CD34- hematopoietic stem cells: current concepts and controversies.

Area of Science:

• Hematology
• Stem Cell Biology
• Molecular Biology

Background:

• Recent findings challenge the exclusive expression of CD34 on hematopoietic stem cells (HSCs).
• Quiescent murine HSCs are predominantly CD34(-), differentiating into CD34(+) cells upon stimulation.
• The Hoechst dye efflux property identifies a distinct side population (SP) of HSCs.

Purpose of the Study:

• To investigate the role of ATP-binding cassette (ABC) transporters in HSC identification.
• To explore the relationship between CD34(-) and CD34(+) HSCs in mice and humans.
• To evaluate the potential of SP cells and Bcrp/ABCG2 expression for HSC isolation and characterization.

Main Methods:

• Utilized Hoechst dye efflux to identify side population (SP) cells.
• Employed semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) to assess gene expression.
• Analyzed ATP-binding cassette (ABC) transporter expression, specifically breast cancer resistance protein (Bcrp)/ABCG2.
• Investigated the effect of enforced ABCG2 expression on SP phenotype and hematopoietic maturation.

Main Results:

• SP cells, predominantly CD34(-), are highly enriched for long-term repopulating HSCs.
• Breast cancer resistance protein (Bcrp)/ABCG2 is highly expressed in SP cells and primitive HSCs, decreasing with differentiation.
• Enforced ABCG2 expression induced an SP phenotype and impaired hematopoietic maturation.

Conclusions:

• The Bcrp/ABCG2 gene is critical for generating the SP phenotype, enabling the selection of immature, pluripotent HSCs.
• Isolation of Bcrp/ABCG2(+) cells offers a promising method for analyzing and purifying HSCs.
• Understanding CD34(-) HSCs and their relationship with CD34(+) HSCs is essential for stem cell research and potential clinical applications.