Cellular and molecular pathways of renal repair after acute kidney injury

The acutely injured mammalian kidney mounts a cellular and molecular response to repair itself. However, in patchy regions such intrinsic processes are impaired and dysregulated leading to chronic kidney disease. Currently, no therapy exists to treat established acute kidney injury per se. Strategies to augment human endogenous repair processes and retard associated profibrotic responses are urgently required. Recent studies have identified injury-induced activation of the intrinsic molecular driver of epithelial regeneration and induction of partial epithelial to the mesenchymal state, respectively. Activation of key developmental transcription factors drive such processes; however, whether these recruit comparable gene regulatory networks with target genes similar to those in nephrogenesis is unclear. Extensive complex molecular cross-talk between the nephron epithelia and immune, interstitial, and endothelial cells regulate renal recovery. In vitro–based M1/M2 macrophage subtypes have been increasingly linked to renal repair; however, the precise contribution of in vivo macrophage plasticity to repair responses is poorly understood. Endothelial cell–pericyte intimacy, balance of the angiocrine/antiangiocrine system, and endothelial cell–regulated inflammatory processes have an impact on renal recovery and fibrosis. Close scrutiny of cellular and molecular pathways in repairing human kidneys is imperative for the identification of promising therapeutic targets and biomarker of human renal repair processes. The acutely injured mammalian kidney mounts a cellular and molecular response to repair itself. However, in patchy regions such intrinsic processes are impaired and dysregulated leading to chronic kidney disease. Currently, no therapy exists to treat established acute kidney injury per se. Strategies to augment human endogenous repair processes and retard associated profibrotic responses are urgently required. Recent studies have identified injury-induced activation of the intrinsic molecular driver of epithelial regeneration and induction of partial epithelial to the mesenchymal state, respectively. Activation of key developmental transcription factors drive such processes; however, whether these recruit comparable gene regulatory networks with target genes similar to those in nephrogenesis is unclear. Extensive complex molecular cross-talk between the nephron epithelia and immune, interstitial, and endothelial cells regulate renal recovery. In vitro–based M1/M2 macrophage subtypes have been increasingly linked to renal repair; however, the precise contribution of in vivo macrophage plasticity to repair responses is poorly understood. Endothelial cell–pericyte intimacy, balance of the angiocrine/antiangiocrine system, and endothelial cell–regulated inflammatory processes have an impact on renal recovery and fibrosis. Close scrutiny of cellular and molecular pathways in repairing human kidneys is imperative for the identification of promising therapeutic targets and biomarker of human renal repair processes. Despite advances in medical care, acute kidney injury (AKI) remains an independent predictor of in-hospital mortality. Depending on the clinical setting and underlying comorbidities, in-hospital mortality rates can approach 50% to 70%.1Chertow G.M. Burdick E. Honour M. et al.Acute kidney injury, mortality, length of stay, and costs in hospitalized patients.J Am Soc Nephrol. 