Regulating Dopamine And 5 - Hydroxytryptamine Mechanism In The Expression Of Male C. Elegans Tail Ray

Distinct neuronal fates, reflected by the production of appropriate neurotransmitter, selection of proper axon pathfinding, and formation of specific types of synaptic connectivity, are specified by the concerted action of many factors. Elucidation of the genetic circuitry that coordinately regulates the generation of these generic and specific phenotypes of neurons with different types and origins is a fundamental issue in understanding the formation of the exquisitely complex nerve circuits in various organisms.One important feature of differentiated neurons is the distinct neurotransmitter produced. Common examples of neurotransmitters include dopamine (DA) and serotonin (5-HT), which control multiple behavior processes. Abnormal functioning of dopaminergic and serotonergic neurons cause mental and neurological disorders [1] . For example, selective degeneration of dopaminergic neurons in midbrain leads to Parkinson's disease, while abnormal function of 5-HT neurons has been implicated in depression. Selection of neurotransmitter phenotype involves the interactions of multiple extrinsic and intrinsic factors. For example, secreted signaling molecules Sonic hedgehog (Shh) and FGF8, and the activity of transcription factors Lmx1b, Nurr1, and Pitx3, are required for specification and maintenance of midbrain dopaminergic neurons, while 5HT neurons in the hindbrain are induced by the concerted action of extracellular signaling FGF4, Shh, FGF8, and intracellular factors, including Nkx2.2, Gata3, and Pet1 [2,3 ]. The mechanisms by which the neurotransmitters are specified in other regions and how the production of specific neurotransmitter is coordinately regulated with the development of other aspects of neuronal identities are largely unknown.The development of C. elegans male ray neurons provides a simple system for exploring how distinct neurotransmitters are specified and how discrete developmental programs of neurons are coordinately regulated [4] . Each finger-like sensory ray, protruding in the mail tail, consists of the dendritic endings of two ultra-structurally distinct neurons, an A-type neuron (RnA) and an B-type neuron (RnB), wrapped in the process of a glial-like structural cell (Rnst) (where n stands for rays 1 to 9). Each ray has a unique identity due to the expression of distinct constellation of characteristics in their constituent cells: morphology, neurotransmitter choice, and axon trajectory. A subset of A-type neurons, R5A, R7A, and R9A, express the neurotransmitter dopamine, while a subset of B-type neurons, R1B, R3B, and R9B, express serotonin. We have shown previously that a TGF-βsignaling pathway and the Hox gene egl-5 (ortholog of Drosophila Abd-B) play important roles in the specification of dopaminergic and serotonergic ray neurons [5,6]. Mutations in components of TGF-βsignaling result in loss of dopamine expression in R5, 7, 9 and serotonin expression in R9. In the egl-5 mutant, dopaminergic and serotonergic fates are lost in rays derived from the epidermal seam cell V6 (rays 2 to 6), in which egl-5 is expressed. Identification of additional genes that specify dopaminergic and serotonergic neurons may contribute to our understanding of the molecular mechanisms underlying the formation of neurons with defined neurotransmitters in other organisms.Polycomb group proteins (PcG) are most studied for their evolutionary roles in specifying positional identity through their transcriptional repression of Hox genes [7]. In PcG mutants, Hox genes are ectopically expressed, resulting in homeotic transformations. Consistent with their roles in transcriptional repression, PcG proteins mediate the formation of repressive chromatin structures by modifying the histone tails [8] . For example, the ESC-E(Z) complex contains a methyltransferase activity for histone 3 (H3), the PRC1 complex functions as an E3 ubiquitin ligase for H2A, and the PhoRC complex contains binding activity for methylated H3K9 and H4K20 [9,10] . PcG genes also regulate the expression of non-Hox genes, including factors involved in development and differentiation processes, such as TGF-βand Wnt [11] . In addition to acting as transcriptional repressors, the mammalian ESC/E(Z) PcG complex, EED/EZH2, functions in the cytoplasm in regulating actin polymerization and also in transducing an integrin receptor signal [12,13] . The roles of PcG genes in neuronal specification have yet to be determined.In this study we demonstrated that the C. elegans PcG genes, sop-2 and sor-3, are involved in specifying neurotransmitter phenotype and several other neuronal properties, including axon pathfinding. Specification of certain neuronal identities by PcG genes involves regulation of non-Hox gene targets. Our studies revealed a key role of epigenetic regulation of chromatin structures in specifying neuronal fate and may contribute to the design of new strategies for engineering dopaminergic neurons for treating human diseases, such as Parkinson's disease.