Extracellular adenosine as a novel neuroimmune modulator

Chiu, Gabriel

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https://hdl.handle.net/2142/49647 Copy

Description

Title

Extracellular adenosine as a novel neuroimmune modulator

Author(s)

Chiu, Gabriel

Issue Date

2014-05-30T17:02:45Z

Director of Research (if dissertation) or Advisor (if thesis)

Freund, Gregory G.

Doctoral Committee Chair(s)

Johnson, Rodney W.

Committee Member(s)

• Freund, Gregory G.
• Woods, Jeffrey A.
• Dilger, Ryan N.

Department of Study

Nutritional Sciences

Discipline

Nutritional Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Date of Ingest

2014-05-30T17:02:45Z

Keyword(s)

• Adenosine
• Interleukin-1
• Caspase-1
• Neuroimmunology

Abstract

Adenosine is a pleotropic purine nucleoside with actions ranging from neurotransmission to immunomodulation. Pathologic processes that trigger cellular stress and damage also cause significant increases in local and systemic concentrations of adenosine. Coinciding with elevation in adenosine is an increase in brain-based IL-1. Perturbed cognition appears to be a sequela of acute hypoxia but the mechanisms by which cognitive functions are impacted are not clearly defined. In this study, we first examined the impact of acute hypoxia on learning and memory and its relationship to the inflammasome. Mice were exposed to normobaric 6% oxygen/ 94% nitrogen for 2 hours (hypoxia) or ambient air (normoxia). Mice required 6 hrs of reoxygenation before they could be successfully trained in novel object recognition post-hypoxia. Mice also lost the ability to be fear conditioned if trained post-hypoxia. When mice were administered interleukin-1 receptor antagonist prior to hypoxia, mice could be successfully trained in novel object recognition 4 hrs post-hypoxia. Post hypoxia learning also recovered in 4 hrs in IL-1 receptor 1 knock-out mice. Importantly, when the caspase-1 inhibitor, YVAD-CMK, was administered by intracerebroventricular (i.c.v.) injection into the brain, post-hypoxia learning recovered in 4 hrs. Similarly, the administration of the adenosine receptor (AR) antagonist, caffeine, expedited recovery from anterograde amnesia. Furthermore, hypoxia/reoxygenation more than doubled brain caspase 1 activity, while adenosine alone was able to elicit a similar response. This reoxygenation-dependent activation of caspase 1 was prevented by administration of either caffeine or by targeted antagonism of A1/A2A ARs. These findings suggest that anterograde amnesia after hypoxia/reoxygenation is reliant, in part, on IL-1β generated by AR-dependent activation of caspase 1 after reoxygenation. Next, we set out to investigate the signaling pathway in which adenosine can trigger caspase-1 activation. Here, we show that mice perfused with 50 μm adenosine demonstrate a brain-based increase in IL-1β and caspase-1 activity coupled to increases in intracellular cAMP and PKA activity. Co-perfusion of adenosine with the adenosine A2A receptor antagonist, 8-(3-Chlorostyryl)-caffeine, prevented adenosine-dependent activation of caspase-1 as did use of adenosine A2A receptor knockout (KO) mice. Co-perfusion of adenosine with the KATP channel blocker, glyburide, also blocked adenosine-dependent activation of caspase-1. Finally, mice i.p. injected with 50 μm adenosine displayed reduced locomotor activity and food intake and increase anxiety-like bahviors. These adenosine-dependent behaviors were prevented in caspase-1, IL-1R1 and adenosine A2A receptor KO mice. Furthermore, when mice were injected with the caspase-1 inhibitor, Ac-YVAD-CMK, i.c.v., they were protected against a peripheral injection of adenosine, suggesting that brain-based IL-1β induction is required after adenosine administration. Taken together, our data indicate that adenosine & the adenosine A2A receptor promotes immobility, anorexia, and anxiety via PKA-mediated KATP channel opening and subsequent K+ efflux-dependent activation of the inflammasome that is unique to the brain. Lastly, we tested the interaction between hypoxia/reoxygenation and adenosine dependent anxiety-like behaviors. Similar to before, mice were exposed to 6% oxygen for 2 hours and allowed to recover. We see that hypoxia/reoxygenation elicited over 2.5-fold increase in circulating adenosine levels, which returned to normal 2 hours after hypoxia. This increase was mirrored with an elevation in active caspase-1 staining in neurons of the amygdala but not the hippocampus, suggesting a region specific upregulation. Furthermore, we see that hypoxia/reoxygenation and subsequent increase in circulating adenosine was anxiogenic in WT animals, while A2A AR KO animals were protected, further suggesting the role of adenosine in hypoxia/reoxygenation-induced anxiety-like behaviors. Taken together, the data suggests that hypoxia/reoxygenation induces an upregulation in neuron-based caspase-1 via A2A AR signaling in the amygdala, which subsequently can cause amnesia and anxiety.

Graduation Semester

2014-05

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http://hdl.handle.net/2142/49647

Copyright and License Information

Copyright 2014 Gabriel S. Chiu

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