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

Transient Interdomain Interactions Modulate the Monomeric Structural Ensemble and Self-Assembly of Huntingtin Exon 1

Polyglutamine (polyQ) tract length expansion (≥ 36 residues) within the N-terminal exon-1 of Huntingtin (Httex1) leads to Huntington's disease, a neurodegenerative condition marked by the presence of intranuclear Htt inclusions. Notably, the polyQ tract in Httex1 is flanked by an N-terminal coiled-coil domain -N17 (17 amino acids), which promotes the formation of soluble oligomers and brings the aggregation-prone polyQ tracts in close proximity. However, the molecular mechanisms underlying the conversion of soluble oligomers into insoluble β-rich aggregates with increasing polyQ length, remain unclear. In this study, extensive atomistic molecular dynamics (MD) simulations (aggregate time ≈0.7 milliseconds) are performed to uncover the interplay between structural transformation and domain "cross-talk" on the conformational ensemble and oligomerization of Httex1 due to polyQ expansion. Notably, MD-derived ensembles of N17-Qn-P5 monomers validated against NMR indicated that in addition to elevated α-helicity, polyQ expansion also favored transient, interdomain (N17/polyQ) interactions which resulted in the emergence of β-sheet conformations. Further, interdomain interactions modulated the stability of N17-mediated polyQ dimers and promoted a heterogeneous dimerization landscape. Finally, it is observed that the intact C-terminal proline-rich domain (PRD) promoted condensation of Httex1 through self-interactions involving its P10/P11 tracts while also interacting with N17 to suppress its α-helicity.

Aberrant Short Tandem Repeats: Pathogenicity, Mechanisms, Detection, and Roles in Neuropsychiatric Disorders

Short tandem repeat (STR) sequences are highly variable DNA segments that significantly contribute to human neurodegenerative disorders, highlighting their crucial role in neuropsychiatric conditions. This article examines the pathogenicity of abnormal STRs and classifies tandem repeat expansion disorders(TREDs), emphasizing their genetic characteristics, mechanisms of action, detection methods, and associated animal models. STR expansions exhibit complex genetic patterns that affect the age of onset and symptom severity. These expansions disrupt gene function through mechanisms such as gene silencing, toxic gain-of-function mutations leading to RNA and protein toxicity, and the generation of toxic peptides via repeat-associated non-AUG (RAN) translation. Advances in sequencing technologies-from traditional PCR and Southern blotting to next-generation and long-read sequencing-have enhanced the accuracy of STR variation detection. Research utilizing these technologies has linked STR expansions to a range of neuropsychiatric disorders, including autism spectrum disorders and schizophrenia, highlighting their contribution to disease risk and phenotypic expression through effects on genes involved in neurodevelopment, synaptic function, and neuronal signaling. Therefore, further investigation is essential to elucidate the intricate interplay between STRs and neuropsychiatric diseases, paving the way for improved diagnostic and therapeutic strategies.

Multiple Copper Ions Bind to and Promote the Oligomerization of Huntingtin Protein with Nonpathological Repeat Expansions

Huntington's disease (HD) is a fatal neurodegenerative disease characterized by the expression of huntingtin protein (htt) that has a polyglutamine (CAG; polyQ) repeat domain consisting of 36 or more glutamines (mhtt). Historically, mhtt is more broadly associated with HD severity, as are elevated metal levels observed in HD patients. The depletion of wild-type (WT) htt (fewer than 36Qs) is also recognized as a contributing factor to HD progression; however, many questions remain about the interactions of biorelevant metals with WT htt and the impact of the interactions on protein aggregation. In the present work, we utilize a combination of biochemical assays and spectroscopic techniques to provide insights into the interaction of copper with an in vitro htt model (N171-17Q). Herein, we use sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and dynamic light scattering to show that the addition of equimolar or higher concentrations of Cu(II) to htt induces time- and temperature-dependent protein oligomerization/aggregation. Additionally, chelation assays, trapped ion mobility spectrometry, and mass spectrometry confirm the (i) rapid reduction of Cu(II) in the presence of N171-17Q htt, (ii) direct binding of multiple copper ions per protein, and (iii) complex Cu:htt speciation profile with a preference for three distinct Cu:htt states. These findings contribute to our molecular level understanding of copper's role in the depletion and oligomerization/aggregation of WT htt while underscoring the physiological significance of our work, its potential relevance to metal binding in mhtt, and its significance for identifying new avenues for biomarker exploration and therapeutic design strategies.

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.

Corticostriatal maldevelopment in the R6/2 mouse model of juvenile Huntington's disease

There is a growing consensus that brain development in Huntington's disease (HD) is abnormal, leading to the idea that HD is not only a neurodegenerative but also a neurodevelopmental disorder. Indeed, structural and functional abnormalities have been observed during brain development in both humans and animal models of HD. However, a concurrent study of cortical and striatal development in a genetic model of HD is still lacking. Here we report significant alterations of corticostriatal development in the R6/2 mouse model of juvenile HD. We examined wildtype (WT) and R6/2 mice at postnatal (P) days 7, 14, and 21. Morphological examination demonstrated early structural and cellular alterations reminiscent of malformations of cortical development, and ex vivo electrophysiological recordings of cortical pyramidal neurons (CPNs) demonstrated significant age- and genotype-dependent changes of intrinsic membrane and synaptic properties. In general, R6/2 CPNs had reduced cell membrane capacitance and increased input resistance (P7 and P14), along with reduced frequency of spontaneous excitatory and inhibitory synaptic events during early development (P7), suggesting delayed cortical maturation. This was confirmed by increased occurrence of GABAA receptor-mediated giant depolarizing potentials at P7. At P14, the rheobase of CPNs was significantly reduced, along with increased excitability. Altered membrane and synaptic properties of R6/2 CPNs recovered progressively, and by P21 they were similar to WT CPNs. In striatal medium-sized spiny neurons (MSNs), a different picture emerged. Intrinsic membrane properties were relatively normal throughout development, except for a transient increase in membrane capacitance at P14. The first alterations in MSNs synaptic activity were observed at P14 and consisted of significant deficits in GABAergic inputs, however, these also were normalized by P21. In contrast, excitatory inputs began to decrease at this age. We conclude that the developing HD brain is capable of compensating for early developmental abnormalities and that cortical alterations precede and are a main contributor of striatal changes. Addressing cortical maldevelopment could help prevent or delay disease manifestations.

Insights into RNA-mediated pathology in new mouse models of Huntington's disease

Huntington's disease (HD) is a neurodegenerative polyglutamine (polyQ) disease resulting from the expansion of CAG repeats located in the ORF of the huntingtin gene (HTT). The extent to which mutant mRNA-driven disruptions contribute to HD pathogenesis, particularly in comparison to the dominant mechanisms related to the gain-of-function effects of the mutant polyQ protein, is still debatable. To evaluate this contribution in vivo, we generated two mouse models through a knock-in strategy at the Rosa26 locus. These models expressed distinct variants of human mutant HTT cDNA fragment: a translated variant (HD/100Q model, serving as a reference) and a nontranslated variant (HD/100CAG model). The cohorts of animals were subjected to a broad spectrum of molecular, behavioral, and cognitive analysis for 21 months. Behavioral testing revealed alterations in both models, with the HD/100Q model exhibiting late disease phenotype. The rotarod, static rod, and open-field tests showed some motor deficits in HD/100CAG and HD/100Q model mice during the light phase, while ActiMot indicated hyperkinesis during the dark phase. Both models also exhibited certain gene deregulations in the striatum that are related to disrupted pathways and phenotype alterations observed in HD. In conclusion, we provide in vivo evidence for a minor contributory role of mutant RNA in HD pathogenesis. The separated effects resulting from the presence of mutant RNA in the HD/100CAG model led to less severe but, to some extent, similar types of impairments as in the HD/100Q model. Increased anxiety was one of the most substantial effects caused by mutant HTT RNA.

