Understanding Podocyte Biology to Develop Novel Kidney Therapeutics

Role of the β2-adrenergic receptor in podocyte injury and recovery

BACKGROUND

Podocytes have a remarkable ability to recover from injury; however, little is known about the recovery mechanisms involved in this process. We recently showed that formoterol, a long-acting β2-adrenergic receptor (β2-AR) agonist, induced mitochondrial biogenesis (MB) in podocytes and led to renoprotection in mice. However, it is not clear whether this effect was mediated by formoterol acting through the β2-AR or if it occurred through "off-target" effects.

METHODS

We genetically deleted the β2-AR specifically in murine podocytes and used these mice to determine whether formoterol acting through the podocyte β2-AR alone is sufficient for recovery of renal filtration function following injury. The podocyte-specific β2-AR knockout mice (β2-ARfl/fl/PodCre) were generated by crossing β2-AR floxed mice with podocin Cre (B6.Cg-Tg(NPHS2-cre)295Lbh/J) mice. These mice were then subjected to both acute and chronic glomerular injury using nephrotoxic serum (NTS) and adriamycin (ADR), respectively. The extent of injury was evaluated by measuring albuminuria and histological and immunostaining analysis of the murine kidney sections.

RESULTS

A similar level of injury was observed in β2-AR knockout and control mice; however, the β2-ARfl/fl/PodCre mice failed to recover in response to formoterol. Functional evaluation of the β2-ARfl/fl/PodCre mice following injury plus formoterol showed similar albuminuria and glomerular injury to control mice that were not treated with formoterol.

CONCLUSIONS

These results indicate that the podocyte β2-AR is a critical component of the recovery mechanism and may serve as a novel therapeutic target for treating podocytopathies.

Urinary exosome proteins PAK6 and EGFR as noninvasive diagnostic biomarkers of diabetic nephropathy

OBJECTIVE

The actin cytoskeleton plays an essential role in maintaining podocyte functions. However, whether the urinary exosome proteins related to the regulation of the actin cytoskeleton are changed in diabetic nephropathy (DN) is still unknown. This study was to investigate the possibility that related proteins can be applied as diagnostic biomarkers for DN.

METHODS

Urinary exosomes were obtained from 144 participants (Discovery phase: n = 72; Validation phase: n = 72) by size exclusion chromatography methods. Proteomic analysis of urinary exosome by LC-MS/MS. Western blot and ELISA were applied to validate the selected urinary exosome proteins. The clinical value of selected urinary exosome proteins was evaluated using correlation and receiver operating characteristic curve analyses.

RESULTS

Fifteen urinary proteins related to the regulation of the actin cytoskeleton were identified in urinary exosomes. Three upregulated proteins were selected, including Serine/threonine-protein kinase PAK6 (PAK6), Epidermal growth factor receptor (EGFR), and SHC-transforming protein 1(SHC1). The expression level of PAK6 and EGFR was negatively correlated with estimated glomerular filtration rate and positively correlated with serum creatinine levels. For diagnosing DN in the discovery phase: the area under curve (AUC) of PAK6 was 0.903, EGFR was 0.842, and the combination of two proteins was 0.912. These better performances were also observed in the validation phase (For PAK6: AUC = 0.829; For EGFR: AUC = 0.797; For PAK6 + EGFR: AUC = 0.897).

CONCLUSIONS

Urinary exosome proteins PAK6 and EGFR may be promising and noninvasive biomarkers for diagnosing DN.

Obesity-Related Kidney Disease: Current Understanding and Future Perspectives

Obesity is a serious chronic disease and an independent risk factor for the new onset and progression of chronic kidney disease (CKD). CKD prevalence is expected to increase, at least partly due to the continuous rise in the prevalence of obesity. The concept of obesity-related kidney disease (OKD) has been introduced to describe the still incompletely understood interplay between obesity, CKD, and other cardiometabolic conditions, including risk factors for OKD and cardiovascular disease, such as diabetes and hypertension. Current therapeutics target obesity and CKD individually. Non-pharmacological interventions play a major part, but the efficacy and clinical applicability of lifestyle changes and metabolic surgery remain debatable, because the strategies do not benefit everyone, and it remains questionable whether lifestyle changes can be sustained in the long term. Pharmacological interventions, such as sodium-glucose co-transporter 2 inhibitors and the non-steroidal mineralocorticoid receptor antagonist finerenone, provide kidney protection but have limited or no impact on body weight. Medicines based on glucagon-like peptide-1 (GLP-1) induce clinically relevant weight loss and may also offer kidney benefits. An urgent medical need remains for investigations to better understand the intertwined pathophysiologies in OKD, paving the way for the best possible therapeutic strategies in this increasingly prevalent disease complex.

