Threshold concentration and random collision determine the growth of the phase-separated huntingtin inclusion from a stable core

Abstract

The processes underlying formation and growth of unfolded protein inclusions are relevant to neurodegenerative diseases. In S. cerevisiae, inclusion bodies formed by mutant huntingtin have characteristics of phase-separated compartments: they are mobile, ovoid, and the contents are diffusible. We have used molecular genetics and quantitative confocal microscopy to probe the relationship between concentration and inclusion growth in vivo. Our analysis and modeling of the growth of mutant huntingtin inclusion bodies (mHtt IBs) suggests that there is a cytoplasmic threshold concentration that triggers the formation of an IB, regardless of proteasome capacity, and that reduction in cytoplasmic mHtt causes IBs to shrink. These findings confirm that the IB is a phase-separated compartment that continuously exchanges material with the cytoplasm. The growth rate of the IB is most consistent with a model in which material is incorporated through collision with the IB. A small remnant of the IB is relatively long-lasting, suggesting that the IB contains a core that is structurally distinct, and which may serve to nucleate it.