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DISC1, GABA Together Regulate Neurogenesis and Schizophrenia Risk

6 March 2012. Interplay between disrupted-in-schizophrenia 1 (DISC1) and GABA signaling regulates dendritic development during adult hippocampal neurogenesis in the mouse, according to a March 2 report in Cell. Led by Guo-li Ming, of Johns Hopkins University in Baltimore, Maryland, this study associates two important factors implicated in schizophrenia, DISC1 and inhibitory signaling, within a shared pathway that controls neuronal development. In addition, the researchers demonstrate an epistatic interaction between DISC1 and GABA signaling in risk for schizophrenia.

DISC1 regulates multiple facets of neuronal development, during both early postnatal and adult neurogenesis, and has been implicated as a major susceptibility factor for schizophrenia in at least one Scottish family (see SRF related news story; Ming and Song, 2009). DISC1 knockdown quickens the pace of adult hippocampal neurogenesis, causing neural progenitor cells to exit the cell cycle early (Mao et al., 2009) and accelerating the dendritic development of newly born neurons (Duan et al., 2007). Signaling through the inhibitory neurotransmitter GABA, too, is no stranger to the schizophrenia stage (see SRF related news story), and also regulates adult neurogenesis (Ge et al., 2007). While the effect of GABA is hyperpolarizing in mature neurons, GABA depolarizes immature neurons due to high levels of the sodium-chloride co-transporter NKCC1, which brings chloride into the cell. Downregulation of NKCC1 in immature neurons abolishes this depolarization, producing defects in dendritic growth (Ge et al., 2006). Given that both DISC1 and GABA signaling affect dendritic development, Ming’s group hypothesized that DISC1 interacts with GABA signaling to regulate neurogenesis.

Probing DISC1 and GABA in adult neurogenesis
First author Ju Young Kim and colleagues used a retrovirus expressing a short hairpin RNA against DISC1 (shRNA-D1) to knock down its expression. Consistent with a prior report from the same group (Duan et al., 2007), injection of this retrovirus into the dentate gyrus of postnatal day 42 mice resulted in accelerated dendritic growth compared to injection of a control virus. In contrast, expression of an shRNA against NKCC1 (shRNA-NK1) that eliminates GABA-induced depolarization of new neurons (Ge et al., 2006) resulted in reduced dendritic length and complexity.

To probe the interplay between DISC1 and NKCC1-dependent GABA signaling, Kim and colleagues performed a double knockdown using both retroviruses. The cells that expressed both shRNA-D1 and shRNA-NK1 exhibited levels of dendritic growth similar to a control retrovirus, indicating that the deficit in dendritic growth by DISC1 knockdown was rescued by reduction of NKCC1, and therefore mediated by GABA. An shRNA against the critical γ2 subunit of GABAA receptors also rescued the effect of DISC1 on dendritic length, confirming that GABA, not chloride, signaling is required for the DISC1-dependent regulation of dendritic development.

The researchers then looked for a mechanistic link between DISC1 and GABA signaling, and found evidence for AKT-mTOR pathway involvement. AKT is a known target of DISC1, and DISC1 knockdown produced increases of pAKT and downstream effector pS6. Knockdown of NKCC1 or GABAA γ2 significantly attenuated this increase, suggesting that depolarizing GABA signaling is important in DISC1’s regulation of the AKT-mTOR pathway and dendritic growth during neurogenesis. Enhancement of tonic GABA signaling using a GABA transaminase inhibitor augmented the accelerated dendritic growth of shRNA-D1-infected neurons, as well as increased pAKT and pS6 levels, and similar effects were observed when a GABAA agonist was used to increase synaptic GABA. These data suggest a synergism between DISC1 and GABA signaling.

