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Gene knockout animal models of depression, anxiety and obsessive compulsive disorders

Scherma, Mariaa; Giunti, Elisaa; Fratta, Waltera; Fadda, Paolaa,,b,,c,,d

doi: 10.1097/YPG.0000000000000238
Review Articles
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In the past decades, the improving knowledge of genes implicated in the pathogenesis of psychiatric disorders together with the advancements in genetic engineering has led to the creation of mice in which one or more genes are inactivated or ‘knocked out’. Knockout mice are extensively used to better investigate the molecular and cellular mechanisms underlying these diseases as well as the biological role of specific genes. Moreover, they are also useful tools for developing new therapeutic strategies. The success of using knockout mice is possible due to availability of several models used to mimic some clinical manifestations reported in psychiatric patients. In the present review, we will give an update of the most used gene knockout models in the field of psychiatric disorders including depression, anxiety, and obsessive-compulsive disorder.

aDivision of Neuroscience and Clinical Pharmacology, Department of Biomedical Sciences, University of Cagliari, Cittadella Universitaria di Monserrato

bCentre of Excellence ‘Neurobiology of Addiction’, University of Cagliari

cInstitute of Neuroscience – Cagliari, National Research Council, Cagliari

dNational Neuroscience Institute, Torino, Italy

Received 21 June 2019 Accepted 31 July 2019

Correspondence to Paola Fadda, Department of Biomedical Sciences, Division of Neuroscience and Clinical Pharmacology, University of Cagliari, Cittadella Universitaria di Monserrato, 09042, Monserrato (Cagliari), Italy Tel: +39 070/675 4326; fax: 0039 070 675 4312; e-mail: pfadda@unica.it

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Introduction

Experimental animal models are of fundamental importance in the preclinical research. Animal models offer an invaluable tool for studying and understanding several human pathologies. In the field of psychiatric disorders is extremely difficult, if not impossible, to reproduce the complexity of a mental disorder and the human behavior in an animal (Nestler and Hyman, 2010). Generally, animal models of psychiatric disorders reproduce one or some symptoms comparable to those present in the human pathology, for instance: the presence of altered locomotor activity or anhedonia in the animal models of depressive disorders or, vice versa, a motor hyperactivity in animal models of manic syndrome. Therefore, it is possible to test whether therapies able to reverse the animal symptoms are also effective to relief symptoms of the human pathology (Fernando and Robbins, 2011). In the past decades, the technique of targeted inactivation of individual genes in mice has become a precious experimental tool for reproducing genetic disorders and understanding gene functions (Leung and Jia, 2016). A knockout mouse is a mouse in which one or more genes are inactivated or “knocked out”. The resulting phenotype of a knockout mouse represents a direct link between the function of a gene and the animal physiology and behavior (Doyle et al., 2012). Indeed, gene knockout strategies constitute a very important progress for understanding which genes are involved in the etiopathogenesis of psychiatric disorders and in the psychopathological processes that occur with these disorders (Sukoff Rizzo and Crawley, 2017). Furthermore, lack of effects or altered response to a drug when one or more genes are knocked out, strongly indicates that those genes are crucial for the response. Therefore, knockout mice are useful tools for developing new drugs or novel therapeutic strategies. The 2007 Nobel Prize in physiology or medicine was awarded to M.C. Capecchi, M.J. Evans and O. Smithies for their discoveries of principle for introducing specific gene modifications in mice by homologous recombination in embryonic stem cells. However, this conventional method takes up to months to a year to generate knockout mice for experimental studies of gene function. Recently, advancements in genetic engineering have led to the development of the CRISPR-Cas9 technology that in addition to producing genetically modified mice in a shorter time, can be applied to both cell lines and whole organisms (Klimke et al., 2019). In the present review, we will give an update of the most used gene knockout models in the field of psychiatric disorders including depression, anxiety and obsessive-compulsive disorders.

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Search methods

A PubMed database search was performed using the combination of keyword: ‘Depression, Anxiety, Obsessive Compulsive Disorder, Monoaminergic system, Brain-derived neurotrophic factor, hypothalamic-pituitary-adrenocortical (HPA) axis, Corticotropin-releasing hormone, Animal models of Depression, Animal models of Anxiety, Animal models of Obsessive Compulsive Disorder, Knockout mouse’. The search was carried out considering only animal studies articles.

