The renin-angiotensin-aldosterone system in podocytes

The role of perirenal adipose tissue deposition in chronic kidney disease progression: Mechanisms and therapeutic implications

Chronic kidney disease (CKD) represents a significant and escalating global health challenge, with morbidity and mortality rates rising steadily. Evidence increasingly implicates perirenal adipose tissue (PRAT) deposition as a contributing factor in the pathogenesis of CKD. This review explores how PRAT deposition may exert deleterious effects on renal structure and function. The anatomical proximity of PRAT to the kidneys not only potentially causes mechanical compression but also leads to the dysregulated secretion of adipokines and inflammatory mediators, such as adiponectin, leptin, visfatin, tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and exosomes. Additionally, PRAT deposition may contribute to renal lipotoxicity through elevated levels of free fatty acids (FFA), triglycerides (TAG), diacylglycerol (DAG), and ceramides (Cer). PRAT deposition is also linked to the hyperactivation of the renin-angiotensin-aldosterone system (RAAS), which further exacerbates CKD progression. Recognizing PRAT deposition as an independent risk factor for CKD underscores the potential of targeting PRAT as a novel strategy for the prevention and management of CKD. This review further discusses interventions that could include measuring PRAT thickness to establish a baseline, managing metabolic risk factors that promote its deposition, and inhibiting key PRAT-induced signaling pathways.

Mechanism of protective actions of sparsentan in the kidney: lessons from studies in models of chronic kidney disease

Simultaneous inhibition of angiotensin II AT1 and endothelin ETA receptors has emerged as a promising approach for treatment of chronic progressive kidney disease. This therapeutic approach has been advanced by the introduction of sparsentan, the first dual AT1 and ETA receptor antagonist. Sparsentan is a single molecule with high affinity for both receptors. It is US Food and Drug Administration approved for immunoglobulin A nephropathy (IgAN) and is currently being developed as a treatment for rare kidney diseases, such as focal segmental glomerulosclerosis. Clinical studies have demonstrated the efficacy and safety of sparsentan in these conditions. In parallel with clinical development, studies have been conducted to elucidate the mechanisms of action of sparsentan and its position in the context of published evidence characterizing the nephroprotective effects of dual ETA and AT1 receptor inhibition. This review summarizes this evidence, documenting beneficial anti-inflammatory, antifibrotic, and hemodynamic actions of sparsentan in the kidney and protective actions in glomerular endothelial cells, mesangial cells, the tubulointerstitium, and podocytes, thus providing the rationale for the use of sparsentan as therapy for focal segmental glomerulosclerosis and IgAN and suggesting potential benefits in other renal diseases, such as Alport syndrome.

The Impact of Early-Stage Chronic Kidney Disease on Weight Loss Outcomes After Gastric Bypass

PURPOSE

Weight loss achieved through bariatric metabolic surgery was demonstrated to be effective at reversing chronic kidney dysfunction associated with obesity-related glomerulopathy. However, robust data on how pre-operative kidney status impacts on bariatric metabolic surgery weight loss outcomes is still lacking. The aim of this study was to evaluate the impact of kidney dysfunction on weight loss outcomes after bariatric metabolic surgery.

METHODS

Patients with obesity to be submitted to gastric bypass surgery underwent a pre-operative evaluation of creatinine clearance, estimated glomerular filtration rate (eGFR), proteinuria, and albuminuria in 24-hour urine. Body mass index (BMI), % total weight loss (%TWL), and % excess BMI loss (%EBMIL) were assessed at 6 and 12 months after surgery.

RESULTS

Before surgery, patients (N=127) had a mean BMI of 39.6 ± 3.0 kg/m2, and 56.7% (n=72) had a creatinine clearance > 130 mL/min, 23.6% (n= 30) presented proteinuria > 150 mg/24h, and 15.0% (n= 19) presented albuminuria > 30 mg/24h. After surgery, the mean BMI was 27.7 kg/m2 and 25.0 kg/m2 at 6 and 12 months, respectively (p<0.0001). The %TWL was lower in patients with pre-operative eGFR < percentile 25 (34.4 ± 5.8% vs 39.4 ± 4.9%, p=0.0007, at 12 months). There were no significant correlations between weight loss metrics and pre-operative creatinine clearance rate, proteinuria, or albuminuria.

CONCLUSION

Early-stage chronic kidney disease (G2) has a negative impact on short-term weight loss outcomes after bariatric metabolic surgery, albeit in a magnitude inferior to the clinically relevant threshold.

Renal Dysfunction Phenotypes in Patients Undergoing Obesity Surgery

Obesity surgery candidates are at an increased risk of kidney injury, but pre-operative evaluation usually neglects kidney function assessment. This study aimed to identify renal dysfunction in candidates for bariatric surgery. To reduce the sources of bias, subjects with diabetes, prediabetes under metformin treatment, neoplastic or inflammatory diseases were excluded. Patients' (n = 192) average body mass index was 41.7 ± 5.4 kg/m². Among these, 51% (n = 94) had creatinine clearance over 140 mL/min, 22.4% (n = 43) had proteinuria over 150 mg/day and 14.6% (n = 28) albuminuria over 30 mg/day. A creatinine clearance higher than 140 mL/min was associated with higher levels of proteinuria and albuminuria. Univariate analysis identified sex, glycated hemoglobin, uric acid, HDL and VLDL cholesterol as being associated with albuminuria, but not with proteinuria. On multivariate analysis, glycated hemoglobin and creatinine clearance as continuous variables were significantly associated with albuminuria. In summary, in our patient population prediabetes, lipid abnormalities and hyperuricemia were associated with albuminuria, but not with proteinuria, suggesting different disease mechanisms might be implicated. Data suggest that in obesity-associated kidney disease, tubulointerstitial injury precedes glomerulopathy. A significant proportion of obesity surgery candidates present clinically relevant albuminuria and proteinuria along with renal hyperfiltration, suggesting that routine pre-operative assessment of these parameters should be considered.

Upregulation of NADH/NADPH oxidase 4 by angiotensin II induces podocyte apoptosis

Background: Angiotensin II induces glomerular and podocyte injury via systemic and local vasoconstrictive or non-hemodynamic effects including oxidative stress. The release of reactive oxygen species (ROS) from podocytes may participate in the development of glomerular injury and proteinuria. We studied the role of oxidative stress in angiotensin II-induced podocyte apoptosis.Methods: Mouse podocytes were incubated in media containing various concentrations of angiotensin II at different incubation times and were transfected with NADH/NADPH oxidase 4 (Nox4) or angiotensin II type 1 receptor for 24 hours. The changes in intracellular and mitochondrial ROS production and podocyte apoptosis were measured according to the presence of angiotensin II.Results: Angiotensin II increased the generation of mitochondrial superoxide anions and ROS levels but suppressed superoxide dismutase activity in a dose- and time-dependent manner that was reversed by probucol, an antioxidant. Angiotensin II increased Nox4 protein and expression by a transcriptional mechanism that was also reversed by probucol. In addition, the suppression of Nox4 by small interfering RNA (siRNA) reduced the oxidative stress induced by angiotensin II. Angiotensin II treatment also upregulated AT1R protein. Furthermore, angiotensin II promoted podocyte apoptosis, which was reduced significantly by probucol and Nox4 siRNA and also recovered by angiotensin II type 1 receptor siRNA.Conclusion: Our findings suggest that angiotensin II increases the generation of mitochondrial superoxide anions and ROS levels via the upregulation of Nox4 and angiotensin II type 1 receptor. This can be prevented by Nox4 inhibition and/or antagonizing angiotensin II type 1 receptor as well as use of antioxidants.

