A NEW APPROACH TO IMAGING THE BRAIN DURING EARLY-STAGE NEURODEGENERATION
Evan Gallagher is a recent graduate of Penn’s neuroscience graduate program, and the lead author on this study. He is broadly interested in using neuroimaging approaches like magnetic resonance imaging (MRI) and positron emission tomography (PET) to study complex biological processes in living animals and people. Ultimately, he hopes that his work allows us to better understand—and eventually treat—major neurological disorders like Alzheimer’s and Parkinson’s.
or technically,
Positron emission tomography with [18F]ROStrace reveals progressive elevations in oxidative stress in a mouse model of alpha-synucleinopathy
[See Original Abstract on Pubmed]
Authors of the study: Evan Gallagher, Catherine Hou, Yi Zhu, Chia-Ju Hsieh, Hsiaoju Lee, Shihong Li, Kuiying Xu, Patrick Henderson, Rea Chroneos, Malkah Sheldon, Shaipreeah Riley, Kelvin C. Luk, Robert H. Mach and Meagan J. McManus
Neurodegenerative diseases, like Alzheimer’s and Parkinson’s, impact 15% of the world’s population [1]. This percentage has climbed considerably over the last 30 years and is expected to continue rising as the global population ages [2]. Still, the process of diagnosing (and therefore treating) neurodegenerative diseases remains quite challenging. The difficulty arises because brain changes that play a central role in the disease process begin years or even decades before people experience symptoms [3]. In other words, by the time patients notice physical changes (e.g., memory loss, difficulties with movement, etc.), their brain is already dramatically and permanently affected.
What if we had some way to detect and visualize very early neurodegeneration-related brain changes? Doctors could then identify people at risk for developing neurodegenerative diseases and intervene with treatments before things escalate beyond repair. This is the vision that inspired recent work by Neuroscience Graduate Group (NGG) alum Dr. Evan Gallagher in collaboration with the labs of Dr. Robert Mach and Dr. Meagan McManus at the University of Pennsylvania.
In order to find early signs of trouble, the team needed something that serves as a “red flag” for neurodegeneration as well as a way to find and measure it in the living brain. They landed on reactive oxygen species (ROS), which are molecules that exist naturally in the body, but that can lead to problems if they build up over time. “Excessive production of reactive oxygen species is an important, central process in many neurodegenerative diseases,” explains Dr. Gallagher. “It’s also a very early process. This means that if we’re able to detect and quantify reactive oxygen species in the brain, we may be able to identify that there’s a problem much earlier than we are currently doing.” To find and label reactive oxygen species in the brain, chemists in Dr. Bob Mach’s lab engineered a chemical called ROStrace [4]. ROStrace is a positron emission tomography (PET) imaging radiotracer. When injected into the body, ROStrace travels through the bloodstream and into the brain where it targets and tags reactive oxygen species, giving off a radioactive signal that can be detected. This means that researchers can perform a ROStrace PET imaging scan and get out an image of the brain that looks and works a lot like a heatmap with the color and pattern of the image relaying information about the number of reactive oxygen species present across brain regions. With these PET imaging heatmaps, researchers are able to find “hot spots” of highly-concentrated reactive oxygen species, which could be indicators of places in the brain where the groundwork for neurodegeneration is being laid.
To validate that their ROStrace technology works and prove that it can be used to detect early signs of neurodegeneration, the Mach and McManus labs performed a series of well-controlled experiments in a mouse model. In particular, they used a mouse model with a mutated form of the human alpha synuclein protein. The mutation causes alpha synuclein to accumulate as mice age, the same process that happens in a number of human neurodegenerative diseases. Dr. Gallagher performed ROStrace PET imaging on these animals when they were either 6 or 12 months old (middle and old age for this type of mouse) and compared the results to healthy mice in the same age groups. He found that the mutant alpha synuclein mice had higher levels of reactive oxygen species than the healthy animals at both timepoints. However, it wasn’t just the overall counts of reactive oxygen species that differed. Looking at the ROStrace brain images, Dr. Gallagher could identify a clear spatial pattern with which reactive oxygen species spread across the brain over time in the mutant alpha synuclein animals. It was the same pattern he found when he took brain tissue from mutant animals and stained it with special chemicals to label reactive oxygen species. In other words, it seemed like his PET scan brain images matched the reality of reactive oxygen spread as seen in mouse brains under the microscope. ROStrace worked! “It’s really exciting,” says Dr. Gallagher. “The fact that our PET images matched up with what we found in tissue means we actually have a way of detecting reactive oxygen species in the brains of living animals.”
