Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS

Single-Cell Transcriptional Signatures of Glomerular Disease in Transgenic Mice with APOL1 Variants

Key Points

Background

Apolipoprotein L1 (APOL1) high-risk variants contribute to kidney disease among individuals with African ancestry. We sought to describe cell-specific APOL1 variant–induced pathways using two mouse models.

Methods

We characterized bacterial artificial chromosome/APOL1 transgenic mice crossed with HIV-associated nephropathy (HIVAN) Tg26 mice and bacterial artificial chromosome/APOL1 transgenic mice given IFN-γ.

Results

Both mouse models showed more severe glomerular disease in APOL1-G1 compared with APOL1-G0 mice. Synergistic podocyte-damaging pathways activated by APOL1-G1 and by the HIV transgene were identified by glomerular bulk RNA sequencing (RNA-seq) of HIVAN model. Single-nuclear RNA-seq revealed podocyte-specific patterns of differentially expressed genes as a function of APOL1 alleles. Shared activated pathways, for example, mammalian target of rapamycin, and differentially expressed genes, for example, Ccn2, in podocytes in both models suggest novel markers of APOL1-associated kidney disease. HIVAN mouse-model podocyte single-nuclear RNA-seq data showed similarity to human focal segmental glomerulosclerosis glomerular RNA-seq data. Differential effects of the APOL1-G1 variant on the eukaryotic initiation factor 2 pathway highlighted differences between the two models.

Conclusions

These findings in two mouse models demonstrated both shared and distinct cell type–specific transcriptomic signatures induced by APOL1 variants. These findings suggest novel therapeutic opportunities for APOL1 glomerulopathies.

A new index for the outcome of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a common pathological form of nephrotic syndrome. This study analyzed the value of pathological lesions and clinical prognosis of different segmental glomerulosclerosis ratios in FSGS. Two hundred and six FSGS patients were collected from Dec 2013 to Apr 2016. The patients were divided into two groups according to the proportion of glomerular segmental sclerosis: F1 (SSR ≤ 15%, n = 133) and F2 (SSR > 15%, n = 73). The clinical and pathological data were recorded and analyzed, and statistical differences were observed between the serum uric acid level and the percentage of chronic renal failure. The pathological results showed significant differences in interstitial fibrosis and tubular atrophy (IFTA), degree of mesangial hyperplasia, vascular lesions, synaptopodin intensity, and foot process effacement between the two groups. Multivariate logistic regression analysis showed significant differences in creatinine (OR: 1.008) and F2 group (OR: 1.19). In all patients, the prognoses of urine protein and serum creatinine levels were statistically different. Multivariate Cox regression analysis revealed that F2 (hazard ratio: 2.306, 95% CI 1.022-5.207) was associated with a risk of ESRD (end stage renal disease). The proportion of segmental glomerulosclerosis provides a guiding value in the pathological diagnosis and clinical prognosis of FSGS.

The CXCR4-AT1 axis plays a vital role in glomerular injury via mediating the crosstalk between podocyte and mesangial cell

Glomeruli stand at the center of nephrons to accomplish filtration and albumin interception. Podocytes and mesangial cells are the major constituents in the glomeruli. However, their interdependency in glomerular injury has rarely been reported. Herein, we investigated the role of C-X-C chemokine receptor type 4 (CXCR4) in mediating the crosstalk between podocytes and mesangial cells. We found CXCR4 and angiotensin II (AngII) increased primarily in injured podocytes. However, type-1 receptor of angiotensin II (AT1) and stromal cell-derived factor 1α (SDF-1α), a ligand of CXCR4, were evidently upregulated in mesangial cells following the progression of podocyte injury. Ectopic expression of CXCR4 in 5/6 nephrectomy mice increased the decline of renal function and glomerular injury, accelerated podocyte injury and mesangial cell activation, and initiated CXCR4-AT1 axis signals. Additionally, treatment with losartan, an AT1 blocker, interrupted the cycle of podocyte injury and mesangial matrix deposition triggered by CXCR4. Podocyte-specific ablation of CXCR4 gene blocked podocyte injury and mesangial cell activation. In vitro, CXCR4 overexpression induced oxidative stress and renin angiotensin system (RAS) activation in podocytes, and triggered the communication between podocytes and mesangial cells. In cultured mesangial cells, AngII treatment induced the expression of SDF-1α, which was secreted into the supernatant to further promote oxidative stress and cell injury in podocytes. Collectively, these results demonstrate that the CXCR4-AT1 axis plays a vital role in glomerular injury via mediating pathologic crosstalk between podocytes and mesangial cells. Our findings uncover a novel pathogenic mechanism by which the CXCR4-AT1 axis promotes glomerular injury.