Is preclinical diabetic retinopathy in diabetic nephropathy individuals more severe?

Purpose

To analyse the retinal vessel density and thickness characteristics of diabetic nephropathy (DN) individuals with preclinical diabetic retinopathy (DR) using optical coherence tomography angiography (OCTA).

Methods

This retrospective case-control study included 88 eyes of 88 type 2 DM patients with preclinical DR [44 non-DN (NDN) and 44 DN]. OCTA images and data were acquired using AngioVue 2.0 of the spectral domain OCT device. The foveal avascular zone (FAZ) area, superficial capillary plexus (SCP) and deep capillary plexus vessel densities, ganglion cell complex (GCC) and full retinal thicknesses, peripapillary capillary density and nerve fibre layer (RNFL) thickness were compared between the NDN and DN groups. The relationship between each renal function parameter and each OCTA parameter was analysed.

Results

SCP vessel density, GCC thickness and full retinal thickness were significantly reduced in DN individuals compared to NDN individuals [(NDN versus DN) SCP vessel density: 46.65 ± 3.84% versus 44.35 ± 5.25%, p=0.030; GCC thickness: 100.79 ± 5.92 μm versus 93.28 ± 8.66 μm, p<0.001; full retinal thickness: whole area: 287.04 ± 13.62 μm versus 277.71 ± 15.10 μm, p=0.005). Within the peripapillary area, capillary density was also significantly reduced in the whole area (50.19 ± 3.10% versus 47.46 ± 5.93%, p=0.016) and some sectors in the DN group, though RNFL thickness was only decreased in some sectors. For all individuals, estimated glomerular filtration rate (eGFR) correlated significantly with most OCTA parameters and then showed a significantly negative correlation with FAZ area (β=-16.43, p=0.039) in multivariate linear regression analysis. In the NDN group, eGFR showed a significantly negative correlation with FAZ area (β=-18.746, p=0.048) and a significantly positive correlation with SCP vessel density (β=0.580, p=0.036).

Conclusion

Preclinical DR may be more severe in DN individuals than in NDN individuals with regard to microvascular and microstructural impairment. Moreover, eGFR may be a good indicator for retinal microvascular impairment.

Tracing angiotensin II's yin-yang effects on cardiovascular-renal pathophysiology

Adverse changes in cardiovascular and renal systems are major contributors to overall morbidity and mortality. Human cardiovascular and renal systems exhibit a complex network of positive and negative feedback that is reflected in the control of vascular tone via angiotensin II (Ang II) based signaling. This review will examine in some depth, the multiple components and processes that control the status and reflect the health of these various cardiovascular and renal systems, such as pathways associated to monomeric G proteins, RhoA/Rho kinase system and ERK, oxidative stress and NO balance. It will specifically emphasize the "yin-yang" nature of Ang II signaling by comparing and contrasting the effects and activity of various systems, pathways and components found in hypertension to those found in Gitelman's and Bartter's syndromes (GS/BS), two rare autosomal recessive tubulopathies characterized by electrolytic imbalance, metabolic alkalosis, sodium wasting and prominent activation of the renin-angiotensin-aldosterone system. Notwithstanding the activation of the renin-angiotensin-aldosterone system, GS/BS are normo-hypotensive and protected from cardiovascular-renal remodeling and therefore can be considered the mirror image, the opposite of hypertension.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Crescents, an Independent Risk Factor for the Progression of Type 2 Diabetic Kidney Disease

CONTEXT

Crescents have been noticed in pathologic changes in patients with diabetic kidney disease (DKD). However, the clinical significance of crescents is still not well recognized.

OBJECTIVE

The main objective was to investigate the association between crescents and the prognoses of type 2 DKD (T2DKD) patients, and, secondly, to analyze the relationship between crescents and clinicopathologic features.

METHODS

A retrospective cohort study of 155 patients with T2DKD diagnosed by renal biopsy was carried out in a single center. Clinicopathologic features of patients with or without crescents were analyzed. Cox regression models and meta-analysis were used to determine the prognostic values of crescents for T2DKD. A nomogram was constructed to provide a simple estimation method of 1, 3, and 5-year renal survival for patients with T2DKD.

RESULTS

Compared with T2DKD patients without crescents, patients with crescents had higher 24-hour proteinuria and serum creatinine levels, as well as more severe Kimmelstiel-Wilson (K-W) nodules, segmental sclerosis (SS), and mesangiolysis (all P < .05). Furthermore, the crescents were positively correlated with serum creatinine, 24-hour proteinuria, K-W nodules, SS, mesangiolysis, and complement 3 deposition. Multivariate Cox models showed that crescents were an independent prognostic risk factor for renal survival (hazard ratio [HR] 2.68, 95% CI 1.27-5.64). The meta-analyzed results of 4 studies on crescents in T2DKD confirmed that patients with crescents had a significantly higher HR for renal progression.

CONCLUSION

Patients with crescents in T2DKD have more severe clinicopathologic changes and worse prognoses. The crescent can serve as an independent risk factor for T2DKD progression.

Up-Date on Diabetic Nephropathy

Diabetes is one of the leading causes of kidney disease. Diabetic kidney disease (DKD) is a major cause of end-stage kidney disease (ESKD) worldwide, and it is linked to an increase in cardiovascular (CV) risk. Diabetic nephropathy (DN) increases morbidity and mortality among people living with diabetes. Risk factors for DN are chronic hyperglycemia and high blood pressure; the renin-angiotensin-aldosterone system blockade improves glomerular function and CV risk in these patients. Recently, new antidiabetic drugs, including sodium-glucose transport protein 2 inhibitors and glucagon-like peptide-1 agonists, have demonstrated additional contribution in delaying the progression of kidney disease and enhancing CV outcomes. The therapeutic goal is regression of albuminuria, but an atypical form of non-proteinuric diabetic nephropathy (NP-DN) is also described. In this review, we provide a state-of-the-art evaluation of current treatment strategies and promising emerging treatments.

Klotho-derived peptide 6 ameliorates diabetic kidney disease by targeting Wnt/β-catenin signaling

Diabetic kidney disease (DKD) is one of the most common and devastating complications of diabetic mellitus, and its prevalence is rising worldwide. Klotho, an anti-aging protein, is kidney protective in DKD. However, its large size, prohibitive cost and structural complexity hamper its potential utility in clinics. Here we report that Klotho-derived peptide 6 (KP6) mimics Klotho function and ameliorates DKD. In either an accelerated model of DKD induced by streptozotocin and advanced oxidation protein products in unilateral nephrectomized mice or db/db mice genetically prone to diabetes, chronic infusion of KP6 reversed established proteinuria, attenuated glomerular hypertrophy, mitigated podocyte damage, and ameliorated glomerulosclerosis and interstitial fibrotic lesions, but did not affect serum phosphorus and calcium levels. KP6 inhibited β-catenin activation in vivo and blocked the expression of its downstream target genes in glomerular podocytes and tubular epithelial cells. In vitro, KP6 prevented podocyte injury and inhibited β-catenin activation induced by high glucose without affecting Wnt expression. Co-immunoprecipitation revealed that KP6 bound to Wnt ligands and disrupted the engagement of Wnts with low density lipoprotein receptor-related protein 6, thereby interrupting Wnt/β-catenin signaling. Mutated KP6 with a scrambled amino acid sequence failed to bind Wnts and did not alleviate DKD in db/db mice. Thus, our studies identified KP6 as a novel Klotho-derived peptide that ameliorated DKD by blocking Wnt/β-catenin. Hence, our findings also suggest a new therapeutic strategy for the treatment of patients with DKD.

