Nephrotic syndrome in a dish: recent developments in modeling in vitro

Abstract

Nephrotic syndrome is a heterogeneous disease, and one of the most frequent glomerular disorders among children. Depending on the etiology, it may result in end-stage renal disease and the need for renal replacement therapy. A dysfunctional glomerular filtration barrier, comprising of endothelial cells, the glomerular basement membrane and podocytes, characterizes nephrotic syndrome. Podocytes are often the primary target cells in the pathogenesis, in which not only the podocyte function but also their crosstalk with other glomerular cell types can be disturbed due to a myriad of factors. The pathophysiology of nephrotic syndrome is highly complex and studying molecular mechanisms in vitro requires state-of-the-art cell-based models resembling the in vivo situation and preferably a fully functional glomerular filtration barrier. Current advances in stem cell biology and microfluidic platforms have heralded a new era of three-dimensional (3D) cultures that might have the potential to recapitulate the glomerular filtration barrier in vitro. Here, we highlight the molecular basis of nephrotic syndrome and discuss requirements to accurately study nephrotic syndrome in vitro, including an overview of specific podocyte markers, cutting-edge stem cell organoids, and the implementation of microfluidic platforms. The development of (patho) physiologically relevant glomerular models will accelerate the identification of molecular targets involved in nephrotic syndrome and may be the harbinger of a new era of therapeutic avenues.

Keywords: Glomerular filtration barrier; Kidney; Nephrotic syndrome; Organoids; Podocytes; Stem cells.

Fig. 1

Schematic illustration depicting glomerular filtration barrier and molecular overview of podocytes. The three-layered glomerular filtration barrier, consisting of fenestrated endothelial cells, glomerular basement membrane (GBM), and podocytes. Molecules depicted are relevant for the function and characterization of podocytes. These molecules include slit diaphragm, cytoskeleton, and foot processes molecules. Furthermore, hypothesized paracrine signaling molecules (ANGPTLs, VEGF-A, and ET-1) and their receptors, Tie-2, VEGFR2, and ETA, respectively, that are responsible for the crosstalk between endothelial cells and podocytes are shown as well. Finally, podocyte injury markers, such as GLEPP-1, B7-1, CR1, and ezrin are included in the illustration. GBM, glomerular basement membrane; PECs, parietal epithelial cells; ANGPTLs, angiopoietins; CD2AP, CD2-associated protein; MAGI1, membrane-associated guanylate kinase; Nck1/2, non-catalytic region of tyrosine kinase adaptor protein 1/2; NEPH1, same as kin of IRRE-like protein 1 (KIRREL); NPHS1, nephrin; PMAT, plasma membrane monoamine transporter; TRPC6, Transient Receptor Potential Cation Channel Subfamily C Member 6; NHERF-2, Sodium-hydrogen exchange regulatory cofactor NHE-RF2; GLEPP-1, glomerular epithelial protein 1; VEGF-A, vascular endothelial growth factor A; VEGFR2, vascular endothelial growth factor A; ZO-1, zonula occludens-1; α-ACTN4, alpha actinin 4; Ca²⁺, calcium; ET-1, endothelin 1; ETA, endothelial ET-1 receptor A; Tie-2, tyrosine kinase receptor 2; uPAR, urokinase plasminogen activator receptor

Fig. 2

Schematic diagram of iPS cultures towards podocyte or organoid differentiation. a Differentiation from pluripotency towards intermediate mesoderm is usually performed in 2D. b Suspension and air-liquid cultures using Transwell® filter inserts to induce self-organizing kidney organoids. c Organoids contain multiple nephrons and glomeruli consist of podocytes, endothelial cells, glomerular basement membrane, and parietal epithelial cells, though not fully mature. d Differentiation of podocyte-like cells in 2D or e in a glomerulus-on-a-chip