Expression Of Platelet-activating Factor Acetylhydrolase Research Articles

Platelet-Activating Factor Acetylhydrolase Expression in BRCA1 Mutant Ovarian Cancer as a Protective Factor and Potential Negative Regulator of the Wnt Signaling Pathway.

Aberrantly activated Wnt/β-catenin signaling pathway, as well as platelet-activating factor (PAF), contribute to cancer progression and metastasis of many cancer entities. Nonetheless, the role of the degradation enzyme named platelet-activating factor acetylhydrolase (PLA2G7/PAF-AH) in ovarian cancer etiology is still unclear. This study investigated the functional impact of platelet-activating factor acetylhydrolase on BRCA1 mutant ovarian cancer biology and its crosstalk with the Wnt signaling pathway. PAF-AH, pGSK3β, and β-catenin expressions were analyzed in 156 ovarian cancer specimens by immunohistochemistry. PAF-AH expression was investigated in ovarian cancer tissue, serum of BRCA1-mutated patients, and in vitro in four ovarian cancer cell lines. Functional assays were performed after PLA2G7 silencing. The association of PAF-AH and β-catenin was examined by immunocytochemistry. In an established ovarian carcinoma collective, we identified PAF-AH as an independent positive prognostic factor for overall survival (median 59.9 vs. 27.4 months; p = 0.016). PAF-AH correlated strongly with the Wnt signaling proteins pGSK3β (Y216; nuclear: cc = 0.494, p < 0.001; cytoplasmic: cc = 0.488, p < 0.001) and β-catenin (nuclear: cc = 0.267, p = 0.001; cytoplasmic: cc = 0.291, p < 0.001). In particular, high levels of PAF-AH were found in tumor tissue and in the serum of BRCA1 mutation carriers. By in vitro expression analysis, a relevant gene and protein expression of PLA2G7/PAF-AH was detected exclusively in the BRCA1-negative ovarian cancer cell line UWB1.289 (p < 0.05). Functional assays showed enhanced viability, proliferation, and motility of UWB1.289 cells when PLA2G7/PAF-AH was downregulated, which underlines its protective character. Interestingly, by siRNA knockdown of PLA2G7/PAF-AH, the immunocytochemistry staining pattern of β-catenin changed from a predominantly membranous expression to a nuclear one, suggesting a negative regulatory role of PAF-AH on the Wnt/β-catenin pathway. Our data provide evidence that PAF-AH is a positive prognostic factor with functional impact, which seems particularly relevant in BRCA1 mutant ovarian cancer. For the first time, we show that its protective character may be mediated by a negative regulation of the Wnt/β-catenin pathway. Further studies need to specify this effect. Potential use of PAF-AH as a biomarker for predicting the disease risk of BRCA1 mutation carriers and for the prognosis of patients with BRCA1-negative ovarian cancer should be explored.

Exercise intervention alters HDL subclass distribution and function in obese women

BackgroundObesity is associated with a change in high-density lipoprotein (HDL) function and subclass. Exercise training reduces cardiovascular risk in obese patients. We aimed to explore the effect of an exercise training stimulus on HDL functionality and subclass in obese women.MethodsThirty-two obese black South African women were randomly assigned to exercise (combined aerobic and resistance exercise) or control (no exercise) conditions for 12-weeks. Pre- and post-testing included venous blood sampling for analysis of lipid profile and HDL functionality, by measuring cellular cholesterol efflux capacity, reduction in endothelial vascular cell adhesion molecule (VCAM) expression (anti-inflammatory function), paraoxonase (PON) (antioxidative function) and platelet activating factor acetylhydrolase (PAF-AH) activities (anti-thrombotic function). PON-1 and PAF-AH expression were determined in serum and in isolated HDL using Western blotting. Levels of large, intermediate and small HDL subclasses were measured using the Lipoprint® system.ResultsExercise training resulted in a decrease in body mass index (− 1.0 ± 0.5% vs + 1.2 ± 0.6%, p = 0.010), PON activity (− 8.7 ± 2.4% vs + 1.1 ± 3.0%, p = 0.021), PAF-AH serum expression (− 22.1 ± 8.0% vs + 16.9 ± 9.8, p = 0.002), and the distribution of small HDL subclasses (− 10.1 ± 5.4% vs + 15.7 ± 6.6%, p = 0.004) compared to controls. Exercise did not alter HDL cellular cholesterol efflux capacity and anti-inflammatory function.ConclusionsThese results demonstrate the potential for exercise training to modify HDL subclass distribution and HDL function in obese women.Trial registrationClinical trials number: PACTR201711002789113.

