequencing of renal cells from type 2 diabetes mice (BTBR ob/ob) at the early stage of DN.

RESULTS

Analysis of differentially expressed genes revealed glucose-independent responses in glomerular cell types. The gene regulatory network upstream of glomerular cell programs suggested the activation of mechanosensitive transcriptional pathway MRTF-SRF predominantly taking place in mesangial cells. Importantly, activation of MRTF-SRF transcriptional pathway was also identified in DN glomeruli in independent patient cohort datasets. Furthermore, ex vivo kidney perfusion suggested that the regulation of MRTF-SRF is a common mechanism in response to glomerular hyperfiltration.

CONCLUSIONS

Overall, our study presents a comprehensive single-cell transcriptomic landscape of early DN, highlighting mechanosensitive signaling pathways as novel targets of diabetic glomerulopathy.

Molecular consequences of SARS-CoV-2 liver tropism

Extrapulmonary manifestations of COVID-19 have gained attention due to their links to clinical outcomes and their potential long-term sequelae¹. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) displays tropism towards several organs, including the heart and kidney. Whether it also directly affects the liver has been debated^2,3. Here we provide clinical, histopathological, molecular and bioinformatic evidence for the hepatic tropism of SARS-CoV-2. We find that liver injury, indicated by a high frequency of abnormal liver function tests, is a common clinical feature of COVID-19 in two independent cohorts of patients with COVID-19 requiring hospitalization. Using autopsy samples obtained from a third patient cohort, we provide multiple levels of evidence for SARS-CoV-2 liver tropism, including viral RNA detection in 69% of autopsy liver specimens, and successful isolation of infectious SARS-CoV-2 from liver tissue postmortem. Furthermore, we identify transcription-, proteomic- and transcription factor-based activity profiles in hepatic autopsy samples, revealing similarities to the signatures associated with multiple other viral infections of the human liver. Together, we provide a comprehensive multimodal analysis of SARS-CoV-2 liver tropism, which increases our understanding of the molecular consequences of severe COVID-19 and could be useful for the identification of organ-specific pharmacological targets.

Studies on the Role of the Transcription Factor Tcf21 in the Transdifferentiation of Parietal Epithelial Cells into Podocyte-Like Cells

BACKGROUND/AIMS

Podocyte differentiation is essential for proper blood filtration in the kidney. It is well known that transcription factors play an essential role to maintain the differentiation of podocytes. The present study is focused on the basic helix-loop-helix (bHLH) transcription factor Tcf21 (Pod1) which is essential for the development of podocytes in vivo. Since parietal epithelial cells (PECs) are still under debate to be progenitor cells which can differentiate into podocytes, we wanted to find out whether the expression of Tcf21 induces a transition of PECs into podocytes.

METHODS

We transfected PECs with Tcf21-GFP and analyzed the expression of PEC- and podocyte-specific markers. Furthermore, we performed ChIP-Seq analysis to identify new putative interaction partners and target genes of Tcf21.

RESULTS

By gene arrays analysis, we found that podocytes express high levels of Tcf21 in vivo in contrast to cultured podocytes and parietal epithelial cells (PECs) in vitro. After the expression of Tcf21 in PECs, we observed a downregulation of specific PEC markers like caveolin‑1, β-catenin and Pax2. Additionally, we found that the upregulation of Tcf21 induced multi-lobulation of cell nuclei, budding and a formation of micronuclei (MBM). Furthermore, a high number of PECs showed a tetraploid set of chromosomes. By qRT-PCR and Western blot analysis, we revealed that the transcription factor YY1 is downregulated by Tcf21. Interestingly, co-expression of YY1 and Tcf21 rescues MBM and reduced tetraploidy. By ChIP-Seq analysis, we identified a genome-wide Tcf21-binding site (CAGCTG), which matched the CANNTG sequence, a common E-box binding motif used by bHLH transcription factors. Using this technique, we identified additional Tcf21 targets genes that are involved in the regulation of the cell cycle (e.g. Mdm2, Cdc45, Cyclin D1, Cyclin D2), on the stability of microtubules (e.g. Mapt) as well as chromosome segregation.

CONCLUSION

Taken together, we demonstrate that Tcf21 inhibits the expression of PEC-specific markers and of the transcription factor YY1, induces MBM as well as regulates the cell cycle suggesting that Tcf21 might be important for PEC differentiation into podocyte-like cells.

The role of filamins in mechanically stressed podocytes

Glomerular hypertension induces mechanical load to podocytes, often resulting in podocyte detachment and the development of glomerulosclerosis. Although it is well known that podocytes are mechanosensitive, the mechanosensors and mechanotransducers are still unknown. Since filamin A, an actin-binding protein, is already described to be a mechanosensor and mechanotransducer, we hypothesized that filamins could be important for the outside-in signaling as well as the actin cytoskeleton of podocytes under mechanical stress. In this study, we demonstrate that filamin A is the main isoform of the filamin family that is expressed in cultured podocytes. Together with filamin B, filamin A was significantly up-regulated during mechanical stretch (3 days, 0.5 Hz, and 5% extension). To study the role of filamin A in cultured podocytes under mechanical stress, filamin A was knocked down (Flna KD) by specific siRNA. Additionally, we established a filamin A knockout podocyte cell line (Flna KO) by CRISPR/Cas9. Knockdown and knockout of filamin A influenced the expression of synaptopodin, a podocyte-specific protein, focal adhesions as well as the morphology of the actin cytoskeleton. Moreover, the cell motility of Flna KO podocytes was significantly increased. Since the knockout of filamin A has had no effect on cell adhesion of podocytes during mechanical stress, we simultaneously knocked down the expression of filamin A and B. Thereby, we observed a significant loss of podocytes during mechanical stress indicating a compensatory mechanism. Analyzing hypertensive mice kidneys as well as biopsies of patients suffering from diabetic nephropathy, we found an up-regulation of filamin A in podocytes in contrast to the control. In summary, filamin A and B mediate matrix-actin cytoskeleton interactions which are essential for the adaptation of cultured podocyte to mechanical stress.

Pro-cachectic factors link experimental and human chronic kidney disease to skeletal muscle wasting programs

Skeletal muscle wasting is commonly associated with chronic kidney disease (CKD), resulting in increased morbidity and mortality. However, the link between kidney and muscle function remains poorly understood. Here, we took a complementary interorgan approach to investigate skeletal muscle wasting in CKD. We identified increased production and elevated blood levels of soluble pro-cachectic factors, including activin A, directly linking experimental and human CKD to skeletal muscle wasting programs. Single-cell sequencing data identified the expression of activin A in specific kidney cell populations of fibroblasts and cells of the juxtaglomerular apparatus. We propose that persistent and increased kidney production of pro-cachectic factors, combined with a lack of kidney clearance, facilitates a vicious kidney/muscle signaling cycle, leading to exacerbated blood accumulation and, thereby, skeletal muscle wasting. Systemic pharmacological blockade of activin A using soluble activin receptor type IIB ligand trap as well as muscle-specific adeno-associated virus-mediated downregulation of its receptor ACVR2A/B prevented muscle wasting in different mouse models of experimental CKD, suggesting that activin A is a key factor in CKD-induced cachexia. In summary, we uncovered a crosstalk between kidney and muscle and propose modulation of activin signaling as a potential therapeutic strategy for skeletal muscle wasting in CKD.

