Tau seeding without tauopathy.

Neurodegenerative tauopathies such as Alzheimer's disease (AD) are caused by brain accumulation of tau assemblies. Evidence suggests tau functions as a prion, and cells and animals can efficiently propagate unique, transmissible tau pathologies. This suggests a dedicated cellular replication machinery, potentially reflecting a normal physiologic function for tau seeds. Consequently, we hypothesized that healthy control brains would contain seeding activity. We have recently developed a novel monoclonal antibody (MD3.1) specific for tau seeds. We used this antibody to immunopurify tau from the parietal and cerebellar cortices of 19 healthy subjects without any neuropathology, ranging 19 to 65 years. We detected seeding in lysates from the parietal cortex, but not in the cerebellum. We also detected no seeding in brain homogenates from wildtype or human tau knockin mice, suggesting that cellular/genetic context dictates development of seed-competent tau. Seeding did not correlate with subject age or brain tau levels. We confirmed our essential findings using an orthogonal assay, real-time quaking-induced conversion, which amplifies tau seeds in vitro. Dot blot analyses revealed no AT8 immunoreactivity above background levels in parietal and cerebellar extracts and ∼1/100 of that present in AD. Based on binding to a panel of antibodies, the conformational characteristics of control seeds differed from AD, suggesting a unique underlying assembly, or structural ensemble. Tau's ability to adopt self-replicating conformations under nonpathogenic conditions may reflect a normal function that goes awry in disease states.

Anti-tau antibodies targeting a conformation-dependent epitope selectively bind seeds.

Neurodegenerative tauopathies are caused by the transition of tau protein from a monomer to a toxic aggregate. They include Alzheimer disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick disease (PiD). We have previously proposed that tau monomer exists in two conformational ensembles: an inert form (Mi), which does not self-assemble, and seed-competent form (Ms), which self-assembles and templates ordered assembly growth. We proposed that cis/trans isomerization of tau at P301, the site of dominant disease-associated S/L missense mutations, might underlie the transition of wild-type tau to a seed-competent state. Consequently, we created monoclonal antibodies using non-natural antigens consisting of fluorinated proline (P∗) at the analogous P270 in repeat 1 (R1), biased toward the trans-configuration at either the R1/R2 (TENLKHQP∗GGGKVQIINKK) or the R1/R3 (TENLKHQP∗GGGKVQIVYK) interfaces. Two antibodies, MD2.2 and MD3.1, efficiently immunoprecipitated soluble seeds from AD and PSP but not CBD or PiD brain samples. The antibodies efficiently stained brain samples of AD, PSP, and PiD, but not CBD. They did not immunoprecipitate or immunostain tau from the control brain. Creation of potent anti-seed antibodies based on the trans-proline epitope implicates local unfolding around P301 in pathogenesis. MD2.2 and MD3.1 may also be useful for therapy and diagnosis.

Ultrasensitive tau biosensor cells detect no seeding in Alzheimer's disease CSF.

Tau protein forms self-replicating assemblies (seeds) that may underlie progression of pathology in Alzheimer's disease (AD) and related tauopathies. Seeding in recombinant protein preparations and brain homogenates has been quantified with "biosensor" cell lines that express tau with a disease-associated mutation (P301S) fused to complementary fluorescent proteins. Quantification of induced aggregation in cells that score positive by fluorescence resonance energy transfer (FRET) is accomplished by cell imaging or flow cytometry. Several groups have reported seeding activity in antemortem cerebrospinal fluid (CSF) using various methods, but these findings are not yet widely replicated. To address this question, we created two improved FRET-based biosensor cell lines based on tau expression, termed version 2 low (v2L) and version 2 high (v2H). We determined that v2H cells are ~ 100-fold more sensitive to AD-derived tau seeds than our original lines, and coupled with immunoprecipitation reliably detect seeding from samples containing as little as 100 attomoles of recombinant tau fibrils or ~ 32 pg of total protein from AD brain homogenate. We tested antemortem CSF from 11 subjects with a clinical diagnosis of AD, 9 confirmed by validated CSF biomarkers. We used immunoprecipitation coupled with seed detection in v2H cells and detected no tau seeding in any sample. Thus we cannot confirm prior reports of tau seeding activity in the CSF of AD patients. This next generation of ultra-sensitive tau biosensors may nonetheless be useful to the research community to quantify tau pathology as sensitively and specifically as possible.