Flexible and multifaceted: the plasticity of renin-expressing cells

The protease renin, the key enzyme of the renin–angiotensin–aldosterone system, is mainly produced and secreted by juxtaglomerular cells in the kidney, which are located in the walls of the afferent arterioles at their entrance into the glomeruli. When the body’s demand for renin rises, the renin production capacity of the kidneys commonly increases by induction of renin expression in vascular smooth muscle cells and in extraglomerular mesangial cells. These cells undergo a reversible metaplastic cellular transformation in order to produce renin. Juxtaglomerular cells of the renin lineage have also been described to migrate into the glomerulus and differentiate into podocytes, epithelial cells or mesangial cells to restore damaged cells in states of glomerular disease. More recently, it could be shown that renin cells can also undergo an endocrine and metaplastic switch to erythropoietin-producing cells. This review aims to describe the high degree of plasticity of renin-producing cells of the kidneys and to analyze the underlying mechanisms.

Diabetic Nephropathy: Challenges in Pathogenesis, Diagnosis, and Treatment

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease worldwide. Chronic hyperglycemia and high blood pressure are the main risk factors for the development of DN. In general, screening for microalbuminuria should be performed annually, starting 5 years after diagnosis in type 1 diabetes and at diagnosis and annually thereafter in type 2 diabetes. Standard therapy is blood glucose and blood pressure control using the renin-angiotensin system blockade, targeting A1c < 7%, and <130/80 mmHg. Regression of albuminuria remains an important therapeutic goal. However, there are problems in diagnosis and treatment of nonproteinuric DN (NP-DN), which does not follow the classic pattern of DN. In fact, the prevalence of DN continues to increase, and additional therapy is needed to prevent or ameliorate the condition. In addition to conventional therapies, vitamin D receptor activators, incretin-related drugs, and therapies that target inflammation may also be promising for the prevention of DN progression. This review focuses on the role of inflammation and oxidative stress in the pathogenesis of DN, approaches to diagnosis in classic and NP-DN, and current and emerging therapeutic interventions.

The Pivotal Role of Oxidative Stress in the Pathophysiology of Cardiovascular-Renal Remodeling in Kidney Disease

The excessive activation of the renin-angiotensin system in kidney disease leads to alteration of intracellular pathways which concur altogether to the induction of cardiovascular and renal remodeling, exposing these patients since the very beginning of the renal injury to chronic kidney disease and progression to end stage renal disease, a very harmful and life threatening clinical condition. Oxidative stress plays a pivotal role in the pathophysiology of renal injury and cardiovascular-renal remodeling, the long-term consequence of its effect. This review will examine the role of oxidative stress in the most significant pathways involved in cardiovascular and renal remodeling with a focus on the detrimental effects of oxidative stress-mediated renal abnormalities on the progression of the disease and of its complications. Food for thoughts on possible therapeutic target are proposed on the basis of experimental evidences.

Calpastatin prevents Angiotensin II-mediated podocyte injury through maintenance of autophagy

The strong predictive value of proteinuria in chronic glomerulopathies is firmly established as well as the pathogenic role of angiotensin II promoting progression of glomerular disease with an altered glomerular filtration barrier, podocyte injury and scarring of glomeruli. Here we found that chronic angiotensin II-induced hypertension inhibited autophagy flux in mouse glomeruli. Deletion of Atg5 (a gene encoding a protein involved autophagy) specifically in the podocyte resulted in accelerated angiotensin II-induced podocytopathy, accentuated albuminuria and glomerulosclerosis. This indicates that autophagy is a key protective mechanism in the podocyte in this condition. Angiotensin-II induced calpain activity in podocytes inhibits autophagy flux. Podocytes from mice with transgenic expression of the endogenous calpain inhibitor calpastatin displayed higher podocyte autophagy at baseline that was resistant to angiotensin II-dependent inhibition. Also, sustained autophagy with calpastatin limited podocyte damage and albuminuria. These findings suggest that hypertension has pathogenic effects on the glomerular structure and function, in part through activation of calpains leading to blockade of podocyte autophagy. These findings uncover an original mechanism whereby angiotensin II-mediated hypertension inhibits autophagy via calcium-induced recruitment of calpain with pathogenic consequences in case of imbalance by calpastatin activity. Thus, preventing a calpain-mediated decrease in autophagy may be a promising new therapeutic strategy for nephropathies associated with high renin-angiotensin system activity.

Efficacy and safety of Shenyankangfu Tablet, a Chinese patent medicine, for primary glomerulonephritis: A multicenter randomized controlled trial

BACKGROUND

Shenyankangfu Tablet (SYKFT) is a Chinese patent medicine that has been used widely to decrease proteinuria and the progression of chronic kidney disease.

OBJECTIVE

This trial compared the efficacy and safety of SYKFT, for the control of proteinuria in primary glomerulonephritis patients, against the standard drug, losartan potassium.

DESIGN, SETTING, PARTICIPANTS AND INTERVENTION

This was a multicenter, double-blind, randomized, controlled clinical trial. Primary glomerulonephritis patients, aged 18-70 years, with blood pressure ≤ 140/90 mmHg, estimated glomerular filtration rate (eGFR) ≥ 45 mL/min per 1.73 m², and 24-hour proteinuria level of 0.5-3.0 g, were recruited in 41 hospitals across 19 provinces in China and were randomly divided into five groups: SYKFT, losartan potassium 50 mg or 100 mg, SYKFT plus losartan potassium 50 mg or 100 mg.

MAIN OUTCOME MEASURES

The primary outcome was change in the 24-hour proteinuria level, after 48 weeks of treatment.

RESULTS

A total of 735 participants were enrolled. The percent decline of urine protein quantification in the SYKFT group after 48 weeks was 8.78% ± 2.56% (P = 0.006) more than that in the losartan 50 mg group, which was 0.51% ± 2.54% (P = 1.000) less than that in the losartan 100 mg group. Compared with the losartan potassium 50 mg group, the SYKFT plus losartan potassium 50 mg group had a 13.39% ± 2.49% (P < 0.001) greater reduction in urine protein level. Compared with the losartan potassium 100 mg group, the SYKFT plus losartan potassium 100 mg group had a 9.77% ± 2.52% (P = 0.001) greater reduction in urine protein. With a superiority threshold of 15%, neither was statistically significant. eGFR, serum creatinine and serum albumin from the baseline did not change statistically significant. The average change in TCM syndrome score between the patients who took SYKFT (-3.00 [-6.00, -2.00]) and who did not take SYKFT (-2.00 [-5.00, 0]) was statistically significant (P = 0.003). No obvious adverse reactions were observed in any group.

CONCLUSION

SYKFT decreased the proteinuria and improved the TCM syndrome scores of primary glomerulonephritis patients, with no change in the rate of decrease in the eGFR. SYKFT plus losartan potassium therapy decreased proteinuria more than losartan potassium therapy alone.

TRIAL REGISTRATION NUMBER

NCT02063100 on ClinicalTrials.gov.

