Aß/Amyloid Precursor Protein-Induced Hyperexcitability and Dysregulation of Homeostatic Synaptic Plasticity in Neuron Models of Alzheimer's Disease.

Alzheimer's disease (AD) is increasingly seen as a disease of synapses and diverse evidence has implicated the amyloid-ß peptide (Aß) in synapse damage. The molecular and cellular mechanism(s) by which Aß and/or its precursor protein, the amyloid precursor protein (APP) can affect synapses remains unclear. Interestingly, early hyperexcitability has been described in human AD and mouse models of AD, which precedes later hypoactivity. Here we show that neurons in culture with either elevated levels of Aß or with human APP mutated to prevent Aß generation can both induce hyperactivity as detected by elevated calcium transient frequency and amplitude. Since homeostatic synaptic plasticity (HSP) mechanisms normally maintain a setpoint of activity, we examined whether HSP was altered in AD transgenic neurons. Using methods known to induce HSP, we demonstrate that APP protein levels are regulated by chronic modulation of activity and that AD transgenic neurons have an impaired adaptation of calcium transients to global changes in activity. Further, AD transgenic compared to WT neurons failed to adjust the length of their axon initial segments (AIS), an adaptation known to alter excitability. Thus, we show that both APP and Aß influence neuronal activity and that mechanisms of HSP are disrupted in primary neuron models of AD.

Neuronal spreading and plaque induction of intracellular Aß and its disruption of Aß homeostasis.

The amyloid-beta peptide (Aß) is thought to have prion-like properties promoting its spread throughout the brain in Alzheimer's disease (AD). However, the cellular mechanism(s) of this spread remains unclear. Here, we show an important role of intracellular Aß in its prion-like spread. We demonstrate that an intracellular source of Aß can induce amyloid plaques in vivo via hippocampal injection. We show that hippocampal injection of mouse AD brain homogenate not only induces plaques, but also damages interneurons and affects intracellular Aß levels in synaptically connected brain areas, paralleling cellular changes seen in AD. Furthermore, in a primary neuron AD model, exposure of picomolar amounts of brain-derived Aß leads to an apparent redistribution of Aß from soma to processes and dystrophic neurites. We also observe that such neuritic dystrophies associate with plaque formation in AD-transgenic mice. Finally, using cellular models, we propose a mechanism for how intracellular accumulation of Aß disturbs homeostatic control of Aß levels and can contribute to the up to 10,000-fold increase of Aß in the AD brain. Our data indicate an essential role for intracellular prion-like Aß and its synaptic spread in the pathogenesis of AD.

Amyloid Structural Changes Studied by Infrared Microspectroscopy in Bigenic Cellular Models of Alzheimer's Disease.

Alzheimer's disease affects millions of lives worldwide. This terminal disease is characterized by the formation of amyloid aggregates, so-called amyloid oligomers. These oligomers are composed of ß-sheet structures, which are believed to be neurotoxic. However, the actual secondary structure that contributes most to neurotoxicity remains unknown. This lack of knowledge is due to the challenging nature of characterizing the secondary structure of amyloids in cells. To overcome this and investigate the molecular changes in proteins directly in cells, we used synchrotron-based infrared microspectroscopy, a label-free and non-destructive technique available for in situ molecular imaging, to detect structural changes in proteins and lipids. Specifically, we evaluated the formation of ß-sheet structures in different monogenic and bigenic cellular models of Alzheimer's disease that we generated for this study. We report on the possibility to discern different amyloid signatures directly in cells using infrared microspectroscopy and demonstrate that bigenic (amyloid-ß, α-synuclein) and (amyloid-ß, Tau) neuron-like cells display changes in ß-sheet load. Altogether, our findings support the notion that different molecular mechanisms of amyloid aggregation, as opposed to a common mechanism, are triggered by the specific cellular environment and, therefore, that various mechanisms lead to the development of Alzheimer's disease.