2005; 16: 3365-3370Crossref PubMed Scopus (1558) Google Scholar, 2Palevsky P.M. Zhang J.H. et al.VA/NIH, Acute Renal Failure Trial NetworkIntensity of renal support in critically ill patients with acute kidney injury.N Engl J Med. 2008; 359: 7-20Crossref PubMed Scopus (889) Google Scholar AKI frequently leads to chronic kidney disease (CKD),3Chawla L.S. Eggers P.W. Star R.A. et al.Acute kidney injury and chronic kidney disease as interconnected syndromes.N Engl J Med. 2014; 371: 58-66Crossref PubMed Scopus (300) Google Scholar with AKI survivors at ∼9-fold, 3-fold, and 2-fold increased risk of progression to CKD, end-stage renal disease, and mortality, respectively, compared with patients without AKI.4Coca S.G. Singanamala S. Parikh C.R. Chronic kidney disease after acute kidney injury: a systematic review and meta-analysis.Kidney Int. 2012; 81: 442-448Abstract Full Text Full Text PDF PubMed Scopus (536) Google Scholar Currently, no definite therapies exist to prevent or treat established AKI per se. Proximal tubular epithelial cell (PTEC) death is the most common cause of AKI, and frequently occurs due to ischemic, toxic, septic, or obstructive insults.5Thadhani R. Pascual M. Bonventre J.V. Acute renal failure.N Engl J Med. 1996; 334: 1448-1460Crossref PubMed Google Scholar PTECs are exquisitely sensitive to such insults, although distal tubular epithelial cells (TECs) also undergo apoptosis in human AKI.6Oberbauer R. Rohrmoser M. Regele H. et al.Apoptosis of tubular epithelial cells in donor kidney biopsies predicts early renal allograft function.J Am Soc Nephrol. 1999; 10: 2006-2013Crossref PubMed Google Scholar The histologic features of human AKI include loss of brush border typical of PTECs, flattening and focal loss of renal TECs, and infiltration of inflammatory cells with the appearance of Tamm-Horsfall protein-rich urinary casts.7Bonventre J.V. Yang L. Cellular pathophysiology of ischemic acute kidney injury.J Clin Invest. 2011; 121: 4210-4221Crossref PubMed Scopus (560) Google Scholar After injury, PTEC regeneration leads to the restoration of epithelial morphology and kidney function (Figure 1). However, particularly in regions of persistent or severe injury, the regenerative processes are frequently inefficient, impaired, and dysregulated, resulting in extensive tissue remodeling and fibrosis. These foci demonstrate persistently flattened tubular epithelium, chronic interstitial inflammation, microvascular dropout “capillary rarefaction,” and the presence of α-smooth muscle actin–positive (α-SMA+) myofibroblasts8Venkatachalam M.A. Weinberg J.M. Kriz W. et al.Failed Tubule Recovery, AKI-CKD Transition, and Kidney Disease Progression.J Am Soc Nephrol. 2015; 26: 1765-1776Crossref PubMed Scopus (69) Google Scholar (Figure 1). The injury-induced profibrotic milieu sets up a self-perpetuating vicious cycle of tubular atrophy, further nephron loss, and fibrosis, inexorably transforming AKI to CKD. The Oxford English Dictionary’s definition of repair is to “restore (something damaged, faulty or worn) to a good condition.” In contrast, regeneration is to generate again to its original form and function. Therefore, in this review, the word “regeneration” is used for the proximal tubular epithelium generated after injury, with complete restoration of basolateral polarity and acquisition of brush border. Of note, nephron epithelial regeneration can only be confirmed by genetic lineage tracing strategy, where progeny demarcating regenerated nephron epithelia display reattainment of the brush border and basolateral polarity. Harnessing and augmenting the human kidney’s intrinsic regenerative processes and suppressing concomitant profibrotic responses is one of the approaches to treat established human AKI per se. To this end, deep critical understanding of the cellular and molecular cross-talk of the repairing kidney is essential to identify high-value therapeutic targets to induce/enhance regeneration where repair is the norm. Here, the focus is on the cellular and molecular pathways of repair responses of the acutely injured mammalian kidney. In this basic science review, diagnosis of murine AKI is based on a significant increase in serum creatinine with histologic changes of acute PTEC injury. The injured epithelium exhibits adenosine triphosphate depletion, mitochondrial dysfunction, disruption of apicobasal polarity, and cytoskeleton with a subset enduring apoptotic or necrotic cell death.7Bonventre J.V. Yang L. Cellular pathophysiology of ischemic acute kidney injury.J Clin Invest. 