Towards a Comprehensive Optical Connectome at Single Synapse Resolution via Expansion Microscopy

Area of Science:

• Neuroscience
• Molecular Biology
• Microscopy

Background:

• Mapping neuronal circuits requires visualizing individual synapses, a challenge due to their small size and high density.
• Electron microscopy (EM) offers nanoscale resolution but is limited to small samples and involves tedious data analysis.
• Expansion microscopy (ExM) provides a way to image larger biological samples at nanoscale resolution.

Purpose of the Study:

• To review advanced methods and molecular probes for fluorescence staining of synapses compatible with ExM.
• To highlight genetically encoded probes for synaptic labeling in various model organisms.
• To discuss experimental considerations and limitations for using these tools.

Main Methods:

• Review of current literature on expansion microscopy (ExM) techniques.
• Analysis of genetically encoded probes for synaptic labeling (postsynaptic scaffolds, receptors, presynaptic terminals, vesicles).
• Discussion of molecular probe delivery vectors and experimental considerations for ExM.

Main Results:

• ExM enables nanoscale imaging of large biological samples for neuronal circuit tracing.
• Advanced molecular probes allow multicolor labeling of synaptic components and visualization of synaptic activity.
• Genetically encoded probes are available for model organisms including mice, zebrafish, Drosophila, and C. elegans.

Conclusions:

• ExM combined with novel molecular probes offers an unprecedented capability for mapping neuronal circuits.
• These tools facilitate automated tracing and analysis of synaptic connectivity and function.
• Emerging tools promise further advancements in understanding brain circuitry.