Nuclear localization of enzymatically active green fluorescent protein-CTP:phosphocholine cytidylyltransferase alpha fusion protein is independent of cell cycle conditions and cell types

From yeast to humans - roles of the Kennedy pathway for phosphatidylcholine synthesis

The major phospholipid present in most eukaryotic membranes is phosphatidylcholine (PC), comprising ~ 50% of phospholipid content. PC metabolic pathways are highly conserved from yeast to humans. The main pathway for the synthesis of PC is the Kennedy (CDP-choline) pathway. In this pathway, choline is converted to phosphocholine by choline kinase, phosphocholine is metabolized to CDP-choline by the rate-determining enzyme for this pathway, CTP:phosphocholine cytidylyltransferase, and cholinephosphotransferase condenses CDP-choline with diacylglycerol to produce PC. This Review discusses how PC synthesis via the Kennedy pathway is regulated, its role in cellular and biological processes, as well as diseases known to be associated with defects in PC synthesis. Finally, we present the first model for the making of a membrane via PC synthesis.

Phosphatidylcholine: Greasing the Cholesterol Transport Machinery

Negative feedback regulation of cholesterol metabolism in mammalian cells ensures a proper balance of cholesterol with other membrane lipids, principal among these being the major phospholipid phosphatidylcholine (PC). Processes such as cholesterol biosynthesis and efflux, cholesteryl ester storage in lipid droplets, and uptake of plasma lipoproteins are tuned to the cholesterol/PC ratio. Cholesterol-loaded macrophages in atherosclerotic lesions display increased PC biosynthesis that buffers against elevated cholesterol levels and may also facilitate cholesterol trafficking to enhance cholesterol sensing and efflux. These same mechanisms could play a generic role in homeostatic responses to acute changes in membrane free cholesterol levels. Here, I discuss the established and emerging roles of PC metabolism in promoting intracellular cholesterol trafficking and membrane lipid homeostasis.

Participation of prostaglandin D2 in the mobilization of the nuclear-localized CTP:phosphocholine cytidylyltransferase alpha in renal epithelial cells

Phosphatidylcholine (PC) is the main constituent of mammalian cell membranes. Consequently, preservation of membrane PC content and composition - PC homeostasis - is crucial to maintain cellular life. PC biosynthetic pathway is generally controlled by CTP:phosphocholine cytidylyltransferase (CCT), which is considered the rate-limiting enzyme. CCTα is an amphitropic protein, whose enzymatic activity is commonly associated with endoplasmic reticulum redistribution. However, most of the enzyme is located inside the nuclei. Here, we demonstrate that CCTα is the most abundant isoform in renal collecting duct cells, and its redistribution is dependent on endogenous prostaglandins. Previously we have demonstrated that PC synthesis was inhibited by indomethacin (Indo) treatment, and this effect was reverted by exogenous PGD(2). In this work we found that Indo induced CCTα distribution into intranuclear Lamin A/C foci. Exogenous PGD(2) reverted this effect by inducing CCTα redistribution to nuclear envelope, suggesting that PGD(2) maintains PC synthesis by CCTα mobilization. Interestingly, we found that the effect of PGD(2) was dependent on ERK1/2 activation. In conclusion, our previous observations and the present results lead us to suggest that papillary cells possess the ability to maintain their structural integrity through the synthesis of their own survival molecule, PGD(2), by modulating CCTα intracellular location.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Phosphatidylcholine and the CDP-choline cycle

The CDP-choline pathway of phosphatidylcholine (PtdCho) biosynthesis was first described more than 50 years ago. Investigation of the CDP-choline pathway in yeast provides a basis for understanding the CDP-choline pathway in mammals. PtdCho is considered as an intermediate in a cycle of synthesis and degradation, and the activity of a CDP-choline cycle is linked to subcellular membrane lipid movement. The components of the mammalian CDP-choline pathway include choline transport, choline kinase, phosphocholine cytidylyltransferase, and choline phosphotransferase activities. The protein isoforms and biochemical mechanisms of regulation of the pathway enzymes are related to their cell- and tissue-specific functions. Regulated PtdCho turnover mediated by phospholipases or neuropathy target esterase participates in the mammalian CDP-choline cycle. Knockout mouse models define the biological functions of the CDP-choline cycle in mammalian cells and tissues. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Phosphatidylcholine synthesis for lipid droplet expansion is mediated by localized activation of CTP:phosphocholine cytidylyltransferase

Lipid droplets (LDs) are cellular storage organelles for neutral lipids that vary in size and abundance according to cellular needs. Physiological conditions that promote lipid storage rapidly and markedly increase LD volume and surface. How the need for surface phospholipids is sensed and balanced during this process is unknown. Here, we show that phosphatidylcholine (PC) acts as a surfactant to prevent LD coalescence, which otherwise yields large, lipolysis-resistant LDs and triglyceride (TG) accumulation. The need for additional PC to coat the enlarging surface during LD expansion is provided by the Kennedy pathway, which is activated by reversible targeting of the rate-limiting enzyme, CTP:phosphocholine cytidylyltransferase (CCT), to growing LD surfaces. The requirement, targeting, and activation of CCT to growing LDs were similar in cells of Drosophila and mice. Our results reveal a mechanism to maintain PC homeostasis at the expanding LD monolayer through targeted activation of a key PC synthesis enzyme.

