用遺傳分析探討線蟲微管蛋白對於軸突運輸的影響

Abstract

微管是維持神經結構的重要的構造，在神經的軸突運輸當中，微管扮演軌道的角色，讓運動蛋白可以在微管上移動，進而運送特定的貨物，像是蛋白或突觸小泡。除了當作軌道之外，微管更可以藉由微管蛋白上的後轉譯修飾等方式來主動控制貨物的運送方向。我們的研究發現，微管蛋白上特定位置的胺基酸也可以藉由和運動蛋白之間不同的親合性，來控制突觸小泡的運送方向。在模式生物Caenorhabditis elegans線蟲當中，α-微管蛋白基因mec-12會特定地表現在線蟲觸覺神經中。當這個MEC-12蛋白功能喪失時，觸覺神經上的突觸小泡就無法被運送到突觸，而聚集在觸覺神經細胞體內。本篇研究發現一個新的mec-12突變，不只是讓突觸小泡聚集在細胞體內，更將其反向運送到觸覺神經PLM的後端，我們將這個突變稱之為gm379。gm379也使得觸覺神經的軸突上出現大的三角形突起。透過RNAi以及SNP mapping的實驗，我們發現gm379將MEC-12上一個高度保留的glycine突變成為glutamate，而這樣一個突變造成的軸突三角形突起和突觸小泡運送方向的改變是一種增益效果的突變。透過免疫染色的實驗我們發現，在gm379中微管的後轉譯修飾並沒有受到影響。有趣的是，當負責順向運送的運動蛋白kinesin產生突變，會造成更多的突觸小泡錯誤地被運送到PLM的後端。相反地，去除反向運送的運動蛋白dynein之功能，可以減少突觸小泡的錯誤運送。這兩種運動蛋白相互競爭在微管上的接合位，所以我們假設gm379改變了微管對於這兩種運動蛋白的親合性，進而造成突觸小泡的運送方向改變。

Microtubules play essential functions in axon transport by serving as tracks for motor proteins and their cargos. They also play a more active role by directing cargo transport through tubulin posttranslational modification. Here we propose yet another level of regulation: specific residues on tubulins directly regulate synaptic vesicle transport by binding motor proteins with differential affinities. The C. elegans gene mec-12 encodes an α-tubulin that is uniquely enriched in the six touch receptor neurons. Complete loss of mec-12 causes loss of synaptic vesicles at synaptic regions and their accumulation in neuronal cell bodies. We identified a missense mutation of mec-12, gm379, which not only prevents synaptic vesicles from reaching synaptic regions, but also redirects them to non-axon compartment of the PLM touch neuron (synaptic vesicle mistargeting). This mutation also triggered extensive axon blebbing in the touch neurons. gm379 alters a conserved C-terminus glycine residue and behaves as a neomorphic gain-of-function mutation, as axon blebbing and synaptic vesicle mistargeting were not seen in the mec-12 null and could be completely abolished by mec-12 RNAi. Immunostaining for various microtubule posttranslational modifications revealed no obvious changes in the gm379 mutant, and synaptic vesicle mistargeting was not seen after RNAi knockdown of genes encoding enzymes for microtubule posttranslational modifications. Interestingly, reducing UNC-104/Kinesin 3/KIF1A functions aggravated vesicle mistargeting, and excess UNC-104 partially rescued it. By contrast, elimination of dynein heavy chain DHC-1 partially suppressed synaptic vesicle defects in gm379, mimicking the effects of UNC-104 overexpression. The glycine residue mutated in gm379 resides in an exposed helix-loop region on the tubulin polymers, which had been shown to be a common binding site for KIF1A and dynein heavy chain. We hypothesize that gm379 switches motor binding affinity of microtubules towards dynein, resulting in transport defects and mistargeting of synaptic vesicles.