Are associated with polyneuropathy. It is established that the Indolactam V site mutations in the TTR gene destabilize the SPDP web native homotetramer [27,28], which is accompanied by formation of toxic oligomers and later mature amyloid [29]. Previous studies have shown that similarly to the wildtype TTR, clinical mutants (e.g. TTRV30M) form stable tetramers in vitro unless incubated under mildly acidic conditions. We have constructed two TTR mutants, one in the edge region comprising the short b-strand D, denoted TTR-D (G53S/E54D/L55S) [30] and the other in the neighboring b-strand A, denoted TTR-A (V14N/V16E) [31]. These mutants are excellent tools for studies of amyloid-induced cellular toxicity since they spontaneously form protofibrils in a reasonable time period at physiological pH. In this work, we have shown that SAP has a protective effect in cell culture during early aggregate formation, which protects from TTR-induced cell death. To determine the role of SAP in TTRinduced toxicity, we complemented the in vitro studies with a genetic approach in a Drosophila model for TTR-associated amyloidosis [32,33]. In the fruit fly, overexpression of the mutated variant TTR-A in secreted form leads to a complex neurological phenotype that reflects several features of the human pathology, including progressive neurodegeneration, accumulation of insoluble TTR, locomotor dysfunction, and premature death. We have found an increased aggregation rate and toxicity of TTR-A in the fruit fly, which results in an abnormal wing posture termed “dragged wings”. This phenotype is significantly suppressed in crosses with transgenic SAP flies. In addition, we have found in vivo that binding of SAP to mutated TTR-A in the eye of Drosophila protects retinal structure from the deleterious effects of aggregating amyloidogenic TTR.Results Binding of SAP to Pre-fibrillar Aggregates of TTRIt is well established that SAP is commonly found in different types of amyloid deposits and that it has also a calciumdependent affinity for binding to isolated mature amyloid fibrils. In previous work, we showed that the toxic effect found in cell culture correlates best with the early stages of fibril formation and that the mature full-length TTR fibrils represent an inert end stage [34]. In order to investigate whether binding of SAP occurs early, before the fibrils are formed, we subjected recombinant TTRs to aggregation at physiological pH in the presence of SAP for 4 days at 37uC. Under these conditions, TTR-D and TTR-A mutants are known to form pre-fibrillar aggregates in vitro, whereas TTRwt and TTRV30M stay soluble unless treated with low-pH buffer [30,31]. The complexes were immunoprecipitated with a SAP-specific antibody and the amount of SAP either bound to the aggregates or free in the remaining supernatants was determined. As shown Figure 1A, SAP co-incubated with pre-aggregated TTR also had the ability to bind pre-fibrillar aggregates of TTR formed in vitro at physiological pH by the TTR-D and TTR-A mutants. The state of these aggregates has been described in detail previously [30,31]. Briefly, amorphous pre-fibrillar intermediates were formed, which transformed into mature fibrils similar in morphology to ex vivo-isolated material from the vitreous body. Immunoprecipitation using an anti-SAP antibody, followed by immunodetection with an anti-TTR antibody, revealed that SAP bound to TTR-D and to TTR-A in 1407003 pre-fibrillar states and the complexes were found in the pellet, while TTRwt an.Are associated with polyneuropathy. It is established that the mutations in the TTR gene destabilize the native homotetramer [27,28], which is accompanied by formation of toxic oligomers and later mature amyloid [29]. Previous studies have shown that similarly to the wildtype TTR, clinical mutants (e.g. TTRV30M) form stable tetramers in vitro unless incubated under mildly acidic conditions. We have constructed two TTR mutants, one in the edge region comprising the short b-strand D, denoted TTR-D (G53S/E54D/L55S) [30] and the other in the neighboring b-strand A, denoted TTR-A (V14N/V16E) [31]. These mutants are excellent tools for studies of amyloid-induced cellular toxicity since they spontaneously form protofibrils in a reasonable time period at physiological pH. In this work, we have shown that SAP has a protective effect in cell culture during early aggregate formation, which protects from TTR-induced cell death. To determine the role of SAP in TTRinduced toxicity, we complemented the in vitro studies with a genetic approach in a Drosophila model for TTR-associated amyloidosis [32,33]. In the fruit fly, overexpression of the mutated variant TTR-A in secreted form leads to a complex neurological phenotype that reflects several features of the human pathology, including progressive neurodegeneration, accumulation of insoluble TTR, locomotor dysfunction, and premature death. We have found an increased aggregation rate and toxicity of TTR-A in the fruit fly, which results in an abnormal wing posture termed “dragged wings”. This phenotype is significantly suppressed in crosses with transgenic SAP flies. In addition, we have found in vivo that binding of SAP to mutated TTR-A in the eye of Drosophila protects retinal structure from the deleterious effects of aggregating amyloidogenic TTR.Results Binding of SAP to Pre-fibrillar Aggregates of TTRIt is well established that SAP is commonly found in different types of amyloid deposits and that it has also a calciumdependent affinity for binding to isolated mature amyloid fibrils. In previous work, we showed that the toxic effect found in cell culture correlates best with the early stages of fibril formation and that the mature full-length TTR fibrils represent an inert end stage [34]. In order to investigate whether binding of SAP occurs early, before the fibrils are formed, we subjected recombinant TTRs to aggregation at physiological pH in the presence of SAP for 4 days at 37uC. Under these conditions, TTR-D and TTR-A mutants are known to form pre-fibrillar aggregates in vitro, whereas TTRwt and TTRV30M stay soluble unless treated with low-pH buffer [30,31]. The complexes were immunoprecipitated with a SAP-specific antibody and the amount of SAP either bound to the aggregates or free in the remaining supernatants was determined. As shown Figure 1A, SAP co-incubated with pre-aggregated TTR also had the ability to bind pre-fibrillar aggregates of TTR formed in vitro at physiological pH by the TTR-D and TTR-A mutants. The state of these aggregates has been described in detail previously [30,31]. Briefly, amorphous pre-fibrillar intermediates were formed, which transformed into mature fibrils similar in morphology to ex vivo-isolated material from the vitreous body. Immunoprecipitation using an anti-SAP antibody, followed by immunodetection with an anti-TTR antibody, revealed that SAP bound to TTR-D and to TTR-A in 1407003 pre-fibrillar states and the complexes were found in the pellet, while TTRwt an.