Increased translation as a novel pathogenic mechanism in Huntington's disease

Progressive alterations in polysomal architecture and activation of ribosome stalling relief factors in a mouse model of Huntington's disease

Given their highly polarized morphology and functional singularity, neurons require precise spatial and temporal control of protein synthesis. Alterations in protein translation have been implicated in the development and progression of a wide range of neurological and neurodegenerative disorders, including Huntington's disease (HD). In this study we examined the architecture of polysomes in their native brain context in striatal tissue from the zQ175 knock-in mouse model of HD. We performed 3D electron tomography of high-pressure frozen and freeze-substituted striatal tissue from HD models and corresponding controls at different ages. Electron tomography results revealed progressive remodelling towards a more compacted polysomal architecture in the mouse model, an effect that coincided with the emergence and progression of HD related symptoms. The aberrant polysomal architecture is compatible with ribosome stalling phenomena. In fact, we also detected in the zQ175 model an increase in the striatal expression of the stalling relief factor EIF5A2 and an increase in the accumulation of eIF5A1, eIF5A2 and hypusinated eIF5A1, the active form of eIF5A1. Polysomal sedimentation gradients showed differences in the relative accumulation of 40S ribosomal subunits and in polysomal distribution in striatal samples of the zQ175 model. These findings indicate that changes in the architecture of the protein synthesis machinery may underlie translational alterations associated with HD, opening new avenues for understanding the progression of the disease.

Polyglutamine-mediated ribotoxicity disrupts proteostasis and stress responses in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by expansion of a CAG trinucleotide repeat in the Huntingtin (HTT) gene, encoding a homopolymeric polyglutamine (polyQ) tract. Although mutant HTT (mHTT) protein is known to aggregate, the links between aggregation and neurotoxicity remain unclear. Here we show that both translation and aggregation of wild-type HTT and mHTT are regulated by a stress-responsive upstream open reading frame and that polyQ expansions cause abortive translation termination and release of truncated, aggregation-prone mHTT fragments. Notably, we find that mHTT depletes translation elongation factor eIF5A in brains of symptomatic HD mice and cultured HD cells, leading to pervasive ribosome pausing and collisions. Loss of eIF5A disrupts homeostatic controls and impairs recovery from acute stress. Importantly, drugs that inhibit translation initiation reduce premature termination and mitigate this escalating cascade of ribotoxic stress and dysfunction in HD.

TYROBP/DAP12 knockout in Huntington's disease Q175 mice cell-autonomously decreases microglial expression of disease-associated genes and non-cell-autonomously mitigates astrogliosis and motor deterioration

INTRODUCTION

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in the Huntingtin gene (HTT). Immune activation is abundant in the striatum of HD patients. Detection of active microglia at presymptomatic stages suggests that microgliosis is a key early driver of neuronal dysfunction and degeneration. Recent studies showed that deletion of Tyrobp, a microglial protein, ameliorates neuronal dysfunction in Alzheimer's disease amyloidopathy and tauopathy mouse models while decreasing components of the complement subnetwork.

OBJECTIVE

While TYROBP/DAP12-mediated microglial activation is detrimental for some diseases such as peripheral nerve injury, it is beneficial for other diseases. We sought to determine whether the TYROBP network is implicated in HD and whether Tyrobp deletion impacts HD striatal function and transcriptomics.

METHODS

To test the hypothesis that Tyrobp deficiency would be beneficial in an HD model, we placed the Q175 HD mouse model on a Tyrobp-null background. We characterized these mice with a combination of behavioral testing, immunohistochemistry, transcriptomic and proteomic profiling. Further, we evaluated the gene signature in isolated Q175 striatal microglia, with and without Tyrobp.

RESULTS

Comprehensive analysis of publicly available human HD transcriptomic data revealed that the TYROBP network is overactivated in the HD putamen. The Q175 mice showed morphologic microglial activation, reduced levels of post-synaptic density-95 protein and motor deficits at 6 and 9 months of age, all of which were ameliorated on the Tyrobp-null background. Gene expression analysis revealed that lack of Tyrobp in the Q175 model does not prevent the decrease in the expression of striatal neuronal genes but reduces pro-inflammatory pathways that are specifically active in HD human brain, including genes identified as detrimental in neurodegenerative diseases, e.g. C1q and members of the Ccr5 signaling pathway. Integration of transcriptomic and proteomic data revealed that astrogliosis and complement system pathway were reduced after Tyrobp deletion, which was further validated by immunofluorescence analysis.

