Pretangle pathology within cholinergic nucleus basalis neurons coincides with neurotrophic and neurotransmitter receptor gene dysregulation during the progression of Alzheimer's disease.

Cholinergic basal forebrain neurons of the nucleus basalis of Meynert (nbM) regulate attentional and memory function and are exquisitely prone to tau pathology and neurofibrillary tangle (NFT) formation during the progression of Alzheimer's disease (AD). nbM neurons require the neurotrophin nerve growth factor (NGF), its cognate receptor TrkA, and the pan-neurotrophin receptor p75NTR for their maintenance and survival. Additionally, nbM neuronal activity and cholinergic tone are regulated by the expression of nicotinic (nAChR) and muscarinic (mAChR) acetylcholine receptors as well as receptors modulating glutamatergic and catecholaminergic afferent signaling. To date, the molecular and cellular relationships between the evolution of tau pathology and nbM neuronal survival remain unknown. To address this knowledge gap, we profiled cholinotrophic pathway genes within nbM neurons immunostained for pS422, a pretangle phosphorylation event preceding tau C-terminal truncation at D421, or dual-labeled for pS422 and TauC3, a later stage tau neo-epitope revealed by this same C-terminal truncation event, via single-population custom microarray analysis. nbM neurons were obtained from postmortem tissues from subjects who died with an antemortem clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI), or mild/moderate AD. Quantitative analysis revealed significant downregulation of mRNAs encoding TrkA as well as TrkB, TrkC, and the Trk-mediated downstream pro-survival kinase Akt in pS422+ compared to unlabeled, pS422-negative nbM neurons. In addition, pS422+ neurons displayed a downregulation of transcripts encoding NMDA receptor subunit 2B, metabotropic glutamate receptor 2, D2 dopamine receptor, and ß1 adrenoceptor. By contrast, transcripts encoding p75NTR were downregulated in dual-labeled pS422+/TauC3+ neurons. Appearance of the TauC3 epitope was also associated with an upregulation of the α7 nAChR subunit and differential downregulation of the ß2 nAChR subunit. Notably, we found that gene expression patterns for each cell phenotype did not differ with clinical diagnosis. However, linear regression revealed that global cognition and Braak stage were predictors of select transcript changes within both unlabeled and pS422+/TauC3- neurons. Taken together, these cell phenotype-specific gene expression profiling data suggest that dysregulation of neurotrophic and neurotransmitter signaling is an early pathogenic mechanism associated with NFT formation in vulnerable nbM neurons and cognitive decline in AD, which may be amenable to therapeutic intervention early in the disease process.

Neuropathological and transcriptomic characteristics of the aged brain.

As more people live longer, age-related neurodegenerative diseases are an increasingly important societal health issue. Treatments targeting specific pathologies such as amyloid beta in Alzheimer's disease (AD) have not led to effective treatments, and there is increasing evidence of a disconnect between traditional pathology and cognitive abilities with advancing age, indicative of individual variation in resilience to pathology. Here, we generated a comprehensive neuropathological, molecular, and transcriptomic characterization of hippocampus and two regions cortex in 107 aged donors (median = 90) from the Adult Changes in Thought (ACT) study as a freely-available resource (http://aging.brain-map.org/). We confirm established associations between AD pathology and dementia, albeit with increased, presumably aging-related variability, and identify sets of co-expressed genes correlated with pathological tau and inflammation markers. Finally, we demonstrate a relationship between dementia and RNA quality, and find common gene signatures, highlighting the importance of properly controlling for RNA quality when studying dementia.

Protein homeostasis gene dysregulation in pretangle-bearing nucleus basalis neurons during the progression of Alzheimer's disease.

Conformational phosphorylation and cleavage events drive the tau protein from a soluble, monomeric state to a relatively insoluble, polymeric state that precipitates the formation of neurofibrillary tangles (NFTs) in projection neurons in Alzheimer's disease (AD), including the magnocellular perikarya located in the nucleus basalis of Meynert (NBM) complex of the basal forebrain. Whether these structural changes in the tau protein are associated with pathogenic changes at the molecular and cellular level remains undetermined during the onset of AD. Here, we examined alterations in gene expression within individual NBM neurons immunostained for pS422, an early tau phosphorylation event, or dual labeled for pS422 and TauC3, a later stage tau neoepitope, from tissue obtained postmortem from subjects who died with an antemortem clinical diagnosis of no cognitive impairment, mild cognitive impairment, or mild/moderate AD. Specifically, pS422-positive pretangles displayed an upregulation of select gene transcripts subserving protein quality control. On the other hand, late-stage TauC3-positive NFTs exhibited upregulation of messenger RNAs involved in protein degradation but also cell survival. Taken together, these results suggest that molecular pathways regulating protein homeostasis are altered during the evolution of NFT pathology in the NBM. These changes likely contribute to the disruption of protein turnover and neuronal survival of these vulnerable NBM neurons during the progression of AD.

