Interstitial fibrosis in mice with overload proteinuria: deficiency of TIMP-1 is not protective

Cellular uptake and in vivo distribution of mesenchymal-stem-cell-derived extracellular vesicles are protein corona dependent

Extracellular vesicles (EVs) derived from mesenchymal stem cells are promising nanotherapeutics in liver diseases due to their regenerative and immunomodulatory properties. Nevertheless, a concern has been raised regarding the rapid clearance of exogenous EVs by phagocytic cells. Here we explore the impact of protein corona on EVs derived from two culturing conditions in which specific proteins acquired from media were simultaneously adsorbed on the EV surface. Additionally, by incubating EVs with serum, simulating protein corona formation upon systemic delivery, further resolved protein corona-EV complex patterns were investigated. Our findings reveal the potential influences of corona composition on EVs under in vitro conditions and their in vivo kinetics. Our data suggest that bound albumin creates an EV signature that can retarget EVs from hepatic macrophages. This results in markedly improved cellular uptake by hepatocytes, liver sinusoidal endothelial cells and hepatic stellate cells. This phenomenon can be applied as a camouflage strategy by precoating EVs with albumin to fabricate the albumin-enriched protein corona-EV complex, enhancing non-phagocytic uptake in the liver. This work addresses a critical challenge facing intravenously administered EVs for liver therapy by tailoring the protein corona-EV complex for liver cell targeting and immune evasion.

Renal Function Parameters in Distinctive Molecular Subtypes of Prostate Cancer

Prostate cancer is a prevalent malignancy in male patients, having diverse clinical outcomes. The follow-up of patients diagnosed with prostate cancer involves the evaluation of renal function, because its impairment reduces patient survival rates and adds complexity to their treatment and clinical care. This study aimed to investigate the relationship between renal function parameters and distinctive molecular subtypes of prostate adenocarcinomas, defined by the immunoexpression of the SPINK1, ERG, HOXB13, and TFF3 markers. The study group comprised 72 patients with prostate cancer and associated chronic kidney disease (CKD) who underwent radical prostatectomy. Histopathological, molecular, and renal parameters were analyzed. Patients were categorized based on ERG/SPINK1 and HOXB13/TFF3 status, and correlations with renal function and prognostic grade groups were assessed. The ERG+/SPINK1+ subgroup exhibited significantly higher postoperative CKD stages and serum creatinine levels compared to the ERG+/SPINK1- subgroup. This suggests an intricate relationship between SPINK1 overexpression and renal function dynamics. The HOXB13-/TFF3+ subgroup displayed higher preoperative serum creatinine levels and CKD stages than the HOXB13-/TFF3- subgroup, aligning with TFF3's potential role in renal function. Furthermore, the study revealed associations between CKD stages and prognostic grade groups in different molecular subtypes, pointing out an intricate interplay between renal function and tumor behavior. Although the molecular classification of prostate acinar ADK is not yet implemented, this research underscores the variability of renal function parameters in different molecular subtypes, offering potential insights into patient prognosis.

TNFR2 as a Potential Biomarker for Early Detection and Progression of CKD

The inflammatory pathway driven by TNF-α, through its receptors TNFR1 and TNFR2, is a common feature in the pathogenesis of chronic kidney disease (CKD), regardless of the initial disease cause. Evidence correlates the chronic inflammatory status with decreased renal function. Our aim was to evaluate the potential of TNF receptors as biomarkers for CKD diagnosis and staging, as well as their association with the progression of renal lesions, in rat models of early and moderate CKD. We analyzed the circulating levels of inflammatory molecules—tumor necrosis factor-alpha (TNF-α), tumor necrosis factor receptor 1 (TNFR1) and 2 (TNFR2) and tissue inhibitor of metalloproteinase-1 (TIMP-1)—and studied their associations with TNFR1 and TNFR2 renal expression, glomerular and tubulointerstitial lesions, and with biomarkers of renal (dys)function. An increase in all inflammatory markers was observed in moderate CKD, as compared to controls, but only circulating levels of both TNFR1 and TNFR2 were significantly increased in the early disease; TNFR2 serum levels were negatively correlated with eGFR. However, only TNFR2 renal expression increased with CKD severity and showed correlations with the score of mild and advanced tubular lesions. Our findings suggest that renal TNFR2 plays a role in CKD development, and has potential to be used as a biomarker for the early detection and progression of the disease. Still, the potential value of this biomarker in disease progression warrants further investigation.

Relaxin in fibrotic ligament diseases: Its regulatory role and mechanism

Fibrotic ligament diseases (FLDs) are diseases caused by the pathological accumulation of periarticular fibrotic tissue, leading to functional disability around joint and poor life quality. Relaxin (RLX) has been reported to be involved in the development of fibrotic lung and liver diseases. Previous studies have shown that RLX can block pro-fibrotic process by reducing the excess extracellular matrix (ECM) formation and accelerating collagen degradation in vitro and in vivo. Recent studies have shown that RLX can attenuate connective tissue fibrosis by suppressing TGF-β/Smads signaling pathways to inhibit the activation of myofibroblasts. However, the specific roles and mechanisms of RLX in FLDs remain unclear. Therefore, in this review, we confirmed the protective effect of RLX in FLDs and summarized its mechanism including cells, key cytokines and signaling pathways involved. In this article, we outline the potential therapeutic role of RLX and look forward to the application of RLX in the clinical translation of FLDs.

TRPC6 Inactivation Reduces Albuminuria Induced by Protein Overload in Sprague Dawley Rats

Canonical transient receptor potential-6 (TRPC6) channels have been implicated in familial and acquired forms of focal and segmental glomerulosclerosis (FSGS), and in renal fibrosis following ureteral obstruction in mice. TRPC6 channels also appear to play a role in driving glomerular disease in aging and in autoimmune glomerulonephritis. In the present study, we examine the role of TRPC6 in the proteinuric state caused by prolonged albumin overload (AO) in Sprague Dawley rats induced by daily injections of exogenous albumin. This was assessed in rats with a global and constitutive inactivation of TRPC6 channels (Trpc6del/del rats) and in wild-type littermates (Trpc6wt/wt rats). AO for 14 and 28 days caused increased urine albumin excretion that was significantly attenuated in Trpc6del/del rats compared to Trpc6wt/wt controls. AO overload did not induce significant glomerulosclerosis or azotemia in either genotype. AO induced mild tubulointerstitial disease characterized by fibrosis, hypercellularity and increased expression of markers of fibrosis and inflammation. Those changes were equally severe in Trpc6wt/wt and Trpc6del/del rats. Immunoblot analysis of renal cortex indicated that AO increased the abundances of TRPC3 and TRPC6, and caused a nearly complete loss of TRPC5 in Trpc6wt/wt rats. The increase in TRPC3 and the loss of TRPC5 occurred to the same extent in Trpc6del/del rats. These data also suggest that TRPC6 plays a role in the normal function of the glomerular filtration barrier. However, whether TRPC6 inactivation protects the tubulointerstitial compartments in Sprague Dawley rats depends on the disease model examined.

Albumin-induced premature senescence in human renal proximal tubular cells and its relationship with intercellular fibrosis

The presence of senescent cells is associated with renal fibrosis. This study aims to investigate the effect of albumin-induced premature senescence on tubulointerstitial fibrosis and its possible mechanism in vitro. Different concentrations of bovine serum albumim (BSA) with or without si-p21 are used to stimulate HK-2 cells for 72 h, and SA-β-gal activity, senescence-associated secretory phenotypes (SASPs), LaminB1 are used as markers of senescence. Immunofluorescence staining is performed to characterize the G2/M phase arrest between the control and BSA groups. Alterations in the DNA damage marker γ-H2AX, fibrogenesis, and associated proteins at the G2/M phase, such as p21, p-CDC25C and p-CDK1, are evaluated. Compared with those in the control group, the SA-β-gal activity, SASP, and γ-H2AX levels are increased in the BSA group, while the level of LaminB1 is decreased. Meanwhile, HK-2 cells blocked at the G2/M phase are significantly increased under the stimulation of BSA, and the levels of p21, p-CDC25C and p-CDK1, as well as fibrogenesis are also increased. When p21 expression is inhibited, the levels of p-CDC25C and p-CDK1 are decreased and the G2/M phase arrest is improved, which decreases the production of fibrogenesis. In conclusion, BSA induces renal tubular epithelial cell premature senescence, which regulates the G2/M phase through the CDC25C/CDK1 pathway, leading to tubulointerstitial fibrosis.

