Transferrin (TF) is a bilobed 80kD glycoprotein with N- and C-lobe iron binding sites. TF circulates as four forms: unbound to iron (apo-TF), iron bound to the N-lobe (monoferric N-TF), the C-lobe (monoferric-C), or to both lobes (diferric-TF). Most circulating TF under physiological conditions is monoferric. The iron-bound TF forms interact with TF receptor-1 (TFR1), which is ubiquitously expressed and serves as the main mechanism for cellular iron delivery. Iron-bound TF also interacts with TF receptor-2 (TFR2) which is expressed on hepatocytes, erythroblasts, and bone cells. Whereas TFR1 serves primarily as a cargo receptor, TFR2 serves primarily to influence cellular signaling events regulating hepcidin expression, erythropoiesis, and bone formation. We proposed that different transferrin forms provide differential signaling properties in this regulation. We thus generated TF mutant mice in which all iron-containing TF was either monoferric N (Tf monoN) or monoferric C (Tf monoC). Compared with Tf monoC mice, the Tf monoN mice demonstrated increased RBC production and increased hepcidin expression relative to iron status (Parrow et al. Blood). Based on observations in β-thalassemic mice treated with exogenous TF (Li et al. Nat Med), we hypothesized that β-thalassemic mice obligate for monoN TF would demonstrate improved erythropoietic and iron parameters compared with β-thalassemic mice obligate for monoC TF.

To address this hypothesis, we crossed Hbb th3/+ mice (a mouse model of β-thalassemia intermedia) with Tf monoN and Tf monoC mice. Compared with Hbb th3Tf +/+mice, in Hbb th3/+Tf monoN mice demonstrated significantly increased RBC counts, elevated hemoglobin, improved erythrocyte morphology (Figure 1A-B), decreased splenomegaly, fewer bone marrow erythroblasts, and improvement of ineffective erythropoiesis (as measured by the ratio of progenitors to RBC in the bone marrow). Additionally, serum ERFE was significantly reduced and hepcidin levels were increased in Hbb th3/+Tf monoN relative to Hbb th3/+Tf +/+controls. Conversely, hematological parameters from Hbb th3/+Tf monoC mice were comparable to Hbb th3/+Tf +/+ mice. Similarly, Hbb th3/+Tf monoCmice had no improvements in markers of ineffective erythropoiesis in the bone marrow compared with Hbb th3/+Tf +/+ mice.

In summary, we demonstrate that the differential regulatory effects of monoN and monoC TF on erythropoiesis are relevant not only in steady-state, but also in the ineffective erythropoiesis that is characteristic of β-thalassemia. Because both monoN and monoC TF forms can deliver only one iron atom per TF-TFR1 binding event, our findings suggest that the improvements observed only in the Hbb th3/+Tf monoN mice were not due to iron restriction alone. We are now elucidating the mechanisms by which the two TF lobes exert their differential effects on ineffective erythropoiesis and exploring the translational potential of obligate monoN TF in the treatment of β-thalassemia.

Figure 1
Figure 1.
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Disclosures

Rivella: Ionis Pharmaceuticals: Consultancy; Meira GTx: Consultancy.

© 2021 by The American Society of Hematology
2021
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