Identification of a gain-of-function mutation in a Golgi P-type ATPase that enhances Mn2+ efflux and protects against toxicity

Zinc and manganese homeostasis closely interact in mammalian cells

Understanding the homeostatic interactions among essential trace metals is important for explaining their roles in cellular systems. Recent studies in vertebrates suggest that cellular Mn metabolism is related to Zn metabolism in multifarious cellular processes. However, the underlying mechanism remains unclear. In this study, we examined the changes in the expression of proteins involved in cellular Zn and/or Mn homeostatic control and measured the Mn as well as Zn contents and Zn enzyme activities to elucidate the effects of Mn and Zn homeostasis on each other. Mn treatment decreased the expression of the Zn homeostatic proteins metallothionein (MT) and ZNT1 and reduced Zn enzyme activities, which were attributed to the decreased Zn content. Moreover, loss of Mn efflux transport protein decreased MT and ZNT1 expression and Zn enzyme activity without changing extracellular Mn content. This reduction was not observed when supplementing with the same Cu concentrations and in cells lacking Cu efflux proteins. Furthermore, cellular Zn homeostasis was oppositely regulated in cells expressing Zn and Mn importer ZIP8, depending on whether Zn or Mn concentration was elevated in the extracellular milieu. Our results provide novel insights into the intricate interactions between Mn and Zn homeostasis in mammalian cells and facilitate our understanding of the physiopathology of Mn, which may lead to the development of treatment strategies for Mn-related diseases in the future.

Consequences of Disturbing Manganese Homeostasis

Manganese (Mn) is an essential trace element with unique functions in the body; it acts as a cofactor for many enzymes involved in energy metabolism, the endogenous antioxidant enzyme systems, neurotransmitter production, and the regulation of reproductive hormones. However, overexposure to Mn is toxic, particularly to the central nervous system (CNS) due to it causing the progressive destruction of nerve cells. Exposure to manganese is widespread and occurs by inhalation, ingestion, or dermal contact. Associations have been observed between Mn accumulation and neurodegenerative diseases such as manganism, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. People with genetic diseases associated with a mutation in the gene associated with impaired Mn excretion, kidney disease, iron deficiency, or a vegetarian diet are at particular risk of excessive exposure to Mn. This review has collected data on the current knowledge of the source of Mn exposure, the experimental data supporting the dispersive accumulation of Mn in the brain, the controversies surrounding the reference values of biomarkers related to Mn status in different matrices, and the competitiveness of Mn with other metals, such as iron (Fe), magnesium (Mg), zinc (Zn), copper (Cu), lead (Pb), calcium (Ca). The disturbed homeostasis of Mn in the body has been connected with susceptibility to neurodegenerative diseases, fertility, and infectious diseases. The current evidence on the involvement of Mn in metabolic diseases, such as type 2 diabetes mellitus/insulin resistance, osteoporosis, obesity, atherosclerosis, and non-alcoholic fatty liver disease, was collected and discussed.

Case of Congenital Hemolytic Anemia with ATP11C and ANK1 Variants

A male infant of Han descent, with a G1P1 mother and gestational age of 40+4 weeks, was born via cesarean section owing to his mother having pregnancy complications, including premature rupture of membranes, chorioamnionitis, and gestational diabetes. On the first day after birth, routine blood examination showed that his total red blood cells count was 2.32 × 1012/L, hemoglobin count was 77 g/L, and C-reactive protein count was 48.99 mg/L. After receiving an anti-infection treatment for 10 days and two blood transfusions (100 mL in total), he was discharged from a neonatal intensive care unit (NICU). Accessory examinations showed that reticulocytes in the peripheral blood were significantly increased, the morphology of red blood cells was normal, and all hemolysis-related examinations were normal; bone marrow examinations showed that the proliferation of the red blood cell system was low and serum ferritin and vitamin B12 levels were elevated. Because of the unexplained hemolysis, a whole-exome sequencing examination was performed. The results showed a hemizygous variant of the ATP11C gene (c.3136a>t/p ile 1046phe) and a frame-shift variant of the ANK1 gene (c.937del/pala313 leufs*19). After a six-month follow-up, the serum ferritin and vitamin B12 levels had gradually decreased to normal levels, and hemoglobin and reticulocyte values were 97 g/L and 7.17%, respectively, in the peripheral blood. No splenomegaly was found in physical examination.

Insights into the regulation of cellular Mn2+ homeostasis via TMEM165

Golgi cation homeostasis is known to be crucial for many cellular processes including vesicular fusion events, protein secretion, as well as for the activity of Golgi glycosyltransferases and glycosidases. TMEM165 was identified in 2012 as the first cation transporter related to human glycosylation diseases, namely the Congenital Disorders of Glycosylation (CDG). Interestingly, divalent manganese (Mn) supplementation has been shown to suppress the observed glycosylation defects in TMEM165-deficient cell lines, thus suggesting that TMEM165 is involved in cellular Mn homeostasis. This paper demonstrates that the origin of the Golgi glycosylation defects arises from impaired Golgi Mn homeostasis in TMEM165-depleted cells. We show that Mn supplementation fully rescues the Mn content in the secretory pathway/organelles of TMEM165-depleted cells and hence the glycosylation process. Strong cytosolic and organellar Mn accumulations can also be observed in TMEM165- and SPCA1-depleted cells upon incubation with increasing Mn concentrations, thus demonstrating the crucial involvement of these two proteins in cellular Mn homeostasis. Interestingly, our results show that the cellular Mn homeostasis maintenance in control cells is correlated with the presence of TMEM165 and that the Mn-detoxifying capacities of cells, through the activity of SPCA1, rely on the Mn-induced degradation mechanism of TMEM165. Finally, this paper highlights that TMEM165 is essential in secretory pathway/organelles Mn homeostasis maintenance to ensure both Golgi glycosylation enzyme activities and cytosolic Mn detoxification.

Cryo-EM structures of human SPCA1a reveal the mechanism of Ca2+/Mn2+ transport into the Golgi apparatus

Secretory pathway Ca²⁺/Mn²⁺ ATPase 1 (SPCA1) actively transports cytosolic Ca²⁺ and Mn²⁺ into the Golgi lumen, playing a crucial role in cellular calcium and manganese homeostasis. Detrimental mutations of the ATP2C1 gene encoding SPCA1 cause Hailey-Hailey disease. Here, using nanobody/megabody technologies, we determined cryo-electron microscopy structures of human SPCA1a in the ATP and Ca²⁺/Mn²⁺-bound (E1-ATP) state and the metal-free phosphorylated (E2P) state at 3.1- to 3.3-Å resolutions. The structures revealed that Ca²⁺ and Mn²⁺ share the same metal ion-binding pocket with similar but notably different coordination geometries in the transmembrane domain, corresponding to the second Ca²⁺-binding site in sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA). In the E1-ATP to E2P transition, SPCA1a undergoes similar domain rearrangements to those of SERCA. Meanwhile, SPCA1a shows larger conformational and positional flexibility of the second and sixth transmembrane helices, possibly explaining its wider metal ion specificity. These structural findings illuminate the unique mechanisms of SPCA1a-mediated Ca²⁺/Mn²⁺ transport.

A genetically encoded fluorescent sensor for manganese(II), engineered from lanmodulin

The design of selective metal-binding sites is a challenge in both small-molecule and macromolecular chemistry. Selective recognition of manganese (II)-the first-row transition metal ion that tends to bind with the lowest affinity to ligands, as described by the Irving-Williams series-is particularly difficult. As a result, there is a dearth of chemical biology tools with which to study manganese physiology in live cells, which would advance understanding of photosynthesis, host-pathogen interactions, and neurobiology. Here we report the rational re-engineering of the lanthanide-binding protein, lanmodulin, into genetically encoded fluorescent sensors for MnII, MnLaMP1 and MnLaMP2. These sensors with effective Kd(MnII) of 29 and 7 µM, respectively, defy the Irving-Williams series to selectively detect MnII in vitro and in vivo. We apply both sensors to visualize kinetics of bacterial labile manganese pools. Biophysical studies indicate the importance of coordinated solvent and hydrophobic interactions in the sensors' selectivity. Our results establish lanmodulin as a versatile scaffold for design of selective protein-based biosensors and chelators for metals beyond the f-block.

