A team of Berlin-based researchers led by Jana Kroll and Christian Rosenmund has captured the fleeting moment a nerve cell releases its neurotransmitters into the synaptic cleft. Their microscopic images and description of the process are published in "Nature Communications."
Joint press release by Charité – Universitätsmedizin Berlin and the Max Delbrück Center
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 Professor Christian Rosenmund of Charité – Universitätsmedizin Berlin has captured this critical moment of brain function in microscopic images. They describe their achievement in the journal "Nature Communications."
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 Professor 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," Kroll explains.
"With technology we developed over five years, it was possible for the first time to observe synapses in action without disrupting them," adds senior author Professor Christian Rosenmund, Deputy Director of the Institute for Neurophysiology at Charité. "Jana Kroll truly did pioneering work here," says Rosenmund, who is also a board member of the NeuroCure Cluster of Excellence.
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.
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 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," 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 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," 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."
Further information
Literature
Jana Kroll, et al. (2025): "Dynamic nanoscale architecture of synaptic vesicle 2 fusion in mouse hippocampal neurons." Nature Communications, 16, 11131. DOI: https://doi.org/10.1038/s41467-025-67291-6
Max Delbrück Center
The Max Delbrück Center for Molecular Medicine in the Helmholtz Association lays the foundation for the medicine of tomorrow through our discoveries of today. At locations in Berlin-Buch, Berlin-Mitte, Heidelberg, and Mannheim, interdisciplinary teams investigate the complexity of disease at the systems level – from molecules and cells to organs and entire organisms. Together with academic, clinical, and industry partners, and as part of global networks, we turn biological insights into innovations for early detection, personalized therapies, and disease prevention. Founded in 1992, the Max Delbrück Center is home to a vibrant, international research community of around 1,800 people from over 70 countries. We are 90 percent funded by the German federal government and 10 percent by the state of Berlin.
Charité - Universitätsmedizin Berlin
With more than 100 departments and institutes across four campuses and 3,293 beds, Charité – Universitätsmedizin Berlin is one of Europe's largest university medical centers. At Charité, the areas of research, teaching, and medical and patient care are closely interconnected. Averaging about 20,600 employees Charité-wide and some 24,300 across the entire group of companies, Berlin's university medicine organization remained one of the capital city's largest employers in 2024. Charité is a leader in diagnosis and treatment of particularly severe, complex, and rare diseases and health conditions. A medical school and university medical center in one, Charité enjoys an outstanding reputation worldwide, combining first-class patient care with excellence in research and innovation, state-of-the-art teaching, and high-quality training and education. Everything Charité does revolves around people and their health. Charité pursues translational research in which scientific findings are applied to prevention, diagnostics, and treatment and clinical observations inform new approaches in research in turn. At Charité, the goal is to actively help shape the medicine of the future to benefit patients. www.charite.de/en/