Cardiac and renal protective effects of 2,5-dimethylcelecoxib in angiotensin II and high-salt-induced hypertension model mice

BACKGROUND

We reported that 2,5-dimethylcelecoxib (DM-celecoxib), a celecoxib derivative that is unable to inhibit cyclooxygenase-2, prevented cardiac remodeling induced by sarcomeric gene mutation, left ventricular pressure overload, or β-adrenergic receptor stimulation. This effect seemed to be mediated by the inhibition of the canonical Wnt/β-catenin signaling pathway, which has been suggested to play a key role in the development of chronic kidney disease and chronic heart failure.

METHOD

We investigated the effect of DM-celecoxib on cardiac remodeling and kidney injury in hypertension model mice induced by angiotensin II infusion in the absence or presence of high-salt load.

RESULTS

DM-celecoxib prevented cardiac remodeling and markedly reduced urinary albumin excretion without altering blood pressure in those mice. Moreover, DM-celecoxib prevented podocyte injury, glomerulosclerosis, and interstitial fibrosis in the kidney of mice loaded with angiotensin II and high-salt load. DM-celecoxib reduced the phosphorylation level of Akt and activated glycogen synthase kinase-3, which led to the suppression of the Wnt/β-catenin signal in the heart and kidney. DM-celecoxib also reduced the expression level of snail, a key transcription factor for the epithelial-mesenchymal transition and of which gene is a target of the Wnt/β-catenin signal.

CONCLUSION

Results of the current study suggested that DM-celecoxib could be beneficial for patients with hypertensive heart and kidney diseases.

LIM and SH3 protein 1 (LASP-1): A novel link between the slit membrane and actin cytoskeleton dynamics in podocytes

The foot processes of podocytes exhibit a dynamic actin cytoskeleton, which maintains their complex cell structure and antagonizes the elastic forces of the glomerular capillary. Interdigitating secondary foot processes form a highly selective filter for proteins in the kidney, the slit membrane. Knockdown of slit membrane components such as Nephrin or Neph1 and cytoskeletal adaptor proteins such as CD2AP in mice leads to breakdown of the filtration barrier with foot process effacement, proteinuria, and early death of the mice. Less is known about the crosstalk between the slit membrane-associated proteins and cytoskeletal components inside the podocyte foot processes. Our study shows that LASP-1, an actin-binding protein, is highly expressed in podocytes. Electron microscopy studies demonstrate that LASP-1 is found at the slit membrane suggesting a role in anchoring slit membrane components to the actin cytoskeleton. Live cell imaging experiments with transfected podocytes reveal that LASP-1 is either part of a highly dynamic granular complex or a static, actin cytoskeleton-bound protein. We identify CD2AP as a novel LASP-1 binding partner that regulates its association with the actin cytoskeleton. Activation of the renin-angiotensin-aldosterone system, which is crucial for podocyte function, leads to phosphorylation and altered localization of LASP-1. In vivo studies using the Drosophila nephrocyte model indicate that Lasp is necessary for the slit membrane integrity and functional filtration.

DUET: A Phase 2 Study Evaluating the Efficacy and Safety of Sparsentan in Patients with FSGS

BACKGROUND

We evaluated and compared the effects of sparsentan, a dual endothelin type A (ET_A) and angiotensin II type 1 receptor antagonist, with those of the angiotensin II type 1 receptor antagonist irbesartan in patients with primary FSGS.

METHODS

In this phase 2, randomized, double-blind, active-control Efficacy and Safety of Sparsentan (RE-021), a Dual Endothelin Receptor and Angiotensin Receptor Blocker, in Patients with Focal Segmental Glomerulosclerosis (FSGS): A Randomized, Double-blind, Active-Control, Dose-Escalation Study (DUET), patients aged 8-75 years with biopsy-proven FSGS, eGFR>30 ml/min per 1.73 m², and urinary protein-to-creatinine ratio (UP/C) ≥1.0 g/g received sparsentan (200, 400, or 800 mg/d) or irbesartan (300 mg/d) for 8 weeks, followed by open-label sparsentan only. End points at week 8 were reduction from baseline in UP/C (primary) and proportion of patients achieving FSGS partial remission end point (FPRE) (UP/C: ≤1.5 g/g and >40% reduction [secondary]).

RESULTS

Of 109 patients randomized, 96 received study drugs and had baseline and week 8 UP/C measurements. Sparsentan-treated patients had greater reductions in UP/C than irbesartan-treated patients did when all doses (45% versus 19%; P=0.006) or the 400 and 800 mg doses (47% versus 19%; P=0.01) were pooled for analysis. The FSGS partial remission end point was achieved in 28% of sparsentan-treated and 9% of irbesartan-treated patients (P=0.04). After 8 weeks of treatment, BP was reduced with sparsentan but not irbesartan, and eGFR was stable with both treatments. Overall, the incidence of adverse events was similar between groups. Hypotension and edema were more common among sparsentan-treated patients but did not result in study withdrawals.

CONCLUSIONS

Patients with FSGS achieved significantly greater reductions in proteinuria after 8 weeks of sparsentan versus irbesartan. Sparsentan was safe and well tolerated.

Cellular and molecular mechanisms of kidney fibrosis

Renal fibrosis is the final pathological process common to any ongoing, chronic kidney injury or maladaptive repair. It is considered as the underlying pathological process of chronic kidney disease (CKD), which affects more than 10% of world population and for which treatment options are limited. Renal fibrosis is defined by excessive deposition of extracellular matrix, which disrupts and replaces the functional parenchyma that leads to organ failure. Kidney's histological structure can be divided into three main compartments, all of which can be affected by fibrosis, specifically termed glomerulosclerosis in glomeruli, interstitial fibrosis in tubulointerstitium and arteriosclerosis and perivascular fibrosis in vasculature. In this review, we summarized the different appearance, cellular origin and major emerging processes and mediators of fibrosis in each compartment. We also depicted and discussed the challenges in translation of anti-fibrotic treatment to clinical practice and discuss possible solutions and future directions.

Glucocorticoids in the treatment of patients with primary focal segmental glomerulosclerosis and moderate proteinuria

BACKGROUND

To compare the efficacy of glucocorticoids in primary focal segmental glomerulosclerosis (pFSGS) patients with moderate proteinuria. Registered at http://www.chictr.org.cn/ , study No. ChiCTR-OPN-17012789.

METHODS

pFSGS patients with urine protein between 1.0 and 3.5 g/24 h were recruited from 2006 to 2016. No decline in urine protein > 50% was observed after 2 months of run-in angiotensin-converting enzyme inhibitors/angiotensin-receptor blockers (ACEI/ARB) treatment. Patients were assigned to study group (glucocorticoids with ACEI/ARB) or control group (ACEI/ARB without glucocorticoids). Variables including 24-h urinary protein, serum albumin and serum creatinine during the trial were recorded. Remission was defined as proteinuria < 0.3 g/24 h or declined > 50%, and our composite end point as > 30% decrease of eGFR or eGFR < 30 ml/min.

RESULTS

A total of 102 patients were enrolled (study group N = 52, control group N = 50), and the median follow-up time was 36 (12-117) months without significant difference between groups. During the 12-month follow-up, the remission rate was significantly higher in study group [73.1 vs 50.0% (P = 0.01)], and the initial median response time was 3 months in the study group while 6 in the control group. The end point was reached by 22.2% cases in study group, and 42.0% in control. The medium survival times were study group 72 months and control 57 (P = 0.03). Minor adverse reactions were observed in 10 patients (study group N = 8, control group N = 2).

