The Mechanical Microenvironment Regulates Axon Diameters Visualized by Cryo-Electron Tomography

Area of Science:

• Neuroscience
• Cell Biology
• Biophysics

Background:

• Axonal varicosities are enlarged axon structures affecting neural function.
• Their presence in the healthy brain and underlying causes are poorly understood.
• Varicosities are often studied in neuropathology.

Purpose of the Study:

• To investigate the causes of axonal varicosities in the normal central nervous system (CNS).
• To explore the role of the microenvironment in regulating axon ultrastructure.

Main Methods:

• Confocal microscopy and cryo-electron tomography (Cryo-ET).
• In vivo and in vitro experimental systems using mouse models and EM grids.
• Analysis of axonal diameter variation and varicosity formation.

Main Results:

• Non-uniform mechanical interactions with the microenvironment cause significant axon diameter variation (up to 10-fold).
• CNS axons formed varicosities in specific microenvironmental contexts (e.g., holes in EM grids) without microtubule breakage.
• Varicosities contained organelles like mitochondria and multivesicular bodies (MVBs).
• Axon fasciculation reduced varicosity levels.

Conclusions:

• The mechanical microenvironment extrinsically regulates the 3D ultrastructure of CNS axons.
• Axon varicosities can form under physiological conditions due to mechanical forces.
• Understanding these extrinsic factors is crucial for comprehending normal brain structure and function.