Autophagy can repair endoplasmic reticulum stress damage of the passive Heymann nephritis model as revealed by proteomics analysis

Crocin improves the renal autophagy in rat experimental membranous nephropathy via regulating the SIRT1/Nrf2/HO-1 signaling pathway

Membranous nephropathy (MN) is a glomerular disease. Crocin is isolated from saffron and gardenia. Its antioxidant, anti-inflammatory, anti-hyperlipidemic, anti-atherosclerotic, anti-tumor, free-radical scavenging and neuroprotective activities have been well established. We investigated the biological functions of crocin and its related mechanisms in MN. We established an experimental passive Heymann nephritis (PHN) rat model induced by anti-Fx1A antiserum. The rats were divided into sham, sham + crocin, PHN, PHN + crocin, and PHN + enalapril groups. Blood samples and kidneys of rats were collected for estimation of biochemical parameters in serum and oxidative stress indicators in kidney tissues. Histopathological changes of renal tissues were evaluated by hematoxylin and eosin, periodic acid-Schiff (PAS) and Masson staining. The podocyte number was estimated by immunohistochemistry staining of Wilms tumor type 1 (WT1). The deposition of rat anti-rabbit IgG antibodies, complement C3 and C5b-9 was detected by immunofluorescence staining. Western blotting was performed to measure the levels of Sirtuin 1 (Sirt1), nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase 1 (HO-1) and apoptosis-related proteins. The total cholesterol, triglycerides, creatinine, blood urea nitrogen, urine volume and urine albumin of PMN rats were significantly reduced by crocin. Additionally, crocin attenuated the renal histopathological changes. Moreover, the oxidative stress damage and podocyte loss and immune injury were relieved by crocin in PHN rats. Mechanistically, crocin administration activated the Sirt1/Nrf2/HO-1 pathways. The results provide a scientific basis that crocin could alleviate MN by inhibiting immune injury and podocyte damage through activating the Sirt1/Nrf2/HO-1 pathways.

Reduction of anaerobic glycolysis contributes to angiotensin II-induced podocyte injury with foot process effacement

Activation of the renin-angiotensin system is associated with podocyte injury and has been well demonstrated as a pivotal factor in the progression of chronic kidney disease. Podocyte energy metabolism is crucial for maintaining their physiological functions. However, whether renin-angiotensin system activation promotes chronic kidney disease progression by disturbing the energy metabolism of podocytes has not been elucidated. Angiotensin II, the main active molecule of the renin-angiotensin system, plays a crucial role in chronic kidney disease initiation and progression, but its impact on podocyte metabolism remains unclear. Here, we demonstrate a rapid decrease in the expression of pyruvate kinase M2, a key glycolytic enzyme, and reduced glycolytic flux in podocytes exposed to angiotensin II in vivo and in vitro. Podocyte-specific deletion of pyruvate kinase M2 in mice aggravated angiotensin II-induced glomerular and podocyte injury with foot process effacement and proteinuria. The inhibition of glycolysis was accompanied by adenosine triphosphate deficiency, cytoskeletal remodeling and podocyte apoptosis. Mechanistically, we found that angiotensin II-induced glycolysis impairment contributed to an insufficient energy supply to the foot process, leading to podocyte injury. Additionally, pyruvate kinase M2 expression was found to be reduced in podocytes from kidney biopsies of patients with hypertensive nephropathy and diabetic kidney disease. Thus, our findings suggest that glycolysis activation is a potential therapeutic strategy for podocyte injury.

Revisiting immunological and clinical aspects of membranous nephropathy

Idiopathic or primary membranous nephropathy (IMN) is one of the most frequent causes of nephrotic syndrome in adults and the elderly. It is characterized by a thickening of the wall of the glomerular capillaries due to the presence of immune complex deposits. 85% of membranous nephropathy cases are classified as primary or idiopathic (IMN). The rest are of secondary origin (SMN), caused by autoimmune conditions or malignant tumors as lung cancer, colon and melanomas. It is an organ-specific autoimmune disease in which the complement system plays an important role with the formation of the membrane attack complex (MAC; C5b-9), which produces an alteration of the podocyte structure. The antigen responsible for 70-80% of IMN is a podocyte protein called M-type phospholipase A2 receptor (PLA2R). More recently, another podocyte antigen has been identified, the "Thrombospondin type-1 domain-containing 7A" (THSD7A), which is responsible for 10% of the cases of negative IMN for anti- PLA2R.

