Genetic predisposition to Alzheimer's disease alters inflammasome activity after traumatic brain injury

Abstract

Traumatic Brain Injury (TBI) is a major cause of death and disability in the US and a recognized risk factor for the development of Alzheimer's disease (AD). The relationship between these conditions is not completely understood, but the conditions may share additive or synergistic pathological hallmarks that may serve as novel therapeutic targets. Heightened inflammasome signaling plays a critical role in the pathogenesis of central nervous system injury (CNS) and the release of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) speck from neurons and activated microglia contribute significantly to TBI and AD pathology. This study investigated whether inflammasome signaling after TBI was augmented in AD and whether this signaling pathway impacted biochemical and neuropathological outcomes and overall cognitive function. Five-month-old, 3xTg mice and respective wild type controls were randomized and underwent moderate controlled cortical impact (CCI) injury or served as sham/uninjured controls. Animals were sacrificed at 1 hour, 1 day, or 1 week after TBI to assess acute pathology or at 12 weeks after assessing cognitive function. The ipsilateral cerebral cortex was processed for inflammasome protein expression by immunoblotting. Mice were evaluated for behavior by open field (3 days), novel object recognition (2 weeks), and Morris water maze (6 weeks) testing after TBI. There was a statistically significant increase in the expression of inflammasome signaling proteins Caspase-1, Caspase-8, ASC, and interleukin (IL)-1β after TBI in both wild type and 3xTg animals. At 1-day post injury, significant increases in ASC and IL-1β protein expression were measured in AD TBI mice compared to WT TBI. Behavioral testing showed that injured AD mice had altered cognitive function when compared to injured WT mice. Elevated Aβ was seen in the ipsilateral cortex and hippocampus of sham and injured AD when compared to respective groups at 12 weeks post injury. Moreover, treatment of injured AD mice with IC100, an anti-ASC monoclonal antibody, inhibited the inflammasome, as evidenced by IL-1β reduction in the injured cortex at 1-week post injury. These findings show that the inflammasome response is heightened in mice genetically predisposed to AD and suggests that AD may exacerbate TBI pathology. Thus, dampening inflammasome signaling may offer a novel approach for the treatment of AD and TBI.

Keywords: ASC; Alzheimer's disease; Caspase-1; Inflammasome; Neurodegeneration; Traumatic brain injury.