Jane M. Flinn

Evidence That the ZNT3 Protein Controls the Total Amount of Elemental Zinc in Synaptic Vesicles

The ZNT3 protein decorates the presynaptic vesicles of central neurons harboring vesicular zinc, and deletion of this protein removes staining for zinc. However, it has been unclear whether only histochemically reactive zinc is lacking or if, indeed, total elemental zinc is missing from neurons lacking the Slc30a3 gene, which encodes the ZNT3 protein. The limitations of conventional histochemical procedures have contributed to this enigma. However, a novel technique, microprobe synchrotron X-ray fluorescence, reveals that the normal 2- to 3-fold elevation of zinc concentration normally present in the hippocampal mossy fibers is absent in Slc30a3 knockout (ZNT3) mice. Thus, the ZNT3 protein evidently controls not only the “stainability” but also the actual mass of zinc in mossy-fiber synaptic vesicles. This work thus confirms the metal-transporting role of the ZNT3 protein in the brain.

Enhanced zinc consumption causes memory deficits and increased brain levels of zinc

Zinc deficiency has been shown to impair cognitive functioning, but little work has been done on the effects of elevated zinc. This research examined the effect on memory of raising Sprague-Dawley rats on enhanced levels of zinc (10 ppm ZnCO3; 0.153 mM) in the drinking water for periods of 3 or 9 months, both pre- and postnatally. Controls were raised on lab water. Memory was tested in a series of Morris Water Maze (MWM) experiments, and zinc-treated rats were found to have impairments in both reference and working memory. They were significantly slower to find a stationary platform and showed greater thigmotaxicity, a measure of anxiety. On a working memory task, where the platform was moved each day, zinc-treated animals had longer latencies over both trials and days, swam further from the platform, and showed greater thigmotaxicity. On trials using an Atlantis platform, which remained in one place but was lowered on probe trials, the zinc-treated animals had significantly fewer platform crossings, spent less time in the target quadrant, and did not swim as close to the platform position. They had significantly greater latency on nonprobe trials. Microprobe synchrotron X-ray fluorescence (microSXRF) confirmed that brain zinc levels were increased by adding ZnCO3 to the drinking water. These data show that long-term dietary administration of zinc can lead to impairments in cognitive function.

Alterations in fear response and spatial memory in pre- and post-natal zinc supplemented rats: remediation by copper.

The role of zinc in the nervous system is receiving increased attention. At a time when dietary fortification and supplementation have increased the amount of zinc being consumed, little work has been done on the effects of enhanced zinc on behavior. Both zinc and copper are essential trace minerals that are acquired from the diet; under normal conditions the body protects against zinc overload, but at excessive dosages, copper deficiency has been seen. In order to examine the effect of enhanced metal administration on learning and memory, Sprague Dawley rats were given water supplemented with 10ppm Zn, 10ppm Zn+0.25ppm Cu, or normal lab water, during pre- and post-natal development. Fear conditioning tests at 4months showed significantly higher freezing rates during contextual retention and extinction and cued extinction for rats drinking water supplemented with zinc, suggesting increased anxiety compared to controls raised on lab water. During the MWM task at 9months, zinc-enhanced rats had significantly longer latencies to reach the platform compared to controls. The addition of copper to the zinc supplemented water brought freezing and latency levels closer to that of controls. These data demonstrate the importance of maintaining appropriate intake of both metals simultaneously, and show that long-term supplementation with zinc may cause alterations in memory.

Identification of hydroxyapatite spherules provides new insight into subretinal pigment epithelial deposit formation in the aging eye

