Neurobiology
A combinatorial native MS and LC-MS/MS approach reveals high intrinsic phosphorylation of human Tau but minimal levels of other key modifications
Abnormal changes of neuronal Tau protein, such as phosphorylation and aggregation, are considered hallmarks of cognitive deficits in Alzheimer's disease. Abnormal phosphorylation is thought to precede aggregation and therefore to promote aggregation, but the nature and extent of phosphorylation remain ill-defined. Tau contains ∼85 potential phosphorylation sites, which can be phosphorylated by various kinases because the unfolded structure of Tau makes them accessible. However, methodological limitations (e.g.Truncation of Tau selectively facilitates its pathological activities
Neurofibrillary tangles of abnormally hyperphosphorylated Tau are a hallmark of Alzheimer's disease (AD) and related tauopathies. Tau is truncated at multiple sites by various proteases in AD brain. Although many studies have reported the effect of truncation on the aggregation of Tau, these studies mostly employed highly artificial conditions, using heparin sulfate or arachidonic acid to induce aggregation. Here, we report for the first time the pathological activities of various truncations of Tau, including site-specific phosphorylation, self-aggregation, binding to hyperphosphorylated and oligomeric Tau isolated from AD brain tissue (AD O-Tau), and aggregation seeded by AD O-Tau.Quantitative propagation of assembled human Tau from Alzheimer's disease brain in microfluidic neuronal cultures
Tau aggregation and hyperphosphorylation is a key neuropathological hallmark of Alzheimer's disease (AD), and the temporospatial spread of Tau observed during clinical manifestation suggests that Tau pathology may spread along the axonal network and propagate between synaptically connected neurons. Here, we have developed a cellular model that allows the study of human AD-derived Tau propagation from neuron to neuron using microfluidic devices. We show by using high-content imaging techniques and an in-house developed interactive computer program that human AD-derived Tau seeds rodent Tau that propagates trans-neuronally in a quantifiable manner in a microfluidic culture model.A synthetic heparinoid blocks Tau aggregate cell uptake and amplification
Tau aggregation underlies neurodegeneration in Alzheimer's disease and related tauopathies. We and others have proposed that transcellular propagation of pathology is mediated by Tau prions, which are ordered protein assemblies that faithfully replicate in vivo and cause specific biological effects. The prion model predicts the release of aggregates from a first-order cell and subsequent uptake into a second-order cell. The assemblies then serve as templates for their own replication, a process termed “seeding.” We have previously observed that heparan sulfate proteoglycans on the cell surface mediate the cellular uptake of Tau aggregates.CNS cell type–specific gene profiling of P301S tau transgenic mice identifies genes dysregulated by progressive tau accumulation
The microtubule-associated protein tau undergoes aberrant modification resulting in insoluble brain deposits in various neurodegenerative diseases, including frontotemporal dementia (FTD), progressive supranuclear palsy, and corticobasal degeneration. Tau aggregates can form in different cell types of the central nervous system (CNS) but are most prevalent in neurons. We have previously recapitulated aspects of human FTD in mouse models by overexpressing mutant human tau in CNS neurons, including a P301S tau variant in TAU58/2 mice, characterized by early-onset and progressive behavioral deficits and FTD-like neuropathology.Spleen tyrosine kinase (SYK) blocks autophagic Tau degradation in vitro and in vivo
Spleen tyrosine kinase (SYK) plays a major role in inflammation and in adaptive immune responses and could therefore contribute to the neuroinflammation observed in various neurodegenerative diseases. Indeed, previously we have reported that SYK also regulates β-amyloid (Aβ) production and hyperphosphorylation of Tau protein involved in these diseases. Moreover, SYK hyperactivation occurs in a subset of activated microglia, in dystrophic neurites surrounding Aβ deposits, and in neurons affected by Tau pathology both in individuals with Alzheimer’s disease (AD) and in AD mouse models.Elucidating Tau function and dysfunction in the era of cryo-EM
Tau is a microtubule-associated protein involved in the regulation of axonal microtubules in neurons. In pathological conditions, it forms fibrils that are molecular hallmarks of neurological disorders known as tauopathies. In the last 2 years, cryo-EM has given unprecedented high-resolution views of Tau in both physiological and pathological conditions. We review here these new findings and put them into the context of the knowledge about Tau before this structural breakthrough. The first structures of Tau fibrils, a molecular hallmark of Alzheimer's disease (AD), were based on fibrils from the brain of an individual with AD and, along with similar patient-derived structures, have set the gold standard for the field.Specific glycosaminoglycan chain length and sulfation patterns are required for cell uptake of tau versus α-synuclein and β-amyloid aggregates