Association between ethnicity and obesity with high-density lipoprotein (HDL) function and subclass distribution.

BackgroundObesity and low high-density lipoprotein-cholesterol (HDL-C) levels are associated with cardiovascular risk. Surprisingly, despite a greater prevalence of obesity and lower HDL concentrations than white women, black South African women are relatively protected against ischaemic heart disease.MethodsWe investigated whether this apparent discrepancy may be related to different HDL function and subclass distribution in black and white, normal-weight and obese South African women (n = 40). HDL functionality was assessed by measuring paraoxonase (PON) activity, platelet activating factor acetylhydrolase (PAF-AH) activity, Oxygen Radical Absorbance Capacity (ORAC) and quantification of the expression of vascular cell adhesion molecule in endothelial cells. PON-1 and PAF-AH expression was determined in isolated HDL and serum using Western blotting. Levels of large, intermediate and small HDL subclasses were measured using the Lipoprint® system.ResultsPON activity was lower in white compared to black women (0.49 ± 0.09 U/L vs 0.78 ± 0.10 U/L, p < 0.05), regardless of PON-1 protein levels. Obese black women had lower PAF-AH activity (9.34 ± 1.15 U/L vs 13.89 ± 1.21 U/L, p <0.05) and HDL-associated PAF-AH expression compared to obese white women. Compared to normal-weight women, obese women had lower large HDL, greater intermediate and small HDL; an effect that was more pronounced in white women than black women. There were no differences in antioxidant capacity or anti-inflammatory function across groups.ConclusionsOur data show that both obesity and ethnicity are associated with differences in HDL functionality, while obesity was associated with decreases in large HDL subclass distribution. Measuring HDL functionality and subclass may, therefore, be important factors to consider when assessing cardiovascular risk.Electronic supplementary materialThe online version of this article (doi:10.1186/s12944-016-0257-9) contains supplementary material, which is available to authorized users.

Diverse Functions of Plasma PAF-AH in Tumorigenesis.

This chapter is focused on the role of the plasma form of platelet-activating factor-acetylhydrolase (PAF-AH), heretofore referred to as PAF-AH, in tumorigenic responses. Biochemical and other properties of this enzyme were discussed in detail in chapter "Plasma PAF-AH (PLA2G7): Biochemical Properties, Association with LDLs and HDLs, and Regulation of Expression" by Stafforini and in other chapters. Although phospholipases tend not to be drivers of tumorigenesis themselves, these enzymes and the lipid mediators whose levels they regulate interact with a variety of oncogenes and tumor suppressors [1]. Like other phospholipases, the functions of PAF-AH in cancer likely are related to its ability to regulate the levels of lipid mediators that participate in cellular processes related to initial tumorigenic events (e.g., proliferation, growth, inflammation) and/or spreading of the disease (e.g., matrix metalloproteinase secretion, actin cytoskeleton reorganization, migration, and angiogenesis) [1]. The importance of substrates and products of PAF-AH on key cellular functions has been evaluated in cell-based analyses which revealed that these metabolites can have pro- and antitumorigenic functions. Studies in genetically engineered mice lacking PAF-AH expression and genetic manipulation of PAF-AH levels in cancer cells demonstrated diverse functions of the protein in models of melanoma, prostate cancer, colon cancer, and others. The following sections highlight lessons learned from studies in cell lines and in mouse models regarding the diversity of functions of PAF-AH in cancer, and the potential of PAFAH transcripts, protein, and/or activity levels to become cancer biomarkers and therapeutic targets.

Biological role of Trichoderma harzianum-derived platelet-activating factor acetylhydrolase (PAF-AH) on stress response and antagonism.

We investigated the properties of platelet-activating factor acetylhydrolase (PAF-AH) derived from Trichoderma harzianum. The enzyme, comprised of 572 amino acids, shares high homology with PAF-AH proteins from T. koningii and other microbial species. The optimum enzymatic activity of PAF-AH occurred at pH 6 in the absence of Ca2+ and it localized in the cytoplasm, and we observed the upregulation of PAF-AH expression in response to carbon starvation and strong heat shock. Furthermore, PAF-AH knockout transformant growth occurred more slowly than wild type cells and over-expression strains grown in SM medium at 37°C and 42°C. In addition, PAF-AH expression significantly increased under a series of maize root induction assay. Eicosanoic acid and ergosterol levels decreased in the PAF-AH knockouts compared to wild type cells, as revealed by GC/MS analysis. We also determined stress responses mediated by PAF-AH were related to proteins HEX1, Cu/Zn superoxide dismutase, and cytochrome c. Finally, PAF-AH exhibited antagonistic activity against Rhizoctonia solani in plate confrontation assays. Our results indicate PAF-AH may play an important role in T. harzianum stress response and antagonism under diverse environmental conditions.

