When an injury occurs, the body reacts immediately. Cells in the vicinity of the trauma signal first-responder cells known as fibroblasts to secrete extracellular matrix, a type of connective tissue rich in collagens that acts as a scaffold for rebuilding the damaged tissue.
The common name for this biological process is scarring. In most cases, scarring helps us recover from the injury by stabilizing the wound and promoting healing. At worst, we have a good story to tell. However, when the injury occurs to vital organs – like the heart – scarring becomes a survival mechanism.
When scar tissue replaces muscle tissue after a heart attack, for example, the heart’s capacity to contract and relax is significantly reduced, limiting blood flow to the rest of the body, which leads to heart disease. Controlling the process of scarring, and identifying therapeutic pathways, could lead to more complete recovery for heart attack victims, more effective healing for people who suffer from poor wound closure, and better outcomes for patients with certain forms of liver and lung diseases.
Naturally, researchers around the world are exploring the possibility of building a more perfect scar, an intervention that could revolutionize treatment for many of our leading causes of death, including heart disease. Jennifer Davis PhD, Assistant Professor of Bioengineering and Pathology, and an Associate Director of the Institute for Stem Cell and Regenerative Medicine (ISCRM), leads a team that has been studying the basic mechanisms of scarring for more than a decade.
Now, Davis is the lead investigator of a study, published this week in the journal Cell Stem Cell, that reveals illuminating details about the role an RNA binding protein (MBNL1) plays in the steps that lead to scarring. The lead author of the paper is Darrian Bugg, PhD, a postdoctoral fellow in the Davis Lab. Bugg sums up the underlying purpose of the research. “The long-term goal is to harness the system [of scarring] for better clinical outcomes. But before we can start changing the system, we have to understand how the system works.”
The system Davis and Bugg want to understand involves the translation of genetic information, encoded in our DNA, into instructions that regulate the activity and expression of genes, specifically genes that regulate scarring. The molecule that translates DNA into marching orders for proteins is RNA. When it comes to scarring, MBNL1 helps to ensure that genes are properly modified to mount the proper fibrotic response.
The premise of the investigation is that MBNL1 promotes the maturation of RNAs that control the ability of fibroblasts to make scar. It does this by transitioning these cells from a quiescent to a highly activated (or secretory) myofibroblast state – from undifferentiated progenitor cells to cardiac fibroblasts to scar-producing myofibroblasts. The researchers hoped to understand how fibroblasts move through these states and whether control over MBNL1 could alter myofibroblast activity, which is to say, scarring.
In their investigation, Davis and Bugg used a mouse model to demonstrate that modulating MBNL1 only in fibroblasts could in fact lead to different fibrotic outcomes. “If you want to show that your faucet handle controls water flow, you turn it one way and see what happens, and then turn it the other way and see if the opposite happens. That’s more or less what we did here,” says Davis. “We knocked out the gene, then we overexpressed it. And the results tell us that MBNL1 is indeed a control switch for scarring. Although there were surprises.”
Modulating a Gene Leads to Unexpected Findings
Knocking out MBNL1 in cardiac fibroblasts led to limited scar production, a result the researchers expected. And single-cell sequencing, performed in ISCRM’s Genomics Core, revealed a pleasant surprise. Turning off the gene reverted a population of the fibroblasts to a more stem cell-like state, indicating a higher degree of plasticity in fibroblasts than was previously assumed.
On the other hand, overexpressing MBNL1 did promote the transition of fibroblasts into myofibroblasts. However, rather unexpectedly, upregulating the gene did not produce more scarring. Instead, Bugg and Davis saw reduced fibroblast proliferation
Davis explains the significance of the finding. “The overexpression of MBNL1 transitioned all of the heart’s fibroblasts to their most activated myofibroblast state. This left them less capable of proliferating, which is why we didn’t see more scarring, despite the enhanced activity of the cells. That tells us that the fibroblasts communicate directly with the muscle during wound healing, which reinforces the importance of learning to modulate these cells.”
Bugg explains the importance of the findings. “The most exciting result is showing that MBNL1 is required for fibroblasts to transition between functional states that underlie cardiac scarring. MBNL1 does this through stabilizing its target RNAs so that they can be turned into the proteins needed for making extracellular matrix. It’s a gatekeeper for fibroblast states. It stabilizes transcripts and makes sure the genes are actually turned into proteins.”
While the researchers primarily used a mouse model for the study, parallel experiments in cells from heart disease patients, provided by ISCRM faculty member April Stempien-Otero, MD, an Associate Professor of Medicine/Cardiology, showed upregulated levels of MBNL1, validating that the observations seen in mice are relevant to human heart failure. (ISCRM faculty member Cole Deforest, PhD, is also an author of the study.) Bugg and Davis next want to explore whether MBNL1 plays a similar role in other forms of disease.
Reflecting on the key takeaways from the study, Bugg emphasizes that discoveries now will pave the way for future breakthroughs that could someday help patients recover from injuries to the heart and other organs. “We are exploring the basic tenets of fibroblast biology. We’re learning which genes are necessary for cells to transition and what causes a fibroblast to awaken and become activated. The implications are definitely exciting.”
This work was supported by grants from the National Institutes of Health for J.D. (HL141187 and HL142624) and C.A.D. (R35GM138036), Biological Mechanisms of Healthy Aging T32 for D.B. (T32AG066574), and Graduate Research Fellowships from the National Science Foundation for R.C.B. and I.M.R. (DGE-1762114).