CONCLUSIONS

Additional glucocorticoids therapy is more efficacious compared to ACEI/ARB alone in the treatment of patients with pFSGS and moderate proteinuria.

A Microfluidic Platform for Investigating Transmembrane Pressure-Induced Glomerular Leakage

Transmembrane pressure across the glomerular filter barrier may underlie renal failure. However, studies of renal failure have been difficult owing to a lack of in vitro models to capture the transmembrane pressure in a controlled approach. Here we report a microfluidic platform of podocyte culture to investigate transmembrane pressure induced glomerular leakage. Podocytes, the glomerular epithelial cells essential for filtration function, were cultivated on a porous membrane supplied with transmembrane pressure ΔP. An anodic aluminum oxide membrane with collagen coating was used as the porous membrane, and the filtration function was evaluated using dextrans of different sizes. The results show that dextran in 20 kDa and 70 kDa can penetrate the podocyte membrane, whereas dextran in 500 kDa was blocked until ΔP ≥ 60 mmHg, which resembles the filtration function when ΔP was in the range of a healthy kidney (ΔP < 60 mmHg) as well as the hypertension-induced glomerular leakage (ΔP ≥ 60 mmHg). Additionally, analysis showed that synaptopodin and actin were also downregulated when ΔP > 30 mmHg, indicating that the dysfunction of renal filtration is correlated with the reduction of synaptopodin expression and disorganized actin cytoskeleton. Taking together, our microfluidic platform enables the investigation of transmembrane pressure in glomerular filter membrane, with potential implications for drug development in the future.

Angiotensin II Modulates Podocyte Glucose Transport

Podocytes play a central role in the maintenance of the glomerular filtration barrier and are cellular targets of angiotensin II (AngII). Non-hemodynamic pathways of AngII signaling regulate cellular function and mediate podocyte abnormalities that are associated with various glomerulopathies, including diabetic kidney disease. In this study we investigated the capacity of AngII to modulate glucose uptake in mouse podocytes expressing the human AT1 receptor (AT1R+) after 5 days of exposure to normal (NG, 5.6 mmol/L) or to high (HG, 30 mmol/L) glucose. Short (30 min) as well as long-term (24 h) incubations with AngII markedly enhanced glucose transport in both NG and HG cells. In podocytes cultured under NG conditions, AngII inhibited insulin-stimulated glucose uptake. Regardless of the presence or absence of AngII, no effect of insulin on glucose uptake was observed in HG cells. Stimulation of glucose transport by AngII was mediated by protein kinase C and by phosphoinositide 3-kinase. Glucose dependent surface expression of the glucose transporters GLUT1, GLUT2, and GLUT4 was modulated by AngII in a time and glucose concentration dependent manner. Furthermore, despite its inhibitory effect on insulin's action, AngII elevated the number of podocyte insulin receptors in both NG and HG cultured cells. These findings demonstrate that AngII modulates podocyte basal, as well as insulin-dependent glucose uptake by regulating glucose transporters and insulin signaling.

Hypertensive nephropathy. Moving from classic to emerging pathogenetic mechanisms

Hypertensive kidney disease classically entails nephroangiosclerosis and hyalinosis with glomerular damage. However, in recent years, several evidences showed that high blood pressure also injures tubular cells, inducing epithelial-to-mesenchymal transition and tubulointerstitial fibrosis. Recently investigated mechanisms are also podocyte effacement and loss, which lead to denudation of the glomerular basement membrane and focal adhesion of the tufts to the Bowman's capsule, with reduced filtration and scars. Starting from the classic concept of nephroangiosclerosis, this review examines the recently emerged knowledge of new biochemical and molecular mechanisms underlying the kidney damage in hypertension and discusses how viable podocytes or podocyte-deriving proteins are promising tools for early diagnosis of renal remodelling in hypertension.

Angiotensin II regulates phosphorylation of actin-associated proteins in human podocytes

Within the kidney, angiotensin II (AngII) targets different cell types in the vasculature, tubuli, and glomeruli. An important part of the renal filtration barrier is composed of podocytes with their actin-rich foot processes. In this study, we used stable isotope labeling with amino acids in cell culture coupled to mass spectrometry to characterize relative changes in the phosphoproteome of human podocytes in response to short-term treatment with AngII. In 4 replicates, we identified a total of 17,956 peptides that were traceable to 2081 distinct proteins. Bioinformatic analyses revealed that among the increasingly phosphorylated peptides are predominantly peptides that are related to actin filaments, cytoskeleton, lamellipodia, mammalian target of rapamycin, and MAPK signaling. Among others, this screening approach highlighted the increased phosphorylation of actin-bundling protein, l-plastin (LCP1). AngII-dependent phosphorylation of LCP1 in cultured podocytes was mediated by the kinases ERK, p90 ribosomal S6 kinase, PKA, or PKC. LCP1 phosphorylation increased filopodia formation. In addition, treatment with AngII led to LCP1 redistribution to the cell margins, membrane ruffling, and formation of lamellipodia. Our data highlight the importance of AngII-triggered actin cytoskeleton-associated signal transduction in podocytes.-Schenk, L. K., Möller-Kerutt, A., Klosowski, R., Wolters, D., Schaffner-Reckinger, E., Weide, T., Pavenstädt, H., Vollenbröker, B. Angiotensin II regulates phosphorylation of actin-associated proteins in human podocytes.

Deficiency of the Angiotensinase Aminopeptidase A Increases Susceptibility to Glomerular Injury

Aminopeptidase A (APA) is expressed in glomerular podocytes and tubular epithelia and metabolizes angiotensin II (AngII), a peptide known to promote glomerulosclerosis. In this study, we tested whether APA expression changes in response to progressive nephron loss or whether APA exerts a protective role against glomerular damage and during AngII-mediated hypertensive kidney injury. At advanced stages of FSGS, fawn-hooded hypertensive rat kidneys exhibited distinctly increased APA staining in areas of intact glomerular capillary loops. Moreover, BALB/c APA-knockout (KO) mice injected with a nephrotoxic serum showed persistent glomerular hyalinosis and albuminuria 96 hours after injection, whereas wild-type controls achieved virtually full recovery. We then tested the effect of 4-week infusion of AngII (400 ng/kg per minute) in APA-KO and wild-type mice. Although we observed no significant difference in achieved systolic BP, AngII-treated APA-KO mice developed a significant rise in albuminuria not observed in AngII-treated wild-type mice along with increased segmental and global sclerosis and/or collapse of juxtamedullary glomeruli, microcystic tubular dilation, and tubulointerstitial fibrosis. In parallel, AngII treatment significantly increased the kidney AngII content and attenuated the expression of podocyte nephrin in APA-KO mice but not in wild-type controls. These data show that deficiency of APA increases susceptibility to glomerular injury in BALB/c mice. The augmented AngII-mediated kidney injury observed in association with increased intrarenal AngII accumulation in the absence of APA suggests a protective metabolizing role of APA in AngII-mediated glomerular diseases.

Glomerular cell crosstalk

Purpose of review

Glomerular filtration occurs in specialized, microscopic organelles. Each glomerulus contains unique cells and these cooperate to maintain normal filtration. Phenomenal adaptation is required for the glomerulus to respond to variable mechanical loads and this adaptation requires efficient communication between the resident cells. This review will focus on the latest discoveries related to signalling events that mediate the crosstalk between glomerular cells, and detail how disease processes can influence normal regulation.

