N-terminal Huntingtin Fragments Research Articles

Solid-state nuclear magnetic resonance in the structural study of polyglutamine aggregation.

The aggregation of proteins into amyloid-like fibrils is seen in many neurodegenerative diseases. Recent years have seen much progress in our understanding of these misfolded protein inclusions, thanks to advances in techniques such as solid-state nuclear magnetic resonance (ssNMR) spectroscopy and cryogenic electron microscopy (cryo-EM). However, multiple repeat-expansion-related disorders have presented special challenges to structural elucidation. This review discusses the special role of ssNMR analysis in the study of protein aggregates associated with CAG repeat expansion disorders. In these diseases, the misfolding and aggregation affect mutant proteins with expanded polyglutamine segments. The most common disorder, Huntington's disease (HD), is connected to the mutation of the huntingtin protein. Since the discovery of the genetic causes for HD in the 1990s, steady progress in our understanding of the role of protein aggregation has depended on the integrative and interdisciplinary use of multiple types of structural techniques. The heterogeneous and dynamic features of polyQ protein fibrils, and in particular those formed by huntingtin N-terminal fragments, have made these aggregates into challenging targets for structural analysis. ssNMR has offered unique insights into many aspects of these amyloid-like aggregates. These include the atomic-level structure of the polyglutamine core, but also measurements of dynamics and solvent accessibility of the non-core flanking domains of these fibrils' fuzzy coats. The obtained structural insights shed new light on pathogenic mechanisms behind this and other protein misfolding diseases.

Nuclear translocation of STAT5 initiates iron overload in huntington's disease by up-regulating IRP1 expression.

Mutant huntingtin (mHtt) proteins interact to form aggregates, disrupting cellular functions including transcriptional dysregulation and iron imbalance in patients with Huntington's disease (HD) and mouse disease models. Previous studies have indicated that mHtt may lead to abnormal iron homeostasis by upregulating the expression of iron response protein 1 (IRP1) in the striatum and cortex of N171-82Q HD transgenic mice, as well as in HEK293 cells expressing the N-terminal fragment of mHtt containing 160 CAG repeats. However, the mechanism underlying the upregulation of IRP1 remains unclear. We investigated the levels and phosphorylation status of signal transducer and activator of transcription 5 (STAT5) in the brains of N171-82Q HD transgenic mice using immunohistochemistry staining. We also assessed the nuclear localization of STAT5 protein through western blot and immunofluorescence, and measured the relative RNA expression levels of STAT5 and IRP1 using RT-PCR in both N171-82Q HD transgenic mice and HEK293 cells expressing the N-terminal fragment of huntingtin. Our findings demonstrate that the transcription factor STAT5 regulates the transcription of the IPR1 gene in HEK293 cells. Notably, both the brains of N171-82Q mice and 160Q HEK293 cells exhibited increased nuclear content of STAT5, despite unchanged total STAT5 expression. These results suggest that mHtt promotes the nuclear translocation of STAT5, leading to enhanced expression of IRP1. The nuclear translocation of STAT5 initiates abnormal iron homeostatic pathways, characterized by elevated IRP1 expression, increased levels of transferrin and transferrin receptor, and iron accumulation in the brains of HD mice. These findings provide valuable insights into potential therapeutic strategies targeting iron homeostasis in HD.

Molecular basis of Q-length selectivity for the MW1 antibody–huntingtin interaction

Huntington's disease is caused by a polyglutamine (polyQ) expansion in the huntingtin protein. Huntingtin exon 1 (Httex1), as well as other naturally occurring N-terminal huntingtin fragments with expanded polyQ are prone to aggregation, forming potentially cytotoxic oligomers and fibrils. Antibodies and other N-terminal huntingtin binders are widely explored as biomarkers and possible aggregation-inhibiting therapeutics. A monoclonal antibody, MW1, is known to preferentially bind to huntingtin fragments with expanded polyQ lengths, but the molecular basis of the polyQ length specificity remains poorly understood. Using solution NMR, electron paramagnetic resonance, and other biophysical methods, we investigated the structural features of the Httex1-MW1 interaction. Rather than recognizing residual α-helical structure, which is promoted by expanded Q-lengths, MW1 caused the formation of a new, non-native, conformation in which the entire polyQ is largely extended. This non-native polyQ structure allowed the formation of large mixed Httex1-MW1 multimers (600-2900 kD), when Httex1 with pathogenic Q-length (Q46) was used. We propose that these multivalent, entropically favored interactions, are available only to proteins with longer Q-lengths and represent a major factor governing the Q-length preference of MW1. The present study reveals that it is possible to target proteins with longer Q-lengths without having to stabilize a natively favored conformation. Such mechanisms could be exploited in the design of other Q-length specific binders.

