Clearly imaging and quantifying the kidney in 3D

For decades, measurements of kidney microanatomy using 2-dimensional sections has provided us with a detailed knowledge of kidney morphology under physiological and pathological conditions. However, the rapid development of tissue clearing methods in recent years, in combination with the development of novel 3-dimensional imaging modalities have provided new insights into kidney structure and function. This review article describes a range of novel insights into kidney development and disease obtained recently using these new methodological approaches. For example, in the developing kidney these approaches have provided new understandings of ureteric branching morphogenesis, nephron progenitor cell proliferation and commitment, interactions between ureteric tip cells and nephron progenitor cells, and the establishment of nephron segmentation. In whole adult mouse kidneys, tissue clearing combined with light sheet microscopy can image and quantify the total number of glomeruli, a major breakthrough in the field. Similar approaches have provided new insights into the structure of the renal vasculature and innervation, tubulointerstitial remodeling, podocyte loss and hypertrophy, cyst formation, the evolution of cellular crescents, and the structure of the glomerular filtration barrier. Many more advances in the understanding of kidney biology and pathology can be expected as additional clearing and imaging techniques are developed and adopted by more investigators.

Apolipoprotein L1 and mechanisms of kidney disease susceptibility

PURPOSE OF REVIEW

Allelic variants in the gene for apolipoprotein L1 (APOL1), found only in individuals of African ancestry, explain a majority of the excess risk of kidney disease in African Americans. However, a clear understanding how the disease-associated APOL1 variants cause kidney injury and the identity of environmental stressors that trigger the injury process have not been determined.

RECENT FINDINGS

Basic mechanistic studies of APOL1 biochemistry and cell biology, bolstered by new antibody reagents and inducible pluripotent stem cell-derived cell systems, have focused on the cytotoxic effect of the risk variants when APOL1 gene expression is induced. Since the APOL1 variants evolved to alter a key protein-protein interaction with the trypanosome serum resistance-associated protein, additional studies have begun to address differences in APOL1 interactions with other proteins expressed in podocytes, including new observations that APOL1 variants may alter podocyte cytoskeleton dynamics.

SUMMARY

A unified mechanism of pathogenesis for the various APOL1 nephropathies still remains unclear and controversial. As ongoing studies have consistently implicated the pathogenic gain-of-function effects of the variant proteins, novel therapeutic development inhibiting the synthesis or function of APOL1 proteins is moving toward clinical trials.

Hyperoside ameliorates diabetic nephropathy induced by STZ via targeting the miR-499-5p/APC axis

Diabetic nephropathy is a serious complication of diabetes. Hyperoside has been widely reported to ameliorate diabetes-associated disease. The current study is designed to explore the mechanism of hyperoside in diabetic nephropathy. In the present study, high glucose was used to treat podocytes. Diabetic nephropathy mice models were established by high-fat feeding followed by multiple low dose injections of streptozocin. Western blot analysis was conducted for detection of extracellular matrix accumulation, inflammatory response and cell apoptosis. We found out that hyperoside improved high glucose-induced cell injury. Additionally, hyperoside prevented mice with diabetic nephropathy from diabetic symptoms and renal dysfunction. Mechanistically, hyperoside inhibited the mRNA and protein expression of APC. MiR-499-5p was found to be an upstream negative mediator of APC, and hyperoside induced the upregulation of miR-499-5p. MiR-499-5p bound with the 3' untranslated region of APC to inhibit its expression. Finally, rescue assays revealed that the suppressive effects of miR-499-5p overexpression on renal dysfunction were rescued by upregulation of APC in mice with diabetic nephropathy. In conclusion, these findings indicated that hyperoside ameliorates diabetic nephropathy via targeting the miR-499-5p/APC axis, suggesting that hyperoside may offer a potential tactic for diabetic nephropathy treatment.

