Recruitment of Podocytes from Glomerular Parietal Epithelial Cells

Loss of a critical number of podocytes from the glomerular tuft leads to glomerulosclerosis. Even in health, some podocytes are lost into the urine. Because podocytes themselves cannot regenerate, we postulated that glomerular parietal epithelial cells (PECs), which proliferate throughout life and adjoin podocytes, may migrate to the glomerular tuft and differentiate into podocytes. Here, we describe transitional cells at the glomerular vascular stalk that exhibit features of both PECs and podocytes. Metabolic labeling in juvenile rats suggested that PECs migrate to become podocytes. To prove this, we generated triple-transgenic mice that allowed specific and irreversible labeling of PECs upon administration of doxycycline. PECs were followed in juvenile mice beginning from either postnatal day 5 or after nephrogenesis had ceased at postnatal day 10. In both cases, the number of genetically labeled cells increased over time. All genetically labeled cells coexpressed podocyte marker proteins. In conclusion, we demonstrate for the first time recruitment of podocytes from PECs in juvenile mice. Unraveling the mechanisms of PEC recruitment onto the glomerular tuft may lead to novel therapeutic approaches to renal injury.Chronic kidney disease, resulting in renal failure and the need for lifelong renal replacement therapy, has become a significant problem worldwide. In the United States, approximately 7% of the total Medicare budget is spent on the treatment of ESRD, and projections suggest that the amount spent will increase by another 50% by 2020.¹Most renal pathologies that ultimately lead to ESRD originate within the glomerulus. It has now been established that a depletion of podocytes, the visceral epithelium of the capillary convolute (Figure 1), is central in this process. As soon as damage to the glomerular podocytes exceeds a certain threshold (approximately 30%), glomerulosclerosis ensues.² Indeed, in patients with a surgical reduction of ≥75% of renal mass, a relative lack of podocytes (podocytopenia) and subsequent FSGS in the originally healthy remnant kidney can lead to renal failure.³ Glomerulosclerosis is also the common final pathway of all glomerular diseases leading to ESRD.⁴ In glomerular diseases such as diabetic nephropathy, glomerulonephritides, or preeclampsia, significant numbers of podocytes are lost as a result of apoptosis, necrosis or excretion of living cells into the urine. Even in normal individuals, low numbers of living podocytes are continuously shed into the urine.^5–7 These numbers are too high to be compatible with renal survival for 80 yr, suggesting the existence of a regenerative mechanism. Also, the reversal of early glomerular damage in animal models and humans^8–10 argues for the existence of such a mechanism; however, podocytes are postmitotic cells that cannot undergo complete cell divisions and are therefore unable to regenerate themselves.^8–10 A potential mechanism for podocyte replacement from bone marrow–derived stem cells has been described in the Alport mouse model as well as in kidney transplants.^11–13 Nevertheless, most studies concluded that regeneration occurs predominantly from an as-yet-unknown source of resident renal cells.^12,14–16Open in a separate windowFigure 1.Renal glomerulus. The glomerular epithelium consists of PECs (red) and podocytes (Pod; blue), which reside on the capillary convolute. Both epithelia adjoin directly at the vascular pole (VP; arrow). At the tubular pole (TP), the parietal epithelium is connected to the epithelium of the proximal tubule. In male mice, this transition from PECs to proximal tubular cells often occurs within the glomerulus. The glomerular basement membrane (black) forms a continuous barrier between the glomerular epithelium and the endocapillary compartment that contains mesangial cells (shaded) and endothelial cells of the glomerular capillaries (*). Primary urine is filtered across the three-layered filtration barrier (endothelial cells, glomerular basement membrane, and Pod) into Bowman''s space (BS).In this study, we tested the hypothesis that glomerular parietal epithelial cells (PECs) lining the inner aspect of Bowman''s capsule migrate onto the glomerular tuft and differentiate into podocytes. Several arguments support this hypothesis. PECs are present in all species whose kidneys contain glomeruli. They are located within the same compartment and are in direct continuity with podocytes at the glomerular vascular stalk, so PECs do not have to cross an anatomic barrier such as the glomerular basement membrane, as was suggested for bone marrow–derived stem cells.^11–13 PECs proliferate lifelong at a relatively low frequency,¹⁷ express several stem cell marker proteins, and could be transdifferentiated in vitro into other cell types such as adipocytes or neuronal cells, suggesting that these cells retain multipotency.^9,18,19 In rodents, PECs do not express any known specific marker protein, which has so far precluded a detailed analysis of the function of these cells.In this work, we provide the first evidence that PECs possess the capability to migrate onto the glomerular tuft via the vascular stalk, where they differentiate into podocytes. This establishes that PECs represent an intrinsic cell population from which podocytes can be recruited.