Cizio, K. | bioRxiv Science

bioRxiv2025

Fis1 is required for the development of the dendritic mitochondrial network in pyramidal cortical neurons

Mitochondrial ATP production and calcium buffering are critical for metabolic regulation and neurotransmission making the formation and maintenance of the mitochondrial network a critical component of neuronal health. Cortical pyramidal neurons contain compartment-specific mitochondrial morphologies that result from distinct axonal and dendritic mitochondrial fission and fusion profiles. We previously showed that axonal mitochondria are maintained at a small size as a result of high axonal mitochondrial fission factor (Mff) activity. However, loss of Mff activity had little effect on cortical dendritic mitochondria, raising the question of how fission/fusion balance is controlled in the dendrites. Thus, we sought to investigate the role of another fission factor, fission 1 (Fis1), on mitochondrial morphology, dynamics and function in cortical neurons. We knocked down Fis1 in cortical neurons both in primary culture and in vivo, and unexpectedly found that Fis1 depletion decreased mitochondrial length in the dendrites, without affecting mitochondrial size in the axon. Further, loss of Fis1 activity resulted in both increased mitochondrial motility and dynamics in the dendrites. These results argue Fis1 exhibits dendrite selectivity and plays a more complex role in neuronal mitochondrial dynamics than previously reported. Functionally, Fis1 loss resulted in reduced mitochondrial membrane potential, increased sensitivity to complex III blockade, and decreased mitochondrial calcium uptake during neuronal activity. The altered mitochondrial network culminated in elevated resting calcium levels that increased dendritic branching but reduced spine density. We conclude that Fis1 regulates morphological and functional mitochondrial characteristics that influence dendritic tree arborization and connectivity.

cell biology↗

bioRxiv2024

Combining in vivo 2-photon imaging with photoactivatable fluorescent labeling reveals strikingly low rates of mitochondrial dynamics in skeletal muscle.

Mitochondrial dynamics involve two distinct and opposing processes, fusion and fission. Traditionally we assess fusion and fission by snapshots of protein markers at distinct time points or in vitro models to infer outcomes in vivo. Recent technological advancements enable visualization of mitochondrial dynamics in vivo using fluorescent microscopy. Our study modified this technique to evaluate mitochondrial dynamics in skeletal muscle, comparing young (6mo) and old (24mo) mice in vivo and contrasting this to ex vivo and in vitro models. We hypothesized that in vitro and ex vivo models would have higher rates of dynamics than in vivo models and that young animals would have higher rates than old animals. We electroporated mitochondrial matrix-targeted photo-activatable GFP into the tibialis anterior (TA) of young and old C57Bl6 mice and imaged using multiphoton microscopy. We also measured rates of mitochondrial dynamics using single fibers isolated from the TA of the electroporated mice, as well as C2C12 myotubes transfected with the same plasmids. We found that the rates of dynamic events in vivo are slower than previously indicated, with the C2C12 myoblasts having the fastest rates of dynamic events across all models. We also observed that dynamic rates are slower in old animals compared to young animals. Finally, we found that rates of dynamic events were higher in old animals after an acute bout of exercise. Our data demonstrate it is possible to directly measure rates of mitochondrial dynamics in vivo. This technique provides a powerful tool to answer experimental questions about mitochondrial dynamics of skeletal muscle.

physiology↗