Inhibition of Human Islet Amyloid Polypeptide Aggregation in Type 2 Diabetes by Hsp70 Molecular Chaperones
Authors: Ladjimi Moncef , Chaari Ali , Eliezer David 
Published: July 2019

Protein misfolding, aggregation and amyloid formation play an important role in more than 30 different human diseases, including Alzheimer's, Parkinson's and type 2 diabetes (T2D). T2D is an age-dependent progressive disorder that represents 90% of all diabetes cases. Two major hormones are involved in diabetes and secreted in the β-cell pancreatic islet of Langerhans: insulin and Islet Amyloid PolyPeptide (IAPP or amylin). In comparison with insulin that has been studied thoroughly, much less is known regarding IAPP and its role in the ontogenesis of the disease. Indeed, human IAPP (hIAPP), a 37 amino acid polypeptide characterized by its C-terminal amidation and its disulfide bridge, forms fibrillar amyloids in the pancreas of patients suffering T2D, and is considered to be one of the most amyloidogenic polypeptides. Oligomers generated during the fibrillogenesis process have been linked to increased β-cell dysfunction and possible contribution to β-cell death. In fact, epidemiological studies revealed that up to 95% of patients with T2D are shown to have pancreatic hIAPP amyloid deposits, as detected in post-mortem samples. Thus, hIAPP aggregation is highly cytotoxic, plays a key role in the death of β-pancreatic cells, and correlate with the severity of the disease.

Molecular chaperones, on the other hand, are known to constitute the first line of defense against protein misfolding and aggregation. Many of the molecular chaperones are Heat Shock Proteins, or HSPs, produced in response to various stresses, including heat shock. Not surprisingly, HSPs which function by sequestering proteins in order to prevent inter-molecular interactions associated with aggregation, have specifically been shown to be beneficial in the case of amyloid diseases associated with toxic aggregation processes, and information is available regarding mechanisms by which various HSPs bind and sequester substrates. However the precise mechanisms by which molecular chaperones interact with amyloid proteins and their various intermediates specifically to prevent their toxicity and/or further aggregation, as well as the structural basis of this interaction, remain unclear.

In this work, the effect of HSP70 on hIAPP aggregation was monitored. The results show that increasing concentrations of HSP70 lead to a decrease in Thioflavine T (ThT) fluorescence indicating that the inhibitory effect of HSP70 is concentration dependent. The inhibitory effect of HSP70 is observed with concentrations as low as 1/100 relative to hIAPP and the maximum of the inhibition effect (96%) is observed with a ratio of 1/1 of [HSP70]/[hIAPP]. The length of the lag phase preceding hIAPP aggregation is increased as a function of HSP70 concentrations. These results were confirmed by Dynamic light Scattering (DLS), which indicated that the size, as determined by the Hydrodynamic Radius, RH, of the particles present at 36 h in absence of HSP70, has been reduced in the presence of different concentrations of HSP70, and low molecular weight aggregates were obtained. Thus HSP70 is clearly able to efficiently inhibit hIAPP aggregation and even suppress it at stoichiometric concentrations, as expected for a molecular chaperone having one binding site and working in absence of ATP.

These results on the inhibition of the aggregation process by HSP70 are encouraging, and may be considered as an attractive avenue for further investigation, with the long-term goal of developing inhibitors for therapeutic intervention.