The Anti-Prion Antibody 15B3 Detects Toxic Amyloid-β Oligomers

Re-Engineering Therapeutic Anti-Aβ Monoclonal Antibody to Target Amyloid Light Chain

Amyloidosis involves the deposition of misfolded proteins. Even though it is caused by different pathogenic mechanisms, in aggregate, it shares similar features. Here, we tested and confirmed a hypothesis that an amyloid antibody can be engineered by a few mutations to target a different species. Amyloid light chain (AL) and β-amyloid peptide (Aβ) are two therapeutic targets that are implicated in amyloid light chain amyloidosis and Alzheimer's disease, respectively. Though crenezumab, an anti-Aβ antibody, is currently unsuccessful, we chose it as a model to computationally design and prepare crenezumab variants, aiming to discover a novel antibody with high affinity to AL fibrils and to establish a technology platform for repurposing amyloid monoclonal antibodies. We successfully re-engineered crenezumab to bind both Aβ42 oligomers and AL fibrils with high binding affinities. It is capable of reversing Aβ42-oligomers-induced cytotoxicity, decreasing the formation of AL fibrils, and alleviating AL-fibrils-induced cytotoxicity in vitro. Our research demonstrated that an amyloid antibody could be engineered by a few mutations to bind new amyloid sequences, providing an efficient way to reposition a therapeutic antibody to target different amyloid diseases.

Are fibrinaloid microclots a cause of autoimmunity in Long Covid and other post-infection diseases?

It is now well established that the blood-clotting protein fibrinogen can polymerise into an anomalous form of fibrin that is amyloid in character; the resultant clots and microclots entrap many other molecules, stain with fluorogenic amyloid stains, are rather resistant to fibrinolysis, can block up microcapillaries, are implicated in a variety of diseases including Long COVID, and have been referred to as fibrinaloids. A necessary corollary of this anomalous polymerisation is the generation of novel epitopes in proteins that would normally be seen as 'self', and otherwise immunologically silent. The precise conformation of the resulting fibrinaloid clots (that, as with prions and classical amyloid proteins, can adopt multiple, stable conformations) must depend on the existing small molecules and metal ions that the fibrinogen may (and is some cases is known to) have bound before polymerisation. Any such novel epitopes, however, are likely to lead to the generation of autoantibodies. A convergent phenomenology, including distinct conformations and seeding of the anomalous form for initiation and propagation, is emerging to link knowledge in prions, prionoids, amyloids and now fibrinaloids. We here summarise the evidence for the above reasoning, which has substantial implications for our understanding of the genesis of autoimmunity (and the possible prevention thereof) based on the primary process of fibrinaloid formation.

Rational Design of a Peptidomimetic Inhibitor of Gelsolin Amyloid Aggregation

Gelsolin amyloidosis (AGel) is characterized by multiple systemic and ophthalmic features resulting from pathological tissue deposition of the gelsolin (GSN) protein. To date, no cure is available for the treatment of any form of AGel. More than ten single-point substitutions in the GSN gene are responsible for the occurrence of the disease and, among them, D187N/Y is the most widespread variant. These substitutions undergo an aberrant proteolytic cascade, producing aggregation-prone peptides of 5 and 8 kDa, containing the Gelsolin Amyloidogenic Core, spanning residues 182-192 (GAC_182-192). Following a structure-based approach, we designed and synthesized three novel sequence-specific peptidomimetics (LB-5, LB-6, and LB-7) built on a piperidine-pyrrolidine unnatural amino acid. LB-5 and LB-6, but not LB-7, efficiently inhibit the aggregation of the GAC_182-192 amyloidogenic peptides at sub-stoichiometric concentrations. These peptidomimetics resulted also effective in vivo, in a C. elegans-based assay, in counteracting the proteotoxicity of aggregated GAC_182-192. These data pave the way to a novel pharmacological strategy against AGel and also validate a toolbox exploitable in other amyloidogenic diseases.

A novel hotspot of gelsolin instability triggers an alternative mechanism of amyloid aggregation

Gelsolin comprises six homologous domains, named G1 to G6. Single point substitutions in this protein are responsible for AGel amyloidosis, a hereditary disease causing progressive corneal lattice dystrophy, cutis laxa, and polyneuropathy. Although several different amyloidogenic variants of gelsolin have been identified, only the most common mutants present in the G2 domain have been thoroughly characterized, leading to clarification of the functional mechanism. The molecular events underlying the pathological aggregation of 3 recently identified mutations, namely A551P, E553K and M517R, all localized at the interface between G4 and G5, are here explored for the first time. Structural studies point to destabilization of the interface between G4 and G5 due to three structural determinants: β-strand breaking, steric hindrance and/or charge repulsion, all implying impairment of interdomain contacts. Such rearrangements decrease the temperature and pressure stability of gelsolin but do not alter its susceptibility to furin cleavage, the first event in the canonical aggregation pathway. These variants also have a greater tendency to aggregate in the unproteolysed forms and exhibit higher proteotoxicity in a C. elegans-based assay. Our data suggest that aggregation of G4G5 variants follows an alternative, likely proteolysis-independent, pathway.