Transient interdomain interactions modulate the monomeric structural ensemble and self-assembly of Huntingtin Exon 1

Polyglutamine expansion (≥ 36 residues) within the N-terminal exon-1 of Huntingtin (Httex1) leads to Huntington's disease, a neurodegenerative condition marked by the presence of intranuclear Htt inclusions. Notably, the polyglutamine tract in Httex1 is flanked by an N-terminal coiled-coil domain - N17 (17 amino acids), which undergoes self-association to promote the formation of soluble Httex1 oligomers and brings the aggregation-prone polyQ tracts in close spatial proximity. However, the mechanisms underlying the subsequent conversion of soluble oligomers into insoluble β-rich aggregates with increasing polyQ length, remain unclear. Current knowledge suggests that expansion of the polyQ tract increases its helicity, and this favors its oligomerization and aggregation. In addition, studies utilizing photocrosslinking, conformation-specific antibodies and a stable coiled-coil heterotetrametric system fused to polyQ indicate that domain "cross-talk" (i.e., interdomain interactions) may play a role in the emergence of toxic conformations and the conversion of Httex1 oligomers into fibrillar aggregates. Here, we performed extensive atomistic molecular dynamics (MD) simulations (aggregate time ~ 0.7 ms) to uncover the interplay between structural transformation and domain "cross-talk" on the conformational ensemble and oligomerization landscape of Httex1. Notably, our MD-derived ensembles of N17-polyQ monomers validated against 13C NMR chemical shifts indicated that in addition to elevated α-helicity, polyQ expansion also favors transient, interdomain (N17-polyQ) interactions which result in the emergence of β-sheet conformations. Further, interdomain interactions competed with increased polyQ tract α-helicity to modulate the stability of N17-mediated dimers and thereby promoted a heterogenous dimerization landscape. Finally, we observed that the C-terminal proline-rich domain (PRD) promoted condensation of Httex1 through self-interactions involving its P10/P11 tracts while also interacting with N17 to suppress its α-helicity. In summary, our study demonstrates a significant role for domain "cross-talk" in modulating the monomeric structural ensemble and self-assembly of Httex1.

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.

Pathogenic TDP-43 accelerates the generation of toxic exon1 HTT in Huntington's disease knock-in mice

Huntington's disease (HD) is caused by a CAG repeat expansion in exon1 of the HTT gene that encodes a polyglutamine tract in huntingtin protein. The formation of HTT exon1 fragments with an expanded polyglutamine repeat has been implicated as a key step in the pathogenesis of HD. It was reported that the CAG repeat length-dependent aberrant splicing of exon1 HTT results in a short polyadenylated mRNA that is translated into an exon1 HTT protein. Under normal conditions, TDP-43 is predominantly found in the nucleus, where it regulates gene expression. However, in various pathological conditions, TDP-43 is mislocalized in the cytoplasm. By investigating HD knock-in mice, we explore whether the pathogenic TDP-43 in the cytoplasm contributes to HD pathogenesis, through expressing the cytoplasmic TDP-43 without nuclear localization signal. We found that the cytoplasmic TDP-43 is increased in the HD mouse brain and that its mislocalization could deteriorate the motor and gait behavior. Importantly, the cytoplasmic TDP-43, via its binding to the intron1 sequence (GU/UG)n of the mouse Htt pre-mRNA, promotes the transport of exon1-intron1 Htt onto ribosome, resulting in the aberrant generation of exon1 Htt. Our findings suggest that cytoplasmic TDP-43 contributes to HD pathogenesis via its binding to and transport of nuclear un-spliced mRNA to the ribosome for the generation of a toxic protein product.

UBL3 Interacts with PolyQ-Expanded Huntingtin Fragments and Modifies Their Intracellular Sorting

BACKGROUND/OBJECTIVES

UBL3 (Ubiquitin-like 3) is a protein that plays a crucial role in post-translational modifications, particularly in regulating protein transport within small extracellular vesicles. While previous research has predominantly focused on its interactions with α-synuclein, this study investigates UBL3's role in Huntington's disease (HD). HD is characterized by movement disorders and cognitive impairments, with its pathogenesis linked to toxic, polyglutamine (polyQ)-expanded mutant huntingtin fragments (mHTT). However, the mechanisms underlying the interaction between UBL3 and mHTT remain poorly understood.

METHODS

To elucidate this relationship, we performed hematoxylin and eosin (HE) staining and immunohistochemistry (IHC) on postmortem brain tissue from HD patients. Gaussia princeps-based split-luciferase complementation assay and co-immunoprecipitation were employed to confirm the interaction between UBL3 and mHTT. Additionally, we conducted a HiBiT lytic detection assay to assess the influence of UBL3 on the intracellular sorting of mHTT. Finally, immunocytochemical staining was utilized to validate the colocalization and distribution of these proteins.

RESULTS

Our findings revealed UBL3-positive inclusions in the cytoplasm and nuclei of neurons throughout the striatum of HD patients. We discovered that UBL3 colocalizes and interacts with mHTT and modulates its intracellular sorting.

CONCLUSIONS

These results suggest that UBL3 may play a significant role in the interaction and sorting of mHTT, contributing to the understanding of its potential implications in the pathophysiology of Huntington's disease.

The Huntington's disease drug pipeline: a review of small molecules and their therapeutic targets

Huntington's disease (HD) is a progressive neurodegenerative condition resulting from a CAG repeat expansion in the huntingtin gene (HTT). Recent advancements in understanding HD's cellular and molecular pathways have paved the way for identifying various effective small-molecule candidates to treat the disorder. Two small molecules, Tetrabenazine and Deutetrabenazine, are approved for managing chorea associated with HD, and several others are under clinical trials. Notably, the field of small-molecule therapeutics targeting HD is rapidly progressing, and there is anticipation of their approval in the foreseeable future. This chapter provides a comprehensive overview of the emergence of small-molecule therapeutics in various stages of clinical development for HD therapy. The emphasis is placed on detailing their structural design, therapeutic effects, and specific mechanisms of action. Additionally, exploring key drivers implicated in HD pathogenesis offers valuable insights, as a foundational principle for designing prospective anti-HD therapeutic leads.

