Abstract
Forces generated by the actomyosin cytoskeleton are key contributors to many morphogenetic processes. The actomyosin cytoskeleton organises in different types of networks depending on intracellular signals and on cell-cell and cell-extracellular matrix (ECM) interactions. However, actomyosin networks are not static and transitions between them have been proposed to drive morphogenesis. Still, little is known about the mechanisms that regulate the dynamics of actomyosin networks during morphogenesis. This work uses the Drosophila follicular epithelium, real-time imaging, laser ablation and quantitative analysis to study the role of integrins on the regulation of basal actomyosin networks organisation and dynamics and the potential contribution of this role to cell shape. We find that elimination of integrins from follicle cells impairs F-actin recruitment to basal medial actomyosin stress fibers. The available F-actin redistributes to the so-called whip-like structures, present at tricellular junctions, and into a new type of actin-rich protrusions that emanate from the basal cortex and project towards the medial region. These F-actin protrusions are dynamic and changes in total protrusion area correlate with periodic cycles of basal myosin accumulation and constriction pulses of the cell membrane. Finally, we find that follicle cells lacking integrin function show increased membrane tension and reduced basal surface. Furthermore, the actin-rich protrusions are responsible for these phenotypes as their elimination in integrin mutant follicle cells rescues both tension and basal surface defects. We thus propose that the role of integrins as regulators of stress fibers plays a key role on controlling epithelial cell shape, as integrin disruption promotes reorganisation into other types of actomyosin networks, in a manner that interferes with proper expansion of epithelial basal surfaces.
Highlights
Forces generated by F-actin networks are important contributors to the generation of cell and tissue shape
We found that mysXG43 mutant follicle cells (FCs) showed increased numbers of basal actin-rich protrusions resembling whip-like structures compared to controls (FCs analysed for each cell type, n = 24; egg chambers analysed, ec = 8, Fig 1B and 1B’)
We found that the ectopic basal actin-rich protrusions observed in mys FCs were whip-like structures, as they moved like flagella against the direction of rotation (S1 Movie) and did not contain myosin
Summary
Forces generated by F-actin networks are important contributors to the generation of cell and tissue shape. The molecular composition of contractile actin networks and bundles is highly conserved among eukaryotic species [2]. Their organisation and dynamics change across different cell types, their position within the cell and the differentiation state of the cell. There are two main ways in which actomyosin networks can be organised within the cell, as a cortical meshwork below the plasma membrane or as stress fibers. While pulsatile contraction of cortical actomyosin networks has been mainly implicated in the cell shape changes underlying key morphogenetic processes, such as gastrulation and neural tube formation (reviewed in [3]), stress fibers have been largely involved in cell adhesion, migration and mechanosensing [4]. Little is known about the mechanisms that guarantee controlled transitions during morphogenesis
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