Soluble Beta-Amyloid Modifies Presynaptic Membrane Cholesterol and Rigidity

Abstract

Cholesterol is influential for the physicochemical properties of lipid membranes in mammalian cells. In neurons, synaptic vesicles that mediate neurotransmitter release have the highest membrane cholesterol content (∼40mol%). Cellular and physiological analyses have shown that vesicular cholesterol is essential for vesicle release and retrieval. However, little is known about its distribution, trafficking and regulation during neurotransmission and how that affects presynaptic membrane architecture. Using new fluorescent ratiometric reporters for cholesterol concentration and membrane fluidity, we uncovered the correlation between vesicle turnover and cholesterol trafficking, illustrated resulted changes in presynaptic membrane fluidity, and elucidated the homeostatic adjustment of membrane cholesterol under variable neuronal activities. Structural studies have suggested that beta-amyloid peptides (Abeta) pathologically linked to Alzheimer's disease (AD) can interact with membrane-embedded cholesterol. Furthermore, three major secretases that cleave amyloid precursor protein (APP) to produce beta-amyloid peptides (Abeta) exhibit distinct cholesterol-sensitivity. To test if APP and/or Abeta modify synaptic cholesterol and synaptic vesicle turnover, and to understand the cause of synaptic dysfunction found in early stage of AD, we applied soluble Abeta1-40 (Ab40) and measured neurotransmitter release, vesicle recycling, cholesterol distribution, membrane dynamics during and after electric stimuli. We have found that extracellular Ab40 binds to and sequesters synaptic membrane cholesterol, disrupts its homeostasis and impairs synaptic vesicle release and retrieval. Consequently, a gradual decrease in the sustainability of neurotransmission emerged after the chronic treatment of near pathological concentration of Ab40. In conclusion, synaptic vesicle cholesterol is homeostatically regulated despite activity variance, and APP or Abeta may be the physiological modulator for synaptic cholesterol homeostasis.