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Gene knockout mice and depression

Depression is a serious mental illnesses characterized by persistent irritability, melancholy and sadness together with a diminished interest or pleasure in all, or almost all, daily activities, for at least two weeks (American Psychiatric Association, 2013). Additionally, depression includes other symptoms such as difficulty concentrating or making decisions, subjective feelings of fatigue and guilt, and recurrent thoughts of death or suicidal ideation (American Psychiatric Association, 2013). Globally, depression affects more than 350 million people worldwide, with a higher risk for females than males (World Health Organization, 2016). The etiology of depression is complex and not yet completely understood. At present, three main hypotheses have been postulated about its molecular and biochemical mechanisms. The so-called monoamine hypothesis of depression, formulated over 50 years ago, was based on the fact that symptoms of depression can be alleviated by agents that, acting by various mechanisms, increase synaptic concentrations of the monoamines noradrenaline, 5-hydroxytriptamine (5-HT) and dopamine (Ferrari and Villa, 2017; Perez-Caballero et al., 2019). The first-line medications currently prescribed include the selective inhibitors of noradrenaline transporter (NAT) (sNRIs) (e.g., reboxetine), 5-HT transporter (SERT) (SSRIs) (e.g., fluoxetine), and inhibitors of both NAT and SERT (SNRIs) (e.g., venlafaxin). Moreover, some antidepressant drugs inhibit both NAT and dopamine transporter (DAT) (e.g., nomifensin and bupropion) as well as of all three monoamine transporters (e.g., nefazodone which is also a 5-HT2 receptor antagonist). Converging lines of evidence also implicate alterations of neurotrophins, including brain-derived neurotrophic factor (BDNF) (Sen et al., 2008; Castrén and Kojima, 2017). According to the ‘neurotrophic theory of depression’ (introduced by Duman et al., 1997), serum BDNF levels are lower in depressed subjects, and these levels are elevated following a course of antidepressant treatment (Sen et al., 2008). Moreover, treatment with antidepressant drugs increases BNDF expression and protein levels in both animals and humans (Nibuya et al., 1995; Russo-Neustadt et al., 1999; Chen et al., 2001). On the other hand, the ‘corticosteroid receptor hypothesis of depression’ includes hyperactivation of the hypothalamic-pituitary-adrenocortical (HPA) axis, resulting among other changes, in increased production and secretion of corticotropin-releasing hormone (CRH, also frequently abbreviated CRF) in various brain regions postulated to be involved in the causality of depression as well as in elevated levels of adrenocorticotropic hormone (ACTH) and cortisol (Stetler and Miller, 2011; Waters et al., 2015). Finally, various methodological approaches (analysis of candidate genes, genome-wide association analysis, genome-wide sequencing) confirm the overall role of genetic factors in depression (Sullivan et al., 2000; Wray et al., 2018). A powerful strategy to investigate the validity of these hypotheses is to modulate the expression of genes that encode for various components of these systems and to study their role in the pathophysiology of depression. Based on the ‘monoamine hypothesis’, many genetic studies focused on the analysis of polymorphisms in genes associated with noradrenaline, dopamine, and 5-HT neurotransmission, and several mice with targeted mutagenesis on different components of monoaminergic systems (i.e., monoamine transporters and receptors) have been generated (Table 1). The characterization of the basal phenotype of these mutant mice is evaluated by different behavioral tests used to mimic some clinical manifestations present in depressed patients, such as depressive-like state, cognitive and sleep deficits, alteration in locomotion, overactivation of the HPA axis and anhedonia. Most of these paradigms involve exposure to relatively acute or subchronic stress (Nestler and Hyman, 2010). Among them, the forced-swim and the tail suspension tests (FST and TST, respectively) (in which the animal is subjected to an inescapable stress and typically responds with alternating bouts of escape-oriented