An ER phospholipid hydrolase drives ER-associated mitochondrial constriction for fission and fusion
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eLife assessment
The authors have used state of the art tools to discover and visualize the role of a known ER-localized lipid hydrolase/acyl transferase (which they call Aphyd) in creating lipids that facilitate the localization of proteins required for mitochondrial fission and fusion at nodal points of interaction between the ER and mitochondria. The data are clear, quantitative and compelling in respect to the role of this protein in the processes of mitochondrial constriction, fission and fusion.
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Abstract
Mitochondria are dynamic organelles that undergo cycles of fission and fusion at a unified platform defined by endoplasmic reticulum (ER)-mitochondria membrane contact sites (MCSs). These MCSs or nodes co-localize fission and fusion machinery. We set out to identify how ER-associated mitochondrial nodes can regulate both fission and fusion machinery assembly. We have used a promiscuous biotin ligase linked to the fusion machinery, Mfn1, and proteomics to identify an ER membrane protein, ABHD16A, as a major regulator of node formation. In the absence of ABHD16A, fission and fusion machineries fail to recruit to ER-associated mitochondrial nodes, and fission and fusion rates are significantly reduced. ABHD16A contains an acyltransferase motif and an α/β hydrolase domain, and point mutations in critical residues of these regions fail to rescue the formation of ER-associated mitochondrial hot spots. These data suggest a mechanism whereby ABHD16A functions by altering phospholipid composition at ER-mitochondria MCSs. Our data present the first example of an ER membrane protein that regulates the recruitment of both fission and fusion machineries to mitochondria.
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eLife assessment
The authors have used state of the art tools to discover and visualize the role of a known ER-localized lipid hydrolase/acyl transferase (which they call Aphyd) in creating lipids that facilitate the localization of proteins required for mitochondrial fission and fusion at nodal points of interaction between the ER and mitochondria. The data are clear, quantitative and compelling in respect to the role of this protein in the processes of mitochondrial constriction, fission and fusion.
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Reviewer #1 (Public Review):
The Voeltz lab in previous work has established a physical connection between the endoplasmic reticulum and mitochondria as an organizing principle in mitochondrial fission and fusion. A key cytoplasmic protein, Drp1 and a mitochondrial outer membrane protein, Mfn1, are known to localize to nodes of interaction between the ER and mitochondria but until now, no ER membrane proteins required in this process have been described. Using a Turbo ID fused to Mfn1, the authors have identified a known ER membrane protein that interacts with functional Mfn. This ER membrane protein, which they call Aphyd1, contains putative acyl hydrolase and acyltransferase domains. Further, in compelling work combining high resolution fluorescence microscopy and gene function analysis they have described the role of this protein in …
Reviewer #1 (Public Review):
The Voeltz lab in previous work has established a physical connection between the endoplasmic reticulum and mitochondria as an organizing principle in mitochondrial fission and fusion. A key cytoplasmic protein, Drp1 and a mitochondrial outer membrane protein, Mfn1, are known to localize to nodes of interaction between the ER and mitochondria but until now, no ER membrane proteins required in this process have been described. Using a Turbo ID fused to Mfn1, the authors have identified a known ER membrane protein that interacts with functional Mfn. This ER membrane protein, which they call Aphyd1, contains putative acyl hydrolase and acyltransferase domains. Further, in compelling work combining high resolution fluorescence microscopy and gene function analysis they have described the role of this protein in the recruitment of Drp1 and Mnn1 to nodes of interaction between the ER and mitochondria and in the sequential processes of mitochondrial constriction, fission and fusion. The work is clear and nicely quantitative and adds a new molecular element to the important question of how the ER serves to organize the division and fusion of mitochondria.
