Acute sleep loss decreases CSF-to-blood clearance of Alzheimer's disease biomarkers

INTRODUCTION

Sleep deprivation increases cerebrospinal fluid (CSF) amyloid beta (Aβ) and tau levels; however, sleep's effect on Aβ and tau in plasma is unknown.

METHODS

In a cross-over design, CSF Aβ and tau concentrations were measured in five cognitively normal individuals who had blood and CSF collected every 2 hours for 36 hours during sleep-deprived and normal sleep control conditions.

RESULTS

Aβ40, Aβ42, unphosphorylated tau threonine181 (T181), unphosphorylated tau threonine-217 (T217), and phosphorylated T181 (pT181) concentrations increased ∼35% to 55% in CSF and decreased ∼5% to 15% in plasma during sleep deprivation. CSF/plasma ratios of all Alzheimer's disease (AD) biomarkers increased during sleep deprivation while the CSF/plasma albumin ratio, a measure of blood-CSF barrier permeability, decreased. CSF and plasma Aβ42/40, pT181/T181, and pT181/Aβ42 ratios were stable longitudinally in both groups.

DISCUSSION

These findings show that sleep loss alters some plasma AD biomarkers by lowering brain clearance mechanisms and needs to be taken into account when interpreting individual plasma AD biomarkers but not ratios.

Apolipoprotein E O-glycosylation is associated with amyloid plaques and APOE genotype

Although the APOE ε4 allele is the strongest genetic risk factor for sporadic Alzheimer's disease (AD), the relationship between apolipoprotein (apoE) and AD pathophysiology is not yet fully understood. Relatively little is known about the apoE protein species, including post-translational modifications, that exist in the human periphery and CNS. To better understand these apoE species, we developed a LC-MS/MS assay that simultaneously quantifies both unmodified and O-glycosylated apoE peptides. The study cohort included 47 older individuals (age 75.6 ± 5.7 years [mean ± standard deviation]), including 23 individuals (49%) with cognitive impairment. Paired plasma and cerebrospinal fluid samples underwent analysis. We quantified O-glycosylation of two apoE protein residues - one in the hinge region and one in the C-terminal region - and found that glycosylation occupancy of the hinge region in the plasma was significantly correlated with plasma total apoE levels, APOE genotype and amyloid status as determined by CSF Aβ42/Aβ40. A model with plasma glycosylation occupancy, plasma total apoE concentration, and APOE genotype distinguished amyloid status with an AUROC of 0.89. These results suggest that plasma apoE glycosylation levels could be a marker of brain amyloidosis, and that apoE glycosylation may play a role in the pathophysiology of AD.

Suvorexant acutely decreases tau phosphorylation and Aβ in the human CNS

OBJECTIVE

In Alzheimer's disease, hyperphosphorylated tau is associated with formation of insoluble paired helical filaments that aggregate as neurofibrillary tau tangles and are associated with neuronal loss and cognitive symptoms. Dual orexin receptor antagonists decrease soluble amyloid-β levels and amyloid plaques in mouse models over-expressing amyloid-β, but have not been reported to affect tau phosphorylation. In this randomized controlled trial, we tested the acute effect of suvorexant, a dual orexin receptor antagonist, on amyloid-β, tau, and phospho-tau.

METHODS

Thirty-eight cognitively unimpaired participants aged 45-65 years were randomized to placebo (N = 13), suvorexant 10 mg (N = 13), and suvorexant 20 mg (N = 12). Six milliliters of cerebrospinal fluid was collected via an indwelling lumbar catheter every 2 hours for 36 hours starting at 20:00. Participants received placebo or suvorexant at 21:00. All samples were processed and measured for multiple forms of amyloid-β, tau, and phospho-tau via immunoprecipitation and liquid chromatography-mass spectrometry.

RESULTS

The ratio of phosphorylated-tau-threonine-181 to unphosphorylated-tau-threonine-181, a measure of phosphorylation at this tau phosphosite, decreased ~10-15% in participants treated with suvorexant 20 mg compared to placebo. However, phosphorylation at tau-serine-202 and tau-threonine-217 were not decreased by suvorexant. Suvorexant decreased amyloid-β ~10-20% compared to placebo starting 5 hours after drug administration.

INTERPRETATION

In this study, suvorexant acutely decreased tau phosphorylation and amyloid-β concentrations in the central nervous system. Suvorexant is approved by the Food and Drug Administration to treatment insomnia and may have potential as a repurposed drug for the prevention of Alzheimer's disease, however future studies with chronic treatment are needed. This article is protected by copyright. All rights reserved.

Predicting continuous amyloid PET values with CSF and plasma Aβ42/Aβ40

Introduction

Continuous measures of amyloid burden as measured by positron emission tomography (PET) are being used increasingly to stage Alzheimer's disease (AD). This study examined whether cerebrospinal fluid (CSF) and plasma amyloid beta (Aβ)42/Aβ40 could predict continuous values for amyloid PET.

Methods

CSF Aβ42 and Aβ40 were measured with automated immunoassays. Plasma Aβ42 and Aβ40 were measured with an immunoprecipitation-mass spectrometry assay. Amyloid PET was performed with Pittsburgh compound B (PiB). The continuous relationships of CSF and plasma Aβ42/Aβ40 with amyloid PET burden were modeled.

Results

Most participants were cognitively normal (427 of 491 [87%]) and the mean age was 69.0 ± 8.8 years. CSF Aβ42/Aβ40 predicted amyloid PET burden until a relatively high level of amyloid accumulation (69.8 Centiloids), whereas plasma Aβ42/Aβ40 predicted amyloid PET burden until a lower level (33.4 Centiloids).

Discussion

CSF Aβ42/Aβ40 predicts the continuous level of amyloid plaque burden over a wider range than plasma Aβ42/Aβ40 and may be useful in AD staging.

Highlights

Cerebrospinal fluid (CSF) amyloid beta (Aβ)42/Aβ40 predicts continuous amyloid positron emission tomography (PET) values up to a relatively high burden.Plasma Aβ42/Aβ40 is a comparatively dichotomous measure of brain amyloidosis.Models can predict regional amyloid PET burden based on CSF Aβ42/Aβ40.CSF Aβ42/Aβ40 may be useful in staging AD.

