Tau represents the subunit protein of one of the major hallmarks of Alzheimer disease (AD), the neurofibrillary tangles, and is therefore of major interest as an indicator of disease mechanisms. Many ...of the unusual properties of Tau can be explained by its nature as a natively unfolded protein. Examples are the large number of structural conformations and biochemical modifications (phosphorylation, proteolysis, glycosylation, and others), the multitude of interaction partners (mainly microtubules, but also other cytoskeletal proteins, kinases, and phosphatases, motor proteins, chaperones, and membrane proteins). The pathological aggregation of Tau is counterintuitive, given its high solubility, but can be rationalized by short hydrophobic motifs forming β structures. The aggregation of Tau is toxic in cell and animal models, but can be reversed by suppressing expression or by aggregation inhibitors. This review summarizes some of the structural, biochemical, and cell biological properties of Tau and Tau fibers. Further aspects of Tau as a diagnostic marker and therapeutic target, its involvement in other Tau-based diseases, and its histopathology are covered by other chapters in this volume.
We studied the effect of microtubule-associated tau protein on trafficking of vesicles and organelles in primary cortical neurons, retinal ganglion cells, and neuroblastoma cells. Tau inhibits ...kinesin-dependent transport of peroxisomes, neurofilaments, and Golgi-derived vesicles into neurites. Loss of peroxisomes makes cells vulnerable to oxidative stress and leads to degeneration. In particular, tau inhibits transport of amyloid precursor protein (APP) into axons and dendrites, causing its accumulation in the cell body. APP tagged with yellow fluorescent protein and transfected by adenovirus associates with vesicles moving rapidly forward in the axon (∼80%) and slowly back (∼20%). Both movements are strongly inhibited by cotransfection with fluorescently tagged tau (cyan fluorescent protein-tau) as seen by two-color confocal microscopy. The data suggests a linkage between tau and APP trafficking, which may be significant in Alzheimer's disease.
Loss of synapses and dying back of axons are considered early events in brain degeneration during Alzheimer’s disease. This is accompanied by an aberrant behavior of the microtubule-associated ...protein tau (hyperphosphorylation, aggregation). Since microtubules are the tracks for axonal transport, we are testing the hypothesis that tau plays a role in the malfunctioning of transport. Experiments with various neuronal and non-neuronal cells show that tau is capable of reducing net anterograde transport of vesicles and cell organelles by blocking the microtubule tracks. Thus, a misregulation of tau could cause the starvation of synapses and enhanced oxidative stress, long before tau detaches from microtubules and aggregates into Alzheimer neurofibrillary tangles. In particular, the transport of amyloid precursor protein is retarded when tau is elevated, suggesting a possible link between the two key proteins that show abnormal behavior in Alzheimer’s disease.
Tau is a highly soluble protein, yet it aggregates abnormally in Alzheimer's disease. Here, we address the question of proteolytic processing of tau and the nucleation of aggregates by tau fragments. ...We show in neuronal cell models that fragments of the repeat domain of tau containing mutations of FTDP17 (frontotemporal dementia with parkinsonism linked to chromosome 17), produced by endogenous proteases, can induce the aggregation of full-length tau. Fragments are generated by successive cleavages, first N-terminally between K257 and S258, then C-terminally around residues 353-364; conversely, when the N-terminal cleavage is inhibited, no fragmentation and aggregation takes place. The C-terminal truncation and the coaggregation of fragments with full-length tau depends on the propensity for β-structure. The aggregation is modulated by phosphorylation but does not depend on it. Aggregation but not fragmentation as such is toxic to cells; conversely, toxicity can be prevented by inhibiting either aggregation or proteolysis. The results reveal a novel pathway of abnormal tau aggregation in neuronal cells.
We have studied biochemical and structural parameters of several missense and deletion mutants of tau protein (G272V, N279K, ΔK280, P301L, V337M, R406W) found in frontotemporal dementia and ...parkinsonism linked to chromosome 17 (FTDP-17). The mutant proteins were expressed on the basis of both full-length tau (htau40) and constructs derived from the repeat domain. They were analyzed with respect to the capacity to enhance microtubule assembly, binding of tau to microtubules, secondary structure content, and aggregation into Alzheimer-like paired helical or straight filaments. We find that the mutations cause a moderate decrease in microtubule interactions and stabilization, and they show no gross structural changes compared with the natively unfolded conformation of the wild-type protein, but the aggregation into PHFs is strongly enhanced, particularly for the mutants ΔK280 and P301L. This gain of pathological aggregation would be consistent with the autosomal dominant nature of the disease.
Tauopathies with parkinsonism represent a spectrum of disease entities unified by the pathologic accumulation of hyperphosphorylated tau protein fragments within the central nervous system. These ...pathologic characteristics suggest shared pathogenetic pathways and possible molecular targets for disease‐modifying therapeutic interventions. Natural history studies, for instance, in progressive supranuclear palsy, frontotemporal dementia with parkinsonism linked to chromosome 17, corticobasal degeneration, and Niemann‐Pick disease type C as well as in amyotrophic lateral sclerosis/Parkinson–dementia complex permit clinical characterization of the disease phenotypes and are crucial to the development and validation of biological markers for differential diagnostics and disease monitoring, for example, by use of neuroimaging or proteomic approaches. The wide pathologic and clinical spectrum of the tauopathies with parkinsonism is reviewed in this article, and perspectives on future advances in the understanding of the pathogenesis are given, together with potential therapeutic strategies.