behavior and immobility) together with the sucrose preference test (used to measure anhedonia, the lack of pleasure of animal of rate to consume sucrose) are currently the simplest and most widely used. For example, results from the FST and TST indicate that mice generated by the deletion of the NAT or DAT genes, behave like antidepressant-treated animals exhibiting a depressive-resistant phenotype (Xu et al., 2000; Spielewoy et al., 2000; Dziedzicka-Wasylewska et al., 2006; Haenisch et al., 2009). In addition, NAT knockout mice have been found resistant to stress-induced depressive-like changes in behavior, highlight the importance of noradrenergic pathways in the maintenance of depression (Haenisch et al., 2009). Both DAT and NAT knockout mice showed an increase sucrose preference indicating reduced anhedonia (Perona et al., 2008; Haenisch et al., 2009). While in NAT knockout mice locomotor activity did not differ from wild-type controls (Xu et al., 2000; Haenisch et al., 2009), DAT knockout mice have been found very hyperactive without developing locomotor habituation after repeated testing (Giros et al., 1996; Pogorelov et al., 2005). Depending on the genetic background, SERT knockout mice showed different behavioral phenotypes. While mice of 129S6 or CD1 genetic background show an increase in immobility in the FST when compared with wild-type mice but an antidepressant-like behavior in the TST, mice generated from C57BL/6J strain show no baseline antidepressant-related phenotype (Holmes et al., 2002; Lira et al., 2003; Perona et al., 2008; Popa et al., 2008; Haenisch and Bönisch, 2011). However, SERT knockout mice of C57BL/6J background presented an increase immobility in both FST and TST after repeated exposure (Wellman et al., 2007). SERT knockout mice of 129/Sv background also exhibited an increased extent of helpless behavior in the learned helplessness paradigm (Lira et al., 2003). As expected, SSRI did not have any effect on SERT knockout mice in the TST, whereas the impact of desipramine was conserved (Holmes et al., 2002). Moreover, in SERT knockout mice of C57BL/6J or C57BL/6J-129SvJ background, sucrose consumption was not changed (Kalueff et al., 2007; Perona et al., 2008). On the contrary, SERT knockout mice of CD1 background, displayed anhedonia (Popa et al., 2008). All the results found in SERT knockout mice seem to counteract with the expectation of a depressive-resistant phenotype. As discussed below, this could depend on neurochemical changes present in these animals as a result of the deletion of the SERT gene. Indeed, it is well established that different neurochemical changes are present in monoamine transporters deficient mice (Haenisch and Bönisch, 2011). In DAT knockout mice, as a consequence of elevated dopamine levels, the coding mRNA for the two major dopamine receptors, Dl and D2, was downregulated while the density of D3 receptors was increased (Giros et al., 1996; Gainetdinov et al., 1998; Fauchey et al., 2000). Moreover, the mRNA levels for BDNF were significantly reduced in the frontal cortex of these mice at postnatal day 15, a change that persisted into adulthood (Fumagalli et al., 2003). Decreased BDNF protein levels were confirmed in the fontal cortex of DAT knockout mice by Li et al. (2010). Elevated synaptic noradrenaline concentration in NAT knockout mice downregulates the postsynaptic α1-adrenoceptors in different brain regions, including the hippocampus and the cerebral cortex (Bohn et al., 2000; Xu et al., 2000; Dziedzicka-Wasylewska et al., 2006). On the contrary, the expression of α2A-adrenoceptor and the α2C-adrenoceptor mRNAs were significantly increased in the brainstem. In addition the α2C-adrenoceptor mRNA resulted upregulated in the hippocampus and in the striatum (Gilsbach et al., 2006). It has been demonstrated that chronic treatment with the selective noradrenaline reuptake inhibitor reboxetine led to a reduction of the ß1 adrenergic receptor-binding sites and a similar effect was found in NAT knockout mice further highlighting the depression-resistant phenotype that characterize these animals (Gould et al., 2003; Dziedzicka-Wasylewska et al., 2006). Despite the evidence of increased extracellular 5-HT levels during embryonic development, marked