An in vivo functional analysis system for renal gene discovery in Drosophila pericardial nephrocytes

The nutrient sensor CRTC and Sarcalumenin/thinman represent an alternate pathway in cardiac hypertrophy

CREB-regulated transcription co-activator (CRTC) is activated by Calcineurin (CaN) to regulate gluconeogenic genes. CaN also has roles in cardiac hypertrophy. Here, we explore a cardiac-autonomous role for CRTC in cardiac hypertrophy. In Drosophila, CRTC mutants exhibit severe cardiac restriction, myofibrillar disorganization, fibrosis, and tachycardia. Cardiac-specific CRTC knockdown (KD) phenocopies mutants, and cardiac overexpression causes hypertrophy. CaN-induced hypertrophy in Drosophila is reduced in CRTC mutants, suggesting that CRTC mediates the effects. RNA sequencing (RNA-seq) of CRTC-KD and -overexpressing hearts reveals contraregulation of metabolic genes. Genes with conserved CREB sites include the fly ortholog of Sarcalumenin, a Ca2+-binding protein. Cardiac manipulation of this gene recapitulates the CRTC-KD and -overexpression phenotypes. CRTC KD in zebrafish also causes cardiac restriction, and CRTC KD in human induced cardiomyocytes causes a reduction in Srl expression and increased action potential duration. Our data from three model systems suggest that CaN-CRTC-Sarcalumenin signaling represents an alternate, conserved pathway underlying cardiac function and hypertrophy.

HIV-1 Nef acts in synergy with APOL1-G1 to induce nephrocyte cell death in a new Drosophila model of HIV-related kidney diseases

Background: People carrying two APOL1 risk alleles (RA) G1 or G2 are at greater risk of developing HIV-associated nephropathy (HIVAN). Studies in transgenic mice showed that the expression of HIV-1 genes in podocytes, and nef in particular, led to HIVAN. However, it remains unclear whether APOL1-RA and HIV-1 Nef interact to induce podocyte cell death. Method: We generated transgenic (Tg) flies that express APOL1-G1 (derived from a child with HIVAN) and HIV-1 nef specifically in the nephrocytes, the fly equivalent of mammalian podocytes, and assessed their individual and combined effects on the nephrocyte filtration structure and function. Results: We found that HIV-1 Nef acts in synergy with APOL1-G1 resulting in nephrocyte structural and functional defects. Specifically, HIV-1 Nef itself can induce endoplasmic reticulum (ER) stress without affecting autophagy. Furthermore, Nef exacerbates the organelle acidification defects and autophagy reduction induced by APOL1-G1. The synergy between HIV-1 Nef and APOL1-G1 is built on their joint effects on elevating ER stress, triggering nephrocyte dysfunction and ultimately cell death. Conclusions: Using a new Drosophila model of HIV-1-related kidney diseases, we identified ER stress as the converging point for the synergy between HIV-1 Nef and APOL1-G1 in inducing nephrocyte cell death. Given the high relevance between Drosophila nephrocytes and human podocytes, this finding suggests ER stress as a new therapeutic target for HIV-1 and APOL1-associated nephropathies.

Promoting mitochondrial dynamics by inhibiting the PINK1-PRKN pathway to relieve diabetic nephropathy

Diabetes is a metabolic disorder characterized by high blood glucose levels and is a leading cause of kidney disease. Diabetic nephropathy has been attributed to dysfunctional mitochondria. However, many questions remain about the exact mechanism. The structure, function and molecular pathways are highly conserved between mammalian podocytes and Drosophila nephrocytes; therefore, we used flies on a high-sucrose diet to model type 2 diabetic nephropathy. The nephrocytes from flies on a high-sucrose diet showed a significant functional decline and decreased cell size, associated with a shortened lifespan. Structurally, the nephrocyte filtration structure, known as the slit diaphragm, was disorganized. At the cellular level, we found altered mitochondrial dynamics and dysfunctional mitochondria. Regulating mitochondrial dynamics by either genetic modification of the Pink1-Park (mammalian PINK1-PRKN) pathway or treatment with BGP-15, mitigated the mitochondrial defects and nephrocyte functional decline. These findings support a role for Pink1-Park-mediated mitophagy and associated control of mitochondrial dynamics in diabetic nephropathy, and demonstrate that targeting this pathway might provide therapeutic benefits for type 2 diabetic nephropathy.

A Drosophila model to screen Alport syndrome COL4A5 variants for their functional pathogenicity

Alport syndrome is a hereditary chronic kidney disease, attributed to rare pathogenic variants in either of three collagen genes (COL4A3/4/5) with most localized in COL4A5. Trimeric type IV Collagen α3α4α5 is essential for the glomerular basement membrane that forms the kidney filtration barrier. A means to functionally assess the many candidate variants and determine pathogenicity is urgently needed. We used Drosophila, an established model for kidney disease, and identify Col4a1 as the functional homolog of human COL4A5 in the fly nephrocyte (equivalent of human podocyte). Fly nephrocytes deficient for Col4a1 showed an irregular and thickened basement membrane and significantly reduced nephrocyte filtration function. This phenotype was restored by expressing human reference (wildtype) COL4A5, but not by COL4A5 carrying any of three established pathogenic patient-derived variants. We then screened seven additional patient COL4A5 variants; their ClinVar classification was either likely pathogenic or of uncertain significance. The findings support pathogenicity for four of these variants; the three others were found benign. Thus, demonstrating the effectiveness of this Drosophila in vivo kidney platform in providing the urgently needed variant-level functional validation.

The adhesion G-protein-coupled receptor mayo/CG11318 controls midgut development in Drosophila

Adhesion G-protein-coupled receptors (aGPCRs) form a large family of cell surface molecules with versatile tasks in organ development. Many aGPCRs still await their functional and pharmacological deorphanization. Here, we characterized the orphan aGPCR CG11318/mayo of Drosophila melanogaster and found it expressed in specific regions of the gastrointestinal canal and anal plates, epithelial specializations that control ion homeostasis. Genetic removal of mayo results in tachycardia, which is caused by hyperkalemia of the larval hemolymph. The hyperkalemic effect can be mimicked by a raise in ambient potassium concentration, while normal potassium levels in mayoKO mutants can be restored by pharmacological inhibition of potassium channels. Intriguingly, hyperkalemia and tachycardia are caused non-cell autonomously through mayo-dependent control of enterocyte proliferation in the larval midgut, which is the primary function of this aGPCR. These findings characterize the ancestral aGPCR Mayo as a homeostatic regulator of gut development.

APOL1-G2 accelerates nephrocyte cell death by inhibiting the autophagy pathway

People of African ancestry who carry the APOL1 risk alleles G1 or G2 are at high risk of developing kidney diseases through not fully understood mechanisms that impair the function of podocytes. It is also not clear whether the APOL1-G1 and APOL1-G2 risk alleles affect these cells through similar mechanisms. Previously, we have developed transgenic Drosophila melanogaster lines expressing either the human APOL1 reference allele (G0) or APOL1-G1 specifically in nephrocytes, the cells homologous to mammalian podocytes. We have found that nephrocytes that expressed the APOL1-G1 risk allele display accelerated cell death, in a manner similar to that of cultured human podocytes and APOL1 transgenic mouse models. Here, to compare how the APOL1-G1 and APOL1-G2 risk alleles affect the structure and function of nephrocytes in vivo, we generated nephrocyte-specific transgenic flies that either expressed the APOL1-G2 or both G1 and G2 (G1G2) risk alleles on the same allele. We found that APOL1-G2- and APOL1-G1G2-expressing nephrocytes developed more severe changes in autophagic pathways, acidification of organelles and the structure of the slit diaphragm, compared to G1-expressing nephrocytes, leading to their premature death. We conclude that both risk alleles affect similar key cell trafficking pathways, leading to reduced autophagy and suggesting new therapeutic targets to prevent APOL1 kidney diseases.

