The Combined Inactivation of Intestinal and Hepatic ZIP14 Exacerbates Manganese Overload in Mice

The Role of Solute Carrier Family Transporters in Hepatic Steatosis and Hepatic Fibrosis

Solute carrier (SLC) family transporters are crucial transmembrane proteins responsible for transporting various molecules, including amino acids, electrolytes, fatty acids, and nucleotides. To date, more than fifty SLC transporter subfamilies have been identified, many of which are linked to the progression of hepatic steatosis and fibrosis. These conditions are often caused by factors such as non-alcoholic fatty liver disease and non-alcoholic steatohepatitis, which are major contributors to the global liver disease burden. The activity of SLC members regulates the transport of substrates across biological membranes, playing key roles in lipid synthesis and metabolism, mitochondrial function, and ferroptosis. These processes, in turn, influence the function of hepatocytes, hepatic stellate cells, and macrophages, thereby contributing to the development of hepatic steatosis and fibrosis. Additionally, some SLC transporters are involved in drug transport, acting as critical regulators of drug-induced hepatic steatosis. Beyond substrate transport, certain SLC members also exhibit additional functions. Given the pivotal role of the SLC family in hepatic steatosis and fibrosis, this review aimed to summarize the molecular mechanisms through which SLC transporters influence these conditions.

Manganese exposure leads to depressive-like behavior through disruption of the Gln-Glu-GABA metabolic cycle

There is a correlation between long-term manganese (Mn) exposure and the Parkinson's-like disease (PD), with depression as an early symptom of PD. However, the direct relationship between Mn exposure and depression, and the mechanisms involved, remain unclear. We found that Mn exposure led to depressive-like behavior and mild cognitive impairment in mice, with Mn primarily accumulating in the cornu ammonis 3 (CA3) area of the hippocampus. Mice displayed a reduction in neuronal dendritic spines and damage to astrocytes specifically in the CA3 area. Spatial metabolomics revealed that Mn downregulated glutamic acid decarboxylase 1 (GAD1) expression in astrocytes, disrupting the Glutamine-Glutamate-γ-aminobutyric acid (GlnGluGABA) metabolic cycle in the hippocampus, leading to neurotoxicity. We established an in vitro astrocyte Gad1 overexpression (OEX) model and found that the cultured medium from Gad1 OEX astrocytes reversed neuronal synaptic damage and the expression of gamma-aminobutyric acid (GABA) related receptors. Using the astrocyte Gad1 OEX mouse model, results showed that OEX of Gad1 ameliorated depressive-like behavior and cognitive dysfunction in mice. These findings provide new insight into the important role of GAD1 mediated GlnGluGABA metabolism disorder in Mn exposure induced depressive-like behavior. This study offers a novel sight to understanding abnormal emotional states following central nervous system damage induced by Mn exposure.

ZIP8 Is Upregulated in the Testis of Zip14-/- Mice

BACKGROUND/OBJECTIVES

Manganese is an essential nutrient involved in various biological processes, including reproductive health, yet the mechanisms regulating its homeostasis in the testis remain poorly understood.

METHODS AND RESULTS

In this study, we investigated the expression and regulation of key manganese transporters-ZIP8, ZIP14, and ZnT10-in mouse testes. Immunoblotting analyses revealed that ZIP8 is expressed in the testes, while ZIP14 and ZnT10 were undetectable. Using Zip14 knockout (Zip14-/-) mice, which exhibit systemic manganese overload, we discovered a significant increase in manganese levels in the testis, accompanied by an upregulation of ZIP8. Importantly, the levels of other essential metals, such as iron, zinc, and copper, remained unchanged.

CONCLUSIONS

Our findings suggest that ZIP8 plays a critical role in manganese transport in the testis, and its increased expression may contribute to manganese accumulation in the absence of ZIP14. This study advances our understanding of manganese homeostasis in the testis and its potential impact on male reproductive health.