Nonphosphorylated tau slows down Aβ1-42 aggregation, binds to Aβ1-42 oligomers, and reduces Aβ1-42 toxicity

The formation of neurofibrillary tangles and amyloid plaques accompanies the progression of Alzheimer's disease. Tangles are made of fibrillar aggregates formed by the microtubule-associated protein tau, whereas plaques comprise fibrillar forms of amyloid-beta (Aβ). Both form toxic oligomers during aggregation and are thought to interact synergistically to each promote the accumulation of the other. Recent in vitro studies have suggested that the monomeric nonphosphorylated full-length tau protein hinders the aggregation of Aβ_1–40 peptide, but whether the same is true for the more aggregation-prone Aβ_1–42 was not determined. We used in vitro and in vivo techniques to explore this question. We have monitored the aggregation kinetics of Aβ_1–42 by thioflavine T fluorescence in the presence or the absence of different concentrations of nonphosphorylated tau. We observed that elongation of Aβ_1–42 fibrils was inhibited by tau in a dose-dependent manner. Interestingly, the fibrils were structurally different in the presence of tau but did not incorporate tau. Surface plasmon resonance indicated that tau monomers bound to Aβ_1–42 oligomers (but not monomers) and hindered their interaction with the anti-Aβ antibody 4G8, suggesting that tau binds to the hydrophobic central core of Aβ recognized by 4G8. Tau monomers also antagonized the toxic effects of Aβ oligomers in Caenorhabditis elegans. This suggests that nonphosphorylated tau might have a neuroprotective effect by binding Aβ_1–42 oligomers formed during the aggregation and shielding their hydrophobic patches.

C. elegans detects toxicity of traumatic brain injury generated tau

Traumatic brain injury (TBI) is associated with widespread tau pathology in about 30% of patients surviving late after injury. We previously found that TBI in mice induces the formation of an abnormal form of tau (tau^TBI) which progressively spreads from the site of injury to remote brain regions. Intracerebral inoculation of TBI brain homogenates into naïve mice induced progressive tau pathology, synaptic loss and late cognitive decline, suggesting a pivotal role of tau^TBI in post-TBI neurodegeneration. However, the possibility that tau^TBI was a marker of TBI-associated neurodegeneration rather than a toxic driver of functional decline could not be excluded.

Here we employed the nematode C. elegans as a biosensor to test the pathogenic role of TBI generated tau. The motility of this nematode depends on efficient neuromuscular transmission and is exceptionally sensitive to the toxicity of amyloidogenic proteins, providing a tractable model for our tests. We found that worms exposed to brain homogenates from chronic but not acute TBI mice, or from mice in which tau^TBI had been transmitted by intracerebral inoculation, had impaired motility and neuromuscular synaptic transmission. Results were similar when worms were given brain homogenates from transgenic mice overexpressing tau P301L, a tauopathy mouse model, suggesting that TBI-induced and mutant tau have similar toxic properties. P301L brain homogenate toxicity was similar in wild-type and ptl-1 knock-out worms, indicating that the nematode tau homolog protein PTL-1 was not required to mediate the toxic effect. Harsh protease digestion to eliminate the protein component of the homogenates, pre-incubation with anti-tau antibodies or tau depletion by immunoprecipitation, abolished the toxicity. Homogenates of chronic TBI brains from tau knock-out mice were not toxic to C. elegans, whereas oligomeric recombinant tau was sufficient to impair their motility.

This study indicates that tau^TBI impairs motor activity and synaptic transmission in C. elegans and supports a pathogenic role of tau^TBI in the long-term consequences of TBI. It also sets the groundwork for the development of a C. elegans-based platform for screening anti-tau compounds.

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Traumatic brain injury (TBI) in mice induces a progressive tau pathology.

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Brain-injured tissue from chronic but not acute TBI mice impairs C. elegans motility.

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TBI tissue immunodepleted of tau or from tau knock-out mice has no toxic effect.

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Brain-injured tissue from TBI mice impairs neuromuscular transmission in worms.

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C. elegans is a tractable model for investigating tau toxicity generated by TBI.

Caenorhabditis elegans Models to Investigate the Mechanisms Underlying Tau Toxicity in Tauopathies

The understanding of the genetic, biochemical, and structural determinants underlying tau aggregation is pivotal in the elucidation of the pathogenic process driving tauopathies and the design of effective therapies. Relevant information on the molecular basis of human neurodegeneration in vivo can be obtained using the nematode Caenorhabditis elegans (C. elegans). To this end, two main approaches can be applied: the overexpression of genes/proteins leading to neuronal dysfunction and death, and studies in which proteins prone to misfolding are exogenously administered to induce a neurotoxic phenotype. Thanks to the easy generation of transgenic strains expressing human disease genes, C. elegans allows the identification of genes and/or proteins specifically associated with pathology and the specific disruptions of cellular processes involved in disease. Several transgenic strains expressing human wild-type or mutated tau have been developed and offer significant information concerning whether transgene expression regulates protein production and aggregation in soluble or insoluble form, onset of the disease, and the degenerative process. C. elegans is able to specifically react to the toxic assemblies of tau, thus developing a neurodegenerative phenotype that, even when exogenously administered, opens up the use of this assay to investigate in vivo the relationship between the tau sequence, its folding, and its proteotoxicity. These approaches can be employed to screen drugs and small molecules that can interact with the biogenesis and dynamics of formation of tau aggregates and to analyze their interactions with other cellular proteins.

