Abstract
Mice harboring Notch2 mutations replicating Hajdu-Cheney syndrome (Notch2(tm1.1ECan)) have osteopenia and exhibit an increase in splenic marginal zone B cells with a decrease in follicular B cells. Whether the altered B-cell allocation is responsible for the osteopenia of Notch2(tm1.1ECan) mutants is unknown. To determine the effect of NOTCH2 activation in B cells on splenic B-cell allocation and skeletal phenotype, a conditional-by-inversion (COIN) Hajdu-Cheney syndrome allele of Notch2 (Notch2([ΔPEST]COIN)) was used. Cre recombination generates a permanent Notch2(ΔPEST) allele expressing a transcript for which sequences coding for the proline, glutamic acid, serine, and threonine-rich (PEST) domain are replaced by a stop codon. CD19-Cre drivers were backcrossed into Notch2([ΔPEST]COIN/[ΔPEST]COIN) to generate CD19-specific Notch2(ΔPEST/ΔPEST) mutants and control Notch2([ΔPEST]COIN/[ΔPEST]COIN) littermates. There was an increase in marginal zone B cells and a decrease in follicular B cells in the spleen of CD19(Cre/WT);Notch2(ΔPEST/ΔPEST) mice, recapitulating the splenic phenotype of Notch2(tm1.1ECan) mice. The effect was reproduced when the NOTCH1 intracellular domain was induced in CD19-expressing cells (CD19(Cre/WT);Rosa(Notch1/WT) mice). However, neither CD19(Cre/WT);Notch2(ΔPEST/ΔPEST) nor CD19(Cre/WT);Rosa(Notch1/WT) mice had a skeletal phenotype. Moreover, splenectomies in Notch2(tm1.1ECan) mice did not reverse their osteopenic phenotype. In conclusion, Notch2 activation in CD19-expressing cells determines B-cell allocation in the spleen but has no skeletal consequences.