Supplementary Figure 1. Viability of corticostriatal organotypic cultures obtained from wild-typeHdhQ7/Q7 mice (a)Representative images of corticostriatal slices from 8 months-old wild-type mice labeled with calcein AM (green) as a vital dye and propidium iodide (red) as death marker. Slices show large areas of calcein-AM labeled cells with some non-viable cells (red) in which nucleus are stained with propidium iodide. Scale bar, 10 m. (b) Representative images of the mouse striatum infected with GFP-AAV p75shRNA showing GFP expression (green) at 4 weeks post-injection. Each animal received bilateral injections (1,53 x 109 GC) into the striatum according to Bergman coordinates. Coronal sections were co-stained with DAPI (blue) to label nuclei. A high number of cells are transduced in this region as shown by GFP fluorescence. Pictures were taken using epifluorescence microscope (a) 10x magnification (b) 40x magnification

Supplementary Figure 2. Overexpression of p75NTR in wild-typeSTHdh7/7Q and mutant STHdh111/111Q cells.(a) Representative immunoblot showing TrkB levels and -tubulin as a loading control in striatal extracts obtained from wild type (HdhQ7/7) and mutant (HdhQ111/111) mice, from stably transfected TrkB murine cell line T48 and from SN56 cellsthat express negligible TrkB. (b) Representative immunoblots showing TrkB and p75NTR levels and -tubulin as a loading control in GFP and GFP-p75 transfected wild-type (ST7/7Q) and mutant (ST111/111Q) huntingtin striatal cells. Notice that all mutant huntingtintransfected cells exhibit lower TrkB levels compared with wild-type cells as we previously reported (Gines et al. 2010). No basal expression of p75NTR was detected.(c) Representative immunoblots showing levels of Akt, p-Akt, ERK1/2, p-ERK1/2 and -tubulin as a loading control in wild-type (ST7/7Q) and mutant (ST111/111Q) huntingtin cells transfected with GFP or GFP-p75. Histograms represent the relative p-Akt/Akt and p-ERK/ERK ratios expressed as percentage of control cells (vehicle). Values are given as mean ± SEM of five independent experiments. Data was analyzed by Student’st-test.**P< 0.01 vswild-type cells.

Supplementary Figure 3 Schematic summary of the signaling cascades activated by BDNF and NMDA in mutant huntingtin striatal cells. (a) The imbalance between p75NTR and TrkB in mutant huntingtin striatal cells promotes BDNF activation of both pro-apoptotic (JNK signaling) and pro-survival (Akt signaling) cascades that in concertwith the lack of BDNF-mediated activation of ERK1/2 phosphorylation (3) might contribute to striatal cell death. (b) This scenario can still be more complex since a dysfunctional crosstalk between TrkB, p75NTR and NMDA was demonstrated.As a result, PP1 levels were increased leading to reduction on Akt phosphorylation which was associated withNMDA-induced striatal cell death revealed by caspase-3 activation. This model is supported by the ability of PP1 inhibition to reverse the decrease on Akt phosphorylation and prevents NMDA and/or BDNF-mediated striatal cell death.

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