A SNARE protective pool antagonizes APOL1 renal toxicity in Drosophila nephrocytes

BACKGROUND

People of Sub-Saharan African ancestry are at higher risk of developing chronic kidney disease (CKD), attributed to the Apolipoprotein L1 (APOL1) gene risk alleles (RA) G1 and G2. The underlying mechanisms by which the APOL1-RA precipitate CKD remain elusive, hindering the development of potential treatments.

RESULTS

Using a Drosophila genetic modifier screen, we found that SNARE proteins (Syx7, Ykt6, and Syb) play an important role in preventing APOL1 cytotoxicity. Reducing the expression of these SNARE proteins significantly increased APOL1 cytotoxicity in fly nephrocytes, the equivalent of mammalian podocytes, whereas overexpression of Syx7, Ykt6, or Syb attenuated their toxicity in nephrocytes. These SNARE proteins bound to APOL1-G0 with higher affinity than APOL1-G1/G2, and attenuated APOL1-G0 cytotoxicity to a greater extent than either APOL1-RA.

CONCLUSIONS

Using a Drosophila screen, we identified SNARE proteins (Syx7, Ykt6, and Syb) as antagonists of APOL1-induced cytotoxicity by directly binding APOL1. These data uncovered a new potential protective role for certain SNARE proteins in the pathogenesis of APOL1-CKD and provide novel therapeutic targets for APOL1-associated nephropathies.

Single-cell profiling of the developing embryonic heart in Drosophila

Drosophila is an important model for studying heart development and disease. Yet, single-cell transcriptomic data of its developing heart have not been performed. Here, we report single-cell profiling of the entire fly heart using ∼3000 Hand-GFP embryos collected at five consecutive developmental stages, ranging from bilateral migrating rows of cardiac progenitors to a fused heart tube. The data revealed six distinct cardiac cell types in the embryonic fly heart: cardioblasts, both Svp+ and Tin+ subtypes; and five types of pericardial cell (PC) that can be distinguished by four key transcription factors (Eve, Odd, Ct and Tin) and include the newly described end of the line PC. Notably, the embryonic fly heart combines transcriptional signatures of the mammalian first and second heart fields. Using unique markers for each heart cell type, we defined their number and location during heart development to build a comprehensive 3D cell map. These data provide a resource to track the expression of any gene in the developing fly heart, which can serve as a reference to study genetic perturbations and cardiac diseases.

Mapping genomic regulation of kidney disease and traits through high-resolution and interpretable eQTLs

Expression quantitative trait locus (eQTL) studies illuminate genomic variants that regulate specific genes and contribute to fine-mapped loci discovered via genome-wide association studies (GWAS). Efforts to maximize their accuracy are ongoing. Using 240 glomerular (GLOM) and 311 tubulointerstitial (TUBE) micro-dissected samples from human kidney biopsies, we discovered 5371 GLOM and 9787 TUBE genes with at least one variant significantly associated with expression (eGene) by incorporating kidney single-nucleus open chromatin data and transcription start site distance as an "integrative prior" for Bayesian statistical fine-mapping. The use of an integrative prior resulted in higher resolution eQTLs illustrated by (1) smaller numbers of variants in credible sets with greater confidence, (2) increased enrichment of partitioned heritability for GWAS of two kidney traits, (3) an increased number of variants colocalized with the GWAS loci, and (4) enrichment of computationally predicted functional regulatory variants. A subset of variants and genes were validated experimentally in vitro and using a Drosophila nephrocyte model. More broadly, this study demonstrates that tissue-specific eQTL maps informed by single-nucleus open chromatin data have enhanced utility for diverse downstream analyses.

Modeling of ACTN4-Based Podocytopathy Using Drosophila Nephrocytes

Introduction

Genetic disorders are among the most prevalent causes leading to progressive glomerular disease and, ultimately, end-stage renal disease (ESRD) in children and adolescents. Identification of underlying genetic causes is indispensable for targeted treatment strategies and counseling of affected patients and their families.

Methods

Here, we report on a boy who presented at 4 years of age with proteinuria and biopsy-proven focal segmental glomerulosclerosis (FSGS) that was temporarily responsive to treatment with ciclosporin A. Molecular genetic testing identified a novel mutation in alpha-actinin-4 (p.M240T). We describe a feasible and efficient experimental approach to test its pathogenicity by combining in silico, in vitro, and in vivo analyses.

Results

The de novo p.M240T mutation led to decreased alpha-actinin-4 stability as well as protein mislocalization and actin cytoskeleton rearrangements. Transgenic expression of wild-type human alpha-actinin-4 in Drosophila melanogaster nephrocytes was able to ameliorate phenotypes associated with the knockdown of endogenous actinin. In contrast, p.M240T, as well as other established disease variants p.W59R and p.K255E, failed to rescue these phenotypes, underlining the pathogenicity of the novel alpha-actinin-4 variant.

Conclusion

Our data highlight that the newly identified alpha-actinin-4 mutation indeed encodes for a disease-causing variant of the protein and promote the Drosophila model as a simple and convenient tool to study monogenic kidney disease in vivo.

JiangShi(僵尸): a widely distributed Mucin-like protein essential for Drosophila development

Epithelia exposed to elements of the environment are protected by a mucus barrier in mammals. This barrier also serves to lubricate during organ movements and to mediate substance exchanges between the environmental milieu and internal organs. A major component of the mucus barrier is a class of glycosylated proteins called Mucin. Mucin and mucin-related proteins are widely present in the animal kingdom. Mucin mis-regulation has been reported in many diseases such as cancers and ones involving the digestive and respiratory tracts. Although the biophysical properties of isolated Mucins have been extensively studied, in vivo models remain scarce for the study of their functions and regulations. Here, we characterize the Mucin-like JiangShi protein and its mutations in the fruit fly Drosophila. JiangShi is an extracellular glycoprotein with domain features reminiscent of mammalian nonmembranous Mucins, and one of the most widely distributed Mucin-like proteins studied in Drosophila. Both loss and over-production of JiangShi lead to terminal defects in adult structures and organismal death. Although the physiological function of JiangShi remains poorly defined, we present a genetically tractable model system for the in vivo studies of Mucin-like molecules.

Drosophila melanogaster: a simple genetic model of kidney structure, function and disease

Although the genetic basis of many kidney diseases is being rapidly elucidated, their experimental study remains problematic owing to the lack of suitable models. The fruitfly Drosophila melanogaster provides a rapid, ethical and cost-effective model system of the kidney. The unique advantages of D. melanogaster include ease and low cost of maintenance, comprehensive availability of genetic mutants and powerful transgenic technologies, and less onerous regulation, as compared with mammalian systems. Renal and excretory functions in D. melanogaster reside in three main tissues - the transporting renal (Malpighian) tubules, the reabsorptive hindgut and the endocytic nephrocytes. Tubules contain multiple cell types and regions and generate a primary urine by transcellular transport rather than filtration, which is then subjected to selective reabsorption in the hindgut. By contrast, the nephrocytes are specialized for uptake of macromolecules and equipped with a filtering slit diaphragm resembling that of podocytes. Many genes with key roles in the human kidney have D. melanogaster orthologues that are enriched and functionally relevant in fly renal tissues. This similarity has allowed investigations of epithelial transport, kidney stone formation and podocyte and proximal tubule function. Furthermore, a range of unique quantitative phenotypes are available to measure function in both wild type and disease-modelling flies.

Transcriptional and functional motifs defining renal function revealed by single-nucleus RNA sequencing

Recent advances in single-cell sequencing provide a unique opportunity to gain novel insights into the diversity, lineage, and functions of cell types constituting a tissue/organ. Here, we performed a single-nucleus study of the adult Drosophila renal system, consisting of Malpighian tubules and nephrocytes, which shares similarities with the mammalian kidney. We identified 11 distinct clusters representing renal stem cells, stellate cells, regionally specific principal cells, garland nephrocyte cells, and pericardial nephrocytes. Characterization of the transcription factors specific to each cluster identified fruitless (fru) as playing a role in stem cell regeneration and Hepatocyte nuclear factor 4 (Hnf4) in regulating glycogen and triglyceride metabolism. In addition, we identified a number of genes, including Rho guanine nucleotide exchange factor at 64C (RhoGEF64c), Frequenin 2 (Frq2), Prip, and CG1093 that are involved in regulating the unusual star shape of stellate cells. Importantly, the single-nucleus dataset allows visualization of the expression at the organ level of genes involved in ion transport and junctional permeability, providing a systems-level view of the organization and physiological roles of the tubules. Finally, a cross-species analysis allowed us to match the fly kidney cell types to mouse kidney cell types and planarian protonephridia, knowledge that will help the generation of kidney disease models. Altogether, our study provides a comprehensive resource for studying the fly kidney.

