Our work is currently concentrating on two distinct but broadly related areas: Regulation and functions of a group of signaling enzymes called phospholipase D (PLD) and the mechanisms, enzymes and pathways involved in the synthesis and inactivation of a receptor active lipid mediator, lysophosphatidic acid (LPA).

PLD activities are found in all organisms. These enzymes catalyze the hydrolysis of the major membrane phospholipid phosphatidylcholine. In mammalian cells, activity of two PLD isoenzymes, PLD1 and PLD2 is under tight regulation by cell surface receptors and via mechanisms that involve GTP binding proteins and protein kinases. The primary lipid product of phospholipase D is phosphatidic acid (PA) which we hypothesize functions as an intracellular second messenger that regulates the activity of lipid and protein kinases. For example, Phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) activity can be stimulated by PA in vitro and we and our collaborators have found that this enzyme localizes with a specific PLD isoform, PLD2 at sites within several different cell types (most notably fibroblasts) where phosphoinositide synthesis is activated. This is an interesting finding because the product of PI4P5K, phosphatidylinositol 4,5-bisphosphate (PI4,5P2) is a lipid messenger that regulates the membrane association and activity of a number of important proteins. Our data suggest that this PLD/PIPkinase pathway may control PI4,5P2 synthesis leading to changes in organization of the actin cytoskeleton. Our research in this area is currently focused two main aims. Building on insights gained into PLD structure and regulation by mutagenesis experiments, we are attempting to determine the high resolution three dimensional structure of a catalytically active regulated fragment of PLD1. We are also exploring the structural basis for the selective interaction of protein targets with PA and hope to develop specific protein probes that can be used to monitor changes in PA levels and localization in living cells.

LPA is a receptor active phospholipid that is released by many different cells. LPA plays a central role in a wide variety of processes that include control of cell growth, differentiation, development, motility, blood clotting and wound healing. LPA exerts these widely studied effects by binding to G-protein coupled cell surface receptors. We are interested in understanding the enzymes, biochemical pathways and cellular mechanisms responsible for the synthesis and inactivation of LPA. Although we use a number of different cell types in our research, our work on LPA synthesis and inactivation focuses primarily on ovarian cancer cells because autocrine actions of LPA appear to contribute to the poor prognosis of this disease by promoting ovarian cancer cell growth, invasiveness and resistance to chemotherapeutics. We therefore postulate that targeting the synthesis and actions of LPA may therefore provide a novel treatment strategy for pharmacological intervention in ovarian cancer. Our research in this area is addressing two major aims. Firstly, using a novel enzymatic assay for LPA, we have found that blood platelets and certain other cell types release LPA both constitutively and in response to agonist stimulation. This process appears to involve the concerted actions of phospholipases and the exocytotic release of membrane microvesicles. Using genetic and pharmacological strategies we are attempting to identify roles for specific PLD and phospholipase A2 enzymes in this process. We are particularly interested in defining the role of a novel lysophosphatidylcholine specific PLD activity in LPA synthesis. Most excitingly, we have cloned genes encoding a number of enzymes that inactivate LPA by dephosphorylation. One class of these are cell surface lipid phosphate phosphatases. Cells over expressing these enzymes become remarkably refractory to stimulation by LPA. In the absence of pharmacological antagonists for LPA receptors, these enzymes promise to be powerful tools to manipulate LPA signaling.

Smyth SS, Sciorra VA, Sigal YJ, Pamulkar Z, Wang Z, Xu Y, Prestwich GD, Morris AJ. Lipid phosphate phosphatases regulate lysophosphatidic acid production and signaling in platelets: Studies using chemical inhibitors of lipid phosphate phosphatase activity. J Biol Chem. (2003) in press.

Escalante-Alcade, D., Hernandez. L. LeStunff, H., Maeda, R., Gang-Cheng, J., Sciorra, V.A., Daar, I. Spiegel, S., Morris, A.J. and Stewart, C.L. Bioactive phospholipids regulate embryonic vasculogenesis and axis patterning. 2003 Development in press.

Luquain C, Singh A, Wang L, Natarajan V, Morris AJ. Role of phospholipase D in agonist stimulated Lysophosphatidic acid synthesis by ovarian cancer cells. J Lipid Res. 2003 in press.

Bankaitis VA, Morris AJ. Lipids and the exocytotic machinery of eukaryotic cells. Curr Opin Cell Biol. 2003 Aug;15(4):389-95.

Luquain C, Sciorra VA, Morris AJ. Lysophosphatidic acid signaling: how a small lipid does big things. Trends Biochem Sci. 2003 Jul;28(7):377-83.

Du G, Altshuller YM, Vitale N, Huang P, Chasserot-Golaz S, Morris AJ, Bader MF, Frohman MA. Regulation of phospholipase D1 subcellular cycling through coordination of multiple membrane association motifs. J Cell Biol. 2003 Jul 21;162(2):305-15.

Sciorra VA, Rudge SA, Wang J, McLaughlin S, Engebrecht J, Morris AJ. Dual role for phosphoinositides in regulation of yeast and mammalian phospholipase D enzymes. J Cell Biol. 2002 Dec 23;159(6):1039-49.

For more of Dr. Morris' publications click here.