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DISC1 Continued: Mitofilin as a Mitochondrial Mechanism

2 November 2010. In a paper published in the October 12 issue of PNAS, a team of researchers at Pohang University of Science and Technology in Korea propose that the schizophrenia susceptibility gene DISC1 interacts with the mitochondrial inner-membrane protein mitofilin, and that underexpression or mutation of DISC1 may cause mitochondrial defects underlying some of the neurodevelopmental and neurophysiological phenotypes seen in schizophrenia.

Mitochondria serve such diverse and vital cellular roles that any compromise in their function may have profound effects. Besides their well-known job as ATP-generating “power plants,” mitochondria are involved in cell differentiation and growth, apoptosis, calcium regulation, and more. Researchers have identified a number of intriguing correlations between various forms of mitochondrial dysfunction and schizophrenia, but there has been little evidence of causal links.

Puzzle pieces
Members of David Porteous’s University of Edinburgh lab, where DISC1 was cloned in 2000 (Millar et al., 2000), reported four years later that DISC1 protein is primarily expressed in mitochondria (James et al., 2004), and they proposed that DISC1 mutations affecting the normal function of this essential organelle could play a fundamental role in schizophrenia.

At about the same time, Akira Sawa’s group at Johns Hopkins showed that DISC1 exerts its effects by forming complexes with proteins including the cytoskeletal protein NUDEL, or NDEL1. Mice carrying a truncated form of DISC1 that did not bind with NDEL1 exhibited abnormal neurite outgrowth and cortical development (Ozeki et al., 2003; also see SRF related news story). In a subsequent genomic study of patients, Sawa, Anil Malhotra, and colleagues reported an association between a DISC1 allele that reduces NDEL1 binding and schizophrenia (Burdick et al., 2008).

In parallel, many differences in mitochondrial structure and function have been observed in populations of patients with schizophrenia and with bipolar disorder. These findings, cataloged in a forthcoming review by Christine Kondradi and colleagues at Vanderbilt University (Clay et al., 2010), include reduced mitochondrial respiration; morphological abnormalities; more frequent mutations in mitochondrial DNA; and higher pH levels in postmortem brain samples.

Mark Vawter’s group at the University of California, Irvine, reported recently that synonymous base-pair substitutions in mitochondrial DNA were 22 percent more common in samples of dorsolateral prefrontal cortex from patients with schizophrenia compared to controls. In another postmortem study, Rosalinda Roberts and colleagues found that the overall number of mitochondria in striatal cells was similar in controls and patients with schizophrenia, but the number of mitochondria at striatal synapses was significantly reduced in patients (Somerville et al., 2010). (However, the researchers could not rule out that this difference is not an effect of antipsychotic medication or a compensatory response to the greater numbers of striatal synapses in patients with schizophrenia that they had reported in an earlier postmortem study.)

This accumulating evidence for an important mitochondrial role in schizophrenia is accompanied by a compelling reverse correlation: the physical symptoms in mitochondrial diseases are often accompanied by psychiatric disorders (Fattal et al., 2006).

Mighty mitofilin
First identified and characterized in the 1990s (Odgren et al., 1996; Gieffers et al., 1997), mitofilin is essential to normal mitochondrial function and morphology; knockdown of mitofilin results in reduced cell proliferation, increased apoptosis, and disorganization of the distinctive folds of the mitochondrial inner membrane known as cristae (John et al., 2005).

In the new work, led by Sang Ki Park, the researchers used several assays to screen for proteins that interact with NDEL1, then tested whether any of these proteins also interact with DISC1. One, mitofilin, interacted strongly with a region of DISC1, a finding that was corroborated by several additional assays. The group then established that a DISC1-mitofilin complex was selectively expressed in mitochondria.

