Taichi Uetsuki

Structure and Expression of the Mouse Necdin Gene IDENTIFICATION OF A POSTMITOTIC NEURON-RESTRICTIVE CORE PROMOTER

Necdin is a 325 amino acid residue protein encoded by a cDNA clone isolated from neurally differentiated embryonal carcinoma cells. In situ hybridization histochemistry revealed that necdin mRNA-containing cells in vivo coincided with postmitotic neurons in the mouse brain from early periods of neurogenesis until adulthood. To study the regulation of necdin gene expression, we have isolated and characterized the necdin gene from a mouse genomic DNA library. The necdin gene contains no intron, and its upstream region lacks canonical TATA and CAAT boxes. To assess promoter activity, the 5′-flanking sequence (844 base pairs) of the necdin gene was fused to the LacZ reporter gene and transiently transfected into retinoic acid-treated P19 embryonal carcinoma cells. Most of the transfectants expressing high levels of LacZ immunoreactivity were postmitotic neurons differentiated from P19 cells. Deletion analysis using luciferase reporter genes demonstrated that a neuron-restrictive core promoter is localized to positions −80 to −35, in which a G+C-rich domain and a putative binding site for transcription factors with PAS (per, arnt, and single-minded) dimerization domain are comprised. These results suggest that postmitotic neuron-restrictive expression of the necdin gene is mediated by the specific cis-acting elements and that this promoter is applicable to postmitotic neuron-targeted expression of various transgenic systems.

Disruption of the paternal necdin gene diminishes TrkA signaling for sensory neuron survival.

Necdin is a multifunctional signaling protein that stabilizes terminal differentiation of postmitotic neurons. The human necdin gene in chromosome 15q11-q12 is maternally imprinted, paternally transcribed, and not expressed in Prader-Willi syndrome, a human genomic imprinting-associated neurodevelopmental disorder. Although necdin-deficient mice display several abnormal phenotypes reminiscent of this syndrome, little is known about molecular mechanisms that lead to the neurodevelopmental defects. Here, we demonstrate that paternally expressed necdin is required for physiological development of nerve growth factor (NGF)-dependent sensory neurons. Mouse embryos defective in the paternal necdin allele displayed absent necdin expression in the dorsal root ganglia, in which the tropomyosin-related kinase A (TrkA) receptor tyrosine kinase and the p75 neurotrophin receptor were expressed in a normal manner. Necdin interacted with both TrkA and p75 to facilitate the association between these receptors. NGF-induced phosphorylation of TrkA and mitogen-activated protein kinase was significantly diminished in the necdin-null sensory ganglia. Furthermore, the mice lacking the paternal necdin allele displayed augmented apoptosis in the sensory ganglia in vivo and had a reduced population of substance P-containing neurons. These mutant mice showed significantly high tolerance to thermal pain, which is often seen in individuals with Prader-Willi syndrome. These results suggest that paternally expressed necdin facilitates TrkA signaling to promote the survival of NGF-dependent nociceptive neurons.

Activation of calpain in cultured neurons overexpressing Alzheimer amyloid precursor protein.

We have previously reported that overexpression of wild-type amyloid precursor protein (APP) in postmitotic neurons induces cleavage-dependent activation of caspase-3 both in vivo and in vitro. In this study, we investigated the mechanism underlying APP-induced caspase-3 activation using adenovirus-mediated gene transfer into postmitotic neurons derived from human embryonal carcinoma NT2 cells. Overexpression of wild-type APP significantly increased intracellular (45)Ca(2+) content prior to the activation of caspase-3 in NT2-derived neurons. Chelation of intracellular Ca(2+) markedly suppressed APP-induced activation of caspase-3. Furthermore, calpain, a Ca(2+)-dependent cysteine protease, was activated in neurons overexpressing APP as assessed by increased levels of calpain-cleaved alpha-fodrin and autolytic mu-calpain fragments. Neither calpain nor caspase-3 was activated in neurons expressing an APP mutant defective in the Abeta(1-20) domain. Calpain inhibitors almost completely suppressed APP-induced activation of neuronal caspase-3. E64d, a membrane permeable inhibitor of calpain, significantly suppressed APP-induced neuronal death. These results suggest that overexpression of wild-type APP activates calpain that mediates caspase-3 activation in postmitotic neurons.

