- Email: [email protected]
Cellular regeneration of podocytes from parietal cells: the debate is still open
commentary
preventing diabetic ketoacidosis while reducing risk of hypoglycemia. The brief report by Bally et al. has several limitations, beginning with those inherent in performing a post hoc subgroup analysis in a small patient population in a brief trial. Relevant secondary end points such as hospital length of stay are omitted. Validation data such as mean absolute relative difference, which are of special interest related to interstitial edema and fluid shifts in the patient receiving dialysis, are not included. Details regarding device calibration, frequency of sensor readings and insulin adjustments, concurrent s.c. insulin injections, management of glycemia in the control group, and cost analysis are not provided. Technical issues related to the algorithm device, which included calibration errors, dislodgments, and failure of the tablet device to communicate with the pump in the larger study, are not included in this report. Increasingly, the work of an expert clinical nephrologist necessitates awareness and, in many cases, comanagement of dysglycemia in the patient with diabetes. Features of the patient with diabetes familiar to the nephrologist, including avoidance of extreme hyperglycemia, risk of hypoglycemia with unawareness, and the impact of hemodialysis treatments, suggest that the population would be well served by new closed-loop technology. Definitive testing in the hospital setting is a good place to begin. DISCLOSURE The author declared no competing interests. REFERENCES 1. American Diabetes Association. 15. Diabetes care in the hospital: Standards of Medical Care in Diabetes – 2019. Diabetes Care. 2019;42: S173–S181. 2. Fonseca VA, Grunberger G, Anhalt H, et al. Continuous glucose monitoring: a consensus conference of the American Association of Clinical Endocrinologists and American College of Endocrinology. Endocr Pract. 2016;22:1008–1021. 3. Garg R, Williams ME. Diabetes management in the kidney patient. Med Clin North Am. 2013;97:135–156. 4. United States Renal Data System. 2018 USRDS Annual Data Report: Epidemiology of Kidney Disease in the United States. Bethesda, MD:
542
National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases; 2018. 5. American Diabetes Association. 7. Diabetes technology: Standards of Medical Care in Diabetes – 2019. Diabetes Care. 2019;42:S71– S80. 6. Bally L, Gubler P, Thabit H, et al. Fully closedloop insulin delivery improves glucose control of inpatients with type 2 diabetes receiving hemodialysis. Kidney Int. 2019;96:593–596.
7. Bally L, Thabit H, Hartnell S, et al. Closed-loop insulin delivery for glycemic control in noncritical care. N Engl J Med. 2018;379:547–556. 8. Danne T, Nimri R, Battelino T, et al. International consensus on use of continuous glucose monitoring. Diabetes Care. 2017;40: 1631–1640. 9. Joubert M. Effectiveness of continuous glucose monitoring in dialysis patients with diabetes: the DIALYDIAB pilot study. Diabetes Res Clin Pract. 2015;107:348–354.
Cellular regeneration of podocytes from parietal cells: the debate is still open Marcus J. Moeller1 and Pierre-Louis Tharaux2 The study by Kaverina et al. in this issue addresses an important question: can podocytes be replenished by parietal epithelial cells (PECs)? The authors use a complex transgenic mouse model in which podocytes are labeled with GFP and PECs are simultaneously labeled with tdTomato. When Kaverina and colleagues induce focal segmental glomerulosclerosis (FSGS), they find that individual PECs are doubly labeled, coexpress podocyte markers, and form structures similar to foot processes, suggesting that these PECs may have transdifferentiated into podocytes. Kidney International (2019) 96, 542–544; https://doi.org/10.1016/j.kint.2019.04.038 Copyright ª 2019, International Society of Nephrology. Published by Elsevier Inc. All rights reserved.
