Nicholas Ktistakis Tel. (01223) 496323

• Contact via email


• Recent, selected Publications

Signalling pathways regulated by phosphatidic acid

Signalling pathways regulate communication between and within cells. Key proteins in the pathways act very broadly to maintain cells in the appropriate functional state. Very small changes in the genes encoding these proteins can lead to drastic and wide-reaching problems. We are interested in cellular traffic and lipid signalling with special emphasis on pathways and proteins regulated by phosphatidic acid (PA). PA is an essential intermediate in lipid biosynthesis but it can also be formed in a signal-dependent way following activation of the enzyme phospholipase D. To gain insight into pathways of PA regulation we set out to identify proteins that interact with PA directly. Two such proteins have been identified recently: sphingosine kinase 1 (SK1) and OPI1.

SK1 phosphorylates sphingosine to generate sphingosine 1-phosphate. A useful generalisation is that whereas elevated levels of sphingosine contribute to cell death via apoptotic pathways, elevation of sphingosine1-phosphate levels has a protective effect. Thus, SK1 contributes to important life/death decisions in the cell. In addition, SK1 has been implicated in calcium mobilisation either during phagocytosis or as a downstream response to receptor crosslinking in immune cells. We have discovered that SK1 is a PA effector: it binds pure PA coupled to a solid support and it translocates in vivo and in vitro to membranes and liposomes enriched in PA. Our working model for the significance of the SK1-PA interaction is shown in Figure 1, and future work aims to identify the molecular basis of the SK1-PA interaction.

Figure 1 (Click to enlarge)


The SK1-PA interaction in the context of two signalling pathways. SK1 can translocate to PA-enriched membranes downstream of PLD activation (A), or it can sense life/death cellular decisions reflected on phospholipid biosynthesis using PA as an indicator (B).

OPI1 is a transcriptional repressor of various genes involved in lipid metabolism in yeast. Inositol regulates OPI1: yeast cells growing without inositol contain an endoplasmic reticulum (ER) bound form of OPI1 whereas addition of inositol rapidly translocates OPI1 to the nucleus. In collaboration with Tim Levine (Institute of Ophthalmology, UCL, London) and Susan Henry (Cornell University, New York) we found that OPI1 is a PA binding protein and may constitute a PA sensor regulating lipid biosynthesis in yeast. In this view, cells growing under normal conditions and with limited inositol would contain abundant ER-localised PA thus keeping OPI1 in the ER. Upon inositol addition, PA would be rapidly consumed and OPI1 would translocate to the nucleus. See Figure 2.

Figure 2 (Click to enlarge)

OPI1 shuttles between cytosol and nucleus in a PA-dependent way. Under conditions of low inositol, increased flux of PA on ER membranes would keep OPI1 bound there (A). Upon inositol stimulation, ER levels of PA would drop and OPI1 would translocate to the nucleus for gene repression (B).

Recent, selected publications

Stace CL, Ktistakis NT (2006) Phosphatidic acid- and phosphatidylserine-binding proteins.
Biochimica et Biophysica Acta - Molecular and Cell Biology of Lipids 1761 913-926
http://dx.doi.org/10.1016/j.bbalip.2006.03.006

Delon C, Manifava M, Wood EJ, Thompson D, Krugmann S, Pyne S, Ktistakis NT (2004) Sphingosine kinase 1 is an intracellular effector of phosphatidic acid.
Journal of Biological Chemistry 279 44763-44774
http://dx.doi.org/10.1074/jbc.M405771200

Loewen CJR, Gaspar ML, Jesch SA, Delon C, Ktistakis NT, Henry SA, Levine TP (2004) Phospholipid metabolism regulated by a transcription factor sensing phosphatidic acid.
Science 304 1644-1647
http://dx.doi.org/10.1126/science.1096083

Ktistakis NT, Delon C, Manifava M, Wood E, Ganley I, Sugars JM (2003) Phospholipase D1 and potential targets of its hydrolysis product, phosphatidic acid.
Biochemical Society Transactions 31 94-97
http://www.biochemsoctrans.org/bst/031/bst0310094.htm

Sugars JM, Cellek S, Manifava M, Coadwell WJ, Ktistakis NT (2002) Hierarchy of membrane-targeting signals of phospholipase D1 involving lipid modification of a pleckstrin homology domain.
Journal of Biological Chemistry 277 29152-29161
http://dx.doi.org/10.1074/jbc.M112169200

Manifava M, Thuring JWJF, Lim Z-Y, Packman L, Holmes AB, Ktistakis NT (2001) Differential binding of traffic-related proteins to phosphatidic acid- or phosphatidylinositol (4,5)-bisphosphate-coupled affinity reagents.
Journal of Biological Chemistry 276 8987-8994
http://dx.doi.org/10.1074/jbc.M010308200

Ridley SH, Ktistakis NT, Davidson K, Anderson KE, Manifava M, Ellson CD, Lipp P, Bootman MD, Coadwell WJ, Nazarian A, Erdjument-Bromage H, Tempst P, Cooper MA, Thuring JWJF, Lim Z-Y, Holmes AB, Stephens LR, Hawkins PT (2001) FENS-1 and DFCP1 are FYVE domain-containing proteins with distinct functions in the endosomal and Golgi compartments.
Journal of Cell Science 114 3991-4000
http://jcs.biologists.org/cgi/content/full/114/22/3991