Live or Let Die: Is There any Cell Death in Podocytes?

Investigating EGF and PAG1 as necroptosis-related biomarkers for diabetic nephropathy: an in silico and in vitro validation study

The current study aims to understand the mechanisms behind regulated cell death (RCD) in diabetic nephropathy and identify related biomarkers through bioinformatics and experimental validation. Datasets of bulk and single-cell RNA sequencing were obtained from public databases and analyzed using gene set variation analysis (GSVA) with gene sets related to RCD, including autophagy, necroptosis, pyroptosis, apoptosis, and ferroptosis. RCD-related gene biomarkers were identified using weighted gene correlation network analysis (WGCNA). The results were verified through experiments with an independent cohort and in vitro experiments. The GSVA revealed higher necroptosis scores in diabetic nephropathy. Three necroptosis-related biomarkers, EGF, PAG1, and ZFP36, were identified and showed strong diagnostic ability for diabetic kidney disease. In vitro experiments showed high levels of necroptotic markers in HK-2 cells treated with high glucose. Bioinformatics and experimental validation have thus identified EGF and PAG1 as necroptosis-related biomarkers for diabetic nephropathy.

Astragaloside IV Inhibited Podocyte Pyroptosis in Diabetic Kidney Disease by Regulating SIRT6/HIF-1α Axis

To investigate the effect of astragaloside IV (AS) on podocytes pyroptosis in diabetic kidney disease (DKD). Forty male Sprague-Dawley rats were randomly divided into normal group (n = 10) and model group (n = 30). Rats in model group were intraperitoneally injected streptozotocin (60 mg/kg) for 3 days to induce DKD. Then rats were divided into DKD group, AS group, and UBCS group. The AS group was given 40 mg/kg/d of AS by gavage, and UBCS group was given 50 mg/kg/d of UBCS039 by gavage, and normal group and DKD group were given the same amount saline for 8 weeks, once a day. Hematoxylin-eosin and masson staining were used to observe pathology of kidney. Rat podocytes were divided into normal group, mannitol hypertonic group, high-glucose group, UBCS group, OSS group, and AS group. Western blotting, quantitative real-time polymerase chain reaction, immunofluorescence, and flow cytometry were used to analyze pyroptosis-related markers and reactive oxygen species (ROS) levels. Results showed that AS inhibited ROS and alleviated podocytes pyroptosis in rats by increasing expression of sirtuin 6 (SIRT6) and decreasing expression of hypoxia inducible factor 1 subunit alpha (HIF-1α). UBCS039 and AS enhanced SIRT6 level, decreased HIF-1α level, and finally improved pyroptosis of podocytes in vitro, whereas OSS-128167 showed the opposite effect for podocytes pyroptosis. AS improved podocytes pyroptosis in DKD by regulating SIRT6/HIF-1α pathway, thereby alleviating injury of DKD.

Upregulated PD-1 signaling antagonizes glomerular health in aged kidneys and disease

With an aging population, kidney health becomes an important medical and socioeconomic factor. Kidney aging mechanisms are not well understood. We previously showed that podocytes isolated from aged mice exhibit increased expression of programmed cell death protein 1 (PD-1) surface receptor and its 2 ligands (PD-L1 and PD-L2). PDCD1 transcript increased with age in microdissected human glomeruli, which correlated with lower estimated glomerular filtration rate and higher segmental glomerulosclerosis and vascular arterial intima-to-lumen ratio. In vitro studies in podocytes demonstrated a critical role for PD-1 signaling in cell survival and in the induction of a senescence-associated secretory phenotype. To prove PD-1 signaling was critical to podocyte aging, aged mice were injected with anti-PD-1 antibody. Treatment significantly improved the aging phenotype in both kidney and liver. In the glomerulus, it increased the life span of podocytes, but not that of parietal epithelial, mesangial, or endothelial cells. Transcriptomic and immunohistochemistry studies demonstrated that anti-PD-1 antibody treatment improved the health span of podocytes. Administering the same anti-PD-1 antibody to young mice with experimental focal segmental glomerulosclerosis (FSGS) lowered proteinuria and improved podocyte number. These results suggest a critical contribution of increased PD-1 signaling toward both kidney and liver aging and in FSGS.

