Zinc homeostasis in the immune system
Lothar Rink
Instiute of Immunology, University Hospital, RWTH-Aachen University, Aachen,
Germany
Zinc is an essential trace element for the immune system and zinc deficiency
compromises the immune function of all cells of the immune system. However, an
access of zinc have negative effects on the immune system too. Therefore zinc
homeostasis must be delicately regulated for an effective immune response.
Zinc deficiency in accompanied by signs of chronic inflammation. The
overexpression of proinflammatory cytokines is due to epigenetic changes in the
promoter regions of proinflammatory cytokines during zinc deficiency. Whereas zinc
deficiency alters the promoter regions into a more accessible form, a
supplementation with zinc restructured the promoter to a closed form. However, this
is not a complete suppression of proinflammatory cytokine expression, but a
regulation of the threshold of gene activation. This resulted in a higher activity of
monocytes under zinc deficient conditions.
For T cell immunity the regulation by zinc is the other way around. Zinc
supplementation induces a shift towards TH1 immune reactions, which is controlled
by regulatory T cells. The production of interferon-2 increases with
zinc supplementation. However, the induction of regulatory T cells limits this effect
and is dependent on the zinc concentration and on the stimuli used. A regulation of
immune responses in dependence of the zinc concentration could be shown in
different models like transplantation, allergy and ageing.
Taken together, the immune response is directly regulated by the zinc level and
could be manipulated by zinc supplementation in vivo. Therefore, depending on the
dose, zinc could be used as immunostimulant as well as immunosuppressant.
Effects of other metals on the expression of zinc-responsive genes
Dianne Ford (Newcastle University)
We reported previously that expression of the zinc transporter ZnT5 (SLC30A5) is
repressed at both the mRNA and protein levels at high extracellular concentrations
of zinc, using the human intestinal Caco-2 cell line model, consistent with
observations we made in human participants in response to an oral zinc
supplement1,2. Studies into the mechanism of this response identified a specific
sequence element – the zinc-responsive element (ZRE) – in the SLC30A5 promoter
as mediating transcriptional repression in response to zinc3,4. Other human genes
that include the ZRE close to the transcription start site include SLC30A10 – coding
for the zinc transporter ZnT10 – and the CBDW genes – coding for COBW domaincontaining proteins. In prokaryotes, CBDW homologues are found within operons
regulated by the zinc-dependent transcriptional repressor Zur along with genes
coding metal-dependent enzymes, and their products appear to have roles in metal
homeostasis. The function of the CBDW proteins in mammals is unknown. We
have demonstrated ZRE-dependent transcriptional repression by zinc of the
SLC30A10* and CBDW promoters, as well as the SLC30A5 promoter, using reporter
constructs expressed in Caco-2 cells. Our recent data indicate that levels of
SLC30A10 and CBDW mRNA are affected by the availability of divalent metals other
than zinc. CBDW mRNA was reduced in Caco-2 cells in response to high
extracellular concentrations of cobalt and nickel and SLC30A10 mRNA* was
reduced in response to high extracellular concentrations of zinc, cobalt and copper II,
suggesting that these proteins may play a complex role in a coordinated cellular
response to multiple metals.
*In collaboration with Ruth Valentine’s laboratory (Newcastle University)
References
1. Cragg RA et al (2002) Journal of Biological Chemistry 277: 22789-97
2. Cragg RA et al (2005) Gut 54: 469-78
3. Jackson KA et al (2007) Journal of Biological Chemistry 282: 10423-31
4. Jackson KA et al (2008) Biochemical Society Transactions 36:1262-6
Acknowledgements
Funded by BBSRC grants BB/D01669X/2 (to Ruth Valentine) and BB/F019637/1 (to
Dianne Ford)
Laboratory work was carried out by Dr John Tyson, Ogo Agbor Ogo and Helen
Bosomworth.
DIFFERENTIAL SUBCELLULAR LOCALIZATION OF THE SPLICE VARIANTS
OF THE ZINC TRANSPORTER ZnT5
Ruth Valentine (Newcastle University)
Zinc is emerging as an important intracellular signaling molecule, as well as fulfilling
essential structural and catalytic functions through incorporation in a myriad of zinc
metalloproteins so it is important to elucidate the molecular mechanisms of zinc
homeostasis, including the subcellular localizations of zinc transporters. Two splice
variants of the human SLC30A5 Zn transporter gene (ZnT5) have been reported in
the literature. These variants differ at their N- and C-terminal regions, corresponding
with the use of different 5’ and 3’ exons. We confirm expression of both variant A
and variant B in a range of untreated human tissues by splice variant-specific RTPCR. Using N- or C-terminal GFP or FLAG fusions of both isoforms of ZnT5 we
identify that the differential subcellular localization to the Golgi apparatus and ER
respectively is a function of their alternative C-terminal sequences. These different
C-terminal regions result from the incorporation into the mature transcript of either
the whole of exon 14 (variant B) or only the 5’ region of exon 14 plus exons 15-17
(variant A). We thus propose that exons 15 to 17 confer functions relating to zinc
delivery to zinc-containing proteins in the Golgi apparatus and that the 3’ end of exon
14 may confer a specialized role in intracellular zinc signaling through mediating
uptake into the ER, well positioned for ZIP7-mediated zinc release into the cytoplasm
in the form of a ‘zinc wave’.
