Deletion of the huntingtin proline-rich region does not significantly affect normal huntingtin function in mice

A Targetable Self-association Surface of the Huntingtin exon1 Helical Tetramer Required for Assembly of Amyloid Pre-nucleation Oligomers

Polyglutamine (polyQ) sequences undergo repeat-length dependent formation of disease-associated, amyloid-like cross-β core structures with kinetics and aggregate morphologies often influenced by the flanking sequences. In Huntington's disease (HD), the httNT segment on the polyQ's N-terminal flank enhances aggregation rates by changing amyloid nucleation from a classical homogeneous mechanism to a two-step process requiring an ɑ-helix-rich oligomeric intermediate. A folded, helix-rich httNT tetrameric structure suggested to be this critical intermediate was recently reported. Here we employ single alanine replacements along the httNT sequence to assess this proposed structure and refine the mechanistic model. We find that Ala replacement of hydrophobic residues within simple httNT peptides greatly suppresses helicity, supporting the tetramer model. These same helix-disruptive replacements in the httNT segment of an exon-1 analog greatly reduce aggregation kinetics, suggesting that an ɑ-helix rich multimer - either the tetramer or a larger multimer - plays an on-pathway role in nucleation. Surprisingly, several other Ala replacements actually enhance helicity and/or amyloid aggregation. The spatial localization of these residues on the tetramer surface suggests a self-association interface responsible for formation of the octomers and higher-order multimers most likely required for polyQ amyloid nucleation. Multimer docking of the tetramer, using the protein-protein docking algorithm ClusPro, predicts this symmetric surface to be a viable tetramer dimerization interface. Intriguingly, octomer formation brings the emerging polyQ chains into closer proximity at this tetramer-tetramer interface. Further supporting the potential importance of tetramer super-assembly, computational docking with a known exon-1 aggregation inhibitor predicts ligand contacts with residues at this interface.

Huntingtin exon 1 deletion does not alter the subcellular distribution of huntingtin and gene transcription in mice

Huntington disease (HD) is caused by the expansion of CAG triplet repeats in exon 1 of the huntingtin (HTT) gene, which also encodes the first 17 amino acids (N-17) that can modulate the toxicity of the expanded polyQ repeat. N-17 are conserved in a wide range of species and are found to influence the subcellular distribution of mutant Htt. Moreover, N-17 is subject to many posttranslational modifications that may regulate the function, stability, and distribution of HTT. However, the function of Htt exon 1 and its influence on the normal Htt remains to be fully investigated. By investigating a knock-in mouse model that lacks Htt exon1, we found that deletion of Htt exon1 does not affect the survival of mice and differentiation of cultured mouse neurons. Furthermore, the lack of Htt exon 1 does not alter the subcellular distribution of Htt, autophagy protein expression, and global gene transcription in the mouse brain. These results suggest that removing the entire exon 1 of Htt could be a therapeutic approach to eliminate expanded polyQ toxicity.