Lipopolysaccharide and platelet-activating factor stimulate expression of platelet-activating factor acetylhydrolase via distinct signaling pathways.

This study was designed to investigate and characterize the ability of platelet-activating factor (PAF) to induce the expression of platelet-activating factor acetylhydrolase (PAF-AH). Ribonuclease protection assays and quantitative real-time PCR were used to investigate the ability of lipopolysaccharide (LPS) and PAF to regulate PAF-AH mRNA expression in human monocyte-macrophage 6 (MM6) cells. Pharmacological inhibitors of mitogen activated protein kinases (MAPK) and PAF receptor antagonists were used to investigate the mechanism of regulation of PAF-AH. PAF-AH mRNA levels were increased upon exposure to LPS or PAF in a dose-dependent manner. LPS elicited a more potent and rapid increase in PAF-AH expression than the PAF-stimulated response. However, when administered concomitantly, PAF augmented the LPS-stimulated response. LPS-stimulated PAF-AH expression was susceptible to partial inhibition by a p38 MAPK inhibitor and PAF receptor antagonists. PAF-induced up-regulation of PAF-AH levels was solely mediated via the PAF receptor and was p38 MAPK-independent. The proinflammatory mediators, LPS and PAF, increased levels of PAF-AH mRNA via distinct signaling pathways.

Novel mechanism for regulation of plasma platelet-activating factor acetylhydrolase expression in mammalian cells

The plasma form of PAF-AH [PAF (platelet-activating factor) acetylhydrolase; also known as LpPLA(2) (lipopoprotein-associated phospholipase A(2)), PLA(2)G7] catalyses the release of sn-2 fatty acyl residues from PAF, oxidatively fragmented phospholipids, and esterified isoprostanes. The plasma levels of this enzyme vary widely among mammalian species, including mice and humans, but the mechanisms that account for these differences are largely unknown. We investigated the basis for these variations using molecular and biochemical approaches. We identified an N-terminal domain that played key roles in the determination of steady-state expression levels. The mouse N-terminal domain robustly enhanced protein expression levels, possibly owing to its ability to adopt a globular conformation that is absent in the human protein. We investigated the mechanism(s) whereby the N-terminal stretch modulated PAF-AH levels and found that differential expression was not due to variations in the efficiency of transcription, translation, or mRNA stability. Studies designed to evaluate the ability of precursor forms of PAF-AH to mature to fully active proteins indicated that the N-terminal end of human and mouse PAF-AH played important and opposite roles in this process. These domains also modulated the levels of expression of an unrelated polypeptide by affecting the stability of precursor forms of the protein. These studies provide insights that contribute to our understanding of the molecular features and mechanisms that contribute to differential expression of plasma PAF-AH in mammals.

Differential expression of platelet-activating factor acetylhydrolase in lung macrophages

Platelet-activating factor (PAF) acetylhydrolase plays a crucial role inactivating the potent inflammatory mediator, PAF. PAF is implicated in the initiation and propagation of acute lung injury. Although PAF acetylhydrolase is a constitutively active plasma protein, increased PAF production during inflammatory events may necessitate an increase in PAF acetylhydrolase activity in the local environment. A series of experiments were conducted to determine whether the systemic administration of LPS to Sprague-Dawley rats resulted in enhanced expression of PAF acetylhydrolase in lung tissue. Ribonuclease protection assays revealed a dramatic increase in PAF acetylhydrolase mRNA, which peaked at 24 h following in vivo LPS administration. The increase in PAF acetylhydrolase mRNA was dose dependent and was detected when as little as 10 microg/kg of LPS was administered. Western blot analyses of lung tissue homogenates confirmed an increased production of PAF acetylhydrolase protein in response to LPS. In addition, Western blot analyses revealed the rat PAF acetylhydrolase protein exhibited heterogeneous molecular weights with predominant species migrating at 63 and 67 kDa. Some of the molecular weight heterogeneity likely resulted from extensive glycosylation of the secreted protein. Immunohistochemical analyses of lung tissue sections and colocalization experiments revealed a heterogenous population of cells that express the plasma-type PAF acetylhydrolase. Lung interstitial macrophages were PAF acetylhydrolase positive, but surprisingly, alveolar macrophages did not increase expression of PAF acetylhydrolase in response to systemic LPS administration. In addition, rat granulocytes consisting primarily of neutrophils were strongly positive for PAF acetylhydrolase in the LPS-exposed lung tissue. The absence of immunoreactive PAF acetylhydrolase in alveolar macrophages obtained from bronchial alveolar lavage confirmed that systemic LPS administration resulted in enhanced PAF acetylhydrolase expression only in a subset of lung macrophages.

