Highly Specific and Sensitive Target Binding by the Humanized pS396-Tau Antibody hC10.2 Across a Wide Spectrum of Alzheimer's Disease and Primary Tauopathy Postmortem Brains.

BACKGROUND: Deposits of hyperphosphorylated tau fibrils are hallmarks of a broad spectrum of tauopathies, including Alzheimer's disease (AD). OBJECTIVE: To investigate heterogeneity of tau pathology across brain extracts from a broad selection of different tauopathies and examine the binding properties of the humanized pS396-tau antibody hC10.2 and six other anti-tau antibodies. METHODS: 76 individual tauopathy tissue samples were analyzed in a battery of assays: immunohistochemistry, ELISA, tau aggregation assay, western blot, [3H]PI-2620 and [3H]MK-6240 tau tracer binding, and aggregated seeding activity in RD_P301S HEK293T Biosensor cells. The efficiency of seven anti-tau antibodies to engage with pathological tau species was directly compared. RESULTS: Our data indicate that a strong correlation existed between the tau tracer binding, amount of tau aggregates, pS396-tau phosphorylation, and seeding activity. The hC10.2 antibody, which has entered clinical development, effectively engaged with its epitope across all individual cases of mid-stage and late AD, and primary tauopathies. hC10.2 was superior compared to other phospho- and total tau antibodies to prevent seeded tau aggregation in the biosensor cells. hC10.2 effectively depleted hyperphosphorylated and aggregated tau species across all tauopathy samples proportionally to the amount of tau aggregates. In AD samples, hC10.2 bound to ghost tangles which represent extracellular pathological tau species. CONCLUSION: S396 hyperphosphorylation is a feature of the formation of seeding-competent tau across different tauopathies and it is present both in intra- and extracellular pathological tau. hC10.2 represents an excellent candidate for a hyperphosphorylation-selective therapeutic tau antibody for the treatment of AD and primary tauopathies.

Antibody-mediated clearance of tau in primary mouse microglial cultures requires Fcγ-receptor binding and functional lysosomes.

Neurodegenerative diseases such as Alzheimer's disease are characterized by the progressive spreading and accumulation of hyper-phosphorylated tau protein in the brain. Anti-tau antibodies have been shown to reduce tau pathology in in vivo models and antibody-mediated clearance of tau exerted by microglia has been proposed as a contributing factor. By subjecting primary microglia cultured in vitro to anti-phospho-tau antibodies in complex with pathological tau, we show that microglia internalise and degrade tau in a manner that is dependent on FcγR interaction and functional lysosomes. It has recently been discussed if anti-tau antibody effector-functions are required for induction of tau clearance. Using antibodies with compromised FcγR binding and non-compromised control antibodies we show that antibody effector functions are required for induction of microglial clearance of tau. Understanding the inflammatory consequences of targeting microglia using therapeutic antibodies is important when developing these molecules for clinical use. Using RNA sequencing, we show that treatment with anti-tau antibodies increases transcription of mRNA encoding pro-inflammatory markers, but that the mRNA expression profile of antibody-treated cells differ from the profile of LPS activated microglia. We further demonstrate that microglia activation alone is not sufficient to induce significant tau clearance.

Highly specific and selective anti-pS396-tau antibody C10.2 targets seeding-competent tau.

INTRODUCTION: The abnormal hyperphosphorylation of the microtubule-associated protein tau plays a crucial role in neurodegeneration in Alzheimer's disease (AD) and other tauopathies. METHODS: Highly specific and selective anti-pS396-tau antibodies have been generated using peptide immunization with screening against pathologic hyperphosphorylated tau from rTg4510 mouse and AD brains and selection in in vitro and in vivo tau seeding assays. RESULTS: The antibody C10.2 bound specifically to pS396-tau with an IC50 of 104 pM and detected preferentially hyperphosphorylated tau aggregates from AD brain with an IC50 of 1.2 nM. C10.2 significantly reduced tau seeding of P301L human tau in HEK293 cells, murine cortical neurons, and mice. AD brain extracts depleted with C10.2 were not able to seed tau in vitro and in vivo, demonstrating that C10.2 specifically recognized pathologic seeding-competent tau. DISCUSSION: Targeting pS396-tau with an antibody like C10.2 may provide therapeutic benefit in AD and other tauopathies.

A neuroprotective role for microglia in prion diseases.

