Imagine standing on earth and peering across the universe toward a star that is billions of miles away. Along the way to this star, your telescope’s path would encounter atmosphere, debris, and numerous other objects that would distort your image and create aberrations. Yet, somehow, we can get surprisingly high-resolution images of celestial objects that are unimaginably far away. This is through the power of a technology called adaptive optics. Now come back down to earth and think instead about using a microscope to visualize a tiny bouton on a dendrite of a neuron deep inside the brain. Using your microscope, you would again encounter distortions, this time of proteins, lipids, and nucleic acids. Is it possible to use the same technology that we use to visualize stars in far-away galaxies to examine neurons inside of the brain? Thanks to the work of Professor Na Ji and others, this and other groundbreaking methods of visualizing the brain are now at our finger-tips.
Dr. Na Ji is an associate professor in the Physics and Molecular & Cell Biology departments at UC Berkeley. Her laboratory aims to develop and apply novel imaging methods to understand the brain. Before she joined the UC Berkeley faculty in 2016, she was at the Janelia Research Campus, of the Howard Hughes Medical Institute. Among other advances at while at Janelia, she pioneered using the aforementioned adaptive optics for in vivo fluorescence microscopy to obtain high resolution images of neurons at depths previously inaccessible.
Let’s take a step back and examine how adaptive optics works. In a recent perspective in Nature Methods, Dr. Ji outlines the physical principles behind the technology. In summary, whether looking through a telescope or a microscope, one encounters image aberrations resulting from the medium one is peering through. However, if the character of the aberrations is a known quantity, these aberrations can be corrected for by the production of a compensatory distortion. This compensation results in the production of a more faithful image. In order to characterize the degree of distortion seen by a telescope examining distant cosmos, astronomers use something called a reference star, which can be either a real star, or an “artificial star” that is actually a laser beam of a known wavelength projected in space. In a biological sample, distortions of microscopic images can be measured by using an object such as a fluorescent bead placed in or below the sample. Dr. Ji’s group demonstrated the power of this technique in one study in 2015, published in Nature Communications, where they presented adaptive optical fluorescence microscopy that was able to obtain clear 2-photon images of neurons up to 700µm deep in the brain.
Dr. Ji’s research has not only tackled the problem of depth of brain imaging, but also how one can visualize the activity of many neurons at once in a live animal. To solve this, the imaging method not only needs to be clear, but it also needs to be fast. In one of her most recent publications “Video-rate volumetric functional imaging of the brain at synaptic resolution,” published in 2017 in Nature Neuroscience, Dr. Ji and authors present a system that can perform volumetric imaging of the brain at sub second temporal resolution. To achieve this, they developed a system that can be integrated into a standard 2-photon laser-scanning microscope (2PLSM). This system uses a type of axially elongated beam called a Bessel beam, which allows for better lateral imaging and depth of field. Because neurons remain relatively stationary during in vivo imaging, instead of constantly tracking the position of neurons in 3D, they used super-fast 2D imaging to reconstruct a 3D image at a 30 Hz rate. They then showed various applications for this imaging strategy, including studying the responses of different inhibitory interneuron subtypes in the mouse visual cortex to various stimuli. With this technique, they could image the activity of up to 71 different interneurons at once! It will be exciting for the future of Neuroscience to see the applications of this and other innovative new imaging technologies developed by Dr. Na Ji.
Shannan McClain is a 1st year Neuroscience PhD student in the laboratory of Dr. Matthew Banghart.