Podocyte migration during nephrotic syndrome requires a coordinated interplay between cathepsin L and alpha3 integrin

Integrin Trafficking, Fibronectin Architecture, and Glomerular Injury upon Adiponectin Receptor 1 Depletion

Key Points

Glomerular expression of adiponectin receptor 1 (AdipoR1) was lower in people with type 2 diabetes and correlates with podocyte loss. AdipoR1 knockout induced glomerular injury and fibrosis in mice, predominantly in males. AdipoR1 knockdown podocytes showed impaired trafficking of active integrin β 1, fibronectin accumulation, impaired adhesion, and increased apoptosis.

Background

Deficiency of adiponectin and its downstream signaling may contribute to the pathogenesis of kidney injury in type 2 diabetes. Adiponectin activates intracellular signaling using adiponectin receptor 1 (AdipoR1) and adiponectin receptor 2, but the role of adiponectin receptor–mediated signaling in glomerular injury in type 2 diabetes remains unknown.

Methods

The expression of AdipoR1 in the kidneys of people with type 2 diabetes and the expression of podocyte proteins or injury markers in the kidneys of AdipoR1 knockout (AdipoR1-KO) mice and immortalized AdipoR1-deficient human podocytes were investigated by immunohistochemistry and immunoblotting. The functional role of AdipoR1 was studied in AdipoR1-deficient podocytes by performing assays for apoptosis, cytokine secretion, mechanical stress, adhesion, and endocytic trafficking.

Results

Glomerular AdipoR1 expression was lower in type 2 diabetes and associated kidney disease, correlating with higher body mass index and podocyte loss. Male AdipoR1-KO mice showed typical signs of early diabetic kidney disease, including albuminuria, glomerular structural abnormalities, and lower expression of central podocyte proteins; females were less affected. Podocyte apoptosis increased in female and male AdipoR1-KO mice, and excessive podocyte loss, potentially due to detachment, was detected in males. AdipoR1 deficiency impaired the yes-associated protein–mediated mechanoresponse and induced accumulation of the extracellular matrix (ECM) protein fibronectin in the glomeruli in vivo and podocytes in vitro . Functionally, AdipoR1 deficiency impaired endocytosis of the ECM receptor active integrin β 1, disturbed focal adhesion turnover, and remodulated podocyte-derived ECM, thereby reducing podocyte adhesion.

Conclusions

AdipoR1 deficiency in mice resulted in the development of kidney injury predominantly in males. Mechanistically, AdipoR1 loss in podocytes impaired endocytosis of active integrin β 1, which plausibly compromised focal adhesion dynamics, disturbed fibronectin matrix turnover, and hindered podocyte adhesion.

Hyperglycemia induced cathepsin L maturation linked to diabetic comorbidities and COVID-19 mortality

Diabetes, a prevalent chronic condition, significantly increases the risk of mortality from COVID-19, yet the underlying mechanisms remain elusive. Emerging evidence implicates Cathepsin L (CTSL) in diabetic complications, including nephropathy and retinopathy. Our previous research identified CTSL as a pivotal protease promoting SARS-CoV-2 infection. Here, we demonstrate elevated blood CTSL levels in individuals with diabetes, facilitating SARS-CoV-2 infection. Chronic hyperglycemia correlates positively with CTSL concentration and activity in diabetic patients, while acute hyperglycemia augments CTSL activity in healthy individuals. In vitro studies reveal high glucose, but not insulin, promotes SARS-CoV-2 infection in wild-type cells, with CTSL knockout cells displaying reduced susceptibility. Utilizing lung tissue samples from diabetic and non-diabetic patients, alongside Leprdb/dbmice and Leprdb/+mice, we illustrate increased CTSL activity in both humans and mice under diabetic conditions. Mechanistically, high glucose levels promote CTSL maturation and translocation from the endoplasmic reticulum (ER) to the lysosome via the ER-Golgi-lysosome axis. Our findings underscore the pivotal role of hyperglycemia-induced CTSL maturation in diabetic comorbidities and complications.

The Protective Role of KANK1 in Podocyte Injury

Approximately 30% of steroid-resistant nephrotic syndromes are attributed to monogenic disorders that involve 27 genes. Mutations in KANK family members have also been linked to nephrotic syndrome; however, the precise mechanism remains elusive. To investigate this, podocyte-specific Kank1 knockout mice were generated to examine phenotypic changes. In the initial assessment under normal conditions, Kank1 knockout mice showed no significant differences in the urinary albumin-creatinine ratio, blood urea nitrogen, serum creatinine levels, or histological features compared to controls. However, following kidney injury with adriamycin, podocyte-specific Kank1 knockout mice exhibited a significantly higher albumin-creatinine ratio and a significantly greater sclerotic index than control mice. Electron microscopy revealed more extensive foot process effacement in the knockout mice than in control mice. In addition, KANK1-deficient human podocytes showed increased detachment and apoptosis following adriamycin exposure. These findings suggest that KANK1 may play a protective role in mitigating podocyte damage under pathological conditions.

The Role of α3β1 Integrin Modulation on Fabry Disease Podocyte Injury and Kidney Impairment

Podocyte dysfunction plays a crucial role in renal injury and is identified as a key contributor to proteinuria in Fabry disease (FD), primarily impacting glomerular filtration function (GFF). The α3β1 integrins are important for podocyte adhesion to the glomerular basement membrane, and disturbances in these integrins can lead to podocyte injury. Therefore, this study aimed to assess the effects of chloroquine (CQ) on podocytes, as this drug can be used to obtain an in vitro condition analogous to the FD. Murine podocytes were employed in our experiments. The results revealed a dose-dependent reduction in cell viability. CQ at a sub-lethal concentration (1.0 µg/mL) induced lysosomal accumulation significantly (p < 0.0001). Morphological changes were evident through scanning electron microscopy and immunofluorescence, highlighting alterations in F-actin and nucleus morphology. No significant changes were observed in the gene expression of α3β1 integrins via RT-qPCR. Protein expression of α3 integrin was evaluated with Western Blotting and immunofluorescence, demonstrating its lower detection in podocytes exposed to CQ. Our findings propose a novel in vitro model for exploring secondary Fabry nephropathy, indicating a modulation of α3β1 integrin and morphological alterations in podocytes under the influence of CQ.

Enhanced podocyte differentiation and changing drug toxicity sensitivity through pressure-controlled mechanical filtration stress on a glomerulus-on-a-chip

Podocytes, localized in the glomerulus, are a prognostic factor of proteinuria in kidney disease and are exposed to distinct physiological stimuli from basal to apical filtration flow. Research studies on drug discovery and disease modeling for glomerulopathy have developed a glomerulus-on-a-chip and studied podocyte mechanobiology to realize alternative methods to animal experiments. However, the effect of filtration stimulus on podocytes has remained unclear. Herein, we report the successful development of a user-friendly filtration culture device and system that can precisely control the filtration flow using air pressure control by incorporating a commercially available culture insert. It allows mouse podocytes to be cultured under filtration conditions for three days with a guarantee of maintaining the integrity of the podocyte layer. Using our system, this study demonstrated that podocyte damage caused by hyperfiltration resulting from glomerular hypertension, a common pathophysiology of many glomerulopathies, was successfully recapitulated and that filtration stimulus promotes the maturation of podocytes in terms of their morphology and gene expression. Furthermore, we demonstrated that filtration stimulus induced different drug responsiveness in podocytes than those seen under static conditions, and that the difference in drug responsiveness was dependent on the pharmacological mechanism. Overall, this study has revealed differentiating and pharmacodynamic properties of filtration stimulus and brings new insights into the research field of podocyte mechanobiology towards the realization of glomerulus-on-a-chip.

