I certainly like the idea that this season might go down in the Alzheimer research history as the summer of calcium, with four major studies recently forging new links between calcium problems in neurons and Alzheimer disease (AD). However, a major issue is how AD-related, deranged calcium signals lead to neuron dysfunction and death.

We have shown a few days ago (Sanz-Blasco et al., 2008) that Aβ oligomers (but not fibrils) promote Ca2+ influx into primary neurons (but not glia). This influx is followed by mitochondrial calcium overload as monitored by photon counting imaging of low-affinity aequorin targeted to mitochondria. The relevance of this finding is that prevention of mitochondrial calcium overload using low concentrations of mitochondrial uncoupler protects neurons against Aβ-induced ROS production, permeability transition, cytochrome c release, and apoptosis and cell death.

Moreover, we found that a series of carboxylic, non-steroidal anti-inflammatory drugs including R-flurbiprofen prevent the mitochondrial calcium overload, acting as mitochondrial...  Read more

I certainly like the idea that this season might go down in the Alzheimer research history as the summer of calcium, with four major studies recently forging new links between calcium problems in neurons and Alzheimer disease (AD). However, a major issue is how AD-related, deranged calcium signals lead to neuron dysfunction and death.

We have shown a few days ago (Sanz-Blasco et al., 2008) that Aβ oligomers (but not fibrils) promote Ca2+ influx into primary neurons (but not glia). This influx is followed by mitochondrial calcium overload as monitored by photon counting imaging of low-affinity aequorin targeted to mitochondria. The relevance of this finding is that prevention of mitochondrial calcium overload using low concentrations of mitochondrial uncoupler protects neurons against Aβ-induced ROS production, permeability transition, cytochrome c release, and apoptosis and cell death.

Moreover, we found that a series of carboxylic, non-steroidal anti-inflammatory drugs including R-flurbiprofen prevent the mitochondrial calcium overload, acting as mitochondrial uncouplers and protecting against cell death. These effects are achieved at NSAID concentrations in the low microM range, well below the range required for targeting γ-secretase. Therefore, mitochondrial calcium overload contributes to cell death induced by Aβ oligomers.

In addition, the long-debated mechanism of neuroprotection by NSAIDs could be related to the calcium hypothesis of Alzheimer disease rather than to their ability to target inflammation or secretases. Whether mitochondrial calcium overload is also involved in cell death induced by excess calcium release promoted by either loss of ER calcium leak or IP3 receptor modulation remains to be established.

References:
Sanz-Blasco S, Valero RA, Rodríguez-Crespo I, Villalobos C, Núñez L. Mitochondrial Ca2+ overload underlies Abeta oligomers neurotoxicity providing an unexpected mechanism of neuroprotection by NSAIDs. PLoS ONE. 2008 Jul 23;3(7):e2718. Abstract

View all comments by Carlos Villalobos

This is a very interesting article. From our perspective, there are several important points: first, it highlights a plaque-specific alteration of neural function, but emphasizes that this occurs in the vicinity around plaques, suggesting a halo effect of a potentially soluble mediator, which is critical to alter activity. Second, it uses an alternative technical strategy to the one utilized by my colleague Brian Bacskai (Kuchibhotla et al., 2008), which used gene transfer of a genetically encoded Ca2+ reporter to observe neurites near plaques, as opposed to loading of an AM molecular dye to monitor neuronal perikarya, to come to a similar conclusion—that neurons in the vicinity of plaques have a profound dyshomeostasis of calcium regulation. Third, that neural systems are markedly disrupted by the presence of plaques, with functional alterations leading to both increased and decreased activity affecting a large percentage of neurons. This neural system collapse provides a plausible biological underpinning for the cognitive impairment...  Read more

This is a very interesting article. From our perspective, there are several important points: first, it highlights a plaque-specific alteration of neural function, but emphasizes that this occurs in the vicinity around plaques, suggesting a halo effect of a potentially soluble mediator, which is critical to alter activity. Second, it uses an alternative technical strategy to the one utilized by my colleague Brian Bacskai (Kuchibhotla et al., 2008), which used gene transfer of a genetically encoded Ca2+ reporter to observe neurites near plaques, as opposed to loading of an AM molecular dye to monitor neuronal perikarya, to come to a similar conclusion—that neurons in the vicinity of plaques have a profound dyshomeostasis of calcium regulation. Third, that neural systems are markedly disrupted by the presence of plaques, with functional alterations leading to both increased and decreased activity affecting a large percentage of neurons. This neural system collapse provides a plausible biological underpinning for the cognitive impairment in AD.

View all comments by Bradley Hyman

This is a great paper that confirms our and others’ work that senile plaques are a focal source of toxicity. The implications of the hyperactivity are unknown; however, it is apparent that a disruption to the normal network activity in the brain is obvious. These data are in line with our own measurements of altered resting calcium concentration in neurons in animals with senile plaques and suggest that a more detailed examination of the calcium hypothesis of AD is warranted.

We have not looked at spontaneous activity of neuronal calcium signaling in the brain, so we cannot confirm these results as yet, but it would be interesting to probe whether these observations share similarities to physiologically relevant evoked responses.

