Neuronal and astrocytic contributions to Huntington's disease dissected with zinc finger protein transcriptional repressors

Long somatic DNA-repeat expansion drives neurodegeneration in Huntington's disease

In Huntington's disease (HD), striatal projection neurons (SPNs) degenerate during midlife; the core biological question involves how the disease-causing DNA repeat (CAG)n in the huntingtin (HTT) gene leads to neurodegeneration after decades of biological latency. We developed a single-cell method for measuring this repeat's length alongside genome-wide RNA expression. We found that the HTT CAG repeat expands somatically from 40-45 to 100-500+ CAGs in SPNs. Somatic expansion from 40 to 150 CAGs had no apparent cell-autonomous effect, but SPNs with 150-500+ CAGs lost positive and then negative features of neuronal identity, de-repressed senescence/apoptosis genes, and were lost. Our results suggest that somatic repeat expansion beyond 150 CAGs causes SPNs to degenerate quickly and asynchronously. We conclude that in HD, at any one time, most neurons have an innocuous but unstable HTT gene and that HD pathogenesis is a DNA process for almost all of a neuron's life.

Homeostasis and metabolism of iron and other metal ions in neurodegenerative diseases

As essential micronutrients, metal ions such as iron, manganese, copper, and zinc, are required for a wide range of physiological processes in the brain. However, an imbalance in metal ions, whether excessive or insufficient, is detrimental and can contribute to neuronal death through oxidative stress, ferroptosis, cuproptosis, cell senescence, or neuroinflammation. These processes have been found to be involved in the pathological mechanisms of neurodegenerative diseases. In this review, the research history and milestone events of studying metal ions, including iron, manganese, copper, and zinc in neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), will be introduced. Then, the upstream regulators, downstream effector, and crosstalk of mental ions under both physiologic and pathologic conditions will be summarized. Finally, the therapeutic effects of metal ion chelators, such as clioquinol, quercetin, curcumin, coumarin, and their derivatives for the treatment of neurodegenerative diseases will be discussed. Additionally, the promising results and limitations observed in clinical trials of these metal ion chelators will also be addressed. This review will not only provide a comprehensive understanding of the role of metal ions in disease development but also offer perspectives on their modulation for the prevention or treatment of neurodegenerative diseases.

Metabolic dysregulation in Huntington's disease: Neuronal and glial perspectives

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutant huntingtin protein with an abnormal CAG/polyQ expansion in the N-terminus of HTT exon 1. HD is characterized by progressive neurodegeneration and metabolic abnormalities, particularly in the brain, which accounts for approximately 20 % of the body's resting metabolic rate. Dysregulation of energy homeostasis in HD includes impaired glucose transporters, abnormal functions of glycolytic enzymes, changes in tricarboxylic acid (TCA) cycle activity and enzyme expression in the basal ganglia and cortical regions of both HD mouse models and HD patients. However, current understanding of brain cell behavior during energy dysregulation and its impact on neuron-glia crosstalk in HD remains limited. This review provides a comprehensive summary of the current understanding of the differences in glucose metabolism between neurons and glial cells in HD and how these differences contribute to disease development compared with normal conditions. We also discuss the potential impact of metabolic shifts on neuron-glia communication in HD. A deeper understanding of these metabolic alterations may reveal potential therapeutic targets for future drug development.

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.

Multi-omic analysis of Huntington's disease reveals a compensatory astrocyte state

The mechanisms underlying the selective regional vulnerability to neurodegeneration in Huntington's disease (HD) have not been fully defined. To explore the role of astrocytes in this phenomenon, we used single-nucleus and bulk RNAseq, lipidomics, HTT gene CAG repeat-length measurements, and multiplexed immunofluorescence on HD and control post-mortem brains. We identified genes that correlated with CAG repeat length, which were enriched in astrocyte genes, and lipidomic signatures that implicated poly-unsaturated fatty acids in sensitizing neurons to cell death. Because astrocytes play essential roles in lipid metabolism, we explored the heterogeneity of astrocytic states in both protoplasmic and fibrous-like (CD44+) astrocytes. Significantly, one protoplasmic astrocyte state showed high levels of metallothioneins and was correlated with the selective vulnerability of distinct striatal neuronal populations. When modeled in vitro, this state improved the viability of HD-patient-derived spiny projection neurons. Our findings uncover key roles of astrocytic states in protecting against neurodegeneration in HD.

