Publication Q&A: Adaptations in Hippo-Yap Signaling and Myofibroblast Fate Underlie Scar-Free Ear Appendage Wound Healing in Spiny Mice

October 2021 – Researchers from the Majesky Lab and Millen Lab recently published a paper in Developmental Cell reporting the first cellular and molecular mechanisms contributing to tissue regenerative wound healing properties in Acomys. 

Seattle Children’s lead authors for the publication include:

• Mark Majesky, PhD, Center for Developmental Biology and Regenerative Medicine
• Kathleen J. Millen, PhD, Norcliffe Foundation Center for Integrative Brain Research
• Branden R. Nelson, PhD, Norcliffe Foundation Center for Integrative Brain Research

Read this paper in Developmental Cell.

Dr. Branden Nelson shares insight from the paper below.

What are the significant findings in this paper?

We report the first cellular and molecular mechanisms contributing to amazing tissue regenerative wound healing properties in spiny mice. These animals heal tissue by regenerative wound healing instead of scarring. By learning how these animals naturally accomplish this, we aim to identify new therapeutics for human wound healing.

The major findings we report are:

1. Myofibroblasts (MFs) appear rapidly after injury in both spiny mice (Acomys cahirinus) and house mice (Mus musculus) ear tissue, but persist only in Mus.
2. Although isolated, Acomys fibroblasts can sense injury signals, such as the activating fibrokine TGF-β1, and they resist the cellular changes leading to Mus and human myofibroblasts.
3. Acomys activated fibroblasts maintain Hippo-Yap activity due to species-specific PP2A-dependent Yap dephosphorylation kinetics in vitro, by contrast to Mus and human myofibroblasts.
4. Acomys regenerating ear tissue in vivo also exhibits transcriptional downregulation of Mst1 (Hippo) that leads to nuclear Yap localization and target gene upregulation, in contrast to cytosolic Yap and fibrotic scarring in Mus.
5. Yap activity is necessary for preventing initial fibrotic scarring, directing later cell fate decisions, and regulating correct tissue patterning.
6. Acomys from both sexes can still regenerate in old age, albeit slightly delayed.
7. Translating insights from Acomys novel Yap regulation to human fibroblasts in vitro both prevented and rescued TGF-β1-mediated myofibroblast differentiation.

What does this research tell us that we didn’t know before?

Most adult mammals, including humans, evolved to rapidly heal traumatic skin tissue injuries by activating local fibroblasts to turn into myofibroblasts. Myofibroblasts are not only associated with scarring in the skin, but also can be activated throughout all internal organs due to other traumatic injuries and diseases that can lead to organ failure. Our study, together with other recent studies, reveals that while myofibroblasts may contribute to initial fibrosis pathways to maintain tissue integrity, they can also exhibit cellular plasticity and could contribute to regeneration in mammals. Further, the adaptions promoting Yap activity during mammalian regeneration in Acomys ear tissue are the same adaptions underlying complex sensory system regeneration in lower vertebrates, and normal tissue proliferation and morphogenesis during Drosophila development. Finally, our studies indicate that fibrotic cells, skin tissue, and scars might be treatable in adult humans using the natural solutions that evolved in other regenerative models.

What are the broad implications of this research?

Since fibrosis impacts all tissues, our findings will extend across all human medical fields. Yet, emerging data also reveal that fibroblasts exhibit tissue-specific functions and differential myofibroblast capacity and signaling states. Hence, a more complete understanding of Acomys fibroblasts across different tissues and injury states could lead to rapid identification of additional signaling adaptations that could be used to remodel genomic responses in human and mouse myofibroblasts from similar tissues. More broadly, our finding that even aged Acomys retains regenerative healing capacity in the skin provides hope for developing similar regenerative treatments in elderly humans, who can progressively lose tissue integrity and can suffer horrendous trauma after falling.

What are the next steps and long-term goals for this research? Next steps include understanding the diversity of cell types present in Acomys injured tissue, how Acomys cell identities correlate with Yap activity during initial tissue proliferative regeneration and later cell fate patterning decisions, what genomic targets are regulated by Yap, how Yap protein is differentially regulated, and why this pathway is blocked in adult Mus and human fibroblasts. Our long-term goal is to develop pro-regenerative ant-fibrosis treatments for human injuries and diseases. Hence, we developed an in vivo inducible viral Yap gene therapy approach that prevented and even rescued human fibroblasts in a dish from turning into myofibroblasts. We also translated Acomys novel Yap biological activity back into Mus ear wounds using this approach, which promising pilot studies indicate can lead to successful regeneration over fibrosis in vivo.