Mapping of the human adenosine A2a receptor gene: relationship to potential schizophrenia loci on chromosome 22q and exclusion from the CATCH 22 region

Glial Purinergic Signaling-Mediated Oxidative Stress (GPOS) in Neuropsychiatric Disorders

Oxidative stress (OS) has been implicated in the progression of multiple neuropsychiatric disorders, including schizophrenia (SZ), major depressive disorder (MDD), bipolar disorder, and autism. However, whether glial purinergic signaling interaction with oxidative/antioxidative system displays an important role in neuropsychiatric disorders is still unclear. In this review, we firstly summarize the oxidative/antioxidative pathways shared in different glial cells and highlight the cell type-specific difference in response to OS. Then, we collect the evidence showing the regulation of purinergic signaling in OS with an emphasis on adenosine and its receptors, P2Y1 receptor in the P2Y family and P2X7receptor in the P2X family. Available data shows that the activation of P1 receptors and P2X accelerates the OS; reversely, the activation of the P2Y family (P2Y1) causes protective effect against OS. Finally, we discuss current findings demonstrating the contribution of the purinergic signaling system to neuropsychiatric disorders and point out the potential role of OS in this process to propose a “glial purinergic-oxidative stress” (“GPOS”) hypothesis for future development of therapeutic strategies against a variety of neuropsychiatric disorders.

Association between ADORA2A gene polymorphisms and schizophrenia in the North Chinese Han population

BACKGROUND

A large number of studies have shown a close relationship between ADORA2A and the pathological mechanism of schizophrenia. However, to our knowledge, there has been no studies examining the association between the ADORA2A gene and schizophrenia in Chinese Han population.

PURPOSE

The objective of this study was to examine the relationship between adenosine A2A receptor (ADORA2A) single nucleotide polymorphisms and schizophrenia in the North Chinese Han population.

PATIENTS AND METHODS

We detected ADORA2A single nucleotide polymorphisms (SNPs) using polymerase chain reaction-restriction fragment length polymorphism analyses and summarized our results using SPSS statistical software and Haploview in schizophrenia case group (n=398) and healthy control group (n=535).

RESULTS

The frequency of the CC homozygote genotype of SNP rs2298383T/C were significantly higher in the case than the control group (p=0.005, OR=1.712, 95% CI=1.172-2.502). After linkage disequilibrium analysis, SNPs rs5996696A/C and rs2298383T/C displayed strong linkage disequilibrium. We found that the frequencies of haplotypes TA (χ²=6.268, p=0.0123) and CA (χ²=7.012, p=0.0081) were significantly higher in the case group than in the control group.

CONCLUSION

In conclusion, SNPs in the ADORA2A gene may be associated with schizophrenia in the northern Chinese Han population.

Deletion of adenosine A2A receptors from astrocytes disrupts glutamate homeostasis leading to psychomotor and cognitive impairment: relevance to schizophrenia

BACKGROUND

Adenosine A2A receptors (A2AR) modulate dopamine and glutamate signaling and thereby may influence some of the psychomotor and cognitive processes associated with schizophrenia. Because astroglial A2AR regulate the availability of glutamate, we hypothesized that they might play an unprecedented role in some of the processes leading to the development of schizophrenia, which we investigated using a mouse line with a selective deletion of A2AR in astrocytes (Gfa2-A2AR knockout [KO] mice].

METHODS

We examined Gfa2-A2AR KO mice for behaviors thought to recapitulate some features of schizophrenia, namely enhanced MK-801 psychomotor response (positive symptoms) and decreased working memory (cognitive symptoms). In addition, we probed for neurochemical alterations in the glutamatergic circuitry, evaluating glutamate uptake and release and the levels of key proteins defining glutamatergic signaling (glutamate transporter-I [GLT-I], N-methyl-D-aspartate receptors [NMDA-R] and α-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors [AMPA-R]) to provide a mechanistic understanding of the phenotype encountered.

RESULTS

We show that Gfa2-A2AR KO mice exhibited enhanced MK-801 psychomotor response and decreased working memory; this was accompanied by a disruption of glutamate homeostasis characterized by aberrant GLT-I activity, increased presynaptic glutamate release, NMDA-R 2B subunit upregulation, and increased internalization of AMPA-R. Accordingly, selective GLT-I inhibition or blockade of GluR1/2 endocytosis prevented the psychomotor and cognitive phenotypes in Gfa2-A2AR KO mice, namely in the nucleus accumbens.

