New methodology spotlights synaptic plasticity in residing mice | Spectrum
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Using a new line of mice that express a synaptic membrane receptor with a fluorescent tag, researchers have followed changes in the strength of the synapses in the brain over several days. The method could provide information about the altered synaptic function in people with autism and the neural mechanisms that are responsible for the characteristics of the disease.
Synapses are highly dynamic connections between neurons that can strengthen or weaken over time depending on their use, a process known as synaptic plasticity. For example, when neurons are stimulated, the synapses between them enlarge and recruit proteins called AMPA receptors to the cell surfaces.
Researchers have traditionally visualized plasticity in living mice by injecting a plasmid carrying the gene for a fluorescently labeled version of the AMPA receptor into neurons, which causes those neurons to express the labeled receptor. However, microinjecting DNA into neurons is time consuming and inefficient because it imparts receptor fluorescence to only a few neurons. In addition, injected cells overexpress the gene, which can change the function of the receptor.
The new mice, developed by Richard Huganir and his colleagues at Johns Hopkins University in Baltimore, Maryland, make it possible to visualize AMPA receptors throughout the brain. The team genetically engineered mice to attach the animals’ AMPA receptors to a fluorescent tag. The day is pH-sensitive and only glows in the neutral environment of the cell surface, not in the acidic interior of the cell. This means that only receptors on the cell membrane are visible.
When the researchers electrically stimulated neurons in brain tissue sections of the mice, the fluorescence increased as expected. Measuring the electrical excitability of the neurons with a method known as patch-clamp recording confirmed that the fluorescence signal correlated with functionally stronger synapses.
Green glow: A fluorescent tag that lights up proteins in synapses allows researchers to measure changes in the strength of neural connections over time.
B.By surgically replacing the skulls of the animals with transparent material, the researchers obtained a window into the cerebral cortex. High resolution fluorescence microscopy showed a constellation of millions of synapses in exquisite detail. The team developed an algorithm to detect and measure the fluorescence at each synapse within the extensive data set, with an accuracy comparable to expert annotators, but with results in a fraction of the time.
Huganir’s team also tracked changes in the same synapses before and after the rodents’ whiskers were stimulated – a manipulation that increases the synapse strength in a specific region of the somatosensory cortex, previous work has shown. As expected, the whisker stimulation increased the fluorescence in the same brain region in the new mouse line, suggesting that the synapses were strengthened by the recruitment of AMPA receptors to the surfaces of the neurons. The results were published in eLife in October.
In an unpublished work that was presented virtually at the Society for Neuroscience 2021 conference in November, the mice went through fear conditioning and learned to associate being in a specific cage with a mild electric shock. Using a system based on deep learning, the group could see that each time the animals were imaged over a period of two weeks, receptors were added to the synapses, confirming that their learned fear response was dependent on synaptic plasticity.
The ability to map individual synapses across entire brain regions has promising implications for autism research, says Huganir, professor of brain sciences as well as psychology and brain sciences. Several genes associated with autism regulate synaptic plasticity. It is believed that variations in these genes disrupt the strength and abundance of synapses, leading to large-scale changes in neural connectivity. Despite the heterogeneity of autism, different forms of the disease converge on a common path that includes altered synapse homeostasis.
Huganir’s lab plans to crossbreed the fluorescent mice with mouse models that express variations in synaptic genes associated with autism, allowing further study of receptor trafficking, altered transmission, and synapse composition in the autism brain.
The approach could also provide clues to the molecular basis of autism traits, including altered social communication and sensory perception. Since these differences involve communication between multiple brain regions, imaging the entire brain is critical to understanding the signaling pathways involved, says Huganir.
Cite this article: https://doi.org/10.53053/UOEU4921
