Autophagy is an intracellular membrane trafficking process responsible for recycling of macromolecules and organelles and overall cell survival under physiological and pathophysiological conditions.(1) It is mediated by a distinct set of core autophagic factors termed ATGs together with general intracellular trafficking factors such as SNAREs, tethering complexes, etc. This adaptive process of formation of a de novo double-membrane organelle is induced under different metabolic and stress conditions, including nutrient deprivation, growth factor depletion, infection, and hypoxia.(1) In yeast, the pathway begins at a perivacuolar phagophore assembly site (PAS) with nucleation of a cup-shaped phagophore membrane that expands within minutes. The membrane than seals, thus capturing the engulfed cytoplasm within a double-membrane autophagosome. The outer autophagosomal membrane fuses with the vacuole, whereas the inner membrane-bound autophagic body and its cargo are degraded by vacuolar enzymes.(2) Multiple organelles are implicated in autophagosome formation, yet the mechanism of autophagic membrane construction in general and phospholipid delivery in particular remains elusive, with evidence pointing at the contribution of lipids from vesicles carrying the transmembrane protein Atg9 and from the endoplasmic reticulum (ER) via Atg2-mediated lipid transport in cooperation with Atg18.(3) An experimentally challenging outstanding issue is the requirement for in situ lipid synthesis on autophagic membranes. The Graef group now employs elegant complementary approaches to demonstrate a role in the formation of autophagic membranes for de novo phospholipid synthesis and autophagy-associated synthesis of the phospholipid precursor coenzyme A (CoA)-activated fatty acid (acyl-CoA).(4) As the authors observe an autophagic membrane localization of FAA1, which synthesizes acyl-CoA from CoA and free fatty acids, they aim to investigate the possible involvement of acyl-CoA activation in autophagy. Upon knockout of the FAA1 paralogs FAA3 and FAA4 with pharmaceutical inhibition of acyl-CoA synthesis from glucose by fatty acid synthetase (FAS) (Figure 1), FAA1 remains for cellular acyl-CoA activation and is essential for survival. Under this condition, artificial tethering of FAA1 to the plasma membrane (PM-FAA1) maintains viability but reduces the level of de novo phospholipids synthesis and impairs autophagy, which is not rescued by supplementation with exogenous fatty acids. Figure 1. FAA1 location and function contribute to autophagosome expansion. By utilizing time lapse microscopy to follow different stages of autophagosome biogenesis, Schutter et al. found that mislocalization of FAA1 impairs primarily phagophore expansion rather than nucleation and is rescued by docking of FAA1 to ER exit sites (ERES) but not the vacuole, suggesting a physical link among fatty acid activation, phagophore membrane elongation, and phagophore-apposing ERES. Schutter et al. go on to determine that mislocalization of acyl-CoA activity does not alter the lipid composition of either whole-cell or autophagic membranes yet abrogates the incorporation of de novo-synthesized phospholipids into nascent autophagic membranes, as neatly demonstrated by pulse chasing of exogenously supplied fatty acid. To study the autophagic requirement for de novo phospholipid synthesis from an alternative approach, the authors opted to inactivate the conversion of acyl-CoA into phospholipids in an inducible manner. Indeed, auxin-induced degradation of the acyltransferase Ale1 or Sct1, in combination with knockout of the functional homologue Slc1 or Gpt2, respectively, impairs phagophore expansion, consistent with a role for newly synthesized phospholipids in autophagy. It is noteworthy that these experiments were also performed under conditions that inhibit FAS activity. Overall, the study supports a model in which FAA1 is recruited to the growing phagophore for local activation of fatty acids, channeled into sites for the synthesis of phospholipids, which in turn translocate back to the phagophore to promote efficient membrane elongation (Figure 1). Consequently, vacuolar fusion of the complete autophagosome leads to the degradation of trapped FAA1. How FAA1 is shuttled to autophagic membranes and the location and manner of transport of acyl-CoA to the phospholipid synthesis site and those of newly synthesized phospholipids back to the phagophore remain unknown. In line with previous studies, the authors suggest that phagophores nucleate from Atg9 vesicles, while nascent phospholipids are delivered to the phagophore from the ER by the Atg2–Atg18 complex. The study ushers in additional questions. Given the vacuolar fusion of the outer autophagosomal membrane and degradation of the inner membrane, it would be interesting to determine whether local lipid metabolism described in this study contributes to the well-established distinct composition of these functionally and structurally asymmetric membranes. Moreover, as metabolic analysis by Schutter et al. distinguishes the lipid composition of autophagic membranes from that of other cellular membranes, future studies should elucidate how different organelles and de novo phospholipid synthesis are coordinated in the contribution of lipids to the autophagosomal membrane upon different physiological triggers of selective and nonselective autophagy. A particular emphasis is placed on differences in lipid metabolism between growth and starvation and distinct putative roles of different phospholipid species in the engulfment of different cargo, especially of organelles that actively metabolize lipids. Interestingly, the localization of FAA1 and FAA4 to autophagic membranes leads to their autophagy-dependent degradation, in addition to the recently reported preferential autophagic degradation of the parallel FAS pathway enzymes.(5) The degradation of these factors (FAA1, FAA4, and FAS) by autophagy may raise a possible role for FAS activity in membrane expansion, which is similar to the findings of the study presented here. This in turn may serve to mildly downregulate acyl-CoA activation and phospholipid synthesis during starvation, thus potentially acting as negative feedback. Finally, phospholipids can be synthesized in different organelles and can be transformed by their contact sites; hence, the identity and localization of the enzymes that synthesize phospholipids for autophagosome formation, together with the mechanism for channeling of the phospholipid into autophagic membranes, are yet to be revealed. Z.E. is the incumbent of the Harold Korda Chair of Biology. The authors are grateful for funding from the Israel Science Foundation (Grant 215/19), the Legacy Heritage Fund (Grant 1935/16), the Sagol Longevity Foundation, and the Yeda-Sela Center for Basic Research. The authors declare no competing financial interest. This article references 5 other publications.

