Mechanism of zinc uptake by microvilli isolated from human term placenta

Role of zinc in female reproduction

Zinc is a critical component in a number of conserved processes that regulate female germ cell growth, fertility, and pregnancy. During follicle development, a sufficient intracellular concentration of zinc in the oocyte maintains meiotic arrest at prophase I until the germ cell is ready to undergo maturation. An adequate supply of zinc is necessary for the oocyte to form a fertilization-competent egg as dietary zinc deficiency or chelation of zinc disrupts maturation and reduces the oocyte quality. Following sperm fusion to the egg to initiate the acrosomal reaction, a quick release of zinc, known as the zinc spark, induces egg activation in addition to facilitating zona pellucida hardening and reducing sperm motility to prevent polyspermy. Symmetric division, proliferation, and differentiation of the preimplantation embryo rely on zinc availability, both during the oocyte development and post-fertilization. Further, the fetal contribution to the placenta, fetal limb growth, and neural tube development are hindered in females challenged with zinc deficiency during pregnancy. In this review, we discuss the role of zinc in germ cell development, fertilization, and pregnancy with a focus on recent studies in mammalian females. We further detail the fundamental zinc-mediated reproductive processes that have only been explored in non-mammalian species and speculate on the role of zinc in similar mechanisms of female mammals. The evidence collected over the last decade highlights the necessity of zinc for normal fertility and healthy pregnancy outcomes, which suggests zinc supplementation should be considered for reproductive age women at risk of zinc deficiency.

Role of zinc in neonatal growth and brain growth: review and scoping review

This manuscript includes (1) a narrative review of Zinc as an essential nutrient for fetal and neonatal growth and brain growth and development and (2) a scoping review of studies assessing the effects of Zinc supplementation on survival, growth, brain growth, and neurodevelopment in neonates. Very preterm infants and small for gestational age infants are at risk for Zinc deficiency. Zinc deficiency can cause several complications including periorificial lesions, delayed wound healing, hair loss, diarrhea, immune deficiency, growth failure with stunting, and brain atrophy and dysfunction. Zinc is considered essential for oligodendrogenesis, neurogenesis, neuronal differentiation, white matter growth, and multiple biological and physiological roles in neurobiology. Data support the possibility that the critical period of Zinc delivery for brain growth in the mouse starts at 18 days of a 20-21-day pregnancy and extends during lactation and in human may start at 26 weeks of gestation and extend until at least 44 weeks of postmenstrual age. Studies are needed to better elucidate Zinc requirement in extremely low gestational age neonates to minimize morbidity, optimize growth, and brain growth, prevent periventricular leukomalacia and optimize neurodevelopment. IMPACT: Zinc is essential for growth and brain growth and development. In the USA, very preterm small for gestational age infants are at risk for Zinc deficiency. Data support the possibility that the critical period of Zinc delivery for brain growth in the mouse starts at 18 days of a 20-21-day pregnancy and extends during lactation and in human may start at 26 weeks' gestation and extend until at least 44 weeks of postmenstrual age. Several randomized trials of Zinc supplementation in neonates have shown improvement in growth when using high enough dose, for long duration in patients likely to or proven to have a Zinc deficiency. Studies are needed to better elucidate Zinc requirement in extremely low gestational age neonates to minimize morbidity, optimize growth and brain growth, prevent periventricular leukomalacia and optimize neurodevelopment.

ATSDR evaluation of the health effects of zinc and relevance to public health

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites, which have the greatest public health impact. These profiles comprehensively summarise toxicological and environmental information. This article constitutes the release of portions of the Toxicological Profile for Zinc. The primary purpose of this article is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of zinc. It contains descriptions and evaluations of toxicological studies and epidemiological investigations, and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health. Toxicology and Industrial Health 2006; 22: 423-493.

