Phospholipid subcellular localization and dynamics

Abstract

Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.

Keywords: flippase; glycerophospholipid; lipid transfer proteins; membrane contact sites; membrane trafficking; nonvesicular transport; organelle; phosphatidylinositol; phospholipase; phospholipid metabolism; phospholipids; scramblase; sphingolipid; vesicular transport.

Figure 1.

Glycerophospholipid composition of organelles. The bar graph highlights the subcellular distribution of glycerophospholipids (GPLs) between the different organelles in baby hamster kidney cells (128). The pie charts display the relative abundance of each class of GPL in organelles based on composite data from rat hepatocytes (1) and for lipid droplets from murine hepatocytes (129). Except for the mitochondrial PtdGly (PG) and CL, the other GPLs are present in all organelle membranes but display heterogeneity. PtdCho (PC) is the most abundant, comprising 45–55 mol % of the GPL in the cell. PtdEtn (PE) is the second most abundant GPL and is mainly enriched in inner membranes of mitochondria (∼35–40 mol %), although it is less prominent in other organelles (∼17–25 mol %). PtdSer (PS) is a precursor for the mitochondrial PE, and as a result of repaid consumption, its abundance is very low in the inner mitochondrial membranes (1 mol %). PS is also a minor component of lysosomes (∼1 mol %), the endoplasmic reticulum (∼4 mol %), nucleus (∼6 mol %), Golgi (∼4 mol %), and plasma membrane (∼4%). Post-Golgi apparatus organelles are enriched in sphingomyelin (SM) with it being most abundant (∼23%) in the PM. The designation Other includes essential precursors and signaling lipids such as PA, DAG, and lysolipids. The schematic highlights the vesicular and nonvesicular pathways responsible for the intracellular trafficking of lipids. The endoplasmic reticulum (ER) is the principal site of synthesis for most lipid species. The extensively branched reticular network of the ER facilitates the establishment of MCS with other organelles, including the Golgi apparatus, mitochondria, endosomes, lysosomes, lipid droplets, and plasma membrane. These MCS bring donor and acceptor membranes in proximity (∼30 nm) where the exchange of GPLs and cholesterol can occur. Most organelles are interconnected via the vesicular transport pathways. GPLs are essential for the formation of vesicles that transport transmembrane and lumenal proteins throughout the cell. Thus, a by-product of vesicular transport is the movement of GPLs and other lipids.

Figure 2.

CDP-DAG and Kennedy pathways. The CDP-DAG pathway begins with the consumption of phosphatidate (PA) on the cytosolic leaflet of the endoplasmic reticulum (ER) by either CDP-DAG synthase (CDS) 1 or 2 to produce cytidine diphosphate diacylglycerol (CDP-DAG). The CDP-DAG is then used as a substrate by the phosphatidylinositol synthase enzyme (PIS) to catalyze the production of phosphatidylinositol (PtdIns) from CDP-DAG. CDP-DAG is also used in the mitochondria by the phosphatidylglycerophosphate synthase (PGPS) to produce phosphatidylglycerol (PtdGly)-phosphate, which is, in turn, dephosphorylated to produce PtdGly. Phosphatidylserine (PtdSer) synthesis is catalyzed by PtdSer synthase 1 (PSS1). A significant fraction of the newly synthesized PtdSer is transported from the ER to the mitochondria. There it serves as a substrate for the enzyme PtdSer decarboxylase (PSD) to produce a mitochondrial pool of phosphatidylethanolamine (PtdEtn). In the Kennedy pathway, choline (Cho) and ethanolamine (Etn) are first activated for phosphorylation by choline kinase (CK) and ethanolamine kinase (ETNK), respectively. Next, the phosphobase serves as substrates for the rate-limiting step of the pathway catalyzed by CTP:phosphocholine cytidyltransferase (CCT) and CTP:phosphoethanolamine cytidyltransferase (ECT), respectively, yielding CDP-Cho and CDP-Etn. The final step of the pathway is catalyzed by two homologous proteins, the phosphatidylcholine (PtdCho)-specific CPT and the PtdCho/PtdEtn producing CEPT. Finally, PtdEtn can be converted to PtdCho by three successive methylation reactions catalyzed by PtdEtn methyltransferases, PEMT1 and -2, in the liver of mammals. PAP, phosphatidate phosphatase.

Figure 3.

Phosphatidylinositol cycle. The synthesis of PtdIns occurs in the ER or possibly in ER-derived vesicles termed PIPEROsomes. First, glycerol 3-phosphate (G3P) is dually acylated by the actions of acyltransferases glycerol-3-phosphate O-acyltransferase (GPAT) and 1-acylglycerol-3-phosphate O-acyltransferase (LPAT), which forms lysophosphatidic acid (LPA) and produces phosphatidic acid (PA), respectively. Next, PA in the ER or the PIPEROsomes is converted to CDP-diacylglycerol (CDP-DAG) by the enzyme CDP-DAG synthases (CDS). Phosphatidylinositol synthase (PIS) in the ER of PIPEROsome catalyzes the coupling of CDP-DAG to myo-inositol to form PtdIns. Once synthesized in the ER or the ER-derived vesicles, PtdIns is delivered to the PM by the secretory pathway (not depicted) or by the actions of either nonselective (TMEM24, E-Syts) or PtdIns (Nir2, PITP). The PtdIns serves as a substrate for generating the plasmalemmal phosphoinositides. PI4,5P₂ is vital to facilitate many of the plasmalemmal transactions such as signaling in response to growth factors, exocytosis, endocytosis, and the polymerization of cortical actin. The activation of PLC isoforms converts PI4,5P₂ into DAG, which can then be converted back to PA by one of 10 DAG kinases. To prevent the accumulation of PA and to replenish the plasmalemmal PtdIns pool, PA is transferred back to the ER via nonvesicular lipid transport proteins. Red arrows represent metabolic reactions; blue arrows represent intracellular transport process; enzymes. DGK, diacylglycerol kinase.

Figure 4.

Phospholipid translocases in lipid bilayers. The spontaneous flip-flop of GPLs between leaflets of a bilayer is energetically unfavorable. Scramblase is the term used to describe a variety of proteins (i.e. TMEM16F, Xlr4, select GPCRs) that can in an energy-independent manner mediate the bidirectional transfer of GPLs between leaflets thereby collapsing the symmetry of the PM. Conversely, Flippase (inward movement) and Floppase (outward movement) are energy-dependent proteins that couple the consumption of ATP with the movement of lipids across the bilayer.