Inhibition of the Wnt/β-catenin signaling pathway reduces autophagy levels in complement treated podocytes

In idiopathic membranous nephropathy, the complement membrane attack complex, more commonly referred to as complement 5b-9 (C5b-9), induces glomerular epithelial cell injury and proteinuria. C5b-9 can also activate numerous mechanisms that restrict or facilitate injury. Recent studies suggest that autophagy and the canonical Wnt signaling pathway serve an important role in repairing podocyte injury. However, the effect of C5b-9 on these pathways and the relationship between them remains unclear. The aim of the present study was to show the effect of C5b-9 on the Wnt/β-catenin signaling pathway and autophagy in podocytes in vitro. Levels of relevant indicators were detected by immunofluorescence staining and capillary western immunoassay. C5b-9 serum significantly activated the Wnt/β-catenin signaling pathway and promoted autophagy. Treatment with Dickkopf-related protein 1 (DKK1), a Wnt/β-catenin pathway blocker, protected podocytes from injury and significantly inhibited autophagy. The results indicated that inhibition of the Wnt/β-catenin pathway physiologically activated autophagy. The results indicated that C5b-9 resulted in a decrease in Akt in podocytes. However, the podocytes preincubated with DKK1 and then attacked by C5b-9 showed an increase in Akt levels. This may explain the observation that blocking the Wnt/β-catenin signaling pathway attenuated C5b-9 podocyte damage, while inhibiting autophagy. The results of the present study also suggest that regulation of these two pathways may serve as a novel method for the treatment of idiopathic membranous nephropathy.

Podocyte Autophagy in Homeostasis and Disease

Autophagy is a protective mechanism that removes dysfunctional components and provides nutrition for cells. Podocytes are terminally differentiated specialized epithelial cells that wrap around the capillaries of the glomerular filtration barrier and show high autophagy level at the baseline. Here, we provide an overview of cellular autophagy and its regulation in homeostasis with specific reference to podocytes. We discuss recent data that have focused on the functional role and regulation of autophagy during podocyte injury in experimental and clinical glomerular diseases. A thorough understanding of podocyte autophagy could shed novel insights into podocyte survival mechanisms with injury and offer potential targets for novel therapeutics for glomerular disease.

The role of extracellular vesicles in podocyte autophagy in kidney disease

Podocytes are the key cells involved in protein filtration in the glomerulus. Once proteins appear in the urine when podocytes fail, patients will end with renal failure due to the progression of glomerular damage if no proper treatment is applied. The injury and loss of podocytes can be attributed to diverse factors, such as genetic, immunologic, toxic, or metabolic disorders. Recently, autophagy has emerged as a key mechanism to eliminate the unwanted cytoplasmic materials and to prolong the lifespan of podocytes by alleviating cell damage and stress. Typically, the fundamental function of extracellular vesicles (EVs) is to mediate the intercellular communication. Recent studies have suggested that, EVs, especially exosomes, play a certain role in information transfer by communicating proteins, mRNAs, and microRNAs with recipient cells. Under physiological and pathological conditions, EVs assist in the bioinformation interchange between kidneys and other organs. It is suggested that EVs are related to the pathogenesis of acute kidney injury and chronic kidney disease, including glomerular disease, diabetic nephropathy, renal fibrosis and end-stage renal disease. However, the role of EVs in podocyte autophagy remains unclear so far. Here, this study integrated the existing information about the relevancy, diagnostic value and therapeutic potential of EVs in a variety of podocytes-related diseases. The accumulating evidence highlighted that autophagy played a critical role in the homeostasis of podocytes in glomerular disease.

Curcumin Improves the Renal Autophagy in Rat Experimental Membranous Nephropathy via Regulating the PI3K/AKT/mTOR and Nrf2/HO-1 Signaling Pathways

