The creation of a new microdevice that allows generation of three-dimensional (3-D) axonal structures from human stem cell-derived neurons should facilitate future studies on axon development and allow drug screening for diseases including amyotrophic lateral sclerosis (ALS), a new study has found.

A collaboration between Japanese and American researchers at the University of Tokyo and Harvard University led to the development of the new microdevice, with the study being reported in the Oct. 26, 2017, edition of Stem Cell Reports.

Axons are the cellular structures through which neurons transmit information to other cells. In the body, axons assemble to form small bundles known as fascicles.

"Axons form fascicles in many different areas of the body, including in the motor and sensory nerves, and in the central nervous system," study leader Yoshiho Ikeuchi, a lecturer at the Institute of Industrial Science at the University of Tokyo, told BioWorld.

"Axons or axonal fascicles transmit action potentials to operate our bodies," he said. "In the present study, we focused on motor nerves, which transmit electrical signals from the spinal cord to the skeletal muscle."

Several technologies allow scientists to generate and study single axons in the laboratory, but no techniques have so far succeeded in creating nerve fascicles.

The newly developed microdevice is expected to provide important insights into brain development and disease management.

"We know that growing axons form fascicles, but we do not know how fascicles form," said Ikeuchi, noting that while many scientists have examined axon development and degeneration in two-dimensional systems, it is increasingly clear that the fascicle's 3-D structure plays an essential role in axonal function.

Because fascicles are disrupted in many neurodegenerative diseases such as ALS, the researchers theorized that understanding their formation could give clues on the prevention of ALS and other diseases.

"Besides ALS, this new technology should prove useful in the treatment of other motor neuron diseases and perhaps even aging itself," said Ikeuchi. "Hopefully, in the future we would like to explore the possibility of using the axon fascicles in regenerative medicine."

The researchers created a microdevice, into which a spheroid comprising human neurons derived from induced pluripotent stem (iPS) cells was placed. A microchannel narrow enough to align axons and allow them to bind to each other, led to formation of fascicles showing properties consistent with those seen in human brains.

Spheroids were placed in the microdevice's chamber, from which axons grew, with some axons spontaneously entering the microchannels. The nature of the molecular signaling causing the entry of axons into the microchannels remains unknown, but fascicles were detected in more than 90 percent of experiments, establishing the value of the microdevice design.

"This is the first stem cell-based technique to have successfully created nerve fascicles in a controlled manner," noted Ikeuchi. "By creating a structure and microenvironment similar to that of neurons cultured outside the body, we should have a better way of analyzing physiological changes and responses of axon fascicles to external stimuli.

"The device gives us a means to investigate which factors are responsible for fascicle assembly," he added.

The researchers then simulated neurodegenerative conditions by introducing peroxide into the microchannels, to which the fascicles responded with morphological changes.

"Use of peroxide to simulate neurodegenerative conditions is a standard method used to damage neurons in a way that mimics the physiological damages in our bodies, albeit on an accelerated time scale," explained Ikeuchi.

"Specifically, we observed that the surface of the axon fascicles became significantly rougher after peroxide treatment, in which presumably the axons were broken," he said.

"It is very easy to quantify surface roughness with pre-existing image analysis software, which makes the technique particularly suitable for drug screening," he noted.

Those new findings, together with the relative ease of the experiments, suggest that the new microdevice will be applicable for testing experimental drug compounds that might prevent fascicle degeneration caused by disease.

'Such motor nerve organoids can be used for drug screening," said Ikeuchi. "We intend to use this new technology to promote drug discovery, make motor nerve organoids mimic actual nerve functions more closely, and use it to mimic the body's other axonal fascicles."

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