Canonical genetic signatures of the adult human brain.

The structure and function of the human brain are highly stereotyped, implying a conserved molecular program responsible for its development, cellular structure and function. We applied a correlation-based metric called differential stability to assess reproducibility of gene expression patterning across 132 structures in six individual brains, revealing mesoscale genetic organization. The genes with the highest differential stability are highly biologically relevant, with enrichment for brain-related annotations, disease associations, drug targets and literature citations. Using genes with high differential stability, we identified 32 anatomically diverse and reproducible gene expression signatures, which represent distinct cell types, intracellular components and/or associations with neurodevelopmental and neurodegenerative disorders. Genes in neuron-associated compared to non-neuronal networks showed higher preservation between human and mouse; however, many diversely patterned genes displayed marked shifts in regulation between species. Finally, highly consistent transcriptional architecture in neocortex is correlated with resting state functional connectivity, suggesting a link between conserved gene expression and functionally relevant circuitry.

An anatomically comprehensive atlas of the adult human brain transcriptome.

Neuroanatomically precise, genome-wide maps of transcript distributions are critical resources to complement genomic sequence data and to correlate functional and genetic brain architecture. Here we describe the generation and analysis of a transcriptional atlas of the adult human brain, comprising extensive histological analysis and comprehensive microarray profiling of ∼900 neuroanatomically precise subdivisions in two individuals. Transcriptional regulation varies enormously by anatomical location, with different regions and their constituent cell types displaying robust molecular signatures that are highly conserved between individuals. Analysis of differential gene expression and gene co-expression relationships demonstrates that brain-wide variation strongly reflects the distributions of major cell classes such as neurons, oligodendrocytes, astrocytes and microglia. Local neighbourhood relationships between fine anatomical subdivisions are associated with discrete neuronal subtypes and genes involved with synaptic transmission. The neocortex displays a relatively homogeneous transcriptional pattern, but with distinct features associated selectively with primary sensorimotor cortices and with enriched frontal lobe expression. Notably, the spatial topography of the neocortex is strongly reflected in its molecular topography-the closer two cortical regions, the more similar their transcriptomes. This freely accessible online data resource forms a high-resolution transcriptional baseline for neurogenetic studies of normal and abnormal human brain function.

Large-scale cellular-resolution gene profiling in human neocortex reveals species-specific molecular signatures.

Although there have been major advances in elucidating the functional biology of the human brain, relatively little is known of its cellular and molecular organization. Here we report a large-scale characterization of the expression of ∼1,000 genes important for neural functions by in situ hybridization at a cellular resolution in visual and temporal cortices of adult human brains. These data reveal diverse gene expression patterns and remarkable conservation of each individual gene's expression among individuals (95%), cortical areas (84%), and between human and mouse (79%). A small but substantial number of genes (21%) exhibited species-differential expression. Distinct molecular signatures, comprised of genes both common between species and unique to each, were identified for each major cortical cell type. The data suggest that gene expression profile changes may contribute to differential cortical function across species, and in particular, a shift from corticosubcortical to more predominant corticocortical communications in the human brain.

X-ray diffraction from intact tau aggregates in human brain tissue.

We describe an instrument to record x-ray diffraction patterns from diseased regions of human brain tissue by combining an in-line visible light fluorescence microscope with an x-ray diffraction microprobe. We use thiazine red fluorescence to specifically label and detect the filamentous tau protein pathology associated with Pick's disease, as several labs have done previously. We demonstrate that thiazine red-enhanced regions within the tissue show periodic structure in x-ray diffraction that is not observed in healthy tissue. One observed periodicity (4.2 Å) is characteristic of cross-beta sheet structure, consistent with previous results from powder diffraction studies performed on purified, dried tau protein.

Tau 6D and 6P isoforms inhibit polymerization of full-length tau in vitro.