CONCLUSIONS

These results show that the dysfunction of astrocytic A2AR, by controlling GLT-I activity, triggers an astrocyte-to-neuron wave of communication resulting in disrupted glutamate homeostasis, thought to underlie several endophenotypes relevant to schizophrenia.

Purinergic receptors in psychiatric disorders

Psychiatric disorders describe different mental or behavioral patterns, causing suffering or poor coping of ordinary life with manifold presentations. Multifactorial processes can contribute to their development and progression. Purinergic neurotransmission and neuromodulation in the brain have attracted increasing therapeutic interest in the field of psychiatry. Purine nucleotides and nucleosides are well recognized as signaling molecules mediating cell to cell communication. The actions of ATP are mediated by ionotropic P2X and metabotropic P2Y receptor subfamilies, whilst the actions of adenosine are mediated by P1 (A1 or A2) adenosine receptors. Purinergic mechanisms and specific receptor subtypes have been shown to be linked to the regulation of many aspects of behavior and mood and to dysregulation in pathological processes of brain function. In this review the recent knowledge on the role of purinergic receptors in the two most frequent psychiatric diseases, major depression and schizophrenia, as well as on related animal models is summarized. At present the most promising data for therapeutic strategies derive from investigations of the adenosine system emphasizing a unique function of A2A receptors at neurons and astrocytes in these disorders. Among the P2 receptor family, in particular P2X7 and P2Y1 receptors were related to disturbances in major depression and schizophrenia, respectively. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Novel Therapeutic GPCRs for Psychiatric Disorders

G protein-coupled receptors (GPCRs) are the most common targets of the neuropharmacological drugs in the central nervous system (CNS). GPCRs are activated by manifold neurotransmitters, and their activation in turn evokes slow synaptic transmission. They are deeply involved in multiple neurological and psychiatric disorders such as Parkinson’s disease and schizophrenia. In the brain, the striatum is strongly innervated by the ventral tegmental area (VTA) and plays a central role in manifestation of psychiatric disorders. Recently, anatomical and comprehensive transcriptome analysis of the non-odorant GPCR superfamily revealed that the orphan GPCRs GPR88, GPR6, and GPR52, as well as dopamine D1 and D2 receptors and the adenosine A2a receptor, are the most highly enriched in the rodent striatum. Genetically engineered animal models and molecular biological studies have suggested that these striatally enriched GPCRs have a potential to be therapeutic psychiatric receptors. This review summarizes the current understanding of the therapeutic GPCR candidates for psychiatric disorders.

Reduced striatal adenosine A2A receptor levels define a molecular subgroup in schizophrenia

Schizophrenia (SZ) is a mental disorder of unknown origin. Some scientific evidence seems to indicate that SZ is not a single disease entity, since there are patient groups with clear symptomatic, course and biomarker differences. SZ is characterized by a hyperdopaminergic state related to high dopamine D2 receptor activity. It has also been proposed that there is a hypoadenosynergic state. Adenosine is a nucleoside widely distributed in the organism with neuromodulative and neuroprotective activity in the central nervous system. In the brain, the most abundant adenosine receptors are A1R and A2AR. In the present report, we characterize the presence of both receptors in human postmortem putamens of patients suffering SZ with real time TaqMan PCR, western blotting and radioligand binding assay. We show that A1R levels remain unchanged with respect to age-matched controls, whereas nearly fifty percent of patients have reduced A2AR, at the transcriptional and translational levels. Moreover, we describe how DNA methylation plays a role in the pathological A2AR levels with the bisulfite-sequencing technique. In fact, an increase in 5-methylcytosine percentage in the 5' UTR region of ADORA2A was found in those SZ patients with reduced A2AR levels. Interestingly, there was a relationship between the A2A/β-actin ratio and motor disturbances as assessed with some items of the PANSS, AIMS and SAS scales. Therefore, there may be a subgroup of SZ patients with reduced striatal A2AR levels accompanied by an altered motor phenotype.

