REVIEW CURRENT OPINION Can podocytes be regenerated in adults? Stuart J. Shankland, Benjamin S. Freedman, and Jeffrey W. Pippin Purpose of review Podocytes are critical components of the nephron filtration barrier and are depleted in many kidney injuries and disease states. Terminally differentiated adult podocytes are highly specialized, postmitotic cells, raising the question of whether the body has any ability to regenerate lost podocytes. This timely question has recently been illuminated by a series of innovative studies. Here, we review recent progress on this topic of significant interest and debate. Recent findings The innovation of genetic labeling techniques enables fate tracing of individual podocytes, providing the strongest evidence yet that podocytes can be replaced by nearby progenitor cells. In particular, two progenitor pools have recently been identified in multiple studies: parietal epithelial cells and cells of renin lineage. These studies furthermore suggest that podocyte regeneration can be enhanced using ex-vivo or pharmacological interventions. Summary Recent studies indicate that the podocyte compartment is more dynamic than previously believed. Bidirectional exchange with neighboring cellular compartments provides a mechanism for podocyte replacement. Based on these findings, we propose a set of criteria for evaluating podocyte regeneration and suggest that restoration of podocyte number to a subsclerotic threshold be targeted as a potentially achievable clinical goal. Keywords glomerulosclerosis, glomerulus, parietal epithelial cell, progenitor, renin cells, stem cell INTRODUCTION: PODOCYTE ABSOLUTE AND RELATIVE PODOCYTE REGENERATION IN THE ADULT KIDNEY NUMBER The current review is intended to provide a bal- This principle segues to the notion of absolute anced view on the broad topic of adult podocyte podocyte number versus relative podocyte number regeneration. Podocyte differentiation and regen- [5,6&,7]. The latter, also known as podocyte density, eration during kidney development is well dis- is simply a measure of absolute podocyte number cussed by others [1,2]. For context, nephron divided by the glomerular volume it occupies. The progenitor cells (NPCs), which give rise to the prox- denominator, glomerular volume, is influenced by imal nephron including podocytes during kidney age, nephron number, obesity and other factors [8]. embryogenesis, cease to exist in mammalian kid- A decrease in podocyte number or density below a neys shortly after birth [3,4]. NPCs, also known as certain threshold has clearly been shown to underlie metanephric mesenchyme cells, express SIX2 and the onset of proteinuria and the development of CITED1 and self-renew only during kidney develop- glomerulosclerosis [9]. Moreover, the lower the ment [3,4]. podocyte number/density, the more severe the glo- We will use the dictionary definition of regen- merular scarring [9,10]. eration (biology), which is ‘the ability to recreate lost or damaged tissues, organs and limbs’. We Division of Nephrology, Department of Medicine, University of would argue therefore, in states in which the kidney Washington School of Medicine, Seattle, Washington, USA and/or adult podocytes are damaged, that a simple Correspondence to Stuart J. Shankland, Division of Nephrology, Depart- requirement for podocyte regeneration is an actual ment of Medicine, University of Washington School of Medicine, Box decrease in the number of podocytes. Stated differ- 358058, 750 Republican Street, Seattle, WA 98109, USA. ently, we do not expect podocyte regeneration in Tel: +1 206 543 2346; fax: +1 206 685 8661; e-mail: [email protected] diseases in which podocyte number has not been Curr Opin Nephrol Hypertens 2017, 26:154–164 reduced. DOI:10.1097/MNH.0000000000000311 www.co-nephrolhypertens.com Volume 26 Number 3 May 2017 Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved. Podocyte replacement Shankland et al. density will decrease [25]. This occurs following KEY POINTS nephrectomy, in the transplanted kidney, in aging, Adult podocytes are unable to self-renew as they are obesity and certain glomerular diseases following unable to proliferate. the scarring of injured glomeruli [8]. The increased glomerular volume leads to a mismatch in coverage Replacement of adult podocytes that have been lost in by podocytes of underlying glomerular capillaries disease needs to derive from stem/progenitor cells. [8]. This is initially compensated for by hypertrophy Glomerular parietal epithelial cells and cells of renin of podocytes (i.e. an increase in their size without an lineage are two local sources of adult podocyte stem/ increase in their number), that over time becomes progenitor cells. paradoxically maladaptive [8]. Under these circum- To