Coordinated regulation of cholinergic motor neuron traits through a conserved terminal selector gene

Cholinergic motor neurons are defined by the co-expression of a battery of genes which encode proteins that act sequentially to synthesize, package and degrade acetylcholine and reuptake its breakdown product, choline. How expression of these critical motor neuron identity determinants is controlled and coordinated is not understood. We show here that in the nematode Caenorhabditis elegans all members of the cholinergic gene battery, as well as many other markers of terminal motor neuron fate, are co-regulated by a shared cis-regulatory signature and a common trans-acting factor, the phylogenetically conserved COE (Collier/Olf/EBF)-type transcription factor UNC-3. UNC-3 initiates and maintains expression of cholinergic fate markers and is sufficient to induce cholinergic fate in other neuron types. UNC-3 furthermore operates in negative feedforward loops to induce the expression of transcription factors that repress individual, UNC-3-induced terminal fate markers resulting in diversification of motor neuron differentiation programs in specific motor neuron subtypes. A chordate ortholog of UNC-3, Ciona intestinalis COE, is also both required and sufficient for inducing a cholinergic fate. Thus, UNC-3 is a terminal selector for cholinergic motor neuron differentiation whose function is conserved across phylogeny.