Hematopoietic stem cells (HSCs) are the best-characterized tissue-specific stem cells, yet the experimental study of HSCs remains challenging, due to the fact that they are exceedingly rare and methods to purify them are cumbersome, and vary between different laboratories. Moreover, genetic tools for specifically addressing issues related to HSC biology are lacking. To address these issues we sought to identify genes uniquely expressed in HSCs within the hematopoietic system, and use such information to develop a reporter strain that specifically labels HSCs. Microarray expression profiling of the murine hematopoietic system identified a number of genes with HSC-restricted expression. Generation of mice with targeted reporter knock-in/knock-out alleles of three of the identified genes, Clec1a, Fgd5, and, Sult1a1 revealed that HSCs isolated from these mice functioned normally, and though Fgd5 was required for embryonic development, it was not required for definitive hematopoiesis or sustained HSC function. Fgd5-reporter expression almost exclusively labeled cells that expressed a panel of markers consistent with HSCs, and bone marrow cells isolated based solely on reporter signal showed potent HSC activity that was comparable to HSCs purified by immunophenotypic means. The labeled fraction of the Fgd5-reporter mice contained all HSC activity, and HSC-specific labeling was retained after transplantation. In summary, reporter expression from the Fgd5 locus permits identification and purification of HSCs based on single color fluorescence.