Low expression of HIV genes in podocytes accelerates the progression of diabetic kidney disease in mice

With the widespread use combination antiretroviral therapy, there has been a dramatic decrease in HIV-associated nephropathy. However, although the patients living with HIV have low or undetectable viral load, the prevalence of chronic kidney disease (CKD) in this population remains high. Additionally, improved survival is associated with aging-related comorbidities such as diabetes and cardiovascular disease. A faster progression of CKD is associated with concurrent HIV infection and diabetes than with HIV infection or diabetes alone. To explore the potential pathogenic mechanisms that synergistically drive CKD progression by diabetes and HIV infection, we generated a new mouse model with a relatively low expression of HIV-1 proviral genes specifically in podocytes (pod-HIV mice) to better mimic the setting of kidney injury in patients living with HIV. While no apparent kidney phenotypes were observed at baseline in pod-HIV mice, the induction of mild diabetic kidney disease with streptozotocin led to significant worsening of albuminuria, glomerular injury, podocyte loss, and kidney dysfunction as compared to the mice with diabetes alone. Mechanistically, diabetes and HIV-1 synergistically increased the glomerular expression of microRNA-34a (miR-34a), thereby reducing the expression of Sirtuin-1 (SIRT1) deacetylase. These changes were also associated with increased acetylation and activation of p53 and p65 NF-κB and with enhanced expression of senescence and inflammatory markers. The treatment of diabetic pod-HIV mice with the specific Sirtuin-1 agonist BF175 significantly attenuated albuminuria and glomerulopathy. Thus, our study highlights the reduction in Sirtuin-1 as a major basis of CKD progression in diabetic patients living with HIV and suggests Sirtuin-1 agonists as a potential therapy.

Glomerular podocyte dysfunction in inherited renal tubular disease

BACKGROUND

Hereditary renal tubular disease can cause hypercalciuria, acid-base imbalance, hypokalemia, hypomagnesemia, rickets, kidney stones, etc. If these diseases are not diagnosed or treated in time, they can cause kidney damage and electrolyte disturbances, which can be detrimental to the maturation and development of the child. Glomerular involvement in renal tubular disease patients has only been considered recently.

METHODS

We screened 71 papers (including experimental research, clinical research, etc.) about Dent's disease, Gitelman syndrome, and cystinosis from PubMed, and made reference.

RESULTS

Glomerular disease was initially underestimated among the clinical signs of renal tubular disease or was treated merely as a consequence of the tubular damage. Renal tubular diseases affect glomerular podocytes through certain mechanisms resulting in functional damage, morphological changes, and glomerular lesions.

CONCLUSIONS

This article focuses on the progress of changes in glomerular podocyte function in Dent disease, Gitelman syndrome, and cystinosis for the purposes of facilitating clinically accurate diagnosis and scientific treatment and improving prognosis.

APX-115, a pan-NADPH oxidase inhibitor, protects development of diabetic nephropathy in podocyte specific NOX5 transgenic mice

NADPH oxidases (NOXs) are comprised of different isoforms, NOX1 to 5 and Duox1 and 2, and they trigger diabetic nephropathy (DN) in the patients with diabetes mellitus. Recently, it was shown that, compared to the other isoforms, the expression of NOX5 was increased in the patients with DN and, NOX5 has been suggested to be important in the development of therapeutic agents. The effect of pan-NOX inhibition by APX-115 has also been investigated in type 2 diabetic mice. However, since NOX5 is absent in mice, we evaluated the effect of pan-NOX inhibition by APX-115 in Nox5 transgenic mouse. Wild type and renal podocyte specific NOX5 transgenic mice (NOX5 pod+) were fed with high-fat diet (60% kcal fat) and treated with APX-115 (60 mg/kg) by oral gavage for 14 weeks. APX-115 significantly improved pancreatic beta cell function by decreased fasting blood glucose levels and increased insulin levels. Further, the total serum cholesterol, triglycerides, and urinary albumin/creatinine levels were also significantly decreased by APX-115 treatment. Increased NOX5 mRNA expressions, increased desmin levels, and reduced podocin protein expressions in the kidney of NOX5 pod + mice were also significantly restored to normal levels by APX-115 treatment. Moreover, APX-115 inhibited the expression of inflammation-related proteins such as TRAF6. Collectively, these data suggest that APX-115 might be a promising therapeutic agent for the treatment of DN because of its pan-NOX inhibitory activity, including its NOX5 inhibitory activity, and also owing to its anti-inflammatory effect.