Deletion of striatal adenosine A(2A) receptor spares latent inhibition and prepulse inhibition but impairs active avoidance learning

Following early clinical leads, the adenosine A(2A)R receptor (A(2A)R) has continued to attract attention as a potential novel target for treating schizophrenia, especially against the negative and cognitive symptoms of the disease because of A(2A)R's unique modulatory action over glutamatergic in addition to dopaminergic signaling. Through (i) the antagonistic interaction with the dopamine D(2) receptor, and (ii) the regulation of glutamate release and N-methyl-d-aspartate receptor function, striatal A(2A)R is ideally positioned to fine-tune the dopamine-glutamate balance, the disturbance of which is implicated in the pathophysiology of schizophrenia. However, the precise function of striatal A(2A)Rs in the regulation of schizophrenia-relevant behavior is poorly understood. Here, we tested the impact of conditional striatum-specific A(2A)R knockout (st-A(2A)R-KO) on latent inhibition (LI) and prepulse inhibition (PPI) - behavior that is tightly regulated by striatal dopamine and glutamate. These are two common cross-species translational tests for the assessment of selective attention and sensorimotor gating deficits reported in schizophrenia patients; and enhanced performance in these tests is associated with antipsychotic drug action. We found that neither LI nor PPI was significantly affected in st-A(2A)R-KO mice, although a deficit in active avoidance learning was identified in these animals. The latter phenotype, however, was not replicated in another form of aversive conditioning - namely, conditioned taste aversion. Hence, the present study shows that neither learned inattention (as measured by LI) nor sensory gating (as indexed by PPI) requires the integrity of striatal A(2A)Rs - a finding that may undermine the hypothesized importance of A(2A)R in the genesis and/or treatment of schizophrenia.

Adenosine hypothesis of schizophrenia--opportunities for pharmacotherapy

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-D-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor-dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities, it represents a promising candidate for reversing the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctions characteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A(1)R and A(2A)R), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

Adenosine A(2A) receptors in psychopharmacology: modulators of behavior, mood and cognition

The adenosine A(2A) receptor (A(2A)R) is in the center of a neuromodulatory network affecting a wide range of neuropsychiatric functions by interacting with and integrating several neurotransmitter systems, especially dopaminergic and glutamatergic neurotransmission. These interactions and integrations occur at multiple levels, including (1) direct receptor- receptor cross-talk at the cell membrane, (2) intracellular second messenger systems, (3) trans-synaptic actions via striatal collaterals or interneurons in the striatum, (4) and interactions at the network level of the basal ganglia. Consequently, A(2A)Rs constitute a novel target to modulate various psychiatric conditions. In the present review we will first summarize the molecular interaction of adenosine receptors with other neurotransmitter systems and then discuss the potential applications of A(2A)R agonists and antagonists in physiological and pathophysiological conditions, such as psychostimulant action, drug addiction, anxiety, depression, schizophrenia and learning and memory.

RASD2, MYH9, and CACNG2 genes at chromosome 22q12 associated with the subgroup of schizophrenia with non-deficit in sustained attention and executive function

BACKGROUND

In a previous linkage study of schizophrenia that included Taiwanese samples, the marker D22S278 (22q12.3) was significantly linked to schizophrenia (p = .001).

METHODS

We conducted fine mapping of the implicated genomic region, with 47 validated single nucleotide polymorphism (SNP) markers around 1 Mb of D22S278, in a Taiwanese sample of 218 pedigrees with at least 2 siblings affected with schizophrenia. We examined the association of these SNPs and their haplotypes with schizophrenia and with subgroups defined by the presence and absence of deficits in sustained attention as assessed by undegraded and degraded continuous performance tests (CPTs). We also examined subgroups defined by deficits in categories achieved in the Wisconsin Card Sort Test (WCST).

RESULTS

Three of five candidate vulnerability genes (RASD2, APOL5, MYH9, EIF3S7, and CACNG2), which had marginally significant associations with schizophrenia, had significant associations with schizophrenic patients who did not have deficits in sustained attention on the undegraded CPT (RASD2 gene SNP rs736212; p = .0008 with single locus analysis) and the degraded CPT (MYH9 gene haplotype 1-1-1-1 of SNP rs3752463 - rs1557540 - rs713839 - rs739097; p = .0059 with haplotype analysis). We also found a significant association for patients who showed no deficits in executive function as measured by categories achieved in the WCST (CACNG2 gene haplotype 2-1-1-1 of SNP rs2267360 - rs140526 - rs1883987 - rs916269; p = .0163 with haplotype analysis).

CONCLUSIONS

The genes RASD2, MYH9, and CACNG2 might be vulnerability genes for neuropsychologically defined subgroups of schizophrenic patients.

