Aggregation of amyloid-␤ (A␤) and Tau protein are hallmarks of Alzheimer's disease (AD), and according to the A␤-cascade hypothesis, A␤ is considered toxic for neurons and Tau a downstream target of A␤. We have investigated differentiated primary hippocampal neurons for early localized changes following exposure to A␤ oligomers. Initial events become evident by missorting of endogenous Tau into the somatodendritic compartment, in contrast to axonal sorting in normal neurons. In missorted dendritic regions there is a depletion of spines and local increase in Ca 2ϩ , and breakdown of microtubules. Tau in these regions shows elevated phosphorylation at certain sites diagnostic of AD-Tau (e.g., epitope of antibody 12E8, whose phosphorylation causes detachment of Tau from microtubules, and AT8 epitope), and local elevation of certain kinase activities (e.g., MARK/par-1, BRSK/SADK, p70S6K, cdk5, but not GSK3␤, JNK, MAPK). These local effects occur without global changes in Tau, tubulin, or kinase levels. Somatodendritic missorting occurs not only with Tau, but also with other axonal proteins such as neurofilaments, and correlates with pronounced depletion of microtubules and mitochondria. The A␤-induced effects on microtubule and mitochondria depletion, Tau missorting, and loss of spines are prevented by taxol, indicating that A␤-induced microtubule destabilization and corresponding traffic defects are key factors in incipient degeneration. By contrast, the rise in Ca 2ϩ levels, kinase activities, and Tau phosphorylation cannot be prevented by taxol. Incipient and local changes similar to those of A␤ oligomers can be evoked by cell stressors (e.g., H 2 O 2 , glutamate, serum deprivation), suggesting some common mechanism of signaling.

Mislocalization and aggregation of Ab and Tau combined with loss of synapses and microtubules (MTs) are hallmarks of Alzheimer disease. We exposed mature primary neurons to Ab oligomers and analysed changes in the Tau/ MT system. MT breakdown occurs in dendrites invaded by Tau (Tau missorting) and is mediated by spastin, an MT-severing enzyme. Spastin is recruited by MT polyglutamylation, induced by Tau missorting triggered translocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like-6) into dendrites. Consequences are spine loss and mitochondria and neurofilament mislocalization. Missorted Tau is not axonally derived, as shown by axonal retention of photoconvertible Dendra2-Tau, but newly synthesized. Recovery from Ab insult occurs after Ab oligomers lose their toxicity and requires the kinase MARK (Microtubule-AffinityRegulating-Kinase). In neurons derived from Tau-knockout mice, MTs and synapses are resistant to Ab toxicity because TTLL6 mislocalization and MT polyglutamylation are prevented; hence no spastin recruitment and no MT breakdown occur, enabling faster recovery. Reintroduction of Tau re-establishes Ab-induced toxicity in TauKO IntroductionAlzheimer disease (AD) is characterized by two types of abnormal protein aggregates in the brain, 'amyloid plaques' made up of Ab peptides, and 'neurofibrillary tangles' assembled from Tau protein Mucke and Selkoe, 2012). In familial AD, genetic evidence points to a master role of Ab aggregation, leading to downstream events including Tau hyperphosphorylation and aggregation (Haass and Selkoe, 2007;Karran et al, 2011). On the other hand, the distribution of Tau aggregates in the brain correlates better with the clinical progression of AD (Braak stages; Braak and Braak, 1991), and experiments with transgenic mice suggest a critical role for Tau in mediating Ab-induced toxicity.A key to understand the normal and toxic roles of Tau lies in the neuronal cytoskeleton, notably microtubules (MTs), the tracks of intracellular transport, and their interaction partners. Tau changes during development include (i) a shift from shorter fetal to longer adult isoforms, (ii) decreased phosphorylation (from a higher fetal to a lower adult level), and (iii) changed intracellular distribution (from ubiquitous to axonal) (Mandell and Banker, 1995;Bullmann et al, 2009). Notably, the early cytoskeletal changes in AD, such as lower MT stability, hyperphosphorylation, and missorting of Tau to the somatodendritic compartment, are reminiscent of the fetal state. In TauKO mice, the lack of Tau during development can be compensated by upregulation of other neuronal microtubule associated proteins (MAPs), for example, MAP1A (Harada et al, 1994), which may explain the mild phenotype. Depending on experimental conditions, the absence of Tau may have detrimental effects (inefficient axon outgrowth and neurodegeneration; Dawson et al, 2001;Dawson et al, 2010), or beneficial ones (reduction in Ab and excitotoxicity; King et al, 2006;Roberson et al, 2007;Ittner et al, 2010). In neuronal cell culture, ...

Missorting of Tau from axons to the somatodendritic compartment of neurons is a hallmark of Alzheimer's disease, but the mechanisms underlying normal sorting and pathological failure are poorly understood. Here, we used several Tau constructs labelled with photoconvertible Dendra2 to analyse its mobility in polarized neurons. This revealed a novel mechanism of sorting-a retrograde barrier in the axon initial segment (AIS) operating as cellular rectifier. It allows anterograde flow of axonal Tau but prevents retrograde flow back into soma and dendrites. The barrier requires binding of Tau to microtubules but does not require F-actin and thus is distinct from the sorting of membrane-associated proteins at the AIS. The barrier breaks down when Tau is phosphorylated in its repeat domain and detached from microtubules, for example, by the kinase MARK/Par1. These observations link the pathological hallmarks of Tau missorting and hyperphosphorylation in neurodegenerative diseases.