Core fucosylation involvement in the paracrine regulation of proteinuria-induced renal interstitial fibrosis evaluated with the use of a microfluidic chip

Proteinuria is a clinical manifestation of chronic kidney disease that aggravates renal interstitial fibrosis (RIF), in which injury of peritubular microvessels is an important event. However, the changes in peritubular microvessels induced by proteinuria and their molecular mechanisms remain unclear. Thus, we aimed to develop a co-culture microfluidic device that contains renal tubules and peritubular microvessels to create a proteinuria model. We found that protein overload in the renal tubule induced trans-differentiation and apoptosis of endothelial cells (ECs) and pericytes. Moreover, profiling of secreted proteins in this model revealed that a paracrine network between tubules and microvessels was activated in proteinuria-induced microvascular injury. Multiple cytokine receptors in this paracrine network were core-fucosylated. Inhibition of core fucosylation significantly reduced ligand-receptor binding ability and blocked downstream pathways, alleviating trans-differentiation and apoptosis of ECs and pericytes. Furthermore, the protective effect of genetic FUT8 deficiency on proteinuria overload-induced RIF and pericyte-myofibroblast trans-differentiation was validated in FUT8 knockout heterozygous mice. In conclusion, we constructed and used a multiple-unit integrated microfluidic device to uncover the mechanism of proteinuria-induced RIF. Furthermore, FUT8 may serve as a hub-like therapeutic target to alleviate peritubular microvascular injury in RIF. STATEMENT OF SIGNIFICANCE: In this study, we constructed a multiple-unit integrated renal tubule-vascular chip. We reproduced human proteinuria on the chip and found that multiple receptors were modified by FUT8-catalyzed core fucosylation (CF) involved in the cross-talk between renal tubules and peritubular microvessels in proteinuria-induced RIF, and inhibiting the FUT8 of receptors could block the tubule-microvessel paracrine network and reverse the damage of peritubular microvessels and renal interstitial fibrosis. This tubule-vascular chip may provide a prospective platform to facilitate future investigations into the mechanisms of kidney diseases, and target-FUT8 inhibition may be an innovative and potential therapeutic strategy for RIF induced by proteinuria.

Key metalloproteinase-mediated pathways in the kidney

Matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinases (ADAMs) belong to the metzincin family of zinc-containing multidomain molecules, and can act as soluble or membrane-bound proteases. These enzymes inactivate or activate other soluble or membrane-expressed mediator molecules, which enables them to control developmental processes, tissue remodelling, inflammatory responses and proliferative signalling pathways. The dysregulation of MMPs and ADAMs has long been recognized in acute kidney injury and in chronic kidney disease, and genetic targeting of selected MMPs and ADAMs in different mouse models of kidney disease showed that they can have detrimental and protective roles. In particular, MMP-2, MMP-7, MMP-9, ADAM10 and ADAM17 have been shown to have a mainly profibrotic effect and might therefore represent therapeutic targets. Each of these proteases has been associated with a different profibrotic pathway that involves tissue remodelling, Wnt-β-catenin signalling, stem cell factor-c-kit signalling, IL-6 trans-signalling or epidermal growth factor receptor (EGFR) signalling. Broad-spectrum metalloproteinase inhibitors have been used to treat fibrotic kidney diseases experimentally but more targeted approaches have since been developed, including inhibitory antibodies, to avoid the toxic side effects initially observed with broad-spectrum inhibitors. These advances not only provide a solid foundation for additional preclinical studies but also encourage further translation into clinical research.

Oxidative stress increases 1-deoxysphingolipid levels in chronic kidney disease

Chronic kidney disease (CKD) leads to deep changes in lipid metabolism and obvious dyslipidemia. The dysregulation of lipid metabolism in turn results in CKD progression and the complications of cardiovascular diseases. To obtain a profound insight into the associated dyslipidemia in CKD, we performed lipidomic analysis to measure lipid metabolites in the serum from a rat 5/6 nephrectomy (5/6 Nx) model of CKD as well as in the serum from CKD patients. HK-2 cells were also used to examine oxidative stress-induced sphingolipid changes. Totally 182 lipid species were identified in 5/6 Nx rats. We found glycerolipids, total free fatty acids, and sphingolipids levels were significantly upregulated in 5/6 Nx rats. The atypical sphingolipids, 1-deoxysphingolipids, were significantly altered in both CKD animals and human CKD patients. The levels of 1-deoxysphingolipids directly relevant to the level of oxidative stress in vivo and in vitro. These results demonstrate that 1-deoxysphingolipid levels are increased in CKD and this increase directly correlates with increased kidney oxidative stress.

Genetic or pharmacologic Nrf2 activation increases proteinuria in chronic kidney disease in mice

The nuclear factor erythroid 2-related factor 2 (Nrf2) pathway upregulates key cellular defenses. Clinical trials are utilizing pharmacologic Nrf2 inducers such as bardoxolone methyl to treat chronic kidney disease, but Nrf2 activation has been linked to a paradoxical increase in proteinuria. To understand this effect, we examined genetically engineered mice with elevated Nrf2 signaling due to reduced expression of the Nrf2 inhibitor, Kelch-like ECH-associated protein 1 (Keap1). These Keap1^FA/FA mice lacked baseline proteinuria but exhibited increased proteinuria in experimental models evoked by adriamycin, angiotensin II, or protein overload. After injury, Keap1^FA/FA mice had increased glomerulosclerosis, nephrin disruption and shedding, podocyte injury, foot process effacement, and interstitial fibrosis. Keap1^FA/FA mice also had higher daytime blood pressures and lower heart rates measured by radiotelemetry. Conversely, Nrf2 knockout mice were protected from proteinuria. We also examined the pharmacologic Nrf2 inducer CDDO-Im. Compared to angiotensin II alone, the combination of angiotensin II and CDDO-Im significantly increased proteinuria, a phenomenon not observed in Nrf2 knockout mice. This effect was not accompanied by additional increases in blood pressure. Finally, Nrf2 was found to be upregulated in the glomeruli of patients with focal segmental glomerulosclerosis, diabetic nephropathy, fibrillary glomerulonephritis, and membranous nephropathy. Thus, our studies demonstrate that Nrf2 induction in mice may exacerbate proteinuria in chronic kidney disease.

The Integrated RNA Landscape of Renal Preconditioning against Ischemia-Reperfusion Injury

BACKGROUND

Although AKI lacks effective therapeutic approaches, preventive strategies using preconditioning protocols, including caloric restriction and hypoxic preconditioning, have been shown to prevent injury in animal models. A better understanding of the molecular mechanisms that underlie the enhanced resistance to AKI conferred by such approaches is needed to facilitate clinical use. We hypothesized that these preconditioning strategies use similar pathways to augment cellular stress resistance.

METHODS

To identify genes and pathways shared by caloric restriction and hypoxic preconditioning, we used RNA-sequencing transcriptome profiling to compare the transcriptional response with both modes of preconditioning in mice before and after renal ischemia-reperfusion injury.

RESULTS

The gene expression signatures induced by both preconditioning strategies involve distinct common genes and pathways that overlap significantly with the transcriptional changes observed after ischemia-reperfusion injury. These changes primarily affect oxidation-reduction processes and have a major effect on mitochondrial processes. We found that 16 of the genes differentially regulated by both modes of preconditioning were strongly correlated with clinical outcome; most of these genes had not previously been directly linked to AKI.

CONCLUSIONS

This comparative analysis of the gene expression signatures in preconditioning strategies shows overlapping patterns in caloric restriction and hypoxic preconditioning, pointing toward common molecular mechanisms. Our analysis identified a limited set of target genes not previously known to be associated with AKI; further study of their potential to provide the basis for novel preventive strategies is warranted. To allow for optimal interactive usability of the data by the kidney research community, we provide an online interface for user-defined interrogation of the gene expression datasets (http://shiny.cecad.uni-koeln.de:3838/IRaP/).