PI(4,5)P2 controls slit diaphragm formation and endocytosis in Drosophila nephrocytes

Drosophila nephrocytes are an emerging model system for mammalian podocytes and proximal tubules as well as for the investigation of kidney diseases. Like podocytes, nephrocytes exhibit characteristics of epithelial cells, but the role of phospholipids in polarization of these cells is yet unclear. In epithelia, phosphatidylinositol(4,5)bisphosphate (PI(4,5)P2) and phosphatidylinositol(3,4,5)-trisphosphate (PI(3,4,5)P3) are asymmetrically distributed in the plasma membrane and determine apical–basal polarity. Here, we demonstrate that both phospholipids are present in the plasma membrane of nephrocytes, but only PI(4,5)P2 accumulates at slit diaphragms. Knockdown of Skittles, a phosphatidylinositol(4)phosphate 5-kinase, which produces PI(4,5)P2, abolished slit diaphragm formation and led to strongly reduced endocytosis. Notably, reduction in PI(3,4,5)P3 by overexpression of PTEN or expression of a dominant-negative phosphatidylinositol-3-kinase did not affect nephrocyte function, whereas enhanced formation of PI(3,4,5)P3 by constitutively active phosphatidylinositol-3-kinase resulted in strong slit diaphragm and endocytosis defects by ectopic activation of the Akt/mTOR pathway. Thus, PI(4,5)P2 but not PI(3,4,5)P3 is essential for slit diaphragm formation and nephrocyte function. However, PI(3,4,5)P3 has to be tightly controlled to ensure nephrocyte development.

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.

Using Drosophila Nephrocytes to Understand the Formation and Maintenance of the Podocyte Slit Diaphragm

The podocyte slit diaphragm (SD) is an essential component of the glomerular filtration barrier and its disruption is a common cause of proteinuria and many types of kidney disease. Therefore, better understanding of the pathways and proteins that play key roles in SD formation and maintenance has been of great interest. Podocyte and SD biology have been mainly studied using mouse and other vertebrate models. However, vertebrates are limited by inherent properties and technically challenging in vivo access to the podocytes. Drosophila is a relatively new alternative model system but it has already made great strides. Past the initial obvious differences, mammalian podocytes and fly nephrocytes are remarkably similar at the genetic, molecular and functional levels. This review discusses SD formation and maintenance, and their dependence on cell polarity, the cytoskeleton, and endo- and exocytosis, as learned from studies in fly nephrocytes and mammalian podocytes. In addition, it reflects on the remaining gaps in our knowledge, the physiological implications for glomerular diseases and how we can leverage the advantages Drosophila has to offer to further our understanding.

Physiological ROS controls Upd3-dependent modeling of ECM to support cardiac function in Drosophila

Despite their highly reactive nature, reactive oxygen species (ROS) at the physiological level serve as signaling molecules regulating diverse biological processes. While ROS usually act autonomously, they also function as local paracrine signals by diffusing out of the cells producing them. Using in vivo molecular genetic analyses in Drosophila, we provide evidence for ROS-dependent paracrine signaling that does not entail ROS release. We show that elevated levels of physiological ROS within the pericardial cells activate a signaling cascade transduced by Ask1, c-Jun N-terminal kinase, and p38 to regulate the expression of the cytokine Unpaired 3 (Upd3). Upd3 released by the pericardial cells controls fat body-specific expression of the extracellular matrix (ECM) protein Pericardin, essential for cardiac function and healthy life span. Therefore, our work reveals an unexpected inter-organ communication circuitry wherein high physiological levels of ROS regulate cytokine-dependent modulation of cardiac ECM with implications in normal and pathophysiological conditions.

The Pathophysiologic Role of Gelsolin in Chronic Kidney Disease: Focus on Podocytes

Chronic kidney disease (CKD) is normally related to proteinuria, a common finding in a compromised glomerular filtration barrier (GFB). GFB is a structure composed of glomerular endothelial cells, the basement membrane, and the podocytes. CKD with podocyte damage may be associated with actin cytoskeleton reorganization, resulting in podocyte effacement. Gelsolin plays a critical role in several diseases, including cardiovascular diseases and cancer. Our current study aimed to determine the connection between gelsolin and podocyte, and thus the mechanism underlying podocyte injury in CKD. Experiments were carried out on Drosophila to demonstrate whether gelsolin had a physiological role in maintaining podocyte. Furthermore, the survival rate of gelsolin-knocked down Drosophila larvae was extensively reduced after AgNO₃ exposure. Secondly, the in vitro podocytes treated with puromycin aminonucleoside (PAN) enhanced the gelsolin protein expression, as well as small GTPase RhoA and Rac1, which also regulated actin dynamic expression incrementally with the PAN concentrations. Thirdly, we further demonstrated in vivo that GSN was highly expressed inside the glomeruli with mitochondrial dysfunction in a CKD mouse model. Our findings suggest that an excess of gelsolin may contribute to podocytes damage in glomeruli.

dCubilin- or dAMN-mediated protein reabsorption in Drosophila nephrocytes modulates longevity

Aging is a multifaceted process regulated by multiple cellular pathways, including the proteostasis network. Pharmacological or genetic enhancement of the intracellular proteostasis network extends lifespan and prevents age-related diseases. However, how proteostasis is regulated in different tissues throughout the aging process remains unclear. Here, we show that Drosophila homologs of Cubilin- and Amnionless (dCubilin and dAMN, respectively)-mediated protein reabsorption (CAMPR) from hemolymph insect blood by nephrocytes modulate longevity through regulating proteostasis in muscle and brain tissues. We find that overexpression of dAMN receptor in nephrocytes extends lifespan, whereas nephrocyte-specific dCubilin or dAMN RNAi knockdown shortens lifespan. We also show that CAMPR in nephrocytes regulates proteostasis in hemolymph and improves healthspan. In addition, we show that enhanced CAMPR in nephrocytes slows down the aging process in muscle and brain by maintaining the proteostasis network in these tissues. Altogether, our work has revealed an inter-organ communication network across nephrocytes and muscle/neuronal tissue that is essential for maintaining proteostasis, and to delay senescence in these organs. These findings provide insight into the role of renal protein reabsorption in the aging process via this tele-proteostasis network.

Summary: dCubilin- or dAMN-mediated protein reabsorption in nephrocytes from hemolymph modulates Drosophila lifespan through regulating proteostasis and aging in muscle and brain tissues via the tele-proteostasis mechanism.

The Basolateral Polarity Module Promotes Slit Diaphragm Formation in Drosophila Nephrocytes, a Model of Vertebrate Podocytes

BACKGROUND

Podocyte slit diaphragms (SDs) are intercellular junctions that function as size-selective filters, excluding most proteins from urine. Abnormalities in SDs cause proteinuria and nephrotic syndrome. Podocytes exhibit apicobasal polarity, which can affect fundamental aspects of cell biology, including morphology, intercellular junction formation, and asymmetric protein distribution along the plasma membrane. Apical polarity protein mutations cause nephrotic syndrome, and data suggest apical polarity proteins regulate SD formation. However, there is no evidence that basolateral polarity proteins regulate SDs. Thus, the role of apicobasal polarity in podocytes remains unclear.

METHODS

Genetic manipulations and transgenic reporters determined the effects of disrupting apicobasal polarity proteins in Drosophila nephrocytes, which have SDs similar to those of mammalian podocytes. Confocal and electron microscopy were used to characterize SD integrity after loss of basolateral polarity proteins, and genetic-interaction studies illuminated relationships among apicobasal polarity proteins.