Alzheimer's disease and other tauopathies are characterized by the intracellular accumulation of insoluble filaments of the microtubule-associated protein tau. The six canonical tau isoforms in the adult brain consist of an N-terminal "projection" domain followed by a proline-rich region, a microtubule-binding repeat region, and a C-terminal tail. However, alternative splicing in exon 6 produces an additional set of tau isoforms, termed 6D and 6P, which contain only the N-terminus and part of the proline-rich region. We have previously shown that constructs representing N-terminal fragments of tau, which resemble the naturally occurring 6P and 6D isoforms, inhibit polymerization of the full-length protein in an in vitro filament formation assay and traced the inhibitory activity to amino acids 18-42. Here we report that 6P and 6D tau isoforms inhibit polymerization of full-length tau (hTau40) in a similar manner, likely by stabilizing full-length tau in a soluble conformation. The absence of exons 2 and 3 decreased the effectiveness of the 6D isoforms but not the 6P variants or the N-terminal tau fragments from our previous study, indicating that the 18-42 region is not the sole determinant of inhibitory ability. Finally, this paper demonstrates that inhibition is blocked by pseudophosphorylation of tyrosines 18 and 29, providing a potential link between tyrosine phosphorylation and disease progression. Taken together, these results indicate that the 6P/6D isoforms are potential endogenous inhibitors of tau filament formation and suggest a mechanism by which this ability may be disrupted in disease.

A possible link between astrocyte activation and tau nitration in Alzheimer's disease.

Alzheimer's disease (AD) pathology has been characterized, in part, by the self-assembly of the tau molecule into neurofibrillary tangles (NFT). While different post-translational modifications have been identified that accelerate tau aggregation, nitration at tyrosine residues prevents or slows tau filament formation in vitro. Of the five tyrosine residues within the molecule, nitration at the first tyrosine residue (Tyr 18) results in a profound inhibition of filament self-assembly. To determine whether nitration at Tyr 18 occurs in AD pathology, monoclonal antibodies were raised against a synthetic tau peptide nitrated at Tyr 18. A clone, termed Tau-nY18, reacts specifically with tau proteins nitrated at Tyr 18 and fails to cross-react with other nitrated tyrosine residues spanning the length of the molecule or with other proteins known to be nitrated in neurodegenerative diseases. In situ, Tau-nY18 sparsely labels the neuronal pathological hallmarks of the disease, including NFT and dystrophic neurites. Surprisingly however, Tau-nY18 robustly labels nitrated tau within activated, GFAP positive astrocytes intimately associated with amyloid plaques. Furthermore, this antibody detects nitrated tau in soluble preparations from both severe AD brains (Braak stage V, VI) and age-matched controls. Collectively, these findings suggest that nitration at Tyr 18 may be linked to astrocyte activation, an early event associated with amyloid plaque formation.

Accumulation of aspartic acid421- and glutamic acid391-cleaved tau in neurofibrillary tangles correlates with progression in Alzheimer disease.

Truncations of tau protein at aspartic acid421 (D421) and glutamic acid391 (E391) residues are associated with neurofibrillary tangles (NFTs) in the brains of Alzheimer disease (AD) patients. Using immunohistochemistry with antibodies to D421- and E391-truncated tau (Tau-C3 and MN423, respectively), we correlated the presence of NFTs composed of these truncated tau proteins with clinical and neuropathologic parameters in 17 AD and 23 non-AD control brains. The densities of NFTs composed of D421- or E391-truncated tau correlated with clinical dementia index and Braak staging in AD. Glutamic acid391 tau truncation was prominent in the entorhinal cortex, whereas D421 truncation was prominent in the subiculum, suggesting that NFTs composed of either D421- or E391-truncated tau may be formed mutually exclusively in these areas. Both truncations were associated with the prevalence of the apolipoprotein E epsilon4 allele. By double labeling, intact tau in NFTs was commonly associated with D421-cleaved tau but not with E391-truncated tau; D421-cleaved tau was never associated with E391-truncated tau. These results indicate that tau is not randomly proteolyzed at different domains, and that proteolysis occurs sequentially from the C-terminus to inner regions of tau in AD progression. Identification of NFTs composed of tau at different stages of truncation may facilitate assessment of neurofibrillary pathology in AD.

Phosphorylation and cleavage of tau in non-AD tauopathies.