Cyr61 Mediates Angiotensin II-Induced Podocyte Apoptosis via the Upregulation of TXNIP

Purpose. It is well documented that angiotensin II (Ang II) elevation promotes apoptosis of podocytes in vivo and vitro, but the potential mechanism is still oscular. The current study is aimed at probing into the assignment of cysteine-rich protein 61 (Cyr61) in Ang II-induced podocyte apoptosis. Methods. Podocytes were treated with Ang II (10-6 mol/L) for 48 hours to establish an injury model in vitro. Western blot assays were detected the expression of Cyr61, Cyt-c, Bax, and Bcl-2. Gene microarray was used to analyze the expression of mRNAs after treatment with Ang II. CRISPR/Cas9 technology was used to knock down Cyr61 and overexpress TXNIP gene, respectively. Results. The expression of Cyr61, TXNIP, Cyt-c, and Bax in podocytes treated with Ang II were upregulated, but the expression and apoptotic rates of Bcl-2 in podocytes were inhibited. The level of the above factors was not significantly different after the knockdown of Cyr61 with Ang II in podocytes. In Ang II group, when knocked down Cyr61, the expressed level of TXNIP, Cyt-c, and Bax was diminished after Ang II treatment; interestingly Bcl-2 expression and podocyte apoptotic rate were reduced. Under the stimulation of Ang II, the expression of Cyt-c and Bax were growing, whereas Bcl-2 was reduced, and the apoptotic rates were higher in the TXNIP overexpression group. Cyt-c and Bax were put on, whereas that of Bcl-2 was to be cut down when the Cyr61 was knockdown, and the apoptotic rates were gained in the TXNIP overexpression+Cyr61 knockdown group. Conclusions. The results of the study extrapolate that Cyr61 plays a dominant role in Ang II-induced podocyte apoptosis. Additionally, Cyr61 may mediate the Ang II-induced podocyte apoptosis by promoting the expression of TNXIP.

Research progress on exosomes in podocyte injury associated with diabetic kidney disease

Diabetic kidney disease (DKD), a common cause of end-stage renal disease, is a serious complication that develops with the progression of chronic diabetes. Its main clinical manifestations are persistent proteinuria and/or a progressive decline in the estimated glomerular filtration rate. Podocytes, terminally differentiated glomerular visceral epithelial cells, constitute the glomerular filtration barrier together with the basement membrane and endothelial cells, and the structural and functional barrier integrity is closely related to proteinuria. In recent years, an increasing number of studies have confirmed that podocyte injury is the central target of the occurrence and development of DKD, and research on exosomes in podocyte injury associated with DKD has also made great progress. The aim of this review is to comprehensively describe the potential diagnostic value of exosomes in podocyte injury associated with DKD, analyze the mechanism by which exosomes realize the communication between podocytes and other types of cells and discuss the possibility of exosomes as targeted therapy drug carriers to provide new targets for and insights into delaying the progression of and treating DKD.

Qing-Re-Xiao-Zheng-Yi-Qi formula relieves kidney damage and activates mitophagy in diabetic kidney disease

Introduction: Qing-Re-Xiao-Zheng-Yi-Qi Formula is an effective prescription in diabetic kidney disease treatment, we have confirmed the efficacy of Qing-Re-Xiao-Zheng therapy in diabetic kidney disease through clinical trials. In this study, we investigated the mechanisms of Qing-Re-Xiao-Zheng-Yi-Qi Formula in the treatment of diabetic kidney disease. Methods: We used Vanquish UHPLC^TM to analyze the chemical profiling of Qing-Re-Xiao-Zheng-Yi-Qi Formula freeze-dried powder. We constructed diabetic kidney disease rat models induced by unilateral nephrectomy and high-dose streptozocin injection. We examined blood urea nitrogen, serum creatinine, serum glucose, total cholesterol, triglyceride, serum total protein, albumin, alanine aminotransferase, aspartate aminotransferase and 24 h urinary total protein in diabetic kidney disease rats. The renal pathological changes were observed by HE, Masson, PAS stanning and transmission electron microscopy. The levels of fibrosis-related proteins and mitophagy-related proteins were detected by western blot analysis. We also conducted an immunofluorescence co-localization analysis on podocytes to further investigate the effect of Qing-Re-Xiao-Zheng-Yi-Qi Formula treatment on mitophagy. Results: A total of 27 constituents in Qing-Re-Xiao-Zheng-Yi-Qi Formula were tentatively identified. We found PINK1/Parkin-mediated mitophagy was inhibited in diabetic kidney disease. Qing-Re-Xiao-Zheng-Yi-Qi Formula treatment could raise body weight and reduce renal index, reduce proteinuria, improve glycolipid metabolic disorders, ameliorate renal fibrosis, and reduce the expression of Col Ⅳ and TGF-β1 in diabetic kidney disease rats. Qing-Re-Xiao-Zheng-Yi-Qi Formula treatment could also increase the expression of nephrin, activate mitophagy and protect podocytes in diabetic kidney disease rats and high glucose cultured podocytes. Conclusion: PINK1/Parkin-mediated mitophagy was inhibited in diabetic kidney disease, and Qing-Re-Xiao-Zheng-Yi-Qi Formula treatment could not only ameliorate pathological damage, but also promote mitophagy to protect podocytes in diabetic kidney disease.