Characterization of genomic organization of the adenosine A2A receptor gene by molecular and bioinformatics analyses

The adenosine A(2A) receptor (A(2A)R) is abundantly expressed in brain and emerging as an important therapeutic target for Parkinson's disease and potentially other neuropsychiatric disorders. To understand the molecular mechanisms of A(2A)R gene expression, we have characterized the genomic organization of the mouse and human A(2A)R genes by molecular and bioinformatic analyses. Three new exons (m1A, m1B and m1C) encoding the 5' untranslated regions (5'-UTRs) of mouse A(2A)R mRNA were identified by rapid amplification of 5' cDNA end (5' RACE), RT-PCR analysis and genome sequence analyses. Similar bioinformatics analysis also suggested six variants of the non-coding "exon 1" (h1A, h1B, h1C, h1D, h1E and h1F) in the human A(2A)R gene, which were confirmed by RT-PCR analysis, while three of the human exon 1 variants (h1D, h1E and h1F) were likewise verified by 5' oligonucleotide capping analysis suggesting multiple transcription start sites. Importantly, RT-PCR and quantitative PCR analysis demonstrated that the A(2A)R transcripts with different exon 1 variants displayed tissue-specific expression patterns. For instance, the mouse exon m1A mRNA was detected only in brain (specifically striatum) and the human exon h1D mRNA in lymphoreticular system. Furthermore, the determination of the three new transcription start sites of human A(2A)R gene by 5' oligonucleotide capping and bioinformatics analyses led to the identification of three corresponding promoter regions which contain several important cis elements, providing additional target for further molecular dissection of A(2A)R gene expression. Finally, our analysis indicates that A(2A)R mRNA and a novel transcript partially overlapping with the 3' exon h3, but in opposite orientation to the A(2A)R gene, could conceivably form duplexes to mutually regulate transcript expression. Thus, combined molecular and bioinformatics analyses revealed a new A(2A)R genomic structure, with conserved coding exons 2 and 3 and divergent, tissue-specific exon 1 variants encoding for 5'-UTR. This raises the possibility of generating multiple tissue-specific A(2A)R mRNA species by alternative promoters with varying regulatory susceptibility.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

Identification of candidate genes for psychosis in rat models, and possible association between schizophrenia and the 14-3-3eta gene

Although the genetic contribution to schizophrenia is substantial, positive findings in whole-genome linkage scans have not been consistently replicated. We analyzed gene expression in various rat conditions to identify novel candidate genes for schizophrenia. Suppression subtraction hybridization (SSH), with polyA mRNA from temporal and frontal cortex of rats, was used to identify differentially expressed genes. Expression of mRNA was compared between adult Lewis and Fischer 344 (F344) rats, adult and postnatal day 6 (d6) F344, and adult F344 treated with haloperidol or control vehicle. These groups were chosen because each highlights a particular aspect of schizophrenia: differences in strain vulnerability to behavioral analogs of psychosis; factors that may relate to disease onset in relation to CNS development; and improvement of symptoms by haloperidol. The 14-3-3 gene family, as represented by 14-3-3gamma and 14-3-3zeta isoforms in the SSH study, and SNAP-25 were among the candidate genes. Genetic association between schizophrenia and the 14-3-3eta gene, positioned close to a genomic locus implicated in schizophrenia, and SNAP-25 genes was analyzed in 168 schizophrenia probands and their families. These findings address three different genes in the 14-3-3 family. We find a significant association with schizophrenia for two polymorphisms in the 14-3-3eta gene: a 7 bp variable number of tandem repeats in the 5' noncoding region (P=0.036, 1 df), and a 3' untranslated region SNP (753G/A) that is an RFLP visualized with Ava II (P=0.028). There was no significant genetic association with SNAP-25. The candidate genes identified may be of functional importance in the etiology, pathophysiology or treatment response of schizophrenia or psychotic symptoms. This is to our knowledge the first report of a significant association between the 14-3-3eta-chain gene and schizophrenia in a family-based sample, strengthening prior association reports in case-control studies and microarray gene expression studies.

Genetic association analysis of behavioral inhibition using candidate loci from mouse models

Genes influence the development of anxiety disorders, but the specific loci involved are not known. Genetic association studies of anxiety disorders are complicated by the complexity of the phenotypes and the difficulty in identifying appropriate candidate loci. We have begun to examine the genetics of behavioral inhibition to the unfamiliar (BI), a heritable temperamental predisposition that is a developmental and familial risk factor for panic and phobic disorders. Specific loci associated with homologous phenotypes in mouse models provide compelling candidate genes for human BI. We conducted family-based association analyses of BI using four genes derived from genetic studies of mouse models with features of behavioral inhibition. The sample included families of 72 children classified as inhibited by structured behavioral assessments. We observed modest evidence of association (P = 0.05) between BI and the glutamic acid decarboxylase gene (65 kDA isoform), which encodes an enzyme involved in GABA synthesis. No significant evidence of association was observed for the genes encoding the adenosine A(1A) receptor, the adenosine A(2A) receptor, or preproenkephalin. This study illustrates the potential utility of using candidate genes derived from mouse models to dissect the genetic basis of BI, a possible intermediate phenotype for panic and phobic disorders.

