In Huntington disease, polyglutamine expansion of the protein huntingtin (Htt) leads to selective neurodegenerative loss of medium spiny neurons throughout the striatum by an unknown apoptotic mechanism. Binding of Hip-1, a protein normally associated with Htt, is reduced by polyglutamine expansion. Free Hip-1 binds to a hitherto unknown polypeptide, Hippi (Hip-1 protein interactor), which has partial sequence homology to Hip-1 and similar tissue and subcellular distribution. The availability of free Hip-1 is modulated by polyglutamine length within Htt, with disease-associated polyglutamine expansion favouring the formation of pro-apoptotic Hippi–Hip-1 heterodimers. This heterodimer can recruit procaspase-8 into a complex of Hippi, Hip-1 and procaspase-8, and launch apoptosis through components of the 'extrinsic' cell-death pathway. We propose that Htt polyglutamine expansion liberates Hip-1 so that it can form a caspase-8 recruitment complex with Hippi. This novel non-receptor-mediated pathway for activating caspase-8 might contribute to neuronal death in Huntington disease.

Huntington's disease is a neurodegenerative disorder caused by CAG expansion that results in expansion of a polyglutamine tract at the extreme N terminus of huntingtin (htt). htt with polyglutamine expansion is proapoptotic in different cell types. Here, we show that caspase inhibitors diminish the toxicity of htt. Additionally, we define htt itself as an important caspase substrate by generating a site-directed htt mutant that is resistant to caspase-3 cleavage at positions 513 and 530 and to caspase-6 cleavage at position 586. In contrast to cleavable htt, caspase-resistant htt with an expanded polyglutamine tract has reduced toxicity in apoptotically stressed neuronal and nonneuronal cells and forms aggregates at a much reduced frequency. These results suggest that inhibiting caspase cleavage of htt may therefore be of potential therapeutic benefit in Huntington's disease.

Expansion of a polyglutamine sequence in the N terminus of huntingtin is the gain-of-function event that causes Huntington's disease. This mutation affects primarily the medium-size spiny neurons of the striatum. Huntingtin is expressed in many neuronal and non-neuronal cell types, implying a more general function for the wild-type protein. Here we report that wild-type huntingtin acts by protecting CNS cells from a variety of apoptotic stimuli, including serum withdrawal, death receptors, and pro-apoptotic Bcl-2 homologs. This protection may take place at the level of caspase-9 activation. The full-length protein also modulates the toxicity of the poly-Q expansion. Cells expressing full-length mutant protein are susceptible to fewer death stimuli than cells expressing truncated mutant huntingtin.

Clinical GeneticsVolume 56, Issue 2 p. 110-111 HotSpots - Insufficient death: autoimmune lymphoproliferative syndrome Cheryl Wellington, Cheryl Wellington Centre for Molecular Medicine and Therapeutics, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, CanadaTel: +1 604 875 3535; fax: +1 604 875 3840; e-mail: mrh@cmmt.ubc.caSearch for more papers by this authorAbigail S. Hackam, Abigail S. Hackam Centre for Molecular Medicine and Therapeutics, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, CanadaTel: +1 604 875 3535; fax: +1 604 875 3840; e-mail: mrh@cmmt.ubc.caSearch for more papers by this author Cheryl Wellington, Cheryl Wellington Centre for Molecular Medicine and Therapeutics, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, CanadaTel: +1 604 875 3535; fax: +1 604 875 3840; e-mail: mrh@cmmt.ubc.caSearch for more papers by this authorAbigail S. Hackam, Abigail S. Hackam Centre for Molecular Medicine and Therapeutics, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, CanadaTel: +1 604 875 3535; fax: +1 604 875 3840; e-mail: mrh@cmmt.ubc.caSearch for more papers by this author First published: 24 December 2001 https://doi.org/10.1034/j.1399-0004.1999.560202.xRead the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume56, Issue2August 1999Pages 110-111 RelatedInformation

