Tmprss6, An Inhibitor of Hepatic Bmp/Smad Signaling, Is Required for Hepcidin Suppression and Iron Loading In a Mouse Model of β-Thalassemia

Abstract 164

TMPRSS6, a transmembrane protease produced by the liver, is an essential regulator of mammalian iron homeostasis. TMPRSS6 inhibits the expression of hepcidin, a circulating peptide that decreases intestinal iron absorption and macrophage iron release, by down-regulating hepatic BMP/SMAD signaling for hepcidin production. Accordingly, TMPRSS6 mutations result in elevated hepcidin levels, impaired absorption of dietary iron, and systemic iron deficiency. Interestingly, in congenital iron loading anemias such as β-thalassemia, hepcidin levels are inappropriately low relative to body iron stores, a finding that has been postulated to result from the production of a hepcidin-repressing factor in the setting of ineffective erythropoiesis. Here we asked if Tmprss6 is required to achieve the hepcidin suppression present in Hbb^th3/+ mice, a model of β-thalassemia intermedia. To test this, we bred Hbb^th3/+ mice to mice harboring a targeted disruption of the Tmprss6 serine protease domain. We generated mice of various Hbb-Tmprss6 genotype combinations and compared parameters of systemic iron homeostasis at 8 weeks of age. Consistent with prior studies of Hbb^th3/+ mice, Hbb^th3/+ mice harboring 2 wild-type Tmprss6 alleles (Hbb^th3/+Tmprss6^+/+ mice) showed non-heme iron concentrations of liver, spleen, and kidney that were significantly elevated compared to wild-type controls. Homozygosity for Tmprss6 mutation, however, ameliorated the iron overload phenotype of Hbb^th3/+ mice and led to systemic iron deficiency. Tissue non-heme iron concentrations were markedly reduced in Hbb^th3/+Tmprss6^−/− mice as compared to Hbb^th3/+Tmprss6^+/+ mice and were similar to levels observed in Tmprss6^−/− mice harboring 2 wild-type Hbb alleles. Hbb^th3/+Tmprss6^−/− mice had hemoglobin levels similar to the thalassemic levels present in Hbb^th3/+Tmprss6^+/+ mice. However, compared to Hbb^th3/+Tmprss6^+/+ mice, Hbb^th3/+Tmprss6^−/− mice showed markedly reduced erythrocyte mean corpuscular volume and serum transferrin saturation, as well as increased red blood cell count. Interestingly, homozygous loss of Tmprss6 in Hbb^th3/+ mice also led to marked reduction in splenomegaly and improvement in peripheral red blood cell morphology. Consistent with prior studies of Hbb^th3/+ mice, Hbb^th3/+Tmprss6^+/+ mice displayed hepatic hepcidin mRNA levels that were similar to wild-type and were, therefore, inappropriately decreased relative to their increased hepatic iron stores. Hepatic mRNA levels of Bmp6, encoding a Bmp ligand that is transcriptionally regulated by iron and acts as a key regulator of hepcidin expression in vivo, were significantly elevated in Hbb^th3/+Tmprss6^+/+ mice, suggesting that their relative hepcidin deficiency does not result from impaired Bmp6 transcription. While Hbb^th3/+Tmprss6^+/+ mice showed suppressed hepcidin levels, homozygous loss of Tmprss6 alleviated their hepcidin suppression and led to elevated hepcidin mRNA levels similar to Tmprss6^−/− mice harboring 2 wild-type Hbb alleles. Hbb^th3/+Tmprss6^−/− mice also showed elevated hepatic mRNA encoding Id1, another transcriptional target of Bmp/Smad signaling. These findings indicate that Tmprss6 is required to achieve the suppression of hepatic hepcidin expression that underlies systemic iron overload in Hbb^th3/+ mice. Furthermore, our results demonstrate that, by up-regulating hepatic Bmp/Smad signaling for hepcidin production, genetic loss of Tmprss6 induces profound changes in systemic iron homeostasis in this thalassemia model.