Platelet-activating factor acetylhydrolase and transacetylase activities in human aorta and mammary artery

Platelet-activating factor (PAF), the potent phospholipid mediator of inflammation, is involved in atherosclerosis. Platelet-activating factor-acetylhydrolase (PAF-AH), the enzyme that inactivates PAF bioactivity, possesses both acetylhydrolase and transacetylase activities. In the present study, we measured acetylhydrolase and transacetylase activities in human atherogenic aorta and nonatherogenic mammary arteries. Immunohistochemistry analysis showed PAF-AH expression in the intima and the media of the aorta and in the media of mammary arteries. Acetylhydrolase and transacetylase activities were (mean ± SE, n = 38): acetylhydrolase of aorta, 2.8 ± 0.5 pmol/min/mg of tissue; transacetylase of aorta, 3.3 ± 0.7 pmol/min/mg of tissue; acetylhydrolase of mammary artery, 1.4 ± 0.3 pmol/min/mg of tissue (P < 0.004 as compared with acetylhydrolase of aorta); transacetylase of mammary artery, 0.8 ± 0.2 pmol/min/mg of tissue (P < 0.03 as compared with acetylhydrolase of mammary artery). Lyso-PAF accumulation and an increase in PAF bioactivity were observed in the aorta of some patients. Reverse-phase HPLC and electrospray ionization mass spectrometry analysis revealed that 1-O-hexadecyl-2 acetyl-sn glycero-3-phosphocholine accounted for 60% of the PAF bioactivity and 1-O-hexadecyl-2-butanoyl-sn-glycerol-3-phosphocholine for 40% of the PAF bioactivity. The nonatherogenic properties of mammary arteries may in part be due to low PAF formation regulated by PAF-AH activity. In atherogenic aortas, an imbalance between PAF-AH and transacetylase activity, as well as lyso-PAF accumulation, may lead to unregulated PAF formation and to progression of atherosclerosis.

Release of Free F2-isoprostanes from Esterified Phospholipids Is Catalyzed by Intracellular and Plasma Platelet-activating Factor Acetylhydrolases

F2-isoprostanes are produced in vivo by nonenzymatic peroxidation of arachidonic acid esterified in phospholipids. Increased urinary and plasma F2-isoprostane levels are associated with a number of human diseases. These metabolites are regarded as excellent markers of oxidant stress in vivo. Isoprostanes are initially generated in situ, i.e. when the arachidonate precursor is esterified in phospholipids, and they are subsequently released in free form. Although the mechanism(s) responsible for the release of free isoprostanes after in situ generation in membrane phospholipids is, for the most part, unknown, this process is likely mediated by phospholipase A2 activity(ies). Here we reported that human plasma contains an enzymatic activity that catalyzes this reaction. The activity associates with high density and low density lipoprotein and comigrates with platelet-activating factor (PAF) acetylhydrolase on KBr density gradients. Plasma samples from subjects deficient in PAF acetylhydrolase do not release F2-isoprostanes from esterified precursors. The intracellular PAF acetylhydrolase II, which shares homology to the plasma enzyme, also catalyzes this reaction. We found that both the intracellular and plasma PAF acetylhydrolases have high affinity for esterified F2-isoprostanes. However, the rate of esterified F2-isoprostane hydrolysis is much slower compared with the rate of hydrolysis of other substrates utilized by these enzymes. Studies using PAF acetylhydrolase transgenic mice indicated that these animals have a higher capacity to release F2-isoprostanes compared with nontransgenic littermates. Our results suggested that PAF acetylhydrolases play key roles in the hydrolysis of F2-isoprostanes esterified on phospholipids in vivo.