Therapeutic approaches targeting aging and cellular senescence in Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease that is manifested by a gradual loss of physical, cognitive, and mental abilities. As the disease advances, age has a major impact on the pathogenic signature of mutant huntingtin (mHTT) protein aggregation. This review aims to explore the intricate relationship between aging, mHTT toxicity, and cellular senescence in HD. Scientific data on the interplay between aging, mHTT, and cellular senescence in HD were collected from several academic databases, including PubMed, Google Scholar, Google, and ScienceDirect. The search terms employed were "AGING," "HUNTINGTON'S DISEASE," "MUTANT HUNTINGTIN," and "CELLULAR SENESCENCE." Additionally, to gather information on the molecular mechanisms and potential therapeutic targets, the search was extended to include relevant terms such as "DNA DAMAGE," "OXIDATIVE STRESS," and "AUTOPHAGY." According to research, aging leads to worsening HD pathophysiology through some processes. As a result of the mHTT accumulation, cellular senescence is promoted, which causes DNA damage, oxidative stress, decreased autophagy, and increased inflammatory responses. Pro-inflammatory cytokines and other substances are released by senescent cells, which may worsen the neuronal damage and the course of the disease. It has been shown that treatments directed at these pathways reduce some of the HD symptoms and enhance longevity in experimental animals, pointing to a new possibility of treating the condition. Through their amplification of the harmful effects of mHTT, aging and cellular senescence play crucial roles in the development of HD. Comprehending these interplays creates novel opportunities for therapeutic measures targeted at alleviating cellular aging and enhancing HD patients' quality of life.

Oleuropein enhances proteasomal activity and reduces mutant huntingtin-induced cytotoxicity

Introduction

Huntington's disease (HD) is a hereditary neurodegenerative disorder that primarily affects the striatum, a brain region responsible for movement control. The disease is characterized by the mutant huntingtin (mHtt) proteins with an extended polyQ stretch, which are prone to aggregation. These mHtt aggregates accumulate in neurons and are the primary cause of the neuropathology associated with HD. To date, no effective cure for HD has been developed.

Methods

The immortalized STHdh Q111/Q111 striatal cell line, the mHtt-transfected wild-type STHdh Q7/Q7 striatal cell line, and N2a cells were used as Huntington's disease cell models. Flow cytometry was used to assess cellular reactive oxygen species and transfection efficiency. The CCK-8 assay was used to measure cell viability, while fluorescence microscopy was used to quantify aggregates. Immunoblotting analyses were used to evaluate the effects on protein expression.

Results

Polyphenols are natural antioxidants that offer neuroprotection in neurological disorders. In this study, we provide evidence that oleuropein, the primary polyphenol in olive leaves and olive oil, enhances cell viability in HD cell models, including. STHdh Q7/Q7 STHdh Q7/Q7 striatal cells, N2a cells ectopically expressing the truncated mHtt, and STHdh Q111/Q111 striatal cells expressing the full-length mHtt. Oleuropein effectively reduced both soluble and aggregated forms of mHtt protein in these HD model cells. Notably, the reduction of mHtt aggregates associated with oleuropein was linked to increased proteasome activity rather than changes in autophagic flux. Oleuropein seems to modulate proteasome activity through an unidentified pathway, as it did not affect the 20S proteasome catalytic β subunits, the proteasome regulator PA28γ, or multiple MAPK pathways.

Discussion

We demonstrated that oleuropein enhances the degradation of mHtt by increasing proteasomal protease activities and alleviates mHtt-induced cytotoxicity. Hence, we propose that oleuropein and potentially other polyphenols hold promise as a candidate for alleviating Huntington's disease.

Advances in animal models of Parkinson's disease

Parkinson's disease is a complex neurodegenerative disease characterized by progressive movement impairments. Predominant symptoms encompass resting tremor, bradykinesia, limb rigidity, and postural instability. In addition, it also includes a series of non-motor symptoms such as sleep disorders, hyposmia, gastrointestinal dysfunction, autonomic dysfunction and cognitive impairment. Pathologically, the disease manifests through dopaminergic neuronal loss and the presence of Lewy bodies. At present, no significant breakthrough has been achieved in clinical Parkinson's disease treatment. Exploring treatment modalities necessitate the establishment of scientifically sound animal models. In recent years, researchers have focused on replicating the symptoms of human Parkinson's disease, resulting in the establishment of various experimental animal models primarily through drugs and transgenic methods to mimic relevant pathologies and identify more effective treatments. This review examines traditional neurotoxin and transgenic animal models as well as α-synuclein pre-formed fibrils models, non-human primate models and non-mammalian specie models. Additionally, it introduces emerging models, including models based on optogenetics, induced pluripotent stem cells, and gene editing, aiming to provide a reference for the utilization of experimental animal models and clinical research for researchers in this field.

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.

Toxic effects of mutant huntingtin in axons are mediated by its proline-rich domain

Huntington's disease results from expansion of a polyglutamine tract (polyQ) in mutant huntingtin (mHTT) protein, but mechanisms underlying polyQ expansion-mediated toxic gain-of-mHTT function remain elusive. Here, deletion and antibody-based experiments revealed that a proline-rich domain (PRD) adjacent to the polyQ tract is necessary for mHTT to inhibit fast axonal transport and promote axonal pathology in cultured mammalian neurons. Further, polypeptides corresponding to subregions of the PRD sufficed to elicit the toxic effect on fast axonal transport, which was mediated by c-Jun N-terminal kinases (JNKs) and involved PRD binding to one or more SH3-domain containing proteins. Collectively, these data suggested a mechanism whereby polyQ tract expansion in mHTT promotes aberrant PRD exposure and interactions of this domain with SH3 domain-containing proteins including some involved in activation of JNKs. In support, biochemical and immunohistochemical experiments linked aberrant PRD exposure to increased JNK activation in striatal tissues of the zQ175 mouse model and from post-mortem Huntington's disease patients. Together, these findings support a critical role of PRD on mHTT toxicity, suggesting a novel framework for the potential development of therapies aimed to halt or reduce axonal pathology in Huntington's disease.

TRIM37 is a primate-specific E3 ligase for Huntingtin and accounts for the striatal degeneration in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease characterized by preferential neuronal loss in the striatum. The mechanism underlying striatal selective neurodegeneration remains unclear, making it difficult to develop effective treatments for HD. In the brains of nonhuman primates, we examined the expression of Huntingtin (HTT), the gene responsible for HD. We found that HTT protein is highly expressed in striatal neurons due to its slow degradation in the striatum. We also identified tripartite motif-containing 37 (TRIM37) as a primate-specific protein that interacts with HTT and is selectively reduced in the primate striatum. TRIM37 promotes the ubiquitination and degradation of mutant HTT (mHTT) in vitro and modulates mHTT aggregation in mouse and monkey brains. Our findings suggest that nonhuman primates are crucial for understanding the mechanisms of human diseases such as HD and support TRIM37 as a potential therapeutic target for treating HD.

HAP40 modulates mutant Huntingtin aggregation and toxicity in Huntington's disease mice

Huntington's disease (HD) is a monogenic neurodegenerative disease, caused by the CAG trinucleotide repeat expansion in exon 1 of the Huntingtin (HTT) gene. The HTT gene encodes a large protein known to interact with many proteins. Huntingtin-associated protein 40 (HAP40) is one that shows high binding affinity with HTT and functions to maintain HTT conformation in vitro. However, the potential role of HAP40 in HD pathogenesis remains unknown. In this study, we found that the expression level of HAP40 is in parallel with HTT but inversely correlates with mutant HTT aggregates in mouse brains. Depletion of endogenous HAP40 in the striatum of HD140Q knock-in (KI) mice leads to enhanced mutant HTT aggregation and neuronal loss. Consistently, overexpression of HAP40 in the striatum of HD140Q KI mice reduced mutant HTT aggregation and ameliorated the behavioral deficits. Mechanistically, HAP40 preferentially binds to mutant HTT and promotes Lysine 48-linked ubiquitination of mutant HTT. Our results revealed that HAP40 is an important regulator of HTT protein homeostasis in vivo and hinted at HAP40 as a therapeutic target in HD treatment.

