Observing synapses in action
17.12.2025
A team of researchers from Charité – Universitätsmedizin Berlin and the Max Delbrück Center has captured the fleeting moment a nerve cell releases its neurotransmitters into the synaptic cleft. Their microscopic images and description of the process are now published in Nature Communications.*
It takes just a few milliseconds: A vesicle, only a few nanometers in size and filled with neurotransmitters, approaches a cell membrane, fuses with it, and releases its chemical messengers into the synaptic cleft – making them available to bind to the next nerve cell. A team led by Prof. Christian Rosenmund, Deputy Director of the Institute for Neurophysiology at Charité, NeuroCure PI and senior author of the study, has captured this critical moment of brain function in microscopic images.
Watching neurons fire — the research team achieved this using an optogenetic technique. The neurons, shown here in purple, release neurotransmitters in response to a light pulse, causing the synapses between them to light up yellow. The team plunge-froze the samples to examine the transmitter release in detail. © Charité/Max Delbrück Center | Jana Kroll
Point-shaped connections
“Until now, no one knew the exact steps of how synaptic vesicles fuse with the cell membrane,” says Dr. Jana Kroll, first author of the study and now a researcher in the Structural Biology of Membrane-Associated Processes lab headed by Prof. Oliver Daumke at the Max Delbrück Center. “In our experiments with mouse neurons, we were able to show that initially, the process begins with the formation of a point-shaped connection. This tiny stalk then expands into a pore through which neurotransmitters enter the synaptic cleft,” she explains.
“With technology we developed over five years, it was possible for the first time to observe synapses in action without disrupting them,” adds Christian Rosenmund. “Jana Kroll truly did pioneering work here,” says Rosenmund, who is also a board member of the NeuroCure Cluster of Excellence.
Flash-frozen in ethane
To observe synapses in action, the team used mouse neurons genetically modified through optogenetics so they could be activated by a flash of light – prompting them to secrete neurotransmitters immediately. One to two milliseconds after a light pulse, the researchers flash-froze the neurons in liquid ethane at minus 180°C. “All cellular activity stops instantly with this ‘plunge freezing’ method, allowing us to visualize the structures using electron microscopy,” explains Jana Kroll.
The method revealed another intriguing detail: “We found that most of the fusing vesicles were connected by tiny filaments to at least one other vesicle. As soon as one vesicle fuses with the membrane, the next one is already in position,” Jana Kroll reports. “We believe that this direct form of vesicle recruitment enables neurons to send signals over a longer period of time and thus maintain their communication.”
Toward better epilepsy treatment
The vesicle fusion process visualized by the team takes place millions of times a minute in the human brain. Understanding it in detail has important clinical implications. “In many people with epilepsy or other synaptic disorders, mutations have been found in proteins involved in vesicle fusion,” explains Christian Rosenmund. “If we can clarify the precise role of these proteins, it will be easier to develop targeted therapies for these so-called synaptopathies.”
“The time-resolved cryo-electron microscopy approach using light, as we’ve presented here, isn’t limited to neurons,” Jana Kroll adds. “It can be applied across many areas of structural and cell biology.” She now plans to repeat the experiments at the Max Delbrück Center using human neurons derived from stem cells. That won’t be easy, she notes: “In the lab, it takes about five weeks for the cells to develop their first synapses – and they are extremely fragile.”
*Kroll J et al. Dynamic nanoscale architecture of synaptic vesicle 2 fusion in mouse hippocampal neurons. Nat Comm 2025 13. doi: 10.1038/s41467-025-67291-6
About the study
The images were produced at the CFcryo-EM (Core Facility for cryo-Electron Microscopy), a joint technology platform operated by Charité, the Max Delbrück Center, and the Leibniz Research Institute for Molecular Pharmacology (FMP) that is directed by Dr. Christoph Diebolder. Also central to the study were Professor Misha Kudryashev, head of the In Situ Structural Biology lab at the Max Delbrück Center, and Dr. Magdalena Schacherl, Project Leader of the Structural Enzymology group at Charité.
Source: Joint press release by Charité and the Max Delbrück Center
Links
Contact:
Prof. Christian Rosenmund
Institute of Neurophysiology
Charité – Universitätsmedizin Berlin
T: +49 30 450 539 145