A new era in regenerative treatment of liver diseases

Liver transplantation is one of the most common organ transplants. According to the 2024 National Transplantation Program in Croatia, there were 254 patients on the waiting list, but only 116 received a new liver. It is known that the liver has the ability to regenerate, but sometimes the damage is too severe, and a full organ transplant is needed. Due to a shortage of donors and many complications related to transplantation, more and more research is focused on new ways to treat and regenerate the liver. One of the most promising research areas is liver organoids—laboratory-grown 3D models that closely mimic liver function.

Current models for liver disease research

Research is conducted using mice or liver cells grown in vitro. However, only 35% of genes expressed in the human liver show similar expression in mice, and in vitro models cannot fully reproduce the structure and function of the liver in a living organism.

Structure of the liver

The main functional unit of the liver is the liver lobule, which has a hexagonal shape. In its center is the central vein, and at the corners are portal triads made up of a hepatic portal arteriole, a hepatic portal venule, and a bile duct. Hepatocytes (liver cells) spread out radially from the central vein and are arranged in plates with bile canaliculi running between them.

Hepatocytes are divided into three zones based on their location – the distance from the central vein or portal venule: zone 1 (periportal), zone 2 (middle), and zone 3 (pericentral). All liver functions – which are many – are distributed across these three zones. For example, gluconeogenesis and fat oxidation happen in zone 1, while fat synthesis occurs in zone 3.

Why are hepatocyte zones important?

Because of this zonal distribution, metabolic diseases usually appear in the zone where a disruption occurs. Although the zoning pattern is mostly similar in humans and rodents, there are enough genetic and molecular differences to make mice unreliable as in vivo models. To better understand how liver diseases develop and how to treat them, we need more precise human models that maintain liver zoning and complexity.

Is that possible?

Recently – yes!

Studies have shown that zoning is maintained thanks to the availability of nutrients and oxygen levels, which depend on the central and portal veins. However, the main transcription factors that control zoning are still unknown, which makes it harder to create accurate human liver models.

Considering the importance of ascorbate and bilirubin for the function of certain hepatocyte zones, it was assumed that these compounds might encourage the development of specific zones—which turned out to be true. Ascorbate increases the expression of genes specific to zone 1 and boosts functions like gluconeogenesis and fat oxidation. Bilirubin activates CYP enzymes, which are typical of zone 3.

Development of multi-zonal liver organoids (mZ-HLO)

By differentiating human induced pluripotent stem cells with the addition of ascorbate and bilirubin, researchers created periportal (Z1) and pericentral (Z3) hepatocytes. When combined, these Z1 and Z3 cells formed multi-zonal liver organoids that show realistic zoning and contain various types of cells. These organoids are the most realistic in vitro model of the human liver.

When transplanted into the livers of immunogenic mice with bile duct damage, mZ-HLO organoids kept their zone-specific functions and helped reduce the symptoms of liver failure.

What does this mean for us?

mZ–HLO is an advanced human in vitro liver model. It offers much better insight into the progression of liver diseases and allows testing of new treatments under realistic conditions. Although the technology is still developing, multi-zonal liver organoids could become a key part of regenerative treatment – without the need for a full liver transplant.

Translated by: Josip Kokanović