Speaker: Joachim Spatz
Department Max Planck institute for medical research department of cellular biophysics
Subject: mechanotransduction in collective cell migration and its synthetic mimic
Location: TU Delft (BN Seminar)
Author: Nemo Andrea
Joachim Spatz is a researcher from Germany who is working on a range of topics ranging from more traditional cell migration to microfluidics. We were informed that he was interested in the TU Delft’s BaSyC project. His talk today was a lecture in two parts with the first part focusing on cell migration and the latter on a new microfluidics-based approach to sequentially add components to a bilipid membrane.
Part 1 – Cell migration
The talk started by introducing the motivation for his current work on migration: collective movement. Collective movement, as seen on the macro scale in flocks of birds and fish, is also seen in biology. Retinal cells are the textbook example of a collective form of cell migration during the formation of the eye. The two behaviors (on the macro and microscale) are very much the same behavior and are worth exploring from a physics point of view. Elucidating the principles that lead to collective motion is therefore of strong interest.
To introduce his work on migration, we first had to be introduced to a few techniques, namely Traction Force Microscopy (TFM). This technique allows one to determine the forces exerted on the ECM by cells migrating on or through the material. From this, one can then also infer the stress between cells themselves (versus cells vs ECM). This can be done because the epithelial cells that they used exhibit a form of collective migration, where the cells are linked through various proteins. From these tractions and stresses, they were able to infer that each cell has a force correlation length that corresponds to about 10 cell lengths. This is then the maximum distance that cells can ‘’affect’’ other cells by exerting force/traction.
The behavior they were interested in is related to wound healing. They spatially constrained the cells in a rectangular cutout, which could then be removed after which the cells would spread. Interestingly, they observed that cells do not spread out homogenously (equal dilation everywhere), but rather that certain cells would move outwards first and that these cells would drag others with them. These cells, dubbed ‘’leader cells’’, and their dynamics where studied. They wondered what the rules where for this system and how this leader cell first emerges. To do this, they observed the stress and force exerted by cells before the leader cell appears/can be identified. They found that the mean traction in the regions where a leader cell will appear is significantly higher than in cells that will not produce a leader cell, and similarly for stress. They then wished to find the rules for the spacing of leader cells, as they observed the spacing between leader cells (along the cell boundary) had a minimum value. This was not just an artifact, as when the aritificially patterned the cells in a way where the leader cells were spaced closer together, they would return to the natural spacing distance.
They found that the spacing of the leader cells depended on the ECM stiffness (which affects the traction exerted by each cell). This was verified by adding factors that increase or decrease internal actomyosin contraction, which similarly affects cell traction. An increase in traction was accompanied by an increase in leader cell spacing. They postulate is because with greater traction a leader cell can affect larger number of cells due to increased force correlation length.
Part 2 – Microfluidics
In line with their work on migration, they wished to make bilipid membranes that had integrins in the membrane. Such vesicles could then adhere to the environment and could work as an artificial model for some aspects of cell migration. In their attempts to create these they ran into problems with the low mechanical strength and the low yield of the fabrication process. They then developed a new system: water droplets in oil with the water droplets being stabilized by a polymer shell. Through this strong polymer shell (which can be moved using standard microfluidics platforms) they can inject many different proteins using a series of picoinjectors. As there is no limit to how many picoinjects can be used, and each picoinjector is placed after the other, one has the advantage of being able to sequentially add elements to the vesicle. This is a massive advantage, as various systems will not properly self-assemble if they are all ‘’’thrown in at once’’. Not only does this allow for precise control, the system also had very high throughput with the picoinjectors being able to handle roughly 500 cells per second.
image taken from a presentation by Marian Weiss titled ‘‘Droplet-Based Microfluidics for Sequential Bottom-Up Assembly of Functional Cell-Like Compartments”
Joachim showed us some beautiful demonstrations, with three differently labeled fluorescent actin types being injected into a single cell, and even adding actin and myosin into a vesicle. This then allowed the myosin to contract the actin, making the vesicles active in a real sense. As they did this they observed that the active actin vesicles were slowly rotating. They were able to turn this rotation into motion across a line using another bead attached to the outside of the vesicle, making essentially a very crude form of a migrating cell. (although mechanically completely different from natural cell migration).
After this, they also added lipids to the vesicle, which, in the right concentration, allowed for the formation of a lipid bilayer in the polymer vesicle. This a remarkable feat, as making a membrane isn’t easy with traditional methods. Next to this, they also demonstrated ester formation and dynamics inside these vesicles in some truly breathtaking video fragments. Importantly, they also demonstrated that is was very easy to remove the polymer shell and just leave the bilipid membrane. To get back to their original goal, they managed to add integrins into this membrane using this technique. Depending on the concentration, they could modulate the extent to which cells adhered to fibrinogen. They are currently working on making artificial mitochondria, and were able to get ATPase and bacterial rhodopsin to work in this artificial context, which I consider to be a remarkable achievement.
I found this to be the most excited seminar I have attended to date. The first part was interesting to me because I knew a fair amount about cell migration due to my Honours Programme Project, and the second part was extremely fascinating due to the wonderful things that they were able to do. I am very excited for work on the BaSyC project of the TU, so seeing a new promising method like this makes me very, very excited about what is to come.