CONCLUSIONS

Our data provide molecular and functional support demonstrating that Tyrobp deletion prevents many of the abnormalities in the HD Q175 mouse model, suggesting that the Tyrobp pathway is a potential therapeutic candidate for Huntington's disease.

Protein Translation in the Pathogenesis of Parkinson's Disease

In recent years, research into Parkinson's disease and similar neurodegenerative disorders has increasingly suggested that these conditions are synonymous with failures in proteostasis. However, the spotlight of this research has remained firmly focused on the tail end of proteostasis, primarily aggregation, misfolding, and degradation, with protein translation being comparatively overlooked. Now, there is an increasing body of evidence supporting a potential role for translation in the pathogenesis of PD, and its dysregulation is already established in other similar neurodegenerative conditions. In this paper, we consider how altered protein translation fits into the broader picture of PD pathogenesis, working hand in hand to compound the stress placed on neurons, until this becomes irrecoverable. We will also consider molecular players of interest, recent evidence that suggests that aggregates may directly influence translation in PD progression, and the implications for the role of protein translation in our development of clinically useful diagnostics and therapeutics.

Gelation of cytoplasmic expanded CAG RNA repeats suppresses global protein synthesis

RNA molecules with the expanded CAG repeat (eCAGr) may undergo sol-gel phase transitions, but the functional impact of RNA gelation is completely unknown. Here, we demonstrate that the eCAGr RNA may form cytoplasmic gel-like foci that are rapidly degraded by lysosomes. These RNA foci may significantly reduce the global protein synthesis rate, possibly by sequestering the translation elongation factor eEF2. Disrupting the eCAGr RNA gelation restored the global protein synthesis rate, whereas enhanced gelation exacerbated this phenotype. eEF2 puncta were significantly enhanced in brain slices from a knock-in mouse model and from patients with Huntington's disease, which is a CAG expansion disorder expressing eCAGr RNA. Finally, neuronal expression of the eCAGr RNA by adeno-associated virus injection caused significant behavioral deficits in mice. Our study demonstrates the existence of RNA gelation inside the cells and reveals its functional impact, providing insights into repeat expansion diseases and functional impacts of RNA phase transition.

VPS13A knockdown impairs corticostriatal synaptic plasticity and locomotor behavior in a new mouse model of chorea-acanthocytosis

Chorea-acanthocytosis (ChAc) is an inherited neurodegenerative movement disorder caused by VPS13A gene mutations leading to the absence of protein expression. The striatum is the most affected brain region in ChAc patients. However, the study of the VPS13A function in the brain has been poorly addressed. Here we generated a VPS13A knockdown (KD) model and aimed to elucidate the contribution of VPS13A to synaptic plasticity and neuronal communication in the corticostriatal circuit. First, we infected primary cortical neurons with miR30-shRNA against VPS13A and analyzed its effects on neuronal plasticity. VPS13A-KD neurons showed a higher degree of branching than controls, accompanied by decreased BDNF and PSD-95 levels, indicative of synaptic alterations. We then injected AAV-KD bilaterally in the frontal cortex and two different regions of the striatum of mice and analyzed the effects of VPS13A-KD on animal behavior and synaptic plasticity. VPS13A-KD mice showed modification of the locomotor behavior pattern, with increased exploratory behavior and hyperlocomotion. Corticostriatal dysfunction in VPS13A-KD mice was evidenced by impaired striatal long-term depression (LTD) after stimulation of cortical afferents, which was partially recovered by BDNF administration. VPS13A-KD did not lead to neuronal loss in the cortex or the striatum but induced a decrease in the neuronal release of CX3CL1 and triggered a microglial reaction, especially in the striatum. Notably, CX3CL1 administration partially restored the impaired corticostriatal LTD in VPS13A-KD mice. Our results unveil the involvement of VPS13A in neuronal connectivity modifying BDNF and CX3CL1 release. Moreover, the involvement of VPS13A in synaptic plasticity and motor behavior provides key information to further understand not only ChAc pathophysiology but also other neurological disorders.

Customization of the translational complex regulates mRNA-specific translation to control CNS regeneration

In the adult mammalian central nervous system (CNS), axons fail to regenerate spontaneously after injury because of a combination of extrinsic and intrinsic factors. Despite recent advances targeting the intrinsic regenerative properties of adult neurons, the molecular mechanisms underlying axon regeneration are not fully understood. Here, we uncover a regulatory mechanism that controls the expression of key proteins involved in regeneration at the translational level. Our results show that mRNA-specific translation is critical for promoting axon regeneration. Indeed, we demonstrate that specific ribosome-interacting proteins, such as the protein Huntingtin (HTT), selectively control the translation of a specific subset of mRNAs. Moreover, modulating the expression of these translationally regulated mRNAs is crucial for promoting axon regeneration. Altogether, our findings highlight that selective translation through the customization of the translational complex is a key mechanism of axon regeneration with major implications in the development of therapeutic strategies for CNS repair.