Glucose-mediated changes in intracellular free Zn2+ in pancreatic β cells are
regulated by the granular zinc transporter ZnT8
Gargi Meur (Imperial College London)
The secretory granule zinc transporter ZnT8, expressed almost exclusively in the
endocrine pancreas in mammals, and is the predominant zinc transporter in the beta
cell. A risk allele in the gene (SLC30A8) encoding ZnT8 predisposes carriers to type
2 diabetes. In the beta cells Zn2+ ions coordinate with insulin peptides to form insulin
crystals inside granule. These granules contain extremely high zinc level (~μM)
compared to other compartments or cytosol. Here we have used FRET-based zinc
sensors of different affinities to estimate free Zn2+ in intracellular compartments in
primary beta cells and changes in response to glucose. Further, combining zinc
measurements using these sensors with a model system where zinc homeostasis is
disrupted, such as a global ZnT8 knock-out mice revealed that absence of the
granular zinc importer not only causes loss of dense cores in the granule, but also
lowers steady-state cytosolic free Zn2+ in the beta cells. Further studies will be
required to understand the impact of ZnT8 perturbation on β cell zinc homeostasis,
particularly on gene expression and oxidative stress response; nevertheless it is
apparent that compartmental free zinc level is dynamically regulated by exchanges
between stores.
Plant MTP Zinc Transporters
Dorina Podar, Judith Scherer, Zeenat Noordally, Pawel Herzyk, Dietrich Nies,
Dale Sanders and Tony Miller (John Innes Centre, Norwich)
One strategy to combat nutritional zinc deficiency is that of biofortification of cereals
through enhancing zinc accumulation in the part of the grain – the endosperm –
which is most commonly eaten. Biofortification might be accomplished through
expression of transporters that sequester the metal into an intracellular
compartment. Therefore, insights into metal selectivity of such transporters are an
essential first step in achieving this goal. Metal Tolerance Proteins (MTPs) are plant
members of the Cation Diffusion Facilitator (CDF) transporter family involved in
cellular metal homeostasis. Members of the CDF family are ubiquitously found in all
living entities and show principal selectivity for Zn2+, Mn2+ and Fe2+. Little is known
with respect to metal selectivity determinants of CDFs. We identified a novel cereal
member of CDFs in barley termed HvMTP1 that localizes to the vacuolar membrane.
Structure/function analysis has been used to identify the metal selectivity
determinants in these plant transporters.
Zinc Fluxes through Calcium Channels
Irina Korichneva, Department of Pharmacie, UJF – Grenoble 1, Grenoble,
FRANCE
Zinc is responsible for a wide range of biological functions, and the intracellular free
zinc concentration must be tightly controlled on the level of uptake, intracellular
sequestration, redistribution, storage and elimination. ZnT transporters facilitate zinc
efflux from the cytoplasm either into various intracellular compartments, or across
the plasma membrane. Zip transporters promote extracellular zinc uptake and
vesicular zinc release. Curiously, it becomes increasingly evident that Ca-specific
channels may allow zinc permeation and contribute significantly to elevation of
intracellular free zinc. As such, one of the major routes of zinc entry into most
excitable cells is L-type calcium channel. Recently, similar role has been uncovered
for the member of the transient receptor potential protein family TRPM7 channel.
Interplay of zinc and calcium signals may be an important way to regulate stress
response and calcium overload, particularly crucial in muscle cells. We performed
the analysis of different technical approaches to simultaneous measurements of the
two ions using fluorescent microscopy. Along with the importance of the
experimental setting the data will demonstrate the zinc and calcium fluxes in
cardiomyocytes and skeletal muscles.
Posters:
Imaging Insulin release using a fluorescent Zn2+ probe – ZIMIR
Elisa A Bellomo, Daliang Li, Shiuhwei Chen, Andrei I Tarasov, Wen-hong Li and Guy
A Rutter (Imperial College London)
Have laboratory Drosophila melanogaster stocks adapted to a low zinc
economy?
Negar Afshar, Bilge Argunhan & Fanis Missirlis (Queen Mary University, London)
Modular 'click' sensors for zinc and their application in vivo
Kajally Jobe, Stephen M. Goldup and Michael Watkinson (Queen Mary University,
London)
Effect of zinc on fenestrae formation in cultured endothelial cells
Sabrina Cahyadi, Bhavika J. Patel, Meihua Ju, Alexia Phedonos, Anishka
Wilwaraarachchi, Imre Lengyel (UCL Institute of Ophthalmology)
Gene expression changes in aged rat eyes following oral zinc supplementation
Neda Barzegar-Befroei, Jane Flinn, Gretchen Knaack, Christer Hogstrand, Arthur A.
Bergen and Imre Lengyel (UCL Institute of Ophthalmology)
Bruch’s membrane changes in the APP/PS1 transgenic mice model of
Alzheimer’s disease
Ashraf Gango, Sabrina Cahyadi, Zhi-Hong Chi, Zhan-You Wang Tunde Peto and
Imre Lengyel (UCL Institute of Ophthalmology)
Selective zinc labelling of short wavelength cone outer segments
Imre Lengyel and Nketchi Onwochei (UCL Institute of Ophthalmology)