Alzheimer's disease (AD) is characterized by amyloid-beta (Abeta) and tau deposition in brain. It has emerged that Abeta toxicity is tau dependent, although mechanistically this link remains unclear. Here, we show that tau, known as axonal protein, has a dendritic function in postsynaptic targeting of the Src kinase Fyn, a substrate of which is the NMDA receptor (NR). Missorting of tau in transgenic mice expressing truncated tau (Deltatau) and absence of tau in tau(-/-) mice both disrupt postsynaptic targeting of Fyn. This uncouples NR-mediated excitotoxicity and hence mitigates Abeta toxicity. Deltatau expression and tau deficiency prevent memory deficits and improve survival in Abeta-forming APP23 mice, a model of AD. These deficits are also fully rescued with a peptide that uncouples the Fyn-mediated interaction of NR and PSD-95 in vivo. Our findings suggest that this dendritic role of tau confers Abeta toxicity at the postsynapse with direct implications for pathogenesis and treatment of AD.

Amyloid-β and tau are the two hallmark proteins in Alzheimer's disease. Although both amyloid-β and tau have been extensively studied individually with regard to their separate modes of toxicity, more recently new light has been shed on their possible interactions and synergistic effects in Alzheimer's disease. Here, we review novel findings that have shifted our understanding of the role of tau in the pathogenesis of Alzheimer's disease towards being a crucial partner of amyloid-β. As we gain a deeper understanding of the different cellular functions of tau, the focus shifts from the axon, where tau has a principal role as a microtubule-associated protein, to the dendrite, where it mediates amyloid-β toxicity.

Alzheimer's disease (AD) is characterized by amyloid-beta (A␤)-containing plaques, neurofibrillary tangles, and neuron and synapse loss. Tangle formation has been reproduced in P301L tau transgenic pR5 mice, whereas APP sw PS2 N141I double-transgenic APP152 mice develop A␤ plaques. Cross-breeding generates triple transgenic ( triple AD) mice that combine both pathologies in one model. To determine functional consequences of the combined A␤ and tau pathologies, we performed a proteomic analysis followed by functional validation. Specifically, we obtained vesicular preparations from triple AD mice, the parental strains, and nontransgenic mice, followed by the quantitative mass-tag labeling proteomic technique iTRAQ and mass spectrometry. Within 1,275 quantified proteins, we found a massive deregulation of 24 proteins, of which one-third were mitochondrial proteins mainly related to complexes I and IV of the oxidative phosphorylation system (OXPHOS). Notably, deregulation of complex I was tau dependent, whereas deregulation of complex IV was A␤ dependent, both at the protein and activity levels. Synergistic effects of A␤ and tau were evident in 8-month-old triple AD mice as only they showed a reduction of the mitochondrial membrane potential at this early age. At the age of 12 months, the strongest defects on OXPHOS, synthesis of ATP, and reactive oxygen species were exhibited in the triple AD mice, again emphasizing synergistic, age-associated effects of A␤ and tau in perishing mitochondria. Our study establishes a molecular link between A␤ and tau protein in AD pathology in vivo, illustrating the potential of quantitative proteomics.amyloid-beta peptide ͉ electron transport chain ͉ energy metabolism ͉ mitochondrial complexes ͉ tau protein A lzheimer's disease (AD) is a devastating neurodegenerative disorder affecting Ͼ15 million people worldwide (1). The key histopathological features are amyloid-beta (A␤)-containing plaques and microtubule-associated protein tau-containing neurofibrillary tangles (NFTs), along with neuronal and synapse loss in selected brain areas (2, 3). In determining the role of distinct proteins in these processes, traditionally, candidate-driven approaches have been pursued, linking neuronal dysfunction to the distribution of known proteins in healthy compared with degenerating neurons, or in transgenic compared with control brain. In comparison, proteomics offers a powerful nonbiased approach as shown by us previously (4, 5).APP152 (APP/PS2) double-transgenic mice model the A␤ plaque pathology of AD (6); they coexpress the N141I mutant form of PS2 together with the APP sw mutant found in familial cases of AD. The mice display age-related cognitive deficits associated with discrete brain A␤ deposition and inflammation (6). pR5 mice model the tangle pathology of AD (7-9). They express P301L mutant tau found in familial cases of frontotemporal dementia (FTD), a dementia related to AD. The pR5 mice show a hippocampus-and amygdala-dependent behavioral impairment related to AD (10). Crossing of ...

Insoluble protein aggregates have been linked to Alzheimer's disease (AD) and frontotemporal dementia (FTD). Recent work in transgenic mice has shed light on the role of these aggregates by identifying soluble oligomeric species that may interfere with essential cellular mechanisms at an early disease stage. This review summarizes what we have learned about the roles of these proteins from transgenic mice and invertebrate species such as flies and worms. Proteomic and transcriptomic analyses of tissue from these animal models have identified new molecules with crucial roles in disease. Moreover, transgenic animals have been instrumental in defining drug targets and designing novel therapeutic strategies. With advanced imaging techniques that can be used in both humans and mice an early, preclinical diagnosis of AD and FTD could be within reach.

Amyloid-β (Aβ) toxicity in Alzheimer's disease (AD) is considered to be mediated by phosphorylated tau protein. In contrast, we found that, at least in early disease, site-specific phosphorylation of tau inhibited Aβ toxicity. This specific tau phosphorylation was mediated by the neuronal p38 mitogen-activated protein kinase p38γ and interfered with postsynaptic excitotoxic signaling complexes engaged by Aβ. Accordingly, depletion of p38γ exacerbated neuronal circuit aberrations, cognitive deficits, and premature lethality in a mouse model of AD, whereas increasing the activity of p38γ abolished these deficits. Furthermore, mimicking site-specific tau phosphorylation alleviated Aβ-induced neuronal death and offered protection from excitotoxicity. Our work provides insights into postsynaptic processes in AD pathogenesis and challenges a purely pathogenic role of tau phosphorylation in neuronal toxicity.