Neuron NeuroResource Network Organization of the Huntingtin Proteomic Interactome in Mammalian Brain Dyna I. Shirasaki,1,8,10 Erin R. Greiner,1,8,10 Ismael Al-Ramahi,9 Michelle Gray,1,6,7,11 Pinmanee Boontheung,8 Daniel H. Geschwind,1,2,3,6,7 Juan Botas,9 Giovanni Coppola,1,3,6,7 Steve Horvath,2,4 Joseph A. Loo,5,6,8,* and X. William Yang1,6,7,* 1Center for Neurobehavioral Genetics, Semel Institute for Neuroscience and Human Behavior, Department of Psychiatry and Biobehavioral Sciences 2Department of Human Genetics 3Department of Neurology, Program in Neurogenetics 4Department of Biostatistics 5Department of Biological Chemistry 6David Geffen School of Medicine at UCLA 7Brain Research Institute 8Department of Chemistry and Biochemistry University of California, Los Angeles, CA 90095, USA 9Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX 77030, USA 10These authors contributed equally to the work 11Present address: Department of Neurology, Center for Neurodegeneration and Experimental Therapeutics, University of Alabama at Birmingham, Birmingham AL 35294, USA *Correspondence: [email protected] (J.A.L.), [email protected] (X.W.Y.) http://dx.doi.org/10.1016/j.neuron.2012.05.024 SUMMARY of the striatal medium spiny neurons (MSNs) and, to a lesser extent, the deep layer cortical pyramidal neurons (Vonsattel We used affinity-purification mass spectrometry to and DiFiglia, 1998). The disease is caused by a CAG repeat identify 747 candidate proteins that are complexed expansion resulting in an elongated polyglutamine (polyQ) with Huntingtin (Htt) in distinct brain regions and stretch near the N terminus of Huntingtin (Htt) (The Huntington’s ages in Huntington’s disease (HD) and wild-type Disease Collaborative Research Group, 1993). HD is one of nine mouse brains. To gain a systems-level view of the polyQ disorders with shared molecular genetic features, such as Htt interactome, we applied Weighted Correlation an inverse relationship between the expanded repeat length and the age of disease onset, and evidence for toxic gain-of-function Network Analysis to the entire proteomic data set as a result of the polyQ expansion (Orr and Zoghbi, 2007). to unveil a verifiable rank of Htt-correlated proteins However, each of the polyQ disorders appears to target a distinct and a network of Htt-interacting protein modules, subset of neurons in the brain leading to disease-specific symp- with each module highlighting distinct aspects of toms. Hence, it is postulated that molecular determinants Htt biology. Importantly, the Htt-containing module beyond the polyQ repeat itself may be critical to disease patho- is highly enriched with proteins involved in 14-3-3 genesis (Orr and Zoghbi, 2007). signaling, microtubule-based transport, and proteo- Protein-interacting cis-domains (Lim et al., 2008) and post- stasis. Top-ranked proteins in this module were vali- translational modifications (PTMs) of polyQ proteins (Emamian dated as Htt interactors and genetic modifiers in an et al., 2003; Gu et al., 2009) can significantly modify disease path- HD Drosophila model. Our study provides a compen- ogenesis in vivo. Thus, studying the proteins that interact with dium of spatiotemporal Htt-interacting proteins in domains beyond the polyQ region may provide important clues to disease mechanisms. In the case of HD, several hundred puta- the mammalian brain and presents an approach for tive Htt interactors have been discovered using ex vivo methods, analyzing proteomic interactome data sets to build such as yeast two-hybrid (Y2H) or in vitro affinity pull-down in vivo protein networks in complex tissues, such assays, utilizing only small N-terminal fragments of Htt (Goehler as the brain. et al., 2004; Kaltenbach et al., 2007). Such studies have provided insight into Htt’s normal function as a scaffolding protein involved in vesicular and axonal transport and nuclear transcription (Cav- INTRODUCTION iston and Holzbaur, 2009; Li and Li, 2006). The caveats of the prior Htt interactome studies include the exclusive use of small Htt Huntington’s disease (HD) is one of the most common domi- N-terminal fragments as baits and the isolation of interactors nantly inherited neurodegenerative disorders clinically charac- ex vivo. Hence, it is not known which proteins can complex terized by a triad of movement disorder, cognitive dysfunction, with full-length Htt (fl-Htt) in vivo within distinct brain regions and psychiatric impairment (Bates et al., 2002). HD neuropa- and at different ages. Such information may shed light on age- thology is characterized by selective and massive degeneration dependent, selective neuropathogenesis in HD. Neuron 75, 41–57, July 12, 2012 ª2012 Elsevier Inc. 41 Neuron Huntingtin Interactome Network in Mammalian Brain Figure 