How Transient Synapses Shape The Brain

Researchers in Japan have revealed that subplate neurons form transient synapses with newborn neurons and send signals to control their migration.

AsianScientist (May 8, 2018) – In a study published in the journal Science, a team of scientists in Japan has shown that transient connections have an influential role in shaping the developing brain.

During neocortical development in the fetus, neurons are born deep within the brain from repeated cell divisions of neural progenitor cells. Subplate neurons are the first neurons born in the neocortex, and they form a layer called the subplate layer. After the generation of subplate neurons, neural progenitors next generate enormous numbers of excitatory neurons, which then migrate toward the brain surface where they form the different layers of the neocortex.

When excitatory neurons are first born, they are star-shaped, or multipolar, and migrate in a slow, meandering manner without a set direction. This type of migration is referred to as multipolar migration. However, at some point, multipolar neurons suddenly change into a spindle shape with two protrusions and begin migrating quickly towards the brain surface in a process called locomotion or radial neuronal migration. The mechanism regulating this switch has been unknown.

In this study, the research team led by Dr. Chiaki Ohtaka-Maruyama of the Tokyo Metropolitan Institute of Medical Science, Japan, found that subplate neurons form transient synapses with newborn neurons and send signals to control their migration.

The researchers first observed that newborn neurons in mouse embryos switch from multipolar migration to locomotion at the subplate layer. From this, they hypothesized that subplate neurons could be involved in organizing neuronal migration. They found that subplate neurons actively extend processes to form transient synapses on newly born multipolar migrating neurons.

Treating newborn neurons with the neurotransmitter glutamate, which mimics synaptic activity, also enhances radial migration. These results suggest that subplate neurons may function like organizers at a massively overcrowded time-trial race, where organizers decide which racers will run at what time.

“We found that a special type of neurons, subplate neurons, control migration of newborn neurons by communicating with them through synapses. We were really surprised by this since synapses are thought to be structures used by mature neurons. This is the first time that synapses have been found so early in development,” said Ohtaka-Maruyama.

Various mental disorders such as autism and schizophrenia are associated with defects in locomotion and radial migration. Thus, these findings may reveal further insights into the causes of neurodegenerative diseases. Clues about how a massively complex structure like the human neocortex evolved may also be uncovered in the future.

The article can be found at: Ohtaka-Maruyama et al. (2018) Synaptic Transmission from Subplate Neurons Controls Radial Migration of Neocortical Neurons.

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Source: Tokyo Metropolitan Institute of Medical Science; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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