Unconventional function of an Achaete-Scute homolog as a terminal selector of nociceptive neuron identity

Abstract

Proneural genes are among the most early-acting genes in nervous system development, instructing blast cells to commit to a neuronal fate. Drosophila Atonal and Achaete-Scute complex (AS-C) genes, as well as their vertebrate orthologs, are basic helix-loop-helix (bHLH) transcription factors with such proneural activity. We show here that a C. elegans AS-C homolog, hlh-4, functions in a fundamentally different manner. In the embryonic, larval, and adult nervous systems, hlh-4 is expressed exclusively in a single nociceptive neuron class, ADL, and its expression in ADL is maintained via transcriptional autoregulation throughout the life of the animal. However, in hlh-4 null mutants, the ADL neuron is generated and still appears neuronal in overall morphology and expression of panneuronal and pansensory features. Rather than acting as a proneural gene, we find that hlh-4 is required for the ADL neuron to function properly, to adopt its correct morphology, to express its unusually large repertoire of olfactory receptor–encoding genes, and to express other known features of terminal ADL identity, including neurotransmitter phenotype, neuropeptides, ion channels, and electrical synapse proteins. hlh-4 is sufficient to induce ADL identity features upon ectopic expression in other neuron types. The expression of ADL terminal identity features is directly controlled by HLH-4 via a phylogenetically conserved E-box motif, which, through bioinformatic analysis, we find to constitute a predictive feature of ADL-expressed terminal identity markers. The lineage that produces the ADL neuron was previously shown to require the conventional, transient proneural activity of another AS-C homolog, hlh-14, demonstrating sequential activities of distinct AS-C-type bHLH genes in neuronal specification. Taken together, we have defined here an unconventional function of an AS-C-type bHLH gene as a terminal selector of neuronal identity and we speculate that such function could be reflective of an ancestral function of an “ur-” bHLH gene.

Transcription factor autoregulation is required for acquisition and maintenance of neuronal identity

“…All known terminal selectors are continuously expressed throughout the life of the respective neuron type (Hobert, 2016). Although many terminal selector indeed autoregulate (Baumeister et al, 1996;Hobert et al, 1997;Masoudi et al, 2018;Way and Chalfie, 1989), the functional necessity of the autoregulatory phenomenon per se has not yet been examined.…”

Inherency of Form and Function in Animal Development and Evolution

“…However, several phenomena, partly overlapping, contribute to “developmental system drift” that can disconnect cell types from their originating GRNs (True and Haag, 2001; Halfon, 2017). These include, as mentioned above, variable enhancer use across species, the fact that enhancers and promoters do not have entirely disjoint roles and can substitute for one another, the fact that developmental TFs have high concentrations of IDP domains and consequently operate at different cis-acting elements in different contexts, including different stages in the establishment of a cell-type lineage (Niklas et al, 2015; Masoudi et al, 2018), and the fact that a TF can act in entirely opposite fashions depending on the context (Murgan et al, 2015; Lukoseviciute et al, 2018).…”

A new method of recording from the giant fiber of Drosophila melanogaster shows that the strength of its auditory inputs remains constant with age

“…Rapid turnover of their molecular constituents means that synapses are evanescent structures unless maintained [73]. The pattern, and strength, of a neuron's synaptic connections are critically important aspects of its identity and, as such, are maintained in part by the persistent expression of "terminal selector" transcription factors [74][75][76] via self-maintaining mechanisms such as transcriptional autoregulation [77]. It often seems to be the case that these transcription factors are used early in development for patterning the body or nervous system, and then are re-used later in life for maintenance of neuronal connectivity [76,78,79].…”

“…The pattern, and strength, of a neuron's synaptic connections are critically important aspects of its identity and, as such, are maintained in part by the persistent expression of "terminal selector" transcription factors [74][75][76] via self-maintaining mechanisms such as transcriptional autoregulation [77]. It often seems to be the case that these transcription factors are used early in development for patterning the body or nervous system, and then are re-used later in life for maintenance of neuronal connectivity [76,78,79]. Whatever the molecular mechanisms for JON-GF synapse maintenance, keeping them operational in "middle-aged" flies is presumably adaptive, and therefore worth the metabolic costs, since males and females are still able to reproduce (albeit less successfully) until about 40 d of age [50,51,53].…”