Hsieh and colleagues provide exciting results on a major question in AD research: how β amyloid induces synaptic dysfunction. In well-controlled experiments, they show that the mechanisms of Aβ effects on synapses parallel those involved in LTD. They confirm that Aβ causes internalization of AMPA receptors, suggesting that this is required for subsequent alterations in NMDA receptors. Importantly, an endocytosis-defective GluR2 construct prevents the physiological alterations induced by Aβ. This work strengthens the idea that Aβ has a role in synaptic biology, although the molecular mechanisms whereby Aβ causes internalization of AMPA receptors and promotes pathology at synapses in AD remain unclear.

View all comments by Claudia Almeida
View all comments by Gunnar K. Gouras

Our findings reported in Neuron indicate that removal of synaptic AMPAR by Aβ leads to loss of spines and loss of NMDA responses. In other cases, removal of synaptic AMPAR does not lead to loss of NMDA responses (e.g., Shi et al., 2001; expression of GluR2 c-tail leads to selective decrease in AMPAR responses). In the Beique et al. paper, which is a very nice study, they find that in PSD95 knockout animals, there are some large spines with no AMPAR responses. So I would conclude that removal of AMPAR from synapses can lead to spine loss, but it appears to be dependent on how those receptors are lost. There may be AMPAR-associated molecules, which normally stabilize spines, that are also removed by Aβ but can persist in PSD95 KO animals.

References:
Shi S, Hayashi Y, Esteban JA, Malinow R. Subunit-specific rules governing AMPA receptor trafficking to synapses in hippocampal pyramidal neurons. Cell. 2001 May 4;105(3):331-43. Abstract

View all comments by Roberto Malinow

The study by Malinow and colleagues provides compelling evidence for a direct link between the synaptic deficits associated with Aβ production and known cellular pathways for physiological synaptic plasticity. This work is important, as it points to several well-established molecular mechanisms of glutamate receptor trafficking as potential early mediators of amyloid-induced synaptic dysfunction.

The study by Huganir and colleagues sheds new light on the fundamental molecular events regulating synaptic transmission and excitatory synapses in the brain. The authors generated mutant mice lacking a critical protein component of excitatory synapses, the scaffold molecule PSD-95. In brain slices from these animals, synaptic transmission was impaired, but plasticity was actually increased. These findings are important because they pinpoint the functional defects in synaptic transmission produced when synapses lack PSD-95. Together with the results of Malinow and colleagues, one can begin to envision an unraveling of the molecular mechanisms underlying synaptic failure in Alzheimer...  Read more

The study by Malinow and colleagues provides compelling evidence for a direct link between the synaptic deficits associated with Aβ production and known cellular pathways for physiological synaptic plasticity. This work is important, as it points to several well-established molecular mechanisms of glutamate receptor trafficking as potential early mediators of amyloid-induced synaptic dysfunction.

The study by Huganir and colleagues sheds new light on the fundamental molecular events regulating synaptic transmission and excitatory synapses in the brain. The authors generated mutant mice lacking a critical protein component of excitatory synapses, the scaffold molecule PSD-95. In brain slices from these animals, synaptic transmission was impaired, but plasticity was actually increased. These findings are important because they pinpoint the functional defects in synaptic transmission produced when synapses lack PSD-95. Together with the results of Malinow and colleagues, one can begin to envision an unraveling of the molecular mechanisms underlying synaptic failure in Alzheimer disease and other disorders of memory and cognition.

View all comments by Michael Ehlers

This is a landmark paper on how Aβ peptides might induce synaptic depression by reducing the number of AMPA receptors at postsynaptic sites of hippocampal neurons. The authors propose that Aβ peptides, either generated endogenously from APP, or added exogenously, induce endocytosis mechanisms that are thought to underlie long-term depression. Two questions, which I imagine the authors are now poised to answer, are (i) the identity of the active Aβ peptide (is it a monomer dimer, oligomer, or Aβ*?) and (ii) how it acts on the postsynaptic membrane. My guess is that the active Aβs must be either monomers or dimers, since the effect is seen when Aβs are generated from endogenous APP, and there is no reason to expect large amounts of Aβ to be generated at concentrations needed to oligomerize. Synthetic Aβ peptides added to the medium of the hippocampal slices need not act only on the external surface of the membrane, since they were added at concentrations (micromolar) high enough to enter and cross the lipid bilayer, giving them access to the cytoplasmic surface. If present at high...  Read more

