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ICOSR 2009—Interneurons Take the Stage

As part of our ongoing coverage of the 2009 International Congress on Schizophrenia Research (ICOSR), 28 March to 1 April 2009, in San Diego, California, we bring you session summaries from some of the Young Investigator travel award winners. We are grateful for this summary by Julie Markham of the University of Maryland.


21 April 2009. On Monday, 30 March, Patricio O’Donnell of the University of Maryland, Baltimore, chaired an afternoon oral session on "Altered cortical interneurons in schizophrenia: from animal models to human studies." [Editor's note: See the recently updated SRF Animal Models compendium, maintained by Markham and her colleagues in Jim Koenig's group, for summaries of many of the animal models of schizophrenia covered in these presentations.]

Michel Cuénod of the University Hospital of Lausanne, Switzerland, delivered the presentation, "Early glutathione deficit impairs parvalbumin expression in GABA interneurons and kainite-induced γ oscillations," as scheduled speaker Jan Cabungcal was unable to join us in San Diego. Cuénod spent the first half of the talk reviewing some of the evidence behind the hypothesis that oxidative stress due to dysfunctional glutathione (GSH) metabolism contributes to the pathophysiology of schizophrenia, a hypothesis that drives the research group headed by his wife and colleague, Kim Q. Do. A reduction in GSH leads to an increase in reactive oxygen species, and has been observed in both the CSF and postmortem tissue (PFC and striatum) from patients with schizophrenia. Although the findings were not presented in the talk, Do and colleagues have found the gene for the key GSH synthesizing enzyme subunit (GCLM, or glutamate cysteine ligase modifier) to be associated with schizophrenia in two case-control and one family study (see SRF related news story). Also, a number of environmental factors linked to schizophrenia, such as obstetric complications, malnutrition, psychosocial stress, and infections, are known to increase oxidative stress.

The latter portion of the talk was devoted to presenting the lab’s findings in their animal preparation, the GCLM knockout mouse. In the anterior cingulate cortex (often termed the rodent mPFC) of the KO mice, they have found an expected increase in oxidative stress and, interestingly, a reduction in MBP (myelin basic protein, a key component of myelin in the nervous system) at postnatal day 20. A reduction in parvalbumin (PV) positive neurons was observed at postnatal day 10 but was returned to normal by day 20. However, if dopamine was elevated postnatally (by delivery of the DA reuptake inhibitor GBR12909) in these animals, the reduction in PV+ neurons was still present at the day 20 time point. Finally, the perineuronal net, which surrounds PV neurons and modulates their connectivity, was reportedly completely absent in the KO mouse cortex.

In the ventral hippocampus, the GCLM KO mouse showed oxidative stress in the hilus and region CA2/3—the same regions that showed a reduction in PV+ cells—but neither finding was present in area CA1. No evidence for either oxidative stress or alterations in PV+ cells was found anywhere in the dorsal hippocampus. Similarly, impairment of kainite-induced γ oscillations was found in ventral, but not dorsal, area CA3.The pattern of changes might be linked to findings that the KO mice appear to show deficits on tasks that rely more heavily on the ventral hippocampus compared to the dorsal.

These findings support the hypothesis that oxidative stress due to impaired GSH metabolism may contribute to the pathophysiology of interneurons similar to that observed in schizophrenia.

The second speaker, Margarita Behrens, began her talk, "Interleukin-6 mediates the increase in NADPH-oxidase in the ketamine model of schizophrenia," by announcing her recent move to the Computational Neurobiology Laboratory at the Salk Institute. The approach Behrens used to generate the findings covered in this talk was to obtain primary cultures from the prelimbic region of the adult mouse mPFC and expose the cells to ketamine, an NMDA receptor antagonist, at various time points. Ketamine exposure is often used to study schizophrenia because it can mimic many aspects of the illness in both humans and animals. As they do in vivo, parvalbumin (PV) interneurons mature by week 3 in this preparation. After a single exposure to ketamine, PV neurons recover (show normal expression levels of the GABA synthesizing enzyme GAD67) after ~24 hours. However, if ketamine is administered twice, the observed reduction in GAD67 takes a much longer 10-12 days to recover to pre-exposure levels. In slices of the rat PFC obtained in collaboration with John Lisman’s group at Brandeis University, in which ketamine treatment resulted in a reduction in mini-IPSC amplitude and frequency and increased pyramidal neuron firing frequency (Zhang et al., 2008), Behrens demonstrated a concomitant reduction in GAD67. This study was important because, although ketamine exposure had previously been shown to cause a reduction in GAD67, direct evidence linking this to functional inhibition had previously been lacking.