see basic research on page 597
T
he study by Kaverina et al. in this issue addresses an important and controversial topic: is it possible to regenerate our limited pool of postmitotic podocytes?1 The answer is relevant for the direction of our search for novel therapeutic concepts: should we focus on boosting cellular regeneration or rather on slowing progression? The academic field of research appears undecided as well: a spontaneous poll on 1 Division of Nephrology and Clinical Immunology, Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University, Aachen, Germany; and 2 Universite; de Paris, Paris Cardiovascular Centre PARCC, Institut National de la Santé et de la Recherche Médicale (INSERM), Paris, France
Correspondence: Marcus J. Moeller, Department of Nephrology and Clinical Immunology, RheinischWestfälische Technische Hochschule (RWTH) University Hospital Aachen, Pauwelsstr. 30, 52074 Aachen, Germany. E-mail: [email protected]
the 11th international podocyte conference in Jerusalem in 2016 showed that about half of the community believes in boosting cellular regeneration versus targeting the sclerotic process as the most promising approach to slow or cure progression into chronic kidney disease. From our group of sceptic investigators only 2 consecutive reports have been published that show that limited cellular replenishment of the podocyte (Pod) pool may occur during early life.2,3 At birth, a limited number of additional committed Pods reside on Bowman’s capsule, which coexpress Pod markers (synaptopodin) as well as PEC markers, and which are labeled by both transgenic driver mouse lines to label Pods (NPHS2-rtTA) or PEC (PECrtTA). As the glomeruli grow, these additional Pods are recruited to the capillary tuft. This mechanism might Kidney International (2019) 96, 540–554
commentary
b
a
Lesion formation by PECs
Normal glomerulus Disease
Proliferative lesion
c
Sclerotic lesion
PEC to Pod transdifferentiation
Disease and regeneration
Figure 1 | (a) In this issue, Kaverina et al.1 have genetically labeled parietal epithelial cells (PECs) in red and podocytes (Pods) in green simultaneously. (b) Induction of glomerular disease may lead to the formation of proliferative and/or sclerotic lesions. Most researchers agree that that these epithelial lesions are formed predominantly by PECs, and accumulating experimental data support that activated PECs are a therapeutic target to prevent loss of renal function. (c) In experimental disease, Kaverina et al. observed that some PECs (red) also can be labeled in green, suggesting transdifferentiation toward the podocyte lineage (arrows with tails). Some of the doubly labeled cells also formed foot processes and expressed Pod markers (arrows).
explain in part the increase of Pod numbers in juvenile humans by about 20%.4 These committed Pods disappeared from Bowman’s capsule with increasing age. Consistent with that, we found that no cellular regeneration occurs from PECs in adult mice even after removal of 50% or up to 83% of the renal mass. Another group combining genetic fate mapping with efficient podocyte isolation protocols to quantify podocyte turnover and regeneration found no significant regeneration in a genetic podocyte ablation model.5 In the present study, Kaverina et al. use a complex transgenic mouse model in which Pods are labeled with GFP (green) and simultaneously PECs with tdTomato (red). To induce the FSGS model, the mice were injected with a Kidney International (2019) 96, 540–554
polyclonal antiserum raised against cultured Pods. The resulting disease was focal (affecting 20% of glomeruli) and segmental in nature, inducing a mixture of segmental sclerotic as well as proliferative lesions (i.e., cellular crescents). Such a mixture of glomerular lesions can be observed in human biopsies also, because PEC activation can lead to both types of phenotypes. Kaverina et al. define Pods that have been regenerated from PECs as cells that coexpress both markers (resulting in yellow staining). The authors found that Pod density decreased by 20% and 17% in affected/ injured glomeruli 28 and 56 days after injection of the antiserum (the increase between both time points was not statistically significant). Regarding the
role of PECs in driving progression, Kaverina et al. confirm that upon induction of FSGS, PECs become activated and migrate to the capillary tuft into sclerotic lesions or proliferate to form cellular crescents. This is in line with an increasing number of independent groups that have confirmed an essential and deleterious role of PECs for lesion formation and progression in FSGS (see Lazareth et al.6), and this aspect of PEC biology has become generally accepted in the field (Figure 1a and b). Regarding the role of PECs in cellular regeneration 28 days after induction of the FSGS model, PECs were genetically marked in red in only about 60% of the injured glomeruli (i.e., 12% of all glomeruli). Of these glomeruli, 543
