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Delta-Notch signaling plays an essential role in cell fate determination in many tissue types, including the central nervous system. Although the signaling mechanism of Notch has been extensively studied, the behaviors of its ligands are not well understood. In the present study, we found that, in the developing neural tube, Dll1(Delta-like 1) was mainly localized on the processes extending from nascent neurons toward both the pia and the ventricle and accumulated at apical termini, where adherens junctions (AJs) were formed. To understand the mechanism of Dll1 localization, we searched for binding proteins for Dll1 and identified a scaffolding molecule, MAGI1. In the developing spinal cord, MAGI1 mRNA was highly expressed in the ventricular zone, where Dll1 mRNA was expressed. MAGI1 protein accumulated at the AJs formed around the termini of apically extending processes and was partially colocalized with Dll1. MAGI1 bound not only to Dll1 but also to N-cadherin-β-catenin complexes. In cultured AJ-forming fibroblasts, MAGI1 was localized at AJs, and Dll1 was recruited to these AJs through binding to MAGI1. In addition, Dll1 was stabilized on the cell surface by MAGI1. Taken together, these results suggest that Dll1 is presented on the surface of AJs formed at the apical termini of processes through interaction with MAGI1 to activate Notch on neighboring cells in the developing central nervous system. [ABSTRACT FROM AUTHOR]

During development, neuronal identity is determined by a combination of numerous transcription factors. However, the mechanisms of synergistic action of these factors in transcriptional regulation and subsequent cell fate specification are largely unknown. In this study, we identified a novel gene, Corl1, encoding a nuclear protein with homology to the Ski oncoprotein. Corl1 was highly selectively expressed in the central nervous system (CNS). In the embryonic CNS, Corl1 was expressed in a certain subset of postmitotic neurons generated posterior to the midbrain-hindbrain border. In the developing spinal cord, Corl1 was selectively expressed in the dorsal horn interneurons where a homeodomain transcription factor, Lbx1, is required for proper specification. Corl1 was localized in a nuclear dot-like structure and interacted with general transcriptional corepressors. In addition, Corl1 showed transcriptional repression activity in the GAL4-fusion system, indicating its involvement in the regulation of transcriptional repression. Furthermore, Corl1 interacted with Lbx1 and cooperatively repressed transcription, suggesting that it acts as a transcriptional corepressor for Lbx1 in regulating cell fate determination in the dorsal spinal cord. Corl1 corepressor activity did not depend on Gro/TLE activity, and Gro/TLE also functioned as a corepressor for Lbx1. Thus, Lbx1 can select two independent partners, Corl1 and Gro/TLE, as corepressors. Identification of a novel transcriptional corepressor with neuronal subtype-restricted expression might provide insights into the mechanisms of cell fate determination in neurons. [ABSTRACT FROM AUTHOR]

Neuronal differentiation is regulated by many basic-helix-loop-helix (bHLH) family transcriptional activators and repressors, and the balance of activity between these factors is important for the differentiation process. Here, we report the identification of a novel transcriptional repressor, designated Helt. Helt encoded a Hey-related bHLH protein containing the bHLH and Orange domains. Helt could homodimerize, and heterodimerize with Hes5 or Hey2. Both the bHLH and Orange domains were involved in the homodimerization. In contrast, only the bHLH domain was required for the heterodimerization with Hey2, whereas only the Orange domain mediated the interaction between Helt and Hes5. Thus, Helt has two dimerization domains, and these domains independently select a partner. Identification of preferred recognition sequences by CASTing experiments revealed that Helt bound to the E box, which was distinct from the Hes1 optimal sequence around the E box core. Not only the core sequence but also sequences flanking the E box were essential for the recognition by Helt and Hes1. Furthermore, Helt repressed transcription from an artificial promoter through binding to the optimal E box elements, as well as transcription from its own promoter. Using in situ hybridization and immunohistochemistry, Helt expression in embryos was investigated. Helt was mainly expressed in undifferentiated neural progenitors in some of the developing brain regions, including the mesencephalon and diencephalon, at the neurogenesis stage. These results suggest that Helt acts as a transcriptional repressor to regulate neuronal differentiation and/or identity. [ABSTRACT FROM AUTHOR]