Microglial activation is a hallmark of most neurodegenerative disorders, and is particularly conspicuous in prion diseases. However, the role of microglia, which function as both primary immune effector cells and professional phagocytes in the central nervous system, remains contentious in the context of neurodegeneration. Here, we evaluated the effect of microglial depletion/deficiency on prion pathogenesis. We found that ganciclovir-mediated microglial ablation on tga20/CD11b-thymidine kinase of Herpes simplex virus (HSVTK) cerebellar organotypic cultured slices markedly aggravated prion-induced neurotoxicity. A similar deterioration of disease was recapitulated in in vivo microglial depletion in prion-infected tga20/CD11b-HSVTK mice. Additionally, deficiency of microglia in interleukin 34 knockout (IL34(-/-)) mice again resulted in significantly augmented proteinase K-resistant prion protein deposition and accelerated prion disease progression. These results provide unambiguous evidence for a general protective role of microglia in prion pathogenesis.

Correction: Prion infections and anti-PrP antibodies trigger converging neurotoxic pathways.

[This corrects the article DOI: 10.1371/journal.ppat.1004662.].

Prion infections and anti-PrP antibodies trigger converging neurotoxic pathways.

Prions induce lethal neurodegeneration and consist of PrPSc, an aggregated conformer of the cellular prion protein PrPC. Antibody-derived ligands to the globular domain of PrPC (collectively termed GDL) are also neurotoxic. Here we show that GDL and prion infections activate the same pathways. Firstly, both GDL and prion infection of cerebellar organotypic cultured slices (COCS) induced the production of reactive oxygen species (ROS). Accordingly, ROS scavenging, which counteracts GDL toxicity in vitro and in vivo, prolonged the lifespan of prion-infected mice and protected prion-infected COCS from neurodegeneration. Instead, neither glutamate receptor antagonists nor inhibitors of endoplasmic reticulum calcium channels abolished neurotoxicity in either model. Secondly, antibodies against the flexible tail (FT) of PrPC reduced neurotoxicity in both GDL-exposed and prion-infected COCS, suggesting that the FT executes toxicity in both paradigms. Thirdly, the PERK pathway of the unfolded protein response was activated in both models. Finally, 80% of transcriptionally downregulated genes overlapped between prion-infected and GDL-treated COCS. We conclude that GDL mimic the interaction of PrPSc with PrPC, thereby triggering the downstream events characteristic of prion infection.

The role of the NADPH oxidase NOX2 in prion pathogenesis.

Prion infections cause neurodegeneration, which often goes along with oxidative stress. However, the cellular source of reactive oxygen species (ROS) and their pathogenetic significance are unclear. Here we analyzed the contribution of NOX2, a prominent NADPH oxidase, to prion diseases. We found that NOX2 is markedly upregulated in microglia within affected brain regions of patients with Creutzfeldt-Jakob disease (CJD). Similarly, NOX2 expression was upregulated in prion-inoculated mouse brains and in murine cerebellar organotypic cultured slices (COCS). We then removed microglia from COCS using a ganciclovir-dependent lineage ablation strategy. NOX2 became undetectable in ganciclovir-treated COCS, confirming its microglial origin. Upon challenge with prions, NOX2-deficient mice showed delayed onset of motor deficits and a modest, but significant prolongation of survival. Dihydroethidium assays demonstrated a conspicuous ROS burst at the terminal stage of disease in wild-type mice, but not in NOX2-ablated mice. Interestingly, the improved motor performance in NOX2 deficient mice was already measurable at earlier stages of the disease, between 13 and 16 weeks post-inoculation. We conclude that NOX2 is a major source of ROS in prion diseases and can affect prion pathogenesis.

The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein.

Prion infections cause lethal neurodegeneration. This process requires the cellular prion protein (PrP(C); ref. 1), which contains a globular domain hinged to a long amino-proximal flexible tail. Here we describe rapid neurotoxicity in mice and cerebellar organotypic cultured slices exposed to ligands targeting the α1 and α3 helices of the PrP(C) globular domain. Ligands included seven distinct monoclonal antibodies, monovalent Fab1 fragments and recombinant single-chain variable fragment miniantibodies. Similar to prion infections, the toxicity of globular domain ligands required neuronal PrP(C), was exacerbated by PrP(C) overexpression, was associated with calpain activation and was antagonized by calpain inhibitors. Neurodegeneration was accompanied by a burst of reactive oxygen species, and was suppressed by antioxidants. Furthermore, genetic ablation of the superoxide-producing enzyme NOX2 (also known as CYBB) protected mice from globular domain ligand toxicity. We also found that neurotoxicity was prevented by deletions of the octapeptide repeats within the flexible tail. These deletions did not appreciably compromise globular domain antibody binding, suggesting that the flexible tail is required to transmit toxic signals that originate from the globular domain and trigger oxidative stress and calpain activation. Supporting this view, various octapeptide ligands were not only innocuous to both cerebellar organotypic cultured slices and mice, but also prevented the toxicity of globular domain ligands while not interfering with their binding. We conclude that PrP(C) consists of two functionally distinct modules, with the globular domain and the flexible tail exerting regulatory and executive functions, respectively. Octapeptide ligands also prolonged the life of mice expressing the toxic PrP(C) mutant, PrP(Δ94-134), indicating that the flexible tail mediates toxicity in two distinct PrP(C)-related conditions. Flexible tail-mediated toxicity may conceivably play a role in further prion pathologies, such as familial Creutzfeldt-Jakob disease in humans bearing supernumerary octapeptides.

