Morphological Adaptations of Mitochondria in Spatially Restricted Axons Revealed by Cryo-Electron Tomography

Abstract Neurons project axons to local and distal sites that can display heterogeneous morphologies with restricted physical dimensions that may influence the transport of large organelles, such as mitochondria. Although transport of mitochondria has been widely studied, whether spatial constraints within axons determines the distribution or morphological characteristics of mitochondria has not been systematically examined. The current study provides insight into the spatial environments of mitochondria within axons and presynaptic varicosities of cryopreserved hippocampal neurons imaged in their native state using 3D cryo-electron tomography (cryo-ET). Segmented tomographic reconstructions revealed major morphological adaptations of mitochondria due to restricted physical dimensions at the boundary between presynaptic varicosities and axons. Furthermore, segmentation of individual mitochondrial membranes revealed distinct structural changes between the inner and outer membranes during this modification, suggesting independent regulation of each membrane to adapt to the spatial constraints within axons. These findings offer a new perspective on the remarkable structural plasticity of mitochondrial membranes and the adaptability of mitochondrial morphology to navigate limited available space necessary for axonal transport.