Perspectives in systems nephrology

Chronic kidney diseases (CKD) are a major health problem affecting approximately 10% of the world’s population and posing increasing challenges to the healthcare system. While CKD encompasses a broad spectrum of pathological processes and diverse etiologies, the classification of kidney disease is currently based on clinical findings or histopathological categorizations. This descriptive classification is agnostic towards the underlying disease mechanisms and has limited progress towards the ability to predict disease prognosis and treatment responses. To gain better insight into the complex and heterogeneous disease pathophysiology of CKD, a systems biology approach can be transformative. Rather than examining one factor or pathway at a time, as in the reductionist approach, with this strategy a broad spectrum of information is integrated, including comprehensive multi-omics data, clinical phenotypic information, and clinicopathological parameters. In recent years, rapid advances in mathematical, statistical, computational, and artificial intelligence methods enable the mapping of diverse big data sets. This holistic approach aims to identify the molecular basis of CKD subtypes as well as individual determinants of disease manifestation in a given patient. The emerging mechanism-based patient stratification and disease classification will lead to improved prognostic and predictive diagnostics and the discovery of novel molecular disease-specific therapies.

Endothelial-Specific Deletion of CD146 Protects Against Experimental Glomerulonephritis in Mice

[Figure: see text].

TrkC Is Essential for Nephron Function and Trans-Activates Igf1R Signaling

BACKGROUND

Injury to kidney podocytes often results in chronic glomerular disease and consecutive nephron malfunction. For most glomerular diseases, targeted therapies are lacking. Thus, it is important to identify novel signaling pathways contributing to glomerular disease. Neurotrophic tyrosine kinase receptor 3 (TrkC) is expressed in podocytes and the protein transmits signals to the podocyte actin cytoskeleton.

METHODS

Nephron-specific TrkC knockout (TrkC-KO) and nephron-specific TrkC-overexpressing (TrkC-OE) mice were generated to dissect the role of TrkC in nephron development and maintenance.

RESULTS

Both TrkC-KO and TrkC-OE mice exhibited enlarged glomeruli, mesangial proliferation, basement membrane thickening, albuminuria, podocyte loss, and aspects of FSGS during aging. Igf1 receptor (Igf1R)-associated gene expression was dysregulated in TrkC-KO mouse glomeruli. Phosphoproteins associated with insulin, erb-b2 receptor tyrosine kinase (Erbb), and Toll-like receptor signaling were enriched in lysates of podocytes treated with the TrkC ligand neurotrophin-3 (Nt-3). Activation of TrkC by Nt-3 resulted in phosphorylation of the Igf1R on activating tyrosine residues in podocytes. Igf1R phosphorylation was increased in TrkC-OE mouse kidneys while it was decreased in TrkC-KO kidneys. Furthermore, TrkC expression was elevated in glomerular tissue of patients with diabetic kidney disease compared with control glomerular tissue.

CONCLUSIONS

Our results show that TrkC is essential for maintaining glomerular integrity. Furthermore, TrkC modulates Igf-related signaling in podocytes.

C-C chemokine receptor type 2 mediates glomerular injury and interstitial fibrosis in focal segmental glomerulosclerosis

BACKGROUND

Glomerulosclerosis and tubulointerstitial fibrosis are hallmarks of chronic kidney injury leading to end-stage renal disease. Inflammatory mechanisms contribute to glomerular and interstitial scarring, including chemokine-mediated recruitment of leucocytes. In particular, accumulation of C-C chemokine receptor type 2 (CCR2)-expressing macrophages promotes renal injury and fibrotic remodelling in diseases like glomerulonephritis and diabetic nephropathy. The functional role of CCR2 in the initiation and progression of primary glomerulosclerosis induced by podocyte injury remains to be characterized.

METHODS

We analysed glomerular expression of CCR2 and its chemokine ligand C-C motif chemokine ligand 2 (CCL2) in human focal segmental glomerulosclerosis (FSGS). Additionally, CCL2 expression was determined in stimulated murine glomeruli and glomerular cells in vitro. To explore pro-inflammatory and profibrotic functions of CCR2 we induced adriamycin nephropathy, a murine model of FSGS, in BALB/c wild-type and Ccr2-deficient mice.

RESULTS

Glomerular expression of CCR2 and CCL2 significantly increased in human FSGS. In adriamycin-induced FSGS, progressive glomerular scarring and reduced glomerular nephrin expression was paralleled by induced glomerular expression of CCL2. Adriamycin exposure stimulated secretion of CCL2 and tumour necrosis factor-α (TNF) in isolated glomeruli and mesangial cells and CCL2 in parietal epithelial cells. In addition, TNF induced CCL2 expression in all glomerular cell populations, most prominently in podocytes. In vivo, Ccr2-deficient mice with adriamycin nephropathy showed reduced injury, macrophage and fibrocyte infiltration and inflammation in glomeruli and the tubulointerstitium. Importantly, glomerulosclerosis and tubulointerstitial fibrosis were significantly ameliorated.

CONCLUSIONS

Our data indicate that CCR2 is an important mediator of glomerular injury and progression of FSGS. CCR2- targeting therapies may represent a novel approach for its treatment.

Pathogen-induced tissue-resident memory TH17 (TRM17) cells amplify autoimmune kidney disease

Although it is well established that microbial infections predispose to autoimmune diseases, the underlying mechanisms remain poorly understood. After infection, tissue-resident memory T (T_RM) cells persist in peripheral organs and provide immune protection against reinfection. However, whether T_RM cells participate in responses unrelated to the primary infection, such as autoimmune inflammation, is unknown. By using high-dimensional single-cell analysis, we identified CD4⁺ T_RM cells with a T_H17 signature (termed T_RM17 cells) in kidneys of patients with ANCA-associated glomerulonephritis. Experimental models demonstrated that renal T_RM17 cells were induced by pathogens infecting the kidney, such as Staphylococcus aureus, Candida albicans, and uropathogenic Escherichia coli, and persisted after the clearance of infections. Upon induction of experimental glomerulonephritis, these kidney T_RM17 cells rapidly responded to local proinflammatory cytokines by producing IL-17A and thereby exacerbate renal pathology. Thus, our data show that pathogen-induced T_RM17 cells have a previously unrecognized function in aggravating autoimmune disease.

The Antioxidative Role of Cytoglobin in Podocytes: Implications for a Role in Chronic Kidney Disease

Aims: Cytoglobin (CYGB) is a member of the mammalian globin family of respiratory proteins. Despite extensive research efforts, its physiological role remains largely unknown, but potential functions include reactive oxygen species (ROS) detoxification and signaling. Accumulating evidence suggests that ROS play a crucial role in podocyte detachment and apoptosis during diabetic kidney disease. This study aimed to explore the potential antioxidative renal role of CYGB both in vivo and in vitro. Results: Using a Cygb-deficient mouse model, we demonstrate a Cygb-dependent reduction in renal function, coinciding with a reduced number of podocytes. To specifically assess the putative antioxidative function of CYGB in podocytes, we first confirmed high endogenous CYGB expression levels in two human podocyte cell lines and subsequently generated short hairpin RNA-mediated stable CYGB knockdown podocyte models. CYGB-deficient podocytes displayed increased cell death and accumulation of ROS as assessed by 2'7'-dichlorodihydrofluorescein diacetate assays and the redox-sensitive probe roGFP2-Orp1. CYGB-deficient cells also exhibited an impaired cellular bioenergetic status. Consistently, analysis of the CYGB-dependent transcriptome identified dysregulation of multiple genes involved in redox balance, apoptosis, as well as in chronic kidney disease (CKD). Finally, genome-wide association studies and expression studies in nephropathy biopsies indicate an association of CYGB with CKD. Innovation: This study demonstrates a podocyte-related renal role of Cygb, confirms abundant CYGB expression in human podocyte cell lines, and describes for the first time an association between CYGB and CKD. Conclusion: Our results provide evidence for an antioxidative role of CYGB in podocytes.

