Rowan Naidoo has uncovered the mechanism that allows plasma membrane architecture and the actomyosin cytoskeleton to work together to generate tension during early Drosophila fruit fly development.
He has published this part of his PhD work in the Harris lab in Journal of Cell Biology as “Actomyosin cortex integration with complex plasma membrane topography in the early Drosophila embryo.”
Naidoo co-authored the paper with Rebecca Tam, now a postdoctoral researcher at Rockefeller University, whose previous work from the lab explored how the walls between nuclear chambers form in the multinucleated Drosophila embryo.
Naidoo was intrigued by the fact that the function of the complex membrane topography that exists before these chambers arise remained unclear.
Understanding complex folds
The early Drosophila embryo has a plasma membrane that is highly folded; Naidoo’s work reveals how actomyosin networks integrate with these folds to generate tension across the embryo surface.
“This work really contrasts with what I learned from the textbook as an undergraduate,” notes Naidoo. “That depiction of the plasma membrane was as a flat sheet, with all the material underneath.”
“With my first assay with a lipid probe, I looked under the microscope and the plasma membrane looked extremely complex and hard to connect to the textbook image.”
Surface tension is important for the physical integrity of embryos, but materials typically fold because of compression. Thus, it was unclear how actomyosin networks generate embryo surface tension across a highly folded plasma membrane.
Naidoo’s research provided evidence of actomyosin networks integrating throughout the folds to form a composite material that produces surface tension and indicates a role for the Arp2/3 protein complex that drives network structure.
A diversity of techniques unveils what happens at the folds
Naidoo’s research led him to master a number of techniques as he studied early stage Drosophila embryos.
To determine whether the folded surface was under tension, Naidoo used laser ablation to cut the membrane and measured membrane recoil from the wound. When myosin levels were low, membrane recoil was substantially decreased, indicating that actomyosin networks generate tension across the folded embryo surface.
Genetic perturbations and drug treatments demonstrated that myosin activity is indeed required for generating the surface tension, and that actin filaments interconnect the composite material.
Using dual live-cell imaging, Naidoo visualized the plasma membrane alongside cytoskeletal molecules in living embryos. He observed that plasma membrane folds condense and expand during cycles of embryo growth, while cortical myosin accumulates and dissipates in synchrony with these changes.
He also revealed that cyclic condensations are preceded by periods of expanded spacing between plasma membrane infoldings driven by the proteins of the Arp2/3 network.
Overall, these techniques provided evidence of the membrane architecture and actomyosin cytoskeleton forming a composite material that generates and transmits force across the embryo cortex. This surface tension is crucial for development of the Drosophila embryo.
The study expands understanding of the early embryo surface.
When viewed together, Naidoo’s findings reveal that plasma membrane folds act as a physical template for actomyosin assembly and activity. Additionally, distinct Arp2/3-dependent actin networks expand through the folds, and regulate their association with actomyosin networks.
The findings suggest that plasma membrane topography plays a fundamental role in the generation of cortical tension during development. Early mammalian embryos also rely on myosin-based surface tension and exhibit plasma membrane folding so this advance has potential relevance to screening in assisted reproductive technologies.
The deep folds of the plasma membrane imply that there could be interactions with other membranes within the cell. The team is now interested in looking at how these folds might also engage the endoplasmic reticulum found below the embryo surface.
Congratulations on this impressive insight into the biophysics of the early embryo!