An unusual imaging technique and curiosity piqued by worms has revealed new insights into the nanoscale structure of mammalian brain cells, specifically the arm-like axons that pass signals between neurons.
“Axons are the cables that connect our brain tissue, enabling learning, memory and other functions,” says senior author Shigeki Watanabe of John Hopkins University.
These structures are about 100 times smaller than the width of a human hair.
For at least a century, scientists have assumed that axons are shaped like tubes with a mostly constant diameter. Indeed, traditional imaging techniques like light and electron microscopy consistently show relatively uniform axons.
However, these imaging techniques typically require tissue fixation and dehydration steps known to introduce artefacts. Instead, Watanabe and colleagues used high pressure freezing electron microscopy to image neurons from mouse brains.
“To see nanoscale structures with standard electron microscopy, we fix and dehydrate the tissues, but freezing them retains their shape — similar to freezing a grape rather than dehydrating it into a raisin,” says Watanabe.
The new images revealed bubble-like bulges all along the axons, which made them look like beads on a string. The researchers dubbed the phenomena “axon pearling”. (This study focused on nonmyelinated neurons, which do not have an outer myelin sheath.)
Watanabe had previously seen axon pearling in worm nervous systems, which prompted the search for it in mammalian neurons.
“These findings challenge a century of understanding about axon structure,” says Watanabe.
Mathematical models and experiments on mouse brain samples suggest that axon pearling can be explained by biophysical forces acting on the cell membrane. The team found that these could be altered by experimentally changing the sugar and cholesterol concentrations around the axon.
Removing cholesterol reduced pearling and, as a result, reduced the ability of the axon to transmit electrical signals. “A wider space in the axons allows ions to pass through more quickly and avoid traffic jams,” explains Watanabe.
The results have implications for our understanding of how neuron structure impacts brain function. For example, the team found that high frequency electrical stimulation caused the pearl-like structures to swell temporarily, increasing the speed of signalling.
Watanabe and colleagues plan to study human axons next, particularly from patients with neurodegenerative diseases.
The research is published in Nature Neuroscience.