Full-length huntingtin is palmitoylated at multiple sites and post-translationally myristoylated following caspase-cleavage.

Introduction: Huntington disease is an autosomal dominant neurodegenerative disorder which is caused by a CAG repeat expansion in the HTT gene that codes for an elongated polyglutamine tract in the huntingtin (HTT) protein. Huntingtin is subjected to multiple post-translational modifications which regulate its cellular functions and degradation. We have previously identified a palmitoylation site at cysteine 214 (C214), catalyzed by the enzymes ZDHHC17 and ZDHHC13. Reduced palmitoylation level of mutant huntingtin is linked to toxicity and loss of function. Moreover, we have described N-terminal myristoylation by the N-myristoyltransferases of a short fragment of huntingtin (HTT553-586) at glycine 553 (G553) following proteolysis at aspartate 552 (D552). Results: Here, we show that huntingtin is palmitoylated at numerous cysteines: C105, C433, C3134 and C3144. In addition, we confirm that full-length huntingtin is cleaved at D552 and post-translationally myristoylated at G553. Importantly, blocking caspase cleavage at the critical and pathogenic aspartate 586 (D586) significantly increases posttranslational myristoylation of huntingtin. In turn, myristoylation of huntingtin promotes the co-interaction between C-terminal and N-terminal huntingtin fragments, which is also protective. Discussion: This suggests that the protective effect of inhibiting caspase-cleavage at D586 may be mediated through post-translational myristoylation of huntingtin at G553.

Gut Bacteria Regulate the Pathogenesis of Huntington's Disease in Drosophila Model.

Changes in the composition of gut microbiota are implicated in the pathogenesis of several neurodegenerative disorders. Here, we investigated whether gut bacteria affect the progression of Huntington’s disease (HD) in transgenic Drosophila melanogaster (fruit fly) models expressing full-length or N-terminal fragments of human mutant huntingtin (HTT) protein. We find that elimination of commensal gut bacteria by antibiotics reduces the aggregation of amyloidogenic N-terminal fragments of HTT and delays the development of motor defects. Conversely, colonization of HD flies with Escherichia coli (E. coli), a known pathobiont of human gut with links to neurodegeneration and other morbidities, accelerates HTT aggregation, aggravates immobility, and shortens lifespan. Similar to antibiotics, treatment of HD flies with small compounds such as luteolin, a flavone, or crocin a beta-carotenoid, ameliorates disease phenotypes, and promotes survival. Crocin prevents colonization of E. coli in the gut and alters the levels of commensal bacteria, which may be linked to its protective effects. The opposing effects of E. coli and crocin on HTT aggregation, motor defects, and survival in transgenic Drosophila models support the involvement of gut-brain networks in the pathogenesis of HD.

The Proteasome Activators Blm10/PA200 Enhance the Proteasomal Degradation of N-Terminal Huntingtin.

The Blm10/PA200 family of proteasome activators modulates the peptidase activity of the core particle (20S CP). They participate in opening the 20S CP gate, thus facilitating the degradation of unstructured proteins such as tau and Dnm1 in a ubiquitin- and ATP-independent manner. Furthermore, PA200 also participates in the degradation of acetylated histones. In our study, we use a combination of yeast and human cell systems to investigate the role of Blm10/PA200 in the degradation of N-terminal Huntingtin fragments (N-Htt). We demonstrate that the human PA200 binds to N-Htt. The loss of Blm10 in yeast or PA200 in human cells results in increased mutant N-Htt aggregate formation and elevated cellular toxicity. Furthermore, Blm10 in vitro accelerates the proteasomal degradation of soluble N-Htt. Collectively, our data suggest N-Htt as a new substrate for Blm10/PA200-proteasomes and point to new approaches in Huntington’s disease (HD) research.

TBK1 phosphorylates mutant Huntingtin and suppresses its aggregation and toxicity in Huntington's disease models.

