| Abstract |
Factors that drive immune responses have emerged as potent regulators of neurodevelopment and function. The Toll-like receptor (TLR) superfamily of proteins are linchpins of innate immunity in both vertebrates and invertebrates; however, their roles in the developing nervous system are just coming into focus. Using Drosophila melanogaster as a model, we uncovered novel roles for a TLR, Toll-6, in two distinct neurodevelopmental contexts.
First, we find that Toll-6 receptor engages non-canonical signaling partners in motoneurons. We present molecular and genetic evidence that the Toll-6 receptor, the Toll/interleukin-1 receptor domain adaptor dSARM, and the FoxO transcription factor function in a linear genetic pathway to promote dynamic microtubules in motoneurons. To provide mechanistic insight into pathway function, we screened known cytoskeletal regulators for altered expression in pathway mutants. We identified that Toll-6-FoxO signaling represses MKLP1/Pavarotti (Pav-KLP), a mitotic kinesin that functions to brake microtubule dynamics during cytokinesis and has emerging roles in postmitotic neurons. We next sought to define in vivo functions of this pathway. We show that impaired Toll-6-FoxO signaling leads to enhanced microtubule stability in the presynaptic compartment. The elevated MT stability observed in Toll-6-FoxO pathway mutants leads to impaired activity-dependent structural plasticity. Furthermore, presynaptic plasticity is fully restored by either reducing levels of Pav-KLP or by pharmacologically destabilizing microtubules in pathway mutants. These results indicate that dynamic microtubules are critical for presynaptic structural plasticity. Thus, a Toll-6-FoxO pathway promotes structural plasticity via a novel microtubule-based mechanism.
We next extend the neurodevelopmental functions of Toll-6-FoxO signaling to glia. We demonstrate that Toll-6 functions cell-autonomously in cortex glia to direct a critical neuron-glia interaction. Specifically, dying neurons and glia communicate via Toll-6-mediated innate immune signaling. In particular, we provide evidence that neuronal apoptosis triggers Furin-dependent processing and activation of Toll-6’s ligand, Spatzle5. This neuron-derived cue activates a FoxO-mediated Toll-6 transcriptional pathway in glia to drive expression of the Draper engulfment receptor. Live-imaging and genetic analyses reveal that blocking neuronal Spatzle5 or cortex glial Toll-6-FoxO signaling leads to impaired Draper-dependent phagocytic clearance of cell corpses in the developing CNS. Additionally, we show that sustained loss of Toll-6-FoxO signaling in cortex glia leads to early-onset neurodegeneration in adult flies. Our work identifies a novel priming signal that functions upstream of the Draper-mediated “eat-me” cue to ready cortex glia for phagocytosis. Therefore, a Toll receptor-directed innate immune pathway promotes a developmental neuron-glia interaction important for brain health and homeostasis.
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