Global impact of HRI-mediated gene expression in vivo in primary erythroblasts during ID. Global impact of HRI-mediated in vivo translation in primary BasoE was investigated by Ribo-seq and mRNA-seq, as illustrated in Figure 3. An in vivo heme deficiency mouse model via diet-induced systemic ID was used.¹²  HRI is activated by heme deficiency and phosphorylates eIF2α. First, eIF2αP inhibits general protein synthesis in cytoplasm and mitochondria through inhibiting the translation of ribosomal protein mRNAs in the cytosolic and mitochondrial cellular compartments. In the absence of HRI, continued protein synthesis results in the accumulation of cytoplasmic and mitochondrial unfolded proteins leading to proteotoxicity and mitochondrial dysfunction. Second, eIF2αP selectively enhances the translation of Atf4 mRNA. ATF4 then induces gene expression of 3 pathways in activating UPR^mt, reprogramming mitochondrial metabolism and reducing oxidative stress, all of which enable adaptation to ID and erythroid differentiation. Last, HRI-ISR suppresses mTORC1 signaling, which is activated by elevated Epo levels in ID, via ATF4-induced GRB10 expression. Overall, global genome-wide gene expression assessment of primary erythroblasts in vivo reveals that HRI-ISR contributes most significantly to adaptation to iron-restricted erythropoiesis. Reprinted from Zhang et al with permission.⁴⁶