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Pathogenic disruption of DISC1-serine racemase binding elicits schizophrenia-like behavior via D-serine depletion.

Ma TM, Abazyan S, Abazyan B, Nomura J, Yang C, Seshadri S, Sawa A, Snyder SH, Pletnikov MV - Mol. Psychiatry (2012)

Bottom Line: Expression of mutant DISC1 downregulates endogenous DISC1 and decreases protein but not mRNA levels of SR, resulting in diminished production of D-serine.In contrast, mutant DISC1 does not alter levels of ALDH1L1, connexins, GLT-1 or binding partners of DISC1 and SR, LIS1 or PICK1.These findings support a model wherein mutant DISC1 leads to SR degradation via dominant negative effects, resulting in D-serine deficiency that diminishes NMDA neurotransmission thus linking DISC1 and NMDA pathophysiological mechanisms in mental illness.

View Article: PubMed Central - PubMed

Affiliation: The Solomon H. Snyder Department of Neuroscience, Baltimore, MD, USA.

ABSTRACT
Perturbation of Disrupted-In-Schizophrenia-1 (DISC1) and D-serine/NMDA receptor hypofunction have both been implicated in the pathophysiology of schizophrenia and other psychiatric disorders. In the present study, we demonstrate that these two pathways intersect with behavioral consequences. DISC1 binds to and stabilizes serine racemase (SR), the enzyme that generates D-serine, an endogenous co-agonist of the NMDA receptor. Mutant DISC1 fails to bind to SR, facilitating ubiquitination and degradation of SR and a decrease in D-serine production. To elucidate DISC1-SR interactions in vivo, we generated a mouse model of selective and inducible expression of mutant DISC1 in astrocytes, the main source of D-serine in the brain. Expression of mutant DISC1 downregulates endogenous DISC1 and decreases protein but not mRNA levels of SR, resulting in diminished production of D-serine. In contrast, mutant DISC1 does not alter levels of ALDH1L1, connexins, GLT-1 or binding partners of DISC1 and SR, LIS1 or PICK1. Adult male and female mice with lifelong expression of mutant DISC1 exhibit behavioral abnormalities consistent with hypofunction of NMDA neurotransmission. Specifically, mutant mice display greater responses to an NMDA antagonist, MK-801, in open field and pre-pulse inhibition of the acoustic startle tests and are significantly more sensitive to the ameliorative effects of D-serine. These findings support a model wherein mutant DISC1 leads to SR degradation via dominant negative effects, resulting in D-serine deficiency that diminishes NMDA neurotransmission thus linking DISC1 and NMDA pathophysiological mechanisms in mental illness.

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Mutant DISC1 depletes SR protein by increasing its ubiquitination level[a] Cycloheximide chase assay. HEK-293 cells were treated with 70 mg/ml cycloheximide 24–36 h after transfection and SR level was quantified. Protein levels were expressed as percentage of the level at time 0. The half-life of SR is significantly shorter when co-expressed with mutant DISC1.[b,c]In vivo ubiquitination assay of mouse [b] and human SR [c] in HEK-293 cells.The levels of mouse and human SR ubiquitination are higher when co-expressed with mutant DISC1 as quantified in [d] and [e], respectively.Primary astrocytes [f] were treated with the proteasome inhibitor MG-132 (30 µM for 10 h), and relative SR protein levels are quantified below the blots. The value of SR level in primary astrocytes from control mice before treatment has been normalized to 1.0.
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Figure 4: Mutant DISC1 depletes SR protein by increasing its ubiquitination level[a] Cycloheximide chase assay. HEK-293 cells were treated with 70 mg/ml cycloheximide 24–36 h after transfection and SR level was quantified. Protein levels were expressed as percentage of the level at time 0. The half-life of SR is significantly shorter when co-expressed with mutant DISC1.[b,c]In vivo ubiquitination assay of mouse [b] and human SR [c] in HEK-293 cells.The levels of mouse and human SR ubiquitination are higher when co-expressed with mutant DISC1 as quantified in [d] and [e], respectively.Primary astrocytes [f] were treated with the proteasome inhibitor MG-132 (30 µM for 10 h), and relative SR protein levels are quantified below the blots. The value of SR level in primary astrocytes from control mice before treatment has been normalized to 1.0.

Mentions: We explored mechanisms whereby mutant DISC1 affects levels of SR. Mutant DISC1 does not appear to affect transcription of SR, as mRNA levels of SR in primary mutant astrocytes are unaltered (Figure 3i). To assess a potential influence of DISC1 upon the stability of SR, we monitored the rate of decline of SR in HEK-293 cells treated with cycloheximide (Figure 4a). SR levels decline much more rapidly in the presence of mutant than wild-type DISC1. Thus, 24 h after treatment, SR protein levels decline by 50% in cells transfected with mutant DISC1 compared to a 15% decrease in SR levels in cells that express wild-type DISC1.