In another study, Behrens found that simultaneous delivery of muscimol (a GABAA agonist) with ketamine prevented the reduction of GAD67.. Similarly, a ketamine-induced increase in neuronal superoxide production, mediated by activation of NADPH-oxidase, was prevented by concurrent muscimol administration. The next question Behrens set out to answer was, What is the mechanism that leads to NADPH-oxidase activation by ketamine? Her follow-up studies found that ketamine induced an increase in IL-6 (but not IL-1β or TNFα) mRNA expression in cultured neurons, and that blocking IL-6 (using anti-IL-6 blocking antibodies) prevented ketamine from reducing GAD67 expression in cultured neurons. This effect was also found to be absent in IL-6 deficient mice (Behrens et al., 2008).

This series of studies strengthens the link between antagonism of the NMDA receptor during cortical development and the pathology of inhibitory interneurons, and suggests that this is mediated through the inflammatory cytokine IL-6.

Jun Nomura, hailing from Mikhail Pletnikov’s lab at Hopkins, described the approach this group has developed to study gene by environment interactions at the cellular level ("A new cell model to study the mechanisms of gene-environment interactions relevant to schizophrenia"). Because DISC1 is a strong candidate gene for schizophrenia, the group had previously generated transgenic mice with inducible expression of the mutant human DISC1 (hDISC1) that is forebrain-specific (see SRF related news story). The findings presented in the talk were obtained from primary cortical cultures taken from E18.5 of DISC1 transgenic or control mice. On days 3 and 7 after plating, IL-6 was administered to the cultures, which were then harvested at day 10.

Compared to cells from normal animals, neurite extension was reduced in cells from DISC1 mutant mice, and short neurites appeared to be especially affected. In control cells IL-6 exposure increased the complexity of long neurites, whereas in mutant cells it increased the extension of short neurites. In cultured cells from control mice, IL-6 upregulated the expression of gp130 and mGluR3, but did not impact DISC1 expression. In contrast, in cultured cells from DISC1 mutant mice, IL-6 administration downregulated gp130 but did not affect expression of mGluR3. DISC1 expression was increased in cells from mutant mice following the first IL-6 administration on day 3, but was returned to baseline levels following the second administration on day 7.

The findings presented are the first from a promising cellular model that the Pletnikov lab is using to study the interaction of illness-relevant environmental factors with expression of a risk gene for schizophrenia.

In this talk, "Developmental vitamin D deficiency (DVD) and brain dopamine ontogeny," Darryl Eyles describes some of the work he has been conducting in John McGrath’s lab at the University of Queensland in Australia. The group’s hypothesis, advanced by Eyles in the talk, is that limiting pregnant mice to low vitamin D levels impacts adversely on brain development in their offspring, causing them to display behaviors relevant to schizophrenia. The rationale for the DVD model comes in part from the robust (but small effect size) link between winter birth and increased risk for both schizophrenia and bipolar disorder. In the DVD model, pregnant mice are fed a diet deficient in vitamin D beginning six weeks prior to conception until birth, at which time dams and their pups are fed a normal diet. Adult offspring show hyperlocomotion in response to amphetamine and MK-801, effects that are sensitive to antipsychotic drug treatment. The brains of DVD animals show altered developmental gene expression and increased lateral ventricle volume which persists into adulthood.

The specific question addressed by studies presented in the talk by Eyles was whether DVD affects development of the brain dopamine (DA) system. Colocalization of tyrosine hydroxylase (TH) with the vitamin D receptor (VDR) was not observed at embryonic day 12 but began around E15 in the developing mesencephalon. DA turnover was disrupted in the neonatal DVD rat brain; specifically, COMT expression was reduced without a change in MAOA expression (thus increasing the DOPAC/HVA ratio). In adult animals, DVD impacted males and females differently. Females showed a doubling of DAT (dopamine transporter) in the nucleus accumbens and striatum but not the PFC, whereas in males these measures were unaffected. Conversely, males but not females showed a reduction in TH+ neurons. Both sexes showed an increase in amphetamine-induced locomotion and, although the interaction between sex and treatment was not statistically significant, the effect appeared to be greater in females, which Eyles suggested might be relevant to the fact that only females showed increased DAT expression.

