Dr Peter Dahlberg
Stanford University, USA
Numerous critical events in cell biology depend on rare ( 1 copy per cell) and small (<500 nm in diameter) structures. Observation of these processes at high resolution using cryogenic electron tomography (CryoET) presents challenges, as the structures must first be precisely targeted within thin sections through focused ion beam milling. In this presentation, I will introduce my group’s work on a tri-coincident system that integrates light, ion, and electron microscopy at a single focal point. This approach enables real-time monitoring of the milling process and makes two different modes of guidance possible that require no addition of fluorescent fiducials or image registration and whose accuracy far exceeds the optical diffraction limit. I will discuss both guidance modes in detail and then describe their application to the study of the in situ structure of the NLRP3 inflammasome. Despite its central role in innate immunity as a master regulator responsible for proinflammatory cytokine maturation and cell death, its in situ structure has remained elusive due to challenges in capturing the small singular punctum it forms per cell in thin sections compatible with CryoET — precisely the kind of target the tri-coincident system was developed for. Using our guidance approach, our cryo-tomograms revealed that the NLRP3 inflammasome forms a dense condensate within and around the microtubule-organizing center. At a later stage after activation, we saw further growth of the condensate, and the cells underwent pyroptosis with widespread mitochondrial damage and autophagy. Our study revealed new insights into NLRP3 structure and other organelle alterations during inflammation.