Calcium and the Ca-ATPase SPCA1 modulate plasma membrane abundance of ZIP8 and ZIP14 to regulate Mn(II) uptake in brain microvascular endothelial cells

Manganese (II) accumulation in human brain microvascular endothelial cells is mediated by the metal-ion transporters ZRT IRT-like protein 8 (ZIP8) and ZRT IRT-like protein 14 (ZIP14). The plasma membrane occupancy of ZIP14, in particular, is increased in cells treated with Mn²⁺, lipopolysaccharide, or IL-6, but the mechanism of this regulation has not been elucidated. The calcium-transporting type 2C member 1 ATPase, SPCA1, is a Golgi-localized Ca²⁺-uptake transporter thought to support Golgi uptake of Mn²⁺ also. Here, we show using surface protein biotinylation, indirect immunofluorescence, and GFP-tagged proteins that cytoplasmic Ca²⁺ regulates ZIP8- and ZIP14-mediated manganese accumulation in human brain microvascular endothelial cells by increasing the plasma membrane localization of these transporters. We demonstrate that RNAi knockdown of SPCA1 expression results in an increase in cytoplasmic Ca²⁺ levels. In turn, we found increased cytoplasmic Ca²⁺ enhances membrane-localized ZIP8 and ZIP14 and a subsequent increase in ⁵⁴Mn²⁺ uptake. Furthermore, overexpression of WT SPCA1 or a gain-of-function mutant resulted in a decrease in cytoplasmic Ca²⁺ and ⁵⁴Mn²⁺ accumulation. While addition of Ca²⁺ positively regulated ZIP-mediated ⁵⁴Mn²⁺ uptake, we show chelation of Ca²⁺ diminished manganese transport. In conclusion, the modulation of ZIP8 and ZIP14 membrane cycling by cytoplasmic calcium is a novel finding and provides new insight into the regulation of the uptake of Mn²⁺ and other divalent metal ions–mediated ZIP metal transporters.

The Role of the Metabolism of Zinc and Manganese Ions in Human Cancerogenesis

Metal ion homeostasis is fundamental for life. Specifically, transition metals iron, manganese and zinc play a pivotal role in mitochondrial metabolism and energy generation, anti-oxidation defense, transcriptional regulation and the immune response. The misregulation of expression or mutations in ion carriers and the corresponding changes in Mn²⁺ and Zn²⁺ levels suggest that these ions play a pivotal role in cancer progression. Moreover, coordinated changes in Mn²⁺ and Zn²⁺ ion carriers have been detected, suggesting that particular mechanisms influenced by both ions might be required for the growth of cancer cells, metastasis and immune evasion. Here, we present a review of zinc and manganese pathophysiology suggesting that these ions might cooperatively regulate cancerogenesis. Zn and Mn effects converge on mitochondria-induced apoptosis, transcriptional regulation and the cGAS-STING signaling pathway, mediating the immune response. Both Zn and Mn influence cancer progression and impact treatment efficacy in animal models and clinical trials. We predict that novel strategies targeting the regulation of both Zn and Mn in cancer will complement current therapeutic strategies.

Golgi Metal Ion Homeostasis in Human Health and Diseases

The Golgi apparatus is a membrane organelle located in the center of the protein processing and trafficking pathway. It consists of sub-compartments with distinct biochemical compositions and functions. Main functions of the Golgi, including membrane trafficking, protein glycosylation, and sorting, require a well-maintained stable microenvironment in the sub-compartments of the Golgi, along with metal ion homeostasis. Metal ions, such as Ca²⁺, Mn²⁺, Zn²⁺, and Cu²⁺, are important cofactors of many Golgi resident glycosylation enzymes. The homeostasis of metal ions in the secretory pathway, which is required for proper function and stress response of the Golgi, is tightly regulated and maintained by transporters. Mutations in the transporters cause human diseases. Here we provide a review specifically focusing on the transporters that maintain Golgi metal ion homeostasis under physiological conditions and their alterations in diseases.

Up-regulation of the manganese transporter SLC30A10 by hypoxia-inducible factors defines a homeostatic response to manganese toxicity

Manganese (Mn) is an essential metal that induces incurable parkinsonism at elevated levels. However, unlike other essential metals, mechanisms that regulate mammalian Mn homeostasis are poorly understood, which has limited therapeutic development. Here, we discovered that the exposure of mice to a translationally relevant oral Mn regimen up-regulated expression of SLC30A10, a critical Mn efflux transporter, in the liver and intestines. Mechanistic studies in cell culture, including primary human hepatocytes, revealed that 1) elevated Mn transcriptionally up-regulated SLC30A10, 2) a hypoxia response element in the SLC30A10 promoter was necessary, 3) the transcriptional activities of hypoxia-inducible factor (HIF) 1 or HIF2 were required and sufficient for the SLC30A10 response, 4) elevated Mn activated HIF1/HIF2 by blocking the prolyl hydroxylation of HIF proteins necessary for their degradation, and 5) blocking the Mn-induced up-regulation of SLC30A10 increased intracellular Mn levels and enhanced Mn toxicity. Finally, prolyl hydroxylase inhibitors that stabilize HIF proteins and are in advanced clinical trials for other diseases reduced intracellular Mn levels and afforded cellular protection against Mn toxicity and also ameliorated the in vivo Mn-induced neuromotor deficits in mice. These findings define a fundamental homeostatic protective response to Mn toxicity-elevated Mn levels activate HIF1 and HIF2 to up-regulate SLC30A10, which in turn reduces cellular and organismal Mn levels, and further indicate that it may be possible to repurpose prolyl hydroxylase inhibitors for the management of Mn neurotoxicity.

Assessment of Concentrations of Heavy Metals in Postmyocardial Infarction Patients and Patients Free from Cardiovascular Event

Cardiovascular diseases (CVDs) constitute the first cause of death among the population of developing and developed countries. Atherosclerosis, which is a disorder with multifactorial etiopathogenesis, underlies most CVDs. The available literature includes ample research studies on the influence of classic cardiovascular (CV) risk factors. However, environmental exposure to heavy metals, among other substances, is still an unappreciated risk factor of CVDs. This study aimed to assess the concentration of some heavy metals (copper (Cu), zinc (Zn), manganese (Mn), cobalt (Co), and iron (Fe)) in the blood serum of postmyocardial infarction (post-MI) patients and patients free from myocardial infarction (MI) as well as estimate the relationship between the occurrence of MI and increased concentration of heavy metals. The concentration of heavy metals (Cu, Zn, Mn, Co, and Fe) was assessed using the inductively coupled plasma mass spectrometry technique in a group of 146 respondents divided into two groups: post-MI group (study group (SG), n = 74) and group without cardiovascular event (CVE) having a low CV risk (control group (CG), n = 72). The concentration of the analyzed heavy metals was higher in SG. All the heavy metals showed a significant diagnostic value (p < 0.001). The highest value of area under the curve (AUC) was observed for manganese (Mn) (0.955; 95% confidence interval (CI) = 0.922–0.988), while the lowest value was found for zinc (Zn) (0.691; 95% CI = 0.599–0.782). In one-dimensional models, high concentrations of each of the analyzed heavy metals significantly increased the chances of having MI from 7-fold (Cu) to 128-fold (Mn). All the models containing a particular metal showed a significant and high discrimination value for MI occurrence (AUC 0.72–0.92). Higher concentrations of Cu, Zn, Mn, Co, and Fe were found to considerably increase the chances of having MI. Considering the increasingly higher environmental exposure to heavy metals in recent times, their concentrations can be distinguished as a potential risk factor of CVDs.