Mitochondria and Brain Disease: A Comprehensive Review of Pathological Mechanisms and Therapeutic Opportunities

Mitochondria play a vital role in maintaining cellular energy homeostasis, regulating apoptosis, and controlling redox signaling. Dysfunction of mitochondria has been implicated in the pathogenesis of various brain diseases, including neurodegenerative disorders, stroke, and psychiatric illnesses. This review paper provides a comprehensive overview of the intricate relationship between mitochondria and brain disease, focusing on the underlying pathological mechanisms and exploring potential therapeutic opportunities. The review covers key topics such as mitochondrial DNA mutations, impaired oxidative phosphorylation, mitochondrial dynamics, calcium dysregulation, and reactive oxygen species generation in the context of brain disease. Additionally, it discusses emerging strategies targeting mitochondrial dysfunction, including mitochondrial protective agents, metabolic modulators, and gene therapy approaches. By critically analysing the existing literature and recent advancements, this review aims to enhance our understanding of the multifaceted role of mitochondria in brain disease and shed light on novel therapeutic interventions.

Brain-Derived Neurotrophic Factor Dysregulation as an Essential Pathological Feature in Huntington's Disease: Mechanisms and Potential Therapeutics

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin whose loss or interruption is well established to have numerous intersections with the pathogenesis of progressive neurological disorders. There is perhaps no greater example of disease pathogenesis resulting from the dysregulation of BDNF signaling than Huntington's disease (HD)-an inherited neurodegenerative disorder characterized by motor, psychiatric, and cognitive impairments associated with basal ganglia dysfunction and the ultimate death of striatal projection neurons. Investigation of the collection of mechanisms leading to BDNF loss in HD highlights this neurotrophin's importance to neuronal viability and calls attention to opportunities for therapeutic interventions. Using electronic database searches of existing and forthcoming research, we constructed a literature review with the overarching goal of exploring the diverse set of molecular events that trigger BDNF dysregulation within HD. We highlighted research that investigated these major mechanisms in preclinical models of HD and connected these studies to those evaluating similar endpoints in human HD subjects. We also included a special focus on the growing body of literature detailing key transcriptomic and epigenetic alterations that affect BDNF abundance in HD. Finally, we offer critical evaluation of proposed neurotrophin-directed therapies and assessed clinical trials seeking to correct BDNF expression in HD individuals.

DEAD-Box Helicase 17 Promotes Amyloidogenesis by Regulating BACE1 Translation

Amyloidogenesis is one of the key pathophysiological changes in Alzheimer's disease (AD). Accumulation of the toxic Aβ results from the catalytic processing of β-amyloid precursor protein (APP) associated β-amyloid converting enzyme 1 (BACE1) activity. It is reported that dead-box helicase 17 (DDX17) controls RNA metabolism and is involved in the development of multiple diseases. However, whether DDX17 might play a role in amyloidogenesis has not been documented. In the present study, we found that DDX17 protein level was significantly increased in HEK and SH-SY5Y cells that stably express full-length APP (HEK-APP and Y5Y-APP) and in the brain of APP/PS1 mice, an animal model of AD. DDX17 knockdown, as opposed to DDX17 overexpression, markedly reduced the protein levels of BACE1 and the β-amyloid peptide (Aβ) in Y5Y-APP cells. We further found that DDX17-mediated enhancement of BACE1 was selectively attenuated by translation inhibitors. Specifically, DDX17 selectively interacted with the 5' untranslated region (5'UTR) of BACE1 mRNA, and deletion of the 5'UTR abolished the effect of DDX17 on luciferase activity or protein level of BACE1. Here, we show that the enhanced expression of DDX17 in AD was associated with amyloidogenesis; through the 5'UTR-dependent BACE1 translation, DDX17 might serve as an important mediator contributing to the progression of AD.