This is a landmark paper on how Aβ peptides might induce synaptic depression by reducing the number of AMPA receptors at postsynaptic sites of hippocampal neurons. The authors propose that Aβ peptides, either generated endogenously from APP, or added exogenously, induce endocytosis mechanisms that are thought to underlie long-term depression. Two questions, which I imagine the authors are now poised to answer, are (i) the identity of the active Aβ peptide (is it a monomer dimer, oligomer, or Aβ*?) and (ii) how it acts on the postsynaptic membrane. My guess is that the active Aβs must be either monomers or dimers, since the effect is seen when Aβs are generated from endogenous APP, and there is no reason to expect large amounts of Aβ to be generated at concentrations needed to oligomerize. Synthetic Aβ peptides added to the medium of the hippocampal slices need not act only on the external surface of the membrane, since they were added at concentrations (micromolar) high enough to enter and cross the lipid bilayer, giving them access to the cytoplasmic surface. If present at high enough concentration, they might affect in some way the functioning of the cytoskeletal network which must play a role in the endocytosis process.

Alternatively, they could partition within the lipid bilayer, as was proposed earlier (1). If a fraction of the Aβ peptide pool remained within the lipid bilayer, their hydrophobic domains, which contain GxxxG/A sequences, could conceivably interact with comparable domains in the TARP proteins, all of which have two hydrophobic and putative transmembrane segments that are similar but not identical to the Aβ domains. A comparison between Aβ and one of the T-M domains is shown below:

stargazin
     MGLFDRGVQMLLTTVGAFAAFSLMTIAVGTDYWLYSRGVCK
g3   MRMCDRGIQMLITTVGAFAAFSLMTIAVGTDYWLYSRGVCRTK
g4   MRMCDRGIQMLITTVGAFAAFSLMTIAVGTDYWLYSRGVCR
g8            VLLTTIGAFAAFGLMTIAISTDYWLYTR
Aβ KLVFFAEDVGSNKGAIIGLMVGGVVIA

Interactions between Aβ peptides and T-M segments of TARP proteins could affect the AMPA receptor trafficking mechanisms proposed earlier (2), and this would be an alternative way in which Aβ peptides might reduce the AMPA receptor population. A test of this idea would be to express Aβ alone in hippocampal neurons with a vector that incorporates an appropriate signal sequence along with the Aβ segment and some anchoring sequence at its C-terminus.

References:
1. Marchesi VT. An alternative interpretation of the amyloid Abeta hypothesis with regard to the pathogenesis of Alzheimer's disease. Proc Natl Acad Sci U S A. 2005 Jun 28;102(26):9093-8. Epub 2005 Jun 20. Abstract

2. Nicoll RA, Tomita S, Bredt DS. Auxiliary subunits assist AMPA-type glutamate receptors. Science. 2006 Mar 3;311(5765):1253-6. Review. Abstract

View all comments by Vincent Marchesi

The classical view of amyloid action in the pathogenesis of Alzheimer disease centers around the neurotoxic properties of aggregated peptides. Recent studies have, however, been challenging this as the exclusive mechanism, suggesting direct modulatory functions for Aβ; compelling evidence that supports this view is now provided by the paper by Hsieh et al., who offer some novel mechanistic insights. They use a clever combination of imaging (with synaptopHluorin tagged receptors) and electrophysiological (electrophysiological tagging of AMPA receptors) techniques. The authors show that application of synthetic Aβ or the overexpression of C99 causes endocytosis of GluR1 and Glur2 receptors from synapses. Furthermore, Hsieh et al. show the involvement of p38/MAPK and calcineurin in GluR2 endocytosis, and detect an increase in the phosphorylation of the cytoplasmic tail of GluR2 after Aβ treatment. Based on these findings, Hsieh et al. suggest that Aβ induces GluR2 endocytosis through a pathway that may be shared with LTD induction. In fact, they also show that Aβ can mimic and...  Read more

