In C. elegans, the rectal epithelial Y cell transdifferentiates into the PDA motor neuron (Jarriault et al., 2008). Transdifferentiation (Td) has been suggested to occur in discrete steps: the Y cell loses rectal contacts and de-differentiates, migrates away from the rectum, and re-differentiates into a motoneuron. Using a membrane-tagged fluorescent reporter, we now observe a more complex pattern of events. While the cell body migrates away from the rectal slit, the Y cell maintains rectal tube contacts, marked with the apical junction protein AJM-1. From the rectal contact site, the cell then extends a process that matures into the PDA axon. Rectal apical junctions are lost later. Previous studies revealed that SEM-4 and EGL-5 transcription factors are required for erasure of Y cell epithelial identity and acquisition of neuronal identity. We identified two novel Td transcriptional regulators, the bHLH proteins NGN-1 and HLH-16, also required for these steps.
ngn-1 expression increases as the Y cell migrates away from the rectum and changes cellular morphology. From reporter fusion studies, we found that SEM-4 represses
ngn-1 expression, whereas EGL-5 and HLH-16 promote
ngn-1 expression. Although
ngn-1 induction correlates with the timing of Y cell migration, precocious expression of
ngn-1 in
sem-4 mutants does not change migration timing, suggesting involvement of other regulators. We also identified three cytoskeletal organizing proteins required for acquisition of neuronal identity:
unc-44,
unc-119, and
unc-33. These proteins appear to act downstream of NGN-1 and HLH-16 and are required for PDA axon extension. Y-to-PDA Td bears striking morphological and molecular similarities to vertebrate spinal-cord pMN-domain motoneuron formation, and to islet cell formation in the pancreas. During development, motoneuron and pancreatic beta cell precursors originate from an epithelium. These precursors then lose their junctions and delaminate from the epithelium. As they migrate and mature, the cells initiate transcription of motoneuron-associated genes, become innervated, and secrete neurotransmitters or insulin, respectively. In both the spinal cord and pancreas, NGN-1 and HLH-16 homologs (Ngn2/Olig2 and Ngn3/bHLH4, respectively) are required for epithelial identity erasure. Furthermore,
unc-44/Ankyrin promotes neuronal and beta cell maturation. Thus, our findings may provide mechanistic insight into the differentiation of these important vertebrate cell types.