Scientists can now identify proteins inside the cells of a living brain, a breakthrough that could aid investigations of brain diseases like Parkinson's and Alzheimer's, according to a study published Wednesday in the journal Nature Communications.

Proteins are the lifeblood of cellular biology, as they allow cells to communicate and perform complex functions. But proteins are also infinitely complex, with the brain's cells alone deploying millions of distinct proteins.

To identify proteins inside different types of brain cells, scientists programmed a virus to deliver an enzyme to a precise location inside the brain of a living mouse. Once delivered, the enzyme, which is derived from soybeans, genetically tags neighboring proteins.

When scientists attempted to image the tagged proteins using fluorescence and electron microscopy, they were elated to find images of the target cell's proteome, its full arsenal of proteins.

Using mass spectroscopy, researchers were able to perform a postmortem analysis and identify the various proteins.

"Similar work has been done before in cellular cultures. But cells in a dish do not work the same way they do in a brain, and they don't have the same proteins in the same places doing the same things," senior study author Yevgenia Kozorovitskiy said in a press release.

"It's a lot more challenging to do this work in the complex tissue of a mouse brain. Now we can take that proteomics prowess and put it into more realistic neural circuits with excellent genetic traction," said Kozorovitskiy, associate professor of neurobiology at Northwestern University.

By tagging and imaging the full slate of proteins expressed by a single cell's genome, researchers can gain insights into how proteins work in concert.

In addition to the soybean enzyme, the cell-targeting virus also delivers a green fluorescent protein.

"The virus essentially acts as a message that we deliver," Kozorovitskiy said. "In this case, the message carried this special soybean enzyme. Then, in a separate message, we sent the green fluorescent protein to show us which neurons were tagged. If the neurons are green, then we know the soybean enzyme was expressed in those neurons."

Genetic tagging has led to great advances in the study of DNA and RNA, but scientists have struggled to target and tag proteins.

Genes and RNA can be sequenced and amplified in order to identify a cell's genetic building blocks, but the same techniques don't work for proteins.

"We have been able to gain a lot of traction with genetic and RNA sequencing, but proteins have been out of the loop," Kozorovitskiy said.

"Yet everyone recognizes the importance of proteins. Proteins are the ultimate effectors in our cells. Understanding where proteins are, how they work and how they work relative to each other is really important," Kozorovitskiy said.

Researchers estimate their new technique, mass spectroscopy-based proteomics, will allow neurobiologists to identify the full spectrum of proteins at work along vital brain circuits.

In followup studies, scientists plan to deploy their new protein-tagging technology to study proteome changes in mouse models with neurological diseases.

"We look forward to taking this to models related to brain diseases and connect those studies to postmortem proteomics work in the human brain," Kozorovitskiy said. "It's ready to be applied to those models, and we can't wait to get started."

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