reductions in 5-HT concentrations have been found in several brain regions of adult SERT knockout mice which could account for the depressive phenotype mentioned above (Bengel et al., 1998). Moreover, adult SERT knockout showed reduced serotonergic cell number and firing rate in the dorsal raphe nucleus (Lira et al., 2003). Alterations of different subtypes of 5HT receptors were also observed in SERT knockout mice. 5-HT1A autoreceptor density has been found decreased in the dorsal raphe nucleus, hypothalamus, amygdala and septum and increased in the hippocampus while no changes were observed in the cortex (Li et al., 1999, Fabre et al., 2000, Bouali et al., 2003). In addition, 5-HT1B binding and functional coupling were markedly reduced in the globus pallidus and in the substantia nigra (Fabre et al., 2000, Shanahan et al., 2009). Moreover, 5-HT2A receptor density was significantly increased in the hypothalamus and septum, but reduced in the striatum (Rioux et al., 1999, Li et al., 2003). Finally, the 5-HT2C receptor density was significantly increased in the amygdala and choroid plexus, but not in other brain regions of these mutant mice (Li et al., 2003). Alterations in the function and expression of several components of the HPA axis have been also reported in SERT knockout mice. For example, CRH expression and basal plasma corticosterone levels were reduced relative to wild-type mice (Jiang et al., 2009). Moreover, CRH type 1 receptors density was elevated and the expression of glucocorticoid receptors was significantly diminished (Jiang et al., 2009). Furthermore, stress augmented the glucocorticoid receptors expression (Jiang et al., 2009). On the other hand, SERT knockout mice showed no differences in BDNF protein levels compared with wild-type animals (Szapacs et al., 2004). Mice with deletion of the genes that encode for monoaminergic receptors are also used to assess the complex role of monoaminergic systems in depression. For example, the loss of the α2-adrenergic receptor elicited a depressive response in the FST and α2-adrenergic receptor knockout mice also show a disrupted sleep/wake cycle as often observed in human depression (Schramm et al., 2001; Lähdesmäki et al., 2002). This suggests that the α2-adrenergic receptor may play a protective role in depression. In addition, the α2-adrenergic receptor seems to mediate the antidepressant effects of imipramine (Schramm et al., 2001). In contrast, the lack of α 2C-subtype receptor decreased the immobility in the FST and also attenuated the elevation of plasma corticosterone after different stressors (Salline et al., 1999). The role of 5-HT receptors in depressive behaviors has been also extensively explored. Among them, the 5-HT1A receptor was the most studied. 5-HT1A receptors are predominantly somatodendritic autoreceptors of serotonergic neurons and their activation put down serotonergic neuronal activity (Gobbi and Blier, 2005). In addition, postsynaptic 5-HT1A receptors are expressed in numerous serotonergic projection sites such as the cerebral cortex, septal nuclei, hippocampus and amygdala (Pompeiano et al., 1992). 5-HT1A knockout mice exhibited antidepressant-like responses in the FST and TST, a phenotype that may result from a disinhibiton of serotonergic neuronal activity due to the lack of autoregulatory control (Heisler et al., 1998, Mayorga et al., 2001). As expected, fluoxetine had no effect on the 5-HT1A knockout immobility, suggesting that these receptors could be very important for the expression of the antidepressant-like behavioral responses to SSRIs (Mayorga et al., 2001). On the contrary, the NAT inhibitor desipramine further reduced the immobility in 5-HT1A knockout, thus exerting an antidepressant-like effect (Mayorga et al., 2001). In contrast, the lack of 5-HT1B receptors had no consequence in the TST and 5-HT1B knockout behave like wild-type mice (Mayorga et al., 2001). However, the effect of low doses of fluoxetine was significantly augmented in the 5-HT1B knockout mice compared with wild-type mice (Mayorga et al., 2001). On the contrary, in the FST, the SSRI paroxetine decreased immobility in wild-type mice but not in 5-HT1B knockout (Trillat et al., 1998). These differences could be due to different testing procedures.