Postnatal Conditional Deletion of Bcl11b in Striatal Projection Neurons Mimics the Transcriptional Signature of Huntington's Disease

The dysregulation of striatal gene expression and function is linked to multiple diseases, including Huntington's disease (HD), Parkinson's disease, X-linked dystonia-parkinsonism (XDP), addiction, autism, and schizophrenia. Striatal medium spiny neurons (MSNs) make up 90% of the neurons in the striatum and are critical to motor control. The transcription factor, Bcl11b (also known as Ctip2), is required for striatal development, but the function of Bcl11b in adult MSNs in vivo has not been investigated. We conditionally deleted Bcl11b specifically in postnatal MSNs and performed a transcriptomic and behavioral analysis on these mice. Multiple enrichment analyses showed that the D9-Cre-Bcl11btm1.1Leid transcriptional profile was similar to the HD gene expression in mouse and human data sets. A Gene Ontology enrichment analysis linked D9-Cre-Bcl11btm1.1Leid to calcium, synapse organization, specifically including the dopaminergic synapse, protein dephosphorylation, and HDAC-signaling, commonly dysregulated pathways in HD. D9-Cre-Bcl11btm1.1Leid mice had decreased DARPP-32/Ppp1r1b in MSNs and behavioral deficits, demonstrating the dysregulation of a subtype of the dopamine D2 receptor expressing MSNs. Finally, in human HD isogenic MSNs, the mislocalization of BCL11B into nuclear aggregates points to a mechanism for BCL11B loss of function in HD. Our results suggest that BCL11B is important for the function and maintenance of mature MSNs and Bcl11b loss of function drives, in part, the transcriptomic and functional changes in HD.

Altered m6A RNA methylation contributes to hippocampal memory deficits in Huntington's disease mice

N6-methyladenosine (m6A) regulates many aspects of RNA metabolism and is involved in learning and memory processes. Yet, the impact of a dysregulation of post-transcriptional m6A editing on synaptic impairments in neurodegenerative disorders remains unknown. Here we investigated the m6A methylation pattern in the hippocampus of Huntington’s disease (HD) mice and the potential role of the m6A RNA modification in HD cognitive symptomatology. m6A modifications were evaluated in HD mice subjected to a hippocampal cognitive training task through m6A immunoprecipitation sequencing (MeRIP-seq) and the relative levels of m6A-modifying proteins (FTO and METTL14) by subcellular fractionation and Western blot analysis. Stereotaxic CA1 hippocampal delivery of AAV-shFTO was performed to investigate the effect of RNA m6A dysregulation in HD memory deficits. Our results reveal a m6A hypermethylation in relevant HD and synaptic related genes in the hippocampal transcriptome of Hdh^+/Q111 mice. Conversely, m6A is aberrantly regulated in an experience-dependent manner in the HD hippocampus leading to demethylation of important components of synapse organization. Notably, the levels of RNA demethylase (FTO) and methyltransferase (METTL14) were modulated after training in the hippocampus of WT mice but not in Hdh^+/Q111 mice. Finally, inhibition of FTO expression in the hippocampal CA1 region restored memory disturbances in symptomatic Hdh^+/Q111 mice. Altogether, our results suggest that a differential RNA methylation landscape contributes to HD cognitive symptoms and uncover a role of m6A as a novel hallmark of HD.

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) Protects Striatal Cells and Improves Motor Function in Huntington’s Disease Models: Role of PAC1 Receptor

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by the expression of mutant huntingtin (mHtt). One of the main features of HD is the degeneration of the striatum that leads to motor discoordination. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that acts through three receptors named PAC1R, VPAC1R, and VPAC2R. In the present study, we first investigated the effect of PACAP on STHdhQ7/Q7 and STHdhQ111/Q111 cells that express wild-type Htt with 7 and mHtt with 111 glutamines, respectively. Then we explored the capacity of PACAP to rescue motor symptoms in the R6/1, a murine model of HD. We found that PACAP treatment (10^–7 M) for 24 h protects STHdhQ111/Q111 cells from mHtt-induced apoptosis. This effect is associated with an increase in PAC1R transcription, phosphorylation of ERK and Akt, and an increase of intracellular c-fos, egr1, CBP, and BDNF protein content. Moreover, the use of pharmacological inhibitors revealed that activation of ERK and Akt mediates these antiapoptotic and neurotrophic effects of PACAP. To find out PAC1R implication, we treated STHdh cells with vasoactive intestinal peptide (VIP), which exhibits equal affinity for VPAC1R and VPAC2R, but lower affinity for PAC1R, in contrast to PACAP which has same affinity for the three receptors. VIP reduced cleaved caspase-3 protein level, without promoting the expression of c-fos, egr1, CBP, and the neurotrophin BDNF. We next measured the protein level of PACAP receptors in the striatum and cortex of R6/1 mice. We observed a specific reduction of PAC1R at the onset of motor symptoms. Importantly, the intranasal administration of PACAP to R6/1 animals restored the motor function and increased the striatal levels of PAC1R, CBP, and BDNF. In conclusion, PACAP exerts antiapoptotic and neurotrophic effects in striatal neurons mainly through PAC1R. This effect in HD striatum allows the recovery of motor function and point out PAC1R as a therapeutic target for treatment of HD.