RESULTS

The study identified four novel regulators of nephrocyte SDs: Dlg, Lgl, Scrib, and Par-1. These proteins comprise the basolateral polarity module and its effector kinase. The data suggest these proteins work together, with apical polarity proteins, to regulate SDs by promoting normal endocytosis and trafficking of SD proteins.

CONCLUSIONS

Given the recognized importance of apical polarity proteins and SD protein trafficking in podocytopathies, the findings connecting basolateral polarity proteins to these processes significantly advance our understanding of SD regulation.

Slit diaphragm maintenance requires dynamic clathrin-mediated endocytosis facilitated by AP-2, Lap, Aux and Hsc70-4 in nephrocytes

Background

The Slit diaphragm (SD) is the key filtration structure in human glomerular kidney that is affected in many types of renal diseases. SD proteins are known to undergo endocytosis and recycling to maintain the integrity of the filtration structure. However, the key components of this pathway remain unclear.

Methods

Using the Drosophila nephrocyte as a genetic screen platform, we screened most genes involved in endocytosis and cell trafficking, and identified the key components of the cell trafficking pathway required for SD protein endocytosis and recycling.

Results

We discovered that the SD protein endocytosis and recycling pathway contains clathrin, dynamin, AP-2 complex, like-AP180 (Lap), auxilin and Hsc70-4 (the endocytosis part) followed by Rab11 and the exocyst complex (the recycling part). Disrupting any component in this pathway led to disrupted SD on the cell surface and intracellular accumulation of mislocalized SD proteins. We also showed the first in vivo evidence of trapped SD proteins in clathrin-coated pits at the plasma membrane when this pathway is disrupted.

Conclusions

All genes in this SD protein endocytosis and recycling pathway, as well as SD proteins themselves, are highly conserved from flies to humans. Thus, our results suggest that the SD proteins in human kidney undergo the same endocytosis and recycling pathway to maintain the filtration structure, and mutations in any genes in this pathway could lead to abnormal SD and renal diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00595-4.

Adipose triglyceride lipase protects renal cell endocytosis in a Drosophila dietary model of chronic kidney disease

Obesity-related renal lipotoxicity and chronic kidney disease (CKD) are prevalent pathologies with complex aetiologies. One hallmark of renal lipotoxicity is the ectopic accumulation of lipid droplets in kidney podocytes and in proximal tubule cells. Renal lipid droplets are observed in human CKD patients and in high-fat diet (HFD) rodent models, but their precise role remains unclear. Here, we establish a HFD model in Drosophila that recapitulates renal lipid droplets and several other aspects of mammalian CKD. Cell type-specific genetic manipulations show that lipid can overflow from adipose tissue and is taken up by renal cells called nephrocytes. A HFD drives nephrocyte lipid uptake via the multiligand receptor Cubilin (Cubn), leading to the ectopic accumulation of lipid droplets. These nephrocyte lipid droplets correlate with endoplasmic reticulum (ER) and mitochondrial deficits, as well as with impaired macromolecular endocytosis, a key conserved function of renal cells. Nephrocyte knockdown of diglyceride acyltransferase 1 (DGAT1), overexpression of adipose triglyceride lipase (ATGL), and epistasis tests together reveal that fatty acid flux through the lipid droplet triglyceride compartment protects the ER, mitochondria, and endocytosis of renal cells. Strikingly, boosting nephrocyte expression of the lipid droplet resident enzyme ATGL is sufficient to rescue HFD-induced defects in renal endocytosis. Moreover, endocytic rescue requires a conserved mitochondrial regulator, peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC1α). This study demonstrates that lipid droplet lipolysis counteracts the harmful effects of a HFD via a mitochondrial pathway that protects renal endocytosis. It also provides a genetic strategy for determining whether lipid droplets in different biological contexts function primarily to release beneficial or to sequester toxic lipids.

Apical-basal polarity regulators are essential for slit diaphragm assembly and endocytosis in Drosophila nephrocytes

Apical-basal polarity is a key feature of most epithelial cells and it is regulated by highly conserved protein complexes. In mammalian podocytes, which emerge from columnar epithelial cells, this polarity is preserved and the tight junctions are converted to the slit diaphragms, establishing the filtration barrier. In Drosophila, nephrocytes show several structural and functional similarities with mammalian podocytes and proximal tubular cells. However, in contrast to podocytes, little is known about the role of apical-basal polarity regulators in these cells. In this study, we used expansion microscopy and found the apical polarity determinants of the PAR/aPKC and Crb-complexes to be predominantly targeted to the cell cortex in proximity to the nephrocyte diaphragm, whereas basolateral regulators also accumulate intracellularly. Knockdown of PAR-complex proteins results in severe endocytosis and nephrocyte diaphragm defects, which is due to impaired aPKC recruitment to the plasma membrane. Similar, downregulation of most basolateral polarity regulators disrupts Nephrin localization but had surprisingly divergent effects on endocytosis. Our findings suggest that morphology and slit diaphragm assembly/maintenance of nephrocytes is regulated by classical apical-basal polarity regulators, which have distinct functions in endocytosis.

Molecular Basis for Autosomal-Dominant Renal Fanconi Syndrome Caused by HNF4A

HNF4A is a nuclear hormone receptor that binds DNA as an obligate homodimer. While all known human heterozygous mutations are associated with the autosomal-dominant diabetes form MODY1, one particular mutation (p.R85W) in the DNA-binding domain (DBD) causes additional renal Fanconi syndrome (FRTS). Here, we find that expression of the conserved fly ortholog dHNF4 harboring the FRTS mutation in Drosophila nephrocytes caused nuclear depletion and cytosolic aggregation of a wild-type dHNF4 reporter protein. While the nuclear depletion led to mitochondrial defects and lipid droplet accumulation, the cytosolic aggregates triggered the expansion of the endoplasmic reticulum (ER), autophagy, and eventually cell death. The latter effects could be fully rescued by preventing nuclear export through interfering with serine phosphorylation in the DBD. Our data describe a genomic and a non-genomic mechanism for FRTS in HNF4A-associated MODY1 with important implications for the renal proximal tubule and the regulation of other nuclear hormone receptors.

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HNF4 controls lipid metabolism in Drosophila nephrocytes

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The kidney disease mutation R85W shows dominant-negative effects in nephrocytes

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Dephosphorylation at S87 prevents the dominant-negative effects

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R85W mutation causes mitochondrial dysfunction in reprogrammed renal epithelial cells

Rabphilin involvement in filtration and molecular uptake in Drosophila nephrocytes suggests a similar role in human podocytes

Drosophila nephrocytes share functional, structural and molecular similarities with human podocytes. It is known that podocytes express the rabphilin 3A (RPH3A)-RAB3A complex, and its expression is altered in mouse and human proteinuric disease. Furthermore, we previously identified a polymorphism that suggested a role for RPH3A protein in the development of urinary albumin excretion. As endocytosis and vesicle trafficking are fundamental pathways for nephrocytes, the objective of this study was to assess the role of the RPH3A orthologue in Drosophila, Rabphilin (Rph), in the structure and function of nephrocytes. We confirmed that Rph is required for the correct function of the endocytic pathway in pericardial Drosophila nephrocytes. Knockdown of Rph reduced the expression of the cubilin and stick and stones genes, which encode proteins that are involved in protein uptake and filtration. We also found that reduced Rph expression resulted in a disappearance of the labyrinthine channel structure and a reduction in the number of endosomes, which ultimately leads to changes in the number and volume of nephrocytes. Finally, we demonstrated that the administration of retinoic acid to IR-Rph nephrocytes rescued some altered aspects, such as filtration and molecular uptake, as well as the maintenance of cell fate. According to our data, Rph is crucial for nephrocyte filtration and reabsorption, and it is required for the maintenance of the ultrastructure, integrity and differentiation of the nephrocyte.

