Nuclear and cytoplasmic huntingtin inclusions exhibit distinct biochemical composition, interactome and ultrastructural properties

Pleiotropic effects of mutant huntingtin on retinopathy in two mouse models of Huntington's disease

Huntington's disease (HD) is caused by the expansion of a CAG repeat, encoding a string of glutamines (polyQ) in the first exon of the huntingtin gene (HTTex1). This mutant huntingtin protein (mHTT) with extended polyQ forms aggregates in cortical and striatal neurons, causing cell damage and death. The retina is part of the central nervous system (CNS), and visual deficits and structural abnormalities in the retina of HD patients have been observed. Defects in retinal structure and function are also present in the R6/2 and R6/1 HD transgenic mouse models that contain a gene fragment to express mHTTex1. We investigated whether these defects extend to the zQ175KI mouse model which is thought to be more representative of the human condition because it was engineered to contain the extended CAG repeat within the endogenous HTT locus. We found qualitatively similar phenotypes between R6/1 and zQ175KI retinae that include the presence of mHTT aggregates in retinal neurons, cone loss, downregulation of rod signaling proteins and abnormally elongated photoreceptor connecting cilia. In addition, we present novel findings that mHTT disrupts cell polarity in the photoreceptor cell layer and the retinal pigment epithelium (RPE). Furthermore, we show that the RPE cells from R6/1 mice contain mHTT nuclear inclusions, adding to the list of non-neuronal cells with mHTT aggregates and pathology. Thus, the eye may serve as a useful system to track disease progression and to test therapeutic intervention strategies for HD.

A nuclear protein quality control system for elimination of nucleolus-related inclusions

The identification of pathways that control elimination of protein inclusions is essential to understand the cellular response to proteotoxicity, particularly in the nuclear compartment, for which our knowledge is limited. We report that stress-induced nuclear inclusions related to the nucleolus are eliminated upon stress alleviation during the recovery period. This process is independent of autophagy/lysosome and CRM1-mediated nuclear export pathways, but strictly depends on the ubiquitin-activating E1 enzyme, UBA1, and on nuclear proteasomes that are recruited into the formed inclusions. UBA1 activity is essential only for the recovery process but dispensable for nuclear inclusion formation. Furthermore, the E3 ligase HUWE1 and HSP70 are components of the ubiquitin/chaperone systems that promote inclusion elimination. The recovery process also requires RNA Pol I-dependent production of the lncRNA IGS42 during stress. IGS42 localises within the formed inclusions and promotes their elimination by preserving the mobility of resident proteins. These findings reveal a protein quality control system that operates within the nucleus for the elimination of stress-induced nucleolus-related inclusions.

Engineering a membrane protein chaperone to ameliorate the proteotoxicity of mutant huntingtin

Toxic protein aggregates are associated with various neurodegenerative diseases, including Huntington's disease (HD). Since no current treatment delays the progression of HD, we develop a mechanistic approach to prevent mutant huntingtin (mHttex1) aggregation. Here, we engineer the ATP-independent cytosolic chaperone PEX19, which targets peroxisomal membrane proteins to peroxisomes, to remove mHttex1 aggregates. Using yeast toxicity-based screening with a random mutant library, we identify two yeast PEX19 variants and engineer equivalent mutations into human PEX19 (hsPEX19). These variants effectively delay mHttex1 aggregation in vitro and in cellular HD models. The mutated hydrophobic residue in the α4 helix of hsPEX19 variants binds to the N17 domain of mHttex1, thereby inhibiting the initial aggregation process. Overexpression of the hsPEX19-FV variant rescues HD-associated phenotypes in primary striatal neurons and in Drosophila. Overall, our data reveal that engineering ATP-independent membrane protein chaperones is a promising therapeutic approach for rational targeting of mHttex1 aggregation in HD.

The N17 domain of huntingtin as a multifaceted player in Huntington's disease

Huntington's disease (HD) is primarily caused by the aberrant aggregation of the N-terminal exon 1 fragment of mutant huntingtin protein (mHttex1) with expanded polyglutamine (polyQ) repeats in neurons. The first 17 amino acids of the N-terminus of Httex1 (N17 domain) immediately preceding the polyQ repeat domain are evolutionarily conserved across vertebrates and play multifaceted roles in the pathogenesis of HD. Due to its amphipathic helical properties, the N17 domain, both alone and when membrane-associated, promotes mHttEx1 aggregation. Diverse post-translational modifications (PTMs) in the N17 domain alter the aggregation state, thus modulating the cellular toxicity of mHttex1. Furthermore, the N17 domain serves as a nuclear export signal (NES) and mediates the cytoplasmic localization of mHttex1. This review summarizes the four main roles of the N17 domain in regulating HD pathology and discusses potential therapeutic approaches targeting this N17 domain to mitigate HD progression.

mTOR inhibition in Q175 Huntington's disease model mice facilitates neuronal autophagy and mutant huntingtin clearance

Huntington's disease (HD) is caused by expansion of the polyglutamine stretch in huntingtin protein (HTT) resulting in hallmark aggresomes/inclusion bodies (IBs) composed of mutant huntingtin protein (mHTT) and its fragments. Stimulating autophagy to enhance mHTT clearance is considered a potential therapeutic strategy for HD. Our recent evaluation of the autophagic-lysosomal pathway (ALP) in human HD brain reveals upregulated lysosomal biogenesis and relatively normal autophagy flux in early Vonsattel grade brains, but impaired autolysosome clearance in late grade brains, suggesting that autophagy stimulation could have therapeutic benefits as an earlier clinical intervention. Here, we tested this hypothesis by crossing the Q175 HD knock-in model with our autophagy reporter mouse TRGL ( T hy-1- R FP- G FP- L C3) to investigate in vivo neuronal ALP dynamics. In the Q175 and/or TRGL/Q175 mice, mHTT was detected in autophagic vacuoles and also exhibited high level colocalization with autophagy receptors p62/SQSTM1 and ubiquitin in the IBs. Compared to the robust lysosomal pathology in late-stage human HD striatum, ALP alterations in Q175 models are also late-onset but milder that included a lowered phospho-p70S6K level, lysosome depletion and autolysosome elevation including more poorly acidified autolysosomes and larger-sized lipofuscin granules, reflecting impaired autophagic flux. Administration of a mTOR inhibitor to 6-mo-old TRGL/Q175 normalized lysosome number, ameliorated aggresome pathology while reducing mHTT-, p62- and ubiquitin-immunoreactivities, suggesting beneficial potential of autophagy modulation at early stages of disease progression.

Developmental and physiological impacts of pathogenic human huntingtin protein in the nervous system

Huntington's Disease (HD) is a neurodegenerative disorder, part of the nine identified inherited polyglutamine (polyQ) diseases. Most commonly, HD pathophysiology manifests in middle-aged adults with symptoms including progressive loss of motor control, cognitive decline, and psychiatric disturbances. Associated with the pathophysiology of HD is the formation of insoluble fragments of the huntingtin protein (htt) that tend to aggregate in the nucleus and cytoplasm of neurons. To track both the intracellular progression of the aggregation phenotype as well as the physiological deficits associated with mutant htt, two constructs of human HTT were expressed in the Drosophila melanogaster nervous system with varying polyQ lengths, non-pathogenic-htt (NP-htt) and pathogenic-htt (P-htt), with an N-terminal RFP tag for in vivo visualization. P-htt aggregates accumulate in the ventral nerve cord cell bodies as early as 24 h post hatching and significant aggregates form in the segmental nerve branches at 48 h post hatching. Organelle trafficking up- and downstream of aggregates formed in motor neurons showed severe deficits in trafficking dynamics. To explore putative downstream deficits of htt aggregation, ultrastructural changes of presynaptic motor neurons and muscles were assessed, but no significant effects were observed. However, the force and kinetics of muscle contractions were severely affected in P-htt animals, reminiscent of human chorea. Reduced muscle force production translated to altered locomotory behavior. A novel HD aggregation model was established to track htt aggregation throughout adulthood in the wing, showing similar aggregation patterns with larvae. Expressing P-htt in the adult nervous system resulted in significantly reduced lifespan, which could be partially rescued by feeding flies the mTOR inhibitor rapamycin. These findings advance our understanding of htt aggregate progression as well the downstream physiological impacts on the nervous system and peripheral tissues.