Nucleolar stress controls mutant Huntington toxicity and monitors Huntington's disease progression

Transcriptional and cellular-stress surveillance deficits are hallmarks of Huntington's disease (HD), a fatal autosomal-dominant neurodegenerative disorder caused by a pathological expansion of CAG repeats in the Huntingtin (HTT) gene. The nucleolus, a dynamic nuclear biomolecular condensate and the site of ribosomal RNA (rRNA) transcription, is implicated in the cellular stress response and in protein quality control. While the exact pathomechanisms of HD are still unclear, the impact of nucleolar dysfunction on HD pathophysiology in vivo remains elusive. Here we identified aberrant maturation of rRNA and decreased translational rate in association with human mutant Huntingtin (mHTT) expression. The protein nucleophosmin 1 (NPM1), important for nucleolar integrity and rRNA maturation, loses its prominent nucleolar localization. Genetic disruption of nucleolar integrity in vulnerable striatal neurons of the R6/2 HD mouse model decreases the distribution of mHTT in a disperse state in the nucleus, exacerbating motor deficits. We confirmed NPM1 delocalization in the gradually progressing zQ175 knock-in HD mouse model: in the striatum at a presymptomatic stage and in the skeletal muscle at an early symptomatic stage. In Huntington's patient skeletal muscle biopsies, we found a selective redistribution of NPM1, similar to that in the zQ175 model. Taken together, our study demonstrates that nucleolar integrity regulates the formation of mHTT inclusions in vivo, and identifies NPM1 as a novel, readily detectable peripheral histopathological marker of HD progression.

Regulation of mRNA translation in stem cells; links to brain disorders

Translational control of gene expression is emerging as a cardinal step in the regulation of protein abundance. Especially for embryonic (ESC) and neuronal stem cells (NSC), regulation of mRNA translation is involved in the maintenance of pluripotency but also differentiation. For neuronal stem cells this regulation is linked to the various neuronal subtypes that arise in the developing brain and is linked to numerous brain disorders. Herein, we review translational control mechanisms in ESCs and NSCs during development and differentiation, and briefly discuss their link to brain disorders.

Mammalian/mechanistic target of rapamycin (mTOR) complexes in neurodegeneration

Novel targets to arrest neurodegeneration in several dementing conditions involving misfolded protein accumulations may be found in the diverse signaling pathways of the Mammalian/mechanistic target of rapamycin (mTOR). As a nutrient sensor, mTOR has important homeostatic functions to regulate energy metabolism and support neuronal growth and plasticity. However, in Alzheimer's disease (AD), mTOR alternately plays important pathogenic roles by inhibiting both insulin signaling and autophagic removal of β-amyloid (Aβ) and phospho-tau (ptau) aggregates. It also plays a role in the cerebrovascular dysfunction of AD. mTOR is a serine/threonine kinase residing at the core in either of two multiprotein complexes termed mTORC1 and mTORC2. Recent data suggest that their balanced actions also have implications for Parkinson's disease (PD) and Huntington's disease (HD), Frontotemporal dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). Beyond rapamycin; an mTOR inhibitor, there are rapalogs having greater tolerability and micro delivery modes, that hold promise in arresting these age dependent conditions.

Huntington's disease brain-derived small RNAs recapitulate associated neuropathology in mice

Progressive motor alterations and selective death of striatal medium spiny neurons (MSNs) are key pathological hallmarks of Huntington's disease (HD), a neurodegenerative condition caused by a CAG trinucleotide repeat expansion in the coding region of the huntingtin (HTT) gene. Most research has focused on the pathogenic effects of the resultant protein product(s); however, growing evidence indicates that expanded CAG repeats within mutant HTT mRNA and derived small CAG repeat RNAs (sCAG) participate in HD pathophysiology. The individual contribution of protein versus RNA toxicity to HD pathophysiology remains largely uncharacterized and the role of other classes of small RNAs (sRNA) that are strongly perturbed in HD is uncertain. Here, we demonstrate that sRNA produced in the putamen of HD patients (HD-sRNA-PT) are sufficient to induce HD pathology in vivo. Mice injected with HD-sRNA-PT show motor abnormalities, decreased levels of striatal HD-related proteins, disruption of the indirect pathway, and strong transcriptional abnormalities, paralleling human HD pathology. Importantly, we show that the specific blockage of sCAG mitigates HD-sRNA-PT neurotoxicity only to a limited extent. This observation prompted us to identify other sRNA species enriched in HD putamen with neurotoxic potential. We detected high levels of tRNA fragments (tRFs) in HD putamen, and we validated the neurotoxic potential of an Alanine derived tRF in vitro. These results highlight that HD-sRNA-PT are neurotoxic, and suggest that multiple sRNA species contribute to striatal dysfunction and general transcriptomic changes, favoring therapeutic strategies based on the blockage of sRNA-mediated toxicity.

