PSD95 nanoclusters are postsynaptic building blocks in hippocampus circuits.

Area of Science:

• Neuroscience
• Cell Biology
• Structural Biology

Background:

• Synaptic molecular organization is crucial for learning and cognition.
• The sub-synaptic organization of synaptic molecules, particularly in the hippocampus, is not well understood.
• Understanding synapse structure at the molecular level is key to advancing neuroscience.

Purpose of the Study:

• To investigate the sub-synaptic organization of Post Synaptic Density 95 protein (PSD95) in hippocampal excitatory synapses.
• To characterize the structural diversity of synapses based on PSD95 nanocluster arrangement.
• To determine if PSD95 nanoclusters represent a fundamental unit of synaptic organization.

Main Methods:

• Mice engineered to express PSD95 fused to fluorescent proteins (eGFP or mEos2).
• Super-resolution microscopy techniques: gated stimulated emission depletion (g-STED) and photoactivated localization microscopy (PALM).
• Large-scale quantitative analysis of approximately 100,000 synapses across 7 hippocampal sub-regions.

Main Results:

• Hippocampal synapses are composed of discrete PSD95 nanoclusters organized into single and multi-nanocluster postsynaptic densities (PSDs).
• Synapse diversity, characterized by the number of nanoclusters per synapse, varies across hippocampal sub-regions, cell types, and dendritic locations.
• Multi-nanocluster synapses are prevalent in CA3 and dentate gyrus regions, correlating with large thorny excrescence synapses.
• Individual nanocluster structure is conserved, but PSD95 packing density varies between sub-regions.

Conclusions:

• PSD95 nanoclusters serve as fundamental structural units ('building blocks') of excitatory synapses.
• The number of PSD95 nanoclusters per synapse is a key determinant of synapse size and structural diversity.
• These findings provide a new framework for understanding synaptic organization and its role in hippocampal function.