Recent findings

New data indicate that the crosstalk between glomerular cells involves an increasing number of secreted signalling ligands that act in an autocrine or paracrine fashion. Furthermore, extended roles for some of the classical signalling molecules have been described and there is emerging evidence of therapeutic strategies to manipulate cellular crosstalk. The glomerular extracellular matrix harbours many of these signalling ligands, acting as a reservoir and presenting ligands to cell surface receptors. Signals can also be transferred between cells by extracellular vesicles and this is an emerging concept in cellular crosstalk.

Summary

Recent discoveries are building our understanding about glomerular cell crosstalk, and this review focuses on growth factors and signalling peptides, methods of delivery to target cells, and the potential for developing new therapies for glomerular disease.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Podocyte Depletion in Thin GBM and Alport Syndrome

The proximate genetic cause of both Thin GBM and Alport Syndrome (AS) is abnormal α3, 4 and 5 collagen IV chains resulting in abnormal glomerular basement membrane (GBM) structure/function. We previously reported that podocyte detachment rate measured in urine is increased in AS, suggesting that podocyte depletion could play a role in causing progressive loss of kidney function. To test this hypothesis podometric parameters were measured in 26 kidney biopsies from 21 patients aged 2–17 years with a clinic-pathologic diagnosis including both classic Alport Syndrome with thin and thick GBM segments and lamellated lamina densa [n = 15] and Thin GBM cases [n = 6]. Protocol biopsies from deceased donor kidneys were used as age-matched controls. Podocyte depletion was present in AS biopsies prior to detectable histologic abnormalities. No abnormality was detected by light microscopy at <30% podocyte depletion, minor pathologic changes (mesangial expansion and adhesions to Bowman’s capsule) were present at 30–50% podocyte depletion, and FSGS was progressively present above 50% podocyte depletion. eGFR did not change measurably until >70% podocyte depletion. Low level proteinuria was an early event at about 25% podocyte depletion and increased in proportion to podocyte depletion. These quantitative data parallel those from model systems where podocyte depletion is the causative event. This result supports a hypothesis that in AS podocyte adherence to the GBM is defective resulting in accelerated podocyte detachment causing progressive podocyte depletion leading to FSGS-like pathologic changes and eventual End Stage Kidney Disease. Early intervention to reduce podocyte depletion is projected to prolong kidney survival in AS.

Dual inhibition of renin-angiotensin-aldosterone system and endothelin-1 in treatment of chronic kidney disease

Inhibition of the renin-angiotensin-aldosterone system (RAAS) plays a pivotal role in treatment of chronic kidney diseases (CKD). However, reversal of the course of CKD or at least long-term stabilization of renal function are often difficult to achieve, and many patients still progress to end-stage renal disease. New treatments are needed to enhance protective actions of RAAS inhibitors (RAASis), such as angiotensin-converting enzyme (ACE) inhibitors (ACEIs) or angiotensin receptor blockers (ARBs), and improve prognosis in CKD patients. Inhibition of endothelin (ET) system in combination with established RAASis may represent such an approach. There are complex interactions between both systems and similarities in their renal physiological and pathophysiological actions that provide theoretical rationale for combined inhibition. This view is supported by some experimental studies in models of both diabetic and nondiabetic CKD showing that a combination of RAASis with ET receptor antagonists (ERAs) ameliorate proteinuria, renal structural changes, and molecular markers of glomerulosclerosis, renal fibrosis, or inflammation more effectively than RAASis or ERAs alone. Practically all clinical studies exploring the effects of RAASis and ERAs combination in nephroprotection have thus far applied add-on designs, in which an ERA is added to baseline treatment with ACEIs or ARBs. These studies, conducted mostly in patients with diabetic nephropathy, have shown that ERAs effectively reduce residual proteinuria in patients with baseline RAASis treatment. Long-term studies are currently being conducted to determine whether promising antiproteinuric effects of the dual blockade will be translated in long-term nephroprotection with acceptable safety profile.

Renin-Angiotensin-Aldosterone System Inhibition Increases Podocyte Derivation from Cells of Renin Lineage

Because adult podocytes cannot proliferate and are therefore unable to self-renew, replacement of these cells depends on stem/progenitor cells. Although podocyte number is higher after renin-angiotensin-aldosterone system (RAAS) inhibition in glomerular diseases, the events explaining this increase are unclear. Cells of renin lineage (CoRL) have marked plasticity, including the ability to acquire a podocyte phenotype. To test the hypothesis that RAAS inhibition partially replenishes adult podocytes by increasing CoRL number, migration, and/or transdifferentiation, we administered tamoxifen to Ren1cCreERxRs-tdTomato-R CoRL reporter mice to induce permanent labeling of CoRL with red fluorescent protein variant tdTomato. We then induced experimental FSGS, typified by abrupt podocyte depletion, with a cytopathic antipodocyte antibody. RAAS inhibition by enalapril (angiotensin-converting enzyme inhibitor) or losartan (angiotensin-receptor blocker) in FSGS mice stimulated the proliferation of CoRL, increasing the reservoir of these cells in the juxtaglomerular compartment (JGC). Compared with water or hydralazine, RAAS inhibition significantly increased the migration of CoRL from the JGC to the intraglomerular compartment (IGC), with more glomeruli containing RFP⁺CoRL and, within these glomeruli, more RFP⁺CoRL. Moreover, RAAS inhibition in FSGS mice increased RFP⁺CoRL transdifferentiation in the IGC to phenotypes, consistent with those of podocytes (coexpression of synaptopodin and Wilms tumor protein), parietal epithelial cells (PAX 8), and mesangial cells (α8 integrin). These results show that in the context of podocyte depletion in FSGS, RAAS inhibition augments CoRL proliferation and plasticity toward three different glomerular cell lineages.

Reactive oxygen species-initiated autophagy opposes aldosterone-induced podocyte injury

Evidence has demonstrated that aldosterone (Aldo) is involved in the development and progression of chronic kidney diseases. The purpose of the present study was to investigate the role of autophagy in Aldo-induced podocyte damage and the underlying mechanism. Mouse podocytes were treated with Aldo in the presence or absence of 3-methyladenine and N-acetylcysteine. Cell apoptosis was investigated by detecting annexin V conjugates, apoptotic bodies, caspase-3 activity, and alterations of the podocyte protein nephrin. Autophagy was evaluated by measuring the expressions of light chain 3, p62, beclin-1, and autophagy-related gene 5. Aldo (10^-7 mol/l) induced podocyte apoptosis, autophagy, and downregulation of nephrin protein in a time-dependent manner. Aldo-induced apoptosis was further promoted by the inhibition of autophagy via 3-methyladenine and autophagy-related gene 5 small interfering RNA pretreatment. Moreover, Aldo time dependently increased ROS generation, and H₂O₂ (10^-4 mol/l) application remarkably elevated podocyte autophagy. After treatment with N-acetylcysteine, the autophagy induced by Aldo or H₂O₂ was markedly attenuated, suggesting a key role of ROS in mediating autophagy formation in podocytes. Inhibition of ROS could also lessen Aldo-induced podocyte injury. Taken together, our findings suggest that ROS-triggered autophagy played a protective role against Aldo-induced podocyte injury, and targeting autophagy in podocytes may represent a new therapeutic strategy for the treatment of podocytopathy.