Transcellular propagation of protein aggregate “seeds” has been proposed to mediate the progression of neurodegenerative diseases in tauopathies and α-synucleinopathies. We previously reported that tau and α-synuclein aggregates bind heparan sulfate proteoglycans (HSPGs) on the cell surface, promoting cellular uptake and intracellular seeding. However, the specificity and binding mode of these protein aggregates to HSPGs remain unknown. Here, we measured direct interaction with modified heparins to determine the size and sulfation requirements for tau, α-synuclein, and β-amyloid (Aβ) aggregate binding to glycosaminoglycans (GAGs).Phosphorylation of Tau protein correlates with changes in hippocampal theta oscillations and reduces hippocampal excitability in Alzheimer's model
Tau hyperphosphorylation at several sites, including those close to the microtubule domain region (MDr), is considered a key pathological event in the development of Alzheimer’s disease (AD). Recent studies indicate that at the very early stage of this disease, increased phosphorylation in Tau’s MDr domain correlates with reduced levels of neuronal excitability. Mechanistically, we show that pyramidal neurons and some parvalbumin-positive interneurons in 1-month-old triple-transgenic AD mice accumulate hyperphosphorylated Tau protein and that this accumulation correlates with changes in theta oscillations in hippocampal neurons.An N-terminal motif unique to primate tau enables differential protein–protein interactions
Compared with other mammalian species, humans are particularly susceptible to tau-mediated neurodegenerative disorders. Differential interactions of the tau protein with other proteins are critical for mediating tau’s physiological functions as well as tau-associated pathological processes. Primate tau harbors an 11-amino acid-long motif in its N-terminal region (residues 18–28), which is not present in non-primate species and whose function is unknown. Here, we used deletion mutagenesis to remove this sequence region from the longest human tau isoform, followed by glutathione S-transferase (GST) pulldown assays paired with isobaric tags for relative and absolute quantitation (iTRAQ) multiplex labeling, a quantitative method to measure protein abundance by mass spectrometry.Galectin-8–mediated selective autophagy protects against seeded tau aggregation
Assembled tau can transfer between cells and seed the aggregation of soluble tau. This process is thought to underlie the amplification and propagation of tau inclusions throughout the brain in neurodegenerative diseases, including Alzheimer's disease. An understanding of the mechanisms involved may provide strategies for limiting assembled tau propagation. Here, we sought to determine how assembled tau seeds gain access to the cytosol and whether this access triggers cellular defenses. We show that tau assemblies enter cells through clathrin-independent endocytosis and escape from damaged endomembranes into the cytosol, where they seed the aggregation of soluble tau.Distinct differences in prion-like seeding and aggregation between Tau protein variants provide mechanistic insights into tauopathies
The accumulation of aberrantly aggregated MAPT (microtubule-associated protein Tau) defines a spectrum of tauopathies, including Alzheimer's disease. Mutations in the MAPT gene cause frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), characterized by neuronal pathological Tau inclusions in the form of neurofibrillary tangles and Pick bodies and in some cases glial Tau pathology. Increasing evidence points to the importance of prion-like seeding as a mechanism for the pathological spread in tauopathy and other neurodegenerative diseases.Isoform-independent and -dependent phosphorylation of microtubule-associated protein tau in mouse brain during postnatal development
Tau is a microtubule (MT)-associated protein that regulates MT dynamics in the axons of neurons. Tau binds to MTs via its C-terminal MT-binding repeats. There are two types of tau, those with three (3R) or four (4R) MT-binding repeats; 4R tau has a stronger MT-stabilizing activity than 3R tau. The MT-stabilizing activity of tau is regulated by phosphorylation. Interestingly, both the isoform and phosphorylation change at the time of neuronal circuit formation during postnatal development; highly phosphorylated 3R tau is replaced with 4R tau, which is less phosphorylated.An acetylation–phosphorylation switch that regulates tau aggregation propensity and function
The aberrant accumulation of tau protein is a pathological hallmark of a class of neurodegenerative diseases known as tauopathies, including Alzheimer's disease and related dementias. On the basis of previous observations that tau is a direct substrate of histone deacetylase 6 (HDAC6), we sought to map all HDAC6-responsive sites in tau and determine how acetylation in a site-specific manner affects tau's biophysical properties in vitro. Our findings indicate that several acetylation sites in tau are responsive to HDAC6 and that acetylation on Lys-321 (within a KCGS motif) is both essential for acetylation-mediated inhibition of tau aggregation in vitro and a molecular tactic for preventing phosphorylation on the downstream Ser-324 residue.Axodendritic sorting and pathological missorting of Tau are isoform-specific and determined by axon initial segment architecture
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.Modification of Tau by 8-Nitroguanosine 3′,5′-Cyclic Monophosphate (8-Nitro-cGMP): EFFECTS OF NITRIC OXIDE-LINKED CHEMICAL MODIFICATION ON TAU AGGREGATION