Expression of Manganese Transporters ZIP8, ZIP14, and ZnT10 in Brain Barrier Tissues

Manganese (Mn) is an essential trace mineral for brain function, but excessive accumulation can cause irreversible nervous system damage, highlighting the need for proper Mn balance. ZIP14, ZnT10, and ZIP8 are key transporters involved in maintaining Mn homeostasis, particularly in the absorption and excretion of Mn in the intestine and liver. However, their roles in the brain are less understood. The blood-cerebrospinal fluid barrier and the blood-brain barrier, formed by the choroid plexus and brain blood vessels, respectively, are critical for brain protection and brain metal homeostasis. This study identified ZIP14 on the choroid plexus epithelium, and ZIP8 and ZnT10 in brain microvascular tissue. We show that despite significant Mn accumulation in the CSF of Znt10 knockout mice, ZIP14 expression levels in the blood-cerebrospinal fluid barrier remain unchanged, indicating that ZIP14 does not have a compensatory mechanism for regulating Mn uptake in the brain in vivo. Additionally, Mn still enters the CSF without ZIP14 when systemic levels rise. This indicates that alternative transport mechanisms or compensatory pathways ensure Mn balance in the CSF, shedding light on potential strategies for managing Mn-related disorders.

Hepatic HIF2 is a key determinant of manganese excess and polycythemia in SLC30A10 deficiency

Manganese is an essential yet potentially toxic metal. Initially reported in 2012, mutations in SLC30A10 are the first known inherited cause of manganese excess. SLC30A10 is an apical membrane protein that exports manganese from hepatocytes into bile and from enterocytes into the lumen of the gastrointestinal tract. SLC30A10 deficiency results in impaired gastrointestinal manganese excretion, leading to manganese excess, neurologic deficits, liver cirrhosis, polycythemia, and erythropoietin excess. Neurologic and liver disease are attributed to manganese toxicity. Polycythemia is attributed to erythropoietin excess. The goal of this study was to determine the basis of erythropoietin excess in SLC30A10 deficiency. Here, we demonstrate that transcription factors hypoxia-inducible factor 1a (Hif1a) and 2a (Hif2a), key mediators of the cellular response to hypoxia, are both upregulated in livers of Slc30a10-deficient mice. Hepatic Hif2a deficiency corrected erythropoietin expression and polycythemia and attenuated aberrant hepatic gene expression in Slc30a10-deficient mice, while hepatic Hif1a deficiency had no discernible impact. Hepatic Hif2a deficiency also attenuated manganese excess, though the underlying cause of this is not clear at this time. Overall, our results indicate that hepatic HIF2 is a key determinant of pathophysiology in SLC30A10 deficiency and expand our understanding of the contribution of HIFs to human disease.

MnO2 nanoparticles trigger hepatic lipotoxicity and mitophagy via mtROS-dependent Hsf1Ser326 phosphorylation

Manganese (Mn) is an essential element for maintaining normal metabolism in vertebrates. Mn dioxide nanoparticles (MnO2 NPs), a novel Mn source, have shown great potentials in biological and biomedical applications due to their distinct physical and chemical properties. However, little is known about potential adverse effects on animal or cellular metabolism. Here, we investigated whether and how dietary MnO2 NPs affect hepatic lipid metabolism in vertebrates. We found that, excessive MnO2 NPs intake increased hepatic and mitochondrial Mn content, promoted hepatic lipotoxic disease and lipogenesis, and inhibited hepatic lipolysis and fatty acid β-oxidation. Moreover, excessive MnO2 NPs intake induced hepatic mitochondrial oxidative stress, damaged mitochondrial function, disrupted mitochondrial dynamics and activated mitophagy. Importantly, we uncovered that mtROS-activated phosphorylation of heat shock factor 1 (Hsf1) at Ser326 residue mediated MnO2 NPs-induced hepatic lipotoxic disease and mitophagy. Mechanistically, MnO2 NPs-induced lipotoxicity and mitophagy were via mtROS-induced phosphorylation and nucleus translocation of Hsf1 and its DNA binding capacity to plin2/dgat1 and bnip3 promoters, respectively. Overall, our findings uncover novel mechanisms by which mtROS-mediated mitochondrial dysfunction and phosphorylation of Hsf1S326 contribute to MnO2 NPs-induced hepatic lipotoxicity and mitophagy, which provide new insights into the effects of metal oxides nanoparticles on hepatotoxicity in vertebrates.