Select Septate Junction Proteins Direct ROS-Mediated Paracrine Regulation of Drosophila Cardiac Function

Septate junction (SJ) complex proteins act in unison to provide a paracellular barrier and maintain structural integrity. Here, we identify a non-barrier role of two individual SJ proteins, Coracle (Cora) and Kune-kune (Kune). Reactive oxygen species (ROS)-p38 MAPK signaling in non-myocytic pericardial cells (PCs) is important for maintaining normal cardiac physiology in Drosophila. However, the underlying mechanisms remain unknown. We find that in PCs, Cora and Kune are altered in abundance in response to manipulations of ROS-p38 signaling. Genetic analyses establish Cora and Kune as key effectors of ROS-p38 signaling in PCs on proper heart function. We further determine that Cora regulates normal Kune levels in PCs, which in turn modulates normal Kune levels in the cardiomyocytes essential for proper heart function. Our results thereby reveal select SJ proteins Cora and Kune as signaling mediators of the PC-derived ROS regulation of cardiac physiology.

Extracellular matrix induced by steroids and aging through a G-protein-coupled receptor in a Drosophila model of renal fibrosis

Aldosterone is produced by the mammalian adrenal cortex to modulate blood pressure and fluid balance; however, excessive, prolonged aldosterone promotes fibrosis and kidney failure. How aldosterone triggers disease may involve actions independent of its canonical mineralocorticoid receptor. Here, we present a Drosophila model of renal pathology caused by excess extracellular matrix formation, stimulated by exogenous aldosterone and by insect ecdysone. Chronic administration of aldosterone or ecdysone induces expression and accumulation of collagen-like Pericardin in adult nephrocytes - podocyte-like cells that filter circulating hemolymph. Excess Pericardin deposition disrupts nephrocyte (glomerular) filtration and causes proteinuria in Drosophila, hallmarks of mammalian kidney failure. Steroid-induced Pericardin production arises from cardiomyocytes associated with nephrocytes, potentially reflecting an analogous role of mammalian myofibroblasts in fibrotic disease. Remarkably, the canonical ecdysteroid nuclear hormone receptor, Ecdysone receptor (EcR), is not required for aldosterone or ecdysone to stimulate Pericardin production or associated renal pathology. Instead, these hormones require a cardiomyocyte-associated G-protein-coupled receptor, Dopamine-EcR (DopEcR), a membrane-associated receptor previously characterized in the fly brain to affect behavior. DopEcR in the brain is known to affect behavior through interactions with the Drosophila Epidermal growth factor receptor (Egfr), referred to as dEGFR. Here, we find that the steroids ecdysone and aldosterone require dEGFR in cardiomyocytes to induce fibrosis of the cardiac-renal system. In addition, endogenous ecdysone that becomes elevated with age is found to foster age-associated fibrosis, and to require both cardiomyocyte DopEcR and dEGFR. This Drosophila renal disease model reveals a novel signaling pathway through which steroids may modulate mammalian fibrosis through potential orthologs of DopEcR.

Disease Modeling To Understand the Pathomechanisms of Human Genetic Kidney Disorders

The class of human genetic kidney diseases is extremely broad and heterogeneous. Accordingly, the range of associated disease phenotypes is highly variable. Many children and adults affected by inherited kidney disease will progress to ESKD at some point in life. Extensive research has been performed on various different disease models to investigate the underlying causes of genetic kidney disease and to identify disease mechanisms that are amenable to therapy. We review some of the research highlights that, by modeling inherited kidney disease, contributed to a better understanding of the underlying pathomechanisms, leading to the identification of novel genetic causes, new therapeutic targets, and to the development of new treatments. We also discuss how the implementation of more efficient genome-editing techniques and tissue-culture methods for kidney research is providing us with personalized models for a precision-medicine approach that takes into account the specificities of the patient and the underlying disease. We focus on the most common model systems used in kidney research and discuss how, according to their specific features, they can differentially contribute to biomedical research. Unfortunately, no definitive treatment exists for most inherited kidney disorders, warranting further exploitation of the existing disease models, as well as the implementation of novel, complex, human patient-specific models to deliver research breakthroughs.

Using Genetic and Species Diversity to Tackle Kidney Disease

Progress in the identification of causal genes and understanding of the mechanism underlying kidney disease is hindered by the almost exclusive use of a few animal models with restrictive monogenic backgrounds that may be more resistant to kidney disease compared with humans and, therefore, poor models. Exploring the large genetic diversity in classical animal models, such as mice and rats, and leveraging species diversity will allow us to use the genetic advantages of zebrafish, Drosophila, and other species, to develop both new animal models that are more relevant to the study of human kidney disease and potential therapies.

Exocyst Genes Are Essential for Recycling Membrane Proteins and Maintaining Slit Diaphragm in Drosophila Nephrocytes

BACKGROUND

Studies have linked mutations in genes encoding the eight-protein exocyst protein complex to kidney disease, but the underlying mechanism is unclear. Because Drosophila nephrocytes share molecular and structural features with mammalian podocytes, they provide an efficient model for studying this issue.

METHODS

We silenced genes encoding exocyst complex proteins specifically in Drosophila nephrocytes and studied the effects on protein reabsorption by lacuna channels and filtration by the slit diaphragm. We performed nephrocyte functional assays, carried out super-resolution confocal microscopy of slit diaphragm proteins, and used transmission electron microscopy to analyze ultrastructural changes. We also examined the colocalization of slit diaphragm proteins with exocyst protein Sec15 and with endocytosis and recycling regulators Rab5, Rab7, and Rab11.

RESULTS

Silencing exocyst genes in nephrocytes led to profound changes in structure and function. Abolition of cellular accumulation of hemolymph proteins with dramatically reduced lacuna channel membrane invaginations offered a strong indication of reabsorption defects. Moreover, the slit diaphragm's highly organized surface structure-essential for filtration-was disrupted, and key proteins were mislocalized. Ultrastructural analysis revealed that exocyst gene silencing led to the striking appearance of novel electron-dense structures that we named "exocyst rods," which likely represent accumulated membrane proteins following defective exocytosis or recycling. The slit diaphragm proteins partially colocalized with Sec15, Rab5, and Rab11.

CONCLUSIONS

Our findings suggest that the slit diaphragm of Drosophila nephrocytes requires balanced endocytosis and recycling to maintain its structural integrity and that impairment of the exocyst complex leads to disruption of the slit diaphragm and nephrocyte malfunction. This model may help identify therapeutic targets for treating kidney diseases featuring molecular defects in vesicle endocytosis, exocytosis, and recycling.

Proteome Analysis of Isolated Podocytes Reveals Stress Responses in Glomerular Sclerosis

BACKGROUND

Understanding podocyte-specific responses to injury at a systems level is difficult because injury leads to podocyte loss or an increase of extracellular matrix, altering glomerular cellular composition. Finding a window into early podocyte injury might help identify molecular pathways involved in the podocyte stress response.

METHODS

We developed an approach to apply proteome analysis to very small samples of purified podocyte fractions. To examine podocytes in early disease states in FSGS mouse models, we used podocyte fractions isolated from individual mice after chemical induction of glomerular disease (with Doxorubicin or LPS). We also applied single-glomerular proteome analysis to tissue from patients with FSGS.

RESULTS

Transcriptome and proteome analysis of glomeruli from patients with FSGS revealed an underrepresentation of podocyte-specific genes and proteins in late-stage disease. Proteome analysis of purified podocyte fractions from FSGS mouse models showed an early stress response that includes perturbations of metabolic, mechanical, and proteostasis proteins. Additional analysis revealed a high correlation between the amount of proteinuria and expression levels of the mechanosensor protein Filamin-B. Increased expression of Filamin-B in podocytes in biopsy samples from patients with FSGS, in single glomeruli from proteinuric rats, and in podocytes undergoing mechanical stress suggests that this protein has a role in detrimental stress responses. In Drosophila, nephrocytes with reduced filamin homolog Cher displayed altered filtration capacity, but exhibited no change in slit diaphragm structure.

CONCLUSIONS

We identified conserved mechanisms of the podocyte stress response through ultrasensitive proteome analysis of human glomerular FSGS tissue and purified native mouse podocytes during early disease stages. This approach enables systematic comparisons of large-scale proteomics data and phenotype-to-protein correlation.