Neurofibrillar tangles caused by intracellular hyperphosphorylated tau inclusion and extracellular amyloid β peptide deposition are hallmarks of Alzheimer's disease. Tau contains one or two cysteine residues in three or four repeats of the microtubule binding region following alternative splicing of exon 10, and formation of intermolecular cysteine disulfide bonds accelerates tau aggregation. 8-Nitroguanosine 3′,5′-cyclic monophosphate (8-nitro-cGMP) acts as a novel second messenger of nitric oxide (NO) by covalently binding cGMP to cysteine residues by electrophilic properties, a process termed protein S-guanylation.The Effect of Fragmented Pathogenic α-Synuclein Seeds on Prion-like Propagation
Aggregates of abnormal proteins are widely observed in neuronal and glial cells of patients with various neurodegenerative diseases, and it has been proposed that prion-like behavior of these proteins can account for not only the onset but also the progression of these diseases. However, it is not yet clear which abnormal protein structures function most efficiently as seeds for prion-like propagation. In this study, we aimed to identify the most pathogenic species of α-synuclein (α-syn), the main component of the Lewy bodies and Lewy neurites that are observed in α-synucleinopathies.α-Synuclein Fibrils Exhibit Gain of Toxic Function, Promoting Tau Aggregation and Inhibiting Microtubule Assembly
α-Synuclein is the major component of Lewy bodies and Lewy neurites in Parkinson disease and dementia with Lewy bodies and of glial cytoplasmic inclusions in multiple system atrophy. It has been suggested that α-synuclein fibrils or intermediate protofibrils in the process of fibril formation may have a toxic effect on neuronal cells. In this study, we investigated the ability of soluble monomeric α-synuclein to promote microtubule assembly and the effects of conformational changes of α-synuclein on Tau-promoted microtubule assembly.Extracellular Vesicles Isolated from the Brains of rTg4510 Mice Seed Tau Protein Aggregation in a Threshold-dependent Manner
The microtubule-associated protein tau has a critical role in Alzheimer disease and related tauopathies. There is accumulating evidence that tau aggregates spread and replicate in a prion-like manner, with the uptake of pathological tau seeds causing misfolding and aggregation of monomeric tau in recipient cells. Here we focused on small extracellular vesicles enriched for exosomes that were isolated from the brains of tau transgenic rTg4510 and control mice. We found that these extracellular vesicles contained tau, although the levels were significantly higher in transgenic mice that have a pronounced tau pathology.Co-immunoprecipitation with Tau Isoform-specific Antibodies Reveals Distinct Protein Interactions and Highlights a Putative Role for 2N Tau in Disease
Alternative splicing generates multiple isoforms of the microtubule-associated protein Tau, but little is known about their specific function. In the adult mouse brain, three Tau isoforms are expressed that contain either 0, 1, or 2 N-terminal inserts (0N, 1N, and 2N). We generated Tau isoform-specific antibodies and performed co-immunoprecipitations followed by tandem mass tag multiplexed quantitative mass spectrometry. We identified novel Tau-interacting proteins of which one-half comprised membrane-bound proteins, localized to the plasma membrane, mitochondria, and other organelles.Glycogen Synthase Kinase 3β-mediated Phosphorylation in the Most C-terminal Region of Protein Interacting with C Kinase 1 (PICK1) Regulates the Binding of PICK1 to Glutamate Receptor Subunit GluA2
Background: Despite extensive research, the mechanisms regulating the interaction between protein interacting with C kinase 1 (PICK1) and GluA2 are still unclear.Results: Glycogen synthase kinase-3β (GSK-3β) phosphorylates PICK1. Phosphorylated PICK1 binds to GluA2.Conclusion: GSK-3β regulates the GluA2-PICK1 interaction.Significance: This study contributes to our understanding of the mechanisms of long-term depression, an Alzheimer disease-related event.Distinct Therapeutic Mechanisms of Tau Antibodies: PROMOTING MICROGLIAL CLEARANCE VERSUS BLOCKING NEURONAL UPTAKE
Background: Vaccination against Tau reduces pathology in vivo; however, the mechanism of action remains unclear.Results: Antibodies promote uptake of Tau fibrils in microglia or block uptake in neurons in a size- and epitope-dependent manner.Conclusion: Antibodies have multiple potential mechanisms.Significance: Establishing specific mechanisms of antibody activity may help in design and optimization of more effective agents.Isoform-selective Genetic Inhibition of Constitutive Cytosolic Hsp70 Activity Promotes Client Tau Degradation Using an Altered Co-chaperone Complement
The constitutively expressed heat shock protein 70 kDa (Hsc70) is a major chaperone protein responsible for maintaining proteostasis, yet how its structure translates into functional decisions regarding client fate is still unclear. We previously showed that Hsc70 preserved aberrant Tau, but it remained unknown if selective inhibition of the activity of this Hsp70 isoform could facilitate Tau clearance. Using single point mutations in the nucleotide binding domain, we assessed the effect of several mutations on the functions of human Hsc70.
