The large GTPase Sey1/atlastin mediates lipid droplet- and FadL-dependent intracellular fatty acid metabolism of Legionella pneumophila
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Curated by eLife
eLife assessment:
This important study advances our understanding of host-derived lipid droplets' interaction with intracellular pathogens. The use of amoeba species Dictyostelium discoideum as a host for Legionella pneumophila infection is compelling and goes beyond the current state of the art, but the strength of evidence is incomplete, and the main claims are only partially supported by the data. With the experimental part strengthened, this paper would be of interest to cell biologists and microbiologists working on the interaction of microbes with host cells.
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Abstract
The amoeba-resistant bacterium Legionella pneumophila causes Legionnaires’ disease and employs a type IV secretion system (T4SS) to replicate in the unique, ER-associated Legionella -containing vacuole (LCV). The large fusion GTPase Sey1/atlastin is implicated in ER dynamics, ER-derived lipid droplet (LD) formation, and LCV maturation. Here, we employ cryo-electron tomography, confocal microscopy, proteomics, and isotopologue profiling to analyze LCV-LD interactions in the genetically tractable amoeba Dictyostelium discoideum . Dually fluorescence-labeled D. discoideum producing LCV and LD markers revealed that Sey1 as well as the L. pneumophila T4SS and the Ran GTPase activator LegG1 promote LCV-LD interactions. In vitro reconstitution using purified LCVs and LDs from parental or Δ sey1 mutant D. discoideum indicated that Sey1 and GTP promote this process. Sey1 and the L. pneumophila fatty acid transporter FadL were implicated in palmitate catabolism and palmitate-dependent intracellular growth. Taken together, our results reveal that Sey1 and LegG1 mediate LD- and FadL-dependent fatty acid metabolism of intracellular L. pneumophila .
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eLife assessment:
This important study advances our understanding of host-derived lipid droplets' interaction with intracellular pathogens. The use of amoeba species Dictyostelium discoideum as a host for Legionella pneumophila infection is compelling and goes beyond the current state of the art, but the strength of evidence is incomplete, and the main claims are only partially supported by the data. With the experimental part strengthened, this paper would be of interest to cell biologists and microbiologists working on the interaction of microbes with host cells.
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Reviewer #1 (Public Review):
The authors propose that the ER-resident large GTPase Sey1, a homolog of mammalian atlastin, localizes to LDs and promotes their association with the Legionella-containing vacuole (LCV); They also propose that the effector LegG1 contributes to this process by activating the host GTPase RanA on the LCV surface. Once LDs associate with the LCV, the authors favor a model where LDs are taken up into the LCV lumen where they are consumed by L. pneumophila as a carbon source. They propose that the fatty acid transporter FadL, Lpg1810, is involved in the transport of palmitate across the bacterial membrane.
Strong points of this study are the use of Dictyostelium as a genetically tractable model system, the finding that FadL and the addition of exogenous palmitate positively affect intracellular bacterial growth, …
Reviewer #1 (Public Review):
The authors propose that the ER-resident large GTPase Sey1, a homolog of mammalian atlastin, localizes to LDs and promotes their association with the Legionella-containing vacuole (LCV); They also propose that the effector LegG1 contributes to this process by activating the host GTPase RanA on the LCV surface. Once LDs associate with the LCV, the authors favor a model where LDs are taken up into the LCV lumen where they are consumed by L. pneumophila as a carbon source. They propose that the fatty acid transporter FadL, Lpg1810, is involved in the transport of palmitate across the bacterial membrane.
Strong points of this study are the use of Dictyostelium as a genetically tractable model system, the finding that FadL and the addition of exogenous palmitate positively affect intracellular bacterial growth, and the fact that LDs can be detected within LCVs which, if confirmed, would be of significant biological importance.
The main concern is that the molecular mechanism underlying LCV-LD dynamics and LD uptake have only been superficially described. It needs to be determined how exactly proteins like Sey1 or LegG1 promote LD recruitment to LCVs. Does this process really depend on Ran GTPases and if so, do constitutively inactive Ran mutants phenocopy the defects? And by what mechanism are LDs delivered across the LCV membrane into their lumen? The authors themselves raise that question in the discussion, but provide no explanation or supporting data. How commonly can LD uptake into LCVs be observed across a population of cells? And are the phenotypes observed upon deletion of Sey1 direct effects, or are global changes in the ER/host cell protein or lipid landscape indirectly causing those phenotypes? These are some of the questions that, once addressed, would improve the impact of this study.
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Reviewer #2 (Public Review):
In this manuscript, Hüsler et al. aimed to evaluate the contribution of LDs, Sey1, and FadL to intracellular replication and palmitate catabolism of L. pneumophila in D. discoideum. The authors found that Sey1 regulates LD proteome composition and promotes Icm/Dot-dependent LCV-LD interactions as well as FadL-dependent fatty acid metabolism of intracellular L. pneumophila. The study is in general well-designed and performed. The data are clearly presented and valuable in enhancing awareness of the mechanisms of L. pneumophila infection. The evidence supporting the claims of the authors is solid, although the inclusion of additional controls and clarifications would have strengthened the study.
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