Cellular and Molecular Mechanisms of Chronic Kidney Disease with Diabetes Mellitus and Cardiovascular Diseases as Its Comorbidities

Chronic kidney disease (CKD), diabetes mellitus (DM), and cardiovascular diseases (CVD) are complex disorders of partly unknown genesis and mostly known progression factors. CVD and DM are the risk factors of CKD and are strongly intertwined since DM can lead to both CKD and/or CVD, and CVD can lead to kidney disease. In recent years, our knowledge of CKD, DM, and CVD has been expanded and several important experimental, clinical, and epidemiological associations have been reported. The tight cellular and molecular interactions between the renal, diabetic, and cardiovascular systems in acute or chronic disease settings are becoming increasingly evident. However, the (patho-) physiological basis of the interactions of CKD, DM, and CVD with involvement of multiple endogenous and environmental factors is highly complex and our knowledge is still at its infancy. Not only single pathways and mediators of progression of these diseases have to be considered in these processes but also the mutual interactions of these factors are essential. The recent advances in proteomics and integrative analysis technologies have allowed rapid progress in analyzing complex disorders and clearly show the opportunity for new efficient and specific therapies. More than a dozen pathways have been identified so far, including hyperactivity of the renin–angiotensin (RAS)–aldosterone system, osmotic sodium retention, endothelial dysfunction, dyslipidemia, RAS/RAF/extracellular-signal-regulated kinase pathway, modification of the purinergic system, phosphatidylinositol 3-kinase (PI 3-kinase)-dependent signaling pathways, and inflammation, all leading to histomorphological alterations of the kidney and vessels of diabetic and non-diabetic patients. Since a better understanding of the common cellular and molecular mechanisms of these diseases may be a key to successful identification of new therapeutic targets, we review in this paper the current literature about cellular and molecular mechanisms of CKD.

AT1R blockade in adverse milieus: role of SMRT and corepressor complexes

ANG II type 1 receptor blockade (AT1R-BLK) is used extensively to slow down the progression of proteinuric kidney diseases. We hypothesized that AT1R-BLK provides podocyte protection through regulation of silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and vitamin D receptor (VDR) expression under adverse milieus such as high glucose and human immunodeficiency virus infection. Both AT1R-BLK and VDR agonists (VDAs) stimulated VDR complex formation that differed not only in their composition but also in their functionality. AT1R-BLK-induced VDR complexes contained predominantly unliganded VDR, SMRT, and phosphorylated histone deacetylase 3, whereas VDA-VDR complexes were constituted by liganded VDR and CREB-binding protein/p300. AT1R-BLK-induced complexes attenuated podocyte acetyl-histone 3 levels as well as cytochrome P-450 family 24A1 expression, thus indicating their deacetylating and repressive properties. On the other hand, VDA-VDR complexes not only increased podocyte acetyl-histone 3 levels but also enhanced cytochrome P-450 family 24A1 expression, thus suggesting their acetylating and gene activation properties. AT1R-BLK- induced podocyte SMRT inhibited expression of the proapoptotic gene BAX through downregulation of Wip1 and phosphorylation of checkpoint kinase 2 in high-glucose milieu. Since SMRT-depleted podocytes lacked AT1R-BLK-mediated protection against DNA damage, it appears that SMRT is necessary for DNA repairs during AT1R-BLK. We conclude that AT1R-BLK provides podocyte protection in adverse milieus predominantly through SMRT expression and partly through unliganded VDR expression in 1,25(OH)2D-deficient states; on the other hand, AT1R-BLK contributes to liganded VDR expression in 1,25(OH)2D-sufficient states.

Wnt/β-catenin signalling and podocyte dysfunction in proteinuric kidney disease

Podocytes are unique, highly specialized, terminally differentiated cells that are integral components of the kidney glomerular filtration barrier. Podocytes are vulnerable to a variety of injuries and in response they undergo a series of changes ranging from hypertrophy, autophagy, dedifferentiation, mesenchymal transition and detachment to apoptosis, depending on the nature and extent of the insult. Emerging evidence indicates that Wnt/β-catenin signalling has a central role in mediating podocyte dysfunction and proteinuria. Wnts are induced and β-catenin is activated in podocytes in various proteinuric kidney diseases. Genetic or pharmacologic activation of β-catenin is sufficient to impair podocyte integrity and causes proteinuria in healthy mice, whereas podocyte-specific ablation of β-catenin protects against proteinuria after kidney injury. Mechanistically, Wnt/β-catenin controls the expression of several key mediators implicated in podocytopathies, including Snail1, the renin-angiotensin system and matrix metalloproteinase 7. Wnt/β-catenin also negatively regulates Wilms tumour protein, a crucial transcription factor that safeguards podocyte integrity. Targeted inhibition of Wnt/β-catenin signalling preserves podocyte integrity and ameliorates proteinuria in animal models. This Review highlights advances in our understanding of the pathomechanisms of Wnt/β-catenin signalling in mediating podocyte injury, and describes the therapeutic potential of targeting this pathway for the treatment of proteinuric kidney disease.

The Hippo pathway is controlled by Angiotensin II signaling and its reactivation induces apoptosis in podocytes

The Hippo pathway fulfills a crucial function in controlling the balance between proliferation, differentiation and apoptosis in cells. Recent studies showed that G protein-coupled receptors (GPCRs) serve as upstream regulators of Hippo signaling, that either activate or inactivate the Hippo pathway via the large tumor suppressor kinase (LATS) and its substrate, the co-transcription factor Yes-associated protein (YAP). In this study, we focused on the Angiotensin II type 1 receptor (AT1R), which belongs to the GPCR family and has an essential role in the control of blood pressure and water homeostasis. We found that Angiotensin II (Ang II) inactivates the pathway by decreasing the activity of LATS kinase; therefore, leading to an enhanced nuclear shuttling of unphosphorylated YAP in HEK293T cells. This shuttling of YAP is actin-dependent as disruption of the actin cytoskeleton inhibited dephosphorylation of LATS and YAP. Interestingly, in contrast to HEK293T cells, podocytes, which are a crucial component of the glomerular filtration barrier, display a predominant nuclear YAP localization in vivo and in vitro. Moreover, stimulation with Ang II did not alter Hippo pathway activity in podocytes, which show a deactivated pathway. Reactivation of the LATS kinase activity in podocytes resulted in an increased cytoplasmic YAP localization accompanied by a strong induction of apoptosis. Thus, our work indicates that the control of LATS activation and subsequent YAP localization is important for podocyte homeostasis and survival.

A possible mechanism for the progression of chronic renal disease and congestive heart failure

Chronic neurologic diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, as well as various forms of chronic renal disease and systolic congestive heart failure, are among the most common progressive degenerative disorders encountered in medicine. Each disease follows a nearly relentless course, albeit at varying rates, driven by progressive cell dysfunction and drop-out. The neurologic diseases are characterized by the progressive spread of disease-causing proteins (prion-like proteins) from cell to cell. Recent evidence indicates that cell autonomous renin angiotensin systems operate in heart and kidney, and it is known that functional intracrine proteins can also spread between cells. This then suggests that certain progressive degenerative cardiovascular disorders such as forms of chronic renal insufficiency and systolic congestive heart failure result from dysfunctional renin angiotensin system intracrine action spreading in kidney or myocardium.

ACE-inhibition increases podocyte number in experimental glomerular disease independent of proliferation

Objective

The objective of this article is to test the effects of angiotensin-converting enzyme (ACE)-inhibition on glomerular epithelial cell number in an inducible experimental model of focal segmental glomerulosclerosis (FSGS).

Background

Although ACE-inhibition has been shown to limit podocyte loss by enhancing survival, little is known about its effect on podocyte number following an abrupt decline in disease.

