Importantly, co-localization of scFv-6E fluorobody with httex1-72Q inclusions could be suppressed by blocking aggregation using a second intrabody, scFv-C4, that binds within the first 17 amino-terminal residues of huntingtin. In the presence of scFv-C4, fibril-specific scFv-6E fluorobody shifted from a punctate localization to a diffuse pattern, suggesting that scFv-C4 intrabody blocked the formation of misfolded httex1 epitopes normally recognized by scFv-6E. Colocalized perfectly with non-aggregated httex1-72Q in the cytoplasm, demonstrating that our fluorobody design generates functional fluorescently-tagged intrabodies. Moreover, a nonspecific control fluorobody selected against botulinum neurotoxin light chain protease failed to co-localize with aggregates of fluorescently-labeled httex1-72Q in ST14A cells, suggesting that the observed co-localization of scFv-6E fluorobody with polyglutamine inclusions is direct. In total, these co-localization experiments support our hypothesis that scFv-6E is a conformation-specific antibody fragment capable of distinguishing disease-related amyloid proteins with filamentous structure inside cells. Similar increases in particle height were observed when mature a-synuclein fibrils, but not purified oligomeric or monomeric a-synuclein species, were incubated with scFv-6E in vitro. These results support prior conclusions that misfolded huntingtin and a-synuclein exhibit common structural amyloid epitopes, as first revealed by others using conformation-specific conventional antibodies. To confirm our AFM findings and test whether scFv-6E may also be specific for soluble conformers of mutant protein, we next assayed for direct antibody-antigen interactions in ST14A cells using subcellular re-localization assays. Echinatin intracellular protein aggregation is linked to the onset and progression of a variety of human amyloid disorders, although whether or not visible protein inclusions are innately toxic remains a matter of contention. Accumulating evidence from HD and other amyloidogenic disease models has led to the “toxic soluble precursor” hypothesis, which proposes that toxic intermediates formed during amyloidogenesis might be “detoxified” through their polymerization into insoluble amyloid-like fibrils. As a result, visible protein aggregates are increasingly thought to fulfill a protective cellular response. This model is indirectly supported by cytoprotective evidence obtained using small molecules that promote the intracellular aggregation of huntingtin as well as asynuclein via an undetermined mechanism. In this study, we demonstrate a direct enhancement of huntingtin and ataxin-3 aggregation using 
a fibril-specific scFv antibody that discriminates intracellular aggregates in situ. This conformationspecific scFv recognizes a fibrillar epitope also characteristic of asynuclein, which scFv-6E was originally selected against. Since little sequence homology exists between a-synuclein, huntingtin, and ataxin-3, we speculate that scFv-6E recognizes a structural epitope common to amyloid morphologies such as the cross Alprostadil b-sheet motif. As a result, conformation-specific scFv antibodies such as 6E have broad research potential for a variety of human amyloid disorders. Using scFv-6E as a kinetic tool for enhancing amyloidogenesis, we show that targeting aggregation of mutant huntingtin in striatal cells is not protective, but rather promotes oxidative stress and cell death. These results are consistent with prior findings demonstrating a role for misfolded huntingtin in eliciting mitochondrial dysfunction and oxidative stress.