Nuclear poly-glutamine aggregates rupture the nuclear envelope and hinder its repair

Huntington's disease (HD) is caused by a polyglutamine expansion of the huntingtin protein, resulting in the formation of polyglutamine aggregates. The mechanisms of toxicity that result in the complex HD pathology remain only partially understood. Here, we show that nuclear polyglutamine aggregates induce nuclear envelope (NE) blebbing and ruptures that are often repaired incompletely. These ruptures coincide with disruptions of the nuclear lamina and lead to lamina scar formation. Expansion microscopy enabled resolving the ultrastructure of nuclear aggregates and revealed polyglutamine fibrils sticking into the cytosol at rupture sites, suggesting a mechanism for incomplete repair. Furthermore, we found that NE repair factors often accumulated near nuclear aggregates, consistent with stalled repair. These findings implicate nuclear polyQ aggregate-induced loss of NE integrity as a potential contributing factor to Huntington's disease and other polyglutamine diseases.

Unveiling intracellular phase separation: advances in optical imaging of biomolecular condensates

Intracellular biomolecular condensates, which form via phase separation, display a highly organized ultrastructure and complex properties. Recent advances in optical imaging techniques, including super-resolution microscopy and innovative microscopic methods that leverage the intrinsic properties of the molecules observed, have transcended the limitations of conventional microscopies. These advances facilitate the exploration of condensates at finer scales and in greater detail. The deployment of these emerging but sophisticated imaging tools allows for precise observations of the multiphasic organization and physicochemical properties of these condensates, shedding light on their functions in cellular processes. In this review, we highlight recent progress in methodological innovations and their profound implications for understanding the organization and dynamics of intracellular biomolecular condensates.

Luteolin as potential treatment for Huntington's disease: Insights from a transgenic mouse model

AIMS

The study aimed to evaluate the potential benefits of luteolin treatment in Huntington's disease (HD), an inherited progressive neurodegenerative disorder.

METHODS

HD N171-82Q transgenic and WT mice received luteolin or vehicle for treatment at 6 weeks of age. The mice's body weight changes and survival rates were monitored throughout the study, and a series of motor functional tests were conducted. Serum level of the marker NfL was also determined. Immunohistochemical staining and western blotting were utilized to assess the expression of huntingtin aggregates.

RESULTS

Luteolin treatment enhanced survival and prevented weight loss in HD mice compared to the vehicle-treated HD group. Furthermore, the luteolin-treated HD mice exhibited enhanced motor coordination and balance and significantly reduced motor dysfunction. Also, luteolin decreased serum NfL levels in HD mice. Notably, the accumulation of huntingtin aggregates was significantly reduced in the brain's cortex, hippocampus, and striatum of luteolin-treated HD mice compared to the vehicle-treated HD group.

CONCLUSION

Luteolin holds promise as a therapeutic agent for improving survival outcomes, managing motor dysfunction, and reducing huntingtin aggregates in HD. The findings are of significance as currently, there are no approved therapeutic interventions that reverse HD pathology or slow down its progression.

TDP43 and huntingtin Exon-1 undergo a conformationally specific interaction that strongly alters the fibril formation of both proteins

Protein aggregation is a common feature of many neurodegenerative diseases. In Huntington's disease, mutant huntingtin is the primary aggregating protein, but the aggregation of other proteins, such as TDP43, is likely to further contribute to toxicity. Moreover, mutant huntingtin is also a risk factor for TDP pathology in ALS. Despite this co-pathology of huntingtin and TDP43, it remains unknown whether these amyloidogenic proteins directly interact with each other. Using a combination of biophysical methods, we show that the aggregation-prone regions of both proteins, huntingtin exon-1 (Httex1) and the TDP43 low complexity domain (TDP43-LCD), interact in a conformationally specific manner. This interaction significantly slows Httex1 aggregation, while it accelerates TDP43-LCD aggregation. A key intermediate responsible for both effects is a complex formed by liquid TDP43-LCD condensates and Httex1 fibrils. This complex shields seeding competent surfaces of Httex1 fibrils from Httex1 monomers, which are excluded from the condensates. In contrast, TDP43-LCD condensates undergo an accelerated liquid-to-solid transition upon exposure to Httex1 fibrils. Cellular studies show co-aggregation of untagged Httex1 with TDP43. This interaction causes mislocalization of TDP43, which has been linked to TDP43 toxicity. The protection from Httex1 aggregation in lieu of TDP43-LCD aggregation is interesting, as it mirrors what has been found in disease models, namely that TDP43 can protect from huntingtin toxicity, while mutant huntingtin can promote TDP43 pathology. These results suggest that direct protein interaction could, at least in part, be responsible for the linked pathologies of both proteins.

Developmental and physiological impacts of pathogenic human huntingtin protein in the nervous system

Huntington's Disease (HD) is a neurodegenerative disorder, part of the nine identified inherited polyglutamine (polyQ) diseases. Most commonly, HD pathophysiology manifests in middle-aged adults with symptoms including progressive loss of motor control, cognitive decline, and psychiatric disturbances. Associated with the pathophysiology of HD is the formation of insoluble fragments of the huntingtin protein (htt) that tend to aggregate in the nucleus and cytoplasm of neurons. To track both the intracellular progression of the aggregation phenotype as well as the physiological deficits associated with mutant htt, two constructs of human HTT were expressed with varying polyQ lengths, non-pathogenic-htt (Q15, NP-htt) and pathogenic-htt (Q138, P-htt), with an N-terminal RFP tag for in vivo visualization. P-htt aggregates accumulate in the ventral nerve cord cell bodies as early as 24 hours post hatching and significant aggregates form in the segmental nerve branches at 48 hours post hatching. Organelle trafficking up-and downstream of aggregates formed in motor neurons showed severe deficits in trafficking dynamics. To explore putative downstream deficits of htt aggregation, ultrastructural changes of presynaptic motor neurons and muscles were assessed, but no significant effects were observed. However, the force and kinetics of muscle contractions were severely affected in P-htt animals, reminiscent of human chorea. Reduced muscle force production translated to altered locomotory behavior. A novel HD aggregation model was established to track htt aggregation throughout adulthood in the wing, showing similar aggregation patterns with larvae. Expressing P-htt in the adult nervous system resulted in significantly reduced lifespan, which could be partially rescued by feeding flies the mTOR inhibitor rapamycin. These findings advance our understanding of htt aggregate progression as well the downstream physiological impacts on the nervous system and peripheral tissues.

Structural Mapping of Protein Aggregates in Live Cells Modeling Huntington's Disease

While protein aggregation is a hallmark of many neurodegenerative diseases, acquiring structural information on protein aggregates inside live cells remains challenging. Traditional microscopy does not provide structural information on protein systems. Routinely used fluorescent protein tags, such as Green Fluorescent Protein (GFP), might perturb native structures. Here, we report a counter-propagating mid-infrared photothermal imaging approach enabling mapping of secondary structure of protein aggregates in live cells modeling Huntington's disease. By comparing mid-infrared photothermal spectra of label-free and GFP-tagged huntingtin inclusions, we demonstrate that GFP fusions indeed perturb the secondary structure of aggregates. By implementing spectra with small spatial step for dissecting spectral features within sub-micrometer distances, we reveal that huntingtin inclusions partition into a β-sheet-rich core and a ɑ-helix-rich shell. We further demonstrate that this structural partition exists only in cells with the [RNQ+] prion state, while [rnq-] cells only carry smaller β-rich non-toxic aggregates. Collectively, our methodology has the potential to unveil detailed structural information on protein assemblies in live cells, enabling high-throughput structural screenings of macromolecular assemblies.

