As the human body’s largest solid organ, the liver produces many substances that the body requires every day, including most of the proteins needed and substances that regulate blood clotting. The liver also breaks down nutrients from food when needed and produces the bile that digests fat and absorbs vitamins. Liver failure can occur as a result of infection or complications from specific medications (acute liver failure) or it can be the result of a long term problem, such as alcoholism, hepatitis, or certain inherited diseases (chronic liver failure). In the U.S. alone, 11,000 patients were added to the liver donor list in 2013 and only 6,000 patients received the life-saving transplant. Simply put, the huge demand outweighs the number of donor livers available for transplantation.
Read MoreCellular scaffolding
One of the most promising potential solutions to the lack of livers for transplantation has been developed in the field of tissue engineering. Recent advances in whole-organ scaffolding have made the idea of creating a substitute organ feasible. Scaffolding in human tissues generally mimics the functions of the body’s “extracellular matrix” (ECM), which provides an “anchor” for the cells and a framework within which to grow cells. All cells require an ECM to carry out their proper functions – influencing their environment, reorganizing themselves, and retaining a recognizable order .
Many scientists are studying the applications of various substances and configurations wherein cells from another source are “seeded” into the scaffold, and are able to reproduce as they do in nature. This scaffold creates 3-dimensional tissues that can be used as a graft, or even – in theory – an entire organ.
The problem with these grafts is that developing substitutes for more vascular-intensive organs like the liver is very complex, and such a graft must provide for the delivery of oxygen and nutrients to large populations of cells. Although the liver has a well-developed ability to regenerate itself, its demand for blood seems to be a major reason that renders it as an unsuitable target for tissue engineering – much to the detriment of the tens of thousands of U.S. patients awaiting liver transplantation.
Liver engineering
MGH investigators are working to develop a new and more effective liver graft – from a structure that is already present in nature. If the liver cells (hepatocytes) could be cleared without damaging the extracellular and vascular structure of the organ, then the remaining framework could be used as a perfect cellular scaffold. The MGH team has made significant advances towards creating a new organ using these acellular scaffolds.
Human hepatocytes derived from human embryonic stem cells
The basic methodology behind this work is simple. Taking organs deemed unsuitable for transplantation, the MGH investigators remove cells by flowing a detergent (sodium dodecylsulfate, SDS) through the blood vessels resulting in the acellular organ scaffolds. Using this slow and gentle decellularization process, all of the cells can be removed, leaving behind only the extracellular skeleton of the native organ, a superstructure composed of proteins and sugars, and the blood vessels. Using this approach, the cells can be removed while preserving the architecture of the organ, including its vascular structure. Once this has been accomplished, healthy cells derived from pluripotent stem cells (master cells that are able to produce any cell or tissue that the body needs) can be reintroduced to the system. The presence of this vascular network designed by nature allows the researchers to easily provide the new cells with oxygen and nutrients as they grow into healthy tissues and, ultimately, a new and fully functional organ.
Moving the field forward
Network of blood vessels in the liver is preserved upon decellularization; venous (blue) and portal (red)
This de-cellularization/re-cellularization methodology published in 2010 has inspired numerous other investigators in the field. The most promising results of these new studies demonstrate that the reconstructed grafts remained functional when transplanted into animal models. In one model, after removing a kidney, the investigators transplanted the reconstructed liver heterotopically using the native renal blood supply and found that re-profusion resulted in only minimal damage to the reconstructed liver grafts. The study demonstrated, for the first time, that it was feasible to repopulate a decellularized liver with healthy cells, retain the cellular functions, and transplant the reconstructed liver graft into a living system with minimal cellular damage caused by ischemia.
The MGH investigators are working to resolve a number of critical issues with the liver grafts that, as yet, prevent them from working sustainably within the animal models. The 2010 study highlighted in particular the need to establish populations of other cells within the liver grafts. Although hepatocytes account for the majority of liver cells, nonparenchymal cells including liver sinusoidal endothelial cells, stellate cells, biliary epithelial cells and Kupffer cells also play a vital role in the varied functions performed by the liver. Establishing these cells in appropriate distribution within the grafts is not only important for the purpose of establishing full liver function, but also for ensuring complete vascular function.
The scientific progress in this area has taken giant steps forward, yet there remains challenges to overcome before clinical trials in patients can be pondered. Although the survival time of the recipient animals is still measured in days, it is promising that an avenue of investigation that had previously stalled – by the difficulties in providing adequate oxygen and nutriment to liver cells – has been revitalized.
Relevant publications
Uygun BE, Yarmush ML. Engineered liver for transplantation. Curr Opin Biotechnol. 2013 Oct;24(5):893-9. Review. PubMed PMID: 23791465; PubMed Central PMCID: PMC3783566
Uygun BE, Yarmush ML, Uygun K. Application of whole-organ tissue engineering in hepatology. Nat Rev Gastroenterol Hepatol. 2012 Dec;9(12):738-44. PubMed PMID: 22890112; PubMed Central PMCID: PMC3732057
Uygun BE, Soto-Gutierrez A, Yagi H, Izamis ML, Guzzardi MA, Shulman C, Milwid J, Kobayashi N, Tilles A, Berthiaume F, Hertl M, Nahmias Y, Yarmush ML, Uygun K. Organ reengineering through development of a transplantable recellularized liver graft using decellularized liver matrix. Nat Med. 2010 Jul;16(7):814-20. PubMed PMID: 20543851; PubMed Central PMCID: PMC2930603
Parashurama N, Nahmias Y, Cho CH, van Poll D, Tilles AW, Berthiaume F, Yarmush ML. Activin alters the kinetics of endoderm induction in embryonic stem cells cultured on collagen gels. Stem Cells. 2008 Feb;26(2):474-84. PubMed PMID: 18065398; PubMed Central PMCID: PMC2802581
Cho CH, Parashurama N, Park EY, Suganuma K, Nahmias Y, Park J, Tilles AW, Berthiaume F, Yarmush ML. Homogeneous differentiation of hepatocyte-like cells from embryonic stem cells: applications for the treatment of liver failure. FASEB J. 2008 Mar;22(3):898-909. Epub 2007 Oct 17. PubMed PMID: 17942827
Contact
Basak E. Uygun, PhD
[email protected]
617-371-4879
Martin L. Yarmush, MD, PhD
[email protected]
617-371-4882