Islet amyloid toxicity: From genesis to counteracting mechanisms.

Islet amyloid polypeptide (IAPP or amylin) is a hormone co-secreted with insulin by pancreatic ß-cells and is the major component of islet amyloid. Islet amyloid is found in the pancreas of patients with type 2 diabetes (T2D) and may be involved in ß-cell dysfunction and death, observed in this disease. Thus, investigating the aspects related to amyloid formation is relevant to the development of strategies towards ß-cell protection. In this sense, IAPP misprocessing, IAPP overproduction, and disturbances in intra- and extracellular environments seem to be decisive for IAPP to form islet amyloid. Islet amyloid toxicity in ß-cells may be triggered in intra- and/or extracellular sites by membrane damage, endoplasmic reticulum stress, autophagy disruption, mitochondrial dysfunction, inflammation, and apoptosis. Importantly, different approaches have been suggested to prevent islet amyloid cytotoxicity, from inhibition of IAPP aggregation to attenuation of cell death mechanisms. Such approaches have improved ß-cell function and prevented the development of hyperglycemia in animals. Therefore, counteracting islet amyloid may be a promising therapy for T2D treatment.

Alzheimer's disease and type 2 diabetes mellitus: Pathophysiologic and pharmacotherapeutics links.

At present, Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM) are two highly prevalent disorders worldwide, especially among elderly individuals. T2DM appears to be associated with cognitive dysfunction, with a higher risk of developing neurocognitive disorders, including AD. These diseases have been observed to share various pathophysiological mechanisms, including alterations in insulin signaling, defects in glucose transporters (GLUTs), and mitochondrial dysfunctions in the brain. Therefore, the aim of this review is to summarize the current knowledge regarding the molecular mechanisms implicated in the association of these pathologies as well as recent therapeutic alternatives. In this context, the hyperphosphorylation of tau and the formation of neurofibrillary tangles have been associated with the dysfunction of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways in the nervous tissues as well as the decrease in the expression of GLUT-1 and GLUT-3 in the different areas of the brain, increase in reactive oxygen species, and production of mitochondrial alterations that occur in T2DM. These findings have contributed to the implementation of overlapping pharmacological interventions based on the use of insulin and antidiabetic drugs, or, more recently, azeliragon, amylin, among others, which have shown possible beneficial effects in diabetic patients diagnosed with AD.

Heterotropic Modulation of Amylin Fibrillation by Small Molecules: Implications for Formulative Designs.

Control of amylin agglomeration is of interest for both the study of pathophysiology and the design of amylin-based pharmaceutical products. Here we report the effects of a large set of common buffering agents, aminoacids and nucleoside phosphates over the amylin amyloid aggregation. Circular dichroism showed no apparent effects of the co-solutes over the secondary-structure of soluble amylin. Instead, we found a large dependence of the fibrillation process on the total amount of co-solute charged groups. The amyloid nature of the aggregates was confirmed by transmission electron microscopy, X-ray diffraction and infrared spectroscopy. While acidic pH and low-ionic co-solutes shows the largest size effect in hampering aggregation, no further effect was observed that could identify a single compound as a major direct heterotropic determinants of the amyloid process. These data suggest a more physico-chemical effect of co-solutes over the modulation of amylin instead of a chemical entity-related causal factor.

The toxic nature of murine amylin and the immune responsivity of pancreatic islet to conformational antibody in mice.

The human amylin is a pancreatic peptide hormone found in hyperhormonemic state along with insulin in subclinical diabetes. Amylin has been associated with the pathology of type 2 diabetes, particularly due to its ability to assembly into toxic oligomers and amyloid specimens. On the other hand, some variants such as murine amylin has been described as non-amyloidogenic, either in vitro or in vivo. Recent data have demonstrated the amyloid propensity of murine amylin and the therapeutic analogue pramlintide, suggesting a universality for amylin amyloidosis. Here, we report the amyloidogenesis of murine amylin, which showed lower responsivity to the fluorescent probe thioflavin T compared to human amylin, but presented highly organized fibrilar amyloid material. The aggregation of murine amylin also resulted in the formation of cytotoxic specimens, as evaluated in vitro in INS-1 cells. The aggregation product from murine amylin was responsive to a specific antibody raised against amyloid oligomers, the A11 oligomer antibody. Pancreatic islets of wild-type Swiss male mice have also shown responsivity for the anti-oligomer, indicating the natural abundance of such specimen in rodents. These data provide for the first time evidences for the toxic nature of oligomeric assemblies of murine amylin and its existence in wild-type, non-transgenic mice.

Regulation of the assembly and amyloid aggregation of murine amylin by zinc.

The secretory granule of the pancreatic ß-cells is a zinc-rich environment copopulated with the hormones amylin and insulin. The human amylin is shown to interact with zinc ions with major contribution from the single histidine residue, which is absent in amylin from other species such as cat, rhesus and rodents. We report here the interaction of murine amylin with zinc ions in vitro. The self-assembly of murine amylin is tightly regulated by zinc and pH. Ion mobility mass spectrometry revealed zinc interaction with monomers and oligomers. Nuclear magnetic resonance confirms the binding of zinc to murine amylin. The aggregation process of murine amylin into amyloid fibrils is accelerated by zinc. Collectively these data suggest a general role of zinc in the modulation of amylin variants oligomerization and amyloid fibril formation.

Stepwise oligomerization of murine amylin and assembly of amyloid fibrils.

Amylin is a pancreatic hormone co-secreted with insulin. Human amylin has been shown to form dimers and exhibit high propensity for amyloid fibril formation. We observed the ability of the water-soluble murine amylin to aggregate in water resulting in an insoluble material with Thioflavin T binding properties. Infrared spectroscopy analysis revealed beta-sheet components in the aggregated murine amylin. Morphological analysis by transmission electron microscopy and atomic force microscopy provided access to the fibril nature of the murine amylin aggregate which is similar to amyloid fibrils from human amylin. X-ray diffraction of the murine amylin fibrils showed peaks at 4.7Å and 10Å, a fingerprint for amyloid fibrils. Electron spray ionization-ion mobility spectroscopy-mass spectrometry (ESI-IMS-MS) analysis and crosslinking assays revealed self-association intermediates of murine amylin into high order oligomeric assemblies. These data demonstrate the stepwise association mechanism of murine amylin into stable oligomers, which ultimately converges to its organization into amyloid fibrils.