Daisuke Ibi

RAGE-mediated signaling contributes to intraneuronal transport of amyloid-β and neuronal dysfunction

Intracellular amyloid-β peptide (Aβ) has been implicated in neuronal death associated with Alzheimers disease. Although Aβ is predominantly secreted into the extracellular space, mechanisms of Aβ transport at the level of the neuronal cell membrane remain to be fully elucidated. We demonstrate that receptor for advanced glycation end products (RAGE) contributes to transport of Aβ from the cell surface to the intracellular space. Mouse cortical neurons exposed to extracellular human Aβ subsequently showed detectable peptide intracellularly in the cytosol and mitochondria by confocal microscope and immunogold electron microscopy. Pretreatment of cultured neurons from wild-type mice with neutralizing antibody to RAGE, and neurons from RAGE knockout mice displayed decreased uptake of Aβ and protection from Aβ-mediated mitochondrial dysfunction. Aβ activated p38 MAPK, but not SAPK/JNK, and then stimulated intracellular uptake of Aβ-RAGE complex. Similar intraneuronal co-localization of Aβ and RAGE was observed in the hippocampus of transgenic mice overexpressing mutant amyloid precursor protein. These findings indicate that RAGE contributes to mechanisms involved in the translocation of Aβ from the extracellular to the intracellular space, thereby enhancing Aβ cytotoxicity.

Social isolation rearing-induced impairment of the hippocampal neurogenesis is associated with deficits in spatial memory and emotion-related behaviors in juvenile mice.

Experiences during brain development may influence the pathogenesis of developmental disorders. Thus, social isolation (SI) rearing after weaning is a useful animal model for studying the pathological mechanisms of such psychiatric diseases. In this study, we examined the effect of SI on neurogenesis in the hippocampal dentate gyrus (DG) relating to memory and emotion‐related behaviors. When newly divided cells were labeled with 5‐bromo‐2′‐deoxyuridine (BrdU) before SI, the number of BrdU‐positive cells and the rate of differentiation into neurons were significantly decreased after 4‐week SI compared with those in group‐housed mice. Repeated treatment of fluoxetine prevented the SI‐induced impairment of survival of newly divided cells and ameliorated spatial memory impairment and part of aggression in SI mice. Furthermore, we investigated the changes in gene expression in the DG of SI mice by using DNA microarray and real‐time PCR. We finally found that SI reduced the expression of development‐related genes Nurr1 and Npas4. These findings suggest that communication in juvenile is important in the survival and differentiation of newly divided cells, which may be associated with memory and aggression, and raise the possibility that the reduced expression of Nurr1 and/or Npas4 may contribute to the impairment of neurogenesis and memory and aggression induced by SI.

Behavioral abnormality and pharmacologic response in social isolation-reared mice

Social isolation (SI) rearing in rodents causes a variety of behavioral changes, including hyperlocomotion, anxiety, impulsivity, aggression, and learning and memory deficits. These behavioral abnormalities in rodents may be related to the symptoms in patients with neuropsychiatric disorders, such as attention-deficit hyperactivity disorder, obsessive-compulsive disorder, autism, schizophrenia and depression. In this study, we examined the effect of long-term SI rearing after weaning on emotional behaviors and cognitive function in mice. Furthermore, the effects of methylphenidate (MPH), clozapine (CLZ) and fluoxetine (FLX) on SI-induced behavioral changes were examined to measure the predictive validity of SI-reared mice as an animal model for these neuropsychiatric disorders. MPH improved SI-induced anxiety-like behavior in the elevated-plus maze test, but had no effect on aggressive behavior. In contrast, CLZ ameliorated aggressive behavior, but not anxiety-like behavior in SI-reared mice. Repeated FLX treatment prevented SI-induced aggressive behavior and social interaction deficits. These findings suggest that SI-induced behavioral abnormality is a psychobehavioral complex relevant to various clinical symptoms observed in neuropsychiatric disorders and that SI-reared mice are a useful animal model to study the pathophysiology/pathogenesis of these diseases.