Resident microbes shape the vaginal epithelial glycan landscape

Epithelial cells are covered in carbohydrates (glycans). This glycan coat or "glycocalyx" interfaces directly with microbes, providing a protective barrier against potential pathogens. Bacterial vaginosis (BV) is a condition associated with adverse health outcomes in which bacteria reside in direct proximity to the vaginal epithelium. Some of these bacteria, including Gardnerella, produce glycosyl hydrolase enzymes. However, glycans of the human vaginal epithelial surface have not been studied in detail. Here, we elucidate key characteristics of the "normal" vaginal epithelial glycan landscape and analyze the impact of resident microbes on the surface glycocalyx. In human BV, glycocalyx staining was visibly diminished in electron micrographs compared to controls. Biochemical and mass spectrometric analysis showed that, compared to normal vaginal epithelial cells, BV cells were depleted of sialylated N- and O-glycans, with underlying galactose residues exposed on the surface. Treatment of primary epithelial cells from BV-negative women with recombinant Gardnerella sialidases generated BV-like glycan phenotypes. Exposure of cultured VK2 vaginal epithelial cells to recombinant Gardnerella sialidase led to desialylation of glycans and induction of pathways regulating cell death, differentiation, and inflammatory responses. These data provide evidence that vaginal epithelial cells exhibit an altered glycan landscape in BV and suggest that BV-associated glycosidic enzymes may lead to changes in epithelial gene transcription that promote cell turnover and regulate responses toward the resident microbiome.

The monosialoganglioside GM1a protects against complement attack

The complement system is a part of the innate immune system in the fluid phase and efficiently eliminates pathogens. However, its activation requires tight regulation on the host cell surface in order not to compromise cellular viability. Previously, we showed that loss of placental cell surface sialylation in mice in vivo leads to a maternal complement attack at the fetal-maternal interface, ultimately resulting in loss of pregnancy. To gain insight into the regulatory function of sialylation in complement activation, we here generated trophoblast stem cells (TSC) devoid of sialylation, which also revealed complement sensitivity and cell death in vitro. Glycolipid-analysis by multiplexed capillary gel electrophoresis coupled to laser-induced fluorescence detection (xCGE-LIF) allowed us to identify the monosialoganglioside GM1a as a key element of cell surface complement regulation. Exogenously administered GM1a integrated into the plasma membrane of trophoblasts, substantially increased binding of complement factor H (FH) and was sufficient to protect the cells from complement attack and cell death. GM1a treatment also rescued human endothelial cells and erythrocytes from complement attack in a concentration dependent manner. Furthermore, GM1a significantly reduced complement mediated hemolysis of erythrocytes from a patient with Paroxysmal nocturnal hemoglobinuria (PNH). This study demonstrates the complement regulatory potential of exogenously administered gangliosides and paves the way for sialoglycotherapeutics as a novel substance class for membrane-targeted complement regulators.

Severe kidney dysfunction in sialidosis mice reveals an essential role for neuraminidase 1 in reabsorption

Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.

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.