Abstract : X‐linked spinal and bulbar muscular atrophy (SBMA), Kennedy's disease, is a degenerative disease of the motor neurons that is associated with an increase in the number of CAG repeats encoding a polyglutamine stretch within the androgen receptor (AR). Recent work has demonstrated that the gene products associated with open reading frame triplet repeat expansions may be substrates for the cysteine protease cell death executioners, the caspases. However, the role that caspase cleavage plays in the cytotoxicity associated with expression of the disease‐associated alleles is unknown. Here, we report the first conclusive evidence that caspase cleavage is a critical step in cytotoxicity ; the expression of the AR with an expanded polyglutamine stretch enhances its ability to induce apoptosis when compared with the normal AR. The AR is cleaved by a caspase‐3 subfamily protease at Asp146, and this cleavage is increased during apoptosis. Cleavage of the AR at Asp146 is critical for the induction of apoptosis by AR, as mutation of the cleavage site blocks the ability of the AR to induce cell death. Further, mutation of the caspase cleavage site at Asp146 blocks the ability of the SBMA AR to form perinuclear aggregates. These studies define a fundamental role for caspase cleavage in the induction of neural cell death by proteins displaying expanded polyglutamine tracts, and therefore suggest a strategy that may be useful to treat neurodegenrative diseases associated with polyglutamine repeat expansions.

A unifying feature of the CAG expansion diseases is the formation of intracellular aggregates composed of the mutant polyglutamine-expanded protein. Despite the presence of aggregates in affected patients, the precise relationship between aggregates and disease pathogenesis is unresolved. Results from in vivo and in vitro studies of mutant huntingtin have lead to the hypothesis that nuclear localization of aggregates is critical for the pathology of Huntington's disease (HD). We tested this hypothesis using a 293T cell culture model system that compared the frequency and toxicity of cytoplasmic and nuclear huntingtin aggregates. We first assessed the mode of nuclear transport of N-terminal fragments of huntingtin, and show that the predicted endogenous NLS is not functional, providing data in support of passive nuclear transport. This result suggests that proteolysis is a necessary step for nuclear entry of huntingtin. Additionally, insertion of nuclear import or export sequences into huntingtin fragments containing 548 or 151 amino acids was used to reverse the normal localization of these proteins. Changing the subcellular localization of the fragments did not influence their total aggregate frequency. There were also no significant differences in toxicity associated with the presence of nuclear compared with cytoplasmic aggregates. The findings of nuclear and cytoplasmic aggregates in affected brains, together with these in vitro data, support the nucleus and cytosol as subcellular sites for pathogenesis in HD.

It is unclear how polyglutamine expansion is associated with the pathogenesis of Huntington disease (HD). Here. we provide evidence that polyglutamine expansion leads to the formation of large intracellular aggregates in vitro and in vivo. In vitro these huntingtin-containing aggregates disrupt normal cellular architecture and increase in frequency with polyglutamine length. Huntingtin truncated at nucleotide 1955, close to the caspase-3 cleavage site, forms perinuclear aggregates more readily than full-length huntingtin and increases the susceptibility of cells to death following apoptotic stimuli. Further truncation of huntingtin to nucleotide 436 results in both intranuclear and perinuclear aggregates. For a given protein size, increasing polyglutamine length is associated with increased cellular toxicity. Asymptomatic transgenic mice expressing full-length huntingtin with 138 polyglutamines form exclusively perinuclear aggregates in neurons. These data support the hypothesis that proteolytic cleavage of mutant huntingtin leads to the development of aggregates which compromise cell viability, and that their localization is influenced by protein length.

The GABA rho 1 subunit, cloned from a human retina library, can form homooligomeric receptors with properties similar to GABAc receptors characterized in retinal cells. The divalent cation Zn2+, abundant in the CNS and retina, was found to inhibit GABA rho 1 receptors in a voltage-independent manner. Varying the extracellular pH from 7.4 to 5.6 significantly reduced this inhibitory effect. This pH profile suggested that one or more histidine residues might play a role in the interaction between Zn2+ and the GABA rho 1 receptor. Site-directed mutagenesis revealed that a single histidine residue (His 156) in the putative extracellular domain of rho 1 was critical for Zn2+ sensitivity. Substitution of this amino acid with tyrosine (H156Y) created a functional GABA receptor with agonist and channel properties indistinguishable from wildtype. However, the H156Y mutant was insensitive to Zn2+, even at concentrations as high as 1 mM. Mutation to aspartic acid, an amino acid that can interact with Zn2+ in other proteins, preserved sensitivity to Zn2+ but abolished the pH-dependent effect. This histidine residue is also involved in Ni2+ and Cd2+ interaction since the H156Y mutation completely suppressed the inhibition effects of these two cations. These data demonstrate that an extracellular histidine residue is critical for transition metal cation sensitivity of GABA rho 1 receptors.