“The ability to make aligned muscle in a lab setting means that we can develop model tissues for understanding muscle in healthy and diseased states and for developing and testing new therapies that combat muscle injury or disease,” says senior author Ritu Raman, an MIT engineer. A better understanding of the rules that govern muscle growth could also have applications in robotics, she adds.

In a previous investigation, Raman and colleagues found that “exercising” muscle fibers by making them contract in response to electrical stimulation for 30 minutes a day over the course of 10 days made the fibers stronger. This time, the researchers wanted to explore whether mechanically stimulating the muscle fibers over the same time frame (rather than letting them respond on their own) would have the same result. To investigate, they developed a method to mechanically stimulate muscle tissue that differs from typical lab techniques.

“Generally, when people want to mechanically stimulate tissues in a lab environment, they grasp the tissue at both ends and move it back and forth, stretching and compressing the whole tissue,” said Raman. “But this doesn’t really mimic how cells talk to each other in our bodies. We wanted to spatially control the forces between cells within a tissue, matching native systems.”

To stimulate the muscle cells in a more true-to-life way, Raman and her team grew cells in a Petri dish on a soft gel that contained magnetic particles. When they would move a magnet back and forth under the gel, the particles moved back and forth, too, which “flexed” the cells. The researchers could precisely control the way the gel moved, and, in turn, the magnitude and direction of the forces the cells within experienced, by changing the strength and orientation of the magnet. To measure the alignment of the muscle fibers within the tissues and whether they contracted in synchrony, the team’s collaborators at Boston University developed a custom software that automatically tracked videos of the muscle and generated graphs of its movement.

“We were very surprised by the findings of our study,” said Raman. While mechanically stimulating the muscle fibers over the 10-day period did not seem to make them any stronger, it did cause them all to grow in the same direction.

“Furthermore, we were excited to find that, when we triggered muscle contraction, aligned muscle was beating synchronously, whereas non-aligned muscle was not beating rhythmically,” said Raman. “This confirmed our understanding that the form and function of muscle are intrinsically connected, and that controlling form can help us control function.”

Raman and colleagues plan to take the study further by investigating how different mechanical stimulation regimens impact both healthy and diseased muscle fibers. Additionally, they plan to study how mechanical stimulation affects other types of cells.

“Our work clearly showed that cord formation is important for infection and why this highly ordered architecture might be important for pathogenesis,” says senior author Vivek Thacker, who led the work at the Global Health Institute at École Polytechnique Fédérale de Lausannen (EPFL) in Switzerland and is now based at the Department of Infectious Diseases at Heidelberg University in Germany.

The study used a unique combination of technologies to address the role of MTB cord formation. One was a lung-on-chip model, which allowed the researchers to get a direct look at “first contact” between MTB and host cells at the air-liquid interface in the lungs. This revealed that cord formation is prominent in early infection. The researchers also used a mouse model that develops pathologies mimicking human tuberculosis, allowing them to obtain tissue that could be studied using confocal imaging and confirming that cording also occurs early in infection in vivo.

The work yielded several new findings about how these cords interact with and compress the cell nucleus, how this compression affects the immune system and connections between host cells and epithelial cells, and how cord formation affects the alveoli in the lungs. The study also revealed how these cords retain their structural integrity and how they increase tolerance to antibiotic therapy.

“There is an increasing understanding that these mechanical forces influence cellular behavior and responses, but this aspect has been overlooked since traditional cell culture models do not recapitulate the mechanical environment of a tissue,” says Melanie Hannebelle, formerly at EPFL’s Global Health Institute and now at Stanford University. “Understanding how forces at the cellular and tissue level or crowding at the molecular level affects cell and tissue function is therefore important to develop a complete picture of how biosystems work.”

“By thinking of MTB in infection as aggregates and not single bacteria, we can imagine new interactions with host proteins for known effectors of MTB pathogenesis and a new paradigm in pathogenesis where forces from bacterial architectures affect host function,” says Thacker.

Future research will focus on understanding whether cord formation enables new functionality to known effectors of MTB pathogenesis, many of which are located on the MTB cell wall. In addition, it will look at the consequence of tight-packing on the bacteria within the clump and how this may lead to a protective effect against antibiotics.

“Antibiotic therapy is the mainstay of treatment for tuberculosis infections, but therapeutic regimens are long and complicated, with an increasing threat of drug resistance,” says Richa Mishra, the other first author who is currently at EPFL’s Global Health Institute. “There is a recognized need for host-directed therapies or therapies that inhibit specific virulence mechanisms that can shorten and improve antibiotic therapy.”

The discovery of the burst, named FRB 20220610A, was made in June last year by the ASKAP radio telescope in Australia [1] and it smashed the team’s previous distance record by 50 percent.

“Using ASKAP’s array of dishes, we were able to determine precisely where the burst came from,” says Stuart Ryder, an astronomer from Macquarie University in Australia and the co-lead author of the study published today in Science. “Then we used [ESO’s VLT] in Chile to search for the source galaxy, [2] finding it to be older and further away than any other FRB source found to date and likely within a small group of merging galaxies.”

The discovery confirms that FRBs can be used to measure the ‘missing’ matter between galaxies, providing a new way to ‘weigh’ the Universe.

Current methods of estimating the mass of the Universe are giving conflicting answers and challenging the standard model of cosmology. “If we count up the amount of normal matter in the Universe — the atoms that we are all made of — we find that more than half of what should be there today is missing,” says Ryan Shannon, a professor at the Swinburne University of Technology in Australia, who also co-led the study. “We think that the missing matter is hiding in the space between galaxies, but it may just be so hot and diffuse that it’s impossible to see using normal techniques.”

“Fast radio bursts sense this ionised material. Even in space that is nearly perfectly empty they can ‘see’ all the electrons, and that allows us to measure how much stuff is between the galaxies,” Shannon says.

Finding distant FRBs is key to accurately measuring the Universe’s missing matter, as shown by the late Australian astronomer Jean-Pierre (‘J-P’) Macquart in 2020. “J-P showed that the further away a fast radio burst is, the more diffuse gas it reveals between the galaxies. This is now known as the Macquart relation. Some recent fast radio bursts appeared to break this relationship. Our measurements confirm the Macquart relation holds out to beyond half the known Universe,” says Ryder.

“While we still don’t know what causes these massive bursts of energy, the paper confirms that fast radio bursts are common events in the cosmos and that we will be able to use them to detect matter between galaxies, and better understand the structure of the Universe,” says Shannon.

The result represents the limit of what is achievable with telescopes today, although astronomers will soon have the tools to detect even older and more distant bursts, pin down their source galaxies and measure the Universe’s missing matter. The international Square Kilometre Array Observatory is currently building two radio telescopes in South Africa and Australia that will be capable of finding thousands of FRBs, including very distant ones that cannot be detected with current facilities. ESO’s Extremely Large Telescope, a 39-metre telescope under construction in the Chilean Atacama Desert, will be one of the few telescopes able to study the source galaxies of bursts even further away than FRB 20220610A.

Notes

[1] The ASKAP telescope is owned and operated by CSIRO, Australia’s national science agency, on Wajarri Yamaji Country in Western Australia.

[2] The team used data obtained with the FOcal Reducer and low dispersion Spectrograph 2 (FORS2), the X-shooter and the High Acuity Wide-field K-band Imager (HAWK-I) instruments on ESO’s VLT. Data from the Keck Observatory in Hawai’i, US, was also used in the study.