Yoshio Wakamatsu

NUMB localizes in the basal cortex of mitotic avian neuroepithelial cells and modulates neuronal differentiation by binding to NOTCH-1.

The importance of lateral inhibition mediated by NOTCH signaling is well demonstrated to control neurogenesis both in invertebrates and vertebrates. We have identified the chicken homolog of Drosophila numb, which suppresses NOTCH signaling. We show that chicken NUMB (c-NUMB) protein is localized to the basal cortex of mitotic neuroepithelial cells, suggesting that c-NUMB regulates neurogenesis by the modification of NOTCH signaling through asymmetrical cell division. Consistent with this suggestion, we show (1) that c-NUMB interferes with the nuclear translocation of activated c-NOTCH-1 through direct binding to the PEST sequence in the cytoplasmic domain of c-NOTCH-1 and (2) that c-NUMB interferes with c-NOTCH-1-mediated inhibition of neuronal differentiation.

Transition of localization of the N-Myc protein from nucleus to cytoplasm in differentiating neurons

N-myc is a developmentally regulated proto-oncogene encoding a putative sequence-specific DNA-binding protein. Previous studies on tissue distribution of N-myc transcripts indicated that one of the major sites of N-myc expression is the CNS and neural crest derivatives in developing embryos. We investigated N-Myc protein expression in embryonic neural tissues and found that the protein was usually localized in the nucleus, but accumulated in the cytoplasm upon differentiation of specific classes of neurons, e.g., retinal ganglion cells, neurons of spinal ganglia, and Purkinje cells of the cerebellum. The change of localization of N-Myc from the nucleus to the cytoplasm indicates a novel feature of regulation of myc family proteins and suggests functions of N-myc in the cytoplasm of maturing neurons.

Regionalized expression of the Dbx family homeobox genes in the embryonic CNS of the mouse

Here we report the identification of a novel homeobox gene family Dbx in mouse, which consists of Dbx and Dbx2. The two genes share similar structural organization and are encoded by different chromosomes. The predicted Dbx and Dbx2 proteins share 85% identity in their homeodomain amino acid sequences, but otherwise showed no significant similarity. Characterization of the expression of these two genes in the embryos suggested their role in the development of the CNS. In the forebrain, Dbx is expressed in various regions, while Dbx2 showed a more restricted pattern of expression. In the midbrain, the expression domains of Dbx and Dbx2 overlap along the dorso-lateral wall of the ventricle. In the hindbrain and spinal cord, both genes are expressed in the boundary separating the basal and alar plates, which seems to correspond to the sulcus limitans. Expression of the Dbx/Dbx2 genes is restricted to the ventricular region of the embryonic CNS except for that of Dbx in the septum of the telencephalon. Together these observations indicate possible participation of the members of the Dbx family in regionalization of the CNS. While the expression of Dbx was restricted to the CNS, Dbx2 was also expressed in some of the mesenchymal cells, such as limb buds and tooth germs.

Tissue Distribution of N-myc Expression in the Early Organogenesis Period of the Mouse Embryo

N‐myc expression in the mouse embryo was examined in its organogenesis period. Northern blot analysis of total RNA of embryos from 9.5 days to 17.5 days of gestation indicated that N‐myc mRNA level was the highest at 9.5 days and decreased as development proceeded. Tissue distribution of N‐myc expression in 9.5 day embryos was histologically analyzed by in situ hybridization of the transcripts and immunofluorescent staining of N‐myc protein. In addition to the central nervous system indicated in previous studies on embryos of different stages, we found N‐myc expression in various developing tissues. Neural crest‐derived tissues generally expressed N‐myc transcripts and proteins to significant levels, e.g. facial primordia, visceral arches and dorsal root ganglia. Among mesodermal tissues, N‐myc expression was especially high in the migrating sclerotomes derived from caudal halves of the somites, primitive nephric tubules, and mesenchymes condensed around the digestive tract and in the limb buds. Expression in endodermal tissues, however, was very low. In situ hybridization and immunohistology gave consistent results, confirming the authenticity of the detection of N‐myc expression.

Organization and expression of mouse Hox3 cluster genes

We determined the physical linkage of six mouse Hox3 homeobox sequences, including a new homeobox sequence (Hox3.5), by analysis of overlapping genomic clones. Additionally, we defined the locations of Hox 1.7 and Hox 1.8 in the Hox 1 cluster. Analysis of the expression patterns of Hox 3.6 and Hox 3.5 during embryogenesis revealed that the relationship between relative position in the Hox 3 cluster and expression domain along antero-posterior axis appears similar to that seen for members of the other Hox clusters.