Complement C3 released from neurotoxic reactive astrocytes impairs learning and memory in the mouse model of postoperative neurodegenerative disorders

Perioperative neurocognitive disorder (PNDs) can commonly occur after major surgery in at risk patients and its occurrence increases medical healthcare burdens and even mortality. Accumulating evidence points to neuroinflammation being pivotal to the pathogenesis of these conditions. The complement cascade contributes to neuroinflammatory responses in the central nervous system (CNS) and complement C3 has been implicated in the manifestation of cognitive deficits in several neurological conditions. Neurotoxic reactive astrocytes function differently to their non-activated counterparts and release complement components in response to pathological triggers. We observed previously that surgery induces a rapid rise and then fall in cytokines but a more sustained glial activation response that coincided with postoperative cognitive impairment. In this study, we explored the relationship between the expression of complement C3, glial activation, and cognitive deficits. Using a murine model of surgery, we characterized the transcriptional profiles of hippocampal astrocytes after surgery and examined the effects of C3 suppression on the neuroinflammatory response and cognitive performance. There was a delayed but sustained rise in hippocampal C3 of astrocytic in origin after surgery which corresponded with the onset of cognitive decline. Furthermore, the A1 or the neurotoxic phenotype predominated in this postoperative astrocytic activation, and these cells have a distinct transcriptional profile including C3 upregulation. Suppression of C3 inhibited synaptic phagocytosis by microglia and attenuated postoperative cognitive impairment. Therefore, C3 from reactive astrocytes appear central to the development of cognitive dysfunction associated with postoperative neuroinflammation.