Kerin K. Higa

Reduced dopamine transporter functioning induces high-reward risk-preference consistent with bipolar disorder.

Individuals with bipolar disorder (BD) exhibit deleterious decision making, negatively impacting their lives. Such aberrant decision making can be quantified using the Iowa Gambling Task (IGT), which requires choosing between advantageous and disadvantageous options based on different reward/punishment schedules. The mechanisms underlying this behavioral deficit are unknown, but may include the reduced dopamine transporter (DAT) functioning reported in BD patients. Using both human and mouse IGTs, we tested whether reduced DAT functioning would recreate patterns of deficient decision making of BD patients. We assessed the IGT performance of 16 BD subjects (7 female) and 17 healthy control (HC) subjects (12 female). We recorded standard IGT performance measures and novel post-reward and post-punishment decision-making strategies. We characterized a novel single-session mouse IGT using C57BL/6J mice (n=44). The BD and HC IGT performances were compared with the effects of chronic (genetic knockdown (KD; n=31) and wild-type (n=28) mice) and acute (C57BL/6J mice (n=89) treated with the DAT inhibitor GBR12909) reductions of DAT functioning in mice performing this novel IGT. BD patients exhibited impaired decision making compared with HC subjects. Both the good-performing DAT KD and GBR12909-treated mice exhibited poor decision making in the mouse IGT. The deficit of each population was driven by high-reward sensitivity. The single-session mouse IGT measures dynamic risk-based decision making similar to humans. Chronic and acute reductions of DAT functioning in mice impaired decision-making consistent with poor IGT performance of BD patients. Hyperdopaminergia caused by reduced DAT may impact poor decision making in BD patients, which should be confirmed in future studies.

Isolation rearing effects on probabilistic learning and cognitive flexibility in rats

Isolation rearing is a neurodevelopmental manipulation that produces neurochemical, structural, and behavioral alterations in rodents that in many ways are consistent with schizophrenia. Symptoms induced by isolation rearing that mirror clinically relevant aspects of schizophrenia, such as cognitive deficits, open up the possibility of testing putative therapeutics in isolation-reared animals prior to clinical development. We investigated what effect isolation rearing would have on cognitive flexibility, a cognitive function characteristically disrupted in schizophrenia. For this purpose, we assessed cognitive flexibility using between- and within-session probabilistic reversal-learning tasks based on clinical tests. Isolation-reared rats required more sessions, though not more task trials, to acquire criterion performance in the reversal phase of the task, and were slower to adjust their task strategy after reward contingencies were switched. Isolation-reared rats also completed fewer trials and exhibited lower levels of overall activity in the probabilistic reversal-learning task than did the socially reared rats. This finding contrasted with the elevated levels of unconditioned investigatory activity and reduced levels of locomotor habituation that isolation-reared rats displayed in the behavioral pattern monitor. Finally, isolation-reared rats also exhibited sensorimotor gating deficits, reflected by decreased prepulse inhibition of the startle response, consistent with previous studies. We concluded that isolation rearing constitutes a valuable, noninvasive manipulation for modeling schizophrenia-like cognitive deficits and assessing putative therapeutics.

GlyT-1 Inhibition Attenuates Attentional But Not Learning or Motivational Deficits of the Sp4 Hypomorphic Mouse Model Relevant to Psychiatric Disorders

Serious mental illness occurs in 25% of the general population, with many disorders being neurodevelopmental, lifelong, and debilitating. The wide variation and overlap in symptoms across disorders increases the difficulty of research and treatment development. The NIMH Research Domain of Criteria initiative aims to improve our understanding of the molecular and behavioral consequences of specific neurodevelopmental mechanisms across disorders, enabling targeted treatment development. The transcription factor Specificity Protein 4 (SP4) is important for neurodevelopment and is genetically associated with both schizophrenia and bipolar disorder. Reduced Sp4 expression in mice (hypomorphic) reproduces several characteristics of psychiatric disorders. We further tested the utility of Sp4 hypomorphic mice as a model organism relevant to psychiatric disorders by assessing cognitive control plus effort and decision-making aspects of approach motivation using cross-species-relevant tests. Sp4 hypomorphic mice exhibited impaired attention as measured by the 5-Choice Continuous Performance Test, an effect that was attenuated by glycine type-1 transporter (GlyT-1) inhibition. Hypomorphic mice also exhibited reduced motivation to work for a reward and impaired probabilistic learning. These deficits may stem from affected anticipatory reward, analogous to anhedonia in patients with schizophrenia and other psychiatric disorders. Neither positive valence deficit was attenuated by GlyT-1 treatment, suggesting that these and the attentional deficits stem from different underlying mechanisms. Given the association of SP4 gene with schizophrenia and bipolar disorder, the present studies provide support that personalized GlyT-1 inhibition may treat attentional deficits in neuropsychiatric patients with low SP4 levels.

