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Spastin tethers lipid droplets to peroxisomes and directs fatty acid trafficking through ESCRT-III

Chi-Lun Chang, Aubrey V. Weigel, Maria S. Ioannou, H. Amalia Pasolli, C. Shan Xu, David R. Peale, Gleb Shtengel, Melanie Freeman, Herald F. Hess, Craig Blackstone, Jennifer Lippincott-Schwartz

Preprint posted on February 14, 2019 https://www.biorxiv.org/content/10.1101/544023v1

Connecting lipids in the cell: The tether between peroxisomes and lipid droplets unveiled

Selected by Pablo Ranea Robles

Categories: cell biology

Background

Lipid metabolism is essential for eukaryotic cells to normally function. To adapt to different nutritional and environmental changes, lipids are distributed to different organelles, depending on their cellular role. For example, lipids are distributed to mitochondria and peroxisomes for beta-oxidation, to the endoplasmic reticulum for lipid synthesis, the lysosome for the degradation and recycling of complex lipids, and lipid droplets for storage. These organelles are connected to each other, via tightly coordinated interactions that allow the cell to control lipid metabolism. One of the interactions that remains obscure is the nature of the contact sites between lipid droplets and peroxisomes. In this preprint, the team of Lippincott-Schwartz at Janelia Research Campus describes the molecular component of the tether between peroxisomes and lipid droplets (1).

Lipid droplets store neutral lipids that, when needed, are transported for their use to other locations in the cells. The peroxisomes are crucial organelles for lipid metabolism, involved in the beta-oxidation of fatty acids (FAs), ether lipid synthesis and branched-chain fatty acid oxidation, among others. The interaction between peroxisomes and lipid droplets has already been reported (2), but the molecular tether that allow lipid transport between them had been a mystery until now.

Key findings

The group of Lippincott-Schwartz combined state-of-the-art imaging techniques, one the distinctive signatures of the Janelia research campus, to describe the nature of this tether. M1 Spastin, an isoform of the Spastin protein, and ABCD1, a peroxisomal transporter of fatty acids whose mutation causes X-linked adrenoleukodystrophy, forms a tether between peroxisomes and lipid droplets. This was observed by the increased number of contacts between peroxisomes and lipid droplets after M1 Spastin overexpression. A meticulous study of the function of the different domains of M1 Spastin led them to reveal an essential role of the MIT (microtubule interacting domain) domain of M1 Spastin in FA trafficking from lipid droplets to the peroxisome, and the AAA ATPase domain in the generation of the contact sites between these two organelles. They show that M1 Spastin gets inserted into the lipid monolayer of lipid droplets, but not into the peroxisome.

Besides the role of M1 Spastin in the tethering of peroxisomes and lipid droplets, they also gain mechanistic insight into how fatty acids are transported from lipid droplets to peroxisomes. This is mediated by the ESCRT-III proteins, IST1 (Increased sodium tolerance 1) and CHMP1B (Charged multivesicular body protein 1B), that somehow may change the curvature of the lipid droplet membrane. These ESCRT-III proteins are recruited to lipid droplets by the MIT domain of M1 Spastin.

Finally, they link defects in formation of this complex to disease. Mutations in M1 Spastin causes hereditary spastic paraplegia (HSP) (3), with a phenotype that partially overlaps the phenotype of X-linked adrenoleukodystrophy patients, caused by mutations in ABCD1, the peroxisomal component of this tether complex. They observed that the formation of the contact site between peroxisomes and lipid droplets, and the trafficking of fatty acids between them were abolished in cells expressing the pathogenic M1 SpastinK388R mutation. Moreover, lipid peroxidation was increased in cells expressing this mutation, and decreased when M1 Spastin was overexpressed. This led the authors to hypothesize that the trafficking of fatty acids from lipid droplets to peroxisomes is important to eliminate the excess of lipid peroxidation, which has been demonstrated to be detrimental to different cellular functions, and involved in the pathogenesis of many diseases (4).

What I liked about the study

What I really liked about this study is that it reveals the identity of the components of the tether between peroxisomes and lipid droplets (Figure 1). This is a milestone for the study of the pathogenesis of many diseases related to fatty acid metabolism, which involves peroxisomal and/or lipid droplet dysfunction. Together, a combination of astonishing microscopy and elegant experimental design led to an important discovery for the lipid metabolism field.

 

Figure 1. Model for the tethering between peroxisomes and lipid droplets, proposed by the authors (corresponds to Figure 7E in the preprint). Obtained with the permission from the authors.

