The podocyte as a direct target of glucocorticoids in nephrotic syndrome

Loss of Nup160 dysregulates Cdc42 in the podocytes of podocyte-specific Nup160 knockout mice

Mutations in four genes encoding the outer ring complex of nuclear pore complexes (NPCs), NUP85, NUP107, NUP133 and NUP160, cause monogenic steroid-resistant nephrotic syndrome (SRNS). Knockout of NUP85, NUP107, or NUP133 in immortalized human podocytes activates CDC42, an important effector of SRNS pathogenesis. However, it is unknown whether or not loss of NUP160 dysregulates CDC42 in the podocytes. Here, we generated a podocyte-specific Nup160 knockout mouse model with double-fluorescent (mT/mG) Cre reporter genes using CRISPR/Cas9 and Cre/loxP technologies. We investigated nephrotic syndrome-associated phenotypes in the Nup160podo-/- mice, and performed single-cell transcriptomic and proteomic analysis of glomerular suspension cells and cultured primary podocytes, respectively. The Nup160podo-/- mice exhibited progressive proteinuria and fusion of podocyte foot processes. We found decreased Cdc42 protein and normal Cdc42 transcriptional level in the podocytes of the Nup160podo-/- mice using analysis of single-cell transcriptomes and proteomes. We subsequently observed that Cdc42 protein decreased in both kidney tissues and cultured primary podocytes of the Nup160podo-/- mice, although Cdc42 mRNA levels were elevated in the cultured primary podocytes of the Nup160podo-/- mice. We also found that Cdc42 activity was significantly reduced in the cultured primary podocytes of the Nup160podo-/- mice. In conclusion, loss of Nup160 dysregulated Cdc42 in the podocytes of the Nup160podo-/- mice with proteinuria and fusion of podocyte foot processes. Our findings suggest that the dysregulation of CDC42 may contribute to the pathogenesis of SRNS in patients with mutations in NUP160.

The Spectrum of Minimal Change Disease/Focal Segmental Glomerulosclerosis: From Pathogenesis to Proteomic Biomarker Research

Podocyte injury plays a central role in both focal segmental glomerulosclerosis (FSGS) and minimal change disease (MCD). Pathogenic mechanisms are diverse and incompletely understood, partially overlap between FSGS and MCD, and are not reflected by kidney biopsy. In order to optimize the current variable response to treatment, personalized management should rely on pathogenesis. One promising approach involves identifying biomarkers associated with specific pathogenic pathways. With the advancement of technology, proteomic studies could be a valuable tool to improve knowledge in this area and define valid biomarkers, as they have in other areas of glomerular disease. This work attempts to cover and discuss the main mechanisms of podocyte injury, followed by a review of the recent literature on proteomic biomarker studies in podocytopathies. Most of these studies have been conducted on biofluids, while tissue proteomic studies applied to podocytopathies remain limited. While we recognize the importance of non-invasive biofluid biomarkers, we propose a sequential approach for their development: tissue proteomics could first identify proteins with increased expression that may reflect underlying disease mechanisms; subsequently, the validation of these proteins in urine or plasma could pave the way to a diagnostic and prognostic biomarker-based approach.

The Ca2+-actin-cytoskeleton axis in podocytes is an important, non-immunologic target of immunosuppressive therapy in proteinuric kidney diseases

The integrity of the filtration barrier of the kidney relies on the proper composition of podocyte interdigitating foot processes. Their architecture is supported by a complex actin-cytoskeleton. Following podocyte stress or injury, podocytes encounter structural changes, including rearrangement of the actin network and subsequent effacement of the foot processes. Immunosuppressive drugs, which are currently used as treatment in proteinuric kidney diseases, have been shown to exert not only immune-mediated effects. This review will focus on the direct effects of glucocorticoids, cyclosporine A, tacrolimus, mycophenolate mofetil, and rituximab on podocytes by regulation of Ca2+ ion channels and consecutive downstream signaling which prevent cytoskeletal rearrangements and ultimately proteinuria. In addition, the efficacy of these drugs in genetic nephrotic syndrome will be discussed.

