Mechanotransductie in hersencellen
Oscar Stassen & Elly Hol, Nederlands Instituut voor Neurowetenschappen

Op alle cellen in een organisme worden mechanische krachten uitgeoefend. Deze worden via een ingenieuze verbinding van membraaneiwitten, cytoskeleteiwitten en signaaleiwitten vertaald in een moleculaire response, een proces dat mechanotransductie heet. Astrocyten zijn hersencellen, die sterk reageren op hersenschade, o.a. geďnduceerd door een mechanische stimulus. In ons onderzoek bestuderen we astrocyten, die blootgesteld worden aan mechanisch letsel, als model voor traumatisch hersenletsel. We analyseren de reactie van de cel met behulp van een transcriptoomanalyse om te onderzoeken hoe het cytoskelet betrokken is bij het verwerken van mechanische krachten. Onze eerste data laten zien dat een kracht uitgeoefend op astrocyten resulteert in de activering van snelle reactiegenen, zoals c-FOS. De komende maanden zullen we een beeld krijgen van de transcriptieveranderingen van het volledige genoom. De analyse zal een belangrijke bijdrage leveren aan ons begrip van mechanotransductie en hoe een (hersen)cel omgaat met een kracht toegediend vanuit de omgeving.

Figure: Links een fragment van een transcriptoomanalyse van 25.000 genen. De intensiteit van een spotje in groen of rood geeft de verandering van hoeveelheid transcript van een gen na behandeling. Rechts een astrocyt met zijn cytoskelet in groen en de celkern in rood.


Fiber-top ECMscope: make room for a new instrument in cell mechanics
Tomek Watering & Davide Iannuzzi, Vrije Universiteit Amsterdam

The goal of our program is to elucidate the mechanisms with which cells react to external mechanical stimuli and exert forces on the surrounding environment. As part of this effort, the collaboration is developing new tools that could provide information on the behavior of individual cells embedded in their natural environment: the extracellular matrix.
The most natural way to apply or measure a force is by touching the object of interest with a force sensor. However, a cell in an extracellular matrix is a microscopic object immersed in a jungle of small filaments. In order to reach the cell and touch it, one should fabricate a force sensor on the tip of an extremely thin wire. To further complicate things, the experiment takes place in electrically conductive liquids: electronic force sensors are not an option.
To solve this issue, our group has recently designed and tested the first fiber-top ECMscope. The idea is to fabricate a flexural spring (a miniaturized diving board called “cantilever”) on the tip of a 15 microns diameter optical fiber. When the end of the flexural spring touches an object, the spring deflects. Light coupled from the opposite end of the fiber allows one to measure, from remote position, the amplitude of the deflection, which, in turn, provides information on the force exerted by and on the spring. The diameter of the sensor is sufficiently small to permit the sensor to reach and touch a cell lying inside the first few layers of the extracellular matrix.

Figure: In the figure, we report the scanning electron microscope image of a fiber-top ECMscope. Its working principle has been tested in air and water by touching the force sensor with a sharp needle and looking at the mechanical response of the spring from the opposite side of the fiber (see figure). The force sensitivity is estimated to be in the order of 1 nN over a 35 kHz bandwidth. Work is under way to reduce the spring constant and soon reach the sub-nN regime. The sensor will then be tested on cells and used to answer the questions of our research program.


Mechanosensing and mechanotransduction of cells.

Many cellular reactions are controlled or mediated by mechanical forces. Cells probe the mechanical properties of their environment and subsequently transduce this information accurately into a specific molecular response: mechanical cues can determine the fate of stem cells, modulate the function of entire tissues and play a key role in various pathologies. Cells also alter their motility and metabolic functions depending on the mechanics of their surroundings. Strikingly, cells are not passive observers of the mechanical properties – many cells actively manipulate their surroundings either by the generation of new extracellular or pericellular materials or, even by exerting forces on the outside world. The physical mechanisms by which cells sense tissue rigidity, respond to it, and apply forces themselves are poorly understood and has been little studied by physicists. We are aiming to elucidate fundamental principles of force production and mechanosensing of cells.