Transplantation of metanephroi to sites within the abdominal cavity

Life-supporting functional kidney replacement by integration of embryonic metanephros-bladder composite tissue transplants

Novel transplantable organs need to be developed to address the global organ shortage. Transplantation of embryonic kidney tissue, or metanephros, facilitates glomerular and tubular maturation and offers partial organ functional support. However, adult environments do not permit exponential growth in size, limiting the life-supporting functionality and organ replacement effect of this approach. Here, we developed a novel strategy that combines the fusion of embryonic bladders with multiple anastomoses to the host ureter, enabling a significant increase in metanephros transplantation and urinary tract integration. By surgically anastomosing divided bladder segments, we reconstructed the excretory pathways by merging four metanephroi into each bladder and integrating them with the host ureter. Following the transplantation and integration of 20 metanephroi at the para-aortic region, anephric rats survived for over a month and generated approximately 50,000 nephrons in vivo. Ultrastructural and single-cell-transcriptomic analyses revealed that the maturity of the transplanted metanephroi was comparable to that of adult kidneys, although their small size likely contributed to their decreased urine concentration ability. Postoperative support helped normalize physiological homeostasis, including solute clearance, acid-base balance, electrolyte levels, and kidney hormone levels, within vital ranges. Our findings underscore the functional maturation capacity and dose-dependent therapeutic efficacy of embryonic kidney tissue, suggesting its potential as a transplantable organ system.

Current Status and Future of Artificial Kidney in Humans

The number of patients needing renal replacement therapy (RRT) is increasing rapidly with an increase in lifestyle diseases such as diabetes, hypertension, and metabolic syndrome. Kidney transplantation, whenever feasible, is the most preferred mode of RRT. However, there is a growing shortage of donor kidneys for transplantation. While dialysis is partially able to perform the filtration and excretion function of the kidneys, it is still not able to perform the other renal tubular and endocrine functions of a normal kidney and has quality-of-life issues with significant long-term morbidity. The need of the hour is to develop an ideal artificial kidney that would be wearable or implantable and would be able to perform the complete excretory, filtration, tubular, endocrine, and metabolic functions of the kidney while preserving the quality of life and minimizing complications. In this review, we discuss the characteristics of an ideal artificial kidney, the challenges of developing such a device, a brief description of the past and current work on this topic, and what the artificial kidney of the future should look like.

Timing urinary tract reconstruction in rats to avoid hydronephrosis and fibrosis in the transplanted fetal metanephros as assessed using imaging

Chronic kidney disease leads to high morbidity rates among humans. Kidney transplantation is often necessary for severe symptoms; however, options for new curative treatments are desired because of donor shortage. For example, it has been established that the kidneys can efficiently generate urine after transplantation of the metanephros, ureter, and bladder as a group. After transplantation, the urine can indirectly flow into the recipient's bladder using a stepwise peristaltic ureter system method where the anastomosis is created via the recipient's ureter for urinary tract reconstruction. However, the growth of the regenerated metanephros varies significantly, whereas the time window for successful completion of the stepwise peristaltic ureter system that does not cause hydronephrosis of the metanephros with bladder (ureter) is quite narrow. Therefore, this study was conducted to periodically and noninvasively evaluate the growth of the transplanted metanephros, ureter, and bladder in rats through computed tomography and ultrasonography. The ultrasonographic findings highly correlated to the computed tomography findings and clearly showed the metanephros and bladder. We found that the degree of growth of the metanephros and the bladder after transplantation differed in each case. Most of the rats were ready for urinary tract reconstruction within 21 days after transplantation. Optimizing the urinary tract reconstruction using ultrasonography allowed for interventions to reduce long-term tubular dilation of the metanephros due to inhibited overdilation of the fetal bladder, thereby decreasing the fibrosis caused possibly by transforming growth factor-β1. These results may be significantly related to the long-term maturation of the fetal metanephros and can provide new insights into the physiology of transplant regeneration of the metanephros in higher animals. Thus, this study contributes to the evidence base for the possibility of kidney regeneration in human clinical trials.

Stem cell-based treatment of kidney diseases

IMPACT STATEMENT

Stem cells hold great promise in regenerative medicine. Pluripotent stem cells have been differentiated into kidney organoids to understand human kidney development and to dissect renal disease mechanisms. Meanwhile, recent studies have explored the treatment of kidney diseases using a variety of cells, including mesenchymal stem cells and renal derivatives. This mini-review discusses the diverse mechanisms underlying current renal disease treatment via stem cell therapy. We postulate that clinical applications of stem cell therapy for kidney diseases can be readily achieved in the near future.

From organoids to transplantable artificial kidneys

It is difficult to restore kidney function following chronic kidney damage. Although dialysis is currently used to treat patients with chronic kidney disease, it does not cure the disease, while severely restricting the patient's daily and social activities. Kidney transplantation is an alternative and curative therapy, but donor numbers remain limited. However, the generation of kidney organoids from human induced pluripotent stem cells represents an important recent advance in regenerative medicine. Kidney organoids are expected to be used for disease modeling and drug discovery, and may eventually be applicable for transplantation. In this review, we describe the current status of kidney organoids and discuss the hurdles that need to be overcome to generate transplantable artificial kidneys.