Modulating FKBP5/FKBP51 and autophagy lowers HTT (huntingtin) levels

Current disease-modifying therapies for Huntington disease (HD) focus on lowering mutant HTT (huntingtin; mHTT) levels, and the immunosuppressant drug rapamycin is an intriguing therapeutic for aging and neurological disorders. Rapamycin interacts with FKBP1A/FKBP12 and FKBP5/FKBP51, inhibiting the MTORC1 complex and increasing cellular clearance mechanisms. Whether the levels of FKBP (FK506 binding protein) family members are altered in HD models and if these proteins are potential therapeutic targets for HD have not been investigated. Here, we found levels of FKBP5 are significantly reduced in HD R6/2 and zQ175 mouse models and human HD isogenic neural stem cells and medium spiny neurons derived from induced pluripotent stem cells. Moreover, FKBP5 interacts and colocalizes with HTT in the striatum and cortex of zQ175 mice and controls. Importantly, when we decreased FKBP5 levels or activity by genetic or pharmacological approaches, we observed reduced levels of mHTT in our isogenic human HD stem cell model. Decreasing FKBP5 levels by siRNA or pharmacological inhibition increased LC3-II levels and macroautophagic/autophagic flux, suggesting autophagic cellular clearance mechanisms are responsible for mHTT lowering. Unlike rapamycin, the effect of pharmacological inhibition with SAFit2, an inhibitor of FKBP5, is MTOR independent. Further, in vivo treatment for 2 weeks with SAFit2, results in reduced HTT levels in both HD R6/2 and zQ175 mouse models. Our studies establish FKBP5 as a protein involved in the pathogenesis of HD and identify FKBP5 as a potential therapeutic target for HD.Abbreviations : ACTB/β-actin: actin beta; AD: Alzheimer disease; BafA1: bafilomycin A1; BCA: bicinchoninic acid; BBB: blood brain barrier; BSA: bovine serum albumin; CoIP: co-immunoprecipitation; DMSO: dimethyl sulfoxide; DTT: dithiothreitol; FKBPs: FK506 binding proteins; HD: Huntington disease; HTT: huntingtin; iPSC: induced pluripotent stem cells; MAP1LC3/LC3:microtubule associated protein 1 light chain 3; MAPT/tau: microtubule associated protein tau; MES: 2-ethanesulfonic acid; MOPS: 3-(N-morphorlino)propanesulfonic acid); MSN: medium spiny neurons; mHTT: mutant huntingtin; MTOR: mechanistic target of rapamycin kinase; NSC: neural stem cells; ON: overnight; PD: Parkinson disease; PPIase: peptidyl-prolyl cis/trans-isomerases; polyQ: polyglutamine; PPP1R1B/DARPP-32: protein phosphatase 1 regulatory inhibitor subunit 1B; PTSD: post-traumatic stress disorder; RT: room temperature; SQSTM1/p62: sequestosome 1; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; TBST:Tris-buffered saline, 0.1% Tween 20; TUBA: tubulin; ULK1: unc-51 like autophagy activating kinase 1; VCL: vinculin; WT: littermate controls.

An Expanded Polyproline Domain Maintains Mutant Huntingtin Soluble in vivo and During Aging

Huntington's disease is a dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat, encoding for the amino acid glutamine (Q), present in the first exon of the protein huntingtin. Over the threshold of Q39 HTT exon 1 (HTTEx1) tends to misfold and aggregate into large intracellular structures, but whether these end-stage aggregates or their on-pathway intermediates are responsible for cytotoxicity is still debated. HTTEx1 can be separated into three domains: an N-terminal 17 amino acid region, the polyglutamine (polyQ) expansion and a C-terminal proline rich domain (PRD). Alongside the expanded polyQ, these flanking domains influence the aggregation propensity of HTTEx1: with the N17 initiating and promoting aggregation, and the PRD modulating it. In this study we focus on the first 11 amino acids of the PRD, a stretch of pure prolines, which are an evolutionary recent addition to the expanding polyQ region. We hypothesize that this proline region is expanding alongside the polyQ to counteract its ability to misfold and cause toxicity, and that expanding this proline region would be overall beneficial. We generated HTTEx1 mutants lacking both flanking domains singularly, missing the first 11 prolines of the PRD, or with this stretch of prolines expanded. We then followed their aggregation landscape in vitro with a battery of biochemical assays, and in vivo in novel models of C. elegans expressing the HTTEx1 mutants pan-neuronally. Employing fluorescence lifetime imaging we could observe the aggregation propensity of all HTTEx1 mutants during aging and correlate this with toxicity via various phenotypic assays. We found that the presence of an expanded proline stretch is beneficial in maintaining HTTEx1 soluble over time, regardless of polyQ length. However, the expanded prolines were only advantageous in promoting the survival and fitness of an organism carrying a pathogenic stretch of Q48 but were extremely deleterious to the nematode expressing a physiological stretch of Q23. Our results reveal the unique importance of the prolines which have and still are evolving alongside expanding glutamines to promote the function of HTTEx1 and avoid pathology.

Characterization of a Knock-In Mouse Model with a Huntingtin Exon 1 Deletion

BACKGROUND

The Huntingtin (HTT) N-terminal domains encoded by Huntingtin's (HTT) exon 1 consist of an N17 domain, the polyglutamine (polyQ) stretch and a proline-rich region (PRR). These domains are conserved in mammals and have been hypothesized to modulate HTT's functions in the developing and adult CNS, including DNA damage repair and autophagy.

OBJECTIVE

This study longitudinally characterizes the in vivo consequences of deleting the murine Htt N-terminal domains encoded by Htt exon 1.

METHODS

Knock-in mice with a deletion of Htt exon 1 sequences (HttΔE1) were generated and bred into the C57BL/6J congenic genetic background. Their behavior, DNA damage response, basal autophagy, and glutamatergic synapse numbers were evaluated.