Solasonine alleviates high glucose-induced podocyte injury through increasing Nrf2-medicated inhibition of NLRP3 activation

The worldwide high prevalence of diabetic nephropathy is one of the common causes of renal failure in diabetic patients. Hyperglycemia-caused podocyte injury is considered as a major contributor to diabetic kidney disease, accompanied by a chronic inflammatory condition. Pyroptosis, a characterized inflammatory form of programmed cell death, is believed to be involved in the pathogenesis of diabetic nephropathy. Solasonine (SS) is a natural alkaloid and received attention as a potential anticancer agent. However, its protective effect against hyperglycemia-caused podocyte injury remains to be determined. Our study found that SS alleviates cell apoptosis, and reduces pyroptosis and oxidative damage in high glucose (HG)-treated MPC5 podocytes. Pro-inflammatory cytokines, including interleukin (IL)-1β and IL-18, and caspase-1 activity were markedly suppressed by SS in HG-treated MPC5 podocytes. SS also reduced HG-induced oxidative damage in MPC5 podocytes. Nrf2 expression was activated by SS in vitro under a HG condition. In addition, Nrf2 silencing attenuated the protective effect of SS against apoptosis, pro-inflammatory cytokines release, caspase-1 activity, and oxidative damage in MPC5 podocytes under a HG condition. Taken together, our findings revealed for the first time that SS alleviated high glucose-induced podocyte apoptosis, pyroptosis, and oxidative damage via regulating the Nrf2/NLRP3 signaling pathway. Our results indicate that SS has the potential as a therapeutic agent for podocyte injury in diabetic nephropathy.

What do we actually know about exosomal microRNAs in kidney diseases?

There are several types of kidney diseases with complex causes. If left untreated, these diseases irreversibly progress to end-stage renal disease. Thus, their early diagnosis and targeted treatment are important. Exosomes—extracellular vesicles released by a variety of cells—are ideal carriers for DNA, RNA, proteins, and other metabolites owing to their bilayer membranes. Studies have shown that almost all renal cells can secrete exosomes. While research on exosomal microRNAs in the context of renal diseases begun only recently, rapid progress has been achieved. This review summarizes the changes in exosomal microRNA expression in different kidney diseases. Thus, it highlights the diagnostic and prognostic value of these exosomal microRNAs. Further, this review analyzes their roles in the development of different kidney diseases, guiding research on molecular mechanisms and therapeutic strategies.

Asparaginyl endopeptidase protects against podocyte injury in diabetic nephropathy through cleaving cofilin-1

Podocyte injury and loss are critical events in diabetic nephropathy (DN); however, the underlying molecular mechanisms remain unclear. Here, we demonstrate that asparaginyl endopeptidase (AEP) protects against podocyte injury through modulating the dynamics of the cytoskeleton. AEP was highly upregulated in diabetic glomeruli and hyperglycemic stimuli treated-podocytes; however, AEP gene knockout and its compound inhibitor treatment accelerated DN in streptozotocin-induced diabetic mice, whereas specific induction of AEP in glomerular cells attenuated podocyte injury and renal function deterioration. In vitro, elevated AEP was involved in actin cytoskeleton maintenance and anti-apoptosis effects. Mechanistically, we found that AEP directly cleaved the actin-binding protein cofilin-1 after the asparagine 138 (N138) site. The protein levels of endogenous cofilin-1 1-138 fragments were upregulated in diabetic podocytes, consistent with the changes in AEP levels. Importantly, we found that cofilin-1 1-138 fragments were remarkably unphosphorylated than full-length cofilin-1, indicating the enhanced cytoskeleton maintenance activity of cofilin-1 1-138. Then we validated cofilin-1 1-138 could rescue podocytes from cytoskeleton disarrangement and injury in diabetic conditions. Taken together, our data suggest a protective role of elevated AEP in podocyte injury during DN progression through cleaving cofilin-1 to maintain podocyte cytoskeleton dynamics and defend damage.

Retinoic acid receptor responder1 promotes development of glomerular diseases via the Nuclear Factor-κB signaling pathway

Inflammatory pathways are activated in most glomerular diseases but molecular mechanisms driving them in kidney tissue are poorly known. We identified retinoic acid receptor responder 1 (Rarres1) as a highly podocyte-enriched protein in healthy kidneys. Studies in podocyte-specific knockout animals indicated that Rarres1 was not needed for the normal development or maintenance of the glomerulus filtration barrier and did not modulate the outcome of kidney disease in a model of glomerulonephritis. Interestingly, we detected an induction of Rarres1 expression in glomerular and peritubular capillary endothelial cells in IgA and diabetic kidney disease, as well as in ANCA-associated vasculitis. Analysis of publicly available RNA data sets showed that the induction of Rarres1 expression was a common molecular mechanism in chronic kidney diseases. A conditional knock-in mouse line, overexpressing Rarres1 specifically in endothelial cells, did not show any obvious kidney phenotype. However, the overexpression promoted the progression of kidney damage in a model of glomerulonephritis. In line with this, conditional knock-out mice, lacking Rarres1 in endothelial cells, were partially protected in the disease model. Mechanistically, Rarres1 promoted inflammation and fibrosis via transcription factor Nuclear Factor-κB signaling pathway by activating receptor tyrosine kinase Axl. Thus, induction of Rarres1 expression in endothelial cells is a prevalent molecular mechanism in human glomerulopathies and this seems to have a pathogenic role in driving inflammation and fibrosis via the Nuclear Factor-κB signaling pathway.

