Cryo-electron tomography and correlative super-resolution light microscopy of neurons

This project will use state-of-the-art optical and electron microscopy at Diamond Light Source (DLS) to provide molecular information on complex neuronal processes.

In the project we want to investigate a dynamin-like structure surrounding and interconnecting synaptic vesicles. Synaptic vesicles are located in the synapse, the end terminal part of a neuron responsible for signal transduction. Synaptic vesicles contain small signalling molecules – neurotransmitter – that upon fusion with the plasma membrane release their content into the synaptic cleft and further to the neighbouring postsynapse. The vesicles are interconnected by so-called connectors of partly unknown protein composition and one potentially being dynamin.

In order to investigate the structure of function of these novel complexes, a vastly different approach will be required. Primary neurons will be grown on electron microscope (EM) support film (aka EM grids) with astrocytes as a feeder layer for support of neuronal growth. This part of the project will be carried out at the Mary Lyon’s mouse facility with Michelle Stewart as Co-Supervisor. Following a 14-day growth period on the EM grid the neurons can be vitrified. Vitrification is the process that turns water into an amorphous glass-like state by rapid freezing at -196°C leaving the neurons in a close-to-native state. After vitrification neurons can directly be imaged at the EM as they are thin enough to be transparent to 300 kV electrons. The grids will be screened for thin areas in which synapses and the respective postsynapses will have formed. Synapse and postsynapse form a functional unit required for signal transduction hence, only those synapses which possess a postsynapse will be considered for analysis. The synapse will then be imaged by cryo-electron tomography (cryo-ET). In cryo-ET the sample is tilted from -60 to 60° and an image is acquired every 2-3°. The reconstruction of those data provides 3D structural information in the cellular context. The achievable resolution of a tomogram is about 5 nm. In order to unambiguously determine the structure of these rings subtomogram averaging (STA) will subsequently be used to improve the resolution, possibly as good as 1 nm at a thickness of 250 nm (Sanchez, Zhang, Chen, Dietrich, & Kudryashev, 2020). In STA the region of interest is boxed out as a 3D volume and run through a software to classify similar structures and average those to improve the resolution.

Additionally, during the project, there will be time to investigate and learn about other microscopes present throughout DLS facilities if interested. The project could potentially be modified to include working with a cryo-light microscope, a cryo-light super-resolution microscope, and/or a cryo-focused ion beam microscope. Diamond offers vast opportunities for testing and learning about the latest generation microscopes at different beamlines.