Global transcriptomic changes occur in aged mouse podocytes

Glomerular podocytes undergo structural and functional changes with advanced age, that increase susceptibility of aging kidneys to worse outcomes following superimposed glomerular diseases. To delineate transcriptional changes in podocytes in aged mice, RNA-seq was performed on isolated populations of reporter-labeled (tdTomato) podocytes from multiple young (two to three months) and advanced aged mice (22 to 24 months, equivalent to 70 plus year old humans). Of the 2,494 differentially expressed genes, 1,219 were higher and 1,275 were lower in aged podocytes. Pathway enrichment showed that major biological processes increased in aged podocytes included immune responses, non-coding RNA metabolism, gene silencing and MAP kinase signaling. Conversely, aged podocytes showed downregulation of developmental, morphogenesis and metabolic processes. Canonical podocyte marker gene expression decreased in aged podocytes, with increases in apoptotic and senescence genes providing a mechanism for the progressive loss of podocytes seen with aging. In addition, we revealed aberrations in the podocyte autocrine signaling network, identified the top transcription factors perturbed in aged podocytes, and uncovered candidate gene modulations that might promote healthy aging in podocytes. The transcriptional signature of aging is distinct from other kidney diseases. Thus, our study provides insights into biomarker discovery and molecular targeting of the aging process itself within podocytes.

The pathogenesis of human membranous nephropathy: we are (almost) there

Primary membranous nephropathy is an autoimmune disease usually associated with antibody to phospholipase A2 receptor (anti-PLA2R). The study by Meyer-Schwesinger et al. describes the first mouse model induced using a PLA2R system to study the pathogenicity of anti-PLA2R. Hyperimmune rabbit anti-PLA2R IgG can induce a primary membranous nephropathy-like glomerulopathy with proteinuria in mice. However, to conclusively establish the pathogenicity of human anti-PLA2R will require additional studies using PLA2R and anti-PLA2R of human origin.

[Role of microRNA-17-5p in the pathogenesis of pediatric nephrotic syndrome and related mechanisms]

OBJECTIVE

To study the role of microRNA-17-5p (miR-17-5p) in the pathogenesis of pediatric nephrotic syndrome (NS) and its effect on renal podocyte apoptosis via the activin A (ActA)/Smads pathway.

METHODS

An analysis was performed on 55 children with NS (NS group) who were admitted from March 2018 to March 2019. Fifty healthy children who underwent physical examination during the same period of time were enrolled as the control group. The mRNA expression of miR-17-5p in peripheral blood was measured and compared between the two groups. Human renal podocytes were transfected with antisense oligonucleotide recombinant plasmid containing miR-17-5p (inhibition group) or control vector containing nonsense random sequence (negative control group), and untreated human renal podocytes were used as the blank group. These groups were compared in terms of cell apoptosis and the mRNA and protein expression of miR-17-5p, ActA, and Smads after transfection.

RESULTS

The NS group had a significantly higher level of miR-17-5p in peripheral blood than the control group (P<0.001). Compared with the blank and negative control groups, the inhibition group had significantly lower apoptosis rate and relative mRNA expression of miR-17-5p and significantly higher relative mRNA and protein expression of ActA, Smad2, and Smad3 (P<0.001).

CONCLUSIONS

There is an increase in the content of miR-17-5p in peripheral blood in children with NS. Low expression of miR-17-5p can inhibit the apoptosis of human renal podocytes, which may be associated with the upregulation of the mRNA and protein expression of ActA, Smad2 and Smad3.