Table 1

Table 1

The neurotrophic hypothesis of depression has been addressed in mice with altered BDNF expression. Because homozygous BDNF knockout mice die shortly after birth which prevent their use, most of the behavioral studies have been conducted with heterozygous BDNF (knockout/+) mice which have about half the levels of BDNF compared with wild type (Ernfors et al., 1994; Conover et al., 1995; MacQueen et al., 2001; Ibarguen-Vargas et al., 2009). Nevertheless, the behavior of these mice did not differ from wild-type counterpart in the FST and TST, thus demonstrating that the reduced levels of BDNF were not low enough to produce a depressive phenotype (MacQueen et al., 2001; Saarelainen et al., 2003; Ibarguen-Vargas et al., 2009), However, they showed a depression-like behavior in the learned helplessness test (MacQueen et al., 2001). Furthermore, it has been reported that BDNF (knockout/+) mice were resistant to the effects of antidepressants, such as imipramine, in both FST and TST indicating that BDNF-mediated trkB activation is required for the behavioral changes typically produced by these drugs (Saarelainen et al., 2003; Ibarguen-Vargas et al., 2009). On the other hand, when exposed to chronic stress, BDNF (knockout/+) mice showed a depression-like behavioral phenotype suggesting a synergic interaction between stress and BDNF deficit (Duman et al., 2007). Thus, a BDNF-related mechanism could be involved in the actions of stress and turns out in depressive behavioral changes. Moreover, even if the loss of forebrain BDNF per se was not sufficient to produce a depression-like phenotype, no significant decrease in immobility in the FST was observed when BDNF (knockout/+) mice were injected with the antidepressant desipramine, suggesting the involvement of BDNF in antidepressant efficacy (Monteggia et al., 2004). However, when the deletion of the trkB gene is restricted to the forebrain and occurs exclusively during postnatal development, behavioral analysis shows that these mice did not differ from wild-type controls in the FST and did not show HPA axis alterations under normal and stressful conditions (Zörner et al., 2003). However, like heterozygous BDNF (knockout/+) mice, they were resistant to the effects of antidepressants in the FST, indicating that antidepressants directly influence despair behavior in mice via the BDNF-TrkB signaling pathway (Saarelainen et al., 2003).

Finally, to validate the ‘corticosteroid receptor hypothesis of depression’, knockout mice with inactivation of the genes encoding different components of the HPA axis have been generated (Müller and Holsboer, 2006). Among them, mice deficient for CRH, CRH receptors 1 and 2 (individually and together) were the most used. Because these mutant mice mainly show an anxiety-related behavior and anxiety is a cardinal symptom with high prevalence in depression, their physiological and behavioral features has been described in detail in the section below.

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Gene knockout mice and anxiety disorders

Anxiety disorders is a generic term which refer to different common and disabling conditions including anxiety, panic and social anxiety disorders, and represent the most prevalent mental illness in the world with high social costs (Merikangas et al., 2003; Craske and Stein, 2016). Although the pathogenesis underlying these disorders has not yet been completely clarified, it has been widely acknowledged that genetic factors contribute to their course and progression (Shimada-Sugimoto et al., 2015). Indeed, different lines of knockout mice have been generated to better understand the pathogenic pathway of these disorders (Table 2; Jacobson and Cryan, 2010). The anxiety-like phenotype commonly present in these animals have been detected and quantified in different paradigms based, in most cases, on conditioned response to an aversive stimulus such as the elevated plus-maze, the open field, the light/dark exploratory transitions and the social interaction test. Noteworthy, these paradigms are responsive to anxiolytics drugs, thus showing a reliability and predictive validity (Cryan and Holmes 2005). Based on the assumption that the 5-HT signaling is impaired in anxiety disorders and that SSRI act enhancing 5-HT release, ablation of the gene encoding for the 5-HT1A receptor leads to the generation of knockout mice with a strong anxiety-like behavioral phenotype (Ramboz et al.,1998; Toth, 2003). In the elevated zero maze, 5-HT1A knockout mice spent significantly less time in the open arms and entered into the open arms significantly less often than wild types. Also, in the open-field test, 5-HT1A knockout displayed less exploratory activity in the center (Heisler et al., 1998; Ramboz et al., 1998). Because the anxiety-like phenotype was present in both homozygote and heterozygote 5-HT1A knockout mice, a partial receptor deficit seems sufficient to elicit anxiety (Toth, 2003). On the other hand, SERT knockout mice also displayed a robust anxiety-like behavioral phenotype with reduced exploratory locomotion in the light/dark exploratory, elevated plus-maze and open field tests (Holmes et al., 2003a; Ansorge et al., 2004; Kalueff et al., 2007). Moreover, the selective 5-HT1A receptor antagonist, WAY 100635 produced a significant anxiolytic-like effect in the SERT knockout mice, but not in the wild-type controls suggesting a role of this receptor in mediating anxiety-related phenotype in these null mutants (Holmes et al., 2003b).