References

  1.     Chang C-L, Weigel A V., Ioannou MS, Pasolli HA, Xu CS, Peale DR, Shtengel G, Freeman M, Hess HF, Blackstone C, Lippincott-Schwartz J (2019) Spastin tethers lipid droplets to peroxisomes and directs fatty acid trafficking through ESCRT-III. bioRxiv:544023.
  2.     Valm AM, Cohen S, Legant WR, Melunis J, Hershberg U, Wait E, Cohen AR, Davidson MW, Betzig E, Lippincott-Schwartz J (2017) Applying systems-level spectral imaging and analysis to reveal the organelle interactome. Nature 546(7656):162–167.
  3.     Blackstone C (2018) Hereditary spastic paraplegia. Handbook of Clinical Neurology, pp 633–652.
  4.     Mylonas C, Kouretas D Lipid peroxidation and tissue damage. In Vivo 13(3):295–309.

 

Tags: cell biology, fat, lipids, organelle, peroxisome

Posted on: 22nd April 2019 , updated on: 23rd April 2019

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  • Author's response

    The author team shared

    • Are other peroxisomal transporters (other ABCD proteins for example) involved in LD-peroxisome contacts, or this is an exclusive role for ABCD1?

    It is very likely that other ABCD proteins are involved in LD-peroxisome contacts since ABCD proteins (ABCD1-3) are half transporters and require homo- or hetero-dimerization with another half transporter to become functional.  

    • What is the explanation for the elongated peroxisome shape that you observed close to the lipid droplet, which is different compared to the usual peroxisomal shape?

    The elongated morphology of peroxisome seems to be a common feature at LD-peroxisome contacts. This morphology was first observed in differentiated 3T3-L1 adipocytes (1-3), and later in COS-7 cells (4) and in yeast Saccharomyces cerevisiae (5). We speculate that the elongated morphology of peroxisome is involved in fatty acid metabolism. Furthermore, elongated peroxisomes are associated with a cytoprotective role against reactive oxygen species (6), which is consistent with our observation that LD-peroxisome contacts play an important role in clearance of peroxidated lipids

    1. A. B. Novikoff, P. M. Novikoff, Microperoxisomes. J Histochem Cytochem 21, 963-966 (1973).
    2. P. M. Novikoff, A. B. Novikoff, N. Quintana, C. Davis, Studies on microperoxisomes.3. Observations on human and rat hepatocytes. J Histochem Cytochem 21, 540-558 (1973).
    3. A. B. Novikoff, P. M. Novikoff, O. M. Rosen, C. S. Rubin, Organelle relationships in cultured 3T3-L1 preadipocytes. J Cell Biol 87, 180-196 (1980).
    4. M. Schrader, Tubulo-reticular clusters of peroxisomes in living COS-7 cells: dynamic behavior
    and association with lipid droplets. J Histochem Cytochem 49, 1421-1429 (2001).
    5. D. Binns et al., An intimate collaboration between peroxisomes and lipid bodies. J Cell Biol 173, 719-731 (2006).
    6. M. Schrader, H. D. Fahimi, Peroxisomes and oxidative stress. Biochim Biophys Acta 1763, 1755-1766 (2006).

    • You already reported decreased lipid droplet-peroxisome contacts after oleic acid treatment, due to increased lysosomal degradation of lipid droplets content (Valm et al 2017). How do you reconcile these with the effects you see with oleic acid here? Is this the reason behind the different concentrations of oleic acid used in Fig. 1 and Fig. 2 (15 vs 300 uM)?

    We speculate that the discrepancy between our prior and current studies is most likely due to M1 Spastin expression levels. As we demonstrated in the current study, M1 Spastin functions as a tether to bridge LD to peroxisome. Thus, overexpression of M1 Spastin may override the endogenous regulation of LD-peroxisome contacts during oleic acid treatment.

    The purpose of oleic acid treatment in this study was simply to help us identify lipid droplets and analyze the association of lipid droplets with the ER or peroxisomes in high resolution imaging (i.e. super-res and EM). There is no particular reason behind different concentrations of oleic acid. In fact, we simply realized that 15-30 uM oleic acid serves our purpose well so we used this concentration in our later experiment instead of 300 uM.

    • In the assay in which you measure fatty acid trafficking from the lipid droplets to the peroxisome, were the cells starved prior or during the pulse and chase, to induce lipolysis?

    No, we did not starve or induce lipolysis in our experiment. It would be interesting to look at fatty acid trafficking and LD-peroxisome contacts under these conditions.

    • You observed fatty acid trafficking from lipid droplets to the peroxisome with the fluorescent NBD-C12. Did you detect any transport of fatty acids from the peroxisome to lipid droplets through this contact site?

    We were unable to examine peroxisome-to-lipid droplet fatty acid trafficking because fluorescence fatty acid analogs are initially incorporated into lipid droplets. To address this question, we need a fatty acid probe that can be directly incorporated into peroxisomes, which is not currently available. We do speculate that lipids and/or fatty acids can be exchanged bi-directionally at LD-peroxisome contacts.

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