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Where to go? Cellular Migration requires coordinated Transitions of Actin Cortex

Plants, Bacteria, and Fungi possess a rigid cell wall that protects the cell and gives it shape. Animal cells, such as mammalian cells, have no outer wall, which exposes their plasma membrane to the environment. Still, they have a distinct shape, can easily alter their shape, and even change their shape to move around. This is possible thanks to a flexible “inner wall” that is composed of the actin cortex and the cytoskeleton.

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Shape of Animal Cells

Animal cells contain a complex and dense network of filamentous proteins (protein fibers), called the cytoskeleton [1]. A cytoskeleton is found in both procaryotes and eucaryotes. It is composed of three types of fibers: microtubules (ca 25nm in diameter), actin filaments (ca 7nm), and intermediate filaments - a large group of protein fibers whose diameter ranges between microtubules and actin filaments. This network is not static, but can be modified and rearranged by other proteins.

At the interface of the plasma membrane, the cytoskeleton is connected to a layer of actin filaments (actin cortex) [2], that are lining the membrane and connect the membrane to the cytoskeleton, imparting shape and structure from the cytoskeleton to the cell outside.

Polarized Migration of Retinal Pigmented Epithelial Cells

Efficient cell migration requires the proper polarization of motility and signaling components. While most research has focused on the role of branched or myosin-associated actin, Anjali Bisaria—a graduate student in the laboratory of Dr. Tobias Meyer (California, USA) —is trying to study if and how the actin cortex may polarize to facilitate migration. This group had previously used both widefield and confocal systems to assess the behavior of actin cortex, but realized that TIRF (Total Internal Reflection Fluorescence microscopy) is the best way to clearly visualize it (Fig 1).

For a cell to migrate, both cytoskeleton and actin cortex undergo a coordinated modification, which allows polarized re-shaping and dislocation of the cell. TIRF microscopy allowed Anjali Bisaria [3] to focus on the cortex/membrane interface for multiposition movies with high spatial (using 100x objective) and temporal (every minute) resolution (Fig 2). Without cytoplasmic background, TIRF produced images that were much easier to interpret. Below (Fig 1) is an example of a live cell imaged in epifluorescence (left panel), and in TIRF (right panel). The Leica DMi8 S with Infinity TIRF was used to acquire these images using a 100x Plan Apo objective.

Cell Migration in TIRF Timelapse

Fig 2: RPE-1 Cell migration on fibronectin. Details as in Fig 1. Movie courtesy of Anjali Bisaria, Dr. Tobias Meyer’s laboratory in California, USA.