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Stochastic kinetics of intracellular huntingtin aggregate formation

Nature Chemical Biology volume 2, pages 319–323 (2006)Cite this article

Abstract

Neurodegeneration in Huntington disease is described by neuronal loss in which the probability of cell death remains constant with time1. However, the quantitative connection between the kinetics of cell death and the molecular mechanism initiating neurodegeneration remains unclear. One hypothesis is that nucleation of protein aggregates containing exon I fragments of the mutant huntingtin protein (mhttex1), which contains an expanded polyglutamine region in patients with the disease, is the explanation for the infrequent but steady occurrence of neuronal death, resulting in adult onset of the disease2. Recent in vitro evidence suggests that sufficiently long polyglutamine peptides undergo a unimolecular conformational change to form a nucleus that seeds aggregation3. Here we use this nucleation mechanism as the basis to derive a stochastic mathematical model describing the probability of aggregate formation in cells as a function of time and mhttex1 protein concentration, and validate the model experimentally. These findings suggest that therapeutic strategies for Huntington disease predicated on reducing the rate of mhttex1 aggregation need only make modest reductions in huntingtin expression level to substantially increase the delay time until aggregate formation.

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Figure 1: Hypothesized mechanism of httex1 aggregate formation3 (Models 1, 1a and 1b in Supplementary Methods).
Figure 2: FACS sorting and fluorescence microscopy of cell populations were used to determine rates of aggregate formation.
Figure 3: Rate of httex1-GFP aggregate formation as a function of time, concentration and polyglutamine length.
Figure 4: Simulated fraction of neurons that do not experience an aggregate nucleation event as a function of concentration (10, 30, 100 or 300 nM) and age for httex1Q47, based on Model 1a or 2, and assuming neurons of volume equivalent to cultured ST14A cells.

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Acknowledgements

We thank the US National Science Foundation Graduate Research Fellowship program (D.W.C.), the Hereditary Disease Foundation and the HighQ Foundation for financial support of this work.

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Authors and Affiliations

  1. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA

    David W Colby & K Dane Wittrup

  2. Department of Biology, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA

    John P Cassady, Grace C Lin & Vernon M Ingram

  3. Biological Engineering Division, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA

    K Dane Wittrup

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Correspondence to K Dane Wittrup.

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Supplementary information

Supplementary Fig. 1

Concentration of httex1Q72-GFP in individual cells during the 18 hour period following sorting, measured by time-lapse fluorescence microscopy. (PDF 456 kb)

Supplementary Methods (PDF 1461 kb)

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Colby, D., Cassady, J., Lin, G. et al. Stochastic kinetics of intracellular huntingtin aggregate formation. Nat Chem Biol 2, 319–323 (2006). https://doi.org/10.1038/nchembio792

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