Greg Guild

Shaping Drosophila cells with the actin cytoskeleton

Cross-linked bundles of polarized actin filaments are common in eukaryotic cells and provide the scaffolding for cell movement and cell shape. We use Drosophila bristle cells as a model system to investigate actin bundle assembly and cell shape architecture by employing molecular and genetic tools to modify actin bundle assembly and confocal and electron microscopic techniques to evaluate the cell biological consequences at high resolution. We are interested in two problems at the moment.

How are extremely long actin bundles constructed? These bundles are an engineering marvel. They can be up to 400 µm in length and taper 20-fold with 500 filaments at their base and 25 filaments at their tip. Amazingly, these structures are build by grafting together much shorter modules. Our approach has been to genetically add or remove components of this system and determine the structural consequences during development by imaging.

Is bundle construction dynamic? We were very surprised to learn that these bundles are constructed by “trial and error” engineering. Our discovery of modules led us to rationalize long bundle construction as being similar to building a skyscraper - one floor at a time and one floor on top of another. While this may be the end result, recent experiments indicate that the cell actually polymerizes and crossbridges filaments at a tremendous rate throughout the cytoplasm. However, the only filaments that survive to contribute to the bundle structure are stabilized by either membrane attachment or by crossbridging to previously stabilized filaments. It seems amazing that such an engineering approach leads to the production of such stereotypical bundle structures and cell shapes. Our approach here involves time-lapse imaging of both wild-type and mutant GFP-labeled cells during development after noninvasive perturbations like photobleaching.

Images of Drosophila bristles. From left to right: Adult fly; Live developing pupa expressing GFP-actin in bristle and socket cells; Actin bundles and other structures in a sprouting bristle; Sprouting bristle stained for actin (red) and the forked crossbridging protein (green); Live wild-type (straight) and mutant (branched) bristles expressing GFP-actin.