Examination of the zinc transporter gene, SLC39A12

Bilirubin-Induced Transcriptomic Imprinting in Neonatal Hyperbilirubinemia

Recent findings indicated aberrant epigenetic control of the central nervous system (CNS) development in hyperbilirubinemic Gunn rats as an additional cause of cerebellar hypoplasia, the landmark of bilirubin neurotoxicity in rodents. Because the symptoms in severely hyperbilirubinemic human neonates suggest other regions as privileged targets of bilirubin neurotoxicity, we expanded the study of the potential impact of bilirubin on the control of postnatal brain development to regions correlating with human symptoms. Histology, transcriptomic, gene correlation, and behavioral studies were performed. The histology revealed widespread perturbation 9 days after birth, restoring in adulthood. At the genetic level, regional differences were noticed. Bilirubin affected synaptogenesis, repair, differentiation, energy, extracellular matrix development, etc., with transient alterations in the hippocampus (memory, learning, and cognition) and inferior colliculi (auditory functions) but permanent changes in the parietal cortex. Behavioral tests confirmed the presence of a permanent motor disability. The data correlate well both with the clinic description of neonatal bilirubin-induced neurotoxicity, as well as with the neurologic syndromes reported in adults that suffered neonatal hyperbilirubinemia. The results pave the way for better deciphering the neurotoxic features of bilirubin and evaluating deeply the efficacy of new therapeutic approaches against the acute and long-lasting sequels of bilirubin neurotoxicity.

A role for zinc transporter gene SLC39A12 in the nervous system and beyond

The SLC39A12 gene encodes the zinc transporter protein ZIP12, which is expressed across many tissues and is highly abundant in the vertebrate nervous system. As a zinc transporter, ZIP12 functions to transport zinc across cellular membranes, including cellular zinc influx across the plasma membrane. Genome-wide association and exome sequencing studies have shown that brain susceptibility-weighted magnetic resonance imaging (MRI) intensity is associated with ZIP12 polymorphisms and rare mutations. ZIP12 is required for neural tube closure and embryonic development in Xenopus tropicalis. Frog embryos depleted of ZIP12 by antisense morpholinos develop an anterior neural tube defect and lack viability. ZIP12 is also necessary for neurite outgrowth and mitochondrial function in mouse neural cells. ZIP12 mRNA is increased in brain regions of schizophrenic patients. Outside of the nervous system, hypoxia induces ZIP12 expression in multiple mammalian species, including humans, which leads to endothelial and smooth muscle thickening in the lung and contributes towards pulmonary hypertension. Other studies have associated ZIP12 with other diseases such as cancer. Given that ZIP12 is highly expressed in the brain and that susceptibility-weighted MRI is associated with brain metal content, ZIP12 may affect neurological diseases and psychiatric illnesses such as Parkinson's disease, Alzheimer's disease, and schizophrenia. Furthermore, the induction of ZIP12 and resultant zinc uptake under pathophysiological conditions may be a critical component of disease pathology, such as in pulmonary hypertension. Drug compounds that bind metals like zinc may be able to treat diseases associated with impaired zinc homeostasis and altered ZIP12 function.

The response of zinc transporter gene expression of selected tissues in a pig model of subclinical zinc deficiency

This study compared the relative mRNA expression of all mammal zinc (Zn) transporter genes in selected tissues of weaned piglets challenged with short-term subclinical Zn deficiency (SZD). The dietary model involved restrictive feeding (450 g/animal*day^-1) of a high-phytate diet (9 g/kg) supplemented with varying amounts of zinc from ZnSO₄*7H₂O ranging from deficient to sufficient supply levels (total diet Zn: 28.1, 33.6, 38.8, 42.7, 47.5, 58.2, 67.8, 88.0 mg Zn/kg). Total RNA preparations comprised jejunal and colonic mucosa as well as hepatic and nephric tissue. Statistical modelling involved broken-line regression (P≤.05). ZIP10 and ZIP12 mRNAs were not detected in any tissue and ZnT3 mRNA was only identified in the kidney. All other genes were expressed in all tissues but only a few gene expression patterns allowed a significant (P<.0001) fitting of broken-line regression models, indicating homeostatic regulation under the present experimental conditions. Interestingly, these genes could be subcategorized by showing significant turnarounds in their response patterns, either at ~40 or ~60 mg Zn/kg diet (P<.0001). In conclusion, the present study showed clear differences in Zn transporter gene expression in response to SZD compared to the present literature on clinical models. We recognized that certain Zn transporter genes were regulated under the present experimental conditions by two distinct homeostatic networks. For the best of our knowledge, this represents the first comprehensive screening of Zn transporter gene expression in a highly translational model to human physiology.

