Watching nerve cells fire – this is what the research team achieved using an optogenetic technique. The neurons, shown here in purple, release neurotransmitters in response to a light pulse, causing the synapses between them to glow yellow. Using plunge freezing, the team froze the processes and examined the release of the neurotransmitters in detail. © Charité/Max Delbrück Center | Jana Kroll
Berlin, December 17, 2025
The moment a nerve cell releases its neurotransmitters into the synaptic cleft is extremely brief. Researchers at Charité – Universitätsmedizin Berlin and the Max Delbrück Center have succeeded in capturing this moment microscopically. They have now published images of the merging vesicles in the journal Nature Communications*.
The process lasts only a few milliseconds: A vesicle, filled with neurotransmitters and only a few nanometers in size, approaches the cell membrane, fuses with it, and releases its messenger substances into the synaptic cleft – allowing them to attach to the next nerve cell. A team led by Prof. Christian Rosenmund, senior author of the publication and deputy director of the Institute of Neurophysiology at Charité, has captured this crucial moment for brain function in microscopic images.
Point-like connections
“Until now, no one knew in detail how the fusion of synaptic vesicles with the cell membrane occurs,” says the study’s first author, Dr. Jana Kroll, who now conducts research in Prof. Oliver Daumke’s “Structural Biology of Membrane-Associated Processes” working group at the Max Delbrück Center, explains, “In our experiments with mouse neurons, we were able to show that a point-like connection initially forms. This tiny stalk then expands into a pore through which the neurotransmitters enter the synaptic cleft.”
“Using the technology developed over five years, it has been possible for the first time to observe synapses at work without disturbing them,” adds Christian Rosenmund. “Jana Kroll has done truly pioneering work here,” says the scientist, who is also a member of the board of the NeuroCure Cluster of Excellence.
Shock-frozen in ethane
To observe the synapses in real time, the researchers used nerve cells from mice that they had previously modified using optogenetics so that the cells were activated by a light signal—and then immediately began releasing neurotransmitters. Within one to two milliseconds, the team then flash-froze the neurons in ethane at minus 180 degrees Celsius. “In this method, called plunge freezing, all cellular processes immediately cease and can be visualized using an electron microscope,” explains Jana Kroll.
The scientists stumbled upon another interesting detail: “We were able to see that most of the fusing vesicles are connected to at least one other vesicle via small filaments – as soon as one vesicle fuses with the cell membrane, the next one is already waiting,” reports Jana Kroll. “We assume that this direct form of vesicle recruitment allows neurons to send signals over extended periods and thus maintain their communication.”
Better Treatment of Epilepsy
The vesicle fusion that the team visualized occurs millions of times every minute in our brains. Understanding the process in detail is also important for medical purposes: “Many people with epilepsy or other synaptic disorders are known to have mutations in proteins involved in vesicle fusion,” explains Christian Rosenmund. “If we uncover the precise role of these proteins, we can more easily develop targeted therapies for such synaptic disorders.”
“The approach we presented for time-resolved cryo-electron microscopy using light is not limited to neurons, but can be applied in many areas of structural and cell biology,” adds Jana Kroll. She herself now plans to repeat her experiments at the Max Delbrück Center, initially using human neurons derived from stem cells. However, this will not be an easy task, the researcher warns: “The cells require about five weeks in the lab to develop their first synapses, and they are extremely sensitive in the process.”
*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 acquired at CFcryo-EM (Core Facility for cryo-Electron Microscopy), the joint technology platform of Charité, the Max Delbrück Center, and the FMP (Leibniz Research Institute for Molecular Pharmacology), headed by Dr. Christoph Diebolder. Also significantly involved in the study were Prof. Misha Kudryashev, head of the “In Situ Structural Biology” research group at the Max Delbrück Center, and Dr. Magdalena Schacherl, head of the “Structural Enzymology” research group at Charité.