Dynamic zinc pools in mouse choroid plexus

Restoration of metal homeostasis: a potential strategy against neurodegenerative diseases

Metal homeostasis is critical to normal neurophysiological activity. Metal ions are involved in the development, metabolism, redox and neurotransmitter transmission of the central nervous system (CNS). Thus, disturbance of homeostasis (such as metal deficiency or excess) can result in serious consequences, including neurooxidative stress, excitotoxicity, neuroinflammation, and nerve cell death. The uptake, transport and metabolism of metal ions are highly regulated by ion channels. There is growing evidence that metal ion disorders and/or the dysfunction of ion channels contribute to the progression of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Therefore, metal homeostasis-related signaling pathways are emerging as promising therapeutic targets for diverse neurological diseases. This review summarizes recent advances in the studies regarding the physiological and pathophysiological functions of metal ions and their channels, as well as their role in neurodegenerative diseases. In addition, currently available metal ion modulators and in vivo quantitative metal ion imaging methods are also discussed. Current work provides certain recommendations based on literatures and in-depth reflections to improve neurodegenerative diseases. Future studies should turn to crosstalk and interactions between different metal ions and their channels. Concomitant pharmacological interventions for two or more metal signaling pathways may offer clinical advantages in treating the neurodegenerative diseases.

GSH and Zinc Supplementation Attenuate Cadmium-Induced Cellular Stress and Stimulation of Choline Uptake in Cultured Neonatal Rat Choroid Plexus Epithelia

Choroid plexus (CP) sequesters cadmium and other metals, protecting the brain from these neurotoxins. These metals can induce cellular stress and modulate homeostatic functions of CP, such as solute transport. We previously showed in primary cultured neonatal rat CP epithelial cells (CPECs) that cadmium induced cellular stress and stimulated choline uptake at the apical membrane, which interfaces with cerebrospinal fluid in situ. Here, in CPECs, we characterized the roles of glutathione (GSH) and Zinc supplementation in the adaptive stress response to cadmium. Cadmium increased GSH and decreased the reduced GSH-to-oxidized GSH (GSSG) ratio. Heat shock protein-70 (Hsp70), heme oxygenase (HO-1), and metallothionein (Mt-1) were induced along with the catalytic and modifier subunits of glutamate cysteine ligase (GCL), the rate-limiting enzyme in GSH synthesis. Inhibition of GCL by l-buthionine sulfoximine (BSO) enhanced stress protein induction and stimulation of choline uptake by cadmium. Zinc alone did not induce Hsp70, HO-1, or GCL subunits, or modulate choline uptake. Zinc supplementation during cadmium exposure attenuated stress protein induction and stimulation of choline uptake; this effect persisted despite inhibition of GSH synthesis. These data indicated up-regulation of GSH synthesis promotes adaptation to cadmium-induced cellular stress in CP, but Zinc may confer cytoprotection independent of GSH.

Mapping Alterations to the Endogenous Elemental Distribution within the Lateral Ventricles and Choroid Plexus in Brain Disorders Using X-Ray Fluorescence Imaging

The choroid plexus and cerebral ventricles are critical structures for the production of cerebral spinal fluid (CSF) and play an important role in regulating ion and metal transport in the brain, however many aspects of its roles in normal physiology and disease states, such as psychiatric illness, remain unknown. The choroid plexus is difficult to examine in vivo, and in situ ex vivo, and as such has typically been examined indirectly with radiolabeled tracers or ex vivo stains, making measurements of the endogenous K+, Cl-, and Ca+ distributions unreliable. In the present study, we directly examined the distribution of endogenous ions and biologically relevant transition metals in the choroid plexus and regions surrounding the ventricles (ventricle wall, cortex, corpus callosum, striatum) using X-ray fluorescence imaging (XFI). We find that the choroid plexus was rich in Cl- and Fe while K+ levels increase further from the ventricle as Cl- levels decrease, consistent with the known role of ion transporters in the choroid plexus CSF production. A polyI:C offspring displayed enlarged ventricles, elevated Cl- surrounding the ventricles, and intraventricular calcifications. These observations fit with clinical findings in patients with schizophrenia and suggest maternal treatment with polyI:C may lead to dysfunctional ion regulation in offspring. This study demonstrates the power of XFI for examining the endogenous elemental distributions of the ventricular system in healthy brain tissue as well as disease models.