Prion pathogenesis is faithfully reproduced in cerebellar organotypic slice cultures.

Prions cause neurodegeneration in vivo, yet prion-infected cultured cells do not show cytotoxicity. This has hampered mechanistic studies of prion-induced neurodegeneration. Here we report that prion-infected cultured organotypic cerebellar slices (COCS) experienced progressive spongiform neurodegeneration closely reproducing prion disease, with three different prion strains giving rise to three distinct patterns of prion protein deposition. Neurodegeneration did not occur when PrP was genetically removed from neurons, and a comprehensive pharmacological screen indicated that neurodegeneration was abrogated by compounds known to antagonize prion replication. Prion infection of COCS and mice led to enhanced fodrin cleavage, suggesting the involvement of calpains or caspases in pathogenesis. Accordingly, neurotoxicity and fodrin cleavage were prevented by calpain inhibitors but not by caspase inhibitors, whereas prion replication proceeded unimpeded. Hence calpain inhibition can uncouple prion replication from its neurotoxic sequelae. These data validate COCS as a powerful model system that faithfully reproduces most morphological hallmarks of prion infections. The exquisite accessibility of COCS to pharmacological manipulations was instrumental in recognizing the role of calpains in neurotoxicity, and significantly extends the collection of tools necessary for rigorously dissecting prion pathogenesis.

Prion propagation, toxicity and degradation.

Prion science has been on a rollercoaster for two decades. In the mid 1990s, the specter of mad cow disease (bovine spongiform encephalopathy, BSE) provoked an unprecedented public scare that was first precipitated by the realization that this animal prion disease could be transmitted to humans and then rekindled by the evidence that BSE-infected humans could pass on the infection through blood transfusions. Along with the gradual disappearance of BSE, the interest in prions has waned with the general public, funding agencies and prospective PhD students. In the past few years, however, a bewildering variety of diseases have been found to share features with prion infections, including cell-to-cell transmission. Here we review these developments and summarize those open questions that we currently deem most interesting in prion biology: how do prions damage their hosts, and how do hosts attempt to neutralize invading prions?

Polythiophenes inhibit prion propagation by stabilizing prion protein (PrP) aggregates.

Luminescent conjugated polymers (LCPs) interact with ordered protein aggregates and sensitively detect amyloids of many different proteins, suggesting that they may possess antiprion properties. Here, we show that a variety of anionic, cationic, and zwitterionic LCPs reduced the infectivity of prion-containing brain homogenates and of prion-infected cerebellar organotypic cultured slices and decreased the amount of scrapie isoform of PrP(C) (PrP(Sc)) oligomers that could be captured in an avidity assay. Paradoxically, treatment enhanced the resistance of PrP(Sc) to proteolysis, triggered the compaction, and enhanced the resistance to proteolysis of recombinant mouse PrP(23-231) fibers. These results suggest that LCPs act as antiprion agents by transitioning PrP aggregates into structures with reduced frangibility. Moreover, ELISA on cerebellar organotypic cultured slices and in vitro conversion assays with mouse PrP(23-231) indicated that poly(thiophene-3-acetic acid) may additionally interfere with the generation of PrP(Sc) by stabilizing the conformation of PrP(C) or of a transition intermediate. Therefore, LCPs represent a novel class of antiprion agents whose mode of action appears to rely on hyperstabilization, rather than destabilization, of PrP(Sc) deposits.

Engulfment of cerebral apoptotic bodies controls the course of prion disease in a mouse strain-dependent manner.