APOL1 kidney risk variants in glomerular diseases modeled in transgenic mice

APOL1 high-risk variants partially explain the high kidney disease prevalence among African ancestry individuals. Many mechanisms have been reported in cell culture models, but few have been demonstrated in mouse models. Here we characterize two models: (1) HIV-associated nephropathy (HIVAN) Tg26 mice crossed with bacterial artificial chromosome (BAC)/APOL1 transgenic mice and (2) interferon-γ administered to BAC/APOL1 mice. Both models showed exacerbated glomerular disease in APOL1-G1 compared to APOL1-G0 mice. HIVAN model glomerular bulk RNA-seq identified synergistic podocyte-damaging pathways activated by the APOL1-G1 allele and by HIV transgenes. Single-nuclear RNA-seq revealed podocyte-specific patterns of differentially-expressed genes as a function of APOL1 alleles. Eukaryotic Initiation factor-2 pathway was the most activated pathway in the interferon-γ model and the most deactivated pathway in the HIVAN model. HIVAN mouse model podocyte single-nuclear RNA-seq data showed similarity to human focal segmental glomerulosclerosis (FSGS) glomerular bulk RNA-seq data. Furthermore, single-nuclear RNA-seq data from interferon-γ mouse model podocytes (in vivo) showed similarity to human FSGS single-cell RNA-seq data from urine podocytes (ex vivo) and from human podocyte cell lines (in vitro) using bulk RNA-seq. These data highlight differences in the transcriptional effects of the APOL1-G1 risk variant in a model specific manner. Shared differentially expressed genes in podocytes in both mouse models suggest possible novel glomerular damage markers in APOL1 variant-induced diseases. Transcription factor Zbtb16 was downregulated in podocytes and endothelial cells in both models, possibly contributing to glucocorticoid-resistance. In summary, these findings in two mouse models suggest both shared and distinct therapeutic opportunities for APOL1 glomerulopathies.

Loss of surface transport is a main cellular pathomechanism of CRB2 variants causing podocytopathies

Crumbs2 (CRB2) is a central component of the renal filtration barrier and part of the slit diaphragm, a unique cell contact formed by glomerular podocytes. Some CRB2 variants cause recessive inherited forms of steroid-resistant nephrotic syndrome. However, the disease-causing potential of numerous CRB2 variants remains unknown. Here, we report the establishment of a live-cell imaging-based assay, allowing a quantitative evaluation of the pathogenic potential of so far non-categorized CRB2 variants. Based on in silico data analysis and protein prediction software, putative disease-associated CRB2 missense variants were selected, expressed as CRB2-GFP fusion proteins, and analyzed in reporter cell lines with BFP-labeled plasma membrane. We found that in comparison with PM-localized WT, disease-associated CRB2 variants remained predominantly at the ER. Accumulation at the ER was also present for several non-characterized CRB2 variants and variants in which putative disulfide bridge-forming cysteines were replaced. Strikingly, WT CRB2 retained inside the ER in cells lacking protein disulfide isomerase A3, indicating that posttranslational modification, especially the formation of disulfide bridges, is a crucial step for the CRB2 PM transport.

Mass spectrometry-based N-glycosylation analysis in kidney disease

Kidney disease is a global health concern with an enormous expense. It is estimated that more than 10% of the population worldwide is affected by kidney disease and millions of patients would progress to death prematurely and unnecessarily. Although creatinine detection and renal biopsy are well-established tools for kidney disease diagnosis, they are limited by several inevitable defects. Therefore, diagnostic tools need to be upgraded, especially for the early stage of the disease and possible progression. As one of the most common post-translational modifications of proteins, N-glycosylation plays a vital role in renal structure and function. Deepening research on N-glycosylation in kidney disease provides new insights into the pathophysiology and paves the way for clinical application. In this study, we reviewed recent N-glycosylation studies on several kidney diseases. We also summarized the development of mass spectrometric methods in the field of N-glycoproteomics and N-glycomics.