Membranous nephropathy (MN, also known as membranous glomerulopathy) is one of the many glomerular diseases causing nephrotic syndrome. The literature indicates that autophagy is associated with the homeostasis of podocytes in glomeruli. Curcumin, the main active component in turmeric, has drawn attention for its effective bioactivities against chronic kidney disease. The current study was aimed at assessing the effects of curcumin and exploring the underlying mechanism that mediates autophagy in an animal model of passive Heymann nephritis (PHN) in rats. Passive Heymann nephritis (PHN) was induced in male SD rats by intraperitoneal injection of anti-Fx1A serum. The rats were divided into 3 groups: control (n = 10, normal diet), model group (n = 10, 0.5% sodium carboxymethylcellulose), and curcumin (n = 10, 300 mg/kg/d). The kidney function and oxidative stress indicators were measured using commercial diagnostic kits, and the histomorphology of renal tissues was observed. The number of podocytes was measured by immunohistochemistry. Meanwhile, the autophagosomes in podocyte were analyzed by transmission electron microscopy and the immunofluorescence assay pointing to p62, an autophagic marker. Western blot analyzed the levels of apoptosis, autophagy, PI3K/AKT/mTOR, and Nrf2/HO-1 pathway-associated proteins. The total cholesterol (TC), triglycerides (TG), creatinine (Scr), blood urea nitrogen (BUN), urine volume, and urine albumin of PHN rats were significantly reduced by the administration of curcumin and attenuated renal histomorphological changes in model rats. Meanwhile, curcumin improved the oxidative stress response by decreasing MDA and increasing SOD, GSH, and CAT levels in the kidney of PHN rats. Furthermore, curcumin significantly ameliorated the podocyte loss, along with the fusion, and increased the autophagic vacuoles compared to the PHN control rats. In addition, curcumin downregulated the expression of Bax, Caspase-3, p62, PI3K, p-AKT, and p-mTOR proteins and upregulated the Bcl-2, beclin1, LC3, Nrf2, and HO-1 levels in this animal model. The results provide a scientific basis that curcumin could significantly alleviate the development of MN by inducing autophagy and alleviating renal oxidative stress through the PI3K/AKT/mTOR and Nrf2/HO-1 pathways.

HDAC6-mediated α-tubulin deacetylation suppresses autophagy and enhances motility of podocytes in diabetic nephropathy

Histone deacetylase 6 (HDAC6) is the specific subtype of HDACs which preferentially located in the cytoplasm, and is crucial in insulin signalling. However, the role of HDAC6 in type 2 diabetic nephropathy (DN) remains undefined. In current study, we observed that HDAC6 was markedly activated in the kidneys of type 2 diabetic patients and db/db mice with albuminuria, along with the advanced glycation end products (AGE)‐treated podocytes. Selective inhibition of HDAC6 activity protected kidneys from hyperglycaemia in db/db mice. Notably, overexpressing HDAC6 inhibited autophagy and promoted motility aside from the apoptosis of podocytes exposed to AGE. We further determined that HDAC6 regulated the autophagy partially by decreasing the acetylation of α‐tubulin at the residue of lysine 40. In contrast, we confirmed that there was no interaction of HDAC6 with α‐tubulin at the sites of lysine 112 and lysine 352. Consistently, inhibiting HDAC6 by siRNA or the selective inhibitor, tubacin, restored the autophagy level and motility of podocytes and rescued podocytes from AGE stimulation. We provide strong evidence of an unexpected role of HDAC6 in the cascade that modulates podocytes autophagy and motility, enlightening that HDAC6 may be a promising therapeutic target for DN treatment.

miR-181b-5p suppresses starvation-induced cardiomyocyte autophagy by targeting Hspa5

This study aimed to investigate the role of microRNA-181b-5p (miR-181b-5p) in starvation-induced cardiomyocyte autophagy by targeting heat shock protein family A member 5 (Hspa5). For this purpose, H9c2 cardiomyocytes and neonatal rat ventricular myocytes (NRVMs) were glucose-starved in Earle's Balanced Salt Solution (EBSS) for different periods of time (0, 2, 4, 6 and 8 h). RT-qPCR analysis was performed to examine the expression of miR-181b-5p in the different groups. Immunofluorescence was performed to detect the expression of LC3. In addition, the H9c2 cardiomyo-cytes and NRVMs were transfected with miR-181b-5p mimic, miR-181b-5p inhibitor, siHspa5 or their respective controls. An MTT assay was performed to measure cell proliferation in the different groups. Western blot analysis was performed to determine the expression of Beclin-1, Hspa5, phosphorylated phosphoinositide 3-kinase PI3K (p-PI3K), phosphorylated Akt (p-Akt), phosphorylated mammalian target of rapamycin (p-mTOR), Bcl-2, Bax and cleaved caspase-3. Flow cytometry was performed to assess cell apoptosis. A luciferase reporter assay was performed to determine whether Hspa5 is a direct target of miR-181b-5p. The results revealed that the down-regulation of miR-181b-5p promoted cell autophagy in the cardiomyocytes. Moreover, miR-181b-5p negatively regulated Beclin-1 and Hspa5. Beclin-1 is a well-known autophagy- and apoptosis-related protein. In addition, cell apoptosis was attenuated by the decreased expression of miR-181b-5p in the cardiomyocytes. Bcl-2 prevented apoptosis and autophagy by binding to Bax and Bcl-2, respectively. The upregulation of miR-181b-5p inhibited autophagy and promoted apoptosis via Hspa5. miR-181b-5p inhibition promoted p-mTOR, p-Akt and p-PI3K expression via Hspa5. The results of luciferase reporter assay also confirmed that Hspa5 is a direct target of miR-181b-5p. On the whole, the findings of this study suggest that miR-181b-5p contributes to starvation-induced autophagy and apoptosis in cardiomyocytes by directly targeting Hspa5 via the PI3K/Akt/mTOR signaling pathway.