The classical view of amyloid action in the pathogenesis of Alzheimer disease centers around the neurotoxic properties of aggregated peptides. Recent studies have, however, been challenging this as the exclusive mechanism, suggesting direct modulatory functions for Aβ; compelling evidence that supports this view is now provided by the paper by Hsieh et al., who offer some novel mechanistic insights. They use a clever combination of imaging (with synaptopHluorin tagged receptors) and electrophysiological (electrophysiological tagging of AMPA receptors) techniques. The authors show that application of synthetic Aβ or the overexpression of C99 causes endocytosis of GluR1 and Glur2 receptors from synapses. Furthermore, Hsieh et al. show the involvement of p38/MAPK and calcineurin in GluR2 endocytosis, and detect an increase in the phosphorylation of the cytoplasmic tail of GluR2 after Aβ treatment. Based on these findings, Hsieh et al. suggest that Aβ induces GluR2 endocytosis through a pathway that may be shared with LTD induction. In fact, they also show that Aβ can mimic and partly occlude mGluR1-induced LTD. Finally, Hsieh et al. bridge the difficult gap between electrophysiological and morphological plasticity and show that Aβ treatment, as well as C99 overexpression, leads to synaptic spine loss, an effect that can be mimicked by expressing AMPAR, which bears mutations that result in enhanced endocytosis; in doing the reverse experiment, they demonstrate that the same mutated AMPAR attenuates Aβ-induced spine loss. On the basis of previous data from this same group (Kamenetz et al., 2003) as well as from Cirrito et al. (2005), it is now suggested that Aβ cleavage is dependent upon synaptic activity which, together with the data from Hsieh et al., strongly backs the view of Aβ as modulator of synaptic activity. In summary, the studies by Hsieh et al. indicate that Aβ might operate in a negative feedback loop to homeostatically regulate synaptic strength.

View all comments by Osborne Almeida
View all comments by Francesco Roselli

Hsieh et al. conducted carefully executed experiments addressing the mechanisms underlying Aβ-induced depression of glutamatergic synaptic transmission. Their work provides convincing evidence that Aβ causes internalization of AMPA receptors which results in synaptic dysfunction and dendritic spine loss. These results further support the link between Aβ and glutamate receptor function during pathological onset in AD.

We wish to learn more about how AMPA receptor internalization leads to the loss of functional NMDA receptors. Specifically, the mechanisms by which loss of AMPA receptors leads to the loss of NMDA receptors remain unclear. There is undoubtedly a correlation between the two events because of their anatomical location and functional relation; however, there are several steps in the process that have yet to be revealed.

View all comments by Hongxin Dong
View all comments by Carla Yuede

This paper confirms recent studies that demonstrate a decrease in AMPA receptor activity as a consequence of exposure to Aβ peptides, but it is more than just confirmatory. The earlier studies employed exogenous Aβ at relatively high concentrations, experiments that are always open to question. This new work suggests that endogenous Aβ is the likely agent responsible for the decrease in synaptic transmission. Their use of a mutant APP incapable of generating Aβ is a new approach that has great potential for further studies.

View all comments by Vincent Marchesi

Ting et al. provide an interesting and well-done analysis of how endogenous Abeta may depress synaptic transmission, namely by depressing AMPA receptor-mediated EPSCs. Also, the authors find subtle presynaptic deficits in synaptic vesicle cycling with unknown consequences for synaptic communication. The key here is the possibility that cellularly derived Abeta may be causing these effects, thereby bypassing problems related to Abeta concentration or Abeta conformation typically associated with exogenously applied Abeta. It will eventually be useful to know the specific types of Abeta that are responsible for this phenomenon.