BACE1-/- mice exhibit seizure activity that does not correlate with sodium channel level or axonal localization.

BACKGROUND: BACE1 is a key enzyme in the generation of the Abeta peptide that plays a central role in the pathogenesis of Alzheimer's disease. While BACE1 is an attractive therapeutic target, its normal physiological function remains largely unknown. Examination of BACE1-/- mice can provide insight into this function and also help anticipate consequences of BACE1 inhibition. Here we report a seizure-susceptibility phenotype that we have identified and characterized in BACE1-/- mice. RESULTS: We find that electroencephalographic recordings reveal epileptiform abnormalities in some BACE1-/- mice, occasionally including generalized tonic-clonic and absence seizures. In addition, we find that kainic acid injection induces seizures of greater severity in BACE1-/- mice relative to BACE1+/+ littermates, and causes excitotoxic cell death in a subset of BACE1-/- mice. This hyperexcitability phenotype is variable and appears to be manifest in approximately 30% of BACE1-/- mice. Finally, examination of the expression and localization of the voltage-gated sodium channel alpha-subunit Nav1.2 reveals no correlation with BACE1 genotype or any measure of seizure susceptibility. CONCLUSIONS: Our data indicate that BACE1 deficiency predisposes mice to spontaneous and pharmacologically-induced seizure activity. This finding has implications for the development of safe therapeutic strategies for reducing Abeta levels in Alzheimer's disease. Further, we demonstrate that altered sodium channel expression and axonal localization are insufficient to account for the observed effect, warranting investigation of alternative mechanisms.

SERCA pump activity is physiologically regulated by presenilin and regulates amyloid beta production.

In addition to disrupting the regulated intramembraneous proteolysis of key substrates, mutations in the presenilins also alter calcium homeostasis, but the mechanism linking presenilins and calcium regulation is unresolved. At rest, cytosolic Ca(2+) is maintained at low levels by pumping Ca(2+) into stores in the endoplasmic reticulum (ER) via the sarco ER Ca(2+)-ATPase (SERCA) pumps. We show that SERCA activity is diminished in fibroblasts lacking both PS1 and PS2 genes, despite elevated SERCA2b steady-state levels, and we show that presenilins and SERCA physically interact. Enhancing presenilin levels in Xenopus laevis oocytes accelerates clearance of cytosolic Ca(2+), whereas higher levels of SERCA2b phenocopy PS1 overexpression, accelerating Ca(2+) clearance and exaggerating inositol 1,4,5-trisphosphate-mediated Ca(2+) liberation. The critical role that SERCA2b plays in the pathogenesis of Alzheimer's disease is underscored by our findings that modulating SERCA activity alters amyloid beta production. Our results point to a physiological role for the presenilins in Ca(2+) signaling via regulation of the SERCA pump.

Presenilin regulates capacitative calcium entry dependently and independently of gamma-secretase activity.

Mutations in presenilin-1 and 2 (PS) lead to increased intracellular calcium stores and an attenuation in the refilling mechanism known as capacitative calcium entry (CCE). Previous studies have shown that the mechanism by which PS modulates intracellular calcium signaling is dependent on gamma-secretase activity. Although the modulation of intracellular calcium signaling can lead to alterations in CCE, it is plausible that PS can also directly affect CCE independent of the effect it exerts on intracellular stores. To investigate this possibility, we studied the effects of the dominant negative variant of PS1 known as DeltaTM1-2, which lacks the first two transmembrane domains of PS1 and in which gamma-secretase activity is abrogated. We demonstrate that, like other dominant negative isoforms of PS1, DeltaTM1-2 expression leads to reduced intracellular calcium. However, unlike other dominant negative isoforms, DeltaTM1-2 leads to a deficit rather than a potentiation of CCE. These data suggest that changes in the structural components of presenilin can modulate CCE independent of its function in gamma-secretase activity and intracellular calcium stores.