The performance of plasma amyloid beta measurements in identifying amyloid plaques in Alzheimer's disease: a literature review

The extracellular buildup of amyloid beta (Aβ) plaques in the brain is a hallmark of Alzheimer's disease (AD). Detection of Aβ pathology is essential for AD diagnosis and for identifying and recruiting research participants for clinical trials evaluating disease-modifying therapies. Currently, AD diagnoses are usually made by clinical assessments, although detection of AD pathology with positron emission tomography (PET) scans or cerebrospinal fluid (CSF) analysis can be used by specialty clinics. These measures of Aβ aggregation, e.g. plaques, protofibrils, and oligomers, are medically invasive and often only available at specialized medical centers or not covered by medical insurance, and PET scans are costly. Therefore, a major goal in recent years has been to identify blood-based biomarkers that can accurately detect AD pathology with cost-effective, minimally invasive procedures.To assess the performance of plasma Aβ assays in predicting amyloid burden in the central nervous system (CNS), this review compares twenty-one different manuscripts that used measurements of 42 and 40 amino acid-long Aβ (Aβ42 and Aβ40) in plasma to predict CNS amyloid status. Methodologies that quantitate Aβ42 and 40 peptides in blood via immunoassay or immunoprecipitation-mass spectrometry (IP-MS) were considered, and their ability to distinguish participants with amyloidosis compared to amyloid PET and CSF Aβ measures as reference standards was evaluated. Recent studies indicate that some IP-MS assays perform well in accurately and precisely measuring Aβ and detecting brain amyloid aggregates.

Comparative analytical performance of multiple plasma Aβ42 and Aβ40 assays and their ability to predict positron emission tomography amyloid positivity

INTRODUCTION

This report details the approach taken to providing a dataset allowing for analyses on the performance of recently developed assays of amyloid beta (Aβ) peptides in plasma and the extent to which they improve the prediction of amyloid positivity.

METHODS

Alzheimer's Disease Neuroimaging Initiative plasma samples with corresponding amyloid positron emission tomography (PET) data were run on six plasma Aβ assays. Statistical tests were performed to determine whether the plasma Aβ measures significantly improved the area under the receiver operating characteristic curve for predicting amyloid PET status compared to age and apolipoprotein E (APOE) genotype.

RESULTS

The age and APOE genotype model predicted amyloid status with an area under the curve (AUC) of 0.75. Three assays improved AUCs to 0.81, 0.81, and 0.84 (P < .05, uncorrected for multiple comparisons).

DISCUSSION

Measurement of Aβ in plasma contributes to addressing the amyloid component of the ATN (amyloid/tau/neurodegeneration) framework and could be a first step before or in place of a PET or cerebrospinal fluid screening study.

HIGHLIGHTS

The Foundation of the National Institutes of Health Biomarkers Consortium evaluated six plasma amyloid beta (Aβ) assays using Alzheimer's Disease Neuroimaging Initiative samples. Three assays improved prediction of amyloid status over age and apolipoprotein E (APOE) genotype. Plasma Aβ42/40 predicted amyloid positron emission tomography status better than Aβ42 or Aβ40 alone.

Validation of Plasma Amyloid-β 42/40 for Detecting Alzheimer Disease Amyloid Plaques

BACKGROUND AND OBJECTIVES

To determine the diagnostic accuracy of a plasma Aβ42/Aβ40 assay in classifying amyloid PET status across global research studies using samples collected by multiple centers that utilize different blood collection and processing protocols.

METHODS

Plasma samples (n = 465) were obtained from 3 large Alzheimer disease (AD) research cohorts in the United States (n = 182), Australia (n = 183), and Sweden (n = 100). Plasma Aβ42/Aβ40 was measured by a high precision immunoprecipitation mass spectrometry (IPMS) assay and compared to the reference standards of amyloid PET and CSF Aβ42/Aβ40.

RESULTS

In the combined cohort of 465 participants, plasma Aβ42/Aβ40 had good concordance with amyloid PET status (receiver operating characteristic area under the curve [AUC] 0.84, 95% confidence interval [CI] 0.80-0.87); concordance improved with the inclusion of APOE ε4 carrier status (AUC 0.88, 95% CI 0.85-0.91). The AUC of plasma Aβ42/Aβ40 with CSF amyloid status was 0.85 (95% CI 0.78-0.91) and improved to 0.93 (95% CI 0.89-0.97) with APOE ε4 status. These findings were consistent across the 3 cohorts, despite differences in protocols. The assay performed similarly in both cognitively unimpaired and impaired individuals.

DISCUSSION

Plasma Aβ42/Aβ40 is a robust measure for detecting amyloid plaques and can be utilized to aid in the diagnosis of AD, identify those at risk for future dementia due to AD, and improve the diversity of populations enrolled in AD research and clinical trials.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that plasma Aβ42/Aβ40, as measured by a high precision IPMS assay, accurately diagnoses brain amyloidosis in both cognitively unimpaired and impaired research participants.

The Skyline ecosystem: Informatics for quantitative mass spectrometry proteomics

Skyline is a freely available, open-source Windows client application for accelerating targeted proteomics experimentation, with an emphasis on the proteomics and mass spectrometry community as users and as contributors. This review covers the informatics encompassed by the Skyline ecosystem, from computationally assisted targeted mass spectrometry method development, to raw acquisition file data processing, and quantitative analysis and results sharing.

High-precision plasma β-amyloid 42/40 predicts current and future brain amyloidosis

OBJECTIVE

We examined whether plasma β-amyloid (Aβ)42/Aβ40, as measured by a high-precision assay, accurately diagnosed brain amyloidosis using amyloid PET or CSF p-tau181/Aβ42 as reference standards.

METHODS

Using an immunoprecipitation and liquid chromatography-mass spectrometry assay, we measured Aβ42/Aβ40 in plasma and CSF samples from 158 mostly cognitively normal individuals that were collected within 18 months of an amyloid PET scan.

RESULTS

Plasma Aβ42/Aβ40 had a high correspondence with amyloid PET status (receiver operating characteristic area under the curve [AUC] 0.88, 95% confidence interval [CI] 0.82-0.93) and CSF p-tau181/Aβ42 (AUC 0.85, 95% CI 0.79-0.92). The combination of plasma Aβ42/Aβ40, age, and APOE ε4 status had a very high correspondence with amyloid PET (AUC 0.94, 95% CI 0.90-0.97). Individuals with a negative amyloid PET scan at baseline and a positive plasma Aβ42/Aβ40 (<0.1218) had a 15-fold greater risk of conversion to amyloid PET-positive compared to individuals with a negative plasma Aβ42/Aβ40 (p = 0.01).

CONCLUSIONS

Plasma Aβ42/Aβ40, especially when combined with age and APOE ε4 status, accurately diagnoses brain amyloidosis and can be used to screen cognitively normal individuals for brain amyloidosis. Individuals with a negative amyloid PET scan and positive plasma Aβ42/Aβ40 are at increased risk for converting to amyloid PET-positive. Plasma Aβ42/Aβ40 could be used in prevention trials to screen for individuals likely to be amyloid PET-positive and at risk for Alzheimer disease dementia.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that plasma Aβ42/Aβ40 levels accurately determine amyloid PET status in cognitively normal research participants.

