In vivo imaging reveals mitophagy independence in the maintenance of axonal mitochondria during normal aging.

In vivo imaging reveals mitophagy independence in the maintenance of axonal mitochondria during normal aging.

Mitophagy is thought to be a critical mitochondrial quality control mechanism in neurons and has been extensively studied in neurological disorders such as Parkinson's disease. However, little is known about how mitochondria are maintained in the lengthy neuronal axons in the context of physiological aging. Here, we utilized the unique Drosophila wing nerve model and in vivo imaging to rigorously profile changes in axonal mitochondria during aging. We revealed that mitochondria became fragmented and accumulated in aged axons. However, lack of Pink1 or Parkin did not lead to the accumulation of axonal mitochondria or axonal degeneration. Further, unlike in in vitro cultured neurons, we found that mitophagy rarely occurred in intact axons in vivo, even in aged animals. Furthermore, blocking overall mitophagy by knockdown of the core autophagy genes Atg12 or Atg17 had little effect on the turnover of axonal mitochondria or axonal integrity, suggesting that mitophagy is not required for axonal maintenance; this is regardless of whether the mitophagy is PINK1‐Parkin dependent or independent. In contrast, downregulation of mitochondrial fission–fusion genes caused age‐dependent axonal degeneration. Moreover, Opa1 expression in the fly head was significantly decreased with age, which may underlie the accumulation of fragmented mitochondria in aged axons. Finally, we showed that adult‐onset, neuronal downregulation of the fission–fusion, but not mitophagy genes, dramatically accelerated features of aging. We propose that axonal mitochondria are maintained independently of mitophagy and that mitophagy‐independent mechanisms such as fission–fusion may be central to the maintenance of axonal mitochondria and neural integrity during normal aging.

线粒体自噬被认为是神经元中一种关键的线粒体质量控制机制，并且在帕金森病等神经系统疾病中已被广泛研究。然而，在生理衰老的背景下，对于线粒体如何在长长的神经元轴突中得以维持却知之甚少。在此，我们利用独特的果蝇翅神经模型和体内成像技术，严格地描述了衰老过程中轴突线粒体的变化。我们发现，在衰老的轴突中，线粒体变得碎片化并积累。然而，Pink1或Parkin的缺失并没有导致轴突线粒体的积累或轴突退变。此外，与体外培养的神经元不同，我们发现即使在老年动物体内，完整轴突中也很少发生线粒体自噬。再者，通过敲低核心自噬基因Atg12或Atg17来阻断整体线粒体自噬，对轴突线粒体的周转或轴突完整性几乎没有影响，这表明轴突的维持不需要线粒体自噬，无论这种线粒体自噬是依赖于PINK1 - Parkin还是不依赖。相反，线粒体分裂 - 融合基因的下调导致了年龄依赖性轴突退变。而且，果蝇头部的Opa1表达随着年龄的增长显著降低，这可能是衰老轴突中碎片化线粒体积累的原因。最后，我们表明，在成年期开始对分裂 - 融合基因（而非线粒体自噬基因）进行神经元下调，会显著加速衰老特征。我们提出，轴突线粒体的维持不依赖于线粒体自噬，并且在正常衰老过程中，诸如分裂 - 融合等不依赖于线粒体自噬的机制可能对轴突线粒体的维持和神经完整性至关重要。