Constructing intricate morphological body plans requires the coordination of biophysical and biochemical signals to oversee the dramatic transitions in cell behaviors that are required. At the cell level, biomechanical forces influence these behaviors by regulating plasma membrane–cortical cytoskeleton machineries, and failure to do so leads to disrupted development and/or diseases, ranging from muscular dystrophies and cardiomyopathies to cancer progression/ metastasis. We are using the highly conserved cell wound repair process in the early Drosophila embryo as a model in which to investigate the means by which cells sense then respond to mechanical signals and/or stresses.

Roles of branched and linear actin filament coordination in actomyosin ring formation/contraction during cell wound repair.  Branched actin acts as a structural scaffold to assemble and tether the linear actin cable at the wound edge. Without branched actin, subunits of the actin cable release from the wound edge and form actin circles as a result of myosin contraction and bending. No actin structures are formed when both branched and linear actin are removed. Scale bar: 10µm.

We have shown that the assembly and function of the actomyosin ring requires both branched and linear actin networks for optimal cell wound repair. This is consistent with studies of in vitro generated actomyosin rings consisting of only linear or only branched actin that similarly exhibit impaired contraction. We find that branched actin networks contribute to cell wound repair in several ways: 1) as a scaffold that influences filament orientation and length, 2) as a foundation for assembling and tethering the actomyosin ring, and 3) as a platform for anchoring the cortical cytoskeleton to the overlying plasma membrane. We find that linear actin networks contribute to the generation of the actomyosin contractile ring.

We find that a cytoskeleton consisting primarily of branched actin leads to the formation of dispersed ill-defined actin structures. These structures are likely small unproductive aggregates of branched networks that cannot organize into force-producing actin assemblies. Conversely, a cytoskeleton consisting primarily of linear actin does not assemble a functional cable. Instead, long linear actin filament bundles are formed in the wound center. Interestingly, we recently identified an exciting new mechanism of actin ring constriction involving the chiral (counter-clockwise) movement of linear actin filament arrays that occurs in the absence of branched actin and canonical myosin II motors. This pattern of chiral actin filament movement is reminiscent of microtubule-based cytoplasmic flows wherein kinesin motors slide microtubules past one another, leading to the possibility that the force production for actin ring contractility in this context is the result of non-canonical actin-based motors.

We are currently using a combination of cell biological, molecular, biochemical, and advanced dynamic high-resolution imaging approaches with the Drosophila cell wound repair model, to delineate how actin associated proteins influence the dynamic organization of the cortical actomyosin assemblies used to generate the precise intracellular forces needed to carry out normal cellular events or to repair cell wounds.

Hui J, Stjepić V, Nakamura M and Parkhurst SM (2022). (invited review). Wrangling Actin Assemblies: Actin Ring Dynamics in Cell Wound Repair. Cells 11(18): 2777; https://doi.org/10.3390/cells11182777.

Nakamura M, Hui J, Stjepić V and Parkhurst SM (2023). Scar/WAVE has Rac GTPase-independent functions during cell wound repair. Sci. Rep. 13(1): 4763. doi: 10.1038/s41598-023-31973-2.

Nakamura M, Hui J and Parkhurst SM (2023). (invited review). Bending actin filaments: twists of fate. Faculty Reviews 12(7) https://doi.org/10.12703/r/12-7.   URL: https://facultyopinions.com/prime/reports/b/12/7/

Hui J, Nakamura M, Dubrulle J, Parkhurst SM. (2023).  Coordinated efforts of different actin filament populations are needed for optimal cell wound repair. Mol. Biol. Cell 34(3): ar15. doi: 10.1091/mbc.E22-05-0155. PMID: 36598808 [preprint posted on BioRxiv]. URL:  https://doi.org/10.1091/mbc.E22-05-0155