Thus, development of the brain DA system does appear to be impacted by DVD, which has been linked to risk for schizophrenia. Future work relating the sex-specific impact of this developmental manipulation to sex differences observed in schizophrenia would be especially interesting.

Ken Fish discussed some of his work on properties of inhibitory neurons, which is being conducted in Dave Lewis’s lab at the University of Pittsburgh. Fish began his talk, "Differential terminal expression of GAD67/GAD65—relevance to schizophrenia," with a review of alterations in the GABAergic system in schizophrenia, including the now much-replicated finding that GABA-related transcripts such as GAD67 are reduced in the PFC in schizophrenia. Although his studies are conducted in (healthy) monkey neocortex, the findings they generate can be informative regarding the relevance of dysfunction in GABA neurotransmission to psychiatric illness, he argued.

He began discussion of his work with a picture illustrating how the anatomical arrangement of inhibitory interneurons with pyramidal cells supports their functional inter-dependence. A “cartridge” of terminals, containing both GAT-1 and GAD67 and originating from a PV+ interneuron was shown to be neatly lined up with the axon initial segment (AIS) of a pyramidal neuron. Before presenting the findings of his double- and triple-label immunofluorescence studies, Fish took a few moments to describe the automated segmentation methodology used to quantify individual immunoreactive puncta, to which he has added a clever size-gating step (a description of this method can be found in Fish et al., 2008). He went on to demonstrate that although GAD67 and GAD65 puncta are colocalized in over 90 percent of terminals in cortical layers 3 and 4, only GAD67 is expressed in the cartridges on pyramidal neuron AISs. The new segmentation methodology made it possible to confirm earlier evidence that 21 terminals form each cartridge.

Fish went on to describe the GAD67/65 ratio in the three subtypes of cortical inhibitory interneurons—the PV+ chandelier neuron (PVch), the PV+ basket cell (PVb), and the CB1+ basket cell (CB1b). This ratio was higher in terminals of PVch relative to PVb terminals and was much lower in CB1 positive (relative to CB1 negative) cells. His studies indicate that the three cell types have distinct terminal ratios of GAD67 and GAD65 in the primate dorsolateral PFC. The punch line here is that, because PVch neurons have the highest ratio of GAD67 to 65, these cells stand to be most affected by the loss of GAD67 that occurs in schizophrenia. His plan now is to turn his attention and these newly developed quantification techniques to examining these ratios in postmortem tissue from individuals with schizophrenia.

Lynn Selemon presented findings from her lab at Yale, "A longitudinal study of the emergency of neuroanatomical abnormalities in non-human primates exposed to irradiation in utero: a neurodevelopmental model of schizophrenia," in place of Kristina Aldridge who was unable to join us in San Diego this week. In her studies, low-dose radiation is applied to the abdomen of a pregnant monkey during embryonic days 32-43 (within the first trimester), a timing which overlaps with neurogenesis of thalamic cells and of dopamine cell groups, but precedes neocortical neurogenesis. Previous findings in this model include an adult-onset reduction in performance of a delayed response task, reduced thalamic volume and reduced neuron number in the mediodorsal nucleus of the thalamus, reduced volume of cerebral white matter, and a trend for reduced cerebral gray matter volume (Selemon et al., 2005).

For the work presented in her talk, a cohort of monkeys, five from the experimental (irradiated) group and four from the control group (the mothers of which were sham anesthetized, handled, etc.) were followed from infancy through puberty (around two to three years) and adulthood (last time point examined, five years). There were approximately equal numbers of males and females in each group. Before presenting her findings, Selemon explained that results are not corrected for total brain size—because prenatal irradiation is known to cause a reduction in overall brain size, correcting for brain size would have the effect of masking all the group differences of interest. Body weight of prenatally irradiated animals was reduced at age six months but was not different from controls by adulthood. The timing of volumetric changes in response to prenatal irradiation was region-specific, with a reduction in cortical volume only detectable in adult animals, whereas a reduction in thalamic volume was observed just after puberty. While volume of the caudate nucleus was unaffected by prenatal irradiation, reduced volume of the putamen was observed across the time points examined (with the biggest difference between groups observed at age three years), and reduced volume of the nucleus accumbens was observed in adult animals. Across all measures, the degree of inter-subject variability was greater in the group that experienced prenatal irradiation as compared to the control subjects.

