The role of Imp and Syp RBPs in precise neuronal elimination by apoptosis through the regulation of TFs

Abstract

SUMMARYNeuronal stem cells produce a finite and stereotyped number of neuronal progenies. This process must be finely regulated during development and adult stages to ensure proper brain function. In Drosophila, stem cells, called Neuroblasts, produce an invariant number of neurons. Two RNA binding proteins, Imp and Syp, play a central role in controlling the speed of division and the end of the proliferative phase of individual NBs, two parameters that influences the final number of neurons produced. Here, we have discovered a novel function for Imp and Syp, where both RBPs also shape the number of neurons produced by a stem cell by controlling program cell death (PCD) in immature neurons. By studying a neuroblast lineage, called Lin A/15, which produces motoneurons (MNs) and glia, we have demonstrated that Lin A/15 stem cell spends 40% of its time producing immature MNs which are eliminated by apoptosis. We have revealed that only the first born MNs (Imp +) survive while the last born MNs (Imp-Syp+) are eliminated by apoptosis. Both RBPs play a central role in neuronal survival, Imp promotes neuronal survival while Syp promotes cell death in immature motoneurons. Interestingly their opposite temporal gradient in Lin A/15 stem cell also determines the end of Lin A/15 stem cell neurogenesis by PCD. Both RNA binding proteins are conserved in vertebrates and seem to play a central role in the number of neurons produce during development. The Drosophila model and its genetic tools offer a unique chance to decipher their function in neural stem cell versus immature neurons.

Post-transcriptional regulation of transcription factor codes in immature neurons drives neuronal diversity

“…To investigate further post-transcriptional regulation, we investigated the expression dynamics and function on muscle innervation of two known RBPs, Imp and Syp, that are major players in the temporal specification of VNC and central brain lineages in Drosophila (Liu et al, 2015;Ren et al, 2017;Syed et al, 2017). In Lin A/15 NB, both Imp and Syp proteins follow opposing temporal expression gradients in accordance with previously described studies of other NBs in the CNS (Guan et al, 2021).…”

“…However, both proteins might also determine other aspects of MN fate. In our parallel study, we have demonstrated that both RBPs act in iMNs to determine the survival of Lin A/15 MNs (Guan et al, 2021). Syp prevents the survival of iMNs, whereas Imp promotes it.…”

“…This demonstrates that, up to the end of L3, iMNs maintain distinct spatial positions that are dictated by their birth dates. Importantly for our study, the most ventral iMNs, which are the last-born iMNs, undergo apoptosis during the pupal stages (Guan et al, 2021), whereas the first-born 29 iMNs morphologically differentiate during metamorphosis (Figure 1L). This clear relationship between birth order and the spatial organization of iMNs was then used to understand how iMNs are morphologically specified by TFs.…”

“…Interestingly, we observed more axonal branches than normal in the distal region of the femur in Syp À/À , suggesting that abolishing Syp function is sufficient to re-direct axonal targeting from distal tibia to distal femur (Figures 5B, 5G, and 5K; Table S2). In Syp À/À clones, more neurons are produced (Figure S6), probably due to the survival of the supernumerary and a late decommissioning of the NB as observed in Syp RNAi LinA/15 (Guan et al, 2021). The ectopic expression of Imp in all Lin A/15 cells, including the Syp + cells (which normally express a low level of Imp), induced a similar innervation phenotype and an increase of the MN produced to that observed in Syp À/À MARCM clones (Figures 5C, 5H, and 5K; Table S2).…”

“…PCCD reveals that 15 out of the 16 differentially expressed TFs are expressed in combinations in the 29 first-born Lin A/15 iMNs (Figure 2M). One TF (Eyeless) was only expressed in the supernumerary iMNs eliminated by apoptosis during pupal stages (Guan et al, 2021). To validate and refine this analysis, 12 co-immunostainings for different TFs were performed (Figure 2N illustrates Chinmo and Jim co-staining; Figure S2 shows all co-stainings).…”