Search

Signal-induced proliferation-associated 1 (SIPA1)-like 1 (SIPA1L1; also known as SPAR1) has been proposed to regulate synaptic functions that are important in maintaining normal neuronal activities, such as regulating spine growth and synaptic scaling, as a component of the PSD-95/NMDA-R-complex. However, its physiological role remains poorly understood. Here, we performed expression analyses using super-resolution microscopy (SRM) in mouse brain and demonstrated that SIPA1L1 is mainly localized to general submembranous regions in neurons, but surprisingly, not to PSD. Our screening for physiological interactors of SIPA1L1 in mouse brain identified spinophilin and neurabin-1, regulators of G-protein-coupled receptor (GPCR) signaling, but rejected PSD-95/NMDA-R-complex components. Furthermore,Sipa1l1−/−mice showed normal spine size distribution and NMDA-R-dependent synaptic plasticity. Nevertheless,Sipa1l1−/−mice showed aberrant responses to α2-adrenergic receptor (a spinophilin target) or adenosine A1 receptor (a neurabin-1 target) agonist stimulation, and striking behavioral anomalies, such as hyperactivity, enhanced anxiety, learning impairments, social interaction deficits, and enhanced epileptic seizure susceptibility. Male mice were used for all experiments. Our findings revealed unexpected properties of SIPA1L1, suggesting a possible association of SIPA1L1 deficiency with neuropsychiatric disorders related to dysregulated GPCR signaling, such as epilepsy, attention deficit hyperactivity disorder (ADHD), autism, or fragile X syndrome (FXS).SIGNIFICANCE STATEMENTSignal-induced proliferation-associated 1 (SIPA1)-like 1 (SIPA1L1) is thought to regulate essential synaptic functions as a component of the PSD-95/NMDA-R-complex. In our screening for physiological SIPA1L1-interactors, we identified G-protein-coupled receptor (GPCR)-signaling regulators. Moreover, SIPA1L1 knock-out (KO) mice showed striking behavioral anomalies, which may be relevant to GPCR signaling. Our findings revealed an unexpected role of SIPA1L1, which may open new avenues for research on neuropsychiatric disorders that involve dysregulated GPCR signaling. Another important aspect of this paper is that we showed effective methods for checking PSD association and identifying native protein interactors that are difficult to solubilize. These results may serve as a caution for future claims about interacting proteins and PSD proteins, which could eventually save time and resources for researchers and avoid confusion in the field.

SIPA1L1 (also known as SPAR1) has been proposed to regulate synaptic functions that are important in maintaining normal neuronal activities, such as regulating spine growth and synaptic scaling, as a component of the postsynaptic density (PSD)-95/N-methyl-D-aspartate receptor (NMDA-R)-complex. However, contrary to this view, our super-resolution and immunoelectron microscopic analyses demonstrate that SIPA1L1 is mainly localized to general submembranous and cytoplasmic regions in neurons, but scarcely to PSD. Our screening for native interactors of SIPA1L1 identified spinophilin and neurabin-1, regulators of G protein-coupled receptor (GPCR) signaling, but rejected PSD-95/NMDA-R-complex components. Furthermore, Sipa1l1-/- mice showed normal spine size distribution and NMDA-R-dependent synaptic plasticity. Nevertheless, Sipa1l1-/- mice showed aberrant responses to α2-adrenergic receptor (a spinophilin target) or adenosine A1 receptor (a neurabin-1 target) agonist stimulation and striking behavioral anomalies, such as hyperactivity, enhanced anxiety, learning impairments, social interaction deficits, and enhanced epileptic seizure susceptibility. Our findings revealed unexpected properties of SIPA1L1, suggesting a possible association of SIPA1L1 deficiency with neuropsychiatric disorders related to dysregulated GPCR signaling, such as epilepsy, attention deficit hyperactivity disorder (ADHD), autism, or fragile X syndrome.

