Scientists at the European Molecular Biology Laboratory (EMBL) used optogenetics to drive structural changes in tissue development in Drosophila (fruit fly) embryos. Their experimental results showed that optogenetics can be used to inhibit the naturally occurring process of invagination. To form internal organs, groups of cells must move toward the inside of an embryo. During this process, called invagination, the surface of the cell grouping contracts and causes tissue to fold inward. The cell grouping initiates the force needed to cause the tissue invagination. Abnormalities in this process lead to problems in tissue and organ development. Top view of modified and natural invagination. While the left tissue is modified with optogenetics and does not invaginate, the right tissue folds toward the embryo's inside and creates a pouch. Courtesy of Daniel Krüger/EMBL. Using optogenetics, the researchers controlled myosin-II levels at the basal (i.e., inner) surface of invaginating cells during the gastrulation phase of embryonic development in fruit flies. They found that while basal myosin-II was lost during ventral furrow formation, use of optogenetics techniques allowed preinvagination levels of myosin-II to be maintained over time. The scientists used optogenetics to stiffen the part of the tissue’s surface that folds inward during invagination. Quantitative imaging showed that optogenetic activation prior to tissue bending slowed cell elongation and blocked the invagination process. When optogenetic activation occurred after cell elongation and tissue bending were initiated, it inhibited cell shortening and folding of the furrow. The experiments demonstrated the need for myosin-II polarization and basal relaxation throughout the invagination process, and showed that it was possible to stop invagination before it happened and also midprocess. “If cells are not allowed to relax their bases, they cannot constrict their apices efficiently, and tissue invagination stops,” said researcher Stefano De Renzis. “It's like when you squeeze the top and the bottom of a balloon simultaneously. The inner pressure becomes higher and the balloon can’t fold inwards anymore.” While scientists have previously speculated about the importance of the tissue’s basal surface, experimental techniques were not advanced enough to test their theories until now. Using optogenetics, the EMBL team can modify protein activity without damaging the cells, while still being able to activate and deactivate modifications made to the cells. The team's experimental results could help explain morphological abnormalities during embryonic development. Although the experiments were done in fruit fly embryos, De Renzis believes the results and methods used could be applied to other organisms. In the future, optogenetics techniques such as those employed by the EMBL team could be used to create and shape artificial tissues or to control tissue development in regenerative medicine. The research was published in The EMBO Journal (http://emboj.embopress.org/content/early/2018/11/13/embj.2018100170). Side view of invagination process. The outer/apical surface of the cells contracts, while the inner/basal surface relaxes. This coordinated process generates a force that drives the cell towards the inside of the embryo. This is the first step towards the development of an organ. Courtesy of Daniel Krüger/EMBL.