Kenta Nakajima

Cellular prion protein prevents brain damage after encephalomyocarditis virus infection in mice

Cellular prion protein (PrPC), a cell-surface glycoprotein normally associated with neurons, is also expressed in other cell types such as glia and lymphocytes. To further elucidate these roles of PrPC, wild-type prion protein gene (Prnp+/+) mice and Prnp-deficient (Prnp−/−) mice were infected with encephalomyocarditis virus B variant (EMCV-B) via an intracranial route. EMCV-B causes encephalitis and apoptotic cell death in vivo. Histopathological studies revealed that Prnp+/+ mice infected with 600 pfu of EMCV-B showed more severe infiltration of inflammatory cells, accompanied by higher activation of microglia cells around the hippocampus, than Prnp−/− mice; viz., no differences in the brain virus titer between these two lines of mice. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP, nick end-labeling (TUNEL) staining of the brain specimens revealed that the CA1 hippocampal pyramidal cells showed a larger number of apoptotic neurons in Prnp−/− than Prnp+/+ mice. Based on all these findings, PrPC may play certain roles in the induction of inflammation and inhibition of apoptosis in vivo.

Species-specific anti-apoptotic activity of cellular prion protein in a mouse PrP-deficient neuronal cell line transfected with mouse, hamster, and bovine Prnp

The neuroprotective function of prion protein (PrP) was revealed first by the fact that reintroduction of the mouse prion protein gene (Prnp) into a mouse Prnp(-/-) neuronal cell line, HpL3-4, could prevent apoptosis induced by serum deprivation. In the present study, the anti-apoptotic activities of mouse, hamster, and bovine PrP were compared by expressing mouse PrP (MoPrP), hamster PrP (HaPrP), and bovine PrP (BoPrP) in HpL3-4 cells, respectively. Morphological analysis and DNA fragmentation assays demonstrated that HpL3-4 cells expressing HaPrP, BoPrP, and empty vector (EM) showed the typical features of apoptosis with DNA laddering and apoptotic bodies after serum deprivation, whereas HpL3-4 cells expressing MoPrP showed decreased levels of apoptosis in comparison. The levels of histone-associated DNA fragments (mono- and oligonucleosomes) in the cytosol fractions of the cells correlated with the levels of DNA laddering. These results indicate a species-specific anti-apoptotic function of PrP exists, suggesting that the interaction of the mouse PrP with mouse host factors is required for its anti-apoptotic activity.

Improvements in Calibration of GSO Scintillators in the Suzaku Hard X-Ray Detector

Improvements of the in-orbit calibration of GSO scintillators in the Hard X-ray Detector aboard Suzaku are reported. To resolve an apparent change in the energy scale of GSO, which appeared across the launch for unknown reasons, consistent and thorough re-analyses of both pre-launch and in-orbit data have been performed. With laboratory experiments using spare hardware, the pulse-height offset, corresponding to zero energy input, was found to change by ~0.5% of the full analog voltage scale, depending on the power supply. Furthermore, by carefully calculating all of the light outputs of secondaries from activation lines used in the in-orbit gain determination, their energy deposits in GSO were found to be effectively lower, by several percent, than their nominal energies. Taking both of these effects into account, the in-orbit data agree with the on-ground measurements within ~5%, without employing the artificial correction introduced in previous work (Kokubun et al. 2007, PASJ, 59, S53). With this knowledge, we updated the data processing, the response, and the auxiliary files of GSO, and reproduced the HXD-PIN and HXD-GSO spectra of the Crab Nebula over 12–300 keV by a broken power-law with a break energy of ~110 keV.

Monte Carlo simulation study of in-orbit background for the soft gamma-ray detector on-board ASTRO-H

The Soft Gamma-ray Detector onboard the ASTRO-H satellite, scheduled for launch in 2014, is a Si/CdTe Compton telescope surrounded by a thick BGO active shield. The SGD covers the energy range from 40 to 600 keV and studies non-thermal phenomena in the universe with high sensitivity. For the success of the SGD mission, careful examination of the expected performance, particularly the instrumental background in orbit, and optimization of the detector configuration are essential. We are developing a Geant4-based Monte Carlo simulation framework on the ANL++ platform, employing the MGGPOD software suite to predict the radioactivation in orbit. A detailed validation of the simulator through the comparison with literature and the beam test data is summarized. Our system will be integrated into the ASTRO-H simulation framework.