Application and perspective of CRISPR/Cas9 genome editing technology in human diseases modeling and gene therapy

The Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) mediated Cas9 nuclease system has been extensively used for genome editing and gene modification in eukaryotic cells. CRISPR/Cas9 technology holds great potential for various applications, including the correction of genetic defects or mutations within the human genome. The application of CRISPR/Cas9 genome editing system in human disease research is anticipated to solve a multitude of intricate molecular biology challenges encountered in life science research. Here, we review the fundamental principles underlying CRISPR/Cas9 technology and its recent application in neurodegenerative diseases, cardiovascular diseases, autoimmune related diseases, and cancer, focusing on the disease modeling and gene therapy potential of CRISPR/Cas9 in these diseases. Finally, we provide an overview of the limitations and future prospects associated with employing CRISPR/Cas9 technology for diseases study and treatment.

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.

Dietary fasting and time-restricted eating in Huntington's disease: therapeutic potential and underlying mechanisms

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by aggregation of the mutant huntingtin (mHTT) protein, resulting from a CAG repeat expansion in the huntingtin gene HTT. HD is characterized by a variety of debilitating symptoms including involuntary movements, cognitive impairment, and psychiatric disturbances. Despite considerable efforts, effective disease-modifying treatments for HD remain elusive, necessitating exploration of novel therapeutic approaches, including lifestyle modifications that could delay symptom onset and disease progression. Recent studies suggest that time-restricted eating (TRE), a form of intermittent fasting involving daily caloric intake within a limited time window, may hold promise in the treatment of neurodegenerative diseases, including HD. TRE has been shown to improve mitochondrial function, upregulate autophagy, reduce oxidative stress, regulate the sleep-wake cycle, and enhance cognitive function. In this review, we explore the potential therapeutic role of TRE in HD, focusing on its underlying physiological mechanisms. We discuss how TRE might enhance the clearance of mHTT, recover striatal brain-derived neurotrophic factor levels, improve mitochondrial function and stress-response pathways, and synchronize circadian rhythm activity. Understanding these mechanisms is critical for the development of targeted lifestyle interventions to mitigate HD pathology and improve patient outcomes. While the potential benefits of TRE in HD animal models are encouraging, future comprehensive clinical trials will be necessary to evaluate its safety, feasibility, and efficacy in persons with HD.

Neuropathogenesis-on-chips for neurodegenerative diseases

Developing diagnostics and treatments for neurodegenerative diseases (NDs) is challenging due to multifactorial pathogenesis that progresses gradually. Advanced in vitro systems that recapitulate patient-like pathophysiology are emerging as alternatives to conventional animal-based models. In this review, we explore the interconnected pathogenic features of different types of ND, discuss the general strategy to modelling NDs using a microfluidic chip, and introduce the organoid-on-a-chip as the next advanced relevant model. Lastly, we overview how these models are being applied in academic and industrial drug development. The integration of microfluidic chips, stem cells, and biotechnological devices promises to provide valuable insights for biomedical research and developing diagnostic and therapeutic solutions for NDs.

Transmission-selective muscle pathology induced by the active propagation of mutant huntingtin across the human neuromuscular synapse

Neuron-to-neuron transmission of aggregation-prone, misfolded proteins may potentially explain the spatiotemporal accumulation of pathological lesions in the brains of patients with neurodegenerative protein-misfolding diseases (PMDs). However, little is known about protein transmission from the central nervous system to the periphery, or how this propagation contributes to PMD pathology. To deepen our understanding of these processes, we established two functional neuromuscular systems derived from human iPSCs. One was suitable for long-term high-throughput live-cell imaging and the other was adapted to a microfluidic system assuring that connectivity between motor neurons and muscle cells was restricted to the neuromuscular junction. We show that the Huntington's disease (HD)-associated mutant HTT exon 1 protein (mHTTEx1) is transmitted from neurons to muscle cells across the human neuromuscular junction. We found that transmission is an active and dynamic process that starts before aggregate formation and is regulated by synaptic activity. We further found that transmitted mHTTEx1 causes HD-relevant pathology at both molecular and functional levels in human muscle cells, even in the presence of the ubiquitous expression of mHTTEx1. In conclusion, we have uncovered a causal link between mHTTEx1 synaptic transmission and HD pathology, highlighting the therapeutic potential of blocking toxic protein transmission in PMDs.

Interaction between huntingtin exon 1 and HEAT repeat structure probed by chimeric model proteins

Huntington disease (HD) is associated with aggregation of huntingtin (HTT) protein containing over 35 continuous Q residues within the N-terminal exon 1 encoded region. The C-terminal of the HTT protein consists mainly of HEAT repeat structure which serves as a scaffold for multiple cellular activities. Structural and biochemical analysis of the intact HTT protein has been hampered by its huge size (~300 kDa) and most in vitro studies to date have focused on the properties of the exon 1 region. To explore the interaction between HTT exon 1 and the HEAT repeat structure, we constructed chimeric proteins containing the N-terminal HTT exon 1 region and the HEAT repeat protein PR65/A. The results indicate that HTT exon 1 slightly destabilizes the downstream HEAT repeat structure and endows the HEAT repeat structure with more conformational flexibility. Wild-type and pathological lengths of polyQ did not show differences in the interaction between HTT exon 1 and the HEAT repeats. With the C-terminal fusion of PR65/A, HTT exon 1 containing pathological lengths of polyQ could still form amyloid fibrils, but the higher-order architecture of fibrils and kinetics of fibril formation were affected by the C-terminal fusion of HEAT repeats. This indicates that interaction between HTT exon 1 and HEAT repeat structure is compatible with both normal function of HTT protein and the pathogenesis of HD, and this study provides a potential model for further exploration.

A Specific Mini-Intrabody Mediates Lysosome Degradation of Mutant Huntingtin

Accumulation of misfolded proteins leads to many neurodegenerative diseases that can be treated by lowering or removing mutant proteins. Huntington's disease (HD) is characterized by the intracellular accumulation of mutant huntingtin (mHTT) that can be soluble and aggregated in the central nervous system and causes neuronal damage and death. Here, an intracellular antibody (intrabody) fragment is generated that can specifically bind mHTT and link to the lysosome for degradation. It is found that delivery of this peptide by either brain injection or intravenous administration can efficiently clear the soluble and aggregated mHTT by activating the lysosomal degradation pathway, resulting in amelioration of gliosis and dyskinesia in HD knock-in (KI-140Q) mice. These findings suggest that the small intrabody peptide linked to lysosomes can effectively lower mutant proteins and provide a new approach for treating neurodegenerative diseases that are caused by the accumulation of mutant proteins.

