Transplantation of developing kidneys

Assembling Kidney Tissues from Cells: The Long Road from Organoids to Organs

The field of regenerative medicine has witnessed significant advances that can pave the way to creating de novo organs. Organoids of brain, heart, intestine, liver, lung and also kidney have been developed by directed differentiation of pluripotent stem cells. While the success in producing tissue-specific units and organoids has been remarkable, the maintenance of an aggregation of such units in vitro is still a major challenge. While cell cultures are maintained by diffusion of oxygen and nutrients, three- dimensional in vitro organoids are generally limited in lifespan, size, and maturation due to the lack of a vascular system. Several groups have attempted to improve vascularization of organoids. Upon transplantation into a host, ramification of blood supply of host origin was observed within these organoids. Moreover, sustained circulation allows cells of an in vitro established renal organoid to mature and gain functionality in terms of absorption, secretion and filtration. Thus, the coordination of tissue differentiation and vascularization within developing organoids is an impending necessity to ensure survival, maturation, and functionality in vitro and tissue integration in vivo. In this review, we inquire how the foundation of circulation is laid down during the course of organogenesis, with special focus on the kidney. We will discuss whether nature offers a clue to assist the generation of a nephro-vascular unit that can attain functionality even prior to receiving external blood supply from a host. We revisit the steps that have been taken to induce nephrons and provide vascularity in lab grown tissues. We also discuss the possibilities offered by advancements in the field of vascular biology and developmental nephrology in order to achieve the long-term goal of producing transplantable kidneys in vitro.

Pancreas and kidney transplantation using embryonic donor organs

One novel solution to the shortage of human organs available for transplantation envisions 'growing' new organs in situ. This can be accomplished by transplantation of developing organ anlagen/primordia. We and others have shown that renal anlagen (metanephroi) transplanted into animal hosts undergo differentiation and growth, become vascularized by blood vessels of host origin and exhibit excretory function. Metanephroi can be stored for up to 3 days in vitro prior to transplantation with no impairment in growth or function post-implantation. Metanephroi can be transplanted across both concordant (rat to mouse) and highly disparate (pig to rodent) xenogeneic barriers. Similarly, pancreatic anlagen can be transplanted across concordant and highly disparate barriers, and undergo growth, differentiation and secrete insulin in a physiological manner following intra-peritoneal placement. Implantation of the embryonic pancreas, is followed by selective differentiation of islet components. Here we review studies exploring the potential therapeutic applicability for organogenesis of the kidney or endocrine pancreas.

Prolongation of life in anephric rats following de novo renal organogenesis

One solution to the shortage of human organs available for transplantation envisions growing new organs in situ. This can be accomplished by transplantation of developing organ anlagen/primordia. Allotransplantation of embryonic day 15 metanephroi into the omentum of adult hosts is followed by differentiation, growth, vascularization and function of the implants. Here we show that survival of rats with all native renal mass removed can be increased by prior metanephros transplantation and ureteroureterostomy. Excretion of urine formed by metanephroi is prerequisite for enhanced survival. This is the first demonstration that life can be extended following de novo renal organogenesis.

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.

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.

Improvement of kidney failure with fetal kidney precursor cell transplantation

BACKGROUND

Current therapies for end-stage renal disease have severe limitations. Dialysis is only a temporary treatment and does not restore kidney function. Transplantation is limited by donor organ shortage and immune-related problems. Here, we show that the transplantation of fetal kidney precursor cells reconstitutes kidney tissues, reduces uremic symptoms, and provides life-saving metabolic support in kidney failure animal models.

METHODS

Kidney failure was surgically induced by resecting kidneys, leaving approximately 1/6 of the total kidney mass (5/6 nephrectomy). Fetal kidney precursor cells were isolated from metanephroi of E17.5 rat fetuses using collagenase/dispase digestion. Five weeks after the nephrectomy procedure, isolated fetal kidney precursor cells were transplanted under the kidney capsule of rats using fibrin gel matrix. Six and ten weeks after transplantation, animals were analyzed biochemically and the grafts were retrieved for histological analyses.

RESULTS

Five weeks after the nephrectomy, glomerular hypertrophy, and increased blood urea nitrogen and serum creatinine levels were observed. The cell transplantation into the kidneys of kidney failure-induced rats resulted in kidney tissue reconstitution and the transplanted cells were observed in the reconstitution region of the kidneys as evidenced by the presence of fluorescently labeled cells. In addition, biochemical parameters from serum and urine samples showed improved kidney functions compared with non-treated group without severe immune response after ten weeks.

CONCLUSION

Transplanting fetal kidney precursor cells showed the potential for the partial augmentation of kidney structure and function in the treatment of kidney failure.

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.

Windows of opportunity for organogenesis

Growing new organs in situ by implanting developing animal organ anlagen/primordia 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. We and others have shown that renal anlagen transplanted into animal hosts undergo differentiation and growth, become vascularized by blood vessels of host origin, exhibit excretory function and support life in otherwise anephric hosts. Renal anlagen can be transplanted across both concordant (rat to mouse) and highly disparate (pig to rodent) xenogeneic barriers. Similarly, pancreatic anlagen can be transplanted across concordant and highly disparate barriers, and undergo growth, differentiation and secrete insulin in a physiological manner following intra-peritoneal placement. 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. Here we review studies that delineate such developmental windows of opportunity for kidney and pancreas.

