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.

Caspase-6 activity in a BACHD mouse modulates steady-state levels of mutant huntingtin protein but is not necessary for production of a 586 amino acid proteolytic fragment.

Huntington's disease (HD) is caused by a mutation in the huntingtin (htt) gene encoding an expansion of glutamine repeats at the N terminus of the Htt protein. Proteolysis of Htt has been identified as a critical pathological event in HD models. In particular, it has been postulated that proteolysis of Htt at the putative caspase-6 cleavage site (at amino acid Asp-586) plays a critical role in disease progression and pathogenesis. However, whether caspase-6 is indeed the essential enzyme that cleaves Htt at this site in vivo has not been determined. To evaluate, we crossed the BACHD mouse model with a caspase-6 knock-out mouse (Casp6(-/-)). Western blot and immunocytochemistry confirmed the lack of caspase-6 protein in Casp6(-/-) mice, regardless of HD genotype. We predicted the Casp6(-/-) mouse would have reduced levels of caspase-6 Htt fragments and increased levels of full-length Htt protein. In contrast, we found a significant reduction of full-length mutant Htt (mHtt) and fragments in the striatum of BACHD Casp6(-/-) mice. Importantly, we detected the presence of Htt fragments consistent with cleavage at amino acid Asp-586 of Htt in the BACHD Casp6(-/-) mouse, indicating that caspase-6 activity cannot fully account for the generation of the Htt 586 fragment in vivo. Our data are not consistent with the hypothesis that caspase-6 activity is critical in generating a potentially toxic 586 aa Htt fragment in vivo. However, our studies do suggest a role for caspase-6 activity in clearance pathways for mHtt protein.

Proteolysis of mutant huntingtin produces an exon 1 fragment that accumulates as an aggregated protein in neuronal nuclei in Huntington disease.

Huntingtin proteolysis has been implicated in the molecular pathogenesis of Huntington disease (HD). Despite an intense effort, the identity of the pathogenic smallest N-terminal fragment has not been determined. Using a panel of anti-huntingtin antibodies, we employed an unbiased approach to generate proteolytic cleavage maps of mutant and wild-type huntingtin in the HdhQ150 knock-in mouse model of HD. We identified 14 prominent N-terminal fragments, which, in addition to the full-length protein, can be readily detected in cytoplasmic but not nuclear fractions. These fragments were detected at all ages and are not a consequence of the pathogenic process. We demonstrated that the smallest fragment is an exon 1 huntingtin protein, known to contain a potent nuclear export signal. Prior to the onset of behavioral phenotypes, the exon 1 protein, and possibly other small fragments, accumulate in neuronal nuclei in the form of a detergent insoluble complex, visualized as diffuse granular nuclear staining in tissue sections. This methodology can be used to validate the inhibition of specific proteases as therapeutic targets for HD by pharmacological or genetic approaches.

Calpain-1 cleaves and activates caspase-7.

Caspase-7 is an executioner caspase that plays a key role in apoptosis, cancer, and a number of neurodegenerative diseases. The mechanism of caspase-7 activation by granzyme B and caspase-3 has been well characterized. However, whether other proteases such as calpains activate or inactivate caspase-7 is not known. Here, we present that recombinant caspase-7 is directly cleaved by calpain-1 within the large subunit of caspase-7 to produce two novel products, large subunit p18 and p17. This new form of caspase-7 has a 6-fold increase in V(max) when compared with the previously characterized p20/p12 form. Zymography revealed that the smaller caspase-7 product (p17) is 18-fold more active than either the caspase-3-cleaved product (p20) or the larger calpain-1 product of caspase-7 (p18). Mass spectrometry and site-directed mutagenesis identified the calpain cleavage sites within the caspase-7 large subunit at amino acid 36 and 45/47. These proteolysis events occur in vivo as indicated by the accumulation of caspase-7 p18 and p17 subunits in cortical neurons undergoing Ca(2+) dysregulation. Further, cleavage at amino acid 45/47 of caspase-7 by calpain results in a reduction in nuclear localization when compared with the caspase-3 cleavage product of caspase-7 (p20). Our studies suggest the calpain-activated form of caspase-7 has unique enzymatic activity, localization, and binding affinity when compared with the caspase-activated form.

