Investigating the significant roles of PKR on systemic inflammation triggered neuroinflammation and cognitive dysfunctions

Abstract

Increasing lines of evidence have shown that systemic inflammation may contribute to neuroinflammation and accelerate the progression of neurodegenerative diseases. Double-stranded RNA-dependent protein kinase (PKR) is a key signaling molecule that regulates immune responses by regulating macrophage activation, various inflammatory pathways, and formation of inflammasomes. This study aims at investigating whether PKR can be a pharmacological target to prevent both systemic inflammation-triggered neuroinflammation and cognitive dysfunctions.

In this study, a non-bacterial endotoxin mouse model of laparotomy is adopted to address the systemic inflammation triggered by surgery. For the first part of our study, both male wild-type C57BL/6J and C57BL/6-Tg(CD68-EGFP)1Drg/J mice were assigned into 2 groups: laparotomy under sevoflurane anesthesia and control group under sevoflurane anesthesia to study the effects of laparotomy on peripheral and neural inflammation, tau phosphorylation and cognitive functions. In the second part, PKR-/- mice were exposed to laparotomy with sevoflurane anesthesia or sevoflurane anesthesia to examine the role of PKR in regulating systemic inflammation-triggered neuroinflammation. For both parts of our study, systemic and neuroinflammatory responses, tau phosphorylation, and cognitions were examined. For the third part, intracerebroventricular injection of rAAV-DIO-PKR-K296R into the right lateral ventricle of ChAT-IRES-Cre-eGFP mice was performed to inhibit PKR activation in cholinergic neurons. Mice were then subjected to laparotomy with sevoflurane or sevoflurane anesthesia respectively. The effects of blocking PKR in cholinergic neurons on modulating glucose metabolism and cognitive functions were examined in an inflammatory context.

Considering the laparotomy model, both the systemic and neural immune responses developed rapidly after the surgery. Significant increases in the cytokine mRNA expression in the liver (IL-1β and MCP-1) and hippocampus (IL-1β) were observed at 4 h after the laparotomy. The level of IL-1β expression remained high in the hippocampus on post-operative day (POD) 1. Persistent activation of microglia/ macrophage was observed on POD 14 by immunohistochemical staining and flow cytometry. Moreover, abnormal phosphorylation of tau and cognitive deficits were also observed in the wild-type mice following laparotomy. By genetic knockout of PKR in mice, it potently downregulated the increased IL-1β expression induced by laparotomy in both the liver and frontal cortex at 4 h and POD 1. Meanwhile, knockout of PKR significantly ameliorated the microglial activation in the frontal cortex and hippocampus; and reduced the laparotomy-induced tau protein phosphorylation (AT8 and pS416) in the frontal cortex. Furthermore, knockout of PKR could ameliorate the laparotomy-induced cognitive impairment, as evidenced by improved problem-solving skills, short-term and long-term memory. Additionally, inhibition of PKR in cholinergic neurons in mice could rescue the laparotomy-induced brain glucose hypometabolism and cognitive deficits.

In summary, the laparotomy model was a viable and appropriate model for studying neuroimmune responses triggered by systemic inflammation, as evidenced by microgliosis, abnormal tau phosphorylation, and cognitive dysfunction. Our results suggested the critical role of PKR in regulating peripheral and neural inflammation, tau phosphorylation and cognition. Furthermore, PKR plays an essential role in cholinergic neurons on modulating glucose metabolism and cognitive functions in the inflammatory context. Therefore, PKR could be a pharmacological target for treating systemic inflammation-induced neuroinflammation and cognitive deficits.