Significance Proteins and lipids accumulating in deposits external to the retinal pigment epithelium (RPE) represent a barrier to metabolic exchange between the retina and the choroidal capillaries. With time, these deposits can lead to age-related macular degeneration (AMD), the most common cause of blindness in the elderly in the developed world. It remains unclear how sub-RPE deposits are initiated and grow to clinically relevant features. Using a combination of high-resolution analytical techniques, we found that tiny hydroxyapatite (bone mineral) spherules with cholesterol-containing cores are present in all examined sub-RPE deposits, providing a scaffold to which proteins adhere. If the spherules are important in initiating sub-RPE deposit formation, this finding may provide attractive new approaches for early identification and treatment of AMD. Accumulation of protein- and lipid-containing deposits external to the retinal pigment epithelium (RPE) is common in the aging eye, and has long been viewed as the hallmark of age-related macular degeneration (AMD). The cause for the accumulation and retention of molecules in the sub-RPE space, however, remains an enigma. Here, we present fluorescence microscopy and X-ray diffraction evidence for the formation of small (0.5–20 μm in diameter), hollow, hydroxyapatite (HAP) spherules in Bruch’s membrane in human eyes. These spherules are distinct in form, placement, and staining from the well-known calcification of the elastin layer of the aging Bruch’s membrane. Secondary ion mass spectrometry (SIMS) imaging confirmed the presence of calcium phosphate in the spherules and identified cholesterol enrichment in their core. Using HAP-selective fluorescent dyes, we show that all types of sub-RPE deposits in the macula, as well as in the periphery, contain numerous HAP spherules. Immunohistochemical labeling for proteins characteristic of sub-RPE deposits, such as complement factor H, vitronectin, and amyloid beta, revealed that HAP spherules were coated with these proteins. HAP spherules were also found outside the sub-RPE deposits, ready to bind proteins at the RPE/choroid interface. Based on these results, we propose a novel mechanism for the growth, and possibly even the formation, of sub-RPE deposits, namely, that the deposit growth and formation begin with the deposition of insoluble HAP shells around naturally occurring, cholesterol-containing extracellular lipid droplets at the RPE/choroid interface; proteins and lipids then attach to these shells, initiating or supporting the growth of sub-RPE deposits.

The Effect of Metals on Spatial Memory in a Transgenic Mouse Model of Alzheimer's Disease

The amyloid-β protein (Aβ) is a metalloprotein with affinity for the metal ions zinc (Zn), copper (Cu), and iron (Fe), which are found in high concentrations in the plaques of Alzheimers disease (AD). Increasing attention is focused on the role of these metals in AD, and much of the evidence suggests a dyshomeostasis between these metal ions may significantly affect Aβ aggregation and deposition in the brain. While the effect of these metals on Aβ has been shown in vitro, there is less behavioral data supporting a direct role in cognitive impairment. In order to investigate the cognitive consequences of metal dyshomeostasis, we sought to directly increase metal levels in the brain by dietary means in a transgenic mouse model (Tg2576). We have now examined the effect of increased Zn (10 ppm) and Fe (10 ppm) levels in the drinking water in the Tg2576 mouse. Since increased dietary Zn can lead to Cu deficiency, a Zn group supplemented with copper was also examined (Zn (10 ppm)+Cu (0.025 ppm)). Significant increases in latency and fewer platform crossings on probe trials, which are considered measures of spatial memory impairment, were seen in both Fe and Zn supplemented transgenic mice, compared to those raised on lab water. No significant differences were seen between the Zn + Cu group and in transgenic mice raised on lab water. These data suggest that the negative consequences of Zn may be due to a reduction in copper levels and, therefore, an imbalance between these metal ions rather than a direct effect of increased Zn.

Effect of practice and training in spatial skills on embedded figures scores of males and females.

The effect of practice and training in spatial skills on scores obtained by male and female students on the Embedded Figures Test was examined. Forms A and B were administered 6 wk. apart to three groups of subjects (ns = 28, 27, 27) enrolled in drafting, mathematics, and liberal arts courses. During the pretest-posttest period the drafting students received training while the other two groups served as controls. Analysis indicated (1) no initial sex difference in test scores; (2) liberal arts students differed significantly from drafting and mathematics students, but there was no significant difference between the last two groups; (3) all groups improved with practice; (4) women receiving training improved more than women who did not; (5) there was a trend toward women receiving spatial training scoring more poorly than males receiving training on the pretest, but there was no significant difference on the posttest. These results suggest that sex differences in embedded-figures scores found by many previous experimenters may have been associated with differences in prior experience in spatial skills and by a confounding of sex with area of academic study.

