Abstract The accumulation of insoluble protein aggregates containing amyloid fibrils has been observed in many different human protein misfolding diseases 1,2 , and their pathological features have been recapitulated in diverse model systems 3 . In vitro kinetic studies have provided a quantitative understanding of how the fundamental molecular level processes of nucleation and growth lead to amyloid formation 4 . However, it is not yet clear to what extent these basic biophysical processes translate to amyloid formation in vivo , given the complexity of the cellular and organismal environment. Here we show that the aggregation of a fluorescently tagged polyglutamine (polyQ) protein into µm-sized inclusions in the muscle tissue of living C. elegans can be quantitatively described by a molecular model where stochastic nucleation occurs independently in each cell, followed by rapid aggregate growth. Global fitting of the image-based aggregation kinetics reveals a nucleation rate corresponding to 0.01 h -1 per cell at 1 mM intracellular protein concentration, and shows that the intrinsic stochasticity of nucleation accounts for a significant fraction of the observed animal-to-animal variation. Our results are consistent with observations for the aggregation of polyQ proteins in vitro 5 and in cell culture 6 , and highlight how nucleation events control the overall progression of aggregation in the organism through the spatial confinement into individual cells. The key finding that the biophysical principles associated with protein aggregation in small volumes remain the governing factors, even in the complex environment of a living organism, will be critical for the interpretation of in vivo data from a wide range of protein aggregation diseases.

Kinetic analysis reveals the rates and mechanisms of protein aggregation in a multicellular organism