Maternal regulation of iron metabolism resolved? Free

In this issue of Blood, Fielding and colleagues demonstrate that TMPRSS6 may contribute to the regulation of maternal iron homeostasis during pregnancy.¹ This finding not only improves our understanding of the basic mechanisms of iron biology, but also confirms the central role of maternal hepcidin in determining the placental and fetal iron status necessary to support fetal development.

In healthy adults, the daily absorption of iron from diet is approximately 1 to 2 mg, which is necessary to compensate for daily losses. The baseline iron requirements in women of reproductive age exceed those of men due to menstruation. However, these requirements can increase up to 7 mg a day during the third trimester of pregnancy to sustain maternal red blood cell production, as well as the placental and fetal iron needs.^2,3 Dietary iron absorption is controlled by the liver-produced hormone hepcidin, which binds to, occludes, and degrades ferroportin, the only known cellular exporter of iron from enterocytes and macrophages within the reticuloendothelial system.⁴ In healthy human pregnancies, the maternal production of hepcidin is repressed during the second and third trimesters by an as yet unidentified “pregnancy specific regulator of hepcidin” that facilitates the release of iron into the circulation.³ Failure to repress hepcidin, or excessive hepcidin production due to inflammation, can result in maternal iron restriction and anemia, lower placental and embryo weight, embryo anemia, an increased risk of neurobehavioral impairments, and embryo mortality.

The expression of hepcidin is primarily regulated by the bone morphogenetic protein sons of mother against decapentaplegic (BMP-SMAD) pathway through the influence of the ligands BMP2 and 6, as well as a number of coreceptors and accessory proteins.⁵ Among these, transmembrane serine protease 6 (TMPRSS6) is a membrane protease that represses hepcidin by cleaving the BMP-coreceptor hemojuvelin, although recent evidence suggests that proteolytic activity may be dispensable for hepcidin regulation.⁶ In human, mutations in the TMPRSS6 gene are associated with elevated hepcidin levels causing iron-refractory iron-deficiency anemia. Tmprss6^−/− dams are infertile, and heterozygous animals are usually bred to obtain Tmprss6^−/− embryos that present with high hepcidin, iron deficiency, and anemia at embryonic day E17.5; however, no sign of embryonic lethality was reported.^6,7 Fielding and colleagues therefore investigated the iron-regulatory function of TMPRSS6 during pregnancy. As Tmprss6 is primarily expressed by hepatocytes, the investigators circumvented the infertility of the Tmprss6^−/− mice by using a potent N-acetylgalactosamine-conjugated silencing RNA that, through its binding to the asialoglycoprotein receptor, is effectively endocytosed in hepatocytes. As RNA interference showed a suppressive effect lasting up to 21 days, dams were administered a Tmprss6 or control small interfering RNA (siRNA) a day prior to the mating.

In wild-type mice, a downregulation of hepcidin, comparable to that occurring in pregnant women, was observed between embryonic days E8.5 and E10.5 without any obvious change in hepatic mRNA expression of Tmprss6 and BMP-target genes Id1, Smad7, and Atoh8. The siRNA-mediated inhibition of Tmprss6 resulted in increased hepcidin levels, iron retention in the spleen, iron deficiency and anemia, decreased placental and embryo iron levels, and reduced embryo weight in comparison with dams treated with the control siRNA. These findings are consistent with those reported in pregnant mice that were administered exogenous hepcidin mimetics,⁸ thus confirming that inappropriately high hepcidin has adverse pregnancy outcomes.

Interestingly, similarly to dams treated with the control siRNA, the induction in hepcidin transcription caused by the silencing of Tmprss6 was mildly reduced at embryonic day 14.5, suggesting that hepcidin can still be somewhat repressed during pregnancy even in absence of Tmprss6. Therefore, although spleen iron was elevated by the silencing of Tmprss6, the residual reduction in hepcidin may explain the mobilization of iron for the hepatic stores. Notably, supraphysiological hepcidin production was also preserved in Tmprss6^−/− mice subjected to erythropoietin treatment and erythroid expansion.⁹ Hepcidin regulators and BMP antagonists may thus exert little control over hepcidin transcription and the BMP-SMAD signaling when the pathway is constitutively activated upon Tmprss6 inhibition. Testing this hypothesis, the authors further found that maternal hepcidin was still suppressed in wild-type dams when fed an iron-enriched or an iron-deficient diet, indicating that maternal hepcidin regulation is maintained when the pathway is inhibited or mildly stimulated. Of note, in these experimental settings, liver iron, liver and plasma hepcidin levels, and hepatic expression of BMP target genes were not significantly increased in mice fed a high-iron diet, suggesting that the activation of the BMP-SMAD pathway was superior when Tmprss6 was silenced. It remains to be tested whether a comparable induction of the pathway could effectively blunt the hepcidin response during pregnancy in the presence of TMPRSS6. Although a modulation of the BMP-SMAD signaling would be anticipated if TMPRSS6 were the pregnancy regulator of hepcidin, the pathway was also marginally affected in murine models of anemia induced by hemorrhage or after the administration of a single dose of erythropoietin.¹⁰ As it is now well established that erythroferrone antagonizes the BMP-SMAD signaling in these settings, it is plausible that the expression of the BMP target genes may be rapidly restored when hepcidin downregulation increases iron uptake in the plasma.

In summary, the present study demonstrates that maternal TMPRSS6 is necessary to ensure the adequate regulation of maternal hepcidin and iron status. It also confirms that an increase in maternal hepcidin has a deleterious effect on fetal iron status and fetal development. The confirmation of the critical role of TMPRSS6 in regulating maternal hepcidin will require the development of specific tools to assess the activation of TMPRSS6 protein in vivo. Further research is needed to identify the pregnancy specific regulator of hepcidin.

Conflict-of-interest disclosure: L.K. declares no competing financial interests.