A fly's eye view of zinc homeostasis: Novel insights into the genetic control of zinc metabolism from Drosophila

Modelling host-pathogen interactions: Galleria mellonella as a platform to study Pseudomonas aeruginosa response to host-imposed zinc starvation

Nutritional immunity, a key component of the vertebrate innate immune response, involves the modulation of zinc availability to limit the growth of pathogens. Pseudomonas aeruginosa counteracts host-imposed zinc starvation through metabolic adaptations, including reprogramming of gene expression and activating efficient metal uptake systems. To unravel how zinc shortage contributes to the complexity of bacterial adaptation to the host environment, it is critical to use model systems that mimic fundamental features of P. aeruginosa-related diseases in humans. Among available animal models, Galleria mellonella has recently emerged as a promising alternative to mammalian hosts. This study aims to evaluate whether G. mellonella can recapitulate the zinc-related nutritional immunity responses observed in mammalian infections. Our results show that, upon P. aeruginosa infection, the larvae upregulate several zinc transporters, suggesting an active redistribution of the metal in response to the pathogen. Additionally, P. aeruginosa colonizing the larvae induces Zn uptake regulator-controlled genes, consistent with bacterial adaptation to zinc starvation. Disruption of bacterial zinc uptake capability significantly reduces P. aeruginosa virulence, underscoring the importance of zinc acquisition in pathogenesis also within this model host. As a proof of concept, we also demonstrate that this in vivo model can serve as a viable preliminary screening tool to unveil novel players involved in P. aeruginosa response to zinc starvation, offering valuable insights into the host-pathogen battle for micronutrients.

Paternal Zinc Deficiency and Its Transgenerational Effects on Zinc Transporters in Drosophila

The nutritional status of fathers plays a significant role in influencing the growth, metabolism, and susceptibility to diseases in their offspring. Paternal zinc deficiency can lead to developmental programming effects on the offspring's zinc homeostasis. This study investigated the effects of paternal zinc deficiency on the zinc homeostasis of offspring in a Drosophila melanogaster (fruit fly) model. Male flies were reared on a diet supplemented with a zinc-chelator, N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), from the egg stage until adulthood, and their offspring were subsequently reared on a normal diet for 7 d. Body zinc status and zinc transporters were assessed afterwards. The results indicated that the prenatal zinc deficiency significantly lowered total body zinc levels (p<0.05) compared to the controls. Additionally, the mRNA levels of zinc transporters, dZip42C.1, dZnT63C, and dZnT35C, were lower in the zinc-deficient male parents (p<0.05) and their male offspring (p<0.05). These findings suggested that paternal zinc deficiency can alter offspring zinc homeostasis, even when the offspring was fed a zinc-sufficient diet. This is an important finding, as zinc is an essential nutrient that is required for a variety of bodily functions. Further research is needed to better understand the mechanisms by which zinc deficiency in the male parent affects the health of the offspring and to develop strategies to prevent this from happening.

Drosophila melanogaster: a simple genetic model of kidney structure, function and disease

Although the genetic basis of many kidney diseases is being rapidly elucidated, their experimental study remains problematic owing to the lack of suitable models. The fruitfly Drosophila melanogaster provides a rapid, ethical and cost-effective model system of the kidney. The unique advantages of D. melanogaster include ease and low cost of maintenance, comprehensive availability of genetic mutants and powerful transgenic technologies, and less onerous regulation, as compared with mammalian systems. Renal and excretory functions in D. melanogaster reside in three main tissues - the transporting renal (Malpighian) tubules, the reabsorptive hindgut and the endocytic nephrocytes. Tubules contain multiple cell types and regions and generate a primary urine by transcellular transport rather than filtration, which is then subjected to selective reabsorption in the hindgut. By contrast, the nephrocytes are specialized for uptake of macromolecules and equipped with a filtering slit diaphragm resembling that of podocytes. Many genes with key roles in the human kidney have D. melanogaster orthologues that are enriched and functionally relevant in fly renal tissues. This similarity has allowed investigations of epithelial transport, kidney stone formation and podocyte and proximal tubule function. Furthermore, a range of unique quantitative phenotypes are available to measure function in both wild type and disease-modelling flies.

