A new study in the journal Neuron describes a whole new way of mapping the brain at the resolution of individual neurons.
This new method was demonstrated successfully in the mouse brain. Anthony Zador, MD, PhD, professor at Cold Spring Harbor Laboratory, said that their method is different than the usual methods that map out brain wiring. However, it does not have the potential for human use, Zador said in our interview with him.
He said that usually scientists inject a tracer in one brain area, which then spreads to other brain areas. The common way to do it is with green fluorescent protein (GFP).
One injects the GFP into one part of the brain and then waits a few days. That green color indicates that those neurons send signals to that place.
Zador said that the problem with that method is that the everything is labeled green, so you cannot tell one neuron goes to one place, and a different neuron goes to a different place.
The team of researchers is doing something fundamentally different by using high-throughput DNA sequencing, a new technology that is revolutionizing other branches of biology, but hasn’t been used in this way before. This is the same technology that was used to map the human genome. This allows people to map their genomes for under $1000.
The researchers are using the same technology, using DNA “barcodes” instead of colors. This allows researchers to do in a single experiment what would before require months or years of work. It is also much less costly, not only because of the labor saved.
The cost of sequencing is decreasing rapidly. Originally, fifteen years ago, it cost over one billion dollars to sequence the human genome, while today it costs under $1000.
The method is called Multiplexed Analysis of Projections by Sequencing (MAPseq). Each injection contains a deactivated virus that has been engineered to contain massive pools of individually unique RNA molecules, each of whose sequence, consisting of combinations of 30 letters, is taken up by a single neuron.
To demonstrate MAPseq’s capabilities, the team of researchers injected the deactivated virus into a part of the mouse brain called locus coeruleus, which is in the brain stem. After about two days, the cortex was divided into 22 pieces, dissected and sequenced for RNA barcodes. The sequence readouts were matched with barcodes of cells in the source region, establishing paths of neurons.
We talked to Dr. Zador, and he said he doesn’t think this has much potential for use in humans because it requires “injecting a barcoded virus into the brain” and “extracting the virus in the brain tissue.”
“There is no technology on the horizon that would allow this to be done non-destructively,” Dr. Zador told us.
Although it can’t be used in humans, he said that this could still have relevance to humans.
“That said, I believe that the basic microcircuitry of mammalian brain, especially cortex, is largely conserved across mammalian evolution, and right now there is so much we do not know about how single neurons are wired up to make cortical circuits, in any species, that what we don’t know completely swamps any differences between humans and related mammals.”