The paper follows on Cirrito and colleagues’ earlier work showing that synaptic activity regulates Aβ levels in interstitial fluid (ISF) surrounding cells in the brain (see ARF related news story). Since amyloid precursor protein (APP) is processed in endosomal vesicles after uptake from the cell surface, the authors hypothesized that endocytosis induced by synaptic activity might serve to deliver more APP into the processing pathway, and that that might account for the increased Aβ production as a result of synaptic activity.

To test this idea, Cirrito used in-vivo microdialysis to measure Aβ over time in the ISF of the hippocampus in living, moving three-month-old Tg2576 APP transgenic mice. The implanted dialysis electrodes allowed the investigators to administer and measure the effects of pharmacological modulators of endocytosis and synaptic activity. For starters, inhibition of clathrin-mediated endocytosis with a dynamin dominant-negative peptide (dynamin-DN) inhibitor caused a 70 percent drop in Aβ within four hours. This is similar to what is seen in cell culture experiments when endocytosis is blocked. The effect on Aβ was not merely due to a depression of synaptic activity, since dynamin peptide itself did not affect synaptic transmission as measured by spontaneous or evoked electrical activity. Dynamin-DN did not alter Aβ clearance, the authors show, suggesting that the decrease resulted from lower production.

Next, the investigators asked whether elevation of Aβ production by synaptic activity required endocytosis. They found that stimulating synaptic transmission with either picrotoxin or a metabotropic glutamate receptor antagonist increased production of Aβ, and this was blocked by the dynamin-DN peptide. They found a similar role for endocytosis in the production of Aβ from endogenous APP in non-transgenic mice; this supported the idea that this phenomenon is part of a normal pathway for Aβ production, and not a consequence of the transgene, the authors write.

To determine the relative contribution of endocytosis and synaptic activity to Aβ production, the researchers measured Aβ levels after co-administering the neurotransmission blocker tetrodotoxin (TTX) and the dynamin-DN peptide. They found that when they gave the endocytosis inhibitor first, Aβ levels plummeted and they saw no further decrease upon adding TTX. However, when they administered TTX first, subsequent inhibition of endocytosis was able to decrease Aβ slightly more. These results suggest that of all the endocytosis-dependent Aβ production, most but not all occurs in response to synaptic activity.

“We estimate that the majority of ISF Aβ in vivo is generated through endocytosis of APP and that, in the brain, synaptic activity plays a primary role in that process,” the authors conclude. The results are consistent with observations in people that Aβ accumulation is enhanced by seizure activity (Mackenzie and Miller, 1994; Gouras et al., 1997) and occurs most conspicuously in highly active regions of the brain (see ARF related news story). Further work will be required to understand the conformation of Aβ that is released via this pathway, its physiological role, and its relation to aging and AD.—Pat McCaffrey.

Reference:
Cirrito JR, Kang JE, Lee J, Stewart FR, Verges DK, Silverio LM, Bu G, Mennerick S, Holtzman DM. Endocytosis is required for synaptic activity-dependent release of amyloid-beta in vivo. Neuron. 2008 Apr 10;58(1):42-51. Abstract