Modeling cognitive endophenotypes of schizophrenia in mice

Abstract

Schizophrenia is a complex mental disorder that is still characterized by its symptoms rather than by biological markers because we have only a limited knowledge of its underlying molecular basis. In the past two decades, however, technical advances in genetics and brain imaging have provided new insights into the biology of the disease. Based on these advances we are now in a position to develop animal models that can be used to test specific hypotheses of the disease and explore mechanisms of pathogenesis. Here, we consider some of the insights that have emerged from studying in mice the relationship between defined genetic and molecular alterations and the cognitive endophenotypes of schizophrenia.

Figure 1

Studying the 22q11 deletion in mice. The top shows the 1.5 megabase region of chromosome (chr.) 22 that is deleted in 22q11 carriers in humans. The syntenic region in the mouse is located on chromosome 16. The names of the genes in both regions are colored to emphasize the difference in the organization of the loci. Chromosomal deletions: seven chromosomal 16 deletions of different lengths that have been generated in the mouse and analyzed for behavior are shown [–40]. Individual gene knockouts: eight knockout mice have been analyzed for behavior [41,77,120,121]. Behavioral analysis: comparative summary of the behavioral analysis of the 15 different mouse models. Because only one copy is deleted in the 22q11 deletion, the behavioral analysis of heterozygous mice are shown unless only homozygous mice (—/—) have been evaluated. * Denotes that a deficit in pre-pulse inhibition was observed in Tbx1 heterozygous mutant mice when backcrossed to C57b6 but not on a mixed genetic background.

Figure 2

Studying the function of Disc1 in the mouse. Top left shows the structure of the Disc1 gene on mouse chromosome 8. Below the genomic loci or the transgenic constructs of seven genetically modified mice are depicted. Mutations in the mouse Disc1 locus: three mouse models carry mutations in the mouse Disc1 locus. Two models carry missense mutations in exon 2 introduced by a chemical-induced mutagenesis screen (Disc1^Rgsc1390 and Disc1^Rgsc1393) [62]. The third model, Disc1^Tm1Kara [63,64], features a stop codon in exon 8 and a polyadenylation signal (pA) after this exon to model the breakpoint of the human translocation. However, because this mutation was generated in a 129S6/SveV background it also carries an intrinsic stop codon in exon 7 resulting in an N-terminal peptide that ends at amino acid 542. This peptide is 50 amino acid smaller than the 597 amino acid long N-terminal peptide that could potentially be expressed in t(1;11) carriers. Transgenic mouse models: four transgenic mouse models have been generated that all overexpress truncated DISC1 proteins or peptides in neurons of the brain of the mouse. One expresses the truncated DISC1 protein suggested to be present in t(1;11) carriers from the endogenous Disc1 mouse locus using a bacterial artificial chromosome that spans 148 kb of the Disc1 gene [60]. This strategy prevents ectopic expression of the transgene. In this mouse DISC1 is fused to enhanced green fluorescent protein (GFP) to visualize protein expression. Three models use the CamKIIa promoter to restrict expression to the forebrain. Two models overexpress the N-terminal 597 amino acid peptide: Tg(CamK2a-Disc1)10Asaw [57] and Tg(CamK2a-tTA)1Mmay x Tg(TRE-CMV-hDisc1) [58]. Note that for the second mouse model the expression of transgenic DISC1 can be regulated at the level of transcription using the tetracycline transactivator (tTA) system. The tTA transcription factor drives expression from the tetO promoter in the absence of doxycycline. When animals are fed with a doxycycline-supplemented diet, tTA dissociates from the promoter and the transgene is switched off. The third mouse model expresses a C-terminal peptide designed to work as a transdominant negative protein by sequestering out the binding partners NUDEL or LIS1 [59]. The peptide is fused to a mutated ligand-binding domain of the estrogen receptor, permitting regulation of the activity of the fusion protein. Treating the animals with the synthetic steroid tamoxifen activates the fusion protein. Behavioral analysis: the right side gives a comparative summary of the behavioral analysis performed with the six mouse models. * Denotes that the deficit in spatial reference memory of Tg(CamK2a-tTA)1Mmay x Tg(TRE-CMV-hDisc1) mice was observed in females but not in males.

Figure 3

Overexpression (OE) of dopamine D₂ receptors (D2R) selectively in the striatum leads to cognitive and negative-like symptoms. (a) Selective overexpression of dopamine D₂ receptors in the striatum using the tTA system (see legend for Figure 2). Although the tTA transcription factor is expressed in the whole forebrain, the tetO-D2R construct response is restricted to the striatum with very limited expression outside of the striatum (gene on) [21]. The transgene can be switched off by feeding the mice doxycycline (Dox; gene off). (b) Cognitive symptoms: excess D₂ receptors in the striatum leads to impairments in two prefrontal-dependent cognitive endophenotypes that are also impaired in patients with schizophrenia: working memory and conditioned associative learning [18,21]. Neither deficit is reversed by switching off the transgene in the adult animal. Negative symptoms: an excess of D₂ receptors in the striatum also induces impairment in incentive motivation. When required to lever press for food rewards in a progressive ratio schedule, striatal D2R-overexpressing mice stop performing much sooner than control littermates [101]. In contrast to the cognitive deficits, this deficit is reversed by switching off the transgene in the adult animal.