Resveratrol induces autophagy-dependent apoptosis in HL-60 cells

BACKGROUND

All known mechanisms of apoptosis induced by resveratrol act through cell cycle arrest and changes in mitochondrial membrane potential. It is currently unknown whether resveratrol-induced apoptosis is associated with other physiological processes, such as autophagy.

METHODS

Apoptosis-related markers involved in the intrinsic and extrinsic apoptotic pathways, and autophagic markers were detected by using western blotting and immunofluorescence. Mitochondrial membrane potential was assayed by flow cytometry. Pharmaceutical or genetic inhibition of autophagy involved were carried by 3- methyladenine or knockdown of autophagy-related (Atg) genes by siRNA. Differences between two values were tested by Student's unpaired t test.

RESULTS

We show that resveratrol-induced apoptosis occurs through both the intrinsic and extrinsic apoptotic pathways. Mitochondrial membrane potential and apoptosis-related markers, such as an increased Bax/Bcl-2 ratio, and cleaved forms of caspase-8 and caspase-3, arise following resveratrol addition. Moreover, we find that resveratrol increases both the levels of microtubule-associated protein 1 light chain 3-II and the number of autophagosomes, and further demonstrate that resveratrol-induced autophagy depends on the LKB1-AMPK-mTOR pathway. We next reveal that some apoptosis-related markers induced by resveratrol are further attenuated by the inhibition of autophagy with 3-methyladenine or knockdown of autophagy-related (Atg) genes by siRNA.

CONCLUSIONS

These results suggest that resveratrol induced apoptotic cell death of HL-60 cells depends on the autophagy activated through both the LKB1-AMPK and PI3K/AKT-regulated mTOR signaling pathways.

Blockage of the lysosome-dependent autophagic pathway contributes to complement membrane attack complex-induced podocyte injury in idiopathic membranous nephropathy

Dysregulation of autophagy-mediated podocyte homeostasis is proposed to play a role in idiopathic membranous nephropathy (IMN). In the present study, autophagic activity and lysosomal alterations were investigated in podocytes of IMN patients and in cultured podocytes exposed to sublytic terminal complement complex, C5b-9. C5b-9 upregulated the number of LC3 positive puncta and the expression of p62 in patient podocytes and in C5b-9 injuried podocyte model. The lysosomal turnover of LC3-II was not influenced, although the BECN1 expression level was upregulated after exposure of podocytes to C5b-9. C5b-9 also caused a significant increase in the number of autophagosomes but not autolysosomes, suggesting that C5b-9 impairs the lysosomal degration of autophagosomes. Moreover, C5b-9 exacerbated the apoptosis of podocytes, which could be mimicked by chloroquine treatment, indicating that C5b-9 triggered podocyte injury, at least partially through inhibiting autophagy. Subsequent studies revealed that C5b-9 triggered lysosomal membrane permeabilization, which likely caused the decrease in enzymatic activity, defective acidification of lysosomes, and suppression of DQ-ovalbumin degradation. Taken together, our results suggest that the lysosomal-dependent autophagic pathway is blocked by C5b-9, which may play a key role in podocyte injury during the development of IMN.

MicroRNA-199a acts as a potential suppressor of cardiomyocyte autophagy through targeting Hspa5

Autophagy is an adaptive response to cardiomyocytes survival under stress conditions. MicroRNAs (miRNAs, miR) have been described to act as potent modulators of autophagy. To investigate whether and how miR-199a modulated autophagy in vitro, primary cardiomyocytes were treated under starvation to induce autophagy. Results showed that down-regulation of miR-199a was sufficient to activate cardiomyocytes autophagy. MiR-199a suppressed cardiomyocytes autophagy through direct inhibiting heat shock protein family A member 5 (Hspa5). Forced overexpression of Hspa5 recovered the inhibitory effect of miR-199a in autophagy activation. Our results suggested miR-199a as an effective suppressor of starvation-induced cardiomyocytes autophagy and that Hspa5 was a direct target during this process. These results extend the understanding of the role and pathway of miR-199a in cardiomyocytes autophagy, and may introduce a potential therapeutic strategy for the protection of cardiomyocytes in myocardial infarction or ischemic heart disease.