Table 2

Table 2

Given the role of benzodiazepine-site ligands in alleviating anxiety, knockout mice with specific inactivation of the genes encoding for the γ-Aminobutyric acid (GABA) A receptor are frequently used (Smith and Rudolph, 2012). Because the γ2 subunit is required for normal single-channel conductance of this receptor and modulation by benzodiazepines, disruption of the γ2 subunit gene represented the first approach. Unlikely, the disruption of the γ2 subunit gene has been found lethal in the perinatal period (Günther et al., 1995). However, the experiments done in the surviving mice have shown that diazepam was behaviorally inactive in these animals and that the conductance level of the channel was reduced. Moreover, the disruption of the γ2 subunit gene resulted in a severe deficit in benzodiazepine binding sites (~94% reduction compared to control) (Günther et al., 1995). On the other hand, compared with wild-type controls, heterozygous γ2 (knockout/+) mice displayed a 20% reduction of benzodiazepine binding sites together with a reduced single-channel conductance, mainly marked in brain areas known to be affected in anxiety disorders in humans such as hippocampus and orbitofrontal cortex (Crestani et al., 1999). Moreover, these mice showed an anxiety-like phenotype in both elevated plus maze and the light/dark box as well as enhanced behavioral reactivity to natural aversive stimuli (Crestani et al., 1999). On the other hand, deletion of the α1 subunit is not lethal and α1 knockout exhibited an anxiety-like behavior that did not differ from wild types when evaluated with the elevate plus maze and an anxiolytic effect of diazepam was detected in both genotypes (Kralic et al., 2002). On the contrary, mice globally lacking the α2 subunit of the GABA A receptor (α2 GABA A knockout) showed anxiety-like phenotype when compared with wild types but were insensitive to the anxiolytic effect of diazepam (Löw et al., 2000). All together these results suggest that α2-, but not α1-subunit, mediates the anxiolytic effect of diazepam. A further involvement of the GABA in anxiety has also been evaluated through the study of the metabotropic GABA receptor, GABA B. This receptor is a heterodimer made up of two subunits, GABA B (1) and GABA B (2), both necessary for its functionality. Targeted deletion of GABA B (1) subunit has been found to increase anxiety in several behavioral paradigms and abolished the response to benzodiazepines in the light–dark box test suggesting a genetic evidence of a role for GABAB receptors in the modulation of anxiety-like behavior (Mombereau et al., 2004).

The involvement of the HPA axis in the stress response has led to the generation of knockout mice with inactivation of the genes regulating this system, such as knockout mutants of CRH, CRH receptors 1 and 2 (individually and together) and the glucocorticoid receptor. Despite a decreased anxiety has been expected, mice with targeted deletion of the CRH gene exhibit normal anxiety-related behaviors in several paradigms (e.g., elevated plus maze) as well as normal stress-induced behavioral changes (Weninger et al., 1999). On the contrary, mice lacking CRH receptor 1 showed a blunted HPA activation in response to stress and displayed a reduced anxiety-like response in both elevated plus maze and light–dark box (Timpl et al., 1998; Smith et al., 1998). These results suggest that the CRH receptor 1 plays an important role in the expression of stress-related responses and its block leads to a reduction of anxiety. With regard to CRH receptor 2 deficiency, both male and female knockout mice were found hypersensitive to stress and displayed increased anxiety-like behavior (Bale et al., 2000). Moreover, Kishimoto et al. (2000) showed that the enhanced anxiety of CRH receptor 2 knockout mice was not due to changes in HPA axis activity, but rather reflects impaired responses in specific brain regions involved in emotional function. The authors proposed that in contrast to the anxiogenic effect mediated by CRH receptor 1, CRH receptor 2 mediates a central anxiolytic effect (Kishimoto et al., 2000).