Dysregulated mesenchymal PDGFR-β drives kidney fibrosis

Kidney fibrosis is characterized by expansion and activation of platelet-derived growth factor receptor-β (PDGFR-β)-positive mesenchymal cells. To study the consequences of PDGFR-β activation, we developed a model of primary renal fibrosis using transgenic mice with PDGFR-β activation specifically in renal mesenchymal cells, driving their pathological proliferation and phenotypic switch toward myofibroblasts. This resulted in progressive mesangioproliferative glomerulonephritis, mesangial sclerosis, and interstitial fibrosis with progressive anemia due to loss of erythropoietin production by fibroblasts. Fibrosis induced secondary tubular epithelial injury at later stages, coinciding with microinflammation, and aggravated the progression of hypertensive and obstructive nephropathy. Inhibition of PDGFR activation reversed fibrosis more effectively in the tubulointerstitium compared to glomeruli. Gene expression signatures in mice with PDGFR-β activation resembled those found in patients. In conclusion, PDGFR-β activation alone is sufficient to induce progressive renal fibrosis and failure, mimicking key aspects of chronic kidney disease in humans. Our data provide direct proof that fibrosis per se can drive chronic organ damage and establish a model of primary fibrosis allowing specific studies targeting fibrosis progression and regression.

Fibronectin is up-regulated in podocytes by mechanical stress

Hypertension is one of the central causes of kidney damage. In the past it was shown that glomerular hypertension leads to morphologic changes of podocytes and effacement and is responsible for detachment of these postmitotic cells. Because we have shown that podocytes are mechanosensitive and respond to mechanical stress by reorganization of the actin cytoskeleton in vitro, we look for mechanotransducers in podocytes. In this study, we demonstrate that the extracellular matrix protein fibronectin (Fn1) might be a potential candidate. The present study shows that Fn1 is essential for the attachment of podocytes during mechanical stress. By real-time quantitative PCR as well as by liquid chromatography-mass spectrometry, we found a significant up-regulation of Fn1 caused by mechanical stretch (3 d, 0.5 Hz, and 5% extension). To study the role of Fn1 in cultured podocytes under mechanical stress, Fn1 was knocked down (Fn1 KD) by a specific small interfering RNA. Additionally, we established a Fn1 knockout (KO) podocyte cell line (Fn1 KO) by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). During mechanical stress, a significant loss of podocytes (>80%) was observed in Fn1 KD as well as Fn1 KO podocytes compared with control cells. Furthermore, Fn1 KO podocytes showed a significant down-regulation of the focal adhesion proteins talin, vinculin, and paxillin and a reduced cell spreading, indicating an important role of Fn1 in adhesion. Analyses of kidney sections from patients with diabetic nephropathy have shown a significant up-regulation of FN1 in contrast to control biopsies. In summary, we show that Fn1 plays an important role in the adaptation of podocytes to mechanical stress.-Kliewe, F., Kaling, S., Lötzsch, H., Artelt, N., Schindler, M., Rogge, H., Schröder, S., Scharf, C., Amann, K., Daniel, C., Lindenmeyer, M. T., Cohen, C. D., Endlich, K., Endlich, N. Fibronectin is up-regulated in podocytes by mechanical stress.

mTOR-mediated podocyte hypertrophy regulates glomerular integrity in mice and humans

The cellular origins of glomerulosclerosis involve activation of parietal epithelial cells (PECs) and progressive podocyte depletion. While mammalian target of rapamycin-mediated (mTOR-mediated) podocyte hypertrophy is recognized as an important signaling pathway in the context of glomerular disease, the role of podocyte hypertrophy as a compensatory mechanism preventing PEC activation and glomerulosclerosis remains poorly understood. In this study, we show that glomerular mTOR and PEC activation-related genes were both upregulated and intercorrelated in biopsies from patients with focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, suggesting both compensatory and pathological roles. Advanced morphometric analyses in murine and human tissues identified podocyte hypertrophy as a compensatory mechanism aiming to regulate glomerular functional integrity in response to somatic growth, podocyte depletion, and even glomerulosclerosis - all of this in the absence of detectable podocyte regeneration. In mice, pharmacological inhibition of mTOR signaling during acute podocyte loss impaired hypertrophy of remaining podocytes, resulting in unexpected albuminuria, PEC activation, and glomerulosclerosis. Exacerbated and persistent podocyte hypertrophy enabled a vicious cycle of podocyte loss and PEC activation, suggesting a limit to its beneficial effects. In summary, our data highlight a critical protective role of mTOR-mediated podocyte hypertrophy following podocyte loss in order to preserve glomerular integrity, preventing PEC activation and glomerulosclerosis.

A Novel Role for GATA3 in Mesangial Cells in Glomerular Development and Injury

BACKGROUND

GATA3 is a dual-zinc finger transcription factor that regulates gene expression in many developing tissues. In the kidney, GATA3 is essential for ureteric bud branching, and mice without it fail to develop kidneys. In humans, autosomal dominant GATA3 mutations can cause renal aplasia as part of the hypoparathyroidism, renal dysplasia, deafness (HDR) syndrome that includes mesangioproliferative GN. This suggests that GATA3 may have a previously unrecognized role in glomerular development or injury.

METHODS

To determine GATA3's role in glomerular development or injury, we assessed GATA3 expression in developing and mature kidneys from Gata3 heterozygous (⁺/-) knockout mice, as well as injured human and rodent kidneys.

RESULTS

We show that GATA3 is expressed by FOXD1 lineage stromal progenitor cells, and a subset of these cells mature into mesangial cells (MCs) that continue to express GATA3 in adult kidneys. In mice, we uncover that GATA3 is essential for normal glomerular development, and mice with haploinsufficiency of Gata3 have too few MC precursors and glomerular abnormalities. Expression of GATA3 is maintained in MCs of adult kidneys and is markedly increased in rodent models of mesangioproliferative GN and in IgA nephropathy, suggesting that GATA3 plays a critical role in the maintenance of glomerular homeostasis.

CONCLUSIONS

These results provide new insights on the role GATA3 plays in MC development and response to injury. It also shows that GATA3 may be a novel and robust nuclear marker for identifying MCs in tissue sections.