2011; 121: 4210-4221Crossref PubMed Scopus (560) Google Scholar Brisk TEC replication replaces the lost cells and repairs the injured epithelium, a process elegantly described by Jean Oliver as early as the 1900s.9Oliver J. The Histogenesis of Chronic Uranium Nephritis with Especial Reference to Epithelial Regeneration.J Exp Med. 1915; 21: 425-450Crossref PubMed Google Scholar Robust conclusions founded on genetic fate-mapping strategies coupled with a murine model of AKI ruled out contribution of cellular populations residing outside the nephron tubular compartment to epithelial repair, thereby reconfirming the earlier observations that surviving TECs proliferate and repair the injured nephron epithelia, a process also likely to be used by its human counterpart.10Duffield J.S. Park K.M. Hsiao L.L. et al.Restoration of tubular epithelial cells during repair of the postischemic kidney occurs independently of bone marrow-derived stem cells.J Clin Invest. 2005; 115: 1743-1755Crossref PubMed Scopus (454) Google Scholar, 11Humphreys B.D. Valerius M.T. Kobayashi A. et al.Intrinsic epithelial cells repair the kidney after injury.Cell Stem Cell. 2008; 2: 284-291Abstract Full Text Full Text PDF PubMed Scopus (462) Google Scholar However, the possibility of epithelial repair mediated by a resident tubular progenitor cell remained. Whether repair is a general capacity shared by surviving cells, or a unique property possessed by a small distinct subset of fixed, resident, identifiable epithelial cells has engendered considerable debate.12Kusaba T. Humphreys B.D. Controversies on the origin of proliferating epithelial cells after kidney injury.Pediatr Nephrol. 2014; 29: 673-679Crossref PubMed Google Scholar, 13Smeets B. Boor P. Dijkman H. et al.Proximal tubular cells contain a phenotypically distinct, scattered cell population involved in tubular regeneration.J Pathol. 2013; 229: 645-659Crossref PubMed Scopus (81) Google Scholar, 14Romagnani P. Lasagni L. Remuzzi G. Renal progenitors: an evolutionary conserved strategy for kidney regeneration.Nat Rev Nephrol. 2013; 9: 137-146Crossref PubMed Scopus (77) Google Scholar The identification of a resident CD133+CD24+ cell as the cellular mediator of human and mouse proximal tubular repair raised the possibility of a fixed population of intratubular progenitor cells.15Sagrinati C. Netti G.S. Mazzinghi B. et al.Isolation and characterization of multipotent progenitor cells from the Bowman's capsule of adult human kidneys.J Am Soc Nephrol. 2006; 17: 2443-2456Crossref PubMed Scopus (402) Google Scholar These cells were demonstrated in vitro to possess an exaggerated self-renewal capacity and could be induced to multiple lineages. The cell culture systems may induce plasticity that may not be shared by the corresponding endogenous cells in vivo. Recently, this was elegantly exemplified by a study that showed pericytes of a majority of organs in vivo did not exhibit mesenchymal stem cell–like properties in contrast to those observed in in vitro plasticity.16Guimaraes-Camboa N. Cattaneo P. Sun Y. et al.Pericytes of Multiple Organs Do Not Behave as Mesenchymal Stem Cells In.Vivo. Cell Stem Cell. 2017; 20: 345-359.e345Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar Of note, identification of such a population in human kidneys and not in mouse kidneys has precluded genetic lineage tracing studies, the latter being the most reliable strategy to map the fate of a distinct cellular population in vivo. However, Corbeil et al.17Corbeil D. Fargeas C.A. Jaszai J. CD133 might be a pan marker of epithelial cells with dedifferentiation capacity.Proc Natl Acad Sci U S A. 2014; 111: E1451-E1452Crossref PubMed Scopus (0) Google Scholar showed abundant expression of CD133 in the proximal tubules of healthy mouse kidneys, and thus questioned the proposed stem/progenitor marker of this cellular population. Therefore, the inability to fate-map human CD133+CD24+ cells in vivo, the controversy regarding its expression in mouse kidneys, and the lack of a precise molecular signature has lent ambiguity to their precise identity and role in renal repair after injury.12Kusaba T. Humphreys B.D. Controversies on the origin of proliferating epithelial cells after kidney injury.Pediatr Nephrol. 