Sulfation of sialic acid is ubiquitous and essential for vertebrate development

Glycosylation of proteins and lipids occurs in vertebrates, usually terminating with sialylation, which regulates the physicochemical and biological properties of these glycoconjugates. Although less commonly known, sialic acid residues also undergo various modifications, such as acetylation, methylation, and sulfation. However, except for acetylation, the enzymes or functions of the other modification processes are unknown. To the best of our knowledge, this study is the first to demonstrate the ubiquitous occurrence of sulfated sialic acids and two genes encoding the sialate: O-sulfotransferases 1 and 2 in vertebrates. These two enzymes showed about 50% amino acid sequence identity, and appeared to be complementary to each other in acceptor substrate preferences. Gene targeting experiments showed that the deficiency of these genes was lethal for medaka fish during young fry development and accompanied by different phenotypes. Thus, the sulfation of sialic acids is essential for the vertebrate development.

Rodent models to study sodium retention in experimental nephrotic syndrome

Sodium retention and edema are hallmarks of nephrotic syndrome (NS). Different experimental rodent models have been established for simulating NS, however, not all of them feature sodium retention which requires proteinuria to exceed a certain threshold. In rats, puromycin aminonucleoside nephrosis (PAN) is a classic NS model introduced in 1955 that was adopted as doxorubicin-induced nephropathy (DIN) in 129S1/SvImJ mice. In recent years, mice with inducible podocin deletion (Nphs2^Δipod ) or podocyte apoptosis (POD-ATTAC) have been developed. In these models, sodium retention is thought to be caused by activation of the epithelial sodium channel (ENaC) in the distal nephron through aberrantly filtered serine proteases or proteasuria. Strikingly, rodent NS models follow an identical chronological time course after the development of proteinuria featuring sodium retention within days and spontaneous reversal thereafter. In DIN and Nphs2^Δipod mice, inhibition of ENaC by amiloride or urinary serine protease activity by aprotinin prevents sodium retention, opening up new and promising therapeutic approaches that could be translated into the treatment of nephrotic patients. However, the essential serine protease(s) responsible for ENaC activation is (are) still unknown. With the use of nephrotic rodent models, there is the possibility that this (these) will be identified in the future. This review summarizes the various rodent models used to study experimental nephrotic syndrome and the insights gained from these models with regard to the pathophysiology of sodium retention.

Edible Bird Nest Protects the Kidney From Gentamicin Induced Acute Tubular Necrosis

Every year, there are about 13.3 million cases of acute kidney injury (AKI). Although AKI is a preventable and treatable disease, if left untreated, it has high risk of multiple organ failure and progression to end stage kidney disease. Acute tubular necrosis (ATN) has been recognised as one of the major causes of AKI. Till to date, there is no effective supplement or medication in treating or reversing AKI. Most of the treatment strategies involve preventative measure to minimise the occurrence of AKI or to reverse the cause of AKI. Hence one of the primary area of research interests is to explore the potential treatment for AKI. Edible bird nests (EBN) are edible food produce by the swiftlet’s saliva, which is rich in sialic acids. Sialic acids are monosaccharides that play a vital role in maintaining the integrity and proper function of the human organs, including kidneys. EBN also contains epidermal growth factor, which is widely believed to have rejuvenation and tissue repairing properties. We initiate this study to study the potential reno-protective effect of edible bird’s nests by studying the Wistar rat model of gentamicin-induced AKI. Besides renal profiles, renal histology was also semiquantitatively assessed. In our study, pre-treatment with EBN prevented and ameliorated the gentamicin-induced AKI. To a lesser extent, post-treatment with EBN also protected the kidney from the toxic effect of gentamicin. Our findings are highly indicative that EBN possesses reno-protective properties.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

Anti-apoptosis mechanism of triptolide based on network pharmacology in focal segmental glomerulosclerosis rats

