The Notch-signalling pathway is important in establishing metameric pattern during somitogenesis. In mice, the lack of either of two molecules involved in the Notch-signalling pathway, Mesp2 or presenilin-1 (Ps1), results in contrasting phenotypes: caudalized versus rostralized vertebra. Here we adopt a genetic approach to analyse the molecular mechanism underlying the establishment of rostro-caudal polarity in somites. By focusing on the fact that expression of a Notch ligand, Dll1, is important for prefiguring somite identity, we found that Mesp2 initiates establishment of rostro-caudal polarity by controlling two Notch-signalling pathways. Initially, Mesp2 activates a Ps1-independent Notch-signalling cascade to suppress Dll1 expression and specify the rostral half of the somite. Ps1-mediated Notch-signalling is required to induce Dll1 expression in the caudal half of the somite. Therefore, Mesp2- and Ps1-dependent activation of Notch-signalling pathways might differentially regulate Dll1 expression, resulting in the establishment of the rostro-caudal polarity of somites.

Recent studies have implicated presenilin-1 (PS-1) in the processing of the amyloid precursor protein and Notch-1. We show that PS-1 has biological effects on differentiation and cell cycle control of neuronal precursor cells in vivo using PS-1-deficient mice. The expression of Class III ␤-tubulin was upregulated throughout the neocortical primordia of PS-1-deficient E14 embryos, especially on the ventricular surface. The increased speed of migration of the immature neurons from the ventricular zone outward in the PS-1-deficient neocortical primordia was indicated by an in vivo bromodeoxyuridine (BrdU)-labeling assay and a DiI-labeling assay in slice culture. Furthermore, we investigated the cell cycle of neuronal precursor cells in the neocortical ventricular zone using an in vivo cumulative BrdU-labeling assay. The length of the cell cycle in the neocortical precursor cells of wild-type mice was 11.4 hr whereas that of the PS-1-deficient mice was 15.4 hr. Among all phases of the cell cycle, S-phase exhibited the most prominent change in length, increasing from 2.4 hr in the wild-type mice to 7.4 hr in the mutant mice. The distribution of ␤-catenin was specifically affected in the ventricular zone of the PS-1-deficient mice. These findings suggest that PS-1 is involved in the differentiation and the cell cycle control of neuronal precursor cells in the ventricular proliferating zone of the neocortical primordium.

The Notch signalling pathway plays essential roles during the specification of the rostral and caudal somite halves and subsequent segmentation of the paraxial mesoderm. We have re-investigated the role of presenilin 1 (Ps1; encoded by Psen1) during segmentation using newly generated alleles of the Psen1 mutation. In Psen1-deficient mice, proteolytic activation of Notch1 was significantly affected and the expression of several genes involved in the Notch signalling pathway was altered, including Δ-like3, Hes5, lunatic fringe (Lfng) and Mesp2. Thus, Ps1-dependent activation of the Notch pathway is essential for caudal half somite development. We observed defects in Notch signalling in both the caudal and rostral region of the presomitic mesoderm. In the caudal presomitic mesoderm, Ps1 was involved in maintaining the amplitude of cyclic activation of the Notch pathway, as represented by significant reduction of Lfng expression in Psen1-deficient mice. In the rostral presomitic mesoderm, rapid downregulation of the Mesp2 expression in the presumptive caudal half somite depends on Ps1 and is a prerequisite for caudal somite half specification. Chimaera analysis between Psen1-deficient and wild-type cells revealed that condensation of the wild-type cells in the caudal half somite was concordant with the formation of segment boundaries, while mutant and wild-type cells intermingled in the presomitic mesoderm. This implies that periodic activation of the Notch pathway in the presomitic mesoderm is still latent to segregate the presumptive rostral and caudal somite. A transient episode of Mesp2 expression might be needed for Notch activation by Ps1 to confer rostral or caudal properties. In summary, we propose that Ps1 is involved in the functional manifestation of the segmentation clock in the presomitic mesoderm.