The rate-limiting enzyme in phosphatidylcholine synthesis is associated with nuclear speckles under stress conditions

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in eukaryotic membranes and its biosynthetic pathway is generally controlled by CTP:Phosphocholine Cytidylyltransferase (CCT), which is considered the rate-limiting enzyme. CCT is an amphitropic protein, whose enzymatic activity is commonly associated with endoplasmic reticulum (ER) translocation; however, most of the enzyme is intranuclearly located. Here we demonstrate that CCTα is concentrated in the nucleoplasm of MDCK cells. Confocal immunofluorescence revealed that extracellular hypertonicity shifted the diffuse intranuclear distribution of the enzyme to intranuclear domains in a foci pattern. One population of CCTα foci colocalised and interacted with lamin A/C speckles, which also contained the pre-mRNA processing factor SC-35, and was resistant to detergent and salt extraction. The lamin A/C silencing allowed us to visualise a second more labile population of CCTα foci that consisted of lamin A/C-independent foci non-resistant to extraction. We demonstrated that CCTα translocation is not restricted to its redistribution from the nucleus to the ER and that intranuclear redistribution must thus be considered. We suggest that the intranuclear organelle distribution of CCTα is a novel mechanism for the regulation of enzyme activity.

DNA binding to zwitterionic model membranes

This study shows that DNA (linearized plasmid, 4331 base pairs and salmon sperm, 2000 base pairs, respectively) adsorbs to model membranes of zwitterionic liquid crystalline phospholipid bilayers in solutions containing divalent Ca(2+) cations, and also in solutions containing monovalent Na(+). The interaction between DNA and surface-supported model membranes was followed in situ using null ellipsometry, quartz crystal microbalance with dissipation, as well as neutron reflectometry. In the presence of Na(+) (in the absence of multivalent ions), DNA adopts an extended coil conformation upon adsorption. The solvent content in the adsorbed layer is high, and DNA is positioned on top of the membrane. In the presence of divalent Ca(2+), the driving force for the adsorption of DNA is electrostatic, and the adsorbed DNA film is not as dilute as in a solution containing Na(+). Cryo-TEM and SANS were further used to investigate the interaction in bulk solution using vesicles as model membrane systems. DNA adsorption could not be identified in the presence of Na(+) using SANS, but cryo-TEM indicates the presence of DNA between neighboring unilamellar vesicles. In the presence of Ca(2+), DNA induces the formation of multilamellar vesicles in which DNA intercalates the lamellae. Possible electrostatic and hydrophobic mechanisms for the adsorption of DNA in solutions containing monovalent salt are discussed and compared to the observations in divalent salt.

Specific interaction between E2F1 and Sp1 regulates the expression of murine CTP:phosphocholine cytidylyltransferase alpha during the S phase

CTP:phosphocholine cytidylyltransferase alpha (CCTalpha) is a key enzyme for phosphatidylcholine biosynthesis in mammalian cells. This enzyme plays an essential role in all processes that require membrane biosynthesis such as cell proliferation and viability. Thus, CCTalpha activity and expression fluctuate during the cell cycle to achieve PtdCho requirements. We demonstrated, for the first time, that CCTalpha is localized in the nucleus in cells transiting the S phase, whereas it is localized in the cytoplasm of G(0)-arrested cells, suggesting a specific role of nuclear CCTalpha during the S phase. We also investigated how E2F1 influences the regulation of the CCTalpha-promoter during the S phase; we demonstrated that E2F1 is necessary, but not sufficient, to activate CCTalpha expression when this factor is over-expressed. However, when E2F1 and Sp1 were over-expressed, the transcription from the CCTalpha-promoter reporter construct was super-activated. Transient transfection studies demonstrated that E2F1 could super-activate Sp1-dependent transcription in a promoter containing only the Sp1 binding sites "B" or "C", and that Sp1 could activate Sp1-dependent transcription in a promoter containing the E2F site, thus, further demonstrating a functional interaction of these factors. In conclusion, the present results allowed us to portray the clearest picture of the CCTalpha-gene expression in proliferating cells, and understand the mechanism by which cells coordinate cell cycle progression with the requirement for phosphatidylcholine.

14-3-3zeta escorts CCTalpha for calcium-activated nuclear import in lung epithelia

Integrity of animal biomembranes is critical to preserve normal cellular functions and viability. Phosphatidylcholine, an indispensible membrane component, requires the enzyme CCTalpha for its biosynthesis. Nuclear expression of CCTalpha is needed for expansion of the nuclear membrane network, but mechanisms for CCTalpha nuclear import are unknown. Herein, we show that in epithelia, extracellular Ca(2+) triggers CCTalpha cytoplasmic-nuclear translocation. CCTalpha nuclear import was associated with binding to 14-3-3zeta, a key regulator of protein trafficking. 14-3-3zeta was both sufficient and required for CCTalpha nuclear import. Helix G within the 14-3-3zeta binding groove interacts with a putative molecular signature within the CCTalpha carboxyl-terminal phosphoserine motif (residues 328-343). 14-3-3zeta was critically involved in preserving phosphatidylcholine synthesis and cell viability in a model of Pseudomonas aeruginosa infection where Ca(2+) concentrations increase within epithelia. Thus, 14-3-3zeta controls CCTalpha nuclear import in response to calcium signals, thereby regulating mammalian phospholipid synthesis. Agassandian, M., Chen, B. B., Schuster, C. C., Houtman, J. C. D., Mallampalli, R. K. 14-3-3zeta escorts CCTalpha for calcium-activated nuclear import in lung epithelia.

Functions of membrane binding domain of CTP:phosphocholine cytidylyltransferase in alveolar type II cells

