Development and Application of Technology for Neural Circuit Visualization - Secondary Publication.

The dynamics of neurite extension and synaptic connections are central issues in neural circuit research. The development of technologies for labeling purified cytoskeletal proteins with fluorescent dyes and introducing them into living neurons using microinjection greatly facilitated our understanding of cytoskeletal dynamics in neuronal axons. Imaging data showed that the cytoskeleton repeatedly polymerized and depolymerized within the axon, and elongation was driven by the new cytoskeleton formed at the axon tip. This finding significantly revised previously proposed models that explained slow axonal transport. After the discovery of green fluorescent protein (GFP), its potential application to the live imaging of neurons was recognized in the 1990s, and a new method for visualizing synapses using GFP-tagged postsynaptic scaffolding molecules was established. This method revealed the continuous turnover of synapses during development, which overturned the established theory that synapses are highly stable once they are formed. Live imaging of synapses also demonstrated that the molecular composition of synapses changes rapidly, driven by the rapid replacement of synaptic molecules. Fluorescence measurement of single GFP molecules enabled estimation of the absolute number of postsynaptic molecules in a single synapse. Furthermore, in multiple mouse models of autism spectrum disorders (ASDs), enhanced synapse turnover was detected as a common circuit-level phenotype. This study provides solid experimental evidence that an increase in synapse dynamics underlies the pathophysiology in mouse models of ASDs. The introduction of fluorescence imaging in neurobiology revealed that the neuronal cytoskeleton and synaptic structure are not static but dynamic cellular components. Imaging technology is expected to further advance our understanding of the dynamic properties of neurons and neural circuits.