Figure 2. PolyQ expansion destabilizes diffuse mHtt^ex1 and full-length Htt.

(a) Automated optical pulse label of striatal neurons expressing wild-type (WT) or disease-associated Htt^ex1 polyQ stretches. Scale bars, 50 μm. (b) Clearance curves of red fluorescence emitted by photoswitched mHtt^ex1-Q₂₅-Dendra2 and mHtt^ex1-Q₄₆-Dendra2. Diffuse mHtt has a shortened mean lifetime, as shown by decay of red fluorescence. There were three representative neurons per group. (c) Mean lifetimes of wild-type Htt^ex1 and mHtt^ex1 vary between neurons. P < 0.001 (Student's t-test); data represent mean ± s.d. (Supplementary Fig. 2a–c); n > 100 striatal neurons analyzed from three independent experiments. Neurons that formed inclusion bodies during the experiment were excluded from analysis. (d) PolyQ expansion shortens the mean lifetime of Htt in neurons. Previous work showed no detectable aggregation of mHtt in embryonic forebrain neurons from Hdh150 knock-in mice during a short window after birth. Homozygous mutant and wild-type cultures were subjected to metabolic labeling and pulse-chase analysis. The more rapid disappearance of mutant relative to wild-type Htt indicates that mHtt has a shorter mean lifetime. The mean lifetimes (92 h for wild-type Htt and 39 h for mHtt) are averages from two experiments.