Necdin acts as a transcriptional repressor that interacts with multiple guanosine clusters

Necdin is a growth suppressor expressed predominantly in postmitotic neurons, and ectopic expression of this protein suppresses cell growth. Here we report that Necdin directly binds to specific DNA sequences and serves as a transcriptional repressor. Polyhistidine-tagged Necdin was used for selection of random-sequence oligonucleotides by polymerase chain reaction-based amplification. Necdin recognized guanosine (G)-rich sequences that encompass multiple G clusters and intervening mono- or di-nucleotides of A, T and C. These sequences, termed GN boxes, resemble multiply aligned forms of the canonical GC box which is recognized by Sp family members. Necdin directly bound to a GN box consisting of contiguous two GC boxes with four G clusters, but not to a single GC box with two G clusters, whereas Sp1 bound to both. In a reporter system using Drosophila Schneider Line 2 cells, Necdin repressed Sp1-dependent activity of mouse c-myc P1 promoter that contains a typical GN box. Deletion of the GN box from the c-myc P1 promoter or its conversion to the single GC box abolished the Necdin-dependent repression. These results suggest that Necdin modulates gene transcription via the GN box that is potentially recognized by GC box-targeting Sp family members.

Caspase-3 activation and inflammatory responses in rat hippocampus inoculated with a recombinant adenovirus expressing the Alzheimer amyloid precursor protein

To elucidate the mechanism of neuronal death in Alzheimers disease, we investigated the effects of overexpression of wild-type Alzheimer amyloid precursor protein (APP) on neuronal cells and glial cells in vivo. When an APP695-expressing adenovirus was injected into the dorsal hippocampal region, a number of neurons in remote areas were positively stained with anti-APP monoclonal antibody, and underwent severe degeneration from 3 to 7 days after viral inoculation. Most degenerating neurons were immunopositive with both APP and activated caspase-3, but some neurons that expressed activated caspase-3 were not expressing APP from 7 to 14 days after virus injection. In the neighborhood of the degenerating neurons, activated microglia/macrophages, which were identified by the phenotypic marker C3bi receptor (CD11b/c; OX-42), were observed, and some of them appeared to phagocytose the caspase-3-immunopositive degenerating neurons. In addition to microglia/macrophages, infiltrating leukocytes expressing CD45 or CD4 were also detected. These results suggest that the increased accumulation of APP induced not only caspase-3-mediated death machinery, but also inflammatory responses including microglial activation. These inflammatory responses might cause further neurodegeneration through the alternative pathway that might activate the caspase-3-mediated death machinery without APP expression.

Shp‐2 Specifically Regulates Several Tyrosine‐Phosphorylated Proteins in Brain‐Derived Neurotrophic Factor Signal

Abstract: Brain‐derived neurotrophic factor (BDNF), a member of the neurotrophins, promotes differentiation and survival and regulates plasticity of various types of neurons. BDNF binds to TrkB, a receptor tyrosine kinase, which results in the activation of a variety of signaling molecules to exert the various functions of BDNF. Shp‐2, a Src homology 2 domain‐containing cytoplasmic tyrosine phosphatase, is involved in neurotrophin signaling in PC12 cells and cultured cerebral cortical neurons. To examine the roles of Shp‐2 in BDNF signaling in cultured rat cerebral cortical neurons, the wild‐type and phosphatase‐inactive mutant (C/S mutant) forms of Shp‐2 were ectopically expressed in cultured neurons using recombinant adenovirus vectors. We found that several proteins tyrosinephosphorylated in response to BDNF showed enhanced levels of tyrosine phosphorylation in cultured neurons infected with C/S mutant adenovirus in comparison with those infected with the wild‐type Shp‐2 adenovirus. In addition, in immunoprecipitates with anti‐Shp‐2 antibody, we also observed at least four proteins that displayed enhanced phosphorylation in response to BDNF in cultured neurons infected with the C/S mutant adenovirus. We found that the Shp‐2‐binding protein, brain immunoglobulin‐like molecule with tyrosine‐based activation motifs (BIT), was strongly tyrosine‐phosphorylated in response to BDNF in cultured neurons expressing the C/S mutant of Shp‐2. In contrast, the level of BDNF‐induced phosphorylation of mitogen‐activated protein kinase and coprecipitated proteins with anti‐Trk and Grb2 antibodies did not show any difference between neurons infected with these two types of Shp‐2. Furthermore, the survival effect of BDNF was enhanced by the wild type of Shp‐2, although it was not influenced by the C/S mutant of Shp‐2. These results indicated that in cultured cerebral cortical neurons Shp‐2 is specifically involved in the regulation of several tyrosine‐phosphorylated proteins, including BIT, in the BDNF signaling pathway. In addition, the phosphatase Shp‐2 may not influence the level of BDNF‐induced activation of mitogen‐activated protein kinase in cultured cortical neurons. Further, Shp‐2 may have potential to positively regulate BDNF‐promoting neuronal survival.

