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The critical function of Tfr1 in hematopoiesis
that downregulation of liver hepcidin expression could be attributed to iron overload resulted from depletion of Bmp2 or Bmp6 in LSEC. On the contrary, our Tfr1fl/fl;Vav- Cre mice had increased levels of liver and serum hepcidin. Interestingly, Stat5a/bf/f;TC (Tie2-Cre driven Stat5a/b gene deletion in HSC and endothelial cells) mutant mice dis- played a 50% reduction of Tfr1 mRNA and protein expres- sion, higher liver and serum iron levels, and elevated trans- ferrin saturation,40 which phenotypes are similar to those observed in our Tfr1fl/fl;Vav-Cre mice. Therefore, it is possi- ble that serum iron and organ iron could be attributed to concomitant deletion of Tfr1 in both hematopoietic and endothelial cells.
Interestingly, we found that iron storage is similar between cKO embryos and control embryos, indicating that the maternal transfer of iron is unaffected by the loss of Tfr1 in the embryo’s HSC. Previous studies showed that neonatal iron status is significantly correlated with three placental heme iron transporters41 and that newborn absorb a significantly higher fraction of heme iron com- pared to non-heme iron.42 In this respect, it is interesting to note that our in vitro colony-forming assay revealed that heme may serve as an alternative source of iron during early embryonic development.
Given that intracellular iron homeostasis is controlled by a complex molecular network, it is possible that trans- porters of non-transferrin bound iron43-46 may also play a role in hematopoiesis. Several studies revealed compelling evidence of other iron transporters in hematopoiesis. For example, the putative heme transporter FLVCR (group C feline leukemia virus receptor) did not appear to be essen- tial for HSC function.47 Slc39a14-/- mice had no effect on hematological parameters,48 either. In addition, Slc11a2-/- mice only showed impaired erythropoiesis.49 While, Tfr2-deficient mice had an increased red blood cell count and terminal erythropoiesis in the BM.50 In general, none
of the aforementioned knockout mouse models present with hematopoietic impairments as severe as our HSC- specific Tfr1-deficient mice. Thus, although other current- ly unknown transporters and/or other modes of iron deliv- ery may play a role in iron uptake in HSPC, Tfr1 is clearly the predominant pathways for iron uptake, particularly in the downstream differentiation of progenitor cells.
In addition to its central role in facilitating cellular iron uptake, Tfr1 also appears to play a role in signal transduc- tion in intestinal homeostasis.16 We found that overex- pressing of wild-type Tfr1 but not the R654A mutant res- cued both differentiation and proliferation. Interestingly, the L622A mutant only partially rescued the defects in cKO cells, possibly because Tfr1-Hfe may also regulate hematopoiesis. These results indicate that iron uptake but not signal transduction is the main mechanism by which Tfr1 promotes the differentiation and survival of hematopoietic cells.
In conclusion, we provide direct in vivo evidence that Tfr1 plays an essential role in hematopoiesis. In particular, we show that Tfr1 is required for the differentiation of HSPC into a range of mature cell types. Importantly, our results indicate that iron uptake appears to be the principal mechanism by which Tfr1 mediates the differentiation and survival of hematopoietic cells, thereby underscoring the important role that intracellular iron homeostasis plays in hematopoiesis.
Acknowledgments
The authors would like to thank the grants from the National Natural Science Foundation of China (31930057 and 31530034 to FW, 31570791 and 91542205 to JM, and 31701034 to QW) and the National Key R&D Program of China (2018YFA0507802 to FW and 2018YFA0507801 to JM). The authors thank the members of the Wang and Min lab- oratory for helpful discussions.
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