Bing Zhou

Liver fibrosis, driven by excessive collagen synthesis following hepatic injury, poses a significant health challenge. SLC39A13/ZIP13, a recently characterized intracellular iron transporter, is shown to provide iron to the ER/Golgi to help catalyze procollagen hydroxylation during collagen maturation. Here, we investigate whether ZIP13 plays a role during hepatic fibrogenesis modeled by CCl ₄ stress or other inducers. ZIP13 expression is induced during liver fibrosis. Germline disruption of Zip13 dramatically reduces fibrosis. Surprisingly, these mice do not benefit from ZIP13 loss after CCl ₄ challenge; instead, they are more susceptible to CCl ₄ toxicity even with substantially less fibrosis development. This elevated vulnerability turns out to be a consequence of ferroptosis in the hepatocyte due to increased cytosolic iron after ZIP13 loss. Tissue‐specific knockout (KO) reveals that hepatic stellate cell (HSC) KO of Zip13 attenuates liver fibrosis progression without adverse effects. Leveraging these findings, HSC‐targeted delivery of Zip13 ‐siRNA demonstrates robust efficacy and safety in preclinical fibrosis models. These results provide critical insights into the complex role of iron in liver fibrosis, and indicate that targeting iron homeostasis via ZIP13 in the HSC may be effective to mitigate fibrogenesis by simultaneously suppressing the synthesis of multiple kinds of collagen while minimizing possible side effects.

BACKGROUND: Iron is a necessary trace element for multiple reactions but is toxic in excess. Its intracellular balance is delicately maintained. We previously found that the loss of SLC39A13 (solute carrier family 39 member 13)/ZIP13 (zinc-iron permease 13), a newly identified endoplasmic reticulum/Golgi–resident iron transporter, impacted iron homeostasis in multiple tissues. The purpose of this study is to investigate the role of ZIP13 in regulating cardiac functions and the precise mechanism of cardiac injury caused by ZIP13 deficiency. METHODS: Cardio-specific knockout of Zip13 ( Zip13-CKO ), tamoxifen-inducible Zip13 knockout ( Zip13-iKO ), and systemic (germline) Zip13 knockout mouse model were used to study the effect of Zip13 deletion on cardiac functions. These mice were analyzed for growth, cardiac systolic function, mitochondrial morphology, mitochondrial iron metabolism, and mitochondrial biogenesis and activity. We also generated cardio-specific ferroportin 1 ( Fpn1-CKO ) and Zip13&Fpn1 ( Zip13&Fpn1-CKO ) double-knockout mice to compare with Zip13-CKO mice. Mouse embryonic fibroblasts and primary cardiomyocytes were used for in vitro experiments. RESULTS: Zip13-CKO mice displayed severe cardiac systolic dysfunctions. The mitochondrial function and morphology were markedly abnormal in Zip13-CKO cardiomyocytes, accompanied by cytosolic iron increase and mitochondrial iron decrease. These were also confirmed in vitro with mouse embryonic fibroblasts and primary cardiomyocytes. Moreover, iron supplementation or overexpressing MFRN1 (mitoferrin 1), a mitochondrial iron importer, could substantially restore the mitochondrial iron homeostasis and function of ZIP13-deficient primary cardiomyocytes, indicating mitochondrial iron dyshomeostasis underlies the observed cardiac abnormality. The Zip13-CKO did not wholly resemble that of Fpn1-CKO , which was associated with elevated cytosolic iron, but no statistically significant change was observed in mitochondrial iron. Zip13&Fpn1-CKO mice presented a more severe heart defect than either single mutant alone, likely due to a further aggravated iron accumulation in the cytosol of cardiomyocytes. CONCLUSIONS: We propose that ZIP13 and FPN1 are both required to maintain cardiac functions via overlapping but different manners; FPN1 maintains the cytosolic iron by exporting iron out of the cells, while ZIP13 helps balance the iron equilibrium between the cytosol and the organellar network system, including the mitochondrion. These findings establish the critical role of ZIP13 in maintaining mitochondrial iron homeostasis and activity, enabling cardiomyocytes to perform effectively their essential roles.