Xuebijing injection and its bioactive components alleviate nephrotic syndrome by inhibiting podocyte inflammatory injury

Xuebijing injection (XBJ) is widely used to treat nephrotic syndrome (NS) in clinic, but its bioactive components and therapeutic mechanism are still unclear. In this study, the bioactive components of XBJ were determined by ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF/MS). The therapeutic effect of XBJ on NS was evaluated in BALB/c mice induced by adriamycin (ADR, 10 mg/kg) via a single tail vein. The protective effect of XBJ and its bioactive components on podocytes was demonstrated using mouse podocytes (MPC-5) induced by lipopolysaccharide (LPS, 4 μg/mL). The results show that 33 components of XBJ were identified. Furthermore, 12 bioactive components were detected in blood, including protocatechuic acid, salvianolic acid C, benzoyloxypaeoniflorin, danshensu, salvianolic acid A, salvianolic acid B, catechin, caffeic acid, galloylpaeoniflorin, oxypaeoniflorin, hydroxysafflor yellow A, rosmarinic acid. The relative content (%) of the bioactive components were 59.32, 16.01, 9.97, 9.73, 8.72, 8.31, 7.92, 6.54, 1.54, 1.30, 0.68 and 0.59 in this order. After XBJ treatment, the renal function, hyperlipidemia and renal pathological damage were improved in NS model mice. Moreover, the levels of nephrin and desmin which are functional proteins in podocytes were reversed, and the levels of pro-inflammatory factors were reduced by XBJ. Interestingly, protocatechuic acid and salvianolic acid C also showed good protective effects on podocyte function and reduced the level of inflammation in LPS-induced MPC-5. The study is the first time to elucidate the bioactive components of XBJ and its potential therapeutic mechanism for treating NS by protecting podocyte function.

Understanding the podocyte immune responses in proteinuric kidney diseases: from pathogenesis to therapy

The glomerular filtration barrier, comprising the inner layer of capillary fenestrated endothelial cells, outermost podocytes, and the glomerular basement membrane between them, plays a pivotal role in kidney function. Podocytes, terminally differentiated epithelial cells, are challenging to regenerate once injured. They are essential for maintaining the integrity of the glomerular filtration barrier. Damage to podocytes, resulting from intrinsic or extrinsic factors, leads to proteinuria in the early stages and eventually progresses to chronic kidney disease (CKD). Immune-mediated podocyte injury is a primary pathogenic mechanism in proteinuric glomerular diseases, including minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy, and lupus nephritis with podocyte involvement. An extensive body of evidence indicates that podocytes not only contribute significantly to the maintenance of the glomerular filtration barrier and serve as targets of immune responses but also exhibit immune cell-like characteristics, participating in both innate and adaptive immunity. They play a pivotal role in mediating glomerular injury and represent potential therapeutic targets for CKD. This review aims to systematically elucidate the mechanisms of podocyte immune injury in various podocyte lesions and provide an overview of recent advances in podocyte immunotherapy. It offers valuable insights for a deeper understanding of the role of podocytes in proteinuric glomerular diseases, and the identification of new therapeutic targets, and has significant implications for the future clinical diagnosis and treatment of podocyte-related disorders.

Cytoskeleton Rearrangement in Podocytopathies: An Update

Podocyte injury can disrupt the glomerular filtration barrier (GFB), leading to podocytopathies that emphasize podocytes as the glomerulus's key organizer. The coordinated cytoskeleton is essential for supporting the elegant structure and complete functions of podocytes. Therefore, cytoskeleton rearrangement is closely related to the pathogenesis of podocytopathies. In podocytopathies, the rearrangement of the cytoskeleton refers to significant alterations in a string of slit diaphragm (SD) and focal adhesion proteins such as the signaling node nephrin, calcium influx via transient receptor potential channel 6 (TRPC6), and regulation of the Rho family, eventually leading to the disorganization of the original cytoskeletal architecture. Thus, it is imperative to focus on these proteins and signaling pathways to probe the cytoskeleton rearrangement in podocytopathies. In this review, we describe podocytopathies and the podocyte cytoskeleton, then discuss the molecular mechanisms involved in cytoskeleton rearrangement in podocytopathies and summarize the effects of currently existing drugs on regulating the podocyte cytoskeleton.

Levamisole Modulation of Podocytes' Actin Cytoskeleton in Nephrotic Syndrome

Podocytes play a central role in glomerular diseases such as (idiopathic) nephrotic syndrome (iNS). Glucocorticoids are the gold standard therapy for iNS. Nevertheless, frequent relapses are common. In children with iNS, steroid-sparing agents are used to avoid prolonged steroid use and reduce steroid toxicity. Levamisole is one of these steroid-sparing drugs and although clinical effectiveness has been demonstrated, the molecular mechanisms of how levamisole exerts its beneficial effects remains poorly studied. Apart from immunomodulatory capacities, nonimmunological effects of levamisole on podocytes have also been suggested. We aimed to elaborate on the effects of levamisole on human podocytes in iNS. RNA sequencing data from a human podocyte cell line treated with levamisole showed that levamisole modulates the expression of various genes involved in actin cytoskeleton stabilization and remodeling. Functional experiments showed that podocytes exposed to puromycin aminonucleoside (PAN), lipopolysaccharides (LPS), and NS patient plasma resulted in significant actin cytoskeleton derangement, reduced cell motility, and impaired cellular adhesion when compared to controls, effects that could be restored by levamisole. Mechanistic studies revealed that levamisole exerts its beneficial effects on podocytes by signaling through the glucocorticoid receptor and by regulating the activity of Rho GTPases. In summary, our data show that levamisole exerts beneficial effects on podocytes by stabilizing the actin cytoskeleton in a glucocorticoid receptor-dependent manner.