Peripheral sequestration of huntingtin delays neuronal death and depends on N-terminal ubiquitination

Huntington's disease (HD) is caused by a glutamine repeat expansion in the protein huntingtin. Mutated huntingtin (mHtt) forms aggregates whose impacts on neuronal survival are still debated. Using weeks-long, continual imaging of cortical neurons, we find that mHtt is gradually sequestrated into peripheral, mainly axonal aggregates, concomitant with dramatic reductions in cytosolic mHtt levels and enhanced neuronal survival. in-situ pulse-chase imaging reveals that aggregates continually gain and lose mHtt, in line with these acting as mHtt sinks at equilibrium with cytosolic pools. Mutating two N-terminal lysines found to be ubiquitinated in HD animal models suppresses peripheral aggregate formation and reductions in cytosolic mHtt, promotes nuclear aggregate formation, stabilizes aggregates and leads to pervasive neuronal death. These findings demonstrate the capacity of aggregates formed at peripheral locations to sequester away cytosolic, presumably toxic mHtt forms and support a crucial role for N-terminal ubiquitination in promoting these processes and delaying neuronal death.

Microglia contribute to polyG-dependent neurodegeneration in neuronal intranuclear inclusion disease

Neuronal intranuclear inclusion disease (NIID) is a neurodegenerative disorder caused by the expansion of GGC trinucleotide repeats in NOTCH2NLC gene. Despite identifying uN2CpolyG, a toxic polyglycine (polyG) protein translated by expanded GGC repeats, the exact pathogenic mechanisms of NIID remain unclear. In this study, we investigated the role of polyG by expressing various forms of NOTCH2NLC in mice: the wild-type, the expanded form with 100 GGC repeats (either translating or not translating into uN2CpolyG), and the mutated form that encodes a pure polyG without GGC-repeat RNA and the C-terminal stretch (uN2CpolyG-dCT). Both uN2CpolyG and uN2CpolyG-dCT induced the formation of inclusions composed by filamentous materials and resulted in neurodegenerative phenotypes in mice, including impaired motor and cognitive performance, shortened lifespan, and pathologic lesions such as white-matter lesions, microgliosis, and astrogliosis. In contrast, expressing GGC-repeat RNA alone was non-pathogenic. Through bulk and single-nuclei RNA sequencing, we identified common molecular signatures linked to the expression of uN2CpolyG and uN2CpolyG-dCT, particularly the upregulation of inflammation and microglia markers, and the downregulation of immediate early genes and splicing factors. Importantly, microglia-mediated inflammation was visualized in NIID patients using positron emission tomography, correlating with levels of white-matter atrophy. Furthermore, microglia ablation ameliorated neurodegenerative phenotypes and transcriptional alterations in uN2CpolyG-expressing mice but did not affect polyG inclusions. Together, these results demonstrate that polyG is crucial for the pathogenesis of NIID and highlight the significant role of microglia in polyG-induced neurodegeneration.

In vivo CRISPR base editing for treatment of Huntington's disease

Huntington's disease (HD) is an inherited and ultimately fatal neurodegenerative disorder caused by an expanded polyglutamine-encoding CAG repeat within exon 1 of the huntingtin (HTT) gene, which produces a mutant protein that destroys striatal and cortical neurons. Importantly, a critical event in the pathogenesis of HD is the proteolytic cleavage of the mutant HTT protein by caspase-6, which generates fragments of the N-terminal domain of the protein that form highly toxic aggregates. Given the role that proteolysis of the mutant HTT protein plays in HD, strategies for preventing this process hold potential for treating the disorder. By screening 141 CRISPR base editor variants targeting splice elements in the HTT gene, we identified platforms capable of producing HTT protein isoforms resistant to caspase-6-mediated proteolysis via editing of the splice acceptor sequence for exon 13. When delivered to the striatum of a rodent HD model, these base editors induced efficient exon skipping and decreased the formation of the N-terminal fragments, which in turn reduced HTT protein aggregation and attenuated striatal and cortical atrophy. Collectively, these results illustrate the potential for CRISPR base editing to decrease the toxicity of the mutant HTT protein for HD.

Pathobiology of the autophagy-lysosomal pathway in the Huntington's disease brain

Accumulated levels of mutant huntingtin protein (mHTT) and its fragments are considered contributors to the pathogenesis of Huntington's disease (HD). Although lowering mHTT by stimulating autophagy has been considered a possible therapeutic strategy, the role and competence of autophagy-lysosomal pathway (ALP) during HD progression in the human disease remains largely unknown. Here, we used multiplex confocal and ultrastructural immunocytochemical analyses of ALP functional markers in relation to mHTT aggresome pathology in striatum and the less affected cortex of HD brains staged from HD2 to HD4 by Vonsattel neuropathological criteria compared to controls. Immunolabeling revealed the localization of HTT/mHTT in ALP vesicular compartments labeled by autophagy-related adaptor proteins p62/SQSTM1 and ubiquitin, and cathepsin D (CTSD) as well as HTT-positive inclusions. Although comparatively normal at HD2, neurons at later HD stages exhibited progressive enlargement and clustering of CTSD-immunoreactive autolysosomes/lysosomes and, ultrastructurally, autophagic vacuole/lipofuscin granules accumulated progressively, more prominently in striatum than cortex. These changes were accompanied by rises in levels of HTT/mHTT and p62/SQSTM1, particularly their fragments, in striatum but not in the cortex, and by increases of LAMP1 and LAMP2 RNA and LAMP1 protein. Importantly, no blockage in autophagosome formation and autophagosome-lysosome fusion was detected, thus pinpointing autophagy substrate clearance deficits as a basis for autophagic flux declines. The findings collectively suggest that upregulated lysosomal biogenesis and preserved proteolysis maintain autophagic clearance in early-stage HD, but failure at advanced stages contributes to progressive HTT build-up and potential neurotoxicity. These findings support the prospect that ALP stimulation applied at early disease stages, when clearance machinery is fully competent, may have therapeutic benefits in HD patients.

Label-Free Techniques for Probing Biomolecular Condensates

Biomolecular condensates play important roles in a wide array of fundamental biological processes, such as cellular compartmentalization, cellular regulation, and other biochemical reactions. Since their discovery and first observations, an extensive and expansive library of tools has been developed to investigate various aspects and properties, encompassing structural and compositional information, material properties, and their evolution throughout the life cycle from formation to eventual dissolution. This Review presents an overview of the expanded set of tools and methods that researchers use to probe the properties of biomolecular condensates across diverse scales of length, concentration, stiffness, and time. In particular, we review recent years' exciting development of label-free techniques and methodologies. We broadly organize the set of tools into 3 categories: (1) imaging-based techniques, such as transmitted-light microscopy (TLM) and Brillouin microscopy (BM), (2) force spectroscopy techniques, such as atomic force microscopy (AFM) and the optical tweezer (OT), and (3) microfluidic platforms and emerging technologies. We point out the tools' key opportunities, challenges, and future perspectives and analyze their correlative potential as well as compatibility with other techniques. Additionally, we review emerging techniques, namely, differential dynamic microscopy (DDM) and interferometric scattering microscopy (iSCAT), that have huge potential for future applications in studying biomolecular condensates. Finally, we highlight how some of these techniques can be translated for diagnostics and therapy purposes. We hope this Review serves as a useful guide for new researchers in this field and aids in advancing the development of new biophysical tools to study biomolecular condensates.