With these findings in hand, the team generated short hairpin RNAs (shRNAs) to suppress DISC1 or mitofilin expression in a mouse neural cell line. Either shRNA significantly reduced activity of mitochondrial NADH hydrogenase and sharply reduced levels of ATP in cell samples, effects also seen in mice carrying a truncated form of DISC1. These disruptions of mitochondrial metabolism could be partially rescued by the coexpression of human DISC1 or mitofilin. The shRNAs against DISC1 and mitofilin also significantly reduced monoamine oxidase A (MAO-A) activity, but MAO-A activity was not significantly lower in cells with the truncated form of DISC1.

Because of the known effects of reduced expression of mitofilin on cell proliferation and apoptosis, the authors ruled out that the lower levels of ATP, NADH hydrogenase, or MAO-A might be due to smaller cell populations in the tested samples. Despite observations of abnormal mitochondrial morphology, they found no reduction in the number of cells or increase in apoptotic activity compared to controls.

Finally, the group used time-lapse imaging to measure DISC1-mitofilin effects on mitochondrial calcium regulation. When the researchers induced rapid increases in intracellular calcium concentration, calcium levels smoothly decreased to basal levels within five minutes. But shRNA-transfected cells exhibited an abnormal pattern of fluctuating calcium levels and a significantly delayed return to basal levels.

Of these various findings, the authors single out the reduction in MAO-A activity as the most interesting in a clinical context, since this observation dovetails well with the dopamine hypothesis of schizophrenia. “Downregulation of MAO-A by compromised functioning of the DISC1-mitofilin complex, which is likely to cause upregulated monoamine contents in monoaminergic neurons, may help explain the cellular basis underlying schizophrenia-associated neurochemical disturbances,” they write. “Thus, a direct link between abnormalities of DISC1 and dopamine homeostasis should be of immediate interest.”—Pete Farley.

Reference:
Park YU, Jeong J, Lee H, Mun JY, Kim JH, Lee JS, Nguyen MD, Han SS, Suh PG, Park SK. Disrupted-in-schizophrenia 1 (DISC1) plays essential roles in mitochondria in collaboration with Mitofilin. Proc Natl Acad Sci U S A. 2010 Oct 12;107(41):17785-90. Abstract

Comments on News and Primary Papers
Comment by:  Christine Konradi
Submitted 2 November 2010
Posted 2 November 2010

Novel findings on the role of DISC1 and mitochondrial function
One of the most interesting genetic leads for schizophrenia and affective disorders is a balanced translocation on chromosome 1, leading to the disruption of DISC1 (disrupted-in-schizophrenia 1). The translocation is observed in a Scottish family with a history of major psychiatric disorders, and the linkage with psychiatric disorders has been thoroughly studied and confirmed (Blackwood et al., 2001). While the function of DISC1 is not entirely known, it has a strong connection with mitochondria. Animal models and studies in cell lines and cortical cultures showed that the protein localizes predominantly to mitochondria (Brandon et al., 2005; James et al., 2004; Morris et al., 2003). Expression of truncated DISC1 in cell lines, mimicking the translocation breakpoint in the Scottish pedigree, led to decreased mitochondrial localization (Brandon et al., 2005; Millar et al., 2005). Furthermore, overexpression of truncated DISC1 isoforms induced abnormal mitochondrial morphologies, and affected mitochondrial fission and fusion (Millar et al., 2005).

Mitochondrial pathology has been implicated in schizophrenia as well as affective disorders, and has been verified in a variety of experimental paradigms (for a recent review, see Clay et al., 2010). A recent article by Park et al. (Park et al., 2010) further elaborates on the link between mitochondrial function and DISC1. The authors examined the interaction of DISC1 with mitofilin, a transmembrane protein of the inner mitochondrial membrane with critical functions in mitochondrial morphology, mitochondrial fission, and fusion. In previous publications, mitofilin has been shown to interact with DISC1 (see, e.g., Camargo et al., 2007), but Park et al. took it one step further. In their study, the authors demonstrate that DISC1 affects protein levels of mitofilin by increasing ubiquitination and proteasome-mediated degradation. Reduction or truncation of DISC1 affected the activity of the electron transport chain and led to a decrease in ATP levels. Moreover, reduction in DISC1 caused abnormal Ca2+ buffering dynamics and reduced the activity of monoamine oxidase A. Thus, the authors present a mechanism by which DISC1 is connected to mitochondrial location and mitochondrial function, providing further evidence that mitochondrial dysfunction can be an important factor in major psychiatric disorders.