Podocyte stress and detachment measured in urine are related to mean arterial pressure in healthy humans

Hypertension-associated progressive glomerulosclerosis is a significant driver of both de novo and all-cause chronic kidney disease leading to end-stage kidney failure. The progression of glomerular disease proceeds via continuing depletion of podocytes from the glomeruli into the ultrafiltrate. To non-invasively assess injury patterns associated with mean arterial pressure (MAP), we conducted an observational study of 87 healthy normotensive individuals who were cleared for living kidney donation. Urine pellet podocin and aquaporin2 mRNAs normalized to the urine creatinine concentration (UPod:Creat ratio and UAqp2:Creat ratio) were used as markers of podocyte detachment and tubular injury, respectively. The ratio of two podocyte mRNA markers, podocin to nephrin (UPod:Neph) as well as the ratio of podocin to the tubular marker aquaporin2 (UPod:Aqp2) estimated the relative rates of podocyte stress and glomerular vs. tubular injury. The MAP was positively correlated with the UPod:Neph and UPod:Aqp2, thereby confirming the relationship of MAP with podocyte stress and the preferential targeting of the glomerulus by higher MAP. In multivariable linear regression analysis, both UPod:Neph and UPod:Creat, but not UAqp2:Creat or proteinuria, were both significantly related to a range of normal MAP (70 to 110 mm Hg). Systolic, as opposed to diastolic or pulse pressure was associated with UPod:Creat. Thus, higher podocyte stress and detachment into the urine are associated with MAP even in a relatively "normal" range of MAP. Hence, urine pellet mRNA monitoring can potentially identify progression risk before the onset of overt hypertension, proteinuria or chronic kidney disease.

Inhibition of Ceramide Accumulation in Podocytes by Myriocin Prevents Diabetic Nephropathy

BACKGROUND

Ceramides are associated with metabolic complications including diabetic nephropathy in patients with diabetes. Recent studies have reported that podocytes play a pivotal role in the progression of diabetic nephropathy. Also, mitochondrial dysfunction is known to be an early event in podocyte injury. Thus, we tested the hypothesis that ceramide accumulation in podocytes induces mitochondrial damage through reactive oxygen species (ROS) production in patients with diabetic nephropathy.

METHODS

We used Otsuka Long Evans Tokushima Fatty (OLETF) rats and high-fat diet (HFD)-fed mice. We fed the animals either a control- or a myriocin-containing diet to evaluate the effects of the ceramide. Also, we assessed the effects of ceramide on intracellular ROS generation and on podocyte autophagy in cultured podocytes.

RESULTS

OLETF rats and HFD-fed mice showed albuminuria, histologic features of diabetic nephropathy, and podocyte injury, whereas myriocin treatment effectively treated these abnormalities. Cultured podocytes exposed to agents predicted to be risk factors (high glucose, high free fatty acid, and angiotensin II in combination [GFA]) showed an increase in ceramide accumulation and ROS generation in podocyte mitochondria. Pretreatment with myriocin reversed GFA-induced mitochondrial ROS generation and prevented cell death. Myriocin-pretreated cells were protected from GFA-induced disruption of mitochondrial integrity.

CONCLUSION

We showed that mitochondrial ceramide accumulation may result in podocyte damage through ROS production. Therefore, this signaling pathway could become a pharmacological target to abate the development of diabetic kidney disease.

Identification of Podocyte Cargo Proteins by Proteomic Analysis of Clathrin-Coated Vesicles

Background

Clathrin-mediated endocytosis (CME) plays a fundamental role in podocyte health. Genetic ablation of genes implicated in CME has been shown to cause severe proteinuria and foot process effacement in mice. However, little is known about the cargo of clathrin-coated vesicles (CCVs) in podocytes. The goal of this study was to isolate CCVs from podocytes and identify their cargo by proteomic analysis.

Methods

Glomeruli isolated from Podocin-Cre Rosa-DTRflox mouse kidneys were seeded and treated with diphtheria toxin to obtain pure primary podocyte cultures. CCVs were isolated by differential gradient ultracentrifugation, and enrichment of CCVs was assessed by immunoblotting and electron microscopy (EM). Liquid chromatography-mass spectrometry (LC-MS) was performed for proteomic analysis. Proteins with higher abundance than transferrin receptor protein 1 were evaluated for CCV cargo potential against previously published literature. Immunofluorescence staining of identified cargo proteins and CCVs was performed in podocytes for further verification.