Immunological aspects of central neurodegeneration

The etiology of various neurodegenerative disorders that mainly affect the central nervous system including (but not limited to) Alzheimer's disease, Parkinson's disease and Huntington's disease has classically been attributed to neuronal defects that culminate with the loss of specific neuronal populations. However, accumulating evidence suggests that numerous immune effector cells and the products thereof (including cytokines and other soluble mediators) have a major impact on the pathogenesis and/or severity of these and other neurodegenerative syndromes. These observations not only add to our understanding of neurodegenerative conditions but also imply that (at least in some cases) therapeutic strategies targeting immune cells or their products may mediate clinically relevant neuroprotective effects. Here, we critically discuss immunological mechanisms of central neurodegeneration and propose potential strategies to correct neurodegeneration-associated immunological dysfunction with therapeutic purposes.

Astrocytes in selective vulnerability to neurodegenerative disease

Selective vulnerability of specific brain regions and cell populations is a hallmark of neurodegenerative disorders. Mechanisms of selective vulnerability involve neuronal heterogeneity, functional specializations, and differential sensitivities to stressors and pathogenic factors. In this review we discuss the growing body of literature suggesting that, like neurons, astrocytes are heterogeneous and specialized, respond to and integrate diverse inputs, and induce selective effects on brain function. In disease, astrocytes undergo specific, context-dependent changes that promote different pathogenic trajectories and functional outcomes. We propose that astrocytes contribute to selective vulnerability through maladaptive transitions to context-divergent phenotypes that impair specific brain regions and functions. Further studies on the multifaceted roles of astrocytes in disease may provide new therapeutic approaches to enhance resilience against neurodegenerative disorders.

A concerted neuron-astrocyte program declines in ageing and schizophrenia

Human brains vary across people and over time; such variation is not yet understood in cellular terms. Here we describe a relationship between people's cortical neurons and cortical astrocytes. We used single-nucleus RNA sequencing to analyse the prefrontal cortex of 191 human donors aged 22-97 years, including healthy individuals and people with schizophrenia. Latent-factor analysis of these data revealed that, in people whose cortical neurons more strongly expressed genes encoding synaptic components, cortical astrocytes more strongly expressed distinct genes with synaptic functions and genes for synthesizing cholesterol, an astrocyte-supplied component of synaptic membranes. We call this relationship the synaptic neuron and astrocyte program (SNAP). In schizophrenia and ageing-two conditions that involve declines in cognitive flexibility and plasticity1,2-cells divested from SNAP: astrocytes, glutamatergic (excitatory) neurons and GABAergic (inhibitory) neurons all showed reduced SNAP expression to corresponding degrees. The distinct astrocytic and neuronal components of SNAP both involved genes in which genetic risk factors for schizophrenia were strongly concentrated. SNAP, which varies quantitatively even among healthy people of similar age, may underlie many aspects of normal human interindividual differences and may be an important point of convergence for multiple kinds of pathophysiology.

Astrocyte morphogenesis requires self-recognition

Self-recognition is a fundamental cellular process across evolution and forms the basis of neuronal self-avoidance1-4. Clustered protocadherins (Pcdh), comprising a large family of isoform-specific homophilic recognition molecules, play a pivotal role in neuronal self-avoidance required for mammalian brain development5-7. The probabilistic expression of different Pcdh isoforms confers unique identities upon neurons and forms the basis for neuronal processes to discriminate between self and non-self5,6,8. Whether this self-recognition mechanism exists in astrocytes, the other predominant cell type of the brain, remains unknown. Here, we report that a specific isoform in the Pcdhγ cluster, γC3, is highly enriched in human and murine astrocytes. Through genetic manipulation, we demonstrate that γC3 acts autonomously to regulate astrocyte morphogenesis in the mouse visual cortex. To determine if γC3 proteins act by promoting recognition between processes of the same astrocyte, we generated pairs of γC3 chimeric proteins capable of heterophilic binding to each other, but incapable of homophilic binding. Co-expressing complementary heterophilic binding isoform pairs in the same γC3 null astrocyte restored normal morphology. By contrast, chimeric γC3 proteins individually expressed in single γC3 null mutant astrocytes did not. These data establish that self-recognition is essential for astrocyte development in the mammalian brain and that, by contrast to neuronal self-recognition, a single Pcdh isoform is both necessary and sufficient for this process.