Absence of Slc39a14/Zip14 in mouse pancreatic beta cells results in hyperinsulinemia

Zinc is an essential component of the insulin protein complex synthesized in β cells. The intracellular compartmentalization and distribution of zinc are controlled by 24 transmembrane zinc transporters belonging to the ZnT or Zrt/Irt-like protein (ZIP) family. Downregulation of SLC39A14/ZIP14 has been reported in pancreatic islets of patients with type 2 diabetes (T2D) as well as mouse models of high-fat diet (HFD)- or db/db-induced obesity. Our previous studies observed mild hyperinsulinemia in mice with whole body knockout of Slc39a14 (Zip14 KO). Based on our current secondary data analysis from an integrative single-cell RNA-seq dataset of human whole pancreatic tissue, SLC39A14 (coding ZIP14) is the only other zinc transporter expressed abundantly in human β cells besides well-known zinc transporter SLC30A8 (coding ZnT8). In the present work, using pancreatic β cell-specific knockout of Slc39a14 (β-Zip14 KO), we investigated the role of SLC39A14/ZIP14-mediated intracellular zinc trafficking in glucose-stimulated insulin secretion and subsequent metabolic responses. Glucose-stimulated insulin secretion, zinc concentrations, and cellular localization of ZIP14 were assessed using in vivo, ex vivo, and in vitro assays using β-Zip14 KO, isolated islets, and murine cell line MIN6. Metabolic evaluations were done on both chow- and HFD-fed mice using time-domain nuclear magnetic resonance and a comprehensive laboratory animal monitoring system. ZIP14 localizes on the endoplasmic reticulum regulating intracellular zinc trafficking in β cells and serves as a negative regulator of glucose-stimulated insulin secretion. Deletion of Zip14 resulted in greater glucose-stimulated insulin secretion, increased energy expenditure, and shifted energy metabolism toward fatty acid utilization. HFD caused β-Zip14 KO mice to develop greater islet hyperplasia, compensatory hyperinsulinemia, and mild insulin resistance and hyperglycemia. This study provided new insights into the contribution of metal transporter ZIP14-mediated intracellular zinc trafficking in glucose-stimulated insulin secretion and subsequent metabolic responses.NEW & NOTEWORTHY Metal transporter SLC39A14/ZIP14 is downregulated in pancreatic islets of patients with T2D and mouse models of HFD- or db/db-induced obesity. However, the function of ZIP14-mediated intracellular zinc trafficking in β cells is unknown. Our analyses revealed that SLC39A14 is the only Zn transporter expressed abundantly in human β cells besides SLC30A8. Within the β cells, ZIP14 is localized on the endoplasmic reticulum and serves as a negative regulator of insulin secretion, providing a potential therapeutic target for T2D.

Hepatic and intestinal manganese excretion are both required to regulate brain manganese during elevated manganese exposure