Viral-based animal models in polyglutamine disorders

Polyglutamine disorders are a complex group of incurable neurodegenerative disorders caused by an abnormal expansion in the trinucleotide cytosine-adenine-guanine tract of the affected gene. To better understand these disorders, our dependence on animal models persists, primarily relying on transgenic models. In an effort to complement and deepen our knowledge, researchers have also developed animal models of polyglutamine disorders employing viral vectors. Viral vectors have been extensively used to deliver genes to the brain, not only for therapeutic purposes but also for the development of animal models, given their remarkable flexibility. In a time- and cost-effective manner, it is possible to use different transgenes, at varying doses, in diverse targeted tissues, at different ages, and in different species, to recreate polyglutamine pathology. This paper aims to showcase the utility of viral vectors in disease modelling, share essential considerations for developing animal models with viral vectors, and provide a comprehensive review of existing viral-based animal models for polyglutamine disorders.

Role of TFEB in Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by an expansion of the CAG trinucleotide repeat in exon 1 of the huntingtin (HTT) gene. This expansion leads to a polyglutamine (polyQ) tract at the N-terminal end of HTT, which reduces the solubility of the protein and promotes its accumulation. Inefficient clearance of mutant HTT (mHTT) by the proteasome or autophagy-lysosomal system leads to accumulation of oligomers and toxic protein aggregates in neurons, resulting in impaired proteolytic systems, transcriptional dysregulation, impaired axonal transport, mitochondrial dysfunction and cellular energy imbalance. Growing evidence suggests that the accumulation of mHTT aggregates and autophagic and/or lysosomal dysfunction are the major pathogenic mechanisms underlying HD. In this context, enhancing autophagy may be an effective therapeutic strategy to remove protein aggregates and improve cell function. Transcription factor EB (TFEB), a master transcriptional regulator of autophagy, controls the expression of genes critical for autophagosome formation, lysosomal biogenesis, lysosomal function and autophagic flux. Consequently, the induction of TFEB activity to promote intracellular clearance may be a therapeutic strategy for HD. However, while some studies have shown that overexpression of TFEB facilitates the clearance of mHTT aggregates and ameliorates the disease phenotype, others indicate such overexpression may lead to mHTT co-aggregation and worsen disease progression. Further studies are necessary to confirm whether TFEB modulation could be an effective therapeutic strategy against mHTT-mediated toxicity in different disease models.

The polyglutamine domain is the primary driver of seeding in huntingtin aggregation

Huntington's Disease (HD) is a fatal, neurodegenerative disease caused by aggregation of the huntingtin protein (htt) with an expanded polyglutamine (polyQ) domain into amyloid fibrils. Htt aggregation is modified by flanking sequences surrounding the polyQ domain as well as the binding of htt to lipid membranes. Upon fibrillization, htt fibrils are able to template the aggregation of monomers into fibrils in a phenomenon known as seeding, and this process appears to play a critical role in cell-to-cell spread of HD. Here, exposure of C. elegans expressing a nonpathogenic N-terminal htt fragment (15-repeat glutamine residues) to preformed htt-exon1 fibrils induced inclusion formation and resulted in decreased viability in a dose dependent manner, demonstrating that seeding can induce toxic aggregation of nonpathogenic forms of htt. To better understand this seeding process, the impact of flanking sequences adjacent to the polyQ stretch, polyQ length, and the presence of model lipid membranes on htt seeding was investigated. Htt seeding readily occurred across polyQ lengths and was independent of flanking sequence, suggesting that the structured polyQ domain within fibrils is the key contributor to the seeding phenomenon. However, the addition of lipid vesicles modified seeding efficiency in a manner suggesting that seeding primarily occurs in bulk solution and not at the membrane interface. In addition, fibrils formed in the presence of lipid membranes displayed similar seeding efficiencies. Collectively, this suggests that the polyQ domain that forms the amyloid fibril core is the main driver of seeding in htt aggregation.

Pathogenetic Contributions and Therapeutic Implications of Transglutaminase 2 in Neurodegenerative Diseases

Neurodegenerative diseases encompass a heterogeneous group of disorders that afflict millions of people worldwide. Characteristic protein aggregates are histopathological hallmark features of these disorders, including Amyloid β (Aβ)-containing plaques and tau-containing neurofibrillary tangles in Alzheimer's disease, α-Synuclein (α-Syn)-containing Lewy bodies and Lewy neurites in Parkinson's disease and dementia with Lewy bodies, and mutant huntingtin (mHTT) in nuclear inclusions in Huntington's disease. These various aggregates are found in specific brain regions that are impacted by neurodegeneration and associated with clinical manifestations. Transglutaminase (TG2) (also known as tissue transglutaminase) is the most ubiquitously expressed member of the transglutaminase family with protein crosslinking activity. To date, Aβ, tau, α-Syn, and mHTT have been determined to be substrates of TG2, leading to their aggregation and implicating the involvement of TG2 in several pathophysiological events in neurodegenerative disorders. In this review, we summarize the biochemistry and physiologic functions of TG2 and describe recent advances in the pathogenetic role of TG2 in these diseases. We also review TG2 inhibitors tested in clinical trials and discuss recent TG2-targeting approaches, which offer new perspectives for the design of future highly potent and selective drugs with improved brain delivery as a disease-modifying treatment for neurodegenerative disorders.

Subcellular localization and ER-mediated cytotoxic function of α1A and α1ACT in spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a polyglutamine (polyQ) disease, which is caused by the elongation of CAG repeats encoding polyQ in the CACNA1A gene. The CACNA1A gene encodes two proteins, namely, α1A (a subunit of the plasma membrane calcium channel), which is translated in its entire length, and α1ACT, which is translated from the second cistron, and both proteins have a polyQ tract. The α1A-polyQ and α1ACT-polyQ proteins with an elongated polyQ stretch have been reported to form aggregates in cells and induce neuronal cell death, but the subcellular localization of these proteins and their cytotoxic properties remain unclear. In this study, we first analyzed SCA6 model mice and found that α1A-polyQlong localized mainly to the Golgi apparatus, whereas a portion of α1ACT-polyQlong localized to the nucleus. Analysis using Neuro2a cells also showed similar subcellular localizations of these proteins, and a proportion of both proteins localized to the endoplasmic reticulum (ER). Cytotoxic studies demonstrated that both proteins induce both the ER stress response and apoptosis, indicating that they are able to induce ER stress-induced apoptosis.

Defining a Lewy Body: Running Up the Hill of Shifting Definitions and Evolving Concepts

Lewy bodies (LBs) are pathological hallmarks of Parkinson's disease and dementia with Lewy bodies, characterized by the accumulation of α-synuclein (αSyn) protein in the brain. While LBs were first described a century ago, their formation and morphogenesis mechanisms remain incompletely understood. Here, we present a historical overview of LB definitions and highlight the importance of semantic clarity and precise definitions when describing brain inclusions. Recent breakthroughs in imaging revealed shared features within LB subsets and the enrichment of membrane-bound organelles in these structures, challenging the conventional LB formation model. We discuss the involvement of emerging concepts of liquid-liquid phase separation, where biomolecules demix from a solution to form dense condensates, as a potential LB formation mechanism. Finally, we emphasize the need for the operational definitions of LBs based on morphological characteristics and detection protocols, particularly in studies investigating LB formation mechanisms. A better understanding of LB organization and ultrastructure can contribute to the development of targeted therapeutic strategies for synucleinopathies.

Fluorescent protein tagging promotes phase separation and alters the aggregation pathway of huntingtin exon-1

Fluorescent protein tags are convenient tools for tracking the aggregation states of amyloidogenic or phase separating proteins, but the effect of the tags is often not well understood. Here, we investigated the impact of a C-terminal red fluorescent protein (RFP) tag on the phase separation of huntingtin exon-1 (Httex1), an N-terminal portion of the huntingtin protein that aggregates in Huntington's disease. We found that the RFP-tagged Httex1 rapidly formed micron-sized, phase separated states in the presence of a crowding agent. The formed structures had a rounded appearance and were highly dynamic according to electron paramagnetic resonance and fluorescence recovery after photobleaching, suggesting that the phase separated state was largely liquid in nature. Remarkably, the untagged protein did not undergo any detectable liquid condensate formation under the same conditions. In addition to strongly promoting liquid-liquid phase separation, the RFP tag also facilitated fibril formation, as the tag-dependent liquid condensates rapidly underwent a liquid-to-solid transition. The rate of fibril formation under these conditions was significantly faster than that of the untagged protein. When expressed in cells, the RFP-tagged Httex1 formed larger aggregates with different antibody staining patterns compared to untagged Httex1. Collectively, these data reveal that the addition of a fluorescent protein tag significantly impacts liquid and solid phase separations of Httex1 in vitro and leads to altered aggregation in cells. Considering that the tagged Httex1 is commonly used to study the mechanisms of Httex1 misfolding and toxicity, our findings highlight the importance to validate the conclusions with untagged protein.

