Mutations in six nephrosis genes delineate a pathogenic pathway amenable to treatment

Nephron progenitor fate is modulated by angiotensin type 1 receptor signaling in human kidney organoids

The renin-angiotensin system (RAS) is essential for normal kidney development. Dysregulation of the RAS during embryogenesis can result in kidney abnormalities. To explore how angiotensin type 1 receptor (AT1R) signaling modulates nephron progenitor (NP) fate specification, we used induced pluripotent stem cell (iPSC) derived human kidney organoids treated with angiotensin II (Ang II) or the AT1R blocker losartan during differentiation. Ang II promoted NP proliferation and differentiation preferentially toward a podocyte fate, depleted the podocyte precursor population, and accelerated glomerular maturation. By contrast, losartan expanded the podocyte precursor population, delayed podocyte differentiation, and regressed the transcriptional signature to a more immature fetal state. Overall, using various in silico approaches with validation by RNAscope, we identified a role for AT1R signaling in regulating NP fate during nephrogenesis in kidney organoids. Our work supports the use of RAS modulators to improve organoid maturation and suggests that RAS may be a determinant of nephron endowment in vivo.

The intelligent podocyte: sensing and responding to a complex microenvironment

Podocytes are key components of the glomerular filtration barrier - a specialized structure that is responsible for the filtration of blood by the kidneys. They therefore exist in a unique microenvironment exposed to mechanical force and the myriad molecules that cross the filtration barrier. To survive and thrive, podocytes must continually sense and respond to their ever-changing microenvironment. Sensing is achieved by interactions with the surrounding extracellular matrix and neighbouring cells, through a variety of pathways, to sense changes in environmental factors such as nutrient levels including glucose and lipids, oxygen levels, pH and pressure. The response mechanisms similarly involve a range of processes, including signalling pathways and the actions of specific organelles that initiate and regulate appropriate responses, including alterations in cell metabolism, immune regulation and changes in podocyte structure and cognate functions. These functions ultimately affect glomerular and kidney health. Imbalances in these processes can lead to inflammation, podocyte loss and glomerular disease.

Genetic aspects of pediatric nephrotic syndrome and anti-nephrin antibodies

Nephrotic syndrome is the most common glomerular disease in children, and various hypotheses regarding its etiology have been proposed, primarily focusing on immune-related mechanisms. Nephrotic syndrome can manifest as a monogenic disease caused by deleterious variants in genes such as NPHS1, which encodes nephrin. In steroid-sensitive nephrotic syndrome, HLA class II and immune-related genes have been identified as susceptibility genes. Moreover, NPHS1 is a susceptibility gene for steroid-sensitive nephrotic syndrome in patients from East Asian populations. Anti-nephrin antibodies have been identified as a significant factor in the pathogenesis of nephrotic syndrome. These discoveries have substantially advanced our understanding of nephrotic syndrome. However, the mechanisms underlying the production of anti-nephrin antibodies and their association with genetic backgrounds have remained unclear and warrant further investigation.

Mechanisms of podocyte injury in genetic kidney disease

Glomerular diseases are a leading cause of chronic kidney disease worldwide. Both acquired and hereditary glomerulopathies frequently share a common final disease mechanism: disruption of the glomerular filtration barrier, podocyte injury, and ultimately podocyte death and detachment. Over 70 monogenic causes of proteinuric kidney disease have been identified, and most of these genes are highly expressed in podocytes, regulating key processes such as maintenance of the slit diaphragm, regulation of actin cytoskeleton remodeling, and modulation of downstream transcriptional pathways. Collectively, these are increasingly being referred to as hereditary "podocytopathies," in which podocyte injury is the central feature driving proteinuria and kidney dysfunction. In this review, we provide an overview of the monogenic podocytopathies and discuss the molecular mechanisms by which single-gene defects lead to podocyte injury and ultimately glomerulosclerosis. We review how advances in genomic technology and a better understanding of the cell biological basis of disease have led to the development of more targeted and personalized therapeutic strategies, including an overview of small molecule and gene therapy approaches.

Advances in focal segmental glomerulosclerosis research: genetic causes to non-coding RNAs

Focal Segmental Glomerulosclerosis (FSGS) is a clinicopathological illness characterized by podocyte damage, impairing glomerular filtration, and substantial proteinuria, which often results in end-stage renal disease (ESRD). Divided into primary, secondary, genetic, and idiopathic categories, its diverse origin highlights the intricacy of its diagnosis and treatment. The existing dependence on immunosuppressive medicines highlights their side effects and inconsistent efficacy, underscoring the pressing necessity for innovative, focused treatments. Recent advancements in genomics and molecular biology have shown the significant involvement of genetic alterations, especially in podocyte-associated proteins, in the pathogenesis of FSGS. Identifying possible novel biomarkers for diagnosing FSGS and monitoring disease activity has revitalized interest in this condition. Recent data underscores the significance of non-coding RNAs, including microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs (lncRNAs), in the modulation of gene expression and podocyte functionality. Please check and confirm that the authors and their respective affiliations have been correctly identified and amend if necessary. Particular dysregulated miRNAs and circRNAs have demonstrated potential as biomarkers for early diagnosis and disease monitoring. Furthermore, understanding lncRNA-mediated pathways provides novel therapeutic targets. This review consolidates current progress in elucidating the genetic and molecular processes of FSGS, emphasizing biomarker identification and treatment innovation.

When should the nephrologist think about genetics in patients with glomerular diseases?

This review discusses the significance of genetics in diagnosing glomerular diseases. Advances in genetic testing, particularly next-generation sequencing, have improved the accessibility and accuracy of diagnosing monogenic diseases, allowing for targeted gene panels and whole-exome/genome sequencing to identify genetic variants associated with glomerular diseases. Key indicators for considering a genetic cause include the age of onset, extrarenal features, family history, and inconclusive kidney biopsy results. Early-onset diseases, for instance, have a higher likelihood of being genetically caused, while extrarenal manifestations can also suggest an underlying genetic condition. A thorough family history can reveal patterns of inheritance that point to monogenic causes, although complexities like incomplete penetrance, skewed X inactivation and mosaicism can complicate the assessment. Also, autosomal recessive conditions imply asymptomatic parents, making genetic suspicion less likely, while de novo mutations can occur without any family history, further obscuring genetic assessment. Focal segmental glomerulosclerosis (FSGS) is characterized by podocyte injury and depletion, presenting in various forms, including primary, genetic, and secondary FSGS. Accurate classification of FSGS patients based on clinical and histological features is essential for guiding treatment decisions, optimizing therapeutic plans, avoiding unnecessary immunosuppression, and predicting relapse risk after kidney transplantation. Overall, a clinicopathological approach, enriched by genetic testing, offers a precise framework for diagnosis and management in glomerular diseases. Future directions for research and clinical practice include potential advancements in genetic testing and personalized medicine, which could further improve diagnostic precision and individualized treatment strategies.

Membranous Nephropathy Target Antigens Display Podocyte-Specific and Non-Specific Expression in Healthy Kidneys

BACKGROUND/OBJECTIVES

Autoimmunity towards podocyte antigens causes membranous nephropathy (MN). Numerous MN target antigens (MNTAgs) have been reported, including PLA2R1, THSD7A, NTNG1, TGFBR3, HTRA1, NDNF, SEMA3B, FAT1, EXT1, CNTN1, NELL1, PCDH7, EXT2, PCSK6, and NCAM1, but their podocyte expression has not been thoroughly studied.

METHODS

We screened CZ CELLxGene single-cell RNA (scRNA) sequence datasets for those of adult, fetal, and mouse kidneys and analyzed the above MNTAgs' expression.

