Global Rhes knockout in the Q175 Huntington’s disease mouse model

Huntington’s disease (HD) results from an expansion mutation in the polyglutamine tract in huntingtin. Although huntingtin is ubiquitously expressed in the body, the striatum suffers the most severe pathology. Rhes is a Ras-related small GTP-binding protein highly expressed in the striatum that has been reported to modulate mTOR and sumoylation of mutant huntingtin to alter HD mouse model pathogenesis. Reports have varied on whether Rhes reduction is desirable for HD. Here we characterize multiple behavioral and molecular endpoints in the Q175 HD mouse model with genetic Rhes knockout (KO). Genetic RhesKO in the Q175 female mouse resulted in both subtle attenuation of Q175 phenotypic features, and detrimental effects on other kinematic features. The Q175 females exhibited measurable pathogenic deficits, as measured by MRI, MRS and DARPP32, however, RhesKO had no effect on these readouts. Additionally, RhesKO in Q175 mixed gender mice deficits did not affect mTOR signaling, autophagy or mutant huntingtin levels. We conclude that global RhesKO does not substantially ameliorate or exacerbate HD mouse phenotypes in Q175 mice.

Membrane Interactions Accelerate the Self-Aggregation of Huntingtin Exon 1 Fragments in a Polyglutamine Length-Dependent Manner

The accumulation of aggregated protein is a typical hallmark of many human neurodegenerative disorders, including polyglutamine-related diseases such as chorea Huntington. Misfolding of the amyloidogenic proteins gives rise to self-assembled complexes and fibres. The huntingtin protein is characterised by a segment of consecutive glutamines which, when exceeding ~ 37 residues, results in the occurrence of the disease. Furthermore, it has also been demonstrated that the 17-residue amino-terminal domain of the protein (htt17), located upstream of this polyglutamine tract, strongly correlates with aggregate formation and pathology. Here, we demonstrate that membrane interactions strongly accelerate the oligomerisation and β-amyloid fibril formation of htt17-polyglutamine segments. By using a combination of biophysical approaches, the kinetics of fibre formation is investigated and found to be strongly dependent on the presence of lipids, the length of the polyQ expansion, and the polypeptide-to-lipid ratio. Finally, the implications for therapeutic approaches are discussed.

Htt is a repressor of Abl activity required for APP-induced axonal growth

Huntington’s disease is a progressive autosomal dominant neurodegenerative disorder caused by the expansion of a polyglutamine tract at the N-terminus of a large cytoplasmic protein. The Drosophila huntingtin (htt) gene is widely expressed during all developmental stages from embryos to adults. However, Drosophila htt mutant individuals are viable with no obvious developmental defects. We asked if such defects could be detected in htt mutants in a background that had been genetically sensitized to reveal cryptic developmental functions. Amyloid precursor protein (APP) is linked to Alzheimer’s disease. Appl is the Drosophila APP ortholog and Appl signaling modulates axon outgrowth in the mushroom bodies (MBs), the learning and memory center in the fly, in part by recruiting Abl tyrosine kinase. Here, we find that htt mutations suppress axon outgrowth defects of αβ neurons in Appl mutant MB by derepressing the activity of Abl. We show that Abl is required in MB αβ neurons for their axon outgrowth. Importantly, both Abl overexpression and lack of expression produce similar phenotypes in the MBs, indicating the necessity of tightly regulating Abl activity. We find that Htt behaves genetically as a repressor of Abl activity, and consistent with this, in vivo FRET-based measurements reveal a significant increase in Abl kinase activity in the MBs when Htt levels are reduced. Thus, Appl and Htt have essential but opposing roles in MB development, promoting and suppressing Abl kinase activity, respectively, to maintain the appropriate intermediate level necessary for axon growth.

