Study of the evolution of terminal differentiation programs using c. Elegans serotonergic neurons as a model system

Resum

Very little is known about the molecular mechanisms that lead to the appearance of new neuronal types. It is thought that new cell types may evolve after changes in genetic networks that lead to functional novelties; however, the number of examples supporting this hypothesis is still small and none of them focus on neuronal type diversification. We intend to study the changes in regulatory networks that have led to cell identity switches throughout Caenorhabditis evolution. C. elegans hermaphrodite serotonergic system comprises nine neurons of six different types: two NSMs, two ADFs, a single RIH neuron, two AIMs and the hermaphrodite-specific HSNs. VC4 and VC5 motorneurons are not considered serotonergic in C. elegans, although they stain weakly and sporadically for 5-HT and express the vesicular monoamine transporter, cat-1. Interestingly, in a small group of Caenorhabditis species, including C. angaria, VC4 and VC5 consistently stain for serotonin. In our work we aim to identify the molecular mechanisms underlying serotonergic fate acquisition by VC4/VC5 neurons. We found that in C. elegans strains defective for heterochromatin formation, such as met-2, a histone methyltransferase, or lin-61 and hpl-2, which bind to methylated H3K9, VC4 and VC5 are robustly serotonin immunoreactive. We also found that HLH-3, a bHLH transcription factor (TF) and UNC-4, a Zn Finger TF, are both required for cat-1 expression in VC4/5 wildtype animals and also for the 5HT new fate acquisition in heterochromatin mutants. Our results suggest that VC4/5 de novo serotonergic neurons do not acquire the ability to synthesize 5HT but reuptake it from the neighbouring HSN 5HT cell. This work provides new insights into the regulatory mechanisms that underlie cell type diversification.