Dipping contacts - a novel type of contact site at the interface between membraneless organelles and membranes

Liquid-liquid phase separation is a major mechanism for organizing macromolecules, particularly proteins with intrinsically disordered regions, in compartments not limited by a membrane or a scaffold. The cell can therefore be perceived as a complex emulsion containing many of these membraneless organelles, also referred to as biomolecular condensates, together with numerous membrane-bound organelles. It is currently unclear how such a complex concoction operates to allow for intracellular trafficking, signaling and metabolic processes to occur with high spatiotemporal precision. Based on experimental observations of synaptic vesicle condensates - a membraneless organelle that is in fact packed with membranes - we present here the framework of dipping contacts: a novel type of contact site between membraneless organelles and membranes. In this Hypothesis, we propose that our framework of dipping contacts can serve as a foundation to investigate the interface that couples the diffusion and material properties of condensates to biochemical processes occurring in membranes. The identity and regulation of this interface is especially critical in the case of neurodegenerative diseases, where aberrant inclusions of misfolded proteins and damaged organelles underlie cellular pathology.

The ubiquitin thioesterase YOD1 ameliorates mutant Huntingtin induced pathology in Drosophila

Huntington's disease (HD) is a neurodegenerative disorder caused by a dominant gain-of-function mutation in the huntingtin gene, resulting in an elongated polyglutamine repeat in the mutant Huntingtin (mHtt) that mediates aberrant protein interactions. Previous studies implicated the ubiquitin-proteasome system in HD, suggesting that restoring cellular proteostasis might be a key element in suppressing pathology. We applied genetic interaction tests in a Drosophila model to ask whether modulating the levels of deubiquitinase enzymes affect HD pathology. By testing 32 deubiquitinase genes we found that overexpression of Yod1 ameliorated all analyzed phenotypes, including neurodegeneration, motor activity, viability, and longevity. Yod1 did not have a similar effect in amyloid beta overexpressing flies, suggesting that the observed effects might be specific to mHtt. Yod1 overexpression did not alter the number of mHtt aggregates but moderately increased the ratio of larger aggregates. Transcriptome analysis showed that Yod1 suppressed the transcriptional effects of mHtt and restored the expression of genes involved in neuronal plasticity, vesicular transport, antimicrobial defense, and protein synthesis, modifications, and clearance. Furthermore, Yod1 overexpression in HD flies leads to the upregulation of genes involved in transcriptional regulation and synaptic transmission, which might be part of a response mechanism to mHtt-induced stress.

Structure-function relationships in protein homorepeats

Homorepeats (or polyX), protein segments containing repetitions of the same amino acid, are abundant in proteomes from all kingdoms of life and are involved in crucial biological functions as well as several neurodegenerative and developmental diseases. Mainly inserted in disordered segments of proteins, the structure/function relationships of homorepeats remain largely unexplored. In this review, we summarize present knowledge for the most abundant homorepeats, highlighting the role of the inherent structure and the conformational influence exerted by their flanking regions. Recent experimental and computational methods enable residue-specific investigations of these regions and promise novel structural and dynamic information for this elusive group of proteins. This information should increase our knowledge about the structural bases of phenomena such as liquid-liquid phase separation and trinucleotide repeat disorders.

Biochemical Deconstruction and Reconstruction of Nuclear Matrix Reveals the Layers of Nuclear Organization

Nuclear matrix (NuMat) is the fraction of the eukaryotic nucleus insoluble to detergents and high-salt extractions that manifests as a pan-nuclear fiber-granule network. NuMat consists of ribonucleoprotein complexes, members of crucial nuclear functional modules, and DNA fragments. Although NuMat captures the organization of nonchromatin nuclear space, very little is known about components organization within NuMat. To understand the organization of NuMat components, we subfractionated it with increasing concentrations of the chaotrope guanidinium hydrochloride (GdnHCl) and analyzed the proteomic makeup of the fractions. We observe that the solubilization of proteins at different concentrations of GdnHCl is finite and independent of the broad biophysical properties of the protein sequences. Looking at the extraction pattern of the nuclear envelope and nuclear pore complex, we surmise that this fractionation represents easily solubilized/loosely bound and difficultly solubilized/tightly bound components of NuMat. Microscopic analyses of the localization of key NuMat proteins across sequential GdnHCl extractions of in situ NuMat further elaborate on the divergent extraction patterns. Furthermore, we solubilized NuMat in 8M GdnHCl and upon removal of GdnHCl through dialysis, en masse renaturation leads to RNA-dependent self-assembly of fibrous structures. The major proteome component of the self-assembled fibers comes from the difficultly solubilized, tightly bound component. This fractionation of the NuMat reveals different organizational levels within it which may reflect the structural and functional organization of nuclear architecture.

Charge within Nt17 peptides modulates huntingtin aggregation and initial lipid binding events

Toxic aggregation of pathogenic huntingtin protein (htt) is implicated in Huntington's disease and influenced by various factors, including the first seventeen amino acids at the N-terminus (Nt17) and the presence of lipid membranes. Nt17 has a propensity to form an amphipathic α-helix in the presence of binding partners, which promotes α-helix rich oligomer formation and facilitates htt/lipid interactions. Within Nt17 are multiple sites that are subject to post-translational modification, including acetylation and phosphorylation. Acetylation can occur at lysine 6, 9, and/or 15 while phosphorylation can occur at threonine 3, serine 13, and/or serine 16. Such modifications impact aggregation and lipid binding through the alteration of various intra- and intermolecular interactions. When incubated with htt-exon1(46Q), free Nt17 peptides containing point mutations mimicking acetylation or phosphorylation reduced fibril formation and altered oligomer morphologies. Upon exposure to lipid vesicles, changes to peptide/lipid complexation were observed and peptide-containing oligomers demonstrated reduced lipid interactions.

Label-free identification of protein aggregates using deep learning

Protein misfolding and aggregation play central roles in the pathogenesis of various neurodegenerative diseases (NDDs), including Huntington's disease, which is caused by a genetic mutation in exon 1 of the Huntingtin protein (Httex1). The fluorescent labels commonly used to visualize and monitor the dynamics of protein expression have been shown to alter the biophysical properties of proteins and the final ultrastructure, composition, and toxic properties of the formed aggregates. To overcome this limitation, we present a method for label-free identification of NDD-associated aggregates (LINA). Our approach utilizes deep learning to detect unlabeled and unaltered Httex1 aggregates in living cells from transmitted-light images, without the need for fluorescent labeling. Our models are robust across imaging conditions and on aggregates formed by different constructs of Httex1. LINA enables the dynamic identification of label-free aggregates and measurement of their dry mass and area changes during their growth process, offering high speed, specificity, and simplicity to analyze protein aggregation dynamics and obtain high-fidelity information.

