Soluble Aβ oligomer-induced synaptopathy: c-Jun N-terminal kinase's role

Dear Editor, Among the neurodegenerative diseases, Alzheimer disease (AD) is the most common and severe age-related dementia for which there is currently no available treatment. Many studies support the assumption that AD is a spine pathology (Selkoe, 2002; Sivanesan et al., 2013) and that soluble amyloid-b (Ab) oligomers are causative of AD synaptopathy. Diverse lines of evidence indicate that Aboligomers induce formation of pore-like structures on the membrane (Arispe et al., 1993; Lashuel et al., 2002) and interfere with glutamatergic transmission. The Ab oligomers result in a decreased number of AMPA receptors (AMPA-r) and NMDA receptors (NMDA-r), as well as PSD-95 at the postsynaptic membrane, and thus reduce the strength and plasticity of excitatory synapses (Chapman et al., 1999; Walsh et al., 2002). However, the underlying intracellular mechanisms regulating synaptic changes are only partially known. By understanding the pathophysiological mechanisms leading to synaptic dysfunction and the progression of this dysfunction, better interference in the pathogenesis of AD can be achieved. We present an in vitro model to study the temporal sequence of dendritic spine modifications induced by soluble Ab oligomers, and to analyse the intracellular signalling pathways leading to AD synaptopathy. This model allows synaptic alterations to be followed in living neurons before and after treatment and reduces bias due to cell variability. This model also permits testing of pharmaceuticals that are designed to reverse the biochemical and structural alterations of synapses induced by Ab oligomers. Brainbow hippocampal neurons, which express fluorescent proteins, were used to visualize dendritic spines and study synaptic plasticity (Figure 1A and B). To obtain isolated cells, fluorescent neurons were seeded on a layer of non-fluorescent neurons (ratio: 1/16) (Figure 1A and B). In this way we avoided the need for transfection, infection protocols, and low density cultures that are not well tolerated by neurons. Neurons were treated with a subtoxic dose (Figure 1E) of soluble Ab1–42 oligomers in order to induce synaptic changes without any signs of neuronal death (Figure 1E). The preparation of synthetic Ab1–42 that were used to induce in vitro synaptic dysfunction had been previously characterized. Oligomeric assemblies were only observed in peptide preparations after a 24-h incubation at 48C (referred to as oligomers). Immediately after dissolution, the majority of Ab1–42 remained as unassembled monomeric structures (Figure 1C and D and Supplementary Figure S1). Subtoxic concentrations of soluble Ab oligomers induced alterations in the postsynaptic density (PSD) composition of dendritic spines, while monomers had no effect on synaptic plasticity (Supplementary Figure S1F). Exposing the neurons for 3 h to 1 mM Ab oligomers induced changes in the PSD region, leading to a 68% and 61% drop of GluN2A and GluN2B subunits of NMDA-r, respectively; a decrease of 70% and 65% of GluA1 and GluA2 subunits of AMPA-r, respectively; a 53% loss of PSD-95; and a 76% loss of drebrin (Figure 1F and Supplementary Figure S2A). To assess dendritic spine modifications in vitro, we analysed changes in spine density and morphology in neurons exposed to soluble Ab oligomers. Application of soluble Ab oligomers (1 mM) for 3 h caused a 25% decrease in total spine number compared with that before Ab application (Figure 1J and K). The decrease involved all types of spines in a proportional manner. The number of mushroom, stubby, and thin spines decreased by 32%, 25%, and 22%, respectively (Figure 1J and L). Moreover, Ab oligomer treatment induced a 67% reduction in new spine formation compared with control conditions, and led to spine shrinkage (Supplementary Figure S2E). The number of mushroom spines that became stubby or thin was significantly increased by Ab oligomer treatment, while the number of spines that became mushroom was decreased (Supplementary Figure S2E). The morphological changes were consistent with the observed biochemical alterations since thin and stubby spines have a less extended PSD region and lower contents of glutamate receptors as well as postsynaptic markers, in comparison with mushroom spines (Tackenberg et al., 2009). To analyse the pathways involved in Ab oligomer-induced synaptopathy, we evaluated synaptic changes induced after 30 min and 3 h of Ab oligomer exposure (1 mM) and correlated them to the activation of two stress signalling pathways, c-Jun N-terminal kinase (JNK) and caspase-3. After 30 min there was no sign of molecular changes: NMDA-r and AMPA-r subunits, PSD-95, and drebrin levels were unaffected by the Ab oligomer treatment (Figure 1F). However, JNK was already activated at this stage, as indicated by a 2.36-fold increase of the P-JNK/JNK ratio compared with control conditions (Figure 1G and Supplementary Figure S2B). There was no indication of caspase-3 cleavage after 30 min of Ab oligomer exposure (Figure 1H and Supplementary Figure S2C). Exposing neurons to soluble Ab oligomers for 3 h induced a biochemical perturbation of PSD (Figure 1F) with a 4.33-fold increase of caspase-3 cleavage (Figure 1H) (Li et al., 2010; D’Amelio et al., 2011), while JNK activity remained elevated by 2.26 folds (Figure 1G). Our results showed that JNK activation was triggered by Ab oligomers before PSD alterations were induced and JNK activation persisted up to 3 h, at which doi:10.1093/jmcb/mjt015 Journal of Molecular Cell Biology (2013), 5, 277–279 | 277 Published online April 18, 2013