A new type of brain cell, called a glutamatergic astrocyte, has been discovered. It could explain how a variety of neurodegenerative conditions, such as Parkinson’s, develop.
Brain cells can largely be categorised into two types: neurons and glia. Neurons are typically considered to communicate with each other across the synapses, or junctions, between them, whereas glia don’t use this type of signalling.
Synaptic transmission occurs when a neuron is electrically excited and releases a chemical, called a neurotransmitter, into the gap between itself and another neuron, which leads to the activation of the second neuron. This ability was largely thought to be unique to neurons.
But two decades ago, Andrea Volterra, now at the University of Lausanne in Switzerland, and his colleagues announced that they had discovered that some glia could also use synaptic-like transmission to communicate with other cells. However, the findings proved controversial as other researchers struggled to replicate the findings.
Now, Volterra and a different team have used modern techniques to finally put this controversy to bed.
The researchers analysed data on the production by genes in mouse cells of RNA molecules, which are intermediates in protein production, to see if they could find the protein complexes required for synaptic transmission in cells other than neurons. The team specifically looked at cells in the brain’s hippocampus region, because this is where the previous research claimed to have spotted non-neuronal synaptic transmission.
The analysis revealed several clusters of astrocytes, a type of glia, that appeared to also possess the ability to take part in synaptic transmission. The cells seemed to release the neurotransmitter glutamate, which is the most common neurotransmitter in the brain. The researchers then confirmed the presence of the genes involved in this by studying brain slices from adult mice. The researchers have coined these cells glutamatergic astrocytes.
“These cells are a little bit like astrocytes and a little bit like neurons,” says Volterra. “They are secreting neurotransmitters with a mechanism and speed that are usually only linked to neurons. It’s why we call it a kind of hybrid cell.”
The researchers then used a type of fluorescent microscopy technique called two-photon imaging to study glutamate release by these cells in the brains of the mice. “The signals that we see are in the order of speed similar to neurons,” says Volterra.
He and his colleagues also found similar protein signatures of synaptic transmission in non-neuronal cells in humans by looking at existing datasets. “The findings suggest that these cells are conserved [in people],” says team member Ludovic Telley, also at the University of Lausanne.
The researchers don’t know how many of these cells can be found in the brain, or if they are mainly in the hippocampus.
It is unclear why the brain needs glia that communicate via synaptic transmission, says Volterra. He speculates that it could lead to a greater coordination of signals. “Often, we have neuronal information that needs to spread to larger ensembles and neurons are not very good for the coordination of this,” he says. One astrocyte can be in contact with 100,000 synapses in mice, which could mean the signals go further in a more coordinated fashion, he says. They can reach millions of synapses in humans.
These cells also appear to be in brain circuits involved in movement, which degenerate in Parkinson’s disease, says Volterra. A better understanding of the cells could give us a greater insight into how to tackle the condition, he says.