New Process May Accelerate ALS Drug Development

A microfluidic device that replicates the neuromuscular junction has been developed my MIT engineers that may lead to new treatments for amyotrophic lateral sclerosis (ALS), and other neuromuscular-related conditions.

The device, which is about the size of a US quarter, contains a single muscle strip and a small set of motor neurons. The study’s findings were published in Science Advances.

“The neuromuscular junction is involved in a lot of very incapacitating, sometimes brutal and fatal disorders, for which a lot has yet to be discovered,” said lead researcher Sebastien Uzel. “The hope is, being able to form neuromuscular junctions in vitro will help us understand how certain diseases function.”

During the study, researchers genetically modified neurons in the device to respond to light. When light is shown directly on the neurons, they can precisely stimulate these cells, which ends up sending signals to excite the muscle fiber.

Furthermore, researchers measured the force the muscle exerts within the devices as it twitches or contracts in response.

Although scientists have been coming up with different ways to stimulate the neuromuscular junction in the lab since the 1970s, it is very different than the processes that occur in the body. Most of the experiments would involve growing muscle and nerve cells in petri dishes or on small glass substrates, environments that are vastly different from the human body.

“Think of a giraffe,” Uzel said. “Neurons that live in the spinal cord send axons across very large distances to connect with muscles in the leg.”

For the study, researchers wanted to recreate more realistic in vitro neuromuscular junctions. To do this, they fabricated a microfluidic device that has 2 key features: a 3-dimensional environment and compartments that separate muscles from nerves to mimic their natural separation in the body.

The muscle and neuron cells were then suspended in millimeter-sized compartments, which researchers filled with gel in order to replicate a 3-dimensional environment. Next, researchers used muscle precursor cells that came from mice, and differentiated them into muscle cells to grow the needed muscle fiber.

The cells were injected into the microfluidic compartment, and the cells began to grow and fuse to form a single muscle strip. Researchers also differentiated motor neurons from a cluster of stem cells, and placed the resulting aggregate of neural cells in the second compartment.

Before differentiating both cell types, researchers genetically modified the neural cells to respond to light by using the technique optogenetics.

“(Light) gives you pinpoint control of what cells you want to activate,” said study co-author Roger Kamm.

This is opposed to using electrodes, which can inadvertently stimulate cells other than the targeted neural cells when they are in confined spaces. The last feature that was added to the device was force-sensing.

Researchers fabricated 2 tiny, flexible pillars within the muscle cell compartment, where the growing muscle fiber can wrap so they could measure muscle contraction. As the muscle contracts, the pillars squeeze together, creating a displacement that researchers can measure and convert to mechanical force.

Once the device was being tested, researchers found that the neurons extended axons towards the muscle fiber within the 3-dimensional region. Once the axon made the connection, researchers stimulated the neuron with a tiny burst of blue light and instantly observed a muscle contraction.

“You flash a light, you get a twitch,” Kamm said.

The findings suggest that the microfluidic device may serve as a testing ground for drugs to treat neuromuscular disorders.

“You could potentially take pluripotent cells from an ALS patient, differentiate them into muscle and nerve cells, and make the whole system for that particular patient,” Kamm said. “Then you could replicate it as many times as you want, and try different drugs or combinations of therapies to see which is most effective in improving the connection between nerves and muscles.”

Authors noted that another way the device may be useful is in modeling exercise protocols. For example, when the muscle fibers are stimulated at various frequencies, scientists can study how repeated stress affects muscle performance.

“Now with all these new microfluidic approaches people are developing, you can start to model more complex systems with neurons and muscles,” Kamm said. “The neuromuscular junction is another unit people can now incorporate into those testing modalities.”