Distinct subcellular localization of three isoforms of insulinoma-associated protein 2β in neuroendocrine tissues

AIMS Insulinoma-associated protein 2β (IA-2β) is considered to play a significant role in regulated secretion. Recent studies have shown that the mouse brain expresses three major isoforms of IA-2β, named IA-2β60, IA-2β64, and IA-2β71. In this study, we analyzed the tissue-, cell- and organelle-specific distributions of IA-2β isoforms in mice. MAIN METHODS To localize IA-2β expression in mouse tissues and cells, western blot and immunohistochemical analyses were carried out. The subcellular distribution of IA-2β isoforms was assessed by sedimentation of mouse brain homogenates in a discontinuous sucrose density gradient. KEY FINDINGS IA-2β60 was abundant in the cerebrum, cerebellum, medulla oblongata, pancreas, adrenal gland, and pituitary, and in the muscular and mucosal layers of the digestive organs. In contrast, the expression of IA-2β64 and IA-2β71 was restricted to the cerebrum, cerebellum, medulla oblongata, and pituitary, and the muscular layers of the digestive organs. Immunohistochemical analysis of mouse pancreatic islets revealed that pancreatic beta cells expressed IA-2β60 exclusively, whereas alpha and delta cells expressed all three isoforms. By the sedimentation of mouse brain homogenates, it was shown that IA-2β64 and IA-2β71 were co-localized with IA-2 on secretory granules, but were absent from synaptic vesicles (SVs). On the other hand, IA-2β60 was co-localized with synaptophisin on SVs, but was absent from secretory granules. SIGNIFICANCE The tissue-, cell- and organelle-specific distributions of IA-2β isoforms suggest that IA-2β60 has a role in secretion from SVs, whereas IA-2β64 and IA-2β71 are involved in secretion from secretory granules.

Development and calibration of fine collimators for the ASTRO-H Soft Gamma-ray Detector

The Soft Gamma-ray Detector (SGD) is a Si/CdTe Compton telescope surrounded by a thick BGO active shield and is scheduled to be onboard the ASTRO-H satellite when it is launched in 2015. The SGD covers the energy range from 40 to 600 keV with high sensitivity, which allows us to study nonthermal phenomena in the universe. The SGD uses a Compton camera with the narrow field-of-view (FOV) concept to reduce the non-Xray background (NXB) and improve the sensitivity. Since the SGD is essentially a nonimaging instrument, it also has to cope with the cosmic X-ray background (CXB) within the FOV. The SGD adopts passive shields called “fine collimators” (FCs) to restrict the FOV to ≤ 0.6° for low-energy photons (≤ 100 keV), which reduces contamination from CXB to less than what is expected due to NXB. Although the FC concept was already adopted by the Hard X-ray Detector onboard Suzaku, FCs for the SGD are about four times larger in size and are technically more difficult to operate. We developed FCs for the SGD and confirmed that the prototypes function as required by subjecting them to an X-ray test and environmental tests, such as vibration tests. We also developed an autocollimator system, which uses visible light to determine the transmittance and the optical axis, and calibrated it against data from the X-ray test. The acceptance tests of flight models started in December 2013: five out of six FCs were deemed acceptable, and one more unit is currently being produced. The activation properties were studied based on a proton-beam test and the results were used to estimate the in-orbit NXB.

The Soft Gamma-ray Detector for the ASTRO-H mission

The Soft Gamma-ray Detector (SGD) on board ASTRO-H (Japanese next high-energy astrophysics mission) is a Compton telescope with narrow fleld-of-view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor detector system which consists of silicon and CdTe (cadmium telluride) detectors. It can detect photons in a wide energy band (50-600 keV) at a background level 10 times better than that of the Suzaku Hard X-ray Detector, and is complimentary to the Hard X-ray Imager on board ASTRO-H with an energy coverage of 5-80 keV. Excellent energy resolution is the key feature of the SGD, allowing it to achieve good background rejection capability taking advantage of good angular resolution. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of gamma-ray emission processes. Here we describe the instrument design of the SGD, its expected performance, and its development status.

Development of BGO active shield for the ASTRO-H soft Gamma-ray Detector

Soft Gamma-ray Detector (SGD:40-600 keV) will be mounted on the 6th Japanese X-ray observatory ASTROH to be launched in 2014. The main part of the SGD is a Compton camera with a narrow field of view and surrounded by BGO active shields (SGD-BGO). Via this combination, the SGD can achieve sensitivity more than ten times superior to the Suzaku/HXD. The BGO active shield will also function as a gamma-ray burst monitor as proven by the wide-band all-sky monitor (WAM) of the Suzaku/HXD. Avalanche Photodiodes (APDs) are used to read out scintillation lights from the BGO. The size of the former also means we need to focus on collecting light from large, complex-shaped BGO blocks. The significant leakage current of the APD means a lower temperature is preferred to minimize the noise while a higher temperature is preferred to simplify the cooling system. To optimize the BGO shape and the operating temperature, we tested the performance of the BGO readout system with various BGO shapes under different operating temperatures. We also apply waveform sampling by flash-ADC and digital filter instead of a conventional analog filter and ADC scheme to reduce the space and power of the circuit with increased flexibilities. As an active shield, we need to achieve a threshold level of 50-100 keV. Here, we report on the studies of threshold energy of active shield under various conditions and signal processings.