Heavy mouse enables hard-hitting research

Scientists from the University of Cambridge have created the world’s first animal enriched with heavy but non-radioactive isotopes. The ‘heavy’ mouse has enabled the scientists to capture in unprecedented detail the molecular structure of natural tissue by reading the magnetism inherent in the isotopes.

The team utilised nuclear magnetic resonance spectroscopy (NMR) - a technique which they said is “useful to determine molecular structure in tissues grown in vitro only if their fidelity, relative to native tissue, can be established”. NMR can read the magnetic nuclei found in certain isotopes, such as carbon-13 - but as carbon-13 makes up only 1% of the carbon in our bodies, the researchers sought to get the carbon of a mouse up to 20% carbon-13.

“We used mouse feed rich in carbon-13 and let the mouse eat as much as it liked,” said study leader Dr Melinda Duer. “No one had thought to do it before. Maybe everyone had assumed it wouldn’t work; I certainly got some odd looks from colleagues.

“Isotope-enriched proteins and cells are fairly commonplace now, but the leap to a whole animal is a big one.”

C57BL/6 mice, the common strain of laboratory mouse, in a Cambridge animal house. This breed of animal was used in the 'heavy mouse' study. Credit: University of Cambridge.

Using NMR analysis of the mouse tissue to map the distance between the carbon atoms and reveal atomic structures, researchers were able to create a ‘gold standard’ reference for growing tissue in the lab - a fingerprint of the atomic networks that are the basis of proteins in our biology.

The team shared this with scientists Rakesh Rajan and Dr Roger Brooks at the university’s Department of Surgery, who used the NMR spectra maps to refine cell cultures and produce lab-grown tissue that looks near identical to real tissue. When comparing microscopy images of the new lab-grown tissue with native tissue, Dr Duer says she could not find a biologist who could tell the difference.

Unlike ordinary tissue, the lab-grown tissue can be manipulated and analysed in ways impossible with natural samples. Furthermore, the scientists can conduct tissue analysis beyond the nanoscopic scale and into the atomic.

“We could see signals in the NMR data for our lab-grown tissue, extra intensities that - when matched with the heavy mouse data - revealed where proteins hadn’t folded up properly,” Dr Brooks said. This kind of ‘misfolding’ could result in host rejection if the tissue were to be implanted.

“Through a process of repeat NMR comparisons, we were able to modify the lab tissue until it looked near identical with NMR and under the microscope,” Dr Brooks continued.

In an early experiment, published in the journal Science, the scientists grew collagen tissue in the lab and discovered signs of molecules that they didn’t think should be there. They revealed that the molecule was poly(ADP ribose) (PAR), previously only thought to exist inside a cell for the purpose of repairing DNA. According to Dr Duer, “It was crazy to see PAR behaving in this way.

“Not only is PAR there, and leaving the cells entirely, but once it’s in the surrounding matrix it’s perfectly designed to start pulling together the calcium and phosphate that make up bone crystals,” Dr Duer continued.

This was happening at the exact same time the cells started laying down the organic matrix to house the mineral crystals that form bone, said Dr Duer. After conducting a staining test for PAR, the team found that the molecule works as an engine of bone production.

“The fact that we are already making such remarkable discoveries using the techniques that have been developed as a result of the heavy mouse is hugely exciting, and shows the enormous potential of this approach,” said Dr Duer.

“We’re now looking at blood vessels to see if lab-grown tissue could be used for replacement arteries and heart valves - and to see if we can find the molecules that trigger calcification of the arteries, as well as calcification of bone.”

The text in this work is licensed under a Creative Commons Licence (CC BY-NC-SA 3.0). The original article can be found here.