Results

Immunoblotting for multiple protein markers of CME revealed enrichment in the CCV fraction. Enrichment of CCVs among other small vesicles was observed via EM. Proteomics yielded a total of >1200 significant proteins. Multiple-step data analysis revealed 36 CCV-associated proteins, of which 10 represent novel, highly abundant cargo proteins in podocytes. Colocalization of cargo proteins and CCVs on immunostaining was observed.

Conclusions

Our identification of podocyte CCV cargo proteins helps to elucidate the importance of endocytic trafficking for podocyte health and maintenance of the glomerular environment.

Cellular regeneration of podocytes from parietal cells: the debate is still open

The study by Kaverina et al. in this issue addresses an important question: can podocytes be replenished by parietal epithelial cells (PECs)? The authors use a complex transgenic mouse model in which podocytes are labeled with GFP and PECs are simultaneously labeled with tdTomato. When Kaverina and colleagues induce focal segmental glomerulosclerosis (FSGS), they find that individual PECs are doubly labeled, coexpress podocyte markers, and form structures similar to foot processes, suggesting that these PECs may have transdifferentiated into podocytes.

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.

High salt-induced weakness of anti-oxidative function of natriuretic peptide receptor-C and podocyte damage in the kidneys of Dahl rats

BACKGROUND

Atrial natriuretic peptide (ANP) and its natriuretic peptide receptors A (NPR-A) and C (NPR-C) are involved in the regulation of physiological and pathophysiological process of blood pressure. The present study aimed to determine the role of NPR-C in the development of salt-sensitive hypertension.

METHODS

The Dahl salt-sensitive (DS) and salt-resistant (DR) rats were used in this study. Animals were matched according to their age and weight, and then placed on either a high-salt (HS, 8%) or a normal-salt (NS, 0.4%) diet for 6 weeks randomly using random number table. The systolic blood pressure (SBP), plasmatic sodium concentration (PLNa), urinary sodium excretion (UVNa), and serum creatinine concentration (Scr) were measured. The concentration of ANP in blood and tissues (heart and kidney) was detected by enzyme-linked immunosorbent assay. The expression of ANP, NPR-A, and NPR-C in kidney was evaluated with western blot analysis. Regarding renal redox state, the concentration changes in malondialdehyde (MDA), lipofuscin, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox), and nitric oxide synthase (NOS) in kidney were detected by a spectrophotometric method. The kidney damage was evaluated using pathological techniques and the succinodehydrogenase (SDHase) examination. Furthermore, after an intra-peritoneal injection of C-atrial natriuretic peptide (ANP)4-23 (C-ANP4-23), an NPR-C receptor agonist, the SBP, biochemical values in blood and urine, and renal redox state were evaluated. The paired Student's t test and analysis of variance followed by the Bonferroni test were performed for statistical analyses of the comparisons between two groups and multiple groups, respectively.

RESULTS

The baseline SBP in all groups was within the normal range. At the end of the 6-week experiment, HS diet significantly increased the SBP in DS rats from 116.63 ± 2.90 mmHg to 162.25 ± 2.15 mmHg (t = -10.213, P < 0.001). The changes of SBP were not significant in DS rats on an NS diet and DR rats on an NS diet or on an HS diet (all P > 0.05). The significant increase of PLNa, UVNa, and Scr related to an HS diet was found in both DS and DR rats (all P < 0.05). However, significant changes in the concentration (t = -21.915, P < 0.001) and expression of renal ANP (t = -3.566, P = 0.016) and the expression of renal NPR-C (t = 5.864, P = 0.002) were only observed in DS hypertensive rats. The significantly higher desmin immunochemical staining score (t = -5.715, P = 0.005) and mitochondrial injury score (t = -6.325, P = 0.003) accompanied by the lower SDHase concentration (t = 3.972, P = 0.017) revealed mitochondrial pathologic abnormalities in podocytes in DS rats with an HS diet. The distinct increases of MDA (t = -4.685, P = 0.009), lipofuscin (t = -8.195, P = 0.001), and Nox (t = -12.733, P < 0.001) but not NOS (t = -0.328, P = 0.764) in kidneys were also found in DS hypertensive rats. C-ANP4-23 treatment significantly decreased the SBP induced by HS in DS rats (P < 0.05), which was still higher than NS groups with the vehicle or C-ANP4-23 treatment (P < 0.05). Moreover, the HS-induced increase of MDA, lipofuscin, Nox concentrations, and Nox4 expression in DS rats was significantly attenuated by C-ANP4-23 treatment as compared with those with HS diet and vehicle injection (all P < 0.05).