RNA Localization and Local Translation in Glia in Neurological and Neurodegenerative Diseases: Lessons from Neurons

Cell polarity is crucial for almost every cell in our body to establish distinct structural and functional domains. Polarized cells have an asymmetrical morphology and therefore their proteins need to be asymmetrically distributed to support their function. Subcellular protein distribution is typically achieved by localization peptides within the protein sequence. However, protein delivery to distinct cellular compartments can rely, not only on the transport of the protein itself but also on the transport of the mRNA that is then translated at target sites. This phenomenon is known as local protein synthesis. Local protein synthesis relies on the transport of mRNAs to subcellular domains and their translation to proteins at target sites by the also localized translation machinery. Neurons and glia specially depend upon the accurate subcellular distribution of their proteome to fulfil their polarized functions. In this sense, local protein synthesis has revealed itself as a crucial mechanism that regulates proper protein homeostasis in subcellular compartments. Thus, deregulation of mRNA transport and/or of localized translation can lead to neurological and neurodegenerative diseases. Local translation has been more extensively studied in neurons than in glia. In this review article, we will summarize the state-of-the art research on local protein synthesis in neuronal function and dysfunction, and we will discuss the possibility that local translation in glia and deregulation thereof contributes to neurological and neurodegenerative diseases.

Mutant Huntingtin stalls ribosomes and represses protein synthesis in a cellular model of Huntington disease

The polyglutamine expansion of huntingtin (mHTT) causes Huntington disease (HD) and neurodegeneration, but the mechanisms remain unclear. Here, we found that mHtt promotes ribosome stalling and suppresses protein synthesis in mouse HD striatal neuronal cells. Depletion of mHtt enhances protein synthesis and increases the speed of ribosomal translocation, while mHtt directly inhibits protein synthesis in vitro. Fmrp, a known regulator of ribosome stalling, is upregulated in HD, but its depletion has no discernible effect on protein synthesis or ribosome stalling in HD cells. We found interactions of ribosomal proteins and translating ribosomes with mHtt. High-resolution global ribosome footprint profiling (Ribo-Seq) and mRNA-Seq indicates a widespread shift in ribosome occupancy toward the 5' and 3' end and unique single-codon pauses on selected mRNA targets in HD cells, compared to controls. Thus, mHtt impedes ribosomal translocation during translation elongation, a mechanistic defect that can be exploited for HD therapeutics.

Neuron type-specific increase in lamin B1 contributes to nuclear dysfunction in Huntington's disease

Lamins are crucial proteins for nuclear functionality. Here, we provide new evidence showing that increased lamin B1 levels contribute to the pathophysiology of Huntington's disease (HD), a CAG repeat-associated neurodegenerative disorder. Through fluorescence-activated nuclear suspension imaging, we show that nucleus from striatal medium-sized spiny and CA1 hippocampal neurons display increased lamin B1 levels, in correlation with altered nuclear morphology and nucleocytoplasmic transport disruption. Moreover, ChIP-sequencing analysis shows an alteration of lamin-associated chromatin domains in hippocampal nuclei, accompanied by changes in chromatin accessibility and transcriptional dysregulation. Supporting lamin B1 alterations as a causal role in mutant huntingtin-mediated neurodegeneration, pharmacological normalization of lamin B1 levels in the hippocampus of the R6/1 mouse model of HD by betulinic acid administration restored nuclear homeostasis and prevented motor and cognitive dysfunction. Collectively, our work points increased lamin B1 levels as a new pathogenic mechanism in HD and provides a novel target for its intervention.

