Investigating the roles of protein kinase R (PKR) to modulate the effects of systemic inflammation on the brain

Abstract

Systemic inflammation induces neuroinflammation and alters neural activity and functions, ultimately leading to sickness behavior (fever, anorexia, motor impairments, decreased exploratory activity, and depressed mood). While these changes collectively help the body to recover from infections and injuries, increasing lines of evidence have indicated that their dysregulation may be involved in depression and delirium. Therefore, a more thorough understanding about their regulation will be beneficial. The aims of this study are to investigate the roles of protein kinase R (PKR), an immune-regulatory kinase that has also been implicated to modulate neurons, in systemic inflammation-induced neuroinflammation, sickness behavior, and neuronal responses.

Three-month old female wild type (WT) and PKR knockout (-/-) mice were subcutaneously injected with live Escherichia coli (E. coli) or phosphate buffered saline (PBS). Central and peripheral inflammation were assessed by determining the mRNA expression levels of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), cyclooxygenase-2 (COX-2), and inducible nitric oxide synthase (iNOS) in the brain (hypothalamus and hippocampus) and the liver.

Additionally, the percentage of neutrophils was quantified from blood smears as another indicator of systemic inflammation. Moreover, sickness behavior was monitored by recording the changes in rectal temperature and food consumption, and by several behavioral tasks (burrowing, open field test, object investigation test, social interaction test, and forced swimming test). Finally, neuronal responses to systemic inflammation were studied in terms of neural activity changes at the paraventricular nucleus (PVN), i.e. indirectly by immunohistochemical staining for c-fos (a neural activity marker), and also in terms of protein level changes of several synaptic proteins, i.e. postsynaptic density 95 (PSD95), NMDA receptor 2B (NMDAR2B), and NMDA receptor 1 (NMDAR1), in the hypothalamus and hippocampus.

My data indicate that E. coli induced inflammatory changes in the brain and at the periphery in both genotypes of mice. Loss of PKR effectively attenuated the inflammatory changes at the periphery, whereas it slightly inhibited the neuroinflammatory responses in the hypothalamus, and it had no effect on those in the hippocampus. Unexpectedly, the decrease of systemic inflammation in PKR -/- mice was not associated with reduced sickness behavior. Instead PKR -/- mice displayed exacerbated sickness behavior when compared WT mice. Regarding the neuronal response changes to systemic inflammation, E. coli increased the number of c-fos immunopositive cells at the PVN of WT mice, but this was not observed in PKR -/- mice. Although E. coli did not affect the expression of NMDAR1 and PSD95 (hypothalamus and hippocampus) and NMDAR2B (hypothalamus) in both genotypes, it lowered NMDAR2B expression in the hippocampus of PKR -/- mice but not in WT mice.

Taken together, these findings suggest that (1) deficiency of PKR could effectively suppress peripheral inflammatory responses but it has little effect on brain inflammatory changes, (2) loss of PKR could exacerbate sickness behavior despite diminishing systemic inflammation, suggesting that PKR likely regulates sickness behavior through a non-inflammation mechanism, and (3) PKR could modulate neuronal responses to systemic inflammation. Future studies should address whether the alteration of neuronal responses could increase sickness in PKR -/- mice.