Current strategies and challenges in engineering a bioartificial kidney

Renal replacement therapy was an early pioneer in both extra-corporeal organ replacement and whole organ transplantation. Today, the success of this pioneering work is directly demonstrated in the millions of patients worldwide successfully treated with dialysis and kidney transplantation. However, there remain significant shortcomings to current treatment modalities that limit clinical outcomes and quality of life. To address these problems, researchers have turned to using cell-based therapies for the development of a bioartificial kidney. These approaches aim to recapitulate the numerous functions of the healthy kidney including solute clearance, fluid homeostasis and metabolic and endocrine functions. This review will examine the state-of-the-art in kidney bioengineering by evaluating the various techniques currently being utilized to create a bioartificial kidney. These promising new technologies, however, still need to address key issues that may limit the widespread adoption of cell therapy including cell sourcing, organ scaffolding, and immune response. Additionally, while these new methods have shown success in animal models, it remains to be seen whether these techniques can be successfully adapted for clinical treatment in humans.

Organogenesis of kidney and endocrine pancreas: the window opens

Growing new organs in situ by implanting developing animal organ primordia (organogenesis) represents a novel solution to the problem of limited supply for human donor organs that offers advantages relative to transplanting embryonic stem (ES) cells or xenotransplantation of developed organs. Successful transplantation of organ primordia depends on obtaining them at defined windows during embryonic development within which the risk of teratogenicity is eliminated, growth potential is maximized, and immunogenicity is reduced. We and others have shown that renal primordia transplanted into the mesentery undergo differentiation and growth, become vascularized by blood vessels of host origin, exhibit excretory function and support life in otherwise anephric hosts. Renal primordia can be transplanted across isogeneic, allogeneic or xenogeneic barriers. Pancreatic primordia can be transplanted across the same barriers undergo growth, and differentiation of endocrine components only and secrete insulin in a physiological manner following mesenteric placement. Insulin-secreting cells originating from embryonic day (E) 28 (E28) pig pancreatic primordia transplanted into the mesentery of streptozotocin-diabetic (type 1) Lewis rats or ZDF diabetic (type 2) rats or STZ-diabetic rhesus macaques engraft without the need for host immune-suppression. Our findings in diabetic macaques represent the first steps in the opening of a window for a novel treatment of diabetes in humans.

The effect of metanephros transplantation on blood pressure in anephric rats with induced acute hypotension

BACKGROUND

The kidney is an important organ for maintaining blood pressure. We have previously reported that transplanted metanephroi can reproduce some kidney functions. The aim of the present study was to determine the metabolic function of transplanted metanephroi with particular reference to maintaining blood pressure.

METHODS

Male Wistar rats were transplanted with metanephroi (transplanted group, n = 28), following unilateral nephrectomy. For comparison, we performed unilateral nephrectomy without transplantation in 32 rats (non-transplanted group, n = 18; haeminephrectomy control group, n = 14). The remaining kidney was removed 2 weeks after the initial operation, while control rats had a sham operation. Hypotension was induced by intravenous infusion of diltiazem hydrochloride or rapid withdrawal of blood. Mean arterial blood pressure (MAP) was invasively monitored and plasma renin activity (PRA) was analysed at multiple time points. Renin expression by metanephroi was evaluated by real-time polymerase chain reaction and immunohistochemistry.

RESULTS

Metanephroi in the transplanted group expressed renin messenger RNA. Metanephros transplantation significantly raised PRA and maintained MAP compared with the non-transplanted group. No significant differences between the transplanted and control groups were found with respect to PRA or MAP. PRA was positively correlated with metanephroi weight as well as MAP in the transplanted group.

CONCLUSION

The present study shows that transplantation of metanephroi produces PRA and contributes to raising MAP in a rat model of acute hypotension.

Metanephros transplantation inhibits the progression of vascular calcification in rats with adenine-induced renal failure

BACKGROUND/AIM

Recent research has shown that transplanted metanephroi form primitive vascularized kidneys with histologically recognizable renal features. The aim of the present study was to determine the metabolic function of transplanted metanephroi in rats with chronic renal failure (CRF), with particular reference to secondary hyperparathyroidism and vascular calcification.

METHODS

CRF was induced in 11-week-old male Wistar rats by maintaining them on a 0.75% adenine diet for 4 weeks, followed by normal diet for an additional 2 weeks. At the end of adenine loading, whole metanephroi from embryonic day 15 rats were transplanted into the omentum and epididymis of the transplantation group. Vascular calcification was evaluated 2 weeks after metanephroi transplantation.