RESULTS

In adult kidneys, most MNTAgs are present in podocytes, except PCSK6 and NCAM1. PLA2R1 is expressed significantly more than other MNTAgs in podocytes and is a major podocyte marker, consistent with PLA2R1 as the dominant MNTAg. Additionally, PLA2R1 is a top-upregulated gene in the podocytes of chronic kidney disease, acute kidney injury, and diabetic nephropathy, indicating its general role in causing podocyte injury. PLA2R1, NTNG1, HTRA1, and NDNF display podocyte-enriched expression along with elevated chromatin accessibility in podocytes, suggesting transcription initiation contributing to their preference expression in podocytes. In the fetal kidney, most MNTAgs are expressed in podocytes. While PLA2R1 is weakly present in podocytes, SEMA3B is abundantly expressed in immature and mature podocytes, supporting SEMA3B as a childhood MNTAg. In mouse kidneys, Thsd7a is the only MNTAg with a prominent level and podocyte-specific expression. Conclusions: Most MNTAgs are present in podocytes in adults and during renal development. In adults, PLA2R1 expression is highly enriched in podocytes and significantly upregulated in multiple kidney diseases accompanied by proteinuria. In mouse kidneys, Thsd7a is specifically expressed in podocytes at an elevated level.

Enhancing diagnostic outcomes in kidney genetic disorders: the KidGen national kidney genomics study protocol

BACKGROUND

Genetic kidney disease (GKD) significantly affects the community and is responsible for a notable portion of adult kidney disease cases and about half of cases in paediatric patients. It substantially impacts the quality of life and life expectancy for affected children and adults across all stages of kidney disease. Precise genetic diagnosis in GKD promises to improve patient outcomes, provide access to targeted treatments, and reduce the disease burden for individuals, families, and healthcare systems. Genetic investigations are increasingly used in nephrology practice; however, many patients who undergo testing still lack a definitive diagnosis.

METHODS

The KidGen National Kidney Genomics Study aims to increase diagnostic yield for those with suspected monogenic kidney disease without a diagnosis after standard diagnostic genetic testing. The program will seek to enrol up to 200 families from KidGen Collaborative kidney genetics clinics across Australia who have yet to receive conclusive diagnoses despite prior testing. Participants will undergo a personalised pathway of research genomic investigations. These include re-analysing existing data and/or undergoing advanced genomic testing methods, including short and long-read whole-genome sequencing, RNA sequencing, and functional genomics strategies using mouse modelling or kidney organoids.

DISCUSSION

The KidGen National Kidney Genomics Study is a coordinated, multidisciplinary extension of previous research projects that aims to assess the diagnostic yield of advanced genomic approaches. The study's evidence will drive changes to current diagnostic pathways, including identifying which chronic kidney disease patients are most likely to benefit from a more comprehensive genomic approach to diagnosis.

Comprehensive circular RNA profiling in various sheep tissues

Despite the scientific relevance of circular RNAs (circRNAs), the study of these RNAs in non-model organisms, especially in sheep, is still in its infancy. On the other hand, while some studies have focused on sheep circRNA identification in a limited number of tissues, there is a lack of comprehensive analysis that profile circRNA expression patterns across the tissues not yet investigated. In this study, 61 public RNA sequencing datasets from 12 different tissues were uniformly analyzed to identify circRNAs, profile their expression and investigate their various characteristics. We reported for the first time a circRNA expression landscape with functional annotation in sheep tissues not yet investigated including hippocampus, BonMarrowMacrophage, left-ventricle, thymus, ileum, reticulum and 23-day-embryo. A stringent computational pipeline was employed and 8919 exon-derived circRNAs with high confidence were identified, including 88 novel circRNAs. Tissue-specificity analysis revealed that 3059 circRNAs were tissue-specific, which were also more specific to the tissues than linear RNAs. The highest number of tissue-specific circRNAs was found in kidney, hippocampus and thymus, respectively. Co-expression analysis revealed that expression of circRNAs may not be affected by their host genes. While most of the host genes produced more than one isoform, only one isoform had dominant expression across the tissues. The host genes of the tissue-specific circRNAs were significantly enriched in biological/pathways terms linked to the important functions of their corresponding tissues, suggesting potential roles of circRNAs in modulating physiological activity of those tissues. Interestingly, functional terms related to the regulation and various signaling pathways were significantly enriched in all tissues, suggesting some common regulatory mechanisms of circRNAs to modulate the physiological functions of tissues. Finding of the present study provide a valuable resource for depicting the complexity of circRNAs expression across tissues of sheep, which can be useful for the field of sheep genomic and veterinary research.

Automated scoring of glomerular injury in TNS2-deficient nephropathy

Several artificial intelligence (AI) systems have been developed for glomerular pathology analysis in clinical settings. However, the application of AI systems in nonclinical fields remains limited. In this study, we trained a convolutional neural network model, which is an AI algorithm, to classify the severity of Tensin 2 (TNS2)-deficient nephropathy into seven categories. A dataset consisting of 803 glomerular images was generated from kidney sections of TNS2-deficient and wild-type mice. Manual evaluations of the images were conducted to assess their glomerular injury scores. The trained AI achieved approximately 70% accuracy in predicting the glomerular injury score for TNS2-deficient nephropathy. However, the AI achieved approximately 100% accuracy when considering predictions within one score of the true label as correct. The AI's predicted mean score closely matched the true mean score. In conclusion, while the AI model may not replace human judgment entirely, it can serve as a reliable second assessor in scoring glomerular injury, offering potential benefits in enhancing the accuracy and objectivity of such assessments.

Nephrotic Syndrome Throughout Childhood: Diagnosing Podocytopathies From the Womb to the Dorm

The etiologies of podocyte dysfunction that lead to pediatric nephrotic syndrome (NS) are vast and vary with age at presentation. The discovery of numerous novel genetic podocytopathies and the evolution of diagnostic technologies has transformed the investigation of steroid-resistant NS while simultaneously promoting the replacement of traditional morphology-based disease classifications with a mechanistic approach. Podocytopathies associated with primary and secondary steroid-resistant NS manifest as diffuse mesangial sclerosis, minimal change disease, focal segmental glomerulosclerosis, and collapsing glomerulopathy. Molecular testing, once an ancillary option, has become a vital component of the clinical investigation and when paired with kidney biopsy findings, provides data that can optimize treatment and prognosis. This review focuses on the causes including selected monogenic defects, clinical phenotypes, histopathologic findings, and age-appropriate differential diagnoses of nephrotic syndrome in the pediatric population with an emphasis on podocytopathies.

Focal Segmental Glomerulosclerosis

Focal segmental glomerular sclerosis (FSGS) is a histological lesion characterized by sclerosis in sections (segmental) of some glomeruli (focal) in association with podocyte injury. Historically, FSGS has often been characterized as a disease, but it is a heterogeneous entity based on etiology, clinical course, and therapeutic approach. A unifying feature is podocyte injury and loss, which can be primary or the result of secondary maladaptive responses to glomerular stressors. FSGS has been demonstrated over time to carry a large health burden and remains a leading glomerular cause of ESRD globally. Recent clinical practice guidelines highlight the unmet scientific need for better understanding of disease pathogenesis, particularly for immunologic etiologies, as well as more targeted therapeutic drug development. In this review, we will discuss the current FSGS classification scheme, pathophysiologic mechanisms of injury, and treatment guidelines, along with emerging and investigational therapeutics.

Role of biophysics and mechanobiology in podocyte physiology

Podocytes form the backbone of the glomerular filtration barrier and are exposed to various mechanical forces throughout the lifetime of an individual. The highly dynamic biomechanical environment of the glomerular capillaries greatly influences the cell biology of podocytes and their pathophysiology. Throughout the past two decades, a holistic picture of podocyte cell biology has emerged, highlighting mechanobiological signalling pathways, cytoskeletal dynamics and cellular adhesion as key determinants of biomechanical resilience in podocytes. This biomechanical resilience is essential for the physiological function of podocytes, including the formation and maintenance of the glomerular filtration barrier. Podocytes integrate diverse biomechanical stimuli from their environment and adapt their biophysical properties accordingly. However, perturbations in biomechanical cues or the underlying podocyte mechanobiology can lead to glomerular dysfunction with severe clinical consequences, including proteinuria and glomerulosclerosis. As our mechanistic understanding of podocyte mechanobiology and its role in the pathogenesis of glomerular disease increases, new targets for podocyte-specific therapeutics will emerge. Treating glomerular diseases by targeting podocyte mechanobiology might improve therapeutic precision and efficacy, with potential to reduce the burden of chronic kidney disease on individuals and health-care systems alike.

