Living Brains Made Transparent: New Reagent Enables Deeper Imaging

The ability to visualize the inner workings of a living brain, once confined to the realm of science fiction, is moving closer to reality. Researchers at Kyushu University have developed a new reagent, SeeDB-Live, that renders living brain tissue transparent, allowing for deeper and brighter imaging of neural activity without disrupting biological function. This breakthrough, published in Nature Methods, could revolutionize our understanding of complex brain processes like memory and thought.

Seeing Through the Complexity

Traditionally, studying the brain’s intricate network of neurons has been hampered by its opacity. While examining brain slices provides some insight, it doesn’t fully capture the dynamic interactions occurring within a living brain. Existing tissue-clearing techniques often alter the brain’s biology, compromising the accuracy of observations. SeeDB-Live overcomes these limitations by utilizing albumin—a protein naturally found in blood serum—to adjust the refractive index of the tissue. This process minimizes light scattering, effectively making the brain transparent while preserving cellular function.

“This is the first time tissue clearing has been achieved without altering its biology,” explains Takeshi Imai, professor at Kyushu University’s Faculty of Medical Sciences and the study’s senior author. The reagent allows scientists to observe structures deeper within the brain, both ex vivo (in brain slices) and in vivo (in living mice), with unprecedented clarity.

The challenge lies in the way light travels through different materials. Just as a glass marble appears to disappear when submerged in oil, light bends and scatters when passing through brain tissue due to variations in refractive indices. SeeDB-Live works by reducing these mismatches, allowing light to travel more uniformly and reveal hidden structures. The team discovered that achieving a refractive index of 1.36–1.37 is optimal for transparency.

The Unexpected Key: Albumin

Finding a non-toxic method to reach this refractive index while maintaining the delicate balance of the cellular environment proved hard. Previous attempts using substances like sugar created osmotic imbalances, causing cells to shrink or swell. The breakthrough came unexpectedly when Assistant Professor Shigenori Inagaki revisited the basic properties of polymers.

“I tested it three or four times before I believed it,” Inagaki recalled, describing the moment he discovered that bovine serum albumin (BSA) possessed the necessary properties. “Of all things, we never expected it would arrive down to this.” BSA, a readily available laboratory reagent, offered the lowest osmotic pressure at the desired refractive index, effectively clearing the tissue without harming the cells.

When applied to living mouse brains, SeeDB-Live increased the brightness of fluorescence signals from deep neurons threefold. This allows for clearer visualization of layer 5 of the cerebral cortex, a region crucial for processing information and translating neural activity into action. The reagent is washed away naturally within hours, restoring the tissue to its original state, enabling repeated imaging of the same brain over time.

Implications for Brain Research and Beyond

The development of SeeDB-Live represents a significant advancement in neuroimaging. By enabling deeper and brighter visualization of neural activity, it promises to enhance our understanding of brain function in both health and disease. Researchers anticipate that this technique will be particularly valuable for studying complex processes like learning, memory, and the development of neurological disorders.

The potential applications extend beyond basic neuroscience. The researchers suggest that SeeDB-Live could also be used to evaluate 3D tissues and brain organoids for drug discovery, offering a more accurate assessment of drug efficacy and toxicity. However, the team acknowledges that delivering the reagent to other organs remains a challenge due to biological barriers, and accessing the brain still requires a surgical procedure that can cause stress to the animal.

“I feel we have not yet fully materialized its potential,” says Inagaki, adding that future research will focus on developing less invasive delivery methods to improve penetration and enhance functional analysis of brain activity. Imai reflects on the decade-long journey, recalling how he repeatedly dismissed the possibility of live tissue clearing. “But 10 years later, here we are. When something seems unachievable, if you keep thinking about it, you may eventually identify a way.”

SeeDB-Live builds upon previous work by Imai’s team, including the development of SeeDB in 2013 and SeeDB2 in 2016, both designed for use with fixed tissue. This latest innovation marks a crucial step towards unlocking the secrets of the living brain and opens new avenues for neurological research.