commentary
PECs coexpressing the podocyte reporter GFP were observed in 38% (i.e., 4.5% of all glomeruli), indicating that these PECs had transdifferentiated into Pods. The authors present individual immunofluorescent stainings to show that transdifferentiated cells express Pod markers (p57, podocin, VEGF) and that they form structures resembling foot processes (using tissue expansion microscopy) (Figure 1c, arrows). On average, 2.1 transdifferentiated PECs were observed per glomerular cross section. Similar numbers were obtained 56 days after induction of FSGS. No transdifferentiation of PECS into Pods was observed in glomeruli that were not affected by this focal FSGS model (all groups reported similar results in unaffected glomeruli). In a previous article, Eng and colleagues reported an initial fall and subsequent rise of Pod numbers in the same FSGS model. In addition, they showed PEC migration onto the glomerular tuft. However, at that time it was technically not possible to show that migrated PECs had differentiated into Pods because the endogenous betagalactosidase in Pods cannot be differentiated from the transgenic reporter protein beta-galactosidase in PECs using immunofluorescent stainings.7 PEC to Pod transdifferentiation in glomerular disease was reported in several studies by Romoli et al. in human biopsies and using a Pax2 tamoxifeninducible mouse line.8 The insignificant rise in Pod density in the present study may not necessarily be a consequence of cellular regeneration. It may also reflect transient downregulation of Pod marker proteins during the acute phase of the disease. Transient downregulation of marker proteins is an important alternative mechanism when studying regeneration of Pods, and it has been proposed in previous studies. However, so far it has not been experimentally proven to occur in Pods. The method of simultaneous transgenic tagging of PECs and Pods in red and green in the present study is complex and involves 5 transgenes (see Figure 1
544
in Kaverina et al.1 for a schematic). As a limitation, a constitutively active flippase line Neph1-FLPo was used to mark (and thus define) presumptive Pods. For this reason, it cannot be ruled out that PECs may be aberrantly labeled as Pods during the course of the disease model. The authors show coexpression of endogenous Pod genes in exemplary colabeled cells, which is reassuring. In their article, Figure 3f shows a beautiful example of a cell coexpressing tdTomato and GFP showing a Pod morphology. However, the same glomerulus as well as several other figure panels show that virtually all PECs on Bowman’s capsule are also coexpressing GFP. At later stages of this disease model, or in human patients with FSGS, should one therefore expect to see glomeruli with parietal Pods? The morphology of these doubly labeled cells resembles partially differentiated Pods, whereas several are more reminiscent of PECs and appear to be arranged in several layers. Therefore, the authors note that the potential contribution of ectopic direct labeling to the pool of regenerated Pods cannot be determined experimentally at this point. Why could ectopic activation of the transgene Nphs1-FLPo occur in PECs? The line Nphs1-FLPo was generated by random integration of the transgene. It can never be assumed with certainty that the expression pattern of any transgene recapitulates the expression pattern of the endogenous gene product (here: NPHS1, nephrin). For example, the NPHS1 3ʹ promoter region may miss important regulatory elements, and the surrounding genome may be different and interfere with gene expression (because of the random insertion). A knock-in strategy to place the transgene into the endogenous gene locus would reduce these issues, but they can never be ruled out because even the endogenous gene nephrin might be expressed “aberrantly/ectopically” in PECs at low levels during the course of the disease. For this reason, cell fate–tracing experiments require the use of an inducible system.9 The question and relevance of repair mechanisms that might exist in the