Recessive variants in the intergenic NOS1AP-C1orf226 locus cause monogenic kidney disease responsive to anti-proteinuric treatment

In genetic disease, an accurate expression landscape of disease genes and faithful animal models will enable precise genetic diagnoses and therapeutic discoveries, respectively. We previously discovered that variants in NOS1AP , encoding nitric oxide synthase 1 (NOS1) adaptor protein, cause monogenic nephrotic syndrome (NS). Here, we determined that an intergenic splice product of N OS1AP / Nos1ap and neighboring C1orf226/Gm7694 , which precludes NOS1 binding, is the predominant isoform in mammalian kidney transcriptional and proteomic data. Gm7694 -/- mice, whose allele exclusively disrupts the intergenic product, developed NS phenotypes. In two human NS subjects, we identified causative NOS1AP splice variants, including one predicted to abrogate intergenic splicing but initially misclassified as benign based on the canonical transcript. Finally, by modifying genetic background, we generated a faithful mouse model of NOS1AP -associated NS, which responded to anti-proteinuric treatment. This study highlights the importance of intergenic splicing and a potential treatment avenue in a mendelian disorder.

FilGAP controls cell-extracellular matrix adhesion and process formation of kidney podocytes

The function of kidney podocytes is closely associated with actin cytoskeleton regulated by Rho small GTPases. Loss of actin-driven cell adhesions and processes is connected to podocyte dysfunction, proteinuria, and kidney diseases. FilGAP, a GTPase-activating protein for Rho small GTPase Rac1, is abundantly expressed in kidney podocytes, and its gene is linked to diseases in a family with focal segmental glomerulosclerosis. In this study, we have studied the role of FilGAP in podocytes in vitro. Depletion of FilGAP in cultured podocytes induced loss of actin stress fibers and increased Rac1 activity. Conversely, forced expression of FilGAP increased stress fiber formation whereas Rac1 activation significantly reduced its formation. FilGAP localizes at the focal adhesion (FA), an integrin-based protein complex closely associated with stress fibers, that mediates cell-extracellular matrix (ECM) adhesion, and FilGAP depletion decreased FA formation and impaired attachment to the ECM. Moreover, in unique podocyte cell cultures capable of inducing the formation of highly organized processes including major processes and foot process-like projections, FilGAP depletion or Rac1 activation decreased the formation of these processes. The reduction of FAs and process formations in FilGAP-depleted podocyte cells was rescued by inhibition of Rac1 or P21-activated kinase 1 (PAK1), a downstream effector of Rac1, and PAK1 activation inhibited their formations. Thus, FilGAP contributes to both cell-ECM adhesion and process formation of podocytes by suppressing Rac1/PAK1 signaling.

Novel variants in CRB2 targeting the malfunction of slit diaphragm related to focal segmental glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a leading cause of steroid-resistant nephrotic syndrome (SRNS) that predominantly affects the podocytes. While mutations in genes causing pediatric SRNS have enhanced our understanding of FSGS, the disease's etiology remains complex and poorly understood.

METHODS

Whole exome sequencing (WES) was performed on a 9-year-old girl with SRNS associated with FSGS (SRNS-FSGS). We analyzed the expression of CRB2, slit diaphragm (SD)-associated proteins, and sphingosine 1-phosphate receptor 1 (S1PR1) in the proband and CRB2 knock-down podocytes.

RESULTS

In this study, we identified two novel compound heterozygous mutations in the Crumbs homolog 2 (CRB2) gene (c.2905delinsGCCACCTCGCGCTGGCTG, p.T969Afs*179 and c.3268C > G, p.R1090G) in a family with early-onset SRNS-FSGS. Our findings demonstrate that these CRB2 abnormalities were the underlying cause of SRNS-FSGS. CRB2 defects led to the dysfunction of podocyte SD-related proteins, including podocin, nephrin, and zonula occludens-1 (ZO-1), by reducing the phosphorylation level of S1PR1. Interestingly, the podocytic cytoskeleton remained unaffected, as demonstrated by normal expression and localization of synaptopodin. Our study also revealed a secondary decrease in CRB2 expression in idiopathic FSGS patients, indicating that CRB2 mutations may cause FSGS through a previously unknown mechanism involving SD-related proteins.

CONCLUSIONS

Overall, our findings shed new light on the pathogenesis of SRNS-FSGS and revealed that the novel pathogenic mutations in CRB2 contribute to the development of FSGS through a previously unknown mechanism involving SD-related proteins. A higher resolution version of the Graphical abstract is available as Supplementary information.

"Idiopathic" minimal change nephrotic syndrome: a podocyte mystery nears the end

The discovery of zinc fingers and homeoboxes (ZHX) transcriptional factors and the upregulation of hyposialylated angiopoietin-like 4 (ANGPTL4) in podocytes have been crucial in explaining the cardinal manifestations of human minimal change nephrotic syndrome (MCNS). Recently, uncovered genomic defects upstream of ZHX2 induce a ZHX2 hypomorph state that makes podocytes inherently susceptible to mild cytokine storms resulting from a common cold. In ZHX2 hypomorph podocytes, ZHX proteins are redistributed away from normal transmembrane partners like aminopeptidase A (APA) toward alternative binding partners like IL-4Rα. During disease relapse, high plasma soluble IL-4Rα (sIL-4Rα) associated with chronic atopy complements the cytokine milieu of a common cold to displace ZHX1 from podocyte transmembrane IL-4Rα toward the podocyte nucleus. Nuclear ZHX1 induces severe upregulation of ANGPTL4, resulting in incomplete sialylation of part of the ANGPTL4 protein, secretion of hyposialylated ANGPTL4, and hyposialylation-related injury in the glomerulus. This pattern of injury induces many of the classic manifestations of human minimal change disease (MCD), including massive and selective proteinuria, podocyte foot process effacement, and loss of glomerular basement membrane charge. Administration of glucocorticoids reduces ANGPTL4 upregulation, which reduces hyposialylation injury to improve the clinical phenotype. Improving sialylation of podocyte-secreted ANGPTL4 also reduces proteinuria and improves experimental MCD. Neutralizing circulating TNF-α, IL-6, or sIL-4Rα after the induction of the cytokine storm in Zhx2 hypomorph mice reduces albuminuria, suggesting potential new therapeutic targets for clinical trials to prevent MCD relapse. These studies collectively lay to rest prior suggestions of a role of single cytokines or soluble proteins in triggering MCD relapse.

MAGI2 ameliorates podocyte apoptosis of diabetic kidney disease through communication with TGF-β-Smad3/nephrin pathway

Podocytes, the key component of the glomerular filtration barrier (GFB), are gradually lost during the progression of diabetic kidney disease (DKD), severely compromising kidney functionality. The molecular mechanisms regulating the survival of podocytes in DKD are incompletely understood. Here, we show that membrane-associated guanylate kinase inverted 2 (MAGI2) is specifically expressed in renal podocytes, and promotes podocyte survival in DKD. We found that MAGI2 expression was downregulated in podocytes cultured with high-glucose in vitro, and in kidneys of db/db mice as well as DKD patients. Conversely, we found enforced expression of MAGI2 via AAV transduction protected podocytes from apoptosis, with concomitant improvement of renal functions. Mechanistically, we found that MAGI2 deficiency induced by high glucose levels activates TGF-β signaling to decrease the expression of anti-apoptotic proteins. These results indicate that MAGI2 protects podocytes from cell death, and can be harnessed therapeutically to improve renal function in diabetic kidney disease.

