Figure 2: Prion-like transfer of HttQ91 aggregates from ORNs to the cytoplasm of glia.

(a) HttQ25 aggregates when co-expressed with HttQ91 in glia. Maximum intensity Z-projections of one antennal lobe from 7-day-old adult flies expressing HttQ91-mCherry alone, HttQ25-YFP alone, HttQ91-mCherry and HttQ25-GFP together, or HttQ91-mCherry and GFP together in glia. The approximate boundary of each antennal lobe (‘AL’) is indicated by the dotted lines. Scale bars, 10 μm. (b) Induced aggregation of HttQ25 in glia by expression of HttQ91 in DA1 ORNs. Maximum intensity Z-projections of the antennal lobe from adult flies of the indicated age expressing HttQ91-mCherry in DA1 ORNs and HttQ25-YFP in glia, without or with antennal injury performed on day 1. Approximate boundaries of antennal lobes (AL; dotted line) and DA1 glomeruli (DA1g; solid line) are drawn. Because larger HttQ91 aggregates lacking HttQ25 dominate the red signal, co-localization between HttQ25 and smaller HttQ91 aggregates is underrepresented in the merged images. See d for a demonstration of co-localization of HttQ25 with HttQ91 in a single Z-slice. Scale bars, 10 μm. (c) Quantification of glial HttQ25 puncta that were present as the flies aged. The average numbers of HttQ25-YFP puncta detected in each antennal lobe without or with antennal injury were graphed as a function of time (ND, not determined; n≥10, ±s.e.m., *P<0.05, ***P<0.0005, Student’s t-test). (d) A single 1.0-μm optical Z-slice from the DA1 glomerulus of a 7-day-old fly with the same genotype as in (b,c). Arrows indicate HttQ25 aggregates (green) that co-localize with HttQ91 signal (red). Scale bar=5 μm. (e) YFP and mCherry fluorescence from a single HttQ25- and HttQ91-containing aggregate before (top) and after (bottom) photobleaching of the mCherry signal. Pixel-by-pixel FRET efficiency is shown in the heat map image and associated histogram. The mean FRET efficiency of the aggregate shown was ∼28%. Scale bar, 1 μm. (f) Histogram showing the distribution of diameters for all HttQ91-mCherry puncta detected in the DA1 glomerulus of 7-day-old flies (n=204, red bars), and for those HttQ91-mCherry puncta that co-localized with HttQ25-YFP (green bars).