Stress-induced stimulation of choline transport in cultured choroid plexus epithelium exposed to low concentrations of cadmium

The choroid plexus epithelium forms the blood-cerebrospinal fluid barrier and accumulates essential minerals and heavy metals. Choroid plexus is cited as being a "sink" for heavy metals and excess minerals, serving to minimize accumulation of these potentially toxic agents in the brain. An understanding of how low doses of contaminant metals might alter transport of other solutes in the choroid plexus is limited. Using primary cultures of epithelial cells isolated from neonatal rat choroid plexus, our objective was to characterize modulation of apical uptake of the model organic cation choline elicited by low concentrations of the contaminant metal cadmium (CdCl₂). At 50-1,000 nM, cadmium did not directly decrease or increase 30-min apical uptake of 10 μM [(3)H]choline. However, extended exposure to 250-500 nM cadmium increased [(3)H]choline uptake by as much as 75% without marked cytotoxicity. In addition, cadmium induced heat shock protein 70 and heme oxygenase-1 protein expression and markedly induced metallothionein gene expression. The antioxidant N-acetylcysteine attenuated stimulation of choline uptake and induction of stress proteins. Conversely, an inhibitor of glutathione synthesis l-buthionine-sulfoximine (BSO) enhanced stimulation of choline uptake and induction of stress proteins. Cadmium also activated ERK1/2 MAP kinase. The MEK1 inhibitor PD98059 diminished ERK1/2 activation and attenuated stimulation of choline uptake. Furthermore, inhibition of ERK1/2 activation abated stimulation of choline uptake in cells exposed to cadmium with BSO. These data indicate that in the choroid plexus, exposure to low concentrations of cadmium may induce oxidative stress and consequently stimulate apical choline transport through activation of ERK1/2 MAP kinase.

Upregulation of zinc transporter 2 in the blood-CSF barrier following lead exposure

Zinc (Zn) is an essential element for normal brain function; an abnormal Zn homeostasis in brain and the cerebrospinal fluid (CSF) has been implied in the etiology of Alzheimer's disease (AD). However, the mechanisms that regulate Zn transport in the blood-brain interface remain unknown. This study was designed to investigate Zn transport by the blood-CSF barrier (BCB) in the choroid plexus, with a particular focus on Zn transporter-2 (ZnT2), and to understand if lead (Pb) accumulation in the choroid plexus disturbed the Zn regulatory function in the BCB. Confocal microscopy, quantitative PCR and western blot demonstrated the presence of ZnT2 in the choroidal epithelia; ZnT2 was primarily in cytosol in freshly isolated plexus tissues but more toward the peripheral membrane in established choroidal Z310 cells. Exposure of rats to Pb (single ip injection of 50 mg Pb acetate/kg) for 24 h increased ZnT2 fluorescent signals in plexus tissues by confocal imaging and protein expression by western blot. Similar results were obtained by in vitro experiments using Z310 cells. Further studies using cultured cells and a two-chamber Transwell device showed that Pb treatment significantly reduced the cellular Zn concentration and led to an increased transport of Zn across the BCB, the effect that may be due to the increased ZnT2 by Pb exposure. Taken together, these results indicate that ZnT2 is present in the BCB; Pb exposure increases the ZnT2 expression in choroidal epithelial cells by a yet unknown mechanism and as a result, more Zn ions may be deposited into the intracellular Zn pool, leading to a relative Zn deficiency state in the cytoplasm at the BCB.

Expression of zinc transporter ZnT7 in mouse superior cervical ganglion

The superior cervical ganglion (SCG) neurons contain a considerable amount of zinc ions, but little is known about the zinc homeostasis in the SCG. It is known that zinc transporter 7 (ZnT7, Slc30a7), a member of the Slc30 ZnT family, is involved in mobilizing zinc ions from the cytoplasm into the Golgi apparatus. In the present study, we examined the expression and localization of ZnT7 and labile zinc ions in the mouse SCG using immunohistochemistry, Western blot and in vivo zinc selenium autometallography (AMG). Our immunohistochemical analysis revealed that the ZnT7 immunoreactivity in the SCG neurons was predominately present in the perinuclear region of the neurons, suggesting an affiliation to the Golgi apparatus. The Western blot results verified that ZnT7 protein was expressed in the mouse SCGs. The AMG reaction product was shown to have a similar distribution as ZnT7 immunoreactivity. These observations support the notion that ZnT7 may participate in zinc transport, storage, and incorporation of zinc into zinc-binding proteins in the Golgi apparatus of mouse SCG neurons.

Abundant expression of zinc transporters in the amyloid plaques of Alzheimer's disease brain

The pathological key features of Alzheimer's disease (AD) are beta-amyloid peptide (Abeta)-containing senile plaques (SP) and neurofibrillary tangles. Previous studies have suggested that an extracellular elevation of the zinc concentration can initiate the deposition of Abeta and lead to the formation of SP. In the present study, we present data showing a correlation between zinc ions, zinc transporters (ZNTs) and AD, using immersion autometallography (AMG) and double immunofluorescence for the ZNTs and Abeta. We found that all the ZNTs tested (ZNT1, 3, 4, 5, 6, 7) were extensively present in the Abeta-positive plaques in the cortex of human AD brains, and the density of autometallographic silver enhanced zinc-sulphur nanoparticles were much higher in the plaques than in the surrounding zinc enriched (ZEN) terminals. Moreover, we found an abundant expression of ZNT3 and autometallographic grains in the amyloid angiopathic vessels. The subcellular localization of ZNTs and zinc ions were not detected, due to the limited tissue preservation in the present study. In conclusion, our data provided significant morphological evidence of zinc ions and ZNTs being actively involved in the pathological processes that lead to plaque formation.