Small Molecule Pytren-4QMn Metal Complex Slows down Huntington's Disease Progression in Male zQ175 Transgenic Mice

Huntington's disease (HD) is an inherited neurodegenerative disorder considered a rare disease with a prevalence of 5.7 per 100,000 people. It is caused by an autosomal dominant mutation consisting of expansions of trinucleotide repeats that translate into poly-glutamine enlarged mutant huntingtin proteins (mHTT), which are particularly deleterious in brain tissues. Since there is no cure for this progressive fatal disease, searches for new therapeutic approaches are much needed. The small molecule pytren-4QMn (4QMn), a highly water-soluble mimic of the enzyme superoxide dismutase, has shown in vivo beneficial anti-inflammatory activity in mice and was able to remove mHTT deposits in a C. elegans model of HD. In this study, we assessed 4QMn therapeutic potential in zQ175 neo-deleted knock-in mice, a model of HD that closely mimics the heterozygosity, genetic injury, and progressive nature of the human disease. We provide evidence that 4QMn has good acute and chronic tolerability, and can cross the blood-brain barrier, and in male, but not female, zQ175 mice moderately ameliorate HD-altered gene expression, mHtt aggregation, and HD disease phenotype. Our data highlight the importance of considering sex-specific differences when testing new therapies using animal models and postulate 4QMn as a potential novel type of small water-soluble metal complex that could be worth further investigating for its therapeutic potential in HD, as well as in other polyglutamine diseases.

Huntingtin Interacting Proteins and Pathological Implications

Huntington's disease (HD) is caused by an expansion of a CAG repeat in the gene that encodes the huntingtin protein (HTT). The exact function of HTT is still not fully understood, and previous studies have mainly focused on identifying proteins that interact with HTT to gain insights into its function. Numerous HTT-interacting proteins have been discovered, shedding light on the functions and structure of HTT. Most of these proteins interact with the N-terminal region of HTT. Among the various HTT-interacting proteins, huntingtin-associated protein 1 (HAP1) and HTT-interacting protein 1 (HIP1) have been extensively studied. Recent research has uncovered differences in the distribution of HAP1 in monkey and human brains compared with mice. This finding suggests that there may be species-specific variations in the regulation and function of HTT-interacting proteins. Understanding these differences could provide crucial insights into the development of HD. In this review, we will focus on the recent advancements in the study of HTT-interacting proteins, with particular attention to the differential distributions of HTT and HAP1 in larger animal models.

Rhes depletion promotes striatal accumulation and aggregation of mutant huntingtin in a presymptomatic HD mouse model

Introduction

Huntington's disease (HD) is caused by CAG trinucleotide repeats in the HTT gene. Selective neurodegeneration in the striatum is prominent in HD, despite widespread expression of mutant HTT (mHTT). Ras homolog enriched in the striatum (Rhes) is a GTP-binding protein enriched in the striatum, involved in dopamine-related behaviors and autophagy regulation. Growing evidence suggests Rhes plays a critical role in the selective striatal degeneration in HD, but its specific function in this context remains complex and controversial.

Methods

In this study, we utilized CRISPR/Cas9 to knockdown Rhes at different disease stages through adeno-associated virus (AAV) transduction in HD knock-in (KI) mice. Immunoblotting and immunofluorescence were employed to assess the impact of Rhes depletion on mHTT levels, neuronal loss, astrogliosis and autophagy activity.

Results

Rhes depletion in 22-week-old HD KI mice (representing the presymptomatic stage) led to mHTT accumulation, reduced neuronal cell staining, and increased astrogliosis. However, no such effects were observed in 36-week-old HD KI mice (representing the symptomatic stage). Additionally, Rhes deletion in 22-week-old HD KI mice resulted in increased P62 levels, reduced LC3-II levels, and unchanged phosphorylation of mTOR and beclin-1, unchanged mTOR protein level, except for a decrease in beclin-1.

Discussion

Our findings suggest that knockdown Rhes promotes striatal aggregation of mutant huntingtin by reducing autophagy activity in a mTOR-independent manner. Rhes plays a protective role during the presymptomatic stage of HD KI mice.

Mutant HTT does not affect glial development but impairs myelination in the early disease stage

Introduction

Huntington's disease (HD) is caused by expanded CAG repeats in the huntingtin gene (HTT) and is characterized by late-onset neurodegeneration that primarily affects the striatum. Several studies have shown that mutant HTT can also affect neuronal development, contributing to the late-onset neurodegeneration. However, it is currently unclear whether mutant HTT impairs the development of glial cells, which is important for understanding whether mutant HTT affects glial cells during early brain development.

Methods

Using HD knock-in mice that express full-length mutant HTT with a 140 glutamine repeat at the endogenous level, we analyzed the numbers of astrocytes and oligodendrocytes from postnatal day 1 to 3 months of age via Western blotting and immunocytochemistry. We also performed electron microscopy, RNAseq analysis, and quantitative RT-PCR.

Results

The numbers of astrocytes and oligodendrocytes were not significantly altered in postnatal HD KI mice compared to wild type (WT) mice. Consistently, glial protein expression levels were not significantly different between HD KI and WT mice. However, at 3 months of age, myelin protein expression was reduced in HD KI mice, as evidenced by Western blotting and immunocytochemical results. Electron microscopy revealed a slight but significant reduction in myelin thickness of axons in the HD KI mouse brain at 3 months of age. RNAseq analysis did not show significant reductions in myelin-related genes in postnatal HD KI mice.

Conclusion

These data suggest that cytoplasmic mutant HTT, rather than nuclear mutant HTT, mediates myelination defects in the early stages of the disease without impacting the differentiation and maturation of glial cells.

The person-to-person transmission landscape of the gut and oral microbiomes

The human microbiome is an integral component of the human body and a co-determinant of several health conditions1,2. However, the extent to which interpersonal relations shape the individual genetic makeup of the microbiome and its transmission within and across populations remains largely unknown3,4. Here, capitalizing on more than 9,700 human metagenomes and computational strain-level profiling, we detected extensive bacterial strain sharing across individuals (more than 10 million instances) with distinct mother-to-infant, intra-household and intra-population transmission patterns. Mother-to-infant gut microbiome transmission was considerable and stable during infancy (around 50% of the same strains among shared species (strain-sharing rate)) and remained detectable at older ages. By contrast, the transmission of the oral microbiome occurred largely horizontally and was enhanced by the duration of cohabitation. There was substantial strain sharing among cohabiting individuals, with 12% and 32% median strain-sharing rates for the gut and oral microbiomes, and time since cohabitation affected strain sharing more than age or genetics did. Bacterial strain sharing additionally recapitulated host population structures better than species-level profiles did. Finally, distinct taxa appeared as efficient spreaders across transmission modes and were associated with different predicted bacterial phenotypes linked with out-of-host survival capabilities. The extent of microorganism transmission that we describe underscores its relevance in human microbiome studies5, especially those on non-infectious, microbiome-associated diseases.

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.

CRISPR-Based Genome-Editing Tools for Huntington's Disease Research and Therapy

Huntington's disease (HD) is an autosomal dominantly-inherited neurodegenerative disease, which is caused by CAG trinucleotide expansion in exon 1 of the Huntingtin (HTT) gene. Although HD is a rare disease, its monogenic nature makes it an ideal model in which to understand pathogenic mechanisms and to develop therapeutic strategies for neurodegenerative diseases. Clustered regularly-interspaced short palindromic repeats (CRISPR) is the latest technology for genome editing. Being simple to use and highly efficient, CRISPR-based genome-editing tools are rapidly gaining popularity in biomedical research and opening up new avenues for disease treatment. Here, we review the development of CRISPR-based genome-editing tools and their applications in HD research to offer a translational perspective on advancing the genome-editing technology to HD treatment.