Phosphorylation of the N‐terminal domain of the huntingtin (HTT) protein has emerged as an important regulator of its localization, structure, aggregation, clearance and toxicity. However, validation of the effect of bona fide phosphorylation in vivo and assessing the therapeutic potential of targeting phosphorylation for the treatment of Huntington's disease (HD) require the identification of the enzymes that regulate HTT phosphorylation. Herein, we report the discovery and validation of a kinase, TANK‐binding kinase 1 (TBK1), that efficiently phosphorylates full‐length and N‐terminal HTT fragments in vitro (at S13/S16), in cells (at S13) and in vivo. TBK1 expression in HD models (cells, primary neurons, and Caenorhabditis elegans) increases mutant HTT exon 1 phosphorylation and reduces its aggregation and cytotoxicity. We demonstrate that the TBK1‐mediated neuroprotective effects are due to phosphorylation‐dependent inhibition of mutant HTT exon 1 aggregation and an increase in autophagic clearance of mutant HTT. These findings suggest that upregulation and/or activation of TBK1 represents a viable strategy for the treatment of HD by simultaneously lowering mutant HTT levels and blocking its aggregation.

Truncation of mutant huntingtin in knock-in mice demonstrates exon1 huntingtin is a key pathogenic form

Polyglutamine expansion in proteins can cause selective neurodegeneration, although the mechanisms are not fully understood. In Huntington’s disease (HD), proteolytic processing generates toxic N-terminal huntingtin (HTT) fragments that preferentially kill striatal neurons. Here, using CRISPR/Cas9 to truncate full-length mutant HTT in HD140Q knock-in (KI) mice, we show that exon 1 HTT is stably present in the brain, regardless of truncation sites in full-length HTT. This N-terminal HTT leads to similar HD-like phenotypes and age-dependent HTT accumulation in the striatum in different KI mice. We find that exon 1 HTT is constantly generated but its selective accumulation in the striatum is associated with the age-dependent expression of striatum-enriched HspBP1, a chaperone inhibitory protein. Our findings suggest that tissue-specific chaperone function contributes to the selective neuropathology in HD, and highlight the therapeutic potential in blocking generation of exon 1 HTT.

Nucleic Acid Therapeutics in Huntington's Disease.

Protein misfolding is a critical factor in the progression of a large number of neurodegenerative diseases. The incorrectly folded protein is prone to aggregation, leading to aberrant interaction with other cellular proteins, elevated oxidative stress, impaired cellular machinery, finally resulting in cell death. Due to its monogenic origin, Huntington's disease (HD) is a poster child of protein misfolding neurodegenerative disorders. The presence of neuronal inclusions of mutant huntingtin N-terminal fragments, mainly in the cortex and striatum, is a neuropathological hallmark of HD. Inhibition of protein misfolding and aggregation has been attempted using a variety of conventional protein stabilizers. This review describes how, in recent times, nucleic acid therapeutics has emerged as a selective tool to downregulate the aberrant transcript and reduce expression of mutant huntingtin, thereby alleviating protein aggregation. Different strategies of use of nucleic acids, including antisense oligonucleotides, short inhibitory RNA sequences and aptamers have been discussed. The following patent databases were consulted: European Patent Office (EPO), the United States Patent and Trademark Office (USPTO), Patent scope Search International and National Patent Collections (WIPO) and Google Patents. Tools such as RNA interference (RNAi) and antisense oligonucleotides (ASOs) are potential therapeutic agents which target the post-transcriptional step, accelerating mRNA degradation and inhibiting the production of the mutant protein. These nucleic acid sequences not only target the elongated CAG triplet repeat translating to an expanded polyglutamine tract in the mutant protein, but have also been used to target single nucleotide polymorphisms associated with the mutant allele. The therapeutic sequences have been investigated in a number of cells and animal models of HD. One antisense sequence, with desirable safety properties, has recently shown downregulation of huntingtin protein in a limited clinical trial. RNA aptamers have also shown promising results in inhibiting protein aggregation in a yeast model of HD. Novel drug delivery techniques have been employed to overcome the blood brain barrier for the use of these therapeutic sequences. The selectivity and specificity imparted by nucleic acids, along with novel delivery techniques, make them hopeful candidates for the development of a curative strategy for HD.

Identification of distinct conformations associated with monomers and fibril assemblies of mutant huntingtin.

