In neurons, mitochondria are transported by molecular motors throughout the cell to form and maintain functional neural connections. These organelles have many critical functions in neurons and are of high interest as their dysfunction is associated with disease. While the mechanics and impact of anterograde mitochondrial movement toward axon terminals are beginning to be understood, the frequency and function of retrograde (cell body directed) mitochondrial transport in neurons are still largely unexplored. While existing evidence indicates that some mitochondria are retrogradely transported for degradation in the cell body, the precise impact of disrupting retrograde transport on the organelles and the axon was unknown. Using long-term, in vivo imaging, we examined mitochondrial motility in zebrafish sensory and motor axons. We show that retrograde transport of mitochondria from axon terminals allows replacement of the axon terminal population within a day. By tracking these organelles, we show that not all mitochondria that leave the axon terminal are degraded; rather, they persist over several days. Disrupting retrograde mitochondrial flux in neurons leads to accumulation of aged organelles in axon terminals and loss of cell body mitochondria. Assays of neural circuit activity demonstrated that disrupting mitochondrial transport and function has no effect on sensory axon terminal activity but does negatively impact motor neuron axons. Taken together, our work supports a previously unappreciated role for retrograde mitochondrial transport in the maintenance of a homeostatic distribution of mitochondria in neurons and illustrates the downstream effects of disrupting this process on sensory and motor circuits.

SIGNIFICANCE STATEMENT Disrupted mitochondrial transport has been linked to neurodegenerative disease. Retrograde transport of this organelle has been implicated in turnover of aged organelles through lysosomal degradation in the cell body. Consistent with this, we provide evidence that retrograde mitochondrial transport is important for removing aged organelles from axons; however, we show that these organelles are not solely degraded, rather they persist in neurons for days. Disrupting retrograde mitochondrial transport impacts the homeostatic distribution of mitochondria throughout the neuron and the function of motor, but not sensory, axon synapses. Together, our work shows the conserved reliance on retrograde mitochondrial transport for maintaining a healthy mitochondrial pool in neurons and illustrates the disparate effects of disrupting this process on sensory versus motor circuits.

在神经元中，线粒体通过分子马达转运至整个细胞，形成并维持功能性神经连接。这些细胞器在神经元中具有许多关键功能，并且由于它们的功能障碍与疾病有关而引起人们的高度关注。虽然人们开始了解顺行线粒体向轴突末端运动的机制和影响，但神经元中逆行（细胞体定向）线粒体运输的频率和功能仍未得到充分研究。尽管现有证据表明某些线粒体被逆行运输以降解细胞体，但破坏逆行运输对细胞器和轴突的确切影响尚不清楚。长期体内使用成像，我们检查了斑马鱼的感觉和运动轴突中的线粒体运动。我们表明，从轴突终端线粒体的逆行运输允许在一天之内更换轴突终端人口。通过跟踪这些细胞器，我们显示出并非所有离开轴突末端的线粒体都被降解。相反，它们会持续几天。破坏神经元中逆行的线粒体通量会导致衰老的细胞器在轴突末端积累，并导致细胞线粒体丢失。神经回路活动的分析表明，破坏线粒体的运输和功能对感觉轴突终末活性没有影响，但会对运动神经元轴突产生负面影响。在一起

重要性声明线粒体运输中断与神经退行性疾病有关。该细胞器的逆行运输与通过细胞体中的溶酶体降解引起的老年细胞器的更新有关。与此相一致，我们提供证据表明逆行线粒体运输对于从轴突中去除衰老的细胞器很重要。但是，我们证明这些细胞器不仅会降解，而且会在神经元中持续数天。逆行线粒体运输的中断会影响整个神经元线粒体的稳态分布，并影响运动轴突的功能，而不是感觉轴突的功能。一起，