Diversification of Caenorhabditis elegans motor neuron identity via selective effector gene repression

A common organizational feature of any nervous system is the existence of groups of neurons that share a set of common traits but that can be further divided into individual neuron types and subtypes. Understanding the mechanistic basis of neuron type and subtype diversification processes will constitute a major step toward understanding brain development and evolution. In this dissertation, I have explored the mechanistic basis for the specification of motor neuron classes in the nematode C. elegans which serves as a paradigm for neuron diversification processes. Cholinergic motor neurons in the C. elegans ventral nerve cord share common traits, but are also comprised of many distinct classes, each characterized by unique patterns of effector gene expression (e.g. motor neuron class-specific ion channels, signaling molecules, and neurotransmitter receptors). Both the common as well as class-specific traits are directly activated by the terminal selector of cholinergic motor neuron identity, the EBF/COE-like transcription factor UNC-3. Via forward genetic screens to identify mutants that are defective in class specification, I have discovered that the diversification of UNC-3/EBF-dependent cholinergic motor neurons is controlled by distinct sets of phylogenetically conserved, motor neuron class-specific transcriptional repressors. One such repressor is in fact a novel gene previously uncharacterized in C. elegans or any nervous systems and is now named bnc-1. By molecularly dissecting the cis-regulatory region of effector genes, I found that the repressor proteins prevent UNC-3/EBF from activating class-specific effector genes in specific motor neuron subsets via discrete binding sites that are adjacent to those of UNC-3/EBF. And by using CRISPR/Cas9-mediated genome engineering to tag repressor proteins with inducible degrons, I demonstrate that these repressors share the important feature of being continuously required throughout the life of the animal to counteract, in a class-specific manner, the function of the UNC-3/EBF terminal selector that is active in all motor neuron classes. I propose that the strategy of antagonizing the activity of broadly acting terminal selectors of neuron identity in a neuron subtype-specific manner may constitute a general principle of neuron subtype diversification.