SLC39A13 Regulates Heart Function via Mitochondrial Iron Homeostasis Maintenance.

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 (ZIP13), 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. 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. 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. 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.