1. Generation and Characterization of a Full-Length Htt In Vivo Proteomic Interactome from BACHD and Wild-Type Mouse Brains (A) Immunoprecipitation using a human Htt-selective antibody (aa 1,844–2,131, clone HDB4E10) followed by western blot analysis using a human and mouse Htt antibody (MAB2166) confirms full-length Htt is pulled down in both 2-month WT and 2- and 12-month BACHD soluble cortical extracts. A negative control (no Htt antibody) and 5% input loading control were also included. (B) The affinity purification followed by mass spectrometry (AP-MS) proteomic approach used to identify endogenous fl-Htt protein interactors: (1) The cortex, striatum, and cerebellum of 2- and 12-month WT and BACHD mice were homogenized; (2) the soluble lysates were subjected to anti-Htt IP using HDB4E10 and Protein G Dynabeads; (3) the immunoprecipitates were eluted with LDS loading buffer, separated on an SDS-PAGE gel, each gel was cut into 24–27 gel bands, and each gel band was subjected to in-gel trypsin digestion; (4) approximately 750 independent LC-MS/MS analysis were performed, and (5) all mass spectra were searched using MASCOT (Matrix Science) to obtain a total of 747 high-scoring, putative Htt-interacting proteins. 42 Neuron 75, 41–57, July 12, 2012 ª2012 Elsevier Inc. Neuron Huntingtin Interactome Network in Mammalian Brain Immunoaffinity purification of native protein complexes AP-MS strategy of using HDB4E10 should be considered as a followed by identification of its individual components using survey of in vivo Htt-complexed proteins regardless of Htt polyQ mass spectrometry (MS) has emerged as a powerful tool for length. This is a reasonable strategy since full-length human deciphering in vivo neuronal signaling (Husi et al., 2000), and mHtt can fully substitute the essential function of murine Htt in synaptic and disease-related interactomes (Selimi et al., 2009; murine embryonic development (Gray et al., 2008), and hence Ferna´ ndez et al., 2009). Although a ‘‘shotgun’’ proteomic both forms should share the majority of in vivo interacting approach is useful in creating a list of native-interacting protein proteins. candidates from relevant mammalian tissues, formidable chal- Human and/or murine fl-Htt complexes were immunoaffinity lenges exist in the unbiased bioinformatic analyses of such purified from the cortex, striatum, and cerebellum of BACHD complex proteomic data sets to identify high-confidence inter- and WT mice at 2 or 12 months of age using HDB4E10 actors and to build accurate, endogenous protein interaction (Figure 1B). As a negative control, a mock IP for each sample networks (Liao et al., 2009). condition (defined by specific brain region, age, and genotype) In this study, we performed a spatiotemporal in vivo proteomic was performed in the absence of the Htt antibody. Therefore, interactome study of fl-Htt using dissected brain regions from a total of 30 independent IP experiments were performed, and a mouse model for HD and wild-type controls. The BACHD approximately 765 trypsin-digested gel slices were subjected mouse model used in the study expresses full-length human to LC-MS/MS analysis (Figure 1C). Using the MASCOT (Matrix mutant Htt (mHtt) with 97Q under the control of human Htt Science) sequence database-searching tool, we identified a total genomic regulatory elements on a BAC transgene (Gray et al., of 747 high-scoring, putative Htt-interacting proteins from 2008). BACHD mice exhibit multiple disease-like phenotypes BACHD and WT mouse brains (Table S1 available online). over the course of 12 months, including progressive motor, Consistent with the claim that we are surveying fl-Htt-interacting cognitive, and psychiatric-like deficits and selective cortical proteins, we identified Htt peptides spanning the entire and striatal atrophy (Gray et al., 2008; Menalled et al., 2009). sequence of Htt in both BACHD and WT samples (Figure 1D Our multidimensional affinity purification-mass spectrometry and Table S2). As confirmation that HDB4E10 more efficiently (AP-MS) study uncovered a total of 747 candidate proteins com- immunoprecipitates human Htt than murine Htt, we observed plexed with fl-Htt in the mammalian brain. Moreover, we applied more Htt peptides and more extensive sequence coverage of WGCNA to analyze the entire fl-Htt interactome data set to fl-Htt in BACHD mice compared to WT mice. define a verifiable rank of Htt-interacting proteins and to uncover the organization of in vivo fl-Htt-interacting protein networks The In Vivo fl-Htt Interactome Is Enriched with Known in the mammalian brain. Htt Interactors and Proteins Involved in Pathways Relevant to HD RESULTS We next performed several standard bioinformatic analyses to determine the validity of our approach in isolating Htt interactors