The tau protein, well known as the primary component of neurofibrillary tangles, also comprises the Pick bodies found in Pick's disease (PiD) and the glial lesions associated with progressive supranuclear palsy (PSP) and cortico-basal ganglionic degeneration (CBD). Many of the tau alterations that are characteristic of Alzheimer's disease have also been identified in PSP and CBD. In this report, we examine three non-AD tauopathies (PSP, CBD, and PiD) for the presence of two specific tau alterations, phosphorylation at Ser422 and truncation at Asp421. We find that truncation at Asp421 is an alteration that is unique to neuronal lesions, occurring in Pick bodies as well as in neurofibrillary tangles, but not in lesions associated with glia. Conversely, phosphorylation at Ser422 is not only present in all these lesions, but identifies additional glial and neuronal pathology in disease-susceptible cortical regions. These results suggest that the molecular alterations of tau that occur during the initial process of tangle formation in AD are similar in non-AD tauopathies, but the middle and later changes are not common to all diseases.

Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer's disease mutations: potential factors in amyloid plaque formation.

Mutations in the genes for amyloid precursor protein (APP) and presenilins (PS1, PS2) increase production of beta-amyloid 42 (Abeta42) and cause familial Alzheimer's disease (FAD). Transgenic mice that express FAD mutant APP and PS1 overproduce Abeta42 and exhibit amyloid plaque pathology similar to that found in AD, but most transgenic models develop plaques slowly. To accelerate plaque development and investigate the effects of very high cerebral Abeta42 levels, we generated APP/PS1 double transgenic mice that coexpress five FAD mutations (5XFAD mice) and additively increase Abeta42 production. 5XFAD mice generate Abeta42 almost exclusively and rapidly accumulate massive cerebral Abeta42 levels. Amyloid deposition (and gliosis) begins at 2 months and reaches a very large burden, especially in subiculum and deep cortical layers. Intraneuronal Abeta42 accumulates in 5XFAD brain starting at 1.5 months of age (before plaques form), is aggregated (as determined by thioflavin S staining), and occurs within neuron soma and neurites. Some amyloid deposits originate within morphologically abnormal neuron soma that contain intraneuronal Abeta. Synaptic markers synaptophysin, syntaxin, and postsynaptic density-95 decrease with age in 5XFAD brain, and large pyramidal neurons in cortical layer 5 and subiculum are lost. In addition, levels of the activation subunit of cyclin-dependent kinase 5, p25, are elevated significantly at 9 months in 5XFAD brain, although an upward trend is observed by 3 months of age, before significant neurodegeneration or neuron loss. Finally, 5XFAD mice have impaired memory in the Y-maze. Thus, 5XFAD mice rapidly recapitulate major features of AD amyloid pathology and may be useful models of intraneuronal Abeta42-induced neurodegeneration and amyloid plaque formation.

Tau nitration occurs at tyrosine 29 in the fibrillar lesions of Alzheimer's disease and other tauopathies.

The neurodegenerative tauopathies are a clinically diverse group of diseases typified by the pathological self-assembly of the microtubule-associated tau protein. Although tau nitration is believed to influence the pathogenesis of these diseases, the precise residues modified, and the resulting effects on tau function, remain enigmatic. Previously, we demonstrated that nitration at residue Tyr29 markedly inhibits the ability of tau to self-associate and stabilize the microtubule lattice (Reynolds et al., 2005b, 2006). Here, we report the first monoclonal antibody to detect nitration in a protein-specific and site-selective manner. This reagent, termed Tau-nY29, recognizes tau only when nitrated at residue Tyr29. It does not cross-react with wild-type tau, tau mutants singly nitrated at Tyr18, Tyr197, and Tyr394, or other proteins known to be nitrated in neurodegenerative diseases. By Western blot analysis, Tau-nY29 detects soluble tau and paired helical filament tau from severely affected Alzheimer's brain but fails to recognize tau from normal aged brain. This observation suggests that nitration at Tyr29 is a disease-related event that may alter the intrinsic ability of tau to self-polymerize. In Alzheimer's brain, Tau-nY29 labels the fibrillar triad of tau lesions, including neurofibrillary tangles, neuritic plaques, and, to a lesser extent, neuropil threads. Intriguingly, although Tau-nY29 stains both the neuronal and glial tau pathology of Pick disease, it detects only the neuronal pathology in corticobasal degeneration and progressive supranuclear palsy without labeling the predominant glial pathology. Collectively, our findings provide the first direct evidence that site-specific tau nitration is linked to the progression of the neurodegenerative tauopathies.