RESULTS

Progeny from HttΔE1/+ intercrosses are born at the expected Mendelian frequency but with a distorted male to female ratio in both the HttΔE1/ΔE1 and Htt+/+ offspring. HttΔE1/ΔE1 adults exhibit a modest deficit in accelerating rotarod performance, and an earlier increase in cortical and striatal DNA damage with elevated neuronal pan-nuclear 53bp1 levels compared to Htt+/+ mice. However, a normal response to induced DNA damage, normal levels of basal autophagy markers, and no significant differences in corticocortical, corticostriatal, thalamocortical, or thalamostriatal synapses numbers were observed compared to controls.

CONCLUSION

Our results suggest that deletion of the Htt N-terminus encoded by the Htt exon 1 does not affect Htt's critical role during embryogenesis, but instead, may have a modest effect on certain motor tasks, basal levels of DNA damage in the brain, and Htt function in the testis.

Huntingtin protein maintains balanced energetics in mouse cardiomyocytes

Huntingtin (HTT) is a multifunctional protein crucial for proper embryogenesis and nervous system development. Mutation of a single allele in gene coded this protein results in the Huntington׳s disease (HD). There is growing evidence of cardiovascular system pathologies coexisting with the neurological symptoms in HD patients. Thus, this study aims to establish the role of huntingtin protein in cardiomyocytes cellular energy and nucleotides metabolism. We used HTT KO mice embryonic stem cells (ESC) obtained with CRISPR method, wild type mice ESC treated by CRISPR with Scramble control sequence (SCR) as well as wild type (WT) mice ESC and differentiate it into cardiomyocytes. Analysis of intracellular concentration of ATP, ADP, and NAD⁺, as well as nucleotide catabolites were performed with HPLC. We noted that HTT null cardiomyocytes showed diminished intracellular ATP (4.9 ± 0.5; 6.7 ± 0.4 nmol/mg protein HTT KO vs. SCR) and NAD⁺ (0.9 ± 0.1; 1.6 ± 0.1 nmol/mg HTT KO vs. SCR). We noted also reduced cellular medium concentration of total purines pool (17.1 ± 1.7; 24.7 ± 2.7 nmol/ml HTT KO vs. SCR) as well as IMP concentration (7.7 ± 0.6; 10.2 ± 0.4 nmol/ml HTT KO vs. SCR). This study indicates that HTT plays an important role in cellular energy balance as well as in nucleotide metabolism in cardiomyocytes. Furthermore, our findings underline that the deterioration in energy metabolism observed in HD may be caused not only by cellular mutant HTT accumulation but also by the loss of HTT function.

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.

The mechanisms by which mutant Huntington protein Htt leads to selective neurodegeneration are not fully understood. Here, using gene editing in HD140Q knock-in mice, the authors show that exon1 Htt is a critical pathological form of the protein.

Molecular Dynamics Simulations Applied to Structural and Dynamical Transitions of the Huntingtin Protein: A Review

Over the recent years, Huntington's disease (HD) has become widely discussed in the scientific literature especially because at the mutant level there are several contradictions regarding the aggregation mechanism. The specific role of the physiological huntingtin protein remains unknown, due to the lack of characterization of its entire crystallographic structure, making the experimental and theoretical research even harder when taking into consideration its involvement in multiple biological functions and its high affinity for different interacting partners. Different types of models, containing fewer (not more than 35 Qs) polyglutamine residues for the WT structure and above 35 Qs for the mutants, were subjected to classical or advanced MD simulations to establish the proteins' structural stability by evaluating their conformational changes. Outside the polyQ tract, there are two other regions of interest (the N17 domain and the polyP rich domain) considered to be essential for the aggregation kinetics at the mutant level. The polymerization process is considered to be dependent on the polyQ length. As the polyQ tract's dimension increases, the structures present more β-sheet conformations. Contrarily, it is also considered that the aggregation stability is not necessarily dependent on the number of Qs, while the initial stage of the aggregation seed might play the decisive role. A general assumption regarding the polyP domain is that it might preserve the polyQ structures soluble by acting as an antagonist for β-sheet formation.