Advanced Techniques for Detecting Protein Misfolding and Aggregation in Cellular Environments

Protein misfolding and aggregation, a key contributor to the progression of numerous neurodegenerative diseases, results in functional deficiencies and the creation of harmful intermediates. Detailed visualization of this misfolding process is of paramount importance for improving our understanding of disease mechanisms and for the development of potential therapeutic strategies. While in vitro studies using purified proteins have been instrumental in delivering significant insights into protein misfolding, the behavior of these proteins in the complex milieu of living cells often diverges significantly from such simplified environments. Biomedical imaging performed in cell provides cellular-level information with high physiological and pathological relevance, often surpassing the depth of information attainable through in vitro methods. This review highlights a variety of methodologies used to scrutinize protein misfolding within biological systems. This includes optical-based methods, strategies leaning on mass spectrometry, in-cell nuclear magnetic resonance, and cryo-electron microscopy. Recent advancements in these techniques have notably deepened our understanding of protein misfolding processes and the features of the resulting misfolded species within living cells. The progression in these fields promises to catalyze further breakthroughs in our comprehension of neurodegenerative disease mechanisms and potential therapeutic interventions.

Elegant Nematodes Improve Our Understanding of Human Neuronal Diseases, the Role of Pollutants and Strategies of Resilience

The prevalence of neurodegenerative disorders such as Alzheimer's and Parkinson's disease are rising globally. The role of environmental pollution in neurodegeneration is largely unknown. Thus, this perspective advocates exposome research in C. elegans models of human diseases. The models express amyloid proteins such as Aβ, recapitulate the degeneration of specifically vulnerable neurons and allow for correlated neurobehavioral phenotyping throughout the entire life span of the nematode. Neurobehavioral traits like locomotion gaits, rigidity, or cognitive decline are quantifiable and carefully mimic key aspects of the human diseases. Underlying molecular pathways of neurodegeneration are elucidated in pollutant-exposed C. elegans Alzheimer's or Parkinson's models by transcriptomics (RNA-seq), mass spectrometry-based proteomics and omics addressing other biochemical traits. Validation of the identified disease pathways can be achieved by genome-wide association studies in matching human cohorts. A consistent One Health approach includes isolation of nematodes from contaminated sites and their comparative investigation by imaging, neurobehavioral profiling and single worm proteomics. C. elegans models of neurodegenerative diseases are likewise well-suited for high throughput methods that provide a promising strategy to identify resilience pathways of neurosafety and keep up with the number of pollutants, nonchemical exposome factors, and their interactions.

Interactions of amyloidogenic proteins with mitochondrial protein import machinery in aging-related neurodegenerative diseases

Most mitochondrial proteins are targeted to the organelle by N-terminal mitochondrial targeting sequences (MTSs, or "presequences") that are recognized by the import machinery and subsequently cleaved to yield the mature protein. MTSs do not have conserved amino acid compositions, but share common physicochemical properties, including the ability to form amphipathic α-helical structures enriched with basic and hydrophobic residues on alternating faces. The lack of strict sequence conservation implies that some polypeptides can be mistargeted to mitochondria, especially under cellular stress. The pathogenic accumulation of proteins within mitochondria is implicated in many aging-related neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases. Mechanistically, these diseases may originate in part from mitochondrial interactions with amyloid-β precursor protein (APP) or its cleavage product amyloid-β (Aβ), α-synuclein (α-syn), and mutant forms of huntingtin (mHtt), respectively, that are mediated in part through their associations with the mitochondrial protein import machinery. Emerging evidence suggests that these amyloidogenic proteins may present cryptic targeting signals that act as MTS mimetics and can be recognized by mitochondrial import receptors and transported into different mitochondrial compartments. Accumulation of these mistargeted proteins could overwhelm the import machinery and its associated quality control mechanisms, thereby contributing to neurological disease progression. Alternatively, the uptake of amyloidogenic proteins into mitochondria may be part of a protein quality control mechanism for clearance of cytotoxic proteins. Here we review the pathomechanisms of these diseases as they relate to mitochondrial protein import and effects on mitochondrial function, what features of APP/Aβ, α-syn and mHtt make them suitable substrates for the import machinery, and how this information can be leveraged for the development of therapeutic interventions.

Neurodegenerative disease-associated inclusion bodies are cleared by selective autophagy in budding yeast

Protein misfolding, aggregation, and accumulation cause neurodegenerative disorders. One such disorder, Huntington's disease, is caused by an increased number of glutamine-encoding trinucleotide repeats CAG in the first exon of the huntingtin (HTT) gene. Mutant proteins of Htt exon 1 with polyglutamine expansion are prone to aggregation and form pathological inclusion bodies in neurons. Extensive studies have shown that misfolded proteins are cleared by the ubiquitin-proteasome system or autophagy to alleviate their cytotoxicity. Misfolded proteins can form small soluble aggregates or large insoluble inclusion bodies. Previous works have elucidated the role of autophagy in the clearance of misfolded protein aggregates, but autophagic clearance of inclusion bodies remains poorly characterized. Here we use mutant Htt exon 1 with 103 polyglutamine (Htt103QP) as a model substrate to study the autophagic clearance of inclusion bodies in budding yeast. We found that the core autophagy-related proteins were required for Htt103QP inclusion body autophagy. Moreover, our evidence indicates that the autophagy of Htt103QP inclusion bodies is selective. Interestingly, Cue5/Tollip, a known autophagy receptor for aggrephagy, is dispensable for this inclusion body autophagy. From the known selective autophagy receptors in budding yeast, we identified three that are essential for inclusion body autophagy. Amyloid beta peptide (Aβ42) is a major component of amyloid plaques found in Alzheimer's disease brains. Interestingly, a similar selective autophagy pathway contributes to the clearance of Aβ42 inclusion bodies in budding yeast. Therefore, our results reveal a novel autophagic pathway specific for inclusion bodies associated with neurodegenerative diseases, which we have termed IBophagy.

Suboptimal Mitochondrial Activity Facilitates Nuclear Heat Shock Responses for Proteostasis and Genome Stability

Thermal stress induces dynamic changes in nuclear proteins and relevant physiology as a part of the heat shock response (HSR). However, how the nuclear HSR is fine-tuned for cellular homeostasis remains elusive. Here, we show that mitochondrial activity plays an important role in nuclear proteostasis and genome stability through two distinct HSR pathways. Mitochondrial ribosomal protein (MRP) depletion enhanced the nucleolar granule formation of HSP70 and ubiquitin during HSR while facilitating the recovery of damaged nuclear proteins and impaired nucleocytoplasmic transport. Treatment of the mitochondrial proton gradient uncoupler masked MRP-depletion effects, implicating oxidative phosphorylation in these nuclear HSRs. On the other hand, MRP depletion and a reactive oxygen species (ROS) scavenger non-additively decreased mitochondrial ROS generation during HSR, thereby protecting the nuclear genome from DNA damage. These results suggest that suboptimal mitochondrial activity sustains nuclear homeostasis under cellular stress, providing plausible evidence for optimal endosymbiotic evolution via mitochondria-to-nuclear communication.

The structural plasticity of polyglutamine repeats

From yeast to humans, polyglutamine (polyQ) repeat tracts are found frequently in the proteome and are particularly prominent in the activation domains of transcription factors. PolyQ is a polymorphic motif that modulates functional protein-protein interactions and aberrant self-assembly. Expansion of the polyQ repeated sequences beyond critical physiological repeat length thresholds triggers self-assembly and is linked to severe pathological implications. This review provides an overview of the current knowledge on the structures of polyQ tracts in the soluble and aggregated states and discusses the influence of neighboring regions on polyQ secondary structure, aggregation, and fibril morphologies. The influence of the genetic context of the polyQ-encoding trinucleotides is briefly discussed as a challenge for future endeavors in this field.