Adeno-associated virus gene therapy prevents progression of kidney disease in genetic models of nephrotic syndrome

Gene therapy for kidney diseases has proven challenging. Adeno-associated virus (AAV) is used as a vector for gene therapy targeting other organs, with particular success demonstrated in monogenic diseases. We aimed to establish gene therapy for the kidney by targeting a monogenic disease of the kidney podocyte. The most common cause of childhood genetic nephrotic syndrome is mutations in the podocyte gene NPHS2, encoding podocin. We used AAV-based gene therapy to rescue this genetic defect in human and mouse models of disease. In vitro transduction studies identified the AAV-LK03 serotype as a highly efficient transducer of human podocytes. AAV-LK03-mediated transduction of podocin in mutant human podocytes resulted in functional rescue in vitro, and AAV 2/9-mediated gene transfer in both the inducible podocin knockout and knock-in mouse models resulted in successful amelioration of kidney disease. A prophylactic approach of AAV 2/9 gene transfer before induction of disease in conditional knockout mice demonstrated improvements in albuminuria, plasma creatinine, plasma urea, plasma cholesterol, histological changes, and long-term survival. A therapeutic approach of AAV 2/9 gene transfer 2 weeks after disease induction in proteinuric conditional knock-in mice demonstrated improvement in urinary albuminuria at days 42 and 56 after disease induction, with corresponding improvements in plasma albumin. Therefore, we have demonstrated successful AAV-mediated gene rescue in a monogenic renal disease and established the podocyte as a tractable target for gene therapy approaches.

State of the art in childhood nephrotic syndrome: concrete discoveries and unmet needs

Nephrotic syndrome (NS) is a clinical entity characterized by proteinuria, hypoalbuminemia, and peripheral edema. NS affects about 2-7 per 100,000 children aged below 18 years old yearly and is classified, based on the response to drugs, into steroid sensitive (SSNS), steroid dependent, (SDNS), multidrug dependent (MDNS), and multidrug resistant (MRNS). Forms of NS that are more difficult to treat are associated with a worse outcome with respect to renal function. In particular, MRNS commonly progresses to end stage renal failure requiring renal transplantation, with recurrence of the original disease in half of the cases. Histological presentations of NS may vary from minimal glomerular lesions (MCD) to focal segmental glomerulosclerosis (FSGS) and, of relevance, the histological patterns do not correlate with the response to treatments. Moreover, around half of MRNS cases are secondary to causative pathogenic variants in genes involved in maintaining the glomerular structure. The pathogenesis of NS is still poorly understood and therapeutic approaches are mostly based on clinical experience. Understanding of pathogenetic mechanisms of NS is one of the 'unmet needs' in nephrology and represents a significant challenge for the scientific community. The scope of the present review includes exploring relevant findings, identifying unmet needs, and reviewing therapeutic developments that characterize NS in the last decades. The main aim is to provide a basis for new perspectives and mechanistic studies in NS.

Recent Advances in Proteinuric Kidney Disease/Nephrotic Syndrome: Lessons from Knockout/Transgenic Mouse Models

Proteinuria is known to be associated with all-cause and cardiovascular mortality, and nephrotic syndrome is defined by the level of proteinuria and hypoalbuminemia. With advances in medicine, new causative genes for genetic kidney diseases are being discovered increasingly frequently. We reviewed articles on proteinuria/nephrotic syndrome, focal segmental glomerulosclerosis, membranous nephropathy, diabetic kidney disease/nephropathy, hypertension/nephrosclerosis, Alport syndrome, and rare diseases, which have been studied in mouse models. Significant progress has been made in understanding the genetics and pathophysiology of kidney diseases thanks to advances in science, but research in this area is ongoing. In the future, genetic analyses of patients with proteinuric kidney disease/nephrotic syndrome may ultimately lead to personalized treatment options.

Tensins in Kidney Function and Diseases

Tensins are focal adhesion proteins that regulate various biological processes, such as mechanical sensing, cell adhesion, migration, invasion, and proliferation, through their multiple binding activities that transduce critical signals across the plasma membrane. When these molecular interactions and/or mediated signaling are disrupted, cellular activities and tissue functions are compromised, leading to disease development. Here, we focus on the significance of the tensin family in renal function and diseases. The expression pattern of each tensin in the kidney, their roles in chronic kidney diseases, renal cell carcinoma, and their potentials as prognostic markers and/or therapeutic targets are discussed in this review.

Cytokine storm-based mechanisms for extrapulmonary manifestations of SARS-CoV-2 infection

Viral illnesses like SARS-CoV-2 have pathologic effects on nonrespiratory organs in the absence of direct viral infection. We injected mice with cocktails of rodent equivalents of human cytokine storms resulting from SARS-CoV-2/COVID-19 or rhinovirus common cold infection. At low doses, COVID-19 cocktails induced glomerular injury and albuminuria in zinc fingers and homeoboxes 2 (Zhx2) hypomorph and Zhx2+/+ mice to mimic COVID-19-related proteinuria. Common Cold cocktail induced albuminuria selectively in Zhx2 hypomorph mice to model relapse of minimal change disease, which improved after depletion of TNF-α, soluble IL-4Rα, or IL-6. The Zhx2 hypomorph state increased cell membrane to nuclear migration of podocyte ZHX proteins in vivo (both cocktails) and lowered phosphorylated STAT6 activation (COVID-19 cocktail) in vitro. At higher doses, COVID-19 cocktails induced acute heart injury, myocarditis, pericarditis, acute liver injury, acute kidney injury, and high mortality in Zhx2+/+ mice, whereas Zhx2 hypomorph mice were relatively protected, due in part to early, asynchronous activation of STAT5 and STAT6 pathways in these organs. Dual depletion of cytokine combinations of TNF-α with IL-2, IL-13, or IL-4 in Zhx2+/+ mice reduced multiorgan injury and eliminated mortality. Using genome sequencing and CRISPR/Cas9, an insertion upstream of ZHX2 was identified as a cause of the human ZHX2 hypomorph state.

Phosphorylation-dependent recognition of diverse protein targets by the cryptic GK domain of MAGI MAGUKs

Dynamic signal transduction requires the rapid assembly and disassembly of signaling complexes, often mediated by phosphoprotein binding modules. The guanylate kinase-like (GK) domain of the membrane-associated guanylate kinases (MAGUKs) is such a module orchestrating signaling at cellular junctions. The MAGI subfamily of MAGUKs contains a truncated GK domain with unknown structure and function, although they participate in diverse physiological and pathological processes. Here, we demonstrate that the truncated GK domain of MAGI2 interacts with its adjacent PDZ0 domain to form a structural supramodule capable of recognizing phosphoproteins. A conserved phosphorylation-dependent binding motif for PDZ0-GK is delineated, which leads to identification of a set of previously unknown binding partners. We explore the structure and function of the MAGI2-target complex with an inhibitory peptide derived from the consensus motif. Our work reveals an action mechanism of the cryptic MAGI GKs and broadens our understanding of the target recognition rules of phosphoprotein binding modules.

Spotlight on Genetic Kidney Diseases: A Call for Drug Delivery and Nanomedicine Solutions

Nanoparticles as drug delivery carriers have benefited diseases, including cancer, since the 1990s, and more recently, their promise to quickly and efficiently be mobilized to fight against global diseases such as in the COVID-19 pandemic have been proven. Despite these success stories, there are limited nanomedicine efforts for chronic kidney diseases (CKDs), which affect 844 million people worldwide and can be linked to a variety of genetic kidney diseases. In this Perspective, we provide a brief overview of the clinical status of genetic kidney diseases, background on kidney physiology and a summary of nanoparticle design that enable kidney access and targeting, and emerging technological strategies that can be applied for genetic kidney diseases, including rare and congenital kidney diseases. Finally, we conclude by discussing gaps in knowledge remaining in both genetic kidney diseases and kidney nanomedicine and collective efforts that are needed to bring together stakeholders from diverse expertise and industries to enable the development of the most relevant drug delivery strategies that can make an impact in the clinic.