Current landscape of gene-editing technology in biomedicine: Applications, advantages, challenges, and perspectives

The expanding genome editing toolbox has revolutionized life science research ranging from the bench to the bedside. These "molecular scissors" have offered us unprecedented abilities to manipulate nucleic acid sequences precisely in living cells from diverse species. Continued advances in genome editing exponentially broaden our knowledge of human genetics, epigenetics, molecular biology, and pathology. Currently, gene editing-mediated therapies have led to impressive responses in patients with hematological diseases, including sickle cell disease and thalassemia. With the discovery of more efficient, precise and sophisticated gene-editing tools, more therapeutic gene-editing approaches will enter the clinic to treat various diseases, such as acquired immunodeficiency sydrome (AIDS), hematologic malignancies, and even severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. These initial successes have spurred the further innovation and development of gene-editing technology. In this review, we will introduce the architecture and mechanism of the current gene-editing tools, including clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated nuclease-based tools and other protein-based DNA targeting systems, and we summarize the meaningful applications of diverse technologies in preclinical studies, focusing on the establishment of disease models and diagnostic techniques. Finally, we provide a comprehensive overview of clinical information using gene-editing therapeutics for treating various human diseases and emphasize the opportunities and challenges.

Construction of a breast cancer prognosis model based on alternative splicing and immune infiltration

BACKGROUND

Breast cancer (BC) is the most common malignancy among women in the world. Alternative splicing (AS) is an important mechanism for regulating gene expression and producing proteome diversity, which is closely related to tumorigenesis. Understanding the role of AS in BC may be helpful to reveal new therapeutic targets for clinical interventions.

METHODS

RNA-seq, clinical and AS data of TCGA-BRCA were downloaded from TCGA and TCGA SpliceSeq databases. AS events associated with prognosis were filtered by univariate Cox regression. The AS risk model of BC was built by Lasso regression, random forest and multivariate Cox regression. The accuracy of the AS risk model and clinicopathological factors were evaluated by time-dependent receiver operating characteristic (ROC) curves. The significant factors were used to construct the nomogram model. Tumor microenvironment analysis, immune infiltration and immune checkpoint analysis were performed to show the differences between the high and low AS risk groups. The expression differences of genes of AS events constituting the risk model in tumor tissues and normal tissues were analyzed, the genes with significant differences were screened, and their relationship with prognosis, tumor microenvironment, immune infiltration and immune checkpoint were analyzed. Finally, Pearson correlation analysis was used to calculate the correlation coefficient between splicing factors (SF) and prognostic AS events in TCGA-BRCA. The results were imported into Cytoscape, and the associated network was constructed.

RESULTS

A total of 21,232 genes had 45,421 AS events occurring in TCGA-BRCA, while 1604 AS events were found to be significantly correlated with survival. The BRCA risk model consisted of 5 AS events, (TTC39C|44853|AT*- 2.67) + (HSPBP1|52052|AP*- 4.28) + (MAZ|35942|ES*2.34) + (ANK3|11845|AP*1.18) + (ZC3HAV1|81940|AT*1.59), which were confirmed to be valuable for predicting BRCA prognosis to a certain degree, including ROC curve, survival analysis, tumor microenvironment analysis, immune infiltration and immune checkpoint analysis. Based on this, we constructed a nomogram prediction model composed of clinicopathological features and the AS risk signature. Furthermore, we found that MAZ was a core gene indicating the connection of tumor prognosis and AS events. Ultimately, a network of SF-AS regulation was established to reveal the relationship between them.

CONCLUSIONS

We constructed a nomogram model combined with clinicopathological features and AS risk score to predict the prognosis of BC. The detailed analysis of tumor microenvironment and immune infiltration in the AS risk model may further reveal the potential mechanisms of BC recurrence and development.

Time-resolved FRET screening identifies small molecular modifiers of mutant Huntingtin conformational inflexibility in patient-derived cells

Huntington's disease (HD) is the most common monogenic neurodegenerative disease and is fatal. CAG repeat expansions in mutant Huntingtin (mHTT) exon 1 encode for polyglutamine (polyQ) stretches and influence age of onset and disease severity, depending on their length. mHTT is more structured compared to wild-type (wt) HTT, resulting in a decreased N-terminal conformational flexibility. mHTT inflexibility may contribute to both gain of function toxicity, due to increased mHTT aggregation propensity, but also to loss of function phenotypes, due to decreased interactions with binding partners. High-throughput-screening techniques to identify mHTT flexibility states and potential flexibility modifying small molecules are currently lacking. Here, we propose a novel approach for identifying small molecules that restore mHTT's conformational flexibility in human patient fibroblasts. We have applied a well-established antibody-based time-resolved Förster resonance energy transfer (TR-FRET) immunoassay, which measures endogenous HTT flexibility using two validated HTT-specific antibodies, to a high-throughput screening platform. By performing a small-scale compound screen, we identified several small molecules that can partially rescue mHTT inflexibility, presumably by altering HTT post-translational modifications. Thus, we demonstrated that the HTT TR-FRET immunoassay can be miniaturized and applied to a compound screening workflow in patient cells. This automated assay can now be used in large screening campaigns to identify previously unknown HD drugs and drug targets.

Advances in Modeling Polyglutamine Diseases Using Genome Editing Tools

Polyglutamine (polyQ) diseases, including Huntington’s disease, are a group of late-onset progressive neurological disorders caused by CAG repeat expansions. Although recently, many studies have investigated the pathological features and development of polyQ diseases, many questions remain unanswered. The advancement of new gene-editing technologies, especially the CRISPR-Cas9 technique, has undeniable value for the generation of relevant polyQ models, which substantially support the research process. Here, we review how these tools have been used to correct disease-causing mutations or create isogenic cell lines with different numbers of CAG repeats. We characterize various cellular models such as HEK 293 cells, patient-derived fibroblasts, human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs) and animal models generated with the use of genome-editing technology.

Polyglutamine diseases

Polyglutamine diseases are a collection of nine CAG trinucleotide expansion disorders, presenting with a spectrum of neurological and clinical phenotypes. Recent human, mouse and cell studies of Huntington's disease have highlighted the role of DNA repair genes in somatic expansion of the CAG repeat region, modifying disease pathogenesis. Incomplete splicing of the HTT gene has also been shown to occur in humans, with the resulting exon 1 fragment most probably contributing to the Huntington's disease phenotype. In the spinocerebellar ataxias, studies have converged on transcriptional dysregulation of ion channels as a key disease modifier. In addition, advances have been made in understanding how increased levels of toxic, polyglutamine-expanded proteins can arise in the spinocerebellar ataxias through post-transcriptional and -translational modifications and autophagic mechanisms. Recent studies in spinal and bulbar muscular atrophy implicate similar pathogenic pathways to the more common polyglutamine diseases, highlighting autophagy stimulation as a potential therapeutic target. Finally, the therapeutic use of antisense oligonucleotides in several polyglutamine diseases has shown preclinical benefits and serves as potential future therapies in humans.

