Digging Deeper: Advancements in Visualization of Inhibitory Synapses in Neurodegenerative Disorders

Abstract

Recent research has provided strong evidence that neurodegeneration may develop from an imbalance between synaptic structural components in the brain. Lately, inhibitory synapses communicating via the neurotransmitters GABA or glycine have come to the center of attention. Increasing evidence suggests that imbalance in the structural composition of inhibitory synapses affect deeply the ability of neurons to communicate effectively over synaptic connections. Progressive failure of synaptic plasticity and memory are thus hallmarks of neurodegenerative diseases. In order to prove that structural changes at synapses contribute to neurodegeneration, we need to visualize single-molecule interactions at synaptic sites in an exact spatial and time frame. This visualization has been restricted in terms of spatial and temporal resolution. New developments in electron microscopy and super-resolution microscopy have improved spatial and time resolution tremendously, opening up numerous possibilities. Here we critically review current and recently developed methods for high-resolution visualization of inhibitory synapses in the context of neurodegenerative diseases. We present advantages, strengths, weaknesses, and current limitations for selected methods in research, as well as present a future perspective. A range of new options has become available that will soon help understand the involvement of inhibitory synapses in neurodegenerative disorders.

Bringing synapses into focus: Recent advances in synaptic imaging and mass-spectrometry for studying synaptopathy

“…While AT uses physical dissection to improve image resolution, another approach is to exploit super resolution microscopy techniques ( Eisenstein, 2015 ; Lambert and Waters, 2017 ; Raduloviæ et al, 2021 ; Fuhrmann et al, 2022 ) such as stochastic optical reconstruction microscopy (STORM, Rust et al, 2006 ), stimulated emission depletion (STED, Hell and Wichmann, 1994 ), photoactivated localization microscopy [PALM, ( Betzig et al, 2006 )], and structured illumination microscopy (SIM, Gustafsson, 2000 ). Briefly, these super-resolution imaging techniques achieve low nanometer resolutions by utilizing different ways to localize fluorophores more accurately in a sample, for instance by switching the fluorescent state of single molecules one at a time and determining their precise localization or by using two different lasers in a donut formation to selectively switch some fluorophores on while depleting others around them.…”