Transferrin Receptors TfR1 and TfR2 Bind Transferrin through Differing Mechanisms

Abstract

Hereditary hemochromatosis (HH), a disease marked by chronic iron overload from insufficient expression of the hormone hepcidin, is one of the most common genetic diseases. One form of HH (Type III) results from mutations in the transferrin receptor-2 (TfR2). TfR2 is postulated to be a part of signaling system that is capable of modulating hepcidin expression. The molecular details of TfR2’s role in this system remain unclear, however. TfR2 is predicted to bind the iron carrier transferrin (Tf) when the iron-saturation of Tf is high. To better understand the nature of these TfR-Tf interactions, a binding study with the full-length receptors was conducted. In agreement with previous studies with truncated forms of the receptors, holo-Tf binds to the homolog, TfR1, significantly stronger than TfR2. However, the binding constant for Tf-TfR2 is still far above that of physiological holo-Tf levels, inconsistent with the hypothetical model and suggests that other factors mediate the interaction. One possible factor, Apo-Tf, only weakly binds TfR2 at serum pH, thus will not be able to effectively compete with holo-Tf. Tf-binding to a TfR2 chimera containing the TfR1-helical domain indicates that the differences in the helical domain account for differences in on-rate of Tf, and non-conserved inter-receptor interactions are necessary for the stabilization of the complex. Conserved residues at one possible site of stabilization, the apical arm junction, are not important for TfR1-Tf binding, but are critical for the TfR2-Tf interaction. Our results highlight the differences in Tf interactions with the two TfRs.

Identification and in vivo characterization of a brain-penetrating nanobody

“…There are several explanations possible why most of the anti-mTfR nanobodies do not cross in vivo. For instance their epitopes might be shielded in vivo by transferrin which has a micromolar plasma concentration and a nanomolar affinity for its receptor [57]. It is known that binding to the apical domain of the TfR can lead to brain uptake of monoclonal antibodies [31].…”

Biological Activity of the Carrier as a Factor in Immunogen Design for Haptens

“…So, these motifs can be introduced to the carriers of vaccines against the hapten (for instance opioid drugs), and facilitate the development of more efficient and safer vaccines. Recently, some work has focused on different TFR1 and TFR2 and their differential substrate specificity [ 16 ]. So, while different receptors of the same species may have differential ligand specifities, it must be noted that a careful selection of TFR-binding motifs or carriers is required, since TFR of different species may have differential binding patterns.…”

“…Iron-free TF (apo-TF) has an affinity to TFR at this pH. Once the TFR is recycled and returns to the cell membrane, apo-TF is released to the extracellular matrix [ 15 , 16 ]. Since this cycle is vital for the cell, TFR is used as a proliferation marker for the cells where highly proliferating cells express more TFR, and serves as a target for therapy.…”

Regulation of Iron Homeostasis and Related Diseases

“…As the iron concentration in the blood, it is sensed by the liver through the Tf-Fe competing with HFE binding to TfR (TfR1/TfR2) on the hepatocyte cytomembrane [63]. The difference in the ability of Tf and HFE to bind to TfR transmits a signal of concentration of the blood iron in hepatocyte [64]. The capability of Tf-Fe combined with TfR1 is stronger than HFE, and Tf-Fe combined with TfR1 is far stronger than TfR2 [65].…”