Manganese in the Diet: Bioaccessibility, Adequate Intake, and Neurotoxicological Effects

Manganese (Mn) is an essential element that participates in several biological processes. Mn serves as a cofactor for several enzymes, such as glutamine synthetase and oxidoreductases, that have an important role in the defense of the organisms against oxidative stress. The diet is the main source of Mn intake for humans, and adequate daily intake levels for this metal change with age. Moreover, in higher amounts, Mn may be toxic, mainly to the brain. Here, we provide an overview of Mn occurrence in food, addressing its bioaccessibility and discussing the dietary standard and recommended intake of Mn consumption. In addition, we review some mechanisms underlying Mn-induced neurotoxicity.

iTRAQ-based comparative proteomic analysis reveals high temperature accelerated leaf senescence of tobacco (Nicotiana tabacum L.) during flue-curing

Tobacco (Nicotiana tabacum) is extensively cultivated all over the world for its economic value. During curing and storage, senescence occurs, which is associated with physiological and biochemical changes in postharvest plant organs. However, the molecular mechanisms involved in accelerated senescence due to high temperatures in tobacco leaves during curing need further elaboration. We studied molecular mechanisms of senescence in tobacco leaves exposed to high temperature during curing (Fresh, 38 °C and 42 °C), revealed by isobaric tags for relative and absolute quantification (iTRAQ) for the proteomic profiles of cultivar Bi'na1. In total, 8903 proteins were identified, and 2034 (1150 up-regulated and 1074 down-regulated) differentially abundant proteins (DAPs) were obtained from tobacco leaf samples. These DAPs were mainly involved in posttranslational modification, protein turnover, energy production and conversion. Sugar- and energy-related metabolic biological processes and pathways might be critical regulators of tobacco leaves exposed to high temperature during senescence. High-temperature stress accelerated tobacco leaf senescence mainly by down-regulating photosynthesis-related pathways and degrading cellular constituents to maintain cell viability and nutrient recycling. Our findings provide a valuable inventory of novel proteins involved in senescence physiology and elucidate the protein regulatory network in postharvest organs exposed to high temperatures during flue-curing.

Investigating the functional link between TMEM165 and SPCA1

TMEM165 was highlighted in 2012 as the first member of the Uncharacterized Protein Family 0016 (UPF0016) related to human glycosylation diseases. Defects in TMEM165 are associated with strong Golgi glycosylation abnormalities. Our previous work has shown that TMEM165 rapidly degrades with supraphysiological manganese supplementation. In this paper, we establish a functional link between TMEM165 and SPCA1, the Golgi Ca2+/Mn2+ P-type ATPase pump. A nearly complete loss of TMEM165 was observed in SPCA1-deficient Hap1 cells. We demonstrate that TMEM165 was constitutively degraded in lysosomes in the absence of SPCA1. Complementation studies showed that TMEM165 abundance was directly dependent on SPCA1's function and more specifically its capacity to pump Mn2+ from the cytosol into the Golgi lumen. Among SPCA1 mutants that differentially impair Mn2+ and Ca2+ transport, only the Q747A mutant that favors Mn2+ pumping rescues the abundance and Golgi subcellular localization of TMEM165. Interestingly, the overexpression of SERCA2b also rescues the expression of TMEM165. Finally, this paper highlights that TMEM165 expression is linked to the function of SPCA1.

Biometals and glycosylation in humans: Congenital disorders of glycosylation shed lights into the crucial role of Golgi manganese homeostasis

About half of the eukaryotic proteins bind biometals that participate in their structure and functions in virtually all physiological processes, including glycosylation. After reviewing the biological roles and transport mechanisms of calcium, magnesium, manganese, zinc and cobalt acting as cofactors of the metalloproteins involved in sugar metabolism and/or glycosylation, the paper will outline the pathologies resulting from a dysregulation of these metals homeostasis and more particularly Congenital Disorders of Glycosylation (CDGs) caused by ion transporter defects. Highlighting of CDGs due to defects in SLC39A8 (ZIP8) and TMEM165, two proteins transporting manganese from the extracellular space to cytosol and from cytosol to the Golgi lumen, respectively, has emphasized the importance of manganese homeostasis for glycosylation. Based on our current knowledge of TMEM165 structure and functions, this review will draw a picture of known and putative mechanisms regulating manganese homeostasis in the secretory pathway.

Targeting the Early Endosome-to-Golgi Transport of Shiga Toxins as a Therapeutic Strategy

Shiga toxin (STx) produced by Shigella and closely related Shiga toxin 1 and 2 (STx1 and STx2) synthesized by Shiga toxin-producing Escherichia coli (STEC) are bacterial AB₅ toxins. All three toxins target kidney cells and may cause life-threatening renal disease. While Shigella infections can be treated with antibiotics, resistance is increasing. Moreover, antibiotic therapy is contraindicated for STEC, and there are no definitive treatments for STEC-induced disease. To exert cellular toxicity, STx, STx1, and STx2 must undergo retrograde trafficking to reach their cytosolic target, ribosomes. Direct transport from early endosomes to the Golgi apparatus is an essential step that allows the toxins to bypass degradative late endosomes and lysosomes. The essentiality of this transport step also makes it an ideal target for the development of small-molecule inhibitors of toxin trafficking as potential therapeutics. Here, we review the recent advances in understanding the molecular mechanisms of the early endosome-to-Golgi transport of STx, STx1, and STx2, as well as the development of small-molecule inhibitors of toxin trafficking that act at the endosome/Golgi interface.

The impact of manganese on neurotransmitter systems

BACKGROUND

Manganese (Mn) is a metal ubiquitously present in nature and essential for many living organisms. As a trace element, it is required in small amounts for the proper functioning of several important enzymes, and reports of Mn deficiency are indeed rare.

METHODS

This mini-review will cover aspects of Mn toxicokinetics and its impact on brain neurotransmission, as well as its Janus-faced effects on humans and other animal's health.

RESULTS

The estimated safe upper limit of intracellular Mn for physiological function is in anarrow range of 20-53 μM.Therefore, intake of higher levels of Mn and the outcomes, especially to the nervous system, have been well documented.

CONCLUSION

The metal affects mostly the brain by accumulating in specific areas, altering cognitive functions and locomotion, thus severely impacting the health of the exposed organisms.

Brain manganese and the balance between essential roles and neurotoxicity

Manganese (Mn) is an essential micronutrient required for the normal development of many organs, including the brain. Although its roles as a cofactor in several enzymes and in maintaining optimal physiology are well-known, the overall biological functions of Mn are rather poorly understood. Alterations in body Mn status are associated with altered neuronal physiology and cognition in humans, and either overexposure or (more rarely) insufficiency can cause neurological dysfunction. The resultant balancing act can be viewed as a hormetic U-shaped relationship for biological Mn status and optimal brain health, with changes in the brain leading to physiological effects throughout the body and vice versa. This review discusses Mn homeostasis, biomarkers, molecular mechanisms of cellular transport, and neuropathological changes associated with disruptions of Mn homeostasis, especially in its excess, and identifies gaps in our understanding of the molecular and biochemical mechanisms underlying Mn homeostasis and neurotoxicity.

SPCA1 governs the stability of TMEM165 in Hailey-Hailey disease

TMEM165 is a Golgi protein whose deficiency causes a Congenital Disorder of Glycosylation (CDG). We have demonstrated that Mn²⁺ supplementation could suppress the glycosylation defects observed in TMEM165-deficient cells and that TMEM165 was a Mn²⁺-sensitive protein. In the Golgi, the other transmembrane protein capable to regulate Mn²⁺/Ca²⁺ homeostasis is SPCA1, encoded by the ATP2C1 gene. A loss of one copy of the ATP2C1 gene leads to Hailey-Hailey Disease (HHD), an acantholytic skin disorder in Humans. Our latest results suggest an unexpected functional link between SPCA1 and TMEM165. In order to clarify this link in case of partial SPCA1 deficiency, HHD fibroblasts were used to assess TMEM165 expression, subcellular localization and Mn²⁺-induced degradation. No differences were observed regarding TMEM165 expression and localization in HHD patients' fibroblasts compared to control fibroblasts. Nevertheless, we demonstrated both for fibroblasts and keratinocytes that TMEM165 expression is more sensitive to MnCl₂ exposure in HHD cells than in control cells. We linked, using ICP-MS and GPP130 as a Golgi Mn²⁺ sensor, this higher Mn²⁺-induced sensitivity to a cytosolic Mn accumulation in MnCl₂ supplemented HHD fibroblasts. Altogether, these results link the function of SPCA1 to the stability of TMEM165 in a pathological context of Hailey-Hailey disease.

ZIP14 is degraded in response to manganese exposure

Manganese (Mn) is an essential element necessary for proper development and brain function. Circulating Mn levels are regulated by hepatobiliary clearance to limit toxic levels and prevent tissue deposition. To characterize mechanisms involved in hepatocyte Mn uptake, polarized human HepaRG cells were used for this study. Western blot analysis and immunofluorescence microscopy showed the Mn transporter ZIP14 was expressed and localized to the basolateral surface of polarized HepaRG cells. HepaRG cells took up 54Mn in a time- and temperature-dependent manner but uptake was reduced after exposure to Mn. This loss in transport activity was associated with decreased ZIP14 protein levels in response to Mn exposure. Mn-induced degradation of ZIP14 was blocked by bafilomycin A1, which increased localization of the transporter in Lamp1-positive vesicles. Mn exposure also down-regulated the Golgi proteins TMEM165 and GPP130 while the ER stress marker BiP was induced. These results indicate that Mn exposure decreases ZIP14 protein levels to limit subsequent uptake of Mn as a cytoprotective response. Thus, high levels of Mn may compromise first-pass-hepatic clearance mechanisms.

