Exploring the Role of Primary Nucleation and Secondary Processes During Insulin Amyloid Fibril Formation

There has been immense interest in the assembly of amyloid fibrils due to their implication in many diseases such as Alzheimer?s Disease and Type-2 diabetes. Fibril formation has historically been explained via a nucleation-dependent process. A key feature of this model is the presence of an often long quiescent period preceding the appearance of fibrils known as the lag phase. This lag time has been attributed to the length of time needed to form a critical nucleus from which fibrils can grow. This phase of fibril assembly has garnered particular interest due to the observation that pre-fibril oligomeric species present during this time may be the cytotoxic agents responsible for amyloid associated pathologies. Studies of the lag time in the past have been fraught with irreproducibility and the appearance of large variation in the measured lag time distributions. This subsequently has been attributed to the stochastic nature of nucleation. However, a recent model has been proposed which regards the primary nucleation event as a small contributor to the duration of the lag time (Knowles, 2009). In this model, secondary processes, particularly fragmentation, dominate the formation and growth behaviour of amyloid fibrils. By employing methods and materials which significantly reduce the previously noted variation in lag time, we have undertaken an extensive investigation of amyloid fibril formation of bovine insulin via Thioflavin T fluorescence assays in the presence and absence of agitation. We observe two distinct protein concentration regimes characterised by a discontinuity in the growth rate of fibrils. Additionally, we look at the effect of varying salt concentration on the growth kinetics to further elucidate the mechanisms of fibril assembly. I will discuss discrepancies between our data and the Knowles model and how, via computer simulations, we are attempting to explain the observed behaviour.