Cellular transplantation of nephrons

Cellular transplantation of nephrons. Embryonic renal cellular primordia transplanted into animal hosts undergo nephrogenesis in situ, 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, and both concordant (rat to mouse) and highly disparate (pig to rodent) xenogeneic barriers. Here I review studies exploring the therapeutic potential for renal organogenesis posttransplantation of cellular kidney primordia.

Organogenesis of kidneys following transplantation of renal progenitor cells

One novel solution to the shortage of human organs available for transplantation envisions 'growing' new organs in situ via xenotransplantation of developing anlagen from animal embryos. We and others have shown that renal progenitor cells (metanephroi) transplanted into animal hosts undergo organogenesis (differentiation and growth), become vascularized by blood vessels of host origin and exhibit excretory function. Metanephroi can be stored for up to 3 days in vitro prior to transplantation with no impairment in growth or function post-implantation. Metanephroi can be transplanted across both concordant (rat to mouse) and highly disparate (pig to rodent) xenogeneic barriers. Here we review studies exploring the potential therapeutic use of embryonic kidney transplantation as a means to achieve renal organogenesis.

Transplantation of embryonic organs - kidney and pancreas

One novel solution to the shortage of human organs available for transplantation envisions 'growing' new organs in situ via xenotransplantation of developing anlagen from animal embryos. We and others have shown that renal anlagen (metanephroi) transplanted into animal hosts undergo differentiation and growth, become vascularized by blood vessels of host origin and exhibit excretory function. Metanephroi can be stored for up to 3 days in vitro prior to transplantation with no impairment in growth or function post-implantation. Metanephroi can be transplanted across both concordant (rat to mouse) and highly disparate (pig to rodent) xenogeneic barriers. Similarly, pancreatic anlagen undergo growth, differentiation and secrete insulin in a physiological manner following intraperitoneal placement. Implantation of the embryonic pancreas, is followed by selective differentiation of islet as compared to acinar components. Here we review studies exploring the potential therapeutic use of embryonic kidney or pancreas transplantation.

Tissue engineering the kidney

The means by which kidney function can be replaced in humans include dialysis and renal allotransplantation. Dialytic therapies are lifesaving, but often poorly tolerated. Transplantation of human kidneys is limited by the availability of donor organs. During the past decades, a number of different approaches have been applied toward tissue engineering the kidney as a means to replace renal function. The goals of one or another of them included the recapitulation of renal filtration, reabsorptive and secretory functions, and replacement of endocrine/metabolic activities. This review will delineate the progress to date recorded for five approaches: (1) integration of new nephrons into the kidney; (2) growing new kidneys in situ; (3) use of stem cells; (4) generation of histocompatible tissues using nuclear transplantation; and (5) bioengineering of an artificial kidney. All five approaches utilize cellular therapy. The first four employ transplantation as well, and the fifth uses dialysis.

Transplantation of pig metanephroi

To determine whether pig metanephroi grow and differentiate after allotransplantation or xenotransplantation across a highly disparate barrier, we implanted metanephroi from embryonic day 28 (E28) pig embryos into the omentum of unilaterally nephrectomized adult pigs or C57Bl/6J mice (hosts). Some mouse hosts received anti-CD45RB, anti-CD154, and anti-CD11a (costimulatory blockade). E28 pig metanephroi were < 0.2 mm in diameter and contained only metanephric blastema and segments of ureteric bud. Pig metanephroi transplanted into pigs underwent growth and differentiation of nephrons over a 2 week period without the need for costimulatory blockade of hosts. In contrast, pig metanephroi did not grow or differentiate in mice that received no costimulatory blockade. However, by 2 weeks posttransplantation in mice in which costimulation was blocked, metanephroi from E28 pigs had enlarged, become vascularized, and had formed mature tubules and glomeruli. By 3 weeks posttransplantation in mice, metanephroi had grown to the point that they were approximately half the volume of the native mouse kidney. Here we show that growth and development of pig metanephroi occurs posttransplantation across an allogeneic or highly disparate xenogeneic barrier.

Xenotransplantation of developing kidneys

The number of kidney transplants performed per year is limited by the availability of donor organs. One novel solution to this shortage envisions "growing" new kidneys in situ via xenotransplantation of renal anlagen. We have shown that developing metanephroi transplanted into the omentum of animal hosts undergo differentiation and growth, become vascularized by blood vessels of host origin, and exhibit excretory function. Metanephroi can be stored for up to 3 days in vitro before transplantation with no impairment in growth or function postimplantation. Metanephroi can be transplanted across both concordant (rat --> mouse) and discordant/highly disparate (pig --> rodent) xenogeneic barriers. This review summarizes experimental data relating to the transplantation of developing kidneys.