CONCLUSIONS

The results indicated that the renal NPR-C might be involved in the salt-sensitive hypertension through the damage of mitochondria in podocytes and the reduction of the anti-oxidative function. Hence, C-ANP4-23 might serve as a therapeutic agent in treating salt-sensitive hypertension.

Assessment of increased glomerular permeability associated with recurrent focal segmental glomerulosclerosis using an in vitro model of the glomerular filtration barrier

The presence of circulating permeability factors (cPFs) has been hypothesized to be associated with recurrence of focal segmental glomerulosclerosis (rFSGS) in renal allografts. The available methods to detect cPFs are complex, not easily repeatable and inappropriate to represent the anatomical characteristics of the three-layer glomerular filtration barrier (GFB). Here we describe a novel method which measures the permeability to bovine serum albumin (BSA) through a three-layer device (3LD). The 3 layers comprise: (1) conditionally immortalized human podocytes (HCiPodo), (2) collagen type IV coated porous membrane and (3) human glomerular endothelial cells (HCiGEnC). Using this method, we found that sera from all rFSGS patients increased albumin permeability, while sera from non recurrent (nrFSGS) and genetic (gFSGS) forms of FSGS did not. The mechanisms underlying the increase of albumin permeability are probably due to endothelial cell damage as an initial event, which was demonstrated by the decrease of Platelet endothelial cell adhesion molecule (PECAM-1 or CD31), while the podocytes' expressions of synaptopodin and podocin were normal. Furthermore, we also found that the plasmapheretic treatment (PPT) eliminated the effect of increasing BSA permeability in sera from rFSGS patients. These preliminary data suggest that our in vitro GFB model could not only be useful in predicting the recurrence of FSGS after renal transplantation (RTx), but also be a valuable in vitro model to study podocyte and endothelial cell biology.

Podocyte RhoGTPases: new therapeutic targets for nephrotic syndrome?

Podocytes, or glomerular epithelial cells, form the final layer in the glomerular capillary wall of the kidney. Along with the glomerular basement membrane and glomerular endothelial cells, they make up the glomerular filtration barrier which allows the passage of water and small molecules and, in healthy individuals, prevents the passage of albumin and other key proteins. The podocyte is a specialised and terminally differentiated cell with a specific cell morphology that is largely dependent on a highly dynamic underlying cytoskeletal network and that is essential for maintaining glomerular function and integrity in healthy kidneys. The RhoGTPases (RhoA, Rac1 and Cdc42), which act as molecular switches that regulate actin dynamics, are known to play a crucial role in maintaining the cytoskeletal and molecular integrity of the podocyte foot processes in a dynamic manner. Recently, novel protein interaction networks that regulate the RhoGTPases in the podocyte and that are altered by disease have been discovered. This review will discuss these networks and their potential as novel therapeutic targets in nephrotic syndrome. It will also discuss the evidence that they are direct targets for (a) steroids, the first-line agents for the treatment of nephrotic syndrome, and (b) certain kinase inhibitors used in cancer treatment, leading to nephrotoxicity.

Mutations in KIRREL1, a slit diaphragm component, cause steroid-resistant nephrotic syndrome

Steroid-resistant nephrotic syndrome is a frequent cause of chronic kidney disease almost inevitably progressing to end-stage renal disease. More than 58 monogenic causes of SRNS have been discovered and majority of known steroid-resistant nephrotic syndrome causing genes are predominantly expressed in glomerular podocytes, placing them at the center of disease pathogenesis. Herein, we describe two unrelated families with steroid-resistant nephrotic syndrome with homozygous mutations in the KIRREL1 gene. One mutation showed high frequency in the European population (minor allele frequency 0.0011) and this patient achieved complete remission following treatment, but later progressed to chronic kidney disease. We found that mutant KIRREL1 proteins failed to localize to the podocyte cell membrane, indicating defective trafficking and impaired podocytes function. Thus, the KIRREL1 gene product has an important role in modulating the integrity of the slit diaphragm and maintaining glomerular filtration function.