No evidence for linkage by transmission disequilibrium test analysis of microsatellite marker D22S278 and schizophrenia in a Palestinian Arab and in a German population

Linkage for a schizophrenia susceptibility locus on chromosome region 22q12-q13 was initially suggested by independent studies from two groups and confirmed in a combined analysis of data for the microsatellite marker D22S278 in multiply affected schizophrenic families derived from 11 independent research groups worldwide. In addition to these reports of linkage to schizophrenia on chromosome 22, bipolar disorder has also been linked to markers in this chromosomal region. We now report results from an analysis of 223 Palestinian Arab trios from three different centers in Israel and Palestine using the allele-wise extended transmission disequilibrium test for multiallelic markers. No evidence for linkage is observed in the entire group or in any of the three centers (entire group: chi-square = 5.59, P = 0.78, df = 9; Afula: chi-square = 6.51, P = 0.48, df = 7; Bethlehem: chi-square = 14.11, P = 0.12, df = 9; Beersheva: chi-square = 7.04, P = 0.32, df = 6). Additionally, we examined D22S278 in a group of 114 schizophrenic German triads and failed to observe evidence for linkage (chi-square = 8.13, P = 0.42, df = 8df).

Localization of genes modulating the predisposition to schizophrenia: a revision

The genetics of schizophrenia or bipolar affective disorder has advanced greatly at the molecular level since the introduction of probes for the localization of specific genes. Research on gene candidates for susceptibility to schizophrenia can broadly be divided into two types, i.e., linkage studies, where a gene is found near a specific DNA marker on a specific chromosome, and association studies, when a condition is associated with a specific allele of a specific gene. This review covers a decade of publications in this area, from the 1988 works of Bassett et al. and Sherrington et al. on a gene localized on the long arm of chromosome 5 at the 5q11-13 loci, to the 1997 work of Lin et al. pointing to the 13q14.1-q32 loci of chromosome 13 and to the 1998 work of Wright et al. on an HLA DRB1 gene locus on chromosome 6 at 6p21-3. The most replicated loci were those in the long arm of chromosome 22 (22q12-q13.1) and on the short arm of chromosome 6 (6p24-22). In this critical review of the molecular genetic studies involved in the localization of genes which modulate the predisposition to schizophrenia the high variability in the results obtained by different workers suggests that multiple loci are involved in the predisposition to this illness.

Schizophrenia: a purinergic hypothesis

Knowledge of the physiological roles of the purinergic system and its influence on other neurotransmitter systems has greatly advanced. In this article, a purinergic model is proposed as an attempt to integrate several findings in schizophrenia. According to this hypothesis, a purinergic system dysfunction would mainly result in reduced adenosinergic activity. This model also addresses the systemic aspects of schizophrenia, based on peripheral roles of purines, such as modulation of the immune system.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Adenosine A2A receptor interactions with receptors for other neurotransmitters and neuromodulators

Adenosine, by activating adenosine A2A receptors, seems to have a crucial function in regulating the activation of multiple receptors that affect neurotransmitter release and/or synaptic transmission, in particular receptors for neuropeptides (calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP)), and NMDA receptors, metabotropic glutamate receptors, nicotinic autofacilitatory receptors, dopamine receptors and adenosine A1 receptors. The manner in which these A2A receptors are involved in interactions with the receptors for other neurotransmitters and or neuromodulators opens novel avenues for the action of this 'omnipresent' nucleoside. Either by direct receptor-receptor modulation or by post-receptor mechanisms, adenosine, in its 'obsession' to protect cells from insults, uses as many receptor systems as possible to synchronize synaptic transmission, in order to exert what seems to be the 'destiny' of this nucleoside--protection of the nervous system.

