飲食、內質網壓力、脂肪衡定在線蟲內的交互關係

Abstract

根據世界衛生組織(WHO)統計，截至2016 年底，全球人口的過胖比例較1975年以來上升近三倍，而導致其結果的原因與不適當的飲食有關，調查結果亦發現體重過重的人比擁有正常體重的人有更高的風險罹患第二型糖尿病、心血管疾病及部分癌症。然而食物中的成分到底如何影響生物個體的生理行為，其詳細的分子機制尚未明瞭。因此我們利用秀麗隱桿線蟲(C.elegans)作為模式生物，欲探究不同的飲食是如何調控脂肪的代謝以及其他生理現象。透過O.R.O染取中性脂質，我們實驗室先前發現餵食Comamonas DA1877的線蟲其體內脂肪的含量較餵食標準飲食Escherichia coli OP50的線蟲來得少。根據我的研究結果發現，相較於餵食OP50的線蟲，餵食DA1877會引起線蟲體內的內質網壓力(ER stress)。而當抑制ER stress的hsp-4功能缺失時，餵食OP50的線蟲其脂肪含量會上升。然而參與在調控ER stress的ire-1及xbp-1突變株中，線蟲不管是在餵食OP50或是DA1877都可以造成脂肪含量的下降。這說明了飲食會影響ER stress的啟動而參與在調控ER stress的基因會影響脂肪的含量。另外，我發現ER stress啟動的另一條下游路徑，其中重要的脂肪生成因子sbp-1經由ER stress的啟動造成的抑制效果可能是餵食DA1877與OP50線蟲脂肪含量差異的重要原因。 我也發現飲食是經由mdt-15參與的機制來調控ER stress與造成脂肪含量的變化。我更進一步利用混菌的方式，發現DA1877飲食中含有特別的成分造成線蟲體內脂肪含量的下降。根據先前的研究，DA1877可以合成OP50所沒有的維他命B12，而B12 可以促進磷脂醯膽鹼(PC)的生成。透過實驗證明 PC的提升可以顯著地造成脂肪含量的下降，同時也會促進ER stress的產生。這說明飲食亦可以透過改變脂肪的平衡來影響ER stress的啟動。在我們實驗室先前的研究中亦發現，餵食DA1877的線蟲有較佳的移動能力，而移動的快慢與神經傳導物質釋放有關。同樣地，利用混菌及乙醯膽鹼脂酶抑制劑alidicarb，我發現DA1877飲食中所含有的特別成分會造成線蟲神經傳導物質釋放增加；實驗證明，DA1877 飲食所造成的PC含量增加亦是造成此現象的主要原因。最後我直接使用阿茲海默症及杭丁頓舞蹈症線蟲疾病模型來研究不同飲食對疾病發展所造成的影響，發現在氟尿苷(FUdR)的共同作用下，阿茲海默症疾病模型在餵食 OP50時有較良好的生理活動；而在杭丁頓舞蹈症疾病模型當中，不管餵食OP50或是DA1877其活動能力沒有顯著差異。這說明飲食對於不同疾病有不同的影響。我的實驗結果顯示，不同飲食可以調控線蟲的諸多生理現象，其中ER stress如何調控脂肪的代謝還需要更深入的分子機制研究。

As of 2016, worldwide obesity had nearly tripled since 1975 according to the statistics of World Health Organization (WHO), and the causes were associated with inappropriate diets. Surveys also revealed that people who were overweight have higher risk of suffering type two diabetes, cardiovascular diseases as well as part of cancers relative to people with normal weight. Nevertheless, the molecular mechanisms underlying how dietary components impact on physiological behaviors in organisms are still unclear. Therefore, we took advantage of Caenorhabditis elegans as model organism to investigate how different diets mediate lipid metabolism and other physiological phenomenon. Our previous data indicated that C. elegans fed on Comamonas DA1877 showed less lipid content compared to worms fed on standard diet, Escherichia coli OP50, through O.R.O staining. According to my investigation, DA1877-fed worms showed induced ER stress compared to OP50-fed worms. Loss of hsp-4, which inhibits ER stress, in OP50-fed worms exhibited increased lipid content compared to DA-fed worms. However, loss of ire-1 or xbp-1, both of which are involved in mediating ER stress, in worms showed decreased lipid content regardless of feeding OP50 or DA1877. The results reveal that diets play a role in affecting the induction of ER stress and genes involved in mediating ER stress can further affect lipid content. In addition, I found that sbp-1, a pivotal factor of lipogenesis, which is inhibited by the induction of the alternative downstream pathway of ER stress, may be the crucial reason why worms fed on OP50 or DA1877 showed different lipid contents. I also discovered that diets mediate ER stress as well as lead to lipid content changes through the mdt-15 involved mechanism. I further found that special ingredients in DA1877 diet results in the decreased lipid content in worms by the means of bacteria mixture. According to previous research, B12, which are absent in OP50 diets, can be synthesized in DA1877 diets, and B12 can facilitate the synthesis of phosphatidylcholine (PC). In my experiments, the increase of PC level could significantly decline lipid content and meanwhile trigger ER stress. The results reveal that diets could also affect ER stress induction via altering lipid homeostasis. Previous data in our lab also indicated that C. elegans fed on DA1877 showed better locomotion compared to worms fed on OP50, and the locomotion ability is related to the release of neurotransmitter. Likewise, I found that special ingredients in DA1877 diet lead to the increase of neurotransmitter release by the means of bacteria mixture and acetylcholinesterase inhibitor, aldicarb. The increase of PC level due to DA1877 diet is also the major reason of resulting in the increase of neurotransmitter release in my experiments. Finally, I directly used Alzheimer’s disease as well as Huntington’s disease model of worms to investigate how different diets have impact on the progression of disease. With floxuridine (FUdR) treatment, paralysis in OP50-fed Alzheimer’s disease model exhibited better motility compared to DA1877-fed worms while Huntington’s disease model fed on OP50 or DA1877 diets showed no difference in motility. The results reveal that diets have different impact on different diseases. My works indicate that different diets could mediate lots of physiological phenomenon in C. elegans, and how ER stress regulate lipid metabolism still needs profound investigation of molecular mechanism.