探討粒線體在線蟲PVD神經中不同區域的運送

Abstract

神經元(neuron)分成兩個主要構造: 樹突(dendrite)與軸突(axon)，粒線體(mitochondria)能夠提供神經元生長與傳導訊息所需的能量，並且可在微管(microtubule)上藉由動力蛋白(motor protein)運輸到突觸(synapse)或是受傷區域來提供能量。運輸粒線體的動力蛋白分為兩類: 往微管正極移動的驅動蛋白(kinesin)和往微管負極移動的動力蛋白(dynein)。先前的研究指出，如果粒線體在神經元運輸的過程受損，會導致許多神經退化性疾病，像是阿茲海默氏症，但是粒線體在神經元當中運輸的機制仍然不是很清楚。在我的研究當中，我觀察粒線體在線蟲PVD神經當中是如何分布的，並且將PVD神經分成樹突正極和負極區域、與軸突共三區，並且統計粒線體在神經分布的總量、在樹突與軸突分布的比例與在較長的負極樹突中分布的距離。我發現在一般情況下往正極移動的驅動蛋白在PVD神經除了會調控粒腺體送往軸突，也會調控粒線體送到負極的樹突區域，而往負極移動的動力蛋白則會調控粒線體送往PVD上最傾向負極的突觸區域。此外，我發現調控驅動蛋白運輸胞器的UNC-33蛋白對於調控粒線體運送到軸突扮演很重要的角色。我進一步的研究發現UNC-33蛋白主要是經由調控驅動蛋白-1(kinesin-1)和其他驅動蛋白來促進粒線體運送到軸突區域。我將UNC-33L蛋白表現在PVD神經後發現能夠回復失去UNC-33在神經造成型態與粒線體運輸的缺陷。最後，我進行了與粒線體運輸有關的基因檢測(forward genetic screening)，並且發現了許多可能會和UNC-33一起調控粒線體運輸的基因。總結以上的結果，UNC-33蛋白會調控神經構造的形成並且調控驅動蛋白來促進粒線體運輸到軸突。

Nervous systems transmit signals in animal bodies that is mediated by neurons with two different compartments: the somatodendritic and the axonal compartment. In neurons, energy is supplied by mitochondria, which needs to be located at some places like synapses or injury loci where energy is demanded. It has been shown that impaired mitochondrial transport is associated to many neurodegenerative diseases. However, the mechanisms of proper transport and distribution of mitochondria in neurons remain unclear. Mitochondria are transported by microtubule (MT)-based motor proteins such as kinesin superfamily proteins (KIFs) and cytoplasmic dyneins. In my study, I used PVD neurons in C. elegans and studied mitochondrial distribution in three parts of PVDs: anterior MT minus-end-out dendrites, posterior plus-end-out dendrites, and axon. In order to understand how motors regulate mitochondrial transport, I quantified the amount of mitochondria in different neurites and analyzed their distribution on dendrites or the axon, as well as transport distance on the long minus-end dendrites. Surprisingly, I found that kinesin-1 is required not only for axonal mitochondrial transport but also mitochondrial distribution in MT minus-end-out dendrites, while dynein preferably mediates mitochondrial transport to the minus-end dendrites. After examining basic roles of motors, I observed that in kinesin-1 regulator unc-33 mutant, the axonal mitochondria were absent, suggesting the essential role of UNC-33 for axonal mitochondrial transport. Furthermore, I demonstrated that UNC-33 functions through kinesin motors in mitochondrial transport. Only expression of the UNC-33L isoform in PVD neuron could rescue neuronal morphogenesis and mitochondrial transport defects of unc-33 but not the other two isoforms (UNC-33M and UNC-33S). Finally, I performed a forward genetic screening, and found several candidates that might cooperate with UNC-33 to regulate mitochondrial transport. Together, my data suggest that UNC-33 plays important roles in neuronal morphogenesis and regulates mitochondrial transport through motor proteins.