Mitochondrial specialization in erythroblasts is important for efficient heme synthesis, with defects or reduced expression of several mitochondrial proteins causing anemia. Trafficking kinesin binding 2 (TRAK2) is known to participate in mitochondrial movement along microtubule by interacting with kinesin motor protein and making a complex with Miro that is localized on the mitochondrial outer membrane. Transcriptome data suggest that TRAK2 is highly and specifically expressed in early erythroid cells. Here the role of TRAK2 was studied among human CD34+ cells that were grown in ex vivo serum-free cultures supplemented with erythropoietin (EPO, total culture period 21 days). Quantitative PCR studies indicated that TRAK2 expression is highly regulated during erythropoiesis. Its expression pattern was nearly identical to aminolevulinate synthase 2, the erythroid specific enzyme for the committed step of the heme biosynthetic pathway, and mitoferrin 1, the erythroid specific mitochondrial iron transporter. Western analyses revealed that TRAK2 protein is detected as a doublet band with molecular weights of 130kD and 105kD. Mitochondrial co-localization of TRAK2 was verified by confocal microscopy in TRAK2-overexpressing K562 cells. To study a potential role of TRAK2 in erythropoiesis, TRAK2 expression was reduced in cultured human erythroid cells using lentiviral shRNA transduction. TRAK2 knockdown (TRAK2-KD) was confirmed by Western analysis in K562 cells. In primary erythroblasts, TRAK2-KD caused slight reduction of CD36+ immature erythroblasts at culture day 7 prior to the addition of EPO (CD36+ population 58% in control vs 40% in TRAK2-KD). After the addition of erythropoietin to the culture medium, TRAK2-KD severely restricted erythroblast proliferation (5.0 million cells/ml in control vs 0.25 million cells/ml in TRAK2-KD on culture day 18). Flow cytometric analyses showed that <1% of the CD36+ progenitors cells differentiated into glycophorin A erythroblasts compared with >90% in control cultures. Annexin-V staining indicated that more than 90% of cells had undergone apoptosis by day 14. These data suggest that TRAK2 expression is required for erythroid differentiation. As such, defects in TRAK2 expression should be considered in cases of unexplained anemia. The data also support the notion that mitochondrial location or mobility within erythroblasts may be important for iron trafficking or heme synthesis.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.