Inhibition of protein translation by the DISC1-Boymaw fusion gene from a Scottish family with major psychiatric disorders

The t(1; 11) translocation appears to be the causal genetic lesion with 70% penetrance for schizophrenia, major depression and other psychiatric disorders in a Scottish family. Molecular studies identified the disruption of the disrupted-in-schizophrenia 1 (DISC1) gene by chromosome translocation at chromosome 1q42. Our previous studies, however, revealed that the translocation also disrupted another gene, Boymaw (also termed DISC1FP1), on chromosome 11. After translocation, two fusion genes [the DISC1-Boymaw (DB7) and the Boymaw-DISC1 (BD13)] are generated between the DISC1 and Boymaw genes. In the present study, we report that expression of the DB7 fusion gene inhibits both intracellular NADH oxidoreductase activities and protein translation. We generated humanized DISC1-Boymaw mice with gene targeting to examine the in vivo functions of the fusion genes. Consistent with the in vitro studies on the DB7 fusion gene, protein translation activity is decreased in the hippocampus and in cultured primary neurons from the brains of the humanized mice. Expression of Gad67, Nmdar1 and Psd95 proteins are also reduced. The humanized mice display prolonged and increased responses to the NMDA receptor antagonist, ketamine, on various mouse genetic backgrounds. Abnormal information processing of acoustic startle and depressive-like behaviors are also observed. In addition, the humanized mice display abnormal erythropoiesis, which was reported to associate with depression in humans. Expression of the DB7 fusion gene may reduce protein translation to impair brain functions and thereby contribute to the pathogenesis of major psychiatric disorders.

Striatal dopamine D1 receptor suppression impairs reward-associative learning.

ABSTRACT Dopamine (DA) is required for reinforcement learning. Hence, disruptions in DA signaling may contribute to the learning deficits associated with psychiatric disorders. The DA D1 receptor (D1R) has been linked to learning and is a target for cognitive/motivational enhancement in patients with schizophrenia. Separating the striatal D1R contribution to learning vs. motivation, however, has been challenging. We suppressed striatal D1R expression in mice using a D1R‐targeting short hairpin RNA (shRNA), delivered locally to the striatum via an adeno‐associated virus (AAV). We then assessed reward‐ and punishment‐associative learning using a probabilistic learning task and motivation using a progressive‐ratio breakpoint procedure. We confirmed suppression of striatal D1Rs immunohistochemically and by testing locomotor activity after the administration of (+)‐doxanthrine, a full D1R agonist, in control mice and those treated with the D1RshRNA. D1RshRNA‐treated mice exhibited impaired reward‐associative learning, while punishment‐associative learning was spared. This deficit was unrelated to general learning impairments or amotivation, because the D1shRNA‐treated mice exhibited normal Barnes maze learning and normal motivation in the progressive‐ratio breakpoint procedure. Suppression of striatal D1Rs selectively impaired reward‐associative learning whereas punishment‐associative learning, aversion‐motivated learning, and appetitive motivation were spared. Because patients with schizophrenia exhibit similar reward‐associative learning deficits, D1R‐targeted treatments should be investigated to improve reward learning in these patients.

Boymaw, Overexpressed in Brains With Major Psychiatric Disorders, May Encode a Small Protein to Inhibit Mitochondrial Function and Protein Translation

The t(1,11) chromosome translocation co‐segregates with major psychiatric disorders in a large Scottish family. The translocation disrupts the DISC1and Boymaw (DISC1FP1) genes on chromosomes 1 and 11, respectively. After translocation, two fusion genes are generated. Our recent studies found that the DISC1‐Boymaw fusion protein is localized in mitochondria and inhibits oxidoreductase activity, rRNA expression, and protein translation. Mice carrying the DISC1‐Boymaw fusion genes display intermediate behavioral phenotypes related to major psychiatric disorders. Here, we report that the Boymaw gene may encode a small protein predominantly localized in mitochondria. The Boymaw protein inhibits oxidoreductase activity, rRNA expression, and protein translation in the same way as the DISC1‐Boymaw fusion protein. Interestingly, Boymaw expression is up‐regulated by different stressors at RNA and/or protein translational levels. In addition, we found that Boymaw RNA expression is significantly increased in the postmortem brains of patients with major psychiatric disorders. Our studies therefore suggest that the Boymaw gene could potentially be a susceptibility gene for major psychiatric disorders in both the Scottish t(1,11) family and the general population of patients.

