# The oncogene protein kinase PIM1 regulates mammalian erythroblast enucleation

## TL;DR

The PIM1 protein, known for promoting cancer, also controls a unique step in blood cell development called enucleation in mammals.

## Contribution

PIM1's novel role in erythroblast enucleation is revealed, independent of its known functions in cell growth and survival.

## Key findings

- PIM1 knockdown in human and mouse erythroid cells inhibits enucleation without affecting differentiation or apoptosis.
- PIM1 deficiency reduces phosphorylation of GTPase-associated proteins involved in actin assembly and vesicle trafficking.
- PIM1 deletion leads to abnormal F-actin and endocytic vesicle distribution in enucleating cells.

## Abstract

Erythroblast enucleation is a unique process during mammalian erythropoiesis, yet its regulatory mechanisms remain largely elusive. Here, we demonstrate the specific regulatory role of the oncogene PIM1, the most highly expressed protein kinase in orthochromatic erythroblasts, in enucleation. Unlike its well-established roles in cancer cell proliferation and survival, knockdown of PIM1 in human erythroid cells does not affect cell growth or apoptosis, but specifically inhibits erythroblast enucleation without altering differentiation. To elucidate the functional conservation of PIM1 in mammalian erythropoiesis, we generate Pim1fl/flEpoRCre mice in which Pim1 is deleted in erythroid cells. Consistent with human erythropoiesis, deletion of Pim1 in mice has no detectable effect on apoptosis or differentiation of erythroid cells, but specifically inhibits erythroblast enucleation. Phosphoproteomic analysis reveals that PIM1 deficiency causes a pronounced decrease in phosphorylation of GTPase-associated proteins involved in actin assembly and vesicle trafficking. Functionally, this perturbation results in an aberrant distribution of F-actin and endocytic vesicles within enucleating cells. These findings reveal the unexpected role of PIM1 in normal erythropoiesis and enhance our understanding of mammalian erythroblast enucleation.

The oncogene PIM1 regulates erythroblast enucleation via GTPase-dependent cytoskeletal remodeling and vesicle trafficking, independent of its canonical roles in cell proliferation and apoptosis.