Inflammatory Mechanisms After Thromboembolic Ischemic Stroke in Mice

Stroke induces multiple pathological sequelae directly affecting neuronal survival and eliciting short and long-term deficits in behavioral outcome. Most of stroke models utilized to investigate these pathological consequences are based on pure cerebral ischemia models. However, human thromboembolic stroke is characterized by a complex multifactorial response that involves the activation of the cerebral microcirculation by the occluding thrombus. Here, we have characterized a novel mouse model of tromboembolic stroke that mimics most of the clinical aspects of the human pathology. The common carotid artery thrombosis (CCAT) model produces consistent and reproducible infarcts and triggers an inflammatory response comparable to other well established models of stroke. Several of the pathological consequences of cerebral ischemia are triggered by focal inflammatory processes that occur early after the ischemic event. Cerebral inflammation is initiated by an early release of pro-inflammatory cytokines. These active cytokines promote the recruitment of inflammatory cells from the blood cerebral circulation into the brain parenchyma and subsequent release of additional amounts of inflammatory cytokines. This exacerbated cytokine response result in further irreversible neuronal and histopathological damage. Cytokines interleukyne-1 beta (IL-1 beta) and interleukyne-18 (IL-18) maturation requires the presence of active caspase-1. Activation of caspase-1 in the peripheral immune response involves the recruitment of several caspase-1 molecules into a macromolecular complex termed the inflammasome. Cerebral ischemia triggers the synthesis and activation of caspase-1. However, the cellular mechanisms associated to the activation of caspase-1 in the ischemic brain remain to be elucidated. In this study, we demonstrate that the NLRP1-inflammasome composed by capase-1, ASC (apoptosis-associated speck-like protein containing a caspase-activating recruitment domain) and NLRP1 (NLR (nucleotide binding, leucine-rich repeat) is assembled following the ischemic event. Moreover, we have characterized the cellular distribution of the inflammasome proteins in the normal and the ischemic brain. Data from this investigation suggest that six to twenty four hours following CCAT the inflammasome complex is assembled in neurons while microglia, macrophages and astrocytes form this complex at 7 days following cerebral ischemia. On the basis of these findings we next investigated whether inhibition of the inflammasome complex reduces the inflammatory response after ischemia. Neutralization of NLRP1 utilizing a specific antibody, revealed decreased activation of caspase-1 and IL-1 beta and reduced histopathological damage within the ischemic brain. Thus, the inflammasome complex is a major contributor of the inflammatory response following cerebral ischemia and inhibition of this complex may be a novel therapeutic target for reducing the pathological consequences of stroke