Maternal zinc intakes and homeostatic adjustments during pregnancy and lactation

Zinc plays critical roles during embryogenesis, fetal growth, and milk secretion, which increase the zinc need for pregnancy and lactation. Increased needs can be met by increasing the dietary zinc intake, along with making homeostatic adjustments in zinc utilization. Potential homeostatic adjustments include changes in circulating zinc, increased zinc absorption, decreased zinc losses, and changes in whole body zinc kinetics. Although severe zinc deficiency during pregnancy has devastating effects, systematic reviews and meta-analysis of the effect of maternal zinc supplementation on pregnancy outcomes have consistently shown a limited benefit. We hypothesize, therefore, that zinc homeostatic adjustments during pregnancy and lactation improve zinc utilization sufficiently to provide the increased zinc needs in these stages and, therefore, mitigate immediate detrimental effects due to a low zinc intake. The specific questions addressed are the following: How is zinc utilization altered during pregnancy and lactation? Are those homeostatic adjustments influenced by maternal zinc status, dietary zinc, or zinc supplementation? These questions are addressed by critically reviewing results from published human studies on zinc homeostasis during pregnancy and lactation carried out in different populations worldwide.

Assessment of essential and nonessential metals and different metal exposure biomarkers in the human placenta in a population from the south of Portugal

The general population is exposed to metals as trace amounts of metallic compounds are present in air, water, and food. Information on background exposures and biomarker concentrations of environmental chemicals in the general Portuguese population is limited. Therefore, the purpose of this study was to determine the levels of important nonessential metals with recognized toxicity cadmium (Cd) and lead (Pb) and essential metals copper (Cu), nickel (Ni), chromium (Cr), and zinc (Zn) in placentas of mothers living in south Portugal (Algarve). Due to the difficulty in establishing the effects of chemicals in a complex and variable environment, this study also aimed to examine the response of biomarkers, such as biochemical changes that occurs at subcellular levels in the presence of contaminants. The investigated biomarkers in placentas indicative of metal exposure or damage included the metallothioneins (MT), delta-aminolevulinic acid dehydratase (ALAD) (specific for Pb), and lipid peroxidation (LPO) as an index of oxidative stress damage. Moreover, HJ-BIPLOT was applied in order to identify and categorize mothers vulnerable to environmental contamination in this region. Metal concentrations in the placenta were not excessive but within the range found in most European studies. In general, the biomarkers MT and LPO were positively correlated with metal levels, while with ALAD the opposite occurred, indicating the selected battery of biomarkers were suitable to study the effects of metals on human placenta. Further, the application of multivariate analysis with HJ-BIPLOT showed that most significant factors contributing to maternal and fetal exposures via placenta were dietary and smoking habits.

Metals content in placentas from moderate cigarette consumers: correlation with newborn birth weight

Cigarette consumption during pregnancy produces deleterious effects in both, mother and fetus, some of them due to the presence of toxic elements in cigarette smoke, such as cadmium. Placenta constitutes a dual-purpose specimen for evaluating the pollutant burden exerted on the mother as well as on the fetus. The main objective of this study was to establish a correlation between placental concentration and distribution of some metal elements and birth weight of neonates delivered by mothers, who were either moderate smokers or nonsmokers. Forty nonsmoking and moderate smoking pregnant women paired per age, parity, weight, height and body mass index were selected. Smoking was assessed by self-reported cigarette consumption during pregnancy and urine cotinine concentration before delivery. Placental metal concentrations were evaluated by atomic absorption spectrometry (copper and cadmium) and neutron activation analysis (zinc and iron). Newborns from smokers had lower birth weights compared to infants from nonsmokers. Birth weights were correlated with placental cadmium concentrations in both, smokers and nonsmokers. Placental zinc and cadmium of smokers were mainly located at the maternal side and their levels were higher than those found in nonsmoker's placentas. In addition, all metal nutrient/pollutant ratios were decreased in the smoker group. In this first study performed in our region, we found that moderate smoking mothers deliver neonates with decreased birth weight and highly correlated to placental cadmium concentration. Decreased metal nutrient/pollutant ratios, a condition here found in smokers, may indicate a placental dysfunction, contributing to impair birth weight.