Functional Characterisation of the Circular RNA, circHTT(2-6), in Huntington's Disease

Trinucleotide repeat disorders comprise ~20 severe, inherited, human neuromuscular and neurodegenerative disorders, which result from an abnormal expansion of repetitive sequences in the DNA. The most common of these, Huntington's disease (HD), results from expansion of the CAG repeat region in exon 1 of the HTT gene via an unknown mechanism. Since non-coding RNAs have been implicated in the initiation and progression of many diseases, herein we focused on a circular RNA (circRNA) molecule arising from non-canonical splicing (backsplicing) of HTT pre-mRNA. The most abundant circRNA from HTT, circHTT(2-6), was found to be more highly expressed in the frontal cortex of HD patients, compared with healthy controls, and positively correlated with CAG repeat tract length. Furthermore, the mouse orthologue (mmu_circHTT(2-6)) was found to be enriched within the brain and specifically the striatum, a region enriched for medium spiny neurons that are preferentially lost in HD. Transgenic overexpression of circHTT(2-6) in two human cell lines-SH-SY5Y and HEK293-reduced cell proliferation and nuclear size without affecting cell cycle progression or cellular size, or altering the CAG repeat region length within HTT. CircHTT(2-6) overexpression did not alter total HTT protein levels, but reduced its nuclear localisation. As these phenotypic and genotypic changes resemble those observed in HD patients, our results suggest that circHTT(2-6) may play a functional role in the pathophysiology of this disease.

Sphingolipids and impaired hypoxic stress responses in Huntington disease

Huntington disease (HD) is a debilitating, currently incurable disease. Protein aggregation and metabolic deficits are pathological hallmarks but their link to neurodegeneration and symptoms remains debated. Here, we summarize alterations in the levels of different sphingolipids in an attempt to characterize sphingolipid patterns specific to HD, an additional molecular hallmark of the disease. Based on the crucial role of sphingolipids in maintaining cellular homeostasis, the dynamic regulation of sphingolipids upon insults and their involvement in cellular stress responses, we hypothesize that maladaptations or blunted adaptations, especially following cellular stress due to reduced oxygen supply (hypoxia) contribute to the development of pathology in HD. We review how sphingolipids shape cellular energy metabolism and control proteostasis and suggest how these functions may fail in HD and in combination with additional insults. Finally, we evaluate the potential of improving cellular resilience in HD by conditioning approaches (improving the efficiency of cellular stress responses) and the role of sphingolipids therein. Sphingolipid metabolism is crucial for cellular homeostasis and for adaptations following cellular stress, including hypoxia. Inadequate cellular management of hypoxic stress likely contributes to HD progression, and sphingolipids are potential mediators. Targeting sphingolipids and the hypoxic stress response are novel treatment strategies for HD.

Tumor suppressor p53 regulates heat shock factor 1 protein degradation in Huntington's disease

p53 and HSF1 are two major transcription factors involved in cell proliferation and apoptosis, whose dysregulation contributes to cancer and neurodegeneration. Contrary to most cancers, p53 is increased in Huntington's disease (HD) and other neurodegenerative diseases, while HSF1 is decreased. p53 and HSF1 reciprocal regulation has been shown in different contexts, but their connection in neurodegeneration remains understudied. Using cellular and animal models of HD, we show that mutant HTT stabilized p53 by abrogating the interaction between p53 and E3 ligase MDM2. Stabilized p53 promotes protein kinase CK2 alpha prime and E3 ligase FBXW7 transcription, both of which are responsible for HSF1 degradation. Consequently, p53 deletion in striatal neurons of zQ175 HD mice restores HSF1 abundance and decrease HTT aggregation and striatal pathology. Our work shows the mechanism connecting p53 stabilization with HSF1 degradation and pathophysiology in HD and sheds light on the broader molecular differences and commonalities between cancer and neurodegeneration.

The structure of pathogenic huntingtin exon 1 defines the bases of its aggregation propensity

Huntington's disease is a neurodegenerative disorder caused by a CAG expansion in the first exon of the HTT gene, resulting in an extended polyglutamine (poly-Q) tract in huntingtin (httex1). The structural changes occurring to the poly-Q when increasing its length remain poorly understood due to its intrinsic flexibility and the strong compositional bias. The systematic application of site-specific isotopic labeling has enabled residue-specific NMR investigations of the poly-Q tract of pathogenic httex1 variants with 46 and 66 consecutive glutamines. Integrative data analysis reveals that the poly-Q tract adopts long α-helical conformations propagated and stabilized by glutamine side chain to backbone hydrogen bonds. We show that α-helical stability is a stronger signature in defining aggregation kinetics and the structure of the resulting fibrils than the number of glutamines. Our observations provide a structural perspective of the pathogenicity of expanded httex1 and pave the way to a deeper understanding of poly-Q-related diseases.

Treatment with THI, an inhibitor of sphingosine-1-phosphate lyase, modulates glycosphingolipid metabolism and results therapeutically effective in experimental models of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disorder with no effective cure currently available. Over the past few years our research has shown that alterations in sphingolipid metabolism represent a critical determinant in HD pathogenesis. In particular, aberrant metabolism of sphingosine-1-phosphate (S1P) has been reported in multiple disease settings, including human postmortem brains from HD patients. In this study, we investigate the potential therapeutic effect of the inhibition of S1P degradative enzyme SGPL1, by the chronic administration of the 2-acetyl-5-tetrahydroxybutyl imidazole (THI) inhibitor. We show that THI mitigated motor dysfunctions in both mouse and fly models of HD. The compound evoked the activation of pro-survival pathways, normalized levels of brain-derived neurotrophic factor, preserved white matter integrity, and stimulated synaptic functions in HD mice. Metabolically, THI restored normal levels of hexosylceramides and stimulated the autophagic and lysosomal machinery, facilitating the reduction of nuclear inclusions of both wild-type and mutant huntingtin proteins.

Pathological tau signatures and nuclear alterations in neurons, astrocytes and microglia in Alzheimer's disease, progressive supranuclear palsy, and dementia with Lewy bodies

Accumulation of pathological tau aggregates is a prominent feature in tauopathies that leads during the course of the diseases to neuronal dysfunction before and cell death after. Microglia and astrocytes have been described as playing important roles in synaptic spreading of toxic tau in several neurodegenerative diseases (NDs). Here, we have investigated the immunological and biochemical properties of aggregated tau species in different brain cell types in tau-induced neurodegenerative diseases such as Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and dementia with Lewy bodies (DLB). Additionally, we examined nuclear size, nuclear density, and chromatin compaction in neuronal and glial cells from diseased brain tissues. Microscopic-histological examination was performed using in-house mouse monoclonal antibodies for toxic tau conformers (TTC-M1 and TTC-M2) and tau oligomers (TOMA1-4). By immunohistochemistry and co-immunofluorescence assays using TOMA/TTC-Ms and cell-type specific markers for neurons, astrocytes, and microglia, we observed that TOMA/TTC-Ms were immunoreactive to diverse tau species in different cell types. Analysis of colocalization coefficients indicated an increased pathological tau deposition mainly in the neurons. Western blot analysis of brain homogenates using TOMA/TTC-Ms revealed distinct patterns of tau aggregation in each disease, suggesting that TOMA/TTC-Ms can distinguish between different tau aggregates present in different tauopathies. Additionally, using DAPI staining, we observed that neuronal and astrocytic nuclei had significantly greater nuclear area and increased chromatin compaction in AD cortices compared to non-demented controls. In contrast, reduction in nuclear density/area and more relaxed chromatin was noticed in DLB neurons, astrocytes and microglia and PSP astrocytes and microglia. Cell-type specific tropism of toxic tau species in tauopathies will provide a greater understanding of the involvement of different brain cell types in tau pathology. In this study, we observed that each disease presented cell-type specific nuclear phenotype and tau deposition pattern.

Ubiquitin-modifying enzymes in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the N-terminus of the HTT gene. The CAG repeat expansion translates into a polyglutamine expansion in the mutant HTT (mHTT) protein, resulting in intracellular aggregation and neurotoxicity. Lowering the mHTT protein by reducing synthesis or improving degradation would delay or prevent the onset of HD, and the ubiquitin-proteasome system (UPS) could be an important pathway to clear the mHTT proteins prior to aggregation. The UPS is not impaired in HD, and proteasomes can degrade mHTT entirely when HTT is targeted for degradation. However, the mHTT protein is differently ubiquitinated when compared to wild-type HTT (wtHTT), suggesting that the polyQ expansion affects interaction with (de) ubiquitinating enzymes and subsequent targeting for degradation. The soluble mHTT protein is associated with several ubiquitin-modifying enzymes, and various ubiquitin-modifying enzymes have been identified that are linked to Huntington's disease, either by improving mHTT turnover or affecting overall homeostasis. Here we describe their potential mechanism of action toward improved mHTT targeting towards the proteostasis machinery.