Molecular physiology of zinc in Drosophila melanogaster

New research in Drosophila melanogaster has revealed the molecular mechanisms of zinc involvement in many biological processes. A newly discovered Metallothionein is predicted to have a higher zinc specificity than the other isoforms. Zinc negatively regulates tyrosine hydroxylase activity by antagonizing iron binding, thus rendering the enzyme ineffective or non-functional. The identification of a new chaperone of the protein disulfide isomerase family provided mechanistic insight into the protein trafficking defects caused by zinc dyshomeostasis in the secretory pathway. Insect models of tumor pathogenesis indicate that zinc regulates the structural stabilization of cells by transcriptionally regulating matrix metalloproteinases while zinc dyshomeostasis in the secretory pathway modulates cell signaling through endoplastic recticulum stress.

Zinc antagonizes iron-regulation of tyrosine hydroxylase activity and dopamine production in Drosophila melanogaster

Background

Dopamine (DA) is a neurotransmitter that plays roles in movement, cognition, attention, and reward responses, and deficient DA signaling is associated with the progression of a number of neurological diseases, such as Parkinson’s disease. Due to its critical functions, DA expression levels in the brain are tightly controlled, with one important and rate-limiting step in its biosynthetic pathway being catalyzed by tyrosine hydroxylase (TH), an enzyme that uses iron ion (Fe²⁺) as a cofactor. A role for metal ions has additionally been associated with the etiology of Parkinson’s disease. However, the way dopamine synthesis is regulated in vivo or whether regulation of metal ion levels is a component of DA synthesis is not fully understood. Here, we analyze the role of Catsup, the Drosophila ortholog of the mammalian zinc transporter SLC39A7 (ZIP7), in regulating dopamine levels.

Results

We found that Catsup is a functional zinc transporter that regulates intracellular zinc distribution between the ER/Golgi and the cytosol. Loss-of-function of Catsup leads to increased DA levels, and we showed that the increased dopamine production is due to a reduction in zinc levels in the cytosol. Zinc ion (Zn²⁺) negatively regulates dopamine synthesis through direct inhibition of TH activity, by antagonizing Fe²⁺ binding to TH, thus rendering the enzyme ineffective or non-functional.

Conclusions

Our findings uncovered a previously unknown mechanism underlying the control of cellular dopamine expression, with normal levels of dopamine synthesis being maintained through a balance between Fe²⁺ and Zn²⁺ ions. The findings also provide support for metal modulation as a possible therapeutic strategy in the treatment of Parkinson’s disease and other dopamine-related diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01168-0.

The Effects of Essential and Non-Essential Metal Toxicity in the Drosophila melanogaster Insect Model: A Review

The biological effects of environmental metal contamination are important issues in an industrialized, resource-dependent world. Different metals have different roles in biology and can be classified as essential if they are required by a living organism (e.g., as cofactors), or as non-essential metals if they are not. While essential metal ions have been well studied in many eukaryotic species, less is known about the effects of non-essential metals, even though essential and non-essential metals are often chemically similar and can bind to the same biological ligands. Insects are often exposed to a variety of contaminated environments and associated essential and non-essential metal toxicity, but many questions regarding their response to toxicity remain unanswered. Drosophila melanogaster is an excellent insect model species in which to study the effects of toxic metal due to the extensive experimental and genetic resources available for this species. Here, we review the current understanding of the impact of a suite of essential and non-essential metals (Cu, Fe, Zn, Hg, Pb, Cd, and Ni) on the D. melanogaster metal response system, highlighting the knowledge gaps between essential and non-essential metals in D. melanogaster. This review emphasizes the need to use multiple metals, multiple genetic backgrounds, and both sexes in future studies to help guide future research towards better understanding the effects of metal contamination in general.

Regulation and biological function of metal ions in Drosophila

A conceptual framework is offered for critically approaching the formidable ability of insects to segregate metal ions to their multiple destinations in proteins and subcellular compartments. New research in Drosophila melanogaster suggests that nuclear iron regulatory proteins and oxidative stress transcription factors mediate metal-responsive gene expression. Identification of a zinc-regulated chaperone in the endoplasmic reticulum potentially explains membrane protein trafficking defects observed in zinc transporter mutants. Compartmentalized zinc is utilized in fertilization, embryogenesis and for the activation of zinc-finger transcription factors - the latter function demonstrated during muscle development, while dietary zinc is sensed through gating of a chloride channel. Another emerging theme in cellular metal homeostasis is that transporters and related proteins meet at endoplasmic reticulum-mitochondria associated membranes with physiologically relevant consequences during aging.