[Diagnosis and treatment of focal-segmental glomerulosclerosis-2023]

The histopathological term focal-segmental glomerulosclerosis comprises different pathogenic processes with the unifying features of a high proteinuria and the name-giving glomerular lesion pattern seen on light microscopy. A differentiation according to the underlying cause into primary, secondary and genetic forms is therefore of utmost importance. The pathogenesis of primary focal-segmental glomerulosclerosis remains unknown but, like minimal-change disease, an autoimmune-mediated process leading to podocyte damage is assumed. Consequently, the unifying term "podocytopathy" is increasingly being used for both entities. Supportive treatment measures to preserve kidney function are important in all subtypes. In contrast, immunosuppressive treatment is only indicated in primary focal-segmental glomerulosclerosis. Steroid-dependence, steroid-resistance and frequently relapsing disease often complicate disease management and necessitate alternative treatment strategies. Here, the Austrian Society of Nephrology (ÖGN) provides consensus recommendations on how to best diagnose and manage patients with focal-segmental glomerulosclerosis.

Human pluripotent stem cell-derived kidney organoids for personalized congenital and idiopathic nephrotic syndrome modeling

Nephrotic syndrome (NS) is characterized by severe proteinuria as a consequence of kidney glomerular injury due to podocyte damage. In vitro models mimicking in vivo podocyte characteristics are a prerequisite to resolve NS pathogenesis. The detailed characterization of organoid podocytes resulting from a hybrid culture protocol showed a podocyte population that resembles adult podocytes and was superior compared with 2D counterparts, based on single-cell RNA sequencing, super-resolution imaging and electron microscopy. In this study, these next-generation podocytes in kidney organoids enabled personalized idiopathic nephrotic syndrome modeling, as shown by activated slit diaphragm signaling and podocyte injury following protamine sulfate, puromycin aminonucleoside treatment and exposure to NS plasma containing pathogenic permeability factors. Organoids cultured from cells of a patient with heterozygous NPHS2 mutations showed poor NPHS2 expression and aberrant NPHS1 localization, which was reversible after genetic correction. Repaired organoids displayed increased VEGFA pathway activity and transcription factor activity known to be essential for podocyte physiology, as shown by RNA sequencing. This study shows that organoids are the preferred model of choice to study idiopathic and congenital podocytopathies.

Summary: Kidney organoid podocytes generated from human pluripotent stem cells using a hybrid differentiation protocol allow podocyte pathophysiology modeling that leads to congenital as well as idiopathic nephrotic syndrome in patients.

Idiopathic Nephrotic Syndrome in Pediatrics: An Up-to-date

BACKGROUND

Idiopathic or Primary Nephrotic Syndrome (INS) is a common glomerular disease in pediatric population, characterized by proteinuria, edema and hypoalbuminemia with variable findings in renal histopathology.

OBJECTIVE

This review aims to summarize current data on the etiopathogenesis diagnosis, protocols of treatment and potential therapeutic advances in INS.

METHODS

This narrative review searched for articles on histopathology, physiopathology, genetic causes, diagnosis and treatment of INS in pediatric patients. The databases evaluated were PubMed and Scopus.

RESULTS

INS is caused by an alteration in the permeability of the glomerular filtration barrier with unknown etiology. There are several gaps in the etiopathogenesis, response to treatment and clinical course of INS that justify further investigation. Novel advances include the recent understanding of the role of podocytes in INS and the identification of genes associated with the disease. The role of immune system cells and molecules has also been investigated. The diagnosis relies on clinical findings, laboratory exams and renal histology for selected cases. The treatment is primarily based on steroids administration. In case of failure, other medications should be tried. Recent studies have also searched for novel biomarkers for diagnosis and alternative therapeutic approaches.

CONCLUSION

The therapeutic response to corticosteroids still remains the main predictive factor for the prognosis of the disease. Genetic and pharmacogenomics tools may allow the identification of cases not responsive to immunosuppressive medications.