Abnormal glomerular basement membrane maturation impairs mesangial cell differentiation during murine postnatal nephrogenesis

Although mesangial cell-glomerular basement membrane (GBM) connections play a key role in maintaining the glomerular capillary loop structure, information remains limited about how these connections are formed during glomerulogenesis. We have previously shown that weakened podocyte-GBM interactions owing to tensin 2 (Tns2) deficiency lead to abnormal GBM maturation during postnatal glomerulogenesis. Here, we investigated whether abnormal GBM maturation affected mesangial cell-GBM connections and mesangial cell differentiation. Histological analysis of the outer cortical glomeruli in Tns2-deficient mice revealed that GBM materials overproduced by stressed immature podocytes accumulated in the mesangium and interrupted the formation of mesangial cell-GBM connections, resulting in fewer capillary loops compared with that of normal glomeruli. In addition, expression of α-smooth muscle actin, an immature mesangial cell marker, persisted in mesangial cells of Tns2-deficient outer cortical glomeruli even after glomerulogenesis was completed, resulting in mesangial expansion. Furthermore, analysis of mouse primary mesangial cells revealed that mesangial cell differentiation depended on the type of extracellular matrix components to which the cells adhered, suggesting the participation of mesangial cell-GBM connections in mesangial cell differentiation. These findings suggest that abnormal GBM maturation affects mesangial cell differentiation by impairing mesangial cell-GBM connections.NEW & NOTEWORTHY Mesangial cell-glomerular basement membrane (GBM) connections play an important role in maintaining the structural integrity of the glomerular tuft. However, information remains scarce about how GBM maturation affects the formation of these connections during glomerular development. Here, we show that abnormal GBM maturation due to tensin 2 deficiency affects mesangial cell differentiation by impairing mesangial cell-GBM connections during postnatal glomerulogenesis.

The magic of MAGI-1: A scaffolding protein with multi signalosomes and functional plasticity

MAGI-1 is a critical cellular scaffolding protein with over 110 different cellular and microbial protein interactors. Since the discovery of MAGI-1 in 1997, MAGI-1 has been implicated in diverse cellular functions such as polarity, cell-cell communication, neurological processes, kidney function, and a host of diseases including cancer and microbial infection. Additionally, MAGI-1 has undergone nomenclature changes in response to the discovery of an additional PDZ domain, leading to lack of continuity in the literature. We address the nomenclature of MAGI-1 as well as summarize many of the critical functions of the known interactions. Given the importance of many of the interactors, such as human papillomavirus E6, the Coxsackievirus and adenovirus receptor (CAR), and PTEN, the enhancement or disruption of MAGI-based interactions has the potential to affect cellular functions that can potentially be harnessed as a therapeutic strategy for a variety of diseases.

Tensin 2-deficient nephropathy - mechanosensitive nephropathy, genetic susceptibility

Tensin 2 (TNS2), a focal adhesion protein, is considered to anchor focal adhesion proteins to β integrin as an integrin adaptor protein and/or serve as a scaffold to facilitate the interactions of these proteins. In the kidney, TNS2 localizes to the basolateral surface of glomerular epithelial cells, i.e., podocytes. Loss of TNS2 leads to the development of glomerular basement membrane lesions and abnormal accumulation of extracellular matrix in maturing glomeruli during the early postnatal stages. It subsequently results in podocyte foot process effacement, eventually leading to glomerulosclerosis. Histopathological features of the affected glomeruli in the middle stage of the disease include expansion of the mesangial matrix without mesangial cell proliferation. In this review, we provide an overview of TNS2-deficient nephropathy and discuss the potential mechanism underlying this mechanosensitive nephropathy, which may be applicable to other glomerulonephropathies, such as CD151-deficient nephropathy and Alport syndrome. The onset of TNS2-deficient nephropathy strictly depends on the genetic background, indicating the presence of critical modifier genes. A better understanding of molecular mechanisms of mechanosensitive nephropathy may open new avenues for the management of patients with glomerulonephropathies.

Six years' accomplishment of the Initiative on Rare and Undiagnosed Diseases: nationwide project in Japan to discover causes, mechanisms, and cures

The identification of causative genetic variants for hereditary diseases has revolutionized clinical medicine and an extensive collaborative framework with international cooperation has become a global trend to understand rare disorders. The Initiative on Rare and Undiagnosed Diseases (IRUD) was established in Japan to provide accurate diagnosis, discover causes, and ultimately provide cures for rare and undiagnosed diseases. The fundamental IRUD system consists of three pillars: IRUD diagnostic coordination, analysis centers (IRUD-ACs), and a data center (IRUD-DC). IRUD diagnostic coordination consists of clinical centers (IRUD-CLs) and clinical specialty subgroups (IRUD-CSSs). In addition, the IRUD coordinating center (IRUD-CC) manages the entire IRUD system and temporarily operates the IRUD resource center (IRUD-RC). By the end of March 2021, 6301 pedigrees consisting of 18,136 individuals were registered in the IRUD. The whole-exome sequencing method was completed in 5136 pedigrees, and a final diagnosis was established in 2247 pedigrees (43.8%). The total number of aberrated genes and pathogenic variants was 657 and 1718, among which 1113 (64.8%) were novel. In addition, 39 novel disease entities or phenotypes with 41 aberrated genes were identified. The 6-year endeavor of IRUD has been an overwhelming success, establishing an all-Japan comprehensive diagnostic and research system covering all geographic areas and clinical specialties/subspecialties. IRUD has accurately diagnosed diseases, identified novel aberrated genes or disease entities, discovered many candidate genes, and enriched phenotypic and pathogenic variant databases. Further promotion of the IRUD is essential for determining causes and developing cures for rare and undiagnosed diseases.

Rap1 Activity Is Essential for Focal Adhesion and Slit Diaphragm Integrity

Glomerular podocytes build, with their intercellular junctions, part of the kidney filter. The podocyte cell adhesion protein, nephrin, is essential for developing and maintaining slit diaphragms as functional loss in humans results in heavy proteinuria. Nephrin expression and function are also altered in many adult-onset glomerulopathies. Nephrin signals from the slit diaphragm to the actin cytoskeleton and integrin β1 at focal adhesions by recruiting Crk family proteins, which can interact with the Rap guanine nucleotide exchange factor 1 C3G. As Rap1 activity affects focal adhesion formation, we hypothesize that nephrin signals via Rap1 to integrin β. To address this issue, we combined Drosophila in vivo and mammalian cell culture experiments. We find that Rap1 is necessary for correct targeting of integrin β to focal adhesions in Drosophila nephrocytes, which also form slit diaphragm-like structures. In the fly, the Rap1 activity is important for signaling of the nephrin ortholog to integrin β, as well as for nephrin-dependent slit diaphragm integrity. We show by genetic interaction experiments that Rap1 functions downstream of nephrin signaling to integrin β and downstream of nephrin signaling necessary for slit diaphragm integrity. Similarly, in human podocyte culture, nephrin activation results in increased activation of Rap1. Thus, Rap1 is necessary for downstream signal transduction of nephrin to integrin β.

A systematic review of the natural history and biomarkers of primary lecithin:cholesterol acyltransferase deficiency

Syndromes associated with LCAT deficiency, a rare autosomal recessive condition, include fish-eye disease (FED) and familial LCAT deficiency (FLD). FLD is more severe and characterized by early and progressive chronic kidney disease (CKD). No treatment is currently available for FLD, but novel therapeutics are under development. Furthermore, although biomarkers of LCAT deficiency have been identified, their suitability to monitor disease progression and therapeutic efficacy is unclear, as little data exist on the rate of progression of renal disease. Here, we systematically review observational studies of FLD, FED, and heterozygous subjects, which summarize available evidence on the natural history and biomarkers of LCAT deficiency, in order to guide the development of novel therapeutics. We identified 146 FLD and 53 FED patients from 219 publications, showing that both syndromes are characterized by early corneal opacity and markedly reduced HDL-C levels. Proteinuria/hematuria were the first signs of renal impairment in FLD, followed by rapid decline of renal function. Furthermore, LCAT activity toward endogenous substrates and the percentage of circulating esterified cholesterol (EC%) were the best discriminators between these two syndromes. In FLD, higher levels of total, non-HDL, and unesterified cholesterol were associated with severe CKD. We reveal a nonlinear association between LCAT activity and EC% levels, in which subnormal levels of LCAT activity were associated with normal EC%. This review provides the first step toward the identification of disease biomarkers to be used in clinical trials and suggests that restoring LCAT activity to subnormal levels may be sufficient to prevent renal disease progression.

