Biochemical characterization of human ZIP13 protein: a homo-dimerized zinc transporter involved in the spondylocheiro dysplastic Ehlers-Danlos syndrome

[Role of zinc in type 2 diabetes]

Pancreatic β cells contain the highest amount of zinc among cells within the human body, and hence, the relationship between zinc and diabetes has been of great interest. To date, many studies of zinc and diabetes have been reported, including studies demonstrating that diabetic patients and mice have a decreased amount of zinc in the pancreas. Zinc may counteract the deleterious effects of oxidative stress, which contributes to reduced insulin resistance, and may also protect pancreatic β cells from glucolipotoxicity. Recently, we have shown that SLC30A8/zinc transporter 8, which is a transporter expressed on the surface of insulin granules, plays a key role in zinc transport into insulin granules and in the regulation of hepatic insulin clearance. Here, we review the role of zinc in whole-body maintenance and the latest information on the relationship between zinc and diabetes.

Current understanding of ZIP and ZnT zinc transporters in human health and diseases

Zinc transporters, the Zrt-, Irt-like protein (ZIP) family and the Zn transporter (ZnT) family transporters, are found in all aspects of life. Increasing evidence has clarified the molecular mechanism, in which both transporters play critical roles in cellular and physiological functions via mobilizing zinc across the cellular membrane. In the last decade, mutations in ZIP and ZnT transporter genes have been shown to be implicated in a number of inherited human diseases. Moreover, dysregulation of expression and activity of both transporters has been suggested to be involved in the pathogenesis and progression of chronic diseases including cancer, immunological impairment, and neurodegenerative diseases, although comprehensive understanding is far from complete. The diverse phenotypes of diseases related to ZIP and ZnT transporters reflect the multifarious biological functions of both transporters. The present review summarizes the current understanding of ZIP and ZnT transporter functions from the standpoint of human health and diseases. The study of zinc transporters is currently of great clinical interest.

Pumping iron

The primary role of the ZIP13 metal transporter in flies is to move iron ions out of cells, rather than moving zinc ions into cells, as is the case in human cells.

Molecular pathogenesis of spondylocheirodysplastic Ehlers-Danlos syndrome caused by mutant ZIP13 proteins

The zinc transporter protein ZIP13 plays critical roles in bone, tooth, and connective tissue development, and its dysfunction is responsible for the spondylocheirodysplastic form of Ehlers-Danlos syndrome (SCD-EDS, OMIM 612350). Here, we report the molecular pathogenic mechanism of SCD-EDS caused by two different mutant ZIP13 proteins found in human patients: ZIP13(G64D), in which Gly at amino acid position 64 is replaced by Asp, and ZIP13(ΔFLA), which contains a deletion of Phe-Leu-Ala. We demonstrated that both the ZIP13(G64D) and ZIP13(ΔFLA) protein levels are decreased by degradation via the valosin-containing protein (VCP)-linked ubiquitin proteasome pathway. The inhibition of degradation pathways rescued the protein expression levels, resulting in improved intracellular Zn homeostasis. Our findings uncover the pathogenic mechanisms elicited by mutant ZIP13 proteins. Further elucidation of these degradation processes may lead to novel therapeutic targets for SCD-EDS.

The metal transporter ZIP13 supplies iron into the secretory pathway in Drosophila melanogaster

The intracellular iron transfer process is not well understood, and the identity of the iron transporter responsible for iron delivery to the secretory compartments remains elusive. In this study, we show Drosophila ZIP13 (Slc39a13), a presumed zinc importer, fulfills the iron effluxing role. Interfering with dZIP13 expression causes iron-rescuable iron absorption defect, simultaneous iron increase in the cytosol and decrease in the secretory compartments, failure of ferritin iron loading, and abnormal collagen secretion. dZIP13 expression in E. coli confers upon the host iron-dependent growth and iron resistance. Importantly, time-coursed transport assays using an iron isotope indicated a potent iron exporting activity of dZIP13. The identification of dZIP13 as an iron transporter suggests that the spondylocheiro dysplastic form of Ehlers–Danlos syndrome, in which hZIP13 is defective, is likely due to a failure of iron delivery to the secretory compartments. Our results also broaden our knowledge of the scope of defects from iron dyshomeostasis.

