Abstract
The presynaptic compartment of the chemical synapse is a small, yet extremely complex structure. Considering its size, most methods of optical microscopy are not able to resolve its nanoarchitecture and dynamics. Thus, its ultrastructure could only be studied by electron microscopy. In the last decade, new methods of optical superresolution microscopy have emerged allowing the study of cellular structures and processes at the nanometer scale. While this is a welcome addition to the experimental arsenal, it has necessitated careful analysis and interpretation to ensure the data obtained remains artifact-free. In this article we review the application of nanoscopic techniques to the study of the synapse and the progress made over the last decade with a particular focus on the presynapse. We find to our surprise that progress has been limited, calling for imaging techniques and probes that allow dense labeling, multiplexing, longer imaging times, higher temporal resolution, while at least maintaining the spatial resolution achieved thus far.
Highlights
The classical chemical synapse in the central nervous system (CNS) of vertebrates is a discontinuous structure consisting of a presynapse formed by the signal transducing neuron and a postsynapse formed by the receiving neuron
A direct interaction of, e.g., Munc13-1 and voltage gated Ca2+ channels (VGCCs) has been shown (Calloway et al, 2015) but attributed to control of VGCC function rather than recruitment. 3D stochastic optical reconstruction microscopy (STORM) revealed that Munc13-1 molecules form multiple supramolecular clusters that serve as independent synaptic vesicles (SVs) release sites by recruiting Syntaxin1A, one of the target SNARE proteins (t-SNARE) in the presynaptic plasma membrane (PM) (Sakamoto et al, 2018)
Presynapses harbor distinct SV fusion sites that are defined by a complex interplay between cytomatrix of the presynaptic active zone (CAZ) proteins, VGCCs, Munc13 and t-SNARES and significant contributions from SR microscopy has helped shed to more light on active zone (AZ) architecture and organization of SV release sites
Summary
The classical chemical synapse in the central nervous system (CNS) of vertebrates is a discontinuous structure consisting of a presynapse formed by the signal transducing neuron and a postsynapse formed by the receiving neuron. The two halves of the synapse are separated by a synaptic cleft with a width of approximately 15–20 nm (De Robertis and Bennett, 1955; Palay and Palade, 1955) and the presynaptic swelling or bouton is densely filled with granular structures designated as synaptic vesicles (SVs). The advent of nanoscopic light microscopy techniques more than a decade ago, held the particular promise that nanometer resolution in combination with highly efficient protein labeling strategies, either by immunostaining or genetically encoded fluorescent proteins will greatly increase our understanding of the presynaptic nano-architecture and protein networks far beyond. After briefly summarizing the previous results made by EM, we ascertain the advances in our understanding of the presynaptic nano-architecture driven by the application of nanoscopic techniques
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