Demonstration of prion-like properties of mutant huntingtin fibrils in both in vitro and in vivo paradigms

In recent years, evidence has accumulated to suggest that mutant huntingtin protein (mHTT) can spread into healthy tissue in a prion-like fashion. This theory, however, remains controversial. To fully address this concept and to understand the possible consequences of mHTT spreading to Huntington’s disease pathology, we investigated the effects of exogenous human fibrillar mHTT (Q48) and huntingtin (HTT) (Q25) N-terminal fragments in three cellular models and three distinct animal paradigms. For in vitro experiments, human neuronal cells [induced pluripotent stem cell-derived GABA neurons (iGABA) and (SH-SY5Y)] as well as human THP1-derived macrophages, were incubated with recombinant mHTT fibrils. Recombinant mHTT and HTT fibrils were taken up by all cell types, inducing cell morphology changes and death. Variations in HTT aggregation were further observed following incubation with fibrils in both THP1 and SH-SY5Y cells. For in vivo experiments, adult wild-type (WT) mice received a unilateral intracerebral cortical injection and R6/2 and WT pups were administered fibrils via bilateral intraventricular injections. In both protocols, the injection of Q48 fibrils resulted in cognitive deficits and increased anxiety-like behavior. Post-mortem analysis of adult WT mice indicated that most fibrils had been degraded/cleared from the brain by 14 months post-surgery. Despite the absence of fibrils at these later time points, a change in the staining pattern of endogenous HTT was detected. A similar change was revealed in post-mortem analysis of the R6/2 mice. These effects were specific to central administration of fibrils, as mice receiving intravenous injections were not characterized by behavioral changes. In fact, peripheral administration resulted in an immune response mounting against the fibrils. Together, the in vitro and in vivo data indicate that exogenously administered mHTT is capable of both causing and exacerbating disease pathology.

Structural insights into the aggregation mechanism of huntingtin exon 1 protein fragment with different polyQ-lengths

Huntington disease is a neurodegenerative disorder caused by the expansion of polyglutamine (polyQ) at the N-terminal of the huntingtin exon 1 protein. The detailed structure and the mechanism behind this aggregation remain unclear and it is assumed that the polyQ undergoes a conformational transition to the β-sheet structure when it aggregates. Investigating the misfolding of polyQ facilitates the determination of the molecular mechanism of aggregation and can potentially help in developing a novel approach to inhibit polyQ aggregation. Moreover, the flanking sequences of the polyQ region play a vital role in structural changes and the aggregation mechanism. We performed all-atom molecular dynamics simulations to gain structural insights into the aggregation mechanism using eight different models with glutamine repeat lengths Q₂₇ , Q₂₇ P₁₁ , Q₃₄ , Q₃₅ , Q₃₆ , Q₄₀ , Q₅₀ , and Q₅₀ P₁₁ . In the models without flanking polyPs, we noticed that the transformation of a random coil to β-sheet occurs when the number of Q increases. We also found that the flanking polyPs prevent aggregation by decreasing the probability of forming a β-sheet structure. When polyQ length increases, the 17 N-terminal flanking residues are more likely to adopt a β-sheet conformation from α-helix and coil. From our simulations, we suggest that at least 34 glutamines are required for initiating aggregation and 40 residues length is critical for the aggregation of huntingtin exon 1 protein for disease onset. This study provides structural insights into misfolding and the role of flanking sequences in huntingtin aggregation which will further help in developing therapeutic strategies for Huntington's disease.

Generation and Characterization of Knock-in Mouse Models Expressing Versions of Huntingtin with Either an N17 or a Combined PolyQ and Proline-Rich Region Deletion

Background: The polyglutamine (polyQ) stretch of the Huntingtin protein (HTT) in mammals is flanked by a highly conserved 17 amino acid N-terminal domain (N17), and a proline-rich region (PRR). The PRR is a binding site for many HTT-interacting proteins, and the N17 domain regulates several normal HTT functions, including HTT’s ability to associate with membranes and organelles.

Objective: This study investigates the consequence of deleting mouse Huntingtin’s (Htt’s) N17 domain or a combination of its polyQ stretch and PRR (QP) on normal Htt function in mice.

Methods: Knock-in mice expressing versions of Htt lacking either the N17 domain (Htt^ΔN17) or both the polyQ and PRR domains (Htt^ΔQP) were generated, and their behavior, autophagy function, and neuropathology were evaluated.

Results: Homozygous and hemizygous Htt^ΔQP/ΔQP, Htt^ΔN17/ΔN17, Htt^ΔQP/–, and Htt^ΔN17/– mice were generated at the expected Mendelian frequency. Htt^ΔQP/ΔQP mutants exhibit improvements in motor coordination compared to controls (Htt^+/+). In contrast, Htt^ΔN17/ΔN17 mutants do not exhibit any changes in motor coordination, but they do display variable changes in spatial learning that are dependent on their age at testing. Neither mutant exhibited any changes in basal autophagy in comparison to controls, but thalamostriatal synapses in the dorsal striatum of 24-month-old Htt^ΔN17/ΔN17 mice were decreased compared to controls.

