Neuroscientists aim to enhance their understanding of how the brain operates, but they have faced challenges when trying to observe neuronal activity deep within the brain. To tackle this issue, scientists have utilized rational molecular engineering to create photoacoustic probes capable of functioning in the depths of brain tissue, allowing for the labeling and visualization of neurons. This novel imaging technique significantly broadens the observational capabilities beyond what traditional light microscopy can achieve, offering insights into deep neuron activity and overall brain function.
Understanding the brain better requires innovative techniques to observe its activities.
At the core of this aim is a molecular engineering initiative led by two research teams from the European Molecular Biology Laboratory (EMBL). Their work has resulted in a groundbreaking technique to produce photoacoustic probes specifically for neuroscience, with their results published in the Journal of the American Chemical Society.
“Photoacoustics allow us to visualize a complete mouse brain, but we previously lacked optimal probes for observing neuronal activity,” stated Robert Prevedel, an EMBL group leader and one of the senior authors of the study. To address this technological hurdle, he collaborated with Claire Deo, another EMBL group leader and senior author, who specializes in chemical engineering.
“We have demonstrated our ability to label neurons in specific areas of the brain with sufficiently bright probes, detectable by our custom photoacoustic microscope,” Prevedel explained.
Researchers can gain insights into biological processes by monitoring specific chemicals, such as ions or biomolecules. Photoacoustic probes serve as ‘reporters’ for challenging-to-detect chemicals by selectively binding to them. When illuminated by lasers, these probes absorb light and emit sound waves that specialized imaging technology can capture. However, thus far, researchers have struggled to create targeted reporters that can visualize brain functions tailored for photoacoustic applications.
Although some scientists have tried using synthetic dyes as photoacoustic reporters for neuronal activity, there have been challenges in controlling their placement and what they label. While proteins have proven helpful for tagging particular molecules, they have yet to result in effective photoacoustic probes for monitoring overall neural activity across the brain.
“In our research, we combined the strengths of both types of sensors—a protein and a thoughtfully designed synthetic dye—to label and visualize neurons in specific targeted areas,” remarked Alexander Cook, the first author of the study and a predoctoral fellow in Deo’s group. Rational design approaches utilize existing knowledge and concepts to construct molecules with desired properties, rather than randomly creating and testing various compounds. “Moreover, our probe doesn’t just provide a static view; it offers a reversible dynamic response to calcium, which indicates neuron activity,” Cook added.
According to Deo, the development of this technology faced significant obstacles. Since photoacoustic probes had not been extensively researched, the team lacked a method to assess the probes they were designing.
To address this, the project launched with the help of Nikita Kaydanov, a co-author and predoctoral fellow in the Prevedel Group, who created a custom spectroscopy setup. “There is currently no ready-made setup available to measure photoacoustic signals from probes in test tubes or cuvettes, so we built our own,” Kaydanov explained. “We designed a photoacoustic spectrometer to evaluate and refine the probes we developed.”
“This facilitated the evaluation and characterization of our probes, allowing us to assess key questions,” Deo noted. “Did they generate a detectable photoacoustic signal? Are they sufficiently sensitive? This guided our next steps.”
However, the researchers did not want to stop at developing probes that only function in a test vial. Their goal was to investigate the performance of the probes in real-world applications. They successfully figured out how to introduce the probes into a mouse brain and detected photoacoustic signals from neurons in the targeted areas.
“While we are thrilled with our advancements, we must emphasize that this is merely the initial generation of these probes,” Deo stated. “They represent a promising pathway, yet we have extensive work ahead of us. This serves as a strong preliminary demonstration of what our system can facilitate and its potential to enhance our understanding of brain functions.”
Looking ahead, the next steps involve refining the dye delivery system and verifying their effectiveness for dynamic imaging within cells.
“One of EMBL’s advantages is its capacity to bring together experts from various fields,” Prevedel highlighted. “We’re both developers in distinct ways—my group focuses on instrumentation, while Claire’s group concentrates on molecular tools. Collaborating with neuroscientists who actively test these tools creates a unique research environment, one that can only be found at EMBL.”