The neuronal microtubule-associated protein tau plays an important role in establishing cell polarity by stabilizing axonal microtubules that serve as tracks for motor-protein-driven transport ...processes. To investigate the role of tau in intracellular transport, we studied the effects of tau expression in stably transfected CHO cells and differentiated neuroblastoma N2a cells. Tau causes a change in cell shape, retards cell growth, and dramatically alters the distribution of various organelles, known to be transported via microtubule-dependent motor proteins. Mitochondria fail to be transported to peripheral cell compartments and cluster in the vicinity of the microtubule-organizing center. The endoplasmic reticulum becomes less dense and no longer extends to the cell periphery. In differentiated N2a cells, the overexpression of tau leads to the disappearance of mitochondria from the neurites. These effects are caused by tau's binding to microtubules and slowing down intracellular transport by preferential impairment of plus-end-directed transport mediated by kinesin-like motor proteins. Since in Alzheimer's disease tau protein is elevated and mislocalized, these observations point to a possible cause for the gradual degeneration of neurons.
One of the hallmarks of Alzheimer's disease is the abnormal state of the microtubule-associated protein tau in neurons. It is both highly phosphorylated and aggregated into paired helical filaments, ...and it is commonly assumed that the hyperphosphorylation of tau causes its detachment from microtubules and promotes its assembly into PHFs. We have studied the relationship between the phosphorylation of tau by several kinases (MARK, PKA, MAPK, GSK3) and its assembly into PHFs. The proline-directed kinases MAPK and GSK3 are known to phosphorylate most Ser-Pro or Thr-Pro motifs in the regions flanking the repeat domain of tau: they induce the reaction with several antibodies diagnostic of Alzheimer PHFs, but this type of phosphorylation has only a weak effect on tau−microtubule interactions and on PHF assembly. By contrast, MARK and PKA phosphorylate several sites within the repeats (notably the KXGS motifs including Ser262, Ser324, and Ser356, plus Ser320); in addition PKA phosphorylates some sites in the flanking domains, notably Ser214. This type of phosphorylation strongly reduces tau's affinity for microtubules, and at the same time inhibits tau's assembly into PHFs. Thus, contrary to expectations, the phosphorylation that detaches tau from microtubules does not prime it for PHF assembly, but rather inhibits it. Likewise, although the phosphorylation sites on Ser-Pro or Thr-Pro motifs are the most prominent ones on Alzheimer PHFs (by antibody labeling), they are only weakly inhibitory to PHF assembly. This implies that the hyperphosphorylation of tau in Alzheimer's disease is not directly responsible for the pathological aggregation into PHFs; on the contrary, phosphorylation protects tau against aggregation.
We have searched for a minimal interaction motif in τ protein that supports the aggregation into Alzheimer-like paired helical filaments. Digestion of the repeat domain with different proteases ...yields a GluC-induced fragment comprising 43 residues (termed PHF43), which represents the third repeat of τ plus some flanking residues. This fragment self assembles readily into thin filaments without a paired helical appearance, but these filaments are highly competent to nucleate bona fide PHFs from full-length τ . Probing the interactions of PHF43 with overlapping peptides derived from the full τ sequence yields a minimal hexapeptide interaction motif of306VQIVYK311at the beginning of the third internal repeat. This motif coincides with the highest predicted β -structure potential in τ . CD and Fourier transform infrared spectroscopy shows that PHF43 acquires pronounced β structure in conditions of self assembly. Point mutations in the hexapeptide region by proline-scanning mutagenesis prevent the aggregation. The data indicate that PHF assembly is initiated by a short fragment containing the minimal interaction motif forming a local β structure embedded in a largely random-coil protein.
Kinases of the MARK/Par-1 family of S/T protein kinases are regulators of diverse cellular processes in Caenorhabditis elegans, Drosophila, yeast, and mammalian cells. They are involved in nematode ...embryogenesis, epithelial cell polarization, cell signaling, and neuronal differentiation. MARK phosphorylates microtubule-associated proteins such as tau and is a key regulator of microtubule-based intracellular transport. Hyperphosphorylation of tau causes defects in neuronal transport and may induce abnormal aggregation of tau in Alzheimer disease and other tauopathies. Recent high-resolution structure analysis of MARK fragments covering the kinase domain and accessory regulatory domains has revealed important details regarding the autoregulation of MARK, but their interpretation has remained controversial. Here we focus on the structural aspects of MARK activity and autoregulation. Comparison of the available MARK structures with related kinases of the AMPK family and with new structures of MARK isoforms (MARK2 and 3) reveals unexpected structural similarities between these kinases that may help to resolve the existing controversies.-- Marx, A., Nugoor, C., Panneerselvam, S., Mandelkow, E. Structure and function of polarity-inducing kinase family MARK/Par-1 within the branch of AMPK/Snf1-related kinases.
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