Methods

Experimental FSGS was induced with cytotoxic antipodocyte antibody. Following induction, groups were randomized to receive the ACE-inhibitor enalapril, the smooth muscle relaxant hydralazine (blood pressure control) or drinking water. Blood pressure, kidney function and histology were measured seven and 14 days following disease induction.

Results

Both glomerulosclerosis and urinary albumin-to-creatinine ratio were less in the ACE-inhibition arm at day 14. At day 7 of disease, mean podocyte numbers were 26% and 29% lower in the enalapril and hydralazine arms, respectively, compared to normal mice in which no antibody was injected. At day 14, the mean podocyte number was only 18% lower in the enalapril arm, but was 39% lower in the hydralazine arm compared to normal mice. Podocyte proliferation did not occur at any time in any group. Compared to water- or hydralazine-treated mice with FSGS, the enalapril arm had a higher mean number of glomerular parietal epithelial cells that co-expressed the podocyte proteins WT-1 and synaptopodin, as well as phospho-ERK.

Conclusion

The results show following an abrupt decline in podocyte number, the initiation of ACE-inhibition but not hydralazine, was accompanied by higher podocyte number in the absence of proliferation. This was accompanied by a higher number of parietal epithelial cells that co-express podocyte proteins. Increasing podocyte number appears to be accompanied by reduced glomerulosclerosis.

Protective effects of aliskiren and valsartan in mice with diabetic nephropathy

AIM

We investigated whether aliskiren, a direct renin inhibitor, provided protection in a model of diabetic nephropathy in mice and compared its protective effects to valsartan, an angiotensin II type 1 receptor blocker.

MATERIALS AND METHODS

Hyperglycemia was induced with streptozotocin (STZ, 40 mg/kg/day × 5 days) injection in DBA/2J mice fed on a high fat diet. Mice were treated with either aliskiren (25 mg/kg/day) or valsartan (8 mg/kg/day) for 6 weeks.

RESULTS

Aliskiren and/or valsartan treatment significantly attenuated albuminuria, urinary nephrin excretion and glomerulosclerosis. Aliskiren and/or valsartan prevented reduction of podocin and WT1 protein abundance in diabetic mice. Aliskiren and/or valsartan significantly prevented increased expression of profibrotic growth factors (TGFβ, CTGF and PAI-1), proinflammatory cytokines (MCP-1, TNFα and IL-1β), endoplasmic reticulum (ER) stress markers (CHOP and XBP-1) and lipid accumulation in the kidney of diabetic animals. Aliskiren showed similar efficacy compared to valsartan therapy and dual treatment in some aspects has synergistic protective effects.

CONCLUSION

Our study indicates that aliskiren and/or valsartan protects against diabetic kidney disease through multiple mechanisms, including decreasing podocyte injury, activation of profibrotic growth factors and proinflammatory cytokines, ER stress and accumulation of lipids.

H(2)S inhibits hyperglycemia-induced intrarenal renin-angiotensin system activation via attenuation of reactive oxygen species generation

Decrease in endogenous hydrogen sulfide (H₂S) was reported to participate in the pathogenesis of diabetic nephropathy (DN). This study is aimed at exploring the relationship between the abnormalities in H₂S metabolism, hyperglycemia-induced oxidative stress and the activation of intrarenal renin-angiotensin system (RAS). Cultured renal mesangial cells (MCs) and streptozotocin (STZ) induced diabetic rats were used for the studies. The expressions of angiotensinogen (AGT), angiotensin converting enzyme (ACE), angiotensin II (Ang II) type I receptor (AT1), transforming growth factor-β1 (TGF-β1) and collagen IV were measured by real time PCR and Western blot. Reactive oxygen species (ROS) production was assessed by fluorescent probe assays. Cell proliferation was analyzed by 5'-bromo-2'-deoxyuridine incorporation assay. Ang II concentration was measured by an enzyme immunoassay. AGT, ACE and AT1 receptor mRNA levels and Ang II concentration were increased in high glucose (HG) -treated MCs, the cell proliferation rate and the production of TGF-β1 and of collagen IV productions were also increased. The NADPH oxidase inhibitor diphenylenechloride iodonium (DPI) was able to reverse the HG-induced RAS activation and the changes in cell proliferation and collagen synthesis. Supplementation of H₂S attenuated HG-induced elevations in ROS and RAS activation. Blockade on H₂S biosynthesis from cystathione-γ-lyase (CSE) by DL-propargylglycine (PPG) resulted in effects similar to that of HG treatment. In STZ-induced diabetic rats, the changes in RAS were also reversed by H₂S supplementation without affecting blood glucose concentration. These data suggested that the decrease in H₂S under hyperglycemic condition leads to an imbalance between oxidative and reductive species. The increased oxidative species results in intrarenal RAS activation, which, in turn, contributes to the pathogenesis of renal dysfunction.

Role of Rho-GTPases and their regulatory proteins in glomerular podocyte function

Podocytes play a critical role in maintaining glomerular permselectivity. It has been long recognized that their intricate actin-based structures are tightly associated with their normal function; however, the precise mechanisms by which podocytes form and maintain their complex structure had been poorly understood until the intensive investigations on podocyte biology began in 1998, triggered by the breakthrough discovery of nephrin. This review summarizes the recent discoveries of the molecular mechanisms by which the actin cytoskeleton is regulated in podocytes. A particular focus will be on the role of the Rho-family of small GTPases, represented by RhoA, Rac1, and Cdc42. Rho-GTPases are known for their versatile cellular functions, most importantly for the actin regulatory roles. We will also discuss the potential roles of the 3 groups of proteins known to regulate Rho-GTPases, namely GTPase-activating proteins, guanine nucleotide exchange factors, and guanine nucleotide dissociation inhibitors.

Mitochondrial dysfunction is an early event in aldosterone-induced podocyte injury

We previously showed that mitochondrial dysfunction (MtD) is involved in an aldosterone (Aldo)-induced podocyte injury. Here, the potential role of MtD in the initiation of podocyte damage was investigated. We detected the dynamic changes of urinary protein, urinary F2-isoprostane and renal malondialdehyde levels, kidney ultrastructure morphology, mitochondrial DNA (mtDNA) copy number, mitochondrial membrane potential (ΔΨm), and nephrin and podocin expressions in Aldo-infused mice. Aldo infusion first induced renal oxidative stress, as evidenced by increased levels of urinary F2-isoprostane and renal malondialdehyde, and MtD, as demonstrated by reduced mtDNA, ΔΨm, and ATP production. Later, at 5 days after Aldo infusion, proteinuria and podocyte injury began to appear. In cultured podocytes, Aldo or hydrogen peroxide (H2O2) induced MtD after 2-8 h of treatment, whereas the podocyte damage, as shown by decreased nephrin and podocin expressions, occurred later after 12 h of treatment. Thus Aldo treatment both in vitro and in vivo indicated that MtD occurred before podocyte damage. Additionally, MtDNA depletion by ethidium bromide or mitochondrial transcription factor A (TFAM) RNAi induced MtD, further promoting podocyte damage. TFAM expression was found to be reduced in Aldo-infused mice and Aldo-treated podocytes. Adenoviral vector-mediated overexpression of TFAM prevented Aldo-induced MtD and protected against podocyte injury. Together, these findings support MtD as an early event in podocyte injury, and manipulation of TFAM may be a novel strategy for treatment of glomerular diseases such as podocytopathy.