Generation of endoplasmic reticulum stress-dependent reactive oxygen species mediates TGF-β1-induced podocyte migration

Podocyte migration results in proteinuria and glomerulonephropathy. Transforming growth factor-β1 (TGF-β1), endoplasmic reticulum (ER) stress and reactive oxygen species (ROS) can mediate podocyte migration; however, the crosstalk between them is unclear. ThisGraphical Abstract study determined the relationships between these factors. ER stress biomarkers (GRP78, p-eIF2α or CHOP), intracellular ROS generation, integrin-β3 and cell adhesion and migration were studied in a treatment of experiment using TGF-β1 with and without the ER stress inhibitors: 4-phenylbutyric acid (4-PBA, a chemical chaperone), salubrinal (an eIF2α dephosphorylation inhibitor) and N-acetylcysteine (NAC, an antioxidant). ER stress biomarkers (p-eIF2α/eIF2α and GRP78), ROS generation and intergrin-β3 expression increased after TGF-β1 treatment. NAC down-regulated the expression of GRP78 after TGF-β1 treatment. 4-PBA attenuated TGF-β1-induced p-eIF2α/eIF2α, CHOP, ROS generation and intergrin-β3 expression. However, salubrinal did not inhibit TGF-β1-induced p-eIF2α/eIF2α, CHOP, ROS generation or integrin-β3 expression. NAC abrogated TGF-β1-induced integrin-β3 expression. At 24 h after treatment with TGF-β1, podocyte adhesion and migration increased. Furthermore, NAC, 4-PBA and an anti-interin-β3 antibody attenuated TGF-β1-induced podocyte adhesion and migration. This study demonstrated that TGF-β1-induced ER stress potentiates the generation of intracellular ROS to a high degree through the PERK/eIF2α/CHOP pathway. This intracellular ROS then mediates integrin-β3 expression, which regulates podocyte migration.

A novel ITGA3 homozygous splice mutation in an ILNEB syndrome child with slow progression

BACKGROUND AND AIMS

ILNEB (interstitial lung disease, nephrotic syndrome, epidermolysis bullosa) syndrome is caused by ITGA3 mutations. Demises usually happened at infancy. This study reports a complete ILNEB syndrome child with slow disease progression.

MATERIALS AND METHODS

Clinical data and related specimens were collected. Genomic DNA was extracted for genetic sequencing. Integrin α3 expression was detected by western blotting and immunofluorescence staining.

RESULTS

The patient was male. He experienced recurrent rashes shortly after birth. His sparse eyebrows and eyelashes gradually lost. The patient was vulnerable to respiratory infections and had recurrent fever after vaccine immunization after 4 years. He was found with nephrotic syndrome and polycystic renal dysplasia at 8 years and progressed to end-stage renal disease at 12 years. A chest Computed Tomography revealed intestinal lung disease at 8 years. Continuous oxygen supplementation was needed at 13 years. Counts of lymphocyte subsets revealed elevated percentage of double-negative T cells and activated T cells. Next-generation sequencing revealed a novel homozygous splice mutation c.2219 + 4A > Cin ITGA3 that was predicted to be deleterious. The mutation resulted in exon17 skipping with the loss of 80 bp in the mRNA. The aberrant integrin α3 mRNA level was lower compared to the healthy control. Integrin α3 protein was not detected in urine epithelial cells and skin of the patient.

CONCLUSIONS

We report a patient harboring a novel ITGA3 homozygous splice mutation who presented with complete ILNEB syndrome but slow disease progression. Immune disorders were suspected.

Deiodinase-3 is a thyrostat to regulate podocyte homeostasis

BACKGROUND

Nephrotic syndrome (NS) is associated with kidney podocyte injury and may occur as part of thyroid autoimmunity such as Graves' disease. Therefore, the present study was designed to ascertain if and how podocytes respond to and regulate the input of biologically active thyroid hormone (TH), 3,5,3'-triiodothyronine (T3); and also to decipher the pathophysiological role of type 3 deiodinase (D3), a membrane-bound selenoenzyme that inactivates TH, in kidney disease.

METHODS

To study D3 function in healthy and injured (PAN, puromycin aminonucleoside and LPS, Lipopolysaccharide-mediated) podocytes, immunofluorescence, qPCR and podocyte-specific D3 knockout mouse were used. Surface plasmon resonance (SPR), co-immunoprecipitation and Proximity Ligation Assay (PLA) were used for the interaction studies.

FINDINGS

Healthy podocytes expressed D3 as the predominant deiodinase isoform. Upon podocyte injury, levels of Dio3 transcript and D3 protein were dramatically reduced both in vitro and in the LPS mouse model of podocyte damage. D3 was no longer directed to the cell membrane, it accumulated in the Golgi and nucleus instead. Further, depleting D3 from the mouse podocytes resulted in foot process effacement and proteinuria. Treatment of mouse podocytes with T3 phenocopied the absence of D3 and elicited activation of αvβ3 integrin signaling, which led to podocyte injury. We also confirmed presence of an active thyroid stimulating hormone receptor (TSH-R) on mouse podocytes, engagement and activation of which resulted in podocyte injury.

INTERPRETATION

The study provided a mechanistic insight into how D3-αvβ3 integrin interaction can minimize T3-dependent integrin activation, illustrating how D3 could act as a renoprotective thyrostat in podocytes. Further, injury caused by binding of TSH-R with TSH-R antibody, as found in patients with Graves' disease, explained a plausible link between thyroid disorder and NS.

FUNDING

This work was supported by American Thyroid Association (ATA-2018-050.R1).

The PKGIα/VASP pathway is involved in insulin- and high glucose-dependent regulation of albumin permeability in cultured rat podocytes

Podocytes, the principal component of the glomerular filtration barrier, regulate glomerular permeability to albumin via their contractile properties. Both insulin- and high glucose (HG)-dependent activation of protein kinase G type Iα (PKGIα) cause reorganization of the actin cytoskeleton and podocyte disruption. Vasodilator-stimulated phosphoprotein (VASP) is a substrate for PKGIα and involved in the regulation of actin cytoskeleton dynamics. We investigated the role of the PKGIα/VASP pathway in the regulation of podocyte permeability to albumin. We evaluated changes in high insulin- and/or HG-induced transepithelial albumin flux in cultured rat podocyte monolayers. Expression of PKGIα and downstream proteins was confirmed by western blot and immunofluorescence. We demonstrate that insulin and HG induce changes in the podocyte contractile apparatus via PKGIα-dependent regulation of the VASP phosphorylation state, increase VASP colocalization with PKGIα, and alter the subcellular localization of these proteins in podocytes. Moreover, VASP was implicated in the insulin- and HG-dependent dynamic remodelling of the actin cytoskeleton and, consequently, increased podocyte permeability to albumin under hyperinsulinaemic and hyperglycaemic conditions. These results indicate that insulin- and HG-dependent regulation of albumin permeability is mediated by the PKGIα/VASP pathway in cultured rat podocytes. This molecular mechanism may explain podocytopathy and albuminuria in diabetes.

Immunopathogenesis of idiopathic nephrotic syndrome in children: two sides of the coin

BACKGROUND

Idiopathic nephrotic syndrome is a common form of glomerular nephropathy in children, with an incidence rate of 1.15-16.9/100,000 depending on different nationalities and ethnicities. The etiological factors and mechanisms of childhood idiopathic nephrotic syndrome have not yet been fully elucidated. This review summarizes the progress of the immunopathogenesis of idiopathic nephrotic syndrome in children.

DATA SOURCES

We review the literature on the immunopathogenesis of idiopathic nephrotic syndrome in children. Databases including Medline, Scopus, and Web of Science were searched for studies published in any language with the terms "children", "idiopathic nephrotic syndrome", "immunopathogenesis", "T cells", "circulating permeability factors", and "B cells".

RESULTS

Dysfunction in T lymphocytes and pathogenic circulatory factors were indicated to play key roles in the pathogenesis of idiopathic nephrotic syndrome. Recently, some studies have shown that cellular immune dysfunction may also be involved in the pathogenesis of idiopathic nephrotic syndrome.

CONCLUSIONS

Both T- and B-cell dysfunction may play significant roles in the pathogenesis of idiopathic nephrotic syndrome, like two sides of one coin, but the role of B cell seems more important than T cells.