Genomic defects with large effect size can help elucidate unknown pathologic architecture of mental disorders. We previously reported on a patient with schizophrenia and a balanced translocation between chromosomes 4 and 13 and found that the breakpoint within chromosome 4 is located near the LDB2 gene. We show here that Ldb2 knockout (KO) mice displayed multiple deficits relevant to mental disorders. In particular, Ldb2 KO mice exhibited deficits in the fear‐conditioning paradigm. Analysis of the amygdala suggested that dysregulation of synaptic activities controlled by the immediate early gene Arc is involved in the phenotypes. We show that LDB2 forms protein complexes with known transcription factors. Consistently, ChIP‐seq analyses indicated that LDB2 binds to > 10,000 genomic sites in human neurospheres. We found that many of those sites, including the promoter region of ARC, are occupied by EGR transcription factors. Our previous study showed an association of the EGR family genes with schizophrenia. Collectively, the findings suggest that dysregulation in the gene expression controlled by the LDB2‐EGR axis underlies a pathogenesis of subset of mental disorders., The LDB2 gene is mapped at the breakpoint of a balanced chromosomal translocation seen in a patient with schizophrenia. This study investigates the role of LDB2 and transcriptional regulation exerted by the “LDB2‐EGR axis” in the pathogenesis of mental disorders.

Down syndrome is a complex genetic disorder caused by the presence of three copies of the chromosome 21 in humans. The most common models, carrying extra-copies of overlapping fragments of mouse chromosome 16 that is syntenic to human chromosome 21, are Ts2Cje, Ts1Cje and Ts1Rhr mice. In electrophysiological analyses using hippocampal slices, we found that the later phase of the depolarization during tetanic stimulation, which was regulated by GABAB receptors, was significantly smaller in Ts1Cje and Ts2Cje mice than that in WT controls but not in Ts1Rhr mice. Furthermore, isolated GABAB receptor-mediated inhibitory synaptic responses were larger in Ts1Cje mice. To our knowledge, this is the first report that directly shows the enhancement of GABAB receptor-mediated synaptic currents in Ts1Cje mice. These results suggest that GABAB receptor-mediated synaptic inhibition was enhanced in Ts1Cje and Ts2Cje mice but not in Ts1Rhr mice. The Cbr1 gene, which is present in three copies in Ts1Cje and Ts2Cje but not in Ts1Rhr, encodes carbonyl reductase that may facilitate GABAB-receptor activity through a reduction of prostaglandin E2 (PGE2). Interestingly, we found that a reduction of PGE2 and an memory impairment in Ts1Cje mice were alleviated when only Cbr1 was set back to two copies (Ts1Cje;Cbr1+/+/−). However, the GABAB receptor-dependent enhancement of synaptic inhibition in Ts1Cje was unaltered in Ts1Cje;Cbr1+/+/− mice. These results indicate that Cbr1 is one of the genes responsible for DS cognitive impairments and the gene(s) other than Cbr1, which is included in Ts1Cje but not in Ts1Rhr, is responsible for the GABAB receptor-dependent over-inhibition.

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

Phosphorylation of neural proteins in response to a diverse array of external stimuli is one of the main mechanisms underlying dynamic changes in neural circuitry. The NR2B subunit of the NMDA receptor is tyrosine-phosphorylated in the brain, with Tyr-1472 its major phosphorylation site. Here, we generate mice with a knockin mutation of the Tyr-1472 site to phenylalanine (Y1472F) and show that Tyr-1472 phosphorylation is essential for fear learning and amygdaloid synaptic plasticity. The knockin mice show impaired fear-related learning and reduced amygdaloid long-term potentiation. NMDA receptor-mediated CaMKII signaling is impaired in YF/YF mice. Electron microscopic analyses reveal that the Y1472F mutant of the NR2B subunit shows improper localization at synapses in the amygdala. We thus identify Tyr-1472 phosphorylation as a key mediator of fear learning and amygdaloid synaptic plasticity.

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.