The N-terminal fragments of mutant huntingtin (mHTT) misfold and assemble into oligomers, which ultimately bundle into insoluble fibrils. Conformations unique to various assemblies of mHTT remain unknown. Knowledge on the half-life of various multimeric structures of mHTT is also scarce. Using a panel of four new antibodies named PHP1-4, we have identified new conformations in monomers and assembled structures of mHTT. PHP1 and PHP2 bind to epitopes within the proline-rich domain (PRD), whereas PHP3 and PHP4 interact with motifs formed at the junction of polyglutamine (polyQ) and polyproline (polyP) repeats of HTT. The PHP1- and PHP2-reactive epitopes are exposed in fibrils of mHTT exon1 (mHTTx1) generated from recombinant proteins and mHTT assemblies, which progressively accumulate in the nuclei, cell bodies and neuropils in the brains of HD mouse models. Notably, electron microscopic examination of brain sections of HD mice revealed that PHP1- and PHP2-reactive mHTT assemblies are present in myelin sheath and in vesicle-like structures. Moreover, PHP1 and PHP2 antibodies block seeding and subsequent fibril assembly of mHTTx1 in vitro and in a cell culture model of HD. PHP3 and PHP4 bind to epitopes in full-length and N-terminal fragments of monomeric mHTT and binding diminishes as the mHTTx1 assembles into fibrils. Interestingly, PHP3 and PHP4 also prevent the aggregation of mHTTx1 in vitro highlighting a regulatory function for the polyQ-polyP motifs. These newly detected conformations may affect fibril assembly, stability and intercellular transport of mHTT.

Profilin reduces aggregation and phase separation of huntingtin N-terminal fragments by preferentially binding to soluble monomers and oligomers

Huntingtin N-terminal fragments (Htt-NTFs) with expanded polyglutamine tracts form a range of neurotoxic aggregates that are associated with Huntington's disease. Here, we show that aggregation of Htt-NTFs, irrespective of polyglutamine length, yields at least three phases (designated M, S, and F) that are delineated by sharp concentration thresholds and distinct aggregate sizes and morphologies. We found that monomers and oligomers make up the soluble M phase, ∼25-nm spheres dominate in the soluble S phase, and long, linear fibrils make up the insoluble F phase. Previous studies showed that profilin, an abundant cellular protein, reduces Htt-NTF aggregation and toxicity in cells. We confirm that profilin achieves its cellular effects through direct binding to the C-terminal proline-rich region of Htt-NTFs. We show that profilin preferentially binds to Htt-NTF M-phase species and destabilizes aggregation and phase separation by shifting the concentration boundaries for phase separation to higher values through a process known as polyphasic linkage. Our experiments, aided by coarse-grained computer simulations and theoretical analysis, suggest that preferential binding of profilin to the M-phase species of Htt-NTFs is enhanced through a combination of specific interactions between profilin and polyproline segments and auxiliary interactions between profilin and polyglutamine tracts. Polyphasic linkage may be a general strategy that cells utilize to regulate phase behavior of aggregation-prone proteins. Accordingly, detailed knowledge of phase behavior and an understanding of how ligands modulate phase boundaries may pave the way for developing new therapeutics against a variety of aggregation-prone proteins.

Herp Promotes Degradation of Mutant Huntingtin: Involvement of the Proteasome and Molecular Chaperones.