Role of zinc transporter ZIP12 in susceptibility-weighted brain magnetic resonance imaging (MRI) phenotypes and mitochondrial function

Brain zinc dysregulation is linked to many neurological disorders. However, the mechanisms regulating brain zinc homeostasis are poorly understood. We performed secondary analyses of brain MRI GWAS and exome sequencing data from adults in the UK Biobank. Coding ZIP12 polymorphisms in zinc transporter ZIP12 (SLC39A12) were associated with altered brain susceptibility weighted MRI (swMRI). Conditional and joint association analyses revealed independent GWAS signals in linkage disequilibrium with 2 missense ZIP12 polymorphisms, rs10764176 and rs72778328, with reduced zinc transport activity. ZIP12 rare coding variants predicted to be deleterious were associated with similar impacts on brain swMRI. In Neuro-2a cells, ZIP12 deficiency by short hairpin RNA (shRNA) depletion or CRISPR/Cas9 genome editing resulted in impaired mitochondrial function, increased superoxide presence, and detectable protein carbonylation. Inhibition of Complexes I and IV of the electron transport chain reduced neurite outgrowth in ZIP12 deficient cells. Transcriptional coactivator PGC-1α, mitochondrial superoxide dismutase (SOD2), and chemical antioxidants α-tocopherol, MitoTEMPO, and MitoQ restored neurite extension impaired by ZIP12 deficiency. Mutant forms of α-synuclein and tau linked to familial Parkinson's disease and frontotemporal dementia, respectively, reduced neurite outgrowth in cells deficient in ZIP12. Zinc and ZIP12 may confer resilience against neurological diseases or premature aging of the brain.

Brain Cell Type Specific Gene Expression and Co-expression Network Architectures

Elucidating brain cell type specific gene expression patterns is critical towards a better understanding of how cell-cell communications may influence brain functions and dysfunctions. We set out to compare and contrast five human and murine cell type-specific transcriptome-wide RNA expression data sets that were generated within the past several years. We defined three measures of brain cell type-relative expression including specificity, enrichment, and absolute expression and identified corresponding consensus brain cell “signatures,” which were well conserved across data sets. We validated that the relative expression of top cell type markers are associated with proxies for cell type proportions in bulk RNA expression data from postmortem human brain samples. We further validated novel marker genes using an orthogonal ATAC-seq dataset. We performed multiscale coexpression network analysis of the single cell data sets and identified robust cell-specific gene modules. To facilitate the use of the cell type-specific genes for cell type proportion estimation and deconvolution from bulk brain gene expression data, we developed an R package, BRETIGEA. In summary, we identified a set of novel brain cell consensus signatures and robust networks from the integration of multiple datasets and therefore transcend limitations related to technical issues characteristic of each individual study.

Zinc as a Gatekeeper of Immune Function

After the discovery of zinc deficiency in the 1960s, it soon became clear that zinc is essential for the function of the immune system. Zinc ions are involved in regulating intracellular signaling pathways in innate and adaptive immune cells. Zinc homeostasis is largely controlled via the expression and action of zinc "importers" (ZIP 1-14), zinc "exporters" (ZnT 1-10), and zinc-binding proteins. Anti-inflammatory and anti-oxidant properties of zinc have long been documented, however, underlying mechanisms are still not entirely clear. Here, we report molecular mechanisms underlying the development of a pro-inflammatory phenotype during zinc deficiency. Furthermore, we describe links between altered zinc homeostasis and disease development. Consequently, the benefits of zinc supplementation for a malfunctioning immune system become clear. This article will focus on underlying mechanisms responsible for the regulation of cellular signaling by alterations in zinc homeostasis. Effects of fast zinc flux, intermediate "zinc waves", and late homeostatic zinc signals will be discriminated. Description of zinc homeostasis-related effects on the activation of key signaling molecules, as well as on epigenetic modifications, are included to emphasize the role of zinc as a gatekeeper of immune function.