The effect of amiloride in decreasing albuminuria in patients with diabetic kidney diseases: a prospective, crossover, open-label study

BACKGROUND

Diabetic kidney diseases (DKD) were the leading cause of End-stage renal diseases worldwide. Albuminuria was a target for treatment in DKD and decreasing albuminuria was particularly important for improving its prognosis. However, there is still a lack of specific treatment for DKD.

METHODS

We conducted a prospective, crossover, open-label study to investigate the effect of amiloride in patients with DKD. Safety and efficacy were assessed by monitoring urine protein creatinine ratio(uPCR), urinary albumin creatinine ratio (uACR), blood pressure, weight, serum sodium, serum potassium, cholesterol, triglyceride, uric acid, serum soluble urokinase-type plasminogen activator receptor (suPAR) and urinary suPAR. Ten subjects were enrolled in the trial.

RESULTS

In this prospective, crossover, open-label design, amiloride could induce a significant decrease of uACR in DKD. The decrease of serum and urinary suPAR in the amiloride/hydrochlorothiazide (HCTZ) group was also significant compared with those patients using HCTZ as the control group. Correlation analysis showed that the levels of urinary suPAR were positively associated with uPCR and uACR. No significant difference in blood pressure, weight, serum sodium, serum potassium, cholesterol, triglyceride, uric acid was seen between the amiloride/HCTZ group and the control group.

CONCLUSION

In summary, among patients with DKD, amiloride could decrease albuminuria without severe side effects, which was accompanied by the significant decline of urinary suPAR.

Role of γ-adducin in actin cytoskeleton rearrangements in podocyte pathophysiology

We recently reported that the enhanced susceptibility to chronic kidney disease (CKD) in the fawn-hooded hypertensive (FHH) rat is caused, at least in part, by a mutation in γ-adducin (ADD3) that attenuates renal vascular function. The present study explored whether Add3 contributes to the modulation of podocyte structure and function using FHH and FHH.Add3 transgenic rats. The expression of ADD3 on the membrane of primary podocytes isolated from FHH was reduced compared with FHH.Add3 transgenic rats. We found that F-actin nets, which are typically localized in the lamellipodia, replaced unbranched stress fibers in conditionally immortalized mouse podocytes transfected with Add3 Dicer-substrate short interfering RNA (DsiRNA) and primary podocytes isolated from FHH rats. There were increased F/G-actin ratios and expression of the Arp2/3 complexes throughout FHH podocytes in association with reduced synaptopodin and RhoA but enhanced Rac1 and CDC42 expression in the renal cortex, glomeruli, and podocytes of FHH rats. The expression of nephrin at the slit diaphragm and the levels of focal adhesion proteins integrin-α3 and integrin-β1 were decreased in the glomeruli of FHH rats. Cell migration was enhanced and adhesion was reduced in podocytes of FHH rats as well as in immortalized mouse podocytes transfected with Add3 DsiRNA. Mean arterial pressures were similar in FHH and FHH.Add3 transgenic rats at 16 wk of age; however, FHH rats exhibited enhanced proteinuria associated with podocyte foot process effacement. These results demonstrate that reduced ADD3 function in FHH rats alters baseline podocyte pathophysiology by rearrangement of the actin cytoskeleton at the onset of proteinuria in young animals.

Bidirectional, non-necrotizing glomerular crescents are the critical pathology in X-linked Alport syndrome mouse model harboring nonsense mutation of human COL4A5

X-linked Alport syndrome (XLAS) is a progressive kidney disease caused by genetic abnormalities of COL4A5. Lack of collagen IV α5 chain staining and “basket-weave” by electron microscopy (EM) in glomerular basement membrane (GBM) are its typical pathology. However, the causal relationship between GBM defects and progressive nephropathy is unknown. We analyzed sequential pathology in a mouse model of XLAS harboring a human nonsense mutation of COL4A5. In mutant mice, nephropathy commenced from focal GBM irregularity by EM at 6 weeks of age, prior to exclusive crescents at 13 weeks of age. Low-vacuum scanning EM demonstrated substantial ragged features in GBM, and crescents were closely associated with fibrinoid exudate, despite lack of GBM break and podocyte depletion at 13 weeks of age. Crescents were derived from two sites by different cellular components. One was CD44 + cells, often with fibrinoid exudate in the urinary space, and the other was accumulation of α-SMA + cells in the thickened Bowman’s capsule. These changes finally coalesced, leading to global obliteration. In conclusion, vulnerability of glomerular and capsular barriers to the structural defect in collagen IV may cause non-necrotizing crescents via activation of PECs and migration of interstitial fibroblasts, promoting kidney disease in this model.

Urinary cathepsin L is predictive of changes in albuminuria and correlates with glucosepane in patients with type 2 diabetes in a closed-cohort study

AIMS

Cathepsin D (CTSD) and L (CTSL) are lysosomal proteases which degrade and detoxify advanced glycation end product (AGE)-modified proteins which are predictive of the development of diabetic nephropathy. We aimed to quantify cathepsin levels in urine from patients with type 2 diabetes and to relate these to the amount of urinary free AGEs at baseline and with kidney function after four years of follow-up in this closed cohort study.

METHODS

We established and validated a LC MS/MS method for the quantification of CTSD and CTSL in urine. Patients with type 2 diabetes were screened for diabetic kidney disease and 141 patients were seen at baseline and after four years. CTSD and CTSL and free AGEs were quantified in urine by LC MS/MS at baseline in these patients.

RESULTS

The detection limit of CTSD and CTSL in urine was 2.4 ng/l and 19.1 ng/l, respectively. CTSD (p < 0.0001, r = 0.555) and CTSL (p < 0.0001, r = 0.608) correlated positively with albuminuria at time of recruitment. In addition levels of the proteases but not albuminuria correlated with urinary levels of the major cross-linking AGE glucosepane (CTSD: p = 0.012, r = 0.225; CTSL: p < 0.001, r = 0.376). A strong non-linear association between CTSD (r = 0.568), CTSL (r = 0.588) and change in albuminuria over four years was present. High levels of CTSL (p = 0.007, beta = -0.366) were associated with an improvement of albuminuria after four years.

CONCLUSIONS

A sensitive LC MS/MS assay for the quantification of CTSD and CTSL in urine was established. High CTSL baseline levels were associated with an improvement in albuminuria at follow-up. An increased excretion and thus detoxification of the free form of the pathogenic cross-linking AGE glucosepane could explain the positive predictive value of high CTSL levels on albuminuria.

Cathepsin C is a novel mediator of podocyte and renal injury induced by hyperglycemia

A growing body of evidence suggests a role of proteolytic enzymes in the development of diabetic nephropathy. Cathepsin C (CatC) is a well-known regulator of inflammatory responses, but its involvement in podocyte and renal injury remains obscure. We used Zucker rats, a genetic model of metabolic syndrome and insulin resistance, to determine the presence, quantity, and activity of CatC in the urine. In addition to the animal study, we used two cellular models, immortalized human podocytes and primary rat podocytes, to determine mRNA and protein expression levels via RT-PCR, Western blot, and confocal microscopy, and to evaluate CatC activity. The role of CatC was analyzed in CatC-depleted podocytes using siRNA and glycolytic flux parameters were obtained from extracellular acidification rate (ECAR) measurements. In functional analyses, podocyte and glomerular permeability to albumin was determined. We found that podocytes express and secrete CatC, and a hyperglycemic environment increases CatC levels and activity. Both high glucose and non-specific activator of CatC phorbol 12-myristate 13-acetate (PMA) diminished nephrin, cofilin, and GLUT4 levels and induced cytoskeletal rearrangements, increasing albumin permeability in podocytes. These negative effects were completely reversed in CatC-depleted podocytes. Moreover, PMA, but not high glucose, increased glycolytic flux in podocytes. Finally, we demonstrated that CatC expression and activity are increased in the urine of diabetic Zucker rats. We propose a novel mechanism of podocyte injury in diabetes, providing deeper insight into the role of CatC in podocyte biology.

