Molecular genetic models related to schizophrenia and psychotic illness: heuristics and challenges

Translational relevance of forward genetic screens in animal models for the study of psychiatric disease

Psychiatric disorders represent a significant burden in our societies. Despite the convincing evidence pointing at gene and gene-environment interaction contributions, the role of genetics in the etiology of psychiatric disease is still poorly understood. Forward genetic screens in animal models have helped elucidate causal links. Here we discuss the application of mutagenesis-based forward genetic approaches in common animal model species: two invertebrates, nematodes (Caenorhabditis elegans) and fruit flies (Drosophila sp.); and two vertebrates, zebrafish (Danio rerio) and mice (Mus musculus), in relation to psychiatric disease. We also discuss the use of large scale genomic studies in human populations. Despite the advances using data from human populations, animal models coupled with next-generation sequencing strategies are still needed. Although with its own limitations, zebrafish possess characteristics that make them especially well-suited to forward genetic studies exploring the etiology of psychiatric disorders.

Animal models of depression: molecular perspectives

Much of the current understanding about the pathogenesis of altered mood, impaired concentration and neurovegetative symptoms in major depression has come from animal models. However, because of the unique and complex features of human depression, the generation of valid and insightful depression models has been less straightforward than modeling other disabling diseases like cancer or autoimmune conditions. Today's popular depression models creatively merge ethologically valid behavioral assays with the latest technological advances in molecular biology and automated video-tracking. This chapter reviews depression assays involving acute stress (e.g., forced swim test), models consisting of prolonged physical or social stress (e.g., social defeat), models of secondary depression, genetic models, and experiments designed to elucidate the mechanisms of antidepressant action. These paradigms are critically evaluated in relation to their ease, validity and replicability, the molecular insights that they have provided, and their capacity to offer the next generation of therapeutics for depression.

Effects of antipsychotics on dentate gyrus stem cell proliferation and survival in animal models: a critical update

Schizophrenia is a complex psychiatric disorder. Although a number of different hypotheses have been developed to explain its aetiopathogenesis, we are far from understanding it. There is clinical and experimental evidence indicating that neurodevelopmental factors play a major role. Disturbances in neurodevelopment might result in alterations of neuroanatomy and neurochemistry, leading to the typical symptoms observed in schizophrenia. The present paper will critically address the neurodevelopmental models underlying schizophrenia by discussing the effects of typical and atypical antipsychotics in animal models. We will specifically discuss the vitamin D deficiency model, the poly I:C model, the ketamine model, and the postnatal ventral hippocampal lesion model, all of which reflect core neurodevelopmental issues underlying schizophrenia onset.

New insights into behaviour using mouse ENU mutagenesis

Identifying genes involved in behavioural disorders in man is a challenge as the cause is often multigenic and the phenotype is modulated by environmental cues. Mouse mutants are a valuable tool for identifying novel pathways underlying specific neurological phenotypes and exploring the influence both genetic and non-genetic factors. Many human variants causing behavioural disorders are not gene deletions but changes in levels of expression or activity of a gene product; consequently, large-scale mouse ENU mutagenesis has the advantage over the study of null mutants in that it generates a range of point mutations that frequently mirror the subtlety and heterogeneity of human genetic lesions. ENU mutants have provided novel and clinically relevant functional information on genes that influence many aspects of mammalian behaviour, from neuropsychiatric endophenotypes to circadian rhythms. This review will highlight some of the most important findings that have been made using this method in several key areas of neurological disease research.

Phenotypic effects of repeated psychosocial stress during adolescence in mice mutant for the schizophrenia risk gene neuregulin-1: a putative model of gene × environment interaction

