Fundamental properties of unperturbed haematopoiesis from stem cells in vivo
11.02.2015
Busch K, Klapproth K, Barile M, Flossdorf M, Holland-Letz T, Schlenner SM, Reth M, Höfer T, Rodewald HR.
Nature. 2015 Feb 26;518(7540):542-6
The numbers of productive Haematopoietic stem cells (HSCs) during normal haematopoiesis, and the flux of differentiating progeny remain unknown. In this publication researchers from the German Cancer Research Center (DKFZ) in Heidelberg devise a mouse model allowing inducible genetic labelling of HSCs in bone marrow. They insert MerCreMer cDNA into a gene locus that is only expressed in haematopoietic stem cells (HSCs), which give rise to all the other blood cells. The research team then crosses the MerCreMer mice with those carrying a conditional yellow fluorescence protein (YFP) gene with a “floxed” stop cassette. Application of tamoxifen to these mice results in YFP expression and thus marked haematopoietic stem cells in an inducible and temporally controlled fashion. The time to approach equilibrium between labelled HSCs and their progeny is surprisingly long, a time scale that would exceed the mouse’s life. Indeed, the team finds that adult haematopoiesis is largely sustained by previously designated ‘shortterm’ stem cells downstream of HSCs that nearly fully self-renew, and receive rare but polyclonal HSC input. By contrast, in fetal and early postnatal life, HSCs are rapidly used to establish the immune and blood system. In the adult mouse, 5-fluoruracil-induced leukopenia enhances the output of HSCs and of downstream compartments, thus accelerating haematopoietic flux. Label tracing also identifies a strong lineage bias in adult mice. The MerCreMer technology has been developed in an earlier work of the group of the BIOSS researcher Prof. Dr. Michael Reth that has shown that the DNA-recombinase Cre can be regulated remotely and induced by the substance tamoxifen. The researchers attached the tamoxifen-binding Mer domains to Cre’s N- and C-terminal part, thus generating the MerCreMer chimera. Cre is a DNA recombinase that can recognize and delete genes flanked by LoxP sites, the recognition sites for Cre activity. Such “flanked by LoxP site” or “floxed” genes can be altered in the presence of Cre. With a similar strategy one can also activate genes whose expression is inhibited by a floxed stop cassette. Reth’s group has shown that the MerCreMer enzyme only becomes active in cells in the presence of the inducer tamoxifen.