Tacrolimus Protects Podocytes from Apoptosis via Downregulation of TRPC6 in Diabetic Nephropathy

Podocyte injury plays an important role in diabetic nephropathy (DN), and apoptosis is one of its mechanisms. The transient receptor potential channel 6 (TRPC6) is expressed in podocytes and mediates podocyte injury induced by high glucose levels. Tacrolimus is a novel immunosuppressive agent that is reported to play an important role in podocyte protection. The purpose of this study was to investigate the potential mechanism of podocyte protection by tacrolimus in a type 2 diabetic mellitus (T2DM) rat model and in immortalized mouse podocytes (MPC5). Transmission electron microcopy was used to evaluate renal injury morphology. After treatment with FK506, we measured 24-hour urinary albumin-to-creatinine ratios and creatinine clearance rates as well as major biochemical parameters such as glucose, insulin, serum creatinine, urea nitrogen, total cholesterol, triglycerides, alanine transaminase, and aspartate aminotransferase. Nephrin and TRPC6 protein expression and podocyte apoptotic rates in vivo and in vitro were measured using immunohistochemical staining, TUNEL assays, and flow cytometry, respectively. Western blot was used to measure expression of cleaved-caspase-3 and bax/bcl-2. Exposed to high glucose (HG), DM rats exhibited disrupted biochemical conditions and impaired podocyte structure. Decreased expression of nephrin and increased expression of TRPC6, cleaved-caspase-3, and bax/bcl-2 ratios were found in podocytes, along with higher apoptotic percentage, while tacrolimus intervention counteracted the effect of HG on podocytes. Our results suggest that tacrolimus protects podocytes during the progression of type 2 diabetic nephropathy, possibly ameliorating podocyte apoptosis by downregulating the expression of TRPC6.

Inhibition of NLRP3 inflammasome ameliorates podocyte damage by suppressing lipid accumulation in diabetic nephropathy

BACKGROUND

Nucleotide leukin-rich polypeptide 3 (NLRP3) inflammasome is documented as a potent target for treating metabolic diseases and inflammatory disorders. Our recent work demonstrated that inhibition of NLRP3 inflammasome activation inhibits renal inflammation and fibrosis in diabetic nephropathy. This study was to investigate the effect of NLRP3 inflammasome on podocyte injury and the underlying mechanism in diabetic nephropathy.

METHODS

In vivo, db/db mice were treated with MCC950, a NLRP3 inflammasome specific inhibitor. NLRP3 knockout (NKO) mice were induced to diabetes by intraperitoneal injections of streptozotocin (STZ). We assessed renal function, albuminuria, podocyte injury and glomerular lipid accumulation in diabetic mice. In vitro, apoptosis, cytoskeleton change, lipid accumulation, NF-κB p65 activation and reactive oxygen species (ROS) generation were evaluated in podocytes interfered with NLRP3 siRNA or MCC950 under high glucose (HG) conditions. In addition, the effect and mechanism of IL-1β on lipid accumulation was explored in podocytes exposed to normal glucose (NG) or HG.

RESULTS

MCC950 treatment improved renal function, attenuated albuminuria, mesangial expansion, podocyte loss, as well as glomerular lipid accumulation in db/db mice. The diabetes-induced podocyte loss and glomerular lipid accumulation were reversed in NLRP3 knockout mice. The increased expression of sterol regulatory element-binding protein1 (SREBP1) and SREBP2, and decreased expression of ATP-binding cassette A1 (ABCA1) in podocytes were reversed by MCC950 treatment or NLRP3 knockout in diabetic mice. In vitro, NLRP3 siRNA or MCC950 treatment markedly inhibited HG-induced apoptosis, cytoskeleton change, lipid accumulation, NF-κB p65 activation, and mitochondrial ROS production in cultured podocytes. In addition, BAY11-7082 or tempol treatment inhibited HG-induced lipid accumulation in podocytes. Moreover, exposure of IL-1β to podocytes induced lipid accumulation, NF-κB p65 activation and mitochondrial ROS generation.

CONCLUSION

Inhibition of NLRP3 inflammasome protects against podocyte damage through suppression of lipid accumulation in diabetic nephropathy. IL-1β/ROS/NF-κB p65 mediates diabetes-associated lipid accumulation in podocytes. The suppression of NLRP3 inflammasome activation may be an effective therapeutic approach to diabetic nephropathy.

Single-cell RNA sequencing reveals the mesangial identity and species diversity of glomerular cell transcriptomes

Molecular characterization of the individual cell types in human kidney as well as model organisms are critical in defining organ function and understanding translational aspects of biomedical research. Previous studies have uncovered gene expression profiles of several kidney glomerular cell types, however, important cells, including mesangial (MCs) and glomerular parietal epithelial cells (PECs), are missing or incompletely described, and a systematic comparison between mouse and human kidney is lacking. To this end, we use Smart-seq2 to profile 4332 individual glomerulus-associated cells isolated from human living donor renal biopsies and mouse kidney. The analysis reveals genetic programs for all four glomerular cell types (podocytes, glomerular endothelial cells, MCs and PECs) as well as rare glomerulus-associated macula densa cells. Importantly, we detect heterogeneity in glomerulus-associated Pdgfrb-expressing cells, including bona fide intraglomerular MCs with the functionally active phagocytic molecular machinery, as well as a unique mural cell type located in the central stalk region of the glomerulus tuft. Furthermore, we observe remarkable species differences in the individual gene expression profiles of defined glomerular cell types that highlight translational challenges in the field and provide a guide to design translational studies.

