Nonhuman IAPP Variants Inhibit Human IAPP Aggregation

AIM

To identify naturally occurring variants of IAPP capable of inhibiting the aggregation of human IAPP and protecting living cells from the toxic effects of human IAPP.

BACKGROUND

The loss of insulin-producing β-cells and the overall progression of type 2 diabetes appears to be linked to the formation of toxic human IAPP (hIAPP, Islet Amyloid Polypeptide, amylin) amyloid in the pancreas. Inhibiting the initial aggregation of hIAPP has the potential to slow, if not stop entirely, the loss of β-cells and halt the progression of the disease.

OBJECTIVE

To identify and characterize naturally occurring variants of IAPP capable of inhibiting human IAPP aggregation.

METHODS

Synthetic human IAPP was incubated with synthetic IAPP variants identified from natural sources under conditions known to promote amyloid-based aggregation. To identify IAPP variants capable of inhibiting human IAPP aggregation, Thioflavin T-binding fluorescence, atomic force microscopy, and cell-rescue assays were performed.

RESULTS

While most IAPP variants showed little to no ability to inhibit human IAPP aggregation, several variants showed some ability to inhibit aggregation, with two variants showing substantial inhibitory potential.

CONCLUSION

Several naturally occurring IAPP variants capable of inhibiting human IAPP aggregation were identified and characterized.

Amyloidogenicity of naturally occurring full-length animal IAPP variants

The aggregation of the 37-amino acid polypeptide human islet amyloid polypeptide (hIAPP), as either insoluble amyloid or as small oligomers, appears to play a direct role in the death of human pancreatic β-islet cells in type 2 diabetes. hIAPP is considered to be one of the most amyloidogenic proteins known. The quick aggregation of hIAPP leads to the formation of toxic species, such as oligomers and fibers, that damage mammalian cells (both human and rat pancreatic cells). Whether this toxicity is necessary for the progression of type 2 diabetes or merely a side effect of the disease remains unclear. If hIAPP aggregation into toxic amyloid is on-path for developing type 2 diabetes in humans, islet amyloid polypeptide (IAPP) aggregation would likely need to play a similar role within other organisms known to develop the disease. In this work, we compared the aggregation potential and cellular toxicity of full-length IAPP from several diabetic and nondiabetic organisms whose aggregation propensities had not yet been determined for full-length IAPP.

Identification of Plant Extracts that Inhibit the Formation of Diabetes-Linked IAPP Amyloid

The extracts of 27 vegetables, spices and herbs were screened for their functional ability to inhibit the aggregation of islet amyloid polypeptide (IAPP, amylin) into toxic amyloid aggregates. The aggregation of IAPP has been directly linked to the death of pancreatic β-islet cells in type 2 diabetes. Inhibiting the aggregation of IAPP is believed to have the potential to slow, if not prevent entirely, the progression of this disease. As vegetables, spices and herbs are known to possess many different positive health effects, the extracts of 27 plants (abundant within the United States and spanning several plant families) were screened for their ability to inhibit the formation of toxic IAPP aggregates. Their anti-amyloid activities were assessed through (1) thioflavin T binding assays, (2) visualization of amyloid fibers using atomic force microscopy and (3) cell rescue studies. From this research, mint, peppermint, red bell pepper and thyme emerged as possessing the greatest anti-amyloid activity.

IAPP aggregation and cellular toxicity are inhibited by 1,2,3,4,6-penta-O-galloyl-β-D-glucose

The polyphenol, 1,2,3,4,6-penta-O-galloyl-β-D-glucose (PGG) has been found to exhibit a host of positive pharmacologic activities, including anti-cancer and anti-diabetic. Little is known about the mode of action of PGG in yielding these positive activities. We show here that PGG is a potent inhibitor of IAPP (islet amyloid polypeptide, amylin) aggregation. Preventing the initial aggregation event of IAPP is one strategy for slowing, and possibly preventing, the toxic effects of IAPP oligomeric intermediates. Equal molar ratios of PGG to IAPP substantially reduced the ability of IAPP to bind thioflavin T. Atomic force microscopy revealed that PGG prevented amyloid-based fiber formation under rigorous conditions conducive to forming IAPP aggregates. PGG was also found to protect PC12 rat cells from toxic IAPP. PGG was compared to the known amyloid inhibitors (and structural relatives); tannic acid and gallic acid. In every test, PGG was far superior to tannic and gallic acids at inhibiting amyloid aggregation. These results indicate that PGG is a potent inhibitor of IAPP amyloid aggregation and a potential lead molecule for development of an amyloid inhibiting therapeutic.

