中国科学院昆明动物研究所机构知识库

机体的能量摄入和消耗决定了机体的能量水平，能量储存则处于这两者之间决定了机体的能量平衡。机体中能量主要以脂肪的形式存在，脂肪则主要是以中性脂(三酰甘油，胆固醇酯)的形式储存在脂滴（Lipid droplets, LDs）中。脂滴是一种由单层磷脂分子形成的极性表面包裹着中性脂疏水核心组成多功能细胞器，其在细胞内调控中性脂的存储和利用。随着脂滴生物学的研究深入，脂滴作为一种储存能量的细胞器，其与能量代谢之间的到底有怎样的关系是我们非常期待研究的方向。确切的说，我们的科学问题就是当机体能量代谢状态不平衡的时候机体将会做出什么响应，脂滴作为一种储存能量的细胞器又会发生什么变化。在细胞内合成代谢是最耗能的反应，而核糖体的合成又被认为是最需要能量的合成反应。核糖体的生物合成是一个十分复杂的过程，包括rDNA的转录，前体-rRNA的剪切，加工，修饰，随后再募集各种核糖体蛋白形成成熟的核糖体大亚基和小亚基。核糖体合成或者功能受到干扰就会使得细胞处于一种“核仁应激”（nucleolar stress）状态。因为核糖体合成将消耗细胞内80%左右的能量，因此当核糖体合成受到影响，而使得细胞处于核仁应激状态，必然会导致能量代谢的失衡。首先，我们想知道由于这种核仁应激导致的能量代谢失衡是否会影响机体的脂质代谢。其次，这种由于核糖体合成受到影响导致的过剩能量会不会促进脂肪的存储。这种核仁应激与能量代谢的联系是否依赖于已有的通路，还是有新的调控因子等等一系列问题仍然不清楚。在本研究中，我们利用秀丽线虫（Caenorhabditis elegnas）进行全基因组遗传诱变筛选得到突变体kun54，并呈现明显的高脂肪含量的表型。通过遗传图谱定位以及基因组重测序分析，我们确定了该突变基因为rrp-8（ribosomal RNA processing 8）。通过对线虫的RRP-8的功能研究，我们发现与酵母中相似，其主要参与26S rRNA特定位点的甲基化修饰，并影响前体rRNA（Pre-rRNA）的剪切加工。随后，核糖体图谱结果表明突变体rrp-8(kun54)中核糖体水平受到明显的抑制。我们的研究进一步发现rrp-8突变导致的高脂肪含量表型并不是特异性的，因为通过能够特异性抑制rDNA转录的化学物质放线菌素D（Actinomycin D，AD）处理线虫以及rRNA加工处理相关的突变体的观察和脂肪含量检测，发现均能明显升高脂肪含量。放线菌素D处理的线虫和rRNA加工处理相关突变体中核糖体图谱均呈现明显降低。因此，我们认为核仁应激能够影响脂质代谢并促进线虫体内脂肪的堆积。为进一步探索核仁应激与脂质代谢之间的内在关系，我们利用秀丽线虫进行一系列的遗传操作来阐明核仁应激促进脂肪储存的内在分子机制以及生理学意义。首先我们发现经典的核仁应激响应因子CEP-1/p53能够响应线虫中的核仁应激，但是并不能够调控核仁应激诱导的脂肪储存。随后，我们发现一个新的响应因子PHA-4/FoxA能够响应核仁应激，并且敲除pha-4能够抑制核仁应激诱导的脂肪储存。通过转录组测序分析发现在核仁应激状态下，脂肪合成相关基因fasn-1，pod-2以及dgat-2表达水平呈现明显上升。更重要的是，PHA-4作为一个转录因子，能够在转录水平调控这些脂肪合成相关基因的表达，促进脂肪合成。为了探索核仁应激促进脂肪储存的生理学意义，我们还发现在核仁应激状态下，过度的脂肪堆积有利于线虫抵抗饥饿环境。综上所述，我们发现在线虫中核仁应激能够引起脂肪的过度储存；PHA-4作为一个转录因子能够响应核仁应激并调控下游脂肪合成相关基因的表达；核仁应激诱导的脂肪储存有利于线虫抵抗饥饿的环境。 

Energy uptake and consumption determine the energy homeostasis in organism. Lipid is the best type for energy storage and primarily store in lipid droplets (LDs) in the form of neutral lipid (triacylglycerols and sterol esters). LDs are dynamic cytoplasmic organelles which contain a hydrophobic core surrounded with a surface of monolayer phospholipids. LDs serve multiple functions to regulate lipid synthesis, lipolysis and storage. With the study of LDs, we wonder links between lipid droplets and energy metabolism since LDs as organelles to store energy. Actually, the scientific question is what response to organism under unbalance energy metabolism and what occurs to LDs at the same circumstance.Anabolism is the major energy-consuming reaction in organism and ribosomal biogenesis was reckoned as most energetically demanding in anabolic reactions. Biogenesis of ribosomes is an extraordinary complex process which contains rDNA transcription, pre-rRNA splicing, modification and sequential recruitment of ribosomal proteins. Failures in ribosome biogenesis or function result in a condition termed nucleolar stress, which ultimately leads to disruptions in cell homeostasis as ribosome biogenesis consuming approximately eighty percent of a cellular energy. Firstly, we interested in whether ribosomal stress will affect lipid metabolism in multicellular organism is remain obscure. Furthermore, if the superfluous energy results from ribosomal stress promote fat accumulation, whether there novel response mechanism connects nucleolar stress and lipid droplets is still unknown.In this study, we performed whole genome ethyl methane sulphonate (EMS) screening and isolated a mutant kun54 which presented larger and more abundance LDs in the worm body. We found a mutation on rrp-8 through performing a rapid single nucleotide polymorphism (SNP) mapping and whole genome sequencing. RRP-8 also possesses methyltransferase activity like in yeast as previously reported. Subsequently, ribosomal profile analysis indicated that rrp-8(kun54) mutants markedly decreased ribosomes levels. Furthermore, we found that high fat accumulation phenotype in rrp-8(kun54) may be a general phenomenon, because decreasing ribosomes levels through specifically inhibiting rDNA transcription induced by actinomycin D (AD) treatment or mutants associated with rRNA processing are all present over load lipids. AD treatment or mutants associated with rRNA processing also decreased ribosomes levels like rrp-8(kun54). Altogether, these findings indicate that nucleolar stress triggered by the perturbation of ribosomal biogenesis results in excessive lipid accumulation.To further investigate connects between nucleolar stress and lipid metabolism, we perform a serial of genetic experiments to illuminate this mechanism and physiological significance. Firstly, we found that CEP-1/p53 as a nucleolar stress sensor did not regulate nucleolar stress induced fat accumulation. Secondly, we found a novel nucleolar stress sensor PHA-4/FoxA response to nucleolar stress and PHA-4 knockout markedly suppress the nucleolar stress induced lipid accumulation. The transcription factor PHA-4/FoxA acts as a sensor of nucleolar stress to bind to and transactivate the expression of the lipogenic genes pod-2, fasn-1 and dgat-2, consequently promoting lipid accumulation.Altogether, we discover that nucleoar stress could induce lipid accumulation in C. elegans; transcription factor PHA-4 response to nucleolar stress and regulate lipogenic genes expression; nucleolar stress induced lipid accumulation is benefit for starvation resistance.