The molecular identity of renal glomerular cells is poorly characterized and rodent glomerulopathy models translate poorly to humans. Here, the authors show molecular signatures of glomerulus-associated cells using single cell RNA sequencing and highlight differences between mouse and human cells.

Non-Coding RNAs as Biomarkers and Therapeutic Targets for Diabetic Kidney Disease

Diabetic kidney disease (DKD) is the most common diabetic complication and is a leading cause of end-stage kidney disease. Increasing evidence shows that DKD is regulated not only by many classical signaling pathways but also by epigenetic mechanisms involving chromatin histone modifications, DNA methylation, and non-coding RNA (ncRNAs). In this review, we focus on our current understanding of the role and mechanisms of ncRNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) in the pathogenesis of DKD. Of them, the regulatory role of TGF-β/Smad3-dependent miRNAs and lncRNAs in DKD is highlighted. Importantly, miRNAs and lncRNAs as biomarkers and therapeutic targets for DKD are also described, and the perspective of ncRNAs as a novel therapeutic approach for combating diabetic nephropathy is also discussed.

Long noncoding RNA Kcnq1ot1 promotes sC5b-9-induced podocyte pyroptosis by inhibiting miR-486a-3p and upregulating NLRP3

Podocytes are epithelial cells adhering glomerular capillaries, which regulate the integrity of glomerular filtration barrier. Irreversible podocyte injury induces glomerular inflammation and causes chronic renal diseases. Kcnq1ot1, a long noncoding RNA, participates in the pathogenesis of diabetic retinopathy and cardiomyopathy. However, its function in podocyte injury is elusive. Pyroptosis of murine podocyte MPC5 was triggered by sublytic complement C5b-9 (sC5b-9) for subsequent in vitro functional and mechanistic investigation. Gain/loss-of-function analysis was conducted to examine the functional role of Kcnq1ot1 in podocyte pyroptosis. Meanwhile, the molecular mechanism of Kcnq1ot1's effect on podocyte injury was explored by identifying downstream molecules and their intermediate interactions. Kcnq1ot1 was upregulated in sC5b-9-induced podocytes, and silencing Kcnq1ot1 could inhibit sC5b-9's effect on podocyte pyroptosis. We also identified the interaction between Kcnq1ot1 and miR-486a-3p, through which Kcnq1ot1 mediated miR-486a-3p inhibition by sC5b-9. Furthermore, miR-486a-3p reduced the transcriptional activity of NLRP3, while the overexpression of NLRP3 enhanced sC5b-9's effect on podocyte pyroptosis through activating NLRP3 inflammasome. sC5b-9 induces pyroptosis in podocytes through modulating the Kcnq1ot1/miR-486a-3p/NLRP3 regulatory axis, and these uncovered key molecules might facilitate podocyte-targeted treatment for renal inflammatory diseases.

Inactivation of mediator complex protein 22 in podocytes results in intracellular vacuole formation, podocyte loss and premature death

Podocytes are critical for the maintenance of kidney ultrafiltration barrier and play a key role in the progression of glomerular diseases. Although mediator complex proteins have been shown to be important for many physiological and pathological processes, their role in kidney tissue has not been studied. In this study, we identified a mediator complex protein 22 (Med22) as a renal podocyte cell-enriched molecule. Podocyte-specific Med22 knockout mouse showed that Med22 was not needed for normal podocyte maturation. However, it was critical for the maintenance of podocyte health as the mice developed progressive glomerular disease and died due to renal failure. Detailed morphological analyses showed that Med22-deficiency in podocytes resulted in intracellular vacuole formation followed by podocyte loss. Moreover, Med22-deficiency in younger mice promoted the progression of glomerular disease, suggesting Med22-mediated processes may have a role in the development of glomerulopathies. This study shows for the first time that mediator complex has a critical role in kidney physiology.

M2 macrophage-derived exosomal miR-25-3p improves high glucose-induced podocytes injury through activation autophagy via inhibiting DUSP1 expression

Diabetic nephropathy (DN) is the primary reason of chronic kidney disease. The aim of our study is to explore the role and action mechanism of M2 macrophage-derived exosomes in high glucose (HG)-induced podocytes injury. Here, 30 mmol/L of HG was used to induce podocytes injury. Annexin V-FITC/PI double staining was performed to measure podocytes apoptosis, and western blot was carried out to ensure proteins expression. The shape of exosomes was identified using TEM. Besides, the expression of miR-25-3p was determined by qRT-PCR, FAM-labeled miR-25-5p combined with DiI-labeled exosomes were utilized to explore the uptake of podocytes to exosomes. Relationship between miR-25-3p and DUSP family members was ensued by luciferase activity assay. In the beginning, we found that M2 macrophage ameliorated HG-induced podocytes apoptosis and epithelial-mesenchymal transition through secreting exosomes. Subsequently, highly expressed miR-25-3p was found in M2 macrophage-derived exosomes that effectively improved HG-induced podocytes injury. Furthermore, inhibition of miR-25-3p in M2 macrophage inefficiently repressed HG-induced podocytes injury, thus we proposed that M2 macrophage attenuated podocytes injury through secreting exosomal miR-25-3p. Then, we used an autophagy inhibitor to stimulate podocytes, and demonstrated that M2 macrophage-derived exosomal miR-25-3p improved HG-induced podocytes injury through activating autophagy. Finally, DUSP1 was proved to be a downstream target and mediated the inhibition of exosomal miR-25-3p to HG-induced podocytes injury. Our results indicated that M2 macrophage could improve HG-induced podocytes injury via secreting exosomal miR-25-3p to activate autophagy of the cells through suppressing DUSP1 expression. We proved a newly potential therapy strategy for DN treatment.

