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Formation and regulation of filopodia

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Coupling Geometric PDEs with Physics for Cell Morphology, Motility and Pattern Formation

Filopodia are finger-like actin-rich protrusions from cells and their number, length and turnover rates are important for their functions. Their roles are as diverse as direction sensing by neuronal growth cone filopodia, targeting signaling during morphogenesis by cytonemes, and detecting sound through the stereocilia in the ear. We are using a two-pronged approach to elucidate the molecular basis of filopodia formation: in vivo imaging of filopodia in developing Drosophila and a cell-free system of filopodia-like structures.

Drosophila embryos display similar phases of differentiation and movement to vertebrate muscles. In addition, development is external (unlike mammals), live in vivo imaging is experimentally tractable, there is a wide molecular biology and genetic toolkit, and Drosophila typically have less redundancy in gene isoforms compared to vertebrates. Timelapse confocal imaging of developing muscles in Drosophila shows intense filopodial activity during migration which diminishes as the muscles attaches to tendon cells in the epidermis. We show that integrins localise to these filopodia and signaling through integrins controls filopodia length and dynamics, which, in turn is needed for the arrest of migration when muscles reach tendon cell attachment sites.

The cell-free system uses PI(4,5)P2-containing supported lipid bilayers as a plasma membrane mimic and frog egg extracts are used to mimic cytosol. Adding extracts to the supported lipid bilayers causes the nucleation of actin foci on the surface and the growth of long actin bundles up from the surface. The cell-free system offers the ability to subtract and add back extracts, fractions of extracts and purified proteins, and is highly amenable to microscopy. We have found that initiation, but not elongation, of filopodia-like structures is driven by formation of the stable tip complex of actin regulators. Elongation is driven by dynamic proteins that are in exchange with the tip complex.

This combination of biochemical dissection, microscopy and genetics allows us to elucidate how developmental programs and membrane environment control actin regulators to orchestrate cell architecture and dynamics.

This talk is part of the Isaac Newton Institute Seminar Series series.

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