A novel isoform of tensin-1 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 underlying molecular regulatory mechanisms remain poorly understood. We used publicly available RNA sequencing and proteomic data sets to mine for actin-regulatory factors differentially expressed during human erythroid differentiation and discovered that a focal adhesion (FA) protein, tensin-1 (TNS1), dramatically increases in expression late in differentiation. Remarkably, we found that differentiating human CD34+ cells express a novel truncated form of TNS1 (erythroid TNS1 [eTNS1]; Mr ∼125 kDa) missing the N-terminal half of the protein containing the actin-binding domain, due to an internal messenger RNA 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 nonhuman 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. Knockout of 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.