The innate immune system senses the invasion of pathogenic microorganisms and tissue injury through Toll-like receptors (TLR), a mechanism thought to be limited to immune cells. We now report that neurons express several TLRs, and that the levels of TLR2 and -4 are increased in neurons in response to IFN-␥ stimulation and energy deprivation. Neurons from both TLR2 knockout and -4 mutant mice were protected against energy deprivation-induced cell death, which was associated with decreased activation of a proapoptotic signaling cascade involving jun N-terminal kinase and the transcription factor AP-1. TLR2 and -4 expression was increased in cerebral cortical neurons in response to ischemia/reperfusion injury, and the amount of brain damage and neurological deficits caused by a stroke were significantly less in mice deficient in TLR2 or -4 compared with WT control mice. Our findings establish a proapoptotic signaling pathway for TLR2 and -4 in neurons that may render them vulnerable to ischemic death.AP-1 ͉ apoptosis ͉ innate immunity ͉ ischemic stroke ͉ microglia T oll-like receptors (TLRs) are a family of at least 11 proteins that function as key mediators of innate immunity, responding to diverse microbial products and injury-induced endogenous ligands (1). Activation of TLRs initiates signal transduction cascades that involve kinases including atypical forms of protein kinase C and the transcription factors activator protein-1 (AP-1) and NF-B, which induce the expression of genes encoding inflammation-associated molecules and cytokines (2-4). Although TLRs are expressed in leukocytes where their functions have been established, recent findings suggest they can also be expressed in nonimmune cells, including hepatocytes and muscle cells (5,6). TLRs are present in the brain, where their expression is believed to be limited to glial cells (microglia, astrocytes, and oligodentrocytes) (7,8). However, recent findings suggest that neurons may express at least some TLRs responsive to viral RNA or bacterial proteins (9, 10). Ischemic injury to the brain (stroke) is a major cause of morbidity and mortality for which effective treatments are lacking. Studies have shown that TLR2 and -4 are up-regulated in response to ischemia in the kidney and heart, suggesting these two TLRs may play important roles in ischemic tissue injury (11-13). However, neither the specific cells in which TLRs are activated in response to ischemia nor the consequences of TLR signaling for the clinical outcome of an ischemic event have been established. Here we use TLR2 and -4 mutant mice and primary neuronal cell culture and in vivo models of ischemic stroke to elucidate roles for neuronal TLR2 and -4 signaling in the pathogenesis of stroke. Results Neurons Express TLRs and Respond to IFN␥ Stimulation.Previous studies suggested that neurons do not express TLRs (14), whereas others have suggested the presence of TLR3 and -8 in neurons (10). By using multiple technologies, we found that primary mouse cortical neurons express TLR2, -3, and -4. First,...

Sequential cleavages of the β‐amyloid precursor protein cleaving enzyme 1 (BACE1) by β‐secretase and γ‐secretase generate the amyloid β‐peptides, believed to be responsible of synaptic dysfunction and neuronal cell death in Alzheimer’s disease (AD). Levels of BACE1 are increased in vulnerable regions of the AD brain, but the underlying mechanism is unknown. Here we show that oxidative stress (OS) stimulates BACE1 expression by a mechanism requiring γ‐secretase activity involving the c‐jun N‐terminal kinase (JNK)/c‐jun pathway. BACE1 levels are increased in response to OS in normal cells, but not in cells lacking presenilins or amyloid precursor protein. Moreover, BACE1 is induced in association with OS in the brains of mice subjected to cerebral ischaemia/reperfusion. The OS‐induced BACE1 expression correlates with an activation of JNK and c‐jun, but is absent in cultured cells or mice lacking JNK. Our findings suggest a mechanism by which OS induces BACE1 transcription, thereby promoting production of pathological levels of amyloid β in AD.

Mice transgenic for antisense Notch and normal mice treated with inhibitors of the Notch-activating enzyme gamma-secretase showed reduced damage to brain cells and improved functional outcome in a model of focal ischemic stroke. Notch endangers neurons by modulating pathways that increase their vulnerability to apoptosis, and by activating microglial cells and stimulating the infiltration of proinflammatory leukocytes. These findings suggest that Notch signaling may be a therapeutic target for treatment of stroke and related neurodegenerative conditions.