Genetically modified large animal models for investigating neurodegenerative diseases

Neurodegenerative diseases represent a large group of neurological disorders including Alzheimer’s disease, amyotrophic lateral sclerosis, Parkinson’s disease, and Huntington’s disease. Although this group of diseases show heterogeneous clinical and pathological phenotypes, they share important pathological features characterized by the age-dependent and progressive degeneration of nerve cells that is caused by the accumulation of misfolded proteins. The association of genetic mutations with neurodegeneration diseases has enabled the establishment of various types of animal models that mimic genetic defects and have provided important insights into the pathogenesis. However, most of genetically modified rodent models lack the overt and selective neurodegeneration seen in the patient brains, making it difficult to use the small animal models to validate the effective treatment on neurodegeneration. Recent studies of pig and monkey models suggest that large animals can more faithfully recapitulate pathological features of neurodegenerative diseases. In this review, we discuss the important differences in animal models for modeling pathological features of neurodegenerative diseases, aiming to assist the use of animal models to better understand the pathogenesis and to develop effective therapeutic strategies.

Nanotechnology-Based Drug Delivery Strategies to Repair the Mitochondrial Function in Neuroinflammatory and Neurodegenerative Diseases

Mitochondria are vital organelles in eukaryotic cells that control diverse physiological processes related to energy production, calcium homeostasis, the generation of reactive oxygen species, and cell death. Several studies have demonstrated that structural and functional mitochondrial disturbances are involved in the development of different neuroinflammatory (NI) and neurodegenerative (ND) diseases (NI&NDDs) such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Remarkably, counteracting mitochondrial impairment by genetic or pharmacologic treatment ameliorates neurodegeneration and clinical disability in animal models of these diseases. Therefore, the development of nanosystems enabling the sustained and selective delivery of mitochondria-targeted drugs is a novel and effective strategy to tackle NI&NDDs. In this review, we outline the impact of mitochondrial dysfunction associated with unbalanced mitochondrial dynamics, altered mitophagy, oxidative stress, energy deficit, and proteinopathies in NI&NDDs. In addition, we review different strategies for selective mitochondria-specific ligand targeting and discuss novel nanomaterials, nanozymes, and drug-loaded nanosystems developed to repair mitochondrial function and their therapeutic benefits protecting against oxidative stress, restoring cell energy production, preventing cell death, inhibiting protein aggregates, and improving motor and cognitive disability in cellular and animal models of different NI&NDDs.

Non-Cell Autonomous and Epigenetic Mechanisms of Huntington's Disease

Huntington's disease (HD) is a rare neurodegenerative disorder caused by an expansion of CAG trinucleotide repeat located in the exon 1 of Huntingtin (HTT) gene in human chromosome 4. The HTT protein is ubiquitously expressed in the brain. Specifically, mutant HTT (mHTT) protein-mediated toxicity leads to a dramatic degeneration of the striatum among many regions of the brain. HD symptoms exhibit a major involuntary movement followed by cognitive and psychiatric dysfunctions. In this review, we address the conventional role of wild type HTT (wtHTT) and how mHTT protein disrupts the function of medium spiny neurons (MSNs). We also discuss how mHTT modulates epigenetic modifications and transcriptional pathways in MSNs. In addition, we define how non-cell autonomous pathways lead to damage and death of MSNs under HD pathological conditions. Lastly, we overview therapeutic approaches for HD. Together, understanding of precise neuropathological mechanisms of HD may improve therapeutic approaches to treat the onset and progression of HD.

Molecular Strategies to Target Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by the aggregation of the mutant huntingtin (mHTT) protein in nerve cells. mHTT self-aggregates to form soluble oligomers and insoluble fibrils, which interfere in a number of key cellular functions. This leads to cell quiescence and ultimately cell death. There are currently still no treatments available for HD, but approaches targeting the HTT levels offer systematic, mechanism-driven routes towards curing HD and other neurodegenerative diseases. This review summarizes the current state of knowledge of the mRNA targeting approaches such as antisense oligonucleotides and RNAi system; and the novel methods targeting mHTT and aggregates for degradation via the ubiquitin proteasome or the autophagy-lysosomal systems. These methods include the proteolysis-targeting chimera, Trim-Away, autophagosome-tethering compound, autophagy-targeting chimera, lysosome-targeting chimera and approach targeting mHTT for chaperone-mediated autophagy. These molecular strategies provide a knowledge-based approach to target HD and other neurodegenerative diseases at the origin.

Altered microRNA expression in animal models of Huntington's disease and potential therapeutic strategies

A review of recent animal models of Huntington's disease showed many microRNAs had altered expression levels in the striatum and cerebral cortex, and which were mostly downregulated. Among the altered microRNAs were miR-9/9*, miR-29b, miR-124a, miR-132, miR-128, miR-139, miR-122, miR-138, miR-23b, miR-135b, miR-181 (all downregulated) and miR-448 (upregulated), and similar changes had been previously found in Huntington's disease patients. In the animal cell studies, the altered microRNAs included miR-9, miR-9*, miR-135b, miR-222 (all downregulated) and miR-214 (upregulated). In the animal models, overexpression of miR-155 and miR-196a caused a decrease in mutant huntingtin mRNA and protein level, lowered the mutant huntingtin aggregates in striatum and cortex, and improved performance in behavioral tests. Improved performance in behavioral tests also occurred with overexpression of miR-132 and miR-124. In the animal cell models, overexpression of miR-22 increased the viability of rat primary cortical and striatal neurons infected with mutant huntingtin and decreased huntingtin -enriched foci of ≥ 2 µm. Also, overexpression of miR-22 enhanced the survival of rat primary striatal neurons treated with 3-nitropropionic acid. Exogenous expression of miR-214, miR-146a, miR-150, and miR-125b decreased endogenous expression of huntingtin mRNA and protein in Hdh^Q111/Hdh^Q111 cells. Further studies with animal models of Huntington's disease are warranted to validate these findings and identify specific microRNAs whose overexpression inhibits the production of mutant huntingtin protein and other harmful processes and may provide a more effective means of treating Huntington's disease in patients and slowing its progression.

N-alpha-acetylation of Huntingtin protein increases its propensity to aggregate

Huntington’s disease (HD) is a neurodegenerative disorder caused by a poly-CAG expansion in the first exon of the HTT gene, resulting in an extended poly-glutamine tract in the N-terminal domain of the Huntingtin (Htt) protein product. Proteolytic fragments of the poly-glutamine–containing N-terminal domain form intranuclear aggregates that are correlated with HD. Post-translational modification of Htt has been shown to alter its function and aggregation properties. However, the effect of N-terminal Htt acetylation has not yet been considered. Here, we developed a bacterial system to produce unmodified or N-terminally acetylated and aggregation-inducible Htt protein. We used this system together with biochemical, biophysical, and imaging studies to confirm that the Htt N-terminus is an in vitro substrate for the NatA N-terminal acetyltransferase and show that N-terminal acetylation promotes aggregation. These studies represent the first link between N-terminal acetylation and the promotion of a neurodegenerative disease and implicates NatA-mediated Htt acetylation as a new potential therapeutic target in HD.

