In an interview for a postdoctoral position in 1992, I presented my dissertation work on the calcium-destablizing effects of the glial protein S100B. The lab to which I had applied was focused on Alzheimer's disease, so I attempted to make my work relevant to their interests by highlighting the role of calcium in excitotoxicity. I was very nearly laughed off the dais by a senior scientist in the audience: "It's silly to think that excitotoxicity—a phenomenon that kills neurons within minutes to hours—could be involved in a neurodegenerative condition that progresses over years." At about the same time, Mark Mattson was beginning to publish his findings that excitotoxicity need not culminate in the death of the entire cell. His work showed that at lower concentrations or at shorter times, glutamate receptor agonists could cause pruning of dendrites only. Indeed, even subtler treatments would have the effect of simply slowing the outgrowth of dendrites. Thus, there came to be an appreciation of an overlap between glutamate's toxicity and its normal roles in development and...  Read more

In an interview for a postdoctoral position in 1992, I presented my dissertation work on the calcium-destablizing effects of the glial protein S100B. The lab to which I had applied was focused on Alzheimer's disease, so I attempted to make my work relevant to their interests by highlighting the role of calcium in excitotoxicity. I was very nearly laughed off the dais by a senior scientist in the audience: "It's silly to think that excitotoxicity—a phenomenon that kills neurons within minutes to hours—could be involved in a neurodegenerative condition that progresses over years." At about the same time, Mark Mattson was beginning to publish his findings that excitotoxicity need not culminate in the death of the entire cell. His work showed that at lower concentrations or at shorter times, glutamate receptor agonists could cause pruning of dendrites only. Indeed, even subtler treatments would have the effect of simply slowing the outgrowth of dendrites. Thus, there came to be an appreciation of an overlap between glutamate's toxicity and its normal roles in development and plasticity. The former seemed to be almost an exaggeration of the latter.

Mark went on to show that the mechanisms by which excessive glutamate could have this limited impact on dendritic compartments (perhaps, single spines) include pathways formerly studied only in the field of apoptosis. Not only could excitotoxity be synapse-limited, but so could caspase-dependent "degeneration" (which would be better called "structural long-term depression, LTD"). If the biochemical changes manifest as long-term potentiation (LTP) can occasionally give rise to more lasting potentiations that are structural in nature, why couldn't synaptic depressions likewise make the transition from LTD to a structural change, i.e., removal of the synapse?

In addition to the claim that excitotoxicity is temporally inconsistent with AD, there is another caveat related to the effects of excessive glutamatergic stimulation which may be more difficult to shake. In AD, or any other in vivo setting, it has been argued that the efficiency of astrocytic transporters in clearing the synaptic cleft makes a glutamate elevation irrelevant if not impossible. Indeed, it is difficult to believe that Aβ could effect a dramatic change in synaptic glutamate levels by inhibiting neuronal transporters alone. Molecular biology approaches and pharmacology both point to astrocytic transporters as being nearly the whole story in clearing synapses of glutamate (Anderson et al, 2000). In the paper at hand, Li et al. present data suggesting that Aβ inhibits a neuronal glutamate transporter rather than astrocytic uptake (Figs. 5H and S3C). One of their arguments is that glutamate uptake into synaptosomes was inhibited by Aβ; however, synaptosomes are well known to contain astrocytic elements (Henn et al., 1976; Chicurel et al, 1993). Another point made by Li et al. is that DHK, an inhibitor of one of the "glial" glutamate transporters, created LTD that was distinct from that of Aβ’s. This is not definitive, however; one of the studies making a case for the significance of neuronal uptake demonstrates exquisite sensitivity of neuronal transporters to DHK (Wang et al., 1998). But reporting an inhibition of glial transporters by Aβ might have lacked sufficient novelty: Marni Harris showed this effect when she was a graduate student, almost 15 years ago (Harris et al., 1995)!

Finally, it is worth considering that the effects of Aβ on extracellular glutamate levels may not involve sodium-dependent transporters at all. Aβ can elicit glutamate release via the xc- transport system, a glutamate/cystine exchanger activated by oxidative stress. Although the effects of fibrillar Aβ on this system that we initially reported were modest (Barger & Basile, 2001), we have subsequently seen much larger increases with oligomeric preparations. This mechanism has relevance to the metabotropic glutamate receptor (mGluR) angle emphasized by Li et al. An important role for Group II mGluRs has been documented in the connection of xc- transport to cocaine relapse (Kau et al., 2008). The possible involvement of xc- transporters is perhaps more worthy of consideration given that almost all the data presented by Li et al. were obtained in tissue slices, where soluble agents applied to the bath can readily access glia at both extra- and intrasynaptic sites.

