When you hear stress fibres most people tend to think of muscles, these two are similar in certain ways, they both expand and contract, however stress fibres are in fact found exclusivity in non- muscle cells. We know many things about these stress fibres, yet, their most important feature, the method and nature of their contraction is still relatively unknown.
When you hear stress fibres most people tend to think of muscles, these two are similar in certain ways, they both expand and contract, however stress fibres are in fact found exclusivity in non- muscle cells.
The job of stress fibres, and why they are of interest to us, is to enable cell movement. Cell movement is facilitated through the expansion and contraction of these fibres within the cell. Stress fibres are made up of dense bundles of microscopic filaments and it is the interactions between these filaments that generates the expansion/contraction we see in nature.
However, within stress fibres these structures are highly packed and cannot be tracked using microscopy. Despite this, we can use other methods to deduce many facts about the internal structure of these fibres. However, one topic that still remains relatively unknow is how these stress fibres are able to contract as we see in nature. There have been many theories but the one that we looked at in our research was “early motor drop-offs”, which we will explain later in more detail. The aim of the research was to determine, using simulations and mathematical models, what effect do “early motor drop-offs” have on contraction.
To begin we designed a simple 1D linear model of a stress fibre (see above). We have three key components in our model. Actin filaments, which make up the backbone of the stress fibre and have a barbed and pointed end (positive/negative polarity purely indicates that the filament is facing to the right/left). Myosin motors, which move along the filaments, pulling the filaments barbed ends towards themselves. And Focal adhesions, which are the end points of the stress fibre.
Normally, when the motors are interacting with the filaments, the motors will drop off and stop exerting force when they reach the barbed ends of the filaments. However in our model we wish to implement “early motor drop-offs” which means that instead of the motors dropping off at the barbed end, the motors will drop off some distance away from the barbed end, i.e. earlier.
To determine what effect this will have on contraction we run simulations of these fibres with and without “early motor drop-offs”. After many simulated fibres it was found that without these early motor drop offs the fibres on average would always end up expanding, which does not match the contraction seen in nature. However, by implementing drop-offs it was found that if the drop off distance was at least one quarter of a filament length away from the barbed end then the fibres would on average contract, with the maximum contraction being when the motors dropped off at the middle of the filaments. So, we have the contraction we wanted, success!
Keep in mind that this is just one of the many theorised methods, and every day we are learning more and more about these curious structures.
Timothy Ryall
University of Queensland
