Caractérisation du rôle de l’amyline (IAPP) dans le diabète de type 2 : études de dérivés peptidiques et de composés inhibiteurs de la formation d’amyloïde

Amyloidosis is a progressive and, as of now, incurable disease caused by the deposition of insoluble proteins. Amyloid research over the past decades focused on the characterization of the substantive biological variability of amyloid deposits. Amyloidosis encompasses a diversity of pathological manifestations, explained by the diversity of underlying causal proteins. In the pancreas of susceptible species, islet amyloid polypeptide (IAPP) is a precursor for an amyloid protein (AIAPP), which has a characteristic fibrillar structure and resistance to physical agents. This folded protein deposits in the islets of Langerhans of patients with type 2 diabetes or islet cell tumors (insulinoma). Amyloid deposits have also been well characterized, anatomically, in feline and non-human primate species. Amyloid fibril formation contributes to the pathogenesis of diabetes mellitus but the precise pathophysiologic factors involved in the fibrillization of IAPP as well as resultant islet injury remain to be elucidated. Further understanding of the causative factors in the fibrillogenesis of IAPP will be requisite in the development of therapeutic strategies to disrupt the amyloidosis process.  This project hypothesizes that the specific variations found in IAPP peptide sequences among different animal species are critical for IAPP fibrillization. Also, some aromatic/polyphenolic compounds can abrogate fibrillization. The main objective forms the basis for development of new therapeutic tactics to impede amyloid formation and associated cellular injury. Thus this project has two specific aims. The first specific aim was to identify critical variations in IAPP amino acid sequences from different animal species and to assess their amyloidogenic potential. To accomplish this, the IAPP gene was isolated and sequenced from paraffin-embedded tissues from various animals (40 species). The aggregation potency was assessed for each sequence using in silico analysis. A library of 23 peptides was prepared from sequences that were distinctly different and their amyloidogenic potential was assessed in vitro using physicochemical analysis (thioflavin-T assay, Congo red binding assay, far-UV circular dichroism, transmission electron microscopy) and cytotoxicity assays (insulinoma cell line INS-1). Among this peptide library, three were non-amyloidogenic and corresponded to the following animal species: golden lion tamarin (Leontopithecus rosalia), commun bottlenose dolphin (Tursiops truncates) and alpaca (Vicugna pacos). Segment 8-20 of the peptide was critical for amyloid formation and the substitution of the NFLVH motif found in this region by a DFLGR or KFLIR motif impeded fibrillization. The amino acids 29P, 14K and 18R were also demonstrated to abrogate fibril formation. The second objective consisted in abrogating IAPP fibrillogenesis using conceptualized aromatic/polyphenolic structures, specifically hypoglycemic, non-steroidal anti-inflammatory and aryl ethyl urea agents. This part of the project involved molecular screening of more than 60 compounds. Their efficacy at inhibiting amyloid formation was assessed in vitro on human IAPP, which exhibits the highest amyloidogenic potential. Techniques included the above-mentioned methods, with the addition of photo-induced cross-linking of unmodified proteins (PICUP). A total of 11 compounds showed potential in abrogating IAPP aggregation. Among the hypoglycemic agents evaluated, glyburide, repaglinide and troglitazone showed the highest potency in reducing fibril formation. The NSAIDs that displayed anti-amyloidogenic activity were diclofenac, meloxicam, phenylbutazone, sulindac and tenoxicam. EU-362 and EU-418 were the hit compounds resulting from the screening of the aryl ethyl urea (EU) class. Additionally, these anti-amyloidogenic molecules conferred a protection against fibril cytotoxicity. All of the active molecules bear a commun motif composed of benzene ring with electron donor moieties, such as alcohol, amine or halide. To conclude, this project characterized IAPP in several animal species in which it has not been previously described and improves our understanding of the amyloidogenesis process. Moreover, the therapeutic potential of hypoglycemic, non-steroidal anti-inflammatory and aryl ethyl ureas agents as anti-amyloidogenic compounds was evaluated. It is conceivable that the additional information hereby gained on the regulation of amyloidogenesis may point towards new therapeutic strategies for diabetic patients.

Keyphrases: amyloid, islet amyloid polypeptide (IAPP), inhibitors of fibrillization, pancreas, islets of Langerhans, peptide analogues, type 2 diabetes