cells Review Urine-Derived Epithelial Cells as Models for Genetic Kidney Diseases Tjessa Bondue 1 , Fanny O. Arcolino 1 , Koenraad R. P. Veys 1,2, Oyindamola C. Adebayo 1,3, Elena Levtchenko 1,2, Lambertus P. van den Heuvel 1,4 and Mohamed A. Elmonem 5,* 1 Department of Development and Regeneration, KU Leuven, 3000 Leuven, Belgium; [email protected] (T.B.); [email protected] (F.O.A.); [email protected] (K.R.P.V.); [email protected] (O.C.A.); [email protected] (E.L.); [email protected] (L.P.v.d.H.) 2 Department of Pediatrics, Division of Pediatric Nephrology, University Hospitals Leuven, 3000 Leuven, Belgium 3 Centre for Molecular and Vascular Biology, Department of Cardiovascular Sciences, KU Leuven, 3000 Leuven, Belgium 4 Department of Pediatric Nephrology, Radboud University Medical Center, 6500 Nijmegen, The Netherlands 5 Department of Clinical and Chemical Pathology, Faculty of Medicine, Cairo University, Cairo 11628, Egypt * Correspondence: [email protected] Abstract: Epithelial cells exfoliated in human urine can include cells anywhere from the urinary tract and kidneys; however, podocytes and proximal tubular epithelial cells (PTECs) are by far the most relevant cell types for the study of genetic kidney diseases. When maintained in vitro, they have been proven extremely valuable for discovering disease mechanisms and for the development of new therapies. Furthermore, cultured patient cells can individually represent their human sources and their specific variants for personalized medicine studies, which are recently gaining much Citation: Bondue, T.; Arcolino, F.O.; interest. In this review, we summarize the methodology for establishing human podocyte and PTEC Veys, K.R.P.; Adebayo, O.C.; cell lines from urine and highlight their importance as kidney disease cell models. We explore Levtchenko, E.; van den Heuvel, L.P.; the well-established and recent techniques of cell isolation, quantification, immortalization and Elmonem, M.A. Urine-Derived characterization, and we describe their current and future applications. Epithelial Cells as Models for Genetic Kidney Diseases. Cells 2021, 10, 1413. Keywords: podocytes; proximal tubular epithelial cells; PTECs; urine-derived cells; glomerular https://doi.org/10.3390/cells10061413 diseases; Fanconi syndrome; renal tubular acidosis; inherited renal disorders; kidney disease cellu- lar models Academic Editor: Laura Perin Received: 4 May 2021 Accepted: 2 June 2021 1. Introduction Published: 6 June 2021 Expansion of urine-derived epithelial cells in vitro was developed almost 50 years Publisher’s Note: MDPI stays neutral ago [1–3]. Although the initial focus for such research was mainly to study cancer cells with regard to jurisdictional claims in originating from the urinary tract [4,5], it expanded rapidly to cover all types of kidney published maps and institutional affil- disorders, particularly genetic kidney diseases, as cells isolated from urine carry the iations. genotypic background of patients [6–9]. The easy and non-invasive approach of harvesting epithelial cells from urine has lured scientists to master cell expansion in culture to use them as a powerful tool to study the pathogenesis of various renal diseases and to screen for new therapeutic modalities [10]. Recently, the potential therapeutic effect of some of the isolated urinary cells has been even suggested [11]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Epithelial cells voided in human urine can include cells anywhere from the urinary This article is an open access article tract and kidneys. In women, vaginal epithelial cells are also normally shed in urine. Fine distributed under the terms and adjustment of culture conditions is necessary to properly isolate and expand each type of conditions of the Creative Commons the target epithelial cells. Next, in depth characterization of viable cells lost in urine helps Attribution (CC BY) license (https:// to identify their origin by studying the expression of cell surface markers, gene and protein creativecommons.org/licenses/by/ expressions and their functionality [12]. 