脂滴是广泛存在的储存中性脂的细胞器，它的生成、生长、维持和降解需要多种蛋白的调控，并处在高度的动态之中，其功能的失调与多种代谢疾病相关。脂质从内质网到脂滴的过程对控制脂滴的生长成熟和细胞内脂质平衡具有重要意义。然而，对于脂滴成熟过程的调控及介导内质网与脂滴之间相互交流的分子媒介目前仍不清楚。在本文的研究中，我们确定了Rab18是一个调控脂滴生长的重要的Rab蛋白。Rab18的缺失会导致细胞内成熟脂滴数目减少以及脂质储存能力的降低，并伴随着内质网应激的增加。经过实验我们证实Rab18并不影响脂滴的生成，这种成熟脂滴数目的减少是由于内质网上合成的中性脂无法正常掺入到初生脂滴所导致的。Rab18在脂滴生成之初便富集到内质网的脂滴生成位点，调控脂滴的生长。内质网上的中性脂合成途径对于Rab18介导的脂滴成熟过程是必须的。由此我们发现，内质网定位的SNARE蛋白 (Syntaxin18, Use1, BNIP1) 对Rab18发挥作用非常重要，Rab18通过NRZ (NAG-RINT1-ZW10) 复合体招募SNARE复合体到脂滴周围，进而形成内质网-脂滴（ER-LD）接触，进而调控脂滴的生长。NRZ/SNARE复合功能缺陷同样会导致细胞成熟脂滴数目和脂质储存减少。此外，Rab18的GEF Rab3GAP1/2复合体通过激活并调控Rab18定位到脂滴上促进脂滴的生长，参与脂代谢调节过程。综上，我们的数据显示，Rab18-NRZ-SNARE复合体是建立ER-LD接触的关键蛋白机器，Rab18通过建立ER-LD接触，实现对脂滴的动态调控，以促进脂滴的成熟过程。此外，脂肪细胞中Rab18可以定位在脂滴上抑制脂滴的融合，Rab18的脂滴定位受到异丙肾上腺素（Iso）的调控。Iso处理的脂肪细胞中性脂交换速率及脂滴融合比例都明显下降。此外，Rab18可以被PKA磷酸化，磷酸化的功能还有待进一步研究。
Lipid droplet (LD) is widespread storage organelle of neutral lipid. Its formation, growth, maintenance and degradation require the regulation of multiple proteins. It is highly dynamic and its dysfunction is associated with a variety of metabolic diseases. Lipid incorporation from endoplasmic reticulum (ER) to LD is important in controlling LD growth and intracellular lipid homeostasis. However, the molecular link mediating ER and LD cross talk remains elusive.Here, we identified Rab18 as an important Rab protein in controlling LD growth and maturation. Rab18 deficiency resulted in a drastically reduced number of mature LDs and decreased lipid storage, and was accompanied by increased ER stress. Our results show that Rab18 deficiency did not affect LD biogenesis and nascent LD formation but controlled the growth of nascent LDs into mature LDs. At the beginning of LD formation, Rab18 was enriched at the LD formation site of ER to control the LD growth. We demonstrated that Rab18-mediated LD growth and maturation are dependent on TAG synthesis on ER. Therefore, we found that ER- localized SNARE protein (Syntaxin18, Use1, BNIP1) plays a very important role on Rab18 function. Rab18 recruits the SNARE complex to LD through NRZ (NAG-RINT1-ZW10) complex, and then forms ER-LD contact, regulating the growth of LDs. NRZ/SNARE complex functional defects also lead to reduced number of mature LDs and lipid storage. Furthermore, Rab3GAP1/2, the GEF of Rab18, promoted LD growth by activating and targeting Rab18 to LDs. Overall, our data reveal that the Rab18-NRZ-SNARE complex is critical protein machinery for tethering ER–LD and establishing ER–LD contact to promote LD growth.In addition, Rab18 in mature adipocytes will inhibit the fusion of LDs, and the LD localization of Rab18 is regulated by isoproterenol (Iso). The neutral lipid exchange rate and the percentage of LD fusion in adipocytes treated by Iso are significantly reduced. Moreover, Rab18 can be phosphorylated by PKA, and the phosphorylation function remains to be further studied.
