Date Published: June 1, 2017
Publisher: Public Library of Science
Author(s): Sabrina J. Nolan, Julia D. Romano, Isabelle Coppens, Oliver Billker.
Toxoplasma is an obligate intracellular parasite that replicates in mammalian cells within a parasitophorous vacuole (PV) that does not fuse with any host organelles. One mechanism developed by the parasite for nutrient acquisition is the attraction of host organelles to the PV. Here, we examined the exploitation of host lipid droplets (LD), ubiquitous fat storage organelles, by Toxoplasma. We show that Toxoplasma replication is reduced in host cells that are depleted of LD, or impaired in TAG lipolysis or fatty acid catabolism. In infected cells, the number of host LD and the expression of host LD-associated genes (ADRP, DGAT2), progressively increase until the onset of parasite replication. Throughout infection, the PV are surrounded by host LD. Toxoplasma is capable of accessing lipids stored in host LD and incorporates these lipids into its own membranes and LD. Exogenous addition of oleic acid stimulates LD biogenesis in the host cell and results in the overaccumulation of neutral lipids in very large LD inside the parasite. To access LD-derived lipids, Toxoplasma intercepts and internalizes within the PV host LD, some of which remaining associated with Rab7, which become wrapped by an intravacuolar network of membranes (IVN). Mutant parasites impaired in IVN formation display diminished capacity of lipid uptake from host LD. Moreover, parasites lacking an IVN-localized phospholipase A2 are less proficient in salvaging lipids from host LD in the PV, suggesting a major contribution of the IVN for host LD processing in the PV and, thus lipid content release. Interestingly, gavage of parasites with lipids unveils, for the first time, the presence in Toxoplasma of endocytic-like structures containing lipidic material originating from the PV lumen. This study highlights the reliance of Toxoplasma on host LD for its intracellular development and the parasite’s capability in scavenging neutral lipids from host LD.
In mammalian cells, lipid droplets (LD) are cytoplasmic structures containing a diverse array of lipids and proteins. LD consist of an organic core, comprising neutral lipids (mostly triacylglycerols (TAG) and cholesteryl esters (CE) with mixed fatty acid composition) bounded by a monolayer of phospholipids . Inserted onto the outer phospholipid monolayer are structural proteins (e.g., proteins of the perilipin family, such as Adipose differentiation-related protein (ADRP) or adipophilin), lipid biosynthetic enzymes (e.g., acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2), and acyl-CoA synthetase), lipolytic enzymes (e.g., adipose tissue triacylglycerol lipase (ATGL) and membrane-trafficking proteins (e.g., Rab7, Rab18 and ARF1) [2–4]. LD display canonical lipid-related functions ranging from energy storage to lipid homeostasis. In addition, these structures are engaged in various cellular processes, depending on cell type and activation conditions, such as cell signaling during inflammation, innate immunity, RNA metabolism, cytoskeletal organization, nuclear transcription and histone modulation [5–7]. Pertinent to the role of LD in immune responses, LD produce inflammation mediators (e.g., prostaglandins and leukotrienes), regulate the MHC-I antigen presentation pathway, and are assembling platforms for effectors involved in interferon response .
Toxoplasma gondii needs to salvage many essential metabolites, including lipids, from host cells to replicate and produce infectious progeny . This parasite has evolved unique strategies to access the content of host organelles, which represent potential reservoirs of nutrients. For example, Toxoplasma intercepts host vesicular transport pathways, e.g., endo-lysosomes filled with LDL-cholesterol and Golgi Rab vesicles loaded with sphingolipids, and retrieves their lipid cargo [21, 24, 76]. Among lipid-enriched structures present in mammalian cells, lipid droplets (LD) represent an opportune source of various neutral lipids for any lipid scavengers. In this study, we demonstrated that Toxoplasma exploits host LD by relocating them to the PV and salvaging neutral lipids stored in these structures. Along with the exploitation of cytokines and lipoxins present in host LD as previously shown [9, 25, 26, 77], Toxoplasma also targets host LD for their neutral lipid stores. Additionally, we provide evidence that host LD are important for parasite infectivity: first, Toxoplasma growth is reduced in mutant MEF largely depleted of LD via the genetic ablation of DGAT1 and DGAT2 (D1D2KO); and second, parasites repetitively cultivated in D1D2KO cells, partially regain their growth capability when transferred to WT MEF.