N-terminal fragments of tau inhibit full-length tau polymerization in vitro.

The polymerization of the microtubule-associated protein, tau, into insoluble filaments is a common thread in Alzheimer's disease and in a variety of frontotemporal dementias. The conformational change required for tau to transition from an extended monomeric state to a filamentous state with a high beta-sheet content involves the extreme N-terminus coming into contact with distal portions of the molecule; however, these exact interactions are incompletely understood. Here we report that a construct representing amino acids 1-196 (Tau196), which itself does not polymerize, inhibits polymerization of full-length tau (hTau40) in vitro. In addition, we trace the inhibitory effect of Tau196 to amino acids 18-42 of the construct. We also provide evidence that the N-terminal tau fragments require a specific C-terminal region of tau (residues 392-421) to exert their inhibitory effect. The fragments are most effective at inhibiting polymerization when present during the initial 5 min; they remain in the soluble fraction of the polymerization reaction, and they increase the amount of soluble hTau40. The fragments also reduce the number and average length of filaments that are formed. Taken together, these results suggest that the N-terminal tau fragments inhibit hTau40 polymerization by interacting with a specific C-terminal sequence, thereby stabilizing a soluble conformation of tau.

Formation of phospho-SAPK/JNK granules in the hippocampus is an early event in Alzheimer disease.

The mitogen-activated protein (MAP) kinase SAPK/JNK phosphorylates tau protein at many of its proline-directed serine/threonine residues in vitro and is a likely candidate kinase to phosphorylate the pathologically relevant S422 site on tau. Since phosphorylation of tau, particularly at S422, is a relatively early marker of AD and seems to precede tangle formation, it appears likely that an early form of activated SAPK/JNK might be detected by immunohistochemical means around the time that tau begins to aggregate into tangles. We report here that an antibody to phospho-SAPK/JNK (p-SAPK/JNK) reacts with several types of lesions including granular bodies in limbic areas; NFTs in limbic cortex and temporal neocortex; occasional neuritic plaques in temporal neocortex; and select axons in the hippocampus, entorhinal cortex, and inferior temporal cortex. In order to characterize the appearance of granular p-SAPK/JNK and determine if it appears early in disease, we employed an immunohistochemical study of postmortem limbic tissue from 20 cases ranging from Braak stages I-VI. By co-staining with anti-tau antibodies specific to different molecular events that occur during tangle evolution, we were able to identify the appearance of p-SAPK/JNK in early Braak stages with an increased elevation during the limbic stages of AD and during the early stages of the formation of individual hippocampal tangles.

Pseudophosphorylation of tau at serine 422 inhibits caspase cleavage: in vitro evidence and implications for tangle formation in vivo.

The tangles of Alzheimer's disease (AD) are comprised of the tau protein displaying numerous alterations, including phosphorylation at serine 422 (S422) and truncation at aspartic acid 421 (D421). Truncation at the latter site appears to result from activation of caspases, a class of proteases that cleave specifically at aspartic acid residues. It has been proposed that phosphorylation at or near caspase cleavage sites could regulate the ability of the protease to cleave at those sites. Here, we use tau pseudophosphorylated at S422 (S422E) to examine the effects of tau phosphorylation on its cleavage by caspase 3. We find that S422E tau is more resistant to proteolysis by caspase 3 than non-pseudophosphorylated tau. Additionally, we use antibodies directed against the phosphorylation site and against the truncation epitope to assess the presence of these epitopes in neurofibrillary tangles in the aged human brain. We show that phosphorylation precedes truncation during tangle maturation. Moreover, the distribution of the two epitopes suggests that a significant length of time (perhaps as much as two decades) elapses between S422 phosphorylation and cleavage at D421. We further conclude that tau phosphorylation at S422 may be a protective mechanism that inhibits cleavage in vivo.

Cholinesterases: roles in the brain during health and disease.