TGFB-INHB/activin signaling regulates age-dependent autophagy and cardiac health through inhibition of MTORC2

Age-related impairment of macroautophagy/autophagy and loss of cardiac tissue homeostasis contribute significantly to cardiovascular diseases later in life. MTOR (mechanistic target of rapamycin kinase) signaling is the most well-known regulator of autophagy, cellular homeostasis, and longevity. The MTOR signaling consists of two structurally and functionally distinct multiprotein complexes, MTORC1 and MTORC2. While MTORC1 is well characterized but the role of MTORC2 in aging and autophagy remains poorly understood. Here we identified TGFB-INHB/activin signaling as a novel upstream regulator of MTORC2 to control autophagy and cardiac health during aging. Using Drosophila heart as a model system, we show that cardiac-specific knockdown of TGFB-INHB/activin-like protein daw induces autophagy and alleviates age-related heart dysfunction, including cardiac arrhythmias and bradycardia. Interestingly, the downregulation of daw activates TORC2 signaling to regulate cardiac autophagy. Activation of TORC2 alone through overexpressing its subunit protein rictor promotes autophagic flux and preserves cardiac function with aging. In contrast, activation of TORC1 does not block autophagy induction in daw knockdown flies. Lastly, either daw knockdown or rictor overexpression in fly hearts prolongs lifespan, suggesting that manipulation of these pathways in the heart has systemic effects on longevity control. Thus, our studies discover the TGFB-INHB/activin-mediated inhibition of TORC2 as a novel mechanism for age-dependent decreases in autophagic activity and cardiac health. Abbreviations: AI: arrhythmia index; BafA1: bafilomycin A₁; BMP: bone morphogenetic protein; CQ: chloroquine; CVD: cardiovascular diseases; DI: diastolic interval; ER: endoplasmic reticulum; HP: heart period; HR: heart rate; MTOR: mechanistic target of rapamycin kinase; NGS: normal goat serum; PBST: PBS with 0.1% Triton X-100; PDPK1: 3-phosphoinositide dependent protein kinase 1; RICTOR: RPTOR independent companion of MTOR complex 2; ROI: region of interest; ROUT: robust regression and outlier removal; ROS: reactive oxygen species; R-SMAD: receptor-activated SMAD; SI: systolic interval; SOHA: semi-automatic optical heartbeat analysis; TGFB: transformation growth factor beta; TSC1: TSC complex subunit 1.

A Multi-layered Quantitative In Vivo Expression Atlas of the Podocyte Unravels Kidney Disease Candidate Genes

Damage to and loss of glomerular podocytes has been identified as the culprit lesion in progressive kidney diseases. Here, we combine mass spectrometry-based proteomics with mRNA sequencing, bioinformatics, and hypothesis-driven studies to provide a comprehensive and quantitative map of mammalian podocytes that identifies unanticipated signaling pathways. Comparison of the in vivo datasets with proteomics data from podocyte cell cultures showed a limited value of available cell culture models. Moreover, in vivo stable isotope labeling by amino acids uncovered surprisingly rapid synthesis of mitochondrial proteins under steady-state conditions that was perturbed under autophagy-deficient, disease-susceptible conditions. Integration of acquired omics dimensions suggested FARP1 as a candidate essential for podocyte function, which could be substantiated by genetic analysis in humans and knockdown experiments in zebrafish. This work exemplifies how the integration of multi-omics datasets can identify a framework of cell-type-specific features relevant for organ health and disease.

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Deep proteome and transcriptome analyses of native podocytes unravel druggable targets

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Static and dynamic proteomics uncover features of podocyte identity and proteostasis

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Candidate genes for nephrotic syndrome were predicted based on multi-omic integration

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FARP1 is a previously unreported candidate gene for human proteinuric kidney disease

Forward genetic screen in human podocytes identifies diphthamide biosynthesis genes as regulators of adhesion

Podocyte function is tightly linked to the complex organization of its cytoskeleton and adhesion to the underlying glomerular basement membrane. Adhesion of cultured podocytes to a variety of substrates is reported to correlate with podocyte health. To identify novel genes that are important for podocyte function, we designed an in vitro genetic screen based on podocyte adhesion to plates coated with either fibronectin or soluble Fms-like tyrosine kinase-1 (sFLT1)/Fc. A genome-scale pooled RNA interference screen on immortalized human podocytes identified 77 genes that increased adhesion to fibronectin, 101 genes that increased adhesion to sFLT1/Fc, and 44 genes that increased adhesion to both substrates when knocked down. Multiple shRNAs against diphthamide biosynthesis protein 1-4 (DPH1-DPH4) were top hits for increased adhesion. Immortalized human podocyte cells stably expressing these hairpins displayed increased adhesion to both substrates. We then used CRISPR-Cas9 to generate podocyte knockout cells for DPH1, DPH2, or DPH3, which also displayed increased adhesion to both fibronectin and sFLT1/Fc, as well as a spreading defect. Finally, we showed that Drosophila nephrocyte-specific knockdown of Dph1, Dph2, and Dph4 resulted in altered nephrocyte function. In summary, we report here a novel high-throughput method to identify genes important for podocyte function. Given the central role of podocyte adhesion as a marker of podocyte health, these data are a rich source of candidate regulators of glomerular disease.

Pericardin, a Drosophila collagen, facilitates accumulation of hemocytes at the heart

Hematopoietic cell lineages support organismal needs by responding to positional and systemic signals that balance proliferative and differentiation events. Drosophila provides an excellent genetic model to dissect these signals, where the activity of cues in the hemolymph or substrate can be traced to determination and differentiation events of well characterized hemocyte types. Plasmatocytes in third instar larvae increase in number in response to infection and in anticipation of metamorphosis. Here we characterize hemocyte clustering, proliferation and transdifferentiation on the heart or dorsal vessel. Hemocytes accumulate on the inner foldings of the heart basement membrane, where they move with heart contraction, and are in proximity to the heart ostia and pericardial nephrocytes. The numbers of hemocytes vary, but increase transiently before pupariation, and decrease by 4 h before pupa formation. During their accumulation at the heart, plasmatocytes can proliferate and can transdifferentiate into crystal cells. Serrate expressing cells as well as lamellocyte-like, Atilla expressing ensheathing cells are associated with some, but not all hemocyte clusters. Hemocyte aggregation is enhanced by the presence of a heart specific Collagen, Pericardin, but not the associated pericardial cells. The varied and transient number of hemocytes in the pericardial compartment suggests that this is not a hematopoietic hub, but a niche supporting differentiation and rapid dispersal in response to systemic signals.

Proteomics of Asrij Perturbation in Drosophila Lymph Glands for Identification of New Regulators of Hematopoiesis

Hematopoiesis is the process of differentiation of precursor blood cells into mature blood cells that is controlled by a complex set of molecular interactions. Understanding hematopoiesis is important for the study of hematological disorders. However, a comprehensive understanding of how physiological and genetic mechanisms regulate blood cell precursor maintenance and differentiation is lacking. Owing to simplicity and ease of genetic analysis, the Drosophila melanogaster lymph gland (LG) is an excellent model to study hematopoiesis. Here, we quantitatively analyzed the LG proteome under genetic conditions that either maintain precursors or promote their differentiation in vivo, by perturbing expression of Asrij, a conserved endosomal regulator of hematopoiesis. Using iTRAQ-based quantitative proteomics, we determined the relative expression levels of proteins in Asrij-knockout and overexpressing LGs from 1500 larval dissections compared with wild type. Our data showed that at least 6.5% of the Drosophila proteome is expressed in wild type LGs. Of the 2133 proteins identified, 780 and 208 proteins were common to previously reported cardiac tube and hemolymph proteomes, respectively, resulting in the identification of 1238 proteins exclusive to the LG. Perturbation of Asrij levels led to differential expression of 619 proteins, of which 27% have human homologs implicated in various diseases. Proteins regulating metabolism, immune system, signal transduction and vesicle-mediated transport were significantly enriched. Immunostaining of representative candidates from the enriched categories and previous reports confirmed 73% of our results, indicating the validity of our LG proteome. Our study provides, for the first time, an in vivo proteomics resource for identifying novel regulators of hematopoiesis that will also be applicable to understanding vertebrate blood cell development.