Induction of zinc transporters by forskolin in human trophoblast BeWo cells

During pregnancy, the zinc level in fetal serum is up to two-fold higher than that in the maternal serum at the end of pregnancy, but the mechanism of zinc release from the placenta into fetal circulation is not well understood. In this study, we determined the expression profiles of zinc transporters in human trophoblast BeWo cells, a representative human trophoblast cell line. Zn transporter 1-8 (ZnT1-8), Zrt/IRT-like protein 1 and Zn transporter-like transporter 1 were detected in BeWo cells by reverse transcription-polymerase chain reaction. Forskolin (FK) is a representative inducer of differentiation of BeWo cells cytotrophoblast into syncytiotrophoblast. Treatment of BeWo cells with FK resulted in morphological changes of BeWo cells into syncytiotrophoblast cells and secretion of human chorionic gonadotropin, which is a characteristic of syncytiotrophoblast cells. Treatment of BeWo cells with FK elevated ZnT1, 2 and 4 mRNA levels. These data about expression profiles of ZnTs may be useful for further investigation of placental biology.

Expression profiles of zinc transporters in rodent placental models

Zinc is a vital metal that is a structural and functional component of many proteins. The precise mechanism of zinc transport in the placenta remains unclear. In this study, we investigated the expression of zinc transporters (ZnTs) in the mouse placenta and in two rat trophoblast cell lines, TR-TBT cells, which are syncytiotrophoblast cells of the labyrinth zone, and Rcho-1 cells, which retain trophoblast cell features and differentiate into trophoblast giant cells of the junctional zone. All of the ZnTs that have been identified in mice (ZnT1-7) were detected in the mouse placenta by RT-PCR. The expression profiles of ZnTs in the placenta during pregnancy were different. The mRNA levels of ZnTs, with the exception of ZnT7, did not change during pregnancy. The ZnT7 mRNA level in placenta was elevated during pregnancy. In TR-TBT cells, ZnT1, ZnT3 and ZnT4 were detected by RT-PCR analysis. In Rcho-1 cells, all of the ZnTs that have been identified in rats (ZnT1-4) were detected by RT-PCR analysis. There were no differences between the mRNA expression levels of ZnT family members in undifferentiated Rcho-1 cells and differentiated Rcho-1 cells. This is the first report of expression profiles of ZnTs during differentiation of the placenta in the mouse placenta and rat placental cell models.

Intestinal and placental zinc transport pathways

Mammalian members of the cation diffusion facilitator (CDF) and zrt-, irt-like protein (ZIP) families of Zn transporters, initially identified in Saccharomyces cerevisiae and Arabidopsis thalania spp., have been cloned during the last 8 years and have been classified as families SLC30 and SLC39 respectively. The cloning of human Zn transporters ZnT-like transporter 1 (hZTL1)/ZnT5 (SLC30A5) and hZIP4 (SLC39A4) were major advances in the understanding of the molecular mechanisms of dietary Zn absorption. Both transporters are localised at the enterocyte apical membrane and are, therefore, potentially of fundamental importance in dietary Zn uptake. hZTL1 mediates Zn uptake when expressed in Xenopus laevis oocytes and hZIP4 is mutated in most cases of the inherited Zn deficiency disease acrodermatitis enteropathica. Localisation of hZTL1/ZnT5 at the apical membrane of the placental syncytiotrophoblast indicates a fundamental role in the transfer of Slc30 Zn to the foetus. Observations in rodent models indicate that in the intestine increased Zn availability increases expression of Zn transporters. Human intestinal Caco-2 cells show a similar response to increasing the Zn2+ concentration of the nutrient medium in relation to the expression of mRNA corresponding to several Zn transporters and that of ZnT1 (SLC30A1) and hZTL1/ZnT5 proteins. In the human placental cell line JAR, however, expression at the mRNA level of a number of Zn transporters is not modified by Zn availability, whilst ZnT1 and hZTL1/ZnT5 proteins are reduced under Zn-supplemented conditions. These differences between Caco-2 and JAR cells in Zn transporter gene responses to Zn supply may reflect the different extracellular Zn concentrations encountered by the corresponding cell types in vitro.