Role of cathepsin L in idiopathic nephrotic syndrome in children

Nephrotic syndrome (NS) is one of the most common glomerular diseases in children. Glomerular podocyte dysfunction can result in proteinuria, the presence of a large amount of protein in the urine. Podocytes are unique epithelial cells that divide into 3 separate structural and functional segments: a cell body, major processes, and foot processes. Since synaptopodin, dynamin, and actin are crucial components of the podocyte cytoskeleton, degradation of these proteins is associated with cytoskeleton instability, resulting in the development of proteinuria. Cathepsin L (CatL), a cysteine proteinase, plays a crucial role in various renal diseases. CatL expression is elevated in rats with puromycin aminonucleoside-induced nephropathy, which is used as a model of minimal change NS. In CatL-deficient mice, which do not develop proteinuria, dynamin is retained through the escape of CatL-mediated decomposition, resulting in no changes in the filtration barrier of podocytes. However, there is limited information on the roles of CatL in NS. Based on these data, CatL might play an important role in the development of proteinuria. Furthermore, identifying the functions of CatL may contribute to a better understanding of the pathogenesis of childhood-onset NS. We hypothesize that high levels of CatL can lead to cytoskeletal instability of podocytes, resulting in proteinuria in childhood-onset NS.

A Review of Small Molecule Inhibitors and Functional Probes of Human Cathepsin L

Human cathepsin L belongs to the cathepsin family of proteolytic enzymes with primarily an endopeptidase activity. Although its primary functions were originally thought to be only of a housekeeping enzyme that degraded intracellular and endocytosed proteins in lysosome, numerous recent studies suggest that it plays many critical and specific roles in diverse cellular settings. Not surprisingly, the dysregulated function of cathepsin L has manifested itself in several human diseases, making it an attractive target for drug development. Unfortunately, several redundant and isoform-specific functions have recently emerged, adding complexities to the drug discovery process. To address this, a series of chemical biology tools have been developed that helped define cathepsin L biology with exquisite precision in specific cellular contexts. This review elaborates on the recently developed small molecule inhibitors and probes of human cathepsin L, outlining their mechanisms of action, and describing their potential utilities in dissecting unknown function.

FSGS-Causing INF2 Mutation Impairs Cleaved INF2 N-Fragment Functions in Podocytes

BACKGROUND

Mutations in the gene encoding inverted formin-2 (INF2), a member of the formin family of actin regulatory proteins, are among the most common causes of autosomal dominant FSGS. INF2 is regulated by interaction between its N-terminal diaphanous inhibitory domain (DID) and its C-terminal diaphanous autoregulatory domain (DAD). INF2 also modulates activity of other formins, such as the mDIA subfamily, and promotes stable microtubule assembly. Why the disease-causing mutations are restricted to the N terminus and how they cause human disease has been unclear.

METHODS

We examined INF2 isoforms present in podocytes and evaluated INF2 cleavage as an explanation for immunoblot findings. We evaluated the expression of INF2 N- and C-terminal fragments in human kidney disease conditions. We also investigated the localization and functions of the DID-containing N-terminal fragment in podocytes and assessed whether the FSGS-associated R218Q mutation impairs INF2 cleavage or the function of the N-fragment.

RESULTS

The INF2-CAAX isoform is the predominant isoform in podocytes. INF2 is proteolytically cleaved, a process mediated by cathepsin proteases, liberating the N-terminal DID to function independently. Although the N-terminal region normally localizes to podocyte foot processes, it does not do so in the presence of FSGS-associated INF2 mutations. The C-terminal fragment localizes to the cell body irrespective of INF2 mutations. In podocytes, the N-fragment localizes to the plasma membrane, binds mDIA1, and promotes cell spreading in a cleavage-dependent way. The disease-associated R218Q mutation impairs these N-fragment functions but not INF2 cleavage.

CONCLUSIONS

INF2 is cleaved into an N-terminal DID-containing fragment and a C-terminal DAD-containing fragment. Cleavage allows the N-terminal fragment to function independently and helps explain the clustering of FSGS-associated mutations.

Human Th17 cells produce a soluble mediator that increases podocyte motility via signaling pathways that mimic PAR-1 activation

The specific pathogenesis of idiopathic nephrotic syndrome (NS) is poorly understood, and the role of immune mediators remains contentious. However, there is good evidence for the role of a circulating factor, and we recently postulated circulating proteases as candidate factors. Immunosuppressive therapy with glucocorticoids (GCs) and T cell inhibitors are widely used in the clinical treatment of NS. Given that T helper (CD4⁺) cells expressing IL-17A (so-called Th17 cells) have recently been reported to be resistant to GC treatment, and GC resistance remains a major challenge in the management of NS, we hypothesized that Th17 cells produce a circulating factor that is capable of signaling to the podocyte and inducing deleterious phenotypic changes. To test this, we generated human Th17 cells from healthy volunteers and added the supernatants from these T cell cultures to conditionally immortalized human podocytes in vitro. This demonstrated that podocytes treated with Th17 cell culture supernatant, as well as with patient disease plasma, showed significant stimulation of JNK and p38 MAPK pathways and an increase in motility, which was blocked using a JNK inhibitor. We have previously shown that nephrotic plasma elicits a podocyte response via protease-activated receptor-1 (PAR-1). Stimulation of PAR-1 in podocytes elicited the same signaling response as Th17 cell culture supernatant treatment. Equally, protease inhibitors with Th17 cell culture treatment blocked the signaling response. This was not replicated by the reagents added to Th17 cell cultures or by IL-17A. Hence, we conclude that an undefined soluble mediator produced by Th17 cells mimics the deleterious effect of PAR-1 activation in vitro. Given the association between pathogenic subsets of Th17 cells and GC resistance, these observations have potential therapeutic relevance for patients with NS.

The human nephrin Y1139RSL motif is essential for podocyte foot process organization and slit diaphragm formation during glomerular development

Nephrin is an immunoglobulin-type cell-adhesion molecule with a key role in the glomerular interpodocyte slit diaphragm. Mutations in the nephrin gene are associated with defects in the slit diaphragm, leading to early-onset nephrotic syndrome, typically resistant to treatment. Although the endocytic trafficking of nephrin is essential for the assembly of the slit diaphragm, nephrin's specific endocytic motifs remain unknown. To search for endocytic motifs, here we performed a multisequence alignment of nephrin and identified a canonical YXXØ-type motif, Y¹¹³⁹RSL, in the nephrin cytoplasmic tail, expressed only in primates. Using site-directed mutagenesis, various biochemical methods, single-plane illumination microscopy, a human podocyte line, and a human nephrin-expressing zebrafish model, we found that Y¹¹³⁹RSL is a novel endocytic motif and a structural element for clathrin-mediated nephrin endocytosis that functions as a phosphorylation-sensitive signal. We observed that Y¹¹³⁹RSL motif-mediated endocytosis helps to localize nephrin to specialized plasma membrane domains in podocytes and is essential for normal foot process organization into a functional slit diaphragm between neighboring foot processes in zebrafish. The importance of nephrin Y¹¹³⁹RSL for healthy podocyte development was supported by population-level analyses of genetic variations at this motif, revealing that such variations are very rare, suggesting that mutations in this motif have autosomal-recessive negative effects on kidney health. These findings expand our understanding of the mechanism underlying nephrin endocytosis and may lead to improved diagnostic tools or therapeutic strategies for managing early-onset, treatment-resistant nephrotic syndrome.

Increased autophagy is cytoprotective against podocyte injury induced by antibody and interferon-α in lupus nephritis

OBJECTIVE

More recent studies suggested that defects in autophagy contribute to the pathogenesis of SLE, especially in adaptive immunity. Occurrence and progression of lupus nephritis (LN) is the end result of complex interactions between regulation of immune responses and pathological process by renal resident cells, but there is still a lot of missing information for an establishment on the role of autophagy in pathogenesis of LN and as a therapy target.