Calpain activation in Huntington's disease.

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG expansion that results in elongation of the polyglutamine tract at the N terminus of huntingtin (Htt). Abnormal proteolytic processing of mutant Htt has been implicated as a critical step in the initiation of HD. The protease(s) involved in this process has not been fully characterized. Here we report that activated calpain was detected in the caudate of human HD tissue but not in age-matched controls. In addition, one of the major N-terminal Htt proteolytic fragments found in human HD tissue appears to be derived from calpain cleavage. Htt fragments in HD lysates were similar in size to those produced by exposure of in vitro-translated Htt to exogenous calpain. Incubation of in vitro-translated Htt with calpain generated a cascade of cleavage events with an initial intermediate cleavage product at 72 kDa and a final cleavage product at 47 kDa. The rate of cleavage of Htt by calpain was polyglutamine-length-dependent. These results suggest that cleavage of Htt in human HD tissue is mediated in part by the Ca2+-activated neutral protease, calpain.

Caspase cleavage of mutant huntingtin precedes neurodegeneration in Huntington's disease.

Huntington's disease (HD) results from polyglutamine expansion in huntingtin (htt), a protein with several consensus caspase cleavage sites. Despite the identification of htt fragments in the brain, it has not been shown conclusively that htt is cleaved by caspases in vivo. Furthermore, no study has addressed when htt cleavage occurs with respect to the onset of neurodegeneration. Using antibodies that detect only caspase-cleaved htt, we demonstrate that htt is cleaved in vivo specifically at the caspase consensus site at amino acid 552. We detect caspase-cleaved htt in control human brain as well as in HD brains with early grade neuropathology, including one homozygote. Cleaved htt is also seen in wild-type and HD transgenic mouse brains before the onset of neurodegeneration. These results suggest that caspase cleavage of htt may be a normal physiological event. However, in HD, cleavage of mutant htt would release N-terminal fragments with the potential for increased toxicity and accumulation caused by the presence of the expanded polyglutamine tract. Furthermore, htt fragments were detected most abundantly in cortical projection neurons, suggesting that accumulation of expanded htt fragments in these neurons may lead to corticostriatal dysfunction as an early event in the pathogenesis of HD.

Non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95) amplifies ionotropic glutamate receptor signaling in embryonic cerebellar granule neurons by a mechanism involving ryanodine receptors.

The mechanisms by which non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95) and rapamycin interact with ryanodine receptor (RyR) complexes to alter Ca2+ signaling, were explored in intact cerebellar granule neurons. PCB 95 (10 microM, 20 min) significantly increased the number of neurons responding to caffeine. PCB 95 sensitization of RyR-mediated responses was further supported by the observations that ryanodine pretreatment blocked response to caffeine and coplanar 2,4,4',5-tetrachlorobiphenyl (PCB 66), which lacks RyR activity, failed to sensitize neurons. PCB 95 did not significantly alter levels of resting cytosolic Ca2+ nor thapsigargin-sensitive Ca2+ stores, suggesting a more complex mechanism than sensitization from increased cytosolic Ca2+ or an increased endoplasmic reticulum/cytosolic Ca2+ gradient. The immunosuppressant, rapamycin, sensitized neurons to caffeine in a manner similar to PCB 95, suggesting a common mechanism. PCB 95 or rapamycin significantly enhanced Ca2+ responses following N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl4-isoxasolepropiate (AMPA) receptor activation. Store depletion or direct block of RyR with ryanodine enhanced responses to NMDA. PCB 95 further enhanced these responses to NMDA. These results suggest that PCB 95 and rapamycin enhance NMDA- and AMPA-mediated Ca2+ signals by modifying a functional association of the FKBP12/RyR complex that results in amplification of glutamate signaling in cultured cerebellar granule neurons in culture.

