End-stage liver failure causes about 30,000 deaths in the U.S. annually.So far, the only definitive treatment is liver transplantation, but the lack of donor livers is a limiting factor.The second cause of liver failure results from progressive damage caused by alcoholic and nonalcoholic fatty liver disease(NAFLD).The number of people with this condition isgrowing rapidly in parallel with epidemics of obesity and diabetes. To understand how this disease progresses, researchers need models that mimic how the disease occurs in humans. But I think we may have solved that problem. Researchers in my lab have figured out how to grow a mini human liver.

I am a physician-scientist and my lab studies new approaches to understand and treat liver disease. I first became interested in the liver during medical school in Mexico when a member of my family died of end-stage liver failure.

After medical school, I earned a Ph.D. in liver tissue engineering and regeneration. I then trained in China and Sweden to learn how to develop cell-based artificial organs. Finally, I studied at Harvard Medical School, where I learned how to salvage organs that were not useful for transplantation and use them to manipulate liver tissue in the lab. After allthose years, I got a job at the University of Pittsburgh, a worldwide hotbed of liver research.

How to grow a liver in the laboratory

The liver is a particularly unusual organ in the human body because it's the only one that can regenerate itself. It also performs about 500 different functions, including processing chemicals or drugs, fat and all the nutrients you eat. And it manufactures many essential molecules.

For the first time, my colleagues and I genetically manipulated whole mini-human livers using induced pluripotent stem cells (iPS), a type of stem cell that can be generated from adult skin or blood cells.

You may wonder how you "grow" a human mini-organ from stem cells. So let me explain. First we collect adult skin cells from a healthy person and grow them in the lab. Then we genetically modify these cells to deactivate a gene called SIRT1, which in the normal liver is responsible for regulating fat metabolism and controlling inflammation in theliver.

The next step is a second genetic modification in which we add four specific genes that convert these adult skin cells into iPS cells that have the potential to differentiate into almost any cell type in the body.

Once these cells have reverted to stem cells, we multiply them in large flasks until we have millions. Then we expose them to different growth factors to trigger their transformation into liver cells in a Petri dish.

Finally, we took the manipulated liver cells and introduced them into a mouse liver in which all the mouse cells were removed, leaving only a structural scaffold made of a natural substance called collagen. This provides a framework in which human liver cells can grow and form a solid organ in a chamber made to support organ growth, knownHere, we add other human cells in the bioreactor to induce tissue and vessel formation in the mini-organ. This process takes about 28 days.

When we are done, we have a mini liver that measures between 5 and 7 centimeters in diameter. It is very exciting to see this form in real time. Left: The bioreactor. Right: A mouse liver skeleton that has been filled with human liver, vascular and inflammatory cells. (UPMC , CC BY-SA)

Mini livers are similar to normal livers

The valuable aspect of our lab-grown mini-livers is that they mimic many aspects of human NAFLD and its progression to a more serious condition known as non-alcoholic steatohepatitis, or NASH. This will allow us and other liver researchers to study the disease process and figure out how to intervene.

Because we genetically modified liver cells to decrease the activity of the SIRT1 gene - which normally regulates fat storage and metabolism - mini human livers began to mimic the metabolic dysfunction seen in the tissues of patients with fatty liver disease. These organs began to accumulate fat, turning yellow as fat levels rosein the cells. For me, watching that organ change was the most exciting part.

After four days of SIRT1 gene suppression in the mini-livers, we performed several tests to understand how fat is processed, how other fat processing genes behave, and what liver cells look like under the microscope.

We found that about 80% of the fat processing deficiencies seen in patients with fatty liver disease were present in our modified human mini fatty livers. I think this is exciting because it means we can create realistic livers similar to patients' livers, which we can use to test new therapies or find new markers ofdisease.

Using laboratory livers for drug testing

So what's the point of growing a mini-liver? My colleagues and I believe it will be a valuable tool for testing candidate drugs. In some cases, these mini-livers may be more accurate than mice in figuring out whether a drug will be effective in humans.

For example, several years ago in preclinical trials, animals with fatty liver disease were given the pharmacological agent resveratrol, a molecule found in grapes and red wine that enhances the functions of the SIRT1 gene. This successfully reduced fat accumulation in mice and suggested that it could do the same in humans with fatty livers.

However, human clinical trials were inconclusive. But when our human mini fatty livers were treated with resveratrol, we found it had no effect on fat accumulation. The conflicting results in humans and mice may be due to interspecies differences between the disease in people and the disease in actual animal models of non-fatty liver disease.This underscores the value of lab-grown organs created with human cells.

Ultimately, the ability to generate human diseased liver tissue using genetically modifiable iPS cells from different human populations is important. Humans are born with different genetic variations that can predispose to different diseases. Thus, generating different human mini-livers with different genetic variations is a powerful resource that allows, for the firstturn, explore the role of these genetic variations in the disease.

My group designed the present study to modify the expression of only one gene, simplifying this complex disease, to understand non-alcoholic fatty liver disease and its progression to NASH. In future experiments, I plan to control the function of many genes simultaneously.

Our mini livers are not perfect replicas of the human liver. We were not able to mimic all the important features of the NASH, and these mini livers have not fully matured when compared to an adult human liver. So, my colleagues and I will continue to explore how to modify these livers to make more accurate replicas of this amazing organ.

By Alejandro Soto-Gutiérrez, Professor of Pathology, University of Pittsburgh.

This article was originally published on The Conversation.

You can read the original article here.