glomerulus is highly relevant and challenging. The present study by Kaverina et al. presents an impressive amount of high-quality experimental data, which will likely trigger more lively discussions in our struggle to identify the relevant mechanisms. Of note, the 2 options, boosting cellular regeneration and blocking PECs pathogenicity, should not be seen as antagonistic but, rather, complementary. We invite curious investigators using similar innovative experimental approaches to join our efforts to ultimately deliver the best possible answer to the question of glomerular scarring and repair. DISCLOSURE All the authors declared no competing interests. REFERENCES 1. Kaverina NV, Eng DG, Freedman BS, et al. Dual lineage tracing shows that glomerular parietal epithelial cells can transdifferentiate toward the adult podocyte fate. Kidney Int. 2019;96: 597–611. 2. Appel D, Kershaw DB, Smeets B, et al. Recruitment of podocytes from glomerular parietal epithelial cells. J Am Soc Nephrol. 2009;20:333–343. 3. Berger K, Schulte K, Boor P, et al. The regenerative potential of parietal epithelial cells in adult mice. J Am Soc Nephrol. 2014;25: 693–705. 4. Puelles VG, Douglas-Denton RN, CullenMcEwen LA, et al. Podocyte number in children and adults: associations with glomerular size and numbers of other glomerular resident cells. J Am Soc Nephrol. 2015;26:2277–2288. 5. Wanner N, Hartleben B, Herbach N, et al. Unraveling the role of podocyte turnover in glomerular aging and injury. J Am Soc Nephrol. 2014;25:707–716. 6. Lazareth H, Henique C, Lenoir O, et al. The tetraspanin CD9 controls migration and proliferation of parietal epithelial cells and glomerular disease progression. Nat Commun. 2019;10:3303. 7. Eng DG, Sunseri MW, Kaverina NV, et al. Glomerular parietal epithelial cells contribute to adult podocyte regeneration in experimental focal segmental glomerulosclerosis. Kidney Int. 2015;88:999–1012. 8. Romoli S, Angelotti ML, Antonelli G, et al. CXCL12 blockade preferentially regenerates lost podocytes in cortical nephrons by targeting an intrinsic podocyte-progenitor feedback mechanism. Kidney Int. 2018;94: 1111–1126. 9. Humphreys BD, DiRocco DP. Lineage-tracing methods and the kidney. Kidney Int. 2014;86: 481–488.
Kidney International (2019) 96, 540–554
preventing diabetic ketoacidosis while reducing risk of hypoglycemia. The brief report by Bally et al. has several limitations, beginning with those inherent in performing a post hoc subgroup analysis in a small patient population in a brief trial. Relevant secondary end points such as hospital length of stay are omitted. Validation data such as mean absolute relative difference, which are of special interest related to interstitial edema and fluid shifts in the patient receiving dialysis, are not included. Details regarding device calibration, frequency of sensor readings and insulin adjustments, concurrent s.c. insulin injections, management of glycemia in the control group, and cost analysis are not provided. Technical issues related to the algorithm device, which included calibration errors, dislodgments, and failure of the tablet device to communicate with the pump in the larger study, are not included in this report. Increasingly, the work of an expert clinical nephrologist necessitates awareness and, in many cases, comanagement of dysglycemia in the patient with diabetes. Features of the patient with diabetes familiar to the nephrologist, including avoidance of extreme hyperglycemia, risk of hypoglycemia with unawareness, and the impact of hemodialysis treatments, suggest that the population would be well served by new closed-loop technology. Definitive testing in the hospital setting is a good place to begin. DISCLOSURE The author declared no competing interests. REFERENCES 1. American Diabetes Association. 15. Diabetes care in the hospital: Standards of Medical Care in Diabetes – 2019. Diabetes Care. 2019;42: S173–S181. 2. Fonseca VA, Grunberger G, Anhalt H, et al. Continuous glucose monitoring: a consensus conference of the American Association of Clinical Endocrinologists and American College of Endocrinology. Endocr Pract. 2016;22:1008–1021. 3. Garg R, Williams ME. Diabetes management in the kidney patient. Med Clin North Am. 2013;97:135–156. 4. United States Renal Data System. 2018 USRDS Annual Data Report: Epidemiology of Kidney Disease in the United States. Bethesda, MD:
542
National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases; 2018. 5. American Diabetes Association. 7. Diabetes technology: Standards of Medical Care in Diabetes – 2019. Diabetes Care. 2019;42:S71– S80. 6. Bally L, Gubler P, Thabit H, et al. Fully closedloop insulin delivery improves glucose control of inpatients with type 2 diabetes receiving hemodialysis. Kidney Int. 2019;96:593–596.