In neurodegenerative diseases, pathogenic proteins tend to misfold and form aggregates that are difficult to remove and able to induce excessive endoplasmic reticulum (ER) stress, leading to neuronal injury and apoptosis. Homocysteine-induced endoplasmic reticulum protein (Herp), an E3 ubiquitin ligase, is an important early marker of ER stress and is involved in the ubiquitination and degradation of many neurodegenerative proteins. However, in Huntington's disease (HD), a typical polyglutamine disease, whether Herp is also involved in the metabolism and degradation of the pathogenic protein, mutant huntingtin, has not been reported. Therefore, we studied the relationship between Herp and N-terminal fragments of huntingtin (HttN-20Q and HttN-160Q). We found that Herp was able to bind to the overexpressed Htt N-terminal, and this interaction was enhanced by expansion of the polyQ fragment. Confocal microscopy demonstrated that Herp was co-localized with the HttN-160Q aggregates in the cytoplasm and tightly surrounded the aggregates. Overexpression of Herp significantly decreased the amount of soluble and insoluble HttN-160Q, promoted its ubiquitination, and inhibited its cytotoxicity. In contrast, knockdown of Herp resulted in more HttN-160Q protein, less ubiquitination, and stronger cytotoxicity. Inhibition of the autophagy-lysosomal pathway (ALP) had no effect on the function of Herp. However, blocking the ubiquitin-proteasome pathway (UPP) inhibited the reduction in soluble HttN-160Q caused by Herp. Interestingly, blocking the UPP did not weaken the ability of Herp to reduce HttN-160Q aggregates. Deletions of the N-terminal of Herp weakened its ability to inhibit HttN-160Q aggregation but did not result in a significant increase in its soluble form. However, loss of the C-terminal led to a significant increase in soluble HttN-160Q, but Herp still maintained the ability to inhibit aggregate formation. We further found that the expression level of Herp was significantly increased in HD animal and cell models. Our findings suggest that Herp is a newly identified huntingtin-interacting protein that is able to reduce the cytotoxicity of mutant huntingtin by inhibiting its aggregation and promoting its degradation. The N-terminal of Herp serves as the molecular chaperone to inhibit protein aggregation, while its C-terminal functions as an E3 ubiquitin ligase to promote the degradation of misfolded proteins through the UPP. Increased expression of Herp in HD models may be a pro-survival mechanism under stress.

Assessing Autophagic Activity and Aggregate Formation of Mutant Huntingtin in Mammalian Cells.

The accumulation of mutant aggregate-prone proteins is a hallmark of the majority of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. Autophagy, a cytosolic bulk degradation system, is the major clearance pathway for several aggregate-prone proteins, such as mutant huntingtin. The autophagosome-associated protein LC3-II is a specific marker of autophagic flux within cells, whereas aggregate formation of mutant huntingtin represents a good readout for studying autophagy modulation. Here we describe the method of assessing autophagic flux using LC3-II western blotting and substrate clearance by expressing the N-terminal fragment of huntingtin (htt exon 1) containing an expanded polyglutamine tract in mammalian cells.

Effect of post-mortem delay on N-terminal huntingtin protein fragments in human control and Huntington disease brain lysates.

Huntington disease is associated with elongation of a CAG repeat in the HTT gene that results in a mutant huntingtin protein. Several studies have implicated N-terminal huntingtin protein fragments in Huntington disease pathogenesis. Ideally, these fragments are studied in human brain tissue. However, the use of human brain tissue comes with certain unavoidable variables such as post mortem delay, artefacts from freeze-thaw cycles and subject-to-subject variation. Knowledge on how these variables might affect N-terminal huntingtin protein fragments in post mortem human brain is important for a proper interpretation of study results. The effect of post mortem delay on protein in human brain is known to vary depending on the protein of interest. In the present study, we have assessed the effect of post mortem delay on N-terminal huntingtin protein fragments using western blot. We mimicked post mortem delay in one individual control case and one individual Huntington disease case with low initial post mortem delay. The influence of subject-to-subject variation on N-terminal huntingtin fragments was assessed in human cortex and human striatum using two cohorts of control and Huntington disease subjects. Our results show that effects of post mortem delay on N-terminal huntingtin protein fragments are minor in our individual subjects. Additionally, one freeze-thaw cycle decreases the huntingtin western blot signal intensity in the cortex control subject, but does not introduce additional N-terminal huntingtin fragments. Our results suggest that subject-to-subject variation contributes more to variability in N-terminal huntingtin fragments than post mortem delay.

Polyglutamine tracts regulate beclin 1-dependent autophagy.