The cholinergic hypothesis of decline in dementia, whereby deficits in learning, memory and behavior are caused, at least in part, by decreased levels of acetylcholine (ACh) in the brain, first emerged more than 20 years ago. The role for acetylcholinesterase (AChE) and its inhibition in this scheme has long been accepted, but findings from preclinical experiments and clinical trials have placed butyrylcholinesterase (BuChE) alongside AChE as an important contributor to the occurrence, symptoms, progression and responses to treatment in dementia. A number of new lines of evidence suggest that both cholinesterase inhibitors (ChEs) may have broader functions in the CNS than previously thought, which relate to both 'classical' esterase activities of the enzymes as well as non-classical actions unrelated to their enzymatic function. Data suggest involvement of the ChEs in modulating glial activation, cerebral blood flow, the amyloid cascade, and tau phosphorylation. It has therefore been speculated that some actions of the ChEs could affect the underlying disease processes in Alzheimer's disease (AD), and that pharmacological manipulation with ChE inhibitors may affect long-term disease progression. Focusing on new findings relating to BuChE, we review recent evidence that has extended knowledge into the roles of ChEs in health, disease and aging.

Tau truncation during neurofibrillary tangle evolution in Alzheimer's disease.

The microtubule-associated protein, tau, is a highly soluble molecule that is nonetheless capable of self-association into filamentous deposits characteristic of a number of neurodegenerative diseases. This state change is thought to be driven by phosphorylation and/or C-terminal truncation events resulting in intracellular inclusions, such as the neurofibrillary tangles (NFTs) in Alzheimer's disease (AD). Previously, we reported the existence of a novel truncation event, cleavage at aspartic acid(421), presumably by a caspase, and also described a monoclonal antibody (Tau-C3) specific for tau cleaved at this site. Here, we report the timing of this cleavage event relative to other antibody-targeted alterations in the tau molecule during the course of NFT evolution in AD. Immunohistochemical studies indicate that cleavage at aspartic acid(421) occurs after formation of the Alz50 epitope but prior to formation of the Tau-66 epitope and truncation at glutamic acid(391) (formation of the MN423 epitope). Thus, creation of the Tau-C3 epitope appears to occur relatively early in the disease state, contemporaneous with the initial Alz50 folding event that heralds the appearance of filamentous tau in NFTs, neuropil threads, and the dystrophic neurites surrounding amyloid plaques.

Tau, tangles, and Alzheimer's disease.

Neurofibrillary tangles (NFT) are comprised of the microtubule-associated protein tau, in the form of filamentous aggregates. In addition to the well-known changes in phosphorylation state, tau undergoes multiple truncations and shifts in conformation as it transforms from an unfolded monomer to the structured polymer characteristic of NFT. Truncations at both the amino- and carboxy-termini directly influence the conformation into which the molecule folds, and hence the ability of tau to polymerize into fibrils. Certain of these truncations may be due to cleavage by caspases as part of the apoptotic cascade. In this review, we discuss evidence that strongly suggests that these truncations occur in an orderly pattern and directly influence the ability of tau to polymerize into filaments.

Early N-terminal changes and caspase-6 cleavage of tau in Alzheimer's disease.

Alzheimer's disease (AD) is a progressive amnestic dementia that involves post-translational hyperphosphorylation, enzymatic cleavage, and conformational alterations of the microtubule-associated protein tau. The truncation state of tau influences many of its pathologic characteristics, including its ability to assume AD-related conformations and to assemble into filaments. Cleavage also appears to be an important marker in AD progression. Although C-terminal truncation of tau at D421 has recently been attributed to the apoptotic enzyme caspase-3, N-terminal processing of the protein remains mostly uncharacterized. Here, we report immunohistochemical staining in a cohort of 35 cases ranging from noncognitively impaired to early AD with a panel of three N-terminal anti-tau antibodies: Tau-12, 5A6, and 9G3-pY18. Of these three, the phosphorylation-independent epitope of 5A6 was the earliest to emerge in the pathological lesions of tau, followed by the appearance of the Tau-12 epitope. The unmasking of the Tau-12 epitope in more mature 5A6-positive tangles was not correlated with tau phosphorylation at tyrosine 18 (9G3-pY18). Still, later in the course of tangle evolution, the extreme N terminus of tau was lost, correlating temporally with the appearance of a C-terminal caspase-truncated epitope lacking residues 422-441. In addition, caspase-6 cleaved the N terminus of tau in vitro, preventing immunoreactivity with both Tau-12 and 5A6. Mass spectrometry confirmed that the in vitro caspase-6 truncation site is D13, a semicanonical and hitherto undescribed caspase cleavage site in tau. Collectively, these results suggest a role for caspase-6 and N-terminal truncation of tau during neurofibrillary tangle evolution and the progression of Alzheimer's disease.