Mitochondrial specialization in erythroblasts is important for efficient heme synthesis, with defects or reduced expression of several mitochondrial proteins that cause anemia. Trafficking binding to kinesin 2 (TRAK2) is known to participate in mitochondrial movement along microtubules by interacting with the kinesin motor protein and forming a complex with Miro that is located in 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 serum-free ex vivo 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 that of aminolevulinate synthase 2, the erythroid-specific enzyme for the compromised passage of the heme biosynthetic pathway, and mitoferrin 1, the erythroid-specific mitochondrial iron transporter. Western analysis revealed that the TRAK2 protein is detected as a double band with molecular weights of 130 kD and 105 kD.

The mitochondrial co-localization of TRAK2 was verified by confocal microscopy in K562 cells that overexpress TRAK2. To study the potential role of TRAK2 in erythropoiesis, TRAK2 expression was reduced in cultured human erythroid cells by lentiviral shRNA transduction. The fall of TRAK2 (TRAK2-KD) was confirmed by Western analysis in K562 cells. In primary erythroblasts, TRAK2-KD caused a slight reduction in CD36 + immature erythroblasts on day 7 of culture before 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 the control vs 0.25 million cells/ml in TRAK2-KD on day 18 of culture). Flow cytometric analyzes showed that <1% of CD36 + progenitor cells differentiated into glycophorin A erythroblasts compared to> 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 idea that mitochondrial location or mobility within erythroblasts may be important for iron trafficking or heme synthesis.


There are no relevant conflicts of interest to declare.