Manganese Mapping Using a Fluorescent Mn2+ Sensor and Nanosynchrotron X-ray Fluorescence Reveals the Role of the Golgi Apparatus as a Manganese Storage Site

Elucidating dynamics in transition-metal distribution and localization under physiological and pathophysiological conditions is central to our understanding of metal-ion regulation. In this Forum Article, we focus on manganese and specifically recent developments that point to the relevance of the Golgi apparatus in manganese detoxification when this essential metal ion is overaccumulated because of either environmental exposure or mutations in manganese efflux transporters. In order to further evaluate the role of the Golgi apparatus as a manganese-ion storage compartment under subcytotoxic manganese levels, we use a combination of confocal microscopy using a sensitive "turn-on" fluorescent manganese sensor, M1, and nanosynchrotron X-ray fluorescence imaging to show that manganese ions are stored in the Golgi apparatus under micromolar manganese exposure concentrations. Our results, along with previous reports on manganese accumulation, now indicate a central role of the Golgi apparatus in manganese storage and trafficking under subcytotoxic manganese levels and hint toward a possible role of the Golgi apparatus in manganese storage even under physiological conditions.

Characterization of Hailey-Hailey Disease-mutants in presence and absence of wild type SPCA1 using Saccharomyces cerevisiae as model organism

Hailey-Hailey disease is an autosomal genetic disease caused by mutations in one of the two ATP2C1 alleles encoding the secretory pathway Ca²⁺/Mn²⁺-ATPase, hSPCA1. The disease almost exclusively affects epidermis, where it mainly results in acantholysis of the suprabasal layers. The etiology of the disease is complex and not well understood. We applied a yeast based complementation system to characterize fourteen disease-causing ATP2C1 missense mutations in presence or absence of wild type ATP2C1 or ATP2A2, encoding SERCA2. In our yeast model system, mutations in ATP2C1 affected Mn²⁺ transport more than Ca²⁺ transport as twelve out of fourteen mutations were unable to complement Mn²⁺ sensitivity while thirteen out of fourteen to some extent complemented the high Ca²⁺requirement. Nine out of fourteen mutations conferred a cold sensitive complementation capacity. In absence of a wild type ATP2C1 allele, twelve out of fourteen mutations induced an unfolded protein response indicating that in vivo folding of hSPCA1 is sensitive to disease causing amino acid substitutions and four of the fourteen mutations caused the hSPCA1 protein to accumulate in the vacuolar membrane. Co-expression of either wild type ATP2C1 or ATP2A2 prevented induction of the unfolded protein response and hSPCA1 mis-localization.

Overexpression of human Atp13a2Isoform-1 protein protects cells against manganese and starvation-induced toxicity

Mutations in ATP13A2 cause Kufor-Rakeb syndrome (KRS), a juvenile form of Parkinson's disease (PD) with dementia. However, the mechanisms by which mutations in ATP13A2 cause KRS is not understood. The mutations lead to misfolding of the translated Atp13a2 protein and its premature degradation in the endoplasmic reticulum, never reaching the lysosome where the protein is thought to function. Atp13a2 is a P-type ATPase, a class of proteins that function in ion transport. Indeed, studies of human, mouse, and yeast Atp13a2 proteins suggest a possible involvement in regulation of heavy metal toxicity. Here we report on the cytoprotective function of Atp13a2 on HeLa cells and dopamine neurons of Caenorhabditis elegans (C. elegans). HeLa cells stably overexpressing V5- tagged Atp13a2Isoform-1 protein were more resistant to elevated manganese exposure and to starvation-induced cell death compared to cells not overexpressing the protein. Because PD is characterized by loss of dopamine neurons, we generated transgenic C. elegans expressing GFP-tagged human Atp13a2 protein in dopamine neurons. The transgenic animals exhibited higher resistance to dopamine neuron degeneration after acute exposure to manganese compared to nematodes that expressed GFP alone. The results suggest Atp13a2 Isoform-1 protein confers cytoprotection against toxic insults, including those that cause PD syndromes.

A Mn-sensing riboswitch activates expression of a Mn2+/Ca2+ ATPase transporter in Streptococcus

Maintaining manganese (Mn) homeostasis is important for the virulence of numerous bacteria. In the human respiratory pathogen Streptococcus pneumoniae, the Mn-specific importer PsaBCA, exporter MntE, and transcriptional regulator PsaR establish Mn homeostasis. In other bacteria, Mn homeostasis is controlled by yybP-ykoY family riboswitches. Here, we characterize a yybP-ykoY family riboswitch upstream of the mgtA gene encoding a PII-type ATPase in S. pneumoniae, suggested previously to function in Ca2+ efflux. We show that the mgtA riboswitch aptamer domain adopts a canonical yybP-ykoY structure containing a three-way junction that is compacted in the presence of Ca2+ or Mn2+ at a physiological Mg2+ concentration. Although Ca2+ binds to the RNA aptamer with higher affinity than Mn2+, in vitro activation of transcription read-through of mgtA by Mn2+ is much greater than by Ca2+. Consistent with this result, mgtA mRNA and protein levels increase ≈5-fold during cellular Mn stress, but only in genetic backgrounds of S. pneumoniae and Bacillus subtilis that exhibit Mn2+ sensitivity, revealing that this riboswitch functions as a failsafe 'on' signal to prevent Mn2+ toxicity in the presence of high cellular Mn2+. In addition, our results suggest that the S. pneumoniae yybP-ykoY riboswitch functions to regulate Ca2+ efflux under these conditions.

Putative metal binding site in the transmembrane domain of the manganese transporter SLC30A10 is different from that of related zinc transporters

SLC30 proteins belong to the cation diffusion facilitator (CDF) superfamily of metal transporters. SLC30A10 mediates manganese efflux, while other SLC30 members transport zinc. Metal specificity of CDFs may be conferred by amino acids that form a transmembrane metal binding site (Site A). Site A of zinc-transporting CDFs, such as SLC30A1/ZnT1, have a HXXXD motif, but manganese transporters, such as SLC30A10, harbor a NXXXD motif. This critical histidine-to-asparagine substitution, at residue 43, was proposed to underlie manganese transport specificity of SLC30A10. However, we recently discovered that asparagine-43 was dispensable for manganese efflux in HeLa cells; instead, glutamate-25, aspartate-40, asparagine-127, and aspartate-248 were required. In contrast, another group reported that asparagine-43 was required in a chicken cell line. The goal of this study was to resolve the divergent results about the requirement of the crucial asparagine-43 residue. For this, we compared the manganese efflux activity of four cell types that stably over-expressed SLC30A10wild-type (WT), SLC30A10N43A or SLC30A10E25A: physiologically-relevant hepatic HepG2 and neuronal AF5 cells, HEK cells, and embryonic fibroblasts from Slc30a10-/- mice. In all cell types, manganese efflux activity of SLC30A10N43A was comparable to WT, while SLC30A10E25A lacked activity. Importantly, unlike SLC30A10, the histidine residue of the HXXXD motif of SLC30A1/ZnT1 was required for zinc transport. These results imply that the mechanisms of ion coordination within the transmembrane domain of SLC30A10 substantially differ from previously-studied CDFs, suggest that factors beyond Site A residues may confer metal specificity to CDFs, and improve understanding of the pathobiology of manganese toxicity due to mutations in SLC30A10.