There is a paucity of animal models by which the contributions of environmental and genetic factors to the pathobiology of psychosis can be investigated. This study examined the individual and combined effects of chronic social stress during adolescence and deletion of the schizophrenia risk gene neuregulin-1 (NRG1) on adult mouse phenotype. Mice were exposed to repeated social defeat stress during adolescence and assessed for exploratory behaviour, working memory, sucrose preference, social behaviour and prepulse inhibition in adulthood. Thereafter, in vitro cytokine responses to mitogen stimulation and corticosterone inhibition were assayed in spleen cells, with measurement of cytokine and brain-derived neurotrophic factor (BDNF) mRNA in frontal cortex, hippocampus and striatum. NRG1 mutants exhibited hyperactivity, decreased anxiety, impaired sensorimotor gating and reduced preference for social novelty. The effects of stress on exploratory/anxiety-related parameters, spatial working memory, sucrose preference and basal cytokine levels were modified by NRG1 deletion. Stress also exerted varied effect on spleen cytokine response to concanavalin A and brain cytokine and BDNF mRNA expression in NRG1 mutants. The experience of psychosocial stress during adolescence may trigger further pathobiological features that contribute to the development of schizophrenia, particularly in those with underlying NRG1 gene abnormalities. This model elaborates the importance of gene × environment interactions in the etiology of schizophrenia.

Disruption of exploratory and habituation behavior in mice with mutation of DISC1: an ethologically based analysis

Disrupted-in-schizophrenia-1 (DISC1) is a gene that has been functionally linked with neurodevelopmental processes and structural plasticity in the brain. Clinical genetic investigations have implicated DISC1 as a genetic risk factor for schizophrenia and related psychoses. Studies using mutant mouse models of DISC1 gene function have demonstrated schizophrenia-related anatomical and behavioral endophenotypes. In the present study, ethologically based assessment of exploratory and habituation behavior in the open field was conducted in DISC1 (L100P), wild-type (WT), heterozygous (HET), and homozygous (HOM) mutant mice of both sexes. Ethological assessment was conducted in an open-field environment to explore specific topographies of murine exploratory behavior across the extended course of interaction from initial exploration through subsequent habituation (the ethogram). During initial exploration, HET and HOM DISC1 mutants evidenced increased levels of locomotion and rearing to wall compared with WT. A HOM-specific increase in total rearing and a HET-specific increase in sifting behavior and reduction in rearing seated were also observed. Over subsequent habituation, locomotion, sniffing, total rearing, rearing to wall, rearing free, and rearing seated were increased in HET and HOM mutants vs. WT. Overall, grooming was increased in HOM relative to other genotypes. HET mice displayed a selective decrease in habituation of sifting behavior. These data demonstrate impairment in both initial exploratory and habituation of exploration in a novel environment in mice with mutation of DISC1. This is discussed in the context of the functional role of the gene vis à vis a schizophrenia phenotype as well as the value of ethologically based approaches to behavioral phenotyping.

Mutant mouse models in evaluating novel approaches to antipsychotic treatment

In this review we consider the application of mutant mouse phenotypes to the study of psychotic illness in general and schizophrenia in particular, as they relate to behavioral, psychopharmacological, and cellular phenotypes of putative import for antipsychotic drug development. Mutant models appear to be heuristic at two main levels; firstly, by indicating the functional roles of neuronal components thought to be of relevance to the putative pathobiology of psychotic illness, they help resolve overt behavioral and underlying cellular processes regulated by those neuronal components; secondly, by indicating the functional roles of genes associated with risk for psychotic illness, they help resolve overt behavioral and underlying cellular processes regulated by those risk genes. We focus initially on models of dopaminergic and glutamatergic dysfunction. Then, we consider advances in the genetics of schizophrenia and mutant models relating to replicable risk genes. Lastly, we extend this discussion by exemplifying two new variant approaches in mutant mice that may serve as prototypes for advancing antipsychotic drug development. There is continuing need not only to address numerous technical challenges but also to develop more "real-world" paradigms that reflect the milieu of gene × environment and gene × gene interactions that characterize psychotic illness and its response to antipsychotic drugs.

Genetic models of sensorimotor gating: relevance to neuropsychiatric disorders

Sensorimotor gating, or the ability of a sensory event to suppress a motor response, can be measured operationally via prepulse inhibition (PPI) of the startle response. PPI is deficient in schizophrenia patients as well as other neuropsychiatric disorders, can be measured across species, and has been used widely as a translational tool in preclinical neuropharmacological and genetic research. First developed to assess drug effects in pharmacological and developmental models, PPI has become one of the standard behavioral measures in genetic models of schizophrenia and other neuropsychiatric disorders that exhibit PPI deficits. In this chapter we review the literature on genetic models of sensorimotor gating and discuss the utility of PPI as a tool in phenotyping mutant mouse models. We highlight the approaches to genetic mouse models of neuropsychiatric disease, discuss some of the important caveats to these approaches, and provide a comprehensive table covering the more recent genetic models that have evaluated PPI.