Myricetin Inhibits Islet Amyloid Polypeptide (IAPP) Aggregation and Rescues Living Mammalian Cells from IAPP Toxicity

The aggregation of the amyloidogenic polypeptide IAPP (Islet Amyloid Polypeptide, amylin) is believed to play a direct role in the death of pancreatic β-islet cells in type II diabetes. Preventing the initial aggregation event of IAPP is one strategy for slowing, and possibly preventing, the progression of this disease. Here, we investigate myricetin's potential as an inhibitor of IAPP aggregation. We show that myricetin prevented thioflavin T binding in a concentration dependent manner. Atomic force microscopy revealed that myricetin prevented fiber formation under rigorous conditions conducive to forming IAPP aggregates. Using an IAPP-EGFP (Enhanced Green Fluorescent Protein) protein construct, we find that high concentrations of myricetin slowed the in vivo aggregation of IAPP-EGFP. Myricetin was also found to rescue living mammalian cells from the toxic effects of IAPP. These results indicate that myricetin is a strong inhibitor of IAPP amyloid aggregation and a potential lead molecule for the development of an amyloid inhibiting therapeutic.

Selection for nonamyloidogenic mutants of islet amyloid polypeptide (IAPP) identifies an extended region for amyloidogenicity

The aggregation of the 37-residue protein, islet amyloid polypeptide (IAPP), as either insoluble amyloid or as small oligomers, appears to play a direct role in the death of pancreatic beta-islet cells in type II diabetes. While IAPP has been known to be the primary component of type II diabetes amyloid, the molecular interactions responsible for this aggregation have not been identified. To identify the aggregation-prone region(s), we constructed a library of randomly generated point mutants of IAPP. This mutant IAPP library was expressed in Escherichia coli as genetic fusions to the reporter protein enhanced green fluorescent protein (EGFP). Because IAPP aggregates rapidly, both independently and when fused to EGFP, the fusion protein does not yield a functional, fluorescent EGFP. However, mutations of IAPP that result in nonamyloidogenic sequences remain soluble and allow EGFP to fold and fluoresce. Using this screen, we identified 22 single mutations, 4 double mutations, and 2 triple mutations of IAPP that appear to be less amyloidogenic than wild-type human IAPP. A comparison of these sequences suggests residues 13 and 15-17 comprise an additional aggregation-prone region outside of the main amyloidogenic region of IAPP.

Inhibition of Abeta42 aggregation using peptides selected from combinatorial libraries

Increasing evidence suggests that the aggregation of the small peptide Abeta42 plays an important role in the development of Alzheimer's disease. Inhibiting the initial aggregation of Abeta42 may be an effective treatment for preventing, or slowing, the onset of the disease. Using an in vivo screen based on the enzyme EGFP, we have searched through two combinatorially diverse peptide libraries to identify peptides capable of inhibiting Abeta42 aggregation. From this initial screen, three candidate peptides were selected and characterized. ThT studies indicated that the selected peptides were capable of inhibiting amyloid aggregation. Additional ThT studies showed that one of the selected peptides was capable of disaggregating preformed Abeta42 fibers.

Subcellular localization and light-regulated expression of protoporphyrinogen IX oxidase and ferrochelatase in Chlamydomonas reinhardtii

Protoporphyrinogen IX oxidase (PPO) catalyzes the last common step in chlorophyll and heme synthesis, and ferrochelatase (FeC) catalyzes the last step of the heme synthesis pathway. In plants, each of these two enzymes is encoded by two or more genes, and the enzymes have been reported to be located in the chloroplasts or in the mitochondria. We report that in the green alga Chlamydomonas reinhardtii, PPO and FeC are each encoded by a single gene. Phylogenetic analysis indicates that C. reinhardtii PPO and FeC are most closely related to plant counterparts that are located only in chloroplasts. Immunoblotting results suggest that C. reinhardtii PPO and FeC are targeted exclusively to the chloroplast, where they are associated with membranes. These results indicate that cellular needs for heme in this photosynthetic eukaryote can be met by heme that is synthesized in the chloroplast. It is proposed that the multiplicity of genes for PPO and FeC in higher plants could be related to differential expression in differently developing tissues rather than to targeting of different gene products to different organelles. The FeC content is higher in C. reinhardtii cells growing in continuous light than in cells growing in the dark, whereas the content of PPO does not significantly differ in light- and dark-grown cells. In cells synchronized to a light/dark cycle, the level of neither enzyme varied significantly with the phase of the cycle. These results indicate that heme synthesis is not directly regulated by the levels of PPO and FeC in C. reinhardtii.

