A Hypercaloric Diet Induces Early Podocyte Damage in Aged, Non-Diabetic Rats

Background/Aims: The number of patients of older age with metabolic syndrome, obesity, and associated kidney disease, which is characterized by podocyte damage, glomerular hypertrophy, and focal segmental glomerulosclerosis (FSGS), is increasing worldwide. Animal models that would reflect the development of such kidney diseases could facilitate the testing of drugs. We investigated the renal effects of a long-term high caloric diet in aged rats and the potential effects of drugs used to treat metabolic syndrome. Methods: We analyzed nine-month-old male and female Sprague Dawley rats fed five months with a normal diet (control group) or high-fat-high-carbohydrate diet (HFHCD group). Two additional groups were fed with HFHCD and treated with drugs used in patients with metabolic syndrome, i.e., the glucagon-like peptide receptor 1 agonist liraglutide (HFHCD+liraglutide group) or metformin (HFHCD+metformin group). Results: Except an increase of waist circumference as a sign of visceral obesity, the HFHCD diet did not induce metabolic syndrome or obesity. There were no significant changes in kidney function and all groups showed similar indices of glomerular injury, i.e., no differences in glomerular size or the number of glomeruli with FSGS or with FSGS-precursor lesions quantified by CD44 expression as a marker of parietal epithelial cell (PEC) activation. Analysis of ultrastructural morphology revealed mild podocyte stress and a decrease of glomerular nestin expression in the HFHCD group, whereas podocin and desmin were not altered. HFHCD did not promote fibrogenesis, however, treatment with liraglutide led to a slightly increased tubulointerstitial damage, immune cell infiltration, and collagen IV expression compared to the control and HFHCD groups. Conclusion: A five-month feeding with HFHCD in aged rats induced mild podocyte injury and microinflammation, which was not alleviated by liraglutide or metformin.

Dexamethasone reduces podocyte injury through the PTEN-PI3K/Akt signaling pathway

Objective: To study the role of Akt and its downstream molecules in the PTEN-PI3K/Akt signaling pathway, namely,GSK3β and Bad, in the Dexamethasone(DEX)-mediated regulation of PAN-induced glomerular podocyte injury and to elucidate the molecular mechanism of podocyte injury regulation. Methods: Glomerular podocytes (MPC5) were cultured in vitro and divided into four groups: the control group, PTEN silencing group (siPTEN group), puromycin group (PAN group), and puromycin group + DEX group (PAN+ DEX group). The cells in each group were treated for 8h, 24h, and 48h, and then, the experiment was carried out. The cells in the control group were cultured in RPMI 1640 with 0.02% DMSO, the cells in the siPTEN group were used to construct a silencing kit, the podocytes in the PAN group were treated with puromycin (final concentration of 50μg/ml), and the podocytes in the DEX+PAN group were pretreated with 0.1μmol/L DEX followed by PAN (final concentration of 50μg/ml). An inverted phase contrast microscope was used to observe the morphological changes in the podocytes and the changes in the cell body area in each group, laser confocal microscopy was used to detect the expression and distribution of the PTEN protein, and flow cytometry was used to detect and analyze the apoptosis rate and mitochondrial membrane potential of each group of podocytes. Western blot was used to detect the expression of the PTEN, P-Akt, Akt, P-GSK3β and GSK3β proteins in each group of podocytes, and transmission electron microscopy was used to observe the changes in the morphology and structure of each group of podocytes. Results: After PAN was used to injure the podocytes, the expression of the PTEN protein decreased, the rate of apoptosis increased, and the flux of autophagy was inhibited. DEX treatment reversed the changes described above.After PAN was used to injure the podocytes, the expression of p-Ak and p-GSK3β decreased, and DEX reversed these effects on the expression of p-Akt and p-GSK3β in the podocytes. Compared with the control group, in the PAN group, the mitochondria gradually swelled and rounded, mitochondrial cristae arrangement became disordered, mitochondrial autophagy was inhibited; DEX reversed the changes described above after the PTEN gene was silenced. Conclusion: This study confirmed that PAN can inhibit podocyte autophagy and induce podocyte damage. DEX can reduce the PAN-induced suppression of podocyte autophagy, enhance podocyte autophagy, and ameliorate podocyte damage. The protective mechanism may be through the upregulation of PTEN expression, which is achieved by inhibiting the activation of the PI3K/Akt signaling pathway.

Indirect podocyte injury manifested in a partial podocytectomy mouse model

In progressive glomerular diseases, segmental podocyte injury often expands, leading to global glomerulosclerosis by unclear mechanisms. To study the expansion of podocyte injury, we established a new mosaic mouse model in which a fraction of podocytes express hCD25 and can be injured by the immunotoxin LMB2. hCD25-positive (+) and negative (-) podocytes were designed to express tdTomato and EGFP, respectively, which enabled cell sorting analysis of podocytes. After the injection of LMB2, mosaic mice developed proteinuria and glomerulosclerosis. Not only tdTomato(+) podocytes but also EGFP(+) podocytes were decreased in number and showed damage, as evidenced by a decrease in nephrin and an increase in desmin at both the protein and RNA levels. Transcriptomics analysis found a decrease in Glcci1 and an increase in Thbs4, Hbegf, and Tgfb2 in EGFP(+) podocytes; these genes may be candidate mediators of secondary podocyte damage. Pathway analysis suggested that focal adhesion, integrin-mediated cell adhesion, and focal adhesion-PI3K-Akt-mTOR signaling are involved in the secondary podocyte injury. Finally, treatment of mosaic mice with angiotensin II receptor blocker markedly ameliorated secondary podocyte injury. This mosaic podocyte injury model has distinctly demonstrated that damaged podocytes cause secondary podocyte damage, which may be a promising therapeutic target in progressive kidney diseases.

