Research to understand Alzheimer’s disease (AD) pathogenesis has involved the use of model systems to reproduce various aspects of the disease process, particularly the coordinated processing of the amyloid precursor protein (APP) by α-, β-, and γ-secretases to generate amyloid beta (Aβ)- containing plaques, a process termed the amyloid hypothesis of AD. In recent years, a variety of invertebrate systems including Drosophila melanogaster and Caenorhabditis elegans have been exploited to study the pathogenesis of AD but it is not clear as to whether lower invertebrate species can provide a direct comparison in understanding human neurological diseases. Additionally, the absence of a functional Aβ sequence and lack of equivalent β-secretases in current invertebrate models have limited efforts in studying the amyloid hypothesis in vivo. As invertebrate chordates belonging to the subphylum Urochordata, the closest living sister group of vertebrates, the ascidian system will complement current AD research models by providing the experimental advantages of invertebrates in a chordate body plan.

Here, we describe the generation of an ascidian model to study various aspects of the amyloid hypothesis including APP processing, Aβ plaque formation, and Aβ-mediated neurotoxicity. In transgenic ascidian larvae that express human APP695 (hAPP695) alone, Aβ peptides are produced that can aggregate to form Aβ-containing plaques detectable within 23 hours post fertilization. In support for conservation of the APP processing cascade, familial AD-associated mutations in hAPP695 result in a significant increase in plaque formation in vivo. Furthermore, co-expression with ascidian CiBace leads to increased amyloid deposition indicating that ascidians possess a functional β-secretase orthologue, a finding unique among other invertebrates. Nervous system-specific expression of the processed Aβ peptide causes observable alterations in neuromuscular functioning and larval behavior, indicative by a reduction in the frequency of tail twitching and the ability to respond to light respectively. Importantly, the treatment of our AD ascidian model with an inhibitor of amyloid aggregation reduces plaque load and improves both neuromuscular function and the phototactic response. Overall, this study introduces the ascidian as an animal model that rapidly exhibits AD-like pathologies and provides the framework for understanding and possibly treating this disease and other neurological disorders.