Nephrin Signaling Results in Integrin β1 Activation

BACKGROUND

Patients with certain mutations in the gene encoding the slit diaphragm protein Nephrin fail to develop functional slit diaphragms and display severe proteinuria. Many adult-onset glomerulopathies also feature alterations in Nephrin expression and function. Nephrin signals from the podocyte slit diaphragm to the Actin cytoskeleton by recruiting proteins that can interact with C3G, a guanine nucleotide exchange factor of the small GTPase Rap1. Because Rap activity affects formation of focal adhesions, we hypothesized that Nephrin transmits signals to the Integrin receptor complex, which mediates podocyte adhesion to the extracellular matrix.

METHODS

To investigate Nephrin's role in transmitting signals to the Integrin receptor complex, we conducted genetic studies in Drosophila nephrocytes and validated findings from Drosophila in a cultured human podocyte model.

RESULTS

Drosophila nephrocytes form a slit diaphragm-like filtration barrier and express the Nephrin ortholog Sticks and stones (Sns). A genetic screen identified c3g as necessary for nephrocyte function. In vivo, nephrocyte-specific gene silencing of sns or c3g compromised nephrocyte filtration and caused nephrocyte diaphragm defects. Nephrocytes with impaired Sns or C3G expression displayed an altered localization of Integrin and the Integrin-associated protein Talin. Furthermore, gene silencing of c3g partly rescued nephrocyte diaphragm defects of an sns overexpression phenotype, pointing to genetic interaction of sns and c3g in nephrocytes. We also found that activated Nephrin recruited phosphorylated C3G and resulted in activation of Integrin β1 in cultured podocytes.

CONCLUSIONS

Our findings suggest that Nephrin can mediate a signaling pathway that results in activation of Integrin β1 at focal adhesions, which may affect podocyte attachment to the extracellular matrix.

Drosophila melanogaster and its nephrocytes: A versatile model for glomerular research

Glomerular disorders are a predominant cause of chronic kidney diseases and end-stage renal failure. Especially podocytes, epithelial cells which represent the outermost part of the filtration barrier, are affected by disease and experience a gradual loss of function. Despite recent advances in identifying potential pathways underlying podocyte injury, treatment remains challenging. It is therefore desirable to employ suitable model organisms in order to study glomerular disease and elucidate affected pathways. Due to its diverse ways of genetic manipulation and high genomic conservation, Drosophila melanogaster is a powerful model organism for biomedical research. The fly was recently used to assess podocytopathies by exploiting the nephrocyte system. Nephrocytes are spherical cells within the body cavity of the fly responsible for detoxification and clearance of unwanted substances. More importantly, they share many characteristics with mammalian podocytes. Here, we summarize how to use Drosophila as a model organism for podocyte research. We discuss examples of techniques that can be used to genetically manipulate nephrocytes and provide protocols for nephrocyte isolation and for morphological as well as functional analysis.

Mutations in NUP160 Are Implicated in Steroid-Resistant Nephrotic Syndrome

BACKGROUND

Studies have identified mutations in >50 genes that can lead to monogenic steroid-resistant nephrotic syndrome (SRNS). The NUP160 gene, which encodes one of the protein components of the nuclear pore complex nucleoporin 160 kD (Nup160), is expressed in both human and mouse kidney cells. Knockdown of NUP160 impairs mouse podocytes in cell culture. Recently, siblings with SRNS and proteinuria in a nonconsanguineous family were found to carry compound-heterozygous mutations in NUP160.

METHODS

We identified NUP160 mutations by whole-exome and Sanger sequencing of genomic DNA from a young girl with familial SRNS and FSGS who did not carry mutations in other genes known to be associated with SRNS. We performed in vivo functional validation studies on the NUP160 mutations using a Drosophila model.

RESULTS

We identified two compound-heterozygous NUP160 mutations, NUP160^R1173× and NUP160^E803K . We showed that silencing of Drosophila NUP160 specifically in nephrocytes (fly renal cells) led to functional abnormalities, reduced cell size and nuclear volume, and disorganized nuclear membrane structure. These defects were completely rescued by expression of the wild-type human NUP160 gene in nephrocytes. By contrast, expression of the NUP160 mutant allele NUP160^R1173× completely failed to rescue nephrocyte phenotypes, and mutant allele NUP160^E803K rescued only nuclear pore complex and nuclear lamin localization defects.

CONCLUSIONS

Mutations in NUP160 are implicated in SRNS. Our findings indicate that NUP160 should be included in the SRNS diagnostic gene panel to identify additional patients with SRNS and homozygous or compound-heterozygous NUP160 mutations and further strengthen the evidence that NUP160 mutations can cause SRNS.

Dissection of Nidogen function in Drosophila reveals tissue-specific mechanisms of basement membrane assembly

Basement membranes (BMs) are thin sheet-like specialized extracellular matrices found at the basal surface of epithelia and endothelial tissues. They have been conserved across evolution and are required for proper tissue growth, organization, differentiation and maintenance. The major constituents of BMs are two independent networks of Laminin and Type IV Collagen in addition to the proteoglycan Perlecan and the glycoprotein Nidogen/entactin (Ndg). The ability of Ndg to bind in vitro Collagen IV and Laminin, both with key functions during embryogenesis, anticipated an essential role for Ndg in morphogenesis linking the Laminin and Collagen IV networks. This was supported by results from cultured embryonic tissue experiments. However, the fact that elimination of Ndg in C. elegans and mice did not affect survival strongly questioned this proposed linking role. Here, we have isolated mutations in the only Ndg gene present in Drosophila. We find that while, similar to C.elegans and mice, Ndg is not essential for overall organogenesis or viability, it is required for appropriate fertility. We also find, alike in mice, tissue-specific requirements of Ndg for proper assembly and maintenance of certain BMs, namely those of the adipose tissue and flight muscles. In addition, we have performed a thorough functional analysis of the different Ndg domains in vivo. Our results support an essential requirement of the G3 domain for Ndg function and unravel a new key role for the Rod domain in regulating Ndg incorporation into BMs. Furthermore, uncoupling of the Laminin and Collagen IV networks is clearly observed in the larval adipose tissue in the absence of Ndg, indeed supporting a linking role. In light of our findings, we propose that BM assembly and/or maintenance is tissue-specific, which could explain the diverse requirements of a ubiquitous conserved BM component like Nidogen.

Basement membranes (BMs) are thin layers of specialized extracellular matrices present in every tissue of the human body. Its main constituents are two networks of laminin and Type IV Collagen linked by Nidogen (Ndg) and proteoglycans. They form an organized scaffold that regulates organ morphogenesis and function. Mutations affecting BM components are associated with organ dysfunction and several congenital diseases. Thus, a better comprehension of BM assembly and maintenance will not only help to learn more about organogenesis but also to a better understanding and, hopefully, treatment of these diseases. Here, we have used the fruit fly Drosophila to analyse the role of Ndg in BM formation in vivo. Elimination of Ndg in worms and mice does not affect survival, strongly questioning its proposed linking role, derived from in vitro experiments. Here, we show that in the fly, Ndg is dispensable for BM assembly and preservation in many tissues, but absolutely required in others. Furthermore, our functional study of the different Ndg domains challenges the significance of some interactions between BM components derived from in vitro experiments, while confirming others, and reveals a new key requirement for the Rod domain in Ndg function and incorporation into BMs.