4.0/). Cells 2021, 10, 1413. https://doi.org/10.3390/cells10061413 https://www.mdpi.com/journal/cells Cells 2021, 10, 1413 2 of 28 Cells 2021, 10, 1413 2 of 28 For the study of genetic kidney disorders two types of epithelial cells stand out: po- docytes forming the glomerular tuft and proximal tubular epithelial cells (PTECs) layer- For the study of genetic kidney disorders two types of epithelial cells stand out: ing the proximal part of the renal tubules. Both cell types are actively involved in the podocytes forming the glomerular tuft and proximal tubular epithelial cells (PTECs) lay- pathogenesisering the proximal of numerous part of inherited the renal tubules.kidney diso Bothrders[10]. cell types This areactively review involvedpresents inthe the recent andpathogenesis the most well-established of numerous inherited techniques kidney for disorders isolation, [10 handling]. This review and characterization presents the re- of thesecent cells and theand most some well-established applications of techniques these cellular for isolation, models handling for the study and characterization of genetic kidney diseases.of these cells and some applications of these cellular models for the study of genetic kidneyPodocytes diseases. are the main cellular components of the renal glomerular filtration barrier. They arePodocytes highly aredifferentiated the main cellular cells componentspreserved throughout of the renal glomerularthe animal filtration kingdom barrier. with the mainThey function are highly of retaining differentiated plasma cells proteins preserved and throughout blood cells the [13]. animal The kingdom glomerular with filtration the barriermain is function responsible of retaining for the plasma selective proteins filtration and bloodof blood cells coming [13]. The through glomerular the afferent filtration arte- barrier is responsible for the selective filtration of blood coming through the afferent ar- riole to the Bowman’s space. The glomerular capillaries are lined with a fenestrated endo- teriole to the Bowman’s space. The glomerular capillaries are lined with a fenestrated theliumendothelium sitting sittingon the on glomerular the glomerular basement basement membrane membrane (GBM). (GBM). The The glomerular glomerular endothe- en- liumdothelium is covered is covered by the byglycocalyx, the glycocalyx, a surface a surface layerlayer comprising comprising glycosaminoglycans, glycosaminoglycans, prote- oglycans,proteoglycans, glycoproteins glycoproteins and glycolipids and glycolipids that thatcontribute contribute to the to the glomerular glomerular permselectivity permselec- [14].tivity Podocytes [14]. Podocytes are attached are attached to the to GBM the GBM via viahighly highly organized organized foot foot processes processes [[15].15]. InIn be- tweenbetween the thefoot foot processes processes are are the the slit slit diaphragms diaphragms (Figure(Figure1 ),1), which which are are the the final final barrier barrier preventingpreventing the the passage passage of of proteins proteins intointo the the urinary urinary filtrate. filtrate. FigureFigure 1. Podocytes: 1. Podocytes: key key element element of of the the glomerular glomerular filtrationfiltration barrier. barrier. Podocytes Podocytes are are highly highly differentiated differentiated cells with cells a specificwith a spe- cific cellularcellular architecture, architecture, comprising comprising primary primary and and secondary secondary foot processesfoot processes that wrap that aroundwrap around the glomerular the glomerular capillaries, capillaries, and and express essentialessential proteins proteins that that define define the the slit slit diaphragm diaphragm and and permselectivity permselectivity of the of glomerular the glomerular filtration filtration barrier. barrier. Kidney Kid- ney diseases affectingaffecting the the podocytes podocytes (podocytopathies) (podocytopathies) can can be characterized be characterized by the by loss the of loss the slitof the diaphragm, slit diaphragm, podocyte podocyte foot foot process effacement effacement and and detachment, detachment, which which clinically clinically manifests manifests as glomerular as glomerular proteinuria. proteinuria. TheThe filtrate filtrate that that passes passes into thethe Bowman’sBowman’s space space continues continues into into the the proximal proximal tubule tubule andand loop loop of of Henle, Henle, distal distal tubules tubules andand collectingcollecting ducts ducts for for further further processing processing [16]. [16]. ThereThere are are currently currently over over 60 genes,genes, whichwhich produce produce important important proteins proteins that that are are re- re- ported to affect podocyte function and cause human glomerular disease when deficient. ported to affect podocyte function and cause human glomerular disease when deficient. Pathogenic variants in these genes alter the dynamics of the glomerular filtration barrier Pathogenic variants in these genes alter the dynamics of the glomerular filtration barrier changing podocyte morphology and their natural connection to each other, usually causing changingwhat is knownpodocyte as podocytemorphology foot and process their effacement natural connection associated withto each widening other, usually of the slit caus- ingdiaphragmatic what is known spaces as podocyte