Aging and amyloid β oligomers enhance TLR4 expression and are required for LPS-induced Ca2+ responses and neuron cell death in rat hippocampal neurons

Abstract

Toll-like receptors (TLRs) are transmembrane pattern-recognition receptors of the innate immune system recognizing diverse pathogen-derived and tissue damage-related ligands, and evidences suggest that TLR signaling contributes to the pathogenesis of age-related neurodegenerative diseases, including Alzheimer’s disease (AD). AD is associated to oligomers of the amyloid β peptide (Aβo) that cause intracellular Ca2+ dishomeostasis and neuron cell death in rat hippocampal neurons. Here we addressed the possible interplay between inflammation and Aβo in long-term cultures of rat hippocampal neurons, a model of in vitro aging. For this end, we used Ca2+ imaging and annexin V immunofluorescence staining in short-term and long-term cultures of rat hippocampal neurons to test the effects of the TLR4 agonist Lipopolysaccharide (LPS) and Aβo on cytosolic [Ca2+] and apoptosis. Expression of TLR4 was tested using immunofluorescence. We found that LPS increases cytosolic [Ca2+] and promotes apoptosis in long-term cultures of rat hippocampal neurons (considered aged neurons) but not in short-term cultures reflecting young neurons. Interestingly, the TLR4 antagonist CAY10614 inhibited the effects of LPS on cytosolic [Ca2+] and on apoptosis. We also found that TLR4 is expressed in hippocampal neurons and the level of expression increases with age in culture. Treatment of aged neurons with Aβo further increases TLR4 expression, LPS-induced Ca2+ responses and neuron cell death. In summary, LPS increases cytosolic [Ca2+] and promotes apoptosis in rat hippocampal neurons aged in vitro. This effect is associated to the age-dependent increase in TLR4 expression that is further enhanced by Aβo, the most likely neurotoxin in AD. We conclude that TLR4 and Aβo cross talk in neuron cell death related to aging and AD.