METHODS

Systemic and organ-specific aetiologies of autophagy were first evaluated by autophagy protein quantification in tissue homogenates in MRL lpr/lpr lupus prone and female C57BL mice. Analysis of gene expression was also adopted in human blood and urine sediments. Then, some key mediators of the disease, including complement inactivated serum, IgG from patients with LN (IgG-LN) and interferon (IFN)-α were chosen to induce podocyte autophagy. Podocyte injuries including apoptosis, podocin derangement, albumin filtration and wound healing were monitored simultaneously with autophagy steady-state and flux.

RESULTS

Elevated LC3B in kidney homogenates and increased autophagosomes in podocyte from MRL lpr/lpr were observed. In humans, mRNA levels of some key autophagy genes were increased in blood and urinary sediments, and podocyte autophagosomes were observed in renal biopsies from patients with LN. Complement inactivated serum, IgG-LN and IFN-α could induce podocyte autophagy in a time-dependent and dosage-dependent manner, and by reactive oxygen species production and mTORC1 inhibition, respectively. Autophagy inhibition aggravated podocyte damage whereas its inducer relieved the injury.

CONCLUSION

Podocyte autophagy is activated in lupus-prone mice and patients with lupus nephritis. Increased autophagy is cytoprotective against antibody and interferon-α induced podocyte injury.

Regulation of cofilin phosphorylation in glomerular podocytes by testis specific kinase 1 (TESK1)

Expression of a constitutively active Rho A (V14Rho) in podocytes in vivo induces albuminuria and foot process (FP) effacement. These effects may be mediated by the Rho A effector Rho kinase (ROK); but inhibition of ROK with Y27632 failed to attenuate albuminuria or FP effacement in V14Rho mice. ROK activates LIM kinases (LIMKs), which phosphorylate and inhibit the actin depolymerizing factor cofilin 1 (CFL1). Sustained phosphorylation of CFL1 is implicated in human nephrotic diseases, but Y27632 did not inhibit phosphorylation of CFL1 in vivo, despite effective ROK inhibition. CFL1 is also phosphorylated by testis-specific kinase 1 (TESK1) on the same serine residue. TESK1 was expressed in podocytes, and, similar to the in vivo situation, Y27632 had little effect on phospho-CFL1 (pCFL1) levels in cultured podocytes. In contrast, Y27632 reduced pCFL1 levels in TESK1 knockout (KO) cells. ROK inhibition enhanced podocyte motility but, the motility promoting effect of Y27632 was absent in TESK1 KO podocytes. Thus, TESK1 regulates podocyte cytoskeletal dynamics in glomerular podocytes and may play an important role in regulating glomerular filtration barrier integrity in glomerular disease processes.

Expression of Cathepsin L and Its Intrinsic Inhibitors in Glomeruli of Rats With Puromycin Aminonucleoside Nephrosis

Cathepsin L, a lysosomal cysteine proteinase, may have a key role in various biological and disease processes by intracellular and extracellular degradation of proteins. We examined the levels of cathepsin L and its intrinsic inhibitors in glomeruli of rats with puromycin aminonucleoside (PAN) nephrosis. In contrast to the weak levels of cathepsin L in normal glomeruli, on days 4 and 8, strong immunostaining was detected in almost all podocytes when proteinuria and pathological changes of the podocytes developed. Cathepsin L was reduced after day 28, but remained in a focal and segmental manner. Cystatin β, an intracellular inhibitor, was not detected in podocytes. However, cystatin C, an extracellular inhibitor, was detected in podocytes after day 4, coincident with cathepsin L. Cystatin C levels were gradually reduced but sustained in many podocytes on day 28, while cystatin C was not detected in podocytes sustained cathepsin L. These results demonstrated that cathepsin L levels are not always accompanied by the levels of its inhibitors in podocytes of PAN nephrosis, suggesting a potential role of cathepsin L in podocyte injury, which is a critical process for the development and progression of tuft adhesion and sclerosis.

A Novel CLCN5 Mutation Associated With Focal Segmental Glomerulosclerosis and Podocyte Injury

Introduction

Tubular dysfunction is characteristic of Dent’s disease; however, focal segmental glomerulosclerosis (FSGS) can also be present. Glomerulosclerosis could be secondary to tubular injury, but it remains uncertain whether the CLCN5 gene, which encodes an endosomal chloride and/or hydrogen exchanger, plays a role in podocyte biology. Here, we implicate a role for CLCN5 in podocyte function and pathophysiology.

Methods

Whole exome capture and sequencing of the proband and 5 maternally-related family members was conducted to identify X-linked mutations associated with biopsy-proven FSGS. Human podocyte cultures were used to characterize the mutant phenotype on podocyte function.

Results

We identified a novel mutation (L521F) in CLCN5 in 2 members of a Hispanic family who presented with a histologic diagnosis of FSGS and low-molecular-weight proteinuria without hypercalciuria. Presence of CLCN5 was confirmed in cultured human podocytes. Podocytes transfected with the wild-type or the mutant (L521F) CLCN5 constructs showed differential localization. CLCN5 knockdown in podocytes resulted in defective transferrin endocytosis and was associated with decreased cell proliferation and increased cell migration, which are hallmarks of podocyte injury.

Conclusions

The CLCN5 mutation, which causes Dent’s disease, may be associated with FSGS without hyercalcuria and nepthrolithiasis. The present findings supported the hypothesis that CLCN5 participates in protein trafficking in podocytes and plays a critical role in organizing the components of the podocyte slit diaphragm to help maintain normal cell physiology and a functional filtration barrier. In addition to tubular dysfunction, mutations in CLCN5 may also lead to podocyte dysfunction, which results in a histologic picture of FSGS that may be a primary event and not a consequence of tubular damage.

The Role of Glucocorticoid Receptors in Podocytes and Nephrotic Syndrome

Glucocorticoid receptor (GC), a founding member of the nuclear hormone receptor superfamily, is a glucocorticoid-activated transcription factor that regulates gene expression and controls the development and homeostasis of human podocytes. Synthetic glucocorticoids are the standard treatment regimens for proteinuria (protein in the urine) and nephrotic syndrome (NS) caused by kidney diseases. These include minimal change disease (MCD), focal segmental glomerulosclerosis (FSGS), membranous nephropathy (MN) and immunoglobulin A nephropathy (IgAN) or subsequent complications due to diabetes mellitus or HIV infection. However, unwanted side effects and steroid-resistance remain major issues for their long-term use. Furthermore, the mechanism by which glucocorticoids elicit their renoprotective activity in podocyte and glomeruli is poorly understood. Podocytes are highly differentiated epithelial cells that contribute to the integrity of kidney glomerular filtration barrier. Injury or loss of podocytes leads to proteinuria and nephrotic syndrome. Recent studies in multiple experimental models have begun to explore the mechanism of GC action in podocytes. This review will discuss progress in our understanding of the role of glucocorticoid receptor and glucocorticoids in podocyte physiology and their renoprotective activity in nephrotic syndrome.

IL-6 increases podocyte motility via MLC-mediated focal adhesion impairment and cytoskeleton disassembly

The disturbance of podocyte motility is an essential pathogenic mechanisms of foot process effacement during proteinuric diseases, and myosin light chain (MLC) is a pivotal component in regulating the motility of podocytes. Inflammatory cytokine interleukin-6 (IL-6) has been reported to induce podocyte abnormalities by various mechanisms, however, whether aberrant cell motility contributes to the IL-6-induced podocyte injury remains unknown. Here, by wound healing, transwell, and cell migration assays, we confirmed that IL-6 accelerates the motility of podocyte. Simultaneously, the phosphorylation of MLC is elevated along with perturbed focal adhesion (FAs) and cytoskeleton. Next, via genetic and pharmacologic interruption of MLC or its phosphorylation we revealed that the activation of MLC is implicated in IL-6-mediated podocyte hypermotility as well as the disassembly of FAs and F-actin. By using stattic, an inhibitor for STAT3 phosphorylation, we uncovered that STAT3 activation is the upstream event for MLC phosphorylation and the following aberrant motility of podocytes. Additionally, we found that calcitriol markedly attenuates podocyte hypermotility via blocking STAT3-MLC. In conclusion, our study demonstrated that IL-6 interrupts FAs dynamic, cytoskeleton organization, and eventually leads to podocyte hypermotility via STAT3/MLC, whereas calcitriol exerts its protective role by inhibiting this pathway. These findings enrich the mechanisms accounting for IL-6-mediated podocyte injury from the standpoint of cell motility and provide a novel therapeutic target for podocyte disorders.

