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

Divergent disruptive effects of soluble recombinant tau assemblies on synaptic plasticity in vivo

Neurofibrillary tangles (NFTs), composed of tau protein fibrils, together with brain inflammation and synaptic loss, are neuropathological hallmarks of several neurodegenerative diseases, including Alzheimer's disease. Compared with tau fibrils, more water-soluble assemblies of either recombinant or patient-derived tau have been reported to exert relatively potent rapid synaptotoxic effects, including inhibition of synaptic long-term potentiation (LTP) in the hippocampus. Less is known regarding the action of exogenous tau soluble assemblies on the opposite form of synaptic plasticity, long-term depression (LTD). We compared the synaptic plasticity actions of two relatively standard preparations of soluble recombinant tau assemblies, (i) fibril-derived soluble sonicated tau aggregates (SτAs) and (ii) oligomer-enriched tau (oTau) prepared from monomers. Consistent with previous findings, intracerebroventricular injections of either SτAs or oTau acutely inhibited high-frequency stimulation-induced LTP at CA3-to-CA1 synapses in the anaesthetized rat hippocampus. However, LTP inhibition by oTau, but not SτAs, was prevented by co-injection with the conformational anti-tau monoclonal antibody, TOMA1. Furthermore, in contrast to SτAs, which inhibited LTD, injection of oTau potently facilitated LTD, decreasing the threshold for LTD induction by low-frequency stimulation. To test the role of pro-inflammatory cytokines in mediating the disruptive effects of the two forms of soluble tau on synaptic plasticity we pre-injected etanercept, a decoy receptor for tumor necrosis factor alpha (TNFα). Etanercept reduced the disruption of synaptic plasticity by oTau but not by SτAs. Moreover, injection of exogenous TNFα mimicked the facilitation of LTD by oTau, consistent with a role of this pro-inflammatory cytokine in LTD facilitation.These data provide evidence that preparations of soluble tau containing either monomer- or fibril-derived assemblies disrupt LTP and LTD via different mechanisms. Intriguingly, when oTau and SτAs were applied together, LTD block predominated. Thus, if similar synaptotoxic soluble tau assemblies are present together later during the disease process, as seems likely, inhibition of synaptic weakening processes is predicted to predominate. Equally, reducing TNFα would be expected to be more effective when the monomer-derived soluble tau assemblies are the dominant synaptotoxic species. We conclude that oTau and SτAs provide useful means to explore ways of targeting different synaptotoxic soluble tau species in tauopathies.

β-Amyloid Induces Microglial Expression of GPC4 and APOE Leading to Increased Neuronal Tau Pathology and Toxicity

To elucidate the impact of Aβ pathology on microglia in Alzheimer's disease pathogenesis, we profiled the microglia surfaceome following treatment with Aβ fibrils. Our findings reveal that Aβ-associated human microglia upregulate Glypican 4 (GPC4), a GPI-anchored heparan sulfate proteoglycan (HSPG). In a Drosophila amyloidosis model, glial GPC4 expression exacerbates motor deficits and reduces lifespan, indicating that glial GPC4 contributes to a toxic cellular program during neurodegeneration. In cell culture, GPC4 enhances microglia phagocytosis of tau aggregates, and shed GPC4 can act in trans to facilitate tau aggregate uptake and seeding in neurons. Additionally, our data demonstrate that GPC4-mediated effects are amplified in the presence of APOE. These studies offer a mechanistic framework linking Aβ and tau pathology through microglial HSPGs and APOE.

Epitope-specific antibody fragments block aggregation of AGelD187N, an aberrant peptide in gelsolin amyloidosis

Aggregation of aberrant fragment of plasma gelsolin, AGelD187N, is a crucial event underlying the pathophysiology of Finnish gelsolin amyloidosis, an inherited form of systemic amyloidosis. The amyloidogenic gelsolin fragment AGelD187N does not play any physiological role in the body, unlike most aggregating proteins related to other protein misfolding diseases. However, no therapeutic agents that specifically and effectively target and neutralize AGelD187N exist. We used phage display technology to identify novel single-chain variable fragments that bind to different epitopes in the monomeric AGelD187N that were further maturated by variable domain shuffling and converted to antigen-binding fragment (Fab) antibodies. The generated antibody fragments had nanomolar binding affinity for full-length AGelD187N, as evaluated by biolayer interferometry. Importantly, all four Fabs selected for functional studies efficiently inhibited the amyloid formation of full-length AGelD187N as examined by thioflavin fluorescence assay and transmission electron microscopy. Two Fabs, neither of which bound to the previously proposed fibril-forming region of AGelD187N, completely blocked the amyloid formation of AGelD187N. Moreover, no small soluble aggregates, which are considered pathogenic species in protein misfolding diseases, were formed after successful inhibition of amyloid formation by the most promising aggregation inhibitor, as investigated by size-exclusion chromatography combined with multiangle light scattering. We conclude that all regions of the full-length AGelD187N are important in modulating its assembly into fibrils and that the discovered epitope-specific anti-AGelD187N antibody fragments provide a promising starting point for a disease-modifying therapy for gelsolin amyloidosis, which is currently lacking.

Nasal tau immunotherapy clears intracellular tau pathology and improves cognitive functions in aged tauopathy mice

Pathological tau aggregates cause cognitive decline in neurodegenerative tauopathies, including Alzheimer's disease (AD). These aggregates are prevalent within intracellular compartments. Current tau immunotherapies have shown limited efficacy in clearing intracellular tau aggregates and improving cognition in clinical trials. In this study, we developed toxic tau conformation-specific monoclonal antibody-2 (TTCM2), which selectively recognized pathological tau aggregates in brain tissues from patients with AD, dementia with Lewy bodies (DLB), and progressive supranuclear palsy (PSP). TTCM2 potently inhibited tau-seeding activity, an essential mechanism underlying tauopathy progression. To effectively target intracellular tau aggregates and ensure rapid delivery to the brain, TTCM2 was loaded in micelles (TTCM2-ms) and administered through the intranasal route. We found that intranasally administered TTCM2-ms efficiently entered the brain in hTau-tauopathy mice, targeting pathological tau in intracellular compartments. Moreover, a single intranasal dose of TTCM2-ms effectively cleared pathological tau, elevated synaptic proteins, and improved cognitive functions in aged tauopathy mice. Mechanistic studies revealed that TTCM2-ms cleared intracellular, synaptic, and seed-competent tau aggregates through tripartite motif-containing 21 (TRIM21), an intracellular antibody receptor and E3 ubiquitin ligase known to facilitate proteasomal degradation of cytosolic antibody-bound proteins. TRIM21 was found to be essential for TTCM2-ms-mediated clearance of tau pathology. Our study collectively provides evidence of the effectiveness of nasal tau immunotherapy in targeting and clearing intracellular tau pathology through TRIM21 and enhancing cognition in aged tauopathy mice. This study could be valuable in designing effective tau immunotherapies for AD and other tauopathies.

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.