Numerous proteins are properly folded by binding with zinc during their itinerary in the biosynthetic-secretory pathway. Several transporters have been implicated in the zinc entry into secretory compartments from cytosol, but their precise roles are poorly understood. We report here that two zinc transporters (ZnT5 and ZnT7) localized in the secretory apparatus are responsible for loading zinc to alkaline phosphatases (ALPs) that are glycosylphosphatidylinositol-anchored membrane proteins exposed to the extracellular site. Disruption of the ZnT5 gene in DT40 cells decreased the ALP activity to 45% of that in the wild-type cells. Disruption of the ZnT7 gene lowered the ALP activity only by 20%. Disruption of both genes markedly decreased the ALP activity to <5%. Overexpression of human ZnT5 or ZnT7 in DT40 cells deficient in both ZnT5 and ZnT7 genes recovered the ALP activity to the level comparable to that in the wild-type cells. The inactive ALP protein in DT40 cells deficient in both ZnT5 and ZnT7 genes was transported to cytoplasmic membrane like the active ALP protein in the wild-type cells. Thus both ZnT5 and ZnT7 contribute to the conversion of apo-ALP to holo-ALP.

SUMMARY1. Allosteric potentiation of the ionotropic quisqualate (iQA) receptor by a nootropic drug aniracetam (1 -p-anisoyl-2-pyrrolidinone) was investigated using Xenopus oocytes injected with rat brain mRNA and rat hippocampal slices.2. Aniracetam potentiates the iQA responses induced in Xenopus oocytes by rat brain mRNA in a reversible manner. This effect was observed above the concentrations of 0.1 mm. Kainate, N-methyl-D-aspartate and y-aminobutyric acid responses induced in the same oocytes were not affected.3. The specific potentiation of iQA responses was accompanied by an increase in the conductance change of iQA and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses, but the affinity of receptors for agonist and the ion-selectivity of the channels (reversal potentials) were not changed. 4. Aniracetam reversibly potentiated the iQA responses recorded intracellularly from the pyramidal cells in the CAI region of rat hippocampal slices. The excitatory postsynaptic potentials (EPSPs) in Schaffer collateral-commissural-CAl synapses were also potentiated by aniracetam.5. Population EPSPs recorded in the mossy fibre-CA3 synapses as well as Schaffer-commissural synapses were also potentiated by aniracetam. The amplitudes of the potentiation were not changed by the formation of long-term potentiation.

Two subsets of neurones expressing either the transgenic or the endogenous MOR28 target their axons to two separate glomeruli based on the differences in the genetic backgrounds, nature of tagging, and chromosomal locations. In contrast, neurones expressing a maternal or paternal allele share the same glomeruli, but tend to target to segregated areas within the glomerular structure. The segregation was more prominent with increased differences in the genetic background between the two alleles.

Chronological changes in the chromosome aberration rates of splenocytes from specific-pathogen-free (SPF) mice after continuous and long-term exposure to low-dose-rate gamma rays were studied. Incidences of dicentrics plus centric rings (Dic+Rc), detected by conventional Giemsa staining, and dicentric chromosomes, detected by fluorescence in situ hybridization (Dic by FISH) using a centromere probe, showed an essentially linear increase up to a total accumulated dose of 8000 mGy after irradiation for about 400 days at a low dose rate of 20 mGy/day. For comparison, acute high-dose-rate and medium-dose-rate irradiation were performed. The values of the alpha coefficients in the linear regression lines for these unstable-type aberrations decreased as the dose rates were lowered from medium dose rates (200 and 400 mGy/day) to low dose rates (1 and 20 mGy/day). The dose and dose-rate effectiveness factor (DDREF), estimated by the ratio of calculated incidences using the best-fit regression lines at a high dose rate (890 mGy/min) and low dose rate (20 mGy/day), was 4.5 for Dic by FISH and 5.2 for Dic+Rc, respectively, at the same dose of 100 mGy, while different DDREFs were obtained for different accumulated doses. This is the first study to provide information regarding the effects of long-term exposure to low-dose-rate radiation on chromosomes.

This review summarizes the results of experiments conducted in the Institute for Environmental Sciences for the past 21 years, focusing on the biological effects of long-term low dose-rate radiation exposure on mice. Mice were chronically exposed to gamma rays at dose-rates of 0.05, 1 or 20 mGy/day for 400 days to total doses of 20, 400 or 8000 mGy, respectively. The dose rate 0.05 mGy/day is comparable to the dose limit for radiation workers. The parameters examined were lifespan, neoplasm incidence, antineoplasm immunity, body weight, chromosome aberration(s), gene mutation(s), alterations in mRNA and protein levels and trans-generational effects. At 20 mGy/day, all biological endpoints were significantly altered except neoplasm incidence in the offspring of exposed males. Slight but statistically significant changes in lifespan, neoplasm incidences, chromosome abnormalities and gene expressions were observed at 1 mGy/day. Except for transient alterations in the mRNA levels of some genes and increased liver neoplasm incidence attributed to radiation exposure, the remaining biological endpoints were not influenced after exposure to 0.05 mGy/day. Results suggest that chronic low dose-rate exposure may induce small biological effects.