Intestinal zinc uptake in freshwater rainbow trout: evidence for apical pathways associated with potassium efflux and modified by calcium

Understanding the mechanisms of intestinal zinc uptake in fish is of considerable interest from both nutritional and toxicological perspectives. In this study, properties of zinc transport across the apical membrane of freshwater rainbow trout intestinal epithelia were examined using right-side-out brush border membrane vesicles (BBMV's). Extravesicular calcium was found to have complex actions on zinc uptake. At a low zinc concentration of 1 microM, calcium (0.1-2 mM) significantly stimulated zinc uptake. In contrast, calcium inhibited zinc uptake at higher zinc levels (100 microM). Lanthanum and cadmium in the external medium did not block zinc uptake, suggesting that interactions between zinc and calcium were not exerted at a calcium channel. Copper also failed to exercise any inhibitory action. Zinc association with the BBMV's was enhanced by an outward potassium gradient. This stimulatory effect was only present at a zinc concentration of 100 microM. The potassium channel blocker, tetraethylammonium chloride inhibited zinc uptake at this relatively high zinc concentration, suggesting the presence of a low affinity zinc uptake pathway linked to potassium efflux. The present study provides evidence that the mechanism of intestinal zinc uptake in rainbow trout is pharmacologically very different from that of the piscine gill and the mammalian intestine.

Effects of dissolved metals and other hydrominerals on in vivo intestinal zinc uptake in freshwater rainbow trout

For aquatic organisms, zinc is both an essential nutrient and an environmental contaminant. The intestine is potentially the most important route of zinc absorption, yet little is known regarding this uptake pathway for zinc in fish. A recently developed in vivo perfusion system was used to investigate the effect of luminal composition upon intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss). Perfusate cadmium and copper had specific, yet distinct, antagonistic effects upon lumen to tissue zinc movement. Copper significantly reduced the proportion of zinc taken up from the perfusate, and concomitantly limited the passage of zinc into the circulation and beyond. Conversely, cadmium decreased subepithelial zinc accumulation, with rates falling to 29 nmol g(-1) h(-1) from the control (zinc alone) values of 53 nmol g(-1) h(-1). Calcium had a similar action to copper, also reducing post-intestinal zinc accumulation from 0.06 to 0.02 nmol g(-1) h(-1), an effect attributed to interactions between calcium and the zinc uptake pathway. In addition to these effects, luminal composition also had a marked influence upon epithelial response to zinc. Calcium, copper and magnesium all greatly reduced zinc-induced mucus secretion. Cadmium, a toxic metal, significantly increased mucus secretion. It is proposed that these modifications were related to the essentiality of each element, and their potential mechanisms of uptake. Despite changes at the epithelium, the post-epithelial accumulation of zinc was dependent mainly upon the nature of the competing cation. Intestinal saline ion substitution experiments suggested a potential link of potassium ion efflux to zinc uptake. The effect of pH buffering of luminal solutions was also investigated.

Zinc uptake into MCF-10A cells is inhibited by cholesterol depletion

The mechanism for cellular Zn uptake was investigated by depleting cell cholesterol levels, a treatment that disrupts lipid rafts/caveolae-dependent processes and inhibits coated-pit budding. Incubation of MCF-10A human breast epithelial cells with hydroxypropyl-beta-cyclodextrin significantly lowered cell cholesterol levels and significantly inhibited cellular zinc uptake measured at 10 min, but had no effect on 2-deoxyglucose uptake. Replacing potassium for sodium in the uptake buffer significantly stimulated Zn uptake by 20%. The effects of potassium depletion and chlorpromazine on Zn uptake were investigated to determine the contribution of coated-pit endocytosis. Potassium depletion following hypotonic shock significantly inhibited Zn uptake into MCF-10A cells approximately 15%. Chlorpromazine at 20 microg/ml inhibited uptake approximately 30%. The data support the hypothesis that Zn uptake into MCF-10A cells involves lipid rafts/caveolae. The relatively mild effects of potassium depletion and chlorpromazine suggest that a small portion of Zn uptake may require coated pit endocytosis.