CTP:phosphocholine cytidylyltransferase (CCTalpha) plays a key role in the biosynthesis of surfactant phosphatidylcholine. In this study, we investigated the role of its membrane-binding (M) domain in modulating its structure, function, and cellular distribution. Multiple enhanced green fluorescent protein-CCTalpha constructs were generated to evaluate the subcellular distribution in A549 cells. The M domain targeted CCTalpha to the perinuclear (membrane-rich) region. Microinjections with glutathione-S-transferase fusion protein containing the M domain corroborated the perinuclear targeting. Deletion of the M domain or substitutions of the hydrophobic residues with arginine/serine in the VEEKS(267-277) motif of the M domain resulted in a nuclear appearance and indented nuclei. Membrane binding of CCTalpha decreased gradually as the number of positively charged arginine residues increased in the VEEKS motif. To identify whether membrane-protein interactions cause structural alterations in CCTalpha, we visualized the protein in the absence and presence of lipids by transmission electron microscopy. These studies revealed that CCTalpha forms a dimer-like complex that condenses upon binding to lipid vesicles, but not lipid monolayers. The influence of the M domain on CCTalpha activity was assessed in transgenic mice overexpressing the N-terminal catalytic domain (CCTalpha(1-239)), N-terminal catalytic plus M domain (CCTalpha(1-290)), or full-length CCTalpha(1-367) in fetal type II cells by using the surfactant protein C promoter. Only overexpression of CCTalpha(1-367) increased surfactant phosphatidylcholine synthesis. Thus, the M domain influences membrane binding, cellular distribution, and topology of CCTalpha, but the domain alone is not sufficient to confer CCT activity in alveolar type II cells in vivo.

Masking of a nuclear signal motif by monoubiquitination leads to mislocalization and degradation of the regulatory enzyme cytidylyltransferase

Monoubiquitination aids in the nuclear export and entrance of proteins into the lysosomal degradative pathway, although the mechanisms are unknown. Cytidylyltransferase (CCTalpha) is a proteolytically sensitive lipogenic enzyme containing an NH(2)-terminal nuclear localization signal (NLS). We show here that CCTalpha is monoubiquitinated at a molecular site (K(57)) juxtaposed near its NLS, resulting in disruption of its interaction with importin-alpha, nuclear exclusion, and subsequent degradation within the lysosome. Cellular expression of a CCTalpha-ubiquitin fusion protein that mimics the monoubiquitinated enzyme resulted in cytoplasmic retention. A CCTalpha K(57R) mutant exhibited an extended half-life, was retained in the nucleus, and displayed proteolytic resistance. Importantly, by using CCTalpha-ubiquitin hybrid constructs that vary in the intermolecular distance between ubiquitin and the NLS, we show that CCTalpha monoubiquitination masks its NLS, resulting in cytoplasmic retention. These results unravel a unique molecular mechanism whereby monoubiquitination governs the trafficking and life span of a critical regulatory enzyme in vivo.

Expansion of the nucleoplasmic reticulum requires the coordinated activity of lamins and CTP:phosphocholine cytidylyltransferase alpha

The nucleoplasmic reticulum (NR), a nuclear membrane network implicated in signaling and transport, is formed by the biosynthetic and membrane curvature-inducing properties of the rate-limiting enzyme in phosphatidylcholine synthesis, CTP:phosphocholine cytidylyltransferase (CCT) alpha. The NR is formed by invagination of the nuclear envelope and has an underlying lamina that may contribute to membrane tubule formation or stability. In this study we investigated the role of lamins A and B in NR formation in response to expression and activation of endogenous and fluorescent protein-tagged CCTalpha. Similarly to endogenous CCTalpha, CCT-green fluorescent protein (GFP) reversibly translocated to nuclear tubules projecting from the NE in response to oleate, a lipid promoter of CCT membrane binding. Coexpression and RNA interference experiments revealed that both CCTalpha and lamin A and B were necessary for NR proliferation. Expression of CCT-GFP mutants with compromised membrane-binding affinity produced fewer nuclear tubules, indicating that the membrane-binding function of CCTalpha promotes the expansion of the NR. Proliferation of atypical bundles of nuclear membrane tubules by a CCTalpha mutant that constitutively associated with membranes revealed that expansion of the double-bilayer NR requires the coordinated assembly of an underlying lamin scaffold and induction of membrane curvature by CCTalpha.

Phospholipid Biosynthesis Program Underlying Membrane Expansion during B-lymphocyte Differentiation

Stimulated B-lymphocytes differentiate into plasma cells committed to antibody production. Expansion of the endoplasmic reticulum and Golgi compartments is a prerequisite for high rate synthesis, assembly, and secretion of immunoglobulins. The bacterial cell wall component lipopolysaccharide (LPS) stimulates murine B-cells to proliferate and differentiate into antibody-secreting cells that morphologically resemble plasma cells. LPS activation of CH12 B-cells augmented phospholipid production and initiated a genetic program, including elevated expression of the genes for the synthesis, elongation, and desaturation of fatty acids that supply the phospholipid acyl moieties. Likewise, many of the genes in phospholipid biosynthesis were up-regulated, most notably those encoding Lipin1 and choline phosphotransferase. In contrast, CTP:phosphocholine cytidylyltransferase alpha (CCTalpha) protein, a key control point in phosphatidylcholine biosynthesis, increased because of stabilization of protein turnover rather than transcriptional activation. Furthermore, an elevation in cellular diacylglycerol and fatty acid correlated with enhanced allosteric activation of CCTalpha by the membrane lipids. This work defines a genetic and biochemical program for membrane phospholipid biogenesis that correlates with an increase in the phospholipid components of the endoplasmic reticulum and Golgi compartments in LPS-stimulated B-cells.

Phospholipids are synthesized in the G2/M phase of the cell cycle

Very little is known about the metabolism of phospholipids in the G2 and M phases of the cell cycle, but limited studies have led to the postulation that phospholipid synthesis ceases during this period. To investigate whether phospholipids are synthesized in the G2/M phase of the cell cycle, protocols were developed to produce synchronized MCF-7 cell populations with greater than 80% of the cells in G1/S or G2/M phases that moved in synchrony following removal of the blocking agent. Analysis of the activities of key phosphatidylcholine and phosphatidylethanolamine biosynthetic enzymes in subcellular fractions obtained from MCF-7 cells at different cell cycle phases revealed that there was robust activity of key enzymes in the fractions prepared from MCF-7 cells in G2/M phase. Radiolabeled choline and ethanolamine were rapidly incorporated into cells maintained at G2/M phase with nocodazole, and the rates of incorporation were similar to those obtained in cells allowed to progress into the G1 phase. Furthermore, radiolabeled glycerol was incorporated into phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and phosphatidic acid in MCF-7 cells maintained at G2/M phase with nocodazole. Similar results were obtained in CHO cells. These results demonstrate that glycerophospholipid synthesis is very active in the G2/M phase of these cells. Therefore, the postulated cessation of phospholipid synthesis in G2/M phases is not applicable to all cell types.