A podocyte view of membranous nephropathy: from Heymann nephritis to the childhood human disease

Membranous nephropathy (MN) is characterized by an accumulation of immune deposits on the subepithelial side of the glomerular basement membrane, which results in complement activation and proteinuria. Since 2002, several major antigens of the podocyte have been identified in human MN, the first one being neutral endopeptidase (NEP), the alloantigen involved in neonatal cases of MN that occur in newborns from NEP-deficient mothers. This discovery opened the field to the major advances that have occurred since then in the pathophysiology and treatment of MN. It is remarkable that experimental models such as Heymann nephritis and cationic bovine serum albumin-induced MN in the rabbit predicted the pathomechanisms of the human glomerulopathy. The podocyte is at the center of the pathogenesis of MN either by providing a source of endogenous antigens or by creating an environment favorable to deposition and accumulation of immune complexes containing exogenous (non-podocyte) antigens. The podocyte is also a victim of complement activation and antibody blocking activity against enzymes or receptors. A search for innovative drugs aimed at protecting this cell against complement activation and the effects of prolonged ER stress has become a priority.

Association of podocyte autophagosome numbers with idiopathic membranous nephropathy and secondary membranous nephropathy

PURPOSE

This study was to investigate the relation between the number of autophagosomes in podocytes and the syndromes of idiopathic membranous nephropathy (IMN) and secondary membranous nephropathy (sMN).

METHODS

The pathological changes in the kidney tissues of patients were detected with the hematoxylin and eosin staining, the periodic acid-Schiff reagent treatment, the Masson's trichrome staining and the immunofluorescence detection (IF). Meanwhile, the autophagosomes in podocyte were analyzed by transmission electron microscopy and the IF assay pointing to LC3-II, an autophagic marker. Clinical data, including age, sex, edema, serum creatinine, estimated glomerular filtration rate, hematuria, urine protein excretion and serum albumin, were collected from in-patient medical records. Finally, the association of podocyte autophagosome numbers with idiopathic membranous nephropathy and secondary membranous nephropathy was studied.

RESULTS

Fewer autophagosomes were observed in podocytes of nephropathy group compared with the control group. Moreover, there was a significant difference in the autophagosome number between the two types of MN and each kind of nephropathy demonstrated distinct characteristics. Although the reduced autophagosome number in the IMN cases was not related to sex, this trend was exacerbated along with the progression from pathological stage I to II. In contrast, fewer autophagosomes were observed in class II and V LN patients compared with the controls, while greater numbers were detected in class III and IV LN patients.

CONCLUSIONS

The results indicated that the autophagy participated in the podocyte injury in IMN and sMN and the number of autophagosomes in podocytes was related to the pathological classification.

How many podocyte autophagosomes are there in immunoglobulin A nephropathy and idiopathic membranous nephropathy?

PURPOSE

The aim of this study was to investigate the number of autophagosomes in podocyte from kidney tissue of immunoglobulin A nephropathy (IgAN) and idiopathic membranous nephropathy (IMN).

METHODS

The changes in kidney tissue pathology were detected after hematoxylin and eosin, periodic acid-Schiff, Masson's trichrome and immunofluorescence. The autophagosomes of podocyte were analyzed by transmission electron microscopy. Clinical data, including age, gender, edema, serum creatinine, estimated glomerular filtrating rate (eGFR), hematuria, urine protein excretion and serum albumin, were collected from inpatient medical record.

RESULTS

It was found that the number of autophagosomes in podocyte of nephropathy group was lower when compared to the control group. At the same time, we did not find the difference of the parameter between these two kinds of nephropathy. Further study showed that the index was affected by two factors: eGFR and gender. The cases with worse eGFR (eGFR < 60 ml/min) and male patients presented more autophagosomes. Furthermore, each nephropathy had its own character. The phenomena of reduced autophagosomes were found in IMN cases, did not change from male to female, and further aggravated from pathological stage I to II. By contrast, IgAN cases with less eGFR exhibited more autophagosomes.