7. Bally L, Thabit H, Hartnell S, et al. Closed-loop insulin delivery for glycemic control in noncritical care. N Engl J Med. 2018;379:547–556. 8. Danne T, Nimri R, Battelino T, et al. International consensus on use of continuous glucose monitoring. Diabetes Care. 2017;40: 1631–1640. 9. Joubert M. Effectiveness of continuous glucose monitoring in dialysis patients with diabetes: the DIALYDIAB pilot study. Diabetes Res Clin Pract. 2015;107:348–354.
Cellular regeneration of podocytes from parietal cells: the debate is still open Marcus J. Moeller1 and Pierre-Louis Tharaux2 The study by Kaverina et al. in this issue addresses an important question: can podocytes be replenished by parietal epithelial cells (PECs)? The authors use a complex transgenic mouse model in which podocytes are labeled with GFP and PECs are simultaneously labeled with tdTomato. When Kaverina and colleagues induce focal segmental glomerulosclerosis (FSGS), they find that individual PECs are doubly labeled, coexpress podocyte markers, and form structures similar to foot processes, suggesting that these PECs may have transdifferentiated into podocytes. Kidney International (2019) 96, 542–544; https://doi.org/10.1016/j.kint.2019.04.038 Copyright ª 2019, International Society of Nephrology. Published by Elsevier Inc. All rights reserved.
see basic research on page 597
T
he study by Kaverina et al. in this issue addresses an important and controversial topic: is it possible to regenerate our limited pool of postmitotic podocytes?1 The answer is relevant for the direction of our search for novel therapeutic concepts: should we focus on boosting cellular regeneration or rather on slowing progression? The academic field of research appears undecided as well: a spontaneous poll on 1 Division of Nephrology and Clinical Immunology, Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University, Aachen, Germany; and 2 Universite; de Paris, Paris Cardiovascular Centre PARCC, Institut National de la Santé et de la Recherche Médicale (INSERM), Paris, France
Correspondence: Marcus J. Moeller, Department of Nephrology and Clinical Immunology, RheinischWestfälische Technische Hochschule (RWTH) University Hospital Aachen, Pauwelsstr. 30, 52074 Aachen, Germany. E-mail: [email protected]
the 11th international podocyte conference in Jerusalem in 2016 showed that about half of the community believes in boosting cellular regeneration versus targeting the sclerotic process as the most promising approach to slow or cure progression into chronic kidney disease. From our group of sceptic investigators only 2 consecutive reports have been published that show that limited cellular replenishment of the podocyte (Pod) pool may occur during early life.2,3 At birth, a limited number of additional committed Pods reside on Bowman’s capsule, which coexpress Pod markers (synaptopodin) as well as PEC markers, and which are labeled by both transgenic driver mouse lines to label Pods (NPHS2-rtTA) or PEC (PECrtTA). As the glomeruli grow, these additional Pods are recruited to the capillary tuft. This mechanism might Kidney International (2019) 96, 540–554
commentary
b
a
Lesion formation by PECs
Normal glomerulus Disease
Proliferative lesion
c
Sclerotic lesion
PEC to Pod transdifferentiation
Disease and regeneration
Figure 1 | (a) In this issue, Kaverina et al.1 have genetically labeled parietal epithelial cells (PECs) in red and podocytes (Pods) in green simultaneously. (b) Induction of glomerular disease may lead to the formation of proliferative and/or sclerotic lesions. Most researchers agree that that these epithelial lesions are formed predominantly by PECs, and accumulating experimental data support that activated PECs are a therapeutic target to prevent loss of renal function. (c) In experimental disease, Kaverina et al. observed that some PECs (red) also can be labeled in green, suggesting transdifferentiation toward the podocyte lineage (arrows with tails). Some of the doubly labeled cells also formed foot processes and expressed Pod markers (arrows).