Mitochondrial Oxidative Stress and Cell Death in Podocytopathies

Podocytopathies are kidney diseases that are driven by podocyte injury with proteinuria and proteinuria-related symptoms as the main clinical presentations. Albeit podocytopathies are the major contributors to end-stage kidney disease, the underlying molecular mechanisms of podocyte injury remain to be elucidated. Mitochondrial oxidative stress is associated with kidney diseases, and increasing evidence suggests that oxidative stress plays a vital role in the pathogenesis of podocytopathies. Accumulating evidence has placed mitochondrial oxidative stress in the focus of cell death research. Excessive generated reactive oxygen species over antioxidant defense under pathological conditions lead to oxidative damage to cellular components and regulate cell death in the podocyte. Conversely, exogenous antioxidants can protect podocyte from cell death. This review provides an overview of the role of mitochondrial oxidative stress in podocytopathies and discusses its role in the cell death of the podocyte, aiming to identify the novel targets to improve the treatment of patients with podocytopathies.

Podocyte Aging: Why and How Getting Old Matters

The effects of healthy aging on the kidney, and how these effects intersect with superimposed diseases, are highly relevant in the context of the population's increasing longevity. Age-associated changes to podocytes, which are terminally differentiated glomerular epithelial cells, adversely affect kidney health. This review discusses the molecular and cellular mechanisms underlying podocyte aging, how these mechanisms might be augmented by disease in the aged kidney, and approaches to mitigate progressive damage to podocytes. Furthermore, we address how biologic pathways such as those associated with cellular growth confound aging in humans and rodents.

Histone deacetylase 3 (HDAC3) as an important epigenetic regulator of kidney diseases

Development and progression of many kidney diseases are substantially influenced by aberrant protein acetylation modifications of gene expression crucial for kidney functions. Histone deacetylase (HDAC) expression alterations are detected from renal samples of patients and animal models of various kidney diseases, and the administrations of HDAC inhibitors display impressive renal protective effects in vitro and in vivo. However, when the expression alterations of multiple HDACs occur, not all the HDACs causally affect the disease onset or progression. Identification of a single HDAC as a disease-causing factor will allow subtype-targeted intervention with less side effect. HDAC3 is a unique HDAC with distinct structural and subcellular distribution features and co-repressor dependency. HDAC3 is required for kidney development and its aberrations actively participate in many pathological processes, such as cancer, cardiovascular diseases, diabetes, and neurodegenerative disorders, and contribute significantly to the pathogenesis of kidney diseases. This review will discuss the recent studies that investigate the critical roles of HDAC3 aberrations in kidney development, renal aging, renal cell carcinoma, renal fibrosis, chronic kidney disease, polycystic kidney disease, glomerular podocyte injury, and diabetic nephropathy. These studies reveal the distinct characters of HDAC3 aberrations that act on different molecules/signaling pathways under various renal pathological conditions, which might shed lights into the epigenetic mechanisms of renal diseases and the potentially therapeutic strategies.

A mathematical estimation of the physical forces driving podocyte detachment

Loss of podocytes, possibly through the detachment of viable cells, is a hallmark of progressive glomerular disease. Podocytes are exposed to considerable physical forces due to pressure and flow resulting in circumferential wall stress and tangential shear stress exerted on the podocyte cell body, which have been proposed to contribute to podocyte depletion. However, estimations of in vivo alterations of physical forces in glomerular disease have been hampered by a lack of quantitative functional and morphological data. Here, we used ultra-resolution data and computational analyses in a mouse model of human disease, hereditary late-onset focal segmental glomerular sclerosis, to calculate increased mechanical stress upon podocyte injury. Transversal shear stress on the lateral walls of the foot processes was prominently increased during the initial stages of podocyte detachment. Thus, our study highlights the importance of targeting glomerular hemodynamics to treat glomerular disease.