Fear-of-intimacy-mediated zinc transport controls fat body cell dissociation through modulating Mmp activity in Drosophila

Matrix metalloproteinases (Mmps) are pivotal extracellular proteinases that have been implicated in tumour invasion and metastasis. Drosophila fat body is important for energy storage and utilization, as well as biosynthetic and metabolic activities. The fat body undergoes remodelling during metamorphosis which is characterized by the dissociation of the fat body into individual cells. Mmps play important roles in the regulation of fat body cell dissociation. Here we show that a zinc transporter fear-of-intimacy (foi) is necessary for the cell dissociation of fat body in Drosophila. The progression of fat body cell dissociation was delayed by fat body-specific foi knockdown while it was accelerated by foi overexpression (OE). In essence, these phenotypes are closely associated with intracellular zinc homeostasis, which can be modulated by dietary zinc intervention or genetic modulation of other zinc transporters. Further study indicated that Mmp1 and Mmp2 levels could be transcriptionally regulated by zinc in vivo. Consistently, the retarded fat body cell dissociation caused by Mmp1 or Mmp2 RNAi could be regulated by modulating the expression of foi. Further, by using Drosophila models of malignant tumour Raf^GOFscrib^−/− and Ras^V12lgl^−/−, we showed that the tumour growth, invasion and migration could be markedly inhibited by foi knockdown. These findings demonstrate a close connection between zinc levels and cell dissociation in vivo, and also suggest that manipulation of zinc levels may provide a novel therapeutic strategy for cancer.

Determination of Metal Content in Drosophila melanogaster During Metal Exposure

Trace metal elements, such as zinc, iron, copper, and manganese, play catalytic or structural roles in many enzymes and numerous proteins, and accordingly, contribute to a variety of fundamental biological processes. During the past decade, the fruit fly (Drosophila melanogaster) has become an important model organism for elucidating metal homeostasis in metazoan. We have been using Drosophila as a model to study metal metabolism for many years and have optimized simple and robust assays for determining the metal content in Drosophila, such as inductively coupled plasma mass spectrometry (ICP-MS), the activity assay of enzymes dependent on metals, and staining metal ions in tissues of Drosophila. In this chapter, we present the step-by-step detailed methods for detecting the metal content in Drosophila melanogaster during metal toxicity study.

Accumulation of different metals in oyster Crassostrea gigas: Significance and specificity of SLC39A (ZIP) and SLC30A (ZnT) gene families and polymorphism variation

The Zrt/Irt-like proteins (ZIP, SLC39A) and zinc transporters (ZnT, SLC30A) are the two major gene families responsible for the import/export of Zn and other metals. In this study, the mRNA expression levels and genetic variations of eight ZnTs and 14 ZIPs were identified in Crassostrea gigas after exposure to Zn, Cd, Cu, Hg, and Pb. Metal exposure induced reactive oxygen species (ROS) and malondialdehyde (MDA) accumulation and antioxidant enzyme expression. The expanded gene numbers of superoxide dismutase (SOD) in the oysters exhibited diverse expression under exposure to the five metals, and the contrasting expressions of both ZnTs and ZIPs under different metal exposures were observed, revealing their ion-specific responses. Zn and Cu have similar transporters and induce high expression levels of ZnT1, 2, 7, and 9 and ZIP1, 3, 6, 9, 10, 11, and 14. Pb induced high expression levels of ZIP7, and 13 and ZnT5, 6, and 7, which are mainly expressed in the endoplasmic reticulum (ER). Cd induced high expression levels of ZnT1, 2, and 7 and ZIP1, 6, 9, 10, 11, and 13. Hg exposure was found to have little effect on the ZIP and ZnT expression levels. Based on 3784 single nucleotide polymorphisms (SNPs) within the ZnTs and ZIPs, genetic association analysis for Zn accumulation was conducted on 427 oyster samples. The 38 SNPs, which were located within 12 genes, were identified to be associated with Zn content (p < 0.01), explaining the phenotypic variation from 1.61% to 3.37%. One nonsynonymous mutation and related haplotypes were identified within ZIP1, explaining 1.69% of the variation in Zn. Its high expression under Zn exposure revealed its important role in Zn transportation. To the best of our knowledge, this study is the first comprehensive investigation of the transportation mechanisms of ZIPs and ZnTs under different metal exposures and the genetic effect of Zn accumulation in oysters, and provides valuable biomarkers and genetic resources to evaluate environmental metal pollution.