Absence of the Lectin Activation Pathway of Complement Ameliorates Proteinuria-Induced Renal Injury

Proteinuria is an adverse prognostic feature in renal diseases. In proteinuric nephropathies, filtered proteins exert an injurious effect on the renal tubulointerstitium, resulting in inflammation and fibrosis. In the present study, we assessed to what extent complement activation via the lectin pathway may contribute to renal injury in response to proteinuria-related stress in proximal tubular cells. We used the well-established mouse model of protein overload proteinuria (POP) to assess the effect of lectin pathway inhibition on renal injury and fibrotic changes characteristic of proteinuric nephropathy. To this end, we compared experimental outcomes in wild type mice with MASP-2-deficient mice or wild type mice treated with MASP-2 inhibitor to block lectin pathway functional activity. Multiple markers of renal injury were assessed including renal function, proteinuria, macrophage infiltration, and cytokine release profiles. Both MASP-2-deficient and MASP-2 inhibitor-treated wild type mice exhibited renoprotection from proteinuria with significantly less tubulointerstitial injury when compared to isotype control antibody treated mice. This indicates that therapeutic targeting of MASP-2 in proteinuric nephropathies may offer a useful strategy in the clinical management of proteinuria associated pathologies in a variety of different underlying renal diseases.

Intracellular albumin overload elicits endoplasmic reticulum stress and PKC-delta/p38 MAPK pathway activation to induce podocyte apoptosis

Podocyte injury is closely related to proteinuria and the progression of chronic kidney disease (CKD). Currently, there is no conclusive understanding about the mechanisms involved in albumin overload and podocyte apoptosis response. In this study, we sought to explore the ways by which intracellular albumin can mediate podocyte apoptosis. Here, immortalized mouse podocytes were treated with bovine serum albumin (BSA) at different times and concentrations, in the presence or absence of SB203580 (0.1 µM, inhibitor of mitogen-activated-protein kinase – p38MAPK). Using immunofluorescence images, flow cytometry and immunoblotting, we observed a time-dependent intracellular accumulation of fluorescent albumin-FITC-BSA, followed by concentration-and time-dependent effect of intracellular albumin overload on podocyte apoptosis, which was mediated by increased expression of the chaperone glucose-regulated-protein 78 (GRP 78) and phosphorylated inositol-requiring enzyme 1 alpha (pIRE1-α), as well as protein kinase C delta (PKC-δ), p38MAPK and cleaved caspase 12 expression. SB203580 prevented the cleavage of caspase 12 and the albumin-mediated podocyte apoptosis. These results suggest that intracellular albumin overload is associated with endoplasmic reticulum (ER) stress and upregulation of PKC-δ/p38MAPK/caspase 12 pathway, which may be a target for future therapeutic of albumin-induced podocyte apoptosis.

NIX-mediated mitophagy protects against proteinuria-induced tubular cell apoptosis and renal injury

Proteinuria, the most common symptom of renal injury, is an independent factor for renal tubular injury. However, the underlying mechanism remains to be fully elucidated. Mitochondrion is an important target for proteinuria-induced renal tubular cell injury. Insufficient mitophagy exacerbates cell injury by initiating mitochondrial dysfunction-related cell apoptosis. In the experiment, the role of NIP3-like protein X (NIX)-mediated mitophagy was investigated in proteinuria-induced renal injury. In this study, we demonstrated that NIX expression was reduced in renal tubules and correlated with the decline of estimated glomerular filtration rate and increase of the proteinuria in patients. In proteinuric mice, NIX-mediated mitophagy was significantly suppressed. Meanwhile, the proteinuric mice exhibited renal dysfunction, increased mitochondrial fragmentation, and tubular cell apoptosis. Overexpression of NIX attenuated those disruptions in proteinuric mice. In cultured renal tubular epithelial cells, albumin induced a decrease in NIX-mediated mitophagy and an increase in cell apoptosis. Overexpression of NIX attenuated albumin-induced cell apoptosis, whereas NIX siRNA aggravated these perturbations. These results indicate that proteinuria suppresses NIX-mediated mitophagy in the renal tubular epithelial cell, which triggers the cell undergoing mitochondria-dependent cell apoptosis. Collectively, our finding suggests that restoration of NIX-mediated mitophagy might be a novel therapeutic target for alleviating proteinuria-induced kidney injury.

Kidney Proximal Tubule Lipoapoptosis Is Regulated by Fatty Acid Transporter-2 (FATP2)

Albuminuria and tubular atrophy are among the highest risks for CKD progression to ESRD. A parsimonious mechanism involves leakage of albumin-bound nonesterified fatty acids (NEFAs) across the damaged glomerular filtration barrier and subsequent reabsorption by the downstream proximal tubule, causing lipoapoptosis. We sought to identify the apical proximal tubule transporter that mediates NEFA uptake and cytotoxicity. We observed transporter-mediated uptake of fluorescently labeled NEFA in cultured proximal tubule cells and microperfused rat proximal tubules, with greater uptake from the apical surface than from the basolateral surface. Protein and mRNA expression analyses revealed that kidney proximal tubules express transmembrane fatty acid transporter-2 (FATP2), encoded by Slc27a2, but not the other candidate transporters CD36 and free fatty acid receptor 1. Kidney FATP2 localized exclusively to proximal tubule epithelial cells along the apical but not the basolateral membrane. Treatment of mice with lipidated albumin to induce proteinuria caused a decrease in the proportion of tubular epithelial cells and an increase in the proportion of interstitial space in kidneys from wild-type but not Slc27a2^-/- mice. Ex vivo microperfusion and in vitro experiments with NEFA-bound albumin at concentrations that mimic apical proximal tubule exposure during glomerular injury revealed significantly reduced NEFA uptake and palmitate-induced apoptosis in microperfused Slc27a2^-/- proximal tubules and Slc27a2^-/- or FATP2 shRNA-treated proximal tubule cell lines compared with wild-type or scrambled oligonucleotide-treated cells, respectively. We conclude that FATP2 is a major apical proximal tubule NEFA transporter that regulates lipoapoptosis and may be an amenable target for the prevention of CKD progression.

Role of albumin and its modifications in glomerular injury

Albuminuria is both a characteristic hallmark and a known risk factor for progressive glomerular disease. Although the molecular basis for a potential causative role for albuminuria in progressive chronic kidney disease remains poorly understood, there have been several recent advances in our understanding of the role of albumin, and its molecular modifications, in the development and progression of glomerular disease. This review discusses recent findings related to the ability of albumin and its associated factors to directly induce podocyte and glomerular injury. Additional recent studies confirming the ability and mechanisms by which podocytes endocytose albumin are also discussed. Lastly, we present several known molecular modifications in the albumin molecule itself, as well as substances bound to it, which may be important and potentially clinically relevant mediators of albumin-induced glomerular injury. These recent findings may create entirely new opportunities to develop novel future therapies directed at albumin that could potentially help reduce podocyte and renal tubular injury and slow the progression of chronic glomerular disease.

Matrix Metalloproteinases in Kidney Disease: Role in Pathogenesis and Potential as a Therapeutic Target

Matrix metalloproteinases (MMPs) are large family of proteinases. In addition to a fundamental role in the remodeling of the extracellular matrix, they also cleave a number of cell surface proteins and are involved in multiple cellular processes. MMP activity is regulated via numerous mechanisms, including inhibition by endogenous tissue inhibitors of metalloproteinases (TIMPs). Similar to MMPs, a role for TIMPs has been established in multiple cell signaling pathways. Aberrant expression of MMPs and TIMPS in renal pathophysiology has long been recognized, and with the generation of specific knockout mice, the mechanistic role of several MMPs and TIMPs is becoming more understood and has revealed both pathogenic and protective roles. This chapter will focus on the expression and localization of MMPs and TIMPs in the kidney, as well as summarizing the current information linking these proteins to acute kidney injury and chronic kidney disease. In addition, we will summarize studies suggesting that MMPs and TIMPs may be biomarkers of renal dysfunction and represent novel therapeutic targets to attenuate kidney disease.