Concerted neuron-astrocyte gene expression declines in aging and schizophrenia

Human brains vary across people and over time; such variation is not yet understood in cellular terms. Here we describe a striking relationship between people's cortical neurons and cortical astrocytes. We used single-nucleus RNA-seq to analyze the prefrontal cortex of 191 human donors ages 22-97 years, including healthy individuals and persons with schizophrenia. Latent-factor analysis of these data revealed that in persons whose cortical neurons more strongly expressed genes for synaptic components, cortical astrocytes more strongly expressed distinct genes with synaptic functions and genes for synthesizing cholesterol, an astrocyte-supplied component of synaptic membranes. We call this relationship the Synaptic Neuron-and-Astrocyte Program (SNAP). In schizophrenia and aging - two conditions that involve declines in cognitive flexibility and plasticity 1,2 - cells had divested from SNAP: astrocytes, glutamatergic (excitatory) neurons, and GABAergic (inhibitory) neurons all reduced SNAP expression to corresponding degrees. The distinct astrocytic and neuronal components of SNAP both involved genes in which genetic risk factors for schizophrenia were strongly concentrated. SNAP, which varies quantitatively even among healthy persons of similar age, may underlie many aspects of normal human interindividual differences and be an important point of convergence for multiple kinds of pathophysiology.

An enquiry to the role of CB1 receptors in neurodegeneration

Neurodegenerative disorders are debilitating conditions that impair patient quality of life and that represent heavy social-economic burdens to society. Whereas the root of some of these brain illnesses lies in autosomal inheritance, the origin of most of these neuropathologies is scantly understood. Similarly, the cellular and molecular substrates explaining the progressive loss of brain functions remains to be fully described too. Indeed, the study of brain neurodegeneration has resulted in a complex picture, composed of a myriad of altered processes that include broken brain bioenergetics, widespread neuroinflammation and aberrant activity of signaling pathways. In this context, several lines of research have shown that the endocannabinoid system (ECS) and its main signaling hub, the type-1 cannabinoid (CB1) receptor are altered in diverse neurodegenerative disorders. However, some of these data are conflictive or poorly described. In this review, we summarize the findings about the alterations in ECS and CB1 receptors signaling in three representative brain illnesses, the Alzheimer's, Parkinson's and Huntington's diseases, and we discuss the relevance of these studies in understanding neurodegeneration development and progression, with a special focus on astrocyte function. Noteworthy, the analysis of ECS defects in neurodegeneration warrant much more studies, as our conceptual understanding of ECS function has evolved quickly in the last years, which now include glia cells and the subcellular-specific CB1 receptors signaling as critical players of brain functions.

Cell-Type Specific Regulation of Cholesterogenesis by CYP46A1 Re-Expression in zQ175 HD Mouse Striatum

Cholesterol metabolism dysregulation is associated with several neurological disorders. In Huntington's disease (HD), several enzymes involved in cholesterol metabolism are downregulated, among which the neuronal cholesterol 24-hydroxylase, CYP46A1, is of particular interest. The restoration of CYP46A1 expression in striatal neurons of HD mouse models is beneficial for motor behavior, cholesterol metabolism, transcriptomic activity, and alleviates neuropathological hallmarks induced by mHTT. Among the genes regulated after CYP46A1 restoration, those involved in cholesterol synthesis and efflux may explain the positive effect of CYP46A1 on cholesterol precursor metabolites. Since cholesterol homeostasis results from a fine-tuning between neurons and astrocytes, we quantified the distribution of key genes regulating cholesterol metabolism and efflux in astrocytes and neurons using in situ hybridization coupled with S100β and NeuN immunostaining, respectively. Neuronal expression of CYP46A1 in the striatum of HD zQ175 mice increased key cholesterol synthesis driver genes (Hmgcr, Dhcr24), specifically in neurons. This effect was associated with an increase of the srebp2 transcription factor gene that regulates most of the genes encoding for cholesterol enzymes. However, the cholesterol efflux gene, ApoE, was specifically upregulated in astrocytes by CYP46A1, probably though a paracrine effect. In summary, the neuronal expression of CYP46A1 has a dual and specific effect on neurons and astrocytes, regulating cholesterol metabolism. The neuronal restoration of CYP46A1 in HD paves the way for future strategies to compensate for mHTT toxicity.

