Fluorescence super-resolution microscopy has allowed unprecedented insight into the workings of biological systems below the diffraction limit of light. Over the past decade, it has overcome several challenges to deliver 3D, multi-color and faster imaging techniques, which have found many applications in deciphering the workings and structure of organelles â specialized cellular compartments whose shape and dynamics serve their specific functions. However, most super-resolution techniques are laborious and slow, and there are existing challenges in achieving sufficient throughput for collection of large datasets and capturing dynamic processes in living cells. The aim of this thesis is to address issues of limited throughput, temporal resolution and live-cell compatibility, with application to centriole architecture and mitochondrial dynamics.
The design of a novel flat-fielding module for multi-focal excitation which â once integrated into an instant structured illumination microscope â allows the capture of larger fields of view with uniform illumination and hence results in increased throughput. This flat-fielded microscope allows multi-color, 3-dimensional imaging at double the diffraction-limited resolution of up to 100 mammalian cells within 1-2 minutes. Furthermore, combined with advanced sample preparation using expansion microscopy, the setup achieves an effective resolution of ~35 nm, comparable to some of the more powerful super-resolution techniques, but at 100-1000 times increased throughput. To showcase this improvement, we study the distribution of post-translational modifications within the centriole, revealing previously unobserved molecular organization.
Second, with fast, live-cell imaging, we investigate the role of membrane bending energy and tension in mitochondrial division. While many molecular factors involved in this process have been identified, little is known about its physical requirements. By analyzing mitochondrial constrictions enriched in Drp1 that divide successfully or reverse, we establish that mitochondrial constrictions with higher bending energy are more likely to divide. Furthermore, by analyzing changes in mitochondrial shape and employing a novel membrane tension sensor, we observe that conditions under which mitochondrial membrane tension is increased, result in more successful divisions. These observations are supported by a model in which membrane bending energy increases the elastic energy at the constriction site, bringing it closer to the energy barrier to fission, while membrane tension acts as a fluctuation to overcome the remaining residual energy.
Finally, to overcome existing trade-offs between temporal resolution and imaging duration, we develop an acquisition framework for smart and adaptive temporal sampling. By adapting the temporal resolution is in response to events of interest, they are captured at high imaging speed, while preserving the sample during a lack thereof. Using the accumulation of Drp1 to detect mitochondrial constriction sites allowed us to adapt the imaging speed to capture mitochondrial constrictions at high temporal resolution while preserving the sample when such events are lacking. Overall, this framework provides both high temporal resolution and long imaging times when needed, decreasing resulting phototoxicity, photobleaching and amount of redundant data.
EPFL2020