Morroniside Inhibits H2O2-Induced Podocyte Apoptosis by Down-Regulating NOX4 Expression Controlled by Autophagy In Vitro

Podocyte apoptosis is the common pathological basis for the progression of various kidney diseases. The overexpression of NOX4, a key enzyme involved in oxidative stress, has been proved to participate in the occurrence of podocyte apoptosis. Autophagy is a kind of adaptive response of cells under stress. However, as a “double-edged sword”, the effect of autophagy on apoptosis in different cells and conditions is complex and variable, which has not been fully explained yet. Morroniside, extracted from the traditional medicinal plant Cornus officinalis, has remarkable antioxidant and anti-apoptosis effects, and has been proven to inhibit the overexpression of NOX4 in kidney tissue. Therefore, H₂O₂ was used in this study to explore the effects of autophagy on podocyte NOX4 overexpression and apoptosis induced by oxidative stress, as well as the protection mechanism of morroniside in podocytes. The results showed that the autophagy activator rapamycin, as well as the autophagy inhibitor chloroquine, could induce podocyte apoptosis cultured in normal condition, and chloroquine could also significantly increase the NOX4 expression. The NOX4 expression and apoptosis rate of podocytes increased after H₂O₂ treatment, the expression of LC3-II decreased, and the expressions of p62, mTOR, and p-mTOR increased. The intervention of morroniside and rapamycin improved autophagy activity and inhibited NOX4 overexpression and apoptosis induced by H₂O₂. And chloroquine reversed the inhibitory effect of morroniside on NOX4 overexpression and podocyte apoptosis. Taken together, our results suggest that the expression level of NOX4 in podocytes is regulated by autophagy activity. Morroniside can reduce oxidative stress induced podocyte apoptosis by restoring the damaged autophagy flux and inhibit the overexpression of NOX4.

Angiotensin II promotes podocyte injury by activating Arf6-Erk1/2-Nox4 signaling pathway

Angiotensin II (Ang II) is a key contributor to glomerular disease by predominantly resulting in podocyte injury, whereas the underlying molecular mechanisms has not been fully understood. This study aimed to investigate if and how ADP-ribosylation factor 6 (Arf6), a small GTP-binding protein, involves Ang II-induced cellular injury in cultured human podocytes. Cellular injury was evaluated with caspase 3 activity, reactive oxygen species (ROS) level and TUNEL assay. Arf6 activity was measured using an Arf6-GTP Pull-Down Assay. Ang II significantly enhanced Arf6 expressions accompanied by increase of Arf6-GTP. The TUNEL-positive cells as well as activated caspase 3, NADPH oxidase 4 protein (Nox4) and ROS levels were dramatically increased in Ang II-treated podocytes, which was prevented by secinH3, an Arf6 activity inhibitor. Induction of ROS by Ang II was inhibited in podocytes with Nox4 knockdown. Ang II-induced elevation of Nox4 and ROS was prevented by Arf6 knockdown. Phpspho-Erk1/2^{Thr202/Tyr204} levels were upregulated remarkably following Ang II treatment, and Erk inhibitor LY3214996 significantly downregulated Nox4 expression. In addition, Ang II decreased CD2AP expression. Overexpression of CD2AP prevented Ang II-induced upregulation of Arf6-GTP. Our data demonstrated that Ang II promotes ROS production and podocytes injury through activation of Arf6-Erk1/2-Nox4 signaling. We also provided evidence that Ang II activates Arf6 by degradation of CD2AP.

Catalpol Ameliorates Podocyte Injury by Stabilizing Cytoskeleton and Enhancing Autophagy in Diabetic Nephropathy

Catalpol, an iridoid glycoside extracted from Rehmannia glutinosa, has been found to ameliorate diabetic nephropathy (DN), but the mechanism has not been clarified. Podocyte injury play a key role in the pathogenesis of DN. This study mainly investigated the protective effect and potential mechanism of catalpol on podocyte injury of DN in vivo and in vitro. The results indicated that the pathological features of DN in mice were markedly ameliorated after treatment with catalpol. Moreover, podocyte foot process effacement, and down-regulation of nephrin and synaptopodin expression in DN mice were also significantly improved after treatment with catalpol. In vitro, catalpol rescued disrupted cytoskeleton and increased migration ratio in podocytes induced by high glucose, the effect might be attributable to the inhibition of RhoA and Cdc42 activities but not Rac1. Furthermore, the impaired podocyte autophagy in DN mice was significantly enhanced after catalpol treatment. And catalpol also enhanced autophagy and lysosome biogenesis in cultured podocytes under high glucose condition. In addition, we found that catalpol could inhibit mTOR activity and promote TFEB nuclear translocation in vivo and in vitro experiments. Our study demonstrated that catalpol could ameliorate podocyte injury in DN, and the protective effect of catalpol might be attributed to the stabilization of podocyte cytoskeleton and the improvement of impaired podocyte autophagy.