Combined effect of neonatal immune activation and mutant DISC1 on phenotypic changes in adulthood

Gene-environment interaction may play a role in the etiology of schizophrenia. Transgenic mice expressing dominant-negative DISC1 (DN-DISC1 mice) show some histological and behavioral endophenotypes relevant to schizophrenia. Viral infection during neurodevelopment provides a major environmental risk for schizophrenia. Neonatal injection of polyriboinosinic-polyribocytidylic acid (polyI:C), which mimics innate immune responses elicited by viral infection, leads to schizophrenia-like behavioral alteration in mice after puberty. To study how gene-environmental interaction during neurodevelopment results in phenotypic changes in adulthood, we treated DN-DISC1 mice or wild-type littermates with injection of polyI:C during the neonatal stage, according to the published method, respectively, and the behavioral and histological phenotypes were examined in adulthood. We demonstrated that neonatal polyI:C treatment in DN-DISC1 mice resulted in the deficits of short-term, object recognition, and hippocampus-dependent fear memories after puberty, although polyI:C treatment by itself had smaller influences on wild-type mice. Furthermore, polyI:C-treated DN-DISC1 mice exhibited signs of impairment of social recognition and interaction, and augmented susceptibility to MK-801-induced hyperactivity as compared with vehicle-treated wild-type mice. Of most importance, additive effects of polyI:C and DN-DISC1 were observed by a marked decrease in parvalbumin-positive interneurons in the medial prefrontal cortex. These results suggest that combined effect of neonatal polyI:C treatment and DN-DISC1 affects some behavioral and histological phenotypes in adulthood.

Neonatal polyI:C treatment in mice results in schizophrenia-like behavioral and neurochemical abnormalities in adulthood

It has been reported that viral infection in the first and second trimesters of pregnancy in humans increases the risk of subsequently developing schizophrenia. To develop a mouse model of immune activation during the early postnatal period, neonatal ICR mice were repeatedly injected with polyriboinosinic-polyribocytidilic acid (polyI:C; an inducer of strong innate immune responses) for 5 days (postnatal day 2-6) which may correspond, in terms of brain development, to the early second trimester in human. Cognitive and emotional behavior as well as the extracellular level of glutamate in the hippocampus were analyzed at the age of 10-12 weeks old. PolyI:C-treated mice showed anxiety-like behavior, impairment of object recognition memory and social behavior, and sensorimotor gating deficits, as compared to the saline-treated control group. Depolarization-evoked glutamate release in the hippocampus was impaired in polyI:C-treated mice compared to saline-treated control mice. Furthermore, to investigate the effect of neonatal immune activation on the expression levels of schizophrenia-related genes, we analyzed mRNA levels in the hippocampus 2 and 24h after polyI:C treatment. No significant differences or only transient and marginal changes were observed between polyI:C-treated and saline-treated control mice in the expression levels of schizophrenia-related genes in the hippocampus.

Matrix Metalloproteinase-9 Contributes to Kindled Seizure Development in Pentylenetetrazole-Treated Mice by Converting Pro-BDNF to Mature BDNF in the Hippocampus

Recurrent seizure activity has been shown to induce a variety of permanent structural changes in the brain. Matrix metalloproteinases (MMPs) function to promote neuronal plasticity, primarily through cleavage of extracellular matrix proteins. Here, we investigated the role of MMP-9 in the development of pentylenetetrazole (PTZ)-induced kindled seizure in mice. Repeated treatment with PTZ (40 mg/kg) produced kindled seizure, which was accompanied by enhanced MMP-9 activity and expression in the hippocampus. No change in MMP-9 activity was observed in the hippocampi of mice with generalized tonic seizure following single administration of PTZ (60 mg/kg). MMP-9 colocalized with the neuronal marker NeuN and the glial marker GFAP in the dentate gyrus of the kindled mouse hippocampus. Coadministration of diazepam or MK-801 with PTZ inhibited the development of kindling and the increased MMP-9 levels in the hippocampus. Marked suppression of kindled seizure progression in response to repeated PTZ treatment was observed in MMP-9(−/−) mice compared with wild-type mice, an observation that was accompanied by decreased hippocampal levels of mature brain-derived neurotrophic factor. Microinjecting the BDNF scavenger TrkB-Fc into the right ventricle before each PTZ treatment significantly suppressed the development of kindling in wild-type mice, whereas no effect was observed in MMP-9(−/−) mice. On the other hand, bilateral injections of pro-BDNF into the hippocampal dentate gyrus significantly enhanced kindling in wild-type mice but not MMP-9(−/−) mice. These findings suggest that MMP-9 is involved in the progression of behavioral phenotypes in kindled mice because of conversion of pro-BDNF to mature BDNF in the hippocampus.