Amyloid plaques arise from zinc-enriched cortical layers in APP/PS1 transgenic mice and are paradoxically enlarged with dietary zinc deficiency

The ZnT3 zinc transporter is uniquely expressed in cortical glutamatergic synapses where it organizes zinc release into the synaptic cleft and mediates beta-amyloid deposition in transgenic mice. We studied the association of zinc in plaques in relation to cytoarchitectural zinc localization in the APP/PS1 transgenic mouse model of Alzheimer's disease. The effects of low dietary zinc for 3 months upon brain pathology were also studied. We determined that synaptic zinc distribution within cortical layers is paralleled by amyloid burden, which is heaviest for both in layers 2-3 and 5. ZnT3 immunoreactivity is prominent in dystrophic neurites within amyloid plaques. Low dietary zinc caused a significant 25% increase in total plaque volume in Alzheimer's mice using stereological measures. The level of oxidized proteins in brain tissue did not changed in animals on a zinc-deficient diet compared with controls. No obvious changes were observed in the autometallographic pattern of zinc-enriched terminals in the neocortex or in the expression levels of zinc transporters, zinc importers or metallothioneins. A small decrease in plasma zinc induced by the low-zinc diet was consistent with the subclinical zinc deficiency that is common in older human populations. While the mechanism remains uncertain, our findings indicate that subclinical zinc deficiency may be a risk factor for Alzheimer's pathology.

Use of Z310 cells as an in vitro blood-cerebrospinal fluid barrier model: tight junction proteins and transport properties

Immortalized rat choroidal epithelial Z310 cells have the potential to become an in vitro model for studying transport of materials at blood-cerebrospinal fluid barrier (BCB) (Shi and Zheng, 2005) [Shi, L.Z., Zheng, W., 2005. Establishment of an in vitro brain barrier epithelial transport system for pharmacological and toxicological study. Brain Research 1057, 37-48]. This study was designed to demonstrate the presence of tight junction properties in Z310 cells and the functionality of Z310 monolayer in transport of selected model compounds. Western blot analyses revealed the presence of claudin-1, ZO-1, and occludin in Z310 cells. Transmission electron microscopy showed a "tight junction" type of structure in the sub-apical lateral membranes between adjacent Z310 cells. Real-time RT-PCR revealed that Z310 cells expressed representative transporters such as DMT1, MTP1, TfR, p-glycoprotein, ATP7A, ZnT1, ABCC1, Oat3, OCT1 and OB-Ra. Moreover, Z310 cells cultured in a two-chamber Transwell device possessed the ability to transport zidovudine (anionic drug), thyroxine (hormone), thymidine (nucleoside), and leptin (large polypeptide) with kinetic properties similar to those obtained from the in vitro model based on primary culture of choroidal epithelial cells. Taken together, these data indicate that the Z310 BCB model expresses major tight junction proteins and forms a tight barrier in vitro. The model also exhibits the ability to transport substances of various categories across the barrier.

Imunoreactivity of zinc transporter 7 (ZNT7) in mouse dorsal root ganglia

In the present study, we showed for the first time the localization of ZNT7 immunoreactivity in the mouse dorsal root ganglion (DRG) by means of immunohistochemistry and confocal laser scanning microscopy. Our results revealed that ZNT7 immunoreactivity was abundantly expressed in the nerve cells of the mouse DRG. Strong ZNT7 immunoreactivity was predominantly distributed in the perinuclear region of positive cells, while the nuclei were devoid of staining. Double immunofluorescence labeling of ZNT7 and TGN38 revealed a colocalization of the two antigens in the Golgi apparatus. In addition, the presence of labile zinc ions was detected with in vivo zinc selenium autometallography (AMG). AMG observations showed that the zinc staining pattern was also predominately located in the perinuclear Golgi area, like the ZNT7 immunostaining pattern in the DRG. These observations strongly suggest that ZNT7 may play an important role in facilitating zinc transport into the Golgi apparatus from the cytosol in the mouse DRG.

Zinc transporter 7 is located in the cis-Golgi apparatus of mouse choroid epithelial cells

The cellular localization of zinc transporter 7 protein in the mouse choroid plexus was examined in this study. Zinc transporter 7 immunoreactive cells were detected in the third, lateral, and fourth ventricles of CD-1 mouse brain. Distinct zinc transporter 7 immunoreactivity was concentrated in the perinuclear regions of the positive cells. The results from zinc autometallography showed that zinc-positive grains were also predominantly located in the perinuclear areas. Ultrastructural localization showed that zinc transporter 7 immunostaining was predominantly present in the membrane and cisternae of the cis-Golgi networks and some vesicle compartments. The results support the notion that zinc transporter 7 may participate in the transport of the cytoplasmic zinc into the Golgi apparatus, and may be involved in local packaging of zinc-binding proteins in the mouse choroid plexus.