2014; 29: 673-679Crossref PubMed Google Scholar An aldehyde dehydrogenase enzyme expression–based isolation strategy was used to isolate putative breast and liver cancer stem cells.18Douville J. Beaulieu R. Balicki D. ALDH1 as a functional marker of cancer stem and progenitor cells.Stem Cells Dev. 2009; 18: 17-25Crossref PubMed Scopus (171) Google Scholar, 19Chute J.P. Muramoto G.G. Whitesides J. et al.Inhibition of aldehyde dehydrogenase and retinoid signaling induces the expansion of human hematopoietic stem cells.Proc Natl Acad Sci U S A. 2006; 103: 11707-11712Crossref PubMed Scopus (267) Google Scholar A similar approach co-opted to the human kidney revealed a high aldehyde dehydrogenase population that also expressed CD133, CD24, keratin19, Bcl2, and vimentin.20Lindgren D. Bostrom A.K. Nilsson K. et al.Isolation and characterization of progenitor-like cells from human renal proximal tubules.Am J Pathol. 2011; 178: 828-837Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar The latter 3 are known to be upregulated by the injured epithelia. In the human kidney, CD133+CD24+ cells expressed vimentin and were shown to have an indistinct brush border with less cytoplasm and fewer mitochondria compared with the CD133−CD24− population. These cells were predominantly detected after AKI and expressed kidney injury molecule-1 (Kim-1+). The fully differentiated mature adult epithelia demarcated by tamoxifen-inducible Slc34a1 (a sodium phosphate cotransporter) CRE-driver upregulated CD24, CD133, Kim-1, and vimentin after mice ischemia reperfusion injury (IRI).21Kusaba T. Lalli M. Kramann R. et al.Differentiated kidney epithelial cells repair injured proximal tubule.Proc Natl Acad Sci U S A. 2014; 111: 1527-1532Crossref PubMed Scopus (113) Google Scholar Taken together, these findings suggest that perhaps a CD133+CD24+ cell represents an injured TEC. An obvious question is why do they exist in the normal uninjured kidney? Nephron tubular epithelia are continuously exposed to potentially toxic products of cellular metabolism contributing to the exfoliation of TECs and their appearance in the urine at the rate of ∼50,000 to 72,000 cells/hour.22Prescott L.F. The normal urinary excretion rates of renal tubular cells, leucocytes and red blood cells.Clin Sci. 1966; 31: 425-435PubMed Google Scholar, 23Dorrenhaus A. Muller J.I. Golka K. et al.Cultures of exfoliated epithelial cells from different locations of the human urinary tract and the renal tubular system.Arch Toxicol. 2000; 74: 618-626Crossref PubMed Scopus (0) Google Scholar It is plausible that such microinjuries invoke CD133+CD24+Kim-1+vimentin+ cell types, as noted by Smeets et al.13Smeets B. Boor P. Dijkman H. et al.Proximal tubular cells contain a phenotypically distinct, scattered cell population involved in tubular regeneration.J Pathol. 2013; 229: 645-659Crossref PubMed Scopus (81) Google Scholar A promoter initially designed to reproduce the expression pattern of the endogenous podocalyxin gene within podocytes fortuitously labeled parietal epithelial cells and random TECs.24Appel D. Kershaw D.B. Smeets B. et al.Recruitment of podocytes from glomerular parietal epithelial cells.J Am Soc Nephrol. 2009; 20: 333-343Crossref PubMed Scopus (0) Google Scholar This observation led to the fate-mapping studies of this scattered TEC population.13Smeets B. Boor P. Dijkman H. et al.Proximal tubular cells contain a phenotypically distinct, scattered cell population involved in tubular regeneration.J Pathol. 2013; 229: 645-659Crossref PubMed Scopus (81) Google Scholar, 25Berger K. Bangen J.M. Hammerich L. et al.Origin of regenerating tubular cells after acute kidney injury.Proc Natl Acad Sci U S A. 2014; 111: 1533-1538Crossref PubMed Scopus (0) Google Scholar The TECs labeled before injury did not increase in number; in contrast, cells labeled by a continuous doxycycline-labeling strategy used postinjury were shown to increase significantly. Injury activated the promoter; therefore, postinjury, a continuous labeling strategy demarcated random surviving injured TECs. Although such a labeling strategy may not unambiguously