ZnT7 RNAi favors RafGOFscrib-/--induced tumor growth and invasion in Drosophila through JNK signaling pathway

The disruption of zinc homeostasis has been identified in patients suffering from various cancers, but a causative relationship has not yet been established. Drosophila melanogaster has become a powerful model to study cancer biology. Here using a Drosophila model of malignant tumor Raf^GOFscrib^-/-, we observed that the tumor growth, invasion and migration were enhanced by silencing dZnT7, a zinc transporter localized on the Golgi apparatus. Further study indicated that the zinc deficiency in Golgi of dZnT7 RNAi resulted in ER stress which could activate the c-Jun-N-terminal Kinase (JNK) signaling and this process is mediated by Atg9. Lastly, we demonstrated that the exacerbation of dZnT7 RNAi on tumor was promoted by JNK signaling-dependent cell autonomous and non-autonomous autophagy. These findings suggest that zinc homeostasis in secretory compartments may provide a new therapeutic target for tumor treatment.

Zinc Deficiency Leads to Lipid Changes in Drosophila Brain Similar to Cognitive-Impairing Drugs: An Imaging Mass Spectrometry Study

Several diseases and disorders have been suggested to be associated with zinc deficiency, especially learning and memory impairment. To have better understanding about the connection between lipid changes and cognitive impairments, we investigated the effects of a zinc-chelated diet on certain brain lipids of Drosophila melanogaster by using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The data revealed that there are increases in the levels of phosphatidylcholine and phosphatidylinositol in the central brains of the zinc-deficient flies compared to the control flies. In contrast, the abundance of phosphatidylethanolamine in the brains of the zinc-deficient flies is lower. These data are consistent with that of cognitive-diminishing drugs, thus providing insight into the biological and molecular effects of zinc deficiency on the major brain lipids and opening a new treatment target for cognitive deficit in zinc deficiency.

Intestinal response to dietary manganese depletion inDrosophila

Metabolic adaptations to manganese deficiency.

Soil Available Water Holding Capacity Can Alter the Reproductive Performance of Mountain Pine Beetle (Coleoptera: Curculionidae) in Jack Pine (Pinales: Pinaceae) Through Phloem Nitrogen Concentration

Mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae, Scolytidae) has recently invaded novel jack pine (Pinus banksiana Lamb., Pinales: Pinaceae) forests in western Canada. Jack pine seems to be a suitable host for mountain pine beetle, but how growing conditions influence jack pine's quality as a host, and hence, its susceptibility for mountain pine beetle, is unknown. Specifically, how soil nutrient concentrations and available water holding capacity (AWHC) affect jack pine quality should be investigated. Host plant quality is an important determinant of mountain pine beetle host colonization and reproduction and is usually assessed by primary (nutrients) and secondary (defense chemistry) constituents of host phloem. We evaluated mountain pine beetle host acceptance and brood production by recording the percentage of female mountain pine beetle that entered the phloem and oviposited in 30 jack pine bolts from two sites that differed in soil nutrient concentrations and AWHC. We also compared the concentrations of phloem nutrients and defense monoterpenes among the selected trees and found that trees at the Low AWHC site had higher amounts of nitrogen, phosphorus, and potassium. Monoterpene concentrations did not differ among trees at the two sites. Host acceptance by and brood production of mountain pine beetle were greater in bolts from the Low AWHC site. We conclude that AWHC of the soil may influence mountain pine beetle host acceptance and brood production through altering host plant quality, particularly nitrogen in the phloem, and will potentially influence any further range expansion of the beetle in eastern North America.

Cloning, function, and localization of human, canine, and Drosophila ZIP10 (SLC39A10), a Zn2+ transporter

Zinc (Zn²⁺) is the second most abundant trace element, but is considered a micronutrient, as it is a cofactor for many enzymes and transcription factors. Whereas Zn²⁺ deficiency can cause cognitive immune or metabolic dysfunction and infertility, excess Zn²⁺ is nephrotoxic. As for other ions and solutes, Zn²⁺ is moved into and out of cells by specific membrane transporters: ZnT, Zip, and NRAMP/DMT proteins. ZIP10 is reported to be localized at the apical membrane of renal proximal tubules in rats, where it is believed to play a role in Zn²⁺ import. Renal regulation of Zn²⁺ is of particular interest in light of growing evidence that Zn²⁺ may play a role in kidney stone formation. The objective of this study was to show that ZIP10 homologs transport Zn²⁺, as well as ZIP10, kidney localization across species. We cloned ZIP10 from dog, human, and Drosophila ( CG10006), tested clones for Zn²⁺ uptake in Xenopus oocytes and localized the protein in renal structures. CG10006, rather than foi (fear-of-intimacy, CG6817) is the primary ZIP10 homolog found in Drosophila Malpighian tubules. The ZIP10 antibody recognizes recombinant dog, human, and Drosophila ZIP10 proteins. Immunohistochemistry reveals that ZIP10 in higher mammals is found not only in the proximal tubule, but also in the collecting duct system. These ZIP10 proteins show Zn²⁺ transport. Together, these studies reveal ZIP10 kidney localization, a role in renal Zn²⁺ transport, and indicates that CG10006 is a Drosophila homolog of ZIP10.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

