Lipoprotein receptor-related protein-6 protects the brain from ischemic injury.

ABSTRACT

BACKGROUND AND PURPOSE: Loss-of-function mutations of the lipoprotein receptor-related protein-6 (LRP6), a coreceptor in the Wingless-related integration site-ß-catenin prosurvival pathway, have been implicated in myocardial ischemia and neurodegeneration. However, it remains to be established whether LRP6 is also involved in ischemic brain injury. We used LRP6+/- mice to examine the role of this receptor in the mechanisms of focal cerebral ischemia. METHODS: Focal cerebral ischemia was induced by transient occlusion of the middle cerebral artery. Motor deficits and infarct volume were assessed 3 days later. Glycogen-synthase-kinase-3ß (GSK-3ß) phosphorylation was examined by Western blotting with phosphospecific antibodies, and the mitochondrial membrane potential changes induced by Ca2+ were also assessed. RESULTS: LRP6+/- mice have larger stroke and more severe motor deficits, effects that were independent of intraischemic cerebral blood flow, vascular factors, or cytosolic ß-catenin levels. Rather, LRP6 haploinsufficiency increased the activating phosphorylation and decreased the inhibitory phosphorylation of GSK-3ß, a kinase involved in proinflammatory signaling and mitochondrial dysfunction. Accordingly, postischemic inflammatory gene expression was enhanced in LRP6+/- mice. Furthermore, the association of mitochondria with activated GSK-3ß was increased in LRP6+/- mice, resulting in a reduction in the Ca2+ handling ability of mitochondria. The mitochondrial dysfunction was reversed by pharmacological inhibition of GSK-3ß. CONCLUSIONS: LRP6 activates an endogenous neuroprotective pathway that acts independently of ß-catenin by controlling GSK-3ß activity and preventing its deleterious mitochondrial and proinflammatory effects. The findings raise the possibility that emerging treatment strategies for diseases attributable to LRP6 loss-of-function mutations could also lead to new therapeutic avenues for ischemic stroke.