confirm true expansion of this scattered TEC population, but it lends support to the notion that repair is most likely conducted by random, surviving injured epithelial cells because a subset of such cells was Kim-1+Ki67+. The TECs strictly preserve their tubular identity during epithelial homeostasis and after injury. The fate-mapping of single epithelial cells via tamoxifen-inducible ActinCreERT2;R26VT2/GK3 mice revealed that clones maintain the fate of a single renal lineage and tubule-type during epithelial homeostasis and repair.26Rinkevich Y. Montoro D.T. Contreras-Trujillo H. et al.In vivo clonal analysis reveals lineage-restricted progenitor characteristics in mammalian kidney development, maintenance, and regeneration.Cell Rep. 2014; 7: 1270-1283.27Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar During epithelial homeostasis, Wnt-responsive cells (Axin2+ cells, Axin2 a readout of active canonical Wnt-β catenin signaling) demonstrated substantially greater proliferative capacity compared with ActinCre+ cells, with Wnt-responsive cells generating large clones (as many as 11 cells) in the renal cortex and medulla. In a rhabdomyolysis model of AKI, Axin2+ cells displayed a similar greater clonal expansion over 2-month period after injury. The precise identity of the cell type generated by the descendants of Wnt-responsive cells after AKI and the source of Wnt ligand is unclear. Whether such cells are essential to epithelial regeneration after injury remains unclear as genetic perturbation of this cell type and its consequent impact on renal repair were not examined. Of note, postinjury induction of Wnt4 within PTECs first highlighted the possible link between Wnt-β catenin signaling to renal repair.27Terada Y. Tanaka H. Okado T. et al.Expression and function of the developmental gene Wnt-4 during experimental acute renal failure in rats.J Am Soc Nephrol. 2003; 14: 1223-1233Crossref PubMed Scopus (92) Google Scholar However, Wnt4 detection was based on immunostaining, and, to date, no robust, reliable antibodies are available to detect Wnt proteins. Subsequent genetic fate-mapping studies did not detect Wnt4 expression within the nephron epithelial compartment. Instead, Wnt4 was reactivated in the medullary myofibroblast after IRI, and removal of Wnt4 had no effect on fibrosis or myofibroblast proliferation.28DiRocco D.P. Kobayashi A. Taketo M.M. et al.Wnt4/beta-catenin signaling in medullary kidney myofibroblasts.J Am Soc Nephrol. 2013; 24: 1399-1412Crossref PubMed Scopus (54) Google Scholar Taken together, currently, no conclusive data exist that points to the existence of a bona fide stem-cell or progenitor-cell population in the nephron. Nevertheless, the possibility of unique TECs with an exaggerated replicative capacity in vivo is exciting as it demarcates a potentially high-value cell type for drug discovery to augment endogenous repair processes. While the existence of unique cell types in the nephron epithelia was being debated, the identity of the key intrinsic molecular driver of epithelial regenerative responses after AKI remained elusive until recently. To this end, we identified injury-induced activation of Sox9 as a key regenerative response.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar The Translating Ribosome Affinity Purification approach used to obtain deep, cell type–specific injury-responsive molecular programs from the Six2-derived nephron compartment revealed Sox9 as one of the highest upregulated transcription factor 24 hours after IRI.30Liu J. Krautzberger A.M. Sui S.H. et al.Cell-specific translational profiling in acute kidney injury.J Clin Invest. 2014; 124: 1242-1254Crossref PubMed Scopus (123) Google Scholar A rare Sox9+ cell was identified in the proximal tubule of the normal uninjured kidney, whereas clusters of Sox9+ cells were expressed in the calbindin-28dk+ distal convoluted TECs.