Immune-mediated entities of (primary) focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) represents a glomerular scar formation downstream of various different mechanisms leading to podocytopathy and podocyte loss. Recently, significant advances were made in understanding genetic factors, podocyte intrinsic mechanisms, and adaptive mechanisms causing FSGS. However, while most cases of nephrotic FSGS are being treated with immunosuppressants, the underlying immune dysregulation, involved immune cells, and soluble factors are only incompletely understood. Thus, we here summarize the current knowledge of proposed immune effector cells, secreted soluble factors, and podocyte response in immune-mediated (primary) FSGS.

In situ evaluation of podocytes in patients with focal segmental glomerulosclerosis and minimal change disease

Podocyte injury in focal segmental glomerulosclerosis (FSGS) and minimal change disease (MCD) results from the imbalance between adaptive responses that maintain homeostasis and cellular dysfunction that can culminate in cell death. Therefore, an in situ analysis was performed to detect morphological changes related to cell death and autophagy in renal biopsies from adult patients with podocytopathies. Forty-nine renal biopsies from patients with FSGS (n = 22) and MCD (n = 27) were selected. In situ expression of Wilms Tumor 1 protein (WT1), light chain microtubule 1-associated protein (LC3) and caspase-3 protein were evaluated by immunohistochemistry. The foot process effacement and morphological alterations related to podocyte cell death and autophagy were analyzed with transmission electronic microscopy. Reduction in the density of WT1-labeled podocytes was observed for FSGS and MCD cases as compared to controls. Foot process width (FPW) in control group was lower than in cases of podocytopathies. In FSGS group, FPW was significantly higher than in MCD group and correlated with proteinuria. A density of LC3-labeled podocytes and the number of autophagosomes in podocytes/ pedicels were higher in the MCD group than in the FSGS group. The number of autophagosomes correlated positively with the estimated glomerular filtration rate in cases of MCD. The density of caspase-3-labeled podocytes in FSGS and MCD was higher than control group, and a higher number of podocytes with an evidence of necrosis was detected in FSGS cases than in MCD and control cases. Podocytes from patients diagnosed with FSGS showed more morphological and functional alterations resulting from a larger number of lesions and reduced cell adaptation.

B-cell lymphoma/leukaemia 10 and angiotensin II-induced kidney injury

AIMS

B-cell lymphoma/leukaemia 10 (Bcl10) is a member of the CARMA-Bcl10-MALT1 signalosome, linking angiotensin (Ang) II, and antigen-dependent immune-cell activation to nuclear factor kappa-B signalling. We showed earlier that Bcl10 plays a role in Ang II-induced cardiac fibrosis and remodelling, independent of blood pressure. We now investigated the role of Bcl10 in Ang II-induced renal damage.

METHODS AND RESULTS

Bcl10 knockout mice (Bcl10 KO) and wild-type (WT) controls were given 1% NaCl in the drinking water and Ang II (1.44 mg/kg/day) for 14 days. Additionally, Bcl10 KO or WT kidneys were transplanted onto WT mice that were challenged by the same protocol for 7 days. Kidneys of Ang II-treated Bcl10 KO mice developed less fibrosis and showed fewer infiltrating cells. Nevertheless, neutrophil gelatinase-associated lipocalin (Ngal) and kidney injury molecule (Kim)1 expression was higher in the kidneys of Ang II-treated Bcl10 KO mice, indicating exacerbated tubular damage. Furthermore, albuminuria was significantly higher in Ang II-treated Bcl10 KO mice accompanied by reduced glomerular nephrin expression and podocyte number. Ang II-treated WT mice transplanted with Bcl10 KO kidney showed more albuminuria and renal Ngal, compared to WT- > WT kidney-transplanted mice, as well as lower podocyte number but similar fibrosis and cell infiltration. Interestingly, mice lacking Bcl10 in the kidney exhibited less Ang II-induced cardiac hypertrophy than controls.