Non-invasive quantification of collagen turnover in renal transplant recipients

Kidney allograft failure due to chronic injury/rejection remains the main cause of graft loss in renal transplant recipients (RTR). Here, we investigated whether specific biomarkers of extracellular matrix (ECM) turnover are associated with allograft function and chronic kidney disease (CKD) stage in RTR. Seventy-eight patients who attended the University Medical Center Groningen for a routine check-up after kidney transplantation were enrolled in the study. Plasma and/or 24h-urine samples were collected and specific matrix-metalloproteinase-generated neo-epitope fragments of collagens were measured by enzyme-linked immunosorbent assay. Our results demonstrated that urinary levels of C3M, a marker for collagen type III degradation, correlated with estimated glomerular filtration rate (eGFR; r = 0.58, p<0.0001), with lower levels detected in the urine of patients with advanced CKD. In addition, plasma levels of Pro-C6, a marker for collagen type VI formation, significantly increased with disease progression and correlated with eGFR (r = -0.72, p<0.0001). Conversely, plasma C3M and urinary Pro-C6 levels showed no correlation with renal function. We identified two neo-epitope biomarkers of tissue turnover associated with ECM remodeling and fibrosis that can stratify patients by CKD stage. This is as promising first step towards non-invasive monitoring of ECM turnover in the kidneys.

Endoplasmic reticulum stress inhibition limits the progression of chronic kidney disease in the Dahl salt-sensitive rat

Proteinuria is one of the primary risk factors for the progression of chronic kidney disease (CKD) and has been implicated in the induction of endoplasmic reticulum (ER) stress. We hypothesized that the suppression of ER stress with a low molecular weight chemical chaperone, 4-phenylbutyric acid (4-PBA), would reduce the severity of CKD and proteinuria in the Dahl salt-sensitive (SS) hypertensive rat. To induce hypertension and CKD, 12-wk-old male rats were placed on a high-salt (HS) diet for 4 wk with or without 4-PBA treatment. We assessed blood pressure and markers of CKD, including proteinuria, albuminuria, and renal pathology. Furthermore, we determined if HS feeding resulted in an impaired myogenic response, subsequent to ER stress. 4-PBA treatment reduced salt-induced hypertension, proteinuria, and albuminuria and preserved myogenic constriction. Furthermore, renal pathology was reduced with 4-PBA treatment, as indicated by lowered expression of profibrotic markers and fewer intratubular protein casts. In addition, ER stress in the glomerulus was reduced, and the integrity of the glomerular filtration barrier was preserved. These results suggest that 4-PBA treatment protects against proteinuria in the SS rat by preserving the myogenic response and by preventing ER stress, which led to a breakdown in the glomerular filtration barrier. As such, alleviating ER stress serves as a viable therapeutic strategy to preserve kidney function and to delay the progression of CKD in the animal model under study.

The significance of tubulointerstitial lesions in childhood Henoch-Schönlein nephritis

BACKGROUND

Little information is currently available on the development of tubulointerstitial lesions in children with Henoch-Schönlein nephritis (HSN). To identify the impact of the development of tubulointerstitial changes in HSN, we retrospectively analyzed renal biopsies obtained from children with HSN.

METHODS

Twenty-eight children with HSN from whom serial renal biopsies had been obtained before and after immunosuppressive therapy were enrolled in the study. The patients were divided into two groups according to the observed change in tubulointerstitial lesion development: group I (n = 15), with stable or improved tubulointerstitial lesions, and group II (n = 13), with worsened tubulointerstitial lesions. Group II patients had longer duration of proteinuria than group I patients (3.7 ± 3.7 years vs. 1.7 ± 1.7 years, p = 0.052).

RESULTS

The change in serum albumin level was negatively correlated with the change in tubulointerstitial scores before and after treatment (γ = -0.444, p = 0.018). Group II patients showed a significant decrease in immunoglobulin G (IgG) and IgA deposits after treatment (p = 0.039 and 0.003, respectively), while group II patients did not (p = 0.458 and 0.506, respectively).

CONCLUSIONS

Although the International Study of Kidney Disease in Children classification of HSN does not include tubulointerstitial lesions, they can progress during treatment and could have significant clinical implications in association with the duration of proteinuria.

Up-Regulation of CD74 Expression in Parietal Epithelial Cells in a Mouse Model of Focal Segmental Glomerulosclerosis

BACKGROUND/AIMS

De novo expression of CD44 is considered as a marker of parietal epithelial cell (PEC) activation. The aim of our study was to explore CD74 expression, which can form a complex with CD44, in PECs during the progression of focal segmental glomerulosclerosis (FSGS). To clarify the role of CD74 expression and of its interaction with CD44, we generated a new mouse model with enhanced PEC activation through lipopolysaccharide (LPS) application to adriamycin (ADR)-induced nephropathy mice (LPS-treated ADR mice).

METHODS

As a new model, LPS was intraperitoneally injected into the mice 3 weeks after ADR injection. The mice were divided into 3 categories: control mice, ADR mice and LPS-treated ADR mice. Renal function parameters, histologic changes and immunohistochemical expression of CD74 and other PEC activation markers were analyzed after LPS application.

RESULTS

After LPS stimulation, the glomeruli were characterized by enlarged epithelial cells with strong CD74 expression, followed by pseudo-crescent formation. By double staining, CD74-positive enlarged cells showed co-expression of classical PEC markers, but not of Lotus tetragonolobus lectin (marker of proximal tubular cells), suggesting amplification of PEC activation. Time-course analysis displayed marked upregulation of CD74 expression during rapid PEC activation compared with CD44. Additionally, the time-dependent change in ERK phosphorylation showed a similar pattern to CD74.

CONCLUSION

Our results indicate that CD74 can be a marker for PEC activation in FSGS. By modifying the ADR mouse model through LPS treatment, we found that CD74 upregulation better reflects a rapid amplification of PEC activation than CD44 expression.

A previously unrecognized role of C3a in proteinuric progressive nephropathy

Podocyte loss is the initial event in the development of glomerulosclerosis, the structural hallmark of progressive proteinuric nephropathies. Understanding mechanisms underlying glomerular injury is the key challenge for identifying novel therapeutic targets. In mice with protein-overload induced by bovine serum albumin (BSA), we evaluated whether the alternative pathway (AP) of complement mediated podocyte depletion and podocyte-dependent parietal epithelial cell (PEC) activation causing glomerulosclerosis. Factor H (Cfh^−/−) or factor B-deficient mice were studied in comparison with wild-type (WT) littermates. WT+BSA mice showed podocyte depletion accompanied by glomerular complement C3 and C3a deposits, PEC migration to capillary tuft, proliferation, and glomerulosclerosis. These changes were more prominent in Cfh^−/− +BSA mice. The pathogenic role of AP was documented by data that factor B deficiency preserved glomerular integrity. In protein-overload mice, PEC dysregulation was associated with upregulation of CXCR4 and GDNF/c-Ret axis. In vitro studies provided additional evidence of a direct action of C3a on proliferation and CXCR4-related migration of PECs. These effects were enhanced by podocyte-derived GDNF. In patients with proteinuric nephropathy, glomerular C3/C3a paralleled PEC activation, CXCR4 and GDNF upregulation. These results indicate that mechanistically uncontrolled AP complement activation is not dispensable for podocyte-dependent PEC activation resulting in glomerulosclerosis.

Downregulation of renal tubular Wnt/β-catenin signaling by Dickkopf-3 induces tubular cell death in proteinuric nephropathy

Studies on the role of Wnt/β-catenin signaling in different forms of kidney disease have yielded discrepant results. Here, we report the biphasic change of renal β-catenin expression in mice with overload proteinuria in which β-catenin was upregulated at the early stage (4 weeks after disease induction) but abrogated at the late phase (8 weeks). Acute albuminuria was observed at 1 week after bovine serum albumin injection, followed by partial remission at 4 weeks that coincided with overexpression of renal tubular β-catenin. Interestingly, a rebound in albuminuria at 8 weeks was accompanied by downregulated tubular β-catenin expression and heightened tubular apoptosis. In addition, there was an inverse relationship between Dickkopf-3 (Dkk-3) and renal tubular β-catenin expression at these time points. In vitro, a similar trend in β-catenin expression was observed in human kidney-2 (HK-2) cells with acute (upregulation) and prolonged (downregulation) exposure to albumin. Induction of a proapoptotic phenotype by albumin was significantly enhanced by silencing β-catenin in HK-2 cells. Finally, Dkk-3 expression and secretion was increased after prolonged exposure to albumin, leading to the suppression of intracellular β-catenin signaling pathway. The effect of Dkk-3 on β-catenin signaling was confirmed by incubation with exogenous Dkk-3 in HK-2 cells. Taken together, these data suggest that downregulation of tubular β-catenin signaling induced by Dkk-3 has a detrimental role in chronic proteinuria, partially through the increase in apoptosis.

