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Publication Date

2012-08-03

Availability

UM campus only

Embargo Period

2012-08-03

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Neuroscience (Medicine)

Date of Defense

2012-06-20

First Committee Member

Robert W. Keane

Second Committee Member

Gerhard Dahl

Third Committee Member

W. Dalton Dietrich

Fourth Committee Member

M. Ross Bullock

Fifth Committee Member

Patrick M. Kochanek

Abstract

The innate immune system is the body’s first line of defense against danger, either from pathogens or damaged host tissue. The central nervous system (CNS), once believed to be a victim of chemical warfare between pathogens and professional immune cells, is now recognized as an active player in the innate immune defense against infection and injured tissue. Here, I investigate the innate immune response of neonatal cortical neurons in an in vitro model of viral infection and evaluate the potential of innate immune effector proteins to serve as diagnostic and prognostic biomarkers of traumatic brain injury (TBI). I show that neurons express a functional, deoxyribonucleic acid (DNA)-responsive absent in melanoma 2 (AIM2) inflammasome that activates caspase-1. AIM2 inflammasome activation induces pyroptosis, a proinflammatory cell death mechanism characterized by: (a) oligomerization of apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC); (b) caspase-1-dependent cell death; (c) formation of discrete pores in the plasma membrane; and (d) release of the inflammatory cytokine interluekin-1β (IL-1β). Probenecid, an inhibitor of the pannexin1 channel, prevents AIM2 inflammasome-mediated cell death, identifying pannexin1 as the discrete plasma membrane pore responsible for cell death during pyroptosis and probenecid as a novel pyroptosis inhibitor. My findings suggest neuronal pyroptosis is an important cell death mechanism during viral encephalitis that may be attenuated by probenecid. I also show patients with severe or moderate cranial trauma exhibit significantly higher cerebrospinal fluid (CSF) levels of the inflammasome proteins ASC, caspase-1, and NAcht leucine-rich-repeat protein-1 (NALP-1) compared to non-trauma controls. Expression of each protein correlated significantly with Glasgow Outcome Score (GOS) at five months post-injury. ASC, caspase-1, and NALP-1 were significantly higher in the CSF of patients with unfavorable outcomes, including death and severe disability. NALP-1 inflammasome proteins are potential biomarkers to assess TBI severity, outcome, and the secondary injury mechanisms impeding recovery, serving as adjuncts to clinical predictors. Collectively, my findings indicate CNS inflammasome complexes mediate neuroinflammatory responses to infection and injury.

Keywords

pyroptosis; inflammasome; innate immunity; neuron; traumatic brain injury; inflammation

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