Glial cells play an essential role in defending brain health and managing neuronal stress and damage. Neurodegeneration triggers robust glial immune responses, including changes in cytoskeletal dynamics, glial cell migration, and increased phagocytic activity. Timely removal and degradation of degenerating axons and neuronal debris by glia confers neuroprotection in the brain. Despite the importance of glial responses to axon injury, we still know surprisingly little about how damaged neurons invoke immune reactions in glial cells. What signals are released from degenerating neurons? What prompts the release of these injury cues? Finally, how are these signals translated by glia to carry out efficient immune responses to damage? We are using the fruit fly Drosophila melanogaster as a tractable model to investigate the immune communication relays that exist between neurons and glial cells in vivo. The fly nervous system contains distinct glial subtypes that are molecularly and functionally similar to vertebrate glia. Moreover, well-established axotomy assays in the adult olfactory system and the adult wing reveal that Drosophila axons undergo a classic Wallerian degeneration (WD) program, which includes increased intra-axonal calcium waves, axon fragmentation, and subsequent clearance by phagocytic glia. Notably, our lab has recently shown that axon degeneration triggers activation of the insulin-like signaling (ILS) pathway in reactive ensheathing glia, which, in turn, elicits essential glial immune responses, including transcriptional upregulation of immune genes (e.g. the engulfment receptor Draper) and phagocytic activity. We hypothesize that neuropeptide-containing dense core vesicles (DCVs) are broadly released from severed axons to trigger immune responses in local glial cells. Here, we propose to use static and live confocal imaging, transcriptional profiling, and newly developed in vivo reporters to investigate how neuropeptide signaling between neurons and glia informs glial immune responses to nerve injury. Specifically, we will 1) monitor DCV dynamics and release in adult severed nerves, 2) utilize novel single transcript labeling methods to visualize local translation of immune mRNA transcripts in glial extensions at sites of injury, and 3) determine how neuropeptide signaling between discrete glial subtypes ensures that glial responses to degenerating axons are properly carried out. Together, these findings will offer exciting molecular and cellular insight into how neuropeptide signaling between neurons and glia govern immune responses in both acute and chronic degenerative conditions.
Glial cells, the primary immune responders in the brain, respond swiftly to neurodegeneration, and abnormal innate immune responses are coupled to the progression of devastating neurological conditions, as well as poor recovery from acute traumatic events. This proposal is relevant to public health because it will provide critically needed insight into the molecular and cellular signaling pathways that govern glial responses to neural injury. These discoveries will advance our understanding of neuron-glia communication in the brain, provide a springboard for the development of treatments to reduce the burden of neurological disorders and, thus, will be highly relevant to the NIH mission.