Cloning and characterization of a novel mammalian zinc transporter, zinc transporter 5, abundantly expressed in pancreatic beta cells

Intracellular homeostasis for zinc is achieved through the coordinate regulation of specific transporters engaged in zinc influx, efflux, and intracellular compartmentalization. We have identified a novel mammalian zinc transporter, zinc transporter 5 (ZnT-5), by virtue of its similarity to ZRC1, a zinc transporter of Saccharomyces cerevisiae, a member of the cation diffusion facilitator family. Human ZnT-5 (hZnT-5) cDNA encodes a 765-amino acid protein with 15 predicted membrane-spanning domains. hZnT-5 was ubiquitously expressed in all tested human tissues and abundantly expressed in the pancreas. In the human pancreas, hZnT-5 was expressed abundantly in insulin-containing beta cells that contain zinc at the highest level in the body. The hZnT-5 immunoreactivity was found to be associated with secretory granules by electron microscopy. The hZnT-5-derived zinc transport activity was detected using the Golgi-enriched vesicles prepared from hZnT-5-induced HeLa/hZnT-5 cells in which exogenous hZnT-5 expression is inducible by the Tet-on gene regulation system. This activity was dependent on time, temperature, and concentration and was saturable. Moreover, zinc at a high concentration (10 mm) inhibited the growth of yeast expressing hZnT-5. These results suggest that ZnT-5 plays an important role for transporting zinc into secretory granules in pancreatic beta cells.

Human fetal endothelial cells acquire zinc(II) from both the protein bound and nonprotein bound pools in serum

To help determine physiologically important routes by which zinc (Zn) is acquired by human fetal vascular endothelium, the authors incubated cultured umbilical vein endothelial cells with 65Zn(II)-tracer labeled human fetal whole serum, ultrafiltrate (containing low molecular mass serum zinc complexes), and dialyzed serum (containing protein-bound zinc). Zinc from whole serum and from both serum fractions entered a rapidly labeled cellular compartment, removable by edetic acid (EDTA), representing Zn bound to the outside cell surface, and accumulatively, an EDTA-resistant compartment-probably largely internalized Zn. Entry of Zn into the EDTA-resistant pool from both serum fractions was strongly temperature-dependent, and was not via the EDTA-sensitive pool. Entry from the ultrafiltrate was resolvable into high affinity saturable, and non- (or hardly-) saturable components. Transfer from the dialyzed serum fraction was not significantly saturable, but only partially accounted for by nonspecific pinocytosis. Thus, Zn is obtained by fetal vascular endothelium partly from low molecular mass serum species, probably through at least one carrier-mediated membrane transport system; but also from Zn complexed with serum protein, via at least one metabolism-related route.

Two major pathways of zinc(II) acquisition by human placental syncytiotrophoblast

Uptake of zinc into placental villous syncytiotrophoblast is the first step in its transfer from mother to fetus. To help characterise physiologically significant pathways of zinc accumulation by these cells, we incubated cultured layers of syncytiotrophoblast cells derived from human near-term placental tissue with serum ultrafiltrate (containing the zinc complexed with low molecular mass serum constituents), dialysed serum (containing the zinc bound to the serum proteins) and whole serum, each of whose endogenous zinc was tracer-labelled with 65Zn(II). Zinc label from both fractions of serum readily entered a rapidly labelled EDTA-sensitive cellular compartment, probably representing zinc bound to the outside cell surface and in accumulative fashion, an EDTA-resistant compartment, probably consisting largely of internalised cellular zinc. Movement of zinc into the EDTA-resistant pool was strongly temperature-dependent and did not occur via the EDTA-sensitive pool from either serum source. Transfer of zinc from the low molecular mass serum fraction into the EDTA-resistant pool was saturable, the concentration giving half-maximal rate being 1.2 mumol/l nonprotein-bound zinc. No nonsaturable component was detected. Zinc from the serum protein-bound fraction entered by a saturable component, already saturated at physiological total protein-bound zinc concentration, and by an apparently nonsaturable component, not appreciably accounted for by nonspecific fluid-phase endocytosis. The results show that zinc is acquired by placental syncytiotrophoblast from the low molecular mass serum zinc pool probably by a carrier-mediated process, and at least as importantly, from the zinc bound to serum protein, possibly by an endocytic mechanism.