Stem Cells: Encouraging cartilage production

A long non-coding RNA called GRASLND is essential to help stem cells create stable cartilage.
  1. H Scott Stadler  Is a corresponding author
  1. Orthopaedics and Rehabilitation, Oregon Health Science University, United States
  2. Skeletal Biology Research Center, Shriners Hospital for Children, United States

Recently watching a rerun of the 2016 Olympics gymnastics finals, I could not help marveling at the way the joints of the athletes could withstand so many gravity-defying leaps, twists, and landings. These feats are possible because the ends of our bones are covered by articular cartilage, a smooth tissue that allows fluid, pain-free movement. This tissue is made by specialized cells secreting proteins that trap water and form an extracellular matrix which cushions joints.

When articular cartilage wears away, for example in degenerative diseases such as osteoarthritis, movements become painful and quality of life drops severely. Yet, these conditions are increasingly common – in the United States alone, it is predicted that more than 78 million people could be affected by 2040 (Hootman et al., 2016).

Cartilage is not connected to the nervous system or to blood and lymphatic vessels, which means the tissue heals poorly when damaged. Most therapies for osteoarthritis therefore work by preserving the remaining cartilage or preventing further loss. Once the cartilage is lost, few interventions exist: surgeons can carefully damage the bone to promote the creation of new tissue, they can graft bone and cartilage obtained from a donor, or they can completely replace the joints with artificial ones (Steadman et al., 2001; Toh et al., 2014; Bugbee et al., 2016; Migliorini et al., 2020). However, these interventions may not be durable, and they are limited by factors such as the availability of donor tissue and the age or health condition of the patient.

Another, lab-based approach is to harvest mesenchymal stem cells or chondroprogenitor cells from patients, and then 'coax' these to create cartilage that can be implanted in the individual (Migliorini et al., 2020). However, one challenge associated with this method is the stability of the resulting cartilage: over time, it can change into bone, reducing the function of the repaired joint.

Long non-coding RNAs are molecules that regulate an array of genetic events in the cell, and it was reported recently that these sequences are essential to keep cartilage stable: for instance, several long non-coding RNAs are activated in mesenchymal stem cells that produce cartilage (Barter et al., 2017; Huynh et al., 2019). Now in eLife, Farshid Guilak and colleagues – including Nguyen Hyunh as first author – report having identified a long non-coding RNA called GRASLND which encourages mesenchymal stem cells to produce molecules that form cartilage (Huynh et al., 2020).

First, the team (which is based at Washington University in St. Louis, the St. Louis Shriners Hospital, Duke University and Vanderbilt University) designed RNA molecules that were used to deactivate GRASLND in mesenchymal stem cells. As a result, the production of cartilage decreased and these cells started to show a molecular profile associated with bone formation. These results demonstrate that, in these cells, GRASLND is required to maintain a cartilage-forming program (Figure 1).

GRASLND helps mesenchymal stem cells to create cartilage by suppressing IFN-signaling.

(A) Exposing mesenchymal stem cells (MSCs) to the growth factor TGFβ3 activates the expression of the Sox9 gene, which triggers the production of a long non-coding RNA called GRASLND. (B) GRASLND binds to the kinase EIF2AK2 (blue), which blocks the inhibitory phosphorylation of the protein EIF2A (green). This, in turn, promotes the expression of 'prochondrogenic factors' that encourage the production of molecules, such as proteoglycans, which form cartilage; the cell is said to have a 'chondrogenic' phenotype. (C) When GRASLND is depleted from mesenchymal stem cells, the kinase EIF2AK2 probably phosphorylates EIF2A (represented here by the ‘-P*’). This activates the Type II IFN-γ response, which ultimately leads to a reduction in proteoglycan expression and a loss of the chondrogenic phenotype. GRASLND: glycosaminoglycan regulatory associated long non-coding RNA; EIF2A: eukaryotic translation initiation factor two alpha; EIF2AK2: EIF2A kinase; TGFβ3: transforming growth factor beta 3. Figure created using BioRender (BioRender.com).

Further experiments showed that GRASLND interacts with EIF2AK2, a kinase that normally inhibits a protein known as EIF2A, which triggers a molecular cascade called the type II IFN-γ signaling pathway (Samuel, 1979; Platanias, 2005). This pathway is essential for the immune system, but some of its elements, such as a cytokine called IFN-γ, also help to stimulate bone formation (Duque et al., 2011).

