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L. Crisafulli et al.
present in Flk2–MPP although at lower levels (Figure 2B and C and Online Supplementary Table S1). However: 1) none of them was DE in HSC from our self-renewal defec- tive mouse model; or 2) the downregulation was less than 3-fold. Among other miRNA whose function in the hematopoietic system has been previously established, miR-29a and miR-126 were shown to play a role in HSC self-renewal and to be differentially expressed in HSC compared to lineage-committed progenitors, but still highly expressed at the MPP stage,11,45 whereas miR-99 was recently found to be DE in the HSC-to-MPP transi- tion.20 Interestingly, these miRNA regulate HSC pool maintenance through different biological mechanisms. miR-126 restrains HSC cell cycle entry, while miR-99 and miR-29a limit HSC maturation, similar to miR-127. However, the accelerated differentiation displayed by miR-127DR cells was not associated to increased apopto- sis or altered cell cycle. This might explain why the effect of miR-127DR was only observed with secondary trans- plants, since HSC depletion was likely due to a gradual exit of HSC from the self-renewing pool. Likewise, it is not surprising that the accelerated differentiation did not result in an increased differentiated progeny in vivo, since: a) differentiation was not associated with increased prolif-
eration; and b) miR-127 is not normally expressed beyond the HSC stage, therefore an effect of miR-127DR in more mature cell population is not expected.
In conclusion, we identified miR-127 as novel player for preserving the HSC pool. Our discovery has potential implications for several translational aspects of experi- mental hematology, including hematopoietic malignan- cies, BM transplantation, and regenerative medicine.
Acknowledgments
The authors would like to thank Dario Strina, Stefano Mantero and Lucia Susani for technical assistance, Massimiliano Mirolo and Ciro Menale for technical suggestions, and Paolo Vezzoni for helpful discussions and for critically read- ing the manuscript.
Funding
We acknowledge support from Marie Curie IRG 256452, from Ricerca Finalizzata GR 2010-2307975 and from AIRC- Fondazione Cariplo (TRIDEO 15882) to FF, from CNR National Program Aging Project to AV, and NIH grant CA116606 to ML. LC was recipient of a fellowship from Fondazione Nicola del Roscio; SM was recipient of a fellowship from Fondazione Damiano per l’Ematologia.
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