David J. Salant

Purpose of review: This review is prompted by substantial new information on the composition and structure of the podocyte slit-diaphragm and the identification of anti-nephrin antibodies in patients with acquired diseases of the glomerular podocyte, including steroid-sensitive nephrotic syndrome of childhood, minimal change disease and some cases of primary focal and segmental glomerulosclerosis (FSGS). Recent findings: New methodologies including multi-epitope affinity purification, high resolution proteomics and cryo-electron tomography reveal that the slit-diaphragm consists of a multi-layered fishnet-like structure made up by nephrin and neph1 homodimers as well as several co-assembled proteins with signaling properties. Insights from clinical studies and experimental models support the concept that pathogenic antibodies engaging the extracellular domain of nephrin disrupt the integrity of the slit-diaphragm and signal changes to the podocyte cytoskeleton that lead to foot process effacement and proteinuria. Summary: These findings raise several questions to be answered experimentally, including the nephrin epitopes targeted by the antibodies and the way in which they alter the slit-diaphragm and podocyte architecture and induce nephrin endocytosis. They also highlight the need for a reliable and widely available assay for anti-nephrin antibodies, which would have diagnostic and therapeutic impact on diagnosis, prognosis and therapy.

To date, this is the first study to demonstrate that intercellular bridges form between podocytes and parietal epithelial cells in the setting of rapid podocyte loss in subtypes of glomerulonephritis and FSGS.

Activation and proliferation of parietal epithelial cells (PECs), located along the inner rim of Bowman’s capsule, drives disease progression in subtypes of glomerulonephritis and focal segmental glomerulosclerosis. In examining the mechanisms contributing to PEC activation two established mouse models were utilized in this study, nephrotoxic serum nephritis (transient model) and podocyte-specific Klf4 knockout (progressive model). A role for transcription factor FRA2 ( Fosl2 ) was uncovered through single nuclear multiomic approaches relating to the regulation of PEC transcriptional/chromatin dynamics. Co-immunoprecipitation followed by mass spectrometry assessed the FRA2 protein interactome in cultured PECs, revealing a potential role for FRA2 in alternative splicing. Fosl2 expression was then blunted through CRISPR-Cas9 gene editing in cultured PECs, revealing reduced proliferative capacity and the downregulation of myofibroblast markers. In-vivo genetic lineage tracing of PECs after nephrotoxic serum revealed PEC-to-myofibroblast trans-differentiation events. Finally, immunostaining of human kidney biopsies with varied subtypes of glomerulonephritis confirmed Fosl2 expression in activated PECs within crescentic lesions, with single cell deconvolution strategies assigning PEC-skewed proportion ratios to bulk RNA-seq patient data from the NEPTUNE consortium. These results suggest that FRA2 ( Fosl2 ) directs a conserved molecular program of PEC-specific responses in subtypes of glomerulonephritis and focal segmental glomerulosclerosis.

Key Points: Detrimental effects of Krüppel-like factor 4 knockdown in podocytes were eliminated with the inhibition of signal transducer and activator of transcription 3 (STAT3) signaling specifically in podocytes. Human kidney biopsies with renal vasculitis demonstrated a glomerular enrichment of STAT3 downstream genes, which negatively correlated with eGFR. Deconvolution of the bulk RNA-seq from Nephrotic Syndrome Study Network showed an enrichment of STAT3 downstream genes in podocytes as compared with other cell clusters. Background: Podocyte loss and parietal epithelial cell activation are features of subtypes of glomerulonephritis and FSGS. We recently reported that the podocyte-specific loss of Krüppel-like factor 4 ( Klf4 ^ΔPod ) triggers dysregulated glomerular signal transducer and activator of transcription 3 (STAT3) activation, podocyte loss with parietal epithelial cell activation and proliferation, leading to FSGS. Although pharmacologic systemic STAT3 inhibition attenuated this phenotype, it remains unclear whether the detrimental effects of Klf4 loss are primarily a result of dysregulated STAT3 activation intrinsically in podocytes. Methods: Mice with the concurrent and conditional knockdown of Stat3 and Klf4 ( Klf4 ^ΔPod Stat3 ^ΔPod ) were generated and characterized. Expression arrays from kidney biopsies with various types of glomerular diseases, deposited in Nephroseq, were interrogated for glomerular expression of genes downstream of STAT3 signaling. Cell-specific modulation of STAT3 genes was determined using single-cell RNA sequencing–based proportional cell type deconvolution of bulk RNA-seq obtained from the Nephrotic Syndrome Study Network (NEPTUNE) FSGS and healthy controls. Results: Klf4 ^ΔPod Stat3 ^ΔPod mice demonstrated no significant podocyte loss, parietal epithelial cell activation and proliferation, FSGS lesions, albuminuria, kidney dysfunction, and tubulointerstitial fibrosis and inflammation compared with the Klf4 ^ΔPod mice. Klf4 ^ΔPod Stat3 ^ΔPod mice also exhibited less glomerular myofibroblasts (+ α -smooth muscle actin) as compared with Klf4 ^ΔPod mice. Overall survival was restored in Klf4 ^ΔPod Stat3 ^ΔPod mice as compared with Klf4 ^ΔPod mice. Interrogation of expression arrays from human kidney biopsies with renal vasculitis demonstrated a glomerular enrichment of genes involved in canonical STAT3 signaling as compared with healthy controls, which negatively correlated with eGFR. Deconvolution of the bulk RNA-seq data from NEPTUNE showed an enrichment of these STAT3 genes in podocytes as compared with other glomerular cell clusters. Conclusions: Collectively, these data demonstrate that inhibiting podocyte-specific STAT3 signaling was sufficient to counter the detrimental effects of Klf4 loss in podocytes and prevented albuminuria, accelerated podocyte loss, activation and proliferation of parietal epithelial cells, FSGS lesions, and kidney failure.

