Structural diversity of mitochondria in the neuromuscular system across development

Mitochondria are highly dynamic organelles that can alter their shape and position according to the needs such as synaptic transmission. Previously, there have been active studies on mitochondrial structure in neurons or muscles. However, prior research has neither examined mitochondria structure within an entire neuromuscular system nor explored its changes across development. Using 3D electron microscopy (EM), we developed semi-automated methods for reconstructing mitochondria in C. elegans EM images using deep learning. Consequently, we collected mitochondria reconstructions of normal reproductive stages and dauer, enabling comparative study on morphology and spatial arrangement of mitochondria in different stages. As previous reports, we have validated spatial distribution of mitochondria in neurons is related to the spatial distribution of presynaptic sites. By inspecting the motor neurons, it has been revealed mitochondria adjacent to the postsynaptic sites are longer than mitochondria near the presynaptic sites, consistent with previous findings in mammalian cortical neurons. As motor neurons innervate muscles, we further explored how mitochondrial structure affects behavior. While it is normal to exhibit a high turning rate when C. elegans are placed away from the food, we discovered the turning rate significantly decreases in drp-1 mutants, where mitochondrial structure is disrupted. To test this is due to specific motor neurons, we showed neuron-specific rescue of SMD neurons recovered the normal behavior. Moreover, we discovered stage-specific properties of mitochondrial structure as well. We discovered the size of the mitochondria in neuronal cells of the dauer stage are larger, especially in motor neurons and interneurons regulating head and neck movement. We also found mitochondria in dauer body wall muscles exhibit unique stranded network structure. We believe these deviations in mitochondrial morphology observed from C. elegans dauer could be due to dauer-specific characteristics.