Mass spectrometry determination of endonuclear phospholipid composition and dynamics

Mammalian cell lipid analyses using tandem electrospray ionization mass spectrometry, in conjunction with stable isotope labeling, permit unparalleled access to membrane phospholipid molecular species compositions and turnover. Lipidomic data from isolable compartments of lipid second messenger generation, such as membrane-free nuclei, can provide dynamic insights into the topology of phospholipid turnover. For example, ESI-MS/MS precursor scans of characteristic phosphocholine m/z 184(+) fragments reveal a highly saturated endonuclear phosphatidylcholine pool with homeostatic maintenance properties. A spatially distinct CDPcholine pathway yields, within minutes of choline-d(9) labeling, unsaturated endonuclear phosphatidylcholines progressively remodeled to more saturated species evidenced by tracking the deuteriated headgroup through precursor scans of phosphocholine-d(9) (m/z 193(+) fragment). Among the other endonuclear phospholipids, diacyl phosphatidylethanolamines (neutral loss of m/z 141(+)) are also highly saturated compared with those of whole cell whereas, phophatidylinositols (precursor scans of m/z 241(-) fragment) are essentially identical in nuclei and whole cells. Moreover, the pattern of myo-inositol-d(6) acquisition into endonuclear phosphatidylinositol (precursor scans of m/z 247(-) fragment) is inconsistent with compartment-specific synthesis. Endonuclear sphingomyelins (seen in precursor scans of m/z 184(+) and confirmed from precursor scans of m/z 168(-) fragments) are enriched but similar in composition to whole cell species whereas endonuclear phosphatidylserines (neutral loss of m/z 87(-)) are more saturated than their whole cell counterparts. The focus of described methodologies emphasize their value in probing the compositions and dynamics of endonuclear phospholipids, but in principle may be extended to exploration of other isolable compartments including ER or plasma membranes.

Induction of apoptosis by lipophilic activators of CTP:phosphocholine cytidylyltransferase alpha (CCTalpha)

Farnesol (FOH) inhibits the CDP-choline pathway for PtdCho (phosphatidylcholine) synthesis, an activity that is involved in subsequent induction of apoptosis. Interestingly, the rate-limiting enzyme in this pathway, CCTalpha (CTP:phosphocholine cytidylyltransferase alpha), is rapidly activated, cleaved by caspases and exported from the nucleus during FOH-induced apoptosis. The purpose of the present study was to determine how CCTalpha activity and PtdCho synthesis contributed to induction of apoptosis by FOH and oleyl alcohol. Contrary to previous reports, we show that the initial effect of FOH and oleyl alcohol was a rapid (10-30 min) and transient activation of PtdCho synthesis. During this period, the mass of DAG (diacylglycerol) decreased by 40%, indicating that subsequent CDP-choline accumulation and inhibition of PtdCho synthesis could be due to substrate depletion. At later time points (>1 h), FOH and oleyl alcohol promoted caspase cleavage and nuclear export of CCTalpha, which was prevented by treatment with oleate or DiC8 (dioctanoylglycerol). Protection from FOH-induced apoptosis required CCTalpha activity and PtdCho synthesis since (i) DiC8 and oleate restored PtdCho synthesis, but not endogenous DAG levels, and (ii) partial resistance was conferred by stable overexpression of CCTalpha and increased PtdCho synthesis in CCTalpha-deficient MT58 cells. These results show that DAG depletion by FOH or oleyl alcohol could be involved in inhibition of PtdCho synthesis. However, decreased DAG was not sufficient to induce apoptosis provided nuclear CCTalpha and PtdCho syntheses were sustained.

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

Dynamic lipidomics of the nucleus

Once nuclear envelope membranes have been removed from isolated nuclei, around 6% of mammalian cell phospholipid is retained within the nuclear matrix, which calculations suggest may occupy 10% of the volume of this subcellular compartment. It is now acknowledged that endonuclear phospholipid, largely ignored for the past 40 years, provides substrate for lipid-mediated signaling events. However, given its abundance, it likely also has other as yet incompletely defined roles. Endonuclear phosphatidylcholine is the predominant phospholipid comprising distinct and highly saturated molecular species compared with that of the whole cell. Moreover, this unusual composition is subject to tight homeostatic maintenance even under conditions of extreme dietary manipulation, presumably reflecting a functional requirement for highly saturated endonuclear phosphatidylcholine. Recent application of new lipidomic technologies exploiting tandem electrospray ionization mass spectrometry in conjunction with deuterium stable isotope labeling have permitted us to probe not just molecular species compositions but endonuclear phospholipid acquisition and turnover with unparalleled sensitivity and specificity. What emerges is a picture of a dynamic pool of endonuclear phospholipid subject to autonomous regulation with respect to bulk cellular phospholipid metabolism. Compartmental biosynthesis de novo of endonuclear phosphatidylcholine contrasts with import of phosphatidylinositol synthesized elsewhere. However, irrespective of the precise temporal-spatial-dynamic relationships underpinning phospholipid acquisition, derangement of endonuclear lipid-mediated signaling from these parental phospholipids halts cell growth and division indicating a pivotal control point. This in turn defines the manipulation of compartmentalized endonuclear phospholipid acquisition and metabolism as intriguing potential targets for the development of future antiproliferative strategies.