Cardiorenal fibrosis and dysfunction in aging: Imbalance in mediators and regulators of collagen

Cardiorenal fibrosis is a biological process that increases with age and contributes to dysfunction of the heart and kidney. While numerous circulating and tissue hormones, cytokines and enzymes have been identified in the development of cardiorenal fibrosis, several reports have suggested that the anti-fibrotic natriuretic peptide system (NPS), pro-fibrotic renin-angiotensin-aldosterone system (RAAS), transforming growth factor-beta 1 (TGF-β1), matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMPs) are fundamental regulators and mediators of this process. However, the simultaneous assessment of these components in the development of age-mediated cardiorenal fibrotic remodeling is not completely understood. Thus, we assessed cardiorenal structure and function, the circulating NPS and RAAS and the cardiorenal tissue gene expression of collagen (Col) I, Col III, TGF-β1, MMP-9 and TIMP-1 in 2 and 20 month old Fischer rats. Our studies determined that aging was characterized by an increase in cardiorenal fibrosis that was accompanied with cardiorenal dysfunction. These alterations were associated with lower circulating atrial and C-type natriuretic peptides and higher angiotensin II and aldosterone levels in the aged rats. Moreover, we observed a decrease in Col I and III and an increase in TIMP- mRNA expressions in the aged heart and kidney, while TGF-β1 expression increased and MMP-9 decreased only in the aged kidney. We conclude that the age-mediated alterations in these fibrotic regulator and mediator profiles favors collagen accumulation due to an imbalance between the NPS and RAAS as well as a decline in the degradative pathway, thus suggesting a therapeutic opportunity to target these components.

Shank2 Regulates Renal Albumin Endocytosis

Albuminuria is a strong and independent predictor of kidney disease progression but the mechanisms of albumin handling by the kidney remain to be fully defined. Previous studies have shown that podocytes endocytose albumin. Here we demonstrate that Shank2, a large scaffolding protein originally identified at the neuronal postsynaptic density, is expressed in podocytes in vivo and in vitro and plays an important role in albumin endocytosis in podocytes. Knockdown of Shank2 in cultured human podocytes decreased albumin uptake, but the decrease was not statistically significant likely due to residual Shank2 still present in the knockdown podocytes. Complete knockout of Shank2 in podocytes significantly diminished albumin uptake in vitro. Shank2 knockout mice develop proteinuria by 8 weeks of age. To examine albumin handling in vivo in wild-type and Shank2 knockout mice we used multiphoton intravital imaging. While FITC-labeled albumin was rapidly seen in the renal tubules of wild-type mice after injection, little albumin was seen in the tubules of Shank2 knockout mice indicating dysregulated renal albumin trafficking in the Shank2 knockouts. We have previously found that caveolin-1 is required for albumin endocytosis in cultured podocytes. Shank2 knockout mice had significantly decreased expression and altered localization of caveolin-1 in podocytes suggesting that disruption of albumin endocytosis in Shank2 knockouts is mediated via caveolin-1. In summary, we have identified Shank2 as another component of the albumin endocytic pathway in podocytes.

A glimpse of matrix metalloproteinases in diabetic nephropathy

Matrix metalloproteinases (MMPs) are proteolytic enzymes belonging to the family of zinc-dependent endopeptidases that are capable of degrading almost all the proteinaceous components of the extracellular matrix (ECM). It is known that MMPs play a role in a number of renal diseases, such as, various forms of glomerulonephritis and tubular diseases, including some of the inherited kidney diseases. In this regard, ECM accumulation is considered to be a hallmark morphologic finding of diabetic nephropathy, which not only is related to the excessive synthesis of matrix proteins, but also to their decreased degradation by the MMPs. In recent years, increasing evidence suggest that there is a good correlation between the activity or expression of MMPs and progression of renal disease in patients with diabetic nephropathy and in various experimental animal models. In such a diabetic milieu, the expression of MMPs is modulated by high glucose, advanced glycation end products (AGEs), TGF-β, reactive oxygen species (ROS), transcription factors and some of the microRNAs. In this review, we focused on the structure and functions of MMPs, and their role in the pathogenesis of diabetic nephropathy.

Extracellular Superoxide Dismutase Protects against Proteinuric Kidney Disease

Extracellular superoxide dismutase (EC-SOD), also known as SOD3, is an antioxidant expressed at high levels in normal adult kidneys. Because oxidative stress contributes to a variety of kidney injuries, we hypothesized that EC-SOD may be protective in CKD progression. To study this hypothesis, we used a murine model of ADR nephropathy characterized by albuminuria and renal dysfunction. We found that levels of EC-SOD diminished throughout the course of disease progression and were associated with increased levels of NADPH oxidase and oxidative stress markers. EC-SOD null mice were sensitized to ADR injury, as evidenced by increases in albuminuria, serum creatinine, histologic damage, and oxidative stress. The absence of EC-SOD led to increased levels of NADPH oxidase and an increase in β-catenin signaling, which has been shown to be pathologic in a variety of kidney injuries. Exposure of EC-SOD null mice to either chronic angiotensin II infusion or to daily albumin injections also caused increased proteinuria. In contrast, EC-SOD null mice subjected to nonproteinuric CKD induced by unilateral ureteral obstruction exhibited no differences compared with wild-type mice. Finally, we also found a decrease in EC-SOD in human CKD biopsy samples, similar to our findings in mice. Therefore, we conclude that EC-SOD is protective in CKDs characterized by proteinuria.

Diverse functions of matrix metalloproteinases during fibrosis

Fibrosis--a debilitating condition that can occur in most organs - is characterized by excess deposition of a collagen-rich extracellular matrix (ECM). At first sight, the activities of proteinases that can degrade matrix, such as matrix metalloproteinases (MMPs), might be expected to be under-expressed in fibrosis or, if present, could function to resolve the excess matrix. However, as we review here, some MMPs are indeed anti-fibrotic, whereas others can have pro-fibrotic functions. MMPs modulate a range of biological processes, especially processes related to immunity and tissue repair and/or remodeling. Although we do not yet know precisely how MMPs function during fibrosis--that is, the protein substrate or substrates that an individual MMP acts on to effect a specific process--experiments in mouse models demonstrate that MMP-dependent functions during fibrosis are not limited to effects on ECM turnover. Rather, data from diverse models indicate that these proteinases influence cellular activities as varied as proliferation and survival, gene expression, and multiple aspects of inflammation that, in turn, impact outcomes related to fibrosis.

NLRP3 inflammasome mediates albumin-induced renal tubular injury through impaired mitochondrial function

Proteinuria serves as a direct causative factor of renal tubular cell injury and is highly associated with the progression of chronic kidney disease via uncertain mechanisms. Recently, evidence demonstrated that both NLRP3 inflammasome and mitochondria are involved in the chronic kidney disease progression. The present study was undertaken to examine the role of NLRP3 inflammasome/mitochondria axis in albumin-induced renal tubular injury. In patients with proteinuria, NLRP3 was significantly up-regulated in tubular epithelial cells and was positively correlated with the severity of proteinuria. In agreement with these results, albumin remarkably activated NLRP3 inflammasome in both in vitro renal tubular cells and in vivo kidneys in parallel with significant epithelial cell phenotypic alteration and cell apoptosis. Genetic disruption of NLRP3 inflammasome remarkably attenuated albumin-induced cell apoptosis and phenotypic changes under both in vitro and in vivo conditions. In addition, albumin treatment resulted in a significant mitochondrial abnormality as evidenced by the impaired function and morphology, which was markedly reversed by invalidation of NLRP3/caspase-1 signaling pathway. Interestingly, protection of mitochondria function by Mn(III)tetrakis (4-benzoic acid) porphyrin (MnTBAP) or cyclosporin A (CsA) robustly attenuated albumin-induced injury in mouse proximal tubular cells. Collectively, these findings demonstrated a pathogenic role of NLRP3 inflammasome/caspase-1/mitochondria axis in mediating albumin-induced renal tubular injury. The discovery of this novel axis provides some potential targets for the treatment of proteinuria-associated renal injury.