DOI:http://dx.doi.org/10.7554/eLife.03191.001

Iron is essential for life. Amongst its many important roles, iron is crucial for producing collagen—the protein that provides both strength and elasticity to bones, tendons, ligaments, and skin. Like many other proteins, collagens are produced inside the endoplasmic reticulum—an organelle inside the cell that is enclosed by a membrane that is similar to the plasma membrane that surrounds the cell itself.

Two enzymes that are critical for producing collagen need to bind with iron in order to work correctly. To do this, iron in the cytoplasm of the cell has to cross the membrane that surrounds the endoplasmic reticulum. Small molecules are commonly transported across membranes by proteins called transporters, which tend to work on specific types of ions or molecules. However, researchers did not know the identity of the membrane transporter responsible for moving iron into the secretory pathway—including the endoplasmic reticulum—to bind with the enzymes that produce collagen.

Xiao, Wan et al. have now investigated the function of the transporter ZIP13 in the fruit fly Drosophila. This transporter was thought to transport zinc across membranes and into the cytoplasm. Instead, Xiao, Wan et al. found that ZIP13 transports iron out of the cytoplasm and into the endoplasmic reticulum.

Ehlers–Danlos syndrome is a condition that causes individuals to suffer from frequent joint dislocations, bone deformities, and fragile skin as a result of their body producing collagen incorrectly. One form of Ehlers–Danlos syndrome is caused by ZIP13 transporters working incorrectly. However, this was difficult to understand when it was thought that ZIP13 only transports zinc. The discovery that ZIP13 mostly transports iron rather than zinc can explain the link between this transporter and Ehlers–Danlos syndrome: if ZIP13 doesn't work, the collagen-building enzymes cannot get the iron they need to work properly.

Disorders caused by iron deficiencies are normally identified by a few tell-tale symptoms, such as anemia, but these are not seen in Ehlers–Danlos syndrome. Xiao, Wan et al. suggest that iron transport problems could therefore be behind a wider range of diseases and disorders than is currently known.

DOI:http://dx.doi.org/10.7554/eLife.03191.002

The zinc transporter, Slc39a7 (Zip7) is implicated in glycaemic control in skeletal muscle cells

Dysfunctional zinc signaling is implicated in disease processes including cardiovascular disease, Alzheimer's disease and diabetes. Of the twenty-four mammalian zinc transporters, ZIP7 has been identified as an important mediator of the ‘zinc wave’ and in cellular signaling. Utilizing siRNA targeting Zip7 mRNA we have identified that Zip7 regulates glucose metabolism in skeletal muscle cells. An siRNA targeting Zip7 mRNA down regulated Zip7 mRNA 4.6-fold (p = 0.0006) when compared to a scramble control. This was concomitant with a reduction in the expression of genes involved in glucose metabolism including Agl, Dlst, Galm, Gbe1, Idh3g, Pck2, Pgam2, Pgm2, Phkb, Pygm, Tpi1, Gusb and Glut4. Glut4 protein expression was also reduced and insulin-stimulated glycogen synthesis was decreased. This was associated with a reduction in the mRNA expression of Insr, Irs1 and Irs2, and the phosphorylation of Akt. These studies provide a novel role for Zip7 in glucose metabolism in skeletal muscle and highlight the importance of this transporter in contributing to glycaemic control in this tissue.

The ZIP5 ectodomain co-localizes with PrP and may acquire a PrP-like fold that assembles into a dimer