DISC1 knockdown in early postnatal neurogenesis
In contrast to the results observed during adult neurogenesis, injection of a retrovirus expressing shRNA-D1 at postnatal day 10 did not produce accelerated dendritic growth. What could explain this discrepancy between early postnatal development and adult neurogenesis? One factor that differs between the two processes is the time point at which GABA polarity reverses, switching from a depolarizing to a hyperpolarizing effect due to NKCC1 downregulation and chloride extruder KCC2 upregulation. While this polarity switch doesn’t occur until 14 days post-injection during adult neurogenesis, the researchers found that it began much earlier during postnatal neurogenesis, starting at seven days post-injection. Thus, the lack of effect of DISC1 knockdown during postnatal neurogenesis may be due to a reduced time window of GABA depolarization.

To test this hypothesis, Kim and colleagues created a retrovirus expressing an shRNA against KCC2 (shRNA-K2) that was associated with a significantly greater Ca2+ rise (indicative of a depolarizing effect of GABA) at seven days post-injection during postnatal neurogenesis than a control retrovirus. KCC2 downregulation increased dendritic length and complexity, and consistent with the researchers’ hypothesis, coexpression with shRNA-D1 resulted in an even greater increase in dendritic growth. Thus, an extension of the time window of GABA depolarization was required for DISC1-dependent regulation of dendritic growth during postnatal neurogenesis. Interestingly, the increases in dendritic growth associated with shRNA-K2 alone, and with shRNA-D1 coexpression, were abolished using an mTOR inhibitor, similar to the effect of the inhibitor on DISC1 knockdown during adult neurogenesis (Kim et al., 2009). These data suggest a common molecular mechanism during both postnatal and adult neurogenesis.

Reminiscent of the effect of shRNA-K2 expression, maternal separation stress also elicited a delay in the polarity switch of GABA and produced accelerated dendritic growth after shRNA-D1 expression during postnatal neurogenesis. Interestingly, the absence of an effect on dendritic growth with DISC1 knockdown or maternal stress alone suggests an interaction between genetic susceptibility and environmental risk factors in the control of neuronal development. Given that the etiology of schizophrenia is thought to be mediated by a combination of genetic and environmental factors, the present data may represent an example of how such factors can combine to produce a neurodevelopmental effect.

Stress and DISC1 unite to regulate dendrites. On its own, DISC1 knockdown doesn't affect dendrite length in postnatal neurogenesis (D1 vs. C1, top), but in combination with maternal deprivation stress, the loss of DISC1 permits "exuberant" dendritic growth (D1 vs. C1, bottom). Image detail from Fig 6b. Image credit: Kim et al. and Cell

DISC1 and NKCC1 interact in risk for schizophrenia
Kim and colleagues also examined DISC1-GABA interactions in three independent cohorts of schizophrenia and healthy control subjects, amounting to a total of 2,961 individuals. The researchers observed that a single nucleotide polymorphism (SNP) in DISC1 (rs1000731) and one in SLC12A2 (rs10089), the gene encoding NKCC1, showed an interaction in the two largest cohorts. Although neither SNP was significantly associated with schizophrenia on its own in the three combined cohorts, minor allele carriers of both SNPs had a significantly increased risk for the illness. Moreover, using a public database containing information about association of SNPs with transcript expression (Heinzen et al., 2008), the researchers found that both SNPs predict the expression of regions within their respective genes, providing evidence that these SNPs may impact gene function.

In summary, Kim and colleagues show evidence that DISC1 targets depolarizing GABAergic signaling in regulating dendritic development during neurogenesis in both adulthood and in early postnatal development after stress. Moreover, DISC1 and GABA signaling appear to act synergistically to confer risk for schizophrenia. Although the exact mechanism by which DISC1 and inhibitory signaling may interact in schizophrenia remains to be determined, according to the authors, their data “suggest a context-dependent role for a prominent schizophrenia gene in neuronal development.”—Allison A. Curley.

Reference:
Kim JY, Liu CY, Zhang F, Duan X, Wen Z, Song J, Feighery E, Lu B, Rujescu D, St Clair D, Christian K, Callicott JH, Weinberger DR, Song H, Ming GL. Interplay between DISC1 and GABA Signaling Regulates Neurogenesis in Mice and Risk for Schizophrenia. Cell . 2012 Mar 2 ; 148(5):1051-64. Abstract

 
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