Interleukin-1β Inhibition for Chronic Kidney Disease in Obese Mice With Type 2 Diabetes

Inflammasome-driven release of interleukin(IL)-1β is a central element of many forms of sterile inflammation and has been evident to promote the onset and progression of diabetic kidney disease. We microdissected glomerular and tubulointerstitial samples from kidney biopsies of patients with diabetic kidney disease and found expression of IL-1β mRNA. Immunostaining of such kidney biopsies across a broad spectrum of diabetic kidney disease stages revealed IL-1β positivity in a small subset of infiltrating immune cell. Thus, we speculated on a potential of IL-1β as a therapeutic target and neutralizing the biological effects of murine IL-1β with a novel monoclonal antibody in uninephrectomized diabetic db/db mice with progressive type 2 diabetes- and obesity-related single nephron hyperfiltration, podocyte loss, proteinuria, and progressive decline of total glomerular filtration rate (GFR). At 18 weeks albuminuric mice were randomized to intraperitoneal injections with either anti-IL-1β or control IgG once weekly for 8 weeks. During this period, anti-IL-1β IgG had no effect on food or fluid intake, body weight, and fasting glucose levels. At week 26, anti-IL-1β IgG had reduced renal mRNA expression of kidney injury markers (Ngal) and fibrosis (Col1, a-Sma), significantly attenuated the progressive decline of GFR in hyperfiltrating diabetic mice, and preserved podocyte number without affecting albuminuria or indicators of single nephron hyperfiltration. No adverse effect were observed. Thus, IL-1β contributes to the progression of chronic kidney disease in type 2 diabetes and might therefore be a valuable therapeutic target, potentially in combination with drugs with different mechanisms-of-action such as RAS and SGLT2 inhibitors.

Vangl2, a planar cell polarity molecule, is implicated in irreversible and reversible kidney glomerular injury

Planar cell polarity (PCP) pathways control the orientation and alignment of epithelial cells within tissues. Van Gogh-like 2 (Vangl2) is a key PCP protein that is required for the normal differentiation of kidney glomeruli and tubules. Vangl2 has also been implicated in modifying the course of acquired glomerular disease, and here, we further explored how Vangl2 impacts on glomerular pathobiology in this context. Targeted genetic deletion of Vangl2 in mouse glomerular epithelial podocytes enhanced the severity of not only irreversible accelerated nephrotoxic nephritis but also lipopolysaccharide-induced reversible glomerular damage. In each proteinuric model, genetic deletion of Vangl2 in podocytes was associated with an increased ratio of active-MMP9 to inactive MMP9, an enzyme involved in tissue remodelling. In addition, by interrogating microarray data from two cohorts of renal patients, we report increased VANGL2 transcript levels in the glomeruli of individuals with focal segmental glomerulosclerosis, suggesting that the molecule may also be involved in certain human glomerular diseases. These observations support the conclusion that Vangl2 modulates glomerular injury, at least in part by acting as a brake on MMP9, a potentially harmful endogenous enzyme. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

BDNF: mRNA expression in urine cells of patients with chronic kidney disease and its role in kidney function

Podocyte loss and changes to the complex morphology are major causes of chronic kidney disease (CKD). As the incidence is continuously increasing over the last decades without sufficient treatment, it is important to find predicting biomarkers. Therefore, we measured urinary mRNA levels of podocyte genes NPHS1, NPHS2, PODXL and BDNF, KIM‐1, CTSL by qRT‐PCR of 120 CKD patients. We showed a strong correlation between BDNF and the kidney injury marker KIM‐1, which were also correlated with NPHS1, suggesting podocytes as a contributing source. In human biopsies, BDNF was localized in the cell body and major processes of podocytes. In glomeruli of diabetic nephropathy patients, we found a strong BDNF signal in the remaining podocytes. An inhibition of the BDNF receptor TrkB resulted in enhanced podocyte dedifferentiation. The knockdown of the orthologue resulted in pericardial oedema formation and lowered viability of zebrafish larvae. We found an enlarged Bowman's space, dilated glomerular capillaries, podocyte loss and an impaired glomerular filtration. We demonstrated that BDNF is essential for glomerular development, morphology and function and the expression of BDNF and KIM‐1 is highly correlated in urine cells of CKD patients. Therefore, BDNF mRNA in urine cells could serve as a potential CKD biomarker.

Metabolic pathways and immunometabolism in rare kidney diseases

OBJECTIVES

To characterise renal tissue metabolic pathway gene expression in different forms of glomerulonephritis.

METHODS

Patients with nephrotic syndrome (NS), antineutrophil cytoplasmic antibody-associated vasculitis (AAV), systemic lupus erythematosus (SLE) and healthy living donors (LD) were studied. Clinically indicated renal biopsies were obtained at time of diagnosis and microdissected into glomerular and tubulointerstitial compartments. Microarray-derived differential gene expression of 88 genes representing critical enzymes of metabolic pathways and 25 genes related to immune cell markers was compared between disease groups. Correlation analyses measured relationships between metabolic pathways, kidney function and cytokine production.

RESULTS

Reduced steady state levels of mRNA species were enriched in pathways of oxidative phosphorylation and increased in the pentose phosphate pathway (PPP) with maximal perturbation in AAV and SLE followed by NS, and least in LD. Transcript regulation was isozymes specific with robust regulation in hexokinases, enolases and glucose transporters. Intercorrelation networks were observed between enzymes of the PPP (eg, transketolase) and macrophage markers (eg, CD68) (r=0.49, p<0.01). Increased PPP transcript levels were associated with reduced glomerular filtration rate in the glomerular (r=-0.49, p<0.01) and tubulointerstitial (r=-0.41, p<0.01) compartments. PPP expression and tumour necrosis factor activation were tightly co-expressed (r=0.70, p<0.01).

CONCLUSION

This study demonstrated concordant alterations of the renal transcriptome consistent with metabolic reprogramming across different forms of glomerulonephritis. Activation of the PPP was tightly linked with intrarenal macrophage marker expression, reduced kidney function and increased production of cytokines. Modulation of glucose metabolism may offer novel immune-modulatory therapeutic approaches in rare kidney diseases.

The transcription factor Dach1 is essential for podocyte function

Dedifferentiation and loss of podocytes are the major cause of chronic kidney disease. Dach1, a transcription factor that is essential for cell fate, was found in genome‐wide association studies to be associated with the glomerular filtration rate. We found that podocytes express high levels of Dach1 in vivo and to a much lower extent in vitro. Parietal epithelial cells (PECs) that are still under debate to be a type of progenitor cell for podocytes expressed Dach1 only at low levels. The transfection of PECs with a plasmid encoding for Dach1 induced the expression of synaptopodin, a podocyte‐specific protein, demonstrated by immunocytochemistry and Western blot. Furthermore, synaptopodin was located along actin fibres in a punctate pattern in Dach1‐expressing PECs comparable with differentiated podocytes. Moreover, dedifferentiating podocytes of isolated glomeruli showed a significant reduction in the expression of Dach1 together with synaptopodin after 9 days in cell culture. To study the role of Dach1 in vivo, we used the zebrafish larva as an animal model. Knockdown of the zebrafish ortholog Dachd by morpholino injection into fertilized eggs resulted in a severe renal phenotype. The glomeruli of the zebrafish larvae showed morphological changes of the glomerulus accompanied by down‐regulation of nephrin and leakage of the filtration barrier. Interestingly, glomeruli of biopsies from patients suffering from diabetic nephropathy showed also a significant reduction of Dach1 and synaptopodin in contrast to control biopsies. Taken together, Dach1 is a transcription factor that is important for podocyte differentiation and proper kidney function.