CONCLUSIONS

Therefore, the results of the present study indicate that autophagy participates in podocyte injury and the progression of IgAN and IMN.

Bioinformatics analysis of proteomics profiles in senescent human primary proximal tubule epithelial cells

Background

Dysfunction of renal tubule epithelial cells is associated with renal tubulointerstitial fibrosis. Exploration of the proteomic profiles of senesced tubule epithelial cells is essential to elucidate the mechanism of tubulointerstitium development.

Methods

Primary human proximal tubule epithelial cells from passage 3 (P3) and passage 6 (P6) were selected for evaluation. EdU and SA-β-galactosidase staining were used to detect cell senescence. p53, p21, and p16 were detected by Western blot analysis. Liquid chromatography mass spectrometry (LC-MS) was used to examine differentially expressed proteins (DEPs) between P6 and P3 cells. The expression of DEPs was examined by Western blot analysis. Bioinformatics analysis was performed by protein-protein interaction and gene ontology analyses.

Results

The majority of tubule cells from passage 6 (P6) stained positive for SA-β-galactosidase, whereas passage 3 (P3) cells were negative. Senescence biomarkers, including p53, p21, and p16, were upregulated in P6 cells relative to P3 cells. EdU staining results showed a lower rate of EdU positive cells in P6 cells than in P3 cells. LC-MS was used to examine DEPs between P6 and P3 cells. These DEPs are involved in glycolysis, response to stress, cytoskeleton regulation, oxidative reduction, ATP binding, and oxidative stress. Using Western blot analysis, we validated the down-regulation of AKR1B1, EEF2, EEF1A1, and HSP90 and the up-regulation of VIM in P6 cells seen in the LC-MS data. More importantly, we built the molecular network based on biological functions and protein-protein interactions and found that the DEPs are involved in translation elongation, stress, and glycolysis, and that they are all associated with cytoskeleton regulation, which regulates senescent cell activities such as apoptosis and EMT in tubule epithelial cells.

Conclusions

We explored proteomic profile changes in cell culture-induced senescent cells and built senescence-associated molecular networks, which will help to elucidate the mechanisms of senescence in human proximal tubule epithelial cells.

Autophagy protects podocytes from sublytic complement induced injury

Podocyte injury induced by sublytic complement attack is the main feature of membranous nephropathy (MN). This study aimed at investigating the impact of sublytic complement attack-related autophagy on podocyte injury in vitro. Here, we show that sublytic complement attack enhances MPC5 podocyte autophagy in vitro. Inhibition of autophagy by treatment with 3-methyladenine (3-MA) significantly increased sublytic complement attack-induced changes in the injury-related morphology, stress fiber, and podocyte apoptosis, but decreased the survival and adhesion of MPC5 podocytes. In contrast, promotion of autophagy by treatment with rapamycin mitigated sublytic complement attack-induced changes in the injury-related morphology, stress fiber, and podocyte apoptosis, but increased the survival and adhesion of MPC5 podocytes. These data suggest that autophagy may protect podocytes from sublytic complement attack-induced injury in vitro.

Proteomic analysis of the kidney filtration barrier--Problems and perspectives

Diseases of the glomerular filter of the kidney are a leading cause of end-stage renal failure. The kidney filter is localized within the renal glomeruli, small microvascular units that are responsible for ultrafiltration of about 180 liters of primary urine every day. The renal filter consists of three layers, fenestrated endothelial cells, glomerular basement membrane, and the podocytes, terminally differentiated, arborized epithelial cells. This review demonstrates the use of proteomics to generate insights into the regulation of the renal filtration barrier at a molecular level. The advantages and disadvantages of different glomerular purification methods are examined, and the technical limitations that have been significantly improved by in silico or biochemical approaches are presented. We also comment on phosphoproteomic studies that have generated considerable molecular-level understanding of the physiological regulation of the kidney filter. Lastly, we conclude with an analysis of urinary exosomes as a potential filter-derived resource for the noninvasive discovery of glomerular disease mechanisms.