Polyploid formation via chromosome duplication induced by CTP:phosphocholine cytidylyltransferase deficiency and Bcl-2 overexpression: identification of two novel endogenous factors

Polyploidy is a profound phenotype found in tumors and its mechanism is unknown. We report here that when B-cell lymphoma gene-2 (Bcl-2) was overexpressed in a Chinese hamster ovary cell line that was deficient in CTP:phosphocholine cytidylyltransferase (CT), cellular DNA content doubled. The higher DNA content was due to a permanent conversion from diploid cells to tetraploid cells. The mechanism of polyploid formation could be attributed to the duplication of 18 parental chromosomes. The rate of conversion from diploid to tetraploid was Bcl-2 dose dependent. The diploid genome was not affected by Bcl-2 expression or by CT deficiency alone. Endogenous CT or expression of recombinant rat liver CTalpha prior to Bcl-2 expression prevented the formation of polyploid cells. This conversion was irreversible even when both initiating factors were removed. In this study, we have identified Bcl-2 as a positive regulator and CTalpha as a negative regulator of polyploid formation.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

The rate-limiting enzyme in phosphatidylcholine synthesis regulates proliferation of the nucleoplasmic reticulum

The nucleus contains a network of tubular invaginations of the nuclear envelope (NE), termed the nucleoplasmic reticulum (NR), implicated in transport, gene expression, and calcium homeostasis. Here, we show that proliferation of the NR, measured by the frequency of NE invaginations and tubules, is regulated by CTP:phosphocholine cytidylyltransferase-alpha (CCTalpha), the nuclear and rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis. In Chinese hamster ovary (CHO)-K1 cells, fatty acids triggered activation and translocation of CCTalpha onto intranuclear tubules characteristic of the NR. This was accompanied by a twofold increase in NR tubules quantified by immunostaining for lamin A/C or the NE. CHO MT58 cells expressing a temperature-sensitive CCTalpha allele displayed reduced PtdCho synthesis and CCTalpha expression and minimal proliferation of the NR in response to oleate compared with CHO MT58 cells stably expressing CCTalpha. Expression of CCTalpha mutants in CHO58 cells revealed that both enzyme activity and membrane binding promoted NR proliferation. In support of a direct role for membrane binding in NR tubule formation, recombinant CCTalpha caused the deformation of liposomes into tubules in vitro. This demonstrates that a key nuclear enzyme in PtdCho synthesis coordinates lipid synthesis and membrane deformation to promote formation of a dynamic nuclear-cytoplasmic interface.

Lipidomic analysis of the molecular specificity of a cholinephosphotransferase in situ

Dynamic lipidomics using ESI–MS (tandem electrospray ionization mass spectrometry) of 9-deuterated choline (choline-d9) incorporation into mammalian cell PtdCho (phosphatidylcholine) permits assessment of the molecular specificity of synthesis. Bulk cell PtdCho synthesis occurs in spatially distinct locations, using separate CPTs (1,2 diacylglycerol CDP:choline cholinephosphotransferases). We assessed whether in vitro molecular selectivity of DAG (diacylglycerol) incorporation between CPTs is manifest in situ, by monitoring choline-d9 incorporation into PtdCho and lyso-PtdCho molecular species over 3 h in control cells and in CHO-K1 cells overexpressing hCEPT1. Compared with controls, the basal molecular species composition of hCEPT1 overexpressors was significantly enriched in arachidonate. This was not due to net accretion of cellular PtdCho arguing against effects of inadequate unsaturated PtdCho degradation or remodelling. Rather, time-course analyses of PtdCho and lyso-PtdCho pools showed that both arachidonate-containing DAG incorporation and turnover of PtdCho is increased in hCEPT1 overexpressors. Increased choline-d9 incorporation into arachidonyl lyso-PtdCho shows that both phospholipase A1- and A2-mediated turnover is involved. Spatially distinct molecular specificity of DAG incorporation into cellular PtdCho at the level of hCEPT1 exists in situ.

Surfactant lipid synthesis and lamellar body formation in glycogen-laden type II cells

Pulmonary surfactant is a lipoprotein complex that functions to reduce surface tension at the air liquid interface in the alveolus of the mature lung. In late gestation glycogen-laden type II cells shift their metabolic program toward the synthesis of surfactant, of which phosphatidylcholine (PC) is by far the most abundant lipid. To investigate the cellular site of surfactant PC synthesis in these cells we determined the subcellular localization of two key enzymes for PC biosynthesis, fatty acid synthase (FAS) and CTP:phosphocholine cytidylyltransferase-alpha (CCT-alpha), and compared their localization with that of surfactant storage organelles, the lamellar bodies (LBs), and surfactant proteins (SPs) in fetal mouse lung. Ultrastructural analysis showed that immature and mature LBs were present within the glycogen pools of fetal type II cells. Multivesicular bodies were noted only in the cytoplasm. Immunogold electron microscopy (EM) revealed that the glycogen pools were the prominent cellular sites for FAS and CCT-alpha. Energy-filtering EM demonstrated that CCT-alpha bound to phosphorus-rich (phospholipid) structures in the glycogen. SP-B and SP-C, but not SP-A, localized predominantly to the glycogen stores. Collectively, these data suggest that the glycogen stores in fetal type II cells are a cellular site for surfactant PC synthesis and LB formation/maturation consistent with the idea that the glycogen is a unique substrate for surfactant lipids.

