A neuroprotective locus modulates ischemic stroke infarction independent of collateral vessel anatomy

Abstract

Ischemic stroke is caused by a disruption of the blood supply to the brain leading to neuronal cell death. Genetic studies of ischemic stroke have identified numerous gene variants that increase the risk to develop stroke. In stark contrast, genetic studies of stroke outcomes, such as the infarct territory size, are confounded by many uncontrollable variables, leading to a paucity of gene targets for treatment of an incipient stroke. Using genetically diverse inbred strains of mice and a surgically-induced model of ischemic stroke, we used quantitative trait locus mapping to identify novel gene targets modulating infarct size, which varies greatly across inbred strains. Although infarct size is largely determined by the extent of collateral vessel connection between arteries in the brain that enables reperfusion of the ischemic territory, we have identified strain pairs that do not vary in this phenotype, but which nonetheless exhibit large differences in infarct size. In this study we performed QTL mapping in mice from an intercross between two such strains, WSB/EiJ and C57BL/6J. We identified a strong locus on chromosome 8 that overlaps with a locus of similar direction and effect previously mapped in an intercross between C3H/HeJ and C57BL/6J strains. To identify causative genes within the overlapping genetic interval, we surveyed nonsynonymous coding SNPs and performed RNA sequencing data analysis for all three mapping strains. We identified Macrophage Scavenger Receptor 1 (Msr1) as a strong candidate gene that harbors multiple coding SNPs predicted to be damaging. Using Msr1-deficient mice, we demonstrated that cerebral infarct volume after stroke induction is dramatically increased in a strain background where reperfusion effects due to collateral vessels is blunted. Significantly, the identification of neuroprotective genes such as Msr1 provides new genes for future mechanistic studies of infarction following ischemic stroke and provides novel gene/protein targets for therapeutic development. Author summaryThe most common form of stroke arises when a blockage occurs in a blood vessel of the brain, thereby preventing delivery of oxygen and nutrients to areas supplied by the affected vessel, leading to tissue death. The main treatment for this form of stroke is medication to dissolve the blockage; however, more treatment options are required to better reduce the death and disability associated with stroke. In this study, we sought to identify genes that can decrease the amount of damage to brain tissue following a stroke, with a specific focus on examining genes that work to directly protect the neurons, rather than returning blood flow to the affected area. Since it is impossible to precisely control the nature of stroke and the genetic variability in humans, we used mice identify a genetic region that is associated with the amount of tissue damage following stroke. Within this genetic region, we identified a list of candidate genes, including the gene Msr1, which we found is important for controlling tissue damage in one genetic population of mice after stroke. The genes identified here require further follow-up to determine the impact on stroke outcomes and the usefulness of these candidates as therapeutic targets.