Helt, a novel basic-helix-loop-helix transcriptional repressor expressed in the developing central nervous system

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.