explain in part the increase of Pod numbers in juvenile humans by about 20%.4 These committed Pods disappeared from Bowman’s capsule with increasing age. Consistent with that, we found that no cellular regeneration occurs from PECs in adult mice even after removal of 50% or up to 83% of the renal mass. Another group combining genetic fate mapping with efficient podocyte isolation protocols to quantify podocyte turnover and regeneration found no significant regeneration in a genetic podocyte ablation model.5 In the present study, Kaverina et al. use a complex transgenic mouse model in which Pods are labeled with GFP (green) and simultaneously PECs with tdTomato (red). To induce the FSGS model, the mice were injected with a Kidney International (2019) 96, 540–554
polyclonal antiserum raised against cultured Pods. The resulting disease was focal (affecting 20% of glomeruli) and segmental in nature, inducing a mixture of segmental sclerotic as well as proliferative lesions (i.e., cellular crescents). Such a mixture of glomerular lesions can be observed in human biopsies also, because PEC activation can lead to both types of phenotypes. Kaverina et al. define Pods that have been regenerated from PECs as cells that coexpress both markers (resulting in yellow staining). The authors found that Pod density decreased by 20% and 17% in affected/ injured glomeruli 28 and 56 days after injection of the antiserum (the increase between both time points was not statistically significant). Regarding the
role of PECs in driving progression, Kaverina et al. confirm that upon induction of FSGS, PECs become activated and migrate to the capillary tuft into sclerotic lesions or proliferate to form cellular crescents. This is in line with an increasing number of independent groups that have confirmed an essential and deleterious role of PECs for lesion formation and progression in FSGS (see Lazareth et al.6), and this aspect of PEC biology has become generally accepted in the field (Figure 1a and b). Regarding the role of PECs in cellular regeneration 28 days after induction of the FSGS model, PECs were genetically marked in red in only about 60% of the injured glomeruli (i.e., 12% of all glomeruli). Of these glomeruli, 543
commentary
PECs coexpressing the podocyte reporter GFP were observed in 38% (i.e., 4.5% of all glomeruli), indicating that these PECs had transdifferentiated into Pods. The authors present individual immunofluorescent stainings to show that transdifferentiated cells express Pod markers (p57, podocin, VEGF) and that they form structures resembling foot processes (using tissue expansion microscopy) (Figure 1c, arrows). On average, 2.1 transdifferentiated PECs were observed per glomerular cross section. Similar numbers were obtained 56 days after induction of FSGS. No transdifferentiation of PECS into Pods was observed in glomeruli that were not affected by this focal FSGS model (all groups reported similar results in unaffected glomeruli). In a previous article, Eng and colleagues reported an initial fall and subsequent rise of Pod numbers in the same FSGS model. In addition, they showed PEC migration onto the glomerular tuft. However, at that time it was technically not possible to show that migrated PECs had differentiated into Pods because the endogenous betagalactosidase in Pods cannot be differentiated from the transgenic reporter protein beta-galactosidase in PECs using immunofluorescent stainings.7 PEC to Pod transdifferentiation in glomerular disease was reported in several studies by Romoli et al. in human biopsies and using a Pax2 tamoxifeninducible mouse line.8 The insignificant rise in Pod density in the present study may not necessarily be a consequence of cellular regeneration. It may also reflect transient downregulation of Pod marker proteins during the acute phase of the disease. Transient downregulation of marker proteins is an important alternative mechanism when studying regeneration of Pods, and it has been proposed in previous studies. However, so far it has not been experimentally proven to occur in Pods. The method of simultaneous transgenic tagging of PECs and Pods in red and green in the present study is complex and involves 5 transgenes (see Figure 1
544
