Soluble Aβ oligomers have been shown to impair synaptic functions but the underlying mechanisms remain to be fully understood. At the postsynaptic side of excitatory synapses, Aβ-induced internalization of neurotransmitter receptors has been considered to contribute to reduced synaptic strength, but how Aβ oligomers reduce surface receptors is unclear. In this study, we used live-cell imaging to investigate the acute effects of soluble Aβ molecules on AMPAR trafficking at the postsynaptic terminal and the potential contribution of mitochondria. This study was partially inspired by our previous findings that soluble Aβ molecules acutely inhibit mitochondrial movement in hippocampal neurons, independent of cell death and other drastic alternations of cellular structures [34
]. Given that mitochondria are a crucial organelle for energy supply and intracellular Ca2+
regulation, impaired mitochondrial movement could disrupt their proper localization to synaptic sites, thus contributing to synaptic deficits elicited by Aβ molecules. Taking advantage of the pH-dependent fluorescence emission of SEP-GluR1, we were able to quantitatively analyze surface AMPARs, their trafficking during cLTP, and the effects of Aβ oligomers at single spine level. Such an imaging-based approach has allowed us to perform detailed analysis of changes associated with individual spine. For instance, we were able to show that Aβ-induced removal of surface AMPARs was not a consequence of spine loss, thus supporting a relatively direct action of Aβ on AMPAR trafficking [24
]. Furthermore, when combined with mitochondrial imaging, we were able to reveal a positive correlation between spine localization of mitochondria and AMAPR trafficking. It is quite intriguing to see that local presence of mitochondria appears to favor AMPAR insertion during synaptic potentiation and make them less prone to Aβ inhibition.
While our findings on Aβ-induced removal of surface AMPARs and inhibition of insertion during synaptic potentiation were based on imaging of exogenously expressed SEP-GluR1, we have performed surface staining using an anti-GluR1 antibody and confirmed the live imaging results (unpublished results). Furthermore, our results are consistent with previous studies employing electrophysiology, immunostaining, and live-cell imaging in which Aβ was shown to reduce surface AMPARs [24
]. Aβ-induced reduction of surface AMPARs has been shown to share a common pathway with long term depression (LTD) and to involve Ca2+
signaling through calcineurin for clathrin-mediated endocytosis of AMPARs [24
]. On the other hand, how Aβ inhibits AMPAR insertion during cLTP is unclear. Given that AMPAR insertion during LTP depends on Ca2+
-dependent exocytosis, Aβ-elicited LTD pathway and elevated AMPAR endocytosis could jeopardize LTP signaling cascades to impair AMPAR insertion. While we considered the increase in SEP-GluR1 fluorescence after TEA-cLTP a result of increased AMPAR insertion, our data could not rule out the possibility of decreased AMPAR internalization by TEA. Nonetheless, our study here has provided an intriguing possibility that Aβ impairment of mitochondrial trafficking might contribute to Aβ inhibition on AMPARs. Localization of mitochondria to both pre- and post-synaptic terminals has been observed and likely plays a crucial role for synaptic transmission and remodeling [31
]. The rapid inhibition of mitochondrial movement observed previously [34
] could potentially disrupt the synaptic localization of mitochondria to adversely affect synaptic functions. Indeed we found that a brief exposure of hippocampal neurons to Aβ oligomers inhibited mitochondrial translocation into spines induced by repetitive membrane depolarization. Based on our correlation analysis, the lack of mitochondrial association appears to facilitate the inhibition of AMPAR trafficking by Aβ oligomers.
How do mitochondria contribute to AMPAR trafficking? Potentially, the local production of ATP by mitochondria is required for vesicular fusion and insertion of AMPARs to the postsynaptic surface. Mitochondria could also be involved in local regulation of intracellular Ca2+
concentrations that are crucial for numerous synaptic activities including synaptic transmission, LTP and LTD, and endo/exocytotic trafficking of membrane proteins. In particular, both LTP and LTD depend on Ca2+
signaling to control synaptic receptor trafficking: the former requires a high Ca2+
elevation for activating CaMKII and downstream effectors for AMPA insertion whereas the latter needs small Ca2+
signals to activate calcineurin phosphatase for AMAPR removal from the surface [20
]. The lack of mitochondria at the postsynaptic terminal could alter local Ca2+
signals to favor the LTD pathway for AMPAR removal [24
], thus impeding the LTP-induced AMPAR insertion. Certainly, many other synaptic activities, such as ATP-driven ion pumps and local protein synthesis could also depend on the local presence of mitochondria, which could be disrupted by Aβ oligomers. While Aβ disruption of mitochondrial trafficking and localization to synapses might not directly or solely cause AMPAR trafficking defects, it could significantly contribute to postsynaptic defects in coordination and synergy with other Aβ-elicited events (e.g. Aβ induced internalization of synaptic receptors). While direct evaluation of this mitochondrial hypothesis requires selective disruption of mitochondrial localization to spines or of specific mitochondrial function(s) at spines, our findings that inhibition of GSK3β mitigate Aβ impairment of trafficking of both AMPAR and mitochondria suggest that these two events could be linked in contributing to Aβ-induced synaptic inhibition.
In conclusion, our studies showed that soluble Aβ oligomers exert acute inhibition on the trafficking of both mitochondria and synaptic receptors. The postsynaptically localized mitochondria appear to be important for the maintenance of AMPARs on postsynaptic surface as well as for AMPAR insertion during synaptic potentiation. Intriguingly, our correlation analysis suggests that impairment of mitochondrial trafficking might contribute to the adverse effects of Aβ oligomers on AMPARs on the postsynaptic surface. Future studies that employ selective targeting of mitochondrial movement could provide more definite answers regarding the precise role of mitochondria in synaptic receptor trafficking, as well as its precise contribution to synaptic defects in AD brains.