Steroid-sensitive nephrotic syndrome candidate gene CLVS1 regulates podocyte oxidative stress and endocytosis

We performed next-generation sequencing in patients with familial steroid-sensitive nephrotic syndrome (SSNS) and identified a homozygous segregating variant (p.H310Y) in the gene encoding clavesin-1 (CLVS1) in a consanguineous family with 3 affected individuals. Knockdown of the clavesin gene in zebrafish (clvs2) produced edema phenotypes due to disruption of podocyte structure and loss of glomerular filtration barrier integrity that could be rescued by WT CLVS1 but not the p.H310Y variant. Analysis of cultured human podocytes with CRISPR/Cas9-mediated CLVS1 knockout or homozygous H310Y knockin revealed deficits in clathrin-mediated endocytosis and increased susceptibility to apoptosis that could be rescued with corticosteroid treatment, mimicking the steroid responsiveness observed in patients with SSNS. The p.H310Y variant also disrupted binding of clavesin-1 to α-tocopherol transfer protein, resulting in increased reactive oxygen species (ROS) accumulation in CLVS1-deficient podocytes. Treatment of CLVS1-knockout or homozygous H310Y-knockin podocytes with pharmacological ROS inhibitors restored viability to control levels. Taken together, these data identify CLVS1 as a candidate gene for SSNS, provide insight into therapeutic effects of corticosteroids on podocyte cellular dynamics, and add to the growing evidence of the importance of endocytosis and oxidative stress regulation to podocyte function.

Rho GTPases in kidney physiology and diseases

Rho family GTPases are molecular switches best known for their pivotal role in dynamic regulation of the actin cytoskeleton, but also of cellular morphology, motility, adhesion and proliferation. The prototypic members of this family (RhoA, Rac1 and Cdc42) also contribute to the normal kidney function and play important roles in the structure and function of various kidney cells including tubular epithelial cells, mesangial cells and podocytes. The kidney's vital filtration function depends on the structural integrity of the glomerulus, the proximal portion of the nephron. Within the glomerulus, the architecturally actin-based cytoskeleton podocyte forms the final cellular barrier to filtration. The glomerulus appears as a highly dynamic signalling hub that is capable of integrating intracellular cues from its individual structural components. Dynamic regulation of the podocyte cytoskeleton is required for efficient barrier function of the kidney. As master regulators of actin cytoskeletal dynamics, Rho GTPases are therefore of critical importance for sustained kidney barrier function. Dysregulated activities of the Rho GTPases and of their effectors are implicated in the pathogenesis of both hereditary and idiopathic forms of kidney diseases. Diabetic nephropathy is a progressive kidney disease that is caused by injury to kidney glomeruli. High glucose activates RhoA/Rho-kinase in mesangial cells, leading to excessive extracellular matrix production (glomerulosclerosis). This RhoA/Rho-kinase pathway also seems involved in the post-transplant hypertension frequently observed during treatment with calcineurin inhibitors, whereas Rac1 activation was observed in post-transplant ischaemic acute kidney injury.

An updated view of the pathogenesis of steroid-sensitive nephrotic syndrome

Idiopathic nephrotic syndrome is the most common childhood glomerular disease. Most forms of this syndrome respond to corticosteroids at standard doses and are, therefore, defined as steroid-sensitive nephrotic syndrome (SSNS). Immunological mechanisms and subsequent podocyte disorders play a pivotal role in SSNS and have been studied for years; however, the precise pathogenesis remains unclear. With recent advances in genetic techniques, an exhaustive hypothesis-free approach called a genome-wide association study (GWAS) has been conducted in various populations. GWASs in pediatric SSNS peaked in the human leukocyte antigen class II region in various populations. Additionally, an association of immune-related CALHM6/FAM26F, PARM1, BTNL2, and TNFSF15 genes, as well as NPHS1, which encodes nephrin expressed in podocytes, has been identified as a locus that achieves genome-wide significance in pediatric SSNS. However, the specific mechanism of SSNS development requires elucidation. This review describes an updated view of SSNS pathogenesis from immunological and genetic aspects, including interactions with infections or allergies, production of circulating factors, and an autoantibody hypothesis.

A critical re-analysis of cases of post-transplantation recurrence in genetic nephrotic syndrome

BACKGROUND

Genetic defects in podocyte proteins account for up to 30% of steroid-resistant nephrotic syndrome (SRNS) in the paediatric population. Most children with genetic SRNS are resistant to immunosuppression and at high risk of progression to stage 5 chronic kidney disease. Kidney transplantation is often the treatment of choice. The possibility of post-transplantation disease recurrence in genetic SRNS remains controversial, and poses fundamental questions about disease biology.

METHODS

We critically evaluated the published cases of post-transplantation recurrence in genetic patients, particularly testing 'mutations' against the most recent population variant databases, in order to clarify the diagnoses, and compare the clinical courses and responses to therapy.

RESULTS

Biallelic pathogenic variants in NPHS1 leading to a complete absence of nephrin were the most commonly reported and best understood instance of nephrotic syndrome occurring post-transplantation. This is an immune-mediated process driven by antibody production against the novel nephrin protein in the allograft. We also identified a number of plausible reported cases of post-transplantation recurrence involving pathogenic variants in NPHS2 (8 patients, biallelic), one in WT1 (monoallelic) and one in NUP93 (biallelic). However, the mechanism for recurrence in these cases remains unclear. Other instances of recurrence in genetic disease were difficult to interpret due to differing clinical criteria, inclusion of patients without true pathogenic variants or the influence of other factors on renal outcome.

CONCLUSIONS

Overall, post-transplantation recurrence remains very rare in patients with genetic SRNS. It appears to occur later after transplantation than in other patients and usually responds well to plasmapheresis with a good renal outcome.

Deciphering the Q-markers of nourishing kidney-yin of Cortex Phellodendri amurense from ZhibaiDihuang pill based on Chinmedomics strategy

BACKGROUND

Cortex Phellodendri amurensis (CPA) has high medicinal value in the treatment of kidney-yin deficiency diseases. However, due to the lack of research on the therapeutic material basis of CPA, the current quality control standard for CPA is defective, and the effect of the nourishing kidney-yin of CPA was limited.

PURPOSE

Based on the principle of correspondence between the syndrome and prescriptions, we studied the CPA in ZhibaiDihuang pill (ZBDH) to identify quality markers (Q-markers) of CPA in ZBDH for treating kidney-yin deficiency and seek the potential Q-markers of CPA under nourishing kidney-yin effect combined with the analysis of single CPA.

METHODS

Taking Chinmedomics as the core strategy, metabonomics analysis and effective component identification were performed by UPLC-MS.

RESULTS

A total of 121 chemical components of ZBDH were identified, among which the contents of berberine, palmatine, jatrorrhizine and magnoflorine changed the most obviously with the addition of CPA. Forty-five components were identified in the blood in the markedly effective state, including berberine, palmatine, jatrorrhizine and magnoflorine. The therapeutic material basis of ZBDH in the treatment of kidney-yin deficiency was found, and 6 components were found to derive from CPA, including magnoflorine and jatrorrhizine. In addition, seventeen components were identified in the blood in the single CPA treatment, including berberine, palmatine, jatrorrhizine and magnoflorine.

CONCLUSIONS

Magnoflorine and jatrorrhizine were the Q-markers of CPA for treating kidney-yin deficiency in the formula of ZBDH and they were also potential Q-markers of the nourishing kidney-yin of CPA.

