Summary: EPSILON, a new experimental technique, lets researchers map the proteins that underlie memory formation with unprecedented precision. By tracking AMPA-type glutamate receptors (AMPARs) over time, EPSILON reveals how synaptic strength changes during learning and how those changes are organized across the brain.
Using sequential fluorescence labeling and high-resolution microscopy in behaving mice, the research team followed memory-linked synaptic changes during learning tasks. The approach exposes rules that determine which synapses are strengthened or weakened, offering a clearer molecular view of how the brain encodes, consolidates, and stores memories. These insights may guide future strategies for treating memory disorders such as Alzheimer’s disease and other forms of dementia.
Key Facts:
- Memory-mapping advance: EPSILON tracks AMPAR trafficking during synaptic plasticity in living brains.
- High-resolution temporal mapping: The method produces synapse-level maps of receptor exocytosis across defined time windows.
- Clinical relevance: The technique offers new perspectives on synaptic dysfunction in memory disorders and may inform therapeutic development.
Source: Harvard
Harvard researchers have introduced a method that maps the molecular events of learning and memory, opening a path to deeper understanding of cognitive function and potential therapies for neurological disease.
“This technique provides a lens into the synaptic architecture of memory, something previously unattainable in such detail,” said Adam Cohen, professor of chemistry and chemical biology and of physics, and senior co-author of the study published in Nature Neuroscience.
Memories are encoded within a dense network of neurons, and synaptic plasticity—the strengthening or weakening of connections between neurons—underpins our ability to learn and remember. EPSILON, short for Extracellular Protein Surface Labeling in Neurons, was developed to map changes in the proteins that mediate synaptic signaling during defined time windows of behavior.
The method focuses on AMPARs, receptor proteins central to synaptic transmission and plasticity. By applying sequential pulse-chase labeling with membrane-impermeable fluorescent dyes, EPSILON labels surface AMPARs at different time points so researchers can track receptor insertion (exocytosis) and movement with synaptic resolution in genetically targeted neurons in vivo.
Combining this labeling strategy with advanced fluorescence microscopy, the team visualized where and when AMPARs were added to synapses as animals learned. This noninvasive, temporally precise approach reveals the spatial distribution and history of synaptic potentiation that earlier methods could not capture as cleanly.
The study team includes members of Cohen’s laboratory—Griffin GSAS student Doyeon Kim and postdoctoral scholars Pojeong Park, Xiuyuan Li, J. David Wong-Campos, He Tian, and Eric M. Moult—alongside collaborators from the Howard Hughes Medical Institute.
One of EPSILON’s early applications was to map AMPAR exocytosis in the hippocampus of mice subjected to contextual fear conditioning, a standard model for studying associative memory. The researchers compared synapse-level AMPAR trafficking in CA1 pyramidal neurons with cell-level expression of the immediate early gene product cFos, a commonly used marker of neuronal activation and putative engram cells.
They found a strong correlation between AMPAR exocytosis at synapses and cFos expression in cells, suggesting that receptor trafficking contributes to the formation of enduring memory traces—or engrams—by strengthening specific synaptic connections on neurons activated by learning.
Doyeon Kim described the value of the method: “Our most important advance is the ability to map the past history of synaptic plasticity in the living brain. We can identify where and how much potentiation occurred during a defined time window of memory formation.” By sampling multiple time points, EPSILON reveals the dynamics of synaptic change and can be adapted to study different memory types that show distinct patterns of plasticity.
Cohen emphasized how long-term basic research enabled the new method. For example, HaloTag protein-labeling technology—used to attach dyes to proteins—traces back to a gene discovered in bacteria decades ago. “Progress depends on supporting the entire arc of basic science that ultimately leads to tools with medical relevance,” he said.
Looking ahead, the researchers plan to distribute the molecular tools broadly and encourage other labs to apply EPSILON to questions across memory, perception, and behavior. The approach can also be extended to study trafficking of other transmembrane proteins, offering a general toolkit for mapping functional protein dynamics at synapses.
Funding: This work was partially supported by the National Institutes of Health.
About this memory and neurotech research news
Author: Yahya Chaudhry
Source: Harvard
Contact: Yahya Chaudhry – Harvard
Image credit: Neuroscience News
Original Research: Closed access. “EPSILON: a method for pulse-chase labeling to probe synaptic AMPAR exocytosis during memory formation” by Adam Cohen et al., Nature Neuroscience.
Abstract
EPSILON: a method for pulse-chase labeling to probe synaptic AMPAR exocytosis during memory formation
Tools that map changes in synaptic strength during defined behavioral windows offer powerful insight into the cellular mechanisms of learning and memory. Here the authors develop Extracellular Protein Surface Labeling in Neurons (EPSILON), a pulse-chase labeling method that tracks surface AMPAR exocytosis in vivo using sequential application of membrane-impermeable dyes.
EPSILON produces synapse-resolution maps of AMPAR insertion—a proxy for synaptic potentiation—in genetically targeted neurons during memory formation. Applied to contextual fear conditioning, the method reveals a strong relationship between synapse-level AMPAR exocytosis in CA1 pyramidal neurons and cell-level cFos expression, linking receptor trafficking to putative engram cells. EPSILON is a versatile tool for mapping synaptic plasticity and can be adapted to study trafficking of other membrane proteins involved in neural function.