Protein production is an early biomarker for RNA-targeted therapies

Objectives

Clinical trials for progressive neurodegenerative disorders such as Alzheimer's Disease and Amyotrophic Lateral Sclerosis have been hindered due to the absence of effective pharmacodynamics markers to assay target engagement. We tested whether measurements of new protein production would be a viable pharmacodynamics tool for RNA-targeted therapies.

Methods

Transgenic animal models expressing human proteins implicated in neurodegenerative disorders - microtubule-associated protein tau (hTau) or superoxide dismutase-1 (hSOD1) - were treated with antisense oligonucleotides (ASOs) delivered to the central nervous system to target these human mRNA transcripts. Simultaneously, animals were administered ¹³C₆-leucine via drinking water to measure new protein synthesis after ASO treatment. Measures of new protein synthesis and protein concentration were assayed at designated time points after ASO treatment using targeted proteomics.

Results

ASO treatment lowered hTau mRNA and protein production (measured by ¹³C₆-leucine-labeled hTau protein) earlier than total hTau protein concentration in transgenic mouse cortex. In the CSF of hSOD1 transgenic rats, ASO treatment lowered newly generated hSOD1 protein driven by decreases in newly synthesized hSOD1 protein, not overall protein concentration, 30 days after treatment. At later time points, decreases in newly generated protein were still observed after mRNA lowering reached a steady state after ASO treatment.

Interpretation

Measures of newly generated protein show earlier pharmacodynamics changes for RNA-lowering therapeutics compared with total protein concentration. Early in ASO treatment, decreases in newly generated protein are driven by changes in newly synthesized protein. Measuring new protein production in CSF may be a promising early pharmacodynamics marker for RNA-targeted therapeutics.

PECAN: library-free peptide detection for data-independent acquisition tandem mass spectrometry data

Data-independent acquisition (DIA) is an emerging mass spectrometry (MS)-based technique for unbiased and reproducible measurement of protein mixtures. DIA tandem mass spectrometry spectra are often highly multiplexed, containing product ions from multiple cofragmenting precursors. Detecting peptides directly from DIA data is therefore challenging; most DIA data analyses require spectral libraries. Here we present PECAN (http://pecan.maccosslab.org), a library-free, peptide-centric tool that robustly and accurately detects peptides directly from DIA data. PECAN reports evidence of detection based on product ion scoring, which enables detection of low-abundance analytes with poor precursor ion signal. We demonstrate the chromatographic peak picking accuracy and peptide detection capability of PECAN, and we further validate its detection with data-dependent acquisition and targeted analyses. Lastly, we used PECAN to build a plasma proteome library from DIA data and to query known sequence variants.

Identification of Epithelial Phospholipase A2 Receptor 1 as a Potential Target in Asthma

Secreted phospholipase A₂s (sPLA₂s) regulate eicosanoid formation and have been implicated in asthma. Although sPLA₂s function as enzymes, some of the sPLA₂s bind with high affinity to a C-type lectin receptor, called PLA2R1, which has functions in both cellular signaling and clearance of sPLA₂s. We sought to examine the expression of PLA2R1 in the airway epithelium of human subjects with asthma and the function of the murine Pla2r1 gene in a model of asthma. Expression of PLA2R1 in epithelial brushings was assessed in two distinct cohorts of children with asthma by microarray and quantitative PCR, and immunostaining for PLA2R1 was conducted on endobronchial tissue and epithelial brushings from adults with asthma. C57BL/129 mice deficient in Pla2r1 (Pla2r1^-/-) were characterized in an ovalbumin (OVA) model of allergic asthma. PLA2R1 was differentially overexpressed in epithelial brushings of children with atopic asthma in both cohorts. Immunostaining for PLA2R1 in endobronchial tissue localized to submucosal glandular epithelium and columnar epithelial cells. After OVA sensitization and challenge, Pla2r1^-/- mice had increased airway hyperresponsiveness, as well as an increase in cellular trafficking of eosinophils to the peribronchial space and bronchoalveolar lavage fluid, and an increase in airway permeability. In addition, Pla2r1^-/- mice had more dendritic cells in the lung, higher levels of OVA-specific IgG, and increased production of both type-1 and type-2 cytokines by lung leukocytes. PLA2R1 is increased in the airway epithelium in asthma, and serves as a regulator of airway hyperresponsiveness, airway permeability, antigen sensitization, and airway inflammation.

Quantification by nano liquid chromatography parallel reaction monitoring mass spectrometry of human apolipoprotein A-I, apolipoprotein B, and hemoglobin A1c in dried blood spots

PURPOSE

Proteomic analysis of blood proteins in dried blood spots (DBS) is gaining attention as a possible replacement for measurements in plasma/serum collected by venipuncture. We aimed to develop and provisionally validate a nanoflow LC-PRM-MS method for clinical use.

EXPERIMENTAL DESIGN

We used Skyline to develop a nanoflow LC-PRM-MS method to quantify glycated hemoglobin-β, apolipoprotein A-I, and apolipoprotein B in DBS. Precision, linearity, interferences, and stability were determined and the method was used to analyze samples from 36 human volunteers. The method was compared with clinically validated measurements in paired blood collected via venipuncture.

RESULTS

The method was relatively precise for these proteins (10-11% CV) and linear across the normal concentration ranges of these proteins. Interference from high total serum protein concentration (>8 g/dL) was noted for apolipoprotein A-I and apolipoprotein B. Proteins in DBS were stable for 14 days at temperatures below 25°C and trypsinized samples were stable for 48 h at 7°C. There was moderate correlation with clinical methods (r = 0.783-0.858) and significant bias in individual samples.

CONCLUSIONS AND CLINICAL RELEVANCE

Although the method had adequate precision and linearity for a biomarker, the accuracy compared with clinically validated assays raises concerns regarding the use of DBS compared with venipuncture for clinical use.

Automated Trapping Column Exchanger for High-Throughput Nanoflow Liquid Chromatography

As compared to conventional high-performance liquid chromatography (HPLC) techniques, nanoflow HPLC exhibits improved sensitivity and limits of detection. However, nanoflow HPLC suffers from low throughput due to instrument failure (e.g., fitting fatigue and trapping column failure), limiting the utility of the technique for clinical and industrial applications. To increase the robustness of nanoflow HPLC, we have developed and tested a trapping column exchanging robot for autonomous interchange of trapping columns. This robot makes reproducible, automated connections between the active trapping column and the rest of the HPLC system. The intertrapping column retention time is shown to be sufficiently reproducible for scheduled selected reaction monitoring assays to be performed on different trapping columns without rescheduling the selection windows.

