COLUMBUS, Ohio – New research reveals how a class of neurons that help coordinate communication in the brain link up with their target cells, identifying two molecules that must be present before synapses, the structures that carry signals between these partners, can form on the target neurons.
These cells are inhibitory interneurons that connect to a specific location on target excitatory neurons, regulating information processing and maintaining proper balance in brain circuits by controlling how active the excitatory neurons become. Loss of coordination between these two types of cells, which leads to circuit malfunction, is associated with such disorders as epilepsy, depression, autism and schizophrenia.
The research team determined that the presence of two different types of proteins in precise locations on each of the cells enables a "handshake" between the neurons – a step called innervation, which generates synapses on the excitatory neurons that set up a lifelong communication channel between the two cell types.
"These inhibitory interneurons shape and balance local circuit activity – they are the modulators, coordinators, the conductors of the orchestra," said first author Yasufumi Hayano , a postdoctoral scholar in pathology at The Ohio State University College of Medicine . "From our results, we've concluded that interaction between two specific proteins regulates the specificity of their synapse formation.
"This is basic neuroscience, but there might be an impact for neuronal disorders. If this process is disrupted, what happens? If we lose those genes, which neuronal disorder might occur? We still don't know, but those possibilities should be explored."
The study is published online in the Journal of Neuroscience .
Hayano works in the lab directed by senior study author Hiroki Taniguchi , associate professor of pathology at Ohio State and an investigator in the Chronic Brain Injury Program . Taniguchi's neurobiology research program focuses on the mechanisms underlying the development of circuitry in the cortex, the brain's outermost layer, with a goal of identifying potential targets for therapeutic intervention in neuronal disorders.
This study involved chandelier cells, inhibitory interneurons in the cortex of the brain that are named for their spray of synapses (called synaptic cartridges) that resemble candles of a traditional chandelier, a pattern that gives them inhibitory control over hundreds of cells at a time.
Chandelier cells coordinate the activity of cells called excitatory pyramidal neurons in the cortex, and their synaptic reduction has been implicated in brain disorders such as schizophrenia and epilepsy. The two cell types' association, though known to exist, is one of many in this critical area of the brain that are not well understood.
Based on their previous research with chandelier cells, the team knew these cells connected to their partner cells through synapse specificity – hooking up at a precise junction point for optimum communication – and that cell adhesion molecules were the likely protein type to facilitate the handshake.
The connection to excitatory pyramidal cells takes place in an area called the axon initial segment, which is located close to where an axon, a long, slender extension of the nerve cell that transmits messages, sprouts from the cell body. Because the axon initial segment is the site that generates action potentials, neuronal signals that encode information and are used for communication between neurons, chandelier cells are thought to be the most powerfully influencing activity patterns in excitatory neuron networks among inhibitory interneuron types.
"The axon initial segment is like a faucet that releases information instead of water," Hayano said. "Chandelier cells have hundreds of hands that grab the faucet handles of surrounding pyramidal neurons. If the chandelier cells turn off the faucet, the pyramidal neurons are unable to send information to other neurons."
Using RNA sequencing analysis systematically comparing gene expression levels among distinct inhibitory interneuron types, the team identified gliomedin as a cell surface molecule that is enriched in chandelier cells and known as a receptor for neurofascin-186, which is localized in the axon initial segment.
Visualizations using dyes to label and detect cells in the brains of very young mice – roughly corresponding to the timing of brain development that occurs in human teenagers – confirmed this connection, which led to the placement by chandelier cells of cartridges, or synapses, on the axon initial segments of pyramidal neurons.
A series of experiments in the mice involving deleting or overexpressing the genes that carry the instructions for building both proteins showed a reduction in synapse formation when the genes were missing and increased synapse formation when the genes were highly expressed.
"This enabled us to watch brain development as it happens and manipulate conditions to test what the mechanisms are, and showed that gliomedin in chandelier cells and neurofascin-186 at the axon initial segment are both essential for development of the chandelier synapse formation," Hayano said.
"This is the mechanism of how our brain can specify the synapses at these tiny, tiny segments within the crowded brain. There are so many neurons, and they cannot see axon initial segments, but they can still make a specific connection. It speaks to the beauty of the brain circuit."
While pyramidal neurons are the main excitatory cells in the brain, there are other subtypes of inhibitory interneurons that regulate excitatory and inhibitory balance in the brain with similarly mysterious mechanisms underlying their circuitry organization.
"In the future, we can use a similar strategy to explore other interneurons, especially those that we predict would have different types of molecular mechanisms," Hayano said.
This work was supported by the National Institutes of Health, the Max Planck Society, The Ohio State University Wexner Medical Center and Ohio State's Chronic Brain Injury Program.
Additional co-authors include Yugo Ishino of the Max Planck Florida Institute for Neuroscience, Manzoor Bhat of UT Health San Antonio and Elior Peles of the Weizmann Institute of Science in Israel.
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