Nine neurodegenerative diseases are caused by expanded polyglutamine (polyQ) tracts in different proteins, like huntingtin in Huntington’s disease (HD) and ataxin-3 in spinocerebellar ataxia type 3 (SCA3)1, 2. Age-at-onset decreases with increasing polyglutamine length in these proteins and the normal length is also polymorphic3. PolyQ expansions drive pathogenesis in these diseases, as isolated polyQ tracts are toxic, and an N-terminal huntingtin fragment comprising exon 1, which occurs in vivo due to alternative splicing4, causes toxicity. While such mutant proteins are aggregate-prone5, toxicity is also associated with soluble forms of the proteins6. The function of the polyQ tracts in many normal/wild-type cytoplasmic proteins is unclear. One such protein is the deubiquitinating enzyme ataxin 37, 8, which is widely expressed in the brain9, 10. Here we show that the polyQ domain in wild-type ataxin-3 enables its interaction with beclin 1, a key autophagy initiator11. This interaction allows the deubiquitinase activity of ataxin-3 to protect beclin 1 from proteasome-mediated degradation and thus enables autophagy. Starvation-induced autophagy, which is regulated by beclin 1, was particularly inhibited in ataxin-3-depleted human cell-lines, primary neurons and in-vivo. This activity of ataxin-3 and its interaction with beclin 1 mediated by its polyQ domain was competed by other soluble proteins with polyQ tracts in a length-dependent fashion. This resulted in impaired starvation-induced autophagy in cells expressing mutant huntingtin exon 1, which was also recapitulated in the brain of HD mouse model and in patient cells. A similar phenomenon was also seen with other polyQ disease proteins, including mutant ataxin-3 itself. Our data thus describe a specific function for a wild-type polyQ tract which is abrogated by a competing longer polyQ mutation in a disease protein. This also reveals a deleterious function of such mutations distinct from their aggregation propensity.

Inclusions of R6/2 Mice Are Not Amyloid and Differ Structurally from Those of Huntington Disease Brain.

R6/2 mice contain an N-terminal fragment of human huntingtin with an expanded polyQ and develop a neurological disease resembling Huntington disease. Although the brain of R6/2 mice contains numerous inclusions, there is very little neuronal death. In that respect, R6/2 mice differ from patients with Huntington disease whose striatum and cerebral cortex develop inclusions associated with extensive neuronal loss. We have previously demonstrated using synchrotron-based infrared microspectroscopy that the striatum and the cortex of patients with Huntington disease contained inclusions specifically enriched in amyloid β-sheets. We had concluded that the presence of an amyloid motif conferred toxicity to the inclusions. We demonstrate here by synchrotron based infrared microspectroscopy in transmission and attenuated total reflectance mode that the inclusions of R6/2 mice possess no detectable amyloid and are composed of proteins whose structure is not distinguishable from that of the surrounding soluble proteins. The difference in structure between the inclusions of patients affected by Huntington disease and those of R6/2 mice might explain why the former but not the latter cause neuronal death.

Lattice Models for Modeling Sequence-Specific Phase Behavior of Low Complexity Domains

Low complexity domains (LCDs), enriched in charged, polar, and aromatic residues are drivers of aggregation and phase separation in a variety of proteins including those that drive the formation of RNA-protein granules. Polyglutamine (polyQ) tracts are a distinct archetype of LCDs. Mutational expansion of polyQ tracts in specific proteins is associated with onset and progression of at least nine different neurodegenerative disorders including Huntington's disease. The latter is associated with an expanded polyQ tract in the protein huntingtin. Recent experiments have uncovered complex thermodynamic and kinetic phase diagrams for N-terminal fragments of huntingtin. These phase diagrams are modulated by polyQ length, sequence contexts, concentration, and solution conditions. The details of the molecular and supramolecular interactions that give rise to emergent phase behavior of polyQ containing at distinct length scales remains poorly understood. Computations play an important role in predicting the phase behavior of LCDs. These calculations require simulations encompassing several hundreds of thousands of protein molecules spanning several orders of magnitude in protein concentrations. Such simulations are beyond the scope of all atom models. Since phase behavior results from the synergistic interactions amongst small numbers of collective coordinates, simulations designed to uncover sequence-encoded phase behavior can be pursued using either systematic coarse-graining or phenomenological models. Recent work has demonstrated the utility of both approaches. However, these models lack the requisite molecular detail to enable a complete understanding of sequence-specific contributions that determine similarities and differences in sequence-encoded phase behavior of LCDs. Here we present a lattice model that is tailored for modeling sequence-specific phase behaviors of LCDs. The model serves as an intermediate description between ultra coarse-graining and all-atom extremes. In the lattice model, a single bead on a 3-dimensional lattice site describes each residue. Each bead has multiple faces and this helps us capture the directionality of backbone hydrogen bonding and sidechain-specific interactions. Monte Carlo simulations based on these lattice models help us uncover the interplay amongst different sequence-encoded energy scales and phase behaviors of LCDs with polyQ tracts. Specifically, we focus on explaining the role of spherical mesophases as mediators of fibril formation in polyQ containing proteins. We also show that the model can be deployed for understanding the interplay amongst liquids, fibrils, and gels for other LCDs.