Human Central Nervous System (CNS) ApoE Isoforms Are Increased by Age, Differentially Altered by Amyloidosis, and Relative Amounts Reversed in the CNS Compared with Plasma

The risk of Alzheimer's disease (AD) is highly dependent on apolipoprotein-E (apoE) genotype. The reasons for apoE isoform-selective risk are uncertain; however, both the amounts and structure of human apoE isoforms have been hypothesized to lead to amyloidosis increasing the risk for AD. To address the hypothesis that amounts of apoE isoforms are different in the human CNS, we developed a novel isoform-specific method to accurately quantify apoE isoforms in clinically relevant samples. The method utilizes an antibody-free enrichment step and isotope-labeled physiologically relevant lipoprotein particle standards produced by immortalized astrocytes. We applied this method to a cohort of well characterized clinical samples and observed the following findings. The apoE isoform amounts are not different in cerebrospinal fluid (CSF) from young normal controls, suggesting that the amount of apoE isoforms is not the reason for risk of amyloidosis prior to the onset of advanced age. We did, however, observe an age-related increase in both apoE isoforms. In contrast to normal aging, the presence of amyloid increased apoE3, whereas apoE4 was unchanged or decreased. Importantly, for heterozygotes, the apoE4/apoE3 isoform ratio was increased in the CNS, although the reverse was true in the periphery. Finally, CSF apoE levels, but not plasma apoE levels, correlated with CSF β-amyloid levels. Collectively, these findings support the hypothesis that CNS and peripheral apoE are separate pools and differentially regulated. Furthermore, these results suggest that apoE mechanisms for the risk of amyloidosis and AD are related to an interaction between apoE, aging, and the amount of amyloid burden.

Selecting Optimal Peptides for Targeted Proteomic Experiments in Human Plasma Using In Vitro Synthesized Proteins as Analytical Standards

In targeted proteomics, the development of robust methodologies is dependent upon the selection of a set of optimal peptides for each protein-of-interest. Unfortunately, predicting which peptides and respective product ion transitions provide the greatest signal-to-noise ratio in a particular assay matrix is complicated. Using in vitro synthesized proteins as analytical standards, we report here an empirically driven method for the selection of said peptides in a human plasma assay matrix.

A targeted proteomic strategy for the measurement of oral cancer candidate biomarkers in human saliva

Head and neck cancers, including oral squamous cell carcinoma (OSCC), are the sixth most common malignancy in the world and are characterized by poor prognosis and a low survival rate. Saliva is oral fluid with intimate contact with OSCC. Besides non-invasive, simple, and rapid to collect, saliva is a potential source of biomarkers. In this study, we build an SRM assay that targets fourteen OSCC candidate biomarker proteins, which were evaluated in a set of clinically-derived saliva samples. Using Skyline software package, we demonstrated a statistically significant higher abundance of the C1R, LCN2, SLPI, FAM49B, TAGLN2, CFB, C3, C4B, LRG1, SERPINA1 candidate biomarkers in the saliva of OSCC patients. Furthermore, our study also demonstrated that CFB, C3, C4B, SERPINA1 and LRG1 are associated with the risk of developing OSCC. Overall, this study successfully used targeted proteomics to measure in saliva a panel of biomarker candidates for OSCC.

Using Data Independent Acquisition (DIA) to Model High-responding Peptides for Targeted Proteomics Experiments

Targeted mass spectrometry is an essential tool for detecting quantitative changes in low abundant proteins throughout the proteome. Although selected reaction monitoring (SRM) is the preferred method for quantifying peptides in complex samples, the process of designing SRM assays is laborious. Peptides have widely varying signal responses dictated by sequence-specific physiochemical properties; one major challenge is in selecting representative peptides to target as a proxy for protein abundance. Here we present PREGO, a software tool that predicts high-responding peptides for SRM experiments. PREGO predicts peptide responses with an artificial neural network trained using 11 minimally redundant, maximally relevant properties. Crucial to its success, PREGO is trained using fragment ion intensities of equimolar synthetic peptides extracted from data independent acquisition experiments. Because of similarities in instrumentation and the nature of data collection, relative peptide responses from data independent acquisition experiments are a suitable substitute for SRM experiments because they both make quantitative measurements from integrated fragment ion chromatograms. Using an SRM experiment containing 12,973 peptides from 724 synthetic proteins, PREGO exhibits a 40-85% improvement over previously published approaches at selecting high-responding peptides. These results also represent a dramatic improvement over the rules-based peptide selection approaches commonly used in the literature.

Regulation and function of epithelial secreted phospholipase A2 group X in asthma

RATIONALE

Indirect airway hyperresponsiveness (AHR) is a fundamental feature of asthma that is manifest as exercise-induced bronchoconstriction (EIB). Secreted phospholipase A2 group X (sPLA2-X) plays a key role in regulating eicosanoid formation and the development of inflammation and AHR in murine models.

OBJECTIVES

We sought to examine sPLA2-X in the airway epithelium and airway wall of patients with asthma, the relationship to AHR in humans, and the regulation and function of sPLA2-X within the epithelium.

METHODS

We precisely phenotyped 34 patients with asthma (19 with and 15 without EIB) and 10 normal control subjects to examine in vivo differences in epithelial gene expression, quantitative morphometry of endobronchial biopsies, and levels of secreted protein. The regulation of sPLA2-X gene (PLA2G10) expression was examined in primary airway epithelial cell cultures. The function of epithelial sPLA2-X in eicosanoid formation was examined using PLA2 inhibitors and murine tracheal epithelial cells with Pla2g10 deletion.

MEASUREMENTS AND MAIN RESULTS

We found that sPLA2-X protein is increased in the airways of patients with asthma and that epithelial-derived sPLA2-X may be increased in association with indirect AHR. The expression of sPLA2-X increases during in vitro epithelial differentiation; is regulated by inflammatory signals including tumor necrosis factor, IL-13, and IL-17; and is both secreted from the epithelium and directly participates in the release of arachidonic acid by epithelial cells.

CONCLUSIONS

These data reveal a relationship between epithelial-derived sPLA2-X and indirect AHR in asthma and that sPLA2-X serves as an epithelial regulator of inflammatory eicosanoid formation. Therapies targeting epithelial sPLA2-X may be useful in asthma.