Selemon concluded from these findings that prenatal irradiation causes a reduction in volume of brain structures that is region- and age-specific, and that disruption of brain development during the early prenatal time period can result in post-pubertal onset of brain changes, which may be relevant to the post-pubertal onset of psychosis observed in many individuals with schizophrenia.

Paula Moran of the University of Nottingham presented her findings on the "Dissociable effects of D-amphetamine in latent inhibition and locomotor activity in D2 receptor knockout mice" (said mice obtained from John Waddington, Royal College of Surgeons in Ireland). She began by explaining the rationale for examining the effects of amphetamine in animal models relevant to schizophrenia—amphetamine induces psychosis in humans and psychosis-relevant behavioral effects in animals, and treatment with antipsychotic drugs (D2 receptor antagonists) reverses these effects in both humans and animals. Specifically, in rodents amphetamine induces hyperactivity and impairs prepulse inhibition (PPI). In contrast, D2 receptor antagonists induce hypoactivity and improved PPI. If amphetamine and a D2 antagonist are concurrently administered, the behavior of animals remains indistinguishable from controls.

Moran sought to extend these findings to a latent inhibition (LI) paradigm. As described in her recent paper (Bay-Richter et al., 2009), LI is “reduced learning to a stimulus that has previously been experienced without consequence.” In rats, LI is impaired by amphetamine and enhanced by antipsychotic drugs, but the relative contribution of action at the D1 and D2 receptors was unclear (antipsychotic drugs are not clean for receptor subtype). Therefore, LI was examined in D1 receptor KO (D1 KO) and D2 receptor KO (D2 KO) mice. In the paradigm employed by Moran, water-deprived mice were trained to drink from a sipper in a Skinner box. A foot shock and a tone were paired, and conditioned suppression of drinking was used as a measure of learning. In the “pre-exposed” group, animals were exposed to the tone prior to it being paired with the foot shock, and demonstrated the LI effect of reduced suppression of drinking (relative to animals who did not receive pre-exposure) in response to the tone even after it had been paired with the foot shock.

D2 KO mice were hypoactive but showed normal PPI; neither behavior showed sensitivity to amphetamine. LI was enhanced in D2 KO mice (i.e., they showed a higher suppression ratio), which supports what is known about the pharmacology of antipsychotic drugs. However, amphetamine disruption of LI was preserved in D2 KO mice, which was unexpected and suggests that amphetamine’s effects on locomotion and LI may be mediated through different mechanisms. In the recently published study from the group, D1 female but not male mice also showed an enhancement of LI, a sex-specific pattern that was not found in the locomotor behavioral measures. Collectively these findings suggest that the dopaminergic mechanism underlying LI potentiation may involve D2 receptors in males but both D1 and D2 receptors in females.

In her talk, "The differential effect of clozapine compared to other antipsychotic drugs on cortical and striatal EGF-ERK cell signaling: a novel antipsychotic drug mechanism?," Avril Pereira of the Mental Health Research Institute of Victoria in Australia began with a discussion of the intracellular signaling pathways that could potentially be involved in clozapine’s actions in the central nervous system. Diagrams outlining four mitogen-activated protein kinase (MAPK) signaling cascades were shown, and the rationale given for choosing to examine the extracellular signal-regulated kinase (ERK) cascade was its role in synaptic plasticity, which may be disrupted in psychosis. Covering findings included in her recently published work (Pereira et al., 2009), she presented data demonstrating that clozapine activates (phosphorylates) ERK via mitogen-activated protein kinase (MEK), and that epidermal growth factor (EGF) receptor inhibition prevents ERK activation by clozapine. She then presented the work she has done delineating the first evidence for the recruitment of EGF signaling by clozapine to affect ERK, and showing that antipsychotic drugs (in addition to clozapine, haloperidol, quietapine, and aripiprazole were examined) exert distinct region- and temporal-specific profiles of ERK stimulation.

Yael Piontkewitz, who traveled from Tel Aviv University, brought us a report of the work he’s been conducting in the Weiner lab in the prenatal immune activation rat model of schizophrenia ("Schizophrenia-like behavioral abnormalities following prenatal maternal immune system activation are prevented by pre-treatment with risperidone"). In this model, pregnant dams are injected with Poly I:C, which activates the immune system due to its virus-like properties (structurally, it resembles double-stranded RNA). The work of this group and others has previously shown that prenatal immune activation results in a post-pubertal emergence of disrupted prepulse inhibition (PPI), latent inhibition (LI), and reversal learning, as well as hyperlocomotion in response to amphetamine and NMDA receptor antagonists.