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.

Identification and evaluation of small molecule pan-caspase inhibitors in Huntington's disease models.

Huntington's Disease (HD) is characterized by a mutation in the huntingtin (Htt) gene encoding an expansion of glutamine repeats on the N terminus of the Htt protein. Numerous studies have identified Htt proteolysis as a critical pathological event in HD postmortem human tissue and mouse HD models, and proteases known as caspases have emerged as attractive HD therapeutic targets. We report the use of the substrate activity screening method against caspase-3 and -6 to identify three novel, pan-caspase inhibitors that block proteolysis of Htt at caspase-3 and -6 cleavage sites. In HD models these irreversible inhibitors suppressed Hdh(111Q/111Q)-mediated toxicity and rescued rat striatal and cortical neurons from cell death. In this study, the identified nonpeptidic caspase inhibitors were used to confirm the role of caspase-mediated Htt proteolysis in HD. These results further implicate caspases as promising targets for HD 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.

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.

FGF-2 promotes neurogenesis and neuroprotection and prolongs survival in a transgenic mouse model of Huntington's disease.

There is no satisfactory treatment for Huntington's disease (HD), a hereditary neurodegenerative disorder that produces chorea, dementia, and death. One potential treatment strategy involves the replacement of dead neurons by stimulating the proliferation of endogenous neuronal precursors (neurogenesis) and their migration into damaged regions of the brain. Because growth factors are neuroprotective in some settings and can also stimulate neurogenesis, we treated HD transgenic R6/2 mice from 8 weeks of age until death by s.c. administration of FGF-2. FGF-2 increased the number of proliferating cells in the subventricular zone by approximately 30% in wild-type mice, and by approximately 150% in HD transgenic R6/2 mice. FGF-2 also induced the recruitment of new neurons from the subventricular zone into the neostriatum and cerebral cortex of HD transgenic R6/2 mice. In the striatum, these neurons were DARPP-32-expressing medium spiny neurons, consistent with the phenotype of neurons lost in HD. FGF-2 was neuroprotective as well, because it blocked cell death induced by mutant expanded Htt in primary striatal cultures. FGF-2 also reduced polyglutamine aggregates, improved motor performance, and extended lifespan by approximately 20%. We conclude that FGF-2 improves neurological deficits and longevity in a transgenic mouse model of HD, and that its neuroprotective and neuroproliferative effects may contribute to this improvement.

Inhibition of calpain cleavage of huntingtin reduces toxicity: accumulation of calpain/caspase fragments in the nucleus.

Huntington's disease (HD) is a neurodegenerative disorder caused by a polyglutamine (polyQ) tract expansion near the N terminus of huntingtin (Htt). Proteolytic processing of mutant Htt and abnormal calcium signaling may play a critical role in disease progression and pathogenesis. Recent work indicates that calpains may participate in the increased and/or altered patterns of Htt proteolysis leading to the selective toxicity observed in HD striatum. Here, we identify two calpain cleavage sites in Htt and show that mutation of these sites renders the polyQ expanded Htt less susceptible to proteolysis and aggregation, resulting in decreased toxicity in an in vitro cell culture model. In addition, we found that calpain- and caspase-derived Htt fragments preferentially accumulate in the nucleus without the requirement of further cleavage into smaller fragments. Calpain family members, calpain-1, -5, -7, and -10, have increased levels or are activated in HD tissue culture and transgenic mouse models, suggesting they may play a key role in Htt proteolysis and disease pathology. Interestingly, calpain-1, -5, -7, and -10 localize to the cytoplasm and the nucleus, whereas the activated forms of calpain-7 and -10 are found only in the nucleus. These results support the role of calpain-derived Htt fragmentation in HD and suggest that aberrant activation of calpains may play a role in HD pathogenesis.