References:
Anderson CM, Swanson RA. Astrocyte glutamate transport: review of properties, regulation, and physiological functions. Glia. 2000 Oct;32(1):1-14. Abstract

Barger SW, Basile AS. Activation of microglia by secreted amyloid precursor protein evokes release of glutamate by cystine exchange and attenuates synaptic function. J Neurochem. 2001 Feb;76(3):846-54. Abstract

Bridges, R.J., Kavanaugh, M.P., Chamberlin, A.R. A pharmacological review of competitive inhibitors and substrates of high affinity, sodium-dependent glutamate transport in the central nervous system. Curr Pharm Des. 1999 May;5(5):363-79. Abstract

Chicurel ME, Terrian DM, Potter H. mRNA at the synapse: analysis of a synaptosomal preparation enriched in hippocampal dendritic spines. Neurosci. 1993 Sep;13(9):4054-63. Abstract

Harris ME, Carney JM, Cole PS, Hensley K, Howard BJ, Martin L, Bummer P, Wang Y, Pedigo NW Jr, Butterfield DA. beta-Amyloid peptide-derived, oxygen-dependent free radicals inhibit glutamate uptake in cultured astrocytes: implications for Alzheimer's disease. Neuroreport. 1995 Oct 2;6(14):1875-9. Abstract

Henn, F.A., Anderson, D.J., Rustad, D.G. Glial contamination of synaptosomal fractions. Brain Res. 1976 Jan 16;101(2):341-4. Abstract

Kau KS, Madayag A, Mantsch JR, Grier MD, Abdulhameed O, Baker DA. Blunted cystine-glutamate antiporter function in the nucleus accumbens promotes cocaine-induced drug seeking. Neuroscience. 2008 Aug 13;155(2):530-7. Abstract

Wang GJ, Chung HJ, Schnuer J, Pratt K, Zable AC, Kavanaugh MP, Rosenberg PA. High affinity glutamate transport in rat cortical neurons in culture. Mol Pharmacol. 1998 Jan;53(1):88-96. Abstract

View all comments by Steve Barger

Two new reports released this week (Villemagne et al., 2010; McDonald et al., 2010) document the prevalence of Aβ dimers in blood and brain samples, respectively, from individuals diagnosed with AD.

The first group used an elegant ProteinChip® array using affinity surfaces coated with various Aβ antibodies including 4G8 or WO2 to measure the levels of species bound to cellular membranes of blood cells in a large human cohort (n = 118). Using this approach, the authors found elevated levels of Aβ monomers and dimers in specimens from AD patients as compared to age-matched controls, though there were large overlaps between clinical groups. They also found that the levels of Aβ dimers strongly correlated with those of monomeric Aβ42. Interestingly, Aβ dimers were not detected when a 40-end specific antibody to Aβ was used as capture agent.

Finally, the authors performed correlation analyses among various clinical and neuroimaging variables, revealing modest but significant correlations between Aβ dimers and cognitive decline. Overall, these findings support the notion that...  Read more

Two new reports released this week (Villemagne et al., 2010; McDonald et al., 2010) document the prevalence of Aβ dimers in blood and brain samples, respectively, from individuals diagnosed with AD.

The first group used an elegant ProteinChip® array using affinity surfaces coated with various Aβ antibodies including 4G8 or WO2 to measure the levels of species bound to cellular membranes of blood cells in a large human cohort (n = 118). Using this approach, the authors found elevated levels of Aβ monomers and dimers in specimens from AD patients as compared to age-matched controls, though there were large overlaps between clinical groups. They also found that the levels of Aβ dimers strongly correlated with those of monomeric Aβ42. Interestingly, Aβ dimers were not detected when a 40-end specific antibody to Aβ was used as capture agent.

Finally, the authors performed correlation analyses among various clinical and neuroimaging variables, revealing modest but significant correlations between Aβ dimers and cognitive decline. Overall, these findings support the notion that Aβ dimers are elevated in AD compared to healthy controls as first reported by Shankar et al., 2008. However, this new report also documents the presence of Aβ dimers in biological samples from cognitively intact controls; this differs from the aforementioned study. Finally, due to the considerable overlap in the levels of Aβ dimers across tested clinical groups, it is unlikely that solely measuring Aβ dimers will represent a confident diagnostic tool for the prognosis of Alzheimer disease. This is disappointing news.

The second study led by Dominic Walsh’s and Dennis Selkoe’s groups can be viewed as a study extending the findings reported by Shankar and colleagues (2008). Here, McDonald et al. determined the levels of monomeric and dimeric Aβ levels in 43 brain specimens using a combination of immunoprecipitation/Western blotting techniques coupled to infrared detection for enhanced sensitivity. The authors report that soluble Aβ monomers, dimers, trimers, and occasionally tetramers were detected in their cohort. Unfortunately, no other oligomers (including Aβ*56) were observed due to the presence of non-specific bands masking potential oligomeric Aβ assemblies between 30 and 75 kDa. Consistent with their previous findings, Aβ dimers were only detected within the AD group compared to the controls, and their calculated concentration rose sharply in the AD group. One possible explanation for this segregation might be explained by differences in postmortem interval delays (24, 18, and 18 hours for the ND, DNAD, and AD groups, respectively) as well as in apparent age at death among groups (means of 81, 92, and 87.5 years). It would be interesting to see whether these variables have an impact on our biochemical analyses of Aβ oligomers.

Finally, the authors identified an association between the levels of Aβ monomers + dimers and intermediate to high brain amyloid loads. Altogether, these findings suggest that the concentration of brain-soluble Aβ dimers might be related to the extent of amyloid deposition in brain tissues.

Granted that both studies used very different biological samples and reported extremely different segregation profiles between controls and AD groups, blood or brain levels of Aβ dimers do appear elevated in AD.

View all comments by Sylvain Lesne