Biosensor mouse lights up the 'skeleton' within cells

Australian and UK scientists have developed a glow-in-the-dark biosensor mouse that gives a real-time readout of the rapidly changing ‘skeleton’ within cells in living tissues, enabling the researchers to watch cells respond dynamically to their surrounding environment.

“The biosensor mouse gives us unprecedented insight into how cells respond to the environment that surrounds them — both in normal tissues and in cancer,” said Associate Professor Paul Timpson of Sydney’s Garvan Institute of Medical Research, who co-led the study.

“Each of the cells in our body has its own tiny skeleton — and with the biosensor, we can see what that minuscule skeleton is doing inside individual cells in a living animal, and see how it responds and becomes addicted to its environment as diseases such as pancreatic cancer progress.”

The mouse contains a fluorescent version of a protein called RhoA, which controls the shape of the cell’s skeleton. RhoA is thought be particularly important in ‘mechanosensing’, the process by which cells remodel their own skeleton in response to the softness or rigidity of their surrounding environment. For this reason, RhoA and its signalling pathway within cells is becoming an attractive new drug target in cancer.

“We’re particularly interested in RhoA because it gives us a readout of how cells reorganise themselves as their environment alters, and we know that the cell’s immediate environs are crucial in cancer, where a ‘stiffer’ tumour environment promotes growth and tumour spreading,” said study co-leader Professor Kurt Anderson, from the Francis Crick Institute, UK.

“Biosensors such as this one also give us the ability to sense where and when drugs hit their targets, which is important for discovering new drugs.

“We also know that forces acting on the cell are crucial in normal tissue such as bone, where cells called osteocytes sense and respond to weight-bearing exercise.”

RhoA oscillates rapidly between two states — active and inactive — in response to chemical and physical cues in the environment. When RhoA is active the biosensor glows blue-green; when inactive, the biosensor glows red.

Using sophisticated intravital imaging technology, the researchers can watch where RhoA is, and measure how its activity changes, in any organ in the body. They have already used the biosensor mouse to explore the progression of breast cancer and pancreatic cancer in detail.

“We’ve been able to watch RhoA through the entire process of pancreatic cancer — from early disease, right through to fully developed cancer and then to metastasis, when the cancer spreads beyond the pancreas to the liver. And what we see is that RhoA switches off and on depending on the organ or stage of disease — this knowledge could be key in helping to guide the timing of new drug schedules and interventions for this aggressive disease,” Associate Professor Timpson said.

“We’ve also visualised the whole cycle of mammary development and watched how cells change during the progression of breast cancer.”

The researchers went on to show that the activity of anticancer drugs could be readily assessed in the biosensor mouse, allowing the team to monitor drug activity in cancers in living tissue.

“And the readout from the mouse is readily interpretable,” said Associate Professor Timpson. “It’s almost like a traffic light — the biosensor molecule is one colour when the cancer’s been effectively attacked, and another if it hasn’t.”

Beyond the cancer findings, the research team investigated the activity of RhoA in a range of healthy tissues — including bone and the immune system. According to Associate Professor Timpson, this enables the researchers to see things that “have never been visualised before — deep inside the cells in living bone”.

“We’ve been able to watch RhoA activity in the small protrusions that extend out from osteocytes (a type of bone cell), and to see what happens when a force is applied — just as it would be during weight-bearing exercise,” he continued.

“Remarkably, we can see clear colour differences between protrusions that extend in different directions — so we think that the protrusions, through RhoA, are giving the cell feedback on which direction a force is coming from. We think that, in the future, the biosensor mouse could aid our understanding of how weight-bearing exercise could impact on osteoporosis and other bone disorders.”

Associate Professor Timpson noted that the new biosensor mouse, and others created by his and Professor Anderson’s labs, is available to researchers worldwide. Their open-access study is also available in the journal Cell Reports.

“There are already a number of groups worldwide that are driving new understanding of a range of cancers through these biosensor mice — and we’d love to work with others to advance understanding and, ultimately, to uncover new therapeutic approaches to cancer,” Associate Professor Timpson said.

Image caption: A glow-in-the-dark breast tumour (green, blue and yellow) in living tissue, fed by blood vessels (red) and surrounded by dense collagen matrix (purple).