References:

Blackwood, D. H., Fordyce, A., Walker, M. T., St Clair, D. M., Porteous, D. J., Muir, W. J., 2001. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet. 69, 428-33. Abstract

Brandon, N. J., Schurov, I., Camargo, L. M., Handford, E. J., Duran-Jimeniz, B., Hunt, P., Millar, J. K., Porteous, D. J., Shearman, M. S., Whiting, P. J., 2005. Subcellular targeting of DISC1 is dependent on a domain independent from the Nudel binding site. Mol Cell Neurosci. 28, 613-24. Abstract

Camargo, L. M., Collura, V., Rain, J. C., Mizuguchi, K., Hermjakob, H., Kerrien, S., Bonnert, T. P., Whiting, P. J., Brandon, N. J., 2007. Disrupted in Schizophrenia 1 Interactome: evidence for the close connectivity of risk genes and a potential synaptic basis for schizophrenia. Mol Psychiatry. 12, 74-86. Abstract

Clay, H. B., Sillivan, S., Konradi, C., 2010. Mitochondrial dysfunction and pathology in bipolar disorder and schizophrenia. Int J Dev Neurosci. Abstract

James, R., Adams, R. R., Christie, S., Buchanan, S. R., Porteous, D. J., Millar, J. K., 2004. Disrupted in Schizophrenia 1 (DISC1) is a multicompartmentalized protein that predominantly localizes to mitochondria. Mol Cell Neurosci. 26, 112-22. Abstract

Millar, J. K., James, R., Christie, S., Porteous, D. J., 2005. Disrupted in schizophrenia 1 (DISC1): subcellular targeting and induction of ring mitochondria. Mol Cell Neurosci. 30, 477-84. Abstract

Morris, J. A., Kandpal, G., Ma, L., Austin, C. P., 2003. DISC1 (Disrupted-In-Schizophrenia 1) is a centrosome-associated protein that interacts with MAP1A, MIPT3, ATF4/5 and NUDEL: regulation and loss of interaction with mutation. Hum Mol Genet. 12, 1591-608. Abstract

Park, Y. U., Jeong, J., Lee, H., Mun, J. Y., Kim, J. H., Lee, J. S., Nguyen, M. D., Han, S. S., Suh, P. G., Park, S. K., 2010. Disrupted-in-schizophrenia 1 (DISC1) plays essential roles in mitochondria in collaboration with Mitofilin. Proc Natl Acad Sci U S A. 107, 17785-90. Abstract

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Comments on Related News


Related News: DISC1 Delivers—Genetic, Molecular Studies Link Protein to Axonal Transport

Comment by:  Akira Sawa, SRF Advisor
Submitted 12 January 2007
Posted 12 January 2007

Although DISC1 is multifunctional, its role for neurite outgrowth has been substantially characterized for the past couple of years (Ozeki et al., 2003; Miyoshi et al., 2003; Kamiya et al., 2006). These studies indicated that DISC1 is involved in neurite outgrowth by more than one mechanism, such as interactions with NUDEL/NDEL1 and FEZ1.

These two papers from Kaibuchi’s lab provide further understanding of how DISC1 is involved in neuronal outgrowth. Kaibuchi’s group identified kinesin heavy chain of kinesin-1 as a novel interactor of DISC1. In their papers, a novel role for DISC1, to link kinesin-1 (microtubule-dependent and plus-end directed motor) to several cellular molecules, including NUDEL, LIS1, 14-3-3, and Grb2, is reported. DISC1 and kinesin-1 are, therefore, responsible to sort Grb2 to the distal part of axons where Grb2 functions as an adaptor and plays a role in NT-3-induced phosphorylation of ERK1/2. This mechanism well explains our previous work, led by Ryota Hashimoto, reporting that knockdown of DISC1 expression results in decreased levels of phosphorylation of ERK1/2 and Akt in primary cortical neurons (Hashimoto et al., 2006).