Podoplanin mediates the renoprotective effect of berberine on diabetic kidney disease in mice

Hyperglycemia-caused podocyte injury plays a crucial role in the progress of diabetic kidney disease. Podoplanin, one of the podocyte-associated molecules, is closely related to the integrity of the glomerular filtration barrier. A number of studies demonstrate that berberine could ameliorate renal dysfunction in diabetic mice with nephropathy, but the molecular mechanisms have not been fully elucidated. In this study, we explored the relationship between the renoprotective effect of berberine and podoplanin expression in streptozotocin (STZ)-induced diabetic mice as well as mouse podocytes (MPC5 cells) cultured in high glucose (HG, 30 mM) medium. We found that the expression levels of podoplanin were significantly decreased both in the renal glomerulus of STZ-induced diabetic mice and HG-cultured MPC5 cells. We also demonstrated that NF-κB signaling pathway was activated in MPC5 cells under HG condition, which downregulated the expression level of podoplanin, thus leading to increased podocyte apoptosis. Administration of berberine (100, 200 mg/kg every day, ig, for 8 weeks) significantly improved hyperglycemia and the renal function of STZ-induced diabetic mice and restored the expression level of podoplanin in renal glomerulus. In high glucose-cultured MPC5 cells, treatment with berberine (30–120 μM) dose-dependently decreased the apoptosis rate, increased the expression of podoplanin, and inhibited the activation of NF-κB signaling pathway. When podoplanin expression was silenced with shRNA, berberine treatment still inhibited the NF-κB signaling pathway, but its antiapoptotic effect on podocytes almost disappeared. Our results suggest that berberine inhibits the activation of NF-κB signaling pathway, thus increasing the podoplanin expression to exert renoprotective effects.

Critical roles of PI3K/Akt/NF‑κB survival axis in angiotensin II‑induced podocyte injury

Numerous studies have reported that angiotensin (Ang) II, nephrin, and podocin serve pivotal roles in podocyte injury, and thus can lead to the occurrence of proteinuria and the progression of kidney diseases. This study aimed to investigate the effects of Ang II on the production of nephrin and podocin, and their relationship with podocyte injury. We also aimed to determine whether nephrin, podocin and caspase-9 production depends on the PI3K/Akt/nuclear factor (NF)-κB signaling pathway in cultured mouse podocytes. We treated mouse podocytes with different doses of Ang II (10⁻⁹, 10⁻⁸, 10⁻⁷ and 10⁻⁶ mol/l) for 12, 24, and 48 h to analyse cell viability, and at 10⁻⁶ mol/l Ang II for 12, 24, and 48 h to evaluate cell apoptosis. Cells were treated with 10⁻⁶ mol/l of Ang II and/or LY294002 (inhibitor of Akt) or 740Y-P (activator of PI3K) for 48 h to detect Akt, phosphorylated (phospho)-Akt, p65 NF-κB, and phospho-p65 NF-κB, nephrin, podocin and caspase-9 expression, and podocyte apoptosis. Treatment with Ang II suppressed the viability and promoted the apoptosis of podocytes in a dose- and time-dependent manner. Ang II decreased phospho-Akt, phospho-p65 NF-κB, nephrin, and podocin and increased caspase-9 expression, while podocyte apoptosis was promoted. LY294002 further enhanced Ang II-induced downregulation of Akt and p65 NF-κB activation, as well as upregulation of caspase-9 mRNA and protein, and promoted the apoptosis of podocytes. Of note, 740Y-P restored Ang II-induced downregulation of Akt and p65 NF-κB activation, and upregulation of caspase-9, and decreased podocyte apoptosis. Interestingly, LY294002 and 740Y-P were determined to have no notable effects on the expression of nephrin and podocin. The data suggested that Ang II could regulate the expression of nephrin, podocin and caspase-9. Collectively, our findings suggested that the PI3K/Akt/NF-κB survival axis may serve a pivotal role in podocyte injury.

