Interplay between human islet amyloid polypeptide aggregates and micro-heterogeneous membranes

Metabolic perturbations associated with hIAPP-induced insulin resistance in skeletal muscles: Implications to the development of type 2 diabetes

The human islet amyloid polypeptide (hIAPP) tends to misfold and self-assemble to form amyloid fibrils, which has been associated with the loss of function and viability of pancreatic β-cells in type 2 diabetes mellitus (T2DM). The role of hIAPP in the development of insulin resistance (a hallmark of T2DM) in skeletal muscles - the major sites for glucose utilization - needs further investigation. Even though, insulin-resistant conditions have been known to stimulate hIAPP aggregation, the events that lead to the development of insulin resistance due to hIAPP aggregation in skeletal muscles remain unidentified. Here, we have attempted to identify metabolic perturbations in L6 myotubes that were exposed to increasing concentrations of recombinant hIAPP for different time durations. It was observed that hIAPP exposure was associated with increased mitochondrial and cellular ROS levels, loss in mitochondrial membrane potential and viability of the myotubes. Metabolomic investigations of hIAPP-treated myotubes revealed significant perturbations in o-phosphocholine, sn-glycero-3-phosphocholine and dimethylamine levels (p < 0.05). Therefore, we anticipate that defects in glycerophospholipid metabolism and the associated oxidative stress and membrane damage may play key roles in the development of insulin resistance due to protein misfolding in skeletal muscles. In summary, the perturbed metabolites and their pathways have not only the potential to be used as early biomarkers to predict the onset of insulin resistance and T2DM but also as therapeutic targets for the effective management of the same.

Structural diversity in the membrane-bound hIAPP dimer correlated with distinct membrane disruption mechanisms

Amyloid deposits of the human islet amyloid polypeptide (hIAPP) have been identified in 90% of patients with type II diabetes. Cellular membranes accelerate the hIAPP fibrillation, and the integrity of membranes is also disrupted at the same time, leading to the apoptosis of β cells in pancreas. The molecular mechanism of hIAPP-induced membrane disruption, especially during the initial membrane disruption stage, has not been well understood yet. Herein, we carried out extensive all-atom molecular dynamics simulations investigating the hIAPP dimerization process in the anionic POPG membrane, to provide the detailed molecular mechanisms during the initial hIAPP aggregation stage in the membrane environment. Compared to the hIAPP monomer on the membrane, we observed not only an increase of α-helical structures, but also a substantial increase of β-sheet structures upon spontaneous dimerization. Moreover, the random coiled and α-helical dimer structures insert deep into the membrane interior with a few inter-chain contacts at the C-terminal region, while the β-sheet-rich structures reside on the membrane surface accompanied by strong inter-chain hydrophobic interactions. The coexistence of α and β structures constitutes a diverse structural ensemble of the membrane-bound hIAPP dimer. From α-helical to β-sheet structures, the degree of membrane disruption decreases gradually, and thus the membrane damage induced by random coiled and α-helical structures precedes that induced by β-sheet structures. We speculate that insertion of random coiled and α-helical structures contributes to the initial stage of membrane damage, while β-sheet structures on the membrane surface are more involved in the later stage of fibril-induced membrane disruption.

Dissecting how ALS-associated D290V mutation enhances pathogenic aggregation of hnRNPA2286-291 peptides: Dynamics and conformational ensembles

The aggregation of RNA binding proteins, including hnRNPA1/2, TDP-43 and FUS, is heavily implicated in causing or increasing disease risk for a series of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). A recent experimental study demonstrated that an ALS-related D290V mutation in the low complexity domain (LCD) of hnRNPA2 can enhance the aggregation propensity of wild type (WT) hnRNPA2_286-291 peptide. However, the underlying molecular mechanisms remain elusive. Herein, we investigated effects of D290V mutation on aggregation dynamics of hnRNPA2_286-291 peptide and the conformational ensemble of hnRNPA2_286-291 oligomers by performing all-atom molecular dynamic and replica-exchange molecular dynamic simulations. Our simulations demonstrate that D290V mutation greatly reduces the dynamics of hnRNPA2_286-291 peptide and that D290V oligomers possess higher compactness and β-sheet content than WT, indicative of mutation-enhanced aggregation capability. Specifically, D290V mutation strengthens inter-peptide hydrophobic, main-chain hydrogen bonding and side-chain aromatic stacking interactions. Those interactions collectively lead to the enhancement of aggregation capability of hnRNPA2_286-291 peptides. Overall, our study provides insights into the dynamics and thermodynamic mechanisms underlying D290V-induced disease-causing aggregation of hnRNPA2_286-291, which could contribute to better understanding of the transitions from reversible condensates to irreversible pathogenic aggregates of hnRNPA2 LCD in ALS-related diseases.

Recent Advances in the Discovery of Therapeutics to Curtail Islet Amyloid Polypeptide Aggregation for Type 2 Diabetes Treatment

In humans with type 2 diabetes, at least 70% of patients exhibit islet amyloid plaques formed by misfolding islet amyloid polypeptides (IAPP). The oligomeric conformation and accumulation of the IAPP plaques lead to a panoply of cytotoxic effects on the islet β-cells. Currently, no marketed therapies for the prevention or elimination of these amyloid deposits exist, and therefore significant efforts are required to address this gap. To date, most of the experimental treatments are limited to only in vitro stages of testing. In general, the proposed therapeutics use various targeting strategies, such as binding to the N-terminal region of islet amyloid polypeptide on residues 1-19 or the hydrophobic region of IAPP. Other strategies include targeting the peptide self-assembly through π-stacking. These methods are realized by using several different families of compounds, four of which are highlighted in this review: naturally occurring products, small molecules, organometallic compounds, and nanoparticles. Each of these categories holds immense potential to optimize and develop inhibitor(s) of pancreatic amyloidosis in the near future.