Spatial memory deficits in a mouse model of late-onset Alzheimer’s disease are caused by zinc supplementation and correlate with amyloid-beta levels

Much of the research in Alzheimer’s disease (AD) that uses mouse models focuses on the early-onset form of the disease, which accounts for less than 5% of cases. In contrast, this study used a late-onset AD model to examine the interaction between increased dietary zinc (Zn) and the apolipoprotein E (ApoE) gene. ApoE ε4 is overrepresented in late-onset AD and enhances Zn binding to amyloid-β (Aβ). This study sought to determine if elevated dietary Zn would impair spatial memory in CRND8 mice (CRND8), as well as mice who carry both the mutated human amyloid precursor protein (APP) and ApoE ε4 genes (CRND8/E4). Mice were provided with either lab tap water or water enhanced with 10 ppm Zn (ZnCO3) for 4 months. At 6 months of age, spatial memory was measured by the Barnes maze. CRND8 mice exhibited significant memory deficits compared to WT mice, as shown by an increased latency to reach the escape box. For the CRND8/E4, but not the CRND8 mice, those given Zn water made significantly more errors than those on lab water. During the probe trial for the WT group, those on Zn water spent significantly less time in the target quadrant than those on lab water. These data suggest that increased dietary Zn can significantly impair spatial memory in CRND8/E4. WT mice given Zn water were also impaired on the 24-h probe trial when compared to lab water WTs. Within the CRND8/E4 group only, levels of soluble Aβ were significantly correlated with average primary latencies. Within the Zn-treated CRND8/E4 group, there was a significant correlation between insoluble Aβ and average primary errors. Levels of the zinc transporter 3, ZnT3, were negatively correlated with soluble Aβ (p < 0.01). These findings are particularly relevant because increased intake of dietary supplements, such as Zn, are common in the elderly—a population already at risk for AD. Given the effects observed in the CRND8/E4 mice, ApoE status should be taken into consideration when evaluating the efficacy of therapies targeting metals.

Correlations in distribution and concentration of calcium, copper and iron with zinc in isolated extracellular deposits associated with age-related macular degeneration

Zinc (Zn) is abundantly enriched in sub-retinal pigment epithelial (RPE) deposits, the hallmarks of age-related macular degeneration (AMD), and is thought to play a role in the formation of these deposits. However, it is not known whether Zn is the only metal relevant for sub-RPE deposit formation. Because of their involvement in the pathogenesis of AMD, we determined the concentration and distribution of calcium (Ca), iron (Fe) and copper (Cu) and compared these with Zn in isolated and sectioned macular (MSD), equatorial (PHD) and far peripheral (FPD) sub-RPE deposits from an 86 year old donor eye with post mortem diagnosis of early AMD. The sections were mounted on Zn free microscopy slides and analyzed by microprobe synchrotron X-ray fluorescence (μSXRF). Metal concentrations were determined using spiked sectioned sheep brain matrix standards, prepared the same way as the samples. The heterogeneity of metal distributions was examined using pixel by pixel comparison. The orders of metal concentrations were Ca ⋙ Zn > Fe in all three types of deposits but Cu levels were not distinguishable from background values. Zinc and Ca were consistently present in all deposits but reached highest concentration in MSD. Iron was present in some but not all deposits and was especially enriched in FPD. Correlation analysis indicated considerable variation in metal distribution within and between sub-RPE deposits. The results suggest that Zn and Ca are the most likely contributors to deposit formation especially in MSD, the characteristic risk factor for the development of AMD in the human eye.

Mechanism of volume viscosity in the liquid metal system lead‐bismuth

Ultrasonic measurements in the Pb–Bi liquid metal system show that volume viscosity in these alloys differs from that observed in the pure metals Pb and Bi. The excess absorption in the alloys exceeds that in either of the pure metals at low temperatures and decreases with increasing temperature. We suggest that this absorption is due to two effects: (1) a structural volume viscosity similar to that observed in pure Pb and Bi; and (2) a volume viscosity due to concentration fluctuations.

Mechanism of Volume Viscosity in Molten Bismuth and Lead

Ultrasonic absorption and velocity have been measured in molten bismuth and lead from the melting point to 650°C. Both liquids were found to exhibit a volume viscosity which increased with increasing temperature. Near the melting point the ratio of volume to shear viscosity was 0.6 and 4.6 for lead and bismuth, respectively. A two‐state model using the experimental coordination number is proposed which gives the proper magnitude and temperature dependence of the volume viscosity in these liquids. The theory is also found to be applicable to molten salts.