Inflammation and NFκB activation is decreased by hypothermia following global cerebral ischemia
Introduction
The neuroprotective effects of mild hypothermia have been well documented in experimental models (see reviews Krieger and Yenari, 2004; Liu and Yenari, 2007; Lyden et al., 2006). Furthermore, there is now clinical evidence showing that mild hypothermia significantly protects against neurological damage following cardiac arrest (Bernard et al., 2002, Hypothermia after Cardiac Arrest Study Group, 2002). The precise mechanism(s) by which mild hypothermia protects brain cells remains to be elucidated, but it is likely that hypothermia acts upon multiple pathways to ultimately prevent cell death (Liu and Yenari, 2007, Lyden et al., 2006). Hypothermia has been reported to attenuate cytochrome c release, apoptosis inducing factor induction, excitatory amino acid accumulation, free radical generation and increased BBB permeability (Zhao et al., 2007). There is a growing literature on the damaging nature of the inflammatory response to brain ischemia (Tang and Yenari, 2006; Wang et al., 2007).
Earlier findings in studies of various models of brain ischemia, especially focal cerebral ischemia (FCI, a model of stroke) have focused on the notion that hypothermia mitigates damage following ischemia by decreasing metabolic rate (Erecinska et al., 2003, Lanier, 1995, Yenari et al., 2004) and improving ion homeostasis (Sick et al., 1999). However, hypothermia appears to mitigate a variety of toxic factors such as decreasing reactive oxygen species (ROS) (Maier et al., 2002), suppressing the infiltration of neutrophils, and decreasing activation of microglia (Ishikawa et al., 1999, Wang et al., 2002). However, the role in inflammation and especially the effect of hypothermia on inflammation has been studied to a lesser extent in global cerebral ischemia (GCI), an experimental correlate of cardiac arrest.
Inflammation is an orchestrated response involving the rapid upregulation and activation of a variety of genes. Nuclear factor-kappa B (NFκB) is a major transcription factor involved in this response. NFκB is normally sequestered in the cytoplasm where it is bound to a family of inhibitory proteins known as the inhibitor of NFκB (IκB). A major IκB and the one most often studied is IκB-α. Inflammatory stimuli activate a family of upstream kinases (IκB kinase, IKK) which phosphorylates IκB leading to its degradation and the liberation of NFκB to enter the nucleus and induce gene expression (Rothwarf and Karin, 1999).
Previously, we showed that NFκB activation is reduced by mild hypothermia following focal cerebral ischemia (Han et al., 2003), and this reduction may explain some of the anti-inflammatory effects of hypothermia. In the FCI model, we found that hypothermia inhibited NFκB's activating kinases, IKKγ and IKKβ. Here, we characterize the inflammatory response in the brain following GCI, and how it is mitigated by hypothermia.
Section snippets
In vivo models of brain ischemia and inflammation
All experimental protocols carried out on animals were approved by the Stanford University Administrative Panel on Laboratory Animal Care and San Francisco VA Medical Center's Animal Care Facility and were in accordance with NIH guidelines.
The inflammatory response following GCI
GCI increased microglial reactivity for OX42 in the rat. 72 h post GCI, OX42 staining was increased in intensity with increased size and numbers of microglia. Microglia were especially noticeable between the blades of the dentate as well as around the CA1 region of the hippocampus (Figs. 1A–C). Myeloperoxidase staining (Figs. 1D, E) did not convincingly demonstrate the presence of neutrophils, although rare positive cells were observed. There was also no evidence of Evan's blue dye
Discussion
At both the laboratory and clinical levels, mild hypothermia has been shown by several labs to improve neurological outcome from the devastating effects of global cerebral ischemic brain injury at both the histological and behavioral levels (Colbourne and Corbett, 1994, Colbourne and Corbett, 1995; Colbourne et al., 1999, Dietrich et al., 1993, Lyden et al., 2006). Several mechanisms by which mild hypothermia exerts its protective effects have been characterized, including salutary effects on
Acknowledgments
This work was made possible by the following institutions and grants: NIH RO1 NS40516 (MAY), AHA EIA0540066N (MAY), NIH P50 NS14543 and P01 NS37520 (RGG, MAY), NIH R01 NS53898 (RGG).
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2017, Progress in NeurobiologyCitation Excerpt :By reducing the number of leukocytes, there is a reduction in the amount of TNF-α released, a potent cytokine that inhibits Complex-I of the ETC (Drury et al., 2010). It has been shown that there is also significant reduction in tumor necrosis factor receptor-1 that further limit the effects of TNF-α (Webster et al., 2009; Yenari and Han, 2006; Yenari and Han, 2012). These immune and inflammatory responses do not occur until relatively late following the HIE and because they take time to develop there is a clear therapeutic window; for these reasons, hypothermia is especially effective in for dealing with immune and inflammatory responses (Polderman, 2009).
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Contributed equally to the work presented.