Generation of CD34 Fluorescent Reporter Human Induced Pluripotent Stem Cells for Monitoring Hematopoietic Differentiation.

Hematopoietic stem and progenitor cells (HSPCs) derived from human induced pluripotent stem cells (hiPSCs) hold great promise for disease modeling， drug screens， and eventually cell therapy approaches. During in vitro differentiation of hiPSCs into hematoendothelial progenitors， the emergence of CD34-positive cells indicates a critical step of lineage specification. To facilitate the monitoring of hematopoietic differentiation of hiPSCs， we established fluorescent reporter cells for the stem and progenitor cell marker CD34. An IRES-GFP (internal ribosome entry site green fluorescent protein) construct was introduced by CRISPR/Cas9 into the 3' untranslated region of one endogenous CD34 allele. Single-cell clones were generated after excision of the floxed puromycin resistance cassette by Cre recombination and correct insertion was confirmed by genotyping polymerase chain reaction and Southern blot. To validate their functionality， the reporter hiPSCs were in vitro differentiated toward CD34 cells using the STEMdiff Hematopoietic Kit combined with short-term inhibition of GSK3 (glycogen synthase kinase 3). All cells expressing nuclear GFP were positive for cell surface CD34， thus allowing the direct monitoring of the differentiation of hiPSCs into CD34 cells either by flow cytometry or confocal microscopy. After fluorescence-activated cell sorting， cells displaying high GFP expression exhibited increased colony-forming potential in the MethoCult colony-forming unit assays as compared with CD34 cells obtained by magnetic-activated cell sorting. In summary， we have generated functional CD34 GFP reporter hiPSCs， which not only permit label-free separation of HSPCs， but also tracing of the emergence and fate of CD34 progenitors at the single-cell level.