Rac1 in podocytes promotes glomerular repair and limits the formation of sclerosis

Rac1, a Rho family member, is ubiquitously expressed and participates in various biological processes. Rac1 expression is induced early in podocyte injury, but its role in repair is unclear. To investigate the role of Rac1 expression in podocytes under pathological conditions, we used podocyte-specific Rac1 conditional knock-out (cKO) mice administered adriamycin (ADR), which causes nephrosis and glomerulosclerosis. Larger areas of detached podocytes, more adhesion of the GBM to Bowman’s capsule, and a higher ratio of sclerotic glomeruli were observed in Rac1 cKO mice than in control mice, whereas no differences were observed in glomerular podocyte numbers in both groups after ADR treatment. The mammalian target of rapamycin (mTOR) pathway, which regulates the cell size, was more strongly suppressed in the podocytes of Rac1 cKO mice than in those of control mice under pathological conditions. In accordance with this result, the volumes of podocytes in Rac1 cKO mice were significantly reduced compared with those of control mice. Experiments using in vitro ADR-administered Rac1 knockdown podocytes also supported that a reduction in Rac1 suppressed mTOR activity in injured podocytes. Taken together, these data indicate that Rac1-associated mTOR activation in podocytes plays an important role in preventing the kidneys from developing glomerulosclerosis.

Lipid peroxidation regulates podocyte migration and cytoskeletal structure through redox sensitive RhoA signaling

Early podocyte loss is characteristic of chronic kidney diseases (CKD) in obesity and diabetes. Since treatments for hyperglycemia and hypertension do not prevent podocyte loss, there must be additional factors causing podocyte depletion. The role of oxidative stress has been implicated in CKD but it is not known how exactly free radicals affect podocyte physiology. To assess this relationship, we investigated the effects of lipid radicals on podocytes, as lipid peroxidation is a major form of oxidative stress in diabetes. We found that lipid radicals govern changes in podocyte homeostasis through redox sensitive RhoA signaling: lipid radicals inhibit migration and cause loss of F-actin fibers. These effects were prevented by mutating the redox sensitive cysteines of RhoA. We therefore suggest that in diseases associated with increased lipid peroxidation, lipid radicals can determine podocyte function with potentially pathogenic consequences for kidney physiology.

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Lipid peroxyl radicals impact podocyte motility and cytoskeletal F-actin arrangement.

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Lipid peroxyl radicals activate the small GTPase RhoA.

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When the Cys residues of RhoA are mutated, lipid peroxyl radicals do not affect podocytes.

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Lipid peroxidation likely contributes to podocyte injury.

Update on Genetic Conditions Affecting the Skin and the Kidneys

Genetic conditions affecting the skin and kidney are clinically and genetically heterogeneous, and target molecular components present in both organs. The molecular pathology involves defects of cell-matrix adhesion, metabolic or signaling pathways, as well as tumor suppressor genes. This article gives a clinically oriented overview of this group of disorders, highlighting entities which have been recently described, as well as the progress made in understanding well-known entities. The genetic bases as well as molecular cell biological mechanisms are described, with therapeutic applications.

The Multifaceted Role of the Lysosomal Protease Cathepsins in Kidney Disease

Kidney disease is worldwide the 12th leading cause of death affecting 8–16% of the entire population. Kidney disease encompasses acute (short-lasting episode) and chronic (developing over years) pathologies both leading to renal failure. Since specific treatments for acute or chronic kidney disease are limited, more than 2 million people a year require dialysis or kidney transplantation. Several recent evidences identified lysosomal proteases cathepsins as key players in kidney pathophysiology. Cathepsins, originally found in the lysosomes, exert important functions also in the cytosol and nucleus of cells as well as in the extracellular space, thus participating in a wide range of physiological and pathological processes. Based on their catalytic active site residue, the 15 human cathepsins identified up to now are classified in three different families: serine (cathepsins A and G), aspartate (cathepsins D and E), or cysteine (cathepsins B, C, F, H, K, L, O, S, V, X, and W) proteases. Specifically in the kidney, cathepsins B, D, L and S have been shown to regulate extracellular matrix homeostasis, autophagy, apoptosis, glomerular permeability, endothelial function, and inflammation. Dysregulation of their expression/activity has been associated to the onset and progression of kidney disease. This review summarizes most of the recent findings that highlight the critical role of cathepsins in kidney disease development and progression. A better understanding of the signaling pathways governed by cathepsins in kidney physiopathology may yield novel selective biomarkers or therapeutic targets for developing specific treatments against kidney disease.

Chronic cathepsin inhibition by E-64 in Dahl salt-sensitive rats

Cysteine cathepsins are lysosomal enzymes expressed in the kidneys and other tissues, and are involved in the maturation and breakdown of cellular proteins. They have been shown to be integrally involved in the progression of many cardiovascular and renal diseases. The goal of this study was to determine the involvement of cysteine cathepsins in the development of salt‐sensitive hypertension and associated kidney damage. In our experiments, Dahl salt‐sensitive (SS) rats were fed an 8% high salt NaCl diet and intravenously infused with the irreversible cysteine cathepsin inhibitor E‐64 (1 mg/day) or the vehicle (control). Both the control and E‐64 infused groups developed significant hypertension and kidney damage, and no difference of the mean arterial pressure and the hypertension‐associated albuminuria was observed between the groups. We next tested basal calcium levels in the podocytes of both control and infused groups using confocal calcium imaging. Basal calcium did not differ between the groups, indicative of the lack of a protective or aggravating influence by the cathepsin inhibition. The efficacy of E‐64 was tested in Western blotting. Our findings corresponded to the previously reported, E‐64 induced increase in cathepsin B and L abundance. We conclude that the inhibition of cysteine cathepsins by E‐64 does not have any effects on the blood pressure development and kidney damage, at least under the studied conditions of this model of SS hypertension.

High-content screening assay-based discovery of paullones as novel podocyte-protective agents

Podocyte dysfunction and loss is an early event and a hallmark of proteinuric kidney diseases. A podocyte's normal function is maintained via its unique cellular architecture that relies on an intracellular network of filaments, including filamentous actin (F-actin) and microtubules, that provides mechanical support. Damage to this filamentous network leads to changes in cellular morphology and results in podocyte injury, dysfunction, and death. Conversely, stabilization of this network protects podocytes and ameliorates proteinuria. This suggests that stabilization of podocyte architecture via its filamentous network could be a key therapeutic strategy for proteinuric kidney diseases. However, development of podocyte-directed therapeutics, especially those that target the cell's filamentous network, is still lacking, partly because of unavailability of appropriate cellular assays for use in a drug discovery environment. Here, we describe a new high-content screening-based methodology and its implementation on podocytes to identify paullone derivatives as a novel group of podocyte-protective compounds. We find that three compounds, i.e., kenpaullone, 1-azakenpaullone, and alsterpaullone, dose dependently protect podocytes from puromycin aminonucleoside (PAN)-mediated injury in vitro by reducing PAN-induced changes in both the filamentous actin and microtubules, with alsterpaullone providing maximal protection. Mechanistic studies further show that alsterpaullone suppressed PAN-induced activation of signaling downstream of GSK3β and p38 mitogen-activated protein kinase. In vivo it reduced ADR-induced glomerular injury in a zebrafish model. Together, these results identify paullone derivatives as novel podocyte-protective agents for future therapeutic development.