Lymphotoxin expression in human and murine renal allografts

The kidney is the most frequently transplanted solid organ. Recruitment of inflammatory cells, ranging from diffuse to nodular accumulations with defined microarchitecture, is a hallmark of acute and chronic renal allograft injury. Lymphotoxins (LTs) mediate the communication of lymphocytes and stromal cells and play a pivotal role in chronic inflammation and formation of lymphoid tissue. The aim of this study was to assess the expression of members of the LT system in acute rejection (AR) and chronic renal allograft injury such as transplant glomerulopathy (TG) and interstitial fibrosis/tubular atrophy (IFTA). We investigated differentially regulated components in transcriptomes of human renal allograft biopsies. By microarray analysis, we found the upregulation of LTβ, LIGHT, HVEM and TNF receptors 1 and 2 in AR and IFTA in human renal allograft biopsies. In addition, there was clear evidence for the activation of the NFκB pathway, most likely a consequence of LTβ receptor stimulation. In human renal allograft biopsies with transplant glomerulopathy (TG) two distinct transcriptional patterns of LT activation were revealed. By quantitative RT-PCR robust upregulation of LTα, LTβ and LIGHT was shown in biopsies with borderline lesions and AR. Immunohistochemistry revealed expression of LTβ in tubular epithelial cells and inflammatory infiltrates in transplant biopsies with AR and IFTA. Finally, activation of LT signaling was reproduced in a murine model of renal transplantation with AR. In summary, our results indicate a potential role of the LT system in acute renal allograft rejection and chronic transplant injury. Activation of the LT system in allograft rejection in rodents indicates a species independent mechanism. The functional role of the LT system in acute renal allograft rejection and chronic injury remains to be determined.

Studying the role of fascin-1 in mechanically stressed podocytes

Glomerular hypertension causes glomerulosclerosis via the loss of podocytes, which are challenged by increased mechanical load. We have demonstrated that podocytes are mechanosensitive. However, the response of podocytes to mechanical stretching remains incompletely understood. Here we demonstrate that the actin-bundling protein fascin-1 plays an important role in podocytes that are exposed to mechanical stress. Immunofluorescence staining revealed colocalization of fascin-1 and nephrin in mouse kidney sections. In cultured mouse podocytes fascin-1 was localized along actin fibers and filopodia in stretched and unstretched podocytes. The mRNA and protein levels of fascin-1 were not affected by mechanical stress. By Western blot and 2D-gelelectrophoresis we observed that phospho-fascin-1 was significantly downregulated after mechanical stretching. It is known that phosphorylation at serine 39 (S39) regulates the bundling activity of fascin-1, e.g. required for filopodia formation. Podocytes expressing wild type GFP-fascin-1 and non-phosphorylatable GFP-fascin-1-S39A showed marked filopodia formation, being absent in podocytes expressing phosphomimetic GFP-fascin-1-S39D. Finally, the immunofluorescence signal of phosphorylated fascin-1 was strongly reduced in glomeruli of patients with diabetic nephropathy compared to healthy controls. In summary, mechanical stress dephosphorylates fascin-1 in podocytes in vitro and in vivo thereby fascin-1 may play an important role in the adaptation of podocytes to mechanical forces.

Renal biopsy-driven molecular target identification in glomerular disease

Chronic kidney disease has severe impacts on the patient and represents a major burden to the health care systems worldwide. Despite an increased knowledge of pathophysiological processes involved in kidney diseases, the progress in defining novel treatment strategies has been limited. One reason is the descriptive disease categorization used in nephrology based on clinical findings or histopathological categories irrespective of potential different molecular disease mechanisms. To accelerate progress toward a targeted treatment, a definition of human disease extending from phenotypic disease classification to mechanism-based disease definitions is needed. In recent years, we have witnessed a major transition in biomedical research from a single gene research to an information rich and collaborative science. Tissue-based analysis in renal disease allows to link structure to molecular function. In our review, we introduce the concept of precision medicine in nephrology, describe several large cohort studies established for molecular analysis of kidney diseases, and highlight examples of renal biopsy-driven target identification by integrative systems biology approaches. Furthermore, we give an outlook on how the new disease definitions can be used for patient stratification in clinical trial design. Finally, we introduce the concept of an informational commons of renal precision medicine for joint analyses of large-scale data sets in renal failure.

Keratins are novel markers of renal epithelial cell injury

Keratins, the intermediate filaments of the epithelial cell cytoskeleton, are up-regulated and post-translationally modified in stress situations. Renal tubular epithelial cell stress is a common finding in progressive kidney diseases, but little is known about keratin expression and phosphorylation. Here, we comprehensively describe keratin expression in healthy and diseased kidneys. In healthy mice, the major renal keratins, K7, K8, K18, and K19, were expressed in the collecting ducts and K8, K18 in the glomerular parietal epithelial cells. Tubular expression of all 4 keratins increased by 20- to 40-fold in 5 different models of renal tubular injury as assessed by immunohistochemistry, Western blot, and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). The up-regulation became significant early after disease induction, increased with disease progression, was found de novo in distal tubules and was accompanied by altered subcellular localization. Phosphorylation of K8 and K18 increased under stress. In humans, injured tubules also exhibited increased keratin expression. Urinary K18 was only detected in mice and patients with tubular cell injury. Keratins labeled glomerular parietal epithelial cells forming crescents in patients and animals. Thus, all 4 major renal keratins are significantly, early, and progressively up-regulated upon tubular injury regardless of the underlying disease and may be novel sensitive markers of renal tubular cell stress.

Cathepsin S Cleavage of Protease-Activated Receptor-2 on Endothelial Cells Promotes Microvascular Diabetes Complications

Endothelial dysfunction is a central pathomechanism in diabetes-associated complications. We hypothesized a pathogenic role in this dysfunction of cathepsin S (Cat-S), a cysteine protease that degrades elastic fibers and activates the protease-activated receptor-2 (PAR2) on endothelial cells. We found that injection of mice with recombinant Cat-S induced albuminuria and glomerular endothelial cell injury in a PAR2-dependent manner. In vivo microscopy confirmed a role for intrinsic Cat-S/PAR2 in ischemia-induced microvascular permeability. In vitro transcriptome analysis and experiments using siRNA or specific Cat-S and PAR2 antagonists revealed that Cat-S specifically impaired the integrity and barrier function of glomerular endothelial cells selectively through PAR2. In human and mouse type 2 diabetic nephropathy, only CD68(+) intrarenal monocytes expressed Cat-S mRNA, whereas Cat-S protein was present along endothelial cells and inside proximal tubular epithelial cells also. In contrast, the cysteine protease inhibitor cystatin C was expressed only in tubules. Delayed treatment of type 2 diabetic db/db mice with Cat-S or PAR2 inhibitors attenuated albuminuria and glomerulosclerosis (indicators of diabetic nephropathy) and attenuated albumin leakage into the retina and other structural markers of diabetic retinopathy. These data identify Cat-S as a monocyte/macrophage-derived circulating PAR2 agonist and mediator of endothelial dysfunction-related microvascular diabetes complications. Thus, Cat-S or PAR2 inhibition might be a novel strategy to prevent microvascular disease in diabetes and other diseases.