In the studies discussed in the talk, Poly I:C was administered to dams at embryonic day 15 and the offspring of these animals were compared with those of controls as adults. Behavioral tests were completed at postnatal day 90, followed by brain MRI scans at day 120. Half of the animals were treated with risperidone during the peri-adolescent time period (postnatal days 34-47). A low dose of the drug (0.045 ng/kg) successfully attenuated the disruption in LI observed in animals that experienced prenatal immune activation, and it raised the number of trials to criterion on the reversal learning task to that exhibited by controls (prenatal immune activation causes a reduction in this measure—faster reversal learning—presumably because initial learning was weaker and therefore more easily reversed). Adult animals exposed to prenatal maternal immune activation also showed a reduction in total hippocampal volume and an increase in the volume of the lateral ventricles—both changes were prevented by peri-adolescent treatment with risperidone (previous work in this model has shown a similar effect of clozapine).

These findings were nicely complemented by work presented by Neil Richtand (University of Cincinnati) on Sunday as part of symposium 1-5, in a talk entitled “Pharmacological intervention on a moving target: animal models with developmental relevance to schizophrenia.” In Richtand’s studies, a low dose of antipsychotic drug (multiple were tested) was delivered between postnatal days 35 and 70, and prevented the postpubertal emergence of hypersensitivity to amphetamine observed in animals that experienced prenatal immune activation but did not receive the treatment. Together, these findings present a strong case that the adverse effects of an insult that occurs very early in brain development can be prevented by peri-adolescent treatment with antipsychotic drugs, as opposed to treatment during adulthood which must rescue adverse consequences already in effect. While all very exciting, there is a cautionary aspect to this promising story, as a third group has shown that although peri-adolescent treatment with antipsychotic drugs normalizes the behavior of animals exposed to prenatal maternal immune activation, it exerts numerous deleterious effects on the normal development of control animals (Meyer et al., 2008). Therefore, while this work suggests a promising avenue for preventive pharmacotherapy of psychotic illness in high-risk individuals, it also indicates there could be a substantial cost to diagnostic false positives.

William O’Connor, of the University of Limerick, concluded the marathon session with his talk, "Neuroplasticity changes in animal models of schizophrenia as targets for innovative treatments." O'Conner uses dual probe microdialysis (in awake animals) in two rat preparations to elucidate prefrontal cortical (PFC) regulation of the ventral tegmental area (VTA) which may be relevant to brain dysfunction observed in schizophrenia. The first paradigm is maternal deprivation, in which the pups are separated from their mother for 24 hours on postnatal day 9, and the second is postweaning social isolation, in which pups are single housed for 55 days beginning at postnatal day 25. Both preparations result in animals that show disrupted prepulse inhibition (PPI). Animals housed alone after weaning also show enhanced locomotor sensitivity to amphetamine and (in males) increased aggressive behavior.

O’Connor began his talk with a discussion of the functional connectivity between the PFC and the VTA; the VTA sends dopaminergic (DA) fibers to the PFC and thereby regulates PFC blood flow, while the PFC sends reciprocal glutamatergic fibers back to the VTA. He then presented Arvid Carlsson’s model of PFC regulation (and, due to the reciprocal connectivity, ultimately self-regulation) of the VTA, which involves a “brake” on activation of the system via glutamatergic synapses onto GABAergic interneurons in the VTA and an “accelerator” on activation via glutamatergic synapses onto pyramidal neurons in the VTA (Carlsson et al., 2000). The model is attractive in that it can help to synthesize the DA, glutamate, and GABA hypotheses of schizophrenia (see SRF related news story).

Under baseline conditions, GABA levels in the VTA are reduced in socially isolated animals, and O’Connor’s interpretation is that these animals are lacking the “brake” part of the circuit, which may explain their abnormal PPI (both effects are reversed by treatment with clozapine). Meanwhile, glutamate at baseline is dramatically decreased in the PFC of these animals.

In response to delivery of a D2 agonist into the PFC, control animals showed a substantial reduction in glutamate and a modest reduction in GABA in the VTA, both of which were restored to normal following 10 days of treatment with clozapine. In socially isolated animals, D2 activation in the PFC did not change glutamate levels in the VTA but did dramatically increase GABA in the VTA. The VTA GABA response to PFC D2 activation was indistinguishable from controls following clozapine treatment. In animals that experienced early maternal deprivation, the responses of VTA GABA and glutamate to PFC D2 activation were normal. O’Connor concludes that, in socially isolated animals, D2 activation in the PFC switches the “brake” in the system off, and this is reversed by antipsychotic drug treatment.