PrP-grafted antibodies bind certain amyloid β-protein aggregates, but do not prevent toxicity

BACKGROUND

The prion protein (PrP) is known to bind certain soluble aggregates of the amyloid β-protein (Aβ), and two regions of PrP, one centered around residues 19-33, and the other around 87-112, are thought to be particularly important for this interaction. When either of these sequences are grafted into a human IgG the resulting antibodies react with disease-associated PrP conformers, whereas the parental b12 IgG does not.

METHODS

Human antibodies containing grafts of PrP 19-33 or 87-112 were prepared as before (Solforosi et al., 2007) and tested for their ability to recognize synthetic and Alzheimer's disease (AD) brain-derived Aβ. Since aqueous extracts of AD brain contain a complex mixture of active and inactive Aβ species, we also assessed whether PrP-grafted antibodies could protect against neuritotoxicity mediated by AD brain-derived Aβ. For these experiments, human iPSC-derived neurons were grown in 96-well plates at 5000 cells per well and on post-induction day 21, AD brain extracts were added +/- test antibodies. Neurons were imaged for 3 days using an IncuCyte live-cell imaging system, and neurite number and density quantified.

RESULTS

Grafted antibodies bound a significant portion of aggregated Aβ in aqueous AD extracts, but when these antibodies were co-incubated with neurons treated with brain extracts they did not reduce toxicity. By contrast, the PrP fragment N1 did protect against Aβ.

CONCLUSIONS

These results further demonstrate that not all Aβ oligomers are toxic and suggest that PrP derivatives may allow development of agents that differentially recognize toxic and innocuous Aβ aggregates.

Antibodies towards Tyrosine Amyloid-Like Fibrils Allow Toxicity Modulation and Cellular Imaging of the Assemblies

The amino acid tyrosine forms cytotoxic amyloid-like fibrils by molecular self-assembly. However, the production of antibodies towards tyrosine assemblies, reflecting their presentation to the immune system, was not demonstrated yet. Here, we describe the production of antibodies that specifically recognize tyrosine in its fibrillated form. The antibodies were demonstrated to specifically bind self-assembled tyrosine, in contrast to its non-aggregated form or disintegrated fibrils. The antibodies could be used for immunostaining of tyrosine fibrils in cultured cells. Furthermore, confocal microscopy allowed a demonstration of the intracellular presence of the metabolite amyloids in a neuroblastoma cell model. Finally, pre-incubation of tyrosine fibrils with the antibodies resulted in significant reduction in their cytotoxicity. Taken together, we provide an experimental proof for the immunogenicity of tyrosine amyloid fibrillary assemblies. These specific antibodies against tyrosine structures could be further used as a research tool to study the dynamics, toxicity and cellular localization of the assemblies.

A novel approach for scrapie-associated prion (PrPSc) detection in blood using the competitive affinity of an aggregate-specific antibody and streptavidin to PrPSc

Scrapie is a fatal neurodegenerative disorder affecting sheep and goats, originating from exposure to disease-associated prions (PrP^Sc). An ante-mortem screening test that can detect native PrP^Sc in body fluids remains unavailable due to insufficient sensitivity of current detection methods that involve proteinase or denaturation treatments. We adopted an approach to detect PrP^Sc in whole blood using a simple proteinase- and denaturation-independent immunoassay, based on the competitive affinity of an aggregate-specific monoclonal antibody and streptavidin to PrP^Sc. First, we demonstrated the ability of native PrP^Sc to bind to streptavidin and the inhibition of this interaction by 15B3 antibody (P<0.05). This led to a new two-step assay that involved capturing native prions from infected blood on a solid-state matrix and detection of PrP^Sc aggregates by evaluating the conformation-dependent conjugate catalytic activity ratio in samples against a pre-determined threshold. This test showed capacity for detecting scrapie prions in 500μl of sheep whole blood spiked with scrapie brain homogenate containing approximately 5ng of total brain protein, and estimated to have 500fg of PrP^Sc. The test also discriminated between blood samples from scrapie-negative (6 sheep, 4 goats) and scrapie-infected animals (3 experimentally infected sheep, 7 naturally infected goats). Collectively, with the proposed high-throughput sample-processing platform, these initial studies provide insights into the development of a large-scale screening test for the routine diagnosis of scrapie.