Nuclear lipids: key signaling effectors in the nervous system and other tissues

Lipids have long been recognized as quantitatively minor components of the nucleus, where they were initially thought to have little functional importance; but they now command growing interest, with recognition of their diverse signaling and modulating properties in that organelle. This applies to the lipid-poor compartments of the nucleoplasm as well as the relatively lipid-rich nuclear envelope. Phosphoglycerides and sphingomyelin, as the predominant lipids, have attracted the most interest among researchers, but some of the less-abundant lipids such as gangliosides, sphingosine, and sphingosine phosphate are now becoming recognized as functionally important nuclear constituents. Among recent advances in this emerging field are detailed findings on the metabolic enzymes that synthesize and catabolize nuclear lipids; the fact that these are localized primarily within the nucleus itself indicates considerable autonomy with respect to lipid metabolism. Current studies suggest several key processes involving RNA and DNA reactivity that are dependent on these lipid-initiated events. Neural cell nuclei have been the subject of such investigations, with results that closely parallel the more numerous studies on nuclei of extraneural cells. This review attempts to outline some of the major findings on nuclear lipids of diverse cell types; results with nonneural nuclei will hopefully provide useful guideposts to further studies of neural systems.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Both acidic and basic amino acids in an amphitropic enzyme, CTP:phosphocholine cytidylyltransferase, dictate its selectivity for anionic membranes

Amphitropic proteins are regulated by reversible membrane interaction. Anionic phospholipids generally promote membrane binding of such proteins via electrostatics between the negatively charged lipid headgroups and clusters of basic groups on the proteins. In this study of one amphitropic protein, a cytidylyltransferase (CT) that regulates phosphatidylcholine synthesis, we found that substitution of lysines to glutamine along both interfacial strips of the membrane-binding amphipathic helix eliminated electrostatic binding. Unexpectedly, three glutamates also participate in the selectivity for anionic membrane surfaces. These glutamates become protonated in the low pH milieu at the surface of anionic, but not zwitterionic membranes, increasing protein positive charge and hydrophobicity. The binding and insertion into lipid vesicles of a synthetic peptide containing the three glutamates was pH-dependent with an apparent pK(a) that varied with anionic lipid content. Glutamate to glutamine substitution eliminated the pH dependence of the membrane interaction, and reduced anionic membrane selectivity of both the peptide and the whole CT enzyme examined in cells. Thus anionic lipids, working via surface-localized pH effects, can promote membrane binding by modifying protein charge and hydrophobicity, and this novel mechanism contributes to the membrane selectivity of CT in vivo.

Phosphatidylcholine biosynthesis at low temperature: differential expression of CTP:phosphorylcholine cytidylyltransferase isogenes in Arabidopsis thaliana

We cloned the gene and a cDNA for a second CTP: phosphorylcholine cytidylyltransferase (CCT, EC 2.7.7.15) annotated in chromosome 4 by the Arabidopsis genome project, and designated the gene AtCCT2 to discriminate it from the isogene AtCCT1 in chromosome 2. When Arabidopsis plants were chilled at 2 degrees C for 12 h, the level of AtCCT2 transcripts in the rosettes increased about 6-fold over that before 2 degrees C treatment. By contrast, no significant change occurred in the level of AtCCT1 transcripts during 7 d of 2 degrees C treatment. Immunoblot analysis revealed that the level of AtCCT2 in the rosettes chilled at 2 degrees C increased, and that the level of AtCCT1 showed minor changes, when compared with those before cold treatment. Total CCT activity measured at 2 degrees C increased in plants subjected to 2 degrees C treatment, and this increase was sufficient to account for lipid changes induced by the 2 degrees C treatment. We therefore concluded that Arabidopsis utilizes two distinct CCT isozymes for CDP-choline synthesis during cold acclimation. Our findings are important in understanding the physiological functions of CCT isozymes in Arabidopsis and will also stimulate efforts to understand the physiological significance of phosphatidylcholine at low temperatures.

The structure of phosphatidylinositol transfer protein alpha reveals sites for phospholipid binding and membrane association with major implications for its function

Elucidation of the three-dimensional structure of phosphatidylinositol transfer protein alpha (PI-TPalpha) void of phospholipid revealed a site of membrane association connected to a channel for phospholipid binding. Near the top of the channel specific binding sites for the phosphorylcholine and phosphorylinositol head groups were identified. The structure of this open form suggests a mechanism by which PI-TPalpha preferentially binds PI from a membrane interface. Modeling predicts that upon association of PI-TPalpha with the membrane the inositol moiety of bound PI is accessible from the medium. Upon release from the membrane PI-TPalpha adopts a closed structure with the phospholipid bound fully encapsulated. This structure provides new insights as to how PI-TPalpha may play a role in PI metabolism.

A comparison of the molecular specificities of whole cell and endonuclear phosphatidylcholine synthesis

Deuterated choline-d(9) labelling of IMR-32 cells enabled comparison of the molecular specificities of whole cell and endonuclear phosphatidylcholine synthesis after 96 h polyunsaturated fatty acid supplementation. Surprisingly, while cell phosphatidylcholine synthesis and remodelling reflected a pattern of polyunsaturated fatty acid accretion, the saturated endonuclear phosphatidylcholine pool was only transiently labelled with polyunsaturates. Periodic endonuclear accumulations of the lipid second messenger diacylglycerol, mobilised from unsaturated phosphatidylinositol or saturated phosphatidylcholine, accompany cell proliferation. Non-specific incorporation into endonuclear phosphatidylcholine and selective removal or remodelling of unsaturated molecular species may form part of a single 'off switch' recycling all endonuclear diacylglycerol accumulations.

The major sites of cellular phospholipid synthesis and molecular determinants of Fatty Acid and lipid head group specificity

