Over the last decade, the notion that stem cell biology can be harnessed to generate transplantable human kidneys has received much attention, even if progress has been slow.1 In the last 2 years, however, 2 independent approaches toward generating transplantable kidneys have demonstrated major progress. In 1 strategy, pluripotent human cells have been shown to possess the capacity to differentiate into nephron-like structures in a dish—hundreds of them, in fact.2 These in vitro kidney organoids self-assemble once the proper ratio of progenitor cell populations is generated, and evidence suggests that they can respond to injurious stimuli much as mammalian nephrons do.
In another approach, stem cells or metanephroi are transplanted into a donor using the organogenic niche method. The metanephroi are placed in the omentum, for example, where they mature and grow to a size suitable for transplantation.3,4 Kidneys generated in this fashion also display mature kidney cell types, chimeric vascularization and production of urine. These remarkable advances have made the once fanciful possibility that transplantable kidneys might be grown in a dish a very real possibility.
An important limitation to either approach has been the challenge of draining urine formed by the transplanted kidney, essentially a plumbing problem. Although progenitor populations from the ureteric bud and metanephric mesenchyme self-assemble into nephrons in vivo, there has been no robust method to connect the collecting system to a somewhat physiological outflow. Thus, once neokidneys are transplanted into a host in preclinical studies, they ultimately develop hydronephrosis due to a lack of a collecting system and subsequently fail.5
In a recent publication in the Proceedings of the National Academy of Sciences, Yokote and colleagues from the Yokoo group6 provide a new and exciting solution to this plumbing problem. Termed the stepwise peristaltic ureter (SWPU) system, the approach involves transplantation of not only primordial metanphroi, which develop into kidneys, but also the cloaca which develops into ureter and bladder. To prevent rejection and establish a kidney size closer to human needs, the investigators used both rats but also cloned pigs as a source of metanephroi and cloacas. Metanephroi and cloaca transplanted into the pig parasplenic artery region developed into filtering kidneys including a bladder by weeks. The distended bladder had then been connected to the contralateral ureter, preventing obstruction-induced kidney fibrosis that had limited previous attempts to grow kidneys in an organogenic niche (Figure 1).
By 8 weeks, urine from the cloaca-derived bladder was discharged into the recipient bladder demonstrated by intravenous urography. This novel approach prevented tubular dilation and interstitial fibrosis seen with cloacas that were transplanted but not later connected to the host urinary system. Further evidence of true filtration was provided by the observation that urea nitrogen and creatinine levels were well above systemic levels, demonstrating that the neo-kidneys were functionally filtering. When the SWPU approach had been performed in anephric rats, survival was longer although not by much less than 24 hours—compared to anephric rats without the SWPU.
This important proof-of-principle study provides one solution for stem cell-derived kidneys and will have a positive impact in the kidney regenerative medicine field. The application of the approach to cloned pigs represents another significant advance. This development brings the SWPU system one step closer to being clinically useful, as the growth of large human derived organs may be more feasible in larger model organisms.
In the last 2 years, however, 2 independent approaches toward generating transplantable kidneys have demonstrated major progress.
Although this report proposes a solution for urine excretion from neokidneys, it does not solve the issue of vascularization. Previous studies reported that the vascularization system of differentiated kidneys contained a chimeric mix of recipient and donor cells.5,7 The study by Yokoo et al used cloned animals, thus bypassing the issue of chimerism. As a next step, the authors suggested repeating this study in humans, using the blastocyst complementation system in pig embryos, thus avoiding the clinical limitations of their current approach. That way, transplanted metanephroi/cloacas may develop in the human host integrating into human vasculature. Even though blastocyst complementation is an exciting area of regenerative medicine, potentially yielding many organs that are transplant-ready in the future, the use of human derived cells raises many ethical questions. Little research has been done in larger model organisms using the blastocyst complementation method, especially using human induced pluripotent stem cells (iPSCs). Potential chimeric tissue from these methods may have limitation in function or be rejected by the transplant recipient.6,8 The authors have proposed creating a Pax2−/− and Pax8−/− pig model that would lack both metanphroi and cloacas, so that all components of the renal system were derived from the human iPSC. Clearly, more work needs to be done to confirm that no chimeric tissue exists before human transplantation.
Finally, the SWPU system had been analyzed over a brief period (8 weeks). Any kidney transplanted clinically needs to be viable for many years, thus durable renal function needs to be established. Moreover, for transplantation involving kidneys derived from, stem cell-derived sources, potential tumorigenicity must also be addressed when using iPSC or embryonic stem cells.
Overall, recent years have seen a dramatic progress of stem cell approaches in providing renewable sources of transplantable kidneys. The stepwise peristaltic ureter demonstration brings us closer to that important goal and represents an important milestone in our efforts to rebuild the renal system.
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