The S20G substitution in hIAPP is more amyloidogenic and cytotoxic than wild-type hIAPP in mouse islets

Abstract

Aims/hypothesis The S20G human islet amyloid polypeptide (hIAPP) substitution is associated with an earlier onset of type 2 diabetes in humans. Studies of synthetic S20G hIAPP in cell-free systems and immortalised beta cells have suggested that this may be due to increased hIAPP amyloidogenicity and cytotoxicity. Thus, using primary islets from mice with endogenous S20G hIAPP expression, we sought to determine whether the S20G gene mutation leads to increased amyloid-induced toxicity, beta cell loss and reduced beta cell function. Methods Islets from mice in which mouse Iapp was replaced with human wild-type or S20G hIAPP were isolated and cultured in vitro under amyloid-forming conditions. Levels of insulin and hIAPP mRNA and protein, amyloid deposition and beta cell apoptosis and area, as well as glucose-stimulated insulin and hIAPP secretion, were quantified. Results Islets expressing S20G hIAPP cultured in 16.7 mmol/l glucose demonstrated increased amyloid deposition and beta cell apoptosis, reduced beta cell area, decreased insulin content and diminished glucose-stimulated insulin secretion, compared with islets expressing wild-type hIAPP. Amyloid deposition and beta cell apoptosis were also increased when S20G islets were cultured in 11.1 mmol/l glucose (the concentration that is thought to be physiological for mouse islets). Conclusions/interpretation S20G hIAPP reduces beta cell number and function, thereby possibly explaining the earlier onset of type 2 diabetes in individuals carrying this gene mutation.

Atomic structures of fibrillar segments of hIAPP suggest tightly mated β-sheets are important for cytotoxicity

“…To verify the cytotoxic effects of each sample, we examined the morphology of the treated cells under a light microscope. Additionally, in the context of residues 19–29, the S20G segment is significantly more cytotoxic than the WT segment, consistent with parent full-length hIAPP (Sakagashira et al, 2000; Meier et al, 2016) (Figure 5B). We did not detect any oligomers present in the 15–25 WT or 19–29 S20G fibril samples using the LOC antibody (Figure 5—figure supplement 1).…”

“…Moreover, mice can be induced to develop islet amyloid and T2D when they are engineered to express human IAPP and fed a high fat diet (Verchere et al, 1996; Westermark et al, 2000). Perhaps the strongest support for a link is the mutation in hIAPP, hIAPP-S20G; segments that contain this mutation aggregate more quickly, contribute to increased pancreatic β-cell apoptosis, and are associated with early onset T2D in families who carry this lesion (Sakagashira et al, 2000; Cao et al, 2012; Meier et al, 2016; Sakagashira et al, 1996; Lee et al, 2001; Morita et al, 2011). …”

β-Hairpin of Islet Amyloid Polypeptide Bound to an Aggregation Inhibitor

“…Interestingly, a high sensitivity of IAPP amyloidogenicity and toxicity to mutations at residues 20, located at the center of the β-hairpin turn, has been reported 25 26 27 . The Ser-20 to Gly substitution is the only known polymorphism of human IAPP and is associated with earlier onset of T2D.…”

Amyloid fibril structure of islet amyloid polypeptide by cryo-electron microscopy reveals similarities with amyloid beta

“…A serine-to-glycine substitution at position 20 (S20G), the only known IAPP genetic polymorphism in humans, is associated with early onset of T2D 40,41 . The S20G substitution enhances aggregation and toxicity of IAPP and leads to increased beta cell apoptosis [42][43][44][45] . Substitution of serine with glycine was suggested to promote turn formation at residue 20, favoring the amyloid fibril conformation 46,47 .…”