RESULTS

Metanephros transplantation significantly reduced vascular calcium and phosphorus content and suppressed the progression of vascular calcification as indicated by von Kossa staining of the media of the thoracic aorta. However, no significant differences between the adenine-fed control and transplantation groups were found regarding the serum levels of 1,25(OH)2D3, calcium or phosphorus or the calcium × phosphorus product.

CONCLUSION

The present study has shown that transplantation of metanephroi suppresses the progression of vascular calcification via a mechanism that is independent of calcium-phosphorus dynamics.

Immunosuppression is essential for successful allogeneic transplantation of the metanephros

BACKGROUND

Transplanted metanephroi vascularize and develop features of mature kidney. One group reported the intriguing finding that metanephric allografts and congenic, major histocompatibility complex-mismatched grafts developed without rejection in the absence of immunosuppression. Our experiments aim to investigate the hypothesis that metanephroi lack immunogenicity and identify immunosuppressives that do not inhibit development.

METHODS

We transplanted syngeneic metanephric grafts, allografts, and class II mismatched transplants to adult rats along with control grafts to nude recipients. glomerular filtration rates (GFRs) were measured where possible and transplants assessed by histology, immunohistochemistry, electron microscopy, and polymerase chain reaction.

RESULTS

Allografts underwent reliable growth and vascularization followed by vigorous rejection (n>200). Rejection was conserved across a class II-mismatched strain and when the earliest dissectable metanephric structures were transplanted. Immunosuppressive drugs other than cyclosporine demonstrated no in vivo toxicity to transplants and treatment with FTY720 and tacrolimus could ablate histologic evidence of allograft rejection. Syngeneic transplants exhibited function of up to 8% of a normal GFR. Renal mass reduction and growth factor treatment was associated with higher GFR than controls. The anatomical site of implantation was also linked strongly with achieved function.

CONCLUSIONS

Fetal kidney rudiments can provide a source of functioning renal tissue. These results suggest that such structures are no less immunogenic than mature organs, but the observed rejection is controllable.

Transplantation of renal primordia: renal organogenesis

Dialysis and allotransplantation of human kidneys represent effective therapies to replace kidney function, but the former replaces only a small component of renal function, and the latter is limited by lack of organ availability. Xenotransplantation of whole kidneys from nonprimate donors is complicated by humoral and severe cellular rejection. The use of individual cells or groups of cells to repair damaged tissue (cellular therapies) offers an alternative for renal tissue replacement. However, recapitulation of complex functions such glomerular filtration and reabsorption and secretion of solutes that are dependent on a three-dimensionally integrated kidney structure are beyond the scope of most cellular replacement therapies. The use of nonvascularized embryonic renal primordia for transplantation circumvents humoral rejection of xenogeneic tissue and ameliorates cellular rejection. Renal primordia are preprogrammed to attract a vasculature and differentiate into a kidney and in this manner undergo organogenesis after transplantation into the mesentery of hosts. Here we review a decade's progress in renal organogenesis.

Increasing renal mass improves survival in anephric rats following metanephros transplantation

Renal failure and end-stage renal disease are prevalent diseases associated with high levels of morbidity and mortality, the preferred treatment for which is kidney transplantation. However, the gulf between supply and demand for kidneys remains high and is growing every year. A potential alternative to the transplantation of mature adult kidneys is the transplantation of the developing renal primordium, the metanephros. It has been shown previously, in rodent models, that transplantation of a metanephros can provide renal function capable of prolonging survival in anephric animals. The aim of the present study was to determine whether increasing the mass of transplanted tissue can prolong survival further. Embryonic day 15 rat metanephroi were transplanted into the peritoneum of anaesthetized adult rat recipients. Twenty-one days later, the transplanted metanephroi were anastomosed to the recipient's urinary system, and 35 days following anastomosis the animal's native renal mass was removed. Survival times and composition of the excreted fluid were determined. Rats with single metanephros transplants survived 29 h longer than anephric controls (P < 0.001); animals with two metanephroi survived 44 h longer (P < 0.001). A dilute urine was formed, with low concentrations of sodium, potassium and urea; potassium and urea concentrations were elevated in terminal serum samples, but sodium concentration and osmolality were comparable to control values. These data show that survival time is proportional to the mass of functional renal tissue. While transplanted metanephroi cannot currently provide life-sustaining renal function, this approach may have therapeutic benefit in the future.

Cellular therapies for kidney failure

Dialysis and transplantation of human kidneys represent effective therapies to replace kidney function, but each has limitations. Xenotransplantation of whole kidneys from non-primate donors is complicated by humoral and severe cellular rejection. The use of individual cells or groups of cells to regenerate or repair damaged tissue (cellular therapies) offers an alternative for renal replacement. Cellular strategies include: incorporation of new nephrons into the kidney; growing new kidneys in situ/renal organogenesis; use of embryonic or adult stem cells; and nuclear transplantation/therapeutic cloning. These approaches circumvent humoral rejection of xenogeneic tissue. Cellular rejection is ameliorated if embryonic cells are transplanted. It is likely that replacement of renal function via one or more cellular approach will constitute a part of future mainstream medical practice.