Highly infectious prions are not directly neurotoxic.

Prions are infectious agents which cause rapidly lethal neurodegenerative diseases in humans and animals following long, clinically silent incubation periods. They are composed of multichain assemblies of misfolded cellular prion protein. While it has long been assumed that prions are themselves neurotoxic, recent development of methods to obtain exceptionally pure prions from mouse brain with maintained strain characteristics, and in which defined structures-paired rod-like double helical fibers-can be definitively correlated with infectivity, allowed a direct test of this assertion. Here we report that while brain homogenates from symptomatic prion-infected mice are highly toxic to cultured neurons, exceptionally pure intact high-titer infectious prions are not directly neurotoxic. We further show that treatment of brain homogenates from prion-infected mice with sodium lauroylsarcosine destroys toxicity without diminishing infectivity. This is consistent with models in which prion propagation and toxicity can be mechanistically uncoupled.

Distinct responses of neurons and astrocytes to TDP-43 proteinopathy in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a fatal and incurable neurodegenerative disease caused by motor neuron loss, resulting in muscle wasting, paralysis and eventual death. A key pathological feature of ALS is cytoplasmically mislocalized and aggregated TDP-43 protein in >95% of cases, which is considered to have prion-like properties. Historical studies have predominantly focused on genetic forms of ALS, which represent ∼10% of cases, leaving the remaining 90% of sporadic ALS relatively understudied. Additionally, the role of astrocytes in ALS and their relationship with TDP-43 pathology is also not currently well understood. We have therefore used highly enriched human induced pluripotent stem cell (iPSC)-derived motor neurons and astrocytes to model early cell type-specific features of sporadic ALS. We first demonstrate seeded aggregation of TDP-43 by exposing human iPSC-derived motor neurons to serially passaged sporadic ALS post-mortem tissue (spALS) extracts. Next, we show that human iPSC-derived motor neurons are more vulnerable to TDP-43 aggregation and toxicity compared with their astrocyte counterparts. We demonstrate that these TDP-43 aggregates can more readily propagate from motor neurons into astrocytes in co-culture paradigms. We next found that astrocytes are neuroprotective to seeded aggregation within motor neurons by reducing (mislocalized) cytoplasmic TDP-43, TDP-43 aggregation and cell toxicity. Furthermore, we detected TDP-43 oligomers in these spALS spinal cord extracts, and as such demonstrated that highly purified recombinant TDP-43 oligomers can reproduce this observed cell-type specific toxicity, providing further support to a protein oligomer-mediated toxicity hypothesis in ALS. In summary, we have developed a human, clinically relevant, and cell-type specific modelling platform that recapitulates key aspects of sporadic ALS and uncovers both an initial neuroprotective role for astrocytes and the cell type-specific toxic effect of TDP-43 oligomers.

Identification of a Compound That Disrupts Binding of Amyloid-ß to the Prion Protein Using a Novel Fluorescence-based Assay.

The prion protein (PrP) has been implicated both in prion diseases such as Creutzfeldt-Jakob disease, where its monomeric cellular isoform (PrP(C)) is recruited into pathogenic self-propagating polymers of misfolded protein, and in Alzheimer disease, where PrP(C) may act as a receptor for synaptotoxic oligomeric forms of amyloid-ß (Aß). There has been considerable interest in identification of compounds that bind to PrP(C), stabilizing its native fold and thereby acting as pharmacological chaperones to block prion propagation and pathogenesis. However, compounds binding PrP(C) could also inhibit the binding of toxic Aß species and may have a role in treating Alzheimer disease, a highly prevalent dementia for which there are currently no disease-modifying treatments. However, the absence of a unitary, readily measurable, physiological function of PrP makes screening for ligands challenging, and the highly heterogeneous nature of Aß oligomer preparations makes conventional competition binding assays difficult to interpret. We have therefore developed a high-throughput screen that utilizes site-specifically fluorescently labeled protein to identify compounds that bind to PrP and inhibit both Aß binding and prion propagation. Following a screen of 1,200 approved drugs, we identified Chicago Sky Blue 6B as the first small molecule PrP ligand capable of inhibiting Aß binding, demonstrating the feasibility of development of drugs to block this interaction. The interaction of Chicago Sky Blue 6B was characterized by isothermal titration calorimetry, and its ability to inhibit Aß binding and reduce prion levels was established in cell-based assays.

N-terminal domain of prion protein directs its oligomeric association.

The self-association of prion protein (PrP) is a critical step in the pathology of prion diseases. It is increasingly recognized that small non-fibrillar ß-sheet-rich oligomers of PrP may be of crucial importance in the prion disease process. Here, we characterize the structure of a well defined ß-sheet-rich oligomer, containing ∼12 PrP molecules, and often enclosing a central cavity, formed using full-length recombinant PrP. The N-terminal region of prion protein (residues 23-90) is required for the formation of this distinct oligomer; a truncated form comprising residues 91-231 forms a broad distribution of aggregated species. No infectivity or toxicity was found using cell and animal model systems. This study demonstrates that examination of the full repertoire of conformers and assembly states that can be accessed by PrP under specific experimental conditions should ideally be done using the full-length protein.

Pharmacological chaperone for the structured domain of human prion protein.

