Alexander Dunn and his colleagues look at how fibroblasts – cells that make up ‘connective tissue’ – generate force to deform their surrounding (3D) environment.
This kind of work might help us to better understand how metastatic cancer cells squeeze themselves through tiny gaps in tissues, or how tissues rearrange themselves in development. A molecular clutch mechanism is found to be the key in the process of matrix deformation and cell movement in 3 dimensions.
By plating human foreskin fibroblasts in fibrin gel and looking at matrix deformation using time-lapse confocal microscopy, they uncovered that a 3D cytoskeletal clutch system is used by cells to propel forward. To do that, cytoskeletal forces within cells were transmitted in the matrix around them through distinct cell adhesions along cell protrusions.
During processes such as development and cancer metastasis, cells deform and remodel the three-dimensional fibrous matrix in order to migrate forward. Although extensive research has been carried out to understand the process of force transduction into the matrix, most of this work has been performed with cells growing on continuous stiff 2D surfaces (such as glass, plastic). However, this does not mimic the environment that cells normally reside in – which is three-dimensional, fibrous and restraining. Previous studies have also shown differences in signaling and morphology in 2D or 3D environments but this is the first study that simultaneously investigates how both cytoskeletal and adhesion dynamics cooperate to deform matrices in 3D.
In this study, the fibroblasts in fibrin gel were studied using various microscopy techniques. Human fibroblasts are frequently found in wounds or tumours where they often modify the nearby matrix to migrate. It was observed that cells embedded in soft matrix generated large protrusions, pushing the matrix around them.
There were differences between how the cells behave in 3D compared to 2D environments. Traction stresses – areas of force generation, where the cell is pushing or pulling the matrix – became localised at spatially distinct area on the cell surface when cells were in soft matrices. In 2D environments traction stresses were restricted to cell edges. In 3D, protrusions were also present, composed of contractile actin bundles (also called stress fibres), anchored to distinct points in the matrix through large cellular adhesion complexes (focal adhesions), known to be intimately involved mechanical-chemical information transfer.
Such cell adhesions observed here often serve as linkages that transmit mechanical signals between the cell and the extracellular matrix. By tagging various parts of the adhesion, the team also discovered that there is a temporal coordination between the movements of actin, focal adhesions and the surrounding matrix: the molecular clutch exists in 3D.
This study provides a 3D molecular clutch mechanism by which cells transduce force to their surroundings when in a soft matrix, where force transduction is mediated by stress fibres and focal adhesions. Mechanical forces exerted by cells are extremely important for many processes, such as differentiation and migration and this study has shown that it is possible to study focal adhesion and cytoskeletal dynamics in a much more biologically relevant environment.
Paper can be found here
Written by Grace Chan