Getting a Notch closer to renal dysfunction: activated Notch suppresses expression of the adaptor protein Disabled-2 in tubular epithelial cells

Reactivation of Notch signaling in kidneys of animal models and patients with chronic kidney disease (CKD) has been shown to contribute to epithelial injury and fibrosis development. Here, we investigated the mechanisms of Notch-induced injury in renal epithelial cells. We performed genome-wide transcriptome analysis to identify Notch target genes using an in vitro system of cultured tubular epithelial cells expressing the intracellular domain of Notch1. One of the top downregulated genes was Disabled-2 ( Dab2). With the use of Drosophila nephrocytes as a model system, we found that Dab (the Drosophila homolog of Dab2) knockdown resulted in a significant filtration defect, indicating that loss of Dab2 plays a functional role in kidney disease development. We showed that Dab2 expression in cultured tubular epithelial cells is involved in endocytic regulation and that it also protects cells from TGF-β-induced epithelial-to-mesenchymal transition. In vivo correlation studies indicated its additional role in renal ischemia-induced injury. Together, these data suggest that Dab2 plays a versatile role in the kidney and may impact on acute and CKDs.-Schütte-Nütgen, K., Edeling, M., Mendl, G., Krahn, M. P., Edemir, B., Weide, T., Kremerskothen, J., Michgehl, U., Pavenstädt, H. Getting a Notch closer to renal dysfunction: activated Notch suppresses expression of the adaptor protein Disabled-2 in tubular epithelial cells.

Experimental Models to Study Podocyte Biology: Stock-Taking the Toolbox of Glomerular Research

Diseases affecting the glomeruli of the kidney, the renal filtration units, are a leading cause of chronic kidney disease and end-stage renal failure. Despite recent advances in the understanding of glomerular biology, treatment of these disorders has remained extraordinarily challenging in many cases. The use of experimental models has proven invaluable to study renal, and in particular, glomerular biology and disease. Over the past 15 years, studies identified different and very distinct pathogenic mechanisms that result in damage, loss of glomerular visceral epithelial cells (podocytes) and progressive renal disease. However, animal studies and, in particular, mouse studies are often protracted and cumbersome due to the long reproductive cycle and high keeping costs. Transgenic and heterologous expression models have been speeded-up by novel gene editing techniques, yet they still take months. In addition, given the complex cellular biology of the filtration barrier, certain questions may not be directly addressed using mouse models due to the limited accessibility of podocytes for analysis and imaging. In this review, we will describe alternative models to study podocyte biology experimentally. We specifically discuss current podocyte cell culture models, their role in experimental strategies to analyze pathophysiologic mechanisms as well as limitations with regard to transferability of results. We introduce current models in Caenorhabditis elegans, Drosophila melanogaster, and Danio rerio that allow for analysis of protein interactions, and principle signaling pathways in functional biological structures, and enable high-throughput transgenic expression or compound screens in multicellular organisms.

Networks that link cytoskeletal regulators and diaphragm proteins underpin filtration function in Drosophila nephrocytes

Insect nephrocytes provide a valuable model for kidney disease, as they are structurally and functionally homologous to mammalian kidney podocytes. They possess an exceptional macromolecular assembly, the nephrocyte diaphragm (ND), which serves as a filtration barrier and helps maintain tissue homeostasis by filtering out wastes and toxic products. However, the elements that maintain nephrocyte architecture and the ND are not understood. We show that Drosophila nephrocytes have a unique cytoplasmic cluster of F-actin, which is maintained by the microtubule cytoskeleton and Rho-GTPases. A balance of Rac1 and Cdc42 activity as well as proper microtubule organization and endoplasmic reticulum structure, are required to position the actin cluster. Further, ND proteins Sns and Duf also localize to this cluster and regulate organization of the actin and microtubule cytoskeleton. Perturbation of any of these inter-dependent components impairs nephrocyte ultrafiltration. Thus cytoskeletal components, Rho-GTPases and ND proteins work in concert to maintain the specialized nephrocyte architecture and function.

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Drosophila nephrocytes have a unique cytoplasmic cluster of F-actin.

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Microtubules, Rho-GTPases and endoplasmic reticulum position the actin cluster.

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Nephrocyte diaphragm proteins localize to and regulate actin cluster organization.

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Perturbation of any of these inter-dependent components impairs ultrafiltration.

Distinct functions of Crumbs regulating slit diaphragms and endocytosis in Drosophila nephrocytes

Mammalian podocytes, the key determinants of the kidney's filtration barrier, differentiate from columnar epithelial cells and several key determinants of apical-basal polarity in the conventional epithelia have been shown to regulate podocyte morphogenesis and function. However, little is known about the role of Crumbs, a conserved polarity regulator in many epithelia, for slit-diaphragm formation and podocyte function. In this study, we used Drosophila nephrocytes as model system for mammalian podocytes and identified a conserved function of Crumbs proteins for cellular morphogenesis, nephrocyte diaphragm assembly/maintenance, and endocytosis. Nephrocyte-specific knock-down of Crumbs results in disturbed nephrocyte diaphragm assembly/maintenance and decreased endocytosis, which can be rescued by Drosophila Crumbs as well as human Crumbs2 and Crumbs3, which were both expressed in human podocytes. In contrast to the extracellular domain, which facilitates nephrocyte diaphragm assembly/maintenance, the intracellular FERM-interaction motif of Crumbs is essential for regulating endocytosis. Moreover, Moesin, which binds to the FERM-binding domain of Crumbs, is essential for efficient endocytosis. Thus, we describe here a new mechanism of nephrocyte development and function, which is likely to be conserved in mammalian podocytes.

A Drosophila model system to assess the function of human monogenic podocyte mutations that cause nephrotic syndrome

Many genetic mutations have been identified as monogenic causes of nephrotic syndrome (NS), but important knowledge gaps exist in the roles of these genes in kidney cell biology and renal diseases. More animal models are needed to assess the functions of these genes in vivo, and to determine how they cause NS in a timely manner. Drosophila nephrocytes and human podocytes share striking similarities, but to what degree these known NS genes play conserved roles in nephrocytes remains unknown. Here we systematically studied 40 genes associated with NS, including 7 that have not previously been analysed for renal function in an animal model. We found that 85% of these genes are required for nephrocyte functions, suggesting that a majority of human genes known to be associated with NS play conserved roles in renal function from flies to humans. To investigate functional conservation in more detail, we focused on Cindr, the fly homolog of the human NS gene CD2AP. Silencing Cindr in nephrocytes led to dramatic nephrocyte functional impairment and shortened life span, as well as collapse of nephrocyte lacunar channels and effacement of nephrocyte slit diaphragms. These phenotypes could be rescued by expression of a wild-type human CD2AP gene, but not a mutant allele derived from a patient with CD2AP-associated NS. We conclude that the Drosophila nephrocyte can be used to elucidate clinically relevant molecular mechanisms underlying the pathogenesis of most monogenic forms of NS, and to efficiently generate personalized in vivo models of genetic renal diseases bearing patient-specific mutations.

A Personalized Model of COQ2 Nephropathy Rescued by the Wild-Type COQ2 Allele or Dietary Coenzyme Q10 Supplementation

Clinical studies have identified patients with nephrotic syndrome caused by mutations in genes involved in the biosynthesis of coenzyme Q₁₀ (CoQ₁₀), a lipid component of the mitochondrial electron transport chain and an important antioxidant. However, the cellular mechanisms through which these mutations induce podocyte injury remain obscure. Here, we exploited the striking similarities between Drosophila nephrocytes and human podocytes to develop a Drosophila model of these renal diseases, and performed a systematic in vivo analysis assessing the role of CoQ₁₀ pathway genes in renal function. Nephrocyte-specific silencing of Coq2, Coq6, and Coq8, which are genes involved in the CoQ₁₀ pathway that have been associated with genetic nephrotic syndrome in humans, induced dramatic adverse changes in these cells. In particular, silencing of Coq2 led to an abnormal localization of slit diaphragms, collapse of lacunar channels, and more dysmorphic mitochondria. In addition, Coq2-deficient nephrocytes showed elevated levels of autophagy and mitophagy, increased levels of reactive oxygen species, and increased sensitivity to oxidative stress. Dietary supplementation with CoQ₁₀ at least partially rescued these defects. Furthermore, expressing the wild-type human COQ2 gene specifically in nephrocytes rescued the defective protein uptake, but expressing the mutant allele derived from a patient with COQ2 nephropathy did not. We conclude that transgenic Drosophila lines carrying mutations in the CoQ₁₀ pathway genes are clinically relevant models with which to explore the pathogenesis of podocyte injury and could serve as a new platform to test novel therapeutic approaches.