by George Taniwaki
There is an extreme shortage of kidneys available for transplantation with over 85,000 people on the UNOS transplant waiting list and an additional 300,000 on dialysis who are not on the waiting list but who could still benefit from improvements in renal replacement therapy. Although it is possible for a patient with end-stage renal disease (ESRD) to live several years on dialysis, it is not ideal.
A May 2010 blog post discussed ways to extend the shelf life of organs donated for transplant. Today’s blog post describes technologies in an exciting area of research called regenerative medicine that may provide significantly better outcomes than dialysis and alleviate the shortage in transplantable organs. Regenerative medicine consists of therapies that use live cells, mostly grown from stem cells, to replace a patient’s nonfunctional organ.
Preparing a scaffold for solid organs
Every organ in the body consists of three primary parts. First is a protein scaffold, a framework that defines the shape, mechanical properties, and organization of the cells in the organ. Second, is the network of blood vessels that feed the organ. Finally, there are the various cells within the organ that interact with the blood.
In solid organs, like the heart, the cells do not interact very much with the blood. Thus the requirements for an artificial heart are more clearly defined, and are more mechanical rather than biochemical. In a paper published in Nature Medicine Jan 2008 and summarized in Tech. Rev. Jan 2008, Doris Taylor, a researcher at the Stem Cell Institute at the Univ Minnesota, and her colleagues describe a process to create a scaffold for a heart. In experiments with rats, they start with a cadaver heart and decellularize it using detergents. Then they seeded the acellular matrix with either neonatal cardiac cells or rat aortic endothelial cells (the cells that line the blood vessels). Afterwards, the muscles in these bioengineered hearts would beat when stimulated.
Decellularizing a heart. Image from Nature Medicine
Other organs have been created using a similar process. Working with rat livers, several researchers at Massachusetts General Hospital published a paper in Nature Medicine Jun 2010 (subscription required). They started with a matrix created by removing the cells from an adult cadaver liver and then seeded it with fetal liver cells and endothelial cells. The resulting organ survived and functioned in culture for 10 days. A good description of the work is provided in Tech. Rev. Jun 2010 and includes a video.
Decellularizing a liver. Image from Tech. Rev.
In another experiment using rats, researchers created a lung by adding fetal lung cells and blood vessel cells to a matrix created from a decellularized cadaver lung. The work was conducted by Laura Niklason and other researchers at Yale. It was reported in the Science Jul 2010 and publicized in the Wall St. J. Jun 2010 (subscription required) and Tech. Rev. Jun 2010, which also has a video. Dr. Niklason has formed a company called Humacyte to commercialize human derived acellular matrices.
All of the artificial organs described above start with a scaffold made from an existing organ from a cadaver. There is also work underway to develop a man-made scaffold using polymers that mimic the behavior of natural proteins. One advance is reported in Nature Materials Nov 2008 (subscription required) for a honeycomb shaped scaffold that combines flexibility with strength. The polymer is made from poly(glycerol sebacate), a biodegradable elastomer. The work is described in Tech. Rev. Nov 2008.
Another scaffold material, this one made from the same fibronectin protein that serves as the framework for natural organs, is described in Nano Letters Jun 2010 (subscription required) and summarized in Tech. Rev. Aug 2010. The process, developed by Kevin Kit Parker of Harvard, starts by depositing fibronectin molecules on a chilled surface made of a hydrophobic polymer. This causes the protein to relax. Then the fibronectin is transferred to a sheet of glass coated with a water-soluble, hydrophilic polymer. Adding room temperature water causes to fibronectin to crosslink and also dissolves the hydrophilic polymer. This leaves the fabric which is ready to use.
Protein nanofabric. Image from Nano Letters
Organs without scaffolding
It may be possible to eliminate the need for an existing scaffolding by suspending cells in a hydrogel containing iron oxide particles and held in a magnetic field to create 3D shapes. The technique is described in Nature Nanotech. Apr 2010 and summarized in Tech. Rev. Mar 2010.
Finally, it may be possible to build up an organ without a scaffold by using a 3D printer. Tom Boland and other researchers at Clemson University reproduced a heart using an off-the-shelf ink jet printer filled with cells suspended in a hydrogel. Their results were reported at the Amer. Assoc. Advan. Sci.Conf. 2007.
An experiment involving mice shows the first steps in creating an artificial pancreas without the use of scaffolding. The work was done by a company called ViaCyte (formerly Novocell). First, stem cells are encapsulated in a membrane. The membrane is porous enough to allow blood and glucose to enter, but fine enough to prevent the cells from leaking into the body. The stem cells are induced to become insulin-producing pancreas cells. Finally, the encapsulated cells are implanted in the mouse. The work was publicized at the Int. Soc. Stem Cell Res. 2010 and reported in Tech. Rev. Jun 2010.
Creating an artificial kidney is much more difficult than forming other organs because the kidneys have a complex internal structure that includes items like tubules and glomeruli. However, it may not be necessary to reproduce these features to make a useful therapy. In addition to its well-known filtering functions, the kidneys are also part of the endocrine system. They produce and regulate the level of various hormones, the best known of which is erythropoietin (EPO), which stimulates the production of red blood cells.
Currently, all dialysis patients get injections of EPO as part of their renal replacement therapy, to avoid anemia. But there may be other hormones that they are missing. David Humes at the Univ. of Michigan has shown that an external device filled with kidney cells can be used to regulate the hormone levels of dialysis patients. The work is described in Tech. Rev. Nov 2006. A company named RenaMed Biologics was formed to commercialize the product. The company partnered with Genzyme to perform clinical trials of this renal assist device, but testing was suspended, MassHighTech Oct 2006.
James Yoo and other researchers and Wake Forest University report in Tissue Eng. Feb 2009 (subscription required) that they were able to generate three-dimensional renal structures resembling tubules and glomeruli in vitro using primary kidney cells. These structures produced a liquid that resembled urine. A company called Tengion has licensed the technology and is working on a neo-kidney augment product. However, it is not yet in clinical development and is not commercially available.
Optimistically, all of these techniques for regenerative medicine will come to market within ten years. Bioengineered organs have the potential to reduce the need for live donor organs, allow more deceased donor organs to be used rather than discarded, and shorten the waiting list for transplants. Further, assuming that the patient’s own stem cells are used to seed the acellular matrix, they will ensure HLA compatibility and eliminate the need for the patients to take immunosuppressant medications which should reduce the risk of side effects.