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21 May 2008. Mutations in disrupted in schizophrenia (DISC1), a putative schizophrenia gene, interfere with the production and maturation of new neurons in the adult mouse brain and may interrupt brain circuits in the hippocampus and the prefrontal cortex, two regions associated with schizophrenia, according to a new study in the May 13 PNAS. The findings, reported by Joseph Gogos, Maria Karayiorgou, and colleagues at Columbia University, New York, may help explain why people with a chromosomal rearrangement in the middle of the DISC1 gene are at high risk for major mental illnesses such as schizophrenia and bipolar disorder.
The original, and best, evidence that DISC1 is involved in schizophrenia comes from an extended Scottish family whose members carry the chromosomal rearrangement leading to a disruption of the DISC1 gene. The disruption is a translocation of a piece of DNA from the middle of the DISC1 gene, resulting in a loss of several major DISC1 isoforms, and the possible production of a truncated protein.
The Columbia researchers set out to recapitulate that same mutation in a mouse model by adding a stop codon in exon 8 and a polyadenylation signal downstream to make the mouse gene behave more like the mutated human gene. According to Gogos, during the course of this work they unexpectedly discovered a natural deletion in the middle of the mouse Disc1 gene that spontaneously arose in the 129S6 strain. The researchers reported two years ago that the spontaneous mutation, a deletion of 25 bases, introduces a stop codon in exon 7 that truncates the mouse 129S6 Disc1 protein (see SRF related news story). As a result, the genetically engineered mouse strain contains two termination codons and a polyadenylation site near the translocation breakpoint of the human gene.
Kvajo and colleagues report that their protein expression analysis, using antibodies against the N-terminal and C-terminal domains of Disc1, confirmed that key and well-characterized long isoforms of Disc1 are eliminated by the introduced mutations. The authors argue that this is in contrast to a recent study by a collective of other DISC1 research groups (Ishizuka et al., 2007), which reported that long Disc1 isoforms are expressed in these mice. In e-mail interviews with SRF, Gogos and colleagues suggested that the antibodies to Disc1 used by other research groups may not be ideal to probe complex brain extracts.
Overall, the researchers say, the genetic lesion introduced into the mouse Disc1 gene closely models the Scottish mutation by virtue of where the truncating lesion is and because it only affects a subset of Disc1 isoforms. (Long Disc1 isoforms are eliminated, while short N-terminal isoforms of the gene, unaffected by the translocation, are preserved, they say).
In the current paper, joint first authors Mirna Kvajo, Heather McKellar, and Alexander Arguello have characterized aspects of the anatomical and behavioral phenotype emerging as a result of this genetic lesion. Their data suggest that Disc1 may be important for proper maturation of new neurons in the adult brain and for maintenance of neuronal circuitry that is involved in working memory.
Effects on form
The authors examined mouse brain for signs of morphological changes in the Disc1 mutants. They found no gross morphological changes in either the prefrontal cortex (PFC) or the hippocampus, though they did find a statistically significant decrease (14 percent) in PFC volume compared to control mice. At a higher level of resolution, the researchers found significant differences in the immature and mature neurons in the dentate gyrus, a site of adult neurogenesis. There, immature (i.e., doublecortin-positive) neurons seemed to migrate farther than normal, with a greater fraction reaching the outer granule cell layer. The apical dendrites in these neurons were also abnormal. Apical dendrites usually lie perpendicular to the surface of the subgranular zone of the dentate gyrus, where new neurons form. But dendrites in about 30 percent of immature neurons in mutant Disc1 mice were statistically outside the normal orientation. It is not clear if this reorientation of the dendrites is related to their ability to migrate farther, but that is a possibility given that the maturation of newly born neurons is tightly coupled to their migration. The researchers also found that the numbers of immature neurons were lower in Disc1 mutants (by ~20 percent), and that neurogenesis was down by about the same degree (as judged by mitotic incorporation of bromodeoxyuridine).