The cellular prion protein (PrP(C)) was recently observed to co-purify with members of the LIV-1 subfamily of ZIP zinc transporters (LZTs), precipitating the surprising discovery that the prion gene family descended from an ancestral LZT gene. Here, we compared the subcellular distribution and biophysical characteristics of LZTs and their PrP-like ectodomains. When expressed in neuroblastoma cells, the ZIP5 member of the LZT subfamily was observed to be largely directed to the same subcellular locations as PrP(C) and both proteins were seen to be endocytosed through vesicles decorated with the Rab5 marker protein. When recombinantly expressed, the PrP-like domain of ZIP5 could be obtained with yields and levels of purity sufficient for structural analyses but it tended to aggregate, thereby precluding attempts to study its structure. These obstacles were overcome by moving to a mammalian cell expression system. The subsequent biophysical characterization of a homogeneous preparation of the ZIP5 PrP-like ectodomain shows that this protein acquires a dimeric, largely globular fold with an α-helical content similar to that of mammalian PrP(C). The use of a mammalian cell expression system also allowed for the expression and purification of stable preparations of Takifugu rubripes PrP-1, thereby overcoming a key hindrance to high-resolution work on a fish PrP(C).

Essential role of the zinc transporter ZIP9/SLC39A9 in regulating the activations of Akt and Erk in B-cell receptor signaling pathway in DT40 cells

The essential trace element zinc is important for all living organisms. Zinc functions not only as a nutritional factor, but also as a second messenger. However, the effects of intracellular zinc on the B cell-receptor (BCR) signaling pathway remain poorly understood. Here, we present data indicating that the increase in intracellular zinc level induced by ZIP9/SLC39A9 (a ZIP Zrt-/Irt-like protein) plays an important role in the activation of Akt and Erk in response to BCR activation. In DT40 cells, the enhancement of Akt and Erk phosphorylation following BCR activation requires intracellular zinc. To clarify this event, we used chicken ZnT5/6/7-gene-triple-knockout DT40 (TKO) cells and chicken Zip9-knockout DT40 (cZip9KO) cells. The levels of Akt and ERK phosphorylation significantly decreased in cZip9KO cells. In addition, the enzymatic activity of protein tyrosine phosphatase (PTPase) increased in cZip9KO cells. These biochemical events were restored by overexpressing the human Zip9 (hZip9) gene. Moreover, we found that the increase in intracellular zinc level depends on the expression of ZIP9. This observation is in agreement with the increased levels of Akt and Erk phosphorylation and the inhibition of total PTPase activity. We concluded that ZIP9 regulates cytosolic zinc level, resulting in the enhancement of Akt and Erk phosphorylation. Our observations provide new mechanistic insights into the BCR signaling pathway underlying the regulation of intracellular zinc level by ZIP9 in response to the BCR activation.

Zinc signal: a new player in osteobiology

Disturbed zinc (Zn) homeostasis in mammals is mainly characterized by impaired bone generation accompanied with growth retardation. However, the underlying mechanisms that determine how Zn controls bone homeostasis remain to be defined. Zn homeostasis is tightly controlled by Zn transporter families. Recent studies have shown that Zn transporter-mediated Zn ion (Zn(2+)) behaves as a signaling factor, called Zn signal, that exerts a multiple function in cellular events, showing why Zn has a vital effect on mammalian bone growth and regeneration. This perspective put importance on the principal mechanisms of Zn participation in mammalian bone homeostasis, shifting our focus on the role of Zn from simply a nutrient to a signaling molecule that fine-tunes intracellular signaling events.

Functional role of intracellular labile zinc in pulmonary endothelium

After iron, zinc is the most abundant essential trace metal. Intracellular zinc ([Zn]_i) is maintained across a wide range of cells and species in a tight quota (100 to 500 μM) by a dynamic process of transport, intracellular vesicular storage, and binding to a large number of proteins (estimated at 3-10% of human proteome). As such, zinc is an integral component of numerous metalloenzymes, structural proteins, and transcription factors. It is generally assumed that a vanishingly small component of [Zn]_i, referred to as free or labile zinc, and operationally defined as the pool sensitive to chelation (by agents such as N, N, N’, N’-tetrakis [2-pyridylmethyl] ethylenediamine [TPEN]) and capable of detection by a variety of chemical and genetic sensors, participates in signal transduction pathways. Zinc deficiencies, per se, can arise from acquired (malnutrition, alcoholism) or genetic (mutations in molecules affecting zinc homeostasis, the informative and first example being acrodermatitis enteropathica) factors or as a component of various diseases (e.g., sickle cell disease, cystic fibrosis, sepsis). Hypozincemia has profound effects on developing humans, and all facets of physiological function (neuronal, endocrine, immunological) are affected, although considerably less is known regarding cardiovascular pathophysiology. In this review, we provide an update on current knowledge of molecular and cellular aspects of zinc homeostasis and then focus on implications of zinc signaling in pulmonary endothelium as it relates to programmed cell death, altered contractility, and septic and aseptic injury to this segment of the lung.

