Scientists discover a mysterious kind of human brain cell that’s not found in mice

Rosehip neuron
This is a digital reconstruction of a rosehip neuron from a human brain. (Tamás Lab, University of Szeged)

A gene-by-gene, neuron-by-neuron search has turned up a new breed of brain cell that may serve as a fine-scale “volume control” for neural activity in humans.

The novel type of brain cell, known as a rosehip neuron, is described in a study published today by Nature Neuroscience.

“It’s very rare, and you only see it, so far, in a human,” study co-author Ed Lein, an investigator at the Seattle-based Allen Institute for Brain Science, told GeekWire.

Lein’s group at the Allen Institute and a Hungarian research team at the University of Szeged, headed by Gábor Tamás, narrowed in on the neurons using two different lines of inquiry.

Tamás’ team uncovered the neurons as they sifted through samples taken from the brains of two men in their 50s who had died and donated their bodies to science. Researchers found the rosehip neurons in the outermost layer of the human neocortex, known as Layer 1. That layer is proportionately thicker in humans than in other species, and it’s thought to play a key role in human consciousness.

The neurons were given the “rosehip” name because each cell’s central axon had the bulbous look of a rose after it has shed its petals. They’re inhibitory neurons, which means they can tamp down electrical impulses coming in from other brain cells.

In a news release, Tamás compared inhibitory neurons to the brake pedals in a car, and said that rosehip neurons appear to be a specialized kind of brake. “This particular cell type — or car type — can stop at places other cell types cannot stop,” he said. “The car or cell types participating in the traffic of a rodent brain cannot stop in these places.”

Rosehip and pyramidal neurons
A rosehip neuron (top of image) forms synaptic connections with a pyramidal neuron (bottom of image) in a different part of the brain (Tamás Lab, University of Szeged)

Lein added another analogy, comparing the neurons to the customized volume controls on a stereo sound system’s equalizer.

He and other researchers from the Allen Institute zeroed in on the rosehip neurons using genetics: In collaboration with neuroscientists from the J. Craig Venter Institute and other research centers, they looked for gene expression patterns that distinguished different types of brain cells in humans and in mice.

There was at least one type of brain cell that had no analog in the mouse brain, and when Tamás visited the Allen Institute to discuss his research, the researchers from Hungary and Seattle realized they were aiming at the same target.

“It was a really great match,” Lein said.

Study co-author Rebecca Hodge, a senior scientist at the Allen Institute, said combining classical neuroscience with genetics gave a boost to the discovery process.

“Alone, these techniques are all powerful, but they give you an incomplete picture of what the cell might be doing,” she said. “Together, they tell you complementary things about a cell that can potentially tell you how it functions in the brain.”

The researchers don’t yet know the precise role that rosehip neurons play in the human brain, other than that it’s an inhibitory role. And they can’t yet say whether or not rosehip neurons are in the brains of other species, such as apes and other primates. “It has not been described in the literature,” Lein said.

In any case, the discovery demonstrates that mouse studies can’t possibly provide a complete picture of how the human brain works.

“Many of our organs can be reasonably modeled in an animal model,” Tamás said. “But what sets us apart from the rest of the animal kingdom is the capacity and the output of our brain. That makes us human. So it turns out humanity is very difficult to model in an animal system.”

Going forward, the researchers plan to intensify the search for specialized cell types in human brain tissue. They’ll also try to examine a wider spectrum of brain tissue, including postmortem samples from patients with neuropsychiatric disorders, to see whether disease affects the function of rosehip neurons and other specialized cells.

But Lein doesn’t expect any one single cell type to be the defining element for consciousness or “humanness.”

“There’s not going to be a magic-bullet cell type, any more than there’s a magic gene for humanity or for the most complex diseases,” he told GeekWire. “This is one small suggestion that there’s going to be lots of specialized circuits in the human brain.”

Lein, Tamás and Hodge are among 34 authors of “Transcriptomic and Morphophysiological Evidence for a Specialized Human Cortical GABAergic Cell Type.” The principal authors are Eszter Boldog, Trygve Bakken and Rebecca Hodge.