Apelin promotes diabetic nephropathy by inducing podocyte dysfunction via inhibiting proteasome activities

Podocyte injuries are associated with progression of diabetic nephropathy (DN). Apelin, an adipocyte-derived peptide, has been reported to be a promoting factor for DN. In this study, we aim to determine whether apelin promotes progression of DN by inducing podocyte dysfunction. kk-Ay mice were used as models for DN. Apelin and its antagonist, F13A were intraperitoneally administered for 4 weeks, respectively. Renal function and foot process proteins were analysed to evaluate the effects of apelin on kk-Ay mice and podocytes. Apelin increased albuminuria and decreased podocyte foot process proteins expression in kk-Ay mice, which is consistent with the results that apelin receptor (APLNR) levels increased in glomeruli of patients or mice with DN. In cultured podocytes, high glucose increased APLNR expression and apelin administration was associated with increased permeability and decreased foot process proteins levels. All these dysfunctions were associated with decreased 26S proteasome activities and increased polyubiquitinated proteins in both kk-Ay mice and cultured podocytes, as demonstrated by 26S proteasome activation with cyclic adenosine monophosphate (cAMP) or oleuropein. These effects seemed to be related to endoplasmic reticulum (ER) stress, as apelin increased C/EBP homologous protein (CHOP) and peiFα levels while cAMP or oleuropein reduced it in high glucose and apelin treated podocytes. These results suggest that apelin induces podocyte dysfunction in DN through ER stress which was induced by decreased proteasome activities in podocytes.

Endoplasmic Reticulum Stress in the Diabetic Kidney, the Good, the Bad and the Ugly

Diabetic kidney disease is the leading worldwide cause of end stage kidney disease and a growing public health challenge. The diabetic kidney is exposed to many environmental stressors and each cell type has developed intricate signaling systems designed to restore optimal cellular function. The unfolded protein response (UPR) is a homeostatic pathway that regulates endoplasmic reticulum (ER) membrane structure and secretory function. Studies suggest that the UPR is activated in the diabetic kidney to restore normal ER function and viability. However, when the cell is continuously stressed in an environment that lies outside of its normal physiological range, then the UPR is known as the ER stress response. The UPR reduces protein synthesis, augments the ER folding capacity and downregulates mRNA expression of genes by multiple pathways. Aberrant activation of ER stress can also induce inflammation and cellular apoptosis, and modify signaling of protective processes such as autophagy and mTORC activation. The following review will discuss our current understanding of ER stress in the diabetic kidney and explore novel means of modulating ER stress and its interacting signaling cascades with the overall goal of identifying therapeutic strategies that will improve outcomes in diabetic nephropathy.

The application of urinary proteomics for the detection of biomarkers of kidney diseases

Urine is a biological material that can be easily obtained in the clinic. The identification of proteins excreted in urine provides useful biological information about the kidney as well as a unique opportunity to examine physiological and pathological changes in the kidney in a noninvasive manner. Recent technological advances in urinary proteomic profiling have provided the foundation for a number of urinary proteomic studies directed at identifying markers of kidney disease diagnosis, prognosis, or responsiveness to therapy. In this review, we describe the strengths of different urinary proteomic methods for the discovery of potential biomarkers of kidney diseases. We also highlight the limitations and future goals of these approaches.

Autophagy modulates endoplasmic reticulum stress-induced cell death in podocytes: a protective role

Endoplasmic reticulum stress occurs in a variety of patho-physiological mechanisms and there has been great interest in managing this pathway for the treatment of clinical diseases. Autophagy is closely interconnected with endoplasmic reticulum stress to counteract the possible injurious effects related with the impairment of protein folding. Studies have shown that glomerular podocytes exhibit high rate of autophagy to maintain as terminally differentiated cells. In this study, podocytes were exposed to tunicamycin and thapsigargin to induce endoplasmic reticulum stress. Thapsigargin/tunicamycin treatment induced a significant increase in endoplasmic reticulum stress and of cell death, represented by higher GADD153 and GRP78 expression and propidium iodide flow cytometry, respectively. However, thapsigargin/tunicamycin stimulation also enhanced autophagy development, demonstrated by monodansylcadaverine assay and LC3 conversion. To evaluate the regulatory effects of autophagy on endoplasmic reticulum stress-induced cell death, rapamycin (Rap) or 3-methyladenine (3-MA) was added to enhance or inhibit autophagosome formation. Endoplasmic reticulum stress-induced cell death was decreased at 6 h, but was not reduced at 24 h after Rap+TG or Rap+TM treatment. In contrast, endoplasmic reticulum stress-induced cell death increased at 6 and 24 h after 3-MA+TG or 3-MA+TM treatment. Our study demonstrated that thapsigargin/tunicamycin treatment induced endoplasmic reticulum stress which resulted in podocytes death. Autophagy, which counteracted the induced endoplasmic reticulum stress, was simultaneously enhanced. The salvational role of autophagy was supported by adding Rap/3-MA to mechanistically regulate the expression of autophagy and autophagosome formation. In summary, autophagy helps the podocytes from cell death and may contribute to sustain the longevity as a highly differentiated cell lineage.

