Engineering genetic reporters for in vivo photoacoustic imaging of mammalian tissues

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After gaining much understand about how individual cells work, biologists are moving their attention to understand how many different cells work together in tissues and organs. Imaging cells functioning in a living animal gives us tremendous insight into how these complex systems work. This is particularly true if the imaging not only tells us where a particular cell is but also gives us some information about what is going on inside it over time. With this grant, we plan to use a new technique called photoacoustic imaging along with genetic engineering to so we can image particular cells functioning within a living animal. Currently, we can image clumps of cells in small animals using imaging techniques used in medicine. Magnetic resonance imaging (MRI) and computed tomography (CT) can produce detailed imaging of a whole small animal. However, it is difficult to identify rare cells we are particular interested in. On the other scale, researchers can use microscopes to observe cells in living animals. Considerable information has been gained by this kind of work. Although allowing very detailed images, this kind of imaging can only observe a small portion of tissue at a time. Further, very often surgery must be performed on the animal to allow the piece of tissue of interest to fit into the microscope apparatus. What we really need is something which can image at resolutions between microscopy and whole animal imaging so we can observe a whole section of tissue function in a non-invasive way. We are currently working on a technique called 'photoacoustic imaging' which can meet these requirements. This non-invasive technique is a unique mixture of optical imaging and ultrasound. We first shine laser light into animal tissue. This light is not powerful enough to cause any damage, but it causes tiny shock waves within the tissues whenever the laser hits something which absorbs light. For instance, small blood vessel packed full of red blood cells absorb light very well. These small shock waves are very similar to the signals received by Ultrasound imagers, and we use an ultrasound receiver to generate 3D images of a section of tissue in a living small animal. The scale of the imaging is precisely what we desire to study complex tissues in action. While we can see small blood vessels clearly with this technique, up until recently we have had to inject chemical dyes into the animals to try and label certain cell types. This is difficult to do as the dyes do not always go just to the cells of interest, and in a living animal the dyes are broken down. We have thought of the idea of genetically engineering cells we are interested in studying so that they make their own dyes. We have taken two genes from coral and introduced them into bacteria. These genes instruct the cells to make a blue pigment allowing us to image these particular cells. We have performed many experiments to show that these blue proteins should be ideal for photoacoustic imaging, particularly since the red light they absorb can penetrate deeply into tissues and is not absorbed so much by red blood cells. Before this can become practically useful, a lot of work needs to be done to modify these coral proteins and establish exactly how they behave when imaged. For example, these coral proteins have to be engineered so that they can be more easily made by mammalian cells and not aggregate or clump. Next, again by engineering, we can alter the colour of these proteins potentially allowing us to image many different cell types at the same time. Next, we can alter these proteins to act as sensors, so they only have colour when a particular event is happening within the cell. Finally, we need to perform a lot of work with cells expressing these proteins in small animals to establish how best to use them. These new proteins will greatly increase the usefulness of photoacoustic imaging and will help us understand how complex collections of cells work together.