Department of Integrative Biology, University of Wisconsin-Madison, Madison, Wisconsin 53706.
Genetics Training Program, University of Wisconsin-Madison, Madison, Wisconsin 53706.
J Neurosci. 2024 Aug 28;44(35):e0879242024. doi: 10.1523/JNEUROSCI.0879-24.2024.
Mitochondrial population maintenance in neurons is essential for neuron function and survival. Contact sites between mitochondria and the endoplasmic reticulum (ER) are poised to regulate mitochondrial homeostasis in neurons. These contact sites can facilitate transfer of calcium and lipids between the organelles and have been shown to regulate aspects of mitochondrial dynamics. Vesicle-associated membrane protein-associated protein B (VapB) is an ER membrane protein present at a subset of ER-mitochondrial contact sites. A proline-to-serine mutation in VapB at amino acid 56 (P56S) correlates with susceptibility to amyotrophic lateral sclerosis (ALS) type 8. Given the relationship between failed mitochondrial health and neurodegenerative disease, we investigated the function of VapB in mitochondrial population maintenance. We demonstrated that transgenic expression of VapB in zebrafish larvae (sex undetermined) increased mitochondrial biogenesis, causing increased mitochondrial population size in the axon terminal. Expression of wild-type VapB did not alter biogenesis but, instead, increased mitophagy in the axon terminal. Using genetic manipulations to independently increase mitochondrial biogenesis, we show that biogenesis is normally balanced by mitophagy to maintain a constant mitochondrial population size. VapB transgenics fail to increase mitophagy to compensate for the increase in mitochondrial biogenesis, suggesting an impaired mitophagic response. Finally, using a synthetic ER-mitochondrial tether, we show that VapB's function in mitochondrial turnover is likely independent of ER-mitochondrial tethering by contact sites. Our findings demonstrate that VapB can control mitochondrial turnover in the axon terminal, and this function is altered by the P56S ALS-linked mutation.
神经元中线粒体群体的维持对于神经元的功能和存活至关重要。线粒体与内质网（ER）之间的接触位点能够调节神经元中线粒体的动态平衡。这些接触位点可以促进细胞器之间钙和脂质的转移，并已被证明可以调节线粒体动力学的某些方面。囊泡相关膜蛋白相关蛋白 B（VapB）是内质网膜蛋白，存在于一部分内质网-线粒体接触位点中。VapB 中第 56 位氨基酸脯氨酸到丝氨酸的突变（P56S）与肌萎缩侧索硬化症（ALS）8 型的易感性相关。鉴于线粒体健康受损与神经退行性疾病之间的关系，我们研究了 VapB 在维持线粒体群体中的功能。我们证明，VapB 在斑马鱼幼虫（性别不确定）中的转基因表达增加了线粒体生物发生，导致轴突末端的线粒体群体大小增加。野生型 VapB 的表达不会改变生物发生，但会增加轴突末端的线粒体自噬。通过遗传操作独立增加线粒体生物发生，我们表明生物发生通常通过线粒体自噬来平衡，以维持线粒体群体大小的恒定。VapB 转基因无法增加线粒体自噬来补偿线粒体生物发生的增加，这表明存在受损的线粒体自噬反应。最后，使用合成的内质网-线粒体连接体，我们表明 VapB 在线粒体周转中的功能可能独立于通过接触点的内质网-线粒体连接。我们的研究结果表明，VapB 可以控制轴突末端的线粒体周转，而这种功能被与 ALS 相关的 P56S 突变改变。