Complement-mediated cellular injury

Complement activation and recruitment of inflammatory leukocytes is an important defense mechanism against bacterial infection. However, complement also can mediate cellular injury and contribute to the pathogenesis of various diseases. With the appreciation that the C5b-9 membrane attack complex can injure cells in the absence of leukocytes, a role for the terminal complement pathway in inducing cell injury and kidney disease was shown in several experimental models, including the rat passive Heymann nephritis model of human membranous nephropathy. In podocytes, sublytic C5b-9 activates a variety of downstream pathways including protein kinases, lipid metabolism, reactive oxygen species, growth factors/gene transcription, endoplasmic reticulum stress, and the ubiquitin-proteasome system, and it impacts the integrity of the cytoskeleton and slit diaphragm proteins. C5b-9 also injures other kidney cells, including mesangial, glomerular endothelial, and tubular epithelial cells, and it contributes to the pathogenesis of mesangial-proliferative glomerulonephritis, thrombotic microangiopathy, and acute kidney injury. Conversely, certain C5b-9 signals limit complement-induced injury, or promote recovery of cells. In addition to C5b-9, complement cleavage products, such as C5a and C1q, can injure kidney cells. Thus, the complement system contributes to various kidney pathologies by causing cellular damage in both an inflammation-dependent and inflammation-independent manner.

Immunopathogenesis of membranous nephropathy: an update

Membranous nephropathy (MN) is a non-inflammatory organ-specific autoimmune disease which affects the kidney glomerulus, resulting in the formation of immune deposits on the outer aspect of the glomerular basement membrane, complement-mediated proteinuria, and severe renal failure in 30% of patients. In the last 10 years, substantial advances have been made in the understanding of the molecular bases of MN, with the identification of several antigens and predisposing genes in children and adults. These ground-breaking findings already have a major impact on diagnosis and monitoring and to some extent on therapies. However, there is evidence that the disease is more complex and involves a variety of antigen-antibody systems and genes involved in immune response, progression, recovery, and protective mechanisms. We herein review these recent findings which open new perspectives of research. Understanding the complex pathogenesis of MN will offer many opportunities for future therapeutic interventions and will hopefully have a major impact on patient care. New insights into the molecular mechanisms of MN may also enlighten the pathogenesis of organ-specific autoimmune diseases.

Autophagy in kidney health and disease

SIGNIFICANCE

Autophagy is emerging as an important pathway in many biological processes and diseases. This review summarizes the current progress on the role of autophagy in renal physiology and pathology.

RECENT ADVANCES

Studies from renal cells in culture, human kidney tissues, and experimental animal models implicate that autophagy regulates many critical aspects of normal and disease conditions in the kidney, such as diabetic nephropathy and other glomerular diseases, tubular injuries, kidney development and aging, cancer, and genetic diseases associated with the kidney.

CRITICAL ISSUES

The importance of autophagy in the kidney has just started to be elucidated. How the process of autophagy is altered in the pathogenesis of kidney diseases and how this alteration is beneficial or detrimental to kidney functions still need to be fully understood.

FUTURE DIRECTIONS

Investigations that uncover the precise mechanism and regulation of autophagy in various kidney diseases may lead to new strategies for therapeutic modulation.

The primary glomerulonephritides: a systems biology approach

Our understanding of the pathogenesis of most primary glomerular diseases, including IgA nephropathy, membranous nephropathy and focal segmental glomerulosclerosis, is limited. Advances in molecular technology now permit genome-wide, high-throughput characterization of genes and gene products from biological samples. Comprehensive examinations of the genome, transcriptome, proteome and metabolome (collectively known as omics analyses), have been applied to the study of IgA nephropathy, membranous nephropathy and focal segmental glomerulosclerosis in both animal models and human patients. However, most omics studies of primary glomerular diseases, with the exception of large genomic studies, have been limited by inadequate sample sizes and the lack of kidney-specific data sets derived from kidney biopsy samples. Collaborative efforts to develop a standardized approach for prospective recruitment of patients, scheduled monitoring of clinical outcomes, and protocols for sampling of kidney tissues will be instrumental in uncovering the mechanisms that drive these diseases. Integration of molecular data sets with the results of clinical and histopathological studies will ultimately enable these diseases to be characterized in a comprehensive and systematic manner, and is expected to improve the diagnosis and treatment of these diseases.