A Novel Strategy for Introducing Exogenous Bcl-2 into Neuronal Cells: The Cre/loxP System-Mediated Activation of Bcl-2 for Preventing Programmed Cell Death Using Recombinant Adenoviruses

We have established a novel strategy for introducing exogenous Bcl-2 into neuronal cells that is mediated by Cre/loxP recombination using recombinant adenoviral vectors. An on/off-switching cassette for Bcl-2 (CALNLbcl-2) was designed to express Bcl-2 by recombinase Cre-mediated excisional deletion of a spacer DNA flanked by a pair of loxP sites. Exogenous Bcl-2 was clearly induced in PC12 cell lines carrying CALNLbcl-2 after infection with recombinant adenovirus producing recombinase Cre (AxCANCre). Dual infection with both AxCANCre and a recombinant adenovirus bearing CALNLbcl-2 showed efficient delivery of exogenous Bcl-2 into a hybrid motoneuronal cell line and primary chicken spinal motoneurons. The delivery of foreign Bcl-2 promoted survival of motoneurons in medium either containing or lacking trophic support. Thus, this strategy for delivery of exogenous Bcl-2 will be useful for studying neuronal death as well as for introducing foreign genes into postmitotic neurons under the control of recombinase Cre.

Neurotoxic and neuroprotective effects of glutamate are enhanced by introduction of amyloid precursor protein cDNA

The physiological role of amyloid precursor protein (APP), whose anomalous metabolite is a putative pathogen for Alzheimer disease, remains unclear. From the enhanced responsiveness to glutamate in cultured hippocampal neurons after the introduction of cDNA of APP695 (an isoform of APP dominant in human brain) using an adenovirus vector, we have recently raised the hypothesis that APP modulates neuronal sensitivity to glutamate. To test this hypothesis, we utilized here the unique effects of glutamate on the survival of different types of neurons. It is known that hippocampal neurons undergo deterioration in 24 h after application of glutamate in a dose-dependent manner. This vulnerability was increased in the cells transfected with adenovirus carrying cDNA of APP695. By contrast, it is known that cerebellar granule neurons require for their survival the supplementation of NMDA to the medium. The dose of NMDA required for survival was reduced after the transfection of the APP-adenovirus to cerebellar granule neurons. These enhancing effects of APP on the glutamate-induced vulnerability in hippocampal neurons and the glutamate (NMDA)-dependent survival in cerebellar neurons were blocked by glutamate receptor inhibitors, and were not seen after application of a control adenovirus carrying cDNA of beta-galactosidase. Since the effects of glutamate were enhanced in both directions, the hypothesis became more likely that one of the physiological functions of cellular APP is the regulation of glutamate receptors.

Glutamate responsiveness enhanced in neurones expressing amyloid precursor protein.

ALTHOUGH the neurotoxicity of β-amyloid peptide is well established, the cellular functions of amyloid precursor protein (APP) remain unclear. Using an adenoviral vector, we introduced cDNA encoding human APP holoprotein into rat hippocampal neurones in culture. Neurones expressing the membrane-bound form of APP showed greater responsiveness to applied glutamate than non-expressing control neurones, as revealed by Ca2+ fluorometry. This increased responsiveness was not the result of secreted APP, as confirmed by observations of closely spaced APP-expressing and non-expressing cells in the same culture dish. These data suggest that one function of APP may be the regulation of glutamate receptors in neurones.

Both cytotoxic and cytoprotective effects of glutamate are enhanced by expression of amyloid precursor protein

Depts. of Anatomy & Neurobiology, *Orthopedics, Kyoto Prefectural University of Medicine. KawaramachiHirokoji. Kamigyo-ku. Kyoto 602-0841 One of the most important approaches to study the function of molecules in the central nervous system is to visualize directly the molecules functioning in living cells. Tagging expressed proteins with the green fluorescent protein(GFP) from jellyfish Aequorea victoria is a very specific and sensitive technique for examining intracellular dynamics of proteins and organelles. In the present study, a chimera protein of GFP and steroid hormone receptors has been developed. To observe dynamic changes of the chimera protein in living single neurons or glial cells, we have established a real time imaging system using a Photometrics high-resolution, cooled charge-coupled device (CCD) camera(Sensys 1400) attached to a Olympus IXL70 microscope equipped with an epifluorescence attachment settled in the temperature-controlled dark room. Data are processed to time-lapse movie and evaluated by scientific imaging software IPLab Spectrum (Signal Analytical Corporation). This imaging system combined with GFP chimera protein is a powerful tool allowing us to visualize dynamics of proteins and other functional moleculesin livingcells which cannot be directly detected in the fixedcells.