[Overview of and update on the physiological functions of mammalian zinc transporters]

In recent years, a number of mammalian zinc transporters have been molecularly characterized. This has brought about major advances in our understanding of the tight regulation of cellular zinc homeostasis and the pivotal roles zinc transporters play in a variety of biological events. Mammalian zinc transporters are classified into two families: the ZRT, IRT-like protein (ZIP) family and the Zn transporter (ZnT) family. The ZIP family consists of 14 members and facilitates zinc influx into the cytosol from the extracellular and intracellular compartments. The ZnT family consists of 9 members and facilitates zinc efflux from the cytosol to the extracellular and intracellular compartments. Coordinated zinc mobilization across the cellular membrane by both transporter families is indispensable for diverse physiological functions. In this review, the features of the ZIP and ZnT families are briefly reviewed from the perspective of zinc physiology, with emphasis on recent progress.

Promotion of vesicular zinc efflux by ZIP13 and its implications for spondylocheiro dysplastic Ehlers-Danlos syndrome

Zinc is essential but potentially toxic, so intracellular zinc levels are tightly controlled. A key strategy used by many organisms to buffer cytosolic zinc is to store it within vesicles and organelles.It is yet unknown whether vesicular or organellar sites perform this function in mammals. Human ZIP13, a member of the Zrt/Irt-like protein (ZIP) metal transporter family, might provide an answer to this question. Mutations in the ZIP13 gene, SLC39A13, previously were found to cause the spondylocheiro dysplastic form of Ehlers–Danlos syndrome (SCD-EDS), a heritable connective tissue disorder.Those previous studies suggested that ZIP13 transports excess zinc out of the early secretory pathway and that zinc overload in the endoplasmic reticulum (ER) occurs in SCD-EDS patients. In contrast,this study indicates that ZIP13’s role is to release labile zinc from vesicular stores for use in the ER and other compartments. We propose that SCD-EDS is the result of vesicular zinc trapping and ER zinc deficiency rather than overload.

Zinc transporters, mechanisms of action and therapeutic utility: implications for type 2 diabetes mellitus

Zinc is an essential trace element that plays a vital role in maintaining many biological processes and cellular homeostasis. Dysfunctional zinc signaling is associated with a number of chronic disease states including cancer, cardiovascular disease, Alzheimer's disease, and diabetes. Cellular homeostasis requires mechanisms that tightly control the uptake, storage, and distribution of zinc. This is achieved through the coordinated actions of zinc transporters and metallothioneins. Evidence on the role of these proteins in type 2 diabetes mellitus (T2DM) is now emerging. Zinc plays a key role in the synthesis, secretion and action of insulin in both physiological and pathophysiological states. Moreover, recent studies highlight zinc's dynamic role as a "cellular second messenger" in the control of insulin signaling and glucose homeostasis. This suggests that zinc plays an unidentified role as a novel second messenger that augments insulin activity. This previously unexplored concept would raise a whole new area of research into the pathophysiology of insulin resistance and introduce a new class of drug target with utility for diabetes pharmacotherapy.

The ZIP-prion connection

The evolutionary origins of vertebrate prion genes had remained elusive until recently when multiple lines of evidence converged to the proposition that members of the prion gene family represent an ancient branch of a larger family of ZIP metal ion transporters. (1) A follow-up investigation which explored the mechanism of evolution in more detail led to the surprising conclusion that the emergence of the prion founder gene likely involved the reverse transcription of a spliced transcript of a LIV-1 ZIP predecessor gene. (2) The objective of this perspective is to discuss the possible significance of this reunion of ZIP and prion gene subfamilies for understanding the biology of the prion protein in health and disease. While a recent review article broadly introduced this area of research, (3) the emphasis here is to comment on some of the more pertinent concepts, experimental paradigms, ongoing developments and challenges.