Trends in pediatric nephrotic syndrome

Nephrotic syndrome (NS) is relatively common in children, with most of its histological types being minimal changed disease. Its etiology has long been attributed to lymphocyte (especially T-cell) dysfunction, while T-cell-mediated vascular hyperpermeability increases protein permeability in glomerular capillaries, leading to proteinuria and hypoproteinemia. Based on this etiology, steroids and immunosuppressive drugs that are effective against this disease have also been considered to correct T-cell dysfunction. However, in recent years, this has been questioned. The primary cause of NS has been considered damage to glomerular epithelial cells and podocyte-related proteins. Therefore, we first describe the changes in expression of molecules involved in NS etiology, and then describe the mechanism by which abnormal expression of these molecules induces proteinuria. Finally, we consider the mechanism by which infection causes the recurrence of NS.

Identification of Genetic Causes of Focal Segmental Glomerulosclerosis Increases With Proper Patient Selection

OBJECTIVE

To increase the likelihood of finding a causative genetic variant in patients with a focal segmental glomerulosclerosis (FSGS) lesion, clinical and histologic characteristics were analyzed.

PATIENTS AND METHODS

Individuals 18 years and older with an FSGS lesion on kidney biopsy evaluated at Mayo Clinic from November 1, 1999, through October 31, 2019, were divided into 4 groups based on clinical and histologic characteristics: primary FSGS, secondary FSGS with known cause, secondary FSGS without known cause, and undetermined FSGS. A targeted gene panel and a customized gene panel retrieved from exome sequencing were performed.

RESULTS

The overall rate of detection of a monogenic cause was 42.9% (21/49). Individuals with undetermined FSGS had the highest rate of positivity (87.5%; 7/8) followed by secondary FSGS without an identifiable cause (61.5%; 8/13) and secondary FSGS with known cause (33.3%; 5/15). Four of 5 (80%) individuals in the latter group who had positive genetic testing results also had a family history of kidney disease. Univariate analysis showed that family history of kidney disease (odds ratio [OR], 13.8; 95% CI, 3.7 to 62.4; P<.001), absence of nephrotic syndrome (OR, 8.2; 95% CI, 1.9 to 58.1; P=.004), and female sex (OR, 5.1; 95% CI, 1.5 to 19.9; P=.01) were strong predictors of finding a causative genetic variant in the entire cohort. The most common variants were in the collagen genes (52.4%; 11/21), followed by the podocyte genes (38.1%; 8/21).

CONCLUSION

In adults with FSGS lesions, proper selection of patients increases the rate of positive genetic testing significantly. The majority of individuals with undetermined FSGS in whom the clinical presentation and histologic parameters are discordant had a genetic diagnosis.

Therapeutic trials in adult FSGS: lessons learned and the road forward

Focal segmental glomerulosclerosis (FSGS) is not a specific disease entity but a lesion that primarily targets the podocyte. In a broad sense, the causes of the lesion can be divided into those triggered by a presumed circulating permeability factor, those that occur secondary to a process that might originate outside the kidneys, those caused by a genetic mutation in a podocyte or glomerular basement membrane protein, and those that arise through an as yet unidentifiable process, seemingly unrelated to a circulating permeability factor. A careful attempt to correctly stratify patients with FSGS based on their clinical presentation and pathological findings on kidney biopsy is essential for sound treatment decisions in individual patients. However, it is also essential for the rational design of therapeutic trials in FSGS. Greater recognition of the pathophysiology underlying podocyte stress and damage in FSGS will increase the likelihood that the cause of an FSGS lesion is properly identified and enable stratification of patients in future interventional trials. Such efforts will facilitate the identification of effective therapeutic agents.

Genomics Integration Into Nephrology Practice

RATIONALE & OBJECTIVE

The etiology of kidney disease remains unknown in many individuals with chronic kidney disease (CKD). We created the Mayo Clinic Nephrology Genomics Clinic to improve our ability to integrate genomic and clinical data to identify the etiology of unexplained CKD.

STUDY DESIGN

Retrospective study.

SETTING & PARTICIPANTS

An essential component of our program is the Nephrology Genomics Board which consists of nephrologists, geneticists, pathologists, translational omics scientists, and trainees who interpret the patient's clinical and genetic data. Since September 2016, the Board has reviewed 163 cases (15 cystic, 100 glomerular, 6 congenital anomalies of kidney and urinary tract (CAKUT), 20 stones, 15 tubulointerstitial, and 13 other).

ANALYTICAL APPROACH

Testing was performed with targeted panels, single gene analysis, or analysis of kidney-related genes from exome sequencing. Variant classification was obtained based on the 2015 American College of Medical Genetics and Genomics and the Association for Molecular Pathology guidelines.

RESULTS

A definitive genetic diagnosis was achieved for 50 families (30.7%). The highest diagnostic yield was obtained in individuals with tubulointerstitial diseases (53.3%), followed by congenital anomalies of the kidney and urological tract (33.3%), glomerular (31%), cysts (26.7%), stones (25%), and others (15.4%). A further 20 (12.3%) patients had variants of interest, and variant segregation, and research activities (exome, genome, or transcriptome sequencing) are ongoing for 44 (40%) unresolved families.

LIMITATIONS

Possible overestimation of diagnostic rate due to inclusion of individuals with variants with evidence of pathogenicity but classified as of uncertain significance by the clinical laboratory.

CONCLUSIONS

Integration of genomic and research testing and multidisciplinary evaluation in a nephrology cohort with CKD of unknown etiology or suspected monogenic disease provided a diagnosis in a third of families. These diagnoses had prognostic implications, and often changes in management were implemented.

Novel insights in the genetics of steroid-sensitive nephrotic syndrome in childhood

Steroid-sensitive nephrotic syndrome (SSNS) is the most common form of nephrotic syndrome in childhood and there is growing evidence that genetics play a role in the susceptibility for the disease. Familial clustering has been observed and has led to several studies on familial SSNS trying to identify a monogenic cause of the disease. Until now, however, none of these have provided convincing evidence for Mendelian inheritance. This and the phenotypic variability within SSNS suggest a complex inheritance pattern, where multiple variants and interactions between those and the environment play roles in disease development. Genome-wide association studies (GWASs) have been used to investigate this complex disease. We herein highlight new insights in the genetics of the disease provided by GWAS and identify how these insights fit into our understanding of the pathogenesis of SSNS.

Phase Separation of MAGI2-Mediated Complex Underlies Formation of Slit Diaphragm Complex in Glomerular Filtration Barrier

BACKGROUND

Slit diaphragm is a specialized adhesion junction between the opposing podocytes, establishing the final filtration barrier to urinary protein loss. At the cytoplasmic insertion site of each slit diaphragm there is an electron-dense and protein-rich cellular compartment that is essential for slit diaphragm integrity and signal transduction. Mutations in genes that encode components of this membrane-less compartment have been associated with glomerular diseases. However, the molecular mechanism governing formation of compartmentalized slit diaphragm assembly remains elusive.

METHODS

We systematically investigated the interactions between key components at slit diaphragm, such as MAGI2, Dendrin, and CD2AP, through a combination of biochemical, biophysical, and cell biologic approaches.

RESULTS

We demonstrated that MAGI2, a unique MAGUK family scaffold protein at slit diaphragm, can autonomously undergo liquid-liquid phase separation. Multivalent interactions among the MAGI2-Dendrin-CD2AP complex drive the formation of the highly dense slit diaphragm condensates at physiologic conditions. The reconstituted slit diaphragm condensates can effectively recruit Nephrin. A nephrotic syndrome-associated mutation of MAGI2 interfered with formation of the slit diaphragm condensates, thus leading to impaired enrichment of Nephrin.

CONCLUSIONS

Key components at slit diaphragm (e.g., MAGI2 and its complex) can spontaneously undergo phase separation. The reconstituted slit diaphragm condensates can be enriched in adhesion molecules and cytoskeletal adaptor proteins. Therefore, the electron-dense slit diaphragm assembly might form via phase separation of core components of the slit diaphragm in podocytes.

