A genome-scale RNA-interference screen identifies RRAS signaling as a pathologic feature of Huntington's disease.

A genome-scale RNAi screen was performed in a mammalian cell-based assay to identify modifiers of mutant huntingtin toxicity. Ontology analysis of suppressor data identified processes previously implicated in Huntington's disease, including proteolysis, glutamate excitotoxicity, and mitochondrial dysfunction. In addition to established mechanisms, the screen identified multiple components of the RRAS signaling pathway as loss-of-function suppressors of mutant huntingtin toxicity in human and mouse cell models. Loss-of-function in orthologous RRAS pathway members also suppressed motor dysfunction in a Drosophila model of Huntington's disease. Abnormal activation of RRAS and a down-stream effector, RAF1, was observed in cellular models and a mouse model of Huntington's disease. We also observe co-localization of RRAS and mutant huntingtin in cells and in mouse striatum, suggesting that activation of R-Ras may occur through protein interaction. These data indicate that mutant huntingtin exerts a pathogenic effect on this pathway that can be corrected at multiple intervention points including RRAS, FNTA/B, PIN1, and PLK1. Consistent with these results, chemical inhibition of farnesyltransferase can also suppress mutant huntingtin toxicity. These data suggest that pharmacological inhibition of RRAS signaling may confer therapeutic benefit in Huntington's disease.

Huntingtin interacting proteins are genetic modifiers of neurodegeneration.

Huntington's disease (HD) is a fatal neurodegenerative condition caused by expansion of the polyglutamine tract in the huntingtin (Htt) protein. Neuronal toxicity in HD is thought to be, at least in part, a consequence of protein interactions involving mutant Htt. We therefore hypothesized that genetic modifiers of HD neurodegeneration should be enriched among Htt protein interactors. To test this idea, we identified a comprehensive set of Htt interactors using two complementary approaches: high-throughput yeast two-hybrid screening and affinity pull down followed by mass spectrometry. This effort led to the identification of 234 high-confidence Htt-associated proteins, 104 of which were found with the yeast method and 130 with the pull downs. We then tested an arbitrary set of 60 genes encoding interacting proteins for their ability to behave as genetic modifiers of neurodegeneration in a Drosophila model of HD. This high-content validation assay showed that 27 of 60 orthologs tested were high-confidence genetic modifiers, as modification was observed with more than one allele. The 45% hit rate for genetic modifiers seen among the interactors is an order of magnitude higher than the 1%-4% typically observed in unbiased genetic screens. Genetic modifiers were similarly represented among proteins discovered using yeast two-hybrid and pull-down/mass spectrometry methods, supporting the notion that these complementary technologies are equally useful in identifying biologically relevant proteins. Interacting proteins confirmed as modifiers of the neurodegeneration phenotype represent a diverse array of biological functions, including synaptic transmission, cytoskeletal organization, signal transduction, and transcription. Among the modifiers were 17 loss-of-function suppressors of neurodegeneration, which can be considered potential targets for therapeutic intervention. Finally, we show that seven interacting proteins from among 11 tested were able to co-immunoprecipitate with full-length Htt from mouse brain. These studies demonstrate that high-throughput screening for protein interactions combined with genetic validation in a model organism is a powerful approach for identifying novel candidate modifiers of polyglutamine toxicity.

Matrix metalloproteinases are modifiers of huntingtin proteolysis and toxicity in Huntington's disease.

Proteolytic cleavage of huntingtin (Htt) is known to be a key event in the pathogenesis of Huntington's disease (HD). Our understanding of proteolytic processing of Htt has thus far focused on the protease families-caspases and calpains. Identifying critical proteases involved in Htt proteolysis and toxicity using an unbiased approach has not been reported. To accomplish this, we designed a high-throughput western blot-based screen to examine the generation of the smallest N-terminal polyglutamine-containing Htt fragment. We screened 514 siRNAs targeting the repertoire of human protease genes. This screen identified 11 proteases that, when inhibited, reduced Htt fragment accumulation. Three of these belonged to the matrix metalloproteinase (MMP) family. One family member, MMP-10, directly cleaves Htt and prevents cell death when knocked down in striatal Hdh(111Q/111Q) cells. Correspondingly, MMPs are activated in HD mouse models, and loss of function of Drosophila homologs of MMPs suppresses Htt-induced neuronal dysfunction in vivo.

Huntingtin phosphorylation sites mapped by mass spectrometry. Modulation of cleavage and toxicity.

Huntingtin (Htt) is a large protein of 3144 amino acids, whose function and regulation have not been well defined. Polyglutamine (polyQ) expansion in the N terminus of Htt causes the neurodegenerative disorder Huntington disease (HD). The cytotoxicity of mutant Htt is modulated by proteolytic cleavage with caspases and calpains generating N-terminal polyQ-containing fragments. We hypothesized that phosphorylation of Htt may modulate cleavage and cytotoxicity. In the present study, we have mapped the major phosphorylation sites of Htt using cell culture models (293T and PC12 cells) expressing full-length myc-tagged Htt constructs containing 23Q or 148Q repeats. Purified myc-tagged Htt was subjected to mass spectrometric analysis including matrix-assisted laser desorption/ionization mass spectrometry and nano-HPLC tandem mass spectrometry, used in conjunction with on-target alkaline phosphatase and protease digestions. We have identified more than six novel serine phosphorylation sites within Htt, one of which lies in the proteolytic susceptibility domain. Three of the sites have the consensus sequence for ERK1 phosphorylation, and addition of ERK1 inhibitor blocks phosphorylation at those sites. Other observed phosphorylation sites are possibly substrates for CDK5/CDC2 kinases. Mutation of amino acid Ser-536, which is located in the proteolytic susceptibility domain, to aspartic acid, inhibited calpain cleavage and reduced mutant Htt toxicity. The results presented here represent the first detailed mapping of the phosphorylation sites in full-length Htt. Dissection of phosphorylation modifications in Htt may provide clues to Huntington disease pathogenesis and targets for therapeutic development.

Progressive phenotype and nuclear accumulation of an amino-terminal cleavage fragment in a transgenic mouse model with inducible expression of full-length mutant huntingtin.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized behaviorally by chorea, incoordination, and shortened lifespan and neuropathologically by huntingtin inclusions and neuronal degeneration. In order to facilitate studies of pathogenesis and therapeutics, we have generated a new inducible mouse model of HD expressing full-length huntingtin (Htt) using a tetracycline-regulated promoter. In double transgenic mice Htt was expressed widely in the brain under the control of the tet-transactivator (tTA) driven by the prion promoter PrP (in the absence of doxycycline). Mice expressing full-length mutant Htt, but not full-length normal Htt, displayed a progressive behavioral phenotype, consisting of slowed and irregular voluntary movements, gait ataxia, tremor and jerky movements, incoordination, and weight loss, with a shortened lifespan. Neuropathology included prominent intranuclear inclusions in cortex and striatum as well as cytoplasmic aggregates. This phenotype is very similar to the phenotypes of previous transgenic mice expressing N-terminal fragments of mutant Htt. The current HD-transgenic mice had nuclear accumulation of Htt, particularly an approximately 60-kDa fragment, which appears to represent an N-terminal cleavage product. This fragment is smaller than calpain or caspase-derived cleavage products of Htt, but it is comparable to a product, termed cp-A, which accumulates in nuclei of cells in a previously described cell model. This new mouse model may be useful in the future for pathogenic and preclinical therapeutic studies related to HD. The data suggest that proteolytic processing could be a part of the pathogenesis of HD, potentially representing an attractive therapeutic target.