Lack of Annexin A6 Exacerbates Liver Dysfunction and Reduces Lifespan of Niemann-Pick Type C Protein-Deficient Mice

Niemann-Pick type C (NPC) disease is a lysosomal storage disorder characterized by cholesterol accumulation caused by loss-of-function mutations in the Npc1 gene. NPC disease primarily affects the brain, causing neuronal damage and affecting motor coordination. In addition, considerable liver malfunction in NPC disease is common. Recently, we found that the depletion of annexin A6 (ANXA6), which is most abundant in the liver and involved in cholesterol transport, ameliorated cholesterol accumulation in Npc1 mutant cells. To evaluate the potential contribution of ANXA6 in the progression of NPC disease, double-knockout mice (Npc1^-/-/Anxa6^-/-) were generated and examined for lifespan, neurologic and hepatic functions, as well as liver histology and ultrastructure. Interestingly, lack of ANXA6 in NPC1-deficient animals did not prevent the cerebellar degeneration phenotype, but further deteriorated their compromised hepatic functions and reduced their lifespan. Moreover, livers of Npc1^-/-/Anxa6^-/- mice contained a significantly elevated number of foam cells congesting the sinusoidal space, a feature commonly associated with inflammation. We hypothesize that ANXA6 deficiency in Npc1^-/- mice not only does not reverse neurologic and motor dysfunction, but further worsens overall liver function, exacerbating hepatic failure in NPC disease.

M2 cortex-dorsolateral striatum stimulation reverses motor symptoms and synaptic deficits in Huntington's disease

Huntington’s disease (HD) is a neurological disorder characterized by motor disturbances. HD pathology is most prominent in the striatum, the central hub of the basal ganglia. The cerebral cortex is the main striatal afferent, and progressive cortico-striatal disconnection characterizes HD. We mapped striatal network dysfunction in HD mice to ultimately modulate the activity of a specific cortico-striatal circuit to ameliorate motor symptoms and recover synaptic plasticity. Multimodal MRI in vivo indicates cortico-striatal and thalamo-striatal functional network deficits and reduced glutamate/glutamine ratio in the striatum of HD mice. Moreover, optogenetically-induced glutamate release from M2 cortex terminals in the dorsolateral striatum (DLS) was undetectable in HD mice and striatal neurons show blunted electrophysiological responses. Remarkably, repeated M2-DLS optogenetic stimulation normalized motor behavior in HD mice and evoked a sustained increase of synaptic plasticity. Overall, these results reveal that selective stimulation of the M2-DLS pathway can become an effective therapeutic strategy in HD.

Nitrosative stress inhibits aminoacylation and editing activities of mitochondrial threonyl-tRNA synthetase by S-nitrosation

Structure and/or function of proteins are frequently affected by oxidative/nitrosative stress via posttranslational modifications. Aminoacyl-tRNA synthetases (aaRSs) constitute a class of ubiquitously expressed enzymes that control cellular protein homeostasis. Here, we found the activity of human mitochondrial (mt) threonyl-tRNA synthetase (hmtThrRS) is resistant to oxidative stress (H₂O₂) but profoundly sensitive to nitrosative stress (S-nitrosoglutathione, GSNO). Further study showed four Cys residues in hmtThrRS were modified by S-nitrosation upon GSNO treatment, and one residue was one of synthetic active sites. We analyzed the effect of modification at individual Cys residue on aminoacylation and editing activities of hmtThrRS in vitro and found that both activities were decreased. We further confirmed that S-nitrosation of mtThrRS could be readily detected in vivo in both human cells and various mouse tissues, and we systematically identified dozens of S-nitrosation-modified sites in most aaRSs, thus establishing both mitochondrial and cytoplasmic aaRS species with S-nitrosation ex vivo and in vivo, respectively. Interestingly, a decrease in the S-nitrosation modification level of mtThrRS was observed in a Huntington disease mouse model. Overall, our results establish, for the first time, a comprehensive S-nitrosation-modified aaRS network and a previously unknown mechanism on the basis of the inhibitory effect of S-nitrosation on hmtThrRS.

Brain energy rescue: an emerging therapeutic concept for neurodegenerative disorders of ageing

The brain requires a continuous supply of energy in the form of ATP, most of which is produced from glucose by oxidative phosphorylation in mitochondria, complemented by aerobic glycolysis in the cytoplasm. When glucose levels are limited, ketone bodies generated in the liver and lactate derived from exercising skeletal muscle can also become important energy substrates for the brain. In neurodegenerative disorders of ageing, brain glucose metabolism deteriorates in a progressive, region-specific and disease-specific manner - a problem that is best characterized in Alzheimer disease, where it begins presymptomatically. This Review discusses the status and prospects of therapeutic strategies for countering neurodegenerative disorders of ageing by improving, preserving or rescuing brain energetics. The approaches described include restoring oxidative phosphorylation and glycolysis, increasing insulin sensitivity, correcting mitochondrial dysfunction, ketone-based interventions, acting via hormones that modulate cerebral energetics, RNA therapeutics and complementary multimodal lifestyle changes.