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar Sox9+ cells demarcate the injured proliferating PTEC subtype after ischemic and obstructive AKI, with ∼40 to 45% coexpressing Ki67 at 48 hours after injury. Coupling genetic lineage tracing with conditional removal of Sox9 from the proximal tubules in a survival compatible AKI to CKD model, confirmed Sox9 as a key intrinsic molecular response mounted by an injured nephron epithelia to regenerate itself.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar The fate-mapping studies of cells that activated Sox9 early after injury (demarcated by a single, low-dose tamoxifen labeling strategy immediately after reperfusion) revealed that the descendants of such cells regenerated a functional tubular epithelium (Figure 2).29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar The progeny regained apicobasal polarity similar to the uninjured tubular epithelium, thereby confirming successful regeneration of epithelia by such cells.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar The mice that underwent conditional removal of Sox9 within the proximal tubules demonstrated impaired early repair responses and renal function recovery with significantly greater fibrosis 4 weeks after renal IRI compared with the control mice. Four weeks after IRI, Sox9 expression returned to baseline in majority of the proximal tubules that regained a normal lotus tetragonolobus lectin expression pattern with complete resolution of injury (Kim-1−) expression. However, distinct proximal tubular domains continued to express Kim-1, and such Kim-1+ PTECs were found to mount Sox9+ response thereby demarcating PTECs subtype with “unresolved injury-repair” responses (Figure 2). Such cell types may play a role in transitioning of AKI to CKD.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar Sox9 also plays a role in repairing nephron epithelia after nephrotoxic AKI. Using a combination of genetic lineage tracing of Sox9+ cells and removal of Sox9 from the nephron epithelia in the setting of folic acid–induced AKI, Kang et al.31Kang H.M. Huang S. Reidy K. et al.Sox9-Positive Progenitor Cells Play a Key Role in Renal Tubule Epithelial Regeneration in Mice.Cell Rep. 2016; 14: 861-871Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar demonstrated Sox9 to be a key molecular response that repairs the acutely injured kidney after nephrotoxic insult. In this study, the relative contribution of the cells that activate Sox9+ after AKI or the resident Sox9+ cell to the overall repair responses is not clear because the relationship of a tamoxifen-dependent labeling strategy with the induction of folic acid–induced AKI was not detailed. In contrast to the Sox9CreERT2 “knockin” mice that we used, Kang et al.31Kang H.M. Huang S. Reidy K. et al.Sox9-Positive Progenitor Cells Play a Key Role in Renal Tubule Epithelial Regeneration in Mice.Cell Rep. 2016; 14: 861-871Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar used the Sox9CreER BAC transgenic mice, thereby precluding direct comparison of tamoxifen regimens and resulting recombination efficiency in the kidneys. Sox9 represents the first robust direct epithelial intrinsic link between the formation and regeneration of the nephron epithelia. Sox9 plays a key role in human and mice nephrogenesis. Inactivating mutations on 1 SOX9 allele leads to its haploinsufficiency causing campomelic dysplasia syndrome.32Foster J.W. Dominguez-Steglich M.A. Guioli S. et al.Campomelic dysplasia and autosomal sex reversal caused by mutations in an SRY-related gene.Nature. 1994; 372: 525-530Crossref PubMed Scopus (0) Google Scholar, 33Wagner T. Wirth J. Meyer J. et al.Autosomal sex reversal and campomelic dysplasia are caused by mutations in and around the SRY-related gene SOX9.Cell. 1994; 79: 1111-1120Abstract Full Text PDF PubMed Scopus (1084) Google Scholar A perinatal lethal syndrome in majority of the cases, campomelic dysplasia syndrome is characterized by wide-ranging skeletal malformations and autosomal sex reversal. Interestingly, renal defects included hydronephrosis, and renal hypoplasia.34Houston C.S. Opitz J.M. Spranger J.W. et al.The campomelic syndrome: review, report of 17 cases, and follow-up on the currently 17-year-old boy first reported by Maroteaux, et al in 1971.Am J Med Genet. 