Progressive accumulation of PrP(Sc), a hallmark of prion diseases, occurs when conversion of PrP(C) into PrP(Sc) is faster than PrP(Sc) clearance. Engulfment of apoptotic bodies by phagocytes is mediated by Mfge8 (milk fat globule epidermal growth factor 8). In this study, we show that brain Mfge8 is primarily produced by astrocytes. Mfge8 ablation induced accelerated prion disease and reduced clearance of cerebellar apoptotic bodies in vivo, as well as excessive PrP(Sc) accumulation and increased prion titers in prion-infected C57BL/6 × 129Sv mice and organotypic cerebellar slices derived therefrom. These phenotypes correlated with the presence of 129Sv genomic markers in hybrid mice and were not observed in inbred C57BL/6 Mfge8(-/-) mice, suggesting the existence of additional strain-specific genetic modifiers. Because Mfge8 receptors are expressed by microglia and depletion of microglia increases PrP(Sc) accumulation in organotypic cerebellar slices, we conclude that engulfment of apoptotic bodies by microglia may be an important pathway of prion clearance controlled by astrocyte-borne Mfge8.

Prion protein and Abeta-related synaptic toxicity impairment.

Alzheimer's disease (AD), the most common neurodegenerative disorder, goes along with extracellular amyloid-beta (Abeta) deposits. The cognitive decline observed during AD progression correlates with damaged spines, dendrites and synapses in hippocampus and cortex. Numerous studies have shown that Abeta oligomers, both synthetic and derived from cultures and AD brains, potently impair synaptic structure and functions. The cellular prion protein (PrP(C)) was proposed to mediate this effect. We report that ablation or overexpression of PrP(C) had no effect on the impairment of hippocampal synaptic plasticity in a transgenic model of AD. These findings challenge the role of PrP(C) as a mediator of Abeta toxicity.

De novo generation of a transmissible spongiform encephalopathy by mouse transgenesis.

Most transmissible spongiform encephalopathies arise either spontaneously or by infection. Mutations of PRNP, which encodes the prion protein, PrP, segregate with phenotypically similar diseases. Here we report that moderate overexpression in transgenic mice of mPrP(170N,174T), a mouse PrP with two point mutations that subtly affect the structure of its globular domain, causes a fully penetrant lethal spongiform encephalopathy with cerebral PrP plaques. This genetic disease was reproduced with 100% attack rate by intracerebral inoculation of brain homogenate to tga20 mice overexpressing WT PrP, and from the latter to WT mice, but not to PrP-deficient mice. Upon successive transmissions, the incubation periods decreased and PrP became more protease-resistant, indicating the presence of a strain barrier that was gradually overcome by repeated passaging. This shows that expression of a subtly altered prion protein, with known 3D structure, efficiently generates a prion disease.

The prion organotypic slice culture assay--POSCA.

Methods enabling prion replication ex vivo are important for advancing prion science. However, few such technologies exist and many prion strains are intractable with them. Here, we describe a prion organotypic slice culture assay (POSCA), which allows for prion amplification and titration ex vivo under conditions that closely resemble intracerebral infection. Organotypic slices are incubated with infectious inoculum as free-floating sections, washed and cultured for up to 8 weeks. Slice cultures are a rich source of protein or RNA and allow for stringent comparisons between uninfected and prion-infected samples generated from the same mouse. Thirty-five days after contact with prions, cerebellar slices have amplified PrP(Sc) quantitatively similar to that seen in vivo, but accelerated fivefold. The POSCA detects replication of specific prion strains from disparate sources, including bovines and ovines, with variable efficiency. The culture procedure and prion infection can be performed in 8 h.

Molecular basis for detection of invading pathogens in the brain.

Classical immunology textbooks have described the central nervous system as an immune-privileged site, i.e., as devoid of inflammatory and host-vs.-graft immunoreactions. This view has been refined, since we now know that hematopoietic cells infiltrate the CNS under certain circumstances and that CNS-resident cells are capable of launching an innate immune response. Microglia cells express an extensive repertoire of pattern-recognition receptors and act as sentinels surveilling the CNS for possible damage or infection. Astrocytes are the most abundant cell type in the brain, and they are capable of launching a strong supportive innate immune response. Novel findings show that both astrocytes and, surprisingly, even neurons express pattern-recognition receptors. Activation of these receptors leads to a functional response, indicating that cells other than microglia are capable of initiating a primary innate immune response against CNS-invading pathogens. Here, we put these findings into context with what has been learned from recent in vitro and in vivo experiments about the initiation of an innate immune response in the brain.

A versatile prion replication assay in organotypic brain slices.

