Notch- and WNT-signalling pathways are both known to have a role in regulating HSC self-renewal. However, little is known about the distinct contributions of these two signalling pathways to discrete cellular requirements for self-renewal — inhibition of differentiation and induction of proliferation. To investigate the role of Notch signalling in HSC function, Duncan et al. generated a Notch-reporter-transgenic mouse, in which expression of green fluorescent protein (GFP) is induced by Notch signalling. Immunofluorescence staining showed that a substantial proportion of cells expressing the HSC marker KIT in the bone-marrow HSC niche were transducing Notch signals. Further analysis indicated that Notch signalling was more prevalent among HSCs than among lineage-committed cells, both when cells were analysed ex vivo and when HSCs were differentiated in vitro. Consistent with the hypothesis that Notch signalling is a marker of the most primitive cells, a greater proportion of GFP+ HSCs had multi-lineage potential when cultured in vitro. Inhibition of the Notch-signalling pathway accelerated differentiation of HSCs in vitro and markedly reduced long-term HSC reconstitution of lethally irradiated mice, providing evidence of a role for Notch signalling in maintaining HSCs in an undifferentiated state.
The role of Notch signalling relative to other signalling pathways was studied using mice expressing reporters of both Notch and WNT signalling, and it was shown that a high proportion of cells in the HSC niche transduced both Notch and WNT signals. Interestingly, although both signalling pathways were active in these cells and WNT3A-induced signalling could promote the survival and growth of HSCs in which the Notch-signalling pathway was inhibited, WNT signalling was unable to maintain these HSCs in an undifferentiated state.
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