Breakthrough method creates functional liver organoids

Replicating the liver's complexity

While organoids aim to mimic human organs, the liver's repertoire of complex functions – and thus the energy it needs to operate – have made it challenging for researchers to grow organoids that proliferate and fully function, says Sato. When prioritizing growth and survival in laboratory settings, hepatocytes, the liver's main cells, eventually transformed into cells resembling cholangiocytes, which line the bile duct. Hepatocyte functions only lasted 1-2 weeks at most. 

The study team, led by Ryo Igarashi and Mayumi Oda at the Keio University School of Medicine, generated hepatocyte organoids from cryopreserved human adult hepatocytes taken directly from patients. Treating these with oncostatin M, a signaling protein involved in inflammation, led to a million-fold proliferation of the organoids, in contrast to previous studies which saw little cell growth. The organoids continued growing for three months and survived half a year without losing the ability to differentiate. 

The researchers also developed a new method for inducing differentiation using hormones that regulate hepatocyte functions. After the cells differentiated through this method, the organoids began to show all major functions of the liver, producing compounds including glucose, urea, bile acid, cholesterol and triglycerol. The secretion of some compounds like albumin – a protein in the bloodstream for maintaining osmotic balance – exceeded previous studies, reaching levels comparable to hepatocytes in the human body. In addition, they formed networks of small canals that allowed bile acid to pass. 

Discovering the role of oncostatin M in organoid development was a key breakthrough, says Sato. "We only know a few molecules that unlock a stem cell's potential to grow into organoids and proliferate," he says. "This one is brand-new and opens up opportunities for developing new types of organoids that researchers have struggled to create."

Accelerating liver research

When the team injected the human hepatocyte organoids into mice with compromised immune systems and dysfunctional livers, the cells eventually replaced the mice's own liver cells and restored liver function.

This has long-term implications on liver regeneration. Though the liver is one of the most sought-after organs for transplants, they are limited in availability as they degrade quickly after harvesting and must be transplanted within a short time frame. While there are initiatives to extract and freeze hepatocytes for later transplantation, the method has had limited success. 

Sato believes that turning frozen cells into organoids may revitalize their proliferation ability, which would make them better material for regeneration therapies. "Our study demonstrated that liver organoid transplants can be successful in mice. But to regenerate a human liver, organoid growth has to scale up to thousands of millions, because the human body is larger," he says. "If realized, this approach could be a game-changer for patients awaiting transplantation."

In the more immediate future, the study helps streamline drug development for liver disease by cutting toxicity testing costs. Due to significant species differences in the liver, the standard pharmaceutical practice is to use human hepatocytes harvested directly from donors. Though they lose their functions within days, these hepatocytes cost 100-300 thousand Japanese yen (670-2000 USD) per vial. In addition, there is high variability in a batch's ability to survive and function. Meanwhile, hepatocyte organoids serve as more consistent research material. 

The organoids also offer better models for liver disease. In the study, they generated their own lipids, which disappeared after being treated with drugs for MASLD. The approach better resembles conditions like MASLD compared to studies that artificially inject lipids. Furthermore, the team succeeded in editing genes to replicate ornithine transcarbamylase deficiency, a genetic disorder disrupting the urea cycle. This achievement advances methods for modeling genetic liver diseases.

Sato says that work on proliferating organoids by a greater order of magnitude, as well as including other types of liver cells in the organoids, are crucial next steps for bolstering the effectiveness of organoids in medical research.