Wet or dry: translatable “water mazes” for mice and humans

Although the cognitive and biological characteristics of Alzheimers disease (AD) are well known and mouse models of AD are available, current treatments for AD-related cognitive deficits have quite limited efficacy. The development of tasks with cross-species validity may enable better prediction of the efficacy of potential new treatments. In this issue of the JCI, Possin et al. present a virtual version of the Morris water maze (a common test of spatial learning and memory for rodents) that is designed for use with humans. The authors tested a mouse model of AD (transgenic mice expressing human amyloid precursor protein [hAPP]) and patients in the earlier mild cognitive impairment (MCI) stage of AD in their respective versions of the maze. Using novel statistical methods, they detected similar deficits across species, providing support for the hAPP model and use of the virtual water maze. Importantly, this work enabled recommendations for appropriate sample sizes when developing potential therapeutics for AD.

Restoration of Sp4 in forebrain GABAergic neurons rescues hypersensitivity to ketamine in Sp4 hypomorphic mice

Background: Ketamine produces schizophrenia-like behavioral phenotypes in healthy people. Prolonged ketamine effects and exacerbation of symptoms after the administration of ketamine have been observed in patients with schizophrenia. More recently, ketamine has been used as a potent antidepressant to treat patients with major depression. The genes and neurons that regulate behavioral responses to ketamine, however, remain poorly understood. Sp4 is a transcription factor for which gene expression is restricted to neuronal cells in the brain. Our previous studies demonstrated that Sp4 hypomorphic mice display several behavioral phenotypes relevant to psychiatric disorders, consistent with human SP4 gene associations with schizophrenia, bipolar disorder, and major depression. Among those behavioral phenotypes, hypersensitivity to ketamine-induced hyperlocomotion has been observed in Sp4 hypomorphic mice. Methods: In the present study, we used the Cre-LoxP system to restore Sp4 gene expression, specifically in either forebrain excitatory or GABAergic inhibitory neurons in Sp4 hypomorphic mice. Mouse behavioral phenotypes related to psychiatric disorders were examined in these distinct rescue mice. Results: Restoration of Sp4 in forebrain excitatory neurons did not rescue deficient sensorimotor gating nor ketamine-induced hyperlocomotion. Restoration of Sp4 in forebrain GABAergic neurons, however, rescued ketamine-induced hyperlocomotion, but did not rescue deficient sensorimotor gating. Conclusions: Our studies suggest that the Sp4 gene in forebrain GABAergic neurons regulates ketamine-induced hyperlocomotion.

Nicotine improves probabilistic reward learning in wildtype but not alpha7 nAChR null mutants, yet alpha7 nAChR agonists do not improve probabilistic learning

Cognitive impairments, e.g., reward learning, are present in various psychiatric disorders and warrant treatment. Improving reward-related learning could synergistically enhance psychosocial treatments and cognition generally. A critical first step is to understand the mechanisms underlying reward learning. The dopamine system has been implicated in such learning, but less known is how indirect activation of this system may affect reward learning. We determined the role of alpha7 nicotinic acetylcholine receptors (nAChR) on a probabilistic reversal learning task (PRLT) in mice that includes reward and punishment. Male alpha7 knockout (KO), heterozygous (HT), and wildtype (WT) littermate mice (n = 84) were treated with vehicle, 0.03, or 0.3 mg/kg nicotine. Two cohorts of C57BL/6NJ male mice were treated with various alpha7 nAChR ligands, including the full agonists PNU282877 and AR-R-17779, the positive allosteric modulator CCMI, the partial agonist SSR180711, and the antagonist methyllycaconitine. All mice were then tested in the PRLT. Nicotine (0.3 mg/kg) significantly improved initial reward learning in alpha7 WT and HT mice but did not improve learning in KO mice, suggesting an involvement of the alpha7 nAChR in the pro-learning effects of nicotine. Neither alpha7 nAChR treatments (PNU282987, AR-R-17779, CCMI, SSR180711, nor methyllycaconitine) affected mouse PRLT performance however. Nicotine improved reward learning via a mechanism that may include alpha7 nAChRs. This improvement unlikely relied solely on alpha7 nAChRs however, since no alpha7 nAChR ligand improved reward learning in normal mice. Future assessments of the effects of other nAChR subtypes on reward learning are needed.

A novel animal model for neuroinflammation and white matter degeneration

Small interference RNA has been widely used to suppress gene expression. Three different short hairpin RNAs (shRNAs) against dopamine D1 receptor (Drd1), driven by mouse U6 promoter in self-complementary AAV8 vector (scAAV8), were used to silence mouse striatal Drd1 expression. Transduction of mouse striatum with all three scAAV8-D1shRNA viruses, but not the control scAAV8 virus, causes extensive neuroinflammation, demyelination, and axon degeneration. RNA interference is known to be coupled to the innate immune system as a host cell defense against virus infection. Activation of the innate immune system may play a causal role in the development of neuroinflammation and white matter degeneration, providing a novel animal model for multiple sclerosis (MS) and other neuroinflammatory diseases.