in Kaverina et al.1 for a schematic). As a limitation, a constitutively active flippase line Neph1-FLPo was used to mark (and thus define) presumptive Pods. For this reason, it cannot be ruled out that PECs may be aberrantly labeled as Pods during the course of the disease model. The authors show coexpression of endogenous Pod genes in exemplary colabeled cells, which is reassuring. In their article, Figure 3f shows a beautiful example of a cell coexpressing tdTomato and GFP showing a Pod morphology. However, the same glomerulus as well as several other figure panels show that virtually all PECs on Bowman’s capsule are also coexpressing GFP. At later stages of this disease model, or in human patients with FSGS, should one therefore expect to see glomeruli with parietal Pods? The morphology of these doubly labeled cells resembles partially differentiated Pods, whereas several are more reminiscent of PECs and appear to be arranged in several layers. Therefore, the authors note that the potential contribution of ectopic direct labeling to the pool of regenerated Pods cannot be determined experimentally at this point. Why could ectopic activation of the transgene Nphs1-FLPo occur in PECs? The line Nphs1-FLPo was generated by random integration of the transgene. It can never be assumed with certainty that the expression pattern of any transgene recapitulates the expression pattern of the endogenous gene product (here: NPHS1, nephrin). For example, the NPHS1 3ʹ promoter region may miss important regulatory elements, and the surrounding genome may be different and interfere with gene expression (because of the random insertion). A knock-in strategy to place the transgene into the endogenous gene locus would reduce these issues, but they can never be ruled out because even the endogenous gene nephrin might be expressed “aberrantly/ectopically” in PECs at low levels during the course of the disease. For this reason, cell fate–tracing experiments require the use of an inducible system.9 The question and relevance of repair mechanisms that might exist in the
glomerulus is highly relevant and challenging. The present study by Kaverina et al. presents an impressive amount of high-quality experimental data, which will likely trigger more lively discussions in our struggle to identify the relevant mechanisms. Of note, the 2 options, boosting cellular regeneration and blocking PECs pathogenicity, should not be seen as antagonistic but, rather, complementary. We invite curious investigators using similar innovative experimental approaches to join our efforts to ultimately deliver the best possible answer to the question of glomerular scarring and repair. DISCLOSURE All the authors declared no competing interests. REFERENCES 1. Kaverina NV, Eng DG, Freedman BS, et al. Dual lineage tracing shows that glomerular parietal epithelial cells can transdifferentiate toward the adult podocyte fate. Kidney Int. 2019;96: 597–611. 2. Appel D, Kershaw DB, Smeets B, et al. Recruitment of podocytes from glomerular parietal epithelial cells. J Am Soc Nephrol. 2009;20:333–343. 3. Berger K, Schulte K, Boor P, et al. The regenerative potential of parietal epithelial cells in adult mice. J Am Soc Nephrol. 2014;25: 693–705. 4. Puelles VG, Douglas-Denton RN, CullenMcEwen LA, et al. Podocyte number in children and adults: associations with glomerular size and numbers of other glomerular resident cells. J Am Soc Nephrol. 2015;26:2277–2288. 5. Wanner N, Hartleben B, Herbach N, et al. Unraveling the role of podocyte turnover in glomerular aging and injury. J Am Soc Nephrol. 2014;25:707–716. 6. Lazareth H, Henique C, Lenoir O, et al. The tetraspanin CD9 controls migration and proliferation of parietal epithelial cells and glomerular disease progression. Nat Commun. 2019;10:3303. 7. Eng DG, Sunseri MW, Kaverina NV, et al. Glomerular parietal epithelial cells contribute to adult podocyte regeneration in experimental focal segmental glomerulosclerosis. Kidney Int. 2015;88:999–1012. 8. Romoli S, Angelotti ML, Antonelli G, et al. CXCL12 blockade preferentially regenerates lost podocytes in cortical nephrons by targeting an intrinsic podocyte-progenitor feedback mechanism. Kidney Int. 2018;94: 1111–1126. 9. Humphreys BD, DiRocco DP. Lineage-tracing methods and the kidney. Kidney Int. 2014;86: 481–488.
Kidney International (2019) 96, 540–554