Morphological changes are not restricted to immature neurons. Kvajo, McKellar, and colleagues found that mature neurons are also compromised in the mutant mouse dentate gyrus (DG). Again, dendrite orientation was off by up to 40 percent, and twice as many mature neurons had dendrite angles outside the normal range. Dendrite lengths were also decreased by about 25 percent in mutant animals, though this was much more pronounced in the outer layers of the granule cell layer of the DG. This again, suggests that Disc1-compromised neurons may have problems maturing. “The lack of an effect in the bottom layer is consistent with our observation of near-normal dendritic growth in newly born neurons and suggests that impaired Disc1 function specifically leads to a halting of dendritic growth during the postnatal maturation of granule cells in the DG,” write the authors.
Effects on function
Behaviorally, the Disc1 mutant mice show deficits in working memory that suggest a problem in the prefrontal cortex. When the authors tested the mice in several hippocampal-dependent paradigms, including the Morris water maze, novel object recognition, and contextual fear conditioning, they found no difference from controls. However, when they used a two-choice, delayed nonmatch (DNMP) to position task, the mutant mice, both heterozygotes and homozygotes, performed statistically worse. Arguello explained that, although the DNMP task demands that the animals retain information over shorter periods of time, there are several trials a day and the two samples within a trial require them to update working memory. As with other spatial working memory tasks in rodents, this test depends on interactions between the PFC and hippocampus, but the exact role played by each is controversial. “This task creates a potential conflict because animals may be confused about the choices they are facing due to irrelevant information from within the trial or the previous trial,” explained Arguello in an interview with SRF. “Resolving this type of interference is something the PFC does and is part of the executive control of the PFC,” he said. Problems in the PFC are believed to be at least partly responsible for deficits in a whole range of working memory tasks in schizophrenia patients.
What does this model tell us about the role of DISC1? “If we look in the adult CA1 we don’t see a morphological deficit, we see an electrophysiological deficit, so maybe this selectivity says something about the necessity for Disc1 to be present in adulthood in the dentate gyrus, where, incidentally, Disc1 expression is strongest,” Kvajo told SRF. The authors found that under baseline conditions synaptic transmission in the CA1 of the hippocampus was normal, as was paired-pulse facilitation and long-term potentiation, both measures of neuronal plasticity. But the authors did find that short-term potentiation was significantly reduced in mutant mice.
So if Disc1 seems so crucial for the dentate gyrus and the hippocampus, then why are the mice deficient in tasks that depend on the PFC? Arguello offered a few possibilities, though he cautioned that they are highly speculative. The first is that the dentate gyrus plays a role in memory that has not been fully appreciated. “There is some recent evidence indicating that in the dentate gyrus, functionally knocking out NMDA receptors can affect working memory performance,” he said. Another possibility is that the primary deficit is in the hippocampus and that across the course of development the abnormal structural changes in the hippocampus can functionally affect the PFC. “So the idea is that a primary structural abnormality in the hippocampus can cause a secondary functional abnormality in PFC,” he said. Arguello said that they are planning to test these theories using various behavioral tasks that differentially engage cognitive networks of the brain and by looking for physiological changes.
Whether these findings in mice will have any bearing on the human condition remains to be determined. “While there is some evidence that adult neurogenesis may be implicated in mental disorders, it is hard to say exactly how these changes relate to the human condition because, in this respect, we know too little about the situation in humans. However, the really important thing about our model, and what makes it stand out, is that we have a model that is as close to the human mutation as can be. We have used a disease-oriented approach, which is important if you want to figure out what DISC1 does in schizophrenia specifically, or in mental disorders,” said Kvajo.—Tom Fagan.
Reference:
Kvajo M, McKellar H, Arguello PA, Drew LJ, Moore H, MacDermott AB, Karayiorgou M, Gogos JA. A mutation in mouse Disc1 that models a schizophrenia risk allele leads to specific alterations in neuronal architecture and cognition. Proc Natl Acad Sci U S A. 2008 May 13;105(19):7076-81. Epub 2008 May 5. Abstract
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