Microbacterium diet rich in branched chain fatty acids promotes lipid droplet enlargement and impacts organismal health in C. elegans

Understanding how dietary nutrients alter lipid metabolism in the host is important in deciphering the physiological mechanisms that regulate organismal health. Using the nematode Caenorhabditis elegans as a model organism, I found that a novel gram-positive monomethyl branched-chain fatty acid (mmBCFA) rich bacteria, Microbacterium sp., isolated from C. elegans natural habit can induce supersized lipid droplets (LDs) up to 30 µm in diameter in older animals and adversely affect their lifespan, development rate and brood size. Lipidomic analysis indicated that Microbacterium sp.-fed worms primarily accumulate dietary mmBCFAs in the triacylglycerides stored within LDs. Transcriptomic analysis discovered that the mmBCFA-rich bacteria diet would enhance fatty acid desaturase fat-7 expression and therefore polyunsaturated fatty acids (PUFAs) production to support LD biosynthesis and fusion processes. Additionally, the diet from Microbacterium sp. significantly inhibits creatine kinase argk-1, which results in the suppression of the AMP-activated protein kinase pathway, leading to a halt in mitochondrial beta-oxidation, enhanced lipid de novo synthesis and activation of a mitochondrial unfolded protein response (UPRmt) in C. elegans. Furthermore, using this enlarged LD-inducing bacteria diet, a gain-of-function missense mutation in scav-4 is identified to exacerbate the LD expansion phenotype while the loss of scav-4 results in diminished lipid storage in the worms. Lipidomic analysis showed that the L462F missense mutation nearly doubles the uptake of dietary mmBCFAs, whereas deletion mutants exhibit a reduced proportion of dietary mmBCFAs in total lipids. The localization of SCAV-4 on the intestinal apical membrane and its phylogenetic similarity to the human CD36 fatty acid transporter suggest that SCAV-4 is a homolog of CD36. Structural analysis indicates that the missense mutation is likely located within the transport tunnel, potentially affecting lipid transport. Among the six SCAV family members in C. elegans, SCAV-6 appears to be a less critical paralog compared to SCAV-4, sharing similar expression patterns and showing reduced lipid storage upon deletion. Notably, the transcriptomic analysis reveals that the SCAV-4 missense mutation does not cause much transcriptomic changes under the E. coli OP50 diet but affects over 3,000 genes in response to the Microbacterium sp. diet, suggesting that animals with SCAV-4(L462F) mutation is much more sensitive to the mmBCFAs-rich diet than the wild-type animals. This finding highlights the influence of host genetic variations in the interactions with dietary nutrients, which has strong impact on host metabolism. Overall design: To study the gene expression changes of different strain of scav-4 mutants under E. coli OP50 diet and Microbacterium sp. JUb74 diet

解析调控机体健康的生理机制，阐明膳食营养素如何改变宿主脂质代谢，具有重要研究价值。本研究以秀丽隐杆线虫（Caenorhabditis elegans）为模式生物，发现从线虫自然栖息地分离得到的一株新型富含甲基支链脂肪酸（monomethyl branched-chain fatty acid, mmBCFA）的革兰氏阳性细菌——微杆菌属（Microbacterium sp.），可诱导老龄线虫形成直径可达30微米的超大脂滴（lipid droplets, LDs），并对其寿命、发育速率与产卵量产生负面影响。脂质组学分析显示，喂食微杆菌属细菌的线虫，其脂滴内储存的甘油三酯中主要积累了膳食来源的甲基支链脂肪酸。转录组学分析发现，富含甲基支链脂肪酸的膳食可上调脂肪酸去饱和酶fat-7的表达，进而促进多不饱和脂肪酸（polyunsaturated fatty acids, PUFAs）的生成，以支持脂滴的生物合成与融合过程。此外，微杆菌属细菌的膳食会显著抑制肌酸激酶argk-1的活性，进而抑制AMP活化蛋白激酶通路，导致线虫线粒体β氧化受阻、脂质从头合成增强，并激活线粒体未折叠蛋白反应（mitochondrial unfolded protein response, UPRmt）。进一步利用该可诱导脂滴增大的细菌膳食，研究人员发现scav-4基因的功能获得性错义突变会加剧脂滴扩张表型，而scav-4基因缺失则会降低线虫的脂质储存水平。脂质组学分析显示，L462F错义突变可使线虫对膳食来源甲基支链脂肪酸的摄取量提升近一倍，而缺失突变体的总脂质中膳食甲基支链脂肪酸的占比则有所降低。SCAV-4在肠道顶膜的定位，以及其与人类CD36脂肪酸转运蛋白的系统发育同源性，表明SCAV-4是CD36的同源蛋白。结构分析显示，该错义突变位点可能位于转运通道内，或会影响脂质转运过程。在线虫的6个SCAV家族成员中，与SCAV-4相比，SCAV-6的功能冗余性更强：二者表达模式相似，且SCAV-6缺失后线虫的脂质储存水平也会降低。值得注意的是，转录组学分析显示，在喂食大肠杆菌OP50的条件下，SCAV-4(L462F)错义突变并未引起显著的转录组变化；但在喂食微杆菌属细菌的条件下，该突变会影响超过3000个基因的表达，表明携带SCAV-4(L462F)突变的线虫，相较于野生型线虫，对富含甲基支链脂肪酸的膳食更为敏感。该研究结果凸显了宿主遗传变异在与膳食营养素互作中的作用，其对宿主代谢具有显著影响。实验整体设计：探究不同scav-4突变体菌株在喂食大肠杆菌OP50与微杆菌属JUb74菌株膳食时的基因表达变化。