Fibrillar amyloid-beta peptides and activated astroglia kill human primary neurons via neutral sphingomyelinase

Alzheimer's disease (AD) is the most common human neurodegenerative disease and fibrillar Amyloid-beta peptides are etiological reagents of this disorder. This study underscores the importance of fibrillar Abeta peptide in inducing the production of ceramide, an important inducer of apoptosis, in human primary neurons. Fibrillar Abeta1--42 induced the activation of sphingomyelinases and production of ceramide in human primary neurons. Antisense knockdown of the neutral sphingomyelinase (N-SMase) but not the acidic sphingomyelinase (A-SMase) markedly inhibited Abeta-mediated apoptosis suggesting that the activation of N-SMase but not the A-SMase plays a cardinal role in Abeta-mediated neuronal apoptosis. Furthermore, ceramide production was redox-sensitive as reactive oxygen species (ROS) were involved in the activation of N-SMase. Abeta1--42 peptides induced the activation of N-SMase via hydrogen peroxide, produced from superoxide generated from NADPH oxidase. Consistently, antisense knockdown of p22phox, a subunit of NADPH oxidase, inhibited Abeta1--42-induced neuronal apoptosis and cell death.; Similarly, we found that two other neurotoxins, HIV-1 coat protein gp120 (glycoprotein 120) and HIV-1 regulatory protein Tat, also provoked neuronal apoptosis via N-SMase activation and ceramide generation. HIV-1 gp120-induced neuronal apoptosis is mediated through the CXCR4-NADPH oxidase-superoxide-N-SMase-ceramide pathway.; In addition to the direct killing of neurons by fibrillar Abeta1--42, neurons are also killed through glial activation. Consistently, in neuron-astrocyte coculture, (Abeta + IL-1beta)activated human primary astrocytes induced the activation of the neuronal N-SMase and cell death. Antisense knockdown of neuronal N-SMase rescued them from (Abeta + IL-1beta)activated astroglia. Moreover, knocking down of astroglial N-SMase prevented the release of soluble cytotoxic products, including TNF-alpha and LT-alpha, from activated astrocytes. Consistently, fibrillar Abeta peptides also induced the activation of N-SMase in vivo in the cortex of C57BL/6 mice. Most importantly, antisense oligonucleotides against N-SMase decreased the activation of astroglia and protected cholinergic neurons from fibrillar Abeta toxicity in vivo in the cortex. Taken together, these findings suggest that fibrillar Abeta itself and Abeta-activated glia kill neurons in cell culture as well as in vivo in the brain via N-SMase. Therefore, inhibitors of N-SMase may have therapeutic importance in AD.