Final Report Summary - MORPHORCE (The input of mechanical forces to morphogenesis and wound healing: a systematic dissection)
All our tissues and organs initially resemble a ball of cells when they first emerge before reaching their final shape during embryogenesis. This shape depends on the final function of the organ: a cavity for the bladder or the stomach, a complex branched tubular structure for the lungs or a simpler tube for the intestine, a rather flat sheet for the ear and the skin, etc. Among the many biological processes that contribute to such dramatic changes stand mechanical forces. The general objective of the project was to understand how a mechanical force generated within the tissue can deform a ball of cells into a complex organ.
Three main cellular structures contribute to define the shape of a cell: junctional complexes, which attach cells to their neighbors or to a gel that surrounds the cell (the extracellular matrix); cytoskeletal components (long labile polymers made of actin or tubulin, which are anchored to the aforementioned junctions); the extracellular matrix. Each of these structures should be seen as being very labile and easily modified. Of course, they can also be torn if the mechanical forces become too strenuous.
We wanted to relate the mechanical forces to processes that modify these structures and to bring an integrated physical, cellular and molecular understanding of such modifications. We mainly relied on a very simple organism to do so, the nematode C. elegans, which has very few cells. The nematode embryo extends its length by a factor of four over a period of less than three hours and we wanted to understand how. Our main results can be understood as follows.
1/ The initial elongation requires the force to be directional (physicists say anisotropic). This anisotropy is achieved by having contractile cells (which shrink) next to more rigid cells containing stiff actin polymers along the embryo circumference. In addition, we found that the outer extracellular matrix distributes the tension generated at local positions. One can think of snake squeezing its prey.
2/ A second stage of deformation next requires a mechanical cooperation between two tissues within the embryo, its muscles and the external epithelium (the skin). We found that the constant muscle micro-contractions deform the embryo through a ratchet mechanism, whereby cell-cell junctions get a bit longer at each contraction while a locking system exerted on the actin cytoskeleton prevents tissue elasticity to bring the cell back to each initial shape.
3/ We identified a protein that acts as a molecular switch: upon sensing tension at the level of junctions it interacts with several other proteins that will exert their effects on the cytoskeleton and on junctions. One of them is a complex that normally produces forces, except that in this case it appears to consolidate the junction without its force component.
4/ We demonstrated that some tiny membrane invaginations called caveolae act as a mechanical buffer at the cell membrane to help replenish junctions with the extracellular matrix when tension is exerted on human cells. This requires some of the same proteins used in nematodes.
5/ To achieve these results, we used advanced imaging techniques such as lightsheet microscopy, evanescent wave microscopy, nano-scissors and we treated cells like special material exposed to external and internal stress.
Overall with this project, we have accounted on how a complex tissue made of different cell types deforms under the influence of mechanical forces and we have advanced our biophysical and cellular understanding of how they do so.
Three main cellular structures contribute to define the shape of a cell: junctional complexes, which attach cells to their neighbors or to a gel that surrounds the cell (the extracellular matrix); cytoskeletal components (long labile polymers made of actin or tubulin, which are anchored to the aforementioned junctions); the extracellular matrix. Each of these structures should be seen as being very labile and easily modified. Of course, they can also be torn if the mechanical forces become too strenuous.
We wanted to relate the mechanical forces to processes that modify these structures and to bring an integrated physical, cellular and molecular understanding of such modifications. We mainly relied on a very simple organism to do so, the nematode C. elegans, which has very few cells. The nematode embryo extends its length by a factor of four over a period of less than three hours and we wanted to understand how. Our main results can be understood as follows.
1/ The initial elongation requires the force to be directional (physicists say anisotropic). This anisotropy is achieved by having contractile cells (which shrink) next to more rigid cells containing stiff actin polymers along the embryo circumference. In addition, we found that the outer extracellular matrix distributes the tension generated at local positions. One can think of snake squeezing its prey.
2/ A second stage of deformation next requires a mechanical cooperation between two tissues within the embryo, its muscles and the external epithelium (the skin). We found that the constant muscle micro-contractions deform the embryo through a ratchet mechanism, whereby cell-cell junctions get a bit longer at each contraction while a locking system exerted on the actin cytoskeleton prevents tissue elasticity to bring the cell back to each initial shape.
3/ We identified a protein that acts as a molecular switch: upon sensing tension at the level of junctions it interacts with several other proteins that will exert their effects on the cytoskeleton and on junctions. One of them is a complex that normally produces forces, except that in this case it appears to consolidate the junction without its force component.
4/ We demonstrated that some tiny membrane invaginations called caveolae act as a mechanical buffer at the cell membrane to help replenish junctions with the extracellular matrix when tension is exerted on human cells. This requires some of the same proteins used in nematodes.
5/ To achieve these results, we used advanced imaging techniques such as lightsheet microscopy, evanescent wave microscopy, nano-scissors and we treated cells like special material exposed to external and internal stress.
Overall with this project, we have accounted on how a complex tissue made of different cell types deforms under the influence of mechanical forces and we have advanced our biophysical and cellular understanding of how they do so.