'Side Population' cells in adult mouse testis express Bcrp1 gene and are enriched in spermatogonia and germinal stem cells

Stem cells in various somatic tissues (bone marrow, skeletal muscle) can be identified by the 'Side Population' marker based on Hoechst 33342 efflux. We show that mouse testicular cells also display a 'Side Population' that express Bcrp1 mRNA, the ABC transporter responsible for Hoechst efflux in hematopoietic cells. Inhibition of Hoechst efflux by specific BCRP1 inhibitor Ko143 show that germinal 'Side Population' phenotype is dependent on BCRP1 activity. Analysis of two well-defined models of altered spermatogenesis (W/W-v mutants and cryptorchid male mice) and RNA expression studies of differentiation markers demonstrate that germinal 'Side Population' contains spermatogonial cells. In addition, alpha(6)-integrin and Stra8 germinal stem cell markers, are expressed in the 'Side Population'. In vivo repopulation. assay clearly establishes that testis 'Side Population' in adult mice is highly enriched in male germ stem cells.