Triptolide (TPL), the active component of Tripterygium wilfordii, exhibits anti-cancer and antioxidant functions. We aimed to explore the anti-apoptosis mechanism of TPL based on network pharmacology and in vivo and in vitro research validation using a rat model of focal segmental glomerulosclerosis (FSGS). The chemical structures and pharmacological activities of the compounds reported in T. wilfordii were determined and used to perform the network pharmacology analysis. The Traditional Chinese Medicine Systems Pharmacology Database (TCMSP) was then used to identify the network targets for 16 compounds from Tripterygium wilfordii. Our results showed that 47 overlapping genes obtained from the GeneCards and OMIM databases were involved in the occurrence and development of FSGS and used to construct the protein–protein interaction (PPI) network using the STRING database. Hub genes were identified via the MCODE plug-in of the Cytoscape software. IL4 was the target gene of TPL in FSGS and was mainly enriched in the cell apoptosis term and p53 signaling pathway, according to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. TPL inhibited FSGS-induced cell apoptosis in rats and regulated IL4, nephrin, podocin, and p53 protein levels via using CCK8, TUNEL, and Western blot assays. The effects of IL4 overexpression, including inhibition of cell viability and promotion of apoptosis, were reversed by TPL. TPL treatment increased the expression of nephrin and podocin and decreased p53 expression in rat podocytes. In conclusion, TPL inhibited podocyte apoptosis by targeting IL4 to alleviate kidney injury in FSGS rats.

Mice lacking core 1-derived O-glycan in podocytes develop transient proteinuria, resulting in focal segmental glomerulosclerosis

The glomerular filtration barrier is composed of podocytes, glomerular basement membrane, and endothelial cells. Disruption of these structures causes several glomerular injuries, such as focal segmental glomerulosclerosis (FSGS). The surface of podocyte apical membranes is coated by negatively charged sialic acids on core 1-derived mucin-type O-glycans. Here, we aimed to investigate the physiological role of core 1-derived O-glycans in the podocytes using adult mice lacking podocyte-specific core 1-derived O-glycans (iPod-Cos). iPod-Cos mice exhibited early and transient proteinuria with foot process effacements and developed typical FSGS-like disease symptoms. To identify the key molecules responsible for the FSGS-like phenotype, we focused on podocalyxin and podoplanin, which possess mucin-type O-glycans. Expression and localization of podocalyxin did not change in iPod-Cos glomeruli. Besides, western blot analysis revealed significantly lower levels of intact podocalyxin in isolated glomeruli of iPod-Cos mice, and high levels of processed forms in iPod-Cos glomeruli, as compared to that in control glomeruli. Conversely, podoplanin mRNA, and protein levels were lower in iPod-Cos mice than in control mice. These results demonstrated that core 1-derived O-glycan on podocytes is required for normal glomerular filtration and may contribute to the stable expression of podocalyxin and podoplanin.

Cosmc-dependent mucin-type O-linked glycosylation is essential for podocyte function

Mucin-type O-linked glycosylation, a posttranslational modification affecting the stability and biophysical characteristics of proteins, requires C1GalT1 (T synthase) and its obligate, X-linked chaperone Cosmc. Hypomorphic C1GalT1 mutations cause renal failure via not yet established mechanisms. We hypothesize that impaired Cosmc-dependent O-glycosylation in podocytes is sufficient to cause disease. Podocyte-specific Cosmc knockout mice were generated and phenotyped to test this hypothesis. Female heterozygous mice displaying mosaic inactivation of Cosmc in podocytes due to random X-linked inactivation were also examined. Mice with podocyte-specific Cosmc deletion develop profound albuminuria, foot process effacement, glomerular sclerosis, progressive renal failure, and impaired survival. Glomerular transcriptome analysis reveals early changes in cell adhesion, extracellular matrix organization, and chemokine-mediated signaling pathways, coupled with podocyte loss. Expression of the O-glycoprotein podoplanin was lost, while Tn antigen, representing immature O-glycans, was most abundantly found on podocalyxin. In contrast to hemizygous male and homozygous female animals, heterozygous female mosaic animals developed only mild albuminuria, focal foot process effacement, and nonprogressive kidney disease. Ultrastructurally, Cosmc-deficient podocytes formed Tn antigen-positive foot processes interdigitating with those of normal podocytes but not with other Cosmc-deficient cells. This suggests a cell nonautonomous mechanism for mucin-type O-glycoproteins in maintaining podocyte function. In summary, our findings demonstrated an essential and likely cell nonautonomous role for mucin-type O-glycosylation for podocyte function.