Terminal Erythroid Differentiation Is Regulated By Mitochondrial Translation Via Maintenance of Iron Homeostasis

Erythropoiesis is a process that continuously produces red blood cells, the most abundant cell type in the body which actively synthesize hemoglobin protein. Modification of transfer RNA (tRNA) regulates its function such as protein translation and other cellular processes (Chujo et al, FEBS J. 2021). Lack of mitochondrial tRNA (mt-tRNA) taurine modification mediated by mitochondrial tRNA translation optimization 1 (Mto1) has been recently shown to induce unfolded protein response (UPR) in ES cells (Fakruddin et al, Cell Rep. 2018). However, how UPR is induced by mt-tRNA dysfunction and its relevance in tissue homeostasis remains unclear. Since aberrant protein translation induces cellular stress, we hypothesized that Mto1 dysfunction might result in defective hematopoiesis, especially erythropoiesis which produces a substantial amount of protein such as hemoglobin via mitochondria. Yet, the role of any modification of mt-tRNAs in hematopoiesis including taurine has not been elucidated at all.

To elucidate the biological function of Mto1 in hematopoiesis, we generated hematopoietic-specific Mto1 conditional KO (cKO) mice by a cross with Vav-Cre mice. The Mto1cKO mouse was embryonic lethal and the fetus had a paler appearance than the wild-type (WT) control, indicating severe anemia. Total fetal liver cellularity was significantly reduced and erythrocyte-related parameters were marked reduced in a peripheral blood parameter analysis in Mto1 cKO fetuses compared to WT. Further erythroblast subpopulation analysis revealed that absolute number of erythroblasts decreased significantly in almost all differentiation stages in Mto1 KO fetal liver compared to WT, especially most decrease in number at polychromatic erythroblast stage. Analysis in adult hematopoiesis using Mx1-Cre system also showed similar anemic phenotype in Mto1 cKO mouse. Consistent with these results, Mto1 mRNA expression and taurine modification levels were markedly up-regulated at the polychromatic erythroblast stage in the WT fetal liver, suggesting essential role of Mto1 in terminal erythroid differentiation at the polychromatic erythroblast stage.

Previous study showed Mto1 dysfunction suppressed mitochondrial translation which impaired the formation of respiratory complexes (Fakruddin et al, Cell Rep. 2018). Consistent with this finding, Mto1 cKO fetal liver polychromatic erythroblasts showed marked suppression of complex-I formation. Impaired formation of complex-I which contains abundant iron-sulfur clusters is supposed to affect intracellular iron distribution, from mitochondria to cytoplasm. In line with this hypothesis, Mto1 cKO fetal liver polychromatic erythroblasts showed cytoplasmic iron accumulation which upregulated ALAS2 protein translation and heme biosynthesis. Proteome analysis of Mto1 cKO fetal liver polychromatic erythroblasts revealed significant upregulation of embryonic hemoglobin proteins expression compared with WT.

Upregulation of heme and unstable embryonic hemoglobin biosynthesis is supposed to induce cellular stress. Consistent with this hypothesis, Xbp1 splicing-mediated UPR was significantly upregulated in Mto1 cKO fetal liver polychromatic erythroblasts. Moreover, both specific inhibitor of Xbp1 splicing and iron chelator rescued terminal erythroid differentiation in the Mto1 KO fetal liver in vitro, suggesting that Xbp1 splicing-mediated UPR via cytoplasmic iron overload derived from Mto1 deficiency is a cause of defect in terminal erythroid differentiation in Mto1 cKO mouse.

This study demonstrated for the first time, an indispensable role of mt-tRNA taurine modification in erythroid terminal differentiation and suggests the biological role of mitochondria in iron homeostasis and erythropoiesis. Our findings propose a novel, non-energy-related molecular mechanism which links primary mitochondrial protein deficiency with erythroid differentiation block via imbalanced iron homeostasis. This important but unappreciated molecular mechanism might make a breakthrough by bringing a new biological aspect to hematology.

No relevant conflicts of interest to declare.