Pathogenic disruption of DISC1-serine racemase binding elicits schizophrenia-like behavior via D-serine depletion.

Ma TM, Abazyan S, Abazyan B, Nomura J, Yang C, Seshadri S, Sawa A, Snyder SH, Pletnikov MV - Mol. Psychiatry (2012)

Mutant DISC1 depletes SR protein by increasing its ubiquitination level[a] Cycloheximide chase assay. HEK-293 cells were treated with 70 mg/ml cycloheximide 24–36 h after transfection and SR level was quantified. Protein levels were expressed as percentage of the level at time 0. The half-life of SR is significantly shorter when co-expressed with mutant DISC1.[b,c]In vivo ubiquitination assay of mouse [b] and human SR [c] in HEK-293 cells.The levels of mouse and human SR ubiquitination are higher when co-expressed with mutant DISC1 as quantified in [d] and [e], respectively.Primary astrocytes [f] were treated with the proteasome inhibitor MG-132 (30 µM for 10 h), and relative SR protein levels are quantified below the blots. The value of SR level in primary astrocytes from control mice before treatment has been normalized to 1.0.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3475769&req=5

Figure 4: Mutant DISC1 depletes SR protein by increasing its ubiquitination level[a] Cycloheximide chase assay. HEK-293 cells were treated with 70 mg/ml cycloheximide 24–36 h after transfection and SR level was quantified. Protein levels were expressed as percentage of the level at time 0. The half-life of SR is significantly shorter when co-expressed with mutant DISC1.[b,c]In vivo ubiquitination assay of mouse [b] and human SR [c] in HEK-293 cells.The levels of mouse and human SR ubiquitination are higher when co-expressed with mutant DISC1 as quantified in [d] and [e], respectively.Primary astrocytes [f] were treated with the proteasome inhibitor MG-132 (30 µM for 10 h), and relative SR protein levels are quantified below the blots. The value of SR level in primary astrocytes from control mice before treatment has been normalized to 1.0.
Mentions: We explored mechanisms whereby mutant DISC1 affects levels of SR. Mutant DISC1 does not appear to affect transcription of SR, as mRNA levels of SR in primary mutant astrocytes are unaltered (Figure 3i). To assess a potential influence of DISC1 upon the stability of SR, we monitored the rate of decline of SR in HEK-293 cells treated with cycloheximide (Figure 4a). SR levels decline much more rapidly in the presence of mutant than wild-type DISC1. Thus, 24 h after treatment, SR protein levels decline by 50% in cells transfected with mutant DISC1 compared to a 15% decrease in SR levels in cells that express wild-type DISC1.

Bottom Line: Expression of mutant DISC1 downregulates endogenous DISC1 and decreases protein but not mRNA levels of SR, resulting in diminished production of D-serine.In contrast, mutant DISC1 does not alter levels of ALDH1L1, connexins, GLT-1 or binding partners of DISC1 and SR, LIS1 or PICK1.These findings support a model wherein mutant DISC1 leads to SR degradation via dominant negative effects, resulting in D-serine deficiency that diminishes NMDA neurotransmission thus linking DISC1 and NMDA pathophysiological mechanisms in mental illness.

View Article: PubMed Central - PubMed

Affiliation: The Solomon H. Snyder Department of Neuroscience, Baltimore, MD, USA.

ABSTRACT
Perturbation of Disrupted-In-Schizophrenia-1 (DISC1) and D-serine/NMDA receptor hypofunction have both been implicated in the pathophysiology of schizophrenia and other psychiatric disorders. In the present study, we demonstrate that these two pathways intersect with behavioral consequences. DISC1 binds to and stabilizes serine racemase (SR), the enzyme that generates D-serine, an endogenous co-agonist of the NMDA receptor. Mutant DISC1 fails to bind to SR, facilitating ubiquitination and degradation of SR and a decrease in D-serine production. To elucidate DISC1-SR interactions in vivo, we generated a mouse model of selective and inducible expression of mutant DISC1 in astrocytes, the main source of D-serine in the brain. Expression of mutant DISC1 downregulates endogenous DISC1 and decreases protein but not mRNA levels of SR, resulting in diminished production of D-serine. In contrast, mutant DISC1 does not alter levels of ALDH1L1, connexins, GLT-1 or binding partners of DISC1 and SR, LIS1 or PICK1. Adult male and female mice with lifelong expression of mutant DISC1 exhibit behavioral abnormalities consistent with hypofunction of NMDA neurotransmission. Specifically, mutant mice display greater responses to an NMDA antagonist, MK-801, in open field and pre-pulse inhibition of the acoustic startle tests and are significantly more sensitive to the ameliorative effects of D-serine. These findings support a model wherein mutant DISC1 leads to SR degradation via dominant negative effects, resulting in D-serine deficiency that diminishes NMDA neurotransmission thus linking DISC1 and NMDA pathophysiological mechanisms in mental illness.

Show MeSH
Related in: MedlinePlus