HLH-14 is aC. elegansAchaete-Scute protein that promotes neurogenesis through asymmetric cell division

Abstract

Achaete-Scute basic helix-loop-helix (bHLH) proteins promote neurogenesis during metazoan development. In this study, we characterize a C. elegans Achaete-Scute homolog, HLH-14. We find that a number of neuroblasts express HLH-14 in the C. elegans embryo, including the PVQ/HSN/PHB neuroblast, a cell that generates the PVQ interneuron, the HSN motoneuron and the PHB sensory neuron. hlh-14 mutants lack all three of these neurons. The fact that HLH-14 promotes all three classes of neuron indicates that C. elegans proneural bHLH factors may act less specifically than their fly and mammalian homologs. Furthermore, neural loss in hlh-14 mutants results from a defect in an asymmetric cell division: the PVQ/HSN/PHB neuroblast inappropriately assumes characteristics of its sister cell, the hyp7/T blast cell. We argue that bHLH proteins, which control various aspects of metazoan development, can control cell fate choices in C. elegans by regulating asymmetric cell divisions. Finally, a reduction in the function of hlh-2, which encodes the C. elegans E/Daughterless bHLH homolog, results in similar neuron loss as hlh-14 mutants and enhances the effects of partially reducing hlh-14 function. We propose that HLH-14 and HLH-2 act together to specify neuroblast lineages and promote neuronal fate.

The T-Box Gene tbx-2, the Homeobox Gene egl-5 and the Asymmetric Cell Division Gene ham-1 Specify Neural Fate in the HSN/PHB Lineage

“…LGI: dpy-5(e61) (Brenner 1974) LGII: bli-2(e768) (Brenner 1974); gmIs20 [hlh-14Tgfp] (Frank et al 2003); gmIs13 (this study).…”

“…This phenotype can be caused by a failure to produce HSNs (Frank et al 2003) or by the death of the HSNs (Desai et al 1988), their fate in males. Alternatively, the HSNs can be produced, but then fail to migrate (Desai et al 1988).…”

“…We assayed the PHB neurons in gm111 and tbx-2(ok529) L1 arrested hermaphrodites using two different GFP markers: gmIs13, which expresses GFP in PHAs and PHBs (Frank et al 2003), and gmIs22(nlp-1TGFP), which expresses GFP in the PHBs but not the PHAs Frank et al 2003). In both of these reporter backgrounds, tbx-2(gm111) and tbx-2(ok529) arrested animals had sides with missing cells (Figures 2C9 and 5A) suggesting that the mutation specifically affected PHB.…”

“…We tested this hypothesis by asking whether the HSN/PHB precursor expressed the egl-5TlacZ transgene, muIs13 (Wang et al 1993). The GFP reporter gmIs20 (hlh-14TGFP) is expressed in the HSN/PHB neuroblast as well as its daughter cell, the HSN/PHB precursor, providing a marker for cells in the lineage (Frank et al 2003). We generated a muIs13 (egl-5TlacZ); gmIs20 (hlh14TGFP) strain, and costained the embryos with anti-b-galactosidase and anti-GFP antibodies.…”

“…hlh-14 mutants lack HSNs, PHBs, and PVQ interneurons because the fate of the neuroblast that produces these cells is altered (Frank et al 2003). To test whether the gm111 gm345 L1 arrested animals also lacked PVQ neurons, we analyzed these mutant animals using the kyIs39 (sra-6Tgfp) transgene marker (Troemel et al 1995).…”