In prion diseases, the misfolded protein aggregates are derived from cellular prion protein (PrP(C)). Numerous ligands have been reported to bind to human PrP(C) (huPrP), but none to the structured region with the affinity required for a pharmacological chaperone. Using equilibrium dialysis, we screened molecules previously suggested to interact with PrP to discriminate between those which did not interact with PrP, behaved as nonspecific polyionic aggregates or formed a genuine interaction. Those that bind could potentially act as pharmacological chaperones. Here we report that a cationic tetrapyrrole [Fe(III)-TMPyP], which displays potent antiprion activity, binds to the structured region of huPrP. Using a battery of biophysical techniques, we demonstrate that Fe(III)-TMPyP forms a 11 complex via the structured C terminus of huPrP with a K(d) of 4.5 ± 2 µM, which is in the range of its IC(50) for curing prion-infected cells of 1.6 ± 0.4 µM and the concentration required to inhibit protein-misfolding cyclic amplification. Therefore, this molecule tests the hypothesis that stabilization of huPrP(C), as a principle, could be used in the treatment of human prion disease. The identification of a binding site with a defined 3D structure opens up the possibility of designing small molecules that stabilize huPrP and prevent its conversion into the disease-associated form.

Loss of Residues 119-136, Including the First ß-strand of Human Prion Protein, Generates an Aggregation-competent Partially "Open" Form.

In prion replication, the cellular form of prion protein (PrPC) must undergo a full conformational transition to its disease-associated fibrillar form. Transmembrane forms of PrP have been implicated in this structural conversion. The cooperative unfolding of a structural core in PrPC presents a substantial energy barrier to prion formation, with membrane insertion and detachment of parts of PrP presenting a plausible route to its reduction. Here, we examined the removal of residues 119-136 of PrP, a region which includes the first ß-strand and a substantial portion of the conserved hydrophobic region of PrP, a region which associates with the ER membrane, on the structure, stability and self-association of the folded domain of PrPC. We see an "open" native-like conformer with increased solvent exposure which fibrilises more readily than the native state. These data suggest a stepwise folding transition, which is initiated by the conformational switch to this "open" form of PrPC.

Amyloid beta-protein dimers rapidly form stable synaptotoxic protofibrils.

Nonfibrillar, water-soluble low-molecular weight assemblies of the amyloid ß-protein (Aß) are believed to play an important role in Alzheimer's disease (AD). Aqueous extracts of human brain contain Aß assemblies that migrate on SDS-polyacrylamide gels and elute from size exclusion as dimers (∼8 kDa) and can block long-term potentiation and impair memory consolidation in the rat. Such species are detected specifically and sensitively in extracts of Alzheimer brain suggesting that SDS-stable dimers may be the basic building blocks of AD-associated synaptotoxic assemblies. Consequently, understanding the structure and properties of Aß dimers is of great interest. In the absence of sufficient brain-derived dimer to facilitate biophysical analysis, we generated synthetic dimers designed to mimic the natural species. For this, Aß(1-40) containing cysteine in place of serine 26 was used to produce disulphide cross-linked dimer, (AßS26C)2. Such dimers had no detectable secondary structure, produced an analytical ultracentrifugation profile consistent for an ∼8.6 kDa protein, and had no effect on hippocampal long-term potentiation (LTP). However, (AßS26C)2 aggregated more rapidly than either AßS26C or wild-type monomers and formed parastable ß-sheet rich, thioflavin T-positive, protofibril-like assemblies. Whereas wild-type Aß aggregated to form typical amyloid fibrils, the protofibril-like structures formed by (AßS26C)2 persisted for prolonged periods and potently inhibited LTP in mouse hippocampus. These data support the idea that Aß dimers may stabilize the formation of fibril intermediates by a process distinct from that available to Aß monomer and that higher molecular weight prefibrillar assemblies are the proximate mediators of Aß toxicity.

Amyloid-ß nanotubes are associated with prion protein-dependent synaptotoxicity.

Growing evidence suggests water-soluble, non-fibrillar forms of amyloid-ß protein (Aß) have important roles in Alzheimer's disease with toxicities mimicked by synthetic Aß(1-42). However, no defined toxic structures acting via specific receptors have been identified and roles of proposed receptors, such as prion protein (PrP), remain controversial. Here we quantify binding to PrP of Aß(1-42) after different durations of aggregation. We show PrP-binding and PrP-dependent inhibition of long-term potentiation (LTP) correlate with the presence of protofibrils. Globular oligomers bind less avidly to PrP and do not inhibit LTP, whereas fibrils inhibit LTP in a PrP-independent manner. That only certain transient Aß assemblies cause PrP-dependent toxicity explains conflicting reports regarding the involvement of PrP in Aß-induced impairments. We show that these protofibrils contain a defined nanotubular structure with a previously unidentified triple helical conformation. Blocking the formation of Aß nanotubes or their interaction with PrP might have a role in treatment of Alzheimer's disease.

Interaction between prion protein and toxic amyloid ß assemblies can be therapeutically targeted at multiple sites.

A role for PrP in the toxic effect of oligomeric forms of Aß, implicated in Alzheimer's disease (AD), has been suggested but remains controversial. Here we show that PrP is required for the plasticity-impairing effects of ex vivo material from human AD brain and that standardized Aß-derived diffusible ligand (ADDL) preparations disrupt hippocampal synaptic plasticity in a PrP-dependent manner. We screened a panel of anti-PrP antibodies for their ability to disrupt the ADDL-PrP interaction. Antibodies directed to the principal PrP/Aß-binding site and to PrP helix-1, were able to block Aß binding to PrP suggesting that the toxic Aß species are of relatively high molecular mass and/or may bind multiple PrP molecules. Two representative and extensively characterized monoclonal antibodies directed to these regions, ICSM-35 and ICSM-18, were shown to block the Aß-mediated disruption of synaptic plasticity validating these antibodies as candidate therapeutics for AD either individually or in combination.