Spiraling inward to heal: a new mechanism for wound closure

From the Parkhurst Lab, Basic Sciences Division

You’ve been wounded; nothing serious thankfully, but you watch your scrape go through the stereotypical stages of healing: first it’s raw, perhaps a bit bloody, but then the wound begins to close and scab. Like our bodies, most of our cells are also subjected to everyday wear-and-tear and ever more traumatic injuries. This repair process is essential for maintaining healthy cells and tissue integrity. But how does a cell heal a wound? The mechanisms governing this microscopic wound healing process are precisely what the researchers in the lab of Dr. Susan Parkhurst investigate. In a recent study led by Justin Hui, the Parkhurst lab uncovered a new mechanism for cell wound repair which involves a spiraling movement of linear actin filaments. This finding was published in Molecular Biology of the Cell.

The authors explained that when a cell gets injured, “there is an influx of calcium that acts as a start signal to initiate the repair process. Soon after, actin and myosin (the cytoskeletal filaments responsible for maintaining and controlling cell shape and architecture) are recruited to the wound edge to form an actomyosin ring. The recruited actin forms either branched actin (mesh) or linear actin (cables) that we show generates either a scaffold or tensile cable, respectively. The actomyosin ring then contracts, driven by myosin, to pull the rest of the cortex together and eventually closes the injury site.” The process looks a bit like tightly closing up a draw string bag. The Parkhurst lab had previously found that Rho family GTPases are vital for regulation of the cytoskeleton during cell wound repair, including branched and linear actin nucleators. In their recent study, Hui et al. investigated what roles each nucleator plays in actin organization during wound closure.

To do so, they used laser ablation to create holes in early Drosophila embryos and various fluorescent dyes or reporters to visualize how cytoskeletal filaments move to close a wound. The Parkhurst group first characterized the localization and function of linear and branched actin nucleation factors during wound repair. They partnered with Dr. Julien Dubrulle in the Cellular Imaging Core for his help with image analysis techniques that enabled them to investigate the actin dynamics during wound closure. The research team found that linear actin factors DAAM and Dia, as well as WAS family branched actin nucleation factors, WASp and SCAR, all exhibit distinct spatiotemporal localization to the wound site. Knocking down these factors individually or in combination revealed that DAAM/Dia and WAS proteins play both overlapping and non-redundant roles during wound repair. Taking a closer look at this healing process, the authors used super-resolution microscopy to examine actin dynamics upon knockdown of linear and branched actin nucleation factors. They add, “With our new super resolution images, we needed robust analytical methods to tease apart their differences, and Julien helped us do this.” Their work revealed that knockdown of either branched or linear actin causes severe- but distinct- disruptions to actin filament architecture of the contractile actomyosin ring at the wound’s edge. For instance, knockdown of linear actin filament resulted in altered actin ring architecture with a reduced variance of actin organization, while loss of branched actin resulted in formation of striking circular actin structures in the unwounded state with pronounced long, linear actin filaments around the wound.

linear actin closes a wound with spiral movement

The authors continue to push the cell’s repair process limits: “As we were conducting our experiments, we wondered how linear/branched actin and a major motor protein (non-muscle myosin), three major components of the cytoskeleton, coordinate together to form the actomyosin ring. We were surprised by the cell’s resilience to repair a wound despite the lack of each component and the way in which it was doing so. In particular, a cell without branched actin and non-muscle myosin uses an amazing concerted counterclockwise movement of linear actin filaments to close the wound!” The researchers made this discovery using a photoconvertible actin reporter that enabled them to track the movement and direction of the linear actin filaments during wound repair, where they found these filaments consistently moved in a counterclockwise direction, indicating there is a specific chirality to this newly uncovered wound closure mechanism. The authors stated, “Our findings demonstrate the importance of different actin structures in the optimal formation of the dynamic actomyosin ring during wound repair. Uncovering this new wound repair mechanism showed us how we can potentially treat vulnerable cells in certain disease states. Perhaps in these fragile cell conditions it may be advantageous to remove more elements of the cytoskeleton to induce a different wound repair response.”

This work highlights the resiliency cells have and their ability to find creative ways to repair their injuries. “Even when the cell is missing some key components to their cytoskeleton, it will use what is at hand to form a structure to repair the wound, albeit a little slower,” the authors explained. Moving forward, the Parkhurst Lab will continue to investigate this new spiral wound repair mechanism, and they are looking into what is generating the force needed to drive this concerted movement of actin filaments.

This work was supported by the National Institutes of Health, the National Institute of General Medical Sciences, the National Institute of Child Health & Human Development, the National Cancer Institute and the Mark Groudine Chair for Outstanding Achievements in Science and Service.

Fred Hutch/UW Cancer Consortium member Susan Parkhurst contributed to this work.

Hui J, Nakamura M, Dubrulle J, Parkhurst SM. Coordinated efforts of different actin filament populations are needed for optimal cell wound repair. Mol Biol Cell. 2023 Jan 4:mbcE22050155. doi: 10.1091/mbc.E22-05-0155. Epub ahead of print. PMID: 36598808.