Cellular levels of glutamyl-tRNA reductase and glutamate-1-semialdehyde aminotransferase do not control chlorophyll synthesis in Chlamydomonas reinhardtii

5-Aminolevulinic acid (ALA) is the first committed universal precursor in the tetrapyrrole biosynthesis pathway. In plants, algae, and most bacteria, ALA is generated from glutamate. First, glutamyl-tRNA synthetase activates glutamate by ligating it to tRNA(Glu). Activated glutamate is then converted to glutamate 1-semialdehyde (GSA) by glutamyl-tRNA reductase (GTR). Finally, GSA is rearranged to ALA by GSA aminotransferase (GSAT). In the unicellular green alga Chlamydomonas reinhardtii, GTR and GSAT were found in the chloroplasts and were not detected in the mitochondria by immunoblotting. The levels of both proteins (assayed by immunoblotting) and their mRNAs (assayed by RNA blotting) were approximately equally abundant in cells growing in continuous dark or continuous light (fluorescent tubes, 80 micromol photons s(-1) m(-2)), consistent with the ability of the cells to form chlorophyll under both conditions. In cells synchronized to a 12-h-light/12-h-dark cycle, chlorophyll accumulated only during the light phase. However, GTR and GSAT were present at all phases of the cycle. The GTR mRNA level increased in the light and peaked about 2-fold at 2 h into the light phase, and GTR protein levels also increased and peaked 2-fold at 4 to 6 h into the light phase. In contrast, although the GSAT mRNA level increased severalfold at 2 h into the light phase, the level of GSAT protein remained approximately constant in the light and dark phases. Under all growth conditions, the cells contained significantly more GSAT than GTR on a molar basis. Our results indicate that the rate of chlorophyll synthesis in C. reinhardtii is not directly controlled by the expression levels of the mRNAs for GTR or GSAT, or by the cellular abundance of these enzyme proteins.

The Chlamydomonas reinhardtii gtr gene encoding the tetrapyrrole biosynthetic enzyme glutamyl-trna reductase: structure of the gene and properties of the expressed enzyme

Plants, algae, cyanobacteria and many other bacteria synthesize the tetrapyrrole precursor, delta-aminolevulinic acid (ALA), from glutamate by means of a tRNAGlu-mediated pathway. The enzyme glutamyl-tRNA reductase (GTR) catalyzes the first committed step in this pathway, which is the reduction of tRNA-bound glutamate to produce glutamate 1-semialdehyde. Chlamydomonas reinhardtii mRNA encoding gtr was sequenced from a cDNA and genomic libraries. The 3179-bp gtr cDNA contains a 1566-bp open reading frame that encodes a 522-amino acid polypeptide. After removal of the predicted transit peptide, the mature 480-residue GTR has a calculated molecular weight of 52,502. The deduced C. reinhardtii mature GTR amino acid sequence has more than 55% identity to a GTR sequence of Arabidopsis thaliana, and significant similarity to GTR proteins of other plants and prokaryotes. Southern blot analysis of C. reinhardtii genomic DNA indicates that C. reinhardtii has only one gtr gene. Genomic DNA sequencing revealed the presence of a small intron near the putative transit peptide cleavage site. Expression constructs for the full-length initial gtr translation product, the mature protein after transit peptide removal, and the coding sequence of the second exon were cloned into expression vector that also introduced a C-terminal His6 tag. All of these constructs were expressed in E. coli, and both the mature protein and the exon 2 translation product complemented a hemA mutation. The expressed proteins were purified by Ni-affinity column chromatography to yield active GTR. Purified mature GTR was not inhibited by heme, but heme inhibition was restored upon addition of C. reinhardtii soluble proteins.

Physical and kinetic interactions between glutamyl-tRNA reductase and glutamate-1-semialdehyde aminotransferase of Chlamydomonas reinhardtii

In plants, algae, and most bacteria, the heme and chlorophyll precursor 5-aminolevulinic acid (ALA) is formed from glutamate in a three-step process. First, glutamate is ligated to its cognate tRNA by glutamyl-tRNA synthetase. Activated glutamate is then converted to a glutamate 1-semialdehyde (GSA) by glutamyl-tRNA reductase (GTR) in an NADPH-dependent reaction. Subsequently, GSA is rearranged to ALA by glutamate-1-semialdehyde aminotransferase (GSAT). The intermediate GSA is highly unstable under physiological conditions. We have used purified recombinant GTR and GSAT from the unicellular alga Chlamydomonas reinhardtii to show that GTR and GSAT form a physical and functional complex that allows channeling of GSA between the enzymes. Co-immunoprecipitation and sucrose gradient ultracentrifugation results indicate that recombinant GTR and GSAT enzymes specifically interact. In vivo cross-linking results support the in vitro results and demonstrate that GTR and GSAT are components of a high molecular mass complex in C. reinhardtii cells. In a coupled enzyme assay containing GTR and wild-type GSAT, addition of inactive mutant GSAT inhibited ALA formation from glutamyl-tRNA. Mutant GSAT did not inhibit ALA formation from GSA by wild-type GSAT. These results suggest that there is competition between wild-type and mutant GSAT for binding to GTR and channeling GSA from GTR to GSAT. Further evidence supporting kinetic interaction of GTR and GSAT is the observation that both wild-type and mutant GSAT stimulate glutamyl-tRNA-dependent NADPH oxidation by GTR.