LC/ESI-MS/MS detection of FAs by charge reversal derivatization with more than four orders of magnitude improvement in sensitivity

Quantitative analysis of fatty acids (FAs) is an important area of analytical biochemistry. Ultra high sensitivity FA analysis usually is done with gas chromatography of pentafluorobenzyl esters coupled to an electron-capture detector. With the popularity of electrospray ionization (ESI) mass spectrometers coupled to liquid chromatography, it would be convenient to develop a method for ultra high sensitivity FA detection using this equipment. Although FAs can be analyzed by ESI in negative ion mode, this method is not very sensitive. In this study, we demonstrate a new method of FA analysis based on conversion of the carboxylic acid to an amide bearing a permanent positive charge, N-(4-aminomethylphenyl)pyridinium (AMPP) combined with analysis on a reverse-phase liquid chromatography column coupled to an ESI mass spectrometer operating in positive ion mode. This leads to an ∼60,000-fold increase in sensitivity compared with the same method carried out with underivatized FAs. The new method is about 10-fold more sensitive than the existing method of gas chromatography/electron-capture mass spectrometry of FA pentafluorobenzyl esters. Furthermore, significant fragmentation of the precursor ions in the nontag portion improves analytical specificity. We show that a large number of FA molecular species can be analyzed with this method in complex biological samples such as mouse serum.

Key role of group v secreted phospholipase A2 in Th2 cytokine and dendritic cell-driven airway hyperresponsiveness and remodeling

Background

Previous work has shown that disruption of the gene for group X secreted phospholipase A₂ (sPLA₂-X) markedly diminishes airway hyperresponsiveness and remodeling in a mouse asthma model. With the large number of additional sPLA₂s in the mammalian genome, the involvement of other sPLA₂s in the asthma model is possible – in particular, the group V sPLA₂ (sPLA₂-V) that like sPLA₂-X is highly active at hydrolyzing membranes of mammalian cells.

Methodology and Principal Findings

The allergen-driven asthma phenotype was significantly reduced in sPLA₂-V-deficient mice but to a lesser extent than observed previously in sPLA₂-X-deficient mice. The most striking difference observed between the sPLA₂-V and sPLA₂-X knockouts was the significant impairment of the primary immune response to the allergen ovalbumin (OVA) in the sPLA₂-V^−/− mice. The impairment in eicosanoid generation and dendritic cell activation in sPLA2-V^−/− mice diminishes Th2 cytokine responses in the airways.

Conclusions

This paper illustrates the diverse roles of sPLA₂s in the immunopathogenesis of the asthma phenotype and directs attention to developing specific inhibitors of sPLA₂-V as a potential new therapy to treat asthma and other allergic disorders.

Blockade of human group X secreted phospholipase A2 (GX-sPLA2)-induced airway inflammation and hyperresponsiveness in a mouse asthma model by a selective GX-sPLA2 inhibitor

Group X (GX) phospholipase A(2), a member of a large group of secreted phospholipases A(2) (sPLA(2)s), has recently been demonstrated to play an important in vivo role in the release of arachidonic acid and subsequent formation of eicosanoids. In a Th2 cytokine-driven mouse asthma model, deficiency of mouse GX (mGX)-sPLA(2) significantly impairs development of the asthma phenotype. In this study, we generated mGX-sPLA(2)(-/-) mice with knock-in of human GX (hGX)-sPLA(2) (i.e. hGX-sPLA(2)(+/+) knock-in mice) to understand more fully the role of GX-sPLA(2) in these allergic pulmonary responses and to assess the effect of pharmacological blockade of the GX-sPLA(2)-mediated responses. Knock-in of hGX-sPLA(2) in mGX-sPLA(2)(-/-) mice restored the allergen-induced airway infiltration by inflammatory cells, including eosinophils, goblet cell metaplasia, and hyperresponsiveness to methacholine in the mGX-sPLA(2)-deficient mice. This knock-in mouse model enabled the use of a highly potent indole-based inhibitor of hGX-sPLA(2), RO061606 (which is ineffective against mGX-sPLA(2)), to assess the potential utility of GX-sPLA(2) blockade as a therapeutic intervention in asthma. Delivery of RO061606 via mini-osmotic pumps enabled the maintenance in vivo in the mouse asthma model of plasma inhibitor concentrations near 10 μm, markedly higher than the IC(50) for inhibition of hGX-sPLA(2) in vitro. RO061606 significantly decreased allergen-induced airway inflammation, mucus hypersecretion, and hyperresponsiveness in the hGX-sPLA(2)(+/+) knock-in mouse. Thus, development of specific hGX-sPLA(2) inhibitors may provide a new pharmacological opportunity for the treatment of patients with asthma.

Improved sensitivity mass spectrometric detection of eicosanoids by charge reversal derivatization

Combined liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) is a powerful method for the analysis of oxygenated metabolites of polyunsaturated fatty acids including eicosanoids. Here we describe the synthesis of a new derivatization reagent N-(4-aminomethylphenyl)pyridinium (AMPP) that can be coupled to eicosanoids via an amide linkage in quantitative yield. Conversion of the carboxylic acid of eicosanoids to a cationic AMPP amide improves sensitivity of detection by 10- to 20-fold compared to negative mode electrospray ionization detection of underivatized analytes. This charge reversal derivatization allows detection of cations rather than anions in the electrospray ionization mass spectrometer, which enhances sensitivity. Another factor is that AMPP amides undergo considerable collision-induced dissociation in the analyte portion rather than exclusively in the cationic tag portion, which allows isobaric derivatives to be distinguished by tandem mass spectrometry, and this further enhances sensitivity and specificity. This simple derivatization method allows prostaglandins, thromboxane B(2), leukotriene B(4), hydroxyeicosatetraenoic acid isomers, and arachidonic acid to be quantified in complex biological samples with limits of quantification in the 200-900 fg range. One can anticipate that the AMPP derivatization method can be extended to other carboxylic acid analytes for enhanced sensitivity detection.

Eosinophil cysteinyl leukotriene synthesis mediated by exogenous secreted phospholipase A2 group X

Secreted phospholipase A(2) group X (sPLA(2)-X) has recently been identified in the airways of patients with asthma and may participate in cysteinyl leukotriene (CysLT; C(4), D(4), and E(4)) synthesis. We examined CysLT synthesis and arachidonic acid (AA) and lysophospholipid release by eosinophils mediated by recombinant human sPLA(2)-X. We found that recombinant sPLA(2)-X caused marked AA release and a rapid onset of CysLT synthesis in human eosinophils that was blocked by a selective sPLA(2)-X inhibitor. Exogenous sPLA(2)-X released lysophospholipid species that arise from phospholipids enriched in AA in eosinophils, including phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine as well as plasmenyl phosphatidylcholine and phosphatidylethanolamine. CysLT synthesis mediated by sPLA(2)-X but not AA release could be suppressed by inhibition of cPLA(2)α. Exogenous sPLA(2)-X initiated Ser(505) phosphorylation of cPLA(2)α, an intracellular Ca(2+) flux, and translocation of cPLA(2)α and 5-lipoxygenase in eosinophils. Synthesis of CysLTs in response to sPLA(2)-X or lysophosphatidylcholine was inhibited by p38 or JNK inhibitors but not by a MEK 1/2 inhibitor. A further increase in CysLT synthesis was induced by the addition of sPLA(2)-X to eosinophils under conditions of N-formyl-methionyl-leucyl-phenylalanine-mediated cPLA(2)α activation. These results indicate that sPLA(2)-X participates in AA and lysophospholipid release, resulting in CysLT synthesis in eosinophils through a mechanism involving p38 and JNK MAPK, cPLA(2)α, and 5-lipoxygenase activation and resulting in the amplification of CysLT synthesis during cPLA(2)α activation. Transactivation of eosinophils by sPLA(2)-X may be an important mechanism leading to CysLT formation in the airways of patients with asthma.