SLC30A10 Mutation Involved in Parkinsonism Results in Manganese Accumulation within Nanovesicles of the Golgi Apparatus

Manganese (Mn) is an essential metal that can be neurotoxic when elevated exposition occurs leading to parkinsonian-like syndromes. Mutations in the Slc30a10 gene have been identified in new forms of familial parkinsonism. SLC30A10 is a cell surface protein involved in the efflux of Mn and protects the cell against Mn toxicity. Disease-causing mutations block the efflux activity of SLC30A10, resulting in Mn accumulation. Determining the intracellular localization of Mn when disease-causing SLC30A10 mutants are expressed is essential to elucidate the mechanisms of Mn neurotoxicity. Here, using organelle fluorescence microscopy and synchrotron X-ray fluorescence (SXRF) imaging, we found that Mn accumulates in the Golgi apparatus of human cells transfected with the disease-causing SLC30A10-Δ105-107 mutant under physiological conditions and after exposure to Mn. In cells expressing the wild-type SLC30A10 protein, cellular Mn content was low after all exposure conditions, confirming efficient Mn efflux. In nontransfected cells that do not express endogenous SLC30A10 and in mock transfected cells, Mn was located in the Golgi apparatus, similarly to its distribution in cells expressing the mutant protein, confirming deficient Mn efflux. The newly developed SXRF cryogenic nanoimaging (<50 nm resolution) indicated that Mn was trapped in single vesicles within the Golgi apparatus. Our results confirm the role of SLC30A10 in Mn efflux and the accumulation of Mn in cells expressing the disease-causing SLC30A10-Δ105-107 mutation. Moreover, we identified suborganelle Golgi nanovesicles as the main compartment of Mn accumulation in SLC30A10 mutants, suggesting interactions with the vesicular trafficking machinery as a cause of the disease.

Involvement of thapsigargin- and cyclopiazonic acid-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn2

Congenital disorders of glycosylation are severe inherited diseases in which aberrant protein glycosylation is a hallmark. Transmembrane protein 165 (TMEM165) is a novel Golgi transmembrane protein involved in type II congenital disorders of glycosylation. Although its biologic function is still a controversial issue, we have demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi Mn²⁺ homeostasis defect. The goal of this study was to delineate the cellular pathway by which extracellular Mn²⁺ rescues N-glycosylation in TMEM165 knockout (KO) cells. We first demonstrated that after extracellular exposure, Mn²⁺ uptake by HEK293 cells at the plasma membrane did not rely on endocytosis but was likely done by plasma membrane transporters. Second, we showed that the secretory pathway Ca²⁺-ATPase 1, also known to mediate the influx of cytosolic Mn²⁺ into the lumen of the Golgi apparatus, is not crucial for the Mn²⁺-induced rescue glycosylation of lysosomal-associated membrane protein 2 (LAMP2). In contrast, our results demonstrate the involvement of cyclopiazonic acid- and thapsigargin (Tg)-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn²⁺. Interestingly, overexpression of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) 2b isoform in TMEM165 KO cells partially rescues the observed LAMP2 glycosylation defect. Overall, this study indicates that the rescue of Golgi N-glycosylation defects in TMEM165 KO cells by extracellular Mn²⁺ involves the activity of Tg and cyclopiazonic acid-sensitive pumps, probably the SERCA pumps.-Houdou, M., Lebredonchel, E., Garat, A., Duvet, S., Legrand, D., Decool, V., Klein, A., Ouzzine, M., Gasnier, B., Potelle, S., Foulquier, F. Involvement of thapsigargin- and cyclopiazonic acid-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn²⁺.

Manganese transport and toxicity in polarized WIF-B hepatocytes

Manganese (Mn) toxicity arises from nutritional problems, community and occupational exposures, and genetic risks. Mn blood levels are controlled by hepatobiliary clearance. The goals of this study were to determine the cellular distribution of Mn transporters in polarized hepatocytes, to establish an in vitro assay for hepatocyte Mn efflux, and to examine possible roles the Mn transporters would play in metal import and export. For these experiments, hepatocytoma WIF-B cells were grown for 12-14 days to achieve maximal polarity. Immunoblots showed that Mn transporters ZIP8, ZnT10, ferroportin (Fpn), and ZIP14 were present. Indirect immunofluorescence microscopy localized Fpn and ZIP14 to WIF-B cell basolateral domains whereas ZnT10 and ZIP8 associated with intracellular vesicular compartments. ZIP8-positive structures were distributed uniformly throughout the cytoplasm, but ZnT10-positive vesicles were adjacent to apical bile compartments. WIF-B cells were sensitive to Mn toxicity, showing decreased viability after 16 h exposure to >250 μM MnCl₂. However, the hepatocytes were resistant to 4-h exposures of up to 500 μM MnCl₂ despite 50-fold increased Mn content. Washout experiments showed time-dependent efflux with 80% Mn released after a 4 h chase period. Hepcidin reduced levels of Fpn in WIF-B cells, clearing Fpn from the cell surface, but Mn efflux was unaffected. The secretory inhibitor, brefeldin A, did block release of Mn from WIF-B cells, suggesting vesicle fusion may be involved in export. These results point to a possible role of ZnT10 to import Mn into vesicles that subsequently fuse with the apical membrane and empty their contents into bile. NEW & NOTEWORTHY Polarized WIF-B hepatocytes express manganese (Mn) transporters ZIP8, ZnT10, ferroportin (Fpn), and ZIP14. Fpn and ZIP14 localize to basolateral domains. ZnT10-positive vesicles were adjacent to apical bile compartments, and ZIP8-positive vesicles were distributed uniformly throughout the cytoplasm. WIF-B hepatocyte Mn export was resistant to hepcidin but inhibited by brefeldin A, pointing to an efflux mechanism involving ZnT10-mediated uptake of Mn into vesicles that subsequently fuse with and empty their contents across the apical bile canalicular membrane.

Influence of iron metabolism on manganese transport and toxicity

Although manganese (Mn) is critical for the proper functioning of various metabolic enzymes and cofactors, excess Mn in the brain causes neurotoxicity. While the exact transport mechanism of Mn has not been fully understood, several importers and exporters for Mn have been identified over the past decade. In addition to Mn-specific transporters, it has been demonstrated that iron transporters can mediate Mn transport in the brain and peripheral tissues. However, while the expression of iron transporters is regulated by body iron stores, whether or not disorders of iron metabolism modify Mn homeostasis has not been systematically discussed. The present review will provide an update on the role of altered iron status in the transport and toxicity of Mn.

Genome-wide siRNA screen identifies UNC50 as a regulator of Shiga toxin 2 trafficking

Toxins produced by Shigella bacteria undergo endosome-to-Golgi retrograde trafficking to evade degradation in the lysosome and reach the cytosol. Selyunin et al. performed a genome-wide siRNA screen and identify host factors required for the transport and toxicity of Shiga toxins.

Shiga toxins 1 and 2 (STx1 and STx2) undergo retrograde trafficking to reach the cytosol. Early endosome-to-Golgi transport allows the toxins to evade degradation in lysosomes. Targeting this trafficking step has therapeutic promise, but the mechanism of trafficking for the more potent toxin STx2 is unclear. To identify host factors required for early endosome-to-Golgi trafficking of STx2, we performed a viability-based genome-wide siRNA screen in HeLa cells. 564, 535, and 196 genes were found to be required for toxicity induced by STx1 only, STx2 only, and both toxins, respectively. We focused on validating endosome/Golgi-localized hits specific for STx2 and found that depletion of UNC50 blocked early endosome-to-Golgi trafficking and induced lysosomal degradation of STx2. UNC50 acted by recruiting GBF1, an ADP ribosylation factor–guanine nucleotide exchange factor (ARF-GEF), to the Golgi. These results provide new information about STx2 trafficking mechanisms and may advance efforts to generate therapeutically viable toxin-trafficking inhibitors.

Hypothyroidism induced by loss of the manganese efflux transporter SLC30A10 may be explained by reduced thyroxine production

SLC30A10 and SLC39A14 are manganese efflux and influx transporters, respectively. Loss-of-function mutations in genes encoding either transporter induce hereditary manganese toxicity. Patients have elevated manganese in the blood and brain and develop neurotoxicity. Liver manganese is increased in patients lacking SLC30A10 but not SLC39A14. These organ-specific changes in manganese were recently recapitulated in knockout mice. Surprisingly, Slc30a10 knockouts also had elevated thyroid manganese and developed hypothyroidism. To determine the mechanisms of manganese-induced hypothyroidism and understand how SLC30A10 and SLC39A14 cooperatively mediate manganese detoxification, here we produced Slc39a14 single and Slc30a10/Slc39a14 double knockout mice and compared their phenotypes with that of Slc30a10 single knockouts. Compared with wild-type controls, Slc39a14 single and Slc30a10/Slc39a14 double knockouts had higher manganese levels in the blood and brain but not in the liver. In contrast, Slc30a10 single knockouts had elevated manganese levels in the liver as well as in the blood and brain. Furthermore, SLC30A10 and SLC39A14 localized to the canalicular and basolateral domains of polarized hepatic cells, respectively. Thus, transport activities of both SLC39A14 and SLC30A10 are required for hepatic manganese excretion. Compared with Slc30a10 single knockouts, Slc39a14 single and Slc30a10/Slc39a14 double knockouts had lower thyroid manganese levels and normal thyroid function. Moreover, intrathyroid thyroxine levels of Slc30a10 single knockouts were lower than those of controls. Thus, the hypothyroidism phenotype of Slc30a10 single knockouts is induced by elevated thyroid manganese, which blocks thyroxine production. These findings provide new insights into the mechanisms of manganese detoxification and manganese-induced thyroid dysfunction.