Identification of Candidate Iron Transporters From the ZIP/ZnT Gene Families in the Mosquito Aedes aegypti

Mosquito-transmitted viral pathogens, such as dengue and Zika, afflict tens of thousands of people every year. These viruses are transmitted during the blood-feeding process that is required for mosquito reproduction, the most important vector being Aedes aegypti. While vertebrate blood is rich in protein, its high iron content is potentially toxic to mosquitoes. Although iron transport and sequestration are essential in the reproduction of vector mosquitoes, we discovered that culicine mosquitoes lack homologs of the common iron transporter NRAMP. Using a novel cell-based screen, we identified two ZIP and one ZnT genes as candidate iron transporters in the mosquito A. aegypti, the vector of dengue, Zika, and chikungunya. We determined the organ-specific expression pattern of these genes at critical time points in early reproduction. The result indicates modulation of these genes upon blood feeding, especially a ZIP13 homolog that is highly up-regulated after blood feeding, suggesting its importance in iron mobilization during blood digestion and reproduction. Gene silencing resulted in differential iron accumulation in the midgut and ovaries. This study sets a foundation for further investigation of iron transport and control strategies of this viral vector.

Biogenesis of zinc storage granules in Drosophila melanogaster

Membrane transporters and sequestration mechanisms concentrate metal ions differentially into discrete subcellular microenvironments for use in protein cofactors, signalling, storage or excretion. Here we identify zinc storage granules as the insect's major zinc reservoir in principal Malpighian tubule epithelial cells of Drosophila melanogaster The concerted action of Adaptor Protein-3, Rab32, HOPS and BLOC complexes as well as of the white-scarlet (ABCG2-like) and ZnT35C (ZnT2/ZnT3/ZnT8-like) transporters is required for zinc storage granule biogenesis. Due to lysosome-related organelle defects caused by mutations in the homologous human genes, patients with Hermansky-Pudlak syndrome may lack zinc granules in beta pancreatic cells, intestinal paneth cells and presynaptic vesicles of hippocampal mossy fibers.

Zinc Detoxification: A Functional Genomics and Transcriptomics Analysis in Drosophila melanogaster Cultured Cells

Cells require some metals, such as zinc and manganese, but excess levels of these metals can be toxic. As a result, cells have evolved complex mechanisms for maintaining metal homeostasis and surviving metal intoxication. Here, we present the results of a large-scale functional genomic screen in Drosophila cultured cells for modifiers of zinc chloride toxicity, together with transcriptomics data for wild-type or genetically zinc-sensitized cells challenged with mild zinc chloride supplementation. Altogether, we identified 47 genes for which knockdown conferred sensitivity or resistance to toxic zinc or manganese chloride treatment, and >1800 putative zinc-responsive genes. Analysis of the 'omics data points to the relevance of ion transporters, glutathione (GSH)-related factors, and conserved disease-associated genes in zinc detoxification. Specific genes identified in the zinc screen include orthologs of human disease-associated genes CTNS, PTPRN (also known as IA-2), and ATP13A2 (also known as PARK9). We show that knockdown of red dog mine (rdog; CG11897), a candidate zinc detoxification gene encoding an ABCC-type transporter family protein related to yeast cadmium factor (YCF1), confers sensitivity to zinc intoxication in cultured cells, and that rdog is transcriptionally upregulated in response to zinc stress. As there are many links between the biology of zinc and other metals and human health, the 'omics data sets presented here provide a resource that will allow researchers to explore metal biology in the context of diverse health-relevant processes.

