HOW CELLS COMMUNICATE WHEN THEY MIGRATE COLLECTIVELY

Recently, scientists at BIOC, in collaboration with the Institut Curie and the Mechanobiology Institute in Singapore, have identified a new mechanism of cellular coordination during collective cell migration. Collective migration is essential for development as well as for wound healing in adults; it is also involved in pathological processes, such as the spread of cancer cells.
During collective migration, layers of cells gradually polarize in the direction of migration. This asymmetric polarization overlays the apical-basal polarity of epithelial cells, thereby adding an additional level of complexity to this phenomenon. Cell movement and polarization, as well as their integration into a monolayer, are controlled by branched actin networks generated by the Arp2/3 complex once it is activated.
Using bioinformatics screening, the authors identified several proteins associated with branched actin. They then performed a functional CRISPR screen, after which cell behavior was characterized using PIV (Particle Image Velocimetry), a method commonly used in physics to analyze fluid motion.

This approach involves dividing the field of view into several sub-images and using a mathematical algorithm to determine the displacements at each instant in order to generate velocity fields. Migration parameters can then be extracted from these velocity fields and analyzed quantitatively.

Les paramètres de migration peuvent ensuite être extraits de ces champs de vitesse et être analysés quantitativement. 

“Thanks to this physics-based approach, which we learned from Pascal Silberzan’s team at the Institut Curie, we were able to quantitatively analyze many aspects of multicellular behavior when the genes of interest were inactivated by CRISPR,” explains Artem Fokin, the paper’s first author, who works in Alexis Gautreau’s team. “We found that depleting one of these genes, PKN2, led to a disruption in cell movement, as if the cells were disconnected from one another.” Indeed, PKN2 was found to be associated with lateral junctions between cells in the moving monolayer, and its depletion led to a loss of E-cadherin enrichment at the cell junctions. In collaboration with Jie Yan of the Mechanobiology Institute in Singapore, the authors ultimately demonstrated that cells lacking PKN2 exhibited a loss of E-cadherin-mediated intercellular adhesion.

Knocking out the second identified candidate, MOB4, produced the most unusual collective behavior phenotype, which the authors termed the “streaming” phenotype. This term captures two phenomena observed in these cells: a loss of directionality in the following cells and an excessive increase in their movements. Together, these two phenomena produce characteristic “streaming lines” in the vector fields obtained by PIV analysis.

The authors then showed that MOB4 tends to localize at the front of the follower cells, which is consistent with its proposed role in the perception or transmission of a directional signal. Through a series of experiments, they demonstrated that MOB4 controls collective cell migration by activating the transcription factor YAP1 in the follower cells.