A defining characteristic of all cells is the plasma membrane, a thin layer of lipids that separates the interior of the cell from the outside environment. The plasma membrane is supported by the cytoskeleton, a network of proteins that control the shape of the membrane and allow the cell to move, which together form the cell cortex. Throughout their lifetime, cells experience mechanical stresses that have the potential to rupture the cortex. Without a robust wound repair process, a cortical injury could quickly lead to efflux of cellular material, infection, or cell death. A critical step after cell cortex injury is the formation of an actomyosin ring that pulls the cytoskeleton and overlying plasma membrane closed. In a recent paper in Cell Reports, Dr. Susan Parkhurst and her team identified two distinct complexes containing Septin family proteins that assemble, contract and remodel the actomyosin ring during the repair process.
How the actomyosin ring forms around a cortical wound to repair the damaged membrane and underlying cytoskeleton is not well understood. “Our previous studies identified several molecules involved in the regulation of actin polymerization and bundling that are indispensable for the formation of the actomyosin ring. However, little is known about how linear actin filaments generated by polymerization and bundling are bent and integrated into the ring structure,” says Dr. Mitsutoshi Nakamura, co-first author on the paper. “We identified Septins, which have F-actin bending activity, as a new player in cell wound repair from a genetic screen and led us to start this exciting project.”
Septins are a family guanosine triphosphate (GTP) binding proteins conserved across eukaryotes except plants that form hetero-oligomeric complexes with each other and bind actin, microtubules, and lipid membranes. They play a role in many processes involving cortex remodeling such as cytokinesis and wound healing and are associated with neurodegenerative diseases and cancer, where they are dysregulated. There are five Septins, grouped into three classes in flies, a tractable model system to study cell wound repair and the focus of the Parkhurst lab’s study. Sep1 and Sep4 form a class, known as SEPT2. Sep2 and Sep5 form the SEPT6 class and Pnut belongs the SEPT7 class. Previously, it was thought that Septins within the same class were interchangeable and occupy the same position within a Septin complex. “Unexpectedly, we found that all five Septins are needed for cell wound repair and that they form two distinct complexes (Sep1-Sep2-Pnut and Sep4-Sep5-Pnut) that are concomitantly recruited to different regions around the wound edge,” says Dr. Nakamura. While individual Septins can bind to and bundle actin, it is the formation of Septin complexes that bends the bundled actin filaments into curved architectures. “Surprisingly, we find that while the two distinct Septin complexes have F-actin bending activities, they bend F-actin to different degrees allowing them to regulate the assembly and disassembly of different portions of the actomyosin ring during cell wound repair,” says Dr. Nakamura. Studies from other investigators had highlighted the role of Anillin in Septin recruitment and actomyosin ring formation. Anillin binds both actin and Septins and is thought to crosslink them. In the absence of Anillin, Sep1 and Sep2 were no longer recruited to the wound edge, but there were no significant changes in Sep4, Sep5 or Pnut, further highlighting the differences between the Sep1-Sep2-Pnut and Sep4-Sep5-Pnut complexes.
Looking to the future, several new questions have now emerged from the Parkhurst lab’s latest findings. How do two distinct Septin complexes form in different regions during cell wound repair and confer different degrees of F-actin bending? Individual Septins do not have much F-actin bending activity. How do Septin complexes change F-actin’s conformation? “Super-resolution microscopy and cryo-EM studies will help us to answer those questions,” says Dr. Nakamura.
The spotlighted research was funded by the National Institutes of Health.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium member Dr. Susan Parkhurst contributed to this work.
Stjepic V, Nakamura M, Hui J, Parkhurst SM. 2024. Two Septin complexes mediate actin dynamics during cell wound repair. Cell Rep. 43(5):114215.