Stabilization of AMPA receptors turns out to be a specialty of the synaptic protein PSD95, according to another new paper from Richard Huganir and colleagues at Johns Hopkins University School of Medicine, Baltimore, Maryland, and the Howard Hughes Medical Institute. Their results show that PSD95 knockout mice lose AMPA receptors, but only in a subset of synapses. PSD95 is a potential target for Aβ’s action on synapses, but Huganir’s results show that getting rid of PSD95 creates a different phenotype than Aβ treatment. In contrast to Aβ’s effects on spine density, the loss of AMPAR in response to PSD95 deletion did not affect spine morphology.

The publication of the Malinow paper follows first author Helen Hsieh’s presentation of some of the results at SfN 2005 (see ARF related news story). There, she showed that expressing APP in hippocampal neurons led to the depression of AMPA and NMDA receptor-related neurotransmission in a way that mimics LTD. Hsieh’s results showed that production of Aβ occluded LTD, which suggests the two use a common pathway. Like LTD, the internalization stimulated by Aβ required calcineurin and p38 MAP kinase signaling.

In their paper, they present all this data and add the link to dendritic spine loss. The system they use, organotypic cultures of hippocampal pyramidal neurons, is amenable to transfection experiments. They show that overexpression of APP and the ensuing production of Aβ, or treatment of cells with added Aβ, reduced spine density by 30 percent in the cultures. They used a variety of mutants in the AMPA receptor GluR2 subunit to probe the mechanism of spine loss. As they had shown for synaptic depression, spine loss required internalization of the AMPA receptor—neither occurred in cells expressing an endocytosis-resistant mutant of the GluR2 subunit of the AMPA receptor. These results suggested that the removal of AMPAR by endocytosis is necessary to see spine loss.

To test if AMPAR removal was sufficient to trigger synaptic loss, they used a phosphomimetic GluR2 mutant (R607Q;S880E) that is constitutively internalized. Phosphorylation of S880 is required to stimulate receptor endocytosis during LTD. Expression of the mutant led to a decrease in both AMPAR- and NMDAR-mediated neurotransmission and dendritic spine loss, showing that removal of AMPA receptors can deal a fatal blow to synapses. Tying this result back to Aβ, they show that in cells expressing GluR2, Aβ treatment results in a small but significant increase in phosphorylation at S880.

From this, Hsieh and colleagues conclude that the loss of synaptic AMPA receptors is driven by Aβ-induced phosphorylation of GluR2, and leads to spine loss. This loss, in turn, may account for the observed decrease in NMDAR-mediated synaptic transmission. “Our results indicate that Aβ drives the removal of synaptic AMPA receptors and this plays a key role in the toxic effects of Aβ on spines,” they write. Their findings suggest that strategies to stabilize AMPAR may be one way to treat AD.

How Aβ taps into the LTD pathway is unclear, but one idea that has been around for a while is that Aβ destabilizes synapses via its interaction with the synaptic protein PSD95. This protein, which regulates expression of both NMDA and AMPA glutamate receptors, has been shown by Bill Klein’s work to colocalize with Aβ in synapses (see Lacor et al., 2004). Work from Claudia Almeida and Gunnar Gouras earlier this year showed that in neuronal cultures from AD mice, loss of AMPAR was preceded by a decrease in the PSD95 protein.

Now, Huganir and colleagues show that in PSD95 knockout mice, there is a loss of AMPA receptors, but only in selected synapses. Previous PSD95 knockouts did not cause total loss of protein function, so first author Jean-Claude Beique and coworkers generated a true null allele. They found that AMPAR-mediated synaptic transmission was reduced in homozygous knockout mice, but there was no change in NMDA function. When they analyzed the function of individual synapses using a caged glutamate compound, they found that, curiously, the knockout had synapse-specific effects. Many synapses were unaffected by the knockout, while in others AMPAR-mediated neurotransmission was absent.

The loss of PSD95 did not affect the ability to enhance AMPAR numbers during LTP. On the contrary, the mice showed enhanced LTP compared to wild-type mice. The researchers speculate that this reflects the increased number of AMPAR-negative synapses at baseline in the knockout mice. The results indicate that PSD95 mainly plays a role in maintaining the stability of synaptic AMPARs.

In contrast to the Malinow observations, AMPAR-lacking synapses were found on normal, mature spines, and the knockout mice showed no changes in spine volume. These results suggest that while disruption of PSD95 or addition of Aβ can both downregulate AMPAR levels and synaptic function, the ramifications appear very different for the health of dendritic spines.—Pat McCaffrey.

Reference:
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

Beique JC, Lin DT, Kang MG, Aizawa H, Takamiya K, Huganir RL. Synapse-specific regulation of AMPA receptor function by PSD-95. Proc Natl Acad Sci U S A. 2006 Dec 5; [Epub ahead of print] Abstract