DNA is the blueprint for life: the genetic code for every protein of each cell is contained in units of DNA called genes. We have the same DNA in all of our cells yet brain, liver and muscle cells look and behave differently. This is because some genes are turned on in liver cells but switched off in brain and muscle cells and others switched off in liver and muscle cells but turned on in brain cells. These molecular switches are called transcription factors and studying what signals activate them in a living organism helps us understand how the system works and why things go wrong. These studies have been difficult to conduct in living organisms but using the tools of gene therapy we have developed a system to do just this (see figure below).
Inside each cell of every living organism is an intricate network of chemical pathways. At the start of these pathways are molecules which act as sensors which respond to cues from both inside and outside the cell. These messages are transported through chemical and protein signals to the nucleus and switches on or off certain genes which produce specific proteins that respond to changes sensed in the environment. The signaling pathways that govern which genes are switched on or off are ultimately controlled by proteins called transcription factors which directly bind DNA and initiate the production of specific proteins via an intermediary called RNA. An example of this in action is when one of our cells detects bacteria on its surface. Signaling pathways are triggered which ultimately result in transcription factors switching on several genes to make proteins that fight the bacteria.
Studying how these signaling pathways work, and how they sometimes don’t work, is one of the best ways in which biologists understanding how diseases are caused, and what sort of drugs could treat them. These pathways can be studied in cells in a culture dish or they can be studied in cells or tissue taken from patients. However neither of these is a particularly good representation of what happens in a whole body. Therefore mice are often used as good “model” organism. However, it still isn’t easy to study these pathways in living mice.
Based upon the tools that are used in gene therapy, we have been working on a new technology where we can transplant the firefly gene, luciferase, into newborn mice. The luciferase protein makes light as part of a chemical reaction when it is activated. We can select and design the transplanted gene so that it makes light when a specific signaling pathway is activated, and we can target the gene to a range of different organs so that reaction is only assessed in that particular organ. Just as with fireflies, the mice emit light, and the light is proportional to the activity of the pathway we have chosen. The light is normally too dim to be seen with the naked eye however it can be captured, and measured, using a highly sensitive camera. An example where we have used this is to study a mouse model of bacterial inflammation in the liver. The mice glowed more as the bacteria-sensing pathway was switched on and the glow subsided as the inflammation resolved. We recently published this in the journal Scientific Reports (Buckley et al., 2015).
This new technology will help scientists learn more about diseases and possible treatments. It also improves the welfare of animals in research, in line with the principles of the National Centre for Reduction, Refinement and Replacement (NC3Rs; http://www.nc3rs.org.uk). It becomes possible to make glowing mice only when required (rather than having to breed a colony of them as with some other technologies; Zinn et al., 2008). Rather than having to take repeated blood samples, or even culling the mice to analyse their tissues, it is sufficient simply to measure emitted light to measure pathway activity.
This blog article was co-authored by Dr. Tristan R. McKay (St. Georges, University of London) & Dr. Simon N. Waddington (University College London).
Buckley, S.M., Delhove, J.M. et al., (2015). In vivo bioimaging with tissue-specific transcription factor activated luciferase reporters. Scientific Reports. 5, 11842.
Zinn, K.R., Chaudhuri, T.R. et al., (2008). Noninvasive bioluminescence imaging in small animals. ILAR Journal. 49 (1), 103-115.