A pooled CRISPR screen reveals genes critical for erythroblast enucleation

Terminal erythroid differentiation involves dramatic cellular remodeling that culminates in the expulsion of the nucleus, a process known as enucleation. While enucleation is conserved across mammals and is crucial for the generation of fully functional erythrocytes, the mechanisms governing this process have remained largely unknown, in part because the absence of genetic material in mature, enucleated red blood cells hinders genetic experimentation. Here, we performed a pooled, forward-genetic CRISPR-Cas9 screen in enucleated red blood cells derived from primary human hematopoietic stem cells to identify genes required for enucleation. We found that Chloride Intracellular Channel 3 (CLIC3) and Vesicle-associated membrane protein 8 (VAMP8) are both necessary for terminal erythroid differentiation, yet likely act through different mechanisms. Knockdown of CLIC3 led to a delay in erythroblast differentiation, culminating in impaired enucleation. We found that the knockdown cells had increased p53 and p21 and exhibited cell cycle alterations, suggesting CLIC3 plays a crucial role in coordinating cell cycle progression during erythropoiesis. In comparison, VAMP8-depleted cells initially appear to undergo accelerated differentiation but then display a specific defect in enucleation. Transcriptional analysis of the VAMP8-knockdown cells suggested dysregulation of pathways for vesicle trafficking and actin binding, and imaging of late-stage erythroblasts revealed impaired nuclear polarization and disorganized actin. This work provides a new approach for functional genomics in enucleated cells and reveals novel factors important for terminal erythroid differentiation and enucleation.