Fibrosis: How collagen becomes ‘stiff’
Oxidative stress following a lung injury can alter the structure of collagen, causing it to stiffen and trigger the formation of a fibrotic scar that further hardens the tissue.
- Department of Medicine, University of Pennsylvania, United States
When our bodies heal following an injury, fibroblasts and other cells deposit components of the extracellular matrix, such as collagen, that form a ‘scar’ over the damaged area. This process, known as fibrosis, leads to a build-up of stiff fibrotic tissue that interferes with the activity of the underlying organ.
In healthy organs, fibroblasts lay down just enough collagen to maintain the integrity of the tissue (D’Urso and Kurniawan, 2020). However, if the tissue begins to stiffen, these cells produce more collagen, disrupting this equilibrium (Tschumperlin et al., 2018). This increases the rigidity of the organ, triggering the fibroblasts to release even more collagen. While this explains how fibrosis progresses, it is less clear how the cycle starts.
Previous studies showed that fibrosis is initiated by changes in the mechanical properties of collagen that arise from modifications to its structure (Jones et al., 2018; Georges et al., 2007). The experiments found that, after a tissue injury, a family of enzymes called lysyl oxidases (or LOXs for short) increase the bonds between individual collagen fibers in the organ. This modified structure makes the tissue stiffer, driving the fibrotic process. Now, in eLife, Mark Jones from the University of Southampton and co-workers – including Christopher Brereton and Liudi Yao as joint first authors – report on the pathway that leads to this altered collagen architecture in patients with pulmonary fibrosis (Brereton et al., 2022).
First, the team (who are based at various institutes in the United Kingdom, the United States, Austria, Italy and Ireland) studied lung tissue from individuals with idiopathic pulmonary fibrosis, a disease of unknown cause in which patient’s lungs are severely scarred (Richeldi et al., 2017). They found that the genes coding for two enzymes that sequentially modify collagen (Yamauchi and Sricholpech, 2012) – lysyl hydroxylase PLOD2 and LOXL2 – were highly expressed in the same lung cells at the same time. The gene for the most commonly found type of collagen in fibrosis (called type I) was also activated in the lung tissue, but its pattern of expression did not correlate with the genes for PLOD2 or LOXL2. This suggests that it is regulated independently from these two enzymes.
Next, Brereton, Yao et al. extracted fibroblasts from patients with idiopathic pulmonary fibrosis and cultured them in the laboratory for six weeks, treating the cells with several growth factors associated with fibrosis. The experiments showed that a family of proteins called Hypoxia Inducible Factors (HIFs), which regulate the body’s response to varying oxygen levels, activated the genes for PLOD2 and LOXL2. This increased the number of cross-links between the collagen fibers and made the fibers stiffer. Imaging the collagen produced by these cells with an electron microscope revealed that the fibers had a smaller diameter, as had previously been observed in patients with idiopathic pulmonary fibrosis (Jones et al., 2018). This suggests that HIFs regulate tissue mechanics not by increasing the amount of collagen, but by altering collagen’s structure.
HIF is negatively regulated by another protein called FIH (short for Factor Inhibiting HIF), which is inactivated by the unstable byproducts of oxygen metabolism, also known as oxidative stress (Lando et al., 2002). Loss of FIH causes cells to enter ‘pseudohypoxia’, a state in which cells behave as if oxygen levels are low despite being in normal conditions (Masson et al., 2012). Brereton, Yao et al. found that reducing FIH – both in cells cultured in the laboratory as well as in lung tissue from patients with idiopathic pulmonary fibrosis – led to higher levels of HIF, which increased the activity of PLOD2 and LOX2, causing changes to collagen’s architecture and fibrosis.
These findings suggest that the pseudohypoxia state induced by oxidative stress is central to the pathology of idiopathic pulmonary fibrosis, and may help to explain how the fibrosis cycle starts (Figure 1): injuring or damaging the lung leads to a rise in oxidative stress, which dampens the level of FIH. This allows HIF to activate the genes for the enzymes PLOD2 and LOXL2, which then sequentially alter the structure of collagen. These changes increase the stiffness of the tissue, initiating the fibrosis cycle.
Figure 1
The pathway to fibrosis.
When cells in the lung become injured or damaged (left), the resulting increase in oxidative stress leads to a decrease in the expression of the genes that code for a protein called FIH (purple triangle). This results in a pseudohypoxia state which increases the activity of the protein HIF (pink circle), allowing it to upregulate the expression of the genes that code for two enzymes: PLOD2 and LOXL2. These enzymes act in sequence to increase and strengthen the bonds, or ‘cross-links’, between fibers of collagen. The altered structure stiffens the tissue, triggering cells in the lung to deposit more collagen and start the process of fibrosis. HIF: Hypoxia Inducible Factor; FIH: Factor Inhibiting HIF.