At the conclusion of this talk, Sherry Leonard (University of Colorado-Denver) asked if O’Connor had examined the effect of nicotine on his dialysate measures, given that nicotine administration improves PPI deficits in both socially isolated and maternally deprived animal models. He had not, but suggested that it could exert its action by its known ability to increase glutamate levels in the PFC, which he found are dramatically reduced in socially isolated animals.—Julie Marhkam.

Comments on Related News


Related News: Inducing Schizophrenic Behavior? Researchers Roll Out New DISC1 Mouse

Comment by:  John RoderSteven Clapcote
Submitted 17 September 2007
Posted 17 September 2007

This is a useful model from Pletnikov, Ross, and colleagues, but like all models, it has some limitations. Since DISC1 is known to have a strong role in development and physiology, the development of inducible mutants is necessary to separate the two.

In the TeT-off system used in the paper, mice must be treated with doxycycline for their entire lives to keep the expression of this gene off. Doxycycline must be used at high levels and may have side effects when used this long. The TeT-on system is better because doxycycline is only used transiently for 1 week for maximum induction then washed away. The TeT-on system is also available for the same promoter used in the paper, that of the CaMKII gene.

The phenotype of reduced neurite length was obtained from in vitro neuron cultures, which are prone to artifacts. There are ways of labeling these neurons in vivo for measuring neurite length and spines. The brain phenotype was obtained by MRI. There are ways, such as adding manganese, of enhancing active pathways. This has been done in the bird brain to map song pathways.

The behavioral phenotype was similar to the recent paper from the Sawa group (Hikida et al., 2007) in that it also analyzed a transgenic mouse expressing the same C-terminal truncation of the human DISC1 gene, using the same CaMKII promoter. An important difference in the findings was a reduction of murine DISC1 (50 percent at protein level) in the Pletnikov et al. mice but none in the Sawa group mice. This issue is important because of a recent paper in Cell by the Song group (Duan et al., 2007). In that paper, RNAi was used to reduce wild-type native murine DISC1. Individual neurons with targeted DISC1 knockdown showed accelerated neurite development, greater synapse formation and enhanced excitability. Hippocampal granule cells showed accelerated morphological integration resulting in mispositioning. Unfortunately, in the Song paper they analyzed only cells with complete or no knockdown of DISC1. Partial knockdown vectors were made that achieved 75 percent reduction at the protein level but were not analyzed. Only then would it be possible to compare these morphological data with those from Pletnikov et al., which was a 50 percent reduction. Another difference was that the Song group found that DISC1 needed to interact with Nudel. Pletnikov et al. found normal levels of Nudel in the mice but lower LIS1, which could explain the brain development phenotype.

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Related News: Schizophrenia: a Case of Faulty Redox Detox?

Comment by:  Richard Deth
Submitted 25 November 2007
Posted 28 November 2007
  I recommend the Primary Papers

Identification of a limitation in the capacity for glutathione (GSH) synthesis by Gysin and colleagues raises several questions: "How could a redox problem (i.e., oxidative stress) lead to schizophrenia?” and "Does this finding mesh with other hypotheses?"

All cells must maintain sufficient levels of GSH to survive collateral damage from oxidative metabolism, and a number of adaptive mechanisms have evolved to meet this need. One important example is inhibition of the enzyme methionine synthase by oxidative stress. The higher the oxidative stress level, the greater the inhibition of its methylation of homocysteine to methionine, allowing the accumulating homocysteine to be diverted to GSH synthesis via the trans-sulfuration pathway. Homocysteine levels are elevated in schizophrenia, especially, but not exclusively, in first-episode males (Regland et al., 1995; Haidemenos et al., 2007), implying that methionine synthase is inhibited, perhaps by oxidative stress. Importantly, methionine synthase activity is also critical for dopamine-stimulated methylation of membrane phospholipids, a unique activity of the D4 dopamine receptor discovered by our lab in 1999 (Sharma et al., 1999). Thus, oxidative stress caused by impaired GSH synthesis will adversely affect this dopaminergic mechanism.