Macrophage Migration Inhibitory Factor Mediates Proliferative GN via CD74

Pathologic proliferation of mesangial and parietal epithelial cells (PECs) is a hallmark of various glomerulonephritides. Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine that mediates inflammation by engagement of a receptor complex involving the components CD74, CD44, CXCR2, and CXCR4. The proliferative effects of MIF may involve CD74 together with the coreceptor and PEC activation marker CD44. Herein, we analyzed the effects of local glomerular MIF/CD74/CD44 signaling in proliferative glomerulonephritides. MIF, CD74, and CD44 were upregulated in the glomeruli of patients and mice with proliferative glomerulonephritides. During disease, CD74 and CD44 were expressed de novo in PECs and colocalized in both PECs and mesangial cells. Stress stimuli induced MIF secretion from glomerular cells in vitro and in vivo, in particular from podocytes, and MIF stimulation induced proliferation of PECs and mesangial cells via CD74. In murine crescentic GN, Mif-deficient mice were almost completely protected from glomerular injury, the development of cellular crescents, and the activation and proliferation of PECs and mesangial cells, whereas wild-type mice were not. Bone marrow reconstitution studies showed that deficiency of both nonmyeloid and bone marrow-derived Mif reduced glomerular cell proliferation and injury. In contrast to wild-type mice, Cd74-deficient mice also were protected from glomerular injury and ensuing activation and proliferation of PECs and mesangial cells. Our data suggest a novel molecular mechanism and glomerular cell crosstalk by which local upregulation of MIF and its receptor complex CD74/CD44 mediate glomerular injury and pathologic proliferation in GN.

Destruction of a distal hypoxia response element abolishes trans-activation of the PAG1 gene mediated by HIF-independent chromatin looping

A crucial step in the cellular adaptation to oxygen deficiency is the binding of hypoxia-inducible factors (HIFs) to hypoxia response elements (HREs) of oxygen-regulated genes. Genome-wide HIF-1α/2α/β DNA-binding studies revealed that the majority of HREs reside distant to the promoter regions, but the function of these distal HREs has only been marginally studied in the genomic context. We used chromatin immunoprecipitation (ChIP), gene editing (TALEN) and chromosome conformation capture (3C) to localize and functionally characterize a 82 kb upstream HRE that solely drives oxygen-regulated expression of the newly identified HIF target gene PAG1. PAG1, a transmembrane adaptor protein involved in Src signalling, was hypoxically induced in various cell lines and mouse tissues. ChIP and reporter gene assays demonstrated that the -82 kb HRE regulates PAG1, but not an equally distant gene further upstream, by direct interaction with HIF. Ablation of the consensus HRE motif abolished the hypoxic induction of PAG1 but not general oxygen signalling. 3C assays revealed that the -82 kb HRE physically associates with the PAG1 promoter region, independent of HIF-DNA interaction. These results demonstrate a constitutive interaction between the -82 kb HRE and the PAG1 promoter, suggesting a physiologically important rapid response to hypoxia.

The Rho-GTPase binding protein IQGAP2 is required for the glomerular filtration barrier

Podocyte dysfunction impairs the size selectivity of the glomerular filter, leading to proteinuria, hypoalbuminuria, and edema, clinically defined as nephrotic syndrome. Hereditary forms of nephrotic syndrome are linked to mutations in podocyte-specific genes. To identify genes contributing to podocyte dysfunction in acquired nephrotic syndrome, we studied human glomerular gene expression data sets for glomerular-enriched gene transcripts differentially regulated between pretransplant biopsy samples and biopsies from patients with nephrotic syndrome. Candidate genes were screened by in situ hybridization for expression in the zebrafish pronephros, an easy-to-use in vivo assay system to assess podocyte function. One glomerulus-enriched product was the Rho-GTPase binding protein, IQGAP2. Immunohistochemistry found a strong presence of IQGAP2 in normal human and zebrafish podocytes. In zebrafish larvae, morpholino-based knockdown of iqgap2 caused a mild foot process effacement of zebrafish podocytes and a cystic dilation of the urinary space of Bowman's capsule upon onset of urinary filtration. Moreover, the glomerulus of zebrafish morphants showed a glomerular permeability for injected high-molecular-weight dextrans, indicating an impaired size selectivity of the glomerular filter. Thus, IQGAP2 is a Rho-GTPase binding protein, highly abundant in human and zebrafish podocytes, which controls normal podocyte structure and function as evidenced in the zebrafish pronephros.

Alterations in the ubiquitin proteasome system in persistent but not reversible proteinuric diseases

Podocytes are the key cells affected in nephrotic glomerular kidney diseases, and they respond uniformly to injury with cytoskeletal rearrangement. In nephrotic diseases, such as membranous nephropathy and FSGS, persistent injury often leads to irreversible structural damage, whereas in minimal change disease, structural alterations are mostly transient. The factors leading to persistent podocyte injury are currently unknown. Proteolysis is an irreversible process and could trigger persistent podocyte injury through degradation of podocyte-specific proteins. We, therefore, analyzed the expression and functional consequence of the two most prominent proteolytic systems, the ubiquitin proteasome system (UPS) and the autophagosomal/lysosomal system, in persistent and transient podocyte injuries. We show that differential upregulation of both proteolytic systems occurs in persistent human and rodent podocyte injury. The expression of specific UPS proteins in podocytes differentiated children with minimal change disease from children with FSGS and correlated with poor clinical outcome. Degradation of the podocyte-specific protein α-actinin-4 by the UPS depended on oxidative modification in membranous nephropathy. Notably, the UPS was overwhelmed in podocytes during experimental glomerular disease, resulting in abnormal protein accumulation and compensatory upregulation of the autophagosomal/lysosomal system. Accordingly, inhibition of both proteolytic systems enhanced proteinuria in persistent nephrotic disease. This study identifies altered proteolysis as a feature of persistent podocyte injury. In the future, specific UPS proteins may serve as new biomarkers or therapeutic targets in persistent nephrotic syndrome.

UCH-L1 induces podocyte hypertrophy in membranous nephropathy by protein accumulation

Podocytes are terminally differentiated cells of the glomerular filtration barrier that react with hypertrophy in the course of injury such as in membranous nephropathy (MGN). The neuronal deubiquitinase ubiquitin C-terminal hydrolase L1 (UCH-L1) is expressed and activated in podocytes of human and rodent MGN. UCH-L1 regulates the mono-ubiquitin pool and induces accumulation of poly-ubiquitinated proteins in affected podocytes. Here, we investigated the role of UCH-L1 in podocyte hypertrophy and in the homeostasis of the hypertrophy associated "model protein" p27(Kip1). A better understanding of the basic mechanisms leading to podocyte hypertrophy is crucial for the development of specific therapies in MGN. In human and rat MGN, hypertrophic podocytes exhibited a simultaneous up-regulation of UCH-L1 and of cytoplasmic p27(Kip1) content. Functionally, inhibition of UCH-L1 activity and knockdown or inhibition of UCH-L1 attenuated podocyte hypertrophy by decreasing the total protein content in isolated glomeruli and in cultured podocytes. In contrast, UCH-L1 levels and activity increased podocyte hypertrophy and total protein content in culture, specifically of cytoplasmic p27(Kip1). UCH-L1 enhanced cytoplasmic p27(Kip1) levels by nuclear export and decreased poly-ubiquitination and proteasomal degradation of p27(Kip1). In parallel, UCH-L1 increased podocyte turnover, migration and cytoskeletal rearrangement, which are associated with known oncogenic functions of cytoplasmic p27(Kip1) in cancer. We propose that UCH-L1 induces podocyte hypertrophy in MGN by increasing the total protein content through altered degradation and accumulation of proteins such as p27(Kip1) in the cytoplasm of podocytes. Modification of both UCH-L1 activity and levels could be a new therapeutic avenue to podocyte hypertrophy in MGN.