Interfacial kinetic and binding properties of mammalian group IVB phospholipase A2 (cPLA2beta) and comparison with the other cPLA2 isoforms

The cytosolic (group IV) phospholipase A(2) (cPLA(2)s) family contains six members. We have prepared recombinant proteins for human α, mouse β, human γ, human δ, human ε, and mouse ζ cPLA(2)s and have studied their interfacial kinetic and binding properties in vitro. Mouse cPLA(2)β action on phosphatidylcholine vesicles is activated by anionic phosphoinositides and cardiolipin but displays a requirement for Ca(2+) only in the presence of cardiolipin. This activation pattern is explained by the effects of anionic phospholipids and Ca(2+) on the interfacial binding of mouse cPLA(2)β and its C2 domain to vesicles. Ca(2+)-dependent binding of mouse cPLA(2)β to cardiolipin-containing vesicles requires a patch of basic residues near the Ca(2+)-binding surface loops of the C2 domain, but binding to phosphoinositide-containing vesicles does not depend on any specific cluster of basic residues. Human cPLA(2)δ also displays Ca(2+)- and cardiolipin-enhanced interfacial binding and activity. The lysophospholipase, phospholipase A(1), and phospholipase A(2) activities of the full set of mammalian cPLA(2)s were quantified. The relative level of these activities is very different among the isoforms, and human cPLA(2)δ stands out as having relatively high phospholipase A(1) activity. We also tested the susceptibility of all cPLA(2) family members to a panel of previously reported inhibitors of human cPLA(2)α and analogs of these compounds. This led to the discovery of a potent and selective inhibitor of mouse cPLA(2)β. These in vitro studies help determine the regulation and function of the cPLA(2) family members.

Improved method for the quantification of lysophospholipids including enol ether species by liquid chromatography-tandem mass spectrometry

LC/ESI-MS/MS has been previously demonstrated to be a powerful method to detect and quantify molecular species of glycerophospholipids including lysophospholipids. In this study, we provide an improved pre-mass spectrometry lipid extraction procedure that avoids the acid-catalyzed decomposition of plasmenyl phospholipids that is problematic with previously reported methods. We show that the use of lysophospholipid internal standards with perdeuterated fatty acyl chains avoids isobar problems associated with the use of internal standards containing odd carbon number fatty acyl chains. We also show that LC prior to MS is required to avoid numerous problems associated with isobars and with MS in-source decomposition of lysophosphatidylserine. The reported method of using normal phase chromatography/ESI-MS is used to quantify lysophospholipids in serum and to quantify lysophospholipids produced in mammalian cells by human group X secreted phospholipase A(2). The latter shows that group X phospholipase A(2) added exogenously to cells generates a different set of lysophospholipids compared with enzyme produced endogenously in cells, which supports earlier studies showing that this phospholipase A(2) can act on cell membranes prior to externalization from cells.

Group X phospholipase A2 stimulates the proliferation of colon cancer cells by producing various lipid mediators

Among mammalian secreted phospholipases A2 (sPLA(2)s), the group X enzyme has the most potent hydrolyzing capacity toward phosphatidylcholine, the major phospholipid of cell membrane and lipoproteins. This enzyme has recently been implicated in chronic inflammatory diseases such as atherosclerosis and asthma and may also play a role in colon tumorigenesis. We show here that group X sPLA(2) [mouse (m)GX] is one of the most highly expressed PLA(2) in the mouse colon and that recombinant mouse and human enzymes stimulate proliferation and mitogen-activated protein kinase activation of various colon cell lines, including Colon-26 cancer cells. Among various recombinant sPLA(2)s, mGX is the most potent enzyme to stimulate cell proliferation. Based on the use of sPLA(2) inhibitors, catalytic site mutants, and small interfering RNA silencing of cytosolic PLA(2)alpha and M-type sPLA(2) receptor, we demonstrate that mGX promotes cell proliferation independently of the receptor and via its intrinsic catalytic activity and production of free arachidonic acid and lysophospholipids, which are mitogenic by themselves. mGX can also elicit the production of large amounts of prostaglandin E2 and other eicosanoids from Colon-26 cells, but these lipid mediators do not play a role in mGX-induced cell proliferation because inhibitors of cyclooxygenases and lipoxygenases do not prevent sPLA(2) mitogenic effects. Together, our results indicate that group X sPLA(2) may play an important role in colon tumorigenesis by promoting cancer cell proliferation and releasing various lipid mediators involved in other key events in cancer progression.

Role of phosphorylation and basic residues in the catalytic domain of cytosolic phospholipase A2alpha in regulating interfacial kinetics and binding and cellular function

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) is regulated by phosphorylation and calcium-induced translocation to membranes. Immortalized mouse lung fibroblasts lacking endogenous cPLA(2)alpha (IMLF(-/-)) were reconstituted with wild type and cPLA(2)alpha mutants to investigate how calcium, phosphorylation, and the putative phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding site regulate translocation and arachidonic acid (AA) release. Agonists that elicit distinct modes of calcium mobilization were used. Serum induced cPLA(2)alpha translocation to Golgi within seconds that temporally paralleled the initial calcium transient. However, the subsequent influx of extracellular calcium was essential for stable binding of cPLA(2)alpha to Golgi and AA release. In contrast, phorbol 12-myristate 13-acetate induced low amplitude calcium oscillations, slower translocation of cPLA(2)alpha to Golgi, and much less AA release, which were blocked by chelating extracellular calcium. AA release from IMLF(-/-) expressing phosphorylation site (S505A) and PIP(2) binding site (K488N/K543N/K544N) mutants was partially reduced compared with cells expressing wild type cPLA(2)alpha, but calcium-induced translocation was not impaired. Consistent with these results, Ser-505 phosphorylation did not change the calcium requirement for interfacial binding and catalysis in vitro but increased activity by 2-fold. Mutations in basic residues in the catalytic domain of cPLA(2)alpha reduced activation by PIP(2) but did not affect the concentration of calcium required for interfacial binding or phospholipid hydrolysis. The results demonstrate that Ser-505 phosphorylation and basic residues in the catalytic domain principally act to regulate cPLA(2)alpha hydrolytic activity.