Fluc-EGFP reporter mice reveal differential alterations of neuronal proteostasis in aging and disease

The cellular protein quality control machinery is important for preventing protein misfolding and aggregation. Declining protein homeostasis (proteostasis) is believed to play a crucial role in age‐related neurodegenerative disorders. However, how neuronal proteostasis capacity changes in different diseases is not yet sufficiently understood, and progress in this area has been hampered by the lack of tools to monitor proteostasis in mammalian models. Here, we have developed reporter mice for in vivo analysis of neuronal proteostasis. The mice express EGFP‐fused firefly luciferase (Fluc‐EGFP), a conformationally unstable protein that requires chaperones for proper folding, and that reacts to proteotoxic stress by formation of intracellular Fluc‐EGFP foci and by reduced luciferase activity. Using these mice, we provide evidence for proteostasis decline in the aging brain. Moreover, we find a marked reaction of the Fluc‐EGFP sensor in a mouse model of tauopathy, but not in mouse models of Huntington’s disease. Mechanistic investigations in primary neuronal cultures demonstrate that different types of protein aggregates have distinct effects on the cellular protein quality control. Thus, Fluc‐EGFP reporter mice enable new insights into proteostasis alterations in different diseases.

The regulatory roles of microRNAs toward pathogenesis and treatments in Huntington's disease

Huntington's disease (HD) is one of neurodegenerative diseases, and is defined as a monogenetic disease due to the mutation of Huntingtin gene. This disease affects several cellular functions in neurons, and further influences motor and cognitive ability, leading to the suffering of devastating symptoms in HD patients. MicroRNA (miRNA) is a non-coding RNA, and is responsible for gene regulation at post-transcriptional levels in cells. Since one miRNA targets to several downstream genes, it may regulate different pathways simultaneously. As a result, it raises a potential therapy for different diseases using miRNAs, especially for inherited diseases. In this review, we will not only introduce the update information of HD and miRNA, but also discuss the development of potential miRNA-based therapy in HD. With the understanding toward the progression of miRNA studies in HD, we anticipate it may provide an insight to treat this devastating disease, even applying to other genetic diseases.

Accumulation of Endogenous Mutant Huntingtin in Astrocytes Exacerbates Neuropathology of Huntington Disease in Mice

Selective neuronal accumulation of misfolded proteins is a key step toward neurodegeneration in a wide range of neurodegenerative diseases, including Huntington's (HD) diseases. Our recent studies suggest that Hsp70-binding protein 1 (HspBP1), an Hsp70/CHIP inhibitor that reduces protein folding, is highly expressed in neuronal cells and accounts for the accumulation of the HD protein huntingtin (HTT) in neuronal cells. To further determine the role of HspBP1 in regulation of mutant protein accumulation, we investigated whether increasing expression of HspBP1 in glial cells can also induce the accumulation of endogenous mutant HTT in glial cells and yield non-cell-autonomous toxic effects. We performed stereotaxic injection of AAV to selectively express HspBP1 in astrocytes in the brains of HD140Q knock-in (KI) mice that express mutant HTT ubiquitously but do not display obvious neurodegeneration. However, HspBP1 expression in HD140Q astrocytes led to the increased accumulation of endogenous mutant HTT and robust neuronal loss in the striatum of HD140Q KI mice. In transgenic HD mice that selectively express mutant HTT in astrocytes, increased accumulation of mutant HTT in astrocytes via HspBP1 expression did not elicit neurodegeneration but could exacerbate neurological symptoms. Consistently, suppressing the expression of endogenous HspBp1 in the striatum of HD140Q KI mice via CRISPR/Cas9 led to a significant reduction of mutant HTT accumulation. Our findings suggest that although endogenous mutant HTT in astrocytes can exacerbate neurological symptoms, it mediates neurodegeneration only when mutant HTT is also accumulated in neuronal cells.

Lack of association of somatic CAG repeat expansion with striatal neurodegeneration in HD knock-in animal models

Our previous work has established a huntingtin knock-in (KI) pig model that displays striatal neuronal loss, allowing us to examine if somatic CAG expansion in striatum accounts for the preferential neurodegeneration in Huntington disease (HD). We found that HD KI pigs do not display somatic CAG expansion in striatum as HD KI mice and that the majority of polyQ repeats in exon 1 HTT in the striatum of HD KI mice are fairly stable. We also found that striatal MSH2 and MLH3, which are involved in DNA repair, are more abundant in mouse brains than pig brains. Consistently inhibiting MSH2 and MLH3 reduced the somatic CAG expansion in HD KI mouse striatum with no influence on neuropathology. Our findings suggest that somatic CAG expansion is species-dependent, occurs in a small fraction of the HD gene in mice, and does not critically contribute to HD neuropathology.

Selective suppression of polyglutamine-expanded protein by lipid nanoparticle-delivered siRNA targeting CAG expansions in the mouse CNS

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by expansion of cytosine-adenine-guanine (CAG)-trinucleotide repeats in causative genes. These diseases include spinal and bulbar muscular atrophy (SBMA), Huntington’s disease, dentatorubral-pallidoluysian atrophy, and spinocerebellar ataxias. Targeting expanded CAG repeats is a common therapeutic approach to polyQ diseases, but concomitant silencing of genes with normal CAG repeats may lead to toxicity. Previous studies have shown that CAG repeat-targeting small interfering RNA duplexes (CAG-siRNAs) have the potential to selectively suppress mutant proteins in in vitro cell models of polyQ diseases. However, in vivo application of these siRNAs has not yet been investigated. In this study, we demonstrate that an unlocked nucleic acid (UNA)-modified CAG-siRNA shows high selectivity for polyQ-expanded androgen receptor (AR) inhibition in in vitro cell models and that lipid nanoparticle (LNP)-mediated delivery of the CAG-siRNA selectively suppresses mutant AR in the central nervous system of an SBMA mouse model. In addition, a subcutaneous injection of the LNP-delivered CAG-siRNA efficiently suppresses mutant AR in the skeletal muscle of the SBMA mouse model. These results support the therapeutic potential of LNP-delivered UNA-modified CAG-siRNAs for selective suppression of mutant proteins in SBMA and other polyQ diseases.

Differential development and electrophysiological activity in cultured cortical neurons from the mouse and cynomolgus monkey

In vitro cultures of primary cortical neurons are widely used to investigate neuronal function. However, it has yet to be fully investigated whether there are significant differences in development and function between cultured rodent and primate cortical neurons, and whether these differences influence the utilization of cultured cortical neurons to model pathological conditions. Using in vitro culture techniques combined with immunofluorescence and electrophysiological methods, our study found that the development and maturation of primary cerebral cortical neurons from cynomolgus monkeys were slower than those from mice. We used a microelectrode array technique to compare the electrophysiological differences in cortical neurons, and found that primary cortical neurons from the mouse brain began to show electrical activity earlier than those from the cynomolgus monkey. Although cultured monkey cortical neurons developed slowly in vitro, they exhibited typical pathological features-revealed by immunofluorescent staining-when infected with adeno-associated viral vectors expressing mutant huntingtin (HTT), the Huntington's disease protein. A quantitative analysis of the cultured monkey cortical neurons also confirmed that mutant HTT significantly reduced the length of neurites. Therefore, compared with the primary cortical neurons of mice, cultured monkey cortical neurons have longer developmental and survival times and greater sustained physiological activity, such as electrophysiological activity. Our findings also suggest that primary cynomolgus monkey neurons cultured in vitro can simulate a cell model of human neurodegenerative disease, and may be useful for investigating time-dependent neuronal death as well as treatment via neuronal regeneration. All mouse experiments and protocols were approved by the Animal Care and Use Committee of Jinan University of China (IACUC Approval No. 20200512-04) on May 12, 2020. All monkey experiments were approved by the IACUC protocol (IACUC Approval No. LDACU 20190820-01) on August 23, 2019 for animal management and use.