In the developing nervous system, proneural genes are among the most-early acting factors required to precisely govern the decision of an undifferentiated cell to become a neuron. bHLH transcription factors such as Drosophila achaete-scute complex (AS-C) genes, their vertebrate orthologues Ascl/Math and C. elegans AS-C homolog
hlh-14 have been reported to carry such proneural activity. Thus far, bHLH-s have been shown to be the only regulatory factors to exhibit the proneural activities in only a handful of neurons in C. elegans. To assess how broadly proneural genes are involved in C. elegans neurogenesis, we undertook an expression pattern analysis. First, we systematically lineaged fosmid-based reporters for eleven of bHLHs. Unexpectedly, we found that one of the C. elegans AS-C homolog,
hlh-4, is expressed and functions in a manner that is fundamentally different from classic proneural bHLH genes: In the developing embryonic, larval and adult nervous system,
hlh-4 is exclusively expressed in a single, nociceptive neuron type, ADL.
hlh-4 expression in ADL is maintained via autoregulation throughout the life of the animal. Moreover, rather than displaying proneural activity,
hlh-4 is required to regulate the expression of ADL's unusually large repertoire of GPCR genes, as well as all other known features of terminal ADL identity, including its nociceptive function. Further, the expression of these terminal identity features is mediated by a conserved E-box motif. We found that loss of the ADL-expressed LIM homeobox gene
lin-11 also controls expression of multiple ADL markers and is also required for ADL-mediated avoidance behavior. This observation suggests that
hlh-4 may act together with
lin-11 to define ADL identity and function. Taken together, we have shown an unusual function for a member of the proneural bHLH family,
hlh-4, to act as a terminal selector of ADL neuronal identity. Considering a previous report from our lab, in which we deciphered a role for
hlh-14, in neurogenesis of ADL-lineage, we conclude that two members of As-C orthologues in C. elegans,
hlh-14 and
hlh-4, act sequentially for neurogenesis (
hlh-14) and ensuing specification of ADL terminal identity (
hlh-4).