Joachim Spatz: How wounds heal

Speaker: Joachim Spatz
Department:
Casimir Research School
Location: Delft
Date: September 8, 2017Transmission electron microscopy image of a biomimetic nuclear pore complex. Stefan Kowalczyk

Subject: Mechanotransduction in Collective Cell Migration and its Synthetic Mimic

Wound healing is a very important part of the medical sciences, so it is not very surprising that scientists have been studying it for a long time. Some 20 years ago, when looking at the way cells in the dermis – a specific layer of the human skin – closes a wound, they noticed a peculiar pattern. Although there is a lot of movement and fluctuations in this layer, some ‘leader cells’ will inevitably arise, evenly spaced and venturing into the void of the wound. These would be followed by many other cells and thus does healing begin.

 

In his talk, Joachim Spatz explained how he discovered the mechanism behind electing these leader cells by means of traction force microscopy. Essentially, he prepared a glass plate, covered with an elastic hydrogel containing a matrix of fluorescent beads to which cells adhere. Following their movement, one can measure the forces these cells exert on each other. The cells themselves were grown in a rectangular stencil on top of this gel. When he removed this stencil, he essentially created a wound, a void for the skin to grow into.

They made two important discoveries. Firstly, the cells are constantly moving around and fluctuating with respect to density and tension forces. They saw areas with high and low forces. This led them to measure the ‘force correlation length’. It’s a distance of about a 160 μm over which the beads are pulled in the same general direction and with about the same force. Secondly, just before a leader cell becomes visually apparent, there is an area of high forces just behind it. This means it is not a single cell’s autonomous decision to become a leader, but rather a consequence of the general population dynamics.

Along a long stretch of skin boundary, multiple leaders will emerge, and the spacing between them is always the same. This is explained by the fixed force correlation length. Next to the area of high tension behind the leader cell, there has got to be an area with low tension in which a leader cannot form. This means that the number of leader cells to form, important for the efficiency of the healing process, does not need to be actively regulated but is a consequence from the apparent chaos of the cells’ movements.

The second part of his seminar treated the creation of a synthetic cell. A subject that is being actively researched in Delft as well. It reminds me of Feynman’s famous quote: “What I cannot create, I do not understand”. The idea being, of course, that we’ll never completely understand how a cell works, until we can create it ourselves. Spatz’s approach was based on forming tiny water droplets in an oil solution. Since the two don’t mix such a water droplet can be the foundation for a synthetic cell. He then injected a mixture of hydrophilic and hydrophobic proteins into the droplet, which formed a membrane, further separating the cell from its environment.

Of course a normal cell has a membrane made of two layers of phospholipids, so he had to find a way to get these and other molecules into the cell. He ended up using an electrode to make the protein membrane porous allowing him to inject whatever he fancied. To make the lipid membrane, he injected a number of smaller vesicles, small membranes of lipid bilayers, and positive ions to destabilize them and fuse them together until they were big enough to form a membrane for the whole cell. FRAP experiments confirmed that these were similar to those of biological cells! Basically, they made the lipids fluorescent so they could see them under a microscope. Then they bleached an area by shining a very bright light on it. A property of biological membranes is that they are ‘liquid’, meaning that the individual phospholipid molecules are constantly moving around. This means that the extinguished phospholipids should diffuse into the greater, non-bleached, area and that other ones (still fluorescent) should be able to take their place. So the bleached area should disappear after some time. This was new to me and I find it very interesting.

But what is the link with the first part of his talk? Well, Spatz was also able to inject myosin and actin into the cell. These are essential to the movement of a cell and can be found in high concentrations in muscle cells, for example. When the cells floated up in the oil, they hit the glass plate and adhered to it. It meant that the cell suddenly had a fixed point of contact and that allowed the myosin to exert forces on the plate, which made the cell rotate. This can be seen through a microscope. If you also fix a little bead to the outside of this membrane, it can stop the rotation when it hits the glass. That is, until a sufficient force is generated to make the cell ‘jump’ over the bead, thereby actually moving the cell instead of rotating in place.

In conclusion, these artificial cells aren’t nearly as complex as their biological counterparts, but the technology is advancing and so does their complexity and capabilities. Joachim Spatz already succeeded in extracting them from the oil into an aqueous medium and he is working on an artificial mitochondrion. Lots of interesting research opportunities. I’m excited to hear what more will be discovered, or to work on it myself.

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