Conclusions: These findings support the hypothesis that Htt’s N17 and QP domains are dispensable for its critical functions during early embryonic development, but are likely more important for Htt functions in CNS development or maintenance.

Huntington Disease as a Neurodevelopmental Disorder and Early Signs of the Disease in Stem Cells

Huntington disease (HD) is a dominantly inherited disorder caused by a CAG expansion mutation in the huntingtin (HTT) gene, which results in the HTT protein that contains an expanded polyglutamine tract. The adult form of HD exhibits a late onset of the fully symptomatic phase. However, there is also a long presymptomatic phase, which has been increasingly investigated and recognized as important for the disease development. Moreover, the juvenile form of HD, evoked by a higher number of CAG repeats, resembles a neurodevelopmental disorder and has recently been the focus of additional interest. Multiple lines of data, such as the developmental necessity of HTT, its role in the cell cycle and neurogenesis, and findings from pluripotent stem cells, suggest the existence of a neurodevelopmental component in HD pathogenesis. Therefore, we discuss the early molecular pathogenesis of HD in pluripotent and neural stem cells, with respect to the neurodevelopmental aspects of HD.

Altered Co-Translational Processing Plays a Role in Huntington's Pathogenesis-A Hypothesis

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG codon repeat region in the HTT gene's first exon that results in huntingtin protein aggregation and neuronal cell death. The development of therapeutic treatments for HD is hindered by the fact that while the etiology and symptoms of HD are understood, the molecular processes connecting this genotype to its phenotype remain unclear. Here, we propose the novel hypothesis that the perturbation of a co-translational process affects mutant huntingtin due to altered translation-elongation kinetics. These altered kinetics arise from the shift of a proline-induced translational pause site away from Htt's localization sequence due to the expansion of the CAG-repeat segment between the poly-proline and localization sequences. Motivation for this hypothesis comes from recent experiments in the field of protein biogenesis that illustrate the critical role that temporal coordination of co-translational processes plays in determining the function, localization, and fate of proteins in cells. We show that our hypothesis is consistent with various experimental observations concerning HD pathology, including the dependence of the age of symptom onset on CAG repeat number. Finally, we suggest three experiments to test our hypothesis.

N-terminal Huntingtin Knock-In Mice: Implications of Removing the N-terminal Region of Huntingtin for Therapy

The Huntington’s disease (HD) protein, huntingtin (HTT), is a large protein consisting of 3144 amino acids and has conserved N-terminal sequences that are followed by a polyglutamine (polyQ) repeat. Loss of Htt is known to cause embryonic lethality in mice, whereas polyQ expansion leads to adult neuronal degeneration. Whether N-terminal HTT is essential for neuronal development or contributes only to late-onset neurodegeneration remains unknown. We established HTT knock-in mice (N160Q-KI) expressing the first 208 amino acids of HTT with 160Q, and they show age-dependent HTT aggregates in the brain and neurological phenotypes. Importantly, the N-terminal mutant HTT also preferentially accumulates in the striatum, the brain region most affected in HD, indicating the importance of N-terminal HTT in selective neuropathology. That said, homozygous N160Q-KI mice are also embryonic lethal, suggesting that N-terminal HTT alone is unable to support embryonic development. Using Htt knockout neurons, we found that loss of Htt selectively affects the survival of developing neuronal cells, but not astrocytes, in culture. This neuronal degeneration could be rescued by a truncated HTT lacking the first 237 amino acids, but not by N-terminal HTT (1–208 amino acids). Also, the rescue effect depends on the region in HTT known to be involved in intracellular trafficking. Thus, the N-terminal HTT region may not be essential for the survival of developing neurons, but when carrying a large polyQ repeat, can cause selective neuropathology. These findings imply a possible therapeutic benefit of removing the N-terminal region of HTT containing the polyQ repeat to treat the neurodegeneration in HD.

The 17 amino acids in the N-terminal region of huntingtin (HTT) are conserved in a wide range of species and are followed by a polyglutamine repeat whose expansion causes selective neurodegeneration in Huntington’s disease (HD). Loss of Htt can affect developing neurons and early embryonic development in mice. Whether N-terminal HTT is important for the survival of developing neurons or contributes mainly to a gain of toxic function in HD remains unknown. In the current study, we generated N-terminal mutant HTT knock-in mice and found that N-terminal HTT with an expanded polyQ repeat is unable to support the early development of mice, but can cause age-dependent neurological phenotypes. Further, we show that a truncated HTT without the N-terminal region can rescue the Htt loss-mediated degeneration of developing neurons. Our studies suggest that removal of the N-terminal region of mutant HTT could be a strategy to abolish the neuronal toxicity of mutant HTT.

