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, ...
Subcellular mislocalization of the microtubule-associated protein Tau is a hallmark of Alzheimer disease (AD) and other tauopathies. Six Tau isoforms, differentiated by the presence or absence of a second repeat or of N-terminal inserts, exist in the human CNS, but their physiological and pathological differences have long remained elusive. Here, we investigated the properties and distributions of human and rodent Tau isoforms in primary forebrain rodent neurons. We found that the Tau diffusion barrier (TDB), located within the axon initial segment (AIS), controls retrograde (axon-to-soma) and anterograde (soma-to-axon) traffic of Tau. Tau isoforms without the N-terminal inserts were sorted efficiently into the axon. However, the longest isoform (2N4R-Tau) was partially retained in cell bodies and dendrites, where it accelerated spine and dendrite growth. The TDB (located within the AIS) was impaired when AIS components (ankyrin G, EB1) were knocked down or when glycogen synthase kinase-3β (GSK3β; an AD-associated kinase tethered to the AIS) was overexpressed. Using superresolution nanoscopy and live-cell imaging, we observed that microtubules within the AIS appeared highly dynamic, a feature essential for the TDB. Pathomechanistically, amyloid-β insult caused cofilin activation and F-actin remodeling and decreased microtubule dynamics in the AIS. Concomitantly with these amyloid-β-induced disruptions, the AIS/TDB sorting function failed, causing AD-like Tau missorting. In summary, we provide evidence that the human and rodent Tau isoforms differ in axodendritic sorting and amyloid-β-induced missorting and that the axodendritic distribution of Tau depends on AIS integrity.
Primary neurons have proved to be an essential tool for investigating neuronal polarity in general and polarized Tau distribution in particular. However, mature primary neurons are notoriously difficult to transfect with nonviral vectors and are very sensitive both to cytoskeletal manipulation and to imaging. Common nonviral transfections require the use of a monolayer of supportive glia or high density cultures, both of which complicate imaging. Here, we provide a simple nonviral transfection method enabling transfection of Tau to achieve expression levels comparable to endogenous Tau. This allows to investigate specific effects on, e.g., distribution and transport of Tau, without grossly affecting other cytoskeleton-based parameters such as microtubule density or microtubule-based transport.
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