Behavioral alterations associated with targeted disruption of exons 2 and 3 of the Disc1 gene in the mouse

Disrupted-In-Schizophrenia 1 (DISC1) is a promising candidate gene for susceptibility to psychiatric disorders, including schizophrenia. DISC1 appears to be involved in neurogenesis, neuronal migration, axon/dendrite formation and synapse formation; during these processes, DISC1 acts as a scaffold protein by interacting with various partners. However, the lack of Disc1 knockout mice and a well-characterized antibody to DISC1 has made it difficult to determine the exact role of DISC1 in vivo. In this study, we generated mice lacking exons 2 and 3 of the Disc1 gene and prepared specific antibodies to the N- and C-termini of DISC1. The Disc1 mutant mice are viable and fertile, and no gross phenotypes, such as disorganization of the brains cytoarchitecture, were observed. Western blot analysis revealed that the DISC1-specific antibodies recognize a protein with an apparent molecular mass of ~100 kDa in brain extracts from wild-type mice but not in brain extracts from DISC1 mutant mice. Immunochemical studies demonstrated that DISC1 is mainly localized to the vicinity of the Golgi apparatus in hippocampal neurons and astrocytes. A deficiency of full-length Disc1 induced a threshold shift in the induction of long-term potentiation in the dentate gyrus. The Disc1 mutant mice displayed abnormal emotional behavior as assessed by the elevated plus-maze and cliff-avoidance tests, thereby suggesting that a deficiency of full-length DISC1 may result in lower anxiety and/or higher impulsivity. Based on these results, we suggest that full-length Disc1-deficient mice and DISC1-specific antibodies are powerful tools for dissecting the pathophysiological functions of DISC1.

Chronic restraint stress impairs neurogenesis and hippocampus‐dependent fear memory in mice: possible involvement of a brain‐specific transcription factor Npas4

J. Neurochem. (2010) 114, 1840–1851.

Involvement of pallidotegmental neurons in methamphetamine- and MK-801-induced impairment of prepulse inhibition of the acoustic startle reflex in mice: reversal by GABAB receptor agonist baclofen.

We have previously demonstrated that pallidotegmental GABAergic neurons play a crucial role in prepulse inhibition (PPI) of the startle reflex in mice through the activation of GABAB receptors in pedunculopontine tegmental neurons. In this study, we investigated whether PPI disruption induced by methamphetamine (METH) or MK-801 is associated with the dysfunction of pallidotegmental neurons. Furthermore, we examined the effects of baclofen, a GABAB receptor agonist, on METH- and MK-801-induced PPI impairment. Acute treatment with METH (3 mg/kg, subcutaneouly (s.c.)) and MK-801 (>0.3 mg/kg, s.c.) significantly disrupted PPI, accompanied by the suppression of c-Fos expression in lateral globus pallidus induced by PPI. Furthermore, acute treatment with METH and MK-801 stimulated c-Fos expression in the caudal pontine reticular nucleus (PnC) in mice subjected to the PPT test, although PPI alone had no effect on c-Fos expression. Repeated treatment with 1 mg/kg METH for 7 days, which did not affect PPI acutely, showed similar effects on PPI and c-Fos expression to acute treatment with METH (3 mg/kg). Baclofen dose-dependently ameliorated PPI impairment induced by acute treatment with METH (3 mg/kg) and MK-801 (1 mg/kg), and decreased METH- and MK-801-stimulated c-Fos expression in PnC to the basal level. These results suggest that dysfunction of pallidotegmental neurons is involved in PPI disruption caused by METH and MK-801 in mice. GABAB receptor may constitute a putative target in treating neuropsychiatric disorders with sensorimotor gating deficits, such as schizophrenia and METH psychosis.

Matrix Metalloprotease-9 Inhibition Improves Amyloid β-Mediated Cognitive Impairment and Neurotoxicity in Mice

In Alzheimers disease (AD), the expression of matrix metalloproteases (MMPs), which are capable of degrading extracellular matrix proteins, is increased in the brain. Previous studies with cultured glial cells have demonstrated that amyloid β (Aβ) protein can induce the expression of MMPs, which could be involved in the degradation of Aβ. In the present study, we investigated the role of MMP-2 and MMP-9 in cognitive impairment induced by the injection of Aβ in mice. The intracerebroventricular injection of Aβ25-35, Aβ1-40, and Aβ1-42, but not Aβ40-1, transiently increased MMP-9, but not MMP-2, activity and protein expression in the hippocampus. Immunohistochemistry revealed the expression of MMP-9 to be increased in both neurons and glial cells in the hippocampus after Aβ treatment. The Aβ-induced cognitive impairment in vivo as well as neurotoxicity in vitro was significantly alleviated in MMP-9 homozygous knockout mice and by treatment with MMP inhibitors. These results suggest the increase in MMP-9 expression in the hippocampus to be involved in the development of cognitive impairment induced by Aβ1-40. Thus, specific inhibitors of MMP-9 may have therapeutic potential for the treatment of AD. Our findings suggest that, as opposed to expectations based on previous findings, MMP-9 plays a causal role in Aβ-induced cognitive impairment and neurotoxicity.