The Evolving Complexity of the Podocyte Cytoskeleton

Podocytes exhibit a unique cytoskeletal architecture that is fundamentally linked to their function in maintaining the kidney filtration barrier. The cytoskeleton regulates podocyte shape, structure, stability, slit diaphragm insertion, adhesion, plasticity, and dynamic response to environmental stimuli. Genetic mutations demonstrate that even slight impairment of the podocyte cytoskeletal apparatus results in proteinuria and glomerular disease. Moreover, mechanisms underpinning all acquired glomerular pathologies converge on disruption of the cytoskeleton, suggesting that this subcellular structure could be targeted for therapeutic purposes. This review summarizes our current understanding of the function of the cytoskeleton in podocytes and the associated implications for pathophysiology.

Loss of the podocyte glucocorticoid receptor exacerbates proteinuria after injury

Nephrotic syndrome is a common disorder in adults and children whose etiology is largely unknown. Glucocorticoids remain the mainstay of therapy in most cases, though their mechanism of action remains poorly understood. Emerging evidence suggests that immunomodulatory therapies used in nephrotic syndrome directly target the podocytes. To study how steroids directly affect the podocytes in the treatment of proteinuria, we created a mouse model with podocyte-specific deletion of the glucocorticoid receptor. The podocyte-specific glucocorticoid receptor (GR) knockout mice had similar renal function and protein excretion compared to wild type. However, after glomerular injury induced by either LPS or nephrotoxic serum, the podocyte GR knockout mice demonstrated worsened proteinuria compared to wild type. Ultrastructural examination of podocytes confirmed more robust foot process effacement in the knockout animals. Expression of several key slit diaphragm protein was down regulated in pGR KO mice. Primary podocytes isolated from wild type and podocyte GR knockout mice showed similar actin stress fiber staining patterns in unstimulated conditions. Yet, when exposed to LPS, GR knockout podocytes demonstrated fewer stress fibers and impaired migration compared to wild type podocytes. We conclude that the podocyte glucocorticoid receptor is important for limiting proteinuria in settings of podocyte injury.

Are podocytes motile?

Podocytes, the postmitotic and highly branched epithelial cells of the glomerulus, play a pivotal role for the function of the glomerular filtration barrier and the development of chronic kidney disease. It has long been discussed whether podocytes in vivo are motile and can laterally migrate in a coordinated way along the capillaries until they reach the position of naked glomerular basement membrane often found in podocytopathies. Such motility would also be the prerequisite for the replacement of lost podocytes by progenitor cells. Additionally, the change of the podocyte foot processes from a normal to an effaced morphology, like it is found in many kidney diseases, would require a dynamic behavior of podocytes. Since the actin cytoskeleton is expressed in podocytes in vitro and in vivo and the morphology of podocytes is highly dependent on actin, actin-associated, and actin-regulating proteins, it was assumed that podocytes are dynamic and motile. After earlier technical limitations had been overcome and novel microscopic techniques like multiphoton microscopy had been developed, it became possible to continuously study the behavior of podocytes in living rodents and zebrafish larvae under physiological and pathological conditions. Recent in vivo microscopic studies in different model organisms suggest that lateral migration of podocytes in situ is a very unlikely event and only dynamic apical cell protrusions can be observed under pathological conditions. This review discusses recent findings concerning different forms of motility (like lateral translocative (LTM), apical translocative (ATM), and stationary motility (SM)) and their role for podocytopathies.

Cathepsin L activity correlates with proteinuria in chronic kidney disease in humans

BACKGROUND

The presence and severity of proteinuria is considered an important prognostic marker in patients with chronic kidney disease (CKD) and is associated with mortality and morbidity. Cathepsin L is highly expressed in the foot processes of podocytes in the kidney, which serves as an ultrafiltration barrier. Cathepsin L is also up-regulated in the setting of inflammation as a feature of CKD. Therefore, we postulated that proteinuria severity in CKD patients might correlate with increased serum levels of cathepsin L.

METHODS AND RESULTS

In this retrospective observational study, a total of 135 patients diagnosed with CKD, 31 renal transplant patients and 48 healthy controls were included. The demographic characteristics and clinical indicators were analyzed. Serum cathepsin L activity was significantly higher in patients with CKD than in renal transplant recipients and healthy controls (P < 0.01). Patients with severe proteinuria had a higher cathepsin L activity compared to those with moderate or mild proteinuria (P < 0.01). Serum cathepsin L activity positively associated with age, body mass index, nitrite level, neutrophil count, high-sensitivity C-reactive protein (hs-CRP), N-terminal pro-brain natriuretic peptide, high-mobility group box-1 protein (HMGB1) and 24-h proteinuria. In the ROC analysis, the sensitivity of cathepsin L activity in diagnosis of moderate and heavy is 0.86 and the specificity is 0.73. Moreover, CKD patients with higher cathepsin L activity had a significantly higher hospital admission rate. The data also showed patients with statin administration present significantly lower cathepsin L activity (P < 0.01), hs-CRP (P < 0.01), HMGB1 (P < 0.01) and proteinuria (P < 0.01) compared to non-statin treatment group.

CONCLUSION

This study revealed that serum cathepsin L activity is significantly elevated in CKD patients and its level correlates with the severity of proteinuria as well as prognosis, suggesting that serum cathepsin L may serve as a potential biomarker for CKD. Further prospective study is needed to explore its clinical implications in the future.

Exogenous cathepsin V protein protects human cardiomyocytes HCM from angiotensin Ⅱ-Induced hypertrophy

Angiotensin (Ang) Ⅱ-induced cardiac hypertrophy can deteriorate to heart failure, a leading cause of mortality. Endogenous Cathepsin V (CTSV) has been reported to be cardioprotective against hypertrophy. However, little is known about the effect of exogenous CTSV on cardiac hypertrophy. We used the human cardiomyocytes HCM as a cell model to investigate the effects of exogenous CTSV on Ang Ⅱ-induced cardiac cell hypertrophy. Cell surface area and expression of classical markers of hypertrophy were analyzed. We further explored the mechanism of CTSV cardioprotective by assessing the levels and activities of PI3K/Akt/mTOR and MAPK signaling pathway proteins. We found that pre-treating cardiomyocytes with CTSV could significantly inhibit Ang Ⅱ-induced hypertrophy. The mRNA expression of hypertrophy markers ANP, BNP and β-MHC was obviously elevated in Ang Ⅱ-treated cardiac cells. Whereas, exogenous CTSV effectively halted this elevation. Further study revealed that the protective effects of exogenous CTSV might be mediated by repressing the phosphorylation of proteins in the PI3K/Akt/mTOR and MAPK pathways. Based on our results, we concluded that exogenous CTSV inhibited Ang Ⅱ-induced hypertrophy in HCM cells by inhibiting PI3K/Akt/mTOR. This study provides experimental evidence for the application of CTSV protein for the treatment of cardiac hypertrophy.

A High-Content Screening Technology for Quantitatively Studying Podocyte Dynamics

Podocytes form the visceral layer of a kidney glomerulus and express a characteristic octopus-like cellular architecture specialized for the ultrafiltration of blood. The cytoskeletal dynamics and structural elasticity of podocytes rely on the self-organization of highly interconnected actin bundles, and the maintenance of these features is important for the intact glomerular filtration. Development of more differentiated podocytes in culture has dramatically increased our understanding of the molecular mechanisms regulating podocyte actin dynamics. Podocytes are damaged in a variety of kidney diseases, and therapies targeting podocytes are being investigated with increasing efforts. Association between podocyte damage and disease severity-or between podocyte recovery and the performance of therapeutic molecules-have been the venues of research for years. In this perspective, more standardized high--content screening has emerged as a powerful tool for visualization and analysis of podocyte morphology. This high-throughput fluorescence microscopy technique is based on an automated image analysis with simultaneous detection of various phenotypes (multiplexing) across multiple phenotypic parameters (multiparametric). Here, we review the principles of high-content screening technology and summarize efforts to carry out small compound screen using podocytes.