The Emerging Role for Zinc in Depression and Psychosis

Zinc participation is essential for all physiological systems, including neural functioning, where it participates in a myriad of cellular processes. Converging clinical, molecular, and genetic discoveries illuminate key roles for zinc homeostasis in association with clinical depression and psychosis which are not yet well appreciated at the clinical interface. Intracellular deficiency may arise from low circulating zinc levels due to dietary insufficiency, or impaired absorption from aging or medical conditions, including alcoholism. A host of medications commonly administered to psychiatric patients, including anticonvulsants, oral medications for diabetes, hormones, antacids, anti-inflammatories and others also impact zinc absorption. Furthermore, inefficient genetic variants in zinc transporter molecules that transport the ion across cellular membranes impede its action even when circulating zinc concentrations is in the normal range. Well powered clinical studies have shown beneficial effects of supplemental zinc in depression and it important to pursue research using zinc as a potential therapeutic option for psychosis as well. Meta-analyses support the adjunctive use of zinc in major depression and a single study now supports zinc for psychotic symptoms. This manuscript reviews the biochemistry and bench top evidence on putative molecular mechanisms of zinc as a psychiatric treatment.

Identification of candidate mimicry proteins involved in parasite-driven phenotypic changes

BACKGROUND

Endoparasites with complex life cycles are faced with several biological challenges, as they need to occupy various ecological niches throughout their development. Host phenotypes that increase the parasite's transmission rate to the next host have been extensively described, but few mechanistic explanations have been proposed to describe their proximate causes. In this study we explore the possibility that host phenotypic changes are triggered by the production of mimicry proteins from the parasite by using an ecological model system consisting of the infection of the threespine stickleback (Gasterosteus aculeatus) by the cestode Schistocephalus solidus.

METHOD

Using RNA-seq data, we assembled 9,093 protein-coding genes from which ORFs were predicted to generate a reference proteome. Based on a previously published method, we built two complementary analysis pipelines to i) establish a general classification of protein similarity among various species (pipeline A) and ii) identify candidate mimicry proteins showing specific host-parasite similarities (pipeline B), a key feature underlying the possibility of molecular mimicry.

RESULTS

Ninety-four tapeworm proteins showed high local sequence homology with stickleback proteins. Four of these candidates correspond to secreted or membrane proteins that could be produced by the parasite and eventually be released in or be in contact with the host to modulate physiological pathways involved in various phenotypes (e.g. behaviors). One of these candidates belongs to the Wnt family, a large group of signaling molecules involved in cell-to-cell interactions and various developmental pathways. The three other candidates are involved in ion transport and post-translational protein modifications. We further confirmed that these four candidates are expressed in three different developmental stages of the cestode by RT-PCR, including the stages found in the host.

CONCLUSION

In this study, we identified mimicry candidate peptides from a behavior-altering cestode showing specific sequence similarity with host proteins. Despite their potential role in modulating host pathways that could lead to parasite-induced phenotypic changes and despite our confirmation that they are expressed in the developmental stage corresponding to the altered host behavior, further investigations will be needed to confirm their mechanistic role in the molecular cross-talk taking place between S. solidus and the threespine stickleback.