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Reviewer #2 (Public Review):
This study by Nguyen and Voeltz uses proximity biotinylation and advanced imaging to elucidate roles for a lipid-metabolizing enzyme in controlling sites of mitochondrial fusion and fission. Using proximity biotinylation, they identify ABHD16A, which they propose to rename Aphyd, as an ER-resident protein close to mitochondrial fusion and fission sites. They find that knockdown and overexpression of this protein affects mitochondrial morphology and rates of mitochondrial constriction and subsequent fusion and fission. They also find that mutation of two different catalytic domains within ABHD16A have similar but non-identical effects in rescue experiments of siRNA-induced phenotypes. Broadly speaking, the hydrolase domain, known to deacylate phospholipids to form lysophospholipids (primarily …
Reviewer #2 (Public Review):
This study by Nguyen and Voeltz uses proximity biotinylation and advanced imaging to elucidate roles for a lipid-metabolizing enzyme in controlling sites of mitochondrial fusion and fission. Using proximity biotinylation, they identify ABHD16A, which they propose to rename Aphyd, as an ER-resident protein close to mitochondrial fusion and fission sites. They find that knockdown and overexpression of this protein affects mitochondrial morphology and rates of mitochondrial constriction and subsequent fusion and fission. They also find that mutation of two different catalytic domains within ABHD16A have similar but non-identical effects in rescue experiments of siRNA-induced phenotypes. Broadly speaking, the hydrolase domain, known to deacylate phospholipids to form lysophospholipids (primarily phosphatidylserine to lysoPS), was more important for the phenotypes, but roles were also required for the acyltransferase domain. Perturbation of lipid transfer protein activity implicated the PS/PI4P transporter ORP8 in this pathway, strongly suggesting a specific role for ABHD16A in modulating PS metabolism at these sites to promote the mitochondrial constriction, fission, and fusion machineries. This focused study builds a rigorous and compelling story centered around the role a lipid-modifying enzyme in an interesting and important cellular behavior, namely how sites of mitochondrial fission and fusion are defined. Overall, this important study presents a compelling new model for understanding how specific local lipid metabolism at ER-mitochondria contact sites could facilitate mitochondrial fission and fusion events.
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Reviewer #3 (Public Review):
In this study, the authors fuse a promiscuous biotin ligase (TurboID) to mitofusin1 to identify new players involved in mitochondrial fission and fusion. They identify an ER membrane protein, ABHD16A, that has been previously established as a phospholipid hydrolase. They rename this protein as Aphyd and go on to study its role in mitochondrial fission and fusion. Using elegant cell biology techniques, their striking images and rigorous analysis convincingly show a key role for Aphyd in recruiting both fission and fusion machineries to ER-associated mitochondrial nodes. Rates of fission and fusion are markedly decreased in the absence of Aphyd. They also show that Aphyd is required for constrictions. The identification of a new player that may regulate mitochondrial fusion and fission is an exciting advance …
Reviewer #3 (Public Review):
In this study, the authors fuse a promiscuous biotin ligase (TurboID) to mitofusin1 to identify new players involved in mitochondrial fission and fusion. They identify an ER membrane protein, ABHD16A, that has been previously established as a phospholipid hydrolase. They rename this protein as Aphyd and go on to study its role in mitochondrial fission and fusion. Using elegant cell biology techniques, their striking images and rigorous analysis convincingly show a key role for Aphyd in recruiting both fission and fusion machineries to ER-associated mitochondrial nodes. Rates of fission and fusion are markedly decreased in the absence of Aphyd. They also show that Aphyd is required for constrictions. The identification of a new player that may regulate mitochondrial fusion and fission is an exciting advance for the field. Going forward, further biochemical analysis of Aphyd's lipid-modifying activities will be needed to shed light on the mechanisms used by Aphyd to deform membranes. In this initial study, the authors provide some tantalizing clues as to how this may occur by showing that versions of Aphyd that have mutations in their lipid-modifying (acyltransferase and hydrolase) domains are impaired in their abilities to generate ER-associated mitochondrial nodes. I look forward to the next chapters of this story to learn more about how Aphyd works.
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