Synaptic RTP801 contributes to motor-learning dysfunction in Huntington's disease

RTP801/REDD1 is a stress-responsive protein that mediates mutant huntingtin (mhtt) toxicity in cellular models and is up regulated in Huntington's disease (HD) patients' putamen. Here, we investigated whether RTP801 is involved in motor impairment in HD by affecting striatal synaptic plasticity. To explore this hypothesis, ectopic mhtt was over expressed in cultured rat primary neurons. Moreover, the protein levels of RTP801 were assessed in homogenates and crude synaptic fractions from human postmortem HD brains and mouse models of HD. Finally, striatal RTP801 expression was knocked down with adeno-associated viral particles containing a shRNA in the R6/1 mouse model of HD and motor learning was then tested. Ectopic mhtt elevated RTP801 in synapses of cultured neurons. RTP801 was also up regulated in striatal synapses from HD patients and mouse models. Knocking down RTP801 in the R6/1 mouse striatum prevented motor-learning impairment. RTP801 silencing normalized the Ser473 Akt hyperphosphorylation by downregulating Rictor and it induced synaptic elevation of calcium permeable GluA1 subunit and TrkB receptor levels, suggesting an enhancement in synaptic plasticity. These results indicate that mhtt-induced RTP801 mediates motor dysfunction in a HD murine model, revealing a potential role in the human disease. These findings open a new therapeutic framework focused on the RTP801/Akt/mTOR axis.

Reduced Fractalkine Levels Lead to Striatal Synaptic Plasticity Deficits in Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder in which the striatum is the most affected brain region. Although a chronic inflammatory microglial reaction that amplifies disease progression has been described in HD patients, some murine models develop symptoms without inflammatory microglial activation. Thus, dysfunction of non-inflammatory microglial activity could also contribute to the early HD pathological process. Here, we show the involvement of microglia and particularly fractalkine signaling in the striatal synaptic dysfunction of R6/1 mice. We found reduced fractalkine gene expression and protein concentration in R6/1 striata from 8 to 20 weeks of age. Consistently, we also observed a down-regulation of fractalkine levels in the putamen of HD patients and in HD patient hiPSC-derived neurons. Automated cell morphology analysis showed a non-inflammatory ramified microglia in the striatum of R6/1 mice. However, we found increased PSD-95-positive puncta inside microglia, indicative of synaptic pruning, before HD motor symptoms start to manifest. Indeed, microglia appeared to be essential for striatal synaptic function, as the inhibition of microglial activity with minocycline impaired the induction of corticostriatal long-term depression (LTD) in wild-type mice. Notably, fractalkine administration restored impaired corticostriatal LTD in R6/1 mice. Our results unveil a role for fractalkine-dependent neuron-microglia interactions in the early striatal synaptic dysfunction characteristic of HD.

The science of puromycin: From studies of ribosome function to applications in biotechnology

Puromycin is a naturally occurring aminonucleoside antibiotic that inhibits protein synthesis by ribosome-catalyzed incorporation into the C-terminus of elongating nascent chains, blocking further extension and resulting in premature termination of translation. It is most commonly known as a selection marker for cell lines genetically engineered to express a resistance transgene, but its additional uses as a probe for protein synthesis have proven invaluable across a wide variety of model systems, ranging from purified ribosomes and cell-free translation to intact cultured cells and whole animals. Puromycin is comprised of a nucleoside covalently bound to an amino acid, mimicking the 3′ end of aminoacylated tRNAs that participate in delivery of amino acids to elongating ribosomes. Both moieties can tolerate some chemical substitutions and modifications without significant loss of activity, generating a diverse toolbox of puromycin-based reagents with added functionality, such as biotin for affinity purification or fluorophores for fluorescent microscopy detection. These reagents, as well as anti-puromycin antibodies, have played a pivotal role in advancing our understanding of the regulation and dysregulation of protein synthesis in normal and pathological processes, including immune response and neurological function. This manuscript reviews the current state of puromycin-based research, including structure and mechanism of action, relevant derivatives, use in advanced methodologies and some of the major insights generated using such techniques both in the lab and the clinic.

What is gained or 'lost in translation' in Huntington's disease

This scientific commentary refers to ‘Increased translation as a novel pathogenic mechanism in Huntington’s disease’, by Jordi Creus-Muncunill et al. (doi:10.1093/brain/awz230).