prevent/limit and even reverse glomerular scarring stances, podocyte density is reduced, but podocyte due to podocyte loss, the goal for replacement does number does not change. Whether this is sufficient not need to be 100%. to trigger podocyte regeneration is not yet clear. EVIDENCE THAT ADULT PODOCYTES CAN Although numbers vary somewhat due to meth- BE PARTIALLY OR FULLY REPLACED IN odology, the mean number of podocytes per human DISEASE STATES glomerulus is 558 (range 263–983), and in mice 72 Several lines of evidence support the notion that (range 70–80) [11&&]. Recent human studies have adult podocytes can be replaced following their shown that absolute podocyte number increases depletion. These include the following: during the adolescent period by up to 20% [12]. In contrast, normal kidney aging is accompanied (1) An increase in podocyte number in disease using cell- by reduced podocyte number in man [11&&] and specific markers: In studies using nongenetic cell rodents [13]. In contrast to absolute podocyte num- fate mapping, the identification of podocytes ber, however, podocyte density is markedly relies on several markers such as WT-1 [26], p57 decreased (more than four-fold) in large glomeruli [23] and others [27]. Several studies have shown in adult humans, compared with children [12]. in experimental models of diabetic and non- diabetic kidney disease that podocyte number ABSOLUTE PODOCYTE NUMBER could be raised/increased when animals were DEPLETION given ACE-I, ARBs, steroids, retinoids or normal- Several mechanisms can lead to podocyte depletion ization of their diabetic state [28–31,32&&,33]. In including apoptosis, detachment, necrosis and DNA all these studies, the increase in podocyte num- damage [14–16]. Indeed, viable podocytes can be ber occurred in the absence of podocyte pro- detected in the urine and can be cultured ex vivo liferation. Changes in podocyte-specific [17,18]. It is estimated that patients with kidney proteins can result secondary to injury, and thus disease slough podocytes into their urine at more the specificity of these changes also needs to than 10 cells/mg creatinine [17]. Clinical pro- be considered. gression is in many cases much slower, and it has (2) An increase in podocyte number in disease using been observed that this suggests a regenerative genetic labeling techniques: Inducible conditional mechanism [19]. reporters enable labeling of podocytes for line- The adult terminally differentiated podocyte is age tracing. For example, the mT/mG mouse unable to proliferate, thereby preventing self- crossed with a podocin-rtTA mouse perma- renewal in the face of ongoing losses [20]. Mechan- nently labels all cells in the body with a red isms underlying the inability of adult podocytes to color. When Cre is turned on by activating the proliferate include an increase in expression of cell podocin promoter with doxycycline, all podo- cycle inhibitors (such as the cyclin kinase inhibitors cytes become green in color, whereas other cells p27 [21,22] and p57 [22,23]), DNA damage resulting remain red [34,35]. Wanner et al. [36&&] used in cell cycle arrest [24], mitotic catastrophe and mT/mG mice crossed with inducible diphtheria deficiencies in M-type cyclins [20]. Thus, to replace toxin receptor (DTR) mice to partially deplete adult podocytes, other cell types need to fulfill this podocytes with the diphtheria toxin. Podocytes biological role. were labeled green and expressed DTR following doxycycline administration; when diphtheria RELATIVE PODOCYTE NUMBER toxin was subsequently administered, there DEPLETION was a decrease in green cells (podocytes). When If the glomerular volume increases without any followed over 4 weeks, red cells appeared on change in absolute podocyte number, podocyte the glomerular tuft and expressed podocyte 1062-4821 Copyright ß 2017 Wolters Kluwer Health, Inc. All rights reserved. www.co-nephrolhypertens.com 155 Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved. Renal immunology and pathology proteins. The authors of the manuscript con- (1) migration from their original location to a new cluded that overall their results suggest a podo- location along the glomerular basement mem- cyte renewal of 38% of ablated cells. brane (GBM) in the glomerular tuft (2) de novo expression of several podocyte Using the same mT/mG mice, Lasagni et al. proteins, coincident with loss of their original [37&&] depleted podocytes with the podocyte toxin cell-specific proteins Adriamycin. When followed over time, the percent- (3) expression of epithelial cell markers age of newly generated podocytes was 6% when all (4) acquisition of classic podocyte ultrastructural podocytes were taken in to account or 30.4% in mice features (such as foot process and