Sam68 mediates high glucose‑induced podocyte apoptosis through modulation of Bax/Bcl‑2

Hyperglycemia promotes podocyte apoptosis and contributes to the pathogenesis of diabetic nephropathy (DN). However, the mechanisms of hyperglycemia-induced podocyte apoptosis remain unknown. Recent studies have implicated Src-associated substrate during mitosis of 68 kDa (Sam68) in various cellular processes including RNA metabolism, apoptosis, signal transduction. This study sought to examine the effect of Sam68 on high glucose (HG)-induced podocytes apoptosis, and the mechanism underlying this effect. Immortalized mouse podocytes were exposed to medium containing normal glucose, or HG and Sam68 siRNA, respectively. The expression of Sam68 in podocytes was determined by fluorescence quantitative PCR (qPCR), immunofluorescence and immunoblotting. The role of Sam68 in HG-induced podocyte apoptosis was further evaluated by inhibiting Sam68 expression by Sam68 siRNA and performing flow cytometry. The mRNA and protein expression of pro-apoptosis gene Bax and anti-apoptotic gene Bcl-2 were assessed by qRCR and immunoblotting. In the present study, it was first demonstrated that Sam68 was upregulated in a time and dose-dependent manner in in vitro HG-treated podocytes. Pretreatment with Sam68 siRNA markedly decreased nuclear Sam68 expression. Moreover, the effects of HG-induced apoptosis were also abrogated by Sam68 knockdown in cultured podocytes. Furthermore, HG increased Bax and decreased Bcl-2 protein expression in cultured podocytes, and this effect was blocked by Sam68 knockdown. The results of the present study revealed that Sam68 mediated HG-induced podocyte apoptosis, probably through the Bax/Bcl-2 signaling pathway, and thus may be a potential therapeutic target for DN.

GPRC5b Modulates Inflammatory Response in Glomerular Diseases via NF-κB Pathway

BACKGROUND

Inflammatory processes play an important role in the pathogenesis of glomerulopathies. Finding novel ways to suppress glomerular inflammation may offer a new way to stop disease progression. However, the molecular mechanisms that initiate and drive inflammation in the glomerulus are still poorly understood.

METHODS

We performed large-scale gene expression profiling of glomerulus-associated G protein-coupled receptors (GPCRs) to identify new potential therapeutic targets for glomerulopathies. The expression of Gprc5b in disease was analyzed using quantitative PCR and immunofluorescence, and by analyzing published microarray data sets. In vivo studies were carried out in a podocyte-specific Gprc5b knockout mouse line. Mechanistic studies were performed in cultured human podocytes.

RESULTS

We identified an orphan GPCR, Gprc5b, as a novel gene highly enriched in podocytes that was significantly upregulated in common human glomerulopathies, including diabetic nephropathy, IgA nephropathy, and lupus nephritis. Similar upregulation of Gprc5b was detected in LPS-induced nephropathy in mice. Studies in podocyte-specific Gprc5b knockout mice showed that Gprc5b was not essential for normal development of the glomerular filtration barrier. However, knockout mice were partially protected from LPS-induced proteinuria and recruitment of inflammatory cells. Mechanistically, RNA sequencing in Gprc5b knockouts mice and experiments in cultured human podocytes showed that Gpr5cb regulated inflammatory response in podocytes via NF-κB signaling.

CONCLUSIONS

GPRC5b is a novel podocyte-specific receptor that regulates inflammatory response in the glomerulus by modulating the NF-κB signaling pathway. Upregulation of Gprc5b in human glomerulopathies suggests that it may play a role in their pathogenesis.

Histone demethylase KDM6B regulates human podocyte differentiation in vitro

Podocytes are terminally differentiated and highly specialized glomerular cells, which have an essential role as a filtration barrier against proteinuria. Histone methylation has been shown to influence cell development, but its role in podocyte differentiation is less understood. In this study, we first examined the expression pattern of histone demethylase KDM6B at different times of cultured human podocytes in vitro We found that the expression of KDM6B and podocyte differentiation markers WT1 and Nephrin are increased in the podocyte differentiation process. In cultured podocytes, KDM6B knockdown with siRNA impaired podocyte differentiation and led to expression down-regulation of WT1 and Nephrin. The treatment of podocytes with GSK-J4, a specific KDM6B inhibitor, can also obtain similar results. Overexpression of WT1 can rescue differentiated phenotype impaired by disruption of KDM6B ChIP (chromatin immunoprecipitation) assay further indicated that KDM6B can bind the promoter region of WT1 and reduce the histone H3K27 methylation. Podocytes in glomeruli from nephrotic patients exhibited increased KDM6B contents and reduced H3K27me3 levels. These data suggest a role for KDM6B as a regulator of podocyte differentiation, which is important for the understanding of podocyte function in kidney development and related diseases.

The podocyte protease web: uncovering the gatekeepers of glomerular disease

Proteases regulate glomerular physiology. The last decade has revealed a multitude of podocyte proteases that govern the glomerular response to numerous chemical, mechanical, and metabolic cues. These proteases form a protein signaling web that integrates stress stimuli and serves as a key controller of the glomerular microenvironment. Both the extracellular and intracellular proteolytic networks are perturbed in focal segmental glomerulosclerosis, as well as hypertensive and diabetic nephropathy. Accordingly, the highly intertwined podocyte protease web is an integrative part of the podocyte's damage response. Novel mass spectrometry-based technologies will help to untangle this proteolytic network: functional readouts acquired from deep podocyte proteomics, single glomerular proteomics, and degradomics have exposed unanticipated protease activity in podocytes. Future efforts should characterize the interdependency and upstream regulation of key proteases, along with their role in promoting tissue heterogeneity in glomerular diseases. These efforts will not only illuminate the machinery of podocyte proteostasis but also reveal avenues for therapeutic intervention in the podocyte protease web.