The human calcium-independent phospholipase A2 gene multiple enzymes with distinct properties from a single gene

Recently, we reported the human 88-kDa calcium-independent phospholipase A2 (iPLA2) cDNA sequence, as well as extensive alternative splicing of the iPLA2 mRNA. In this report we identified the gene coding for iPLA2, which was localized on chromosome 22q13.1. The gene consists of at least 17 exons spanning > 69 kb. Based on the iPLA2 gene organization the splice variants can be explained. The putative promotor for the iPLA2 gene lacks a TATA-box and contains a CpG island as well as several potential Sp-1-binding sites. Furthermore, the 5'-flanking region also contains one medium reiteration frequency repeat (MER53) and an Alu repetitive sequence. Northern blot analysis of iPLA2 mRNA in various human tissues demonstrated tissue-specific expression of four distinct iPLA2 transcripts. The native human 3.2-kb iPLA2 transcript was predominantly expressed in heart, brain, skeletal muscle, prostate, testis, thyroid and spinal cord, and to a lesser extent in peripheral blood leucocytes, stomach, trachea and bone marrow. Studies on the subcellular localization of the native iPLA2 protein were performed in COS-7 cells overexpressing this enzyme. The cytosolic fraction of untransfected and cells overexpressing iPLA2 contained equal amounts of calcium-independent PLA2 activity. However, the membrane fraction displayed a 5.5-fold increased activity in iPLA2 overexpressing cells. This increased calcium-independent PLA2 activity correlated with the presence of iPLA2 immunoreactive protein in the membrane fraction, indicating that this form of iPLA2 protein was membrane associated. Studies of iPLA2 in rat vascular smooth muscle cells verified the membrane association of this form of iPLA2. The major difference between this form of iPLA2 enzyme and the soluble forms of iPLA2 studied previously is the presence of 54 additional amino acid residues derived from exon 9. We suggest that the addition of these 54 amino acids leads to a membrane-associated protein. In summary, these results demonstrate that alternative splicing of the human iPLA2 transcript generates multiple iPLA2 isoforms with distinct tissue distribution and cellular localization.

Indirect modulation of dopamine D2 receptors as potential pharmacotherapy for schizophrenia: I. Adenosine agonists

OBJECTIVE

To review preclinical and clinical information related to pharmacologic modulation of dopamine D2 receptors as potential novel antipsychotic therapy. Specifically, to summarize the data that suggest a modulatory action of adenosine A2A receptors on dopamine D2 receptors and, therefore, a possible rational role of adenosine A2A agonists as novel antipsychotic agents.

DATA SOURCES

Primary and review articles were identified by MEDLINE search (from 1966 to May 1998) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All of the articles identified from the data sources were evaluated and all information deemed relevant was included in this review.

DATA SYNTHESIS

For all of the older and many of the newer antipsychotic agents, there is a strong correlation between clinical antipsychotic activity and affinity for dopamine D2 receptors. Unfortunately, dopamine D2 receptors are believed to also be involved in the adverse effect profile of these agents. The indirect modulation of dopamine D2 receptors, rather than direct block, might produce antipsychotic effects without the usual adverse reactions. Several lines of evidence from animal studies suggest that the use of selective A2A agonists might represent a novel approach to the treatment of psychoses.

CONCLUSIONS

Dopamine receptor modulation might represent a novel antipsychotic approach or adjunct therapy. The data regarding adenosine agonists (particularly selective A2A receptor agonists) are inconclusive at the present time. Direct clinical demonstration of effectiveness is required.

Human adenosine A2a receptor (A2aAR) gene: systematic mutation screening in patients with schizophrenia

Several lines of evidence suggest an involvement of adenosine A2a receptor (A2aAR) mediated adenosinergic neuromodulation in the etiopathogenesis of schizophrenia. We therefore performed a systematic mutation scan of the complete coding region of the human A2aAR gene in a sample of 42 schizophrenic patients. We detected one rare naturally occurring receptor variant (Gly-340-Ser) and two silent mutations (405C/T and 1083C/T). To our knowledge the Gly-340-Ser substitution is the first naturally occurring molecular variant of the A2aAR identified. Determining the frequency of the three variants in 42 unrelated healthy controls, we observed a significant trend towards an overrepresentation of the 1083T variant in patients when compared to controls (p = 0.041). This trend was followed up in a large independent replication sample. However, we were not able to confirm the original trend in the second sample (p = 0.367). The Ser-340 variant was found in a single schizophrenic individual. Investigation of the patient's family revealed independent segregation between the Ser-340 variant and psychiatric illness. Our data suggest that genetically determined structural variation of the A2aAR does not play a major role in the development of schizophrenia.