MAGI1, a Scaffold Protein with Tumor Suppressive and Vascular Functions

MAGI1 is a cytoplasmic scaffolding protein initially identified as a component of cell-to-cell contacts stabilizing cadherin-mediated cell–cell adhesion in epithelial and endothelial cells. Clinical-pathological and experimental evidence indicates that MAGI1 expression is decreased in some inflammatory diseases, and also in several cancers, including hepatocellular carcinoma, colorectal, cervical, breast, brain, and gastric cancers and appears to act as a tumor suppressor, modulating the activity of oncogenic pathways such as the PI3K/AKT and the Wnt/β-catenin pathways. Genomic mutations and other mechanisms such as mechanical stress or inflammation have been described to regulate MAGI1 expression. Intriguingly, in breast and colorectal cancers, MAGI1 expression is induced by non-steroidal anti-inflammatory drugs (NSAIDs), suggesting a role in mediating the tumor suppressive activity of NSAIDs. More recently, MAGI1 was found to localize at mature focal adhesion and to regulate integrin-mediated adhesion and signaling in endothelial cells. Here, we review MAGI1′s role as scaffolding protein, recent developments in the understanding of MAGI1 function as tumor suppressor gene, its role in endothelial cells and its implication in cancer and vascular biology. We also discuss outstanding questions about its regulation and potential translational implications in oncology.

Urine-Derived Epithelial Cells as Models for Genetic Kidney Diseases

Epithelial cells exfoliated in human urine can include cells anywhere from the urinary tract and kidneys; however, podocytes and proximal tubular epithelial cells (PTECs) are by far the most relevant cell types for the study of genetic kidney diseases. When maintained in vitro, they have been proven extremely valuable for discovering disease mechanisms and for the development of new therapies. Furthermore, cultured patient cells can individually represent their human sources and their specific variants for personalized medicine studies, which are recently gaining much interest. In this review, we summarize the methodology for establishing human podocyte and PTEC cell lines from urine and highlight their importance as kidney disease cell models. We explore the well-established and recent techniques of cell isolation, quantification, immortalization and characterization, and we describe their current and future applications.

Tensins - emerging insights into their domain functions, biological roles and disease relevance

Tensins are a family of focal adhesion proteins consisting of four members in mammals (TNS1, TNS2, TNS3 and TNS4). Their multiple domains and activities contribute to the molecular linkage between the extracellular matrix and cytoskeletal networks, as well as mediating signal transduction pathways, leading to a variety of physiological processes, including cell proliferation, attachment, migration and mechanical sensing in a cell. Tensins are required for maintaining normal tissue structures and functions, especially in the kidney and heart, as well as in muscle regeneration, in animals. This Review discusses our current understanding of the domain functions and biological roles of tensins in cells and mice, as well as highlighting their relevance to human diseases.

Generation of Monogenic Candidate Genes for Human Nephrotic Syndrome Using 3 Independent Approaches

INTRODUCTION

Steroid-resistant nephrotic syndrome (SRNS) is the second most common cause of chronic kidney disease during childhood. Identification of 63 monogenic human genes has delineated 12 distinct pathogenic pathways.

METHODS

Here, we generated 2 independent sets of nephrotic syndrome (NS) candidate genes to augment the discovery of additional monogenic causes based on whole-exome sequencing (WES) data from 1382 families with NS.

RESULTS

We first identified 63 known monogenic causes of NS in mice from public databases and scientific publications, and 12 of these genes overlapped with the 63 known human monogenic SRNS genes. Second, we used a set of 64 genes that are regulated by the transcription factor Wilms tumor 1 (WT1), which causes SRNS if mutated. Thirteen of these WT1-regulated genes overlapped with human or murine NS genes. Finally, we overlapped these lists of murine and WT1 candidate genes with our list of 120 candidate genes generated from WES in 1382 NS families, to identify novel candidate genes for monogenic human SRNS. Using this approach, we identified 7 overlapping genes, of which 3 genes were shared by all datasets, including SYNPO. We show that loss-of-function of SYNPO leads to decreased CDC42 activity and reduced podocyte migration rate, both of which are rescued by overexpression of wild-type complementary DNA (cDNA), but not by cDNA representing the patient mutation.

CONCLUSION

Thus, we identified 3 novel candidate genes for human SRNS using 3 independent, nonoverlapping hypotheses, and generated functional evidence for SYNPO as a novel potential monogenic cause of NS.

CD80 Insights as Therapeutic Target in the Current and Future Treatment Options of Frequent-Relapse Minimal Change Disease

Minimal change disease (MCD) is the most common cause of idiopathic nephrotic syndrome in children, and it is well known for its multifactorial causes which are the manifestation of the disease. Proteinuria is an early consequence of podocyte injury and a typical sign of kidney disease. Steroid-sensitive patients react well with glucocorticoids, but there is a high chance of multiple relapses. CD80, also known as B7-1, is generally expressed on antigen-presenting cells (APCs) in steroid-sensitive MCD patients. Various glomerular disease models associated with proteinuria demonstrated that the detection of CD80 with the increase of urinary CD80 was strongly associated closely with frequent-relapse MCD patients. The role of CD80 in MCD became controversial because one contradicts finding. This review covers the treatment alternatives for MCD with the insight of CD80 as a potential therapeutic target. The promising effectiveness of CD20 (rituximab) antibody and CD80 inhibitor (abatacept) encourages further investigation of CD80 as a therapeutic target in frequent-relapse MCD patients. Therapeutic-based antibody towards CD80 (galiximab) had never been investigated in MCD or any kidney-related disease; hence, the role of CD80 is still undetermined. A new therapeutic approach towards MCD is essential to provide broader effective treatment options besides the general immunosuppressive agents with gruesome adverse effects.

Recessive NOS1AP variants impair actin remodeling and cause glomerulopathy in humans and mice

Nephrotic syndrome (NS) is a leading cause of chronic kidney disease. We found recessive NOS1AP variants in two families with early-onset NS by exome sequencing. Overexpression of wild-type (WT) NOS1AP, but not cDNA constructs bearing patient variants, increased active CDC42 and promoted filopodia and podosome formation. Pharmacologic inhibition of CDC42 or its effectors, formin proteins, reduced NOS1AP-induced filopodia formation. NOS1AP knockdown reduced podocyte migration rate (PMR), which was rescued by overexpression of WT Nos1ap but not by constructs bearing patient variants. PMR in NOS1AP knockdown podocytes was also rescued by constitutively active CDC42Q61L or the formin DIAPH3 Modeling a NOS1AP patient variant in knock-in human kidney organoids revealed malformed glomeruli with increased apoptosis. Nos1apEx3-/Ex3- mice recapitulated the human phenotype, exhibiting proteinuria, foot process effacement, and glomerulosclerosis. These findings demonstrate that recessive NOS1AP variants impair CDC42/DIAPH-dependent actin remodeling, cause aberrant organoid glomerulogenesis, and lead to a glomerulopathy in humans and mice.

Genetic Disorders of the Glomerular Filtration Barrier

The glomerular filtration barrier is a highly specialized capillary wall comprising fenestrated endothelial cells, podocytes, and an intervening basement membrane. In glomerular disease, this barrier loses functional integrity, allowing the passage of macromolecules and cells, and there are associated changes in both cell morphology and the extracellular matrix. Over the past 3 decades, there has been a transformation in our understanding about glomerular disease, fueled by genetic discovery, and this is leading to exciting advances in our knowledge about glomerular biology and pathophysiology. In current clinical practice, a genetic diagnosis already has important implications for management, ranging from estimating the risk of disease recurrence post-transplant to the life-changing advances in the treatment of atypical hemolytic uremic syndrome. Improving our understanding about the mechanistic basis of glomerular disease is required for more effective and personalized therapy options. In this review, we describe genotype and phenotype correlations for genetic disorders of the glomerular filtration barrier, with a particular emphasis on how these gene defects cluster by both their ontology and patterns of glomerular pathology.