Inhibition of miR-17~92 Cluster Ameliorates High Glucose-Induced Podocyte Damage

The loss and damage of podocytes is an early feature of diabetic nephropathy (DN). The miR-17∼92 cluster was dysregulated in diabetic and polycystic kidney disease patients, but its role in DN is unclear. Hence, an in vitro study on the high glucose- (HG-) treated mouse podocytes (MPC5) was designed to elucidate the effect of miR-17∼92 cluster downregulation on cell viability, apoptosis, inflammation, fibrosis, and podocyte function. The results suggested that the miR-17∼92 cluster members miR-17-5p, miR-18a, miR-19a, miR-19b, miR-20a, and miR-92a were upregulated in the renal biopsy tissue of DN patients and HG-treated MPC5. The downregulation of the miR-17∼92 cluster effectively suppressed the cell apoptosis, inflammation, fibrosis, and podocyte dysfunction in HG-stimulated MPC5 cells. The bioinformatics analysis and rescue experiments showed that ABCA1 (ATP-binding cassette transporter A1) is an effector of the miR-17~92 cluster. Silence of ABCA1 inhibited the protective effect of the miR-17∼92 cluster downregulation on podocyte damage. In summary, this research indicated that the downregulation of the miR-17∼92 cluster ameliorates HG-induced podocyte damage via targeting ABCA1.

P0143OPPOSING EFFECTS OF P38 MAPK IN PODOCYTES ON ALDOSTERONE-INDUCED GLOMERULAR INJURY IN POD-GC-A CKO MICE

Background and Aims Previously, we demonstrated that uninephrectomized aldosterone-infused, high salt-fed podocyte-specific guanylyl cyclase-A (natriuretic peptide receptor 1) conditional KO (pod-GC-A cKO) mice exhibited glomerular injury and that pharmacological inhibition of p38 MAPK ameliorates podocyte damage. However, the effects of genetic deletion of p38 MAPK in podocytes of pod-GC-A cKO mice have been unclarified. Method We generated p38 MAPK(fl/fl);Nephrin-Cre (pod-p38 MAPK cKO) mice and p38 MAPK(fl/fl);GC-A(fl/fl);Nephrin-Cre (pod-p38MAPK/GC-A DKO) mice. For induction of glomerular injury, we treated them with aldosterone and high salt at 2 months of age for 3 weeks without nephrectomy (B-ALDO). In vitro, we examined the effect of p38 MAPK inhibitor in cultured human podocytes transfected with GC-A siRNA. Results B-ALDO-treated pod-p38 MAPK/GC-A DKO mice resulted in significant elevation of serum Cr (0.29 ± 0.04 mg/dl), massive albuminuria (42,660 ± 20,200 μg/mgCr) and severe foot process effacement in addition to intracapillary fibrin thrombi which indicated endothelial damage. Vehicle-treated DKO mice, B-ALDO-treated pod-GC-A cKO mice, and B-ALDO-treated pod-p38 MAPK cKO showed normal serum Cr levels (0.14 ± 0.01, 0.18 ± 0.02, 0.20 ± 0.01 mg/dl, respectively), mild increase of albuminuria (223 ± 6.5, 1,496 ± 592, 649 ± 303 μg/mgCr, respectively) and only segmental foot process effacement. Blood pressure was not elevated in either mutant mice compared with that of B-ALDO control mice. Furthermore, glomerular mRNA expressions of MCP-1, PAI-1, and FN were upregulated and that of VEGF-A was downregulated in DKO mice. In vitro, suppression of GC-A mRNA by siRNA in combination with p38 MAPK inhibitor downregulated VEGF mRNA in human cultured podocytes. Our previous works showed that pharmacological inhibition of p38 MAPK in the whole body ameliorated podocyte damage, whereas our current result showed that genetic deletion of p38 MAPK in podocytes aggravated renal injury. In order to explain the discrepancy in these results, we added an analysis of podocyte specific GC-A fl/fl p38 fl/+ cKO mice. Pod GC-A fl/fl p38 fl/+ cKO mice exhibited considerably milder renal damage than pod GC-A fl/fl p38 fl/fl double cKO mice. Conclusion Genetic complete p38 MAPK deletion in GC-A-nul podocytes exacerbated aldosterone-induced glomerular endothelial cell injury as well as podocytes, and resulted in renal dysfunction, probably through VEGF downregulation, whereas partial p38 MAPK inhibition in podocytes ameliorated aldosterone-induced glomerular injury in pod-GC-A cKO mice. These results suggest a certain level of p38 MAPK in podocytes is necessary to protect endothelial and epithelial cells from aldosterone-induced renal injury.