In vivo proof-of-concept of removal of the huntingtin caspase cleavage motif-encoding exon 12 approach in the YAC128 mouse model of Huntington’s disease

Huntington's disease (HD) is a progressive autosomal dominant disease, caused by a CAG repeat expansion in the HTT gene, resulting in an expanded polyglutamine stretch at the N-terminal of the huntingtin protein. An important event in HD pathogenesis appears to be the proteolysis of the mutant protein, which forms N-terminal huntingtin fragments. These fragments form insoluble aggregates and are found in nuclei and cytoplasm of affected neurons where they interfere with normal cell functioning. Important cleavage sites are encoded by exon 12 of HTT. A novel approach is Htt protein modification through exon skipping, which has recently been proven effective both in vitro and in vivo. Here we report proof-of-concept of AON 12.1 in vivo using the YAC128 mouse model of HD. Our results support and encourage future longitudinal studies exploring the therapeutic effects of sustained infusions in the YAC128 mouse model.

B8 Ablation of p62 modulates levels of soluble and aggregated mutant huntingtin and delays end-stage disease in R6/2 mice

Background N-terminal fragments of the mutant huntingtin (HTT) protein aggregate into oligomeric and fibrillary structures that can be detected as polyubiquitylated inclusion bodies in tissue sections from HD patients and mouse models of HD. Misfolded and aggregated proteins are degraded through two main intracellular protein clearance systems, the ubiquitin–proteasome system (UPS) and the autophagy–lysosome pathway. p62 is an adapter protein that regulates the proteolysis of ubiquitylated proteins via selective autophagy. In both the R6/2 and HdhQ150 knock-in mice, p62 relocates to the nucleus with disease progression where it forms high molecular weight structures and co-localises with HTT inclusions. Aims To investigate the effect of the constitutive ablation of the p62 gene on HD-related phenotypes in the R6/2 mouse model of HD. Methods R6/2 mice that were heterozygous for p62 knock-out (p62Het) to p62Het females to generate WT, p62Het, p62 homozygous knock-out (p62Hom); R6/2, R6/2::p62Het and R6/2::p62Hom progeny. The effect of p62 ablation was assessed by multiple biochemical, immunohistochemical and behavioural approaches. Results We have been able to demonstrate that the levels of soluble and aggregated HTT are decreased in multiple brain regions of R6/2::p62Hom mice, as compared to their R6/2 littermates and that this effect becomes more pronounced with disease progression. Ablation of p62 in R6/2 mice did not modify the failure of R6/2 mice to gain body weight, or impairments in rotarod performance, grip strength or activity but did delay end-stage disease as previously reported (Kurosawa et al., 2014). Similarly, ablation of p62 did not improve dysregulated transcriptional profiles. Conclusion Unravelling the mechanism by which the manipulation of p62 modulates soluble and aggregated HTT levels will lead to a greater understanding of the aggregation and clearance of mutant HTT. Funding CHDI Foundation

B6 Super-resolution fluorescence imaging of the seeding and polymerizatoin of the huntingtin exon 1 protein

Background Huntington’s disease (HD) is caused by the expansion of the CAG repeat in exon 1 of the gene coding for the huntingtin (HTT) protein. The mutant version of the polyglutamine tract can result in misfolding and aggregation of N-terminal fragments of HTT under physiological conditions. Therefore, it’s crucial to understand the mechanisms underlying the seeding and polymerization of HTT exon 1. However, the formation of HTT aggregates has been difficult to investigate by conventional fluorescence microscopy due to their small size. Recent advances in super-resolution microscope have made this possible by permitting researchers to investigate protein aggregation in much greater detail. Aims To use highly novel super-resolution microscope techniques to understand the mechanisms underlying the seeding and polymerization of HTT exon 1 Methods/techniques Purify native HTT exon 1 protein from newly designed constructs and use two-colour TIRF (total internal reflection fluorescence microscope)-SIM (Structured illumination microscopy) microscope to directly observe the growth of HTT exon 1 at the single fibril level. Results/outcome We have designed and cloned new constructs ideal for making native HTT exon 1 protein by precise protease cleavage and fluorescent dye labelling for microscope imaging. The characterisation and application of these constructs will be presented. Conclusions Our methods will enable us to directly visualise the dynamics of the HTT exon 1 aggregation and study the aggregation kinetics which is important to HD therapeutics.