Familial manganese-induced neurotoxicity due to mutations in SLC30A10 or SLC39A14

Over the last few years, two rare, familial diseases that lead to the onset of manganese (Mn)-induced neurotoxicity have been discovered. Loss-of-function mutations in SLC30A10, a Mn efflux transporter, or SLC39A14, a Mn influx transporter, increase Mn levels in blood and brain, and induce severe neurotoxicity. The discoveries of these genetic diseases have transformed our understanding of Mn homeostasis, detoxification, and neurotoxicity. Current knowledge about the mechanisms by which mutations in these transporters alter Mn homeostasis to induce human disease is reviewed here.

Manganese-induced trafficking and turnover of GPP130 is mediated by sortilin

By binding and directing the cycling Golgi protein GPP130 to lysosomes, the sorting receptor sortilin mediates the manganese-induced GPP130 down-regulation that protects against Shiga toxicosis.

Elevated, nontoxic doses of manganese (Mn) protect against Shiga toxin-1–induced cell death via down-regulation of GPP130, a cycling Golgi membrane protein that serves as an endosome-to-Golgi trafficking receptor for the toxin. Mn binds to GPP130 in the Golgi and causes GPP130 to oligomerize/aggregate, and the complexes are diverted to lysosomes. In fact, based on experiments using the self-interacting FM domain, it appears generally true that aggregation of a Golgi protein leads to its lysosomal degradation. How such oligomers are selectively sorted out of the Golgi is unknown. Here we provide evidence that Mn-induced exit of GPP130 from the trans-Golgi network (TGN) toward lysosomes is mediated by the sorting receptor sortilin interacting with the lumenal stem domain of GPP130. In contrast, FM-induced lysosomal trafficking of the Golgi protein galactosyltransferase was sortilin independent and occurred even in the absence of its native lumenal domain. Thus sortilin-dependent as well as sortilin-independent sorting mechanisms target aggregated Golgi membrane proteins for lysosomal degradation.

Zebrafish slc30a10 deficiency revealed a novel compensatory mechanism of Atp2c1 in maintaining manganese homeostasis

Recent studies found that mutations in the human SLC30A10 gene, which encodes a manganese (Mn) efflux transporter, are associated with hypermanganesemia with dystonia, polycythemia, and cirrhosis (HMDPC). However, the relationship between Mn metabolism and HMDPC is poorly understood, and no specific treatments are available for this disorder. Here, we generated two zebrafish slc30a10 mutant lines using the CRISPR/Cas9 system. Compared to wild-type animals, mutant adult animals developed significantly higher systemic Mn levels, and Mn accumulated in the brain and liver of mutant embryos in response to exogenous Mn. Interestingly, slc30a10 mutants developed neurological deficits in adulthood, as well as environmental Mn-induced manganism in the embryonic stage; moreover, mutant animals had impaired dopaminergic and GABAergic signaling. Finally, mutant animals developed steatosis, liver fibrosis, and polycythemia accompanied by increased epo expression. This phenotype was rescued partially by EDTA- CaNa₂ chelation therapy and iron supplementation. Interestingly, prior to the onset of slc30a10 expression, expressing ATP2C1 (ATPase secretory pathway Ca²⁺ transporting 1) protected mutant embryos from Mn exposure, suggesting a compensatory role for Atp2c1 in the absence of Slc30a10. Notably, expressing either wild-type or mutant forms of SLC30A10 was sufficient to inhibit the effect of ATP2C1 in response to Mn challenge in both zebrafish embryos and HeLa cells. These findings suggest that either activating ATP2C1 or restoring the Mn-induced trafficking of ATP2C1 can reduce Mn accumulation, providing a possible target for treating HMDPC.

Impaired function of the manganese transporter SLC30A10 has been implicated in HMDPC (hypermanganesemia with dystonia, polycythemia, and cirrhosis), an early-onset metabolic disorder clinically characterized by increased systemic Mn levels, neurological impairment, polycythemia, and hepatic injury. No specific treatment is currently available for HMDPC. Moreover, the mechanisms that underlie Mn metabolism are poorly understood, thereby hindering the development of effective treatments. To investigate the physiological processes underlying Mn metabolism and to develop new disease models of HMDPC, we generated two zebrafish slc30a10 mutant lines using the CRISPR/Cas9 system and found that these mutants develop clinical deficits typically associated with HMDPC. Furthermore, we identified a putative compensatory role for ATP2C1 in the absence of SLC30A10 with respect to modulating Mn metabolism. These findings provide a valuable tool for investigating the role of manganese dysregulation in neurological degenerative diseases and which can be used to develop new pharmacological approaches for managing Mn accumulation.

Deficiency in the manganese efflux transporter SLC30A10 induces severe hypothyroidism in mice

Manganese is an essential metal that becomes toxic at elevated levels. Loss-of-function mutations in SLC30A10, a cell-surface-localized manganese efflux transporter, cause a heritable manganese metabolism disorder resulting in elevated manganese levels and parkinsonian-like movement deficits. The underlying disease mechanisms are unclear; therefore, treatment is challenging. To understand the consequences of loss of SLC30A10 function at the organism level, we generated Slc30a10 knock-out mice. During early development, knock-outs were indistinguishable from controls. Surprisingly, however, after weaning and compared with controls, knock-out mice failed to gain weight, were smaller, and died prematurely (by ∼6-8 weeks of age). At 6 weeks, manganese levels in the brain, blood, and liver of the knock-outs were ∼20-60-fold higher than controls. Unexpectedly, histological analyses revealed that the brain and liver of the knock-outs were largely unaffected, but their thyroid exhibited extensive alterations. Because hypothyroidism leads to growth defects and premature death in mice, we assayed for changes in thyroid and pituitary hormones. At 6 weeks and compared with controls, the knock-outs had markedly reduced thyroxine levels (∼50-80%) and profoundly increased thyroid-stimulating hormone levels (∼800-1000-fold), indicating that Slc30a10 knock-out mice develop hypothyroidism. Importantly, a low-manganese diet produced lower tissue manganese levels in the knock-outs and rescued the phenotype, suggesting that manganese toxicity was the underlying cause. Our unanticipated discovery highlights the importance of determining the role of thyroid dysfunction in the onset and progression of manganese-induced disease and identifies Slc30a10 knock-out mice as a new model for studying thyroid biology.

Manganese-induced turnover of TMEM165

TMEM165 deficiencies lead to one of the congenital disorders of glycosylation (CDG), a group of inherited diseases where the glycosylation process is altered. We recently demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi manganese homeostasis defect and that Mn²⁺ supplementation was sufficient to rescue normal glycosylation. In the present paper, we highlight TMEM165 as a novel Golgi protein sensitive to manganese. When cells were exposed to high Mn²⁺ concentrations, TMEM165 was degraded in lysosomes. Remarkably, while the variant R126H was sensitive upon manganese exposure, the variant E108G, recently identified in a novel TMEM165-CDG patient, was found to be insensitive. We also showed that the E108G mutation did not abolish the function of TMEM165 in Golgi glycosylation. Altogether, the present study identified the Golgi protein TMEM165 as a novel Mn²⁺-sensitive protein in mammalian cells and pointed to the crucial importance of the glutamic acid (E108) in the cytosolic ELGDK motif in Mn²⁺-induced degradation of TMEM165.

Inherited Disorders of Manganese Metabolism

While the neurotoxic effects of manganese were recognized in 1837, the first genetic disorder of manganese metabolism was described only in 2012 when homozygous mutations in SLC30A10 were reported to cause manganese-induced neurotoxicity. Two other genetic disorders of manganese metabolism have now been described - mutations in SLC39A14 cause manganese toxicity, while mutations in SLC39A8 cause manganese and zinc deficiency. Study of rare genetic disorders often provides unique insights into disease pathobiology, and the discoveries of these three inherited disorders of manganese metabolism are already transforming our understanding of manganese homeostasis, detoxification, and neurotoxicity. Here, we review the mechanisms by which mutations in SLC30A10, SLC39A14, and SLC39A8 impact manganese homeostasis to cause human disease.

