Emerging roles of ARHGAP33 in intracellular trafficking of TrkB and pathophysiology of neuropsychiatric disorders

Intracellular trafficking of receptor proteins is essential for neurons to detect various extracellular factors during the formation and refinement of neural circuits. However, the precise mechanisms underlying the trafficking of neurotrophin receptors to synapses remain elusive. Here, we demonstrate that a brain-enriched sorting nexin, ARHGAP33, is a new type of regulator for the intracellular trafficking of TrkB, a high-affinity receptor for brain-derived neurotrophic factor. ARHGAP33 knockout (KO) mice exhibit reduced expression of synaptic TrkB, impaired spine development and neuropsychiatric disorder-related behavioural abnormalities. These deficits are rescued by specific pharmacological enhancement of TrkB signalling in ARHGAP33 KO mice. Mechanistically, ARHGAP33 interacts with SORT1 to cooperatively regulate TrkB trafficking. Human ARHGAP33 is associated with brain phenotypes and reduced SORT1 expression is found in patients with schizophrenia. We propose that ARHGAP33/SORT1-mediated TrkB trafficking is essential for synapse development and that the dysfunction of this mechanism may be a new molecular pathology of neuropsychiatric disorders

Correlated Alterations in Serotonergic and Dopaminergic Modulations at the Hippocampal Mossy Fiber Synapse in Mice Lacking Dysbindin

Dysbindin-1 (dystrobrevin-binding protein 1, DTNBP1) is one of the promising schizophrenia susceptibility genes. Dysbindin protein is abundantly expressed in synaptic regions of the hippocampus, including the terminal field of the mossy fibers, and this hippocampal expression of dysbindin is strongly reduced in patients with schizophrenia. In the present study, we examined the functional role of dysbindin in hippocampal mossy fiber-CA3 synaptic transmission and its modulation using the sandy mouse, a spontaneous mutant with deletion in the dysbindin gene. Electrophysiological recordings were made in hippocampal slices prepared from adult male sandy mice and their wild-type littermates. Basic properties of the mossy fiber synaptic transmission in the mutant mice were generally normal except for slightly reduced frequency facilitation. Serotonin and dopamine, two major neuromodulators implicated in the pathophysiology of schizophrenia, can potentiate mossy fiber synaptic transmission probably via an increase in cAMP levels. Synaptic potentiation induced by serotonin and dopamine was very variable in magnitude in the mutant mice, with some mice showing prominent enhancement as compared with the wild-type mice. In addition, the magnitude of potentiation induced by these monoamines significantly correlated with each other in the mutant mice, indicating that a subpopulation of sandy mice has marked hypersensitivity to both serotonin and dopamine. While direct activation of the cAMP cascade by forskolin induced robust synaptic potentiation in both wild-type and mutant mice, this forskolin-induced potentaition correlated in magnitude with the serotonin-induced potentiation only in the mutant mice, suggesting a possible change in coupling of receptor activation to downstream signaling. These results suggest that the dysbindin deficiency could be an essential genetic factor that causes synaptic hypersensitivity to dopamine and serotonin. The altered monoaminergic modulation at the mossy fiber synapse could be a candidate pathophysiological basis for impairment of hippocampus-dependent brain functions in schizophrenia

The p250GAP Gene Is Associated with Risk for Schizophrenia and Schizotypal Personality Traits

BACKGROUND: Hypofunction of the glutamate N-Methyl-d-aspartate (NMDA) receptor has been implicated in the pathophysiology of schizophrenia. p250GAP is a brain-enriched NMDA receptor-interacting RhoGAP. p250GAP is involved in spine morphology, and spine morphology has been shown to be altered in the post-mortem brains of patients with schizophrenia. Schizotypal personality disorder has a strong familial relationship with schizophrenia. Several susceptibility genes for schizophrenia have been related to schizotypal traits. METHODS: We first investigated the association of eight linkage disequilibrium-tagging single-nucleotide polymorphisms (SNPs) that cover the p250GAP gene with schizophrenia in a Japanese sample of 431 schizophrenia patients and 572 controls. We then investigated the impact of the risk genetic variant in the p250GAP gene on schizotypal personality traits in 180 healthy subjects using the Schizotypal Personality Questionnaire. RESULTS: We found a significant difference in genotype frequency between the patients and the controls in rs2298599 (χ(2) = 17.6, p = 0.00015). The minor A/A genotype frequency of rs2298599 was higher in the patients (18%) than in the controls (9%) (χ(2) = 15.5, p = 0.000083). Moreover, we found that subjects with the rs2298599 risk A/A genotype, compared with G allele carriers, had higher scores of schizotypal traits (F(1,178) = 4.08, p = 0.045), particularly the interpersonal factor (F(1,178) = 5.85, p = 0.017). DISCUSSION: These results suggest that a genetic variation in the p250GAP gene might increase susceptibility not only for schizophrenia but also for schizotypal personality traits. We concluded that the p250GAP gene might be a new candidate gene for susceptibility to schizophrenia

Basic properties of synaptic transmission at mossy fiber synapse.