Methods enabling prion replication ex vivo are important for advancing prion studies. However, few such technologies exist, and many prion strains are not amenable to them. Here we describe a prion organotypic slice culture assay (POSCA) that allows prion amplification and titration ex vivo under conditions that closely resemble intracerebral infection. Thirty-five days after contact with prions, mouse cerebellar slices had amplified the abnormal isoform of prion protein, PrP(Sc), >10(5)-fold. This is quantitatively similar to amplification in vivo, but fivefold faster. PrP(Sc) accumulated predominantly in the molecular layer, as in infected mice. The POSCA detected replication of prion strains from disparate sources, including bovines and ovines, with variable detection efficiency. Pharmacogenetic ablation of microglia from POSCA slices led to a 15-fold increase in prion titers and PrP(Sc) concentrations over those in microglia-containing slices, as well as an increase in susceptibility to infection. This suggests that the extensive microglial activation accompanying prion diseases represents an efficacious defensive reaction.

Chemical and biophysical insights into the propagation of prion strains.

Transmissible spongiform encephalopathies (TSEs) are lethal infectious neurodegenerative diseases. TSEs are caused by prions, infectious agents lacking informational nucleic acids, and possibly identical with higher-order aggregates of the cellular glycolipoprotein PrP(C). Prion strains are derived from TSE isolates that, even after inoculation into genetically identical hosts, cause disease with distinct patterns of protein aggregate deposition, incubation times, morphology of the characteristic brain damage, and cellular tropism. Most of these traits are relatively stable across serial passages. Here we review current techniques for studying prion strain differences in vivo and in cells, and discuss the strain phenomena in the general context of the knowledge gained from modeling prion fibril growth in vitro and in simple organisms.

Wide spectrum modulation by KP-544 in models relevant for neuronal survival.

Reduced neurotrophic signalling has been proposed as a part of the pathophysiology behind neuronal death and dysfunction. The small molecule KP-544 was developed with the intention to enhance nerve growth factor signalling. To characterize the actions of KP-544 pharmacologically, we used four diverse models with relevance for neuronal function and survival. We found that 300-1000 nM KP-544 enhanced the neurite outgrowth in PC12 cells in response to a suboptimal concentration of nerve growth factor. KP-544 also protected the cerebellar granule cells from excitotoxicity apoptosis induced by the mitochondrial toxin methyl-phenyl-pyridinium, and modulated inflammation by inhibiting interleukin-6 production in primary astrocytes. Chronic treatment of rats with KP-544 prevented the hyper-responsiveness to amphetamine of animals treated with methylazoxymethanol acetate, a recently described neurodevelopmental model of schizophrenia.

The dynamics of the LPS triggered inflammatory response of murine microglia under different culture and in vivo conditions.

Overall, the inflammatory potential of lipopolysaccharide (LPS) in vitro and in vivo was investigated using different omics technologies. We investigated the hippocampal response to intracerebroventricular (i.c.v) LPS in vivo, at both the transcriptional and protein level. Here, a time course analysis of interleukin-6 (IL-6) and monocyte chemotactic protein-1 (MCP-1) showed a sharp peak at 4 h and a return to baseline at 16 h. The expression of inflammatory mediators was not temporally correlated with expression of the microglia marker F4/80, which did not peak until 2 days after LPS injection. Of 480 inflammation-related genes present on a microarray, 29 transcripts were robustly up-regulated and 90% of them were also detected in LPS stimulated primary microglia (PM) cultures. Further in vitro to in vivo comparison showed that the counter regulation response observed in vivo was less evident in vitro, as transcript levels in PM decreased relatively little over 16 h. This apparent deficiency of homeostatic control of the innate immune response in cultures may also explain why a group of genes comprising tnf receptor associated factor-1, endothelin-1 and schlafen-1 were regulated strongly in vitro, but not in vivo. When the overall LPS-induced transcriptional response of PM was examined on a large Affymetrix chip, chemokines and cytokines constituted the most strongly regulated and largest groups. Interesting new microglia markers included interferon-induced protein with tetratricopeptide repeat (ifit), immune responsive gene-1 (irg-1) and thymidylate kinase family LPS-inducible member (tyki). The regulation of the former two was confirmed on the protein level in a proteomics study. Furthermore, conspicuous regulation of several gene clusters was identified, for instance that of genes pertaining to the extra-cellular matrix and enzymatic regulation thereof. Although most inflammatory genes induced in vitro were transferable to our in vivo model, the observed discrepancy for some genes potentially represents regulatory factors present in the central nervous system (CNS) but not in vitro.