12 August 2009. Do we use it or lose it, or use it and lose it? If synaptic activity spurs Aβ production, why do education, environmental stimulation, and socialization all protect against AD? A possible explanation comes from Gunnar Gouras and colleagues of Weill Medical College of Cornell University, New York, with data that suggest a more positive effect of synaptic activity on Aβ dynamics. Writing in the August 5 Journal of Neuroscience, they report that even while synaptic activity boosts extracellular Aβ levels, it reduces intraneuronal Aβ, and that reduction is enough to ameliorate synaptic toxicity. The results support the idea that where Aβ is concerned, neuronal activity benefits neurons. The work also bolsters the theory that intraneuronal Aβ can bring down synapses, and paints a more complex picture of activity-induced Aβ regulation, which includes the shuttling of amyloid precursor protein (APP) to synapses, and neprilysin-mediated Aβ degradation.
Previous work showed that synaptic activity drives Aβ secretion (see ARF related news story on Kamenetz et al., 2003 and ARF related news story on Cirrito et al., 2005), and accounts for the majority of APP endocytosis, processing, and Aβ production in the brain (see ARF related news story on Cirrito et al., 2008). Early Aβ deposition also seems to occur preferentially in places of high neuronal activity (see ARF related news story on Buckner et al., 2009). However, the new work suggests that synaptic activity has another side, too, where it reduces the intraneuronal pool of Aβ and normalizes synapses, Gouras told ARF. “This goes against the idea that the activity-dependent secretion of Aβ is a bad thing and suggests there is a balance.”
In the study, first author Davide Tampellini and colleagues focused on the effects synaptic activity has not on extracellular, but on the intraneuronal pool of Aβ, which is increasingly viewed by Gouras and others as critical to Aβ toxicity. The researchers activated cultured primary neurons from mutant APP-overexpressing Tg2576 mice with a chemical cocktail that mimics the stimulus for long-term potentiation, and measured a 38 percent decrease in levels of intraneuronal Aβ40 and Aβ42 by ELISA, along with the expected increase in secreted Aβ. They also saw a reduction in immunofluorescent Aβ in dendrites. The same reduction was seen in potassium chloride (KCl)-activated hippocampal slices from another mutant APP transgenic mouse, Tg19959 (see ARF related news story on Li et al., 2004). In the latter, they saw an increase in Aβ levels in inactive neurons in the whisker barrel cortex after chronic understimulation caused by removal of the whisker bulb afferents. This suggests that synaptic activity negatively regulates intraneuronal Aβ in vivo.
With extracellular Aβ going up, and intracellular Aβ going down, the researchers asked whether, on balance, synaptic activity would help or harm the neurons. They looked at the impact of activation on synapse structure by staining neurons in culture for the synaptic protein PSD-95. The protein is normally decreased in Tg2576 neurons compared to wild-type, but after activation the researchers found levels rebounded to wild-type, whereas activation of wild-type neurons had no effect. This suggests that synaptic activity has an overall protective effect, and that the positive effects of reducing intraneuronal Aβ override the potential negative effects of extracellular Aβ release.
The connection between intracellular and extracellular Aβ is complex: It has been established that extracellular Aβ can trigger the production of new intracellular Aβ, and that this production is required for synaptic downregulation (see Yang et al., 1999 and ARF related news story). Outside-in effects appeared important for the synaptic effects of exogenous Aβ in this system as well. Using γ-secretase inhibitors or APP knockout mice, the researchers showed that new processing and production of intraneuronal Aβ are required for externally added Aβ to bring down PSD-95 levels.
In other experiments, the researchers revealed more details of the effects of synaptic activity on the natural history of Aβ. Using live cell imaging, they watched APP while applying the activating solution to cultured cells. Gouras said that within 10 to 20 seconds, they could see APP-carrying vesicles that were being transported away from synapses actually turn and go back. Cell labeling experiments showed that APP increased on the surface of neurons after activation, except in synapses. This work supports the model that upon activation, APP travels to synapses, where it is then internalized, and processed. The destruction of Aβ, on the other hand, depended on neprilysin. Gouras said they focused on that protease because it is the most efficient degrader of Aβ, and it is increased in the brains of animals after environmental enrichment (see ARF related news story on Lazarov et al. 2005). Using a neprilysin inhibitor or neprilysin knockout mice, they showed that loss of the protease prevented the reduction in Aβ42 by synaptic activity.
When it comes to Aβ and synaptic activity, Gouras concludes, “We tend to say something is good or bad, but that’s too simple. Synaptic activity has a good side. Our results suggest it is good to get rid of intracellular Aβ, and not a problem to get out some extracellular Aβ.” However, he says, “For some reason people get more vulnerable with aging, and very active areas are a setup for AD to start.”
In the same vein, he says, “We have to be careful saying extracellular Aβ is bad,” pointing to the recent work that in patients with severe head trauma, extracellular Aβ is very low, and goes up when they begin to improve their cognitive function (see ARF related news story on Brody et al. 2008). In addition, there is evidence that very low concentrations of extracellular Aβ might play a physiological role in learning and memory (see ARF related news story on Puzzo et al., 2008), although Gouras notes this is controversial.—Pat McCaffrey.
Tampellini D, Rahman N, Gallo EF, Huang Z, Dumont M, Capetillo-Zarate E, Ma T, Zheng R, Lu B, Nanus DM, Lin MT, Gouras GK. Synaptic activity reduces intraneuronal Aβ, promotes APP transport to synapses, and protects against Aβ-related synaptic alterations. J. Neurosci. 2009 August 5; 29(31):9704-9713. Abstract