How do cells know where they are? It’s an important question we often take for granted, especially in situations like wound healing where cells that are closer to the wound need to respond differently than cells that are farther away. If cells don’t respond correctly it can lead to chronic wounds (wounds that don’t heal), excessive scarring, and even cancer. Here I highlight a study recently published in Currently Biology that demonstrates how hemocytes* find their way to a wound. The Paul Martin Lab at the University of Bristol and the Michael P.H. Stumpf** group at the Imperial College London worked together using a combination of in vivo (meaning, experiments done in a living organism) wound data and computational simulations to answer the question of how hemocytes know which direction to travel following an injury in Drosophila (fly)*** embryos.
One way that cells figure out where they are is by using information from the wounded cells themselves. Injured cells release DAMPs (damage associated molecular patterns) which tell the neighboring healthy cells how far they are from the wound. To illustrate how DAMPs work, imagine you are sitting in the stands of the Shamu show (or what used to be the Shamu show, at least) at SeaWorld and you want to figure out how close you are to the tank. It should be easy enough, but here’s the catch: you can’t open your eyes and you can’t move. One way you could figure out how close you are is by measuring how wet you get when Shamu splashes the audience. If you are close to the tank, you will get soaked. If you are farther from the tank, you will stay dry. DAMPs work in a similar way. Cells that are closer to the wound receive a lot of DAMPs, and cells that are further away receive less. However, scientists still aren’t sure what exactly these DAMPs are and how they travel across cells to activate them to initiate a healing response. Here, Martin and Stumpf determine how DAMPs lead hemocytes to an injury.
Following an injury, hemocytes travel to the site of the wound to begin the healing process by following a gradient of DAMPs. However, the specific identity of these DAMPs and how they travel remains unknown. Since the technology to physically measure gradients formed by DAMPs is yet to be fully developed, the Martin and Stumpf groups developed a mathematical model to predict how the DAMP gradient forms based on how hemocytes traveled to a wound following an injury. They began by injuring flies and quantifying how hemocytes moved towards the wound. Quantifying hemocyte movement involved several parameters including how long it took the hemocyte to reach the wound, when the hemocyte began to move after the wound, and how many times the hemocyte changed directions while moving towards the wound.
The results of their experimental data combined with the computational model show 3 main points that differ from the current theory of DAMPs.
- The Martin and Stumpf labs found that DAMPs are released from the cells immediately surrounding the wound. This differs from prior predictions that DAMPs are released from the wounded cells themselves. The current theory of DAMP release is kind of like a water balloon that explodes when punctured. All of the water comes and gets anything in close proximity wet.
- The model revealed that there is only a single “wave” of DAMPS that lasts for only 30 minutes (as opposed to multiple releases over an extended period of time). This is significant when you think about how long a wound takes to heal-days and even weeks! That’s like taking a 2-week long road relying only on directions given to you in the first 30 minutes.
- DAMPs that recruit hemocytes to a wound travel by diffusion. That is, the healthy cells surrounding the wound do not use any sort of active mechanism to ensure that the DAMPs travel to the appropriate distance.
The Martin and Stumpf groups were also able to use their model to determine how hemocyte recruitment changes with multiple wounds, closely spaced wounds, and large wounds.
Multiple Wounds. For multiple wounds, they found that hemocytes become desensitized, or no longer respond, if the wound takes place less than 3 hours following the first wound. This makes sense when you think about how an organism needs to efficiently allocate its resources. If all of the hemocytes went to the new wound, then the old wound would not be able to continue healing. However, this might imply decreased healing for wounds occurring close to the first wound within a short time period. They also examined how hemocytes travel to wounds that are varying distances from each other.
Closely Spaced Wounds. Wounds that are very close together are essentially seen as “one wound” by hemocytes whereas wounds that are far apart are seen as two separate wounds. However, wounds at an intermediary distance displayed an interesting hemocyte behavior. Here, hemocytes between the wounds struggled to decide which wound to travel to, resulting in a decreased number of hemocytes at both wounds. It would be interesting to study whether this behavior causes wounds located at intermediary distances to heal less efficiently, but as of yet, no one has.
Large Wounds. The Martin and Stumpf groups studied the effects of very large wounds on hemocyte recruitment. Interestingly, they found that some large wounds healed appropriately while others did not. This is similar to clinical observations where chronic wounds cannot always be determined at the initial stage of wounding (meaning, you can’t tell if a wound is going to be chronic based simply by looking at it). This is the first time that a chronic wound model has been shown and will be of interest to future scientists to study what factors impact whether a wound will be chronic or not.
So what does this mean in the broader world of science??
One of the most interesting thing that the Martin and Stumpf accomplished here was their ability to develop a mathematical model to predict the formation of the DAMP gradient. Mathematical models, although used heavily in other sciences such as physics and chemistry, have been slow to be adopted in biology. Biology can get messy, and therefore mathematical models can be difficult to implement. However, when technology to directly observe biological phenomena is still lacking, mathematical models can be a powerful tool. Although their model cannot tell you what the exact DAMPs released from the wound are (that will require more experiments), it provides a lot of clues to where researchers should be looking. This is a great example of the power of quantitative biology to solve biological problems.
*Hemocytes is the term for blood cells in invertebrates. In this case the invertebrate is a fly. Hemocytes are a part of the immune system and are required in the wound healing process.
**Weavers et al., Systems Analysis of the Dynamic Inflammatory Response to Tissue Damage Reveals Spatiotemporal Properties of the Wound Attractant Gradient, Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.06.012
***Why flies? The use of Drosphila embryos as a model is common in wound studies. They are transparent, making them easy to image under microscopes. Furthermore, flies are easy to genetically alter and, as far as immune response goes, work similarly to humans.