Metabolomics reveals signature of mitochondrial dysfunction in diabetic kidney disease

Diabetic kidney disease is the leading cause of ESRD, but few biomarkers of diabetic kidney disease are available. This study used gas chromatography-mass spectrometry to quantify 94 urine metabolites in screening and validation cohorts of patients with diabetes mellitus (DM) and CKD(DM+CKD), in patients with DM without CKD (DM-CKD), and in healthy controls. Compared with levels in healthy controls, 13 metabolites were significantly reduced in the DM+CKD cohorts (P≤0.001), and 12 of the 13 remained significant when compared with the DM-CKD cohort. Many of the differentially expressed metabolites were water-soluble organic anions. Notably, organic anion transporter-1 (OAT1) knockout mice expressed a similar pattern of reduced levels of urinary organic acids, and human kidney tissue from patients with diabetic nephropathy demonstrated lower gene expression of OAT1 and OAT3. Analysis of bioinformatics data indicated that 12 of the 13 differentially expressed metabolites are linked to mitochondrial metabolism and suggested global suppression of mitochondrial activity in diabetic kidney disease. Supporting this analysis, human diabetic kidney sections expressed less mitochondrial protein, urine exosomes from patients with diabetes and CKD had less mitochondrial DNA, and kidney tissues from patients with diabetic kidney disease had lower gene expression of PGC1α (a master regulator of mitochondrial biogenesis). We conclude that urine metabolomics is a reliable source for biomarkers of diabetic complications, and our data suggest that renal organic ion transport and mitochondrial function are dysregulated in diabetic kidney disease.

Susceptibility of podocytes to palmitic acid is regulated by stearoyl-CoA desaturases 1 and 2

Type 2 diabetes mellitus is characterized by dyslipidemia with elevated free fatty acids (FFAs). Loss of podocytes is a hallmark of diabetic nephropathy, and podocytes are highly susceptible to saturated FFAs but not to protective, monounsaturated FFAs. We report that patients with diabetic nephropathy develop alterations in glomerular gene expression of enzymes involved in fatty acid metabolism, including induction of stearoyl-CoA desaturase (SCD)-1, which converts saturated to monounsaturated FFAs. By IHC of human renal biopsy specimens, glomerular SCD-1 induction was observed in podocytes of patients with diabetic nephropathy. Functionally, the liver X receptor agonists TO901317 and GW3965, two known inducers of SCD, increased Scd-1 and Scd-2 expression in cultured podocytes and reduced palmitic acid-induced cell death. Similarly, overexpression of Scd-1 attenuated palmitic acid-induced cell death. The protective effect of TO901317 was associated with a reduction of endoplasmic reticulum stress. It was lost after gene silencing of Scd-1/-2, thereby confirming that the protective effect of TO901317 is mediated by Scd-1/-2. TO901317 also shifted palmitic acid-derived FFAs into biologically inactive triglycerides. In summary, SCD-1 up-regulation in diabetic nephropathy may be part of a protective mechanism against saturated FFA-derived toxic metabolites that drive endoplasmic reticulum stress and podocyte death.

Distinct contributions of TNF receptor 1 and 2 to TNF-induced glomerular inflammation in mice

TNF is an important mediator of glomerulonephritis. The two TNF-receptors TNFR1 and TNFR2 contribute differently to glomerular inflammation in vivo, but specific mechanisms of TNFR-mediated inflammatory responses in glomeruli are unknown. We investigated their expression and function in murine kidneys, isolated glomeruli ex vivo, and glomerular cells in vitro. In normal kidney TNFR1 and TNFR2 were preferentially expressed in glomeruli. Expression of both TNFRs and TNF-induced upregulation of TNFR2 mRNA was confirmed in murine glomerular endothelial and mesangial cell lines. In vivo, TNF exposure rapidly induced glomerular accumulation of leukocytes. To examine TNFR-specific inflammatory responses in intrinsic glomerular cells but not infiltrating leukocytes we performed microarray gene expression profiling on intact glomeruli isolated from wildtype and Tnfr-deficient mice following exposure to soluble TNF ex vivo. Most TNF-induced effects were exclusively mediated by TNFR1, including induced glomerular expression of adhesion molecules, chemokines, complement factors and pro-apoptotic molecules. However, TNFR2 contributed to TNFR1-dependent mRNA expression of inflammatory mediators in glomeruli when exposed to low TNF concentrations. Chemokine secretion was absent in TNF-stimulated Tnfr1-deficient glomeruli, but also significantly decreased in glomeruli lacking TNFR2. In vivo, TNF-induced glomerular leukocyte infiltration was abrogated in Tnfr1-deficient mice, whereas Tnfr2-deficiency decreased mononuclear phagocytes infiltrates, but not neutrophils. These data demonstrate that activation of intrinsic glomerular cells by soluble TNF requires TNFR1, whereas TNFR2 is not essential, but augments TNFR1-dependent effects. Previously described TNFR2-dependent glomerular inflammation may therefore require TNFR2 activation by membrane-bound, but not soluble TNF.

Resistance to ABT-737 in activated T lymphocytes: molecular mechanisms and reversibility by inhibition of the calcineurin-NFAT pathway

Dynamic regulation of the intrinsic apoptosis pathway controls central and peripheral lymphocyte deletion, and may interfere with the pro-apoptotic potency of B-cell lymphoma 2 inhibitors such as ABT-737. By following a T-cell receptor (TCR) transgenic population of alloantigen-specific T cells, we found that sensitivity to ABT-737 radically changed during the course of allo-specific immune responses. Particularly, activated T cells were fully resistant to ABT-737 during the first days after antigen recognition. This phenomenon was caused by a TCR–calcineurin–nuclear factor of activated T cells-dependent upregulation of A1, and was therefore prevented by cyclosporine A (CsA). As a result, exposure to ABT-737 after alloantigen recognition induced selection of alloreactive T cells in vivo, whereas in combination with low-dose CsA, ABT-737 efficiently depleted alloreactive T cells in murine host-versus-graft and graft-versus-host models. Thus, ABT-737 resistance is not a prerogative of neoplastic cells, but it physiologically occurs in T cells after antigen recognition. Reversibility of this process by calcineurin inhibitors opens new pharmacological opportunities to modulate this process in the context of cancer, autoimmunity and transplantation.

