Achaete/Scute (A/S) basic helix-loop-helix (bHLH) proteins are required for neurogenesis in Drosophila and vertebrates. These transcription factors are generally referred to as "proneural factors", as they are both necessary and sufficient for specification of neural precursors and/or neural lineages from embryonic ectoderm. Although A/S proteins are well-characterized in Hydra, jellyfish, several insect phyla, and many vertebrates, the role of A/S proteins in C. elegans neurogenesis is relatively uncharacterized. Based on sequence homology, the C. elegans genome encodes three A/S proteins, identified as HLH-3, HLH-6, and HLH-14. HLH-3 and HLH-14 belong to the A/S type A subfamily, whereas HLH-6 belongs to the A/S type B subfamily. We find that expression of
hlh-6 is restricted to the pharyngeal glands. This is consistent with the glandular expression of other A/S type B genes, such as Mash3 in mouse salivary glands and PcAsh2 in jellyfish secretory cells. Thus, overall, A/S type B genes do not appear to have a role in neurogenesis. In contrast, the A/S type A genes,
hlh-3 and
hlh-14, are widely expressed in the developing nervous system. Previous studies by others have revealed that HLH-14 is required for specification of the PVQ/HSN/PHB neuroblast. Thus, consistent with Drosophila A/S proteins, HLH-14 is a proneural factor. In this study, we characterize HLH-3, an A/S protein most similar to Drosophila Asense and mouse Mash1 based on sequence identity and expression pattern. We find that
hlh-3 null mutants have defects in hermaphrodite-specific motor neuron (HSN) axon pathfinding. As HSN-specific expression of
hlh-3 rescues these defects, we conclude that HLH-3 is required for cell-autonomous interpretation of HSN axon guidance cues. It is known that the UNC-40 netrin receptor mediates cues for attraction of the HSN growth cone. Thus, we propose that HLH-3 regulates the expression of
unc-40 specifically in the HSNs, as
hlh-3 mutants do not exhibit additional
unc-40 like phenotypes. We will report on the expression of
unc-40::gfp in an
hlh-3 null mutant, as well as the penetrance of HSN axon pathfinding defects in an
unc-40;
hlh-3 double mutant. Significantly, our work suggests a novel mechanism by which A/S bHLH proteins are linked to mechanisms of axonal pathfinding.