CONCLUSION

Bcl10 has multi-faceted actions in Ang II-induced renal damage. On the one hand, global Bcl10 deficiency ameliorates renal fibrosis and cell infiltration; on the other hand, lack of renal Bcl10 aggravates albuminuria and podocyte damage. These data suggest that Bcl10 maintains podocyte integrity and renal function.

Activation of Notch1 signaling in podocytes by glucose-derived AGEs contributes to proteinuria

INTRODUCTION

Advanced glycation end-products (AGEs) are implicated in the pathogenesis of diabetic nephropathy (DN). Previous studies have shown that AGEs contribute to glomerulosclerosis and proteinuria. Podocytes, terminally differentiated epithelial cells of the glomerulus and the critical component of the glomerular filtration barrier, express the receptor for AGEs (RAGE). Podocytes are susceptible to severe injury during DN. In this study, we investigated the mechanism by which AGEs contribute to podocyte injury.

RESEARCH DESIGN AND METHODS

Glucose-derived AGEs were prepared in vitro. Reactivation of Notch signaling was examined in AGE-treated human podocytes (in vitro) and glomeruli from AGE-injected mice (in vivo) by quantitative reverse transcription-PCR, western blot analysis, ELISA and immunohistochemical staining. Further, the effects of AGEs on epithelial to mesenchymal transition (EMT) of podocytes and expression of fibrotic markers were evaluated.

RESULTS

Using human podocytes and a mouse model, we demonstrated that AGEs activate Notch1 signaling in podocytes and provoke EMT. Inhibition of RAGE and Notch1 by FPS-ZM1 (N-Benzyl-4-chloro-N-cyclohexylbenzamide) and DAPT (N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenyl glycine t-butylester), respectively, abrogates AGE-induced Notch activation and EMT. Inhibition of RAGE and Notch1 prevents AGE-induced glomerular fibrosis, thickening of the glomerular basement membrane, foot process effacement, and proteinuria. Furthermore, kidney biopsy sections from people with DN revealed the accumulation of AGEs in the glomerulus with elevated RAGE expression and activated Notch signaling.

CONCLUSION

The data suggest that AGEs activate Notch signaling in the glomerular podocytes. Pharmacological inhibition of Notch signaling by DAPT ameliorates AGE-induced podocytopathy and fibrosis. Our observations suggest that AGE-induced Notch reactivation in mature podocytes could be a novel mechanism in glomerular disease and thus could represent a novel therapeutic target.

Cyclin-dependent kinase 5 contributes to endoplasmic reticulum stress induced podocyte apoptosis via promoting MEKK1 phosphorylation at Ser280 in diabetic nephropathy

Endoplasmic reticulum (ER) stress has been reported to be associated with podocyte apoptosis in diabetic nephropathy, but the mechanism of ER signaling in podocyte apoptosis hasn't been fully understood. Our previous studies have demonstrated that Cyclin-dependent kinase 5 (Cdk5) was associated with podocyte apoptosis in diabetic nephropathy. The present study was designed to examine whether and how Cdk5 activity plays a role in ER stress induced podocyte apoptosis in diabetic nephropathy. The results showed that along with induction of Cdk5 and apoptosis, GRP78 and its two sensors as well as CHOP and cleaved caspase-12 were induced in high glucose treated podocytes. These responses were attenuated by treated salubrinal. The ER stress inducer, tunicamycin, also up-regulated the kinase activity and protein expression of Cdk5 in podocytes accompanied with the increasing of GRP78. On the other hand, Cdk5 phosphorylates MEKK1 at Ser280 in tunicamycin treated podocytes, and together, they increase the JNK phosphorylation. Moreover, disruption of this pathway can decrease the podocyte apoptosis induced by tunicamycin. Therefore, our study proved that Cdk5 may play an important role in ER stress induced podocyte apoptosis through MEKK1/JNK pathway in diabetic nephropathy.