Inhibition of lipoprotein-associated phospholipase A2 reduces complex coronary atherosclerotic plaque development

Increased lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) activity is associated with increased risk of cardiac events, but it is not known whether Lp-PLA(2) is a causative agent. Here we show that selective inhibition of Lp-PLA(2) with darapladib reduced development of advanced coronary atherosclerosis in diabetic and hypercholesterolemic swine. Darapladib markedly inhibited plasma and lesion Lp-PLA(2) activity and reduced lesion lysophosphatidylcholine content. Analysis of coronary gene expression showed that darapladib exerted a general anti-inflammatory action, substantially reducing the expression of 24 genes associated with macrophage and T lymphocyte functioning. Darapladib treatment resulted in a considerable decrease in plaque area and, notably, a markedly reduced necrotic core area and reduced medial destruction, resulting in fewer lesions with an unstable phenotype. These data show that selective inhibition of Lp-PLA(2) inhibits progression to advanced coronary atherosclerotic lesions and confirms a crucial role of vascular inflammation independent from hypercholesterolemia in the development of lesions implicated in the pathogenesis of myocardial infarction and stroke.

Importance of group X-secreted phospholipase A2 in allergen-induced airway inflammation and remodeling in a mouse asthma model

Arachidonic acid metabolites, the eicosanoids, are key mediators of allergen-induced airway inflammation and remodeling in asthma. The availability of free arachidonate in cells for subsequent eicosanoid biosynthesis is controlled by phospholipase A₂s (PLA₂s), most notably cytosolic PLA₂-α. 10 secreted PLA₂s (sPLA₂s) have also been identified, but their function in eicosanoid generation is poorly understood. We investigated the role of group X sPLA₂ (sPLA₂-X), the sPLA₂ with the highest in vitro cellular phospholipolysis activity, in acute and chronic mouse asthma models in vivo. The lungs of sPLA₂-X^−/− mice, compared with those of sPLA₂-X^+/+ littermates, had significant reduction in ovalbumin-induced infiltration by CD4⁺ and CD8⁺ T cells and eosinophils, goblet cell metaplasia, smooth muscle cell layer thickening, subepithelial fibrosis, and levels of T helper type 2 cell cytokines and eicosanoids. These data direct attention to sPLA₂-X as a novel therapeutic target for asthma.

C-terminal proteolysis of prenylated proteins in trypanosomatids and RNA interference of enzymes required for the post-translational processing pathway of farnesylated proteins

The C-terminal "CaaX"-motif-containing proteins usually undergo three sequential post-translational processing steps: (1) attachment of a prenyl group to the cysteine residue; (2) proteolytic removal of the last three amino acids "aaX"; (3) methyl esterification of the exposed alpha-carboxyl group of the prenyl-cysteine residue. The Trypanosoma brucei and Leishmania major Ras converting enzyme 1 (RCE1) orthologs of 302 and 285 amino acids-proteins, respectively, have only 13-20% sequence identity to those from other species but contain the critical residues for the activity found in other orthologs. The Trypanosoma brucei a-factor converting enzyme 1 (AFC1) ortholog consists of 427 amino acids with 29-33% sequence identity to those of other species and contains the consensus HExxH zinc-binding motif. The trypanosomatid RCE1 and AFC1 orthologs contain predicted transmembrane regions like other species. Membranes from Sf9 cells expressing the RCE1 ortholog of T. brucei or L. major showed proteolytic activity against farnesylated RAS-CVIM, whereas membranes containing T. brucei AFC1 ortholog were inactive. The results suggest that RCE1 is responsible for proteolytic removal of the C-terminal aaX from prenyl-CaaX proteins in these parasites. All the three enzymatic post-translational processes are thought to be required for proper cellular functioning of CaaX-proteins in eukaryotic cells. We carried out RNA interference experiments in Trypanosoma brucei of the enzymes involved in farnesyl protein post-translational modification to evaluate their importance in cell proliferation. Knockdown of T. brucei PFT beta subunit and RCE1 mRNAs resulted in >20-fold suppression of cell growth and dramatic morphologic changes. Knockdown of PPMT mRNA caused less dramatic effects on growth but induced noticeable changes in cell morphology.

Facile preparation of leukotrienes C(4), D(4) and E(4) containing carbon-13 and nitrogen-15 for quantification of cysteinyl leukotrienes by mass spectrometry

With the recent ability to use combined liquid chromatography/electrospray tandem mass spectrometry to analyze for several eicosanoids in biological samples in a single and rapid experiment, heavy isotope-labeled eicosanoids are needed as internal standards in order to quantify eicosanoid analytes. The present study describes a practical preparation of cysteinyl leukotrienes (leukotriene C(4), D(4) and E(4)) with three (13)C atoms and one (15)N atom in the cysteinyl residue. The method involves solid-phase peptide synthesis to make glutathione with heavy isotopes in the cysteinyl residue and reaction of this tripeptide with commercially available leukotriene A(4) methyl ester to give labeled leukotriene C(4) methyl ester, which is hydrolyzed to labeled leukotriene C(4). Labeled leukotriene E(4) is prepared in the same way with the use of labeled cysteine. Labeled leukotriene D(4) is prepared by treatment of labeled leukotriene C(4) with commercially available γ-glutamyl transpeptidase.

The proinflammatory mediator Platelet Activating Factor is an effective substrate for human group X secreted phospholipase A2

Platelet Activating Factor (PAF) is a potent mediator of inflammation whose biological activity depends on the acetyl group esterified at the sn-2 position of the molecule. PAF-acetylhydrolase (PAF-AH), a secreted calcium-independent phospholipase A(2), is known to inactivate PAF by formation of lyso-PAF and acetate. However, PAF-AH deficient patients are not susceptible to the biological effects of inhaled PAF in airway inflammation, suggesting that other enzymes may regulate extracellular levels of PAF. We therefore examined the hydrolytic activity of the recently described human group X secreted phospholipase A(2) (hGX sPLA(2)) towards PAF. Among different sPLA(2)s, hGX sPLA(2) has the highest affinity towards phosphatidylcholine (PC), the major phospholipid of cellular membranes and plasma lipoproteins. Our results show that unlike group IIA, group V, and the pancreatic group IB sPLA(2), recombinant hGX sPLA(2) can efficiently hydrolyze PAF. The hydrolysis of PAF by hGX sPLA(2) rises abruptly when the concentration of PAF passes through its critical micelle concentration suggesting that the enzyme undergoes interfacial binding and activation to PAF. In conclusion, our study shows that hGX sPLA(2) may be a novel player in PAF regulation during inflammatory processes.