The Role of the EGF Receptor in Sex Differences in Kidney Injury

BACKGROUND

Sex differences mediating predisposition to kidney injury are well known, with evidence indicating lower CKD incidence rates and slower decline in renal function in nondiabetic CKD for premenopausal women compared with men. However, signaling pathways involved have not been elucidated to date. The EGF receptor (EGFR) is widely expressed in the kidney in glomeruli and tubules, and persistent and dysregulated EGFR activation mediates progressive renal injury.

METHODS

To investigate the sex differences in response to renal injury, we examined EGFR expression in mice, in human kidney tissue, and in cultured cell lines.

RESULTS

In wild type mice, renal mRNA and protein EGFR levels were comparable in males and females at postnatal day 7 but were significantly lower in age-matched adult females than in adult males. Similar gender differences in renal EGFR expression were detected in normal adult human kidneys. In Dsk5 mutant mice with a gain-of-function allele that increases basal EGFR kinase activity, males had progressive glomerulopathy, albuminuria, loss of podocytes, and tubulointerstitial fibrosis, but female Dsk5 mice had minimal kidney injury. Oophorectomy had no effect on renal EGFR levels in female Dsk5 mice, while castration protected against the kidney injury in male Dsk5 mice, in association with a reduction in EGFR expression to levels seen in females. Conversely, testosterone increased EGFR expression and renal injury in female Dsk5 mice. Testosterone directly stimulated EGFR expression in cultured kidney cells.

CONCLUSIONS

These studies indicate that differential renal EGFR expression plays a role in the sex differences in susceptibility to progressive kidney injury that may be mediated at least in part by testosterone.

Nephrotic syndrome: immunological mechanisms

Nephrotic Syndrome (NS) is a rare diseases (around 2-7 cases per 100.000 children per year) characterized by proteinuria ≥50 mg/kg/day (or ≥40 mg/m2/h) or a proteinuria/creatininuria ratio >2 (mg/mg); hypoalbuminaemia less than 25 g/l and edema. The protein leakage, with the consequent hypoalbunaemia and edema, due to podocyte alterations may be caused by genetic diseases, immunological mechanisms, infections, toxins or malignancy. However, most commonly the exact etiology is unknow. The idiopathic NS may be classified based on response to corticosteroid therapy or the hytological appearance. The first classification identifies steroid-resistant NS (no response after 4 weeks of steroid therapy); frequently relapsing NS (≥ 2 relapses in first 6 months or ≥4 relapses in 1-year); steroid dependent NS (relapses during steroid decalage or within 2 weeks from steroid therapy interruption). The hystological classification is based on light and electron microscopy after renal biopsy, which is indicated in case of onset disease before 1 year or after 12 years of age. Macroscopic hematuria: persistent hypertension and/or microscopic hematuria and/or low plasma C3 renal failure not related to hypovolemia; steroid resistence: secondary or relatedsyndromes NS. Minimal change disease (MCD) is the most common form of idiopahtic NS in children, with good response to steroid treatment, and it is characterized by normal glomerular appearance on light microscopy and evidence of podocyte foot alterations on electron microscopy, due to immunological related damage. Focal segmental glomerulosclerosis (FSGS) is described inidiopahtic NS, particularly in steroiddependent or steroid-resistant forms, and is characterized by evidence of focal glomerular damage with secondary sclerosis and adhesion with Bowman's capsule; the electron appearance is the same of MCD one. Recent authors hypotizethat the FSGS is an evolution of MCD. These 2 idiopathic NS forms may be expression of the same immunological disease, with 2 different severity grades; so they may be considered different moments of the same disease spectrum. Less common idiopathic NS forms are membrano proliferative glomerulonephritis; membranous nephropathy; IgM-nephropathy; C1q nephropathy and thin basement membrane disease (1, 2, 3).