An improved assay for platelet-activating factor using HPLC-tandem mass spectrometry

We describe an improved assay for platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) using HPLC-tandem mass spectrometry (LC-MS/MS). The present method can readily detect as little as 1 pg (1.9 fmol) of PAF, a significant improvement over previously described LC-MS/MS methods, and gives a linear response up to 1,000 pg of PAF. Our method also overcomes the artifacts from isobaric lipids that have limited the usefulness of certain existing LC-MS/MS assays for PAF. In the course of these studies, we detected three novel lipid species in human neutrophils. One of the novel lipids appears to be a new molecular species of PAF, and the other two have chromatographic and mass spectrometric properties consistent with stearoyl-formyl-glycerophosphocholine and oleoyl-formyl-glycerophosphocholine. These observations identify previously unknown potential interferences in the measurement of PAF by LC-MS/MS. Moreover, our data suggest that the previously described palmitoyl-formyl-glycerophosphocholine is not unique but rather is a member of a new and poorly understood family of formylated lipids.

PAF receptor and PAF acetylhydrolase expression in the endosalpinx of the human Fallopian tube: possible role of embryo-derived PAF in the control of embryo transport to the uterus.

Prostaglandin-E(2) and platelet-activating factor (PAF) are embryonic-derived signals that time embryo passage into the uterus in the mare and hamster respectively. PAF-like activity is detectable in the spent media of preimplantation human embryos and it has been suggested that PAF may be the embryonic signal that controls embryo transport to the uterus in our species. The actions of PAF are regulated at the level of its synthesis and degradation as well as the expression of a specific cell surface receptor (PAFr). The enzyme PAF acetylhydrolase (PAF-AH) degrades PAF. This study was undertaken to examine whether or not PAFr and PAF-AH are expressed in the human Fallopian tube and to identify the cell types in which they are expressed. The presence of PAFr mRNA in tissue extracts was investigated using reverse transcription-polymerase chain reaction. We amplified the predicted amplicon for PAFr mRNA from RNA samples extracted from Fallopian tubes. The expression of PAF-AH was detected by Western blot and the localization of PAFr and PAF-AH proteins was detected by immunohistochemistry. Utilizing antibodies against PAFr and PAF-AH, co-localization of the two proteins in the epithelium and stromal cells were demonstrated. These observations show that the human Fallopian tube expresses PAFr and PAF-AH at a location compatible with the proposed paracrine role of early embryo-derived PAF.

Role of platelet-activating factor in functional alterations induced by xenoreactive antibodies in porcine endothelial cells.

Platelet-activating factor (PAF) is a phospholipid mediator of inflammation which has been implicated in rejection. The interaction of anti-alpha-galactosyl natural antibodies (anti-alpha gal Abs) with endothelial cells is the initial step for the development of xenograft rejection. In our study, we stimulated porcine aortic endothelial cells (PAEC) with anti-alpha gal IgG to investigate the synthesis of PAF from PAEC and its biological consequences. PAF was extracted and chromatographically purified from cultured PAEC stimulated with baboon anti-alpha gal Abs. The Abs induced a dose-dependent synthesis of PAF peaking after 30 min of incubation, and decreasing thereafter. Concomitant cell shape change, motility, and cytoskeleton redistribution were observed. These events were prevented by addition of a panel of PAF-receptor antagonists. An SV40 T-large antigen-immortalized PAEC line was engineered to express PAF acetyl-hydrolase (PAF-AH) cDNA, the major PAF-inactivating enzyme. These transfected cells exposed to anti-alpha gal Abs showed reduced cell contraction and motility compared with empty vector-transfected cells. Moreover, in PAEC stimulated with anti-alpha gal Abs, the synthesis of PAF promoted the adhesion of a monocytic cell line as shown by the inhibitory effect of PAF-receptor antagonists and of PAF-AH expression. Finally, studies on cell monolayer demonstrated an enhanced permeability 48 hr after exposure to anti-alpha gal Abs, and this increase was prevented by PAF-inactivation and by PAF-receptor blockade. These results demonstrate that on stimulation with anti-alpha gal Abs, PAEC synthetize PAF which can contribute to several vascular events involved in xenograft rejection.

