Amyloid precursor protein overexpression depresses excitatory transmission through both presynaptic and postsynaptic mechanisms

Abstract

Overexpression of the amyloid precursor protein (APP) in hippocampal neurons leads to elevated ␤-amyloid peptide (A␤) production and consequent depression of excitatory transmission. The precise mechanisms underlying APP-induced synaptic depression are poorly understood. Uncovering these mechanisms could provide insight into how neuronal function is compromised before cell death during the early stages of Alzheimer's disease. Here we verify that APP up-regulation leads to depression of transmission in cultured hippocampal autapses; and we perform whole-cell recording, FM imaging, and immunocytochemistry to identify the specific mechanisms accounting for this depression. We find that APP overexpression leads to postsynaptic silencing through a selective reduction of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents. This effect is likely mediated by A␤ because expression of mutant APP incapable of producing A␤ did not depress transmission. In addition, although we eliminate presynaptic silencing as a mechanism underlying APP-mediated inhibition of transmission, we did observe an A␤-induced presynaptic deficit in vesicle recycling with sustained stimulation. These findings demonstrate that APP elevation disrupts both presynaptic and postsynaptic compartments.Alzheimer's disease ͉ synaptic transmission ͉ glutamate receptor ͉ synaptic vesicle cycling T he early stages of Alzheimer's disease (AD) are characterized by cognitive deficits that are likely the result of impairments of synaptic function before either plaque formation or cell death (1-4). Identifying these impairments and the mechanisms that underlie them is a critical step in the development of novel therapies aimed at improving cognitive function in AD patients.A critical role for amyloid precursor protein (APP) and its cleavage products is indicated by the finding that mutations in the APP gene are linked to the inherited form of the disease, familial early onset AD (FAD; ref. 5). In addition, animal models that express the human APP protein bearing FAD-linked mutations recapitulate key features of early stage AD pathology (6, 7). Notably, levels of the ␤-amyloid peptide (A␤), which is generated through sequential cleavage of APP by ␤-APPcleaving enzyme 1 (BACE) and ␥-secretase, are greatly elevated in these model mice (7).Three lines of evidence strongly suggest that A␤ has acute effects on synaptic function: (i) Intracerebroventricular injection of naturally secreted A␤ is sufficient to disrupt cognitive function in a rapid and transient manner (8).(ii) Active immunization with A␤ peptide can protect against, or even reverse, AD-like neuropathology and behavioral deficits of AD model mice (9, 10). (iii) Passive immunization with anti-A␤ antibody can rapidly reverse memory deficits of AD model mice (11-13).Consistent with this notion, rapid viral-mediated upregulation of APP and consequent elevation of A␤ production in cultured hippocampal slices has been shown to depress excitatory transmission by decreasing the num...

Beta-amyloid disrupted synaptic vesicle endocytosis in cultured hippocampal neurons

“…Taken collectively, these data suggested that this deficiency emerges when the level or speed of synaptic vesicle recycling is increased under sustained synaptic activity. In contrast, no decrease in dye uptake was observed when hippocampal neurons overexpressing APP were cultured as microislands (Ting et al, 2007). Possible explanations of this discrepancy are the different age (embryonic vs. postnatal), species (rats vs. mice), and culture conditions (dissociated vs. microisland cultures) used in these two studies.…”

“…Aβ could also induced changes in postsynaptic elements that might contribute to synaptic dysfunction. Thus, it has been recently shown that APP over expression led to a selective reduction of α-amino-3-hydroxy-5methyl-4-isoxazole-propionic acid (AMPA) receptors, and hence, disruption in excitatory transmission in hippocampal neurons (Chang et al, 2006;Ting et al, 2007). However, it is unlikely that these defects are mediated by dynamin 1 depletion since these proteins are localized in different subcellular compartments.…”

“…Possible explanations of this discrepancy are the different age (embryonic vs. postnatal), species (rats vs. mice), and culture conditions (dissociated vs. microisland cultures) used in these two studies. More importantly, while we used tightly control amounts of Aβ and aggregation times, Ting and coworkers used cells that overexpressed APP and were kept in culture for 10 to 17 days (Ting et al, 2007). Under these experimental conditions, the undertermined amounts of Aβ could accumulate in the culture medium for long periods of time and could be aggregated beyond the oligomeric forms into the less toxic fibrils, and hence, fail to elicit changes in synaptic vesicle recycling similar to the ones reported in this study.…”

A Specific Role for Ca2+-Dependent Adenylyl Cyclases in Recovery from Adaptive Presynaptic Silencing

“…Role of cAMP signaling and AC1/AC8 in basal and FSK unsilencing Our own and others' previous results have demonstrated that a fraction of hippocampal presynaptic terminals are silent under basal conditions (Rosenmund et al, 2002;Altrock et al, 2003;Moulder et al, 2006;Ting et al, 2007) (Fig. 1 A).…”

“…By a combination of optical and electrophysiological measures, our work shows strong evidence that cAMP signaling activates previously silent presynaptic terminals. We and others have demonstrated previously the existence of presynaptically silent synapses under basal conditions, defined by normal antibody staining for synaptic vesicle markers but lack of FM dye labeling during brief, strong depolarization (Rosenmund et al, 2002;Altrock et al, 2003;Moulder et al, 2006;Ting et al, 2007). There is an important role for cAMP potentiation in the awakening of these basally silent presynaptic terminals.…”