Zinc and its role in immunity and inflammation

Zinc (Zn) nutritional importance has been known for a long time, but in the last decades its importance in immune modulation has arisen. This review aims at describing the mechanisms involved in the regulation of Zn homeostasis and their effects on the immune response focusing on those which are implicated in the physiopathology of rheumatoid arthritis. Zn functions as a modulator of the immune response through its availability, which is tightly regulated by several transporters and regulators. When this mechanism is disturbed, Zn availability is reduced, altering survival, proliferation and differentiation of the cells of different organs and systems and, in particular, cells of the immune system. Zn deficiency affects cells involved in both innate and adaptive immunity at the survival, proliferation and maturation levels. These cells include monocytes, polymorphonuclear-, natural killer-, T-, and B-cells. T cell functions and the balance between the different T helper cell subsets are particularly susceptible to changes in Zn status. While acute Zn deficiency causes a decrease in innate and adaptive immunity, chronic deficiency increases inflammation. During chronic deficiency, the production of pro-inflammatory cytokines increases, influencing the outcome of a large number of inflammatory diseases, including rheumatoid arthritis.

The SLC39 family of zinc transporters

Zinc is a trace element nutrient that is essential for life. This mineral serves as a cofactor for enzymes that are involved in critical biochemical processes and it plays many structural roles as well. At the cellular level, zinc is tightly regulated and disruption of zinc homeostasis results in serious physiological or pathological issues. Despite the high demand for zinc in cells, free or labile zinc must be kept at very low levels. In humans, two major zinc transporter families, the SLC30 (ZnT) family and SLC39 (ZIP) family control cellular zinc homeostasis. This review will focus on the SLC39 transporters. SLC39 transporters primarily serve to pass zinc into the cytoplasm, and play critical roles in maintaining cellular zinc homeostasis. These proteins are also significant at the organismal level, and studies are revealing their link to human diseases. Therefore, we will discuss the function, structure, physiology, and pathology of SLC39 transporters.

Zinc and human disease

The vast knowledge of the physiologic functions of zinc in at least 3000 proteins and the recent recognition of fundamental regulatory functions of zinc(II) ions released from cells or within cells links this nutritionally essential metal ion to numerous diseases. However, this knowledge so far has had remarkably limited impact on diagnosing, preventing, and treating human diseases. One major roadblock is a lack of suitable biomarkers that would detect changes in cellular zinc metabolism and relate them to specific disease outcomes. It is not only the right amount of zinc in the diet that maintains health. At least as important is the proper functioning of the dozens of proteins that control cellular zinc homeostasis, regulate intracellular traffic of zinc between the cytosol and vesicles/organelles, and determine the fluctuations of signaling zinc(II) ions. Cellular zinc deficiencies or overloads, a term referring to zinc concentrations exceeding the cellular zinc buffering capacity, compromise the redox balance. Zinc supplementation may not readily remedy zinc deficiency if other factors limit the capability of a cell to control zinc. The role of zinc in human diseases requires a general understanding of the wide spectrum of functions of zinc, how zinc is controlled, how it interacts with the metabolism of other metal ions, in particular copper and iron, and how perturbation of specific zinc-dependent molecular processes causes disease and influences the progression of disease.

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.

Expression Profiling of Solute Carrier Gene Families at the Blood-CSF Barrier

The choroid plexus (CP) is a highly vascularized tissue in the brain ventricles and acts as the blood-cerebrospinal fluid (CSF) barrier (BCSFB). A main function of the CP is to secrete CSF, which is accomplished by active transport of small ions and water from the blood side to the CSF side. The CP also supplies the brain with certain nutrients, hormones, and metal ions, while removing metabolites and xenobiotics from the CSF. Numerous membrane transporters are expressed in the CP in order to facilitate the solute exchange between the blood and the CSF. The solute carrier (SLC) superfamily represents a major class of transporters in the CP that constitutes the molecular mechanisms for CP function. Recently, we systematically and quantitatively examined Slc gene expression in 20 anatomically comprehensive brain areas in the adult mouse brain using high-quality in situ hybridization data generated by the Allen Brain Atlas. Here we focus our analysis on Slc gene expression at the BCSFB using previously obtained data. Of the 252 Slc genes present in the mouse brain, 202 Slc genes were found at detectable levels in the CP. Unsupervised hierarchical cluster analysis showed that the CP Slc gene expression pattern is substantially different from the other 19 analyzed brain regions. The majority of the Slc genes in the CP are expressed at low to moderate levels, whereas 28 Slc genes are present in the CP at the highest levels. These highly expressed Slc genes encode transporters involved in CSF secretion, energy production, and transport of nutrients, hormones, neurotransmitters, sulfate, and metal ions. In this review, the functional characteristics and potential importance of these Slc transporters in the CP are discussed, with particular emphasis on their localization and physiological functions at the BCSFB.

