When the liver is hurting, it tries to heal. By doing so, scars form within the tissue. In the disease called Alagille syndrome, liver scars are rather atypical. Recent research helps us to understand why and what we can learn from it.
When healing goes wrong
An injury in the liver can cause bile to leak into the surrounding tissue. This poses a problem for cells, as bile is highly toxic. Its main function is to help breakdown fats from the food in the small intestine. However, fat is also an essential component of cell membranes, and digesting these can be deadly to cells. If the bile is released too early or at the wrong spot, it leaves behind a graveyard of dead cells and holes in the tissue.
This alarming situation triggers immune cells in the body to travel to the affected site before it’s too late (inflammation). Immune cells work to restore balance by clearing away dead cells, fighting off intruders, and initiating tissue repair.
Inflammation activates cells that produce fibres such as collagen and fibronectin to build scaffolds around the damaged part (fibrosis), where the repair begins. Different tissues recruit specific cell types for this process, for example fibroblasts in the skin and hepatic stellate cells in the liver.
While inflammation and fibrosis are natural steps in the healing process, they can cause more harm than good if they spiral out of control. For instance, excessive fibrosis over a prolonged period makes the tissue stiff, impairing its ability to function and repair itself. Moreover, liver fibrosis compresses blood vessels and reduces their blood flow, which can subsequently impact other organs, as well.
Peculiarities of Alagille syndrome livers
Deposition of scaffold fibres (also called extracellular matrix) can create different patterns in the tissue depending on the disease. Alagille syndrome is a genetic condition and an example of cholestatic liver disease where there is a problem with the bile flow (cholestasis). Compared to other cholestatic liver diseases, in the livers that are affected by Alagille syndrome, the fibrosis pattern is rather atypical, reminding of a “chicken-wire” pattern (Figure 1), which is milder than other types of fibrotic patterns. This prompted a team of researchers led by Jan Mašek and Emma R. Andersson to ask how immune system and fibrotic response are regulated in this disease (1).
Figure 1: Sirius red staining reveals collagen fibers of "chicken-wire" fibrosis in the liver of an Alagille syndrome mouse model. Image by Noémi Van Hul.
Alagille syndrome is most often driven by a mutation in a single, yet crucial, gene JAG1. This affects communication between cells through the so-called Notch signalling pathway. This signalling is important to instruct cells on what fate (“profession”) they should take. Without these instructions, cells are confused and don’t do what they are supposed to.
What the cells have to say
Because the immune system and liver comprise many different cell types, researchers leveraged the power of high-throughput methods, where hundreds of single cells can be analysed in parallel. Using RNA sequencing, we can peak in into cells’ decision-making process by analysing which genes are being expressed at different timepoints.
Using these methods, researchers found that a key cell type, which constitutes most of the liver mass, struggles to decide its course of action—hepatocytes appear hesitant to mature and fulfil their responsibilities, one of which is to call immune cells to the liver when trouble arises.
The Notch signalling also regulates cell fate decisions in immune cells. The researchers looked into thymus and spleen, organs where immune cells are made, and found that the proportion of different cell types changes in the condition of Alagille syndrome, resulting for example in more T-regulatory cells (Tregs, Figure 2). These cells play a protective role by counteracting fibrosis in cholestatic liver injury. Consequently, the elevated levels of Tregs may help explain the unusual fibrosis patterns observed in Alagille syndrome.
Figure 2: Immunofluoresce image of an injured liver, to which lymphocytes taken from Alagille syndrom mouse were introduced to test their ability to react to injury. The white color shows E-cadherin-positive hepatocytes and bile ducts, while the red signal indicates the presence of Foxp3-positive regulatory T cells (Tregs). Image by Daniel Oliveira and Anna Maria Frontino.
In sum, the study revealed that the Notch signalling pathway, through the gene which is mutated in Alagille syndrome, regulate the injury response in hepatocytes and the immune system development, both of which contribute to the fibrotic process.
What it means for further research
Around a quarter of individuals with Alagille syndrome experience frequent (re)infections (2), suggesting their immune system may not work optimally. To address this, we need more systematic data describing the immune system in a larger cohort of people affected by cholestatic liver diseases. Additionally, since Notch signalling modulates fibrosis through multiple other cell types in the liver, it is crucial to study the interactions between different compartments, such as hepatocytes and immune cells.
Original publication:
References:
Terms explained:
Inflammation – the body’s response to injury that promotes healing
Fibrosis – a deposition of connective tissue that occurs as part of normal healing
Extracellular matrix – a network of large molecules that surrounds cells
Cholestasis - a condition in which the flow of bile is slowed or blocked
Alagille syndrome - a genetic condition caused by a mutation in a single but crucial gene JAG1
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