Chemotaxis describes how cells move towards favourable chemicals (chemoattractants) and away from unfavourable chemicals (chemorepellants). This process requires chemicals outside the cell to bind to proteins on the cell surface called receptors. Different receptors are specific for different chemicals. There are hundreds of receptor types found on cells, and varying cell types rely of different receptors. When a chemical binds to its receptor, this “signal” launches a series of biochemical reactions with the cell. The end result of these reactions is usually some sort of action, such as movement towards or away from a certain chemical.
In the presence of a chemical gradient, bacteria will direct their overall motion based on the gradient. There are two strategies a cell can use to move accordingly: temporal sensing or spatial sensing. When a cell uses temporal sensing it compares the intensity of receptor binding between different time points. For example, at the first time point, a cell may sense a low amount of chemoattractant. The cell will then move a certain distance, and at a second time point it will measure if there is an increase or decrease in the amount of chemoattractant. If there is an increase, then the cell determines it moved in the correct direction. The cell will continue to measure receptor binding at various time points until it reaches the highest amount of chemoattractant. If at any point there is a decrease in the amount of chemoattractant, the cell will change its direction with the help of whip-like structures known as the flagella. In comparison, a cell using spatial sensing senses its external environment by simultaneously measuring the intensity of receptor binding at both ends of the cell. For example, if the receptors at one end of the cell sense a chemorepellant, an internal biochemical reaction will be activated, and the cell will move away from that end.
Scientists previously believed that cell size determined sensing strategy; with larger cells using spatial sensing and smaller cells using temporal sensing. Using computational modeling, however, Tan et al. demonstrated that selecting a sensing strategy is not purely about size. They found the most important factors that determined sensing strategy were the ratio of cell speed to cell diameter, and rate of cellular signaling. This means that a small cell moving quickly will use temporal sensing, whereas a large cell moving slowly will use spatial sensing. Tan et al. have provided a means to predict sensing of a variety of cells, such as small cells that move slowly, and large cells that move quickly.
Bacteria and human immune cells alike rely on chemotaxis to respond to cues in their environment. Some bacteria can move towards nutrients and away from anti-bacterial drugs or toxins. By contrast, immune cells use specific chemicals to lead them to sites of inflammation and infection. For this reason, certain diseases result when chemotaxis is disrupted in cells of the immune system. Our understanding of chemotaxis has certainly improved since 1884 when the phenomenon was first described, but there are still many unanswered questions. The model by Tan et al. will play an important role in predicting the movement of a variety of cell types, which will help researchers to study disease and potential therapeutics.
Summary written by: Emma Finlayson-Trick
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A computational model for how cells choose temporal or spatial sensing during chemotaxis