The interaction of DISC1 and kinesin-1 may also be interesting from the perspective of psychiatric genetics. First, the mechanism proposed in one of the papers (Taya et al., 2007) supports the notion that the C-terminal truncated DISC1 fragment—that occurs due to the balanced translocation in an extended Scottish family—functions as a dominant-negative. Second, the domain of DISC1 responsible for kinesin-1 is overlapped with the haplotype block region(s) that are positive in more than one association study of DISC1 and major mental illnesses.

View all comments by Akira Sawa

Related News: DISC1 Delivers—Genetic, Molecular Studies Link Protein to Axonal Transport

Comment by:  Luiz Miguel Camargo (Disclosure)
Submitted 13 January 2007
Posted 13 January 2007

Two recent back-to-back papers, published this month in Journal of Neuroscience, highlight the value of protein-protein interactions in determining the biological role of a key schizophrenia risk factor, DISC1, in processes that are important for the proper development of neurons.

Key questions need to be addressed once having established a set of interactors for a given protein. First, where do these proteins interact on the target molecule? Second, do these interactions take place at the same time (i.e., do they form a complex)? Third, in what context do these interactions occur (temporal, tissue/cell compartment, signaling), and, fourth, are the biological processes of the interacting molecules affected/regulated by the protein of interest? The Kaibuchi lab, as exemplified in the works by Taya et al. and Shinoda et al., elegantly address some of these questions in the context of DISC1 interactions with Grb2, Nudel (NDEL1), 14-3-3ε, and kinesin-1. The key findings of these papers are as follows:

1. Identification of the interaction sites, or more importantly, which part of DISC1 is involved in particular processes, for example, that axon elongation is dependent on the N-terminal, but not the C-terminal portion of DISC1. This suggests that the DISC1 role in axon elongation is mediated by interactions with the N-terminal portion of DISC1 that could be competed for by the truncated protein in a dominant negative fashion (Camargo et al., 2007).

2. Although a protein may have many interacting partners, such as DISC1, these interactions may not occur at the same time. For example, DISC1 is able to form a ternary complex with kinesin-1 and NDEL1 or with kinesin-1 and Grb2. However, a ternary complex of DISC1-Grb2-NDEL1 is not possible as Grb2 and NDEL1 may be competing for the same interaction site on DISC1.

3. Protein interactions may occur in certain cellular compartments, in the case of DISC1, the cell body and the distal part of axons.

4. Neurotrophin-induced axon elongation requires DISC1.

These papers confirm some of the hypotheses raised by the interactions that we have recently derived for DISC1 and some of its interacting partners (see Camargo et al., 2007). From the DISC1 interactome, we concluded that DISC1 may affect key intracellular transport mechanisms, such as those regulated by kinesins, and that DISC1 may be downstream of neurotrophin receptors, via its interaction with SH3BP5, an adaptor protein, which we found to interact with SOS1, a guanine exchange factor that binds Grb2 and responds to signaling of neurotrophin receptors. These observations have been validated by Taya et al. and Shinoda et al. and demonstrate the value of the DISC1 interactome in understanding the role of DISC1, and as a valuable resource to the wider community.

The molecular function of DISC1, as defined by its structure, still remains elusive, requiring a more dedicated effort on this front. The good news is that, via its protein-protein interactions, significant progress on the role of DISC1 in key biological processes has been achieved, as illustrated by the work of different labs (Brandon et. al., 2004; Millar et al., 2005; Kamiya et al., 2005; and now by Shinoda et al. and Taya et al.).

View all comments by Luiz Miguel Camargo