Podocytes degrade endocytosed albumin primarily in lysosomes

Albuminuria is a strong, independent predictor of chronic kidney disease progression. We hypothesize that podocyte processing of albumin via the lysosome may be an important determinant of podocyte injury and loss. A human urine derived podocyte-like epithelial cell (HUPEC) line was used for in vitro experiments. Albumin uptake was quantified by Western blot after loading HUPECs with fluorescein-labeled (FITC) albumin. Co-localization of albumin with lysosomes was determined by confocal microscopy. Albumin degradation was measured by quantifying FITC-albumin abundance in HUPEC lysates by Western blot. Degradation experiments were repeated using HUPECs treated with chloroquine, a lysosome inhibitor, or MG-132, a proteasome inhibitor. Lysosome activity was measured by fluorescence recovery after photo bleaching (FRAP). Cytokine production was measured by ELISA. Cell death was determined by trypan blue staining. In vivo, staining with lysosome-associated membrane protein-1 (LAMP-1) was performed on tissue from a Denys-Drash trangenic mouse model of nephrotic syndrome. HUPECs endocytosed albumin, which co-localized with lysosomes. Choloroquine, but not MG-132, inhibited albumin degradation, indicating that degradation occurs in lysosomes. Cathepsin B activity, measured by FRAP, significantly decreased in HUPECs exposed to albumin (12.5% of activity in controls) and chloroquine (12.8%), and declined further with exposure to albumin plus chloroquine (8.2%, p<0.05). Cytokine production and cell death were significantly increased in HUPECs exposed to albumin and chloroquine alone, and these effects were potentiated by exposure to albumin plus chloroquine. Compared to wild-type mice, glomerular staining of LAMP-1 was significantly increased in Denys-Drash mice and appeared to be most prominent in podocytes. These data suggest lysosomes are involved in the processing of endocytosed albumin in podocytes, and lysosomal dysfunction may contribute to podocyte injury and glomerulosclerosis in albuminuric diseases. Modifiers of lysosomal activity may have therapeutic potential in slowing the progression of glomerulosclerosis by enhancing the ability of podocytes to process and degrade albumin.

Albumin-induced podocyte injury and protection are associated with regulation of COX-2

Albuminuria is both a hallmark and a risk factor for progressive glomerular disease, and results in increased exposure of podocytes to serum albumin with its associated factors. Here in vivo and in vitro models of serum albumin overload were used to test the hypothesis that albumin-induced proteinuria and podocyte injury directly correlate with COX-2 induction. Albumin induced COX-2, MCP-1, CXCL1 and the stress protein HSP25 in both rat glomeruli and cultured podocytes, while B7-1 and HSP70i were also induced in podocytes. Podocyte exposure to albumin induced both mRNA and protein and enhanced the mRNA stability of COX-2, a key regulator of renal hemodynamics and inflammation, which renders podocytes susceptible to injury. Podocyte exposure to albumin also stimulated several kinases (p38 MAPK, MK2, JNK/SAPK and ERK1/2), inhibitors of which (except JNK/SAPK) down-regulated albumin-induced COX-2. Inhibition of AMPK, PKC and NFκB also down-regulated albumin-induced COX-2. Critically, albumin-induced COX-2 was also inhibited by glucocorticoids and thiazolidinediones, both of which directly protect podocytes against injury. Furthermore, specific albumin-associated fatty acids were identified as important contributors to COX-2 induction, podocyte injury and proteinuria. Thus, COX-2 is associated with podocyte injury during albuminuria, as well as with the known podocyte protection imparted by glucocorticoids and thiazolidinediones. Moreover, COX-2 induction, podocyte damage and albuminuria appear mediated largely by serum albumin-associated fatty acids.

Human podocytes perform polarized, caveolae-dependent albumin endocytosis

The renal glomerulus forms a selective filtration barrier that allows the passage of water, ions, and small solutes into the urinary space while restricting the passage of cells and macromolecules. The three layers of the glomerular filtration barrier include the vascular endothelium, glomerular basement membrane (GBM), and podocyte epithelium. Podocytes are capable of internalizing albumin and are hypothesized to clear proteins that traverse the GBM. The present study followed the fate of FITC-labeled albumin to establish the mechanisms of albumin endocytosis and processing by podocytes. Confocal imaging and total internal reflection fluorescence microscopy of immortalized human podocytes showed FITC-albumin endocytosis occurred preferentially across the basal membrane. Inhibition of clathrin-mediated endocytosis and caveolae-mediated endocytosis demonstrated that the majority of FITC-albumin entered podocytes through caveolae. Once internalized, FITC-albumin colocalized with EEA1 and LAMP1, endocytic markers, and with the neonatal Fc receptor, a marker for transcytosis. After preloading podocytes with FITC-albumin, the majority of loaded FITC-albumin was lost over the subsequent 60 min of incubation. A portion of the loss of albumin occurred via lysosomal degradation as pretreatment with leupeptin, a lysosomal protease inhibitor, partially inhibited the loss of FITC-albumin. Consistent with transcytosis of albumin, preloaded podocytes also progressively released FITC-albumin into the extracellular media. These studies confirm the ability of podocytes to endocytose albumin and provide mechanistic insight into cellular mechanisms and fates of albumin handling in podocytes.

Mesenchymal stem cells modulate albumin-induced renal tubular inflammation and fibrosis

Bone marrow-derived mesenchymal stem cells (BM-MSCs) have recently shown promise as a therapeutic tool in various types of chronic kidney disease (CKD) models. However, the mechanism of action is incompletely understood. As renal prognosis in CKD is largely determined by the degree of renal tubular injury that correlates with residual proteinuria, we hypothesized that BM-MSCs may exert modulatory effects on renal tubular inflammation and epithelial-to-mesenchymal transition (EMT) under a protein-overloaded milieu. Using a co-culture model of human proximal tubular epithelial cells (PTECs) and BM-MSCs, we showed that concomitant stimulation of BM-MSCs by albumin excess was a prerequisite for them to attenuate albumin-induced IL-6, IL-8, TNF-α, CCL-2, CCL-5 overexpression in PTECs, which was partly mediated via deactivation of tubular NF-κB signaling. In addition, albumin induced tubular EMT, as shown by E-cadherin loss and α-SMA, FN and collagen IV overexpression, was also prevented by BM-MSC co-culture. Albumin-overloaded BM-MSCs per se retained their tri-lineage differentiation capacity and overexpressed hepatocyte growth factor (HGF) and TNFα-stimulating gene (TSG)-6 via P38 and NF-κB signaling. Albumin-induced tubular CCL-2, CCL-5 and TNF-α overexpression were suppressed by recombinant HGF treatment, while the upregulation of α-SMA, FN and collagen IV was attenuated by recombinant TSG-6. Neutralizing HGF and TSG-6 abolished the anti-inflammatory and anti-EMT effects of BM-MSC co-culture in albumin-induced PTECs, respectively. In vivo, albumin-overloaded mice treated with mouse BM-MSCs had markedly reduced BUN, tubular CCL-2 and CCL-5 expression, α-SMA and collagen IV accumulation independent of changes in proteinuria. These data suggest anti-inflammatory and anti-fibrotic roles of BM-MSCs on renal tubular cells under a protein overloaded condition, probably mediated via the paracrine action of HGF and TSG-6.

TIMP2 and TIMP3 have divergent roles in early renal tubulointerstitial injury

Tissue inhibitors of metalloproteinases (TIMPs) are endogenous inhibitors of matrix metalloproteinases (MMPs). While TIMP2 and TIMP3 inhibit MMPs, TIMP3 also inhibits activation of pro-MMP2, whereas TIMP2 promotes it. Here we assessed the differential role of TIMP2 and TIMP3 in renal injury using the unilateral ureteral obstruction model. Gene microarray assay showed that post obstruction, the lack of TIMP3 had a greater impact on gene expression of intermediate, late injury- and repair-induced transcripts, kidney selective transcripts, and solute carriers. Renal injury in TIMP3(-/-), but not in TIMP2(-/-), mice increased the expression of collagen type I/III, connective tissue growth factor, transforming growth factor-β, and the downstream Smad2/3 pathway. Interestingly, ureteral obstruction markedly increased MMP2 activation in the kidneys of TIMP3(-/-) mice, which was completely blocked in the kidneys of TIMP2(-/-) mice. These changes are consistent with enhanced renal tubulointerstitial fibrosis in TIMP3(-/-) and its reduction in TIMP2(-/-) mice. The activities of tumor necrosis factor-α-converting enzyme, caspase-3, and mitogen-activated kinases were elevated in the kidneys of TIMP3(-/-) mice but not TIMP2(-/-) mice, suggesting enhanced activation of apoptotic and pathological signaling pathways only in the obstructed kidney of TIMP3(-/-) mice. Thus, TIMP2 and TIMP3 play differential and contrasting roles in renal injury: TIMP3 protects from damage, whereas TIMP2 promotes injury through MMP2 activation.

