Phospholipase cleavage of glycosylphosphatidylinositol reconstituted in liposomal membranes

Phospholipase cleavage of D- and L-chiro-glycosylphosphoinositides asymmetrically incorporated into liposomal membranes

The nature of chiro-inositol-containing inositolphosphoglycans (IPGs), reported to be putative insulin mediators, was studied by examination of the substrate specificities of the phosphatidylinositol-specific phospholipase C (PI-PLC) and the glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) by using a series of synthetic D- and L-chiro-glycosylphosphoinositides. 3-O-alpha-D-Glucosaminyl- (3) and -galactosaminyl-2-phosphatidyl-L-chiro-inositol (4), which show the maximum stereochemical similarity to the 6-O-alpha-D-glucosaminylphosphatidylinositol pseudodisaccharide motifs of GPI anchors, were synthesized and asymmetrically incorporated into phospholipid bilayers in the form of large unilamellar vesicles (LUVs). Similarly, 2-O-alpha-D-glucosaminyl- (5) and -galactosaminyl-1-phosphatidyl-D-chiro-inositol (6), which differ from the corresponding pseudodisaccharide motif of the GPI anchors only in the axial orientation of the phosphatidyl moiety, were also synthesized and asymmetrically inserted into LUVs. The cleavage of these synthetic molecules in the liposomal constructs by PI-PLC from Bacillus cereus and by GPI-PLD from bovine serum was studied with the use of 6-O-alpha-D-glucosaminylphosphatidylinositol (7) and the conserved GPI anchor structure (8) as positive controls. Although PI-PLC cleaved 3 and 4 with about the same efficiency as 7 and 8, this enzyme did not accept 5 or 6. GPI-PLD accepted both the L-chiro- (3 and 4) and the D-chiro- (5 and 6) glycosylinositolphosphoinositides. Therefore, IPGs containing L-chiro-inositol only are expected to be released from chiro-inositol-containing GPIs if the cleavage is effected by a PI-PLC, whereas GPI-PLD cleavage could result in both L-chiro- and D-chiro-inositol-containing IPGs.

Phosphorylation of glycosyl-phosphatidylinositol by phosphatidylinositol 3-kinase changes its properties as a substrate for phospholipases

Phosphatidylinositol 3-kinases (PI3K) phosphorylate the 3-position of the inositol ring of phosphatidylinositol-4,5-bisphosphate to produce phosphatidylinositol-3,4,5-trisphosphate. It is not clear whether PI3K can phosphorylate the inositol group in other biomolecules. We sought to determine whether PI3K was able to use glycosyl-phosphatidylinositol (GPI) as a substrate. This phospholipid may exist either in free form (GPIfree) or forming a lipid anchor (GPIanchor) for the attachment of extracellular proteins to the plasma membrane. We demonstrate the specific PI3K-mediated phosphorylation of the inositol 3-hydroxyl group within both types of GPI by incubating this phospholipid with immunoprecipitated PI3K. The phosphorylated product behaves in HPLC as a derivative of a PI3K lipid product. To our knowledge, this is the first demonstration that PI3K uses lipid substrates other than phosphoinositides. Further, we show that this has potential functional consequences. When GPIfree is phosphorylated, it becomes a poorer substrate for GPI-specific phospholipase D, but a better substrate for phosphatidylinositol-specific phospholipase C. These phosphorylation events may constitute the basis of a previously undescribed signal transduction mechanism.

Tachykinins and tachykinin receptors: a growing family

The peptides of the tachykinin family are widely distributed within the mammalian peripheral and central nervous systems and play a well-recognized role as excitatory neurotransmitters. Currently, the concept that tachykinins act exclusively as neuropeptides is being challenged, since the best known members of the family, substance P, neurokinin A and neurokinin B, are also present in non-neuronal cells and in non-innervated tissues. Moreover, the recently cloned mammalian tachykinins hemokinin-1 and endokinins are primarily expressed in non-neuronal cells, suggesting a widespread distribution and important role for these peptides as intercellular signaling molecules. The biological actions of tachykinins are mediated through three types of receptors denoted NK(1), NK(2) and NK(3) that belong to the family of G protein-coupled receptors. The identification of additional tachykinins has reopened the debate of whether more tachykinin receptors exist. In this review, we summarize the current knowledge of tachykinins and their receptors.

GPI-anchored protein cleavage in the regulation of transmembrane signals

The structure of covalently-linked glycosylphosphatidylinositol (GPI) anchors of membrane proteins displayed on the cell surface is described. Evidence of how the GPI-anchors are sorted into membrane rafts in the plasma membrane is reviewed. Proteins are released by hydrolysis of the linkage to the GPI anchor and phospholipases from different sources involved in this process are characterised. The regulation of protein conformation and function resulting from phospholipase cleavage of the GPI anchor is discussed in the context of its role in signal transduction by insulin. In this signalling system, re-distribution of critical membrane components, including GPI-anchored proteins and non-receptor tyrosine kinases, between different raft domains appears to play a central role in the signal transduction pathway.