Optical pulse labeling studies reveal exogenous seeding slows α-synuclein clearance

The accumulation of α-synuclein (α-syn) in intracellular formations known as Lewy bodies (LBs) is associated with several neurodegenerative diseases including Parkinson's disease and Lewy Body Dementia. There is still limited understanding of how α-syn and LB formation is associated with cellular dysfunction and degeneration in these diseases. To examine the clearance and production dynamics of α-syn we transduced organotypic murine brain slice cultures (BSCs) with recombinant adeno-associated viruses (rAAVs) to express Dendra2-tagged human wild-type (WT) and mutant A53T α-syn, with and without the addition of exogenous α-syn fibrillar seeds and tracked them over several weeks in culture using optical pulse labeling. We found that neurons expressing WT or mutant A53T human α-syn show similar rates of α-syn turnover even when insoluble, phosphorylated Ser129 α-syn has accumulated. Taken together, this data reveals α-syn aggregation and overexpression, pSer129 α-syn, nor the A53T mutation affect α-syn dynamics in this system. Prion-type seeding with exogenous α-syn fibrils significantly slows α-syn turnover, in the absence of toxicity but is associated with the accumulation of anti-p62 immunoreactivity and Thiazin Red positivity. Prion-type induction of α-syn aggregation points towards a potential protein clearance deficit in the presence of fibrillar seeds and the ease of this system to explore precise mechanisms underlying these processes. This system facilitates the exploration of α-syn protein dynamics over long-term culture periods. This platform can further be exploited to provide mechanistic insight on what drives this slowing of α-syn turnover and how therapeutics, other genes or different α-syn mutations may affect α-syn protein dynamics.

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.

Model biomolecular condensates have heterogeneous structure quantitatively dependent on the interaction profile of their constituent macromolecules

Biomolecular condensates play numerous roles in cells by selectively concentrating client proteins while excluding others. These functions are likely to be sensitive to the spatial organization of the scaffold proteins forming the condensate. We use coarse-grained molecular simulations to show that model intrinsically-disordered proteins phase separate into a heterogeneous, structured fluid characterized by a well-defined length scale. The proteins are modelled as semi-flexible polymers with punctate, multifunctional binding sites in good solvent conditions. Their dense phase is highly solvated with a spatial structure that is more sensitive to the separation of the binding sites than their affinity. We introduce graph theoretic measures to quantify their heterogeneity, and find that it increases with increasing binding site number, and exhibits multi-timescale dynamics. The model proteins also swell on passing from the dilute solution to the dense phase. The simulations predict that the structure of the dense phase is modulated by the location and affinity of binding sites distant from the termini of the proteins, while sites near the termini more strongly affect its phase behaviour. The relations uncovered between the arrangement of weak interaction sites on disordered proteins and the material properties of their dense phase can be experimentally tested to give insight into the biophysical properties, pathological effects, and rational design of biomolecular condensates.

HAP40 is a conserved central regulator of Huntingtin and a potential modulator of Huntington's disease pathogenesis

Perturbation of huntingtin (HTT)'s physiological function is one postulated pathogenic factor in Huntington's disease (HD). However, little is known how HTT is regulated in vivo. In a proteomic study, we isolated a novel ~40kDa protein as a strong binding partner of Drosophila HTT and demonstrated it was the functional ortholog of HAP40, an HTT associated protein shown recently to modulate HTT's conformation but with unclear physiological and pathologic roles. We showed that in both flies and human cells, HAP40 maintained conserved physical and functional interactions with HTT. Additionally, loss of HAP40 resulted in similar phenotypes as HTT knockout. More strikingly, HAP40 strongly affected HTT's stability, as depletion of HAP40 significantly reduced the levels of endogenous HTT protein while HAP40 overexpression markedly extended its half-life. Conversely, in the absence of HTT, the majority of HAP40 protein were degraded, likely through the proteasome. Further, the affinity between HTT and HAP40 was not significantly affected by polyglutamine expansion in HTT, and contrary to an early report, there were no abnormal accumulations of endogenous HAP40 protein in HD cells from mouse HD models or human patients. Lastly, when tested in Drosophila models of HD, HAP40 partially modulated the neurodegeneration induced by full-length mutant HTT while showed no apparent effect on the toxicity of mutant HTT exon 1 fragment. Together, our study uncovers a conserved mechanism governing the stability and in vivo functions of HTT and demonstrates that HAP40 is a central and positive regulator of endogenous HTT. Further, our results support that mutant HTT is toxic regardless of the presence of its partner HAP40, and implicate HAP40 as a potential modulator of HD pathogenesis through its multiplex effect on HTT's function, stability and the potency of mutant HTT's toxicity.

Structural Basis of Huntingtin Fibril Polymorphism Revealed by Cryogenic Electron Microscopy of Exon 1 HTT Fibrils

The lack of detailed insight into the structure of aggregates formed by the huntingtin protein (HTT) has hampered the efforts to develop therapeutics and diagnostics targeting pathology formation in the brain of patients with Huntington's disease. To address this knowledge gap, we investigated the structural properties of in vitro-generated fibrils from exon1 of the huntingtin protein by cryogenic electron microscopy and single-particle analyses. We show that wildtype and mutant exon1 of the huntingtin protein form nonhelical fibrils with a polyglutamine amyloid core composed of β-hairpins with unique characteristics that have not been previously observed with other amyloid filaments. The stacks of β-hairpins form long planar β-sheets (protofilaments) which combine inter- and intra-molecular interactions, with variable stacking angles and occasional out-of-register states of individual β-hairpins. These features and the propensity of protofilaments to undergo lateral association result in a high degree of fibril polymorphisms, including fibrils composed of varying numbers of protofilaments. Our results allow us to speculate on how the flanking domains are organized around the polyglutamine core of the fibril and provide insight into how they might affect the huntingtin fibril structure and polymorphism. The removal of the first 17 amino acids at the N-terminus resulted in surprising intra-fibril structural heterogeneity and reduced fibril's propensity to lateral associations. Overall, this work provides valuable insights that could help guide future mechanistic studies to elucidate the sequence and structural determinants of huntingtin aggregation, as well as for cryo-EM and structural studies of fibrils derived from huntingtin protein and other disease-associated polyglutamine-containing proteins.

Protein interaction networks in neurodegenerative diseases: from physiological function to aggregation

The accumulation of protein inclusions is linked to many neurodegenerative diseases that typically develop in older individuals, due to a combination of genetic and environmental factors. In rare familial neurodegenerative disorders, genes encoding for aggregation-prone proteins are often mutated. While the underlying mechanism leading to these diseases still remains to be fully elucidated, efforts in the past 20 years revealed a vast network of protein–protein interactions that play a major role in regulating the aggregation of key proteins associated with neurodegeneration. Misfolded proteins that can oligomerize and form insoluble aggregates associate with molecular chaperones and other elements of the proteolytic machineries that are the frontline workers attempting to protect the cells by promoting clearance and preventing aggregation. Proteins that are normally bound to aggregation-prone proteins can become sequestered and mislocalized in protein inclusions, leading to their loss of function. In contrast, mutations, posttranslational modifications, or misfolding of aggregation-prone proteins can lead to gain of function by inducing novel or altered protein interactions, which in turn can impact numerous essential cellular processes and organelles, such as vesicle trafficking and the mitochondria. This review examines our current knowledge of protein–protein interactions involving several key aggregation-prone proteins that are associated with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, or amyotrophic lateral sclerosis. We aim to provide an overview of the protein interaction networks that play a central role in driving or mitigating inclusion formation, while highlighting some of the key proteomic studies that helped to uncover the extent of these networks.