When GRASLND binds EIF2AK2, it probably stops this kinase from acting on EIF2A; this suppresses IFN activity while allowing the genes that promote the production of cartilage to be expressed (Figure 1B). On the other hand, Hyunh et al. find that removing GRASLND is associated with an increase in the expression of genes under the control of IFN-γ (Figure 1C). As IFN-γ promotes bone formation, these findings explain why depleting mesenchymal stem cells of GRASLND leads to more bone production.

Finally, Hyunh et al. used data mining to show that, in diseased cartilage, genes regulated by IFN are expressed more abundantly. This suggests that IFN-signaling may be directly responsible for the production of the abnormal, bony nodules that are often present in osteoarthritic cartilage.

Overall, these results indicate that — at least in vitro — GRASLND is an important modulator of type II IFN-γ signaling that is necessary for cartilage differentiation. They also highlight that this pathway may be involved in diseases of the cartilage. If so, the interaction between GRASLND and EIF2AK2 could be an important pharmacological target. Exploring this possibility will first require comparing the expression of GRASLND in healthy and diseased cartilage.

GRASLND has only been found in primates, but related long non-coding RNAs could be identified in other species by spotting the motifs that GRASLND needs to interact with EIF2AK2. In turn, this knowledge could pave the way for better animal models to study how this class of long non-coding RNAs is involved in degenerative joint diseases.

References

Article and author information

Author details

  1. H Scott Stadler

    H Scott Stadler is in the Department of Orthopaedics and Rehabilitation at the Oregon Health Science University, and the Skeletal Biology Research Center at the Shriners Hospital for Children, Portland, United States

    For correspondence
    stadlers@ohsu.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-2504-5876

Publication history

  1. Version of Record published: May 6, 2020 (version 1)

Copyright

© 2020, Stadler

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.

Metrics

  • 799
    Page views
  • 46
    Downloads
  • 1
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

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)

  1. H Scott Stadler
(2020)
Stem Cells: Encouraging cartilage production
eLife 9:e57239.
https://doi.org/10.7554/eLife.57239

Further reading

    1. Stem Cells and Regenerative Medicine
    Ece Yildiz, Gaby El Alam ... Kristina Schoonjans
    Research Article Updated

    During severe or chronic hepatic injury, biliary epithelial cells (BECs) undergo rapid activation into proliferating progenitors, a crucial step required to establish a regenerative process known as ductular reaction (DR). While DR is a hallmark of chronic liver diseases, including advanced stages of non-alcoholic fatty liver disease (NAFLD), the early events underlying BEC activation are largely unknown. Here, we demonstrate that BECs readily accumulate lipids during high-fat diet feeding in mice and upon fatty acid treatment in BEC-derived organoids. Lipid overload induces metabolic rewiring to support the conversion of adult cholangiocytes into reactive BECs. Mechanistically, we found that lipid overload activates the E2F transcription factors in BECs, which drive cell cycle progression while promoting glycolytic metabolism. These findings demonstrate that fat overload is sufficient to reprogram BECs into progenitor cells in the early stages of NAFLD and provide new insights into the mechanistic basis of this process, revealing unexpected connections between lipid metabolism, stemness, and regeneration.

    1. Stem Cells and Regenerative Medicine
    Dennis May, Sangwon Yun ... Valentina Greco
    Research Article Updated

    Stem cell differentiation requires dramatic changes in gene expression and global remodeling of chromatin architecture. How and when chromatin remodels relative to the transcriptional, behavioral, and morphological changes during differentiation remain unclear, particularly in an intact tissue context. Here, we develop a quantitative pipeline which leverages fluorescently-tagged histones and longitudinal imaging to track large-scale chromatin compaction changes within individual cells in a live mouse. Applying this pipeline to epidermal stem cells, we reveal that cell-to-cell chromatin compaction heterogeneity within the stem cell compartment emerges independent of cell cycle status, and instead is reflective of differentiation status. Chromatin compaction state gradually transitions over days as differentiating cells exit the stem cell compartment. Moreover, establishing live imaging of Keratin-10 (K10) nascent RNA, which marks the onset of stem cell differentiation, we find that Keratin-10 transcription is highly dynamic and largely precedes the global chromatin compaction changes associated with differentiation. Together, these analyses reveal that stem cell differentiation involves dynamic transcriptional states and gradual chromatin rearrangement.