Reply to Gaiano et al.: Expression of Notch Proteins in Pyramidal Neurons in Vivo

This is a response to a letter by Gaiano et al. (1)

Gaiano et al. (1) argued that the lack of reduction of Notch1 and Notch2 mRNAs and proteins in the hippocampus and neocortex of conditional knock-out mice at 2 months of age or older could be due to the specific αCaMKII-Cre line used in our study (2) or possible inefficient Cre-mediated recombination at the Notch1 and Notch2 loci. Compared with the T29-1 line of αCaMKII-Cre mice (3) used in their study (4), in which Cre-mediated recombination is restricted specifically to pyramidal neurons in hippocampal area CA1 (3), Cre-mediated recombination occurs broadly in pyramidal neurons of the entire hippocampus and neocortex in our αCaMKII-Cre mice (5), which have been used to inactivate effectively many genes, including presenilin-1 (5), nicastrin, CREB-binding protein, amyloid precursor protein, APLP1, and APLP2, by 2 months of age. Furthermore, Notch1 and Notch2 loci can be excised rapidly and efficiently by Cre recombinase, as shown by our results that Cre-mediated recombination and degradation of existing Notch mRNAs and proteins are complete within 2–3 days of introduction of the Cre cDNA (2). Our findings are consistent with in situ hybridization data showing predominant localization of Notch1 (6) and Notch2 (2) mRNAs in the germinal areas rather than the hippocampal CA1 and CA3 areas. It is puzzling that Alberi et al. (4) found that Notch1 mRNA is expressed abundantly in pyramidal neurons of the entire hippocampus in wild-type mice and that its mRNA expression is eliminated in the pyramidal neurons of both hippocampal CA1 and CA3 areas using this highly restrictive T29-1 line of αCaMKII-Cre mice (3).