)
The research team around 2020’s Award Winner Stefan Pfeffer aims to reveal the inner workings of the cell at an unprecedented level of detail using an innovative imaging approach, cryo-electron tomography, which is uniquely suited to study the molecular structure and three-dimensional distribution of large macromolecules within intact frozen cells. This is achieved by an approach conceptually highly similar to medical CT imaging: different orientational views of an area of interest are sampled at very high magnification in an electron microscope and subsequently merged into a tomographic reconstruction of the sample area (see image). The resulting cellular tomograms are three-dimensional snapshots of the native molecular landscape of a cell, containing an almost inexhaustible amount of structural information.
One major conceptual limitation for structural studies in intact cells using cryo-electron tomography is that specimen thickness must not exceed 200 nanometers to allow the imaging electrons to pass through the sample. Thus, most human cells are almost an order of magnitude too thick to be directly imaged using cryo-electron tomography and solutions for artifact-free thinning of frozen cells had to be developed. Manual cutting of frozen cells using a diamond knife, as done for decades for dehydrated plastic-embedded samples, was out of the question due to mechanical compression of the specimen during cutting and the risk of melting the sample. A solution to the problem was found after many years of technical development and optimization in rasterizing a focused beam of Gallium ions across the frozen sample to mill away material in a highly targeted manner, while the rest of the specimen remains natively frozen.
This process requires highly specialized technical instrumentation, the development of which is still ongoing. Funding received from the Aventis Foundation for the Life Sciences Bridge Award was instrumental in upgrading the focused ion beam system in Stefan Pfeffer’s group to the latest standards, now enabling much more robust specimen preparation at even higher-throughput, which considerably accelerated research in his group.
Three-dimensional visualization of the cellular environment at molecular resolution. The machinery for cellular protein synthesis (light blue) is shown in the context of different cellular membranes.