Next-generation sequencing identifies TGF-β1-associated gene expression profiles in renal epithelial cells reiterated in human diabetic nephropathy

Transforming growth factor-beta (TGF-β1) is implicated in the onset and progression of renal fibrosis and diabetic nephropathy (DN), leading to a loss of epithelial characteristics of tubular cells. The transcriptional profile of renal tubular epithelial cells stimulated with TGF-β1 was assessed using RNA-Seq, with 2027 differentially expressed genes identified. Promoter analysis of transcription factor binding sites in the TGF-β1 responsive gene set predicted activation of multiple transcriptional networks, including NFκB. Comparison of RNA-Seq with microarray data from identical experimental conditions identified low abundance transcripts exclusive to RNA-Seq data. We compared these findings to human disease by analyzing transcriptomic data from renal biopsies of patients with DN versus control groups, identifying a shared subset of 179 regulated genes. ARK5, encoding an AMP-related kinase, and TGFBI - encoding transforming growth factor, beta-induced protein were induced by TGF-β1 and also upregulated in human DN. Suppression of ARK5 attenuated fibrotic responses of renal epithelia to TGF-β1 exposure; and silencing of TGFBI induced expression of the epithelial cell marker - E-cadherin. We identified low abundance transcripts in sequence data and validated expression levels of several transcripts (ANKRD56, ENTPD8) in tubular enriched kidney biopsies of DN patients versus living donors. In conclusion, we have defined a TGF-β1-driven pro-fibrotic signal in renal epithelial cells that is also evident in the DN renal transcriptome.

Anti-GBM glomerulonephritis involves IL-1 but is independent of NLRP3/ASC inflammasome-mediated activation of caspase-1

IL-1β and IL-18 are proinflammatory cytokines that contribute to renal immune complex disease, but whether IL-1β and IL-18 are mediators of intrinsic glomerular inflammation is unknown. In contrast to other cytokines the secretion of IL-1β and IL-18 requires a second stimulus that activates the inflammasome-ASC-caspase-1 pathway to cleave pro-IL-1β and -IL-18 into their mature and secretable forms. As the NLRP3 inflammasome and caspase-1 were shown to contribute to postischemic and postobstructive tubulointerstitial inflammation, we hypothesized a similar role for NLRP3, ASC, and caspase-1 in glomerular immunopathology. This concept was supported by the finding that lack of IL-1R1 reduced antiserum-induced focal segmental necrosis, crescent formation, and tubular atrophy when compared to wildtype mice. Lack of IL-18 reduced tubular atrophy only. However, NLRP3-, ASC- or caspase-1-deficiency had no significant effect on renal histopathology or proteinuria of serum nephritis. In vitro studies with mouse glomeruli or mesangial cells, glomerular endothelial cells, and podocytes did not reveal any pro-IL-1β induction upon LPS stimulation and no caspase-1 activation after an additional exposure to the NLRP3 agonist ATP. Only renal dendritic cells, which reside mainly in the tubulointerstitium, expressed pro-IL-1β and were able to activate the NLRP3-caspase-1 axis and secrete mature IL-1β. Together, the NLRP3-ASC-caspase-1 axis does not contribute to intrinsic glomerular inflammation via glomerular parenchymal cells as these cannot produce IL-1β during sterile inflammation.

Periostin is induced in glomerular injury and expressed de novo in interstitial renal fibrosis

Matricellular proteins participate in the pathogenesis of chronic kidney diseases. We analyzed glomerular gene expression profiles from patients with proteinuric diseases to identify matricellular proteins contributing to the progression of human nephropathies. Several genes encoding matricellular proteins, such as SPARC, THBS1, and CTGF, were induced in progressive nephropathies, but not in nonprogressive minimal-change disease. Periostin showed the highest induction, and its transcript levels correlated negatively with glomerular filtration rate in both glomerular and tubulointerstitial specimen. In well-preserved renal tissue, periostin localized to the glomerular tuft, the vascular pole, and along Bowman's capsule; no signal was detected in the tubulointerstitial compartment. Biopsies from patients with glomerulopathies and renal dysfunction showed enhanced periostin expression in the mesangium, tubular interstitium, and sites of fibrosis. Periostin staining correlated negatively with renal function. α-smooth muscle actin-positive mesangial and interstitial cells localized close to periostin-positive sites, as indicated by co-immunofluorescence. In vitro stimulation of mesangial cells by external addition of TGF-β1 resulted in robust induction of periostin. Addition of periostin to mesangial cells induced cell proliferation and decreased the number of cells expressing activated caspase-3, a marker of apoptosis. These human data indicate for the first time a role of periostin in glomerular and interstitial injury in acquired nephropathies.

Role of mTOR in podocyte function and diabetic nephropathy in humans and mice

Chronic glomerular diseases, associated with renal failure and cardiovascular morbidity, represent a major health issue. However, they remain poorly understood. Here we have reported that tightly controlled mTOR activity was crucial to maintaining glomerular podocyte function, while dysregulation of mTOR facilitated glomerular diseases. Genetic deletion of mTOR complex 1 (mTORC1) in mouse podocytes induced proteinuria and progressive glomerulosclerosis. Furthermore, simultaneous deletion of both mTORC1 and mTORC2 from mouse podocytes aggravated the glomerular lesions, revealing the importance of both mTOR complexes for podocyte homeostasis. In contrast, increased mTOR activity accompanied human diabetic nephropathy, characterized by early glomerular hypertrophy and hyperfiltration. Curtailing mTORC1 signaling in mice by genetically reducing mTORC1 copy number in podocytes prevented glomerulosclerosis and significantly ameliorated the progression of glomerular disease in diabetic nephropathy. These results demonstrate the requirement for tightly balanced mTOR activity in podocyte homeostasis and suggest that mTOR inhibition can protect podocytes and prevent progressive diabetic nephropathy.

Intrarenal production of B-cell survival factors in human lupus nephritis

The B-cell survival factors APRIL and BLyS are important for B-cell maturation and activation and contribute to human autoimmune diseases. Interference with B-cell function by targeting these molecules is currently being investigated in large clinical trials for systemic lupus erythematosus. The local expression patterns of APRIL and BLyS have not been investigated in detail in kidneys with lupus nephritis. We studied the mRNA expression of APRIL, BLyS, and the corresponding receptors BCMA, TACI, and BAFF-R in microdissected human biopsies with proliferative lupus nephritis (n=25) and compared it with pretransplant biopsies of living donors (n=9). APRIL and BLyS mRNA levels were significantly higher in glomeruli of patients with proliferative lupus nephritis (12- and 30-fold, respectively). Tubulointerstitial expression of APRIL, BLyS, BCMA, and TACI was also significantly elevated. To localize the respective proteins in the kidney, APRIL, BLyS, and BAFF-R were studied by immunohistochemistry in renal biopsies with proliferative (n=21) or membranous (n=8) lupus nephritis. APRIL was prominently expressed in glomeruli with proliferative, but not membranous, lupus nephritis. The staining pattern was consistent with mesangial cells. A prominent accumulation of CD68-positive cells was present in glomeruli in association with APRIL expression. APRIL, BLyS, and BAFF-R were also expressed in interstitial inflammatory cell accumulation. This is the first study, which details local expression of APRIL and BLyS in glomeruli and tubulointerstitium of human proliferative lupus nephritis. This information might help define intrarenal effects of APRIL and BLyS inhibition in human lupus nephritis.