Â鶹ÒùÔº

The amino acid glycine is an important neurotransmitter that regulates memory, reflex, and brain development, and it may also be a biomarker for bacterial virulence. Of the 20 standard amino acids, the building blocks of proteins, glycine is the simplest, the only one lacking a side chain extending from its backbone.

"But, despite these important roles of glycine, there haven't been any tools that can image glycine both inside and outside of living cells," University of Utah chemistry professor Ming Hammond said.

Using engineered strands of RNA called aptamers, her lab created such a tool, unveiled in a study published earlier this month. Through persistent work, Hammond's team created a new aptamer dubbed "Golden Broccoli" that is used alongside the previously existing "Red Broccoli" aptamer to bind with a single dye. The two aptamers fluoresce yellow and red, respectively.

"The yellow signal indicates how much of the RNA sensor is in the cell," said lead author Madeline Bodin, a doctoral student in the Hammond lab. "The red signal indicates how much glycine is present. You can use the ratio between these signals for absolute quantitation."

This research is expected to help advance imaging tools, according to the journal , whose reviewers selected Bodin's paper as a Breakthrough Article, an honor reserved for about 20 articles a year. The paper is titled "Visualizing intracellular glycine with two-dye and single-dye ratiometric RNA-based sensors."

The Hammond Lab seeks to engineer nucleic acids as programmable tools for molecular imaging and gene control, and to illuminate the chemistry and biology of cyclic dinucleotides as signaling molecules in bacteria and mammalian cells.

The new study builds on , a method of measurement that uses the ratio of two signals to determine a value. Instead of measuring the absolute intensity of a fluorescent signal, a ratiometric approach measures two signals at different wavelengths. Their ratio, rather than their raw intensity, is then used to quantify the signal of interest.

Here's how ratiometry applies in Bodin and Hammond's "Broccoli" experiments on glycine.

When the aptamers light up, yellow light and red light are emitted mixed together.

"The two signals have some overlap. Luckily, it turns out you can unmix these signals using mathematical formulas," Hammond said. The key part now is that glycine levels can be read accurately in real time within living cells, without breaking open the cells and killing them in the process.

"Any questions we have about how the amount of glycine in the cell changes during different cellular processes or where glycine is located in the cell at different times now can be answered," Bodin added.

The single-dye approach avoids problems caused by different dyes having different cell permeabilities, solubilities, or dependencies, making measurements more consistent across experiments.

This new tool can help test and improve fundamental models of cell signaling and behavior. One application for more accurate reading of glycine levels is in the human brain, specifically with a glial cell called an astrocyte.

Astrocytes are abundant within the central nervous system, responsible for regulating neuronal activity. They have a sheath-like role in protecting the brain from injury. Recent studies have hypothesized that astrocytes might provide neurotransmitters to the neurons themselves.

"We want to use the biosensor to determine if astrocytes release glycine in a way that could potentially affect neuronal signaling," Bodin said. Imaging of glycine in the brain is currently not possible. But Bodin and Hammond are optimistic that the necessary technological advancements will eventually emerge to achieve such imaging.

"I'm always thinking of how there's more to be done," Bodin said. "Although this was a breakthrough, I hope that in the future, other people can develop even brighter single-dye ratiometric aptamers. And in that sense, there's still more work to be done."

The hope is that Golden Broccoli will lead to advancements that can reveal the big things that the simple little glycine molecule is up to.