U-Shaped Association between Plasma Manganese Levels and Type 2 Diabetes

BACKGROUND

Manganese is both an essential element and a known toxicant, and it plays important roles in many mechanisms in relation to type 2 diabetes (T2D). However, epidemiological studies of this relationship are rare.

OBJECTIVE

We investigated the association between plasma manganese and newly diagnosed T2D as well as whether the association could be modified by manganese superoxide dismutase (MnSOD) polymorphisms.

METHODS

We conducted a case-control study of 3,228 participants in China: 1,614 T2D patients and 1,614 controls. Concentrations of plasma magnesium were measured, and all participants were genotyped for the MnSOD Val16Ala polymorphism (rs4880).

RESULTS

A U-shaped association was observed between plasma manganese and T2D, with increased odds ratios (ORs) in relation to either low or high plasma manganese levels. Compared with the middle tertile, the multivariate-adjusted ORs [95% confidence intervals (CIs)] of T2D associated with the lowest tertile and the highest tertile of plasma manganese were 1.89 (1.53, 2.33) and 1.56 (1.23, 1.97), respectively. In spline analysis, the U-shaped association was consistently indicated, with the lowest odds of T2D at the plasma manganese concentration of 4.95 μg/L. Minor allele frequencies (C allele) of the MnSOD Val16Ala polymorphism (rs4880) in the normal glucose tolerance (NGT) and the T2D groups were 13.57% and 14.50%, respectively. The MnSOD rs4880 polymorphism was not associated with T2D, and no interaction was found between plasma manganese and the MnSOD rs4880 polymorphism in relation to T2D.

CONCLUSIONS

Our results suggested a U-shaped association between plasma manganese and T2D; both low and high levels of plasma manganese were associated with higher odds of newly diagnosed T2D. The U-shaped association was not modified by the MnSOD rs4880 polymorphism. Citation: Shan Z, Chen S, Sun T, Luo C, Guo Y, Yu X, Yang W, Hu FB, Liu L. 2016. U-shaped association between plasma manganese levels and type 2 diabetes. Environ Health Perspect 124:1876-1881; http://dx.doi.org/10.1289/EHP176.

"Manganese-induced neurotoxicity: a review of its behavioral consequences and neuroprotective strategies"

Manganese (Mn) is an essential heavy metal. However, Mn’s nutritional aspects are paralleled by its role as a neurotoxicant upon excessive exposure. In this review, we covered recent advances in identifying mechanisms of Mn uptake and its molecular actions in the brain as well as promising neuroprotective strategies. The authors focused on reporting findings regarding Mn transport mechanisms, Mn effects on cholinergic system, behavioral alterations induced by Mn exposure and studies of neuroprotective strategies against Mn intoxication. We report that exposure to Mn may arise from environmental sources, occupational settings, food, total parenteral nutrition (TPN), methcathinone drug abuse or even genetic factors, such as mutation in the transporter SLC30A10. Accumulation of Mn occurs mainly in the basal ganglia and leads to a syndrome called manganism, whose symptoms of cognitive dysfunction and motor impairment resemble Parkinson’s disease (PD). Various neurotransmitter systems may be impaired due to Mn, especially dopaminergic, but also cholinergic and GABAergic. Several proteins have been identified to transport Mn, including divalent metal tranporter-1 (DMT-1), SLC30A10, transferrin and ferroportin and allow its accumulation in the central nervous system. Parallel to identification of Mn neurotoxic properties, neuroprotective strategies have been reported, and these include endogenous antioxidants (for instance, vitamin E), plant extracts (complex mixtures containing polyphenols and non-characterized components), iron chelating agents, precursors of glutathione (GSH), and synthetic compounds that can experimentally afford protection against Mn-induced neurotoxicity.

Syndesome Therapeutics for Enhancing Diabetic Wound Healing

Chronic wounds represent a major healthcare and economic problem worldwide. Advanced wound dressings that incorporate bioactive compounds have great potential for improving outcomes in patients with chronic wounds but significant challenges in designing treatments that are effective in long-standing, nonhealing wounds. Here, an optimized wound healing gel was developed that delivers syndecan-4 proteoliposomes ("syndesomes") with fibroblast growth factor-2 (FGF-2) to enhance diabetic wound healing. In vitro studies demonstrate that syndesomes markedly increase migration of keratinocytes and fibroblasts isolated from both nondiabetic and diabetic donors. In addition, syndesome treatment leads to increased endocytic processing of FGF-2 that includes enhanced recycling of FGF-2 to the cell surface after uptake. The optimized syndesome formulation was incorporated into an alginate wound dressing and tested in a splinted wound model in diabetic, ob/ob mice. It was found that wounds treated with syndesomes and FGF-2 have markedly enhanced wound closure in comparison to wounds treated with only FGF-2. Moreover, syndesomes have an immunomodulatory effect on wound macrophages, leading to a shift toward the M2 macrophage phenotype and alterations in the wound cytokine profile. Together, these studies show that delivery of exogenous syndecan-4 is an effective method for enhancing wound healing in the long-term diabetic diseased state.

Exposure, epidemiology, and mechanism of the environmental toxicant manganese

It has become increasingly apparent that global manganese (Mn) pollution to air and water is a significant threat to human health. Despite this recognition, research is only beginning to comprehend the detrimental effects of exposure. Mn, while essential, is particularly harmful to the central nervous system, and overexposure is symptomatic of several neurological disorders. At-risk populations have been identified, but it is still unclear whether typical exposure levels have any long-term consequences. Those at an elevated risk have diminished intellectual function, learning and memory, and mental development. While the overall mechanism of toxicity is undetermined, Mn has been found to induce oxidative stress, exacerbate mitochondrial dysfunction, dysregulate autophagy, and promote apoptosis, ultimately enhancing neurodegeneration. Extrapolation of this in vitro and in vivo data to humans is difficult. There is a definite need to correlate epidemiological studies with causative effects. It is imperative that research efforts endure, so threats are appropriately identified and exposure properly regulated.

Structural Elements in the Transmembrane and Cytoplasmic Domains of the Metal Transporter SLC30A10 Are Required for Its Manganese Efflux Activity

Homozygous mutations in SLC30A10 lead to the development of familial manganese-induced parkinsonism. We previously demonstrated that SLC30A10 is a cell surface-localized manganese efflux transporter, and parkinsonism-causing mutations block its trafficking and efflux activity. Interestingly, other transporters in the SLC30 family mediate zinc efflux. Determining the mechanisms that allow SLC30A10 to transport manganese, which are unclear, is essential to understand its role in parkinsonism. Here, we generated a predicted structure of SLC30A10, based on the structure of the bacterial zinc transporter YiiP, and performed functional studies. In YiiP, side chains of residues Asp-45 and Asp-49 in the second and His-153 and Asp-157 in the fifth transmembrane segments coordinate zinc and are required for transport. In SLC30A10, the corresponding residues are Asn-43 and Asp-47 in the second and His-244 and Asp-248 in the fifth transmembrane segments. Surprisingly, although alanine substitution of Asp-248 abolished manganese efflux, that of Asn-43 and Asp-47 did not. Instead, side chains of charged or polar residues adjacent to Asp-248 in the first (Glu-25) or fourth (Asn-127) transmembrane segments were required. Further analyses revealed that residues His-333 and His-350 in the cytoplasmic C-terminal domain were required for full activity. However, the C-terminal domain failed to transfer manganese transport capability to a related zinc transporter. Overall, our results indicate that residues in the transmembrane and C-terminal domains together confer optimal manganese transport capability to SLC30A10 and suggest that the mechanism of ion coordination in the transmembrane domain of SLC30A10 may be substantially different from that in YiiP/other SLC30 proteins.

ATP2C1 gene mutations in Hailey-Hailey disease and possible roles of SPCA1 isoforms in membrane trafficking

ATP2C1 gene codes for the secretory pathway Ca(2+)/Mn(2+)-ATPase pump type 1 (SPCA1) localizing at the golgi apparatus. Mutations on the human ATP2C1 gene, causing decreased levels of the SPCA1 expression, have been identified as the cause of the Hailey-Hailey disease, a rare skin disorder. In the last few years, several mutations have been described, and here we summarize how they are distributed along the gene and how missense mutations affect protein expression. SPCA1 is expressed in four different isoforms through alternative splicing of the ATP2C1 gene and none of these isoforms is differentially affected by any of these mutations. However, a better understanding of the tissue specific expression of the isoforms, their localization along the secretory pathway, their specific binding partners and the role of the C-terminal tail making isoforms different from each other, will be future goals of the research in this field.