Ablation of huntingtin in adult neurons is nondeleterious but its depletion in young mice causes acute pancreatitis

The Huntington's disease (HD) protein, huntingtin (HTT), is essential for early development. Because suppressing the expression of mutant HTT is an important approach to treat the disease, we must first understand the normal function of Htt in adults versus younger animals. Using inducible Htt knockout mice, we found that Htt depletion does not lead to adult neurodegeneration or animal death at >4 mo of age, which was also verified by selectively depleting Htt in neurons. On the other hand, young Htt KO mice die at 2 mo of age of acute pancreatitis due to the degeneration of pancreatic acinar cells. Importantly, Htt interacts with the trypsin inhibitor, serine protease inhibitor Kazal-type 3 (Spink3), to inhibit activation of digestive enzymes in acinar cells in young mice, and transgenic HTT can rescue the early death of Htt KO mice. These findings point out age- and cell type-dependent vital functions of Htt and the safety of knocking down neuronal Htt expression in adult brains as a treatment.

N17 Modifies mutant Huntingtin nuclear pathogenesis and severity of disease in HD BAC transgenic mice

The nucleus is a critical subcellular compartment for the pathogenesis of polyglutamine disorders, including Huntington’s disease (HD). Recent studies suggest the first 17-amino-acid domain (N17) of mutant Huntingtin (mHTT) mediates its nuclear exclusion in cultured cells. Here, we test whether N17 could be a molecular determinant of nuclear mHTT pathogenesis in vivo. BAC transgenic mice expressing mHTT lacking the N17 domain (BACHD-ΔN17) show dramatically accelerated mHTT pathology exclusively in the nucleus, which is associated with HD-like transcriptionopathy. Interestingly, BACHD-ΔN17 mice manifest more overt disease-like phenotypes than the original BACHD mice, including body weight loss, movement deficits, robust striatal neuronal loss, and neuroinflammation. Mechanistically, N17 is necessary for nuclear exclusion of small mHTT fragments that are part of nuclear pathology in HD. Together, our study suggests that N17 modifies nuclear pathogenesis and disease severity in HD mice by regulating subcellular localization of known nuclear pathogenic mHTT species.

A large scale Huntingtin protein interaction network implicates Rho GTPase signaling pathways in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disease caused by a CAG expansion in the HTT gene. Using yeast two-hybrid methods, we identified a large set of proteins that interact with huntingtin (HTT)-interacting proteins. This network, composed of HTT-interacting proteins (HIPs) and proteins interacting with these primary nodes, contains 3235 interactions among 2141 highly interconnected proteins. Analysis of functional annotations of these proteins indicates that primary and secondary HIPs are enriched in pathways implicated in HD, including mammalian target of rapamycin, Rho GTPase signaling, and oxidative stress response. To validate roles for HIPs in mutant HTT toxicity, we show that the Rho GTPase signaling components, BAIAP2, EZR, PIK3R1, PAK2, and RAC1, are modifiers of mutant HTT toxicity. We also demonstrate that Htt co-localizes with BAIAP2 in filopodia and that mutant HTT interferes with filopodial dynamics. These data indicate that HTT is involved directly in membrane dynamics, cell attachment, and motility. Furthermore, they implicate dysregulation in these pathways as pathological mechanisms in HD.

Huntington's disease: the past, present, and future search for disease modifiers

Huntington's disease (HD) is an autosomal dominant genetic disorder that specifically causes neurodegeneration of striatal neurons, resulting in a triad of symptoms that includes emotional, cognitive, and motor disturbances. The HD mutation causes a polyglutamine repeat expansion within the N-terminal of the huntingtin (Htt) protein. This expansion causes aggregate formation within the cytosol and nucleus due to the presence of misfolded mutant Htt, as well as altered interactions with Htt's multiple binding partners, and changes in post-translational Htt modifications. The present review charts efforts toward a therapy that delays age of onset or slows symptom progression in patients affected by HD, as there is currently no effective treatment. Although silencing Htt expression appears promising as a disease modifying treatment, it should be attempted with caution in light of Htt's essential roles in neural maintenance and development. Other therapeutic targets include those that boost aggregate dissolution, target excitotoxicity and metabolic issues, and supplement growth factors.