Image credit: Iris de Jong (CC BY 4.0).
This work provides new insights in to how fibrosis is triggered. Particularly intriguing is the suggestion that intervening at the level FIH or HIF may be better at treating idiopathic pulmonary fibrosis than targeting the machinery that synthesizes collagen, which is currently considered to be the most effective approach. Further work is needed to fully understand how the architecture of collagen becomes dysregulated and to test how substances that block this newly discovered pathway impact fibrosis.
References
-
Mechanical and physical regulation of fibroblast-myofibroblast transition: from cellular mechanoresponse to tissue pathologyFrontiers in Bioengineering and Biotechnology 8:609653.https://doi.org/10.3389/fbioe.2020.609653
-
Increased stiffness of the rat liver precedes matrix deposition: implications for fibrosisAmerican Journal of Physiology. Gastrointestinal and Liver Physiology 293:G1147–G1154.https://doi.org/10.1152/ajpgi.00032.2007
-
Mechanosensing and fibrosisThe Journal of Clinical Investigation 128:74–84.https://doi.org/10.1172/JCI93561
-
Lysine post-translational modifications of collagenEssays in Biochemistry 52:113–133.https://doi.org/10.1042/bse0520113
Article and author information
Author details
Publication history
Copyright
© 2022, Wells
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Fibrosis: How collagen becomes ‘stiff’
eLife 11:e77041.
https://doi.org/10.7554/eLife.77041
Further reading
-
- Cell Biology
Understanding how the brain controls nutrient storage is pivotal. Transient receptor potential (TRP) channels are conserved from insects to humans. They serve in detecting environmental shifts and in acting as internal sensors. Previously, we demonstrated the role of TRPγ in nutrient-sensing behavior (Dhakal et al., 2022). Here, we found that a TRPγ mutant exhibited in Drosophila melanogaster is required for maintaining normal lipid and protein levels. In animals, lipogenesis and lipolysis control lipid levels in response to food availability. Lipids are mostly stored as triacylglycerol in the fat bodies (FBs) of D. melanogaster. Interestingly, trpγ deficient mutants exhibited elevated TAG levels and our genetic data indicated that Dh44 neurons are indispensable for normal lipid storage but not protein storage. The trpγ mutants also exhibited reduced starvation resistance, which was attributed to insufficient lipolysis in the FBs. This could be mitigated by administering lipase or metformin orally, indicating a potential treatment pathway. Gene expression analysis indicated that trpγ knockout downregulated brummer, a key lipolytic gene, resulting in chronic lipolytic deficits in the gut and other fat tissues. The study also highlighted the role of specific proteins, including neuropeptide DH44 and its receptor DH44R2 in lipid regulation. Our findings provide insight into the broader question of how the brain and gut regulate nutrient storage.
-
- Cell Biology
- Immunology and Inflammation
The endothelial blood-brain barrier (BBB) strictly controls immune cell trafficking into the central nervous system (CNS). In neuroinflammatory diseases such as multiple sclerosis, this tight control is, however, disturbed, leading to immune cell infiltration into the CNS. The development of in vitro models of the BBB combined with microfluidic devices has advanced our understanding of the cellular and molecular mechanisms mediating the multistep T-cell extravasation across the BBB. A major bottleneck of these in vitro studies is the absence of a robust and automated pipeline suitable for analyzing and quantifying the sequential interaction steps of different immune cell subsets with the BBB under physiological flow in vitro. Here, we present the under-flow migration tracker (UFMTrack) framework for studying immune cell interactions with endothelial monolayers under physiological flow. We then showcase a pipeline built based on it to study the entire multistep extravasation cascade of immune cells across brain microvascular endothelial cells under physiological flow in vitro. UFMTrack achieves 90% track reconstruction efficiency and allows for scaling due to the reduction of the analysis cost and by eliminating experimenter bias. This allowed for an in-depth analysis of all behavioral regimes involved in the multistep immune cell extravasation cascade. The study summarizes how UFMTrack can be employed to delineate the interactions of CD4+ and CD8+ T cells with the BBB under physiological flow. We also demonstrate its applicability to the other BBB models, showcasing broader applicability of the developed framework to a range of immune cell-endothelial monolayer interaction studies. The UFMTrack framework along with the generated datasets is publicly available in the corresponding repositories.