The Expression and Localization of Plasma Platelet-activating Factor Acetylhydrolase in Endotoxemic Rats

The production of platelet-activating factor (PAF) and PAF-like phospholipids that also bind the PAF receptor are implicated in numerous pathological situations including bacterial endotoxemia and injury-induced oxidative damage. PAF and PAF-like phospholipids are hydrolyzed and inactivated by the enzyme PAF acetylhydrolase. In the intact rat, infusion of lipopolysaccharide (LPS) into a mesenteric vein served as an acute, liver-focused model of endotoxemia. We determined that the liver responds to LPS exposure with the production of plasma-type PAF acetylhydrolase mRNA and protein expression specifically in the resident macrophages of the liver. Liver macrophages, defined immunohistochemically using antibodies against ED1, present in livers from saline-treated animals contained no detectable PAF acetylhydrolase. Twenty-four hours following in vivo LPS administration, immunohistochemistry detected a slight increase in the number of ED1 staining cells and the ED1-positive cells now contained an abundance of PAF acetylhydrolase. The systemic administration of LPS resulted in increased expression of PAF acetylhydrolase in several tissues. Of the tissues examined, the greatest increase in PAF acetylhydrolase expression was observed in lung followed by increases in spleen, liver, kidney, and thymus. Additionally, the expression of PAF acetylhydrolase mRNA increased in circulating leukocytes and in peritoneal macrophages in response to systemic exposure to LPS. We examined the regulation of PAF acetylhydrolase expression and demonstrated the administration of the PAF receptor antagonists, BN 50739 and WEB 2170, inhibited by 50% the increase in PAF acetylhydrolase expression in response to LPS. The up-regulation of the plasma-type PAF acetylhydrolase expression constitutes an important mechanism for elevating the local and systemic ability to inactivate PAF and oxidized phospholipids in order to minimize PAF-mediated pathophysiology consequent from exposure to endotoxin. The abundance of PAF acetylhydrolase production in the liver lobule likely limits endotoxin-mediated tissue damage due to PAF synthesis.

Activated Mast Cells Release Extracellular Type Platelet-activating Factor Acetylhydrolase That Contributes to Autocrine Inactivation of Platelet-activating Factor

IgE-dependent and -independent activation of mouse bone marrow-derived mast cells (BMMC) elicited rapid and transient production of platelet-activating factor (PAF), which reached a maximal level by 2-5 min and was then degraded rapidly, returning to base-line levels by 10-20 min. Inactivation of PAF was preceded by the release of PAF acetylhydrolase (PAF-AH) activity, which reached a plateau by 3-5 min and paralleled the release of beta-hexosaminidase, a marker of mast cell exocytosis. Immunochemical and molecular biological studies revealed that the PAF-AH released from activated mast cells was identical to the plasma-type isoform. In support of the autocrine action of exocytosed PAF-AH, adding exogenous recombinant plasma-type PAF-AH markedly reduced PAF accumulation in activated BMMC. Furthermore, culture of BMMC with a combination of c-kit ligand, interleukin-1beta and interleukin-10 for > 24 h led to an increase in plasma-type PAF-AH expression, accompanied by a reduction in stimulus-initiated PAF production. Collectively, these results suggest that plasma-type PAF-AH released from activated mast cells sequesters proinflammatory PAF produced by these cells, thereby revealing an intriguing anti-inflammatory aspect of mast cells.

Platelet-Activating Factor Acetylhydrolase Expression and Activity Suggest a Link between Neuronal Migration and Platelet-Activating Factor

A hemizygous deletion ofLIS1,the gene encoding αLis1protein, causes Miller–Dieker syndrome (MDS). MDS is a developmental disorder characterized by neuronal migration defects resulting in a disorganization of the cerebral and cerebellar cortices. αLis1binds to two other proteins (β and γ) to form a heterotrimeric cytosolic enzyme which hydrolyzes platelet-activating factor (PAF). The existence of heterotrimers is implicated from copurification and crosslinking studies carried outin vitro.To determine whether such a heterotrimeric complex could be present in tissues, we have investigated whether the αLis1, β, and γ genes are coexpressed in the developing and adult brain. We have isolated murine cDNAs and show byin situhybridization that in developing brain tissues αLis1, β, and γ genes are coexpressed. This suggests that αLis1, β, and γ gene products form heterotrimers in developing neuronal tissues. In the adult brain, αLis1and β mRNAs continue to be coexpressed at high levels while γ gene expression is greatly diminished. This reduction in γ transcript levels is likely to result in a decline of the cellular concentration of αLis1, β, and γ heterotrimers. The developmental expression pattern of αLis1, β, and γ genes is consistent with the neuronal migration defects seen in MDS; regions containing migrating neurons such as the developing cerebral and cerebellar cortices express these genes at a particularly high level. Furthermore, we uncovered a correlation between γ gene expression, granule cell migration, and PAF hydrolytic activity in the cerebellum. In this tissue γ gene expression and PAF hydrolysis peaked at Postnatal Days P5 and P15, a period during which neuronal migration in the cerebellum is most extensive. Mechanisms by which PAF could affect neuronal migration are discussed.