A polyglutamine domain is required for de novo CIZ1 assembly formation at the inactive X chromosome

SummaryCIP1-interacting zinc finger protein 1 (CIZ1) forms large assemblies at the inactive X chromosome (Xi) in female fibroblasts in an Xist lncRNA-dependent manner. Here we address the requirements for assembly formation, and show that CIZ1 interacts directly with Xist via two independent domains in its N- and C-terminus. Interaction with Xist repeat E, assembly at Xi in cells, and the complexity of self-assemblies formed in vitro, are all modulated by alternatively-spliced exons that include two glutamine-rich prion-like domains (PLD1 and PLD2), both conditionally excluded from the N-terminal domain. Exclusion of PLD1 alone is sufficient to abrogate de novo establishment of new CIZ1 assemblies and Xi territories enriched for H3K27me3 in CIZ1-null fibroblasts. Together the data suggest that PLD1-driven CIZ1 assemblies form at Xi, are nucleated by interaction with Xist and amplified by multivalent interaction with RNA, so implicating a polyglutamine tract in the maintenance of epigenetic state.

Membrane interactions accelerate the self-aggregation of huntingtin exon 1 fragments in a polyglutamine length-dependent manner

ABSTRACTThe accumulation of aggregated protein is a typical hallmark of many human neurodegenerative disorders including Huntington’s disease. Misfolding of the amyloidogenic proteins gives rise to self-assembled complexes and fibers. The huntingtin protein is characterized by a segment of consecutive glutamines, which when exceeding a certain number of residues results in the occurrence of the disease. Furthermore, it has also been demonstrated that the 17-residue amino-terminal domain of the protein (htt17), located upstream of this polyglutamine tract, strongly correlates with aggregate formation and pathology. Here we demonstrate that membrane interactions strongly accelerate the oligomerization and β-amyloid fibril formation of htt17-polyglutamine segments. By using a combination of biophysical approaches the kinetics of fibre formation has been quantitatively investigated and found to be strongly dependent to the presence of lipids, the length of the polyQ expansion and the polypeptide-to-lipid ratio. Finally, the implications for therapeutic approaches are discussed.Statement of significanceThe quantitative analysis of the aggregation kinetics of amino-terminal fragments of huntingtin demonstrate the importance of the 17-residue amino-terminal membrane anchor and a resulting dominant effect of membranes in promoting the aggregation of polyglutamines. Other parameters further modulating the association kinetics are the length of the polyglutamine stretch and the peptide concentration. The findings can have important impact on finding new therapies to treat Huntington’s and other polyglutamine related diseases.

THE PHYSIOPATHOLOGICAL ROLES OF ANDROGENS IN MOTONEURONS

The androgen receptor has been purified in the ‘70s and cloned in the ‘80s. It is a member of the steroid receptor superfamily and mediated the most important effects of androgen in androgen dependent or sensitive tissues. Several physiological function of the brain are differentially controlled in the two sexes and androgens play specific role in the processes of sexual differentiation and it is involved in the maintenance of male sex behaviour in adulthood. When mutated, the androgen receptor may impact on many of these androgen-regulated activities because of a loss of androgenic function in target cells. However, in the case of a peculiar type of mutation, the elongation of the polyglutamine tract normally present in its N-terminus, the androgen receptor becomes neurotoxic and induces cells death of a number of motoneurons in the spinal cord, which express very high level of this protein. Here, we will briefly discuss the most important actions of androgen receptor-mediated androgen activity in the brain and the mechanisms by which the mutant androgen receptor may lead to neurodegeneration in Spinal and Bulbar Muscular Atrophy (SBMA).

Bim contributes to the progression of Huntington’s disease-associated phenotypes

Huntington’s disease (HD) is a neurodegenerative disorder caused by an expanded polyglutamine tract in the huntingtin (HTT) protein. Mutant HTT (mHTT) toxicity is caused by its aggregation/oligomerization. The striatum is the most vulnerable region, although all brain regions undergo neuronal degeneration in the disease. Here we show that the levels of Bim, a BH3-only protein, are significantly increased in HD human post-mortem and HD mouse striata, correlating with neuronal death. Bim reduction ameliorates mHTT neurotoxicity in HD cells. In the HD mouse model, heterozygous Bim knockout significantly mitigates mHTT accumulation and neuronal death, ameliorating disease-associated phenotypes and lifespan. Therefore, Bim could contribute to the progression of HD.