Taking things one step further, Dr. Gallagher wanted to verify that the ROStrace signal was highlighting something biologically and clinically useful. That is to say, he wanted to confirm that the reactive oxygen species seen on ROStrace brain images were actually related to alpha synuclein disease pathology. Using brain tissue from the same mutant mouse species, this time he stained it with chemicals to mark reactive oxygen species and alpha synuclein proteins. Dr. Gallagher found that the two chemicals labeled many of the same brain cells and brain regions, suggesting that reactive oxygen species were, in fact, associated with alpha synuclein pathology. Remarkably, the co-occurrence of reactive oxygen species and alpha synuclein in the brain was detectable months before mice showed severe behavioral symptoms. This implies that the ROStrace signal could one day be used to diagnose -- or even to preemptively screen for -- alpha synuclein-related brain diseases, like Parkinson’s or Alzheimer’s, long before patients realize something is wrong.
ROStrace could also have applications beyond serving as a marker for alpha synuclein accumulation and neurodegeneration. “As far as we can tell, just about everything ‘bad’ happening in the body is associated with reactive oxygen species,” offers Dr. Gallagher. That means that the methods used in this study should be generalizable across many other disorders. This generalizability, however, does come with a downside. “One bad thing that could be happening in your body is alpha synuclein aggregation, but that’s certainly not going to be the only bad thing happening in your body at any given time,” Dr. Gallagher continues. “For instance, a stressful event or a bad night of sleep can cause increased levels of reactive oxygen species. ROStrace can pick up on that and it makes interpreting a single scan really tricky.” In the current study, Dr. Gallagher navigated this limitation by using scans from many mice to find differences between sick and healthy groups. However, being able to get interpretable information from one single scan is critically important when we think about using technologies, like ROStrace, in the clinic. So, while this study represents an exciting first step, for now research with ROStrace has been limited to lab animals and there are still hurdles to clear before human subjects are part of the picture.
About the brief writer: Kara McGaughey
Kara is a PhD candidate in Josh Gold’s lab studying how we make decisions in the face of uncertainty and instability. Combining electrophysiology and computational modeling, she’s investigating the neural mechanisms that may underlie this adaptive behavior.
Citations:
Van Schependom, J., & D’haeseleer, M. (2023). Advances in Neurodegenerative Diseases. Journal of Clinical Medicine, 12(5), 1709. https://doi.org/10.3390/jcm12051709
Brown, R. C., Lockwood, A. H., & Sonawane, B. R. (2005). Neurodegenerative Diseases: An Overview of Environmental Risk Factors. Environmental Health Perspectives, 113(9), 1250–1256. https://doi.org/10.1289/ehp.7567
Gallagher, E., Hou, C., Zhu, Y., Hsieh, C.-J., Lee, H., Li, S., Xu, K., Henderson, P., Chroneos, R., Sheldon, M., Riley, S., Luk, K. C., Mach, R. H., & McManus, M. J. (2024). Positron Emission Tomography with [18F]ROStrace Reveals Progressive Elevations in Oxidative Stress in a Mouse Model of Alpha-Synucleinopathy. International Journal of Molecular Sciences, 25(9), 4943. https://doi.org/10.3390/ijms25094943
Hou, C., Hsieh, C.-J., Li, S., Lee, H., Graham, T. J., Xu, K., Weng, C.-C., Doot, R. K., Chu, W., Chakraborty, S. K., Dugan, L. L., Mintun, M. A., & Mach, R. H. (2018). Development of a Positron Emission Tomography Radiotracer for Imaging Elevated Levels of Superoxide in Neuroinflammation. ACS Chemical Neuroscience, 9(3), 578–586. https://doi.org/10.1021/acschemneuro.7b00385