Alzheimer's Disease

All the Tau We Cannot See

Alzheimer’s disease (AD) was described in 1906 as a dementing disease marked by the presence of two types of fibrillar aggregates in the brain: neurofibrillary tangles and senile plaques. The process of aggregation and formation of the aggregates has been a major focus of investigation ever since the discoveries that the tau protein is the predominant protein in tangles and amyloid β is the predominant protein in plaques. The idea that smaller, oligomeric species of amyloid may also be bioactive has now been clearly established. This review examines the possibility that soluble, nonfibrillar, bioactive forms of tau—the “tau we cannot see”—comprise a dominant driver of neurodegeneration in AD.

Alzheimer disease-associated tau post-translational modification mimics impact tau propagation and uptake

As Alzheimer disease (AD) progresses, pathological tau spreads by cell-to-cell propagation of tau. This study aims to elucidate the impact of AD-associated post-translational modifications of tau-on-tau propagation. Tau propagation reporter constructs distinguishing donor cells from recipient cells were developed, and additional constructs were made with tau residues mutated from serine or threonine to aspartate to mimic the negative charge of a phosphorylation and/or from lysine to glutamine to mimic the charge-neutralizing effect of acetylation. Flow cytometry was used to quantify donor and recipient cells. This revealed that the mutations generally tended to reduce tau propagation compared to wildtype tau. Recombinant tau containing either wildtype or posttranslational modification mimicking mutations were used to treat Chinese hamster ovary cells or human induced pluripotent stem cell-derived neurons to quantify tau uptake, revealing that the mutations generally resulted in reduced uptake compared to wildtype tau. Surface plasmon resonance revealed that the mutations had a reduced affinity for lipoprotein receptor-related protein 1 (LRP1), a tau uptake receptor, compared to wildtype tau. Overall, these results suggest that AD-associated posttranslational modification mimicking mutations reduce the cell-to-cell propagation of tau by reducing tau uptake by recipient cells, which may be in part due to reduced binding affinity to LRP1.

Alzheimer proteopathic tau seeds are biochemically a forme fruste of mature paired helical filaments

Aggregation prone molecules, such as tau, form both historically well characterized fibrillar deposits (neurofibrillary tangles) and recently identified phosphate-buffered saline (PBS) extract species called proteopathic seeds. Both can cause normal endogenous tau to undergo templated misfolding. The relationship of these seeds to the fibrils that define tau-related diseases is unknown. We characterized the aqueous extractable and sarkosyl insoluble fibrillar tau species derived from human Alzheimer brain using mass spectrometry and in vitro bioassays. Post-translational modifications (PTMs) including phosphorylation, acetylation and ubiquitination are identified in both preparations. PBS extract seed competent tau can be distinguished from sarkosyl insoluble tau by the presence of overlapping, but less abundant, PTMs and an absence of some PTMs unique to the latter. The presence of ubiquitin and other PTMs on the PBS-extracted tau species correlates with the amount of tau in the seed competent size exclusion fractions, with the bioactivity and with the aggressiveness of clinical disease. These results demonstrate that the PTMs present on bioactive, seed competent PBS extract tau species are closely related to, but distinct from, the PTMs of mature paired helical filaments, consistent with the idea that they are a forme fruste of tau species that ultimately form fibrils.

Alzheimer’s disease patient-derived high-molecular-weight tau impairs bursting in hippocampal neurons

Tau accumulation is closely related to cognitive symptoms in Alzheimer’s disease (AD). However, the cellular drivers of tau-dependent decline of memory-based cognition remain elusive. Here, we employed in vivo Neuropixels and patch-clamp recordings in mouse models and demonstrate that tau, independent of β-amyloid, selectively debilitates complex-spike burst firing of CA1 hippocampal neurons, a fundamental cellular mechanism underpinning learning and memory. Impaired bursting was associated with altered hippocampal network activities that are coupled to burst firing patterns (i.e., theta rhythms and high-frequency ripples) and was concurrent with reduced neuronal expression of CaV2.3 calcium channels, which are essential for burst firing in vivo. We subsequently identify soluble high molecular weight (HMW) tau, isolated from human AD brain, as the tau species responsible for suppression of burst firing. These data provide a cellular mechanism for tau-dependent cognitive decline in AD and implicate a rare species of intracellular HMW tau as a therapeutic target.

APOE2 gene therapy reduces amyloid deposition and improves markers of neuroinflammation and neurodegeneration in a mouse model of Alzheimer disease

Epidemiological studies show that individuals who carry the relatively uncommon APOE ε2 allele rarely develop Alzheimer disease, and if they do, they have a later age of onset, milder clinical course, and less severe neuropathological findings than people without this allele. The contrast is especially stark when compared with the major genetic risk factor for Alzheimer disease, APOE ε4, which has an age of onset several decades earlier, a more aggressive clinical course and more severe neuropathological findings, especially in terms of the amount of amyloid deposition. Here, we demonstrate that brain exposure to APOE ε2 via a gene therapy approach, which bathes the entire cortical mantle in the gene product after transduction of the ependyma, reduces Ab plaque deposition, neurodegenerative synaptic loss, and, remarkably, reduces microglial activation in an APP/PS1 mouse model despite continued expression of human APOE ε4. This result suggests a promising protective effect of exogenous APOE ε2 and reveals a cell nonautonomous effect of the protein on microglial activation, which we show is similar to plaque-associated microglia in the brain of Alzheimer disease patients who inherit APOE ε2. These data increase the potential that an APOE ε2 therapeutic could be effective in Alzheimer disease, even in individuals born with the risky ε4 allele.

