cAMP-Inhibits Cytoplasmic Phospholipase A₂ and Protects Neurons against Amyloid-β-Induced Synapse Damage

A key event in Alzheimer's disease (AD) is the production of amyloid-β (Aβ) peptides and the loss of synapses. In cultured neurons Aβ triggered synapse damage as measured by the loss of synaptic proteins. α-synuclein (αSN), aggregates of which accumulate in Parkinson's disease, also caused synapse damage. Synapse damage was associated with activation of cytoplasmic phospholipase A₂ (cPLA₂), an enzyme that regulates synapse function and structure, and the production of prostaglandin (PG) E₂. In synaptosomes PGE₂ increased concentrations of cyclic adenosine monophosphate (cAMP) which suppressed the activation of cPLA₂ demonstrating an inhibitory feedback system. Thus, Aβ/αSN-induced activated cPLA₂ produces PGE₂ which increases cAMP which in turn suppresses cPLA₂ and, hence, its own production. Neurons pre-treated with pentoxifylline and caffeine (broad spectrum phosphodiesterase (PDE) inhibitors) or the PDE4 specific inhibitor rolipram significantly increased the Aβ/αSN-induced increase in cAMP and consequently protected neurons against synapse damage. The addition of cAMP analogues also inhibited cPLA₂ and protected neurons against synapse damage. These results suggest that drugs that inhibit Aβ-induced activation of cPLA₂ and cross the blood-brain barrier may reduce synapse damage in AD.