Placental GPI-PLD is of maternal origin and its GPI substrate is absent from placentae of pregnancies associated with pre-eclampsia

Pre-eclampsia (PE) is a disorder affecting 5-10% of all pregnancies and is characterised by abnormal trophoblast invasion, maternal endothelial cell dysfunction and a systemic maternal response. A unifying factor responsible for eliciting these effects remains unknown. However, levels of the autocrine insulin mediators, inositolphosphoglycans (IPG), are elevated 3-fold in pre-eclamptic placentae compared with controls and are also elevated 3-fold in maternal urine of pre-eclamptic women, suggesting an abnormal paracrine role of the mediator in the systemic maternal response. At the placental level, IPGs are metabolic second messengers capable of eliciting some of the characteristic features of PE, such as the 10-fold increase in glycogen synthesis and 16-fold increase in the activity of the IPG-dependent enzyme glycogen synthase. IPGs are derived from their lipidic precursors, the glycosylphosphatidylinositols (GPI), in membrane associated caveolae by the action of a GPI-specific phospholipase D whose activity is regulated by its membrane microenvironment. We show that the lipidic GPI precursor was detected in total placental membrane and microvillous membrane from normal placentae. The presence of GPI could not be detected in PE placentae, suggesting that the GPI/IPG signalling system is dysregulated in this disorder. Equivalent amounts of a proteolytically-cleaved 50 kDa GPI-PLD protein is detected in both normal and PE placentae. However, GPI-PLD mRNA is absent, suggesting a mechanism of uptake from maternal serum. Since GPI-PLD, whose presence is required for hydrolysis of GPI and release of free IPG, is detectable with equal activity in both normal and PE placentae, we postulate that dysregulation of the tubular caveolar structure of the microvilli in pre-eclamptic placentae provides an environment which promotes the unregulated hydrolysis of GPI in this disorder.

Stimulation of enveloped virus infection by beta-amyloid fibrils

Alzheimer's disease is characterized by deposition of beta-amyloid peptide (Abeta) into plaques in the brain, leading to neuronal toxicity and dementia. Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system can also cause a dementia, and amyloid deposition in the central nervous system is significantly higher in HIV-1-infected individuals compared with uninfected controls. Here we report that Abeta fibrils stimulated, by 5-20-fold, infection of target cells expressing CD4 and an appropriate coreceptor by multiple HIV-1 isolates but did not permit infection of cells lacking these receptors. Abeta enhanced infection at the stage of virus attachment or entry into the cell. Abeta fibrils also stimulated infection by amphotrophic Moloney leukemia virus, herpes simplex virus, and viruses pseudotyped with the envelope glycoprotein of vesicular stomatitis virus. Other synthetic fibril-forming peptides similarly enhanced viral infection and may be useful in gene delivery applications utilizing retroviral vectors. These data suggest that Abeta deposition may increase the vulnerability of the central nervous system to enveloped viral infection and that amyloidogenic peptides could be useful in enhancing gene transfer by enveloped viral vectors.

Insulin reduces serum glycosylphosphatidylinositol phospholipase D levels in human type I diabetic patients and streptozotocin diabetic rats

The enzyme glycosylphosphatidylinositol phospholipase D has a postulated role in the insulin-mimetic signaling pathway of glycosylphosphatidylinositol compounds. We have investigated enzyme activity in the serum of human type I diabetic patients and plasma and tissues of streptozotocin-induced diabetic rats following insulin administration. In the human diabetic patients serum enzyme activity fell by an average of 10.6% (SEM = 2.7; P = 0.008; n = 20) following administration of insulin. In addition serum enzyme activity appeared to be depleted by 27% (SEM = 8.8; P = 0.011; n = 10) compared to nondiabetic controls. In untreated diabetic rats plasma enzyme activity gradually increased 0.3-fold over a 6-week period (P < 0.001; n = 8), this increase was reversed and activity normalized when these animals were treated with insulin. Cloning of the rat glycosylphosphatidylinositol phospholipase D cDNA enabled confirmation of the liver as the principal organ of synthesis. Analysis of mRNA levels in the livers of the diabetic rats showed that gene expression was reduced in the insulin-treated animals compared to the noninsulin-treated controls by 0.7-fold (P = 0.004; n = 4). Tissue enzyme activity was also reduced in the insulin-treated rats; in skeletal muscle enzyme activity was 0.3-fold lower (P = 0.001; n = 4). Insulin therefore decreases glycosylphosphatidylinositol phospholipase D synthesis in diabetic animals resulting in decreased serum enzyme levels, suggesting a relationship between this enzyme and the function of insulin.

Towards the in vitro reconstitution of caveolae. Asymmetric incorporation of glycosylphosphatidylinositol (GPI) and gangliosides into liposomal membranes

Large unilamellar vesicles consisting of phospholipids with or without cholesterol have been prepared containing GPI and/or gangliosides asymmetrically located in the outer leaflet of the bilayer. Such asymmetric distribution of GPI and gangliosides is found in 'rafts' and caveolae. Using these vesicles, GPI can be readily hydrolysed by phospholipases. Both cholesterol and ganglioside are seen to inhibit, in an additive way, the hydrolytic activity of GPI-specific phospholipase D.