Phosphatidylcholine and phosphatidylethanolamine are the two main phospholipids in eukaryotic cells comprising ~50 and 25% of phospholipid mass, respectively. Phosphatidylcholine is synthesized almost exclusively through the CDP-choline pathway in essentially all mammalian cells. Phosphatidylethanolamine is synthesized through either the CDP-ethanolamine pathway or by the decarboxylation of phosphatidylserine, with the contribution of each pathway being cell type dependent. Two human genes, CEPT1 and CPT1, code for the total compliment of activities that directly synthesize phosphatidylcholine and phosphatidylethanolamine through the CDP-alcohol pathways. CEPT1 transfers a phosphobase from either CDP-choline or CDP-ethanolamine to diacylglycerol to synthesize both phosphatidylcholine and phosphatidylethanolamine, whereas CPT1 synthesizes phosphatidylcholine exclusively. We show through immunofluorescence that brefeldin A treatment relocalizes CPT1, but not CEPT1, implying CPT1 is found in the Golgi. A combination of coimmunofluorescence and subcellular fractionation experiments with various endoplasmic reticulum, Golgi, and nuclear markers confirmed that CPT1 was found in the Golgi and CEPT1 was found in both the endoplasmic reticulum and nuclear membranes. The rate-limiting step for phosphatidylcholine synthesis is catalyzed by the amphitropic CTP:phosphocholine cytidylyltransferase alpha, which is found in the nucleus in most cell types. CTP:phosphocholine cytidylyltransferase alpha is found immediately upstream cholinephosphotransferase, and it translocates from a soluble nuclear location to the nuclear membrane in response to activators of the CDP-choline pathway. Thus, substrate channeling of the CDP-choline produced by CTP:phosphocholine cytidylyltransferase alpha to nuclear located CEPT1 is the mechanism by which upregulation of the CDP-choline pathway increases de novo phosphatidylcholine biosynthesis. In addition, a series of CEPT1 site-directed mutants was generated that allowed for the assignment of specific amino acid residues as structural requirements that directly alter either phospholipid head group or fatty acyl composition. This pinpointed glycine 156 within the catalytic motif as being responsible for the dual CDP-alcohol specificity of CEPT1, whereas mutations within helix 214-228 allowed for the orientation of transmembrane helices surrounding the catalytic site to be definitively positioned.

Caspase processing and nuclear export of CTP:phosphocholine cytidylyltransferase alpha during farnesol-induced apoptosis

CTP:phosphocholine cytidylyltransferase alpha (CCT alpha) is a nuclear enzyme that catalyzes the rate-limiting step in the CDP-choline pathway, the primary route for synthesis of phosphatidylcholine (PtdCho) in eukaryotic cells. Induction of apoptosis by farnesol (FOH) and other cytotoxic drugs has been shown to alter PtdCho synthesis via the CDP-choline pathway. Here we report that FOH-induced apoptosis in CHO cells caused a dose-dependent activation of CCT alpha and inhibition of the final step in the pathway, resulting in a biphasic effect on PtdCho synthesis. Activation of CCT alpha was accompanied by enzyme translocation to the nuclear envelope within 30 min of FOH addition to cells. Following translocation to membranes, CCT alpha was exported from the nucleus and underwent caspase-mediated proteolysis that coincided with poly(ADP-ribose) polymerase cleavage. Site-directed mutagenesis and in vivo and in vitro expression studies mapped a caspase 6 and/or 8 cleavage site to TEED(28 downward arrow)G, the final residue in the CCT alpha nuclear localization signal. Nuclear export of CCT alpha appeared to be an active process in FOH-treated CHO cells that was independent of caspase removal of the nuclear localization signal. Caspase cleavage of CCT alpha occurred during UV or chelerythrine-induced apoptosis; however, nuclear membrane translocation and nuclear export were not evident under these conditions. Thus, caspase cleavage of CCT alpha was a late feature of several apoptotic programs that occurred in the nucleus or at the nuclear envelope. Activation and nuclear export of CCT alpha were early events in FOH-induced apoptosis that contributed to altered PtdCho synthesis and, in conjunction with caspase cleavage, excluded CCT alpha from the nucleus.

Cell cycle regulation of pulmonary phosphatidylcholine synthesis

Pulmonary surfactant phosphatidylcholine (PC) formation increases as alveolar type II cells mature and arrest in G0/G1 state of the cell cycle at late fetal gestation. To determine whether this G0/G1 arrest is responsible for the increase in PC synthesis, we investigated the rates of PC synthesis and the activity, phosphorylation, intracellular distribution, synthesis, and degradation of a key enzyme of PC synthesis, cytidine triphosphate (CTP):phosphocholine cytidylyltransferase (CCTalpha). In synchronized mouse lung epithelial (MLE)-15 cells, PC production and CCTalpha activity peaked at G0/G1, declined during transition to G1/S, and remained low during S and G2/M. The changes in CCTalpha activity were not due to alterations in CCTalpha gene and protein expression. CCTalpha protein degradation also did not change during the cell cycle. Indirect immunofluorescence and immunogold electron microscopy revealed that CCTalpha localized to the cytoplasmic compartment and that its cytosolic localization did not change with the cell cycle. Although immunoblotting suggested no major redistribution of CCTalpha mass from cytosol to endoplasmic reticulum, activity measurements revealed that the ratio of particulate/soluble CCTalpha activity was cell cycle-dependent. The particulate/soluble ratio peaked at G0/G1 and declined with cell-cycle progression. Furthermore, the decrease in CCTalpha activity during exit from G0/G1 was associated with an increase in CCTalpha phosphorylation. These data suggest that the cell-cycle changes in PC synthesis are likely not due to alterations in CCTalpha expression and degradation but are primarily a consequence of changes in CCTalpha activity, phosphorylation, and membrane affinity.

CTP:phosphocholine cytidylyltransferase alpha is a cytosolic protein in pulmonary epithelial cells and tissues

CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in de novo synthesis of phosphatidylcholine (PC). The lung requires a steady synthesis of PC for lung surfactant of which disaturated PC is the essential active agent. Surfactant synthesis occurs in alveolar type II cells. Studies with non-pulmonary cells have suggested that CCT is both a nuclear and cytoplasmic protein. The unusual requirements of the lung for PC synthesis and, therefore, CCT activity suggest a unique mechanism of regulation and possibly localization of CCT. The localization of CCT alpha in lung epithelial cells and, of greater consequence, lung tissues are yet unknown. Three isoforms of CCT have been identified. Herein we investigated the localization of the ubiquitously expressed CCT alpha isoform. To ascertain CCT alpha localization in lungs and lung-related epithelial cells, we employed a number of localization methods. Immunogold electron microscopy using polyclonal antibodies raised to either the carboxyl terminus, catalytic domain, or amino terminus of CCT alpha localized CCT alpha mostly to the exterior plasma membrane or regions of the endoplasmic reticulum (ER) in both A549 and MLE-15 epithelial lung cell lines and primary cultures of fetal rat lung epithelial cells. In contrast to other studies, little or no nuclear labeling was observed. Indirect immunofluorescence of these cells with anti-CCT alpha antibodies resulted in a similar distribution. Indirect visualization of both hemagglutinin- and FLAG-tagged CCT alpha as well as direct visualization of enhanced green fluorescence protein-CCT alpha fusion protein corroborated a cytoplasmic localization of CCT alpha in pulmonary cells. Moreover, analysis of lung tissue from fetal and adult mouse by either immunogold electron microscopy or indirect immunofluorescence yielded a strong cytoplasmic CCT alpha signal with virtually no nuclear localization in epithelial cells lining the airways. The cytoplasmic localization of CCT alpha in type II cells was further substantiated with transgenic mice overexpressing FLAG-tagged CCT alpha using the lung-specific human surfactant protein C (SP-C) promoter. We conclude that CCT alpha does not localize to the nucleus in pulmonary tissues, and, therefore, nuclear localization of CCT alpha is not a universal event.

Effect of D609 on phosphatidylcholine metabolism in the nuclei of LA-N-1 neuroblastoma cells: a key role for diacylglycerol

In our previous studies, TPA treatment of LA-N-1 cells stimulated the production of diacylglycerol in nuclei, probably through the activation of a phospholipase C. Stimulation of the synthesis of nuclear phosphatidylcholine by the activation of CTP:phosphocholine cytidylyltransferase was also observed. The present data show that both effects were inhibited by the pretreatment of the cells with D609, a selective phosphatidylcholine-phospholipase C inhibitor, indicating that the diacylglycerol produced through the hydrolysis of phosphatidylcholine in the nuclei is reutilized for the synthesis of nuclear phosphatidylcholine and is required for the activation of CTP:phosphocholine cytidylyltransferase.

Transcription of the CTP:phosphocholine cytidylyltransferase alpha gene is enhanced during the S phase of the cell cycle

We have studied the transcription of the CTP:phosphocholine cytidylyltransferase alpha (CTalpha) gene in C3H10T1/2 fibroblasts as a function of the cell cycle. The cells were incubated for 48 h with 0.5% fetal bovine serum. The cells were induced into the G(1) phase of the cell cycle by the addition of medium with 10% fetal bovine serum. The cells began the synthesis of DNA after 12 h. At 16 and 20 h there was an increased amount of CTalpha mRNA that coincided with an increase in the expression of CTalpha proximal promoter-luciferase constructs (-201/+38 and -130/+38). Luciferase constructs with the basal promoter (-52/+38) showed no change in activity during the cell cycle. Incorporation of [(3)H]choline into phosphatidylcholine began to increase by 8 h after the addition of serum and peaked at 18 h. The mass of phosphatidylcholine nearly doubled between 8 and 26 h after addition of serum. CT activity increased by 6 h after serum addition and was maintained until 22 h. Thus, the increase of phosphatidylcholine biosynthesis in the G(1) phase of the cell cycle is not due to enhanced transcription of the CTalpha gene. Instead increased transcription of the CTalpha gene occurred during the S phase of the cell cycle in preparation for mitosis.

Highly saturated endonuclear phosphatidylcholine is synthesized in situ and colocated with CDP-choline pathway enzymes

Chromatin-associated phospholipids are well recognized. A report that catalytically active endonuclear CTP:choline-phosphate cytidylyltransferase alpha is necessary for cell survival questions whether endonuclear, CDP-choline pathway phosphatidylcholine synthesis may occur in situ. We report that chromatin from human IMR-32 neuroblastoma cells possesses such a biosynthetic pathway. First, membrane-free nuclei retain all three CDP-choline pathway enzymes in proportions comparable with the content of chromatin-associated phosphatidylcholine. Second, following supplementation of cells with deuterated choline and using electrospray ionization mass spectrometry, both the time course and molecular species labeling pattern of newly synthesized endonuclear and whole cell phosphatidylcholine revealed the operation of spatially separate, compositionally distinct biosynthetic routes. Specifically, endogenous and newly synthesized endonuclear phosphatidylcholine species are both characterized by a high degree of diacyl/alkylacyl chain saturation. This unusual species content and synthetic pattern (evident within 10 min of supplementation) are maintained through cell growth arrest by serum depletion and when proliferation is restored, suggesting that endonuclear disaturated phosphatidylcholine enrichment is essential and closely regulated. We propose that endonuclear phosphatidylcholine synthesis may regulate periodic nuclear accumulations of phosphatidylcholine-derived lipid second messengers. Furthermore, our estimates of saturated phosphatidylcholine nuclear volume occupancy of around 10% may imply a significant additional role in regulating chromatin structure.

Regulation of CTP:phosphocholine cytidylyltransferase by amphitropism and relocalization

Phosphatidylcholine (PC) synthesis in animal cells is generally controlled by cytidine 5'-triphosphate (CTP):phosphocholine cytidylyltransferase (CCT). This enzyme is amphitropic, that is, it can interconvert between a soluble inactive form and a membrane-bound active form. The membrane-binding domain of CCT is a long amphipathic alpha helix that responds to changes in the physical properties of PC-deficient membranes. Binding of this domain to membranes activates CCT by relieving an inhibitory constraint in the catalytic domain. This leads to stimulation of PC synthesis and maintenance of membrane PC content. Surprisingly, the major isoform, CCT alpha, is localized in the nucleus of many cells. Recently, a new level of its regulation has emerged with the discovery that signals that stimulate PC synthesis recruit CCT alpha from an inactive nuclear reservoir to a functional site on the endoplasmic reticulum.