Presence of a light-independent phospholipase A2 in bovine retina but not in rod outer segments

Identification of a lipase-linked cell membrane receptor for pigment epithelium-derived factor

Pigment epithelium-derived factor (PEDF) is an extracellular multifunctional protein belonging to the serpin superfamily with demonstrable neurotrophic, gliastatic, neuronotrophic, antiangiogenic, and antitumorigenic properties. We have previously provided biochemical evidence for high affinity PEDF-binding sites and proteins in plasma membranes of retina, retinoblastoma, and CNS cells. This study was designed to reveal a receptor involved in the biological activities of PEDF. Using a yeast two-hybrid screening, we identified a novel gene from pigment epithelium of the human retina that codes for a PEDF-binding partner, which we term PEDF-R. The derived polypeptide has putative transmembrane, intracellular and extracellular regions, and a phospholipase domain. Recently, PEDF-R (TTS-2.2/independent phospholipase A(2) (PLA(2))zeta and mouse desnutrin/ATGL) has been described in adipose cells as a member of the new calcium-independent PLA(2)/nutrin/patatin-like phospholipase domain-containing 2 (PNPLA2) family that possesses triglyceride lipase and acylglycerol transacylase activities. Here we describe the PEDF-R gene expression in the retina and its heterologous expression by bacterial and eukaryotic systems, and we demonstrate that its protein product has specific and high binding affinity for PEDF, has a potent phospholipase A(2) activity that liberates fatty acids, and is associated with eukaryotic cell membranes. Most importantly, PEDF binding stimulates the enzymatic phospholipase A(2) activity of PEDF-R. In conclusion, we have identified a novel PEDF-R gene in the retina for a phospholipase-linked membrane protein with high affinity for PEDF, suggesting a molecular pathway by which ligand/receptor interaction on the cell surface could generate a cellular signal.

Expression and location of mRNAs encoding multiple forms of secretory phospholipase A2 in the rat retina

Low-molecular-weight secretory phospholipases A(2) (sPLA(2)s) are a subgroup of PLA(2)s, which are secreted, bind to receptors, and may act as intercellular signaling modulators. At least 10 different groups have been characterized in mammals, and there is expanding evidence of the significance of sPLA(2)s in neuronal signaling and survival [Kolko et al. (1996) J. Biol. Chem. 271: 32722-32728]. To date, no retinal sPLA(2)s have been cloned or characterized. We evaluated the existence and abundance of sPLA(2) subtypes in rat retina and explored their possible involvement in light-induced retinal damage. We designed primers to identify the sPLA(2)s in rat retina, based on known sequences of sPLA(2)-specific mRNAs in other tissues. RNA was isolated from rat retina, and cDNA was produced and used for PCR cloning to identify the novel subtypes of sPLA(2). Our study revealed the presence of mRNAs encoding sPLA(2)-IB, -X, -V, -IIE, -IIA, and -IIF in the retina, and quantification by real-time PCR revealed different abundances of the sPLA(2)s. We showed a time-dependent gene induction of sPLA(2)-X, -IB, and -V in light-induced retinal damage. We further explored the location of sPLA(2)-IB by in situ hybridization and immunohistochemistry. This study is the first to reveal the presence, abundance, and induction of mRNAs encoding sPLA(2)s in rat retina. We suggest that these enzymes are themselves intercellular signaling modulators of retinal cell function and perhaps also of retinal degeneration.

Polymorphism of the 1-palmitoyl-2-arachidonoyl-phosphatidyl-ethanolamine/dimyristoyl-phos phatidylmethanol mixture, a phospholipase A2 substrate

The polymorphism of the equimolar mixture of 1-palmitoyl-2-arachidonoyl-phosphatidylethanolamine (PAPE) and dimyristoyl-phosphatidylmethanol (DMPM) was examined by infrared and 31P nuclear magnetic resonance spectroscopy to determine the suitability of this widely used but yet uncharacterized lipid mixture as a phospholipase A2 substrate. The results show that the mixture undergoes a gel-to-liquid crystalline phase transition between 12 and 35 degrees C. The transitions of the individual lipids were also examined. We report that the temperature of the gel-to-liquid crystalline phase transition of DMPM is 40 degrees C. At 2 degrees C, PAPE exists in the fluid lamellar form. This lipid undergoes a lamellar-to-inverted hexagonal phase transition at 31 degrees C. In conclusion, the equimolar PAPE/DMPM mixture forms fluid lamellar phase at the physiological temperature. However, the upper end of the gel-to-liquid crystalline phase transition of the mixture is really close to the physiological temperature and this situation is a serious source of potential artefacts.

Determination of the depth of penetration of the alpha subunit of retinal G protein in membranes: a spectroscopic study

This paper reports the fluorescence quenching of the alpha subunit of retinal rod outer segment G protein (Gtalpha) by vesicles of brominated phospholipids. Two different brominated phospholipids with the bromine quencher groups attached at the 6-7 and 9-10 positions in one of the fatty acyl chains have been used to estimate the depth of penetration of the Gtalpha protein in the lipid vesicles using steady-state fluorescence quenching techniques. Our studies provide evidence of the interaction between Gtalpha protein, in its active conformation, with the lipid vesicles mimicking natural membranes. This study demonstrates that in vitro the distance between fluorescent tryptophan site of Gtalpha and the membrane surface is approximately 6.5 A.

Bovine retinal pigment epithelium contains novel types of phospholipase A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in cells from bovine retinal pigment epithelium (RPE) [Jacob et al. (1996) J. Biol. Chem. 271, 19209-19218]. We report here our results on the characterization of this RPE-PLA2 activity. We show that RPE probably contains two types of PLA2 enzyme, as indicated by the results obtained with different PLA2-active fractions eluted from cation-exchange columns and treated with Ca2+/EGTA, dithiothreitol, p-bromophenacyl bromide or heat. These results, in addition to those from PLA2 assays using different substrates, also suggest that RPE-PLA2 enzymes are different from the well-known secretory, cytoplasmic and Ca2+-independent forms. Sequential extraction of RPE with (1) isotonic, (2) hypertonic and (3) detergent-containing PBS argues for the presence of weakly membrane-associated enzymes. Control experiments using 'back and forth' TLC allowed us to discriminate between PLA2 and phospholipase C/diacylglycerol lipase activity and confirmed that, in our assay conditions, the release of fatty acids was indeed due to PLA2 enzymes. These results, together with those obtained by treating RPE homogenates with H2SO4, guanosine 5'-[gamma-thio]triphosphate, ATP and different protease inhibitors, permitted us to make the first characterization of these RPE-PLA2 enzymes. We conclude that RPE contains novel types of PLA2 that are different from the secretory, cytoplasmic and Ca2+-independent forms.