ZIP13: A Study of Drosophila Offers an Alternative Explanation for the Corresponding Human Disease

The fruit fly Drosophila melanogaster has become an important model organism to investigate metal homeostasis and human diseases. Previously we identified dZIP13 (CG7816), a member of the ZIP transporter family (SLC39A) and presumably a zinc importer, is in fact physiologically primarily responsible to move iron from the cytosol into the secretory compartments in the fly. This review will discuss the implication of this finding for the etiology of Spondylocheirodysplasia-Ehlers-Danlos Syndrome (SCD–EDS), a human disease defective in ZIP13. We propose an entirely different model in that lack of iron in the secretory compartment may underlie SCD-EDS. Altogether three different working models are discussed, supported by relevant findings made in different studies, with uncertainties, and questions remained to be solved. We speculate that the distinct ZIP13 sequence features, different from those of all other ZIP family members, may confer it special transport properties.

The essential roles of metal ions in insect homeostasis and physiology

Metal ions play distinct roles in living organisms, including insects. Some, like sodium and potassium, are central players in osmoregulation and 'blood and guts' transport physiology, and have been implicated in cold adaptation. Calcium is a key player as a second messenger, and as a structural element. Other metals, particularly those with multiple redox states, can be cofactors in many metalloenzymes, but can contribute to toxic oxidative stress on the organism in excess. This short review selects some examples where classical knowledge has been supplemented with recent advances, in order to emphasize the importance of metals as essential nutrients for insect survival.

Natural and laboratory mutations in kuzbanian are associated with zinc stress phenotypes in Drosophila melanogaster

Organisms must cope with altered environmental conditions such as high concentrations of heavy metals. Stress response to heavy metals is mediated by the metal-responsive transcription factor 1 (MTF-1), which is conserved from Drosophila to humans. MTF-1 binds to metal response elements (MREs) and changes the expression of target genes. kuzbanian (kuz), a metalloendopeptidase that activates the evolutionary conserved Notch signaling pathway, has been identified as an MTF-1 target gene. We have previously identified a putatively adaptive transposable element in the Drosophila melanogaster genome, named FBti0019170, inserted in a kuz intron. In this work, we investigated whether a laboratory mutant stock overexpressing kuz is associated with zinc stress phenotypes. We found that both embryos and adult flies overexpressing kuz are more tolerant to zinc compared with wild-type flies. On the other hand, we found that the effect of FBti0019170 on zinc stress tolerance depends on developmental stage and genetic background. Moreover, in the majority of the genetic backgrounds analyzed, FBti0019170 has a deleterious effect in unpolluted environments in pre-adult stages. These results highlight the complexity of natural mutations and suggest that besides laboratory mutations, natural mutations should be studied in order to accurately characterize gene function and evolution.

Functional studies of Drosophila zinc transporters reveal the mechanism for zinc excretion in Malpighian tubules

Background

Zinc is an essential metal involved in many physiological processes. Previous work has identified a set of zinc transporters involved in Drosophila dietary zinc absorption. However, zinc excretion and reabsorption, the other two important processes to maintain zinc homeostasis, are not as well understood. In this work, we screened all the potential zinc transporter Zip (SLC39) and ZnT (SLC30) members for their likely roles in zinc excretion in Malpighian tubules, an insect organ functionally analogous to mammalian kidneys.

Results

Zip71B (CG10006, most homologous to hZIP5), in addition to the previously characterized ZnT35C (CG3994), was identified as being critical in zinc excretion. Tubule-specific knockdown of Zip71B/dZip5 reduces zinc accumulation in the tubules, but increases zinc levels in the body, resulting in survival defect under zinc excess conditions. Zip71B/dZip5 is localized to the plasma membrane at the basolateral side of the tubules, and is functionally epistatic to the apically localized ZnT35C in regulating the tubule zinc homeostasis. Our results indicate that Zip71B/dZip5 is involved in zinc import into the tubular cells from the circulation, and ZnT35C in turn effluxes the tubular zinc out. Notably, mammalian ZIP5, which is expressed in the kidney, functions analogously to Zip71B/dZip5 in the fly while hZIP4 cannot complement the loss of Zip71B/dZip5 function. Furthermore, Zip71B/dZip5 expression is regulated by zinc so that, in response to toxic levels of zinc, the tubules can increase zinc efflux capability. We also characterized the role of dZnT1 (CG17723) in zinc reabsorption in Malpighian tubules. Finally, using a tubule calcification model, we were able to show that knockdown of Zip71B/dZip5 or ZnT35C was able to mitigate stone formation, consistent with their roles in tubular zinc homeostasis.

Conclusions

Our results start to sketch out a relatively complete picture of the zinc excretion process in Drosophila Malpighian tubules, and may provide a reference for relevant mammalian studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0355-9) contains supplementary material, which is available to authorized users.