Striatal spatial heterogeneity, clustering, and white matter association of GFAP+ astrocytes in a mouse model of Huntington's disease

Introduction

Huntington's disease (HD) is a neurodegenerative disease that primarily affects the striatum, a brain region that controls movement and some forms of cognition. Neuronal dysfunction and loss in HD is accompanied by increased astrocyte density and astrocyte pathology. Astrocytes are a heterogeneous population classified into multiple subtypes depending on the expression of different gene markers. Studying whether mutant Huntingtin (HTT) alters specific subtypes of astrocytes is necessary to understand their relative contribution to HD.

Methods

Here, we studied whether astrocytes expressing two different markers; glial fibrillary acidic protein (GFAP), associated with astrocyte activation, and S100 calcium-binding protein B (S100B), a marker of matured astrocytes and inflammation, were differentially altered in HD.

Results

First, we found three distinct populations in the striatum of WT and symptomatic zQ175 mice: GFAP+, S100B+, and dual GFAP+S100B+. The number of GFAP+ and S100B+ astrocytes throughout the striatum was increased in HD mice compared to WT, coinciding with an increase in HTT aggregation. Overlap between GFAP and S100B staining was expected, but dual GFAP+S100B+ astrocytes only accounted for less than 10% of all tested astrocytes and the number of GFAP+S100B+ astrocytes did not differ between WT and HD, suggesting that GFAP+ astrocytes and S100B+ astrocytes are distinct types of astrocytes. Interestingly, a spatial characterization of these astrocyte subtypes in HD mice showed that while S100B+ were homogeneously distributed throughout the striatum, GFAP+ preferentially accumulated in "patches" in the dorsomedial (dm) striatum, a region associated with goal-directed behaviors. In addition, GFAP+ astrocytes in the dm striatum of zQ175 mice showed increased clustering and association with white matter fascicles and were preferentially located in areas with low HTT aggregate load.

Discussion

In summary, we showed that GFAP+ and S100B+ astrocyte subtypes are distinctly affected in HD and exist in distinct spatial arrangements that may offer new insights to the function of these specific astrocytes subtypes and their potential implications in HD pathology.

Astrocytic contributions to Huntington's disease pathophysiology

Huntington's disease (HD) is a fatal, monogenic, autosomal dominant neurodegenerative disease caused by a polyglutamine-encoding CAG expansion in the huntingtin (HTT) gene that results in mutant huntingtin proteins (mHTT) in cells throughout the body. Although large parts of the central nervous system (CNS) are affected, the striatum is especially vulnerable and undergoes marked atrophy. Astrocytes are abundant within the striatum and contain mHTT in HD, as well as in mouse models of the disease. We focus on striatal astrocytes and summarize how they participate in, and contribute to, molecular pathophysiology and disease-related phenotypes in HD model mice. Where possible, reference is made to pertinent astrocyte alterations in human HD. Astrocytic dysfunctions related to cellular morphology, extracellular ion and neurotransmitter homeostasis, and metabolic support all accompany the development and progression of HD, in both transgenic mouse and human cellular and chimeric models of HD. These findings reveal the potential for the therapeutic targeting of astrocytes so as to restore synaptic as well as tissue homeostasis in HD. Elucidation of the mechanisms by which astrocytes contribute to HD pathogenesis may inform a broader understanding of the role of glial pathology in neurodegenerative disorders and, by so doing, enable new strategies of glial-directed therapeutics.