Manganese (Mn) is essential but neurotoxic at elevated levels. Under physiological conditions, Mn is primarily excreted by the liver, with the intestines playing a secondary role. Recent analyses of tissue-specific Slc30a10 or Slc39a14 knockout mice (SLC30A10 and SLC39A14 are Mn transporters) revealed that, under physiological conditions: 1) excretion of Mn by the liver and intestines is a major pathway that regulates brain Mn; and surprisingly, 2) the intestines compensate for loss of hepatic Mn excretion in controlling brain Mn. The unexpected importance of the intestines in controlling physiological brain Mn led us to determine the role of hepatic and intestinal Mn excretion in regulating brain Mn during elevated Mn exposure. We used liver- or intestine-specific Slc30a10 knockout mice as models to inhibit hepatic or intestinal Mn excretion. Compared with littermates, both knockout strains exhibited similar increases in brain Mn after elevated Mn exposure in early or later life. Thus, unlike physiological conditions, both hepatic and intestinal Mn excretion are required to control brain Mn during elevated Mn exposure. However, brain Mn levels of littermates and both knockout strains exposed to elevated Mn only in early life were normalized in later life. Thus, hepatic and intestinal Mn excretion play compensatory roles in clearing brain Mn accumulated by early life Mn exposure. Finally, neuromotor assays provided evidence consistent with a role for hepatic and intestinal Mn excretion in functionally modulating Mn neurotoxicity during Mn exposure. Put together, these findings substantially enhance understanding of the regulation of brain Mn by excretion.NEW & NOTEWORTHY This article shows that, in contrast with expectations from prior studies and physiological conditions, excretion of manganese by the intestines and liver is equally important in controlling brain manganese during human-relevant manganese exposure. The results provide foundational insights about the interorgan mechanisms that control brain manganese homeostasis at the organism level and have important implications for the development of therapeutics to treat manganese-induced neurological disease.

Hypoxia-inducible factor 2 is a key determinant of manganese excess and polycythemia in SLC30A10 deficiency

Manganese is an essential yet potentially toxic metal. Initially reported in 2012, mutations in SLC30A10 are the first known inherited cause of manganese excess. SLC30A10 is an apical membrane transport protein that exports manganese from hepatocytes into bile and from enterocytes into the lumen of the gastrointestinal tract. SLC30A10 deficiency results in impaired gastrointestinal manganese excretion, leading to severe manganese excess, neurologic deficits, liver cirrhosis, polycythemia, and erythropoietin excess. Neurologic and liver disease are attributed to manganese toxicity. Polycythemia is attributed to erythropoietin excess, but the basis of erythropoietin excess in SLC30A10 deficiency has yet to be established. Here we demonstrate that erythropoietin expression is increased in liver but decreased in kidneys in Slc30a10-deficient mice. Using pharmacologic and genetic approaches, we show that liver expression of hypoxia-inducible factor 2 (Hif2), a transcription factor that mediates the cellular response to hypoxia, is essential for erythropoietin excess and polycythemia in Slc30a10-deficient mice, while hypoxia-inducible factor 1 (HIF1) plays no discernible role. RNA-seq analysis determined that Slc30a10-deficient livers exhibit aberrant expression of a large number of genes, most of which align with cell cycle and metabolic processes, while hepatic Hif2 deficiency attenuates differential expression of half of these genes in mutant mice. One such gene downregulated in Slc30a10-deficient mice in a Hif2-dependent manner is hepcidin, a hormonal inhibitor of dietary iron absorption. Our analyses indicate that hepcidin downregulation serves to increase iron absorption to meet the demands of erythropoiesis driven by erythropoietin excess. Finally, we also observed that hepatic Hif2 deficiency attenuates tissue manganese excess, although the underlying cause of this observation is not clear at this time. Overall, our results indicate that HIF2 is a key determinant of pathophysiology in SLC30A10 deficiency.

Recent progress toward understanding the role of ZIP14 in regulating systemic manganese homeostasis

ZIP14 is a metal transporter essential for the regulation of body manganese homeostasis. The physiological functions of ZIP14 have been uncovered mainly through two lines of in vivo studies that examined the phenotypes of ZIP14 loss, including studies of humans with ZIP14 mutations and animals with ZIP14 deficiency. This mini review aims at presenting an updated view of the important advances made towards understanding the genetic and pathological mechanisms of brain manganese overload caused by ZIP14 deficiency.