When the serotonin transporter gene meets adversity: the contribution of animal models to understanding epigenetic mechanisms in affective disorders and resilience

Although converging epidemiological evidence links exposure to stressful life events with increased risk for affective spectrum disorders, there is extraordinary interindividual variability in vulnerability to adversity. The environmentally moderated penetrance of genetic variation is thought to play a major role in determining who will either develop disease or remain resilient. Research on genetic factors in the aetiology of disorders of emotion regulation has, nevertheless, been complicated by a mysterious discrepancy between high heritability estimates and a scarcity of replicable gene-disorder associations. One explanation for this incongruity is that at least some specific gene effects are conditional on environmental cues, i.e. gene-by-environment interaction (G × E) is present. For example, a remarkable number of studies reported an association of variation in the human serotonin (5-HT) transporter gene (SLC6A4, 5-HTT, SERT) with emotional and cognitive traits as well as increased risk for depression in interaction with psychosocial adversity. The results from investigations in non-human primate and mouse support the occurrence of G × E interaction by showing that variation of 5-HTT function is associated with a vulnerability to adversity across the lifespan leading to unfavourable outcomes resembling various neuropsychiatric disorders. The neural and molecular mechanisms by which environmental adversity in early life increases disease risk in adulthood are not known but may include epigenetic programming of gene expression during development. Epigenetic mechanisms, such as DNA methylation and chromatin modification, are dynamic and reversible and may also provide targets for intervention strategies (see Bountra et al., Curr Top Behav Neurosci, 2011). Animal models amenable to genetic manipulation are useful in the identification of molecular mechanisms underlying epigenetic programming by adverse environments and individual differences in resilience to stress. Therefore, deeper insight into the role of epigenetic regulation in the process of neurodevelopmental programmes is likely to result in early diagnosis of affective spectrum disorders and will contribute to the design of innovative treatments targeting neural pathways that foster resilience.

Mouse mutagenesis and disease models for neuropsychiatric disorders

In this chapter, mutant mouse resources which have been developed by classical genetics as well as by modern large-scale mutagenesis projects are summarized. Various spontaneous and induced mouse mutations have been archived since the rediscovery of Mendel's genetics in 1900. Moreover, genome-wide, large-scale mutagenesis efforts have recently been expanding the available mutant mouse resources. Forward genetics projects using ENU mutagenesis in the mouse were started in the mid-1990s. The widespread adoption of reverse genetics, using knockouts and conditional mutagenesis based on gene-targeting technology, followed. ENU mutagenesis has now evolved to provide a further resource for reverse genetics, with multiple point mutations in a single gene and this new approach is described. Researchers now have various options to obtain mutant mice: point mutations, transgenic mouse strains, and constitutional or conditional knockout mice. The established mutant strains have already contributed to modeling human diseases by elucidating the underlying molecular mechanisms as well as by providing preclinical applications. Examples of mutant mice, focusing on neurological and behavioral models for human diseases, are reviewed. Human diseases caused by a single gene or a small number of major genes have been well modeled by corresponding mutant mice. Current evidence suggests that quantitative traits based on polygenes are likely to be associated with a range of psychiatric diseases, and these are now coming within the range of modeling by mouse mutagenesis.

Drosophila as a model organism for the study of neuropsychiatric disorders

The fruitfly Drosophila offers a model system in which powerful genetic tools can be applied to understanding the neurobiological bases of a range of complex behaviors. The Drosophila and human lineages diverged several hundred million years ago, and despite their obvious differences, flies and humans share many fundamental cellular and neurobiological processes. The similarities include fundamental mechanisms of neuronal signaling, a conserved underlying brain architecture and the main classes of neurotransmitter system. Drosophila also have a sophisticated behavioral repertoire that includes extensive abilities to adapt to experience and other circumstances, and is therefore susceptible to the same kinds of insults that can cause neuropsychiatric disorders in humans. Given the different physiologies, lifestyles, and cognitive abilities of flies and humans, many higher order behavioral features of the human disorders cannot be modeled readily in flies. However, an increasing understanding of the genetics of human neuropsychiatric disorders is suggesting parallels with underlying neurobiological mechanisms in flies, thus providing important insights into the possible mechanisms of these poorly understood disorders.