Direct Comparison of Manganese Detoxification/Efflux Proteins and Molecular Characterization of ZnT10 Protein as a Manganese Transporter

Manganese homeostasis involves coordinated regulation of specific proteins involved in manganese influx and efflux. However, the proteins that are involved in detoxification/efflux have not been completely resolved nor has the basis by which they select their metal substrate. Here, we compared six proteins, which were reported to be involved in manganese detoxification/efflux, by evaluating their ability to reduce manganese toxicity in chicken DT40 cells, finding that human ZnT10 (hZnT10) was the most significant contributor. A domain swapping and substitution analysis between hZnT10 and the zinc-specific transporter hZnT1 showed that residue Asn(43), which corresponds to the His residue constituting the potential intramembranous zinc coordination site in other ZnT transporters, is necessary to impart hZnT10's unique manganese mobilization activity; residues Cys(52) and Leu(242) in transmembrane domains II and V play a subtler role in controlling the metal specificity of hZnT10. Interestingly, the His → Asn reversion mutant in hZnT1 conferred manganese transport activity and loss of zinc transport activity. These results provide important information about manganese detoxification/efflux mechanisms in vertebrate cells as well as the molecular characterization of hZnT10 as a manganese transporter.

Glypican-1 nanoliposomes for potentiating growth factor activity in therapeutic angiogenesis

Therapeutic angiogenesis is a highly appealing concept for treating tissues that become ischemic due to vascular disease. A major barrier to the clinical translation of angiogenic therapies is that the patients that are in the greatest need of these treatments often have long term disease states and co-morbidities, such as diabetes and obesity, that make them resistant to angiogenic stimuli. In this study, we identified that human patients with type 2 diabetes have reduced levels of glypican-1 in the blood vessels of their skin. The lack of this key co-receptor in the tissue may make the application of exogenous angiogenic growth factors or cell therapies ineffective. We created a novel therapeutic enhancer for growth factor activity consisting of glypican-1 delivered in a nanoliposomal carrier (a "glypisome"). Here, we demonstrate that glypisomes enhance FGF-2 mediated endothelial cell proliferation, migration and tube formation. In addition, glypisomes enhance FGF-2 trafficking by increasing both uptake and endosomal processing. We encapsulated FGF-2 or FGF-2 with glypisomes in alginate beads and used these to deliver localized growth factor therapy in a murine hind limb ischemia model. Co-delivery of glypisomes with FGF-2 markedly increased the recovery of perfusion and vessel formation in ischemic hind limbs of wild type and diabetic mice in comparison to mice treated with FGF-2 alone. Together, our findings support that glypisomes are effective means for enhancing growth factor activity and may improve the response to local angiogenic growth factor therapies for ischemia.

A Conserved Structural Motif Mediates Retrograde Trafficking of Shiga Toxin Types 1 and 2

Shiga toxin-producing Escherichia coli (STEC) produce two types of Shiga toxin (STx): STx1 and STx2. The toxin A-subunits block protein synthesis, while the B-subunits mediate retrograde trafficking. STEC infections do not have definitive treatments, and there is growing interest in generating toxin transport inhibitors for therapy. However, a comprehensive understanding of the mechanisms of toxin trafficking is essential for drug development. While STx2 is more toxic in vivo, prior studies focused on STx1 B-subunit (STx1B) trafficking. Here, we show that, compared with STx1B, trafficking of the B-subunit of STx2 (STx2B) to the Golgi occurs with slower kinetics. Despite this difference, similar to STx1B, endosome-to-Golgi transport of STx2B does not involve transit through degradative late endosomes and is dependent on dynamin II, epsinR, retromer and syntaxin5. Importantly, additional experiments show that a surface-exposed loop in STx2B (β4-β5 loop) is required for its endosome-to-Golgi trafficking. We previously demonstrated that residues in the corresponding β4-β5 loop of STx1B are required for interaction with GPP130, the STx1B-specific endosomal receptor, and for endosome-to-Golgi transport. Overall, STx1B and STx2B share a common pathway and use a similar structural motif to traffic to the Golgi, suggesting that the underlying mechanisms of endosomal sorting may be evolutionarily conserved.

Prion infection in cells is abolished by a mutated manganese transporter but shows no relation to zinc

The cellular prion protein has been identified as a metalloprotein that binds copper. There have been some suggestions that prion protein also influences zinc and manganese homeostasis. In this study we used a series of cell lines to study the levels of zinc and manganese under different conditions. We overexpressed either the prion protein or known transporters for zinc and manganese to determine relations between the prion protein and both manganese and zinc homeostasis. Our observations supported neither a link between the prion protein and zinc metabolism nor any effect of altered zinc levels on prion protein expression or cellular infection with prions. In contrast we found that a gain of function mutant of a manganese transporter caused reduction of manganese levels in prion infected cells, loss of observable PrP(Sc) in cells and resistance to prion infection. These studies strengthen the link between manganese and prion disease.

Manganese-Induced Parkinsonism and Parkinson's Disease: Shared and Distinguishable Features

Manganese (Mn) is an essential trace element necessary for physiological processes that support development, growth and neuronal function. Secondary to elevated exposure or decreased excretion, Mn accumulates in the basal ganglia region of the brain and may cause a parkinsonian-like syndrome, referred to as manganism. The present review discusses the advances made in understanding the essentiality and neurotoxicity of Mn. We review occupational Mn-induced parkinsonism and the dynamic modes of Mn transport in biological systems, as well as the detection and pharmacokinetic modeling of Mn trafficking. In addition, we review some of the shared similarities, pathologic and clinical distinctions between Mn-induced parkinsonism and Parkinson’s disease. Where possible, we review the influence of Mn toxicity on dopamine, gamma aminobutyric acid (GABA), and glutamate neurotransmitter levels and function. We conclude with a survey of the preventive and treatment strategies for manganism and idiopathic Parkinson’s disease (PD).

Manganese homeostasis in the nervous system

Manganese (Mn) is an essential heavy metal that is naturally found in the environment. Daily intake through dietary sources provides the necessary amount required for several key physiological processes, including antioxidant defense, energy metabolism, immune function and others. However, overexposure from environmental sources can result in a condition known as manganism that features symptomatology similar to Parkinson's disease (PD). This disorder presents with debilitating motor and cognitive deficits that arise from a neurodegenerative process. In order to maintain a balance between its essentiality and neurotoxicity, several mechanisms exist to properly buffer cellular Mn levels. These include transporters involved in Mn uptake, and newly discovered Mn efflux mechanisms. This review will focus on current studies related to mechanisms underlying Mn import and export, primarily the Mn transporters, and their function and roles in Mn-induced neurotoxicity. Though and essential metal, overexposure to manganese may result in neurodegenerative disease analogous to Parkinson's disease. Manganese homeostasis is tightly regulated by transporters, including transmembrane importers (divalent metal transporter 1, transferrin and its receptor, zinc transporters ZIP8 and Zip14, dopamine transporter, calcium channels, choline transporters and citrate transporters) and exporters (ferroportin and SLC30A10), as well as the intracellular trafficking proteins (SPCA1 and ATP12A2). A manganese-specific sensor, GPP130, has been identified, which affords means for monitoring intracellular levels of this metal.

Manganese Is Essential for Neuronal Health

The understanding of manganese (Mn) biology, in particular its cellular regulation and role in neurological disease, is an area of expanding interest. Mn is an essential micronutrient that is required for the activity of a diverse set of enzymatic proteins (e.g., arginase and glutamine synthase). Although necessary for life, Mn is toxic in excess. Thus, maintaining appropriate levels of intracellular Mn is critical. Unlike other essential metals, cell-level homeostatic mechanisms of Mn have not been identified. In this review, we discuss common forms of Mn exposure, absorption, and transport via regulated uptake/exchange at the gut and blood-brain barrier and via biliary excretion. We present the current understanding of cellular uptake and efflux as well as subcellular storage and transport of Mn. In addition, we highlight the Mn-dependent and Mn-responsive pathways implicated in the growing evidence of its role in Parkinson's disease and Huntington's disease. We conclude with suggestions for future focuses of Mn health-related research.