Alteration of histone H3K4 methylation in glomerular podocytes associated with proteinuria in patients with membranous nephropathy

Background

Histone H3K4 trimethylation (H3K4 me3) is found in active euchromatic regions and plays an important role in podocyte function in which actin filaments are abundant in the foot processes. The pathogenesis of membranous nephropathy (MN), the most prevalent cause of primary nephrotic syndrome in the middle-aged and elderly, is podocyte dysfunction.

Methods

We investigated the role of H3K4 me3 in podocyte dysfunction in nephrotic syndrome using cultured podocytes and a mouse proteinuria model induced by LPS. We examined renal biopsy specimens from six patients with nephrotic syndrome caused by Phospholipase-A2-Receptor-positive primary MN.

Results

H3K4 me3 exhibited a pattern of nuclear expression in podocytes of the kidneys from patients with MN. The overlapping expression of H3K4 me3 and cathepsin L (a potent endoprotease causing the breakdown of actin-associated protein within lysosomal compartments in kidney podocytes) were higher in patients with MN compared with the controls. Histone H3K4 me3 in kidney podocytes was negatively correlated with synaptopodin, an actin-associated protein in podocytes, and the expression was positively correlated with the proteinuria levels in patients with MN. Histone H3K4 me3 levels were elevated in podocytes of LPS-treated mice, combined with an increase in podocyte swelling, an elevation of serum creatinine and urine albumin, increased cathepsin L, and decreased synaptopodin expression. Histone H3K4 me3 levels at the cathepsin L promoter were elevated in LPS-exposed mouse kidneys. The administration of shRNA against MLL3 (an H3K4 methyltransferase) to LPS-treated mice and cultured podocytes co-cultured with LPS-stimulated macrophages ameliorated podocyte swelling, an elevation in the serum creatinine and urine albumin levels and an increased expression of histone H3K4 me3 and cathepsin L, and a decreased expression of synaptopodin and increase in histone H3K4 me3 levels at the cathepsin L promoter.

Conclusions

Histone H3K4 me3 upregulation may be involved in podocyte dysfunction and the pathophysiology of MN. Targeting this epigenetic signature of histone H3K4 me3 followed by modulating the actin dynamics may be an effective strategy to ameliorate the consequences of MN.

Human genetics of nephrotic syndrome and the quest for precision medicine

PURPOSE OF REVIEW

In this review, we take a combined membrane biologist's and geneticist's view of the podocyte, to examine how genetics have informed our understanding of membrane receptors, channels, and other signaling molecules affecting podocyte health and disease.

RECENT FINDINGS

An integral part of the kidney, the glomerulus, is responsible for the kidney's filter function. Within the glomerulus, the podocyte is a unique cell serving a critically important role: it is exposed to signals from the urinary space in Bowman's capsule, it receives and transmits signals to/from the basement membrane upon which it elaborates, and it receives signals from the vascular space with which it also communicates, thus exposed to toxins, viruses, chemicals, proteins, and cellular components or debris that flow in the blood stream. Our understanding of how podocytes perform their important role has been largely informed by human genetics, and the recent revolution afforded by exome sequencing has brought a tremendous wealth of new genetic data to light.

SUMMARY

Genetically defined, rare/orphan podocytopathies, as reviewed here, are critically important to study as they may reveal the next generation targets for precision medicine in nephrology.

A small molecule screening to detect potential therapeutic targets in human podocytes

WIDMEIER E, TAN W, AIRIK M, HILDEBRANDT F

A small molecule screening to detect potential therapeutic targets in human podocytes. Am J Physiol Renal Physiol 312: F157-F171, 2017. First published October 19, 2016; doi:10.1152/ajprenal.00386.2016. Steroid-resistant nephrotic syndrome (SRNS) inevitably progresses to end-stage kidney disease, requiring dialysis or transplantation for survival. However, treatment modalities and drug discovery remain limited. Mutations in over 30 genes have been discovered as monogenic causes of SRNS. Most of these genes are predominantly expressed in the glomerular epithelial cell, the podocyte, placing it at the center of the pathogenesis of SRNS. Podocyte migration rate (PMR) represents a relevant intermediate phenotype of disease in monogenic causes of SRNS. We therefore adapted PMR in a high-throughput manner to screen small molecules as potential therapeutic targets for SRNS. We performed a high-throughput drug screening of a National Institutes of Health Clinical Collection (NCC) library (n = 725 compounds) measuring PMR by videomicroscopy. We used the Woundmaker to perform individual 96-well scratch wounds and screened compounds using a quantitative kinetic live cell imaging migration assay using IncuCyte ZOOM technology. Using a normal distribution for the average PMR in wild-type podocytes with a vehicle control (DMSO), we applied a 90% confidence interval to define "distinct" compounds (5% faster/slower PMR) and found that 12 of 725 compounds (at 10 μM) reduced PMR. Clusters of drugs that alter PMR included actin/tubulin modulators such as the azole class of antifungals and antineoplastic vinca-alkaloids. We hereby identify compounds that alter PMR. The PMR assay provides a new avenue to test therapeutics for nephrotic syndrome. Positive results may reveal novel pathways in the study of glomerular diseases such as SRNS.

Podocyte injury and its consequences

Podocytes maintain the glomerular filtration barrier, and the stability of this barrier depends on their highly differentiated postmitotic phenotype, which also defines the particular vulnerability of the glomerulus. Recent podocyte biology and gene disruption studies in vivo indicate a causal relationship between abnormalities of single podocyte molecules and proteinuria and glomerulosclerosis. Podocytes live under various stresses and pathological stimuli. They adapt to maintain homeostasis, but excessive stress leads to maladaptation with complex biological changes including loss of integrity and dysregulation of cellular metabolism. Podocyte injury causes proteinuria and detachment from the glomerular basement membrane. In addition to "sick" podocytes and their detachment, our understanding of glomerular responses following podocyte loss needs to address the pathways from podocyte injury to sclerosis. Studies have found a variety of glomerular responses to podocyte dysfunction in vivo, such as disruption of podocyte-endothelial cross talk and activation of podocyte-parietal cell interactions, all of which help us to understand the complex scenario of podocyte injury and its consequences. This review focuses on the cellular aspects of podocyte dysfunction and the adaptive or maladaptive glomerular responses to podocyte injury that lead to its major consequence, glomerulosclerosis.

The β isoform of GSK3 mediates podocyte autonomous injury in proteinuric glomerulopathy

Converging evidence points to glycogen synthase kinase (GSK) 3 as a key player in the pathogenesis of podocytopathy and proteinuria. However, it remains unclear if GSK3 is involved in podocyte autonomous injury in glomerular disease. In normal kidneys, the β isoform of GSK3 was found to be the major GSK3 expressed in glomeruli and intensely stained in podocytes. GSK3β expression in podocytes was markedly elevated in experimental or human proteinuric glomerulopathy. Podocyte-specific somatic ablation of GSK3β in adult mice attenuated proteinuria and ameliorated podocyte injury and glomerular damage in experimental adriamycin (ADR) nephropathy. Mechanistically, actin cytoskeleton integrity in podocytes was largely preserved in GSK3β knockout mice following ADR insult, concomitant with a correction of podocyte hypermotility and lessened phosphorylation and activation of paxillin, a focal adhesion-associated adaptor protein. In addition, GSK3β knockout diminished ADR-induced NFκB RelA/p65 phosphorylation selectively at serine 467; suppressed de novo expression by podocytes of NFκB-dependent podocytopathic mediators, including B7-1, cathepsin L, and MCP-1; but barely affected the induction of NFκB target pro-survival factors, such as Bcl-xL. Moreover, the ADR-elicited podocytopenia and podocyte death were significantly attenuated in GSK3β knockout mice, associated with protection against podocyte mitochondrial damage and reduced phosphorylation and activation of cyclophilin F, a structural component of mitochondria permeability transition pores. Overall, our findings suggest that the β isoform of GSK3 mediates autonomous podocyte injury in glomerulopathy by integrating multiple podocytopathic signalling pathways.