APOE4 derived from astrocytes leads to blood-brain barrier impairment

Apolipoprotein E (ApoE) is a multifaceted secreted molecule synthesized in the CNS by astrocytes and microglia, and in the periphery largely by the liver. ApoE has been shown to impact the integrity of the blood-brain barrier, and, in humans, the APOE4 allele of the gene is reported to lead to a leaky blood-brain barrier. We used allele specific knock-in mice expressing each of the common (human) ApoE alleles, and longitudinal multiphoton intravital microscopy, to directly monitor the impact of various ApoE isoforms on blood-brain barrier integrity. We found that humanized APOE4, but not APOE2 or APOE3, mice show a leaky blood-brain barrier, increased MMP9, impaired tight junctions, and reduced astrocyte end-foot coverage of blood vessels. Removal of astrocyte-produced ApoE4 led to the amelioration of all phenotypes while the removal of astrocyte-produced ApoE3 had no effect on blood-brain barrier integrity. This work shows a cell specific gain of function effect of ApoE4 in the dysfunction of the BBB and implicates astrocyte production of ApoE4, possibly as a function of astrocytic end foot interactions with vessels, as a key regulator of the integrity of the blood-brain barrier.

Astrocyte transcriptomic changes along the spatiotemporal progression of Alzheimer's disease

Astrocytes are crucial to brain homeostasis, yet their changes along the spatiotemporal progression of Alzheimer’s disease (AD) neuropathology remain unexplored. Here we performed single-nucleus RNA sequencing of 628,943 astrocytes from five brain regions representing the stereotypical progression of AD pathology across 32 donors spanning the entire normal aging to severe AD continuum. We mapped out several unique astrocyte subclusters that exhibited varying responses to neuropathology across the AD-vulnerable neural network (spatial axis) or AD pathology stage (temporal axis). The proportion of homeostatic, intermediate and reactive astrocytes changed only along the spatial axis, whereas two other subclusters changed along the temporal axis. One of these, a trophic factor-rich subcluster, declined along pathology stages, whereas the other increased in the late stage but returned to baseline levels in the end stage, suggesting an exhausted response with chronic exposure to neuropathology. Our study underscores the complex dynamics of astrocytic responses in AD.

Cell-death pathways and tau-associated neuronal vulnerability in Alzheimer's disease

Neuronal loss is the ultimate driver of neural system dysfunction in Alzheimer’s disease (AD). We used single-nucleus RNA sequencing and neuropathological phenotyping to elucidate mechanisms of neurodegeneration in AD by identifying vulnerable neuronal populations and probing for their differentially expressed genes. Evidenced by transcriptomic analyses and quantitative tau immunoassays of human AD and non-AD brain tissue, we identified a neuronal population especially vulnerable to tau pathology. Multiplexed immunohistochemistry and in situ hybridization (CBLN2 and LINC00507) validated the presence of the tau-vulnerable neuronal population and revealed a propensity of this population to bear tau pathology. Differentially expressed genes associated with phospho-tau pathology in these neurons revealed genes involved in apoptosis, cell-component dissociation (e.g., autophagosome maturation and actin filament depolymerization), and regulation of vesicle-mediated transport.

Common mouse models of tauopathy reflect early but not late human disease

Mouse models that overexpress human mutant Tau (P301S and P301L) are commonly used in preclinical studies of Alzheimer’s Disease (AD) and while several drugs showed therapeutic efects in these mice, they were inefective in humans. This leads to the question to which extent the murine models refect human Tau pathology on the molecular level. AD mouse models that overexpress human Tau using risk mutations are a suitable tool for testing drug candidates that aim to intervene in the early formation of insoluble Tau species promoted by increased phosphorylation of Tau.

Cryptic splicing of stathmin-2 and UNC13A mRNAs is a pathological hallmark of TDP-43-associated Alzheimer's disease

Nuclear clearance and cytoplasmic accumulations of the RNA-binding protein TDP-43 are pathological hallmarks in almost all patients with amyotrophic lateral sclerosis (ALS) and up to 50% of patients with frontotemporal dementia (FTD) and Alzheimer’s disease. In Alzheimer’s disease, TDP-43 pathology is predominantly observed in the limbic system and correlates with cognitive decline and reduced hippocampal volume. Disruption of nuclear TDP-43 function leads to abnormal RNA splicing and incorporation of erroneous cryptic exons in numerous transcripts including Stathmin-2 (STMN2, also known as SCG10) and UNC13A, recently reported in tissues from patients with ALS and FTD. Here, we identify both STMN2 and UNC13A cryptic exons in Alzheimer’s disease patients, that correlate with TDP-43 pathology burden, but not with amyloid-β or tau deposits. We also demonstrate that processing of the STMN2 pre-mRNA is more sensitive to TDP-43 loss of function than UNC13A. In addition, full-length RNAs encoding STMN2 and UNC13A are suppressed in large RNA-seq datasets generated from Alzheimer’s disease post-mortem brain tissue. Collectively, these results open exciting new avenues to use STMN2 and UNC13A as potential therapeutic targets in a broad range of neurodegenerative conditions with TDP-43 proteinopathy including Alzheimer’s disease.