Role of TRPC6 in Progression of Diabetic Kidney Disease

PURPOSE OF REVIEW

The underlining goal of this review is to offer a concise, detailed look into current knowledge surrounding transient receptor potential canonical channel 6 (TRPC6) in the progression of diabetic kidney disease (DKD).

RECENT FINDINGS

Mutations and over-activation in TRPC6 channel activity lead to the development of glomeruli injury. Angiotensin II, reactive oxygen species, and other factors in the setting of DKD stimulate drastic increases in calcium influx through the TRPC6 channel, causing podocyte hypertrophy and foot process effacement. Loss of the podocytes further promote deterioration of the glomerular filtration barrier and play a major role in the development of both albuminuria and the renal injury in DKD. Recent genetic manipulation with TRPC6 channels in various rodent models provide additional knowledge about the role of TRPC6 in DKD and are reviewed here. The TRPC6 channel has a pronounced role in the progression of DKD, with deviations in activity yielding detrimental outcomes. The benefits of targeting TRPC6 or its upstream or downstream signaling pathways in DKD are prominent.

Losing their footing: Rac1 signaling causes podocyte detachment and FSGS

Selective modulation of Rho GTPase activity in podocytes recapitulates characteristic features of human nephrosis. Using a mouse model, Robins et al. found that high levels of Rac1 activation in podocytes caused podocyte detachment and glomerulosclerosis. Podocyte Rac1 activity was enhanced in biopsy specimens from patients with nephrosis, and serum from this patient population activated Rac1 in cultured podocytes. These data provide a causal link between podocyte Rac1 activation and human nephrotic diseases.

c-Src is in the effector pathway linking uPAR and podocyte injury

The role of urokinase-type plasminogen activator receptor (uPAR) in kidney physiology and pathology has attracted considerable attention. The protein uPAR has dual functions: as a key regulator of plasmin generation and a component of the innate immune system. In the current issue, Wei and colleagues describe a transgenic mouse expressing Plaur RNA in glomerular podocytes. The mice manifested podocyte injury, including c-Src phosphorylation, proteinuria, and focal segmental glomerulosclerosis (FSGS). Plaur-transgenic mice on a β3 integrin-deficient background were protected from podocyte injury. Renal biopsies from subjects with FSGS, but not those with other glomerular diseases, manifested increased c-Src phosphorylation in podocytes. These findings suggest a novel injury mechanism in FSGS, with possible implications for new treatment strategies.

The natural drug DIAVIT is protective in a type II mouse model of diabetic nephropathy

There is evidence to suggest that abnormal angiogenesis, inflammation, and fibrosis drive diabetic nephropathy (DN). However, there is no specific treatment to counteract these processes. We aimed to determine whether DIAVIT, a natural Vaccinium myrtillus (blueberry) and Hippophae Rhamnoides (sea buckthorn) extract, is protective in a model of type II DN. Diabetic db/db mice were administered DIAVIT in their drinking water for 14 weeks. We assessed the functional, structural, and ultra-structural phenotype of three experimental groups (lean+vehicle, db/db+vehicle, db/db+DIAVIT). We also investigated the angiogenic and fibrotic pathways involved in the mechanism of action of DIAVIT. Diabetic db/db mice developed hyperglycaemia, albuminuria, and an increased glomerular water permeability; the latter two were prevented by DIAVIT. db/db mice developed fibrotic glomeruli, endothelial insult, and glomerular ultra-structural changes, which were not present in DIAVIT-treated mice. Vascular endothelial growth factor A (VEGF-A) splicing was altered in the db/db kidney cortex, increasing the pro-angiogenic VEGF-A₁₆₅ relative to the anti-angiogenic VEGF-A₁₆₅b. This was partially prevented with DIAVIT treatment. Delphinidin, an anthocyanin abundant in DIAVIT, increased the VEGF-A₁₆₅b expression relative to total VEGF-A₁₆₅ in cultured podocytes through phosphorylation of the splice factor SRSF6. DIAVIT, in particular delphinidin, alters VEGF-A splicing in type II DN, rescuing the DN phenotype. This study highlights the therapeutic potential of natural drugs in DN through the manipulation of gene splicing and expression.