The general shortage of organs for organ transplant is of great concern in the medical community. Recent attempts to address shortages have focused on creating all or part of the needed organs to make them more readily available to patients in need of transplant. A team at Massachusetts General Hospital led by Harald Ott has successfully regenerated functional rat kidneys from decellularized extracellular matrices repopulated with native-like kidney cells.
The need for kidneys in patients with end-stage renal disease is a major example of the organ shortage problem. Approximately 100,000 people in the United States await kidney transplants, but only 18,000 or so will receive a transplant in a given year (1). Between five and 10 percent of those on the kidney transplant waiting list die before receiving a transplant, and around 40 percent of those who receive a transplant will die or require another transplant within 10 years (1).
Developments in bioengineering may help to provide better functioning and more readily available kidneys to those with renal failure. Previous successes in increasing renal function have involved renal-assist devices with various strategies of hemofiltration and hemodialysis. These have included portable or implantable devices that have improved the livesof patients unable to receive timely transplants (1).
Previous attempts to bioengineer human tissue have included the successful use of biocompatible matrices and cells from the patient to develop urinary tract tissue to improve bladder function. Such scaffolds allow more effective cultivation of biocompatible cells with appropriate structure and function (1).
Ott’s team is working to create autologous kidney tissue— tissue derived from the patient into whom it will be implanted. They have previously worked on creating extracellular matrix (ECM) scaffolds for hearts and lungs (1, 2).
In their study, published in Nature Medicine, the ECM of the rat kidney was used to create the scaffold for the bioengineered kidney tissue (1). ECM was obtained and made into a whole-organ scaffold by decellularization of kidney tissue with detergent (1). Decellularized tissue consists of a scaffold of collagen and other structural compounds without live cells (2). The scaffold was treated with endothelial and epithelial cells to achieve the cell populations found in native kidneys (1).
Laboratory testing, including immunohistochemical staining, which identifies specific cell types based on proteins characteristic to them, helped to verify that the ECMs contained all necessary components for organ function (1). To see if the ECM creation process might be applicable to other species, this portion of the process was repeated with tissue derived from the kidneys of pigs and humans. Similar scaffold tissue with functional properties was achievable in both species, as evidenced by observing the flow of dye through the ECMs (1).
The cell repopulation process was facilitated by putting the scaffold under a vacuum, as the small pressure gradient on the scaffold helped to improve cell delivery and retention (1). The cells used were derived from rat neonatal kidneys, and were cultured to produce greater amounts in whole-organ biomimetic culture (1). Maturation steroid signals were applied to speed the maturation of these neonatal cells to a functional level (1).
After repopulation and maturation, most cell types displayed the appropriate membrane proteins for their function (1). However, the bioengineered kidney contained only about 70 percent of the cellular glomeruli found in the native kidney (1). The cellular glomeruli are responsible for the filtration function of the kidney, so the bioengineered kidney would theoretically not restore full kidney function.
In in vitro tests, the regenerated kidney was able to produce a steady amount of urine (the output after filtration), although it did produce less than a cadaver control kidney (1). These tests did find that the regenerated kidney was less able to perform glomerular filtration, achieving about 10 percent of the cadaveric filtration, as measured by filtration of creatinine. Increased flow pressure improved this filtration ability (1). The regenerated kidney’s albumin retention and electrolyte and glucose reabsorption abilities were about half that of a control kidney. The researchers hypothesize that these functional indicators would improve if the cells were given additional time to mature (1).
When the regenerated kidneys were transplanted and assessed in vivo for adequate urine production, no serious complications were observed, and urine production was quickly established (1).
Bioengineering kidneys presents the possibility of creating viable transplant alternatives “on demand” for patients with renal failure (1). However, beyond the functional limitations of the regenerated kidneys, expected challenges include the even greater complexity of cell repopulation given a human-sized kidney scaffold and the greater extent of the biomimetic organ culturing required (1). It would also be potentially difficult to obtain, differentiate, and expand the cell types needed from “clinically feasible sources” (1).
Ott has suggested that it might be more advantageous to use decellularized pig kidneys repopulated with human cells, rather than trying to use human ECM scaffolds. The human cells could still be taken from the patient, so the effect would be similar to using the patient’s own native organ (2).
Still, the use of a regenerated kidney means that the ECM scaffold used would be shaped like a natural kidney, rather than artificially designed to look like one (2). This means the regenerated kidney would probably be more readily accepted by a transplant recipient than previously suggested artificial designs.
Regenerating kidneys is an alternative approach to previous strategies to bioengineer kidneys, including one involving a 3D printer proposed by Wake Forest researcher Anthony Atala in 2011 (3). Rather than artificially constructing the needed structure, Ott’s team’s approach works with structures that are already available.
Atala warns that it could be years before such technology is viable for regular human use. Curt Civin of the Stem Cell Biology and Regenerative Medicine Center at the University of Maryland Medical School suggests that in 10 to 20 years there might be human testing of such technology (4). Another researcher in regenerative medicine at the University of Pittsburgh, Stephen Badylak, is more optimistic, suggesting that a process like that used by the Ott team could be “a real option” within seven years (4). However, Badylak believes the lengthy treatment approval process will slow the regenerative organ transplants’ entry into regular practice (4).
Regardless of the timing, the development of organs that mimic the native tissue paints a promising picture of the future of kidney transplantation. Applications of the process could lead to the regeneration of other organs, potentially addressing both the organ shortages and immunorejection problems that plague the organ transplant landscape today.
- H. C. Ott, et al., Nat Med, (2013).
- H. Fountain, Rat Kidneys Made in Lab Point to Aid for Humans (2013). Available at http://www.nytimes.com/2013/04/15/science/rat-kidneys-made-in-lab-seen-as-step-to-human-transplants.html?_r=0 (20 April 2013).
- E. Brown, Bioengineered rat kidney could lead to treatments for people (2013). Available at http://www.latimes.com/news/science/sciencenow/la-sci-sn-bioengineered-kidney-20130414,0,4202332.story (20 April 2013).
- J. Berman, Bioengineered Kidney a Possible Solution to Donor-Organ Crisis (2013). available at http://www.voanews.com/content/bioengineered-kidney-a-possible-solution-to-donor-organ-crisis/1642933.html (20 April 2013).