Lipids

Abstract

We have recently shown that an inhibition of sterol synthesis by fenpropimorph leads to an accumulation of sterol precursors, hydroxypalmitic acid-containing glucosylceramides and detergent resistant membranes in the Golgi bodies instead of the plasma membrane, suggesting that the individual molecules or the microdomains were blocked in the Golgi. These results and others from several eukaryotic models link lipid metabolism with membrane morphodynamics that are involved in membrane trafficking. Focus has been expanded to other lipid families, and numerous evidences are given showing lipids and lipid-modifying enzymes as key regulators of membrane homeostasis which can strongly regulate membrane morphodynamics and therefore trafficking. Beside protein-based machineries, lipid-based machineries are also shown as crucial regulatory forces involved in protein transport and sorting.

In the plant secretory pathway, it was shown that sterols, glucosylceramides, and the unsaturation degree of lipid acyl chains are critical for lipid microdomain formation at the cell surface.1 Inhibition of sterol synthesis by fenpropimorph led to an accumulation of sterol precursors, glucosylceramides and detergent resistant membranes in the Golgi instead of the plasma membrane, suggesting that either individual molecules or microdomains were blocked in the Golgi. These results raised numerous questions and two additional aspects will be addressed here:

1. Are endomembrane morphodynamics linked to lipid metabolism?

2. Can we consider lipids as components of the cellular trafficking machineries?

Endomembrane Morphodynamics and Lipid Metabolism

More and more evidences highlight relationships between lipid metabolism and membrane morphodynamics. Blocking cyclopropylsterol maturation by fenpropimorph induced a fenestration of the Golgi.2 Interestingly, disturbing Golgi morphology by brefeldin A reduced the synthesis of phytosterols.3 Relationships may therefore exist between sterol metabolism and Golgi morphology in plant cells. In other eukaryotes, the sterol structure can affect membrane curvature and prepare a membrane for budding or fusion events.4 For example, cholesterol is involved in the formation of post-Golgi secretory vesicles5 and may help to drive vesicle fusion by virtue of its intrinsic negative curvature.6 However, cholesterol levels in the Golgi membranes must be tightly regulated since an excess of this molecule can vesiculate the Golgi complex itself.7

Besides sterols, several other lipid families may affect membrane organisation. In animal cells, ceramides increase the disassembly of the Golgi induced by brefeldin A, indicating that ceramide levels could be critical for Golgi stability.8 Pharmacological approaches using inhibitors of glucosylceramide synthase have suggested a role for sphingolipids in Golgi architecture.9 High concentrations of sphingosine derived from ceramide hydrolysis can induce Golgi fragmentation.10 Therefore, all these studies highlight a critical role for sphingolipids in Golgi morphodynamics as found for sterols.

Lysophospholipids, formed by phospholipases A₂, can induce positive membrane curvatures and favor membrane tubulation.11 Interestingly, inhibition of the reverse reaction by acyltransferases also leads to Golgi membrane tubulation.12 Phospholipases A₁-related enzymes can also induce dispersion of the Golgi complex and aggregation of ER membranes.13 Inhibition of phospholipase D, which decreases phosphatidic acid (PA) but also phosphoinositide levels, can also affect the structural integrity of the Golgi complex in animal cells.14 Besides the role of PA in stimulating phosphoinositide synthesis, PA and its derivative lyso-PA can regulate membrane curvature and therefore Golgi stability. In animal cells, an isoform of phospholipase D is concentrated to the rims of the Golgi stacks and could be involved in Golgi morphodynamics driving some aspects of vesicular transport from the Golgi.15 In plant cells, inhibition of phospholipase D and therefore formation of PA in the pollen tube highly reduces the number of secretory vesicles, suggesting that PA levels but also phosphoinositides regulate pollen tube growth.16,17 In addition, we have observed that the inhibition of sterol synthesis by fenpropimorph was accompanied by an increase of PA in the endomembranes and especially the Golgi (Fig. 1), suggesting that elevated PA levels may drive the fenestration of Golgi stacks.2 As a consequence, PA (and also diacylglycerol, see below) levels must be tightly controlled, and small variations may have dramatic effects.

Finally, it has recently been shown that the phospholipid-binding protein Nir2p (a Sec14-related protein) is involved in the control of diacylglycerol levels in mammalian Golgi membranes as in yeast,18 and this could be related to the propensity of diacylglycerol to fission membranes. A Sec14p-nodulin PI-transfer protein from Arabidopsis has been shown to be a key sensor of polarized membrane growth in root hairs.19

Therefore lipids and lipid-modifying enzymes must be considered as key molecular regulators of membrane domain formation and homeostasis which strongly regulate membrane morphodynamics and trafficking. A summary of the relationships between Golgi membrane morphodynamics and lipid metabolism is presented in Figure 2.

Lipid Interactions with the Cellular Trafficking Machineries

As highlighted above, lipid-based machineries can regulate membrane structure through the control of membrane curvature, but can also be engaged in critical interactions with cargo proteins or proteins of the cellular trafficking machineries.

Protein sorting towards the secretory pathway may be linked to lipid-based sorting processes and specific interactions with acidic phospholipids.20,21 In this respect, it was interesting to determine a specific PS targeting to ER-derived domains in plant cells,22 and that PS was required in the formation of ER-derived COPII vesicles in vitro.23 In a similar way, short chain ceramides can interact with the binding of ARF to Golgi membranes and therefore affect the formation of COPI vesicles.24 The yeast Oxysterol-binding protein (OSBP) Kes1p is a lipid receptor which may regulate Golgi secretory function through interactions with the ARF and Sec14 pathways.25 In animal cells, OSBP-related proteins were found to interact with a syntaxin-like VAMP-associated protein-A, and to participate in the organisation of the COPII-dependent ER-Golgi pathway involved in protein and ceramide transport.26 Finally, phospholipase D and phospholipase A₁-related enzymes can interact with proteins of the COPII machinery to participate in the formation and organisation of ER export sites.27,28

It has recently been found that different SNAREs (proteins involved in membrane fusion) can have various affinities for specific lipid microdomains such as sterol- and sphingolipid-rich lipid rafts, and that the degree of their association with these domains could condition the efficiency of exocytosis.29,30 In addition, the basic domain of the synaptobrevin VAMP2, also involved in exocytosis, was found to strongly interact with PS.31 Such interactions between lipids and proteins of the cellular transport machineries may contribute to both the specific targeting and function of these proteins.

It must be considered that at the time a lipid is synthesized, its physicochemical properties and structure will spontaneously allow or avoid specific interactions with other molecules (lipids or proteins) that will condition its association (and that of partners) within a microdomain or another, and that these features can be modified during transport through the secretory pathway. Beside protein-based machineries, lipid-based machineries are also crucial regulatory forces involved in protein trafficking and sorting through the secretory pathway.

Figures and Tables

Figure 1 Phosphatidic acid level in endomembranes of A. porrum seedlings treated with fenpropimorph. ER, endoplasmic reticulum; GA, Golgi membranes; PM, plasma membranes.

Figure 2 Lipid metabolism and Golgi morphodynamics. The different variations (up and down) in the concentrations of several lipids which influence the morphology of the Golgi are indicated. Most of the data come from studies carried out in animal models. The effect of fenpropimorph (FEN) on post-Golgi traffic and Golgi fenestration in plant cells is shown. Inhibition of PA formation is also affecting post-Golgi traffic in plant cells. Cer, ceramides; DAG, diacylglycerol; LCB, long chain bases of sphingolipids; LysoPL, lyso-phospholipids; PA, phosphatidic acid; St, sterols.

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