Several groups have demonstrated that synaptic activity can regulate release of Abeta from neurons (Kamenetz et al., 2003, Cirrito et al., 2005 ). Is activity-dependent release of Abeta necessary for this phenomenon, or is Abeta release via other mechanisms sufficient to mediate the effect on AMPA receptors? These questions ultimately address whether Abeta may act...  Read more

Ting et al. provide an interesting and well-done analysis of how endogenous Abeta may depress synaptic transmission, namely by depressing AMPA receptor-mediated EPSCs. Also, the authors find subtle presynaptic deficits in synaptic vesicle cycling with unknown consequences for synaptic communication. The key here is the possibility that cellularly derived Abeta may be causing these effects, thereby bypassing problems related to Abeta concentration or Abeta conformation typically associated with exogenously applied Abeta. It will eventually be useful to know the specific types of Abeta that are responsible for this phenomenon.

Several groups have demonstrated that synaptic activity can regulate release of Abeta from neurons (Kamenetz et al., 2003, Cirrito et al., 2005 ). Is activity-dependent release of Abeta necessary for this phenomenon, or is Abeta release via other mechanisms sufficient to mediate the effect on AMPA receptors? These questions ultimately address whether Abeta may act as a negative feedback signal for synaptic transmission.

APP with a mutation at the BACE cleavage site was a very clever tool to use in these studies. As the authors note, while this vector suggests that Abeta could be a key mediator of the effects seen here, other APP cleavage products are also affected and therefore cannot be excluded.

View all comments by John Cirrito

Our PNAS study identifies deficits in synaptic transmission when APP is overexpressed in neurons. We use Semliki Forest virus to rapidly upregulate APP in autaptic (isolated microisland) cultures of hippocampal neurons, and record synaptic responses 12 to 24 hours after infection. Our finding that AMPA receptor-mediated responses are reduced in neurons overexpressing APP is consistent with a number of recent studies reporting APP- or Aβ-mediated internalization of AMPA receptors (e.g., Almeida et al., 2005; Roselli et al., 2005; Hsieh et al., 2006).

One notable difference between our study and that of Hsieh et al. is that we do not observe a decrease in NMDA receptor-mediated synaptic responses. I believe we fortuitously caught our synapses at a point predicted but not seen by Hsieh et al.—that is, after AMPA receptor removal but prior to spine retraction—by recording a few hours earlier after infection than Hsieh et al. We also identified a presynaptic deficit in synaptic vesicle recycling that has implications for neurotransmission in response to extended trains of action...  Read more

Our PNAS study identifies deficits in synaptic transmission when APP is overexpressed in neurons. We use Semliki Forest virus to rapidly upregulate APP in autaptic (isolated microisland) cultures of hippocampal neurons, and record synaptic responses 12 to 24 hours after infection. Our finding that AMPA receptor-mediated responses are reduced in neurons overexpressing APP is consistent with a number of recent studies reporting APP- or Aβ-mediated internalization of AMPA receptors (e.g., Almeida et al., 2005; Roselli et al., 2005; Hsieh et al., 2006).

One notable difference between our study and that of Hsieh et al. is that we do not observe a decrease in NMDA receptor-mediated synaptic responses. I believe we fortuitously caught our synapses at a point predicted but not seen by Hsieh et al.—that is, after AMPA receptor removal but prior to spine retraction—by recording a few hours earlier after infection than Hsieh et al. We also identified a presynaptic deficit in synaptic vesicle recycling that has implications for neurotransmission in response to extended trains of action potentials.

References:
Almeida CG, Tampellini D, Takahashi RH, Greengard P, Lin MT, Snyder EM, Gouras GK. Beta-amyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses. Neurobiol Dis. 2005 Nov;20(2):187-98. Abstract

Hsieh H, Boehm J, Sato C, Iwatsubo T, Tomita T, Sisodia S, Malinow R. AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss. Neuron. 2006 Dec 7;52(5):831-43. Abstract

Roselli F, Tirard M, Lu J, Hutzler P, Lamberti P, Livrea P, Morabito M, Almeida OF. Soluble beta-amyloid1-40 induces NMDA-dependent degradation of postsynaptic density-95 at glutamatergic synapses. J Neurosci. 2005 Nov 30;25(48):11061-70. Abstract

View all comments by Jane Sullivan