A novel isoform of Tensin1 promotes actin filament assembly for efficient erythroblast enucleation.

Mammalian red blood cells are generated via a terminal erythroid differentiation pathway culminating in cell polarization and enucleation. Actin filament (F-actin) polymerization is critical for enucleation, but the molecular regulatory mechanisms remain poorly understood. We utilized publicly available RNA-seq and proteomics datasets to mine for actin-regulatory factors differentially expressed during human erythroid differentiation and discovered that a focal adhesion protein-Tensin1-dramatically increases in expression late in differentiation. Remarkably, we found that differentiating human CD34+ cells express a novel truncated form of Tensin1 (eTNS1; Mr ~125 kDa) missing the N-terminal half of the protein containing the actin-binding domain, due to an internal mRNA translation start site resulting in a unique exon 1E. The region upstream of eTNS1 has features of an active erythroid promoter, demonstrating increasing chromatin accessibility during terminal differentiation, paralleling increasing gene expression. Sequence comparisons across species indicate that eTNS1 is expressed in humans and non-human primates, but not in zebrafish, mice or other rodents. Confocal microscopy showed that eTNS1 localized to the cytoplasm during terminal erythroid differentiation, but surprisingly, did not appear to form focal adhesions nor to colocalize with F-actin. Knocking out eTNS1 did not affect terminal differentiation or assembly of the spectrin membrane skeleton but led to reduced F-actin assembly and abnormal organization in polarized and enucleating erythroblasts, resulting in impaired enucleation efficiency. We conclude that eTNS1 is a novel regulator of F-actin during human erythroid terminal differentiation that is required for efficient enucleation.