In sum, this paper confirms that calcium imaging as an indicator of neuronal activity, and calcium imaging as an indicator of dyshomeostasis of calcium concentration, is an important future direction for our understanding of the disruption of the cellular and network disruption of neuronal signaling in AD.

View all comments by Brian Bacskai

This is a very interesting paper and consistent with our results from primary hippocampal neurons derived from APPswe transgenic rats. We have recently shown that modest overexpression of APPswe results in increased frequency but unaltered amplitude of spontaneous calcium oscillations in these transgenic neurons (Kloskowska et al., 2008a). Furthermore, we found that the baseline of calcium oscillations was significantly higher than in control neurons (Kloskowska et al., 2008b). Thus, recent data implies that the relationship between Aβ and neuronal activity is more complex than has been thought.

References:
Kloskowska E., Malkiewicz K., Winblad B., Benedikz E., Bruton J.D., 2008. APPswe mutation increases the frequency of spontaneous Ca2+-oscillations in rat hippocampal neurons. Neurosci Lett. 436(2):250-254. Abstract

Kloskowska E., Bruton J.D., Winblad B., Benedikz E., 2008. The APP670/671 mutation alters calcium signaling and response to hyperosmotic stress in rat primary hippocampal neurons. Neurosci Lett. 444(3):275-279. Abstract

View all comments by Ewa Kloskowska

The increase in activity in some of the neurons near the amyloid plaques in the layer of cortical neurons is surprising, but it makes sense when it is framed, just as the authors did, in the “synaptic failure hypothesis.” This finding could very well be related to the observation that endocytosis of glutamate receptors is observed in certain animal models of AD. Dysfunction of Ca2+ kinetics is probably causing the calcium overload the authors mention (seen in another study).

Would it be interesting to look specifically at which neurons are affected? Could it be that GABAergic neurons are being affected the most, and their dysfunction thereby resulting in an overall increase in excitation? The main question still to be answered is why is it that some of the cortical neurons decrease activity, while others increase neuronal activity? In the conclusion, the authors mention anatomical remodeling in synaptic inputs as a possible cause. This explanation, although plausible, does not clarify within which mechanism are the chemical signals from Aβ affecting the neurons. Could it be...  Read more

The increase in activity in some of the neurons near the amyloid plaques in the layer of cortical neurons is surprising, but it makes sense when it is framed, just as the authors did, in the “synaptic failure hypothesis.” This finding could very well be related to the observation that endocytosis of glutamate receptors is observed in certain animal models of AD. Dysfunction of Ca2+ kinetics is probably causing the calcium overload the authors mention (seen in another study).

Would it be interesting to look specifically at which neurons are affected? Could it be that GABAergic neurons are being affected the most, and their dysfunction thereby resulting in an overall increase in excitation? The main question still to be answered is why is it that some of the cortical neurons decrease activity, while others increase neuronal activity? In the conclusion, the authors mention anatomical remodeling in synaptic inputs as a possible cause. This explanation, although plausible, does not clarify within which mechanism are the chemical signals from Aβ affecting the neurons. Could it be that defective neurogenesis is also occurring, adding to the changes in neuronal function?

Interestingly, distance from the plaques seems to be the determining factor of whether increase or decrease in activity is seen (the mechanism AB uses to affect neurons could be similar to what is seen in neural development, where chemical gradients determine the “identity” of a cell). The fact that Aβ is the main factor in the hyperactivation of neurons close to the cortical layer 2/3 was strengthened by the results of the predisposing APP23xPS45 mice study. Unfortunately, the fact that these predisposing mice present normal cortical activity and Ca2+ function rules out the use of these biological markers in pharmacological studies for prevention of AD. However, because the APP mice do present behavioral deficits in the discriminatory water maze and the Y-maze, there may be a possibility that an in between age (between two months and six months) could serve as a model for testing of preventive drugs.

View all comments by Karienn Montgomery

It's of interest that mRNA levels of the calcineurin inhibitor, DSCR1, are also much higher in AD brain (1). The recent study be Lee and colleagues finds that DSCR1 interacts with Tollip and positively modulates IL-1R signalling (2). Tollip is an IRAK-1 inhibitor. This would seem to suggest problems with TLR2/TLR4 signalling in AD. This is supported by the Landreth study finding that CD14 and TLR2 and TLR4 bind Aβ to stimulate microglial activation (3). The KEGG link is below for the TOLL RECEPTOR signaling pathway (4).

References:
1. Ermak G, Morgan TE, Davies KJ. Chronic overexpression of the calcineurin inhibitory gene DSCR1 (Adapt78) is associated with Alzheimer's disease. J Biol Chem. 2001 Oct 19;276(42):38787-94. Abstract

2. Lee JY, Lee HJ, Lee EJ, Jang SH, Kim H, Yoon JH, Chung KC. Down syndrome candidate region-1 protein interacts with Tollip and positively modulates interleukin-1 receptor-mediated signaling. Biochim Biophys Acta. 2009 Dec;1790(12):1673-80. Abstract

3. Reed-Geaghan EG, Savage JC, Hise AG, Landreth GE. CD14 and toll-like receptors 2 and 4 are required for fibrillar A{beta}-stimulated microglial activation. J Neurosci. 2009 Sep 23;29(38):11982-92. Abstract

4. Toll-like receptor signaling pathway—Homo sapiens (human)

View all comments by Mary Reid