Limited capacity of proximal tubular proteolysis in mice with proteinuria

Albuminuria is associated with the additional loss in the urine of small molecular weight proteins normally degraded by the proximal convoluted tubule (PCT), and competition for binding to the megalin/cubilin reuptake system has been considered the likely cause. We have previously reported that deficiency of the intrinsic lysosomal protein Limp-2 causes tubular proteinuria due to reduced fusion of endosomes with lysosomes in the PCT leading to inadequate proteolysis. To determine whether this mechanism also contributes to the tubular proteinuria induced by albumin overload in normal mice, wild-type (WT) mice received daily BSA injections intraperitoneally for 10 days, using untreated Limp-2(-/-) mice as positive controls for inadequate proteolysis. BSA overload induced significant urinary loss of megalin and cubilin ligands in WT mice. Tubular uptake of Alexa-conjugated BSA, administered by intravenous injection, was not reduced in the PCT of mice receiving intraperitoneal BSA. Expression of the tubular protein receptor megalin was also unchanged. There was a delay in proteolysis of reabsorbed proteins in WT mice receiving BSA, evidenced by an increased quantity of retinol-binding protein (RBP) in the kidney cortex, increased basal distribution of endocytosed RBP in cells of the PCT, and persistence of exogenous Alexa-conjugated BSA and RBP after injection. Upregulation of cathepsin L and normal fusion of lysosomes with endosomes were apparently not sufficient to maintain normal clearance of endocytosed proteins. The data suggest that in the presence of competition from albumin overload, reabsorption of filtered proteins is limited by the capacity of lysosomal degradation rather than receptor-mediated endocytosis.

Investigating mechanisms of chronic kidney disease in mouse models

Animal models of chronic kidney disease (CKD) are important experimental tools that are used to investigate novel mechanistic pathways and to validate potential new therapeutic interventions prior to pre-clinical testing in humans. Over the past several years, mouse CKD models have been extensively used for these purposes. Despite significant limitations, the model of unilateral ureteral obstruction (UUO) has essentially become the high-throughput in vivo model, as it recapitulates the fundamental pathogenetic mechanisms that typify all forms of CKD in a relatively short time span. In addition, several alternative mouse models are available that can be used to validate new mechanistic paradigms and/or novel therapies. Here, we review several models-both genetic and experimentally induced-that provide investigators with an opportunity to include renal functional study end-points together with quantitative measures of fibrosis severity, something that is not possible with the UUO model.

Albumin-induced apoptosis of glomerular parietal epithelial cells is modulated by extracellular signal-regulated kinase 1/2

BACKGROUND

The biological role(s) of glomerular parietal epithelial cells (PECs) is not fully understood in health or disease. Given its location, PECs are constantly exposed to low levels of filtered albumin, which is increased in nephrotic states. We tested the hypothesis that PECs internalize albumin and increased uptake results in apoptosis.

METHODS

Confocal microscopy of immunofluorescent staining and immunohistochemistry were used to demonstrate albumin internalization in PECs and to quantitate albumin uptake in normal mice and rats as well as experimental models of membranous nephropathy, minimal change disease/focal segmental glomerulosclerosis and protein overload nephropathy. Fluorescence-activated cell sorting analysis was performed on immortalized cultured PECs exposed to fluorescein isothiocyanate (FITC)-labeled albumin in the presence of an endosomal inhibitor or vehicle. Apoptosis was measured by Hoechst staining in cultured PECs exposed to bovine serum albumin. Levels of phosphorylated extracellular signal-regulated kinase 1 and 2 (p-ERK1/2) were restored by retroviral infection of mitogen-activated protein kinase (MEK) 1/2 and reduced by U0126 in PECs exposed to high albumin levels in culture and apoptosis measured by Hoechst staining.

RESULTS

PECs internalized albumin normally, and this was markedly increased in all of the experimental disease models (P<0.05 versus controls). Cultured immortalized PECs also internalize FITC-labeled albumin, which was reduced by endosomal inhibition. A consequence of increased albumin internalization was PEC apoptosis in vitro and in vivo. Candidate signaling pathways underlying these events were examined. Data showed markedly reduced levels of phosphorylated extracellular signal-regulated kinase 1 and 2 (ERK1/2) in PECs exposed to high albumin levels in nephropathy and in culture. A role for ERK1/2 in limiting albumin-induced apoptosis was shown by restoring p-ERK1/2 by retroviral infection, which reduced apoptosis in cultured PECs, while a forced decrease of p-ERK1/2 through inhibition of MEK 1/2 significantly increased albumin-induced PEC apoptosis.

CONCLUSIONS

A normal role of PECs is to take up filtered albumin. However, this is increased in proteinuric glomerular diseases, leading to apoptosis through changes in ERK1/2.

Can emerging biomarkers of myocardial remodelling identify asymptomatic hypertensive patients at risk for diastolic dysfunction and diastolic heart failure?

AIMS

Hypertension is one of the main drivers of the heart failure (HF) epidemic. The aims of this study were to profile fibro-inflammatory biomarkers across stages of the hypertensive heart disease (HHD) spectrum and to examine whether particular biochemical profiles in asymptomatic patients identify a higher risk of evolution to HF.

METHODS AND RESULTS

This was a cross-sectional observational study involving a population of 275 stable hypertensive patients divided into two different cohorts: Group 1, asymptomatic hypertension (AH) (n= 94); Group 2, HF with preserved ejection fraction  (n= 181). Asymptomatic hypertension patients were further subdivided according to left atrial volume index ≥34 mL/m(2) (n= 30) and <34 mL/m(2) (n= 64). Study assays involved inflammatory markers [interleukin 6 (IL6), interleukin 8 (IL8), monocyte chemoattractant protein 1 (MCP1), and tumour necrosis factor α], collagen 1 and 3 metabolic markers [carboxy-terminal propeptide of collagen 1, amino-terminal propeptide of collagen 1, amino-terminal propeptide of collagen 3 (PIIINP), and carboxy-terminal telopeptide of collagen 1 (CITP)], extra-cellular matrix turnover markers [matrix metalloproteinase 2 (MMP2), matrix metalloproteinase 9 (MMP9), and tissue inhibitor of metalloproteinase 1 (TIMP1)], and the brain natriuretic peptide. Data were adjusted for age, sex, systolic blood pressure, and creatinine. Heart failure with preserved ejection fraction  was associated with an increased inflammatory signal (IL6, IL8, and MCP1), an increased fibrotic signal (PIIINP and CITP), and an increased matrix turnover signal (MMP2 and MMP9). Alterations in MMP and TIMP enzymes were found to be significant indicators of greater degrees of asymptomatic left ventricular diastolic dysfunction.

CONCLUSION

These data define varying fibro-inflammatory profiles throughout different stages of HHD. In particular, the observations on MMP9 and TIMP1 raise the possibility of earlier detection of those at risk of evolution to HF which may help focus effective preventative strategies.

Growth factor Midkine is involved in the pathogenesis of renal injury induced by protein overload containing endotoxin

BACKGROUND

Growth factor Midkine (MK), which expresses on endothelial cells and renal proximal tubules, has been implicated in inflammation-related kidney diseases such as ischemic reperfusion-induced tubulointerstitial injury and diabetic nephropathy. The biological actions of MK are elicited through its chemotactic activity and chemokine-driven inflammatory pathway. Post-infectious glomerulonephritis is caused by the deposition of immune complexes into glomeruli by infiltrating a number of inflammatory cells. Therefore, we investigated whether MK might be involved in the pathogenesis of acute glomerulonephritis.

METHODS

We induced endocapillary proliferative glomerulonephritis in 129/SV mice using intraperitoneal injections of a large amount of protein.