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Gene knockout mice and obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a debilitating psychiatric illness characterized by persistent intrusive thoughts, uncontrollable repetitive rituals and excessive anxiety, with a worldwide prevalence of 2–3% (Ruscio et al., 2010). The etiology of OCD is complex and not yet completely understood which limits the development of effective treatments. However, a genetic risk of OCD has been well established and alterations in different neurotransmitter systems have been implicated in its pathophysiology including alterations of the glutamatergic systems. In agreement, increasing evidence implicate glutamatergic synaptic dysfunction within cortical-striatal-thalamo-cortical circuitry in the etiology of OCD (Ting and Feng, 2008; Wu et al., 2012). On this basis, several knockout mice have been developed with specific genetic mutations affecting glutamate neurotransmission (Table 3). Among them, SAPAP-3 knockout mice are the most widely used. These mice are characterized by a deletion of the gene that encode for the Sapap3 protein, a postsynaptic scaffolding protein at excitatory synapses expressed in corticostriatal circuits, particularly in the striatum. Similar to OCD patients, SAPAP-3 knockout exhibited several OCD-like behaviors, including excessive grooming that led to facial hair loss and skin lesions and anxiety-like phenotype (Welch et al., 2007). Furthermore, SAPAP-3 knockout displayed defects in corticostriatal synapse, including reduced corticostriatal synaptic transmission and defected in the functioning of glutamate receptors, further demonstrating the importance of this protein in regulating corticostriatal synaptic function (Welch et al., 2007). Noteworthy, repeated administration of the first-line OCD treatment fluoxetine prevented the OCD-like behaviors in SAPAP-3 knockout (Welch et al., 2007). Likewise SAPAP-3 knockout, knockout created by a deletion of the neuron-specific transmembrane SLIT and NTRK-like protein-5 (Slitrk5 knockout) also showed excessive self-grooming that led to severe facial lesions and increased anxiety-like behaviors that were alleviated by fluoxetine (Shmelkov et al., 2010). Moreover, in agreement with findings in patients with OCD, these mice exhibited increased neuronal activity specifically in the orbitofrontal cortex as well as anatomical deficits in the striatum and downregulation of glutamate receptors, which led to a reduction in corticostriatal neurotransmission (Shmelkov et al., 2010). Alterations in 5-HT and dopamine neurotransmission were also associated with OCD (Baumgarten and Grozdanovic, 1998; Denys et al., 2004). Indeed, SAPAP-3 knockout mice exhibited serotonergic abnormalities in both cortex and striatum as well as dopaminergic dysregulations in the orbitofrontal cortex (Wood et al., 2018). Moreover, knockout created by a loss of aromatase enzyme (ArKO mice) manifested a range of compulsive behaviors including excessive grooming that was accompanied by concomitant decreases in catechol-O-methyltransferase (COMT), one of the major enzymes involved in the metabolic degradation of catecholamines. In agreement, low activity allele for COMT (COMT-L) has been found significantly associated with higher risk of developing OCD, particularly in males (Karayiorgou et al., 1999). ArKO also showed disruption in prepulse inhibition, a measure of sensorimotor gating that is also impaired in patients with OCD (Kumari et al., 2001).

Table 3

Table 3

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Conclusion

Despite the growing knowledge base of neuropsychiatric disorder neurobiology, a high percentage of patients do not respond to first-line therapeutic interventions. Therefore, there is clearly a need for new, more effective treatments. In this review, we have briefly discussed the use of knockout mice to investigate the molecular and cellular mechanisms underlying depression, anxiety and OCD disorders. We highlighted the importance to modulate the expression of different genes that encode for the various components of different neurotransmitter systems involved in the pathophysiology of these disorders to understand their specific role. Although gene knockout strategy offers a valid method for the investigation of genetic contributions in the neurobiology of neuropsychiatric disorders, we should consider the possibility that compensatory mechanisms are put in place following the loss of the functionality of a specific gene and this can lead to misinterpretation of results. For example, we cannot distinguish between phenotypes emerging from impaired signaling from those resulting from changes that took place during the development. In fact, mice lacking a specific gene may exhibit behavioral changes that may differ from those expected as we have seen for mice that result from the deletion of the SERT gene. However, as mentioned in the introduction, advancements in genetic engineering have led to the development of new technology that in addition to producing genetically modified mice in a shorter time, can be applied to both cell lines and whole organisms. In conclusion, knockout mice generated by a single gene deletion can yield important information about the precise role of specific gene in the pathophysiology of psychiatric disorders, but might limit their applicability to human processes that occur with these disorders.

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Acknowledgements

This work was funded by by the Departmentof Biomedical Sciences Project (RICDIP_2012_Fratta_01) at the University of Cagliari.

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Conflicts of interest

There are no conflicts of interest.

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Keywords:

genetic models; knockout mouse; psychiatric disorders

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