The physiological role of D4 receptor-mediated phospholipid methylation remains to be fully elucidated, but studies indicate a central role in attention and synchronization of neural networks, both of which are impaired in schizophrenia. The seven-repeat variant of the D4 receptor is considered to be the most important genetic risk factor for ADHD (Swanson et al., 2007), and is also associated with lower IQ, alone or in combination with dopamine transporter variants (Mill et al., 2006). MEG studies in subjects without ADHD showed stronger γ synchronized oscillatory activity during attention in subjects possessing the seven-repeat allele (Demiralp et al., 2007). We recently described a molecular mechanism by which dopamine-stimulated phospholipid methylation could tune neural networks to γ frequency during attention (Kuznetsova et al., 2007), and a restriction in GSH synthesis, as described by Gysin and colleagues, could contribute to impairments of synchronization and attention in schizophrenia.

Oxidative stress-induced inhibition of methionine synthesis also causes accumulation of S-adenosylhomocysteine, a general inhibitor of methylation reactions, affecting more than 150 cellular methylation reactions. Lower COMT activity would augment dopamine levels, while lower activity of DNA and histone methyltransferases would alter epigenetic regulation of gene expression. Thus, a putative role for oxidative stress can be integrated with other proposed theories. Indeed, methylation defects in schizophrenia have been recognized for more than 40 years (Spiro et al., 1965), a hypothesis advanced by Seymour Kety (Kety, 1972), including the replicable finding that methionine administration provokes acute psychosis in schizophrenia subjects but is without effect in normal individuals (Cohen et al., 1974). Despite these early clues, defective sulfur metabolism has received only limited attention. Perhaps the illuminating findings of Gysin and colleagues will reinvigorate interest and encourage schizophrenia researchers to invest the time needed to understand and appreciate this important area of biochemistry.

References:

Regland B, Johansson BV, Grenfeldt B, Hjelmgren LT, Medhus M. Homocysteinemia is a common feature of schizophrenia. J Neural Transm Gen Sect. 1995;100(2):165-9. Abstract

Haidemenos A, Kontis D, Gazi A, Kallai E, Allin M, Lucia B. Plasma homocysteine, folate and B12 in chronic schizophrenia. Prog Neuropsychopharmacol Biol Psychiatry. 2007 Aug 15;31(6):1289-96. Abstract

Sharma A, Kramer ML, Wick PF, Liu D, Chari S, Shim S, Tan W, Ouellette D, Nagata M, DuRand CJ, Kotb M, Deth RC. D4 dopamine receptor-mediated phospholipid methylation and its implications for mental illnesses such as schizophrenia. Mol Psychiatry. 1999 May;4(3):235-46. Abstract

Swanson JM, Kinsbourne M, Nigg J, Lanphear B, Stefanatos GA, Volkow N, Taylor E, Casey BJ, Castellanos FX, Wadhwa PD. Etiologic subtypes of attention-deficit/hyperactivity disorder: brain imaging, molecular genetic and environmental factors and the dopamine hypothesis. Neuropsychol Rev. 2007 Mar;17(1):39-59. Abstract

Mill J, Caspi A, Williams BS, Craig I, Taylor A, Polo-Tomas M, Berridge CW, Poulton R, Moffitt TE. Prediction of heterogeneity in intelligence and adult prognosis by genetic polymorphisms in the dopamine system among children with attention-deficit/hyperactivity disorder: evidence from 2 birth cohorts. Arch Gen Psychiatry. 2006 Apr;63(4):462-9. Abstract

Demiralp T, Herrmann CS, Erdal ME, Ergenoglu T, Keskin YH, Ergen M, Beydagi H. DRD4 and DAT1 polymorphisms modulate human gamma band responses. Cereb Cortex. 2007 May;17(5):1007-19. Abstract

Kuznetsova AY, Deth RC. A model for modulation of neuronal synchronization by D4 dopamine receptor-mediated phospholipid methylation. J Comput Neurosci. 2007 Oct 11; [Epub ahead of print] Abstract

Spiro HR, Schimke RN, Welch JP. Schizophrenia in a patient with a defect in methionine metabolism. J Nerv Ment Dis. 1965 Sep;141(3):285-90. Abstract

Kety SS. Toward hypotheses for a biochemical component in the vulnerability to schizophrenia. Semin Psychiatry. 1972 Aug;4(3):233-8. Abstract

Cohen SM, Nichols A, Wyatt R, Pollin W. The administration of methionine to chronic schizophrenic patients: a review of ten studies. Biol Psychiatry. 1974 Apr;8(2):209-25. Abstract

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