Iron is a potent biochemical, and accurate homeostatic control is orchestrated by a network of interacting players at multiple levels. Although our understanding of organismal iron homeostasis has advanced, intracellular iron homeostasis is poorly understood, including coordination between organelles and iron export into the ER/Golgi. Here, we show that SLC39A13 (ZIP13), previously identified as a zinc transporter, promotes intracellular iron transport and reduces intracellular iron levels. ZIP13 loss causes an iron deficiency in the ER/Golgi and other intracellular compartments, such as lysosomes and mitochondria, as well as elevating iron in the cytosol. ZIP13 overexpression has the opposite effect, increasing iron in organellar compartments. We suggest that ZIP13 gatekeeps an iron trafficking route that shunts iron from the cytosol to the ER/Golgi hub. Zip13-knockout male mice have iron deposition in several tissues. These data demonstrate that mammalian ZIP13 is crucial for iron homeostasis and suggest a potential iron transport function.

SLC30A9 (ZnT9) is a mitochondria-resident zinc transporter. Mutations in SLC30A9 have been reported in human patients with a novel cerebro-renal syndrome. Here, we show that ZnT9 is an evolutionarily highly conserved protein, with many regions extremely preserved among evolutionarily distant organisms. In Drosophila melanogaster (the fly), ZnT9 (ZnT49B) knockdown results in acutely impaired movement and drastic mitochondrial deformation. Severe Drosophila ZnT9 (dZnT9) reduction and ZnT9-null mutant flies are pupal lethal. The phenotype of dZnT9 knockdown can be partially rescued by mouse ZnT9 expression or zinc chelator TPEN, indicating the defect of dZnT9 loss is indeed a result of zinc dyshomeostasis. Interestingly, in the mouse, germline loss of Znt9 produces even more extreme phenotypes: the mutant embryos exhibit midgestational lethality with severe development abnormalities. Targeted mutagenesis of Znt9 in the mouse brain leads to serious dwarfism and physical incapacitation, followed by death shortly. Strikingly, the GH/IGF-1 signals are almost non-existent in these tissue-specific knockout mice, consistent with the medical finding in some human patients with severe mitochondrial deficiecny. ZnT9 mutations cause mitochondrial zinc dyshomeostasis, and we demonstrate mechanistically that mitochondrial zinc elevation quickly and potently inhibits the activities of respiration complexes. These results reveal the critical role of ZnT9 and mitochondrial zinc homeostasis in mammalian development. Based on our functional analyses, we finally discussed the possible nature of the so far identified human SLC30A9 mutations.

Background: The rapid reproduction of malaria parasites requires proper iron uptake. However, the process of iron absorption by parasites is rarely studied. Divalent metal transporter (DMT1) is a critical iron transporter responsible for uptaking iron. A homolog of human DMT1 exists in the malaria parasite genome, which in Plasmodium yoelii is hereafter named PyDMT1. Results: PyDMT1 knockout appears to be lethal. Surprisingly, despite dwelling in an iron-rich environment, the parasite cannot afford to lose even partial expression of PyDMT1; PyDMT1 hypomorphs were associated with severe growth defects and quick loss of pathogenicity. Iron supplementation could completely suppress the defect of the PyDMT1 hypomorph during in vitro culturing. Genetic manipulation through host ferritin (Fth1) knockout to increase intracellular iron levels enforced significant growth inhibition in vivo on the normal parasites but not the mutant. In vitro culturing with isolated ferritin knockout mouse erythrocytes completely rescued PyDMT1-hypomorph parasites. Conclusion: A critical iron requirement of malaria parasites at the blood stage as mediated by this newly identified iron importer PyDMT1, and the iron homeostasis in malarial parasites is finely tuned. Tipping the iron balance between the parasite and host will efficiently kill the pathogenicity of the parasite. Lastly, PyDMT1 hypomorph parasites were less sensitive to the action of artemisinin.