Podocytes and parietal epithelial cells (PECs) are among the few principal cell types within the kidney glomerulus, the former serving as a crucial constituent of the kidney filtration barrier and the latter representing a supporting epithelial layer that adorns the inner wall of Bowman's capsule. Podocytes and PECs share a circumscript developmental lineage that only begins to diverge during the S-shaped body stage of nephron formation–occurring immediately before the emergence of the fully mature nephron. These two cell types, therefore, share a highly conserved gene expression program, evidenced by recently discovered intermediate cell types occupying a distinct spatiotemporal gene expression zone between podocytes and PECs. In addition to their homeostatic functions, podocytes and PECs also have roles in kidney pathogenesis. Rapid podocyte loss in diseases, such as rapidly progressive GN and collapsing and cellular subtypes of FSGS, is closely allied with PEC proliferation and migration toward the capillary tuft, resulting in the formation of crescents and pseudocrescents. PECs are thought to contribute to disease progression and severity, and the interdependence between these two cell types during development and in various manifestations of kidney pathology is the primary focus of this review.

FOXD1 ⁺ cell–derived stromal cells give rise to pericytes and fibroblasts that support the kidney vasculature and interstitium but are also major precursors of myofibroblasts. ZEB2 is a SMAD-interacting transcription factor that is expressed in developing kidney stromal progenitors. Here we show that Zeb2 is essential for normal FOXD1 ⁺ stromal progenitor development. Specific conditional knockout of mouse Zeb2 in FOXD1 ⁺ stromal progenitors ( Zeb2 cKO) leads to abnormal interstitial stromal cell development, differentiation, and kidney fibrosis. Immunofluorescent staining analyses revealed abnormal expression of interstitial stromal cell markers MEIS1/2/3, CDKN1C, and CSPG4 (NG2) in newborn and 3-week-old Zeb2 -cKO mouse kidneys. Zeb2 -deficient FOXD1 ⁺ stromal progenitors also took on a myofibroblast fate that led to kidney fibrosis and kidney failure. Cell marker studies further confirmed that these myofibroblasts expressed pericyte and resident fibroblast markers, including PDGFR β , CSPG4, desmin, GLI1, and NT5E. Notably, increased interstitial collagen deposition associated with loss of Zeb2 in FOXD1 ⁺ stromal progenitors was accompanied by increased expression of activated SMAD1/5/8, SMAD2/3, SMAD4, and AXIN2. Thus, our study identifies a key role of ZEB2 in maintaining the cell fate of FOXD1 ⁺ stromal progenitors during kidney development, whereas loss of ZEB2 leads to differentiation of FOXD1 ⁺ stromal progenitors into myofibroblasts and kidney fibrosis.

Integrative omics identifies mediators between podocytes and parietal epithelial cells in proliferative glomerulopathies.

Primary membranous nephropathy (pMN) is a leading cause of nephrotic syndrome in adults. In most cases, this autoimmune kidney disease is associated with autoantibodies against the M-type phospholipase A2 receptor (PLA2R1) expressed on kidney podocytes, but the mechanisms leading to glomerular damage remain elusive. Here, we developed a cell culture model using human podocytes and found that anti-PLA2R1–positive pMN patient sera or isolated IgG4, but not IgG4-depleted sera, induced proteolysis of the 2 essential podocyte proteins synaptopodin and NEPH1 in the presence of complement, resulting in perturbations of the podocyte cytoskeleton. Specific blockade of the lectin pathway prevented degradation of synaptopodin and NEPH1. Anti-PLA2R1 IgG4 directly bound mannose-binding lectin in a glycosylation-dependent manner. In a cohort of pMN patients, we identified increased levels of galactose-deficient IgG4, which correlated with anti-PLA2R1 titers and podocyte damage induced by patient sera. Assembly of the terminal C5b-9 complement complex and activation of the complement receptors C3aR1 or C5aR1 were required to induce proteolysis of synaptopodin and NEPH1 by 2 distinct proteolytic pathways mediated by cysteine and aspartic proteinases, respectively. Together, these results demonstrated a mechanism by which aberrantly glycosylated IgG4 activated the lectin pathway and induced podocyte injury in primary membranous nephropathy.

New treatments, new understanding, and new approaches to translational research are transforming the outlook for patients with kidney diseases. A number of new initiatives dedicated to advancing the field of nephrology—from value-based care to prize competitions—will further improve outcomes of patients with kidney disease. Because of individual nephrologists and kidney organizations in the United States, such as the American Society of Nephrology, the National Kidney Foundation, and the Renal Physicians Association, and international nephrologists and organizations, such as the International Society of Nephrology and the European Renal Association–European Dialysis and Transplant Association, we are beginning to gain traction to invigorate nephrology to meet the pandemic of global kidney diseases. Recognizing the timeliness of this opportunity, the American Society of Nephrology convened a Division Chief Retreat in Dallas, Texas, in June 2019 to address five key issues: (1) asserting the value of nephrology to the health system; (2) productivity and compensation; (3) financial support of faculty’s and divisions’ educational efforts; (4) faculty recruitment, retention, diversity, and inclusion; and (5) ensuring that fellowship programs prepare trainees to provide high-value nephrology care and enhance attraction of trainees to nephrology. Herein, we highlight the outcomes of these discussions and recommendations to the American Society of Nephrology.