Zinc and its transporters play key roles during embryonic development and many biological processes. Ablation of ZIP6 and ZnT1 results in developmental abnormalities in zebrafish and rodents, respectively (
Andrews et al. 2004;
Yamashita et al. 2004). ZIP6 also plays a role in the maturation of immune cells, including dendritic cells and the regulation of T-cell receptor signaling (
Kitamura et al. 2006;
Yu et al. 2011). ZIP13 knockout mice exhibit defects in connective tissue development, via dysregulation of bone morphogenic protein and TGF-β signaling (
Fukada et al. 2008). ZIP14 appears to be required for systemic growth via control of G-protein coupled receptor-mediated signaling (
Hojyo et al. 2011). ZnT3 is essential for pre-synaptic MAP kinase signaling and the development of hippocampus-dependent memory (
Sindreu et al. 2011). Although a key role of zinc during growth and developmental processes has been clearly established in experimental models, in human studies, the data linking zinc to adverse fetal growth and developmental outcomes is conflicting (
Hess et al. 2009). However, interpretation of human studies is hampered by the lack of specific and sensitive biomarkers for zinc status, especially during pregnancy and development (
Shah et al. 2006). This is coupled with a lack of knowledge of the precise mechanisms that regulate zinc delivery from maternal stores to the fetus, especially at early stages of development, and a lack of understanding the mechanistic consequences of zinc deficiency on fetal outcomes. The development of an in vivo model, such as zebrafish, to study the zinc metabolism during embryo development would significantly aid in filling these knowledge gaps and aid in our understanding of the relationship between zinc & its regulatory proteins and developmental processes at a whole organism level.
The ZnT family of transporters functions in zinc efflux from the cytoplasm to either the extracellular space and/or intracellular organelles. The ZIP family of protein functions in zinc uptake from the extracellular matrix and/or intracellular organelles into the cytoplasm (
Lichten et al. 2009). These zinc transporters are expressed in a tissue-specific manner, and respond differentially to dietary zinc levels and physiological conditions. Therefore, a loss of function or dysregulation of certain zinc transporters would result in an impairment of zinc homeostasis and predispose the body to zinc-imbalance-related diseases, such as cancer, asthma, diabetes, and Alzheimer’s disease (
Lichten et al. 2009). For example, in humans, mutations of Zip4 gene is the cause of human acrodermatitis enteropathica, an autosomal recessively inherited disease (
Kury et al. 2002), and polymorphisms in ZnT8 is associated with type 2 diabetes (
Sladek et al. 2007). Dysregulation of ZnT1 has been associated with the genetic disorder epidermodysplasia verruciformia, where patients are highly susceptible to human papillomavirus (HPV) infection (
Orth 2008). Mutations in ZnT2 has been linked to an inability for the mammary gland to secrete zinc during lactation (
Seo et al. 2010). Over-expression of zinc transporters, such as ZIP6 and ZIP4 has also been associated with cancer cell growth in breast, liver and pancreatic cancers (
Li et al. 2007;
Shen et al. 2009;
Weaver et al. 2010). We observed trends for differential zinc transporter expression patterns that were affected by developmental age. This data highlights our ability to monitor zinc transporter expression during zebrafish development and supports studies using this model organism for zinc metabolism and/or functional zinc transporter studies.
The tissue expression, cellular localization and regulation among each zinc transporter are very different. For example, ZnT1 is ubiquitously expressed, but highly expressed in tissues involved in zinc transfer, such as the basolateral side of enterocytes and kidney tubules (
McMahon et al. 1998). ZnT1 in particular appears to play a critical role in dietary zinc absorption, controlling the efflux of zinc out of the enterocyte across the basolateral membrane (
Andrews et al. 2004). It also localizes to the yolk sac membrane in rodents and may facilitate transfer of zinc between mother and fetus (
Liuzzi et al. 2003). The severity lethality of ZnT1 knockdown in mammals also renders it more difficult to identify the precise role of ZnT1 and zinc during development. Relative to the other zinc transporters, ZnT1 and ZnT5 transcript copy number was most abundant at 0 hpf in zebrafish.
Large increases in expression of ZIP3,4,8 and ZnT2 and ZnT8 were apparent across development (). The greatest change in expression levels occurred in ZnT8 mRNA levels across development. We highlight that there is marked increase in expression of ZnT8 at 6 hpf (~646 fold increase) that corresponds with embryonic synthesis of proinsulin (
Kinkel et al. 2009) and peaks at 48hpf (~5000 fold increase) that corresponds with organogenesis. ZnT8 appears to play a critical role in the synthesis and transport of zinc into secretory vesicles and formation of zinc-insulin complexes (
Chistiakov et al. 2009). Loss of ZnT8 in pancreatic β cells reduces insulin content and loss of insulin release (
Fu et al. 2009;
Lemaire et al. 2009). Genome-wide association studies have found that a polymorphism variant in ZnT8, rs1226634 [C/T transition; Arg(325)- Trp (325)] is associated with increased risk of Type 2 diabetes (
Saxena et al. 2007;
Scott et al. 2007;
Pound et al. 2009). Interestingly, ZnT8 is also an auto-antigen in Type I diabetes (
Wenzlau et al. 2007). The insulin producing pancreatic β-cells contain some of the highest levels of zinc in the body. This high zinc requirement is largely due to the critical function of zinc for insulin synthesis, secretion and signaling (
Tallman et al. 1999). High zinc levels in the pancreas may also be necessary to providing protection against oxidative stresses (
Ho et al. 2001). Zinc deficiency may predispose individuals to diabetes and its cardiovascular complications (
Mocchegiani et al. 2008). Interestingly, maternal zinc deficiency increases her offsprings’ susceptibility to increased body mass, glucose intolerance and impaired insulin secretion (
Padmavathi et al. 2009). The use of zebrafish as a model organism for zinc studies is powerful in that for the first time we could quantitatively track zinc transporter expression from the single cell zygote stage through all stages of vertebrate development. Since early stages of vertebrate development are remarkably conserved, these zebrafish studies are highly relevant to human health. Importantly, similar to humans, zebrafish also require zinc and have in place similar zinc regulatory proteins to maintain zinc homeostasis (
Feeney et al. 2005). Both family of zinc transports (ZIP and ZnT) and their control via Metal Transcription Factor and Metal Response Elements have been previously established to occur in zebrafish (
Zheng et al. 2008). Studies in zebrafish gills, also suggest that zebrafish tissues respond to both zinc supplementation and zinc deficiency exposures (Zheng et al.; Zheng et al.). The zebrafish, especially the developing embryo, is an attractive model system for studies of zinc function. Beneficial attributes include its small size (<1 gram body weight), rapid embryonic development (less than 1 week) and short life cycle (
Dodd et al. 2000;
Wixon 2000;
Udvadia et al. 2003). The zebrafish model enables assessment of integrative, whole animal effects. Importantly, fundamental processes and mechanisms of development are conserved across species (
Lein et al. 2005).
Collectively, the current data confirms the expression of zinc transporter proteins in zebrafish, and that they are differentially expression during development. This sets the stage for using zebrafish to aid in defining the specific molecular targets that control zinc homeostasis during development and characterize mechanisms by which zinc status or alterations in transporter expression affects fetal health. Future studies using purified zebrafish diets containing adequate or deficiency levels of zinc will be performed to closely examine the impact of dietary zinc status on embryonic developmental processes. This mechanistic work in zebrafish will provide the framework for translating results to humans and gain a better understanding of importance of zinc during pregnancy and development.