中文翻译：

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De Novo磷脂合成促进高效自噬。

自噬是一种细胞内膜运输过程，负责在生理和病理生理条件下回收大分子和细胞器，以及总体细胞存活。（1）它由一组称为ATG的核心自噬因子与一般的细胞内运输因子（如SNARE）共同介导。系留物等。从头形成的这种自适应过程双膜细胞器是在不同的代谢和应激条件下诱导的，包括营养缺乏，生长因子耗竭，感染和缺氧。（1）在酵母中，该途径始于具有杯形核的周周吞噬细胞装配位点（PAS）。形状的吞噬膜在几分钟内膨胀。膜然后密封，从而将被吞噬的细胞质捕获在双膜自噬体中。自噬体的外膜与液泡融合，而与内膜结合的自噬体及其货物被液泡酶降解。（2）自噬体的形成涉及多个细胞器，但自噬膜的构建机制和磷脂的运输一般特别仍然难以捉摸，自噬膜上的原位脂质合成。Graef集团现在采用优雅的互补方法来证明从头形成自噬膜的作用磷脂前体辅酶A（CoA）活化的脂肪酸（酰基-CoA）的磷脂合成和自噬相关的合成。（4）作者观察到FAA1的自噬膜定位，该结构由CoA和游离脂肪合成酰基-CoA。它们旨在研究酰基辅酶A活化可能与自噬有关。敲除FAA1旁系同源物FAA3和FAA4并通过脂肪酸合成酶（FAS）抑制葡萄糖对酰基辅酶A的合成（图1）后，FAA1仍可用于细胞酰基辅酶A的活化，对于存活至关重要。在这种情况下，FAA1与质膜（PM-FAA1）的人工束缚可保持活力，但会降低从头水平磷脂合成并损害自噬，这不能通过补充外源脂肪酸来挽救。图1. FAA1的位置和功能有助于自噬体的扩增。通过利用延时显微镜观察自噬体生物发生的不同阶段，Schutter等。发现FAA1的错误定位主要损害了荧光团的扩张而不是成核作用，并且通过将FAA1与ER出口位点（ERES）停靠而得以挽救，而不是液泡得以挽救，这表明脂肪酸活化，荧光团膜伸长和具有荧光团的ERES之间存在物理联系。Schutter等。继续确定酰基辅酶A活性的错误定位不会改变全细胞膜或自噬膜的脂质组成，但可消除从头结合-通过脉冲追逐外源供应的脂肪酸清楚地证明了磷脂合成成新生的自噬膜。研究从头对自噬的要求通过另一种方法合成磷脂，作者选择以可诱导的方式使酰基辅酶A转化为磷脂失活。的确，生长素诱导的酰基转移酶Ale1或Sct1的降解，分别与功能同源物Slc1或Gpt2的敲除相结合，会损害吞噬细胞的扩增，这与新合成的磷脂在自噬中的作用一致。值得注意的是，这些实验也是在抑制FAS活性的条件下进行的。总的来说，这项研究支持了一个模型，在该模型中，FAA1被募集到正在生长的荧光团中以进行脂肪酸的局部活化，并被引导进入磷脂的合成位点，然后磷脂又重新定位回该荧光团以促进有效的膜伸长（图1）。所以，完全自噬体的液泡融合导致捕获的FAA1降解。如何将FAA1穿梭到自噬膜上，以及酰基辅酶A转运到磷脂合成位点以及新合成的磷脂返回到噬菌体的位置和方式仍然未知。与以前的研究一致，作者建议吞噬细胞从Atg9囊泡中析出核，而新生的磷脂通过Atg2-Atg18复合物从ER传递到吞噬细胞。该研究提出了其他问题。考虑到外吞噬体膜的液泡融合和内膜的降解，确定本研究中描述的局部脂质代谢是否有助于这些功能和结构上不对称膜的公认组成是很有意思的。此外，如Schutter等人的代谢分析。区分自噬膜的脂质组成与其他细胞膜的脂质组成，未来的研究应阐明不同细胞器和从头在选择性和非选择性自噬的不同生理触发条件下，磷脂合成在脂质对自噬体膜的贡献中得到协调。特别强调的是生长和饥饿之间脂质代谢的差异，以及不同磷脂物种在吞噬不同货物（尤其是活跃地代谢脂质的细胞器）中的独特推定作用。有趣的是，除了最近报道的平行FAS途径酶的优先自噬降解之外，FAA1和FAA4在自噬膜上的定位还导致它们的自噬依赖性降解。（5）这些因素（FAA1，FAA4和FAS）的降解）通过自噬可能会提高FAS活性在膜扩张中的作用，这与此处提出的研究结果相似。反过来，这可能有助于在饥饿期间轻度下调酰基辅酶A的活化和磷脂的合成，从而潜在地充当负反馈。最后，磷脂可以在不同的细胞器中合成，并可以通过它们的接触位点转化。因此，尚未揭示合成用于自噬体形成的磷脂的酶的身份和定位，以及将磷脂引导进入自噬膜的机制。ZE是Harold Korda生物学系主任。作者非常感谢以色列科学基金会（Grant 215/19），传统遗产基金会（Grant 1935/16），Sagol Longevity Foundation和Yeda-Sela基础研究中心的资助。作者宣称没有竞争性的经济利益。本文引用了其他5个出版物。