<p>(A) Input-output relationship at the mossy fiber-CA3 synapse. The amplitude of the presynaptic fiber volley (left) and fEPSP (right) was plotted against the stimulus intensity (n = 15 slices each). (B) Frequency facilitation induced by repetitive stimulation was reduced in mutant mice at the stimulus frequency of 0.2 Hz (+/+: 54.0±2.8% increase, n = 8; −/−: 42.9±3.3% increase, n = 11; <i>p</i> = 0.039), but not at 1 Hz (+/+: n = 7; −/−: n = 10). (C) No significant difference in facilitation of fEPSP induced by paired stimulation (+/+: n = 8; −/−: n = 9).</p

Passive and active somatic membrane properties of dentate granule cells.

<p>(A) Reduced input resistance in mutant mice (+/+: 494.8±48.4 MΩ, n = 12 cells; −/−: 373.6±33.4 MΩ, n = 14 cells; <i>p</i> = 0.0477).v (B) Shorter membrane time constant in mutant mice (+/+: 33.2±2.2 ms; −/−: 24.9±1.4 ms; <i>p</i> = 0.0109). (C–F) There was no significant difference in resting membrane potentials (C), the current intensity to evoke a single spike (D), the maximal number of spikes during sustained depolarization (E), or spike amplitude (F).</p

Synaptic potentiation induced by forskolin at mossy fiber synapse.

<p>(A) Pooled data showing potentiation of mossy fiber synaptic transmission by bath-applied forskolin (10 µM). There was no significant difference in the magnitude of potentiation between wild-type and mutant mice (<i>p</i> = 0.6905, n = 5 each). The effect of serotonin examined in the same mice was shown in (B) and was larger in mutant mice (<i>p</i> = 0.0159). (B) Significant correlation between serotonin- and forskolin-induced synaptic potentiation in mutant mice (r² = 0.8177, <i>p</i> = 0.00351), but not in wild-type mice (r² = 0.5067, <i>p</i> = 0.1775).</p

Reduced hippocampal expression of dopamine D1 receptor in sandy mice.

<p>(A) Expression levels of mRNAs for serotonin 5-HT₄ receptor relative to those of GAPDH in the hippocampus (n = 10 each). (B) Relative expression levels of mRNAs for dopamine D₁ receptor in the hippocampus were reduced in mutant mice as compared with wild-type mice (+/+: 6.04±0.17; −/−: 5.16±0.2, n = 10 each; <i>p</i> = 0.0068).</p

Impact of the genome wide supported NRGN gene on anterior cingulate morphology in schizophrenia.

BACKGROUND: The rs12807809 single-nucleotide polymorphism in NRGN is a genetic risk variant with genome-wide significance for schizophrenia. The frequency of the T allele of rs12807809 is higher in individuals with schizophrenia than in those without the disorder. Reduced immunoreactivity of NRGN, which is expressed exclusively in the brain, has been observed in Brodmann areas (BA) 9 and 32 of the prefrontal cortex in postmortem brains from patients with schizophrenia compared with those in controls. METHODS: Genotype effects of rs12807809 were investigated on gray matter (GM) and white matter (WM) volumes using magnetic resonance imaging (MRI) with a voxel-based morphometry (VBM) technique in a sample of 99 Japanese patients with schizophrenia and 263 healthy controls. RESULTS: Although significant genotype-diagnosis interaction either on GM or WM volume was not observed, there was a trend of genotype-diagnosis interaction on GM volume in the left anterior cingulate cortex (ACC). Thus, the effects of NRGN genotype on GM volume of patients with schizophrenia and healthy controls were separately investigated. In patients with schizophrenia, carriers of the risk T allele had a smaller GM volume in the left ACC (BA32) than did carriers of the non-risk C allele. Significant genotype effect on other regions of the GM or WM was not observed for either the patients or controls. CONCLUSIONS: Our findings suggest that the genome-wide associated genetic risk variant in the NRGN gene may be related to a small GM volume in the ACC in the left hemisphere in patients with schizophrenia