Making the move from 2D to 3D
By human standards, an angstrom is practically nothing. One angstrom unit equals one ten-millionth of a millimeter, making it tinier than a nanometer. An electron microscope can penetrate such scales. It works by bombarding a sample with a beam of accelerated electrons. Because the electron beam travels at very high speed, its wavelength is much shorter than that of visible light. Electron microscopes can therefore achieve a much higher resolution than a light microscope.
Dr. Séverine Kunz became fascinated by this technology when she characterized the sarcolemma – the cell membrane of muscle cells – in muscular dystrophies during her doctorate. Since late last year the 37-year-old has headed the Electron Microscopy Platform at the Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC).
The microscopy bug bit her in Paris
After studying biology in Mainz, Kunz commuted between Berlin and Paris for her doctorate as part of an international research training group. She worked on endocytosis, a cellular process that not only occurs during cell-to-cell communication but also during fatty acid metabolism. Here the cell envelope folds inward, forming small flask-shaped membrane structures called caveolae. These invaginations either remain on the surface of the cell membrane or they pinch off and carry foreign material – such as fatty acids – into the cell interior. She rotated between laboratories at Freie Universität Berlin and the Pierre and Marie Curie University in Paris. Kunz came into contact with electron microscopy (EM) under Professor Jean-Pierre Carteaux in Paris, whose research included signal transmission across synapses. She was immediately hooked. “It was breathtaking to discover the same images from which I had learned the inner workings of cells during my studies,” Kunz says. “Not only to see them with my own eyes, but also to understand how structure and function are interrelated – I found that thrilling.”
She acquired expertise in electron microscopy in Paris and brought this knowledge with her to Berlin. As a doctoral student in Professor Simone Spuler’s lab at the Experimental and Clinical Research Center (ECRC), she was the only member of the team who was well versed in this technology. The ECRC is a joint institution of the MDC and Charité – Universitätsmedizin Berlin. So it was not long before she became familiar with the MDC’s Electron Microscopy Platform. And she soon knew that her heart beats more for this method than for her own research.
Her favorite topic: Membranes and their cargo
With a PhD in hand, she used electron microscopy to observe the cristae in Professor Oliver Daumke’s lab. The cristae are the numerous folds on the inner membrane of mitochondria that give them their characteristic crumpled appearance and where many chemical reactions like cellular respiration take place. In Professor Carmen Birchmeier’s team she watched through the microscope as muscle cells fused together. And then a scientific position opened up in the Electron Microscopy Platform.
Kunz applied and got the job. “I very much enjoy using my expertise to support other scientists. This allows me to gain insights into many different research projects and help move them forward,” she says. Over the past five years she has been involved in many MDC projects as a scientist with the technology platform. She has used an electron microscope to visualize such things as insulin vesicles in the pancreatic islet organ, the neuromuscular end plate in organoids, and protein deposits in mouse brain tissue samples.
Electron microscopy goes 3D
When the platform’s previous director, Dr. Bettina Purfürst, retired last year, Kunz applied to succeed her and beat out external applicants. Her focus, she says, will be on supporting the advance of electron microscopy. Since April 2021 the platform has had a new tool in its imaging arsenal: a scanning electron microscope (SEM). This instrument may provide a slightly lower resolution than the platform’s previous go-to tool – a transmission electron microscope (TEM) – but it is capable of imaging larger volumes at high resolution. It achieves this by employing two other methods in addition to traditional surface imaging. In array tomography, the sample is cut into ultrathin serial sections and then automatically imaged with the electron microscope. This creates two-dimensional image tiles that are reconstructed computationally into three-dimensional volume images. The second method is called “focused ion beam scanning electron microscopy,” or FIB SEM for short. In this method no serial sections are prepared. Instead, a small block is made from the sample and placed inside the microscope. Here a focused ion beam first mills a very thin slice of the sample before the instrument images the smooth surface. The block sample is then repeatedly milled and imaged, producing a series of razor-sharp 3D images. Kunz is convinced that “the transition from 2D to 3D imaging, especially in combination with light microscopy, will greatly advance research at the MDC.”
The scanning electron microscope is a joint acquisition of the MDC and the Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP). “The close collaboration with our neighboring institute is a lot of fun,” enthuses the new platform director. “I would like to further deepen our collaboration in the future. Both research and method development benefit from it.” The same goes for the Core Facility for Cryo-Electron Microscopy (Cryo-EM) at Charité, which is run in cooperation with the MDC and FMP. “I see us as a link between high-resolution cryo-EM, which is used to do structural biology, and functional light microscopy, which enables live imaging.”
An open door policy
As head of the Electron Microscopy Platform, Kunz relies on close cooperation with researchers – just like when she was a staff scientist. Communication is very important to her. “I want our facility to always have an open door,” she says. For the MDC community and beyond.
Text: Jana Ehrhardt-Joswig