The online version of this article (doi:10.1186/s13024-017-0169-9) contains supplementary material, which is available to authorized users.
Dr. Arbel-Ornath and Dr. Hudry are co-first authors.
Amyloid-β oligomers (oAβ) are thought to mediate neurotoxicity in Alzheimer’s disease (AD), and previous studies in AD transgenic mice suggest that calcium dysregulation may contribute to these pathological effects. Even though AD mouse models remain a valuable resource to investigate amyloid neurotoxicity, the concomitant presence of soluble Aβ species, fibrillar Aβ, and fragments of amyloid precursor protein (APP) complicate the interpretation of the phenotypes.
To explore the specific contribution of soluble oligomeric Aβ (oAβ) to calcium dyshomeostasis and synaptic morphological changes, we acutely exposed the healthy mouse brain, at 3 to 6 months of age, to naturally occurring soluble oligomers and investigated their effect on calcium levels using in vivo multiphoton imaging.
We observed a dramatic increase in the levels of neuronal resting calcium, which was dependent upon extracellular calcium influx and activation of NMDA receptors. Ryanodine receptors, previously implicated in AD models, did not appear to be primarily involved using this experimental setting. We used the high resolution cortical volumes acquired in-vivo to measure the effect on synaptic densities and observed that, while spine density remained stable within the first hour of oAβ exposure, a significant decrease in the number of dendritic spines was observed 24 h post treatment, despite restoration of intraneuronal calcium levels at this time point.
These observations demonstrate a specific effect of oAβ on NMDA-mediated calcium influx, which triggers synaptic collapse in vivo. Moreover, this work leverages a method to quantitatively measure calcium concentration at the level of neuronal processes, cell bodies and single synaptic elements repeatedly and thus can be applicable to testing putative drugs and/or other intervention methodologies.
Additional file 1: Figure S1. Biochemical characterization of Aβ oligomers present in Tg conditioned media (TgCM). a Representative profile showing the amount of Aβ40 detected by ELISA in each fraction after separation of the different oligomeric species present in TgCM (red bars) by size-exclusion chromatography (SEC, Superdex 75 SEC columns). The molecular weight markers (kDa) that ran at the same conditions are indicated above (arrowheads). b Measurements of oligomeric Aβ within each SEC fraction show that most of oAβ is of low molecular weight (fractions 12–18).. A smaller peak of high-molecular weight species was also detected in TgCM (fractions 2–6 in both ELISAs). (TIF 1492 kb)
Additional file 2: Figure S2. Time course of oAβ-induced increase in resting calcium levels after acute exposure with TgCM on a healthy brain. C57BL/6 mice were injected with AAV-CBA-YC3.6 into the somatosensory cortex and acutely exposed to TgCM containing oAβ. The changes in YFP/CFP ratios in the neurites were measured at baseline before treatment and 30 min, 1 h and 5 h after topical application of TgCM. A progressive increase in the levels of resting calcium (YFP/CFP ratio and the actual calcium levels on the lower and upper x-axis, respectively) was detected that reached a peak after 1 h (20.6% of overloaded neurites), before returning to baseline levels after 5 h (1.7% overloaded neurites). (TIF 37844 kb)
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- Soluble oligomeric amyloid-β induces calcium dyshomeostasis that precedes synapse loss in the living mouse brain
Josiah R. Boivin
Kishore V. Kuchibhotla
Carli R. Lattarulo
Arianna M. Belcher
Pariss B. Trujillo
Rebecca A. Betensky
Bradley T. Hyman
Brian J. Bacskai
- BioMed Central