RESULTS

In contrast to mice deficient in MK (Mdk (-/-)), Mdk (+/+) mice induced by protein overload demonstrated more diffuse cellular proliferation in the mesangial areas and capillary lumens, eventually leading to glomerular damage and tubulointerstitial injury. This pathological observation could be attributable to neutrophil infiltration through the chemotaxis and stimulation of the MK-macrophage inflammatory protein (MIP)-2 pathway, but appeared to be due to the MK-related immunoglobulin (Ig)G deposition and C3 activation. These findings are often seen in infectious-related glomerular injury. Furthermore, the profile of MK expression was strongly consistent with that of glomerular damage and tubulointersititial injury.

CONCLUSION

This study might provide a new insight into understanding the deleterious role of MK in endocapillary proliferative glomerulonephritis induced by protein overload.

Galectin-3 preserves renal tubules and modulates extracellular matrix remodeling in progressive fibrosis

Renal tubular cell apoptosis is a critical detrimental event that leads to chronic kidney injury in association with renal fibrosis. The present study was designed to investigate the role of galectin-3 (Gal-3), an important regulator of multiple apoptotic pathways, in chronic kidney disease induced by unilateral ureteral obstruction (UUO). After UUO, Gal-3 expression significantly increased compared with basal levels reaching a peak increase of 95-fold by day 7. Upregulated Gal-3 is predominantly tubular at early time points after UUO but shifts to interstitial cells as the injury progresses. On day 14, there was a significant increase in TdT-mediated dUTP nick end labeling-positive cells (129%) and cytochrome c release (29%), and a decrease in BrdU-positive cells (62%) in Gal-3-deficient compared with wild-type mice. The degree of renal damage was more extensive in Gal-3-deficient mice at days 14 and 21, 35 and 21% increase in total collagen, respectively. Despite more severe fibrosis, myofibroblasts were significantly decreased by 58% on day 14 in the Gal-3-deficient compared with wild-type mice. There was also a corresponding 80% decrease in extracellular matrix synthesis in Gal-3-deficient compared with wild-type mice. Endo180 is a recently recognized receptor for intracellular collagen degradation that is expressed by interstitial cells during renal fibrogenesis. Endo180 expression was significantly decreased by greater than 50% in Gal-3-deficient compared with wild-type mice. Taken together, these results suggested that Gal-3 not only protects renal tubules from chronic injury by limiting apoptosis but that it may lead to enhanced matrix remodeling and fibrosis attenuation.

Homocysteine and Hypertension in Diabetes: Does PPARgamma Have a Regulatory Role?

Dysfunction of macro- and microvessels is a major cause of morbidity and mortality in patients with cardio-renovascular diseases such as atherosclerosis, hypertension, and diabetes. Renal failure and impairment of renal function due to vasoconstriction of the glomerular arteriole in diabetic nephropathy leads to renal volume retention and increase in plasma homocysteine level. Homocysteine, which is a nonprotein amino acid, at elevated levels is an independent cardio-renovascular risk factor. Homocysteine induces oxidative injury of vascular endothelial cells, involved in matrix remodeling through modulation of the matrix metalloproteinase (MMP)/tissue inhibitor of metalloproteinase (TIMP) axis, and increased formation and accumulation of extracellular matrix protein, such as collagen. In heart this leads to increased endothelial-myocyte uncoupling resulting in diastolic dysfunction and hypertension. In the kidney, increased matrix accumulation in the glomerulus causes glomerulosclerosis resulting in hypofiltration, increased renal volume retention, and hypertension. PPARγ agonist reduces tissue homocysteine levels and is reported to ameliorate homocysteine-induced deleterious vascular effects in diabetes. This review, in light of current information, focuses on the beneficial effects of PPARγ agonist in homocysteine-associated hypertension and vascular remodeling in diabetes.

Abnormalities in signaling pathways in diabetic nephropathy

Diabetic nephropathy (DN) is characterized by a plethora of signaling abnormalities that together ultimately result in the clinical and pathologic hallmarks of DN, namely progressive albuminuria followed by a gradual decline in glomerular filtration rate leading to kidney failure, and accompanied by podocyte loss, progressive glomerular sclerosis and, ultimately, progressive tubulointerstitial fibrosis. Over the past few years, the general understanding of the abnormalities in signaling pathways that lead to DN has expanded considerably. In this review, some of the important pathways that appear to be involved in driving this process are discussed, with special emphasis on newer findings and insights. Newer concepts regarding signaling changes in bradykinin, mTOR, JAK/STAT, MCP-1, VEGF, endothelial nitric oxide synthase, activated protein C and other pathways are discussed.

Serum levels of MMP-9 and TIMP-1 in primary hypertension and effect of antihypertensive treatment

BACKGROUND

Matrix metalloproteinases, a family of proteolytic enzymes are thought to be involved in extracellular matrix accumulation during development of hypertensive target organ disease. The present study was designed to compare hypertensive patients with normotensive individuals with respect to serum levels of matrix metalloproteinase (MMP)-9 and tissue inhibitor of metalloproteinase (TIMP)-1 and to search for the effect of antihypertensive treatment on the serum enzyme levels.

METHODS

Thirty-three patients with stage 1 primary hypertension and sixteen age- and sexmatched control subjects were enrolled into the study. Serum MMP-9 and TIMP-1 levels were assessed in the hypertensive group before and after a 3-month-antihypertensive treatment (candesartan 8 mg/day to 17 patients and lisinopril 10 mg/day to 16 patients).

RESULTS

Pre-treatment serum MMP-9 levels were higher in the hypertensive group (p=0.309) while serum TIMP-1 levels were lower (p=0.296). Serum MMP-9 levels were decreased (p<0.001) and TIMP-1 levels were increased (p=0.022) after the antihypertensive treatment.

CONCLUSIONS

In hypertensive patients, increased MMP-9 activity could result in increased degradation of elastin relative to collagen and non-elasticity, while decreased TIMP-1 activity could lead to accumulation of poorly cross-linked, immature and unstable fibril degradation products, which result in misdirected deposition of collagen. Our study is important for revealing the role of the MMP enzyme system in the pathogenesis of hypertensive target organ disease.

Loss of TIMP3 enhances interstitial nephritis and fibrosis

The balance of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs) determines the integrity of the extracellular matrix. TIMP3 is the most highly expressed tissue inhibitor of metalloproteinase (TIMP) in the kidney, but its function in renal disease is incompletely understood. In this study, TIMP3-/- mice demonstrated an age-dependent chronic tubulointerstitial fibrosis. After unilateral ureteral obstruction (UUO), young TIMP3-/- mice exhibited increased renal injury (tubular atrophy, cortical and medullary thinning, and vascular damage) compared with wild-type mice. In addition, TIMP3-/- mice had greater interstitial fibrosis; increased synthesis and deposition of type I collagen; increased activation of fibroblasts; enhanced apoptosis; and greater activation of MMP2, but not MMP9, after UUO. TIMP3 deficiency also led to accelerated processing of TNFalpha, demonstrated by significantly higher TACE activity and greater soluble TNFalpha levels by 3 d after UUO. The additional deletion of TNFalpha markedly reduced inflammation, apoptosis, and induction of a number of MMPs. Moreover, inhibition of MMPs in TIMP3-/-/TNFalpha-/- mice further abrogated postobstructive injury and prevented tubulointerestitial fibrosis. In humans, TIMP3 expression increased in the renal arteries and proximal tubules of subjects with diabetic nephropathy or chronic allograft nephropathy. Taken together, these results provide evidence that TIMP3 is an important mediator of kidney injury, and regulating its activity may have therapeutic benefit for patients with kidney disease.

Inducible rodent models of acquired podocyte diseases

Glomerular diseases remain the leading cause of chronic and end-stage kidney disease. Significant advances in our understanding of human glomerular diseases have been enabled by the development and better characterization of animal models. Diseases of the glomerular epithelial cells (podocytes) account for the majority of proteinuric diseases. Rodents have been extensively used experimentally to better define mechanisms of disease induction and progression, as well as to identify potential targets and therapies. The development of podocyte-specific genetically modified mice has energized the research field to better understand which animal models are appropriate to study acquired podocyte diseases. In this review we discuss inducible experimental models of acquired nondiabetic podocyte diseases in rodents, namely, passive Heymann nephritis, puromycin aminonucleoside nephrosis, adriamycin nephrosis, liopolysaccharide, crescentic glomerulonephritis, and protein overload nephropathy models. Details are given on the model backgrounds, how to induce each model, the interpretations of the data, and the benefits and shortcomings of each. Genetic rodent models of podocyte injury are excluded.