20 November 2005. When it rains, it pours, they say, and that may be the case with two major papers, appearing in rapid succession and unleashing a torrent of new information about the role of the DISC1 gene in mental illness. DISC1 (disrupted in schizophrenia 1) has been identified in several families with schizophrenia and other major mental disorders, but just how mutations in this gene precipitate illness is unclear. A new report from Kirsty Millar, David Porteous, and colleagues in Science reveals that DISC1 partners with phosphodiesterase 4B, an enzyme that regulates levels of cAMP in neurons. This link between DISC1 and the cAMP signaling pathway, which functions in cognition, memory, and mood, could help explain how the altered genes can elicit psychiatric symptoms via faulty neurotransmitter signaling.
The pathogenesis of schizophrenia has a developmental component as well, and in another paper, Akira Sawa and colleagues from the US and Japan explore the impact of the DISC1 translocation on brain development. Writing in Nature Cell Biology, the group reports that a carboxy-terminal truncated form of DISC1 disrupts neuronal migration and axon growth by destabilizing the dynein motor and disrupting microtubule dynamics. Their work explains how the translocation involving DISC1 could contribute to the changes in neural architecture that have been reported in the brains of people with schizophrenia.
It was a rearranged chromosome in a large Scottish family with frequent mental illness that led Porteous and colleagues to DISC1 several years ago (Millar et al., 2000). In their new paper, Millar and collaborators in Edinburgh, Glasgow, France, and the UK report the discovery of a different rearrangement in another person with schizophrenia that disrupts two genes, phosphodiesterase 4B (PDE4B) and cadherin 8. Of the two, they chose to focus on PDE4B, an enzyme which degrades and inactivates the second messenger cAMP. They showed that expression of PDE4B was decreased in the patient with the translocation. Add to that a previous observation (Brandon et al., 2004) that DISC1 and PDE4B interact in a two-hybrid screen, and the researchers were off and running, looking for a link between the two proteins in cells.
Several approaches established that DISC1 and PDE4B were directly physically associated in neurons. Standard coexpression and coimmunoprecipitation experiments revealed that multiple isoforms of PDE4B directly interacted with the N-terminal end of DISC1. The endogenous PDE4B1 isoform coprecipitated with a 71 kDa form of DISC1. Cell fractionation showed that both of the proteins were most abundant in the mitochondria-enriched fraction, and fluorescence microscopy revealed an overlapping distribution in that compartment.
PDE4B enzyme activity is increased by elevation of cAMP and activation of PKA, which phosphorylates a regulatory domain on PDE4B. Using pharmacological elevators of cAMP, Millar and coworkers showed that increasing cAMP caused a decrease in the association of PDE4B with DISC1, and this effect required PKA activity. Their results support the idea that DISC1 binds a dephosphorylated, inactive form of PDE4B. When cAMP levels go up, PDE4B becomes phosphorylated and active, and the proteins dissociate. Having shown that chromosomal translocations involving PDE4B or DISC1 decreased expression of their respective proteins by about half, and that the interaction between the two proteins is dynamic, the authors conclude by speculating that “functional variation in DISC1 and/or PDE4 will modulate their interaction and affect mitochondrial cAMP catabolism with a concomitant physiological and psychiatric outcome.”
In a perspective piece in the same issue of Science, Sawa and Solomon Snyder from Johns Hopkins ask whether PDE4B presents a new therapeutic target for schizophrenia. If DISC1 deficiency results in higher PDE4 activity, it would make sense to look for agents that increase binding of DISC1 to PDE4B, or even direct inhibitors of the enzyme, like the antidepressant rolipram (see SRF related news story). But, they point out, it’s hard to predict the effects of elevating cAMP, which serves different functions in different areas of the brain. Nonetheless, they conclude that the work provides a unifying link between schizophrenia and mood disorders, which often occur in the same families, and sometimes in the same individuals, as in schizoaffective disorder.
In his paper in Nature Cell Biology, Sawa has another story to tell about DISC1 and its role in neurodevelopment. In this study, first author Atsushi Kamiya and coworkers show that DISC1 is part of the dynein motor complex, along with the developmentally important proteins NUDEL and LIS1, and stabilizes the complex at the centrosome, contributing to normal microtubule dynamics. The carboxy-terminal truncation of DISC1 that occurs in the Scottish family acts like a dominant negative, knocking normal DISC1 out of the dynein complex and impairing motor activity.
In neurons, the dynein motor and its effects on microtubule organization help drive neuronal migration and axon formation during development. When the researchers used RNAi to lower DISC1 expression in neurons in culture, or expressed the truncated DISC1, they observed decreased neurite outgrowth. Moving in vivo, they performed in utero electroporation to deliver RNAi or mutated DISC1 to fetal rat cortical neurons, and showed decreased migration of neurons during cortical development. Neurons that did make it to their correct positions had impaired orientation, polarity, and dendritic arborization. While nearly complete knockdown of DISC1 with a potent RNAi caused almost full inhibition of migration, the mutant DISC1 produced a partial migration defect, consistent with the subtle developmental changes seen in brains of schizophrenic patients.
Whether the DISC1 gene translocation in people results in a deficiency of protein, or the production of a mutant protein, or both, is still an open question, but either could account for what Sawa and colleagues propose to be a key developmental impairment resulting from a loss of DISC1 function. Importantly, both the PDE4 and dynein connections for DISC1 allow for dose-dependent partial effects that could produce a spectrum of disorders, with various combinations of schizophrenic and affective symptoms, stemming from both developmental and functional roots.—Pat McCaffrey.
Millar JK, Pickard BS, Mackie S, James R, Christie S, Buchanan SR, Malloy MP, Chubb JE, Huston E, Baillie GS, Thomson PA, Hill EV, Brandon NJ, Rain JC, Camargo LM, Whiting PJ, Houslay MD, Blackwood DH, Muir WJ, Porteous DJ. DISC1 and PDE4B Are Interacting Genetic Factors in Schizophrenia That Regulate cAMP Signaling. Science. 2005 Nov 18;310:1187-1191. Abstract
Sawa A, Snyder SH. GENETICS: Two Genes Link Two Distinct Psychoses. Science. 2005 Nov 18;310:1128-1129. Abstract
Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME,Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nature Cell Biology. 2005 November 20. Advance online publication. Abstract