Interfacial binding of bee venom secreted phospholipase A2 to membranes occurs predominantly by a nonelectrostatic mechanism

The secreted phospholipase A(2) from bee venom (bvPLA(2)) contains a membrane binding surface composed mainly of hydrophobic residues and two basic residues that come in close contact with the membrane. Previous studies have shown that the mutant in which these two basic residues (K14 and R23) as well as three other nearby basic residues were collectively changed to glutamate (charge reversal), like wild-type enzyme, binds with high affinity to anionic phospholipid vesicles. In the present study, we have measured the equilibrium constants for the interaction of wild-type bvPLA(2), the charge-reversal mutant (bvPLA(2)-E5), and the mutant in which the five basic residues were changed to neutral glutamine (bvPLA(2)-Q5) with phosphatidylcholine (PC) vesicles containing various amounts of the anionic phosphatidylserine (PS). Remarkably, bvPLA(2)-E5 with an anionic membrane binding surface binds more tightly to vesicles as the mole percent of PS is increased. Computational studies predict that this is due to a significant upward shift in the pK(a) of E14 (and to some extent E23) when the enzyme binds to PC/PS vesicles such that the carboxylate of the glutamate side chain near the membrane surface undergoes protonation. The experimental pH dependence of vesicle binding supports this prediction. bvPLA(2)-E5 binds more weakly to PS/PC vesicles than does wild-type enzyme due to electrostatic protein-vesicle repulsion coupled with the similar energetics of desolvation of basic residues and glutamates that accompanies enzyme-vesicle contact. Studies with bvPLA(2)-Q5 show that only a small fraction of the total bvPLA(2) interfacial binding energy ( approximately 10%) is due to electrostatics.

Inhibition of the complete set of mammalian secreted phospholipases A2 by indole analogues

Structure-guided design was employed in a search for potent and selective inhibitors of mammalian secreted phospholipases A(2) (sPLA(2)s). Using the X-ray structures of human groups IIA and X sPLA(2)s (hGIIA and hGX) as templates, homology structural models were made for the other human and mouse sPLA(2)s (hGIB, mGIB, mGIIA, mGIIC, hGIID, mGIID, hGIIE, mGIIE, hGIIF, mGIIF, hGV, mGV, and mGX). Me-Indoxam is a previously discovered indole analogue that binds tightly to many sPLA(2)s, and the X-ray structure of the hGX-Me-Indoxam complex was determined at a resolution of 2.0 A. Modeling suggests that the residues near the N(1)-substituent of Me-Indoxam vary significantly among the mammalian sPLA(2)s, and therefore a library of 83N(1)-variants was prepared by parallel synthesis. Several Me-Indoxam analogues bearing a 4-(2-oxy-ethanoic acid) side chain were potent inhibitors (IC(50) <0.05 microM) of hGIIA, mGIIA, mGIIC, hGIIE, mGIIE, hGV, and mGV, while they displayed intermediate potency (0.05-5 microM) against hGIB, mGIB, hGX, and mGX, and poorly inhibited (>5 microM) hGIID, mGIID, hGIIF, and mGIIF. Me-Indoxam analogues bearing a 5-(4-oxy-butanoic acid) side chain were generally less potent inhibitors. Although no compounds were found to be highly specific for a single human or mouse sPLA(2), combinations of Me-Indoxam analogues were discovered that could be used to distinguish the action of various sPLA(2)s in cellular events. For example, Me-Indoxam and compound 5 are approximately 5-fold more potent on hGIIA than on hGV, and compound 21 is 10-fold more potent on hGV versus hGIIA.

Novel mammalian group XII secreted phospholipase A2 lacking enzymatic activity

An increasing number of mammalian secreted phospholipases A(2) (sPLA(2)s) has been identified over the past few years. Here, we report the identification and recombinant expression of a novel sPLA(2)-like protein in mouse and human species that has been called group XIIB (GXIIB). The mature protein has a molecular mass of 19.7 kDa and structural features similar to those of the previously identified GXII sPLA(2), now called GXIIA. Strikingly, the GXIIB sPLA(2) has a mutation in the active site, replacing the canonical histidine by a leucine, suggesting that this sPLA(2) is catalytically inactive. Recombinant expression of human (hGXIIB) and mouse (mGXIIB) sPLA(2)s in Escherichia coli indicates that GXIIB sPLA(2)s display no measurable lipolytic activity on various types of phospholipid substrates. Furthermore, these sPLA(2)-like proteins display relatively weak affinity to phospholipid vesicles. Binding experiments indicate that these proteins are also unable to bind to the well-known M-type sPLA(2) receptor. The RNA tissue distribution of GXIIB sPLA(2)s is distinct from that of other sPLA(2)s including the homologous GXIIA. Strong expression was observed in liver, small intestine, and kidney in both human and mouse species. Interestingly, the expression of the novel sPLA(2) is dramatically decreased in human tumors from the same tissues. The absence of enzymatic activity suggests that the GXIIB sPLA(2)-like proteins probably exert their biological roles by acting as ligands for as yet unidentified receptors.

On the Binding Preference of Human Groups IIA and X Phospholipases A2 for Membranes with Anionic Phospholipids

Mammals contain 9-10 secreted phospholipases A(2) (sPLA(2)s) that display widely different affinities for membranes, depending on the phospholipid composition. The much higher enzymatic activity of human group X sPLA(2) (hGX) compared with human group IIA sPLA(2) (hGIIA) on phosphatidylcholine (PC)-rich vesicles is due in large part to the higher affinity of the former enzyme for such vesicles; this result also holds when vesicles contain cholesterol and sphingomyelin. The inclusion of anionic phosphatidylserine in PC vesicles dramatically enhances interfacial binding and catalysis of hGIIA but not of hGX. This is the result of the large number of lysine and arginine residues scattered over the entire surface of hGIIA, which cause the enzyme to form a supramolecular aggregate with multiple vesicles. Thus, high affinity binding of hGIIA to anionic vesicles is a complex process and cannot be attributed to a few basic residues on its interfacial binding surface, as is also evident from mutagenesis studies. The main reason hGIIA binds poorly to PC-rich vesicles is that it lacks a tryptophan residue on its interfacial binding surface, a residue that contributes to the high affinity binding of hGX to PC-rich vesicles. Results show that the lag in the onset of hydrolysis of PC vesicles by hGIIA is due in part to the poor affinity of this enzyme for these vesicles. Binding affinity of hGIIA, hGX, and their mutants to PC-rich vesicles is well correlated to the ability of these enzymes to act on the PC-rich outer plasma membrane of mammalian cells.