lin-14 regulates the timing of synaptic remodelling in Caenorhabditis elegans

In the nematode Caenorhabditis elegans six GABAergic motor neurons, known as DDs, remodel their patterns of synaptic connectivity during larval development. DD remodelling involves a complete reversal of the direction of information flow within nerve processes without marked changes in process morphology. We used a marker localized in vivo to DD presynaptic zones to analyse how the timing of DD remodelling is controlled. In wild-type animals, DDs remodel their synaptic outputs within a 3-5-hour period at the end of the first larval stage. We show that the heterochronic gene lin-14, which controls the timing of stage-specific cell lineages, regulates the timing of DD synaptic output remodelling. In lin-14 loss-of-function mutants, DDs remodel precociously. The degree of precocious remodelling is correlated with the level of lin-14 activity. Expression of lin-14(+) in the DDs of lin-14-null mutants rescues the precocious remodelling, indicating that lin-14 can act cell-autonomously. Consistent with this hypothesis, LIN-14 protein levels decrease in the DDs before remodelling. Our observations reveal a role of heterochronic genes in non-dividing cells, and provide an example of cell-autonomous respecification of neuronal connectivity.