1983; 15: 3-28Crossref PubMed Google Scholar During nephrogenesis, conditional removal of Sox8 and Sox9 leads to hypoplastic kidneys in mice.35Reginensi A. Clarkson M. Neirijnck Y. et al.SOX9 controls epithelial branching by activating RET effector genes during kidney development.Hum Mol Genet. 2011; 20: 1143-1153Crossref PubMed Scopus (55) Google Scholar To determine whether Sox9 combines a role in PTEC repair with development of the proximal tubule, our genetic lineage tracing of Sox9+ cells in the embryonic kidneys revealed that Sox9+ cells gave rise to the bulk of the nephron epithelia, including all the 4 specialized epithelial cell types.29Kumar S. Liu J. Pang P. et al.Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney.Cell Rep. 2015; 12: 1325-1338Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar The aforementioned observations raise 2 important questions: first, does postinjury regenerative action of Sox9 represent reiteration of a Sox9-regulated developmental program, and, second, do Sox9 cells demarcate dedifferentiated cell types? “Repair recapitulates developmental pathways” is an often-mentioned statement based on the observations of reactivation of key developmental gene(s) after injury. However, whether the reactivated gene uses the similar gene regulatory networks to regenerate the injured epithelia, akin to the one it uses during development, is unclear.36Kumar S. Liu J. McMahon A.P. Defining the acute kidney injury and repair transcriptome.Semin Nephrol. 2014; 34: 404-417Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar It is likely that an activated gene that plays a crucial role in organogenesis may recruit disparate gene networks compared with its embryonic counterpart, which may very well be the case in Sox9. Dedifferentiation involves reversion of a differentiated mature cell to its primitive form encountered in that lineage during development. Pax2 has been used as a “tubular epithelial dedifferentiation marker” primarily based on the initial study that reported re-expression of Pax2 after folic acid–induced AKI.37Imgrund M. Grone E. Grone H.J. et al.Re-expression of the developmental gene Pax-2 during experimental acute tubular necrosis in mice 1.Kidney Int. 1999; 56: 1423-1431Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar In the normal adult kidney, Pax2 expression persists in the medullary collecting ducts, and it is believed that after injury, Pax2 is re-expressed in the PTECs. In the setting of AKI, the fate of Pax2-expressing cells and whether the Pax2 requirement per se is essential for epithelial repair remains to be examined. Of note, whether Pax2 or Sox9 truly demarcates a dedifferentiated PTEC subtype is unclear. Although injured TECs exhibits robust fidelity with respect to cell-type identity; few undergo partial epithelial-mesenchymal transition (EMT), thereby demonstrating epithelial plasticity38Lovisa S. LeBleu V.S. Tampe B. et al.Epithelial-to-mesenchymal transition induces cell cycle arrest and parenchymal damage in renal fibrosis.Nat Med. 2015; 21: 998-1009Crossref PubMed Scopus (103) Google Scholar, 39Grande M.T. Sanchez-Laorden B. Lopez-Blau C. et al.Snail1-induced partial epithelial-to-mesenchymal transition drives renal fibrosis in mice and can be targeted to reverse established disease.Nat Med. 2015; 21: 989-997Crossref PubMed Scopus (94) Google Scholar (Figure 2). Rescuing the injured cell from the state of partial EMT may render efficiency to postinjury renal repair processes. On injury, random subsets of TECs undergo partial EMT and acquire a pathologic secretome that instigates a proinflammatory and fibrotic milieu. The injury-activated Snai1 (encoding snail family zinc finger 1), a potent EMT inducer during embryonic development and tumor progression, has been suggested to induce a partial EMT state. The PTEC-specific Snai1 knockout animals displayed significantly reduced inflammation, decreased α-SMA+ myofibroblasts, and fibrosis after unilateral ureteral obstruction– and folic acid–induced AKI compared with the controls.39Grande M.T. Sanchez-Laorden B. Lopez-Blau C. et al.Snail1-in