Mammalian zinc transporters: nutritional and physiologic regulation

Research advances defining how zinc is transported into and out of cells and organelles have increased exponentially within the past five years. Research has progressed through application of molecular techniques including genomic analysis, cell transfection, RNA interference, kinetic analysis of ion transport, and application of cell and animal models including knockout mice. The knowledge base has increased for most of 10 members of the ZnT family and 14 members of the Zrt-, Irt-like protein (ZIP) family. Relative to the handling of dietary zinc is the involvement of ZnT1, ZIP4, and ZIP5 in intestinal zinc transport, involvement of ZIP10 and ZnT1 in renal zinc reabsorption, and the roles of ZIP5, ZnT2, and ZnT1 in pancreatic release of endogenous zinc. These events are major factors in regulation of zinc homeostasis. Other salient findings are the involvement of ZnT2 in lactation, ZIP14 in the hypozincemia of inflammation, ZIP6, ZIP7, and ZIP10 in metastatic breast cancer, and ZnT8 in insulin processing and as an autoantigen in diabetes.

Zinc transporters and cancer: a potential role for ZIP7 as a hub for tyrosine kinase activation

Zinc, which is essential for many cellular processes, is controlled by zinc transporters and through buffering by metallothioneins and glutathione. Although zinc is increasingly implicated in disease states, little is known about how zinc regulates cellular biochemical pathways. Recent seminal articles have revealed discrete zinc-trafficking pathways that are linked to signalling cascades, particularly those involving protein phosphatase inhibition and downstream activation of mitogen-activated protein kinases and tyrosine kinases. Here, we discuss the mechanisms of cellular zinc homeostasis, and we propose an important role for the zinc transporter solute carrier family 39, member 7 (SLC39A7; commonly referred to as ZIP7). ZIP7 releases zinc from the endoplasmic reticulum and might be required for tyrosine kinase activation. These observations position ZIP7 at a critical node in zinc-mediated tyrosine kinase signalling and suggest that this protein might form a novel target for diseases such as cancer where prevention of tyrosine kinase activation would be therapeutically advantageous.

The emerging role of the LIV-1 subfamily of zinc transporters in breast cancer

Zinc transporter LIV-1 (SLC39A6) is estrogen regulated and present in increased amounts in estrogen receptor-positive breast cancer as well as in tumors that spread to the lymph nodes. The LIV-1 subfamily of ZIP zinc transporters consists of nine human sequences that share considerable homology across transmembrane domains. Many of these sequences have been shown to transport zinc and/or other ions across cell membranes. Increasingly, studies have implicated members of the LIV-1 transporter subfamily in a variety of diseases. We review these studies and report our own investigations of the role in breast cancer of the nine LIV-1 zinc transporters. We have documented the response of these transporters to estrogen and antiestrogens, and also their presence in our models of resistance to antiestrogens. Resistance to antiestrogen drugs such as tamoxifen and fulvestrant often occurs in advanced breast cancer. In these models we observed differential expression of individual LIV-1 family members, which may be related to their observed variable tissue expression. We were unable detect ZIP4, which is known to be expressed in the intestine. HKE4/SLC39A7 had elevated expression in both antiestrogen-resistant cell lines, and ZIP8 had elevated expression in fulvestrant-resistant cells. In addition, we investigated the expression of the nine LIV-1 family members in a clinical breast cancer series. Although a number of different LIV-1 family members showed some association with growth factor receptors, LIV-1 was solely associated with estrogen receptor and a variety of growth factors commonly associated with clinical breast cancer. HKE4, however, did show an association with the marker of cell proliferation Ki67 the spread of breast cancer to lymph nodes.