The post-transcriptional regulation of TFs in immature motoneurons shapes the axon-muscle connectome

Temporal factors expressed sequentially in neural stem cells, such as RNA binding proteins (RBPs) or transcription factors (TFs), are key elements in the generation of neuronal diversity. The molecular mechanism underlying how the temporal identity of stem cells is decoded into their progeny to generate neuronal diversity is largely unknown. Here, we used genetic and new computational tools to study with precision the unique fates of the progeny of a stem cell producing 29 morphologically distinct leg motoneurons (MNs) in Drosophila. We identified 40 TFs expressed in this MN lineage, 15 of which are expressed in a combinatorial manner in immature MNs just before their morphological differentiation. By following TF expression patterns at an earlier developmental stages, we discovered 19 combinatorial codes of TFs that were progressively established in immature MNs as a function of their birth order. The comparison of the RNA and protein expression profiles of 6 TFs revealed that post-transcriptional regulation plays an essential role in shaping these TF codes. We found that the two known RBPs, Imp and Syp, expressed sequentially in neuronal stem cells, are upstream regulators of the TF codes. Both RBPs are key players in the construction of axon-muscle connectome through the post-transcriptional regulation of 5 of the 6 TFs examined. By deciphering the function of Imp in the immature MNs with respect to the stem cell of the same lineage, we propose a model where RBPs shape the morphological fates of MNs through post-transcriptional regulation of TF codes in immature MNs. Taken together, our study reveals that immature MNs are plastic cells that have the potential to acquire many morphological fates. The molecular basis of MN plasticity originates in the broad expression of different TF mRNA, that are post-transcriptionally shaped into TF codes by Imp and Syp, and potentially by other RBPs that remain to be discovered, to determine their morphological fates.